This editorial summarizes advances from the Clearance of Interstitial Fluid and Cerebrospinal Fluid (CLIC) group, within the Vascular Professional Interest Area (PIA) of the Alzheimer's Association International Society to Advance Alzheimer's Research and Treatment (ISTAART). The overarching objectives of the CLIC group are to: (1) understand the age-related physiology changes that underlie impaired clearance of interstitial fluid (ISF) and cerebrospinal fluid (CSF) (CLIC); (2) understand the cellular and molecular mechanisms underlying intramural periarterial drainage (IPAD) in the brain; (3) establish novel diagnostic tests for Alzheimer's disease (AD), cerebral amyloid angiopathy (CAA), retinal amyloid vasculopathy, amyloid-related imaging abnormalities (ARIA) of spontaneous and iatrogenic CAA-related inflammation (CAA-ri), and vasomotion; and (4) establish novel therapies that facilitate IPAD to eliminate amyloid β (Aβ) from the aging brain and retina, to prevent or reduce AD and CAA pathology and ARIA side events associated with AD immunotherapy.

Neurofluids is a term introduced to define all fluids in the brain and spine such as blood, cerebrospinal fluid, and interstitial fluid. Neuroscientists in the past millennium have steadily identified the several different fluid environments in the brain and spine that interact in a synchronized harmonious manner to assure a healthy microenvironment required for optimal neuroglial function. Neuroanatomists and biochemists have provided an incredible wealth of evidence revealing the anatomy of perivascular spaces, meninges and glia and their role in drainage of neuronal waste products. Human studies have been limited due to the restricted availability of noninvasive imaging modalities that can provide a high spatiotemporal depiction of the brain neurofluids. Therefore, animal studies have been key in advancing our knowledge of the temporal and spatial dynamics of fluids, for example, by injecting tracers with different molecular weights. Such studies have sparked interest to identify possible disruptions to neurofluids dynamics in human diseases such as small vessel disease, cerebral amyloid angiopathy, and dementia. However, key differences between rodent and human physiology should be considered when extrapolating these findings to understand the human brain. An increasing armamentarium of noninvasive MRI techniques is being built to identify markers of altered drainage pathways. During the three-day workshop organized by the International Society of Magnetic Resonance in Medicine that was held in Rome in September 2022, several of these concepts were discussed by a distinguished international faculty to lay the basis of what is known and where we still lack evidence. We envision that in the next decade, MRI will allow imaging of the physiology of neurofluid dynamics and drainage pathways in the human brain to identify true pathological processes underlying disease and to discover new avenues for early diagnoses and treatments including drug delivery. Evidence level: 1 Technical Efficacy: Stage 3.

Amyloid beta (Aβ) deposition within the brain vasculature is an early hallmark of Alzheimer's disease (AD), which triggers loss of brain vascular smooth muscle cells (BVSMCs) in cerebral arteries, via poorly understood mechanisms, altering cerebral blood flow, brain waste clearance, and promoting cognitive impairment. We have previously shown that, in brain endothelial cells (ECs), vasculotropic Aβ species induce apoptosis through death receptors (DRs) DR4 and DR5 and mitochondria-mediated mechanisms, while FDA-approved carbonic anhydrase inhibitors (CAIs) prevent mitochondria-mediated EC apoptosis in vitro and in vivo. In this study, we analyzed Aβ-induced extrinsic and intrinsic (DR- and mitochondria-mediated) apoptotic pathways in BVSMC, aiming to unveil new therapeutic targets to prevent BVSMC stress and death. We show that both apoptotic pathways are activated in BVSMCs by oligomeric Aβ42 and and mitochondrial respiration is severely impaired. Importantly, the CAIs methazolamide (MTZ) and acetazolamide (ATZ) prevent the pro-apoptotic effects in BVSMCs, while reducing caspase 3 activation and Aβ deposition in the arterial walls of TgSwDI animals, a murine model of cerebral amyloid angiopathy (CAA). This study reveals new molecular targets and a promising therapeutic strategy against BVSMC dysfunction in AD, CAA, and ARIA (amyloid-related imaging abnormalities) complications of recently FDA-approved anti-Aβ antibodies.

Cerebral microbleeds (CMBs) influence long-term prognoses of stroke patients. Streptococcus mutans expressing the collagen-binding protein Cnm induces cerebrovascular inflammation, impairing blood brain barrier integrity and causing cerebral bleeding. Here, we examine the association of Cnm-positive S. mutans with CMBs.

Circulating C-reactive protein (pCRP) concentrations rise dramatically during both acute (e.g., following stroke) or chronic infection and disease (e.g., autoimmune conditions such as lupus), providing complement fixation through C1q protein binding. It is now known, that on exposure to the membranes of activated immune cells (and microvesicles and platelets), or damaged/dysfunctional tissue, it undergoes lysophosphocholine (LPC)-phospholipase-C-dependent dissociation to the monomeric form (mCRP), concomitantly becoming biologically active. We review histological, immunohistochemical, and morphological/topological studies of post-mortem brain tissue from individuals with neuroinflammatory disease, showing that mCRP becomes stably distributed within the parenchyma, and resident in the arterial intima and lumen, being "released" from damaged, hemorrhagic vessels into the extracellular matrix. The possible de novo synthesis via neurons, endothelial cells, and glia is also considered. In vitro, in vivo, and human tissue co-localization analyses have linked mCRP to neurovascular dysfunction, vascular activation resulting in increased permeability, and leakage, compromise of blood brain barrier function, buildup of toxic proteins including tau and beta amyloid (Aβ), association with and capacity to "manufacture" Aβ-mCRP-hybrid plaques, and, greater susceptibility to neurodegeneration and dementia. Recently, several studies linked chronic CRP/mCRP systemic expression in autoimmune disease with increased risk of dementia and the mechanisms through which this occurs are investigated here. The neurovascular unit mediates correct intramural periarterial drainage, evidence is provided here that suggests a critical impact of mCRP on neurovascular elements that could suggest its participation in the earliest stages of dysfunction and conclude that further investigation is warranted. We discuss future therapeutic options aimed at inhibiting the pCRP-LPC mediated dissociation associated with brain pathology, for example, compound 1,6-bis-PC, injected intravenously, prevented mCRP deposition and associated damage, after temporary left anterior descending artery ligation and myocardial infarction in a rat model.

Vascular amyloid beta (Aβ) protein deposits were detected in retinas of mild cognitively impaired (MCI) and Alzheimer's disease (AD) patients. We tested the hypothesis that the retinal vascular tight junctions (TJs) were compromised and linked to disease status.

One of the most complex and challenging developments at the beginning of the third millennium is the alarming increase in demographic aging, mainly-but not exclusively-affecting developed countries. This reality results in one of the harsh medical, social, and economic consequences: the continuously increasing number of people with dementia, including Alzheimer's disease (AD), which accounts for up to 80% of all such types of pathology. Its large and progressive disabling potential, which eventually leads to death, therefore represents an important public health matter, especially because there is no known cure for this disease. Consequently, periodic reappraisals of different therapeutic possibilities are necessary. For this purpose, we conducted this systematic literature review investigating nonpharmacological interventions for AD, including their currently known cellular and molecular action bases. This endeavor was based on the PRISMA method, by which we selected 116 eligible articles published during the last year. Because of the unfortunate lack of effective treatments for AD, it is necessary to enhance efforts toward identifying and improving various therapeutic and rehabilitative approaches, as well as related prophylactic measures.

The Fugl-Meyer Assessment (FMA) scale, which is widely used and highly recommended, is an appropriate tool for evaluating poststroke sensorimotor and other possible somatic deficits. It is also well-suited for capturing a dynamic rehabilitation process. The aim of this study was to first translate the entire sensorimotor FMA scale into Romanian using the transcultural and semantic-linguistic adaptations of its official afferent protocols and to validate it using the preliminary clinical evaluation of inter- and intra-rater reliability and relevant concurrent validity.

Alzheimer's disease is the commonest form of dementia. It is likely that a lack of clearance of amyloid beta (Aβ) results in its accumulation in the parenchyma as Aβ oligomers and insoluble plaques, and within the walls of blood vessels as cerebral amyloid angiopathy (CAA). The drainage of Aβ along the basement membranes of blood vessels as intramural periarterial drainage (IPAD), could be improved if the driving force behind IPAD could be augmented, therefore reducing Aβ accumulation. There are alterations in the composition of the vascular basement membrane in Alzheimer's disease. Lysyl oxidase (LOX) is an enzyme involved in the remodelling of the extracellular matrix and its expression and function is altered in various disease states. The expression of LOX is increased in Alzheimer's disease, but it is unclear whether this is a contributory factor in the impairment of IPAD in Alzheimer's disease. The pharmacological inhibition of LOX may be a strategy to improve IPAD and reduce the accumulation of Aβ in the parenchyma and within the walls of blood vessels.

Interstitial fluid (ISF) from brain drains along the basement membranes of capillaries and arteries as Intramural Periarterial Drainage (IPAD); failure of IPAD results in cerebral amyloid angiopathy (CAA). In this study, we test the hypothesis that IPAD fails after subarachnoid haemorrhage (SAH). The rat SAH model was established using endovascular perforation method. Fluorescence dyes with various molecular weights were injected into cisterna magna of rats, and the pattern of IPAD after SAH was detected using immunofluorescence staining, two-photon fluorescent microscope, transmission electron microscope and magnetic resonance imaging tracking techniques. Our results showed that fluorescence dyes entered the brain along a periarterial compartment and were cleared from brain along the basement membranes of the capillaries, with different patterns based on individual molecular weights. After SAH, there was significant impairment in the IPAD system: marked expansion of perivascular spaces, and ISF clearance rate was significantly decreased, associated with the apoptosis of endothelial cells, activation of astrocytes, over-expression of matrix metalloproteinase 9 and loss of collagen type IV. In conclusion, experimental SAH leads to a failure of IPAD, clinically significant for long term complications such as CAA, following SAH.

Cerebrospinal fluid (CSF), predominantly produced in the ventricles and circulating throughout the brain and spinal cord, is a key protective mechanism of the central nervous system (CNS). Physical cushioning, nutrient delivery, metabolic waste, including protein clearance, are key functions of the CSF in humans. CSF volume and flow dynamics regulate intracranial pressure and are fundamental to diagnosing disorders including normal pressure hydrocephalus, intracranial hypotension, CSF leaks, and possibly Alzheimer's disease (AD). The ability of CSF to clear normal and pathological proteins, such as amyloid-beta (Aβ), tau, alpha synuclein and others, implicates it production, circulation, and composition, in many neuropathologies. Several neuroimaging modalities have been developed to probe CSF fluid dynamics and better relate CSF volume and flow to anatomy and clinical conditions. Approaches include 2-photon microscopic techniques, MRI (tracer-based, gadolinium contrast, endogenous phase-contrast), and dynamic positron emission tomography (PET) using existing approved radiotracers. Here, we discuss CSF flow neuroimaging, from animal models to recent clinical-research advances, summarizing current endeavors to quantify and map CSF flow with implications towards pathophysiology, new biomarkers, and treatments of neurological diseases.

α-Dystrobrevin (α-DB) is a major component of the dystrophin-associated protein complex (DAPC). Knockout (KO) of α-DB in the brain is associated with astrocytic abnormalities and loss of neuronal GABA receptor clustering. Mutations in DAPC proteins are associated with altered dopamine signaling and cognitive and psychiatric disorders, including schizophrenia. This study tested the hypothesis that motivation and associated underlying biological pathways are altered in the absence of α-DB expression. Male wildtype and α-DB KO mice were tested for measures of motivation, executive function and extinction in the rodent touchscreen apparatus. Subsequently, brain tissues were evaluated for mRNA and/or protein levels of dysbindin-1, dopamine transporter and receptor 1 and 2, mu opioid receptor 1 (mOR1) and cannabinoid receptor 1 (CB1). α-DB KO mice had significantly increased motivation for the appetitive reward, while measures of executive function and extinction were unaffected. No differences were observed between wildtype and KO animals on mRNA levels of dysbindin-1 or any of the dopamine markers. mRNA levels of mOR1were significantly decreased in the caudate-putamen and nucleus accumbens of α-DB KO compared to WT animals, but protein levels were unaltered. However, CB1 protein levels were significantly increased in the prefrontal cortex and decreased in the nucleus accumbens of α-DB KO mice. Triple-labelling immunohistochemistry confirmed that changes in CB1 were not specific to astrocytes. These results highlight a novel role for α-DB in the regulation of appetitive motivation that may have implications for other behaviours that involve the dopaminergic and endocannabinoid systems.

In sporadic Alzheimer's disease (AD), brain amyloid-beta (Aβ) deposition is believed to be a consequence of impaired Aβ clearance, but this relationship is not well established in living humans. CSF clearance, a major feature of brain glymphatic clearance (BGC), has been shown to be abnormal in AD murine models. MRI phase contrast and intrathecally delivered contrast studies have reported reduced CSF flow in AD. Using PET and tau tracer F-THK5117, we previously reported that the ventricular CSF clearance of the PET tracer was reduced in AD and associated with elevated brain Aβ levels.

Alpha synuclein has a key role in the pathogenesis of Parkinson's disease (PD), Dementia with Lewy Bodies (LBD) and Multiple System Atrophy (MSA). Immunotherapies aiming at neutralising toxic αSyn species are being investigated in the clinic as potential disease modifying therapies for PD and other synucleinopathies. In this study, the effects of active immunisation against αSyn with the UB-312 vaccine were investigated in the Thy1SNCA/15 mouse model of PD. Young transgenic and wild-type mice received an immunisation regimen over a period of 6 weeks, then observed for an additional 9 weeks. Behavioural assessment was conducted before immunisation and at 15 weeks after the first dose. UB-312 immunisation prevented the development of motor impairment in the wire test and challenging beam test, which was associated with reduced levels of αSyn oligomers in the cerebral cortex, hippocampus and striatum of Thy1SNCA/15 mice. UB-312 immunotherapy resulted in a significant reduction of theαSyn load in the colon, accompanied by a reduction in enteric glial cell reactivity in the colonic ganglia. Our results demonstrate that immunisation with UB-312 prevents functional deficits and both central and peripheral pathology in Thy1SNCA/15 mice.

Perivascular spaces (PVS) are fluid-filled compartments that are part of the cerebral blood vessel wall and represent the conduit for fluid transport in and out of the brain. PVS are considered pathologic when sufficiently enlarged to be visible on MRI. Recent studies have demonstrated that enlarged PVS (ePVS) may have clinical consequences related to cognition. Emerging literature points to arterial stiffening and abnormal protein aggregation in vessel walls as 2 possible mechanisms that drive ePVS formation. We describe the clinical consequences, anatomy, fluid dynamics, physiology, risk factors, and in vivo quantification methods of ePVS. Given competing views of PVS physiology, we detail the 2 most prominent theoretical views and review ePVS associations with other common small vessel disease markers. Because ePVS are a marker of small vessel disease and ePVS burden is higher in Alzheimer disease, a comprehensive understanding about ePVS is essential in developing prevention and treatment strategies.

The human brain functions at the center of a network of systems aimed at providing a structural and immunological layer of protection. The cerebrospinal fluid (CSF) maintains a physiological homeostasis that is of paramount importance to proper neurological activity. CSF is largely produced in the choroid plexus where it is continuous with the brain extracellular fluid and circulates through the ventricles. CSF movement through the central nervous system has been extensively explored. Across numerous animal species, the involvement of various drainage pathways in CSF, including arachnoid granulations, cranial nerves, perivascular pathways, and meningeal lymphatics, has been studied. Among these, there is a proposed CSF clearance route spanning the olfactory nerve and exiting the brain at the cribriform plate and entering lymphatics. While this pathway has been demonstrated in multiple animal species, evidence of a similar CSF egress mechanism involving the nasal cavity in humans remains poorly consolidated. This review will synthesize contemporary evidence surrounding CSF clearance at the nose-brain interface, examining across species this anatomical pathway, and its possible significance to human neurodegenerative disease. Our discussion of a bidirectional nasal pathway includes examination of the immune surveillance in the olfactory region protecting the brain. Overall, we expect that an expanded discussion of the brain-nose pathway and interactions with the environment will contribute to an improved understanding of neurodegenerative and infectious diseases, and potentially to novel prevention and treatment considerations.

Large vessel disease and carotid stenosis are key mechanisms contributing to vascular cognitive impairment (VCI) and dementia. Our previous work, and that of others, using rodent models, demonstrated that bilateral common carotid stenosis (BCAS) leads to cognitive impairment via gradual deterioration of the neuro-glial-vascular unit and accumulation of amyloid-β (Aβ) protein. Since brain-wide drainage pathways (glymphatic) for waste clearance, including Aβ removal, have been implicated in the pathophysiology of VCI via glial mechanisms, we hypothesized that glymphatic function would be impaired in a BCAS model and exacerbated in the presence of Aβ. Male wild-type and Tg-SwDI (model of microvascular amyloid) mice were subjected to BCAS or sham surgery which led to a reduction in cerebral perfusion and impaired spatial learning acquisition and cognitive flexibility. After 3 months survival, glymphatic function was evaluated by cerebrospinal fluid (CSF) fluorescent tracer influx. We demonstrated that BCAS caused a marked regional reduction of CSF tracer influx in the dorsolateral cortex and CA1-DG molecular layer. In parallel to these changes increased reactive astrogliosis was observed post-BCAS. To further investigate the mechanisms that may lead to these changes, we measured the pulsation of cortical vessels. BCAS impaired vascular pulsation in pial arteries in WT and Tg-SwDI mice. Our findings show that BCAS influences VCI and that this is paralleled by impaired glymphatic drainage and reduced vascular pulsation. We propose that these additional targets need to be considered when treating VCI.

Amyloid-related imaging abnormalities with edema/effusion (ARIA-E) are commonly observed with anti-amyloid therapies in Alzheimer's disease. We developed a semi-mechanistic, in silico model to understand the time course of ARIA-E and its dose dependency.

The Fugl-Meyer Assessment (FMA) scale, which is widely used and highly recommended, is an appropriate tool for evaluating poststroke sensorimotor and other possible somatic deficits. It is also well-suited for capturing a dynamic rehabilitation process. The aim of this study was to first translate the entire sensorimotor FMA scale into Romanian using the transcultural and semantic-linguistic adaptations of its official afferent protocols and to validate it using the preliminary clinical evaluation of inter- and intra-rater reliability and relevant concurrent validity.

Alpha synuclein has a key role in the pathogenesis of Parkinson's disease (PD), Dementia with Lewy Bodies (LBD) and Multiple System Atrophy (MSA). Immunotherapies aiming at neutralising toxic αSyn species are being investigated in the clinic as potential disease modifying therapies for PD and other synucleinopathies. In this study, the effects of active immunisation against αSyn with the UB-312 vaccine were investigated in the Thy1SNCA/15 mouse model of PD. Young transgenic and wild-type mice received an immunisation regimen over a period of 6 weeks, then observed for an additional 9 weeks. Behavioural assessment was conducted before immunisation and at 15 weeks after the first dose. UB-312 immunisation prevented the development of motor impairment in the wire test and challenging beam test, which was associated with reduced levels of αSyn oligomers in the cerebral cortex, hippocampus and striatum of Thy1SNCA/15 mice. UB-312 immunotherapy resulted in a significant reduction of theαSyn load in the colon, accompanied by a reduction in enteric glial cell reactivity in the colonic ganglia. Our results demonstrate that immunisation with UB-312 prevents functional deficits and both central and peripheral pathology in Thy1SNCA/15 mice.

Perivascular spaces (PVS) are fluid-filled compartments that are part of the cerebral blood vessel wall and represent the conduit for fluid transport in and out of the brain. PVS are considered pathologic when sufficiently enlarged to be visible on MRI. Recent studies have demonstrated that enlarged PVS (ePVS) may have clinical consequences related to cognition. Emerging literature points to arterial stiffening and abnormal protein aggregation in vessel walls as 2 possible mechanisms that drive ePVS formation. We describe the clinical consequences, anatomy, fluid dynamics, physiology, risk factors, and in vivo quantification methods of ePVS. Given competing views of PVS physiology, we detail the 2 most prominent theoretical views and review ePVS associations with other common small vessel disease markers. Because ePVS are a marker of small vessel disease and ePVS burden is higher in Alzheimer disease, a comprehensive understanding about ePVS is essential in developing prevention and treatment strategies.

The adverse effects of air pollutants on the respiratory and cardiovascular systems are unquestionable. However, in recent years, indications of effects beyond these organ systems have become more evident. Traffic-related air pollution has been linked with neurological diseases, exacerbated cognitive dysfunction, and Alzheimer's disease. However, the exact air pollutant compositions and exposure scenarios leading to these adverse health effects are not known. Although several components of air pollution may be at play, recent experimental studies point to a key role of ultrafine particles (UFPs). While the importance of UFPs has been recognized, almost nothing is known about the smallest fraction of UFPs, and only >23 nm emissions are regulated in the EU. Moreover, the role of the semivolatile fraction of the emissions has been neglected. The Transport-Derived Ultrafines and the Brain Effects (TUBE) project will increase knowledge on harmful ultrafine air pollutants, as well as semivolatile compounds related to adverse health effects. By including all the major current combustion and emission control technologies, the TUBE project aims to provide new information on the adverse health effects of current traffic, as well as information for decision makers to develop more effective emission legislation. Most importantly, the TUBE project will include adverse health effects beyond the respiratory system; TUBE will assess how air pollution affects the brain and how air pollution particles might be removed from the brain. The purpose of this report is to describe the TUBE project, its background, and its goals.

The goal of this work was to investigate the natural history and outcomes after treatment for spontaneous amyloid-related imaging abnormalities (ARIA)-like in cerebral amyloid angiopathy-related inflammation (CAA-ri).

Drainage of interstitial fluid and solutes from the brainstem has not been well studied. To map one drainage pathway in the human brainstem, we took advantage of the focal blood-brain barrier disruption occurring in a multiple sclerosis brainstem lesion, coupled with intravenous injection of gadolinium, which simulates an intraparenchymal injection of gadolinium tracer within the restricted confines of this small brain region. Using high-resolution MRI, we show how it is possible for interstitial fluid to drain into the adjacent trigeminal and oculomotor nerves, in keeping with a pathway of communication between the extracellular spaces of the brainstem and cranial nerve parenchyma.

The extracellular matrix (ECM) of the cerebral vasculature provides a pathway for the flow of interstitial fluid (ISF) and solutes out of the brain by intramural periarterial drainage (IPAD). Failure of IPAD leads to protein elimination failure arteriopathies such as cerebral amyloid angiopathy (CAA). The ECM consists of a complex network of glycoproteins and proteoglycans that form distinct basement membranes (BM) around different vascular cell types. Astrocyte endfeet that are localised against the walls of blood vessels are tethered to these BMs by dystrophin associated protein complex (DPC). Alpha-dystrobrevin (α-DB) is a key dystrophin associated protein within perivascular astrocyte endfeet; its deficiency leads to a reduction in other dystrophin associated proteins, loss of AQP4 and altered ECM. In human dementia cohorts there is a positive correlation between dystrobrevin gene expression and CAA. In the present study, we test the hypotheses that (a) the positive correlation between dystrobrevin gene expression and CAA is associated with elevated expression of α-DB at glial-vascular endfeet and (b) a deficiency in α-DB results in changes to the ECM and failure of IPAD. We used human post-mortem brain tissue with different severities of CAA and transgenic α-DB deficient mice. In human post-mortem tissue we observed a significant increase in vascular α-DB with CAA (CAA vrs. Old p < 0.005, CAA vrs. Young p < 0.005). In the mouse model of α-DB deficiency, there was early modifications to vascular ECM (collagen IV and BM thickening) that translated into reduced IPAD efficiency. Our findings highlight the important role of α-DB in maintaining structure and function of ECM, particularly as a pathway for the flow of ISF and solutes out of the brain by IPAD.

In the last 20 years, research focused on developing retinal imaging as a source of potential biomarkers for Alzheimer's disease and other neurodegenerative diseases, has increased significantly. The Alzheimer's Association and the Alzheimer's & Dementia: Diagnosis, Assessment, Disease Monitoring editorial team (companion journal to Alzheimer's & Dementia) convened an interdisciplinary discussion in 2019 to identify a path to expedite the development of retinal biomarkers capable of identifying biological changes associated with AD, and for tracking progression of disease severity over time. As different retinal imaging modalities provide different types of structural and/or functional information, the discussion reflected on these modalities and their respective strengths and weaknesses. Discussion further focused on the importance of defining the context of use to help guide the development of retinal biomarkers. Moving from research to context of use, and ultimately to clinical evaluation, this article outlines ongoing retinal imaging research today in Alzheimer's and other brain diseases, including a discussion of future directions for this area of study.

Vascular dysregulation and cholinergic basal forebrain degeneration are both early pathological events in the development of Alzheimer's disease (AD). Acetylcholine contributes to localised arterial dilatation and increased cerebral blood flow (CBF) during neurovascular coupling via activation of endothelial nitric oxide synthase (eNOS). Decreased vascular reactivity is suggested to contribute to impaired clearance of β-amyloid (Aβ) along intramural periarterial drainage (IPAD) pathways of the brain, leading to the development of cerebral amyloid angiopathy (CAA). However, the possible relationship between loss of cholinergic innervation, impaired vasoreactivity and reduced clearance of Aβ from the brain has not been previously investigated. In the present study, intracerebroventricular administration of mu-saporin resulted in significant death of cholinergic neurons and fibres in the medial septum, cortex and hippocampus of C57BL/6 mice. Arterial spin labelling MRI revealed a loss of CBF response to stimulation of eNOS by the Rho-kinase inhibitor fasudil hydrochloride in the cortex of denervated mice. By contrast, the hippocampus remained responsive to drug treatment, in association with altered eNOS expression. Fasudil hydrochloride significantly increased IPAD in the hippocampus of both control and saporin-treated mice, while increased clearance from the cortex was only observed in control animals. Administration of mu-saporin in the TetOAPPSweInd mouse model of AD was associated with a significant and selective increase in Aβ40-positive CAA. These findings support the importance of the interrelationship between cholinergic innervation and vascular function in the aetiology and/or progression of CAA and suggest that combined eNOS/cholinergic therapies may improve the efficiency of Aβ removal from the brain and reduce its deposition as CAA.

The cerebral vasculature is made up of highly specialized structures that assure constant brain perfusion necessary to meet the very high demand for oxygen and glucose by neurons and glial cells. A dense, redundant network of arteries is spread over the entire pial surface from which penetrating arteries dive into the cortex to reach the neurovascular units. Besides providing blood to the brain parenchyma, cerebral arteries are key in the drainage of interstitial fluid (ISF) and solutes such as amyloid-beta. This occurs along the basement membranes surrounding vascular smooth muscle cells, toward leptomeningeal arteries and deep cervical lymph nodes. The dense microvasculature is made up of fine capillaries. Capillary walls contain pericytes that have contractile properties and are lined by a highly specialized blood-brain barrier that regulates the entry of solutes and ions and maintains the integrity of the composition of ISF. They are also important for the production of ISF. Capillaries drain into venules that course centrifugally toward the cortex to reach cortical veins and empty into dural venous sinuses. The walls of the venous sinuses are also home to meningeal lymphatic vessels that support the drainage of cerebrospinal fluid, although such pathways are still poorly understood. Damage to macro- and microvasculature will compromise cerebral perfusion, hamper the highly synchronized movement of neurofluids, and affect the drainage of waste products leading to neuronal and glial degeneration. This review will present vascular anatomy, their role in fluid dynamics, and a summary of how their dysfunction can lead to neurodegeneration.

Cerebral amyloid angiopathy (CAA) is characterized by the accumulation of β-amyloid (Aβ) in the walls of cerebral vessels, leading to complications such as intracerebral hemorrhage, convexity subarachnoid hemorrhage and cerebral microinfarcts. Patients with CAA-related intracerebral hemorrhage are more likely to develop dementia and strokes. Several pathological investigations have demonstrated that more than 90% of Alzheimer's disease patients have concomitant CAA, suggesting common pathogenic mechanisms. Potential causes of CAA include impaired Aβ clearance from the brain through the intramural periarterial drainage (IPAD) system. Conversely, CAA causes restriction of IPAD, limiting clearance. Early intervention in CAA could thus prevent Alzheimer's disease progression. Growing evidence has suggested Taxifolin (dihydroquercetin) could be used as an effective therapy for CAA. Taxifolin is a plant flavonoid, widely available as a health supplement product, which has been demonstrated to exhibit anti-oxidative and anti-inflammatory effects, and provide protection against advanced glycation end products and mitochondrial damage. It has also been shown to facilitate disassembly, prevent oligomer formation and increase clearance of Aβ in a mouse model of CAA. Disturbed cerebrovascular reactivity and spatial reference memory impairment in CAA are completely prevented by Taxifolin treatment. These results highlight the need for clinical trials on the efficacy and safety of Taxifolin in patients with CAA.

Alzheimer's disease and Lewy body diseases are the most common causes of neurodegeneration and dementia. Amyloid-beta (Aβ) and alpha-synuclein (αSyn) are two key proteins involved in the pathogenesis of these neurodegenerative diseases. Immunotherapy aims to reduce the harmful effects of protein accumulation by neutralising toxic species and facilitating their removal. The results of the first immunisation trial against Aβ led to a small percentage of meningoencephalitis cases which revolutionised vaccine design, causing a shift in the field of immunotherapy from active to passive immunisation. While the vast majority of immunotherapies have been developed for Aβ and tested in Alzheimer's disease, the field has progressed to targeting other proteins including αSyn. Despite showing some remarkable results in animal models, immunotherapies have largely failed final stages of clinical trials to date, with the exception of Aducanumab recently licenced in the US by the FDA. Neuropathological findings translate quite effectively from animal models to human trials, however, cognitive and functional outcome measures do not. The apparent lack of translation of experimental studies to clinical trials suggests that we are not obtaining a full representation of the effects of immunotherapies from animal studies. Here we provide a background understanding to the key concepts and challenges involved in therapeutic design. This review further provides a comprehensive comparison between experimental and clinical studies in Aβ and αSyn immunotherapy and aims to determine the possible reasons for the disconnection in their outcomes.

The human brain functions at the center of a network of systems aimed at providing a structural and immunological layer of protection. The cerebrospinal fluid (CSF) maintains a physiological homeostasis that is of paramount importance to proper neurological activity. CSF is largely produced in the choroid plexus where it is continuous with the brain extracellular fluid and circulates through the ventricles. CSF movement through the central nervous system has been extensively explored. Across numerous animal species, the involvement of various drainage pathways in CSF, including arachnoid granulations, cranial nerves, perivascular pathways, and meningeal lymphatics, has been studied. Among these, there is a proposed CSF clearance route spanning the olfactory nerve and exiting the brain at the cribriform plate and entering lymphatics. While this pathway has been demonstrated in multiple animal species, evidence of a similar CSF egress mechanism involving the nasal cavity in humans remains poorly consolidated. This review will synthesize contemporary evidence surrounding CSF clearance at the nose-brain interface, examining across species this anatomical pathway, and its possible significance to human neurodegenerative disease. Our discussion of a bidirectional nasal pathway includes examination of the immune surveillance in the olfactory region protecting the brain. Overall, we expect that an expanded discussion of the brain-nose pathway and interactions with the environment will contribute to an improved understanding of neurodegenerative and infectious diseases, and potentially to novel prevention and treatment considerations.

Large vessel disease and carotid stenosis are key mechanisms contributing to vascular cognitive impairment (VCI) and dementia. Our previous work, and that of others, using rodent models, demonstrated that bilateral common carotid stenosis (BCAS) leads to cognitive impairment via gradual deterioration of the neuro-glial-vascular unit and accumulation of amyloid-β (Aβ) protein. Since brain-wide drainage pathways (glymphatic) for waste clearance, including Aβ removal, have been implicated in the pathophysiology of VCI via glial mechanisms, we hypothesized that glymphatic function would be impaired in a BCAS model and exacerbated in the presence of Aβ. Male wild-type and Tg-SwDI (model of microvascular amyloid) mice were subjected to BCAS or sham surgery which led to a reduction in cerebral perfusion and impaired spatial learning acquisition and cognitive flexibility. After 3 months survival, glymphatic function was evaluated by cerebrospinal fluid (CSF) fluorescent tracer influx. We demonstrated that BCAS caused a marked regional reduction of CSF tracer influx in the dorsolateral cortex and CA1-DG molecular layer. In parallel to these changes increased reactive astrogliosis was observed post-BCAS. To further investigate the mechanisms that may lead to these changes, we measured the pulsation of cortical vessels. BCAS impaired vascular pulsation in pial arteries in WT and Tg-SwDI mice. Our findings show that BCAS influences VCI and that this is paralleled by impaired glymphatic drainage and reduced vascular pulsation. We propose that these additional targets need to be considered when treating VCI.

In the last 20 years, research focused on developing retinal imaging as a source of potential biomarkers for Alzheimer's disease and other neurodegenerative diseases, has increased significantly. The Alzheimer's Association and the Alzheimer's & Dementia: Diagnosis, Assessment, Disease Monitoring editorial team (companion journal to Alzheimer's & Dementia) convened an interdisciplinary discussion in 2019 to identify a path to expedite the development of retinal biomarkers capable of identifying biological changes associated with AD, and for tracking progression of disease severity over time. As different retinal imaging modalities provide different types of structural and/or functional information, the discussion reflected on these modalities and their respective strengths and weaknesses. Discussion further focused on the importance of defining the context of use to help guide the development of retinal biomarkers. Moving from research to context of use, and ultimately to clinical evaluation, this article outlines ongoing retinal imaging research today in Alzheimer's and other brain diseases, including a discussion of future directions for this area of study.

Cerebral microbleeds (CMB) are associated with stroke and cognitive impairment. We previously reported a high prevalence of CMB in people with expressing Cnm, a collagen-binding protein in the oral cavity. is a major pathogen responsible for dental caries. Repeated challenge with harboring the gene encoding Cnm induced cerebral bleeding in stroke-prone spontaneously hypertensive rats. The purpose of this longitudinal study is to examine the relationship of -positive to the development of CMB.

Alpha-synuclein (α-Syn) aggregation is the primary characteristic of synucleinopathies including Parkinson's disease (PD), dementia with Lewy bodies (DLB) and multiple system atrophy (MSA). Immunotherapy targeting α-Syn has shown promising results in animal models of the disease. This study investigates the target specificity of three different active vaccines for pathological α-Syn aggregates found in human brain tissue from synucleinopathies.

Drainage of interstitial fluid from the brain occurs via the intramural periarterial drainage (IPAD) pathways along the basement membranes of cerebral capillaries and arteries against the direction of blood flow into the brain. The cerebrovascular smooth muscle cells (SMCs) provide the motive force for driving IPAD, and their decrease in function may explain the deposition of amyloid-beta as cerebral amyloid angiopathy (CAA), a key feature of Alzheimer's disease. The α-adrenoceptor subtype α is abundant in the brain, but its distribution in the cerebral vessels is unclear. We analysed cultured human cerebrovascular SMCs and young, old and CAA human brains for (a) the presence of α receptor and (b) the distribution of the α receptor within the cerebral vessels. The α receptor was present on the wall of cerebrovascular SMCs. No significant changes were observed in the vascular expression of the α-adrenergic receptor in young, old and CAA cases. The pattern of vascular staining appeared less punctate and more diffuse with ageing and CAA. Our results show that the α-adrenergic receptor is preserved in cerebral vessels with ageing and in CAA and is expressed on cerebrovascular smooth muscle cells, suggesting that vascular adrenergic receptors may hold potential for therapeutic targeting of IPAD.

White matter hyperintensities (WMH) occur in association with dementia but the aetiology is unclear. Here we test the hypothesis that there is a combination of impaired elimination of interstitial fluid from the white matter together with a degree of hypoxia in WMH. One of the mechanisms for the elimination of amyloid-β (Aβ) from the brain is along the basement membranes in the walls of capillaries and arteries (Intramural Peri-Arterial Drainage - IPAD). We compared the dynamics of IPAD in the grey matter of the hippocampus and in the white matter of the corpus callosum in 10 week old C57/B16 mice by injecting soluble Aβ as a tracer. The dynamics of IPAD in the white matter were significantly slower compared with the grey matter and this was associated with a lower density of capillaries in the white matter. Exposing cultures of smooth muscle cells to hypercapnia as a model of cerebral hypoperfusion resulted in a reduction in fibronectin and an increase in laminin in the extracellular matrix. Similar changes were detected in the white matter in human WMH suggesting that hypercapnia/hypoxia may play a role in WMH. Employing therapies to enhance both IPAD and blood flow in the white matter may reduce WMH in patients with dementia.

Interest in neurofilaments has risen sharply in recent years with recognition of their potential as biomarkers of brain injury or neurodegeneration in CSF and blood. This is in the context of a growing appreciation for the complexity of the neurobiology of neurofilaments, new recognition of specialized roles for neurofilaments in synapses and a developing understanding of mechanisms responsible for their turnover. Here we will review the neurobiology of neurofilament proteins, describing current understanding of their structure and function, including recently discovered evidence for their roles in synapses. We will explore emerging understanding of the mechanisms of neurofilament degradation and clearance and review new methods for future elucidation of the kinetics of their turnover in humans. Primary roles of neurofilaments in the pathogenesis of human diseases will be described. With this background, we then will review critically evidence supporting use of neurofilament concentration measures as biomarkers of neuronal injury or degeneration. Finally, we will reflect on major challenges for studies of the neurobiology of intermediate filaments with specific attention to identifying what needs to be learned for more precise use and confident interpretation of neurofilament measures as biomarkers of neurodegeneration.

Assessment of outcome in preclinical studies of vascular cognitive impairment (VCI) is heterogenous. Through an ARUK Scottish Network supported questionnaire and workshop (mostly UK-based researchers), we aimed to determine underlying variability and what could be implemented to overcome identified challenges. Twelve UK VCI research centres were identified and invited to complete a questionnaire and attend a one-day workshop. Questionnaire responses demonstrated agreement that outcome assessments in VCI preclinical research vary by group and even those common across groups, may be performed differently. From the workshop, six themes were discussed: issues with preclinical models, reasons for choosing functional assessments, issues in interpretation of functional assessments, describing and reporting functional outcome assessments, sharing resources and expertise, and standardization of outcomes. Eight consensus points emerged demonstrating broadly that the chosen assessment should reflect the deficit being measured, and therefore that one assessment does not suit all models; guidance/standardisation on recording VCI outcome reporting is needed and that uniformity would be aided by a platform to share expertise, material, protocols and procedures thus reducing heterogeneity and so increasing potential for collaboration, comparison and replication. As a result of the workshop, UK wide consensus statements were agreed and future priorities for preclinical research identified.

The accumulation of amyloid-β (Aβ) in the walls of capillaries and arteries as cerebral amyloid angiopathy (CAA) is part of the small vessel disease spectrum, related to a failure of elimination of Aβ from the brain. Aβ is eliminated along basement membranes in walls of cerebral capillaries and arteries (Intramural Peri-Arterial Drainage-IPAD), a pathway that fails with age and ApolipoproteinEε4 (ApoE4) genotype. IPAD is along basement membranes formed by capillary endothelial cells and surrounding astrocytes. Here, we examine (1) the composition of basement membranes synthesised by ApoE4 astrocytes; (2) structural differences between ApoE4 and ApoE3 astrocytes, and (3) how flow of Aβ affects Apo3/4 astrocytes. Using cultured astrocytes expressing ApoE3 or ApoE4, immunofluorescence, confocal, correlative light and electron microscopy (CLEM), and a millifluidic flow system, we show that ApoE4 astrocytes synthesise more fibronectin, possess smaller processes, and become rarefied when Aβ flows over them, as compared to ApoE3 astrocytes. Our results suggest that basement membranes synthesised by ApoE4 astrocytes favour the aggregation of Aβ, its reduced clearance via IPAD, thus promoting cerebral amyloid angiopathy.

Deep perforator arteriopathy (DPA) and cerebral amyloid angiopathy (CAA) are the commonest known cerebral small vessel diseases (CSVD), which cause ischaemic stroke, intracebral haemorrhage (ICH) and vascular cognitive impairment (VCI). While thus far mainly considered as separate entities, we here propose that DPA and CAA share similarities, overlap and interact, so that 'pure' DPA or CAA are extremes along a continuum of age-related small vessel pathologies. We suggest blood-brain barrier (BBB) breakdown, endothelial damage and impaired perivascular β-amyloid (Aβ) drainage are hallmark common mechanisms connecting DPA and CAA. We also suggest a need for new biomarkers (e.g. high-resolution imaging) to deepen understanding of the complex relationships between DPA and CAA.

In the absence of lymphatics, fluid and solutes such as amyloid-β (Aβ) are eliminated from the brain along basement membranes in the walls of cerebral capillaries and arteries-the Intramural Peri-Arterial Drainage (IPAD) pathway. IPAD fails with age and insoluble Aβ is deposited as plaques in the brain and in IPAD pathways as cerebral amyloid angiopathy (CAA); fluid accumulates in the white matter as reflected by hyperintensities (WMH) on MRI. Within the brain, fluid uptake by astrocytes is regulated by aquaporin 4 (AQP4). We test the hypothesis that expression of astrocytic AQP4 increases in grey matter and decreases in white matter with onset of CAA. AQP4 expression was quantitated by immunocytochemistry and confocal microscopy in post-mortem occipital grey and white matter from young and old non-demented human brains, in CAA and in WMH. Results AQP4 expression tended to increase with normal ageing but AQP4 expression in severe CAA was significantly reduced when compared to moderate CAA ( = 0.018). AQP4 expression tended to decline in the white matter with CAA and WMH, both of which are associated with impaired IPAD. Adjusting the level of AQP4 activity may be a valid therapeutic target for restoring homoeostasis in the brain as IPAD fails with age and CAA.

In this issue of Neuron, van Veluw et al. (2020) show that elimination of solutes from the brain along arterial walls is driven by low-frequency arteriolar oscillations and suggest that age-related reduction of this vasomotion may contribute to impaired clearance of Aβ.

Cerebral amyloid angiopathy (CAA) is the accumulation of amyloid beta (Aβ) in the walls of cerebral arterioles, arteries and capillaries. Changes in the white matter in CAA are observed as hyperintensities and dilated perivascular spaces on MRI suggesting impairment of fluid drainage but the pathophysiology behind these changes is poorly understood. We tested the hypothesis that proteins associated with clearance of Aβ peptides are upregulated in the white matter in cases of CAA. In this study, we compare the quantitative proteomic profile of white matter from post-mortem brains of patients with CAA and age-matched controls in order to gain insight into the cellular processes and key molecules involved in the pathophysiology of CAA. Our proteomic analysis resulted in the profiling of 3,734 proteins (peptide FDR p<0.05). Of these, 189 were differentially expressed in CAA vs. control. Bioinformatics analysis of these proteins showed significant enrichment of proteins related to cell adhesion | cell-matrix interaction, mitochondrial dysfunction and hypoxia. Upregulated proteins in CAA included EMILIN2, COL4A2, TLN1, CLU, HSPG2. Downregulated proteins included DSP, IDE, HBG1. The present study reports an in-depth quantitative proteomic profiling of white matter from patients with CAA, highlighting extracellular matrix proteins and clusterin as key molecules in the pathophysiology of white matter changes in cases of CAA.

We performed a clinicopathological study to assess the burden of small vessel disease (SVD) type of pathological changes in elderly demented subjects, who had clinical evidence of autonomic dysfunction, either carotid sinus hypersensitivity or orthostatic hypotension or both or had exhibited unexpected repeated falls. Clinical and neuropathological diagnoses in 112 demented subjects comprised dementia with Lewy bodies (DLB), Parkinson's disease with dementia (PDD), Alzheimer's disease (AD), Mixed dementia (mostly AD-DLB) and vascular dementia (VaD). Of these, 12 DLB subjects had no recorded unexpected falls in life and therefore no evidence of concomitant autonomic dysfunction. A further 17 subjects were assessed as aging controls without significant pathology or signs of autonomic dysfunction. We quantified brain vascular pathological changes and determined severities of neurodegenerative lesions including α-synuclein pathology. We found moderate-severe vascular changes and high-vascular pathology scores (P < 0.01) in all neurodegenerative dementias and as expected in VaD compared to similar age controls. Arteriolosclerosis, perivascular spacing and microinfarcts were frequent in the basal ganglia and frontal white matter (WM) across all dementias, whereas small infarcts (<5 mm) were restricted to VaD. In a sub-set of demented subjects, we found that vascular pathology scores were correlated with WM hyperintensity volumes determined by MRI in life (P < 0.02). Sclerotic index values were increased by ~50% in both the WM and neocortex in all dementias compared to similar age controls. We found no evidence for increased α-synuclein deposition in subjects with autonomic dysfunction. Our findings suggest greater SVD pathological changes occur in the elderly diagnosed with neurodegenerative dementias including DLB and who develop autonomic dysfunction. SVD changes may not necessarily manifest in clinically overt symptoms but they likely confound motor or cognitive dysfunction. We propose dysautonomia promotes chronic cerebral hypoperfusion to impact upon aging-related neurodegenerative disorders and characterize their end-stage clinical syndromes.

Cerebral amyloid angiopathy (CAA) is characterised by the progressive accumulation of β-amyloid (Aβ) in the walls of cerebral capillaries and arteries representing a major cause of haemorrhagic stroke including lobar intracerebral haemorrhage (ICH) and convexity subarachnoid haemorrhage (SAH). Haemorrhaging from CAA predominantly involves smaller arteries rather than arterial aneurysm. Restricted bleeding into the subarachnoid space in CAA results in asymptomatic or mild symptomatic SAH. Herein, we present an autopsied case of massive SAH related to CAA. An 89-year-old male with a history of mild Alzheimer's disease (AD) and advanced pancreatic cancer with liver metastasis developed sudden onset of coma. Head CT illustrated ICH located in the right frontal lobe and right insula, as well as SAH bilaterally spreading from the basal cistern to the Sylvian fissure, with hydrocephalus and brain herniation. He died about 24 h after onset and the post-mortem examination showed no evidence of arterial aneurysm. The substantial accumulation of Aβ in the vessels around the haemorrhagic lesions led to the diagnosis of ICH related to CAA and secondary SAH, which may have been aggravated by old age and malignancy. This case suggests that CAA can cause severe SAH resembling aneurysmal origin and thus may be overlooked when complicated by atypical cerebral haemorrhage.

The cerebral vasculature is made up of highly specialized structures that assure constant brain perfusion necessary to meet the very high demand for oxygen and glucose by neurons and glial cells. A dense, redundant network of arteries is spread over the entire pial surface from which penetrating arteries dive into the cortex to reach the neurovascular units. Besides providing blood to the brain parenchyma, cerebral arteries are key in the drainage of interstitial fluid (ISF) and solutes such as amyloid-beta. This occurs along the basement membranes surrounding vascular smooth muscle cells, toward leptomeningeal arteries and deep cervical lymph nodes. The dense microvasculature is made up of fine capillaries. Capillary walls contain pericytes that have contractile properties and are lined by a highly specialized blood-brain barrier that regulates the entry of solutes and ions and maintains the integrity of the composition of ISF. They are also important for the production of ISF. Capillaries drain into venules that course centrifugally toward the cortex to reach cortical veins and empty into dural venous sinuses. The walls of the venous sinuses are also home to meningeal lymphatic vessels that support the drainage of cerebrospinal fluid, although such pathways are still poorly understood. Damage to macro- and microvasculature will compromise cerebral perfusion, hamper the highly synchronized movement of neurofluids, and affect the drainage of waste products leading to neuronal and glial degeneration. This review will present vascular anatomy, their role in fluid dynamics, and a summary of how their dysfunction can lead to neurodegeneration.

Accumulation of amyloid beta (Aβ), α-synuclein (αSyn), and tau in dementias indicates their age-related failure of elimination from the brain. Aβ is eliminated along basement membranes in walls of cerebral arterioles and leptomeningeal arteries (intramural peri-arterial drainage [IPAD]); IPAD is impaired with age. We test the hypothesis that αSyn and tau are also eliminated from the normal brain along IPAD pathways.

Vascular pulsations, contractions of vascular smooth muscle cells and breathing have been reported to foster movement and clearance of interstitial and cerebrospinal fluids from the brain. The aim of this study was to estimate the contribution of these vital functions. We compared the spread of an injected hydrophilic tracer (Fluoro-Emerald, a 10 kDa fluorescein-coupled dextran amine) in the brains of live anesthetized and sacrificed rats at 30 and 90 min after injection. To determine the overall pattern of distribution of tracers, we created 3D-reconstructions of the horizontal transections of the whole brain. Immunofluorescence staining with laminin and collagen IV was performed to determine the pattern of distribution of tracer in relation to the cerebrovascular basement membranes. We found that diffusion was widely restricted to the periventricular region in sacrificed rats with no spread to the contralateral hemisphere, while the bulk flow occurred along the vasculature and reached the surface and the contralateral hemisphere as soon as 30 min after injection in live anesthetized animals. The tracer appeared to be localized along the vascular basement membranes and along fiber tracts as reported previously. Thus, our data indicate that vital functions are essential for the remote movement of extracellular fluids within the cerebral parenchyma.

Two of the key functions of arteries in the brain are (1) the well-recognized supply of blood via the vascular lumen and (2) the emerging role for the arterial walls as routes for the elimination of interstitial fluid (ISF) and soluble metabolites, such as amyloid beta (Aβ), from the brain and retina. As the brain and retina possess no conventional lymphatic vessels, fluid drainage toward peripheral lymph nodes is mediated via transport along basement membranes in the walls of capillaries and arteries that form the intramural peri-arterial drainage (IPAD) system. IPAD tends to fail as arteries age but the mechanisms underlying the failure are unclear. In some people this is reflected in the accumulation of Aβ plaques in the brain in Alzheimer's disease (AD) and deposition of Aβ within artery walls as cerebral amyloid angiopathy (CAA). Knowledge of the dynamics of IPAD and why it fails with age is essential for establishing diagnostic tests for the early stages of the disease and for devising therapies that promote the clearance of Aβ in the prevention and treatment of AD and CAA. This editorial is intended to introduce the rationale that has led to the establishment of the Clearance of Interstitial Fluid (ISF) and CSF (CLIC) group, within the Vascular Professional Interest Area of the Alzheimer's Association International Society to Advance Alzheimer's Research and Treatment.

The initial disease stages of hypertensive arteriopathy (HA) and cerebral amyloid angiopathy (CAA), the two main forms of sporadic human cerebral small vessel diseases (CSVD), are too subtle to be detectable on clinical routine imaging. Small vessel disease (SVD) is a systemic condition, affecting not only the brain, but also other organs. The retina appears as an ideal marker for the early detection of incipient CSVD. We therefore investigated the retinal microvasculature of the spontaneously hypertensive stroke-prone rat (SHRSP), an animal model of sporadic CSVD. The brains and retinas of 26 male SHRSP (18-44 weeks) were examined histologically and immunohistochemically for the presence of HA phenomena (erythrocyte thrombi, small perivascular bleeds) and amyloid angiopathy (AA). CAA and AA in the retina showed a significant correlation with age (CAA: rho = 0.55, = 0.005; AA: rho = 0.89, < 0.001). The number of erythrocyte thrombi in the brain correlated with the severity of retinal erythrocyte thrombi (rho = 0.46, = 0.023), while the occurrence of CAA correlated with the appearance of AA in the retina (rho = 0.51, = 0.012). Retinal SVD markers predicted CSVD markers with good sensitivity. These results indicate that SVD also occurs in the retinal microvasculature of SHRSP and the prediction of cerebral erythrocyte thrombi and CAA might be possible using retinal biomarkers. This underlines the important role of the investigation of the retina in the early diagnosis of CSVD.

Deep perforator arteriopathy (DPA) and cerebral amyloid angiopathy (CAA) are the commonest known cerebral small vessel diseases (CSVD), which cause ischaemic stroke, intracebral haemorrhage (ICH) and vascular cognitive impairment (VCI). While thus far mainly considered as separate entities, we here propose that DPA and CAA share similarities, overlap and interact, so that 'pure' DPA or CAA are extremes along a continuum of age-related small vessel pathologies. We suggest blood-brain barrier (BBB) breakdown, endothelial damage and impaired perivascular β-amyloid (Aβ) drainage are hallmark common mechanisms connecting DPA and CAA. We also suggest a need for new biomarkers (e.g. high-resolution imaging) to deepen understanding of the complex relationships between DPA and CAA.

We performed a clinicopathological study to assess the burden of small vessel disease (SVD) type of pathological changes in elderly demented subjects, who had clinical evidence of autonomic dysfunction, either carotid sinus hypersensitivity or orthostatic hypotension or both or had exhibited unexpected repeated falls. Clinical and neuropathological diagnoses in 112 demented subjects comprised dementia with Lewy bodies (DLB), Parkinson's disease with dementia (PDD), Alzheimer's disease (AD), Mixed dementia (mostly AD-DLB) and vascular dementia (VaD). Of these, 12 DLB subjects had no recorded unexpected falls in life and therefore no evidence of concomitant autonomic dysfunction. A further 17 subjects were assessed as aging controls without significant pathology or signs of autonomic dysfunction. We quantified brain vascular pathological changes and determined severities of neurodegenerative lesions including α-synuclein pathology. We found moderate-severe vascular changes and high-vascular pathology scores (P < 0.01) in all neurodegenerative dementias and as expected in VaD compared to similar age controls. Arteriolosclerosis, perivascular spacing and microinfarcts were frequent in the basal ganglia and frontal white matter (WM) across all dementias, whereas small infarcts (<5 mm) were restricted to VaD. In a sub-set of demented subjects, we found that vascular pathology scores were correlated with WM hyperintensity volumes determined by MRI in life (P < 0.02). Sclerotic index values were increased by ~50% in both the WM and neocortex in all dementias compared to similar age controls. We found no evidence for increased α-synuclein deposition in subjects with autonomic dysfunction. Our findings suggest greater SVD pathological changes occur in the elderly diagnosed with neurodegenerative dementias including DLB and who develop autonomic dysfunction. SVD changes may not necessarily manifest in clinically overt symptoms but they likely confound motor or cognitive dysfunction. We propose dysautonomia promotes chronic cerebral hypoperfusion to impact upon aging-related neurodegenerative disorders and characterize their end-stage clinical syndromes.

Apolipoprotein A-I (apoA-I), the principal apolipoprotein associated with high-density lipoproteins in the periphery, is also found at high concentrations in the cerebrospinal fluid. Previous studies have reported either no impact or vascular-specific effects of apoA-I knockout (KO) on β-amyloid (Aβ) pathology. However, the putative mechanism(s) by which apoA-I may influence Aβ deposition is unknown.

Tissue inhibitor of metalloproteinase-3 (TIMP-3) is a component of the extracellular environment, where it mediates diverse processes including matrix regulation/turnover, inflammation and angiogenesis. Rare risk alleles and mutations are directly linked with retinopathies such as age-related macular degeneration (AMD) and Sorsby fundus dystrophy, and potentially, through indirect mechanisms, with Alzheimer's disease. Insights into TIMP-3 activities may be gleaned from studying Sorsby-linked mutations. However, recent findings do not fully support the prevailing hypothesis that a gain of function through the dimerisation of mutated TIMP-3 is responsible for retinopathy. Findings from Alzheimer's patients suggest a hitherto poorly studied relationship between TIMP-3 and the Alzheimer's-linked amyloid-beta (A) proteins that warrant further scrutiny. This may also have implications for understanding AMD as aged/diseased retinae contain high levels of A. Findings from knockout and mutant knock-in mice have not led to new treatments, particularly as the latter does not satisfactorily recapitulate the Sorsby phenotype. However, recent advances in stem cell and in vitro approaches offer novel insights into understanding TIMP-3 pathology in the retina-brain axis, which has so far not been collectively examined. We propose that TIMP-3 activities could extend beyond its hitherto supposed functions to cause age-related changes and disease in these organs.

Intrathecal (IT) delivery and pharmacology of antisense oligonucleotides (ASOs) for the CNS have been successfully developed to treat spinal muscular atrophy. However, ASO pharmacokinetic (PK) and pharmacodynamic (PD) properties remain poorly understood in the IT compartment. We applied multimodal imaging techniques to elucidate the IT PK and PD of unlabeled, radioactively labeled, or fluorescently labeled ASOs targeting ubiquitously expressed or neuron-specific RNAs. Following lumbar IT bolus injection in rats, all ASOs spread rostrally along the neuraxis, adhered to meninges, and were partially cleared to peripheral lymph nodes and kidneys. Rapid association with the pia and arterial walls preceded passage of ASOs across the glia limitans, along arterial intramural basement membranes, and along white-matter axonal bundles. Several neuronal and glial cell types accumulated ASOs over time, with evidence of probable glial accumulation preceding neuronal uptake. IT doses of anti-GluR1 and anti-Gabra1 ASOs markedly reduced the mRNA and protein levels of their respective neurotransmitter receptor protein targets by 2 weeks and anti-Gabra1 ASOs also reduced binding of the GABAA receptor PET ligand 18F-flumazenil in the brain over 4 weeks. Our multimodal imaging approaches elucidate multiple transport routes underlying the CNS distribution, clearance, and efficacy of IT-dosed ASOs.

As an alternative to advection, solute transport by shear-augmented dispersion within oscillatory cerebrospinal fluid flow was investigated in small channels representing the basement membranes located between cerebral arterial smooth muscle cells, the paraarterial space surrounding the vessel wall and in large channels modeling the spinal subarachnoid space (SSS).

Increasing evidence recognizes Alzheimer's disease (AD) as a multifactorial and heterogeneous disease with multiple contributors to its pathophysiology, including vascular dysfunction. The recently updated AD Research Framework put forth by the National Institute on Aging-Alzheimer's Association describes a biomarker-based pathologic definition of AD focused on amyloid, tau, and neuronal injury. In response to this article, here we first discussed evidence that vascular dysfunction is an important early event in AD pathophysiology. Next, we examined various imaging sequences that could be easily implemented to evaluate different types of vascular dysfunction associated with, and/or contributing to, AD pathophysiology, including changes in blood-brain barrier integrity and cerebral blood flow. Vascular imaging biomarkers of small vessel disease of the brain, which is responsible for >50% of dementia worldwide, including AD, are already established, well characterized, and easy to recognize. We suggest that these vascular biomarkers should be incorporated into the AD Research Framework to gain a better understanding of AD pathophysiology and aid in treatment efforts.

The human brain is the organ with the highest metabolic activity but it lacks a traditional lymphatic system responsible for clearing waste products. We have demonstrated that the basement membranes of cerebral capillaries and arteries represent the lymphatic pathways of the brain along which intramural periarterial drainage (IPAD) of soluble metabolites occurs. Failure of IPAD could explain the vascular deposition of the amyloid-beta protein as cerebral amyloid angiopathy (CAA), which is a key pathological feature of Alzheimer's disease. The underlying mechanisms of IPAD, including its motive force, have not been clarified, delaying successful therapies for CAA. Although arterial pulsations from the heart were initially considered to be the motive force for IPAD, they are not strong enough for efficient IPAD. This study aims to unravel the driving force for IPAD, by shifting the perspective of a heart-driven clearance of soluble metabolites from the brain to an intrinsic mechanism of cerebral arteries (e.g., vasomotion-driven IPAD). We test the hypothesis that the cerebrovascular smooth muscle cells, whose cycles of contraction and relaxation generate vasomotion, are the drivers of IPAD. A novel multiscale model of arteries, in which we treat the basement membrane as a fluid-filled poroelastic medium deformed by the contractile cerebrovascular smooth muscle cells, is used to test the hypothesis. The vasomotion-induced intramural flow rates suggest that vasomotion-driven IPAD is the only mechanism postulated to date capable of explaining the available experimental observations. The cerebrovascular smooth muscle cells could represent valuable drug targets for prevention and early interventions in CAA.

Increasing evidence supports a role for cerebrovasculature dysfunction in the etiology of Alzheimer's disease (AD). Blood vessels in the brain are composed of a collection of cells and acellular material that comprise the neurovascular unit (NVU). The NVU in the hippocampus and cortex receives innervation from cholinergic neurons that originate in the basal forebrain. Death of these neurons and their nerve fibers is an early feature of AD. However, the effect of the loss of cholinergic innervation on the NVU is not well characterized. The purpose of this study was to evaluate the effect of the loss of cholinergic innervation of components of the NVU at capillaries, arteries and veins in the hippocampus and cortex. Adult male C57BL/6 mice received an intracerebroventricular injection of the immunotoxin p75NTR mu-saporin to induce the loss of cholinergic neurons. Quadruple labeling immunohistochemistry and 3D reconstruction were carried out to characterize specific points of contact between cholinergic fibers and collagen IV, smooth muscle cells and astrocyte endfeet. Innate differences were observed between vessels of the hippocampus and cortex of control mice, including a greater amount of cholinergic contact with perivascular astrocytes in hippocampal capillaries and a thicker basement membrane in hippocampal veins. Saporin treatment induced a loss of cholinergic innervation at the arterial basement membrane and smooth muscle cells of both the hippocampus and the cortex. In the cortex, there was an additional loss of innervation at the astrocytic endfeet. The current results suggest that cortical arteries are more strongly affected by cholinergic denervation than arteries in the hippocampus. This regional variation may have implications for the etiology of the vascular pathology that develops in AD.

There are very few reliable methods in the literature to discern with certainty between cerebral arterioles and venules. Smooth muscle cells (SMC) and pericytes are present in both arterioles and venules, so immunocytochemistry for markers specific to intramural cells (IMC) is unreliable. This study employed transmission electron microscopy (TEM) and a canine brain to produce robust criteria for the correct identification of cerebral arterioles and venules based on lumen:vessel wall area, tested against the less accurate lumen diameter:vessel wall thickness. We first used morphology of IMC to identify two distinct groups of vessels; group 1 with morphology akin to venules and group 2 with morphology akin to arterioles. We then quantitatively assessed these vessels for lumen:vessel wall area ratio and lumen diameter:wall thickness ratio. After assessing 112 vessels, we show two distinct groups of vessels that can be separated using lumen:vessel wall area (group 1, 1.89 -10.96 vs. group 2, 0.27-1.57; < 0.001) but not using lumen diameter:vessel wall thickness where a substantial overlap in ranges between groups occurred (group 1, 1.58-22.66 vs. group 2, 1.40-11.63). We, therefore, conclude that lumen:vessel wall area is a more sensitive and preferred method for distinguishing cerebral arterioles from venules. The significance of this study is wide, as cerebral small vessel disease is a key feature of vascular dementia and understanding the pathogenesis relies on correct identification of vessels.

In recent years, there has been growing discussion to better understand the pathophysiological mechanisms of traumatic brain injury and post-traumatic stress disorder and how they may be linked to an increased risk of neurodegenerative diseases including Alzheimer's disease in veterans.

We aimed to test the hypothesis that in spontaneously hypertensive stroke-prone rats (SHRSP), non-amyloid cerebral small vessel disease/hypertensive arteriopathy (HA) results in vessel wall injury that may promote cerebral amyloid angiopathy (CAA). Our study comprised 21 male SHRSP (age 17-44 weeks) and 10 age- and sex-matched Wistar control rats, that underwent two-photon (2PM) imaging of the arterioles in the parietal cortex using Methoxy-X04, Dextran and cerebral blood flow (CBF) measurements. Our data suggest that HA in SHRSP progresses in a temporal and age-dependent manner, starting from small vessel wall damage (stage 1A), proceeding to CBF reduction (stage 1B), non-occlusive (stage 2), and finally, occlusive thrombi (stage 3). Wistar animals also demonstrated small vessel wall damage, but were free of any of the later HA stages. Nearly half of all SHRSP additionally displayed vascular Methoxy-X04 positivity indicative of cortical CAA. Vascular β-amyloid deposits were found in small vessels characterized by thrombotic occlusions (stage 2 or 3). Post-mortem analysis of the rat brains confirmed the findings derived from intravital 2PM microscopy. Our data thus overall suggest that advanced HA may play a role in CAA development with the two small vessel disease entities might be related to the same pathological spectrum of the aging brain.

Tracers injected into CSF pass into the brain alongside arteries and out again. This has been recently termed the "glymphatic system" that proposes tracers enter the brain along periarterial "spaces" and leave the brain along the walls of veins. The object of the present study is to test the hypothesis that: (1) tracers from the CSF enter the cerebral cortex along pial-glial basement membranes as there are no perivascular "spaces" around cortical arteries, (2) tracers leave the brain along smooth muscle cell basement membranes that form the Intramural Peri-Arterial Drainage (IPAD) pathways for the elimination of interstitial fluid and solutes from the brain. 2 μL of 100 μM soluble, fluorescent fixable amyloid β (Aβ) were injected into the CSF of the cisterna magna of 6-10 and 24-30 month-old male mice and their brains were examined 5 and 30 min later. At 5 min, immunocytochemistry and confocal microscopy revealed Aβ on the outer aspects of cortical arteries colocalized with α-2 laminin in the pial-glial basement membranes. At 30 min, Aβ was colocalised with collagen IV in smooth muscle cell basement membranes in the walls of cortical arteries corresponding to the IPAD pathways. No evidence for drainage along the walls of veins was found. Measurements of the depth of penetration of tracer were taken from 11 regions of the brain. Maximum depths of penetration of tracer into the brain were achieved in the pons and caudoputamen. Conclusions drawn from the present study are that tracers injected into the CSF enter and leave the brain along separate periarterial basement membrane pathways. The exit route is along IPAD pathways in which Aβ accumulates in cerebral amyloid angiopathy (CAA) in Alzheimer's disease. Results from this study suggest that CSF may be a suitable route for delivery of therapies for neurological diseases, including CAA.

Cerebral small vessel disease (SVD) is a major contributor to stroke, cognitive impairment and dementia with limited therapeutic interventions. There is a critical need to provide mechanistic insight and improve translation between pre-clinical research and the clinic. A 2-day workshop was held which brought together experts from several disciplines in cerebrovascular disease, dementia and cardiovascular biology, to highlight current advances in these fields, explore synergies and scope for development. These proceedings provide a summary of key talks at the workshop with a particular focus on animal models of cerebral vascular disease and dementia, mechanisms and approaches to improve translation. The outcomes of discussion groups on related themes to identify the gaps in knowledge and requirements to advance knowledge are summarized.

Meninges that surround the CNS consist of an outer fibrous sheet of dura mater (pachymeninx) that is also the inner periosteum of the skull. Underlying the dura are the arachnoid and pia mater (leptomeninges) that form the boundaries of the subarachnoid space. In this review we (1) examine the development of leptomeninges and their role as barriers and facilitators in the foetal CNS. There are two separate CSF systems during early foetal life, inner CSF in the ventricles and outer CSF in the subarachnoid space. As the foramina of Magendi and Luschka develop, one continuous CSF system evolves. Due to the lack of arachnoid granulations during foetal life, it is most likely that CSF is eliminated by lymphatic drainage pathways passing through the cribriform plate and nasal submucosa. (2) We then review the fine structure of the adult human and rodent leptomeninges to establish their roles as barriers and facilitators for the movement of fluid, cells and pathogens. Leptomeningeal cells line CSF spaces, including arachnoid granulations and lymphatic drainage pathways, and separate elements of extracellular matrix from the CSF. The leptomeningeal lining facilitates the traffic of inflammatory cells within CSF but also allows attachment of bacteria such as Neisseria meningitidis and of tumour cells as CSF metastases. Single layers of leptomeningeal cells extend into the brain closely associated with the walls of arteries so that there are no perivascular spaces around arteries in the cerebral cortex. Perivascular spaces surrounding arteries in the white matter and basal ganglia relate to their two encompassing layers of leptomeninges. (3) Finally we examine the roles of ligands expressed by leptomeningeal cells for the attachment of inflammatory cells, bacteria and tumour cells as understanding these roles may aid the design of therapeutic strategies to manage developmental, autoimmune, infectious and neoplastic diseases relating to the CSF, the leptomeninges and the associated CNS.

The mechanisms behind the clearance of soluble waste from deep within the parenchyma of the brain remain unclear. Experimental evidence reveals that one pathway for clearance of waste, termed intra-mural peri-arterial drainage (IPAD), is the rapid drainage of interstitial fluid along basement membranes (BM) of the smooth muscle cells of cerebral arteries; failure of IPAD is closely associated with the pathology of Alzheimer's disease (AD), but its driving mechanism remains unclear. We have previously shown that arterial pulsations generated by the heart beat are not strong enough to drive IPAD. Here we present computational evidence for a mechanism for clearance of waste from the brain that is driven by functional hyperaemia, that is, the dilatation of cerebral arterioles as a consequence of increased nutrient demand from neurons. This mechanism is based on our model for the flow of fluid through the vascular BM. It accounts for clearance rates observed in mouse experiments, and aligns with pathological observations and recommendations to lower the individual risk of AD, such as mental and physical activity. Thus, our neurovascular hypothesis should act as the new working hypothesis for the driving force behind IPAD.

Elevated neurofilament light chain (NFL) levels within the cerebrospinal fluid (CSF) are a biomarker representing axonal neurodegeneration in rapid progressive neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). It is unclear to what extent the levels of NFL increase in the CSF (CSF-NFL) in a chronic neuroinflammatory process with axonal neurodegeneration, as found in primary progressive multiple sclerosis (PPMS). We used a multicenter approach to statistically compare CSF-NFL levels between PPMS patients ( = 50), ALS patients ( = 50), and healthy controls ( = 50). Clinical findings, including disease duration, expanded disability status scale (EDSS), electrophysiological recordings such as visual evoked potentials or spinal and cerebral MRI, and previously administered treatment were selected as experimental parameters retrospectively. Median [range] CSF-NFL concentrations in PPMS patients were significantly higher than in the controls [1724 (799-4275) pg/ml vs. 1202 (612-2934) pg/ml, = 0.015], and significantly lower compared to ALS patients [1724 (799-4275) pg/ml vs. 10238 (2610-35138) pg/ml, < 0.001]. There was no correlation between CSF-NFL and disease duration ( = 0.5), EDSS ( = 0.2) or treatment ( = 0.3). We conclude that CSF-NFL may mirror the proposed slow axonal degeneration in PPMS, but does not reflect the disease severity.

Aquaporin-4 (AQP4) is the most abundant water channel in the brain, and its inhibition before inducing focal ischemia, using the AQP4 inhibitor TGN-020, has been showed to reduce oedema in imaging studies. Here, we aimed to evaluate, for the first time, the histopathological effects of a single dose of TGN-020 administered after the occlusion of the medial cerebral artery (MCAO). On a rat model of non-reperfusion ischemia, we have assessed vascular densities, albumin extravasation, gliosis, and apoptosis at 3 and 7 days after MCAO. TGN-020 significantly reduced oedema, glial scar, albumin effusion, and apoptosis, at both 3 and 7 days after MCAO. The area of GFAP-positive gliotic rim decreased, and 3D fractal analysis of astrocytic processes revealed a less complex architecture, possibly indicating water accumulating in the cytoplasm. Evaluation of the blood vessels revealed thicker basement membranes colocalizing with exudated albumin in the treated animals, suggesting that inhibition of AQP4 blocks fluid flow towards the parenchyma in the paravascular drainage pathways of the interstitial fluid. These findings suggest that a single dose of an AQP4 inhibitor can reduce brain oedema, even if administered after the onset of ischemia, and AQP4 agonists/antagonists might be effective modulators of the paravascular drainage flow.

Cerebral amyloid angiopathy (CAA) has never been more relevant. The last 5 years have seen a rapid increase in publications and research in the field, with the development of new biomarkers for the disease, thanks to advances in MRI, amyloid positron emission tomography and cerebrospinal fluid biomarker analysis. The inadvertent development of CAA-like pathology in patients treated with amyloid-beta immunotherapy for Alzheimer's disease has highlighted the importance of establishing how and why CAA develops; without this information, the use of these treatments may be unnecessarily restricted. Our understanding of the clinical and radiological spectrum of CAA has continued to evolve, and there are new insights into the independent impact that CAA has on cognition in the context of ageing and intracerebral haemorrhage, as well as in Alzheimer's and other dementias. While the association between CAA and lobar intracerebral haemorrhage (with its high recurrence risk) is now well recognised, a number of management dilemmas remain, particularly when considering the use of antithrombotics, anticoagulants and statins. The Boston criteria for CAA, in use in one form or another for the last 20 years, are now being reviewed to reflect these new wide-ranging clinical and radiological findings. This review aims to provide a 5-year update on these recent advances, as well as a look towards future directions for CAA research and clinical practice.

Dilatation of periarteriolar spaces in MRI of the ageing human brains occurs in white matter (WM), basal ganglia and midbrain but not in cerebral cortex. Perivenous collagenous occurs in periventricular but not in subcortical WM.Here we test the hypotheses that (a) the capacity for dilatation of periarteriolar spaces correlates with the anatomical distribution of leptomeningeal cells coating intracerebral arteries and (b) the regional development of perivenous collagenous in the WM correlates with the population of intramural cells in the walls of veins.The anatomical distribution of leptomeningeal and intramural cells related to cerebral blood vessels is best documented by electron microscopy, requiring perfusion-fixed tissue not available in human material. We therefore analysed perfusion-fixed brain from a 12-year-old Beagle dog as the canine brain represents the anatomical arrangement in the human brain. Results showed regional variation in the arrangement of leptomeningeal cells around blood vessels. Arterioles are enveloped by one complete layer of leptomeninges often with a second incomplete layer in the WM. Venules showed incomplete layers of leptomeningeal cells. Intramural cell expression was higher in the post-capillary venules of the subcortical WM when compared with periventricular WM, suggesting that periventricular collagenosis around venules may be due to a lower resistance in the venular walls. It appears that the regional variation in the capacity for dilatation of arteriolar perivascular spaces in the white WM may be related to the number of perivascular leptomeningeal cells surrounding vessels in different areas of the brain.

Although there are no conventional lymphatic vessels in the brain, fluid and solutes drain along basement membranes (BMs) of cerebral capillaries and arteries towards the subarachnoid space and cervical lymph nodes. Convective influx/glymphatic entry of the cerebrospinal fluid (CSF) into the brain parenchyma occurs along the pial-glial BMs of arteries. This project tested the hypotheses that pial-glial BM of arteries are thicker in the midbrain, allowing more glymphatic entry of CSF. The MRI and PET images were obtained from a 4.2-year-old dog, whereas the post-mortem electron microscopy was performed in a 12-year-old dog. We demonstrated a significant increase in the thickness of the pial-glial BM in the midbrain compared with the same BM in different regions of the brain and an increase in the convective influx of fluid from the subarachnoid space. These results are highly significant for the intrathecal drug delivery into the brain, indicating that the midbrain is better equipped for convective influx/glymphatic entry of the CSF.

Amyloid beta (Aβ) accumulation in the walls of leptomeningeal arteries as cerebral amyloid angiopathy (CAA) is a major feature of Alzheimer's disease. In this study, we used global quantitative proteomic analysis to examine the hypothesis that the leptomeningeal arteries derived from patients with CAA have a distinct endophenotypic profile compared to those from young and elderly controls.

Evidence supporting the hypothesis that reduced cerebrospinal fluid (CSF) clearance is involved in the pathophysiology of Alzheimer disease (AD) comes primarily from rodent models. However, unlike rodents, in which predominant extracranial CSF egress is via olfactory nerves traversing the cribriform plate, human CSF clearance pathways are not well characterized. Dynamic PET with F-THK5117, a tracer for tau pathology, was used to estimate the ventricular CSF time-activity as a biomarker for CSF clearance. We tested 3 hypotheses: extracranial CSF is detected at the superior turbinates; CSF clearance is reduced in AD; and CSF clearance is inversely associated with amyloid deposition. Fifteen subjects, 8 with AD and 7 normal control volunteers, were examined with F-THK5117. Ten subjects additionally underwent C-Pittsburgh compound B (C-PiB) PET scanning, and 8 were C-PiB-positive. Ventricular time-activity curves of F-THK5117 were used to identify highly correlated time-activity curves from extracranial voxels. For all subjects, the greatest density of CSF-positive extracranial voxels was in the nasal turbinates. Tracer concentration analyses validated the superior nasal turbinate CSF signal intensity. AD patients showed ventricular tracer clearance reduced by 23% and 66% fewer superior turbinate CSF egress sites. Ventricular CSF clearance was inversely associated with amyloid deposition. The human nasal turbinate is part of the CSF clearance system. Lateral ventricle and superior nasal turbinate CSF clearance abnormalities are found in AD. Ventricular CSF clearance reductions are associated with increased brain amyloid depositions. These data suggest that PET-measured CSF clearance is a biomarker of potential interest in AD and other neurodegenerative diseases.

Alzheimer's disease (AD) is characterized by amyloid-β (Aβ) peptide deposition in brain parenchyma as plaques and in cerebral blood vessels as cerebral amyloid angiopathy (CAA). CAA deposition leads to several clinical complications, including intracerebral hemorrhage. The underlying molecular mechanisms that regulate plaque and CAA deposition in the vast majority of sporadic AD patients remain unclear. The clusterin () gene is genetically associated with AD and CLU has been shown to alter aggregation, toxicity, and blood-brain barrier transport of Aβ, suggesting it might play a key role in regulating the balance between Aβ deposition and clearance in both brain and blood vessels. Here, we investigated the effect of CLU on Aβ pathology using the amyloid precursor protein/presenilin 1 (APP/PS1) mouse model of AD amyloidosis on a or background. We found a marked decrease in plaque deposition in the brain parenchyma but an equally striking increase in CAA within the cerebrovasculature of APP/PS1; mice. Surprisingly, despite the several-fold increase in CAA levels, APP/PS1; mice had significantly less hemorrhage and inflammation. Mice lacking CLU had impaired clearance of Aβ in vivo and exogenously added CLU significantly prevented Aβ binding to isolated vessels ex vivo. These findings suggest that in the absence of CLU, Aβ clearance shifts to perivascular drainage pathways, resulting in fewer parenchymal plaques but more CAA because of loss of CLU chaperone activity, complicating the potential therapeutic targeting of CLU for AD.

Non-amyloid cerebral small vessel disease (CSVD) and cerebral amyloid angiopathy (CAA) may be interrelated through the damaged basement membranes (BMs) and extracellular matrix changes of small vessels, resulting in a failure of β-amyloid (Aβ) transport and degradation. We analyzed BM changes and the pattern of deposition of Aβ in the walls of blood vessels in spontaneously hypertensive stroke-prone rats (SHRSP), a non-transgenic CSVD model. In 45 SHRSP and 38 Wistar rats aged 18 to 32 weeks: (i) the percentage area immunostained for vascular collagen IV and laminin was quantified; (ii) the capillary BM thickness as well as endothelial and pericyte pathological changes were analysed using transmission electron microscopy (TEM); and (iii) the presence of vascular Aβ was assessed. Compared with controls, SHRSP exhibited a significantly higher percentage area immunostained with collagen IV in the striatum and thalamus. SHRSP also revealed an age-dependent increase of the capillary BM thickness and of endothelial vacuoles (caveolae) within subcortical regions. Endogenous Aβ deposits in the walls of small blood vessels were observed in the cortex (with the highest incidence found within fronto-parietal areas), striatum, thalamus and hippocampus. Vascular β-amyloid accumulations were frequently detected at sites of small vessel wall damage. Our data demonstrate changes in the expression of collagen IV and of the ultrastructure of BMs in the small vessels of SHRSP. Alterations are accompanied by vascular deposits of endogenous Aβ. Impaired β-amyloid clearance along perivascular and endothelial pathways and failure of extracellular Aβ degradation may be the key mechanisms connecting non-amyloid CSVD and CAA.

In this chapter we describe in detail the surgical and imaging techniques employed for the study of the anatomical routes of drainage of cerebrospinal fluid (CSF) and interstitial fluid (ISF) from the brain. The types of tracers, sites of injection, and volumes injected are crucial. For example, when testing the drainage of ISF from the parenchyma, volumes larger than 0.5 μL result in spillage of ISF into the ventricular CSF.

In this chapter we describe in detail the tissue processing techniques we employ for the study of cerebral tissue by transmission electron microscopy (TEM). In particular, we explain a technique that enables quantification of changes in cerebral basement membranes at the ultrastructural level. This is significant, as age related pathological conditions affecting the brain are often accompanied by ultrastructural changes in the cerebral vasculature.Briefly, experimental mice are fixed by perfusion and their brains removed. Brains are then vibratomed into 100 μm slices with regions of interest microdissected and processed for TEM following a protocol optimized for the preservation of cerebral tissue. Changes in the thickness of cerebral basement membranes are then quantified using novel software. Some prior knowledge of general TEM specimen preparation and sectioning will be useful when performing this protocol.

While hypercholesterolemia plays a causative role for the development of ischemic stroke in large vessels, its significance for cerebral small vessel disease (CSVD) remains unclear. We thus aimed to understand the detailed relationship between hypercholesterolemia and CSVD using the well described Ldlr-/- mouse model.

Alzheimer's Disease (AD) is the most common form of dementia and to date there is no cure or efficient prophylaxis. The cognitive decline correlates with the accumulation of amyloid-β (β) in the walls of capillaries and arteries. Our group has demonstrated that interstitial fluid and β are eliminated from the brain along the basement membranes of capillaries and arteries, the intramural periarterial drainage (IPAD) pathway. With advancing age and arteriosclerosis, the stiffness of arterial walls, this pathway fails in its function and β accumulates in the walls of arteries. In this study we tested the hypothesis that arterial pulsations drive IPAD and that a valve mechanism ensures the net drainage in a direction opposite to that of the blood flow. This hypothesis was tested using a mathematical model of the drainage mechanism. We demonstrate firstly that arterial pulsations are not strong enough to produce drainage velocities comparable to experimental observations. Secondly, we demonstrate that a valve mechanism such as directional permeability of the IPAD pathway is necessary to achieve a net reverse flow. The mathematical simulation results are confirmed by assessing the pattern of IPAD in mice using pulse modulators, showing no significant alteration of IPAD. Our results indicate that forces other than the cardiac pulsations are responsible for efficient IPAD.

Amyloid beta (Aβ) accumulation in the walls of leptomeningeal arteries as cerebral amyloid angiopathy (CAA) is a major feature of Alzheimer's disease. In this study, we used global quantitative proteomic analysis to examine the hypothesis that the leptomeningeal arteries derived from patients with CAA have a distinct endophenotypic profile compared to those from young and elderly controls.

Immune privilege of the central nervous system (CNS) has been ascribed to the presence of a blood-brain barrier and the lack of lymphatic vessels within the CNS parenchyma. However, immune reactions occur within the CNS and it is clear that the CNS has a unique relationship with the immune system. Recent developments in high-resolution imaging techniques have prompted a reassessment of the relationships between the CNS and the immune system. This review will take these developments into account in describing our present understanding of the anatomical connections of the CNS fluid drainage pathways towards regional lymph nodes and our current concept of immune cell trafficking into the CNS during immunosurveillance and neuroinflammation. Cerebrospinal fluid (CSF) and interstitial fluid are the two major components that drain from the CNS to regional lymph nodes. CSF drains via lymphatic vessels and appears to carry antigen-presenting cells. Interstitial fluid from the CNS parenchyma, on the other hand, drains to lymph nodes via narrow and restricted basement membrane pathways within the walls of cerebral capillaries and arteries that do not allow traffic of antigen-presenting cells. Lymphocytes targeting the CNS enter by a two-step process entailing receptor-mediated crossing of vascular endothelium and enzyme-mediated penetration of the glia limitans that covers the CNS. The contribution of the pathways into and out of the CNS as initiators or contributors to neurological disorders, such as multiple sclerosis and Alzheimer's disease, will be discussed. Furthermore, we propose a clear nomenclature allowing improved precision when describing the CNS-specific communication pathways with the immune system.

Beta-amyloid accumulation within arterial walls in cerebral amyloid angiopathy is associated with the onset of Alzheimer's disease. However, the mechanism of beta-amyloid clearance along peri-arterial pathways in the brain is not well understood. In this study, we investigate a transport mechanism in the arterial basement membrane consisting of forward-propagating waves and their reflections. The arterial basement membrane is modeled as a periodically deforming annulus filled with an incompressible single-phase Newtonian fluid. A reverse flow, which has been suggested in literature as a beta-amyloid clearance pathway, can be induced by the motion of reflected boundary waves along the annular walls. The wave amplitude and the volume of the annular region govern the flow magnitude and may have important implications for an aging brain. Magnitudes of transport obtained from control volume analysis and numerical solutions of the Navier-Stokes equations are presented.

Accumulation of amyloid-β (Aβ) in plaques in the brain and in artery walls as cerebral amyloid angiopathy indicates a failure of elimination of Aβ from the brain with age and Alzheimer's disease. A major pathway for elimination of Aβ and other soluble metabolites from the brain is along basement membranes within the walls of cerebral arteries that represent the lymphatic drainage pathways for the brain. The motive force for the elimination of Aβ along this perivascular pathway appears to be the contrary (reflection) wave that follows the arterial pulse wave. Following injection into brain parenchyma, Aβ rapidly drains out of the brain along basement membranes in the walls of cerebral arteries; such drainage is impaired in apolipoprotein E ε4 (ApoE4) mice. For drainage of Aβ to occur in a direction contrary to the pulse wave, some form of attachment to basement membrane would be required to prevent reflux of Aβ back into the brain during the passage of the subsequent pulse wave. In this study, we show first that apolipoprotein E co-localizes with Aβ in basement membrane drainage pathways in the walls of arteries. Secondly, we show by Atomic Force Microscopy that attachment of ApoE4/Aβ complexes to basement membrane laminin is significantly weaker than ApoE3/Aβ complexes. These results suggest that perivascular elimination of ApoE4/Aβ complexes would be less efficient than with other isoforms of apolipoprotein E, thus endowing a higher risk for Alzheimer's disease. Therapeutic correction for ApoE4/Aβ/laminin interactions may increase the efficiency of elimination of Aβ in the prevention of Alzheimer's disease. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Deposition of amyloid β (Aβ) in the walls of cerebral arteries as cerebral amyloid angiopathy (CAA) suggests an age-related failure of perivascular drainage of soluble Aβ from the brain. As CAA is associated with Alzheimer's disease and with intracerebral haemorrhage, the present study determines the unique sequence of changes that occur as Aβ accumulates in artery walls. Paraffin sections of post-mortem human occipital cortex were immunostained for collagen IV, fibronectin, nidogen 2, Aβ and smooth muscle actin and the immunostaining was analysed using Image J and confocal microscopy. Results showed that nidogen 2 (entactin) increases with age and decreases in CAA. Confocal microscopy revealed stages in the progression of CAA: Aβ initially deposits in basement membranes in the tunica media, replaces first the smooth muscle cells and then the connective tissue elements to leave artery walls completely or focally replaced by Aβ. The pattern of development of CAA in the human brain suggests expansion of Aβ from the basement membranes to progressively replace all tissue elements in the artery wall. Establishing this full picture of the development of CAA is pivotal in understanding the clinical presentation of CAA and for developing therapies to prevent accumulation of Aβ in artery walls. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Alzheimer's disease is characterized by accumulation of amyloid-β (Aβ) in the brain and in the walls of cerebral arteries. The focus of this work is on clearance of Aβ along artery walls, the failure of which may explain the accumulation of Aβ in Alzheimer's disease. Periarterial basement membranes form continuous channels from cerebral capillaries to major arteries on the surface of the brain. Arterial pressure pulses drive peristaltic flow in the basement membranes in the same direction as blood flow. Here we forward the hypothesis that flexible structures within the basement membrane, if oriented such they present greater resistance to forward than retrograde flow, may cause net reverse flow, advecting Aβ along with it. A solution was obtained for peristaltic flow with low Reynolds number, long wavelength compared to channel height and small channel height compared to vessel radius in a Darcy-Brinkman medium representing a square array of cylinders. Results show that retrograde flow is promoted by high cylinder volume fraction and low peristaltic amplitude. A decrease in cylinder concentration and/or an increase in amplitude, both of which may occur during ageing, can reduce retrograde flow or even cause a transition from retrograde to forward flow. Such changes may explain the accumulation of Aβ in the brain and in artery walls in Alzheimer's disease.

In the absence of conventional lymphatics, drainage of interstitial fluid and solutes from the brain parenchyma to cervical lymph nodes is along basement membranes in the walls of cerebral capillaries and tunica media of arteries. Perivascular pathways are also involved in the entry of CSF into the brain by the convective influx/glymphatic system. The objective of this study is to differentiate the cerebral vascular basement membrane pathways by which fluid passes out of the brain from the pathway by which CSF enters the brain. Experiment 1: 0.5 µl of soluble biotinylated or fluorescent Aβ, or 1 µl 15 nm gold nanoparticles was injected into the mouse hippocampus and their distributions determined at 5 min by transmission electron microscopy. Aβ was distributed within the extracellular spaces of the hippocampus and within basement membranes of capillaries and tunica media of arteries. Nanoparticles did not enter capillary basement membranes from the extracellular spaces. Experiment 2: 2 µl of 15 nm nanoparticles were injected into mouse CSF. Within 5 min, groups of nanoparticles were present in the pial-glial basement membrane on the outer aspect of cortical arteries between the investing layer of pia mater and the glia limitans. The results of this study and previous research suggest that cerebral vascular basement membranes form the pathways by which fluid passes into and out of the brain but that different basement membrane layers are involved. The significance of these findings for neuroimmunology, Alzheimer's disease, drug delivery to the brain and the concept of the Virchow-Robin space are discussed.

The lymphatic clearance pathways of the brain are different compared to the other organs of the body and have been the subject of heated debates. Drainage of brain extracellular fluids, particularly interstitial fluid (ISF) and cerebrospinal fluid (CSF), is not only important for volume regulation, but also for removal of waste products such as amyloid beta (Aβ). CSF plays a special role in clinical medicine, as it is available for analysis of biomarkers for Alzheimer's disease. Despite the lack of a complete anatomical and physiological picture of the communications between the subarachnoid space (SAS) and the brain parenchyma, it is often assumed that Aβ is cleared from the cerebral ISF into the CSF. Recent work suggests that clearance of the brain mainly occurs during sleep, with a specific role for peri- and para-vascular spaces as drainage pathways from the brain parenchyma. However, the direction of flow, the anatomical structures involved and the driving forces remain elusive, with partially conflicting data in literature. The presence of Aβ in the glia limitans in Alzheimer's disease suggests a direct communication of ISF with CSF. Nonetheless, there is also the well-described pathology of cerebral amyloid angiopathy associated with the failure of perivascular drainage of Aβ. Herein, we review the role of the vasculature and the impact of vascular pathology on the peri- and para-vascular clearance pathways of the brain. The different views on the possible routes for ISF drainage of the brain are discussed in the context of pathological significance.

Maternal obesity is associated with increased risk of developing diabetes, obesity and premature death in adult offspring. Mid-life diabetes, hypertension and hypercholesterolaemia are risk factors for the development of sporadic Alzheimer's disease (AD). A key pathogenic feature of AD is the accumulation of β-amyloid (Aβ) in the brain. The purpose of this study was to investigate the effect of high fat diet feeding during early life on Aβ pathology in the Tg2576 mouse model of AD. Female mice were fed a standard (C) or high fat (HF) diet before mating and during gestation and lactation. At weaning, male offspring were fed a C diet. Significantly higher levels of guanidine-soluble Aβ and plaque loads were observed in the hippocampi of 11-month old Tg2576 mice born to mothers fed a HF diet. Changes in the extracellular matrix led to increased retention of Aβ within the parenchyma. These data support a role for maternal and gestational health on the health of the aged brain and pathologies associated with AD and may provide a novel target for both the prevention and treatment of AD.

The aim of this study was to examine hemisphere asymmetry of response to pharmacologic treatment in an Alzheimer's disease mouse model using cilostazol as a chemical stimulus. Eight-month-old mice were assigned to vehicle or cilostazol treatment for three months and hemispheres were analyzed using quantitative proteomics. Bioinformatics interpretation showed that following treatment, aggregation of blood platelets significantly decreased in the right hemisphere whereas neurodegeneration significantly decreased and synaptic transmission increased in the left hemisphere only. Our study provides novel evidence on cerebral laterality of pharmacologic activity, with important implications in deciphering regional pharmacodynamic effects of existing drugs thus uncovering novel hemisphere-specific therapeutic targets.

The accumulation of soluble and insoluble amyloid-β (Aβ) in the brain indicates failure of elimination of Aβ from the brain with age and Alzheimer's disease (AD). There is a variety of mechanisms for elimination of Aβ from the brain. They include the action of microglia and enzymes together with receptor-mediated absorption of Aβ into the blood and periarterial lymphatic drainage of Aβ. Although the brain possesses no conventional lymphatics, experimental studies have shown that fluid and solutes, such as Aβ, are eliminated from the brain along 100 nm wide basement membranes in the walls of cerebral capillaries and arteries. This lymphatic drainage pathway is reflected in the deposition of Aβ in the walls of human arteries with age and AD as cerebral amyloid angiopathy (CAA). Initially, Aβ diffuses through the extracellular spaces of gray matter in the brain and then enters basement membranes in capillaries and arteries to flow out of the brain. Although diffusion through the extracellular spaces of the brain has been well characterized, the exact mechanism whereby perivascular elimination of Aβ occurs has not been resolved. Here we use a computational model to describe the process of periarterial drainage in the context of diffusion in the brain, demonstrating that periarterial drainage along basement membranes is very rapid compared with diffusion. Our results are a validation of experimental data and are significant in the context of failure of periarterial drainage as a mechanism underlying the pathogenesis of AD as well as complications associated with its immunotherapy.

Accumulation of amyloid-β (Aβ) in plaques in the brain and in artery walls as cerebral amyloid angiopathy indicates a failure of elimination of Aβ from the brain with age and Alzheimer's disease. A major pathway for elimination of Aβ and other soluble metabolites from the brain is along basement membranes within the walls of cerebral arteries that represent the lymphatic drainage pathways for the brain. The motive force for the elimination of Aβ along this perivascular pathway appears to be the contrary (reflection) wave that follows the arterial pulse wave. Following injection into brain parenchyma, Aβ rapidly drains out of the brain along basement membranes in the walls of cerebral arteries; such drainage is impaired in apolipoprotein E ε4 (ApoE4) mice. For drainage of Aβ to occur in a direction contrary to the pulse wave, some form of attachment to basement membrane would be required to prevent reflux of Aβ back into the brain during the passage of the subsequent pulse wave. In this study, we show first that apolipoprotein E co-localizes with Aβ in basement membrane drainage pathways in the walls of arteries. Secondly, we show by Atomic Force Microscopy that attachment of ApoE4/Aβ complexes to basement membrane laminin is significantly weaker than ApoE3/Aβ complexes. These results suggest that perivascular elimination of ApoE4/Aβ complexes would be less efficient than with other isoforms of apolipoprotein E, thus endowing a higher risk for Alzheimer's disease. Therapeutic correction for ApoE4/Aβ/laminin interactions may increase the efficiency of elimination of Aβ in the prevention of Alzheimer's disease. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Deposition of amyloid β (Aβ) in the walls of cerebral arteries as cerebral amyloid angiopathy (CAA) suggests an age-related failure of perivascular drainage of soluble Aβ from the brain. As CAA is associated with Alzheimer's disease and with intracerebral haemorrhage, the present study determines the unique sequence of changes that occur as Aβ accumulates in artery walls. Paraffin sections of post-mortem human occipital cortex were immunostained for collagen IV, fibronectin, nidogen 2, Aβ and smooth muscle actin and the immunostaining was analysed using Image J and confocal microscopy. Results showed that nidogen 2 (entactin) increases with age and decreases in CAA. Confocal microscopy revealed stages in the progression of CAA: Aβ initially deposits in basement membranes in the tunica media, replaces first the smooth muscle cells and then the connective tissue elements to leave artery walls completely or focally replaced by Aβ. The pattern of development of CAA in the human brain suggests expansion of Aβ from the basement membranes to progressively replace all tissue elements in the artery wall. Establishing this full picture of the development of CAA is pivotal in understanding the clinical presentation of CAA and for developing therapies to prevent accumulation of Aβ in artery walls. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Alzheimer's disease is characterized by accumulation of amyloid-β (Aβ) in the brain and in the walls of cerebral arteries. The focus of this work is on clearance of Aβ along artery walls, the failure of which may explain the accumulation of Aβ in Alzheimer's disease. Periarterial basement membranes form continuous channels from cerebral capillaries to major arteries on the surface of the brain. Arterial pressure pulses drive peristaltic flow in the basement membranes in the same direction as blood flow. Here we forward the hypothesis that flexible structures within the basement membrane, if oriented such they present greater resistance to forward than retrograde flow, may cause net reverse flow, advecting Aβ along with it. A solution was obtained for peristaltic flow with low Reynolds number, long wavelength compared to channel height and small channel height compared to vessel radius in a Darcy-Brinkman medium representing a square array of cylinders. Results show that retrograde flow is promoted by high cylinder volume fraction and low peristaltic amplitude. A decrease in cylinder concentration and/or an increase in amplitude, both of which may occur during ageing, can reduce retrograde flow or even cause a transition from retrograde to forward flow. Such changes may explain the accumulation of Aβ in the brain and in artery walls in Alzheimer's disease.

Epidemiological studies suggest an association between maternal obesity and adverse neurodevelopmental outcomes in offspring. Our aim was to compare the global proteomic portrait in the cerebral cortex between mice born to mothers on a high-fat or control diet who themselves were fed a high-fat or control diet. Male mice born to dams fed a control (C) or high-fat (H) diet 4 weeks before conception and during gestation, and lactation were assigned to either C or H diet at weaning. Mice were killed at 19 weeks and their cerebral cortices were analysed using a two-dimensional liquid chromatography-mass spectrometry methodology. In total, 6 695 proteins were identified (q<0.01), 10% of which were modulated in at least one of the groups relative to controls. In silico analysis revealed that mice clustered based on the diet of the mother and not their own diet and that maternal high-fat diet was significantly associated with response to hypoxia/oxidative stress and apoptosis in the cerebral cortex of the adult offspring. Maternal high-fat diet resulted in distinct endophenotypic changes of the adult offspring cerebral cortex independent of its current diet. The identified proteins could represent novel therapeutic targets for the prevention of neuropathological features resulting from maternal obesity.

Accumulation of toxic protein aggregates-amyloid-β (Aβ) plaques and hyperphosphorylated tau tangles-is the pathological hallmark of Alzheimer disease (AD). Aβ accumulation has been hypothesized to result from an imbalance between Aβ production and clearance; indeed, Aβ clearance seems to be impaired in both early and late forms of AD. To develop efficient strategies to slow down or halt AD, it is critical to understand how Aβ is cleared from the brain. Extracellular Aβ deposits can be removed from the brain by various clearance systems, most importantly, transport across the blood-brain barrier. Findings from the past few years suggest that astroglial-mediated interstitial fluid (ISF) bulk flow, known as the glymphatic system, might contribute to a larger portion of extracellular Aβ (eAβ) clearance than previously thought. The meningeal lymphatic vessels, discovered in 2015, might provide another clearance route. Because these clearance systems act together to drive eAβ from the brain, any alteration to their function could contribute to AD. An understanding of Aβ clearance might provide strategies to reduce excess Aβ deposits and delay, or even prevent, disease onset. In this Review, we describe the clearance systems of the brain as they relate to proteins implicated in AD pathology, with the main focus on Aβ.

Alzheimer's disease (AD) is characterized by the accumulation of β-amyloid (Aβ) peptides in the extracellular spaces of the brain as plaques and in the walls of blood vessels as cerebral amyloid angiopathy (CAA). Failure of perivascular drainage of Aβ along cerebrovascular basement membranes contributes to the development of CAA. Mid-life hypercholesterolaemia is a risk factor for the development of AD. Maternal obesity is associated with the development of obesity, hypertension and hypercholesterolaemia in adulthood, suggesting that the risk for AD and CAA may also be influenced by the early-life environment. In the present study, we tested the hypothesis that early-life exposure to a high-fat diet results in changes to the cerebrovasculature and failure of Aβ clearance from the brain. We also assessed whether vascular Aβ deposition is greater in the brains of aged humans with a history of hyperlipidaemia, compared to age-matched controls with normal lipidaemia. Using a mouse model of maternal obesity, we found that exposure to a high-fat diet during gestation and lactation induced changes in multiple components of the neurovascular unit, including a down-regulation in collagen IV, fibronectin and apolipoprotein E, an up-regulation in markers of astrocytes and perivascular macrophages and altered blood vessel morphology in the brains of adult mice. Sustained high-fat diet over the entire lifespan resulted in additional decreases in levels of pericytes and impaired perivascular clearance of Aβ from the brain. In humans, vascular Aβ load was significantly increased in the brains of aged individuals with a history of hypercholesterolaemia. These results support a critical role for early dietary influence on the brain vasculature across the lifespan, with consequences for the development of age-related cerebrovascular and neurodegenerative diseases.

White matter abnormalities on magnetic resonance imaging (MRI) are associated with dementia and include white matter hyperintensities (WMH; also termed leukoaraiosis) and visible perivascular spaces (PVS). We review the potential role of impaired drainage of interstitial fluid in the pathogenesis of WMH and PVS. Whereas the volume of extracellular space in the grey matter is tightly controlled, fluid accumulates and expands the extracellular spaces of the white matter in acute hydrocephalus, vasogenic edema and WMH. Although there are no conventional lymphatic vessels in the brain, there is very effective lymphatic drainage for fluid and solutes along restricted pathways in the basement membranes of cerebral capillaries and arteries in young individuals. Lymphatic drainage of the brain is impaired with age and in association with apolipoprotein E ε4, risk factors for Alzheimer's disease and cerebral amyloid angiopathy (CAA). Deposition of proteins in the lymphatic drainage pathways in the walls of cerebral arteries with age is recognized as protein elimination failure angiopathy (PEFA), as in CAA and cerebral autosomal dominant arteriopathy and leukoencephalopathy (CADASIL). Facilitating perivascular lymphatic drainage from the aging brain may play a significant role in the prevention of CAA, WMH and Alzheimer's disease and may enhance the efficacy of immunotherapy for Alzheimer's disease.

APOE ϵ4 - one of three possible allelic variants (ϵ2, ϵ3 and ϵ4) of the polymorphic protein APOE - is well characterized in its role as the strongest risk factor (after old age) for sporadic Alzheimer's disease (AD). Perhaps less well known, and certainly less well characterized, is that this ϵ4 variant of the APOE gene also is a significant risk factor for age-related cognitive decline in nonclinical populations. This article considers APOE ϵ4 effects on cognition in people without dementia, the extent to which such effects may depend on age and on gender and other interactive biological systems that change across the lifespan.

Aβ immunotherapy for Alzheimer's disease (AD) results in the removal of Aβ plaques and increased cerebral amyloid angiopathy (CAA). In current clinical trials, amyloid-related imaging abnormalities (ARIAs), putatively due to exacerbation of CAA, are concerning side effects. We aimed to assess the role of the Aβ transporter apolipoprotein E (apoE) in the exacerbation of CAA and development of CAA-associated vasculopathy after Aβ immunotherapy. 12 Aβ42-immunized AD (iAD; AN1792, Elan Pharmaceuticals) cases were compared with 28 unimmunized AD (cAD) cases. Immunohistochemistry was quantified for Aβ42, apoE, apoE E4 and smooth muscle actin, and CAA-associated vasculopathy was analyzed. Aβ immunotherapy was associated with redistribution of apoE from cortical plaques to cerebral vessel walls, mirroring the altered distribution of Aβ42. Concentric vessel wall splitting was increased threefold in leptomeningeal vessels after immunotherapy (cAD 6.3 vs iAD 20.6 %, P < 0.001), but smooth muscle cell abnormalities did not differ. The findings suggest that apoE is involved in the removal of plaques and transport of Aβ to the cerebral vasculature induced by Aβ immunotherapy. Immunotherapy was not associated with CAA-related vascular smooth muscle damage, but was accompanied by increased splitting of the vessel wall, perhaps reflecting enhanced deposition and subsequent removal of Aβ. ARIA occurring in some current trials of Aβ immunotherapy may reflect an extreme form of these vascular changes.

Brain amyloidosis is a key feature of Alzheimer's disease (AD). It also incorporates cerebrovascular amyloid β (Aβ) in the form of cerebral amyloid angiopathy (CAA) involving neurovascular dysfunction. We have recently shown by retrospective analysis that patients with mild cognitive impairment receiving a vasoactive drug cilostazol, a selective inhibitor of phosphodiesterase (PDE) III, exhibit significantly reduced cognitive decline. Here, we tested whether cilostazol protects against the disruption of the neurovascular unit and facilitates the arterial pulsation-driven perivascular drainage of Aβ in AD/CAA.

In Alzheimer's disease, amyloid-β (Aβ) accumulates as insoluble plaques in the brain and deposits in blood vessel walls as cerebral amyloid angiopathy (CAA). The severity of CAA correlates with the degree of cognitive decline in dementia. The distribution of Aβ in the walls of capillaries and arteries in CAA suggests that Aβ is deposited in the perivascular pathways by which interstitial fluid drains from the brain. Soluble Aβ from the extracellular spaces of gray matter enters the basement membranes of capillaries and drains along the arterial basement membranes that surround smooth muscle cells toward the leptomeningeal arteries. The motive force for perivascular drainage is derived from arterial pulsations combined with the valve effect of proteins present in the arterial basement membranes. Physical and biochemical changes associated with arteriosclerosis, aging and possession of apolipoprotein E4 genotype lead to a failure of perivascular drainage of soluble proteins, including Aβ. Perivascular cells associated with arteries and the lymphocytes recruited in the perivenous spaces contribute to the clearance of Aβ. The failure of perivascular clearance of Aβ may be a major factor in the accumulation of Aβ in CAA and may have significant implications for the design of therapeutics for the treatment of Alzheimer's disease.

Immunological privilege appears to be a product of unique lymphatic drainage systems for the brain and receptor-mediated entry of inflammatory cells through the blood-brain barrier. Most organs of the body have well-defined lymphatic vessels that carry extracellular fluid, antigen presenting cells, lymphocytes, neoplastic cells and even bacteria to regional lymph nodes. The brain has no such conventional lymphatics, but has perivascular pathways that drain interstitial fluid (ISF) from brain parenchyma and cerebrospinal fluid (CSF) from the subarachnoid space to cervical lymph nodes. ISF and solutes drain along narrow, ∼100 nm-thick basement membranes within the walls of cerebral capillaries and arteries to cervical lymph nodes; this pathway does not allow traffic of lymphocytes or antigen presenting cells from brain to lymph nodes. Although CSF drains into blood through arachnoid villi, CSF also drains from the subarachnoid space through channels in the cribriform plate of the ethmoid bone into nasal lymphatics and thence to cervical lymph nodes. This pathway does allow the traffic of lymphocytes and antigen presenting cells from CSF to cervical lymph nodes. Efferent pathways by which lymphocytes enter the brain are regulated by selected integrins on lymphocytes and selective receptors on vascular endothelial cells. Here we review: (1) the structure and function of afferent lymphatic drainage of ISF and CSF, (2) mechanisms involved in the efferent pathways by which lymphocytes enter the brain and (3) the failure of lymphatic drainage of the brain parenchyma with age and the role of such failure in the pathogenesis of Alzheimer's disease.

There is substantial controversy regarding the causative role of amyloid β (Aβ) deposition in Alzheimer's disease (AD). The cerebrovasculature plays an important role in the elimination of Aβ from the brain and hypertension is a well-known risk factor for AD. In spontaneously hypertensive stroke-prone rats (SHRSP), an animal model of chronic arterial hypertension, cerebral small vessel disease (CSVD) leads to age-dependent parenchymal Aβ accumulation similar to that observed in AD. These data approve the neuropathological link between CSVD and AD, confirm the challenge that parenchymal Aβ deposition is a specific marker for AD and disclose the meaning of SHRSP as valid experimental model to investigate the association between hypertension, CSVD, and Aβ plaques.

There is growing evidence that endothelial failure and subsequent blood brain barrier (BBB) breakdown initiate cerebral small vessel disease (CSVD) pathology. In spontaneously hypertensive stroke-prone rats (SHRSP) endothelial damage is indicated by intraluminal accumulations of erythrocytes (erythrocyte thrombi) that are not observed with current magnetic resonance imaging techniques. Two-photon microscopy (2 PM) offers the potential for real-time direct detection of the small vasculature. Thus, within this pilot study we investigated the sensitivity of 2 PM to detect erythrocyte thrombi expressing initiating CSVD phenomena in vivo.

Deposition of amyloid-β (Aβ) in blood vessel walls as cerebral amyloid angiopathy (CAA) is observed in the majority of Alzheimer's disease (AD) brains. Inhibition of the 5-lipoxygenase (5-LOX) pathway has recently been suggested to play a role in reducing parenchymal Aβ deposition. However, products of the 5-LOX pathway also activate the peroxisome proliferator-activated receptor (PPAR) family, which promotes clearance of Aβ from the brain.

Accumulation of amyloid-β (Aβ) and hyperphosphorylated tau (ptau) accompany cerebral small vessel disease (CSVD) in the aging brain and in Alzheimer's disease. CSVD is characterized by a heterogeneous spectrum of histopathological features possibly initiated by an endothelial dysfunction and blood-brain barrier (BBB) breakdown.

Cerebral small vessel disease (CSVD) in spontaneously hypertensive stroke prone rats (SHRSP) is accompanied by parenchymal amyloid-β (Aβ) deposition in the brain and by hypertensive nephropathy with tubulointerstitial damage. N-acetylcysteine (NAC) promotes blood-brain barrier (BBB) breakdown in SHRSP and may thus accelerate the failure of vascular and perivascular clearance of Aβ.

Cerebral amyloid angiopathy (CAA), the accumulation of β-amyloid (Aβ) peptides in the walls of cerebral blood vessels, is observed in the majority of Alzheimer's disease (AD) brains and is thought to be due to a failure of the aging brain to clear Aβ. Perivascular drainage of Aβ along cerebrovascular basement membranes (CVBMs) is one of the mechanisms by which Aβ is removed from the brain. CVBMs are specialized sheets of extracellular matrix that provide structural and functional support for cerebral blood vessels. Changes in CVBM composition and structure are observed in the aged and AD brain and may contribute to the development and progression of CAA. This review summarizes the properties of the CVBM, its role in mediating clearance of interstitial fluids and solutes from the brain, and evidence supporting a role for CVBM in the etiology of CAA.

Immunological privilege appears to be a product of unique lymphatic drainage systems for the brain and receptor-mediated entry of inflammatory cells through the blood-brain barrier. Most organs of the body have well-defined lymphatic vessels that carry extracellular fluid, antigen presenting cells, lymphocytes, neoplastic cells and even bacteria to regional lymph nodes. The brain has no such conventional lymphatics, but has perivascular pathways that drain interstitial fluid (ISF) from brain parenchyma and cerebrospinal fluid (CSF) from the subarachnoid space to cervical lymph nodes. ISF and solutes drain along narrow, ∼100 nm-thick basement membranes within the walls of cerebral capillaries and arteries to cervical lymph nodes; this pathway does not allow traffic of lymphocytes or antigen presenting cells from brain to lymph nodes. Although CSF drains into blood through arachnoid villi, CSF also drains from the subarachnoid space through channels in the cribriform plate of the ethmoid bone into nasal lymphatics and thence to cervical lymph nodes. This pathway does allow the traffic of lymphocytes and antigen presenting cells from CSF to cervical lymph nodes. Efferent pathways by which lymphocytes enter the brain are regulated by selected integrins on lymphocytes and selective receptors on vascular endothelial cells. Here we review: (1) the structure and function of afferent lymphatic drainage of ISF and CSF, (2) mechanisms involved in the efferent pathways by which lymphocytes enter the brain and (3) the failure of lymphatic drainage of the brain parenchyma with age and the role of such failure in the pathogenesis of Alzheimer's disease.

The pathophysiological concept of ischaemic stroke was recently expanded to a more comprehensive perspective, focussing on the vasculature as well as peri- and juxtavascular cells including astrocytes. Increasing evidence also supports a role of the vasculature in Alzheimer's disease (AD), but causal relationships are poorly understood. The purpose of this study was to examine vascular alterations due to cerebral ischaemia in aged wildtype (WT) mice and in the triple-transgenic (3xTg) mouse model of AD. Three- and 12-month-old WT and 3xTg mice underwent permanent middle cerebral artery occlusion. One day after ischaemia onset, expression of collagen IV and laminin as basement membrane constituents, and Solanum tuberosum lectin (STL) as endothelial marker was quantified in the ischaemic neocortex, striatum and hippocampus. Further, CD31- and aquaporin-4-immunoreactivity served for coverage of endothelium and astrocyte endfeet. Ischaemia resulted in strong upregulation of collagen IV and laminin in the neocortex of 3-month-old WT and 3xTg mice, while STL appeared unaffected. Quantification confirmed collagen IV upregulation in the ischaemic neocortex of 3- and 12-month-old WT and 3xTg mice, whereas striatal changes were limited to young WT mice. However, collagen IV expression in the hippocampus appeared nearly unaltered. Qualitative and quantitative data evidenced more severe degeneration of endothelial cells and astrocyte endfeet in 3xTg mice. In conclusion, this study supports the critical impact of the vasculature in the aged and AD brain by showing an age- and genetic background-dependent response of basement membranes to cerebral ischaemia, and a pronounced endothelial and astrocytic degeneration in the AD-like brain.

Failure of elimination of proteins from the brain is a major feature in many neurodegenerative diseases. Insoluble proteins accumulate in brain parenchyma and in walls of cerebral capillaries and arteries. Cerebral amyloid angiopathy (CAA) is a descriptive term for amyloid in vessel walls. Here, we adopt the term protein elimination failure angiopathy (PEFA) to focus on mechanisms involved in the pathogenesis of a spectrum of disorders that exhibit both unique and common features of protein accumulation in blood vessel walls. We review (a) normal pathways and mechanisms by which proteins and other soluble metabolites are eliminated from the brain along 100- to 150-nm-thick basement membranes in walls of cerebral capillaries and arteries that serve as routes for lymphatic drainage of the brain; (b) a spectrum of proteins involved in PEFA; and (c) changes that occur in artery walls and contribute to failure of protein elimination. We use accumulation of amyloid beta (Aβ), prion protein and granular osmiophilic material (GOM) in cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) as examples of different factors involved in the aetiology and pathogenesis of PEFA. Finally, we discuss how knowledge of factors involved in PEFA may help to focus on new therapies for neurodegenerative diseases. When Aβ (following immunotherapy) and prion protein are released from brain parenchyma they deposit in walls of cerebral capillaries and arteries; GOM in CADASIL accumulates primarily in artery walls. Therefore, the focus of therapy for protein clearance in neurodegenerative disease should perhaps be on facilitating perivascular elimination of proteins and reducing PEFA.

Basement membranes in the walls of cerebral capillaries and arteries form a major lymphatic drainage pathway for fluid and solutes from the brain. Amyloid-β (Aβ) draining from the brain is deposited in such perivascular pathways as cerebral amyloid angiopathy (CAA) in Alzheimer's disease (AD). CAA increases in severity when Aβ is removed from the brain parenchyma by immunotherapy for AD. In this study we investigated the consequences of immune complexes in artery walls upon drainage of solutes similar to soluble Aβ. We tested the hypothesis that, following active immunization with ovalbumin, immune complexes form within the walls of cerebral arteries and impair the perivascular drainage of solutes from the brain. Mice were immunized against ovalbumin and then challenged by intracerebral microinjection of ovalbumin. Perivascular drainage of solutes was quantified following intracerebral microinjection of soluble fluorescent 3kDa dextran into the brain at different time intervals after intracerebral challenge with ovalbumin.

Development of cerebral amyloid angiopathy (CAA) and Alzheimer's disease (AD) is associated with failure of elimination of amyloid-β (Aβ) from the brain along perivascular basement membranes that form the pathways for drainage of interstitial fluid and solutes from the brain. In transgenic APP mouse models of AD, the severity of cerebral amyloid angiopathy is greater in the cerebral cortex and hippocampus, intermediate in the thalamus, and least in the striatum. In this study we test the hypothesis that age-related regional variation in (1) vascular basement membranes and (2) perivascular drainage of Aβ contribute to the different regional patterns of CAA in the mouse brain. Quantitative electron microscopy of the brains of 2-, 7-, and 23-month-old mice revealed significant age-related thickening of capillary basement membranes in cerebral cortex, hippocampus, and thalamus, but not in the striatum. Results from Western blotting and immunocytochemistry experiments showed a significant reduction in collagen IV in the cortex and hippocampus with age and a reduction in laminin and nidogen 2 in the cortex and striatum. Injection of soluble Aβ into the hippocampus or thalamus showed an age-related reduction in perivascular drainage from the hippocampus but not from the thalamus. The results of the study suggest that changes in vascular basement membranes and perivascular drainage with age differ between brain regions, in the mouse, in a manner that may help to explain the differential deposition of Aβ in the brain in AD and may facilitate development of improved therapeutic strategies to remove Aβ from the brain in AD.

Deposition of amyloid-β (Aβ) in blood vessel walls as cerebral amyloid angiopathy (CAA) is observed in the majority of Alzheimer's disease (AD) brains. Inhibition of the 5-lipoxygenase (5-LOX) pathway has recently been suggested to play a role in reducing parenchymal Aβ deposition. However, products of the 5-LOX pathway also activate the peroxisome proliferator-activated receptor (PPAR) family, which promotes clearance of Aβ from the brain.

In this study, we investigate the underlying mechanisms of antibody-mediated inflammation in the brain. We show that immune complexes formed in the brain parenchyma generate a robust and long-lasting inflammatory response, characterized by increased expression of the microglia markers CD11b, CD68 and FcRII/III, but no neutrophil recruitment. In addition to these histological changes, we observed transient behavioural changes that coincided with the inflammatory response in the brain. The inflammatory and behavioural changes were absent in Fc-gamma chain (Fcγ)-deficient mice, while C1q-deficient mice were not different from wild-type mice. We conclude that, in the presence of antigen, antibodies can lead to a local immune complex-mediated inflammatory reaction in the brain parenchyma and indirectly induce neuronal tissue damage through recruitment and activation of microglia via Fcγ receptors. These observations may have important implications for the development of therapeutic antibodies directed against neuronal antigens used for therapeutic intervention in neurological diseases.

Failure of elimination of amyloid-β (Aβ) from the brain and vasculature appears to be a key factor in the etiology of sporadic Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). In addition to age, possession of an apolipoprotein E (APOE) ε4 allele is a strong risk factor for the development of sporadic AD. The present study tested the hypothesis that possession of the APOE ε4 allele is associated with disruption of perivascular drainage of Aβ from the brain and with changes in cerebrovascular basement membrane protein levels. Targeted replacement (TR) mice expressing the human APOE3 (TRE3) or APOE4 (TRE4) genes and wildtype mice received intracerebral injections of human Aβ(40). Aβ(40) aggregated in peri-arterial drainage pathways in TRE4 mice, but not in TRE3 or wildtype mice. The number of Aβ deposits was significantly higher in the hippocampi of TRE4 mice than in the TRE3 mice, at both 3- and 16-months of age, suggesting that clearance of Aβ was disrupted in the brains of TRE4 mice. Immunocytochemical and Western blot analysis of vascular basement membrane proteins demonstrated significantly raised levels of collagen IV in 3-month-old TRE4 mice compared with TRE3 and wild type mice. In 16-month-old mice, collagen IV and laminin levels were unchanged between wild type and TRE3 mice, but were lower in TRE4 mice. The results of this study suggest that APOE4 may increase the risk for AD through disruption and impedance of perivascular drainage of soluble Aβ from the brain. This effect may be mediated, in part, by changes in age-related expression of basement membrane proteins in the cerebral vasculature.

The deposition of amyloid-β (Aβ) peptides in the walls of leptomeningeal and cortical blood vessels as cerebral amyloid angiopathy (CAA) is present in normal ageing and the majority of Alzheimer's disease (AD) brains. The failure of clearance mechanisms to eliminate Aβ from the brain contributes to the development of sporadic CAA and AD. Here, we investigated the effects of CAA and ageing on the pattern of perivascular drainage of solutes in the brains of naïve mice and in the Tg2576 mouse model of AD. We report that drainage of small molecular weight dextran along cerebrovascular basement membranes is impaired in the hippocampal capillaries and arteries of 22-month-old wild-type mice compared to 3- and 7-month-old animals, which was associated with age-dependent changes in capillary density. Age-related alterations in the levels of laminin, fibronectin and perlecan in vascular basement membranes were also noted in wild-type mice. Furthermore, dextran was observed in the walls of veins of Tg2576 mice in the presence of CAA, suggesting that deposition of Aβ in vessel walls disrupts the normal route of elimination of solutes from the brain parenchyma. These data support the hypothesis that perivascular solute drainage from the brain is altered both in the ageing brain and as a consequence of CAA. These findings have implications for the success of therapeutic strategies for the treatment of AD that rely upon the health of the ageing cerebral vasculature.

An exploratory investigation is reported into the role of spirituality and religious practice in protecting against depression among older people living in rural villages in Bulgaria and Romania, two neighbouring countries with similar cultural, political and religious histories, but with differing levels of current religiosity.

There is no consensus regarding the most appropriate surgical approach for the treatment of posterior malleolar fractures. The posterolateral approach facilitates more accurate reduction, but the sural nerve is potentially at risk during the approach. The location of this nerve in relation to this approach has not been clearly described in the literature.

In most organs of the body, immunological reactions involve the drainage of antigens and antigen presenting cells (APCs) along defined lymphatic channels to regional lymph nodes. The CNS is considered to be an immunologically privileged organ with no conventional lymphatics. However, immunological reactions do occur in the CNS in response to infections and in immune-mediated disorders such as multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE). Here, we review evidence that cervical lymph nodes play a role in B and T cell mediated immune reactions in the CNS. Then we define the separate pathways by which interstitial fluid (ISF) and CSF drain to cervical lymph nodes. ISF and solutes drain from the brain along the 100-150nm-wide basement membranes in the walls of capillaries and arteries. In humans, this perivascular pathway is outlined by the deposition of insoluble amyloid (Abeta) in capillary and artery walls in cerebral amyloid angiopathy in Alzheimer's disease. The failure of APCs to migrate to lymph nodes along perivascular lymphatic drainage pathways may be a major factor in immunological privilege of the brain. Lymphatic drainage of CSF is predominantly through the cribriform plate into nasal lymphatics. Lymphatic drainage of ISF and CSF and the specialised cervical lymph nodes to which they drain play significant roles in the induction of immunological tolerance and of adaptive immunological responses in the CNS. Understanding the afferent and efferent arms of the CNS lymphatic system will be valuable for the development of therapeutic strategies for diseases such as MS.

The brain has no conventional lymphatics, but solutes injected into it drain along artery walls and reach lymph nodes in the neck. This study seeks to identify cervical lymph nodes related to the human internal carotid artery (ICA) that could act as the first regional lymph nodes for the brain. Bilateral dissections were carried out on four embalmed human heads, from the level of the carotid bifurcation in the neck, to the base of the skull. Lymph nodes from every specimen were processed for histological examination. A total of 51 deep cervical lymph nodes were identified: 12 lymph nodes (confirmed by histological examination) were observed to be in direct relationship with the ICA. These lymph nodes were found within the carotid sheath and had average diameters of 13.5 x 4.8 mm. Solutes and interstitial fluid from the brain may drain along the walls of cerebral arteries and reach these lymph nodes. They may be sites of stimulation of immune responses against antigens from the brain.

In most organs of the body, immunological reactions involve the drainage of antigens and antigen presenting cells (APCs) along defined lymphatic channels to regional lymph nodes. The CNS is considered to be an immunologically privileged organ with no conventional lymphatics. However, immunological reactions do occur in the CNS in response to infections and in immune-mediated disorders such as multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE). Here, we review evidence that cervical lymph nodes play a role in B and T cell mediated immune reactions in the CNS. Then we define the separate pathways by which interstitial fluid (ISF) and CSF drain to cervical lymph nodes. ISF and solutes drain from the brain along the 100-150nm-wide basement membranes in the walls of capillaries and arteries. In humans, this perivascular pathway is outlined by the deposition of insoluble amyloid (Abeta) in capillary and artery walls in cerebral amyloid angiopathy in Alzheimer's disease. The failure of APCs to migrate to lymph nodes along perivascular lymphatic drainage pathways may be a major factor in immunological privilege of the brain. Lymphatic drainage of CSF is predominantly through the cribriform plate into nasal lymphatics. Lymphatic drainage of ISF and CSF and the specialised cervical lymph nodes to which they drain play significant roles in the induction of immunological tolerance and of adaptive immunological responses in the CNS. Understanding the afferent and efferent arms of the CNS lymphatic system will be valuable for the development of therapeutic strategies for diseases such as MS.

Amyloid is deposited in the walls of arteries and capillaries as cerebral amyloid angiopathy (CAA) in the brains of older individuals and of those with Alzheimer disease (AD). CAA in AD reflects an age-related failure of elimination of amyloid-beta (Abeta) from the brain along perivascular lymphatic drainage pathways. In the absence of conventional lymphatic vessel in the brain, interstitial fluid and solutes drain from the brain to cervical lymph nodes along narrow basement membranes in the walls of capillaries and arteries, a pathway that is largely separate from the cerebrospinal fluid. In this review we focus on the pathology and pathogenesis of CAA, its role in the aetiology of AD and its impact on immunotherapy for AD. The motive force for lymphatic drainage of the brain appears to be generated by arterial pulsations. Failure of elimination of Abeta along perivascular pathways coincides with a reduction in enzymic degradation of Abeta, reduced absorption of Abeta into the blood and age-related stiffening of artery walls that appears to reduce the motive force for lymphatic drainage. Reduced clearances of Abeta and CAA are associated with the accumulation of insoluble and soluble Abetas in the brain in AD and the probable loss of homeostasis of the neuronal environment due to retention of soluble metabolites within the brain. Tau metabolism may also be affected. Immunotherapy has been successful in removing insoluble plaques of Abeta from the brain in AD but with little effect on cognitive decline. One major problem is the increase in CAA in immunised patients that probably prevents the complete removal of Abeta from the brain. Increased knowledge of the physiology and structural and genetic aspects of the lymphatic drainage of Abeta from the brain will stimulate the development of therapeutic strategies for the prevention and treatment of AD.

There are no conventional lymphatics in the brain but physiological studies have revealed a substantial and immunologically significant lymphatic drainage from brain to cervical lymph nodes. Cerebrospinal fluid drains via the cribriform plate and nasal mucosa to cervical lymph nodes in rats and sheep and to a lesser extent in humans. More significant for a range of human neurological disorders is the lymphatic drainage of interstitial fluid (ISF) and solutes from brain parenchyma along capillary and artery walls. Tracers injected into grey matter, drain out of the brain along basement membranes in the walls of capillaries and cerebral arteries. Lymphatic drainage of antigens from the brain by this route may play a significant role in the immune response in virus infections, experimental autoimmune encephalomyelitis and multiple sclerosis. Neither antigen-presenting cells nor lymphocytes drain to lymph nodes by the perivascular route and this may be a factor in immunological privilege of the brain. Vessel pulsations appear to be the driving force for the lymphatic drainage along artery walls, and as vessels stiffen with age, amyloid peptides deposit in the drainage pathways as cerebral amyloid angiopathy (CAA). Blockage of lymphatic drainage of ISF and solutes from the brain by CAA may result in loss of homeostasis of the neuronal environment that may contribute to neuronal malfunction and dementia. Facilitating perivascular lymphatic drainage of amyloid-beta (Abeta) in the elderly may prevent the accumulation of Abeta in the brain, maintain homeostasis and provide a therapeutic strategy to help avert cognitive decline in Alzheimer's disease.

Deposition of amyloid-beta (Abeta) in vessel walls of the brain as cerebral amyloid angiopathy (CAA) could be a major factor in the pathogenesis of dementia. Here we investigate the relationship between dementia and the prevalence of CAA in older populations. We searched the literature for prospective population-based epidemiological clinicopathological studies, free of the biases of other sampling techniques, which were used as a comparison.

There are no conventional lymphatics in the brain but physiological studies have revealed a substantial and immunologically significant lymphatic drainage from brain to cervical lymph nodes. Cerebrospinal fluid drains via the cribriform plate and nasal mucosa to cervical lymph nodes in rats and sheep and to a lesser extent in humans. More significant for a range of human neurological disorders is the lymphatic drainage of interstitial fluid (ISF) and solutes from brain parenchyma along capillary and artery walls. Tracers injected into grey matter, drain out of the brain along basement membranes in the walls of capillaries and cerebral arteries. Lymphatic drainage of antigens from the brain by this route may play a significant role in the immune response in virus infections, experimental autoimmune encephalomyelitis and multiple sclerosis. Neither antigen-presenting cells nor lymphocytes drain to lymph nodes by the perivascular route and this may be a factor in immunological privilege of the brain. Vessel pulsations appear to be the driving force for the lymphatic drainage along artery walls, and as vessels stiffen with age, amyloid peptides deposit in the drainage pathways as cerebral amyloid angiopathy (CAA). Blockage of lymphatic drainage of ISF and solutes from the brain by CAA may result in loss of homeostasis of the neuronal environment that may contribute to neuronal malfunction and dementia. Facilitating perivascular lymphatic drainage of amyloid-beta (Abeta) in the elderly may prevent the accumulation of Abeta in the brain, maintain homeostasis and provide a therapeutic strategy to help avert cognitive decline in Alzheimer's disease.

Elimination of interstitial fluid and solutes plays a role in homeostasis in the brain, but the pathways are unclear. Previous work suggests that interstitial fluid drains along the walls of arteries.

Alzheimer's disease is the commonest dementia. One major characteristic of its pathology is accumulation of amyloid-beta (Abeta) as insoluble deposits in brain parenchyma and in blood vessel walls [cerebral amyloid angiopathy (CAA)]. The distribution of Abeta deposits in the basement membranes of cerebral capillaries and arteries corresponds to the perivascular drainage pathways by which interstitial fluid (ISF) and solutes are eliminated from the brain--effectively the lymphatic drainage of the brain. Theoretical models suggest that vessel pulsations supply the motive force for perivascular drainage of ISF and solutes. As arteries stiffen with age, the amplitude of pulsations is reduced and insoluble Abeta is deposited in ISF drainage pathways as CAA, thus, further impeding the drainage of soluble Abeta. Failure of perivascular drainage of Abeta and deposition of Abeta in the walls of arteries has two major consequences: (i) intracerebral hemorrhage associated with rupture of Abeta-laden arteries in CAA; and (ii) Alzheimer's disease in which failure of elimination of ISF, Abeta and other soluble metabolites from the brain alters homeostasis and the neuronal environment resulting in cognitive decline and dementia. Therapeutic strategies that improve elimination of Abeta and other soluble metabolites from the brain may prevent cognitive decline in Alzheimer's disease.