Neurovascular defects and faulty amyloid-β vascular clearance in Alzheimer’s disease

Abstract

The evidence that neurovascular dysfunction is an integral part of Alzheimer’s disease (AD) pathogenesis has continued to emerge in the last decade. Changes in the brain vasculature have been shown to contribute to the onset and progression of the pathological processes associated with AD, such as microvascular reductions, blood brain barrier (BBB) breakdown and faulty clearance of amyloid β-peptide (Aβ) from the brain. Herein, we review the role of the neurovascular unit and molecular mechanisms in cerebral vascular cells behind the pathogenesis of AD. In particular, we focus on molecular pathways within cerebral vascular cells and the systemic circulation that contribute to BBB dysfunction, brain hypoperfusion and impaired clearance of Aβ from the brain. We aim to provide a summary of recent research findings implicated in neurovascular defects and faulty amyloid-β vasular clearance contributing to AD pathogenesis.

INTRODUCTION

Alzheimer’s disease (AD) is a neurodegenerative disorder of the aging brain that results in progressive cognitive decline, and is associated with accumulation of amyloid β-peptide (Aβ) in the brain parenchyma and around cerebral blood vessels [1], intraneuronal tau-related lesions [2, 3] and neurovascular dysfunction and stress [4-7]. Recent studies have elucidated specific cellular and molecular mechanisms within the neurovascular unit and at the blood-brain barrier (BBB) mediating neurovascular defects in AD. In this review, we will discuss these cerebrovascular mechanisms and how they may contribute to disease onset and progression of AD. We will focus on recent studies that have identified molecular changes within the cerebrovascular system suggesting potentially novel cellular and molecular tragets in AD that might ultimately offer hope and provide new therapeutic opportunities for AD.

The neurovascular unit (NVU) consists of different cell types including (i) vascular cells such as brain endothelial cells lining the cerebral vascular tree, pericytes covering microvascular capillaries, and vascular smooth muscle cells (VSMCs) enwrapping cerebral arterioles and arteries, (ii) glial cells such as astrocytes, microglia and oliogodendrocytes, and (iii) neurons [8-10]. The BBB lies within the NVU. In contrast to leaky capillaries in most peripheral organs [11], the BBB restricts entry of polar molecules into the brain except for nutrients, energy metabolites, amino acids and vitamins that cross the BBB via specialized transport systems [12]. Peptides and proteins in general poorly cross the BBB [13, 14], but can be transported if specific peptide transporters and/or receptors are expressed at the BBB [15, 16].

The close proximity of non-neuronal neighboring cells with each other and with neurons allows for effective cell autonomous and non-autonomous communications that are critical for normal functions of the healthy central nervous system including regulation of cerebral blood flow (CBF), transport of oxygen and energy metabolites into the brain, clearance of waste and toxic products of metabolism including various potential neurotoxins from the brain, regulation of BBB permeability, control of inflammatory response, and stimulation of vascular repair and neuronal regeneration [17-20]. Alterations in the physiological signal transduction pathways between different cell types within the NVU has been increasingly recognized as an important factor which may contribute to disease initiation and progression in multiple neurological disorders [8, 9, 17, 18, 20-24].

Neurovascular stress mediates neuronal dysfunction

As described above, the functional integrity of the NVU is essential for normal neuronal and synaptic functioning. Here we will illustrate this concept with a few recent examples. For instance, recent studies have demonstrated that proper signaling between endothelial cells and pericytes mediated by brain endothelial platelet-derived growth factor B (PDGF-B) and platelet-derived growth factor receptor β (PDGFRβ) in pericytes importantly regulates functional and physical properties of the BBB, [25-27]. Studies using transgenic mouse models of pericyte deficiency generated by PDGF-B and/or PDGFRβ deficiency have shown that brain pericytes are essential safeguards of the BBB [25-27] contributing to BBB formation during development and BBB maintenance in the adult brain [20, 26]. Moreover, recent studies have shown that pericyte loss caused by faulty PDGF-B signal transduction from endothelium to PDGFRβ in pericytes leads to BBB breakdown, CBF reductions and hypoxia which in turn initiates age-dependent secondary neuronal and synaptic changes associated with neuronal and synaptic dysfunction [26]. Pericyte detachment from capillary endothelium has been shown to result in leakage of numerous toxic serum proteins, endothelial cell death, capillary regression and reductions in regional CBF [26]. It is also plausible that loss of brain pericytes may deprive the NVU from pericyte-derived growth factors including angioneurins [28], affecting health of the cerebrovascular system and neurons.

Another recent example relates vascular damage and BBB disruption mediated by apolipoprotein E4 (APOE4), a major genetic risk factor for AD, which has been shown to initiate vascular-mediated secondary neuronal dysfunction and neurodegenerative changes [29]. More specifically, using multiple transgenic apoE mouse lines with genetic ablation or pharmacological inhibition of cyclophilin A (CypA), a proinflammatory cytokine, we have identified that astrocyte-secreted human apoE2 and apoE3, but not apoE4, signal pericytes via the low density lipoprotein receptor related protein 1 (LRP1) to maintain low, physiological levels of CypA [29], previously shown that at increased concentrations disrupts the endothelial barrier damage in systemic vessels. In contrast to high affinity of apoE2 and apoE3 binding to LRP1 in vascular cells, poor interaction of astrocyte-derived apoE4 with LRP1 in pericytes activates a proinflammatory CypA pathway which in turn leads to activation of nuclear factor-κB (NF-κB)-matrix metalloproteinase-9 (MMP-9) pathway in pericytes causing degradation of the BBB endothelial tight junction and extracellular matrix proteins and BBB breakdown. This data has implicated CypA as a new target for treating neurovascular damage and the associated secondary neuronal dysfunction in neurological disorders affected by APOE4 genotype, such as AD.

Vascular-specific genes implicated in AD

Mesenchyme homeobox 2

Impaired angiogenesis and ineffective vascular regeneration may lead to degeneration of brain endothelium in AD and AD models. Several studies have reported that focal vascular regression and diminished microvascular density occur in AD [18, 30-32] as well as in AD transgenic mouse models [33]. The anti-angiogenic activity of Aβ which accumulates in the brains of individuals with AD and in AD mouse models, may contribute to such vascular regression [33]. On the other hand, genome-wide transcriptional profiling of brain endothelial cells from patients with AD demonstrated reduced expression of the mesenchyme homeobox gene 2 (MEOX2) [31]. This homeodomain-containing transcription factor, whose expression in the adult brain is restricted to the vascular system, is a member of a subfamily of non-clustered, diverged, antennapedia-like homeobox-containing genes and has been identified as a master regulator of vascular differentiation and remodeling [34]. The low levels of MEOX2 specifically found in brain endothelial cells isolated from AD patients, but not neurologically normal age-matched controls or young individuals, were shown to mediate aberrant angiogenic responses of human brain endothelium to angiogenic factors such as vascular endothelial growth factor (VEGF) and hypoxia both in vivo and in vitro, leading to premature capillary pruning and death due to activation of the forehead transcription factor 4 which is transcriptionally regulated by MEOX2 [31]. Importantly, a recent genome-wide study examining rare copy number variations exclusive to extreme phenotypes of AD identified a rare rearrangement that targets MEOX2 [35]. Moreover, mice that lack one Meox2 allele have been shown to develop primary cerebral endothelial hypoplasia with an intact BBB but a pronounced reductions in vascular desnity and chronic brain hypoperfusion causing secondary neurodegenerative changes [26] (Figure 1). This data suggest that capillary degeneration in AD could be related to ineffective vascular repair and/or remodeling. Some studies have suggested presence of isolated areas of increased focal vascularity in AD brains [36] further supporting a concept that abnormalities in brain angiogenesis are implicated in AD pathogenesis.

Figure 1. Altered expression of vascular-specific genes in AD results in neurovascular dysfunction.

Hypoxia downregulates mesenchyme homeobox gene-2 (MEOX2) in brain endothelial cells (BEC) (Left). Reduced levels of MEOX2 lead to unsuccessful vascular remodeling and vascular regression resulting in a primary endothelial hypoplasia and brain hypoperfusion. On the other hand, reduced levels of MEOX2 stimulate proteosomal degradation of LRP1, a major Aβ clearance receptor, leading to a loss of LRP1 from BEC and reduced Aβ clearance from brain. Hypoxia increases expression of myocardin (MYOCD) in vascular smooth muscle cells (VSMCs) resulting in elevated levels of MYOCD and serum response factor (SRF) (Right). Elevated SRF/MYOCD levels lead to increased expression of several contractile proteins and calcium-regulated channels in VSMCs resulting in a hypercontractile phenotype of small cerebral arteries and brain hypoperfusion. On the other hand, increased SRF/MYOCD activity stimulates directed expression of the sterol binding protein-2 which is a major transcriptional suppressor of LRP1. Loss of LRP1 from VSMCs diminishes Aβ clearance from small cerebral arteries leading to deposition of Aβ and amyloid in the arterial wall known as CAA, cerebral amyloid angiopathy. Changes in the expression of vascular-restricted genes MEOX2 and MYCD can trigger both an Aβ-independent brain hypoperfusion and Aβ accumulation mediating neuronal dysfunction. Interestingly, hypoxia seems to be upstream to both, a diminished MEOX2 expression in BEC and an increased MYOCD expression in VSMCs. Adapted from [174].

Low levels of MEOX2 also promote proteasomal degradation of LRP1 in brain endothelium [31] which has been shown to diminish Aβ clearance at the BBB, as described below. Therefore, in this group of AD patients Aβ may act in concert with low MEOX2 levels at the BBB to focally reduce brain capillary density.

Myocardin and serum response factor

Previously, we identified that a vascular-specific transcriptional co-factor, myocardin (MYOCD) and a ubiquitously expressed transcription factor, serum response factor (SRF) are overexpressed in the cerebral VSMCs of AD patients and contribute to the development of a hypercontractile cerebral arterial phenotype resulting in brain hypoperfusion, diminished functional hyperemia and cerebral amyloid angiopathy (CAA) [37, 38]. The MYOCD-SRF transcriptional switch binds a cis element known as the CArG box to regulate gene transcription [39] and constitute a molecular switch for the VSMC differentiation program. A growing number of essential VSMC-restricted cytocontractile genes and genes regulting Ca2+ flux are downstream targets of SRFMYOCD such as taglin, smooth muscle alpha actin, calponin and myosin heavy chain, sacroplasmic/endoplasmic reticulum calcium ATPase2, calsequestrin 1, myosin light chain kinase, and the miRNA 143/145 cluster, respectively [38, 40-42]. The high levels of MYOCD and SRF in AD VSMCs lead to increased expression of these cytocontractile proteins and channels regulating Ca2+ fluxes ultimately inducing a VSMC “hypercontracile phenotype” and cerebral hypoperfusion [38, 43] (Figure 1). We further hypothesize that this hypoperfusion can decrease passive Aβ drainage due to reductions in arterial pulsatile blood flow, as previously proposed [43, 44].

In addition, it has been shown that increased levels of MYOCD and SRF in AD VSMCs also suppress Aβ clearance directly thus further exacerbating CAA [37]. Namely, elevated levels of MYOCD and SRF in VSMCs have been shown to lead to CArG box-dependent expression of sterol response element binding protein 2 which is a major LRP1 transcriptional suppressor [45] which results in LRP1 depletion and diminished LRP1-mediated Aβ clearance from the vessel wall [37]. Whether similar signaling mechanisms are present in brain pericytes has yet to be determined. Moreover, the exact mechanism leading to increased MYOCD and SRF in AD VSMCs is currently unclear, although it has been shown that hypoxia can increase MYOCD-SRF levels in cerebral VSMCs in vitro and in vivo [37, 38], thus implicating cerebral ischemia and micro or “silent strokes” as a plausible upstream mechanism.

Aβ clearance mechanisms

The major pathological hallmark of AD is the accumulation of neurotoxic Aβ, believed to be caused not by increased production [46] but due to faulty clearance from the brain [18]. The efficiency of Aβ clearance from brain interstitial fluid across the BBB is influenced by Aβ binding transport proteins such as apoE and apoJ (clusterin), and BBB receptors such as LRP1, LRP2 and receptor for advanced glycation end products (RAGE) which control Aβ efflux from brain and influx into the brain, respectively, and Aβ degrading enzymes [47-55] (Figure 2).

Figure 2. The role of blood-brain barrier transport in homeostasis of brain Aβ.

Brain Aβ is regulated by multiple mechanisms including: (1) central and (2) systemic production from its precursor protein APP; (3) oligomerization and aggregation; (4) receptor-mediated re-entry across the BBB into the brain via the receptor for advanced glycation end products (RAGE) (5) receptor-mediated vascular clearance across the BBB via LRP1; (6) Aβ binding to apoE, α2-macroglobulin (α2M) and apoJ in brain interstitial fluid (ISF) which influences Aβ clearance and aggregation; (7) LRP2-mediated efflux of Aβ-apoJ complexes from brain; (8) ABCB1 at the luminal side may contribute to Aβ efflux from endothelium to blood; (9) enzymatic degradation by neprilysin (NEP), insulin-degrading enzyme (IDE), tissue plasminogen activator (tPA), matrix MMPs; (10) cellular degradation by astrocytes and microglia; (11) LRP1- and LRP2-mediated transport across the choroid plexus; (12) slow removal via the ISF–CSF bulk flow; (13) sequestration in plasma by soluble LRP1 (sLRP1), which is a major Aβ binding protein in plasma; (14) removal by the liver and kidneys. Modified from [8].

LRP1

LRP1, a member of the low-density lipoprotein (LDL) receptor family, is both a rapid cargo transporter and a cell signaling receptor [48]. Some of LRP1 functions include regulation of cholesterol and lipoprotein metabolism, regulation of coagulation, cell survival, synaptic transmission, APP trafficking and metabolism, and Aβ clearance [56-58]. LRP1 is essential for early embryonic development and deletion of Lrp1 gene in mice results in failure of embryo implantation into the uterus [59]. LRP1 is synthesized as a single ~600 kDa protein which is cleaved by furin in the trans-Golgi network to produce a 515 kDa entirely extracellular heavy α-chain and a non-covalently associated 85 kDa light β-chain [60]. During biosynthesis of LRP1, the receptor-associated protein (RAP) acts as a chaperone for its proper folding [61]. The α-chain of LRP1 has four ligand-binding domains (clusters I-IV) [62, 63]. Domains II and IV bind over forty structurally and functionally unrelated ligands such as apoE, activated α2-macroglobulin (α2M), tissue plasminogen activator (tPA), proteinase-inhibitors, blood coagulation factor VIII, RAP, prion protein, lactoferrin [48, 64, 65] and Aβ [66]. RAP blocks binding of ligands to LRP1 [67, 68]. The β-chain of LRP1 has an extracellular domain, a transmembrane region, and a short cytoplasmic tail which consists of two NPxY motifs, and one YxxL motif and two di-leucine motifs which are required for rapid endocytosis of LRP1 ligands [66, 69, 70]. The cytoplasmic tail phosphorylated on serine and/or tyrosine residues [71, 72] interacts with different adaptor proteins associated with cell signaling such as disabled-1 (Dab1), FE65 and postsynaptic density protein 95 (PSD-95) [58, 73, 74]. LRP1 expression is controlled at both transcriptional and translational levels [45, 75-77]. Cell surface levels and function of LRP1 are controlled by proteolytic shedding of its ectodomain [77-82]. Intact soluble LRP1 α-chain (sLRP1), consisting of the ligand binding domains of LRP1, is shed into plasma [78, 83-85]. A short membrane spanning and the intracellular fragment of LRP1 left after ectodomain shedding is further processed by the action of γ-secretase, a APP processing enzyme, resulting in release of LRP1 intracellular domain (LRP1-ICD), which can translocate to the nucleus and inhibit the transcription of the genes mediating inflammatory response [86].

Earlier genetic studies have suggested that LRP1 is linked to AD and CAA [87-90], but this has not been confirmed by later studies [91, 92]. It has been shown that LRP1 interacts with APP which influences Aβ generation [93, 94]. LRP1 also mediates Aβ neuronal uptake via α2M and apoE [70, 95-99]. The exact role of these findings for the development of Aβ pathology is not yet clear.

Within the NVU brain endothelial cells, cerebral VCMCs, pericytes, astrocytes and neurons, all express LRP1 [81, 100]. LRP1 internalizes its ligands and directs them to lysosomes for proteolytic degradation. LRP1 and/or another member of the LDL receptor family have been involved in apoE-dependent uptake of Aβ and degradation by astrocytes [101]. Several reports have shown that LRP1 also transports its ligands transcellularly across the BBB including Aβ [66, 102], RAP [103], tPA [104], lipid free and lipidated apoE2 and apoE3 including their complexes with Aβ [70] and a family of Kunitz domain-derived peptides [105].

As shown in Figure 2, numerous studies have demonstrated that LRP1 has a key role in a three-step serial clearance mechanism mediating Aβ elimination from brain and body [48]. In multiple animal models, we and others have shown that binding of Aβ to LRP1 at the abluminal side of the BBB results in its rapid clearance into the blood [66, 70, 102, 106-112]. A decreased expression of LRP1 in the choroid plexus epithelium [113] leads to Aβ accumulation in the choroid plexus [114, 115]. Studies using in vitro BBB models [116, 117] have confirmed the role of LRP1 in Aβ endothelial cellular uptake and endocytosis, respectively, resulting in clearance of Aβ.

LRP1 expression in brain endothelium decreases with normal aging in rodents, non-human primates and humans, as well as in AD models and AD patients [43, 66, 102, 118-120]. LRP1 reductions have been reported in cerebral VSMCs associated with Aβ accumulation in the wall of small pial and intracerebral arteries [37]. Reduced LRP1 levels in brain microvessels correlate with endogenous Aβ deposition in a chronic hydrocephalus model in rats [121] and Aβ cerebrovascular and brain accumulation in AD patients [102, 120]. Therefore, LRP1 downregulation at the BBB and in vascular cells may contribute to cerebrovascular and parenchymal Aβ deposition.

In blood, circulating form of LRP1 (i.e., soluble LRP1, sLRP1) provides a key endogenous peripheral ‘sink’ activity for Aβ as shown in a mouse model of AD [84]. In neurologically healthy humans and mice, sLRP1 binds over 70% of circulating Aβ preventing free Aβ access to the brain [84] (Figure 2). In AD patients and AD transgenic mice, Aβ binding to sLRP1 is compromised by oxidation resulting in increased levels of oxidized sLRP1 which does not bind Aβ [84]. This is associated with elevated levels of free Aβ40 and Aβ42 in plasma [85] which can re-enter the brain via RAGE-mediated transport across the BBB [49, 84, 122-124]. An increased RAGE expression in brain endothelium has been shown in advanced AD compared to early stage AD and/or individuals with mild cognitive impairment (MCI) [125]. This might further contribute to Aβ accumulation in brain via accelerated Aβ influx from blood. We have recently reported that a diminished sLRP1-Aβ peripheral binding precede an increase in the tau/Aβ42 CSF ratio and a drop in global cognitive decline in individuals with MCI converting into AD [85]. Replacement of damaged sLRP1 with recombinant LRP1 ligand binding domains can sequester free Aβ in plasma in AD patients and AD transgenic mice and reduce Aβ-related pathology in brain [84].

LRP1 in the liver mediates rapid peripheral clearance of Aβ [126, 127]. Reduced hepatic LRP levels have been shown to be associated with decreased peripheral Aβ clearance in the aged rats [126, 127]. Regulation of Aβ brain levels by the liver has been recently demonstrated by an independent study [128, 129]. Enhancing expression of LRP1 in brain microvessels and liver and circulating sLRP1 in plasma using plant-derived pharmacologically active compounds from Withania somnifera resulted in rapid reversal of Aβ pathology and improved cognitive function in APP/PS1 transgenic mouse model of AD [130, 131].

RAGE

RAGE is a multiligand receptor of the immunoglobulin (Ig) superfamily [132]. In brain, RAGE is expressed on the cell surface of vascular endothelial cells, pericytes, smooth muscle cells, neurons, and glial cells and acts as a receptor for Aβ [133-135]. RAGE contains one V-type and two C-type immunoglobulin domains which bind to several ligands, and a short cytoplasmic tail that is required for RAGE-mediated cell signaling [134]. The extracellular V domain of RAGE binds to ligands such as AGE proteins, S100/calgranulins, monomeric and oligomeric Aβ, amphoterin, and the family of crossed β-sheet macromolecules [136]. Interaction of ligands with RAGE activates signal transduction pathways leading to increased cellular stress as seen in chronic diseases such as diabetes, inflammation and AD [137-139]. RAGE mediates Aβ-induced neurotoxicity directly by causing oxidant stress and indirectly by activating microglia [133]. In addition, intraneuronal Aβ transport via RAGE leads to mitochondrial dysfunction [140]. RAGE amplifies Aβ-mediated inflammatory response in microglia [133]. Targeted expression of RAGE in neurons in APP transgenic mice accelerated cognitive decline and Aβ-induced neuronal perturbation [141]. Expression of RAGE is increased in cerebrovascular endothelial cells under pathological conditions including those seen in AD models and AD [49, 133, 142].

At the BBB, RAGE mediates transport of circulating Aβ into the brain [49, 143] which results in NF-κB-dependent endothelial cell activation and neuroinflammatory response, from one hand, and generation of endothelin-1 suppressing the CBF, from the other [49]. In addition, expression of RAGE in brain endothelium initiates cellular signaling leading to monocyte trafficking across the BBB [144]. RAGE expression is increased in both neurons and endothelium in an Aβ-rich or AGE-rich environment as in AD [142], which amplifies Aβ-mediated pathogenic responses. The cellular events triggered by RAGE at the BBB, neurons, microglia and VSMCs may be associated with the onset and progression of AD. Therefore, RAGE is a potential therapeutic target in AD and blocking RAGE might contribute to control of Aβ-mediated brain disorder [49, 55, 145].

ApoE

The human apoE is a 34 kDa glycoprotein of 299 amino acids [146]. It plays an important role in lipoprotein transport and cholesterol homeostasis [147]. In humans there are three common isoforms of apoE: apoE2 (cys112, cys158), apoE3 (cys112, arg158) and apoE4 (arg112, arg158), differing from each other by one or two amino acids at position 112 and 158 [148]. The ε4 allele of the APOE gene encoding apoE4 is a major risk factor for early-onset AD [149, 150]. How apoE4 influences onset and progression of AD is not completely understood [51, 53, 151].

In addition to earlier described Aβ-independent pathway of apoE4-mediated vascular defects causing neuronal dysfunction and neurodegeneration [29], several studies have suggested that interaction of apoE with Aβ plays an important role in the pathogenesis of AD [70, 97, 106, 152-155]. Studies by our group showed that binding of Aβ to lipidated or lipid-poor apoE can influence Aβ clearance from the brain [70, 106, 156] or its transport from the circulation across the BBB [156]. Binding of Aβ to apoE reduced Aβ efflux across the BBB compared to Aβ alone [70, 106], and this process was dependent on apoE isoform and the status of lipidation. Namely, apoE4 had a greater disruptive effect on BBB clearance of Aβ than either apoE3 or apoE2 [70]. We have also shown that clearance of Aβ40 and Aβ42 via apoE2 and apoE3 involves mainly LRP1 at the BBB, whereas apoE4 complexes with Aβ40 and Aβ42 have affinity for very low density lipoprotein receptor (VLDLR) which mediates slow internalization of its ligands into the endothelium of the BBB compared to rapid internalization and transcytosis provided by LRP1 [70, 106].

ApoJ

ApoJ, also known as clusterin, is a 75 kDa chaperone protein. It is highly conserved and ubiquitously expressed sulfated glycoprotein [50, 157, 158]. ApoJ is involved in apoptosis, inflammation, cancer and many neurodegenerative disorders including AD [50]. ApoJ is expressed at high levels in the brain and found associated with senile plaques in AD [159]. Recent genome-wide association studies (GWAS) identified clusterin gene (CLU) as one of the risk factor for Alzheimer’s disease [91, 92, 160-166]. How variations in CLU gene influence transcription or loss or gain of protein function, especially how it affects Aβ metabolism in the brain is not completely understood and needs further investigation [166].

ApoJ forms a stable complex with Aβ [106, 167-169]. Our earlier studies have shown that soluble Aβ40-apoJ complexes are taken up at the BBB in vivo [47, 170] by a receptor-mediated transport mechanism involving megalin/gp330 receptor [169]. However, megalin at the BBB is saturated under physiological levels of plasma apoJ which prevents influx of circulating Aβ-apoJ complex into the brain via megalin [169]. In the CNS, megalin is expressed in brain vascular endothelial cells and the epithelial cells of the choroid plexus [171, 172]. In a recent study we found that apoJ facilitates clearance of soluble Aβ across the BBB [106]. ApoJ and apoJ-Aβ42 complexes are rapidly cleared across the BBB via LRP2 and our data suggest that clusterin/megalin pathway at the BBB plays a crucial role in removal of soluble Aβ particularly Aβ42 from the mouse brain [106].

Conclusions

Recent clinical observations provide strong evidence for the link between cerebrovascular disease and AD and the role of vascular risk factors in AD [18]. In this review, we have briefly reviewed literature on dysregulated and diminished CBF, BBB dysfunction and impaired vascular clearance of Aβ from brain supporting an essential role of the neurovascular and BBB mechanisms in AD pathogenesis. Several studies in animal models of AD and more recently in AD patients [46] have demonstrated a diminished Aβ clearance from brain. The recognition of Aβ clearance pathways opens exciting new therapeutic opportunities for AD. It is now established that faulty clearance from brain and across the BBB leads to elevated Aβ levels in brain which in turn have been shown to contribute to formation of neurotoxic Aβ oligomers [173] and the development of Aβ-mediated brain storage disorder and cerebral β-amyloidosis [4].

The activation of neurovascular pathogenic pathways has been shown to compromise synaptic and neuronal functions prior to and/or in parallel with Aβ accumulation and development of intraneuronal tangles, neuronal loss and dementia. Some early molecular targets within the neurovascular pathway include receptors RAGE and LRP1 at the BBB, Aβ chaperone proteins such as apoE and apoJ, and possibly vascular-specific genes MEOX2 and MYOCD. ApoE4, the major genetic risk factor for AD, which affects AD pathogenesis by both Aβ-independent and Aβ-dependent pathways, involves in either case effects on the BBB and the neurovascular mechanisms of disease.

Focusing on comorbidity, vascular risk factors associated with AD such as hypoperfusion, hypertension, mini strokes and/or diabetes might generate useful new models of human dementia according to the vascular two-hit hypothesis of AD [18]. The proposed neurovascular model of AD raises a set of new important questions that require further study, as recently discussed [18]. For example, the molecular basis of the neurovascular link with neurodegenerative disorders is still poorly understood as well as the molecular cues underlying the cross-talks between different cell types of the NVU including vascular and glia cells, and how these cellular interactions influence neuronal activity. Addressing these questions will lead to better understanding of the neurovascular link with neurodegeneration process which will lead to the development of novel neurovascular-based approaches for AD [18].