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Blood-brain barrier alterations in ageing and dementia

Bogdan O. Popescu a,b,⁎, Emil C. Toescu c, Laurenţiu M. Popescu a, Ovidiu Bajenaru b, Dafin F. Muresanu d,Marianne Schultzberg e, Nenad Bogdanovic f

a Laboratory of Molecular Medicine, ‘Victor Babeş’ National Institute of Pathology, Bucharest 050096, Romaniab Department of Neurology, University Hospital Bucharest, ‘Carol Davila’ University of Medicine and Pharmacy, Bucharest 050098, Romaniac Department of Physiology, Division of Medical Sciences, The Medical School, University of Birmingham, Birmingham B15 2TT, UKd Department of Neurology, ‘Iuliu Hatieganu’ University of Medicine and Pharmacy, Cluj Napoca 400023, Romaniae Division of Neurodegeneration, Department of Neurobiology, Care Sciences and Society, Karolinska Institutet, Stockholm SE-141 86, Swedenf Alzheimer's Disease Research Center, Department of Neurobiology, Care Sciences and Society, Karolinska Institutet, Stockholm SE-141 86, Sweden

a b s t r a c ta r t i c l e i n f o

Available online 4 March 2009

Keywords:AgeingAlzheimer's diseaseBlood-brain barrierBrain capillariesMechanisms of neurodegenerationNeurovascular couplingVascular dementiaParkinson's diseaseTrophic factors

The current pathogenic scenarios of different types of dementia are based on a number of commonmechanisms of neurodegeneration, such as accumulation of abnormal proteins (within or outside cells),mitochondrial dysfunction and oxidative stress, calcium homeostasis dysregulation, early synapticdisconnection and late apoptotic cell death. Ageing itself is associated with mild cognitive deterioration,probably due to subtle multifactorial changes resulting in a global decrease of a functional brain reserve.Increased age is a risk factor for neurodegeneration and key pathological features of dementia can also befound in aged brains. One of the underexplored brain structures in ageing and dementia is the blood-brainbarrier (BBB), a complex cellular gate which regulates tightly the transport of molecules into and from thecentral nervous system. Disruption of this barrier is now increasingly documented not only in brain vasculardisease but also in ageing and neurodegenerative disorders. To date, such evidence points mainly at anassociation between various dementia forms and disruption of the BBB. But, in reviewing such results, andtaking into account the exquisite sensitivity of neuronal function to the composition of the interstitial brainfluid (IBF), which is regulated by the BBB, we would like to propose the existence of a possible causal linkbetween alterations of BBB and conditions associated with cognitive decline.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

In order to function properly, the central nervous system (CNS),and the neurones in particular, require the maintenance of ionicconcentrations (Na+, K+ and Ca2+) within narrow ranges and ofadequate metabolic substrates. In addition, the neural function is alsovery sensitive to the effects of a wide variety of circulating substances(both proteins and non-proteins) that are not harmful to theperipheral organs. Such strict separation between the neural millieuand the circulatory space is achieved through the properties of aunique biological structure, essentially composed of capillaryendothelial cells, basal lamina and astrocytes, and acting as a two-way diffusion barrier — the blood-brain barrier (BBB). Pericytes, theleast studied BBB constituents, are mesenchymal-like cells surround-ing the endothelial cells and able to contract brain capillaries and tochange the blood flow [1,2]. The peculiarity of BBB (Fig. 1) is given, atleast in part, by a specific phenotype of the endothelial cells, with

adherens junctions as cell–cell interaction stabilizers, and tightjunctions (TJs) that limit the paracellular flow of water, ions andlarger molecules into the brain (gate function) and organize the cellmembrane in apical–basal domains (fence function) [3]. The ‘sealing’feature of the TJs is conferred by the expression in endothelial cells oftransmembrane proteins such as occludin [4], claudins [5] andjunction adhesion molecules [6], anchored via accessory proteins tothe cytoskeleton [7]. This ‘BBB phenotype’ of endothelial cells ischaracterized by a low pinocytic activity and is also much moreeffective in limiting the paracellular flow of ions as compared to that inperipheral capillaries, as reflected by a ~100 fold higher transen-dothelial electrical resistance in the CNS [8]. Therefore, the moleculartraffic at the BBB level takes place mostly by transcellular means.Gases, such as O2 and CO2, are exchanged through the lipidmembranes, as a result of gradient diffusion, while the influx ofamino acids and glucose is regulated through specific transporters.Larger molecules such as peptides (e.g. leptin, insulin, and transferrin)are transported by receptor-mediated transcytosis. With the excep-tion of the receptor-guided molecules, passage of all other hydrophiliccompounds, including drugs, is highly restricted under normalconditions. Size generally represents another passage criterion,substances with a molecular weight higher than 500 Da being unable

Journal of the Neurological Sciences 283 (2009) 99–106

⁎ Corresponding author. Laboratory of Molecular Medicine, ‘Victor Babeş’ NationalInstitute of Pathology, Bucharest 050096, Romania. Tel.: +40 744 35 34 33; fax: +40 21312 81 02.

E-mail address: bogdan.popescu@jcmm.org (B.O. Popescu).

0022-510X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.jns.2009.02.321

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to cross the BBB [7]. Some of the transporters are specialized forendothelial efflux, for instance P-glycoprotein, which acts as an active,ATP-dependent pump [9]. Water flow into and out of the brain tissueis regulated by aquaporins (AQP), AQP-4 being expressed in astrocyteperivascular endfeet [10] to form water channels and to enable fluidhomeostasis in the brain. In contrast, the brain endothelial cells do notexpress AQP channels [11]. Besides the physical and transport barriersdescribed above, an enzymatic barrier made of nucleotidases andpeptidases is active in the proximity of the capillaries [12].

Another potentially important theoretical issue is the physiologicalregulation of BBB permeability. The current view is that under normalcircumstances the tightness and transport properties of the BBB arerelatively constant, and this is in agreement with the need for aconserved extracellular milieu allowing normal CNS functioning.However, the major features of the endothelial cell phenotype in theBBB are dependent on astrocytes [13], indicated by the fact thatprotein expression and distribution in endothelia are induced byastrocyte-derived signals, and thereby being adaptable. In mixedculture conditions for instance, astrocytes induce the differentiation ofendothelial cells and pericytes partly via secretion of transforminggrowth factor beta 1 (TGFβ1) [14], which also alters the expression ofbasement membrane-related molecules [15]. Moreover, there isevidence showing that other growth factors synthesized and secretedby astrocytes, such as the vascular endothelial growth factor (VEGF)[16,17], the basic fibroblast growth factor (bFGF) [17,18] and the glialcell line-derived neurotrophic factor (GDNF) [19], are involved inmodulation of BBB permeability. Recent reports also suggested thatcalcium signals originating in astrocytes can be transferred to capillary

endothelial cells and mediate BBB transport processes [20]. Mobiliza-tion of cytosolic calcium (Ca2+) in astrocytic endfeet occurs forinstance as a result of purinergic receptors (P2Y) signalling; however,how endothelial cells respond to these Ca2+ waves is not elucidated[21]. Endothelial cells bear also receptors of tumour necrosis factor-α(TNF-α) receptor superfamily, such as TNFR1, Fas and DR5, but theirligands do not trigger cell death in this cell type. Instead, they are ableto upregulate chemokines and adhesion molecules and to activate theextracellular signal regulated kinases [22]. Besides, TNF-α can inducean upregulation of the efflux transporter P-glycoprotein in endothelialcells [23]. However, the regulation of occludin and claudins inendothelial cells and the factors inducing their low pinocyticphenotype, key features of the BBB gate function maintenance, arenot yet clarified.

2. Ageing impact on blood-brain barrier

Dysfunction of the BBB in the ageing brain was documented byvarious studies and is at least in part responsible for pathologicalalterations such as white matter lesions [24], which in turn correlatewith progressive cognitive deterioration [25]. In a recent review,Farrall and Wardlaw extensively document the increase in BBBpermeability with ageing in healthy individuals. The commonly usedmethods to analyze the BBB function in studies in humans are eitherlinked to brain imaging or biochemical. By means of assessment of thecerebrospinal fluid (CSF)/plasma albumin ratio, different studiesdemonstrated significantly higher albumin leakage through the BBB inold healthy as compared to young healthy individuals, and these

Fig. 1. Schematic representation of the blood-brain barrier (BBB) structure, transport and regulation. The capillaries and the brain tissue are separated by a barrier formed byendothelial cells, pericytes and astrocyte end-feet. Endothelial cells are sealed by tight and adherens junctions. The tight junctions are composed of transmembrane proteins such asoccludin, claudins and junction adhesionmolecules (JAM), which grasp actin-anchored cytoplasmic proteins such as ZO1, ZO2 and ZO3. Transport through the BBBmostly takes placeby transcellular means: diffusion of gases (O2, CO2), transporters (e.g. GLUT-1 for glucose), or receptor-mediated transcytosis (e.g. insulin). P-glycoprotein (P-gp) operates theendothelial efflux of different compounds, including drugs, and aquaporin 4 (AQP-4)modulates thewater flow into the brain. Physiological regulation of BBB permeability is probablyregulated by both blood-borne and astrocytic factors, such as growth factors (transforming growth factor beta 1 — TGFβ1, vascular endothelial growth factor — VEGF, the basicfibroblast growth factor— bFGF, glial cell line-derived neurotrophic factor— GDNF), tumour necrosis factor (TNF) family members and calcium signals. Extracellular signal-regulatedkinases (ERK) are activated through TNF receptors and are able to influence transcription via transcription factors. In BBB endothelial cells, localization of many proteins at theluminal and abluminal membranes differs, thus creating a ‘polarization’ (e.g. P-gp is expressed only in luminal capillary membranes; GLUT-1 is present at both cellular versants butasymmetrically). Moreover, there are proteins important for BBB transport expressed only in astrocytes, and not in endothelial cells, and vice versa (e.g. AQP-4 is abundant in glialendfeet but is absent in endothelial cells). Mobilization of cytosolic calcium (Ca2+) in astrocytic endfeet occurs as a result of purinergic receptors (P2Y) signalling; however, howendothelial cells respond to these Ca2+ waves is not elucidated.

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results were also confirmed by studies using brain imaging (computedtomography — CT, magnetic resonance imaging — MRI or positron-emission tomography — PET) [26].

In a rodent model of senescence, morphological changes of BBB[27] and leakage of endogenous albumin [28] and IgG [29] to the brainparenchymawere selectively demonstrated for brain regions involvedin cognition, such as the hippocampus. The senescence-acceleratedprone mouse strain 8 (SAMP8), which shows age-related learning andmemory deficits [27], exhibits increased oxidative stress at an earlyage [30], before other pathological features are reported. Decreasedlevels of superoxide dismutase [31,32] and glutathione peroxidase[33] seem to be responsible for this modification. An age-relatedincrease in amyloid precursor protein (APP) expression withoutplaque formation was described in the brain of the SAMP8 mice [34].These results suggest that oxidative stress in brain parenchyma cantrigger alterations of the BBB, possibly by cell death, gliosis andsignalling changes. Moreover, in aged Wistar rats, which showimpairment in short term-memory, leakage through the BBB wasfound to be associated with microglial activation [35]. The latter is asource of oxidative damage [36] possibly not only to neurones, butalso to glial and endothelial cells. Leakage of BBB can inducemicroglialactivation by letting abnormal molecules pass into the brainparenchyma and in turn free radicals released from the microgliamay further alter the BBB, in a vicious cycle. In addition, passage ofneuroimmune factors to the brain also changes with senescence.Furthermore, increased transport of TNF-α has been shown in somebrain regions of old SAMP8 mice, suggesting that beyond modulatingBBB properties, TNF-α is able to cross the BBB more efficiently withage [37], while passage of interleukin-1-β (IL-1β) through the BBBdecreases with age [38]. It was also suggested that the aged BBB is notable to transfer glucose properly to the brain, since expression of theglucose transporter GLUT-1 is reduced, both in physiologically agedand in SAMP8 mice [39]. Neurotrophin-like peptides are able toreverse this modification [40], suggesting that protein expression atthe BBB level could be influenced by neurotrophines during ageing.

Another effect of brain ageing is the accumulation of iron inastrocytes [41] and one suggested mechanism for this is the reductionof ceruloplasmin expression in the CNS [42]. Iron in excess cangenerate free radicals and harm cells [43], therefore its increase in theperivascular astrocytes could participate in alterations seen in BBBwith ageing. Transferrin mRNA levels have been shown to increase inthe brainwith age, independent to blood-borne factors [44]. However,the reverse scenario, i.e. an increased passage of iron to the brain dueto improper barrier function, cannot be excluded [45].

A different mechanism for BBB dysfunction in the elderly is thedecreased activity of the efflux transporter P-glycoprotein that mayresult in decreased extrusion of toxins from the brain to capillaries.Toornvliet and coauthorsmeasured P-glycoprotein activity inyoung andelderly healthy volunteers by (R)-[(11)C] verapamil andPETand found asignificantly increased grey substance distribution for the ligand in theaged subjects [46], suggesting a decreased P-glycoprotein activity.

A decline in estrogen levels may also be involved in age-relatedmalfunction of BBB and may result in sensitization to neurodegenera-tion. Thus, Bake and Sohrabji found that BBB leakage is 2 to 4 timesincreased in senescent ovariectomized rats as compared to the youngadult controls. Surprisingly, estrogen replacement resulted in a furtherincrease of leakage at the hippocampal level [47]. Moreover, a recentstudy showed that mid-life adiposity factors correlate with BBBleakage in late life and suggested a sex-hormone mediated mechan-ism for this alteration [48]. In turn, an altered transfer through theaged BBB of leptin [49], a hormone of adipose tissue origin linked toobesity, could induce its deleterious central effects, which may bereversible [50]. Furthermore, ageing and obesity are both associatedwith reduced sensitivity to insulin in the brain [51,52], which candecrease the clearance of β-amyloid (Aβ) peptide by a mechanismlinked to BBB [53].

3. Blood-brain barrier in cerebrovascular disease and vasculardementia

Vascular dementia (VD) is a syndrome with different etiologies anddifferent pathogenic mechanisms, which associates with some commonvascular risk factors [54]. Such a risk factor, perhaps the most important,is arterial hypertension. In epidemiological studies, antihypertensivesshowed a robust effect as protectors against development of dementia[55]. It was hypothesized that changes in brain microvasculature inresponse to chronic hypertension, for instance lipohyalinosis andfibrosis,may induce BBB failure, which further contributes to lacunar strokes,leukoaraiosis and dementia [56]. Indeed, a recent report showed thathypertension can induce BBB alterations by modulation of proteinexpression level. In this study, spontaneously hypertensive rats (SHR)displayed lower levels of the TJ protein zonula occludens-2 (ZO-2) andhigher levels of the ion transporter, Na+/H+exchanger 1, as compared tocontrol rats whichmaintained normal blood pressure values. In contrast,the expression of ZO-1, occludin, actin, claudin-5 in endothelial cells atthe BBB level remained unchanged [57]. Themost acute and threateningeffects of hypertension on BBB may occur in hypertension encephalo-pathy, a condition inwhich the cerebral perfusion is elevated beyond thelimits of autoregulation. In Dahl salt-sensitive rats, the most commonlyusedmodel forhypertensionencephalopathy, BBB leakage ismassive andcan be reversed by treatment with δ V1-1, a selective peptide inhibitor ofδ protein kinase C (δPKC) [58,59]. Therefore, δPKC could be a promisingtherapeutic target for theprotection of BBBunder vascular stress [60] andpossibly for the prevention of cognitive impairment in cardiovascularpatients.OtherPKC inhibitorswere alsopreviously reported to reduce theendothelial permeability induced by deleterious factors such as oxidativestress [61,62] and therefore it can be claimed that basic cell signallingmechanisms regulate endothelial transfer properties at least in patho-logical conditions. Besides kinase inhibitors, magnesium sulphate hasbeen shown to reduce BBB permeability and brain oedema in an animalmodel for eclampsia by an AQP-4-independent mechanism [63]. More-over, hypertension is not only able to alter BBB properties per se, but canalso aggravate the BBB leakage and cognitive decline triggered byenvironmental factors, such as heat stress [64].

Type 2 diabetes mellitus appears to be a risk factor for developingVD, particularly if treated with insulin [65,66]. Despite being aperipheral disease, diabetes is able to induce alterations in BBBpermeability [67], and this may be a mechanism for its role in VDcaused by small vessels disease. In a rat model of diabetes it has beenshown that the expression of GLUT-1 and GLUT-3 is downregulated,probably by an adaptive mechanism of protection against sustainedhigh blood glucose levels [68]. Furthermore, in an in vitro BBB modeland in streptozotocin-induced diabetic rats, the efflux transporter P-glycoprotein was found to be upregulated by high glucose through aNF-κB-dependent mechanism [69]. Hawkins and colleagues showedthat in experimental rat diabetes BBB permeability to sucroseincreased in parallel to a decrease in the level of the TJs proteinsoccludin and ZO-1 [70]. Interestingly, occludin expression in neuronesis elevated in VD brains, suggesting an additional biological role forthis protein [71]. Moreover, a recent study showed that in diabetic ratsthe BBB permeability is progressively increased despite normalizationof blood glucose levels by insulin treatment [72]. Based on themolecular weight passage pattern, the authors suggested that the BBBfailure in diabetes mellitus is mainly due to a loosening of theparacellular path. Besides affecting the properties of endothelial cells,diabetes may act on astrocyte phenotype, which is the otherimportant regulator of molecular transport of the BBB [73].

High cholesterol and dyslipidemia are well-known vascular riskfactors that associate with cognitive decline, stroke, VD and Alzheimer'sdisease (AD), even though the molecular basis needs clarifying [74].Brain cholesterol represents a quarter of the total cholesterol in thehuman body. It is synthesized locally and exchange with circulatingcholesterol is normally prevented by the BBB [75]. The main carrier of

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brain cholesterol is apolipoprotein E (apoE),whichensures a continuousmolecular exchange between astrocytes and neurones [76]. However, aconversion of brain cholesterol to 24 S-hydroxycholesterol occurs bymediation of the neuronal cytochrome P-450 species CYP46 and thisform can pass the BBB and be extruded from the brain [77]. In animalmodels, high blood cholesterol levels have been shown to affect thebrain and the BBB. Thus, a long-term cholesterol-enriched diet triggeredBBB disruption and increased the iron deposition and Aβ accumulationin rabbit brains [78]. Furthermore, long-term cholesterol-enriched dietin mice resulted in a marked activation of astrocytes and microgliaincluding an increased expression of inflammatory proteins such ascaspase-1 and the cytokine interleukin-6 (IL-6) [79,80]. In addition, thecholesterol-enriched diet resulted in increased expression of the anti-oxidant protein NAD(P)H:quinone oxidoreductase (NQO1), which waspotentiated in apoE knockout (KO) mice [79], as well as in neuronalaccumulation of hyperphosphorylated tau protein in apoE KOmice [81].Statins, inhibitors of cholesterol synthesis, have been showed to providea protective effect on endothelial permeability in diabetic rats, however,apparently not via inhibition of the cholesterol synthesizing enzyme 3-hydroxy-3-methylglutarylcoenzyme A (HMGCoA), but through a directaction on the endothelium [82]. The protective effect of statins on BBBmay be explained by a decreased interaction of monocytes with theendothelial cells through inhibition of the phosphoinositide 3 kinase/Akt pathway [83].

Stroke is one of the events potentially leading to VD. Cerebralischemia is known to cause increased intracellular calcium levels andexcitotoxicity for neurones, as well as for endothelial cells [84]. Studieson in vitro experimental models, such as hypoxia/aglycemia ofendothelial cells, showed profoundly altered transendothelial elec-trical resistance, an effect that was reversible upon blocking thecalcium channels [85]. Furthermore, it was suggested that themechanism of BBB disruption in stroke is due to the alteration inadherens and tight junction function through modulation of proteinexpression, induced by calcium dyshomeostasis [86]. Moreover,factors released by endothelial cells, such as endothelin-1 and nitricoxide (NO), contribute to changes in the brain microvasculature inboth hypertension and stroke [87] and these changes occur bothbefore and after the ischemic stroke [88]. In this line of evidence,postischemic reperfusion was shown to alter the BBB structure andpermeability [89]. Not only brain ischemia, but also hemorrhagic

stroke seems to be preceded by alterations in BBB properties. In arecent report, Lee and colleagues brought evidence that focallyincreased brain permeability in the SHR model, as demonstrated byMRI, occurred 1 to 2 weeks before the intracerebral hemorrhage [90].

4. Blood-brain barrier in Alzheimer's disease

Even though there is no unifying theory that is able to fully explainthe neuropathological lesions and the progression of the neurodegen-erative process in AD, this disease is still best defined by dementiawitha peculiar accumulation of Aβ peptide and hyperphosphorylated tau inthe brain [91]. The ApoE genotype is a known factor for AD [92], andinflammation [93], vascular factors [94], oxidative stress and synapticdysfunction [95], alterations in cell signalling [96] and sensitization tocell death [97] all appear to be important pathogenic mechanisms.Many of these factors are encountered in other dementia types,mainlyin VD, therefore giving rationale for the concept that most of thedementia cases in fact have amixed origin [54]. The importance of BBBalterations in AD is well established [98] and it is worthy to notice thatmany of the above mentioned disease mechanisms also affect the BBB(e.g. oxidative stress, vascular factors, inflammation). Furthermore,one crucial link between ageing and AD can be an alteredmanagementof Aβ at the BBB level [99]. First, since Aβ is produced not only in thebrain but also in the periphery, leakage of Aβ from capillaries to thebrain may occur both in aged and in pathologically altered BBB.Moreover, it was suggested that the receptor for advanced glycationend products (RAGE), which mediates transfer of Aβ to the brainthrough the endothelial cells [100], can be upregulated during ageing[101]. Second, a diminished extrusion of Aβ from the brain occursduring ageing, by a decreased low-density lipoprotein (LDL) receptor-related protein (LRP)-mediated endothelial transcytosis of Aβ [102].Moreover, in P-glycoprotein null mice, clearance of Aβ from the brainfalls by 50%, suggesting that not only LRP, but also P-glycoprotein isimportant in Aβ clearance [103]. As already mentioned, the activity ofP-glycoprotein decreases with age [46] and therefore excessiveaccumulation of Aβ due to BBB impairment could be one of the linksbetween ageing and AD. Indeed, a proof that BBB alteration precedesaccumulation of senile plaques came from work performed in ADtransgenic mice, in which a breakdown of BBB was demonstrablebefore Aβ deposits were seen in the brain [104]. In current therapeutic

Table 1Molecular mechanisms associated with BBB disruption in ageing and different dementia types and proposed protective treatments.

Mechanism Possible intervention Ageing or disease Model/human References

Increased oxidative stress Antioxidants, gene therapy to target SOD Ageing SAMP 8 mice [30–33]Microglia activation Anti-inflammatory drugs Ageing Physiologically aged rats [35]Increased transport of TNF throughBBB

Anti-TNF antibodies Ageing SAMP 8 mice [37]

Decreased expression of GLUT-1 Neurotrophines, neurotrophic-likepeptides

Ageing, diabetes (risk factor for VD) Physiologically aged and SAMP8 mice, SDR

[39,40,68]

Iron accumulation Iron chelators Ageing, high cholesterol (risk factor for VD), PD Human, physiologically aged mice,cholesterol-enriched rabbits

[41,42,78,119]

Decreased activity of P-gp Gene therapy targeting P-gp Ageing, AD, PD Human, P-gp null transgenic mice [46,103,118]Decreased estrogen level New methods of estrogen replacement Ageing Senescent ovariectomized rats [47]Reduced sensitivity to insulin Insulin, IGF-1 Ageing Human, physiologically aged mice [51,52]Decreased occludin expression Insulin, gene therapy, statins Diabetes (risk factor for VD) SDR [70]Decreased ZO-1 expression Insulin, gene therapy, statins Diabetes (risk factor for VD) SDR [70]Decreased ZO-2 expression Antihypertensives, gene therapy Hypertension (risk factor for both AD and VD) SHR [57]Increased endothelia–monocytesinteraction

Inhibition of PI3K/Akt Diabetes (risk factor for VD) SDR [82,83]

Activation of δPKC δPKC specific inhibitors Hypertension (risk factor for both AD and VD) DSSR [58,59,60]Alteration of adherens and TJsfunction

Calcium blockers Stroke (risk factor for both AD and VD) In vitro BBB model, rats [85,86]

RAGE upregulation RAGE blockers Ageing, AD AD transgenic mice [101]LRP downregulation Gene therapy, other Aβ clearance strategies Ageing, AD AD transgenic mice [102,109]

SOD= superoxide dismutase, SAMP= senescence-accelerated pronemice strain 8, TNF= tumour necrosis factor, BBB= blood-brain barrier, GLUT-1= glucose transporter 1, VD=vascular dementia, SDR= streptozotocin-induced diabetic rats, PD=Parkinson's disease, P-gp= P-glycoprotein, IGF-1= insulin-like growth factor 1, ZO=zonula occludens, SHR=spontaneous hypertensive rats, PI3K = phosphoinositide 3 kinase, PKC = protein kinase C, DSSR = Dahl salt-sensitive rats, RAGE = receptor for advanced glycation end products,LRP = low-density lipoprotein receptor-related protein, Aβ = β-amyloid peptide.

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approaches it has been shown that antibodies to Aβ can induceclearance of Aβ from the brain, through the BBB [105].

ApoE4 is the main genetic risk factor for sporadic AD [92]. Micedeficient in apoE show a compromised BBB function, demonstrated bya spontaneous leakage of dyes into the brain [106], which progresseswith age [107]. Mulder and colleagues showed that a high-fat diet isan important inducer of both atherosclerosis and BBB disruption inapoE KOmice suggesting that protective isoforms of apoE may protectthe brain from pathological lesions induced by high-fat diet and BBBleakage [108]. Interestingly, apoE KO mice also show a decreasedexpression of LRP receptor [109].

A recent systematic review concluded on the existence of BBBdisruption in patients with AD or VD. The authors reported that BBBpermeability, as assessed by biochemical and imaging methods,significantly increases in both AD and VD as compared to ageingcontrols. Furthermore, the BBB leakage was more pronounced in VDpatients as compared to AD patients [26], and this is in agreementwiththe experimental evidence that vascular factors are able to modify theBBB phenotype and properties (see the previous chapter). Bringing anargument that not only vascular factors could affect the BBB, Algotssonand Winblad recently reported on an altered BBB integrity in asubgroup of AD patients without noticeable vascular lesions [110].Interestingly, menwere primarily affected. However, in a recent reportin Neurology, Bowman and colleagues followed a population of ADpatients for one year and found that BBB impairment is present andconstant in a subgroup of AD individuals, but with no correlationwithage, gender, ApoE genotype or Mini-Mental State examination [111].

It is our view that BBB changes found in AD are not explained only onthebasis of vascular lesions. Thedegeneration of neurones andactivationof glial cells may also affect the endothelial cell phenotype and transportproperties. For instance, BBB disruption has been shown to develop in atransgenic mouse model for taupathies [112], supporting the conceptthat the BBB is affected not only by vascular factors, but also bydegeneration of brain-residing-cells. Dickstein and coworkers have alsoshown that expression of an AD-linked mutation in APP in transgenic

mice is sufficient to compromise the BBB integrity [113]. Moreover,astrocytes and neurones secrete trophic factors such as FGF2 and VEGF[114] that regulate BBB function and can be depleted in dementia [115].Thus, it is conceivable that theBBBalteration inAD isdueboth tovascularfactors and to cell degeneration and abnormal signalling. Moreover,disruption of BBB may accelerate the pathology of the disease, resultingin a vicious cycle. Some of the molecular mechanisms associated withBBB disruption in ageing and different dementia types and proposedprotective treatments are presented in Table 1.

5. Blood-brain barrier in Parkinson's disease

Except for AD and VD, there is little known regarding BBB alter-ations in other neurodegenerative diseases associated with cognitivedecline and dementia. However, a recent review analyses malfunctionof BBB in Parkinson's disease (PD) [98]. Recent data from animalexperiments support a pathogenic link between BBB disruption anddegeneration of dopaminergic neurones. Rite and colleagues foundthat increased permeability by injection of VEGF into the substantianigra resulted in a subsequent loss of dopaminergic neurones [116].Moreover, there is increasing interest in inflammation as a pathogenicmechanism in PD and possibly through alterations in BBB properties,as shown in many different experimental paradigms [117]. The firstevidence of increased permeability of the BBB in PD came from a PETstudy, showing a significantly increased uptake of 11C-verapamil in themidbrain of PD patients relative to controls, suggested to be due to adecreased function of P-glycoprotein [118]. It was also speculated thatan increased BBB passage of metals, such as iron or manganese, maybe involved in PD pathogenesis [119].

6. Conclusions and perspectives

In this review we presented some of the evidence that is rapidlyaccumulating and which implicates dysfunctions of the blood-brainbarrier in a variety of instances connected to cognitive decline, from

Fig. 2. Factors that alter the blood-brain barrier (BBB) and affect cognition. BBB can be disrupted by ageing, vascular factors (hypertension, diabetes, high cholesterol) andinflammation. One important question iswhether BBB lesions could trigger, or at least accelerate, brainpathology, particularlywhen superimposed on a favorable genetic background.Nevertheless, there is data showing that neurodegeneration per se (which associates with e.g. lack of growth factors, Aβ, tangles, inflammatorymediators) can induce BBB disruption.Our hypothesis is that an aggression to any component of the neurovascular unit can ultimately result in brain degeneration, pathological lesions and cognitive decline.

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the normal ageing phenotype to various forms of dementia. Althoughthese observations are currently mainly associational, the fact thatboth in ageing and during early neurodegenerative states theneurones are working in regimens of reduced homeostatic reserve[120] and thus become extremely sensitive and vulnerable to analtered composition of the extracellular milieu, raise the potentiallyvery important question of whether BBB lesions could trigger, or atleast accelerate, brain pathology, particularly when superimposed ona favorable genetic background (Fig. 2). This view was until nowmostly applicable to understanding the etiopathology of multiplesclerosis or neuroinfections, rather than dementia. Nevertheless, thereis increasing data showing that neurodegeneration per se can induceBBB disruption. Therefore, we conclude that an aggression to anycomponent of the neurovascular unit can ultimately result in braindegeneration, pathological lesions and cognitive decline. Moreover, inthe future it may be of interest to target the BBB with specifictherapeutic approaches that could be complementary to strategiesdesigned for neuronal protection or glia.

Acknowledgements

This study has been supported by the Romanian Ministry ofEducation and Research, the National Innovation, Research andDevelopment Plan — grants no 41-013/2007, 61-019-P2/2007, PN06.26-01.01/2007, CEEX 31/2005 and VIASAN 456/2004, by grantsfrom Karolinska Institutet research funds, The Swedish ResearchCouncil and Swedish Brain Power. Our gratefulness is towards thefamilies that donated the brain material for scientific research.

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