Apolipoprotein E, amyloid-beta, and blood-brain barrier permeability in Alzheimer disease

There is increasing evidence for blood-brain barrier (BBB) compromise in Alzheimer disease (AD). The presence of the epsilon4 allele of the apolipoprotein E (apoE) gene is a risk factor for sporadic AD. Apolipoprotein E is essential both for maintenance of BBB integrity and for the deposition of fibrillar amyloid-beta (Abeta) that leads to the development of Abeta plaques in AD and to cerebral amyloid angiopathy. This review investigates the relationships between apoE, Abeta, and the BBB in AD. Alterations in the expression and distribution of the BBB Abeta transporters receptor for advanced glycation end-products and low-density lipoprotein receptor-related protein 1 in AD and the potential roles of apoE4 expression in adversely influencing Abeta burden and BBB permeability are also examined. Because both apoE and Abeta are ligands for low-density lipoprotein receptor-related protein 1, all 3 molecules are present in AD plaques, and most AD plaques are located close to the cerebral microvasculature. The interactions of these molecules at the BBB likely influence metabolism and clearance of Abeta and contribute to AD pathogenesis. Therapeutic alternatives targeting apoE/Abeta and sealing a compromised BBB are under development for the treatment of AD.     

A Multifaceted Role for apoE in the Clearance of Beta-Amyloid across the Blood-Brain Barrier

Background: While apolipoprotein E4 (apoE4) is highly correlated with the development of Alzheimer's disease (AD), its role in AD pathology and, in particular, beta-amyloid (Aβ) removal from the brain, is not clearly defined. Objective: To elucidate the influence of apoE on the clearance of Aβ across the blood-brain barrier (BBB). Methods: Aβ(1-42) was intracerebrally administered to transgenic mice expressing human apoE isoforms and examined in the periphery. Results: apoE3 and apoE4 mice had 5 times and 2 times, respectively, more Aβ(1-42) appearing in the plasma than wild-type or apoE knockout mice, indicating an enhanced clearance of Aβ from the brain to the periphery. In vitro, unbound basolateral apoE3 (i.e., not bound to Aβ), and to a lesser extent unbound apoE4, at concentrations ≤10 nM facilitated basolateral-to-apical fluorescein-Aβ(1-42) transcytosis across a BBB model, while apoE isoforms bound to Aβ significantly disrupted Aβ transcytosis. Additionally, following apical exposure to the BBB model, we found that apoE4 bound to Aβ is able to penetrate the BBB more readily than apoE3 bound to Aβ and does so via the RAGE (receptor for advanced glycation end products) transporter. Conclusion: These studies indicate a multifaceted, isoform-dependent role for apoE in the exchange of Aβ across the BBB and may partially explain the association of apoE4 and Aβ brain accumulation in AD.     

Human and murine ApoE markedly alters A beta metabolism before and after plaque formation in a mouse model of Alzheimer's disease

The epsilon4 allele of apolipoprotein E (apoE) is a risk factor for Alzheimer's disease (AD), perhaps through effects on amyloid-beta (Abeta) metabolism. Detailed analyses of various Abeta parameters in aging APP(V717F+/-) transgenic mice expressing mouse apoE, no apoE, or human apoE2, apoE3, or apoE4 demonstrate that apoE facilitates, but is not required for, Abeta fibril formation in vivo. Human apoE isoforms markedly delayed Abeta deposition relative to mouse apoE, with apoE2 (and apoE3 to a lesser extent) having a prolonged ability to prevent Abeta from converting into fibrillar forms. Isoform-specific effects of human apoE on Abeta levels and neuritic plaque formation mimicked that observed in AD (E4 > E3 > E2). Importantly, observation of an apoE-dependent decrease in percent soluble Abeta and enrichment of Abeta in membrane microdomains prior to Abeta deposition indicates that apoE influences Abeta metabolism early in the amyloidogenic process and provides a possible novel mechanism by which apoE affects AD pathogenesis.     

Behavioral deficits in APP(V717F) transgenic mice deficient for the apolipoprotein E gene

Both the beta-amyloid precursor protein (APP) and the apoliprotein E (apoE) genes are involved in the pathogenesis of Alzheimer's disease (AD). We previously showed that mice over-expressing a human mutated form of APP (APP(V717F)) display age-dependent recognition memory deficits associated with the progression of amyloid deposition. Here, we asked whether 10- to 12-month-old APP(V717F) mice lacking the apoE gene, which do not present obvious amyloid deposition, differ from APP(V717F) mice in the object recognition task. The recognition performance is decreased in both transgenic mouse groups compared to control groups. Moreover, some behavioral disturbances displayed by APP mice lacking apoE are even more pronounced than those of APP mice expressing apoE. Our results suggest that the recognition memory deficits are related to high levels of soluble Abeta rather than to amyloid deposits.     

Apolipoprotein E deposition and astrogliosis are associated with maturation of beta-amyloid plaques in betaAPPswe transgenic mouse: Implications for the pathogenesis of Alzheimer's disease

A transgenic mouse expressing the human beta-amyloid precursor protein with the 'Swedish' mutation, Tg2576, was used to investigate the mechanism of beta-amyloid (Abeta) deposition. Previously, we have reported that the major species of Abeta in the amyloid plaques of Tg2576 mice are Abeta1-40 and Abeta1-42. Moreover, Abeta1-42 deposition precedes Abeta1-40 deposition, while Abeta1-40 accumulates in the central part of the plaques later in the pathogenic process. Those data indicate that Abeta deposits in Tg2576 mice have similar characteristics to those in Alzheimer's disease. In the present study, to understand more fully the amyloid deposition mechanism implicating Alzheimer's disease pathogenesis, we examined immunohistochemically the distributions of apolipoprotein E (apoE) and Abeta in amyloid plaques of aged Tg2576 mouse brains. Our findings suggest that Abeta1-42 deposition precedes apoE deposition, and that Abeta1-40 deposition follows apoE deposition during plaque maturation. We next examined the relationship between apoE and astrogliosis associated with amyloid plaques using a double-immunofluorescence method. Extracellular apoE deposits were always associated with reactive astrocytes whose processes showed enhancement of apoE-immunoreactivity. Taken together, the characteristics of amyloid plaques in Tg2576 mice are similar to those in Alzheimer's disease with respect to apoE and astrogliosis. Furthermore, apoE deposition and astrogliosis may be necessary for amyloid plaque maturation.     

Transport pathways for clearance of human Alzheimer's amyloid beta-peptide and apolipoproteins E and J in the mouse central nervous system.

Amyloid beta-peptide (Abeta) clearance from the central nervous system (CNS) maintains its low levels in brain. In Alzheimer's disease, Abeta accumulates in brain possibly because of its faulty CNS clearance and a deficient efflux across the blood-brain barrier (BBB). By using human-specific enzyme-linked immunosorbent assays, we measured a rapid 30 mins efflux at the BBB and transport via the interstitial fluid (ISF) bulk flow of human-unlabeled Abeta and of Abeta transport proteins, apolipoprotein E (apoE) and apoJ in mice. We show (i) Abeta40 is cleared rapidly across the BBB via low-density lipoprotein receptor-related protein (LRP)1 at a rate of 0.21 pmol/min g ISF or 6-fold faster than via the ISF flow; (ii) Abeta42 is removed across the BBB at a rate 1.9-fold slower compared with Abeta40; (iii) apoE, lipid-poor isoform 3, is cleared slowly via the ISF flow and across the BBB (0.03-0.04 pmol/min g ISF), and after lipidation its transport at the BBB becomes barely detectable within 30 mins; (iv) apoJ is eliminated rapidly across the BBB (0.16 pmol/min g ISF) via LRP2. Clearance rates of unlabeled and corresponding 125I-labeled Abeta and apolipoproteins were almost identical, but could not be measured at low physiologic levels by mass spectrometry. Amyloid beta-peptide 40 binding to apoE3 reduced its efflux rate at the BBB by 5.7-fold, whereas Abeta42 binding to apoJ enhanced Abeta42 BBB clearance rate by 83%. Thus, Abeta, apoE, and apoJ are cleared from brain by different transport pathways, and apoE and apoJ may critically modify Abeta clearance at the BBB.     

The nucleation growth and reversibility of Amyloid-beta deposition in vivo

The amyloid-beta (Abeta) peptide is a major constituent of the brain senile plaques that characterize Alzheimer's disease (AD). Converging observations led to the formulation of the amyloid hypothesis whereby the accumulation of soluble aggregates and insoluble Abeta deposits is the primary event in AD pathogenesis. Furthermore, the apoE4 isoform of apolipoprotein E, a major prevalent genetic risk factor of AD, is associated with increased Abeta deposition. To investigate the initial stages of the amyloid cascade in vivo and how this is affected by apoE4, we studied the effects of prolonged inhibition and subsequent reactivation of the Abeta-degrading enzyme, neprilysin, on aggregation and deposition of Abeta in apoE transgenic and control mice. The results revealed that Abeta deposition in vivo is initiated by aggregation of Abeta42, which is followed by reversible deposition of both Abeta42 and Abeta40, along with growth of the deposits, and by their subsequent irreversible fibrillization. The initiation of Abeta42 deposition is accelerated isoform-specifically by apoE4, whereas the growth and dissolution of the Abeta deposits as well as their fibrillization are similarly stimulated by the various apoE isoforms. Interestingly, Abeta deposition was associated with increased gliosis, which may reflect early pathological interactions of beta with the brain's parenchyma.     

Neuroinflammation-induced acceleration of amyloid deposition in the APPV717F transgenic mouse

It has been postulated that neuroinflammation plays a critical role in the pathogenesis of Alzheimer's disease (AD). To directly test whether an inflammatory stimulus can accelerate amyloid deposition in vivo, we chronically administered the bacterial endotoxin, lipopolysaccharide (LPS), intracerebroventricularly (i.c.v.) to 2-month-old APPV717F+/+ transgenic (TG) mice, which overexpress a mutant human amyloid precursor protein (APP 717V-F) with or without apolipoprotein E (apoE) for 2 weeks. Two weeks following central LPS administration a striking global reactive astrocytosis with increased GFAP immunoreactivity was found throughout the brains of all LPS-treated wild-type and transgenic mice including the contralateral brain hemisphere. Localized microglial activation was also evident from lectin immunostaining adjacent to the cannula track of LPS-treated mice. Quantification of thioflavine-S-positive Abeta deposits revealed a marked acceleration of amyloid deposition in LPS-treated APPV717F+/+-apoE+/+ mice compared to nontreated or vehicle-treated APPV717F+/+-apoE+/+ mice (P = 0.005). By contrast, no amyloid deposits were detected by thioflavine-S staining in LPS or vehicle-treated apoE-deficient APPV717F TG mice. Our data suggest that neuroinflammation can accelerate amyloid deposition in the APPV717F+/+ mouse model of AD and that this process requires the expression of apoE.     

Aging increased amyloid peptide and caused amyloid plaques in brain of old APP/V717I transgenic mice by a different mechanism than mutant presenilin1.

Aging of transgenic mice that overexpress the London mutant of amyloid precursor protein (APP/V717I) (Moechars et al., 1999a) was now demonstrated not to affect the normalized levels of alpha- or beta-cleaved secreted APP nor of the beta-C-terminal stubs. This indicated that aging did not markedly disturb either alpha- or beta-secretase cleavage of APP and failed to explain the origin of the massive amounts of amyloid peptides Abeta40 and Abeta42, soluble and precipitated as amyloid plaques in the brain of old APP/V717I transgenic mice. We tested the hypothesis that aging acted on presenilin1 (PS1) to affect gamma-secretase-mediated production of amyloid peptides by comparing aged APP/V717I transgenic mice to double transgenic mice coexpressing human PS1 and APP/V717I. In double transgenic mice with mutant (A246E) but not wild-type human PS1, brain amyloid peptide levels increased and resulted in amyloid plaques when the mice were only 6-9 months old, much earlier than in APP/V717I transgenic mice (12-15 months old). Mutant PS1 increased mainly brain Abeta42 levels, whereas in aged APP/V717I transgenic mice, both Abeta42 and Abeta40 increased. This resulted in a dramatic difference in the Abeta42/Abeta40 ratio of precipitated or plaque-associated amyloid peptides, i.e., 3.11+/-0.22 in double APP/V717I x PS1/A246E transgenic mice compared with 0.43 +/- 0.07 in aged APP/V717I transgenic mice, and demonstrated a clear difference between the effect of aging and the effect of the insertion of a mutant PS1 transgene. In conclusion, we demonstrate that aging did not favor amyloidogenic over nonamyloidogenic processing of APP, nor did it exert a mutant PS1-like effect on gamma-secretase. Therefore, the data are interpreted to suggest that parenchymal and vascular accumulation of amyloid in aging brain resulted from failure to clear the amyloid peptides rather than from increased production.     

Commentary: Abeta N- Terminal Isoforms: Critical contributors in the course of AD pathophysiology

The assessment of protein or amino acid variations across evolution allows one to glean divergent features of disease-specific pathology. Within the Alzheimer's disease (AD) literature, extensive differences in Abeta processing across cell lines and evolution have clearly been observed. In the recent past, increased levels of amyloid beta Abeta1-42 have been heralded to be what distinguishes whether one is prone to the development of AD [59]. However, observations in naturally occurring, non-transgenic animals which display a great deal of parenchymal Abeta1-42 (Abeta found within extracellular plaque deposits) and a complete lack ofbeta1-40 within these same Abeta1-42 plaques raise the issue of whether Abetax-42 (Abeta that is truncated or modified at the N- terminus), rather than Abeta1-42, is instead the critical mediator of Abeta production and pathogenesis [47,49]. Distinct ratios of Abeta N-terminal variants (i.e. Abeta1-x, Abeta3-x, Abeta11-x, beta17-x) have been assessed in human amyloid plaques [18,21,40,41,42,47,48,49,52]. Moreover, ratios of specific Abeta N-terminal variants separate naturally occurring, non-transgenic animals which develop abundant levels of Abetax-42 and not Abetax-40 from human AD participants who harbor plaques that contain both the Abetax-42 and Abetax-40 variants [49]. Next, Teller and colleagues have demonstrated the presence of N-terminal truncated soluble 3kD (likely Abeta17-x) and 3.7kD peptides (in addition to 4kD Abeta) well before the appearance of amyloid plaques in Down Syndrome brain [51], indicating an early contribution of thebeta N-terminus to the formation of amyloid pathology. Additional critical facts concerning the major contribution of the Abeta N-terminus in AD pathogenesis include observations which support thatbeta generated by rodent neurons is predominantly truncated at Abeta11-x [13], the major form of APP C-terminal fragments in mice lacking functional PS1 is AbetaPP11-98 [9], beta11-x expression is increased as a function of BACE expression [55], and an interrelationship between presenilin-1 mutations and increased levels of N-terminally truncatedbeta [40]. This commentary highlights current understanding and potential biochemical, pathological, and cell biological contributions of Abeta N-terminal variants implicated during the course of AD pathogenesis. Although the amyloid beta protein precursor (AbetaPP) gene and Abeta are highly conserved across mammalian species, there are species-specific differences. For instance, the primate, guinea pig, canine, and polar bear share an identical Abeta sequence to that observed in human brain while the rat displays a distinct amino acid sequence with substitutions at residues 5 (Arg), 10 (Tyr), and 13 (His) [24,37]. All of these mammals generate Abeta1-42 via cleavage by at least two enzymes, beta (beta-) secretase and gamma (gamma-) secretase (Fig. 1). The enzyme that liberates the N- terminus of the Abeta peptide ('beta-secretase') is also termed BACE (beta-site AbetaPP cleaving enzyme) [55]. Cathepsin D, which accumulates within AD neurons [15], also cleaves at the N-terminal side of the first aspartate residue of amyloid beta [2].beta-secretase activity is necessary in order to initiate 4kD beta1-x formation by cleaving AbetaPP at the N-terminus and results in the release of a soluble 100kD AbetaPP N- terminal fragment and a 12kD membrane bound C-terminal fragment (C99/C100) [55]. The carboxyl-terminus of the Abetapeptide is liberated through cleavage by the enzyme termed gamma-secretase. In the past, potential AD therapeutic strategies have mainly been geared towards gamma-secretase inhibition. However, such strategies alone no longer appear sound as it is clear that the AbetaPP C99/C100 fragment itself, which requires beta-, but not gamma-, secretase cleavage for generation and includes the entire Abeta peptide, is neurotoxic when evaluated in cultured cells [12,30,34]. Thus, gamma-secretase inhibition alone would not preclude the generation of the neurotoxic C99/C100 fragment.     

beta-amyloid (Abeta)42(43), abeta42, abeta40 and apoE immunostaining of plaques in fatal head injury

beta-Amyloid (Abeta) deposits are found in the brains of approximately one-third of patients who die within days after a severe head injury; their presence correlating strongly with possession of an apolipoprotein E (apoE)-epsilon4 allele. The aim of the study was to investigate the relationship between Abeta42, Abeta40 and apoE immunostaining of Abeta plaques in the cerebral cortex and the relevance of apoE genotype in 23 fatally head-injured patients. These cases were known to have Abeta deposits from a previous study in which they were examined and semiquantified and related to apoE genotype. In the present study, the temporal cortex was probed using four different antibodies that recognize Abeta42(43), Abeta40 and an antibody to apoE. Abeta42(43)-positive plaques were observed in all of the 23 cases and Abeta40 immunoreactivity in only 11 of the 23 cases. In addition, semiquantitative analysis showed that relatively fewer plaques were detected with anti-Abeta40 than anti-Abeta42(43). ApoE-immunoreactive plaques were identified in 18 of the 23 cases. The number of plaques stained for apoE was relatively less than for Abeta42(43) but greater than for Abeta40. Furthermore, the density of Abeta plaques detected using either Abeta42(43), Abeta40 or apoE antibodies was associated with possession of apoE-epsilon4 in an allele dose-dependent manner. The results are consistent with Abeta42(43) as the initially deposited species in brain parenchyma and provide evidence that apoE is involved in the early stages of amyloid deposition. Further, the findings may be of relevance to the role of apoE genotype in influencing outcome after acute brain injury.     

Apolipoprotein E and beta-amyloid levels in the hippocampus and frontal cortex of Alzheimer's disease subjects are disease-related and apolipoprotein E genotype dependent.

The epsilon4 allele of apolipoprotein E (apoE) is associated with increased risk for the development of Alzheimer's disease (AD), possibly due to interactions with the beta-amyloid (Abeta) protein. The mechanism by which these two proteins are linked to AD is still unclear. To further assess their potential relationship with the disease, we have determined levels of apoE and Abeta isoforms from three brain regions of neuropathologically confirmed AD and non-AD tissue. In two brain regions affected by AD neuropathology, the hippocampus and frontal cortex, apoE levels were found to be decreased while Abeta(1-40) levels were increased. Levels of apoE were unchanged in AD cerebellum. Furthermore, levels of apoE and Abeta(1-40) were found to be apoE genotype dependent, with lowest levels of apoE and highest levels of Abeta(1-40) occurring in epsilon4 allele carriers. These results suggest that reduction in apoE levels may give rise to increased deposition of amyloid peptides in AD brain.     

Quantitation of apoE domains in Alzheimer disease brain suggests a role for apoE in Abeta aggregation

Apolipoprotein E (apoE) and apoE-derived proteolytic fragments are present in amyloid deposits in Alzheimer disease (AD) and cerebral amyloid angiopathy (CAA). In this study, we examined which apoE fragments are most strongly associated with amyloid deposits and whether apoE receptor binding domains were present. We found that both apoE2- and apoE4-specific residues were present on plaques and blood vessels in AD and CAA. We quantified Abeta plaque burden and apoE plaque burdens in 5 AD brains. ApoE N-terminal-specific and C-terminal-specific antibodies covered 50% and 74% of Abeta plaque burden, respectively (p < 0.003). Double-labeling demonstrated that the plaque cores contained the entire apoE protein, but that outer regions contained only a C-terminal fragment, suggesting a cleavage in the random coil region of apoE. Presence of N- and C-terminal apoE cleavage fragments in brain extracts was confirmed by immunoblotting. The numbers of plaques identified by the apoE N-terminal-specific antibodies and the apoE C-terminal-specific antibody were equal, but were only approximately 60% of the total Abeta plaque number (p < 0.0001). Analysis of the size distribution of Abeta and apoE deposits demonstrated that most of the Abeta-positive, apoE-negative deposits were the smallest deposits (less than 150 microm2). These data suggest that C-terminal residues of apoE bind to Abeta and that apoE may help aid in the progression of small Abeta deposits to larger deposits. Furthermore, the presence of the apoE receptor binding domain in the center of amyloid deposits could affect surrounding cells via chronic interactions with cell surface apoE receptors.     

Alzheimer disease: mouse models pave the way for therapeutic opportunities

Research into the molecular mechanisms of Alzheimer disease (AD) continues to clarify important issues in aberrant protein processing while seeking to identify therapeutic targets. Mutations of genes on chromosomes 1, 14 (presenilins 1 and 2), and 21 (the amyloid-beta [Abeta] amyloid precursor protein [APP]) cause the familial forms of AD that often begin before age 65. An allelic polymorphism on chromosome 19 (apolipoprotein E ) affects the age of onset of the more common forms of sporadic AD. Multiple studies in transgenic mice provide strong evidence to support the view that Abeta amyloid formation is an early and critical pathogenic event: mice expressing pathogenic human APP mutations develop Abeta deposits; coexpression of mutant presenilin genes accelerates the rate of Abeta deposition; and apolipoprotein E plays a role in this process. Thus, the 3 established genetic causes or risk factors for AD affect Abeta deposition. The fact that elevation of the Abeta42/Abeta40 ratio (differing only in 2 amino acids in length) is also linked to amyloid deposition in the APP mice and is temporally linked to cognitive impairment suggests that Abeta42 may be a principal inducing factor of AD. The exact sequence of events is still unknown, but the transgenic models generated so far have shown their usefulness in clarifying this complex part of the pathology. The continuing progress in elucidation of the molecular pathogenesis of AD suggests a range of rational pharmacological interventions for this disorder. The most promising strategy involves the development of approaches to retard, halt, or prevent Abeta-mediated disease progression, and these can now be tested in transgenic animals.     

Detection of amyloid plaques by radioligands for Abeta40 and Abeta42: potential imaging agents in Alzheimer's patients

Alzheimer s disease (AD) is linked to increased brain deposition of amyloid-beta (Abeta) peptides in senile plaques (SPs), and recent therapeutic efforts have focused on inhibiting the production or enhancing the clearance of Abeta in brain. However, it has not been possible to measure the burden of SPs or assess the effect of potential therapies on brain Abeta levels in patients. Toward that end, we have developed a novel radioligand, [(125)I]TZDM, which binds Abeta fibrils with high affinity, crosses the blood-brain barrier (BBB), and labels amyloid plaques in vivo. Compared to a styrylbenzene probe, [(125)I]IMSB, [(125)I]TZDM showed a 10-fold greater brain penetration and labeled plaques with higher sensitivity for in vivo imaging. However, this ligand also labels white matter, which contributes to undesirable high background regions of the brain. Interestingly, parallel to their differential binding characteristics onto fibrils composed of 40 (Abeta40)- or 42 (Abeta42)-amino-acid-long forms of Abeta peptides, these radioligands displayed differential labeling of SPs in AD brain sections under our experimental conditions. It was observed that [(125)I]IMSB labeled SPs containing Abeta40, amyloid angiopathy (AA), and neurofibrillary tangles, whereas [(125)I]TZDM detected only SPs and Abeta42-positive AA. Since increased production and deposition of Abeta42 relative to Abeta40 may be crucial for the generation of SPs, [(125)I]TZDM and related derivatives may be more attractive probes for in vivo plaque labeling. Further structural modifications of TZDM to lower the background labeling will be needed to optimize the plaque-labeling property.     

Apolipoprotein is required for the formation of filamentous amyloid, but not for amorphous Abeta deposition, in an AbetaPP/PS double transgenic mouse model of Alzheimer's disease

To determine the role of apolipoprotein E (apoE) in the deposition of different forms of Alzheimer amyloid deposit, we studied mice expressing both mutant human amyloid beta-protein precursor (AbetaPP) and presenilin 1 (PS1) that, in addition, were either normal or knocked-out for apoE. By 7 months of age, extensive deposits of amorphous amyloid beta (Abeta) had developed equally in both lines, indicating that, when present in high amounts, Abeta alone is sufficient for such deposition to occur. In contrast, filamentous, thioflavine S-positive amyloid deposition in AbetaPP/PS mice was catalyzed at least 3000 fold by apoE. Electron micrographs further illustrated the filamentous nature of Abeta deposits in mice expressing apoE. These and other behavior data indicate that the primary function of apoE in Alzheimer's disease is to promote the polymerization of Abeta into mature, beta pleated sheet filaments, a process that is necessary for inducing cognitive decline. Thus, preventing apoE from binding to Abeta may prove to be an effective means of therapeutic intervention.     

In vivo effects of apoE and clusterin on amyloid-β metabolism and neuropathology

The epsilon4 allele of apolipoprotein E APOE is a risk factor for Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA), and the epsilon2 allele is associated with a decreased risk for AD. There is strong evidence to suggest that a major, if not the main, mechanism underlying the link between apoE and both AD and CAA is related to the ability of apoE to interact with the amyloid-beta (Abeta) peptide and influence its clearance, aggregation, and conformation. In addition to a number of in vitro studies supporting this concept, in vivo studies with amyloid precursor protein (APP) transgenic mice indicate that apoE and a related molecule, clusterin (also called apolipoprotein J), have profound effects on the onset of Abeta deposition, as well as the local toxicity associated with Abeta deposits both in the brain parenchyma and in cerebral blood vessels. Taken together, these studies suggest that altering the expression of apoE and clusterin in the brain or the interactions between these molecules and Abeta would alter AD pathogenesis and provide new therapeutic avenues for prevention or treatment of CAA and AD.     

LRP and senile plaques in Alzheimer's disease: colocalization with apolipoprotein E and with activated astrocytes

The low density lipoprotein receptor-related protein (LRP) is a multifunctional receptor which is present on senile plaques in Alzheimer's disease (AD). It is suggested to play an important role in the balance between amyloid beta (Abeta) synthesis and clearance mechanisms. One of its ligands, apolipoprotein E (apoE), is also present on senile plaques and has been implicated as a risk factor for AD, potentially affecting the deposition, fibrillogenesis and clearance of Abeta. Using immunohistochemistry we show that LRP was present only on cored, apoE-containing senile plaques, in both PDAPP transgenic mice and human AD brains. We detected strong LRP staining in neurons and in reactive astrocytes, and immunostaining of membrane-bound LRP showed colocalization with fine astrocytic processes surrounding senile plaques. LRP was not present in plaques in young transgenic mice or in plaques of APOE-knockout mice. As LRP ligands associated with Abeta deposits in AD brain may play an important role in inducing levels of LRP in both neurons and astrocytes, our findings support the idea that apoE might be involved in upregulation of LRP (present in fine astrocytic processes) and act as a local scaffolding protein for LRP and Abeta. The upregulation of LRP would allow increased clearance of LRP ligands as well as clearance of Abeta/ApoE complexes.     

Reduced levels of amyloid beta-peptide antibody in Alzheimer disease

OBJECTIVE: To investigate whether it was possible to detect the presence and different levels of naturally occurring anti-beta-amyloid (Abeta) antibodies in the CSF of patients with AD and age-matched controls by employing a sensitive ELISA. BACKGROUND: Immunization with preaggregated amyloid beta-peptide (Abeta(1-42)) and administration of antibodies against Abeta into amyloid precursor protein APP(V717F)- transgenic mice (an animal model of AD) have recently been reported to dramatically reduce amyloid plaque deposition, neuritic dystrophy, and astrogliosis, most likely by enhancing Abeta clearance from brain. METHODS: A sensitive ELISA was performed to detect levels of naturally occurring anti-Abeta antibodies in the CSF of patients with AD and age-matched controls. Additionally, an immunoprecipitation assay was performed to confirm that naturally occurring anti-Abeta antibodies also exist in the human blood. RESULT: - Naturally occurring antibodies directed against Abeta were found in the CSF and plasma of patients with AD and healthy control subjects. Moreover, CSF anti-Abeta antibody titers are significantly lower in patients with AD compared with healthy control subjects. CONCLUSION: Naturally occurring antibodies directed against Abeta exist in human CSF and plasma. The CSF anti-Abeta antibody titers may be helpful in better understanding the effects of future immunologic therapies for AD.     

LRP-mediated clearance of Abeta is inhibited by KPI-containing isoforms of APP

The pathogenesis of Alzheimer's disease (AD) involves the abnormal accumulation and deposition of beta-amyloid in cerebral blood vessels and in the brain parenchyma. Critical in modulating beta-amyloid deposition in brain is the flux of Abeta across the blood brain barrier. The low-density lipoprotein receptor-related protein (LRP), is a large endocytic receptor that mediates the efflux of Abeta out of brain and into the periphery. The first step in the LRP-mediated clearance of Abeta involves the formation of a complex between Abeta and the LRP ligands apolipoprotein E (apoE) or alpha(2)-macroglobulin (alpha(2)M). The Abeta/chaperone complexes then bind to LRP via binding sites on apoE or alpha(2)M. The efflux of Abeta/chaperone complexes out of the neuropil and into the periphery may be attenuated by LRP-ligands that compete with apoE or alpha(2)M for LRP binding. LRP is also the cell surface receptor for Kunitz Protease Inhibitor (KPI) containing isoforms of Abeta's parent protein, the amyloid protein precursor (APP). Protein and mRNA levels of KPI-containing APP isoforms (APP-KPI) are elevated in AD brain and are associated with increased Abeta production. In this study we show that soluble non-amyloidogenic APP-KPI can also inhibit the uptake of Abeta/alpha(2)M in a cell culture model of LRP mediated Abeta clearance. Clearance of Abeta/apoE complexes was not inhibited by APP-KPI. Our findings are consistent with studies showing that apoE and alpha(2)M have discrete binding sites on LRP. Most significantly, our data suggests that the elevated levels of APP-KPI in AD brain may attenuate the clearance of Abeta, the proteins own amyloidogenic catabolic product.