Expression of human apolipoprotein E reduces amyloid-beta deposition in a mouse model of Alzheimer's disease

Immunotherapeutic approaches for Alzheimer's disease in transgenic mouse models

Alzheimer's disease (AD) is a member of a category of neurodegenerative diseases characterized by the conformational change of a normal protein into a pathological conformer with a high beta-sheet content that renders it resistant to degradation and neurotoxic. In the case of AD the normal soluble amyloid beta (sAbeta) peptide is converted into oligomeric/fibrillar Abeta. The oligomeric forms of Abeta are thought to be the most toxic, while fibrillar Abeta becomes deposited as amyloid plaques and congophilic angiopathy, which both serve as neuropathological markers of the disease. In addition, the accumulation of abnormally phosphorylated tau as soluble toxic oligomers and as neurofibrillary tangles is an essential part of the pathology. Many therapeutic interventions are under investigation to prevent and treat AD. The testing of these diverse approaches to ameliorate AD pathology has been made possible by the existence of numerous transgenic mouse models which each mirror different aspects of AD pathology. Perhaps the most exciting of these approaches is immunomodulation. Vaccination is currently being tried for a range of age associated CNS disorders with great success being reported in many transgenic mouse models. However, there is a discrepancy between these results and current human clinical trials which highlights the limitations of current models and also uncertainties in our understanding of the underlying pathogenesis of AD. No current AD Tg mouse model exactly reflects all aspects of the human disease. Since the underlying etiology of sporadic AD is unknown, the process of creating better Tg models is in constant evolution. This is an essential goal since it will be necessary to develop therapeutic approaches which will be highly effective in humans.

Animal models of amyloid-beta-related pathologies in Alzheimer's disease

In the early 1990s, breakthrough discoveries on the genetics of Alzheimer's disease led to the identification of missense mutations in the amyloid-beta precursor protein gene. Research findings quickly followed, giving insights into molecular pathogenesis and possibilities for the development of new types of animal models. The complete toolbox of transgenic techniques, including pronuclear oocyte injection and homologous recombination, has been applied in the Alzheimer's disease field, to produce overexpressors, knockouts, knockins and regulatable transgenics. Transgenic models have dramatically advanced our understanding of pathogenic mechanisms and allowed therapeutic approaches to be tested. Following a brief introduction to Alzheimer's disease, various nontransgenic and transgenic animal models are described in terms of their values and limitations with respect to pathogenic, therapeutic and functional understandings of the human disease.

Increased expression of cholesterol transporter ABCA1 is highly correlated with severity of dementia in AD hippocampus

To gain insight into ATP-binding cassette transporter A1 (ABCA1) function and its potential role in AD pathology, we analyzed the expression of the cholesterol transporter ABCA1 in postmortem hippocampus from persons at different stages of dementia and AD associated neuropathology relative to cognitively intact normal donors by quantitative polymerase chain reaction (qPCR) and Western blot. In this study clinical dementia rating (CDR) scores were used as a measure of dementia severity, whereas, Braak neuropathological staging and neuritic plaque density were used as an index of the neuropathological progression of AD. Correlation analysis showed that ABCA1 mRNA expression was significantly elevated at the earliest recognizable stage of dementia compared to persons with intact cognition. ABCA1 mRNA was also positively correlated with Braak neuropathological stages and neuritic plaque density counts. Additionally, ABCA1 mRNA levels showed robust correlation with dementia severity even after controlling for the confounding contribution of accompanying neuropathological parameters to ABCA1 mRNA expression. Western blot analyses showed that the differential expression observed at the transcriptional level is also reflected at the protein level. Thus, our study provides transcriptional and translational evidence that the expression of ABCA1, a key modulator of cholesterol transport across the plasma membrane, is dysregulated in the AD brain and that this dysregulation is associated with increasing severity of AD, whether measured functionally as dementia severity or neuropathologically as increased neuritic plaque and neurofibrillary tangle density.

Use of genetic variation as biomarkers for mild cognitive impairment and progression of mild cognitive impairment to dementia

Cognitive impairment is prevalent in the elderly. The high estimates of conversion to dementia have spurred the interest in identification of genetic risk factors associated with development of cognitive impairment and or its progression. However, despite notable achievements in human genetics over the years, in particular technological advances in gene mapping and in statistical methods that relate genetic variants to disease, to date only a small proportion of the genetic contribution to late-life cognitive impairment can be explained. A likely explanation for the difficulty in gene identification is that it is a multifactorial disorder with both genetic and environmental components, in which several genes with small effects each are likely to contribute to the quantitative traits associated with the disease. The motivation for identifying the underlying genetic risk factors elderly is clear. Not only could it shed light on disease pathogenesis, but it may also provide potential targets for effective treatment, screening, and prevention. In this article we review the current knowledge on underlying genetic variants and the usefulness of genetic variation as diagnostic tools and biomarkers. In addition, we discuss the potentials and difficulties researchers face in designing appropriate studies for gene discovery.

APP transgenic modeling of Alzheimer's disease: mechanisms of neurodegeneration and aberrant neurogenesis

Neurodegenerative disorders of the aging population affect over 5 million people in the US and Europe alone. The common feature is the progressive accumulation of misfolded proteins with the formation of toxic oligomers. Alzheimer’s disease (AD) is characterized by cognitive impairment, progressive degeneration of neuronal populations in the neocortex and limbic system, and formation of amyloid plaques and neurofibrillary tangles. Amyloid-β (Aβ) is the product of proteolysis of amyloid precursor protein (APP) by β and γ-secretase enzymes. The neurodegenerative process in AD initiates with axonal and synaptic damage and is associated with progressive accumulation of toxic Aβ oligomers in the intracellular and extracellular space. In addition, neurodegeneration in AD is associated with alterations in neurogenesis. Aβ accumulation is the consequence of an altered balance between protein synthesis, aggregation rate, and clearance. Identification of genetic mutations in APP associated with familial forms of AD and gene polymorphisms associated with the more common sporadic variants of AD has led to the development of transgenic (tg) and knock out rodents as well as viral vector driven models of AD. While APP tg murine models with mutations in the N- and C-terminal flanking regions of Aβ are characterized by increased Aβ production with plaque formation, mutations in the mid-segment of Aβ result in increased formation of oligomers, and mutations toward the C-terminus (E22Q) segment results in amyloid angiopathy. Similar to AD, in APP tg models bearing familial mutations, formation of Aβ oligomers results in defective plasticity in the perforant pathway, selective neuronal degeneration, and alterations in neurogenesis. Promising results have been obtained utilizing APP tg models of AD to develop therapies including the use of β- and γ-secretase inhibitors, immunization, and stimulating neurogenesis.

Differential SUMOylation of LXRalpha and LXRbeta mediates transrepression of STAT1 inflammatory signaling in IFN-gamma-stimulated brain astrocytes

To unravel the roles of LXRs in inflammation and immunity, we examined the function of LXRs in development of IFN-gamma-mediated inflammation using cultured rat brain astrocytes. LXR ligands inhibit neither STAT1 phosphorylation nor STAT1 translocation to the nucleus but, rather, inhibit STAT1 binding to promoters and the expression of IRF1, TNFalpha, and IL-6, downstream effectors of STAT1 action. Immunoprecipitation data revealed that LXRbeta formed a trimer with PIAS1-pSTAT1, whereas LXRalpha formed a trimer with HDAC4-pSTAT1, mediated by direct ligand binding to the LXR proteins. In line with the fact that both PIAS1 and HDAC4 belong to the SUMO E3 ligase family, LXRbeta and LXRalpha were SUMO-conjugated by PIAS1 or HDAC4, respectively, and SUMOylation was blocked by transient transfection of appropriate individual siRNAs, reversing LXR-induced suppression of IRF1 and TNFalpha expression. Together, our data show that SUMOylation is required for the suppression of STAT1-dependent inflammatory responses by LXRs in IFN-gamma-stimulated brain astrocytes.

Use of genetic variation as biomarkers for Alzheimer's disease

Late-onset Alzheimer's disease (LOAD) is the most common cause of late-onset dementia in western societies. Despite remarkable achievements in human genetics throughout the years, in particular technological advances in gene mapping and in statistical methods that relate genetic variants to disease, to date only a small proportion of the genetic contribution to LOAD can be explained leaving several remaining genetic risk factors to be identified. A possible explanation for the difficulty in gene identification is that LOAD is a multifactorial complex disorder with both genetic and environmental components. Multiple genes with small effects each ("quantitative trait loci"[QTLs]) are likely to contribute to the quantitative traits associated with the disease, such as memory performance, amyloid/tau pathology, or hippocampal atrophy. The motivation for identifying the genetics of LOAD is clear. Not only could it shed light on disease pathogenesis, but it may also provide potential targets for effective treatment, screening, and prevention. Here, we review the usefulness of genetic variation as diagnostic tools and biomarkers in LOAD and discuss the potentials and difficulties researchers face in designing appropriate studies for gene discovery.

Genome-wide association study identifies variants at CLU and CR1 associated with Alzheimer's disease

The gene encoding apolipoprotein E (APOE) on chromosome 19 is the only confirmed susceptibility locus for late-onset Alzheimer's disease. To identify other risk loci, we conducted a large genome-wide association study of 2,032 individuals from France with Alzheimer's disease (cases) and 5,328 controls. Markers outside APOE with suggestive evidence of association (P < 10(-5)) were examined in collections from Belgium, Finland, Italy and Spain totaling 3,978 Alzheimer's disease cases and 3,297 controls. Two loci gave replicated evidence of association: one within CLU (also called APOJ), encoding clusterin or apolipoprotein J, on chromosome 8 (rs11136000, OR = 0.86, 95% CI 0.81-0.90, P = 7.5 x 10(-9) for combined data) and the other within CR1, encoding the complement component (3b/4b) receptor 1, on chromosome 1 (rs6656401, OR = 1.21, 95% CI 1.14-1.29, P = 3.7 x 10(-9) for combined data). Previous biological studies support roles of CLU and CR1 in the clearance of beta amyloid (Abeta) peptide, the principal constituent of amyloid plaques, which are one of the major brain lesions of individuals with Alzheimer's disease.

The role of apolipoprotein E in Alzheimer's disease

The epsilon4 allele of apolipoprotein E (APOE) is the major genetic risk factor for Alzheimer's disease (AD). Although there have been numerous studies attempting to elucidate the underlying mechanism for this increased risk, how apoE4 influences AD onset and progression has yet to be proven. However, prevailing evidence suggests that the differential effects of apoE isoforms on Abeta aggregation and clearance play the major role in AD pathogenesis. Other potential mechanisms, such as the differential modulation of neurotoxicity and tau phosphorylation by apoE isoforms as well as its role in synaptic plasticity and neuroinflammation, have not been ruled out. Inconsistent results among studies have made it difficult to define whether the APOE epsilon4 allele represents a gain of toxic function, a loss of neuroprotective function, or both. Therapeutic strategies based on apoE propose to reduce the toxic effects of apoE4 or to restore the physiological, protective functions of apoE.

Reelin and apoE actions on signal transduction, synaptic function and memory formation

Low-density-lipoprotein receptors (LDLRs) are an evolutionarily ancient surface protein family with the ability to activate a diversity of extracellular signals across the cellular membrane in the adult central nervous system (CNS). Their intimate roles in modulating synaptic plasticity and their necessity in hippocampal-dependent learning and memory have only recently come to light. Two known LDLR ligands, specifically apolipoprotein E (apoE) and reelin, have been the most widely investigated in this regard. Most of our understanding of synaptic plasticity comes from investigation of both pre- and postsynaptic alterations. Therefore, it is interesting to note that neurons and glia that do not contribute to the synaptic junction in question can secrete signaling molecules that affect synaptic plasticity. Notably, reelin and apoE have been shown to modulate hippocampal long-term potentiation in general, and affect NMDA receptor and AMPA receptor regulation specifically. Furthermore, these receptors and signaling molecules have significant roles in neuronal degenerative diseases such as Alzheimer's disease. The recent production of recombinant proteins, knockout and transgenic mice for receptors and ligands and the development of human ApoE targeted replacement mice have significantly expanded our understanding of the roles LDLRs and their ligands have in certain disease states and the accompanying initiation of specific signaling pathways. This review describes the role LDLRs, apoE and reelin have in the regulation of hippocampal synaptic plasticity.

Reduced levels of human apoE4 protein in an animal model of cognitive impairment

The APOE4 allele is the most common genetic determinant for Alzheimer's disease (AD) in the developed world. APOE genotype specific differences in brain apolipoprotein E protein levels have been observed in numerous studies since the discovery of APOE4's link to AD. Since the human apoE4 targeted replacement mice display characteristics of cognitive impairment we sought to determine if reduced levels of apoE might provide one explanation for this impairment. We developed a novel mass spectrometry method to measure apoE protein levels in plasma. Additionally, we developed an ELISA that replicates the mass spectrometry data and enables the rapid quantitation of apoE in plasma, brain and cerebrospinal fluid. We detected a significant decrease in plasma, brain and cerebrospinal fluid apoE levels in the apoE4 mice compared to apoE2 and E3 mice. We also measured a small (∼19%) decrease in brain apoE levels from aged, non-demented APOE4 carriers. Our findings suggest that a fraction of APOE4-linked AD may be due to insufficient levels of functional apoE required to maintain neuronal health.

Human APOE isoform-dependent effects on brain beta-amyloid levels in PDAPP transgenic mice

To investigate the role of human apolipoprotein E (apoE) on Abeta deposition in vivo, we crossed PDAPP mice lacking mouse Apoe to targeted replacement mice expressing human apoE (PDAPP/TRE2, PDAPP/TRE3, or PDAPP/TRE4). We then measured the levels of apoE protein and Abeta peptides in plasma, CSF, and brain homogenates in these mice at different ages. We also quantified the amount of brain Abeta and amyloid burden in 18-month-old mice. In young PDAPP/TRE4 mice that were analyzed at an age before brain Abeta deposition, we observed a significant decrease in the levels of apoE in CSF and brain when compared with age-matched mice expressing either human E2 or E3. The brain levels of Abeta42 in PDAPP/TRE4 mice were substantially elevated even at this very early time point. In older PDAPP/TRE4 mice, the levels of insoluble apoE protein increased in parallel to the dramatic rise in brain Abeta burden, and the majority of apoE was associated with Abeta. In TRE4 only mice, we also observed a significant decrease in the level of apoE in brain homogenates. Since the relative level of apoE mRNA was equivalent in PDAPP/TRE and TRE only mice, it appears that post-translational mechanisms influence the levels of apoE protein in brain (E4 < E3 << E2), resulting in early and dramatic apoE isoform-dependent effects on brain Abeta levels (E4 >> E3 > E2) that increase with age. Therapeutic strategies aimed at increasing the soluble levels of apoE protein, regardless of isoform, may effectively prevent and (or) treat Alzheimer's disease.

Greasing the wheels of Abeta clearance in Alzheimer's disease: the role of lipids and apolipoprotein E

Although apolipoprotein E (apoE) is the most common genetic risk factor for Alzheimer's Disease (AD), how apoE participates in AD pathogenesis remains incompletely understood. ApoE is also the major carrier of lipids in the brain. Here, we review studies showing that the lipidation status of apoE influences the metabolism of Abeta peptides, which accumulate as amyloid deposits in the neural parenchyma and cerebrovasculature. One effect of apoE is to inhibit the transport of Abeta across the blood-brain-barrier (BBB), particularly when apoE is lipidated. A second effect is to facilitate the proteolytic degradation of Abeta by neprilysin and insulin degrading enzyme (IDE), which is enhanced when apoE is lipidated. We also describe how apoE becomes lipidated and how this impacts Abeta metabolism. Specifically, genetic loss of the cholesterol transporter ABCA1 impairs apoE lipidation and promotes amyloid deposition in AD mouse models. ABCA1 catalyses the ATP-dependent transport of cholesterol and phospholipids from the plasma membrane to lipid-free apolipoproteins including apoE. Conversely, selective overexpression of ABCA1 increases apoE lipidation in the central nervous system (CNS) and eliminates the formation of amyloid plaques in vivo. Deficiency of Liver-X-Receptors (LXRs), transcription factors that stimulate ABCA1 and apoE expression, exacerbates AD pathogenesis in vivo, whereas treatment of AD mice with synthetic LXR agonists reduces amyloid load and improves cognitive performance. These studies provide new insights into the mechanisms by which apoE affects Abeta metabolism, and offer opportunities to develop novel therapeutic approaches to reduce the leading cause of dementia in the elderly.

Macrophage-mediated degradation of beta-amyloid via an apolipoprotein E isoform-dependent mechanism

Recent studies suggest that bone marrow-derived macrophages can effectively reduce beta-amyloid (Abeta) deposition in brain. To further elucidate the mechanisms by which macrophages degrade Abeta, we cultured murine macrophages on top of Abeta plaque-bearing brain sections from transgenic mice expressing PDAPP [human amyloid precursor protein (APP) with the APP(717V>F) mutation driven by the platelet-derived growth factor promoter]. Using this ex vivo assay, we found that macrophages from wild-type mice very efficiently degrade both soluble and insoluble Abeta in a time-dependent manner and markedly eliminate thioflavine-S positive amyloid deposits. Because macrophages express and secrete apolipoprotein E (apoE), we compared the efficiency of Abeta degradation by macrophages prepared from apoE-deficient mice or mice expressing human apoE2, apoE3, or apoE4. Macrophages expressing apoE2 were more efficient at degrading Abeta than apoE3-expressing, apoE4-expressing, or apoE-deficient macrophages. Moreover, macrophage-induced degradation of Abeta was effectively blocked by an anti-apoE antibody and receptor-associated protein, an antagonist of the low-density lipoprotein (LDL) receptor family, suggesting involvement of LDL receptors. Measurement of matrix metalloproteinase-9 (MMP-9) activity in the media from human apoE-expressing macrophages cocultured with Abeta-containing brain sections revealed greater levels of MMP-9 activity in apoE2-expressing than in either apoE3- or apoE4-expressing macrophages. Differences in MMP-9 activity appear to contribute to the isoform-specific differences in Abeta degradation by macrophages. These apoE isoform-dependent effects of macrophages on Abeta degradation suggest a novel "peripheral" mechanism for Abeta clearance from brain that may also, in part, explain the isoform-dependent effects of apoE in determining the genetic risk for Alzheimer's disease.

Apolipoprotein E and its receptors in Alzheimer's disease: pathways, pathogenesis and therapy

The vast majority of Alzheimer's disease (AD) cases are late-onset and their development is probably influenced by both genetic and environmental risk factors. A strong genetic risk factor for late-onset AD is the presence of the epsilon4 allele of the apolipoprotein E (APOE) gene, which encodes a protein with crucial roles in cholesterol metabolism. There is mounting evidence that APOE4 contributes to AD pathogenesis by modulating the metabolism and aggregation of amyloid-beta peptide and by directly regulating brain lipid metabolism and synaptic functions through APOE receptors. Emerging knowledge of the contribution of APOE to the pathophysiology of AD presents new opportunities for AD therapy.

The complex interaction between APOE promoter and AD: an Italian case-control study

The single nucleotide polymorphisms (SNPs) rs449647, rs769446 and rs405509 in the promoter region of the APOE gene have been variously suggested to be epsilon 4-independent risk factors for Alzheimer's disease (AD). A previous Italian study found that the rs449647 was significantly associated with late-onset AD. The aim of this study was to verify whether these APOE promoter SNPs are genetic risk factors for AD and to investigate their interaction with the common APOE polymorphism. A total of 169 clinically diagnosed AD patients and 99 cognitively intact age-matched controls were included in the study. Significant associations with AD independent from sex, age and APOE/epsilon 4 status were found for rs449647 A/A and rs405509 G/G genotypes (positive), and rs449647 A/T and rs405509 T/T genotypes (negative). Haplotype frequency estimation at the APOE locus showed significant associations for the ATG4, ATT4 and ACG3 (positive) and ATT2, ATT3 and TCG3 (negative) haplotypes. Therefore this study confirms the role of the rs449647 A/A genotype as risk factor for AD in Italy and suggests that promoter genotypes and APOE haplotypes might have a complex function in AD-associated genetic risk factors.

Preventing beta-amyloid fibrillization and deposition: beta-sheet breakers and pathological chaperone inhibitors

Central to the pathogenesis of Alzheimer's disease (AD) is the conversion of normal, soluble beta-amyloid (sAbeta) to oligomeric, fibrillar Abeta. This process of conformational conversion can be influenced by interactions with other proteins that can stabilize the disease-associated state; these proteins have been termed 'pathological chaperones'. In a number of AD models, intervention that block soluble Abeta aggregation, including beta-sheet breakers, and compounds that block interactions with pathological chaperones, have been shown to be highly effective. When combined with early pathology detection, these therapeutic strategies hold great promise as effective and relatively toxicity free methods of preventing AD related pathology.

apoE isoform-specific disruption of amyloid beta peptide clearance from mouse brain

Neurotoxic amyloid beta peptide (Abeta) accumulates in the brains of individuals with Alzheimer disease (AD). The APOE4 allele is a major risk factor for sporadic AD and has been associated with increased brain parenchymal and vascular amyloid burden. How apoE isoforms influence Abeta accumulation in the brain has, however, remained unclear. Here, we have shown that apoE disrupts Abeta clearance across the mouse blood-brain barrier (BBB) in an isoform-specific manner (specifically, apoE4 had a greater disruptive effect than either apoE3 or apoE2). Abeta binding to apoE4 redirected the rapid clearance of free Abeta40/42 from the LDL receptor-related protein 1 (LRP1) to the VLDL receptor (VLDLR), which internalized apoE4 and Abeta-apoE4 complexes at the BBB more slowly than LRP1. In contrast, apoE2 and apoE3 as well as Abeta-apoE2 and Abeta-apoE3 complexes were cleared at the BBB via both VLDLR and LRP1 at a substantially faster rate than Abeta-apoE4 complexes. Astrocyte-secreted lipo-apoE2, lipo-apoE3, and lipo-apoE4 as well as their complexes with Abeta were cleared at the BBB by mechanisms similar to those of their respective lipid-poor isoforms but at 2- to 3-fold slower rates. Thus, apoE isoforms differentially regulate Abeta clearance from the brain, and this might contribute to the effects of APOE genotype on the disease process in both individuals with AD and animal models of AD.

Genetically altering Abeta distribution from the brain to the vasculature ameliorates tau pathology

The inheritance of the epsilon4 allele of the apolipoprotein E (apoE) gene is the major genetic risk factor for developing late-onset Alzheimer disease. In transgenic mice overexpressing amyloid precursor protein (APP), replacing the endogenous mouse apoE gene with the human apolipoprotein E4 (apoE4) gene alters the distribution of amyloid-beta (Abeta) deposits from the brain parenchyma to the vasculature. However, the effects of this distribution on the onset and progression of tau pathology remain to be established. To address this issue, we used a genetic approach to replace the endogenous apoE gene with the human apoE4 allele in the 3xTg-AD mice. We showed that changing Abeta distribution from the parenchyma to the vasculature drastically reduces the tau pathology. The 3xTg-AD mice expressing the human apoE4 gene were virtually depleted of any somatodendritic tau deposits. These data strongly suggest that the somatodendritic tau accumulation is dependent on the parenchyma Abeta deposits.

The role of the immune system in clearance of Abeta from the brain

In Alzheimer's disease (AD), there is abnormal accumulation of Abeta and tau proteins in the brain. There is an associated immunological response, but it is still unclear whether this is beneficial or harmful. Inflammation in AD, specifically in the form of microglial activation, has, for many years, been considered to contribute to disease progression. However, two types of evidence suggest that it may be appropriate to revise this view: first, the disappointing results of prospective clinical trials of anti-inflammatory agents and, second, the observation that microglia can clear plaques in AD following Abeta immunization. Although Abeta immunization alters AD pathology, there is limited evidence so far of benefit to cognitive function. Immunization against microorganisms is almost always used as a method of disease prevention rather than to treat a disease process that has already started. In animal models, immunotherapy at an early age can protect against Abeta accumulation and it will be interesting to see if this can usefully be applied to humans to prevent AD.

Apolipoprotein E and Alzheimer's disease: molecular mechanisms and therapeutic opportunities

Multiple genetic and environmental factors are likely to contribute to the development of Alzheimer's disease (AD). The most important known risk factor for AD is presence of the E4 isoform of apolipoprotein E (apoE). Epidemiological studies demonstrated that apoE4 carriers have a higher risk and develop the disease and an early onset. Moreover, apoE4 is the only molecule that has been associated with all the biochemical disturbances characteristic of the disease: amyloid-beta (Abeta) deposition, tangle formation, oxidative stress, lipid homeostasis deregulation, synaptic plasticity loss and cholinergic dysfunction. This large body of evidence suggest that apoE is a key player in the pathogenesis of AD. This short review examines the current facts and hypotheses of the association between apoE4 and AD, as well as the therapeutic possibilities that apoE might offer for the treatment of this disease.

Overview on rodent models of Alzheimer's disease

In Alzheimer's disease (AD), characteristic lesions develop in brain regions that subserve cognitive functions, ultimately leading to dementia. There are now several lesioned or transgenic small-animal models of the disease that model select aspects of cognitive deficits and/or recapitulate many, but not all, of the characteristic pathologic lesions observed in AD. This overview describes the most common approaches used to model AD in rodents, highlights their utility, and discusses some of their deficiencies.

Overexpression of ABCA1 reduces amyloid deposition in the PDAPP mouse model of Alzheimer disease

APOE genotype is a major genetic risk factor for late-onset Alzheimer disease (AD). ABCA1, a member of the ATP-binding cassette family of active transporters, lipidates apoE in the CNS. Abca1(-/-) mice have decreased lipid associated with apoE and increased amyloid deposition in several AD mouse models. We hypothesized that mice overexpressing ABCA1 in the brain would have increased lipidation of apoE-containing lipoproteins and decreased amyloid deposition. To address these hypotheses, we created PrP-mAbca1 Tg mice that overexpress mouse Abca1 throughout the brain under the control of the mouse prion promoter. We bred the PrP-mAbca1 mice to the PDAPP AD mouse model, a transgenic line overexpressing a mutant human amyloid precursor protein. PDAPP/Abca1 Tg mice developed a phenotype remarkably similar to that seen in PDAPP/Apoe(-/-) mice: there was significantly less amyloid beta-peptide (Abeta) deposition, a redistribution of Abeta to the hilus of the dentate gyrus in the hippocampus, and an almost complete absence of thioflavine S-positive amyloid plaques. Analyses of CSF from PrP-mAbca1 Tg mice and media conditioned by PrP-mAbca1 Tg primary astrocytes demonstrated increased lipidation of apoE-containing particles. These data support the conclusions that increased ABCA1-mediated lipidation of apoE in the CNS can reduce amyloid burden and that increasing ABCA1 function may have a therapeutic effect on AD.

Heat shock proteins and amateur chaperones in amyloid-Beta accumulation and clearance in Alzheimer's disease

The pathologic lesions of Alzheimer’s disease (AD) are characterized by accumulation of protein aggregates consisting of intracellular or extracellular misfolded proteins. The amyloid-β (Aβ) protein accumulates extracellularly in senile plaques and cerebral amyloid angiopathy, whereas the hyperphosphorylated tau protein accumulates intracellularly as neurofibrillary tangles. “Professional chaperones”, such as the heat shock protein family, have a function in the prevention of protein misfolding and subsequent aggregation. “Amateur” chaperones, such as apolipoproteins and heparan sulfate proteoglycans, bind amyloidogenic proteins and may affect their aggregation process. Professional and amateur chaperones not only colocalize with the pathological lesions of AD, but may also be involved in conformational changes of Aβ, and in the clearance of Aβ from the brain via phagocytosis or active transport across the blood–brain barrier. Thus, both professional and amateur chaperones may be involved in the aggregation, accumulation, persistence, and clearance of Aβ and tau and in other Aβ-associated reactions such as inflammation associated with AD lesions, and may, therefore, serve as potential targets for therapeutic intervention.

Molecular genetics of Alzheimer's disease: an update

Alzheimer's disease (AD) is a complex disorder of the central nervous system (CNS). Molecular genetic research has provided a wealth of information regarding the genetic etiology of this devastating disease. Identification and functional characterization of autosomal dominant mutations in the amyloid precursor protein gene (APP) and the presenilin genes 1 and 2 (PSEN1 and PSEN2) have contributed substantially to our understanding of the biological mechanisms leading towards CNS neurodegeneration in AD. Nonetheless, a large part of the genetic etiology remains unresolved, especially that of more common, sporadic forms of AD. While substantial efforts were invested in the identification of genetic risk factors underlying sporadic AD, using carefully designed genetic association studies in large patient-control groups, the only firmly established risk factor remains the epsilon4 allele of the apolipoprotein E gene (APOE). Nevertheless, one can expect that with the current availability of high-throughput genotyping platforms and dense maps of single-nucleotide polymorphisms (SNPs), large-scale genetic studies will eventually generate additional knowledge about the genetic risk profile for AD. This review provides an overview of the current understanding in the field of AD genetics, covering both the rare monogenic forms as well as recent developments in the search for novel AD susceptibility genes.

Apolipoprotein E receptors and amyloid expression are modulated in an apolipoprotein E-dependent fashion in response to hippocampal deafferentation in rodent

The entorhinal cortex lesion paradigm is a widely accepted and efficient method to provoke reactive synaptogenesis and terminal remodeling in the adult CNS. This approach has been used successfully to contrast the profile of reactivity from various proteins associated with Alzheimer's disease pathophysiology in wild-type and apolipoprotein E (apoE)-deficient (APOE ko) mice. Results indicate that the production of the beta-amyloid 1-40 peptide (A beta 40) is increased in response to neuronal injury, with a timing that is different between wild-type and APOE ko animals. Moreover, we report that baseline levels of the A beta 40 peptide are significantly higher in the APOE ko mice. The expression of the apolipoprotein E receptor type 2 (apoER2) is also modulated by the deafferentation process in the hippocampus, but only in APOE ko mice. These results provide novel insights as to the molecular mechanisms responsible for the poor plastic response reported in apoE4-expressing and apoE deficient mice in response to hippocampal injury.

Structural basis for the recognition and cross-linking of amyloid fibrils by human apolipoprotein E

Apolipoprotein (apo) E is a well characterized lipid-binding protein in plasma that also exists as a common nonfibrillar component of both cerebral and systemic amyloid deposits. A genetic link between a common isoform of apoE, apoE4, and the incidence of late onset Alzheimer disease has drawn considerable attention to the potential roles of apoE in amyloid-related disease. We examined the interactions of apoE with amyloid fibrils composed of apoC-II and the amyloid-beta (Abeta) peptide. Aggregates of apoE with Abeta and apoC-II are found in Alzheimer and atherosclerotic plaques, respectively. Sedimentation velocity and fibril size distribution analysis showed that apoE3 and E4 isoforms bind and noncovalently cross-link apoC-II fibrils in a similar manner. This ability to cross-link apoC-II fibrils was abolished by the dissociation of the apoE tetramer to monomers or by thrombin cleavage to yield separate N- and C-terminal domains. Preparative ultracentrifuge binding studies indicated that apoE and the isolated N- and C-terminal domains of apoE bind with submicromolar affinities to both apoC-II and Abeta fibrils. Fluorescence quenching and resonance energy transfer experiments confirmed that both domains of apoE interact with apoC-II fibrils and demonstrated that the binding of the isolated N-terminal domain of apoE to apoC-II or Abeta fibrils is accompanied by a significant conformational change with helix three of the domain moving relative to helix one. We propose a model involving the interaction of apoE with patterns of aligned residues that could explain the general ability of apoE to bind to a diverse range of amyloid fibrils.

Genetic heterogeneity of Alzheimer's disease: complexity and advances

Most of what we know about the pathological process of Alzheimer's disease (AD) results from research on the amyloid cascade hypothesis. This hypothesis in turn is derived largely from the characterization of rare disease-causing mutations in three genes, which code for the amyloid precursor protein (APP), presenilin 1 (PS-1) and presenilin 2 (PS-2) and account for most cases of early-onset autosomal dominant familial AD. These genetic findings also suggested that better understanding of the genetic components of AD, even in the late-onset sporadic forms of the disease, might help to identify central pathways of the AD process and lead to the rapid development of active molecules. Twin studies have reinforced the rationale of this approach, for they indicate that more than 50% of the late-onset AD risk may be attributable to genetic factors. The 1993 discovery that the apolipoprotein E4 (ApoE4) allele is genetically associated with increased risk in both sporadic and familial late-onset Alzheimer's disease strongly supports the validity of this genetic approach. Further progress based on this major finding has nonetheless been disappointing and raises questions about it. First, despite intensive researches, the exact role of APOE in the pathophysiological process still remains unknown. Second, the APOE gene is the only gene so far recognized as a consistent genetic determinant of sporadic forms of AD, even though numerous studies have looked for such genes; these disappointing results suggest persistent methodological limitations. However, recent methodologies allowing new strategies may allow important breakthrough.

Effects of human apolipoprotein E isoforms on the amyloid beta-protein concentration and lipid composition in brain low-density membrane domains

Apolipoprotein E4 (apoE4) encoded by epsilon 4 allele is a strong genetic risk factor for Alzheimer's disease (AD). ApoE4 carriers have accelerated amyloid beta-protein (A beta) deposition in their brains, which may account for their unusual susceptibility to AD. We hypothesized that the accelerated A beta deposition in the brain of apoE4 carriers is mediated through cholesterol-enriched low-density membrane (LDM) domains. Thus, the concentrations of A beta and various lipids in LDM domains were quantified in the brains of homozygous apoE3 and apoE4 knock-in (KI) mice, and in the brains of those mice bred with beta-amyloid precursor protein (APP) transgenic mice (Tg2576). The A beta 40 and A beta 42 concentrations and the A beta 42 proportions in LDM domains did not differ between apoE3 and apoE4 KI mice up to 18 months of age. The A beta 40 concentration in the LDM domains was slightly, but significantly higher in apoE3/APP mice than in apoE4/APP mice. The lipid composition of LDM domains was modulated in an apoE isoform-specific manner, but its significance for A beta deposition remains unknown. These data show that the apoE isoform-specific effects on the A beta concentration in LDM domains do not occur in KI mouse models.

Therapeutic approaches for prion and Alzheimer's diseases

Alzheimer's and prion diseases belong to a category of conformational neurodegenerative disorders [Prusiner SB (2001) N Eng J Med344, 1516-1526; Sadowski M & Wisniewski T (2007) Curr Pharm Des 13, 1943-1954; Beekes M (2007) FEBS J 274, 575]. Treatments capable of arresting or at least effectively modifying the course of disease do not yet exist for either one of these diseases. Alzheimer's disease is the major cause of dementia in the elderly and has become an ever greater problem with the aging of Western societies. Unlike Alzheimer's disease, prion diseases are relatively rare. Each year only approximately 300 people in the USA and approximately 100 people in the UK succumb to various forms of prion diseases [Beekes M (2007) FEBS J 274, 575; Sigurdsson EM & Wisniewski T (2005) Exp Rev Vaccines 4, 607-610]. Nevertheless, these disorders have received great scientific and public interest due to the fact that they can be transmissible among humans and in certain conditions from animals to humans. The emergence of variant Creutzfeld-Jakob disease demonstrated the transmissibility of the bovine spongiform encephalopathy to humans [Beekes M (2007) FEBS J 274, 575]. Therefore, the spread of bovine spongiform encephalopathy across Europe and the recently identified cases in North America have put a large human population at risk of prion infection. It is estimated that at least several thousand Britons are asymptomatic carriers of prion infections and may develop variant Creutzfeld-Jakob disease in the future [Hilton DA (2006) J Pathol 208, 134-141]. This delayed emergence of human cases following the near elimination of bovine spongiform encephalopathy in the UK may occur because prion disease have a very prolonged incubation period, ranging from months to decades, which depends on the amount of inoculum, the route of infection and the genetic predisposition of the infected subject [Hilton DA (2006) J Pathol 208, 134-141]. Therefore, there is a great need for effective therapies for both Alzheimer's disease and prion diseases.

Cholesterol metabolism, apolipoprotein E, adenosine triphosphate-binding cassette transporters, and Alzheimer's disease

PURPOSE OF REVIEW

Recent evidence suggests that cholesterol metabolism participates in the pathogenesis of Alzheimer's disease. Apolipoprotein E is the main lipid carrier in the brain and the best-established risk factor for late-onset Alzheimer's disease. Intracellular cholesterol levels influence the generation of amyloid-beta peptides, the toxic species thought to be a primary cause of Alzheimer's disease. Finally, compounds that modulate cholesterol metabolism affect amyloid-beta generation. This review summarizes data linking apolipoprotein E and adenosine triphosphate-binding cassette transporters to aspects of cholesterol metabolism and Alzheimer's disease pathogenesis.

RECENT FINDINGS

In vivo, the lipidation status of apolipoprotein E affects amyloid-beta burden in mice with Alzheimer's disease, which appears to caused by the modulation of amyloid-beta deposition or clearance rather than amyloid-beta production. State-of-the-art in-vivo assays reveal that amyloid-beta is cleared from the brain by multiple pathways. Members of the adenosine triphosphate-binding cassette superfamily of transporters regulate lipid homeostasis and apolipoprotein metabolism in the brain, and may affect Alzheimer's disease pathogenesis by modulating apolipoprotein E lipidation as well as intracellular sterol homeostasis.

SUMMARY

Proteins involved in brain cholesterol metabolism may affect the pathogenesis of Alzheimer's disease. Compounds that modulate the expression of these proteins may be of therapeutic benefit in Alzheimer's disease.

The generation and function of soluble apoE receptors in the CNS

More than a decade has passed since apolipoprotein E4 (APOE-ε4) was identified as a primary risk factor for Alzheimer 's disease (AD), yet researchers are even now struggling to understand how the apolipoprotein system integrates into the puzzle of AD etiology. The specific pathological actions of apoE4, methods of modulating apolipoprotein E4-associated risk, and possible roles of apoE in normal synaptic function are still being debated. These critical questions will never be fully answered without a complete understanding of the life cycle of the apolipoprotein receptors that mediate the uptake, signaling, and degradation of apoE. The present review will focus on apoE receptors as modulators of apoE actions and, in particular, explore the functions of soluble apoE receptors, a field almost entirely overlooked until now.

Effects of apoE isoforms on beta-amyloid-induced matrix metalloproteinase-9 in rat astrocytes

Matrix metalloproteinase-9 (MMP-9) may play a role in the inflammatory glial response during Alzheimer's disease (AD). Astrocytes can degrade beta-amyloid (Abeta) and extracellular proteolysis via MMP-9 may be involved. Because Apolipoprotein E (APOE) genotype is an important factor for AD, we ask whether various apoE isoforms can influence Abeta-induced MMP-9 responses in primary rat astrocytes. Our data show that apoE4 significantly dampens Abeta-induced MMP-9 levels, possibly by downregulating the Rho-Rho kinase (ROCK) pathway. Reduction of astrocytic MMP-9 by apoE4 may affect Abeta clearance and promote Abeta deposition in AD.

Human prions and plasma lipoproteins

Prions are composed solely of an alternatively folded isoform of the prion protein (PrP), designated PrP(Sc). The polyoxometalate phosphotungstic acid has been used to separate PrP(Sc) from its precursor PrP(C) by selective precipitation; notably, native PrP(Sc) has not been solubilized by using nondenaturing detergents. Because of the similarities between PrP(Sc) and lipoproteins with respect to hydrophobicity and formation of phosphotungstic acid complexes, we asked whether these molecules are bound to each other in blood. Here we report that prions from the brains of patients with sporadic Creutzfeldt-Jakob disease (CJD) bind to very low-density (VLDL) and low-density (LDL) lipoproteins but not to high-density lipoproteins (HDL) or other plasma components, as demonstrated both by affinity assay and electron microscopy. Immunoassays demonstrated that apolipoprotein B (apoB), which is the major protein component of VLDL and LDL, bound PrP(Sc) through a highly cooperative process. Approximately 50% of the PrP(Sc) bound to LDL particles was released after exposure to 4 M guanidine hydrochloride at 80 degrees C for 20 min. The apparent binding constants of native human (Hu) PrP(Sc) or denatured recombinant HuPrP(90-231) for apoB and LDL ranged from 28 to 212 pM. Whether detection of PrP(Sc) in VLDL and LDL particles can be adapted into an antemortem diagnostic test for prions in the blood of humans, livestock, and free-ranging cervids remains to be determined.

Mechanism of cerebral beta-amyloid angiopathy: murine and cellular models

Cerebral amyloid angiopathy of the beta-amyloid type (Abeta-CAA) is a risk factor for hemorrhagic stroke and independently is believed to contribute to dementia. Naturally occurring animal models of Abeta-CAA are scarce and not well suited for the laboratory. To this end, a variety of transgenic mouse models have been developed that, similar to cerebral Abeta-amyloidosis in humans, develop either Abeta-CAA only or both Abeta-CAA and parenchymal amyloid, or primarily parenchymal amyloid with only scarce Abeta-CAA. The lessons learned from these mouse models are: i) Abeta-CAA alone is sufficient to induce cerebral hemorrhage and associate pathologies including neuroinflammation, ii) the origin of vascular amyloid is mainly neuronal, iii) Abeta-CAA results largely from impaired Abeta clearance, iv) a high ratio Abeta40:42 favors vascular over parenchymal amyloidosis, and v) genetic risk factors such as ApoE modulate Abeta-CAA and CAA-induced hemorrhages. Therapeutic strategies to inhibit Abeta-CAA are poor at the present time. Once Abeta-CAA is present current Abeta immunotherapy strategies have failed to clear vascular amyloid and even run the risk of serious side effects. Despite this progress in deciphering the pathomechanism of Abeta-CAA, with these first generation mouse models of Abeta-CAA, refining these models is needed and will help to understand the emerging importance of Abeta-CAA for dementia and to develop biomarkers and therapeutic strategies.

Sporadic and familial cerebral amyloid angiopathies

Cerebral amyloid angiopathy (CAA) is the term used to describe deposition of amyloid in the walls of arteries, arterioles and, less often, capillaries and veins of the central nervous system. CAAs are an important cause of cerebral hemorrhage and may also result in ischemic lesions and dementia. A number of amyloid proteins are known to cause CAA. The most common sporadic CAA, caused by A beta deposition, is associated with aging and is a common feature of Alzheimer disease (AD). CAA occurs in several familial conditions, including hereditary cerebral hemorrhage with amyloidosis of Icelandic type caused by deposition of mutant cystatin C, hereditary cerebral hemorrhage with amyloidosis Dutch type and familial AD with deposition of either A beta variants or wild-type A beta, the transthyretin-related meningo-vascular amyloidoses, gelsolin as well as familial prion disease-related CAAs and the recently described BRI2 gene-related CAAs in familial British dementia and familial Danish dementia. This review focuses on the morphological, biochemical, and genetic aspects as well as the clinical significance of CAAs with special emphasis on the BRI2 gene-related cerebrovascular amyloidoses. We also discuss data relevant to the pathomechanism of the different forms of CAA with an emphasis on the most common A beta-related types.

Neuronal or glial expression of human apolipoprotein e4 affects parenchymal and vascular amyloid pathology differentially in different brain regions of double- and triple-transgenic mice

Apolipoprotein E4 (ApoE4) is associated with Alzheimer's disease by unknown mechanisms. We generated six transgenic mice strains expressing human ApoE4 in combination with mutant amyloid precursor protein (APP) and mutant presenilin-1 (PS1) in single-, double-, or triple-transgenic combinations. Diffuse, but not dense, amyloid plaque-load in subiculum and cortex was increased by neuronal but not glial ApoE4 in old (15 months) double-transgenic mice, whereas both diffuse and dense plaques formed in thalamus in both genotypes. Neuronal and glial ApoE4 promoted cerebral amyloid angiopathy as extensively as mutant PS1 but with pronounced regional differences: cortical angiopathy was induced by neuronal ApoE4 while thalamic angiopathy was again independent of ApoE4 source. Angiopathy correlated more strongly with soluble Abeta40 and Abeta42 levels in cortex than in thalamus throughout the six genotypes. Neither neuronal nor glial ApoE4 affected APP proteolytic processing, as opposed to mutant PS1. Neuronal ApoE4 increased soluble amyloid levels more than glial ApoE4, but the Abeta42/40 ratios were similar, although significantly higher than in single APP transgenic mice. We conclude that although the cellular origin of ApoE4 differentially affects regional amyloid pathology, ApoE4 acts on the disposition of amyloid peptides downstream from their excision from APP but without induction of tauopathy.

Human apolipoprotein E4 alters the amyloid-beta 40:42 ratio and promotes the formation of cerebral amyloid angiopathy in an amyloid precursor protein transgenic model

Alzheimer's disease (AD) is characterized by the aggregation and deposition of the normally soluble amyloid-beta (Abeta) peptide in the extracellular spaces of the brain as parenchymal plaques and in the walls of cerebral vessels as cerebral amyloid angiopathy (CAA). CAA is a common cause of brain hemorrhage and is found in most patients with AD. As in AD, the epsilon4 allele of the apolipoprotein E (apoE) gene (APOE) is a risk factor for CAA. To determine the effect of human apoE on CAA in vivo, we bred human APOE3 and APOE4 "knock-in" mice to a transgenic mouse model (Tg2576) that develops amyloid plaques as well as CAA. The expression of both human apoE isoforms resulted in a delay in Abeta deposition of several months relative to murine apoE. Tg2576 mice expressing the more fibrillogenic murine apoE develop parenchymal amyloid plaques and CAA by 9 months of age. At 15 months of age, the expression of human apoE4 led to substantial CAA with very few parenchymal plaques, whereas the expression of human apoE3 resulted in almost no CAA or parenchymal plaques. Additionally, young apoE4-expressing mice had an elevated ratio of Abeta 40:42 in brain extracellular pools and a lower 40:42 ratio in CSF, suggesting that apoE4 results in altered clearance and transport of Abeta species within different brain compartments. These findings demonstrate that, once Abeta fibrillogenesis occurs, apoE4 favors the formation of CAA over parenchymal plaques and suggest that molecules or treatments that increase the ratio of Abeta 40:42 may favor the formation of CAA versus parenchymal plaques.

A deficit in astroglial organization causes the impaired reactive sprouting in human apolipoprotein E4 targeted replacement mice

The epsilon4 allele of apolipoprotein (apo)E associates with an increased risk of developing Alzheimer's disease (AD) as well as an earlier age of onset. However, the exact mechanisms by which apoE4 confers such susceptibility is currently unknown. We used a human apoE targeted replacement (hE-TR) mouse model to investigate the allele-specific response to entorhinal cortex lesion (ECL). We observed a marked impairment in reactive sprouting in hE4 mice compared to hE3 mice. ApoE expression was similar between genotypes at days post-lesion (DPL) 2 and 14. Thirty days post-lesion, hE4 mice had more reactive astrocytes as well as a defective outward migration pattern of the astrocytes in the dentate gyrus. The expression of the anti-inflammatory cytokine IL-1ra was delayed in hE4 mice compared to hE3 mice. ApoE and beta-amyloid (Abeta) 1-40 accumulated at 30 DPL in hE4 mice. These results suggest that the presence of apoE4 delays the astroglial repair process and indirectly compromises synaptic remodeling.

Apolipoprotein E genotype regulates amyloid-beta cytotoxicity

The epsilon4 allele of apolipoprotein E (ApoE) is a risk factor for Alzheimer's disease (AD), whereas the epsilon2 allele may be relatively protective. Both alleles are risk factors for cerebral amyloid angiopathy (CAA)-related hemorrhages. CAA is associated with degeneration of smooth muscle cells and pericytes. Previously, we described that synthetic amyloid-beta1-40 peptide (Abeta1-40) with the 22Glu--> Gln "Dutch" mutation caused pericyte death in vitro by a mechanism that involves Abeta fibril-like assembly at the cell surface. It is known that ApoE binds to Abeta and may modify its biological activities. In the present study, we evaluated the effect of ApoE on Abeta-mediated toxicity of cerebrovascular cells. We observed that cultured cells with an epsilon4/epsilon4 genotype were more vulnerable to Abeta than cultures with an epsilon3/epsilon3 or epsilon3/epsilon4 genotype. The one cell culture with the epsilon2/epsilon3 genotype was relatively resistant to Abeta compared with other cultures. Furthermore, we observed a dose-dependent protective effect of native ApoE against Abeta-mediated toxicity of cerebrovascular cells and, in addition, ApoE epsilon2/epsilon3 cells secreted more ApoE protein compared with cells with other ApoE genotypes, in particular, compared with epsilon4/epsilon4 cells. Thus, the disparity between ApoE genotype and Abeta-mediated toxicity might be related to differences in the cellular capacity to secrete ApoE. The present data suggest that one mechanism by which ApoE may alter the risk for AD is a genotype-dependent regulation of Abeta cytotoxicity, possibly via variations in its secretion levels, whereby extracellular ApoE may bind to Abeta and thereby modify Abeta-mediated cell death.

Selection of peptides binding to the amyloid b-protein reveals potential inhibitors of amyloid formation

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by extracellular amyloid plaques, cerebrovascular amyloid deposits, intracellular neurofibrillary tangles, and neuronal loss. Amyloid deposits are composed of insoluble fibers of a 39-43 amino acid peptide named the amyloid beta-protein (A beta). Neuropathological and genetic studies provide strong evidence of a key role for A beta amyloidosis in the pathogenesis of AD. Therefore, an obvious pharmacological target for treatment of AD is the inhibition of amyloid growth and/or inhibition of amyloid function. We took an unbiased approach to generate new inhibitors of amyloid formation by screening a FliTrx combinatorial peptide library for A beta binding peptides and identified four groups of peptides with different A beta binding motifs. In addition, we designed and examined peptides mimicking the A beta binding domain of transthyretin (TTR). Our results showed that A beta binding peptides selected from FliTrx peptide library and from TTR-peptide analogs are capable of inhibiting A beta aggregation and A beta deposition in vitro. These properties demonstrate that binding of selected peptides to the amyloid beta-protein may provide potent therapeutic compounds for the treatment AD.

The absence of ABCA1 decreases soluble ApoE levels but does not diminish amyloid deposition in two murine models of Alzheimer disease

ABCA1, a cholesterol transporter expressed in the brain, has been shown recently to be required to maintain normal apoE levels and lipidation in the central nervous system. In addition, ABCA1 has been reported to modulate beta-amyloid (Abeta) production in vitro. These observations raise the possibility that ABCA1 may play a role in the pathogenesis of Alzheimer disease. Here we report that the deficiency of ABCA1 does not affect soluble or guanidine-extractable Abeta levels in Tg-SwDI/B or amyloid precursor protein/presenilin 1 (APP/PS1) mice, but rather is associated with a dramatic reduction in soluble apoE levels in brain. Although this reduction in apoE was expected to reduce the amyloid burden in vivo, we observed that the parenchymal and vascular amyloid load was increased in Tg-SwDI/B animals and was not diminished in APP/PS1 mice. Furthermore, we observed an increase in the proportion of apoE retained in the insoluble fraction, particularly in the APP/PS1 model. These data suggested that ABCA1-mediated effects on apoE levels and lipidation influenced amyloidogenesis in vivo.

A 13 kDa carboxy-terminal fragment of ApoE stabilizes Abeta hexamers

The pathological role of ApoE4 in Alzheimer's disease (AD) is not fully elucidated yet but there is strong evidence that ApoE is involved in Abeta deposition, which is an early hallmark of AD neuropathology. Overexpression of ApoE in neuroblastoma cells (Neuro2a) leads to the generation of an intracellular 13 kDa carboxy-terminal fragment of ApoE comparable to fragments seen in brains of AD patients. ApoE4 generates more of this fragment than ApoE2 and E3 suggesting a potential pathological role of these fragments in Alzheimer's disease. Analysis of this intracellular ApoE4 fragment by protease digest followed by MALDI-TOF mass spectrometry showed the proteolytic cleavage site close to residue 187 of ApoE. We have engineered and expressed the corresponding ApoE fragments in vitro. The recombinant 13 kDa carboxy-terminal fragment inhibited fibril formation of Abeta; this contrasts with the full-length ApoE and the corresponding amino-terminal ApoE fragment. Moreover, we show that the 13 kDa carboxy-terminal fragment of ApoE stabilizes the formation of Abeta hexamers. Complexes of Abeta with the 13 kDa carboxy-terminal ApoE fragment show toxicity in PC12 cells comparable to Abeta fibrils. These data suggest that cleavage of ApoE, leading to the generation of this fragment, contributes to the pathogenic effect of ApoE4 in AD.

ApoE and clusterin cooperatively suppress Abeta levels and deposition: evidence that ApoE regulates extracellular Abeta metabolism in vivo

Apolipoprotein E (apoE) and clusterin can influence structure, toxicity, and accumulation of the amyloid-beta (Abeta) peptide in brain. Both molecules may also be involved in Abeta metabolism prior to its deposition. To assess this possibility, we compared PDAPP transgenic mice that develop age-dependent Abeta accumulation in the absence of apoE or clusterin as well as in the absence of both proteins. apoE(-/-) and clusterin(-/-) mice accumulated similar Abeta levels but much less fibrillar Abeta. In contrast, apoE(-/-)/clusterin(-/-) mice had both earlier onset and markedly increased Abeta and amyloid deposition. Both apoE(-/-) and apoE(-/-)/clusterin(-/-) mice had elevated CSF and brain interstitial fluid Abeta, as well as significant differences in the elimination half-life of interstitial fluid Abeta measured by in vivo microdialysis. These findings demonstrate additive effects of apoE and clusterin on influencing Abeta deposition and that apoE plays an important role in regulating extracellular CNS Abeta metabolism independent of Abeta synthesis.

Exacerbation of cerebral amyloid angiopathy-associated microhemorrhage in amyloid precursor protein transgenic mice by immunotherapy is dependent on antibody recognition of deposited forms of amyloid beta

Passive immunization with an antibody directed against the N terminus of amyloid beta (Abeta) has recently been reported to exacerbate cerebral amyloid angiopathy (CAA)-related microhemorrhage in a transgenic animal model. Although the mechanism responsible for the deleterious interaction is unclear, a direct binding event may be required. We characterized the binding properties of several monoclonal anti-Abeta antibodies to deposited Abeta in brain parenchyma and CAA. Biochemical analyses demonstrated that the 3D6 and 10D5, two N-terminally directed antibodies, bound with high affinity to deposited forms of Abeta, whereas 266, a central domain antibody, lacked affinity for deposited Abeta. To determine whether 266 or 3D6 would exacerbate CAA-associated microhemorrhage, we treated aged PDAPP mice with either antibody for 6 weeks. We observed an increase in both the incidence and severity of CAA-associated microhemorrhage when PDAPP transgenic mice were treated with the N-terminally directed 3D6 antibody, whereas mice treated with 266 were unaffected. These results may have important implications for future immune-based therapeutic strategies for Alzheimer's disease.