The role of Abeta peptides in Alzheimer's disease

The Abeta peptide has been identified as central to the onset and development of Alzheimer's disease (AD) and several hypotheses about toxicity involving Abeta peptides have been proposed including mechanisms of oxidative stress and disruption of calcium homeostasis. The biology, structure and physical properties of Abeta peptides are discussed, as well as existing therapeutics and future strategies for the treatment of AD.     

Aging, gender and APOE isotype modulate metabolism of Alzheimer's Abeta peptides and F-isoprostanes in the absence of detectable amyloid deposits

Aging and apolipoprotein E (APOE) isoform are among the most consistent risks for the development of Alzheimer's disease (AD). Metabolic factors that modulate risk have been elusive, though oxidative reactions and their by-products have been implicated in human AD and in transgenic mice with overt histological amyloidosis. We investigated the relationship between the levels of endogenous murine amyloid beta (Abeta) peptides and the levels of a marker of oxidation in mice that never develop histological amyloidosis [i.e. APOE knockout (KO) mice with or without transgenic human APOEepsilon3 or human APOEepsilon4 alleles]. Aging-, gender-, and APOE-genotype-dependent changes were observed for endogenous mouse brain Abeta40 and Abeta42 peptides. Levels of the oxidized lipid F2-isoprostane (F2-isoPs) in the brains of the same animals as those used for the Abeta analyses revealed aging- and gender-dependent changes in APOE KO and in human APOEepsilon4 transgenic KO mice. Human APOEepsilon3 transgenic KO mice did not exhibit aging- or gender-dependent increases in F2-isoPs. In general, the changes in the levels of brain F2-isoPs in mice according to age, gender, and APOE genotype mirrored the changes in brain Abeta levels, which, in turn, paralleled known trends in the risk for human AD. These data indicate that there exists an aging-dependent, APOE-genotype-sensitive rise in murine brain Abeta levels despite the apparent inability of the peptide to form histologically detectable amyloid. Human APOEepsilon3, but not human APOEepsilon4, can apparently prevent the aging-dependent rise in murine brain Abeta levels, consistent with the relative risk for AD associated with these genotypes. The fidelity of the brain Abeta/F2-isoP relationship across multiple relevant variables supports the hypothesis that oxidized lipids play a role in AD pathogenesis, as has been suggested by recent evidence that F2-isoPs can stimulate Abeta generation and aggregation.     

Neurotoxicity and physicochemical properties of Abeta mutant peptides from cerebral amyloid angiopathy: implication for the pathogenesis of cerebral amyloid angiopathy and Alzheimer's disease

Cerebral amyloid angiopathy (CAA) due to beta-amyloid (Abeta) is one of the specific pathological features of familial Alzheimer's disease. Abeta mainly consisting of 40- and 42-mer peptides (Abeta40 and Abeta42) exhibits neurotoxicity and aggregative abilities. All of the variants of Abeta40 and Abeta42 found in CAA were synthesized in a highly pure form and examined for neurotoxicity in PC12 cells and aggregative ability. All of the Abeta40 mutants at positions 22 and 23 showed stronger neurotoxicity than wild-type Abeta40. Similar tendency was observed for Abeta42 mutants at positions 22 and 23 whose neurotoxicity was 50-200 times stronger than that of the corresponding Abeta40 mutants, suggesting that these Abeta42 mutants are mainly involved in the pathogenesis of CAA. Although the aggregation of E22G-Abeta42 and D23N-Abeta42 was similar to that of wild-type Abeta42, E22Q-Abeta42 and E22K-Abeta42 aggregated extensively, supporting the clinical evidence that Dutch and Italian patients are diagnosed as hereditary cerebral hemorrhage with amyloidosis. In contrast, A21G mutation needs alternative explanation with the exception of physicochemical properties of Abeta mutants. Attenuated total reflection-Fourier transform infrared spectroscopy spectra suggested that beta-sheet content of the Abeta mutants correlates with their aggregation. However, beta-turn is also a critical secondary structure because residues at positions 22 and 23 that preferably form two-residue beta-turn significantly enhanced the aggregative ability.     

Aqueous dissolution of Alzheimer's disease Abeta amyloid deposits by biometal depletion.

Zn(II) and Cu(II) precipitate Abeta in vitro into insoluble aggregates that are dissolved by metal chelators. We now report evidence that these biometals also mediate the deposition of Abeta amyloid in Alzheimer's disease, since the solubilization of Abeta from post-mortem brain tissue was significantly increased by the presence of chelators, EGTA, N,N,N',N'-tetrakis(2-pyridyl-methyl) ethylene diamine, and bathocuproine. Efficient extraction of Abeta also required Mg(II) and Ca(II). The chelators were more effective in extracting Abeta from Alzheimer's disease brain tissue than age-matched controls, suggesting that metal ions differentiate the chemical architecture of amyloid in Alzheimer's disease. Agents that specifically chelate copper and zinc ions but preserve Mg(II) and Ca(II) may be of therapeutic value in Alzheimer's disease.     

Linking calcium to Abeta and Alzheimer's disease

Recent developments point to a critical role for calcium dysregulation in the pathogenesis of Alzheimer's disease. A novel calcium-conducting channel called CALHM1 is genetically linked to the disorder and modulates Abeta production. Calcium homeostasis has also been shown to be perturbed in dendritic spines adjacent to amyloid plaques. Finally, new studies have elucidated the role by which presenilins modulate calcium signaling, including effects on SERCA2b and gating of the IP(3) receptor, and lead to Abeta production.     

Aggregation and secondary structure of synthetic amyloid beta A4 peptides of Alzheimer's disease

The deposition of amyloid beta A4 in the brain is a major pathological hallmark of Alzheimer's disease. Amyloid beta A4 is a peptide composed of 42 or 43 amino acid residues. In brain, it appears in the form of highly insoluble, filamentous aggregates. Using synthetic peptides corresponding to the natural beta A4 sequence as well as analog peptides, we demonstrate requirements for filament formation in vitro. We also determine aggregational properties and the secondary structure of beta A4. A comparison of amino-terminally truncated beta A4 peptides identifies a peptide spanning residues 10 to 43 as a prototype for amyloid beta A4. Infrared spectroscopy of beta A4 peptides in the solid state shows that their secondary structure consists of a beta-turn flanked by two strands of antiparallel beta-pleated sheet. Analog peptides containing a disulfide bridge were designed to stabilize different putative beta-turn positions. Limited proteolysis of these analogs allowed a localization of the central beta-turn at residues 26 to 29 of the entire sequence. Purified beta A4 peptides are soluble in water. Size-exclusion chromatography shows that they form dimers that, according to circular dichroism spectroscopy, adopt a beta-sheet conformation. Upon addition of salts, the bulk fraction of peptides precipitates and adopts a beta-sheet structure. Only a small fraction of peptides remains solubilized. They are monomeric and adopt a random coil conformation. This suggests that the formation of aggregates depends upon a hydrophobic effect that leads to intra- and intermolecular interactions between hydrophobic parts of the beta A4 sequence. This model is sustained by the properties of beta A4 analogs in which hydrophobic residues were substituted. These peptides show a markedly increased solubility in salt solutions and have lost the ability to form filaments. In contrast, the substitution of hydrophilic residues leads only to small deviations in the shape of filaments, indicating that hydrophilic residues contribute to the specificity of interactions between beta A4 peptides.     

General dynamic properties of Abeta12-36 amyloid peptide involved in Alzheimer's disease from unfolding simulation

To study the folding/unfolding properties of a beta-amyloid peptide Abeta(12-36) of Alzheimer's disease, five molecular dynamics simulations of Abeta(12-36) in explicit water were done at 450 K starting from a structure that is stable in trifluoroethanol/water at room temperature with two alpha-helices. Due to high temperature, the initial helical structure unfolded during the simulation. The observed aspects of the unfolding were as follows. 1) One helix (helix 1) had a longer life than the other (helix 2), which correlates well with the theoretically computed Phi values. 2) Temporal prolongation of helix 1 was found before unfolding. 3) Hydrophobic cores formed frequently with rearrangement of amino-acid residues in the hydrophobic cores. The formation and rearrangement of the hydrophobic cores may be a general aspect of this peptide in the unfolded state, and the structural changes accompanied by the hydrophobic-core rearrangement may lead the peptide to the most stable structure. 4) Concerted motions (collective modes) appeared to unfold helix 1. The collective modes were similar with those observed in another simulation at 300 K. The analysis implies that the conformation moves according to the collective modes when the peptide is in the initial stage of protein unfolding and in the final stage of protein folding.     

Structural studies of Alzheimer's disease Abeta peptide oligomers

Amyloid beta peptide is recognized as the main constituent of the extracellular amyloid plaques, the major neuropathological hallmark of Alzheimer's disease. Abeta is a small peptide constitutively expressed in normal cells, not toxic in the monomeric form but aggregated Abeta is assumed to be the main if not the only factor causing Alzheimer's disease. Interestingly, the new reports suggest neurotoxicity of soluble Abeta oligomers rather than amyloid fibrils. Because of the fact that fibrils were thought to be the main toxic species in AD, early structural studies focused on fibrils themselves and Abeta monomers, as their building blocks while there is practically no data on oligomer structure and mechanism of neurotoxicity. Using a model peptide spanning residues 10–30 of Abeta, obtained by overexpression in bacteria, we have employed mass spectrometry of noncovalent complexes and disulfide rearrangement assay to gain new insight into structure and dynamics of a prenucleation step of Abeta peptide oligomerisation.     

Transgenic potato expressing Abeta reduce Abeta burden in Alzheimer's disease mouse model

Beta amyloid (Abeta) is believed one of the major pathogens of Alzheimer's disease (AD), and the reduction of Abeta is considered a primary therapeutic target. Immunization with Abeta can reduce Abeta burden and pathological features in transgenic AD model mice. Transgenic potato plants were made using genes encoding 5 tandem repeats of Abeta1-42 peptides with an ER retention signal. Amyloid precursor protein transgenic mice (Tg2576) fed with transgenic potato tubers with adjuvant showed a primary immune response and a partial reduction of Abeta burden in the brain. Thus, Abeta tandem repeats can be expressed in transgenic potato plants to form immunologically functional Abeta, and these potatoes has a potential to be used for the prevention and treatment of AD.     

The role of membrane trafficking in the processing of amyloid precursor protein and production of amyloid peptides in Alzheimer's disease

Alzheimer's disease (AD) is characterized by progressive accumulation of misfolded proteins, which form senile plaques and neurofibrillary tangles, and the release of inflammatory mediators by innate immune responses. β-Amyloid peptide (Aβ) is derived from sequential processing of the amyloid precursor protein (APP) by membrane-bound proteases, namely the β-secretase, BACE1, and γ-secretase. Membrane trafficking plays a key role in the regulation of APP processing as both APP and the processing secretases traffic along distinct pathways. Genome wide sequencing studies have identified several AD susceptibility genes which regulate membrane trafficking events. To understand the pathogenesis of AD it is critical that the cell biology of APP and Aβ production in neurons is well defined. This review discusses recent advances in unravelling the membrane trafficking events associated with the production of Aβ, and how AD susceptible alleles may perturb the sorting and transport of APP and BACE1. Mechanisms whereby inflammation may influence APP processing are also considered.     

Alzheimer's disease: Abeta, tau and synaptic dysfunction

Alzheimer's disease is a progressive neurodegenerative disorder that is characterized by two hallmark lesions: extracellular amyloid plaques and neurofibrillary tangles. The role that these lesions have in the pathogenesis of AD has proven difficult to unravel, in part because of unanticipated challenges of reproducing both pathologic hallmarks in transgenic mice. Recent advances in recapitulating both plaques and tangles in the brains of transgenic mice are leading to novel insights into their role in the degenerative process, including their impact on synaptic activity and plasticity. Transgenic mice that harbor both neuropathological lesions are also facilitating the elucidation of the relationship of these proteinaceous aggregates to one another and providing a crucial in vivo system for developing and evaluating therapies.     

Abeta ion channels. Prospects for treating Alzheimer's disease with Abeta channel blockers

The main pathological features in the Alzheimer's brain are progressive depositions of amyloid protein plaques among nerve cells, and neurofibrillary tangles within the nerve cells. The major components of plaques are Abeta peptides. Numerous reports have provided evidence that Abeta peptides are cytotoxic and may play a role in the pathogenesis of AD. An increasing number of research reports support the concept that the Abeta-membrane interaction event may be followed by the insertion of Abeta into the membrane in a structural configuration which forms an ion channel. This review summarizes experimental procedures which have been designed to test the hypothesis that the interaction of Abeta with a variety of membranes, both artificial and natural, results in the subsequent formation of Abeta ion channels We describe experiments, by ourselves and others, that support the view that Abeta is cytotoxic largely due to the action of Abeta channels in the cell membrane. The interaction of Abeta with the surface of the cell membrane may results in the activation of a chain of processes that, when large enough, become cytotoxic and induce cell death by apoptosis. Remarkably, the blockage of Abeta ion channels at the surface of the cell absolutely prevents the activation of these processes at different intracellular levels, thereby preserving the life of the cells. As a prospect for therapy for Alzheimer's disease, our findings at cellular level may be testable on AD animal models to elucidate the potential role and the magnitude of the contribution of the Abeta channels for induction of the disease.     

Berberine alters the processing of Alzheimer's amyloid precursor protein to decrease Abeta secretion

Berberine is an isoquinoline alkaloid isolated from Coptidis rhizoma, a major herb widely used in Chinese herbal medicine. Berberine's biological activity includes antidiarrheal, antimicrobial, and anti-inflammatory effects. Recent findings show that berberine prevents neuronal damage due to ischemia or oxidative stress and that it might act as a novel cholesterol-lowering compound. The accumulation of amyloid-beta peptide (Abeta) derived from amyloid precursor protein (APP) is a triggering event leading to the pathological cascade of Alzheimer's disease (AD); therefore the inhibition of Abeta production should be a rational therapeutic strategy in the prevention and treatment of AD. Here, we report that berberine reduces Abeta levels by modulating APP processing in human neuroglioma H4 cells stably expressing Swedish-type of APP at the range of berberine concentration without cellular toxicity. Our results indicate that berberine would be a promising candidate for the treatment of AD.     

Oligomerization of amyloid Abeta16-22 peptides using hydrogen bonds and hydrophobicity forces

The 16-22 amino-acid fragment of the beta-amyloid peptide associated with the Alzheimer's disease, Abeta, is capable of forming amyloid fibrils. Here we study the aggregation mechanism of Abeta16-22 peptides by unbiased thermodynamic simulations at the atomic level for systems of one, three, and six Abeta16-22 peptides. We find that the isolated Abeta16-22 peptide is mainly a random coil in the sense that both the alpha-helix and beta-strand contents are low, whereas the three- and six-chain systems form aggregated structures with a high beta-sheet content. Furthermore, in agreement with experiments on Abeta16-22 fibrils, we find that large parallel beta-sheets are unlikely to form. For the six-chain system, the aggregated structures can have many different shapes, but certain particularly stable shapes can be identified.     

Mitochondria dysfunction of Alzheimer's disease cybrids enhances Abeta toxicity

Alzheimer's disease (AD) brain reveals high rates of oxygen consumption and oxidative stress, altered antioxidant defences, increased oxidized polyunsaturated fatty acids, and elevated transition metal ions. Mitochondrial dysfunction in AD is perhaps relevant to these observations, as such may contribute to neurodegenerative cell death through the formation of reactive oxygen species (ROS) and the release of molecules that initiate programmed cell death pathways. In this study, we analyzed the effects of beta-amyloid peptide (Abeta) on human teratocarcinoma (NT2) cells expressing endogenous mitochondrial DNA (mtDNA), mtDNA from AD subjects (AD cybrids), and mtDNA from age-matched control subjects (control cybrids). In addition to finding reduced cytochrome oxidase activity, elevated ROS, and reduced ATP levels in the AD cybrids, when these cell lines were exposed to Abeta 1-40 we observed excessive mitochondrial membrane potential depolarization, increased cytoplasmic cytochrome c, and elevated caspase-3 activity. When exposed to Abeta, events associated with programmed cell death are activated in AD NT2 cybrids to a greater extent than they are in control cybrids or the native NT2 cell line, suggesting a role for mtDNA-derived mitochondrial dysfunction in AD degeneration.     

Cholesterol metabolism is associated with soluble amyloid precursor protein production in Alzheimer's disease

Disturbances of the cholesterol metabolism are associated with Alzheimer's disease (AD) risk and related cerebral pathology. Experimental studies found changing levels of cholesterol and its metabolites 24S‐hydroxycholesterol (24S‐OHC) and 27‐hydroxycholesterol (27‐OHC) to contribute to amyloidogenesis by increasing the production of soluble amyloid precursor protein (sAPP). The aim of this study was to evaluate the relationship between the CSF and circulating cholesterol 24S‐OHC and 27‐OHC, and the sAPP production as measured by CSF concentrations of sAPP forms in humans. The plasma and the CSF concentrations of cholesterol, 24S‐OHC and 27‐OHC, and the CSF concentrations of sAPPα, sAPPβ, and Aß1‐42 were assessed in subjects with AD and controls with normal cognition. In multivariate regression tests including age, gender, albumin ratio, and apolipoprotein E (APOE)ε4 status CSF cholesterol, 24S‐OHC, and 27‐OHC independently predicted the concentrations of sAPPα and sAPPβ. The associations remained significant when analyses were separately performed in the AD group. Furthermore, plasma 27‐OHC concentrations were associated with the CSF sAPP levels. The results suggest that high CSF concentrations of cholesterol, 24S‐OHC, and 27‐OHC are associated with increased production of both sAPP forms in AD.     

Systemic catabolism of Alzheimer's Abeta40 and Abeta42

To better understand the physiologic excretion and/or catabolism of circulating peripheral amyloid beta (Abeta), we labeled human Abeta40 (monomeric, with predominant unordered structure) and Abeta42 (mixture of monomers and oligomers in approximately 50:50 ratio, rich in beta-sheet conformation) with either Na(125)I or (125)I-tyramine cellobiose, also known as the cell-trapping ligand procedure, testing their blood clearance and organ uptake in B6SJLF1/J mice. Irrespective of the labeling protocol, the peptide conformation, and the degree of oligomerization, both Abeta40 and Abeta42 showed a short half-life of 2.5-3.0 min. The liver was the major organ responsible for plasma clearance, accounting for >60% of the peptide uptake, followed by the kidney. In vivo, hepatocytes captured >90% of the radiolabeled peptides which, after endocytosis, were preferentially catabolized and excreted into the bile. Biliary excretion of intact as well as partially degraded Abeta species became obviously relevant at doses above 10 microg. The use of biotin-labeled Abeta allowed the visualization of the interaction with HepG2 cells in culture, whereas competitive inhibition experiments with unlabeled Abeta demonstrated the specificity of the binding. The capability of the liver to uptake, catabolize, and excrete large doses of Abeta, several orders of magnitude above its physiologic concentration, may explain not only the femtomolar plasma levels of Abeta but the little fluctuation observed with age and disease stages.