Aβ Monomers Transiently Sample Oligomer and Fibril-Like Configurations: Ensemble Characterization Using a Combined MD/NMR Approach

Amyloid β (Aβ) peptides are a primary component of fibrils and oligomers implicated in the etiology of Alzheimer's disease (AD). However, the intrinsic flexibility of these peptides has frustrated efforts to investigate the secondary and tertiary structure of Aβ monomers, whose conformational landscapes directly contribute to the kinetics and thermodynamics of Aβ aggregation. In this work, de novo replica exchange molecular dynamics (REMD) simulations on the microseconds-per-replica timescale are used to characterize the structural ensembles of Aβ42, Aβ40, and M35-oxidized Aβ42, three physiologically relevant isoforms with substantially different aggregation properties. J-coupling data calculated from the REMD trajectories were compared to corresponding NMR-derived values acquired through two different pulse sequences, revealing that all simulations converge on the order of hundreds of nanoseconds-per-replica toward ensembles that yield good agreement with experiment. Though all three Aβ species adopt highly heterogeneous ensembles, these are considerably more structured compared to simulations on shorter timescales. Prominent in the C-terminus are antiparallel β-hairpins between L17–A21, A30–L36, and V39–I41, similar to oligomer and fibril intrapeptide models that expose these hydrophobic side chains to solvent and may serve as hotspots for self-association. Compared to reduced Aβ42, the absence of a second β-hairpin in Aβ40 and the sampling of alternate β topologies by M35-oxidized Aβ42 may explain the reduced aggregation rates of these forms. A persistent V24–K28 bend motif, observed in all three species, is stabilized by buried backbone to side-chain hydrogen bonds with D23 and a cross-region salt bridge between E22 and K28, highlighting the role of the familial AD-linked E22 and D23 residues in Aβ monomer folding. These characterizations help illustrate the conformational landscapes of Aβ monomers at atomic resolution and provide insight into the early stages of Aβ aggregation pathways.     

The Osaka FAD mutation E22Δ leads to the formation of a previously unknown type of amyloid β fibrils and modulates Aβ neurotoxicity

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cerebral deposition of amyloid fibrils formed by the amyloid β (Aβ) peptide. Aβ has a length of 39-43 amino acid residues; the predominant Aβ isoforms are Aβ1-40 and Aβ1-42. While the majority of AD cases occur spontaneously, a subset of early-onset familial AD cases is caused by mutations in the genes encoding the Aβ precursor protein or presenilin 1/presenilin 2. Recently, a deletion of glutamic acid at position 22 within the Aβ sequence (E22Δ) was identified in Japanese patients with familial dementia, but the aggregation properties of the deletion variant of Aβ are not well understood. We investigated the aggregation characteristics and neurotoxicity of recombinantly expressed Aβ isoforms 1-40 and 1-42 with and without the E22Δ mutation. We show that the E22Δ mutation strongly accelerates the fibril formation of Aβ1-42 E22Δ compared to Aβ1-42 wild type (wt). In addition, we demonstrate that fibrils of Aβ1-40 E22Δ form a unique quaternary structure characterized by a strong tendency to form fibrillar bundles and a strongly increased thioflavin T binding capacity. Aβ1-40 E22Δ was neurotoxic in rat primary neuron cultures as compared to nontoxic Aβ1-40 wt. Aβ1-42 E22Δ was less toxic than Aβ1-42 wt, but it significantly decreased neurite outgrowth per cell in neuronal primary cultures. Because Aβ1-40 is the major Aβ form in vivo, the gain of toxic function caused by the E22 deletion may explain the development of familial AD in mutation carriers.     

Structure of the 21-30 fragment of amyloid beta-protein

Folding and self-assembly of the 42-residue amyloid beta-protein (Abeta) are linked to Alzheimer's disease (AD). The 21-30 region of Abeta, Abeta(21-30), is resistant to proteolysis and is believed to nucleate the folding of full-length Abeta. The conformational space accessible to the Abeta(21-30) peptide is investigated by using replica exchange molecular dynamics simulations in explicit solvent. Conformations belonging to the global free energy minimum (the "native" state) from simulation are in good agreement with reported NMR structures. These conformations possess a bend motif spanning the central residues V24-K28. This bend is stabilized by a network of hydrogen bonds involving the side chain of residue D23 and the amide hydrogens of adjacent residues G25, S26, N27, and K28, as well as by a salt bridge formed between side chains of K28 and E22. The non-native states of this peptide are compact and retain a native-like bend topology. The persistence of structure in the denatured state may account for the resistance of this peptide to protease degradation and aggregation, even at elevated temperatures.     

Point mutations in Aβ result in the formation of distinct polymorphic aggregates in the presence of lipid bilayers

A hallmark of Alzheimer's disease (AD) is the rearrangement of the β-amyloid (Aβ) peptide to a non-native conformation that promotes the formation of toxic, nanoscale aggregates. Recent studies have pointed to the role of sample preparation in creating polymorphic fibrillar species. One of many potential pathways for Aβ toxicity may be modulation of lipid membrane function on cellular surfaces. There are several mutations clustered around the central hydrophobic core of Aβ near the α-secretase cleavage site (E22G Arctic mutation, E22K Italian mutation, D23N Iowa mutation, and A21G Flemish mutation). These point mutations are associated with hereditary diseases ranging from almost pure cerebral amyloid angiopathy (CAA) to typical Alzheimer's disease pathology with plaques and tangles. We investigated how these point mutations alter Aβ aggregation in the presence of supported lipid membranes comprised of total brain lipid extract. Brain lipid extract bilayers were used as a physiologically relevant model of a neuronal cell surface. Intact lipid bilayers were exposed to predominantly monomeric preparations of Wild Type or different mutant forms of Aβ, and atomic force microscopy was used to monitor aggregate formation and morphology as well as bilayer integrity over a 12 hour period. The goal of this study was to determine how point mutations in Aβ, which alter peptide charge and hydrophobic character, influence interactions between Aβ and the lipid surface. While fibril morphology did not appear to be significantly altered when mutants were prepped similarly and incubated under free solution conditions, aggregation in the lipid membranes resulted in a variety of polymorphic aggregates in a mutation dependent manner. The mutant peptides also had a variable ability to disrupt bilayer integrity.     

Investigating how peptide length and a pathogenic mutation modify the structural ensemble of amyloid beta monomer

The aggregation of amyloid beta (Aβ) peptides plays an important role in the development of Alzheimer's disease. Despite extensive effort, it has been difficult to characterize the secondary and tertiary structure of the Aβ monomer, the starting point for aggregation, due to its hydrophobicity and high aggregation propensity. Here, we employ extensive molecular dynamics simulations with atomistic protein and water models to determine structural ensembles for Aβ(42), Aβ(40), and Aβ(42)-E22K (the Italian mutant) monomers in solution. Sampling of a total of >700 microseconds in all-atom detail with explicit solvent enables us to observe the effects of peptide length and a pathogenic mutation on the disordered Aβ monomer structural ensemble. Aβ(42) and Aβ(40) have crudely similar characteristics but reducing the peptide length from 42 to 40 residues reduces β-hairpin formation near the C-terminus. The pathogenic Italian E22K mutation induces helix formation in the region of residues 20-24. This structural alteration may increase helix-helix interactions between monomers, resulting in altered mechanism and kinetics of Aβ oligomerization.     

Micelle-Like Architecture of the Monomer Ensemble of Alzheimer’s Amyloid-β Peptide in Aqueous Solution and Its Implications for Aβ Aggregation

Aggregation of amyloid-β (Aβ) peptide, a 39- to 43-residue fragment of the amyloid precursor protein, is associated with Alzheimer's disease, the most common form of dementia in the elderly population. Several experimental studies have tried to characterize the atomic details of amyloid fibrils, which are the final product of Aβ aggregation. Much less is known about species forming during the early stages of aggregation, in particular about the monomeric state of the Aβ peptide that may be viewed as the product of the very first step in the hypothesized amyloid cascade. Here, the equilibrium ensembles of monomeric Aβ alloforms Aβ_1-40 and Aβ_1-42 are investigated by Monte Carlo simulations using an atomistic force field and implicit solvent model that have been shown previously to correctly reproduce the ensemble properties of other intrinsically disordered polypeptides.Our simulation results indicate that at physiological temperatures, both alloforms of Aβ assume a largely collapsed globular structure. Conformations feature a fluid hydrophobic core formed, on average, by contacts both within and between the two segments comprising residues 12-21 and 24-40/42, respectively. Furthermore, the 11 N-terminal residues are completely unstructured, and all charged side chains, in particular those of Glu22 and Asp23, remain exposed to solvent. Taken together, these observations indicate a micelle-like† architecture at the monomer level whose implications for oligomerization, as well as fibril formation and elongation, are discussed. We establish quantitatively the intrinsic disorder of Aβ and find the propensity to form regular secondary structure to be low but sequence specific. In the presence of a global and unspecific bias for backbone conformations to populate the β-basin, the β-sheet propensity along the sequence is consistent with the arrangement of the monomer within the fibril, as derived from solid-state NMR data. These observations indicate that the primary sequence partially encodes fibril structure, but that fibril elongation must be thought of as a templated assembly step.     

Mutations enhance the aggregation propensity of the Alzheimer's A beta peptide

Aggregation of the amyloid β (Aβ) peptide plays a key role in the molecular etiology of Alzheimer’s disease. Despite the importance of this process, the relationship between the sequence of Aβ and the propensity of the peptide to aggregate has not been fully elucidated. The sequence determinants of aggregation can be revealed by probing the ability of amino acid substitutions (mutations) to increase or decrease aggregation. Numerous mutations that decrease aggregation have been isolated by laboratory-based studies. In contrast, very few mutations that increase aggregation have been reported, and most of these were isolated from rare individuals with early-onset familial Alzheimer’s disease. To augment the limited data set of clinically derived mutations, we developed an artificial genetic screen to isolate novel mutations that increase aggregation propensity. The screen relies on the expression of Aβ-green fluorescent protein fusion in Escherichia coli. In this fusion, the ability of the green fluorescent protein reporter to fold and fluoresce is inversely correlated with the aggregation propensity of the Aβ sequence. Implementation of this screen enabled the isolation of 20 mutant versions of Aβ with amino acid substitutions at 17 positions in the 42-residue sequence of Aβ. Biophysical studies of synthetic peptides corresponding to sequences isolated by the screen confirm the increased aggregation propensity and amyloidogenic behavior of the mutants. The mutations were isolated using an unbiased screen that makes no assumptions about the sequence determinants of aggregation. Nonetheless, all 16 of the most aggregating mutants contain substitutions that reduce charge and/or increase hydrophobicity. These findings provide compelling evidence supporting the hypothesis that sequence hydrophobicity is a major determinant of Aβ aggregation.     

Amyloid beta-protein monomer folding: free-energy surfaces reveal alloform-specific differences

Alloform-specific differences in structural dynamics between amyloid β-protein (Aβ) 40 and Aβ42 appear to underlie the pathogenesis of Alzheimer's disease. To elucidate these differences, we performed microsecond timescale replica-exchange molecular dynamics simulations to sample the conformational space of the Aβ monomer and constructed its free-energy surface. We find that neither peptide monomer is unstructured, but rather that each may be described as a unique statistical coil in which five relatively independent folding units exist, comprising residues 1-5, 10-13, 17-22, 28-37, and 39-42, which are connected by four turn structures. The free-energy surfaces of both peptides are characterized by two large basins, comprising conformers with either substantial α-helix or β-sheet content. Conformational transitions within and between these basins are rapid. The two additional hydrophobic residues at the Aβ42 C-terminus, Ile41 and Ala42, significantly increase contacts within the C-terminus, and between the C-terminus and the central hydrophobic cluster (Leu17-Ala21). As a result, the β-structure of Aβ42 is more stable than that of Aβ40, and the conformational equilibrium in Aβ42 shifts towards β-structure. These results suggest that drugs stabilizing α-helical Aβ conformers (or destabilizing the β-sheet state) would block formation of neurotoxic oligomers. The atomic-resolution conformer structures determined in our simulations may serve as useful targets for this purpose. The conformers also provide starting points for simulations of Aβ oligomerization—a process postulated to be the key pathogenetic event in Alzheimer's disease.     

Conformational solution studies of the SDS micelle-bound 11-28 fragment of two Alzheimer's beta-amyloid variants (E22K and A21G) using CD, NMR, and MD techniques

The beta-amyloid (Abeta) is the major peptide constituent of neuritic plaques in Alzheimer's disease (AD) and its aggregation is believed to play a central role in the pathogenesis of the disease. Naturally occurring mutations resulting in changes in the Abeta sequence (pos. 21-23) are associated with familial AD-like diseases with extensive cerebrovascular pathology. It was proved that the mutations alter the aggregation ability of Abeta and its neurotoxicity. Among five mutations at positions 21-23 there are two mutations with distinct clinical characteristics and potentially distinct pathogenic mechanism-the Italian (E22K) and the Flemish (A21G) mutations. In our studies we have examined the structures of the 11-28 fragment of the Italian and Flemish Abeta variants. The fragment was chosen because it has been shown to be the most important for amyloid fibril formation. The detailed structure of both variants Abeta(11-28) was determined using CD, 2D NMR, and molecular dynamics techniques under water-SDS micelle conditions. The NMR analysis revealed two distinct sets of proton resonances for the peptides. The studies of both peptides pointed out the existence of well-defined alpha-helical conformation in the Italian mutant, whereas the Flemish was found to be unstructured with the possibility of a bent structure in the central part of the peptide.     

Conversion of Aβ42 into a Folded Soluble Native-like Protein using a Semi-random Library of Amphipathic Helices

The amyloid cascade model hypothesizes that neurotoxic oligomers or aggregates formed by the Alzheimer amyloid peptide (Aβ) cause disease pathology in Alzheimer's disease. Attempted treatment strategies for Alzheimer's disease have involved either inhibiting Aβ oligomerization or aggregation, or dissolving existing aggregates. Blocking such downhill processes, however, has proved daunting. We have used a different approach that targets Aβ before the oligomerization cascade begins. We predicted that an amphipathic helix could convert Aβ into a native-like protein and inhibit initiation of oligomerization and aggregation. This idea was tested with a designed library and genetic screen. We exhaustively screened a library of semi-randomized amphipathic helical sequences, each expressed as a fusion protein with an Aβ42-yellow fluorescent protein sequence serving as a reporter for folding and solubilization. This yielded an amphipathic helix capable of initiating native-like folding in Aβ42 and preventing aggregation. This amphipathic helix has direct application to Alzheimer's disease therapy development.     

Turn nucleation perturbs amyloid β self-assembly and cytotoxicity

The accumulation of senile plaques composed of amyloid β (Aβ) fibrils is a hallmark of Alzheimer's disease, although prefibrillar oligomeric species are believed to be the primary neurotoxic congeners in the pathogenesis of Alzheimer's disease. Uncertainty regarding the mechanistic relationship between Aβ oligomer and fibril formation and the cytotoxicity of these aggregate species persists. β-Turn formation has been proposed to be a potential rate-limiting step during Aβ fibrillogenesis. The effect of turn nucleation on Aβ self-assembly was probed by systematically replacing amino acid pairs in the putative turn region of Aβ (residues 24-27) with d-ProGly ((D)PG), an effective turn-nucleating motif. The kinetic, thermodynamic, and cytotoxic effects of these mutations were characterized. It was found that turn formation dramatically accelerated Aβ fibril self-assembly dependent on the site of turn nucleation. The cytotoxicity of the three (D)PG-containing Aβ variants was significantly lower than that of wild-type Aβ40, presumably due to decreased oligomer populations as a function of a more rapid progression to mature fibrils; oligomer populations were not eliminated, however, suggesting that turn formation is also a feature of oligomer structures. These results indicate that turn nucleation is a critical step in Aβ40 fibril formation.     

Synthesis,aggregation, neurotoxicity,and secondary structure of various A beta 1-42 mutants of familial Alzheimer's disease at positions 21-23

Cerebral amyloid angiopathy (CAA) due to amyloid beta (A beta) deposition is a key pathological feature of Alzheimer's disease (AD), especially in some form of familial Alzheimer's disease (FAD) including hereditary cerebral hemorrhage with amyloidosis-Dutch type. A beta mainly consists of 40- and 42-mer peptides (Abeta 1-40 and A beta 1-42), which accumulate in senile plaques of AD brains and show neurotoxicity for cultured nerve cells. We synthesized all variant forms of A beta 1-42 associated with reported FAD, such as A21G (Flemish), E22Q (Dutch), E22K (Italian), E22G (Arctic), and D23N (Iowa) along with three potential mutants by one point missense mutation (E22A, E22D, and E22V) in a highly pure form, and examined their ability to aggregate and their neurotoxicity in PC12 cells. The mutants at positions 22 and 23 showed potent aggregative ability and neurotoxicity whereas the potential mutants did not, indicating that A beta 1-42 mutants at positions 22 and 23 play a critical role in FAD of Dutch-, Italian-, Arctic-, and Iowa-types. However, Flemish-type FAD needs alternative explanation except the aggregation and neurotoxicity of the corresponding A beta 1-42 mutant.     

Structures and free-energy landscapes of the wild type and mutants of the Abeta(21-30) peptide are determined by an interplay between intrapeptide electrostatic and hydrophobic interactions

The initial events in protein aggregation involve fluctuations that populate monomer conformations, which lead to oligomerization and fibril assembly. The highly populated structures, driven by a balance between hydrophobic and electrostatic interactions in the protease-resistant wild-type Aβ_21-30 peptide and mutants E22Q (Dutch), D23N (Iowa), and K28N, are analyzed using molecular dynamics simulations. Intrapeptide electrostatic interactions were connected to calculated pK_a values that compare well with the experimental estimates. The pK_a values of the titratable residues show that E22 and D23 side chains form salt bridges only infrequently with the K28 side chain. Contacts between E22-K28 are more probable in “dried” salt bridges, whereas D23-K28 contacts are more probable in solvated salt bridges. The strength of the intrapeptide hydrophobic interactions increases as D23N < WT < E22Q < K28A. Free-energy profiles and disconnectivity representation of the energy landscapes show that the monomer structures partition into four distinct basins. The hydrophobic interactions cluster the Aβ_21-30 peptide into two basins, differentiated by the relative position of the DVG(23-25) and GSN(25-27) fragments about the G25 residue. The E22Q mutation increases the population with intact VGSN turn compared to the wild-type (WT) peptide. The increase in the population of the structures in the aggregation-prone Basin I in E22Q, which occurs solely due to the difference in charge states between the Dutch mutant and the WT, gives a structural explanation of the somewhat larger aggregation rate in the mutant. The D23N mutation dramatically reduces the intrapeptide interactions. The K28A mutation increases the intrapeptide hydrophobic interactions that promote population of structures in Basin I and Basin II whose structures are characterized by hydrophobic interaction between V24 and K28 side chains but with well-separated ends of the backbone atoms in the VGSN turn. The intrapeptide electrostatic interactions in the WT and E22Q peptides roughen the free-energy surface compared to the K28A peptide. The D23N mutation has a flat free-energy surface, corresponding to an increased population of random coil-like structures with weak hydrophobic and electrostatic interactions. We propose that mutations or sequences that enhance the probability of occupying Basin I would promote aggregation of Aβ peptides.     

Dutch and arctic mutant peptides of β amyloid1-40 differentially affect the FGF-2 pathway in brain endothelium

Single point mutations of the amyloid precursor protein generate Aβ variants bearing amino acid substitutions at positions 21-23. These mutants are associated with distinct hereditary phenotypes of cerebral amyloid angiopathy, manifesting varying degrees of tropism for brain vessels, and impaired microvessel remodeling and angiogenesis. We examined the differential effects of E22Q (Dutch), and E22G (Arctic) variants in comparison to WT Aβ on brain endothelial cell proliferation, angiogenic phenotype expression triggered by fibroblast growth factor (FGF-2), pseudo-capillary sprouting, and induction of apoptosis. E22Q exhibited a potent anti-angiogenic profile in contrast to E22G, which had a much weaker effect. Investigations on the FGF-2 signaling pathway revealed the greatest differences among the peptides: E22Q and WT peptides suppressed FGF-2 expression while E22G had barely any effect. Phosphorylation of the FGF-2 receptor, FGFR-1, and the survival signal Akt were abolished by E22Q and WT peptides, but not by E22G. The biological dissimilar effect of the mutant and WT peptides on cerebral EC cannot be assigned to a particular Aβ structure, suggesting that the toxic effect of the Aβ assemblies goes beyond mere multimerization.     

Internal and environmental effects on folding and dimerisation of Alzheimer's β-amyloid peptide

Amyloid deposits are a hallmark of many diseases. In the case of Alzheimer's disease, a turn between 21Ala and 30Ala, stabilised by a salt bridge between 22Glu/23Asp and 28Lys, may nucleate folding and aggregation of the amyloid β (Aβ) peptide. In the present paper, we test this hypothesis by studying how salt bridge and turn formation vary with intrinsic and environmental changes, and how these changes affect folding and aggregation of the Aβ-peptide.     

Amyloid-beta (Aβ) D7H mutation increases oligomeric Aβ42 and alters properties of Aβ-zinc/copper assemblies

Amyloid precursor protein (APP) mutations associated with familial Alzheimer's disease (AD) usually lead to increases in amyloid β-protein (Aβ) levels or aggregation. Here, we identified a novel APP mutation, located within the Aβ sequence (Aβ(D7H)), in a Taiwanese family with early onset AD and explored the pathogenicity of this mutation. Cellular and biochemical analysis reveal that this mutation increased Aβ production, Aβ42/40 ratio and prolonged Aβ42 oligomer state with higher neurotoxicity. Because the D7H mutant Aβ has an additional metal ion-coordinating residue, histidine, we speculate that this mutation may promote susceptibility of Aβ to ion. When co-incubated with Zn(2+) or Cu(2+), Aβ(D7H) aggregated into low molecular weight oligomers. Together, the D7H mutation could contribute to AD pathology through a "double punch" effect on elevating both Aβ production and oligomerization. Although the pathogenic nature of this mutation needs further confirmation, our findings suggest that the Aβ N-terminal region potentially modulates APP processing and Aβ aggregation, and further provides a genetic indication of the importance of Zn(2+) and Cu(2+) in the etiology of AD.     

Structural dynamics of the amyloid β-protein monomer folding nucleus

Alzheimer's disease (AD) is linked to the aberrant assembly of the amyloid β-protein (Aβ). The (21)AEDVGSNKGA(30) segment, Aβ(21-30), forms a turn that acts as a monomer folding nucleus. Amino acid substitutions within this nucleus cause familial forms of AD. To determine the biophysical characteristics of the folding nucleus, we studied the biologically relevant acetyl-Aβ(21-30)-amide peptide using experimental techniques (limited proteolysis, thermal denaturation, urea denaturation followed by pulse proteolysis, and electron microscopy) and computational methods (molecular dynamics). Our results reveal a highly stable foldon and suggest new strategies for therapeutic drug development.     

Monte Carlo study of the formation and conformational properties of dimers of Aβ42 variants

Small soluble oligomers, and dimers in particular, of the amyloid β-peptide (Aβ) are believed to play an important pathological role in Alzheimer's disease. Here, we investigate the spontaneous dimerization of Aβ42, with 42 residues, by implicit solvent all-atom Monte Carlo simulations, for the wild-type peptide and the mutants F20E, E22G and E22G/I31E. The observed dimers of these variants share many overall conformational characteristics but differ in several aspects at a detailed level. In all four cases, the most common type of secondary structure is intramolecular antiparallel β-sheets. Parallel, in-register β-sheet structure, as in models for Aβ fibrils, is rare. The primary force driving the formation of dimers is hydrophobic attraction. The conformational differences that we do see involve turns centered in the 20-30 region. The probability of finding turns centered in the 25-30 region, where there is a loop in Aβ fibrils, is found to increase upon dimerization and to correlate with experimentally measured rates of fibril formation for the different Aβ42 variants. Our findings hint at reorganization of this part of the molecule as a potentially critical step in Aβ aggregation.     

The Arctic mutation alters helix length and type in the 11-28 beta-amyloid peptide monomer-CD, NMR and MD studies in an SDS micelle

The beta-amyloid (Abeta) is the major peptide constituent of neuritic plaques in Alzheimer's disease, and its aggregation is believed to play a central role in the pathogenesis of the disease. Naturally occurring mutations resulting in changes in the Abeta sequence (pos. 21-23) are associated with familial Alzheimer's-like diseases with extensive cerebrovascular pathology. It has been demonstrated that such mutations alter the aggregation ability of Abeta and its neurotoxicity. Among the five mutations at positions 21-23 there is one with distinct clinical characteristics and a potentially distinct pathogenic mechanism-the Arctic (E22G) mutation. We have examined the structures of fragment 11-28 of the native peptide and its E22G variant. This fragment was chosen because it has been shown to be a good model for conformational and aggregation studies as it contains the hydrophobic core responsible for aggregation and the residues critical to alpha-secretase cleavage of APP. The detailed structure of the two peptides was determined using CD, 2D NMR and molecular dynamics techniques under water-SDS micelle conditions. Our studies indicated the existence of partially alpha- and 3(10)-helical conformations in the native and mutated peptide, respectively.     

Structural properties of EGCG-induced, nontoxic Alzheimer's disease Aβ oligomers

The green tea compound epigallocatechin-3-gallate (EGCG) inhibits Alzheimer's disease β-amyloid peptide (Aβ) neurotoxicity. Solution-state NMR allows probing initial EGCG-Aβ interactions. We show that EGCG-induced Aβ oligomers adopt a well-defined structure and are amenable for magic angle spinning solid-state NMR investigations. We find that EGCG interferes with the aromatic hydrophobic core of Aβ. The C-terminal part of the Aβ peptide (residues 22-39) adopts a β-sheet conformation, whereas the N-terminus (residues 1-20) is unstructured. The characteristic salt bridge involving residues D23 and K28 is present in the structure of these oligomeric Aβ aggregates as well. The structural analysis of small-molecule-induced amyloid aggregates will open new perspectives for Alzheimer's disease drug development.