Quenched hydrogen/deuterium exchange NMR characterization of amyloid-beta peptide aggregates formed in the presence of Cu2+ or Zn2+

An N-terminal acidic β-sheet domain is responsible for the metal-accumulation properties of amyloid-β protofibrils: a molecular dynamics study

The influence of metal ions on the structure of amyloid- β (Aβ) protofibril models was studied through molecular dynamics to explore the molecular mechanisms underlying metal-induced Aβ aggregation relevant in Alzheimer's disease (AD). The models included 36-, 48-, and 188-mers of the Aβ42 sequence and two disease-modifying variants. Primary structural effects were observed at the N-terminal domain, as it became susceptible to the presence of cations. Specially when β-sheets predominate, this motif orients N-terminal acidic residues toward one single face of the β-sheet, resulting in the formation of an acidic region that attracts cations from the media and promotes the folding of the N-terminal region, with implications in amyloid aggregation. The molecular phenotype of the protofibril models based on Aβ variants shows that the AD-causative D7N mutation promotes the formation of N-terminal β-sheets and accumulates more Zn2+, in contrast to the non-amyloidogenic rodent sequence that hinders the β-sheets and is more selective for Na+ over Zn2+ cations. It is proposed that forming an acidic β-sheet domain and accumulating cations is a plausible molecular mechanism connecting the elevated affinity and concentration of metals in Aβ fibrils to their high content of β-sheet structure at the N-terminal sequence.

Trace elements dyshomeostasis in liver and brain of weanling mice under altered dietary selenium conditions

BACKGROUND

A balanced diet containing selenium (Se) and other trace elements is essential for normal development and growth. Se has been recognized as an essential trace element; however, its interaction with other elements has not been fully investigated. In the present study, sodium (Na), magnesium (Mg), potassium (K), calcium (Ca), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), copper (Cu), zinc (Zn), Se and rubidium (Rb), were analysed in liver and brain regions under altered dietary Se intake in weanling mice to identify major discriminatory elements.

METHODS

The study investigated the effects of different levels of Se intake on the elemental composition in liver and brain tissues of weaned mice. After 24 weeks of feeding with Se adequate, deficient, and excess diets, elemental analysis was performed on the harvested tissues using Inductively coupled plasma mass spectrometry (ICP-MS). Statistical analysis that included analysis of covariance (ANCOVA), correlation coefficient analysis, principal component analysis, and partial least squares discriminant analysis were performed.

RESULTS

The ANCOVA showed statistically significant changes and correlations among the analysed elements under altered dietary Se status. The multivariate analysis showed differential changes in elements in liver and brain regions. The results suggest that long-term dietary Se alternations lead to dyshomeostasis in trace elements that are required in higher concentrations compared to Se. It was observed that changes in the Fe, Co, and Rb levels were similar in all the tissues studied, whereas the changes in Mg, Cr, and Mn levels were different among the tissues under altered dietary Se status. Additionally, the changes in Rb levels correlated with the dietary Se intake but had no relation with the tissue Se levels.

CONCLUSIONS

The findings suggest interactions between Mg, Cr, Mn, Fe, Co, and Se under altered Se status may impact cellular functions during postnatal development. However, the possible biological significance of alterations in Rb levels under different dietary Se paradigms needs to be further explored.

The English (H6R) Mutation of the Alzheimer's Disease Amyloid-β Peptide Modulates Its Zinc-Induced Aggregation

The coordination of zinc ions by histidine residues of amyloid-beta peptide (Aβ) plays a critical role in the zinc-induced Aβ aggregation implicated in Alzheimer's disease (AD) pathogenesis. The histidine to arginine substitution at position 6 of the Aβ sequence (H6R, English mutation) leads to an early onset of AD. Herein, we studied the effects of zinc ions on the aggregation of the Aβ42 peptide and its isoform carrying the H6R mutation (H6R-Aβ42) by circular dichroism spectroscopy, dynamic light scattering, turbidimetric and sedimentation methods, and bis-ANS and thioflavin T fluorescence assays. Zinc ions triggered the occurrence of amorphous aggregates for both Aβ42 and H6R-Aβ42 peptides but with distinct optical properties. The structural difference of the formed Aβ42 and H6R-Aβ42 zinc-induced amorphous aggregates was also supported by the results of the bis-ANS assay. Moreover, while the Aβ42 peptide demonstrated an increase in the random coil and β-sheet content upon complexing with zinc ions, the H6R-Aβ42 peptide showed no appreciable structural changes under the same conditions. These observations were ascribed to the impact of H6R mutation on a mode of zinc/peptide binding. The presented findings further advance the understanding of the pathological role of the H6R mutation and the role of H6 residue in the zinc-induced Aβ aggregation.

Effects of Cu(II) on the aggregation of amyloid-β

Aberrant aggregation of the Aβ protein is a hallmark of Alzheimer's disease (AD), but no complete characterization of the molecular level pathogenesis has been achieved. A promising hypothesis is that dysfunction of metal ion homeostasis, and consequently, the undesired interaction of metal ions with Aβ, may be central to the development of AD. Qualitatively, most data indicate that Cu(II) induces rapid self-assembly of both Aβ40 and Aβ42 during the initial phase of the aggregation, while at longer time scales fibrillation may occur, depending on the experimental conditions. For Aβ40 and Cu(II):Aβ ≤ 1, most data imply that low concentration of Aβ40 favors nucleation and rapid fibril elongation, while high concentration of Aβ40 favors formation of amorphous aggregates. However, there are conflicting reports on this issue. For Aβ42 and Cu(II):Aβ ≤ 1, there is consensus that the lag time is extended upon addition of Cu(II). For Cu(II):Aβ > 1, the lag time is increased upon interaction with Cu(II), and in most cases fibrillation is not observed, presumably because Cu(II) occupies a second more solvent-exposed binding site, which is more prone to form metal ion-bridged species and cause rapid formation of non-fibrillar aggregates. The interesting N-terminally truncated Aβ11-40 with high affinity for Cu(II), exhibits delay of fibrillation upon addition of 0.4 eq. Cu(II). In our view, there are still problems achieving reproducible results in this field, and we provide a shortlist of some of the pitfalls. Finally, we propose a consensus model for the effects of Cu(II) on the aggregation kinetics of Aβ.

His6, His13, and His14 residues in Aβ 1-40 peptide significantly and specifically affect oligomeric equilibria

Oligomers of Aβ peptide are implicated as the most probable causative agent in Alzheimer's disease. However, their structural properties remain elusive due to the dynamic and heterogeneous character of oligomeric species coexisting in solution. Nevertheless, new approaches, mainly based on mass spectrometry, provide unique access to these different structural forms. Using these methods, we previously showed that the N-terminal, non-amyloidogenic region of Aβ is involved in the network of interactions specifically stabilizing oligomers. In the present study, we identified three histidine residues as active participants in this network. Detailed knowledge of the structural features that are potentially important for oligomer-mediated neurotoxicity is a prerequisite for the rational design of oligomerization modifiers.

Heparin Modulates the Kinetics of Zinc-Induced Aggregation of Amyloid-β Peptides

Zinc-induced aggregation of amyloid-β peptides (Aβ) is considered to contribute to the pathogenesis of Alzheimer's disease. While glycosaminoglycans (GAGs) that are commonly present in interneuronal space are known to enhance Aβ self-aggregation in vitro, the impact of GAGs on the formation of zinc-induced amorphous Aβ aggregates has not yet been thoroughly studied. Here, employing dynamic light scattering, bis-ANS fluorimetry, and sedimentation assays, we demonstrate that heparin serving as a representative GAG modulates the kinetics of zinc-induced Aβ42 aggregation in vitro by slowing the rate of aggregate formation and aggregate size growth. By using synthetic Aβ16 peptides to model the Aβ metal-binding domain (MBD), heparin was found to effectively interact with MBDs in complex with zinc ions. We suggest that heparin adsorbs to the surface of growing zinc-induced Aβ42 aggregates via electrostatic interactions, thus creating a steric hindrance that inhibits further inclusion of monomeric and/or oligomeric zinc-Aβ42 complexes. Furthermore, the adsorbed heparin can interfere with the zinc-bridging mechanism of Aβ42 aggregation, requiring the formation of two zinc-mediated interaction interfaces in the MBD. As revealed by computer simulations of the zinc-Aβ16 homodimer complexed with a heparin chain, heparin can interact with the MBD via polar contacts with residues Arg-5 and Tyr-10, resulting in a conformational rearrangement that hampers the formation of the second zinc-mediated interaction in the MBD interface. The findings of this study suggest that GAGs, which are common in the in vivo macromolecular environment, may have a substantial impact on the time course of zinc-induced Aβ aggregation.

Pathways of amyloid-beta absorption and aggregation in a membranous environment

Aggregation of misfolded oligomeric amyloid-beta (Aβ) peptides on lipid membranes has been identified as a primary event in Alzheimer's pathogenesis. However, the structural and dynamical features of this membrane assisted Aβ aggregation have not been well characterized. The microscopic characterization of dynamic molecular-level interactions in peptide aggregation pathways has been challenging both computationally and experimentally. In this work, we explore differential patterns of membrane-induced Aβ 16-22 (K-L-V-F-F-A-E) aggregation from the microscopic perspective of molecular interactions. Physics-based coarse-grained molecular dynamics (CG-MD) simulations were employed to investigate the effect of lipid headgroup charge - zwitterionic (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine: POPC) and anionic (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine: POPS) - on Aβ 16-22 peptide aggregation. Our analyses present an extensive overview of multiple pathways for peptide absorption and biomechanical forces governing peptide folding and aggregation. In agreement with experimental observations, anionic POPS molecules promote extended configurations in Aβ peptides that contribute towards faster emergence of ordered β-sheet-rich peptide assemblies compared to POPC, suggesting faster fibrillation. In addition, lower cumulative rates of peptide aggregation in POPS due to higher peptide-lipid interactions and slower lipid diffusion result in multiple distinct ordered peptide aggregates that can serve as nucleation seeds for subsequent Aβ aggregation. This study provides an in-silico assessment of experimentally observed aggregation patterns, presents new morphological insights and highlights the importance of lipid headgroup chemistry in modulating the peptide absorption and aggregation process.

Selenium, selenoprotein P, and Alzheimer's disease: is there a link?

The essential trace element, selenium (Se), is crucial to the brain but it may be potentially neurotoxic, depending on dosage and speciation; Se has been discussed for decades in relation to Alzheimer's disease (AD). Selenoprotein P (SELENOP) is a secreted heparin-binding glycoprotein which serves as the main Se transport protein in mammals. In vivo studies showed that this protein might have additional functions such as a contribution to redox regulation. The current review focuses on recent research on the possible role of SELENOP in AD pathology, based on model and human studies. The review also briefly summarizes results of epidemiological studies on Se supplementation in relation to brain diseases, including PREADViSE, EVA, and AIBL. Although mainly positive effects of Se are assessed in this review, possible detrimental effects of Se supplementation or exposure, including potential neurotoxicity, are also mentioned. In relation to AD, various roles of SELENOP are discussed, i.e. as the means of Se delivery to neurons, as an antioxidant, in cytoskeleton assembly, in interaction with redox-active metals (copper, iron, and mercury) and with misfolded proteins (amyloid-beta and hyperphosphorylated tau-protein).

Side-chain moieties from the N-terminal region of Aβ are Involved in an oligomer-stabilizing network of interactions

Oligomeric forms of the Aβ peptide represent the most probable neurotoxic agent in Alzheimer’s disease. The dynamic and heterogeneous character of these oligomers makes their structural characterization by classic methods difficult. Native mass spectrometry, when supported by additional gas phase techniques, like ion mobility separation and hydrogen-deuterium exchange (IM-HDX-MS), enable analysis of different oligomers coexisting in the sample and may provide species-specific structural information for each oligomeric form populated in the gas phase. Here, we have combined these three techniques to obtain insight into the structural properties of oligomers of Aβ1–40 and two variants with scrambled sequences. Gas-phase HDX-MS revealed a sequence-specific engagement of the side-chains of residues located at the N-terminal part of the peptide in a network of oligomer-stabilizing interactions. Oligomer-specific interactions were no longer observed in the case of the fully scrambled sequence. Also, the ability to form alternative structures, observed for WT Aβ peptide, was lost upon scrambling. Our data underscore a role for the N-terminal residues in shaping the equilibria of oligomeric forms. Although the peptide lacking the N-terminal 1–16 residues (p3 peptide) is thought to be benign, the role of the N-terminus has not been sufficiently characterized yet. We speculate that the interaction networks revealed here may be crucial for enabling structural transitions necessary to obtain mature parallel cross-β structures from smaller antiparallel oligomers. We provide a hypothetical molecular model of the trajectory that allows a gradual conversion from antiparallel to parallel oligomers without decomposition of oligomers. Oligomer-defining interactions involving the Aβ peptide N-terminus may be important in production of the neurotoxic forms and thus should not be neglected.

The role of histidines in amyloid β fibril assembly

Low pH has a strong stabilising effect on the fibrillar assembly of amyloid β, which is associated with Alzheimer's disease. The stabilising effect is already pronounced at pH 6.0, suggesting that protonation of histidines might mediate this effect. Through the systematic substitution of the three native histidines in Aβ for alanines, we have evaluated their role in fibril stability. Using surface plasmon resonance, we show that at neutral pH the fibrillar forms of all His-Ala variants are destabilised by a factor of 4-12 compared to wild-type Aβ. However, none of the His-Ala Aβ variants impair the stabilising effect of the fibril at low pH.

The phosphorylation of Hsp20 enhances its association with amyloid-β to increase protection against neuronal cell death

Up-regulation of Hsp20 protein levels in response to amyloid fibril formation is considered a key protective response against the onset of Alzheimer's disease (AD). Indeed, the physical interaction between Hsp20 and Aβ is known to prevent Aβ oligomerisation and protects neuronal cells from Aβ mediated toxicity, however, details of the molecular mechanism and regulatory cell signalling events behind this process have remained elusive. Using both conventional MTT end-point assays and novel real time measurement of cell impedance, we show that Hsp20 protects human neuroblastoma SH-SY5Y cells from the neurotoxic effects of Aβ. In an attempt to provide a mechanism for the neuroprotection afforded by Hsp20, we used peptide array, co-immunoprecipitation analysis and NMR techniques to map the interaction between Hsp20 and Aβ and report a binding mode where Hsp20 binds adjacent to the oligomerisation domain of Aβ, preventing aggregation. The Hsp20/Aβ interaction is enhanced by Hsp20 phosphorylation, which serves to increase association with low molecular weight Aβ species and decrease the effective concentration of Hsp20 required to disrupt the formation of amyloid oligomers. Finally, using a novel fluorescent assay for the real time evaluation of morphology-specific Aβ aggregation, we show that phospho-dependency of this effect is more pronounced for fibrils than for globular Aβ forms and that 25mers corresponding to the Hsp20 N-terminal can be used as Aβ aggregate inhibitors. Our report is the first to provide a molecular model for the Hsp20/Aβ complex and the first to suggest that modulation of the cAMP/cGMP pathways could be a novel route to enhance Hsp20-mediated attenuation of Aβ fibril neurotoxicity.

Nanoprobing of the effect of Cu(2+) cations on misfolding, interaction and aggregation of amyloid β peptide

Misfolding and aggregation of the amyloid β-protein (Aβ) are hallmarks of Alzheimer's disease. Both processes are dependent on the environmental conditions, including the presence of divalent cations, such as Cu(2+). Cu(2+) cations regulate early stages of Aβ aggregation, but the molecular mechanism of Cu(2+) regulation is unknown. In this study we applied single molecule AFM force spectroscopy to elucidate the role of Cu(2+) cations on interpeptide interactions. By immobilizing one of two interacting Aβ42 molecules on a mica surface and tethering the counterpart molecule onto the tip, we were able to probe the interpeptide interactions in the presence and absence of Cu(2+) cations at pH 7.4, 6.8, 6.0, 5.0, and 4.0. The results show that the presence of Cu(2+) cations change the pattern of Aβ interactions for pH values between pH 7.4 and pH 5.0. Under these conditions, Cu(2+) cations induce Aβ42 peptide structural changes resulting in N-termini interactions within the dimers. Cu(2+) cations also stabilize the dimers. No effects of Cu(2+) cations on Aβ-Aβ interactions were observed at pH 4.0, suggesting that peptide protonation changes the peptide-cation interaction. The effect of Cu(2+) cations on later stages of Aβ aggregation was studied by AFM topographic images. The results demonstrate that substoichiometric Cu(2+) cations accelerate the formation of fibrils at pH 7.4 and 5.0, whereas no effect of Cu(2+) cations was observed at pH 4.0. Taken together, the combined AFM force spectroscopy and imaging analyses demonstrate that Cu(2+) cations promote both the initial and the elongation stages of Aβ aggregation, but protein protonation diminishes the effect of Cu(2+).

Copper prevents amyloid-β(1-42) from forming amyloid fibrils under near-physiological conditions in vitro

The aggregation and deposition of amyloid-β((1-42) )(Aβ(42)) in the brain is implicated in the aetiology of Alzheimer's disease (AD). While the mechanism underlying its deposition in vivo is unknown its precipitation in vitro is influenced by metal ions. For example, Aβ(42) is known to bind copper, Cu(II), in vitro and binding results in aggregation of the peptide. The biophysical properties of Cu(II)-Aβ(42) aggregates are of significant importance to their putative involvement in the amyloid cascade hypothesis of AD and are currently the subject of strong debate. In particular the question has been raised if sub- and super-stoichiometric concentrations of Cu(II) act in opposing ways in respectively accelerating and preventing amyloid fibril formation by Aβ(42). Herein we have used fluorimetry and transmission electron microscopy to provide unequivocal evidence that under near-physiological conditions both sub- and super-stoichiometric concentrations of Cu(II) prevented the assembly of Aβ(42) into ThT-positive β-sheet rich amyloid fibrils.

Ca2+ enhances Aβ polymerization rate and fibrillar stability in a dynamic manner

Identifying factors that affect the self-assembly of Aβ (amyloid-β peptide) is of utmost importance in the quest to understand the molecular mechanisms causing AD (Alzheimer's disease). Ca2+ has previously been shown to accelerate both Aβ fibril nucleation and maturation, and dysregulated Ca2+ homoeostasis frequently correlates with development of AD. The mechanisms regarding Ca2+ binding, as well as its effect on fibril kinetics, are not fully understood. Using a polymerization assay we show that Ca2+ in a dynamic and reversible manner enhances both the elongation rate and fibrillar stability, where specifically the ‘dock and lock’ phase mechanism is enhanced. Through NMR analysis we found that Ca2+ affects the fibrillar architecture. In addition, and unexpectedly, we found that Ca2+ does not bind the free Aβ monomer. This implies that Ca2+ binding requires an architecture adopted by assembled peptides, and consequently is mediated through intermolecular interactions between adjacent peptides. This gives a mechanistic explanation to the enhancing effect on fibril maturation and indicates structural similarities between prefibrillar structures and mature amyloid. Taken together we show how Ca2+ levels affect the delicate equilibrium between the monomeric and assembled Aβ and how fluctuations in vivo may contribute to development and progression of the disease.

Structures and free energy landscapes of aqueous zinc(II)-bound amyloid-β(1-40) and zinc(II)-bound amyloid-β(1-42) with dynamics

Binding of divalent metal ions with intrinsically disordered fibrillogenic proteins, such as amyloid-β (Aβ), influences the aggregation process and the severity of neurodegenerative diseases. The Aβ monomers and oligomers are the building blocks of the aggregates. In this work, we report the structures and free energy landscapes of the monomeric zinc(II)-bound Aβ40 (Zn:Aβ40) and zinc(II)-bound Aβ42 (Zn:Aβ42) intrinsically disordered fibrillogenic metallopeptides in an aqueous solution by utilizing an approach that employs first principles calculations and parallel tempering molecular dynamics simulations. The structural and thermodynamic properties, including the secondary and tertiary structures and conformational Gibbs free energies of these intrinsically disordered metallopeptide alloforms, are presented. The results show distinct differing characteristics for these metallopeptides. For example, prominent β-sheet formation in the N-terminal region (Asp1, Arg5, and Tyr10) of Zn:Aβ40 is significantly decreased or lacking in Zn:Aβ42. Our findings indicate that blocking multiple reactive residues forming abundant β-sheet structure located in the central hydrophobic core and C-terminal regions of Zn:Aβ42 via antibodies or small organic molecules might help to reduce the aggregation of Zn(II)-bound Aβ42. Furthermore, we find that helix formation increases but β-sheet formation decreases in the C-terminal region upon Zn(II) binding to Aβ. This depressed β-sheet formation in the C-terminal region (Gly33-Gly38) in monomeric Zn:Aβ42 might be linked to the formation of amorphous instead of fibrillar aggregates of Zn:Aβ42.

Biochemistry of amyloid β-protein and amyloid deposits in Alzheimer disease

Progressive cerebral deposition of the amyloid β-protein (Aβ) in brain regions serving memory and cognition is an invariant and defining feature of Alzheimer disease. A highly similar but less robust process accompanies brain aging in many nondemented humans, lower primates, and some other mammals. The discovery of Aβ as the subunit of the amyloid fibrils in meningocerebral blood vessels and parenchymal plaques has led to innumerable studies of its biochemistry and potential cytotoxic properties. Here we will review the discovery of Aβ, numerous aspects of its complex biochemistry, and current attempts to understand how a range of Aβ assemblies, including soluble oligomers and insoluble fibrils, may precipitate and promote neuronal and glial alterations that underlie the development of dementia. Although the role of Aβ as a key molecular factor in the etiology of Alzheimer disease remains controversial, clinical trials of amyloid-lowering agents, reviewed elsewhere in this book, are poised to resolve the question of its pathogenic primacy.

Zn(++) binding disrupts the Asp(23)-Lys(28) salt bridge without altering the hairpin-shaped cross-β Structure of Aβ(42) amyloid aggregates

Observations like high Zn(2+) concentrations in senile plaques found in the brains of Alzheimer's patients and evidences emphasizing the role of Zn(2+) in amyloid-β (Aβ)-induced toxicity have triggered wide interest in understanding the nature of Zn(2+)-Aβ interaction. In vivo and in vitro studies have shown that aggregation kinetics, toxicity, and morphology of Aβ aggregates are perturbed in the presence of Zn(2+). Structural studies have revealed that Zn(2+) has a binding site in the N-terminal region of monomeric Aβ, but not much is precisely known about the nature of binding of Zn(2+) with aggregated forms of Aβ or its effect on the molecular structure of these aggregates. Here, we explore this aspect of the Zn(2+)-Aβ interaction using one- and two-dimensional (13)C and (15)N solid-state NMR. We find that Zn(2+) causes major structural changes in the N-terminal and the loop region connecting the two β-sheets. It breaks the salt bridge between the side chains of Asp(23) and Lys(28) by driving these residues into nonsalt-bridge-forming conformations. However, the cross-β structure of Aβ(42) aggregates remains unperturbed though the fibrillar morphology changes distinctly. We conclude that the salt bridge is not important for defining the characteristic molecular architecture of Aβ(42) but is significant for determining its fibrillar morphology and toxicity.

Aβ peptide fibrillar architectures controlled by conformational constraints of the monomer

Anomalous self-assembly of the Aβ peptide into fibrillar amyloid deposits is strongly correlated with the development of Alzheimer's disease. Aβ fibril extension follows a template guided “dock and lock” mechanism where polymerisation is catalysed by the fibrillar ends. Using surface plasmon resonance (SPR) and quenched hydrogen-deuterium exchange NMR (H/D-exchange NMR), we have analysed the fibrillar structure and polymerisation properties of both the highly aggregation prone Aβ1–40 Glu22Gly (Aβ^40Arc) and wild type Aβ1–40 (Aβ^40WT). The solvent protection patterns from H/D exchange experiments suggest very similar structures of the fibrillar forms. However, through cross-seeding experiments monitored by SPR, we found that the monomeric form of Aβ^40WT is significantly impaired to acquire the fibrillar architecture of Aβ^40Arc. A detailed characterisation demonstrated that Aβ^40WT has a restricted ability to dock and isomerise with high binding affinity onto Aβ^40Arc fibrils. These results have general implications for the process of fibril assembly, where the rate of polymerisation, and consequently the architecture of the formed fibrils, is restricted by conformational constraints of the monomers. Interestingly, we also found that the kinetic rate of fibril formation rather than the thermodynamically lowest energy state determines the overall fibrillar structure.

Molecular-level examination of Cu2+ binding structure for amyloid fibrils of 40-residue Alzheimer's β by solid-state NMR spectroscopy

Cu(2+) binding to Alzheimer's β (Aβ) peptides in amyloid fibrils has attracted broad attention, as it was shown that Cu ion concentration elevates in Alzheimer's senile plaque and such association of Aβ with Cu(2+) triggers the production of neurotoxic reactive oxygen species (ROS) such as H(2)O(2). However, detailed binding sites and binding structures of Cu(2+) to Aβ are still largely unknown for Aβ fibrils or other aggregates of Aβ. In this work, we examined molecular details of Cu(2+) binding to amyloid fibrils by detecting paramagnetic signal quenching in 1D and 2D high-resolution (13)C solid-state NMR (SSNMR) for full-length 40-residue Aβ(1-40). Selective quenching observed in (13)C SSNMR of Cu(2+)-bound Aβ(1-40) suggested that primary Cu(2+) binding sites in Aβ(1-40) fibrils include N(ε) in His-13 and His-14 and carboxyl groups in Val-40 as well as in Glu sidechains (Glu-3, Glu-11, and/or Glu-22). (13)C chemical shift analysis demonstrated no major structural changes upon Cu(2+) binding in the hydrophobic core regions (residues 18-25 and 30-36). Although the ROS production via oxidization of Met-35 in the presence of Cu(2+) has been long suspected, our SSNMR analysis of (13)C(ε)H(3)-S- in M35 showed little changes after Cu(2+) binding, excluding the possibility of Met-35 oxidization by Cu(2+) alone. Preliminary molecular dynamics (MD) simulations on Cu(2+)-Aβ complex in amyloid fibrils confirmed binding sites suggested by the SSNMR results and the stabilities of such bindings. The MD simulations also indicate the coexistence of a variety of Cu(2+)-binding modes unique in Aβ fibril, which are realized by both intra- and intermolecular contacts and highly concentrated coordination sites due to the in-register parallel β-sheet arrangements.

Beta-barrel models of soluble amyloid beta oligomers and annular protofibrils

Both soluble and membrane-bound prefibrillar assemblies of Abeta (Aβ) peptides have been associated with Alzheimer's disease (AD). The size and nature of these assemblies vary greatly and are affected by many factors. Here, we present models of soluble hexameric assemblies of Aβ42 and suggest how they can lead to larger assemblies and eventually to fibrils. The common element in most of these assemblies is a six-stranded β-barrel formed by the last third of Aβ42, which is composed of hydrophobic residues and glycines. The hydrophobic core β-barrels of the hexameric models are shielded from water by the N-terminus and central segments. These more hydrophilic segments were modeled to have either predominantly β or predominantly α secondary structure. Molecular dynamics simulations were performed to analyze stabilities of the models. The hexameric models were used as starting points from which larger soluble assemblies of 12 and 36 subunits were modeled. These models were developed to be consistent with numerous experimental results.

Role of zinc in human islet amyloid polypeptide aggregation

Human Islet Amyloid Polypeptide (hIAPP) is a highly amyloidogenic protein found in islet cells of patients with type II diabetes. Because hIAPP is highly toxic to beta-cells under certain conditions, it has been proposed that hIAPP is linked to the loss of beta-cells and insulin secretion in type II diabetics. One of the interesting questions surrounding this peptide is how the toxic and aggregation prone hIAPP peptide can be maintained in a safe state at the high concentrations that are found in the secretory granule where it is stored. We show here zinc, which is found at millimolar concentrations in the secretory granule, significantly inhibits hIAPP amyloid fibrillogenesis at concentrations similar to those found in the extracellular environment. Zinc has a dual effect on hIAPP fibrillogenesis: it increases the lag-time for fiber formation and decreases the rate of addition of hIAPP to existing fibers at lower concentrations, while having the opposite effect at higher concentrations. Experiments at an acidic pH which partially neutralizes the change in charge upon zinc binding show inhibition is largely due to an electrostatic effect at His18. High-resolution structures of hIAPP determined from NMR experiments confirm zinc binding to His18 and indicate zinc induces localized disruption of the secondary structure of IAPP in the vicinity of His18 of a putative helical intermediate of IAPP. The inhibition of the formation of aggregated and toxic forms of hIAPP by zinc provides a possible mechanism between the recent discovery of linkage between deleterious mutations in the SLC30A8 zinc transporter, which transports zinc into the secretory granule, and type II diabetes.