Atomic Resolution Structure of Monomorphic Aβ42 Amyloid Fibrils

Conformational Ensembles of the Wild-Type and S8C Aβ1-42 Dimers

We characterized the dimer of the amyloid-β wild-type (WT) peptide, Aβ, of 42 residues and its disulfide-bond-locked double mutant (S8C) by replica exchange molecular dynamics simulations. Aβ dimers are known to be the smallest toxic species in Alzheimer's disease, and the S8C mutant has been shown experimentally to form an exclusive homogeneous and neurotoxic dimer. Our 50 μs all-atom simulations reveal similar secondary structures and collision cross-sections but very different intramolecular and intermolecular conformations upon double S8C mutation. Both dimers are very dynamic with hundreds of free-energy minima that differ from the U-shape and S-shape conformations of the peptides in the fibrils. The only common structural feature, shared by both species with a probability of 4% in WT and 12% in S8C-S8C, is a three-stranded β-sheet spanning the 17-23, 29-36, and 39-41 residues, which does not exist in the Aβ40 WT dimers.

Amyloidogenicity at a Distance: How Distal Protein Regions Modulate Aggregation in Disease

The misfolding of proteins to form amyloid is a key pathological feature of several progressive, and currently incurable, diseases. A mechanistic understanding of the pathway from soluble, native protein to insoluble amyloid is crucial for therapeutic design, and recent efforts have helped to elucidate the key molecular events that trigger protein misfolding. Generally, either global or local structural perturbations occur early in amyloidogenesis to expose aggregation-prone regions of the protein that can then self-associate to form toxic oligomers. Surprisingly, these initiating structural changes are often caused or influenced by protein regions distal to the classically amyloidogenic sequences. Understanding the importance of these distal regions in the pathogenic process has highlighted many remaining knowledge gaps regarding the precise molecular events that occur in classic aggregation pathways. In this review, we discuss how these distal regions can influence aggregation in disease and the recent technical and conceptual advances that have allowed this insight.

Quenched hydrogen-deuterium exchange NMR of a disease-relevant Aβ(1-42) amyloid polymorph

Alzheimer’s disease is associated with the aggregation into amyloid fibrils of Aβ(1–42) and Aβ(1–40) peptides. Interestingly, these fibrils often do not obtain one single structure but rather show different morphologies, so-called polymorphs. Here, we compare quenched hydrogen-deuterium (H/D) exchange of a disease-relevant Aβ(1–42) fibril for which the 3D structure has been determined by solid-state NMR with H/D exchange previously determined on another structural polymorph. This comparison reveals secondary structural differences between the two polymorphs suggesting that the two polymorphisms can be classified as segmental polymorphs.

Nucleobindin 1 binds to multiple types of pre-fibrillar amyloid and inhibits fibrillization

During amyloid fibril formation, amyloidogenic polypeptides misfold and self assemble into soluble pre-fibrillar aggregates, i.e., protofibrils, which elongate and mature into insoluble fibrillar aggregates. An emerging class of chaperones, chaperone-like amyloid binding proteins (CLABPs), has been shown to interfere with aggregation of particular misfolded amyloid peptides or proteins. We have discovered that the calcium-binding protein nuclebindin-1 (NUCB1) is a novel CLABP. We show that NUCB1 inhibits aggregation of islet-amyloid polypeptide associated with type 2 diabetes mellitus, a-synuclein associated with Parkinson’s disease, transthyretin V30M mutant associated with familial amyloid polyneuropathy, and Aβ42 associated with Alzheimer’s disease by stabilizing their respective protofibril intermediates. Kinetic studies employing the modeling software AmyloFit show that NUCB1 affects both primary nucleation and secondary nucleation. We hypothesize that NUCB1 binds to the common cross-β-sheet structure of protofibril aggregates to “cap” and stabilize soluble macromolecular complexes. Transmission electron microscopy and atomic force microscopy were employed to characterize the size, shape and volume distribution of multiple sources of NUCB1-capped protofibrils. Interestingly, NUCB1 prevents Aβ42 protofibril toxicity in a cellular assay. NUCB1-stabilized amyloid protofibrils could be used as immunogens to prepare conformation-specific antibodies and as novel tools to develop screens for anti-protofibril diagnostics and therapeutics.

Stereoisomers Probe Steric Zippers in Amyloid-β

Shape complementarity between close-packed residues plays a critical role in the amyloid aggregation process. Here, we probe such "steric zipper" interactions in amyloid-β (Aβ₄₀), whose aggregation is linked to Alzheimer's disease, by replacing natural residues by their stereoisomers. Such mutations are expected to specifically destabilize the shape sensitive "packing" interactions, which may potentially increase their solubility and change other properties. We study the stereomutants DF19 and DL34 and also the DA2/DF4/DH6/DS8 mutant of Aβ₄₀. F19-L34 is a critical contact in a tightly packed region of Aβ, while residues 1-9 are known to be disordered. While both DF19 and DL34 slow down the kinetics of aggregation and form amyloid fibrils efficiently, only DL34 increases the final solubility. DF19 gives rise to additional off-pathway aggregation which results in large, kinetically stable aggregates, and has lower net solubility. DA2/DF4/DH6/DS8 does not have an effect on the kinetics or the solubility. Notably, both DF19 and DL34 oligomers have a significantly lower level of interactions with lipid vesicles and live cells. We conclude that stereoisomers can cause complex site dependent changes in amyloid properties, and provide an effective tool to determine the role of individual residues in shaping the packed interiors of amyloid aggregates.

Crystal Structure of the DFNKF Segment of Human Calcitonin Unveils Aromatic Interactions between Phenylalanines

Although intensively studied, the high-resolution crystal structure of the peptide DFNKF, the core-segment of human calcitonin, has never been described. Here we report how the use of iodination as a strategy to promote crystallisation and facilitate phase determination, allowed us to solve, for the first time, the single-crystal X-ray structure of a DFNKF derivative. Computational studies suggest that both the iodinated and the wild-type peptides populate very similar conformations. Furthermore, the conformer found in the solid-state structure is one of the most populated in solution, making the crystal structure a reliable model for the peptide in solution. The crystal structure of DFNKF(I) confirms the overall features of the amyloid cross-β spine and highlights how aromatic-aromatic interactions are important structural factors in the self-assembly of this peptide. A detailed analysis of such interactions is reported.

Fibrillation-prone conformations of the amyloid-β-42 peptide at the gold/water interface

Proteins in the proximity of inorganic surfaces and nanoparticles may undergo profound adjustments that trigger biomedically relevant processes, such as protein fibrillation. The mechanisms that govern protein-surface interactions at the molecular level are still poorly understood. In this work, we investigate the adsorption onto a gold surface, in water, of an amyloid-β (Aβ) peptide, which is the amyloidogenic peptide involved in Alzheimer's disease. The entire adsorption process, from the peptide in bulk water to its conformational relaxation on the surface, is explored by large-scale atomistic molecular dynamics (MD) simulations. We start by providing a description of the conformational ensemble of Aβ in solution by a 22 μs temperature replica exchange MD simulation, which is consistent with previous results. Then, we obtain a statistical description of how the peptide approaches the gold surface by multiple MD simulations, identifying the preferential gold-binding sites and giving a kinetic picture of the association process. Finally, relaxation of the Aβ conformations at the gold/water interface is performed by a 19 μs Hamiltonian-temperature replica exchange MD simulation. We find that the conformational ensemble of Aβ is strongly perturbed by the presence of the surface. In particular, at the gold/water interface the population of the conformers akin to amyloid fibrils is significantly enriched, suggesting that this extended contact geometry may promote fibrillation.

A Two-Component Adhesive: Tau Fibrils Arise from a Combination of a Well-Defined Motif and Conformationally Flexible Interactions

Fibrillar aggregates of Aβ and Tau in the brain are the major hallmarks of Alzheimer's disease. Most Tau fibers have a twisted appearance, but the twist can be variable and even absent. This ambiguity, which has also been associated with different phenotypes of tauopathies, has led to controversial assumptions about fibril constitution, and it is unclear to-date what the molecular causes of this polymorphism are. To tackle this question, we used solid-state NMR strategies providing assignments of non-seeded three-repeat-domain Tau^3RD with an inherent heterogeneity. This is in contrast to the general approach to characterize the most homogeneous preparations by construct truncation or intricate seeding protocols. Here, carbon and nitrogen chemical-shift conservation between fibrils revealed invariable secondary-structure properties, however, with inter-monomer interactions variable among samples. Residues with variable amide shifts are localized mostly to N- and C-terminal regions within the rigid beta structure in the repeat region of Tau^3RD. By contrast, the hexapeptide motif in repeat R3, a crucial motif for fibril formation, shows strikingly low variability of all NMR parameters: Starting as a nucleation site for monomer-monomer contacts, this six-residue sequence element also turns into a well-defined structural element upon fibril formation. Given the absence of external causes in vitro, the interplay of structurally differently conserved elements in this protein likely reflects an intrinsic property of Tau fibrils.

A computational study on how structure influences the optical properties in model crystal structures of amyloid fibrils

Amyloid fibrils have been shown to have peculiar optical properties since they can exhibit fluorescence in the absence of aromatic residues. In a recent study, we have shown that proton transfer (PT) events along hydrogen bonds (HBs) are coupled to absorption in the near UV range. Here, we gain more insights into the different types of hydrogen bonding interactions that occur in our model systems and the molecular factors that control the susceptibility of the protons to undergo PT and how this couples to the optical properties. In the case of the strong N-C termini interactions, a nearby methionine residue stabilizes the non-zwitterionic NH₂-COOH pair, while zwitterionic NH₃⁺-COO- is stabilized by the proximity of nearby crystallographic water molecules. Proton motion along the hydrogen bonds in the fibril is intimately coupled to the compression of the heavier atoms, similar to what is observed in bulk water. Small changes in the compression of the hydrogen bonds in the protein can lead to significant changes in both the ground and excited state potential energy surfaces associated with PT. Finally, we also reinforce the importance of nuclear quantum fluctuations of protons in the HBs of the amyloid proteins.

Mapping protein-protein interactions by double-REDOR-filtered magic angle spinning NMR spectroscopy

REDOR-based experiments with simultaneous 1H-13C and 1H-15N dipolar dephasing are explored for investigating intermolecular protein-protein interfaces in complexes formed by a U-13C,15N-labeled protein and its natural abundance binding partner. The application of a double-REDOR filter (dREDOR) results in a complete dephasing of proton magnetization in the U-13C,15N-enriched molecule while the proton magnetization of the unlabeled binding partner is not dephased. This retained proton magnetization is then transferred across the intermolecular interface by 1H-13C or 1H-15N cross polarization, permitting to establish the residues of the U-13C,15N-labeled protein, which constitute the binding interface. To assign the interface residues, this dREDOR-CPMAS element is incorporated as a building block into 13C-13C correlation experiments. We established the validity of this approach on U-13C,15N-histidine and on a structurally characterized complex of dynactin's U-13C,15N-CAP-Gly domain with end-binding protein 1 (EB1). The approach introduced here is broadly applicable to the analysis of intermolecular interfaces when one of the binding partners in a complex cannot be isotopically labeled.

Structural variation in amyloid-β fibrils from Alzheimer's disease clinical subtypes

Aggregation of amyloid-β peptides into fibrils or other self-assembled states is central to the pathogenesis of Alzheimer's disease. Fibrils formed in vitro by 40- and 42-residue amyloid-β peptides (Aβ40 and Aβ42) are polymorphic, with variations in molecular structure that depend on fibril growth conditions. Recent experiments suggest that variations in amyloid-β fibril structure in vivo may correlate with variations in Alzheimer's disease phenotype, in analogy to distinct prion strains that are associated with different clinical and pathological phenotypes. Here we investigate correlations between structural variation and Alzheimer's disease phenotype using solid-state nuclear magnetic resonance (ssNMR) measurements on Aβ40 and Aβ42 fibrils prepared by seeded growth from extracts of Alzheimer's disease brain cortex. We compared two atypical Alzheimer's disease clinical subtypes-the rapidly progressive form (r-AD) and the posterior cortical atrophy variant (PCA-AD)-with a typical prolonged-duration form (t-AD). On the basis of ssNMR data from 37 cortical tissue samples from 18 individuals, we find that a single Aβ40 fibril structure is most abundant in samples from patients with t-AD and PCA-AD, whereas Aβ40 fibrils from r-AD samples exhibit a significantly greater proportion of additional structures. Data for Aβ42 fibrils indicate structural heterogeneity in most samples from all patient categories, with at least two prevalent structures. These results demonstrate the existence of a specific predominant Aβ40 fibril structure in t-AD and PCA-AD, suggest that r-AD may relate to additional fibril structures and indicate that there is a qualitative difference between Aβ40 and Aβ42 aggregates in the brain tissue of patients with Alzheimer's disease.

Structural Studies of Amyloid Proteins at the Molecular Level

Dozens of proteins are known to convert to the aggregated amyloid state. These include fibrils associated with systemic and neurodegenerative diseases and cancer, functional amyloid fibrils in microorganisms and animals, and many denatured proteins. Amyloid fibrils can be much more stable than other protein assemblies. In contrast to globular proteins, a single protein sequence can aggregate into several distinctly different amyloid structures, termed polymorphs, and a given polymorph can reproduce itself by seeding. Amyloid polymorphs may be the molecular basis of prion strains. Whereas the Protein Data Bank contains some 100,000 globular protein and 3,000 membrane protein structures, only a few dozen amyloid protein structures have been determined, and most of these are short segments of full amyloid-forming proteins. Regardless, these amyloid structures illuminate the architecture of the amyloid state, including its stability and its capacity for formation of polymorphs.

Line-Broadening in Low-Temperature Solid-State NMR Spectra of Fibrils

The temperature-dependent resonance-line broadening of HET-s(218-289) in its amyloid form is investigated in the range between 110 K and 280 K. Significant differences are observed between residues in the structured hydrophobic triangular core, which are broadened the least and can be detected down to 100 K, and in the solvent-exposed parts, which are broadened the most and often disappear from the observed spectrum around 200 K. Below the freezing of the bulk water, around 273 K, the protein fibrils are still surrounded by a layer of mobile water whose thickness decreases with temperature, leading to drying out of the fibrils.

In silico studies of solvated F19W amyloid β (11–40) trimer

REMD studies shows that F19W mutation does not change in the overall structure of Aβ_11–40 trimer significantly but increases it flexibility, consistent with the observed formation of the same fibril structures at slower rates.

Elucidating the Aβ42 Anti-Aggregation Mechanism of Action of Tramiprosate in Alzheimer's Disease: Integrating Molecular Analytical Methods, Pharmacokinetic and Clinical Data

BACKGROUND

Amyloid beta (Aβ) oligomers play a critical role in the pathogenesis of Alzheimer's disease (AD) and represent a promising target for drug development. Tramiprosate is a small-molecule Aβ anti-aggregation agent that was evaluated in phase III clinical trials for AD but did not meet the primary efficacy endpoints; however, a pre-specified subgroup analysis revealed robust, sustained, and clinically meaningful cognitive and functional effects in patients with AD homozygous for the ε4 allele of apolipoprotein E4 (APOE4/4 homozygotes), who carry an increased risk for the disease. Therefore, to build on this important efficacy attribute and to further improve its pharmaceutical properties, we have developed a prodrug of tramiprosate ALZ-801 that is in advanced stages of clinical development. To elucidate how tramiprosate works, we investigated its molecular mechanism of action (MOA) and the translation to observed clinical outcomes.

OBJECTIVE

The two main objectives of this research were to (1) elucidate and characterize the MOA of tramiprosate via an integrated application of three independent molecular methodologies and (2) present an integrated translational analysis that links the MOA, conformation of the target, stoichiometry, and pharmacokinetic dose exposure to the observed clinical outcome in APOE4/4 homozygote subjects.

METHOD

We used three molecular analytical methods-ion mobility spectrometry-mass spectrometry (IMS-MS), nuclear magnetic resonance (NMR), and molecular dynamics-to characterize the concentration-related interactions of tramiprosate versus Aβ42 monomers and the resultant conformational alterations affecting aggregation into oligomers. The molecular stoichiometry of the tramiprosate versus Aβ42 interaction was further analyzed in the context of clinical pharmacokinetic dose exposure and central nervous system Aβ42 levels (i.e., pharmacokinetic-pharmacodynamic translation in humans).

RESULTS

We observed a multi-ligand interaction of tramiprosate with monomeric Aβ42, which differs from the traditional 1:1 binding. This resulted in the stabilization of Aβ42 monomers and inhibition of oligomer formation and elongation, as demonstrated by IMS-MS and molecular dynamics. Using NMR spectroscopy and molecular dynamics, we also showed that tramiprosate bound to Lys16, Lys28, and Asp23, the key amino acid side chains of Aβ42 that are responsible for both conformational seed formation and neuronal toxicity. The projected molar excess of tramiprosate versus Aβ42 in humans using the dose effective in patients with AD aligned with the molecular stoichiometry of the interaction, providing a clear clinical translation of the MOA. A consistent alignment of these preclinical-to-clinical elements describes a unique example of translational medicine and supports the efficacy seen in symptomatic patients with AD. This unique "enveloping mechanism" of tramiprosate also provides a potential basis for tramiprosate dose selection for patients with homozygous AD at earlier stages of disease.

CONCLUSION

We have identified the molecular mechanism that may account for the observed clinical efficacy of tramiprosate in patients with APOE4/4 homozygous AD. In addition, the integrated application of the molecular methodologies (i.e., IMS-MS, NMR, and thermodynamics analysis) indicates that it is feasible to modulate and control the Aβ42 conformational dynamics landscape by a small molecule, resulting in a favorable Aβ42 conformational change that leads to a clinically relevant amyloid anti-aggregation effect and inhibition of oligomer formation. This novel enveloping MOA of tramiprosate has potential utility in the development of disease-modifying therapies for AD and other neurodegenerative diseases caused by misfolded proteins.

The efficient synthesis and purification of amyloid-β(1–42) using an oligoethylene glycol-containing photocleavable lysine tag

An oligoethylene glycol-containing photocleavable lysine tag was developed to facilitate the efficient synthesis and purification of the Aβ₄₂ peptide.

Replica exchange molecular dynamics study of the truncated amyloid beta (11–40) trimer in solution

The structure of the 3Aβ_11–40 oligomer is determined for the first time using T-REMD simulations.

Cross-seeding between Aβ40 and Aβ42 in Alzheimer's disease

Aβ42 is the major component of parenchymal plaques in the brain of Alzheimer's patients, while Aβ40 is the major component of cerebrovascular plaques. Since Aβ40 and Aβ42 coexist in the brain, understanding the interaction between Aβ40 and Aβ42 during their aggregation is important to delineate the molecular mechanism underlying Alzheimer's disease. Here, we present a rigorous and systematic study of the cross-seeding effects between Aβ40 and Aβ42. We show that Aβ40 fibril seeds can promote Aβ42 aggregation in a concentration-dependent manner, and vice versa. Our results also suggest that seeded aggregation and spontaneous aggregation may be two separate pathways. These findings may partly resolve conflicting observations in the literature regarding the cross-seeding effects between Aβ40 and Aβ42.

Structure of Crenezumab Complex with Aβ Shows Loss of β-Hairpin

Accumulation of amyloid-β (Aβ) peptides and amyloid plaque deposition in brain is postulated as a cause of Alzheimer's disease (AD). The precise pathological species of Aβ remains elusive although evidence suggests soluble oligomers may be primarily responsible for neurotoxicity. Crenezumab is a humanized anti-Aβ monoclonal IgG4 that binds multiple forms of Aβ, with higher affinity for aggregated forms, and that blocks Aβ aggregation, and promotes disaggregation. To understand the structural basis for this binding profile and activity, we determined the crystal structure of crenezumab in complex with Aβ. The structure reveals a sequential epitope and conformational requirements for epitope recognition, which include a subtle but critical element that is likely the basis for crenezumab's versatile binding profile. We find interactions consistent with high affinity for multiple forms of Aβ, particularly oligomers. Of note, crenezumab also sequesters the hydrophobic core of Aβ and breaks an essential salt-bridge characteristic of the β-hairpin conformation, eliminating features characteristic of the basic organization in Aβ oligomers and fibrils, and explains crenezumab's inhibition of aggregation and promotion of disaggregation. These insights highlight crenezumab's unique mechanism of action, particularly regarding Aβ oligomers, and provide a strong rationale for the evaluation of crenezumab as a potential AD therapy.

The activities of amyloids from a structural perspective

The aggregation of proteins into structures known as amyloids is observed in many neurodegenerative diseases, including Alzheimer's disease. Amyloids are composed of pairs of tightly interacting, many stranded and repetitive intermolecular β-sheets, which form the cross-β-sheet structure. This structure enables amyloids to grow by recruitment of the same protein and its repetition can transform a weak biological activity into a potent one through cooperativity and avidity. Amyloids therefore have the potential to self-replicate and can adapt to the environment, yielding cell-to-cell transmissibility, prion infectivity and toxicity.

Fast Motions of Key Methyl Groups in Amyloid-β Fibrils

Amyloid-β (Aβ) peptide is the major component of plaques found in Alzheimer's disease patients. Using solid-state 2H NMR relaxation performed on selectively deuterated methyl groups, we probed the dynamics in the threefold symmetric and twofold symmetric polymorphs of native Aβ as well as the protofibrils of the D23N mutant. Specifically, we investigated the methyl groups of two leucine residues that belong to the hydrophobic core (L17 and L34) as well as M35 residues belonging to the hydrophobic interface between the cross-β subunits, which has been previously found to be water-accessible. Relaxation measurements performed over 310-140 K and two magnetic field strengths provide insights into conformational variability within and between polymorphs. Core packing variations within a single polymorph are similar to what is observed for globular proteins for the core residues, whereas M35 exhibits a larger degree of variability. M35 site is also shown to undergo a solvent-dependent dynamical transition in which slower amplitude motions of methyl axes are activated at high temperature. The motions, modeled as a diffusion of methyl axis, have activation energy by a factor of 2.7 larger in the twofold compared with the threefold polymorph, whereas D23N protofibrils display a value similar to the threefold polymorph. This suggests enhanced flexibility of the hydrophobic interface in the threefold polymorph. This difference is only observed in the hydrated state and is absent in the dry fibrils, highlighting the role of solvent at the cavity. In contrast, the dynamic behavior of the core is hydration-independent.

Covalent Tethering and Residues with Bulky Hydrophobic Side Chains Enable Self-Assembly of Distinct Amyloid Structures

Polymorphism is a common property of amyloid fibers that complicates their detailed structural and functional studies. Here we report experiments illustrating the chemical principles that enable the formation of amyloid polymorphs with distinct stoichiometric composition. Using appropriate covalent tethering we programmed self-assembly of a model peptide corresponding to the [20-41] fragment of human β2-microglobulin into fibers with either trimeric or dimeric amyloid cores. Using a set of biophysical and biochemical methods we demonstrated their distinct structural, morphological, and templating properties. Furthermore, we showed that supramolecular approaches in which the peptide is modified with bulky substituents can also be applied to modulate the formation of different fiber polymorphs. Such strategies, when applied to disease-related peptides and proteins, will greatly help in the evaluation of the biological properties of structurally distinct amyloids.

Few Ramachandran Angle Changes Provide Interaction Strength Increase in Aβ42 versus Aβ40 Amyloid Fibrils

The pathology of Alzheimer's disease can ultimately be traced to the increased aggregation stability of Aβ42 peptides which possess two extra residues (Ile 41 &Ala 42) that the non-pathological strain (Aβ40) lacks. We have found Aβ42 fibrils to exhibit stronger energies in inter-chain interactions and we have also identified the cause for this increase to be the result of different Ramachandran angle values in certain residues of the Aβ42 strain compared to Aβ40. These unique angle configurations result in the peptide planes in the fibril structures to be more vertical along the fibril axis for Aβ42 which thus reduces the inter-atomic distance between interacting atoms on vicinal peptide chains thereby increasing the electrostatic interaction energies. We lastly postulate that these different Ramachandran angle values could possibly be traced to the unique conformational folding avenues sampled by the Aβ42 peptide owing to the presence of its two extra residues.

Hematoxylin Inhibits Amyloid β-Protein Fibrillation and Alleviates Amyloid-Induced Cytotoxicity

Accumulation and aggregation of amyloid β-protein (Aβ) play an important role in the pathogenesis of Alzheimer's disease. There has been increased interest in finding new anti-amyloidogenic compounds to inhibit Aβ aggregation. Herein, thioflavin T fluorescent assay and transmission electron microscopy results showed that hematoxylin, a natural organic molecule extracted from Caesalpinia sappan, was a powerful inhibitor of Aβ42 fibrillogenesis. Circular dichroism studies revealed hematoxylin reduced the β-sheet content of Aβ42 and made it assemble into antiparallel arrangement, which induced Aβ42 to form off-pathway aggregates. As a result, hematoxylin greatly alleviated Aβ42-induced cytotoxicity. Molecular dynamics simulations revealed the detailed interactions between hematoxylin and Aβ42. Four binding sites of hematoxylin on Aβ42 hexamer were identified, including the N-terminal region, S8GY10 region, turn region, and C-terminal region. Notably, abundant hydroxyl groups made hematoxylin prefer to interact with Aβ42 via hydrogen bonds. This also contributed to the formation of π-π stacking and hydrophobic interactions. Taken together, the research proved that hematoxylin was a potential agent against Aβ fibrillogenesis and cytotoxicity.

Coassembly of Peptides Derived from β-Sheet Regions of β-Amyloid

In this paper, we investigate the coassembly of peptides derived from the central and C-terminal regions of the β-amyloid peptide (Aβ). In the preceding paper, J. Am. Chem. Soc.2016, DOI: 10.1021/jacs.6b06000, we established that peptides containing residues 17–23 (LVFFAED) from the central region of Aβ and residues 30–36 (AIIGLMV) from the C-terminal region of Aβ assemble to form homotetramers consisting of two hydrogen-bonded dimers. Here, we mix these tetramer-forming peptides and determine how they coassemble. Incorporation of a single ¹⁵N isotopic label into each peptide provides a spectroscopic probe with which to elucidate the coassembly of the peptides by ¹H,¹⁵N HSQC. Job’s method of continuous variation and nonlinear least-squares fitting reveal that the peptides form a mixture of heterotetramers in 3:1, 2:2, and 1:3 stoichiometries, in addition to the homotetramers. These studies also establish the relative stability of each tetramer and show that the 2:2 heterotetramer predominates. ¹⁵N-Edited NOESY shows the 2:2 heterotetramer comprises two different homodimers, rather than two heterodimers. The peptides within the heterotetramer segregate in forming the homodimer subunits, but the two homodimers coassemble in forming the heterotetramer. These studies show that the central and C-terminal regions of Aβ can preferentially segregate within β-sheets and that the resulting segregated β-sheets can further coassemble.

Assembly of Peptides Derived from β-Sheet Regions of β-Amyloid

In Alzheimer’s disease, aggregation of the β-amyloid peptide (Aβ) results in the formation of oligomers and fibrils that are associated with neurodegeneration. Aggregation of Aβ occurs through interactions between different regions of the peptide. This paper and the accompanying paper constitute a two-part investigation of two key regions of Aβ: the central region and the C-terminal region. These two regions promote aggregation and adopt β-sheet structure in the fibrils, and may also do so in the oligomers. In this paper, we study the assembly of macrocyclic β-sheet peptides that contain residues 17–23 (LVFFAED) from the central region and residues 30–36 (AIIGLMV) from the C-terminal region. These peptides assemble to form tetramers. Each tetramer consists of two hydrogen-bonded dimers that pack through hydrophobic interactions in a sandwich-like fashion. Incorporation of a single ¹⁵N isotopic label into each peptide provides a spectroscopic probe with which to elucidate the β-sheet assembly and interaction: ¹H,¹⁵N HSQC studies facilitate the identification of the monomers and tetramers; ¹⁵N-edited NOESY studies corroborate the pairing of the dimers within the tetramers. In the following paper, J. Am. Chem. Soc.2016, DOI: 10.1021/jacs.6b06001, we will extend these studies to elucidate the coassembly of the peptides to form heterotetramers.

Bioelectromagnetics Research within an Australian Context: The Australian Centre for Electromagnetic Bioeffects Research (ACEBR)

Mobile phone subscriptions continue to increase across the world, with the electromagnetic fields (EMF) emitted by these devices, as well as by related technologies such as Wi-Fi and smart meters, now ubiquitous. This increase in use and consequent exposure to mobile communication (MC)-related EMF has led to concern about possible health effects that could arise from this exposure. Although much research has been conducted since the introduction of these technologies, uncertainty about the impact on health remains. The Australian Centre for Electromagnetic Bioeffects Research (ACEBR) is a National Health and Medical Research Council Centre of Research Excellence that is undertaking research addressing the most important aspects of the MC-EMF health debate, with a strong focus on mechanisms, neurodegenerative diseases, cancer, and exposure dosimetry. This research takes as its starting point the current scientific status quo, but also addresses the adequacy of the evidence for the status quo. Risk communication research complements the above, and aims to ensure that whatever is found, it is communicated effectively and appropriately. This paper provides a summary of this ACEBR research (both completed and ongoing), and discusses the rationale for conducting it in light of the prevailing science.

Structural Polymorphism of Alzheimer's β-Amyloid Fibrils as Controlled by an E22 Switch: A Solid-State NMR Study

The amyloid-β (Aβ) peptide of Alzheimer's disease (AD) forms polymorphic fibrils on the micrometer and molecular scales. Various fibril growth conditions have been identified to cause polymorphism, but the intrinsic amino acid sequence basis for this polymorphism has been unclear. Several single-site mutations in the center of the Aβ sequence cause different disease phenotypes and fibrillization properties. The E22G (Arctic) mutant is found in familial AD and forms protofibrils more rapidly than wild-type Aβ. Here, we use solid-state NMR spectroscopy to investigate the structure, dynamics, hydration and morphology of Arctic E22G Aβ40 fibrils. (13)C, (15)N-labeled synthetic E22G Aβ40 peptides are studied and compared with wild-type and Osaka E22Δ Aβ40 fibrils. Under the same fibrillization conditions, Arctic Aβ40 exhibits a high degree of polymorphism, showing at least four sets of NMR chemical shifts for various residues, while the Osaka and wild-type Aβ40 fibrils show a single or a predominant set of chemical shifts. Thus, structural polymorphism is intrinsic to the Arctic E22G Aβ40 sequence. Chemical shifts and inter-residue contacts obtained from 2D correlation spectra indicate that one of the major Arctic conformers has surprisingly high structural similarity with wild-type Aβ42. (13)C-(1)H dipolar order parameters, (1)H rotating-frame spin-lattice relaxation times and water-to-protein spin diffusion experiments reveal substantial differences in the dynamics and hydration of Arctic, Osaka and wild-type Aβ40 fibrils. Together, these results strongly suggest that electrostatic interactions in the center of the Aβ peptide sequence play a crucial role in the three-dimensional fold of the fibrils, and by inference, fibril-induced neuronal toxicity and AD pathogenesis.