Steric Crowding of the Turn Region Alters the Tertiary Fold of Amyloid-β18-35 and Makes It Soluble

β-Turn mimetic synthetic peptides as amyloid-β aggregation inhibitors

Aggregation of amyloid peptides results in severe neurodegenerative diseases. While the fibril structures of Aβ₄₀ and Aβ₄₂ have been described recently, resolution of the aggregation pathway and evaluation of potent inhibitors still remains elusive, in particular in view of the hairpin-region of Aβ₄₀. We here report the preparation of beta-turn mimetic conjugates containing synthetic turn mimetic structures in the turn region of Aβ₄₀ and Aβ_16-35, replacing 2 amino acids in the turn-region G25 - K28. The structure of the turn mimic induces both, acceleration of fibrillation and the complete inhibition of fibrillation, confirming the importance of the turn region on the aggregation. Replacing position G25-S26 provided the best inhibition effect for both beta-turn mimetics, the bicyclic BTD 1 and the aromatic TAA 2, while positions N27-K28 and V24-G25 showed only weaker or no inhibitory effects. When comparing different turn mimetics at the same position (G25-S26), conjugate 1a bearing the BTD turn showed the best inhibition of Aβ₄₀ aggregation, while 5-amino-valeric acid 4a showed the weakest effect. Thus there is a pronounced impact on fibrillation with the chemical nature of the embedded beta-turn-mimic: the conformationally constrained turns 1 and 2 lead to a significantly reduced fibrillation, even inhibiting fibrillation of native Aβ₄₀ when added in amounts down to 1/10, whereas the more flexible beta-turn-mimics 4-amino-benzoic acid 3a and 5-amino-valeric acid 4a lead to enhanced fibrillation. Toxicity-testing of the most successful conjugate showed only minor toxicity in cell-viability assays using the N2a cell line. Structural downsizing lead to the short fragment BTD/peptide Aβ_16-35 as inhibitor of the aggregation of Aβ₄₀, opening large potential for further small peptide based inhibitors.

Incorporation of the Nonproteinogenic Amino Acid β-Methylamino-alanine Affects Amyloid β Fibril Properties and Toxicity

The nonproteinogenic amino acid β-methylamino alarelevant example for environmental hazards are nonnine (BMAA) is a neurotoxin and represents a potential risk factor for neurodegenerative diseases. Despite intense research over the last years, the pathological mechanism of BMAA is still unclear. One of the main open questions is whether BMAA can be misincorporated into proteins, especially as a substitute for serine, and whether this has structural and functional consequences for the afflicted proteins leading to early onset neurodegeneration. In this study, we hypothesize that BMAA was indeed incorporated into Aβ₄₀ molecules and study the structural and dynamical consequences of such misincorporation along with the effect such mutated Aβ₄₀ peptides have on neuronal cells. We used the synthetic β-amyloid peptide (Aβ₄₀), a known key player in the development of Alzheimer's disease, to incorporate BMAA substitutions at three different positions in the peptide sequence: Ser₈BMAA at the peptide's N-terminus, Phe₁₉BMAA in the hydrophobic core region, and S₂₆BMAA in the flexible turn region of Aβ₄₀ fibrils. We performed a set of biophysical experiments including fluorescence, circular dichroism, solid-state NMR spectroscopy, transmission electron microscopy, and X-ray diffraction to investigate structural and functional aspects of the mutated peptides compared to wildtype Aβ₄₀. All variants showed high structural tolerance to BMAA misincorporation. In contrast, the cellular response and neuronal survival were affected in a mutation site-specific manner. As a consequence, we can state from the physicochemical point of view that, if BMAA was misincorporated into proteins, it could indeed represent a risk factor that could potentially play a role in neurodegeneration. Further research addressing the role of BMAA, especially its protein-associated form, should be performed to obtain a better understanding of neurodegenerative diseases and to develop new therapeutic strategies.

Amyloid β (1-40) Toxicity Depends on the Molecular Contact between Phenylalanine 19 and Leucine 34

The formation of the hydrophobic contact between phenylalanine 19 (F19) and leucine 34 (L34) of amyloid β (1-40) (Aβ(1-40)) is known to be an important step in the fibrillation of Aβ(1-40) peptides. Mutations of this putatively early molecular contact were shown to strongly influence the toxicity of Aβ(1-40) ( Das et al. ( 2015 ) ACS Chem. Neurosci. 6 , 1290 - 1295 ). Any mutation of residue F19 completely abolished the toxicity of Aβ(1-40), suggesting that a proper F19-L34 contact is crucial also for the formation of transient oligomers. In this work, we investigate a series of isomeric substitutions of L34, namely, d-leucine, isoleucine, and valine, to study further details of this molecular contact. These replacements represent very minor alterations in the Aβ(1-40) structure posing the question how these alterations challenge the fibrillation kinetics, structure, dynamics, and toxicity of the Aβ(1-40) aggregates. Our work involves kinetic studies using thioflavin T, transmission electron microscopy, X-ray diffraction for the analysis of the fibril morphology, and nuclear magnetic resonance experiments for local structure and molecular dynamics investigations. Combined with cell toxicity assays of the mutated Aβ(1-40) peptides, the physicochemical and biological importance of the early folding contact between F19 and L34 in Aβ(1-40) is underlined. This implies that the F19-L34 contact influences a broad range of different processes including the initiation of fibrillation, oligomer stability, fibril elongation, local fibril structure, and dynamics and cellular toxicity. These processes do not only cover a broad range of diverse mechanisms, but also proved to be highly sensitive to minor modulations of this crucial contact. Furthermore, our work shows that the contact is not simply mediated by general hydrophobic interactions, but also depends on stereospecific mechanisms.

Glycation induces conformational changes in the amyloid-β peptide and enhances its aggregation propensity: molecular insights

The cytotoxicity of the amyloid beta (Aβ) peptide, implicated in the pathogenesis of Alzheimer's disease (AD), can be enhanced by its post-translational glycation, a series of non-enzymatic reactions with reducing sugars and reactive dicarbonyls. However, little is known about the underlying mechanisms that potentially enhance the cytotoxicity of the advanced glycation modified Aβ. In this work, fully atomistic molecular dynamics (MD) simulations are exploited to obtain direct molecular insights into the process of early Aβ self-assembly in the presence and absence of glycated lysine residues. Analyses of data exceeding cumulative timescales of 1 microsecond for each system reveal that glycation results in a stronger enthalpy of association between Aβ monomers and lower conformational entropy, in addition to a sharp overall increase in the beta-sheet content. Further analyses reveal that the enhanced interactions originate, in large part, due to markedly stronger, as well as new, inter-monomer salt bridging propensities in the glycated variety. Interestingly, these conformational and energetic effects are broadly reflected in preformed protofibrillar forms of Aβ small oligomers modified with glycation. Our combined results imply that glycation consolidates Aβ self-assembly regardless of its point of occurrence in the pathway. They provide a basis for further mechanistic studies and therapeutic endeavors that could potentially result in novel ways of combating AGE related AD progression.

12-Crown-4 Ether Disrupts the Patient Brain-Derived Amyloid-β-Fibril Trimer: Insight from All-Atom Molecular Dynamics Simulations

Recent experimental data elucidated that 12-crown-4 ether molecule can disrupt Aβ40 fibrils but the mechanism of disruption remains elusive. We have performed a series of all-atom molecular dynamics simulations to study the molecular mechanism of Aβ40 fibril disruption by 12-crown-4. In the present study we have used the Aβ40 fibril trimer as it is the smallest unit that maintains a stable U-shaped structure, and serves as the nucleus to form larger fibrils. Our study reveals that 12-crown-4 ether can enter into the hydrophobic core region and form competitive, hydrophobic interactions with key hydrophobic residues; these interactions break the intersheet hydrophobic interactions and lead to the opening of the U-shaped topology and a loss of β-sheet structure. Furthermore, we observed periods of time when 12-crown-4 was in the hydrophobic core and periods of time when it interacted with Lys28 (chain C), a "tug of war"; the 12-crown-4 binding with Lys28 destabilizes the salt-bridge between Asp23 and Lys28. In addition to the two aforementioned binding modes, the 12-crown-4 binds with Lys16, which is known to form a salt-bridge with Glu22 in antiparallel arranged Aβ fibrils. Our results are in good agreement with experimental results and suggest that molecules that have the ability to interact with both the hydrophobic core region and positively charged residues could serve as potential inhibitors of Aβ fibrils.

Hydralazine inhibits amyloid beta (Aβ) aggregation and glycation and ameliorates Aβ1–42 induced neurotoxicity

Top panel (red) depicts the formation beta sheet rich neurotoxic amyloid aggregates and bottom panel (blue) shows disordered non toxic amyloid aggregates formation upon hydralazine treatment.