The protective effect of cHASI is confirmed by cytotoxicity assays. This indicates that only oligomers produced during secondary surface nucleation disrupt membrane integrity.
Remarkably, when Aβ monomers are incubated with cHASI, although Aβ forms amyloid fibrils via primary nucleation and elongation, this pathway to fibrils does not damage the lipid bilayer. By incubating Aβ monomers with lipid vesicles, we show that during assembly of Aβ into amyloid fibrils, oligomers are formed that markedly disrupt the lipid bilayer. Here we use cHASI to efficiently inhibit surface‐catalyzed secondary nucleation process of Aβ in a lipid membrane environment. We have previously designed a short peptide called cyclic helical amyloid surface inhibitor (cHASI) that can selectively bind to the Aβ fibril surface. Pre‐fibril Aβ‐oligomers induced membrane disruption and are crucial to neurotoxicity. Inhibition of amyloid‐β peptide (Aβ) aggregation is a promising therapeutic strategy for Alzheimer's disease (AD), as Aβ aggregation is generally believed to trigger AD pathology. The concentration of monomeric Aβ42 was 3 μM. The insets show the relative numbers of oligomers generated during the aggregation reaction. The blue line corresponds to the situation in the absence of Brichos, and the green dashed lines show predictions for the cases in which primary nucleation (d), fibril elongation (e) or secondary nucleation (f) are each inhibited by the chaperone. (d–f) Time evolution of the nucleation rate calculated from the kinetic analysis. In schematics, C, chaperone blue spheres, Aβ42. The thin dotted lines in (c) are theoretical predictions for the reaction profiles at the intermediate Brichos concentrations using the association and dissociation rate constants determined for its binding by SPR (b). The green dashed lines show predictions for the resulting reaction profiles when primary nucleation (a), fibril elongation (b) and secondary nucleation (c) are inhibited by the chaperone (Supplementary ). The blue dashed line is the integrated rate law for Aβ42 aggregation in the absence of Brichos, according to the rate constants determined previously19. The effect of Brichos saturates at a stoichiometry of approximately one monomer equivalent of Aβ42 (Supplementary ).
The data show averages (points) and standard errors over five technical replicates.
(a–c) Reaction profiles for aggregation in the absence of Brichos and in the presence of 10%, 15%, 35%, 50%, 75% and 100% Aβ42 monomer equivalents of Brichos, from left (blue) to right (green) respectively. Kinetics of Aβ42 aggregation in the presence of Brichos. These results reveal that molecular chaperones can help maintain protein homeostasis by selectively suppressing critical microscopic steps within the complex reaction pathways responsible for the toxic effects of protein misfolding and aggregation. We verify that this mechanism occurs in living mouse brain tissue by cytotoxicity and electrophysiology experiments. We demonstrate in vitro that Brichos achieves this inhibition by binding to the surfaces of fibrils, thereby redirecting the aggregation reaction to a pathway that involves minimal formation of toxic oligomeric intermediates. Here we show that a molecular chaperone, a human Brichos domain, can specifically inhibit this catalytic cycle and limit human Aβ42 toxicity. Recent studies have revealed that once Aβ42 fibrils are generated, their surfaces effectively catalyze the formation of neurotoxic oligomers. Alzheimer's disease is an increasingly prevalent neurodegenerative disorder whose pathogenesis has been associated with aggregation of the amyloid-β peptide ( Aβ42).
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