Amyloid-β forms fibrils by nucleated conformational conversion of oligomers

Abstract

FlAsH fluorescence and thioflavin-to-FlAsH FRET are used to distinguish amyloid-β oligomer formation from fibril formation, supporting rapid oligomer formation prior to fibril formation—consistent with a nucleated conformational conversion mechanism—that can be modulated by certain Alzheimer's disease–linked mutations or lipids. Amyloid-β amyloidogenesis is reported to occur via a nucleated polymerization mechanism. If this is true, the energetically unfavorable oligomeric nucleus should be very hard to detect. However, many laboratories have detected early nonfibrillar amyloid-β oligomers without observing amyloid fibrils, suggesting that a mechanistic revision may be needed. Here we introduce Cys-Cys-amyloid-β1–40, which cannot bind to the latent fluorophore FlAsH as a monomer, but can bind FlAsH as an nonfibrillar oligomer or as a fibril, rendering the conjugates fluorescent. Through FlAsH monitoring of Cys-Cys-amyloid-β1–40 aggregation, we found that amyloid-β1–40 rapidly and efficiently forms spherical oligomers in vitro (85% yield) that are kinetically competent to slowly convert to amyloid fibrils by a nucleated conformational conversion mechanism. This methodology was used to show that plasmalogen ethanolamine vesicles eliminate the proteotoxicity-associated oligomerization phase of amyloid-β amyloidogenesis while allowing fibril formation, rationalizing how low concentrations of plasmalogen ethanolamine in the brain are epidemiologically linked to Alzheimer's disease.