Identification and characterization of key kinetic intermediates in amyloid β-protein fibrillogenesis

Amyloid β-protein (Aβ) assembly into toxic oligomeric and fibrillar structures is a seminal event in Alzheimer’s disease, therefore blocking this process could have significant therapeutic benefit. A rigorous mechanistic understanding of Aβ assembly would facilitate the targeting and design of fibrillogenesis inhibitors. Prior studies have shown that Aβ fibrillogenesis involves conformational changes leading to the formation of extended β-sheets and that an α-helix-containing intermediate may be involved. However, the significance of this intermediate has been a matter of debate. We report here that the formation of an oligomeric, α-helix-containing assembly is a key step in Aβ fibrillogenesis. The generality of this phenomenon was supported by conformational studies of 18 different Aβ peptides, including wild-type Aβ(1–40) and Aβ(1–42), biologically relevant truncated and chemically modified Aβ peptides, and Aβ peptides causing familial forms of cerebral amyloid angiopathy. Without exception, fibrillogenesis of these peptides involved an oligomeric α-helix-containing intermediate and the kinetics of formation of the intermediate and of fibrils was temporally correlated. The kinetics varied depending on amino acid sequence and the extent of peptide N- and C-terminal truncation. The pH dependence of helix formation suggested that Asp and His exerted significant control over this process and over fibrillogenesis in general. Consistent with this idea, Aβ peptides containing Asp → Asn or His → Gln substitutions showed altered fibrillogenesis kinetics. These data emphasize the importance of the dynamic interplay between Aβ monomer conformation and oligomerization state in controlling fibrillogenesis kinetics.