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

Amyloid beta -protein (A beta) 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 A beta assembly would facilitate the targeting and design of fibrillogenesis inhibitors. Prior studies have shown that A beta fibrillogenesis involves conformational changes leading to the formation of extended beta -sheets and that an alpha -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, alpha -helix-containing assembly is a key step in A beta fibrillogenesis. The generality of this phenomenon was supported by conformational studies of 18 different A beta peptides, including wild-type A beta (1-40) and A beta (1-42), biologically relevant truncated and chemically modified AP peptides, and A beta peptides causing familial forms of cerebral amyloid angiopathy. Without exception, fibrillogenesis of these peptides involved an oligomeric alpha -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, A beta peptides containing Asp --> Asn or His --> Gln substitutions showed altered fibrillogenesis kinetics. These data emphasize the importance of the dynamic interplay between A beta monomer conformation and oligomerization state in controlling fibrillogenesis kinetics. (C) 2001 Academic Press.