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

Amyloid beta -protein (A beta) assembly into toxic oligomeric and fibrillar structures is a seminal event in Alzheimer's disease, therefore blocking t his process could have significant therapeutic benefit. A rigorous mechanis tic understanding of A beta assembly would facilitate the targeting and des ign of fibrillogenesis inhibitors. Prior studies have shown that A beta fib rillogenesis involves conformational changes leading to the formation of ex tended 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 -heli x-containing assembly is a key step in A beta fibrillogenesis. The generali ty of this phenomenon was supported by conformational studies of 18 differe nt A beta peptides, including wild-type A beta (1-40) and A beta (1-42), bi ologically relevant truncated and chemically modified AP peptides, and A be ta 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 intermed iate and of fibrils was temporally correlated. The kinetics varied dependin g on amino acid sequence and the extent of peptide N- and C-terminal trunca tion. The pH dependence of helix formation suggested that Asp and His exert ed significant control over this process and over fibrillogenesis in genera l. 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 co nformation and oligomerization state in controlling fibrillogenesis kinetic s. (C) 2001 Academic Press.