Amyloid β-Protein Fibrillogenesis

Abstract

Alzheimer's disease is characterized by extensive cerebral amyloid deposition. Amyloid deposits associated with damaged neuropil and blood vessels contain abundant fibrils formed by the amyloid ␤-protein (A␤). Fibrils, both in vitro and in vivo, are neurotoxic. For this reason, substantial effort has been expended to develop therapeutic approaches to control A␤ production and amyloidogenesis. Achievement of the latter goal is facilitated by a rigorous mechanistic understanding of the fibrillogenesis process. Recently, we discovered a novel intermediate in the pathway of A␤ fibril formation, the amyloid protofibril (Walsh, D. M., Lomakin, A., Benedek, G. B., Condron, M. M., and Teplow, D. B. (1997) J. Biol. Chem. 272, 22364 -22372). We report here results of studies of the assembly, structure, and biological activity of these polymers. We find that protofibrils: 1) are in equilibrium with low molecular weight A␤ (monomeric or dimeric); 2) have a secondary structure characteristic of amyloid fibrils; 3) appear as beaded chains in rotary shadowed preparations examined electron microscopically; 4) give rise to mature amyloid-like fibrils; and 5) affect the normal metabolism of cultured neurons. The implications of these results for the development of therapies for Alzheimer's disease and for our understanding of fibril assembly are discussed. Alzheimer's disease (AD)1 is a progressive neurodegenerative disorder defined histologically by the formation in the brain of intracellular neurofibrillary tangles and extracellular amyloid deposits (1). Particular attention has been focused on the role that the amyloid ␤-protein (A␤), the primary protein constituent of amyloid deposits, plays in development of AD. A␤ molecules are fibrillogenic and exist in a number of forms in vivo (2). Among those forms found in amyloid deposits, 40 and 42 residue long species (A␤(1-40) and A␤(1-42), respectively) are particularly important. Genetic studies of AD have shown that mutations in the gene encoding the precursor of A␤ (the amyloid ␤-protein precursor (␤APP) gene) (3-6), or in genes that regulate the proteolytic processing of ␤APP (7-9), cause AD. The phenotypic effects of these mutations show remarkable consistency, they all result in excessive production of A␤ or in an increased A␤(1-42)/A␤(1-40) ratio, facilitating amyloid deposition (10, 11). In addition, specific haplotypes and mutations in genes involved in the extracellular transport or cleavage of A␤ are risk factors for AD (12,13). In vitro and in vivo studies of A␤ toxicity indicate that fibrillar A␤ can directly kill neurons or initiate a cascade of events leading to neuronal cell death (14 -16). For this reason, therapeutic strategies targeting A␤ fibrillogenesis are being pursued actively (17-20). Unfortunately, key areas of A␤ fibrillogenesis are poorly understood. In particular, the three-dimensional structure and organization of fibril subunits are unknown, as are the steps involved in assembly of nascent, monomeric A␤ first into nuclei, then into higher order oligo...

The Alzheimer’s β amyloid (Aβ1–39) monomer in an implicit solvent

“…The percentage of ordered residues in aqueous A␤ 39 in our simulations agrees reasonably with the NMR data 35,36 that estimate the amount of disordered structure to be 60%-80%, and ␣ helices to be 10%. However, our simulations did not reveal the origin of the 25% ␤ content found in CD studies for disordered A␤ 39 in water.…”

“…Ma and Nussinov 11 suggested that at high temperature fragments A␤ [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] and A␤ form a strand-loop-strand structure with parallel ␤ sheet, with an interior salt bridge between Asp23 and Lys28 residues as a major element of the fibril structure. On the other hand, computational studies using coarse-grained as well as atomistic atom models suggest the formation of antiparallel ␤ sheets by A␤ [16][17][18][19][20][21][22] subpeptides.…”

Structural properties of Aβ protofibrils stabilized by a small molecule

“…P rotofibrils and oligomers are metastable peptide assemblies observed during the growth of amyloid fibrils by a number of peptides, including the Alzheimer's amyloid plaque peptide A␤ (1)(2)(3). These oligomeric assemblies are important for at least two reasons.…”

“…First, it is now believed that such forms, rather than mature fibrils, may be the cytotoxic agents responsible for some amyloid-associated disorders like Alzheimer's and Parkinson's diseases (4,5). Second, it has been postulated that these structures may be intimately involved in the amyloid fibril assembly mechanism, both in amyloid nucleation and fibril elongation (2,3,6).…”

A nontransgenic mouse model shows inducible amyloid-β (Aβ) peptide deposition and elucidates the role of apolipoprotein E in the amyloid cascade

“…Recent findings suggest that brains of individuals with AD also contain soluble and neurotoxic A␤ oligomers, which seem to be an intermediate state in the A␤-aggregation cascade, whose downstream product is the senile plaque (3)(4)(5)(6)(7)(8)(9)(10). There is poor correlation between the severity of dementia and the density of amyloid plaques in AD (11)(12)(13); furthermore, in mice transgenic for the human A␤ precursor protein (APP), synaptic degeneration and cognitive decline are observed even before amyloid is deposited (14)(15)(16).…”