Polymerization of amyloid ␤-peptide (A␤) into amyloid fibrils is a critical step in the pathogenesis of Alzheimer's disease. Here, we show that peptides incorporating a short A␤ fragment (KLVFF; A␤ 16 -20 ) can bind full-length A␤ and prevent its assembly into amyloid fibrils. Through alanine substitution, it was demonstrated that amino acids Lys 16 , Leu 17 , and Phe 20 are critical for binding to A␤ and inhibition of A␤ fibril formation. A mutant A␤ molecule, in which these residues had been substituted, had a markedly reduced capability of forming amyloid fibrils. The present data suggest that residues A␤ 16 -20 serve as a binding sequence during A␤ polymerization and fibril formation. Moreover, the present KLVFF peptide may serve as a lead compound for the development of peptide and nonpeptide agents aimed at inhibiting A␤ amyloidogenesis in vivo.The preeminent neuropathological feature of Alzheimer's disease is the deposition of amyloid in the brain parenchyma and cerebrovasculature (1, 2). The basic components of the amyloid are thin fibrils of a peptide termed A␤ (3, 4). This peptide is a 40-to 42-amino acid-long proteolytic fragment of the Alzheimer amyloid precursor protein (APP), 1 a protein expressed in most tissues (5). Genetic and neuropathological studies provide strong evidence for a central role of A␤ amyloid in the pathogenesis of Alzheimer's disease (6), but the pathophysiological consequences of the amyloid deposition are unclear. However, it has been suggested that A␤ polymers and amyloid are toxic to neurons, either directly or via induction of radicals, and hence cause neurodegeneration (7-9). Previous studies indicate that A␤ polymerization in vivo and in vitro is a specific process that probably involves interactions between binding sequences in the A␤ peptide (10 -12). A rational pharmacological approach for prevention of amyloid formation would therefore be to use drugs that specifically interfere with A␤-A␤ interaction and polymerization. We hypothesized that ligands capable of binding to and blocking such sequences might inhibit amyloid fibril formation as outlined schematically in Fig. 1. Our strategy in searching for an A␤ ligand was to identify binding sequences in A␤ and then, based on their primary structures, synthesize a peptide ligand. Binding sequences were identified by systematically synthesizing short peptides corresponding to sequences of the A␤ molecule. The minimum length of an identified binding sequence was determined by truncating the peptide. Residues critical for binding were identified by alanine scanning. These critical residues were then substituted in an A␤ fragment (A␤ ) that normally is capable of forming amyloid fibrils (13,14) in order to determine if they indeed are important for A␤ amyloid fibril formation. Finally, it was determined if the identified ligand, in addition to binding to the A␤ molecule, was capable of inhibiting fibril formation of A␤ . EXPERIMENTAL PROCEDURES Materials-Synthetic A␤1-40 and all other soluble peptides were synthesized b...

The molecular structure of the amyloid fibril has remained elusive because of the difficulty of growing well diffracting crystals. By using a sequence-designed polypeptide, we have produced crystals of an amyloid fiber. These crystals diffract to high resolution (1 Å) by electron and x-ray diffraction, enabling us to determine a detailed structure for amyloid. The structure reveals that the polypeptides form fibrous crystals composed of antiparallel ␤-sheets in a cross-␤ arrangement, characteristic of all amyloid fibers, and allows us to determine the side-chain packing within an amyloid fiber. The antiparallel ␤-sheets are zipped together by means of -bonding between adjacent phenylalanine rings and salt-bridges between charge pairs (glutamic acid-lysine), thus controlling and stabilizing the structure. These interactions are likely to be important in the formation and stability of other amyloid fibrils.x-ray diffraction ͉ side-chain packing ͉ structure ͉ -bonding ͉ ␤-sheet interaction

Alzheimer’s disease is an increasingly prevalent neurodegenerative disorder whose pathogenesis has been associated with aggregation of the amyloid-β peptide (Aβ42). Recent studies have revealed that once Aβ42 fibrils are generated, their surfaces strongly catalyse the formation of neurotoxic oligomers. Here we show that a molecular chaperone, a Brichos domain, can specifically inhibit this catalytic cycle and limit Aβ42 toxicity. We demonstrate in vitro that Brichos achieves this inhibition by binding to the surfaces of fibrils, thereby redirecting the aggregation reaction to a pathway that involves minimal formation of toxic oligomeric intermediates. We verify that this mechanism occurs in living brain tissue by means of cytotoxicity and electrophysiology experiments. These results reveal that molecular chaperones can help maintain protein homeostasis by selectively suppressing critical microscopic steps within the complex reaction pathways responsible for the toxic effects of protein misfolding and aggregation.