THE TOXICITY OF THE ALZHEIMERS BETA-AMYLOID PEPTIDE CORRELATES WITH ADISTINCT FIBER MORPHOLOGY

In an attempt to elucidate the relationship among aggregation properti es, fiber morphology, and cellular toxicity several beta-amyloid pepti des (A beta) were prepared according to a standardized procedure. Pept ides either carried mutations inside the membrane anchor segment aroun d amino acid position 35 or their carboxy terminus was shortened from 42 to 41, 40, or 39 amino acids. The time-dependent self-assembly of m onomeric A beta into fibers was simultaneously monitored by electron m icroscopy, circular dichroism spectroscopy, analytical ultracentrifuga tion, and A beta-mediated cellular toxicity using the reduction of [4, 5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) to measu re cell viability, The transition of A beta monomers into fibers was a nalyzed by more than 600 electron micrographs. Distinct morphological changes from seed-like structures to immature and mature fibers were o bserved. Seeds were of spherical appearance, Immature fibers were typi cally elongated structures with a rough surface and with varying thick ness depending on the A beta sequence. Mature fibers were characterize d by a periodic variation of their thickness along the fiber axis. The proportion of these different structures and the total amount of aggr egated A beta was amino acid sequence-dependent. Wild-type A beta(1-42 ) and its oxidized derivative carrying a methionine sulfoxide residue at position 35 showed the highest rate of fiber formation and exerted toxic activity in the MTT assay at very low nanomolar concentrations. The fibers formed by these two peptides were predominantly of the matu re type. In contrast, carboxyl-terminus truncated peptides A beta(1-41 ), A beta(1-40), and A beta(1-39) or most A beta(1-42) derivatives mut ated around amino acid position 35 showed a reduced aggregation rate, the immature fibers predominated, and the toxicity was orders of magni tude lower. Thus, a correlation can be drawn among the chemical struct ure, aggregation properties, fiber morphology; and cellular toxicity. (C) 1997 Academic Press.