Spontaneous in vitro formation of supramolecular β-amyloid structures, "βamy balls", by β-amyloid 1-40 peptide

The concentration of beta -amyloid peptide (A beta), x-42 or x-40 amino acids long, increases in brain with the progression Alzheimer's disease (AD). These peptides are deposited extracellularly as highly insoluble fibrils that form densities of amyloid plaques. A beta fibrillization is a complex polymerization process preceded by the formation of oligomeric and prefibrillar A beta intermediates. In some of our in vitro studies, in which the kinetics of intermediate steps of fibril formation were examined, we used concentrations of synthetic A beta that exceed what is normally employed in fibrillization studies, 300-600 muM. At these concentrations, in a cell free system and under physiological conditions, A beta 1-40 peptide (A beta 40) forms fibrils that spontaneously assemble into clearly defined spheres, "beta amy balls", with diameters of similar to 20-200 mum. These supramolecular structures show weak birefringence with Congo red staining and high stability with prolonged incubation times (at least 2 weeks) at 30 degreesC, freezing, and dilution in H2O. At 600 muM, they are detected after incubation for similar to 20 h. A beta peptide 1-42 (A beta 42) lacks the ability to form beta amy balls but accelerates A beta 40 beta amy ball formation at low stoichiometric levels (1:20 A beta 42: A beta 40 ratio). A beta 42 levels above this (=10-50% w/w) impede A beta 40 beta amy ball formation. Using light (LM) and electron microscopy (EM), this study examines the gross morphology and ultrastructure of A beta 40 beta amy balls and their time course of formation, in the absence and presence of A beta 42, along with some stability measures. As spheres of a misfolded protein, beta amy balls resemble both AD A beta senile plaques and neuronal inclusion bodies associated with other neurodegenerative diseases.