Stable Amyloid Oligomers can Seed Fibril Growth Near Physiological Conditions

Abstract

Spontaneous formation and deposition of protein fibrils with cross beta-sheet structure are associated with an increasing number of human disorders including Alzheimer's disease, senile systemic or dialysis-related amyloidosis. The morphological and structural similarities of amyloid deposits and their various intermediates suggest that amyloid diseases share basic aspects of their etiology. Elucidating the molecular mechanisms underlying these similarities has been challenging since biophysical studies of fibril formation in vitro require partial denaturation of native proteins. These conditions are far from the physiological environment relevant to fibril growth in vitro. Here we show that metastable amyloid oligomers and protofibrils of lysozyme, transiently formed under denaturing conditions, can become stabilized at physiological conditions. In addition, they are capable of inducing the conversion of natively folded, monomeric lysozyme into aggregated β-sheet rich fibrils in vitro. There was no discernible threshold concentration of lysozyme monomers below which seeding didn't induce growth.Our data suggest that only the nucleation of amyloid oligomers is a low-probability event requiring partially denaturing conditions. Once formed, these seeds can become stabilized under near physiological conditions and induce seed elongation and fragmentation. The stabilization of amyloidogenic intermediates away from nucleation conditions, their ability to remain soluble or to seed subsequent fibril growth of native monomers appear to be protein-specific. The oligomer/protofibril-induced growth of native lysozyme observed in our experiments bears intriguing similarities with the ability of prion proteins to drive the autocatalytic conversion of their native counterpart into amyloid fibrils.