An ultrastructural study of amyloid intermediates in A beta(1-42) fibrillogenesis

Many attempts have been made to define early stages and intermediates in am yloid fibrillogenesis that may be susceptible to inhibition. We have develo ped an in vitro system based on the use of A beta(1-42) peptides, in which the development of prestages of protofilaments and protofilament and fibril formation could, for the first time, be followed by electron microscopy, s upported by fluorescence spectrometry. The first recognizable ultrastructur es after incubation of A beta(1-42) peptides at 37 degrees C were globular subunits (4-5nm in diameter) that gradually became organized into short pro tofilaments (30-100 nm), which in turn formed fibrils mainly by lateral ass ociation. At this stage, part of the protofilaments were seen first as coll aterals protruding from the fibrils and then, as they were gradually incorp orated, as buds on the fibril surface. A continuous growth of A beta(1-42) fibrils was observed, seemingly originating from a nucleus, which appeared to consist of aggregates of amyloid intermediates. That protofilaments are intermediates also in the in vivo formation of amyloid was supported by the finding that AL fibrils isolated from amyloid tissues also exhibited radia ting protofilaments. The demonstrated globular subunits and early formed pr otofilaments may be attractive targets for inhibition of fibril formation.