Spherulites of Amyloid-β42 In Vitro and in Alzheimer's Disease

Abstract

Several amyloidogenic proteins including insulin, beta-lactoglobulin, and albumin form spherulites in vitro under non-physiological conditions. These micrometer-sized, roughly spherical structures are composed of ordered arrays of amyloid fibrils in radial arrangements which, characteristically, show a typical Maltese cross pattern of light extinction under the polarizing microscope. The physiological significance of amyloid spherulites is unknown though in Alzheimer's disease, senile plaques composed primarily of beta sheets of amyloid-beta (Abeta)42 have, very occasionally, been shown to give a Maltese cross pattern of light extinction under crossed polarizers. Herein we describe the first observation of the formation in vitro of spherulites of Abeta42. They were formed under near-physiological conditions in which the beta sheet conformation of pre-formed aggregates of Abeta42 had been abolished following the addition of an excess of copper. Incubation of these preparations at 37 degrees C for up to 9 months resulted in the formation of globular structures, 5-20 microm in diameter, which exhibited a Maltese cross pattern of light extinction typical of spherulites. Near-identical spherulitic structures were also observed in abundance in 30 microm thick sections of Alzheimer's disease brain tissue. Synchrotron x-ray fluorescence showed that the location of these spherulites in AD tissue coincided with locally elevated concentrations of tissue copper. The formation in vitro of spherulites of Abeta42 which morphologically appeared analogous to spherulitic structures observed in vivo strongly supports the hypothesis that spherulites and senile plaques in AD tissue are one and the same structures and that their ultimate formation may involve copper.