Spherulites of A[beta]~42~ in vitro and in Alzheimer's disease

Abstract

AbstractSeveral amyloid-forming proteins and peptides, including insulin^1^, [beta]-lactoglobulin^2^ and albumin^3^, form spherulites in vitro under non-physiological solution conditions. These micrometer-sized, roughly spherical structures are composed of ordered arrays of [beta] sheets of amyloid fibrils in radial arrangements which, characteristically, show a typical Maltese cross pattern of light extinction under the polarizing microscope. The physiological significance, if any, of these amyloid super assemblies is unknown although in Alzheimer's disease there is the suggestion that senile plaques composed primarily of [beta] sheets of A[beta]~42~ are spherulitic^4^. Herein we describe the first observation of the formation in vitro of spherulites of A[beta]~42~. They were formed under near-physiological conditions in which the [beta] sheet conformation of pre-formed aggregates of A[beta]~42~ had been abolished following the addition of an excess of copper. Incubation of these preparations at 37^o^C for up to 9 months resulted in the formation of spherulites. These were globular in appearance, 5 - 20 microns in diameter, and exhibited the typical Maltese cross pattern of light extinction. Similarly to other amyloid spherulites formed in vitro they bound Congo red without giving apple-green birefringence^5^ while also being thioflavin T-positive when viewed by fluorescence microscopy^3^. Near-identical spherulitic structures were also observed in abundance in 30 micron 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 A[beta]~42~ 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 involves copper.