Spherulites of Aβ42 in vitro and in Alzheimer’s disease

Several amyloid-forming proteins and peptides, including insulin 1 , β-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 β 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 β sheets of Aβ 42 are spherulitic 4. Herein we describe the first observation of the formation in vitro of spherulites of Αβ 42. They were formed under near-physiological conditions in which the β sheet conformation of pre-formed aggregates of Aβ 42 had been abolished following the addition of an excess of copper. Incubation of these preparations at 37°C for up to 9 months resulted in the formation of spherulites. These were globular in appearance, 5 – 20 μm 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