Spherical aggregates of β-amyloid (amylospheroid) show high neurotoxicity and activate tau protein kinase I/glycogen synthase kinase-3β

Abstract

β-Amyloid (Aβ) acquires toxicity by self-aggregation. To identify and characterize the toxic form(s) of Aβ aggregates, we examined in vitro aggregation conditions by using large quantities of homogenous, chemically synthesized Aβ_1–40 peptide. We found that slow rotation of Aβ_1–40 solution reproducibly gave self-aggregated Aβ_1–40 containing a stable and highly toxic moiety. Examination of the aggregates purified by glycerol-gradient centrifugation by atomic force microscopy and transmission electron microscopy revealed that the toxic moiety is a perfect sphere, which we call amylospheroid (ASPD). Other Aβ_1–40 aggregates, including fibrils, were nontoxic. Correlation studies between toxicity and sphere size indicate that 10- to 15-nm ASPD was highly toxic, whereas ASPD 10 nm also appeared to exist when Aβ_1–42 formed ASPD by slow rotation. However, Aβ_1–42-ASPD formed more rapidly, killed neurons at lower concentrations, and showed ≈100-fold-higher toxicity than Aβ_1–40-ASPD. The toxic ASPD was associated with SDS-resistant oligomeric bands in immunoblotting, which were absent in nontoxic ASPD. Because the formation of ASPD was not disturbed by pentapeptides that break β-sheet interactions, Aβ may form ASPD through a pathway that is at least partly distinct from that of fibril formation. Inhibition experiments with lithium suggest the involvement of tau protein kinase I/glycogen synthase kinase-3β in the early stages of ASPD-induced neurodegeneration. Here we describe the identification and characterization of ASPD and discuss its possible role in the neurodegeneration in Alzheimer9s disease.