␤-Amyloid (A␤) acquires toxicity by self-aggregation. To identify and characterize the toxic form(s) of A␤ aggregates, we examined in vitro aggregation conditions by using large quantities of homogenous, chemically synthesized A␤1-40 peptide. We found that slow rotation of A␤1-40 solution reproducibly gave self-aggregated A␤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 A␤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 was nontoxic. A positive correlation between the toxicity and ASPD >10 nm also appeared to exist when A␤1-42 formed ASPD by slow rotation. However, A␤1-42-ASPD formed more rapidly, killed neurons at lower concentrations, and showed Ϸ100-fold-higher toxicity than A␤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 ␤-sheet interactions, A␤ 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 I͞gly-cogen synthase kinase-3␤ 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 Alzheimer's disease.A 40-to 42-residue peptide named ␤-amyloid (A␤) is a major constituent of senile plaques in Alzheimer's disease (AD) (1). Although multiple pathways have been suggested to lead to AD, recent advances indicate a causal link between A␤ and AD (2), and this idea is supported further by findings that vaccination against A␤ ameliorates behavioral deficits in transgenic mice (3-5). Among various in vivo A␤ species, A␤ 1-42 generally is considered as the primary vehicle of toxicity, whereas A␤ 1-40 , a major species under physiological conditions, is considered less harmful and more resistant to the formation of oligomers than A␤ 1-42 (6). However, it remains controversial which A␤ species contributes predominantly to AD pathogenesis, because both in vitro and in vivo studies have confirmed toxicity of A␤ 1-40 aggregates (7,8).It has been widely accepted that toxicity of A␤ requires aggregation of native A␤ monomers (9-11). Besides fibrils, several types of nonfibrillar aggregates have been reported: A␤ 1-40 oligomers from dimers-hexamers (6, 12-14); a mixture of A␤ 1-42 oligomers named A␤-derived diffusible ligands (ADDLs), ranging from trimers-hexamers up to 24-mers (15); and fibril intermediates named protofibrils (PFs) (16,17). All of these aggregates are mixtures of A␤ oligomers with a variety in oligomer size, and precise morphological analysis of each A␤ a...

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