The coexistence of an equal amount of Alzheimer's amyloid-β 40 and 42 forms structurally stable and toxic oligomers through a distinct pathway.

Fibrillar amyloid-β (Aβ) is the major constituent of senile plaques in the brain of patients with Alzheimer's disease (AD). Aβ is a short peptide generated from amyloid precursor protein with two main isoforms, Aβ40 and Aβ42, with the latter having two additional hydrophobic residues at the C-terminus. The two isoforms have distinct characteristics, in which Aβ42 plays a more pathogenic role. Some early-onset familial AD cases possess an elevated Aβ42/Aβ40 level, and biochemical studies show the two species interact with each other. Therefore, understanding structural conversion in the aggregation of mixed Aβ isoforms is essential for elucidating AD pathogenesis. Here, we systematically examined the differences between Aβ42, Aβ40 and various Aβ42/Aβ40 mixtures by monitoring the fibrillization kinetics, epitope changes, assembly, morphology and induced cytotoxicity. We found that the minor Aβ species in different mixing ratios modulated the major aggregation pathway. Size exclusion chromatography, circular dichroism spectroscopy and photo-crosslinking assay showed that soluble Aβ42 oligomers were stabilized after Aβ40 addition, and the equimolar Aβ42/Aβ40 mixture rapidly formed spherical oligomers. These oligomers were the most toxic among those examined as evidenced by neurite degeneration and neuronal toxicity. However, the oligomers were not responsible for intracellular calcium elevation. Overall, our results demonstrated that differently mixed Aβ species repartitioned oligomer intermediates on the major aggregation pathway. Furthermore, the equimolar mixture rapidly formed structurally stable and the most toxic oligomers. These results provided information on the potential pathological mechanisms underlying the elevated Aβ42/Aβ40 ratio in familial AD patients and in the local environment of sporadic AD brains.