Role of aggregation conditions in structure, stability, and toxicity of intermediates in the Aβ fibril formation pathway

beta-amyloid peptide (A beta) is one of the main protein components of senile plaques associated with Alzheimer's disease (AD). A beta readily aggregates to forms fibrils and other aggregated species that have been shown to be toxic in a number of studies. In particular, soluble oligomeric forms are closely related to neurotoxicity. However, the relationship between neurotoxicity and the size of Ab aggregates or oligomers is still under investigation. In this article, we show that different A beta incubation conditions in vitro can affect the rate of A beta fibril formation, the conformation and stability of intermediates in the aggregation pathway, and toxicity of aggregated species formed. When gently agitated, A beta aggregates faster than A beta prepared under quiescent conditions, forming fibrils. The morphology of fibrils formed at the end of aggregation with or without agitation, as observed in electron micrographs, is somewhat different. Interestingly, intermediates or oligomers formed during A beta aggregation differ greatly under agitated and quiescent conditions. Unfolding studies in guanidine hydrochloride indicate that fibrils formed under quiescent conditions are more stable to unfolding in detergent than aggregation associated oligomers or A beta fibrils formed with agitation. In addition, A beta fibrils formed under quiescent conditions were less toxic to differentiated SH-SY5Y cells than the A beta aggregation associated oligomers or fibrils formed with agitation. These results highlight differences between A beta aggregation intermediates formed under different conditions and provide insight into the structure and stability of toxic A beta oligomers.