The formation of neurotoxic aggregates by amyloid-β peptide (Aβ) is considered to be a key step in the onset of Alzheimer's disease. It is widely accepted that oligomers are more neurotoxic than amyloid fibrils in the aqueous-phase aggregation of Aβ. Membrane-mediated amyloidogenesis is also relevant to the pathology, although the relationship between the aggregate size and cytotoxicity has remained elusive. Here, aggregation processes of Aβ on living cells and cytotoxic events were monitored by fluorescence techniques. Aβ formed amyloids after forming oligomers composed of ≈10 Aβ molecules. The formation of amyloids was necessary to activate apoptotic caspase-3 and reduce the ability of the cell to proliferate; this indicated that amyloid formation is a key event in Aβ-induced cytotoxicity.

Aggregational states of amyloid β-protein (Aβ) are critical for its neurotoxicity, although they are not wellcharacterized, particularly after binding to the cell membranes. This is one reason why the mechanisms of Aβ neurotoxicity are controversial and elusive. In this study, the effects of toxic Aβ-(1− 42) fibrils formed in the membrane on cellular processes were investigated using human neuroblastoma SH-SY5Y cells. Consistent with previous observations, fibrillar Aβs formed on the membranes induced activation of caspase-3, the effector caspase for apoptosis. Knockdown analyses of the initiator caspases, caspase-8 and caspase-9, indicated that the apoptosis was induced via activation of caspase-8, followed by activation of caspase-9 and caspase-3. We also found that inflammation signaling pathways including Toll-like receptors and inflammasomes NOD-, LRR-, and pyrin domain-containing protein 3 are involved in the initiation of apoptosis by the Aβ fibrils. These inflammation-related molecules are promising targets for the prevention of apoptotic cell death induced by Aβ.

Both peptides were a-helical in dry state and gradually transitioned to intramolecular and then to intermolecular b-sheet upon exposure to vapor. The onset of b-sheet formation was delayed in AbpE but proceeded rapidly once triggered. AbpE inhibited the transition of Ab to intermolecular b-sheet structure, while Ab shifted the structure of AbpE towards more b-turn and less intermolecular b-sheet structure. Thus, each peptide inhibits intermolecular b-sheet formation in the other. This may explain the increased toxicity of oligomeric Ab þ AbpE peptide combinations as fibrils are less toxic. Additionally, the rapid formation and retention of b-turn structure in AbpE may facilitate its increased toxicity.