Nanoprobing of the Effect of Cu2+ Cations on Misfolding, Interaction and Aggregation of Amyloid β Peptide

Misfolding and aggregation of the amyloid beta-protein (A beta) are hallmarks of Alzheimer's disease. Both processes are dependent on the environmental conditions, including the presence of divalent cations, such as Cu2+. Cu2+ cations regulate early stages of A beta aggregation, but the molecular mechanism of Cu2+ regulation is unknown. In this study we applied single molecule AFM force spectroscopy to elucidate the role of Cu2+ cations on interpeptide interactions. By immobilizing one of two interacting A beta 42 molecules on a mica surface and tethering the counterpart molecule onto the tip, we were able to probe the interpeptide interactions in the presence and absence of Cu2+ cations at pH 7.4, 6.8, 6.0, 5.0, and 4.0. The results show that the presence of Cu2+ cations change the pattern of A beta interactions for pH values between pH 7.4 and pH 5.0. Under these conditions, Cu2+ cations induce A beta 42 peptide structural changes resulting in N-termini interactions within the dimers. Cu2+ cations also stabilize the dimers. No effects of Cu2+ cations on A beta-A beta interactions were observed at pH 4.0, suggesting that peptide protonation changes the peptide-cation interaction. The effect of Cu2+ cations on later stages of A beta aggregation was studied by AFM topographic images. The results demonstrate that substoichiometric Cu2+ cations accelerate the formation of fibrils at pH 7.4 and 5.0, whereas no effect of Cu2+ cations was observed at pH 4.0. Taken together, the combined AFM force spectroscopy and imaging analyses demonstrate that Cu2+ cations promote both the initial and the elongation stages of A beta aggregation, but protein protonation diminishes the effect of Cu2+.