Misfolding and Aggregation of Amyloid Beta Peptide: Single Molecule AFM Force Spectroscopy

Abstract

Misfolding and aggregation of amyloid beta (Aβ) peptide result in development of Alzheimer's disease, and Aβ dimers are considered as the smallest neurotoxic species. The aggregates formed by Aβ-peptides have been characterized by various techniques, but our knowledge on the molecular mechanism underlying the processes of misfolding and the early stages of aggregation of the peptides is limited. We have shown earlier that AFM force spectroscopy is capable of detecting protein misfolded states and characterizing the initial stages of the protein aggregation. Importantly, we showed that α-synuclein dimers are stable transient states playing role of triggers in the process of the protein self-assembly in nanopartiles and fibrils. Here we applied the sample methodology to probing and characterizing of misfolding of Aβ40 peptide. The protein was immobilized on the AFM tip and the surface and the interaction between the proteins was measured in multiple approach-retraction cycles. Using this approach was able to analyze interprotein interactions at single molecule level. The force spectroscopy analysis provided us with the following important information. First, using Dynamic Force Spectroscopy (DFS) approach we characterized pathways of Aβ40 misfolding. The lifetimes of transient Aβ40 dimers can be as long as several seconds suggesting that formation of the states with such a lifetimes can trigger the aggregation. Second, the pathways for the misfolding and aggregation depend on pH leading to a rather complex energy landscape reconstructed from the DFS data. Third, the analysis of the contour lengths supported the conclusions on various aggregation pathways and led to the models for such conformations of Aβ40.This work is supported by DE-FG02-08ER64579 grant from DOE.