Dynamics of Inter-Molecular Interactions Between Single Aβ42 Oligomeric and Aggregate Species by High-Speed Atomic Force Microscopy.

Within the amyloid hypothesis in Alzheimer's disease, current focus has shifted to earlier stages of amyloid beta (Aβ) peptide assembly, involving soluble oligomers and smaller aggregates, which are more toxic to cells compared to their morphological distinct fibril forms. Critical to the Aβ field is unlocking the molecular-level kinetic pathways of oligomerization, leading to the culprit subset or specific species of Aβ oligomer populations responsible for the disease etiology. Here, we apply high-speed atomic force microscopy to enable direct visualization of dynamic interactions between single Aβ42 oligomers and aggregate forms, with combined nanometre structural and millisecond temporal resolution in liquid. Analysis of dimensions revealed up to three main Aβ42 species distributions, in addition to the appearance of monomers that showed fast surface diffusion compared to the larger Aβ42 species. Significantly, we devised a new single-molecule analysis based on image contrast in high-speed atomic force microscopy movies to quantify rate determining kinetic constants for interactions between the different Aβ42 species. The findings revealed that smaller Aβ42 species show an exponential decay of lifetime distribution, indicating that all molecules undergo the same process with a single well-defined energy barrier. In contrast, larger aggregates show randomized lifetimes, indicating a distribution of interactions energies/barriers that must be overcome in order to dissociate. We interpret the latter as being due to "permissive" binding, arising from different conformation states of the aggregates, along with a variety of accessible interacting groups. Inevitably, this may lead to the formation of different complexes or alloforms, which is known to contribute to difficulties in identifying Aβ oligomer toxicity and has implications for mechanisms underlying neuronal death accompanying Alzheimer's disease.