Direct Observation of Amyloid-β 42 Oligomer, Fibril, and Amorphous Aggregate Growth and Dissolution at Biomimetic Concentrations.

The aggregation pathways of Aβ42 peptides are complex and can lead to both amyloids and nonamyloid aggregates. We use atomic force microscopy imaging to monitor the assembly of aggregate structures and their dynamics. Two aggregation pathways emerge, one leading to amyloid fibrils and a second one that includes the formation of oligomers and apparently amorphous aggregates, which we identify as nonamyloid. Whereas the fibrils seem to require elevated peptide concentration to nucleate and grow, oligomers and amorphous aggregates form at near-physiological peptide concentrations. On the time scales of the experiments, the two aggregation pathways do not cross: the oligomers and aggregates do not participate in the fibrillization pathway and, analogously, secondary nucleation assisted by mature fibrils does not produce misfolded aggregates. We show that distinct Aβ42 fibril polymorphs form and coexist under identical conditions. Mature fibrils serve as substrates for secondary nucleation that leads to forked, branched, and thicker fibrils and, importantly, produces new fibril fragments. Aβ42 fibrils accumulate structural defects, with more defects generated at higher peptide concentrations. The defects lead to substantial variations of growth rate both over time and between different fibrils. The average growth rates of Aβ42 fibrils are about 50-fold faster than those of Aβ40 fibrils. Our findings are consistent with the basic premise of the polymorph selection hypothesis, according to which the late onset of Alzheimer's disease, its high clinical variability, and the presence of amyloid plaques in healthy individuals have their origins in differing toxicities and aggregation kinetics of distinct Aβ structural polymorphs.