Heterogeneous aggregation of amyloid-β 42 from single-molecule spectroscopy

Protein aggregation is implicated as the cause of pathology in various diseases such as Alzheimers and Parkinsons disease. Polymorphism in the structure of fibrils formed by aggregation suggests the existence of many different assembly pathways and therefore a heterogeneous ensemble of soluble oligomers. Characterization of this heterogeneity is the key to understanding the aggregation mechanism and toxicity of specific oligomers, but in practice it is extremely difficult because oligomers cannot be readily separated. Here, we investigate highly heterogeneous oligomerization and fibril formation of the 42-residue amyloid-{beta} peptide (A{beta}42). We developed and used new single-molecule fluorescence spectroscopic and fluorescence lifetime imaging methods, combined with deep learning for image analysis. We found that the concentration of oligomers, including dimers, is extremely low and that the dimer is conformationally diverse. Aggregation to form fibrils is also highly heterogeneous in terms of the number of strands in a fibril and the elongation speed and conformation of fibrils. This heterogeneity in all stages of aggregation explains diverse and sometimes irreproducible results of experimental studies of amyloid-{beta}. Based on our observations and analysis, we propose a new model for aggregation of A{beta}42.