Fragmentation-Induced Disassembly and Reaggregation of α-Synuclein Amyloid Fibrils

Aggregation of the protein α-synuclein (α-syn) is a defining pathological characteristic of Parkinson's disease (PD). Kinetic studies have provided increasingly detailed insights into the mechanisms of α-syn aggregation, highlighting the contributions of secondary nucleation and elongation to fibril growth. However, the understanding of the role of fibril breakage (fragmentation) remains sparse. We therefore established a modified thioflavin-T (ThT) kinetic assay in which ultrasonication steps were introduced when the conversion of α-syn monomers into amyloid fibrils had reached the plateau. This triggered, expectedly, fibril fragmentation but also rapid partial dissociation of the α-syn fibrils and subsequent elongation-dominated fibril regrowth, the kinetics of which could be monitored by ThT and were found to proceed until steady state was reestablished. Interestingly, the regrowth of α-syn variants A30P, E46K, and A53T, but not wild type or variant H50Q, resulted in significant increases in ThT fluorescence even though the residual monomer concentration at steady state was unaffected and no new monomers were added to the assayed systems. Furthermore, for these variants, which are all associated with early-onset PD, the residual monomer concentration was consistently higher than for wild-type α-syn and the late-onset variant H50Q, suggesting differences in monomer-fibril equilibria. Altogether, our study shows that α-syn amyloid fibrils are capable of undergoing structural evolution of a type that alters their ThT binding, highlights the role of fragmentation in expediating such maturation processes, and points out a putative connection between propensity of structural conversion, decreased fibril stability, and early onset of Parkinson's disease.