Competing chaperone pathways in α-synuclein disaggregation and aggregation dynamics

The aggregation of the protein α-synuclein into amyloid fibrils and their subsequent deposition into large proteinaceous inclusions is a pathological hallmark of several neurodegenerative diseases, including Parkinson's disease. Molecular chaperones, including the small heat shock proteins (sHsps) and the Hsp70 chaperone system, are known to interact with α-synuclein fibrils, preventing further aggregation and disaggregating fibrillar species respectively. However, it remains unclear if sHsps co-operate with the Hsp70 chaperones to potentially improve the kinetics or effectiveness of Hsp70-mediated disaggregation and how disaggregation kinetics are influenced by aggregation-prone α-synuclein monomers. Using thioflavin-T assays, we demonstrate that the sHsps Hsp27 (HSPB1) and αB-crystallin (HSPB5) do not synergise with the Hsp70 chaperones during α-synuclein seed fibril disaggregation. Moreover, the addition of monomeric α-synuclein with fibril seeds results in increased aggregation that overwhelms Hsp70-mediated disaggregation. Upon addition of sHsps to this system, antagonism between the two chaperone classes is observed, likely due to these chaperones competing for productive binding to the ends of α-synuclein fibrils. Overall, these results suggest that while Hsp70 and sHsp chaperones are independently capable of binding to and inhibiting fibril elongation, they do not have synergistic effects on disaggregation. Furthermore, Hsp70-mediated disaggregation is ineffectual in the presence of physiological concentrations of α-synuclein monomers, conditions that actually lead to further α-synuclein aggregation. Overall, these data may offer insight into factors that lead to the failure of the Hsp70 chaperones to clear cells of α-synuclein aggregates that leads to neurodegenerative disease.