Mechanism-selective inhibition of α-synuclein aggregation by the chaperone-like BRICHOS domain

Current therapeutic approaches for Parkinson's disease (PD) and other synucleinopathies alleviate symptoms but fail to effectively prevent disease progression. As a result, there is an increasing focus on alternative disease-modifying strategies where molecular chaperones are emerging candidates. Recently, the chaperone-like Bri2 BRICHOS domain has been shown to be a promising therapeutic candidate, inhibiting amyloid formation and associated toxicity of multiple amyloidogenic proteins including human α-synuclein (αSyn). To advance the development of Bri2 BRICHOS as a therapeutic, in vivo tests in mice are necessary, which commonly rely on injections of pre-formed fibrils of mouse αSyn. Here, we investigate the inhibitory mechanism of Bri2 BRICHOS on mouse αSyn aggregation and fibril interaction. In contrast to previous results on human αSyn, we found that Bri2 BRICHOS exhibits a very modest inhibitory effect on mouse αSyn aggregation, which is only observed under gentle shaking or quiescent conditions. While Bri2 BRICHOS binds with similar affinities to the respective fibrils, we observed that differences in the underlying nucleation mechanisms of mouse vs human αSyn fibril formation explain the impaired suppression of mouse αSyn fibrillation under strong shaking conditions. The more fragile nature of mouse αSyn fibrils causes stronger contributions of fibril fragmentation processes compared to surface-catalyzed secondary nucleation - the dominant nucleation mechanism for human αSyn. In conclusion, these findings provide molecular insights into the mechanism-of-action of Bri2 BRICHOS-mediated inhibition of αSyn aggregation as a selective chaperone-based inhibitor of surface-catalyzed secondary nucleation pathways, which facilitates informed choices of in vivo model systems for future treatment studies.