Dopamine and Rotenone Modulate α‐Synuclein Phase Separation and Liquid to Solid Transition

ABSTRACT Liquid–liquid phase separation (LLPS) of α‐Synuclein (α‐Syn) is recognized as an early biophysical event driving pathological aggregation in Parkinson's disease (PD). Although 10%–15% of PD cases are due to familial mutations, the remaining cases are sporadic, often linked to various factors, like pesticides and metals. For example, rotenone and dopamine are known to be involved in PD pathology and are suggested to cause changes in α‐Syn protein homeostasis, although the mechanism by which they influence α‐Syn and cellular toxicity is largely unknown. In this work, we demonstrate that both dopamine and rotenone promote the LLPS of α‐Syn. Although rotenone promotes the liquid‐to‐solid transition almost instantaneously, dopamine, however, maintains a liquid state for a long time and rather delays the solidification process, unlike α‐Syn alone. Similarly, exposure to both toxicants resulted in faster LLPS in SH‐SY5Y cells. Interestingly, irrespective of their impact on material properties, rotenone promotes the faster formation of oligomers and amyloid fibrils, while dopamine exhibits oligomer formation and delayed fibrillation, both of which result in higher cytotoxicity. Our results provide new insight into the molecular processes underlying PD by indicating that endogenous dopamine stress and environmental toxicants converge on phase‐separation dynamics as a common mechanism of α‐Syn pathology.