GM1 and OligoGM1 Drive Divergent α-Synuclein Aggregation Pathways

Parkinson's disease (PD) is an age-related disorder characterized by amyloid deposits of the 140-amino-acid protein α-synuclein (AS), which contribute to neuronal dysfunction and degeneration, leading to motor and cognitive impairments. AS fibrillation, the process by which the native soluble protein misfolds into insoluble cross-β-sheet fibrils, involves transient prefibrillar species with distinct biophysical features. <i>In vitro</i> evidence suggests that these oligomeric intermediates, rather than mature fibrils, are the primary pathogenic agents in neurodegenerative diseases. Initial studies demonstrated that AS fibril formation is inhibited upon binding to lipid vesicles containing GM1 ganglioside. Here, we report for the first time the effect of the oligosaccharide portion of GM1 on AS fibrillation under <i>in vitro</i> conditions. To assess the specific contribution of this hydrophilic headgroup, we synthesized the oligosaccharide moiety of GM1 (oligoGM1) and compared its effect with that of GM1 micelles. We show that oligoGM1 stimulates amyloid fibril formation, whereas GM1 reduces or inhibits AS aggregation in a lipid/protein ratio-dependent manner. AFM analysis revealed that fibrils formed in the presence of oligoGM1 were lower in height than control fibrils, and that structures compatible with AS oligomers accumulated in the presence of GM1. Using dot blotting and AFM, we further confirmed that GM1 promotes the formation of oligomeric AS species <i>in vitro</i>. ThT-based aggregation kinetics showed that AS aggregation in the presence of GM1 was markedly slower than with oligoGM1, supporting that oligoGM1 accelerates AS fibrillation. Finally, cytotoxicity assays in SH-SY5Y neuroblastoma cells demonstrated that oligomers generated in the presence of GM1 exhibited higher toxicity than AS fibrils formed in the absence or presence of oligoGM1. These findings highlight the importance of the hydrophobic lipid moiety of GM1 in modulating AS aggregation and support the emerging view that the extent of fibrillar amyloid deposition does not correlate directly with neurodegenerative disease pathogenesis.