Lipid Composition Drives Mutant Huntingtin Dimerization and Membrane Association: Insights from Computational Simulations.

Huntington's disease (HD) is a neurodegenerative disorder caused by the expansion of the CAG trinucleotide in the exon 1 of the huntingtin gmodellerene. This abnormal expansion produces a mutant huntingtin (mHTT) protein with extended polyglutamine (polyQ) tracts. Although the molecular mechanisms underlying HD onset and progression remain poorly understood, aberrant folding, aggregation, and membrane interactions of mHTT are considered central to disease pathogenesis. In this study, we used molecular dynamics (MD) simulations to investigate the structural properties, dimerization propensity, and membrane lipid interaction of mHTT carrying 70 polyQ repeats (mHTT-Q70). Our analyses revealed that mHTT-Q70 retains partially structured α-helical conformations with increased flexibility within the polyQ domain, thus being predisposed to misfolding. Coarse-grained MD simulations further revealed a strong tendency of mHTT-Q70 to dimerize, indicating that early oligomerization may represent a critical step in protein aggregation. Interestingly, we show that membrane cholesterol content dose-dependently promotes dimeric mHTT-Q70-but not monomeric mHTT-Q70-association with neuronal membrane models, which was observed for 70% of simulation time at 40% cholesterol content. Such a cholesterol-dependent membrane binding of dimeric mHTT-Q70 suggests that membrane lipid composition may represent a critical checkpoint in the early stages of mHTT-Q70 aggregation, and of cytotoxicity thereof. Moreover, distinct neuronal membrane lipids like phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine differently contributed to mHTT-Q70 binding, highlighting the complexity of such a lipid-dependent modulation. Taken together, these findings underscore the dynamic interplay between polyQ-driven misfolding, dimerization, and membrane lipids in HD pathogenesis, suggesting that modulation of membrane composition, and in particular of cholesterol levels, may be a novel action point to design therapeutic drugs for HD.