Distinct recognition of mutant huntingtin aggregates by autophagy receptor SQSTM1/p62 versus optineurin has differential effects on cell survival

Huntington's disease (HD) is a neurodegenerative disease caused by a polyglutamine (polyQ) expansion in the N-terminus of huntingtin (HTT). Mutant HTT (mHTT) undergoes misfolding and tends to aggregate, yet these aggregates are not effectively cleared by autophagy thus contributing to neurotoxicity in HD. The polyQ length of HTT in patients with HD varies from 40 to > 90; however, the precise mechanisms of autophagy dysfunction for mHTT with varying polyQ lengths remain unclear. In this study, we developed new HTT-polyQ aggregation sensors based on bimolecular fluorescence complementation (BiFC) to monitor the real-time aggregation process of mHTT with varying polyQ lengths. Using BiFC-based aggregation sensors, we demonstrated that mHTT aggregation kinetics is faster with a longer polyQ length, suggesting a correlation between polyQ length and the onset age of HD. Interestingly, we discovered that the different aggregation kinetics of mHTT may determine the physical properties of the aggregates: mHTT-polyQ43 forms liquid-like protein condensates, whereas mHTT-polyQ103 generates tightly concentrated aggregates. Furthermore, mHTT aggregates with different physical states were selectively recognized by distinct autophagy receptors, which resulted in differential effects on cell viability. The liquid-like mHTT-polyQ43 condensates were recognized by SQSTM1/p62 but failed to proceed through autophagy thereby facilitating cytotoxicity. In contrast, mHTT-polyQ103 aggregates were selectively recognized by optineurin, which led to autophagic degradation and prolonged cell survival. Therefore, our results suggest that different therapeutic strategies should be considered for the HD patients with different polyQ lengths.