Distinct autophagy impairment mechanisms of huntingtin aggregates with different polyQ lengths.

Huntington's disease (HD) is characterized by the aggregation of mutant huntingtin (mHTT) containing elongated polyglutamine (polyQ) tracts. mHTT aggregates that fail to be cleared by autophagy cause neurotoxicity. While the polyQ length in patients with HD ranges from 40 to over 90 repeats, how these varying polyQ lengths affect autophagy impairment remains unclear. Using polyQ aggregation sensors based on bimolecular fluorescence complementation (BiFC), we uncovered distinct autophagy impairment mechanisms: PolyQ103 aggregates evade recognition by autophagy receptor SQSTM1/p62, whereas polyQ43 condensates are recognized by SQSTM1/p62, but their bulky association prevents complete autophagosome formation. Interestingly, overexpression of optineurin (Optn), another autophagy receptor, preferentially binds to polyQ103 aggregates but not polyQ43 condensates, improving cell survival. K63-ubiquitination on polyQ103 aggregates serves as a critical determinant for Optn recruitment via its UBAN domain. These findings reveal polyQ length-dependent pathological mechanisms underlying autophagy impairment of mHTT aggregates, suggesting potential therapeutic strategies for patients with longer polyQ sequences.