Inhibition of calpain cleavage of huntingtin reduces toxicity: accumulation of calpain/caspase fragments in the nucleus.

Huntington's disease (HD) is a neurodegenerative disorder caused by a polyglutamine (polyQ) tract expansion near the N terminus of huntingtin (Htt). Proteolytic processing of mutant Htt and abnormal calcium signaling may play a critical role in disease progression and pathogenesis. Recent work indicates that calpains may participate in the increased and/or altered patterns of Htt proteolysis leading to the selective toxicity observed in HD striatum. Here, we identify two calpain cleavage sites in Htt and show that mutation of these sites renders the polyQ expanded Htt less susceptible to proteolysis and aggregation, resulting in decreased toxicity in an in vitro cell culture model. In addition, we found that calpain- and caspase-derived Htt fragments preferentially accumulate in the nucleus without the requirement of further cleavage into smaller fragments. Calpain family members, calpain-1, -5, -7, and -10, have increased levels or are activated in HD tissue culture and transgenic mouse models, suggesting they may play a key role in Htt proteolysis and disease pathology. Interestingly, calpain-1, -5, -7, and -10 localize to the cytoplasm and the nucleus, whereas the activated forms of calpain-7 and -10 are found only in the nucleus. These results support the role of calpain-derived Htt fragmentation in HD and suggest that aberrant activation of calpains may play a role in HD pathogenesis.