Caspase 3-cleaved N-terminal fragments of wild-type and mutant huntingtin are present in normal and Huntington's disease brains, associate with membranes, and undergo calpain-dependent proteolysis

The Huntington's disease (HD) mutation is a polyglutamine expansion in the N-terminal region of huntingtin (N-htt). How neurons die in HD is unclear. Mutant N-htt aggregates in neurons in the HD brain; expression of mutant N- htt in vitro causes cell death. Other in vitro studies show that proteolysi s by caspase 3 could be important in regulating mutant N-htt function, but there has been no direct evidence for caspase 3-cleaved N-htt fragments in brain. Here, we show that N-htt fragments consistent with the size produced by caspase 3 cleavage in vitro are resident in the cortex, striatum, and c erebellum of normal and adult onset HD brain and are similar in size to the fragments seen after exogenous expression of human huntingtin in mouse clo nal striatal neurons. HD brain extracts treated with active caspase 3 had i ncreased levels of N-htt fragments. Compared with the full-length huntingti n, the caspase 3-cleaved N-htt fragments, especially the mutant fragment, p referentially segregated with the membrane fraction. Partial proteolysis of the human caspase 3-cleaved N-htt fragment by calpain occurred in vitro an d resulted in smaller N-terminal products; products of similar size appeare d when mouse brain protein extracts were treated with calpain. Results supp ort the idea that sequential proteolysis by caspase 3 and calpain may regul ate huntingtin function at membranes and produce N-terminal mutant fragment s that aggregate and cause cellular dysfunction in HD.