Mutant huntingtin N-terminal fragments of specific size mediate aggregation and toxicity in neuronal cells.

Huntingtin proteolysis is implicated in Huntington disease pathogenesis, yet, the nature of huntingtin toxic fragments remains unclear. Huntingtin undergoes proteolysis by calpains and caspases within an N-terminal region between amino acids 460 and 600. We have focused on proteolytic steps producing shorter N-terminal fragments, which we term cp-1 and cp-2 (distinct from previously described cp-A/cp-B). We used HEK293 cells to express the first 511 residues of huntingtin and further define the cp-1 and cp-2 cleavage sites. Based on epitope mapping with huntingtin-specific antibodies, we found that cp-1 cleavage occurs between residues 81 and 129 of huntingtin. Affinity and size exclusion chromatography were used to further purify huntingtin cleavage products and enrich for the cp-1/cp-2 fragments. Using mass spectrometry, we found that the cp-2 fragment is generated by cleavage of huntingtin at position Arg(167). This site was confirmed by deletion analysis and specific detection with a custom-generated cp-2 site neo-epitope antibody. Furthermore, alterations of this cleavage site resulted in a decrease in toxicity and an increase in aggregation of huntingtin in neuronal cells. These data suggest that cleavage of huntingtin at residue Arg(167) may mediate mutant huntingtin toxicity in Huntington disease.

N-terminal proteolysis of full-length mutant huntingtin in an inducible PC12 cell model of Huntington's disease.

Proteolytic cleavage of mutant huntingtin may play a key role in the pathogenesis of Huntington's disease; however the steps in huntingtin proteolysis are not fully understood. Huntingtin was shown to be cleaved by caspases and calpains within a region between 460-600 amino acids from the N-terminus. Two smaller N-terminal fragments produced by unknown protease have been previously described as cp-A and cp-B. To further investigate the huntingtin proteolytic pathway, we used an inducible PC12 cell model expressing full-length huntingtin with either normal or expanded polyglutamine. This cell model recapitulates several steps of huntingtin proteolysis: proteolysis mediated by caspases within the region previously mapped for caspase cleavage, and cleavage generating two novel N-terminal fragments (cp-1 approximately 90-105 residues long and cp-2 extending beyond 115-129 epitope of huntingtin). Interestingly, the deletion of amino acids 105-114 (mapped previously as a cleavage site for cp-A) failed to affect the production of cp-1 or cp-2. Therefore, we conclude that these new fragments are distinct from previously described cp-A and cp-B. We demonstrate that cp-1 and cp-2 fragments are produced and accumulate within nuclear and cytoplasmic inclusions prior to huntingtin-induced cell toxicity, and these fragments can be formed by caspase-independent proteolytic cleavage of huntingtin in PC12 cells. In addition, inhibition of calpains leads to decreased subsequent degradation of cp-1 and cp-2 fragments, and accelerated formation of inclusions. Further delineation of huntingtin cleavage events may lead to novel therapeutic targets for HD.