p21-activated kinase 1 promotes soluble mutant huntingtin self-interaction and enhances toxicity.

Huntington's disease (HD) is caused by a polyglutamine (polyQ) expansion in the huntingtin (htt) protein. While aggregation is a pathological hallmark of HD and related polyQ expansion diseases, the role of aggregates has been disputed. Here we report that p21-activated kinase 1 (Pak1) binds to htt in vivo and in vitro. Pak1 colocalized with mutant htt (muhtt) aggregates in cell models and in human HD brains. Pak1 overexpression enhanced the aggregation of muhtt. Furthermore, we observed SDS-soluble wild-type htt (wthtt)-wthtt, wthtt-muhtt and muhtt-muhtt interactions, which were enhanced by the presence of Pak1. We show that Pak1 overexpression enhanced htt toxicity in cell models and neurons in parallel with its ability to promote aggregation, while Pak1 knockdown suppressed both aggregation and toxicity. Overexpression of either kinase-dead or wild-type Pak enhanced both aggregation and toxicity. Our data reveal a novel mechanism regulating muhtt oligomerization and toxicity and suggest that pathology may be at least partly dependent on soluble muhtt-muhtt interactions.