Huntingtin's neuroprotective activity occurs via inhibition of procaspase-9 processing.

Huntington's Disease is an inherited neurodegenerative disease that affects the medium spiny neurons in the striatum. The disease is caused by the expansion of a polyglutamine sequence in the N terminus of Huntingtin (Htt), a widely expressed protein. Recently, we have found that Htt is an antiapoptotic protein in striatal cells and acts by preventing caspase-3 activity. Here we report that Htt overexpression in other CNS-derived cells can protect them from more than 20 days exposure to fatal stimuli. In particular, we found that cytochrome c continues to be released from mitochondria into the cytosol of cells that overexpress normal Htt. However, procaspase-9 is not processed, indicating that wild-type Htt (wtHtt) acts downstream of cytochrome c release. These data show that Htt inhibits neuronal cell death by interfering with the activity of the apoptosome complex.