Mechanisms of polyglutamine expanded huntingtin induced toxicity

Huntington's Disease (HD) belongs to the CAG repeat family of neurodegenerative diseases and is characterized by the presence of an expanded polyglutamine (polyQ) repeat in the huntingtin (htt) gene product. PolyQ-expanded htt accumulates within large aggregates in various subcellular compartments, but are more often localized within the nucleus. The sequestration of proteins essential to cell viability may be one mechanism that accounts for toxicity generated by polyQ-expanded proteins. Nuclear inclusions containing polyQ-expanded htt recruit the transcriptional cofactor, CREB-binding protein (CBP). PolyQ toxicity appears to involve alterations of gene transcription and reduced neuronal cell viability. In the HT22 hippocampal cell line, we found that toxicity within individual cells induced by polyQ-expanded htt was associated with the localization of the mutant htt within either nuclear or perinuclear aggregates. However, in addition to CBP recruitment, we found that CBP ubiquitylation and degradation can be selectively enhanced by polyQ-expanded htt. Thus, selected substrates may be directed to the ubiquitin/proteasome-dependent protein degradation pathway (UPP) in response to polyQ-expanded htt within the nucleus. While both the polyQ domain and the histone acetyltransferase domain (HAT) of CBP have been found to interact with polyQ-expanded htt, deletion of either domain does not affect its enhanced degradation in the presence of polyQ-expanded htt in HT22 cells. Thus, enhanced degradation of CBP in cells expressing polyQ-expanded htt may not involve a direct interaction between CBP and htt. It seems likely specific enzymes in the UPP may be activated by htt and selectively target proteins such as CBP for degradation. Since molecular chaperones are found in the aggregates containing polyQ-expanded proteins, misfolding of polyQ-expanded proteins may play a key role in polyglutamine disease pathogenesis. In a number of some studies, HDJ-2, a member of DnaJ family molecular chaperones, was found to reduce aggregation and toxicity induced by polyQ-expanded proteins. In contrast, we show that HDJ-2 is unable to rescue aggregate formation of polyQ-expanded htt in transfected HEK293 fibroblast cells, nor is it recruited into these aggregates in vivo in a HD transgenic mouse model. Thus, molecular chaperone effects on polyQ-expanded protein induced toxicity could be cell-type specific or influenced by the developmental state of the culturable cells. These factors must be considered in any attempts to use chaperones as potential therapeutic targets in polyglutamine diseases