Mutant huntingtin interacts with the sterol regulatory element-binding proteins and impairs their nuclear import

Brain cholesterol homeostasis is altered in Huntington's disease (HD), a neurodegenerative disorder caused by the expansion of a CAG nucleotide repeat in the HTT gene. Genes involved in the synthesis of cholesterol and fatty acids were shown to be downregulated shortly after the expression of mutant huntingtin (mHTT) in inducible HD cells. Nuclear levels of the transcription factors that regulate lipid biogenesis, the sterol regulatory element-binding proteins (SREBP1 and SREBP2), were found to be decreased in HD models compared to wild-type, but the underlying causes were not known. SREBPs are synthesized as inactive endoplasmic reticulum-localized precursors. Their mature forms (mSREBPs) are generated upon transport of the SREBP precursors to the Golgi and proteolytic cleavage, and are rapidly imported into the nucleus by binding to importin β. We show that, although SREBP2 processing into mSREBP2 is not affected in YAC128 HD mice, mSREBP2 is mislocalized to the cytoplasm. Chimeric mSREBP2-and mSREBP1-EGFP proteins are also mislocalized to the cytoplasm in immortalized striatal cells expressing mHTT, in YAC128 neurons and in fibroblasts from HD patients. We further show that mHTT binds to the SREBP2/importin β complex required for nuclear import and sequesters it in the cytoplasm. As a result, HD cells fail to upregulate cholesterogenic genes under sterol-depleted conditions. These findings provide mechanistic insight into the downregulation of genes involved in the synthesis of cholesterol and fatty acids in HD models, and have potential implications for other pathways modulated by SREBPs, including autophagy and excitotoxicity.