The CYP27A1 - VDR Feedback Rheostat Controls Mitochondrial Protein Homeostasis and Preserves Hippocampal - Striatal Network Integrity to rescue Cognitive impairment in HD condition

Emerging evidence links vitamin D (VD) deficiency to cognitive and motor dysfunction in Huntingtons disease (HD), yet the underlying mechanism remains unclear. Here, we combine in vitro genetic and 3-nitropropionic acid (3-NP)-induced in vivo models of HD to define the mechanistic basis of VD-mediated neuroprotection. We demonstrate that VD supplementation restores cognitive and motor deficits while improving mitochondrial function and cellular survival. Mechanistically, VD enhances mitochondrial fusion and rescues complex II expression, a key defect in HD pathology. We identify the mitochondrial enzyme CYP27A1 as a critical mediator of this effect, as its expression is reduced in HD models but restored upon VD treatment. Overexpression of CYP27A1 recapitulates the protective effects of VD, confirming its central role in maintaining mitochondrial integrity. Furthermore, VD promotes vitamin D receptor (VDR)-dependent transcriptional activation of CYP27A1 and additional nuclear- and mitochondrial-encoded genes, establishing a regulatory feedback loop that supports mitochondrial biogenesis and function. VD supplementation also improves proteostasis via attenuating endoplasmic reticulum stress. Together, our findings uncover a previously unrecognized VD-CYP27A1 axis that links mitochondrial dysfunction to HD pathology and highlight CYP27A1 as a potential therapeutic target.