Single cell resolution of neurosteroidogenesis in the murine brain: de novo biosynthesis.

Neurosteroids synthesized within the central nervous system play essential roles in modulating neurotransmission, providing neuroprotection, regulating immune responses, influencing behavior and cognition and mediating stress physiology. Despite their broad significance, the specific brain cell types capable of de novo steroid synthesis from cholesterol remain poorly defined. In this study, we analyzed single-cell transcriptomic data to map steroidogenic gene expression across cell populations in the murine brain, focusing on the de novo production of the neurosteroid pregnenolone. Our findings reveal that de novo steroidogenesis, as marked by Cyp11a1 expression, is predominantly confined to specific neuronal subtypes, particularly glutamatergic neurons of the intra- and extra-telencephalic regions and the corticothalamic layer. In contrast, Star expression, which is essential for mitochondrial cholesterol import, was more broadly distributed, occurring in both neuronal and non-neuronal cells (including oligodendrocytes, astrocytes, immune cells and vascular cells). In these non-neuronal populations, Star was notably co-expressed with mitochondrial Cyp27a1, indicative of bile acid synthesis rather than neurosteroidogenesis. This distinction highlights that Star expression alone is not a reliable marker of de novo neurosteroidogenic capacity in the brain, as its functional significance depends on the broader enzymatic context in which it occurs. The resulting single-cell map of de novo neurosteroid biosynthetic capacity across brain regions, including modest sex-associated differences, provides a foundational framework for understanding neurosteroid signaling in distinct cell types and its relevance to brain physiology and pathophysiology.