Single cell resolution of neurosteroidogenesis in the murine brain: intermediary biosynthesis

Neurosteroids synthesized within the central nervous system modulate neurotransmission, enhance neuroprotection, regulate immune responses and influence cognitive and behavioral processes. Beyond de novo synthesis of pregnenolone (PREG) from cholesterol, the brain also engages in intermediary synthesis, converting local or circulating precursors into active neurosteroids. However, the specific cell types and brain regions involved remain poorly defined. In this study, we used single-cell transcriptomic data to map the expression of steroidogenic genes and identify cell populations in the murine brain responsible for intermediary neurosteroid biosynthesis. Our findings reveal that the synthesis of bioactive steroids downstream of PREG is not streamlined but selectively compartmentalized. Notably, cells involved in de novo neurosteroid biosynthesis are largely disjointed from intermediary steps, indicating reliance on regional diffusion and/or systemic sources. Capacity for synthesis of androgens and corticosteroids anew are practically absent. While sterol sulfotransferases are expressed, sterol sulfatase required for desulfation is absent, indicating irreversible sulfonation of PREG and dehydroepiandrosterone (DHEA). Other enzymes involved in bioconversions of pregnanes and androstanes, when expressed, showed cell type- and region-specific ramifications. Although certain limitations exist in fully deciphering these results due to certain gaps in enzyme substrate specificity and isoform catalytic preferences, our study reveals valuable insights into the brain's intermediary neurosteroid pathways. The distinct compartmentalization of these processes suggests precise control over steroid regulation, which could have far-reaching functional implications. By mapping the neurosteroidogenic potential in the murine brain and sex-associated variations, this study sets the stage for future investigations into the roles of neurosteroids in brain function and their therapeutic potential in neurological disorders.