In neurodegenerative diseases, microglia commonly accumulate toxic levels of cholesterol, compromising their ability to resolve the neuroinflammatory response. Here, we explored the role of neurosteroidogenesis, a complex cholesterol-metabolizing pathway that produces anti-inflammatory neurosteroids, in regulating cholesterol dynamics in IL-1β-activated human microglia. Our investigation focused on the Translocator protein (TSPO, 18 kDa), which plays a pivotal role in initiating neurosteroidogenesis, representing its rate-limiting step. Microglia in which TSPO was knocked down (TSPO KD) exhibited a marked reduction in neurosteroid biosynthesis suggesting an impaired cholesterol metabolism. Concurrently, these cells showed upregulated expression of the SREBP/HMGCR/Fdft-1/CEH axis, indicating compensatory activation of the systems aimed at increasing cholesterol availability, and downregulated cholesterol membrane efflux. As a result of these dynamics, excessive cholesterol significantly accumulated, suggesting inadequate cholesterol clearance. This dysregulation was further exacerbated in IL-1β-treated TSPO KD microglia. The impairment of neurosteroidogenic biosynthetic capacity, combined with pronounced downregulation of SREBP/HMGCR/Fdft-1/CEH axis and cholesterol membrane efflux, resulted in a severe cholesterol overload, highlighting a complete disruption of cholesterol trafficking mechanism cross-talks. Noteworthy, these cells exhibited hallmarks of neurodegenerative diseases-associated microglia (MGnD), including heightened inflammatory reactivity. On the other hand, the stimulation of TSPO-mediated neurosteroidogenesis, known to promote a reparative microglial phenotype, significantly reduced cholesterol accumulation. Neurosteroids such as allopregnanolone and estradiol emerged as key mediators in enhancing cholesterol clearance. In conclusion, these findings underscore that native neurosteroidogenesis is a crucial autocrine regulator of cholesterol trafficking in activated human microglia. They further highlight the therapeutic potential of targeting TSPO-mediated neurosteroidogenesis for the treatment of neuroinflammatory-based diseases by restoring cholesterol homeostasis and promoting reparative functions in MGnD.
Angeloni et al. (Tue,) studied this question.