Etifoxine drives macrophage M2 polarization via Schwann cell-derived progesterone activation of PPARγ to accelerate peripheral nerve repair

Background: Peripheral nerve injury (PNI) presents a significant clinical challenge due to limited endogenous regenerative capacity. The translocator protein (TSPO) ligand etifoxine (ETX) has shown promise in promoting nerve repair, but the underlying cellular and molecular mechanisms remain incompletely understood. Methods: Utilizing in vitro co-culture systems with human Schwann cells (HSCs) and THP-1-derived macrophages, TSPO-knockdown HSCs, conditioned medium experiments, and an in vivo rat sciatic nerve crush injury model, we investigated the effects of ETX on cellular crosstalk and macrophage polarization. Molecular analyses included RNA sequencing, western blotting, fatty acid oxidation (FAO) assays, and a Mito-QC reporter system to assess mitophagy. Functional recovery was evaluated through behavioral tests (hindlimb grip strength, mechanical pain threshold), immunofluorescence, and retrograde tracing. Results: ETX specifically activated TSPO on Schwann cells, stimulating progesterone synthesis and secretion. This Schwann cell-derived progesterone acted as a paracrine signal on macrophages, activating the PPARγ-PGC1α axis. This activation triggered dual reprogramming in macrophages: a metabolic shift toward FAO and induction of BNIP3L-mediated mitophagy, both essential for sustaining a pro-regenerative M2 phenotype. These effects were significantly attenuated by the progesterone receptor antagonist RU486 or the PPARγ antagonist GW9662. In vivo, ETX treatment accelerated functional recovery, enhanced axonal regeneration, and increased infiltration of M2 macrophages at the injury site, effects that were partially reversed by RU486 or GW9662 co-administration. Conclusion: ETX facilitates peripheral nerve repair by promoting Schwann cell-derived progesterone, which drives macrophage PPARγ pathway activation, orchestrating metabolic-autophagic reprogramming necessary for sustained M2 polarization. These findings identify a novel Schwann cell-macrophage metabolic crosstalk mechanism and support the therapeutic potential of targeting this axis in PNI.