PD-1 deficiency aggravates spinal cord injury by regulating the reprogramming of NG2 glia and activating the NgR/RhoA/ROCK signaling pathway

Spinal cord injury (SCI) is a devastating disorder and a leading cause of disability in adults worldwide. Multiple studies have reported the upregulation of programmed cell death 1 (PD-1) following SCI. However, the under-lying mechanism of PD-1 deficiency in SCI is not well established. Therefore, we aimed to investigate the role and potential mechanism of PD-1 in SCI pathogenesis. PD-1 Knockout (KO) SCI mouse model was established, and PD-1 expression was evaluated in tissue samples by western blot assay. We then used a series of function gain-and-loss assays to determine the role of PD-1 in SCI pathogenesis. Moreover, mechanistic assays were performed to explore the association between PD-1, neuron-glia antigen-2 (NG2) glia cells, and miR-23b-5p and then investigated the involved signaling pathway. Results illustrated that PD-1 deficiency enhanced the inflammatory response, neuron loss, and functional impairment induced by SCI. We found that NG2 glia depletion aggravated inflammation, reduced neural survival, and suppressed locomotor recovery in murine SCI model. Further analysis indicated that NG2+ cells were increased in the spinal cord of SCI mice, and PD-1 deficiency increased the number of NG2+ cells by activating the Nogo receptor/ras homolog family member A/Rho kinase (NgR/ RhoA/ROCK) signaling. Mechanistically, miR-23b-5p was identified as the negative regulator of PD-1 in NG2 glia. MiR-23b-5p deficiency reduced the expression of inflammatory cytokines, enhanced neural survival, and promoted locomotor recovery in SCI mice, which was counteracted by PD-1 deficiency. In conclusion, PD-1 deficiency exacerbates SCI in vivo by regulating reprogramming of NG2 glia and activating the NgR/RhoA/ ROCK signaling.