Downregulation of USP4 Promotes Activation of Microglia and Subsequent Neuronal Inflammation in Rat Spinal Cord After Injury

NF-κB is involved in the activation of microglia, which induces secondary spinal cord injury (SCI). This process involves the activation of NF-κB signaling pathway by TRAF6 through its polyubiquitination function. We know that deubiquitination of TRAF6 mediated by deubiquitinating enzyme (DUB) significantly inhibits activation of NF-κB pathway. The ubiquitin-specific protease 4 (USP4) belongs to the deubiquitinase family. Therefore, we hypothesize that USP4 is involved in the microglial activation and subsequent neuronal inflammation after SCI. In this study, we examined the expression and the role of USP4 after SCI. Western blot analysis showed that the expression of USP4 was downregulated and the expression of p-p65 was upregulated in the spinal cord after SCI. Immunohistochemical and immunofluorescence staining showed that USP4 was expressed in microglia but its expression decreased after SCI. In vitro LPS-induced activation of microglia showed decreased expression of USP4 and increased expression of p-p65 and TRAF6. USP4 silencing in LPS-induced activation of microglia promoted the expression of p-p65 and TRAF6 and the secretion of TNF-α and IL-1β. In conclusion, our study provides the first evidence that in microglial cells expression of USP4 decreases after SCI in rats. The downregulation of USP4 expression may promote microglial activation and subsequent neuronal inflammation through NF-κB by attenuating the deubiquitination of TRAF6. This mechanism is of great significance in the pathophysiology of secondary SCI.