Microglial depletion impairs glial scar formation and aggravates inflammation partly by inhibiting STAT3 phosphorylation in astrocytes after spinal cord injury

Astrocytes and microglia play an orchestrated role following spinal cord injury; however, the molecular mechanisms through which microglia regulate astrocytes after spinal cord injury are not yet fully understood. Herein, microglia were pharmacologically depleted and the effects on the astrocytic response were examined. We further explored the potential mechanisms involving the signal transducers and activators of transcription 3 (STAT3) pathway. For in vivo experiments, we constructed a contusion spinal cord injury model in C57BL/6 mice. To deplete microglia, all mice were treated with colony-stimulating factor 1 receptor inhibitor PLX3397, starting 2 weeks prior to surgery until they were sacrificed. Cell proliferation was examined by 5-ethynyl-2-deoxyuridine (EdU) and three pivotal inflammatory cytokines were detected by a specific Bio-Plex Pro™ Reagent Kit. Locomotor function, neuroinflammation, astrocyte activation and phosphorylated STAT3 (pSTAT3, a maker of activation of STAT3 signaling) levels were determined. For in vitro experiments, a microglia and astrocyte coculture system was established, and the small molecule STA21, which blocks STAT3 activation, was applied to investigate whether STAT3 signaling is involved in mediating astrocyte proliferation induced by microglia. PLX3397 administration disrupted glial scar formation, increased inflammatory spillover, induced diffuse tissue damage and impaired functional recovery after spinal cord injury. Microglial depletion markedly reduced EdU(+) proliferating cells, especially proliferating astrocytes at 7 days after spinal cord injury. RNA sequencing analysis showed that the JAK/STAT3 pathway was downregulated in mice treated with PLX3397. Double immunofluorescence staining confirmed that PLX3397 significantly decreased STAT3 expression in astrocytes. Importantly, in vitro coculture of astrocytes and microglia showed that microglia-induced astrocyte proliferation was abolished by STA21 administration. These findings suggest that microglial depletion impaired astrocyte proliferation and astrocytic scar formation, and induced inflammatory diffusion partly by inhibiting STAT3 phosphorylation in astrocytes following spinal cord injury.