Inhibition of Mer exacerbates early brain injury by regulating microglia/macrophage phenotype after subarachnoid hemorrhage in mice

Background: Polarization of microglia/macrophages toward the pro-inflammatory phenotype is a crucial contributor to neuroinflammation after subarachnoid hemor-rhage (SAH). Mer belongs to the TAM receptor tyrosine kinases family, which is known to play a significant role in the resolution of inflammation. However, the effect and mechanism of Mer after SAH remain unclear. In this study, we explored the effect of Mer on modulating the microglia/macrophage phenotype and neuro-inflammation and possible potential mechanism after SAH. Method: Endovascular perforation model of SAH was performed. There are 3 parts in this study. Firstly, the time course of Mer expression was determined within 72 hours after SAH. Sec-ondly, the effect of Mer downregulation on brain water content, neurological func-tion, and microglial polarization was evaluated at 24 h after SAH. Thirdly, the neuroprotective effects of pharmacological Mer agonist were assessed. Result: The expression of Mer increased after SAH, and was prominently localized in micro-glia/macrophages. Treatment with Mer siRNA increased pro-inflammatory pheno-type and decreased anti-inflammatory phenotype of microglia/macrophage, thus resulted in exacerbation of neurological deficits and brain edema after SAH. Mecha-nistically, the downregulation of Mer inhibited the downstream anti-inflammatory signals, SOCS1/SOCS3, by decreasing phosphorylated STATs. Conclusion: Mer is involved in the microglia/macrophage polarization and inflammation resolution after SAH, and that mechanism, at least in part, may contribute to the involvement of the STATs/SOCSs pathway.