Involvement of the MEK-ERK/p38-CREB/c-fos signaling pathway in Kir channel inhibition-induced rat retinal Muller cell gliosis

Our previous studies have demonstrated that activation of group I metabotropic glutamate receptors downregulated Kir channels in chronic ocular hypertension (COH) rats, thus contributing to Muller cell gliosis, characterized by upregulated expression of glial fibrillary acidic protein (GFAP). In the present study, we explored possible signaling pathways linking Kir channel inhibition and GFAP upregulation. In normal retinas, intravitreal injection of BaCl2 significantly increased GFAP expression in Muller cells, which was eliminated by co-injecting mitogen-activated protein kinase (MAPK) inhibitor U0126. The protein levels of phosphorylated extracellular signal-regulated protein kinase1/2 (p-ERK1/2) and its upstream regulator, p-MEK, were significantly increased, while the levels of phosphorylated c-Jun N-terminal kinase (p-JNK) and p38 kinase (p-p38) remained unchanged. Furthermore, the protein levels of phosphorylated cAMP response element binding protein (p-CREB) and c-fos were also increased, which were blocked by co-injecting ERK inhibitor FR180204. In purified cultured rat Muller cells, BaCl2 treatment induced similar changes in these protein levels apart from p-p38 levels and the p-p38:p38 ratio showing significant upregulation. Moreover, intravitreal injection of U0126 eliminated the upregulated GFAP expression in COH retinas. Together, these results suggest that Kir channel inhibition-induced Muller cell gliosis is mediated by the MEK-ERK/p38-CREB/c-fos signaling pathway.