Influence of insulin on glutamine synthetase in the Muller glial cells of retina

Glutamine synthetase (GS), a Muller cell specific enzyme in the retina, is the key enzyme involve in glutamate metabolism. The goal of this study was to investigate the expression and regulation of GS by insulin in the cultured rat retinal Muller cells. Immunocytochemical and immunoblotting experiments showed that the cultured Muller cells express GS protein under normal cell culture conditions. Insulin treatments decreased the GS expression both in a time and dose dependent manner. Insulin also decreased the hydrocortisone induced GS expression. Furthermore, we investigated the expression and regulation of two other Muller cell specific enzymes known to be involved in glutamate metabolism, the mitochondrial branched chain aminotransferase (BCATm) and pyruvate carboxylase (PC). Immunoblotting experiments showed that Muller cells expressed both BCATm and PC. Treatments of cells with hydrocortisone or insulin did not influence the BCATm expression level. Hydrocortisone treatment of cells increased the PC expression but this induced expression was suppressed by insulin treatment. Muller cells expressed insulin receptor proteins (IR beta and IRS-1) and insulin activation induced the phosphotyrosine level of insulin receptor proteins. Moreover, hydrocortisone did not influence the expression or activation of these receptor proteins. The data suggests that insulin modulates the GS synthesis and may influence glutamate metabolism in the cultured retinal Muller cells but not by influencing the insulin signaling pathway.