Immunohistochemical Evaluation of Idiopathic Epiretinal Membranes and In Vitro Studies on the Effect of TGF-beta on Müller Cells

PURPOSE. The purpose of this study was to investigate the presence of type VI collagen and glial cells in idiopathic epiretinal membrane (iERM) and the role of TGF-beta in the expression of collagens and alpha-smooth muscle actin (alpha-SMA) in retinal Muller cells.METHODS. Idiopathic ERM samples from vitrectomy were analyzed for glial acidic fibrillary protein (GFAP), cellular retinaldehyde-binding protein (CRALBP), alpha-SMA, and type VI collagen using flat-mount immunohistochemistry. To study intracellular collagen expression in relation to cellular phenotype, spontaneously immortalized human Muller cells (MIO-M1) were treated with TGF-beta 1 for 48 hours, and the expression of alpha-SMA and intracellular type I, II, IV, and VI collagens was studied by using immunocytology. Findings in Muller cells were compared with those in fetal lung fibroblasts and newborn skin fibroblasts.RESULTS. A colocalization of GFAP/CRALBP and GFAP/alpha-SMA was found in iERM, indicating a dynamic process of activation of retinal Muller cells in vivo. Transforming growth factor-beta 1 induced up-regulation of alpha-SMA stress fibers in retinal Muller cells and both types of fibroblasts in vitro. The intracellular staining intensity of type I, II, and VI collagens was decreased in retinal Muller cells containing alpha-SMA stress fibers, whereas the intracellular staining intensity of type I and VI collagens in both types of fibroblasts was not affected.CONCLUSIONS. Type VI collagen and activated retinal Muller cells are present in iERM. Transforming growth factor-beta 1 induces an up-regulation of alpha-SMA stress fibers in retinal Muller cells and fibroblasts and appears to have a cell-specific effect on intracellular collagen expression.