Substrate Elastic Modulus Regulates the Morphology, Focal Adhesions, and alpha-Smooth Muscle Actin Expression of Retinal Muller Cells

PURPOSE. The stiffness of the extracellular matrix has been shown to regulate cell adhesion, migration, and transdifferentiation in fibrotic processes. Retinal Muller cells have been shown to be mechanosensitive; they are involved in fibrotic vitreoretinal diseases. Since fibrosis increases the rigidity of the extracellular matrix, our aim was to develop an in vitro model for studying Muller cell morphology and differentiation state in relation to matrix stiffness. METHODS. A spontaneously immortalized human Muller cell line (MIO-M1) was cultured on type I collagen-coated polyacrylamide gels with Young's moduli ranging from 2 to 92 kPa. Cell surface area, focal adhesion, and the expression and morphology of alpha-smooth muscle actin induced by transforming growth factor beta (TGF-beta [10 ng/mL for 48 hours]) were analyzed by immunocytology. The images were documented by using fluorescence microscopy and confocal scanning laser microscopy. RESULTS. MIO-M1 cells cultured on stiff substrates exhibited a significant increase in cell surface area, stress fiber, and mature focal adhesion formation. Furthermore, Muller cells treated with TGF-beta 1 and TGF-beta 2 and cultured on stiff substrates showed an increased incorporation of alpha-smooth muscle actin into stress fibers when compared to those grown on soft surfaces. CONCLUSIONS. Compliance of the surrounding matrix seems to influence the morphology and contraction of retinal Muller cells in fibrotic conditions. Development of an in vitro model simulating both the normally compliant retinal tissue and the rigid retinal fibrotic tissue helps fill the gap between the results of petri-dish cell culture with rigid surfaces and in vivo findings