TGF-β-SNAIL axis induces Muller glial-mesenchymal transition in the pathogenesis of idiopathic epiretinal membrane

The epithelial-mesenchymal transition (EMT) is a key process in fibrogenic diseases where transdifferentiated myofibroblasts produce excessive amounts of extracellular matrix, resulting in organ dysfunction. Idiopathic epiretinal membrane (iERM) is a vision-threatening disorder characterized by fibrocellular proliferation and contraction on the central retina. Muller glial cells, which regulate retinal physiology and structure, are the major cellular components in the iERM tissue; however, the pathological role of this cell type remains incompletely understood. Here we revealed the involvement of Muller glial-mesenchymal transition (GMT), as an alternative to EMT, in the pathogenesis of iERM lacking epithelial contribution in nature. Of various pro-fibrotic cytokines, transforming growth factor (TGF)-beta 1 stimulation to human Muller glial cells exclusively increased mRNA and protein levels of several EMT-related molecular markers, together with the transcription factor SNAIL but not SLUG or TWIST. TGF-beta 1-stimulated Muller cells also exhibited EMT-related cell motility, while reducing the expression of glutamine synthetase (GS), a Muller glial marker. Notably, all of these TGF-beta-induced EMT features were reversed by SNAI1 knockdown in Muller cells. iERM patient specimens demonstrated co-immunolocalization of SNAIL with TGF-beta 1, GS, and smooth muscle protein 22. Our data implicated a critical role of the TGF-beta-SNAIL axis in Muller GMT to promote iERM formation.