Tractional force generation by human Muller cells: Growth factor responsiveness and integrin receptor involvement

PURPOSE. To assess the ability of human Muller cells to generate tractional forces and to determine the role of growth factors and Collagen binding integrins in this process. METHODS. Muller cells were isolated from papain-DNase-digested human retina. Cell identity and changes in cell phenotype were confirmed by immunodetection of glial fibrillary acidic protein (GFAP), cellular retinaldehyde-binding protein (CRALBP), vimentin, and alpha-smooth muscle actin (alpha-SMA). Generation of tractional force was assessed with a tissue culture assay involving incubation of cells on three-dimensional collagen gels. The effects of contraction-promoting growth factors were examined by adding these directly to the culture medium. Muller cell expression and the involvement of specific integrin receptors were assessed by immunodetection, RTPCR, and subunit-specific blocking antibodies. RESULTS. During maintenance in culture, human Muller cells adopted a fibroblast-like phenotype capable of generating tractional forces. Matrix contraction was stimulated in a dose-dependent fashion by insulin-like growth factor I and platelet-derived growth factor. Modest responses were observed with high concentrations of transforming growth factor (TGF)-beta1 and -beta2. Muler cells express all four integrin subunits that comprise the collagen-binding receptors including alpha1, alpha2, alpha3, and beta1. Blocking antibodies against receptor subunits alpha2 and beta1 significantly reduced the overall rate of matrix contraction. Antibodies against the alpha1 subunit were modestly inhibitory, whereas anti-alpha3 was without effect. CONCLUSIONS. Human Muller cells acquire the capacity to generate tractional forces in vitro and the contraction-promoting growth factors insulin-like growth factor (IGF)-I and platelet-derived growth factor (PDGF) are potent stimuli. Generation of tractional force by Muller cells primarily involves integrin receptors containing alpha2 and beta1 subunits.