The alpha 2 beta 1 integrin regulates collagen-mediated MDCK epithelial membrane remodeling and tubule formation

Previous studies have demonstrated that incubation of MDCK cell epithelial cysts in collagen gel induced a reversal in cell surface polarity that was regulated by beta 1 integrins. Further experiments were done to identify the specific collagen binding integrin involved by applying collagen gel overlays to the apical membrane of subconfluent MDCK monolayers. Cell surface levels of the apical membrane glycoprotein gp135 were monitored by ELISA to quantitate the extent of collagen-mediated membrane remodeling. After an 8 hour incubation with collagen, there was a 35% reduction in gp135 while the cell surface levels of the alpha 2, alpha 3 and beta 1 integrin subunits were not affected. Immunofluorescence microscopy confirmed the loss of gp135 from selected regions of the apical cell surface while the alpha 2 and beta 1 integrin subunits were distributed in small clusters over the entire apical membrane in both control and collagen-treated monolayers. Collagen-mediated loss of gp135 was inhibited by monoclonal antibodies which recognize either the alpha 2 or beta 1 integrin subunits but not by a monoclonal antibody against the alpha 6 beta 1 integrin. These results demonstrated that remodeling of the apical membrane had occurred, allowing the selective retention of beta 1 integrins but not gp135. They were supported by the observation that collagen-mediated loss of apical membrane microvilli was inhibited by the monoclonal antibody against the alpha 2 integrin subunit. Incubation of confluent monolayers with collagen gel induced the formation of polarized epithelial tubules within 16 hours. Epithelial tubule biogenesis was completely inhibited by monoclonal antibodies against either the alpha 2 or beta 1 integrin subunits, providing strong evidence that the alpha 2 beta 1 integrin is essential for collagen-mediated epithelial membrane remodeling and tubule formation.