Integrin regulation of cell-cell adhesion during epithelial tubule formation

The extracellular matrix plays an important role in regulation of epithelia l development and organization. To determine more precisely the function of extracellular matrix in this process, the initial steps in collagen-mediat ed formation of epithelial tubules were studied using a model cell culture system. Previous studies have demonstrated that incubation of Madin-Darby c anine kidney (MDCK) epithelial cells with a collagen gel overlay induces be ta1 integrin-regulated epithelial remodeling accompanied by extensive cell rearrangements and formation of epithelial tubules, During epithelial remod eling there was extensive disruption of the epithelial junctional complex. Progressive opening of tight junctions was observed over 8 hours using tran sepithelial resistance measurements and immunofluorescence microscopy demon strated that tight and adherens junction proteins were dispersed throughout the apical and basolateral membranes. Junction complex disruption allowed the formation of apical cell extensions and subsequent migration of selecte d cell sheets from the epithelial monolayer, Confocal microscopy demonstrat ed the presence of adherens junction (E-cadherin, alpha -catenin, beta -cat enin, plakoglobin) and desmosomal (desmoplakin-1/2, plakoglobin) proteins o n, and within, cell extensions demonstrating that cell junctions had underg one considerable disassembly, However, groups of cell extensions appeared t o be associated by E-cadherin/catenin-mediated interactions. Association of E-cadherin/catenin complexes with the epithelial cytoskeleton was analyzed by differential detergent extraction. SDS-PAGE and immunoblot analysis dem onstrated that adherens junction proteins were primarily cytoskeleton-assoc iated in control cells. During integrin-regulated remodeling, there was a p rogressive reduction in the interaction of adherens junction proteins with the cytoskeleton suggesting that they play an important role in the mainten ance of epithelial integrity. Since loss of transepithelial electrical resi stance and disruption of junctional complexes were inhibited by an antifunc tional integrin antibody, we propose that activation of integrin signaling pathways regulate junctional complex stability, cell-cell interactions and cell migration. These observations provide evidence that integrin-regulated MDCK epithelial tubule formation can serve as a model system for studying rearrangements of epithelial sheets which occur during development.