Integrin regulation of cell-cell adhesion during epithelial tubule formation.

The extracellular matrix plays an important role in regulation of epithelial development and organization. To determine more precisely the function of extracellular matrix in this process, the initial steps in collagen-mediated formation of epithelial tubules were studied using a model cell culture system. Previous studies have demonstrated that incubation of Madin-Darby canine kidney (MDCK) epithelial cells with a collagen gel overlay induces (beta)1 integrin-regulated epithelial remodeling accompanied by extensive cell rearrangements and formation of epithelial tubules. During epithelial remodeling there was extensive disruption of the epithelial junctional complex. Progressive opening of tight junctions was observed over 8 hours using transepithelial resistance measurements and immunofluorescence microscopy demonstrated 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 selected cell sheets from the epithelial monolayer. Confocal microscopy demonstrated the presence of adherens junction (E-cadherin, (alpha)-catenin, (beta)-catenin, plakoglobin) and desmosomal (desmoplakin-1/2, plakoglobin) proteins on, and within, cell extensions demonstrating that cell junctions had undergone considerable disassembly. However, groups of cell extensions appeared to 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 demonstrated that adherens junction proteins were primarily cytoskeleton-associated in control cells. During integrin-regulated remodeling, there was a progressive reduction in the interaction of adherens junction proteins with the cytoskeleton suggesting that they play an important role in the maintenance of epithelial integrity. Since loss of transepithelial electrical resistance and disruption of junctional complexes were inhibited by an antifunctional 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.