Changes in intracellular calcium during the development of epithelial polarity and junctions
The "Ca2+ switch" model with cultured Madin-Darby canine kidney (MDCK) cells is useful in studying the biogenesis of epithelial polarity and junction formation and provides insight into early steps in the morphogenesis of polarized epithelial tissues. When extracellular Ca2+ in the medium is changed from less than 5 microM to 1.8 mM, MDCK cells rapidly change from a nonpolarized state exhibiting little cell-cell contact (with the apical membrane and junctional proteins largely within the cell) to a polarized state with well-formed tight junctions and desmosomes. To examine the role of intracellular Ca2+ in the development of polarity and junctions, we made continuous spectrofluorimetric measurements of intracellular Ca2+ during the "switch," using the fluorescent indicator fura-2. Intracellular Ca2+ increased greater than 10-fold during the switch and gave a complex pattern of increase, decrease, and stabilization. In contrast, intracellular pH [monitored with 2',7'-bis(2-carboxyethyl)-5(and 6)-carboxyfluorescein (BCECF)] did not change during the period studied. When intracellular Ca2+ curves in several cells were compared, considerable heterogeneity in the rate of increase of intracellular Ca2+ levels and in peak levels was evident, perhaps reflecting the heterogeneity among cells in establishing junctions and polarity. The heterogeneity of the process was confirmed by digital imaging of intracellular Ca2+ and was present even in a "clonal" line of MDCK cells, indicating the heterogeneity was intrinsic to the process and not simply a function of slight genetic variation within the population of MDCK cells. In pairs of cells that had barely established cell-cell contact, often one cell exhibited a much greater increase in intracellular Ca2+ than the other cell in the pair. At the site of cell-cell contact, an apparent localized change (an increase over the basal level) in intracellular Ca2+ was frequently present and occasionally appeared to extend beyond the point of cell-cell contact. Since the region of cell-cell contact is also the site where junctions form and where vesicles containing apical membranes fuse during the development of polarity, we postulate a role for global and local changes in intracellular Ca2+ in these events.