MORPHOGENESIS OF THE EPITHELIAL-CELL TRANSPORTING PHENOTYPE - SYNTHESIS AND DISTRIBUTION OF ION CHANNELS

The exchange of substances between higher organisms and the environmen t takes place across epithelia consisting of one or more cell layers. To perform this function, epithelial cells have two basic differentiat ed properties: 1) they form tight junctions (TJs) that seal the extrac ellular space, and 2) they are polarized into an apical and a basolate ral domain, with entirely different structural, biochemical and physio logical properties. Our understanding of the mechanisms involved in th e expression of these properties has been greatly enhanced by the avai lability of epithelial cell lines that form TJs and polarize in vitro under conditions suitable for experimental control. In this article we summarize our studies on the synthesis and polarized expression of io n channels in epithelial cells. MDCK cells have four types of K+ chann els in the apical domain, and a fifth one in the basolateral domain. T he basolateral side also has a population of Cl- channels. Each type o f channel is absolutely polarized. Harvesting with trypsin-EDTA reduce s the area of the plasma membrane by 50% and the channel population by 90%. Upon plating, these channels are recovered within a few hours. W e describe here the main extracellular and intracellular mechanisms in volved in these phenomena