Mechanisms for regulating the cell surface distribution of Na/K-ATPase in polarized epithelial cells

New perspectives on mechanisms involved in generating epithelial cell polarity

Polarized epithelial cells form barriers that separate biological compartments and regulate homeostasis by controlling ion and solute transport between those compartments. Receptors, ion transporters and channels, signal transduction proteins, and cytoskeletal proteins are organized into functionally and structurally distinct domains of the cell surface, termed apical and basolateral, that face these different compartments. This review is about mechanisms involved in the establishment and maintenance of cell polarity. Previous reports and reviews have adopted a Golgi-centric view of how epithelial cell polarity is established, in which the sorting of apical and basolateral membrane proteins in the Golgi complex is a specialized process in polarized cells, and the generation of cell surface polarity is a direct consequence of this process. Here, we argue that events at the cell surface are fundamental to the generation of cell polarity. We propose that the establishment of structural asymmetry in the plasma membrane is the first, critical event, and subsequently, this asymmetry is reinforced and maintained by delivery of proteins that were constitutively sorted in the Golgi. We propose a hierarchy of stages for establishing cell polarity.

Subcellular distribution of tissue kallikrein and Na,K-ATPase alpha-subunit in rat parotid striated duct cells

Intracellular protein distribution and sorting were examined in rat parotid striated duct cells, in which tissue kallikrein is apical, and Na,K-ATPase is basolateral. Electron-microscopic immunogold cytochemistry, with both polyclonal and monoclonal antibodies, demonstrated these enzymes at opposite poles of the cells and in distinct intracellular sites. Kallikrein was found within apical secretory granules, whereas Na,K-ATPase was present on basolateral cell membranes. In addition, kallikrein was localized throughout cisternae of all Golgi profiles, whereas Na,K-ATPase (alpha-subunit) was found only in small peripheral vesicles and/or lateral cisternal extensions of a basal subset of Golgi profiles. These differences in the subcellular distribution of the two marker antigens were most clearly seen with double immunogold labelling. Our results suggest that kallikrein, an apical, regulated secretory protein, and Na,K-ATPase, a basolateral, constitutively transported membrane protein, are segregated at (or prior to) the level of the Golgi apparatus rather than in the trans-Golgi network (TGN), as was expected.

Regulation of cell surface polarity in renal epithelia

In the kidney, polarized epithelial cells play critical roles in ion, fluid and solute reabsorption from the ultrafiltrate to the blood supply. Detailed analysis of protein distributions has revealed that ion channels, transporters and pumps are restricted to distinct domains of the plasma membrane that face either the ultrafiltrate (apical membrane) or the blood supply (basal-lateral membrane). The importance of the development and maintenance of the polarized distributions of these proteins in renal epithelia for normal cell function is demonstrated by the fact that several disease states are characterized by abnormal distributions of proteins; for example in polycystic kidney disease, Na+/K(+)-ATPase has been detected in the apical and lateral membranes, compared with normal cells where Na+/K(+)-ATPase is localized in the basal-lateral membrane domain. Recent studies indicate that the development of restricted distributions of proteins at the cell surface of Madin Darby canine kidney epithelial cells is determined by direct sorting of proteins in the trans Golgi network into vesicles that are delivered vectorially to either the apical or basal-lateral membrane. Upon arrival at the plasma membrane, some proteins, such as Na+/K(+)-ATPase, may be selectively retained by binding to the membrane cytoskeleton.

Abnormal sodium pump distribution during renal tubulogenesis in congenital murine polycystic kidney disease

Congenital polycystic kidney disease is characterized by the formation of large fluid-filled cysts in kidney tubules. It has been postulated that increased epithelial cell proliferation and altered transtubular fluid transport are necessary for cyst formation. To address the latter problem, we have studied the plasma membrane distribution of the alpha 1 and beta 1 subunits of Na+/K(+)-ATPase during progressive stages of proximal and collecting tubular cyst formation in the CPK mouse, a murine model of autosomal recessive polycystic kidney disease. In both control and cystic proximal tubules, Na+/K(+)-ATPase distribution was restricted to the basal-lateral membrane of cells. However, in newborn through day 5 kidney tissue, 16% of control vs. 47% of cystic outer cortical, 6% of control vs. 46% of cystic inner cortical, and 2% of control vs. 63% of cystic medullary collecting tubules demonstrated apical and lateral membrane distribution of Na+/K(+)-ATPase. In all nephrogenic zones, the percentage of control or cystic collecting tubules demonstrating apical membrane distribution of Na+/K(+)-ATPase decreased over time, but the percentage of cystic collecting tubules with apical membrane Na+/K(+)-ATPase remained significantly greater than in developmentally matched controls. No alterations in the normal distributions of other apical or basal-lateral membrane marker proteins were noted at any stage of control or cystic proximal or collecting tubule development. We conclude that apical-lateral membrane Na+/K(+)-ATPase expression is a normal transient feature of early collecting tubule development. However, apical membrane Na+/K(+)-ATPase persists in cystic kidneys, suggesting that such expression may be a manifestation of the relatively undifferentiated phenotype of epithelial cells lining collecting tubule cysts. The persistence of apical membrane Na+/K(+)-ATPase, if the enzyme is functional, may have pathogenic important in abnormal transtubular fluid transport in polycystic kidney disease.