Hensin remodels the apical cytoskeleton and induces columnarization of intercalated epithelial cells: Processes that resemble terminal differentiation

Intercalated epithelial cells exist in a spectrum of phenotypes; at one ext reme, beta cells secrete HCO3 by an apical Cl/HCO3 exchanger and a basolate ral H+ ATPase. When an immortalized beta cell line is seeded at high densit y it deposits in its extracellular matrix (ECM) a new protein, hensin, whic h can reverse the polarity of several proteins including the Cl/HCO3 exchan ger (an alternately spliced form of band 3) and the proton translocating AT Pase. When seeded at low density and allowed to form monolayers these polar ized epithelial cells maintain the original distribution of these two prote ins. Although these cells synthesize and secrete hensin, it is not retained in the ECM, but rather, hensin is present in a large number of intracellul ar vesicles. The apical cytoplasm of low density cells is devoid of actin, villin, and cytokeratin19, Scanning electron microscopy shows that these ce lls have sparse microvilli, whereas high density cells have exuberant apica l surface infolding and microvilli, The apical cytoplasm of high density ce lls contains high levels of actin, cytokeratin19, and villin. The cell shap e of these two phenotypes is different with high density cells being tall w ith a small cross-sectional area, whereas low density cells are low and fla t. This columnarization and the remodeling of the apical cytoplasm is hensi n-dependent; it can be induced by seeding low density cells on filters cond itioned by high density cells and prevented by an antibody to hensin. The c hanges in cell shape and apical cytoskeleton are reminiscent of the process es that occur in terminal differentiation of the intestine and other epithe lia. Hensin is highly expressed in the intestine and prostate (two organs w here there is a continuous process of differentiation). The expression of h ensin in the less differentiated crypt cells of the intestine and the basal cells of the prostate is similar to that of low density cells; i.e., abund ant intracellular vesicles but no localization in the ECM, On the other han d, as in high density cells hensin is located exclusively in the ECM of the terminally differentiated absorptive villus cells and the prostatic lumina l cell, These studies suggest that hensin is a critical new molecule in the terminal differentiation of intercalated cell and perhaps other epithelial cells.