Endocytotic activity of bladder superficial urothelial cells is inversely related to their differentiation stage.

The composition of the apical plasma membrane of bladder superficial urothelial cells is dramatically modified during cell differentiation, which is accompanied by the change in the dynamics of endocytosis. We studied the expression of urothelial differentiation-related proteins uroplakins and consequently the apical plasma membrane molecular composition in relation to the membrane-bound and fluid-phase endocytosis in bladder superficial urothelial cells. By using primary urothelial cultures in the environment without mechanical stimuli, we studied the constitutive endocytosis. Four new findings emerge from our study. First, in highly differentiated superficial urothelial cells with strong uroplakin expression, the endocytosis of fluid-phase endocytotic markers was 43% lower and the endocytosis of membrane-bound markers was 86% lower compared to partially differentiated cells with weak uroplakin expression. Second, superficial urothelial cells have 5-15-times lower endocytotic activity than MDCK cells. Third, in superficial urothelial cells the membrane-bound markers are delivered to lysosomes, while fluid-phase markers are seen only in early endocytotic compartments, suggesting their kiss-and-run recycling. Finally, we provide the first evidence that in highly differentiated cells the uroplakin-positive membrane regions are excluded from internalization, suggesting that uroplakins hinder endocytosis from the apical plasma membrane in superficial urothelial cells and thus maintain optimal permeability barrier function.