Membrane lipids and proteins as modulators of urothelial endocytic vesicles pathways

The increased studies on urinary bladder umbrella cells as an important factor for maintaining the permeability barrier have suggested new pathways for the discoidal/fusiform endocytic vesicles which is one of the main features of the umbrella cells. The biological role of these vesicles was defined, for many years, as a membrane reservoir for the umbrella cell apical plasma membrane which are subject to an increased tension during the filling phase of the micturition cycle and, therefore, the vesicles are fused with the apical membrane. Upon voiding, the added membrane is reinserted via a non-clathrin or caveolin-dependant endocytosis thereby restoring the vesicle cytoplasmic pool. However, in the last decade, new evidence appeared indicating alternative pathways of the endocytic vesicles different than the cycling process of exocytosis/endocytosis. The purpose of this review is to analyze the molecular modulators, such as membrane lipids and proteins, in the permeability of endocytic vesicles, the sorting of endocytosed material to lysosomal degradation pathway and recycling of both membrane and fluid phases.