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Hans Merzendorfer

Epithelia are layers of adherent cells that line the inner and outer surfaces of diverse tissues and organs throughout the body. Their main function is to separate body compartments, and to mediate transport of different substances between them. This is only possible because epithelial cells are ‘polar’ in the sense that their apical membranes (on the cell's outer surface) differ in protein composition from their basolateral membranes (on the inner surface). Many proteins of the apical or basolateral membranes have been recognized as important players in establishing cellular polarity, but the mechanisms that regulate epithelial polarization are still poorly understood. In a recent Nature paper a US/Canadian team of scientists led by Ulrich Tepass have genetically dissected some components of the underlying machinery which regulates epithelial polarity.

The organization of epithelial cell membranes has been intensively studied in Drosophila embryos and several genes that function in epithelial polarity have been discovered. These genes include yrt, cora, Nrx-IV and ATPa, which encode the basolateral proteins Yurt, Coracle, Neurexin IV and the α-subunit of the Na+,K+-ATPase, respectively. To gain more insight into their functions in epithelial polarity, the scientists first generated transgenic flies, which were defective in one or two of the above genes. In a second step they assessed the effects on epithelial polarity by using antibodies to identify the locations of two proteins, ‘Crumbs’ and ‘Discs large’, which served as marker proteins for apical and basolateral membranes, respectively.

When the team examined early Drosophila embryos that lacked the yrt gene, they observed that in addition to being found in the apical membrane, ‘Crumbs’ was mislocalized in the basolateral membrane, indicating that the Yurt protein is required for correct positioning of ‘Crumbs’ and epithelial polarity. In another type of mutant embryo, which lacked a functional yrt gene, but was provided with yrt mRNA to produce Yurt protein by the mother's follicle cells (surrounding the egg), mislocalization of ‘Crumbs’ was far less pronounced. Because the yrt gene was still expressed to some extent, the team could study genetic interactions between yrt and polarity genes that did not show mislocalization of ‘Crumbs’ when they were mutated individually.

The scientists hypothesized that if they observe polarity defects in a double mutant, the deleted genes must be acting synergistically, suggesting that they function in the same pathway of epithelial cell differentiation. From six genes that they investigated in combination with yrt, they observed mislocalization of the ‘Crumbs’ protein in the yrt/ATPa, yrt/Nrx-IV and yrt/cora double mutants, suggesting functions for the Na+,K+-ATPase and Neurexin-IV in establishing epithelial polarity.

Next they carefully analyzed the polarity phenotypes of the various mutants at different developmental stages. In doing so, the team provided genetic evidence that Yurt, Coracle, Neurexin-IV and the Na+,K+-ATPase are a new group of functionally cooperating proteins that are involved in establishing an epithelial cell's polarity, and the proteins seem to act in two partially overlapping pathways.

Together, Tepass's team have provided evidence that epithelial polarity is controlled on multiple levels involving different protein complexes. Yurt seems to play a major role in this process. It appears that Yurt is a general core regulator of epithelial membrane organization, as this function is conserved even in mammalian cells.