Distribution of the tight junction proteins ZO-1, occludin, and claudin-4, -8, and -12 in bladder epithelium.

In mammals, the bladder stores urine without permitting the passage of urine contents into the bloodstream, a function, in part, of the uroepithelial-associated tight junction complex. The protein constituents that make up this high-resistance barrier in the bladder are currently unknown, although the claudins, a multigene family, are thought to govern paracellular transport in other epithelia. Reverse transcriptase-polymerase chain reaction analysis was used to define that mRNA for claudin-2, -4, -8, -12, and -13 was expressed in mouse bladder tissue. The localization of these claudins, as well as the tight junction-associated proteins zonula occludens-1 (ZO-1) and occludin, within the bladder epithelium was determined by immunofluorescence microscopy. As expected, occludin and ZO-1 were localized to the tight junctions of rat, mouse, and rabbit umbrella cells. Intriguingly, ZO-1 in mouse epithelium, ZO-1 in the dome region of rabbit bladders and occludin in rat and mouse bladders were also expressed in the underlying intermediate and basal cell layers. Claudin-4, -8, and -12 were found in the umbrella cell tight junction; however, additional staining of claudin-4 was observed along the sites of cell-cell contact in the underlying cell layers of rat, mouse, and rabbit tissue. No claudin-2 staining was associated with tight junctions in the uroepithelium. Our results indicate that claudin-4, -8, and -12 are expressed in umbrella cells, where they may impart the high-resistance phenotype associated with this cell type, and that in some instances tight junction proteins are also associated at the sites of cell contact of the underlying cell layers, perhaps playing some role in cell-cell adhesion.