The tight junction protein claudin-3 shows conserved expression in the nephric duct and ureteric bud and promotes tubulogenesis in vitro

The claudin family of proteins is required for the formation of tight junctions that are contact points between epithelial cells. Although little is known of the cellular events by which epithelial cells of the ureteric bud form tubules and branch, tubule formation is critical for kidney development. We hypothesize that if claudin-3 (Cldn3) is expressed within tight junctions of the ureteric bud, this will affect ureteric bud cell shape and tubule formation. Using transmission electron microscopy, we identified tight junctions within epithelial cells of the ureteric bud. Whole mount in situ hybridization and immunoassays were performed in the mouse and chick and demonstrated that Cldn3 transcript and protein were expressed in the nephric duct, the ureteric bud, and its derivatives at critical time points during tubule formation and branching. Mouse inner medullary collecting duct cells (mIMCD-3) form tubules when seeded in a type I collagen matrix and were found to coexpress CLDN3 and the tight junction marker zonula occludens-1 in the cell membrane. When these cells were stably transfected with Cldn3 fused to the enhanced green fluorescent protein reporter, multiple clones showed a significant increase in tubule formation compared with controls (P < 0.05) due in part to an increase in cell proliferation (P < 0.01). Cldn3 may therefore promote tubule formation and expansion of the ureteric bud epithelium.