Grainyhead transcription factors control epithelial barriers, tissue morphogenesis, and differentiation, but their role in the kidney is poorly understood. Here, we report that nephric duct, ureteric bud, and collecting duct epithelia express high levels of grainyhead-like homolog 2 (Grhl2) and that nephric duct lumen expansion is defective in Grhl2-deficient mice. In collecting duct epithelial cells, Grhl2 inactivation impaired epithelial barrier formation and inhibited lumen expansion. Molecular analyses showed that GRHL2 acts as a transcriptional activator and strongly associates with histone H3 lysine 4 trimethylation. Integrating genome-wide GRHL2 binding as well as H3 lysine 4 trimethylation chromatin immunoprecipitation sequencing and gene expression data allowed us to derive a high-confidence GRHL2 target set. GRHL2 transactivated a group of genes including Ovol2, encoding the ovo-like 2 zinc finger transcription factor, as well as E-cadherin, claudin 4 (Cldn4), and the small GTPase Rab25. Ovol2 induction alone was sufficient to bypass the requirement of Grhl2 for E-cadherin, Cldn4, and Rab25 expression. Re-expression of either Ovol2 or a combination of Cldn4 and Rab25 was sufficient to rescue lumen expansion and barrier formation in Grhl2-deficient collecting duct cells. Hence, we identified a Grhl2/Ovol2 network controlling Cldn4 and Rab25 expression that facilitates lumen expansion and barrier formation in subtypes of renal epithelia. The renal collecting duct's vital electrolyte and waterregulatory functions are carried out by highly specialized cell populations. 1 The collecting duct itself is composed of a tight epithelium, which separates the urinary compartment from a hypertonic interstitium and maintains a barrier to concentration gradients. The collecting duct derives from the ureteric bud, which emanates from the nephric duct and then undergoes branching morphogenesis in response to signals from the adjacent metanephric mesenchyme. 2 Many of the genes that are critical for aspects of nephric duct development continue to be expressed in the ureteric bud and collecting duct, serving roles in development, differentiation, and maintenance of these cells (e.g., Pax2/8, Gata3, Emx2, and Hnf1b). 3 Little is known about the molecular pathways governing the specific epithelial properties of these cells, although it is clear that these epithelia share several cell biologic properties such as a uniform tubular appearance characterized by a cuboidal epithelium surrounding a fluid-filled lumen and a molecular composition of the apical junctional complex that includes

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