The human apical protein CRB3 (Crb3 in mouse) organizes epithelial cell polarity. Loss of CRB3 expression increases the tumorogenic potential of cultured epithelial cells and favors metastasis formation in nude mice. These data emphasize the need of in vivo models to study CRB3 functions. Here, we report the phenotypic analysis of a novel Crb3 knockout mouse model. Crb3-deficient newborn mice show improper clearance of airways, suffer from respiratory distress and display perinatal lethality. Crb3 is also essential to maintain apical membrane identity in kidney epithelial cells. Numerous kidney cysts accompany these polarity defects. Impaired differentiation of the apical membrane is also observed in a subset of cells of the intestinal epithelium. This results in improper remodeling of adhesive contacts in the developing intestinal epithelium, thereby leading to villus fusion. We also noted a strong increase in cytoplasmic β-catenin levels in intestinal epithelial cells. β-catenin is a mediator of the Wnt signaling pathway, which is overactivated in the majority of colon cancers. In addition to clarifying the physiologic roles of Crb3, our study highlights that further functional analysis of this protein is likely to provide insights into the etiology of diverse pathologies, including respiratory distress syndrome, polycystic kidney disease and cancer.

The transmembrane protein CRB3A controls epithelial cell polarization. Elucidating the molecular mechanisms of CRB3A function is essential as this protein prevents the epithelial-to-mesenchymal transition (EMT), which contributes to tumor progression. To investigate the functional impact of altered CRB3A expression in cancer cells, we expressed CRB3A in HeLa cells, which are devoid of endogenous CRB3A. While control HeLa cells display a patchy F-actin distribution, CRB3A-expressing cells form a circumferential actomyosin belt. This reorganization of the cytoskeleton is accompanied by a transition from an ameboid cell shape to an epithelial-cell-like morphology. In addition, CRB3A increases the cohesion of HeLa cells. To perform these functions, CRB3A recruits p114RhoGEF and its activator Ehm2 to the cell periphery using both functional motifs of its cytoplasmic tail and increases RhoA activation levels. ROCK1 and ROCK2 (ROCK1/2), which are critical effectors of RhoA, are also essential to modulate the cytoskeleton and cell shape downstream of CRB3A. Overall, our study highlights novel roles for CRB3A and deciphers the signaling pathway conferring to CRB3A the ability to fulfill these functions. Thereby, our data will facilitate further investigation of CRB3A functions and increase our understanding of the cellular defects associated with the loss of CRB3A expression in cancer cells.T he physiology of epithelial cells relies on the asymmetric distribution of specific cellular constituents-a structural organization referred to as epithelial polarity (1). Epithelial cell polarization results in the regionalization of the plasma membrane into apical, lateral, and basal domains. In vertebrate epithelial cells, the apical and lateral domains are segregated by tight junctions (TJ), which seal the intercellular space to prevent passive diffusion across the tissue (2). Different groups of apical and lateral proteins cooperate within their respective domains to elaborate membrane territories with specific compositions and functions (3). In addition, the mutual antagonism between apical and lateral protein complexes defines a sharp apicolateral boundary (3). Pioneer studies in Drosophila melanogaster have established that one of these protein complexes is articulated around the transmembrane apical protein Crumbs (Crb) (4-7). The mammalian genome encodes three Crb orthologs, namely, CRB1, CRB2, and CRB3 (8). CRB1 is expressed mainly in the brain, cornea, and retina (9, 10). Mutations in the human CRB1 or mouse Crb1 gene cause retinal dystrophies (11)(12)(13)(14). CRB2 distribution overlaps that of CRB1, but CRB2 is also found in other organs such as kidneys (15). CRB2 is required for retinal integrity and for gastrulation of mouse embryos (16, 17). CRB3 is widely expressed in epithelial tissues and exists as two isoforms, namely, CRB3A and CRB3B (18,19). The latter associates with spindle poles during mitosis or is found in the primary cilium of epithelial cells to control cytokinesis and ciliogenesis (19). CRB3A is apical...