P13. Wnt signaling pathways regulate epithelial polarity and morphogenesis

Abstract

Wnt proteins are highly conserved secreted proteins that are essential for development in all metazoan organisms. Wnt proteins bind to cell surface receptors and activates the β-catenin-dependent and -independent pathways. The β-catenin-dependent pathway regulates cell proliferation and differentiation through the regulation of target gene expression. The β-catenin-independent pathway is involved in cell migration and polarity. Knockout mouse studies revealed that both the β-catenin-dependent and -independent pathways of Wnt signaling are important in the epithelial morphogenesis of tubular organs such as the lungs, kidneys, guts, and mammary glands, but its molecular mechanism is poorly understood. Epithelial cells established apical–basal polarity and formed cyst with central lumen in three-dimensional culture. Epidermal growth factor (EGF) increased the size of cyst through the promotion of cell proliferation. Although Wnt3a, which is a representative ligand that activates the β-catenin-dependent pathway, did not affect cyst morphogenesis, combination of Wnt3a and EGF induced tube-like branching morphogenesis. Treatment with EGF and blebbistatin, a Myosin II inhibitor, induced branching morphogenesis similar to that induced by EGF and Wnt3a. Blebbistatin inhibited phosphorylation of paxillin, which is a component of focal adhesions. Knockdown of β-catenin or LRP6, which is a receptor of Wnt3a, inhibited branching morphogenesis induced by Wnt3a. Wnt5a, which is a representative ligand that activates the β-catenin-independent pathway, did not affect cyst morphology in the presence or absence of EGF. Knockdown of Wnt5a inhibited the formation of apical membrane domain of epithelial cells. ECM signaling from the basal cell surface is important for the decision of apical–basal polarity. Wnt5a was observed at the basal cell surface and Ror2, a Wnt5a receptor specifically localized basolateral cell surface in epithelial cyst. These results indicate that both the β-catenin-dependent and -independent pathways of Wnt signaling are involved in the regulation of epithelial polarity and morphogenesis through the distinct mechanisms.