Planar cell polarity and its molecular regulatory mechanism in mammalian development

Abstract

Planar cell polarity (PCP) is an evolutionarily conserved mechanism controlling collective cell polarity within a cell sheet. The core PCP machinery consists of six key components, including Frizzled and Vang, which form two complexes locating at the opposing sides of the cell. Dysregulation of PCP results in various developmental defects throughout the animal kingdom, as well as a variety of human diseases such as neural tube defects and skeletal dysplasias. PCP is established through the coordination of a local module and a global module. The local module, which is composed of six core proteins, allows the local amplification of the PCP signal, while the global module such as Wnt ligands provides the directional information for proper alignment of PCP regarding the body axis. Wnt gradient has been thought to break the symmetry of the core PCP proteins, including Vang and Frizzled. Wnt5a, a non-canonical Wnt ligand, has been shown to induce Vangl2 phosphorylation in a dose-dependent manner and function as a global cue of PCP. However, the in vivo significance of Vangl2 phosphorylation remains elusive. On the other hand, PCP has recently been shown to be involved in breast cancer, but the role of PCP in mammary gland development remains unknown. In this project, I first investigated the functional role of Vangl2 phosphorylation in PCP during mouse development and found that Vangl2 phosphorylation determines its activity. While Vangl2 with reduced phosphorylation level is hypomorphic and exhibits reduced activity, the phosphor-mutant Vangl2 exhibits dominant negative activities by disrupting the membrane localization and stability of its binding partners. I showed that Vangl2 phosphorylation is necessary for its function in PCP in multiple mammalian tissues. Phosphor-mutant Vangl2 fail to rescue typical PCP defects, including neural tube defects, inner ear hair cell mis-orientation, as well as limb elongation. With the reduction of Vangl gene dosage or Vangl2 phosphorylation level, PCP defects become more severe, indicating a dose-dependent effect. I further found that Casein kinase 1 family members CK1ε/δ were responsible for most of the phosphorylation events upon Wnt5a induction and that they function redundantly in mediating Wnt5a-induced Vangl2 phosphorylation. Another core PCP family member, Dishevelled, can also promote Vangl2 phosphorylation. Taken together, a “Vangl activity gradient” model is proposed in PCP establishment that links Wnt signaling gradient and the PCP core machinery. To understand the role of PCP in mammary gland development, an HA-Vangl2 mouse model is generated for convenient and specific detection of Vangl2 expression pattern in mammary gland. My preliminary data show that Vangl2 exhibits a dynamic expression pattern in mammary gland at different stages. Starting from embryonic stage, it continuously expresses until mid-pregnant stage, decreases at late-pregnant stage and re-expresses at involution stage. Vangl2 expresses in both layers of mammary epithelial cells and exhibits an asymmetrical localization pattern in the luminal epithelial cells. These results suggest that PCP might play an important role in mammary gland development so that further studies would provide new insights into our understanding of mammary gland development and its implication in breast cancer.