Roles of adhesion G protein-coupled receptors during zebrafish gastrulation and neuronal migration
Gastrulation is a fundamental process during embryogenesis when the germ layers and shape of the animal are generated. Although the major cell movements during gastrulation have been characterized, the underlying cellular and molecular mechanisms are only beginning to be understood. Identifying new molecules regulating gastrulation and studying their interactions with known regulators of gastrulation will bring us closer to a full mechanistic understanding of this key developmental process. In this study, I investigated the potential roles of the adhesion G protein-coupled receptor (GPCR) family during gastrulation. Among 30 annotated or partially annotated adhesion GPCRs in the sequenced zebrafish genome, the four members comprising the Group IV adhesion GPCR subfamily exhibit distinct and dynamic expression patterns during embryogenesis. Therefore, I performed functional analyses of members from this subfamily during embryogenesis. Interfering with Gpr124 function caused defects in multiple tissues in the caudal region of the embryo, including the notochord and vasculature, which warrant future studies on the mechanism of Gpr124 function. Central to this thesis, I uncovered a role for Gpr125 during convergence and extension (C&E) gastrulation movements and facial branchiomotor neuron (FBMN) migration. Consistent with a potential role during gastrulation, I showed gpr125 is expressed maternally and at blastula and gastrula stages. Excess Gpr125 in wild-type embryos impaired C&E movements and the underlying cellular and molecular polarities. Whereas interfering with Gpr125 function alone did not affect development, it exacerbated the C&E and FBMN migration defects of embryos with reduced Wnt/planar cell polarity (PCP) signaling. At the cellular level, Gpr125 depletion enhanced the disruption of polarized cell behaviors in embryos heterozygous and homozygous for trilobite/vang-like 2 (tri/vangl2) Wnt/PCP gene. At the molecular level, Gpr125 recruited Dishevelled (Dvl), the signaling hub of Wnt/PCP pathway, to the cell membrane, which fulfils the prerequisite for Wnt/PCP activation. Moreover, Gpr125 and Dvl mutually clustered with one another to form discrete membrane subdomains, and the Gpr125 intracellular domain directly interacted with Dvl in pull-down assays. Intriguingly, Dvl and Gpr125 were able to recruit a subset of PCP components into membrane subdomains, suggesting that Gpr125 may modulate the composition of Wnt/PCP membrane complexes. This study reveals a role for Gpr125 in Wnt/PCP mediated processes and provides mechanistic insight into Gpr125 function and Wnt/PCP signaling.