Investigating the Role of Apelin Receptor Signaling in Zebrafish Myocardial Progenitor Development

In vertebrates, the heart is the first organ to form and function. The basic steps and molecular pathways involved in heart development are highly conserved. Myocardial progenitor-fated cells are among the first cells to migrate during gastrulation away from the primitive streak. These cells move bilaterally to populate the heart-forming region (HFR) in the anterior lateral plate mesoderm (ALPM). Once cells have reached the HFR, they receive the signals necessary to differentiate into myocardial progenitor cells. It is clear that the development of myocardial progenitor cells entails the migration of cells from the lateral embryonic margin to the ALPM. However, it is unclear whether cells are specified for a myocardial progenitor fate early in embryogenesis, a step that may promote their migration specifically to the ALPM, or whether the migration of cells to the ALPM alone is sufficient for differentiation into myocardial progenitor cells. A zebrafish mutant, grinch (grn), was indentified in which there is a defect in the development of myocardial progenitor cells. The mutation resulting in the grn phenotype was mapped to the gene encoding the G protein-coupled receptor Apelin receptor b (Aplnrb). I have used the aplnrb mutant embryo, as well as morpholino-mediated knockdown (morphant embryos) of aplnrb, and its paralog aplnra, to determine the function of Aplnr signaling in myocardial progenitor development. Results demonstrate that Aplnr signaling is necessary for the migration of cells from the lateral embryonic margin of the zebrafish embryo to the heart-forming region. Interestingly, this entails a novel cell-non-autonomous function for Aplnr signaling. Furthermore, both the only identified ligand for the receptor, Apelin, and the canonical mediators of Aplnr signaling, Gαi/o proteins, appear to be dispensable for this process. Loss of Aplnr signaling also appears to affect embryonic patterning of the early embryo through subtle perturbations of Nodal, Wnt, and Bmp signaling and attenuation of Nodal signaling can partially recapitulate the aplnr morphant cardiac phenotype. Taken together, my results suggest that Aplnr signaling plays a role in creating an environment that allows for the migration of cells to the heart-forming region, possibly through the regulation of early embryonic patterning.Ph