THE ROLE OF THE VG1 ZEBRAFISH ORTHOLOG GDF3 IN NODAL-DEPENDENT MESENDODERM FORMATION AND LEFT-RIGHT PATTERNING

Abstract

Nodal signaling plays an important role in mesendoderm formation and the placement of organs during vertebrate embryonic patterning. There are three Nodal (a TGFβ ligand) orthologs in zebrafish; the nodal orthologs squint (sqt or nodal-related 1) and cyclops (cyc or nodal-related 2) are required for mesendoderm formation while the nodal ortholog southpaw (spaw) is required for the organization of organs. Here, I report on the role of another TGFβ ligand, the Vg1 ortholog in zebrafish called Growth differentiation factor 3 (Gdf3). Vg1 orthologs have been implicated in mesoderm formation and left-right patterning in the mouse and Xenopus. However, mesoderm defects had not been observed when Gdf3 was depleted in zebrafish embryos, which suggested the need for a more comprehensive investigation of Gdf3 function during zebrafish embryonic development. Using CRISPR/Cas9 generated mutants and Gdf3 knockdown techniques, we demonstrate that maternal gdf3 is required for Nodal-dependent mesendoderm formation. Furthermore, we show that Gdf3 is important for the formation of the left-right organizer and for Nodal signaling during left-right patterning. Our data suggest that Gdf3 is a facilitator of Nodal signaling. And although Nodal signaling can occur without Gdf3, the signaling potential is insufficient for proper mesendoderm formation and left-right patterning. Interestingly, our work also reveals that Gdf3 augments the function of the Nodal signaling inhibitor Lefty1. Collectively, our data indicate that Gdf3 potentiates the function of the Nodal ligand to promote robust Nodal signaling, while also enhancing the function of Lefty1 to negatively regulate signaling pathways during zebrafish embryonic development.