Sphingosine 1-phosphate signaling in zebrafish heart and endoderm development

One of the first morphogenetic processes in vertebrate heart development is the assembly of the midline primitive heart tube from two fields of precardiac mesodermal cells. How these two groups of anterior lateral mesodermal cells migrate to the embryonic midline to form a single heart tube is a question of great interest. One developmental system that has contributed to our understanding of heart tube assembly is the zebrafish. Numerous zebrafish mutants have been isolated that cause defects in the migration of the precardiac mesodermal cells to the midline, therefore disrupting formation of the primitive heart tube. One such mutant, miles apart (mil), was shown to encode an orthologue of the mammalian G protein-coupled receptor, S1P2. S1P2 is a member of a family of receptors that specifically recognize sphingosine 1-phosphate (S1P) as their ligand. In the first chapter of this dissertation I review the current understanding of how S1P is regulated and then how it mediates signaling by its receptors. I discuss the biology of this receptor family with respect to downstream signaling partners as well as some of the biological processes affected by the receptors. In the second chapter, I present data showing that the precardiac mesoderm migration defect seen in mil mutant zebrafish embryos is due to a defect in the morphogenesis of the anterior endoderm, a tissue known to be required for migration of the precardiac mesoderm in zebrafish. Furthermore, I show that these endodermal defects cause craniofacial development defects in mil mutants. In the third chapter, I introduce another zebrafish mutant two of hearts (toh) and show that toh is a necessary component in signaling via Mil, but not by another related S1P receptor. Furthermore, the toh locus encodes a putative twelve pass transmembrane transporter related to the Drosophila spinster gene. These data represent the first connection between a vertebrate spinster-like gene and a specific signaling pathway. These findings also demonstrate that toh is a novel component in signaling via an S1P receptor.