The spinster homolog, two of hearts, is required for sphingosine 1-phosphate signaling in zebrafish.

The bioactive lipid sphingosine 1-phosphate (S1P) and its G protein-coupled receptors play critical roles in cardiovascular, immunological, and neural development and function. Despite its importance, many questions remain about S1P signaling, including how S1P, which is synthesized intracellularly, is released from cells. Mutations in the zebrafish gene encoding the S1P receptor Miles Apart (Mil)/S1P(2) disrupt the formation of the primitive heart tube. We find that mutations of another zebrafish locus, two of hearts (toh), cause phenotypes that are morphologically indistinguishable from those seen in mil/s1p2 mutants. Positional cloning of toh reveals that it encodes a member of the Spinster-like family of putative transmembrane transporters. The biological functions of these proteins are poorly understood, although phenotypes of the Drosophila spinster and zebrafish not really started mutants suggest that these proteins may play a role in lipid trafficking. Through gain- and loss-of-function analyses, we show that toh is required for signaling by S1P(2). Further evidence indicates that Toh is involved in the trafficking or cellular release of S1P.

Nodal signaling promotes the speed and directional movement of cardiomyocytes in zebrafish.

Members of the Nodal family regulate left-right asymmetry during vertebrate organogenesis, but it is unclear how Nodal signaling controls asymmetric morphogenesis at the cellular level. We used high-resolution time-lapse imaging in zebrafish to compare the movements of cardiomyocytes in the presence or absence of Nodal signaling. Loss of Nodal signaling in late-zygotic mutants for the Nodal co-receptor one-eyed pinhead (LZoep) abolished the leftward movement of cardiomyocytes. Global heart rotation was blocked but cardiomyocyte neighbor relationships were maintained as in wild type. Cardiomyocytes in LZoep mutants moved more slowly and less directionally than their wild-type counterparts. The phenotypes observed in the absence of Nodal signaling strongly resemble abnormalities found in BMP signaling mutants. These results indicate that a Nodal-BMP signaling cascade drives left-right heart morphogenesis by regulating the speed and direction of cardiomyocyte movement.

Endocardium is necessary for cardiomyocyte movement during heart tube assembly.

Embryonic heart formation requires the union of bilateral populations of cardiomyocytes and their reorganization into a simple tube. Little is known about the morphogenetic mechanisms that coordinate assembly of the heart tube and determine its dimensions. Using time-lapse confocal microscopy to track individual cardiomyocyte movements in the zebrafish embryo, we identify two morphologically and genetically separable phases of cell movement that coordinate heart tube assembly. First, all cardiomyocytes undergo coherent medial movement. Next, peripherally located cardiomyocytes change their direction of movement, angling toward the endocardial precursors and thereby establishing the initial circumference of the nascent heart tube. These two phases of cardiomyocyte behavior are independently regulated. Furthermore, we find that myocardial-endocardial interactions influence the second phase by regulating the induction, direction and duration of cardiomyocyte movement. Thus, the endocardium plays a crucial early role in cardiac morphogenesis, organizing cardiomyocytes into a configuration appropriate for heart tube assembly. Together, our data reveal a dynamic cellular mechanism by which tissue interactions establish organ architecture.