S1pr2/Gα13 signaling controls myocardial migration by regulating endoderm convergence

A key process during vertebrate heart development is the migration of bilateral populations of myocardial precursors towards the midline to form the primitive heart tube. In zebrafish, signaling mediated by sphingosine-1-phosphate (S1P) and its cognate G protein-coupled receptor (S1pr2/Mil) is essential for myocardial migration, but the underlying mechanisms remain undefined. Here, we show that suppression of G alpha(13) signaling disrupts myocardial migration, leading to the formation of two bilaterally located hearts (cardia bifida). Genetic studies indicate that G alpha(13) acts downstream of S1pr2 to regulate myocardial migration through a RhoGEF-dependent pathway. Furthermore, disrupting any component of the S1pr2/G alpha(13)/RhoGEF pathway impairs endoderm convergence during segmentation, and the endodermal defects correlate with the extent of cardia bifida. Moreover, endoderm transplantation reveals that the presence of wild-type anterior endodermal cells in G alpha(13)-deficient embryos is sufficient to rescue the endoderm convergence defect and cardia bifida, and, conversely, that the presence of anterior endodermal cells defective for S1pr2 or G alpha(13) in wild-type embryos causes such defects. Thus, S1pr2/G alpha(13) signaling probably acts in the endoderm to regulate myocardial migration. In support of this notion, cardiac-specific expression of G alpha(13) fails to rescue cardia bifida in the context of global G alpha(13) inhibition. Our data demonstrate for the first time that the G alpha(13)/RhoGEF-dependent pathway functions downstream of S1pr2 to regulate convergent movement of the endoderm, an event that is crucial for coordinating myocardial migration.