The Sphingosine-1-phospate receptor 1 mediates S1P action during cardiac development

Background: Sphingosine-1-phosophate (S1P) is a biologically active sphingolipid metabolite that influences cellular events including differentiation, proliferation, and migration. S1P acts through five distinct cell surface receptors designated S1P(1-5)R, with S1P(1)R having the highest expression level in the developing heart. S1P(1)R is critical for vascular maturation, with its loss leading to embryonic death by E14.5; however, its function during early cardiac development is not well known. Our previous studies demonstrated that altered S1P levels adversely affects atrioventricular (AV) canal development in vitro, with reduced levels leading to cell death and elevated levels inhibiting cell migration and endothelial to mesenchymal cell transformation (EMT). Results: We determined, by real-time PCR analysis, that S1P(1)R was expressed at least 10-fold higher than other S1P receptors in the developing heart. Immunohistochemical analysis revealed S1P(1)R protein expression in both endothelial and myocardial cells in the developing atrium and ventricle. Using AV canal cultures, we observed that treatment with either FTY720 (an S1P(1,3,4,5)R agonist) or KRP203 (an S1P(1)R-specific agonist) caused similar effects on AV canal cultures as S1P treatment, including induction of cell rounding, inhibition of cell migration, and inhibition of EMT. In vivo, morphological analysis of embryonic hearts at E10.5 revealed that S1P(1)R-/- hearts were malformed with reduced myocardial tissue. In addition to reduced myocardial tissue, E12.5 S1P(1)R-/- hearts had disrupted morphology of the heart wall and trabeculae, with thickened and disorganized outer compact layer and reduced fibronectin (FN) deposition compared to S1P(1)R+/+ littermates. The reduced myocardium was accompanied by a decrease in cell proliferation but not an increase in apoptosis. Conclusions: These data indicate that S1P(1)R is the primary mediator of S1P action in AV canal cultures and that loss of S1P(1)R expression in vivo leads to malformed embryonic hearts, in part due to reduced fibronectin expression and reduced cell proliferation.