Essential role of Apelin signaling during lymphatic development in zebrafish.

OBJECTIVE Apelin and its cognate receptor Aplnr/Apj are essential for diverse biological processes. However, the function of Apelin signaling in lymphatic development remains to be identified, despite the preferential expression of Apelin and Aplnr within developing blood and lymphatic endothelial cells in vertebrates. In this report, we aim to delineate the functions of Apelin signaling during lymphatic development. APPROACH AND RESULTS We investigated the functions of Apelin signaling during lymphatic development using zebrafish embryos and found that attenuation of Apelin signaling substantially decreased the formation of the parachordal vessel and the number of lymphatic endothelial cells within the developing thoracic duct, indicating an essential role of Apelin signaling during the early phase of lymphatic development. Mechanistically, we found that abrogation of Apelin signaling selectively attenuates lymphatic endothelial serine-threonine kinase Akt 1/2 phosphorylation without affecting the phosphorylation status of extracellular signal-regulated kinase 1/2. Moreover, lymphatic abnormalities caused by the reduction of Apelin signaling were significantly exacerbated by the concomitant partial inhibition of serine-threonine kinase Akt/protein kinase B signaling. Apelin and vascular endothelial growth factor-C (VEGF-C) signaling provide a nonredundant activation of serine-threonine kinase Akt/protein kinase B during lymphatic development because overexpression of VEGF-C or apelin was unable to rescue the lymphatic defects caused by the lack of Apelin or VEGF-C, respectively. CONCLUSIONS Taken together, our data present compelling evidence suggesting that Apelin signaling regulates lymphatic development by promoting serine-threonine kinase Akt/protein kinase B activity in a VEGF-C/VEGF receptor 3-independent manner during zebrafish embryogenesis.