Angiogenesis, the formation of new blood vessels from preexisting vessels, is critical to most physiological processes and many pathological conditions. During zebrafish development, angiogenesis expands the axial vessels into a complex vascular network that is necessary for efficient oxygen delivery. Although the dorsal aorta (DA) and the axial vein (AV) are spatially juxtaposed, the initial angiogenic sprouts from these vessels extend in opposite directions, suggesting that distinct cues may regulate angiogenesis of the axial vessels. In this report, we found that angiogenic sprouts from the DA are dependent on Vegf-A signaling, and do not respond to Bmp signals. In contrast, sprouts from the AV are regulated by Bmp signaling independent of Vegf-A signals, suggesting that Bmp is a vein-specific angiogenic cue during early vascular development. Our results support a paradigm, whereby different signals regulate distinct programs of sprouting angiogenesis from the AV and DA, and suggest that signaling heterogeneity contributes to the complexity of vascular networks.

Rationale The peptide ligand apelin and its receptor APJ constitute a signaling pathway with numerous effects on the cardiovascular system, including cardiovascular development in model organisms such as xenopus and zebrafish. Objective This study aimed to characterize the embryonic lethal phenotype of the Apj−/− mice and define the involved downstream signaling targets. Methods and Results We report the first characterization of the embryonic lethality of the Apj−/− mice. Greater than half of the expected Apj−/− embryos died in utero due to cardiovascular developmental defects. Those succumbing to early embryonic death had markedly deformed vasculature of the yolk sac and the embryo, as well as poorly looped hearts with aberrantly formed right ventricles and defective atrioventricular cushion formation. Apj−/− embryos surviving to later stages demonstrated incomplete vascular maturation due to a deficiency of vascular smooth muscle cells, and impaired myocardial trabeculation and ventricular wall development. The molecular mechanism implicates a novel, non-canonical signaling pathway downstream of apelin-APJ involving Gα13, which induces histone deacetylase (HDAC) 4 and HDAC5 phosphorylation and cytoplasmic translocation, resulting in activation of MEF2 (myocyte enhancer factor 2). Apj−/− mice have greater endocardial Hdac4 and Hdac5 nuclear localization, and reduced expression of the MEF2 transcriptional target Klf2. We identify a number of commonly shared transcriptional targets among apelin-APJ, Gα13, and MEF2 in endothelial cells, which are significantly decreased in the Apj−/− embryos and endothelial cells. Conclusions Our results demonstrate a novel role for apelin-APJ signaling as a potent regulator of endothelial MEF2 function in the developing cardiovascular system.