Apelin-APJ Signaling Is a Critical Regulator of Endothelial MEF2 Activation in Cardiovascular Development

Abstract

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.

Hidden Peptides Encoded by Putative Noncoding RNAs

“…About half of Apela-deficient mice also die during embryogenesis as a result of hypovascularity of the yolk sac and placenta as well as embryonic vascular malformations (Ho et al, 2017), which are again similar to the phenotypes of Aplnr-deficient mice (Kang et al, 2013). Although Aplnr is expressed in the yolk sac mesoderm that gives rise to endothelial cells, the Apela mRNA is not detectable in the endothelial precursors of the yolk sac.…”

The apelin receptor APJ: journey from an orphan to a multifaceted regulator of homeostasis

“…In the periphery, strong receptor binding can be observed in those tissues that exhibit strong ISHH signals, indicating a good correlation between receptor transcription and translation (Pope et al 2012). Recently, in the mouse embryo (E9.5-10.5) cardiovascular system, APJ mRNA has been shown to predominate in the endothelial layers of arteries and veins and in the endocardial layer of the heart (Kang et al 2013), while APJ protein has been shown to be present in mouse hepatocytes and hepatic tissue (Chu et al 2013). …”

“…These effects suggest possible PTXsensitive and -insensitive signalling pathways to be linked to this receptor and indicate that some of the actions of APJ could be mediated by G i/o and/or G q/11 coupling (Szokodi et al 2002). Recently, in human umbilical vein endothelial cells (HUVECs), APJ has been shown to activate Ga 13 , resulting in the cytoplasmic translocation of class II histone deacetylases HDAC4 and HDAC5 and the activation of the transcription factor MEF2, in an apelin-independent manner (Kang et al 2013). Therefore, these studies suggest that apelin signalling may exhibit 'functional selectivity' or 'biased signalling'.…”

“…Cardiovascular development defects have been reported in APJ KO mice, where a loss of homozygous mutants has been described (Charo et al 2009, Kang et al 2013, but not in apelin KO mice (Charo et al 2009), indicating possible ligand-independent effects of the receptor. This effect may perhaps be explained by the recent report that APJ signals independently of apelin in response to cardiac mechanical stretch (Scimia et al 2012).…”

“…APJ KO embryos at E10.5, when lethality begins, have poorly developed vasculature of the yolk sac, delayed formation of the atrioventricular cushion and unusually formed cardinal veins and dorsal aorta (Kang et al 2013). APJ KOs that survive do not reveal any apparent morphological differences , Kang et al 2013; however, they have decreased vascular smooth muscle layer recruitment and myocardial defects including thinning of the myocardium, enlarged right ventricles and ventricular septal defects (Kang et al 2013), suggesting an involvement of apelin/APJ signalling in cardiovascular development.…”