Biological functions of Elabela, a novel endogenous ligand of APJ receptor

The G protein-coupled receptor APJ and its cognate ligand, apelin, are widely expressed throughout human body. They are implicated in different key physiological processes such as angiogenesis, cardiovascular functions, fluid homeostasis, and energy metabolism regulation. Recently, a new endogenous peptidic ligand of APJ, named Elabela, has been identified and shown to play a crucial role in embryonic development. In addition, increasing evidences show that Elabela is also intimate associated with a large number of physiological processes in adulthood. However, a comprehensive summary of Elabela has not been reported to date. In this review, we provide an overview of the biological functions of Elabela. Collectively, Elabela, a potential therapeutic peptide, exerts diverse biological functions in both embryos and adult organisms, such as dysontogenesis, self-renewing of human embryonic stem cells, endoderm differentiation, heart morphogenesis, cardiac dyfunctions, blood pressure control, angiogenesis, blood pressure control, regulation of food and water intake, bone formation, and kidney diseases.

The apelinergic system as an alternative to catecholamines in low-output septic shock

Catecholamines, in concert with fluid resuscitation, have long been recommended in the management of septic shock. However, not all patients respond positively and controversy surrounding the efficacy-to-safety profile of catecholamines has emerged, trending toward decatecholaminization. Contextually, it is time to re-examine the “maintaining blood pressure” paradigm by identifying safer and life-saving alternatives. We put in perspective the emerging and growing knowledge on a promising alternative avenue: the apelinergic system. This target exhibits invaluable pleiotropic properties, including inodilator activity, cardio-renal protection, and control of fluid homeostasis. Taken together, its effects are expected to be greatly beneficial for patients in septic shock.

The mechanism of all-trans retinoic acid in the regulation of apelin expression in vascular endothelial cells

The apelin gene can promote vascular endothelial cell (VEC) proliferation, migration, and angiogenesis. However, the molecular mechanism for regulation of the apelin gene is still unknown. Real-time PCR and Western blotting analysis were employed to detect the effect of all-trans retinoic acid (ATRA) in up-regulating apelin expression in human umbilical vein endothelial cells (HUVECs). Furthermore, the in vivo study also indicated that ATRA could increase apelin expression in balloon-injured arteries of rats, which is consistent with the results from the cultured HUVECs. To ensure whether retinoic acid receptor (RAR) α (RARα) could be induced by ATRA in regulating apelin, the expression of RARα was tested with a siRNA method to knock down RARα or adenovirus vector infection to overexpress RARα. The results showed that ATRA could up-regulate apelin expression time- and dose- dependently in HUVECs. ATRA could induce a RARα increase; however, the expression of RARβ and RARγ were unchanged. The blocking of RARα signaling reduced the response of apelin to ATRA when HUVECs were treated with RARα antagonists (Ro 41-5253) or the use of siRNA against RARα (si-RARα) knockdown RARα expression before using ATRA. In addition, induction of RARα overexpression by infection with pAd-GFP-RARα further increased the induction of apelin by ATRA. These results suggested that ATRA up-regulated apelin expression by promoting RARα signaling.

ELABELA Improves Cardio-Renal Outcome in Fatal Experimental Septic Shock

OBJECTIVES

Apelin-13 was recently proposed as an alternative to the recommended β-adrenergic drugs for supporting endotoxin-induced myocardial dysfunction. Since Apelin-13 signals through its receptor (Apelin peptide jejunum) to exert singular inotropic/vasotropic actions and to optimize body fluid balance, this candidate pathway might benefit septic shock management. Whether the newly discovered ELABELA (ELA), a second endogenous ligand of the Apelin peptide jejunum receptor highly expressed in the kidney, further improves cardio-renal impairment remains unknown.

DESIGN, SETTING, AND SUBJECTS

Interventional study in a rat model of septic shock (128 adult males) to assess the effects of ELA and Apelin-13 on vascular and cardio-renal function. Experiments were performed in a tertiary care University-based research institute.

INTERVENTIONS

Polymicrobial sepsis-induced cardiac dysfunction was produced by cecal ligation puncture to assess hemodynamic efficacy, cardioprotection, and biomechanics under acute or continuous infusions of the apelinergic agonists ELA or Apelin-13 (39 and 15 µg/kg/hr, respectively) versus normal saline.

MEASUREMENTS AND MAIN RESULTS

Apelinergic agonists improved 72-hour survival after sepsis induction, with ELA providing the best clinical outcome after 24 hours. Apelinergic agonist infusion counteracted cecal ligation puncture-induced myocardial dysfunction by improving left ventricular pressure-volume relationship. ELA-treated cecal ligation puncture rats were the only group to 1) display a significant improvement in left ventricular filling as shown by increased E-wave velocity and left ventricular end-diastolic volume, 2) exhibit a higher plasma volume, and 3) limit kidney injury and free-water clearance. These beneficial renal effects were superior to Apelin-13, likely because full-length ELA enabled a distinctive regulation of pituitary vasopressin release.

CONCLUSIONS

Activation of the apelinergic system by exogenous ELA or Apelin-13 infusion improves cardiovascular function and survival after cecal ligation puncture-induced sepsis. However, ELA proved better than Apelin-13 by improving fluid homeostasis, cardiovascular hemodynamics recovery, and limiting kidney dysfunction in a vasopressinergic-dependent manner.

Alternative Progenitor Cells Compensate to Rebuild the Coronary Vasculature in Elabela- and Apj-Deficient Hearts

Organogenesis during embryonic development occurs through the differentiation of progenitor cells. This process is extraordinarily accurate, but the mechanisms ensuring high fidelity are poorly understood. Coronary vessels of the mouse heart derive from at least two progenitor pools, the sinus venosus and endocardium. We find that the ELABELA (ELA)-APJ signaling axis is only required for sinus venosus-derived progenitors. Because they do not depend on ELA-APJ, endocardial progenitors are able to expand and compensate for faulty sinus venosus development in Apj mutants, leading to normal adult heart function. An upregulation of endocardial SOX17 accompanied compensation in Apj mutants, which was also seen in Ccbe1 knockouts, indicating that the endocardium is activated in multiple cases where sinus venosus angiogenesis is stunted. Our data demonstrate that by diversifying their responsivity to growth cues, distinct coronary progenitor pools are able to compensate for each other during coronary development, thereby providing robustness to organ development.