Characterization of apela, a novel endogenous ligand of apelin receptor, in the adult heart

Electrical remodeling of atrioventricular junction: a study on retrogradely perfused chick embryonic heart

Atrioventricular (AV) accessory pathways (APs) provide additional electrical connections between the atria and ventricles, resulting in severe electrical disturbances. It is generally accepted that APs originate in the altered annulus fibrosus maturation in the late prenatal and perinatal period. However, current experimental methods cannot address their development in specific locations around the annulus fibrosus because of the inaccessibility of late fetal hearts for electrophysiological investigation under physiological conditions. In this study, we describe an approach for optical mapping of the retrogradely perfused chick heart in the last third of the incubation period. This system showed stability for electrophysiological measurement for several hours. This feature allowed analysis of the number and functionality of the APs separately in each clinically relevant position. Under physiological conditions, we also recorded the shortening of the AV delay with annulus fibrosus maturation and analyzed ventricular activation patterns after conduction through APs at specific locations. We observed a gradual regression of AP with an area-specific rate (left-sided APs disappeared first). The results also revealed a sudden drop in the number of active APs between embryonic days 16 and 18. Accessory myocardial AV connections were histologically documented in all positions around the annulus fibrosus even after hatching. The fact that no electrically active AP was present at this stage highlights the necessity of electrophysiological evaluation of accessory atrioventricular connections in studying AP formation.NEW & NOTEWORTHY We present the use of retrograde perfusion and optical mapping to investigate, for the first time, the regression of accessory pathways during annulus fibrosus maturation, separately examining each clinically relevant location. The system enables measurements under physiological conditions and demonstrates long-lasting stability compared with other approaches. This study offers applications of the model to investigate electrical and/or functional development in late embryonic development without concern about heart viability.

Endothelial ELABELA improves post-ischemic angiogenesis by upregulating VEGFR2 expression

BACKGROUND

Post-ischemic angiogenesis is critical for perfusion recovery and tissue repair. ELABELA (ELA) plays an essential role in embryonic heart development and vasculogenesis. However, the mechanism of ELA on post-ischemic angiogenesis is poorly characterized.

METHODS

We first assessed ELA expression after hind limb ischemia (HLI) in mice. We then established a HLI model in tamoxifen-inducible endothelial-ELA-specific knockout mice (ELAECKO) and assessed the rate of perfusion recovery, capillary density, and VEGFR2 pathway. Knockdown of ELA with lentivirus or siRNA and exogenous addition of ELA peptides were employed to analyze the effects of ELA on angiogenic capacity and VEGFR2 pathway in endothelial cells in vitro. The serum levels of ELA in healthy people and patients with type 2 diabetes mellitus (T2DM) and diabetic foot ulcer (DFU) were detected by a commercial ELISA kit.

RESULTS

In murine HLI models, ELA was significantly up-regulated in the ischemic hindlimb. Endothelial-specific deletion of ELA impaired perfusion recovery and angiogenesis. In physiologic conditions, no significant difference in VEGFR2 expression was found between ELAECKO mice and ELAWT mice. After ischemia, the expression of VEGFR2, p-VEGFR2, and p-AKT was significantly lower in ELAECKO mice than in ELAWT mice. In cellular experiments, the knockdown of ELA inhibited endothelial cell proliferation and tube formation, and the addition of ELA peptides promoted proliferation and tube formation. Mechanistically, ELA upregulated the expression of VEGFR2, p-VEGFR2, and p-AKT in endothelial cells under hypoxic conditions. In clinical investigations, DFU patients had significantly lower serum levels of ELA compared to T2DM patients.

CONCLUSION

Our results indicated that endothelial ELA is a positive regulator of post-ischemic angiogenesis via upregulating VEGFR2 expression. Targeting ELA may be a potential therapeutic option for peripheral arterial diseases.

Apelin regulates skeletal muscle adaptation to exercise in a high-intensity interval training model

Plasma apelin levels are reduced in aging and muscle wasting conditions. We aimed to investigate the significance of apelin signaling in cardiac and skeletal muscle responses to physiological stress. Apelin knockout (KO) and wild-type (WT) mice were subjected to high-intensity interval training (HIIT) by treadmill running. The effects of apelin on energy metabolism were studied in primary mouse skeletal muscle myotubes and cardiomyocytes. Apelin increased mitochondrial ATP production and mitochondrial coupling efficiency in myotubes and promoted the expression of mitochondrial genes both in primary myotubes and cardiomyocytes. HIIT induced mild concentric cardiac hypertrophy in WT mice, whereas eccentric growth was observed in the left ventricles of apelin KO mice. HIIT did not affect myofiber size in skeletal muscles of WT mice but decreased the myofiber size in apelin KO mice. The decrease in myofiber size resulted from a fiber type switch toward smaller slow-twitch type I fibers. The increased proportion of slow-twitch type I fibers in apelin KO mice was associated with upregulation of myosin heavy chain slow isoform expression, accompanied with upregulated expression of genes related to fatty acid transport and downregulated expression of genes related to glucose metabolism. Mechanistically, skeletal muscles of apelin KO mice showed defective induction of insulin-like growth factor-1 signaling in response to HIIT. In conclusion, apelin is required for proper skeletal and cardiac muscle adaptation to high-intensity exercise. Promoting apelinergic signaling may have benefits in aging- or disease-related muscle wasting conditions.NEW & NOTEWORTHY Apelin levels decline with age. This study demonstrates that in trained mice, apelin deficiency results in a switch from fast type II myofibers to slow oxidative type I myofibers. This is associated with a concomitant change in gene expression profile toward fatty acid utilization, indicating an aged-muscle phenotype in exercised apelin-deficient mice. These data are of importance in the design of exercise programs for aging individuals and could offer therapeutic target to maintain muscle mass.

Expression and in vitro effect of phoenixin-14 on the porcine ovarian granulosa cells

Phoenixin-14 (PNX-14) regulates energy metabolism via the G protein-coupled receptor 173 (GPR173); elevated plasma levels have been described in patients with polycystic ovary syndrome. The aims were to investigate the ovarian expression of PNX-14/GPR173 and the in vitro effect of PNX-14 on granulosa cells (Gc) function. Transcript and protein levels of PNX-14/GRP173 were analysed by real-time PCR, western blot and immunohistochemistry in the porcine ovarian follicles at days 2-3, 10-12 and 16-18 of the oestrous. For in vitro experiments, Gc were isolated from follicles at days 10-12 of the oestrous (4-6 mm) and PNX-14 at doses 1-1000 nM was added for 24-72 h to determine Gc proliferation. Cell cycle progression, E2 secretion, expression of proliferating cells nuclear antigen, cyclins, mitogen-activated kinase (MAP3/1; ERK1/2), protein kinase B (AKT) and signal transducer and activator of transcription 3 (STAT3) were studied. The involvement of these kinases in PNX-14 action on Gc proliferation was analysed using pharmacological inhibitors. Levels of GPR173 were increased in the ovarian follicles with oestrous progression, while only PNX-14 protein was the highest at days 10-12 of the oestrous. Immuno-signal of PNX-14 was detected in Gc and theca cells and oocyte, while GPR173 was mostly in theca. Interestingly, PNX-14 stimulated Gc proliferation, E2 secretion, cell cycle progression and cyclins expression and had a modulatory effect on MAP3/1, AKT and STAT3 activation. Our study suggests that PNX-14 could be an important factor for porcine reproduction by influencing ovarian follicle growth through direct action on Gc function.

The Apelin/APJ System: A Potential Therapeutic Target for Sepsis

Apelin is the native ligand for the G protein-coupled receptor APJ. Numerous studies have demonstrated that the Apelin/APJ system has positive inotropic, anti-inflammatory, and anti-apoptotic effects and regulates fluid homeostasis. The Apelin/APJ system has been demonstrated to play a protective role in sepsis and may serve as a promising therapeutic target for the treatment of sepsis. Better understanding of the mechanisms of the effects of the Apelin/APJ system will aid in the development of novel drugs for the treatment of sepsis. In this review, we provide a brief overview of the physiological role of the Apelin/APJ system and its role in sepsis.

Targeting the apelin system for the treatment of cardiovascular diseases

Cardiovascular disease is the leading cause of death worldwide. Its prevalence is rising due to ageing populations and the increasing incidence of diseases such as chronic kidney disease, obesity, and diabetes that are associated with elevated cardiovascular risk. Despite currently available treatments, there remains a huge burden of cardiovascular disease-associated morbidity for patients and healthcare systems, and newer treatments are needed. The apelin system, comprising the apelin receptor and its two endogenous ligands apelin and elabela, is a broad regulator of physiology that opposes the actions of the renin-angiotensin and vasopressin systems. Activation of the apelin receptor promotes endothelium-dependent vasodilatation and inotropy, lowers blood pressure, and promotes angiogenesis. The apelin system appears to protect against arrhythmias, inhibits thrombosis, and has broad anti-inflammatory and anti-fibrotic actions. It also promotes aqueous diuresis through direct and indirect (central) effects in the kidney. Thus, the apelin system offers therapeutic promise for a range of cardiovascular, kidney, and metabolic diseases. This review will discuss current cardiovascular disease targets of the apelin system and future clinical utility of apelin receptor agonism.

A promising therapeutic peptide and preventive/diagnostic biomarker for age-related diseases: The Elabela/Apela/Toddler peptide

Elabela (ELA), Apela or Toddler peptide is a hormone peptide belonging to the adipokine group and a component of apelinergic system, discovered in 2013-2014. Given its high homology with apelin, the first ligand of APJ receptor, ELA likely mediates similar effects. Increasing evidence shows that ELA has a critical function not only in embryonic development, but also in adulthood, contributing to physiological and pathological conditions, such as the onset of age-related diseases (ARD). However, still little is known about the mechanisms and molecular pathways of ELA, as well as its precise functions in ARD pathophysiology. Here, we report the mechanisms by which ELA/APJ signaling acts in a very complex network of pathways for the maintenance of physiological functions of human tissue and organs, as well as in the onset of some ARD, where it appears to play a central role. Therefore, we describe the possibility to use the ELA/APJ pathway, as novel biomarker (predictive and diagnostic) and target for personalized treatments of ARD. Its potentiality as an optimal peptide candidate for therapeutic ARD treatments is largely described, also detailing potential current limitations.

Cardiovascular aspects of ELABELA: A potential diagnostic biomarker and therapeutic target

ELABELA, an early endogenous ligand for the G protein-coupled receptor APJ (apelin peptide jejunum, apelin receptor), has been known as an important regulator in cardiovascular homeostasis and may be a novel therapeutic target for multiple cardiovascular diseases (CVDs). At the physiological level, ELABELA exhibits angiogenic and vasorelaxant effects and is essential for heart development. At the pathological level, circulating ELABELA levels may be a novel diagnostic biomarker for various CVDs. ELABELA peripherally displays antihypertensive, vascular-protective, and cardioprotective effects, whereas central administration of ELABELA elevated BP and caused cardiovascular remodeling. This review highlights the physiological and pathological roles of ELABELA in the cardiovascular system. Enhancement of the peripheral ELABELA may be a promising pharmacological therapeutic strategy for CVDs.

ELABELA-APJ-Akt/YAP Signaling Axis: A Novel Mechanism of Aerobic Exercise in Cardioprotection of Myocardial Infarction Rats

PURPOSE

The aim of this study was to investigate the function and mechanisms of ELABELA (ELA) in the aerobic exercise-induced antiapoptosis and angiogenesis of ischemic heart.

METHODS

The myocardial infarction (MI) model of Sprague-Dawley rat was established by the ligation of the left anterior descending coronary artery. MI rats underwent 5 wk of Fc-ELA-21 subcutaneous injection and aerobic exercise training using a motorized rodent treadmill. Heart function was evaluated by hemodynamic measures. Cardiac pathological remodeling was evaluated by Masson's staining and the calculation of left ventricular weight index. Cell proliferation, angiogenesis, and Yes-associated protein (YAP) translocation were observed by immunofluorescence staining. Cell apoptosis was analyzed by TUNEL. Cell culture and treatment were used to elucidate the molecular mechanism of ELA. Protein expression was detected by Western blotting. Angiogenesis was observed by tubule formation test. One-way or two-way ANOVA and Student's t -test were used for statistical analysis.

RESULTS

Aerobic exercise stimulated the endogenous ELA expression. Exercise and Fc-ELA-21 intervention significantly activated APJ-Akt-mTOR-P70S6K signaling pathway, kept more cardiomyocytes alive, and increased angiogenesis, so as to inhibit the cardiac pathological remodeling and improved the heart function of MI rats. Fc-ELA-32 also had the cellular and functional cardioprotective activities in vivo . In vitro , ELA-14 peptide regulated the phosphorylation and nucleoplasmic translocation of YAP and activated the APJ-Akt signaling pathway so as to increase the proliferation of H9C2 cells. Moreover, the antiapoptosis and the tubule formation of HUVECs were also enhanced by ELA-14, whereas the inhibition of Akt activity weakened such effects.

CONCLUSIONS

ELA is a potential therapeutic member that plays a key role through APJ-Akt/YAP signaling axis in aerobic exercise-induced cardioprotection of MI rats.

Role of G-protein coupled receptors in cardiovascular diseases

Cardiovascular diseases (CVDs) are the leading cause of death globally, with CVDs accounting for nearly 30% of deaths worldwide each year. G-protein-coupled receptors (GPCRs) are the most prominent family of receptors on the cell surface, and play an essential regulating cellular physiology and pathology. Some GPCR antagonists, such as β-blockers, are standard therapy for the treatment of CVDs. In addition, nearly one-third of the drugs used to treat CVDs target GPCRs. All the evidence demonstrates the crucial role of GPCRs in CVDs. Over the past decades, studies on the structure and function of GPCRs have identified many targets for the treatment of CVDs. In this review, we summarize and discuss the role of GPCRs in the function of the cardiovascular system from both vascular and heart perspectives, then analyze the complex ways in which multiple GPCRs exert regulatory functions in vascular and heart diseases. We hope to provide new ideas for the treatment of CVDs and the development of novel drugs.

APJ as Promising Therapeutic Target of Peptide Analogues in Myocardial Infarction- and Hypertension-Induced Heart Failure

The widely expressed G protein-coupled apelin receptor (APJ) is activated by two bioactive endogenous peptides, apelin and ELABELA (ELA). The apelin/ELA-APJ-related pathway has been found involved in the regulation of many physiological and pathological cardiovascular processes. Increasing studies are deepening the role of the APJ pathway in limiting hypertension and myocardial ischaemia, thus reducing cardiac fibrosis and adverse tissue remodelling, outlining APJ regulation as a potential therapeutic target for heart failure prevention. However, the low plasma half-life of native apelin and ELABELA isoforms lowered their potential for pharmacological applications. In recent years, many research groups focused their attention on studying how APJ ligand modifications could affect receptor structure and dynamics as well as its downstream signalling. This review summarises the novel insights regarding the role of APJ-related pathways in myocardial infarction and hypertension. Furthermore, recent progress in designing synthetic compounds or analogues of APJ ligands able to fully activate the apelinergic pathway is reported. Determining how to exogenously regulate the APJ activation could help to outline a promising therapy for cardiac diseases.

ELABELA RELAXES RAT PULMONARY ARTERY AND TRACHEA VIA BKCa, KV, and KATP CHANNELS

OBJECTIVE

Elabela is a newly discovered peptide hormone. This study aimed to determine the functional effects and mechanisms of action of elabela in rat pulmonary artery and trachea.

MATERIALS AND METHODS

Vascular rings isolated from the pulmonary arteries of male Wistar Albino rats were placed in chambers in the isolated tissue bath system. The resting tension was set to 1g. After the equilibration period, the pulmonary artery rings were contracted with 10^-6M phenylephrine. Once a stable contraction was achieved, elabela was applied cumulatively (10^-10-10^-6M) to the vascular rings. To determine the vasoactive effect mechanisms of elabela, the specified experimental protocol was repeated after the incubation of signaling pathway inhibitors and potassium channel blockers. The effect and mechanisms of action of elabela on tracheal smooth muscle were also determined by a similar protocol.

RESULTS

Elabela exhibited a concentration-dependent relaxation in the precontracted rat pulmonary artery rings (p<.001). Maximal relaxation level was 83% (pEC₅₀: 7.947 CI95(7.824-8.069)). Removal of the endothelium, indomethacin incubation, and dideoxyadenosine incubation significantly decreased the vasorelaxant effect levels of elabela (p<.001). Elabela-induced vasorelaxation levels were significantly reduced after iberiotoxin, glyburide, and 4-Aminopyridine administrations (p<.001). L-NAME, methylene blue, apamin, TRAM-34, anandamide, and BaCl₂ administrations did not cause a significant change in the vasorelaxant effect level of elabela (p=1.000). Elabela showed a relaxing effect on precontracted tracheal rings (p<.001). Maximal relaxation level was 73% (pEC₅₀: 6.978 CI95(6.791-7.153)). The relaxant effect of elabela on tracheal smooth muscle was decreased significantly after indomethacin, dideoxyadenosine, iberiotoxin, glyburide, and 4-Aminopyridine incubations (p<.001).

CONCLUSIONS

Elabela exerted a prominent relaxant effect in the rat pulmonary artery and trachea. Intact endothelium, prostaglandins, cAMP signaling pathway, and potassium channels (BK_Ca, K_V, and K_ATP channels) are involved in the vasorelaxant effect of elabela. Prostaglandins, cAMP signaling pathway, BK_Ca channels, K_V channels, and K_ATP channels also contribute to elabela-induced tracheal smooth muscle relaxant effect.

The impact of the apelinergic system in coronary collateral formation

Background

This study aims to examine the relationship between the development of coronary collateral circulation and serum elabela levels.

Methods

Between January 2020 and December 2021, a total of 50 control individuals (29 males, 21 females; mean age: 63.2±10.0 years; range, 52 to 73 years) with no significant coronary artery disease as confirmed by angiography (Group 1) and 100 patients (55 males, 45 females; mean age: 66.6±9.6 years; range, 56 to 75 years) with coronary artery disease were included. The patients were further divided into two equal groups according to the Rentrop classification as poor (Group 2) and good coronary collateral circulation (Group 3). All groups were compared in terms of several parameters, particularly serum elabela levels.

Results

Serum elabela levels were found to be statistically higher in the group with good collateral than the other groups (p<0.05). Low serum elabela levels increased the risk of developing weak collaterals by 2.43 times.

Conclusion

The elabela protein is directly related to good collateral development and can be considered a potential agent for treatment.

Elabela-APJ axis mediates angiogenesis via YAP/TAZ pathway in cerebral ischemia/reperfusion injury

Angiogenesis helps to improve neurological recovery by repairing damaged brain tissue and restoring cerebral blood flow (CBF). The role of the Elabela (ELA)-Apelin receptor (APJ) system in angiogenesis has gained much attention. We aimed to investigate the function of endothelial ELA on postischemic cerebral angiogenesis. Here, we demonstrated that the endothelial ELA expression was upregulated in the ischemic brain and treatment with ELA-32 mitigated brain injury and enhanced the restoration of CBF and newly formed functional vessels following cerebral ischemia/reperfusion (I/R) injury. Furthermore, ELA-32 incubation potentiated proliferation, migration, and tube formation abilities of the mouse brain endothelial cells (bEnd.3 cells) under oxygen-glucose deprivation/reoxygenation (OGD/R) condition. RNA sequencing analysis indicated that ELA-32 incubation had a role in the Hippo signaling pathway, and improved angiogenesis-related gene expression in OGD/R-exposed bEnd.3 cells. Mechanistically, we depicted that ELA could bind to APJ and subsequently activate YAP/TAZ signaling pathway. Silence of APJ or pharmacological blockade of YAP abolished the pro-angiogenesis effects of ELA-32. Together, these findings highlight the ELA-APJ axis as a potential therapeutic strategy for ischemic stroke by showing how activation of this pathway promotes poststroke angiogenesis.

Elabela-APJ axis attenuates cerebral ischemia/reperfusion injury by inhibiting neuronal ferroptosis

Ferroptosis is a form of non-apoptotic cell death caused by iron-dependent peroxidation of lipids. It contributes to ischemic stroke-induced neuronal damage. Elabela (ELA), a novel endogenous ligand for Apelin receptor (APJ), regulates oxidative stress and exerts a protective role in cardiovascular disease. However, the effect of ELA-APJ axis on cellular ferroptosis in cerebral ischemia/reperfusion (I/R) remains elusive. The present study showed that ELA and APJ were expressed on neurons and increased after cerebral I/R injury. The I/R insult triggered typical molecular and morphological features of neuronal ferroptosis, including iron and MDA accumulation, mitochondrial shrink and membrane rupture, upregulation of positive ferroptosis regulators and downregulation of negative regulators. ELA-32 treatment reduced brain infarction and ameliorated neurobehavioral deficits and cognitive dysfunction. Moreover, ELA-32 administration alleviated neuronal ferroptosis, accompanied by reduced iron deposition, decreased mitochondrial damage, relived lipid peroxidation and glutathione reduction. Such effects of ELA-32 were abolished by AAV-APJ-RNAi or nuclear factor erythroid 2-related factor 2 (NRF2) inhibitor ML385. Mechanistically, ELA was shown to bind to APJ and activate NRF2/ARE anti-oxidative signaling pathway via Gα13. Together, these findings suggested that ELA-APJ axis mitigates neuronal ferroptosis after ischemic stroke and that the ELA-32 peptide may be a putative therapeutic avenue for ischemic stroke.

Cancer apelin receptor suppresses vascular mimicry in malignant melanoma

Several reports indicate that apelin is often over-expressed in tumors, and therefore it has been suggested that the apelin-apelin receptor (APJ) system may induce tumor progression. In contrast, our previous research revealed high expression of the apelin-APJ system in tumor blood vessels, suggesting its involvement in the regulation of tumor vessel formation and normalization, resulting in the suppression of tumor growth by promoting the infiltration of T cells. Thus, the effect of the apelin-APJ system on tumors remains controversial. In this report, to clarify the effect of apelin in tumor cells, we analyzed the function of APJ in tumor cells using APJ knock out (KO) mice. In APJ-KO mice, Apelin overexpression in B16/BL6 (B16) melanoma cells induced greater tumor growth than controls. In an APJ-KO melanoma inoculation model, although angiogenesis is suppressed compared to wild type, no difference is evident in tumor growth. We found that APJ deficiency promoted vascular mimicry in tumors. In vitro, cultured APJ-KO B16 cells demonstrated a spindle-like shape. This phenotypic change was thought to be induced by epithelial-mesenchymal transition (EMT) based on evidence that APJ-KO B16 cells show persistently high levels of the mesenchymal maker, Zeb1; however, we found that EMT did not correlate with the transforming growth factor-β/smad signaling pathway in our model. We propose that apelin-APJ system in cancer cells induces tumor growth but negatively regulates EMT and tumor malignancy.

Elabela: Negative Regulation of Ferroptosis in Trophoblasts via the Ferritinophagy Pathway Implicated in the Pathogenesis of Preeclampsia

Preeclampsia is a leading contributor to increased maternal morbidity and mortality in the perinatal period. Increasing evidence demonstrates that ferroptosis is an essential mechanism for the pathogenesis of preeclampsia. Elabela is a novel small-molecule polypeptide, mainly expressed in embryonic and transplacental tissues, with an ability to promote cell proliferation and invasion. However, its specific regulatory mechanism in preeclampsia has not been completely elucidated. In this study, we first reveal an increased grade of ferroptosis accompanied by a downregulation of the expression of Elabela in preeclampsia placentas. We then confirm the presence of a ferroptosis phenotype in the placenta of the mouse PE-like model, and Elabela can reduce ferroptosis in the placenta and improve adverse pregnancy outcomes. Furthermore, we demonstrate that targeting Elabela alleviates the cellular dysfunction mediated by Erastin promoting increased lipid peroxidation in vitro. Subsequent mechanistic studies suggest that Elabela increases FTH1 levels by inhibiting the ferritinophagy pathway, and consequently chelates the intracellular labile iron pool and eventually arrests ferroptosis. In conclusion, Elabela deficiency exacerbates ferroptosis in the placenta, which is among the potential mechanisms in the pathogenesis of preeclampsia. Targeting the Elabela-ferritinophagy-ferroptosis signaling axis provides a new therapeutic intervention strategy to alleviate preeclampsia.

The emerging role of the apelinergic system in kidney physiology and disease

The apelinergic system (AS) is a novel pleiotropic system with an essential role in renal and cardiovascular physiology and disease, including water homeostasis and blood pressure regulation. It consists of two highly conserved peptide ligands, apelin and apela, and a G-protein-coupled apelin receptor. The two ligands have many isoforms and a short half-life and exert both similar and divergent effects. Vasopressin, apelin and their receptors colocalize in hypothalamic regions essential for body fluid homeostasis and interact at the central and renal levels to regulate water homeostasis and diuresis in inverse directions. In addition, the AS and renin-angiotensin system interact both systemically and in the kidney, with implications for the cardiovascular system. A role for the AS in diverse pathological states, including disorders of sodium and water balance, hypertension, heart failure, pre-eclampsia, acute kidney injury, sepsis and diabetic nephropathy, has recently been reported. Furthermore, several metabolically stable apelin analogues have been developed, with potential applications in diverse diseases. We review here what is currently known about the physiological functions of the AS, focusing on renal, cardiovascular and metabolic homeostasis, and the role of the AS in associated diseases. We also describe several hurdles and research opportunities worthy of the attention of the nephrology community.

Elabela Peptide: An Emerging Target in Therapeutics

Elabela, a bioactive micropeptide, is recognized as the second endogenous ligand for the Apelin receptor and is widely distributed in different tissues and organs. Elabela plays an important role in various physiological processes, such as blood pressure control, heart morphogenesis, apoptosis, angiogenesis, cell proliferation, migration, etc. Elabela is also implicated in pathological conditions, like cardiac dysfunctions, heart failure, hypertension, kidney diseases, cancer and CNS disorders. The association of Elabela with these disease conditions makes it a potential target for their therapy. This review summarizes the physiological role of Elabela peptide as well as its implication in various disease conditions.

Structural insight into apelin receptor-G protein stoichiometry

The technique of cryogenic-electron microscopy (cryo-EM) has revolutionized the field of membrane protein structure and function with a focus on the dominantly observed molecular species. This report describes the structural characterization of a fully active human apelin receptor (APJR) complexed with heterotrimeric G protein observed in both 2:1 and 1:1 stoichiometric ratios. We use cryo-EM single-particle analysis to determine the structural details of both species from the same sample preparation. Protein preparations, in the presence of the endogenous peptide ligand ELA or a synthetic small molecule, both demonstrate these mixed stoichiometric states. Structural differences in G protein engagement between dimeric and monomeric APJR suggest a role for the stoichiometry of G protein-coupled receptor- (GPCR-)G protein coupling on downstream signaling and receptor pharmacology. Furthermore, a small, hydrophobic dimer interface provides a starting framework for additional class A GPCR dimerization studies. Together, these findings uncover a mechanism of versatile regulation through oligomerization by which GPCRs can modulate their signaling.

Serum Elabela level is significantly increased in patients with acromegaly

BACKGROUND

Although the bioactive peptides associated with the apelinergic system are known to be associated with heart failure and ischemic heart disease, there are no data on their association with acromegaly.

AIM

We aimed to investigate the change in serum Elabela levels, a novel peptide of the apelinergic system, in patients with acromegaly.

METHODS

Our study included 30 treatment naive patients who were recently diagnosed with acromegaly, and 50 age-and-sex-matched healthy controls. In addition to routine history, physical examination and laboratory examinations, serum Elabela level was measured. Participants were divided into two groups as individuals with and without acromegaly and compared to each other.

RESULTS

Diastolic blood pressure (DBP) and systolic blood pressure (SBP) were found to be higher in patients with acromegaly. Serum glucose, Hs-CRP, NT-proBNP, insulin-like growth factor-1, growth hormone and serum Elabela levels were higher in patients with acromegaly (p < 0.05 for each). Left ventricular ejection fraction (LV-EF) was found to be lower in patients with acromegaly than the patients in healthy control group (p < 0.05). In multivariate analysis; age, systolic blood pressure, NT-proBNP, Insulin-like growth factor 1 and growth hormone levels were found to be very closely and positively related to serum Elabela level (p < 0.05 for each).

CONCLUSIONS

Serum Elabela level can be used as an early and objective indicator of early cardiovascular involvement in patients with acromegaly. Further research is needed to clarify the role of serum Elabela levels on cardiovascular system in acromegaly patients.

Vascular Functional Effect Mechanisms of Elabela in Rat Thoracic Aorta

BACKGROUND

Elabela is a recently discovered peptide hormone. The present study aims to investigate the vasorelaxant effect mechanisms of elabela in the rat thoracic aorta.

METHODS

The vascular rings obtained from the thoracic aortas of the male Wistar Albino rats were placed in the isolated tissue bath system. Resting tension was set to 1 gram. After the equilibration period, the vessel rings were contracted with phenylephrine or potassium chloride. Once a stable contraction was achieved, elabela-32 was applied cumulatively (10^-9 - 10^-6 molar) to the vascular rings. The experimental protocol was repeated in the presence of specific signaling pathway inhibitors or potassium channel blockers to determine the effect mechanisms of elabela.

RESULTS

Elabela showed a significant vasorelaxant effect in a concentration-dependent manner (p < 0.001). The vasorelaxant effect level of elabela was significantly reduced by the apelin receptor antagonist F13A, cyclooxygenase inhibitor indomethacin, adenosine monophosphate-activated protein kinase inhibitor dorsomorphin, protein kinase C inhibitor bisindolmaleimide, large-conductance calcium-activated potassium channel blocker iberiotoxin, and intermediate-conductance calcium-activated potassium channel blocker TRAM-34 (p < 0.001). However, the vasorelaxant effect level of elabela was not significantly affected by the endothelial nitric oxide synthase inhibitor nitro-L-arginine methyl ester and mitogen-activated protein kinase inhibitor U0126.

CONCLUSIONS

Elabela exhibits a prominent vasodilator effect in rat thoracic aorta. Apelin receptor, prostanoids, adenosine monophosphate-activated protein kinase, protein kinase C, and calcium-activated potassium channels are involved in the vasorelaxant effect mechanisms of elabela.

Elabela ameliorates doxorubicin-induced cardiotoxicity by promoting autophagic flux through TFEB pathway

Doxorubicin (DOX) is a widely used and effective antineoplastic drug; however, its clinical application is limited by cardiotoxicity. A safe and effective strategy to prevent from doxorubicin-induced cardiotoxicity (DIC) is still beyond reach. Elabela (ELA), a new APJ ligand, has exerted cardioprotective effect against multiple cardiovascular diseases. Here, we asked whether ELA alleviates DIC. Mice were injected with DOX to established acute DIC. In vivo studies were assessed with echocardiography, serum cTnT and CK-MB, HW/BW ratio and WGA staining. Cell death and atrophy were measured by AM/PI staining and phalloidin staining respectively in vitro. Autophagic flux was monitored with Transmission electron microscopy in vivo, as well as LysoSensor and mRFP-GFP-LC3 puncta in vitro. Our results showed that ELA improved cardiac dysfunction in DIC mice. ELA administration also attenuated cell death and atrophy in DOX-challenged neonatal rat cardiomyocytes (NRCs). Additionally, we found that ELA restored DOX-induced autophagic flux blockage, which was evidenced by the reverse of p62 and LC3II, improvement of lysosome function and accelerated degradation of accumulated autolysosomes. Chloroquine, a classical autophagic flux inhibitor, blunted the improvement of ELA on cardiac dysfunction. At last, we revealed that ELA reversed DOX-induced downregulation of transcription factor EB (TFEB), and silencing TFEB by siRNA abrogated the effects of ELA on autophagic flux as well as cell death and atrophy in NRCs. In conclusion, this study indicated that ELA ameliorated DIC through enhancing autophagic flux via activating TFEB. ELA may become a potential target against DIC.

The role of potassium channels on vasorelaxant effects of elabela in rat thoracic aorta

Background

This study aims to investigate the roles of potassium channel subtypes in the vasorelaxant effect mechanism of elabela, which is a recently discovered endogenous apelin receptor ligand.

Methods

The vascular rings (4-mm) obtained from the thoracic aortas of 20 male Wistar Albino rats were placed into the isolated tissue bath system. The resting tension was set to 1 g. The aortic rings were contracted with 10-5 molar phenylephrine after the equilibration period (90 min). Elabela was applied cumulatively (10-10-10-6 molar) to the aortic rings in the plateau phase. The experimental protocol was repeated in the presence of specific potassium channel subtype inhibitors to determine the role of potassium channels in the vasorelaxant effect mechanism of elabela.

Results

Elabela induced a concentration-dependent vasorelaxation (p<0.001). The maximum relaxation level was approximately 51% according to phenylephrineinduced contraction. Vasorelaxant effect level of elabela statistically significantly decreased after removal of the endothelium (p<0.05). Tetraethylammonium (1 milimolar), 4-Aminopyridine (1 milimolar), glyburide (10 micromolar), and barium chloride (30 micromolar) statistically significantly decreased the vasorelaxant effect level of elabela (p<0.001, p<0.001, p<0.01, and p<0.05 respectively). However, anandamide (10 micromolar) and apamin (100 nanomolar) did not statistically significantly change the vasorelaxant effect level of elabela.

Conclusion

Our results suggest that large-conductance calciumactivated, voltage-gated, adenosine triphosphate-sensitive, and inward-rectifier potassium channels are involved in the vasorelaxant effect mechanism of elabela in the rat thoracic aorta.

Serum Elabela level is related to endoscopic activity index in patients with active ulcerative colitis

BACKGROUND

In ulcerative colitis patients, Elabela levels and the relation of Elabela with laboratory parameters is unknown.

AIM

The purpose of this study was to investigate the serum Elabela levels in UC patients and its relationship with other clinical and laboratory findings.

METHODS

Forty-three patients with UC and 40 healthy controls (group I) similar in age and gender were included in the study. Routine patient history, physical examination, and laboratory tests were followed by analysis of serum Elabela levels. Endoscopic activity index (EAI) of patients with UC was calculated. There were two groups of patients: those in remission (group II) and with active disease (group III).

RESULTS

Groups I, II, and III had 40, 22, and 21 participants, respectively. Serum Elabela levels were found to be 3.32 ± 1.25 ng/mL in group I, 3.38 ± 0.88 ng/mL in group II, and 5.48 ± 1.61 ng/mL in group III. Comparing the serum Elabela levels, a statistically significant difference was found between three groups (p < 0.001). Serum Elabela level showed a significant and positive correlation with EAI, leukocyte count, and hs-CRP, while a negative correlation was found with hemoglobin levels in univariate analysis (p < 0.001, for each). In linear regression analysis, these parameters were found to be associated with EAI and hs-CRP (p = 0.049, β = 0.337, and p = 0.015, β = 0.396, respectively).

CONCLUSION

Elabela concentrations in patients with active UC was significantly higher and was associated with EAI and hs-CRP. Blood Elabela concentrations can be useful in the diagnosis and follow-up of patients with active UC.

Effects and signaling pathways of Elabela in the cardiovascular system

For a long time, Apelin has been considered as the only endogenous ligand of G protein-coupled receptor APJ. Until recently, the discovery of Elabela (Apela/Toddler) as a new polypeptide that can act through APJ and has a similar function to Apelin broke this situation. Elabela promotes a variety of cell proliferation processes, including embryonic development, and has especially beneficial effects in the cardiovascular system. In this review, we summarize the biological functions of Elabela and review its specific roles in cardiovascular diseases and the signaling pathways mediated by it.

Apelin, APJ, and ELABELA: Role in Placental Function, Pregnancy, and Foetal Development-An Overview

The apelinergic system, which includes the apelin receptor (APJ) as well as its two specific ligands, namely apelin and ELABELA (ELA/APELA/Toddler), have been the subject of many recent studies due to their pleiotropic effects in humans and other animals. Expression of these factors has been investigated in numerous tissues and organs—for example, the lungs, heart, uterus, and ovary. Moreover, a number of studies have been devoted to understanding the role of apelin and the entire apelinergic system in the most important processes in the body, starting from early stages of human life with regulation of placental function and the proper course of pregnancy. Disturbances in the balance of placental processes such as proliferation, apoptosis, angiogenesis, or hormone secretion may lead to specific pregnancy pathologies; therefore, there is a great need to search for substances that would help in their early diagnosis or treatment. A number of studies have indicated that compounds of the apelinergic system could serve this purpose. Hence, in this review, we summarized the most important reports about the role of apelin and the entire apelinergic system in the regulation of placental physiology and pregnancy.

The therapeutic potential of apelin in kidney disease

Chronic kidney disease (CKD) is a leading cause of global morbidity and mortality and is independently associated with cardiovascular disease. The mainstay of treatment for CKD is blockade of the renin-angiotensin-aldosterone system (RAAS), which reduces blood pressure and proteinuria and slows kidney function decline. Despite this treatment, many patients progress to kidney failure, which requires dialysis or kidney transplantation, and/or die as a result of cardiovascular disease. The apelin system is an endogenous physiological regulator that is emerging as a potential therapeutic target for many diseases. This system comprises the apelin receptor and its two families of endogenous ligands, apelin and elabela/toddler. Preclinical and clinical studies show that apelin receptor ligands are endothelium-dependent vasodilators and potent inotropes, and the apelin system has a reciprocal relationship with the RAAS. In preclinical studies, apelin regulates glomerular haemodynamics and acts on the tubule to promote aquaresis. In addition, apelin is protective in several kidney injury models. Although the apelin system has not yet been studied in patients with CKD, the available data suggest that apelin is a promising potential therapeutic target for kidney disease.

Apelin-13 in septic shock: effective in supporting hemodynamics in sheep but compromised by enzymatic breakdown in patients

Sepsis is a prevalent life-threatening condition related to a systemic infection, and with unresolved issues including refractory septic shock and organ failures. Endogenously released catecholamines are often inefficient to maintain blood pressure, and low reactivity to exogenous catecholamines with risk of sympathetic overstimulation is well documented in septic shock. In this context, apelinergics are efficient and safe inotrope and vasoregulator in rodents. However, their utility in a larger animal model as well as the limitations with regards to the enzymatic breakdown during sepsis, need to be investigated. The therapeutic potential and degradation of apelinergics in sepsis were tested experimentally and in a cohort of patients. (1) 36 sheep with or without fecal peritonitis-induced septic shock (a large animal experimental design aimed to mimic the human septic shock paradigm) were evaluated for hemodynamic and renal responsiveness to incremental doses of two dominant apelinergics: apelin-13 (APLN-13) or Elabela (ELA), and (2) 52 subjects (33 patients with sepsis/septic shock and 19 healthy volunteers) were investigated for early levels of endogenous apelinergics in the blood, the related enzymatic degradation profile, and data regarding sepsis outcome. APLN-13 was the only one apelinergic which efficiently improved hemodynamics in both healthy and septic sheep. Endogenous apelinergic levels early rose, and specific enzymatic breakdown activities potentially threatened endogenous apelin system reactivity and negatively impacted the outcome in human sepsis. Short-term exogenous APLN-13 infusion is helpful in stabilizing cardiorenal functions in ovine septic shock; however, this ability might be impaired by specific enzymatic systems triggered during the early time course of human sepsis. Strategies to improve resistance of APLN-13 to degradation and/or to overcome sepsis-induced enzymatic breakdown environment should guide future works.

Targeting the elabela/apelin-apelin receptor axis as a novel therapeutic approach for hypertension

ABSTRACT

Hypertension is the leading risk factor for global mortality and morbidity and those with hypertension are more likely to develop severe symptoms in cardiovascular and cerebrovascular system, which is closely related to abnormal renin-angiotensin system and elabela/apelin-apelin receptor (APJ) axis. The elabela/apelin-APJ axis exerts essential roles in regulating blood pressure levels, vascular tone, and cardiovascular dysfunction in hypertension by counterbalancing the action of the angiotensin II/angiotensin II type 1 receptor axis and enhancing the endothelial nitric oxide (NO) synthase/NO signaling. Furthermore, the elabela/apelin-APJ axis demonstrates beneficial effects in cardiovascular physiology and pathophysiology, including angiogenesis, cellular proliferation, fibrosis, apoptosis, oxidative stress, and cardiovascular remodeling and dysfunction during hypertension. More importantly, effects of the elabela/apelin-APJ axis on vascular tone may depend upon blood vessel type or various pathological conditions. Intriguingly, the broad distribution of elabela/apelin and alternative isoforms implicated its distinct functions in diverse cardiac and vascular cells and tissue types. Finally, both loss-of-function and gain-of-function approaches have defined critical roles of the elabela/apelin-APJ axis in reducing the development and severity of hypertensive diseases. Thus, targeting the elabela/apelin-APJ axis has emerged as a pre-warning biomarker and a novel therapeutic approach against progression of hypertension, and an increased understanding of cardiovascular actions of the elabela/apelin-APJ axis will help to develop effective interventions for hypertension. In this review, we focus on the physiology and biochemistry, diverse actions, and underlying mechanisms of the elabela/apelin-APJ axis, highlighting its role in hypertension and hypertensive cardiovascular injury and dysfunction, with a view to provide a prospective strategy for hypertensive disease therapy.

Plasma Levels of Apelinergic System Components in Patients with Chronic and Acute Coronary Syndromes-A Pilot Study

The effects of the apelinergic system components apelin (AP) and elabela (ELA) in the regulation of human cardiovascular homeostasis, and data concerning the relationship between ELA and AP and coronary artery disease (CAD) are yet unknown. The aim of the study was the evaluation of AP, ELA and APJ-receptor levels in the plasma of patients with chronic coronary syndromes (CCS) and acute coronary syndromes (ACS). The study group consisted of 114 patients with CAD and 33 healthy controls. Patients were divided into two groups: with CCS (n = 30) and ACS (n = 84). Routine laboratory tests and plasma ELA, AP-17, AP-13 and APJ receptor levels were measured. Echocardiographic data were analyzed in all patients. Levels of AP-17 and ELA were significantly lower in CCS than in healthy controls and ACS patients. We demonstrated significant increase of levels of plasma apelinergic system peptides, especially ELA and AP-17 in ACS patients compared with healthy controls and CCS, suggestive of compensating up-regulation mechanisms. There is a relationship between circulating ELA and AP-17 levels and classical, biochemical markers of ischemia and left ventricular ejection faction as well.

The Elabela-APJ axis: a promising therapeutic target for heart failure

Heart failure (HF) is a growing epidemic with high morbidity and mortality at an international scale. The apelin-APJ receptor pathway has been implicated in HF, making it a promising therapeutic target. APJ has been shown to be activated by a novel endogenous peptide ligand known as Elabela (ELA, also called Toddler or Apela), with a critical role in cardiac development and function. Activation of the ELA-APJ receptor axis exerts a wide range of physiological effects, including depressor response, positive inotropic action, diuresis, anti-inflammatory, anti-fibrotic, and anti-remodeling, leading to its cardiovascular protection. The ELA-APJ axis is essential for diverse biological processes and has been shown to regulate fluid homeostasis, myocardial contractility, vasodilation, angiogenesis, cellular differentiation, apoptosis, oxidative stress, cardiorenal fibrosis, and dysfunction. The beneficial effects of the ELA-APJ receptor system are well-established by treating hypertension, myocardial infarction, and HF. Additionally, administration of ELA protects human embryonic stem cells against apoptosis and stress-induced cell death and promotes survival and self-renewal in an APJ-independent manner (X receptor) via the phosphatidylinositol 3-kinase/Akt pathway, which may provide a new therapeutic approach for HF. Thus, targeting the ELA-APJ axis has emerged as a pre-warning biomarker and a novel therapeutic approach against progression of HF. An increased understanding of cardiovascular actions of ELA will help to develop effective interventions. This article gives an overview of the characteristics of the ELA-apelin-APJ axis and summarizes the current knowledge on its cardioprotective roles, potential mechanisms, and prospective application for acute and chronic HF.

The network map of Elabela signaling pathway in physiological and pathological conditions

Elabela (ELA; also called Apela and Toddler) is one of the recently discovered ligand among the two endogenous peptide ligands (Apelin and Elabela) of the apelin receptor (APLNR, also known as APJ). Elabela-induced signaling plays a crucial role in diverse biological processes, including formation of the embryonic cardiovascular system and early placental development by reducing the chances of occurrence of preeclampsia during pregnancy. It also plays the major role in the renoprotection by reducing kidney injury and the inflammatory response and regulation of gene expression associated with heart failure and fibrosis. Elabela may be processed into different active peptides, each of which binds to APLNR and predominantly activates the signals through PI3K/AKT pathway. Owing to its biomedical importance, we developed a consolidated signaling map of Elabela, in accordance with the NetPath criteria. The presented Elabela signaling map comprises 12 activation/inhibition events, 15 catalysis events, 1 molecular association, 34 gene regulation events and 32 protein expression events. The Elabela signaling pathway map is freely made available through the WikiPathways Database ( https://www.wikipathways.org/index.php/Pathway:WP5100 ).

Elabela Protects Spontaneously Hypertensive Rats From Hypertension and Cardiorenal Dysfunctions Exacerbated by Dietary High-Salt Intake

Objectives: Arterial hypertension, when exacerbated by excessive dietary salt intake, worsens the morbidity and mortality rates associated with cardiovascular and renal diseases. Stimulation of the apelinergic system appears to protect against several circulatory system diseases, but it remains unknown if such beneficial effects are conserved in severe hypertension. Therefore, we aimed at determining whether continuous infusion of apelinergic ligands (i.e., Apelin-13 and Elabela) exerted cardiorenal protective effects in spontaneously hypertensive (SHR) rats receiving high-salt diet.

Methods: A combination of echocardiography, binding assay, histology, and biochemical approaches were used to investigate the cardiovascular and renal effects of Apelin-13 or Elabela infusion over 6 weeks in SHR fed with normal-salt or high-salt chow.

Results: High-salt intake upregulated the cardiac and renal expression of APJ receptor in SHR. Importantly, Elabela was more effective than Apelin-13 in reducing high blood pressure, cardiovascular and renal dysfunctions, fibrosis and hypertrophy in high-salt fed SHR. Unlike Apelin-13, the beneficial effects of Elabela were associated with a counter-regulatory role of the ACE/ACE2/neprilysin axis of the renin-angiotensin-aldosterone system (RAAS) in heart and kidneys of salt-loaded SHR. Interestingly, Elabela also displayed higher affinity for APJ in the presence of high salt concentration and better resistance to RAAS enzymes known to cleave Apelin-13.

Conclusion: These findings highlight the protective action of the apelinergic system against salt-induced severe hypertension and cardiorenal failure. As compared with Apelin-13, Elabela displays superior pharmacodynamic and pharmacokinetic properties that warrant further investigation of its therapeutic use in cardiovascular and kidney diseases.

Plasma Level of Elabela in Patients with Coronary Heart Disease and Its Correlation with the Disease Classification

This study aimed to evaluate the concentration of plasma elabela (ELA) in patients with coronary heart disease (CHD) and its correlation with the disease classification.We enrolled 238 patients diagnosed by coronary angiography as CHD and 86 controls. The CHD group was divided into three subgroups: stable angina (SA), unstable angina (UAP), and acute myocardial infarction (AMI). The plasma levels of ELA were measured in all participants and compared among different groups. The relationship between ELA and CHD classification was analyzed.ELA levels were markedly higher by 10.71% in patients with CHD than in controls (P < 0.05). The concentration of ELA in UAP and AMI subgroups were higher than in controls and SA subgroup. The former difference was significant (P < 0.05), but the latter was not. In addition, the ELA concentration was not correlated with SYNTAX score, left ventricular ejection fraction, and other biochemical variables.The newfound hormone, ELA, significantly increased in patients with UAP and AMI. There is a tendency that ELA levels might be correlated with CHD classification, but not with lesion severity. ELA may play a role in acute coronary syndrome.

Association Between Serum Elabela Levels and Chronic Totally Occlusion in Patients with Stable Angina Pectoris

BACKGROUND

The beneficial effects of Elabela on the cardiovascular system have been shown in studies.

OBJECTIVE

To compare serum Elabela levels of chronic total occlusion (CTO) patients with control patients with normal coronary arteries, and to investigate whether there is a correlation with collateral development.

METHODS

The study was planned cross-sectionally and prospectively. Fifty patients (28.0% female, mean age 61.6±7.3years) with CTO in at least one coronary vessel and 50 patients (38% female, mean age 60,7±6.38 years) with normal coronary arteries were included in the study. Patients in the CTO group were divided into two groups as Rentrop 0-1, those with weak collateral development, and Rentrop 2-3 with good collateral development. In addition to the age, sex, demographic characteristics and routine laboratory tests of the patients, Elabela levels were measured.

RESULTS

Demographic characteristics and laboratory values were similar in both groups. While the mean NT-proBNP and troponin were higher in the CTO group, the Elabela mean was lower (p <0.05 for all). In the multivariate regression analysis, NT-proBNP and Elabela levels were found to be independent predictors for CTO. Also, Elabela level was found to be statistically higher in Rentrop class 2-3 patients compared to Rentrop class 0-1 patients (p<0.05).

CONCLUSION

In our study, we showed that the average Elabela level was low in CTO patients compared to normal patients. In addition, we found the level of Elabela to be lower in patients with weak collateral development compared to patients with good collateral development. (Arq Bras Cardiol. 2021; [online].ahead print, PP.0-0).

Serum elabela and apelin levels during different stages of chronic kidney disease

PURPOSE

The association of serum elabela (ELA) and apelin with the progression of chronic kidney disease (CKD) is unknown. We determined if serum ELA and apelin levels were associated with CKD stage.

METHODS

This observational study involved 60 CKD patients and 20 healthy, age-, race-, and gender-matched controls. The participants were grouped according to renal function as follows: normal control group, CKD1 group (stage-1 CKD, 20 patients), CKD3 group (stage-3 CKD, 20 patients), and CKD5 group (stage-5 CKD, 20 patients) in accordance with the Kidney Disease Outcomes - Quality Initiative criteria. We recorded the demographic, clinical, and biochemical data of all participants. Serum ELA and apelin levels were measured using commercially available enzyme-linked immunosorbent assays.

RESULTS

Serum ELA levels gradually and significantly declined with decreases in the estimated glomerular filtration rate (eGFR). Serum ELA showed significant negative correlations with serum creatinine (r = -0.529, p < .001), blood urea nitrogen (r = -0.575, p < .001), systolic blood pressure (r = -0.455, p < .001), and diastolic blood pressure (r = -0.450, p < .001), and significant positive correlations with hemoglobin (r = 0.523, p < .001) and eGFR (r = 0.728, p < .001). Multiple regression analysis showed that eGFR independently influenced serum ELA levels. No significant association was found between serum apelin levels and CKD progression.

CONCLUSION

In CKD patients, serum ELA levels decreased with decreasing eGFR. This finding may provide a new target for the prediction, diagnosis, and staging of CKD.

Circulating levels of Elabela in pregnant women complicated with intrauterine growth restriction

OBJECTIVE

This study aimed to detect Elabela concentrations in the serum of normotensive pregnant women complicated with intrauterine growth restriction (IUGR) and compare them with the uncomplicated healthy pregnancies.

MATERIAL AND METHODS

This prospective case-control study was performed from May 1, 2020 to September 30, 2020. Of the 92 pregnant patients included in the study, we enrolled 49 normotensive patients complicated with IUGR as the study group, and 43 normotensive healthy gestational age-matched and body mass index (BMI)-matched patients without IUGR or additional pregnancy complication as the control group. Demographic and clinical characteristics, and maternal serum Elabela concentrations were recorded.

RESULTS

Maternal serum Elabela levels were significantly lower in IUGR pregnancies (4.02±3.42 ng/mL) compared to healthy pregnant women (14.01±18.38 ng/mL, p<0.001). There was a positive intermediate correlation between maternal serum Elabela levels and the birth weight (r = 0.308, p = 0.004).

CONCLUSION

Maternal circulating levels of Elabela were significantly lower in IUGR pregnancies than in healthy pregnant women. Also, birth weight was positively correlated with maternal serum Elabela levels. We consider that Elabela might be a crucial biomarker of the pathophysiologic process in pregnancies complicated by IUGR.

Interaction between the apelinergic system and ACE2 in the cardiovascular system: therapeutic implications

The apelinergic system is widely expressed and acts through autocrine and paracrine signaling to exert protective effects, including vasodilatory, metabolic, and inotropic effects on the cardiovascular (CV) system. The apelin pathway's dominant physiological role has delineated therapeutic implications for coronary artery disease, heart failure (HF), aortic aneurysm, pulmonary arterial hypertension (PAH), and transplant vasculopathy. Apelin peptides interact with the renin-angiotensin system (RAS) by promoting angiotensin converting enzyme 2 (ACE2) transcription leading to increased ACE2 protein and activity while also antagonizing the effects of angiotensin II (Ang II). Apelin modulation of the RAS by increasing ACE2 action is limited due to its rapid degradation by proteases, including ACE2, neprilysin (NEP), and kallikrein. Apelin peptides are hence tightly regulated in a negative feedback manner by ACE2. Plasma apelin levels are suppressed in pathological conditions, but its diagnostic and prognostic utility requires further clinical exploration. Enhancing the beneficial actions of apelin peptides and ACE2 axes while complementing existing pharmacological blockade of detrimental pathways is an exciting pathway for developing new therapies. In this review, we highlight the interaction between the apelin and ACE2 systems, discuss their pathophysiological roles and potential for treating a wide array of CV diseases (CVDs).

The Effects of Apelin and Elabela Ligands on Apelin Receptor Distinct Signaling Profiles

Apelin and Elabela are endogenous peptide ligands for Apelin receptor (APJ), a widely expressed G protein-coupled receptor. They constitute a spatiotemporal dual ligand system to control APJ signal transduction and function. We investigated the effects of Apelin-13, pGlu₁-apelin-13, Apelin-17, Apelin-36, Elabela-21 and Elabela-32 peptides on APJ signal transduction. Whether different ligands are biased to different APJ mediated signal transduction pathways was studied. We observed the different changes of G protein dependent and β-arrestin dependent signaling pathways after APJ was activated by six peptide ligands. We demonstrated that stimulation with APJ ligands resulted in dose-dependent increases in both G protein dependent [cyclic AMP (cAMP), Ca²⁺ mobilization, and the early phase extracellular related kinase (ERK) activation] and β-arrestin dependent [GRKs, β-arrestin 1, β-arrestin 2, and β2 subunit of the clathrin adaptor AP2] signaling pathways. However, the ligands exhibited distinct signaling profiles. Elabela-32 showed a >1000-fold bias to the β-statin-dependent signaling pathway. These data provide that Apelin-17 was biased toward β-arrestin dependent signaling. Eabela-21 and pGlu₁-Apelin-13 exhibited very distinct activities on the G protein dependent pathway. The activity profiles of these ligands could be valuable for the development of drugs with high selectivity for specific APJ downstream signaling pathways.

Sarcolipin haploinsufficiency prevents dystrophic cardiomyopathy in mdx mice

Sarcolipin (SLN) is an inhibitor of sarco/endoplasmic reticulum (SR) Ca2+-ATPase (SERCA) and expressed at high levels in the ventricles of animal models for and patients with Duchenne muscular dystrophy (DMD). The goal of this study was to determine whether the germline ablation of SLN expression improves cardiac SERCA function and intracellular Ca2+ (Ca2+i) handling and prevents cardiomyopathy in the mdx mouse model of DMD. Wild-type, mdx, SLN-haploinsufficient mdx (mdx:sln+/-), and SLN-deficient mdx (mdx:sln-/-) mice were used for this study. SERCA function and Ca2+i handling were determined by Ca2+ uptake assays and by measuring single-cell Ca2+ transients, respectively. Age-dependent disease progression was determined by histopathological examinations and by echocardiography in 6-, 12-, and 20-mo-old mice. Gene expression changes in the ventricles of mdx:sln+/- mice were determined by RNA-Seq analysis. SERCA function and Ca2+i cycling were improved in the ventricles of mdx:sln+/- mice. Fibrosis and necrosis were significantly decreased, and cardiac function was enhanced in the mdx:sln+/- mice until the study endpoint. The mdx:sln-/- mice also exhibited similar beneficial effects. RNA-Seq analysis identified distinct gene expression changes including the activation of the apelin pathway in the ventricles of mdx:sln+/- mice. Our findings suggest that reducing SLN expression is sufficient to improve cardiac SERCA function and Ca2+i cycling and prevent cardiomyopathy in mdx mice.NEW & NOTEWORTHY First, reducing sarcopolin (SLN) expression improves sarco/endoplasmic reticulum Ca2+ uptake and intracellular Ca2+ handling and prevents cardiomyopathy in mdx mice. Second, reducing SLN expression prevents diastolic dysfunction and improves cardiac contractility in mdx mice Third, reducing SLN expression activates apelin-mediated cardioprotective signaling pathways in mdx heart.

Emerging roles of APLN and APELA in the physiology and pathology of the female reproductive system

APLN, APELA and their common receptor APLNR (composing the apelinergic axis) have been described in various species with extensive body distribution and multiple physiological functions. Recent studies have witnessed emerging intracellular cascades triggered by APLN and APELA which play crucial roles in female reproductive organs, including hypothalamus-pituitary-gonadal axis, ovary, oviduct, uterus and placenta. However, a comprehensive summary of APLN and APELA roles in physiology and pathology of female reproductive system has not been reported to date. In this review, we aim to concentrate on the general characteristics of APLN and APELA, as well as their specific physiological roles in female reproductive system. Meanwhile, the pathological contexts of apelinergic axis dysregulation in the obstetrics and gynecology are also summarized here, suggesting its potential prospect as a diagnostic biomarker and/or therapeutic intervention in the polycystic ovary syndrome, ovarian cancer, preeclampsia and gestational diabetes mellitus.

Apelin Receptor Signaling During Mesoderm Development

The Apelin receptor (Aplnr) is a G-protein coupled receptor which has a wide body distribution and various physiological roles including homeostasis, angiogenesis, cardiovascular and neuroendocrine function. Apelin and Elabela are two peptide components of the Aplnr signaling and are cleaved to give different isoforms which are active in different tissues and organisms.Aplnr signaling is related to several pathologies including obesity, heart disases and cancer in the adult body. However, the developmental role in mammalian embryogenesis is crucial for migration of early cardiac progenitors and cardiac function. Aplnr and peptide components have a role in proliferation, differentiation and movement of endodermal precursors. Although expression of Aplnr signaling is observed in endodermal lineages, the main function is the control of mesoderm cell movement and cardiac development. Mutant of the Aplnr signaling components results in the malformations, defects and lethality mainly due to the deformed heart function. This developmental role share similarity with the cardiovascular functions in the adult body.Determination of Aplnr signaling and underlying mechanisms during mammalian development might enable understanding of regulatory molecular mechanisms which not only control embryonic development process but also control tissue function and disease pathology in the adult body.

Serum Elabela Level Significantly Increased in Patients with Complete Heart Block

Objective

To investigate the change in serum Elabela level, a new apelinergic system peptide, in patients with complete atrioventricular (AV) block and healthy controls.

Methods

The study included 50 patients with planned cardiac pacemaker (PM) implantation due to complete AV block and 50 healthy controls with similar age and gender. Elabela level was measured in addition to routine anamnesis, physical examination, and laboratory tests. Patients were divided into two groups, with and without AV block, and then compared.

Results

In patients with AV block, serum Elabela level was significantly higher and heart rate and cardiac output were significantly lower than in healthy controls. Serum Elabela level was found to be positively correlated with high-sensitive C-reactive protein and N-terminal pro-brain natriuretic peptide (NT-proBNP) levels, but negatively correlated with heart rate, high-density lipoprotein cholesterol, and cardiac output. In linear regression analysis, it was found that these parameters were only closely related to heart rate and NT-proBNP. Serum Elabela level was determined in the patients with AV block independently; an Elabela level > 9.5 ng/ml determined the risk of complete AV-block with 90.2% sensitivity and 88.0% specificity.

Conclusion

In patients with complete AV block, the serum Elabela level increases significantly before the PM implantation procedure. According to the results of our study, it was concluded that serum Elabela level could be used in the early determination of patients with complete AV block.

Expression characteristics and regulatory mechanism of Apela gene in liver of chicken (Gallus gallus)

Apela, a novel endogenous peptide ligand for the G-protein-coupled apelin receptor, was first discovered and identified in human embryonic stem cells in 2013. Apela has showed some biological functions in promoting angiogenesis and inducing vasodilatation of mammals by binding apelin receptor, but little is known about its expression characteristics and regulatory mechanism in chicken. In the present study, the coding sequences of Apela in chicken was cloned. The evolution history and potential function of Apela were analyzed. Subsequently, the spatiotemporal expression characteristics of chicken Apela were investigated. Furthermore, the regulatory mechanism of Apela mRNA responsing to estrogen was explored by in vitro and in vivo experiments. The results showed that the length of the CDs of Apela mRNA was 165 bp and encoded a protein consisting of 54 amino acids residues with a transmembrane domain in chicken. The Apela was derived from the same ancestor of Apelin, and abundantly expressed in liver, kidney and pancreas tissues. The expression levels of Apela in the liver of hens were significantly higher at the peak-laying stage than that at the pre-laying stage (p ≤ 0.05). The Apela mRNA levels were significantly up-regulated in primary hepatocytes treated with 17β-estradiol (p ≤ 0.05), and could be effectively inhibited by estrogen receptor antagonists MPP, ICI 182780 and tamoxifen. It indicated that chicken Apela expression was regulated by estrogen via estrogen receptor α (ERα). In individual levels, both the contents of TG, TC and VLDL-c in serum, and the expression of ApoVLDLII and Apela in liver markedly up-regulated by 17β-estradiol induction at 1mg/kg and 2mg/kg concentrations (p ≤ 0.05). This study lays a foundation for further research on Apela involving in hepatic lipid metabolism.

The Elabela in hypertension, cardiovascular disease, renal disease, and preeclampsia: an update

: Although considerable success has been shown for antihypertensive medications, the resistant hypertension and hypertension-related organ damages are still the important clinical issues and pose as high health and economic pressure. Therefore, novel therapeutic techniques and antihypertensive drugs are needed to advance more effective therapy of hypertension and hypertension-related disease to ameliorate mortality and healthcare costs worldwide. In this review, we highlight the latest progress in supporting the therapeutic potential of Elabela (ELA), a recently discovered early endogenous ligand for G-protein-coupled receptor apelin peptide jejunum, apelin receptor. Systemic administration of ELA exerts vasodilatory, antihypertensive, cardioprotective, and renoprotective effects, whereas central application of ELA increases blood pressure and causes cardiovascular remodeling primarily secondary to the hypertension. In addition, ELA drives extravillous trophoblast differentiation and prevents the pathogenesis of preeclampsia (a gestational hypertensive syndrome) by promoting placental angiogenesis. These findings strongly suggest peripheral ELA's therapeutic potential in preventing and treating hypertension and hypertension-related diseases including cardiovascular disease, kidney disease, and preeclampsia. Since therapeutic use of ELA is mainly limited by its short half-life and parenteral administration, it may be a clinical application candidate for the therapy of hypertension and its complications when fused with a large inert chemicals (e.g. polyethylene glycol, termed polyethylene glycol-ELA-21) or other proteins (e.g. the Fc fragment of IgG and albumin, termed Fc-ELA-21 or albumin-ELA-21), and new delivery methods are encouraged to develop to improve the efficacy of ELA fragments on apelin peptide jejunum or alternative unknown receptors.

Apelin peptides linked to anti-serum albumin domain antibodies retain affinity in vitro and are efficacious receptor agonists in vivo

The apelin receptor is a potential target in the treatment of heart failure and pulmonary arterial hypertension where levels of endogenous apelin peptides are reduced but significant receptor levels remain. Our aim was to characterise the pharmacology of a modified peptide agonist, MM202, designed to have high affinity for the apelin receptor and resistance to peptidase degradation and linked to an anti-serum albumin domain antibody (AlbudAb) to extend half-life in the blood. In competition, binding experiments in human heart MM202-AlbudAb (pKi  = 9.39 ± 0.09) bound with similar high affinity as the endogenous peptides [Pyr1 ]apelin-13 (pKi  = 8.83 ± 0.06) and apelin-17 (pKi  = 9.57 ± 0.08). [Pyr1 ]apelin-13 was tenfold more potent in the cAMP (pD2  = 9.52 ± 0.05) compared to the β-arrestin (pD2  = 8.53 ± 0.03) assay, whereas apelin-17 (pD2  = 10.31 ± 0.28; pD2  = 10.15 ± 0.13, respectively) and MM202-AlbudAb (pD2  = 9.15 ± 0.12; pD2  = 9.26 ± 0.03, respectively) were equipotent in both assays, with MM202-AlbudAb tenfold less potent than apelin-17. MM202-AlbudAb bound to immobilised human serum albumin with high affinity (pKD  = 9.02). In anaesthetised, male Sprague Dawley rats, MM202-AlbudAb (5 nmol, n = 15) significantly reduced left ventricular systolic pressure by 6.61 ± 1.46 mm Hg and systolic arterial pressure by 14.12 ± 3.35 mm Hg and significantly increased cardiac contractility by 533 ± 170 mm Hg/s, cardiac output by 1277 ± 190 RVU/min, stroke volume by 3.09 ± 0.47 RVU and heart rate by 4.64 ± 2.24 bpm. This study demonstrates that conjugating an apelin mimetic peptide to the AlbudAb structure retains receptor and in vivo activity and may be a new strategy for development of apelin peptides as therapeutic agents.

Elabela/Toddler and apelin bind differently to the apelin receptor

Like apelin (pE13F, K17F), Elabela/Toddler is an endogenous ligand of the apelin receptor playing a key role in cardiovascular development. Elabela/Toddler exists as peptide fragments of 32 (Q32P), 22 (K22P) and 11 (C11P) amino acids. In this study, we investigated the possible structural and functional similarities between these endogenous ligands. We performed in vitro pharmacological characterization and biased signaling analyses for apelin and Elabela/Toddler fragments in CHO cells, by assessing binding affinities, the inhibition of cyclic adenosine monophosphate (cAMP) production and the triggering of ß-arrestin 2 recruitment. We also performed Alanine scanning for Elabela/Toddler and structure-function studies based on site-directed mutagenesis of the rat and human apelin receptor, to compare the modes of binding of the different endogenous ligands. Alanine scanning of K22P showed that neither of its cysteine residues were involved in binding or in peptide activity and that its C-terminus carried the key pharmacophore for receptor binding and activation. We showed that Asp²⁸² and Asp²⁸⁴ of rat and human apelin receptor, respectively, were not involved in Elabela/Toddler activity, whereas they are key residues for apelin binding and activity. We found that the structural features of Elabela/Toddler and apelin were different, resulting in different modes of binding of these endogenous ligands to the apelin receptor. These differences should be taken into account in the future development metabolically stable analogs of Elabela/Toddler and apelin as potential therapeutic tools for the treatment of cardiovascular diseases and water retention/hyponatremic disorders.

Apelin/Elabela-APJ: a novel therapeutic target in the cardiovascular system

Apelin and Elabela (ELA) are endogenous ligands of angiotensin domain type 1 receptor-associated proteins (APJ). Apelin/ELA-APJ signal is widely distributed in the cardiovascular system of fetuse and adult. The signal is involved in the development of the fetal heart and blood vessels and regulating vascular tension in adults. This review described the effects of apelin/ELA-APJ on fetal (vasculogenesis and angiogenesis) and adult cardiovascular function [vascular smooth muscle cell (VSMC) proliferation, vasodilation, positive myodynamia], and relative diseases [eclampsia, hypertension, pulmonary hypertension, heart failure (HF), myocardial infarction (MI), atherosclerosis, etc.] in detail. The pathways of apelin/ELA-APJ regulating cardiovascular function and cardiovascular-related diseases are summarized. The drugs developed based on apelin and ELA suggests APJ is a prospective strategy for cardiovascular disease therapy.