Apelin, Elabela/Toddler, and biased agonists as novel therapeutic agents in the cardiovascular system

Targeted delivery of apelin using a novel extracellular vesicle platform for pulmonary arterial hypertension treatment

Pulmonary arterial hypertension (PAH) is a severe disease characterized by endothelial dysfunction, vascular remodeling, and pulmonary artery occlusion, culminating in right ventricular hypertrophy and heart failure. While apelin peptides are promising therapeutic candidates due to their critical role in vascular homeostasis, their efficacy as agonists is limited by insufficient lesion-specific targeting and suboptimal in vivo stability. Here, we developed an engineered extracellular vesicle (EV) platform for precise apelin delivery to PAH lesions, maximizing therapeutic impact. Using interferon-induced transmembrane protein 3 (IFITM3), a type II transmembrane protein, we oriented the apelin peptide on the EV surface with its C-terminus fully exposed, preserving the critical binding interface for functional interaction with the apelin receptor. To further enhance targeting specificity, we integrated the PAH-targeting peptide CARSKNKDC (CAR), which selectively binds to heparan sulfate overexpressed on PAH endothelial cells, into the IFITM3-apelin scaffold, creating CAR-Apelin EVs. This dual-engineered EVs demonstrated exceptional targeting and therapeutic efficacy in PAH models. CAR-Apelin EVs significantly reversed pathological vascular remodeling and improved cardiac function, as evidenced by reduced right ventricular systolic pressure and hypertrophy. Our findings establish CAR-Apelin EVs as a transformative therapeutic strategy, providing a targeted and effective approach to meet critical unmet needs in PAH treatment.

Activation of APJ Receptors by CMF-019, But Not Apelin, Causes Endothelium-Dependent Relaxation of Spontaneously Hypertensive Rat Coronary Arteries

ABSTRACT

Receptors for the vasoactive adipokine apelin, termed APJ receptors, are G-protein-coupled receptors and are widely expressed throughout the cardiovascular system. APJ receptors can also signal through G-protein-independent pathways, including G-protein-coupled receptor kinase 2 (GRK2), which inhibits endothelial nitric oxide synthase (eNOS) activity and nitric oxide production in endothelial cells. Apelin causes endothelium-dependent, nitric oxide-mediated relaxation of coronary arteries from normotensive animals, but the effects of activating APJ receptor signaling pathways in hypertensive coronary arteries are largely unknown. We hypothesized that apelin-induced relaxation is impaired in coronary arteries from spontaneously hypertensive rats (SHR). Western blot and mRNA analysis revealed increased GRK2 expression in cultured SHR coronary endothelial cells. Apelin failed to cause relaxation in isolated SHR coronary arteries but, in the presence of apelin, relaxations to acetylcholine were impaired. Apelin had no effect on relaxation to diethylamine NONOate. The GRK2 inhibitor, CMPD101, increased apelin-induced phosphorylation of Akt and eNOS in SHR endothelial cells and restored relaxation to apelin in SHR arteries. CMPD101 also blocked the inhibitory effect of apelin on ACh-induced relaxation. Relaxations to the APJ receptor-biased agonist, CMF-019, which preferentially activates the G-protein-dependent pathway with minimal effect on GRK2, were similar in SHR and Wistar Kyoto coronary arteries. Immunoblot analysis in SHR coronary endothelial cells demonstrated that CMF-019 increased Akt and eNOS phosphorylation whereas apelin had no effect. Thus, APJ receptor signaling through GRK2 impairs nitric oxide production or release from SHR endothelial cells. APJ receptor-biased agonists, such as CMF-019, may be more effective than apelin in causing vasodilation of SHR coronary arteries.

Advances in the therapeutic potentials of ligands of the apelin receptor APJ

Angiotensin II protein J receptor, APJ, is a type A G protein coupled receptor. Endogenous apelin and elabela peptides stimulate APJ via distinct signalling profiles. A complex signalling map of elabela-stimulated APJ was published in 2022. Dimerization or oligomerization of APJ with itself or other receptor(s) can affect APJ signalling. Apelin has been shown to tolerate mutations and/or modifications at multiple sites without abolishing activity. This offers a great opportunity to design and engineer variants with desired signalling profiles and enhanced resistance to breakdown by peptidases. Several biased agonists with enhanced therapeutic potential have been generated. APJ agonists have therapeutic potential in multiple diseases including cardiovascular, renal, pulmonary and metabolic diseases, and viral infections. APJ antagonists may have therapeutic potential in cancer and retinopathy, and in related diseases in which unwanted angiogenesis is to be halted. A growing understanding of APJ signalling pathways and the robust therapeutic potential of associated ligands for many serious diseases will stimulate the clinical development of APJ ligands.

Structural and functional determination of peptide versus small molecule ligand binding at the apelin receptor

We describe a structural and functional study of the G protein-coupled apelin receptor, which binds two endogenous peptide ligands, apelin and Elabela/Toddler (ELA), to regulate cardiovascular development and function. Characterisation of naturally occurring apelin receptor variants from the UK Genomics England 100,000 Genomes Project, and AlphaFold2 modelling, identifies T892.64 as important in the ELA binding site, and R1684.64 as forming extensive interactions with the C-termini of both peptides. Base editing to introduce an R/H1684.64 variant into human stem cell-derived cardiomyocytes demonstrates that this residue is critical for receptor binding and function. Additionally, we present an apelin receptor crystal structure bound to the G protein-biased, small molecule agonist, CMF-019, which reveals a deeper binding mode versus the endogenous peptides at lipophilic pockets between transmembrane helices associated with GPCR activation. Overall, the data provide proof-of-principle for using genetic variation to identify key sites regulating receptor-ligand engagement.

Endothelial cell-cardiomyocyte cross-talk: understanding bidirectional paracrine signaling in cardiovascular homeostasis and disease

To maintain homeostasis in the heart, endothelial cells and cardiomyocytes engage in dynamic cross-talk through paracrine signals that regulate both cardiac development and function. Here, we review the paracrine signals that endothelial cells release to regulate cardiomyocyte growth, hypertrophy and contractility, and the factors that cardiomyocytes release to influence angiogenesis and vascular tone. Dysregulated communication between these cell types can drive pathophysiology of disease, as seen in ischemia-reperfusion injury, diabetes, maladaptive hypertrophy, and chemotherapy-induced cardiotoxicity. Investingating the role of cross-talk is critical in developing an understanding of tissue homeostasis, regeneration, and disease pathogenesis, with the potential to identify novel targets for diagnostic and therapeutic purposes.

Early Administration of Apelin Could Prevent Heart Failure Following Myocardial Injury; A Systematic Review and Meta-Analysis

Introduction

Heart failure (HF) is a prevalent and advancing cardiovascular disorder that impacts 1-2% of the worldwide population, particularly the elderly. Studies indicate that the intravenous administration of apelin may yield advantageous effects in preventing heart failure subsequent to myocardial injury. This meta-analysis aimed to assess the effects of exogenous apelin administration on heart failure in animal models, in light of the lack of a definitive consensus on the matter.

Method

An extensive search was performed in the Medline (via PubMed), Web of Science, Embase, and Scopus databases till the end of January 2024. Two independent reviewers screened and summarized the relevant articles. Outcomes related to cardiac function, including ejection fraction (EF), maximum and minimum rate of left ventricle systolic pressure (+dp/dt and -dp/dt, respectively), heart rate, left ventricular end-diastolic pressure (LVEDP), and left ventricular systolic pressure (LVSP) were assessed. Findings were reported as a pooled standardized mean difference (SMD) with a 95% confidence interval (95% CI).

Results

12 studies were included. Pooled analysis demonstrated that early treatment with apelin following myocardial injury significantly increases +dp/dt (SMD = 2.36; 95% CI: 1.58 to 3.15; p < 0.001) and decreases -dp/dt (SMD = -3.31; 95% CI: -4.46 to -2.17; p < 0.001). Furthermore, the administration of apelin resulted in a significant increase in EF (SMD = 0.79; 95% CI: 0.15 to 1.44; p = 0.02) and LVSP (SMD = 2.09; 95% CI: 0.82 to 3.36; p < 0.001), while it led to a decrease in LVEDP in the animals (SMD = -1.85; 95% CI: -2.81 to -0.88; p < 0.001). Noteworthy, apelin treatment was shown to have no significant influence on the heart rate of the animals (SMD = -0.12; 95% CI: -0.82 to -0.58; p = 0.73).

Conclusion

The current study demonstrated that the early administration of apelin has the potential to improve cardiac function and mitigate the onset of heart failure subsequent to myocardial injury. Further, in vivo research is essential to lay the groundwork for the integration of apelin into clinical practice.

The biased apelin receptor agonist, MM07, reverses Sugen/hypoxia-induced pulmonary arterial hypertension as effectively as the endothelin antagonist macitentan

Introduction: Pulmonary arterial hypertension (PAH) is characterised by endothelial dysfunction and pathological vascular remodelling, resulting in the occlusion of pulmonary arteries and arterioles, right ventricular hypertrophy, and eventually fatal heart failure. Targeting the apelin receptor with the novel, G protein-biased peptide agonist, MM07, is hypothesised to reverse the developed symptoms of elevated right ventricular systolic pressure and right ventricular hypertrophy. Here, the effects of MM07 were compared with the clinical standard-of-care endothelin receptor antagonist macitentan. Methods: Male Sprague-Dawley rats were randomised and treated with either normoxia/saline, or Sugen/hypoxia (SuHx) to induce an established model of PAH, before subsequent treatment with either saline, macitentan (30 mg/kg), or MM07 (10 mg/kg). Rats were then anaesthetised and catheterised for haemodynamic measurements, and tissues collected for histopathological assessment. Results: The SuHx/saline group presented with significant increases in right ventricular hypertrophy, right ventricular systolic pressure, and muscularization of pulmonary arteries compared to normoxic/saline controls. Critically, MM07 was as at least as effective as macitentan in significantly reversing detrimental structural and haemodynamic changes after 4 weeks of treatment. Discussion: These results support the development of G protein-biased apelin receptor agonists with improved pharmacokinetic profiles for use in human disease.

Serum Elabela expression is decreased in hypertensive patients and could be associated with the progression of hypertensive renal damage

BACKGROUND

Elabela, a recently discovered hormonal peptide containing 32 amino acids, is a ligand for the apelin receptor. It can lower blood pressure and attenuate renal fibrosis. However, the clinicopathological relationship between Elabela level and renal damage caused by benign hypertension (BHT) and malignant hypertension (MHT) has not been elucidated. Therefore, we investigated the clinicopathological correlation between serum Elabela level and renal damage caused by BHT and MHT.

METHODS

The participants comprised 50 patients and 25 age-matched healthy adults. The 50 patients were separated into two groups: MHT (n = 25) and BHT groups (n = 25). We analyzed their medical histories, demographics, and clinical examinations, including physical and laboratory tests.

RESULTS

The results showed that serum Elabela level decreased gradually with a continuous increase in blood pressure from the healthy control group, BHT, to MHT. Moreover, Elabela levels negatively correlated with BMI (R =  - 0.27, P = 0.02), SBP (r =  - 0.64, P < 0.01), DBP (r =  - 0.58, P < 0.01), uric acid (r =  - 0.39, P < 0.01), bun (r =  - 0.53, P < 0.01), and Scr (r =  - 0.53 P < 0.01) but positively correlated with eGFR (r = 0.54, P < 0.01). Stepwise multivariate linear regression analysis showed that SBP was the variable most related to Elabela (t =  - 5.592, P < 0.01).

CONCLUSIONS

Serum Elabela levels decreased in patients with hypertension, especially malignant hypertension, and has the potential to be a marker of hypertension-related kidney damage.

Molecular and functional characterization of the Drosophila melanogaster conserved smORFome

Short polypeptides encoded by small open reading frames (smORFs) are ubiquitously found in eukaryotic genomes and are important regulators of physiology, development, and mitochondrial processes. Here, we focus on a subset of 298 smORFs that are evolutionarily conserved between Drosophila melanogaster and humans. Many of these smORFs are conserved broadly in the bilaterian lineage, and ∼182 are conserved in plants. We observe remarkably heterogeneous spatial and temporal expression patterns of smORF transcripts-indicating wide-spread tissue-specific and stage-specific mitochondrial architectures. In addition, an analysis of annotated functional domains reveals a predicted enrichment of smORF polypeptides localizing to mitochondria. We conduct an embryonic ribosome profiling experiment and find support for translation of 137 of these smORFs during embryogenesis. We further embark on functional characterization using CRISPR knockout/activation, RNAi knockdown, and cDNA overexpression, revealing diverse phenotypes. This study underscores the importance of identifying smORF function in disease and phenotypic diversity.

Advances in the study of ELABELA in renal physiological functions and related diseases

ELABELA (ELA), also known as Toddler or Apela, is a novel endogenous ligand of the angiotensin receptor AT1-related receptor protein (APJ). ELA is highly expressed in human embryonic, cardiac, and renal tissues and involves various biological functions, such as embryonic development, blood circulation regulation, and maintaining body fluid homeostasis. ELA is also closely related to the occurrence and development of acute kidney injury, hypertensive kidney damage, diabetic nephropathy, renal tumors, and other diseases. Understanding the physiological role of ELA and its mechanism of action in kidney-related diseases would provide new targets and directions for the clinical treatment of kidney diseases.

Inducible apelin receptor knockdown reduces differentiation efficiency and contractility of hESC-derived cardiomyocytes

AIMS

The apelin receptor, a G protein-coupled receptor, has emerged as a key regulator of cardiovascular development, physiology, and disease. However, there is a lack of suitable human in vitro models to investigate the apelinergic system in cardiovascular cell types. For the first time we have used human embryonic stem cell-derived cardiomyocytes (hESC-CMs) and a novel inducible knockdown system to examine the role of the apelin receptor in both cardiomyocyte development and to determine the consequences of loss of apelin receptor function as a model of disease.

METHODS AND RESULTS

Expression of the apelin receptor and its ligands in hESCs and hESC-CMs was determined. hESCs carrying a tetracycline-inducible short hairpin RNA targeting the apelin receptor were generated using the sOPTiKD system. Phenotypic assays characterized the consequences of either apelin receptor knockdown before hESC-CM differentiation (early knockdown) or in 3D engineered heart tissues as a disease model (late knockdown). hESC-CMs expressed the apelin signalling system at a similar level to the adult heart. Early apelin receptor knockdown decreased cardiomyocyte differentiation efficiency and prolonged voltage sensing, associated with asynchronous contraction. Late apelin receptor knockdown had detrimental consequences on 3D engineered heart tissue contractile properties, decreasing contractility and increasing stiffness.

CONCLUSIONS

We have successfully knocked down the apelin receptor, using an inducible system, to demonstrate a key role in hESC-CM differentiation. Knockdown in 3D engineered heart tissues recapitulated the phenotype of apelin receptor down-regulation in a failing heart, providing a potential platform for modelling heart failure and testing novel therapeutic strategies.

Signaling Modulation via Minimal C-Terminal Modifications of Apelin-13

Apelin is an endogenous peptide that is involved in many diseases such as cardiovascular diseases, obesity, and cancer, which has made it an attractive target for drug discovery. Herein, we explore the penultimate and final sequence positions of [Pyr1]-apelin-13 (Ape13) via C-terminal N α-alkylated amide bonds and the introduction of positive charges, potentially targeting the allosteric sodium pocket, by assessing the binding affinity and signaling profiles at the apelin receptor (APJ). Synthetic analogues modified within this segment of Ape13 showed high affinity (K i 0.12-0.17 nM vs Ape13 K i 0.7 nM), potent Gαi1 activation (EC50 Gαi1 0.4-0.9 nM vs Ape13 EC50 1.1 nM), partial agonist behavior disfavoring β-arrestin 2 recruitment for positively charged ligands (e.g., 49 (SBL-AP-058), EC50 β-arr2 275 nM, E max 54%) and high plasma stability for N-alkyl ligands (t 1/2 > 7 h vs Ape13 t 1/2 0.5 h). Combining the benefits of the N α-alkylated amide bond with the guanidino substitution in a constrained ligand led to 63 (SBL-AP-049), which displayed increased plasma stability (t 1/2 5.3 h) and strong reduction of β-arrestin 2 signaling with partial maximal efficacy (EC50 β-arr 864 nM, E max 48%), significantly reducing the hypotensive effect in vivo.

The G protein-coupled receptor ligand apelin-13 ameliorates skeletal muscle atrophy induced by chronic kidney disease

BACKGROUND

Targeting of the apelin-apelin receptor (Apj) system may serve as a useful therapeutic intervention for the management of chronic kidney disease (CKD)-induced skeletal muscle atrophy. We investigated the roles and efficacy of the apelin-Apj system in CKD-induced skeletal muscle atrophy.

METHODS

The 5/6-nephrectomized mice were used as CKD models. AST-120, a charcoal adsorbent of uraemic toxins (8 w/w% in diet), or apelin (1 μmol/kg) was administered to CKD mice to investigate the mechanism and therapeutic potential of apelin on CKD-induced skeletal muscle atrophy. The effect of indoxyl sulfate, a uraemic toxin, or apelin on skeletal muscle atrophy was evaluated using mouse myoblast cells (C2C12 cells) in vitro.

RESULTS

Skeletal muscle atrophy developed over time following nephrectomy at 12 weeks, as confirmed by a significant increase of atrogin-1 and myostatin mRNA expression in the gastrocnemius (GA) muscle and a decrease of lower limb skeletal muscle weight (P < 0.05, 0.01 and 0.05, respectively). Apelin expression in GA muscle was significantly decreased (P < 0.05) and elabela, another Apj endogenous ligand, tended to show a non-significant decrease at 12 weeks after nephrectomy. Administration of AST-120 inhibited the decline of muscle weight and increase of atrogin-1 and myostatin expression. Apelin and elabela expression was slightly improved by AST-120 administration but Apj expression was not, suggesting the involvement of uraemic toxins in endogenous Apj ligand expression. The administration of apelin at 1.0 μmol/kg for 4 weeks to CKD mice suppressed the increase of atrogin-1 and myostatin, increased apelin and Apj mRNA expression at 30 min after apelin administration and significantly ameliorated weight loss and a decrease of the cross-sectional area of hindlimb skeletal muscle.

CONCLUSIONS

This study demonstrated for the first time the association of the Apj endogenous ligand-uraemic toxin axis with skeletal muscle atrophy in CKD and the utility of therapeutic targeting of the apelin-Apj system.

Apelin-17 to diagnose idiopathic pulmonary arterial hypertension: A biomarker study

Background: NT-proBNP and GDF-15 are established blood-derived biomarkers for risk assessment in pulmonary hypertension (PH), despite limited sensitivity and specificity. Apelin has a crucial function in endothelial homeostasis, thus it might represent a new biomarker for PH. However, there are numerous circulating apelin isoforms, and their potential role in this setting is unknown. This study evaluated different apelin isoforms in PH patients and prospectively evaluated the role of apelin-17 in comparison with NT-proBNP and GDF-15 as diagnostic marker in idiopathic pulmonary arterial hypertension (IPAH). Methods: Based on our pilot study, we performed a power calculation for apelin-13, apelin-17, apelin-36, as predictor of IPAH vs healthy controls. Apelin-17 provided the best discriminatory power, and accordingly, we enrolled n = 31 patients with IPAH and n = 31 matched healthy controls in a prospective study. NT-proBNP and GDF-15 was determined in all patients. ROC curve analysis was performed to assess the diagnostic value of the markers and their combinations. Results: Apelin-17, NT-proBNP, and GDF-15 were significantly elevated in IPAH patients as compared to controls (p < .001). Apelin-17 detected IPAH with a sensitivity of 68% and a specificity of 93% at a cut-off value of >1,480 pg/ml (AUC 0.86, 95%CI:0.76-0.95) as compared to GDF-15 (sensitivity 86%; specificity 72%, AUC 0.81 (95%CI:0.7-0.92)) and NT-proBNP (sensitivity 86%; specificity 72% (AUC 0.85, 95%CI:0.75-0.95)). Combinations of these markers could be used to increase either specificity or sensitivity. Conclusion: Apelin-17 appears to be suitable blood derived diagnostic marker for idiopathic pulmonary arterial hypertension.

A Novel Peptide Elabela is Associated with Hypertension-Related Subclinical Atherosclerosis

INTRODUCTION

Elabela is a newly identified peptide which, alongside apelin, acts as an endogenous ligand that activates the angiotensin receptor-like 1 receptor. Previous studies have shown the association of elabela with hypertension, but information about the role of elabela in hypertension-related subclinical atherosclerosis is scarce.

AIM

We aimed to determine the elabela levels in hypertensive patients and explore its association with subclinical atherosclerosis.

METHODS

A total of 104 subjects with hypertension were included in the study. Elabela levels were measured using an enzyme-linked immunosorbent assay, by first extracting the peptide following the manufacturer's instructions. Subclinical atherosclerosis was assessed by measuring the carotid intima-media thickness (IMT) using ultrasound.

RESULTS

Compared to stage 1, elabela levels decreased in stage 2 hypertension (0.23 [0.13, 0.45] ng/ml vs. 0.14 [0.09, 0.23] ng/ml; P = 0.000), and in the group with increased carotid IMT compared to normal IMT (0.24 [0.13, 0.38] ng/ml vs. 0.15 [0.10, 0.23] ng/ml; P = 0.005). Additionally, a linear correlation analysis showed that elabela had a significant negative correlation with systolic blood pressure (r = - 0.340, P = 0.000) and carotid IMT (r = - 0.213; P = 0.030). In multivariate analysis, lower elabela levels were associated with a higher cardiovascular risk group in this study (OR 5.0, 95% CI 1.8-13.5, P < 0.001).

CONCLUSIONS

This study demonstrated for the first time that circulating elabela declined in a higher stage of hypertension and hypertensive patients with increased carotid IMT, implicating that elabela may be involved in the pathogenesis of hypertension-associated subclinical atherosclerosis.

Expanding the apelin receptor pharmacological toolbox using novel fluorescent ligands

INTRODUCTION

The apelin receptor binds two distinct endogenous peptides, apelin and ELA, which act in an autocrine/paracrine manner to regulate the human cardiovascular system. As a class A GPCR, targeting the apelin receptor is an attractive therapeutic strategy. With improvements in imaging techniques, and the stability and brightness of dyes, fluorescent ligands are becoming increasingly useful in studying protein targets. Here, we describe the design and validation of four novel fluorescent ligands; two based on [Pyr1]apelin-13 (apelin488 and apelin647), and two based on ELA-14 (ELA488 and ELA647).

METHODS

Fluorescent ligands were pharmacologically assessed using radioligand and functional in vitro assays. Apelin647 was validated in high content imaging and internalisation studies, and in a clinically relevant human embryonic stem cell-derived cardiomyocyte model. Apelin488 and ELA488 were used to visualise apelin receptor binding in human renal tissue.

RESULTS

All four fluorescent ligands retained the ability to bind and activate the apelin receptor and, crucially, triggered receptor internalisation. In high content imaging studies, apelin647 bound specifically to CHO-K1 cells stably expressing apelin receptor, providing proof-of-principle for a platform that could screen novel hits targeting this GPCR. The ligand also bound specifically to endogenous apelin receptor in stem cell-derived cardiomyocytes. Apelin488 and ELA488 bound specifically to apelin receptor, localising to blood vessels and tubules of the renal cortex.

DISCUSSION

Our data indicate that the described novel fluorescent ligands expand the pharmacological toolbox for studying the apelin receptor across multiple platforms to facilitate drug discovery.

Comparative Study of Elabela and Apelin on Apelin Receptor Activation Through β-Arrestin Recruitment

Apelin receptor (APJ) ligands elabela (ELA) and apelin have divergent distributions and function differently in vitro and in vivo. Whether differences exist in their capacity of recruitment of β-arrestins (ARRBs) to APJ remains unknown. The aim of the current study was to investigate the different effects of ELA and apelin on the interaction between APJ and ARRBs in live cells by NanoBiT®. NanoBiT® system is a new technology for studying protein-protein interaction in real-time in live cells, based on the emission of luminescence when two split components of NanoLuc luciferase, large Bit (LgBit) and small Bit (SmBit), complement each other to form an enzymatically active entity. We tagged the APJ and ARRBs with LgBit or SmBit and then evaluated their interactions in transiently transfected HEK293T cells, and determined the signal strength yielded as a result of the interaction. We also investigated the concentration-dependent response of the APJ-ARRB interaction in response to ELA and apelin. Finally, we assessed the effect of F13A, an APJ antagonist which is structurally very similar to apelin-13, on ELA- and apelin-mediated APJ-ARRB interactions. The NanoLuc® luciferase signal was highest in the pair of APJ-LgBit with SmBit-ARRB1 or SmBit-ARRB2. NanoLuc® luciferase signal increased in a concentration-dependent manner from 0.1 nM to 10 μM in response to ELA or apelin. Interestingly, ELA elicited weaker APJ-ARRB interaction signals than apelin. Pre-treatment with F13A potently reduced the APJ-ARRB interaction in response to both ELA and apelin. Our results demonstrated that both ELA and apelin promoted the interaction of APJ and ARRBs in a concentration-dependent manner, and ELA is less efficacious than apelin in inducing the recruitment of ARRBs to APJ, providing a biased functional aspect of ELA vs. apelin at the receptor signaling level. Additionally, ELA and apelin may share the same binding site(s) or pocket(s) at the APJ level.

Sexual Dimorphism in the Polarization of Cardiac ILCs through Elabela

Elabela is a component of the apelinergic system and may exert a cardioprotective role by regulating the innate immune responses. Innate lymphoid cells (ILCs) have a significant role in initiating and progressing immune-inflammatory responses. While ILCs have been intensively investigated during the last decade, little is known about their relationship with the apelinergic system and their cardiac diversity in a gender-based paradigm. In this study, we investigated the polarization of cardiac ILCs by Elabela in males versus females in a mouse model. Using flow cytometry and immunohistochemistry analyses, we showed a potential interplay between Elabela and cardiac ILCs and whether such interactions depend on sexual dimorphism. Our findings showed, for the first time, that Elabela is expressed by cardiac ILCs, and its expression is higher in females' ILC class 3 (ILC3s) compared to males. Females had higher frequencies of ILC1s, and Elabela was able to suppress T-cell activation and the expression of co-stimulatory CD28 in a mixed lymphocyte reaction assay (MLR). In conclusion, our results suggest, for the first time, a protective role for Elabela through its interplay with ILCs and that it can be used as an immunotherapeutic target in the treatment of cardiovascular disorders in a gender-based fashion.

Apelin-Induced Relaxation of Coronary Arteries Is Impaired in a Model of Second-Hand Cigarette Smoke Exposure

ABSTRACT

Apelin, an endogenous ligand for APJ receptors, causes nitric oxide (NO)-dependent relaxation of coronary arteries. Little is known about the effects of apelin/APJ receptor signaling in the coronary circulation under pathological conditions. Here, we tested the hypothesis that the vasorelaxing effect of apelin is impaired by cigarette smoke extract (CSE), an established model for second-hand smoke exposure. Isolated rat coronary arteries were treated with 2% CSE for 4 hours. Apelin-induced relaxation of coronary arteries was abolished by CSE exposure, while relaxations to acetylcholine (ACh) (endothelium-dependent relaxation) and to diethyl amine NONOate (NO donor) were similar in control and CSE-treated arteries. Immunoblot analysis demonstrated that apelin increased eNOS ser1177 phosphorylation under control conditions but had no effect after exposure to CSE. Moreover, GRK2 expression was increased in CSE-exposed coronary endothelial cells. Pretreatment with CMPD101, a GRK2 inhibitor, improved the relaxation response to apelin in CSE-exposed coronary arteries. CSE treatment failed to inhibit relaxations evoked by CMF-019, an APJ receptor biased agonist that has little effect on GRK2. In arteries exposed to CSE, apelin impaired the response to ACh but not to diethyl amine NONOate. ACh-induced relaxation was unaffected by CMF-019 in either control or CSE-treated coronary arteries. The results suggest that APJ receptor signaling using the GRK2 pathway contributes to both loss of relaxation to apelin itself and the ability of apelin to inhibit endothelium-dependent relaxation to ACh in CSE-exposed coronary arteries, likely because of impaired production of NO from endothelial cells. These changes in apelin/APJ receptor signaling under pathological conditions (eg, exposure to second-hand smoke) could create an environment that favors increased vasomotor tone in coronary arteries.

Levels of Serum and Urine Catecholaminergic and Apelinergic System Members in Acute Ischemic Stroke Patients

Objective: To compare levels of catecholaminergic system members, renalase, cerebellin, and their substrates, epinephrine, norepinephrine, and dopamine, and apelinergic system members, apelin, elabela, and nitric oxide in the blood and urine of patients with acute ischemic stroke and healthy controls. Materials and Methods: 42 patients with acute ischemic stroke and 42 age and sex-matched healthy controls were included in the study. Blood and urine samples were collected simultaneously and within the first 24 hours after the onset of acute stroke clinical manifestations and were measured using an ELISA method. Results: The levels of serum and urine cerebellin, renalase, epinephrine, norepinephrine, dopamine, apelin, elebela, and nitric oxide were similar in ischemic stroke and in control groups (P>0.05). Strong correlations were found between renalase, cerebellin, and catecholamine levels in serum and urine (p

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.

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.

Therapeutic Approaches for Treating Pulmonary Arterial Hypertension by Correcting Imbalanced TGF-β Superfamily Signaling

Pulmonary arterial hypertension (PAH) is a rare disease characterized by high blood pressure in the pulmonary circulation driven by pathological remodeling of distal pulmonary arteries, leading typically to death by right ventricular failure. Available treatments improve physical activity and slow disease progression, but they act primarily as vasodilators and have limited effects on the biological cause of the disease—the uncontrolled proliferation of vascular endothelial and smooth muscle cells. Imbalanced signaling by the transforming growth factor-β (TGF-β) superfamily contributes extensively to dysregulated vascular cell proliferation in PAH, with overactive pro-proliferative SMAD2/3 signaling occurring alongside deficient anti-proliferative SMAD1/5/8 signaling. We review the TGF-β superfamily mechanisms underlying PAH pathogenesis, superfamily interactions with inflammation and mechanobiological forces, and therapeutic strategies under development that aim to restore SMAD signaling balance in the diseased pulmonary arterial vessels. These strategies could potentially reverse pulmonary arterial remodeling in PAH by targeting causative mechanisms and therefore hold significant promise for the PAH patient population.

Multi-tumor analysis of cancer-stroma interactomes of patient-derived xenografts unveils the unique homeostatic process in renal cell carcinomas

The patient-derived xenograft (PDX) model is a versatile tool used to study the tumor microenvironment (TME). However, limited studies have described multi-tumor PDX screening strategies to detect hub regulators during cancer-stroma interaction. Transcriptomes of cancer (human) and stroma (mouse) components of 70 PDX samples comprising 9 distinctive tumor types were analyzed in this study. PDX models recapitulated the original tumors' features, including tumor composition and putative signaling. Particularly, kidney renal clear cell carcinoma (KIRC) stood out, with altered hypoxia-related pathways and a high proportion of endothelial cells in the TME. Furthermore, an integrated analysis conducted to predict paracrine effectors in the KIRC cancer-to-stroma communication detected well-established soluble factors responsible for the hypoxia-related reaction and the so-far unestablished soluble factor, apelin (APLN). Subsequent experiments also supported the potential role of APLN in KIRC tumor progression. Therefore, this paper hereby provides an analytical workflow to find hub regulators in cancer-stroma interactions.

Importance of ELABELA in the differential diagnosis of benign and malignant lesions of the thyroid gland

OBJECTIVE

This study investigated whether ELABELA plays a role in the differential diagnosis of benign and malignant lesions of the thyroid gland.

METHODS

Of the 87 patients included in the study, 12 had undergone surgery for benign thyroid diseases, 30 had papillary thyroid cancer without invasion and/or lymph node metastasis in the surrounding tissues in the pathology report, and 45 had papillary thyroid cancer with invasion and/or lymph node metastasis in the surrounding tissues.

RESULTS

In the macrocarcinoma group, the proportion of patients with severe ELABELA staining (61.1%) was higher than that in the adenoma (50%) and microcarcinoma (23.8%) groups, while the proportion of those with mild to moderate staining was lower (p < 0.001). In the microcarcinoma group, the proportion of patients with severe staining was lower than that in the adenoma group, while the proportion of those with mild to moderate staining was higher (p < 0.001). In papillary thyroid carcinomas, the rates of moderate and severe staining in the classical variant, mild staining in the follicular variant, severe staining in the classical + follicular variant, and severe staining in the oncocytic variant were higher.

CONCLUSION

To the best of our knowledge, this study is the first to be conducted on this subject. In this study, ELABELA was not found to be significant in the differential diagnosis of benign and malignant lesions of the thyroid gland. In papillary thyroid carcinomas, severe ELABELA staining patterns were more common in macrocarcinoma patients than in microcarcinoma patients.

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.

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.

Metabolically stable apelin-analogues, incorporating cyclohexylalanine and homoarginine, as potent apelin receptor activators

High blood pressure and consequential cardiovascular diseases are among the top causes of death worldwide. The apelinergic (APJ) system has emerged as a promising target for the treatment of cardiovascular issues, especially prevention of ischemia reperfusion (IR) injury after a heart attack or stroke. However, rapid degradation of the endogenous apelin peptides in vivo limits their use as therapeutic agents. Here, we study the effects of simple homologue substitutions, i.e. incorporation of non-canonical amino acids l-cyclohexylalanine (l-Cha) and l-homoarginine (l-hArg), on the proteolytic stability of pyr-1-apelin-13 and apelin-17 analogues. The modified 13-mers display up to 40 times longer plasma half-life than native apelin-13 and in preliminary in vivo assay show moderate blood pressure-lowering effects. The corresponding apelin-17 analogues show pronounced blood pressure-lowering effects and up to a 340-fold increase in plasma half-life compared to the native apelin-17 isoforms, suggesting their potential use in the design of metabolically stable apelin analogues to prevent IR injury.

Declined ELABELA plasma levels in hypertension patients with atrial fibrillation: a case control study

Background

Atrial fibrillation (AF) is a common arrhythmia in patients with hypertension. ELABELA, which has cardioprotective effects, is decreased in the plasma of patients with hypertension and might be associated with AF in the hypertensive population. This study aims to measure the ELABELA plasma levels in hypertension patients with and without AF and to analyse the related factors.

Methods

A total of 162 hypertension patients with or without AF were recruited for our monocentric observational study. Subjects were excluded if they had a history of valvular heart disease, rheumatic heart disease, cardiomyopathy, thyroid diseases, or heart failure. The patients’ histories were recorded, and laboratory examinations were conducted. Plasma ELABELA was detected by immunoassay. Echocardiographs were performed, and parameters were collected by two experienced doctors. Binary logistic regression analysis was used to identify the association between ELABELA plasma level and AF in patients with hypertension.

Results

Plasma ELABELA levels were lower in hypertension patients with AF than in those without AF (2.0 [1.5, 2.8] vs. 4.0 [3.4, 5.0] ng/ml, P < 0.001). ELABELA levels were correlated with age, heart rate, BNP levels and left atrial dimension. In addition to the left atrial dimension, ELABELA plasma levels were associated with AF in patients with hypertension (OR 0.081, 95% CI 0.029–0.224, P < 0.001). ELABELA levels were further decreased in the persistent AF subgroup compared with the paroxysmal AF subgroup (1.8 [1.4, 2.5] vs. 2.2 [1.8, 3.0] ng/ml, P = 0.012) and correlated with HR, BNP and ESR levels.

Conclusions

ELALABELA levels were decreased in hypertension patients with AF and further lowered in the persistent AF subgroup. Decreased ELABELA plasma levels were associated with AF in hypertension patients and may be an underlying risk factor.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12872-021-02197-x.

The glycoprotein GP130 governs the surface presentation of the G protein-coupled receptor APLNR

Glioblastoma is one of the most lethal forms of adult cancer, with a median survival of ∼15 mo. Targeting glioblastoma stem-like cells (GSCs) at the origin of tumor formation and relapse may prove beneficial. In situ, GSCs are nested within the vascular bed in tight interaction with brain endothelial cells, which positively control their expansion. Because GSCs are notably addicted to apelin (APLN), sourced from the surrounding endothelial stroma, the APLN/APLNR nexus has emerged as a druggable network. However, how this signaling axis operates in gliomagenesis remains underestimated. Here, we find that the glycoprotein GP130 interacts with APLNR at the plasma membrane of GSCs and arbitrates its availability at the surface via ELMOD1, which may further impact on ARF-mediated endovesicular trafficking. From a functional standpoint, interfering with GP130 thwarts APLNR-mediated self-renewal of GSCs ex vivo. Thus, GP130 emerges as an unexpected cicerone to the G protein–coupled APLN receptor, opening new therapeutic perspectives toward the targeting of cancer stem cells.

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.

Diuretic Action of Apelin-13 Mediated by Inhibiting cAMP/PKA/sPRR Pathway

Emerging evidence is showing that apelin plays an important role in regulating salt and water balance by counteracting the antidiuretic action of vasopressin (AVP). However, the underlying mechanism remains unknown. Here, we hypothesized that (pro) renin receptor (PRR)/soluble prorenin receptor (sPRR) might mediate the diuretic action of apelin in the distal nephron. During water deprivation (WD), the urine concentrating capability was impaired by an apelin peptide, apelin-13, accompanied by the suppression of the protein expression of aquaporin 2 (AQP2), NKCC2, PRR/sPRR, renin and nuclear β-catenin levels in the kidney. The upregulated expression of AQP2 or PRR/sPRR both induced by AVP and 8-Br-cAMP was blocked by apelin-13, PKA inhibitor (H89), or β-catenin inhibitor (ICG001). Interestingly, the blockage of apelin-13 on AVP-induced AQP2 protein expression was reversed by exogenous sPRR. Together, the present study has defined the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA)/sPRR pathway in the CD as the molecular target of the diuretic action of apelin.

Gαi-biased apelin analog protects against isoproterenol-induced myocardial dysfunction in rats

Apelin receptor (APJ) activation by apelin-13 (APLN-13) engages both Gαi proteins and β-arrestins, stimulating distinct intracellular pathways and triggering physiological responses like enhanced cardiac contractility. Substituting the C-terminal phenylalanine of APLN-13 with α-methyl-l-phenylalanine [(l-α-Me)Phe] or p-benzoyl-l-phenylalanine (Bpa) generates biased analogs inducing APJ functional selectivity toward Gαi proteins. Using these original analogs, we proposed to investigate how the canonical Gαi signaling of APJ regulates the cardiac function and to assess their therapeutic impact in a rat model of isoproterenol-induced myocardial dysfunction. In vivo and ex vivo infusions of either Bpa or (l-α-Me)Phe analogs failed to enhance rats' left ventricular (LV) contractility compared with APLN-13. Inhibition of Gαi with pertussis toxin injection optimized the cardiotropic effect of APLN-13 and revealed the inotropic impact of Bpa. Moreover, both APLN-13 and Bpa efficiently limited the forskolin-induced and PKA-dependent phosphorylation of phospholamban at the Ser16 in neonatal rat ventricular myocytes. However, only Bpa significantly reduced the inotropic effect of forskolin infusion in isolated-perfused heart, highlighting its efficient bias toward Gαi. Compared with APLN-13, Bpa also markedly improved isoproterenol-induced myocardial systolic and diastolic dysfunctions. Bpa prevented cardiac weight increase, normalized both ANP and BNP mRNA expressions, and decreased LV fibrosis in isoproterenol-treated rats. Our results show that APJ-driven Gαi/adenylyl cyclase signaling is functional in cardiomyocytes and acts as negative feedback of the APLN-APJ-dependent inotropic response. Biased APJ signaling toward Gαi over the β-arrestin pathway offers a promising strategy in the treatment of cardiovascular diseases related to myocardial hypertrophy and high catecholamine levels.NEW & NOTEWORTHY By using more potent Gαi-biased APJ agonists that strongly inhibit cAMP production, these data point to the negative inotropic effect of APJ-mediated Gαi signaling in the heart and highlight the potential protective impact of APJ-dependent Gαi signaling in cardiovascular diseases associated with left ventricular hypertrophy.

Novel Targets for Hypertension Drug Discovery

PURPOSE OF REVIEW

Despite the availability of various medications and prescribing combination therapies, uncontrolled blood pressure and resistance are observed in more than 40% of patients. The purpose of this review is to discuss emerging novel approaches for the treatment of hypertension and propose future research and clinical directions.

RECENT FINDINGS

Hypertension is a common disease of the cardiovascular system which may arise solely or as a comorbidity of other disorders. It is a crucial risk factor for cardiovascular diseases such as coronary artery disease, myocardial infarction, congestive heart failure, renal failure, and stroke. The results from current literature regarding the novel approaches showed several targets that could be explored as potential therapeutic options. These include toll-like receptor 4, a critical regulator of angiotensin II-induced hypertension; protease-activated receptor 2, which promotes collagen deposition and inflammatory responses; chemerin, which causes metabolic and obesity-associated hypertension; apelin receptor; transient receptor potential melastatin; urotensin-II; and Tie2 receptor. This review discusses various targets and pathways that could be emerging pharmacological therapies for hypertension.

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 role of Elabela in kidney disease

Elabela, also known as Toddler or Apela, is a recently discovered hormonal peptide containing 32 amino acids. Elabela is a ligand of the apelin receptor (APJ). APJ is a G protein-coupled receptor widely expressed throughout body, and together with its cognate ligand, apelin, it plays an important role in various physiological processes including cardiovascular functions, angiogenesis and fluid homeostasis. Elabela also participates in embryonic development and pathophysiological processes in adulthood. Elabela is highly expressed in undifferentiated embryonic stem cells and regulates endoderm differentiation and cardiovascular system development. During differentiation, Elabela is highly expressed in pluripotent stem cells and in adult renal collecting ducts and loops, where it functions to maintain water and sodium homeostasis. Other studies have also shown that Elabela plays a crucial role in the pathogenesis of kidney diseases. This review addresses the role of Elabela in kidney diseases including renal ischemia/reperfusion injury, hypertensive nephropathy, diabetic nephropathy, and cardiorenal syndrome.

Zebrafish Vascular Development: General and Tissue-Specific Regulation

Circulation is required for the delivery of oxygen and nutrition to tissues and organs, as well as waste collection. Therefore, the heart and vessels develop first during embryogenesis. The circulatory system consists of the heart, blood vessels, and blood cells, which originate from the mesoderm. The gene expression pattern required for blood vessel development is predetermined by the hierarchical and sequential regulation of genes for the differentiation of mesodermal cells. Herein, we review how blood vessels form distinctly in different tissues or organs of zebrafish and how vessel formation is universally or tissue-specifically regulated by signal transduction pathways and blood flow. In addition, the unsolved issues of mutual contacts and interplay of circulatory organs during embryogenesis are discussed.

Loss of APJ mediated β-arrestin signalling improves high-fat diet induced metabolic dysfunction but does not alter cardiac function in mice

Apelin receptor (APJ) is a G protein-coupled receptor that contributes to many physiological processes and is emerging as a therapeutic target to treat a variety of diseases. For most disease indications the role of G protein vs β-arrestin signalling in mitigating disease pathophysiology remains poorly understood. This hinders the development of G protein biased APJ agonists, which have been proposed to have several advantages over balanced APJ signalling agonists. To elucidate the contribution of APJ β-arrestin signalling, we generated a transgenic mouse harbouring a point mutation (APJ I107A) that maintains full G protein activity but fails to recruit β-arrestin following receptor activation. APJ I107A mutant mice did not alter cardiac function at rest, following exercise challenge or in response to pressure overload induced cardiac hypertrophy. Additionally, APJ I107A mice have comparable body weights, plasma glucose and lipid levels relative to WT mice when fed a chow diet. However, APJ I107A mice showed significantly lower body weight, blood insulin levels, improved glucose tolerance and greater insulin sensitivity when fed a high-fat diet. Furthermore, loss of APJ β-arrestin signalling also affected fat composition and the expression of lipid metabolism related genes in adipose tissue from high-fat fed mice. Taken together, our results suggest that G protein biased APJ activation may be more effective for certain disease indications given that loss of APJ mediated β-arrestin signalling appears to mitigate several aspects of diet induced metabolic dysfunction.

The Role of Apelin/Apelin Receptor in Energy Metabolism and Water Homeostasis: A Comprehensive Narrative Review

The apelin receptor (APJ) is a member of the family A of G-protein-coupled receptors (GPCRs) and is involved in range of physiological and pathological functions, including fluid homeostasis, anxiety, and depression, as well as cardiovascular and metabolic disorders. APJ was classically described as a monomeric transmembrane receptor that forms a ternary complex together with its ligand and associated G proteins. More recently, increasing evidence indicates that APJ may interact with other GPCRs to form heterodimers, which may selectively modulate distinct intracellular signal transduction pathways. Besides, the apelin/APJ system plays important roles in the physiology and pathophysiology of several organs, including regulation of blood pressure, cardiac contractility, angiogenesis, metabolic balance, and cell proliferation, apoptosis, or inflammation. Additionally, the apelin/APJ system is widely expressed in the central nervous system, especially in neurons and oligodendrocytes. This article reviews the role of apelin/APJ in energy metabolism and water homeostasis. Compared with the traditional diuretics, apelin exerts a positive inotropic effect on the heart, while increases water excretion. Therefore, drugs targeting apelin/APJ system undoubtedly provide more therapeutic options for patients with congestive heart failure accompanied with hyponatremia. To provide more precise guidance for the development of clinical drugs, further in-depth studies are warranted on the metabolism and signaling pathways associated with apelin/APJ system.

Apelin/APJ-Manipulated CaMKK/AMPK/GSK3β Signaling Works as an Endogenous Counterinjury Mechanism in Promoting the Vitality of Random-Pattern Skin Flaps

A random-pattern skin flap plays an important role in the field of wound repair; the mechanisms that influence the survival of random-pattern skin flaps have been extensively studied but little attention has been paid to endogenous counterinjury substances and mechanism. Previous reports reveal that the apelin-APJ axis is an endogenous counterinjury mechanism that has considerable function in protecting against infection, inflammation, oxidative stress, necrosis, and apoptosis in various organs. As an in vivo study, our study proved that the apelin/APJ axis protected the skin flap by alleviating vascular oxidative stress and the apelin/APJ axis works as an antioxidant stress factor dependent on CaMKK/AMPK/GSK3β signaling. In addition, the apelin/APJ-manipulated CaMKK/AMPK/GSK3β-dependent mechanism improves HUVECs' resistance to oxygen and glucose deprivation/reperfusion (OGD/R), reduces ROS production and accumulation, maintained the normal mitochondrial membrane potential, and suppresses oxidative stress in vitro. Besides, activation of the apelin/APJ axis promotes vascular migration and angiogenesis under relative hypoxia condition through CaMKK/AMPK/GSK3β signaling. In a word, we provide new evidence that the apelin/APJ axis is an effective antioxidant and can significantly improve the vitality of random flaps, so it has potential be a promising clinical treatment.

The G Protein Biased Small Molecule Apelin Agonist CMF-019 is Disease Modifying in Endothelial Cell Apoptosis In Vitro and Induces Vasodilatation Without Desensitisation In Vivo

Signaling through the apelin receptor is beneficial for a number of diseases including pulmonary arterial hypertension. The endogenous small peptides, apelin and elabela/toddler, are downregulated in pulmonary arterial hypertension but are not suitable for exogenous administration owing to a lack of bioavailability, proteolytic instability and susceptibility to renal clearance. CMF-019, a small molecule apelin agonist that displays strong bias towards G protein signaling over β-arrestin (∼400 fold), may be more suitable. This study demonstrates that in addition to being a positive inotrope, CMF-019 caused dose-dependent vasodilatation in vivo (50 nmol 4.16 ± 1.18 mmHg, **p < 0.01; 500 nmol 6.62 ± 1.85 mmHg, **p < 0.01), without receptor desensitization. Furthermore, CMF-019 rescues human pulmonary artery endothelial cells from apoptosis induced by tumor necrosis factor α and cycloheximide (5.66 ± 0.97%, **p < 0.01) by approximately 50% of that observable with rhVEGF (11.59 ± 1.85%, **p < 0.01), suggesting it has disease-modifying potential in vitro. CMF-019 displays remarkable bias at the apelin receptor for a small molecule and importantly recapitulates all aspects of the cardiovascular responses to the endogenous ligand, [Pyr¹]apelin-13, in vivo. Additionally, it is able to protect human pulmonary artery endothelial cells from apoptosis, suggesting that the beneficial effects observed with apelin agonists extend beyond hemodynamic alleviation and address disease etiology itself. These findings support CMF-019 as a G protein biased small molecule apelin agonist in vitro and in vivo that could form the basis for the design of novel therapeutic agents in chronic diseases, such as, pulmonary arterial hypertension.

Beta-Arrestins and Receptor Signaling in the Vascular Endothelium

The vascular endothelium is the innermost layer of blood vessels and is a key regulator of vascular tone. Endothelial function is controlled by receptor signaling through G protein-coupled receptors, receptor tyrosine kinases and receptor serine-threonine kinases. The β-arrestins, multifunctional adapter proteins, have the potential to regulate all of these receptor families, although it is unclear as to whether they serve to integrate signaling across all of these different axes. Notably, the β-arrestins have been shown to regulate signaling by a number of receptors important in endothelial function, such as chemokine receptors and receptors for vasoactive substances such as angiotensin II, endothelin-1 and prostaglandins. β-arrestin-mediated signaling pathways have been shown to play central roles in pathways that control vasodilation, cell proliferation, migration, and immune function. At this time, the physiological impact of this signaling has not been studied in detail, but a deeper understanding of it could lead to the development of novel therapies for the treatment of vascular disease.

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-potential therapy for COVID-19?

We believe that, in parallel to the attempts for direct blockade of the SARS-CoV-2 penetration into host cell and repurposing drugs, finding new therapeutic strategies for patients with lung injury or cardiovascular complications/coagulopathies associated with COVID-19 should be paid particular attention. Apelin or its receptor agonists are of great potential treatment for COVID-19 through suppressing angiotensin-converting enzyme (ACE) and angiotensin II (Ang-II) production, as well as, down-regulating angiotensin receptor 1 (AT1R) and ACE2 up-regulation. These drugs have potential to improve acute lung injury and cardiovascular/coagulopathy complications in COVID-19 which are associated with elevated Ang-II/Ang(1–7) ratio.

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RAS up-regulation is associated with lung and cardiovascular injuries in COVID-19.

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Apelin can suppress ACE and AT1R, and activate ACE2 which is down-regulated by SARS-CoV-2.

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Apelin may improve Ang-II-mediated inflammation, thrombosis, and vasoconstriction in COVID-19.

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Apelin and its receptor agonists could be trialed in COVID-19 patients.

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.

Formation and Growth of Cardiac Lymphatics during Embryonic Development, Heart Regeneration, and Disease

The lymphatic system plays crucial roles in regulating fluid homeostasis, immune surveillance, and lipid transport. As is in most of the body's organs, the heart possesses an extensive lymphatic network. Moreover, a robust lymphangiogenic response has been shown to take place following myocardial infarction, highlighting cardiac lymphatics as potential targets for therapeutic intervention. Yet, the unique molecular properties and functions of the heart's lymphatic system have only recently begun to be addressed. In this review, we discuss the mechanisms underlying the formation and growth of cardiac lymphatics during embryonic development and describe their characteristics across species. We further summarize recent findings highlighting diverse cellular origins for cardiac lymphatic endothelial cells and how they integrate to form a single functional lymphatic network. Finally, we outline novel therapeutic avenues aimed at enhancing lymphatic vessel formation and integrity following cardiac injury, which hold great promise for promoting healing of the infarcted heart.