The endogenous peptide apelin potently improves cardiac contractility and reduces cardiac loading in vivo

Apelin-13 administration allows for norepinephrine sparing in a rat model of cecal ligation and puncture-induced septic shock

BACKGROUND

Infusion of exogenous catecholamines (i.e., norepinephrine [NE] and dobutamine) is a recommended treatment for septic shock with myocardial dysfunction. However, sustained catecholamine infusion is linked to cardiac toxicity and impaired responsiveness. Several pre-clinical and clinical studies have investigated the use of alternative vasopressors in the treatment of septic shock, with limited benefits and generally no effect on mortality. Apelin-13 (APL-13) is an endogenous positive inotrope and vasoactive peptide and has been demonstrated cardioprotective with vasomodulator and sparing life effects in animal models of septic shock. A primary objective of this study was to evaluate the NE-sparing effect of APL-13 infusion in an experimental sepsis-induced hypotension.

METHODS

For this goal, sepsis was induced by cecal ligation and puncture (CLP) in male rats and the arterial blood pressure (BP) monitored continuously via a carotid catheter. Monitoring, fluid resuscitation and experimental treatments were performed on conscious animals. Based on pilot assays, normal saline fluid resuscitation (2.5 mL/Kg/h) was initiated 3 h post-CLP and maintained up to the endpoint. Thus, titrated doses of NE, with or without fixed-doses of APL-13 or the apelin receptor antagonist F13A co-infusion were started when 20% decrease of systolic BP (SBP) from baseline was achieved, to restore SBP values ≥ 115 ± 1.5 mmHg (baseline average ± SEM).

RESULTS

A reduction in mean NE dose was observed with APL-13 but not F13A co-infusion at pre-determined treatment time of 4.5 ± 0.5 h (17.37 ± 1.74 µg/Kg/h [APL-13] vs. 25.64 ± 2.61 µg/Kg/h [Control NE] vs. 28.60 ± 4.79 µg/Kg/min [F13A], P = 0.0491). A 60% decrease in NE infusion rate over time was observed with APL-13 co-infusion, (p = 0.008 vs NE alone), while F13A co-infusion increased the NE infusion rate over time by 218% (p = 0.003 vs NE + APL-13). Associated improvements in cardiac function are likely mediated by (i) enhanced left ventricular end-diastolic volume (0.18 ± 0.02 mL [Control NE] vs. 0.30 ± 0.03 mL [APL-13], P = 0.0051), stroke volume (0.11 ± 0.01 mL [Control NE] vs. 0.21 ± 0.01 mL [APL-13], P < 0.001) and cardiac output (67.57 ± 8.63 mL/min [Control NE] vs. 112.20 ± 8.53 mL/min [APL-13], P = 0.0036), and (ii) a reduced effective arterial elastance (920.6 ± 81.4 mmHg/mL/min [Control NE] vs. 497.633.44 mmHg/mL/min. [APL-13], P = 0.0002). APL-13 administration was also associated with a decrease in lactate levels compared to animals only receiving NE (7.08 ± 0.40 [Control NE] vs. 4.78 ± 0.60 [APL-13], P < 0.01).

CONCLUSION

APL-13 exhibits NE-sparing benefits in the treatment of sepsis-induced shock, potentially reducing deleterious effects of prolonged exogenous catecholamine administration.

Comprehensive Quality Analysis of Conventional and Novel Biomarkers in Diagnosing and Predicting Prognosis of Coronary Artery Disease, Acute Coronary Syndrome, and Heart Failure, a Comprehensive Literature Review

Coronary artery disease (CAD), acute coronary syndrome (ACS), and heart failure (HF) are major global health issues with high morbidity and mortality rates. Biomarkers like cardiac troponins (cTn) and natriuretic peptides (NPs) are crucial tools in cardiology, but numerous new biomarkers have emerged, proving increasingly valuable in CAD/ACS. These biomarkers are classified based on their mechanisms, such as fibrosis, metabolism, inflammation, and congestion. The integration of established and emerging biomarkers into clinical practice is an ongoing process, and recognizing their strengths and limitations is crucial for their accurate interpretation, incorporation into clinical settings, and improved management of CVD patients. We explored established biomarkers like cTn, NPs, and CRP, alongside newer biomarkers such as Apo-A1, IL-17E, IgA, Gal-3, sST2, GDF-15, MPO, H-FABP, Lp-PLA2, and ncRNAs; provided evidence of their utility in CAD/ACS diagnosis and prognosis; and empowered clinicians to confidently integrate these biomarkers into clinical practice based on solid evidence.

The structural analogue of apelin-12 prevents energy disorders in the heart in experimental type 1 diabetes mellitus

Type 1 diabetes mellitus (T1DM) is the most severe form of diabetes, which is characterized by absolute insulin deficiency induced by the destruction of pancreatic beta cells. The aim of this study was to evaluate the effect of a structural analogue of apelin-12 ((NαMe)Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Nle-Pro-Phe-OH, metilin) on hyperglycemia, mitochondrial (MCh) respiration in permeabilized cardiac left ventricular (LV) fibers, the myocardial energy state, and cardiomyocyte membranes damage in a model of streptozotocin (STZ) diabetes in rats. Metilin was prepared by solid-phase synthesis using the Fmoc strategy and purified using HPLC. Four groups of animals were used: initial state (IS); control (C), diabetic control (D) and diabetic animals additionally treated with metilin (DM). The following parameters have been studied: blood glucose, MCh respiration in LV fibers, the content of cardiac ATP, ADP, AMP, phosphocreatine (PCr) and creatine (Cr), the activity of creatine kinase-MB (CK-MB) and lactate dehydrogenase (LDH) in blood plasma. Administration of metilin to STZ-treated rats decreased blood glucose, increased state 3 oxygen consumption, the respiratory control ratio in MCh of permeabilized LV fibers, and increased the functional coupling of mitochondrial CK (mt-CK) to oxidative phosphorylation compared with these parameters in group D. In STZ-treated animals metilin administration caused an increase in the PCr content and prevention of the loss of total creatine (ΣCr=PCr+Cr) in the diabetic hearts, as well as restoration of the PCr/ATP ratio in the myocardium and a decrease in the activity of CK-MB and LDH in plasma to initial values. Thus, metilin prevented energy disorders disturbances in cardiomyocytes of animals with experimental T1DM.

Apelin-13 reverses bupivacaine-induced cardiotoxicity: an experimental study

INTRODUCTION

Cardiac arrest or arrhythmia caused by bupivacaine may be refractory to treatment. Apelin has been reported to directly increase the frequency of spontaneous activation and the propagation of action potentials, ultimately promoting cardiac contractility. This study aimed to investigate the effects of apelin-13 in reversing cardiac suppression induced by bupivacaine in rats.

METHODS

A rat model of cardiac suppression was established by a 3-min continuous intravenous infusion of bupivacaine at the rate of 5 mg.kg-1.min-1, and serial doses of apelin-13 (50, 150 and 450 μg.kg-1) were administered to rescue cardiac suppression to identify its dose-response relationship. We used F13A, an inhibitor of Angiotensin Receptor-Like 1 (APJ), and Protein Kinase C (PKC) inhibitor chelerythrine to reverse the effects of apelin-13. Moreover, the protein expressions of PKC, Nav1.5, and APJ in ventricular tissues were measured using Western blotting and immunofluorescence assay.

RESULTS

Compared to the control rats, the rats subjected to continuous intravenous administration of bupivacaine had impaired hemodynamic stability. Administration of apelin-13, in a dose-dependent manner, significantly improved hemodynamic parameters in rats with bupivacaine-induced cardiac suppression (p < 0.05), and apelin-13 treatment also significantly upregulated the protein expressions of p-PKC and Nav1.5 (p < 0.05), these effects were abrogated by F13A or chelerythrine (p < 0.05).

CONCLUSION

Exogenous apelin-13, at least in part, activates the PKC signaling pathway through the apelin/APJ system to improve cardiac function in a rat model of bupivacaine-induced cardiac suppression.

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.

Influence of the Apelinergic System on Conduction Disorders in Patients after Myocardial Infarction

BACKGROUND

There is a growing body of evidence for an important role of the apelinergic system in the modulation of cardiovascular homeostasis. The aim of our study was to (1) examine the relationship between apelin serum concentration at index myocardial infarction (MI) and atrioventricular conduction disorders (AVCDs) at 12-month follow-up, and (2) investigate the association between initial apelin concentration and the novel marker of post-MI scar (Q/QRS ratio) at follow-up.

METHODS

In 84 patients with MI with complete revascularization, apelin peptide serum concentrations for apelin-13, apelin-17, elabela (ELA) and apelin receptor (APJ) were measured on day one of hospitalization; at 12-month follow-up, 54 of them underwent thorough examination that included 12-lead electrocardiography (ECG), Holter ECG monitoring and echocardiography.

RESULTS

The mean age was 58.9 years. At 12-month follow-up, AVCDs were diagnosed in 21.4% of subjects, with AV first-degree block in 16.7% and sinoatrial arrest in 3.7%. ELA serum concentration at index MI correlated positively with the occurrence of AVCD (p = 0.003) and heart rate (p = 0.005) at 12-month follow-up. The apelin-13 serum concentration at index MI correlated negatively with the Q/QRS ratio.

CONCLUSIONS

The apelin peptide concentration during an acute phase of MI impacts the development of AVCD and the value of Q/QRS ratio in MI survivors.

Apelin C-Terminal Fragments: Biological Properties and Therapeutic Potential

Creation of bioactive molecules for treatment of cardiovascular diseases based on natural peptides is the focus of intensive experimental research. In the recent years, it has been established that C-terminal fragments of apelin, an endogenous ligand of the APJ receptor, reduce metabolic and functional disorders in experimental heart damage. The review presents literature data and generalized results of our own experiments on the effect of apelin-13, [Pyr]apelin-13, apelin-12, and their chemically modified analogues on the heart under normal and pathophysiological conditions in vitro and in vivo. It has been shown that the spectrum of action of apelin peptides on the damaged myocardium includes decrease in the death of cardiomyocytes from necrosis, reduction of damage to cardiomyocyte membranes, improvement in myocardial metabolic state, and decrease in formation of reactive oxygen species and lipid peroxidation products. The mechanisms of protective action of these peptides associated with activation of the APJ receptor and manifestation of antioxidant properties are discussed. The data presented in the review show promise of the molecular design of APJ receptor peptide agonists, which can serve as the basis for the development of cardioprotectors that affect the processes of free radical oxidation and metabolic adaptation.

Adipokines and Their Role in Heart Failure: A Literature Review

Obesity is a major risk factor for heart failure (HF). The relationship between adipokines and HF has been implicated in many previous studies and reviews. However, this review article summarizes the basic role of major adipokines, such as apelin, adiponectin, chemerin, resistin, retinol-binding protein 4 (RBP4), vaspin, visfatin, plasminogen activator inhibitor-1, monocyte chemotactic protein-1, nesfatin-1, progranulin, leptin, omentin-1, lipocalin-2, and follistatin-like 1 (FSTL1), in the pathogenesis of HF. Apelin is reduced in patients with HF and upregulated following favorable left ventricular (LV) remodeling. Higher levels of adiponectin have been found in patients with HF compared to in control patients. Also, high plasma chemerin levels are linked to a higher risk of HF. Serum resistin is related to the severity of HF and associated with a high risk for adverse cardiac events. Evidence indicates that RBP4 can contribute to inflammation and damage heart muscle cells, potentially leading to HF. Vaspin might stop the progression of cardiac degeneration, fibrosis, and HF according to experiments on rats with experimental isoproterenol-induced chronic HF. The serum concentrations of visfatin are significantly lower in patients with systolic HF. Leptin levels were found to be correlated with low LV mass and myocardial stiffness, both of which are significant risk factors for the development of HF with preserved ejection fraction (HFpEF). Measuring serum omentin-1 levels appears to be a novel prognostic indicator for risk stratification in HF patients. Increased expression of neutrophil gelatinase-associated lipocalin in both systemic circulation and myocardium in clinical and experimental HF suggests that innate immune responses may contribute to the development of HF. FSTL1 was elevated in patients with HF with reduced ejection fraction and associated with an increase in the size of the left ventricle of the heart. However, other adipokines, such as plasminogen activator inhibitor-1, monocyte chemotactic protein-1, nesfatin-1, and progranulin, have not yet been studied for HF.

A First-in-Human Study of AMG 986, a Novel Apelin Receptor Agonist, in Healthy Subjects and Heart Failure Patients

PURPOSE

AMG 986 is a novel apelin receptor (APJ) agonist that improves cardiac contractility in animal models without adversely impacting hemodynamics. This phase 1b study evaluated the safety/tolerability, pharmacokinetics, and pharmacodynamics of AMG 986 in healthy subjects and patients with heart failure (HF).

METHODS

Healthy adults (Parts A/B) and HF patients (Part C) aged 18-85 years were randomized 3:1 to single-dose oral/IV AMG 986 or placebo (Part A); multiple-dose oral/IV AMG 986 or placebo (Part B); or escalating-dose oral AMG 986 or placebo (Part C).

PRIMARY ENDPOINT

treatment-emergent adverse events, laboratory values/vital signs/ECGs; others included AMG 986 pharmacokinetics, left ventricular (LV) function.

RESULTS

Overall, 182 subjects were randomized (AMG 986/healthy: n = 116, placebo, n = 38; AMG 986/HF: n = 20, placebo, n = 8). AMG 986 had acceptable safety profile; no clinically significant dose-related impact on safety parameters up to 650 mg/day was observed. AMG 986 exposures increased nonlinearly with increasing doses; minimal accumulation was observed. In HF with reduced ejection fraction patients, there were numerical increases in percent changes from baseline in LV ejection fraction and stroke volume by volumetric assessment with AMG 986 vs placebo (stroke volume increase not recapitulated by Doppler).

CONCLUSIONS

In healthy subjects and HF patients, short-term AMG 986 treatment was well tolerated. Consistent with this observation, clinically meaningful pharmacodynamic effects in HF patients were not observed. Changes in ejection fraction and stroke volume in HF patients suggest additional studies may be needed to better define the clinical utility and optimal dosing for this molecule.

TRIAL REGISTRATION NUMBER

ClinicalTrials.gov NCT03276728.

DATE OF REGISTRATION

September 8, 2017.

The Lymphatic Vasculature in Cardiac Development and Ischemic Heart Disease

In recent years, the lymphatic system has received increasing attention due to the fast-growing number of findings about its diverse novel functional roles in health and disease. It is well documented that the lymphatic vasculature plays major roles in the maintenance of tissue-fluid balance, the immune response, and in lipid absorption. However, recent studies have identified an additional growing number of novel and sometimes unexpected functional roles of the lymphatic vasculature in normal and pathological conditions in different organs. Among those, cardiac lymphatics have been shown to play important roles in heart development, ischemic cardiac disease, and cardiac disorders. In this review, we will discuss some of those novel functional roles of cardiac lymphatics, as well as the therapeutic potential of targeting lymphatics for the treatment of cardiovascular diseases.

The role of endothelial cell in cardiac hypertrophy: Focusing on angiogenesis and intercellular crosstalk

Cardiac hypertrophy is characterized by cardiac structural remodeling, fibrosis, microvascular rarefaction, and chronic inflammation. The heart is structurally organized by different cell types, including cardiomyocytes, fibroblasts, endothelial cells, and immune cells. These cells highly interact with each other by a number of paracrine or autocrine factors. Cell-cell communication is indispensable for cardiac development, but also plays a vital role in regulating cardiac response to damage. Although cardiomyocytes and fibroblasts are deemed as key regulators of hypertrophic stimulation, other cells, including endothelial cells, also exert important effects on cardiac hypertrophy. More particularly, endothelial cells are the most abundant cells in the heart, which make up the basic structure of blood vessels and are widespread around other cells in the heart, implicating the great and inbuilt advantage of intercellular crosstalk. Cardiac microvascular plexuses are essential for transport of liquids, nutrients, molecules and cells within the heart. Meanwhile, endothelial cell-mediated paracrine signals have multiple positive or negative influences on cardiac hypertrophy. However, a comprehensive discussion of these influences and consequences is required. This review aims to summarize the basic function of endothelial cells in angiogenesis, with an emphasis on angiogenic molecules under hypertrophic conditions. The secondary objective of the research is to fully discuss the key molecules involved in the intercellular crosstalk and the endothelial cell-mediated protective or detrimental effects on other cardiac cells. This review provides a more comprehensive understanding of the overall role of endothelial cells in cardiac hypertrophy and guides the therapeutic approaches and drug development of cardiac hypertrophy.

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.

The Yin and Yang Effect of the Apelinergic System in Oxidative Stress

Apelin is an endogenous ligand for the G protein-coupled receptor APJ and has multiple biological activities in human tissues and organs, including the heart, blood vessels, adipose tissue, central nervous system, lungs, kidneys, and liver. This article reviews the crucial role of apelin in regulating oxidative stress-related processes by promoting prooxidant or antioxidant mechanisms. Following the binding of APJ to different active apelin isoforms and the interaction with several G proteins according to cell types, the apelin/APJ system is able to modulate different intracellular signaling pathways and biological functions, such as vascular tone, platelet aggregation and leukocytes adhesion, myocardial activity, ischemia/reperfusion injury, insulin resistance, inflammation, and cell proliferation and invasion. As a consequence of these multifaceted properties, the role of the apelinergic axis in the pathogenesis of degenerative and proliferative conditions (e.g., Alzheimer's and Parkinson's diseases, osteoporosis, and cancer) is currently investigated. In this view, the dual effect of the apelin/APJ system in the regulation of oxidative stress needs to be more extensively clarified, in order to identify new potential strategies and tools able to selectively modulate this axis according to the tissue-specific profile.

The protective effect of Apelin-13 against cardiac hypertrophy through activating the PI3K-AKT-mTOR signaling pathway

Objectives

To determine the protective effect of Apelin-13 on cardiac hypertrophy through activating the PI3K-AKT-mTOR signaling pathway.

Materials and Methods

The phenylephrine-induced cardiomyocyte hypertrophy model was established in H9C2 cells in vitro. Electroporation transfection technology was utilized to prepare and screen the H9C2 cells inducing low expression of the angiotensin type one receptor-related protein (Si-APJ). H9C2 and Si-APJ cells were divided independently into five groups: the control group, the PE group, the PE+Apelin group, the PE+Rapa group, and the PE+Apelin+Rapa group. RT-PCR was performed to analyze the mRNA expression levels of myosin heavy chain 7 (MYH7). Expression of the PI3K/AKT/mTOR pathway proteins and MYH7 was investigated by western blot.

Results

The expression of PI3K/AKT/mTOR phosphorylated proteins was significantly higher in the PE group compared with the PE+Apelin group in H9C2 cells (P<0.05). Conversely, in Si-APJ H9C2 cells, the expression of PI3K/AKT/mTOR phosphorylated proteins was decreased (P<0.05). In H9C2 cells, the expression of MYH7 protein was increased in the PE group compared with the control group (P<0.05). In the same cell line, the expression of MYH7 in the PE+Apelin group was decreased significantly compared with the PE group (P<0.05). In Si-APJ H9C2 cells, compared with the control group, the expression of MYH7 in the PE group still increased significantly (P<0.05). In contrast, in the same cell line, there was no statistically significant difference in MYH7 expression between the PE+Apelin, PE+Rapa, and PE+Apelin+Rapa groups compared to the PE group (P>0.05).

Conclusion

Apelin-13 reduces PE-induced cardiac hypertrophy by activating the PI3K/AKT/mTOR signaling pathway.

A Review of the Roles of Apelin and ELABELA Peptide Ligands in Cardiovascular Disease, Including Heart Failure and Hypertension

Apelin and ELABELA (ELA), which are peptides belonging to the adipokines group, are endogenous peptide ligands of their receptor, APJ, which together constitute the apelinergic system. The apelinergic system is expressed in numerous human tissues and organs, including the heart, blood vessels, adipose tissue, central nervous system, lungs, kidneys, and liver. Apelin, being the most widely studied member of the apelinergic system, plays a key role in the cardiovascular system and exerts a pleiotropic effect in tissues. Under physiological conditions, the peripheral actions of apelin include augmented cardiac contractility, increased left ventricular stroke volume, vasodilation, increased diuresis, and lowered systemic blood pressure. Multiple studies suggest that activation of the apelinergic system exerts beneficial effects on the treatment of cardiovascular diseases (CVD), including hypertension and heart failure, whereas the silencing of the apelin/APJ axis results in attenuation of inflammatory processes and prevents formation of atherosclerotic plaques. As numerous effects of apelin are not entirely explained, further studies of the cardiovascular actions of apelin and ELA are necessary to help establish effective pharmacological treatments of CVDs. This article aims to review the roles of apelin and elabela peptide ligands in cardiovascular diseases, including heart failure and hypertension.

Design and preparation of N-linked hydroxypyridine-based APJ agonists

Agonism of the apelin receptor (APJ) has demonstrated beneficial effects in models of heart failure. We have previously disclosed compounds such as 4, which showed good APJ agonist activity but were metabolized to the mono-demethylated, non-interconverting atropisomer metabolites. Herein, we detail the design and optimization of a novel series of N-linked APJ agonists with good potency, metabolic stability, and rat pharmacokinetic profile, which are unable to undergo the same metabolic mono-demethylation cleavage.

Prognostic Value of Combining Apelin-12 and Estimated Glomerular Filtration Rate in Patients with ST-Segment Elevation Myocardial Infarction

Background

Apelin-12 and estimated glomerular filtration rate (eGFR) are considered prognostic factors for ST-segment elevation myocardial infarction (STEMI). However, little is known about whether the combined use of these two biomarkers could enhance the prognostic value. This study aimed to investigate the utility of combining apelin-12 and eGFR for STEMI.

Methods

Patients were divided into four groups based on median apelin-12 level and eGFR level: A: low apelin-12, low eGFR; B: low apelin-12, high eGFR; C: high apelin-12, low eGFR; and D: high apelin-12, high eGFR. The Cox regression was used to identify prognostic factors. The Kaplan-Meier and the receiver operating characteristic (ROC) curves were generated to evaluate the prognostic value of apelin-12 combined with eGFR in patients with STEMI.

Results

Among 460 patients, 118 (25.7%) experienced major adverse cardiac events (MACEs) during the entire follow-up of 30 months. The Kaplan-Meier curve analysis revealed that group D had the best prognosis compared with the other three groups. The combination of apelin-12 and eGFR (area under the ROC curve (AUC), 0.699) enhanced the predictive value for MACE compared with either apelin-12 (AUC, 0.617) or eGFR (AUC, 0.596) alone. There was a negative association between apelin-12 and eGFR (r = -0.32, p < 0.001), while no association was observed between the Gensini score and apelin-12 or eGFR.

Conclusions

This study suggests that both low apelin-12 (<0.76 ng/ml) and low eGFR (<94.06 mL/min/1.73 m2) are associated with poor prognosis in STEMI, indicating that the combination of apelin-12 and eGFR could enhance the prognostic value of patients with STEMI.

Associations between adipokines and atrial fibrillation: A systematic review and meta-analysis

AIMS

Although overweight and obesity are associated with increased risk of atrial fibrillation (AF), the underlying mechanisms are not well characterised. Recent data suggest that this link may be partly due to abnormal adipose tissue-derived cytokines or adipokines. However, this relationship is not well clarified. To evaluate the association between adipokines and AF in a systematic review and meta-analysis.

DATA SYNTHESIS

PubMed, Embase, and Web of Science Core Collection were searched from inception through 1^st March 2021. Studies were included if they reported any adipokine and AF, with their quality assessed using the Newcastle-Ottawa scale. Data were independently abstracted, with unadjusted and multivariable adjusted estimates pooled in a random-effects meta-analysis. Data are presented for overall prevalent or incident AF and AF subtypes (paroxysmal, persistent, or non-paroxysmal AF). A total of 34 studies, with 31,479 patients, were included. The following adipokines were significantly associated with AF in the pooled univariate data - apelin (risk ratio for prevalent AF: 0.05 [0.00-0.50], p = 0.01; recurrent AF: 0.21 [0.11-0.42], p < 0.01) and resistin (incident AF: 2.05 [1.02-4.1], p = 0.04; prevalent AF: 2.62 [1.78-3.85], p < 0.01). Pooled analysis of multivariable adjusted effect size estimates showed adiponectin as the sole independent predictor of AF incidence (1.14 [1.02-1.27], p = 0.02). Moreover, adiponectin was associated with non-paroxysmal AF (persistent AF: 1.45 [1.08-1.94, p = 0.01; non-paroxysmal versus paroxysmal AF: 3.14 [1.87-5.27, p < 0.01).

CONCLUSIONS

Adipokines, principally adiponectin, apelin, and resistin, are associated with the risk of atrial fibrillation. However, the association is not seen after multivariate adjustment, likely reflecting the lack of statistical power. Future research should investigate these relationships in larger prospective cohorts and how they can refine AF monitoring strategies.

PROSPERO ID

CRD42020208879.

Apelin pathway in cardiovascular, kidney, and metabolic diseases: Therapeutic role of apelin analogs and apelin receptor agonists

The apelin/apelin receptor (ApelinR) signal transduction pathway exerts essential biological roles, particularly in the cardiovascular system. Disturbances in the apelin/ApelinR axis are linked to vascular, heart, kidney, and metabolic disorders. Therefore, the apelinergic system has surfaced as a critical therapeutic strategy for cardiovascular diseases (including pulmonary arterial hypertension), kidney disease, insulin resistance, hyponatremia, preeclampsia, and erectile dysfunction. However, apelin peptides are susceptible to rapid degradation through endogenous peptidases, limiting their use as therapeutic tools and translational potential. These proteases include angiotensin converting enzyme 2, neutral endopeptidase, and kallikrein thereby linking the apelin pathway with other peptide systems. In this context, apelin analogs with enhanced proteolytic stability and synthetic ApelinR agonists emerged as promising pharmacological alternatives. In this review, we focus on discussing the putative roles of the apelin pathway in various physiological systems from function to dysfunction, and emphasizing the therapeutic potential of newly generated metabolically stable apelin analogs and non-peptide ApelinR agonists.

Identification of a Hydroxypyrimidinone Compound () as a Potent APJ Receptor Agonist for the Potential Treatment of Heart Failure

This paper describes our continued efforts in the area of small-molecule apelin receptor agonists. Recently disclosed compound 2 showed an acceptable metabolic stability but demonstrated monodemethylation of the dimethoxyphenyl group to generate atropisomer metabolites in vitro. In this article, we extended the structure-activity relationship at the C2 position that led to the identification of potent pyrazole analogues with excellent metabolic stability. Due to the increased polarity at C2, the permeability for these compounds decreased. Further adjustment of the polarity by replacing the N1 2,6-dimethoxyphenyl group with a 2,6-diethylphenyl group and reoptimization for the potency of the C5 pyrroloamides resulted in potent compounds with improved permeability. Compound 21 displayed excellent pharmacokinetic profiles in rat, monkey, and dog models and robust pharmacodynamic efficacy in the rodent heart failure model. Compound 21 also showed an acceptable safety profile in preclinical toxicology studies and was selected as a backup development candidate for the program.

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.

Identification of 6-Hydroxypyrimidin-4(1H)-one-3-carboxamides as Potent and Orally Active APJ Receptor Agonists

The apelin receptor (APJ) is a significant regulator of cardiovascular function and is involved in heart failure and other cardiovascular diseases. (Pyr¹)apelin-13 is one of the endogenous agonists of the APJ receptor. Administration of (Pyr¹)apelin-13 increases cardiac output in preclinical models and humans. Recently we disclosed clinical lead BMS-986224 (1), a C3 oxadiazole pyridinone APJ receptor agonist with robust pharmacodynamic effects similar to (Pyr¹)apelin-13 in an acute rat pressure-volume loop model. Herein we describe the structure-activity relationship of the carboxamides as oxadiazole bioisosteres at C3 of the pyridinone core and C5 of the respective pyrimidinone core. This study led to the identification of structurally differentiated 6-hydroxypyrimidin-4(1H)-one-3-carboxamide 14a with pharmacodynamic effects comparable to those of compound 1.

The role of lymphangiogenesis in cardiovascular diseases and heart transplantation

Cardiac lymphangiogenesis plays an important physiological role in the regulation of interstitial fluid homeostasis, inflammatory, and immune responses. Impaired or excessive cardiac lymphatic remodeling and insufficient lymph drainage have been implicated in several cardiovascular diseases including atherosclerosis and myocardial infarction (MI). Although the molecular mechanisms underlying the regulation of functional lymphatics are not fully understood, the interplay between lymphangiogenesis and immune regulation has recently been explored in relation to the initiation and development of these diseases. In this field, experimental therapeutic strategies targeting lymphangiogenesis have shown promise by reducing myocardial inflammation, edema and fibrosis, and improving cardiac function. On the other hand, however, whether lymphangiogenesis is beneficial or detrimental to cardiac transplant survival remains controversial. In the light of recent evidence, cardiac lymphangiogenesis, a thriving and challenging field has been summarized and discussed, which may improve our knowledge in the pathogenesis of cardiovascular diseases and transplant biology.

Apelin improves cardiac function mainly through peripheral vasodilation in a mouse model of dilated cardiomyopathy

There is growing evidence that apelin plays a role in the regulation of the cardiovascular system by increasing myocardial contractility and acting as a vasodilator. However, it remains unclear whether apelin improves cardiac contractility in a load-dependent or independent manner in pathological conditions. For this purpose we investigated the cardiovascular effects of apelin in α-actin transgenic mice (mActin-Tg mice), a model of cardiomyopathy. [Pyr¹]apelin-13 was administered by continuous infusion at 2 mg/kg/d for 3 weeks. Effects on cardiac function were determined by echocardiography and a Pressure-Volume (PV) analysis. mActin-Tg mice showed a dilated cardiomyopathy (DCM) phenotype similar to that encountered in patients expressing the same mutation. Compared to WT animals, mActin-Tg mice displayed cardiac systolic impairment [significant decrease in ejection fraction (EF), cardiac output (CO), and stroke volume (SV)] associated with cardiac ventricular dilation and diastolic dysfunction, characterized by an impairment in mitral flow velocity (E/A) and in deceleration time (DT). Load-independent myocardial contractility was strongly decreased in mActin-Tg mice while total peripheral vascular resistance (TPR) was significantly increased. As compared to vehicle-treated animals, a 3-week treatment with [Pyr¹]apelin-13 significantly improved EF%, SV, E/A, DT and corrected TPR, with no significant effect on load-independent indices of myocardial contractility, blood pressure and heart rate. In conclusion [Pyr¹]apelin-13 displayed no intrinsic contractile effect but improved cardiac function in dilated cardiomyopathy mainly by reducing peripheral vascular resistance, with no change in blood pressure.

The therapeutic potentials of apelin in obesity-associated diseases

Apelin, a peptide with several active isoforms ranging from 36 to 12 amino acids and its receptor APJ, a G-protein-coupled receptor, are widely distributed. However, apelin has emerged as an adipokine more than fifteen years ago, integrating the field of inter-organs interactions. The apelin/APJ system plays important roles in several physiological functions both in rodent and humans such as fluid homeostasis, cardiovascular physiology, angiogenesis, energy metabolism. Thus the apelin/APJ system has generated great interest as a potential therapeutic target in different pathologies. The present review will consider the effects of apelin in metabolic diseases such as obesity and diabetes with a focus on diabetic cardiomyopathy among the complications associated with diabetes and APJ agonists or antagonists of interest in these diseases.

Stress-induced differential gene expression in cardiac tissue

The stress response is adaptive and aims to guarantee survival. However, the persistence of a stressor can culminate in pathology. Catecholamines released as part of the stress response over activate beta adrenoceptors (β-AR) in the heart. Whether and how stress affects the expression of components of the intracellular environment in the heart is still, however, unknown. This paper used microarray to analyze the gene expression in the left ventricle wall of rats submitted to foot shock stress, treated or not treated with the selective β₂-AR antagonist ICI118,551 (ICI), compared to those of non-stressed rats also treated or not with ICI, respectively. The main findings were that stress induces changes in gene expression in the heart and that β₂-AR plays a role in this process. The vast majority of genes disregulated by stress were exclusive for only one of the comparisons, indicating that, in the same stressful situation, the profile of gene expression in the heart is substantially different when the β₂-AR is active or when it is blocked. Stress induced alterations in the expression of such a large number of genes seems to be part of stress-induced adaptive mechanism.

Discovery of a Hydroxypyridinone APJ Receptor Agonist as a Clinical Candidate

Apelin-13 is an endogenous peptidic agonist of the apelin receptor (APJ) receptor with the potential for improving cardiac function in heart failure patients. However, the low plasma stability of apelin-13 necessitates continuous intravenous infusion for therapeutic use. There are several approaches to increase the stability of apelin-13 including attachment of pharmacokinetic enhancing groups, stabilized peptides, and Fc-fusion approaches. We sought a small-molecule APJ receptor agonist approach to target a compound with a pharmacokinetic profile amenable for chronic oral administration. This manuscript describes sequential optimization of the pyrimidinone series, leading to pyridinone 14, with in vitro potency equivalent to the endogenous ligand apelin-13 and with an excellent oral bioavailability and PK profile in multiple preclinical species. Compound 14 exhibited robust pharmacodynamic effects similar to apelin-13 in an acute rat pressure-volume loop model and was advanced as a clinical candidate.

Apelin and Vasopressin: The Yin and Yang of Water Balance

Apelin, a (neuro)vasoactive peptide, plays a prominent role in controlling body fluid homeostasis and cardiovascular functions. Experimental data performed in rodents have shown that apelin has an aquaretic effect via its central and renal actions. In the brain, apelin inhibits the phasic electrical activity of vasopressinergic neurons and the release of vasopressin from the posterior pituitary into the bloodstream and in the kidney, apelin regulates renal microcirculation and counteracts in the collecting duct, the antidiuretic effect of vasopressin occurring via the vasopressin receptor type 2. In humans and rodents, if plasma osmolality is increased by hypertonic saline infusion/water deprivation or decreased by water loading, plasma vasopressin and apelin are conversely regulated to maintain body fluid homeostasis. In patients with the syndrome of inappropriate antidiuresis, in which vasopressin hypersecretion leads to hyponatremia, the balance between apelin and vasopressin is significantly altered. In order to re-establish the correct balance, a metabolically stable apelin-17 analog, LIT01-196, was developed, to overcome the problem of the very short half-life (in the minute range) of apelin in vivo. In a rat experimental model of vasopressin-induced hyponatremia, subcutaneously (s.c.) administered LIT01-196 blocks the antidiuretic effect of vasopressin and the vasopressin-induced increase in urinary osmolality, and induces a progressive improvement in hyponatremia, suggesting that apelin receptor activation constitutes an original approach for hyponatremia treatment.

Aplnr knockout mice display sex-specific changes in conditioned fear

The G-protein-coupled receptor APLNR and its ligands apelin and ELABELA/TODDLER/apela comprise the apelinergic system, a signaling pathway that is critical during development and physiological homeostasis. Targeted regulation of the receptor has been proposed to treat several important diseases including heart failure, pulmonary arterial hypertension and metabolic syndrome. The apelinergic system is widely expressed within the central nervous system (CNS). However, the role of this system in the CNS has not been completely elucidated. Utilizing an Aplnr knockout mouse model, we report here results from tests of sensory ability, locomotion, reward preference, social preference, learning and memory, and anxiety. We find that knockout of Aplnr leads to significant effects on acoustic startle response and sex-specific effects on conditioned fear responses without significant changes in baseline anxiety. In particular, male Aplnr knockout mice display enhanced context- and cue-dependent fear responses. Our results complement previous reports that exogenous Apelin administration reduced conditioned fear and freezing responses in rodent models, and future studies will explore the therapeutic benefit of APLNR-targeted drugs in rodent models of PTSD.

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.

The Lymphatic Vasculature in the 21st Century: Novel Functional Roles in Homeostasis and Disease

Mammals have two specialized vascular circulatory systems: the blood vasculature and the lymphatic vasculature. The lymphatic vasculature is a unidirectional conduit that returns filtered interstitial arterial fluid and tissue metabolites to the blood circulation. It also plays major roles in immune cell trafficking and lipid absorption. As we discuss in this review, the molecular characterization of lymphatic vascular development and our understanding of this vasculature's role in pathophysiological conditions has greatly improved in recent years, changing conventional views about the roles of the lymphatic vasculature in health and disease. Morphological or functional defects in the lymphatic vasculature have now been uncovered in several pathological conditions. We propose that subtle asymptomatic alterations in lymphatic vascular function could underlie the variability seen in the body's response to a wide range of human diseases.

[MeArg1, NLe10]-apelin-12: Optimization of solid-phase synthesis and evaluation of biological properties in vitro and in vivo

Chemically modified peptide apelin-12 ([MeArg¹, NLe¹⁰]-apelin12, peptide M) is able to reduce reactive oxygen species (ROS) formation, cell death, and metabolic and ionic homeostasis disorders in experimental myocardial ischemia-reperfusion injury. These beneficial effects indicate the therapeutic potential of this compound in cardiovascular diseases. The goals of this work were to optimize the synthesis of peptide M, and to study its proteolytic stability and effect on the heart function of rabbits with doxorubicin (Dox) cardiomyopathy. We have developed a rational method of solid-phase synthesis of peptide M using the Fmoc methodology in combination with the temporary protection of the guanidine function of arginine residues by protonation (salt formation) during the formation of the amide bond. It avoids the formation of by-products, and simplifies the post-synthetic procedures, providing an increase in the yield of the final product of higher purity. Comparative evaluation of the proteolytic stability of peptide M and apelin-12 in human blood plasma was carried out using ¹H NMR spectroscopy. It was shown that the half-life of peptide M in plasma is approximately three times longer than that of apelin-12. Intravenous infusion of increasing doses of peptide M caused a gradual increase in left ventricular (LV) fractional shortening and ejection fraction in rabbits after 8 weeks of Dox administration (2 mg/kg weekly). The effect of the modified peptide on LV systolic dysfunction was significantly more pronounced than the effect of apelin-12, which suggests the promise of using this pharmacological agonist of the APJ receptor in patients with heart failure.

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.

Impact of Apelin-13 on the Development of Coronary Artery Ectasia

Background

Coronary artery ectasia (CAE) is the limitation or diffuse expansion of the epicardial coronary artery. In most cases, the pathological basis of CAE is considered to be coronary atherosclerosis. Previous studies have confirmed the association between Apelin and arterial atherosclerosis. Apelin-13 (AP-13) is the main serum Apelin subtype in healthy humans, however the effect of serum AP-13 on CAE has yet to be elucidated. In this research, we analysed the relationship between serum AP-13 levels and CAE.

Methods

One hundred and forty subjects who underwent selective diagnostic coronary angiography were enrolled in this research. We identified and included 40 patients with CAE as the study subjects. Another 50 patients with coronary artery disease (CAD) were randomly selected as the CAD group, and 50 patients without CAD were selected as the normal control group. Serum AP-13 levels were collected for all subjects.

Results

There were no statistically significant differences in baseline data except for gender. After unconditional logistic regression analysis, AP-13 and HDL-c were independent risk factors for CAE (both p < 0.05). The serum AP-13 level was significantly lower in the CAE patients than in the CAD patients (1.86 ± 0.59 vs. 2.49 ± 1.19 ng/mL, p = 0.004). Serum AP-13 levels were slightly lower in the CAD patients than in the controls (2.49 ± 1.19 vs. 3.12 ± 1.64, p = 0.079).

Conclusions

Apelin-13 may have an effect on the development of CAE. Further studies should be performed to elucidate the possible pathogenic role of AP-13 in CAE.

Cardiovascular response to small-molecule APJ activation

Heart failure (HF) remains a grievous illness with poor prognosis even with optimal care. The apelin receptor (APJ) counteracts the pressor effect of angiotensin II, attenuates ischemic injury, and has the potential to be a novel target to treat HF. Intravenous administration of apelin improves cardiac function acutely in patients with HF. However, its short half-life restricts its use to infusion therapy. To identify a longer acting APJ agonist, we conducted a medicinal chemistry campaign, leading to the discovery of potent small-molecule APJ agonists with comparable activity to apelin by mimicking the C-terminal portion of apelin-13. Acute infusion increased systolic function and reduced systemic vascular resistance in 2 rat models of impaired cardiac function. Similar results were obtained in an anesthetized but not a conscious canine HF model. Chronic oral dosing in a rat myocardial infarction model reduced myocardial collagen content and improved diastolic function to a similar extent as losartan, a RAS antagonist standard-of-care therapy, but lacked additivity with coadministration. Collectively, this work demonstrates the feasibility of developing clinical, viable, potent small-molecule agonists that mimic the endogenous APJ ligand with more favorable drug-like properties and highlights potential limitations for APJ agonism for this indication.

Function-based high-throughput screening for antibody antagonists and agonists against G protein-coupled receptors

Hybridoma and phage display are two powerful technologies for isolating target-specific monoclonal antibodies based on the binding. However, for complex membrane proteins, such as G protein-coupled receptors (GPCRs), binding-based screening rarely results in functional antibodies. Here we describe a function-based high-throughput screening method for quickly identifying antibody antagonists and agonists against GPCRs by combining glycosylphosphatidylinositol-anchored antibody cell display with β-arrestin recruitment-based cell sorting and screening. This method links antibody genotype with phenotype and is applicable to all GPCR targets. We validated this method by identifying a panel of antibody antagonists and an antibody agonist to the human apelin receptor from an immune antibody repertoire. In contrast, we obtained only neutral binders and antibody antagonists from the same repertoire by phage display, suggesting that the new approach described here is more efficient than traditional methods in isolating functional antibodies. This new method may create a new paradigm in antibody drug discovery.

Ren et al. develop a function-based high-throughput screening method for identifying antibody antagonists and agonists against GPCRs by combining GPI-anchored antibody cell surface display and β-arrestin recruitment reporter assay. They identify a panel of antibody antagonists and an agonist to the human apelin receptor, which is not obtainable from phage display technology.

Stretch-Induced Biased Signaling in Angiotensin II Type 1 and Apelin Receptors for the Mediation of Cardiac Contractility and Hypertrophy

The myocardium has an intrinsic ability to sense and respond to mechanical load in order to adapt to physiological demands. Primary examples are the augmentation of myocardial contractility in response to increased ventricular filling caused by either increased venous return (Frank-Starling law) or aortic resistance to ejection (the Anrep effect). Sustained mechanical overload, however, can induce pathological hypertrophy and dysfunction, resulting in heart failure and arrhythmias. It has been proposed that angiotensin II type 1 receptor (AT1R) and apelin receptor (APJ) are primary upstream actors in this acute myocardial autoregulation as well as the chronic maladaptive signaling program. These receptors are thought to have mechanosensing capacity through activation of intracellular signaling via G proteins and/or the multifunctional transducer protein, β-arrestin. Importantly, ligand and mechanical stimuli can selectively activate different downstream signaling pathways to promote inotropic, cardioprotective or cardiotoxic signaling. Studies to understand how AT1R and APJ integrate ligand and mechanical stimuli to bias downstream signaling are an important and novel area for the discovery of new therapeutics for heart failure. In this review, we provide an up-to-date understanding of AT1R and APJ signaling pathways activated by ligand versus mechanical stimuli, and their effects on inotropy and adaptive/maladaptive hypertrophy. We also discuss the possibility of targeting these signaling pathways for the development of novel heart failure therapeutics.

Distinct hemodynamic responses to (pyr)apelin-13 in large animal models

This study tested the hypothesis that (pyr)apelin-13 dose-dependently augments myocardial contractility and coronary blood flow, irrespective of changes in systemic hemodynamics. Acute effects of intravenous (pyr)apelin-13 administration (10 to 1,000 nM) on blood pressure, heart rate, left ventricular pressure and volume, and coronary parameters were measured in dogs and pigs. Administration of (pyr)apelin-13 did not influence blood pressure (P = 0.59), dP/dt_max (P = 0.26), or dP/dt_min (P = 0.85) in dogs. However, heart rate dose-dependently increased > 70% (P < 0.01), which was accompanied by a significant increase in coronary blood flow (P < 0.05) and reductions in left ventricular end-diastolic volume and stroke volume (P < 0.001). In contrast, (pyr)apelin-13 did not significantly affect hemodynamics, coronary blood flow, or indexes of contractile function in pigs. Furthermore, swine studies found no effect of intracoronary (pyr)apelin-13 administration on coronary blood flow (P = 0.83) or vasorelaxation in isolated, endothelium-intact (P = 0.89) or denuded (P = 0.38) coronary artery rings. Examination of all data across (pyr)apelin-13 concentrations revealed an exponential increase in cardiac output as peripheral resistance decreased across pigs and dogs (P < 0.001; R² = 0.78). Assessment of the Frank-Starling relationship demonstrated a significant linear relationship between left ventricular end-diastolic volume and stroke volume across species (P < 0.001; R² = 0.70). Taken together, these findings demonstrate that (pyr)apelin-13 does not directly influence myocardial contractility or coronary blood flow in either dogs or pigs.NEW & NOTEWORTHY Our findings provide much needed insight regarding the pharmacological cardiac and coronary effects of (pyr)apelin-13 in larger animal preparations. In particular, data highlight distinct hemodynamic responses of apelin across species, which are independent of any direct effect on myocardial contractility or perfusion.

Insight into adipokines to optimize therapeutic effects of stem cell for tissue regeneration

Stem cell therapy is considered as a promising regenerative medicine for repairing and treating damaged tissues and/or preventing various diseases. But there are still some obstacles such as low cell migration, poor stem cell engraftment and decreased cell survival that need to be overcome before transplantation. Therefore, a large body of studies has focused on improving the efficiency of stem cell therapy. For instance, preconditioning of stem cells has emerged as an effective strategy to reinforce therapeutic efficacy. Adipokines are signaling molecules, secreted by adipose tissue, which regulate a variety of biological processes in adipose tissue and other organs including the brain, liver, and muscle. In this review article, we shed light on the biological effects of some adipokines including apelin, oncostatin M, omentin-1 and vaspin on stem cell therapy and the most recent preclinical advances in our understanding of how these functions ameliorate stem cell therapy outcome.

Structure-guided discovery of a single-domain antibody agonist against human apelin receptor

Developing antibody agonists targeting the human apelin receptor (APJ) is a promising therapeutic approach for the treatment of chronic heart failure. Here, we report the structure-guided discovery of a single-domain antibody (sdAb) agonist JN241-9, based on the cocrystal structure of APJ with an sdAb antagonist JN241, the first cocrystal structure of a class A G protein-coupled receptor (GPCR) with a functional antibody. As revealed by the structure, JN241 binds to the extracellular side of APJ, makes critical contacts with the second extracellular loop, and inserts the CDR3 into the ligand-binding pocket. We converted JN241 into a full agonist JN241-9 by inserting a tyrosine into the CDR3. Modeling and molecular dynamics simulation shed light on JN241-9-stimulated receptor activation, providing structural insights for finding agonistic antibodies against class A GPCRs.

Apj+ Vessels Drive Tumor Growth and Represent a Tractable Therapeutic Target

Identification of cellular surface markers that distinguish tumorous from normal vasculature is important for the development of tumor vessel-targeted therapy. Here, we show that Apj, a G protein-coupled receptor, is highly enriched in tumor endothelial cells but absent from most endothelial cells of adult tissues in homeostasis. By genetic targeting using Apj-CreER and Apj-DTRGFP-Luciferase, we demonstrated that hypoxia-VEGF signaling drives expansion of Apj⁺ tumor vessels and that targeting of these vessels, genetically and pharmacologically, remarkably inhibits tumor angiogenesis and restricts tumor growth. These in vivo findings implicate Apj⁺ vessels as a key driver of pathological angiogenesis and identify Apj⁺ endothelial cells as an important therapeutic target for the anti-angiogenic treatment of tumors.

New insights about the lymphatic vasculature in cardiovascular diseases

The heart contains a complex network of blood and lymphatic vessels. The coronary blood vessels provide the cardiac tissue with oxygen and nutrients and have been the major focus of research for the past few decades. Cardiac lymphatic vessels, which consist of lymphatic capillaries and collecting lymphatic vessels covering all layers of the heart, transport excess fluid from the interstitium and play important roles in maintaining tissue fluid balance. Unlike for the coronary blood vessels, until a few years ago, not much information was available on the origin and function of the cardiac-associated lymphatic vasculature. A growing body of evidence indicates that cardiac lymphatic vessels (lymphatics) may serve as a therapeutic cardiovascular target.

G-Protein-Coupled Receptors in Heart Disease

GPCRs (G-protein [guanine nucleotide-binding protein]-coupled receptors) play a central physiological role in the regulation of cardiac function in both health and disease and thus represent one of the largest class of surface receptors targeted by drugs. Several antagonists of GPCRs, such as βARs (β-adrenergic receptors) and Ang II (angiotensin II) receptors, are now considered standard of therapy for a wide range of cardiovascular disease, such as hypertension, coronary artery disease, and heart failure. Although the mechanism of action for GPCRs was thought to be largely worked out in the 80s and 90s, recent discoveries have brought to the fore new and previously unappreciated mechanisms for GPCR activation and subsequent downstream signaling. In this review, we focus on GPCRs most relevant to the cardiovascular system and discuss traditional components of GPCR signaling and highlight evolving concepts in the field, such as ligand bias, β-arrestin-mediated signaling, and conformational heterogeneity.

International Union of Basic and Clinical Pharmacology. CVII. Structure and Pharmacology of the Apelin Receptor with a Recommendation that Elabela/Toddler Is a Second Endogenous Peptide Ligand

The predicted protein encoded by the APJ gene discovered in 1993 was originally classified as a class A G protein-coupled orphan receptor but was subsequently paired with a novel peptide ligand, apelin-36 in 1998. Substantial research identified a family of shorter peptides activating the apelin receptor, including apelin-17, apelin-13, and [Pyr1]apelin-13, with the latter peptide predominating in human plasma and cardiovascular system. A range of pharmacological tools have been developed, including radiolabeled ligands, analogs with improved plasma stability, peptides, and small molecules including biased agonists and antagonists, leading to the recommendation that the APJ gene be renamed APLNR and encode the apelin receptor protein. Recently, a second endogenous ligand has been identified and called Elabela/Toddler, a 54-amino acid peptide originally identified in the genomes of fish and humans but misclassified as noncoding. This precursor is also able to be cleaved to shorter sequences (32, 21, and 11 amino acids), and all are able to activate the apelin receptor and are blocked by apelin receptor antagonists. This review summarizes the pharmacology of these ligands and the apelin receptor, highlights the emerging physiologic and pathophysiological roles in a number of diseases, and recommends that Elabela/Toddler is a second endogenous peptide ligand of the apelin receptor protein.

APLN/APJ pathway: The key regulator of macrophage functions

Macrophages play key roles during cardiovascular diseases (CVD) and their related complications. Apelin (APLN) is a key molecule, whose roles during CVD have been documented previously. Therefore, it has been hypothesized that APLN may perform its roles via modulation of macrophages. Additionally, due to the widespread distribution of the CVD, more effective therapeutic strategies need to be developed to overcome the related complications. This review article collected recent information regarding the roles of APLN on the macrophages and discusses its potential chance to be a target for molecular/cellular therapy of APLN and the APLN treated macrophages for CVD.