Impaired heart contractility in Apelin gene-deficient mice associated with aging and pressure overload

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

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

B38-CAP is a bacteria-derived ACE2-like enzyme that suppresses hypertension and cardiac dysfunction

Angiotensin-converting enzyme 2 (ACE2) is critically involved in cardiovascular physiology and pathology, and is currently clinically evaluated to treat acute lung failure. Here we show that the B38-CAP, a carboxypeptidase derived from Paenibacillus sp. B38, is an ACE2-like enzyme to decrease angiotensin II levels in mice. In protein 3D structure analysis, B38-CAP homolog shares structural similarity to mammalian ACE2 with low sequence identity. In vitro, recombinant B38-CAP protein catalyzed the conversion of angiotensin II to angiotensin 1–7, as well as other known ACE2 target peptides. Treatment with B38-CAP suppressed angiotensin II-induced hypertension, cardiac hypertrophy, and fibrosis in mice. Moreover, B38-CAP inhibited pressure overload-induced pathological hypertrophy, myocardial fibrosis, and cardiac dysfunction in mice. Our data identify the bacterial B38-CAP as an ACE2-like carboxypeptidase, indicating that evolution has shaped a bacterial carboxypeptidase to a human ACE2-like enzyme. Bacterial engineering could be utilized to design improved protein drugs for hypertension and heart failure.

The enzyme ACE2 is involved in cardiac pathology and can counteract heart failure and other cardio-pulmonary diseases. Here the authors show that bacteria produce an ACE2-like enzyme that is effective in suppressing cardiac hypertrophy and fibrosis in mice.

Apelin Promotes ECM Synthesis by Enhancing Autophagy Flux via TFEB in Human Degenerative NP Cells under Oxidative Stress

Background

Apelin alleviates oxidative stress which contributes to the development of aging. IVDD is a disease closely correlated to aging and oxidative stress which is known to be harmful to NP cells' matrix synthesis. The purpose of the present study was to investigate the role and underlying mechanism of Apelin in NP cells' matrix degradation under oxidative stress.

Methods

First, the mRNA and protein expressions of Apelin were checked by RT-PCR and Western blot in NP from normal and degenerative IVD to explore the relationship between Apelin and IVDD preliminarily. Then, H₂O₂ was used to mimic oxidative stress of NP cells. After treated with Apelin 13 and CQ, the GAG content was assessed by DMMB and the mRNA/protein expressions of NP matrix macromolecules (Collagen II and Aggrecan) and autophagy-related markers (LC3 and p62) were assessed by RT-PCR/Western blot. Finally, TFEB was knocked down by esiRNA-TFEB transfection and the nucleoprotein expression of TFEB and autophagy-related markers (LC3 and p62) were assessed by Western blot to discuss whether TFEB is involved in Apelin regulating autophagy flux in NP cells under oxidative stress.

Results

Our data first confirmed that the mRNA and protein expressions of Apelin were decreased with IVDD. Furthermore, Apelin increased GAG content of NP cells and mRNA/protein expressions of NP matrix macromolecules (Collagen II and Aggrecan) and promoted autophagic flux (LC3II/I increased and p62 decreased) under oxidative stress. Finally, after transfected with esiRNA-TFEB, Apelin cannot promote autophagic flux any more in human degenerative NP cells.

Conclusion

Our data indicated that Apelin promotes ECM synthesis by enhancing autophagy flux via TFEB in human degenerative NP cells under oxidative stress. This viewpoint may provide a new therapeutic idea for IVDD.

Chronic treatment with apelin, losartan and their combination reduces myocardial infarct size and improves cardiac mechanical function

The renin-angiotensin system (RAS) has a deleterious and apelin/APJ system has protective effect on the ischaemic heart. The collaboration between these systems in the pathophysiology of myocardial infarction is not clear. We determined the effect of chronic pretreatment with apelin, losartan and their combination on ischaemia-reperfusion (IR) injury in the isolated perfused rat heart and on the expression of apelin-13 receptor (APJ) and angiotensin type 1 receptor (AT1R) in the myocardium. During 5 days before the induction of IR, saline (vehicle), apelin-13 (Apl), F13A (apelin antagonist), losartan (Los, AT1R antagonist) and the combination of Apl and Los were administered intraperitoneally in rats. Ischaemia was induced by left anterior descending (LAD) artery occlusion for 30 minutes followed by reperfusion for 55 minutes in the Langendorff isolated heart perfusion system. Pretreatment with Apl, Los and the combination of Apl + Los significantly reduced infarct size by about 30, 33 and 48 percent respectively; and significantly improved the left ventricular function indices such as left ventricular developed pressure (LVDP), left ventricular end-diastolic pressure (LVEDP) and rate pressure product (RPP). IR increased AT1R protein level but it did not change APJ significantly. AT1R expression was reduced in groups treated with Apl, Los and Apl + Los. Findings showed that chronic pretreatment with apelin along with AT1R antagonist had more protective effects against IR injury. Combination therapy may diminish the risk of IR-induced heart damage, by reducing AT1R expression, in the heart of patients with coronary artery disease that are at the risk of MI and reperfusion injury.

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.

Apela/Elabela/Toddler: New perspectives in molecular mechanism of heart failure

Background. Despite significant therapeutic advances, heart failure (HF) remains unacceptably high in morbidity and mortality. Additionally, its high-care and costs make HF a deadly and costly disease. First reported independently by two group of researchers, Apela/Elabela/Toddler (ELA) is the second endogenous apelin-receptor ligand discovered which is encoded from a previously classified non-coding gene, and has emerged as a key signalling-pathway in the cardiovascular system.

Aims. To explore and summarise the biological effects and diagnostic potential of ELA as a new biomarker for heart failure.

Results. ELA (prepro-ELA 54 AA) is a molecule with three isoforms (ELA 11,16 and 32), recently identified as the second endogenous ligand to APJ-receptor and functions to mediate early cardiac development during zebrafish embryogenesis by inducing cardiogenesis, vasculogenesis and bone formation. In adults, it enhances cardiac contractility, promotes vasodilatory effects, mediates fluid homeostasis, reduces food intake, limits kidney dysfunction and exerts anti-atherosclerotic as well as anti-oxidative properties.

Conclusion. These results show that ELA, an endogenous agonist of the APJ-receptor exerts cardiovascular effects comparable and potentially more potent than apelin and is found to be downregulated in experimental models and humans with heart failure.

Identifying roles for peptidergic signaling in mice

Despite accumulating evidence demonstrating the essential roles played by neuropeptides, it has proven challenging to use this information to develop therapeutic strategies. Peptidergic signaling can involve juxtacrine, paracrine, endocrine, and neuronal signaling, making it difficult to define physiologically important pathways. One of the final steps in the biosynthesis of many neuropeptides requires a single enzyme, peptidylglycine α-amidating monooxygenase (PAM), and lack of amidation renders most of these peptides biologically inert. PAM, an ancient integral membrane enzyme that traverses the biosynthetic and endocytic pathways, also affects cytoskeletal organization and gene expression. While mice, zebrafish, and flies lacking Pam (Pam^KO/KO ) are not viable, we reasoned that cell type-specific elimination of Pam expression would generate mice that could be screened for physiologically important and tissue-specific deficits. Conditional Pam^cKO/cKO mice, with loxP sites flanking the 2 exons deleted in the global Pam^KO/KO mouse, were indistinguishable from wild-type mice. Eliminating Pam expression in excitatory forebrain neurons reduced anxiety-like behavior, increased locomotor responsiveness to cocaine, and improved thermoregulation in the cold. A number of amidated peptides play essential roles in each of these behaviors. Although atrial natriuretic peptide (ANP) is not amidated, Pam expression in the atrium exceeds levels in any other tissue. Eliminating Pam expression in cardiomyocytes increased anxiety-like behavior and improved thermoregulation. Atrial and serum levels of ANP fell sharply in PAM myosin heavy chain 6 conditional knockout mice, and RNA sequencing analysis identified changes in gene expression in pathways related to cardiac function. Use of this screening platform should facilitate the development of therapeutic approaches targeted to peptidergic pathways.

Apelin inhibition prevents resistance and metastasis associated with anti-angiogenic therapy

Angiogenesis is a hallmark of cancer, promoting growth and metastasis. Anti-angiogenic treatment has limited efficacy due to therapy-induced blood vessel alterations, often followed by local hypoxia, tumor adaptation, progression, and metastasis. It is therefore paramount to overcome therapy-induced resistance. We show that Apelin inhibition potently remodels the tumor microenvironment, reducing angiogenesis, and effectively blunting tumor growth. Functionally, targeting Apelin improves vessel function and reduces polymorphonuclear myeloid-derived suppressor cell infiltration. Importantly, in mammary and lung cancer, Apelin prevents resistance to anti-angiogenic receptor tyrosine kinase (RTK) inhibitor therapy, reducing growth and angiogenesis in lung and breast cancer models without increased hypoxia in the tumor microenvironment. Apelin blockage also prevents RTK inhibitor-induced metastases, and high Apelin levels correlate with poor prognosis of anti-angiogenic therapy patients. These data identify a druggable anti-angiogenic drug target that reduces tumor blood vessel densities and normalizes the tumor vasculature to decrease metastases.

Aplnra/b Sequentially Regulate Organ Left-Right Patterning via Distinct Mechanisms

The G protein-coupled receptor APJ/Aplnr has been widely reported to be involved in heart and vascular development and disease, but whether it contributes to organ left-right patterning is largely unknown. Here, we show that in zebrafish, aplnra/b coordinates organ LR patterning in an apela/apln ligand-dependent manner using distinct mechanisms at different stages. During gastrulation and early somitogenesis, aplnra/b loss of function results in heart and liver LR asymmetry defects, accompanied by disturbed KV/cilia morphogenesis and disrupted left-sided Nodal/spaw expression in the LPM. In this process, only aplnra loss of function results in KV/cilia morphogenesis defect. In addition, only apela works as the early endogenous ligand to regulate KV morphogenesis, which then contributes to left-sided Nodal/spaw expression and subsequent organ LR patterning. The aplnra-apela cascade regulates KV morphogenesis by enhancing the expression of foxj1a, but not fgf8 or dnh9, during KV development. At the late somite stage, both aplnra and aplnrb contribute to the expression of lft1 in the trunk midline but do not regulate KV formation, and this role is possibly mediated by both endogenous ligands, apela and apln. In conclusion, our study is the first to identify a role for aplnra/b and their endogenous ligands apela/apln in LR patterning, and it clarifies the distinct roles of aplnra-apela and aplnra/b-apela/apln in orchestrating organ LR patterning.

Adipose Tissue-Derived Signatures for Obesity and Type 2 Diabetes: Adipokines, Batokines and MicroRNAs

: Obesity is one of the main risk factors for type 2 diabetes mellitus (T2DM). It is closely related to metabolic disturbances in the adipose tissue that primarily functions as a fat reservoir. For this reason, adipose tissue is considered as the primary site for initiation and aggravation of obesity and T2DM. As a key endocrine organ, the adipose tissue communicates with other organs, such as the brain, liver, muscle, and pancreas, for the maintenance of energy homeostasis. Two different types of adipose tissues-the white adipose tissue (WAT) and brown adipose tissue (BAT)-secrete bioactive peptides and proteins, known as "adipokines" and "batokines," respectively. Some of them have beneficial anti-inflammatory effects, while others have harmful inflammatory effects. Recently, "exosomal microRNAs (miRNAs)" were identified as novel adipokines, as adipose tissue-derived exosomal miRNAs can affect other organs. In the present review, we discuss the role of adipose-derived secretory factors-adipokines, batokines, and exosomal miRNA-in obesity and T2DM. It will provide new insights into the pathophysiological mechanisms involved in disturbances of adipose-derived factors and will support the development of adipose-derived factors as potential therapeutic targets for obesity and T2DM.

Catalpol pretreatment attenuates cardiac dysfunction following myocardial infarction in rats

Objective:

To investigate the effects and mechanisms of catalpol on cardiac function in rats with isoproterenol (ISO)-induced myocardial infarction (MI).

Methods:

Adult male Wistar rats were divided into four groups: control group, ISO group, catalpol (L, low dose) group, and catalpol (H, high dose) group. Isoproterenol (85 mg/kg) was injected subcutaneously for 2 consecutive days to induce experimental MI. At the end of experiment, the effects of catalpol on cardiac function; apelin levels; apoptosis index; apelin, APJ, Bcl-2, and Bax protein expression; and caspase-3/9 activities were investigated.

Results:

The rats in the ISO group showed lower left ventricular maximum rate of positive or negative pressure development (±LVdp/dt_max) and left ventricular end-systolic pressure (LVSP) and higher left ventricular end-diastolic pressure (LVEDP) than those in the control group, suggesting severe cardiac dysfunction. Interestingly, catalpol administration significantly ameliorated the ISO-induced cardiac dysfunction. The groups administered low and high dosages catalpol (5 and 10 mg/kg/day, respectively) showed higher ±LVdp/dt_max and LVSP and lower LVEDP than the group administered ISO alone. Catalpol markedly upregulated apelin levels in the plasma and myocardium. Further, catalpol increased the apelin and APJ expression levels in the myocardium of the ISO-treated rats. In addition, catalpol pretreatment inhibited cardiomyocyte apoptosis as indicated by a decrease in the TUNEL-positive cell percentage, alterations in the Bax and Bcl-2 expression levels, and a decline in caspase-3 and caspase-9 activities.

Conclusion:

Our results revealed that catalpol can improve cardiac function. Its protective effects may be linked to the enhancement of myocardium contractility, regulation of the apelin/APJ pathway, and inhibition of cardiomyocyte apoptosis.

(3R)‑5,6,7‑trihydroxy‑3‑isopropyl‑3‑methylisochroman‑1‑one attenuates cardiac dysfunction via the apelin/APJ signaling pathway

Myocardial infarction (MI) is associated with a high risk of mortality and is a major global health concern. The present study aimed to investigate the protective effects of (3R)‑5,6,7‑trihydroxy‑3‑isopropyl‑3‑methylisochroman‑1‑one (TIM) against MI induced by isoproterenol (ISO) in a rat model and the underlying mechanisms. Wistar rats were assigned to 4 groups (n=10): The control group received saline treatment; the ISO group received an intraperitoneal injection of ISO (100 mg/kg); and the TIM (low) and TIM (high) groups received an intraperitoneal injection of ISO, plus a 1 and 2 mg/kg dose of TIM orally, respectively. TIM rats were treated with TIM daily for 12 days and received ISO injections on the final 2 days to induce MI. Cardiac function, apoptosis index and protein expression were subsequently determined. The levels of oxidative stress markers were determined by ELISAs, whereas DNA damage was detected using a Cell Death Detection ELISA kit. Gene and protein expression were determined via reverse transcription‑quantitative polymerase chain reaction and western blot analyses, respectively. Following treatment with ISO, the maximum left ventricular contraction/relaxation velocity and left ventricular systolic pressure were significantly decreased, whereas the left ventricular end‑diastolic pressure was increased; however, treatment with TIM significantly ameliorated ISO‑induced cardiac dysfunction. Additionally, TIM treatment significantly decreased oxidative stress and inhibited the apoptosis of cardiomyocytes, as determined by a decrease in caspase activities, increased expression of B‑cell lymphoma 2 (Bcl‑2) and reduced expression of cleaved caspase‑3, cleaved caspase‑9 and Bcl‑2‑associated X. Furthermore, treatment with TIM upregulated the levels of apelin in the plasma and myocardium of ISO‑treated rats. The results indicated that TIM protected cardiomyocytes against ISO‑induced MI, potentially via the apelin/apelin receptor signaling pathway. The results of the present study suggested that TIM may be a potential novel therapy for the treatment of MI.

Elabela and Apelin actions in healthy and pathological pregnancies

Pregnancy is a dynamic and precisely organized process during which one or more baby develops. Embryonic development relies on the formation of the placenta, allowing nutrient and oxygen exchange between the mother and the fetus. Dysfunction of placental formation lead to pregnancy disorders such as preeclampsia (PE) with serious deleterious consequences for fetal and maternal health. Identifying factors involved in fetoplacental homeostasis could inform better diagnostic and therapeutic strategies for these pathological pregnancies. Here, we summarize actions of elabela, apelin and their common receptor APJ in the fetoplacental unit. Studies indicate that elabela is crucial for embryo cardiovascular system formation and early placental development, while apelin acts in mid/late gestation to modulate fetal angiogenesis and energy homeostasis. Most of these findings, drawn from animal models, indicate a key role of elabela/apelin-APJ system in the fetoplacental unit. This review also provides an overview of clinical studies investigating elabela/apelin-APJ system in pathological complicated pregnancies such as PE and gestational diabetes mellitus (GDM). While elabela-deficient mice display all the features of PE, current clinical studies show no difference in circulating elabela levels between PE and control patients which does not support a role in PE development. Conversely, apelin levels are increased during PE, but the use of apelin as an early PE marker remains to be fully investigated.

The Significance of the Effect of Visceral Adiposity on Left Ventricular Diastolic Function in the General Population

We evaluated the association between visceral adiposity and left ventricular (LV) diastolic function in association with plasma adiponectin levels in 213 subjects without overt cardiac diseases. Abdominal visceral fat area was quantified by computed tomography. Excessive visceral fat was significantly associated with impaired diastolic parameters including E/A, E′ and E/E′. Although serum adiponectin levels decreased with increased visceral adiposity, there was no independent association between serum adiponectin levels and diastolic parameters, which suggest that the role of adiponectin in this association might be indirect.

Adipokine Dysregulation and Insulin Resistance with Atherosclerotic Vascular Disease: Metabolic Syndrome or Independent Sequelae?

Adipokine dysregulation and insulin resistance are two hallmark sequelae attributed to the current clinical definition of metabolic syndrome (MetS) that are also linked to atherosclerotic vascular disease. Here, we critically discuss the underlying pathophysiological mechanisms and the interplay between the two sequelae. Adipokine dysregulation is involved with decreased nitric oxide, vascular inflammation, and insulin resistance in itself to promote atherosclerosis. Insulin resistance is involved with endothelial dysfunction by direct and indirect mechanisms that also promote vascular inflammation and atherosclerosis. These mechanisms are discussed in atherosclerosis irrespective of MetS, and to evaluate the possibility of synergism in MetS. High retinol-binding protein-4 (RBP-4) and low cholesterol efflux in MetS may provide evidence of possible synergism and elevated atherosclerotic risk. An adverse adipokine panel that includes fetuin-A and adiponectin can potentially assess atherosclerotic risk in even those without MetS. Genetic possibilities may exist in atherosclerotic vascular diseases secondary to insulin resistance.

Targeting APLN/APLNR Improves Antiangiogenic Efficiency and Blunts Proinvasive Side Effects of VEGFA/VEGFR2 Blockade in Glioblastoma

Antiangiogenic therapy of glioblastoma (GBM) with bevacizumab, a VEGFA-blocking antibody, may accelerate tumor cell invasion and induce alternative angiogenic pathways. Here we investigate the roles of the proangiogenic apelin receptor APLNR and its cognate ligand apelin in VEGFA/VEGFR2 antiangiogenic therapy against distinct subtypes of GBM. In proneural GBM, apelin levels were downregulated by VEGFA or VEGFR2 blockade. A central role for apelin/APLNR in controlling GBM vascularization was corroborated in a serial implantation model of the angiogenic switch that occurs in human GBM. Apelin and APLNR are broadly expressed in human GBM, and knockdown or knockout of APLN in orthotopic models of proneural or classical GBM subtypes significantly reduced GBM vascularization compared with controls. However, reduction in apelin expression led to accelerated GBM cell invasion. Analysis of stereotactic GBM biopsies from patients as well as from in vitro and in vivo experiments revealed increased dissemination of APLNR-positive tumor cells when apelin levels were reduced. Application of apelin-F13A, a mutant APLNR ligand, blocked tumor angiogenesis and GBM cell invasion. Furthermore, cotargeting VEGFR2 and APLNR synergistically improved survival of mice bearing proneural GBM. In summary, we show that apelin/APLNR signaling controls GBM angiogenesis and invasion and that both pathologic features are blunted by apelin-F13A. We suggest that apelin-F13A can improve the efficiency and reduce the side effects of established antiangiogenic treatments for distinct GBM subtypes. SIGNIFICANCE: Pharmacologic targeting of the APLNR acts synergistically with established antiangiogenic treatments in glioblastoma and blunts therapy resistance to current strategies for antiangiogenesis.See related commentary by Amoozgar et al., p. 2104.

Elevated blood apelin levels in type 2 diabetes mellitus: A systematic review and meta-analysis

AIMS

Apelin is a circulatory blood peptide acting as a ligand for the orphan G protein-coupled receptor known as APJ. Whether apelin blood levels can affect the pathogenesis of type 2 diabetes mellitus is an open question. In the present study, we aimed to assess the levels of circulatory apelin peptide in the type 2 diabetic subjects using systematic review and meta-analysis under random-effects model and standardized mean difference (SMD) as the effect size. For heterogeneity testing, Q and I²% statistic indices as well as meta-regression were applied.

METHODS

Using specialized biomedical online databases of Pubmed, Pubmed Central, Medline, Google scholar, Scopus and Embase databases without the beginning date restriction until July 2018, the systematic review retrieved nine studies for meta-analysis after fulfilling the inclusion and exclusion criteria.

RESULTS

Analysis of Q and I²% statistic indices as well as meta-regression showed a high heterogeneity in the 16 selected studies (737.578 and 96.475, respectively), thus, the random-effects model was chosen. The primary analysis for the main hypothesis on a total number of 1102 cases and 1078 healthy control subjects found that the weighted pooled SMD for the impact of apelin blood concentration in type 2 diabetes mellitus was as follows: SMD = 2.136 (95% confidence interval, 1.580-2.693). The P-value for the significance of the combined SMD examined by the z-test was 0.000 and thus, it was clearly significant.

CONCLUSIONS

This meta-analysis presents evidence that apelin circulatory levels are higher in type 2 diabetic subjects than normal controls.

The biological function of ELABELA and APJ signaling in the cardiovascular system and pre-eclampsia

Pre-eclampsia (PE) is a pregnancy-specific syndrome that is characterized by hypertension and proteinuria. The etiology of PE is not completely understood but is believed to involve placental insufficiency and maternal vascular damage. Growing evidence supports an important role for the apelin receptor (APJ) system in regulating cardiovascular physiology. There are two vertebrate APJ ligands, APELIN and ELABELA, both of which mediate vasodilatory functions. A recent study linked deficient ELABELA signaling and the development of PE, though the molecular mechanism remains largely unknown. In this review, we summarize the biological function of the ELABELA and APJ system in cardiovascular homeostasis and discuss the potential mechanisms by which ELABELA and APJ regulate placenta trophoblast invasion and vascular functions and participate in the development of PE.

Loss of Apelin Augments Angiotensin II-Induced Cardiac Dysfunction and Pathological Remodeling

Apelin is an inotropic and cardioprotective peptide that exhibits beneficial effects through activation of the APJ receptor in the pathology of cardiovascular diseases. Apelin induces the expression of angiotensin-converting enzyme 2 (ACE2) in failing hearts, thereby improving heart function in an angiotensin 1⁻7-dependent manner. Whether apelin antagonizes the over-activation of the renin⁻angiotensin system in the heart remains elusive. In this study we show that the detrimental effects of angiotensin II (Ang II) were exacerbated in the hearts of aged apelin-gene-deficient mice. Ang II-mediated cardiac dysfunction and hypertrophy were augmented in apelin knockout mice. The loss of apelin increased the ratio of angiotensin-converting enzyme (ACE) to ACE2 expression in the Ang II-stressed hearts, and Ang II-induced cardiac fibrosis was markedly enhanced in apelin knockout mice. mRNA expression of pro-fibrotic genes, such as transforming growth-factor beta (TGF-β) signaling, were significantly upregulated in apelin knockout hearts. Consistently, treatment with the ACE-inhibitor Captopril decreased cardiac contractility in apelin knockout mice. In vitro, apelin ameliorated Ang II-induced TGF-β expression in primary cardiomyocytes, accompanied with reduced hypertrophy. These results provide direct evidence that endogenous apelin plays a crucial role in suppressing Ang II-induced cardiac dysfunction and pathological remodeling.

Apelin and Elabela/Toddler; double ligands for APJ/Apelin receptor in heart development, physiology, and pathology

Apelin is an endogenous peptide ligand for the G protein-coupled receptor APJ/AGTRL1/APLNR and is widely expressed throughout human body. In adult hearts Apelin-APJ/Apelin receptor axis is potently inotropic, vasodilatory, and pro-angiogenic and thereby contributes to maintaining homeostasis in normal and pathological hearts. Apelin-APJ/Apelin receptor is also involved in heart development including endoderm differentiation, heart morphogenesis, and coronary vascular formation. APJ/Apelin receptor had been originally identified as an orphan receptor for its sequence similarity to Angiotensin II type 1 receptor, and it was later deorphanized by identification of Apelin in 1998. Both Apelin and Angiotensin II are substrates for Angiotensin converting enzyme 2 (ACE2), which degrades the peptides and thus negatively regulates their agonistic activities. Elabela/Toddler, which shares little sequence homology with Apelin, has been recently identified as a second endogenous APJ ligand. Elabela plays crucial roles in heart development and disease conditions presumably at time points or at areas of the heart different from Apelin. Apelin and Elabela seem to constitute a spatiotemporal double ligand system to control APJ/Apelin receptor signaling in the heart. These expanding knowledges of Apelin systems would further encourage therapeutic applications of Apelin, Elabela, or their synthetic derivatives for cardiovascular diseases.

[The functional role of endogenous APJ agonists; Apelin and Elabela/Toddler in cardiovascular diseases]

Apelin is an endogenous peptide ligand for APJ receptor, which is widely expressed in human body, and exerts various physiological effects such as vasodilation, inotropic effect, water balance, heart development, angiogenesis and energy metabolism. The beneficial effects of Apelin in cardiovascular diseases have been elucidated, and the roles of Apelin in aging-associated diseases are recently implicated. The mechanisms for therapeutic effects of Aplein include an antagonistic action to renin-angiotensin system (RAS) in addition to inotropic and vasodilatory actions. We have revealed that endogenous Apelin negatively regulates RAS via upregulation of Angiotensin converting enzyme 2 (ACE2). In addition, a second ligand for APJ receptor, Elabela/Toddler, was identified as an essential hormone for heart development, and it has been reported to have physiological effects similar to Apelin. We and others have shown that Elabela exerts inotropic and protective effects in the heart. Although the number of heart failure patients is rapidly increasing, the pathophysiology of heart failure remains elusive and further development of new therapeutic option is awaited. Apelin is a unique bifunctional molecule, which has both inotropic and cardioprotective effects in heart failure, and thus further elucidation of the mechanisms for Apelin/Elabela-APJ signaling would contribute to development of a novel therapeutics for heart failure patients.

Genetic Models

Genetically altered rat and mouse models have been instrumental in the functional analysis of genes in a physiological context. In particular, studies on the renin-angiotensin system (RAS) have profited from this technology in the past. In this review, we summarize the existing animal models for the protective axis of the RAS consisting of angiotensin-converting enzyme 2 (ACE2), angiotensin-(1-7)(Ang-(1-7), and its receptor Mas. With the help of models with altered expression of the components of this axis in the brain and cardiovascular organs, its physiological and pathophysiological functions have been elucidated. Thus, novel opportunities for therapeutic interventions in cardiovascular diseases were revealed targeting ACE2 or Mas.

Elabela, a newly discovered APJ ligand: Similarities and differences with Apelin

The Apelin/APJ system is involved in a wide range of biological functions. For a long time, Apelin was thought to be the only ligand for APJ. Recently, a new peptide that acts via APJ and has similar functions, called Elabela, was identified. Elabela has beneficial effects on body fluid homeostasis, cardiovascular health, and renal insufficiency, as well as potential benefits for metabolism and diabetes. In this review, the properties and biological functions of this new peptide are discussed in comparison with those of Apelin. Important areas for future study are also discussed, with the consideration that research on Apelin could guide future research on Elabela.

Blockade of Rostral Ventrolateral Medulla Apelin Receptors Does Not Attenuate Arterial Pressure in SHR and L-NAME-Induced Hypertensive Rats

Dysfunction of the apelinergic system, comprised of the neuropeptide apelin mediating its effects via the G protein-coupled apelin receptor (APJ), may underlie the onset of cardiovascular disease such as hypertension. Apelin expression is increased in the rostral ventrolateral medulla (RVLM) in spontaneously hypertensive rats (SHRs) compared to Wistar-Kyoto (WKY) normotensive rats, however, evidence that the apelinergic system chronically influences mean arterial blood pressure (MABP) under pathophysiological conditions remains to be established. In this study we investigated, in conscious unrestrained rats, whether APJ contributes to MABP and sympathetic vasomotor tone in the progression of two models of hypertension - SHR and _L-NAME-treated rats - and whether APJ contributes to the development of hypertension in pre-hypertensive SHR. In SHR we showed that APJ gene (aplnr) expression was elevated in the RVLM, and there was a greater MABP increase following microinjection of [Pyr¹]apelin-13 to the RVLM of SHR compared to WKY rats. Bilateral microinjection of a lentiviral APJ-specific-shRNA construct into the RVLM of WKY, SHR, and _L-NAME-treated rats, chronically implanted with radiotelemeters to measure MABP, decreased aplnr expression in the RVLM and abolished acute [Pyr¹]apelin-13-induced increases in MABP. However, chronic knockdown of aplnr in the RVLM did not affect MABP in either SHR or _L-NAME-treated rats. Moreover, knockdown of aplnr in the RVLM of prehypertensive SHR did not protect against the development of hypertension. These results show that endogenous apelin, acting via APJ, is not involved in the genesis or maintenance of hypertension in either animal model used in this study.

Apelin/APJ system: A novel promising target for anti-aging intervention

Apelin, an endogenous ligand for the G protein-coupled receptor APJ, is widely expressed in various organs. Recent research has indicated that the Apelin/APJ system plays an important role in aging. Apelin and APJ receptor expression are down-regulated with increasing age. In murine models, Apelin and APJ knockouts exhibit accelerated senescence whereas Apelin-restoration results in enhanced vigor and rejuvenated behavioral and circadian phenotypes. Furthermore, aged Apelin knockout mice develop progressive impairment of cardiac contractility associated with systolic dysfunction. Apelin is crucial to maintain cardiac contractility in aging. Moreover, the Apelin/APJ system appears to be involved in regulation of renin-angiotensin-aldosterone system (RAAS), apoptosis, inflammation and oxidative stress which promotes aging. Likewise, the Apelin/APJ system regulates autophagy, stem cells and the sirtuin family thus contributing to anti-aging. In this review, we describe the relationship between Apelin/APJ system and aging. We elaborate on the role of the Apelin/APJ system in aging stimulators, aging inhibitors and age-related diseases such as obesity, diabetes and cardiovascular disease. We conclude that Apelin/APJ system might become a novel promising therapeutic target for anti-aging.

Comparative evaluation of cannabinoid receptors, apelin and S100A6 protein in the heart of women of different age groups

Background

Recent studies have shown a significant role of the endocannabinoid system, apelin and S100A6 protein in the regulation of cardiovascular system functioning. The aim of the study was to compare and evaluate the distribution of cannabinoid receptors (CB1 and CB2), apelin and S100A6 protein in the heart of healthy women in different age groups.

Methods

The study was conducted on the hearts of 10 women (organ donors) without a history of cardiovascular disease, who were divided into two age groups: women older than 50 years and women under 50 years of age. Paraffin heart sections were processed by immunohistochemistry for detection of cannabinoids receptors (CB1 and CB2), apelin and S100A6 protein.

Results

CB1 and CB2 immunoreactivity in the cytoplasm of cardiomyocytes in the heart of women over 50 was weaker than in younger individuals. There was also strong immunoreactivity of CB1 in intercalated discs (ICDs) of the heart, only in women over 50. The presence of this receptor in this location was not found in women under 50. Apelin- and S100A6-immunoreactivity in the cardiomyocytes was stronger in older women compared to women under 50.The CB1, apelin and S100A6 immunostaining in the endothelium of myocardial vessels was weaker in women over 50 than in younger women, while intensity of CB2- immunoreaction in coronary endothelium was similar in both groups of women.

The results of the study indicate the important role of endocannabinoids, apelin, and S100A6 protein in cardiac muscle function.

Conclusion

This report might contribute to a better understanding of the role of endocannabinoid system, apelin and S100 proteins in heart function as well as shed new light on processes involved in age-related cardiomyopathy.

The ACE2/Angiotensin-(1-7)/MAS Axis of the Renin-Angiotensin System: Focus on Angiotensin-(1-7)

The renin-angiotensin system (RAS) is a key player in the control of the cardiovascular system and hydroelectrolyte balance, with an influence on organs and functions throughout the body. The classical view of this system saw it as a sequence of many enzymatic steps that culminate in the production of a single biologically active metabolite, the octapeptide angiotensin (ANG) II, by the angiotensin converting enzyme (ACE). The past two decades have revealed new functions for some of the intermediate products, beyond their roles as substrates along the classical route. They may be processed in alternative ways by enzymes such as the ACE homolog ACE2. One effect is to establish a second axis through ACE2/ANG-(1-7)/MAS, whose end point is the metabolite ANG-(1-7). ACE2 and other enzymes can form ANG-(1-7) directly or indirectly from either the decapeptide ANG I or from ANG II. In many cases, this second axis appears to counteract or modulate the effects of the classical axis. ANG-(1-7) itself acts on the receptor MAS to influence a range of mechanisms in the heart, kidney, brain, and other tissues. This review highlights the current knowledge about the roles of ANG-(1-7) in physiology and disease, with particular emphasis on the brain.

Repurposing antimalarial aminoquinolines and related compounds for treatment of retinal neovascularization

Neovascularization is the pathological driver of blinding eye diseases such as retinopathy of prematurity, proliferative diabetic retinopathy, and wet age-related macular degeneration. The loss of vision resulting from these diseases significantly impacts the productivity and quality of life of patients, and represents a substantial burden on the health care system. Current standard of care includes biologics that target vascular endothelial growth factor (VEGF), a key mediator of neovascularization. While anti-VGEF therapies have been successful, up to 30% of patients are non-responsive. Therefore, there is a need for new therapeutic targets, and small molecule inhibitors of angiogenesis to complement existing treatments. Apelin and its receptor have recently been shown to play a key role in both developmental and pathological angiogenesis in the eye. Through a cell-based high-throughput screen, we identified 4-aminoquinoline antimalarial drugs as potent selective antagonists of APJ. The prototypical 4-aminoquinoline, amodiaquine was found to be a selective, non-competitive APJ antagonist that inhibited apelin signaling in a concentration-dependent manner. Additionally, amodiaquine suppressed both apelin-and VGEF-induced endothelial tube formation. Intravitreal amodaiquine significantly reduced choroidal neovascularization (CNV) lesion volume in the laser-induced CNV mouse model, and showed no signs of ocular toxicity at the highest doses tested. This work firmly establishes APJ as a novel, chemically tractable therapeutic target for the treatment of ocular neovascularization, and that amodiaquine is a potential candidate for repurposing and further toxicological, and pharmacokinetic evaluation in the clinic.

Apelin: A novel prognostic predictor for atrial fibrillation recurrence after pulmonary vein isolation

Apelin, the ligand for the APJ receptor, is involved in the pathogenesis of atrial fibrillation (AF). However, whether serum apelin can predict the recurrence of AF after pulmonary vein isolation (PVI) has not been determined.A prospective cohort study was performed in patients with AF (but without structural heart disease) who were undergoing first-time PVI. Serum apelin-12 was measured by enzyme-linked immunosorbent assay. Echocardiographic examination was performed at baseline, 3 months, and 6 months after PVI. Patients were followed up for 6 months after PVI, and the association between baseline apelin-12 and AF recurrence (early recurrence: within 3 months after ablation; late recurrence: 3-6 months after ablation) was analyzed.A total of 61 patients were included in the study. Baseline serum level of apelin-12 was significant lower in patients with early (median [interquartile range]: 1844 [1607-2061] vs 2197 [1895-2455] ng/L, P = .01) and late (1639 [1524-1853] vs 1923 [1741-2303] ng/L, P = .02) AF recurrence compared with patients without these events. Results of Cox stepwise multivariate analysis demonstrated that lower baseline apelin-12 (<2265 ng/L) was independently associated with increased AF recurrence within 6 months after PVI (P < .05). The specificity and positive predictive value of apelin-12 for AF recurrence were significantly higher than those of baseline N-terminal brain proBNP (60.4% vs 28.6%, P < .001; 58.8% vs 34.4%, P = .01), although the sensitivity and negative predictive value were similar.Reduced baseline serum apelin-12 may be an independent risk factor for the recurrence of AF after PVI in patients without structural heart disease.

Designer Approaches for G Protein-Coupled Receptor Modulation for Cardiovascular Disease

The new horizon for cardiac therapy may lie beneath the surface, with the downstream mediators of G protein–coupled receptor (GPCR) activity. Targeted approaches have shown that receptor activation may be biased toward signaling through G proteins or through GPCR kinases (GRKs) and β-arrestins, with divergent functional outcomes. In addition to these canonical roles, numerous noncanonical activities of GRKs and β-arrestins have been demonstrated to modulate GPCR signaling at all levels of receptor activation and regulation. Further, research continues to identify novel GRK/effector and β-arrestin/effector complexes with distinct impacts on cardiac function in the normal heart and the diseased heart. Coupled with the identification of once orphan receptors and endogenous ligands with beneficial cardiovascular effects, this expands the repertoire of GPCR targets. Together, this research highlights the potential for focused therapeutic activation of beneficial pathways, with simultaneous exclusion or inhibition of detrimental signaling, and represents a new wave of therapeutic development.

Inhibitory Regulation of Skin Fibrosis in Systemic Sclerosis by Apelin/APJ Signaling

OBJECTIVE

Apelin/APJ signaling has been determined to regulate cardiac and arterial fibrosis and to be involved in the pathogenesis of pulmonary arterial hypertension. Our objective was to elucidate the role of apelin in skin fibrosis in systemic sclerosis (SSc).

METHODS

Expression of apelin/APJ in normal and SSc fibroblasts was compared. Effects of small interfering RNA depletion and the addition of apelin in fibroblasts were analyzed. The effect of apelin injections on bleomycin-induced dermal fibrosis in mice was investigated. We analyzed the effects of the biased agonist of APJ, MM07, on skin fibrosis in vitro and in vivo.

RESULTS

The expression of apelin in SSc fibroblasts was significantly lower than that in normal fibroblasts. Serum apelin levels were negatively correlated with the modified Rodnan skin thickness score in SSc patients. Stimulation with transforming growth factor β1 (TGFβ1) inhibited apelin expression in fibroblasts, suggesting that activation of TGFβ1 signaling in SSc might be responsible for reduced apelin expression in SSc fibroblasts. Small interfering RNA depletion of apelin from fibroblasts significantly enhanced fibrosis-related gene expression, and treatment with apelin protein significantly inhibited TGFβ1 signaling in fibroblasts. Administration of apelin significantly inhibited bleomycin-induced dermal fibrosis in mice. We demonstrated that MM07 had greater potential than apelin to inhibit fibrosis in vivo and in vitro.

CONCLUSION

Collectively, TGFβ1 signaling and apelin signaling may counteract each other in the fibrotic process of SSc. Inhibitory regulation of TGFβ1-induced skin fibrosis by apelin/APJ signaling may be involved in the pathogenesis of SSc and could be a therapeutic target for fibrosis in SSc patients.

Apelin and APJ orchestrate complex tissue-specific control of cardiomyocyte hypertrophy and contractility in the hypertrophy-heart failure transition

The G protein-coupled receptor APJ is a promising therapeutic target for heart failure. Constitutive deletion of APJ in the mouse is protective against the hypertrophy-heart failure transition via elimination of ligand-independent, β-arrestin-dependent stretch transduction. However, the cellular origin of this stretch transduction and the details of its interaction with apelin signaling remain unknown. We generated mice with conditional elimination of APJ in the endothelium (APJendo-/-) and myocardium (APJmyo-/-). No baseline difference was observed in left ventricular function in APJendo-/-, APJmyo-/-, or control (APJendo+/+, APJmyo+/+) mice. After exposure to transaortic constriction, APJendo-/- mice displayed decreased left ventricular systolic function and increased wall thickness, whereas APJmyo-/- mice were protected. At the cellular level, carbon fiber stretch of freshly isolated single cardiomyocytes demonstrated decreased contractile responses to stretch in APJ-/- cardiomyocytes compared with APJ+/+ cardiomyocytes. Ca2+ transients did not change with stretch in either APJ-/- or APJ+/+ cardiomyocytes. Application of apelin to APJ+/+ cardiomyocytes resulted in decreased Ca2+ transients. Furthermore, hearts of mice treated with apelin exhibited decreased phosphorylation in cardiac troponin I NH2-terminal residues (Ser22 and Ser23) consistent with increased Ca2+ sensitivity. These data establish that APJ stretch transduction is mediated specifically by myocardial APJ, that APJ is necessary for stretch-induced increases in contractility, and that apelin opposes APJ's stretch-mediated hypertrophy signaling by lowering Ca2+ transients while maintaining contractility through myofilament Ca2+ sensitization. These findings underscore apelin's unique potential as a therapeutic agent that can simultaneously support cardiac function and protect against the hypertrophy-heart failure transition. NEW & NOTEWORTHY These data address fundamental gaps in our understanding of apelin-APJ signaling in heart failure by localizing APJ's ligand-independent stretch sensing to the myocardium, identifying a novel mechanism of apelin-APJ inotropy via myofilament Ca2+ sensitization, and identifying potential mitigating effects of apelin in APJ stretch-induced hypertrophic signaling.

The exerkine apelin reverses age-associated sarcopenia

Sarcopenia, the degenerative loss of skeletal muscle mass, quality and strength, lacks early diagnostic tools and new therapeutic strategies to prevent the frailty-to-disability transition often responsible for the medical institutionalization of elderly individuals. Herein we report that production of the endogenous peptide apelin, induced by muscle contraction, is reduced in an age-dependent manner in humans and rodents and is positively associated with the beneficial effects of exercise in older persons. Mice deficient in either apelin or its receptor (APLNR) presented dramatic alterations in muscle function with increasing age. Various strategies that restored apelin signaling during aging further demonstrated that this peptide considerably enhanced muscle function by triggering mitochondriogenesis, autophagy and anti-inflammatory pathways in myofibers as well as enhancing the regenerative capacity by targeting muscle stem cells. Taken together, these findings revealed positive regulatory feedback between physical activity, apelin and muscle function and identified apelin both as a tool for diagnosis of early sarcopenia and as the target of an innovative pharmacological strategy to prevent age-associated muscle weakness and restore physical autonomy.

Hepatic and cardiac beneficial effects of a long-acting Fc-apelin fusion protein in diet-induced obese mice

BACKGROUND

Apelin is a peptide ligand of the G-protein-coupled receptor APJ and exhibits anti-diabetes and anti-heart failure activities. However, short serum half-life of the apelin peptide limits its potential clinical applications. This study aimed to develop a long-acting apelin analog.

METHODS

To extend apelin's in vivo half-life, we made a recombinant protein by fusing the IgG Fc fragment to apelin-13 (Fc-apelin-13), conducted pharmacokinetics studies in mice, and determined in vitro biological activities in suppressing cyclic adenosine monophosphate and activating extracellular signal-regulated kinase signalling by reporter assays. We investigated the effects of Fc-apelin-13 on food intake, body weight, fasting blood glucose and insulin levels, glucose tolerance test, hepatic steatosis, and cardiac function and fibrosis by subcutaneous administration of Fc-apelin-13 in diet-induced obese mice for 4 weeks.

RESULTS

The estimated half-life of Fc-apelin-13 in blood was approximately 33 hours. Reporter assays showed that Fc-apelin-13 was active in suppressing cyclic adenosine monophosphate response element and activating serum response element activities. Four weeks of Fc-apelin-13 treatment in obese mice did not affect food intake and body weight, but resulted in a significant improvement of glucose tolerance, and a decrease in hepatic steatosis and fibrosis, as well as in serum alanine transaminase levels. Moreover, cardiac stroke volume and output were increased and cardiac fibrosis was decreased in the treated mice.

CONCLUSIONS

Fc-apelin-13 fusion protein has an extended in vivo half-life and exerts multiple benefits on obese mice with respect to the improvement of glucose disposal, amelioration of liver steatosis and heart fibrosis, and increase of cardiac output. Hence, Fc-apelin-13 is potentially a therapeutic for obesity-associated disease conditions.

Downregulation of the Apelinergic Axis Accelerates Aging, whereas Its Systemic Restoration Improves the Mammalian Healthspan

Aging drives the occurrence of numerous diseases, including cardiovascular disease (CVD). Recent studies indicate that blood from young mice reduces age-associated pathologies. However, the "anti-aging" factors in juvenile circulation remain poorly identified. Here, we characterize the role of the apelinergic axis in mammalian aging and identify apelin as an anti-aging factor. The expression of apelin (apln) and its receptor (aplnr) exhibits an age-dependent decline in multiple organs. Reduced apln signaling perturbs organismal homeostasis; mice harboring genetic deficiency of aplnr or apln exhibit enhanced cardiovascular, renal, and reproductive aging. Genetic or pharmacological abrogation of apln signaling also induces cellular senescence mediated, in part, by the activation of senescence-promoting transcription factors. Conversely, restoration of apln in 15-month-old wild-type mice reduces cardiac hypertrophy and exercise-induced hypertensive response. Additionally, apln-restored mice exhibit enhanced vigor and rejuvenated behavioral and circadian phenotypes. Hence, a declining apelinergic axis promotes aging, whereas its restoration extends the murine healthspan.

The interplay between adipose tissue and the cardiovascular system: is fat always bad?

Obesity is a risk factor for cardiovascular disease (CVD). However, clinical research has revealed a paradoxically protective role for obesity in patients with chronic diseases including CVD, suggesting that the biological 'quality' of adipose tissue (AT) may be more important than overall AT mass or body weight. Importantly, AT is recognised as a dynamic organ secreting a wide range of biologically active adipokines, microRNAs, gaseous messengers, and other metabolites that affect the cardiovascular system in both endocrine and paracrine ways. Despite being able to mediate normal cardiovascular function under physiological conditions, AT undergoes a phenotypic shift characterised by acquisition of pro-oxidant and pro-inflammatory properties in cases of CVD. Crucially, recent evidence suggests that AT depots such as perivascular AT and epicardial AT are able to modify their phenotype in response to local signals of vascular and myocardial origin, respectively. Utilisation of this unique property of certain AT depots to dynamically track cardiovascular biology may reveal novel diagnostic and prognostic tools against CVD. Better understanding of the mechanisms controlling the 'quality' of AT secretome, as well as the communication links between AT and the cardiovascular system, is required for the efficient management of CVD.

Effect of L-Carnitine Supplementation on Apelin and Apelin Receptor (Apj) Expression in Cardiac Muscle of Obese Diabetic Rats

Objective

L-carnitine (LC) has been shown to protect cardiac metabolism in diabetes patients with cardiovascular diseases (CVDs). Apelin, a newly discovered adipocytokines, is an important regulator of cardiac muscle function; however, the role of the level of expression of Apelin axis in improvement of cardiac function by LC in diabetic patients, is not clear. In the present study, obese insulin-resistant rats were used to determine the effect of LC, when given orally with a high-calorie diet, on Apelin and Apelin receptor (Apj) expression in cardiac muscle.

Materials and Methods

In this experimental study, rats were fed with high-fat/high-carbohydrate diet for five weeks and subsequently were injected with streptozotocin 30 mg/kg for induction of obesity and insulin resistance. After confirming the induction of diabetes (serum glucose above 7.5 mmol/L), the animals received LC 300 mg/kg in drinking water for 28 days. On days 0, 14 and 28 after treatment, cardiac Apelin and Apj gene expression was evaluated by real time polymerase chain reaction (PCR) analysis. Serum levels of insulin, Apelin, glucose, tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and the homeostasis model assessment of insulin resistance (HOMA-IR) were also measured using commercial kits.

Results

Cardiac Apelin and Apj expression and serum Apelin were increased in obese rats, while LC supplementation decreased the serum levels of Apelin and down-regulated Apelin and Apj expression in cardiac muscle. These changes were associated with reduced insulin resistance markers and serum inflammatory factors and improved lipid profile.

Conclusion

We concluded that LC supplementation could attenuate the over-expression of Apelin axis in heart of diabetic rats, a novel mechanism by which LC improves cardiovascular complications in diabetic patients.

Loss of Apela Peptide in Mice Causes Low Penetrance Embryonic Lethality and Defects in Early Mesodermal Derivatives

Apela (also known as Elabela, Ende, and Toddler) is a small signaling peptide that activates the G-protein-coupled receptor Aplnr to stimulate cell migration during zebrafish gastrulation. Here, using CRISPR/Cas9 to generate a null, reporter-expressing allele, we study the role of Apela in the developing mouse embryo. We found that loss of Apela results in low-penetrance cardiovascular defects that manifest after the onset of circulation. Three-dimensional micro-computed tomography revealed a higher penetrance of vascular remodeling defects, from which some mutants recover, and identified extraembryonic anomalies as the earliest morphological distinction in Apela mutant embryos. Transcriptomics at late gastrulation identified aberrant upregulation of erythroid and myeloid markers in mutant embryos prior to the appearance of physical malformations. Double-mutant analyses showed that loss of Apela signaling impacts early Aplnr-expressing mesodermal populations independently of the alternative ligand Apelin, leading to lethal cardiac defects in some Apela null embryos.

Cardiac Remodeling: Endothelial Cells Have More to Say Than Just NO

The heart is a highly structured organ consisting of different cell types, including myocytes, endothelial cells, fibroblasts, stem cells, and inflammatory cells. This pluricellularity provides the opportunity of intercellular communication within the organ, with subsequent optimization of its function. Intercellular cross-talk is indispensable during cardiac development, but also plays a substantial modulatory role in the normal and failing heart of adults. More specifically, factors secreted by cardiac microvascular endothelial cells modulate cardiac performance and either positively or negatively affect cardiac remodeling. The role of endothelium-derived small molecules and peptides—for instance NO or endothelin-1—has been extensively studied and is relatively well defined. However, endothelial cells also secrete numerous larger proteins. Information on the role of these proteins in the heart is scattered throughout the literature. In this review, we will link specific proteins that modulate cardiac contractility or cardiac remodeling to their expression by cardiac microvascular endothelial cells. The following proteins will be discussed: IL-6, periostin, tenascin-C, thrombospondin, follistatin-like 1, frizzled-related protein 3, IGF-1, CTGF, dickkopf-3, BMP-2 and−4, apelin, IL-1β, placental growth factor, LIF, WISP-1, midkine, and adrenomedullin. In the future, it is likely that some of these proteins can serve as markers of cardiac remodeling and that the concept of endothelial function and dysfunction might have to be redefined as we learn more about other factors secreted by ECs besides NO.

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

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

Acute inhalation of ozone induces DNA methylation of apelin in lungs of Long-Evans rats

Apelin has cardiopulmonary protective properties that promote vasodilation and maintenance of the endothelial barrier. While reductions in apelin have been identified as a contributor to various lung diseases, including pulmonary edema, its role in the effect of air pollutants has not been examined. Thus, in the current study, we sought to investigate if apelin is a downstream target of inhaled ozone and if such change in expression is related to altered DNA methylation in the lung. Male, Long-Evans rats were exposed to filtered air or 1.0 ppm ozone for 4 h. Ventilation changes were assessed using whole-body plethysmography immediately following exposure, and markers of pulmonary edema and inflammation were assessed in the bronchoaveolar lavage (BAL) fluid. The enzymatic regulators of DNA methylation were measured in the lung, along with methylation and hydroxymethylation of the apelin promoter. Data showed that ozone exposure was associated with increased enhanced pause and protein leakage in the BAL fluid. Ozone exposure reduced DNA cytosine-5-methyltransferase (DNMT) activity and Dnmt3a/b gene expression. Exposure-induced upregulation of proliferating cell nuclear antigen, indicative of DNA damage, repair, and maintenance methylation. Increased methylation and reduced hydroxymethylation were measured on the apelin promoter. These epigenetic modifications accompanied ozone-induced reduction of apelin expression and development of pulmonary edema. In conclusion, epigenetic regulation, specifically increased methylation of the apelin promoter downstream of DNA damage, may lead to reductions in protective signaling of the apelinergic system, contributing to the pulmonary edema observed following the exposure to oxidant air pollution.

The CCR4-NOT deadenylase complex controls Atg7-dependent cell death and heart function

Shortening and removal of the polyadenylate [poly(A)] tail of mRNA, a process called deadenylation, is a key step in mRNA decay that is mediated through the CCR4-NOT (carbon catabolite repression 4-negative on TATA-less) complex. In our investigation of the regulation of mRNA deadenylation in the heart, we found that this complex was required to prevent cell death. Conditional deletion of the CCR4-NOT complex components Cnot1 or Cnot3 resulted in the formation of autophagic vacuoles and cardiomyocyte death, leading to lethal heart failure accompanied by long QT intervals. Cnot3 bound to and shortened the poly(A) tail of the mRNA encoding the key autophagy regulator Atg7. In Cnot3-depleted hearts, Atg7 expression was posttranscriptionally increased. Genetic ablation of Atg7, but not Atg5, increased survival and partially restored cardiac function of Cnot1 or Cnot3 knockout mice. We further showed that in Cnot3-depleted hearts, Atg7 interacted with p53 and modulated p53 activity to induce the expression of genes encoding cell death-promoting factors in cardiomyocytes, indicating that defects in deadenylation in the heart aberrantly activated Atg7 and p53 to promote cell death. Thus, mRNA deadenylation mediated by the CCR4-NOT complex is crucial to prevent Atg7-induced cell death and heart failure, suggesting a role for mRNA deadenylation in targeting autophagy genes to maintain normal cardiac homeostasis.

Apelin Ameliorates High Glucose-Induced Downregulation of Connexin 43 via AMPK-Dependent Pathway in Neonatal Rat Cardiomyocytes

Diabetes Mellitus is a common disorder, with increasing risk of cardiac arrhythmias. Studies have shown that altered connexin expression and gap junction remodeling under hyperglycemia contribute to the high prevalence of cardiac arrhythmias and even sudden death. Connexin 43 (Cx43), a major protein that assembles to form cardiac gap junctions, has been found to be downregulated under high glucose conditions, along with inhibition of gap junctional intercellular communication (GJIC). While, apelin, a beneficial adipokine, increases Cx43 protein expression in mouse and human embryonic stem cells during cardiac differentiation. However, it remains unknown whether apelin influences GJIC capacity in cardiomyocytes. Here, using Western blotting and dye transfer assays, we found that Cx43 protein expression was reduced and GJIC was impaired after treatment with high glucose, which, however, could be abrogated after apelin treatment for 48 h. We also found that apelin increased Cx43 expression under normal glucose. Real-time PCR showed that the Cx43 mRNA was not significantly affected under high glucose conditions in the presence of apelin or high glucose and apelin. High glucose decreased the phosphorylation of AMPKα; however, apelin activated AMPKα. Interestingly, we found that Cx43 expression was increased after treatment with AICAR, an activator of AMPK signaling. AMPKα inhibition mediated with transfection of siRNA-AMPKα1 and siRNA-AMPKα2 abolished the protective effect of apelin on Cx43 expression. Our data suggest that apelin attenuates high glucose-induced Cx43 downregulation and improves the loss of functional gap junctions partly through the AMPK pathway.

Apelin/APJ system: A critical regulator of vascular smooth muscle cell

APJ, an orphan G protein-coupled receptor, is first identified through homology cloning in 1993. Apelin is endogenous ligand of APJ extracted from bovine stomach tissue in 1998. Apelin/APJ system is widely expressed in many kinds of cells such as endothelial cells, cardiomyocytes, especially vascular smooth muscle cell. Vascular smooth muscle cell (VSMC), an integral part of the vascular wall, takes part in many normal physiological processes. Our experiment firstly finds that apelin/APJ system enhances VSMC proliferation by ERK1/2-cyclin D1 signal pathway. Accumulating studies also show that apelin/APJ system plays a pivotal role in mediating the function of VSMC. In this paper, we review the exact role of apelin/APJ system in VSMC, including induction of proliferation and migration, enhance of contraction and relaxation, inhibition of calcification. Furthermore, we discuss the role of apelin/APJ system in vascular diseases, such as atherosclerosis, hypertension, and chronic kidney disease (CKD) from the point of VSMC. Above all, apelin/APJ system is a promising target for managing vascular disease.

Characterization of the Apelin/Elabela Receptors (APLNR) in Chickens, Turtles, and Zebrafish: Identification of a Novel Apelin-Specific Receptor in Teleosts

Apelin receptor(s) (APLNR) are suggested to mediate the actions of apelin and Elabela (ELA) peptides in many physiological processes, including cardiovascular development and food intake in vertebrates. However, the functionality of APLNR has not been examined in most vertebrate groups. Here, we characterized two APLNRs APLNR1, APLNR2) in chickens and red-eared sliders, and three APLNRs in zebrafish (APLNR2a, APLNR2b, APLNR3a), which are homologous to human APLNR. Using luciferase-reporter assays or Western blot, we demonstrated that in chickens, APLNR1 (not APLNR2) expressed in HEK293 cells was potently activated by chicken apelin-36 and ELA-32 and coupled to Gi-cAMP and MAPK/ERK signaling pathways, indicating a crucial role of APLNR1 in mediating apelin/ELA actions; in red-eared sliders, APLNR2 (not APLNR1) was potently activated by apelin-36/ELA-32, suggesting that APLNR2 may mediate apelin/ELA actions; in zebrafish, both APLNR2a and APLNR2b were potently activated by apelin-36/ELA-32 and coupled to Gi-cAMP signaling pathway, as previously proposed, whereas the novel APLNR3a was specifically and potently activated by apelin. Similarly, an apelin-specific receptor (APLNR3b) sharing 57% sequence identity with zebrafish APLNR3a was identified in Nile tilapia. Collectively, our data facilitates the uncovering of the roles of APLNR signaling in different vertebrate groups and suggests a key functional switch between APLNR1 and APLNR2/3 in mediating the actions of ELA and apelin during vertebrate evolution.

Effects of Apelin on Cardiovascular Aging

Apelin is the endogenous ligand of APJ, the orphan G protein-coupled receptor. The apelin-APJ signal transduction pathway is widely expressed in the cardiovascular system and is an important factor in cardiovascular homeostasis. This signal transduction pathway has long been related to diseases with high morbidity in the elderly, such as atherosclerosis, coronary atherosclerotic heart disease, hypertension, calcific aortic valve disease, heart failure and atrial fibrillation. In this review, we discuss the apelin-APJ signal transduction pathway related to age-associated cardiovascular diseases.