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

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.

Therapeutic Benefit and Gene Network Regulation by Combined Gene Transfer of Apelin, FGF2, and SERCA2a into Ischemic Heart

Despite considerable advances in cardiovascular disease treatment, heart failure remains a public health challenge. In this context, gene therapy appears as an attractive approach, but clinical trials using single therapeutic molecules result in moderate benefit. With the objective of improving ischemic heart failure therapy, we designed a combined treatment, aimed to simultaneously stimulate angiogenesis, prevent cardiac remodeling, and restore contractile function. We have previously validated IRES-based vectors as powerful tools to co-express genes of interest. Mono- and multicistronic lentivectors expressing fibroblast growth factor 2 (angiogenesis), apelin (cardioprotection), and/or SERCA2a (contractile function) were produced and administrated by intramyocardial injection into a mouse model of myocardial infarction. Data reveal that combined treatment simultaneously improves vessel number, heart function parameters, and fibrosis prevention, due to FGF2, SERCA2a, and apelin, respectively. Furthermore, addition of SERCA2a in the combination decreases cardiomyocyte hypertrophy. Large-scale transcriptome analysis reveals that the triple treatment is the most efficient in restoring angiogenic balance as well as expression of genes involved in cardiac function and remodeling. Our study validates the concept of combined treatment of ischemic heart disease with apelin, FGF2, and SERCA2a and shows that such therapeutic benefit is mediated by a more effective recovery of gene network regulation.

An apelin receptor antagonist prevents pathological retinal angiogenesis with ischemic retinopathy in mice

Pathological retinal angiogenesis is caused by the progression of ischemic retinal diseases and can result in retinal detachment and irreversible blindness. This neovascularization is initiated from the retinal veins and their associated capillaries and involves the overgrowth of vascular endothelial cells. Since expression of the apelin receptor (APJ) is restricted to the veins and proliferative endothelial cells during physiological retinal angiogenesis, in the present study, we investigated the effect of APJ inhibition on pathological retinal angiogenesis in a mouse model of oxygen-induced retinopathy (OIR). In vitro experiments revealed that ML221, an APJ antagonist, suppressed cultured-endothelial cell proliferation in a dose-dependent manner. Intraperitoneal administration of ML221 inhibited pathological angiogenesis but enhanced the recovery of normal vessels into the ischemic regions in the retina of the OIR model mice. ML221 did not affect the expression levels of vascular endothelial growth factor (VEGF) and its receptor (VEGFR2) in the retina. APJ was highly expressed in the endothelial cells within abnormal vessels but was only detected in small amounts in morphologically normal vessels. These results suggest that APJ inhibitors selectively prevent pathological retinal angiogenesis and that the drugs targeting APJ may be new a candidate for treating ischemic retinopathy.

Emerging role of angiogenesis in adaptive and maladaptive right ventricular remodeling in pulmonary hypertension

Right ventricular (RV) function is the primary prognostic factor for both morbidity and mortality in pulmonary hypertension (PH). RV hypertrophy is initially an adaptive physiological response to increased overload; however, with persistent and/or progressive afterload increase, this response frequently transitions to more pathological maladaptive remodeling. The mechanisms and disease processes underlying this transition are mostly unknown. Angiogenesis has recently emerged as a major modifier of RV adaptation in the setting of pressure overload. A novel paradigm has emerged that suggests that angiogenesis and angiogenic signaling are required for RV adaptation to afterload increases and that impaired and/or insufficient angiogenesis is a major driver of RV decompensation. Here, we summarize our current understanding of the concepts of maladaptive and adaptive RV remodeling, discuss the current literature on angiogenesis in the adapted and failing RV, and identify potential therapeutic approaches targeting angiogenesis in RV failure.

A Tale of Two Elabela Null Mice

Elabela (ELA), the second peptide ligand for the apelin receptor, APLNR, was previously found in lower vertebrates to be crucial for endoderm and cardiac development. Two new studies report on the phenotypes of Ela null mice, ranging from defective embryogenesis to preeclampsia, providing new insights and raising greater intrigue on this cardiometabolic pathway.

Apelin Is a Negative Regulator of Angiotensin II-Mediated Adverse Myocardial Remodeling and Dysfunction

The apelin pathway has emerged as a critical regulator of cardiovascular homeostasis and disease. However, the exact role of pyr1-apelin-13 in angiotensin (Ang) II-mediated heart disease remains unclear. We used apelin-deficient (APLN^-/y) and apolipoprotein E knockout mice to evaluate the regulatory roles of pyr1-apelin-13. The 1-year aged APLN^-/y mice developed myocardial hypertrophy and dysfunction with reduced angiotensin-converting enzyme 2 levels. Ang II infusion (1.5 mg kg^-1 d^-1) for 4 weeks potentiated oxidative stress, pathological hypertrophy, and myocardial fibrosis in young APLN^-/y hearts resulting in exacerbation of cardiac dysfunction. Importantly, daily administration of 100 μg/kg pyr1-apelin-13 resulted in upregulated angiotensin-converting enzyme 2 levels, decreased superoxide production and expression of hypertrophy- and fibrosis-related genes leading to attenuated myocardial hypertrophy, fibrosis, and dysfunction in the Ang II-infused apolipoprotein E knockout mice. In addition, pyr1-apelin-13 treatment largely attenuated Ang II-induced apoptosis and ultrastructural injury in the apolipoprotein E knockout mice by activating Akt and endothelial nitric oxide synthase phosphorylation signaling. In cultured neonatal rat cardiomyocytes and cardiofibroblasts, exposure of Ang II decreased angiotensin-converting enzyme 2 protein and increased superoxide generation, cellular proliferation, and migration, which were rescued by pyr1-apelin-13, and Akt and endothelial nitric oxide synthase agonist stimulation. The increased superoxide generation and apoptosis in cultured cardiofibroblasts in response to Ang II were strikingly prevented by pyr1-apelin-13 which was partially reversed by cotreatment with the Akt inhibitor MK2206. In conclusion, pyr1-apelin-13 peptide pathway is a negative regulator of aging-mediated and Ang II-mediated adverse myocardial remodeling and dysfunction and represents a potential candidate to prevent and treat heart disease.

Load-dependent effects of apelin on murine cardiomyocytes

The apelin peptide is described as one of the most potent inotropic agents, produced endogenously in a wide range of cells, including cardiomyocytes. Despite positive effects on cardiac contractility in multicellular preparations, as well as indications of cardio-protective actions in several diseases, its effects and mechanisms of action at the cellular level are incompletely understood.

Here, we report apelin effects on dynamic mechanical characteristics of single ventricular cardiomyocytes, isolated from mouse models (control, apelin-deficient [Apelin-KO], apelin-receptor KO mouse [APJ-KO]), and rat. Dynamic changes in maximal velocity of cell shortening and relaxation were monitored. In addition, more traditional indicators of inotropic effects, such as maximum shortening (in mechanically unloaded cells) or peak force development (in auxotonic contracting cells, preloaded using the carbon fibre technique) were studied.

The key finding is that, using Apelin-KO cardiomyocytes exposed to different preloads with the 2-carbon fibre technique, we observe a lowering of the slope of the end-diastolic stress-length relation in response to 10 nM apelin, an effect that is preload-dependent. This suggests a positive lusitropic effect of apelin, which could explain earlier counter-intuitive findings on an apelin-induced increase in contractility occurring without matching rise in oxygen consumption.

Expression and functional implications of the renal apelinergic system in rodents

Apelin binds to the G protein-coupled apelin receptor (APJ; gene name aplnr) to modulate diverse physiological systems including cardiovascular function, and hydromineral and metabolic balance. Recently a second endogenous ligand for APJ, named apela, has been discovered. We confirm that apela activates signal transduction pathways (ERK activation) in cells expressing the cloned rat APJ. Previous studies suggest that exogenous apela is diuretic, attributable wholly or in part to an action on renal APJ. Thus far the cellular distribution of apela in the kidney has not been reported. We have utilized in situ hybridization histochemistry to reveal strong apela labelling in the inner medulla (IM), with lower levels observed in the inner stripe of the outer medulla (ISOM), of rat and mouse kidneys. This contrasts with renal aplnr expression where the converse is apparent, with intense labelling in the ISOM (consistent with vasa recta labelling) and low-moderate hybridization in the IM, in addition to labelling of glomeruli. Apelin is found in sparsely distributed cells amongst more prevalent aplnr-labelled cells in extra-tubular regions of the medulla. This expression profile is supported by RNA-Seq data that shows that apela, but not apelin or aplnr, is highly expressed in microdissected rat kidney tubules. If endogenous tubular apela promotes diuresis in the kidney it could conceivably do this by interacting with APJ in vasculature, or via an unknown receptor in the tubules. The comparative distribution of apela, apelin and aplnr in the rodent kidney lays the foundation for future work on how the renal apelinergic system interacts.

Apelin modulates pathological remodeling of lymphatic endothelium after myocardial infarction

Lymphatic endothelium serves as a barrier to control fluid balance and immune cell trafficking to maintain tissue homeostasis. Long-term alteration of lymphatic vasculature promotes edema and fibrosis, which is an aggravating factor in the onset of cardiovascular diseases such as myocardial infarction. Apelin is a bioactive peptide that plays a central role in angiogenesis and cardiac contractility. Despite an established role of apelin in lymphangiogenesis, little is known about its function in the cardiac lymphatic endothelium. Here, we show that apelin and its receptor APJ were exclusively expressed on newly formed lymphatic vasculature in a pathological model of myocardial infarction. Using an apelin-knockout mouse model, we identified morphological and functional defects in lymphatic vasculature associated with a proinflammatory status. Surprisingly, apelin deficiency increased the expression of lymphangiogenic growth factors VEGF-C and VEGF-D and exacerbated lymphangiogenesis after myocardial infarction. Conversely, the overexpression of apelin in ischemic heart was sufficient to restore a functional lymphatic vasculature and to reduce matrix remodeling and inflammation. In vitro, the expression of apelin prevented the alteration of cellular junctions in lymphatic endothelial cells induced by hypoxia. In addition, we demonstrated that apelin controls the secretion of the lipid mediator sphingosine-1-phosphate in lymphatic endothelial cells by regulating the level of expression of sphingosine kinase 2 and the transporter SPNS2. Taken together, our results show that apelin plays a key role in lymphatic vessel maturation and stability in pathological settings. Thus, apelin may represent a novel candidate to prevent pathological lymphatic remodeling in diseases.

[Pyr1]Apelin-13(1-12) Is a Biologically Active ACE2 Metabolite of the Endogenous Cardiovascular Peptide [Pyr1]Apelin-13

Aims: Apelin is a predicted substrate for ACE2, a novel therapeutic target. Our aim was to demonstrate the endogenous presence of the putative ACE2 product [Pyr¹]apelin-13_(1–12) in human cardiovascular tissues and to confirm it retains significant biological activity for the apelin receptor in vitro and in vivo. The minimum active apelin fragment was also investigated.

Methods and Results: [Pyr¹]apelin-13 incubated with recombinant human ACE2 resulted in de novo generation of [Pyr¹]apelin-13_(1–12) identified by mass spectrometry. Endogenous [Pyr¹]apelin-13_(1–12) was detected by immunostaining in human heart and lung localized to the endothelium. Expression was undetectable in lung from patients with pulmonary arterial hypertension. In human heart [Pyr¹]apelin-13_(1–12) (pK_i = 8.04 ± 0.06) and apelin-13(F13A) (pK_i = 8.07 ± 0.24) competed with [¹²⁵I]apelin-13 binding with nanomolar affinity, 4-fold lower than for [Pyr¹]apelin-13 (pK_i = 8.83 ± 0.06) whereas apelin-17 exhibited highest affinity (pK_i = 9.63 ± 0.17). The rank order of potency of peptides to inhibit forskolin-stimulated cAMP was apelin-17 (pD₂ = 10.31 ± 0.28) > [Pyr¹]apelin-13 (pD₂ = 9.67 ± 0.04) ≥ apelin-13(F13A) (pD₂ = 9.54 ± 0.05) > [Pyr¹]apelin-13_(1–12) (pD₂ = 9.30 ± 0.06). The truncated peptide apelin-13(R10M) retained nanomolar potency (pD₂ = 8.70 ± 0.04) but shorter fragments exhibited low micromolar potency. In a β-arrestin recruitment assay the rank order of potency was apelin-17 (pD₂ = 10.26 ± 0.09) >> [Pyr¹]apelin-13 (pD₂ = 8.43 ± 0.08) > apelin-13(R10M) (pD₂ = 8.26 ± 0.17) > apelin-13(F13A) (pD₂ = 7.98 ± 0.04) ≥ [Pyr¹]apelin-13_(1–12) (pD₂ = 7.84 ± 0.06) >> shorter fragments (pD₂ < 6). [Pyr¹]apelin-13_(1–12) and apelin-13(F13A) contracted human saphenous vein with similar sub-nanomolar potencies and [Pyr¹]apelin-13_(1–12) was a potent inotrope in paced mouse right ventricle and human atria. [Pyr¹]apelin-13_(1–12) elicited a dose-dependent decrease in blood pressure in anesthetized rat and dose-dependent increase in forearm blood flow in human volunteers.

Conclusions: We provide evidence that ACE2 cleaves [Pyr¹]apelin-13 to [Pyr¹]apelin-13_(1–12) and this cleavage product is expressed in human cardiovascular tissues. We have demonstrated biological activity of [Pyr¹]apelin-13_(1–12) at the human and rodent apelin receptor in vitro and in vivo. Our data show that reported enhanced ACE2 activity in cardiovascular disease should not significantly compromise the beneficial effects of apelin based therapies for example in PAH.

Vps34 regulates myofibril proteostasis to prevent hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a common heart disease with a prevalence of 1 in 500 in the general population. Several mutations in genes encoding cardiac proteins have been found in HCM patients, but these changes do not predict occurrence or prognosis and the molecular mechanisms underlying HCM remain largely elusive. Here we show that cardiac expression of vacuolar protein sorting 34 (Vps34) is reduced in a subset of HCM patients. In a mouse model, muscle-specific loss of Vps34 led to HCM-like manifestations and sudden death. Vps34-deficient hearts exhibited abnormal histopathologies, including myofibrillar disarray and aggregates containing αB-crystallin (CryAB). These features result from a block in the ESCRT-mediated proteolysis that normally degrades K63-polyubiquitinated CryAB. CryAB deposition was also found in myocardial specimens from a subset of HCM patients whose hearts showed decreased Vps34. Our results identify disruption of the previously unknown Vps34-CryAB axis as a potentially novel etiology of HCM.

Apelin-36 Modulates Blood Glucose and Body Weight Independently of Canonical APJ Receptor Signaling

Apelin-36 was discovered as the endogenous ligand for the previously orphan receptor APJ. Apelin-36 has been linked to two major types of biological activities: cardiovascular (stimulation of cardiac contractility and suppression of blood pressure) and metabolic (improving glucose homeostasis and lowering body weight). It has been assumed that both of these activities are modulated through APJ. Here, we demonstrate that the metabolic activity of apelin-36 can be separated from canonical APJ activation. We developed a series of apelin-36 variants in which evolutionarily conserved residues were mutated, and evaluated their ability to modulate glucose homeostasis and body weight in chronic mouse models. We found that apelin-36(L28A) retains full metabolic activity, but is 100-fold impaired in its ability to activate APJ. In contrast to its full metabolic activity, apelin-36(L28A) lost the ability to suppress blood pressure in spontaneously hypertensive rats (SHR). We took advantage of these findings to develop a longer-acting variant of apelin-36 that could modulate glucose homeostasis without impacting blood pressure (or activating APJ). Apelin-36-[L28C(30kDa-PEG)] is 10,000-fold less potent than apelin-36 at activating the APJ receptor but retains its ability to significantly lower blood glucose and improve glucose tolerance in diet-induced obese mice. Apelin-36-[L28C(30kDa-PEG)] provides a starting point for the development of diabetes therapeutics that are devoid of the blood pressure effects associated with canonical APJ activation.

Development of original metabolically stable apelin-17 analogs with diuretic and cardiovascular effects

Apelin, a (neuro)vasoactive peptide, plays a prominent role in controlling cardiovascular functions and water balance. Because the in vivo apelin half-life is in the minute range, we aimed to identify metabolically stable apelin-17 (K17F) analogs. We generated P92 by classic chemical substitutions and LIT01-196 by original addition of a fluorocarbon chain to the N terminus of K17F. Both analogs were much more stable in plasma (half-life >24 h for LIT01-196) than K17F (4.6 min). Analogs displayed a subnanomolar affinity for the apelin receptor and behaved as full agonists with regard to cAMP production, ERK phosphorylation, and apelin receptor internalization. Ex vivo, these compounds induced vasorelaxation of rat aortas and glomerular arterioles, respectively, precontracted with norepinephrine and angiotensin II, and increased cardiac contractility. In vivo, after intracerebroventricular administration in water-deprived mice, P92 and LIT01-196 were 6 and 160 times, respectively, more efficient at inhibiting systemic vasopressin release than K17F. Administered intravenously (nmol/kg range) in normotensive rats, these analogs potently increased urine output and induced a profound and sustained decrease in arterial blood pressure. In summary, these new compounds, which favor diuresis and improve cardiac contractility while reducing vascular resistances, represent promising candidates for the treatment of heart failure and water retention/hyponatremic disorders.-Gerbier, R., Alvear-Perez, R., Margathe, J.-F., Flahault, A., Couvineau, P., Gao, J., De Mota, N., Dabire, H., Li, B., Ceraudo, E., Hus-Citharel, A., Esteoulle, L., Bisoo, C., Hibert, M., Berdeaux, A., Iturrioz, X., Bonnet, D., Llorens-Cortes, C. Development of original metabolically stable apelin-17 analogs with diuretic and cardiovascular effects.

Resistin-induced cardiomyocyte hypertrophy is inhibited by apelin through the inactivation of extracellular signal-regulated kinase signaling pathway in H9c2 embryonic rat cardiomyocytes

It has been reported that resistin induces, whereas apelin inhibits cardiac hypertrophy. However, the underlying molecular mechanisms of apelin inhibiting resistin-induced cardiac hypertrophy remain unclear. The aim of the current study is to investigate the effects of apelin on resistin-induced cardiomyocyte hypertrophy and elucidate the underlying molecular mechanism. H9c2 cells were used in the present study, and cell surface area and protein synthesis were evaluated. Reverse transcription-quantitative polymerase chain reaction was performed to analyze the expression levels of hypertrophic markers, brain natriuretic peptide (BNP) and β-myosin heavy chain (β-MHC). In addition, western blotting was conducted to examine phosphorylation of extracellular signal-regulated kinase (ERK)1/2. Following treatment of H9c2 cells with resistin, cell surface area, protein synthesis, and BNP and β-MHC mRNA expression levels were increased. Subsequent to co-treatment of H9c2 cells with apelin and resistin, lead to the inhibition of resistin-induced hypertrophic effects by apelin. In addition, treatment with resistin increased phosphorylation of ERK1/2, whereas pretreatment with apelin decreased phosphorylation of ERK1/2, which was increased by resistin. These results indicate that resistin-induced cardiac hypertrophy is inhibited by apelin via inactivation of ERK1/2 cell signaling.

Apelin/APJ system as a therapeutic target in diabetes and its complications

The G-protein-coupled receptor APJ and its endogenous ligand apelin are widely expressed in many peripheral tissues and central nervous system, including adipose tissue, skeletal muscles and hypothalamus. Apelin/APJ system, involved in numerous physiological functions like angiogenesis, fluid homeostasis and energy metabolism regulation, is notably implicated in the development of different pathologies such as diabetes and its complications. Increasing evidence suggests that apelin regulates insulin sensitivity, stimulates glucose utilization and enhances brown adipogenesis in different tissues associated with diabetes. Moreover, apelin is also involved in the regulation of diabetic complications via binding to APJ receptor. Apelin improves diabetes-induced kidney hypertrophia, normalizes obesity-associated cardiac hypertrophy and negatively promotes retinal angiogenesis in diabetic retinopathy. In this review, we provide a comprehensive overview about the role of apelin/APJ system in different tissues related with diabetes. Furthermore, we describe the pathogenesis of diabetic complications associated with apelin/APJ system. Finally, agonists and antagonists targeted to APJ receptor are described in the literature. Thus, we highlight apelin/APJ system as a novel therapeutic target for pharmacological intervention in treating diabetes and its complications.

Apelin-13 administration protects against ischaemia/reperfusion-mediated apoptosis through the FoxO1 pathway in high-fat diet-induced obesity

BACKGROUND AND PURPOSE

Apelin-13, an endogenous ligand for the apelin (APJ) receptor, behaves as a potent modulator of metabolic and cardiovascular disorders. Here, we examined the effects of apelin-13 on myocardial injury in a mouse model combining ischaemia/reperfusion (I/R) and obesity and explored their underlying mechanisms.

EXPERIMENTAL APPROACH

Adult male C57BL/6J mice were fed a normal diet (ND) or high-fat diet (HFD) for 6 months and then subjected to cardiac I/R. The effects of apelin-13 post-treatment on myocardial injury were evaluated in HFD-fed mice after 24 h I/R. Changes in protein abundance, phosphorylation, subcellular localization and mRNA expression were determined in cardiomyoblast cell line H9C2, primary cardiomyocytes and cardiac tissue from ND- and HFD-fed mice. Apoptosis was evaluated by TUNEL staining and caspase-3 activity. Mitochondrial ultrastructure was analysed by electron microscopy.

KEY RESULTS

In HFD-fed mice subjected to cardiac I/R, i.v. administration of apelin-13 significantly reduced infarct size, myocardial apoptosis and mitochondrial damage compared with vehicle-treated animals. In H9C2 cells and primary cardiomyocytes, apelin-13 induced FoxO1 phosphorylation and nuclear exclusion. FoxO1 silencing by siRNA abolished the protective effects of apelin-13 against hypoxia-induced apoptosis and mitochondrial ROS generation. Finally, apelin deficiency in mice fed a HFD resulted in reduced myocardial FoxO1 expression and impaired FoxO1 distribution.

CONCLUSIONS AND IMPLICATIONS

These data reveal apelin as a novel regulator of FoxO1 in cardiac cells and provide evidence for the potential of apelin-13 in prevention of apoptosis and mitochondrial damage in conditions combining I/R injury and obesity.

The Apelin receptor enhances Nodal/TGFβ signaling to ensure proper cardiac development

The Apelin receptor (Aplnr) is essential for heart development, controlling the early migration of cardiac progenitors. Here we demonstrate that in zebrafish Aplnr modulates Nodal/TGFβ signaling, a key pathway essential for mesendoderm induction and migration. Loss of Aplnr function leads to a reduction in Nodal target gene expression whereas activation of Aplnr by a non-peptide agonist increases the expression of these same targets. Furthermore, loss of Aplnr results in a delay in the expression of the cardiogenic transcription factors mespaa/ab. Elevating Nodal levels in aplnra/b morphant and double mutant embryos is sufficient to rescue cardiac differentiation defects. We demonstrate that loss of Aplnr attenuates the activity of a point source of Nodal ligands Squint and Cyclops in a non-cell autonomous manner. Our results favour a model in which Aplnr is required to fine-tune Nodal output, acting as a specific rheostat for the Nodal/TGFβ pathway during the earliest stages of cardiogenesis.

DOI:http://dx.doi.org/10.7554/eLife.13758.001

In one of the first events that happens as an embryo develops, cells become the different stem cell populations that form the body’s organs. So what makes a cell become one stem cell type rather than another? In the case of the heart, the first important event is the activity of a signaling pathway called the Nodal/TGFβ pathway. Nodal signaling can drive cells to become many different stem cell types depending on its level of activity. Many different levels of regulation fine-tune Nodal signaling to produce these activity thresholds.

Zebrafish that have a mutation in the gene that encodes a protein called the Apelin receptor have no heart. The loss of this receptor interferes with how heart stem cells (called cardiac progenitors) are made and how they move to where heart development occurs.

Deshwar et al. have now studied mutant zebrafish in order to investigate how the Apelin receptor influences early heart development. This revealed that Nodal signaling levels are slightly lower in the mutant zebrafish embryos than in normal fish at the time when Nodal activity induces cardiac progenitors to form. When Nodal activity is experimentally boosted in zebrafish that lack the Apelin receptor, they become able to develop hearts.

Deshwar et al. also found that the Apelin receptor does not work in cells that produce or receive Nodal signals. This suggests that the Apelin receptor modulates Nodal signaling levels by acting in cells that lie between the cells that release Nodal signals and the cardiac progenitors. An important question for future work to address is how this modulation works. As Nodal is a key determinant of many cell types in developing embryos, learning how Apelin receptors regulate its activity could help researchers to derive specific cell types from cultured stem cells for use in regenerative medicine.

DOI:http://dx.doi.org/10.7554/eLife.13758.002

Lactation Is a Risk Factor of Postpartum Heart Failure in Mice with Cardiomyocyte-specific Apelin Receptor (APJ) Overexpression

The G protein-coupled receptor APJ and its ligand apelin are highly expressed in cardiovascular tissues and are associated with the regulation of blood pressure and cardiac function. Although accumulating evidence suggests that APJ plays a crucial role in the heart, it remains unclear whether up-regulation of APJ affects cardiac function. Here we generated cardiomyocyte-specific APJ-overexpressing (APJ-TG) mice and investigated the cardiac phenotype in APJ-TG mice. Male and non-pregnant APJ-TG mice showed cardiac hypertrophy, contractile dysfunction, and elevation of B-type natriuretic peptide gene expression in the heart but not cardiac fibrosis and symptoms of heart failure, including breathing abnormality and pleural effusion. We further examined the influence of APJ overexpression in response to physiological stress induced by pregnancy and lactation in the heart. Interestingly, repeating pregnancy and lactation (pregnancy-lactation cycle) exacerbated cardiac hypertrophy and systolic dysfunction and induced cardiac fibrosis, lung congestion, pleural effusion, and abnormal breathing in APJ-TG mice. These data indicate that female APJ-TG mice develop postpartum cardiomyopathy. We showed that lactation, but not parturition, was critical for the onset of postpartum cardiomyopathy in APJ-TG mice. Furthermore, we found that lactating APJ-TG mice showed impaired myocardial angiogenesis and imbalance of pro- and antiangiogenic gene expression in the heart. These results demonstrate that overexpression of APJ in cardiomyocytes has adverse effects on cardiac function in male and non-pregnant mice and that lactation contributes to the development of postpartum cardiomyopathy in the heart with APJ overexpression.

Establishment of a prediction model of changing trends in cardiac hypertrophy disease based on microarray data screening

The aim of the present study was to construct a mathematical model to predict the changing trends of cardiac hypertrophy at gene level. Microarray data were downloaded from Gene Expression Omnibus database (accession, GSE21600), which included 35 samples harvested from the heart of Wistar rats on postoperative days 1 (D1 group), 6 (D6 group) and 42 (D42 group) following aorta ligation and sham operated Wistar rats, respectively. Each group contained six samples, with the exception of the samples harvested from the aorta ligated group after 6 days, where n=5. Differentially expressed genes (DEGs) were identified using a Limma package in R. Hierarchical clustering analysis was performed on common DEGs in order to construct a linear equation between the D1 and D42 groups, using linear discriminant analysis. Subsequent verification was performed using receiver operating characteristic (ROC) curve and the measurement data at day 42. A total of 319, 44 and 57 DEGs were detected in D1, D6 and D42 sample groups, respectively. AKIP1, ANKRD23, LTBP2, TGF-β2 and TNFRSF12A were identified as common DEGs in all groups. The predicted linear equation between D1 and D42 group was calculated to be y=1.526×-186.671. Assessment of the ROC curve demonstrated that the area under the curve was 0.831, with a specificity and sensitivity of 0.8. As compared with the predictive and measurement data at day 42, the consistency of the two sets of data was 76.5%. In conclusion, the present model may contribute to the early prediction of changing trends in cardiac hypertrophy disease at gene level.

Genetic lineage tracing discloses arteriogenesis as the main mechanism for collateral growth in the mouse heart

Aims

Capillary and arterial endothelial cells share many common molecular markers in both the neonatal and adult hearts. Herein, we aim to establish a genetic tool that distinguishes these two types of vessels in order to determine the cellular mechanism underlying collateral artery formation.

Methods and results

Using Apln-GFP and Apln-LacZ reporter mice, we demonstrate that APLN expression is enriched in coronary vascular endothelial cells. However, APLN expression is reduced in coronary arterial endothelial cells. Genetic lineage tracing, using an Apln-CreER mouse line, robustly labelled capillary endothelial cells, but not arterial endothelial cells. We leveraged this differential activity of Apln-CreER to study collateral artery formation following myocardial infarction (MI). In a neonatal heart MI model, we found that Apln-CreER-labelled capillary endothelial cells do not contribute to the large collateral arteries. Instead, these large collateral arteries mainly arise from pre-existing, infrequently labelled coronary arteries, indicative of arteriogenesis. Furthermore, in an adult heart MI model, Apln-CreER activity also distinguishes large and small diameter arteries from capillaries. Lineage tracing in this setting demonstrated that most large and small coronary arteries in the infarcted myocardium and border region are derived not from capillaries, but from pre-existing arteries.

Conclusion

Apln-CreER-mediated lineage tracing distinguishes capillaries from large arteries, in both the neonatal and adult hearts. Through genetic fate mapping, we demonstrate that pre-existing arteries, but not capillaries, extensively contribute to collateral artery formation following myocardial injury. These results suggest that arteriogenesis is the major mechanism underlying collateral vessel formation.

Apelin: an antithrombotic factor that inhibits platelet function

Apelin peptide and its receptor APJ are directly implicated in various physiological processes ranging from cardiovascular homeostasis to immune signaling. Here, we show that apelin is a key player in hemostasis with an ability to inhibit thrombin- and collagen-mediated platelet activation. Mice lacking apelin displayed a shorter bleeding time and a prothrombotic profile. Their platelets exhibited increased adhesion and a reduced occlusion time in venules, and displayed a higher aggregation rate after their activation by thrombin compared with wild-type platelets. Consequently, human and mouse platelets express apelin and its receptor APJ. Apelin directly interferes with thrombin-mediated signaling pathways and platelet activation, secretion, and aggregation, but not with ADP and thromboxane A2-mediated pathways. IV apelin administration induced excessive bleeding and prevented thrombosis in mice. Taken together, these findings suggest that apelin and/or APJ agonists could potentially be useful adducts in antiplatelet therapies and may provide a promising perspective for patients who continue to display adverse thrombotic events with current antiplatelet therapies.

Apelin regulates FoxO3 translocation to mediate cardioprotective responses to myocardial injury and obesity

The increasing incidence of obesity accentuates the importance of identifying mechanisms and optimal therapeutic strategies for patients with heart failure (HF) in relation to obesity status. Here, we investigated the association between plasma level of apelin, an adipocyte-derived factor, and clinicopathological features of obese and non-obese patients with HF. We further explored potential regulatory mechanisms of cardiac cell fate responses in conditions combining myocardial injury and obesity. In a prospective, cross-sectional study involving patients with HF we show that obese patients (BMI ≥ 30 kg/m(2)) have higher left ventricular ejection fraction (LVEF) and greater levels of plasma apelin (p < 0.005) than non-obese patients (< 30 kg/m(2)), independently of ischemic etiology. In a mouse model combining ischemia-reperfusion (I/R) injury and high-fat diet (HFD)-induced obesity, we identify apelin as a novel regulator of FoxO3 trafficking in cardiomyocytes. Confocal microscopy analysis of cardiac cells revealed that apelin prevents nuclear translocation of FoxO3 in response to oxygen deprivation through a PI3K pathway. These findings uncover apelin as a novel regulator of FoxO3 nucleocytoplasmic trafficking in cardiac cells in response to stress and provide insight into its potential clinical relevance in obese patients with HF.

Regulation of the Apelinergic System and Its Potential in Cardiovascular Disease: Peptides and Small Molecules as Tools for Discovery

Apelin peptides and the apelin receptor represent a relatively new therapeutic axis for the potential treatment of cardiovascular disease. Several reports suggest apelin receptor activation with apelin peptides results in cardioprotection as noted through positive ionotropy, angiogenesis, reduction of mean arterial blood pressure, and apoptosis. Considering the potential therapeutic benefit attainable through modulation of the apelinergic system, research is expanding to develop novel therapies that limit the inherent rapid degradation of endogenous apelin peptides and produce metabolically stable small molecule agonists and antagonists to more rigorously interrogate the apelin receptor system. This review details the structure-activity relationships for chemically modified apelin peptides and recent disclosures of small molecule agonists and antagonists and summarizes the peer reviewed and patented literature. Development of metabolically stable ligands of apelin receptor and their effects in various models over the coming years will hopefully lead to establishment of this receptor as a validated target for cardiovascular indications.

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

Apelin and its G protein-coupled receptor (GPCR) have emerged as a key signalling pathway in the cardiovascular system. The peptide is a potent inotropic agent and vasodilator. Remarkably, a peptide, Elabela/Toddler, that has little sequence similarity to apelin, has been proposed as a second endogenous apelin receptor ligand and is encoded by a gene from a region of the genome previously classified as 'non-coding'. Apelin is downregulated in pulmonary arterial hypertension and heart failure. To replace the missing endogenous peptide, 'biased' apelin agonists have been designed that preferentially activate G protein pathways, resulting in reduced β-arrestin recruitment and receptor internalisation, with the additional benefit of attenuating detrimental β-arrestin signalling. Proof-of-concept studies support the clinical potential for apelin receptor biased agonists.

Adipokines in health and disease

Obesity increases the risk for metabolic, cardiovascular, chronic inflammatory, and several malignant diseases and, therefore, may contribute to shortened lifespan. Adipokines are peptides that signal the functional status of adipose tissue to targets in the brain, liver, pancreas, immune system, vasculature, muscle, and other tissues. Secretion of adipokines, including leptin, adiponectin, fibroblast growth factor 21 (FGF21), retinol-binding protein 4 (RBP4), dipeptidyl peptidase 4 (DPP-4), bone morphogenetic protein (BMP)-4, BMP-7, vaspin, apelin, and progranulin, is altered in adipose tissue dysfunction and may contribute to a spectrum of obesity-associated diseases. Adipokines are promising candidates both for novel pharmacological treatment strategies and as diagnostic tools, provided that we can develop a better understanding of the function and molecular targets of the more recently discovered adipokines.

Apela Regulates Fluid Homeostasis by Binding to the APJ Receptor to Activate Gi Signaling

Apela (APJ early endogenous ligand, also known as elabela or toddler) is a recently discovered peptide hormone. Based on genetic studies in zebrafish, apela was found to be important for endoderm differentiation and heart development during embryogenesis. Although common phenotypes of apela and APJ-null zebrafish during embryonic development suggested that apela interacts with the APJ receptor, kinetics of apela binding to APJ and intracellular signaling pathways for apela remain unknown. The role of apela in adults is also uncertain. Using a chimeric apela ligand, we showed direct binding of apela to APJ with high affinity (Kd = 0.51 nm) and the ability of apelin, the known peptide ligand for APJ, to compete for apela binding. Apela, similar to apelin, acts through the inhibitory G protein pathway by inhibiting forskolin-stimulated cAMP production and by inducing ERK1/2 phosphorylation. In adult rats, apela is expressed exclusively in the kidney, unlike the wide tissue distribution of apelin. In vivo studies demonstrated the ability of apela to regulate fluid homeostasis by increasing diuresis and water intake. Dose-response studies further indicated that apela induces 2- and 5-fold higher maximal responses than apelin in ERK1/2 phosphorylation and diuresis/water intake, respectively. After designing an apela antagonist, we further demonstrated the role of endogenous ligand(s) in regulating APJ-mediated fluid homeostasis. Our results identified apela as a potent peptide hormone capable of regulating fluid homeostasis in adult kidney through coupling to the APJ-mediated Gi signaling pathway.

Possible involvement of downregulation of the apelin-APJ system in doxorubicin-induced cardiotoxicity

Apelin peptide is an endogenous ligand of APJ (a putative receptor protein related to the angiotensin II type 1 receptor), which is a member of a G protein-coupled receptor superfamily with seven transmembrane domains. Recent findings have suggested that the apelin-APJ system plays a potential role in cardiac contraction and cardioprotection. In the present study, we show that the apelin-APJ system is disrupted in doxorubicin (Dox)-induced cardiotoxicity. We found downregulation of apelin and APJ mRNA expression in C57Bl/6J mouse hearts on days 1 and 5 after Dox administration (20 mg/kg ip). Plasma apelin levels and cardiac APJ protein expression were significantly decreased on day 5 after Dox injection. Cardiac apelin contents were reduced on day 1 but increased to basal levels on day 5 after Dox injection. We also examined the effects of APJ gene deletion on Dox-induced cardiotoxicity. Compared with wild-type mice, APJ knockout mice showed a significant depression in cardiac contractility on day 5 after Dox (15 mg/kg ip) treatment followed by a decrease in 14-day survival rates. Moreover, Dox-induced myocardial damage, cardiac protein carbonylation, and autophagic dysfunction were accelerated in APJ knockout mice. Rat cardiac H9c2 cells showed Dox-induced decreases in viability, which were prevented by APJ overexpression and the combination with apelin treatment. These results suggest that the suppression of APJ expression after Dox administration can exacerbate Dox-induced cardiotoxicity, which may be responsible for depressed protective function of the endogenous apelin-APJ system. Modulation of the apelin-APJ system may hold promise for the treatment of Dox-induced cardiotoxicity.

The Emerging Potential of the Apelin-APJ System in Heart Failure

The apelin-APJ system is a novel neurohormonal pathway, with studies to date suggesting that it may be of pathophysiologic relevance in heart failure and may indeed be a viable therapeutic target in this syndrome. This interest is driven primarily by the demonstration of its vasodilator, inotropic, and aquaretic actions as well as its apparent antagonistic relationship with the renin-angiotensin system. However, its promise is heightened further by the observation that, unlike other and more established cardioprotective pathways, it appears to be down-regulated in heart failure, suggesting that augmentation of this axis may have a powerful effect on the heart failure syndrome. We review the literature regarding the apelin-APJ system in heart failure and suggest areas requiring further research.

C-Terminal modifications of apelin-13 significantly change ligand binding, receptor signaling, and hypotensive action

Apelin is the endogenous ligand of the APJ receptor, a member of the G protein-coupled receptor family. This system plays an important role in the regulation of blood pressure and cardiovascular functions. To better understand the role of its C-terminal Phe(13) residue on ligand binding, receptor signaling, and hypotension, we report a series of modified analogues in which Phe(13) was substituted by unnatural amino acids. These modifications delivered new compounds exhibiting higher affinity and potency to inhibit cAMP accumulation compared to apelin-13. In particular, analogues Bpa(13) or (α-Me)Phe(13) were 30-fold more potent to inhibit cAMP accumulation than apelin-13. Tyr(OBn)(13) substitution led to a 60-fold improvement in binding affinity and induced stronger and more sustained drop in blood pressure compared to apelin-13. Our study identified new potent analogues of apelin-13, which represent valuable probes to better understand its structure-function relationship.

Elabela-apelin receptor signaling pathway is functional in mammalian systems

Elabela (ELA) or Toddler is a recently discovered hormone which is required for normal development of heart and vasculature through activation of apelin receptor (APJ), a G protein-coupled receptor (GPCR), in zebrafish. The present study explores whether the ELA-APJ signaling pathway is functional in the mammalian system. Using reverse-transcription PCR, we found that ELA is restrictedly expressed in human pluripotent stem cells and adult kidney whereas APJ is more widely expressed. We next studied ELA-APJ signaling pathway in reconstituted mammalian cell systems. Addition of ELA to HEK293 cells over-expressing GFP-AJP fusion protein resulted in rapid internalization of the fusion receptor. In Chinese hamster ovarian (CHO) cells over-expressing human APJ, ELA suppresses cAMP production with EC₅₀ of 11.1 nM, stimulates ERK1/2 phosphorylation with EC₅₀ of 14.3 nM and weakly induces intracellular calcium mobilization. Finally, we tested ELA biological function in human umbilical vascular endothelial cells and showed that ELA induces angiogenesis and relaxes mouse aortic blood vessel in a dose-dependent manner through a mechanism different from apelin. Collectively, we demonstrate that the ELA-AJP signaling pathways are functional in mammalian systems, indicating that ELA likely serves as a hormone regulating the circulation system in adulthood as well as in embryonic development.

The Apelin-APJ System in the Evolution of Heart Failure

Heart failure is a chronic, progressive disease in which the overexpression of biologically active molecules and neurohomonal activation are the key factors of the evolution and natural history. The apelin-APJ system is a newly discovered molecular pathway and the RAAS counterbalance is its principal effect. The apelin is a potent inotrope, vasodilator and diuretic with crucial cardioprotective effects against angiotensin and aldosterone injuries. Intense and prolonged RAAS induces the downregulation of the apelin and its receptor at myocardial level and cancels their protection. Compared to the vasoactive agents used in the treatment of acute heart failure, exogen apelin has unique intropic and vasodilatory effects without deleterious consequences, being a promising therapeutic option.

Platelet endothelial cell adhesion molecule-1 mediates endothelial-cardiomyocyte communication and regulates cardiac function

Background

Dilated cardiomyopathy is characterized by impaired contractility of cardiomyocytes, ventricular chamber dilatation, and systolic dysfunction. Although mutations in genes expressed in the cardiomyocyte are the best described causes of reduced contractility, the importance of endothelial‐cardiomyocyte communication for proper cardiac function is increasingly appreciated. In the present study, we investigate the role of the endothelial adhesion molecule platelet endothelial cell adhesion molecule (PECAM‐1) in the regulation of cardiac function.

Methods and Results

Using cell culture and animal models, we show that PECAM‐1 expressed in endothelial cells (ECs) regulates cardiomyocyte contractility and cardiac function via the neuregulin‐ErbB signaling pathway. Conscious echocardiography revealed left ventricular (LV) chamber dilation and systolic dysfunction in PECAM‐1^−/− mice in the absence of histological abnormalities or defects in cardiac capillary density. Despite deficits in global cardiac function, cardiomyocytes isolated from PECAM‐1^−/− hearts displayed normal baseline and isoproterenol‐stimulated contractility. Mechanistically, absence of PECAM‐1 resulted in elevated NO/ROS signaling and NRG‐1 release from ECs, which resulted in augmented phosphorylation of its receptor ErbB2. Treatment of cardiomyocytes with conditioned media from PECAM‐1^−/− ECs resulted in enhanced ErbB2 activation, which was normalized by pre‐treatment with an NRG‐1 blocking antibody. To determine whether normalization of increased NRG‐1 levels could correct cardiac function, PECAM‐1^−/− mice were treated with the NRG‐1 blocking antibody. Echocardiography showed that treatment significantly improved cardiac function of PECAM‐1^−/− mice, as revealed by increased ejection fraction and fractional shortening.

Conclusions

We identify a novel role for PECAM‐1 in regulating cardiac function via a paracrine NRG1‐ErbB pathway. These data highlight the importance of tightly regulated cellular communication for proper cardiac function.

Genetic targeting of sprouting angiogenesis using Apln-CreER

Under pathophysiological conditions in adults, endothelial cells (ECs) sprout from pre-existing blood vessels to form new ones by a process termed angiogenesis. During embryonic development, Apelin (APLN) is robustly expressed in vascular ECs. In adult mice, however, APLN expression in the vasculature is significantly reduced. Here we show that APLN expression is reactivated in adult ECs after ischaemia insults. In models of both injury ischaemia and tumor angiogenesis, we find that Apln-CreER genetically labels sprouting but not quiescent vasculature. By leveraging this specific activity, we demonstrate that abolishment of the VEGF-VEGFR2 signalling pathway as well as ablation of sprouting ECs diminished tumour vascularization and growth without compromising vascular homeostasis in other organs. Collectively, we show that Apln-CreER distinguishes sprouting vessels from stabilized vessels in multiple pathological settings. The Apln-CreER line described here will greatly aid future mechanistic studies in both vascular developmental biology and adult vascular diseases.

[Pyr1]-Apelin-13 delivery via nano-liposomal encapsulation attenuates pressure overload-induced cardiac dysfunction

Nanoparticle-mediated sustained delivery of therapeutics is one of the highly effective and increasingly utilized applications of nanomedicine. Here, we report the development and application of a drug delivery system consisting of polyethylene glycol (PEG)-conjugated liposomal nanoparticles as an efficient in vivo delivery approach for [Pyr1]-apelin-13 polypeptide. Apelin is an adipokine that regulates a variety of biological functions including cardiac hypertrophy and hypertrophy-induced heart failure. The clinical use of apelin has been greatly impaired by its remarkably short half-life in circulation. Here, we investigate whether [Pyr1]-apelin-13 encapsulation in liposome nanocarriers, conjugated with PEG polymer on their surface, can prolong apelin stability in the blood stream and potentiate apelin beneficial effects in cardiac function. Atomic force microscopy and dynamic light scattering were used to assess the structure and size distribution of drug-laden nanoparticles. [Pyr1]-apelin-13 encapsulation in PEGylated liposomal nanocarriers resulted in sustained and extended drug release both in vitro and in vivo. Moreover, intraperitoneal injection of [Pyr1]-apelin-13 nanocarriers in a mouse model of pressure-overload induced heart failure demonstrated a sustainable long-term effect of [Pyr1]-apelin-13 in preventing cardiac dysfunction. We concluded that this engineered nanocarrier system can serve as a delivery platform for treating heart injuries through sustained bioavailability of cardioprotective therapeutics.

Transition from metabolic adaptation to maladaptation of the heart in obesity: role of apelin

Background/Objectives:

Impaired energy metabolism is the defining characteristic of obesity-related heart failure. The adipocyte-derived peptide apelin has a role in the regulation of cardiovascular and metabolic homeostasis and may contribute to the link between obesity, energy metabolism and cardiac function. Here we investigate the role of apelin in the transition from metabolic adaptation to maladaptation of the heart in obese state.

Methods:

Adult male C57BL/6J, apelin knock-out (KO) or wild-type mice were fed a high-fat diet (HFD) for 18 weeks. To induce heart failure, mice were subjected to pressure overload after 18 weeks of HFD. Long-term effects of apelin on fatty acid (FA) oxidation, glucose metabolism, cardiac function and mitochondrial changes were evaluated in HFD-fed mice after 4 weeks of pressure overload. Cardiomyocytes from HFD-fed mice were isolated for analysis of metabolic responses.

Results:

In HFD-fed mice, pressure overload-induced transition from hypertrophy to heart failure is associated with reduced FA utilization (P<0.05), accelerated glucose oxidation (P<0.05) and mitochondrial damage. Treatment of HFD-fed mice with apelin for 4 weeks prevented pressure overload-induced decline in FA metabolism (P<0.05) and mitochondrial defects. Furthermore, apelin treatment lowered fasting plasma glucose (P<0.01), improved glucose tolerance (P<0.05) and preserved cardiac function (P<0.05) in HFD-fed mice subjected to pressure overload. In apelin KO HFD-fed mice, spontaneous cardiac dysfunction is associated with reduced FA oxidation (P<0.001) and increased glucose oxidation (P<0.05). In isolated cardiomyocytes, apelin stimulated FA oxidation in a dose-dependent manner and this effect was prevented by small interfering RNA sirtuin 3 knockdown.

Conclusions:

These data suggest that obesity-related decline in cardiac function is associated with defective myocardial energy metabolism and mitochondrial abnormalities. Furthermore, our work points for therapeutic potential of apelin to prevent myocardial metabolic abnormalities in heart failure paired with obesity.

Biased signaling favoring gi over β-arrestin promoted by an apelin fragment lacking the C-terminal phenylalanine

Apelin plays a prominent role in body fluid and cardiovascular homeostasis. We previously showed that the C-terminal Phe of apelin 17 (K17F) is crucial for triggering apelin receptor internalization and decreasing blood pressure (BP) but is not required for apelin binding or Gi protein coupling. Based on these findings, we hypothesized that the important role of the C-terminal Phe in BP decrease may be as a Gi-independent but β-arrestin-dependent signaling pathway that could involve MAPKs. For this purpose, we have used apelin fragments K17F and K16P (K17F with the C-terminal Phe deleted), which exhibit opposite profiles on apelin receptor internalization and BP. Using BRET-based biosensors, we showed that whereas K17F activates Gi and promotes β-arrestin recruitment to the receptor, K16P had a much reduced ability to promote β-arrestin recruitment while maintaining its Gi activating property, revealing the biased agonist character of K16P. We further show that both β-arrestin recruitment and apelin receptor internalization contribute to the K17F-stimulated ERK1/2 activity, whereas the K16P-promoted ERK1/2 activity is entirely Gi-dependent. In addition to providing new insights on the structural basis underlying the functional selectivity of apelin peptides, our study indicates that the β-arrestin-dependent ERK1/2 activation and not the Gi-dependent signaling may participate in K17F-induced BP decrease.

Apelin increases cardiac contractility via protein kinase Cε- and extracellular signal-regulated kinase-dependent mechanisms

Background

Apelin, the endogenous ligand for the G protein-coupled apelin receptor, is an important regulator of the cardiovascular homoeostasis. We previously demonstrated that apelin is one of the most potent endogenous stimulators of cardiac contractility; however, its underlying signaling mechanisms remain largely elusive. In this study we characterized the contribution of protein kinase C (PKC), extracellular signal-regulated kinase 1/2 (ERK1/2) and myosin light chain kinase (MLCK) to the positive inotropic effect of apelin.

Methods and Results

In isolated perfused rat hearts, apelin increased contractility in association with activation of prosurvival kinases PKC and ERK1/2. Apelin induced a transient increase in the translocation of PKCε, but not PKCα, from the cytosol to the particulate fraction, and a sustained increase in the phosphorylation of ERK1/2 in the left ventricle. Suppression of ERK1/2 activation diminished the apelin-induced increase in contractility. Although pharmacological inhibition of PKC attenuated the inotropic response to apelin, it had no effect on ERK1/2 phosphorylation. Moreover, the apelin-induced positive inotropic effect was significantly decreased by inhibition of MLCK, a kinase that increases myofilament Ca²⁺ sensitivity.

Conclusions

Apelin increases cardiac contractility through parallel and independent activation of PKCε and ERK1/2 signaling in the adult rat heart. Additionally MLCK activation represents a downstream mechanism in apelin signaling. Our data suggest that, in addition to their role in cytoprotection, modest activation of PKCε and ERK1/2 signaling improve contractile function, therefore these pathways represent attractive possible targets in the treatment of heart failure.

Structure-activity relationship studies toward the discovery of selective apelin receptor agonists

Apelin is the endogenous ligand for the previously orphaned G protein-coupled receptor APJ. Apelin and its receptor are widely distributed in the brain, heart, and vasculature, and are emerging as an important regulator of body fluid homeostasis and cardiovascular functions. To further progress in the pharmacology and the physiological role of the apelin receptor, the development of small, bioavailable agonists and antagonists of the apelin receptor, is crucial. In this context, E339-3D6 (1) was described as the first nonpeptidic apelin receptor agonist. We show here that 1 is actually a mixture of polymethylated species, and we describe an alternative and versatile solid-phase approach that allows access to highly pure 27, the major component of 1. This approach was also applied to prepare a series of derivatives in order to identify the crucial structural determinants required for the ligand to maintain its affinity for the apelin receptor as well as its capacity to promote apelin receptor signaling and internalization. The study of the structure-activity relationships led to the identification of ligands 19, 21, and 38, which display an increased affinity compared to that of 27. The latter and 19 behave as full agonists with regard to cAMP production and apelin receptor internalization, whereas 21 is a biased agonist toward cAMP production. Interestingly, the three ligands display a much higher stability in mouse plasma (T1/2 > 10 h) than the endogenous apelin-17 peptide 2 (T1/2 < 4 min).

HACE1-dependent protein degradation provides cardiac protection in response to haemodynamic stress

The HECT E3 ubiquitin ligase HACE1 is a tumour suppressor known to regulate Rac1 activity under stress conditions. HACE1 is increased in the serum of patients with heart failure. Here we show that HACE1 protects the heart under pressure stress by controlling protein degradation. Hace1 deficiency in mice results in accelerated heart failure and increased mortality under haemodynamic stress. Hearts from Hace1^−/− mice display abnormal cardiac hypertrophy, left ventricular dysfunction, accumulation of LC3, p62 and ubiquitinated proteins enriched for cytoskeletal species, indicating impaired autophagy. Our data suggest that HACE1 mediates p62-dependent selective autophagic turnover of ubiquitinated proteins by its ankyrin repeat domain through protein–protein interaction, which is independent of its E3 ligase activity. This would classify HACE1 as a dual-function E3 ligase. Our finding that HACE1 has a protective function in the heart in response to haemodynamic stress suggests that HACE1 may be a potential diagnostic and therapeutic target for heart disease.

HACE1 is an E3 ubiquitin ligase known to regulate various cell biological processes. Here, Zhang et al. identify HACE1 as a protective factor in the heart, demonstrating that HACE1 inhibits the development of heart failure in response to haemodynamic stress by regulating protein degradation pathways.