Isolation and characterization of a novel endogenous peptide ligand for the human APJ receptor

In the search for an endogenous ligand of the orphan G protein-coupled receptor APJ, the presence of the ligand in various tissue extracts was examined by measuring the increase in extracellular acidification rate of the cells expressing the APJ receptor as a specific signal induced by the interaction of the receptor and ligand. By monitoring this activity, we isolated an APJ receptor ligand, designated apelin, from bovine stomach extracts. The structures of bovine and human apelin preproproteins were deduced from the sequences of the corresponding cDNAs. The preproproteins consisted of 77 amino acid residues, and the apelin sequence was encoded in the C-terminal regions. Synthetic peptides derived from the C-terminal amino acid sequence of bovine preproapelin were capable of specifically promoting the acidification rate in the cells expressing the APJ receptor in a range from 10(-7) to 10(-10) M, indicating that apelin is an endogenous ligand for the APJ receptor.

Characterization of apelin, the ligand for the APJ receptor

The apelin peptide was recently discovered and demonstrated to be the endogenous ligand for the G protein-coupled receptor, APJ. A search of the GenBank databases retrieved a rat expressed sequence tag partially encoding the preproapelin sequence. The GenBank search also revealed a human sequence on chromosome Xq25-26.1, containing the gene encoding preproapelin. We have used the rat sequence to screen a rat brain cDNA library to obtain a cDNA encoding the full-length open reading frame of rat preproapelin. This cDNA encoded a protein of 77 amino acids, sharing an identity of 82% with human preproapelin. Northern and in situ hybridization analyses revealed both human and rat apelin and APJ to be expressed in the brain and periphery. Both sequence and mRNA expression distribution analyses revealed similarities between apelin and angiotensin II, suggesting they that share related physiological roles. A synthetic apelin peptide was injected intravenously into male Wistar rats, resulting in immediate lowering of both systolic and diastolic blood pressure, which persisted for several minutes. Intraperitoneal apelin injections induced an increase in drinking behavior within the first 30 min after injection, with a return to baseline within 1 h.

The novel peptide apelin lowers blood pressure via a nitric oxide-dependent mechanism

Apelin is an endogenous ligand of the human orphan receptor APJ. We detected apelin-like immunoreactivity in the adipocytes, gastric mucosa, and Kupffer cells in the liver. We also detected apelin-like immunoreactivity localized within the endothelia of small arteries in various organs. Further, it was found that mean arterial pressure after the administration of apelin-12, apelin-13, and apelin-36 at a dose of 10 nmol/kg in anaesthetized rats was reduced by 26+/-5, 11+/-4, and 5+/-4 mm Hg, respectively. In the presence of a nitric oxide (NO) synthase inhibitor, the effect of apelin-12 on blood pressure was abolished. Furthermore, the administration of apelin-12 (10 nmol/kg) in rats produced a transitory elevation of the plasma nitrite/nitrate concentration from a basal level of 21.4+/-1.6 to 27.0+/-1.5 microM. Thus, apelin may lower blood pressure via a nitric oxide-dependent mechanism.

Apelin, the novel endogenous ligand of the orphan receptor APJ, regulates cardiac contractility

The orphan receptor APJ and its recently identified endogenous ligand, apelin, exhibit high levels of mRNA expression in the heart. However, the functional importance of apelin in the cardiovascular system is not known. In isolated perfused rat hearts, infusion of apelin (0.01 to 10 nmol/L) induced a dose-dependent positive inotropic effect (EC50: 33.1+/-1.5 pmol/L). Moreover, preload-induced increase in dP/dt(max) was significantly augmented (P<0.05) in the presence of apelin. Inhibition of phospholipase C (PLC) with U-73122 and suppression of protein kinase C (PKC) with staurosporine and GF-109203X markedly attenuated the apelin-induced inotropic effect (P<0.001). In addition, zoniporide, a selective inhibitor of Na+-H+ exchange (NHE) isoform-1, and KB-R7943, a potent inhibitor of the reverse mode Na+-Ca2+ exchange (NCX), significantly suppressed the response to apelin (P<0.001). Perforated patch-clamp recordings showed that apelin did not modulate L-type Ca2+ current or voltage-activated K+ currents in isolated adult rat ventricular myocytes. Apelin mRNA was markedly downregulated in cultured neonatal rat ventricular myocytes subjected to mechanical stretch and in vivo in two models of chronic ventricular pressure overload. The present study provides the first evidence for the physiological significance of apelin in the heart. Our results show that apelin is one of the most potent endogenous positive inotropic substances yet identified and that the inotropic response to apelin may involve activation of PLC, PKC, and sarcolemmal NHE and NCX.

Molecular properties of apelin: tissue distribution and receptor binding

We analyzed the tissue distribution of apelin mRNA in rats by a quantitative reverse transcription-polymerase chain reaction and that of immunoreactive apelin (ir-apelin) by an enzyme immunoassay (EIA) using a monoclonal antibody. The expression levels of apelin mRNA and ir-apelin seemed to be consistent among tissues: they were highly expressed in the lung and mammary gland. By the combination of gel filtration and EIA, we found that the molecular forms of apelin differ among respective tissues: apelin molecules with sizes close to apelin-36 (long forms) were major components in the lung, testis, and uterus, but both long and short (whose sizes were close to [<Glu(65)]apelin-13) forms were detected in the mammary gland. In Scatchard analyses, the radioiodinated apelin-36 analogue bound to the receptor, APJ, with high affinity. In competitive binding assays, apelin-36 and apelin-19 far more efficiently inhibited the binding of the labeled apelin-36 analogue with APJ than [<Glu(65)]apelin-13. In analyses for the dissociation of apelin from APJ, unlabeled apelin-36 replaced more rapidly the labeled apelin-36 analogue bound with APJ than [<Glu(65)]apelin-13. Our results demonstrate that the long and short forms of apelin differently interact with APJ.

Molecular and functional characteristics of APJ. Tissue distribution of mRNA and interaction with the endogenous ligand apelin

We have recently identified apelin as the endogenous ligand for human APJ. In rats, the highest expression of APJ mRNA was detected in the lung, suggesting that APJ and its ligand play an important role in the pulmonary system. When apelin-36 and its pyroglutamylated C-terminal peptide, [<Glu(65)]apelin-13, were compared in microphysiometric analyses, the elevation of extracellular acidification induced in cells expressing APJ by [<Glu(65)]apelin-13 was transient, whereas that by apelin-36 was sustained. These responses were almost completely inhibited by a specific inhibitor for G(i) or that for Na(+)/H(+) exchanger. (125)I -Labeled [<Glu(65)]apelin-13 analogue specifically bound to APJ with a high affinity, and [<Glu(65)]apelin-13 was more potent than apelin-36 in competitive inhibition assays. Because pretreatment with apelin-36 but not [<Glu(65)]apelin-13 drastically reduced the binding of the labeled apelin to APJ, the different patterns of acidification induced by these two peptides appeared to reflect their dissociation rather than association with APJ. Apelin elicited the migration of APJ-expressing cells, and [<Glu(65)]apelin-13 was more potent than apelin-36 in this activity. Heterogeneous molecular forms of apelin corresponding to apelin-36 and [<Glu(65)]apelin-13 were produced in bovine colostrum. Apelin-36 and [<Glu(65)]apelin-13 might have different functions in vivo and in vitro.

Pharmacological and immunohistochemical characterization of the APJ receptor and its endogenous ligand apelin

Apelin peptides have recently been identified to be the endogenous ligands for the G protein-coupled receptor APJ. However, little is known about the physiological roles of this ligand-receptor pairing. In the present study we investigated the pharmacology of several apelin analogues at the human recombinant APJ receptor using radioligand binding and functional assays. This has led to the identification of key residues in the apelin peptide required for functional potency and binding affinity through structure-activity studies. In particular, we have identified that replacement of leucine in position 5, or arginine in position 2 and 4 of the C-terminal apelin peptide, apelin-13, resulted in significant changes in pharmacology. We also investigated the detailed localization of pre-proapelin and APJ receptor mRNA in a wide range of human, rat and mouse tissues using quantitative RT-PCR, and carried out a detailed immunohistochemical study of the distribution of the APJ receptor in rat brain and spinal cord. Interestingly, the APJ receptor was not only co-localized in white matter with GFAP in the spinal cord, but was also clearly localized on neurones in the brain, suggesting that this receptor and its peptide may be involved in a wide range of biological process yet to be determined.

Regulatory roles for APJ, a seven-transmembrane receptor related to angiotensin-type 1 receptor in blood pressure in vivo

APJ is a G-protein-coupled receptor with seven transmembrane domains, and its endogenous ligand, apelin, was identified recently. They are highly expressed in the cardiovascular system, suggesting that APJ is important in the regulation of blood pressure. To investigate the physiological functions of APJ, we have generated mice lacking the gene encoding APJ. The base-line blood pressure of APJ-deficient mice is equivalent to that of wild-type mice in the steady state. The administration of apelin transiently decreased the blood pressure of wild-type mice and a hypertensive model animal, a spontaneously hypertensive rat. On the other hand, this hypotensive response to apelin was abolished in APJ-deficient mice. This apelin-induced response was inhibited by pretreatment with a nitric-oxide synthase inhibitor, and apelin-induced phosphorylation of endothelial nitric-oxide synthase in lung endothelial cells from APJ-deficient mice disappeared. In addition, APJ-deficient mice showed an increased vasopressor response to the most potent vasoconstrictor angiotensin II, and the base-line blood pressure of double mutant mice homozygous for both APJ and angiotensin-type 1a receptor was significantly elevated compared with that of angiotensin-type 1a receptor-deficient mice. These results demonstrate that APJ exerts the hypotensive effect in vivo and plays a counterregulatory role against the pressor action of angiotensin II.

Emerging roles of apelin in biology and medicine

The family of apelin peptides is derived from a single gene and activates the 7-transmembrane G-protein-coupled receptor (GPCR) APJ. Apelins have been shown to be involved in the regulation of cardiovascular function and fluid homeostasis and interestingly represent substrates for ACE2, a carboxypeptidase recently described as a novel key enzyme in the renin-angiotensin-aldosterone system (RAS). APJ has further been reported to be a coreceptor for the infection of CD4-positive cells with HIV in the central nervous system (CNS). Apelin-36 and shorter C-terminal sequences have different potencies and efficacies in regulating these functions. Shorter sequences, especially (Pyr(1))apelin-13, are potent regulators of cardiovascular function, while longer peptides such as apelin-36 are more effective in inhibiting human immunodeficiency virus (HIV) infection by blocking the HIV coreceptor APJ. The pyroglutamate modification characteristic of the short apelin peptide (Pyr(1))apelin-13 indicates paramount biological importance of this peptide. The aim of this review is to compile conclusive evidence for the involvement of apelin/APJ in the regulation of cardiovascular function and HIV pathology, emphasizing the properties of this receptor system that may make it a successful future drug target.

Physiological role of a novel neuropeptide, apelin, and its receptor in the rat brain

Apelin, a peptide recently isolated from bovine stomach tissue extracts, has been identified as the endogenous ligand of the human orphan APJ receptor. We established a stable Chinese hamster ovary (CHO) cell line expressing a gene encoding the rat apelin receptor fused to the enhanced green fluorescent protein, to investigate internalization and the pharmacological profile of the apelin receptor. Stimulation of this receptor by the apelin fragments K17F (Lys1-Phe-Arg-Arg-Gln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe17) and pE13F (pGlu5-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe17) resulted in a dose-dependent inhibition of forskolin-induced cAMP production and promoted its internalization. In contrast, the apelin fragments R10F (Arg8-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe17) and G5F (Gly13-Pro-Met-Pro-Phe17) were inactive. The physiological role of apelin and its receptor was then investigated by showing for the first time in rodent brain: (i) detection of apelin neurons in the supraoptic and paraventricular nuclei by immunohistochemistry with a specific polyclonal anti-apelin K17F antibody; (ii) detection of apelin receptor mRNA in supraoptic vasopressinergic neurons by in situ hybridization and immunohistochemistry; and (iii) a decrease in vasopressin release following intracerebroventricular injection of K17F, or pE13F, but not R10F. Thus, apelin locally synthesized in the supraoptic nucleus could exert a direct inhibitory action on vasopressinergic neuron activity via the apelin receptors synthesized in these cells. Furthermore, central injection of pE13F significantly decreased water intake in dehydrated normotensive rats but did not affect blood pressure. Together, these results suggest that neuronal apelin plays an important role in the central control of body fluid homeostasis.

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

OBJECTIVE

The endogenous peptide apelin is differentially regulated in cardiovascular disease but the nature of its role in cardiac function remains unclear.

METHODS

We investigated the functional relevance of this peptide using ECG and respiration gated magnetic resonance imaging, conductance catheter pressure-volume hemodynamic measurements, and echocardiography in vivo. In addition, we carried out histology and immunohistochemistry to assess cardiac hypertrophy and to localize apelin and APJ in the adult and embryonic mouse heart.

RESULTS

Intraperitoneal injection of apelin (300 microg/kg) resulted in a decrease in left ventricular end diastolic area (pre: 0.122+/-0.007; post: 0.104+/-0.005 cm(2), p=0.006) and an increase in heart rate (pre: 537+/-20; post: 559+/-19 beats per minute, p=0.03). Hemodynamic measurements revealed a marked increase in ventricular elastance (pre: 3.7+/-0.9; post: 6.5+/-1.4 mm Hg/RVU, p=0.018) and preload recruitable stroke work (pre: 27.4+/-8.0; post: 51.8+/-3.1, p=0.059) with little change in diastolic parameters following acute infusion of apelin. Chronic infusion (2 mg/kg/day) resulted in significant increases in the velocity of circumferential shortening (baseline: 5.36+/-0.401; 14 days: 6.85+/-0.358 circ/s, p=0.049) and cardiac output (baseline: 0.142+/-0.019; 14 days: 0.25+/-0.019 l/min, p=0.001) as determined by 15 MHz echocardiography. Post-mortem corrected heart weights were not different between apelin and saline groups (p=0.5) and histology revealed no evidence of cellular hypertrophy in the apelin group (nuclei per unit area, p=0.9). Immunohistochemistry studies revealed APJ staining of myocardial cells in all regions of the adult mouse heart. Antibody staining, as well as quantitative real time polymerase chain reaction identified expression of both APJ and apelin in embryonic myocardium as early as embryonic day 13.5.

CONCLUSIONS

Apelin reduces left ventricular preload and afterload and increases contractile reserve without evidence of hypertrophy. These results associate apelin with a positive hemodynamic profile and suggest it as an attractive target for pharmacotherapy in the setting of heart failure.

Apelin, the natural ligand of the orphan receptor APJ, is abundantly secreted in the colostrum

By using a strategy that we have developed to search for the ligands of orphan seven-transmembrane-domain receptors [S. Hinuma et al., Nature 393 (1998) 272-276], we have recently identified a natural ligand, apelin, for the orphan 7TMR, APJ [K. Tatemoto et al., Biochem. Biophys. Res. Commun. 251 (1998) 471-476]. In this paper, we isolated rat and mouse apelin cDNAs, and analyzed the tissue distribution of apelin mRNA in rats. Although apelin mRNA was widely detected in a variety of tissues, the highest expression of apelin mRNA was detected in the mammary gland of pregnant rats. In the mammary gland, biologically active apelin and its mRNA considerably increased during pregnancy and lactation, and reached a maximal level around parturition. Moreover, a large amount of apelin (14-93 pmol/ml) was found to be secreted in the bovine colostrum, and it was still detectable even in commercial bovine milk. Since apelin partially suppressed cytokine production by mouse spleen cells in response to T cell receptor/CD3 cross-linking, the oral intake of apelin in the colostrum and milk might modulate immune responses in neonates.

Novel role for the potent endogenous inotrope apelin in human cardiac dysfunction

BACKGROUND

Apelin is among the most potent stimulators of cardiac contractility known. However, no physiological or pathological role for apelin-angiotensin receptor-like 1 (APJ) signaling has ever been described.

METHODS AND RESULTS

We performed transcriptional profiling using a spotted cDNA microarray with 12 814 unique clones on paired samples of left ventricle obtained before and after placement of a left ventricular assist device in 11 patients. The significance analysis of microarrays and a novel rank consistency score designed to exploit the paired structure of the data confirmed that natriuretic peptides were among the most significantly downregulated genes after offloading. The most significantly upregulated gene was the G-protein-coupled receptor APJ, the specific receptor for apelin. We demonstrate here using immunoassay and immunohistochemical techniques that apelin is localized primarily in the endothelium of the coronary arteries and is found at a higher concentration in cardiac tissue after mechanical offloading. These findings imply an important paracrine signaling pathway in the heart. We additionally extend the clinical significance of this work by reporting for the first time circulating human apelin levels and demonstrating increases in the plasma level of apelin in patients with left ventricular dysfunction.

CONCLUSIONS

The apelin-APJ signaling pathway emerges as an important novel mediator of cardiovascular control.

The apelin receptor APJ: journey from an orphan to a multifaceted regulator of homeostasis

The apelin receptor (APJ; gene symbol APLNR) is a member of the G protein-coupled receptor gene family. Neural gene expression patterns of APJ, and its cognate ligand apelin, in the brain implicate the apelinergic system in the regulation of a number of physiological processes. APJ and apelin are highly expressed in the hypothalamo-neurohypophysial system, which regulates fluid homeostasis, in the hypothalamic-pituitary-adrenal axis, which controls the neuroendocrine response to stress, and in the forebrain and lower brainstem regions, which are involved in cardiovascular function. Recently, apelin, synthesised and secreted by adipocytes, has been described as a beneficial adipokine related to obesity, and there is growing awareness of a potential role for apelin and APJ in glucose and energy metabolism. In this review we provide a comprehensive overview of the structure, expression pattern and regulation of apelin and its receptor, as well as the main second messengers and signalling proteins activated by apelin. We also highlight the physiological and pathological roles that support this system as a novel therapeutic target for pharmacological intervention in treating conditions related to altered water balance, stress-induced disorders such as anxiety and depression, and cardiovascular and metabolic disorders.

Distribution of mRNA encoding B78/apj, the rat homologue of the human APJ receptor, and its endogenous ligand apelin in brain and peripheral tissues

The human APJ receptor is a G protein-coupled receptor which functions as an efficient alternative co-receptor for a number of human immunodeficiency virus type 1 and simian immunodeficiency virus strains. We have cloned the rat APJ receptor, which we term B78/apj, and have mapped the mRNA distribution of both the receptor and its natural ligand apelin in rat tissues. Northern blot analysis showed a similar pattern of expression for B78/apj and apelin mRNAs with hybridising transcripts seen in the lung, heart, skeletal muscle, kidney, brain and liver. In situ hybridisation histochemistry studies revealed intense B78/apj gene expression in the parenchyma of the lung, a sub-population of glomeruli in the kidney, the corpora lutea of the ovary and isolated cells of the anterior lobe of the pituitary. B78/apj mRNA had a striking and unique distribution within the central nervous system (CNS) where receptor expression was found in cells within the meninges around the brain, in the posterior magnocellular and medial parvocellular areas of the hypothalamic paraventricular nucleus and in the supraoptic nucleus. This hypothalamic distribution offers a possible specific role of this receptor in mediating neuroendocrine responses in the CNS.

Immunocytochemical localization of the endogenous vasoactive peptide apelin to human vascular and endocardial endothelial cells

Apelin, the proposed endogenous peptide ligand of the novel G-protein-coupled receptor APJ, has been shown to possess potent vasodilator and positive inotropic effects in rats and humans in vivo. However, in humans, no endogenous source of apelin has been reported. Therefore, based on the presence of APJ and mRNA encoding apelin in human tissues, we investigated the expression of apelin in fresh-frozen human tissue from right atrium, left ventricle, lung, kidney, adrenal and large conduit vessels using immunocytochemistry. Apelin-like immunoreactivity (apelin-LI) was detected in vascular endothelial cells lining blood vessels in the human heart, kidney, adrenal gland and lung and in endothelial cells of large conduit vessels. Apelin-LI was also present in endocardial endothelial cells lining recesses of the right atrium. Apelin-LI was not present or below the level of detection in cardiomyocytes, Purkinje's cells, pulmonary or renal epithelial cells, secretory cells of the adrenal gland, vascular smooth muscle cells, adipocytes, nerves and connective tissue. The restricted presence of apelin-LI in endothelial cells suggests that endothelial apelin may play a role as a locally secreted cardiovascular mediator acting on APJ receptors present on the vascular smooth muscle and on cardiac myocytes to regulate vascular tone and cardiac contractility.

Immunocytochemical localisation of the apelin receptor, APJ, to human cardiomyocytes, vascular smooth muscle and endothelial cells

The novel G protein-coupled receptor APJ, recently paired with the proposed cognate peptide ligand apelin, mediates potent vasodilator and positive inotropic actions in rats. Radioligand binding showed apelin receptors in rat and human heart and human large conduit vessels. The specific cell types expressing the receptor, however, have not been determined. Apelin, the cognate receptor ligand, is present in endothelial cells. However, the exact pathway of endothelial apelin synthesis and secretion is not known. We therefore investigated the cellular distribution of APJ receptor-like immunoreactivity (APJ-LI) in a range of human tissues using immunocytochemistry and fluorescent double staining confocal microscopy. The same techniques were applied to determine the intracellular localisation of apelin-like immunoreactivity (apelin-LI) in cultured human umbilical vein endothelial cells (HUVECs). APJ-LI is present in endothelial cells, vascular smooth muscle cells and cardiomyocytes. Apelin-LI localises to secretory vesicles and the Golgi complex/endoplasmic reticulum of HUVECs. Apelin-LI does not co-localise with von Willebrand factor in Weibel-Palade bodies, suggesting synthesis of apelin via the constitutive pathway. The proximity of receptor and ligand in the human vasculature, together with evidence for local vascular apelin synthesis, suggests an important role for APJ/apelin as a paracrine cardiovascular regulator system.

APJ acts as a dual receptor in cardiac hypertrophy

Cardiac hypertrophy is initiated as an adaptive response to sustained overload but progresses pathologically as heart failure ensues. Here we report that genetic loss of APJ, a G-protein-coupled receptor, confers resistance to chronic pressure overload by markedly reducing myocardial hypertrophy and heart failure. In contrast, mice lacking apelin (the endogenous APJ ligand) remain sensitive, suggesting an apelin-independent function of APJ. Freshly isolated APJ-null cardiomyocytes exhibit an attenuated response to stretch, indicating that APJ is a mechanosensor. Activation of APJ by stretch increases cardiomyocyte cell size and induces molecular markers of hypertrophy. Whereas apelin stimulates APJ to activate Gαi and elicits a protective response, stretch signals in an APJ-dependent, G-protein-independent fashion to induce hypertrophy. Stretch-mediated hypertrophy is prevented by knockdown of β-arrestins or by pharmacological doses of apelin acting through Gαi. Taken together, our data indicate that APJ is a bifunctional receptor for both mechanical stretch and the endogenous peptide apelin. By sensing the balance between these stimuli, APJ occupies a pivotal point linking sustained overload to cardiomyocyte hypertrophy.

Circulating and cardiac levels of apelin, the novel ligand of the orphan receptor APJ, in patients with heart failure

The orphan receptor APJ and its recently identified endogenous ligand, apelin, are expressed in the heart. However, their importance in the human cardiovascular system is not known. This study shows that apelin-like immunoreactivity is abundantly present in healthy human heart and plasma. Gel filtration HPLC analysis revealed that atrial and plasma levels of high molecular weight apelin, possibly proapelin, were markedly higher than those of mature apelin-36 itself. As assessed by quantitative RT-PCR analysis, left ventricular apelin mRNA levels were increased 4.7-fold in chronic heart failure (CHF) due to coronary heart disease (p<0.01) and 3.3-fold due to idiopathic dilated cardiomyopathy (p<0.05), whereas atrial apelin mRNA levels were unchanged. Atrial and plasma apelin-like immunoreactivity as well as atrial and ventricular APJ receptor mRNA levels were significantly decreased in CHF. Our results suggest that a new cardiac regulatory peptide, apelin, and APJ receptor may contribute to the pathophysiology of human CHF.

[(125)I]-(Pyr(1))Apelin-13 is a novel radioligand for localizing the APJ orphan receptor in human and rat tissues with evidence for a vasoconstrictor role in man

1. We have determined the binding characteristics of [(125)I]-(Pyr(1))Apelin-13, a putative ligand for the APJ orphan receptor in human cardiovascular and rat tissue and investigated the functional properties of (Pyr(1))Apelin-13 in human saphenous vein. 2. The binding of [(125)I]-(Pyr(1))Apelin-13 to sections of human heart tissue was time dependent and rapid at 23 degrees C. Data were fitted to a single site model with an association rate constant (k(obs)) of 0.115 min(-1). [(125)I]-(Pyr(1))Apelin-13 also dissociated from a single site with a dissociation rate constant of 0.0105 min(-1). 3. In saturation binding experiments [(125)I]-(Pyr(1))Apelin-13 bound to human left ventricle with a K(D) value of 0.35+/-0.08 nM, B(max) of 4.3+/-0.9 fmol mg(-1) protein with a Hill slope of 0.97+/-0.04 and to the right atria with a K(D) of 0.33+/-0.09 nM, B(max) of 3.1+/-0.6 fmol mg(-1) protein and a Hill slope of 0.93+/-0.05. 4. [(125)I]-(Pyr(1))Apelin-13 binding sites were localized using autoradiography to human cardiovascular tissue, including coronary artery, aorta and saphenous vein grafts. In rat tissue a high density of receptors were localized to the molecular layer of the rat cerebellum, rat lung, rat heart and low levels in the rat kidney cortex. 2. (Pyr(1))Apelin-13 potently contracted human saphenous vein with a pD(2) value of 8.4+/-0.2 (n=8). The maximum response elicited by the peptide was 22.6+/-6% of 100 mM KCl. 6. We provide the first evidence of APJ receptor expression, relative densities and functional properties of (Pyr(1))Apelin-13 in human cardiovascular tissue.

Cloning, pharmacological characterization and brain distribution of the rat apelin receptor

The peptide apelin, recently isolated from bovine stomach tissue extracts, has been identified as an endogenous ligand of the human putative receptor protein related to the angiotensin receptor AT(1) (APJ). In this article, we report cloning of the rat apelin receptor cDNA. The sequence shares 90% identity with the human APJ receptor and 31% with the rat AT(1A) angiotensin receptor. Subsequently a stable CHO cell line expressing the receptor fused at its C-terminal part with the enhanced green fluorescent protein (EGFP) was established, allowing to verify its cell surface distribution and to determine the affinity of various apelin and angiotensin fragments on the cloned receptor. As shown for the human APJ receptor, the rat apelin receptor expressed in the cell line was negatively coupled to adenylate cyclase. The apelin fragment K17F (Lys(1)-Phe-Arg-Arg-Gln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe(17)) inhibited forskolin-stimulated cAMP production at sub-nanomolar concentrations whereas angiotensin II and angiotensin III were inactive. N-terminal elongation of K17F with a tyrosine or the N-terminal deletion of the first four amino acids did not modify the inhibitory action of K17F on cAMP production. In contrast, deletion of the first seven amino acids of K17F or substitution of phenylalanine by an alanine residue at the C-terminus completely abolished the activity of the peptide. In situ hybridization analysis of apelin receptor mRNA expression in the adult rat brain showed intense labeling in the hypothalamus, especially in the supraoptic and the paraventricular nuclei. The anterior and intermediate lobes of the pituitary were also highly labeled, as well as the pineal gland. Labeling was also found in extrahypothalamic structures such as the piriform cortex, the nucleus of the lateral olfactory tract, the central grey matter, the pars compacta of the substantia nigra, the dorsal raphe nucleus, the entorhinal cortex, the dentate gyrus and the Ammon's horn. The hypothalamic and hypophyseal distribution of the receptor suggests an involvement of apelin in the control of neuro- and adenohypophyseal hormone release, whereas its presence in the pineal gland and in discrete higher brain structures points out to possible roles in the regulation of circadian rhythms and of water and food intake behavior.

An orphan seven-transmembrane domain receptor expressed widely in the brain functions as a coreceptor for human immunodeficiency virus type 1 and simian immunodeficiency virus

Both CD4 and an appropriate coreceptor are necessary for infection of cells by human immunodeficiency virus type 1 (HIV-1) and most strains of HIV-2. The chemokine receptors CCR5 and CXCR4 are the major HIV-1 coreceptors, although some virus strains can also utilize alternative coreceptors such as CCR3 to infect cells. In contrast, most if not all simian immunodeficiency virus (SIV) strains use CCR5 as a coreceptor, and many SIV strains can use CCR5 independently of CD4. In addition, several orphan seven-transmembrane receptors which can serve as HIV-1 and SIV coreceptors have been identified. Here we report that APJ, an orphan seven-transmembrane domain receptor with homology to the angiotensin receptor family, functions as a coreceptor for a number of HIV-1 and SIV strains. APJ was expressed widely in the human brain and in NT2N neurons. APJ transcripts were also detected by reverse transcription-PCR in the CD4-positive T-cell line C8166, but not in peripheral blood leukocytes, microglia, phytohemagglutinin (PHA)- or PHA/interleukin-2-stimulated peripheral blood mononuclear cells, monocytes, or monocyte-derived macrophages. The widespread distribution of APJ in the central nervous system coupled with its use as a coreceptor by some HIV-1 strains indicates that it may play a role in neuropathogenesis.

The orphan seven-transmembrane receptor apj supports the entry of primary T-cell-line-tropic and dualtropic human immunodeficiency virus type 1

Human immunodeficiency virus type 1 (HIV-1) enters target cells by sequential binding to CD4 and specific seven-transmembrane-segment (7TMS) coreceptors. Viruses use the chemokine receptor CCR5 as a coreceptor in the early, asymptomatic stages of HIV-1 infection but can adapt to the use of other receptors such as CXCR4 and CCR3 as the infection proceeds. Here we identify one such coreceptor, Apj, which supported the efficient entry of several primary T-cell-line tropic (T-tropic) and dualtropic HIV-1 isolates and the simian immunodeficiency virus SIVmac316. Another 7TMS protein, CCR9, supported the less efficient entry of one primary T-tropic isolate. mRNAs for both receptors were present in phytohemagglutinin- and interleukin-2-activated peripheral blood mononuclear cells. Apj and CCR9 share with other coreceptors for HIV-1 and SIV an N-terminal region rich in aromatic and acidic residues. These results highlight properties common to 7TMS proteins that can function as HIV-1 coreceptors, and they may contribute to an understanding of viral evolution in infected individuals.

Agonist-independent nuclear localization of the Apelin, angiotensin AT1, and bradykinin B2 receptors

Signaling of the apelin, angiotensin, and bradykinin peptides is mediated by G protein-coupled receptors related through structure and similarities of physiological function. We report nuclear expression as a characteristic of these receptors, including a nuclear localization for the apelin receptor in brain and cerebellum-derived D283 Med cells and the AT(1) and bradykinin B(2) receptors in HEK-293T cells. Immunocytochemical analyses revealed the apelin receptor with localization in neuronal nuclei in cerebellum and hypothalamus, exhibiting expression in neuronal cytoplasm or in both nuclei and cytoplasm. Confocal microscopy of HEK-293T cells revealed the majority of transfected cells displayed constitutive nuclear localization of AT(1) and B(2) receptors, whereas apelin receptors did not show nuclear localization in these cells. The majority of apelin receptor-transfected cerebellum D283 Med cells showed receptor nuclear expression. Immunoblot analyses of subcellular-fractionated D283 Med cells demonstrated endogenous apelin receptor species in nuclear fractions. In addition, an identified nuclear localization signal motif in the third intracellular loop of the apelin receptor was disrupted by a substituted glutamine in place of lysine. This apelin receptor (K242Q) did not exhibit nuclear localization in D283 Med cells. These results demonstrate the following: (i) the apelin receptor exhibits nuclear localization in human brain; (ii) distinct cell-dependent mechanisms for the nuclear transport of apelin, AT(1), and B(2) receptors; and (iii) the disruption of a nuclear localization signal sequence disrupts the nuclear translocation of the apelin receptor. This discovery of apelin, AT(1), and B(2) receptors with agonist-independent nuclear translocation suggests major unanticipated roles for these receptors in cell signaling and function.

Apelin is a positive regulator of ACE2 in failing hearts

Angiotensin converting enzyme 2 (ACE2) is a negative regulator of the renin-angiotensin system (RAS), catalyzing the conversion of Angiotensin II to Angiotensin 1-7. Apelin is a second catalytic substrate for ACE2 and functions as an inotropic and cardioprotective peptide. While an antagonistic relationship between the RAS and apelin has been proposed, such functional interplay remains elusive. Here we found that ACE2 was downregulated in apelin-deficient mice. Pharmacological or genetic inhibition of angiotensin II type 1 receptor (AT1R) rescued the impaired contractility and hypertrophy of apelin mutant mice, which was accompanied by restored ACE2 levels. Importantly, treatment with angiotensin 1-7 rescued hypertrophy and heart dysfunctions of apelin-knockout mice. Moreover, apelin, via activation of its receptor, APJ, increased ACE2 promoter activity in vitro and upregulated ACE2 expression in failing hearts in vivo. Apelin treatment also increased cardiac contractility and ACE2 levels in AT1R-deficient mice. These data demonstrate that ACE2 couples the RAS to the apelin system, adding a conceptual framework for the apelin-ACE2-angiotensin 1-7 axis as a therapeutic target for cardiovascular diseases.