Central and peripheral cardiovascular actions of apelin in conscious rats

The apelin receptor: physiology, pathology, cell signalling, and ligand modulation of a peptide-activated class A GPCR

The apelin receptor (AR or APJ) is a class A (rhodopsin-like) G-protein-coupled receptor with wide distribution throughout the human body. Activation of the AR by its cognate peptide ligand, apelin, induces diverse physiological effects including vasoconstriction and dilation, strengthening of heart muscle contractility, angiogenesis, and regulation of energy metabolism and fluid homeostasis. Recently, another endogenous peptidic activator of the AR, Toddler/ELABELA, was identified as having a crucial role in zebrafish (Danio rerio) embryonic development. The AR is also implicated in pathologies including cardiovascular disease, diabetes, obesity, and cancer, making it a promising therapeutic target. Despite its established importance, the precise roles of AR signalling remain poorly understood. Moreover, little is known about the mechanisms of peptide-AR activation. Additional complexity arises from modulation of the AR by 2 endogenous peptide ligands, both with multiple bioactive isoforms of variable length and distribution. The various apelin and Toddler/ELABELA isoforms may also produce distinct cellular effects. Further complexity arises through formation of functionally distinct heterodimers between the AR and other G-protein-coupled receptors. This minireview outlines key (patho)physiological actions of the AR, addresses what is known about signal transduction downstream of AR activation, and concludes by discussing unique properties of the endogenous peptidic ligands of the AR.

Small expression tags enhance bacterial expression of the first three transmembrane segments of the apelin receptor

G-protein coupled receptors (GPCRs) are inherently dynamic membrane protein modulators of various important cellular signaling cascades. The apelin receptor (AR or APJ) is a class A GPCR involved in numerous physiological processes, implicated in angiogenesis during tumour formation and as a CD4 co-receptor for entry of human immunodeficiency virus type 1 (HIV-1) to cells. Due to the lack of efficient methods to produce full-length GPCRs enriched with nuclear magnetic resonance (NMR) active (15)N, (13)C, and (or) (2)H isotopes, small GPCR fragments typically comprising 1-2 transmembrane segments are frequently studied using NMR spectroscopy. Here, we report successful overexpression of transmembrane segments 1-3 of AR (AR_TM1-3) in the C41(DE3) strain of Escherichia coli using an AT-rich gene tag previously reported to enhance cell-free expression yields. The resulting protein, with 6 additional N-terminal residues due to the expression tag, was purified using high-performance liquid chromatography (HPLC). Far UV circular dichroism spectropolarimetry demonstrates that AR_TM1-3 has the predicted ~40% α-helical character in membrane-mimetic environments. (1)H-(15)N HSQC NMR experiments imply amenability to high-resolution NMR structural characterization and stability in solution for weeks. Notably, this small expression tag approach may also be generally applicable to other membrane proteins that are difficult to express in E. coli.

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.

Apelin acts in the subfornical organ to influence neuronal excitability and cardiovascular function

Apelin is an adipocyte-derived hormone involved in the regulation of water balance, food intake and the cardiovascular system partially through actions in the CNS. The subfornical organ (SFO) is a circumventricular organ with identified roles in body fluid homeostasis, cardiovascular control and energy balance. The SFO lacks a normal blood-brain barrier, and is thus able to detect circulating signalling molecules such as angiotensin II and leptin. In this study, we investigated actions of apelin-13, the predominant apelin isoform in brain and circulatory system, on the excitability of dissociated SFO neurons using electrophysiological approaches, and determined the cardiovascular consequences of direct administration into the SFO of anaesthetized rats. Whole cell current clamp recording revealed that bath-applied 100 nm apelin-13 directly influences the excitability of the majority of SFO neurons by eliciting either depolarizing (31.8%, mean 7.0 ± 0.8 mV) or hyperpolarizing (28.6%, mean -10.4 ± 1.8 mV) responses. Using voltage-clamp techniques, we also identified modulatory actions of apelin-13 on specific ion channels, demonstrating that apelin-13 activates a non-selective cationic conductance to depolarize SFO neurons while activation of the delayed rectifier potassium conductance underlies hyperpolarizing effects. In anaesthetized rats, microinjection of apelin into SFO decreased both blood pressure (BP) (mean area under the curve -1492.3 ± 357.1 mmHg.s, n = 5) and heart rate (HR) (-32.4 ± 10.39 beats, n = 5). Our data suggest that circulating apelin can directly affect BP and HR as a consequence of the ability of this peptide to modulate the excitability of SFO neurons.

Stress-dependent and gender-specific neuroregulatory roles of the apelin receptor in the hypothalamic-pituitary-adrenal axis response to acute stress

The neuropeptide apelin is expressed in hypothalamic paraventricular and supraoptic nuclei and mediates its effects via activation of the apelin receptor (APJ). Evidence suggests a role for apelin and APJ in mediating the neuroendocrine response to stress. To understand the physiological role of APJ in regulation of the hypothalamic-pituitary-adrenal (HPA) axis, we measured ACTH and corticosterone (CORT) plasma levels in male and female mice lacking APJ (APJ knockout, APJ KO) and in wild-type controls, in response to a variety of acute stressors. Exposure to mild restraint, systemic injection of lipopolysaccharide (LPS), insulin-induced hypoglycaemia and forced swim (FS) stressors, elevated plasma ACTH and CORT levels in wild-type mice. Acute mild restraint significantly increased plasma ACTH and CORT to a similar level in APJ KO mice as in wild-type mice. However, an intact APJ was required for a conventional ACTH, but not CORT, response to LPS administration in male mice and to insulin-induced hypoglycaemia in male and female mice. In contrast, APJ KO mice displayed an impaired CORT response to acute FS stress, regardless of gender. These data indicate that APJ has a role in regulation of the HPA axis response to some acute stressors and has a gender-specific function in peripheral immune activation of the HPA axis.

Cardiovascular effects of a PEGylated apelin

Several studies have documented cardiovascular effects of apelin, including enhanced inotropy and vasodilation. However, these cardiovascular effects are short lived due to the predicted short circulating half-life of the apelin peptide. To address this limitation of apelin, we pursued N-terminal PEGylation of apelin and examined the cardiovascular effects of the PEGylated apelin. A 40kDa PEG conjugated apelin-36 (PEG-apelin-36) was successfully produced with N-terminal conjugation, high purity (>98%) and minimum reduction of APJ receptor binding affinity. Using an adenylate cyclase inhibition assay, comparable in vitro bioactivity was observed between the PEG-apelin-36 and unmodified apelin-36. In vivo evaluation of the PEG-apelin-36 was performed in normal rats and rats with myocardial infarction (MI). Cardiac function was assessed via echocardiography before, during a 20 min IV infusion and up to 100 min post peptide infusion. Similar increases in cardiac ejection fraction (EF) were observed during the infusion of PEG-apelin-36 and apelin-36 in normal rats. However, animals that received PEG-apelin-36 maintained significantly increased EF over the 100 min post infusion monitoring period compared to the animals that received unmodified apelin-36. Interestingly, EF increases observed with PEG-apelin-36 and apelin-36 were greater in the MI rats. PEG-apelin-36 had a prolonged circulating life compared to apelin-36 in rats. There were no changes in aortic blood pressure when PEG-apelin-36 or apelin-36 was administered. To our knowledge this is the first report of apelin PEGylation and documentation of its cardiovascular effects.

Adipose tissue as regulator of vascular tone

Adipokines secreted by visceral, subcutaneous, and perivascular adipocytes are involved in the regulation of vascular tone by acting as circulatory hormones (leptin, adiponectin, omentin, visfatin, angiotensin II, resistin, tumor necrosis factor-α, interleukin-6, apelin) and/or via local paracrine factors (perivascular adipocyte-derived relaxing and contractile factors). Vascular tone regulation by adipokines is compromised in obesitas and obesity-related disorders. Hypoxia created in growing adipose tissue dysregulates synthesis of vasoactive adipokines in favor of harmful proinflammatory adipokines, while the levels of the cardioprotective adipokines adiponectin and omentin decrease. Considering the potential of the role of adipokines in obesity-related vascular diseases, strategies to counter these diseases by targeting the adipokines are discussed.

Polymorphisms of ACE2 are associated with blood pressure response to cold pressor test: the GenSalt study

BACKGROUND

Increased blood pressure (BP) reactivity to cold pressor test (CPT) is a risk factor for hypertension. Genetic factors may influence the variation of BP response to CPT among individuals. We explored the association of genetic variants in the apelin system genes (APLN, APLNR and ACE2) and BP response to CPT in a Chinese population.

METHODS

A total of 1,998 Han Chinese participants from the Genetic Epidemiology Network of Salt Sensitivity completed a CPT. The percentage changes of BP right after the end of ice-water immersion were used as the measurement of BP responses to CPT. Twenty-two single nucleotide polymorphisms (SNPs) were selected and genotyped, including both tag and potential functional SNPs of the APLN, APLNR, and ACE2 genes. A mixed-effect linear model was used to assess the association between SNPs and BP responses to CPT.

RESULTS

In women, three SNPs (rs1514283, rs4646176, and rs879922) of the ACE2 gene were significantly associated with the diastolic BP (DBP) response to CPT in the general and recessive genetic models after adjustment for multiple testing (all false discovery rate q < 0.05). There were no significant associations of polymorphisms in APLN and APLNR genes with BP responses to CPT.

CONCLUSIONS

Our study identified genetic variants in the ACE2 gene that were significantly associated with DBP responses to cold stress in the Chinese female population. Future studies are warranted to confirm these findings.

Central and peripheral apelin receptor distribution in the mouse: species differences with rat

The G protein-coupled apelin receptor (APJ) binds the endogenous peptide apelin and has been shown to have roles in many physiological systems. Thus far, distribution studies have predominantly been conducted in the rat and there is limited knowledge of the cellular distribution of APJ in mouse or human tissues. As recent functional studies have been conducted in APJ knock-out mice (APJ KO), in this study we undertook to characterize APJ mRNA and I(125)[Pyr(1)]apelin-13 binding site distribution in mouse tissues to enable correlation of distribution with function. We have utilized in situ hybridization histochemistry (ISHH) using APJ riboprobes, which revealed strong hybridization specifically in the paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamus and in the anterior pituitary, with marginally lower levels in the posterior pituitary. In the periphery, strong hybridization was observed in the lung, heart, adrenal cortex, renal medulla, ovary and uterus. Autoradiographic binding to APJ with I(125)[Pyr(1)]apelin-13 exhibited significant binding in the anterior pituitary, while lower levels were observed in the posterior pituitary and PVN and SON. In the periphery, strong receptor binding was observed in tissues exhibiting intense riboprobe hybridization, indicating a good correlation between receptor transcription and translation. While the distribution of APJ mRNA and functional protein in the mouse shows similarities to that of the rat, we report a species difference in central APJ distribution and in the pituitary gland.

Reduced circulating apelin in essential hypertension and its association with cardiac dysfunction

OBJECTIVE

Apelin--a novel multifunction peptide implicated in regulation of the cardiovascular system, including blood pressure and cardiac function control - has been postulated to be involved in the pathophysiology of hypertension and hypertensive heart disease. We investigated the circulating apelin level and its relationship to left ventricular function in patients with essential hypertension.

METHODS

We enrolled 232 hypertensive patients without concomitant diseases affecting cardiovascular functions and 76 healthy controls. Each patient underwent plasma apelin measurement and echocardiographic assessment of left ventricular systolic and diastolic function using myocardial velocities and deformation parameters, and myocardial reflectivity using calibrated integrated backscatter.

RESULTS

Hypertensive patients demonstrated lower plasma apelin than the controls (265 ± 127 vs. 330 ± 159 pg/ml; P<0.001). Patients with the lowest plasma apelin, that is, from the first tertile, exhibited more severe left ventricular systolic and diastolic function abnormalities than their peers from the other two tertiles. In multivariable regression analysis, apelin was, in addition to patient age, BMI, blood pressure, left ventricular mass index and calibrated integrated backscatter in the basal septum, an independent correlate of left ventricular systolic function parameters (β=0.18; P<0.001 for strain and β=0.12; P<0.03 for systolic strain rate) and diastolic function parameters (β=0.13; P<0.01 for early diastolic strain rate, β=0.11; P<0.04 for early diastolic myocardial velocity, and β=-0.11; P<0.04 for the ratio of mitral inflow to mitral annular early diastolic velocity).

CONCLUSION

In patients with essential hypertension, circulating apelin levels are reduced, and lower plasma apelin is independently associated with more profound left ventricular systolic and diastolic function impairment.

Apelin and pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a devastating disease characterized by pulmonary vasoconstriction, pulmonary arterial remodeling, abnormal angiogenesis and impaired right ventricular function. Despite progress in pharmacological therapy, there is still no cure for PAH. The peptide apelin and the G-protein coupled apelin receptor (APLNR) are expressed in several tissues throughout the organism. Apelin is localized in vascular endothelial cells while the APLNR is localized in both endothelial and smooth muscle cells in vessels and in the heart. Apelin is regulated by hypoxia inducible factor -1α and bone morphogenetic protein receptor-2. Patients with PAH have lower levels of plasma-apelin, and decreased apelin expression in pulmonary endothelial cells. Apelin has therefore been proposed as a potential biomarker for PAH. Furthermore, apelin plays a role in angiogenesis and regulates endothelial and smooth muscle cell apoptosis and proliferation complementary and opposite to vascular endothelial growth factor. In the systemic circulation, apelin modulates endothelial nitric oxide synthase (eNOS) expression, induces eNOS-dependent vasodilatation, counteracts angiotensin-II mediated vasoconstriction, and has positive inotropic and cardioprotective effects. Apelin attenuates vasoconstriction in isolated rat pulmonary arteries, and chronic treatment with apelin attenuates the development of pulmonary hypertension in animal models. The existing literature thus renders APLNR an interesting potential new therapeutic target for PH.

Regulation of vascular tone by adipocytes

Recent studies have shown that adipose tissue is an active endocrine and paracrine organ secreting several mediators called adipokines. Adipokines include hormones, inflammatory cytokines and other proteins. In obesity, adipose tissue becomes dysfunctional, resulting in an overproduction of proinflammatory adipokines and a lower production of anti-inflammatory adipokines. The pathological accumulation of dysfunctional adipose tissue that characterizes obesity is a major risk factor for many other diseases, including type 2 diabetes, cardiovascular disease and hypertension. Multiple physiological roles have been assigned to adipokines, including the regulation of vascular tone. For example, the unidentified adipocyte-derived relaxing factor (ADRF) released from adipose tissue has been shown to relax arteries. Besides ADRF, other adipokines such as adiponectin, omentin and visfatin are vasorelaxants. On the other hand, angiotensin II and resistin are vasoconstrictors released by adipocytes. Reactive oxygen species, leptin, tumour necrosis factor α, interleukin-6 and apelin share both vasorelaxing and constricting properties. Dysregulated synthesis of the vasoactive and proinflammatory adipokines may underlie the compromised vascular reactivity in obesity and obesity-related disorders.

[Genome-wide association study for ischemic stroke based on the Hisayama study]

Ischemic stroke is a major cause of death and disability, and occurs owing to a combination of multiple environmental and genetic risk factors. Although twin studies and family-based studies have suggested the existence of genetic risk factors for ischemic stroke, few candidate genes have been discovered. To identify genes for susceptibility to ischemic stroke, we performed a genome-wide association study using 52,608 single nucleotide polymorphism (SNP) markers. After comparison of allele frequencies between 1,112 patients with ischemic stroke and the same number of age- and sex-matched control subjects, who were selected from among the residents of the town of Hisayama, Japan, we identified three novel candidate genes for ischemic stroke: PRKCH (protein kinase C eta (PKCη)), AGTRL1 (apelin receptor), and ARHGEF10 (Rho guanine nucleotide exchange factor 10). In the functional analyses, we found that PKCη was expressed mainly in vascular endothelial cells and foamy macrophages in human atherosclerotic lesions, and that the kinase activity of PKCη was modified by a nonsynonymous SNP in PRKCH (rs2230500). We also clarified that functional SNPs in AGTRL1 (rs9943582) and ARHGEF10 (rs4376531) affected transcriptional activities owing to the different Sp1-binding affinities. In a 14-year follow-up cohort study of Hisayama residents, all of these SNPs were significantly associated with the development of ischemic stroke. Although the mechanisms of these genes in the pathogenesis of ischemic stroke are still to be elucidated, our findings might contribute to a better understanding of ischemic stroke in the future.

Genetic variants in the apelin system and blood pressure responses to dietary sodium interventions: a family-based association study

OBJECTIVE

We examined the association between genetic variants in the apelin system and blood pressure (BP) responses to low-sodium and high-sodium interventions in the GenSalt Study.

METHODS

A 7-day low-sodium intervention (51.3 mmol sodium per day) followed by a 7-day high-sodium intervention (307.8 mmol sodium per day) was conducted among 1906 participants from 637 Han Chinese families. BP measurements were obtained at baseline and following each intervention using a random-zero sphygmomanometer. Twenty-three single nucleotide polymorphisms (SNPs), including both tag and functional SNPs, were selected from three candidate genes (APLN, APLNR, and ACE2). Single marker and haplotype analyses were conducted using the Family Based Association Test program. The false discovery rate method was used to correct for multiple testing.

RESULTS

SNPs rs2282623 and rs746886 of the APLNR gene were significantly associated with DBP (both P = 0.002) and mean arterial pressure (MAP) (P = 0.001 and 0.005, respectively) responses to low-sodium intervention. Six SNPs of the ACE2 gene were significantly associated with SBP, DBP, or MAP responses to low-sodium intervention. Three of them, rs1514283, rs1514282, and rs4646176, were also significantly associated with MAP response to high-sodium intervention (all P <or= 0.006). Haplotype analysis indicated the A-T-T haplotype of APLNR SNPs rs721608-rs2282623-rs746886 was associated with decreased DBP and MAP responses to low-sodium intervention (P = 0.001 and 0.003, respectively), whereas G-C-C was associated with increased SBP and MAP responses to high-sodium intervention (P = 0.004 and 0.01, respectively).

CONCLUSION

This large family-based study indicates that genetic variants in the APLNR and ACE2 genes are significantly associated with BP responses to dietary sodium intervention.

Validation of genetic association in apelin-AGTRL1 system with hypertension in a larger Han Chinese population

BACKGROUND AND OBJECTIVE

We have recently resequenced apelin and AGTRL1 genes to identify candidate polymorphisms in family-based association with hypertension and related phenotypes. In this study, we aimed to determine and replicate these polymorphisms via a larger, independent case-control study in Shanghai Han Chinese.

METHODS

Two polymorphisms [rs3761581 (A/C) and T-1860C] in apelin gene and two [rs7119375 (G/A), rs10501367 (G/A)] in AGTRL1 gene were genotyped using the TaqMan assay among 969 patients diagnosed with essential hypertension and 980 age and sex-matched controls. Data were analyzed using Haplo.stats program.

RESULTS

In single-locus analysis, significant differences were observed in allele distribution of rs3761581 (P = 0.0156) in men and in rs7119375 (P = 0.0488) genotype distribution in women between patients and controls. Haplotype analysis indicated that haplotypes C-C-G-G (in order of T-1860C, rs3761581, rs7119375 and rs10501367) [adjusted odds ratio (ORadjusted) = 1.67, P = 0.0061] and T-A-A-A (ORadjusted = 1.62, P = 0.0008) conferred an increased risk for hypertension after adjustment for age, onset age, body mass index (BMI) and waist-to-hip ratio, whereas haplotype C-C-A-A (ORadjusted = 0.33, P = 0.0048) conferred a protective effect. In women, increased risk for hypertension was seen for haplotypes T-A-G-G (ORadjusted = 1.30, P = 0.0051), C-C-G-G (ORadjusted = 2.86, P < 0.0001), T-A-A-A (ORadjusted = 1.66, P = 0.0003) and C-C-A-A (ORadjusted = 2.65, P < 0.0001), whereas decreased risk was seen for haplotypes C-A-G-G (ORadjusted = 0.48, P < 0.0001) and T-C-G-G (ORadjusted = 0.40, P < 0.0001). Further haplotype-phenotype analyses indicated the robustness of these associations. For example, haplotype T-A-A-A was significantly associated with obesity index (BMI and waist-to-hip ratio) in both sexes and blood pressures in men (P-sim < 0.05).

CONCLUSION

In this exploratory pilot case-control study, we found robust haplotype-based and haplotype-phenotype associations of four well characterized polymorphisms in apelin-AGTRL1 system with hypertension and related phenotypes.

Pressor effect of apelin-13 in the rostral ventrolateral medulla: role of NAD(P)H oxidase-derived superoxide

Microinjection of apelin-13 into the rostral ventrolateral medulla (RVLM) in the brainstem increases blood pressure in rats. In the present study, we tested the hypotheses that apelin-13 directly stimulates neuronal activity in neurons cultured from the brainstem and that NAD(P)H oxidase-derived reactive oxygen species are involved in this action of apelin-13. Microinjection of apelin-13 into the RVLM resulted in increases in arterial pressure and in renal sympathetic nerve activity in Sprague-Dawley rats. The pressor effect of apelin-13 was attenuated by the specific NAD(P)H-oxidase inhibitor gp91ds-tat. In neurons cultured from the ventral brainstem, spontaneous action potentials were recorded using current-clamp recording. Superfusion of neurons with apelin-13 (100 nM) increased the neuronal firing rate from 0.79 ± 0.14 to 1.45 ± 0.26 Hz (n = 7, P < 0.01) in angiotensin II receptor-like 1-positive neurons, identified with single-cell reverse transcriptase-polymerase chain reaction. Neither the angiotensin II type 1 receptor antagonist losartan nor the angiotensin II type 2 receptor antagonist 1-[[4-(dimethylamino)-3-methylphenyl[methyl]-5-(diphenylacetyl)-4,5,6,7-tetrahydro-1H-imidazo[4,5-c]pyridine-6-carboxylic acid ditrifluoroacetate (PD123319) altered the positive chronotropic effect of apelin-13. Pretreatment of cells with either the reactive oxygen species scavenger superoxide dismutase [polyethylene glycol-superoxide dismutase (PEG-SOD), 25 U/ml] or with gp91ds-tat significantly attenuated the chronotropic action of apelin-13. PEG-SOD and gp91ds-tat alone had no effect on basal neuronal firing. In addition, apelin-13 significantly increased NAD(P)H oxidase activity and elevated intracellular superoxide levels in neuronal cultures. The superoxide generator xanthine-xanthine oxidase also increased neuronal activity in neurons, mimicking the neuronal response to apelin-13. These observations provide the first evidence that apelin-13 directly increases neuronal activity via stimulation of NAD(P)H oxidase-derived superoxide, a cellular signaling mechanism that may be involved in the pressor effect of apelin-13 in the RVLM.

Role of adipokines in obesity-associated hypertension

It has been well documented that obesity is a major risk factor for the development of the hypertensive state. The correlation between body mass index and blood pressure level is well established. Nevertheless, the exact mechanisms which contribute to obesity-related hypertension remain poorly understood. In the last years, we have realized that the white adipose tissue is not just an inert organ for nutrient storage and isolation but rather depending on the body mass index the biggest endocrinological organ. Thus, the possible contribution of adipokines to the blood pressure elevation becomes an attractive hypothesis to explain the hypertensive state that often occurs in obesity. In this review, we consider direct and indirect effects of main adipokines on structural and functional changes in the cardiovascular system.

Hemodynamic effect of apelin in a canine model of acute pulmonary thromboembolism

The peptide apelin is expressed in the pulmonary vasculature and is involved in the pathogenesis of many cardiovascular diseases. It has a biphasic role in the regulation of vasomotor tone related to the vascular endothelium. In this study, we induced acute pulmonary embolism (APE) in dogs with autologous blood clots to assess the effect of apelin on pulmonary and systemic circulation in the acute phase of APE. The expression of apelin mRNA was found to be upregulated in the lung tissue in the early several hours after APE induction and decreased at 24 h. The expression of apelin protein in the pulmonary arteries did not change within 24 h after APE, but significantly increased in the bronchial epithelial cells as early as 1h and decreased at 24 h. In normal anesthetized dogs, intravenous bolus administration of apelin significantly reduced the mean arterial pressure (MAP), but did not significantly affect the mean pulmonary arterial pressure (MPAP). In the dogs with APE, apelin decreased MPAP, whereas its impact on MAP was not significantly different from that in the control group. Taken together, the level of endogenous apelin did not change significantly in the pulmonary arterial wall, whereas its expression in the bronchial epithelium was upregulated in the early stage of APE. The effect of exogenous apelin on vasomotor tone was complicated: it resulted in differential changes in the pulmonary and systemic arterial pressures under different physiological and pathological conditions.

Effect of diet-induced weight loss on plasma apelin and cytokine levels in individuals with the metabolic syndrome

BACKGROUND AND AIMS

Adipose tissue is an active endocrine organ that secretes signaling molecules involved in the regulation of insulin sensitivity, food intake and inflammation. Apelin is a peptide secreted by adipose tissue that has been shown to modulate cardiovascular tone in animals. The aim of this study was to measure abdominal fat, blood pressure and circulating apelin, adiponectin, leptin, ghrelin, TNF-alpha and IL-6 levels in patients with the metabolic syndrome after a diet-induced weight loss.

METHODS AND RESULTS

35 obese individuals with the metabolic syndrome underwent an 8-week very-low-calorie diet (VLCD) and a 6-month weight maintenance period (WM) with 120mg orlistat or placebo administered 3 times daily. VLCD and WM (-15.1+/-1.0kg) decreased mean arterial pressure (MAP), insulin, leptin, triglycerides and visceral and subcutaneous adipose tissue. Moreover, adiponectin increased in response to the weight loss. However, the overall changes in plasma apelin, TNF-alpha and IL-6 were non-significant. A correlation between plasma apelin and TNF-alpha was observed at baseline (0.41, p<0.05), and the minor changes in plasma apelin levels were associated with changes in BMI during VLCD and MAP and TNF-alpha during VLCD and WM periods.

CONCLUSION

Despite reductions in BMI, body adiposity, MAP and enhancement of glucose metabolism and adiponectin in response to weight loss, no significant changes in plasma apelin, TNF-alpha and IL-6 were observed. However, apelin significantly correlated with TNF-alpha and MAP. These results suggest that apelin may not be that strongly correlated with the fat mass as an adipokine like the more abundant adipokines adiponectin or leptin and it might be involved in the regulation of inflammation and cardiovascular tone.

The effects of apelin on hypothalamic-pituitary-adrenal axis neuroendocrine function are mediated through corticotrophin-releasing factor- and vasopressin-dependent mechanisms

The apelinergic system has a widespread expression in the central nervous system (CNS) including the paraventricular nucleus, supraoptic nucleus and median eminence, and isolated cells of the anterior lobe of the pituitary. This pattern of expression in hypothalamic nuclei known to contain corticotrophin-releasing factor (CRF) and vasopressin (AVP) and to co-ordinate endocrine responses to stress has generated interest in a role for apelin in the modulation of stress, perhaps via the regulation of hormone release from the pituitary. In this study, to determine whether apelin has a central role in the regulation of CRF and AVP neurones, we investigated the effect of i.c.v. administration of pGlu-apelin-13 on neuroendocrine function in male mice pre-treated with the CRF receptor antagonist, alpha-helical CRF(9-41), and in mice-lacking functional AVP V1b receptors (V1bR KO). Administration of pGlu-apelin-13 (1 mg/kg i.c.v.) resulted in significant increases in plasma ACTH and corticosterone (CORT), which were significantly reduced by pre-treatment with alpha-helical CRF(9-41), indicating the involvement of a CRF-dependent mechanism. Additionally, pGlu-apelin-13-mediated increases in both plasma ACTH and CORT were significantly attenuated in V1bR KO animals when compared with wild-type controls, indicating a role for the vasopressinergic system in the regulation of the effects of apelin on neuroendocrine function. Together, these data confirm that the in vivo effects of apelin on hypothalamic-pituitary-adrenal neuroendocrine function appear to be mediated through both CRF- and AVP-dependent mechanisms.

Apelin gene transfer into the rostral ventrolateral medulla induces chronic blood pressure elevation in normotensive rats

The peripheral apelin system plays a significant role in cardiovascular homeostasis and in the pathophysiology of cardiovascular diseases. However, the central effect of this neurohormonal system in neural control of cardiovascular function remains poorly understood. Thus, this study was undertaken to evaluate the effect of apelin in the rostral ventrolateral medulla (RVLM) on blood pressure, cardiac function, and sympathetic nerve activity. Apelin mRNA and protein levels were detected with real-time RT-PCR and Western blots, respectively. Expression of apelin was significantly enhanced in the RVLM of spontaneously hypertensive rat (SHR) compared with normotensive Wistar-Kyoto (WKY) rats. To study the functional consequence of upregulated apelin expression, apelin was overexpressed by bilateral microinjection of the AAV2-apelin viral vector into the RVLM of WKY rats. Immunofluorescence staining and Western blots demonstrated that microinjection of AAV2-apelin into the RVLM resulted in a significant increase in apelin expression, which was associated with a chronic elevation in blood pressure and cardiac hypertrophy. In addition, direct microinjection of exogenous apelin-13 (200 pmol in 50 nL) into the RVLM caused a 20 mm Hg elevation in blood pressure and a 24% increase in sympathetic nerve activity. The present study is the first to show that apelin expression is enhanced in the RVLM of SHR versus WKY rats and that overexpression of this gene in the RVLM results in chronic blood pressure elevation and cardiac hypertrophy in normotensive rats. Thus, the apelin system in the RVLM may play a very important role in central blood pressure regulation and in the pathogenesis of hypertension.

Angiotensin-converting enzyme 2 in the brain: properties and future directions

Angiotensin (Ang)-converting enzyme (ACE) 2 cleaves Ang-II into the vasodilator peptide Ang-(1-7), thus acting as a pivotal element in balancing the local effects of these peptides. ACE2 has been identified in various tissues and is supposed to be a modulator of cardiovascular function. Decreases in ACE2 expression and activity have been reported in models of hypertension, heart failure, atherosclerosis, diabetic nephropathy and others. In addition, the expression level and/or activity are affected by other renin-angiotensin system components (e.g., ACE and AT1 receptors). Local inhibition or global deletion of brain ACE2 induces a reduction in baroreflex sensitivity. Moreover, ACE2-null mice have been shown to exhibit either blood pressure or cardiac dysfunction phenotypes. On the other hand, over-expression of ACE2 exerts protective effects in local tissues, including the brain. In this review, we will first summarize the major findings linking ACE2 to cardiovascular function in the periphery then focus on recent discoveries related to ACE2 in the CNS. Finally, we will unveil new tools designed to address the importance of central ACE2 in various diseases, and discuss the potential for this carboxypeptidase as a new target in the treatment of hypertension and other cardiovascular diseases.

The role of apelin in cardiovascular function and heart failure

Apelin is a novel peptide that acts through the APJ receptor, sharing similarities with the angiotensin II-angiotensin II type 1 receptor pathway. It is a peripheral vasodilator, powerful inotrope and may affect central fluid homeostasis. Animal and human studies suggest that it may play a role in the pathogenesis of heart failure by modulating the harmful effects of angiotensin II. Apelin is reduced in patients with heart failure and up regulated following favourable left ventricular remodelling. It is widely distributed in a number of tissues, mainly restricted to vascular endothelium. This comprehensive review of the literature highlights the important studies that have led to the discovery of apelin and its role in cardiovascular function and heart failure.

Injections of angiotensin-converting enzyme 2 inhibitor MLN4760 into nucleus tractus solitarii reduce baroreceptor reflex sensitivity for heart rate control in rats

Injections of the angiotensin(1-7) [Ang(1-7)] antagonist [d-Ala7]-Ang(1-7) into the nucleus of the solitary tract (NTS) of Sprague-Dawley rats reduce baroreceptor reflex sensitivity (BRS) for control of heart rate by approximately 40%, whereas injections of the angiotensin II (Ang II) type 1 receptor antagonist candesartan increase BRS by 40% when reflex bradycardia is assessed. The enzyme angiotensin-converting enzyme 2 (ACE2) is known to convert Ang II to Ang(1-7). We report that ACE2 activity, as well as ACE and neprilysin activities, are present in plasma membrane fractions of the dorsomedial medulla of Sprague-Dawley rats. Moreover, we show that BRS for reflex bradycardia is attenuated (1.16 +/- 0.29 ms mmHg-1 before versus 0.33 +/- 0.11 ms mmHg-1 after; P < 0.05; n = 8) 30-60 min following injection of the selective ACE2 inhibitor MLN4760 (12 pmol in 120 nl) into the NTS. These findings support the concept that within the NTS, local synthesis of Ang(1-7) from Ang II is required for normal sensitivity for the baroreflex control of heart rate in response to increases in arterial pressure.

Jacques Benoit lecture: the neuroendocrine view of the angiotensin and apelin systems

The hyperactivity of the brain renin-angiotensin system (RAS) has been implicated in the development and maintenance of hypertension in several types of experimental and genetic hypertension animal models. Among the main bioactive peptides of the brain RAS, angiotensin (Ang) II and Ang III display the same affinity for type 1 and type 2 Ang II receptors. Both peptides, injected intracerebroventricularly, similarly increase arginine vasopressin (AVP) release and blood pressure (BP); however, because Ang II is converted in vivo to Ang III, the identity of the true effector is unknown. We review new insights into the predominant role of brain Ang III in the control of BP, underlining the fact that brain aminopeptidase A (APA), the enzyme generating brain Ang III, may therefore be an interesting candidate target for the treatment of hypertension. This justifies the development of potent systemically active APA inhibitors, such as RB150, as prototypes of a new class of antihypertensive agents for the treatment of certain forms of hypertension. We also searched for a putative angiotensin receptor subtype specific for Ang III and isolated a seven transmembrane-domain G protein-coupled receptor corresponding to the receptor for apelin, a newly-discovered peptide isolated from bovine stomach. Apelin and its receptor are expressed in magnocellular vasopressinergic neurones in the hypothalamus. The central injection of apelin in lactating rats decreases the phasic electrical activity of vasopressinergic neurones and the systemic secretion of AVP, inducing water diuresis. Apelin is therefore a natural inhibitor of the antidiuretic effect of AVP. In addition, systemic administration of apelin decreases BP, improves cardiac contractility and reduces cardiac loading. The development of nonpeptide agonists of the apelin receptor may provide new therapeutic tools for treating water retention, hyponatraemia and cardiovascular diseases. Angiotensins and apelin thus exert opposing but complementary effects, and are thereby determinant for the maintenance of body fluid homeostasis and cardiovascular functions.

Reduced apelin levels in stable angina

OBJECTIVE

To investigate the change in the plasma apelin level in patients with stable angina.

METHODS

The study enrolled 96 patients with stable angina as the Stable Angina Group and another 78 outpatients with no angina as the Control Group.

SUBJECTS

were excluded if they had a history of acute coronary syndrome, rheumatic heart disease, cardiomyopathy, cardiac arrhythmia, diabetes mellitus, hyperthyroidism, or antecedent hypertension. Plasma apelin levels of all subjects were determined using a commercially available immunoassay. In addition, blood was sampled for measurements of 8-iso-prostaglandin-F2alpha by enzyme-linked immunosorbent assay. The severity of coronary artery stenosis of stable angina patients was evaluated using the Gensini score.

RESULTS

The mean levels of apelin in plasma were significantly lower in subjects with stable angina compared with controls (1.24 vs.1.98 ng/mL, p <0.05). The plasma level of apelin in the stable angina group was negatively correlated with the Gensini score (r =-0.399, p <0.05).

CONCLUSION

Reduced apelin levels were observed in this homogenous population of stable angina subjects and the plasma apelin level was negatively correlated with the degree of coronary stenosis.

Expanding role for the apelin/APJ system in physiopathology

Apelin is a bioactive peptide known as the ligand of the G protein-coupled receptor APJ. Diverse active apelin peptides exist under the form of 13, 17 or 36 amino acids, originated from a common 77-amino-acid precursor. Both apelin and APJ mRNA are widely expressed in several rodent and human tissues and have functional effects in both the central nervous system and peripheral tissues. Apelin has been shown to be involved in the regulation of cardiovascular functions, fluid homeostasis, vessel formation and cell proliferation. More recently, apelin has been described as an adipocyte-secreted factor (adipokine), up-regulated in obesity. By acting as circulating hormone or paracrine factor, adipokines are involved in physiological regulations (fat depot development, energy storage, metabolism or eating behavior) or in the promotion of obesity-associated disorders (type 2 diabetes and cardiovascular dysfunctions). In this regard, expression of apelin gene in adipose tissue is increased by insulin and TNFα. This review will consider the main roles of apelin in physiopathology with particular attention on its role in energy balance regulation and in obesity-associated disorders.

Emerging role of the apelin system in cardiovascular homeostasis

The angiotensin receptor-like 1 (APJ) and its novel ligand, apelin, share similarities in structure and anatomical distribution with that of angiotensin II and the angiotensin II type 1 receptor. However, apelin has positive inotropic, vasodilatory and diuretic properties. Differential expression and synthesis of apelin and the APJ receptor in normal and failing hearts suggest that the apelin system may contribute to the pathophysiology of human heart failure and has potential therapeutic use in treatment of heart failure.

Functional SNP in an Sp1-binding site of AGTRL1 gene is associated with susceptibility to brain infarction

Brain infarction is one of the common causes of death and also a major cause of severe disability. To identify a gene(s) susceptible to brain infarction, we performed a large-scale association study of Japanese patients with brain infarction, using 52608 gene-based single nucleotide polymorphism (SNP) markers. Comparison of allele frequencies between 1112 cases with brain infarction and age- and sex-matched control subjects of the same number found an SNP in the 5'-flanking region of angiotensin receptor like-1 (AGTRL1) gene (rs9943582, - 154G/A) to have a significant association with brain infarction [odds ratio = 1.30, 95% confidence interval (CI) = 1.14-1.47, P = 0.000066]. We also found the binding of Sp1 transcription factor to the region including the susceptible G allele, but not the non-susceptible A allele. Luciferase assay and RT-PCR analysis demonstrated that exogenously introduced Sp1 induced transcription of AGTRL1 and its ligand, apelin, as well, indicating direct regulation of apelin/APJ pathway by Sp1. Furthermore, a 14 year follow-up cohort study in a Japanese community in Hisayama town, Japan revealed that the homozygote of the susceptible G allele of this particular SNP had significantly higher risk of brain infarction (hazard ratio = 2.00, 95% CI = 1.22-3.29, P = 0.006). Our results indicate that the SNP in the AGTRL1 gene is associated with the susceptibility to brain infarction.

Utility of plasma apelin and other indices of cardiac dysfunction in the clinical assessment of patients with dilated cardiomyopathy

Apelin is a recently discovered peptide ligand reported to be involved in the regulation of cardiovascular homeostasis. The exact role of apelin in the pathophysiology of congestive heart failure has remained obscure, and the reported circulating levels of apelin in patients with heart failure have been contradictory. To establish the role of apelin in the assessment of cardiac dysfunction we measured plasma apelin levels in 65 patients with congestive heart failure caused by idiopathic dilated cardiomyopathy (IDC) and 14 healthy volunteers by specific radioimmunoassay. IDC patients were carefully examined including echocardiography, both-sided cardiac catheterization and cardiopulmonary exercise test. In addition, plasma levels of N-terminal pro-brain natriuretic peptide (NT-proBNP), N-terminal pro-atrial natriuretic peptide (NT-proANP), interleukin (IL)-6, tumor necrosis factor alpha (TNF-alpha), epinephrine and norepinephrine were determined. Plasma apelin levels were similar in IDC patients (median 26.5 pg/ml, range<3.40-97.6 pg/ml) and in control subjects (median 24.1 pg/ml, range 19.0-28.7 pg/ml; p=NS). Unlike the levels of NT-proBNP, IL-6, TNF-alpha, and norepinephrine, plasma apelin levels did not reflect the severity of heart failure. Our study demonstrates that although disturbed apelin-APJ signalling in heart may play a role in the pathophysiology of heart failure, circulating apelin levels cannot be applied in the clinical assessment of patients with chronic left ventricular dysfunction.

Effects of central and peripheral injections of apelin on fluid intake and cardiovascular parameters in rats

Previous studies have indicated that apelin, a novel peptide suggested to have some actions related to angiotensin peptides, has either a dipsogenic or an antidipsogenic effect and either increases or decreases blood pressure. The present study attempts to provide replication or understanding of these disparate effects. Neither central (lateral or third cerebral ventricle) nor peripheral (intravenous) administration of apelin induced water intake in sated rats, nor did it decrease water intake in deprived rats. It also had no effect on sodium appetite. Peripherally injected apelin had a hypotensive action in anesthetized rats, but had no consistent effect in awake, unrestrained rats. We conclude that apelin does not have reliable or robust effects on fluid intake or blood pressure in Sprague-Dawley rats under normal conditions, but discuss the possibility for a role of apelin in fluid homeostasis in selected physiological states.

Down-regulation of cardiac apelin system in hypertrophied and failing hearts: Possible role of angiotensin II-angiotensin type 1 receptor system

Cardiac apelin has recently been suggested to contribute to the pathophysiology of heart failure (HF) in humans. In animal experiments, its infusion acutely improved systolic as well as diastolic LV function. Although its deficit could critically determine the cardiac dysfunction, its regulatory mechanism is unknown. Accordingly, we investigated the role and regulation of the cardiac apelin system in the diseased heart using Dahl salt-sensitive rats, which show a distinctive transition from compensatory LV hypertrophy (LVH) to HF. In the compensatory LVH stage, apelin and its receptor APJ mRNA showed no change compared with control animals, while these were markedly down-regulated in the HF stage (72% and 57% decrease, respectively). The rats were chronically treated with telmisartan (angiotensin type 1 receptor blocker [ARB], 5 mg/kg/day, n=9), ONO-4817 (matrix metalloproteinase [MMP] inhibitor, 200 mg/kg/day, n=9), bisoprolol (beta blocker, 3 mg/kg/day, n=6) or vehicle (0.5%CMC, n=9) from the LVH stage. Although the functional improvements were similar among the three treated groups 6 weeks after treatment, restoration of cardiac apelin and APJ expression was observed only in the ARB group. Furthermore, in angiotensin II-infused rats, cardiac apelin mRNA was decreased after 24 h of treatment and its restoration was achieved by treatment with ARB. These results indicate that the cardiac apelin system is markedly down-regulated in experimental HF and may be regulated by the angiotensin II-angiotensin type 1 receptor system directly. Inhibition of the renin-angiotensin system may have beneficial effects, at least in part, through restoration of the cardiac apelin system in the treatment of HF.

Unravelling the roles of the apelin system: prospective therapeutic applications in heart failure and obesity

The apelin receptor was initially classed as an orphan G-protein-coupled receptor, and little was known about its physiological functions until apelin, the endogenous ligand, was identified. Similarities between the structure and anatomical distribution of apelin and its receptor and that of angiotensin II and the angiotensin AT1 receptor provide clues about the physiological functions of this novel signal-transduction system. Now, roles have been established for the apelin system in lowering blood pressure, as a potent cardiac inotrope, in modulating pituitary hormone release and food and water intake, in stress activation, and as a novel adipokine that is excreted from fat cells and regulates insulin. Given its broad array of physiological roles, apelin has attracted much interest as a target for novel therapeutic research and drug design.