Background The apelin system is a novel peptidic pathway widely expressed in the heart and vasculature. In preclinical studies, apelin receptor agonism mediates nitric oxide-dependent vasodilatation, reduces ventricular preload and afterload and potently increases myocardial contractility. In preclinical models of heart failure, expression of the apelin pathway is down regulated but the haemodynamic effects of apelin receptor agonism are preserved. These changes in expression appear to be paralleled in patients with chronic heart failure but the cardiovascular actions of apelin in vivo in man are, to date, unknown. Detailed clinical investigation is therefore required to establish the role of apelin in human cardiovascular physiology and pathophysiology and to explore the therapeutic potential of apelin receptor agonism in patients with heart failure. Objectives Through a series of in vivo clinical studies: 1) to establish the direct vascular actions of apelin in the peripheral venous, peripheral arterial and coronary arterial circulations; 2) to determine the contribution of the endothelium-derived vasodilators, nitric oxide and prostacyclin, to the vascular actions of apelin; 3) to establish the effects of apelin on cardiac contractility and systemic haemodynamics; 4) to compare the direct vascular and systemic haemodynamic effects of the fulllength mature apelin peptide, apelin-36, with a shorter, biologically active carboxyl (C)-terminal fragment, (Pyr1)apelin-13); and 5) to establish whether the local vascular and systemic haemodynamic effects of apelin are altered in patients with chronic heart failure. Methods The cardiovascular effects of apelin were assessed in 32 healthy volunteers, 6 patients undergoing elective diagnostic coronary angiography, 18 patients with stable New York Heart Association (NYHA) class II-III chronic heart failure and 18 age- and sex-matched healthy controls. Dorsal hand vein tone was assessed by the Aellig hand vein technique during local intravenous infusions (0.1-3 nmol/min) of apelin-36, (Pyr1)apelin-13, and sodium nitroprusside (SNP; 0.6 nmol/min). Forearm blood flow was measured by venous occlusion plethysmography during intrabrachial infusions of apelin-36 and (Pyr1)apelin-13 (0.01-30 nmol/min) and subsequently in the presence or absence of a ‘nitric oxide clamp’ (nitric oxide synthase inhibitor, L-NG-monomethylarginine (L-NMMA; 8 μmol/min), co-infused with SNP (90-900 ng/min)), or a single oral dose of aspirin (600 mg) or matched placebo. Coronary blood flow was evaluated by quantitative coronary angiography (QCA) and Doppler flow wire, and left ventricular pressures measured by pressure wire before and after intracoronary injection of apelin-36 (20 and 200 nM), 0.9% saline and glyceryl trinitrate (GTN) (100 μg). Blood pressure, heart rate, cardiac output and peripheral vascular resistance were assessed by sphygmomanometry and thoracic electrical bioimpedance (TEB) during systemic intravenous infusion of apelin-36 and (Pyr1)apelin-13 (30-300 nmol/min). Forearm blood flow and systemic haemodynamic responses to (Pyr1)apelin-13 in patients with chronic heart failure were then compared with age- and sex-matched healthy controls. Results Although SNP caused venodilatation (P<0.0001), apelin-36 and (Pyr1)apelin-13 had no effect on dorsal hand vein diameter (P=0.2). Both apelin isoforms caused vasodilatation in forearm resistance vessels (P<0.0001) but the offset was slower with apelin-36. (Pyr1)apelin-13-mediated vasodilatation was attenuated by the nitric oxide clamp (P=0.004) but unaffected by aspirin (P=0.7). Intracoronary bolus of apelin-36 increased coronary blood flow and the maximum rate of rise in left ventricular pressure, and reduced peak and end-diastolic left ventricular pressures (all P<0.05). Both (Pyr1)apelin-13 and apelin-36 increased heart rate and cardiac output whilst reducing peripheral vascular resistance (P<0.01 for all) with no overall effect on blood pressure. Intrabrachial infusions of (Pyr1)apelin-13, acetylcholine and SNP caused forearm vasodilatation in patients and controls (P<0.0001 for all). Vasodilatation to acetylcholine (P=0.01) but not apelin (P=0.3) or SNP (p=0.9) was attenuated in patients with heart failure. Systemic infusions of (Pyr1)apelin-13 increased cardiac index and lowered mean arterial pressure and peripheral vascular resistance index in patients and matched controls (all P<0.01) but increased heart rate only in controls (P<0.01). Conclusions Although having no apparent effect on venous tone, acute apelin receptor agonism causes peripheral and coronary vasodilatation and increases cardiac contractility and output. Local vascular and systemic haemodynamic responses to apelin are preserved in patients with stable symptomatic chronic heart failure. The apelin system merits further clinical investigation to determine its role in cardiovascular homeostasis and represents a novel potential therapeutic target for patients with heart failure.

Apelin was discovered in 1998 as the endogenous peptide ligand of the orphan APJ receptor. The apelin system is well conserved across vertebrate species and is reported to have cardiovascular effects including positive inotropy, vasodilation, vasoconstriction and cardioprotection during ischaemia. Recent studies in human healthy volunteers and in chronic heart failure patients have highlighted the apelin system as a potential target for drug development. However, the cellular and molecular pathways through which apelin acts remain poorly understood. This study aimed to confirm the inotropic and vasoactive actions of apelin and to further examine the proposed cardioprotective effects of apelin under ischaemic and hypoxic conditions. Cardioprotection is defined as a mechanism, for example induced by a drug, which reduces injury in response to ischaemia or hypoxia. In vivo in the anaesthetised rat, apelin was administered as a bolus dose via the cannulated jugular vein and mean arterial pressure was measured by cannulation of the carotid artery. Pyr-apelin-13 had no effect on heart rate or mean arterial pressure. Apelin-13 decreased mean arterial pressure by approximately 20 mmHg, although the effect was highly variable among animals. Apelin-16 consistently lowered heart rate, but had no effect on mean arterial pressure. In rat isolated mesenteric arteries, studied using wire myography, apelin-13 and apelin-36 had no vasodilator or vasoconstrictor effect. In rat isolated right ventricular papillary muscles and right atrial strips, no change in tension, time to peak or time to relax was observed in response to pyr-apelin-13 despite responding to standard pharmacological stimuli such as noradrenaline and increased calcium concentrations in the bathing medium. In isolated, perfused rat heart (Langendorff), infusion of apelin-16 for 15 minutes did not alter developed pressure, rate of rise or rate of fall detected by an intraventricular balloon positioned in the left ventricle throughout the infusion. As the isolated perfused hearts did not demonstrate an inotropic effect in response to apelin, no cardioprotective studies were carried out in this model. Cardioprotective studies of apelin were performed in zebrafish embryos 3 – 5 days post fertilisation (dpf). I developed a hypoxia-recovery model in which we could test the effect of pharmacological agents, including apelin, on the hypoxia-recovery response. In zebrafish embryos 3 dpf, 2h hypoxia (1% oxygen) reduced heart rate and wall motion amplitude (to approximately 90% of control) and contraction velocity and relaxation velocity (to approximately 80% of control). All parameters recovered during a subsequent 2h in normoxia. Incubation in pyr-apelin-13 for 1h prior to and throughout hypoxia did not affect the depression in heart rate observed on exposure to hypoxia. However, apelin incubation resulted in an improvement in wall motion amplitude and relaxation velocity and a significant improvement in contraction velocity after hypoxia and throughout recovery. Pyr-apelin-13 had no inotropic or chronotropic effect on baseline heart function in embryos 3 dpf or in isolated hearts from embryos. However, apelin knockdown using a morpholino targeting the exon 2/intron 2 boundary of apelin pre-mRNA resulted in a high mortality rate and a severe total body and cardiovascular phenotype, suggesting that endogenous apelin is crucial during development in zebrafish embryos. In order to test pharmacological agents more efficiently, I developed a semi-quantitative scoring method to screen a larger number of embryos in a reduced time period. Heart rate and circulation was defined as normal, reduced or absent after 2h and 4h in hypoxia and during recovery in normoxia. The abundance of apelin and HIF-1α mRNA was measured using quantitative RT-PCR. In zebrafish 5 dpf, a marked decrease in apelin mRNA expression was observed after 4h, but not 2h, hypoxia and this was not accompanied by a change in HIF-1α mRNA expression. In zebrafish 5 dpf, exogenous pyr-apelin-13 did not affect the proportion of embryos with normal heart rate and circulation at any timepoint in this model. However, desferrioxamine (iron chelator) and α-ketoglutarate (metabolite involved in aerobic respiration) significantly increased the proportion of embryos with normal heart rate and circulation during the recovery phase. In summary, apelin-13 and apelin-16 were effective in lowering mean arterial pressure and heart rate, respectively, in the anaesthetised rat. However, apelin-13 did not vasodilate or vasoconstrict rat isolated mesenteric arteries. There was no effect of apelin on contractility parameters in rat isolated papillary muscles or in the isolated, perfused rat heart which made it difficult to pursue a cardioprotective effect in this model. In zebrafish, endogenous apelin appeared to be crucial in the development of the embryo, while exogenous apelin had no inotropic effect on cardiac function. In hypoxia-recovery, we demonstrate a cardioprotective effect of apelin in zebrafish 3 dpf, but not zebrafish 5 dpf.

Apelin ist ein Adipokin, das Einfluß auf die Glukosehomöostase hat und vermutlich eine wichtige Rolle in der Regulation von Adipositas und den damit assoziierten Erkrankungen einnimmt. Die Effekte von Apelin scheinen metabolisch günstig zu sein. In dieser Arbeit wurden zunächst Apelin-Serumkonzentrationen und metabolische Parameter bei 740 Studienteilnehmern bestimmt und in einer Querschnittsstudie (n=629) sowie in drei Interventionsstudien (n=111) dargestellt. In einer Subgruppe (n=161) wurde die mRNA-Expression von Apelin und dessen Rezeptor APJ im viszeralen und subkutanen Fettgewebe bei Patienten mit Typ-2-Diabetes genauer untersucht. Im Rahmen der Interventionsstuden wurde der Einfluß von 12 Wochen körperlichem Training (n=60), 6 Monaten hypokalorischer Mischkost (n=19) und bariatrischer Chirurgie (n=32) auf den Serum-Apelinspiegel sowie Zusammenhänge mit Gewichtsreduktion, verbesserter Insulinsensitivität und subklinischer Inflammation analysiert. Die höchsten Apelin-Serumkonzentrationen fanden sich beim adipösen Typ-2-Diabetiker. Die Apelin-Serumkonzentration korrelierte aber auch unabhängig vom Bodymassindex signifikant mit Parametern für Insulinresistenz und subklinischer Inflammation. Die Apelin-Expression war in den unterschiedlichen Fettgewebsdepots bei normal glukosetoleranten Patienten gleich, beim Typ-2-Diabetiker mit insgesamt höherer Expression überwog sie im viszeralen Fettgewebe. Nach allen Interventionsstudien kam es zur Abnahme der Apelin-Serumkonzentration und korrelierte auch dann signifikant mit einer verbesserten Insulinsensitivität, wenn es zu keiner Gewichtsreduktion kam. Die Apelinkonzentration im Serum sowie die Expression im Fettgewebe ist nicht nur vom Bodymassindex abhängig, sondern steht im direkten Zusammenhang mit Insulinsensitivität und inflammatorischen Prozessen. Die unterschiedliche fettdepotspezifische Regulation unterstreicht die pathogenetische Bedeutung eines „kranken“ viszeralen Fettgewebes in der Entwicklung von Typ-2-Diabetes, wobei Apelin als metabolisch günstiges Adipokin vermutlich eine kompensatorische Rolle einnimmt.

Apelin ist ein Adipokin, das Einfluß auf die Glukosehomöostase hat und vermutlich eine wichtige Rolle in der Regulation von Adipositas und den damit assoziierten Erkrankungen einnimmt. Die Effekte von Apelin scheinen metabolisch günstig zu sein. In dieser Arbeit wurden zunächst Apelin-Serumkonzentrationen und metabolische Parameter bei 740 Studienteilnehmern bestimmt und in einer Querschnittsstudie (n=629) sowie in drei Interventionsstudien (n=111) dargestellt. In einer Subgruppe (n=161) wurde die mRNA-Expression von Apelin und dessen Rezeptor APJ im viszeralen und subkutanen Fettgewebe bei Patienten mit Typ-2-Diabetes genauer untersucht. Im Rahmen der Interventionsstuden wurde der Einfluß von 12 Wochen körperlichem Training (n=60), 6 Monaten hypokalorischer Mischkost (n=19) und bariatrischer Chirurgie (n=32) auf den Serum-Apelinspiegel sowie Zusammenhänge mit Gewichtsreduktion, verbesserter Insulinsensitivität und subklinischer Inflammation analysiert. Die höchsten Apelin-Serumkonzentrationen fanden sich beim adipösen Typ-2-Diabetiker. Die Apelin-Serumkonzentration korrelierte aber auch unabhängig vom Bodymassindex signifikant mit Parametern für Insulinresistenz und subklinischer Inflammation. Die Apelin-Expression war in den unterschiedlichen Fettgewebsdepots bei normal glukosetoleranten Patienten gleich, beim Typ-2-Diabetiker mit insgesamt höherer Expression überwog sie im viszeralen Fettgewebe. Nach allen Interventionsstudien kam es zur Abnahme der Apelin-Serumkonzentration und korrelierte auch dann signifikant mit einer verbesserten Insulinsensitivität, wenn es zu keiner Gewichtsreduktion kam. Die Apelinkonzentration im Serum sowie die Expression im Fettgewebe ist nicht nur vom Bodymassindex abhängig, sondern steht im direkten Zusammenhang mit Insulinsensitivität und inflammatorischen Prozessen. Die unterschiedliche fettdepotspezifische Regulation unterstreicht die pathogenetische Bedeutung eines „kranken“ viszeralen Fettgewebes in der Entwicklung von Typ-2-Diabetes, wobei Apelin als metabolisch günstiges Adipokin vermutlich eine kompensatorische Rolle einnimmt.

The apelin-apelin receptor (APLNR) system is an important regulator of cardiovascular homeostasis both in health and disease. Principal actions of the apelin-APLNR system are positive inotropism, vasodilatation, diuresis and a potential anti-inflammatory role in vascular tissue. The significance of this system is highlighted in heart failure and pulmonary hypertension. Preclinical models of these diseases report downregulation of apelin- APLNR, whilst knockout strains develop more severe phenotypes, more rapidly. Moreover treatment with exogenous apelin retards or prevents disease progression. In man plasma apelin concentrations are reduced in heart failure and vary with disease severity. Initial increases are reported in mild heart failure suggesting a compensatory role, but are depressed in severe heart failure. Limited data profile myocardial APLNR expression in heart failure and in keeping with plasma apelin concentrations, expression is reduced in severe heart failure. Of interest, the APLNR most closely resembles the angiotensin II type 1 receptor (AT1R), sharing similar tissue expression and sequence homology, but mediates opposing physiological actions. Furthermore, emerging preclinical data support receptor interactions between the APLNR and AT1R that modify their native signalling pathways. It is likely that the apelin-APLNR system serves to antagonise the renin-angiotensin system. Given the established role of angiotensin II, arguably the most important peptide in cardiovascular pathophysiology, any system influencing its actions merits further investigation. Current clinical studies are limited to 20 minutes infusions and understanding its cardiovascular effects requires more prolonged administration. There are concerns of tachyphylaxis and interaction with the renin-angiotensin-aldosterone system (RAAS), possibly reducing efficacy of APLNR agonism in clinical settings. In a series of randomised, blinded crossover clinical trials 60 healthy volunteers and 20 patients with chronic stable heart failure were enrolled to assess the effects of (Pyr1)apelin-13 infusion at rest, during acute and subacute infusion, exercise and upregulation of the renin-angiotensin system. I have identified that APLNR agonism is unaffected by prevailing levels of angiotensin II activity in local vascular beds and systemic haemodynamic infusions. Furthermore, the efficacy of (Pyr1)apelin-13 is retained in healthy volunteers and patients with chronic stable heart failure during acute and subacute infusions. Finally, systemic (Pyr1)apelin-13 does not alter exercise performance in healthy individuals. My findings support a role in targeting the APLNR in chronic heart failure and predict that efficacy will be retained in chronic dosing. Future research directed at other patient groups with ventricular dysfunction is merited, in order to further characterise the utility of this system. These studies are encouraging; however, longer term studies may reveal effects beyond haemodynamic alterations and examine the effects on cardiac fibrosis and endothelial function. A long acting agonist is required to fully evaluate the role of APLNR signalling in cardiovascular disease.

Apelin, the endogenous ligand to the apelin receptor, is a small peptide involved with cardiovascular regulation. Using nuclear magnetic resonance (NMR) spectroscopy, I demonstrate that at low temperature, residues R6-L9 and G13-F17 of apelin are more structured than the rest of the peptide. I also study the interactions of apelin with sodium dodecylsulphate (SDS), dodecylphosphocholine (DPC) and 1-palmitoyl-2-hydroxy-sn- glycero-3-[phospho-RAC-(1-glycerol)] (LPPG) micelles. Apelin binds to SDS micelles through residues R6-L9, with structure being induced in this region as well as the C- terminus of the peptide. The binding to micelles along with the corresponding change in structure make it likely that apelin binds to the apelin receptor following the membrane catalysis hypothesis. NMR spectroscopy was used to determine the structure of the N- terminal tail and first transmembrane segment of the apelin receptor (AR55) in DPC micelles. AR55 has two disrupted helices from D14-K25 and from A29-K57. The second helix is the membrane spanning region of AR55 and has a significant kink located at N46. Mutagenesis of the apelin receptor and functional assays indicate that G42, G45 and N46 are essential for the proper trafficking and function of AR. In the N-terminal tail, the functionally critical residues E20 and D23 form an anionic face that could take part in initial binding of apelin to AR. The structure of AR55 was also determined in SDS micelles, LPPG micelles and a 1:1 water: 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) solution. Overall, the micelle spanning region of AR55 has a consistent structure with a kink near N46. The N-terminal tail of AR55 is more variable, having similar structures in the micelle conditions but being largely helical in 50% HFIP. NMR relaxation experiments indicate that the N-terminal tail of AR55 undergoes much more motion in LPPG micelles compared to SDS and DPC micelles. Finally, I created a program named MC-HELAN that characterizes the kinks that occur in protein helices. I used MC- HELAN to analyze all non-redundant membrane protein structures as of March 2010. Membrane protein helix kinks are remarkably common and diverse. Initial attempts to predict membrane protein kinks using only the protein sequence were unsuccessful.

The Apelin-APJ system is transiently up-regulated in murine models of cardiac dysfunction. We have previously shown that Apelin-deficient mice subjected to aortic constriction suffer from severe fibrosis. In turn, we hypothesized that Apelin deficiency will also exaggerate the fibrosis phenotype post experimental myocardial infarction, associated with changes in fibroblast cell activity. Apelin-deficient and wildtype mice were randomly subjected to sham operation or left coronary artery ligation. Apelin deficiency worsened cardiac functionality, enhanced fibrosis-related gene expression and morphology, and enhanced vimentin intermediate filament expression, which may be involved in increasing fibroblast proliferation. MicroRNA target prediction softwares predict that apelin and vimentin 3 ’UTRs are potential targets of microRNA-378 regulation, and were confirmed with Luciferase reporter assays and western blot. Apelin up-regulation may be a useful strategy for attenuating unfavorable fibrosis through down-regulating vimentin-mediated adverse fibroblast activity. MicroRNA-378 regulation may be partly responsible for changes in apelin and vimentin expression.

The Apelin-APJ system is transiently up-regulated in murine models of cardiac dysfunction. We have previously shown that Apelin-deficient mice subjected to aortic constriction suffer from severe fibrosis. In turn, we hypothesized that Apelin deficiency will also exaggerate the fibrosis phenotype post experimental myocardial infarction, associated with changes in fibroblast cell activity. Apelin-deficient and wildtype mice were randomly subjected to sham operation or left coronary artery ligation. Apelin deficiency worsened cardiac functionality, enhanced fibrosis-related gene expression and morphology, and enhanced vimentin intermediate filament expression, which may be involved in increasing fibroblast proliferation. MicroRNA target prediction softwares predict that apelin and vimentin 3 ’UTRs are potential targets of microRNA-378 regulation, and were confirmed with Luciferase reporter assays and western blot. Apelin up-regulation may be a useful strategy for attenuating unfavorable fibrosis through down-regulating vimentin-mediated adverse fibroblast activity. MicroRNA-378 regulation may be partly responsible for changes in apelin and vimentin expression.

Modulation of cardiac rate and contraction through calcium-dependent and independent means are of central import to the ability of an organism to adapt to its environment. Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent calcium-releasing second messenger across a broad range of tissues and organisms. In cardiac myocytes, NAADP is thought to stimulate calcium release from acidic stores which then bolsters filling and release during CICR. Questions remain: as to the potential need for amplification to generate the size of responses observed and the physiological role of the NAADP pathway. In contractile myocytes, photorelease of NAADP caused significant increase in calcium transient amplitude and velocity of transient upstroke and decay. Effects were absent during NAADP photorelease in the presence of Ned-19 or CaMKII inhibitors. Cellular calcium transient responses to &beta;-adrenergic stimulation were significantly reduced in the presence of inhibitors of the NAADP pathway. These data support the hypothesis that NAADP-induced calcium release is relevant during adrenergic stimulation and requires amplification through CaMKII. Rate modulation at the sino-atrial node can occur through the hyperpolarisation-activated current I_(f). Basal cardiac rate is a major determinant in cardiac mortality and compounds which specifically affect rate have clinical utility. A compound currently used to treat inflammatory conditions was found to have a significant rate-reducing effect in sino-atrial node preparations mediated by inhibition of I_(f). Apelin, an endogenous peptide, has been reported to potently generate improved contractility without development of hypertrophy. Study of its effects in single cells have provided conflicting information, at least in part because of the difficulty in working with the compound. A method for the consistent observation of apelin-mediated contractile responses is presented, focusing on the timecourse of cell contraction. These observations suggest a role for apelin in both inotropy and lusitropy and will enable further research.

The apelin system is an evolving transmitter system consisting of the G protein coupled apelin receptor and two endogenous peptide ligands, apelin and elabela. It is implicated as a potential therapeutic for a number of diseases; however, the endogenous peptides are limited by half-life and bioavailability. This study aims to identify and pharmacologically characterise apelin agonists in vitro and in vivo and to evaluate their therapeutic potential in pulmonary arterial hypertension as a model disease. CMF-019 was identified as the first G protein biased apelin agonist. To date, suitable small molecule apelin agonists as experimental tool compounds have been limited and CMF-019 represents an important advance. CMF-019 was active in vivo, producing an increase in cardiac contractility and vasodilatation, similar to apelin. These effects were achieved without receptor desensitisation, supporting the remarkable G protein bias observed in vitro. Furthermore, it was disease-modifying in vitro in an endothelial cell apoptosis assay but despite this, did not prevent pulmonary arterial hypertension in a monocrotaline rat model of the disease. An apelin mimetic peptide possessing an unnatural amino acid, MM202, conjugated chemically via a polyethylene glycol linker to an anti-serum domain antibody (AlbudAb) was also characterised. The product MM202-AlbudAb represents the first time an AlbudAb has been conjugated chemically to an unnatural peptide mimetic, providing protection from proteolysis and glomerular filtration. Importantly, it retained binding to albumin and demonstrated in vitro and in vivo activity at the apelin receptor. In conclusion, this thesis has identified and pharmacologically characterised two novel apelin agonists that possess significant advantages over the endogenous peptides. CMF-019 is suitable as an experimental tool compound and, as the first G protein biased small molecule, provides a starting point for more suitable therapeutics. In addition, MM202-AlbudAb proves that unnatural peptides can be conjugated to AlbudAb, supporting use of this technology in other small-peptide ligand transmitter systems.