Structural Insight into G-Protein Coupled Receptor Binding by Apelin

Discovery and Structure-Activity Relationship of a Bioactive Fragment of ELABELA that Modulates Vascular and Cardiac Functions

ELABELA (ELA) was recently discovered as a novel endogenous ligand of the apelin receptor (APJ), a G protein-coupled receptor. ELA signaling was demonstrated to be crucial for normal heart and vasculature development during embryogenesis. We delineate here ELA's structure-activity relationships and report the identification of analogue 3 (ELA(19-32)), a fragment of ELA that binds to APJ, activates the Gαi1 and β-arrestin-2 signaling pathways, and induces receptor internalization similarly to its parent endogenous peptide. An alanine scan performed on 3 revealed that the C-terminal residues are critical for binding to APJ and signaling. Finally, using isolated-perfused hearts and in vivo hemodynamic and echocardiographic measurements, we demonstrate that ELA and 3 both reduce arterial pressure and exert positive inotropic effects on the heart. Altogether, these results present ELA and 3 as potential therapeutic options in managing cardiovascular diseases.

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

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

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

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

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

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

Design, characterization, and first-in-human study of the vascular actions of a novel biased apelin receptor agonist

Supplemental Digital Content is available in the text.

[Pyr¹]apelin-13 is an endogenous vasodilator and inotrope but is downregulated in pulmonary hypertension and heart failure, making the apelin receptor an attractive therapeutic target. Agonists acting at the same G-protein–coupled receptor can be engineered to stabilize different conformational states and function as biased ligands, selectively stimulating either G-protein or β-arrestin pathways. We used molecular dynamics simulations of apelin/receptor interactions to design cyclic analogues and identified MM07 as a biased agonist. In β-arrestin and internalization assays (G-protein–independent), MM07 was 2 orders of magnitude less potent than [Pyr¹]apelin-13. In a G-protein–dependent saphenous vein contraction assay, both peptides had comparable potency (pD₂:[Pyr¹]apelin-13 9.93±0.24; MM07 9.54±0.42) and maximum responses with a resulting bias for MM07 of ≈350- to 1300-fold for the G-protein pathway. In rats, systemic infusions of MM07 (10-100nmol) caused a dose-dependent increase in cardiac output that was significantly greater than the response to [Pyr¹]apelin-13. Similarly, in human volunteers, MM07 produced a significant dose-dependent increase in forearm blood flow with a maximum dilatation double that is seen with [Pyr¹]apelin-13. Additionally, repeated doses of MM07 produced reproducible increases in forearm blood flow. These responses are consistent with a more efficacious action of the biased agonist. In human hand vein, both peptides reversed an established norepinephrine constrictor response and significantly increased venous flow. Our results suggest that MM07 acting as a biased agonist at the apelin receptor can preferentially stimulate the G-protein pathway, which could translate to improved efficacy in the clinic by selectively stimulating vasodilatation and inotropic actions but avoiding activating detrimental β-arrestin–dependent pathways.

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.

Preferential apelin-13 production by the proprotein convertase PCSK3 is implicated in obesity

The peptide hormone apelin is translated as a 77-residue preproprotein, truncated to the 55-residue proapelin and, subsequently, to 13–36-residue bioactive isoforms named apelin-13 to -36. Proapelin is hypothesized to be cleaved to apelin-36 and then to the shorter isoforms. However, neither the mechanism of proapelin processing nor the endoproteases involved have been determined. We show direct cleavage of proapelin to apelin-13 by proprotein convertase subtilisin/kexin 3 (PCSK3, or furin) in vitro, with no production of longer isoforms. Conversely, neither PCSK1 nor PCSK7 has appreciable proapelin cleavage activity. Furthermore, we show that both proapelin and PCSK3 transcript expression levels are increased in adipose tissue with obesity and during adipogenesis, suggesting that PCSK3 is responsible for proapelin processing in adipose tissue.

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No prohormone processing mechanisms have yet been shown for apelin.

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Proapelin is cleaved into apelin-13 both specifically and preferentially by PCSK3.

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Conversely, PCSK1 and PCSK7 do not cleave proapelin.

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PCSK3 and apelin expression increase with adipocyte differentiation and obesity.

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We show the first evidence of and propose a new theory for apelin bioactivation.

Anopheles gambiae, Anoga-HrTH hormone, free and bound structure--a nuclear magnetic resonance experiment

The spread of malaria by the female mosquito, Anopheles gambiae, is dependent, amongst other things, on its ability to fly. This in turn, is dependent on the adipokinetic hormone, Anoga-HrTH (pGlu-Leu-Thr-Phe-Thr-Pro-Ala-Trp-NH2). No crystal structure of this important neuropeptide is available and hence NMR restrained molecular dynamics was used to investigate its conformational space in aqueous solution and when bound to a membrane surface. The results showed that Anoga-HrTH has an almost cyclic conformation that is stabilized by a hydrogen bond between the C-terminus and Thr3. Upon docking of the agonist to its receptor, this H-bond is broken and the molecule adopts a more extended structure. Preliminary AKHR docking calculations give the free energy of binding to be -47.30 kJ/mol. There is a close correspondence between the structure of the docked ligand and literature structure-activity studies. Information about the 3D structure and binding mode of Anoga-HrTH to its receptor is vital for the design of suitable mimetics which can act as insecticides.

Structural features of the apelin receptor N-terminal tail and first transmembrane segment implicated in ligand binding and receptor trafficking

G-protein coupled receptors (GPCRs) comprise a large family of membrane proteins with rich functional diversity. Signaling through the apelin receptor (AR or APJ) influences the cardiovascular system, central nervous system and glucose regulation. Pathophysiological involvement of apelin has been shown in atherosclerosis, cancer, human immunodeficiency virus-1 (HIV-1) infection and obesity. Here, we present the high-resolution nuclear magnetic resonance (NMR) spectroscopy-based structure of the N-terminus and first transmembrane (TM) segment of AR (residues 1-55, AR55) in dodecylphosphocholine micelles. AR55 consists of two disrupted helices, spanning residues D14-K25 and A29-R55(1.59). Molecular dynamics (MD) simulations of AR built from a hybrid of experimental NMR and homology model-based restraints allowed validation of the AR55 structure in the context of the full-length receptor in a hydrated bilayer. AR55 structural features were functionally probed using mutagenesis in full-length AR through monitoring of apelin-induced extracellular signal-regulated kinase (ERK) phosphorylation in transiently transfected human embryonic kidney (HEK) 293A cells. Residues E20 and D23 form an extracellular anionic face and interact with lipid headgroups during MD simulations in the absence of ligand, producing an ideal binding site for a cationic apelin ligand proximal to the membrane-water interface, lending credence to membrane-catalyzed apelin-AR binding. In the TM region of AR55, N46(1.50) is central to a disruption in helical character. G42(1.46), G45(1.49) and N46(1.50), which are all involved in the TM helical disruption, are essential for proper trafficking of AR. In summary, we introduce a new correlative NMR spectroscopy and computational biochemistry methodology and demonstrate its utility in providing some of the first high-resolution structural information for a peptide-activated GPCR TM domain.

Functional and structural features of adipokine family

In the mid-1990s, the interest in adipose tissue was revived by the discovery of leptin. Since then numerous other hormones have been isolated from white adipose tissue that has no longer considered an inert tissue mainly devoted to energy storage but emerged as an active participant in regulating physiologic and pathologic processes, including immunity and inflammation. Adipose tissue produces and releases a variety of proinflammatory and anti-inflammatory factors, including the adipokines, as well as cytokines and chemokines. Proinflammatory molecules produced by adipose tissue have been implicated as active participants in the development of insulin resistance and the increased risk of cardiovascular disease associated with obesity. In contrast, reduced leptin levels might predispose to increased susceptibility to infection caused by reduced T-cell responses in malnourished individuals. Altered adipokine levels have been observed in a variety of inflammatory conditions, although their pathogenic role has not been completely clarified. In this paper we want to review: (i) the role of adipose tissue in different biological processes, (ii) the functional and structural description of all the known adipokines subdivided in different subfamilies, (iii) the adipokine involvement in obesity and cancers, and (iv) the adipokine interactome.

Neuroprotection of apelin and its signaling pathway

Apelin was initially isolated from bovine stomach and is an endogenous neuropeptide. It is a native ligand of the apelin receptor (APJ). Some research has found that apelin peptides alter blood pressure, feeding behavior, and pituitary hormone release. However, a new neuroprotective effect of apelin peptides was only recently discovered. This review summarizes the evidence of apelin-neuroprotection, which has the potential to cure acute and chronic neurological diseases.

Elucidation of the structure-activity relationships of apelin: influence of unnatural amino acids on binding, signaling, and plasma stability

Apelin is the endogenous ligand of the APJ receptor, a member of the G-protein-coupled receptor family. The apelin-APJ complex has been detected in many tissues and is emerging as a promising target for several pathophysiological conditions. There is currently little information on the structure-activity relationship (SAR) of the apelin hormone. In an effort to better delineate SAR, we synthesized analogues of apelin-13 modified at selected positions with unnatural amino acids, with a particular emphasis on the C-terminal portion. Analogues were then tested in binding and functional assays by evaluating Gi/o-mediated decreases in cAMP levels and by assessing β-arrestin2 recruitment to the APJ receptor. The plasma stability of new compounds was also assessed. Several analogues were found to possess increased binding and higher stability than the parent peptide.

Biophysical characterization of G-protein coupled receptor-peptide ligand binding

G-protein coupled receptors (GPCRs) are ubiquitous membrane proteins allowing intracellular responses to extracellular factors that range from photons of light to small molecules to proteins. Despite extensive exploitation of GPCRs as therapeutic targets, biophysical characterization of GPCR-ligand interactions remains challenging. In this minireview, we focus on techniques that have been successfully used for structural and biophysical characterization of peptide ligands binding to their cognate GPCRs. The techniques reviewed include solution-state nuclear magnetic resonance (NMR) spectroscopy, solid-state NMR, X-ray diffraction, fluorescence spectroscopy and single-molecule fluorescence methods, flow cytometry, surface plasmon resonance, isothermal titration calorimetry, and atomic force microscopy. The goal herein is to provide a cohesive starting point to allow selection of techniques appropriate to the elucidation of a given GPCR-peptide interaction.

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.

Discovery of a competitive apelin receptor (APJ) antagonist

The apelin receptor (APJ) is a class A G-protein-coupled receptor (GPCR) and is a putative target for the treatment of cardiovascular and metabolic diseases. Apelin-13 (NH₂-QRPRLSHKGPMPF-COOH) is a vasoactive peptide and one of the most potent endogenous inotropic agents identified to date. We report the design and discovery of a novel APJ antagonist. By using a bivalent ligand approach, we have designed compounds with two 'affinity' motifs and a short series of linker groups with different conformational and non-bonded interaction properties. One of these, cyclo(1-6)CRPRLC-KH-cyclo(9-14)CRPRLC is a competitive antagonist at APJ. Radioligand binding in CHO cells transfected with human APJ gave a K(i) value of 82 nM, competition binding in human left ventricle gave a K(D) value of 3.2 μM, and cAMP accumulation assays in CHO-K1-APJ cells gave a K(D) value of 1.32 μM.

Membrane catalysis of peptide-receptor binding

The membrane catalysis hypothesis states that a peptide ligand activates its target receptor after an initial interaction with the surrounding membrane. Upon membrane binding and interaction, the ligand is structured such that receptor binding and activation is encouraged. As evidence for this hypothesis, there are numerous studies concerning the conformation that peptides adopt in membrane mimetic environments. This mini-review analyzes the features of ligand peptides with an available high-resolution membrane-induced structure and a characterized membrane-binding region. At the peptide-membrane interface, both amphipathic helices and turn structures are commonly formed in peptide ligands and both hydrophobic and electrostatic interactions can be responsible for membrane binding. Apelin is the ligand to the G-protein coupled receptor (GPCR) named APJ, with various important physiological effects, which we have recently characterized both in solution and bound to anionic micelles. The structural changes that apelin undergoes when binding to micelles provide strong evidence for membrane catalysis of apelin-APJ interactions.

The predictive accuracy of secondary chemical shifts is more affected by protein secondary structure than solvent environment

Biomolecular NMR spectroscopy frequently employs estimates of protein secondary structure using secondary chemical shift (Deltadelta) values, measured as the difference between experimental and random coil chemical shifts (RCCS). Most published random coil data have been determined in aqueous conditions, reasonable for non-membrane proteins, but potentially less relevant for membrane proteins. Two new RCCS sets are presented here, determined in dimethyl sulfoxide (DMSO) and chloroform:methanol:water (4:4:1 by volume) at 298 K. A web-based program, CS-CHEMeleon, has been implemented to determine the accuracy of secondary structure assessment by calculating and comparing Deltadelta values for various RCCS datasets. Using CS-CHEMeleon, Deltadelta predicted versus experimentally determined secondary structures were compared for large datasets of membrane and non-membrane proteins as a function of RCCS dataset, Deltadelta threshold, nucleus, localized parameter averaging and secondary structure type. Optimized Deltadelta thresholds are presented both for published and for the DMSO and chloroform:methanol:water derived RCCS tables. Despite obvious RCCS variations between datasets, prediction of secondary structure was consistently similar. Strikingly, predictive accuracy seems to be most dependent upon the type of secondary structure, with helices being the most accurately predicted by Deltadelta values using five different RCCS tables. We suggest caution when using Deltadelta-based restraints in structure calculations as the underlying dataset may be biased. Comparative assessment of multiple RCCS datasets should be performed, and resulting Deltadelta-based restraints weighted appropriately relative to other experimental restraints.

Peptide T exhibits a well-defined structure in fluorinated solvent at low temperature

The structure of Peptide T was determined by solution NMR spectroscopy, under strong structure-inducing conditions: 40% hexafluoro-2-propanol aqueous solution at 5 degrees C. Under these conditions it was possible to detect medium-range NOEs for the first time for this peptide. This allowed a much better-defined structure to be determined for Peptide T in comparison with earlier NMR and computational studies. Peptide structures consistent with the experimental restraints were generated using a restrained MD simulation with a full empirical force field. Residues 4-8 of Peptide T take on a well-defined structure with a heavy atom RMSD of 0.78 A. The structure is stabilized by hydrogen bonding to side-chain oxygen atoms of Thr 4 and Thr 8, as well as backbone hydrogen bonding between residues 5 and 7 that forms this region into a classic gamma-turn.

Apelin in goldfish (Carassius auratus): cloning, distribution and role in appetite regulation

Apelin is a recently discovered peptide produced by several tissues including brain and adipose tissue. In mammals and zebrafish, apelin regulates cardiovascular functions. Recent evidence in mammals suggest that apelin might also regulate food intake. In this study, we cloned a cDNA encoding apelin and examined apelin mRNA distribution within the brain and in peripheral tissues. We also assessed the effects of fasting on apelin brain mRNA abundance. Apelin mRNA was expressed throughout the brain as well as in several peripheral tissues including brain, spleen, heart and fat. Apelin mRNA abundance in both hypothalamus and telencephalon was significant higher in fasted fish than in fed fish. In order to further characterize apelin in goldfish, we assessed the effects of central (intracerebroventricular, icv) and peripheral (intraperitoneal, ip) injections of apelin-13 on food intake in goldfish. Apelin injected ip at a dose of 100ng/g or icv at a dose of 10ng/g induced a significant increase in food intake compared to saline-injected fish. Our results suggest that apelin acts as an orexigenic factor in goldfish. Its widespread distribution in the brain and the periphery also suggests that apelin might have multiple physiological regulating roles in fish.

Headgroup-dependent membrane catalysis of apelin-receptor interactions is likely

Apelin is the peptidic ligand for the G-protein-coupled receptor APJ. The apelin-APJ system is important in cardiovascular regulation, fluid homeostasis, and angiogenesis, among other roles. In this study, we investigate interactions between apelin and membrane-mimetic micelles of the detergents sodium dodecyl sulfate (SDS), dodecylphosphocholine (DPC), and 1-palmitoyl-2-hydroxy-sn-glycero-3-[phospho-rac-(1-glycerol)] (LPPG). Far-ultraviolet circular dichroism spectropolarimetry and diffusion-ordered spectroscopy indicate that apelin peptides bind to micelles of the anionic detergents SDS and LPPG much more favorably than to zwitterionic DPC micelles. Nuclear magnetic resonance spectroscopy allowed full characterization of the interactions of apelin-17 with SDS micelles. Titration with paramagnetic agents and structural determination of apelin-17 in SDS indicate that R6-K12 is highly structured, with R6-L9 directly interacting with headgroups of the micelle. Type I beta-turns are initiated between R6 and L9, and a well-defined type IV beta-turn is initiated at S10. Furthermore, binding of apelin-17 to SDS micelles causes structuring of M15-F17, with no evidence for direct binding of this region to the micelles. These results are placed into the context of the membrane catalysis hypothesis for peptide-receptor binding, and a hypothetical mechanism of APJ binding and activation by apelin is advanced.