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

A Systematic Approach to Identify Biased Agonists of the Apelin Receptor through High-Throughput Screening

Biased agonists are defined by their ability to selectively activate distinct signaling pathways of a receptor, and they hold enormous promise for the development of novel drugs that specifically elicit only the desired therapeutic response and avoid potential adverse effects. Unfortunately, most high-throughput screening (HTS) assays are designed to detect signaling of G protein-coupled receptors (GPCRs) downstream of either G protein or β-arrestin-mediated signaling but not both. A comprehensive drug discovery program seeking biased agonists must employ assays that report on the activity of each compound at multiple discrete pathways, particularly for HTS campaigns. Here, we report a systematic approach to the identification of biased agonists of human apelin receptor (APJ). We synthesized 448 modified versions of apelin and screened them against a cascade of cell-based assays, including intracellular cAMP and β-arrestin recruitment to APJ, simultaneously. The screen yielded potent and highly selective APJ agonists. Representative hits displaying preferential signaling via either G-protein or β-arrestin were subjected to a battery of confirmation assays. These biased agonists will be useful as tools to probe the function and pharmacology of APJ and provide proof of concept of our systematic approach to the discovery of biased ligands. This approach is likely universally applicable to the search for biased agonists of GPCRs.

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

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

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

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

Elabela/Toddler Is an Endogenous Agonist of the Apelin APJ Receptor in the Adult Cardiovascular System, and Exogenous Administration of the Peptide Compensates for the Downregulation of Its Expression in Pulmonary Arterial Hypertension

Supplemental Digital Content is available in the text.

Background:

Elabela/toddler (ELA) is a critical cardiac developmental peptide that acts through the G-protein–coupled apelin receptor, despite lack of sequence similarity to the established ligand apelin. Our aim was to investigate the receptor pharmacology, expression pattern, and in vivo function of ELA peptides in the adult cardiovascular system, to seek evidence for alteration in pulmonary arterial hypertension (PAH) in which apelin signaling is downregulated, and to demonstrate attenuation of PAH severity with exogenous administration of ELA in a rat model.

Methods:

In silico docking analysis, competition binding experiments, and downstream assays were used to characterize ELA receptor binding in human heart and signaling in cells expressing the apelin receptor. ELA expression in human cardiovascular tissues and plasma was determined using real-time quantitative polymerase chain reaction, dual-labeling immunofluorescent staining, and immunoassays. Acute cardiac effects of ELA-32 and [Pyr¹]apelin-13 were assessed by MRI and cardiac catheterization in anesthetized rats. Cardiopulmonary human and rat tissues from PAH patients and monocrotaline- and Sugen/hypoxia-exposed rats were used to show changes in ELA expression in PAH. The effect of ELA treatment on cardiopulmonary remodeling in PAH was investigated in the monocrotaline rat model.

Results:

ELA competed for binding of apelin in human heart with overlap for the 2 peptides indicated by in silico modeling. ELA activated G-protein– and β-arrestin–dependent pathways. We detected ELA expression in human vascular endothelium and plasma. Comparable to apelin, ELA increased cardiac contractility, ejection fraction, and cardiac output and elicited vasodilatation in rat in vivo. ELA expression was reduced in cardiopulmonary tissues from PAH patients and PAH rat models, respectively. ELA treatment significantly attenuated elevation of right ventricular systolic pressure and right ventricular hypertrophy and pulmonary vascular remodeling in monocrotaline-exposed rats.

Conclusions:

These results show that ELA is an endogenous agonist of the human apelin receptor, exhibits a cardiovascular profile comparable to apelin, and is downregulated in human disease and rodent PAH models, and exogenous peptide can reduce the severity of cardiopulmonary remodeling and function in PAH in rats. This study provides additional proof of principle that an apelin receptor agonist may be of therapeutic use in PAH in humans.

Apelin/APJ system: A novel promising therapy target for pathological angiogenesis

Apelin is the endogenous ligand of the G protein-coupled receptor APJ. Both Apelin and APJ receptor are widely distributed in various tissues such as heart, brain, limbs, retina and liver. Recent research indicates that the Apelin/APJ system plays an important role in pathological angiogenesis which is a progress of new blood branches developing from preexisting vessels via sprouting. In this paper, we review the important role of the Apelin/APJ system in pathological angiogenesis. The Apelin/APJ system promotes angiogenesis in myocardial infarction, ischemic stroke, critical limb ischemia, tumor, retinal angiogenesis diseases, cirrhosis, obesity, diabetes and other related diseases. Furthermore, we illustrate the detailed mechanism of pathological angiogenesis induced by the Apelin/APJ system. In conclusion, the Apelin/APJ system would be a promising therapeutic target for angiogenesis-related diseases.

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

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

Cardiac action of the first G protein biased small molecule apelin agonist

Apelin peptide analogues displaying bias towards G protein signalling pathways have beneficial cardiovascular actions compared with the native peptide in humans in vivo. Our aim was to determine whether small molecule agonists could retain G protein bias. We have identified a biased small molecule, CMF-019, and characterised it in vitro and in vivo. In competition radioligand binding experiments in heart homogenates, CMF-019 bound to the human, rat and mouse apelin receptor with high affinity (pKi=8.58±0.04, 8.49±0.04 and 8.71±0.06 respectively). In cell-based functional assays, whereas, CMF-019 showed similar potency for the Gαi pathway to the endogenous agonist [Pyr(1)]apelin-13 (pD2=10.00±0.13 vs 9.34±0.15), in β-arrestin and internalisation assays it was less potent (pD2=6.65±0.15 vs 8.65±0.10 and pD2=6.16±0.21 vs 9.28±0.10 respectively). Analysis of these data demonstrated a bias of ∼400 for the Gαi over the β-arrestin pathway and ∼6000 over receptor internalisation. CMF-019 was tested for in vivo activity using intravenous injections into anaesthetised male Sprague-Dawley rats fitted with a pressure-volume catheter in the left ventricle. CMF-019 caused a significant increase in cardiac contractility of 606±112mmHg/s (p<0.001) at 500nmol. CMF-019 is the first biased small molecule identified at the apelin receptor and increases cardiac contractility in vivo. We have demonstrated that Gαi over β-arrestin/internalisation bias can be retained in a non-peptide analogue and predict that such bias will have the therapeutic benefit following chronic use. CMF-019 is suitable as a tool compound and provides the basis for design of biased agonists with improved pharmacokinetics for treatment of cardiovascular conditions such as pulmonary arterial hypertension.

[A preliminary investigation of relationship between serum apelin level and pulmonary artery pressure in children with congenital heart disease]

OBJECTIVE

To preliminarily investigate the relationship between serum apelin level and pulmonary artery pressure in children with congenital heart disease.

METHODS

One hundred and twenty-six children with congenital heart disease undergoing surgical treatment were enrolled as subjects. The serum level of apelin was determined before surgery and at 7 days after surgery. The ratio of pulmonary artery systolic pressure to aortic systolic pressure (Pp/Ps) was calculated before extracorporeal circulation. According to the Pp/Ps value, patients were classified into non-pulmonary arterial hypertension (PAH) group, mild PAH group, moderate PAH group, and severe PAH group. Pulmonary artery mean pressure was estimated by echocardiography at 7 days after surgery.

RESULTS

The non-PAH group had the highest serum level of apelin before and after surgery, followed by the mild PAH group, moderate PAH group, and severe PAH group (P<0.05). All groups had significantly increased serum levels of apelin at 7 days after surgery (P<0.05). The serum level of apelin was negatively correlated with pulmonary artery pressure before surgery (r=-0.51, P<0.05) and at 7 days after surgery (r=-0.54, P<0.05).

CONCLUSIONS

The decrease in serum apelin level is associated with the development of pulmonary hypertension in children with congenital heart disease. The significance of serum apelin in predicting the development and degree of pulmonary hypertension in children with congenital heart disease deserves further studies.

[A preliminary investigation of relationship between serum apelin level and pulmonary artery pressure in children with congenital heart disease]

OBJECTIVE

To preliminarily investigate the relationship between serum apelin level and pulmonary artery pressure in children with congenital heart disease.

METHODS

One hundred and twenty-six children with congenital heart disease undergoing surgical treatment were enrolled as subjects. The serum level of apelin was determined before surgery and at 7 days after surgery. The ratio of pulmonary artery systolic pressure to aortic systolic pressure (Pp/Ps) was calculated before extracorporeal circulation. According to the Pp/Ps value, patients were classified into non-pulmonary arterial hypertension (PAH) group, mild PAH group, moderate PAH group, and severe PAH group. Pulmonary artery mean pressure was estimated by echocardiography at 7 days after surgery.

RESULTS

The non-PAH group had the highest serum level of apelin before and after surgery, followed by the mild PAH group, moderate PAH group, and severe PAH group (P<0.05). All groups had significantly increased serum levels of apelin at 7 days after surgery (P<0.05). The serum level of apelin was negatively correlated with pulmonary artery pressure before surgery (r=-0.51, P<0.05) and at 7 days after surgery (r=-0.54, P<0.05).

CONCLUSIONS

The decrease in serum apelin level is associated with the development of pulmonary hypertension in children with congenital heart disease. The significance of serum apelin in predicting the development and degree of pulmonary hypertension in children with congenital heart disease deserves further studies.

In silico Therapeutics for Neurogenic Hypertension and Vasovagal Syncope

Neurocardiovascular diseases (NCVD) are the leading cause of death in the developed world and will remain so till 2020. In these diseases the pathologically changed nervous control of cardiovascular system has the central role. The actual NCV syndromes are neurogenic hypertension, representing the sympathetically mediated disorder, and vasovagal syncope, which is the vagally mediated disorders. Vasovagal syncope, the disease far from its etiological treatment, could benefit from recruiting and application of antimuscarinic drugs used in other parasympathetic disorders. The informational spectrum method (ISM), a method widely applied for the characterization of protein-protein interactions in the field of immunology, endocrinology and anti HIV drug discovery, was applied for the first time in the analysis of neurogenic hypertension and vasovagal syncope therapeutic targets. In silico analysis revealed the potential involvement of apelin in neurogenic hypertension. Applying the EIIP/ISM bioinformatics concept in investigation of drugs for treatment of vasovagal syncope suggests that 78% of tested antimuscarinic drugs could have anti vasovagal syncope effect. The presented results confirm that ISM is a promissing method for investigation of molecular mechanisms underlying pathophysiological proceses of NCV syndromes and discovery of therapeutics targets for their treatment.