Chemical Probe Identification Platform for Orphan GPCRs Using Focused Compound Screening: GPR39 as a Case Example

GPR39: An orphan receptor begging for ligands

Progress in the understanding of the receptor GPR39 is held up by inconsistent pharmacological data. First, the endogenous ligand(s) remain(s) contentious. Data pointing to zinc ions (Zn2+) and/or eicosanoids as endogenous ligands are a matter of debate. Second, there are uncertainties in the specificity of the widely used synthetic ligand (agonist) TC-G 1008. Third, activation of GPR39 has been often proposed as a novel treatment strategy, but new data also support that inhibition might be beneficial in certain disease contexts. Constitutive activity/promiscuous signaling suggests the need for antagonists/inverse agonists in addition to (biased) agonists. Here, we scrutinize data on the signaling and functions of GPR39 and critically assess factors that might have contributed to divergent outcomes and interpretations of investigations on this important receptor.

Synthesis and SAR of novel GPR39 Agonists and Positive Allosteric Modulators

We report a significant decrease in transcription of the G protein-coupled receptor GPR39 in striatal neurons of Parkinson's disease patients compared to healthy controls, suggesting that a positive modulator of GPR39 may beneficially impact neuroprotection. To test this notion, we developed various structurally diverse tool molecules. While we elaborated on previously reported starting points, we also performed an in silico screen which led to completely novel pharmacophores. In vitro studies indicated that GPR39 agonism does not have a profound effect on neuroprotection.

The Zinc-Sensing Receptor GPR39 in Physiology and as a Pharmacological Target

The G-protein coupled receptor GPR39 is abundantly expressed in various tissues and can be activated by changes in extracellular Zn²⁺ in physiological concentrations. Previously, genetically modified rodent models have been able to shed some light on the physiological functions of GPR39, and more recently the utilization of novel synthetic agonists has led to the unraveling of several new functions in the variety of tissues GPR39 is expressed. Indeed, GPR39 seems to be involved in many important metabolic and endocrine functions, but also to play a part in inflammation, cardiovascular diseases, saliva secretion, bone formation, male fertility, addictive and depression disorders and cancer. These new discoveries offer opportunities for the development of novel therapeutic approaches against many diseases where efficient therapeutics are still lacking. This review focuses on Zn²⁺ as an endogenous ligand as well as on the novel synthetic agonists of GPR39, placing special emphasis on the recently discovered physiological functions and discusses their pharmacological potential.

Zinc-induced activation of GPR39 regulates glucose homeostasis through glucose-dependent insulinotropic polypeptide secretion from enteroendocrine K-cells

The role of Zn²⁺-sensing receptor GPR39 on glucose homeostasis and incretin regulation was assessed in enteroendocrine L- and K-cells. Anti-hyperglycaemic, insulinotropic and incretin secreting properties of Zn²⁺ were explored in normal, diabetic and incretin receptor knockout mice. Compared to intraperitoneal injection, oral administration of Zn²⁺ (50 μmol/kg body weight) with glucose (18 mmol/kg) in lean mice reduced the glycaemic excursion by 25-34% (p < 0.05-p < 0.001) and enhanced glucose-induced insulin release by 46-48% (p < 0.05-p < 0.01). In diabetic mice, orally administered Zn²⁺ lowered glucose by 24-31% (p < 0.01) and augmented insulin release by 32% (p < 0.01). In glucagon like peptide-1 (GLP-1) receptor knockout mice, Zn²⁺ reduced glucose by 15-28% (p < 0.05-p < 0.01) and increased insulin release by 35-43% (p < 0.01). In contrast Zn²⁺ had no effect on responses of glucose-dependent insulinotropic polypeptide (GIP) receptor knockout mice. Consistent with this, Zn²⁺ had no effect on circulating total GLP-1 whereas GIP release was stimulated by 26% (p < 0.05) in lean mice. Immunocytochemistry demonstrated GPR39 expression on mouse enteroendocrine L- and K-cells, GLUTag cells and pGIP/Neo STC-1 cells. Zn²⁺ had a direct effect on GIP secretion from pGIPneo STC-1 cells, increasing GIP secretion by 1.3-fold. GPR39 is expressed on intestinal L- and K-cells, and stimulated GIP secretion plays an integral role in mediating enhanced insulin secretion and glucose tolerance following oral administration of Zn²⁺. This suggests development of potent and selective GPR39 agonists as a therapeutic approach for diabetes.

Rho kinase-dependent desensitization of GPR39; a unique mechanism of GPCR downregulation

GPR39, a G-protein-coupled receptor activated by zinc, reportedly activates multiple intracellular signaling pathways via Gs, Gq, G12/13, and β-arrestin, but little is known about downregulation of the receptor upon its activation. To our knowledge, this is the first report on the mechanism of feedback regulation of GPR39 function determined in GPR39-expressing HEK293 cells (HEK293-GPR39) as a model cell system. In HEK293-GPR39 cells, GPR39-C3, which is a positive allosteric modulator, activated cAMP production (downstream of Gs), IP1 accumulation (downstream of Gq), SRF-RE-dependent transcription (downstream of G12/13), and β-arrestin recruitment. GPR39-C3 induced dose- and time-dependent loss of response in cAMP production by second challenge of the compound. This functional desensitization was blocked by the Rho kinase (ROCK) inhibitor, Y-27632, but not by Gq or Gs-pathway inhibitors or inhibition of β-arrestin recruitment. In the receptor localization assay, GPR39-C3 induced internalization of GFP-tagged GPR39. This internalization was also inhibited by Y-27632, which suggested that ROCK activation is critical for internalization and desensitization of GPR39. A novel biased GPR39 positive allosteric modulator, 5-[2-[(2,4-dichlorophenyl)methoxy]phenyl]-2,2-dimethyl-1,3,5,6-tetrahydrobenzo[a]phenanthridin-4-one (GSB-118), which activated cAMP responses and β-arrestin recruitment but showed no effect on SRF-RE-dependent transcription, did not induce desensitization. These results revealed a unique mechanism of desensitization of GPR39.

Neuro-psychopharmacological perspective of Orphan receptors of Rhodopsin (class A) family of G protein-coupled receptors

BACKGROUND

In the central nervous system (CNS), G protein-coupled receptors (GPCRs) are the most fruitful targets for neuropsychopharmacological drug development. Rhodopsin (class A) is the most studied class of GPCR and includes orphan receptors for which the endogenous ligand is not known or is unclear. Characterization of orphan GPCRs has proven to be challenging, and the production pace of GPCR-based drugs has been incredibly slow.

OBJECTIVE

Determination of the functions of these receptors may provide unexpected insight into physiological and neuropathological processes. Advances in various methods and techniques to investigate orphan receptors including in situ hybridization and knockdown/knockout (KD/KO) showed extensive expression of these receptors in the mammalian brain and unmasked their physiological and neuropathological roles. Due to these rapid progress and development, orphan GPCRs are rising as a new and promising class of drug targets for neurodegenerative diseases and psychiatric disorders.

CONCLUSION

This review presents a neuropsychopharmacological perspective of 26 orphan receptors of rhodopsin (class A) family, namely GPR3, GPR6, GPR12, GPR17, GPR26, GPR35, GPR39, GPR48, GPR49, GPR50, GPR52, GPR55, GPR61, GPR62, GPR63, GPR68, GPR75, GPR78, GPR83, GPR84, GPR85, GPR88, GPR153, GPR162, GPR171, and TAAR6. We discussed the expression of these receptors in mammalian brain and their physiological roles. Furthermore, we have briefly highlighted their roles in neurodegenerative diseases and psychiatric disorders including Alzheimer's disease, Parkinson's disease, neuroinflammation, inflammatory pain, bipolar and schizophrenic disorders, epilepsy, anxiety, and depression.

Model-Based Discovery of Synthetic Agonists for the Zn2+-Sensing G-Protein-Coupled Receptor 39 (GPR39) Reveals Novel Biological Functions

The G-protein-coupled receptor 39 (GPR39) is a G-protein-coupled receptor activated by Zn²⁺. We used a homology model-based approach to identify small-molecule pharmacological tool compounds for the receptor. The method focused on a putative binding site in GPR39 for synthetic ligands and knowledge of ligand binding to other receptors with similar binding pockets to select iterative series of minilibraries. These libraries were cherry-picked from all commercially available synthetic compounds. A total of only 520 compounds were tested in vitro, making this method broadly applicable for tool compound development. The compounds of the initial library were inactive when tested alone, but lead compounds were identified using Zn²⁺ as an allosteric enhancer. Highly selective, highly potent Zn²⁺-independent GPR39 agonists were found in subsequent minilibraries. These agonists identified GPR39 as a novel regulator of gastric somatostatin secretion.

Discovery and Characterization of Novel GPR39 Agonists Allosterically Modulated by Zinc

In this study, we identified two previously described kinase inhibitors-3-(4-chloro-2-fluorobenzyl)-2-methyl-N-(3-methyl-1H-pyrazol-5-yl)-8-(morpholinomethyl)imidazo[1,2-b]pyridazin-6-amine (LY2784544) and 1H-benzimidazole-4-carboxylic acid, 2-methyl-1-[[2-methyl-3-(trifluoromethyl)phenyl]methyl]-6-(4-morpholinyl)- (GSK2636771)-as novel GPR39 agonists by unbiased small-molecule-based screening using a β-arrestin recruitment screening approach (PRESTO-Tango). We characterized the signaling of LY2784544 and GSK2636771 and compared their signaling patterns with a previously described "GPR39-selective" agonist N-[3-chloro-4-[[[2-(methylamino)-6-(2-pyridinyl)-4- pyrimidinyl]amino]methyl]phenyl]methanesulfonamide (GPR39-C3) at both canonical and noncanonical signaling pathways. Unexpectedly, all three compounds displayed probe-dependent and pathway-dependent allosteric modulation by concentrations of zinc reported to be physiologic. LY2784544 and GS2636771 at GPR39 in the presence of zinc were generally as potent or more potent than their reported activities against kinases in whole-cell assays. These findings reveal an unexpected role of zinc as an allosteric potentiator of small-molecule-induced activation of GPR39 and expand the list of potential kinase off-targets to include understudied G protein-coupled receptors.

Potential antidepressant-like properties of the TC G-1008, a GPR39 (zinc receptor) agonist

Some forms of depression appear to be more related to the glutamatergic system. G-coupled protein receptor 39 (GPR39) is the metabotropic zinc receptor, which may be involved in the pathophysiology of depression and in the antidepressant response. Its deficiency abolishes the antidepressant response, which means that GPR39 is required to obtain a therapeutic effect in depression. This raises the possibility that agonists of the zinc receptor may have a role in antidepressant treatment. To explore this possibility we investigated animal behaviour in the forced swim test, the tail suspension test (to assess antidepressant-like properties), the light/dark test and the elevated plus maze test (to assess anxiolytic-like properties), following acute administration of a GPR39 agonist (TC G-1008). We found an antidepressant response (as measured by the forced swim test but not by the tail suspension test) in mice following the GPR39 agonist treatment. Additionally, we observed the opposite results in the light/dark box (decreased overall distance; increased time spent in the lit compartment; decreased time spent in the dark compartment; increased freezing time) and elevated plus maze (no significant changes), which may be a consequence of the sedative effect of TC G-1008. We also found hippocampal GPR39 and brain-derived neurotrophic factor (BDNF) up-regulation following administration of the GPR39 agonist, which may be undiscovered so far as a possible novel agent in the treatment of mood disorders.

Novel Zn2+ Modulated GPR39 Receptor Agonists Do Not Drive Acute Insulin Secretion in Rodents

Type 2 diabetes (T2D) occurs when there is insufficient insulin release to control blood glucose, due to insulin resistance and impaired β-cell function. The GPR39 receptor is expressed in metabolic tissues including pancreatic β-cells and has been proposed as a T2D target. Specifically, GPR39 agonists might improve β-cell function leading to more adequate and sustained insulin release and glucose control. The present study aimed to test the hypothesis that GPR39 agonism would improve glucose stimulated insulin secretion in vivo. A high throughput screen, followed by a medicinal chemistry program, identified three novel potent Zn²⁺ modulated GPR39 agonists. These agonists were evaluated in acute rodent glucose tolerance tests. The results showed a lack of glucose lowering and insulinotropic effects not only in lean mice, but also in diet-induced obese (DIO) mice and Zucker fatty rats. It is concluded that Zn²⁺ modulated GPR39 agonists do not acutely stimulate insulin release in rodents.

Pharmacology and physiology of gastrointestinal enteroendocrine cells

Gastrointestinal (GI) polypeptides are secreted from enteroendocrine cells (EECs). Recent technical advances and the identification of endogenous and synthetic ligands have enabled exploration of the pharmacology and physiology of EECs. Enteroendocrine signaling pathways stimulating hormone secretion involve multiple nutrient transporters and G protein-coupled receptors (GPCRs), which are activated simultaneously under prevailing nutrient conditions in the intestine following a meal. The majority of studies investigate hormone secretion from EECs in response to single ligands and although the mechanisms behind how individual signaling pathways generate a hormonal output have been well characterized, our understanding of how these signaling pathways converge to generate a single hormone secretory response is still in its infancy. However, a picture is beginning to emerge of how nutrients and full, partial, or allosteric GPCR ligands differentially regulate the enteroendocrine system and its interaction with the enteric and central nervous system. So far, activation of multiple pathways underlies drug discovery efforts to harness the therapeutic potential of the enteroendocrine system to mimic the phenotypic changes observed in patients who have undergone Roux-en-Y gastric surgery. Typically obese patients exhibit ∼30% weight loss and greater than 80% of obese diabetics show remission of diabetes. Targeting combinations of enteroendocrine signaling pathways that work synergistically may manifest with significant, differentiated EEC secretory efficacy. Furthermore, allosteric modulators with their increased selectivity, self-limiting activity, and structural novelty may translate into more promising enteroendocrine drugs. Together with the potential to bias enteroendocrine GPCR signaling and/or to activate multiple divergent signaling pathways highlights the considerable range of therapeutic possibilities available. Here, we review the pharmacology and physiology of the EEC system.

GPR39 Zn(2+)-sensing receptor: a new target in antidepressant development?

Zinc is a trace element released from glutamatergic terminals, and modulates the pre- and postsynaptic areas, giving a diverse biological response. Zinc is a natural ligand that inhibits the N-methyl-d-aspartate (NMDA) receptor and regulates the excessive release of glutamate. Moreover, zinc exhibits an antidepressant-like profile, as demonstrated in both preclinical and clinical studies. Recent reports indicate that the GPR39 Zn(2+)-sensing receptor is an important target for zinc "transmission" (its activation modulates/induces diverse biochemical pathways involved in neuroprotection). Preclinical studies provide evidence that zinc deficiency leads to depressive-like behavior related to down-regulation of the GPR39 Zn(2+)-sensing receptor. Zinc binds to the GPR39 and triggers signals, leading to CRE-dependent gene transcription, resulting in increases in proteins such as brain-derived neurotrophic factor (BDNF), that plays a pivotal role in antidepressant action. Chronic administration of many antidepressants induces GPR39 up-regulation, which suggests that the Zn(2+)-sensing receptor may be considered as a new target for drug development in the field of depression.

Synthesis of 2-triazolyl-imidazo[1,2-a]pyridine through a one-pot three-component reaction using a nano copper oxide assisted click-catalyst

The syntheses of 2-triazolyl imidazo[1,2-a]pyridine were accomplished through 5-exo dig cyclisation using 5 mol% nanocopper oxide together with 10 mol% sodium ascorbate as a click-catalyst in ethanol at 70 °C.

What can we learn from molecular dynamics simulations for GPCR drug design?

Recent years have seen a tremendous progress in the elucidation of experimental structural information for G-protein coupled receptors (GPCRs). Although for the vast majority of pharmaceutically relevant GPCRs structural information is still accessible only by homology models the steadily increasing amount of structural information fosters the application of structure-based drug design tools for this important class of drug targets. In this article we focus on the application of molecular dynamics (MD) simulations in GPCR drug discovery programs. Typical application scenarios of MD simulations and their scope and limitations will be described on the basis of two selected case studies, namely the binding of small molecule antagonists to the human CC chemokine receptor 3 (CCR3) and a detailed investigation of the interplay between receptor dynamics and solvation for the binding of small molecules to the human muscarinic acetylcholine receptor 3 (hM3R).

Discovery of 2-Pyridylpyrimidines as the First Orally Bioavailable GPR39 Agonists

The identification of highly potent and orally bioavailable GPR39 agonists is reported. Compound 1, found in a phenotypic screening campaign, was transformed into compound 2 with good activity on both the rat and human GPR39 receptor. This compound was further optimized to improve ligand efficiency and pharmacokinetic properties to yield GPR39 agonists for the potential oral treatment of type 2 diabetes. Thus, compound 3 is the first potent GPR39 agonist (EC50s ≤ 1 nM for human and rat receptor) that is orally bioavailable in mice and robustly induced acute GLP-1 levels.