High-throughput identification and characterization of novel, species-selective GPR35 agonists

Biased constitutive signaling of the G protein-coupled receptor GPR35 suppresses gut barrier permeability

Agonist-independent, or constitutive, activity is an integral feature of G protein-coupled receptors, but its relevance in pathophysiological settings is generally poorly explored. GPR35 is a therapeutic target in inflammatory diseases of the lower gut. In colonic organoids from a human GPR35a-expressing transgenic mouse line, the GPR35 inverse agonist CID-2745687 increased barrier permeability substantially, indicating that constitutive receptor activity contributes to maintaining epithelial barrier integrity. High constitutive activity of GPR35 was also observed in both HT-29 and HEPG2 cells that express GPR35 endogenously. Mechanistic investigations in recombinant in vitro systems revealed that the constitutive activity of GPR35a was biased and not equivalent across signaling pathways. Hence, no constitutive interactions of the receptor with arrestin-adaptor proteins or activation of Gαo-containing G protein heterotrimers were detected while, even at low GPR35a expression levels, substantial constitutive activation of heterotrimers containing either Gα12 or Gα13 was observed. Similar biased constitutive activity was observed for the human GPR35b isoform. The extent of constitutive and agonist-mediated activity was dependent on receptor expression level. At high receptor levels, constitutive activation of Gα12 or Gα13 masked any agonist-induced effects while low expression levels with low constitutive activity allowed measurement of agonist-induced responses. These results highlight roles, selectivity, and the extent of constitutive activity of GPR35 in cells and tissues that express this receptor endogenously and highlight the contribution of its constitutive activity to maintaining the colonic epithelial barrier, potentially limiting the development of inflammatory bowel diseases.

Computer-Aided Drug Discovery and Design: Recent Advances and Future Prospects

Computer-aided drug discovery and design involve the use of information technologies to identify and develop, on a rational ground, chemical compounds that align a set of desired physicochemical and biological properties. In its most common form, it involves the identification and/or modification of an active scaffold (or the combination of known active scaffolds), although de novo drug design from scratch is also possible. Traditionally, the drug discovery and design processes have focused on the molecular determinants of the interactions between drug candidates and their known or intended pharmacological target(s). Nevertheless, in modern times, drug discovery and design are conceived as a particularly complex multiparameter optimization task, due to the complicated, often conflicting, property requirements.This chapter provides an updated overview of in silico approaches for identifying active scaffolds and guiding the subsequent optimization process. Recent groundbreaking advances in the field have also analyzed the integration of state-of-the-art machine learning approaches in every step of the drug discovery process (from prediction of target structure to customized molecular docking scoring functions), integration of multilevel omics data, and the use of a diversity of computational approaches to assist target validation and assess plausible binding pockets.

Discovery of a novel GPR35 agonist with high and equipotent species potency for oral treatment of IBD

The G protein-coupled receptor 35 (GPR35) has been identified as a potential target in the treatment of inflammatory bowel disease (IBD). However, the lack of high and equipotent agonists on both human and mouse GPR35 has limited the in vivo study of GPR35 agonists in mouse models of IBD. In this study, structural modifications to lodoxamide provides a series of high and equivalent agonists on human, mouse, and rat GPR35. These molecules eliminate the species selectivity of human to mouse and rat orthologs that have been prevalent with GPR35 agonists including lodoxamide. The cLogP properties are also optimized to make the compounds more obedient to drug-like rules, yielding compound 4b (cLogP = 2.41), which activates human, mouse or rat GPR35 with EC50 values of 76.0, 63.7 and 77.8 nM, respectively. Oral administration of compound 4b at 20 mg/kg alleviates clinical symptoms of DSS-induced IBD in mice, and is slightly more effective than 5-ASA at 200 mg/kg. In summary, it can serve as a new start point for exploiting more potent GPR35 agonists without species differences for the treatment of IBD, and warrants further study.

GPR35 acts a dual role and therapeutic target in inflammation

GPR35 is a G protein-coupled receptor with notable involvement in modulating inflammatory responses. Although the precise role of GPR35 in inflammation is not yet fully understood, studies have suggested that it may have both pro- and anti-inflammatory effects depending on the specific cellular environment. Some studies have shown that GPR35 activation can stimulate the production of pro-inflammatory cytokines and facilitate the movement of immune cells towards inflammatory tissues or infected areas. Conversely, other investigations have suggested that GPR35 may possess anti-inflammatory properties in the gastrointestinal tract, liver and certain other tissues by curbing the generation of inflammatory mediators and endorsing the differentiation of regulatory T cells. The intricate role of GPR35 in inflammation underscores the requirement for more in-depth research to thoroughly comprehend its functional mechanisms and its potential significance as a therapeutic target for inflammatory diseases. The purpose of this review is to concurrently investigate the pro-inflammatory and anti-inflammatory roles of GPR35, thus illuminating both facets of this complex issue.

GPR35: from enigma to therapeutic target

The orphan G-protein-coupled receptor 35 (GPR35), although poorly characterised, is attracting considerable interest as a therapeutic target. Marked differences in pharmacology between human and rodent orthologues of the receptor and a dearth of antagonists with affinity for mouse and rat GPR35 have previously restricted the use of preclinical disease models. The development of improved ligands, novel transgenic knock-in mouse lines, and detailed analysis of the disease relevance of single-nucleotide polymorphisms (SNPs) have greatly enhanced understanding of the key roles of GPR35 and have stimulated efforts towards disease-targeted proof-of-concept studies. In this opinion article, new information on the biology of the receptor is considered, whilst insight into how GPR35 is currently being assessed for therapeutic utility - in areas ranging from inflammatory bowel diseases to nonalcoholic steatohepatitis and various cancers - is also provided.

Recent Progress in Assays for GPCR Drug Discovery

G-protein coupled receptors (GPCRs), also known as 7 transmembrane receptors, are the largest family of cell surface receptors in eukaryotes. There are ~800 GPCRs in human, regulating diverse physiological processes. GPCRs are the most intensively studied drug targets. Drugs that target GPCRs account for about a quarter of the global market share of therapeutic drugs. Therefore, to develop physiologically relevant and robust assays to search new GPCR ligands or modulators remain the major focus of drug discovery research worldwide. Early functional GPCR assays are mainly depend on the measurement of G protein-mediated second messenger generation. Recent development in GPCR biology indicate the signaling of these receptors is much more complex than the oversimplified classical view. GPCRs have been found to activate multiple G proteins simultaneously and induce b-arrestin-mediated signaling. GPCRs have also been found to interacte with other cytosolic scaffolding proteins and form dimer or heteromer with GPCRs or other transmembrane proteins. Here we mainly discuss technologies focused on detecting protein-protein interactions, such as FRET/BRET, NanoBiT, Tango, etc, and their applications in measuring GPCRs interacting with various signaling partners. In the final part, we also discuss the species differences in GPCRs when using animal models to study the in vivofunctions of GPCR ligands, and possible ways to solve this problem with modern genetic tools.

Agonist-induced phosphorylation of orthologues of the orphan receptor GPR35 functions as an activation sensor

G protein-coupled receptor 35 (GPR35) is poorly characterized but nevertheless has been revealed to have diverse roles in areas including lower gut inflammation and pain. The development of novel reagents and tools will greatly enhance analysis of GPR35 functions in health and disease. Here, we used mass spectrometry, mutagenesis, and [³²P] orthophosphate labeling to identify that all five hydroxy-amino acids in the C-terminal tail of human GPR35a became phosphorylated in response to agonist occupancy of the receptor and that, apart from Ser²⁹⁴, each of these contributed to interactions with arretin-3, which inhibits further G protein-coupled receptor signaling. We found that Ser³⁰³ was key to such interactions; the serine corresponding to human GPR35a residue 303 also played a dominant role in arrestin-3 interactions for both mouse and rat GPR35. We also demonstrated that fully phospho-site–deficient mutants of human GPR35a and mouse GPR35 failed to interact effectively with arrestin-3, and the human phospho-deficient variant was not internalized from the surface of cells in response to agonist treatment. Even in cells stably expressing species orthologues of GPR35, a substantial proportion of the expressed protein(s) was determined to be immature. Finally, phospho-site–specific antisera targeting the region encompassing Ser³⁰³ in human (Ser³⁰¹ in mouse) GPR35a identified only the mature forms of GPR35 and provided effective sensors of the activation status of the receptors both in immunoblotting and immunocytochemical studies. Such antisera may be useful tools to evaluate target engagement in drug discovery and target validation programs.

GPR35 in Intestinal Diseases: From Risk Gene to Function

Diet and gut microbial metabolites mediate host immune responses and are central to the maintenance of intestinal health. The metabolite-sensing G-protein coupled receptors (GPCRs) bind metabolites and trigger signals that are important for the host cell function, survival, proliferation and expansion. On the contrary, inadequate signaling of these metabolite-sensing GPCRs most likely participate to the development of diseases including inflammatory bowel diseases (IBD). In the intestine, metabolite-sensing GPCRs are highly expressed by epithelial cells and by specific subsets of immune cells. Such receptors provide an important link between immune system, gut microbiota and metabolic system. Member of these receptors, GPR35, a class A rhodopsin-like GPCR, has been shown to be activated by the metabolites tryptophan-derived kynurenic acid (KYNA), the chemokine CXCL17 and phospholipid derivate lysophosphatidic acid (LPA) species. There have been studies on GPR35 in the context of intestinal diseases since its identification as a risk gene for IBD. In this review, we discuss the pharmacology of GPR35 including its proposed endogenous and synthetic ligands as well as its antagonists. We elaborate on the risk variants of GPR35 implicated in gut-related diseases and the mechanisms by which GPR35 contribute to intestinal homeostasis.

Therapeutic Opportunities and Challenges in Targeting the Orphan G Protein-Coupled Receptor GPR35

GPR35 is a class A, rhodopsin-like G protein-coupled receptor (GPCR) first identified more than 20 years ago. In the intervening period, identification of strong expression in the lower intestine and colon, in a variety of immune cells including monocytes and a variety of dendritic cells, and in dorsal root ganglia has suggested potential therapeutic opportunities in targeting this receptor in a range of conditions. GPR35 is, however, unusual in a variety of ways that challenge routes to translation. These include the following: (i) Although a substantial range and diversity of endogenous ligands have been suggested as agonist partners for this receptor, it officially remains defined as an "orphan" GPCR. (ii) Humans express two distinct protein isoform sequences, while rodents express only a single form. (iii) The pharmacologies of the human and rodent orthologues of GPR35 are very distinct, with variation between rat and mouse GPR35 being as marked as that between either of these species and the human forms. Herein we provide perspectives on each of the topics above as well as suggesting ways to overcome the challenges currently hindering potential translation. These include a better understanding of the extent and molecular basis for species selective GPR35 pharmacology and the production of novel mouse models in which both "on-target" and "off-target" effects of presumptive GPR35 ligands can be better defined, as well as a clear understanding of the human isoform expression profile and its significance at both tissue and individual cell levels.

Lodoxamide Attenuates Hepatic Fibrosis in Mice: Involvement of GPR35

A previous pharmacogenomic analysis identified cromolyn, an anti-allergic drug, as an effective anti-fibrotic agent that acts on hepatocytes and stellate cells. Furthermore, cromolyn was shown to be a G protein-coupled receptor 35 (GPR35) agonist. However, it has not been studied whether anti-fibrotic effects are mediated by GPR35. Therefore, in this study, the role of GPR35 in hepatic fibrosis was investigated through the use of lodoxamide, another anti-allergic drug and a potent GPR35 agonist. Long-term treatment with carbon tetrachloride induced hepatic fibrosis, which was inhibited by treatment with lodoxamide. Furthermore, CID2745687, a specific GPR35 antagonist, reversed lodoxamide-mediated anti-fibrotic effects. In addition, lodoxamide treatment showed significant effects on the mRNA expression of collagen Iα1, collagen Iα2, and TGF-β1 in the extracellular matrix. However, a transforming growth factor α (TGF-α) shedding assay revealed lodoxamide not to be a potent agonist of mouse GPR35 in vitro. Therefore, these results showed anti-fibrotic effects of lodoxamide in mice and raise concerns how lodoxamide protects against liver fibrosis in vivo and whether GPR35 is involved in the action.

Receptor selectivity between the G proteins Gα12 and Gα13 is defined by a single leucine-to-isoleucine variation

Despite recent advances in structural definition of GPCR-G protein complexes, the basis of receptor selectivity between G proteins remains unclear. The Gα₁₂ and Gα₁₃ subtypes together form the least studied group of heterotrimeric G proteins. G protein-coupled receptor 35 (GPR35) has been suggested to couple efficiently to Gα₁₃ but weakly to Gα₁₂. Using combinations of cells genome-edited to not express G proteins and bioluminescence resonance energy transfer-based sensors, we confirmed marked selectivity of GPR35 for Gα₁₃. Incorporating Gα₁₂/Gα₁₃ chimeras and individual residue swap mutations into these sensors defined that selectivity between Gα₁₃ and Gα₁₂ was imbued largely by a single leucine-to-isoleucine variation at position G.H5.23. Indeed, leucine could not be substituted by other amino acids in Gα₁₃ without almost complete loss of GPR35 coupling. The critical importance of leucine at G.H5.23 for GPR35-G protein interaction was further demonstrated by introduction of this leucine into Gα_q, resulting in the gain of coupling to GPR35. These studies demonstrate that Gα₁₃ is markedly the most effective G protein for interaction with GPR35 and that selection between Gα₁₃ and Gα₁₂ is dictated largely by a single conservative amino acid variation.-Mackenzie, A. E., Quon, T., Lin, L.-C., Hauser, A. S., Jenkins, L., Inoue, A., Tobin, A. B., Gloriam, D. E., Hudson, B. D., Milligan, G. Receptor selectivity between the G proteins Gα₁₂ and Gα₁₃ is defined by a single leucine-to-isoleucine variation.

Zaprinast diminished pain and enhanced opioid analgesia in a rat neuropathic pain model

The mechanism of neuropathic pain is complex and unclear. Based on our results, we postulate that an intensification of the kynurenine pathway occurs as a consequence of nerve injury. The G protein-coupled receptor 35 (GPR35) is important for kynurenine pathway activation. Cyclic GMP-specific phosphodiesterase inhibitors have also been shown to have beneficial effects on neuropathic pain. Therefore, the aims of our research were to elucidate how a substance that acts as both an agonist of GPR35 and an inhibitor of phosphodiesterase influences neuropathic pain in a rat model. Here, we demonstrated that preemptive and repeated intrathecal (i.t.) administration (16 h and 1 h before injury and then after nerve ligation daily for 7 days) of zaprinast (1 μg/5 μl) significantly attenuated mechanical (von Frey test) and thermal (cold plate test) hypersensitivity measured on day 7 after chronic constriction injury, and the effect of even a single injection lasted up to 24 h. Our data indicate that zaprinast diminished the number of IBA1-positive cells and consequently attenuated the levels of IL-1beta, IL-6, IL-18, and NOS2 in the lumbar spinal cord and/or dorsal root ganglia. Our results also demonstrated that zaprinast potentiated the analgesic properties of morphine and buprenorphine. In summary, in a neuropathic pain model, zaprinast significantly reduced pain symptoms and enhanced the effectiveness of opioids. Our data provide new evidence that modulation of both GPR35 and phosphodiesterase could be an important strategy for innovative pharmacological treatments designed to decrease hypersensitivity evoked by nerve injury.

The Orphan Receptor GPR35 Contributes to Angiotensin II-Induced Hypertension and Cardiac Dysfunction in Mice

BACKGROUND

The orphan receptor G protein–coupled receptor 35 (GPR35) has been associated with a range of diseases, including cancer, inflammatory bowel disease, diabetes, hypertension, and heart failure. To assess the potential for GPR35 as a therapeutic target in cardiovascular disease, this study investigated the cardiovascular phenotype of a GPR35 knockout mouse under both basal conditions and following pathophysiological stimulation.

METHODS

Blood pressure was monitored in male wild-type and GPR35 knockout mice over 7–14 days using implantable telemetry. Cardiac function and dimensions were assessed using echocardiography, and cardiomyocyte morphology evaluated histologically. Two weeks of angiotensin II (Ang II) infusion was used to investigate the effects of GPR35 deficiency under pathophysiological conditions. Gpr35 messenger RNA expression in cardiovascular tissues was assessed using quantitative polymerase chain reaction.

RESULTS

There were no significant differences in blood pressure, cardiac function, or cardiomyocyte morphology in GPR35 knockout mice compared with wild-type mice. Following Ang II infusion, GPR35 knockout mice were protected from significant increases in systolic, diastolic, and mean arterial blood pressure or impaired left ventricular systolic function, in contrast to wild-type mice. There were no significant differences in Gpr35 messenger RNA expression in heart, kidney, and aorta following Ang II infusion in wild-type mice.

CONCLUSIONS

Although GPR35 does not appear to influence basal cardiovascular regulation, these findings demonstrate that it plays an important pathological role in the development of Ang II–induced hypertension and impaired cardiac function. This suggests that GPR35 is a potential novel drug target for therapeutic intervention in hypertension.

Computer-Aided Drug Design: An Overview

The term drug design describes the search of novel compounds with biological activity, on a systematic basis. In its most common form, it involves modification of a known active scaffold or linking known active scaffolds, although de novo drug design (i.e., from scratch) is also possible. Though highly interrelated, identification of active scaffolds should be conceptually separated from drug design. Traditionally, the drug design process has focused on the molecular determinants of the interactions between the drug and its known or intended molecular target. Nevertheless, current drug design also takes into consideration other relevant processes than influence drug efficacy and safety (e.g., bioavailability, metabolic stability, interaction with antitargets).This chapter provides an overview on possible approaches to identify active scaffolds (including in silico approximations to approach that task) and computational methods to guide the subsequent optimization process. It also discusses in which situations each of the overviewed techniques is more appropriate.

GPR35 mediates lodoxamide-induced migration inhibitory response but not CXCL17-induced migration stimulatory response in THP-1 cells; is GPR35 a receptor for CXCL17?

BACKGROUND AND PURPOSE

GPR35 has long been considered an orphan GPCR, because no endogenous ligand of GPR35 has been discovered. CXCL17 (a chemokine) has been reported to be an endogenous ligand of GPR35, and it has even been suggested that it be called CXCR8. However, at present there is no supporting evidence that CXCL17 does interact with GPR35.

EXPERIMENTAL APPROACH

We applied two assay systems to explore the relationship between CXCL17 and GPR35. An AP-TGF-α shedding assay in GPR35 over-expressing HEK293 cells was used as a gain-of-function assay. GPR35 knock-down by siRNA transfection was performed in endogenously GPR35-expressing THP-1 cells.

KEY RESULTS

In the AP-TGF-α shedding assay, lodoxamide, a well-known synthetic GPR35 agonist, was confirmed to be the most potent agonist among other reported agonists. However, neither human nor mouse CXCL17 had an effect on GPR35. Consistent with previous findings, G proteins Gαi/o and Gα12/13 were found to couple with GPR35. Furthermore, lodoxamide-induced activation of GPR35 was concentration-dependently inhibited by CID2745687 (a selective GPR35 antagonist). In endogenously GPR35-expressing THP-1 cells, lodoxamide concentration-dependently inhibited migration and this inhibitory effect was blocked by CID2745687 treatment or GPR35 siRNA transfection. However, even though CXCL17 stimulated the migration of THP-1 cells, which is consistent with a previous report, this stimulatory effect of CXCL17 was not blocked by CID2745687 or GPR35 siRNA.

CONCLUSIONS AND IMPLICATIONS

The present findings suggest that GPR35 functions as a migration inhibitory receptor, but CXCL17-stimulated migration of THP-1 cells is not dependent on GPR35.

Assessments of cellular melatonin receptor signaling pathways: β-arrestin recruitment, receptor internalization, and impedance variations

Melatonin receptors belong to the family of G-protein coupled receptors. Agonist-induced receptor activation is terminated with the recruitment of β-arrestin, which leads to receptor internalization. Furthermore, agonist binding induces a shift in cellular shape that translates into a change in the electric impedance of the cell. In the present study, we employed engineered cells to study these internalization-related processes in the context of the two melatonin receptors, MT₁ and MT₂. To assess these three receptor internalization-related functions and validate the results, we employed four classical ligands of melatonin receptors: the natural agonist melatonin; the super-agonist 2-iodo-melatonin and the two antagonists luzindole and 4-phenyl-2-propionamidotetralin. The assessments confirmed the nature of the agonistic ligands but showed that 4-phenyl-2-propionamidotetralin, a described antagonist, is a biased partial agonist at MT₂ with poorer affinity for MT₁. The methods are now available to be applied to any receptor system for which multiple signaling pathways must be evaluated for new molecules.

5-Ene-4-thiazolidinones - An efficient tool in medicinal chemistry

The presented review is an attempt to summarize a huge volume of data on 5-ene-4-thiazolidinones being a widely studied class of small molecules used in modern organic and medicinal chemistry. The manuscript covers approaches to the synthesis of 5-ene-4-thiazolidinone derivatives: modification of the C5 position of the basic core; synthesis of the target compounds in the one-pot or multistage reactions or transformation of other related heterocycles. The most prominent pharmacological profiles of 5-ene derivatives of different 4-thiazolidinone subtypes belonging to hit-, lead-compounds, drug-candidates and drugs as well as the most studied targets have been discussed. Currently target compounds (especially 5-en-rhodanines) are assigned as frequent hitters or pan-assay interference compounds (PAINS) within high-throughput screening campaigns. Nevertheless, the crucial impact of the presence/nature of C5 substituent (namely 5-ene) on the pharmacological effects of 5-ene-4-thiazolidinones was confirmed by the numerous listed findings from the original articles. The main directions for active 5-ene-4-thiazolidinones optimization have been shown: i) complication of the fragment in the C5 position; ii) introduction of the substituents in the N3 position (especially fragments with carboxylic group or its derivatives); iii) annealing in complex heterocyclic systems; iv) combination with other pharmacologically attractive fragments within hybrid pharmacophore approach. Moreover, the utilization of 5-ene-4-thiazolidinones in the synthesis of complex compounds with potent pharmacological application is described. The chemical transformations cover mainly the reactions which involve the exocyclic double bond in C5 position of the main core and correspond to the abovementioned direction of the 5-ene-4-thiazolidinone modification.

G protein-coupled receptors not currently in the spotlight: free fatty acid receptor 2 and GPR35

It is widely appreciated that G protein‐coupled receptors have been the most successfully exploited class of targets for the development of small molecule medicines. Despite this, to date, less than 15% of the non‐olfactory G protein‐coupled receptors in the human genome are the targets of a clinically used medicine. In many cases, this is likely to reflect a lack of understanding of the basic underpinning biology of many G protein‐coupled receptors that are not currently in the spotlight, as well as a paucity of pharmacological tool compounds and appropriate animal models to test in vivo function of such G protein‐coupled receptors in both normal physiology and in the context of disease. ‘Open Innovation’ arrangements, in which pharmaceutical companies and public–private partnerships provide wider access to tool compounds identified from ligand screening programmes, alongside enhanced medicinal chemistry support to convert such screening ‘hits’ into useful ‘tool’ compounds will provide important routes to improved understanding. However, in parallel, novel approaches to define and fully appreciate the selectivity and mode of action of such tool compounds, as well as better understanding of potential species orthologue variability in the pharmacology and/or signalling profile of a wide range of currently poorly understood and understudied G protein‐coupled receptors, will be vital to fully exploit the therapeutic potential of this large target class. I consider these themes using as exemplars two G protein‐coupled receptors, free fatty acid receptor 2 and GPR35.

GABAB receptor allosteric modulators exhibit pathway-dependent and species-selective activity

Positive modulation of the GABA_B receptor (GABA_BR) represents a potentially useful therapeutic approach for the treatment of nicotine addiction. The positive allosteric modulators (PAMs) of GABA_BR GS39783 and BHF177 enhance GABA‐stimulated [³⁵S]GTPγS‐binding, and have shown efficacy in a rodent nicotine self‐administration procedure reflecting aspects of nicotine dependence. Interestingly, the structural related analog, NVP998, had no effect on nicotine self‐administration in rats despite demonstrating similar pharmacokinetic properties. Extensive in vitro characterization of GS39783, BHF177, and NVP998 activity on GABA_BR‐regulated signaling events, including modulation of cAMP, intracellular calcium levels, and ERK activation, revealed that these structurally related molecules display distinct pathway‐specific signaling activities that correlate with the dissimilarities observed in rodent models and may be predictive of in vivo efficacy. Furthermore, these GABA_BR allosteric modulators exhibit species‐dependent activity. Collectively, these data will be useful in guiding the development of GABA_BR allosteric modulators that display optimal in vivo efficacy in a preclinical model of nicotine dependence, and will identify those that have the potential to lead to novel antismoking therapies.

G-Protein-Coupled Receptor 35 Mediates Human Saphenous Vein Vascular Smooth Muscle Cell Migration and Endothelial Cell Proliferation

Vascular smooth muscle cell (VSMC) migration and proliferation is central to neointima formation in vein graft failure following coronary artery bypass. However, there are currently no pharmacological interventions that prevent vein graft failure through intimal occlusion. It is hence a therapeutic target. Here, we investigated the contribution of GPR35 to human VSMC and endothelial cell (EC) migration, using a scratch-wound assay, and also the contribution to proliferation, using MTS and BrdU assays, in in vitro models using recently characterized human GPR35 ortholog-selective small-molecule agonists and antagonists. Real-time PCR studies showed GPR35 to be robustly expressed in human VSMCs and ECs. Stimulation of GPR35, with either the human-selective agonist pamoic acid or the reference agonist zaprinast, promoted VSMC migration in the scratch-wound assay. These effects were blocked by coincubation with either of the human GPR35-specific antagonists, CID-2745687 or ML-145. These GPR35-mediated effects were produced by inducing alterations in the actin cytoskeleton via the Rho A/Rho kinase signaling axis. Additionally, the agonist ligands stimulated a proliferative response in ECs. These studies highlight the potential that small molecules that stimulate or block GPR35 activity can modulate vascular proliferation and migration. These data propose GPR35 as a translational therapeutic target in vascular remodeling.

G protein-coupled receptor 35: an emerging target in inflammatory and cardiovascular disease

G protein-coupled receptor 35 (GPR35) is an orphan receptor, discovered in 1998, that has garnered interest as a potential therapeutic target through its association with a range of diseases. However, a lack of pharmacological tools and the absence of convincingly defined endogenous ligands have hampered the understanding of function necessary to exploit it therapeutically. Although several endogenous molecules can activate GPR35 none has yet been confirmed as the key endogenous ligand due to reasons that include lack of biological specificity, non-physiologically relevant potency and species ortholog selectivity. Recent advances have identified several highly potent synthetic agonists and antagonists, as well as agonists with equivalent potency at rodent and human orthologs, which will be useful as tool compounds. Homology modeling and mutagenesis studies have provided insight into the mode of ligand binding and possible reasons for the species selectivity of some ligands. Advances have also been made in determining the role of the receptor in disease. In the past, genome-wide association studies have associated GPR35 with diseases such as inflammatory bowel disease, type 2 diabetes, and coronary artery disease. More recent functional studies have implicated it in processes as diverse as heart failure and hypoxia, inflammation, pain transduction and synaptic transmission. In this review, we summarize the progress made in understanding the molecular pharmacology, downstream signaling and physiological function of GPR35, and discuss its emerging potential applications as a therapeutic target.

G protein-coupled receptors in cardiac biology: old and new receptors

G protein-coupled receptors (GPCRs) are seven-transmembrane-spanning proteins that mediate cellular and physiological responses. They are critical for cardiovascular function and are targeted for the treatment of hypertension and heart failure. Nevertheless, current therapies only target a small fraction of the cardiac GPCR repertoire, indicating that there are many opportunities to investigate unappreciated aspects of heart biology. Here, we offer an update on the contemporary view of GPCRs and the complexities of their signalling, and review the roles of the 'classical' GPCRs in cardiovascular physiology and disease. We then provide insights into other GPCRs that have been less extensively studied in the heart, including orphan, odorant and taste receptors. We contend that these novel cardiac GPCRs contribute to heart function in health and disease and thereby offer exciting opportunities to therapeutically modulate heart function.

Functional G-protein-coupled receptor 35 is expressed by neurons in the CA1 field of the hippocampus

The G-protein-coupled receptor 35 (GPR35) was de-orphanized after the discovery that kynurenic acid (KYNA), an endogenous tryptophan metabolite, acts as an agonist of this receptor. Abundant evidence supports that GPR35 exists primarily in peripheral tissues. Here, we tested the hypothesis that GPR35 exists in the hippocampus and influences the neuronal activity. Fluorescence immunohistochemical staining using an antibody anti-NeuN (a neuronal marker), an antibody anti-GFAP (a glial marker), and an antibody anti-GPR35 revealed that neurons in the stratum oriens, stratum pyramidale, and stratum radiatum of the CA1 field of the hippocampus express GPR35. To determine the presence of functional GPR35 in the neurocircuitry, we tested the effects of various GPR35 agonists on the frequency of spontaneous action potentials recorded as fast current transients (CTs) from stratum radiatum interneurons (SRIs) under cell-attached configuration in rat hippocampal slices. Bath application of the GPR35 agonists zaprinast (1-10 μM), dicumarol (50-100 μM), pamoic acid (500-1000 μM), and amlexanox (3 μM) produced a concentration- and time-dependent reduction in the frequency of CTs. Superfusion of the hippocampal slices with the GPR35 antagonist ML145 (1 μM) increased the frequency of CTs and reduced the inhibitory effect of zaprinast. Bath application of phosphodiesterase 5 inhibitor sildenafil (1 or 5 μM) was ineffective, whereas a subsequent application of zaprinast was effective in reducing the CT frequency. The present results demonstrate for the first time that functional GPR35s are expressed by CA1 neurons and suggest that these receptors can be molecular targets for controlling neuronal activity in the hippocampus.

Hunting for the function of orphan GPCRs - beyond the search for the endogenous ligand

Seven transmembrane-spanning proteins (7TM), also called GPCRs, are among the most versatile and evolutionary successful protein families. Out of the 400 non-odourant members identified in the human genome, approximately 100 remain orphans that have not been matched with an endogenous ligand. Apart from the classical deorphanization strategies, several alternative strategies provided recent new insights into the function of these proteins, which hold promise for high therapeutic potential. These alternative strategies consist of the phenotypical characterization of organisms silenced or overexpressing orphan 7TM proteins, the search for constitutive receptor activity and formation of protein complexes including 7TM proteins as well as the development of synthetic, surrogate ligands. Taken together, a variety of ligand-independent functions can be attributed to orphan 7TM proteins that range from constitutive activity to complex formation with other proteins and include 'true' orphans for which no ligand exist and 'conditional' orphans that behave like orphans in the absence of ligand and as non-orphans in the presence of ligand.

Intercellular Lipid Mediators and GPCR Drug Discovery

G-protein-coupled receptors (GPCR) are the largest superfamily of receptors responsible for signaling between cells and tissues, and because they play important physiological roles in homeostasis, they are major drug targets. New technologies have been developed for the identification of new ligands, new GPCR functions, and for drug discovery purposes. In particular, intercellular lipid mediators, such as, lysophosphatidic acid and sphingosine 1-phosphate have attracted much attention for drug discovery and this has resulted in the development of fingolimod (FTY-720) and AM095. The discovery of new intercellular lipid mediators and their GPCRs are discussed from the perspective of drug development. Lipid GPCRs for lysophospholipids, including lysophosphatidylserine, lysophosphatidylinositol, lysophosphatidylcholine, free fatty acids, fatty acid derivatives, and other lipid mediators are reviewed.

The antiallergic mast cell stabilizers lodoxamide and bufrolin as the first high and equipotent agonists of human and rat GPR35

Lack of high potency agonists has restricted analysis of the G protein-coupled receptor GPR35. Moreover, marked variation in potency and/or affinity of current ligands between human and rodent orthologs of GPR35 has limited their productive use in rodent models of physiology. Based on the reported modest potency of the antiasthma and antiallergic ligands cromolyn disodium and nedocromil sodium, we identified the related compounds lodoxamide and bufrolin as high potency agonists of human GPR35. Unlike previously identified high potency agonists that are highly selective for human GPR35, both lodoxamide and bufrolin displayed equivalent potency at rat GPR35. Further synthetic antiallergic ligands, either sharing features of the standard surrogate agonist zaprinast, or with lodoxamide and bufrolin, were also shown to display agonism at either human or rat GPR35. Because both lodoxamide and bufrolin are symmetric di-acids, their potential mode of binding was explored via mutagenesis based on swapping between the rat and human ortholog nonconserved arginine residues within proximity of a key conserved arginine at position 3.36. Computational modeling and ligand docking predicted the contributions of different arginine residues, other than at 3.36, in human GPR35 for these two ligands and were consistent with selective loss of potency of either bufrolin or lodoxamide at distinct arginine mutants. The computational models also suggested that bufrolin and lodoxamide would display reduced potency at a low-frequency human GPR35 single nucleotide polymorphism. This prediction was confirmed experimentally.

8-Benzamidochromen-4-one-2-carboxylic acids: potent and selective agonists for the orphan G protein-coupled receptor GPR35

8-Amido-chromen-4-one-2-carboxylic acid derivatives were identified as novel agonists at the G protein-coupled orphan receptor GPR35. They were characterized by a β-arrestin recruitment assay and optimized to obtain agonists with nanomolar potency for the human GPR35. The compounds were found to exhibit high selectivity versus the related GPR55. The most potent agonists were 6-bromo-8-(4-methoxybenzamido)-4-oxo-4H-chromene-2-carboxylic acid (85, EC50 12.1 nM) and 6-bromo-8-(2-chloro-4-methoxybenzamido)-4-oxo-4H-chromene-2-carboxylic acid (90, EC50 11.1 nM), both of which were >1700-fold selective versus GPR55. Most compounds were considerably less potent at rat and mouse than at human GPR35. 6-Bromo-8-(2-methoxybenzamido)-4-oxo-4H-chromene-2-carboxylic acid (87) was the only derivative that activated GPR35 of all three species at similar, low micromolar concentration. Compounds 85 and 90 are the most potent agonists at the human GPR35 known to date and might thus serve as powerful pharmacological tools to further elucidate the receptor's (patho)physiological role and its potential as a future drug target.