Orphan G protein-coupled receptors: the ongoing search for a home

G protein-coupled receptors (GPCRs) make up the largest receptor superfamily, accounting for 4% of protein-coding genes. Despite the prevalence of such transmembrane receptors, a significant number remain orphans, lacking identified endogenous ligands. Since their conception, the reverse pharmacology approach has been used to characterize such receptors. However, the multifaceted and nuanced nature of GPCR signaling poses a great challenge to their pharmacological elucidation. Considering their therapeutic relevance, the search for native orphan GPCR ligands continues. Despite limited structural input in terms of 3D crystallized structures, with advances in machine-learning approaches, there has been great progress with respect to accurate ligand prediction. Though such an approach proves valuable given that ligand scarcity is the greatest hurdle to orphan GPCR deorphanization, the future pairings of the remaining orphan GPCRs may not necessarily take a one-size-fits-all approach but should be more comprehensive in accounting for numerous nuanced possibilities to cover the full spectrum of GPCR signaling.

The dark sides of the GPCR tree - research progress on understudied GPCRs

A large portion of the human GPCRome is still in the dark and understudied, consisting even of entire subfamilies of GPCRs such as odorant receptors, class A and C orphans, adhesion GPCRs, Frizzleds and taste receptors. However, it is undeniable that these GPCRs bring an untapped therapeutic potential that should be explored further. Open questions on these GPCRs span diverse topics such as deorphanisation, the development of tool compounds and tools for studying these GPCRs, as well as understanding basic signalling mechanisms. This review gives an overview of the current state of knowledge for each of the diverse subfamilies of understudied receptors regarding their physiological relevance, molecular mechanisms, endogenous ligands and pharmacological tools. Furthermore, it identifies some of the largest knowledge gaps that should be addressed in the foreseeable future and lists some general strategies that might be helpful in this process.

GPR18 and GPR55-related Ligands Serving as Antagonists or Agonists: Current Situation, Challenges and Perspectives

GPCR superfamily, the largest known family of membrane receptors, consists of six classes from A to F. GPR18 and GPR55, δ-branch of A class, had been reported to have no confirmed endogenous ligand and were named as "orphan receptors". Previous studies suggest that both GPR18 and GPR55 are possibly related to the migration and proliferation of cancer cells, macrophages and other inflammation-associated immune cells. Thus, they may be potential targets for inflammation, cancer and analgesia therapy. In this paper, we aimed to summarize the chemical structures and bioactivities of the agonists and antagonists of GPR18 and GPR55; moreover, we have briefly discussed the challenges and future perspectives in this field. This review will be beneficial for further design and synthesis of efficient agonists and antagonists towards GPR18 and GPR55- related disease treatment.

Glycine Effect on the Expression Profile of Orphan Receptors GPR21, GPR26, GPR39, GPR82 and GPR6 in a Model of Inflammation in 3T3-L1 Cells

BACKGROUND

Chronic or low-grade inflammation is a process where various immune cells are recruited from the periphery into adipose tissue. This event gives rise to localised inflammation, in addition to having a close interaction with cardiometabolic pathologies where the mediation of orphan receptors is observed. The aim of this study was to analyse the participation of the orphan receptors GPR21, GPR39, GPR82 and GPR6 in a chronic inflammatory process in 3T3-L1 cells. The 3T3-L1 cells were stimulated with TNF-α (5 ng/mL) for 60 min as an inflammatory model. Gene expression was measured by RT-qPCR.

RESULTS

We showed that the inflammatory stimulus of TNF-α in adipocytes decreased the expression of the orphan receptors GPR21, GPR26, GPR39, GPR82 and GPR6, which are related to low-grade inflammation.

CONCLUSIONS

Our results suggest that GPR21 and GPR82 are modulated by glycine, it shows a possible protective role in the presence of an inflammatory environment in adipocytes, and they could be a therapeutic target to decrease the inflammation in some diseases related to low-grade inflammation such as diabetes, obesity and metabolic syndrome.

Epithelial Nr5a2 heterozygosity cooperates with mutant Kras in the development of pancreatic cystic lesions

Cystic neoplasms of the pancreas are an increasingly important public health problem. The majority of these lesions are benign but some progress to invasive pancreatic ductal adenocarcinoma (PDAC). There is a dearth of mouse models of these conditions. The orphan nuclear receptor NR5A2 regulates development, differentiation, and inflammation. Germline Nr5a2 heterozygosity sensitizes mice to the oncogenic effects of mutant Kras in the pancreas. Here, we show that - unlike constitutive Nr5a2^+/- mice - conditional Nr5a2 heterozygosity in pancreatic epithelial cells, combined with mutant Kras (KPN^+/- ), leads to a dramatic replacement of the pancreatic parenchyma with cystic structures and an accelerated development of high-grade PanINs and PDAC. Timed histopathological analyses indicated that in KPN^+/- mice PanINs precede the formation of cystic lesions and the latter precede PDAC. A single episode of acute caerulein pancreatitis is sufficient to accelerate the development of cystic lesions in KPN^+/- mice. Epithelial cells of cystic lesions of KPN^+/- mice express MUC1, MUC5AC, and MUC6, but lack expression of MUC2, CDX2, and acinar markers, indicative of a pancreato-biliary/gastric phenotype. In accordance with this, in human samples we found a non-significantly decreased expression of NR5A2 in mucinous tumours, compared with conventional PDAC. These results highlight that the effects of loss of one Nr5a2 allele are time- and cell context-dependent. KPN^+/- mice represent a new model to study the formation of cystic pancreatic lesions and their relationship with PanINs and classical PDAC. Our findings suggest that pancreatitis could also contribute to acceleration of cystic tumour progression in patients. © 2020 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Changes in expression of orphan receptors GPR99 and GPR107 during the development and establishment of hypertension in spontaneously hypertensive rats

Hypertension is a disease, which in spite of existing treatments continues to have high morbidity and mortality, which suggests that there are other mechanisms involved in this pathology. In this sense, the orphan receptors are G protein-coupled receptor associated with various pathologies such as GPR99 which has been linked to mice develop left ventricular hypertrophy induced by blood pressure overload while GPR107 with patients with idiopathic pulmonary arterial hypertension. For this reason, the aim of this work was to study if the expression of the orphan receptors GPR99 and GPR107 are modified by arterial hypertension. Male SHR and WKY rats of 6-8 and 10-12 weeks old were used. The weight, systolic blood pressure and heart rate were measured, as well as the mRNA of the receptors GPR99 and GPR107 in the aorta, kidney, heart and brain by RT-PCR, also was realized an in silico analysis to predict which G protein could be coupled the orphan receptor GPR107. Our results showed that receptors GPR99 and GPR107 are expressed in the analyzed tissues and their expression profile tends to change at different ages and with the development of hypertension, for the other hand, the bioinformatics analysis for GPR107 showed that is coupled to Gi protein. Therefore, we do not rule out that GPR99 and GPR107 could be involved in the pathophysiology of hypertension and could be used as targets therapeutic in hypertension.

High Sensitivity Mapping of Cortical Dopamine D2 Receptor Expressing Neurons

Cortical D2 dopamine receptor (Drd2) have mostly been examined in the context of cognitive function regulation and neurotransmission modulation of medial prefrontal cortex by principal neurons and parvalbumin positive, fast-spiking, interneurons in schizophrenia. Early studies suggested the presence of D2 receptors in several cortical areas, albeit with major technical limitations. We used combinations of transgenic reporter systems, recombinase activated viral vectors, quantitative translatome analysis, and high sensitivity in situ hybridization to identify D2 receptor expressing cells and establish a map of their respective projections. Our results identified previously uncharacterized clusters of D2 expressing neurons in limbic and sensory regions of the adult mouse brain cortex. Characterization of these clusters by translatome analysis and cell type specific labeling revealed highly heterogeneous expression of D2 receptors in principal neurons and various populations of interneurons across cortical areas. Transcript enrichment analysis also demonstrated variable levels of D2 receptor expression and several orphan G-protein-coupled receptors coexpression in different neuronal clusters, thus suggesting strategies for genetic and therapeutic targeting of D2 expressing neurons in specific cortical areas. These results pave the way for a thorough re-examination of cortical D2 receptor functions, which could provide information about neuronal circuits involved in psychotic and mood disorders.

Orphan Nuclear Receptor ERRγ Is a Novel Transcriptional Regulator of IL-6 Mediated Hepatic BMP6 Gene Expression in Mice

Bone morphogenetic protein 6 (BMP6) is a multifunctional growth factor involved in organ development and homeostasis. BMP6 controls expression of the liver hormone, hepcidin, and thereby plays a crucial role in regulating iron homeostasis. BMP6 gene transcriptional regulation in liver is largely unknown, but would be of great help to externally modulate iron load in pathologic conditions. Here, we describe a detailed molecular mechanism of hepatic BMP6 gene expression by an orphan nuclear receptor, estrogen-related receptor γ (ERRγ), in response to the pro-inflammatory cytokine interleukin 6 (IL-6). Recombinant IL-6 treatment increases hepatic ERRγ and BMP6 expression. Overexpression of ERRγ is sufficient to increase BMP6 gene expression in hepatocytes, suggesting that IL-6 is upstream of ERRγ. In line, knock-down of ERRγ in cell lines or a hepatocyte specific knock-out of ERRγ in mice significantly decreases IL-6 mediated BMP6 expression. Promoter studies show that ERRγ directly binds to the ERR response element (ERRE) in the mouse BMP6 gene promoter and positively regulates BMP6 gene transcription in IL-6 treatment conditions, which is further confirmed by ERRE mutated mBMP6-luciferase reporter assays. Finally, an inverse agonist of ERRγ, GSK5182, markedly inhibits IL-6 induced hepatic BMP6 expression in vitro and in vivo. Taken together, these results reveal a novel molecular mechanism on ERRγ mediated transcriptional regulation of hepatic BMP6 gene expression in response to IL-6.

Modifications in GPR21 and GPR82 genes expression as a consequence of metabolic syndrome etiology

Metabolic syndrome (MS) has been related with alterations in expression levels of orphan G protein coupled receptors (GPCRs) such as GPR21 and GPR82, which could be involved in some of the elements that characterizes the metabolic syndrome. The aim of this work was to evaluate changes in GPR21 and GPR82 receptors expression in two models of metabolic syndrome: one genetic (Zucker rats), and the other based on a diet (70% fructose for 9 weeks). GPR21 and GPR82 gene expressions were evaluated in brain, heart, aorta, liver and kidney by RT-qPCR. Rats with a high fructose diet, as well as obese Zucker rats, showed initial stages of pancreatic damage and alterations in some biochemical parameters related to the model consistent with the classification of MS. GPR21 and GPR82 receptors expressed in all tissues. The expression of GPR21 decreased in heart, aorta and kidney, but in liver the expression was different: decreased in diet model and increased in genetic model. In contrast, GPR82 expression depended of tissue and metabolic syndrome model. The results highlight the possible role of GPR21 and GPR82 receptors in the development MS. We conclude that the expression of GPR21 and GPR82 in different tissues is related with MS and depend of the origin of the syndrome, so they could be a therapeutic target for that syndrome.

Rare-variant pathogenicity triage and inclusion of synonymous variants improves analysis of disease associations of orphan G protein-coupled receptors

The pace of deorphanization of G protein-coupled receptors (GPCRs) has slowed, and new approaches are required. Small molecule targeting of orphan GPCRs can potentially be of clinical benefit even if the endogenous receptor ligand has not been identified. Many GPCRs lack common variants that lead to reproducible genome-wide disease associations, and rare-variant approaches have emerged as a viable alternative to identify disease associations for such genes. Therefore, our goal was to prioritize orphan GPCRs by determining their associations with human diseases in a large clinical population. We used sequence kernel association tests to assess the disease associations of 85 orphan or understudied GPCRs in an unselected cohort of 51,289 individuals. Using rare loss-of-function variants, missense variants predicted to be pathogenic or likely pathogenic, and a subset of rare synonymous variants that cause large changes in local codon bias as independent data sets, we found strong, phenome-wide disease associations shared by two or more variant categories for 39% of the GPCRs. To validate the bioinformatics and sequence kernel association test analyses, we functionally characterized rare missense and synonymous variants of GPR39, a family A GPCR, revealing altered expression or Zn²⁺-mediated signaling for members of both variant classes. These results support the utility of rare variant analyses for identifying disease associations for GPCRs that lack impactful common variants. We highlight the importance of rare synonymous variants in human physiology and argue for their routine inclusion in any comprehensive analysis of genomic variants as potential causes of disease.

The signaling proteins GPR158 and RGS7 modulate excitability of L2/3 pyramidal neurons and control A-type potassium channel in the prelimbic cortex

Stress profoundly affects physiological properties of neurons across brain circuits and thereby increases the risk for depression. However, the molecular and cellular mechanisms mediating these effects are poorly understood. In this study, we report that chronic physical restraint stress in mice decreases excitability specifically in layer 2/3 of pyramidal neurons within the prelimbic subarea of the prefrontal cortex (PFC) accompanied by the induction of depressive-like behavioral states. We found that a complex between G protein-coupled receptor (GPCR) 158 (GPR158) and regulator of G protein signaling 7 (RGS7), a regulatory GPCR signaling node recently discovered to be a key modulator of affective behaviors, plays a key role in controlling stress-induced changes in excitability in this neuronal population. Deletion of GPR158 or RGS7 enhanced excitability of layer 2/3 PFC neurons and prevented the impact of stress. Investigation of the underlying molecular mechanisms revealed that the A-type potassium channel Kv4.2 subunit is a molecular target of the GPR158-RGS7 complex. We further report that GPR158 physically associates with Kv4.2 channel and promotes its function by suppressing inhibitory modulation by cAMP-protein kinase A (PKA)-mediated phosphorylation. Taken together, our observations reveal a critical mechanism that adjusts neuronal excitability in L2/3 pyramidal neurons of the PFC and may thereby modulate the effects of stress on depression.

GPR3, GPR6, and GPR12 as novel molecular targets: their biological functions and interaction with cannabidiol

The G protein-coupled receptors 3, 6, and 12 (GPR3, GPR6, and GPR12) comprise a family of closely related orphan receptors with no confirmed endogenous ligands. These receptors are constitutively active and capable of signaling through G protein-mediated and non-G protein-mediated mechanisms. These orphan receptors have previously been reported to play important roles in many normal physiological functions and to be involved in a variety of pathological conditions. Although they are orphans, GPR3, GPR6, and GPR12 are phylogenetically most closely related to the cannabinoid receptors. Using β-arrestin2 recruitment and cAMP accumulation assays, we recently found that the nonpsychoactive phytocannabinoid cannabidiol (CBD) is an inverse agonist for GPR3, GPR6, and GPR12. This discovery highlights these orphan receptors as potential new molecular targets for CBD, provides novel mechanisms of action, and suggests new therapeutic uses of CBD for illnesses such as Alzheimer's disease, Parkinson's disease, cancer, and infertility. Furthermore, identification of CBD as a new inverse agonist for GPR3, GPR6, and GPR12 provides the initial chemical scaffolds upon which potent and efficacious agents acting on these receptors can be developed, with the goal of developing chemical tools for studying these orphan receptors and ultimately new therapeutic agents.

Glio- and neuro-protection by prosaposin is mediated by orphan G-protein coupled receptors GPR37L1 and GPR37

Discovery of neuroprotective pathways is one of the major priorities for neuroscience. Astrocytes are natural neuroprotectors and it is likely that brain resilience can be enhanced by mobilizing their protective potential. Among G‐protein coupled receptors expressed by astrocytes, two highly related receptors, GPR37L1 and GPR37, are of particular interest. Previous studies suggested that these receptors are activated by a peptide Saposin C and its neuroactive fragments (prosaptide TX14(A)), which were demonstrated to be neuroprotective in various animal models by several groups. However, pairing of Saposin C or prosaptides with GPR37L1/GPR37 has been challenged and presently GPR37L1/GPR37 have regained their orphan status. Here, we demonstrate that in their natural habitat, astrocytes, these receptors mediate a range of effects of TX14(A), including protection from oxidative stress. The Saposin C/GPR37L1/GPR37 pathway is also involved in the neuroprotective effect of astrocytes on neurons subjected to oxidative stress. The action of TX14(A) is at least partially mediated by Gi‐proteins and the cAMP‐PKA axis. On the other hand, when recombinant GPR37L1 or GPR37 are expressed in HEK293 cells, they are not functional and do not respond to TX14(A), which explains unsuccessful attempts to confirm the ligand‐receptor pairing. Therefore, this study identifies GPR37L1/GPR37 as the receptors for TX14(A), and, by extension of Saposin C, and paves the way for the development of neuroprotective therapeutics acting via these receptors.

A video abstract of this article can be found at: https://www.youtube.com/watch?v=qTn13My9Sz8

GPCRomics: GPCR Expression in Cancer Cells and Tumors Identifies New, Potential Biomarkers and Therapeutic Targets

G protein-coupled receptors (GPCRs), the largest family of targets for approved drugs, are rarely targeted for cancer treatment, except for certain endocrine and hormone-responsive tumors. Limited knowledge regarding GPCR expression in cancer cells likely has contributed to this lack of use of GPCR-targeted drugs as cancer therapeutics. We thus undertook GPCRomic studies to define the expression of endoGPCRs (which respond to endogenous molecules such as hormones, neurotransmitters and metabolites) in multiple types of cancer cells. Using TaqMan qPCR arrays to quantify the mRNA expression of ∼340 such GPCRs, we found that human chronic lymphocytic leukemia (CLL) cells/stromal cells associated with CLL, breast cancer cell lines, colon cancer cell lines, pancreatic ductal adenocarcinoma (PDAC) cells, cancer associated fibroblasts (CAFs), and PDAC tumors express 50 to >100 GPCRs, including many orphan GPCRs (which lack known physiologic agonists). Limited prior data exist regarding the expression or function of most of the highly expressed GPCRs in these cancer cells and tumors. Independent results from public cancer gene expression databases confirm the expression of such GPCRs. We propose that highly expressed GPCRs in cancer cells (for example, GPRC5A in PDAC and colon cancer cells and GPR68 in PDAC CAFs) may contribute to the malignant phenotype, serve as biomarkers and/or may be novel therapeutic targets for the treatment of cancer.

Orphan receptor GPR158 controls stress-induced depression

Stress can be a motivational force for decisive action and adapting to novel environment; whereas, exposure to chronic stress contributes to the development of depression and anxiety. However, the molecular mechanisms underlying stress-responsive behaviors are not fully understood. Here, we identified the orphan receptor GPR158 as a novel regulator operating in the prefrontal cortex (PFC) that links chronic stress to depression. GPR158 is highly upregulated in the PFC of human subjects with major depressive disorder. Exposure of mice to chronic stress also increased GPR158 protein levels in the PFC in a glucocorticoid-dependent manner. Viral overexpression of GPR158 in the PFC induced depressive-like behaviors. In contrast GPR158 ablation, led to a prominent antidepressant-like phenotype and stress resiliency. We found that GPR158 exerts its effects via modulating synaptic strength altering AMPA receptor activity. Taken together, our findings identify a new player in mood regulation and introduce a pharmacological target for managing depression.

An Update on Non-CB1, Non-CB2 Cannabinoid Related G-Protein-Coupled Receptors

The endocannabinoid system (ECS) has been shown to be of great importance in the regulation of numerous physiological and pathological processes. To date, two Class A G-protein-coupled receptors (GPCRs) have been discovered and validated as the main therapeutic targets of this system: the cannabinoid receptor type 1 (CB1), which is the most abundant neuromodulatory receptor in the brain, and the cannabinoid receptor type 2 (CB2), predominantly found in the immune system among other organs and tissues. Endogenous cannabinoid receptor ligands (endocannabinoids) and the enzymes involved in their synthesis, cell uptake, and degradation have also been identified as part of the ECS. However, its complex pharmacology suggests that other GPCRs may also play physiologically relevant roles in this therapeutically promising system. In the last years, GPCRs such as GPR18 and GPR55 have emerged as possible missing members of the cannabinoid family. This categorization still stimulates strong debate due to the lack of pharmacological tools to validate it. Because of their close phylogenetic relationship, the Class A orphan GPCRs, GPR3, GPR6, and GPR12, have also been associated with the cannabinoids. Moreover, certain endo-, phyto-, and synthetic cannabinoid ligands have displayed activity at other well-established GPCRs, including the opioid, adenosine, serotonin, and dopamine receptor families. In addition, the cannabinoid receptors have also been shown to form dimers with other GPCRs triggering cross-talk signaling under specific conditions. In this mini review, we aim to provide insight into the non-CB1, non-CB2 cannabinoid-related GPCRs that have been reported thus far. We consider the physiological relevance of these molecular targets in modulating the ECS.

Identification of agonists for a group of human odorant receptors

Olfaction plays a critical role in several aspects of the human life. Odorants are detected by hundreds of odorant receptors (ORs) which belong to the superfamily of G protein-coupled receptors. These receptors are expressed in the olfactory sensory neurons of the nose. The information provided by the activation of different combinations of ORs in the nose is transmitted to the brain, leading to odorant perception and emotional and behavioral responses. There are ~400 intact human ORs, and to date only a small percentage of these receptors (~10%) have known agonists. The determination of the specificity of the human ORs will contribute to a better understanding of how odorants are discriminated by the olfactory system. In this work, we aimed to identify human specific ORs, that is, ORs that are present in humans but absent from other species, and their corresponding agonists. To do this, we first selected 22 OR gene sequences from the human genome with no counterparts in the mouse, rat or dog genomes. Then we used a heterologous expression system to screen a subset of these human ORs against a panel of odorants of biological relevance, including foodborne aroma volatiles. We found that different types of odorants are able to activate some of these previously uncharacterized human ORs.

Peripheral actions of GPCRs in energy homeostasis: view from the Chair

The roles of G-protein-coupled receptors (GPCRs) in the control of food intake and energy expenditure are being increasingly recognized, and new drug candidates targeting these receptors are making their entry into the clinic. GPCRs exert their action along the various sites of regulation of energy homeostasis control including the central nervous system, the pancreas, the gut and fat cells. Exciting new data about GPCRs recognizing and mediating the effects of lipid mediators and concerning receptors for which no endogenous ligands have been identified yet open new exciting avenues for the validation of additional drug targets. In addition, recently developed paradigms around the concepts of cross-talk regulation and functional selectivity should lead to the development of drugs with improved therapeutic efficacy and reduced undesirable effects. Some of these promising discoveries are discussed in the present article and accompanying papers.