pH-sensing G protein-coupled orphan receptor GPR68 is expressed in human cartilage and correlates with degradation of extracellular matrix during OA progression

Background

Osteoarthritis (OA) is a debilitating joints disease affecting millions of people worldwide. As OA progresses, chondrocytes experience heightened catabolic activity, often accompanied by alterations in the extracellular environment's osmolarity and acidity. Nevertheless, the precise mechanism by which chondrocytes perceive and respond to acidic stress remains unknown. Recently, there has been growing interest in pH-sensing G protein-coupled receptors (GPCRs), such as GPR68, within musculoskeletal tissues. However, function of GPR68 in cartilage during OA progression remains unknown. This study aims to identify the role of GPR68 in regulation of catabolic gene expression utilizing an in vitro model that simulates catabolic processes in OA.

Methods

We examined the expression of GPCR by analyzing high throughput RNA-Seq data in human cartilage isolated from healthy donors and OA patients. De-identified and discarded OA cartilage was obtained from joint arthroplasty and chondrocytes were prepared by enzymatic digestion. Chondrocytes were treated with GPR68 agonist, Ogerin and then stimulated IL1β and RNA isolation was performed using Trizol method. Reverse transcription was done using the cDNA synthesis kit and the expression of GPR68 and OA related catabolic genes was quantified using SYBR® green assays.

Results

The transcriptome analysis revealed that pH sensing GPCR were expressed in human cartilage with a notable increase in the expression of GPR68 in OA cartilage which suggest a potential role for GPR68 in the pathogenesis of OA. Immunohistochemical (IHC) and qPCR analyses in human cartilage representing various stages of OA indicated a progressive increase in GPR68 expression in cartilage associated with higher OA grades, underscoring a correlation between GPR68 expression and the severity of OA. Furthermore, IHC analysis of Gpr68 in murine cartilage subjected to surgically induced OA demonstrated elevated levels of GPR68 in knee cartilage and meniscus. Using IL1β stimulated in vitro model of OA catabolism, our qPCR analysis unveiled a time-dependent increase in GPR68 expression in response to IL1β stimulation, which correlates with the expression of matrix degrading proteases suggesting the role of GPR68 in chondrocytes catabolism and matrix degeneration. Using pharmacological activator of GPR68, our results further showed that GPR68 activation repressed the expression of MMPs in human chondrocytes.

Conclusions

Our results demonstrated that GPR68 was robustly expressed in human cartilage and mice and its expression correlates with matrix degeneration and severity of OA progression in human and surgical model. GPR68 activation in human chondrocytes further repressed the expression of MMPs under OA pathological condition. These results identify GPR68 as a possible therapeutic target in the regulation of matrix degradation during OA.

Research progress on the role of orphan receptor GPR139 in neuropsychiatric behaviours

The study of orphan G protein-coupled receptors (GPCRs) holds much promise for increasing our understanding of neuropsychiatric diseases and for the development of new therapeutic strategies for these diseases. GPR139 is an orphan GPCR expressed in the central nervous system, especially in areas of the brain that control movement, motivation, and reward, and those that regulate neuropsychiatric behaviour. This review provides information about the discovery, tissue expression, signal transduction pathways, and physiological functions of GPR139, as well as how GPR139 interacts with other GPCRs, which form heteromeric complexes that affect their pharmacology and function. We also discuss the utility and therapeutic potential of ligands that target GPR139, including the pharmacological properties of reported agonists and antagonists. Finally, we highlight the pathologic role of GPR139 in neuropsychiatric behaviour and its potential as a therapeutic target in neuropsychiatric disorders.

Inverse agonism of lysophospholipids with cationic head groups at Gi-coupled receptor GPR82

GPR82 is an orphan G protein-coupled receptor (GPCR) that has been implicated in lipid storage in mouse adipocytes. However, the intracellular signaling as well as the specific ligands of GPR82 remain unknown. GPR82 is closely related to GPR34, a GPCR for the bioactive lipid molecule lysophosphatidylserine. In this study, we screened a lipid library using GPR82-transfected cells to search for ligands that act on GPR82. By measuring cyclic adenosine monophosphate levels, we found that GPR82 is an apparently constitutively active GPCR that leads to Gi protein activation. In addition, edelfosine (1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine), an artificial lysophospholipid with a cationic head group that exerts antitumor activity, inhibited the Gi protein activation by GPR82. Two endogenous lysophospholipids with cationic head groups, lysophosphatidylcholine (1-oleoyl-sn-glycero-3-phosphocholine) and lysophosphatidylethanolamine (1-oleoyl-sn-glycero-3-phosphoethanolamine), also exhibited GPR82 inhibitory activity, albeit weaker than edelfosine. Förster resonance energy transfer imaging analysis consistently demonstrated that Gi protein-coupled GPR82 has an apparent constitutive activity that is edelfosine-sensitive. Consistent data were obtained from GPR82-mediated binding analysis of guanosine-5'-O-(3-thiotriphosphate) to cell membranes. Furthermore, in GPR82-transfected cells, edelfosine inhibited insulin-induced extracellular signal-regulated kinase activation, like compounds that function as inverse agonists at other GPCRs. Therefore, edelfosine is likely to act as an inverse agonist of GPR82. Finally, GPR82 expression inhibited adipocyte lipolysis, which was abrogated by edelfosine. Our findings suggested that the cationic lysophospholipids edelfosine, lysophosphatidylcholine and lysophosphatidylethanolamine are novel inverse agonists for Gi-coupled GPR82, which is apparently constitutively active, and has the potential to exert lipolytic effects through GPR82.

Structural insight into the constitutive activity of human orphan receptor GPR12

G protein-coupled receptor 12 (GPR12) is an orphan G protein-coupled receptor that is highly expressed in the thalamus of the brain and plays a vital role in driving thalamocortical functions in short-term memory. GPR12 performs high constitutive activity and couples with Gs, increasing the intracellular cyclic adenosine monophosphate (cAMP) level when it is expressed. However, exploitation for drug development is limited since it is unclear how GPR12 initiates self-activation and signal transduction, and whether it can be modulated by endogenous or synthetic ligands. Here, we report the cryo-electron microscopy structure of the GPR12-Gs complex in the absence of agonists. Our structure reveals the key determinants for the intrinsically high basal activity of GPR12, including extracellular loop 2 partially occupying the orthosteric binding pocket, a tight-packed TM1 and TM7, and unique activation-related residues in TM6 and TM7. Together with mutagenesis data, this study will improve our understanding of the function and self-activation of the orphan receptor GPR12, enable the identification of endogenous ligands, and guide drug discovery efforts that target GPR12.

Functional association of NR4A3 downregulation with impaired differentiation in myeloid leukemogenesis

The coincident downregulation of NR4A1 and NR4A3 has been implicated in myeloid leukemogenesis, but it remains unknown how these two genes function in myeloid cells and how their combined downregulation promotes myeloid leukemogenesis. Since NR4A1 abrogation is thought to confer a survival and proliferation advantage to myeloid cells, we hypothesized that downregulation of NR4A3 may have a complementary effect on myeloid cell differentiation. First, we tested the association between differentiation status of leukemic cells and NR4A3 expression using two large clinical datasets from patients with different acute myeloid leukemia (AML) subtypes. The analysis revealed a close association between differentiation status and different subtypes of AML Then, we probed the effects of differentiation-inducing treatments on NR4A3 expression and NR4A3 knockdown on cell differentiation using two myeloid leukemia cell lines. Differentiation-inducing treatments caused upregulation of NR4A3, while NR4A3 knockdown prevented differentiation in both cell lines. The cell culture findings were validated using samples from chronic myeloid leukemia (CML) patients at chronic, accelerated and blastic phases, and in acute promyelocytic leukemia (APL) patients before and after all trans-retinoic acid (ATRA)-based differentiation therapy. Progressive NR4A3 downregulation was coincident with impairments in differentiation in patients during progression to blastic phase of CML, and NR4A3 expression was increased in APL patients treated with ATRA-based differentiating therapy. Together, our findings demonstrate a tight association between impaired differentiation status and NR4A3 downregulation in myeloid leukemias, providing a plausible mechanistic explanation of how myeloid leukemogenesis might occur upon concurrent downregulation of NR4A1 and NR4A3.

Understudied G Protein-Coupled Receptors in the Kidney

G protein-coupled receptors (GPCRs) are cell surface proteins which play a key role in allowing cells, tissues, and organs to respond to changes in the external environment in order to maintain homeostasis. Despite the fact that GPCRs are known to play key roles in a variety of tissues, there are a large subset of GPCRs that remain poorly studied. In this minireview, we will summarize what is known regarding the "understudied" GPCRs with respect to renal function, and in so doing will highlight the promise represented by studying this gene family.

Ecto-GPR37: a potential biomarker for Parkinson's disease

Objective

α-Synuclein has been studied as a potential biomarker for Parkinson’s disease (PD) with no concluding results. Accordingly, there is an urgent need to find out reliable specific biomarkers for PD. GPR37 is an orphan G protein-coupled receptor that toxically accumulates in autosomal recessive juvenile parkinsonism. Here, we investigated whether GPR37 is upregulated in sporadic PD, and thus a suitable potential biomarker for PD.

Methods

GPR37 protein density and mRNA expression in postmortem substantia nigra (SN) from PD patients were analysed by immunoblot and RT-qPCR, respectively. The presence of peptides from the N-terminus-cleaved domain of GPR37 (i.e. ecto-GPR37) in human cerebrospinal fluid (CSF) was determined by liquid chromatography-mass spectrometric analysis. An engineered in-house nanoluciferase-based immunoassay was used to quantify ecto-GPR37 in CSF samples from neurological control (NC) subjects, PD patients and Alzheimer’s disease (AD) patients.

Results

GPR37 protein density and mRNA expression were significantly augmented in sporadic PD. Increased amounts of ecto-GPR37 peptides in the CSF samples from PD patients were identified by mass spectrometry and quantified by the in-house ELISA method. However, the CSF total α-synuclein level in PD patients did not differ from that in NC subjects. Similarly, the cortical GPR37 mRNA expression and CSF ecto-GPR37 levels in AD patients were also unaltered.

Conclusion

GPR37 expression is increased in SN of sporadic PD patients. The ecto-GPR37 peptides are significantly increased in the CSF of PD patients, but not in AD patients. These results open perspectives and encourage further clinical studies to confirm the validity and utility of ecto-GPR37 as a potential PD biomarker.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40035-021-00232-7.

The History of N/OFQ and the NOP Receptor

The discovery of nociceptin/orphanin FQ (N/OFQ) marks the genuine start of the reverse pharmacology era, when systematic hunting for ligands of orphan receptors began. The choice of this particular target was no coincidence as the orphan receptor ORL-1 displayed high similarity to known opioid receptors, and thus its elusive ligand held promise to find more than a ligand but a missing opioid peptide. N/OFQ indeed turned out to belong to the opioid peptide family, but with significant pharmacological and functional distinctions. The quest for understanding N/OFQ's physiological functions has produced some novel insights into stress regulation and many other body functions but is still ongoing almost 25 years after its discovery. This chapter highlights the early steps of orphan receptor research and some of the protagonists who helped to advance the field.

Structure-activity relationships of imidazothiazinones and analogs as antagonists of the cannabinoid-activated orphan G protein-coupled receptor GPR18

GPR18 is a cannabinoid-activated orphan G protein-coupled receptor (GPCR) that is selectively expressed on immune cells. Despite its significant potential as a drug target for inflammatory diseases and cancer immunotherapy, only very few GPR18 ligands have been described to date. In the present study we investigated the structure-activity relationships (SARs) of (Z)-2-(3-(4-chlorobenzyloxy)benzylidene)-6,7-dihydro-2H-imidazo[2,1-b][1,3]thiazin-3(5H)-one (PSB-CB5, 5), the most potent GPR18 antagonist described to date. Analogs were synthesized that exhibit broad modifications of the heterocyclic core and/or variation of substituents at the benzylidene moiety. The compounds were investigated in β-arrestin recruitment assays as inhibitors of human GPR18 activation by tetrahydrocannabinol (THC). Selectivity was assessed versus the cannabinoid receptors (CB₁ and CB₂) and versus GPR55, another orphan GPCR that interacts with cannabinoids. Phenyloxyalkyloxy-substituted benzylidenethiazinones with long alkyl chains (optimal length: hexamethylene) efficiently blocked GPR18 with similarly high potency as lead structure 5. (Z)-2-(3-(6-(4-Chlorophenoxy)hexyloxy)benzylidene)-6,7-dihydro-2H-imidazo[2,1-b][1,3]thiazin-3(5H)-one (PSB-CB-27, 23) exhibited the best profile: it displayed an IC₅₀ value of 650 nM at GPR18 and showed improved selectivity versus CB receptors as compared to lead structure 5. Importantly, in contrast to 5, which showed only partial inhibition (60%), 23 led to a complete blockade of THC-induced GPR18 activation and is thus a superior tool for target validation. In addition, several compounds, e.g. 18 and 22, were identified as dual GPR18/GPR55 antagonists with similar potency at both targets, and selectivity versus CB receptors.

Cannabidiol, a novel inverse agonist for GPR12

GPR12 is a constitutively active, G_s protein-coupled receptor that currently has no confirmed endogenous ligands. GPR12 may be involved in physiological processes such as maintenance of oocyte meiotic arrest and brain development, as well as pathological conditions such as metastatic cancer. In this study, the potential effects of various classes of cannabinoids on GPR12 were tested using a cAMP accumulation assay. Our data demonstrate that cannabidiol (CBD), a major non-psychoactive phytocannabinoid, acted as an inverse agonist to inhibit cAMP accumulation stimulated by the constitutively active GPR12. Thus, GPR12 is a novel molecular target for CBD. The structure-activity relationship studies of CBD indicate that both the free hydroxyl and the pentyl side chain are crucial for the effects of CBD on GPR12. Furthermore, studies using cholera toxin, which blocks G_s protein and pertussis toxin, which blocks G_i protein, revealed that G_s, but not G_i is involved in the inverse agonism of CBD on GPR12. CBD is a promising novel therapeutic agent for cancer, and GPR12 has been shown to alter viscoelasticity of metastatic cancer cells. Since we have demonstrated that CBD is an inverse agonist for GPR12, this provides novel mechanism of action for CBD, and an initial chemical scaffold upon which highly potent and efficacious agents acting on GPR12 may be developed with the ultimate goal of blocking cancer metastasis.

GPR3 and GPR6, novel molecular targets for cannabidiol

GPR3 and GPR6 are members of a family of constitutively active, Gs protein-coupled receptors. Previously, it has been reported that GPR3 is involved in Alzheimer's disease whereas GPR6 plays potential roles in Parkinson's disease. GPR3 and GPR6 are considered orphan receptors because there are no confirmed endogenous agonists for them. However, GPR3 and GPR6 are phylogenetically related to the cannabinoid receptors. In this study, the activities of endocannabinoids and phytocannabinoids were tested on GPR3 and GPR6 using a β-arrestin2 recruitment assay. Among the variety of cannabinoids tested, cannabidiol (CBD), the major non-psychoactive component of marijuana, significantly reduced β-arrestin2 recruitment to both GPR3 and GPR6. In addition, the inhibitory effects of CBD on β-arrestin2 recruitment were concentration-dependent for both GPR3 and GPR6, with a higher potency for GPR6. These data show that CBD acts as an inverse agonist at both GPR3 and GPR6 receptors. These results demonstrate for the first time that both GPR3 and GPR6 are novel molecular targets for CBD. Our discovery that CBD acts as a novel inverse agonist on both GPR3 and GPR6 indicates that some of the potential therapeutic effects of CBD (e.g. treatment of Alzheimer's disease and Parkinson's disease) may be mediated through these important receptors.

Parkinson's disease-associated GPR37 receptor regulates cocaine-mediated synaptic depression in corticostriatal synapses

GPR37 is an orphan G protein-coupled receptor highly expressed in the brain. The precise function of GPR37 is still unknown, but a number of evidences indicate it modulates the dopaminergic system. Here, we aimed to determine the role of GPR37 on the control of cocaine-mediated electrophysiological effects (synaptic transmission and short-term plasticity) in corticostriatal synapses. Accordingly, we evaluated basal synaptic transmission and paired-pulse stimulation (PPS) in wild-type and GPR37KO mice slices. Regardless of the genotype, a low concentration of cocaine (2μM) did not modify basal synaptic transmission. Conversely, a higher dose of cocaine (30μM) decreased synaptic transmission in both genotypes, although with different intensities: approximately 30% in slices from wild-type mice and 45% in slices from GPR37-KO mice. On the other hand, no differences in PPS ratio were observed between wild-type and GPR37-KO cocaine-treated mice. Overall, our data suggest that GPR37 is involved in cocaine-induced modification of basal synaptic transmission without modifying cocaine effects in short-term plasticity.

GPR18 undergoes a high degree of constitutive trafficking but is unresponsive to N-Arachidonoyl Glycine

The orphan receptor GPR18 has become a research target following the discovery of a putative endogenous agonist, N-arachidonoyl glycine (NAGly). Chemical similarity between NAGly and the endocannabinoid anandamide suggested the hypothesis that GPR18 is a third cannabinoid receptor. GPR18-mediated cellular signalling through inhibition of cyclic adenosine monophosphate (cAMP) and phosphorylation of extracellular signal-regulated kinase (ERK), in addition to physiological consequences such as regulation of cellular migration and proliferation/apoptosis have been described in response to both NAGly and anandamide. However, discordant findings have also been reported. Here we sought to describe the functional consequences of GPR18 activation in heterologously-expressing HEK cells. GPR18 expression was predominantly intracellular in stably transfected cell lines, but moderate cell surface expression could be achieved in transiently transfected cells which also had higher overall expression. Assays were employed to characterise the ability of NAGly or anandamide to inhibit cAMP or induce ERK phosphorylation through GPR18, or induce receptor trafficking. Positive control experiments, which utilised cells expressing hCB1 receptors (hCB1R), were performed to validate assay design and performance. While these functional pathways in GPR18-expressing cells were not modified on treatment with a panel of putative GPR18 ligands, a constitutive phenotype was discovered for this receptor. Our data reveal that GPR18 undergoes rapid constitutive receptor membrane trafficking-several-fold faster than hCB1R, a highly constitutively active receptor. To enhance the likelihood of detecting agonist-mediated receptor signalling responses, we increased GPR18 protein expression (by tagging with a preprolactin signal sequence) and generated a putative constitutively inactive receptor by mutating the hGPR18 gene at amino acid site 108 (alanine to asparagine). This A108N mutant did cause an increase in surface receptor expression (which may argue for reduced constitutive activity), but no ligand-mediated effects were detected. Two glioblastoma multiforme cell lines (which endogenously express GPR18) were assayed for NAGly-induced pERK phosphorylation, with negative results. Despite a lack of ligand-mediated responses in all assays, the constitutive trafficking of GPR18 remains an interesting facet of receptor function and will have consequences for understanding the role of GPR18 in physiology.

Historical overview of nuclear receptors

This review summarizes the birth of the field of nuclear receptors, from Jensen's discovery of estrogen receptor alpha, Gustafsson's discovery of the three-domain structure of the glucocorticoid receptor, the discovery of the glucocorticoid response element and the first partial cloning of the glucocorticoid receptor. Furthermore the discovery of the novel receptors called orphan receptors is described.

GnRH-(1-5) activates matrix metallopeptidase-9 to release epidermal growth factor and promote cellular invasion

In the extracellular space, the gonadotropin-releasing hormone (GnRH) is metabolized by the zinc metalloendopeptidase EC3.4.24.15 (EP24.15) to form the pentapeptide, GnRH-(1-5). GnRH-(1-5) diverges in function and mechanism of action from GnRH in the brain and periphery. GnRH-(1-5) acts on the orphan G protein-coupled receptor 101 (GPR101) to sequentially stimulate epidermal growth factor (EGF) release, phosphorylate the EGF receptor (EGFR), and facilitate cellular migration. These GnRH-(1-5) actions are dependent on matrix metallopeptidase (MMP) activity. Here, we demonstrated that these GnRH-(1-5) effects are dependent on increased MMP-9 enzymatic activity in the Ishikawa and ECC-1 cell lines. Furthermore, the effects of GnRH-(1-5) mediated by GPR101 and the subsequent increase in MMP-9 enzymatic activity lead to an increase in cellular invasion. These results suggest that GnRH-(1-5) and GPR101 regulation of MMP-9 may have physiological relevance in the metastatic potential of endometrial cancer cells.

Adenosine A2A receptor-mediated control of pilocarpine-induced tremulous jaw movements is Parkinson's disease-associated GPR37 receptor-dependent

GPR37, also known as parkin associated endothelin-like receptor (Pael-R), is an orphan GPCR that aggregates intracellularly in a juvenile form of Parkinson's disease. However, little is known about the function of this orphan receptor. Here, using a model for parkisonian tremor, the pilocarpine-induced tremulous jaw movements (TJMs), we show that the deletion of GPR37 attenuated the TJMs in response to this cholinomimetic. Interestingly, the control that adenosine A2A receptor exerted over TJMs was lost in the absence of GPR37, thus pointing to a pivotal role of this orphan receptor in the adenosinergic control of parkinsonian tremor.

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.

The Camerino symposium series (1978-2013): a privileged observatory of receptorology development

The organizers of the Camerino Receptor Symposia survey the development of receptorology. They trace the course from the first Symposium in 1978, which laid the foundation for Pirenzepine, the first selective muscarinic antagonist, to the 2010 Symposium, which highlighted the utility of functional simple domain antibodies (nanobodies) as novel G Protein-Coupled Receptor (GPCR) modulators. This 30-year period sees the acceptance of terms such as G-protein, auto- and heteroreceptors, site-directed mutagenesis, chimeric receptors, constitutive activity, inverse agonism, and orphan receptors. GPCRs are finally a reality and Langley and Ehrlich, if they returned to their laboratories, would be proud of how their intuitions have been realized.

Anaplastic lymphoma kinase: "Ligand Independent Activation" mediated by the PTN/RPTPβ/ζ signaling pathway

Anaplastic lymphoma kinase is essential in early development, differentiation, and maintenance of cell survival; nevertheless, the mechanism to activate ALK has remained elusive. ALK has remained an "Orphan Receptor." The studies cited below describe a unique mechanism termed "Ligand Independent Activation." It is shown that activation of ALK results when the cytokine pleiotrophin (PTN) interacts with its receptor, the receptor protein tyrosine phosphatase β/ζ (RPTPβ/ζ). Pleiotrophin inactivates the catalytic activity of RPTPβ/ζ, which, when not inactivated, dephosphorylates phosphotyrosine sites in the activation domain of ALK; as a consequence of the inactivation of RPTPβ/ζ by PTN, autophosphorylation and autoactivation of ALK rapidly follow. The PTN/RPTPβ/ζ signaling pathway thus regulates the catalytic activity of ALK and tyrosine phosphorylation levels of ALK downstream target proteins. Furthermore, since ALK is only one of the key ALK phosphoproteins targeted by the PTN/RPTPβ/ζ signaling pathway, the PTN/RPTPβ/ζ signaling pathway has the potential to coordinately regulate tyrosine phosphorylation of other different key proteins in multiple cellular compartments. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.