Structural and functional evolution of the P2Y(12)-like receptor group

Modulating vertebrate physiology by genomic fine-tuning of GPCR functions

G protein-coupled receptors (GPCRs) play a crucial role as membrane receptors, facilitating the communication of eukaryotic species with their environment and regulating cellular and organ interactions. Consequently, GPCRs hold immense potential in contributing to adaptation to ecological niches and responding to environmental shifts. Comparative analyses of vertebrate genomes reveal patterns of GPCR gene loss, expansion, and signatures of selection. Integrating these genomic data with insights from functional analyses of gene variants enables the interpretation of genotype-phenotype correlations. This review underscores the involvement of GPCRs in adaptive processes, presenting numerous examples of how alterations in GPCR functionality influence vertebrate physiology or, conversely, how environmental changes impact GPCR functions. The findings demonstrate that modifications in GPCR function contribute to adapting to aquatic, arid, and nocturnal habitats, influencing camouflage strategies, and specializing in particular dietary preferences. Furthermore, the adaptability of GPCR functions provides an effective mechanism in facilitating past, recent, or ongoing adaptations in animal domestication and human evolution and should be considered in therapeutic strategies and drug development.

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.

Reduced urine volume and changed renal sphingolipid metabolism in P2ry14-deficient mice

The UDP-glucose receptor P2RY14, a rhodopsin-like G protein-coupled receptor (GPCR), was previously described as receptor expressed in A-intercalated cells of the mouse kidney. Additionally, we found P2RY14 is abundantly expressed in mouse renal collecting duct principal cells of the papilla and epithelial cells lining the renal papilla. To better understand its physiological function in kidney, we took advantage of a P2ry14 reporter and gene-deficient (KO) mouse strain. Morphometric studies showed that the receptor function contributes to kidney morphology. KO mice had a broader cortex relative to the total kidney area than wild-type (WT) mice. In contrast, the area of the outer stripe of the outer medulla was larger in WT compared to KO mice. Transcriptome comparison of the papilla region of WT and KO mice revealed differences in the gene expression of extracellular matrix proteins (e.g., decorin, fibulin-1, fibulin-7) and proteins involved in sphingolipid metabolism (e.g., small subunit b of the serine palmitoyltransferase) and other related GPCRs (e.g., GPR171). Using mass spectrometry, changes in the sphingolipid composition (e.g., chain length) were detected in the renal papilla of KO mice. At the functional level, we found that KO mice had a reduced urine volume but an unchanged glomerular filtration rate under normal chow and salt diets. Our study revealed P2ry14 as a functionally important GPCR in collecting duct principal cells and cells lining the renal papilla and the possible involvement of P2ry14 in nephroprotection by regulation of decorin.

G protein-coupled receptors (GPCRs) in rotifers and cladocerans: Potential applications in ecotoxicology, ecophysiology, comparative endocrinology, and pharmacology

The G protein-coupled receptor (GPCR) superfamily plays a fundamental role in both sensory functions and the regulation of homeostasis, and is highly conserved across the eukaryote taxa. Its functional diversity is related to a conserved seven-transmembrane core and invariant set of intracellular signaling mechanisms. The interplay between these properties is key to the evolutionary success of GPCR. As this superfamily originated from a common ancestor, GPCR genes have evolved via lineage-specific duplications through the process of adaptation. Here we summarized information on GPCR gene families in rotifers and cladocerans based on their evolutionary position in aquatic invertebrates and their potential application in ecotoxicology, ecophysiology, comparative endocrinology, and pharmacology. Phylogenetic analyses were conducted to examine the evolutionary significance of GPCR gene families and to provide structural insight on their role in aquatic invertebrates. In particular, most GPCR gene families have undergone sporadic evolutionary processes, but some GPCRs are highly conserved across species despite the dynamics of GPCR evolution. Overall, this review provides a better understanding of GPCR evolution in aquatic invertebrates and expand our knowledge of the potential application of these receptors in various fields.

The genome of the minute marine rotifer Proales similis: Genome-wide identification of 401 G protein-coupled receptor (GPCR) genes

The minute marine rotifer Proales similis is a potential model species for ecotoxicological and ecophysiological studies. Therefore, the provision of whole-genome data for P. similis is an easy way to deepen understanding of the molecular mechanisms involved in response to various environmental stressors. In this research, we assembled the whole-genome sequence (32.7 Mb total, N50 = 2.42 Mb) of P. similis, consisting of 15 contigs with 10,785 annotated genes. To understand the ligand-receptor signaling pathway in rotifers in response to environmental cues, we identified 401 G protein-coupled receptor (GPCR) genes in the P. similis genome and compared them with those from other species. The 401 full-length GPCR genes were classified into five distinct classes: A (363), B (18), C (7), F (2), and other (11). Most GPCR gene families have undergone sporadic evolutionary processes. However, some classes were highly conserved between species. Overall, this result provides new information about GPCR-based signaling pathways and the evolution of GPCRs in the minute rotifer P. similis, and it expands our knowledge of ligand-receptor signaling pathways in response to various environmental cues.

Zebrafish for thrombocytopoiesis- and hemostasis-related researches and disorders

Platelets play vital roles in hemostasis, inflammation, and vascular biology. Platelets are also active participants in the immune responses. As vertebrates, zebrafish have a highly conserved hematopoietic system in the developmental, cellular, functional, biochemical, and genetic levels with mammals. Thrombocytes in zebrafish are functional homologs of mammalian platelets. Here, we summarized thrombocyte development, function, and related research techniques in zebrafish, and reviewed available zebrafish models of platelet-associated disorders, including congenital amegakaryocytic thrombocytopenia, inherited thrombocytopenia, essential thrombocythemia, and blood coagulation disorders such as gray platelet syndrome. These elegant zebrafish models and methods are crucial for understanding the molecular and genetic mechanisms of thrombocyte development and function, and provide deep insights into related human disease pathophysiology and drug development.

Update of P2Y receptor pharmacology: IUPHAR Review 27

Eight G protein-coupled P2Y receptor subtypes respond to extracellular adenine and uracil mononucleotides and dinucleotides. P2Y receptors belong to the δ group of rhodopsin-like GPCRs and contain two structurally distinct subfamilies: P2Y₁ , P2Y₂ , P2Y₄ , P2Y₆ , and P2Y₁₁ (principally G_q protein-coupled P2Y₁ -like) and P2Y_12-14 (principally G_i protein-coupled P2Y₁₂ -like) receptors. Brain P2Y receptors occur in neurons, glial cells, and vasculature. Endothelial P2Y₁ , P2Y₂ , P2Y₄ , and P2Y₆ receptors induce vasodilation, while smooth muscle P2Y₂ , P2Y₄ , and P2Y₆ receptor activation leads to vasoconstriction. Pancreatic P2Y₁ and P2Y₆ receptors stimulate while P2Y₁₃ receptors inhibits insulin secretion. Antagonists of P2Y₁₂ receptors, and potentially P2Y₁ receptors, are anti-thrombotic agents, and a P2Y₂ /P2Y₄ receptor agonist treats dry eye syndrome in Asia. P2Y receptor agonists are generally pro-inflammatory, and antagonists may eventually treat inflammatory conditions. This article reviews recent developments in P2Y receptor pharmacology (using synthetic agonists and antagonists), structure and biophysical properties (using X-ray crystallography, mutagenesis and modelling), physiological and pathophysiological roles, and present and potentially future therapeutic targeting.

Extracellular Nucleotides and P2 Receptors in Renal Function

The understanding of the nucleotide/P2 receptor system in the regulation of renal hemodynamics and transport function has grown exponentially over the last 20 yr. This review attempts to integrate the available data while also identifying areas of missing information. First, the determinants of nucleotide concentrations in the interstitial and tubular fluids of the kidney are described, including mechanisms of cellular release of nucleotides and their extracellular breakdown. Then the renal cell membrane expression of P2X and P2Y receptors is discussed in the context of their effects on renal vascular and tubular functions. Attention is paid to effects on the cortical vasculature and intraglomerular structures, autoregulation of renal blood flow, tubuloglomerular feedback, and the control of medullary blood flow. The role of the nucleotide/P2 receptor system in the autocrine/paracrine regulation of sodium and fluid transport in the tubular and collecting duct system is outlined together with its role in integrative sodium and fluid homeostasis and blood pressure control. The final section summarizes the rapidly growing evidence indicating a prominent role of the extracellular nucleotide/P2 receptor system in the pathophysiology of the kidney and aims to identify potential therapeutic opportunities, including hypertension, lithium-induced nephropathy, polycystic kidney disease, and kidney inflammation. We are only beginning to unravel the distinct physiological and pathophysiological influences of the extracellular nucleotide/P2 receptor system and the associated therapeutic perspectives.

Revisiting the classification of adhesion GPCRs

G protein–coupled receptors (GPCRs) are encoded by over 800 genes in the human genome. Motivated by different scientific rationales, the two classification systems that are mainly in use, the ABC and GRAFS systems, organize GPCRs according to their pharmacological features and phylogenetic relations, respectively. Within those systems, adhesion GPCRs (aGPCRs) constitute a group of over 30 mammalian homologs, most of which are still orphans with undefined activating signals and signal transduction properties. Previous efforts have further subdivided mammalian aGPCRs into nine subfamilies to indicate phylogenetic relationships. However, this subclassification scheme has shortcomings and inconsistencies that require attention. Here, we have reassessed the phylogenetic relationships of aGPCRs from vertebrate and invertebrate species. Our findings confirm that secretin receptor–like GPCRs most probably emerged from ancestral aGPCRs. We show that reassignment of several aGPCRs to families essentially requires input from functional data. Our analyses establish the need for introducing novel aGPCR subfamilies due to aGPCR sequences from invertebrate species that are not readily assignable to any existing subfamily. We conclude that the current classification systems ought to be updated to consider an unambiguous taxonomy of a hierarchically organized classification and pharmacological properties, and to accommodate phylogenetic affiliations between aGPCR genes within mammals and across the animal kingdom.

Breakthrough in GPCR Crystallography and Its Impact on Computer-Aided Drug Design

Recent crystallographic structures of G protein-coupled receptors (GPCRs) have greatly advanced our understanding of the recognition of their diverse agonist and antagonist ligands. We illustrate here how this applies to A_2A adenosine receptors (ARs) and to P2Y₁ and P2Y₁₂ receptors (P2YRs) for ADP. These X-ray structures have impacted the medicinal chemistry aimed at discovering new ligands for these two receptor families, including receptors that have not yet been crystallized but are closely related to the known structures. In this Chapter, we discuss recent structure-based drug design projects that led to the discovery of: (a) novel A₃AR agonists based on a highly rigidified (N)-methanocarba scaffold for the treatment of chronic neuropathic pain and other conditions, (b) fluorescent probes of the ARs and P2Y₁₄R, as chemical tools for structural probing of these GPCRs and for improving assay capabilities, and (c) new more drug-like antagonists of the inflammation-related P2Y₁₄R. We also describe the computationally enabled molecular recognition of positive (for A₃AR) and negative (P2Y₁R) allosteric modulators that in some cases are shown to be consistent with structure-activity relationship (SAR) data. Thus, computational modeling has become an essential tool for the design of purine receptor ligands.

The G protein-coupled receptor GPR34 - The past 20 years of a grownup

Research on GPR34, which was discovered in 1999 as an orphan G protein-coupled receptor of the rhodopsin-like class, disclosed its physiologic relevance only piece by piece. Being present in all recent vertebrate genomes analyzed so far it seems to improve the fitness of species although it is not essential for life and reproduction as GPR34-deficient mice demonstrate. However, closer inspection of macrophages and microglia, where it is mainly expressed, revealed its relevance in immune cell function. Recent data clearly demonstrate that GPR34 function is required to arrest microglia in the M0 homeostatic non-phagocytic phenotype. Herein, we summarize the current knowledge on its evolution, genomic and structural organization, physiology, pharmacology and relevance in human diseases including neurodegenerative diseases and cancer, which accumulated over the last 20 years.

P2Y Receptors in Immune Response and Inflammation

Metabotropic pyrimidine and purine nucleotide receptors (P2Y receptors) are expressed in virtually all cells with implications in very diverse biological functions, including the well-established platelet aggregation (P2Y12), but also immune regulation and inflammation. The classical P2Y receptors bind nucleotides and are encoded by eight genes with limited sequence homology, while phylogenetically related receptors (e.g., P2Y12-like) recognize lipids and peptides, but also nucleotide derivatives. Growing lines of evidence suggest an important function of P2Y receptors in immune cell differentiation and maturation, migration, and cell apoptosis. Here, we give a perspective on the P2Y receptors' molecular structure and physiological importance in immune cells, as well as the related diseases and P2Y-targeting therapies. Extensive research is being undertaken to find modulators of P2Y receptors and uncover their physiological roles. We anticipate the medical applications of P2Y modulators and their immune relevance.

Prostaglandin E2 glyceryl ester is an endogenous agonist of the nucleotide receptor P2Y6

Cyclooxygenase-2 catalyses the biosynthesis of prostaglandins from arachidonic acid but also the biosynthesis of prostaglandin glycerol esters (PG-Gs) from 2-arachidonoylglycerol. Previous studies identified PG-Gs as signalling molecules involved in inflammation. Thus, the glyceryl ester of prostaglandin E2, PGE2-G, mobilizes Ca2+ and activates protein kinase C and ERK, suggesting the involvement of a G protein-coupled receptor (GPCR). To identify the endogenous receptor for PGE2-G, we performed a subtractive screening approach where mRNA from PGE2-G response-positive and -negative cell lines was subjected to transcriptome-wide RNA sequencing analysis. We found several GPCRs that are only expressed in the PGE2-G responder cell lines. Using a set of functional readouts in heterologous and endogenous expression systems, we identified the UDP receptor P2Y6 as the specific target of PGE2-G. We show that PGE2-G and UDP are both agonists at P2Y6, but they activate the receptor with extremely different EC50 values of ~1 pM and ~50 nM, respectively. The identification of the PGE2-G/P2Y6 pair uncovers the signalling mode of PG-Gs as previously under-appreciated products of cyclooxygenase-2.

G protein-coupled receptor 34 in ovarian granulosa cells of cattle: changes during follicular development and potential functional implications

Abundance of G protein-coupled receptor 34 (GPR34) mRNA is greater in granulosa cells (GCs) of cystic vs normal follicles of cattle. The present experiments were designed to determine if GPR34 mRNA in granulosa cell [GC] changes during selection and growth of dominant follicles in cattle as well as to investigate the hormonal regulation of GPR34 mRNA in bovine GC in vitro. In Exp. 1, estrous cycles of nonlactating cows were synchronized and then ovariectomized on either day 3-4 or 5-6 after ovulation. GPR34 mRNA abundance in GC was 2.8- to 3.8-fold greater (P < 0.05) in small (1-5 mm) and large (≥8 mm) estrogen-inactive dominant follicles than in large estrogen-active follicles. Also, GPR34 mRNA tended to be greater (P < 0.10) in F2 than F1 follicles on day 3-4 postovulation. In Exp. 2-7, ovaries were collected at an abattoir and GC were isolated and treated in vitro. Expression of GPR34 was increased (P < 0.05) 2.2-fold by IGF1. Tumor necrosis factor (TNF)-α decreased (P < 0.05) the IGF1-induced GPR34 mRNA abundance in small-follicle GC, whereas IGF1 decreased (P < 0.05) GPR34 expression by 45% in large-follicle GC. Treatment of small-follicle GC with either IL-2, prostaglandin E2 or angiogenin decreased (P < 0.05) GPR34 expression, whereas FSH, cortisol, wingless 3A, or hedgehog proteins did not affect (P > 0.10) GPR34 expression. In Exp. 6 and 7, 2 presumed ligands of GPR34, L-a-lysophosphatidylserine (LPPS) and LPP-ethanolamine, increased (P < 0.05) GC numbers and estradiol production by 2-fold or more in small-follicle GC, and this response was only observed in IGF1-treated GC. In conclusion, GPR34 is a developmentally and hormonally regulated gene in GC, and its presumed ligands enhance IGF1-induced proliferation and steroidogenesis of bovine GC.

Regulation of Vascular and Renal Function by Metabolite Receptors

To maintain metabolic homeostasis, the body must be able to monitor the concentration of a large number of substances, including metabolites, in real time and to use that information to regulate the activities of different metabolic pathways. Such regulation is achieved by the presence of sensors, termed metabolite receptors, in various tissues and cells of the body, which in turn convey the information to appropriate regulatory or positive or negative feedback systems. In this review, we cover the unique roles of metabolite receptors in renal and vascular function. These receptors play a wide variety of important roles in maintaining various aspects of homeostasis-from salt and water balance to metabolism-by sensing metabolites from a wide variety of sources. We discuss the role of metabolite sensors in sensing metabolites generated locally, metabolites generated at distant tissues or organs, or even metabolites generated by resident microbes. Metabolite receptors are also involved in various pathophysiological conditions and are being recognized as potential targets for new drugs. By highlighting three receptor families-(a) citric acid cycle intermediate receptors, (b) purinergic receptors, and

Targeting renal purinergic signalling for the treatment of lithium-induced nephrogenic diabetes insipidus

Lithium still retains its critical position in the treatment of bipolar disorder by virtue of its ability to prevent suicidal tendencies. However, chronic use of lithium is often limited by the development of nephrogenic diabetes insipidus (NDI), a debilitating condition. Lithium-induced NDI is due to resistance of the kidney to arginine vasopressin (AVP), leading to polyuria, natriuresis and kaliuresis. Purinergic signalling mediated by extracellular nucleotides (ATP/UTP), acting via P2Y receptors, opposes the action of AVP on renal collecting duct (CD) by decreasing the cellular cAMP and thus AQP2 protein levels. Taking a cue from this phenomenon, we discovered the potential involvement of ATP/UTP-activated P2Y2 receptor in lithium-induced NDI in rats and showed that P2Y2 receptor knockout mice are significantly resistant to Li-induced polyuria, natriuresis and kaliuresis. Extension of these studies revealed that ADP-activated P2Y12 receptor is expressed in the kidney, and its irreversible blockade by the administration of clopidogrel bisulphate (Plavix(®)) ameliorates Li-induced NDI in rodents. Parallel in vitro studies showed that P2Y12 receptor blockade by the reversible antagonist PSB-0739 sensitizes CD to the action of AVP. Thus, our studies unravelled the potential beneficial effects of targeting P2Y2 or P2Y12 receptors to counter AVP resistance in lithium-induced NDI. If established in further studies, our findings may pave the way for the development of better and safer methods for the treatment of NDI by bringing a paradigm shift in the approach from the current therapies that predominantly counter the anti-AVP effects to those that enhance the sensitivity of the kidney to AVP action.

P2Y12 Receptor Localizes in the Renal Collecting Duct and Its Blockade Augments Arginine Vasopressin Action and Alleviates Nephrogenic Diabetes Insipidus

P2Y12 receptor (P2Y12-R) signaling is mediated through Gi, ultimately reducing cellular cAMP levels. Because cAMP is a central modulator of arginine vasopressin (AVP)-induced water transport in the renal collecting duct (CD), we hypothesized that if expressed in the CD, P2Y12-R may play a role in renal handling of water in health and in nephrogenic diabetes insipidus. We found P2Y12-R mRNA expression in rat kidney, and immunolocalized its protein and aquaporin-2 (AQP2) in CD principal cells. Administration of clopidogrel bisulfate, an irreversible inhibitor of P2Y12-R, significantly increased urine concentration and AQP2 protein in the kidneys of Sprague-Dawley rats. Notably, clopidogrel did not alter urine concentration in Brattleboro rats that lack AVP. Clopidogrel administration also significantly ameliorated lithium-induced polyuria, improved urine concentrating ability and AQP2 protein abundance, and reversed the lithium-induced increase in free-water excretion, without decreasing blood or kidney tissue lithium levels. Clopidogrel administration also augmented the lithium-induced increase in urinary AVP excretion and suppressed the lithium-induced increase in urinary nitrates/nitrites (nitric oxide production) and 8-isoprostane (oxidative stress). Furthermore, selective blockade of P2Y12-R by the reversible antagonist PSB-0739 in primary cultures of rat inner medullary CD cells potentiated the expression of AQP2 and AQP3 mRNA, and cAMP production induced by dDAVP (desmopressin). In conclusion, pharmacologic blockade of renal P2Y12-R increases urinary concentrating ability by augmenting the effect of AVP on the kidney and ameliorates lithium-induced NDI by potentiating the action of AVP on the CD. This strategy may offer a novel and effective therapy for lithium-induced NDI.

The G protein-coupled receptor P2Y14 influences insulin release and smooth muscle function in mice

UDP sugars were identified as extracellular signaling molecules, assigning a new function to these compounds in addition to their well defined role in intracellular substrate metabolism and storage. Previously regarded as an orphan receptor, the G protein-coupled receptor P2Y14 (GPR105) was found to bind extracellular UDP and UDP sugars. Little is known about the physiological functions of this G protein-coupled receptor. To study its physiological role, we used a gene-deficient mouse strain expressing the bacterial LacZ reporter gene to monitor the physiological expression pattern of P2Y14. We found that P2Y14 is mainly expressed in pancreas and salivary glands and in subpopulations of smooth muscle cells of the gastrointestinal tract, blood vessels, lung, and uterus. Among other phenotypical differences, knock-out mice showed a significantly impaired glucose tolerance following oral and intraperitoneal glucose application. An unchanged insulin tolerance suggested altered pancreatic islet function. Transcriptome analysis of pancreatic islets showed that P2Y14 deficiency significantly changed expression of components involved in insulin secretion. Insulin secretion tests revealed a reduced insulin release from P2Y14-deficient islets, highlighting P2Y14 as a new modulator of proper insulin secretion.

Identification of GPR99 protein as a potential third cysteinyl leukotriene receptor with a preference for leukotriene E4 ligand

The cysteinyl leukotrienes (cys-LTs), leukotriene C4 (LTC4), a conjugation product of glutathione and eicosatetraenoic acid, and its metabolites, LTD4 and LTE4, are lipid mediators of smooth muscle constriction and inflammation in asthma. LTD4 is the most potent ligand for the type 1 cys-LT receptor (CysLT1R), and LTC4 and LTD4 have similar lesser potency for CysLT2R, whereas LTE4 has little potency for either receptor. Cysltr1/Cysltr2(-/-) mice, lacking the two defined receptors, exhibited a comparable dose-dependent vascular leak to intradermal injection of LTC4 or LTD4 and an augmented response to LTE4 as compared with WT mice. As LTE4 retains a cysteine residue and might provide recognition via a dicarboxylic acid structure, we screened cDNAs within the P2Y nucleotide receptor family containing CysLTRs and dicarboxylic acid receptors with trans-activator reporter gene assays. GPR99, previously described as an oxoglutarate receptor (Oxgr1), showed both a functional and a binding response to LTE4 in these transfectants. We generated Gpr99(-/-) and Gpr99/Cysltr1/Cysltr2(-/-) mice for comparison with WT and Cysltr1/Cysltr2(-/-) mice. Strikingly, GPR99 deficiency in the Cysltr1/Cysltr2(-/-) mice virtually eliminated the vascular leak in response to the cys-LT ligands, indicating GPR99 as a potential CysLT3R active in the Cysltr1/Cysltr2(-/-) mice. Importantly, the Gpr99(-/-) mice showed a dose-dependent loss of LTE4-mediated vascular permeability, but not to LTC4 or LTD4, revealing a preference of GPR99 for LTE4 even when CysLT1R is present. As LTE4 is the predominant cys-LT species in inflamed tissues, GPR99 may provide a new therapeutic target.

A Strategy Combining Differential Low-Throughput Screening and Virtual Screening (DLS-VS) Accelerating the Discovery of new Modulators for the Orphan GPR34 Receptor

The DLS-VS strategy was developed as an integrated method for identifying chemical modulators for orphan GPCRs. It combines differential low-throughput screening (DLS) and virtual screening (VS). The two cascaded techniques offer complementary advantages and allow the experimental testing of a minimal number of compounds. First, DLS identifies modulators specific for the considered receptor among a set of receptors, through the screening of a small library with diverse chemical compounds. Then, an active molecular model of the receptor is built by homology to a validated template, and it is progressively refined by rotamers modification for key side-chains, by VS of the already screened library, and by iterative selection of the model generating the best enrichment. The refined active model is finally used for the VS of a large chemical library and the selection of a small set of compounds for experimental testing. Applied to the orphan receptor GPR34, the DLS-VS strategy combined the experimental screening of 20 000 compounds and the virtual screening of 1 250 000 compounds. It identified one agonist and eight inverse agonists, showing a high chemical diversity. We describe the method. The strategy can be applied to other GPCRs.

The ligand specificity of the G-protein-coupled receptor GPR34

Lyso-PS (lyso-phosphatidylserine) has been shown to activate the G(i/o)-protein-coupled receptor GPR34. Since in vitro and in vivo studies provided controversial results in assigning lyso-PS as the endogenous agonist for GPR34, we investigated the evolutionary conservation of agonist specificity in more detail. Except for some fish GPR34 subtypes, lyso-PS has no or very weak agonistic activity at most vertebrate GPR34 orthologues investigated. Using chimaeras we identified single positions in the second extracellular loop and the transmembrane helix 5 of carp subtype 2a that, if transferred to the human orthologue, enabled lyso-PS to activate the human GPR34. Significant improvement of agonist efficacy by changing only a few positions strongly argues against the hypothesis that nature optimized GPR34 as the receptor for lyso-PS. Phylogenetic analysis revealed several positions in some fish GPR34 orthologues which are under positive selection. These structural changes may indicate functional specification of these orthologues which can explain the species- and subtype-specific pharmacology of lyso-PS. Furthermore, we identified aminoethyl-carbamoyl ATP as an antagonist of carp GPR34, indicating ligand promiscuity with non-lipid compounds. The results of the present study suggest that lyso-PS has only a random agonistic activity at some GPR34 orthologues and the search for the endogenous agonist should consider additional chemical entities.

Reduced food intake and body weight in mice deficient for the G protein-coupled receptor GPR82

G protein-coupled receptors (GPCR) are involved in the regulation of numerous physiological functions. Therefore, GPCR variants may have conferred important selective advantages during periods of human evolution. Indeed, several genomic loci with signatures of recent selection in humans contain GPCR genes among them the X-chromosomally located gene for GPR82. This gene encodes a so-called orphan GPCR with unknown function. To address the functional relevance of GPR82 gene-deficient mice were characterized. GPR82-deficient mice were viable, reproduced normally, and showed no gross anatomical abnormalities. However, GPR82-deficient mice have a reduced body weight and body fat content associated with a lower food intake. Moreover, GPR82-deficient mice showed decreased serum triacylglyceride levels, increased insulin sensitivity and glucose tolerance, most pronounced under Western diet. Because there were no differences in respiratory and metabolic rates between wild-type and GPR82-deficient mice our data suggest that GPR82 function influences food intake and, therefore, energy and body weight balance. GPR82 may represent a thrifty gene most probably representing an advantage during human expansion into new environments.

Molecular evolution of the transmembrane domains of G protein-coupled receptors

G protein-coupled receptors (GPCRs) are a superfamily of integral membrane proteins vital for signaling and are important targets for pharmaceutical intervention in humans. Previously, we identified a group of ten amino acid positions (called key positions), within the seven transmembrane domain (7TM) interhelical region, which had high mutual information with each other and many other positions in the 7TM. Here, we estimated the evolutionary selection pressure at those key positions. We found that the key positions of receptors for small molecule natural ligands were under strong negative selection. Receptors naturally activated by lipids had weaker negative selection in general when compared to small molecule-activated receptors. Selection pressure varied widely in peptide-activated receptors. We used this observation to predict that a subgroup of orphan GPCRs not under strong selection may not possess a natural small-molecule ligand. In the subgroup of MRGX1-type GPCRs, we identified a key position, along with two non-key positions, under statistically significant positive selection.

Evolution of GPCR: change and continuity

Once introduced into the very early eukaryotic blueprint, seven-transmembrane receptors soon became the central and versatile components of the evolutionary highly successful G protein-coupled transmembrane signaling mechanism. In contrast to all other components of this signal transduction pathway, G protein-coupled receptors (GPCR) evolved in various structural families, eventually comprising hundreds of members in vertebrate genomes. Their functional diversity is in contrast to the conserved transmembrane core and the invariant set of intracellular signaling mechanisms, and it may be the interplay of these properties that is the key to the evolutionary success of GPCR. The GPCR repertoires retrieved from extant vertebrate genomes are the recent endpoints of this long evolutionary process. But the shaping of the fine structure and the repertoire of GPCR is still ongoing, and signatures of recent selection acting on GPCR genes can be made visible by modern population genetic methods. The very dynamic evolution of GPCR can be analyzed from different perspectives: at the levels of sequence comparisons between species from different families, orders and classes, and at the level of populations within a species. Here, we summarize the main conclusions from studies at these different levels with a specific focus on the more recent evolutionary dynamics of GPCR.

Molecular pharmacology, physiology, and structure of the P2Y receptors

The P2Y receptors are a widely expressed group of eight nucleotide-activated G protein-coupled receptors (GPCRs). The P2Y(1)(ADP), P2Y(2)(ATP/UTP), P2Y(4)(UTP), P2Y(6)(UDP), and P2Y(11)(ATP) receptors activate G(q) and therefore robustly promote inositol lipid signaling responses. The P2Y(12)(ADP), P2Y(13)(ADP), and P2Y(14)(UDP/UDP-glucose) receptors activate G(i) leading to inhibition of adenylyl cyclase and to Gβγ-mediated activation of a range of effector proteins including phosphoinositide 3-kinase-γ, inward rectifying K(+) (GIRK) channels, phospholipase C-β2 and -β3, and G protein-receptor kinases 2 and 3. A broad range of physiological responses occur downstream of activation of these receptors ranging from Cl(-) secretion by epithelia to aggregation of platelets to neurotransmission. Useful structural models of the P2Y receptors have evolved from extensive genetic analyses coupled with molecular modeling based on three-dimensional structures obtained for rhodopsin and several other GPCRs. Selective ligands have been synthesized for most of the P2Y receptors with the most prominent successes attained with highly selective agonist and antagonist molecules for the ADP-activated P2Y(1) and P2Y(12) receptors. The widely prescribed drug, clopidogrel, which results in irreversible blockade of the platelet P2Y(12) receptor, is the most important therapeutic agent that targets a P2Y receptor.

Mining human genome for novel purinergic P2Y receptors: a sequence analysis and molecular modeling approach

The purinergic P2Y receptors are G-protein coupled receptors (GPCRs) that control many physiological processes by mediating cellular responses to purines, pyrimidines and their analogues. They can be used as potential therapeutic targets in a variety of disease conditions. Therefore, it is critical to identify new members of this family of receptors from the human genome and characterize them for their role in health and disease. In the present work, molecular modeling was carried out for the 21 known P2Y receptors. Binding site analysis was done on the basis of docking and site-directed mutagenesis data. Thus, conserved features of P2Y receptors could be formulated. These features can be used to determine the purinergic nature of potential P2Y receptors in the human genome. We applied this knowledge to human genome GPCR sequences found by sensitive sequence search techniques and identified two orphan receptors, namely GPR34 and GP171 that have all the necessary conserved features of P2Y receptors.

Altered immune response in mice deficient for the G protein-coupled receptor GPR34

The X-chromosomal GPR34 gene encodes an orphan G(i) protein-coupled receptor that is highly conserved among vertebrates. To evaluate the physiological relevance of GPR34, we generated a GPR34-deficient mouse line. GPR34-deficient mice were vital, reproduced normally, and showed no gross abnormalities in anatomical, histological, laboratory chemistry, or behavioral investigations under standard housing. Because GPR34 is highly expressed in mononuclear cells of the immune system, mice were specifically tested for altered functions of these cell types. Following immunization with methylated BSA, the number of granulocytes and macrophages in spleens was significantly lower in GPR34-deficient mice as in wild-type mice. GPR34-deficient mice showed significantly increased paw swelling in the delayed type hypersensitivity test and higher pathogen burden in extrapulmonary tissues after pulmonary infection with Cryptococcus neoformans compared with wild-type mice. The findings in delayed type hypersensitivity and infection tests were accompanied by significantly different basal and stimulated TNF-α, GM-CSF, and IFN-γ levels in GPR34-deficient animals. Our data point toward a functional role of GPR34 in the cellular response to immunological challenges.

Regulation of functionally active P2Y12 ADP receptors by thrombin in human smooth muscle cells and the presence of P2Y12 in carotid artery lesions

OBJECTIVE

The platelet P2Y12 ADP receptor is a well-known target of thienopyridine-type antiplatelet drugs. This study is the first to describe increased transcriptional expression of a functionally active P2Y12 in response to thrombin in human vascular smooth muscle cells (SMC).

METHODS AND RESULTS

On exposure to thrombin, P2Y12 mRNA was transiently increased, whereas total protein and cell surface expression of P2Y12 were markedly increased within 6 hours and remained elevated over 24 hours. This effect was mediated by activation of nuclear factor κB. Preincubation with thrombin significantly enhanced the efficacy of the P2Y receptor agonist 2-methylthio-ADP to induce interleukin 6 expression and SMC mitogenesis. Effects induced by 2-methylthio-ADP were prevented by RNA interference-mediated knockdown of P2Y12 and a selective P2Y12-antagonist R-138727, the active metabolite of prasugrel. In addition, positive P2Y12 immunostaining was shown in SMC of human carotid artery plaques and was found to colocalize with tissue factor, the rate-limiting factor of thrombin formation in vivo.

CONCLUSIONS

These data suggest that the P2Y12 receptor not only is central to ADP-induced platelet activation but also may mediate platelet-independent responses, specifically under conditions of enhanced thrombin formation, such as local vessel injury and atherosclerotic plaque rupture.

Involvement of the V2 vasopressin receptor in adaptation to limited water supply

Mammals adapted to a great variety of habitats with different accessibility to water. In addition to changes in kidney morphology, e.g. the length of the loops of Henle, several hormone systems are involved in adaptation to limited water supply, among them the renal-neurohypophysial vasopressin/vasopressin receptor system. Comparison of over 80 mammalian V2 vasopressin receptor (V2R) orthologs revealed high structural and functional conservation of this key component involved in renal water reabsorption. Although many mammalian species have unlimited access to water there is no evidence for complete loss of V2R function indicating an essential role of V2R activity for survival even of those species. In contrast, several marsupial V2R orthologs show a significant increase in basal receptor activity. An increased vasopressin-independent V2R activity can be interpreted as a shift in the set point of the renal-neurohypophysial hormone circuit to realize sufficient water reabsorption already at low hormone levels. As found in other desert mammals arid-adapted marsupials show high urine osmolalities. The gain of basal V2R function in several marsupials may contribute to the increased urine concentration abilities and, therefore, provide an advantage to maintain water and electrolyte homeostasis under limited water supply conditions.

Emerging lysophospholipid mediators, lysophosphatidylserine, lysophosphatidylthreonine, lysophosphatidylethanolamine and lysophosphatidylglycerol

It is now widely accepted that lysophospholipids (LPLs), a product of the phospholipase A reaction, function as mediators through G-protein-coupled receptors. Notably, recent studies of lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) have revealed their essential roles in vivo. On the other hand, other LPLs such as lysophosphatidylserine (LPS), lysophosphatidylthreonine (LPT), lysophosphatidylethanolamine (LPE), lysophosphatidylinositol (LPI) and lysophosphatidylglycerol (LPG) have been reported to show lipid mediator-like responses both in vivo (LPS and LPT) and in vitro (LPS, LPT, LPE and LPG), while very little is known about their receptor, synthetic enzyme and patho-physiological roles. In this review, we summarize the actions of these LPLs as lipid mediators including LPS, LPT, LPE and LPG.

Toward the three-dimensional structure and lysophosphatidic acid binding characteristics of the LPA(4)/p2y(9)/GPR23 receptor: a homology modeling study

Lysophosphatidic acid (LPA) is a naturally occurring phospholipid that initiates a broad array of biological processes, including those involved in cell proliferation, survival and migration via activation of specific G protein-coupled receptors located on the cell surface. To date, at least five receptor subtypes (LPA(1-5)) have been identified. The LPA(1-3) receptors are members of the endothelial cell differentiation gene (Edg) family. LPA(4), a member of the purinergic receptor family, and the recently identified LPA(5) are structurally distant from the canonical Edg LPA(1-3) receptors. LPA(4) and LPA(5) are linked to G(q), G(12/13) and G(s) but not G(i), while LPA(1-3) all couple to G(i) in addition to G(q) and G(12/13). There is also evidence that LPA(4) and LPA(5) are functionally different from the Edg LPA receptors. Computational modeling has provided useful information on the structure-activity relationship (SAR) of the Edg LPA receptors. In this work, we focus on the initial analysis of the structural and ligand-binding properties of LPA(4), a prototype non-Edg LPA receptor. Three homology models of the LPA(4) receptor were developed based on the X-ray crystal structures of the ground state and photoactivated bovine rhodopsin and the recently determined human beta(2)-adrenergic receptor. Docking studies of LPA in the homology models were then conducted, and plausible LPA binding loci were explored. Based on these analyses, LPA is predicted to bind to LPA(4) in an orientation similar to that reported for LPA(1-3), but through a different network of hydrogen bonds. In LPA(1-3), the ligand polar head group is reported to interact with residues at positions 3.28, 3.29 and 7.36, whereas three non-conserved amino acid residues, S114(3.28), T187(EL2) and Y265(6.51), are predicted to interact with the polar head group in the LPA(4) receptor models.

Involvement of basic amino acid residues in transmembrane regions 6 and 7 in agonist and antagonist recognition of the human platelet P2Y(12)-receptor

The P2Y(12)-receptor plays a prominent role in ADP-induced platelet aggregation. In the present study, we searched for amino acid residues involved in ligand recognition of the human P2Y(12)-receptor. Wild-type or mutated receptors were expressed in 1321N1 astrocytoma cells and Chinese hamster ovary (CHO) cells. There were no major differences in cellular expression of the constructs. Cellular cAMP production and cAMP response element (CRE)-dependent luciferase expression was increased by isoproterenol (astrocytoma cells) or forskolin (CHO cells). In cells expressing wild-type receptors, R256K or S101A mutant constructs, 2-methylthio-ADP inhibited the induced cAMP production with IC(50) concentrations of about 0.3nM. In cells expressing R256A constructs, the IC(50) concentration amounted to 25nM. In cells expressing H253A/R256A, Y259D and K280A constructs, 2-methylthio-ADP failed to affect the cellular cAMP production. Moreover, in cells expressing Y259D and K280A constructs, 2-methylthio-ADP did also not change the forskolin-induced CRE-dependent luciferase expression and caused only small increases in the serum response element-dependent luciferase expression. The antagonist cangrelor had similar potencies at wild-type receptors and R256A constructs (apparent pK(B)-value at wild-type receptors: 9.2). In contrast, reactive blue-2 had a lower potency at the R256A construct (apparent pK(B)-value at wild-type receptors: 7.6). In summary, the data indicate the involvement of Arg256, Tyr259 and, possibly, H253 (transmembrane region TM6) as well as Lys280 (TM7) in the function of the human P2Y(12)-receptor. Arg256 appears to play a role in the recognition of nucleotide agonists and the non-nucleotide antagonist reactive blue-2, but no role in the recognition of the nucleotide antagonist cangrelor.