International Union of Pharmacology. XLVI. G Protein-Coupled Receptor List

Agonist-regulated cleavage of the extracellular domain of parathyroid hormone receptor type 1

The receptor for parathyroid hormone (PTHR) is a main regulator of calcium homeostasis and bone maintenance. As a member of class B of G protein-coupled receptors, it harbors a large extracellular domain, which is required for ligand binding. Here, we demonstrate that the PTHR extracellular domain is cleaved by a protease belonging to the family of extracellular metalloproteinases. We show that the cleavage takes place in a region of the extracellular domain that belongs to an unstructured loop connecting the ligand-binding parts and that the N-terminal 10-kDa fragment is connected to the receptor core by a disulfide bond. Cleaved receptor revealed reduced protein stability compared with noncleaved receptor, suggesting degradation of the whole receptor. In the presence of the agonistic peptides PTH(1-34), PTH(1-14), or PTH(1-31), the processing of the PTHR extracellular domain was inhibited, and receptor protein levels were stabilized. A processed form of the PTHR was also detected in human kidney. These findings suggest a new model of PTHR processing and regulation of its stability.

Structural basis for ligand recognition of incretin receptors

The glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor are homologous G-protein-coupled receptors (GPCRs). Incretin receptor agonists stimulate the synthesis and secretion of insulin from pancreatic β-cells and are therefore promising agents for the treatment of type 2 diabetes. It is well established that the N-terminal extracellular domain (ECD) of incretin receptors is important for ligand binding and ligand specificity, whereas the transmembrane domain is involved in receptor activation. Structures of the ligand-bound ECD of incretin receptors have been solved recently by X-ray crystallography. The crystal structures reveal a similar fold of the ECD and a similar mechanism of ligand binding, where the ligand adopts an α-helical conformation. Residues in the C-terminal part of the ligand interact directly with the ECD and hydrophobic interactions appear to be the main driving force for ligand binding to the ECD of incretin receptors. Obviously, the-still missing-structures of full-length incretin receptors are required to construct a complete picture of receptor function at the molecular level. However, the progress made recently in structural analysis of the ECDs of incretin receptors and related GPCRs has shed new light on the process of ligand recognition and binding and provided a basis to disclose some of the mechanisms underlying receptor activation at high resolution.

Heterodimerization of the GABAB receptor-implications for GPCR signaling and drug discovery

The identification of the molecular nature of the GABA(B) receptor and the demonstration of its heterodimeric structure has led to extensive studies investigating the mechanism of activation and signaling. Phylogenetic studies suggest that the formation of the heterodimer is a relatively recent event arising in conjunction with the evolution of the central nervous system. Heterodimerization has now been demonstrated for many other G-protein-coupled receptors (GPCRs) and plays a role in signaling and trafficking. This presents both challenges and opportunities for GPCR drug discovery. In the case of the GABA(B) receptor the best hope for the development of new drugs directed at this receptor is from allosteric modulators. This chapter summarizes our current understanding of the molecular function of the GABA(B) receptor and recent developments in the identification of allosteric modulators. The broader implication of heterodimerization on GPCR function and drug discovery is also discussed.

Wnt signalling and cancer stem cells

Intracellular signalling mediated by secreted Wnt proteins is essential for the establishment of cell fates and proper tissue patterning during embryo development and for the regulation of tissue homeostasis and stem cell function in adult tissues. Aberrant activation of Wnt signalling pathways has been directly linked to the genesis of different tumours. Here, the components and molecular mechanisms implicated in the transduction of Wnt signal, along with important results supporting a central role for this signalling pathway in stem cell function regulation and carcinogenesis will be briefl y reviewed.

Group II metabotropic glutamate receptors and schizophrenia

Schizophrenia is one of the most common mental illnesses, with hereditary and environmental factors important for its etiology. All antipsychotics have in common a high affinity for monoaminergic receptors. Whereas hallucinations and delusions usually respond to typical (haloperidol-like) and atypical (clozapine-like) monoaminergic antipsychotics, their efficacy in improving negative symptoms and cognitive deficits remains inadequate. In addition, devastating side effects are a common characteristic of monoaminergic antipsychotics. Recent biochemical, preclinical and clinical findings support group II metabotropic glutamate receptors (mGluR2 and mGluR3) as a new approach to treat schizophrenia. This paper reviews the status of general knowledge of mGluR2 and mGluR3 in the psychopharmacology, genetics and neuropathology of schizophrenia.

Metabotropic glutamate receptor-mediated long-term depression: molecular mechanisms

The ability to modify synaptic transmission between neurons is a fundamental process of the nervous system that is involved in development, learning, and disease. Thus, synaptic plasticity is the ability to bidirectionally modify transmission, where long-term potentiation and long-term depression (LTD) represent the best characterized forms of plasticity. In the hippocampus, two main forms of LTD coexist that are mediated by activation of either N-methyl-d-aspartic acid receptors (NMDARs) or metabotropic glutamate receptors (mGluRs). Compared with NMDAR-LTD, mGluR-LTD is less well understood, but recent advances have started to delineate the underlying mechanisms. mGluR-LTD at CA3:CA1 synapses in the hippocampus can be induced either by synaptic stimulation or by bath application of the group I selective agonist (R,S)-3,5-dihydroxyphenylglycine. Multiple signaling mechanisms have been implicated in mGluR-LTD, illustrating the complexity of this form of plasticity. This review provides an overview of recent studies investigating the molecular mechanisms underlying hippocampal mGluR-LTD. It highlights the role of key molecular components and signaling pathways that are involved in the induction and expression of mGluR-LTD and considers how the different signaling pathways may work together to elicit a persistent reduction in synaptic transmission.

beta-Arrestins - scaffolds and signalling elements essential for WNT/Frizzled signalling pathways?

beta-arrestins were originally identified as negative regulators of G protein-coupled receptor signalling. Recent studies have revealed that beta-arrestins serve as intracellular scaffolds and signalling intermediates. Their diverse functions in intracellular signalling pathways provide mechanisms for achieving signal specificity that might be attacked for pharmacological intervention. Here, we summarize the importance of beta-arrestin function for WNT [wingless (from Drosophila) and the oncogene int-1]/Frizzled (FZD) signalling. WNTs are secreted lipoglycoproteins that act through the seven transmembrane-spanning receptors of the FZD family. It recently became evident that beta-arrestins are required for cellular communication by means of WNTs and FZDs both in cellular systems and in vivo. Although the overall importance of arrestin for WNT/FZD signalling remains obscure, interaction with the central phosphoprotein Dishevelled and the endocytic machinery implicates beta-arrestin as a determinant of WNT signalling specificity, a mediator of WNT/FZD desensitization and a regulator of signalling compartmentation.

Crystal structure of glucagon-like peptide-1 in complex with the extracellular domain of the glucagon-like peptide-1 receptor

GLP-1 (glucagon-like peptide-1) is an incretin released from intestinal L-cells in response to food intake. Activation of the GLP-1 receptor potentiates the synthesis and release of insulin from pancreatic β-cells in a glucose-dependent manner. The GLP-1 receptor belongs to class B of the G-protein-coupled receptors, a subfamily characterized by a large N-terminal extracellular ligand binding domain. Exendin-4 and GLP-1 are 50% identical, and exendin-4 is a full agonist with similar affinity and potency for the GLP-1 receptor. We recently solved the crystal structure of the GLP-1 receptor extracellular domain in complex with the competitive antagonist exendin-4(9–39). Interestingly, the isolated extracellular domain binds exendin-4 with much higher affinity than the endogenous agonist GLP-1. Here, we have solved the crystal structure of the extracellular domain in complex with GLP-1 to 2.1 Åresolution. The structure shows that important hydrophobic ligand-receptor interactions are conserved in agonist- and antagonist-bound forms of the extracellular domain, but certain residues in the ligand-binding site adopt a GLP-1-specific conformation. GLP-1 is a kinked but continuous α-helix from Thr¹³ to Val³³ when bound to the extracellular domain. We supplemented the crystal structure with site-directed mutagenesis to link the structural information of the isolated extracellular domain with the binding properties of the full-length receptor. The data support the existence of differences in the binding modes of GLP-1 and exendin-4 on the full-length GLP-1 receptor.

Cloning and characterization of the rhesus monkey nociceptin/orphanin FQ receptor

We succeeded in cloning the rhesus monkey nociceptin/orphanin FQ peptide (NOP) receptor. The nucleotide sequence and amino acid sequence of the rhesus monkey NOP receptor were 95.9% and 97.8%, respectively, identical to the human NOP receptor. There was no significant difference between the rhesus monkey NOP receptor and the human NOP receptor in the binding affinity of [(125)I] [Thy(14)]nociceptin and the binding of [(35)S]guanosine 5'-O-(gamma thio)triphospate ([(35)S]GTPgammaS) stimulated by nociceptin/orphanin FQ (N/OFQ). A selective NOP receptor antagonist, 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one ((+)-J-113397) inhibited the [(35)S]GTPgammaS binding activated by N/OFQ using the membrane of the rhesus monkey NOP receptor. The antagonistic activity of (+)-J-113397 to the rhesus monkey NOP receptor was comparable to that to the human NOP receptor. Thus, N/OFQ acts via activation of the NOP receptor in both human and rhesus monkeys without significant species differences.

In search of analgesia: emerging roles of GPCRs in pain

Of all clinically marketed drugs, greater than thirty percent are modulators of G protein-coupled receptors (GPCRs). Nearly 400 GPCRs (i.e., excluding odorant and light receptors) are encoded within the human genome, but only a small fraction of these seven-transmembrane proteins have been identified as drug targets. Chronic pain affects more than one-third of the population, representing a substantial societal burden in use of health care resources and lost productivity. Furthermore, currently available treatments are often inadequate, underscoring the significant need for better therapeutic strategies. The expansion of the identified human GPCR repertoire, coupled with recent insights into the function and structure of GPCRs, offers new opportunities for the development of novel analgesic therapeutics.

Interactions of histamine H1-receptor agonists and antagonists with the human histamine H4-receptor

The human histamine H(4)-receptor (hH(4)R) possesses high constitutive activity and, like the human H(1)-receptor (hH(1)R), is involved in the pathogenesis of type-I allergic reactions. The study aims were to explore the value of dual H(1)/H(4)R antagonists as antiallergy drugs and to address the question of whether H(1)R ligands bind to hH(4)R. In an acute murine asthma model, the H(1)R antagonist mepyramine and the H(4)R antagonist 1-[(5-chloro-1H-indol-2-yl)carbonyl]-4-methyl-piperazine (JNJ 7777120) exhibited synergistic inhibitory effects on eosinophil accumulation in the bronchoalveolar lavage fluid. At the hH(4)R expressed in Sf9 insect cells, 18 H(1)R antagonists and 22 H(1)R agonists showed lower affinity to hH(4)R than to hH(1)R as assessed in competition binding experiments. For a small number of H(1)R antagonists, hH(4)R partial agonism was observed in the steady-state GTPase assay. Most compounds were neutral antagonists or inverse agonists. Twelve phenylhistamine-type hH(1)R partial agonists were also hH(4)R partial agonists. Four histaprodifen-type hH(1)R partial agonists were hH(4)R inverse agonists. Dimeric histaprodifen was a more efficacious hH(4)R inverse agonist than the reference compound thioperamide. Suprahistaprodifen was the only histaprodifen acting as hH(4)R partial agonist. Suprahistaprodifen was docked into the binding pocket of inactive and active hH(4)R models in two different orientations, predominantly stabilizing the active state of hH(4)R. Collectively, the synergistic effects of H(1)R and H(4)R antagonists in an acute asthma model and the overlapping interaction of structurally diverse H(1)R ligands with hH(1)R and hH(4)R indicate that the development of dual H(1)R/H(4)R antagonists is a worthwhile and technically feasible goal for the treatment of type-I allergic reactions.

Definition of the G protein-coupled receptor transmembrane bundle binding pocket and calculation of receptor similarities for drug design

Recent advances in structural biology for G-protein-coupled receptors (GPCRs) have provided new opportunities to improve the definition of the transmembrane binding pocket. Here a reference set of 44 residue positions accessible for ligand binding was defined through detailed analysis of all currently available crystal structures. This was used to characterize pharmacological relationships of Family A/Rhodopsin family GPCRs, minimizing evolutionary influence from parts of the receptor that do not generally affect ligand binding. The resultant dendogram tended to group receptors according to endogenous ligand types, although it revealed subdivision of certain classes, notably peptide and lipid receptors. The transmembrane binding site reference set, particularly when coupled with a means of identifying the subset of ligand binding residues, provides a general paradigm for understanding the pharmacology/selectivity profile of ligands at Family A GPCRs. This has wide applicability to GPCR drug design problems across many disease areas.

Bile acids as regulatory molecules

In the past, bile acids were considered to be just detergent molecules derived from cholesterol in the liver. They were known to be important for the solubilization of cholesterol in the gallbladder and for stimulating the absorption of cholesterol, fat-soluble vitamins, and lipids from the intestines. However, during the last two decades, it has been discovered that bile acids are regulatory molecules. Bile acids have been discovered to activate specific nuclear receptors (farnesoid X receptor, preganane X receptor, and vitamin D receptor), G protein coupled receptor TGR5 (TGR5), and cell signaling pathways (c-jun N-terminal kinase 1/2, AKT, and ERK 1/2) in cells in the liver and gastrointestinal tract. Activation of nuclear receptors and cell signaling pathways alter the expression of numerous genes encoding enzyme/proteins involved in the regulation of bile acid, glucose, fatty acid, lipoprotein synthesis, metabolism, transport, and energy metabolism. They also play a role in the regulation of serum triglyceride levels in humans and rodents. Bile acids appear to function as nutrient signaling molecules primarily during the feed/fast cycle as there is a flux of these molecules returning from the intestines to the liver following a meal. In this review, we will summarize the current knowledge of how bile acids regulate hepatic lipid and glucose metabolism through the activation of specific nuclear receptors and cell signaling pathways.

Behavioral effects of a synthetic agonist selective for nociceptin/orphanin FQ peptide receptors in monkeys

Behavioral effects of a nonpeptidic NOP (nociceptin/orphanin FQ Peptide) receptor agonist, Ro 64-6198, have not been studied in primate species. The aim of the study was to verify the receptor mechanism underlying the behavioral effects of Ro 64-6198 and to systematically compare behavioral effects of Ro 64-6198 with those of a mu-opioid receptor agonist, alfentanil, in monkeys. Both Ro 64-6198 (0.001-0.06 mg/kg, s.c.) and alfentanil (0.001-0.06 mg/kg, s.c.) produced antinociception against an acute noxious stimulus (50 degrees C water) and capsaicin-induced allodynia. An NOP receptor antagonist, J-113397 (0.01-0.1 mg/kg, s.c.), dose-dependently produced rightward shifts of the dose-response curve of Ro 64-6198-induced antinociception. The apparent pA(2) value of J-113397 was 8.0. Antagonist studies using J-113397 and naltrexone revealed that Ro 64-6198 produced NOP receptor-mediated antinociception independent of mu-opioid receptors. In addition, alfentanil dose-dependently produced respiratory depression and itch/scratching responses, but antinociceptive doses of Ro 64-6198 did not produce such effects. More important, Ro 64-6198 did not produce reinforcing effects comparable with those of alfentanil, cocaine, or methohexital under self-administration procedures in monkeys. These results provide the first functional evidence that the activation of NOP receptors produces antinociception without reinforcing effects in primates. Non-peptidic NOP receptor agonists may have therapeutic value as novel analgesics without abuse liability in humans.

International Union of Pharmacology. LXXII. Recommendations for trace amine receptor nomenclature

Trace amines such as p-tyramine and beta-phenylethylamine are found endogenously as well as in the diet. Concomitant ingestion of these foodstuffs with monoamine oxidase inhibitors may result in the hypertensive crisis known as the "beer, wine, and cheese effect" attributed to their sympathomimetic action. Trace amines have been shown to act on one of a novel group of mammalian seven transmembrane spanning G protein-coupled receptors belonging to the rhodopsin superfamily, cloned in 2001. This receptor encoded by the human TAAR1 gene is also present in rat and mouse genomes (Taar1) and has been shown to be activated by endogenous trace amine ligands, including p-tyramine and beta-phenylethylamine. A number of drugs, most notably amphetamine and its derivatives, act as agonists at this receptor. This review proposes an official nomenclature designating TAAR1 as the trace amine 1 receptor following the convention of naming receptors after the endogenous agonist, abbreviated to TA(1) where necessary. It goes on to discuss briefly the significance of the receptor, agents acting upon it, its distribution, and currently hypothesized physiological and pathophysiological roles. In humans, a further five genes are thought to encode functional receptors (TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9). TAAR3 seems to be a pseudogene in some individuals but not others. TAAR4 is a pseudogene in humans, but occurs with TAAR3 as a functional gene in rodents. Nine further genes are present in rats and mice. The endogenous ligands are not firmly established but some may respond to odorants consistent with their expression in olfactory epithelium.

Islet G protein-coupled receptors as potential targets for treatment of type 2 diabetes

Islet dysfunction - characterized by a combination of defective insulin secretion, inappropriately high glucagon secretion and reduced beta-cell mass - has a central role in the pathophysiology of type 2 diabetes. Several G protein-coupled receptors (GPCRs) expressed in islet beta-cells are known to be involved in the regulation of islet function, and therefore are potential therapeutic targets. This is evident from the recent success of glucagon-like peptide 1 (GLP1) mimetics and dipeptidyl peptidase 4 (DPP4) inhibitors, which promote activation of the GLP1 receptor to stimulate insulin secretion and inhibit glucagon secretion, and also have the potential to increase beta-cell mass. Other islet beta-cell GPCRs that are involved in the regulation of islet function include the glucose-dependent insulinotropic peptide (GIP) receptor, lipid GPCRs, pleiotropic peptide GPCRs and islet biogenic amine GPCRs. This Review summarizes islet GPCR expression, signalling and function, and highlights their potential as targets for the treatment of type 2 diabetes.

TGR5: an emerging bile acid G-protein-coupled receptor target for the potential treatment of metabolic disorders

Over the past decade, new roles for bile acids in paracrine and endocrine regulation of cholesterol homeostasis, lipid and carbohydrate metabolism and immunomodulatory functions have been discovered. Most of the early discoveries focused on the genomic actions of bile acids through the activation of families of nuclear receptors, such as the farnesoid X receptor and vitamin D receptors, until a new chapter in the bile acid receptor discovery unfolded in the form of TGR5; a novel G-protein-coupled receptor mediating several non-genomic functional responses induced by binding of bile acids. The key involvement of TGR5 in mediating energy homeostasis and glucose homeostasis made it an attractive target for the potential treatment of metabolic disorders.

G-protein-coupled receptor screen reveals a role for chemokine receptor CCR5 in suppressing microglial neurotoxicity

G-protein-coupled receptors (GPCRs) form the largest superfamily of membrane proteins, and several GPCRs have been implicated in signaling between neurons and glia to protect neurons from pathological stresses. Here, we have used a screening strategy to investigate GPCRs that are involved in neuronal protection. The real-time PCR was performed using 274 primers targeting nonsensory GPCR mRNAs, which were listed on the database. The cDNAs from control and nerve-injured hypoglossal nuclei of mouse brain were used, and the alterations of PCR products were compared. This screen and the subsequent in situ hybridization screen exhibited six GPCR mRNAs which were prominently and convincingly induced in nerve-injured hypoglossal nuclei. Among these candidates, the chemokine receptor CCR5 was selected, based on the marked induction in CCR5 mRNA in microglia after nerve injury. The mRNA expression of ligands for CCR5, such as regulated on activation normal T-cell expressed and secreted (RANTES/CCL5), MIP-1alpha, and MIP-1beta, were induced in injured motor neurons, indicating that CCR5 and its ligands were expressed in microglia and neurons, respectively, in response to nerve injury. In vitro, lipopolysaccharide (LPS)-induced expression of mRNAs for inflammatory cytokines (IL-1beta, IL-6, and tumor necrosis factor-alpha) and inducible nitric oxide synthase (iNOS) in microglia were all suppressed by RANTES. Those suppressions were not observed in microglia from CCR5 null mice. In addition, nerve injury-induced motor neuron death seen in wild type C56BL/6J mice was accelerated in CCR5 knock-out C57BL/6J. These results may suggest that CCR5-mediated neuron-glia signaling functions to protect neurons by suppressing microglia toxicity.

PROKR2 missense mutations associated with Kallmann syndrome impair receptor signalling activity

Kallmann syndrome (KS) combines hypogonadism due to gonadotropin-releasing hormone deficiency, and anosmia or hyposmia, related to defective olfactory bulb morphogenesis. In a large series of KS patients, ten different missense mutations (p.R85C, p.R85H, p.R164Q, p.L173R, p.W178S, p.Q210R, p.R268C, p.P290S, p.M323I, p.V331M) have been identified in the gene encoding the G protein-coupled receptor prokineticin receptor-2 (PROKR2), most often in the heterozygous state. Many of these mutations were, however, also found in clinically unaffected individuals, thus raising the question of their actual implication in the KS phenotype. We reproduced each of the ten mutations in a recombinant murine Prokr2, and tested their effects on the signalling activity in transfected HEK-293 cells, by measuring intracellular calcium release upon ligand-activation of the receptor. We found that all mutated receptors except one (M323I) had decreased signalling activities. These could be explained by different defective mechanisms. Three mutations (L173R, W178S, P290S) impaired cell surface-targeting of the receptor. One mutation (Q210R) abolished ligand-binding. Finally, five mutations (R85C, R85H, R164Q, R268C, V331M) presumably impaired G protein-coupling of the receptor. In addition, when wild-type and mutant receptors were coexpressed in HEK-293 cells, none of the mutant receptors that were retained within the cells did affect cell surface-targeting of the wild-type receptor, and none of the mutant receptors properly addressed at the plasma membrane did affect wild-type receptor signalling activity. This argues against a dominant negative effect of the mutations in vivo.

Screening technologies for G protein-coupled receptors: from HTS to uHTS

The discovery of drugs for G protein-coupled receptors (GPCRs) has traditionally been very successful, even before the structural nature of these molecular targets was elucidated. Over the years, this family of proteins has become more important in the understanding and treatment of different human pathologies, representing today close to 30% of the molecular targets of all marketed drugs. The sequencing of the human genome unveiled the existence of many new GPCRs and this has increased even more the interest of this family of proteins as potential drug targets. Today the search for compounds that interfere or modulate the function of GPCRs is one of the major focuses of pharmaceutical companies. The understanding of the molecular events that take place upon receptor activation, together with the need of testing large chemical libraries, has resulted in the development of a variety of methods and technologies to measure the activity of these receptors. In this chapter we will review most of the assay technologies currently in use for "in vitro" pharmacological screening, their evolution, their capabilities, and their limitations.

VPAC and PAC receptors: From ligands to function

Vasoactive intestinal peptide (VIP) and the pituitary adenylate cyclase activating polypeptides (PACAPs) share 68% identity at the amino acid level and belong to the secretin peptide family. Following the initial discovery of VIP almost four decades ago a substantial amount of knowledge has been presented describing the mechanisms of action, distribution and pleiotropic functions of these related peptides. It is now known that the physiological actions of these widely distributed peptides are produced through activation of three common G-protein coupled receptors (VPAC(1), VPAC(2) and PAC(1)R) which preferentially stimulate adenylate cyclase and increase intracellular cAMP, although stimulation of other intracellular messengers, including calcium and phospholipase D, has been reported. Using a range of in vitro and in vivo approaches, including cell-based functional assays, transgenic animals and rodent models of disease, VPAC/PAC receptor activation has been associated with numerous physiological processes (e.g. control of circadian rhythms) and clinical conditions (e.g. pulmonary hypertension), which underlies on-going research efforts and makes these peptides and their cognate receptors attractive targets for the pharmaceutical industry. However, despite the considerable interest in VPAC/PAC receptors and the processes which they mediate, there is still a paucity of selective and available, non-peptide ligands, which has hindered further advances in this field both at the basic research and clinical level. This review summarises the current knowledge of VIP/PACAP and the VPAC/PAC receptors with regard to their distribution, pharmacology, signalling pathways, splice variants and finally, the utility of animal models in exploring their physiological roles.

Constitutive internalization of G protein-coupled receptors and G proteins via clathrin-independent endocytosis

Although agonist-dependent endocytosis of G protein-coupled receptors (GPCRs) as a means to modulate receptor signaling has been widely studied, the constitutive endocytosis of GPCRs has received little attention. Here we show that two prototypical class I GPCRs, the beta2 adrenergic and M3 muscarinic receptors, enter cells constitutively by clathrin-independent endocytosis and colocalize with markers of this endosomal pathway on recycling tubular endosomes, indicating that these receptors can subsequently recycle back to the plasma membrane (PM). This constitutive endocytosis of these receptors was not blocked by antagonists, indicating that receptor signaling was not required. Interestingly, the G proteins that these receptors couple to, Galpha(s) and Galpha(q), localized together with their receptors at the plasma membrane and on tubular recycling endosomes. Upon agonist stimulation, Galpha(s) and Galpha(q) remained associated with the PM and these endosomal membranes, whereas beta2 and M3 receptors now entered cells via clathrin-dependent endocytosis. Deletion of the third intracellular loop (i3 loop), which is thought to play a role in agonist-dependent endocytosis of the M3 receptor, had no effect on the constitutive internalization of the receptor. Surprisingly, with agonist, the mutated M3 receptor still internalized and accumulated in cells but through clathrin-independent and not clathrin-dependent endocytosis. These findings demonstrate that GPCRs are versatile PM proteins that can utilize different mechanisms of internalization depending upon ligand activation.

Murine and rat cavernosal responses to endothelin-1 and urotensin-II Vasoactive Peptide Symposium

BACKGROUND: Endothelin-1 (ET-1) and urotensin-II (U-II) are the most potent constrictors of human vessels. Although the cavernosal tissue is higly responsive to ET-1, no information exists on the effects of U-II on cavernosal function. The aim of this study was to characterize ET-1 and U-II responses in corpora cavernosa from rats and mice. METHODS AND RESULTS: Male Wistar rats and C57/BL6 mice were used at 13 weeks. Cumulative concentration-response curves to ET-1, U-II and IRL-1620, an ET(B) agonist, were performed. ET-1 increased force generation in cavernosal strips from mice and rats, but no response to U-II was observed in the presence or absence of L-NAME, or in strips pre-stimulated with 20mM KCl. IRL-1620 did not induce cavernosal contraction even in presence of L-NAME, but induced a cavernosal relaxation which was greater in rats than mice. No relaxation responses to U-II were observed in cavernosal strips pre-contracted with phenylephrine. mRNA expression of ET-1, ET(A), ET(B) and U-II receptors, but not U-II was observed in cavernosal strips. CONCLUSION: ET-1, via ET(A) receptors activation, causes contractile responses in cavernosal strips from rats and mice whereas ET(B) receptor activation produces relaxation. Although the cavernosal tissue expresses U-II receptors, U-II does not induce contractile responses in corpora cavernosa from mice or rats.

The NIFSTD and BIRNLex vocabularies: building comprehensive ontologies for neuroscience

A critical component of the Neuroscience Information Framework (NIF) project is a consistent, flexible terminology for describing and retrieving neuroscience-relevant resources. Although the original NIF specification called for a loosely structured controlled vocabulary for describing neuroscience resources, as the NIF system evolved, the requirement for a formally structured ontology for neuroscience with sufficient granularity to describe and access a diverse collection of information became obvious. This requirement led to the NIF standardized (NIFSTD) ontology, a comprehensive collection of common neuroscience domain terminologies woven into an ontologically consistent, unified representation of the biomedical domains typically used to describe neuroscience data (e.g., anatomy, cell types, techniques), as well as digital resources (tools, databases) being created throughout the neuroscience community. NIFSTD builds upon a structure established by the BIRNLex, a lexicon of concepts covering clinical neuroimaging research developed by the Biomedical Informatics Research Network (BIRN) project. Each distinct domain module is represented using the Web Ontology Language (OWL). As much as has been practical, NIFSTD reuses existing community ontologies that cover the required biomedical domains, building the more specific concepts required to annotate NIF resources. By following this principle, an extensive vocabulary was assembled in a relatively short period of time for NIF information annotation, organization, and retrieval, in a form that promotes easy extension and modification. We report here on the structure of the NIFSTD, and its predecessor BIRNLex, the principles followed in its construction and provide examples of its use within NIF.

IUPHAR-DB: the IUPHAR database of G protein-coupled receptors and ion channels

The IUPHAR database (IUPHAR-DB) integrates peer-reviewed pharmacological, chemical, genetic, functional and anatomical information on the 354 nonsensory G protein-coupled receptors (GPCRs), 71 ligand-gated ion channel subunits and 141 voltage-gated-like ion channel subunits encoded by the human, rat and mouse genomes. These genes represent the targets of approximately one-third of currently approved drugs and are a major focus of drug discovery and development programs in the pharmaceutical industry. IUPHAR-DB provides a comprehensive description of the genes and their functions, with information on protein structure and interactions, ligands, expression patterns, signaling mechanisms, functional assays and biologically important receptor variants (e.g. single nucleotide polymorphisms and splice variants). In addition, the phenotypes resulting from altered gene expression (e.g. in genetically altered animals or in human genetic disorders) are described. The content of the database is peer reviewed by members of the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR); the data are provided through manual curation of the primary literature by a network of over 60 subcommittees of NC-IUPHAR. Links to other bioinformatics resources, such as NCBI, Uniprot, HGNC and the rat and mouse genome databases are provided. IUPHAR-DB is freely available at http://www.iuphar-db.org.

Case files of the University of Massachusetts fellowship in medical toxicology: lethal dose of opioids contained in an elastomeric capsule labeled as vancomycin

A 67 year-old male presented to the emergency department with alteration in mental status. On arrival he had vital signs: pulse 110, BP 173/83, respiratory rate 4, oxygen saturation 57% and temperature 36.1 degrees Celsius. His past medical history included hypertension, vitamin B12 deficiency, hyperlipidemia, and recurrent cellulitis treated with vancomycin. The patient had no response to noxious stimuli, pinpoint pupils, and agonal respirations. Secondary to his wife's vehement denial that he had access to or history of using any narcotics, he was intubated after 2.2mg IV naloxone failed to reverse respiratory depression. Thirty minutes before presentation, however, he had received an intravenous infusion of vancomycin administered by his wife at home. The vancomycin, obtained from a home infusion medication supply company, was contained in one of five sealed elastomeric capsules delivered earlier that day. A qualitative comprehensive toxicology screen of urine for 1043 substances identified morphine, codeine, naloxone, lidocaine and caffeine. The original elastomeric container was not available for testing, but another container from the same delivery was submitted for testing to the state forensic laboratory. This intact container was labeled as Vancomycin 1g in 240mL of normal saline. The forensic laboratory confirmed that the alkaloidal contents of the elastomeric capsule were 10% codeine, 4.4% 6-monoacetyl morphine, and 84% morphine. No vancomycin was identified in the infusion bottles. The case was referred to the local police department and the state department of health drug control board. The home infusion company was also immediately notified to prevent similar occurrence.

CONCLUSION

We are reporting the first known case of opioid overdose from an adulterated elastomeric capsule that was labeled as containing an antimicrobial agent.

Targeting bile-acid signalling for metabolic diseases

Bile acids are increasingly being appreciated as complex metabolic integrators and signalling factors and not just as lipid solubilizers and simple regulators of bile-acid homeostasis. It is therefore not surprising that a number of bile-acid-activated signalling pathways have become attractive therapeutic targets for metabolic disorders. Here, we review how the signalling functions of bile acids can be exploited in the development of drugs for obesity, type 2 diabetes, hypertriglyceridaemia and atherosclerosis, as well as other associated chronic diseases such as non-alcoholic steatohepatitis.

A novel adrenocorticotropin receptor mutation alters its structure and function, causing familial glucocorticoid deficiency

CONTEXT

Familial glucocorticoid deficiency (FGD) is an autosomal recessive disorder characterized by unresponsiveness to ACTH. In this study, two mutations of the ACTH receptor (MC2R) gene are reported in this FGD clinical case.

OBJECTIVE

The objective of the study was to characterize a novel MC2R gene mutation in a compound heterozygous patient with FGD phenotype.

DESIGN

This was a clinical case description, biochemical, molecular, and bioinformatics analysis to describe a novel MC2R gene mutation.

PATIENTS

The subject of the study was a male diagnosed with primary adrenal insufficiency. The family history showed nonconsanguineous healthy parents, three healthy siblings, and one brother affected with FGD.

MAIN OUTCOME MEASURES

The mutant MC2R-Ala126Ser showed significantly lower activity when it was stimulated with ACTH-(1-24) than did cells transfected with wild-type MC2R.

RESULTS

The molecular studies demonstrated the presence of an adenine heterozygous insertion (InsA1347) in the MC2R gene (G217fs) in the patient. This insertion was due to a frame shift mutation in one allele and a premature stop codon codifying an aberrant receptor of 247 residues (27.2 kDa). We also found a novel heterozygous mutation alanine 126 by serine. Molecular dynamic simulations showed that serine 126 side chain fluctuates forming a noncanonical intrahelical hydrogen bond in the transmembrane helix 3 of the mutated receptor. This produces a structural rearrangement of the MC2R internal cavities that may affect the ligand recognition and signal transduction throughout the G protein.

CONCLUSIONS

We propose a molecular explanation for the reduced activity exhibited by the MC2R alanine 126 by serine mutant.

Allosteric modulators of glycoprotein hormone receptors: discovery and therapeutic potential

The glycoprotein hormones, luteinizing hormone, follicle-stimulating hormone and thyroid stimulating hormone, are important regulators of reproductive and metabolic processes. However, because of the nature of their ligand-receptor interactions that contain multiple contact sites, classical small molecule drug discovery strategies have not been successful. However, recent advances in screening and combinatorial chemistry strategies have identified chemical series that act allosterically as positive, negative or mixed modulators of the glycoprotein hormone receptors. This review will discuss the discovery and highlight the currently known series of allosteric modulators to this therapeutically important family of G-protein coupled receptors. Lastly, we will present potential mechanisms whereby the different series could modulate receptor function in the context of currently held theory and known structure of G protein-coupled receptors.

Structure of a beta1-adrenergic G-protein-coupled receptor

G-protein-coupled receptors have a major role in transmembrane signalling in most eukaryotes and many are important drug targets. Here we report the 2.7 A resolution crystal structure of a beta(1)-adrenergic receptor in complex with the high-affinity antagonist cyanopindolol. The modified turkey (Meleagris gallopavo) receptor was selected to be in its antagonist conformation and its thermostability improved by earlier limited mutagenesis. The ligand-binding pocket comprises 15 side chains from amino acid residues in 4 transmembrane alpha-helices and extracellular loop 2. This loop defines the entrance of the ligand-binding pocket and is stabilized by two disulphide bonds and a sodium ion. Binding of cyanopindolol to the beta(1)-adrenergic receptor and binding of carazolol to the beta(2)-adrenergic receptor involve similar interactions. A short well-defined helix in cytoplasmic loop 2, not observed in either rhodopsin or the beta(2)-adrenergic receptor, directly interacts by means of a tyrosine with the highly conserved DRY motif at the end of helix 3 that is essential for receptor activation.

Wnt signaling: the good and the bad

Since the first Wnt gene was identified in 1982, the functions and mechanisms of Wnt signaling have been extensively studied. Wnt signaling is conserved from invertebrates to vertebrates and regulates early embryonic development as well as the homeostasis of adult tissues. In addition, both embryonic stem cells and adult stem cells are regulated by Wnt signaling. Deregulation of Wnt signaling is associated with many human diseases, particularly cancers. In this review, we will discuss in detail the functions of many components involved in the Wnt signal transduction pathway. Then, we will explore what is known about the role of Wnt signaling in stem cells and cancers.

Spatial approximation between secretin residue five and the third extracellular loop of its receptor provides new insight into the molecular basis of natural agonist binding

The amino terminus of class II G protein-coupled receptors plays an important role in ligand binding and receptor activation. Understanding of the conformation of the amino-terminal domain of these receptors has been substantially advanced with the solution of nuclear magnetic resonance and crystal structures of this region of receptors for corticotrophin-releasing factor, pituitary adenylate cyclase-activating polypeptide, and gastric inhibitory polypeptide. However, the orientation of the amino terminus relative to the receptor core and how the receptor gets activated upon ligand binding remain unclear. In this work, we have used photoaffinity labeling to identify a critical spatial approximation between residue five of secretin and a residue within the proposed third extracellular loop of the secretin receptor. This was achieved by purification, deglycosylation, cyanogen bromide cleavage, and sequencing of labeled wild-type and mutant secretin receptors. This constraint has been used to refine our evolving molecular model of secretin docked at the intact receptor, which for the first time includes refined helical bundle and loop regions and reflects a peptide-binding groove within the receptor amino terminus that directs the amino terminus of the peptide toward the receptor body. This model is fully consistent with the endogenous agonist mechanism for class II G protein-coupled receptor activation, where ligand binding promotes the interaction of a portion of the receptor amino terminus with the receptor body to activate it.

Solution structure of urotensin-II receptor extracellular loop III and characterization of its interaction with urotensin-II

Urotensin-II (U-II) is a vasoactive hormone that acts through a G-protein-coupled receptor named UT. Recently, we have shown, using the surface plasmon resonance technology that human U-II (hU-II) interacts with the hUT(281-300) fragment, a segment containing the extracellular loop III (EC-III) and short extensions of the transmembrane domains VI and VII (TM-VI and TM-VII). To further investigate the interaction of UT receptor with U-II, we have determined the solution structure of hUT(281-300) by high-resolution NMR and molecular modeling and we have examined, also using NMR, the binding with hU-II at residue level. In the presence of dodecylphosphocholine micelles, hUT(281-300) exhibited a type III beta-turn (Q285-L288), followed by an -helical structure (A289-L299), the latter including a stretch of transmembrane helix VII. Upon addition of hU-II, significant chemical shift perturbations were observed for residues located just on the N-terminal side of the beta-turn (end of TM-VI/beginning of EC-III) and on one face of the -helix (end of EC-III/beginning of TM-VII). These data, in conjunction with intermolecular NOEs, suggest that the initiation site of EC-III, as well as the upstream portion of helix VII, would be involved in agonist binding and allow to propose points of interaction in the ligand-receptor complex.

Expression analysis of G Protein-Coupled Receptors in mouse macrophages

Background

Monocytes and macrophages express an extensive repertoire of G Protein-Coupled Receptors (GPCRs) that regulate inflammation and immunity. In this study we performed a systematic micro-array analysis of GPCR expression in primary mouse macrophages to identify family members that are either enriched in macrophages compared to a panel of other cell types, or are regulated by an inflammatory stimulus, the bacterial product lipopolysaccharide (LPS).

Results

Several members of the P2RY family had striking expression patterns in macrophages; P2ry6 mRNA was essentially expressed in a macrophage-specific fashion, whilst P2ry1 and P2ry5 mRNA levels were strongly down-regulated by LPS. Expression of several other GPCRs was either restricted to macrophages (e.g. Gpr84) or to both macrophages and neural tissues (e.g. P2ry12, Gpr85). The GPCR repertoire expressed by bone marrow-derived macrophages and thioglycollate-elicited peritoneal macrophages had some commonality, but there were also several GPCRs preferentially expressed by either cell population.

Conclusion

The constitutive or regulated expression in macrophages of several GPCRs identified in this study has not previously been described. Future studies on such GPCRs and their agonists are likely to provide important insights into macrophage biology, as well as novel inflammatory pathways that could be future targets for drug discovery.

Structural diversity of G protein-coupled receptors and significance for drug discovery

G protein-coupled receptors (GPCRs) are the largest family of membrane-bound receptors and also the targets of many drugs. Understanding of the functional significance of the wide structural diversity of GPCRs has been aided considerably in recent years by the sequencing of the human genome and by structural studies, and has important implications for the future therapeutic potential of targeting this receptor family. This article aims to provide a comprehensive overview of the five main human GPCR families--Rhodopsin, Secretin, Adhesion, Glutamate and Frizzled/Taste2--with a focus on gene repertoire, general ligand preference, common and unique structural features, and the potential for future drug discovery.

GPCR genes are preferentially retained after whole genome duplication

One of the most interesting questions in biology is whether certain pathways have been favored during evolution, and if so, what properties could cause such a preference. Due to the lack of experimental evidence, whether select gene families have been preferentially retained over time after duplication in metazoan organisms remains unclear. Here, by syntenic mapping of nonchemosensory G protein-coupled receptor genes (nGPCRs which represent half the receptome for transmembrane signaling) in the vertebrate genomes, we found that, as opposed to the 8–15% retention rate for whole genome duplication (WGD)-derived gene duplicates in the entire genome of pufferfish, greater than 27.8% of WGD-derived nGPCRs which interact with a nonpeptide ligand were retained after WGD in pufferfish Tetraodon nigroviridis. In addition, we show that concurrent duplication of cognate ligand genes by WGD could impose selection of nGPCRs that interact with a polypeptide ligand. Against less than 2.25% probability for parallel retention of a pair of WGD-derived ligands and a pair of cognate receptor duplicates, we found a more than 8.9% retention of WGD-derived ligand-nGPCR pairs–threefold greater than one would surmise. These results demonstrate that gene retention is not uniform after WGD in vertebrates, and suggest a Darwinian selection of GPCR-mediated intercellular communication in metazoan organisms.