Target validation of G-protein coupled receptors

Rhodopsin: structure, signal transduction and oligomerisation

Rhodopsin was the first G protein-coupled receptor (GPCR) for which a high-resolution crystal structure was obtained. Several crystal structures have now been solved representing different activation states of the receptor. These structures, together with those from lower resolution techniques (e.g. electron microscopy), shed light on the stepwise process by which energy from an extracellular photon is transduced across the membrane to the intracellular compartment thereby activating signalling mechanisms responsible for very low-level light detection. Controversy remains in several areas including: (i) transmembrane helix movements responsible for the transduction process, (ii) the stoichiometry of coupling to G proteins and their mode of activation, (iii) the role, if any, of receptor oligomerisation and (iv) the suitability of using structures of this GPCR as templates for modelling the structures of other GPCRs, and their mechanisms of activation.

High yield production of recombinant native and modified peptides exemplified by ligands for G-protein coupled receptors

G-protein coupled receptors (GPCRs) comprise a large family of membrane proteins and attract pharmaceutical interest as therapeutic targets. Two examples of class B GPCRs that are involved in metabolic diseases are the Parathyroid hormone receptor 1 (PTHR1) and the Glucagon-like-peptide-1 receptor (GLP-1R) which play central roles in osteoporosis and diabetes mellitus type II, respectively. Class B GPCRs are characterised by a large extracellular N-terminal domain with a typical disulfide bridge pattern. This domain is responsible for the binding of peptide hormone ligands. Here we report the recombinant expression of these ligands in natural and several modified forms for their use in functional assays, NMR analyses or affinity purification of receptor/ligand complexes for crystallisation. Applying the SUMO system, low cost expression of soluble fusion-proteins is achieved. Moreover, via the SUMO cleavage site, the authentic N-terminal sequence which is essential for ligand-receptor interactions can be obtained. Purification of the peptide by RP-HPLC results in >98% pure preparations. The strategy can also be adopted for many other purposes, especially if small peptides are needed at either large amounts or with specific features like isotope, affinity or fluorescent labels. Furthermore, for the growing demand for therapeutic peptides, this method could represent a straightforward production process.

The evolutionarily conserved G protein-coupled receptor SREB2/GPR85 influences brain size, behavior, and vulnerability to schizophrenia

The G protein-coupled receptor (GPCR) family is highly diversified and involved in many forms of information processing. SREB2 (GPR85) is the most conserved GPCR throughout vertebrate evolution and is expressed abundantly in brain structures exhibiting high levels of plasticity, e.g., the hippocampal dentate gyrus. Here, we show that SREB2 is involved in determining brain size, modulating diverse behaviors, and potentially in vulnerability to schizophrenia. Mild overexpression of SREB2 caused significant brain weight reduction and ventricular enlargement in transgenic (Tg) mice as well as behavioral abnormalities mirroring psychiatric disorders, e.g., decreased social interaction, abnormal sensorimotor gating, and impaired memory. SREB2 KO mice showed a reciprocal phenotype, a significant increase in brain weight accompanying a trend toward enhanced memory without apparent other behavioral abnormalities. In both Tg and KO mice, no gross malformation of brain structures was observed. Because of phenotypic overlap between SREB2 Tg mice and schizophrenia, we sought a possible link between the two. Minor alleles of two SREB2 SNPs, located in intron 2 and in the 3' UTR, were overtransmitted to schizophrenia patients in a family-based sample and showed an allele load association with reduced hippocampal gray matter volume in patients. Our data implicate SREB2 as a potential risk factor for psychiatric disorders and its pathway as a target for psychiatric therapy.

Modelling the structures of G protein-coupled receptors aided by three-dimensional validation

Background

G protein-coupled receptors (GPCRs) are abundant, activate complex signalling and represent the targets for up to ~60% of pharmaceuticals but there is a paucity of structural data. Bovine rhodopsin is the first GPCR for which high-resolution structures have been completed but significant variations in structure are likely to exist among the GPCRs. Because of this, considerable effort has been expended on developing in silico tools for refining structures of individual GPCRs. We have developed REPIMPS, a modification of the inverse-folding software Profiles-3D, to assess and predict the rotational orientation and vertical position of helices within the helix bundle of individual GPCRs. We highlight the value of the method by applying it to the Baldwin GPCR template but the method can, in principle, be applied to any low- or high-resolution membrane protein template or structure.

Results

3D models were built for transmembrane helical segments of 493 GPCRs based on the Baldwin template, and the models were then scored using REPIMPS and Profiles-3D. The compatibility scores increased significantly using REPIMPS because it takes into account the physicochemical properties of the (lipid) environment surrounding the helix bundle. The arrangement of helices in the helix bundle of the 493 models was then altered systematically by rotating the individual helices. For most GPCRs in the set, changes in the rotational position of one or more helices resulted in significant improvement in the compatibility scores. In particular, for most GPCRs, a rotation of helix VII by 240–300° resulted in improved scores. Bovine rhodopsin modelled using this method showed 3.31 Å RMSD to its crystal structure for 198 C^α atom pairs, suggesting the utility of the method even when starting with idealised structures such as the Baldwin template.

Conclusion

We have developed an in silico tool which can be used to test the validity of, and refine, models of GPCRs with respect to helix rotation and vertical position based on the physicochemical properties of amino acids and the surrounding environment. The method can be applied to any multi-pass membrane protein and potentially can be used in combination with other high-throughput methodologies to generate and refine models of membrane proteins.

Application of the BD ACTOne technology for the high-throughput screening of Gs-coupled receptor antagonists

A novel technology for monitoring the changes of 3,'5'-adenosine cyclic monophosphate (cAMP) in live cells suitable for drug screening relies on the use of cyclic nucleotide-gated channels as biosensors coexpressed with the appropriate target receptor. The technique (termed BD ACTOne) offers measurement of cAMP-dependent calcium influx or membrane depolarization with conventional fluorescent methods both in kinetic and in endpoint modes, optimal for high-throughput and subsequent compound screening. The utility of the technique is reported here based on assay development and high-throughput screening for small-molecule antagonists of the peptide parathyroid hormone 2 receptor (PTH2R). The dual-signaling properties of the receptor were retained in the recombinant system, and the observed pharmacological profile corresponded to data from radiolabeled cAMP determination. The membrane-potential-based high-throughput assay produced reproducible actives and led to the identification of several chemical scaffolds with potential utility as PTH2R ligands.

Antagonizing Methuselah to extend life span

A peptide ligand for the receptor Methuselah has been identified that extends fly life span.

A recent report describes the identification through the use of in vitro selection of a peptide that antagonizes Methuselah signaling in Drosophila in vitro and extends fly life span in vivo.

Computational modeling study of functional microdomains in cannabinoid receptor type 1

The seven transmembrane helices (TMH) G-protein-coupled receptors (GPCRs) constitute one of the largest superfamily of signaling proteins found in mammals. Some of its members, in which the cannabinoid (CB) receptors are included, stand out because their functional states can be modulated by a broad spectrum of effector molecules. The relative ligand promiscuity exhibited by these receptors could be related with particular attributes conferred by their molecular architecture and represents a motivating issue to be explored. In this regard, this study represents an effort to investigate the cannabinoid receptor type 1 (CB1) ligand recognition plasticity, using comparative modeling, molecular dynamics (MD) simulations and docking. Our results suggest that a cooperative set of subtle structural rearrangements within the TMHs provide to the CB1 protein the plasticity to reach alternate configurations. These changes include the relaxation of intramolecular constraints, the rotations, translations and kinks of the majority of TMHs and the reorganization of the ligand binding cavities.

Employing Rhodobacter sphaeroides to functionally express and purify human G protein-coupled receptors

G protein-coupled receptors (GPCRs) represent the largest class of cell surface receptors and play crucial roles in many cellular and physiological processes. Functional production of recombinant GPCRs is one of the main bottlenecks to obtaining structural information. Here, we report the use of a novel bacterial expression system based on the photosynthetic bacteriumRhodobacter sphaeroidesfor the production of human recombinant GPCRs. The advantage of employingR. sphaeroidesas a host lies in the fact that it provides much more membrane surface per cell compared to other typical expression hosts. The system was tailored to overexpress recombinant receptors under the control of the moderately strong and highly regulated superoperonic photosynthetic promoterpufQ. We tested this system for the expression of some class A GPCRs, namely, the human adenosine A2a receptor (A2aR), the human angiotensin AT1a receptor (AT1aR) and the human bradykinin B2 receptor (B2R). Several different constructs were examined and functional production of the recombinant receptors was achieved. The best-expressed receptor, AT1aR, was solubilized and affinity-purified. To the best of our knowledge, this is the first report of successful use of a bacterial host –R. sphaeroides– to produce functional recombinant GPCRs under the control of a photosynthetic gene promoter.

beta-arrestin-biased agonism at the beta2-adrenergic receptor

Classically, the beta 2-adrenergic receptor (beta 2AR) and other members of the seven-transmembrane receptor (7TMR) superfamily activate G protein-dependent signaling pathways in response to ligand stimulus. It has recently been discovered, however, that a number of 7TMRs, including beta 2AR, can signal via beta-arrestin-dependent pathways independent of G protein activation. It is currently unclear if among beta 2AR agonists there exist ligands that disproportionately signal via G proteins or beta-arrestins and are hence "biased." Using a variety of approaches that include highly sensitive fluorescence resonance energy transfer-based methodologies, including a novel assay for receptor internalization, we show that the majority of known beta 2AR agonists exhibit relative efficacies for beta-arrestin-associated activities (beta-arrestin membrane translocation and beta 2AR internalization) identical to the irrelative efficacies for G protein-dependent signaling (cyclic AMP generation). However, for three betaAR ligands there is a marked bias toward beta-arrestin signaling; these ligands stimulate beta-arrestin-dependent receptor activities to a much greater extent than would be expected given their efficacy for G protein-dependent activity. Structural comparison of these biased ligands reveals that all three are catecholamines containing an ethyl substitution on the alpha-carbon, a motif absent on all of the other, unbiased ligands tested. Thus, these studies demonstrate the potential for developing a novel class of 7TMR ligands with a distinct bias for beta-arrestin-mediated signaling.

Gene expression profiling for the identification of G-protein coupled receptors in human platelets

INTRODUCTION AND MATERIALS AND METHODS: G-protein coupled receptors (GPCRs) play an important role in platelet aggregation. To identify new platelet GPCRs, a platelet gene expression profile was generated and validated using quantitative real-time PCR.

RESULTS

In total, mRNA of 28 GPCR genes was detected in human platelets. The 12 most abundant platelet GPCR transcripts were: thrombin receptor PAR1 (1865+/-178%), ADP receptor P2Y(12) (459+/-88%), succinate receptor 1 (257+/-48%), ADP receptor P2Y(1) (100%), orphan P2RY(10) (68.2+/-3.3%), lysophosphatidic acid receptors GPR23 (48.2+/-11%) and GPR92 (26.1+/-3.3%), adrenergic receptor alpha(2A) (18.4+/-4.4%), orphan EBI2 (6.31+/-0.42), adenosine receptors A(2A) (2.94+/-0.24%) and A(2B) (2.88+/-0.16%) and lysophosphatidic acid receptor LPA(1) (2.59+/-0.39%) (% relative to the chosen calibrator P2Y(1)). A surprising G-protein coupled receptor redundancy was found: two ADP receptors (P2Y(1) and P2Y(12)), three adenosine receptors (A(2A), A(2B), and A(1)), four lysophosphatidic acid receptors (LPA(1), LPA(3), GPR23 and GPR92), two l-glutamate receptors (mGlu(3) and mGlu(4)) and two serotonin receptors (5-HT(1F) and 5-HT(4)). The adenosine receptor A(2B) gene expression was validated with protein expression and functional studies. Western blot confirmed A(2B) receptor protein expression and platelet flow cytometry demonstrated inhibition of the effect of NECA by the adenosine A(2B)-antagonist MRS1754.

CONCLUSIONS

We have detected several GPCRs not previously known to be expressed in platelets, including a functional adenosine A(2B) receptor. The findings could improve our understanding of platelet aggregation and provide new targets for drug development.

Structural divergence among cannabinoids influences membrane dynamics: A 2H Solid-State NMR analysis

Cannabinoids are compounds that can modulate neuronal functions and immune responses via their activity at the CB(1) receptor. We used (2)H NMR order parameters and relaxation rate determination to delineate the behavior of magnetically aligned phospholipid bilayers in the presence of several structurally distinct cannabinoid ligands. THC (Delta(9)-Tetrahydrocannabinol) and WIN-55,212-2 were found to lower the phase transition temperature of the DMPC and to destabilize their acyl chains leading to a lower average S(CD) ( approximately 0.13), while methanandamide and CP-55,940 exhibited unusual properties within the lipid bilayer resulting in a greater average S(CD) ( approximately 0.14) at the top of the phospholipid upper chain. The CB(1) antagonist AM281 had average S(CD) values that were higher than the pure DMPC lipids, indicating a stabilization of the lipid bilayer. R(1Z) versus |S(CD)|(2) plots indicated that the membrane fluidity is increased in the presence of THC and WIN-55,212-2. The interaction of CP-55,940 with a variety of zwitterionic and charged membranes was also assessed. The unusual effect of CP-55,940 was present only in bicelles composed of DMPC. These studies strongly suggest that cannabinoid action on the membrane depends upon membrane composition as well as the structure of the cannabinoid ligands.

Genome sequence of Aedes aegypti, a major arbovirus vector

We present a draft sequence of the genome of Aedes aegypti, the primary vector for yellow fever and dengue fever, which at approximately 1376 million base pairs is about 5 times the size of the genome of the malaria vector Anopheles gambiae. Nearly 50% of the Ae. aegypti genome consists of transposable elements. These contribute to a factor of approximately 4 to 6 increase in average gene length and in sizes of intergenic regions relative to An. gambiae and Drosophila melanogaster. Nonetheless, chromosomal synteny is generally maintained among all three insects, although conservation of orthologous gene order is higher (by a factor of approximately 2) between the mosquito species than between either of them and the fruit fly. An increase in genes encoding odorant binding, cytochrome P450, and cuticle domains relative to An. gambiae suggests that members of these protein families underpin some of the biological differences between the two mosquito species.

Ligand screening system using fusion proteins of G protein-coupled receptors with G protein alpha subunits

G protein-coupled receptors (GPCRs) constitute one of the largest families of genes in the human genome, and are the largest targets for drug development. Although a large number of GPCR genes have recently been identified, ligands have not yet been identified for many of them. Various assay systems have been employed to identify ligands for orphan GPCRs, but there is still no simple and general method to screen for ligands of such GPCRs, particularly of G(i)-coupled receptors. We have examined whether fusion proteins of GPCRs with G protein alpha subunit (Galpha) could be utilized for ligand screening and showed that the fusion proteins provide an effective method for the purpose. This article focuses on the followings: (1) characterization of GPCR genes and GPCRs, (2) identification of ligands for orphan GPCRs, (3) characterization of GPCR-Galpha fusion proteins, and (4) identification of ligands for orphan GPCRs using GPCR-Galpha fusion proteins.

Functional overexpression and characterization of human bradykinin subtype 2 receptor in insect cells using the baculovirus system

Bradykinin exerts its actions via binding to B1 and B2 receptors (B1R and B2R), which are members of G protein-coupled receptor superfamily. B2R is constitutively expressed in a variety of cells such as endothelial cells, vascular smooth muscle cells, and cardiomyocytes and it is an important drug target for the treatment of cardiovascular disorders. During this study, the human B2R was functionally overexpressed in insect cells using the baculovirus expression system. The maximum expression level in Sf9 cells under optimized condition was 10 pmol/mg. This corresponds to approximately 0.25 mg active receptor per liter culture. The recombinant receptor showed high affinity for its endogenous ligand bradykinin, similar to the B2R expressed in native tissues. Functional coupling of the recombinant receptor to the endogenous G alpha(s) protein was demonstrated via cAMP release assay upon agonist stimulation. Confocal laser scanning microscopy and immunogold-labeling experiment revealed that the recombinant B2R was mainly localized intracellularly and only a minor fraction of the recombinant receptor reached the plasma membrane. To our knowledge, this is the first report of high level expression of recombinant B2R in insect cells and provides a way for large scale production and structural characterization of this receptor.

Multiplexing G protein-coupled receptors in microarrays: A radioligand-binding assay

Multiplexing of G protein-coupled receptors (GPCRs) in microarrays promises to increase the efficiency, reduce the costs, and improve the quality of high-throughput assays. However, this technology is still nascent and has not yet achieved the status of "high throughput" or laid claim to handling a large set of receptors. In addition, the technology has been demonstrated only when using fluorescent ligands to detect binding, limiting its application to a subset of GPCRs. To expand the impact of multiplexing on this receptor class, we have developed a radiometric approach to the microarray assay. In these studies, we considered two receptors in the alpha-adrenergic receptor family, alpha2A and alpha2C, and the 125I-labeled agonist clonidine. We demonstrate that microarrays of these receptors can be readily detected (signal/noise ratio approximately 160) using a Typhoon 9210 PhosphorImager. In addition, biochemical characterization shows that ligand-binding profiles and selectivity are preserved with the selective antagonists BRL44408 and ARC239. Importantly, these microarrays use approximately 200- to 400-fold less membrane preparation required by conventional assay methods and allow two or more receptors to be assayed in an area equivalent to a standard well of a microtiter plate. The impact of this approach on screening in drug discovery is discussed.

Kisspeptins: a multifunctional peptide system with a role in reproduction, cancer and the cardiovascular system

Orphan G-protein-coupled receptors that have recently been paired with their cognate ligand are an often untapped resource for novel drug development. The KISS1 receptor (previously designated GPR54) has been paired with biologically active cleavage peptides of the KiSS-1 gene product, the kisspeptins (KP). The focus of this review is the emerging pharmacology and physiology of the KP. Genetic linkage analysis in humans revealed that mutations in KISS1 (GPR54, AXOR12 or hOT7T175) result in idiopathic hypogonadotrophic hypogonadism and knockout mouse studies confirmed this finding. Identification of KISS1 (GPR54) as a molecular switch for puberty subsequently led to the discovery that KP activate the GnRH cascade. Prior to the role of KISS1 (GPR54) in puberty being described, KP had been shown to be inhibitors of tumour metastasis across a range of cancers. Subsequently the mechanism of this inhibition has been suggested to be via altered cell motility and adhesiveness. PCR detected highest expression of KP and KISS1 (GPR54) in placenta, and changes in KP levels throughout pregnancy and expression in trophoblasts suggests a role in placentation. Placentation and metastasis are invasive processes that require angiogenesis. Investigation of KISS1 (GPR54) and KP in vasculature revealed discrete localisation of KISS1 (GPR54) to blood vessels prone to atherosclerosis and a potent vasoconstrictor action. A role for KP has also been shown in whole body homeostasis. KP are multifunctional peptides and further investigation is required to fully elucidate the complex pathways regulated by these peptides and how these pathways integrate in the whole body system.

Mining the gene repertoire and ESTs for G protein-coupled receptors with evolutionary perspective

The purpose of this article was to review recent progress in mining the gene repertoire and expressed sequence tags (ESTs) for the super-family of G protein-coupled receptors (GPCRs) in the form of a proceeding from the Nordic GPCR meeting held at the Nobel Forum, Karolinska Institute in August 2006. We update and give an overview of the expansion of the main families of GPCRs; Glutamate, Rhodopsin, Adhesion, Frizzled and Secretin (GRAFS) in perspective of fully sequenced genomes. We look into the most recent findings including the work that has been carried out on the spotted green puffer fish (Tetraodon nigroviridis), mouse (Mus musculus), chicken (Gallus gallus), slime mold (Dictyostelium discoideum) and the plant pathogenic fungus Magnaporthe grisea. We use examples from our recent work on chicken GPCRs to highlight the importance of detailed assembly and curation of sequences and how that can affect percentage similarity and phylogeny. ESTs can give valuable information about expression patterns. GPCRs have comparatively low numbers of EST suggesting that GPCRs are in generally expressed in lower amount than other genes. We discuss similarities in the evolution of the trace amine associated receptors with other sensory receptors.

Seven-transmembrane receptors and ubiquitination

Regulation of protein function by posttranslational modification plays an important role in many biological pathways. The most well known among such modifications is protein phosphorylation performed by highly specific protein kinases. In the past decade, however, covalent linkage of the low-molecular-weight protein ubiquitin to substrate proteins (protein ubiquitination) has proven to be yet another widely used mechanism of protein regulation playing a crucial role in virtually all aspects of cellular functions. This review highlights some of the recently discovered and provocative roles for ubiquitination in the regulation of the life cycle and signal transduction properties of 7-transmembrane receptors that serve to integrate many biological functions and play fundamental roles in cardiovascular homeostasis.

Heterologous GPCR expression: a bottleneck to obtaining crystal structures

G protein-coupled receptors (GPCRs) are an important, medically relevant class of integral membrane proteins. Laboratories throughout all disciplines of science devote time and energy into developing practical methods for the discovery, isolation, and characterization of these proteins. Since the crystal structure of rhodopsin was solved 6 years ago, the race to determine high-resolution structures of more GPCRs has gained momentum. Since certain GPCRs are currently produced at sufficient levels for X-ray crystallography trials, it is speculated that heterologous expression of GPCRs may no longer be a bottleneck in obtaining crystal structures. This Review focuses on the current approaches in heterologous expression of GPCRs and explores the problems associated with obtaining crystal structures from GPCRs expressed in different systems. Although milligram amounts of certain GPCRs are attainable, the majority of GPCRs are still either produced at very low levels or not at all. Developing reliable expression techniques for GPCRs is still a major priority for the structural characterization of GPCRs.

Galphai2-mediated signaling events in the endothelium are involved in controlling leukocyte extravasation

The trafficking of leukocytes from the blood to sites of inflammation is the cumulative result of receptor-ligand-mediated signaling events associated with the leukocytes themselves as well as with the underlying vascular endothelium. Our data show that Galpha(i) signaling pathways in the vascular endothelium regulate a critical step required for leukocyte diapedesis. In vivo studies using knockout mice demonstrated that a signaling event in a non-lymphohematopoietic compartment of the lung prevented the recruitment of proinflammatory leukocytes. Intravital microscopy showed that blockade was at the capillary endothelial surface and ex vivo studies of leukocyte trafficking demonstrated that a Galpha(i)-signaling event in endothelial cells was required for transmigration. Collectively, these data suggest that specific Galpha(i2)-mediated signaling between endothelial cells and leukocytes is required for the extravasation of leukocytes and for tissue-specific accumulation.

Entirely artificial signal transduction with adrenaline

Multifunctional transmembrane-building blocks with recognition sites for adrenaline on one end and the reaction partners for an SN2 reaction on the opposite end have been embedded in DPPC-liposomes. These doped vesicles can be quantitatively reduced at their disulfide head groups by externally added reducing agents; their composition and chemical processes taking place within can be monitored by NMR spectroscopy and--with limitations--by UV/Vis spectroscopy. Attempted release of thiopyridine as a second messenger into the interior of the liposome on external adrenaline addition could not be proven unambiguously because the detection system does not fulfill the necessary rigorous specificity and sensitivity requirements.

Differential expression of genes in pyrethroid resistant and susceptible mosquitoes, Culex quinquefasciatus (S.)

The transcriptional regulation of gene expression is a primary means by which insects adapt to a changing environment. The evolution of insecticide resistance is conferred through mechanisms, typically requiring the interaction of multiple genes. Consequently, the characterization of gene regulation and interactions in resistance is fundamental for achieving an understanding of the complex processes responsible for resistance. cDNA macroarray technology offers a promising new approach for investigating the complicated processes responsible for resistance development by revealing the interrelations of all of the elements in a resistant system simultaneously. In the current study, we compared the gene expression profiles of resistant and susceptible Culex quinquefasciatus mosquitoes, using a combination of subtractive PCR hybridization and cDNA microarray technique. By screening of 1500 cDNA clones from a resistant-susceptible mosquito subtractive library, we identified a set of genes with up-regulated expression in insecticide resistant Culex mosquitoes through transcriptional profiling compared to those in susceptible mosquitoes. These genes are vital for cellular and molecular metabolism, signal transduction, vesicular and molecular transport, protein biosynthesis, ubiquitination, and neuronal survival, but most have not previously been implicated in insecticide resistance. Functional studies of resistance-associated up-regulated genes should shed new light on both the molecular basis of resistance and the regulatory pathways that lead to it.

Inhibition of G alpha i2 activation by G alpha i3 in CXCR3-mediated signaling

G protein-coupled receptors (GPCRs) convey extracellular stimulation into dynamic intracellular action, leading to the regulation of cell migration and differentiation. T lymphocytes express G alpha(i2) and G alpha(i3), two members of the G alpha(i/o) protein family, but whether these two G alpha(i) proteins have distinguishable roles guiding T cell migration remains largely unknown because of a lack of member-specific inhibitors. This study details distinct G alpha(i2) and G alpha(i3) effects on chemokine receptor CXCR3-mediated signaling. Our data showed that G alpha(i2) was indispensable for T cell responses to three CXCR3 ligands, CXCL9, CXCL10, and CXCL11, as the lack of G alpha(i2) abolished CXCR3-stimulated migration and guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) incorporation. In sharp contrast, T cells isolated from G alpha(i3) knock-out mice displayed a significant increase in both GTPgammaS incorporation and migration as compared with wild type T cells when stimulated with CXCR3 agonists. The increased GTPgammaS incorporation was blocked by G alpha(i3) protein in a dose-dependent manner. G alpha(i3)-mediated blockade of G alpha(i2) activation did not result from G alpha(i3) activation, but instead resulted from competition or steric hindrance of G alpha(i2) interaction with the CXCR3 receptor via the N terminus of the second intracellular loop. A mutation in this domain abrogated not only G alpha(i2) activation induced by a CXCR3 agonist but also the interaction of G alpha(i3) to the CXCR3 receptor. These findings reveal for the first time an interplay of G alpha(i) proteins in transmitting G protein-coupled receptor signals. This interplay has heretofore been masked by the use of pertussis toxin, a broad inhibitor of the G alpha(i/o) protein family.

Protease-activated receptors: potential therapeutic targets in irritable bowel syndrome?

Protease-activated receptors (PARs) are a family of four G-protein-coupled receptors (PAR-1 to PAR-4) activated by the proteolytic cleavage of their N-terminal extracellular domain. This activation first involves the recognition of the extracellular domain by proteases, such as thrombin, but also trypsin or tryptase which are particularly abundant in the gastrointestinal tract, both under physiological circumstances and in several digestive diseases. Activation of PARs, particularly of PAR-1 and -2, modulates intestinal functions, such as gastrointestinal motility, visceral nociception, mucosal inflammatory response, and epithelial functions (intestinal secretion and permeability). As these physiological properties have been shown to be altered in various extents and combinations in different clinical presentations of irritable bowel syndrome, PARs appear as putative targets for future therapeutic intervention in these patients.

Characteristic amino acid combinations in olfactory G protein-coupled receptors

The human olfactory subgenome has recently been fully characterized with over 1000 genes. Although as many as two thirds of them are expected to be pseudogenes, it still leaves us with about half of all human G protein-coupled receptors being olfactory. It is therefore of great interest to characterize olfactory receptors with high precision. Usually it is done through sequence motifs that are not fully conserved, making an exact characterization difficult. In this paper, we propose a rule-based characterization of olfactory receptors derived from a multiple sequence alignment of human GPCRs. We show that just seven alignment sites are sufficient to characterize 99% of human olfactory GPCRs with one feature, a tyrosine at site 7.41, being of particular importance. We also show dependencies between sites near the extracellular and intracellular region of a membrane-embedded receptor, indicating that olfactory receptors are characterized by a combination of important residues in these two areas, whereas nonolfactory receptors tend to have residues of lower importance at the same sites.

Development of a homogeneous calcium mobilization assay for high throughput screening of mas-related gene receptor agonists

AIM

To develop homogeneous calcium mobilization assay for high-throughput screening (HTS) of mas-related gene (Mrg) receptor agonists.

METHODS

CHO-K1 cells stably expressing the full-length MrgD receptor and a calcium-sensitive dye were used to develop an HTS assay based on intracellular calcium influx. This method was applied to large-scale screening of a library containing 8000 synthetic compounds and natural product extracts. cAMP measurements were carried out to verify the bioactivities of the hits found by the calcium mobilization assay. Similar approaches were also employed in the identification of the MrgA1 receptor agonists following HTS of 16,000 samples.

RESULTS

EC(50) values of the positive control compounds (beta-alanine for MrgD receptor and dynorphin A for MrgA1 receptor) determined by the calcium mobilization assay were consistent with those reported in the literature, and the Z' factors were 0.65 and 0.50 for MrgD and MrgA1 receptor assay, respectively. About 31 compounds for the MrgD receptor and 48 compounds for the MrgA1 receptor showing > or =20% of the maximal agonist activities found in the controls were initially identified as hits. Secondary screening confirmed that 2 compounds for each receptor possessed specific agonist activities. Intracellular cAMP level measurements indicated that the 2 confirmed hits displayed the functionality of the MrgD receptor agonists.

CONCLUSION

A series of validation studies demonstrated that the homogeneous calcium mobilization assay developed was highly efficient, amenable to automation and a robust tool to screen potential MrgD and MrgA1 receptor agonists. Its application may be expanded to other G-protein coupled receptors that mobilize calcium influx upon activation.

Orphan seven transmembrane receptor screening

Drug discovery has successfully exploited the superfamily of seven transmembrane receptors (7TMR), with over 35% of clinically marketed drugs targeting them. However, it is clear that there remains an undefined potential within this protein family for successful drugs of the future. The human genome sequencing project identified approximately 720 genes that belong to the 7TMR superfamily. Around half of these genes encode sensory receptors, while the other half are potential drug targets. Natural ligands have been identified for approximately 215 of these, leaving 155 receptors classified as orphan 7TMRs having no known ligand. Deorphanisation of these receptors by identification of natural ligands has been the traditional method enabling target validation by use of these ligands as tools to define biological relevance and disease association. Such ligands have been paired with their cognate receptor experimentally by screening of small molecule and peptide ligands, reverse pharmacology and the use of bioinformatics to predict candidate ligands. In this manuscript, we review the methodologies developed for the identification of ligands at orphan 7TMRs and exemplify these with case studies.

Therapeutic targets: progress of their exploration and investigation of their characteristics

Modern drug discovery is primarily based on the search and subsequent testing of drug candidates acting on a preselected therapeutic target. Progress in genomics, protein structure, proteomics, and disease mechanisms has led to a growing interest in and effort for finding new targets and more effective exploration of existing targets. The number of reported targets of marketed and investigational drugs has significantly increased in the past 8 years. There are 1535 targets collected in the therapeutic target database compared with approximately 500 targets reported in a 1996 review. Knowledge of these targets is helpful for molecular dissection of the mechanism of action of drugs and for predicting features that guide new drug design and the search for new targets. This article summarizes the progress of target exploration and investigates the characteristics of the currently explored targets to analyze their sequence, structure, family representation, pathway association, tissue distribution, and genome location features for finding clues useful for searching for new targets. Possible "rules" to guide the search for druggable proteins and the feasibility of using a statistical learning method for predicting druggable proteins directly from their sequences are discussed.

G protein signaling in yeast: new components, new connections, new compartments

Signaling by cell surface receptors and heterotrimeric guanine nucleotide-binding proteins (G proteins) is one of the most exhaustively studied processes in the cell but remains a major focus of molecular pharmacology research. The pheromone-response system in yeast (see the Connections Map at Science's Signal Transduction Knowledge Environment) has provided numerous major advances in our understanding of G protein signaling and regulation. However, the basic features of this prototypical pathway have remained largely unchanged since the mid-1990s. New tools available in yeast are beginning to uncover new pathway components and interactions and have revealed signaling in unexpected locations within the cell.

Sensitive and specific method for detecting G protein-coupled receptor mRNAs

G protein-coupled receptors (GPCRs) mediate effects of extracellular signaling molecules in all the body's cells. These receptors are encoded by scarce mRNAs; therefore, detecting their transcripts with conventional microarrays is difficult. We present a method based on multiplex PCR and array detection of amplicons to assay GPCR gene expression with as little as 1 mug of total RNA, and using it, we profiled three human bone marrow stromal cell (BMSC) lines.

Adventures in drug discovery: potent agents based on ligands for cell-surface receptors

How does one go about discovering new drugs? This question is addressed by descriptions of drug discovery research in three project areas that pertain to antagonist ligands for cell-surface receptors. The molecular targets of interest are protease-activated receptor-1 (PAR-1), vasopressin receptors (V1a and V2 subtypes), and the fibrinogen receptor (GPIIb/IIIa). I present different approaches to the identification of high-affinity ligands for these receptors, en route to drug candidates. The PAR-1 project resulted in a pharmacological tool compound that facilitated in vivo proof-of-principle studies, whereas the vasopressin and fibrinogen receptor projects resulted in several preclinical development compounds, three of which advanced into human clinical trials.

Trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) play an integral role in the signal transduction of an enormous array of biological phenomena, thereby serving to modulate at a molecular level almost all components of human biology. This role is nowhere more evident than in cardiovascular biology, where GPCRs regulate such core measures of cardiovascular function as heart rate, contractility, and vascular tone. GPCR/ligand interaction initiates signal transduction cascades, and requires the presence of the receptor at the plasma membrane. Plasma membrane localization is in turn a function of the delivery of a receptor to and removal from the cell surface, a concept defined most broadly as receptor trafficking. This review illuminates our current view of GPCR trafficking, particularly within the cardiovascular system, as well as highlights the recent and provocative finding that components of the GPCR trafficking machinery can facilitate GPCR signaling independent of G protein activation.

A combined ligand-based and target-based drug design approach for G-protein coupled receptors: application to salvinorin A, a selective kappa opioid receptor agonist

Combined ligand-based and target-based drug design approaches provide a synergistic advantage over either method individually. Therefore, we set out to develop a powerful virtual screening model to identify novel molecular scaffolds as potential leads for the human KOP (hKOP) receptor employing a combined approach. Utilizing a set of recently reported derivatives of salvinorin A, a structurally unique KOP receptor agonist, a pharmacophore model was developed that consisted of two hydrogen bond acceptor and three hydrophobic features. The model was cross-validated by randomizing the data using the CatScramble technique. Further validation was carried out using a test set that performed well in classifying active and inactive molecules correctly. Simultaneously, a bovine rhodopsin based "agonist-bound" hKOP receptor model was also generated. The model provided more accurate information about the putative binding site of salvinorin A based ligands. Several protein structure-checking programs were used to validate the model. In addition, this model was in agreement with the mutation experiments carried out on KOP receptor. The predictive ability of the model was evaluated by docking a set of known KOP receptor agonists into the active site of this model. The docked scores correlated reasonably well with experimental pK (i) values. It is hypothesized that the integration of these two independently generated models would enable a swift and reliable identification of new lead compounds that could reduce time and cost of hit finding within the drug discovery and development process, particularly in the case of GPCRs.

Medium-chain fatty acids as ligands for orphan G protein-coupled receptor GPR84

Free fatty acids (FFAs) play important physiological roles in many tissues as an energy source and as signaling molecules in various cellular processes. Elevated levels of circulating FFAs are associated with obesity, dyslipidemia, and diabetes. Here we show that GPR84, a previously orphan G protein-coupled receptor, functions as a receptor for medium-chain FFAs with carbon chain lengths of 9-14. Medium-chain FFAs elicit calcium mobilization, inhibit 3',5'-cyclic AMP production, and stimulate [35S]guanosine 5'-O-(3-thiotriphosphate) binding in a GPR84-dependent manner. The activation of GPR84 by medium-chain FFAs couples primarily to a pertussis toxin-sensitive G(i/o) pathway. In addition, we show that GPR84 is selectively expressed in leukocytes and markedly induced in monocytes/macrophages upon activation by lipopolysaccharide. Furthermore, we demonstrate that medium-chain FFAs amplify lipopolysaccharide-stimulated production of the proinflammatory cytokine interleukin-12 p40 through GPR84. Our results indicate a role for GPR84 in directly linking fatty acid metabolism to immunological regulation.

Identification of novel splice variants of Adhesion G protein-coupled receptors

Alternative splicing is an important mechanism to generate proteome diversity in higher eukaryotic organisms. We searched for splice variants of the human Adhesion family of G protein-coupled receptors (GPCRs) using mRNA sequences and expressed sequence tags. The results presented here describe 53 human splice variants among the 33 Adhesion GPCRs. Many of these variants appear to be coding for "functional" proteins (29) while the others are seemingly "non-functional" (24). Novel functional splice variants were found for: CD97, CELR3, EMR2, EMR3, GPR56, GPR110, GPR112-GPR114, GPR116, GPR123-GPR126, GPR133, HE6, and LEC1-LEC3. Splice variants for GPR116, GPR125, GPR126, and HE6 were found conserved in other species. Several of the functional splice variants lack one or more of the functional domains that are found in the N-termini of these receptors. These functional domains are likely to affect ligand binding or interaction with other proteins and these novel splice variants may have important roles for the specificity of interactions between these receptors and extracellular molecules. Another type of splice variants found here lacks a GPCR proteolytic site (GPS). The GPS domain has been shown to be essential for the proteolytic cleavage of the receptors N-termini and for cellular surface expression. We suggest that these alternative splice variants may be crucial for the function of the receptors while the seemingly non-functional splice variants may be a part of a regulative mechanism.

Zaprinast, a well-known cyclic guanosine monophosphate-specific phosphodiesterase inhibitor, is an agonist for GPR35

We found that zaprinast, a well-known cyclic guanosine monophosphate-specific phosphodiesterase inhibitor, acted as an agonist for a G protein-coupled receptor, GPR35. In our intracellular calcium mobilization assay, zaprinast activated rat GPR35 strongly (geometric mean EC(50) value of 16nM), whereas it activated human GPR35 moderately (geometric mean EC(50) value of 840nM). We also demonstrated that GPR35 acted as a Galpha(i/o)- and Galpha(16)-coupled receptor for zaprinast when heterologously expressed in human embryonic kidney 293 (HEK 293) cells. These findings will facilitate the research on GPR35 and the drug discovery of the GPR35 modulators.

[Voltage-dependent calcium channels at the heart of pain perception]

Voltage-dependent calcium channels represent a major pathway of calcium entry into neurons, where they participate actively to cell excitability and to the molecular processes of synaptic transmission. For that reason, they have been the direct or indirect pharmacological targets of analgesics and this long before their implication in the physiology of nociception had been demonstrated. These last years, the still more refined molecular characterization of these channels and their associated regulatory subunits and the demonstration of their implication in nociceptive processes indicates that these structures are prime pharmacological targets for the management of pain. Herein, we detail the recent breakthroughs on calcium channel structure, function and pharmacology, review the implication of calcium channels in the transmission of nociception, and evaluate their importance as targets for the treatment of pain perception. The search for specific inhibitors of voltage-dependent calcium channels appears as a prelude to the development of new promising analgesic molecules.

Dimethylsulphoxide as a tool to increase functional expression of heterologously produced GPCRs in mammalian cells

High-level overexpression of G protein-coupled receptors GPCRs in mammalian cells remains a difficult task inspite of newly developed virus based expression systems. Here, we show that the functional expression level of the recombinant bradykinin receptor (B(2)R) in mammalian cells can be increased up to sixfold just by the addition of dimethylsulphoxide in the culture medium. Total expression level, cellular localization and binding affinity of the recombinant receptor for its endogenous ligand remains unaltered. The strategy presented here, with recombinant B(2)R as a case example, is applicable to other GPCRs and provides a generic tool to improve the functional expression level of recombinant GPCRs in mammalian cells.

Multiplexing Fluo-4 NW and a GeneBLAzer® Transcriptional Assay for High-Throughput Screening of G-Protein-Coupled Receptors

Activation of G-protein-coupled receptors (GPCRs) leads to a cascade of signaling events, including calcium mobilization and downstream transcriptional activation of various proteins. Two commonly used methods of high-throughput screening for GPCRs include calcium-sensitive dyes, such as Fluo-4 NW, and reporter gene assays, such as beta-lactamase. To determine whether the advantages of each assay format could be combined by multiplexing, Jurkat and CHO-K1 cell lines over-expressing the M1 muscarinic receptor and beta-lactamase under control of an NFAT response element were tested in a multiplexed format. The Jurkat cell line was further screened with a subset of the LOPAC1280 library. The multiplexing assay was compatible with both the CHO-K1 and Jurkat cell lines. For the screen, there was 100% correlation of on-target hits in the multiplexed format, and several false positives with each assay format were identified. Therefore, not only can the assays be multiplexed, but by multiplexing, the false positives associated with each assay format also could be easily identified. In addition to enhanced reliability, this method saves time and money because only half the amount of compounds, cells, and consumables are needed to screen a cell line in a multiplexed mode versus separate screening by both methods.

Comprehensive repertoire and phylogenetic analysis of the G protein-coupled receptors in human and mouse

Understanding differences in the repertoire of orthologous gene pairs is vital for interpretation of pharmacological and physiological experiments if conclusions are conveyed between species. Here we present a comprehensive dataset for G protein-coupled receptors (GPCRs) in both human and mouse with a phylogenetic road map. We performed systematic searches applying several search tools such as BLAST, BLAT, and Hidden Markov models and searches in literature data. We aimed to gather a full-length version of each human or mouse GPCR in only one copy referring to a single chromosomal position. Moreover, we performed detailed phylogenetic analysis of the transmembrane regions of the receptors to establish accurate orthologous pairs. The results show the identity of 495 mouse and 400 human functional nonolfactory GPCRs. Overall, 329 of the receptors are found in one-to-one orthologous pairs, while 119 mouse and 31 human receptors originate from species-specific expansions or deletions. The average percentage similarity of the orthologue pairs is 85%, while it varies between the main GRAFS families from an average of 59 to 94%. The orthologous pairs for the lipid-binding GPCRs had the lowest levels of conservation, while the biogenic amines had highest levels of conservation. Moreover, we searched for expressed sequence tags (ESTs) and identified more than 17,000 ESTs matching GPCRs in mouse and human, providing information about their expression patterns. On the whole, this is the most comprehensive study of the gene repertoire that codes for human and mouse GPCRs. The datasets are available for downloading.

Kynurenic acid as a ligand for orphan G protein-coupled receptor GPR35

Local catabolism of the essential amino acid tryptophan is considered an important mechanism in regulating immunological and neurological responses. The kynurenine pathway is the main route for the non-protein metabolism of tryptophan. The intermediates of the kynurenine pathway are present at micromolar concentrations in blood and are regulated by inflammatory stimuli. Here we show that GPR35, a previously orphan G protein-coupled receptor, functions as a receptor for the kynurenine pathway intermediate kynurenic acid. Kynurenic acid elicits calcium mobilization and inositol phosphate production in a GPR35-dependent manner in the presence of G(qi/o) chimeric G proteins. Kynurenic acid stimulates [35S]guanosine 5'-O-(3-thiotriphosphate) binding in GPR35-expressing cells, an effect abolished by pertussis toxin treatment. Kynurenic acid also induces the internalization of GPR35. Expression analysis indicates that GPR35 is predominantly detected in immune cells and the gastrointestinal tract. Furthermore, we show that kynurenic acid inhibits lipopolysaccharide-induced tumor necrosis factor-alpha secretion in peripheral blood mononuclear cells. Our results suggest unexpected signaling functions for kynurenic acid through GPR35 activation.

New approaches for male fertility control: HE6 as an example of a putative target

Reversible contraceptive methods for males are still not available. During the last few years several marketing studies have clearly shown that men and women would welcome a situation where men could assume responsibility for family planning. Schering AG and Organon are currently collaborating to develop a hormonal method for male fertility control based on the combination of etonogestrel as gestagenic component and testosterone undecanoate. To further optimize male contraceptives in terms of improved efficiency, rapid onset, reversibility, fewer side effects and a convenient method of application, a search for innovative non-hormonal approaches was started. During the last few years, numerous proteins were identified which play a specific role in male fertility. These proteins have first to fulfil a set of indication-specific criteria before a drug discovery process can be initiated. The most important criteria for a putative target protein are tissue-selective expression, crucial biological function in fertility, drugable properties and feasibility of assay development for high-throughput-screening and lead optimization. The G-protein-coupled receptor HE6 was selected as target and the above selection criteria were applied. HE6 displays a preferred epididymis-specific expression pattern and belongs to the superfamily of GPCRs, which are well known to be drugable with small molecules. A knockout mouse was generated which revealed an infertility phenotype with the onset occurring 6 weeks after initiation of spermatogenesis at the latest. Surprisingly, no epididymis-specific phenotype was observed. Instead, the reabsorption of testicular fluid along the efferent ducts was strongly affected. No further obvious side effects were observed in male or female mice. This study with HE6 exemplifies how targets for male contraception have to be validated before drug development can start.