International Union of Pharmacology. LVI. Ghrelin receptor nomenclature, distribution, and function

Ghrelin is a 28-amino acid peptide originally isolated from rat stomach and is cleaved from a 117-amino acid precursor. The sequence of the mature peptide from rats and mice differs by two amino acids from that of human ghrelin. Alternative splicing of the ghrelin gene transcript can result in the translation of a second biologically active peptide, des-Gln14-ghrelin. Both peptides have a unique post-translational modification, octanoylation of Ser3, which is essential for the binding to receptors in hypothalamus and pituitary and stimulating the release of growth hormone from the pituitary. The growth hormone secretagogue receptor (GHS-R1a, Swiss-Prot code Q92847, LocusLink ID 2693), a rhodopsin-like seven transmembrane spanning G protein-coupled receptors belonging to Family A, was cloned in 1996 from the pituitary and hypothalamus and shown to be the target of growth hormone secretagogues (GHS), a class of synthetic peptide and nonpeptide compounds causing growth hormone release from the anterior pituitary. In 1999, ghrelin was identified as the endogenous cognate ligand for this receptor. The purpose of this review is to propose an official International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR) nomenclature designating GHS-R1a as the ghrelin receptor to follow the convention of naming receptors after the endogenous agonist, abbreviated where necessary to GRLN.

Regions in the G Protein γ Subunit Important for Interaction with Receptors and Effectors

G betagamma dimers containing the gamma11 or gamma1 subunits are often less potent and effective in their ability to regulate effectors compared with dimers containing the gamma2 subunit. To explore the regions of the gamma subunit that affect the activity of the betagamma dimer, we constructed eight chimeric gamma subunits from the gamma1 and gamma2 subunits. Two chimeras were made in which the N-terminal regions of gamma1 and gamma2 were exchanged and two in which the C-terminal regions were transposed. Another set of chimeras was made in which the CAAX motifs of the chimeras were altered to direct modification with different prenyl groups. All eight gamma chimeras were expressed in Sf9 cells with the beta1 subunit, G betagamma dimers were purified, and then they were assayed in vitro for their ability to bind to the G alpha(i1) subunit, to couple G alpha(i1) to the A1 adenosine receptor, to stimulate phospholipase C-beta, and to regulate type I or type II adenyl cyclases. Dimers containing the C-terminal sequence of the gamma2 subunit modified with the geranylgeranyl lipid had the highest affinity for G(i1)alpha (range, 0.5-1.2 nM) and were most effective at coupling the G(i1)alpha subunit to receptor. These dimers were most effective at stimulating the phosphatidylinositol-specific phospholipase C-beta isoform and inhibiting type I adenyl cyclase. In contrast, betagamma dimers containing the N-terminal sequence of the gamma2 subunit and a geranylgeranyl group are most effective at activating type II adenyl cyclase. The results indicate that both the N- and C-terminal regions of the gamma subunit impart specificity to receptor and effector interactions.

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

NC-IUPHAR (International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification) and its subcommittees provide authoritative reports on the nomenclature and pharmacology of G protein-coupled receptors (GPCRs) that summarize their structure, pharmacology, and roles in physiology and pathology. These reports are published in Pharmacological Reviews (http://www.iuphar.org/nciuphar_arti.html) and through the International Union of Pharmacology (IUPHAR) Receptor Database web site (http://www.iuphar-db.org/iuphar-rd). The essentially complete sequencing of the human genome has allowed the cataloging of all of the human gene sequences potentially encoding GPCRs. The IUPHAR Receptor List (http://www.iuphar-db.org/iuphar-rd/list/index.htm) presents this catalog giving IUPHAR-approved nomenclature (where available), known ligands, and gene names for all of these potential receptors (excluding sensory receptors and pseudogenes) together with links to curated sequence, descriptive information, and additional links in the Entrez Gene database (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=gene). This list is a major new initiative of NC-IUPHAR that, through continuing curation, defines the target of our ongoing receptor classification and invites further input from the scientific community.

Galanin receptor 1 has anti-proliferative effects in oral squamous cell carcinoma

In the United States, oral cancer accounts for more deaths annually than cervical cancer, leukemias, or Hodgkin's lymphoma. Studies have shown that aberrations of chromosome 18q develop with tumor progression and are associated with significantly decreased survival in head and neck cancer patients. The G-protein-coupled receptor, galanin receptor 1 (GALR1), maps to this region of chromosome 18q. Although the role of GALR1 has been well characterized in neuronal cells, little is known regarding this receptor in non-neuronal cells. In this study, the expression, mitogenic function, and signaling mechanism of GALR1 are investigated in normal and malignant oral epithelial cells. mRNA expression was determined via reverse transcriptase-PCR. Protein quantification was done via immunoblot analysis and enzyme-linked immunosorbent assay. For functional and signaling studies, an inhibitory antibody was generated to the N-terminal ligand binding domain of GALR1. GALR1 protein and mRNA expression and GAL secretion were detected at variable levels in immortalized human oral keratinocytes and human oropharyngeal squamous cell carcinoma cell lines. Upon competitive inhibition of GALR1, proliferation was up-regulated in immortalized and malignant keratinocytes. Furthermore, studies with the inhibitory antibody and U0126, the MAPK inhibitor, show that GALR1 inhibits proliferation in immortalized and malignant keratinocytes by inactivating the MAPK pathway. GALR1s inhibitory effects on proliferation in epithelial cells raises the possibility that inactivation or disregulation of this receptor can lead to uncontrolled proliferation and neoplastic transformation.

Receptor−Antagonist Interactions in the Complexes of Agouti and Agouti-Related Protein with Human Melanocortin 1 and 4 Receptors,

The molecular interactions between human melanocortin receptor-1 and -4 (hMC1R and hMC4R) and their endogenous antagonists, agouti signaling protein (ASIP) and agouti-related protein (AGRP), were assessed by studying the effects of site-directed mutations on the binding affinity of (125)I-ASIP[90-132(L89Y)] and (125)I-AGRP(86-132). Mutations of homologous residues from transmembrane helices (TMHs) 3 and 6 and extracellular loop (EL) 3 (D121A, T124A, F257A, and F277M in hMC1R and D126A, I129A F261A, and M281F in hMC4R) impaired binding of both antagonists to hMC4R and binding of the ASIP fragment to hMC1R. However, the mutations in TMH2 (E94A in hMC1R and E100A in hMC4R), TMH7 (F280A in hMC1R and F284A in hMC4R), and EL2 (Y183S, H184S, and D184H in hMC1R) only significantly affected binding of the ASIP fragment. The dependence of agonist binding on the dithiothreitol concentration followed a monophasic curve for wild-type hMC4R and its C40A, C271A, and C279A mutants and a biphasic curve for hMC1R, suggesting the presence of at least one structurally and functionally essential disulfide bond in both wild-type receptors and the hMC4R mutants. Models of complexes of both receptors with the ASIP fragment and hMC4R with the AGRP fragment were calculated using constraints from the experimental structures of rhodopsin and AGRP fragments, a set of deduced hydrogen bonds, supplemented by two proposed disulfide bridges and receptor-ligand contacts, derived from our mutagenesis data. In the models of the ASIP fragment complexed with both receptors, the core ligand tripeptide, Arg-Phe-Phe, positioned between TMHs 3 and 6, is shifted toward TMHs 2 and 7 relative to its position in the AGRP-hMC4R model, while the N-terminal loop and two central disulfides of the antagonists interact with EL2 of the receptors.

REGULATORs OF G PROTEIN SIGNALING & DRUGS OF ABUSE

Drugs of abuse such as opioids and stimulants share a common dopaminergic reward pathway; however, in response to continual intermittent exposure to such drugs, there are neuronal alterations leading to changes in behavior. Regulators of G protein signaling (RGS) are proteins that negatively regulate G protein signaling and are expressed in brain areas important for the pharmacology of abused drugs. Moreover, the level of expression of several of these proteins is regulated by abused drugs. In this article, we discuss RGS proteins, their regulation by morphine and stimulants, and how altered levels of these proteins affect cell signaling to contribute to the pharmacology and behavioral consequence of abused drugs. Finally, we consider if RGS proteins represent viable targets for drug abuse medications.

Real-time detection of basal and stimulated G protein GTPase activity using fluorescent GTP analogues

Hydrolysis of fluorescent GTP analogues BODIPY FL guanosine 5 '-O-(thiotriphosphate) (BGTPgammaS) and BODIPY FL GTP (BGTP) by Galpha(i1) and Galpha was characterized using on-line capillary electrophoresis (o) laser-induced fluorescence assays in order that changes in sub-strate, substrate-enzyme complex, and product could be monitored separately. Apparent k values (V /[E]) (max cat) steady-state and K(m) values were determined from assays for each substrate-protein pair. When BGTP was the substrate, maximum turnover numbers for Galpha and Galpha(i1) were 8.3 +/- 1 x 10(-3) and 3.0 +/- 0.2 x 10(-2) s(-1), respectively, and K(m) values were 120 +/- 60 and 940 +/- 160 nm. Assays with BGTPgammaS yielded maximum turnover numbers of 1.6 +/- 0.1 x 10(-4) and 5.5 +/- 0.3 x 10(-4) s(-1) for Galpha and Galpha(i1); K(m) values were 14 (o)(+/-)8 and 87 +/- 22 nm. Acceleration of Galpha GTPase activity by regulators of G protein signaling (RGS) was demonstrated in both steady-state and pseudo-single-turnover assay formats with BGTP. Nanomolar RGS increased the rate of enzyme product formation (BODIPY(R) FL GDP (BGDP)) by 117-213% under steady-state conditions and accelerated the rate of G protein-BGTP complex decay by 199 -778% in pseudo-single-turnover assays. Stimulation of GTPase activity by RGS proteins was inhibited 38-81% by 40 mum YJ34, a previously reported peptide RGS inhibitor. Taken together, these results illustrate that Galpha subunits utilize BGTP as a substrate similarly to GTP, making BGTP a useful fluorescent indicator of G protein activity. The unexpected levels of BGTPgammaS hydrolysis detected suggest that caution should be used when interpreting data from fluorescence assays with this probe.

Affinity Assays Using Fluorescence Anisotropy with Capillary Electrophoresis Separation

A novel approach to detecting affinity interactions that combines fluorescence anisotropy with capillary electrophoresis (FACE) was developed. In the method, sample is injected into a capillary filled with buffer that contains a fluorescent probe that possesses low fluorescence anisotropy. If proteins or other large molecules in the sample bind the fluorescent probe, their migration through the capillary can be detected as a positive anisotropy shift. Thus, the method provides both separation and confirmation of binding to the probe. Calculations based on combining the Perrin equation and dissociation constant were used to predict the effect of conditions on aniostropy detection. These calculations predict that low probe concentrations yield the best sensitivity while higher concentrations increase the dynamic range for detection of binding partner. The assay was applied to detection of G proteins using BODIPY FL GTPgammaS as the fluorescent probe. Experimental measurements exhibited trends in anisotropy with varying probe and protein concentrations that were consistent with the calculations. The limit of detection for G(alphai1) was 3 nM when the electrophoresis buffer contained 250 nM BODIPY FL GTPgammaS. FACE affinity assay is envisioned as a method that can quantify selected binding partners and screen complex samples for compounds that possess affinity for a particular small molecule that is used as a probe.

Structure-based design, synthesis, and pharmacologic evaluation of peptide RGS4 inhibitors

Regulators of G-protein signaling (RGS) proteins form a multifunctional signaling family. A key role of RGS proteins is binding to the G-protein Galpha-subunit and acting as GTPase-activating proteins (GAPs), thereby rapidly terminating G protein-coupled receptor (GPCR) signaling. Using the published RGS4-Gialpha1 X-ray structure we have designed and synthesized a series of cyclic peptides, modeled on the Gialpha Switch I region, that inhibit RGS4 GAP activity. These compounds should prove useful for elucidating RGS-mediated activity and serve as a starting point for the development of a novel class of therapeutic agent.

Thrombin and Lysophosphatidic Acid Receptors Utilize Distinct rhoGEFs in Prostate Cancer Cells

Thrombin and lysophosphatidic acid (LPA) receptors play important roles in vascular biology, development, and cancer. These receptors activate rho via G(12/13) family heterotrimeric G proteins, which are known to directly activate three distinct rho guanine nucleotide exchange factors (rhoGEFs) that contain a regulator of G protein signaling (RGS) domain (RGS-rhoGEFs). However, it is not known which, if any, of these RGS-rhoGEFs (LARG (leukemia-associated rhoGEF), p115rhoGEF, or PDZrhoGEF) plays a role in G protein-coupled receptor-stimulated rho signaling. Using oligonucleotide small interfering RNAs that suppress specific RGS-rhoGEF expression, we show that thrombin receptor stimulation of rho is primarily mediated by LARG in HEK293T and PC-3 prostate cancer cell lines. In contrast, the LPA-stimulated rho response in PC-3 cells is dependent on PDZrhoGEF expression. Suppression of p115rhoGEF had no effect. Thus different rhoGEFs (LARG and PDZrhoGEF) mediate downstream rho signaling by the thrombin and LPA receptors.

Detection of G proteins by affinity probe capillary electrophoresis using a fluorescently labeled GTP analogue

An affinity probe capillary electrophoresis (APCE) assay for guanine-nucleotide-binding proteins (G proteins) was developed using BODIPY FL GTPgammaS (BGTPgammaS), a fluorescently labeled GTP analogue, as the affinity probe. In the assay, BGTPgammaS was incubated with samples containing G proteins and the resulting mixtures of BGTPgammaS-G protein complexes and free BGTPgammaS were separated by capillary electrophoresis and detected with laser-induced fluorescence detection. Separations were completed in less than 30 s using 25 mM Tris, 192 mM glycine at pH 8.5 as the electrophoresis buffer and applying 555 V/cm over a 4-cm separation distance. BGTPgammaS-Galpha(o) peak heights increased linearly with Galpha(o) up to approximately 200 nM using a 50 nM BGTPgammaS probe. The detection limit for Galpha(o) was 2 nM, corresponding to a mass detection limit of 3 amol. The high speed of the APCE assays allowed reaction kinetics and the dissociation constant (Kd) to be determined. The on-rate and off-rate of BGTPgammaS to Galpha(o) were 0.0068 +/- 0.0004 and 0.000 23 +/- 0.000 01 s(-1), respectively. The half-life of the BGTPgammaS-Galpha(o) complex was 3060 +/- 240 s and Kd was 8.6 +/- 0.7 nM. The estimates of these parameters are in good agreement with those obtained using established techniques, indicating the suitability of this method for such measurements. Lowering the temperature of the separation improved the detection of the complex, allowing the assay to be performed on a commercial instrument with longer separation times. Additionally, the capability of the technique to detect several G proteins based on their binding to BGTPgammaS was demonstrated with assays for Galpha and Galpha(i1) and for Ras and Rab3A.

Endogenous regulator of G protein signaling proteins suppress Galphao-dependent, mu-opioid agonist-mediated adenylyl cyclase supersensitization

Chronic mu-opioid agonist treatment leads to dependence with withdrawal on removal of agonist. At the cellular level withdrawal is accompanied by a supersensitization of adenylyl cyclase, an effect that requires inhibitory Galpha proteins. Inhibitory Galpha protein action is modulated by regulator of G protein signaling (RGS) proteins that act as GTPase activating proteins and reduce the lifetime of Galpha-GTP. In this article, we use C6 glioma cells expressing the rat mu-opioid receptor (C6mu) to examine the hypothesis that Galphao alone can mediate mu-opioid agonist induced adenylyl cyclase supersensitivity and that endogenous RGS proteins serve to limit the extent of this supersensitization. C6mu cells were stably transfected with pertussis toxin (PTX)-insensitive Galphao that was either sensitive or insensitive to endogenous RGS proteins. Cells were treated with PTX to uncouple endogenous Galpha proteins followed by exposure to the mu-opioid agonists [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin or morphine. Supersensitization was observed in cells expressing wild-type Galpha, but this was lost on PTX treatment. In cells expressing PTX-insensitive Galphao supersensitization was recovered, confirming that Galphao alone can support supersensitization. In cells expressing the RGS-insensitive mutant Galphao, there was a greater degree of supersensitization and the concentration of micro-agonist needed to achieve half-maximal supersensitization was reduced by 10-fold. The amount of supersensitization seen did not directly relate to the degree of acute inhibition of adenylyl cyclase. These results demonstrate a role for Galphao in adenylyl cyclase supersensitization after mu-agonist exposure and show that this action is modulated by endogenous RGS proteins.

Real-time analysis of ternary complex on particles: direct evidence for partial agonism at the agonist-receptor-G protein complex assembly step of signal transduction

We developed a novel and generalized approach to investigate G protein-coupled receptor molecular assemblies. We solubilized a fusion protein consisting of the beta(2)-adrenergic receptor and green fluorescent protein (GFP) for bead-based flow cytometric analysis. beta(2)-Adrenergic receptor GFP bound to dihydroalprenolol-conjugated beads, providing a K(d) for the fusion protein and, in competition with beta(2)-adrenergic receptor ligands, K(d) values for agonists and antagonists. Beads displaying chelated nickel bound purified hexahistidine-tagged G protein heterotrimers and, subsequently, the binary complex of agonist with beta(2)-adrenergic receptor GFP. The dose-response curves of ternary complex formation revealed maximal assembly for ligands previously classified as full agonists and reduced assembly for ligands previously classified as partial agonists. Guanosine 5'-3-O-(thio)triphosphate-induced dissociation rates of the ternary complex were the same for full and partial agonists. Soluble G protein, competing with ternary complexes on beads provided an affinity estimate of agonist-receptor complexes to G protein. When performed simultaneously, the two assemblies discriminated between agonist, antagonist or inactive molecule in a manner appropriate for high throughput, small volume drug discovery. The assemblies can be further generalized to other G protein coupled receptor protein-protein interactions.

Structure-Based Design, Synthesis, and Activity of Peptide Inhibitors of RGS4 GAP Activity

One of the principal roles of the multifunctional regulator of G-protein signaling (RGS) proteins is to terminate G-protein-coupled receptor (GPCR) signaling by binding to the G-protein Galpha subunit, thus acting as GTPase-activating proteins (GAPs). In principle, then, selective inhibitors of this GAP function would have potential as therapeutic agents, as they could be used to augment the effects of endogenous or exogenous GPCR agonists. Using the published RGS4-G(ialpha1) X-ray structure, we have designed and synthesized a series of cyclic peptides, modeled on the G(ialpha) switch I region, that inhibit RGS4 GAP activity, presumably by blocking the interaction between RGS4 and G(ialpha1). These compounds should prove useful for elucidating RGS-mediated activity and serve as a starting point for the development of a novel class of therapeutic agent.

Ribozyme- and siRNA-Mediated Suppression of RGS-Containing RhoGEF Proteins

Given recent efforts to determine the sequence information on thousands of genes in the human genome, the current challenge is to identify the functions of these genes, including those encoding the regulator of G-protein signaling protein gene superfamily, and to establish their roles in particular signaling pathways in a native system. Increasingly, reverse genetic approaches are being used to address these questions. This article compares two powerful approaches [ribozyme and "short interfering" RNA (siRNA) techniques] under identical conditions for the first report on the suppression of endogenous RGS domain-containing RhoGEFs. The siRNA technique was found to be much more potent than ribozyme targeting at the same mRNA site of RGS-RhoGEFs. Also, the three siRNAs targeting LARG, PDZ-RhoGEF, and p115-RhoGEF are able to discriminate the closely related sequences within this RGS-RhoGEF gene family.

RGS-insensitive G-protein mutations to study the role of endogenous RGS proteins

Regulator of G-protein signaling (RGS) proteins are very active GTPase-accelerating proteins (GAPs) in vitro and are expected to reduce signaling by G-protein coupled receptors in vivo. A novel method is presented to assess the in vivo role of RGS proteins in the function of a G protein in which Galpha subunits do not bind to RGS proteins or respond with enhanced GTPase activity. A point mutation in the switch I region of Galpha subunits (G184S Galpha(o) and G183S Galpha(i1)) blocks the interaction with RGS proteins but leaves intact the ability of Galpha to couple to betagamma subunits, receptors, and downstream effectors. Expression of the RGS-insensitive mutant G184S Galpha(o) in C6 glioma cells with the micro-opioid receptor dramatically enhances adenylylcyclase inhibition and activation of extracellular regulated kinase. Introducing the same G184S Galpha(o) protein into embryonic stem (ES) cells by gene targeting allows us to assess the functional importance of the endogenous RGS proteins using in vitro differentiation models and in intact mice. Using ES cell-derived cardiocytes, spontaneous and isoproterenol-stimulated beating rates were not different between wild-type and G184S Galpha(o) mutant cells; however, the bradycardiac response to adenosine A1 receptor agonists was enhanced significantly (seven-fold decrease EC50) in Galpha(o)RGSi mutant cells compared to wild-type Galpha(o), indicating a significant role of endogenous RGS proteins in cardiac automaticity regulation. The approach of using RGS-insensitive Galpha subunit knockins will reveal the role of RGS protein-mediated GAP activity in signaling by a given G(i/o) protein. This will reveal the full extent of RGS regulation and will not be confounded by redundancy in the function of multiple RGS proteins.

Depicting a protein's two faces: GPCR classification by phylogenetic tree-based HMMs

Related proteins with similar biological functions generally share common features, allowing us to extract the common sequence features. These common features enable us to build statistical models that can be used to classify proteins, to predict new members, and to study the sequence-function relationship of this protein function group. Although evolution underlies the basis of multiple sequence analysis methods, most methods ignore phylogenetic relationships and the evolutionary process in building these statistical models. Previously we have shown that a phylogenetic tree-based profile hidden Markov model (T-HMM) is superior in generating a profile for a group of similar proteins. In this study we used the method to generate common features of G protein-coupled receptors (GPCRs). The profile generated by T-HMM gives high accuracy in GPCR function classification, both by ligand and by coupled G protein.

International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. XXXVIII. Update on Terms and Symbols in Quantitative Pharmacology

The recommendations that follow have been updated from the proposals of a Technical Subcommittee set up by the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (Jenkinson DH, Barnard EA, Hoyer D, Humphrey PPA, Leff P, and Shankley NP (1995) International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. IX. Recommendations on terms and symbols in quantitative pharmacology. Pharmacol Rev 47:255-266).

Dimerization in Aminergic G-Protein-Coupled Receptors: Application of a Hidden-Site Class Model of Evolution†

G-Protein-coupled receptors (GPCRs) are an important superfamily of transmembrane proteins involved in cellular communication. Recently, it has been shown that dimerization is a widely occurring phenomenon in the GPCR superfamily, with likely important physiological roles. Here we use a novel hidden-site class model of evolution as a sequence analysis tool to predict possible dimerization interfaces in GPCRs. This model aims to simulate the evolution of proteins at the amino acid level, allowing the analysis of their sequences in an explicitly evolutionary context. Applying this model to aminergic GPCR sequences, we first validate the general reasoning behind the model. We then use the model to perform a family specific analysis of GPCRs. Accounting for the family structure of these proteins, this approach detects different evolutionarily conserved and accessible patches on transmembrane (TM) helices 4-6 in different families. On the basis of these findings, we propose an experimentally testable dimerization mechanism, involving interactions among different combinations of these helices in different families of aminergic GPCRs.

Ligand-receptor-G-protein molecular assemblies on beads for mechanistic studies and screening by flow cytometry

G protein-coupled receptors form a ternary complex of ligand, receptor, and G protein heterotrimer (LRG) during signal transduction from the outside to the inside of a cell. Our goal was to develop a homogeneous, small-volume, bead-based approach compatible with high-throughput flow cytometry that would allow evaluation of G protein coupled receptor molecular assemblies. Dextran beads were derivatized to carry chelated nickel to bind hexahistidine-tagged green fluorescent protein (GFP) and hexahistidine-tagged G proteins. Ternary complexes were assembled on these beads using fluorescent ligand with wild-type receptor or a receptor-Gialpha2 fusion protein, and with a nonfluorescent ligand and receptor-GFP fusion protein. Streptavidin-coated polystyrene beads used biotinylated anti-FLAG antibodies to bind FLAG-tagged G proteins for ternary complex assembly. Validation was achieved by showing time and concentration dependence of ternary complex formation. Affinity measurements of ligand for receptor on particles, of the ligand-receptor complex for G protein on the particles, and receptor-Gialpha2 fusion protein for Gbetagamma, were consistent with comparable assemblies in detergent suspension. Performance was assessed in applications representing the potential of these assemblies for ternary complex mechanisms. We showed the relationship for a family of ligands between LR and LRG affinity and characterized the affinity of both the wild-type and GFP fusion receptors with G protein. We also showed the potential of kinetic measurements to allow observation of individual steps of GTP-induced ternary complex disassembly and discriminated a fast step caused by RG disassembly compared with the slower step of Galphabetagamma disassembly.

Diabetic neuropathy: inhibitory G protein dysfunction involves PKC-dependent phosphorylation of Goalpha

We examined the hypothesis that decreased inhibitory G protein function in diabetic neuropathy is associated with increased protein kinase C (PKC)-dependent phosphorylation of the Goalpha subunit. Streptozotocin-induced diabetic rats were studied between 4 and 8 weeks after onset of diabetes and compared with aged-matched healthy animals as controls. Opioid-mediated inhibition of forskolin-stimulated cyclic AMP was significantly less in dorsal root ganglia (DRGs) from diabetic rats compared with controls. Activation of PKC in DRGs from control rats was associated with a significant decrease in opioid-mediated inhibition of forskolin-stimulated cyclic AMP that was similar to the decrease in inhibition observed in DRGs from diabetic rats. Both basal and PKC-mediated labeling of Goalpha with 32Pi was significantly less in DRGs from diabetic rats, supporting increased endogenous PKC-dependent phosphorylation of Goalpha. Probing of immunoprecipitated Goalpha with an anti-phospho-serine/threonine specific antibody revealed a significant increase in baseline phosphorylation in diabetic DRGs. Activation of PKC produced a significant increase in phosphorylation in control DRGs but no significant increase in Goalpha in diabetic DRGs. Phosphorylation of PKC-alpha was increased, PKC-betaII was unchanged and PKC-delta decreased in diabetic DRGs. These results suggest that diminished inhibitory G protein function observed in DRGs neurons from diabetic rats involves an isoform-specific PKC-dependent pathway.

Stimulation of cellular signaling and G protein subunit dissociation by G protein betagamma subunit-binding peptides

We previously developed peptides that bind to G protein betagamma subunits and selectively block interactions between betagamma subunits and a subset of effectors in vitro (Scott, J. K., Huang, S. F., Gangadhar, B. P., Samoriski, G. M., Clapp, P., Gross, R. A., Taussig, R., and Smrcka, A. V. (2001) EMBO J. 20, 767-776). Here, we created cell-permeating versions of some of these peptides by N-terminal modification with either myristate or the cell permeation sequence from human immunodeficiency virus TAT protein. The myristoylated betagamma-binding peptide (mSIRK) applied to primary rat arterial smooth muscle cells caused rapid activation of extracellular signal-regulated kinase 1/2 in the absence of an agonist. This activation did not occur if the peptide lacked a myristate at the N terminus, if the peptide had a single point mutation to eliminate betagamma subunit binding, or if the cells stably expressed the C terminus of betaARK1. A human immunodeficiency virus TAT-modified peptide (TAT-SIRK) and a myristoylated version of a second peptide (mSCAR) that binds to the same site on betagamma subunits as mSIRK, also caused extracellular signal-regulated kinase activation. mSIRK also stimulated Jun N-terminal kinase phosphorylation, p38 mitogen-activated protein kinase phosphorylation, and phospholipase C activity and caused Ca2+ release from internal stores. When tested with purified G protein subunits in vitro, SIRK promoted alpha subunit dissociation from betagamma subunits without stimulating nucleotide exchange. These data suggest a novel mechanism by which selective betagamma-binding peptides can release G protein betagamma subunits from heterotrimers to stimulate G protein pathways in cells.

Inverse agonist activity of agouti and agouti-related protein

Agouti and agouti-related protein (AgRP) are endogenous antagonists of the melanocortin receptors (MCxR). Previous data showed that recombinant full-length agouti and a synthetic fragment of AgRP, AgRP (83-132), are inverse agonists at the MC1R and MC4R, respectively. This study demonstrates the smaller analogs AgRP (87-120) and ASIP [90-132 (L89Y)], and short peptides Yc[CRFFNAFC]Y and Qc[CRFFRSAC]S are also MC4R inverse agonists. Furthermore, the relative affinity of the series of MC4R ligands for displacement of radiolabeled antagonist 125I-AgRP (86-132) versus radiolabeled agonist 125I-NDP-MSH did not correlate with ligand efficacy, which is more consistent with an induced-fit model than a simple two-state model of MC4R activation. These data shed new light on the determinants and mechanism of inverse agonism at the MC4R.

Endogenous RGS protein action modulates mu-opioid signaling through Galphao. Effects on adenylyl cyclase, extracellular signal-regulated kinases, and intracellular calcium pathways

RGS (regulators of G protein signaling) proteins are GTPase-activating proteins for the Galpha subunits of heterotrimeric G proteins and act to regulate signaling by rapidly cycling G protein. RGS proteins may integrate receptors and signaling pathways by physical or kinetic scaffolding mechanisms. To determine whether this results in enhancement and/or selectivity of agonist signaling, we have prepared C6 cells stably expressing the mu-opioid receptor and either pertussis toxin-insensitive or RGS- and pertussis toxin-insensitive Galpha(o). We have compared the activation of G protein, inhibition of adenylyl cyclase, stimulation of intracellular calcium release, and activation of the ERK1/2 MAPK pathway between cells expressing mutant Galpha(o) that is either RGS-insensitive or RGS-sensitive. The mu-receptor agonist [d-Ala(2),MePhe(4),Gly(5)-ol]enkephalin and partial agonist morphine were much more potent and/or had an increased maximal effect in inhibiting adenylyl cyclase and in activating MAPK in cells expressing RGS-insensitive Galpha(o). In contrast, mu-opioid agonist increases in intracellular calcium were less affected. The results are consistent with the hypothesis that the GTPase-activating protein activity of RGS proteins provides a control that limits agonist action through effector pathways and may contribute to selectivity of activation of intracellular signaling pathways.