Mechanistic hypotheses for the activation of G-protein-coupled receptors

Conformational state of human cardiac 5-HT(4(g)) receptors influences the functional effects of polyclonal anti-5-HT(4) receptor antibodies

The functional effects of the anti-G21V antibody directed against the second extracellular loop of human heart 5-HT(4) receptors can differ when the receptors are expressed in different cell lines. Here, we extend these studies to show variation in the responses of 5-HT(4(g)) receptors to the antibody within the same expression system. In a previous report no effect of the anti-G21V antibodies had been shown upon 5-HT(4(g)) receptors expressed in CHO cells. Here the same antibodies alone or when added before 5-HT had a functional "inverse-agonist like" effect upon 5-HT(4(g)) receptors expressed in a separate line of CHO cells. Although these CHO cells showed a lower efficacy of cAMP production evoked by 5-HT than the previous report they express a similar h5-HT(4(g)) receptor density. Inhibition of either phosphodiesterases or Gi proteins had no effect upon the action of the antibody. Conformational states of the 5-HT(4) receptor and/or equilibrium between different states of receptors may then determine the functional effect of antibodies against this receptor.

G protein-coupled receptor signalling and cross-talk: achieving rapidity and specificity

Activation of a given type of G protein-coupled receptor (GPCR) triggers a limited set of signalling events in a very rapid and specific manner. The classical paradigm of GPCR signalling was rather linear and sequential. Emerging evidence, however, has revealed that this is only a part of the complex signalling mediated by GPCR. Propagation of GPCR signalling involves cross-regulation of many but specific pathways, including cross-talks between different GPCRs as well as with other signalling pathways. Moreover, it is increasingly apparent that GPCRs can activate both heterotrimeric G protein-dependent and G protein-independent signalling pathways. In this review, we discuss how the signallings initiated by GPCRs achieve rapidity as well as specificity, and how the GPCRs can cross-regulate other specific signalling pathways at the same time. New concepts regarding GPCR signalling have been arising to address this issue, which include multiprotein signalling complex and signalling compartment in microdomain concepts that enable close colocalization or even contact among the proteins engaged in the specific signal transduction. The final outcome of a stimulation of GPCR will thus be the sum of its own specific set of intracellular signalling pathways it regulates.

Length analyses of mammalian G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) play a crucial role in mediating effects of extracellular messengers in a wide variety of biological systems, comprising the largest gene superfamily at least in mammals. Mammalian GPCRs are broadly classified into three families based on pharmacological properties and sequence similarities. These sequence similarities are largely confined to the seven transmembrane domains, and much less in the extracellular and intracellular loops and terminals (LTs). Together with the fact that the LTs vary considerably in length and sequence, the LT length of GPCRs has not been studied systematically. Here we have applied a statistical analysis to the length of the LTs of a wide variety of mammalian GPCRs in order to examine the existence of any trends in molecular architecture among a known mammalian GPCR population. Tree diagrams constructed by cluster analyses, using eight length factors in a given GPCR, revealed possible length relations among GPCRs and defined at least three groups. Most samples in Group J (joined) and Group M (minor) had an exceptionally long N-terminal and I3 loop, respectively; and other samples were considered as Group O (other/original). This length-based classification largely coincided with the conventional sequence- and pharmacology-based classification, suggesting that the LT length contains some biological information when analysed at the population level. Principle component analyses suggested the existence of inherent length differences between loops and terminals as well as between extracellular and intracellular LTs. Wilcoxon rank transformation tests unveiled statistically significant differences between Group O and Group J, not only in the N-terminal and I3 loop, but also in the E3 loop. Correlation analyses identified an E1-I2 length-correlation in Group O and Group J and an N-E3 length-correlation in Group J. Taken together, these results suggest a possible functional importance of LT length in the GPCR superfamily.

D2 dopamine receptor-G protein coupling. Cross-regulation of agonist and guanosine nucleotide binding sites

The cross regulation of agonist binding to D2 dopamine receptors and guanosine nucleotide binding to G proteins was studied using membranes of Chinese hamster ovary cells expressing rat D2short dopamine receptors. All guanosine nucleotides studied caused a concentration-dependent loss of high-affinity agonist binding sites of D2 receptors with potencies corresponding to their affinity to bind to G proteins in these membranes. On the other hand, the dopaminergic agonists, but not antagonists, decreased the affinities of guanosine diphosphate and guanosine monophosphate, but not of guanosine 5'-(gamma-thiotriphosphate). The cross regulation of ligand binding to D2 dopamine receptors and G proteins suggests the existence of several conformational states of these proteins during the signal transduction and that the high-affinity state of agonist binding is a transient state of the agonist-receptor-G protein complex, where no nucleotides are bound.

Switching agonist/antagonist properties of opiate alkaloids at the delta opioid receptor using mutations based on the structure of the orphanin FQ receptor

In an earlier study, we have demonstrated that by mutating five amino acid residues to those conserved in the opioid receptors, the OFQ receptor could be converted to a functional receptor that bound many opioid alkaloids with nanomolar affinities. Surprisingly, when the reciprocal mutations, Lys-214 --> Ala (TM5), Ile-277 --> Val/His-278 --> Gln/Ile-279 --> Val (TM6), and Ile-304 --> Thr (TM7), are introduced in the delta receptor, neither the individual mutations nor their various combinations significantly reduce the binding affinities of opioid alkaloids tested. However, these mutations cause profound alterations in the functional characteristics of the mutant receptors as measured in guanosine 5'-3-O-(thio)triphosphate binding assays. Some agonists become antagonists at some constructs as they lose their ability to activate them. Some alkaloid antagonists are transformed into agonists at other constructs, but their agonistic effects can still be blocked by the peptide antagonist TIPP. Even the delta inverse agonist 7-benzylidenenaltrexone becomes an agonist at the mutant containing both the Ile-277 --> Val/His-278 --> Gln/Ile-279 --> Val and Ile-304 --> Thr mutations. Thus, although the mutated residues are thought to be part of the binding pocket, they are critically involved in the control of the delta receptor activation process. These findings shed light on some of the structural bases of ligand efficacy. They are also compatible with the hypothesis that a ligand may achieve high affinity binding in several different ways, each having different effects on receptor activation.

Species-dependent pharmacological properties of the melanocortin-5 receptor

The genes encoding the melanocortin-3 receptor and melanocortin-5 receptor have been cloned from rhesus monkey. Heterologous expression in CHO cells indicated species dependent in vitro pharmacological properties for the human and rhesus melanocortin-5 receptors. Several peptides including NDP-alpha-MSH, alpha-MSH, MT-II and ACTH1-24 are more potent at the rhesus melanocortin-5 receptor than the human melanocortin-5 receptor by more than 10-fold. In contrast, we found no species difference in pharmacological properties between the human and rhesus melanocortin-3 receptors. Such a species-dependent pharmacological difference for melanocortin-5 receptor appears to be an exception compared to other G protein-coupled receptors from human and rhesus monkey.

Modelling G-protein-coupled receptors for drug design

The G-protein coupled receptors form a large and diverse multi-gene superfamily with many important physiological functions. As such, they have become important targets in pharmaceutical research. Molecular modelling and site-directed mutagenesis have played an important role in our increasing understanding of the structural basis of drug action at these receptors. Aspects of this understanding, how these techniques can be used within a drug-design programme, and remaining challenges for the future are reviewed.

Constitutive activation of the delta opioid receptor by mutations in transmembrane domains III and VII

We have investigated whether transmembrane amino acid residues Asp128 (domain III), Tyr129 (domain III) [corrected], and Tyr308 (domain VII) in the mouse delta opioid receptor play a role in receptor activation. To do so, we have used a [35S]GTPgammaS (where GTPgammaS is guanosine 5'-3-O-(thio)triphosphate) binding assay to quantify the activation of recombinant receptors transiently expressed in COS cells and compared functional responses of D128N, D128A, Y129F, Y129A, and Y308F point-mutated receptors to that of the wild-type receptor. In the absence of ligand, [35S]GTPgammaS binding was increased for every mutant receptor under study (1.6-2.6-fold), suggesting that all mutations are able to enhance constitutive activity at the receptor. In support of this finding, the inverse agonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu (where Aib represents alpha-aminobutyric acid) efficiently reduced basal [35S]GTPgammaS binding in the mutated receptor preparations. The potent agonist BW373U86 stimulated [35S]GTPgammaS binding above basal levels with similar (D128N, Y129F, and Y129A) or markedly increased (Y308F) efficacy compared with wild-type receptor. BW373U86 potency was maintained or increased. In conclusion, our results demonstrate that the mutations under study increase functional activity of the receptor. Three-dimensional modeling suggests that Asp128 (III) and Tyr308 (VII) interact with each other and that Tyr129 (III) undergoes H bonding with His278 (VI). Thus, Asp128, Tyr129, and Tyr308 may be involved in a network of interhelical bonds, which contributes to maintain the delta receptor under an inactive conformation. We suggest that the mutations weaken helix-helix interactions and generate a receptor state that favors the active conformation and/or interacts with heterotrimeric G proteins more effectively.

Intracellular signalling pathways induced by chemokines in natural killer cells

Chemokines are small peptides involved in the recruitment of various cell types into inflammatory sites. They are divided into four sub-families depending on the presence of amino acids separating the cysteine residues in their N-terminal region. These are the alpha (CXC), beta (CC), gamma (C) and delta (CX)C) chemokines. In addition, five CXC chemokine (CXCR1-5), nine CC chemokine (CCR1-9), one C chemokine (XCR1) and one C-X3C chemokine (CX3CR1) receptors have been identified. These receptors belong to the seven transmembrane spanning domain family, and are coupled to the heterotrimeric guanine nucleotide binding (G) proteins. Chemokines activate various immune cells, and in particular the anti-viral/anti-tumour effectors, the natural killer (NK) cells by activating members of the heterotrimeric G proteins. The importance of the family of chemokines is highlighted by the ability of its members to inhibit the replication of HIV-1 strains in CD4+ cells, where chemokine receptors act as HIV-1 co-receptors. This review discusses the intracellular signalling pathways induced by chemokines in NK and other cell types, and the relationships to HIV-1 signalling in these cells.

Pharmacological characterization of dopamine-stimulated [35S]-guanosine 5'(gamma-thiotriphosphate) ([35S]GTPgammaS) binding in rat striatal membranes

Functional activation of dopamine receptors in the crude membranes from rat striatum was studied by a [35S]-guanosine 5'-O-(gamma-thiotriphosphate) ([35S]GTPgammaS) binding assay. Binding of [35S]GTPgammaS could be characterized with a dissociation constant (Kd) = 14.6+/-0.8 nM and this did not depend on the presence of dopamine. The displacement of [35S]GTPgammaS binding by GDP could be characterized with an inhibition constant (K(i)) = 78+/-15 microM in the presence of 10 microM of butaclamol, while the presence of 100 microM of dopamine decreased it to a K(i) = 0.13+/-0.02 mM. Dopamine increased the association rate of [35S]GTPgammaS binding in the presence of GDP in a dose-dependent manner with an EC50 = 1.45+/-0.48 microM. Other dopamine receptor agonists studied displayed a potency to stimulate the [35S]GTPgammaS binding in the order R(-)-10,11dihydroxy-N-n-propylnorapomorphine (NPA) > pergolide > or = apomorphine > dopamine approximately quinpirole > R(+)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride (SKF-38393) > S(+)(4aR,10bR)-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]be nzopyrano[4,3-b]-1,4-oxazin-9-ol hydrocholoride (PD 128,907). The dopamine-induced stimulation of [35S]GTPgammaS binding was inhibited by different dopamine receptor antagonists in the potency order: (+)butaclamol > haloperidol approximately clorpromazine > or = raclopride > (-)-sulpride > remoxipride > 5,6-dimethoxy-2-(dipropylamine)indan (U 991944A) > R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzaz epine (SCH-23390). Comparison of the obtained data with the dissociation constants of these ligands to different subtypes of dopamine receptors gave a good correlation only with constants for the D2 subtype, supporting the idea that this subtype is most likely responsible for the dopaminergic activation of [35S]GTPgammaS binding in rat striatal membranes.

Regulation of agonist-receptor binding by G proteins and divalent cations in spermatozoa solubilized A1 adenosine receptors

Solubilized A1 adenosine receptor (A1AR) was used to investigate the effect of several cations on agonist-binding characteristics and GTP hydrolysis. It was shown by Western blot with G beta-M14 that this preparation contains both G proteins and receptor. The role of the receptor molecule is to facilitate the activation of G proteins as alpha-GTP complex, and GTP hydrolysis has important consequences for the basic deactivation mechanism. Divalent cations, such as Mn2+, Ca2+, and Mg2+, potentiated the agonist-specific binding: Mn2+ had the highest apparent affinity with half-maximal effect at 50 microM. Binding assays, performed in the presence of 100 microM Mn2+, showed an increase in the apparent affinity of the binding sites, whereas, in the presence of 1 mM Mg2+, significant alteration of the apparent affinity, but not of the number of sites, was detected. Concentrations of 1 mM Mg2+ and 100 microM Mn2+ enhanced GTPase activity, whereas 5 mM Ca2+ resulted in the increase of Vmax values without significant alterations of K(m). In the presence of A1-specific agonists, Mn2+ and Mg2+ caused a decrease of Vmax values and an increase of GTP affinity. Other cations, such as Co2+, Cd2+, Cu2+, and Zn2+, inhibited the binding capacity but caused almost no changes in GTP hydrolysis kinetics.

Structural requirements for the activation of the human growth hormone secretagogue receptor by peptide and nonpeptide secretagogues

Antibodies raised against an intracellular and extracellular domain of the GH secretagogue receptor (GHS-R) confirmed that its topological orientation in the lipid bilayer is as predicted for G protein-coupled receptors with seven transmembrane domains. A strategy for mapping the agonist-binding site of the human GHS-R was conceived based on our understanding of ligand binding in biogenic amine and peptide hormone G protein-coupled receptors. Using site-directed mutagenesis and molecular modeling, we classified GHS peptide and nonpeptide agonist binding in the context of its receptor environment. All peptide and nonpeptide ligand classes shared a common binding domain in transmembrane (TM) region 3 of the GHS-R. This finding was based on TM-3 mutation E124Q, which eliminated the counter-ion to the shared basic N+ group of all GHSs and resulted in a nonfunctional receptor. Restoration of function for the E124Q mutant was achieved by a complementary change in the MK-0677 ligand through modification of its amine side-chain to the corresponding alcohol. Contacts in other TM domains [TM-2 (D99N), TM-5 (M213K, S117A), TM-6 (H280F), and extracellular loop 1 (C116A)] of the receptor revealed specificity for the different peptide, benzolactam, and spiroindolane GHSs. GHS-R agonism, therefore, does not require identical disposition of all agonist classes at the ligand-binding site. Our results support the hypothesis that the ligand-binding pocket in the GHS-R is spatially disposed similarly to the well characterized catechol-binding site in the beta2-adrenergic receptor.

A G protein-coupled receptor phosphatase required for rhodopsin function

Heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors are phosphorylated by kinases that mediate agonist-dependent receptor deactivation. Although many receptor kinases have been isolated, the corresponding phosphatases, necessary for restoring the ground state of the receptor, have not been identified. Drosophila RDGC (retinal degeneration C) is a phosphatase required for rhodopsin dephosphorylation in vivo. Loss of RDGC caused severe defects in the termination of the light response as well as extensive light-dependent retinal degeneration. These phenotypes resulted from the hyperphosphorylation of rhodopsin because expression of a truncated rhodopsin lacking the phosphorylation sites restored normal photoreceptor function. These results suggest the existence of a family of receptor phosphatases involved in the regulation of G protein-coupled signaling cascades.

Agonist and inverse agonist efficacy at human recombinant serotonin 5-HT1A receptors as a function of receptor:G-protein stoichiometry

Membrane preparations were made from Chinese Hamster Ovary (CHO) cells expressing 1.6 and 4.2 pmol/mg of recombinant human 5-HT1A receptors, as determined by saturation binding with the selective antagonist, [3H]-S 15535 ([3H]-4-(benzodioxan-5-yl)]-(indan-2-yl)piperazine). There was no change in the number of G-proteins activated by the full agonist, serotonin (5-HT; approximately 1.1 pmol/mg in each preparation, measured by [35S]-GTP gamma S saturation binding), therefore increasing the receptor:G-protein ratio from approximately 1.4:1 (RGlow) to approximately 4:1 (RGhigh). Agonist efficacy was measured by stimulation of [35S]-GTP gamma S binding. The serotonergic agonist, eltoprazine, behaved as a partial agonist (Emax = 52.7%) at RGlow membranes but virtually as a full agonist (Emax = 93.2%) at RGhigh membranes, relative to 5-HT (= 100%). The latter exhibited a two-fold shift to the left in its concentration-response curve in RGhigh compared to RGlow membranes (P < 0.01). WAY 100,635 (N-¿2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl¿-N-(2-pyridinyl) -cyclo-hexane-carboxamide), did not alter [35S]-GTP gamma S binding from basal levels in either membrane preparation. In contrast, spiperone displayed inverse agonist activity, decreasing [35S]-GTP gamma S binding from basal levels by 17% in RGlow membranes but by 28% in RGhigh membranes. These data indicate that an increased receptor:G-protein ratio (i) augments the potency of full agonists, (ii) increases the efficacy of partial agonists and (iii) increases the negative efficacy of inverse agonists at recombinant human 5-HT1A receptors. Furthermore, these data suggest that spiperone induces, or stabilises, a G-protein-coupled, but inactive conformation of the receptor.

Substitution of charged amino acid residues in transmembrane regions 6 and 7 affect ligand binding and signal transduction of the prostaglandin EP3 receptor

Expression of the rabbit EP3 receptor isoform 77A in COS1 and HEK293tsA201 cells demonstrated specific binding of [3H]prostaglandin (PG)E2 and receptor-evoked decreases in intracellular cAMP levels. Competition binding with PGE2, PGE2 methyl ester, misoprostol-free acid, misoprostol, and sulprostone suggested that a negative charge at the C1 position is essential for high affinity ligand binding and that the charge at this position is more important than steric bulk. Charged amino acid residues within the transmembrane (TM) domains of the receptor were mutated, and the resulting receptor proteins were analyzed for the effects of these mutations on receptor structure and/or function. Positively charged TM residues are candidates for interaction with the C1 carboxylic acid moiety of prostanoid ligands. Substitution of R329 (TM VII) with either alanine or glutamate resulted in a loss of both detectable [3H]PGE2 binding and receptor activation despite expression of the receptor protein as determined by immunoprecipitation and immunofluorescence. Substitution of K300 (TM V) with alanine had no effect on binding or signal transduction. Substitution of the conserved aspartic acid at position 338 (TM VII) with alanine caused a loss of detectable receptor-evoked inhibition of cAMP generation, although this mutation did not appreciably affect ligand binding. These studies suggest that R329 but not K300 is a key determinant in receptor/ligand interaction. Furthermore, D338 plays a critical role in G1 activation by the EP3 receptor.