A transmembrane domain-derived peptide inhibits D1 dopamine receptor function without affecting receptor oligomerization

In this study, we show that a peptide based on the sequence of transmembrane domain 6 of the D1 dopamine receptor (D1DR) specifically inhibited D1DR binding and function, without affecting receptor oligomerization. It has been shown that an analogous peptide from the beta2-adrenergic receptor disrupted dimerization and adenylyl cyclase activation in the beta2-adrenergic receptor (Hebert, T. E., Moffett, S., Morello, J. P., Loisel, T. P., Bichet, D. G., Barret, C., and Bouvier, M. (1996) J. Biol. Chem. 271, 16384-16392). Treatment of D1DR with the D1DR transmembrane 6 peptide resulted in a dose-dependent, irreversible inhibition of D1DR antagonist binding, an effect not seen in D1DR with peptides based on transmembrane domains of other G protein-coupled receptors. Incubation with the D1DR transmembrane 6 peptide also resulted in a dose-dependent attenuation of both dopamine-induced [35S]guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) binding and receptor-mediated dopamine stimulation of adenylyl cyclase activity. Notably, GTPgammaS binding and cAMP production were reduced to levels below baseline, indicating blockade of ligand-independent, intrinsic receptor activity. Immunoblot analyses of the D1DR revealed the receptor existed as monomers, dimers, and higher order oligomers and that these oligomeric states were unaffected after incubation with the D1DR transmembrane 6 peptide. These findings represent the first demonstration that a peptide based on the transmembrane 6 of the D1DR may represent a novel category of noncompetitive D1DR antagonists.

A conserved tyrosine residue (Y601) in transmembrane domain 5 of the human thyrotropin receptor serves as a molecular switch to determine G-protein coupling

In the human thyroid, the wild-type thyrotropin receptor (TSHR) couples to adenylyl cyclase and phospholipase C and constitutively increases intracellular cAMP levels. The first human TSHR sequence submitted differs from subsequently cloned wild-type receptors by an exchange of a conserved Y residue within transmembrane domain 5 (TM5) for an H residue. We did not detect the Y601H mutant in 263 European individuals, but confirmed the homozygous occurrence of TSHR-Y601. Expression of TSHR-Y601H in COS-7 cells revealed a loss of constitutive cAMP production and selective lack of TSH-induced phosphoinositide hydrolysis, whereas agonist-induced cAMP formation remained unaltered. Analysis of several mutant receptors (Y601A, Y601D, Y601F, Y601K, Y601P, Y601S, Y601W, Y601Delta) did not show restoration of constitutive activity and dual signaling, thus suggesting a functional role of a properly spaced hydroxyl group at position 601. Molecular modeling revealed that the formation of a hydrogen bond between the hydroxyl group of Y601 in TM5 and the carbonyl oxygen of A623 in the peptide backbone of TM6 is critical for the receptor to adopt active conformations that impart wild-type signaling properties. Our findings indicate that multiple active receptor states underlie coupling of a G-protein-coupled receptor to different G-proteins.

Agonist-independent regulation of constitutively active G-protein-coupled receptors

G-protein-coupled receptors constitute one of the largest protein super-families in mammals. Since the cloning of the encoding genes, these important drug targets have been subjected to thorough biochemical and pharmacological studies. It has become clear that G-protein-coupled receptors not only transmit signals after stimulation by agonists but can also spontaneously couple to signal-transduction pathways. Recent findings show that constitutively active G-protein-coupled receptors can also be regulated in an agonist-independent manner, which has important implications for the interpretation of the actions of (inverse) agonists and the results of site-directed-mutagenesis studies.

Mutations within the cholecystokinin-B/gastrin receptor ligand 'pocket' interconvert the functions of nonpeptide agonists and antagonists

We have reported previously that the transmembrane domains of the cholecystokinin-B/gastrin receptor (CCK-BR) comprise a putative ligand binding pocket. In the present study, we examined whether amino acid substitutions within the CCK-BR pocket altered the affinities and/or functional activities of L-365,260 (the prototypical nonpeptide CCK-BR antagonist) and two structural derivatives, YM022 (a higher affinity antagonist) and L-740,093S (a partial agonist). Eight amino acids that project into the CCK-BR pocket were individually replaced by alanine, using site-directed mutagenesis. Affinities for the nonpeptide molecules, as well as ligand-induced inositol phosphate production, were assessed with the wild-type and mutant receptors. For each of the nonpeptide ligands examined, a distinct series of mutations altered the affinity, suggesting that each ligand possessed a characteristic pattern of interactions within the CCK-BR pocket. Basal signaling levels and inositol phosphate formation induced by the full agonist CCK octapeptide were comparable for the wild-type receptor and all of the mutant CCK-BR forms. In contrast to the peptide agonist CCK octapeptide, the functional activities of the nonpeptide molecules were selectively altered by single point mutations within the CCK-BR pocket, resulting in interconversion of agonists and antagonists. These findings suggest that interactions between nonpeptide molecules and transmembrane domain amino acids of the CCK-BR can determine the functional activity and affinity of the ligands.

Extreme C terminus of G protein alpha-subunits contains a site that discriminates between Gi-coupled metabotropic glutamate receptors

Metabotropic glutamate receptors (mGlu receptors), the Ca2+-sensing receptor, gamma-aminobutyric acid type B receptors, and one group of pheromone receptors constitute a unique family (also called family 3) of heptahelical receptors. This original family shares no sequence similarity with any other G protein-coupled receptors. The identification and comparison of the molecular determinants of receptor/G protein coupling within the different receptor families may help identify general rules involved in this protein/protein interaction. In order to detect possible contact sites important for coupling selectivity between family 3 receptors and the G protein alpha-subunits, we examined the coupling of the cyclase-inhibiting mGlu2 and mGlu4 receptors to chimeric alphaq-subunits bearing the 5 extreme C-terminal amino acid residues of either Galphai, Galphao, or Galphaz. Whereas mGlu4 receptor activated all three chimeric G proteins, mGlu2 receptor activated Galphaqi and Galphaqo but not Galphaqz. The mutation of isoleucine -4 of Galphaqz into cysteine was sufficient to recover coupling of the mutant G protein to mGlu2 receptor. Moreover, the mutation of cysteine -4 of Galphaqo into isoleucine was sufficient to suppress the coupling to mGlu2 receptor. Mutations at positions -5 and -1 had an effect on coupling efficiency, but not selectivity. Our results emphasize the importance of the residue -4 of the alpha-subunits in their specific interaction to heptahelical receptors by extending this finding on the third family of G protein-coupled receptors.

Binding of transducin and transducin-derived peptides to rhodopsin studies by attenuated total reflection-Fourier transform infrared difference spectroscopy

Fourier transform infrared difference spectroscopy combined with the attenuated total reflection technique allows the monitoring of the association of transducin with bovine photoreceptor membranes in the dark. Illumination causes infrared absorption changes linked to formation of the light-activated rhodopsin-transducin complex. In addition to the spectral changes normally associated with meta II formation, prominent absorption increases occur at 1735 cm-1, 1640 cm-1, 1550 cm-1, and 1517 cm-1. The D2O sensitivity of the broad carbonyl stretching band around 1735 cm-1 indicates that a carboxylic acid group becomes protonated upon formation of the activated complex. Reconstitution of rhodopsin into phosphatidylcholine vesicles has little influence on the spectral properties of the rhodopsin-transducin complex, whereas pH affects the intensity of the carbonyl stretching band. AC-terminal peptide comprising amino acids 340-350 of the transducin alpha-subunit reproduces the frequencies and isotope sensitivities of several of the transducin-induced bands between 1500 and 1800 cm-1, whereas an N-terminal peptide (aa 8-23) does not. Therefore, the transducin-induced absorption changes can be ascribed mainly to an interaction between the transducin-alpha C-terminus and rhodopsin. The 1735 cm-1 vibration is also seen in the complex with C-terminal peptides devoid of free carboxylic acid groups, indicating that the corresponding carbonyl group is located on rhodopsin.

Mutagenesis of human Mel1a melatonin receptor expressed in yeast reveals domains important for receptor function

A yeast functional colorimetric assay was employed to test the effects of site-directed point mutations on the function of the human Mel1a melatonin receptor. Seven mutants were created in transmembrane domains III, V, and VII of the receptor to test the rhodopsin-based model of melatonin recognition. Two mutants in transmembrane domains III and VI were created to investigate the mechanisms of G protein activation in the melatonin receptor. Mutations in transmembrane domain V either potentiated agonist efficiencies (H195A) or totally abolished all responses to tested compounds (V192T+H195A). Mutation N124A in the conserved NRY motif in the end of transmembrane domain III seriously impaired receptor activation. Several mutants were found to have decreased ability to activate functional responses, reflecting the importance of these residues for receptor function. These data also suggest that activation of the receptor involves interaction of the 5-methoxy group of melatonin with the conserved histidine H195 in transmembrane domain V.

C. The Chemokine Monocyte Chemotactic Protein 1 Triggers Janus Kinase 2 Activation and Tyrosine Phosphorylation of the CCR2B Receptor

The chemokines are a growing family of low m.w., 70- to 80-residue proinflammatory cytokines that operate by interacting with G protein-coupled receptors. Chemokines are involved in cell migration and in the activation of specific leukocyte subsets. Using the Mono Mac 1 monocytic cell line, we show that monocyte chemotactic protein 1 (MCP-1) triggers activation of the Janus kinase 2 (JAK2)/STAT3 pathway and CCR2 receptor tyrosine phosphorylation. Both Ca2+ mobilization and cell migration are blocked in Mono Mac 1 cells by tyrphostin B42, a specific JAK2 kinase inhibitor. Within seconds of MCP-1 activation, JAK2 phosphorylates CCR2 at the Tyr139 position and promotes JAK2/STAT3 complex association to the receptor. This MCP-1-initiated phosphorylation and association to JAK2 is also observed in CCR2B-transfected HEK293 cells. In contrast, when a CCR2B Tyr139Phe mutant is expressed in HEK293 cells, it is not phosphorylated in tyrosine and triggers neither JAK2/STAT3 activation nor Ca2+ mobilization in response to MCP-1. These results implicate the tyrosine kinase pathway in early chemokine signaling, suggesting a key role for this kinase in later events.

[Use of a G-protein-coupled receptor to communicate. An evolutionary success]

Among membrane-bound receptors, the seven transmembrane receptors are the most abundant (several thousand, 1% of the genome). They were the most successful during evolution. They are capable of transducing messages as different as photons, organic odorants, nucleotides, nucleosides, peptides, lipids, proteins, etc. They are catalysts of the GDP/GTP nucleotide exchange on heterotrimeric G proteins. They are therefore also called 'G-protein-coupled receptors' (GPCR). G proteins are composed of three subunits, G alpha and two undissociable subunits, G beta gamma. There are at least three families of GPCR showing no sequence similarity. Among G proteins, some have been crystallized (including under the heterotrimeric form) and their structure as well as their activation mechanisms are well known. The structures of GPCR are less known owing to the difficulty in crystallizing membrane-bound proteins. Indirect studies (mutations, 2D crystallization of rhodopsine, molecular modelling, etc.) lead to a useful model of the 'central core' composed of the seven transmembrane domains and of its structural modifications during activation. The intimate contact zones between GPCR and G proteins include, on the GPCR side, domains of intracellular loops and C-terminal, which are specific for each family and on the G protein side, essentially the N- et C-terminal domains plus the alpha 4-beta 6 loop. GPCR can adopt several 'active' conformations some of them being found in mutated receptors responsible for pathologies.

Functional microdomains in G-protein-coupled receptors. The conserved arginine-cage motif in the gonadotropin-releasing hormone receptor

An Arg present in the third transmembrane domain of all rhodopsin-like G-protein-coupled receptors is required for efficient signal transduction. Mutation of this Arg in the gonadotropin-releasing hormone receptor to Gln, His, or Lys abolished or severely impaired agonist-stimulated inositol phosphate generation, consistent with Arg having a role in receptor activation. To investigate the contribution of the surrounding structural domain in the actions of the conserved Arg, an integrated microdomain modeling and mutagenesis approach has been utilized. Two conserved residues that constrain the Arg side chain to a limited number of conformations have been identified. In the inactive wild-type receptor, the Arg side chain is proposed to form an ionic interaction with Asp3.49(138). Experimental results for the Asp3. 49(138) --> Asn mutant receptor show a modestly enhanced receptor efficiency, consistent with the hypothesis that weakening the Asp3. 49(138)-Arg3.50(139) interaction by protonation of the Asp or by the mutation to Asn favors activation. With activation, the Asp3. 49(138)-Arg3.50(139) ionic bond would break, and the unrestrained Arg would be prevented from orienting itself toward the water phase by a steric clash with Ile3.54(143). The mutation Ile3.54(143) --> Ala, which eliminates this clash in simulations, causes a marked reduction in measured receptor signaling efficiency, implying that solvation of Arg3.50(139) prevents it from functioning in the activation of the receptor. These data are consistent with residues Asp3.49(138) and Ile3.54(143) forming a structural motif, which helps position Arg in its appropriate inactive and active receptor conformations.

Rhodopsin arginine-135 mutants are phosphorylated by rhodopsin kinase and bind arrestin in the absence of 11-cis-retinal

Arginine-135, located at the border between the third transmembrane domain and the second cytoplasmic loop of rhodopsin, is one of the most highly conserved amino acids in the family of G protein-coupled receptors. The effect of mutation at Arg-135 on the ability of rhodopsin to undergo desensitization was investigated. Four mutants, R135K, R135Q, R135A, and R135L, were examined for their ability to be phosphorylated by rhodopsin kinase, to bind arrestin, and to activate the rod cell G protein, transducin (Gt). All of the mutants were phosphorylated, bound arrestin, and were able to activate Gt when reconstituted with 11-cis-retinal. Surprisingly, several of the mutants could be phosphorylated by rhodopsin kinase and could bind arrestin in the absence of 11-cis-retinal but were not able to activate Gt. These observations represent the first demonstration of a mutant G protein-coupled receptor that assumes a conformation able to interact with its G protein-coupled receptor kinase and arrestin, but not with its G protein, in the absence of ligand.

A ligand-mimetic model for constitutive activation of the melanocortin-1 receptor

Dark coat color in the mouse and fox results from constitutively activated melanocortin-1 receptors. Receptor mutations in the mouse (E92K, L98P), cow (L99P), fox (C125R), and sheep (D119N) cluster near the membrane/extracellular junctions of the second and third transmembrane domains, an acidic domain that is the likely site of electrostatic interaction with an arginine residue in the ligand, alpha-MSH. For transmembrane residues E92, D119, and C125, conversion to a basic residue is required for constitutive activation. Unlike constitutively activating mutations in many G protein-coupled receptors that increase agonist efficacy and affinity, these MC1-R mutations have the opposite effect. Therefore, these mutations do not activate the receptor by directly disrupting intramolecular constraints on formation of the active high-affinity state, R*, but do so indirectly by mimicking ligand binding.

Mechanisms governing the activation and trafficking of yeast G protein-coupled receptors

We have addressed the mechanisms governing the activation and trafficking of G protein-coupled receptors (GPCRs) by analyzing constitutively active mating pheromone receptors (Ste2p and Ste3p) of the yeast Saccharomyces cerevisiae. Substitution of the highly conserved proline residue in transmembrane segment VI of these receptors causes constitutive signaling. This proline residue may facilitate folding of GPCRs into native, inactive conformations, and/or mediate agonist-induced structural changes leading to G protein activation. Constitutive signaling by mutant receptors is suppressed upon coexpression with wild-type, but not G protein coupling-defective, receptors. Wild-type receptors may therefore sequester a limiting pool of G proteins; this apparent "precoupling" of receptors and G proteins could facilitate signal production at sites where cell surface projections form during mating partner discrimination. Finally, rather than being expressed mainly at the cell surface, constitutively active pheromone receptors accumulate in post-endoplasmic reticulum compartments. This is in contrast to other defective membrane proteins, which apparently are targeted by default to the vacuole. We suggest that the quality-control mechanism that retains receptors in post-endoplasmic reticulum compartments may normally allow wild-type receptors to fold into their native, fully inactive conformations before reaching the cell surface. This may ensure that receptors do not trigger a response in the absence of agonist.

Molecular mechanisms involved in the activation and regulation of the alpha 1-adrenergic receptor subtypes

The adrenergic receptors (ARs) belong to the superfamily of membrane-bound G protein coupled receptors (GPCRs). Our investigation has focused on the structure-function relationship of the alpha 1b-AR subtype used as the model system for other GPCRs. Site-directed mutagenesis studies have elucidated the structural domains of the alpha 1b-AR involved in ligand binding, G protein coupling or desensitization. In addition, a combined approach using site-directed mutagenesis and molecular dynamics analysis of the alpha 1b-AR has provided information about the potential mechanisms underlying the activation process of the receptor, i.e. its transition from the 'inactive' to the 'active' conformation.

Molecular genetics of mating recognition in basidiomycete fungi

The recognition of compatible mating partners in the basidiomycete fungi requires the coordinated activities of two gene complexes defined as the mating-type genes. One complex encodes members of the homeobox family of transcription factors, which heterodimerize on mating to generate an active transcription regulator. The other complex encodes peptide pheromones and 7-transmembrane receptors that permit intercellular signalling. Remarkably, a single species may have many thousands of cross-compatible mating types because the mating-type genes are multiallelic. Different alleles of both sets of genes are necessary for mating compatibility, and they trigger the initial stages of sexual development--the formation of a specialized filamentous mycelium termed the dikaryon, in which the haploid nuclei remain closely associated in each cell but do not fuse. Three species have been taken as models to describe the molecular structure and organization of the mating-type loci and the genes sequestered within them: the pathogenic smut fungus Ustilago maydis and the mushrooms Coprinus cinereus and Schizophyllum commune. Topics addressed in this review are the roles of the mating-type gene products in regulating sexual development, the molecular basis for multiple mating types, and the molecular interactions that permit different allelic products of the mating type genes to be discriminated. Attention is drawn to the remarkable conservation in the mechanisms that regulate sexual development in basidiomycetes and unicellular ascomycete yeasts, Saccharomyces cerevisiae and Schizosaccharomyces pombe, a theme which is developed in the general conclusion to include the filamentous ascomycetes Neurospora crassa and Podospora anserina.

Chloroethylclonidine binds irreversibly to exposed cysteines in the fifth membrane-spanning domain of the human alpha2A-adrenergic receptor

The alpha2-adrenergic receptors (alpha2-ARs) mediate signals to intracellular second messengers via guanine nucleotide binding proteins. Three human genes encoding alpha2-AR subtypes (alpha2A, alpha2B, alpha2C) have been cloned. Several chemical compounds display subtype differences in their binding and/or functional activity. Site-directed mutagenesis and molecular modeling are new tools with which to investigate the subtype selectivity of ligands. In this study, we introduce a new approach to mapping of the binding site crevice of the human alpha2A-AR. Based on a three-dimensional receptor model, we systematically mutated residues 197-201 and 204 in the fifth transmembrane domain of the human alpha2A-AR to cysteine. Chloroethylclonidine, an alkylating derivative of the alpha2-adrenergic agonist clonidine, binds irreversibly to alpha2A-ARs by forming a covalent bond with the sulfhydryl side chain of a cysteine residue exposed in the binding cavity, leading to inactivation of the receptor. Irreversible binding of chloroethylclonidine was used as a criterion for identifying introduced cysteine residues as being exposed in the binding cavity. The results supported a receptor model in which the fifth transmembrane domain is alpha-helical, with residues Val197, Ser200, Cys201, and Ser204 exposed in the binding pocket. Residues Ile198, Ser199, Ile202, and Gly203 face the lipid bilayer of the plasma membrane. This approach emerges as a powerful tool for structural characterization of the alpha2-ARs.

Ab initio modeling and molecular dynamics simulation of the alpha 1b-adrenergic receptor activation

This work describes the ab initio procedure employed to build an activation model for the alpha 1b-adrenergic receptor (alpha 1b-AR). The first version of the model was progressively modified and complicated by means of a many-step iterative procedure characterized by the employment of experimental validations of the model in each upgrading step. A combined simulated (molecular dynamics) and experimental mutagenesis approach was used to determine the structural and dynamic features characterizing the inactive and active states of alpha 1b-AR. The latest version of the model has been successfully challenged with respect to its ability to interpret and predict the functional properties of a large number of mutants. The iterative approach employed to describe alpha 1b-AR activation in terms of molecular structure and dynamics allows further complications of the model to allow prediction and interpretation of an ever-increasing number of experimental data.

Constitutive activity of glucagon receptor mutants

Increased constitutive activity has been observed in the PTH receptor in association with naturally occurring mutations of two residues that are conserved between members of the glucagon/vasoactive intestinal peptide/calcitonin 7TM receptor family. Here, the corresponding residues of the glucagon receptor, His178 and Thr352, were probed by mutagenesis. An elevated level of basal cAMP production was observed after the exchange of His178 into Arg, but not for the exchange into Lys, Ala, or Glu. However, for all of these His178 substitutions, an increased binding affinity for glucagon was observed [dissociation constant (Kd) ranging from 1.1-6.4 nM, wild type: Kd = 12.0 nM]. A further increase in cAMP production was observed for the [H178R] construct upon stimulation with glucagon, albeit the EC50 surprisingly was increased approximately 10-fold relative to the wild-type receptor. Substitution of Thr352, located at the intracellular end of transmembrane segment VI, with Ala led to a slightly elevated basal cAMP level, while the introduction of Pro or Ser at this position affected rather the binding affinity of glucagon or the EC50 for stimulation of cAMP production. The large extracellular segment, which is essential for glucagon binding, was not required for constitutive activation of the glucagon receptor as the introduction of the [H178R] mutation into an N-terminally truncated construct exhibited an elevated basal level of cAMP production. The analog des-His1-[Glu9]glucagon amide, which in vivo is a glucagon antagonist, was an agonist on both the wild-type and the [H178R] receptor and did not display any activity as an inverse agonist. It is concluded that the various phenotypes displayed by the constitutively active glucagon receptor mutants reflect the existence of multiple agonist-preferring receptor conformers, which include functionally active as well as inactive states. This view agrees with a recent multi-state extension of the ternary complex model for 7TM receptor activation.

Efficacy of inverse agonists in cells overexpressing a constitutively active beta2-adrenoceptor and type II adenylyl cyclase

1 Maximal stimulant output from the adenylyl cyclase cascade in neuroblastoma x glioma hybrid, NG108-15, cells is limited by the levels of expression of isoforms of adenylyl cyclase. Stable expression in these cells of a constitutively active mutant (CAM) version of the human beta2-adrenoceptor resulted in higher basal adenylyl cyclase activity than following expression of the human wild type beta2-adrenoceptor. Isoprenaline acted as a full agonist in membranes from both wild type and CAM beta2-adrenoceptor expressing clones. 2 Expression of type II adenylyl cyclase resulted in a substantially elevated capacity of isoprenaline to stimulate [3H]-forskolin binding, whereas in CAM beta2-adrenoceptor expressing cells the basal high affinity [3H]-forskolin binding represented a markedly greater % of the maximal effect which could be produced by addition of isoprenaline, and the EC50 for isoprenaline was some 10 fold lower than in cells expressing the wild type beta2-adrenoceptor. 3 Further transfection of the CAM beta2-adrenoceptor expressing cells with type II adenylyl cyclase greatly increased both absolute basal and agonist-stimulated levels of adenylyl cyclase activity. 4 Betaxolol, ICI 118,551, sotalol and timolol acted as inverse agonists with varying degrees of efficacy, whereas propranolol functioned as a neutral antagonist and alprenolol as a partial agonist. 5 Pretreatment of the CAM beta2-adrenoceptor and type II adenylyl cyclase expressing clones with the irreversible alkylating agent BAAM (1 microM) did not reduce the efficacy of isoprenaline but eliminated efficacy from all the inverse agonist ligands. This effect was dependent upon the concentration of BAAM employed, with half-maximal effects being produced between 10 nM and 100 nM of the alkylating agent, which is similar to the concentrations required to prevent subsequent ligand access to some 50% of the CAM beta2-adrenoceptor population. 6 These data demonstrate that inverse agonist efficacy can be modulated by receptor availability and also indicate why in physiological systems, inverse agonism can be difficult to detect.

Review: amino acid domains involved in constitutive activation of G-protein-coupled receptors

Guanine nucleotide-binding protein-coupled receptors may attain an active conformation in the absence of agonist by spontaneous isomerization and thus yield constitutive, agonist-independent, activity. This has mainly been demonstrated for isolated membranes and recombinant wild-type receptors, and mutant receptors. They generally show remarkable increases in the sensitivity of a biological response. The location of activating mutations both within a single receptor and across receptors is widespread, with changes reported in the seven-transmembrane domains, the second and third intracellular loop. For most of these receptors, examples of ligands defined as inverse agonists have been documented. Regulation of these receptors by inverse agonists opposite to that observed by agonists, and the therapeutic potential of inverse agonists is underlined.

Inverse agonism and the regulation of receptor number

Inverse agonists are ligands that preferentially stabilize inactive conformations of G protein-coupled receptors. In a range of systems, sustained treatment with inverse agonists can produce substantially greater upregulation of receptor levels than antagonists. The use of constitutively active mutant receptors can exaggerate this effect but may also allow agonists and antagonists to mimic the effect by preventing denaturation of the mutant receptor polypeptide. In this review Graeme Milligan and Richard Bond consider the basis for these effects and their therapeutic implications.

The C-terminal third intracellular loop of the rat AT1A angiotensin receptor plays a key role in G protein coupling specificity and transduction of the mitogenic signal

To identify the role(s) of the third intracellular loop of the angiotensin II (AngII) type 1A (AT1A) receptor in G protein coupling specificity and receptor activation, several chimerae were constructed and characterized. The cDNA sequence encoding the C-terminal segment of the third intracellular loop of the AT1A receptor (residues 234-240) was replaced with the homologous regions of the alpha1B adrenergic (alpha1B-AR), the beta2 adrenergic (beta2-AR), and the AngII type 2 (AT2) receptors. These chimeric receptors were stably expressed in Chinese hamster ovary cells, and their pharmacological and functional properties were characterized, including AngII-induced inositol phosphate and cyclic AMP (cAMP) productions, [3H]thymidine incorporation into DNA, and internalization. The affinities of these chimeric receptors for [Sar1]AngII, [Sar1,Ile8]AngII, and losartan were essentially normal; however, the affinity of these mutants was increased by a factor of 10-40 for the AT2-specific ligand CGP42112A. The functional properties of the alpha1B-AR chimera were essentially identical to those of the wild type AT1A receptor. On the other hand, replacement with the beta2-AR segment produced a partial reduction of the inositol phosphate production, a measurable AngII-induced cAMP accumulation, a reduced internalization, and a total impairment to transduce the mitogenic effect of AngII. The AT2 chimera presented a normal internalization, but was inactive in all the other functional tests. In conclusion, the distal segment of the third intracellular loop of the rat AT1A receptor plays a pivotal role in coupling selectivity and receptor signaling via G protein(s) as well as in the activation of the specific signaling pathways involved in the mitogenic actions of AngII.

Molecular simulation of dynorphin A-(1-10) binding to extracellular loop 2 of the kappa-opioid receptor. A model for receptor activation

The structure of the second extracellular loop region (EL2) of the kappa-opioid receptor has been explored in an effort to understand the structural basis for dynorphin A binding and selectivity. Application of secondary structure prediction methods and homology modeling resulted in a turn-helix motif for the N-terminal region of kappa-EL2. A similar motif was not predicted for EL2 of either delta or mu opioid receptors. The EL2 helix was further shown to be amphiphilic and complementary to the helical component of dynorphin A. Using a model of the kappa-receptor (Metzger et al. Neurochem. Res. 1996, 21, 1287-1294), including the newly predicted EL2 turn-helix domain, a binding mode is proposed based on helix--helix interactions between hydrophobic residues of EL2 and the helical component of dynorphin A-(1-10). Molecular simulations of the receptor--ligand complex yielded structures in which the tyramine moiety or opioid "message" of dynorphin is bound within a conserved aromatic pocket in the transmembrane domain while the helical portion contacted residues in EL2 and in the extracellular end of transmembrane helices 6 and 7. The model is in general agreement with site-directed mutagenesis data and chimera studies that have identified binding domains in both the EL2 and transmembrane regions of dynorphin A. The results confirm the importance of the opioid "message" displayed by many opioid ligands but also suggest a potential mechanism of receptor activation that may be mediated by EL2 through interactions with the "address" component of dynorphin A.

The effect of the molecular mechanism of G protein-coupled receptor activation on the process of signal transduction

A thermodynamic model of signal transduction that incorporates the possibility of multiple conformational states between the inactive and the active forms of the receptor was developed. The obtained equilibrium model is equivalent to the extended ternary complex of Samama et al. (J. Biol. Chem. 268 (1993) 4625-4636) if only two states of the receptor exist. These multiple equilibria between receptor states are modeled by two sets of equilibrium constants: K(piAR) and K(sigma piAR), in the presence of the ligand; and K(piR) and K(sigma piR), in the absence of the ligand. The higher the value of these constants, the more efficiently the active form of the receptor is generated. Intrinsic efficacy of the agonist is defined in the present formulation as the molecular processes induced by ligands in the receptor that lead to the active form of the receptor. Both the energetics (associated to K[piAR]) and mechanism of the process of receptor activation (associated to K[sigma piAR]) are important in eliciting the maximum response. Moreover, analytical expressions of basal activity, potency and maximum response were obtained. These definitions were used to classify the extra cellular ligand as agonists (K[sigma piAR] > K[sigma piR]), inverse agonists (K[sigma piR] > K[sigma piAR] > 0), neutral antagonists (K[sigma piAR] = K[sigma piR]), and pure antagonists.

Selective inhibition of adenylyl cyclase by octopamine via a human cloned alpha 2A-adrenoceptor

1. In this study we have compared the abilities of the enantiomers of the structural isomers of the phenolamines, octopamine and synephrine, and the catecholamines, noradrenaline and adrenaline, to couple selectively a human cloned alpha 2A-adrenoceptor, stably expressed in a Chinese hamster ovary (CHO) cell line, to G-protein linked second messenger pathways mediating an increase and a decrease in cyclic AMP production. 2. The catecholamines couple the alpha 2A-adrenoceptor to both an increase and a decrease in the rate of cyclic AMP production. In the absence of pertussis toxin pretreatment both catecholamines tested showed a dose-dependent decrease with a maximum at 100 nM. After pertussis toxin pretreatment they both produced a dose-dependent increase in cyclic AMP production with a maximum at 10 microM. 3. The phenolamines, octopamine and synephrine were only able to couple the alpha 2A-adrenoceptor to a dose-dependent decrease in cyclic AMP production at concentrations up to 1 mM, with the synephrine isomers being more potent than the corresponding octopamine isomers. The meta-isomers of both phenolamines were more potent than the corresponding para-isomers and the (-)-enantiomers were more potent than the (+)-enantiomers. Thus, (-)-meta-synephrine [(-)-phenylephrine] was the most effective isomer tested with an observable decrease occurring between 100 nM and 1 microM. 4. The effects of octopamine and the catecholamines on the decrease in cyclic AMP production were additive at submaximal concentrations, whilst octopamine reduced the stimulant effect of submaximal concentrations of noradrenaline on cyclic AMP production after pertussis toxin pretreatment. 5. The time courses of the inhibitory effects of both meta-octopamine and noradrenaline were parallel and peaked after a 1 min exposure to the agonist. In contrast, the stimulant effects of noradrenaline after pertussis toxin pretreatment were of a much slower time course with a maximum effect occurring after a 5 min incubation period. 6. Since octopamine and synephrine occur naturally in, and are co-released with catecholamines from, mammalian tissues, the results of the present study suggest that the human cloned alpha 2A-adrenoceptor can be coupled selectively by different endogenous agonists to G-protein pathways mediating the regulation of adenylyl cyclase activity.

Interactions between conserved residues in transmembrane helices 1, 2, and 7 of the thyrotropin-releasing hormone receptor

The roles of conserved residues in transmembrane helices (TMs) of G protein-coupled receptors have not been well established. A computer-generated model of the thyrotropin-releasing hormone receptor (TRH-R) indicated that conserved Asp-71 (TM-2) could interact with conserved asparagines 316 (TM-7) and 43 (TM-1). To test this model, we constructed mutant TRH-Rs containing polar or alanine substitutions of these residues. The maximal activities of N43A and N316A TRH-Rs were diminished, whereas D71A (Perlman, J. H., Nussenzveig, D. R., Osman, R., and Gershengorn, M. C. (1992) J. Biol. Chem. 267, 24413-24417) and N43A/N316A TRH-Rs were inactive. Computer models of D71A and N43A/N316A TRH-Rs show similar changes from native TRH-R in their TM bundle conformations. The inactivity and the similarity of the computer models of D71A and N43A/N316A TRH-Rs are consistent with the idea that Asp-71 bridges Asn-43 and Asn-316 and suggest that activity is critically dependent on these interactions. The conservation of these residues suggests these specific interactions involving TMs 1, 2, and 7 may be structurally important for all members of the rhodopsin/beta-adrenergic receptor subfamily of G protein-coupled receptors.

A mutation changes ligand selectivity and transmembrane signaling preference of the neurokinin-1 receptor

We studied the biochemical properties of a genetically engineered neurokinin-1 receptor (NK1R) in which two residues lying on the extracellular edge of the fourth transmembrane domain were replaced by equivalently located elements of the neurokinin-2 receptor (G166C, Y167F NK1R mutant). The mutation produced two effects. The first is enhancement of the apparent binding affinity for heterologous tachykinins (substance K and neurokinin B) and for N- or C-terminal modified analogues of substance P, but not for substance P itself, its full-length analogues, and several peptide and nonpeptide antagonists. Only two antagonists, as exceptions, were found to exhibit a diminished affinity for the mutant receptor. The second effect is a shift in NK1R preference for distinct G protein-mediated signaling pathways. NK1R-mediated phosphoinositide hydrolysis was enhanced both in transiently and permanently transfected cells, while stimulation of cAMP accumulation did not change in transient expression experiments and was reduced in permanently expressing cells. The effect of the mutation on ligand affinity was not related to any obvious structural commonality, nor to the selectivity for different neurokinin receptors or the agonistic/antagonistic nature of the ligand. However, all ligands responding to the mutation appear to share the ability to induce phosphoinositide signaling more efficiently than cAMP responses when binding to NK1R. We suggest that the mutation shifts the internal equilibria of different functional forms of NK1R. A theoretical analysis according to a multistate allosteric model suggests that the link between binding and biological changes can result from altered stability constants of substates in the conformational space of the receptor.

Mutations in the second cytoplasmic loop of the rat parathyroid hormone (PTH)/PTH-related protein receptor result in selective loss of PTH-stimulated phospholipase C activity

To define the structural requirements of the parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor necessary for activation of phospholipase C (PLC), receptors with random mutations in their second cytoplasmic loop were synthesized, and their properties were assessed. A mutant in which the wild type (WT) rat PTH/PTHrP receptor sequence EKKY (amino acids 317-320) was replaced with DSEL had little or no PTH-stimulated PLC activity when expressed transiently in COS-7 cells, but it retained full capacity to bind ligand and to generate cAMP. This phenotype was confirmed in LLC-PK1 cells stably expressing the DSEL mutant receptor, where both PTH-stimulated PLC activity and sodium-dependent phosphate co-transport were essentially abolished. Individual mutations of these four residues point to a critical role for Lys-319 in receptor-G protein coupling. PTH-generated IPs were reduced to 27 +/- 13% when K319E, compared with the WT receptor, and PLC activation was fully recovered in a receptor revertant in which Glu-319 in the DSEL mutant cassette was restored to the WT residue, Lys. Moreover, the WT receptor and a mutant receptor in which K319R had indistinguishable properties, thus suggesting that a basic amino acid at this position may be important for PLC activation. All of these receptors had unimpaired capacity to bind ligand and to generate cAMP. To ensure adequacy of Galphaq-subunits for transducing the receptor signal, Galphaq was expressed in HEK293 and in LLC-PK1 cells together with either WT receptors or receptors with the DSEL mutant cassette. PTH generated no inositol phosphates (IPs) in either HEK293 or LLC-PK1 cells, when they expressed DSEL mutant receptors together with Galphaq. In contrast, PTH generated 2- and 2. 5-fold increases in IPs, respectively, when these cells co-expressed both the WT receptor and Galphaq. Thus, generation of IPs by the activated PTH/PTHrP receptor can be selectively abolished without affecting its capacity to generate cAMP, and Lys-319 in the second intracellular loop is critical for activating the PLC pathway. Moreover, alpha-subunits of the Gq family, rather than betagamma-subunits, transduce the signal from the activated receptor to PLC, and the PLC, rather than the adenylyl cyclase, pathway mediates sodium-dependent phosphate co-transport in LLC-PK1 cells.

The activation process of the alpha1B-adrenergic receptor: potential role of protonation and hydrophobicity of a highly conserved aspartate

In this study, a quantitative approach was used to investigate the role of D142, which belongs to the highly conserved E/DRY sequence, in the activation process of the alpha1B-adrenergic receptor (alpha1B-AR). Experimental and computer-simulated mutagenesis were performed by substituting all possible natural amino acids at the D142 site. The resulting congeneric set of proteins together with the finding that all the receptor mutants show various levels of constitutive (agonist-independent) activity enabled us to quantitatively analyze the relationships between structural/dynamic features and the extent of constitutive activity. Our results suggest that the hydrophobic/hydrophilic character of D142, which could be regulated by protonation/deprotonation of this residue, is an important modulator of the transition between the inactive (R) and active (R*) state of the alpha1B-AR. Our study represents an example of quantitative structure-activity relationship analysis of the activation process of a G protein-coupled receptor.

Dual agonistic and antagonistic property of nonpeptide angiotensin AT1 ligands: susceptibility to receptor mutations

Two nonpeptide ligands that differ chemically by only a single methyl group but have agonistic (L-162,782) and antagonistic (L-162,389) properties in vivo were characterized on the cloned angiotensin AT1 receptor. Both compounds bound with high affinity (K(I) = 8 and 28 nM, respectively) to the AT1 receptor expressed transiently in COS-7 cells as determined in radioligand competition assays. L-162,782 acted as a powerful partial agonist, stimulating phosphatidylinositol turnover with a bell-shaped dose-response curve to 64% of the maximal level reached in response to angiotensin II. Surprisingly, L-162,389 also stimulated phosphatidylinositol turnover, albeit only to a small percentage of the angiotensin response. The prototype nonpeptide AT1 agonist L-162,313 gave a response of approximately 50%. The apparent EC50 values for all three compounds in stimulating phosphatidylinositol turnover were similar, approximately 30 nM, corresponding to their binding affinity. Each of the three compounds also acted as angiotensin antagonists, yet in this capacity the compounds differed markedly, with IC50 values ranging from 1.05 x 10(-7) M for L-162,389 to 6.5 x 10(-6) for L-162,782. A series of point mutations in the transmembrane segments (TMs) of the AT1 receptor had only minor effect on the binding affinity of the nonpeptide compounds, with the exception of A104V at the top of TM III, which selectively impaired the binding of L-162,782 and L-162,389. Substitutions in the middle of TM III, VI, or VII, which did not affect the binding affinity of the compounds, impaired or eliminated the agonistic efficacy of the nonpeptides but with only minor or no effect on the angiotensin potency or efficacy. Thus, in the N295D rat AT1 construct, L-162,782, L-162,313, and L-162,389 all antagonized the angiotensin-induced phosphatidylinositol turnover with surprisingly similar IC50 values (90-180 nM), and they all bound with unaltered, high affinity (22-36 nM). However, L-162,313 and L-162,782 could stimulate phosphatidylinositol turnover to only 20% of that of angiotensin. It is concluded that minor chemical modifications of either the compound or the receptor can dramatically alter the agonistic efficacy of biphenyl imidazole compounds on the AT1 receptor without affecting their affinity, as determined in binding assays, and that a number of substitutions in the middle of the TM segments affect the efficacy of nonpeptide agonists as opposed to angiotensin.

Mutation of Asn111 in the third transmembrane domain of the AT1A angiotensin II receptor induces its constitutive activation

A preliminary model of the rat AT1A angiotensin II (AII) receptor (Joseph, M. P., Maigret, B., Bonnafous J.-C., Marie, J., and Scheraga, H. A. (1995) J. Protein Chem. 14, 381-398) has predicted an interaction between Asn111 located in transmembrane domain (TM) III and Tyr292 (TM VII) in the nonactivated receptor; a disruption of this interaction upon AII activation would allow Tyr292 to interact with the conserved Asp74 (TM II). The previous verification that Tyr292 is essential for receptor coupling to phospholipase C (Marie, J., Maigret, B., Joseph, M. P., Larguier, R., Nouet, S., Lombard, C., and Bonnafous, J.-C. (1994) J. Biol. Chem. 269, 20815-20818) prompted us to check the possible alterations in receptor properties upon Asn111 --> Ala mutation. The mutated receptor (N111A) displayed: (i) strong constitutive activity, with amplification of the maximal phospholipase C response to AII; (ii) agonist behavior of the AT2-specific ligand CGP 42112A, [Sar1, Ile8]AII, and [Sar1,Ala8]AII, antagonists of the wild-type receptor; (iii) inverse agonism behavior of the non-peptide ligands DuP 753, LF 7-0156, and LF 8-0129. The results are discussed in the light of the allosteric ternary complex models and other described examples of constitutive activation of G protein-coupled receptors.

Amino acids of the alpha1B-adrenergic receptor involved in agonist binding: differences in docking catecholamines to receptor subtypes

Site-directed mutagenesis and molecular dynamics analysis of the 3-D model of the alpha1B-adrenergic receptor (AR) were combined to identify the molecular determinants of the receptor involved in catecholamine binding. Our results indicate that the three conserved serines in the fifth transmembrane domain (TMD) of the alpha1B-AR play a distinct role in catecholamine binding versus receptor activation. In addition to the amino acids D125 in TMDIII and S207 in TMDV directly involved in ligand binding, our findings identify a large number of polar residues playing an important role in the activation process of the alpha1B-AR thus providing new insights into the structure/function relationship of G protein-coupled receptors.

Cloning, expression and pharmacology of the mouse 5-HT(4L) receptor

Since most of our knowledge on pharmacological properties of brain 5-HT4 receptors have been discussed for mouse colliculi neurons, we cloned the corresponding receptor using the RT-PCR approach. As expected, the homology with the already cloned rat 5-HT(4L) receptor was high, revealing only 16 differences at the amino-acid level. One of the differences, proline75 in mouse, alanine75 in the already published rat sequences was not confirmed. Therefore this proline is part of the consensus sequence present in all 5-HT receptor transmembrane domain II (LVMP). Comparing the affinities of 11 agonists and five antagonists for the cloned mouse receptor (5-HT(4L))expressed in LLCPK1 and the corresponding receptor in mouse colliculi shows an excellent correlation. The transfected mouse 5-HT(4L) receptor stimulated cAMP production. When expressed at high density, it exhibited intrinsic activity. In contrast to the previously described distribution, we found that mRNA encoding for both the short (5-HT(4S))and the long form (5-HT(4L)) of 5-HT4 receptors are expressed in all mouse and rat brain areas.