In vitro mutagenesis and the search for structure-function relationships among G protein-coupled receptors.
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Amino Acid Sequence↗
Biomedical subjects
C M Fraser
Publications and source records attributed to C M Fraser.
Radioligand binding studies of the atypical beta 3-adrenergic receptor in rat brown adipose tissue using [3H]CGP 12177.
Two populations of [3H]CGP 12177 binding sites exist in rat interscapular brown adipose tissue (IBAT) plasma membranes. The majority of binding sites are of low affinity with a Kd of 31 nM, a value in close agreement with that for the Kd of [3H]CGP 12177 binding to a cloned rat beta 3-adrenergic receptor (AR) expressed in CHO cells (44 nM). Competition binding studies demonstrate that the Ki values of the cloned rat beta 3-AR and of the low affinity sites in IBAT are 45 and 29 nM, respectively, for BRL 37344 and 1.4 and 1.0 microM, for (-)-propranolol. These findings strongly suggest that the low affinity [3H]CGP 12177 binding site measured in IBAT plasma membranes represents the atypical beta 3-AR in this tissue.
Adipose Tissue, Brown↗
Selective interaction of beta 2- and alpha 2-adrenergic receptors with stimulatory and inhibitory guanine nucleotide-binding proteins.
In Chinese hamster ovary cells expressing recombinant beta 2-adrenergic receptors, isoproterenol enhanced cholera toxin-catalyzed ADP-ribosylation of the large form of G5 alpha. The effect was stereoselectively blocked by the enantiomers of propranolol, indicating receptor mediation. The ADP-ribosylated form of Gs alpha-subunit was resolved into a triplet in gradient gels. beta 2-Adrenergic receptors increased both the labelling and the apparent mass of the slower migrating forms of large Gs alpha, as determined by autoradiography and immunoblotting, suggesting that Gs alpha, can incorporate more than one ADP-ribose per molecule. In cells coexpressing similar amounts of beta 2-adrenergic, alpha 2-adrenergic, and m1 muscarinic receptors, beta 2 receptors stimulated the ADP-ribosylation of only large Gs and alpha 2 receptors that of only Gi; muscarinic receptors had no apparent effect. Thus, in native membranes there appears to be a selectivity for the interaction between adrenergic receptor subtypes and Gs alpha or Gi alpha subunits.
Adenosine Diphosphate Ribose↗
An adipose tissue-specific beta-adrenergic receptor. Molecular cloning and down-regulation in obesity.
Clones encoding an atypical beta-adrenergic receptor were isolated from a rat brown adipose tissue cDNA library. This receptor expressed in Chinese hamster ovary (CHO) cells displays a low affinity for beta-adrenergic antagonists and a high affinity for BRL 37344, an agonist that selectively stimulates lipolysis in adipose tissue. The rank order of potency for agonist-mediated increases in intracellular cAMP in transfected cells correlates with that for agonist-mediated stimulation of lipolysis in brown adipocytes. Northern blot analysis demonstrates that this receptor subtype is expressed only in brown and white adipose tissue where it represents the predominant beta-receptor subtype. The amount of atypical beta-adrenergic receptor present in adipose tissue of obese (fa/fa) Zucker rats is reduced by up to 71% as compared with lean (Fa/Fa) control animals. These findings suggest that a change in the expression of this beta-adrenergic receptor subtype may play a role in obesity.
Adipose Tissue, Brown↗
Changes in beta 1- and beta 2-adrenergic receptor mRNA levels in brown adipose tissue and heart of hypothyroid rats.
The aim of the present work was to study the effect of hypothyroidism on the expression of the beta-adrenergic receptor (beta-AR) in interscapular brown adipose tissue and heart. The total density of plasma membrane beta-AR per tissue is decreased by 44% in hypothyroid rat interscapular brown adipose tissue and by 55% in hypothyroid rat heart compared with euthyroid controls. The effects of hypothyroidism on the density of both beta 1- and beta 2-AR subtypes were also determined in competition displacement experiments. The densities of beta 1- and beta 2-AR per tissue are decreased by 50% and 48% respectively in interscapular brown adipose tissue and by 52% and 54% in the heart. Northern blot analysis of poly(A)+ RNA from hypothyroid rat interscapular brown adipose tissue demonstrated that the levels of beta 1- and beta 2-AR mRNA per tissue are decreased by 73% and 58% respectively, whereas in hypothyroid heart, only the beta 1-AR mRNA is decreased, by 43%. The effect of hypothyroidism on the beta 1-AR mRNA is significantly more marked in the interscapular brown adipose tissue than in the heart. These results indicate that beta-AR mRNA levels are differentially regulated in rat interscapular brown adipose tissue and heart, and suggest that the decrease in beta-AR number in interscapular brown adipose tissue and heart of hypothyroid animals may in part be explained by a decreased steady-state level of beta-AR mRNA.
Adipose Tissue, Brown↗
Molecular biology of adrenergic receptors: model systems for the study of G-protein-mediated signal transduction.
Elucidation of the gene structure of several receptors known to mediate the signal of hormone or transmitter binding to intracellular effector systems through guanine-nucleotide-binding proteins (G proteins) has revealed that these receptors comprise a super-family of related proteins. The hallmark of all G-protein-linked receptors is a presumed topography of 7 membrane-spanning loops, analogous to the structure of bacteriorhodopsin. Members of this gene superfamily contain regions, particularly with the hydrophobic domains, of homologous sequence. The expression of G-protein-linked receptors in heterologous cell systems has allowed for the study of the pharmacological and biochemical properties of individual receptor subtypes in a manner not previously possible with intact tissues containing multiple receptors. Site-directed mutagenesis experiments have identified many conserved amino acids which are involved in ligand binding, receptor activation by agonists and receptor-G protein coupling, and suggest that the conservation of receptor structure throughout this gene family may reflect a conservation of important functional domains within these proteins.
Amino Acid Sequence↗
Genetic mapping of the beta 1 GABA receptor gene to human chromosome 4, using a tetranucleotide repeat polymorphism.
As more coding loci for functional human genes are described, there is a growing need to identify DNA polymorphisms in specific genes. By examining DNA sequences within the introns of the beta 1 subunit of the gamma-aminobutyric acid receptor gene, GABARB1, we found a tetranucleotide repeat sequence (GATA). Amplification of this region by using PCR revealed seven alleles and a high degree of polymorphism (PIC = .75) in human populations. DNAs from the CEPH families were typed for the GABARB1 intron polymorphism and were analyzed with respect to 20 linked markers on chromosome 4. The results permit placement of GABARB1 on the linkage map of chromosome 4, between D4S104 and ALB. These results affirm that sequence analysis of noncoding segments included within or adjacent to functional genes has value as a strategy to detect highly informative polymorphisms.
Alleles↗
Site-directed mutagenesis of alpha 2A-adrenergic receptors: identification of amino acids involved in ligand binding and receptor activation by agonists.
Molecular cloning of the alpha 2A-adrenergic receptor has shown that this receptor is a member of the gene superfamily of guanine nucleotide-binding protein (G protein)-coupled receptors. The alpha 2A-adrenergic receptor expressed in Chinese hamster ovary (CHO) cells attenuates and potentiates forskolin-stimulated cAMP production through independent signaling pathways. To examine the role of three conserved aspartic acid and two conserved serine residues in alpha 2A-adrenergic receptor function, we substituted asparagine for aspartic acid or alanine for serine and characterized the mutant receptors in stably transfected CHO cells. Asn113 mutant alpha 2-adrenergic receptors display no [3H] yohimbine specific binding, at concentrations up to 1000 nM. In transfected cells expressing the Asn113 mutant receptor, agonists, at concentrations up to 0.1 mM, produce small decreases (approximately 10% of wild-type values) in forskolin-stimulated cAMP and potentiate forskolin-stimulated cAMP concentrations in a dose-dependent manner, with EC50 values approximately 500-fold higher than those for the wild-type receptor. These findings suggest that Asp113 may be involved in high affinity binding of agonists and antagonists, as has been previously reported for beta 2-adrenergic and m1 muscarinic acetylcholine receptors. Asn79 mutant alpha 2-adrenergic receptors display high affinity agonist binding that is insensitive to guanine nucleotides, suggesting an altered receptor-G protein coupling. Furthermore, agonist binding to Asn79 mutant receptors elicits no change in forskolin-stimulated cAMP concentrations, similar to earlier findings that the corresponding residue in beta 2-adrenergic and muscarinic receptors is required for effector stimulation. Asp130 appears to influence receptor-G protein coupling. Mutation of this residue eliminates high affinity, guanine nucleotide-sensitive, agonist binding and produces a rightward shift in the dose-response curves for agonist-mediated inhibition of forskolin-stimulated cAMP production, compared with the wild-type receptor. Moreover, agonist potentiation of forskolin-stimulated cAMP levels is abolished if Asp130 is replaced by Asn, supporting the hypothesis that inhibition and potentiation of forskolin-stimulated cAMP production in CHO cells proceed through distinct signaling pathways. Characterization of Ala204 mutant alpha 2A-adrenergic receptors suggests a possible role for Ser204 in hydrogen bond interactions with the para-hydroxyl group of the phenyl ring of the catecholamines, as has been previously described for the corresponding serine in beta 2-adrenergic receptors.(ABSTRACT TRUNCATED AT 400 WORDS)
Adrenergic alpha-Agonists↗
Muscarinic acetylcholine receptor subtypes which selectively couple to phospholipase C: pharmacological and biochemical properties.
The pharmacological and biochemical properties of rat m1 and m3 muscarinic acetylcholine receptors (mAChR) stably transfected into Chinese hamster ovary-K1 (CHO) cells were characterized with ligand binding, affinity labeling and biochemical assays. Both mAChR subtypes display saturable, high affinity binding of [3H]-quinuclidinyl benzilate (QNB) and a rank order of antagonist potency of QNB greater than atropine greater than pirenzepine greater than AF-DX 116. Carbachol displacement of [3H]-QNB binding to the m3 mAChR revealed an approximate 17-fold higher affinity than observed with the m1 mAChR. [3H]-propylbenzilylcholine mustard (PrBCM) labeling of mAChR revealed that m1 and m3 mAChR migrated on SDS-polyacrylamide gels with apparent molecular masses of 80,000 and 94,000 daltons, respectively, consistent with the known differences in their molecular sizes. Both m1 and m3 mAChR elicited dose-dependent increases in the hydrolysis of phosphoinositides; however, the maximal increase in total inositol phosphates elicited with the m1 mAChR was approximately 2-fold greater than that observed in cells expressing similar densities of m3 mAChR. Agonist activation of the m1 mAChR also elicited increases in basal and forskolin-stimulated cAMP, whereas the m3 mAChR had no effect on intracellular cAMP levels. These data suggest that although m1 and m3 mAChR display a considerable degree of structural homology, they exhibit distinct pharmacological and biochemical properties.
Animals↗
Cloning, localization, and permanent expression of a Drosophila octopamine receptor.
A cDNA for a member of the G protein-coupled receptor family was isolated from Drosophila using a probe derived from a human beta 2-adrenergic receptor cDNA. This Drosophila receptor gene is localized at 99A10-B1 on the right arm of chromosome 3 and is preferentially expressed in Drosophila heads. The insect octopamine receptor has been permanently expressed in mammalian cells, where it mediates the attenuation of adenylate cyclase activity and exhibits a pharmacological profile consistent with an octopamine type 1 receptor. Sequence and pharmacological comparisons indicate that the octopamine receptor is unique but closely related to mammalian adrenergic receptors, perhaps as an evolutionary precursor.
Amino Acid Sequence↗
Continuous monitoring of receptor-mediated changes in the metabolic rates of living cells.
Activation of beta-adrenergic or muscarinic acetylcholine receptors expressed in transfected cells or epidermal growth factor receptors in human keratinocytes produces 15% to 200% changes in cellular metabolic rates. Changes in cell metabolism were monitored continuously with a previously described silicon-based microphysiometer that detects small changes in extracellular pH. The amplitude and kinetics of the metabolic changes depend upon several factors including pretreatment of the cells prior to receptor stimulation, the dose of hormone/neurotransmitter used, and the receptor complement of the cells. Responses are receptor specific; cells transfected with receptor genes respond only to the appropriate hormone/transmitter, whereas control (nontransfected) cells or cells transfected with different receptors exhibit no response. The specificity of the responses was further documented by using pharmacological antagonists. In Chinese hamster ovary (CHO) cells transfected with human beta 2-adrenergic receptors, isoproterenol produces a 20-60% increase in the rate of extracellular acidification with an EC50 of 4 nM, a response that is competitively antagonized by (-)-propranolol. The EC50 for the isoproterenol response is shifted from 4 nM to 100 nM in the presence of 3 nM (-)-propranolol. The kinetics of the metabolic response induced by beta-adrenergic receptor stimulation are markedly slower than those elicited by muscarinic receptor agonists. The maximal metabolic response in cells transfected with beta-adrenergic receptors peaks at approximately 12 min as compared with less than 30 sec in cells transfected with muscarinic receptors, perhaps reflecting activation of different second-messenger pathways. These findings illustrate an alternative means of studying cellular responses to hormones and neurotransmitters and suggest that metabolic changes will be generally useful for detecting the consequences of receptor-ligand interactions.
Animals↗
Receptor mechanisms. Structure and molecular biology of transmitter receptors.
Details of receptor structure and function that were unavailable as recently as two years ago are now readily obtainable through the application of molecular biological techniques. Cloning and sequence analysis of neurotransmitter receptor genes have provided information on the primary structure of these proteins, revealing the relationship between pharmacologically diverse families of receptors. Knowledge of the primary structure of receptors has allowed for prediction of secondary structure and the construction of three-dimensional models. Permanent expression of cloned neurotransmitter receptor genes in cultured cells is providing unlimited sources of pure receptor, which allows for pharmacological and biochemical studies on single receptor subtypes. The use of site-directed mutagenesis to elucidate the relationship between protein structure and function has provided considerable information on the role of certain conserved amino acids in receptor function and has suggested possible molecular mechanisms of signal transduction across membranes. The article will review some of these recent developments in the area of neurotransmitter receptors and point out the utility of molecular biology in these endeavors.
Amino Acid Sequence↗
Beta-adrenergic receptors. Relationship of primary structure, receptor function, and regulation.
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Amino Acid Sequence↗
Cloning, sequence analysis and chromosome localization of a Drosophila muscarinic acetylcholine receptor.
Two cDNA clones (3.7 kb and 4.8 kb) encoding a Drosophila muscarinic acetylcholine receptor were isolated from a Drosophila head cDNA library and characterized by automated DNA sequence analysis. The Drosophila muscarinic receptor contains 788 amino acids with a calculated Mr of 84,807 and displays greater than 60% homology with mammalian muscarinic receptors. The muscarinic receptor maps to the tip of the right arm of the second chromosome of the Drosophila genome.
Amino Acid Sequence↗
Cloning, sequence analysis, and permanent expression of a human alpha 2-adrenergic receptor in Chinese hamster ovary cells. Evidence for independent pathways of receptor coupling to adenylate cyclase attenuation and activation.
The gene encoding a human alpha 2-adrenergic receptor was isolated from a human genomic DNA library using a 367-base pair fragment of Drosophila genomic DNA that exhibited 54% identity with the human beta 2-adrenergic receptor and 57% identity with the human alpha 2-adrenergic receptor. The nucleotide sequence of a fragment containing the human alpha 2-receptor gene and 2.076 kilobases of untranslated 5' sequence was determined, and potential upstream regulatory regions were identified. This gene encodes a protein of 450 amino acids and was identified as an alpha 2-adrenergic receptor by homology with published sequences and by pharmacological characterization of the protein expressed in cultured cells. Permanent expression of the alpha 2-receptor was achieved by transfecting Chinese hamster ovary (CHO) cells which lack adrenergic receptors with a 1.5-kilobase NcoI-HindIII fragment of the genomic clone containing the coding region of the gene. The alpha 2-receptor expressed in CHO cells displayed pharmacology characteristic of an alpha 2 A-receptor subtype with a high affinity for yohimbine (Ki = 1 nM) and a low affinity for prazosin (Ki = 10,000 nM). Agonists displayed a rank order of potency in radioligand binding assays of para-aminoclonidine greater than or equal to UK-14304 greater than (-)-epinephrine greater than (-)-norepinephrine greater than (-)-isoproterenol, consistent with the identification of this protein as an alpha 2-receptor. The role of the alpha 2-receptor in modulating intracellular cyclic AMP concentrations was investigated in three transfected cell lines expressing 50, 200, and 1200 fmol of receptor/mg membrane protein. At low concentrations (1-100 nM), (-)-epinephrine attenuated forskolin-stimulated cyclic AMP accumulation by up to 60% in a receptor density-dependent manner. At epinephrine concentrations above 100 nM, cyclic AMP levels were increased up to 140% of the forskolin-stimulated level. Pertussis toxin pretreatment of cells eliminated alpha 2-receptor-mediated attenuation of forskolin-stimulated cyclic AMP levels and enhanced the receptor density-dependent potentiation of forskolin-stimulated cyclic AMP concentrations from 3 to 8-fold. Potentiation of forskolin-stimulated cyclic AMP levels was also elicited by the alpha 2-adrenergic agonists, UK-14304 and para-aminoclonidine, and blocked by the alpha 2-adrenergic antagonist yohimbine, but not by the alpha 1-adrenergic antagonist prazosin or the beta-adrenergic antagonist propranolol.(ABSTRACT TRUNCATED AT 400 WORDS)
Adenylate Cyclase Toxin↗
Site-directed mutagenesis of beta-adrenergic receptors. Identification of conserved cysteine residues that independently affect ligand binding and receptor activation.
Using site-directed mutagenesis of the human beta 2-adrenergic receptor and continuous expression in B-82 cells, the role of 3 conserved cysteines in transmembrane domains and 2 conserved cysteines in the third extracellular domain in receptor function was examined. Cysteine was replaced with serine in each mutant receptor as this amino acid is similar to cysteine in size but it cannot form disulfide linkages. Replacement of cysteine residues 77 and 327, in the second and seventh transmembrane-spanning domains, respectively, had no effect on ligand binding or the ability of the receptor to mediate isoproterenol stimulation of adenylate cyclase. Substitution of cysteine 285, in the sixth transmembrane domain of the receptor, produced a mutant receptor with normal ligand-binding properties but a significantly attenuated ability to mediate stimulation of adenylate cyclase. Mutation of cysteine residues 190 and 191, in the third extracellular loop of the beta 2 receptor, had qualitatively similar effects on ligand binding and isoproterenol-mediated stimulation of adenylate cyclase. Replacement of either of these residues with serine produced mutant receptors that displayed a marked loss in affinity for both beta-adrenergic agonists and antagonists. Replacement of both cysteine 190 and 191 with serine had an even greater effect on the ability of the receptor to bind ligands. Consistent with the loss of Ser190 and/or Ser191 mutant receptor affinity for agonists was a corresponding shift to the right in the dose-response curve for isoproterenol-induced increases in intracellular cyclic AMP concentrations in cells expressing the mutant receptors. These data implicate one of the conserved transmembrane cysteine residues in the human beta 2-adrenergic receptor in receptor activation by agonists and also suggest that conserved cysteine residues in an extracellular domain of the receptor may be involved in ligand binding.
Amino Acid Sequence↗
Molecular biology of adrenergic and muscarinic cholinergic receptors. A perspective.
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Amino Acid Sequence↗
Pharmacological characterization of the M1 muscarinic receptors expressed in murine fibroblast B82 cells.
The muscarinic receptors in a B82 cell line which were transfected with the rat m1 muscarinic receptor gene (cTB10 cells) were studied by using radioligand binding assays. Their possible coupling to the hydrolysis of inositol lipids and cyclic AMP formation were also investigated. [(-)-[3H]Quinuclidinyl benzilate [(-)-[3H]QNB] binding to the intact cTB10 cells was saturable and displaceable by 1 microM atropine sulfate. The Kd and maximum binding values of (-)-[3H]QNB from saturation studies were 12 pM and 17 fmol/10(6) cells, respectively. Inhibition studies of (-)-[3H]QNB binding to intact cTB10 cells suggested that these muscarinic receptors are of the M1 type defined by their high affinity for pirenzepine and low affinity for AF-DX 116 [11-[2-diethylamino methyl-1-piperidinylacetyl]-5,11-dihydro-6H-pyrido(2,3-b) (1,4)benzodiazepine-6-one]. The muscarinic agonist carbachol stimulated [3H]inositol monophosphate accumulation in the cTB10 cells, which could be reversed by the muscarinic antagonists atropine, pirenzepine or AF-DX 116. The rank order of potency of the muscarinic antagonists in inhibiting carbachol-stimulated [3H]inositol monophosphate accumulation was atropine greater than pirenzepine greater than AF-DX 116, in agreement with that from ligand/(-)-[3H]QNB competition experiments. Pertussis toxin and 4 beta-phorbol, 12-beta-myristate, 13-alpha-acetate reduced carbachol-stimulated [3H]inositol monophosphate accumulation. Prostaglandin E1 stimulated cyclic AMP formation in the cTB10 cells. Carbachol at the concentration of 10 mM exhibited no stimulatory or inhibitor effect on the basal or prostaglandin E1-stimulated cyclic AMP formation. These results suggest that the muscarinic receptors encoded by the transfected m1 gene in the cTB10 cells are of the M1 type and are coupled to the hydrolysis of inositol lipids, possibly via a pertussis toxin sensitive G protein.
Alprostadil↗