Primary structure of porcine cardiac muscarinic acetylcholine receptor deduced from the cDNA sequence

From epinephrine to cyclic AMP

Binding of catecholamines to the beta-adrenergic receptor results in the activation of adenylate cyclase and the intracellular formation of adenosine 3',5'-monophosphate (cAMP). In the past 20 years the events that lead from hormone binding at the cell surface receptor site to the synthesis of cAMP at the inner layer of the membrane have been intensively studied. Signal transduction in this system involves the sequential interaction of the beta-adrenergic receptor with the guanine nucleotide-binding protein (Gs) and the adenylate cyclase catalyst (C). The mechanism of signal transduction from the receptor through Gs to C, as well as the role of the adenylate cyclase inhibitory G protein Gi, is discussed.

Differential regulation of PI hydrolysis and adenylyl cyclase by muscarinic receptor subtypes

Muscarinic acetylcholine receptors (mAChRs), like many other neurotransmitter and hormone receptors, transduce agonist signals by activating G proteins to regulate ion channel activity and the generation of second messengers via the phosphoinositide (PI) and adenylyl cyclase systems. Human mAChRs are a family of at least four gene products which have distinct primary structures, ligand-binding properties and patterns of tissue-specific expression. To examine the question of whether functional differences exist between multiple receptor subtypes, we have investigated the ability of each subtype to regulate PI hydrolysis and adenylyl cyclase when expressed individually in a cell lacking endogenous mAChRs. We show that the HM2 and HM3 mAChRs efficiently inhibit adenylyl cyclase activity but poorly activate PI hydrolysis. In contrast, the HM1 and HM4 mAChRs strongly activate PI hydrolysis, but do not inhibit adenylyl cyclase, and in fact can substantially elevate cAMP levels. Interestingly, the subtypes that we find to be functionally similar are also more similar in sequence. Our results indicate that the different receptor subtypes are functionally specialized.

Membrane phospholipid polar heads influence the coupling of M2 muscarinic receptors to G proteins

Treating membranes from rat heart with phospholipase C (phosphatidylcholine choline phosphohydrolase) from Clostridium perfringens increased the affinity of muscarinic acetylcholine receptors (M2) for the agonists carbachol and oxotremorine. The affinity for antagonists was not affected. Phospholipase C activity, i.e., the cleavage of polar heads of membrane phospholipids, led to the disappearance of the guanine nucleotide-dependent rightward shift of the isotherm for agonist binding. The treatment of tracheal smooth muscle with phospholipase C led to a decrease in the maximum contractile effect of muscarinic (M2) stimulation with no modification of the agonist EC50, i.e., to the uncoupling of the stimulation-contraction process. These results demonstrate that when phospholipid polar heads are hydrolysed by phospholipase C, M2 receptors are uncoupled from G proteins, which enhances their affinity for agonists but prevents information transfer.

Site-directed mutagenesis of human beta-adrenergic receptors: substitution of aspartic acid-130 by asparagine produces a receptor with high-affinity agonist binding that is uncoupled from adenylate cyclase

By using oligonucleotide-directed mutagenesis, we have produced a point mutation (guanine to adenine) at nucleotide 388 of the gene for human beta-adrenergic receptor (beta AR) that results in a substitution of asparagine for the highly conserved aspartic acid at position 130 in the putative third transmembrane domain of the human beta AR ([Asn130]beta AR). We have examined the functional significance of this mutation in B-82 cells continuously expressing the mutant [Asn130]beta AR. The mutant [Asn130]beta AR displayed normal antagonist binding but unusually high-affinity agonist binding (5- to 10-fold higher than wild-type beta AR), consistent with a single class of high-affinity binding sites. The mutant beta AR displayed guanine nucleotide-sensitive changes in agonist affinity (3- to 5-fold shift) implying an interaction between the beta AR and the stimulatory guanine nucleotide-binding regulatory protein; however, the ability of guanine nucleotides to alter agonist affinity was attenuated. Addition of saturating concentrations of isoproterenol to cell cultures expressing mutant [Asn130]-beta ARs had no effect on intracellular levels of cAMP, indicating that the mutant beta AR is unable to affect stimulation of adenylate cyclase. These results indicate that substitution of the aspartic acid with asparagine at residue 130 of the human beta AR dissociates the well-characterized guanine nucleotide effects on agonist affinity from those on activation of the stimulatory guanine nucleotide-binding regulatory protein and adenylate cyclase and suggests the existence of two distinct counterions for the amine portion of catecholamines that are associated with high- and low-affinity agonist binding states of beta AR.

Primary structure of porcine muscarinic acetylcholine receptor III and antagonist binding studies

The complete amino acid sequence of porcine muscarinic acetylcholine receptor III has been deduced by cloning and sequencing the genomic DNA. The antagonist binding properties of muscarinic acetylcholine receptor III expressed from the cloned DNA in Xenopus oocytes correspond most closely to those of the pharmacologically defined M2 glandular (III) subtype.

Expression of receptors for cholecystokinin and other Ca2+-mobilizing hormones in Xenopus oocytes

The expression of receptors for cholecystokinin (CCK) and other similar acting Ca2+-mobilizing hormones was studied in Xenopus laevis oocytes. Poly(A)+ RNA was prepared from pancreatic AR42J cells, which normally express receptors for CCK and bombesin and the RNA injected into oocytes. The presence of these pancreatic receptors on the oocytes was then demonstrated by hormone-induced mobilization of 45Ca2+. CCK receptors were present 1 day (maximum, 2 days) after injection of RNA and were generally proportional to the amount of poly(A)+ RNA injected (1-50 ng). Oocyte CCK receptors retained selectivity for CCK analogs (CCK8 greater than unsulfated CCK8 greater than CCK4) and were blocked by the specific CCK receptor antagonist CR 1409. When poly(A)+ RNA was subjected to size fractionation on sucrose gradients, activity-inducing CCK receptors showed a single peak centered at 3 kilobases. The generality of this oocyte system for expressing Ca2+-mobilizing hormone receptors was further shown by expression of a response to bombesin after injection of AR42J cell RNA and a response to vasopressin and angiotensin II when poly(A)+ RNA from rat liver was injected. No response to CCK was demonstrable after injection of liver RNA, demonstrating the specificity of this assay.

Characterization of cholinergic muscarinic receptors in the rabbit iris

The sphincter smooth muscle of the iris is innervated by excitatory parasympathetic nerve fibers, and the activation of these fibers results in the breakdown of phosphatidylinositol 4,5-bisphosphate into its derived second messengers, myosin light chain phosphorylation and muscle contraction. The present study characterizes the muscarinic acetylcholine receptors (mAChRs) of the rabbit iris employing [3H]N-methylscopolamine ([3H]NMS) and L-[3H]quinuclidinyl benzilate ([3H]QNB) as probes. Binding studies indicated that [3H]NMS and [3H]QNB bound to homogeneous populations of mAChRs with apparent Bmax values of 0.67 and 1.09 pmol/mg protein respectively. Binding of radioligands was rapid, saturable, stereospecific, reversible, and inhibited by specific muscarinic agonists and antagonists in a competitive manner. [3H]NMS displayed a lower amount of nonspecific binding and a faster association and dissociation rate than [3H]QNB. The relative potencies for displacement of both radioligands, based on their Ki values, were (-)QNB greater than atropine greater than (+)QNB greater than pirenzepine greater than pilocarpine. Antagonist displacement of the radioligands appeared to obey the law of mass action, indicating interaction with a single binding site. However, displacement of the radioligands by the agonists carbamylcholine and methacholine indicated interaction with both high and low affinity binding sites. Comparison of the displacement of [3H]NMS and [3H]QNB by pirenzepine in microsomal fractions from rabbit iris, ileal muscle and cerebral cortex revealed the presence of a single subtype of mAChR in the iris which had an affinity for PZ that was slightly higher than that of ileal M2 receptors, but lower than that of brain M1 receptors. This suggests that the mAChRs in the iris may represent a subclass of receptors within the M2 subtype, or they may constitute an entirely different subtype of mAChRs.

Cloning and expression of the human and rat m5 muscarinic acetylcholine receptor genes

The human and rat genes for a fifth muscarinic receptor have been cloned and expressed in mammalian cells. The 532 amino acid human protein has 89% sequence identity to the 531 amino acid rat protein and is most closely related to the m3 receptor. Both proteins are encoded by single exons. The receptor has intermediate affinity for pirenzepine and low affinity for AF-DX 116, and it increases metabolism of phosphatidylinositol when stimulated with carbachol. Expression of mRNA has yet to be observed in brain or selected peripheral tissues, suggesting that either it is substantially less abundant than m1-m4 or its distribution is quite different.

Pharmacological characterization of muscarinic receptors involved in McN-A-343-induced effects on intestinal motility and heart rate in conscious dogs

1. Intravenous injection of the muscarinic agonist, McN-A-343, in conscious dogs equipped with an ileal Thiry fistula produced a dose-related inhibition of intestinal phasic contractile activity, and an increase in heart rate. 2. The inhibitory action of McN-A-343 on motility was antagonized with different potencies by antimuscarinic drugs. The non-selective drug, N-methylatropine, blocked the McN-A-343 effect as well as the reflex phasic activity. The M1-selective compound, pirenzepine (1-30 micrograms kg-1), was a potent antagonist of the McN-A-343 effect, whereas the cardioselective M2-antagonist, AF-DX 116, and the smooth muscle selective compound, 4-diphenylacetoxy-N-methyl piperidine (4-DAMP), were completely ineffective at the doses tested. 3. The McN-A-343-induced inhibition of intestinal motility was blocked by locally applied lignocaine, suggesting the involvement of a neural inhibitory pathway. The resistance to hexamethonium and (alpha 1-, alpha 2- and beta-) adrenoceptor blocking drugs excluded transmission through a nicotinic synapse or release of catecholamines. 4. McN-A-343-induced tachycardia was also the result of muscarinic receptor activation. It was very sensitive to antagonism by 4-DAMP, while being completely unaffected by AF-DX 116. Pirenzepine displayed an intermediate profile, reducing tachycardia at doses fully active in reversing the agonist-mediated effect on intestinal motility. Propranolol partially reduced McN-A-343 tachycardia, suggesting catecholamine release. 5. The two McN-A-343 effects investigated in the present study appear to be mediated by different muscarinic receptor subtypes. While the inhibitory action on intestinal motility results from stimulation of Ml-muscarinic receptors, the tachycardia is mediated by receptors blocked selectively by 4-DAMP.

Two affinity states of M1 muscarine receptors

1. The binding of oxotremorine-M to M1 muscarine receptors was examined by measuring competition between the agonist and 3H-pirenzepine, using rabbit hippocampal membranes suspended in 20 mM Tris buffer containing 1 mM Mn2+. 2. Both ligands interacted with a single class of receptors. The receptors could assume two affinity states for oxotremorine-M, with equal numbers of high-affinity (KH) and low-affinity (KL) sites. 3. KH interconverted reversibly to KL in the absence of divalent cations and interconverted reversibly to a state similar to KL in the presence of guanyl 5'-yl imidodiphosphate. 4. The results are compatible with a model in which a pair of receptor molecules can be stabilized by a guanine nucleotide-binding "G protein" and have one site each of KH and KL affinity.

Electrophysiological characterization of cloned m1 muscarinic receptors expressed in A9 L cells

The electrophysiological properties of A9 L cells stably transfected with m1 muscarinic receptor cDNA were examined by using the whole-cell patch-clamp technique. In current-clamp recordings, acetylcholine (AcCho) elicited a hyperpolarization of all transfected cells studied but had no effect on nontransfected A9 L cells. In voltage-clamp recordings, AcCho elicited an outward current at -50 mV accompanied by an increase in conductance. The onset of the current response was consistently delayed by several seconds with respect to the onset of the application of AcCho and could not be accounted for by diffusion. The AcCho-induced currents were reversibly inhibited by the muscarinic receptor antagonist atropine (1 microM) but were unaffected by the nicotinic receptor antagonist tubocurarine (50 microM). Ion-substitution experiments replacing K+ with N-methyl-D-glucamine and Cl- with methanesulfonate indicated that the current was carried mainly by K+, although a minor part appeared to be carried by Cl-. The AcCho-induced current could be blocked by the K+ channel blocking agents tetraethylammonium ion, 4-aminopyridine, apamin, and Ba2+ but not by Cs+. The AcCho-induced current was inhibited when 5 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was included in the patch pipette or when extracellular Cd2+ or Co2+ was applied, indicating a role for intracellular Ca2+ in the generation of the response. Thus, these results show that cloned m1 muscarinic receptors expressed in A9 L cells can activate a Ca2+-dependent K+ conductance, possibly via a second-messenger system.

Muscarinic acetylcholine receptors from avian retina and heart undergo different patterns of molecular maturation

Muscarinic acetylcholine receptors (mAChRs) from the avian CNS exist in two molecular weight forms whose concentrations change during development. Here, we have compared the development of mAChRs from embryonic hearts with those of the CNS. Analysis of [3H]-propylbenzilylcholine mustard (PrBCM)-labeled retina and heart mAChRs by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed two atropine-sensitive peaks for each tissue. Apparent molecular masses of retina mAChRs, 86 +/- 0.7 kilodaltons (kDa) and 72 +/- 0.7 kDa, were different from those of heart mAChRs, 77 +/- 1.0 kDa and 52 +/- 0.9 kDa. During retina development, the major receptor type changed from 86 kDa to 72 kDa. No such change occurred during heart development. Furthermore, the 52-kDa species appeared to be generated by endogenous proteolysis, as prolonged incubation of heart membranes at 37 degrees C increased the amount of 52-kDa peptide with a decrease of 77-kDa peptide. Protease inhibitors blocked this conversion. Incubation of retina membranes at 37 degrees C did not result in a conversion of the 86-kDa peptide into the 72-kDa peptide, but it did cause the appearance of a minor amount of 52-kDa peptide. The proteolysis of retina mAChRs was not enhanced by cohomogenizing them with heart tissue, arguing against the presence of releasable proteases in heart. Membrane-bound retina and heart mAChRs displayed similar sensitivity to exogenous (Staphylococcus aureus V8) protease, indicating that heart receptors were not unusually susceptible to proteolytic attack; analysis of the labeled polypeptides with the V8 protease showed different patterns of digestion for the retina and heart receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Distance between the propylbenzilylcholine mustard attachment site and carbohydrates and thiol groups in muscarinic acetylcholine receptor protein from rat cerebral cortex

When rat cerebral-cortex membranes were labelled with [3H]propylbenzilylcholine mustard ([3H]PrBCM), a single protein of Mr 68,000 was found to carry the atropine-sensitive covalent label. After trypsinolysis of the receptors solubilized in 0.075% SDS, the resulting fragments were submitted to size analysis in combination with wheat-germ agglutinin (WGA)-Sepharose and organomercurial-agarose chromatography. Peptides of Mr 75,000, 50,000, 30,000, 18,000 and 8000 were specifically released from the receptor. All fragments above Mr 8000 were able to bind WGA-Sepharose and therefore the peptide of Mr 18,000 was taken as the upper limit of the distance between the antagonist and the glycan moieties. The limit fragment of Mr 8000 carried chemical groups which were modified by N-ethylmaleimide and reacted with an immobilized organomercurial. About 65-80% of the labelled receptors were adsorbed on concanavalin A-Sepharose with low affinity, generating two further components after sequential application to WGA-Sepharose. About 50% of the receptors were susceptible to neuraminidase treatment, with a concomitant slight modification of the SDS/polyacrylamide-gel-electrophoretic pattern.

The intact human neuroblastoma cell (SH-SY5Y) exhibits high-affinity [3H]pirenzepine binding associated with hydrolysis of phosphatidylinositols

The binding of [3H]pirenzepine to a human neuroblastoma cell line (SH-SY5Y) and its correlation with hydrolysis of phosphatidylinositols were characterized. Specific [3H]pirenzepine binding to intact cells was rapid, reversible, saturable, and of high affinity. Kinetic studies yielded association (k+1) and dissociation (k-1) rate constants of 5.2 +/- 1.4 X 10(6) M-1 min-1 and 1.1 +/- 0.06 X 10(-1) min-1, respectively. Saturation experiments revealed a single class of binding sites (nH = 1.1) for the radioligand with a total binding capacity of 160 +/- 33 fmol/mg protein and an apparent dissociation constant of 13 nM. The specific [3H]pirenzepine binding was inhibited by the presence of selected muscarinic drugs. The order of antagonist potency was atropine sulfate greater than pirenzepine greater than AF-DX 116, with K0.5 of 0.53 nM, 2.2 nM, and 190 nM, respectively. The binding properties of [3H](-)-quinuclidinyl benzilate and its quaternary derivative [3H](-)-methylquinuclidinyl benzilate were also investigated. The muscarinic agonist carbachol stimulated formation of inositol phosphates which could be inhibited by muscarinic antagonists. The inhibition constants of pirenzepine and AF-DX 116 were 11 nM and 190 nM, respectively. In conclusion, we show that the nonclassical muscarinic receptor antagonist [3H]pirenzepine identifies a high-affinity population of muscarinic sites which is associated with hydrolysis of phosphatidylinositols in this human neuroblastoma cell line.

Himbacine recognizes a high affinity subtype of M2 muscarinic cholinergic receptors in the rat cerebral cortex

The in vitro receptor binding properties of a muscarinic antagonist himbacine have been studied in rat cerebral cortical, cardiac and ileal membranes. Himbacine displayed high affinity (KH = 2.94 nM) for 19%, and low affinity (KL = 71.2 nM) for the remaining muscarinic receptors in rat cerebral cortex. This high affinity of himbacine agrees with its affinity for the 62% of cerebral cortical [3H]AF-DX 116 binding sites (KH = 2.30 nM). The affinity of himbacine for cardiac receptors (Ki = 9.06 nM) and ileal receptors (Ki = 12.4 nM) was the same. Therefore, himbacine appears to be a high-affinity M2-selective ligand which recognizes a subtype of M2 receptors in the cerebral cortex.

A partial structure for the gamma-aminobutyric acid (GABAA) receptor is derived from the model for the nicotinic acetylcholine receptor. The anion-exchange protein of cell membranes is related to the GABAA receptor

Based on the nicotinic acetylcholine receptor model [(1987) Eur. J. Biochem. 168, 431-449], a partial model is constructed for the exobilayer portion of the GABAA receptor, an approach justified by the superfamily relationship of the two receptors [(1987) Nature 328, 221-227]. The model predicts successfully the excess positive charge on interior strands which constitute the ligand-responsive portion of the receptor. Binding to GABA expands the exobilayer portion of the receptor, opening a pathway to a chloride channel. Separate binding sites for antianxiolytics (benzodiazepines) and hypnotics (barbiturates) are suggested, with prolongation of chloride entry projected as a consequence of stabilization of the open form. The anion-exchange protein (AEP) of membranes (band 3 of red blood cell membranes) is similar in some respects to the gamma-aminobutyric acid (GABAA) receptor. Both proteins are inhibited and labeled by diisocyanatostilbenedisulfonate (DIDS), both transport Cl- and HCO-3, and both are membrane proteins. Starting with the lysines known to be labeled in band 3 protein, searches of the amino acid sequences of the GABAA receptor alpha- and beta-subunits reveal at least 4 reasonably homologous sequences. The relationship between AEP and GABAA receptor leads to the idea that the chloride/bicarbonate channel may be the ancestor of all ligand-gated channels, with ligand gating by gamma-aminobutyric acid and acetylcholine arising later in evolution.

Chemical modification of rat cerebral cortex M1 muscarinic receptors: role of histidyl residues in antagonist and agonist binding

Chemical modification of muscarinic M1 receptors in a synaptoneurosomal preparation of rat cerebral cortex by a hydrophilic histidyl-group-specific reagent, diethylpyrocarbonate (DEP), reduces the number of [3H]-4NMPB binding sites in a dose-dependent way. The effect can be reversed by hydroxylamine treatment. No such effect is observed when carbethoxylation with 2.5 mM DEP is carried out in the presence of atropine, 4NMPB, pirenzepine or carbachol. These findings indicate that DEP specifically modifies histidyl residue(s) positioned at the binding site in members of the M1 receptor family. However, treatment with 2.5 mM DEP in the presence of various muscarinic ligands significantly disturbs the binding state of agonists. The results suggest that M1 receptors may have more than one histidyl residue of importance in ligand binding.

The striatum and cerebral cortex express different muscarinic receptor mRNAs

The existence of four distinct muscarinic acetylcholine receptor genes (m1-mr) has recently been demonstrated. cDNAs for three of these receptors have been cloned from brain (m1, m3, m4) and one from heart (m2). To gain some understanding of the physiological role of the brain muscarinic receptors, we mapped the distribution of their mRNAs in rat brain by in situ hybridization. These mRNAs are barely detectable in the hindbrain and cerebellum. Within forebrain, each mRNA has a strikingly different pattern of distribution. The highest levels of m1 mRNA are in the cerebral cortex and hippocampus followed by the striatum. m3 mRNA is also prominent in the cerebral cortex, but has very low levels in the striatum. Conversely, the levels of m4 mRNA are highest in the striatum. Since the cognitive effects of muscarinic drugs have been localized to the cerebral cortex and hippocampus, and their psychomotor effects to the striatum, these data suggest that the muscarinic receptors which subserve these responses may be different gene products. Finally, we show that these muscarinic receptors can be distinguished pharmacologically, suggesting that it may be possible to develop drugs for the selective treatment of the psychomotor vs cognitive difficulties of Parkinson's and Alzheimer's disease, respectively.

Identification and sequence of a binding site peptide of the beta 2-adrenergic receptor

p-(Bromoacetamido)benzyl-1-[125I]iodocarazolol (125I-pBABC) is a potent derivative of the beta-adrenergic receptor antagonist p-aminobenzylcarazolol. Treatment of the receptor with 125I-pBABC results in efficient covalent incorporation of the ligand into the receptor binding site. Extensive degradation of 125I-pBABC-labeled beta 2-adrenergic receptor with either cyanogen bromide or Staphylococcus aureus V8 protease results in specifically labeled fragments having Mr's of about 1600 and 3500, respectively. Because the primary structure of the beta 2-adrenergic receptor is known, and these proteolytic reagents are highly sequence specific, the site of 125I-pBABC incorporation may be deduced from the sizes of the specifically labeled fragments. Thus the fragment generated by cyanogen bromide cleavage corresponds to residues 83-96, a region of 14 amino acids included in the second membrane spanning domain (helix II) of the beta 2-adrenergic receptor. This assignment was confirmed by direct amino acid sequencing of this labeled fragment, though the actual amino acid modified could not be determined. These data permit the assignment of a part of the hormone binding region of the beta 2-adrenergic receptor.

Temperature effect on the detection of muscarinic receptor-G protein interactions in ligand binding assays

The ability of guanine nucleotides to lower agonist binding affinity provides a convenient indication of receptor-G protein coupling: guanine nucleotides convert muscarinic receptors from high-affinity states for agonists to low-affinity states. We studied the influence of assay temperature on the demonstration of this coupling in rat brainstem and atrium. Agonist affinity of brainstem receptors increased as temperature was lowered, reflecting a greater proportion of receptors in high-affinity conformations. The influence of 5'-guanylylimidodiphosphate, a stable analog of GTP, on agonist binding, determined directly (using [3H]oxotremorine-M) or indirectly (in [3H]N-methylscopolamine/carbamylcholine competition studies), was greatest from 16 to 20 degrees. Guanine nucleotide sensitivity was much reduced at 0-4 degrees and 37 degrees. Brainstem and atrial muscarinic receptors were similarly affected by temperature. We suggest that high-affinity receptor-G protein complexes are unstable at high temperatures, thereby decreasing agonist affinity and masking the guanine nucleotide effect. At low temperatures, the receptor-G protein complex is stabilized and fails to dissociate in the presence of guanine nucleotides. The optimum temperature for monitoring receptor-G protein interactions in binding assays was 16-20 degrees.

Different binding properties of muscarinic M2-receptor subtypes for agonists and antagonists in porcine gastric smooth muscle and mucosa

The hypothesis that muscarinic receptors on smooth muscle differ from those in epithelial glands was tested by comparing the properties of muscarinic binding sites in porcine fundic smooth muscle with those in mucosal membranes. The binding of agonists and of antagonists was assessed by displacement of [3H]N-methylscopolamine. Pirenzepine (M1-antagonist) labeled low-affinity binding sites in smooth muscle (KD = 229 nM) and in mucosa (KD = 124 nM) consistent with the presence of M2 sites. Carbachol interacted with a high-affinity (KD = 164 nM) and a low-affinity (KD = 18.2 microM) state of binding sites in smooth muscle. Guanyl 5'-yl-imidodiphosphate converted all sites to the low-affinity state. N-ethylmaleimide pretreatment increased the affinity of carbachol and the proportion of high-affinity sites. In clear contrast, only low-affinity sites of carbachol were detectable in mucosa (KD = 17 microM) that were not modulated by N-ethylmaleimide or guanyl 5'-yl-imidodiphosphate. The cardioselective antagonist AF-DX 116 displayed low affinity to mucosal binding sites (KD = 3.4 microM), whereas its affinity to smooth muscle was 503 nM. The antagonist hexahydro-sila-difenidol had a very high affinity (KD = 2.9 nM) to mucosal receptors, whereas its affinity to smooth muscle sites was 88 nM. These data show that muscarinic M2 binding sites in mucosa and smooth muscle can be distinguished by both agonist and antagonist binding experiments, and suggest the existence of different subtypes of M2-binding sites in these tissues.

Sialic acid is selectively involved in the interaction of agonists with M2 muscarinic acetylcholine receptors

Neuraminidase and slight acid hydrolysis were used to investigate the role of sialic acid residues in the binding of muscarinic agonists and antagonists to membranes from tissues rich in M1 and M2 receptors. Membranes were pretreated with neuraminidase at pH 5 and the binding parameters were determined from competitive experiments with (3H)-quinuclidinylbenzylate. The removal of sialic acid residues reduced the affinity of muscarinic agonists for cerebellum, heart and lung membranes (M2), in contrast to striatum (M1). The affinity of antagonists was not affected. Thus, sialic acid is selectively involved in the interaction of agonists with M2 muscarinic receptors.

Screening of cDNA libraries with oligonucleotides as applied to signal transducing G proteins, receptors and effectors

Screening of cDNA libraries constructed in phage or plasmids with oligonucleotide probes has become one of the preferred cloning techniques with the least number of false positive failures. In this article we present our current protocols for designing the procedure to detect cDNA inserts and isolate them. We illustrate with primary screens for G protein subunits and membrane receptors.

Cloning of the cDNA and genes for the hamster and human beta 2-adrenergic receptors

The adenylate cyclase-stimulatory beta 2-adrenergic receptor has been purified to apparent homogeneity from hamster lung. Partial amino acid sequence obtained from isolated CNBr peptides was used to clone the gene and cDNA for this receptor. The predicted amino acid sequence for the hamster beta 2-adrenergic receptor revealed that the protein consists of a single polypeptide chain of 418 aa with consensus N-glycosylation and phosphorylation sites predicted by previous in vitro data. The most striking feature of the receptor protein however, is that it contains seven stretches of hydrophobic residues similar to the proposed seven transmembrane segments of the light receptor rhodopsin. Significant amino acid homology (30-35%) can be found between the hamster beta 2-adrenergic receptor and rhodopsin within these putative membrane spanning regions. Using a hamster beta 2-adrenergic receptor probe, the gene and cDNA for the human beta 2-adrenergic receptor were isolated, revealing a high degree of homology (87%) between the two proteins from different species. Unlike the genes encoding the family of opsin pigments, of which rhodopsin is a member, the genes encoding both hamster and human beta 2-adrenergic receptors are devoid of introns in their coding as well as 5' and 3' untranslated nucleotide sequences. The cloning of the genes and the elucidation of the aa sequences for these G-protein coupled receptors should help to determine the structure-function as well as the evolutionary relationship of these proteins.

Distinct primary structures, ligand-binding properties and tissue-specific expression of four human muscarinic acetylcholine receptors

To investigate the molecular basis for the diversity in muscarinic cholinergic function, we have isolated the genes encoding the human M1 and M2 muscarinic receptors (mAChR) as well as two previously undiscovered mAChR subtypes, designated HM3 and HM4. The amino acid sequence of each subtype reflects a structure consisting of seven, highly conserved transmembrane segments and a large intracellular region unique to each subtype, which may constitute the ligand-binding and effector-coupling domains respectively. Significant differences in affinity for muscarinic ligands were detected in individual mAChR subtypes produced by transfection of mammalian cells. Each subtype exhibited multiple affinity states for agonists; differences among subtypes in the affinities and proportions of such sites suggest the capacity of mAChR subtypes to interact differentially with the cellular effector-coupling apparatus. Subtype-specific mRNA expression was observed in the heart, pancreas and a neuronal cell line, indicating that the regulation of mAChR gene expression contributes to the differentiation of cholinergic activity.

Reconstitution of the purified porcine atrial muscarinic acetylcholine receptor with purified porcine atrial inhibitory guanine nucleotide binding protein

Purified porcine atrial muscarinic receptor (mAcChR) was reconstituted with purified porcine atrial inhibitory guanine nucleotide binding protein (Gi) in a lipid mixture consisting of phosphatidylcholine, phosphatidylserine, and cholesterol (1:1:0.1 w/w). 5'-Guanylyl imidodiphosphate (0.1 mM) had no effect on the binding of the muscarinic antagonist L-quinuclidinyl benzilate but converted high-affinity carbachol binding sites (Kd equal to 1 microM) in the reconstituted preparation to the low-affinity state (Kd equal to about 100 microM). Steady-state kinetic measurements of GTPase activity showed that the turnover number was increased from 0.19 min-1 in the presence of the muscarinic antagonist L-hyoscyamine to 2.11 min-1 for the agonist carbachol. The affinity of Gi for GDP was reduced by about 50-fold upon interaction with the carbachol-mAcChR complex, and the observed rate constant for GDP dissociation was increased by 38-fold from 0.12 to 4.5 min-1. Thus, the increase in steady-state GTPase activity observed for muscarinic agonists is largely, if not exclusively, due to the increase in GDP dissociation from Gi--probably the rate-limiting step in the steady-state mechanism. Carbachol-stimulated GTPase was sensitive to ADP-ribosylation of the reconstituted Gi by pertussis toxin, but the high-affinity agonist binding was uncoupled only when the reconstituted preparation was treated with pertussis toxin in the presence of GTP and the agonist acetylcholine. These results suggest that association with the mAcChR protects Gi from ADP-ribosylation by pertussis toxin.

Phosphorylation of the porcine atrial muscarinic acetylcholine receptor by cyclic AMP dependent protein kinase

cAMP-dependent protein kinase, protein kinase C, cGMP-dependent protein kinase, smooth muscle myosin light-chain kinase, and phosphorylase kinase were examined with respect to their ability to phosphorylate porcine atrial muscarinic receptors (mAcChRs). Experiments were performed both in detergent solution and in a reconstituted system containing the mAcChR alone or in the presence of the purified porcine atrial inhibitor guanine nucleotide binding protein (Gi). Only cAMP-dependent protein kinase was capable of phosphorylating the receptor under any of the experimental conditions examined. Phosphorylation of the mAcChR in the detergent-solubilized state resulted in a loss of ligand binding sites that was reversible upon treatment with calcineurin in the presence of calcium and calmodulin. Upon reconstitution, the apparent stoichiometry of phosphorylation was increased by about 15-fold. Carbachol-stimulated covalent incorporation of phosphate was found only in the reconstituted system in the presence of Gi, suggesting that the large agonist-stimulated increase in phosphorylation observed in vivo [Kwatra, M. M., & Hosey, M. M. (1986) J. Biol. Chem. 261, 12429-12432] may in part result from a unique receptor conformation that occurs upon association with this protein. Ligand binding studies indicated that phosphorylation of the mAcChR in the detergent-solubilized or reconstituted state did not affect its interaction with carbachol or L-quinuclidinyl benzilate in vitro. Carbachol-induced stimulation of the GTPase activity of Gi in the reconstituted system was also unaffected by phosphorylation.

Antimuscarinic action of methoctramine, a new cardioselective M-2 muscarinic receptor antagonist, alone and in combination with atropine and gallamine

The antimuscarinic effects of methoctramine (N,N'-bis[6- [(2-methoxybenzyl)amino]hexyl]-1,8-octanediamine tetrahydrochloride) were investigated in vitro in isolated paced left (force) and spontaneously beating right (force and rate) atria of guinea pigs as well as ileum of guinea pig and rat. Methoctramine was a potent competitive antagonist of M-2 muscarinic receptors in myocardium and pacemaker cells over a wide range of concentrations. The pA2 values ranged from 7.74 to 7.93. They were not significantly different in the two cardiac preparations and were independent of the agonist used (muscarine and carbachol). A combination of methoctramine with atropine resulted in addition of the dose ratios for left atria, which is expected for two antagonists interacting competitively with the same receptor site. In contrast, a combination of methoctramine with gallamine produced a less than additive shift of the dose-response curve for carbachol, confirming that gallamine acts as an allosteric antagonist at cardiac muscarinic receptors. Methoctramine was 54 to 132-fold less potent in ileal than in atrial preparations (pA2 values ranging from 5.81 to 6.20) which makes it the most cardioselective antimuscarinic agent now available. A combination of methoctramine with atropine gave a slight supra-additive antagonism on guinea pig ileum, which suggests that methoctramine interacts to some extent with a second independent site. These results strongly reinforce the view that M-2 muscarinic receptors are not a homogeneous population.

Primary structure of rat cardiac beta-adrenergic and muscarinic cholinergic receptors obtained by automated DNA sequence analysis: further evidence for a multigene family

Two cDNA clones, lambda RHM-MF and lambda RHB-DAR, encoding the muscarinic cholinergic receptor and the beta-adrenergic receptor, respectively, have been isolated from a rat heart cDNA library. The cDNA clones were characterized by restriction mapping and automated DNA sequence analysis utilizing fluorescent dye primers. The rat heart muscarinic receptor consists of 466 amino acids and has a calculated molecular weight of 51,543. The rat heart beta-adrenergic receptor consists of 418 amino acids and has a calculated molecular weight of 46,890. The two cardiac receptors have substantial amino acid homology (27.2% identity, 50.6% with favored substitutions). The rat cardiac beta receptor has 88.0% homology (92.5% with favored substitutions) with the human brain beta receptor and the rat cardiac muscarinic receptor has 94.6% homology (97.6% with favored substitutions) with the porcine cardiac muscarinic receptor. The muscarinic cholinergic and beta-adrenergic receptors appear to be as conserved as hemoglobin and cytochrome c but less conserved than histones and are clearly members of a multigene family. These data support our hypothesis, based upon biochemical and immunological evidence, that suggests considerable structural homology and evolutionary conservation between adrenergic and muscarinic cholinergic receptors. To our knowledge, this is the first report utilizing automated DNA sequence analysis to determine the structure of a gene.

The next frontier in the molecular biology of the opioid system. The opioid receptors

The analgesic and euphoric properties of some plant alkaloids such as morphine have been known and exploited for centuries. In contrast, only during the last twenty years have we begun to unravel the molecular basis by which opiates exert their effects, mechanisms important to our general understanding of the nervous system. The analgesic response to opiates is the result of a cascade of biochemical events that are triggered by the interaction of the opiate with specific macromolecular components found on the membranes of nervous system tissues, the opioid receptors. The endogenous ligands of these receptors are small peptides, the opioid peptides. Although much has been learned about the structures and the mode of synthesis of the opioid peptides, little is understood about the structure of their receptors. The application of molecular genetic techniques was of great importance to the studies of the opioid peptides. It is now expected that this same technology will unravel the physical mysteries of the opioid receptors.

Molecular studies of the neuronal nicotinic acetylcholine receptor family

Nicotinic acetylcholine receptors on neurons are part of a gene family that includes nicotinic acetylcholine receptors on skeletal muscles and neuronal alpha bungarotoxin-binding proteins that in many species, unlike receptors, do not have an acetylcholine-regulated cation channel. This gene superfamily of ligand-gated receptors also includes receptors for glycine and gamma-aminobutyric acid. Rapid progress on neuronal nicotinic receptors has recently been possible using monoclonal antibodies as probes for receptor proteins and cDNAs as probes for receptor genes. These studies are the primary focus of this review, although other aspects of these receptors are also considered. In birds and mammals, there are subtypes of neuronal nicotinic receptors. All of these receptors differ from nicotinic receptors of muscle pharmacologically (none bind alpha bungarotoxin, and some have very high affinity for nicotine), structurally (having only two types of subunits rather than four), and, in some cases, in functional role (some are located presynaptically). However, there are amino acid sequence homologies between the subunits of these receptors that suggest the location of important functional domains. Sequence homologies also suggest that the subunits of the proteins of this family all evolved from a common ancestral protein subunit. The ligand-gated ion channel characteristic of this superfamily is formed from multiple copies of homologous subunits. Conserved domains responsible for strong stereospecific association of the subunits are probably a fundamental organizing principle of the superfamily. Whereas the structure of muscle-type nicotinic receptors appears to have been established by the time of elasmobranchs and has evolved quite conservatively since then, the evolution of neuronal-type nicotinic receptors appears to be in more rapid flux. Certainly, the studies of these receptors are in rapid flux, with the availability of monoclonal antibody probes for localizing, purifying, and characterizing the proteins, and cDNA probes for determining sequences, localizing mRNAs, expressing functional receptors, and studying genetic regulation. The role of nicotinic receptors in neuromuscular transmission is well understood, but the role of nicotinic receptors in brain function is not. The current deluge of data using antibodies and cDNAs is beginning to come together nicely to describe the structure of these receptors. Soon, these techniques may combine with others to better reveal the functional roles of neuronal nicotinic receptors.

A novel subtype of muscarinic receptor identified by homology screening

A new member of the protein superfamily of G-protein coupled receptors has been isolated by homology screening. By virtue of its homology with other muscarinic acetylcholine receptors and its ability to bind muscarinic specific antagonists, this muscarinic receptor subtype is designated M4. The M4 mRNA is preferentially expressed in certain brain regions. The existence of multiple receptor subtypes encoded by distinct genes in the brain has functional implications for the molecular mechanisms underlying information transmission in neuronal networks.