Distinct sequence elements control the specificity of G protein activation by muscarinic acetylcholine receptor subtypes

Muscarinic signaling pathway for calcium release and calcium-activated chloride current in smooth muscle

We investigated the muscarinic activation of Ca(2+)-activated Cl- currents [ICl(Ca)] in voltage-clamped equine tracheal myocytes. The threshold of cytosolic free Ca2+ concentration ([Ca2+]i) required for activation of ICl(Ca) was 202 +/- 22 nM, and full activation of the current occurred at 771 +/- 31 nM. Hexahydro-sila-difenidol (M3 antagonist) inhibited the methacholine-induced phasic [Ca2+]i increase and ICl(Ca) in a concentration-dependent manner, whereas methoctramine (M2 antagonist) only slightly attenuated the [Ca2+]i increase and ICl(Ca) (14.8 and 21.4%, respectively), consistent with incomplete selectivity. Dialysis of heparin (10 mg/ml) blocked methacholine-induced [Ca2+]i and ICl(Ca) but had no effect on the caffeine-induced Ca2+ release or ICl(Ca); inositol 1,4,5-trisphosphate (100 microM) induced ICl(Ca) and blocked the methacholine current. Conversely, ruthenium red (50 microM) prevented the caffeine-induced [Ca2+]i release and ICl(Ca) but had no effect on methacholine-induced [Ca2+]i or current. Intracellular dialysis of the calmodulin antagonist N-(6-aminohexyl)-1-naphthalenesulfonamide (W-7, 500 microM) or the Ca2+/calmodulin-dependent protein kinase inhibitor KN93 (5 microM) had no effect on the [Ca2+]i increase or ICl(Ca). Pertussis toxin (0.5 mg/ml) did not affect the increase in [Ca2+]i or ICl(Ca). Dialysis with antibodies directed against the alpha-subunit of Gq/G11 (Gq alpha/ G alpha 11) blocked the methacholine-induced ICl(Ca) in a concentration-dependent manner, whereas anti-G alpha i-1/G alpha 1-2 antibodies (1:35) and anti-G alpha i-3/G(o) alpha antibodies (1:35) were without effect. The results indicate that stimulation of phospholipase C via M3/Gq proteins is the predominant signaling pathway for the activation of ICl(Ca); at high agonist concentrations, Ca(2+)-induced Ca2+ release does not appear to play a prominent role in muscarinic signaling.

A site-directed mutagenesis study on the conserved alanine residue in the distal third intracellular loops of cholecystokininB and neurotensin receptors

1. An alanine residue at the C-terminal tail of the third intracellular loop is highly conserved among various Gq protein-coupled receptors including rat cholecystokininB (CCKB) and neurotensin receptors. To investigate the functional significance of the conserved alanine in the activation of Gq proteins and phospholipase C (PLC) by CCKB and neurotensin receptors, the alanine residue was mutated in the present study. Subsequently, the ability of resulting mutant receptors to activate PLC was investigated by measuring the formation of inositol phosphates (IP) in COS-7 cells and recording Ca(2+)-activated chloride currents from Xenopus oocytes. 2. Site-directed mutagenesis was performed to mutate alanine at position 332 of rat CCKB receptor to glutamate. When the (A332E) mutant receptor was expressed in COS-7 cells and Xenopus oocytes, the efficacy and the potency of sulphated cholecystokinin octapeptide (CCK-8) to stimulate polyphosphoinositide hydrolysis in COS-7 cells and evoke calcium-dependent Cl- currents in oocytes were not significantly affected. 3. Alanine residue at position 302 of rat neurotensin receptor was also mutated to glutamate. When expressed in COS-7 cells and Xenopus oocytes, the resulting (A302E) mutant receptor was strongly defective in stimulating phosphatidylinositol turnover in COS-7 cells and evoking Ca(2+)-dependent chloride currents in oocytes. 4. In summary, the present study demonstrates that alanine residue at the C-terminus of third cytoplasmic domain is required for the full activation of Gq proteins and PLC by neurotensin receptors. However, in contrast to other Gq protein-coupled receptors, alanine at the distal third intracellular loop does not play a significant role in CCKB receptor activation of PLC.

Differential cholinoceptor subtype-dependent activation of signal transduction pathways in neonatal versus adult rat atria

In this study, we investigated the expression and distribution of muscarinic cholinergic receptors (mAChRs) and the different signaling pathways associated with mAChR activation in atria isolated from adult and neonatal rats. Carbachol stimulation of mAChRs in both neonatal and adult rat atria led to a negative inotropic response with activation of phosphoinositide hydrolysis, an increase in cyclic GMP levels, and a decrease in cyclic AMP production. However, compared with adult atria, neonatal atria showed hypersensitivity in the contractile effect induced by carbachol. Pharmacological analysis with mAChR antagonists indicated that M1 and M2 mAChR subtypes are important mediators of the response to carbachol in neonatal atria. In contrast, in adult atria the effect of the agonist was coupled only to the M2 mAChR subtype. Moreover, an increased number of total mAChRs was labeled in neonatal atrial membranes compared with those of adults. Although a predominant M2 mAChR population is expressed in atria at both stages of development studied, competition binding parameters calculated for carbachol indicated the presence of high-affinity binding sites, with higher affinity in neonates than in adults. These results suggest that the differences observed between neonatal and adult atria in their response to a cholinergic agonist may be related to differential expression of mAChR subtypes and/or changes in functional coupling of mAChR subtypes during development.

The thrombin receptor second cytoplasmic loop confers coupling to Gq-like G proteins in chimeric receptors. Additional evidence for a common transmembrane signaling and G protein coupling mechanism in G protein-coupled receptors

Thrombin activates human platelets and other cells in part by cleaving an unusual G protein-coupled receptor. Thrombin cleavage of this receptor's amino-terminal exodomain unmasks a new amino terminus. This then binds intramolecularly to the body of the receptor to trigger transmembrane signaling and activation of Gi- and Gq-like G proteins. Toward identifying the domains responsible for thrombin receptor-G protein interactions, we examined the signaling properties of chimeric receptors in which thrombin receptor cytoplasmic sequences replaced the cognate sequences in the Gs-coupled beta2-adrenergic receptor (beta2AR) or the Gi-coupled dopamine D2 receptor (D2R). In Xenopus oocytes, a chimeric beta2AR bearing the thrombin receptor second cytoplasmic (C2) loop gained the ability to trigger intracellular Ca2+ release in response to adrenergic agonist, whereas a beta2AR bearing the cognate C2 loop from the D2R did not. Similarly, in COS-7 cells, a chimeric D2R bearing the thrombin receptor C2 loop gained the ability to trigger phosphoinositide hydrolysis in response to dopaminergic agonist, apparently by coupling to a Gq-like G protein. No detectable Gs coupling was seen. Thus, the thrombin receptor C2 loop was able to confer Gq-like coupling in several different receptor contexts. These observations suggest that the thrombin receptor C2 loop specifies Gq coupling by directly contacting Gq or by contributing to a structure required for Gq coupling. The ability of the thrombin receptor C2 loop to function in the context of the D2R and beta2AR strongly suggests that the transmembrane switching and G protein activation strategies used by the thrombin receptor must be very similar to those used by the D2R and beta2AR despite the thrombin receptor's strikingly different liganding mechanism.

Identification of A2a adenosine receptor domains involved in selective coupling to Gs. Analysis of chimeric A1/A2a adenosine receptors

Responses to adenosine are governed by selective activation of distinct G proteins by adenosine receptor (AR) subtypes. The A2aAR couples via Gs to adenylyl cyclase stimulation while the A1AR couples to Gi to inhibit adenylyl cyclase. To determine regions of the A2aAR that selectively couple to Gs, chimeric A1/A2aARs were expressed in Chinese hamster ovary cells and ligand binding and adenylyl cyclase activity analyzed. Replacement of the third intracellular loop of the A2aAR with that of the A1AR reduced maximal adenylyl cyclase stimulation and decreased agonist potency. Restricted chimeras indicated that the NH2-terminal portion of intracellular loop 3 was predominantly responsible for this impairment. Reciprocal chimeras composed primarily of A1AR sequence with limited A2aAR sequence substitution stimulated adenylyl cyclase and thus supported these findings. A lysine and glutamic acid residue were identified as necessary for efficient A2aAR-Gs coupling. Analysis of chimeric receptors in which sequence of intracellular loop 2 was substituted indicated that the nature of amino acids in this domain may indirectly modulate A2aAR-Gs coupling. Replacement of the cytoplasmic tail of the A2aAR with the A1AR tail did not affect adenylyl cyclase stimulation. Thus, selective activation of Gs is predominantly dictated by the NH2-terminal segment of the third intracellular loop of the A2aAR.

Structural basis of receptor/G protein coupling selectivity studied with muscarinic receptors as model systems

Different muscarinic acetylcholine receptor subtypes were used as model systems to study the structural basis of receptor/G protein coupling selectivity. Extensive mutagenesis studies have previously led to the identification of single amino acids on the m3 muscarinic receptor protein (located in the second intracellular loop (i2) and at the N- and C-terminus of the third intracellular loop (i3)) that dictate selective recognition of Gq/11 proteins by this receptor subtype. Based on these results, we proposed a model of the intracellular m3 receptor surface in which the functionally critical residues project into the interior of the transmembrane receptor core. To identify specific regions on the G protein(s) that are contacted by these different, functionally critical receptor sites, we recently employed a novel experimental strategy involving the coexpression of hybrid m2/m3 muscarinic receptors with hybrid G alpha-subunits. Using this approach, we could demonstrate that the C-terminus of G protein alpha i/o-subunits is recognized by a short sequence element in the m2 muscarinic receptor ("VTIL") that is located at the junction between the sixth transmembrane domain (TM VI) and the i3 loop. We could show that this interaction is critically involved in determining coupling selectivity and triggering G protein activation. By using a similar strategy (coexpression of mutant muscarinic receptors with hybrid G alpha-subunits), other major receptor/G protein contact sites are currently being identified. These studies, complemented by biochemical and biophysical approaches, should eventually lead to a detailed structural model of the ligand-receptor-G protein complex.

Subunit stoichiometry of a heteromultimeric G protein-coupled inward-rectifier K+ channel

We investigated the stoichiometry of the heteromultimeric G protein-coupled inward-recitfier K+ channel (GIRK) formed from GIRK1 and GIRK4 subunits. Multimeric GIRK constructs with several concatenated channel subunits were expressed in Xenopus oocytes. Coexpression of various trimeric constructs with different monomers clearly showed that the functional channel has stoichiometry (GIRK1)2(GIRK4)2. Efforts to establish a preferred arrangement of subunits around the channel pore suggest that more than one arrangement may be viable.

AR4-2J cells: a model to study polypeptide hormone receptors

The AR4-2J cell line is derived from a transplantable tumour of the exocrine rat pancreas. Acinar in origin, this cell line contains significant amounts of amylase and can be grown in continuous culture. Many in vitro studies have been done using these cells; these studies were often complemented with in vivo experiments on animals. Particularly, many polypeptide hormones interacting with specific receptors located on the cell membrane have been analysed. The accurate knowledge of the hormone-receptor interactions has allowed to design interesting analogs of these hormones. In several cases, these compounds are powerful antagonists and are able to control cell proliferation induced by the corresponding polypeptide hormones. Other cell lines are useful to understand human pancreatic cancer. These human cell lines (Capan 1, Panc-1 for example) are of ductal origin and differ from AR4-2J cells, especially regarding the distribution of several polypeptide hormone and growth factor receptors. Both models are important for basic studies of neuropeptides, gastrointestinal peptides and their receptors, as well as for a better understanding of the underlying mechanisms of human pancreatic cancer.

Chimeric mutagenesis of putative G-protein coupling domains of the alpha2A-adrenergic receptor. Localization of two redundant and fully competent gi coupling domains

We have investigated potential Gi and Gs coupling domains within the intracellular regions of the alpha2AAR subtype using a series of nine chimeric mutations. The second intracellular loop (ICL2, amino acids 133-149) and the amino- and carboxyl-terminal regions of the third intracellular loop (ICL3, amino acids 218-235 and 355-371, respectively) of the cloned human alpha2AAR were substituted with the analogous sequence from either the Gs-coupled beta2AR or the Gi-coupled serotonin type 1A receptor (5-HT1AR). Mutant and wild type alpha2AAR were stably expressed in Chinese hamster ovary cells and functional coupling of each receptor to Gi and Gs was assessed in membrane adenylyl cyclase assays. Substitution of 5-HT1AR sequence into ICL2 ablated coupling to Gs but not to Gi, whereas substitution of beta2AR sequence significantly depressed coupling to Gi but not to Gs. Thus, the ICL2 of the alpha2AAR contains elements essential for both signaling pathways. Substitution of either the amino- or carboxyl-terminal segments of ICL3 with 5-HT1AR sequence ablated agonist stimulation of adenylyl cyclase activity (without affecting inhibition), suggesting that both domains are necessary for alpha2AAR coupling to Gs. In contrast, individual substitution of beta2AR sequence into ICL3 amino or carboxyl termini had no appreciable effect on Gi coupling. Concomitant substitution of beta2AR sequence into both regions substantially impaired Gi coupling, implying that each is capable of independently supporting functional coupling. Substitution of 5-HT1AR at either locus had no effect on Gi coupling. Thus, for Gs coupling, these two domains within ICL3 are both required for functional coupling. However, for Gi coupling, the alpha2AAR appears to have two distinct regions within ICL3 that are capable of supporting Gi coupling independently. There has been no previous elucidation of a receptor having redundant, fully competent domains for coupling to a single class of G-protein. Such duplicity of functional domains within alpha2AR may suggest strong evolutionary pressure to maintain Gi coupling.

Muscarinic receptors in developing rat colon

Muscarinic receptor subtypes were characterized in fetal (21 day), newborn (3 day), and adult (3 month) rat colon smooth muscle. Saturation binding of the nonselective muscarinic antagonist radioligand [3H]quinuclidinyl benzilate revealed a single class of binding sites in all three age groups. The binding affinities of [3H]quinuclidinyl benzilate were not significantly different among three age groups (KD: 0.19-->0.27 nM). In contrast, the receptor densities (Bmax, fmol/mg protein) showed a significant age-related decrease with fetus (518.9 +/- 7.4) > newborn (480.3 +/- 45.6) >> adult (192.4 +/- 32.8). In both newborn and adult tissues, the muscarinic agonist carbachol bound to two sites with high and low affinities. Although the agonist binding affinities in the newborn tissue were not significantly different from those in the adult tissue, the high-affinity binding sites for carbachol were significantly increased in the later (41%-->61%). Addition of guanosine-5'-O-(3-thio)triphosphate (100 microM) abolished apparent high-affinity binding sites in both newborn and adult tissues. Antagonist competition binding in the newborn tissue indicated a homogeneous population of muscarinic M2 receptors. Unlike in newborn tissues, the heterogeneous binding of pirenzepine and 4-diphenylacetoxy-N-methylpiperidine methobromide in adult tissues revealed coexistence of muscarinic M3 (45%) and M2 (55%) receptors. In accordance, activation of muscarinic receptors in the adult tissue stimulated synthesis of inositol 1,4,5-trisphosphate. These results suggest maturational changes of muscarinic receptor subtypes and their coupling to G proteins in rat colonic smooth muscle. These changes may account, at least in part, for developmental alterations of functional responses in colonic smooth muscle.

The third intracellular domain of the m3 muscarinic receptor determines coupling to calcium influx in transfected Chinese hamster ovary cells

The m2 and m3 muscarinic acetylcholine receptors were expressed in CHO cells and were shown to couple to the release of calcium from intracellular stores. The m3 receptor, but not the m2 receptor, also coupled to calcium influx. Chimeric m2/m3 receptors were used to determine the structural domain of the m3 receptor linked to the regulation of calcium influx. It was found that the third intracellular loop of m3 receptor plays a fundamental role in regulating Ca2+ influx predicted to occur through Ca2+ channels located in the plasma membrane in CHO cells.

Specificity of coupling of muscarinic receptor isoforms to a novel chick inward-rectifying acetylcholine-sensitive K+ channel

The G-protein-gated inward-rectifying K+ channel GIRK1 has been demonstrated in heart and brain. These tissues also both express the M2, M3, and M4, muscarinic acetylcholine receptors (mAChR) (Gadbut, A.P., and Galper, J.B. (1994),J. Biol. Chem. 269,25823-25829). Only the M2 mAChR has been demonstrated to couple to GIRK1 (Kubo, Y., Reuveny, E., Slesinger, P. A., Jan, Y. N., and Jan, L. Y. (1993) Nature 264, 802-806). In this study we determined the specificity of coupling of the M3 and M4 mAChR to a new GIRK1 cloned from a chick brain cDNA library. This clone codes for a 492-amino acid protein that is 93% identical to rat GIRK1 and is expressed in brain, atrium, and ventricle, but not skeletal muscle. In Xenopus laetis oocytes co-expression of GIRK1 with either the chick M2 or M4 mAChR gave carbamylcholine (10 microm)-stimulated K+ currents of 308 +/-26 nA and 298 +/-29 nA, respectively, which were both Ba2+- and pertussis toxin-sensitive. Activation of the M3 receptor produced 2382 +/-478 nA of current which was insensitive to Ba2+ and pertussis toxin, but was 85% inhabitable by the Cl channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (10-20 microm) consistent with coupling to an endogenous Ca2+-activated Cl- channel via a phosphatidylinositol-dependent mechanism. Co-expression of the cardiac inward rectifier CIR with chick M2 or M4 mAChR and GIRK1 increased currents more than 10-fold, but had no effect on specificity of coupling. These data demonstrate a new function for the M4 mAChR and a high degree of specificity for coupling of each receptor subtype to GIRK1.

Constitutive activation of chimeric m2/m5 muscarinic receptors and delineation of G-protein coupling selectivity domains

To derive structure/function relationships for muscarinic receptor/G-protein coupling, the m2 and m5 muscarinic receptors and a series of m2/m5 chimeras were tested for agonist binding and functional responses in a cellular proliferation/transformation assay. m5, which mediates stimulation of phosphatidylinositol turnover, displayed robust activity in the proliferation assay, whereas m2, which mediates inhibition of adenylyl cyclase, was inactive in the proliferation assay. Chimeras that contained m2 sequences in the i2 or i3 loops had impaired activity or were inactive, respectively. Chimeras that contained m2 segments reaching from the N-terminus to TM2, or from TM6 to the C-terminus, had enhanced activity relative to m5, and a chimera with both of these elements was constitutively activated.

Identification of a region required for subtype-specific agonist-induced sequestration of the m2 muscarinic acetylcholine receptor

When the m1 and m2 muscarinic acetylcholine receptors are transiently expressed in JEG-3 cells, the m2, but not the m1, receptor undergoes agonist-induced sequestration. Both receptors exhibit internalization when expressed in Y1 cells. These results suggest that the m1 and m2 receptors use distinct cellular mechanisms or pathways for agonist-induced internalization and that JEG-3 cells are deficient in the mechanism or pathway used by the m1 receptor. Transfection experiments with chimeric receptors indicate that the specificity for agonist-induced internalization for the m2 receptor lies in the carboxyl-terminal fifth of the receptor. The intracellular carboxyl-terminal tail of the m2 receptor is neither sufficient nor required for the m2-specific sequestration. Site-directed mutagenesis demonstrates that two amino acids in the carboxyl-terminal end of the third cytoplasmic loop of the m2 receptor are required for sequestration in JEG-3 cells. In addition, the sixth transmembrane domain, which is adjacent to this cytoplasmic domain, is also required. Thus, m2-specific agonist-induced sequestration requires sequences both in the carboxyl-terminal end of the third cytoplasmic loop and the adjacent transmembrane domain.

Amino acid side chains that define muscarinic receptor/G-protein coupling. Studies of the third intracellular loop

Amino acids in the third intracellular loops of receptors play pivotal roles in G-protein coupling. To define their structural requirements, we have subjected the N- and C-terminal regions of this loop (Ni3 and Ci3, respectively) of the m5 muscarinic receptor to random saturation mutagenesis. (see Burstein, E. S., Spalding, T. A., Hill-Eubanks, D., and Brann, M. R. (1995) J. Biol. Chem. 270, 3141 3146 and Hill-Eubanks, D., Burstein, E. S., Spalding, T. A., Bräuner-Osborne, H., and Brann, M. R. (1996) J. Biol. Chem. 271, 3058 3065). In the present study, we have extended our analysis of Ni3 by constructing libraries of receptors with all possible amino acid substitutions at the residues we previously identified as functionally important and characterizing their functional phenotypes. Numerous hydrophobic substitutions were well tolerated at Ile216 and Thr220 and caused constitutive activation in two cases, establishing that hydrophobicity is structurally favored at these positions and that many amino acid side chains are compatible with this structural role. Similarly, hydrophobic and polar, but not charged, substitutions were observed at Tyr217, but in contrast to results for Thr220, most substitutions at Tyr217 substantially decreased maximum response and increased the EC50 for carbachol, demonstrating that the specific side chain of residue 217 participates in G-protein coupling. Arg223 allowed the widest range of substitutions of the residues tested, but only basic residues were well tolerated. All other substitutions significantly increased (up to 100-fold) the EC50 for carbachol without significantly affecting maximal response. There were no significant changes in the ligand binding properties of these mutant receptors. We conclude that Ile216 and Thr220 fulfill a structural role, forming the foundation of the G-protein-coupling pocket, whereas Tyr217 and Arg223 contact G-proteins through specific side chain interactions. We propose that G-proteins are recruited to receptors by ionic interactions and that hydrophobic residues participate in activation.

Structure of a G-protein-coupling domain of a muscarinic receptor predicted by random saturation mutagenesis

The third intracellular loop (i3) plays a critical role in the coupling of many receptors to G-proteins. In muscarinic receptor subtypes, the N- and C-terminal regions (Ni3 and Ci3) of this loop are sufficient to direct appropriate G-protein coupling. The relative functional contributions of all amino acids within Ni3 was evaluated by constructing libraries of m5 muscarinic receptors containing random mutations in Ni3 and screening them using high throughput assays based on ligand-dependent transformation of NIH 3T3 cells. In receptors that retained a wild type phenotype, the pattern of functionally tolerated substitutions is consistent with the presence of three turns of an alpha helix extending from the transmembrane domain. All of the amino acid positions that tolerate radical substitutions face away from a conserved hydrophobic face that ends with an arginine, and helix-disrupting proline substitutions were not observed. All of the mutant receptors with significantly compromised phenotypes had amino acid substitutions in residues predicted to form the hydrophobic face. Similar data from the Ci3 region (Burstein, E. S., Spalding, T. A., Hill-Eubanks, D., and Brann, M. R. (1995) J. Biol. Chem. 270, 3141-3146) are consistent with the presence of a single helical turn extending from the transmembrane domain, with an alanine that defines G-protein affinity. Functionally critical residues of Ni3 and Ci3 are predicted to be in close proximity where they form the G-protein-coupling domain.

The second intracellular loop of metabotropic glutamate receptor 1 cooperates with the other intracellular domains to control coupling to G-proteins

Metabotropic glutamate receptors (mGluR) share no sequence homology with any other G-protein-coupled receptors (GPCRs). The characterization of their G-protein coupling domains will therefore help define the general rules for receptor-G-protein interaction. To this end, the intracellular domains of mGluR3 and mGluR1, receptors coupled negatively to adenylyl cyclase and positively to phospholipase C, respectively, were systematically exchanged. The ability of these chimeric receptors to induce Ca2+ signals were examined in Xenopus oocytes and HER 293 cells. The chimeric receptors that still possessed the second intracellular loop (i2) of these proteins were targeted correctly to the plasma membrane. Consistent Ca2+ signals could be recorded only with chimeric mGluR3 receptors that contains i2 and at least one other intracellular domains of mGluR3 have to be replaced by their mGluR1 equivalent to produce optimal coupling to G protein. These observations indicate that i2 of mGluR1 is a critical element in determining the transduction mechanism of this receptor. These results suggest that i2 of mGluRs may play a role similar to i3 of most other GPCRs in the specificity of coupling to the G-proteins. Moreover, as in many other GPCRs, our data revealed cooperation between the different mGluR intracellular domains to control efficient coupling to G-proteins.

Identification of domains conferring G protein regulation on inward rectifier potassium channels

Cardiac m2 muscarinic acetylcholine receptors reduce heart rate by coupling to heterotrimeric (alpha beta gamma) guanine nucleotide-binding (G) proteins that activate IKACh, an inward rectifier K+ channel (IRK). Activation of the GIRK subunit of IKACh requires G beta gamma subunits; however, the structural basis of channel regulation is unknown. To determine which sequences confer G beta gamma regulation upon IRKs, we generated chimeric proteins composed of GIRK and RB-IRK2, a related, G protein-insensitive channel. Importantly, a chimeric channel containing the hydrophobic pore region of RB-IRK2 joined to the amino and carboxyl termini of GIRK exhibited voltage- and receptor-dependent activation in Xenopus oocytes. Furthermore, carboxy-terminal sequences specific to this chimera and GIRK bound G beta gamma subunits in vitro. Thus, G beta gamma may regulate IRKs by interacting with sequences adjacent to the putative channel pore.

Identification of a Gs coupling domain in the amino terminus of the third intracellular loop of the alpha 2A-adrenergic receptor. Evidence for distinct structural determinants that confer Gs versus Gi coupling

alpha2-Adrenergic receptors (alpha 2AR) functionally couple not only to Gi but also to Gs. We investigated the amino-terminal portion of the third intracellular loop of the human alpha 2AAR (alpha 2C10) for potential Gs coupling domains using site-directed mutagenesis and recombinant expression in several different cell types. A deletion mutant and four chimeric receptors consisting of the alpha 2AAR with the analogous sequence from the 5-HT1A receptor (a Gi-coupled receptor) and the beta 2AR (a Gs-coupled receptor) were expressed in Chinese hamster ovary cells, Chinese hamster fibroblasts, or COS-7 cells and examined for their ability to mediate stimulation or inhibition of membrane adenylyl cyclase activity or whole cell cAMP accumulation. In stably expressing Chinese hamster ovary cells, deletion of amino acids 221-231, which are in close proximity to the fifth transmembrane domain, eliminated alpha 2C10-mediated stimulation of adenylyl cyclase activity, while alpha 2C10-mediated inhibition was only moderately affected. This suggested that this region is important for Gs coupling, prompting construction of the chimeric receptor mutants. Substitution of amino acids 218-235 with 5-HT1A receptor sequence entirely ablated agonist-promoted Gs coupling, as compared with a 338 +/- 29% stimulation of adenylyl cyclase activity observed with the wild-type alpha 2C10. In contrast, Gi coupling for this mutant remained fully intact (57 +/- 2% versus 52 +/- 1% inhibition for wild-type alpha 2C10). Similar substitution with beta 2AR sequence had no effect on Gi coupling but did reduce Gs coupling. Two additional mutated alpha 2C10 containing smaller substitutions of the amino-terminal region with 5-HT1A receptor sequence at residues 218-228 or 229-235 were then studied. While Gi coupling remained intact with both mutants, Gs coupling was ablated in the former but not the latter mutant receptor. Similar results were obtained using transfected Chinese hamster fibroblasts (which exclusively display alpha 2AR-Gi coupling) and COS-7 cells (which exclusively display alpha 2AR-Gs coupling). Thus, a critical determinant for Gs coupling is contained within 11 amino acids (218-228) of the amino-terminal region of the third intracellular loop localized directly adjacent to the fifth transmembrane domain. Taken together, these studies demonstrate the presence of a discrete structural determinant for agonist-promoted alpha 2AR-Gs coupling, which is distinct and separable from the structural requirements for alpha 2AR-Gi coupling.

Mapping of single amino acid residues required for selective activation of Gq/11 by the m3 muscarinic acetylcholine receptor

Each G protein-coupled receptor can interact only with a limited number of the many structurally similar G proteins expressed within a cell. This study was undertaken to identify single amino acids required for selectively coupling the m3 muscarinic acetylcholine receptor to G proteins of the Gq/11 family. To this goal, distinct intracellular segments/amino acids of the m3 receptor were systematically substituted into the structurally closely related m2 muscarinic receptor, which couples to Gi/o proteins, not Gq/11 proteins. The resultant mutant receptors were expressed in COS-7 cells and studied for their ability to induce agonist-dependent stimulation of phosphatidylinositol hydrolysis, a response known to be mediated by G proteins of the Gq/11 class. Using this approach, we were able to identify four amino acids in the second intracellular loop and four amino acids at the C terminus of the third intracellular loop of the m3 muscarinic receptor that are essential for efficient Gq/11 activation. We could demonstrate that these amino acids, together with a short segment at the N terminus of the third intracellular loop, fully account for the G protein coupling preference of the m3 muscarinic receptor. Taken together, our data strongly suggest that only a limited number of amino acids, located on different intracellular regions, are required to determine the functional profile of a given G protein-coupled receptor.

Influence of second and third cytoplasmic loops on binding, internalization, and coupling of chimeric bombesin/m3 muscarinic receptors

In order to investigate the molecular basis for differences in the characteristics of bombesin (Bn) and m3 muscarinic cholinergic (m3 ACh) receptors, chimeric Bn receptors possessing cytoplasmic domains from the m3 ACh receptor were produced. The receptors were expressed in CHO-K1 cells and binding, structural, and signal transduction characteristics were analyzed. Cell lines bearing chimeric Bn receptors possessing m3 ACh receptor domains in place of either the second cytoplasmic loop (BM2L), the third cytoplasmic loop (BM3L), or both loops (BM23L) each bound 125I-bombesin with a single affinity that was approximately the same as that of the Bn receptor (5-10 nM). However, Bn receptors possessing the m3 ACh third cytoplasmic loop were severely affected in other respects. Internalization of ligand in Bn and BM2L cells was rapid and extensive (> 80% of bound 125I-bombesin was acid-resistant). In contrast, internalization was dramatically reduced in BM3L and BM23L cells (approximately 20% of bound 125I-bombesin was acid-resistant). In Bn or BM2L cells 10 nM bombesin stimulated approximately 10-fold increases in phosphatidylinositol hydrolysis. Activation of Bn receptors also induced an increase in arachidonic acid release (478 +/- 32% of control, n = 3) and large increases in intracellular Ca2+. In contrast, in BM3L or BM23L cells, bombesin had no significant effect on phosphatidylinositol hydrolysis. Furthermore, BM3L receptor activation did not increase arachidonic acid release. However, BM3L and BM23L cells showed a small increase in intracellular Ca2+ at high concentrations of bombesin. These data indicate that the third cytoplasmic loop alone, or together with the second cytoplasmic loop, was not sufficient to transfer the characteristics of G protein interaction between m3 ACh and bombesin receptors. Furthermore, for the Bn receptor, ligand internalization does, whereas formation of the high affinity binding state does not, appear to require activation of G proteins.

Cholecystokinin and gastrin are not equally sensitive to GTP gamma S at CCKB receptors: importance of the sulphated tyrosine

We have shown that gastrin and cholecystokinin octapeptide (CCK-8) are differently coupled to G protein (GTP-binding protein) through type B cholecystokinin receptors in guinea-pig brain membranes and Jurkat cells. Indeed, the gastrin-13 binding affinity is strongly reduced by stable guanyl nucleotides, whereas CCK-8 binding is only slightly affected. In order to determine the structural requirements regulating such coupling, we have synthesized several gastrin and cholecystokinin fragments (sulphated or unsulphated) elongated at the N-terminus of the common C-terminal tetrapeptide. We investigated their interaction with CCKB receptors in guinea pig brain membranes and Jurkat cells and their involvement in the G protein coupling. Their apparent binding affinities to CCKB receptors were measured by inhibition of [125I]Bolton Hunter-CCK-8 (3-[125I]iodo-4-hydroxyphenyl)propionyl-CCK-8) binding in the presence or absence of GTP gamma S (guanosine 5'-O-(3-thio)triphosphate) or aluminum tetrafluoride (AlF4-). Activation of the G proteins by GTP gamma S or AlF4- led to a decrease in binding affinity for the gastrin related peptides, the common CCK-gastrin C-terminal forms, the cholecystokinin hexapeptide and the unsulphated cholecystokinin heptapeptide. Sulphated CCK-7, CCK-8, and cionin apparent binding affinities were not affected. These finding indicated that the sulphated tyrosine in position 7 in CCK (as counted from the C-terminus), provides the cholecystokinin selectivity for the CCKB receptor compared to gastrin. The results are discussed with the aim to better clarify the physiological relevance of gastrin and cholecystokinin toward CCKB receptors and their related intracellular events.

Identification of a domain in the angiotensin II type 1 receptor determining Gq coupling by the use of receptor chimeras

The angiotensin II type 1 (AT1R) and type 2 (AT2R) receptors belong to the seven transmembrane receptor superfamily. Previous studies have suggested that the AT1R couples to a Gq signaling pathway, whereas the AT2R does not associate with Gq. To identify the role that individual intracellular domains play in AT1R function, AT1R/AT2R chimeric receptors were prepared by substitution of intracellular loops. CHO cells expressing these chimeras were used to test angiotensin II-induced c-fos expression and Ca2+ mobilization which are involved in the AT1R signaling pathway through Gq coupling. Substitution of the second intracellular loop (IC2) and the cytoplasmic tail between the two receptors did not affect AT1R function. However, exchange of the third intracellular loop (IC3) resulted in the loss of function in the AT1R and conferred to the AT2R the ability to constitutively activate the fos promoter. These findings suggest that the third intracellular loop of the AT1R is critical for Gq coupling. Substitution of discrete amino acid sequences of the third intracellular loop indicate that its N-terminal and C-terminal portions, especially the seven amino acids 219-225 in the N-terminal portion, are important for AT1R function, and that the intermediate portion of this loop is not required for Gq coupling.

Structural basis of G protein specificity of human endothelin receptors. A study with endothelinA/B chimeras

The endothelin (ET) family of peptides acts via two subtypes of guanine nucleotide-binding regulatory protein (G protein)-coupled receptors termed ETA and ETB. ET-1 stimulated cAMP formation in Chinese hamster ovary (CHO) cells stably expressing human wild-type ETA (CHO/hETA cells) while it inhibited cAMP formation in CHO cells expressing human wild-type ETB (CHO/hETB cells), and pharmacological evidence indicated that the opposite effects were due to the selective coupling of each receptor subtype with G alpha s/G alpha i. To find out a receptor domain(s) that determined the selective coupling, a series of chimeric receptors between hETA and hETB was expressed on CHO cells, and the effect of ET-1 on cAMP formation in each cell line was tested. hETA with the replacement of second and/or third intracellular loop (ICLII and/or -III) to the corresponding region(s) of hETB failed to transmit the stimulatory effect of ET-1. hETB with the replacement of ICLIII to the corresponding region of hETA failed to transmit the inhibitory effect of ET-1. A chimeric receptor with ICLII of hETB and with ICLIII of hETA failed to transmit both effects. In cells expressing chimeric receptors with ICLII of hETA and with ICLIII of hETB, ET-1 inhibited cAMP formation while it stimulated cAMP formation when cells were pretreated with pertussis toxin. These results indicated the roles of ICLII and -III of hETR as a major determinant of the selective coupling of hETA and hETB with G alpha s/G alpha i, respectively. We also demonstrated that each receptor subtype expressed on the same cell could work independently, i.e. for hETA to activate G alpha s and for hETB to activate G alpha i, resulting in dose-dependent dual effects of ET-1 on cAMP formation.

Different alpha 1-adrenergic receptor sequences required for activating different G alpha subunits of Gq class of G proteins

In order to understand the specific interactions between receptors and guanine nucleotide-binding regulatory protein (G proteins), we attempted to delineate the alpha 1 B-adrenergic receptor sequences involved in activation of the alpha subunits of the Gq class of G proteins. A number of specific mutations were introduced into the third inner loop of the receptor, and the mutants were tested for their abilities to activate different G alpha subunits of the Gq class. Our results indicate that the receptor sequences required for activating G alpha q/11, G alpha 14, or G alpha 16 are different. The sequence extending from residues Lys240 to His252 is required for activation of G alpha q/11, but not for activation of G alpha 14 or G alpha 16. Two segments in the third loop of the receptor are required for activation of G alpha 14: one is located at the N terminus of the loop ending at residue Asn226, and the other is located at the C terminus of the loop starting from residue Ser278. The latter contains a BBXXB motif, which is apparently critical for G alpha 14 coupling, but not for G alpha 16 or G alpha q/11 coupling. Furthermore, the three amino acids stretch (Tyr217 to Val219) included in the N-terminal segment is not only required for G alpha 14 coupling, but also for G alpha q/11 coupling. It may be involved to some extent in G alpha 16 coupling as well.

M1 muscarinic receptors heterologously expressed in cardiac myocytes mediate Ras-dependent changes in gene expression

Stimulation of alpha 1-adrenergic receptors in neonatal ventricular cardiomyocytes induces hypertrophic changes including activation of the atrial natriuretic factor (ANF) gene. This receptor couples to Gq to activate phospholipase C (PLC) and protein kinase C, which have been implicated as mediators of the hypertrophic response. To directly determine whether receptor coupling to Gq/PLC is sufficient to induce ANF expression, we expressed wild-type and chimeric muscarinic cholinergic receptors (mAChRs) with altered G-protein coupling properties in cardiac myocytes and examined their ability to activate an ANF promoter/luciferase reporter gene. The cholinergic agonist carbachol failed to induce transcriptional activation of the ANF reporter gene through endogenous Gi-linked M2mAChRs or in cells transfected with M2mAChRs. In contrast, in cells transfected with M1mAChRs, which effectively couple to Gq/PLC, carbachol increased ANF reporter gene expression 10-fold and also increased ANF protein, as determined by immunofluorescence. Carbachol-mediated ANF gene expression was inhibited by the mAChR antagonist pirenzepine with a Ki value characteristic of an M1mAChR. Studies using chimeric M1- and M2mAChRs demonstrated that the N-terminal 21 amino acids of the third intracellular loop of the M1mAChR were required for receptor coupling to ANF gene expression. This region, previously shown to specify receptor coupling to Gq/PLC, also conferred partial activity to a chimeric M2 receptor. We further demonstrated that M1mAChR coupling to ANF gene expression was Ras-dependent since co-expression of dominant-interfering Ala-15 Ras inhibited M1mAChR-induced ANF expression by 60%. In contrast, ANF expression induced by the chimeric M2 receptor was not blocked by dominant-interfering Ras. We suggest that receptor coupling to Gq/PLC is sufficient to induce ANF expression and that a Ras-dependent pathway contributes additional signals required for maximal M1mAChR-mediated ANF gene expression.

Activating and inactivating mutations in N- and C-terminal i3 loop junctions of muscarinic acetylcholine Hm1 receptors

The N- and C-terminal junctions of the third intracellular loop (i3) of G protein-coupled receptors play a role in the coupling process. We had previously constructed two triple point alanine mutants of the i3 junction of the muscarinic Hm1 receptor, W209A/I211A/Y212A and E360A/K362A/T366A, which are defective in mediating carbachol stimulation of phosphatidylinositol (PI) turnover (Moro, O., Lameh, J., Högger, P., and Sadée, W. (1993) J. Biol. Chem. 268, 22273-22276). Each of the corresponding six single point mutations were constructed to determine residues crucial to receptor coupling. Mutants W209A and T366A were similar to or only slightly less effective than wild type Hm1 in stimulating PI turnover. In the N-terminal junction, I211A and Y212A were defective in coupling, and I211A was even more defective than the corresponding triple mutant. Therefore, the triple mutation compensated at least partially for the effect of these two single point mutations. In the C-terminal i3 loop junction, mutant K362A was again more strongly defective than the corresponding triple mutant. In contrast, mutation E360A was found to be activating, leading to elevated PI turnover in the absence of agonist and sensitization toward carbachol activation. Activating mutations in the C-terminal i3 loop junction have been reported previously for the adrenergic receptors, but E360A represents the first muscarinic receptor with substantial basal activity. The effects of the single point mutations observed in this study were not readily predictable from similar mutations from closely related G protein-coupled receptors despite sequence conservation in the i3 loop junctions. Our results caution against defining precise coupling domains in these regions by mutagenesis results.

Gs couples thyrotropin-releasing hormone receptors expressed in Xenopus oocytes to phospholipase C

Coupling of thyrotropin-releasing hormone (TRH) receptors to individual G-proteins has been studied in Xenopus oocytes injected with receptor cRNA and antisense oligonucleotides to mRNA encoding different G-protein alpha- and beta-subunits. Injection of antisenses which target mRNA sequences shared by several G-protein alpha or beta gamma polypeptides effectively blocked Ca(2+)-dependent Cl- currents induced by TRH through activation of phospholipase C. Three different alpha s-specific antisense oligonucleotides complementary to sequences located in different positions along the coding region of the alpha s protein mRNA were highly effective in inhibiting TRH-induced responses. Anti-alpha o, -alpha q, -alpha i, or -alpha z oligonucleotides were not able to modify the TRH-evoked response. In contrast, anti-alpha o, but not anti-alpha s, oligonucleotides blocked the response to serotonin in oocytes injected with serotonin 5-HT1c receptor cRNA. Cholera toxin catalyzed the [32P]ADP-ribosylation of 40-42- and 50-52-kDa proteins in GH3 cell plasma membranes. [32P]ADP-ribosylation of oocyte membranes with the toxin labeled several proteins. These include a single 50-55-kDa substrate, which is clearly diminished in membranes from anti-alpha s-injected oocytes. Amplification of oocyte RNA in a polymerase chain reaction system and sequencing of the amplified products demonstrated that anti-alpha s oligonucleotides selectively recognize the message for the Xenopus alpha s polypeptide. It is concluded that Gs, but not Go, Gq, Gi, or Gz, couples TRH receptors expressed in oocytes to activation of phospholipase C and subsequent inositol 1,4,5-trisphosphate-dependent stimulation of Ca(2+)-dependent Cl- currents.

Structure-function of muscarinic receptor coupling to G proteins. Random saturation mutagenesis identifies a critical determinant of receptor affinity for G proteins

To derive structure-function relationships for receptor-G protein coupling, libraries were created of human m5 muscarinic acetylcholine receptors (m5) randomly mutated in the C-terminal region of the third intracellular loop. Functional receptors were identified based on their ability to amplify NIH 3T3 cells in a ligand-dependent manner. These receptors either had wild-type phenotypes (Group 1) or were functionally impaired (Group 2). No "activated receptors" were identified. Tolerated substitutions in Group 2 receptors were randomly distributed and frequently included prolines and glycines. In contrast, tolerated substitutions in Group 1 receptors exhibited a periodicity proximal to transmembrane domain 6 were proline and glycine substitutions were not observed. These observations are consistent with a short alpha-helical extension of the C-terminal region of the third intracellular loop from transmembrane domain 6. Mutations at Ala-441 were most commonly associated with impaired function of Group 2 receptors. Twelve point mutations at Ala-441 were tested, and all caused marked increases in EC50 values with little effect on maximal response or agonist binding affinity. These results indicate that Ala-441 is a key determinant of m5 receptor affinity for G proteins and exists within the structural context of a short alpha-helix.

G-protein involvement in muscarinic receptor-stimulation of inositol phosphates in longitudinal smooth muscle from the small intestine of the guinea-pig

1. Aluminium fluoride (AlF), pertussis toxin (PTX) and cholera toxin (ChTX) have been used to examine the involvement of G-proteins during muscarinic acetylcholine receptor (AChR) stimulation of inositol phospholipid hydrolysis in fragments of longitudinal smooth muscle from the small intestine of the guinea-pig. 2. Carbachol (CCh) induced time- and concentration-dependent increases in [3H]-inositol monophosphates, [3H]-inositol (1,4) bisphosphate, [3H]-inositol (1,3,4) trisphosphate, [3H]-inositol (1,4,5) trisphosphate ([3H]-Ins (1,4,5)P3) and [3H]-inositol tetrakisphosphates measured by h.p.l.c. These increases were inhibited > 95% in the presence of the muscarinic AChR antagonist atropine (0.5 microM). 3. AlF transiently increased the basal levels of [3H]-Ins (1,4,5)P3 but increases in the levels of the other [3H]-inositol phosphates occurred more slowly. CCh-induced increases in the levels of all the [3H]-inositol phosphates were strongly inhibited in the presence of AlF. 4. PTX had no effect on basal levels of any of the [3H]-inositol phosphates but reduced the effects of CCh on these; ChTX had no effects on either basal or CCh-stimulated levels. 5. It was concluded that muscarinic AChR-stimulated increases in the levels of [3H]-inositol phosphates occur via both a PTX-sensitive G-protein and a PTX-insensitive mechanism. The actions of AlF may suggest the involvement of an inhibitory G-protein in the regulation of muscarinic AChR-stimulated inositol phospholipid turnover.

Endogenous expression of histamine H1 receptors functionally coupled to phosphoinositide hydrolysis in C6 glioma cells: regulation by cyclic AMP

1. The effects of histamine receptor agonists and antagonists on phospholipid hydrolysis in rat-derived C6 glioma cells have been investigated. 2. Histamine H1 receptor-stimulation caused a concentration-dependent increase in the accumulation of total [3H]-inositol phosphates in cells prelabelled with [3H]-myo-inositol. The rank order of agonist potencies was histamine (EC50 = 24 microM) > N alpha-methylhistamine (EC50 = 31 microM) > 2-thiazolylethylamine (EC50 = 91 microM). 3. The response to 0.1 mM histamine was antagonized in a concentration-dependent manner by the H1-antagonists, mepyramine (apparent Kd = 1 nM) and (+)-chlorpheniramine (apparent Kd = 4 nM). In addition, (-)-chlorpheniramine was more than two orders of magnitude less potent than its (+)-stereoisomer. 4. Elevation of intracellular cyclic AMP accumulation with forskolin (10 microM, EC50 = 0.3 microM), isoprenaline (1 microM, EC50 = 4 nM) or rolipram (0.5 mM), significantly reduced the histamine-mediated (0.1 mM) inositol phosphate response by 37%, 43% and 26% respectively. In contrast, 1,9-dideoxyforskolin did not increase cyclic AMP accumulation and had no effect on the phosphoinositide response to histamine. 5. These data indicate the presence of functionally coupled, endogenous histamine H1 receptors in C6 glioma cells. Furthermore, the results also indicate that H1 receptor-mediated phospholipid hydrolysis is inhibited by the elevation of cyclic AMP levels in these cells.

Deletion analysis of the m4 muscarinic acetylcholine receptor. Molecular determinants for activation of but not coupling to the Gi guanine-nucleotide-binding regulatory protein regulate receptor internalization

In order to investigate whether coupling to and/or activation of guanine-nucleotide-binding proteins (G proteins) is involved in agonist-induced internalization of m4 muscarinic acetylcholine receptors (mAChRs), a deletion mutant [des-(264-394)mAChR] was constructed that lacks a substantial portion of the putative third intracellular loop. The wild-type receptor and des-(264-394)mAChR stably expressed in Chinese hamster ovary cells in essentially comparable amounts, exhibited identical antagonist-binding affinities. Consistent with the reported importance of the third cytoplasmic loop for Gi protein activation, the des-(264-394)mAChR showed a drastically reduced potential to mediate agonist-induced inhibition of adenylyl cyclase. In contrast, the ability of the mutant receptor to couple to Gi proteins was not impaired, as demonstrated by a similar guanine-nucleotide-sensitive and pertussis-toxin-sensitive high-affinity agonist-receptor binding for both mAChRs. In contrast, des-(264-394)mAChR was hardly able to stimulate the GTPase activity of G proteins, suggesting impaired activation of Gi proteins rather than ineffective coupling to Gi proteins. Internalization of wild-type receptor and des-(264-394)mAChR was observed with similar agonist concentrations and showed similar maximal values. However, des-(264-394)mAChR displayed a significantly reduced rate of receptor internalization. A similar attenuation of wild-type mAChR internalization was obtained upon pertussis toxin treatment. In conclusion, our data provide evidence that the molecular determinants of the m4 mAChR involved in Gi-protein coupling and activation are not identical and that activation of, but not coupling to, Gi proteins regulates m4 mAChR internalization.

Activation of Gi protein by peptide structures of the muscarinic M2 receptor second intracellular loop

The muscarinic M2 receptor that normally couples via Gi to inhibit adenylyl cyclase was made to couple to Gs by exchange of its third intracellular loop for the comparable domain of the beta 2-adrenoceptor. In HeLa cells transfected with the recombinant M2 beta i-3 cDNA, the chimaeric receptor showed carbachol-mediated activation of adenylyl cyclase (EC50 = 73 nM) that was blocked by atropine, but not by propranolol. The chimaeric receptor was shown to mediate a carbachol-stimulated, Bordetella pertussis toxin-sensitive GTPase activity in membranes of transfected HeLa cells. Interestingly, stimulation of adenylyl cyclase by carbachol was 2-fold higher in transfected cells that had been pretreated with pertussis toxin. These data suggested that the M2 beta i-3 receptor was able to couple to both Gi and Gs, and that the ability to recognise and stimulate Gi did not involve the third cytoplasmic loop of M2. We investigated peptide elements taken from the second intracellular loop of the M2 receptor for their ability to mediate activation of Gi and found that a nine amino acid peptide representing the C-terminal sequence, VKRTTKMAG-NH2 (V9G), was capable of inhibiting forskolin-stimulated adenylyl cyclase by up to 18% and could stimulate high affinity GTPase activity of rat brain membranes by 32%. Further, V9G was shown to cause a doubling of the initial rate of [35S]GTP gamma S binding to purified bovine brain Gi/Go in reconstituted phospholipid vesicles. These data identify a domain on the second intracellular loop of the muscarinic M2 receptor that is involved in the selection of a pertussis toxin-sensitive G protein.

Tyrosine kinase-dependent suppression of a potassium channel by the G protein-coupled m1 muscarinic acetylcholine receptor

Neurotransmitter receptors alter membrane excitability and synaptic efficacy by generating intracellular signals that ultimately change the properties of ion channels. Through expression studies in Xenopus oocytes and mammalian cells, we found that the G protein-coupled m1 muscarinic acetylcholine receptor potently suppresses a cloned delayed rectifier K+ channel through a pathway involving phospholipase C activation and direct tyrosine phosphorylation of the K+ channel. Furthermore, analysis of neuroblastoma cells revealed that a similar tyrosine kinase-dependent pathway links endogenous G protein-coupled receptors to suppression of the native RAK channel. These results suggest a novel mechanism by which neurotransmitters and hormones may regulate a specific type of K+ channel that is widely expressed in the mammalian brain and heart.

Differential signal transduction by five splice variants of the PACAP receptor

The two forms of pituitary adenylyl cyclase-activating polypeptide (PACAP-27 and -38) are neuropeptides of the secretin/glucagon/vasoactive intestinal polypeptide/growth-hormone-releasing hormone family and regulate hormone release from the pituitary and adrenal gland. They may also be involved in spermatogenesis, and PACAP-38 potently stimulates neuritogenesis and survival of cultured rat sympathetic neuroblast and promotes neurite outgrowth of PC-12 cells. The PACAP type-I receptor (found in hypothalamus, brain stem, pituitary, adrenal gland and testes), specific for PACAP, is positively coupled to adenylyl cyclase and phospholipase C. The recently cloned type II receptor does not discriminate between PACAP and vasoactive intestinal polypeptide and is coupled to only adenylyl cyclase. Here we have used a new expression cloning strategy, based on the induction of a reporter gene by cyclic AMP, to isolate a complementary DNA encoding the type-I PACAP receptor. On transfection of this cDNA, both PACAP-27 and -38 stimulate adenylyl cyclase with similar EC50 values (50% effective concentration, 0.1-0.4 nM), whereas only PACAP-38 stimulates phospholipase C with high potency (EC50 = 15 nM). Four other splice variants were isolated with insertions at the C-terminal end of the third intracellular loop. Expression of these cDNAs revealed altered patterns of adenylyl cyclase and phospholipase C stimulation, suggesting a novel mechanism for fine tuning of signal transduction.