Coupling of the expressed alpha 1B-adrenergic receptor to the phospholipase C pathway in Xenopus oocytes. The role of Go

Adrenergic regulation of cardiac ionic channels: role of membrane microdomains in the regulation of kv4 channels

The heart must constantly adapt its activity to the needs of the body. In any potentially dangerous or physically demanding situation the activated sympathetic nervous system leads a very fast cardiac response. Under these circumstances, α1-adrenergic receptors activate intracellular signaling pathways that finally phosphorylate the caveolae-located subpopulation of Kv4 channels and reduce the transient outward K(+) current (Ito) amplitude. This reduction changes the shape of the cardiac action potential and makes the plateau phase to start at higher voltages. This means that there are more calcium ions entering the myocyte and the result is an increase in the strength of the contraction. However, an excessive reduction of Ito could dangerously prolong action potential duration and this could cause arrhythmias when the heart rate is high. This excessive current reduction does not occur because there is a second population of Ito channels located in non-caveolar membrane rafts that are not accessible for α1-AR mediated regulation. Thus, the location of the components of a given transduction signaling pathway in membrane domains determines the correct and safe behavior of the heart. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Transient outward potassium channel regulation in healthy and diabetic heartsThis article is one of a selection of papers from the NATO Advanced Research Workshop on Translational Knowledge for Heart Health (published in part 1 of a 2-part Special Issue)

Diabetic patients have a higher incidence of cardiac arrhythmias, including ventricular fibrillation and sudden death, and show important alterations in the electrocardiogram, most of these related to the repolarization. In myocytes isolated from diabetic hearts, the transient outward K+ current (Ito) is the repolarizing current that is mainly affected. Type 1 diabetes alters Ito at 3 levels: the recovery of inactivation, the responsiveness to physiologic regulators, and the functional expression of the channel. Diabetes slows down Ito recovery of inactivation because it triggers the switching from fast-recovering Kv4.x channels to the slow-recovering Kv1.4. Diabetic animals also have decreased responsiveness of Ito towards the sympathetic nervous system; thus, the diabetic heart develops a resistance to its physiologic regulator. Finally, diabetes impairs support of various trophic factors required for the functional expression of the channel and reduces Ito amplitude by decreasing the amount of Kv4.2 and Kv4.3 proteins.

Role of brain adrenoceptors in the corticortopin-releasing factor-induced central activation of sympatho-adrenomedullary outflow in rats

We investigated the role played by catecholamine-dependent pathways in modulating the ability of centrally administered corticotropin releasing factor (CRF) to activate sympatho-adrenomedullay outflow, using urethane-anesthetized rats. The CRF (1.5 nmol/animal, i.c.v.)-induced elevations of both plasma noradrenaline and adrenaline were attenuated by phentolamine (a non-selective alpha adrenoceptor antagonist) [125 and 250 microg (0.33 and 0.66 micromol)/animal], Heat (a selective alpha(1) adrenoceptor antagonist) [10 and 30 microg (30 and 90 nmol)/animal, i.c.v.] and clonidine (a selective alpha(2) adrenoceptor agonist) [100 microg (0.375 micromol)/animal, i.c.v.]. On the other hand, the CRF (1.5 nmol/animal, i.c.v.)-induced elevation of both catecholamines was not influenced by RS 79948 (a selective alpha(2) adrenoceptor antagonist) [10 and 30 microg (7.2 and 72 nmol)/animal, i.c.v.]. Furthermore, the CRF (1.5 nmol/animal, i.c.v.)-induced elevation of noradrenaline was attenuated by sotalol (a non-selective beta adrenoceptor antagonist) [125 and 250 microg (0.4 and 0.8 micromol)/animal, i.c.v.], while that of adrenaline was not influenced by sotalol. These results suggest that centrally administered CRF-induced elevation of plasma noradrenaline is mediated by an activation of alpha(1) and beta adrenoceptors in the brain, and that of plasma adrenaline is mediated by an activation of alpha(1) adrenoceptors in the brain. Furthermore, central alpha(2) adrenoceptors are involved in modulating the CRF-induced elevation of both plasma catecholamines.

Morituri te salutant? Olfactory signal transduction and the role of phosphoinositides

During the past 150 years, researchers have investigated the cellular, physiological, and molecular mechanisms underlying the sense of smell. Based on these efforts, a conclusive model of olfactory signal transduction in the vertebrate's nose is now available, spanning from G-protein-mediated odorant receptors to ion channels, which are linked by a cyclic adenosine 3',5'-monophosphate-mediated signal transduction cascade. Here we review some historical milestones in the chronology of olfactory research, particularly emphasising the role of cyclic nucleotides and inositol trisphosphate as alternative second messengers in olfactory cells. We will describe the functional anatomy of the nose, outline the cellular composition of the olfactory epithelium, and describe the discovery of the molecular backbone of the olfactory signal transduction cascade. We then summarize our current model, in which cyclic adenosine monophosphate is the sole excitatory second messenger in olfactory sensory neurons. Finally, a possible significance of microvillous olfactory epithelial cells and inositol trisphosphate in olfaction will be discussed.

Differential ion current activation by human 5-HT(1A) receptors in Xenopus oocytes: evidence for agonist-directed trafficking of receptor signalling

The subject of the present study was the functional and pharmacological characterization of human 5-HT(1A) receptor regulation of ion channels in Xenopus oocytes. Activation of the heterologously expressed human 5-HT(1A) receptor induced two distinct currents in Xenopus oocytes, consisting of a smooth inward current (I(smooth)) and an oscillatory calcium-activated chloride current, I(Cl(Ca)). 5-HT(1A) receptor coupling to both ionic responses as well as to co-expressed inward rectifier potassium (GIRK) channels was pharmacologically characterized using 5-HT(1A) receptor agonists. The relative order of efficacy for activation of GIRK current was 5-HT approximately F 13714 approximately L 694,247 approximately LY 228,729>flesinoxan approximately (+/-)8-OH-DPAT. In contrast, flesinoxan and (+/-)8-OH-DPAT typically failed to activate I(Cl(Ca)). The other ligands behaved as full or partial agonists, exhibiting an efficacy rank order of 5-HT approximately L 694,247>F 13714 approximately LY 228,729. The pharmacological profile of I(smooth) activation was completely distinct: flesinoxan and F 13714 were inactive and rather exhibited an inhibition of this current. I(smooth) was activated by the other agonists with an efficacy order of L 694,247>5-HT approximately LY 228,729>(+/-)8-OH-DPAT. Moreover, activation of I(smooth) was not affected by application of pertussis toxin or the non-hydrolyzable GDP-analogue, guanosine-5'-O-(2-thio)-diphosphate (GDP betaS), suggesting a GTP binding protein-independent pathway. Together, these results suggest the existence of distinct and agonist-specific signalling states of this receptor.

Site of action of the general anesthetic propofol in muscarinic M1 receptor-mediated signal transduction

Although a potential target site of general anesthetics is primarily the GABA A receptor, a chloride ion channel, a previous study suggested that the intravenous general anesthetic propofol attenuates the M1 muscarinic acetylcholine receptor (M1 receptor)-mediated signal transduction. In the present study, we examined the target site of propofol in M1 receptor-mediated signal transduction. Two-electrode voltage-clamp method was used in Xenopus oocytes expressing both M1 receptors and associated G protein alpha subunits (Gqalpha). Propofol inhibited M1 receptor-mediated signal transduction in a dose-dependent manner (IC50 = 50 nM). Injection of guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) into oocytes overexpressing Gqalpha was used to investigate direct effects of propofol on G protein coupled with the M1 receptor. Propofol did not affect activation of Gqalpha-mediated signal transduction with the intracellular injection of GTPgammaS. We also studied effects of propofol on l-[N-methyl-3H]scopolamine methyl chloride ([3H]NMS) binding and M1 receptor-mediated signal transduction in mammalian cells expressing M1 receptor. Propofol inhibited the M1 receptor-mediated signal transduction but did not inhibit binding of [3H]NMS. Effects of propofol on Gs- and Gi/o-coupled signal transduction were investigated, using oocytes expressing the beta2 adrenoceptor (beta2 receptor)/cystic fibrosis transmembrane conductance regulator or oocytes expressing the M2 muscarinic acetylcholine receptor (M2 receptor)/Kir3.1 (a member of G protein-gated inwardly rectifying K(+) channels). Neither beta2 receptor-mediated nor M2 receptor-mediated signal transduction was inhibited by a relatively high concentration of propofol (50 microM). These results indicate that propofol inhibits M1 receptor-mediated signal transduction by selectively disrupting interaction between the receptor and associated G protein.

Involvement of G(q/11) in signal transduction in the mammalian vomeronasal organ

Social behaviors of most mammals are profoundly affected by pheromones. Pheromones are detected by G-protein coupled receptors in the vomeronasal organ (VNO). To investigate the role of G alpha(q/11) in vomeronasal signal transduction pathways, microvillar membranes from murine VNO were prepared. Incubation of such membranes from prepubertal females with adult male urine results in an increase in production of inositol-(1,4,5)-trisphosphate (IP(3)). This stimulation is mimicked by GTP gamma S, blocked by GDP beta S and is tissue specific. Furthermore, use of bacterial toxins such as pertussis that lead to ADP-ribosylation of the G-protein alpha subunits of G(o) and G(i2) do not block the increase in IP(3) levels but U-73122, a PLC inhibitor, blocks the production of IP(3). Studies with monospecific antibodies revealed the presence of three G-proteins, G alpha(o), G alpha(i2) and G alpha(q/11)-related protein, in vomeronasal neurons, concentrated on their microvilli. Our observations indicate that pheromones in male urine act on vomeronasal neurons in the female VNO via a receptor-mediated, G alpha(q/11)-protein-dependent increase in IP(3) levels.

Cell surface beta-1,4-galactosyltransferase-I activates G protein-dependent exocytotic signaling

ZP3 is a protein in the mammalian egg coat (zona pellucida) that binds sperm and stimulates acrosomal exocytosis, enabling sperm to penetrate the zona pellucida. The nature of the ZP3 receptor/s on sperm is a matter of considerable debate, but most evidence suggests that ZP3 binds to beta-1,4-galactosyltransferase-I (GalTase) on the sperm surface. It has been suggested that ZP3 induces the acrosome reaction by crosslinking GalTase, activating a heterotrimeric G protein. In this regard, acrosomal exocytosis is sensitive to pertussis toxin and the GalTase cytoplasmic domain can precipitate G(i) from sperm lysates. Sperm from mice that overexpress GalTase bind more soluble ZP3 and show accelerated G protein activation, whereas sperm from mice with a targeted deletion in GalTase have markedly less ability to bind soluble ZP3, undergo the ZP3-induced acrosome reaction, and penetrate the zona pellucida. We have examined the ability of GalTase to function as a ZP3 receptor and to activate heterotrimeric G proteins using Xenopus laevis oocytes as a heterologous expression system. Oocytes that express GalTase bound ZP3 but did not bind other zona pellucida glycoproteins. After oocyte maturation, ZP3 or GalTase antibodies were able to trigger cortical granule exocytosis and activation of GalTase-expressing eggs. Pertussis toxin inhibited GalTase-induced egg activation. Consistent with G protein activation, both ZP3 and anti-GalTase antibodies increased GTP-gamma[(35)S] binding as well as GTPase activity in membranes from eggs expressing GalTase. Finally, mutagenesis of a putative G protein activation motif within the GalTase cytoplasmic domain eliminated G protein activation in response to ZP3 or anti-GalTase antibodies. These results demonstrate directly that GalTase functions as a ZP3 receptor and following aggregation, is capable of activating pertussis toxin-sensitive G proteins leading to exocytosis.

Alpha1-adrenoceptor subtypes

Alpha1-adrenoceptors are one of three subfamilies of receptors (alpha1, alpha2, beta) mediating responses to adrenaline and noradrenaline. Three alpha1-adrenoceptor subtypes are known (alpha1A, alpha1B, alpha1D) which are all members of the G protein coupled receptor family, and splice variants have been reported in the C-terminus of the alpha1A. They are expressed in many tissues, particularly smooth muscle where they mediate contraction. Certain subtype-selective agonists and antagonists are now available, and alpha1A-adrenoceptor selective antagonists are used to treat benign prostatic hypertrophy. All subtypes activate phospholipase C through the G(q/11) family of G proteins, release stored Ca2+, and activate protein kinase C, although with significant differences in coupling efficiency (alpha1A > alpha1B > alpha1D). Other second messenger pathways are also activated by these receptors, including Ca2+ influx, arachidonic acid release, and phospholipase D. Alpha1-adrenoceptors also activate mitogen-activated protein kinase pathways in many cells, and some of these responses are independent of Ca2+ and protein kinase C but involve small G proteins and tyrosine kinases. Direct interactions of alpha1-adrenoceptors with proteins other than G proteins have not yet been reported, however there is a consensus binding motif for the immediate early gene Homer in the C-terminal tail of the alpha1D subtype. Current research is focused on discovering new subtype-selective drugs, identifying non-traditional signaling pathways activated by these receptors, clarifying how multiple signals are integrated, and identifying proteins interacting directly with the receptors to influence their functions.

Identification and expression of Go1 and Go2 alpha-subunit transcripts in human myometrium in relation to pregnancy

The 39-kDa Goalpha protein, the alpha subunit of a major heterotrimeric G protein of brain and neuroendocrine cells, was found to be present in human myometrium. Using three different antisera, we showed its strong expression in myometrium from pregnant patients as compared to nonpregnant ones. This is in agreement with the high expression level of its two isoforms (alphao1 and alphao2), previously described in late pregnancy. To better ascertain the nature of these immunoreactive isoforms, we investigated transcripts of the Goalpha gene in myometrium from pregnant and nonpregnant patients by reverse transcription-polymerase chain reaction (RT-PCR). In this tissue, the amplified cDNA product of a region common to both Go1alpha and Go2alpha mRNA variants was recognized as the Goalpha nucleotide sequence. Transcripts of Go1alpha and Go2alpha were identified by sequencing. A partial cDNA Go2alpha sequence was described, which differed from the Goalpha gene by two nucleotides in exon 8B. Levels of Go1alpha and Go2alpha transcripts analyzed by semi-quantitative RT-PCR were significantly higher in myometrium from pregnant than from nonpregnant patients. It is suggested that Goalpha gene expression in this tissue may contribute to modifications seen in the signaling pathways observed at the end of pregnancy.

Propofol inhibits muscarinic acetylcholine receptor-mediated signal transduction in Xenopus Oocytes expressing the rat M1 receptor

The effects of propofol, 2,6-diisopropylphenol, an intravenous general anesthetic, on signal transduction mediated by the rat M1 muscarinic acetylcholine (ACh) receptor (M1 receptor) were examined in electrophysiological studies by analyzing receptor-stimulated, Ca2+-activated Cl--current responses in the Xenopus oocyte expression system. In oocytes expressing the M1 receptor, ACh induced the Ca2+-activated C1- current, in a dose-dependent manner (EC50= 114 nM). Propofol (5-50 microM) reversibly and dose-dependently inhibited induction of the Ca2+-activated Cl- current by ACh (100 nM) (IC50=5.6 microM). To determine a possible site affected by propofol in this signal transduction, we tested the effects of this anesthetic (10 microM) on the activation of current by injection of CaCl2 and aluminum fluoride (AlF4-). Propofol did not affect activation of the current by the intracellular injected Ca2+, or activation of the current by the intracellular injected AlF4-. These results indicate that propofol does not affect G protein, the inositol phosphate turnover, release of Ca2+ from Ca2+ store or the Ca2+-activated Cl- channel. Propofol apparently inhibits the M1 receptor-mediated signal transduction at the receptor site and/or the site of interaction between the receptor and associated G protein.

Odorants selectively activate distinct G protein subtypes in olfactory cilia

Chemoelectrical signal transduction in olfactory neurons appears to involve intracellular reaction cascades mediated by heterotrimeric GTP-binding proteins. In this study attempts were made to identify the G protein subtype(s) in olfactory cilia that are activated by the primary (odorant) signal. Antibodies directed against the alpha subunits of distinct G protein subtypes interfered specifically with second messenger reponses elicited by defined subsets of odorants; odor-induced cAMP-formation was attenuated by Galphas antibodies, whereas Galphao antibodies blocked odor-induced inositol 1,4, 5-trisphosphate (IP3) formation. Activation-dependent photolabeling of Galpha subunits with [alpha-32P]GTP azidoanilide followed by immunoprecipitation using subtype-specific antibodies enabled identification of particular individual G protein subtypes that were activated upon stimulation of isolated olfactory cilia by chemically distinct odorants. For example odorants that elicited a cAMP response resulted in labeling of a Galphas-like protein, whereas odorants that elicited an IP3 response led to the labeling of a Galphao-like protein. Since odorant-induced IP3 formation was also blocked by Gbeta antibodies, activation of olfactory phospholipase C might be mediated by betagamma subunits of a Go-like G protein. These results indicate that different subsets of odorants selectively trigger distinct reaction cascades and provide evidence for dual transduction pathways in olfactory signaling.

The relationship between the agonist-induced activation and desensitization of the human tachykinin NK2 receptor expressed in Xenopus oocytes

1. Repeated applications of neurokinin A (NKA) to oocytes injected with 25 ng wild-type hNK2 receptor cRNA caused complete attenuation of second and subsequent NKA-induced responses while analogous experiments using repeated applications of GR64349 and [Nle10]NKA(4-10) resulted in no such desensitization. This behaviour has been previously attributed to the ability of the different ligands to stabilize different active conformations of the receptor that have differing susceptibilities to receptor kinases (Nemeth & Chollet. 1995). 2. However, for Xenopus oocytes injected (into the nucleus) with 10 ng wild-type hNK2 receptor cDNA, a single 100 nM concentration of any of the three ligands resulted in complete desensitization to further concentrations. 3. On the other hand, none of the ligands caused any desensitization in oocytes injected with 0.25 ng wild-type hNK2 receptor cRNA. even at concentrations up to 10 microM. 4. The two N-terminally truncated analogues of neurokinin A have a lower efficacy than NKA and it is likely that it is this property which causes the observed differences in desensitization, rather than the formation of alternative active states of the receptor. 5. The peak calcium-dependent chloride current is not a reliable measure of maximal receptor stimulation and efficacy is better measured in this system by studying agonist-induced desensitization. 6. The specific adenylyl cyclase inhibitor SQ22536 can enhance NKA and GR64349-mediated desensitization which suggests that agonist-induced desensitization involves the inhibition of adenylyl cyclase and the subsequent down-regulation of the cyclic AMP-dependent protein kinase, possibly by cross-talk to a second signalling pathway.

Expression of the mammalian calcium signaling response to Trypanosoma cruzi in Xenopus laevis oocytes

Infective stages of the protozoan parasite Trypanosoma cruzi contain a soluble factor that induces elevation in the intracellular free Ca2+ concentration ([Ca2+]i) of mammalian cells. The process is pertussis toxin (PTx)-sensitive, and involves phospholipase C (PLC) activation, inositol 1,4,5-trisphosphate (IP3) formation and Ca2+ release from intracellular stores (Tardieux I, et al. J Exp Med 1994;179:1017-1022; Rodriguez A, et al. J Cell Biol 1995;129:1263-1273). We now report that a molecule exposed on the surface of the target cells is required to trigger the signaling cascade, and that a response with identical characteristics can be induced in Xenopus laevis oocytes injected with mRNA from normal rat kidney (NRK) fibroblasts. Xenopus oocytes do not show an endogenous response to the trypomastigote Ca2+ signaling factor, but a vigorous response in the form of a propagating Ca2+ wave is expressed after injection of NRK cell mRNA. As previously demonstrated for mammalian cells, the response is inhibited when injected oocytes are pretreated with PTx, implicating Galphai or Galphao trimeric G-proteins, and with thapsigargin, which depletes intracellular Ca2+ stores. Moreover, the [Ca2+]i transients triggered by the T. cruzi soluble factor in mRNA-injected oocytes are blocked by the same inhibitors of the parasite oligopeptidase B that abolish the [Ca2+]i response in NRK cells (Burleigh B, Andrews NW. J Biol Chem 1995;270:5172-5180; Burleigh BA et al. J Cell Biol 1997;136:609-620). The NRK mRNA fraction that induces expression of the [Ca2+]i response to the T. cruzi signaling factor contains messages from 1.5 to 2.0 kb, a size range consistent with the family of seven-transmembrane G-protein-coupled receptors.

Role of G alpha q or G alpha o proteins in alpha 1-adrenoceptor subtype-mediated responses in Fischer 344 rat aorta

Previous studies showed that alpha-adrenoceptor (AR) stimulation with norepinephrine is more potent at eliciting contraction in aortas from 1-month-old Fischer 344 rats than from older rats and that this response is mediated by alpha 1b- and alpha 1d-AR subtypes in 1-month-old rats. We examined the G proteins responsible for alpha 1-AR-mediated contractile response and inositol phosphate accumulation in the aortas of 1-month-old Fischer 344 rats. Pertussis toxin (PTX) treatment (2.5 micrograms/ml for 4 hr) of aortic rings partially inhibited phenylephrine (PHE)-stimulated contraction and inositol phosphate accumulation, suggesting the involvement of PTX-sensitive and -insensitive G proteins. Specific antisera directed against G alpha q and G alpha o but not G alpha s and G alpha i precipitated specific alpha 1-AR binding sites labeled with 2-[beta-(4-hydroxy-3-[125I]iodophenyl)ethylaminomethyl]tetralone. The number of 2-[beta-(4-hydroxy-3-[125I]iodophenyl)ethylaminomethyl]tetralone binding sites precipitated by G alpha proteins was increased by activating membrane alpha 1-ARs with PHE. Moreover, PHE stimulated the palmitoylation of G alpha q and G alpha o, and this response was blocked by the alpha 1-AR antagonist prazosin. Characterization of the alpha 1-AR subtypes that couple to G proteins indicates that although aortic alpha 1a-, alpha 1b-, and alpha 1d-ARs were associated with G alpha q, alpha 1b-AR was also linked to G alpha o. These results suggest that alpha 1-ARs mediate the contractile response in rat aorta by coupling to both Gq protein and the PTX-sensitive G(o) protein.

The expression of a cloned Drosophila octopamine/tyramine receptor in Xenopus oocytes

The expression of a cloned Drosophila octopamine/tyramine receptor (OctyR99AB) is described in Xenopus oocytes. Agonist stimulation of OctyR99AB receptors increased intracellular Ca2+ levels monitored as changes in the endogenous inward Ca2+-dependent chloride current. The receptor is preferentially sensitive to biogenic amines with a single hydroxyl on the aromatic ring. The G-protein, Galphai, appears to be involved in the coupling of the receptor to the production of intracellular calcium signals, since the effect is pertussis-toxin sensitive and is blocked or substantially reduced in antisense knockout experiments using oligonucleotides directed against Galphai but not by those directed against Galphao, Galphaq and Galpha11. The increase in intracellular calcium levels induced by activation of the OctyR99AB receptor can potentiate the ability of activation of a co-expressed beta2-adrenergic receptor to increase oocyte cyclic AMP levels. A comparison of the pharmacological coupling of OctyR99AB to different second messenger systems when expressed in Xenopus oocytes with previous studies on the expression of the receptor in a Chinese hamster ovary cell line suggests that the property of agonist-specific coupling of the receptor to different second messenger systems may be cell-specific, depending upon the G-protein environment of any particular cell type.

Specific Gq protein involvement in muscarinic M3 receptor-induced phosphatidylinositol hydrolysis and Ca2+ release in mouse duodenal myocytes

1. Cytosolic Ca2+ concentration ([Ca2+]i) during exposure to acetylcholine or caffeine was measured in mouse duodenal myocytes loaded with fura-2. Acetylcholine evoked a transient increase in [Ca2+]i followed by a sustained rise which was rapidly terminated after drug removal. Although L-type Ca2+ currents participated in the global Ca2+ response induced by acetylcholine, the initial peak in [Ca2+]i was mainly due to release of Ca2+ from intracellular stores. 2. Atropine, 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, a muscarinic M3 antagonist), pirenzepine (a muscarinic M1 antagonist), methoctramine and gallamine (muscarinic M2 antagonists) inhibited the acetylcholine-induced Ca2+ release, with a high affinity for 4-DAMP and atropine and a low affinity for the other antagonists. Selective protection of muscarinic M2 receptors with methoctramine during 4-DAMP mustard alkylation of muscarinic M3 receptors provided no evidence for muscarinic M2 receptor-activated [Ca2+]i increase. 3. Acetylcholine-induced Ca2+ release was blocked by intracellular dialysis with a patch pipette containing either heparin or an anti-phosphatidylinositol antibody and by external application of U73122 (a phospholipase C inhibitor). 4. Acetylcholine-induced Ca2+ release was insensitive to external pretreatment with pertussis toxin, but concentration-dependently inhibited by intracellular dialysis with a patch pipette solution containing an anti-alpha q/alpha 11 antibody. An antisense oligonucleotide approach revealed that only the Gq protein was involved in acetylcholine-induced Ca2+ release. 5. Intracellular applications of either an anti-beta com antibody or a peptide corresponding to the G beta gamma binding domain of the beta-adrenoceptor kinase 1 had no effect on acetylcholine-induced Ca2+ release. 6. Our results show that, in mouse duodenal myocytes, acetylcholine-induced release of Ca2+ from intracellular stores is mediated through activation of muscarinic M3 receptors which couple with a Gq protein to activate a phosphatidylinositol-specific phospholipase C.

Desensitization of inositol 1,4,5-trisphosphate/Ca2+-induced Cl- currents by prolonged activation of G proteins in Xenopus oocytes

Expression of G protein alpha subunits of the Gq family with various G protein-coupled receptors induces activation of an inositol 1,4, 5-trisphosphate (IP3)/Ca2+-mediated Cl- conductance in Xenopus oocytes. Our present data show that two members of this family, the human Galpha16 subunit and the murine homologue Galpha15, can induce both activation and inhibition of these agonist-induced currents. Although extremely low amounts (10-50 pg) of injected Galpha16 subunit cRNA cause modest ( approximately 2-fold) enhancement of ligand-induced Cl- currents in oocytes co-injected with thyrotropin-releasing hormone (TRH) receptor cRNA 48 h postinjection, larger Galpha16 and Galpha15 cRNA injections cause >10-fold inhibition of TRH or 5HT2c receptor responses. The inhibition is analyzed in this study. The inhibited currents are recovered if various Gbetagamma subunit combinations are also expressed with the Galpha subunits. The constitutively active mutant, Galpha16Q212L, also causes a strong attenuation of the ligand-induced Cl- currents, but this inhibition is not recovered by co-expression of Gbetagamma subunits. These results indicate that the free Galpha subunit is responsible for the inhibitory signal. Although expression of TRH receptor alone produces maximum responses approximately 48 h after injection, co-expression of TRH receptor with Galpha16 results in enhanced responses 6-12 h postinjection, followed by complete attenuation at 36 h. Furthermore, injection of Galpha16 cRNA alone at comparable levels gives rise to spontaneous Cl- currents within 6-12 h postinjection, suggesting that the early spontaneous activation underlies the later suppression. Expression of other G protein alpha subunits of the Gq family, at cRNA levels considerably higher than effective for Galpha16, produces both analogous spontaneous Cl- currents and, later, inhibition of ligand-induced Cl- currents. Experiments with direct injection of IP3 and of Ca2+ suggest that this inhibition is consistent with the down-regulation of IP3 receptors. These data indicate that both enhancement and inhibition of signaling through G protein-coupled receptors can be mediated by the expression level and/or activity of an individual G protein.

Purification and G protein subunit regulation of a phospholipase C-beta from Xenopus laevis oocytes

Xenopus oocytes exhibit both pertussis toxin-sensitive and -insensitive inositol lipid signaling responses to G protein-coupled receptor activation. The G protein subunits Galphai, Galphao, Galphaq, Galphas, and Gbetagamma all have been proposed to function as activators of phospholipase C in oocytes. Ma et al. (Ma, H.-W., Blitzer, R. D., Healy, E. C., Premont, R. T., Landau, E. M., and Iyengar, R. J. Biol. Chem. 268, 19915-19918) cloned a Xenopus phospholipase C (PLC-betaX) that exhibits homology to the PLC-beta class of isoenzymes. Although this enzyme was proposed to function as a signaling protein in the pertussis toxin-sensitive inositol lipid signaling pathway of oocytes, its regulation by G protein subunits has not been directly assessed. As such we have utilized baculovirus-promoted overexpression of PLC-betaX in Sf9 insect cells and have purified a recombinant 150-kDa isoenzyme. PLC-betaX catalyzes hydrolysis of phosphatidylinositol(4,5)bisphosphate and phosphatidylinositol(4)monophosphate, and reaction velocity is dependent on Ca2+. Recombinant PLC-betaX was activated by both Galphaq and Gbetagamma. PLC-betaX exhibited a higher apparent affinity for Galphaq than Gbetagamma, and Galphaq was more efficacious than Gbetagamma at lower concentrations of PLC-betaX. Relative to other PLC-beta isoenzymes, PLC-betaX was less sensitive to stimulation by Galphaq than PLC-beta1 but similar to PLC-beta2 and PLC-betaT. PLC-betaX was more sensitive to stimulation by Gbetagamma than PLC-beta1 but less sensitive than PLC-beta2 and PLC-betaT. In contrast PLC-betaX was not activated by the pertussis toxin substrate G proteins Galphai1, Galphai2, Galphai3, or Galphao. These results are consistent with the idea that PLC-betaX is regulated by alpha-subunits of the Gq family and by Gbetagamma and do not support the idea that alpha-subunits of pertussis toxin-sensitive G proteins are directly involved in regulation of this protein.

Molecular cloning and functional expression of the alpha1A-adrenoceptor of Medaka fish, Oryzias latipes

A genomic DNA encoding a subtype adrenoceptor (AR) was cloned from Medaka fish, Oryzias latipes, using an oligonucleotide probe corresponding to the consensus sequence of mammalian alpha-AR and beta-AR. The gene spans at least 9kbp, and the coding region consists of two exons split by an intron of 7.2 kbp located at the same position as those of mammalian alpha1B-AR genes. The gene encodes 470 amino acid residues, the sequence of which shows the highest similarity to that of mammalian alpha1A-AR (61%) and significant but lower similarities to other alpha-AR and beta-AR proteins (31-45%), indicating that the gene encodes a Medaka homolog of alpha1A-AR. To characterize the encoded protein, the mRNA was synthesized in vitro and injected into Xenopus oocytes. As a result, the oocytes responded to 100 nM epinephrine evoking a Ca2 + -dependent C1- current in the order of microamperes, which was not observed for oocytes injected with water alone. The response was reversibly inhibited by an alpha1-selective antagonist, WB4101 (2-[2,6-dimethoxphenoxyethyl]aminomethyl)-1,4-benzodioxane). Similar experiments using several adrenergic agonists revealed that Medaka alpha1A-AR responds to the following agonists in the order: epinephrine > or = (-)norepinephrine > oxymetazoline > or = methoxamine, which is similar to the responses of rat alpha1A receptor expressed in COS cells. The results indicate that fish contains adrenoceptor systems similar to those of mammals in terms of primary structure and pharmacological properties.

The G protein beta gamma subunit transduces the muscarinic receptor signal for Ca2+ release in Xenopus oocytes

At least 30 G protein-linked receptors stimulate phosphatidylinositol 4,5-bisphosphate phosphodiesterase (phospholipase C beta, PLC beta) through G protein subunits to release intracellular calcium from the endoplasmic reticulum (Clapham, D. E. (1995) Cell 80, 259-268). Although both G alpha and G beta gamma G protein subunits have been shown to activate purified PLC beta in vitro, G alpha q has been presumed to mediate the pertussis toxin-insensitive response in vivo. In this study, we show that G beta gamma plays a dominant role in muscarinic-mediated activation of PLC beta by employing the Xenopus oocyte expression system. Antisense nucleotides and antibodies to G alpha q/11 blocked the m3-mediated signal transduction by inhibiting interaction of the muscarinic receptor with the G protein. Agents that specifically bound free G beta gamma subunits (G alpha-GDP and a beta-adrenergic receptor kinase fragment) inhibited acetylcholine-induced signal transduction to PLC beta, and injection of G beta gamma subunits into oocytes directly induced release of intracellular Ca2+. We conclude that receptor coupling specificity of the G alpha q/G beta gamma heterotrimer is determined by G alpha q; G beta gamma is the predominant signaling molecule activating oocyte PLC beta.

G protein regulation of the Na+/H+ antiporter in Xenopus laevis oocytes. Involvement of protein kinases A and C

We have characterized the regulation of the endogenous Na+/H+ exchanger in Xenopus laevis oocytes by G proteins and protein kinases by measuring the ethylisopropylamiloride-sensitive Li+ uptake. Injection of oocytes with the stable GTP analog GTP gamma S stimulated Li+ uptake up to almost 4-fold, an effect blocked by coinjection with the GDP analog, guanyl-5'-yl thiophosphate. Injection into oocytes of beta gamma subunits of the heterotrimeric G protein transducin enhanced Li+ uptake by about 3-fold. This stimulation was blocked by transducin alpha subunits, which by themselves did not influence Li+ uptake. Using various activators and inhibitors of protein kinases, it is demonstrated that the X. laevis oocyte Na+/H+ antiporter can be stimulated by activation of both protein kinase A and C. Stimulation of Na+/H+ exchanger activity by GTP gamma S but not that induced by transducin beta gamma subunits was blocked by the protein kinase A inhibitor H-89. On the other hand, transducin beta gamma subunit-stimulated activity was prevented by the protein kinase C inhibitor, calphostin C. The non-selective protein kinase inhibitor H-7 blocked both GTP gamma S- and transducin beta gamma subunit-stimulated Na+/H+ exchanger activity. The results suggest that the Na+/H+ exchanger of X. laevis oocytes can be activated by G proteins and that this activation is not direct but mediated by protein kinase A- and/or protein kinase C-dependent pathways.

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.

Inositol phospholipid metabolism in Xenopus oocytes mediated by endogenous G(o) and Gi proteins

To characterize G-proteins which mediate the signal transduction from ligand stimulated receptor to phospholipase C (PLC), we injected antisense DNAs complementary to Xenopus G(o) alpha or Gi-l alpha to suppress these endogenous G-proteins, together with the mRNAs encoding metabotropic glutamate receptor 1 (mGluR1), 5 (mGluR5) or with M1 type muscarinic receptor into oocytes. Receptor-stimulated chloride current responses were reduced by the suppression of Xenopus G(o) alpha regardless of the types of receptors. However, injection of Gi-1 antisense DNA resulted in the reduction of M1-stimulated responses but not mGluR-stimulated responses. These results suggested that all these receptors could use G(o) alpha, and M1 receptors, but not mGluRs, could also use Gi-1 proteins, to activate PLC in Xenopus oocytes.

Cloning of a Xenopus laevis muscarinic receptor encoded by an intronless gene

The Xenopus laevis oocyte has endogenous sites that bind muscarinic agonists, which have been pharmacologically characterized as M3 and/or M1 receptor subtypes. In order to define the molecular identify of the receptor protein we have analyzed a Xenopus oocyte cDNA library and cloned a 2.9 kb cDNA fragment encoding a muscarinic receptor (xMR). The deduced amino acid sequence reveals a protein of 484 residues with an apparent molecular weight of 54,188 Da. Amino acid comparison with previously cloned mammalian muscarinic receptors showed a 78% identity with the human m4 subtype, presenting at the same time clustered differences within the amino-terminal region and third intracellular loop Genomic Southern analysis displayed the presence of one main gene belonging to this subtype, and the PCR analysis revealed an intronless gene.

Neuromedin B receptor, expressed in Xenopus laevis oocytes, selectively couples to G alpha q and not G alpha 11

G-proteins of the q family have been implicated as mediators of bombesin receptors action. We cloned Xenopus G alpha q and G alpha 11 and specifically disrupted the synthesis of either protein with selective antisense oligonucleotides. G alpha q antisense inhibited responses mediated by neuromedin B receptor (NMB-R) by 74%, though not by gastrin-releasing peptide receptor (GRP-R). G alpha 11 antisense had little effect on either GRP-R- or NMB-R-mediated responses. This suggests that NMB-R couples to G alpha q, and that GRP-R and NMB-R show distinct G-protein coupling preferences in the Xenopus oocyte.

Impaired expression of Gs alpha protein mRNA in rat ventricular myocardium with aging

We have recently reported that the responsiveness of adrenoceptors is decreased with aging in rat ventricular myocardium. Thus, the current study determined aging-dependent changes in: (a) characteristics of myocardial G proteins as determined by Western blot analysis; (b) steady-state levels of G protein mRNA as determined by Northern blot analysis; and (c) the intropic response to isoproterenol, a beta-adrenoceptor agonist. Cardiac preparations were isolated from male Wistar rats of 6 (adult) and 24 (old) months old. Compared with adults, aging decreased the combined level of the three Gs alpha subunits (45, 47 and 52 kDa) by a total of 23% in ventricular membrane preparations. In contrast, levels of Gi alpha (40/41 kDa), Gq alpha (42 kDa), Go alpha (39 kDa) and G common beta (35/36 kDa) immunoreactivity were not affected by aging in the same membrane preparations. In ventricular myocardium, steady-state levels of Gs alpha mRNA (1.9 kb) decreased by 20-28% between 6 and 24 months of age with no change in Gi alpha mRNA (2.4 kb). An aging-associated decline in beta-adrenergic stimulation was observed in the maximum positive inotropic effect elicited by isoproterenol in the presence of prazosin in left papillary muscles, with no change in ED50 values. These results suggest that age-related changes in cardiac excitation and contraction coupling following beta-adrenoceptor stimulation are mediated, at least in part, by Gs alpha protein dysfunction.