G protein-mediated inhibition of phosphoinositide metabolism evoked by metabotropic glutamate receptors in frog oocytes

A seven-transmembrane receptor that mediates avoidance response to dihydrocaffeic acid, a water-soluble repellent in Caenorhabditis elegans

The ability to detect harmful chemicals rapidly is essential for the survival of all animals. In Caenorhabditis elegans (C. elegans), repellents trigger an avoidance response, causing animals to move away from repellents. Dihydrocaffeic acid (DHCA) is a water-soluble repellent and nonflavonoid catecholic compound that can be found in plant products. Using a Xenopus laevis (X. laevis) oocyte expression system, we identified a candidate dihydrocaffeic acid receptor (DCAR), DCAR-1. DCAR-1 is a novel seven-transmembrane protein that is expressed in the ASH avoidance sensory neurons of C. elegans. dcar-1 mutant animals are defective in avoidance response to DHCA, and cell-specific expression of dcar-1 in the ASH neurons of dcar-1 mutant animals rescued the defect in avoidance response to DHCA. Our findings identify DCAR-1 as the first seven-transmembrane receptor required for avoidance of a water-soluble repellent, DHCA, in C. elegans.

Oxygen/glucose deprivation induces a reduction in synaptic AMPA receptors on hippocampal CA3 neurons mediated by mGluR1 and adenosine A3 receptors

Hippocampal CA1 pyramidal neurons are highly sensitive to ischemic damage, whereas neighboring CA3 pyramidal neurons are less susceptible. It is proposed that switching of AMPA receptor (AMPAR) subunits on CA1 neurons during an in vitro model of ischemia, oxygen/glucose deprivation (OGD), leads to an enhanced permeability of AMPARs to Ca(2+), resulting in delayed cell death. However, it is unclear whether the same mechanisms exist in CA3 neurons and whether this underlies the differential sensitivity to ischemia. Here, we investigated the consequences of OGD for AMPAR function in CA3 neurons using electrophysiological recordings in rat hippocampal slices. Following a 15 min OGD protocol, a substantial depression of AMPAR-mediated synaptic transmission was observed at CA3 associational/commissural and mossy fiber synapses but not CA1 Schaffer collateral synapses. The depression of synaptic transmission following OGD was prevented by metabotropic glutamate receptor 1 (mGluR1) or A(3) receptor antagonists, indicating a role for both glutamate and adenosine release. Inhibition of PLC, PKC, or chelation of intracellular Ca(2+) also prevented the depression of synaptic transmission. Inclusion of peptides to interrupt the interaction between GluA2 and PICK1 or dynamin and amphiphysin prevented the depression of transmission, suggesting a dynamin and PICK1-dependent internalization of AMPARs after OGD. We also show that a reduction in surface and total AMPAR protein levels after OGD was prevented by mGluR1 or A(3) receptor antagonists, indicating that AMPARs are degraded following internalization. Thus, we describe a novel mechanism for the removal of AMPARs in CA3 pyramidal neurons following OGD that has the potential to reduce excitotoxicity and promote neuroprotection.

Virtual Screening for Selective Allosteric mGluR1 Antagonists and Structure–Activity Relationship Investigations for Coumarine Derivatives

A virtual screening study towards novel noncompetitive antagonists of the metabotropic glutamate receptor 1 (mGluR1) is described. Alignment-free topological pharmacophore descriptors (CATS) were used to encode the screening compounds. All virtual hits were characterized with respect to their allosteric antagonistic effect on mGluR1 in both functional and binding assays. Exceptionally high hit rates of up to 26 % were achieved, confirming the applicability of this virtual screening concept. Most of the compounds were found to be moderately active, however, one potent and subtype selective mGluR1 antagonist, 13 (IC(50): 0.362 microM, SEM +/-0.031; K(i): 0.753 microM, SEM +/-0.048), based on a coumarine scaffold was discovered. In a following activity optimization program a series of coumarine derivatives was synthesized. This led to the discovery of potent (60, IC(50): 0.058 microM, SEM +/-0.008; K(i): 0.293 microM, SEM +/-0.022) and subtype selective (rmGluR5 IC(50): 28.6 microM) mGluR1 antagonists. From our homology model of mGluR1 we derived a potential binding mode within the allosteric transmembrane region. Potential interacting patterns are proposed considering the difference of the binding pockets between rat and human receptors. The study demonstrates the applicability of ligand-based virtual screening for noncompetitive antagonists of a G-protein coupled receptor, resulting in novel, potent, and selective agents.

Identification of R(-)-isomer of efonidipine as a selective blocker of T-type Ca2+ channels

Efonidipine, a derivative of dihydropyridine Ca(2+) antagonist, is known to block both L- and T-type Ca(2+) channels. It remains to be clarified, however, whether efonidipine affects other voltage-dependent Ca(2+) channel subtypes such as N-, P/Q- and R-types, and whether the optical isomers of efonidipine have different selectivities in blocking these Ca(2+) channels, including L- and T-types. To address these issues, the effects of efonidipine and its R(-)- and S(+)-isomers on these Ca(2+) channel subtypes were examined electrophysiologically in the expression systems using Xenopus oocytes and baby hamster kidney cells (BHK tk-ts13). Efonidipine, a mixture of R(-)- and S(+)-isomers, exerted blocking actions on L- and T-types, but no effects on N-, P/Q- and R-type Ca(2+) channels. The selective blocking actions on L- and T-type channels were reproduced by the S(+)-efonidipine isomer. By contrast, the R(-)-efonidipine isomer preferentially blocked T-type channels. The blocking actions of efonidipine and its enantiomers were dependent on holding potentials. These findings indicate that the R(-)-isomer of efonidipine is a specific blocker of the T-type Ca(2+) channel.

Induction of Hypoxia-inducible Factor 1 Activity by Muscarinic Acetylcholine Receptor Signaling

Hypoxia-inducible factor-1 (HIF-1) is a master regulator of cellular adaptive responses to hypoxia. Levels of the HIF-1alpha subunit increase under hypoxic conditions. Exposure of cells to growth factors, prostaglandin, and certain nitric oxide donors also induces HIF-1alpha expression under non-hypoxic conditions. We demonstrate that muscarinic acetylcholine signals induce HIF-1alpha expression and transcriptional activity in a receptor subtype-specific manner using HEK293 cells transiently overexpressing each of M1-M4 muscarinic acetylcholine receptors. The muscarinic signaling pathways inhibited HIF-1alpha hydroxylation and degradation and induced HIF-1alpha protein synthesis that was confirmed by pulse labeling studies. Muscarinic signal-induced HIF-1alpha protein and HIF-1-dependent gene expression were blocked by treating cells with inhibitors of phosphatidylinositol 3-kinase, MAP kinase kinase, or tyrosine kinase signaling pathways. Dominant-negative forms of Ras and/or Rac-1 significantly suppressed HIF-1 activation by muscarinic signaling. Signaling via M1- and M3- but not M2- or M4-AchRs promote accumulation and transcriptional activation of HIF-1alpha. We conclude that muscarinic acetylcholine signals activate HIF-1 by both stabilization and synthesis of HIF-1alpha and by inducing the transcriptional activity of HIF-1alpha.

Activation of group I mGluRs elicits different responses in murine CA1 and CA3 pyramidal cells

The group I metabotropic glutamate receptor agonist DHPG has been shown to produce two major effects on CA3 pyramidal cells at rest: a reduction in the background conductance and an activation of a voltage-gated inward current (I(mGluR(V))). Both effects contribute to depolarising CA3 pyramidal cells and the latter has been implicated in eliciting prolonged epileptiform population bursts. We observed that DHPG-induced depolarisation was smaller in CA1 pyramidal cells than in CA3 cells. Voltage clamp studies revealed that while DHPG elicited I(mGluR(V)) in CA3 pyramidal cells, such a response was absent in CA1 pyramidal cells. Both mGluR1 and mGluR5 have been localised in CA3 pyramidal cells, whereas only mGluR5 has been detected in CA1 pyramidal cells. Using mGluR1 knockout mice, we evaluated whether the absence of an I(mGluR(V)) response can be correlated with the absence of mGluR1. In these experiments, DHPG failed to elicit I(mGluR(V)) in CA3 pyramidal cells. This suggests that the smaller depolarising effects of DHPG on wild-type CA1 pyramidal cells is caused, at least in part, by the absence of I(mGluR(V)) in these cells and that the difference in the responses of CA1 and CA3 cells may be attributable to the lack of mGluR1 in CA1 pyramidal cells.

Mutation (Ser284Leu) of neuronal nicotinic acetylcholine receptor alpha 4 subunit associated with frontal lobe epilepsy causes faster desensitization of the rat receptor expressed in oocytes

To date five mutations in two major constituents of neuronal nicotinic acetylcholine receptor (nAChR) in the brain, i.e. alpha4 and beta2 subunits, have been identified to be associated with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). Among them, only Ser284Leu, a point mutation in alpha4 subunit identified in ADNFLE as well as in a sporadic case with nocturnal frontal lobe epilepsy, remains to be characterized electrophysiologically. We examined the properties of rat nAChR harboring Ser284Leu reconstituted on Xenopus oocytes. Currents elicited in response to application of acetylcholine to oocytes expressing wild type or mutant nAChR were measured by a standard two-microelectrode voltage clamp method. Compared with wild-type nAChR, the mutant nAChR had a comparable EC(50) value for acetylcholine whereas it showed faster desensitization and lower Cs(+)/Na(+) permeability ratio. Ser284Phe, a putative mutation constructed for comparison, exhibited similar properties. These findings indicate that Ser(284) plays an important role in gating of nAChR along with Thr(276) and Ser(280), and suggest that mutation at Ser(284) could reduce nAChR activity similar to other mutations of alpha4 subunit found in ADNFLE.

Galpha(14) links a variety of G(i)- and G(s)-coupled receptors to the stimulation of phospholipase C

1. The bovine Galpha(14) is a member of the G(q) subfamily of G proteins that can regulate phospholipase Cbeta isoforms but the extent to which Galpha(14) recognizes different receptor classes is not known. 2. Galpha(14) was cotransfected with a variety of receptors in COS-7 cells, and agonist-induced stimulation of phospholipase C was then measured. 3. Activation of the type 2 but not type 1 somatostatin receptor in cells coexpressing Galpha(14) stimulated the accumulation of inositol phosphates; functional expression of both subtypes of somatostatin receptors was determined by the ability of somatostatin to inhibit cyclic AMP accumulation. 4. Among the three opioid receptors (mu, delta, and kappa), only the delta receptor was capable of stimulating IP formation when coexpressed with Galpha(14) in COS-7 cells. 5. A panel of G(i)- and G(s)-linked receptors was screened for their ability to stimulate IP accumulation via Galpha(14). The adenosine A(1), complement C5a, dopamine D(1), D(2) and D(5), formyl peptide, luteinizing hormone, secretin, and the three subtypes of melatonin (mt1, MT2, and Xenopus) receptors were all incapable of activating Galpha(14), while the alpha(2)- and beta(2)-adrenoceptors were able to do so. 6. Galpha(14)-mediated stimulation of phospholipase Cbeta was agonist dose-dependent. These data demonstrate that although Galpha(14) can interact with different classes of receptors, it is much less promiscuous than Galpha(15) or Galpha(16).

A region of the sulfonylurea receptor critical for a modulation of ATP-sensitive K(+) channels by G-protein betagamma-subunits

To determine the interaction site(s) of ATP-sensitive K(+) (K(ATP)) channels for G-proteins, sulfonylurea receptor (SUR2A or SUR1) and pore-forming (Kir6.2) subunits were reconstituted in the mammalian cell line, COS-7. Intracellular application of the G-protein betagamma2-subunits (G(betagamma)(2)) caused a reduction of ATP-induced inhibition of Kir6.2/SUR channel activities by lessening the ATP sensitivity of the channels. G(betagamma)(2) bound in vitro to both intracellular (loop-NBD) and C-terminal segments of SUR2A, each containing a nucleotide-binding domain (NBD). Furthermore, a single amino acid substitution in the loop-NBD of SUR (Arg656Ala in SUR2A or Arg665Ala in SUR1) abolished the G(betagamma)(2)-dependent alteration of the channel activities. These findings provide evidence that G(betagamma) modulates K(ATP) channels through a direct interaction with the loop-NBD of SUR.

Amino acid residues in the transmembrane domain of the type 1 sigma receptor critical for ligand binding

The type 1 sigma receptor expressed in Xenopus oocytes showed binding abilities for the sigma-1 ligands, [3H](+)pentazocine and [3H]NE-100, with similar kinetic properties as observed in native tissue membranes. Amino acid substitutions (Ser99Ala, Tyr103Phe and di-Leu105,106di-Ala) in the transmembrane domain did not alter the expression levels of the type 1 sigma receptor as determined by immunoblot analysis using an anti-type 1 sigma receptor antiserum. By contrast, ligand binding was significantly suppressed by the substitutions. These findings provide evidence that the transmembrane domain of the type 1 sigma receptor plays a critical role in ligand binding of this receptor.

Differential interactions of the C terminus and the cytoplasmic I-II loop of neuronal Ca2+ channels with G-protein alpha and beta gamma subunits. I. Molecular determination

Interactions of G-protein alpha (Galpha) and beta gamma subunits (Gbeta gamma) with N- (alpha1B) and P/Q-type (alpha1A) Ca2+ channels were investigated using the Xenopus oocyte expression system. Gi3alpha was found to inhibit both N- and P/Q-type channels by receptor agonists, whereas Gbeta1 gamma2 was responsible for prepulse facilitation of N-type channels. L-type channels (alpha1C) were not regulated by Galpha or Gbeta gamma. For N-type, prepulse facilitation mediated via Gbeta gamma was impaired when the cytoplasmic I-II loop (loop 1) was deleted or replaced with the alpha1C loop 1. Galpha-mediated inhibitions were also impaired by substitution of the alpha1C loop 1, but only when the C terminus was deleted. For P/Q-type, by contrast, deletion of the C terminus alone diminished Galpha-mediated inhibition. Moreover, a chimera of L-type with the alpha1B loop 1 gained Gbeta gamma-dependent facilitation, whereas an L-type chimera with the N- or P/Q-type C terminus gained Galpha-mediated inhibition. These findings provide evidence that loop 1 of N-type channels is a regulatory site for Gbeta gamma and the C termini of P/Q- and N-types for Galpha.

Differential interactions of the C terminus and the cytoplasmic I-II loop of neuronal Ca2+ channels with G-protein alpha and beta gamma subunits. II. Evidence for direct binding

The present study was designed to obtain evidence for direct interactions of G-protein alpha (Galpha) and beta gamma subunits (Gbeta gamma) with N- (alpha1B) and P/Q-type (alpha1A) Ca2+ channels, using synthetic peptides and fusion proteins derived from loop 1 (cytoplasmic loop between repeat I and II) and the C terminus of these channels. For N-type, prepulse facilitation as mediated by Gbeta gamma was impaired when a synthetic loop 1 peptide was applied intracellularly. Receptor agonist-induced inhibition of N-type as mediated by Galpha was also impaired by the loop 1 peptide but only when applied in combination with a C-terminal peptide. For P/Q-type channels, by contrast, the Galpha-mediated inhibition was diminished by application of a C-terminal peptide alone. Moreover, in vitro binding analysis for N- and P/Q-type channels revealed direct interaction of Galpha with C-terminal fusion proteins as well as direct interaction of Gbeta gamma with loop 1 fusion proteins. These findings define loop 1 of N- and P/Q-type Ca2+ channels as an interaction site for Gbeta gamma and the C termini for Galpha.

Role of the second and third intracellular loops of metabotropic glutamate receptors in mediating dual signal transduction activation

On the basis of sequence homology and structural similarities, metabotropic glutamate receptors (mGluRs), extracellular Ca2+-sensing receptor, gamma-aminobutyric acid type B receptor, and pheromone receptors are enlisted in a distinct family within the larger G protein-coupled receptor superfamily. When expressed in heterologous systems, group I mGluRs can activate dual signal transduction pathways, phosphoinositides turnover and cAMP production. To investigate the structural basis of these coupling properties, we introduced single amino acid substitutions within the second and third intracellular loops (i2 and i3) of mGluR1alpha. Wild-type and mutant receptors were expressed in human embryonic kidney 293 cells and analyzed for their capacity to stimulate both signaling cascades. Each domain appeared to be critical for the coupling to phospholipase C and adenylyl cyclase. Within i2, Thr695, Lys697, and Ser702 were found to be selectively involved in the interaction with Gq class alpha subunit(s), whereas mutation of Pro698 and the deletion Cys694-Thr695 affected only Gs coupling. Furthermore, the mutation K690A profoundly altered mGluR1alpha signaling properties and imparted to the receptor the ability to couple to the inhibitory cAMP pathway. Within i3, we uncovered two residues, Arg775 and Phe781, that are crucial for coupling to both pathways, since their substitution leads to receptor inactivation.

Enhanced type 1alpha metabotropic glutamate receptor-stimulated phosphoinositide signaling after pertussis toxin treatment

The regulation of phosphoinositide hydrolysis by the type 1alpha metabotropic glutamate receptor (mGluR1alpha) was investigated in stably transfected baby hamster kidney (BHK) cells. Incubation of the cells with L-glutamate, quisqualate, and 1-aminocyclopentane-1S, 3R-dicarboxylic acid resulted in a marked accumulation of [3H]inositol monophosphate (InsP1) and inositol-1,4,5-trisphosphate [Ins(1,4,5)P3] mass in a time- and concentration-dependent manner. Pretreatment of BHK-mGluR1alpha cells with pertussis toxin [ 100 ng/ml, 24 hr] led to a dramatic 12-16-fold increase in the accumulation of [3H]InsP1 and a 2-fold increase in Ins(1,4,5)P3 in the absence of added agonist. Although only very low levels (/=75%, and the EC50 shifted leftward by 65-fold [-log EC50 values (molar), 7.26 +/- 0.23 versus 5.45 +/- 0.07; n = 4) in PTX-treated compared with control cells. In contrast, antagonist effects on agonist-stimulated [3H]InsP1 responses were similar in control and PTX-treated BHK-mGluR1alpha cells. These changes in the concentration-effect curves for mGluR agonists are consistent with a model in which the receptor associates with PTX-sensitive inhibitory (Gi/o) and PTX-insensitive stimulatory (Gq/11) G proteins that can each influence PIC activity. The present observations are consistent with a dual regulation of mGluR1alpha-mediated PIC activity that could be fundamental in controlling the output of phosphoinositide-derived messengers.

Activation of the G protein Gq/11 through tyrosine phosphorylation of the alpha subunit

Various receptors coupled to the heterotrimeric guanine nucleotide-binding protein Gq/11 stimulate formation of inositol-1,4,5-trisphosphate (IP3). Activation of these receptors also induces protein tyrosine phosphorylation. Formation of IP3 in response to stimulated receptors that couple to Gq/11 was blocked by protein tyrosine kinase inhibitors. These inhibitors appeared to act before activation of Gq/11. Moreover, stimulation of receptors coupled to Gq/11 induced phosphorylation on a tyrosine residue (Tyr356) of the Galphaq/11 subunit, and this tyrosine phosphorylation event was essential for Gq/11 activation. Tyrosine phosphorylation of Galphaq/11 induced changes in its interaction with receptors. Therefore, tyrosine phosphorylation of Galphaq/11 appears to regulate the activation of Gq/11 protein.

Signalling functions and biochemical properties of pertussis toxin-resistant G-proteins

Pertussis toxin (PTX) has been widely used as a reagent to characterize the involvement of heterotrimeric G-proteins in signalling. This toxin catalyses the ADP-ribosylation of specific G-protein alpha subunits of the Gi family, and this modification prevents the occurrence of the receptor-G-protein interaction. This review focuses on the biochemical properties and signalling of those G-proteins historically classified as 'PTX-resistant' due to the inability of the toxin to influence signalling through them. These G-proteins include members of the Gq and G12 families and one Gi family member, i.e. Gz. Signalling pathways controlled by these G-proteins are well characterized only for Gq family members, which activate specific isoforms of phospholipase C, resulting in increases in intracellular calcium and activation of protein kinase C (PKC), among other responses. While members of the G12 family have been implicated in processes that regulate cell growth, and Gz has been shown to inhibit adenylate cyclase, the specific downstream targets to these G-proteins in vivo have not been clearly established. Since two of these proteins, G12 alpha and Gz alpha, are excellent substrates for PKC, there is the potential for cross-talk between their signalling and Gq-dependent processes leading to activation of PKC. In tissues that express these G-proteins, a number of guanine-nucleotide-dependent, PTX-resistant, signalling pathways have been defined for which the G-protein involved has not been identified. This review summarizes these pathways and discusses the evidence both for the participation of specific PTX-resistant G-proteins in them and for the regulation of these processes by PKC.

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.

Evidence for a metabostatic opioid kappa-receptor inhibiting pertussis toxin-sensitive metabotropic glutamate receptor-currents in Xenopus oocytes

Glutamate evoked pertussis toxin-sensitive currents in Xenopus oocytes expressing metabotropic glutamate receptor subtype 1 (mGluR1) and exogenous Gi1 alpha. The mGluR1-currents were completely blocked by U-73122, a phospholipase C (PLC) inhibitor and by niflumic acid, a chloride channel blocker. In the oocyte further coinjected with poly(A)+ RNA from the guinea pig cerebellum, the mGluR1-currents were inhibited by U-50488H, an opioid kappa-agonist, and this inhibition was blocked by norbinaltorphimine, an opioid kappa-antagonist. These findings suggest that the mRNA encoding a novel subtype of opioid kappa-receptor which inhibits Gi1-PLC-mediated currents is present in guinea pig cerebellar poly(A)+ fractions.

Determinants of PKC-dependent modulation of a family of neuronal calcium channels

The modulation of Ca2+ channel activity by protein kinases contributes to the dynamic regulation of neuronal physiology. Using the transient expression of a family of neuronal Ca2+ channels, we have identified several factors that contribute to the PKC-dependent modulation of Ca2+ channels. First, the nature of the Ca2+ channel alpha 1 subunit protein is critical. Both alpha 1B and alpha 1E channels exhibit a 30%-40% increase in peak currents after exposure to phorbol esters, whereas neither alpha 1A nor alpha 1C channels are significantly affected. This up-regulation can be mimicked for alpha 1E channels by stimulation of a coexpressed metabotropic glutamate receptor (type 1 alpha) through a PKC-dependent pathway. Second, PKC-stimulated up-regulation is dependent upon coexpression with a Ca2+ channel beta subunit. Third, substitution of the cytoplasmic domain I-II linker from alpha 1B confers PKC sensitivity to alpha 1A channels. The results provide direct evidence for the modulation of a subset of neuronal Ca2+ channels by PKC and implicate alpha 1 and beta subunit interactions in regulating channel activity via second messenger pathways.

An activated mutant of the alpha subunit of G(o) increases neurite outgrowth via protein kinase C

The GTP-binding protein, G(o), is present at very high concentration in the neuronal growth cone membrane. The expression of activated mutants of the a subunit of G(o) increases neurite outgrowth. To determine the intracellular mechanism for this outgrowth, we have examined activated alpha o-dependent outgrowth in the presence of agents which modulate different signal transduction cascades. Activation of protein kinase C with phorbol esters or with diacylglycerol prevents the alpha o-dependent increase in neurite extension. Inhibition of protein kinase C with staurosporine, with H7, or with long-term, high dose phorbol ester treatment resulted in greater neurite elongation, and no further increase after activated alpha o transfection. The protein phosphatase inhibitor, okadaic acid, also blocked the effect of activated alpha o. In contrast, tyrosine kinase inhibitors and agents which alter cAMP levels did not alter activated alpha o-dependent neurite extension. We tested a number of compounds which alter intracellular calcium levels. TMB-8 and thapsigargin prevented an increase in outgrowth by activated alpha o, but diltiazem, Bay K8644 and dantrolene had no effect on activated alpha o-dependent outgrowth. These studies suggest that activated alpha o increases neurite outgrowth by inhibiting protein kinase C and by modulating intracellular calcium release.

Characterization of Gq family G proteins GL1 alpha (G14 alpha), GL2 alpha (G11 alpha), and Gq alpha expressed in the baculovirus-insect cell system

The alpha subunits of Gq family G proteins, GL1 alpha (G14 alpha), GL2 alpha(G11 alpha), and Gq alpha were expressed with G protein beta 1 and gamma 2 subunits in insect cells using a baculovirus system. The trimeric forms of G proteins, GL1 (GL1 alpha beta gamma), GL2 (GL2 alpha beta gamma), and Gq (Gq alpha beta gamma), were solubilized by 1% sodium cholate and purified by sequential chromatography on three kinds of columns. GL1, GL2, and Gq activated phospholipase C-beta purified from bovine brain in the presence of aluminum fluoride to the same extent. Muscarinic acetylcholine receptor m1 subtype stimulated the guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) binding to GL1, GL2, and Gq in the presence of similar concentrations of carbamylcholine. When m1 receptor, G protein, and phospholipase C-beta were reconstituted in lipid vesicles, each subtype of Gq family G proteins mediated the activation of phospholipase C-beta by carbamylcholine in the presence of either 1 microM GTP gamma S or 1 mM GTP. Phospholipase C-beta stimulated the GTPase activity of GL1, GL2, and Gq in the presence of m1 receptor and carbamylcholine but did not stimulate the GTPase activity of GO. Protein kinase C phosphorylated m1 receptor and phospholipase C-beta, but the phosphorylation did not significantly affect the ability of the m1 receptor to stimulate phospholipase C-beta in the reconstitution system of purified proteins.

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.

A region of the muscarinic-gated atrial K+ channel critical for activation by G protein beta gamma subunits

Complementary DNAs encoding two types of inwardly rectifying K+ channels, GIRK1 and IRK1, have been cloned from rat atrium and mouse macrophage, respectively. GIRK1 expressed in Xenopus oocytes was activated by acetylcholine when m2 muscarinic acetylcholine receptor was coexpressed. The acetylcholine-induced activation of GIRK1 was enhanced by coexpression with the G protein beta 1 gamma 2 subunit but not the beta 1 gamma 1 or alpha subunits. Deletion of the C-terminus of GIRK1 impaired the channel activation associated with the beta 1 gamma 2 subunit. Moreover, replacement of the C-terminus of IRK1 with that of GIRK1 produced a chimera channel that was activated by the beta 1 gamma 2 subunit, whereas intact IRK1 was not activated by the beta 1 gamma 2 subunit. These findings define the C-terminus of GIRK1 as a regulatory region for the G protein beta gamma subunit.