Heterotrimeric G proteins containing G alpha i3 regulate multiple effector enzymes in the same cell. Activation of phospholipases C and A2 and inhibition of adenylyl cyclase

A photoswitchable GPCR-based opsin for presynaptic inhibition

Optical manipulations of genetically defined cell types have generated significant insights into the dynamics of neural circuits. While optogenetic activation has been relatively straightforward, rapid and reversible synaptic inhibition has proven more elusive. Here, we leveraged the natural ability of inhibitory presynaptic GPCRs to suppress synaptic transmission and characterize parapinopsin (PPO) as a GPCR-based opsin for terminal inhibition. PPO is a photoswitchable opsin that couples to Gi/o signaling cascades and is rapidly activated by pulsed blue light, switched off with amber light, and effective for repeated, prolonged, and reversible inhibition. PPO rapidly and reversibly inhibits glutamate, GABA, and dopamine release at presynaptic terminals. Furthermore, PPO alters reward behaviors in a time-locked and reversible manner in vivo. These results demonstrate that PPO fills a significant gap in the neuroscience toolkit for rapid and reversible synaptic inhibition and has broad utility for spatiotemporal control of inhibitory GPCR signaling cascades.

Ric-8A-mediated stabilization of the trimeric G protein subunit Gαi is inhibited by pertussis toxin-catalyzed ADP-ribosylation

The heterotrimeric G protein subunit Gαi can be activated by G protein-coupled receptors and the cytosolic protein Ric-8A, the latter of which is also known to prevent ubiquitin-dependent degradation of Gαi. Here we show that the amounts of the three Gαi-related proteins Gαi1, Gαi2, and Gαi3, but not that of Gαq, are rapidly decreased by cell treatment with pertussis toxin (PTX). The decrease appears to be due to ADP-ribosylation of Gαi, because PTX treatment does not affect the amount of a mutant Gαi2 carrying alanine substitution for Cys352, the residue that is ADP-ribosylated by the toxin. The presence of endogenous and exogenous Ric-8A increases Gαi stability as shown in cells treated with the protein synthesis inhibitor cycloheximide; however, Ric-8A fails to efficiently stabilize ADP-ribosylated Gαi. The failure agrees with the inability of Ric-8A to bind to ADP-ribosylated Gαi both in vitro and in vivo. Thus PTX appears to exert its pathological effects at least in part by converting Gαi to an unstable ADP-ribosylated form, in addition to the well-known inability of ADP-ribosylated Gαi to transduce signals triggered by G protein-coupled receptors.

Altered macrophage function in tumor necrosis factor α- and endotoxin-induced tolerance

Pretreatment of rats with a sublethal dose of human recombinant tumor necrosis factor-α (hrTNFα, 10 μg/kg i.p.) or Salmonella enteritidis LPS (100 μg/kg, i.p.) prevented death when a lethal dose of S. enteritidis lipopolysaccharide (LPS, 15 mg/kg i.p.) was administered 24 h later. The resistance to the lethal effect of LPS was associated with similar alterations of the functional phenotype of peritoneal macrophages from both groups. In ex vivo studies, peritoneal macrophages were harvested 24 h after vehicle (control), hrTNFα or LPS injection and stimulated in vitro with LPS. In macrophages collected from control rats, LPS stimulated arachidonic acid (AA) metabolism, as assessed by 6-keto-prostaglandin F1α (6-keto-PGF1α) levels, nitric oxide (NO) production, as assessed by nitrite, and interleukin 6 (IL-6) production. In macrophages from hrTNFα-pretreated or LPS pretreated rats, basal and LPS-stimulated 6-keto-PGF1α production were significantly reduced compared to controls, while nitrite production was increased (P < 0.001). LPS induced IL-6 synthesis was not affected in macrophages from hrTNFα-pretreated rats but was significantly reduced in stimulated macrophages from LPS treated rats. Furthermore, the macrophage membrane content of guanine nucleotide binding regulatory (G) protein subunits was determined. Macrophages collected from hrTNFα-pretreated rats exhibited a marked reduction of the membrane content of the Giα3 subunit compared to control macrophages, whereas the Giα1,2 and Gβ subunits were not significantly affected. The decrease in Giα3 in hrTNFα treated rats is similar to that previously observed in macrophages from LPS tolerant rats. The results demonstrate that hrTNFα induces cross tolerance to the lethal effect of LPS, and that tolerance induced by TNF or LPS is associated with differential changes in peritoneal macrophage mediator production. These changes may, in part, be a consequence of altered signal transduction via specific G proteins.

Novel variants in GNAI3 associated with auriculocondylar syndrome strengthen a common dominant negative effect

Auriculocondylar syndrome is a rare craniofacial disorder comprising core features of micrognathia, condyle dysplasia and question mark ear. Causative variants have been identified in PLCB4, GNAI3 and EDN1, which are predicted to function within the EDN1-EDNRA pathway during early pharyngeal arch patterning. To date, two GNAI3 variants in three families have been reported. Here we report three novel GNAI3 variants, one segregating with affected members in a family previously linked to 1p21.1-q23.3 and two de novo variants in simplex cases. Two variants occur in known functional motifs, the G1 and G4 boxes, and the third variant is one amino acid outside of the G1 box. Structural modeling shows that all five altered GNAI3 residues identified to date cluster in a region involved in GDP/GTP binding. We hypothesize that all GNAI3 variants lead to dominant negative effects.

Characterization of an orphan G protein-coupled receptor, GPR20, that constitutively activates Gi proteins

GPR20 was isolated as an orphan G protein-coupled receptor from genomic DNA by PCR amplification. Although GPR20 was closely related to nucleotide or lipid receptors, the functional role of this receptor, as well as its endogenous ligand, remains unclear. Here we demonstrate that GPR20 is constitutively active in the absence of ligand, leading to continuous activation of its coupled G proteins. When GPR20 was exogenously expressed in HEK293 cells, both the basal level and the prostaglandin E(2)-induced production of cAMP were significantly decreased. A remarkable increase in [(35)S]guanosine 5'-(gamma-thio)triphosphate (GTPgammaS) binding to membrane preparations was also observed in GPR20-expressing cells. These effects of GPR20 overexpression were diminished in cells treated with pertussis toxin, suggesting that the expression of GPR20 results in the activation of G(i/o) proteins. Involvement of GPR20 in the activation of G(i/o) proteins was also supported by evidence that the disruption of a conserved DRY motif in GPR20 attenuated both [(35)S]GTPgammaS incorporation and inhibition of the prostaglandin E(2)-induced cAMP production. Knockdown of GPR20 in PC12h cells resulted in an elevation of the basal cAMP level, suggesting that the endogenous GPR20 achieves a constitutively or spontaneously active conformation. Furthermore, enhancement of [(3)H]thymidine incorporation was also observed in the GPR20-silencing cells, implying that the GPR20 expression seems to attenuate PC12h cell growth. Taken together, these data indicate that GPR20 constitutively activates G(i) proteins without ligand stimulation. The receptor may be involved in cellular processes, including control of intracellular cAMP levels and mitogenic signaling.

Specificity of Gβγ Signaling to Kir3 Channels Depends on the Helical Domain of Pertussis Toxin-sensitive Gα Subunits

Acetylcholine signaling through muscarinic type 2 receptors activates atrial G protein-gated inwardly rectifying K(+) (Kir3) channels via the betagamma subunits of G proteins (Gbetagamma). Different combinations of recombinant Gbetagamma subunits have been shown to activate Kir3 channels in a similar manner. In native systems, however, only Gbetagamma subunits associated with the pertussis toxin-sensitive Galpha(i/o) subunits signal to K(+) channels. Additionally, in vitro binding experiments supported the notion that the C terminus of Kir3 channels interacts preferentially with Galpha(i) over Galpha(q). In this study we confirmed in two heterologous expression systems a preference of Galpha(i) over Galpha(q) in the activation of K(+) currents. To identify determinants of Gbetagamma signaling specificity, we first exchanged domains of Galpha(i) and Galpha(q) subunits responsible for receptor coupling selectivity and swapped their receptor coupling partners. Our results established that the G proteins, regardless of the receptor type to which they coupled, conferred specificity to Kir3 activation. We next tested signaling through chimeras between the Galpha(i) and Galpha(q) subunits in which the N terminus, the helical, or the GTPase domains of the Galpha subunits were exchanged. Our results revealed that the helical domain of Galpha(i) (residues 63-175) in the background of Galpha(q) could support Kir3 activation, whereas the reverse chimera could not. Moreover, the helical domain of the Galpha(i) subunit conferred "Galpha(i)-like" binding of the Kir3 C terminus to the Galpha(q) subunits that contained it. These results implicate the helical domain of Galpha(i) proteins as a critical determinant of Gbetagamma signaling specificity.

Histamine H3-receptor signaling in cardiac sympathetic nerves: Identification of a novel MAPK-PLA2-COX-PGE2-EP3R pathway

We hypothesized that the histamine H(3)-receptor (H(3)R)-mediated attenuation of norepinephrine (NE) exocytosis from cardiac sympathetic nerves results not only from a Galpha(i)-mediated inhibition of the adenylyl cyclase-cAMP-PKA pathway, but also from a Gbetagamma(i)-mediated activation of the MAPK-PLA(2) cascade, culminating in the formation of an arachidonate metabolite with anti-exocytotic characteristics (e.g., PGE(2)). We report that in Langendorff-perfused guinea-pig hearts and isolated sympathetic nerve endings (cardiac synaptosomes), H(3)R-mediated attenuation of K(+)-induced NE exocytosis was prevented by MAPK and PLA(2) inhibitors, and by cyclooxygenase and EP(3)-receptor (EP(3)R) antagonists. Moreover, H(3)R activation resulted in MAPK phosphorylation in H(3)R-transfected SH-SY5Y neuroblastoma cells, and in PLA(2) activation and PGE(2) production in cardiac synaptosomes; H(3)R-induced MAPK phosphorylation was prevented by an anti-betagamma peptide. Synergism between H(3)R and EP(3)R agonists (i.e., imetit and sulprostone, respectively) suggested that PGE(2) may be a downstream effector of the anti-exocytotic effect of H(3)R activation. Furthermore, the anti-exocytotic effect of imetit and sulprostone was potentiated by the N-type Ca(2+)-channel antagonist omega-conotoxin GVIA, and prevented by an anti-Gbetagamma peptide. Our findings imply that an EP(3)R Gbetagamma(i)-induced decrease in Ca(2+) influx through N-type Ca(2+)-channels is involved in the PGE(2)/EP(3)R-mediated attenuation of NE exocytosis elicited by H(3)R activation. Conceivably, activation of the Gbetagamma(i) subunit of H(3)R and EP(3)R may also inhibit Ca(2+) entry directly, independent of MAPK intervention. As heart failure, myocardial ischemia and arrhythmic dysfunction are associated with excessive local NE release, attenuation of NE release by H(3)R activation is cardioprotective. Accordingly, this novel H(3)R signaling pathway may ultimately bear therapeutic significance in hyper-adrenergic states.

G protein betagamma11 complex translocation is induced by Gi, Gq and Gs coupling receptors and is regulated by the alpha subunit type

G protein activation by Gi/Go coupling M2 muscarinic receptors, Gq coupling M3 receptors and Gs coupling beta2 adrenergic receptors causes rapid reversible translocation of the G protein gamma11 subunit from the plasma membrane to the Golgi complex. Co-translocation of the beta1 subunit suggests that gamma11 translocates as a betagamma complex. Pertussis toxin ADP ribosylation of the alphai subunit type or substitution of the C terminal domain of alphao with the corresponding region of alphas inhibits gamma11 translocation demonstrating that alpha subunit interaction with a receptor and its activation are requirements for the translocation. The rate of gamma11 translocation is sensitive to the rate of activation of the G protein alpha subunit. alpha subunit types that show high receptor activated rates of guanine nucleotide exchange in vitro support high rates of gamma11 translocation compared to alpha subunit types that have a relatively lower rate of guanine nucleotide exchange. The results suggest that the receptor induced translocation of gamma11 is controlled by the rate of cycling of the G protein through active and inactive forms. They also demonstrate that imaging of gamma11 translocation can be used as a non-invasive tool to measure the relative activities of wild type or mutant receptor and alpha subunit types in a live cell.

The apelin receptor is coupled to Gi1 or Gi2 protein and is differentially desensitized by apelin fragments

The apelin receptor is a G protein-coupled receptor to which two ligand fragments, apelin-(65-77) and apelin-(42-77), can bind. To address the physiological significance of the existence of dual ligands for a single receptor, we first compared the ability of the apelin fragments to regulate intracellular effectors, to promote G protein coupling, and to desensitize the response in Chinese hamster ovary cells expressing the murine apelin receptor. We found that both apelin fragments inhibited adenylyl cyclase and increased the phosphorylation of ERK or Akt. Using stably transfected cells expressing a pertussis toxin-insensitive alpha(i) subunit, we demonstrated that each apelin fragment promoted coupling of the apelin receptor to either Galpha(i1) or Galpha(i2) but not to Galpha(i3). Although preincubation with each apelin fragment induced a desensitization at the level of the three effectors, preincubation with apelin-(42-77) also increased basal effector activity. In addition, a C-terminal deletion of the apelin receptor decreased the desensitization induced by apelin-(65-77) but did not alter the desensitization pattern induced by apelin-(42-77). Finally, in umbilical endothelial cells, which we have recently shown to express the apelin receptor, the Galpha(i1) and Galpha(i2) subunits are also expressed, ERK and Akt phosphorylation is desensitized after preincubation with apelin-(65-77), and basal levels of Akt phosphorylation are increased after preincubation with apelin-(42-77). In summary, apelin fragments regulate the same effectors, via the preferential coupling of the apelin receptor to G(i1) or G(i2), but they promote a differential desensitization pattern that may be central to their respective physiological roles.

Comparison of the relative efficacy and potency of mu-opioid agonists to activate Galpha(i/o) proteins containing a pertussis toxin-insensitive mutation

Pertussis toxin (PTX)-insensitive mutants of Galpha(i/o) proteins expressed in C6mu cells were used to examine the hypothesis that there are agonist-specific conformational states of the mu-opioid receptor with coupling preferences to different Galpha(i/o) subtypes, as measured by the degree of stimulation of [(35)S]guanosine 5'-O-(3-thio)triphosphate (GTPgammaS) binding. Binding of [(35)S]GTPgammaS to endogenous Galpha(i/o) proteins stimulated by the full mu-opioid agonist [d-Ala(2),MePhe(4),Gly(5)-ol]enkephalin (DAMGO) was completely blocked by overnight treatment with 100 ng/ml PTX. Treatment for 4 h with lower concentrations led to a PTX-dependent reduction in the maximal effect of DAMGO but no alteration in the potency of DAMGO or morphine nor in the relative maximal effect (relative efficacy) of the partial agonists morphine and buprenorphine compared with the full agonist DAMGO. Using PTX-insensitive Galpha mutants in which the PTX-sensitive cysteine was replaced with isoleucine, the potency for a series of mu-opioid agonists was highest in cells expressing Galpha(i3) and Galpha(o) and lowest with Galpha(i1) and Galpha(i2), with no significant change in the order of potency, namely, etorphine >> endomorphin-1 = DAMGO = endomorphin-2 = fentanyl = morphine >> meperidine. The order of agonist relative efficacy, etorphine = DAMGO = endomorphin-1 = endomorphin-2 = fentanyl > or = morphine > or = meperidine > buprenorphine > or = nalbuphine, was also the same across all of the PTX-insensitive Galpha(i/o) subtypes. Highest relative efficacy to stimulate [(35)S]GTPgammaS binding was seen with Galpha(i3). Consequently, reported observations of agonist-directed trafficking at mu-opioid receptors most likely involve non-PTX-sensitive Galpha protein mechanisms.

Regulators of G-protein signaling form a quaternary complex with the agonist, receptor, and G-protein. A novel explanation for the acceleration of signaling activation kinetics

Regulators of G-protein signaling (RGS) proteins modulate signaling through heterotrimeric G-proteins. They act to enhance the intrinsic GTPase activity of the Galpha subunit but paradoxically have also been shown to enhance receptor-stimulated activation. To study this paradox, we used a G-protein gated K+ channel to report the dynamics of the G-protein cycle and fluorescence resonance energy transfer techniques with cyan and yellow fluorescent protein-tagged proteins to report physical interaction. Our data show that the acceleration of the activation kinetics is dissociated from deactivation kinetics and dependent on receptor and RGS type, G-protein isoform, and RGS expression levels. By using fluorescently tagged proteins, fluorescence resonance energy transfer microscopy showed a stable physical interaction between the G-protein alpha subunit and RGS (RGS8 and RGS7) that is independent of the functional state of the G-protein. RGS8 does not directly interact with G-protein-coupled receptors. Our data show participation of the RGS in the ternary complex between agonist-receptor and G-protein to form a "quaternary complex." Thus we propose a novel model for the action of RGS proteins in the G-protein cycle in which the RGS protein appears to enhance the "kinetic efficacy" of the ternary complex, by direct association with the G-protein alpha subunit.

Signal transduction underlying carbachol-induced contraction of rat urinary bladder. I. Phospholipases and Ca2+ sources

We have reexamined the muscarinic receptor subtype mediating carbachol-induced contraction of rat urinary bladder and investigated the role of phospholipase (PL)C, D, and A2 and of intra- and extracellular Ca2+ sources in this effect. Based on the nonsubtype-selective tolterodine, the highly M2 receptor-selective (R)-4-[2-[3-(4-methoxy-benzoylamino)-benzyl]-piperidin-1-ylmethyl]-piperidine-1-carboxylic acid amide (Ro-320-6206), and the highly M3 receptor-selective darifenacin and 3-(1-carbamoyl-1,1-diphenylmethyl)-1-(4-methoxyphenylethyl)pyrrolidine (APP), contraction occurs via M3 receptors. Carbachol stimulated inositol phosphate formation in rat bladder slices, and this was abolished by the phospholipase C inhibitor 1-(6-[([17beta]-3-methoxyestra-1,3,5[10]-trien-17-yl)-amino]hexyl)-1H-pyrrole-2,5-dione (U 73,122; 10 microM). Nevertheless, U 73,122 (1-10 microM) did not significantly affect carbachol-stimulated bladder contraction. Carbachol had only little effect on PLD activity in bladder slices, but the PLD inhibitor butan-1-ol, relative to its negative control butan-2-ol (0.3% each), caused detectable inhibition of carbachol-induced bladder contraction. The cytosolic PLA2 inhibitor arachidonyltrifluoromethyl ketone weakly inhibited carbachol-induced contraction at a concentration of 300 microM, but the cyclooxygenase inhibitor indomethacin (1-10 microM) remained without effect. The Ca2+ entry blocker nifedipine (10-100 nM) almost completely inhibited carbachol-induced bladder contraction. In contrast, 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole HCl (SKF 96,365; 10 microM), an inhibitor of store-operated Ca2+ channels, caused little inhibition. We conclude that carbachol-induced contraction of rat bladder largely depends on Ca2+ entry through nifedipine-sensitive channels and, perhaps, PLD, PLA2, and store-operated Ca2+ channels, whereas cyclooxygenase and, surprisingly, also PLC are not involved to a relevant extent.

Different roles for Gi and Go proteins in modulation of adenylyl cyclase type-2 activity

The effect of Gi/o protein-coupled receptors on adenylyl cyclase type 2 (AC2) has been studied in Sf9 insect cells. Stimulation of cells expressing AC2 with the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) led to a twofold stimulation of cAMP synthesis that could be blocked with the protein kinase C inhibitor GF109203X. Activation of a coexpressed alpha2A-adrenoceptor or muscarinic M4 receptor inhibited the stimulation by TPA almost completely in a pertussis toxin-sensitive manner. Activation of Gs proteins switched the response of the alpha2A-adrenoceptor to potentiation of prestimulated AC2 activity. The potentiation, but not the inhibition, could be blocked by a Gbetagamma scavenger. A novel methodological approach, whereby signalling through endogenous G proteins was ablated, was used to assess specific G protein species in the signal pathway. Expression of Go proteins (alphao1 + beta1gamma2) restored both the inhibition and the potentiation, whereas expression of Gi proteins (alphai1 + beta1gamma2) resulted in a potentiation of both the TPA- and the Gs-stimulated AC2 activity. The data presented supports the view of AC2 as a molecular switch and implicates this isoform as a target for Go protein-linked signalling.

A novel strategy to engineer functional fluorescent inhibitory G-protein alpha subunits

Signaling studies in living cells would be greatly facilitated by the development of functional fluorescently tagged G-protein alpha subunits. We have designed G(i/o)alpha subunits fused to the cyan fluorescent protein and assayed their function by studying the following two signal transduction pathways: the regulation of G-protein-gated inwardly rectifying K(+) channels (Kir3.0 family) and adenylate cyclase. Palmitoylation and myristoylation consensus sites were removed from G(i/o) alpha subunits (G(i1)alpha, G(i2)alpha, G(i3)alpha, and G(oA)alpha) and a mutation introduced at Cys(-4) rendering the subunit resistant to pertussis toxin. This construct was fused in-frame with cyan fluorescent protein containing a short peptide motif from GAP43 that directs palmitoylation and thus membrane targeting. Western blotting confirmed G(i/o)alpha protein expression. Confocal microscopy and biochemical fractionation studies revealed membrane localization. Each mutant G(i/o) alpha subunit significantly reduced basal current density when transiently expressed in a stable cell line expressing Kir3.1 and Kir3.2A, consistent with the sequestration of the Gbetagamma dimer by the mutant Galpha subunit. Moreover, each subunit was able to support A1-mediated and D2S-mediated channel activation when transiently expressed in pertussis toxin-treated cells. Overexpression of tagged G(i3)alpha and G(oA)alpha alpha subunits reduced receptor-mediated and forskolin-induced cAMP mobilization.

Synergistic outside-in regulation of platelet activation by GPIIb/IIIa ligand-induced conformation and oligomerization

Full platelet activation with serotonin secretion and thromboxane A(2) (TxA(2)) formation induced by a low dose of thrombin receptor agonist peptide (TRAP) or high dose ADP requires platelet aggregation. This requirement can be replaced by pretreatment of platelets with a combination of reagents including: GPIIb/IIIa inhibitors yielding ligand-induced binding sites (LIBS), either arginine-glycine-aspartate-serine (RGDS) peptide or Ro 43-5054, cytochalasin to disrupt actin filaments and crosslinking by a GPIIb/IIIa mAb (pl-62). Crosslinking is required since Fab fragments of pl-62 do not support activation. Engagement of the Fc receptor by the mAb Fc domain is not required for pl-62 augmentation, since it is not blocked by the anti-Fc receptor mAb, IV-3. Another GPIIb/IIIa inhibitor, Ro 44-9883, not yielding LIBS epitopes, serves as a negative control and shows a requirement for LIBS in addition to crosslinking. Focal adhesion kinase tyrosine phosphorylation induced by TRAP is blocked by these GPIIb/IIIa antagonists, but restored by pl-62 crosslinking independent of LIBS induction. Tyrosine phosphorylation of a peptide comigrating with p38 MAP kinase is also inhibited by these antagonists and restored by pl-62 crosslinking. However, p38 MAP kinase activation by low dose TRAP is not affected by these aggregation inhibitors. Tyrosine phosphorylation of a 34-kDa phosphoprotein in the absence of aggregation or TxA(2) formation was uniquely augmented by Ro 43-5054 but not Ro 44-9883 under the above activation conditions.

Edg2 receptor functionality: gialpha1 coexpression and fusion protein studies

Recombinant receptor cell lines are widely used in G-protein-coupled receptor selectivity studies. To unequivocally interpret the results of such studies, it is essential that the host cell line does not endogenously express the receptor of interest and in addition is unresponsive to the receptor's natural ligand. Here we describe an approach to overcome such difficulties associated with orphan receptors or, as in the present case, receptors whose endogenous ligand ubiquitously affects mammalian cells. The functional heterologous assay system described is for the hEdg2 receptor, which uses lysophosphatidic acid as its endogenous ligand. Once activated, this receptor mediates its effects via multiple secondary messenger pathways, including a Gi-coupled pathway. We have transiently expressed a pertussis toxin-insensitive hEdg2 receptor-ratGialpha1 fusion protein into human embryonic kidney cells and have monitored the ability of compounds to stimulate [(35)S]GTPgammaS binding in membranes prepared from these cells after pretreatment with toxin. Because the assay conditions used favor Gi-mediated responses and because endogenous Gialpha subunits are rendered inactive, the response measured is, by definition, fusion protein-mediated. Consequently, we have developed an assay that monitors definitively Edg2 receptor-mediated responses in a mammalian cell line. A limited structure activity relationship study suggests that the lysophospholipid carbon chain has a role in receptor activation and in addition indicates that certain modifications to the phosphate group are tolerated.

The role of members of the pertussis toxin-sensitive family of G proteins in coupling receptors to the activation of the G protein-gated inwardly rectifying potassium channel

Inwardly rectifying potassium (K(+)) channels gated by G proteins (Kir3.x family) are widely distributed in neuronal, atrial, and endocrine tissues and play key roles in generating late inhibitory postsynaptic potentials, slowing the heart rate and modulating hormone release. They are directly activated by G(betagamma) subunits released from G protein heterotrimers of the G(i/o) family upon appropriate receptor stimulation. Here we examine the role of isoforms of pertussis toxin (PTx)-sensitive G protein alpha subunits (G(ialpha1-3) and G(oalphaA)) in mediating coupling between various receptor systems (A(1), alpha(2A), D(2S), M(4), GABA(B)1a+2, and GABA(B)1b+2) and the cloned counterpart of the neuronal channel (Kir3.1+3.2A). The expression of mutant PTx-resistant G(i/oalpha) subunits in PTx-treated HEK293 cells stably expressing Kir3.1+3.2A allows us to selectively investigate that coupling. We find that, for those receptors (A(1), alpha(2A)) known to interact with all isoforms, G(ialpha1-3) and G(oalphaA) can all support a significant degree of coupling to Kir3.1+3.2A. The M(4) receptor appears to preferentially couple to G(ialpha2) while another group of receptors (D(2S), GABA(B)1a+2, GABA(B)1b+2) activates the channel predominantly through G(betagamma) liberated from G(oA) heterotrimers. Interestingly, we have also found a distinct difference in G protein coupling between the two splice variants of GABA(B)1. Our data reveal selective pathways of receptor activation through different G(i/oalpha) isoforms for stimulation of the G protein-gated inwardly rectifying K(+) channel.

Increase of [Ca(2+)]i and release of arachidonic acid via activation of M2 receptor coupled to Gi and rho proteins in oesophageal muscle

We have previously shown that acetylcholine-induced contraction of oesophageal circular muscle depends on activation of phosphatidylcholine selective phospholipase C and D, which result in formation of diacylglycerol, and of phospholipase 2 which produces arachidonic acid. Diacylglycerol and arachidonic acid interact synergistically to activate protein kinase C. We have therefore investigated the relationship between cytosolic Ca(2+) and activation of phospholipase A(2) in response to acetylcholine-induced stimulation, by measuring the intracellular free Ca(2+) ([Ca(2+)]i), muscle tension, and [3H] arachidonic acid release. Acetylcholine-induced contraction was associated with increased [Ca(2+)]i and arachidonic acid release in a dose-dependent manner. In Ca(2+)-free medium, acetylcholine did not produce contraction, [Ca(2+)]i increase, and arachidonic acid release. In contrast, after depletion of Ca(2+) stores by thapsigargin (3 microM), acetylcholine caused a normal contraction, [Ca(2+)]i increase and arachidonic acid release. The increase in [Ca(2+)]i and arachidonic acid release were attenuated by the M2 receptor antagonist methoctramine, but not by the M3 receptor antagonist p-fluoro-hexahydro siladifenidol. Increase in [Ca(2+)]i and arachidonic acid release by acetylcholine were inhibited by pertussis toxin and C3 toxin. These findings indicate that contraction and arachidonic acid release are mediated through muscarinic M2 coupled to Gi or rho protein activation and Ca(2+) influx. Acetylcholine-induced contraction and the associated increase in [Ca(2+)]i and release of arachidonic acid were completely reduced by the combination treatment with a phospholipase A(2) inhibitor dimethyleicosadienoic acid and a phospholipase D inhibitor pCMB. They increased by the action of the inhibitor of diacylglycerol kinase R59949, whereas they decreased by a protein kinase C inhibitor chelerythrine. These data suggest that in oesophageal circular muscle acetylcholine-induced [Ca(2+)]i increase and arachidonic acid release are mediated through activation of M2 receptor coupled to Gi or rho protein, resulting in the activation of phospholipase A(2) and phospholipase D to activate protein kinase C.

Distinct roles for Galpha(i)2 and Gbetagamma in signaling to DNA synthesis and Galpha(i)3 in cellular transformation by dopamine D2S receptor activation in BALB/c 3T3 cells

Control of cell proliferation depends on intracellular mediators that determine the cellular response to external cues. In neuroendocrine cells, the dopamine D2 receptor short form (D2S receptor) inhibits cell proliferation, whereas in mesenchymal cells the same receptor enhances cell proliferation. Nontransformed BALB/c 3T3 fibroblast cells were stably transfected with the D2S receptor cDNA to study the G proteins that direct D2S signaling to stimulate cell proliferation. Pertussis toxin inactivates G(i) and G(o) proteins and blocks signaling of the D2S receptor in these cells. D2S receptor signaling was reconstituted by individually transfecting pertussis toxin-resistant Galpha(i/o) subunit mutants and measuring D2-induced responses in pertussis toxin-treated cells. This approach identified Galpha(i)2 and Galpha(i)3 as mediators of the D2S receptor-mediated inhibition of forskolin-stimulated adenylyl cyclase activity; Galpha(i)2-mediated D2S-induced stimulation of p42 and p44 mitogen-activated kinase (MAPK) and DNA synthesis, whereas Galpha(i)3 was required for formation of transformed foci. Transfection of toxin-resistant Galpha(i)1 cDNA induced abnormal cell growth independent of D2S receptor activation, while Galpha(o) inhibited dopamine-induced transformation. The role of Gbetagamma subunits was assessed by ectopic expression of the carboxyl-terminal domain of G protein receptor kinase to selectively antagonize Gbetagamma activity. Mobilization of Gbetagamma subunits was required for D2S-induced calcium mobilization, MAPK activation, and DNA synthesis. These findings reveal a remarkable and distinct G protein specificity for D2S receptor-mediated signaling to initiate DNA synthesis (Galpha(i)2 and Gbetagamma) and oncogenic transformation (Galpha(i)3), and they indicate that acute activation of MAPK correlates with enhanced DNA synthesis but not with transformation.

Degradation of heterotrimeric Galpha(o) subunits via the proteosome pathway is induced by the hsp90-specific compound geldanamycin

One mechanism utilized by cells to maintain signaling pathways is to regulate the levels of specific signal transduction proteins. The compound geldanamycin (GA) specifically interacts with heat shock protein 90 (hsp90) complexes and has been widely utilized to study the role of hsp90 in modulating the function of signaling proteins. In this study, we used GA to demonstrate that levels of heterotrimeric Galpha subunits can be regulated through interactions with hsp90. In a dose-dependent manner, GA significantly reduced the steady state levels of endogenous Galpha(o) expression in two cell lines (PC12 and GH3) and had a similar effect on Galpha(o) transiently expressed in COS cells. Galpha(o) synthesis and degradation was studied in PC12 cells and in transiently transfected COS cells. (35)S labeling followed by immunoprecipitation demonstrated no effect of GA on the rate of Galpha(o) synthesis, but GA accelerated degradation of Galpha(o) in both PC12 cells and COS cells. The use of inhibitors, including lactacystin (a proteosome-specific inhibitor), suggests that Galpha(o) is predominantly degraded through the proteosome pathway. In vitro translated (35)S-labeled Galpha(o) could be detected in hsp90 immunoprecipitates, and this interaction did not require N-terminal myristoylation. Taken together, these results suggest that heterotrimeric Galpha(o) subunits are protected from degradation by interaction with hsp90 and that the interaction of Galpha subunits with heat shock proteins may be a general mechanism for regulating Galpha levels in the cell.

Effect of G protein heterotrimer composition on coupling of neurotransmitter receptors to N-type Ca(2+) channel modulation in sympathetic neurons

Voltage-dependent (VD) inhibition of N-type Ca(2+) channels is mediated primarily by neurotransmitter receptors that couple to pertussis toxin (PTX)-sensitive G proteins (such as G(o) and G(i)). To date, however, the composition of heterotrimeric complexes, i.e., specific Galphabetagamma combinations, capable of coupling receptors to N-type Ca(2+) channels has not been defined. We addressed this question by heterologously expressing identified Galphabetagamma combinations in PTX-treated rat sympathetic neurons and testing for reconstitution of agonist-mediated VD inhibition. The heterologously expressed Galpha subunits were rendered PTX-insensitive by mutating the codon specifying the ADP ribosylation site. The following results were obtained from this approach. (i) Expression of Galpha(oA), Galpha(oB), and Galpha(i2) (along with Gbeta(1)gamma(2)) reconstituted VD inhibition mediated by alpha(2)-adrenergic, adenosine, somatostatin, and prostaglandin E(2) receptors. Conversely, expression of Galpha(i1) and Galpha(i3) was ineffective at restoring coupling. (ii) Coupling efficiency, as determined from the magnitude of reconstituted Ca(2+) current inhibition, depended on both the receptor and Galpha subtype. The following rank order of coupling efficiency was observed: Galpha(oA) = Galpha(oB) > Galpha(i2) for alpha(2)-adrenergic receptor; Galpha(i2) > Galpha(oA) = Galpha(oB) for adenosine and prostaglandin E(2) receptors; and Galpha(oB) = Galpha(i2) > Galpha(oA) for the somatostatin receptor. (iii) In general, varying the Gbetagamma composition of Galpha(oA)-containing heterotrimers had little effect on the coupling of alpha(2)-adrenergic receptors to the VD pathway. Taken together, these results suggest that multiple, diverse Galphabetagamma combinations are capable of coupling neurotransmitter receptors to VD inhibition of N-type Ca(2+) channels. Thus, if exquisite Galphabetagamma-coupling specificity exists in situ, it cannot arise solely from the inherent inability of other Galphabetagamma combinations to form functional signaling complexes.

The G protein alpha subunit has a key role in determining the specificity of coupling to, but not the activation of, G protein-gated inwardly rectifying K(+) channels

In neuronal and atrial tissue, G protein-gated inwardly rectifying K(+) channels (Kir3.x family) are responsible for mediating inhibitory postsynaptic potentials and slowing the heart rate. They are activated by Gbetagamma dimers released in response to the stimulation of receptors coupled to inhibitory G proteins of the G(i/o) family but not receptors coupled to the stimulatory G protein G(s). We have used biochemical, electrophysiological, and molecular biology techniques to examine this specificity of channel activation. In this study we have succeeded in reconstituting such specificity in an heterologous expression system stably expressing a cloned counterpart of the neuronal channel (Kir3.1 and Kir3.2A heteromultimers). The use of pertussis toxin-resistant G protein alpha subunits and chimeras between G(i1) and G(s) indicate a central role for the G protein alpha subunits in determining receptor specificity of coupling to, but not activation of, G protein-gated inwardly rectifying K(+) channels.

A G protein gamma subunit-specific peptide inhibits muscarinic receptor signaling

Muscarinic acetylcholine receptors modulate the function of a variety of effectors through heterotrimeric G proteins. A prenylated peptide specific to the G protein gamma5 subunit type inhibits G protein activation by the M2 muscarinic receptor in a reconstitution assay. Scrambling the amino acid sequence of the peptide significantly reduces the efficacy of the peptide. The peptide does not disrupt the G protein heterotrimer. In cultured sympathetic neurons, the gamma5 peptide inhibits modulation of Ca(2+) current by the M4 receptor. Peptide activity is specific, the scrambled peptide and peptides specific to two other members of the G protein gamma subunit family are significantly less effective. The gamma5 peptide has no effect on Ca(2+) current modulation by the alpha2-adrenergic and somatostatin receptors. In addition, the gamma5 peptide inhibits muscarinic receptor signaling in spinal cord slices with specificity. These results support a specific role for G protein gamma subunit types in signal transduction, most likely at the receptor-G protein interface.

Functional analysis of a human A(1) adenosine receptor/green fluorescent protein/G(i1)alpha fusion protein following stable expression in CHO cells

Fusion proteins between the human A(1) adenosine receptor and the pertussis toxin resistant (Cys351Gly) mutant of the G-protein alpha subunit G(i1)alpha (A1/Gi), and between the human A(1) adenosine receptor, the Aequorea victoria green fluorescent protein (GFP) and Cys351Gly G(i1)alpha (A1/GFP/Gi), were expressed in CHO cells. The agonist NECA caused a stimulation of [(35)S]GTPgammaS binding at both fusion proteins with similar concentration dependence as at the native receptor. However in the presence of pertussis toxin NECA stimulation of [(35)S]GTPgammaS binding was only seen at the A1/GFP/Gi fusion protein. The regulation of the adenylyl cyclase and MAP kinase effector systems by both fusion proteins was attenuated following pertussis toxin treatment. These studies demonstrate for the first time the characterisation of a fusion protein between a G-protein coupled receptor, GFP and a G-protein alpha subunit.

Interaction of heterotrimeric G protein Galphao with Purkinje cell protein-2. Evidence for a novel nucleotide exchange factor

The heterotrimeric G protein Galphao is ubiquitously expressed throughout the central nervous system, but many of its functions remain to be defined. To search for novel proteins that interact with Galphao, a mouse brain library was screened using the yeast two-hybrid interaction system. Pcp2 (Purkinje cell protein-2) was identified as a partner for Galphao in this system. Pcp2 is expressed in cerebellar Purkinje cells and retinal bipolar neurons, two locations where Galphao is also expressed. Pcp2 was first identified as a candidate gene to explain Purkinje cell degeneration in pcd mice (Nordquist, D. T., Kozak, C. A., and Orr, H. T. (1988) J. Neurosci. 8, 4780-4789), but its function remains unknown as Pcp2 knockout mice are normal (Mohn, A. R., Feddersen, R. M., Nguyen, M. S., and Koller, B. H. (1997) Mol. Cell. Neurosci. 9, 63-76). Galphao and Pcp2 binding was confirmed in vitro using glutathione S-transferase-Pcp2 fusion proteins and in vitro translated [35S]methionine-labeled Galphao. In addition, when Galphao and Pcp2 were cotransfected into COS cells, Galphao was detected in immunoprecipitates of Pcp2. To determine whether Pcp2 could modulate Galphao function, kinetic constants kcat and koff of bovine brain Galphao were determined in the presence and absence of Pcp2. Pcp2 stimulates GDP release from Galphao more than 5-fold without affecting kcat. These findings define a novel nucleotide exchange function for Pcp2 and suggest that the interaction between Pcp2 and Galphao is important to Purkinje cell function.

Distinct roles for Galphai2, Galphai3, and Gbeta gamma in modulation offorskolin- or Gs-mediated cAMP accumulation and calcium mobilization by dopamine D2S receptors

Previous studies have shown that a single G protein-coupled receptor can regulate different effector systems by signaling through multiple subtypes of heterotrimeric G proteins. In LD2S fibroblast cells, the dopamine D2S receptor couples to pertussis toxin (PTX)-sensitive Gi/Go proteins to inhibit forskolin- or prostaglandin E1-stimulated cAMP production and to stimulate calcium mobilization. To analyze the role of distinct Galphai/o protein subtypes, LD2S cells were stably transfected with a series of PTX-insensitive Galphai/o protein Cys --> Ser point mutants and assayed for D2S receptor signaling after PTX treatment. The level of expression of the transfected Galpha mutant subunits was similar to the endogenous level of the most abundant Galphai/o proteins (Galphao, Galphai3). D2S receptor-mediated inhibition of forskolin-stimulated cAMP production was retained only in clones expressing mutant Galphai2. In contrast, the D2S receptor utilized Galphai3 to inhibit PGE1-induced (Gs-coupled) enhancement of cAMP production. Following stable or transient transfection, no single or pair set of mutant Galphai/o subtypes rescued the D2S-mediated calcium response following PTX pretreatment. On the other hand, in LD2S cells stably transfected with GRK-CT, a receptor kinase fragment that specifically antagonizes Gbeta gamma subunit activity, D2S receptor-mediated calcium mobilization was blocked. The observed specificity of Galphai2 and Galphai3 for different states of adenylyl cyclase activation suggests a higher level of specificity for interaction of Galphai subunits with forskolin- versus Gs-activated states of adenylyl cyclase than has been previously appreciated.

Roles of Gi and Gq/11 in mediating desensitization of the luteinizing hormone/choriogonadotropin receptor in porcine ovarian follicular membranes

Although desensitization of most guanine nucleotide-binding (G) protein receptors is triggered by phosphorylation of the receptor, desensitization of the LH/CG receptor (-R) in porcine follicular ovarian membranes appears to be independent of LH/CG-R phosphorylation. We therefore evaluated whether desensitization of the LH/CG-R reflected a direct inhibition of adenylyl cyclase (AC) activity by either the alpha-subunit of Gi or betagamma-subunits derived from any of the membrane G proteins activated in response to LH/CG-R activation or whether desensitization reflected a competition between Gs and a G protein that activated phospholipase C for binding sites on the LH/CG-R. The results showed that follicular membrane AC activity was not inhibited upon activation of the LH/CG-R despite evidence that the ACs in follicular membranes, when maximally activated by forskolin, could be inhibited when membrane G proteins were activated by guanyl-5'-yl imidodiphosphate, and that pertussis toxin pretreatment of membranes raised forskolin-stimulated AC activity, consistent with a tonic inhibition of follicular membrane AC activity. Similarly, agonist-stimulated desensitization of LH/CG-R-stimulated AC activity was not inhibited by pertussis toxin. Therefore, desensitization is not the result of inhibition of AC mediated by an inhibitory Gi subunit. Follicular membrane AC was also not inhibited by Gbetagamma subunits freed with activation of Gs Gq/11, or G13, based on the inabilities of exogenous Gbetagamma to promote desensitization and of a protein that sequesters Gbetagamma to inhibit desensitization. Desensitization was also not inhibited by a Gq/11 C-terminal peptide or antiserum directed toward the C-terminus of Gq/11, nor was it reversed with the addition of Gbetagamma to membranes exhibiting desensitized LH/CG-R, suggesting that desensitization is independent of coupling of the LH/CG-R to Gq/11. These results indicate that agonist-dependent desensitization of LH/CG-R-stimulated AC activity is mediated by a unique mechanism.

Modulation of relative intrinsic activity of agonists at the alpha-2A adrenoceptor by mutation of residue 351 of G protein gi1alpha

Compared with epinephrine, the relative intrinsic activity of a series of partial agonists to activate fusion proteins between the porcine alpha-2A adrenoceptor and the alpha-subunit of Gi1 was reduced after a single-point mutation (Cys351Gly) in the G protein. Although UK14304 was close to a full agonist at the fusion construct containing wild-type (Cys351)Gi1alpha, it was a partial agonist at that containing Gly351Gi1alpha. Moreover, although clonidine functioned as a good partial agonist to activate the fusion protein containing Cys351Gi1alpha, it was essentially an antagonist at the Gly351Gi1alpha-containing fusion protein. By contrast, incorporation of Ile351Gi1alpha into the fusion protein resulted in all partial agonists displaying higher intrinsic activity relative to epinephrine to activate this fusion protein than the one containing the wild-type G protein sequence. This is the first demonstration that the relative intrinsic activity of a series of agonists can be modified by a point mutation in a G protein rather than a receptor and indicates that the nature of a key contact site between a G protein and a receptor can selectively regulate partial agonist function. We provide a model for this based on the hydrophobicity of a key receptor-G protein alpha-subunit interaction interface.

Molecular basis of receptor/G-protein-coupling selectivity

Molecular cloning studies have shown that G-protein-coupled receptors form one of the largest protein families found in nature, and it is estimated that approximately 1000 different such receptors exist in mammals. Characteristically, when activated by the appropriate ligand, an individual receptor can recognize and activate only a limited set of the many structurally closely related heterotrimeric G-proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level has become the focus of an ever increasing number of laboratories. This review provides an overview of recent structural, molecular genetic, biochemical, and biophysical studies that have led to novel insights into the molecular mechanisms governing receptor-mediated G-protein activation and receptor/G-protein coupling selectivity.

Vesicle budding on Golgi membranes: regulation by G proteins and myosin motors

One of the main functions of the Golgi complex is to generate transport vesicles for the post-Golgi trafficking of proteins in secretory pathways. Many different populations of vesicles are distinguished by unique sets of structural and regulatory proteins which participate in vesicle budding and fusion. Monomeric and heterotrimeric G proteins regulate vesicle budding and secretory traffic into and out of the Golgi complex. An inventory of G protein alpha subunits associated with Golgi membranes highlights their diverse involvement and potential for coupling Golgi trafficking, through various signal transduction pathways, to cell growth or other more specialized cell functions. Cytoskeletal proteins are now also known to associate specifically with the Golgi complex and Golgi-derived vesicles. Amongst these, conventional and unconventional myosins are recruited to vesicle membranes. Several roles in vesicle budding and vesicle trafficking can be proposed for these actin-based motors.

Quantitative analysis of a cysteine351glycine mutation in the G protein Gi1alpha: effect on alpha2A-adrenoceptor-Gi1alpha fusion protein activation

Fusion proteins were constructed between the porcine alpha2A-adrenoceptor and either wild-type (Cys351) or a pertussis toxin-resistant (Gly351) form of the G protein Gi1alpha. Addition of adrenaline to membranes expressing the fusion proteins resulted in concentration-dependent stimulation of their high affinity GTPase activity. The alpha2A-adrenoceptor-wild type Gi1alpha fusion protein produced substantially higher maximal stimulation of GTPase activity in response to adrenaline than that containing Gly351 Gi1alpha. Treatment of the fusion proteins as agonist-regulated enzymes allowed measurement of Vmax and turnover number for adrenaline-stimulation of the GTPase activity of each fusion construct. The turnover number of the alpha2A-adrenoceptor-Cys351 Gly Gi1alpha fusion protein was only 44'S, of that for the alpha2A-adrenoceptor-wild type Gi1alpha fusion protein. These data provide the first direct quantitative evaluation of the effects of a mutation of a G protein on the capacity of an agonist-occupied receptor to activate the mutant.

Concomitant regulation of Ca2+ mobilization and G13 expression in human erythroleukemia cells

In human erythroleukemia (HEL) cells, stimulation of alpha2-adrenoceptors by adrenaline or neuropeptide Y Y1 receptors by neuropeptide Y, concomitantly inhibit cAMP accumulation and stimulate mobilization of Ca2+ from intracellular stores via pertussis toxin-sensitive G-proteins. Treatment of HEL cells in chemically-defined, serum-free medium with 1.25% dimethylsulfoxide (DMSO) for 4 days, increased alpha2-adrenoceptor number by 120%, while the neuropeptide Y receptor number was not significantly changed. In DMSO-treated HEL cells, Ca2+ elevations by adrenaline or neuropeptide Y were significantly reduced by 28% and 57%, respectively, while basal Ca2+ and elevations by thrombin or thapsigargin were not significantly altered. Adrenaline and neuropeptide Y-induced inhibition of forskolin-stimulated cAMP accumulation was not significantly altered upon DMSO treatment. While immunodetectable alpha-subunits of Gi2 were not significantly changed by DMSO treatment, those of Gi3 were reduced by 27%. Inactivation of pertussis toxin substrates by pertussis toxin treatment and inhibition of adrenaline or neuropeptide Y stimulated Ca2+ elevations were linearly correlated. These data are compatible with the idea that, in HEL cells, alpha2-adrenoceptors and neuropeptide Y receptors couple to inhibition of adenylyl cyclase via Gi2 while they couple to Ca2+ elevations via Gi3.

Rescue of functional interactions between the alpha2A-adrenoreceptor and acylation-resistant forms of Gi1alpha by expressing the proteins from chimeric open reading frames

Co-expression of the alpha2A-adrenoreceptor with a pertussis toxin-resistant (C351G), but not with an also palmitoylation-resistant (C3S/C351G), form of the alpha subunit of Gi1 resulted in agonist-induced, pertussis toxin-independent, GTP hydrolysis. Construction and expression of a chimeric fusion protein between the receptor and C351G Gi1alpha generated a membrane protein in which the G protein element was activated by receptor agonist. An equivalent fusion protein containing C3S/C351G Gi1alpha rescued the ability of receptor agonist to activate this mutant. Fusion proteins of a palmitoylation-resistant (C442A) alpha2A-adrenoreceptor and either C351G or C3S/C351G Gi1alpha also responded effectively to agonist. Myristoylation resistant (G2A/C351G) and combined acylation-resistant (G2A/C3S/C351G) mutants of Gi1alpha are cytosolic proteins. Expression of these as chimeric alpha2A-adrenoreceptor-G protein fusions restored membrane localization and activation of the G protein by receptor agonist. These studies demonstrate the general utility of generating chimeric fusion proteins to examine receptor regulation of G protein function and that the lack of functional activation of acylation-negative G proteins by a co-expressed receptor is related to deficiencies in cellular targeting and location rather than an inherent incapacity to produce appropriate protein-protein interactions and signal transmission.

Control of the expression and activity of the Galpha-interacting protein (GAIP) in human intestinal cells

The Galpha-interacting protein (GAIP) is known to interact with the Galphai3 protein. It has been suggested that, depending on its expression, GAIP can be a regulator of trimeric Gi protein signaling pathways. In the present study we show that the GAIP mRNA content declines during the enterocytic differentiation of two cell lines derived from human colon adenocarcinomas: HT-29 and Caco-2. In undifferentiated HT-29 cells, when the GDP/GTP cycle on the trimeric Gi3 protein is interrupted by either pertussis toxin treatment or by the transfection of a mutant of the Galphai3 protein with no GTPase activity (Q204L), we observed a decrease in the GAIP mRNA content. As these conditions are known to impair the Gi3-dependent lysosomal-autophagic pathway existing in undifferentiated HT-29 cells, we have investigated the role of GAIP in controling the lysosomal-autophagic pathway. Overexpression of GAIP stimulated protein degradation along the macroautophagic pathway. In contrast, overexpression of GAIP did not modify the low rate of macroautophagy in cells expressing the Q204L mutant of the Galphai3 protein. These results show that GAIP regulates a major catabolic pathway and that the expression of GAIP is dependent upon the activity of the Galphai3 protein and the state of enterocytic differentiation of cells.

Betagamma subunits of pertussis toxin-sensitive G proteins mediate A1 adenosine receptor agonist-induced activation of phospholipase C in collaboration with thyrotropin. A novel stimulatory mechanism through the cross-talk of two types of receptors

COS-7 cells were transiently transfected with human thyrotropin receptor and dog A1 adenosine receptor cDNAs. An A1 agonist, N6-(L-2-phenylisopropyl) adenosine (PIA), which is ineffective alone, enhanced the thyrotropin (TSH)-induced inositol phosphate production, reflecting phospholipase C (PLC) activation, but inhibited the TSH-induced cAMP accumulation, reflecting adenylyl cyclase inhibition. These PIA-induced actions were completely inhibited by pertussis toxin (PTX) treatment. Moreover, in the cells expressing a PTX-insensitive mutant of Gi2alpha or Gi3alpha, in which a glycine residue was substituted for a cysteine residue to be ADP-ribosylated by PTX, at the fourth position of the C terminus, PIA effectively exerted both stimulatory and inhibitory effects on the TSH-induced actions although the cells were treated with the toxin. Overexpression of the betagamma subunits of the G proteins enhanced the TSH-induced inositol phosphate production without any significant effect on the cAMP response; under these conditions, PIA did not further increase the elevated inositol phosphate response to TSH. On the contrary, overexpression of a constitutively active mutant of Gi2alpha, in which the guanosine triphosphatase activity is lost, inhibited the TSH-induced cAMP accumulation but hardly affected the inositol phosphate response; under these conditions, PIA never exerted further inhibitory effects on the cAMP response to TSH. In contrast to the case of the TSH-induced inositol phosphate response, the response to a constitutively active G11alpha mutant was not appreciably affected, and that to NaF was rather inhibited by PIA and overexpression of the betagamma subunits. Taken together, these results suggest that a single type of PTX-sensitive G protein mediates the A1 adenosine receptor-linked modulation of two signaling pathways in collaboration with an activated thyrotropin receptor; alpha subunits of the PTX-sensitive G proteins mediate the inhibitory action on adenylyl cyclase, and the betagamma subunits mediate the stimulatory action on PLC. In the case of the latter stimulatory action on PLC, the betagamma subunits may not directly activate PLC. The possible mechanism by which betagamma subunits enhance the TSH-induced PLC activation is discussed.

Phosphorylation and calcium influx are not sufficient for the activation of cytosolic phospholipase A2 in U937 cells: requirement for a Gi alpha-type G-protein

Differentiation with dibutyryl cyclic AMP (dBcAMP) of the human, premonocytic U937 cell line toward a monocyte/granulocyte-like cell results in the cell acquiring an ability to release arachidonate upon stimulation. In contrast, the calcium ionophore ionomycin was able to stimulate phospholipase C, as measured by inositol 1,4,5-trisphosphate formation, to equal extents in both undifferentiated and dBcAMP-differentiated U937 cells. The role and regulation of cytosolic phospholipase A2 (cPLA2) in the production of arachidonate in these cells when either the chemotactic peptide fMLP or ionomycin are used as stimulus were investigated. The ionomycin- and fMLP-stimulated release of arachidonate were sensitive to the cPLA2 inhibitor arachidonyl trifluoromethylketone (IC50 values of 32 and 18 microM, respectively), but were not inhibited by E-6-(bromomethylene)-tetrahydro-3-(1-naphthalenyl)-2 H-pyran-2-one, a bromoenol lactone inhibitor of the calcium-independent phospholipase A2. These results, coupled with the inhibition of ionomycin-induced arachidonate production by electroporation of differentiated cells to introduce an anti-cPLA2, demonstrate that the cPLA2 is the enzyme responsible for arachidonate release in differentiated cells. SDS-PAGE and immunoblot analysis of differentiated cells showed the cells to contain both phosphorylated and unphosphorylated forms of cPLA2 (ratio of about 2: 3). Surprisingly, undifferentiated cells contain 30% more enzyme than differentiated cells and contain a higher percentage (approximately 75%) of the phosphorylated in the absence of stimulation. The inability of undifferentiated cells to produce arachidonate is not due to insufficient intracellular calcium concentrations since ionomycin induces large (820-940 nM) influxes of intracellular calcium in both differentiated and undifferentiated cells. This demonstrates that phosphorylation of cPLA2 andan influx of intracellular calcium are not sufficient to activate the enzyme to produce arachidonate. Instead, activation of a pertussis toxin-sensitive Gi alpha-type G-protein is required as evidenced by the production of arachidonate in undifferentiated cells stimulated with mastoparan, an activator of Gi alpha subunits, in combination with ionomycin. This activation of a Gi alpha-type G-protein is independent of modulations of adenylyl cyclase activity since cellular cAMP levels were not modulated upon treatment with mastoparan and ionomycin.

Targeted inactivation of alphai2 or alphai3 disrupts activation of the cardiac muscarinic K+ channel, IK+Ach, in intact cells

Cardiac muscarinic receptors activate an inwardly rectifying K+ channel, IK+Ach, via pertussis toxin (PT)-sensitive heterotrimeric G proteins (in heart Gi2, Gi3, or Go). We have used embryonic stem cell (ES cell)-derived cardiocytes with targeted inactivations of specific PT-sensitive alpha subunits to determine which G proteins are required for receptor-mediated regulation of IK+Ach in intact cells. The muscarinic agonist carbachol increased IK+Ach activity in ES cell-derived cardiocytes from wild-type cells, in cells lacking alphao, and in cells lacking the PT-insensitive G protein alphaq. In cells with targeted inactivation of alphai2 or alphai3, channel activation by both carbachol and adenosine was blocked. Carbachol-induced channel activation was restored in the alphai2- and alphai3-null cells by reexpressing the previously targeted gene and guanosine 5'-[gamma-thio] triphosphate was able to fully activate IK+Ach in excised membranes patches from these mutants. In contrast, negative chronotropic responses to both carbachol and adenosine were preserved in cells lacking alphai2 or alphai3. Our results show that expression of two specific PT-sensitive alpha subunits (alphai2 and alphai3 but not alphao) is required for normal agonist-dependent activation of IK+Ach and suggest that both alphai2- and alphai3-containing heterotrimeric G proteins may be involved in the signaling process. Also the generation of negative chronotropic responses to muscarinic or adenosine receptor agonists do not require activation of IK+Ach or the expression of alphai2 or alphai3.

Coupling of oxytocin receptor to G proteins in rat myometrium during labor: Gi receptor interaction

Occupancy of oxytocin receptor (OTR) binding sites in pregnant rat myometrial membranes with iodinated oxytocin antagonist (OTA), followed by detergent solubilization and size selection, showed that radioactivity eluted in two distinct peaks: one corresponding in size to the isolated receptor (approximately 60 kDa) and the other ranging from 240 to 320 kDa. The unliganded 240- to 320-kDa fraction contained OTRs coupled to G proteins, as the addition of oxytocin (OT) increased guanosine 35S-labeled 5'-O-(3-thiotriphosphate) binding up to twofold in a dose-dependent manner. The effects of OT were blocked by coincubation with OTA. G protein alpha-subunits associated with OTRs in the 240- to 320-kDa peak were identified by immunoadsorption. Significant amounts of both G alpha q/11 and G alpha i3 were associated with the OTR; a lesser amount of G alpha s was complexed. Using the same approach but with antibodies to effector enzymes, we observed that phospholipase C beta 1 (PLC beta 1) and PLA2 were also associated with the OTR. The results corroborate the well-established interaction of OTR with Gq and further show that Gi coupling might be an important component of OTR signal transduction. To further investigate the interaction of Gi with the OTR, we showed that OT stimulation of guanosine 5'-triphosphatase activity in intact myometrial membranes was inhibited by pertussis toxin. Pertussis toxin-stimulated ADP ribosylation of G alpha i in myometrial membranes was also decreased by OT treatment. These findings with pertussis toxin strongly indicate that OTR is coupled to Gi in rat myometrial membranes. The 60-kDa OTR peak (noncoupled receptor) was demonstrable in the myometrium only before the end of gestation and after parturition and accounted for about one-half the 125I-OTA binding activity. At term, there was about a fivefold increase in binding and almost a complete shift to the 240- to 320-kDa-size complex. Thus the established increased sensitivity of the myometrium to OT at term could be the result of both upregulation of OTRs and an increase in the fraction of receptors coupled to signal transduction components, one of which is Gi.

5-HT1A receptor activates Na+/H+ exchange in CHO-K1 cells through Gialpha2 and Gialpha3

5-HT1A receptors couple to many signaling pathways in CHO-K1 cells through pertussis toxin-sensitive G proteins. The purpose of this study was to determine which members of the Gi/o/z family mediate 5-HT1A receptor-activated Na+/H+ exchange as measured by microphysiometry of cell monolayers. The method was extensively validated, showing that proton efflux was sodium-dependent, inhibited by amiloride analogs, and activated by growth factors, phorbol ester, calcium ionophore, and hypertonic stress. 5-HT and the specific agonist (+/-)-8-hydroxy-2-(di-N-propylamino)tetralin hydrobromide rapidly stimulated proton efflux that was blocked by a specific receptor antagonist, amiloride analogs or pertussis toxin. The activation by 5-HT depended upon extracellular sodium and could be demonstrated under conditions of imposed intracellular acid load, as well as in the presence and absence of glycolytic substrate. Acceleration of proton efflux was not inhibited by sequestration of G protein betagamma-subunits, a maneuver that blocked 5-HT1A receptor activation of mitogen-activated protein kinase. Transfection of Gzalpha and pertussis toxin-resistant mutants of Goalpha and Gialpha1 did not reverse the blockade induced by pertussis toxin. In contrast, pertussis toxin-resistant mutants of Gialpha2 and Gialpha3 "rescued" the ability of 5-HT to increase proton efflux after pertussis toxin treatment. These experiments demonstrate clearly that Gialpha2 and Gialpha3 can specifically mediate rapid agonist-induced acceleration of Na+/H+ exchange.

A new linear V1A vasopressin antagonist and its use in characterizing receptor/G protein interactions

We characterized a new iodinated, high affinity, linear V1a vasopressin antagonist, phenylacetylD-Tyr(Et)Phe-Gln-Asn-Lys-Pro-Arg-Tyr-NH2. The antagonist bound specifically to the V1a vasopressin receptor in crude rat liver membranes with an apparent Kd value of 0.168 nM. This affinity is approximately 1 order of magnitude greater than that of the natural agonist, vasopressin. The inhibitory activity of the antagonist can be demonstrated by its inability to elicit activation and uncoupling of G proteins from the receptor. Thus, after occupancy of receptor sites in rat liver membranes with labeled antagonist and detergent solubilization, the labeled receptor (approximately 60 kDa) was eluted as a stable 400-kDa complex on size-exclusion chromatography. In contrast, when the receptor sites were occupied by the agonist [3H]vasopressin, the receptor eluted as a 60-kDa peak. Coincubation of membranes with iodinated antagonist and an excess of unlabeled vasopressin caused both reduced antagonist binding and a complete shift from the 400-kDa to the 60-kDa peak. The addition of vasopressin to unliganded 400-kDa fractions resulted in a 75% increase in [35S]guanosine-5'-O-(3-thio)triphosphate binding activity, indicating that the 400-kDa fraction contains complexes between the V1a receptor and G proteins. The vasopressin-elicited increase was inhibited by antagonist. Using specific antibodies and immunoadsorption to protein A/Sepharose columns, we found that G protein isotypes G(alpha q/11), G(alpha i3), and G(alpha s), and effector enzymes PLC-beta1, PLC-gamma2 and PLA-2 were associated with the antagonist-labeled receptor in the 400-kDa fraction. Because the 400-kDa complex was found in the absence of ligand, the V1a receptor and the appropriate G proteins and effector enzymes are likely preassociated with each other and do not aggregate after antagonist addition. The association of V1a receptor with the different specific G proteins and effector enzymes is consistent with the multiple actions of vasopressin on liver cells. Antibodies directed against a portion of the carboxyl-terminal domain of the V1a receptor interacted with 60-kDa antagonist-occupied receptor but not with receptor in the 400-kDa complex. These results suggest that the carboxyl-terminal region of the receptor is sterically hindered when coupled to G proteins. The iodinated linear vasopressin antagonist therefore allows stable receptor/G protein complexes and can be an important tool (along with the antisera) for use in the study of factors that control V1a receptor/G protein coupling.

The heterotrimeric G protein G alpha i2 mediates lysophosphatidic acid-stimulated induction of the c-fos gene in mouse fibroblasts

Lysophosphatidic acid (LPA) utilizes a heterotrimeric guanine nucleotide regulatory (G) protein-coupled receptor to activate the mitogen-activated protein kinase pathway and induce mitogenesis in fibroblasts and other cells. A single cell assay system was used to examine the functional interaction of the LPA receptor with G proteins in intact mouse fibroblasts, by measuring LPA-stimulated induction of the immediate-early gene, c-fos, as read out by a stably expressed fos-lacZ reporter gene. Pretreatment of these cells with pertussis toxin at 100 ng/ml almost completely abolished LPA-stimulated c-fos induction. Western blotting revealed that two pertussis toxin (PTX)-sensitive G proteins, G alpha i2 and G alpha i3, were present in membranes prepared from these cells, and Northern blotting confirmed the absence of message for other PTX-sensitive subunits. Microinjection of an alpha il/alpha i2-specific antibody into living cells decreased LPA-stimulated induction of c-fos by 60%, whereas introduction of antibodies to either alpha i3 or alpha 16, a subtype not present in these cells but used as a control, decreased LPA-stimulated c-fos induction by only 19%. In contrast, the alpha i1/alpha i2-specific antibody had no effect on insulin-induced c-fos expression, which is thought to utilize a G protein-independent mechanism of signaling. In addition, cellular expression of an epitope-tagged PTX-resistant mutant of G alpha i2, but not PTX-resistant G alpha i3, restored LPA-stimulated c-fos induction in cells in which endogenous G protein a subunits were uncoupled from the receptor by pretreatment with PTX. Together, these results provide conclusive in vivo evidence that G alpha i2 is the PTX-sensitive G protein a subunit which mediates LPA-stimulated c-fos induction and perhaps mitogenesis in these cells.

alpha-Thrombin upregulates G alpha i3 in human vascular endothelial cells

BACKGROUND AND PURPOSE

During thrombosis, alpha-thrombin becomes sequestered by fibrin and the subendothelial basement membrane, and it is available to interact with the vasculature over prolonged periods. In this study, we investigated the long-term effect of alpha-thrombin on G alpha i3 and G alpha s levels in human vascular endothelial cells (EC). Because obesity is associated with changes in receptor signaling in many animal models, we also explored the influence of this clinical risk factor.

METHODS

Primary cultures of human EC were exposed to alpha-thrombin for 16 hours, and immunologically detectable G alpha i3 and G alpha s levels were measured.

RESULTS

alpha-Thrombin (100 nmol/L) increased G alpha i3 levels in EC derived from the cerebral microvasculature and superficial temporal artery (4.2 +/- 1.2-fold and 2.8 +/- 0.32-fold, respectively) but had no significant effect on EC derived from omental artery (P > .6) or from the superficial temporal artery of obese (body mass index > or = 28 kg/m2) patients (P > .4). The expression of G alpha s was unchanged in all cell types (P > or = .1). Two other circulating peptides, vasoactive intestinal peptide and endothelin-1, failed to alter the expression of either G protein in EC from the cerebral microvasculature, further demonstrating the specificity of the alpha-thrombin effect. However, thrombin receptor activating protein-14 mimicked the alpha-thrombin response and increased G alpha i3 in EC derived from the cerebral microvasculature and superficial temporal artery.

CONCLUSIONS

We conclude that alpha-thrombin increases G alpha i3 expression in some EC through activation of its tethered liganded receptor. Obesity appears to suppress this action of alpha-thrombin.

Guanine nucleotide exchange on heterotrimeric Gi3 protein controls autophagic sequestration in HT-29 cells

Recent results have shown that autophagic sequestration in the human colon cancer cell line HT-29 is controlled by the pertussis toxin-sensitive heterotrimeric Gi3 protein. Here we show that transfection of an antisense oligodeoxynucleotide to the alphai3-subunit markedly inhibits autophagic sequestration, whereas transfection of an antisense oligodeoxynucleotide to the alphai2-subunit does not change the rate of autophagy in HT-29 cells. Autophagic sequestration was arrested in cells transfected with a mutant of the alphai3-subunit (Q204L) that is restricted to the GTP-bound form. In Q204L-expressing cells, 3-methyladenine-sensitive degradation of long lived [14C]valine-labeled proteins was severely impaired and could not be stimulated by nutrient deprivation. Autophagy was also reduced when dissociation of the betagamma dimer from the GTP-bound alphai3-subunit was impaired in cells transfected with the G203A mutant. In contrast, a high rate of pertussis toxin-sensitive autophagy was observed in cells transfected with an alphai3-subunit mutant (S47N) which has an increased guanine nucleotide exchange rate and increased preference for GDP over GTP. Cells that express pertussis toxin-insensitive mutants of either wild-type alphai3-subunit (C351S) or S47N alphai3-subunit (S47N/C351S) exhibit a high rate of autophagy.

Arachidonic acid activates the noncapacitative entry of Ca2+ during [Ca2+]i oscillations

Current models for agonist-activated Ca2+ entry in nonexcitable cells focus on the capacitative mechanism where entry is activated as a downstream result of the sustained depletion of agonist-sensitive stores without any direct requirement for inositol phosphates. This mechanism has been shown to be important for the sustained Ca2+ signals seen in a variety of nonexcitable cells under conditions of maximal stimulation. In contrast, relatively little attention has been given to Ca2+ entry under more physiological levels of agonist where, for example, oscillating Ca2+ responses are common. In recent studies using cells from the exocrine avian nasal gland, we have shown that agonist-activated Ca2+ entry under these conditions demonstrates properties that are inconsistent with current versions of the capacitative model. We now report that activation of this novel noncapacitative Ca2+ entry is via a distinct signaling pathway involving an agonist-induced, phospholipase A2-mediated generation of arachidonic acid.

Biosynthesis of the endogenous cyclic adenosine monophosphate (AMP) antagonist, prostaglandylinositol cyclic phosphate (cyclic PIP), from prostaglandin E and activated inositol polyphosphate in rat liver plasma membranes

The endogenous cyclic adenosine monophosphate (AMP) antagonist, cyclic PIP, has been identified as a prostaglandylinositol cyclic phosphate. It inhibits protein kinase A 100% and activates protein serine phosphatase about sevenfold. It is biosynthesized by an enzyme of the plasma membrane when the assay mixture contains adenosine triphosphate (ATP), Mg2+, prostaglandin E and a novel inositol polyphosphate, which cannot be substituted by commercially available inositol phosphates. This novel inositol polyphosphate is a very labile compound. On anion exchange chromatography it elutes in the range of ATP, which may indicate the presence of three phosphate groups. It adsorbs on charcoal, which suggests the presence of a hydrophobic component, possibly a guanosine. Pyrophosphates obtained from inositol 1,4- and inositol 2,4-bisphosphate are accepted by cyclic PIP synthetase for the synthesis of cyclic PIP. The biosynthesis is characterized by enzyme kinetic parameters like dependence on time, enzyme and substrate concentration. The pH optimum of the enzyme is in the range 7.5-8. The enzyme functions optimally with prostaglandin E and poorly with prostaglandin A as the substrate. The presence of fluoride in the assay causes a three- to fourfold increase in cyclic PIP synthesis, which may be correlated with activation via G proteins. These data support previous reports on the chemical structure and action of cyclic PIP. With respect to the possible isomers of cyclic PIP, these indicate that it is most likely the C4-hydroxyl group of the inositol which binds the C15-hydroxyl group of prostaglandin E. A model of hormone-stimulated synthesis of cyclic PIP is proposed: phospholipase A2 and phospholipase C, activated by G proteins upon alpha-adrenergic stimulation, liberate either unsaturated fatty acids or inositol phosphates, which are transformed to prostaglandins and to novel inositol polyphosphate with an energy-rich bond. The cyclic PIP synthetase combines these two substrates to cyclic PIP.

Alterations in macrophage G proteins are associated with endotoxin tolerance

Previous studies have suggested that endotoxin tolerance induces macrophage desensitization to endotoxin through altered guanine nucleotide regulatory (G) protein function. In the present study the binding characteristics of the nonhydrolyzable GTP analogue GTP gamma [35S] to macrophage membranes from endotoxin tolerant and control rats were determined. Membranes were prepared from peritoneal macrophages harvested from rats 72 h after two sequential daily doses of vehicle or Salmonella enteritidis endotoxin (100 micrograms/kg on day 1 and 500 micrograms/kg on day 2). GTP gamma [35S] bound to a single class of sites that were saturable and displaceable in control and endotoxin tolerant macrophage membranes. The maximum specific binding of GTP gamma [35S] was significantly (P < 0.01) decreased in membranes from tolerant rats compared to control (Bmax = 39 +/- 7 pmol/mg protein in control vs. 11 +/- 2 pmol/mg protein in endotoxin tolerant; n = 5). There were no significant differences in the Kd values. To determine whether the reduced GTP gamma S binding was due to decreases in G proteins, macrophage membrane G protein content was determined by western blotting with specific antisera to Gi1,2 alpha, Gi3 alpha, Gs alpha, and the beta subunit of G. Scanning densitometric analysis demonstrated differential decreases in tolerant macrophage membrane G proteins. Gi3 alpha was reduced the most to 48 +/- 8% of controls (n = 3), and this reduction was significant compared to those of other G proteins. Gi1,2 alpha and G beta were reduced to 73 +/- 5% (n = 3) and 65 +/- 4% (n = 3) of control values, respectively. Gs alpha(L) and Gs alpha(H) were reduced to 61 +/- 5% (n = 3) and 68 +/- 3% (n = 3) of control, respectively. These results demonstrate that endotoxin tolerant macrophages exhibit decreased membrane GTP binding capacity and differential reductions in the content of specific G proteins. The cellular mechanisms leading to such alterations in G proteins and their functional significance in the acquisition of endotoxin tolerance merit further investigation.

Synergistic interactions between human transfected adenosine A1 receptors and endogenous cholecystokinin receptors in CHO cells

The effect of Gi coupled receptor activation (adenosine A1 and 5-HT1B receptors) on cholecystokinin receptor-stimulated inositol phosphate accumulation has been investigated in Chinese hamster ovary cells transfected with the human adenosine A1 receptor cDNA (CHO-A1). CHO cells constitutively express the 5-HT1B receptor [Berg, Clarke, Sailstad, Saltzman and Maayani (1994) Mol. Pharmacol. 46, 477-484]. Our previous studies using CHO-A1 cells have revealed that both the adenosine A1 and 5-HT1B receptor are negatively coupled to adenylyl cyclase activity and stimulate increases in [Ca2+]i, through a pertussis toxin-sensitive pathway. In the present study the selective adenosine A1 receptor agonist N6-cyclopentyladenosine stimulated a pertussis toxin-sensitive increase in total [3H]inositol phosphate accumulation. The sulphated C-terminal octapeptide of cholecystokinin (CCK-8) stimulated a robust and pertussis toxin-insensitive increase in [3H]inositol phosphate accumulation through the activation of CCKA receptors. Co-stimulation of CHO-A1 cells with N6-cyclopentyladenosine and CCK-8 produced a synergistic increase in [3H]inositol phosphate accumulation. The synergistic interaction between N6-cyclopentyladenosine and CCK-8 was abolished in pertussis toxin-treated cells. Synergy between N6-cyclopentyladenosine and CCK-8 still occurred in the absence of extracellular calcium. The 5-HT1B receptor agonist 5-carboxyamidotryptamine did not stimulate a measurable increase in [3H]inositol phosphate accumulation. Furthermore, 5-carboxyamidotryptamine had no significant effect on CCK-8 mediated [3H]inositol phosphate production. Activation of endogenous P2U receptors (Gq/Gll coupled) with ATP gamma S produced a significant increase in [3H]inositol phosphate accumulation. Co-stimulation of CHO-A1 cells with ATP gamma S and CCK-8 produced additive increases in [3H]inositol phosphate accumulation. These data indicate that CHO-A1 cells may prove a useful model system in which to investigate further the mechanisms underlying the intracellular 'cross-talk' between phospholipase C coupled receptors (Gq/Gll linked) and Gi/Go coupled receptors.