Transfected Go1 alpha inhibits the calcium dependence of beta-adrenergic stimulated cAMP accumulation in C6 glioma cells

β adrenergic receptor modulated signaling in glioma models: promoting β adrenergic receptor-β arrestin scaffold-mediated activation of extracellular-regulated kinase 1/2 may prove to be a panacea in the treatment of intracranial and spinal malignancy and extra-neuraxial carcinoma

Neoplastically transformed astrocytes express functionally active cell surface β adrenergic receptors (βARs). Treatment of glioma models in vitro and in vivo with β adrenergic agonists variably amplifies or attenuates cellular proliferation. In the majority of in vivo models, β adrenergic agonists generally reduce cellular proliferation. However, treatment with β adrenergic agonists consistently reduces tumor cell invasive potential, angiogenesis, and metastasis. β adrenergic agonists induced decreases of invasive potential are chiefly mediated through reductions in the expression of matrix metalloproteinases types 2 and 9. Treatment with β adrenergic agonists also clearly reduce tumoral neoangiogenesis, which may represent a putatively useful mechanism to adjuvantly amplify the effects of bevacizumab. Bevacizumab is a monoclonal antibody targeting the vascular endothelial growth factor receptor. We may accordingly designate βagonists to represent an enhancer of bevacizumab. The antiangiogenic effects of β adrenergic agonists may thus effectively render an otherwise borderline effective therapy to generate significant enhancement in clinical outcomes. β adrenergic agonists upregulate expression of the major histocompatibility class II DR alpha gene, effectively potentiating the immunogenicity of tumor cells to tumor surveillance mechanisms. Authors have also demonstrated crossmodal modulation of signaling events downstream from the β adrenergic cell surface receptor and microtubular polymerization and depolymerization. Complex effects and desensitization mechanisms of the β adrenergic signaling may putatively represent promising therapeutic targets. Constant stimulation of the β adrenergic receptor induces its phosphorylation by β adrenergic receptor kinase (βARK), rendering it a suitable substrate for alternate binding by β arrestins 1 or 2. The binding of a β arrestin to βARK phosphorylated βAR promotes receptor mediated internalization and downregulation of cell surface receptor and contemporaneously generates a cell surface scaffold at the βAR. The scaffold mediated activation of extracellular regulated kinase 1/2, compared with protein kinase A mediated activation, preferentially favors cytosolic retention of ERK1/2 and blunting of nuclear translocation and ensuant pro-transcriptional activity. Thus, βAR desensitization and consequent scaffold assembly effectively retains the cytosolic homeostatic functions of ERK1/2 while inhibiting its pro-proliferative effects. We suggest these mechanisms specifically will prove quite promising in developing primary and adjuvant therapies mitigating glioma growth, angiogenesis, invasive potential, and angiogenesis. We suggest generating compounds and targeted mutations of the β adrenergic receptor favoring β arrestin binding and scaffold facilitated activation of ERK1/2 may hold potential promise and therapeutic benefit in adjuvantly treating most or all cancers. We hope our discussion will generate fruitful research endeavors seeking to exploit these mechanisms.

Differential modulation of mu-opioid receptor signaling to adenylyl cyclase by regulators of G protein signaling proteins 4 or 8 and 7 in permeabilised C6 cells is Galpha subtype dependent

Regulators of G protein signaling (RGS) proteins act as GTPase-accelerating protein to negatively modulate G protein signaling and are defined by a conserved RGS domain with considerable amino acid diversity. To determine the effects of specific, purified RGS proteins on mu-opioid signaling, C6 cells stably expressing a mu-opioid receptor were rendered permeable to proteins by treatment with digitonin. Mu-opioid inhibition of forskolin-stimulated adenylyl cyclase by [D-Ala(2),N-Me-Phe(4),Gly-ol]-enkephalin (DAMGO), a mu-specific opioid peptide, remained fully intact in permeabilized cells. Purified RGS domain of RGS4 added to permeabilized cells resulted in a twofold loss in DAMGO potency but had no effect in cells expressing RGS-insensitive G proteins. The inhibitory effect of DAMGO was reduced to the same extent by purified RGS4 and RGS8. In contrast, the RGS domain of RGS7 had no effect and inhibited the action of RGS8 as a result of weak physical association with Galphai2 and minimal GTPase-accelerating protein activity in C6 cell membranes. These data suggest that differences in conserved RGS domains of specific RGS proteins contribute to differential regulation of opioid signaling to adenylyl cyclase and that a permeabilized cell model is useful for studying the effects of specific RGS proteins on aspects of G protein-coupled receptor signaling.

Endogenous regulators of G protein signaling differentially modulate full and partial mu-opioid agonists at adenylyl cyclase as predicted by a collision coupling model

Regulator of G protein signaling (RGS) proteins accelerate the endogenous GTPase activity of Galpha(i/o) proteins to increase the rate of deactivation of active Galpha-GTP and Gbetagamma signaling molecules. Previous studies have suggested that RGS proteins are more effective on less efficiently coupled systems such as with partial agonist responses. To determine the role of endogenous RGS proteins in functional responses to mu-opioid agonists of different intrinsic efficacy, Galpha(i/o) subunits with a mutation at the pertussis toxin (PTX)-sensitive cysteine (C351I) and with or without a mutation at the RGS binding site (G184S) were stably expressed in C6 glioma cells expressing a mu-opioid receptor. Cells were treated overnight with PTX to inactivate endogenous G proteins. Maximal inhibition of forskolin-stimulated adenylyl cyclase by the low-efficacy partial agonists buprenorphine and nalbuphine was increased in cells expressing RGS-insensitive Galpha(o)(CIGS), Galpha(i2)(CIGS), or Galpha(i3)(CIGS) compared with their Galpha(CI) counterparts, but the RGS-insensitive mutation had little or no effect on the maximal inhibition by the higher efficacy agonists DAMGO and morphine. The potency of all the agonists to inhibit forskolin-stimulated adenylyl cyclase was increased in cells expressing RGS-insensitive Galpha(o)(CIGS), Galpha(i2)(CIGS), or Galpha(i3)(CIGS), regardless of efficacy. These data are comparable with predictions based on a collision coupling model. In this model, the rate of G protein inactivation, which is modulated by RGS proteins, and the rate of G protein activation, which is affected by agonist intrinsic efficacy, determine the maximal agonist response and potency at adenylyl cyclase under steady state conditions.

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.

Assays for G-protein-coupled receptor signaling using RGS-insensitive Galpha subunits

Regulators of G-protein signaling (RGS) proteins, by their action on Galpha(i/o) proteins, may enhance receptor-effector signaling by physical or kinetic scaffolding mechanisms. However, more than 30 mammalian proteins with RGS activity have been identified so it is difficult to determine which RGS protein is most relevant to a particular receptor system and in any particular cell. To avoid this problem, one approach is to examine agonist-stimulated second messenger signaling in cells expressing Galpha proteins that are insensitive to the GTPase accelerating property of all RGS proteins. This article describes protocols for the preparation and analysis of C6 rat glioma cells stably expressing RGS- and pertussis toxin-insensitive Galpha subunits; pertussis toxin treatment uncouples endogenous Galpha(i/o) proteins and allows for the determination of the expressed RGS-insensitive Galpha activity. Methods to determine signaling at the level of adenylyl cyclase, the extracellular signal-regulated kinase (ERK1/2) mitogen-activated protein kinase pathway, and intracellular Ca2+ levels are described. As a typical G-protein-coupled receptor, we have used the micro-opioid receptor expressed in C6 cells together with RGS-insensitive Galpha(o). In these cells, agonist inhibition of adenylyl cyclase and stimulation of ERK1/2 phosphorylation were enhanced markedly. In contrast, increases in intracellular calcium were less affected. The altered signaling in cells expressing RGS-insensitive Galpha(o) subunits allows for determination of the role of endogenous RGS proteins to limit and/or direct signaling.

Endogenous regulator of G protein signaling proteins suppress Galphao-dependent, mu-opioid agonist-mediated adenylyl cyclase supersensitization

Chronic mu-opioid agonist treatment leads to dependence with withdrawal on removal of agonist. At the cellular level withdrawal is accompanied by a supersensitization of adenylyl cyclase, an effect that requires inhibitory Galpha proteins. Inhibitory Galpha protein action is modulated by regulator of G protein signaling (RGS) proteins that act as GTPase activating proteins and reduce the lifetime of Galpha-GTP. In this article, we use C6 glioma cells expressing the rat mu-opioid receptor (C6mu) to examine the hypothesis that Galphao alone can mediate mu-opioid agonist induced adenylyl cyclase supersensitivity and that endogenous RGS proteins serve to limit the extent of this supersensitization. C6mu cells were stably transfected with pertussis toxin (PTX)-insensitive Galphao that was either sensitive or insensitive to endogenous RGS proteins. Cells were treated with PTX to uncouple endogenous Galpha proteins followed by exposure to the mu-opioid agonists [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin or morphine. Supersensitization was observed in cells expressing wild-type Galpha, but this was lost on PTX treatment. In cells expressing PTX-insensitive Galphao supersensitization was recovered, confirming that Galphao alone can support supersensitization. In cells expressing the RGS-insensitive mutant Galphao, there was a greater degree of supersensitization and the concentration of micro-agonist needed to achieve half-maximal supersensitization was reduced by 10-fold. The amount of supersensitization seen did not directly relate to the degree of acute inhibition of adenylyl cyclase. These results demonstrate a role for Galphao in adenylyl cyclase supersensitization after mu-agonist exposure and show that this action is modulated by endogenous RGS proteins.

Dissimilar pharmacological responses by a new series of imidazoline derivatives at precoupled and ligand-activated alpha 2A-adrenoceptor states: evidence for effector pathway-dependent differential antagonism

Whereas agonist-directed differential signaling at a single receptor subtype has become an accepted pharmacological concept, distinct behaviors by ligands that are assumed to be antagonists is less documented. The intrinsic activity and capacity of antagonism for a new series of imidazoline-derived adrenergic ligands analogous to dexefaroxan were investigated by measuring two distinct signaling pathways at the recombinant human alpha 2A-adrenoceptor (alpha 2A AR): 1) pertussis toxin-resistant guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTP gamma S) binding responses mediated by either a recombinant G alpha oCys351Ile or G alpha i2Cys352Ile protein in CHO-K1 cells, and 2) inhibition of cAMP formation in a stably transfected C6-glial cell line. Ligands could be differentiated as inverse agonists [i.e., 2-(4-methoxy-2-ethyl-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 851062], neutral antagonists [i.e., 2-(4-hydroxy-2-ethyl-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 851057], partial [i.e., 2-(4-chloro-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 821008], and high-efficacy [i.e., 2-(6,7-dichloro-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 821010] agonists at a precoupled alpha 2A AR state in the copresence of a G alpha oCys351Ile protein but not G alpha i2Cys352Ile protein by monitoring [35S]GTP gamma S binding responses. Neither positive nor negative efficacy was observed for these compounds by monitoring the adenylate cyclase pathway at a presumably low-affinity alpha 2A AR state. The capacity of the dexefaroxan analogs to antagonize the (-)-epinephrine-mediated [35S]GTP gamma S binding response at a G alpha oCys351Ile protein was inversely correlated with their magnitude of intrinsic activity and unrelated to their ligand binding affinity for the alpha 2A AR. On the other hand, their capacity to antagonize either (-)-epinephrine or 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline tartrate (UK 14304)-mediated inhibition of forskolin-stimulated cAMP formation was not related with the rank order of antagonist capacity for the (-)-epinephrine-mediated [35S]GTP gamma S binding response. In conclusion, these data demonstrate that certain alpha2 AR ligands that are assumed to be antagonists, may yield dissimilar pharmacological responses, dependent on the investigated agonist-stimulated effector pathway.

Endogenous RGS protein action modulates mu-opioid signaling through Galphao. Effects on adenylyl cyclase, extracellular signal-regulated kinases, and intracellular calcium pathways

RGS (regulators of G protein signaling) proteins are GTPase-activating proteins for the Galpha subunits of heterotrimeric G proteins and act to regulate signaling by rapidly cycling G protein. RGS proteins may integrate receptors and signaling pathways by physical or kinetic scaffolding mechanisms. To determine whether this results in enhancement and/or selectivity of agonist signaling, we have prepared C6 cells stably expressing the mu-opioid receptor and either pertussis toxin-insensitive or RGS- and pertussis toxin-insensitive Galpha(o). We have compared the activation of G protein, inhibition of adenylyl cyclase, stimulation of intracellular calcium release, and activation of the ERK1/2 MAPK pathway between cells expressing mutant Galpha(o) that is either RGS-insensitive or RGS-sensitive. The mu-receptor agonist [d-Ala(2),MePhe(4),Gly(5)-ol]enkephalin and partial agonist morphine were much more potent and/or had an increased maximal effect in inhibiting adenylyl cyclase and in activating MAPK in cells expressing RGS-insensitive Galpha(o). In contrast, mu-opioid agonist increases in intracellular calcium were less affected. The results are consistent with the hypothesis that the GTPase-activating protein activity of RGS proteins provides a control that limits agonist action through effector pathways and may contribute to selectivity of activation of intracellular signaling pathways.

Pertussis toxin treatment prevents 5-HT(5a) receptor-mediated inhibition of cyclic AMP accumulation in rat C6 glioma cells

We have investigated the functional coupling of the rat 5HT(5a) receptor subtype to adenylate cyclase in a rat C6 glioma cell line. In 5HT(5a) receptor-transfected cells, 5HT caused a concentration-dependent inhibition of forskolin-stimulated cAMP accumulation, with an EC(50) value of 41 nM and a maximal effect of 57% inhibition. This effect was dependent on the concentration of forskolin used to elevate cAMP levels. Methiothepin (1 mcM), which has high affinity for the 5HT(5a) receptor, antagonized the 5HT(5a) receptor-mediated inhibition, and unmasked a stimulation of cAMP formation similar to that observed in untransfected cells, whereas ketanserin (0.1 mcM) enhanced the inhibitory effect of 5HT. Pertussis toxin treatment (0.5 mcg/ml) completely blocked the inhibitory effect of 5HT on cAMP formation, also revealing increase in cAMP accumulation. Pretreatment of the transfected membranes with pertussis toxin abolished subsequent ADP-ribosylation of a 41 kDa protein, correlating the cAMP effect with a functional uncoupling of an inhibitory G protein from its receptor. These results demonstrate an efficient functional coupling of the rat 5HT(5a) receptor to the inhibition of adenylate cyclase via a pertussis toxin-sensitive G[alpha(i)], inhibitory G-protein.

Capacitative Ca2+ entry involves Ca2+ influx factor in rat glioma C6 cells

Capacitative Ca2+ entry exists in rat glioma C6 cells; however, how the information of depletion of Ca2+ in intracellular stores transmits to the plasma membrane is unknown. In the present study, we examined whether Ca2+ influx factor (CIF) causes capacitative Ca2+ entry in C6 cells. CIF was extracted from non-treated (Non-CIF), bombesin-treated (BBS-CIF) and thapsigargin-treated (TG-CIF) C6 cells by a reverse-phase silica cartridge. The addition of BBS-CIF and TG-CIF gradually increased cytoplasmic Ca2+ concentration ([Ca2+]i) but Non-CIF did not increase [Ca2+]i. Neither BBS-CIF nor TG-CIF elevated [Ca2+]i in the absence of extracellular Ca2+. Gd3+ inhibited the increase in [Ca2+]i induced by BBS-CIF and TG-CIF. Genistein abolished an elevation of [Ca2+]i induced by BBS-CIF and TG-CIF. BBS-CIF and TG-CIF did not increase inositol 1,4,5-trisphosphate accumulation. The results suggest that capacitative Ca2+ entry is caused by CIF in rat glioma C6 cells.

Characterization of a Goalpha mutant that binds xanthine nucleotides

Several GTP binding proteins, including EF-Tu, Ypt1, rab-5, and FtsY, and adenylosuccinate synthetase have been reported to bind xanthine nucleotides when the conserved aspartate residue in the NKXD motif was changed to asparagine. However, the corresponding single Goalpha mutant protein (D273N) did not bind either xanthine nucleotides or guanine nucleotides. Interestingly, the introduction of a second mutation to generate the Goalpha subunit D273N/Q205L switched nucleotide binding specificity to xanthine nucleotide. The double mutant protein GoalphaD273N/Q205L (GoalphaX) bound xanthine triphosphate, but not guanine triphosphate. Recombinant GoalphaX (GoalphaD273N/Q205L) formed heterotrimers with betagamma complexes only in the presence of xanthine diphosphate (XDP), and the binding to betagamma was inhibited by xanthine triphosphate (XTP). Furthermore, as a result of binding to XTP, the GoalphaX protein underwent a conformational change similar to that of the activated wild-type Goalpha. In transfected COS-7 cells, we demonstrate that the interaction between GoalphaX and betagamma occurred only when cell membranes were permeabilized to allow the uptake of xanthine diphosphate. This is the first example of a switch in nucleotide binding specificity from guanine to xanthine nucleotides in a heterotrimeric G protein alpha subunit.

Secretion of protease nexin-1 by C6 glioma cells is under the control of a heterotrimeric G protein, Go1

Heterotrimeric Go proteins have recently been described as regulators of vesicular traffic. The Goalpha gene encodes, by alternative splicing, two Goalpha polypeptides, Go1alpha and Go2alpha. By immunofluorescence and electron microscopy, we detected Go1alpha on the membrane of small intracellular vesicles in C6 glioma cells. After stable transfection of these cells, overexpression of Go1alpha but not Go2alpha was followed by a rise in the secretion of a serine protease inhibitor, protease nexin-1 (PN-1). This secretion was enhanced as a function of the amount of expressed Go1alpha. Metabolic cell labeling indicated that this increase in PN-1 secretion was not the result of an enhancement in PN-1 biosynthesis or a decrease in its uptake, but revealed a potential role of Go1alpha in the regulation of vesicular PN-1 trafficking. Furthermore, activators of Go proteins, mastoparan and a peptide derived from the amino terminus of the growth cone-associated protein GAP43, increased PN-1 secretion in parental and Go1alpha-overexpressing cells. Brefeldin A, an inhibitor of vesicular traffic, inhibited both basal and mastoparan-stimulated PN-1 secretions. These results indicate, that in C6 glioma cells, PN-1 secretion could be regulated by both Go1alpha expression and activation.

Physiology of transformed glial cells

Much of our present knowledge of glial cell function stems from studies of glioma cell lines, both rodent (C6, C6 polyploid, and TR33B) and human (1321N1, 138MG, D384, R-111, T67, Tp-276MG, Tp-301MG, Tp-483MG, Tp-387MG, U-118MG, U-251MG, U-373MG, U-787MG, U-1242MG, and UC-11MG). New methods such as patch clamp and Ca2+ imaging have lead to rapid progress the last few years in our knowledge about glial cells, where an unexpected presence and diversity of receptors and ion channels have emerged. Basic mechanisms related to membrane potential and K+ transport and the presence of voltage gated ion channels (Na+, inwardly rectifying K+, Ca(2+)-activated K+, Ca2+, and Cl- channels) have been identified. Receptor function and intracellular signaling for glutamate, acetylcholine, histamine, serotonin, cathecolamines, and a large number of neuropeptides (bradykinin, cholecystokinin, endothelin, opioids, and tachykinins) have been characterized. Such studies are facilitated in cell lines which offer a more homogenous material than primary cultures. Although the expression of ion channels and receptors vary considerably between different cell lines and comparative studies are rare, a few differences (compared to astrocytes in primary culture) have been identified which may turn out to be characteristic for glioma cells. Future identification of specific markers for receptors on glial and glioma cells related to cell type and growth properties may have great potential in clinical diagnosis and therapy.

Capacitative Ca2+ entry exclusively inhibits cAMP synthesis in C6-2B glioma cells. Evidence that physiologically evoked Ca2+ entry regulates Ca(2+)-inhibitable adenylyl cyclase in non-excitable cells

Elevation of cytosolic free Ca2+ inhibits the type VI adenylyl cyclase that predominates in C6-2B cells. However, it is not known whether there is any selective requirement for Ca2+ entry or release for inhibition of cAMP accumulation to occur. In the present study, the effectiveness of intracellular Ca2+ release evoked by three independent methods (thapsigargin, ionomycin, and UTP) was compared with the capacitative Ca2+ entry that was triggered by these treatments. In each situation, only Ca2+ entry could inhibit cAMP accumulation (La3+ ions blocked the effect); Ca2+ release, which was substantial in some cases, was without effect. A moderate inhibition, as was elicited by a modest degree of Ca2+ entry, could be rendered substantial in the absence of phosphodiesterase inhibitors. Such conditions more closely mimic the physiological situation of normal cells. These results are particularly significant, in demonstrating not only that Ca2+ entry mediates the inhibitory effects of Ca2+ on cAMP accumulation, but also that diffuse elevations in [Ca2+]i are ineffective in modulating cAMP synthesis. This property suggests that, as with certain Ca(2+)-sensitive ion channels, Ca(2+)-sensitive adenylyl cyclases may be functionally colocalized with Ca2+ entry channels.

A comparative study on the effects of tetracyclines and lithium on the cyclic AMP second messenger system in rat brain

1. This study was aimed at investigating the effects of demeclocycline (DMC), minocycline (MC), and lithium (Li) in vitro on cyclic AMP (cAMP) accumulation in rat cerebral cortex stimulated by noradrenaline, forskolin, and ouabain. 2. DMC, MC, and Li dose-dependently reduced noradrenaline-stimulated cAMP formation in cortical slices, but only Li inhibited the cAMP formation induced by forskolin. 3. In contrast to Li, DMC and MC did not affect noradrenaline-stimulated adenylate cyclase activity in cortical membranes. 4. In cortical slices, ouabain stimulated the cAMP production (required the presence of extracellular Ca2+ and was blocked by verapamil). Ouabain-stimulated cAMP accumulation in cortical slices was inhibited by DMC, MC, and Li. 5. DMC and MC do not seem to interact directly with the adenylate cyclase as reported for Li. It is concluded that the tetracyclines, DMC and MC, affect the cAMP signaling system in rat brain by mechanisms that differ from that of Li. The decreased receptor agonist-stimulated cAMP production in cortical slices in the presence of DMC and MC may be due to the Ca(2+)-chelating ability of these tetracyclines.

Up-regulation in late pregnancy of both Go1 alpha and Go2 alpha isoforms in human myometrium

The nature of the 39 kDa pertussis toxin substrate previously detected in human pregnant myometrium was investigated. Comparison of membranes from non-pregnant and from 39-40 week pregnant myometrium revealed a higher level at 39 kDa of pertussis toxin ADP-ribosylation and Go alpha immunoreactivity in late pregnancy. Furthermore, quantification of both Go alpha isoforms with specific anti-alpha o1- and alpha o2-antibodies revealed an increase in their expression in late pregnancy. At the same time, only limited changes in the levels of Gi1,2, Gi3 and Gq were observed. In rat myometrial membranes, neither pertussis toxin substrate nor Go alpha immunoreactivity could be detected at the 39 kDa level. These results demonstrate that, in the human myometrium in late pregnancy, there is an increase in the expression of both isoforms of the Go alpha subunits, suggesting a role for these proteins throughout gestation and/or near term for parturition.

Octopamine-sensitive adenylyl cyclase and G proteins in Ceratitis capitata brain during aging

The loss of the ability in signalling transduction constitutes an attractive hypothesis to explain the age-related loss of functions in the nervous system. In this paper we have examined adenylyl cyclase and G proteins in Ceratitis capitata brain during aging. The intermediate level of complexity of the Mediterranean fruit fly and its short lifespan make it a particularly interesting system for aging studies. Adenylyl cyclase basal activity decreased in the course of aging. By contrast, neither guanine nucleotide-induced activation of adenylyl cyclase nor Gs protein levels were modified. However, adenylyl cyclase activation by octopamine, which is a major neurotransmitter, neuromodulator and neurohormone in insects, was lost during aging. This observation correlated with a decrease in octopamine binding to brain plasma membranes that was due to a decrease in both receptor affinity and binding sites. On the other hand, we observed an increase in the expression of C. capitata Go protein with age, as revealed by pertussis toxin-catalysed ADP-ribosylation and immunoblotting experiments, that was not correlated with an increase in beta subunit levels. This report constitutes the first direct evidence for the participation of a Go protein in aging in the nervous system.