Guanosine-5'-O-thiodiphosphate functions as a partial agonist for the receptor-independent stimulation of neural adenylate cyclase

Rapid membrane responses to dihydrotestosterone are sex dependent in growth plate chondrocytes

Sex steroids are important regulators for longitudinal growth, bone mass accrual, and sexual dimorphism of the skeleton. 17β-Estradiol regulates proliferation and differentiation of female chondrocytes via a membrane-associated signaling pathway in addition to its estrogen receptor (ER) mediated effects. In contrast, testosterone does not elicit a similar membrane response, either in male or female cells. Whereas female rat growth plate chondrocytes convert testosterone to 17β-estradiol, male chondrocytes produce 5α-dihydrotestosterone (DHT). Previously DHT was found to mediate sex-specific effects of testosterone in male cells, but it is not known if a membrane-signaling pathway is involved. In this study, we hypothesized that DHT can induce sex-specific rapid membrane effects similar to other steroid hormones. Confluent cultures of chondrocytes isolated from resting zones of growth plates of both male and female rats were treated with 10(-10)-10(-7)M testosterone or DHT for 3, 9, 90 and 270min and protein kinase C (PKC) and phospholipase A2 (PLA2) activities were measured. To examine potential signaling pathways involved in PKC activation, male chondrocytes were treated with 10(-7)M DHT for 9min in the presence or absence of the phospholipase C (PLC) inhibitor U73122, the secretory PLA2 inhibitor quinacrine or the cytosolic PLA2 inhibitor AACOCF3; the Gαi inhibitor pertussis toxin (PTX) or Gαs activator cholera toxin (CTX), and the general G-protein inhibitor GDPβS; thapsigargin, an inhibitor of a Ca-ATPase pump in the endoplasmic reticulum; verapamil and nifedipine, inhibitors of specific L type Ca2+ channels on the cell membrane; and cyproterone acetate (CPA), which is an inhibitor of the classical androgen receptor (AR); as well as the transcription inhibitor actinomycin D, or the translation inhibitor cycloheximide. DHT induced a dose-dependent increase in PKC and PLA2 activity in male cells with the highest increase at 10(-7)M DHT (p<0.05), whereas testosterone had no effect. PKC activity was augmented at 9 and 90 min, and then decreased to baseline at 270min. Neither testosterone nor DHT affected PKC in female cells. U73122, quinacrine, and AACOCF3 inhibited DHT-induced activation of PKC. DHT treatment for 9 min had no effect in [(3)H]-thymidine incorporation in quiescent confluent cultures but caused a dose dependent increase in alkaline phosphatase specific activity. Inhibition of PLC reduced the response of to DHT in a dose dependent manner, indicating that PLC is involved. In conclusion, our study indicates that DHT, but not testosterone, has sex-specific rapid membrane effects in male growth plate chondrocytes involving PLC and PLA2-mediated PKC signaling pathways. Together with previous observations showing that male cells convert testosterone to DHT, these results suggest that DHT might act in the membrane through an autocrine/paracrine mechanism.

Chronic treatment with escitalopram but not R-citalopram translocates Galpha(s) from lipid raft domains and potentiates adenylyl cyclase: a 5-hydroxytryptamine transporter-independent action of this antidepressant compound

Chronic antidepressant treatment has been shown to increase adenylyl cyclase activity, in part, due to translocation of Galpha(s) from lipid rafts to a nonraft fraction of the plasma membrane where they engage in a more facile stimulation of adenylyl cyclase. This effect holds for multiple classes of antidepressants, and for serotonin uptake inhibitors, it occurs in the absence of the serotonin transporter. In the present study, we examined the change in the amount of Galpha(s) in lipid raft and whole cell lysate after exposing C6 cells to escitalopram. The results showed that chronic (but not acute) escitalopram decreased the content of Galpha(s) in lipid rafts, whereas there was no change in overall Galpha(s) content. These effects were drug dose- and exposure time-dependent. Although R-citalopram has been reported to antagonize some effects of escitalopram, this compound was without effect on Galpha(s) localization in lipid rafts, and R-citalopram did not inhibit these actions of escitalopram. Escitalopram treatment increased cAMP accumulation, and this seemed due to increased coupling between Galpha(s) and adenylyl cyclase. Thus, escitalopram is potent, rapid and efficacious in translocating Galpha(s) from lipid rafts, and this effect seems to occur independently of 5-hydroxytryptamine transporters. Our results suggest that, although antidepressants display distinct affinities for well identified targets (e.g., monoamine transporters), several presynaptic and postsynaptic molecules are probably modified during chronic antidepressant treatment, and these additional targets may be required for clinical efficacy of these drugs.

Cytosolic G{alpha}s acts as an intracellular messenger to increase microtubule dynamics and promote neurite outgrowth

It is now evident that Galpha(s) traffics into cytosol following G protein-coupled receptor activation, and alpha subunits of some heterotrimeric G-proteins, including Galpha(s) bind to tubulin in vitro. Nevertheless, many features of G-protein-microtubule interaction and possible intracellular effects of G protein alpha subunits remain unclear. In this study, several biochemical approaches demonstrated that activated Galpha(s) directly bound to tubulin and cellular microtubules, and fluorescence microscopy showed that cholera toxin-activated Galpha(s) colocalized with microtubules. The activated, GTP-bound, Galpha(s) mimicked tubulin in serving as a GTPase activator for beta-tubulin. As a result, activated Galpha(s) made microtubules more dynamic, both in vitro and in cells, decreasing the pool of insoluble microtubules without changing total cellular tubulin content. The amount of acetylated tubulin (an indicator of microtubule stability) was reduced in the presence of Galpha(s) activated by mutation. Previous studies showed that cholera toxin and cAMP analogs may stimulate neurite outgrowth in PC12 cells. However, in this study, overexpression of a constitutively activated Galpha(s) or activation of Galpha(s) with cholera toxin in protein kinase A-deficient PC12 cells promoted neurite outgrowth in a cAMP-independent manner. Thus, it is suggested that activated Galpha(s) acts as an intracellular messenger to regulate directly microtubule dynamics and promote neurite outgrowth. These data serve to link G-protein signaling with modulation of the cytoskeleton and cell morphology.

G protein signaling in a neuronal network is necessary for rhythmic motor pattern production

G protein-coupled receptors are widely recognized as playing important roles in mediating the actions of extrinsic neuromodulatory inputs to motor networks. However, the potential for their direct involvement in rhythmic motor pattern generation has received considerably less attention. Results from this study indicate that G protein signaling appears to be integral to the operation of the central pattern generator (CPG) underlying the escape swim of the mollusk Tritonia diomedea. Blocking G protein signaling in a single CPG neuron, cerebral neuron C2, with intracellular iontophoresis of the guanine nucleotide analogue guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S), prevented the production of the swim motor program. Moreover, tonic activation of G protein signaling in this neuron by iontophoresis of the GTP analogues guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and 5'-guanylyl-imidodiphosphate also inhibited motor pattern production. The possible sites of action of these guanine nucleotide analogues were examined to assess potential mechanisms by which they interfered with motor pattern production. Intracellular iontophoresis of GDP-beta-S into C2 did not affect C2 basal synaptic strength. However, it did reduce heterosynaptic facilitation of C2 synapses caused by the dorsal swim interneurons (DSIs), a set of serotonergic swim CPG neurons. In contrast, GTP-gamma-S directly enhanced C2 synaptic strength onto DFN, mimicking the neuromodulatory effect of the DSIs. GDP-beta-S, but not the GTP analogues, decreased C2 excitability, whereas both GTP analogues, but not GDP-beta-S, blocked the ability of DSI stimulation to increase C2 excitability. The decrease in C2 excitability caused by GDP-beta-S is not likely to be responsible for the inhibition of the swim motor pattern because decreasing C2 firing rate, by injecting hyperpolarizing current, did not prevent the production of the rhythmic motor pattern. Taken together, these data suggest that G protein signaling is a necessary and integral component of the escape swim CPG in Tritonia and that G protein signaling mediates DSI heterosynaptic facilitation of C2 but may not mediate the DSI-evoked enhancement of C2 excitability.

Tubulin stimulates adenylyl cyclase activity in C6 glioma cells by bypassing the beta-adrenergic receptor: a potential mechanism of G protein activation

While the cytoskeleton is known to play several roles in the biology of the cell, one role, which has been revealed only recently, is that of a participant in the signal transduction process. Tubulin binds specifically to the alpha subunits of Gs (stimulatory GTP-binding regulatory protein of adenylyl cyclase), Gi1 (inhibitory protein of adenylyl cyclase), and Gq and transactivates those molecules through direct transfer of GTP. The relevance of this transactivation process to G proteins which are normally activated by a neurotransmitter-occupied receptor is the subject of this study. C6 glioma cells, made permeable with saponin, retained tight coupling between Gs and the beta-adrenergic receptor. Although 5-guanylylimidodiphosphate (GppNHp) was incapable of activating Gs (and subsequently, adenylyl cyclase) in the absence of agonist, tubulin with GppNHp bound (tubulin-GppNHp) activated adenylyl cyclase with an EC(50) of 30 nM. Desensitization of beta-adrenergic receptors by isoproterenol exposure had no effect on the ability of tubulin-GppNHp to activate Gs and adenylyl cyclase. When the photoaffinity GTP analog, azidoanilido GTP (AAGTP; P3(4-azidoanilido)-P1-5'-GTP), was added to C6 membranes or permeable C6 cells, it was only weakly incorporated by G alpha s in the absence of isoproterenol. When the same concentration of dimeric tubulin with AAGTP bound was introduced, AAGTP was transferred from tubulin to G alpha s, activating the latter species. Similar 'preferential' activation of G alpha s by tubulin-AAGTP versus the free nucleotide was seen using purified components. Thus, membrane-associated tubulin may serve to activate G alpha s, independent of signals not normally coupled to that protein. Tubulin may act as an agent to link a variety of membrane-associated signalling systems.

Treatment of C6 glioma cells and rats with antidepressant drugs increases the detergent extraction of G(s alpha) from plasma membrane

Results from previous studies suggested that chronic treatment of rats or C6 glioma cells with antidepressants augments the coupling between Gs and adenylyl cyclase. As these effects on C6 glioma cells are seen in the absence of presynaptic input, several antidepressant drugs may have a direct "postsynaptic" effect on their target cells. It was hypothesized that the target of antidepressant action was some membrane protein that may regulate coupling between G proteins and adenylyl cyclase. To test this, C6 glioma cells were treated with amitriptyline, desipramine, iprindole, or fluoxetine for 3 days. Chlorpromazine served as a control for these treatments. Membrane proteins were extracted sequentially with Triton X-100 and Triton X-114 from C6 glioma cells. Triton X-100 extracted more G(s alpha) in membranes prepared from antidepressant-treated C6 glioma cells than from control groups. In addition, cell fractionation studies revealed that the amount of G(s alpha) in caveolin-enriched domains was reduced after antidepressant treatment and that adenylyl cyclase comigrated with G(s alpha) in the gradients. These data suggest that some postsynaptic component that increases availability of Gs to activate effector molecules, such as adenylyl cyclase, might be a target of antidepressant treatment.

Bradykinin modulates potassium and calcium currents in neuroblastoma hybrid cells via different pertussis toxin-insensitive pathways

In NG108-15 cells, bradykinin (BK) activates a potassium current (IK,BK) and inhibits the voltage-dependent calcium current (ICa,V). BK also stimulates a phosphatidylinositol-specific phospholipase C (PI-PLC). The subsequent release of inositol 1,4,5-trisphosphate and increase in intracellular calcium contribute to IK,BK, through activation of a calcium-dependent potassium current. In membranes from these cells, stimulation of PI-PLC by BK is mediated by Gq and/or G11, two homologous, pertussis toxin-insensitive G proteins. Here, we have investigated the role of Gq/11 in the electrical responses to BK. GTP gamma S mimicked and occluded both actions of BK, and both effects were insensitive to pertussis toxin. Perfusion of an anti-Gq/11 alpha antibody into the pipette suppressed IK,BK, but not the inhibition of ICa,V by BK. Thus, BK couples to IK,BK via Gq/11, but coupling to ICa,V is most likely via a different, pertussis toxin-insensitive G protein.

Adenylate cyclase from Hirudo medicinalis segmental ganglia: modulation by physiological and non-physiological agents

1. In Hirudo medicinalis segmental ganglia GTP is essential for the full expression of the stimulatory action of serotonin on the adenylate cyclase activity. The amine, in turn, increases the overall affinity of the enzymatic system for GTP. 2. GTP gamma S and Gpp(NH)p, non-hydrolysable analogues of GTP, dose-dependently enhance the basal enzyme activity, but impair the stimulatory effect of serotonin. 3. Fluoride ions biphasically modulate the leech adenylate cyclase both in the absence and in the presence of GTP. The ion effect is also influenced by non-physiological guanine nucleotides.

Neurotransmitter-mediated inhibition of post-mortem human brain adenylyl cyclase

The effects of a range of neurotransmitter agonists showing selectivity for receptor types inhibitorily coupled to adenylyl cyclase were compared in membrane preparations of hippocampus, frontal cortex and caudate nucleus/striatum from previously frozen post-mortem human and rat brain. Agonists were tested against basal and forskolin stimulated activities, forskolin being a potent activator of the catalytic sub-unit of the enzyme. Of those agonists tested, only somatostatin (100 microM) and neuropeptide Y (10 microM) gave consistent inhibitions of basal and forskolin stimulated enzyme activities in all three regions of both human and rat brain. Somatostatin-mediated inhibition of human brain adenylyl cyclase was reduced in the absence of GTP and in the presence of the guanine nucleotide partial agonist, guanosine 5'-O-thiodiposphate, consistent with a G-protein-linked receptor. No such GTP-dependence was found for the neuropeptide Y-mediated adenylyl cyclase inhibition. GTP-dependent somatostatin mediated inhibitions of human brain adenylyl cyclase activity were of highest magnitude in the thalamus, intermediate magnitude in the hippocampus and caudate nucleus and lowest magnitude in the frontal cortex. It is concluded that of a range of neurotransmitter receptor agonists tested, only somatostatin gives robust, GTP-dependent responses that are reproducible enough to be used with post-mortem tissue for the comparison of receptor function in human brain disorders.

Different mechanisms of beta-adrenoceptor down-regulation by chronic imipramine and electroconvulsive treatment: possible role for protein kinase C

The aim of this study was to find out how protein kinase C (PKC) is involved in down-regulation of the beta-adrenoceptor in cortical slices of rats subjected to antidepressant treatments. The responses of the cyclic AMP generating system to forskolin, isoproterenol, and noradrenaline were tested in the absence and presence of a PKC activator, 12-O-tetradecanoylphorbol 13-acetate (TPA). The antidepressive treatments applied were chronic administration of imipramine and electroconvulsive shock. The potentiating effect of the phorbol ester on cyclic AMP response to isoproterenol was retained in imipramine-treated animals and even accentuated in rats subjected to electroconvulsive treatment; the TPA effect on noradrenaline-induced cyclic AMP response was blunted in rats receiving imipramine, but augmented in those receiving electroconvulsive treatment. In imipramine-treated rats the beta-down-regulation was still evident in the presence of TPA; after electroconvulsive treatment the phorbol ester-induced potentiation was so high that no significant beta-down-regulation could be observed. No procedure affected the response to forskolin. The beta-down-regulation that develops during chronic imipramine treatment differs from that caused by chronic electroconvulsive treatment; in both cases it is not related to the direct effect on adenylate cyclase.

Muscarinic cholinergic stimulation of exogenous phosphatidylinositol hydrolysis is regulated by guanine nucleotides in rabbit brain cortical membranes

Rabbit brain cortical membranes, which have been extracted with 2 M KCl, hydrolyze exogenously added [3H]phosphatidylinositol [( 3H]PI) in a guanine nucleotide- and carbachol-dependent manner. Both oxotremorine-M and carbachol are full agonists with EC50 values of 8 and 73 microM, respectively. Pirenzepine and atropine inhibit carbachol-stimulated [3H]PI hydrolysis. The hydrolysis-resistant guanine nucleotide analog guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) is the most potent in supporting carbachol-stimulated hydrolysis of PI. There is no effect of carbachol in the absence of guanine nucleotides or in the presence of 100 microM adenosine 5'-O-(3-thiotriphosphate), adenosine-5'-(beta, gamma-imido)triphosphate, or sodium pyrophosphate. Guanylyl-5'-(beta,gamma-imido)triphosphate [Gpp(NH)p] in the presence of carbachol also stimulates PI hydrolysis although much less than that seen with GTP gamma S. GDP and Gpp(NH)p are potent antagonists of the GTP gamma S-dependent carbachol response. Optimal stimulation by carbachol and GTP gamma S was observed at 0.3-1 microM free Ca2+ and 6 mM MgCl2. Limited trypsinization resulted in loss of receptor-regulated PI breakdown and a slight decrease in basal activity. These results demonstrate that phospholipase C hydrolysis of exogenous PI by rabbit cortical membranes may be stimulated by carbachol in a guanine nucleotide-dependent manner.

Assay of a phosphatidylinositol bisphosphate phospholipase C activity in postmortem human brain

The activity of a phospholipase C which hydrolyses exogenous phosphatidylinositol-4,5-bisphosphate [( 3H]PtdIns(4,5)P2) in membranes prepared from frozen postmortem human brain and rat brain was investigated. Enzyme characteristics were essentially similar in membranes prepared from frozen postmortem brain and fresh or frozen rat brain. The [3H]PtdIns(4,5)P2 solubilization and assay procedure employed resulted in an efficient availability of the substrate for the enzyme. The non-hydrolysable guanosine triphosphate analogue guanosine 5'-[beta gamma-imido]diphosphate (Gpp[NH]p) stimulated hydrolysis rapidly with a half maximum activity of approximately 25 microM. This stimulation was not specific for guanine nucleotides as ATP, imidodiphosphate and pyrophosphate also caused enzyme activation. However these activation effects could be distinguished by the polyanion spermine. The non-hydrolysable guanine dinucleotide analogue guanosine 5'-[beta-thio]diphosphate acted as a partial agonist thereby inhibiting the stimulatory effect of Gpp[NH]p. Gpp[NH]p-stimulated enzyme activity showed a maximum response in the presence of 1 mM deoxycholate and displayed a pH optima in the range 7.0-7.5. PtdIns(4,5)P2 hydrolysis was observed in the absence of added calcium, but hydrolytic cleavage was inhibited in the presence of divalent ion chelators. Magnesium inhibited PtdIns(4,5)P2 hydrolysis in a concentration-dependent manner. Elucidation of these aspects of the phosphatidylinositol cycle in normal human postmortem brain will permit comparative studies in CNS disease states.

Chronic electroconvulsive treatment augments coupling of the GTP-binding protein Gs to the catalytic moiety of adenylyl cyclase in a manner similar to that seen with chronic antidepressant drugs

A significant increase of guanylylimidodiphosphate (GppNHp)-, fluoride-, and forskolin-stimulated adenylyl cyclase was observed in synaptic membrane preparations from rat cerebral cortex subsequent to chronic electroconvulsive shock (ECS) treatment. This effect required at least five treatments over a course of 10 days. The inhibition of adenylyl cyclase induced by GppNHp was not affected by these treatments. The dissociation constant (KD) and maximal binding for the photoaffinity GTP analog, [32P]P3-(4-azidoanilido)-P1-5'-GTP [( 32P]AAGTP), to each of the synaptic membrane G proteins also were unchanged after ECS treatment. Nonetheless, the transfer of [32P]AAGTP from Gi to Gs, which we suggest is indicative of the coupling between Gs and the adenylyl cyclase catalytic moiety, was accelerated by chronic ECS treatment but not by acute or sham treatment. Furthermore, chemical uncoupling of Gs from adenylyl cyclase rendered membranes from treated animals indistinguishable from controls. Finally, in all cases tested, membranes prepared from animals subjected to chronic treatment with amitriptyline or iprindole showed similar changes in the Gs-mediated activation of adenylyl cyclase. Acute treatments produced effects similar to controls, and liver and kidney membranes from animals receiving chronic treatment showed no changes in adenylyl cyclase despite the marked changes seen in brain. These results suggest that chronic administration of ECS enhances coupling between Gs and adenylyl cyclase enzyme and modifies interactions between Gs and Gi.

Supra-additive activation of guinea-pig superior cervical ganglion adenylate cyclase by PGE2 and D-Ala2-Met-enkephalinamide: role of GTP

The effects of guanine nucleotides were tested on basal and agonist-modulated adenylate cyclase in guinea-pig superior cervical ganglion crude membrane preparations. GTP gamma S and Gpp(NH)p dose-dependently stimulate, while GDP beta S inhibits, both the basal and the prostaglandin E2-stimulated enzyme activity. Low GTP doses, up to 10(-5) M, stimulate, while higher doses inhibit, the ganglionic adenylate cyclase. The GTP-induced diphasic pattern is maintained also in the presence of prostaglandin E2, D-Ala2-Met-enkephalinamide, or a combination of the two drugs. However, the opioid inhibits the enzyme activity, but only at high GTP doses, while the prostaglandin stimulates the enzyme at all GTP concentrations. The effect is potentiated by a combination of prostaglandin and enkephalin. The enhancing effect of the prostaglandin and of the combination with enkephalin is maximally expressed at high, almost physiological, GTP doses.