Heterotrimeric Gi protein is associated with the inositol 1,4,5-trisphosphate receptor complex and modulates calcium flux

Opioid elevation of intracellular free calcium: possible mechanisms and physiological relevance

Opioid receptors are seven transmembrane domain Gi/G0 protein-coupled receptors, the activation of which stimulates a variety of intracellular signalling mechanisms including activation of inwardly rectifying potassium channels, and inhibition of both voltage-operated N-type Ca2+ channels and adenylyl cyclase activity. It is now apparent that like many other Gi/G0-coupled receptors, opioid receptor activation can significantly elevate intracellular free Ca2+ ([Ca2+]i), although the mechanism underlying this phenomenon is not well understood. In some cases opioid receptor activation alone appears to elevate [Ca2+]i, but in many cases it requires concomitant activation of Gq-coupled receptors, which themselves stimulate Ca2+ release from intracellular stores via the inositol phosphate pathway. Given the number of Ca2+-sensitive processes known to occur in cells, there are therefore a myriad of situations in which opioid receptor-mediated elevations of [Ca2+](i) may be important. Here, we review the literature documenting opioid receptor-mediated elevations of [Ca2+]i, discussing both the possible mechanisms underlying this phenomenon and its potential physiological relevance.

Manipulation of olfactory ensheathing cell signaling mechanisms: effects on their support for neurite regrowth from adult CNS neurons in coculture

Pretreatment of olfactory ensheathing cells (OECs) with Pertussis toxin increased the number of subsequently cocultured adult retinal ganglion cells (RGCs) regrowing neurites without affecting neuronal survival. Pertussis toxin (PTx) inactivated an OEC G(i/o) protein as pretreating OECs with the PTx B-oligomer subunit had no effect on RGC neurite regrowth. However, the B-oligomer was responsible for decreasing the marked orientation of neurite regrowth on the OEC substrate. Simultaneous incubation of OECs with PTx and a depolarizing concentration of KCl abolished the increase in neurite regrowth from cocultured RGCs, but exposure to a depolarizing KCl concentration after OECs had been PTx-treated had no effect. Our evidence supports the hypothesis that G-protein-regulated calcium signaling plays a significant role in OEC support for CNS axonal regeneration.

Calcium signaling dysfunction in schizophrenia: a unifying approach

The present paper demonstrates a remarkable pervasiveness of underlying Ca(2+) signaling motifs among the available biochemical findings in schizophrenic patients and among the major molecular hypotheses of this disease. In addition, the paper reviews the findings suggesting that Ca(2+) is capable of inducing structural and cognitive deficits seen in schizophrenia. The evidence of the ability of antipsychotic drugs to affect Ca(2+) signaling is also presented. Based on these data, it is proposed that altered Ca(2+) signaling may constitute the central unifying molecular pathology in schizophrenia. According to this hypothesis schizophrenia can result from alterations in multiple proteins and other molecules as long as these alterations lead to abnormalities in certain key aspects of intracellular Ca(2+) signaling cascades.

Co-incident signalling between mu-opioid and M3 muscarinic receptors at the level of Ca2+ release from intracellular stores: lack of evidence for Ins(1,4,5)P3 receptor sensitization

Activation of G(i)/G(o)-coupled opioid receptors increases [Ca2+]i (intracellular free-Ca2+ concentration), but only if there is concomitant G(q)-coupled receptor activation. This G(i)/G(o)-coupled receptor-mediated [Ca2+]i increase does not appear to result from further production of Ins P3 [Ins(1,4,5) P3] in SH-SY5Y cells. In the present study, fast-scanning confocal microscopy revealed that activation of mu-opioid receptors alone by 1 muM DAMGO ([L-Ala, NMe-Phe, Gly-ol]-enkephalin) did not stimulate the Ins P3-dependent elementary Ca2+-signalling events (Ca2+ puffs), whereas DAMGO did evoke Ca2+ puffs when applied during concomitant activation of M3 muscarinic receptors with 1 muM carbachol. We next determined whether mu-opioid receptor activation might increase [Ca2+]i by sensitizing the Ins P3 receptor to Ins P3. DAMGO did not potentiate the amplitude of the [Ca2+]i increase evoked by flash photolysis of the caged Ins P3 receptor agonist, caged 2,3-isopropylidene-Ins P3, whereas the Ins P3 receptor sensitizing agent, thimerosal (10 muM), did potentiate this response. DAMGO also did not prolong the rate of decay of the increase in [Ca2+]i evoked by flash photolysis of caged 2,3-isopropylidene-Ins P3. Furthermore, DAMGO did not increase [Ca2+]i in the presence of the cell-membrane-permeable Ins P3 receptor agonist, Ins P3 hexakis(butyryloxymethyl) ester. Therefore it appears that mu-opioid receptors do not increase [Ca2+]i through either Ins P3 receptor sensitization, enhancing the releasable pool of Ca2+ or inhibition of Ca2+ removal from the cytoplasm.

AT2 receptor activation regulates myocardial eNOS expression via the calcineurin-NF-AT pathway

The role of AT2-receptors has recently been subject of considerable debate. We investigated the influence of AT2-stimulation/inhibition on myocardial endothelial NO-synthase (eNOS, NOS-III) promoter activity and eNOS protein expression. Stimulation of rat cardiomyocytes with angiotensin II (AngII) increased eNOS protein expression 3.3-fold. This was blocked by Cyclosporin A (CsA). Inhibition of the AT1-receptor did not reduce AngII-mediated eNOS protein expression, whereas AT2 stimulation increased it 2.4-fold and AT2 inhibition suppressed it. The modulatory effects of the AT2-receptor on eNOS expression was confirmed in mice with a genetic deletion of the AT2-receptor (AT2-KO). In gel shift assays two putative NF-AT sites in a 1.6 kb eNOS promoter fragment showed NF-AT binding and a supershift by NF-AT2(-c1)-specific antibodies. Stimulation of transfected cells with AngII or specific AT2-receptor agonists resulted in a significant increase in eNOS promoter activity, which was blocked by CsA, MCIP1, and mutation of an upstream NF-AT site.

CONCLUSION

1) AngII-stimulation of the myocardium, both in vivo and in vitro, is accompanied by increased expression of eNOS. 2) This effect is mediated by the calcineurin pathway and is induced by the AT2-receptor. 3) These results define a calcineurin/NF-AT/eNOS pathway as downstream effector of AT2-receptor activation in the myocardium.

Parathyroid hormone increases the sensitivity of inositol trisphosphate receptors by a mechanism that is independent of cyclic AMP

1 In fura 2-loaded HEK-293 cells stably expressing human type 1 parathyroid hormone (PTH) receptors, PTH potentiated the Ca(2+) mobilization evoked by carbachol by >4 fold without itself increasing the intracellular [Ca(2+)]. 2 PTH potentiated the Ca(2+) release evoked by a cell-permeant analogue of inositol 1,4,5-trisphosphate (InsP(3)BM). 3 Prolonged incubation with InsP(3)BM emptied the Ca(2+) stores as effectively as PTH in combination with a maximal concentration of carbachol, indicating that PTH did not increase the size of the InsP(3)-sensitive Ca(2+) pool. 4 Responses to PTH were unaffected by disruption of the cytoskeleton. 5 The EC(50) for carbachol-evoked Ca(2+) release and InsP(3) formation were indistinguishable (approximately 40 microM), consistent with even the highest concentrations of carbachol generating insufficient InsP(3) to release the entire InsP(3)-sensitive Ca(2+) pool. 6 Inhibition of cyclic AMP-dependent protein kinase A (PKA), using H89 or CMIQ, did not affect potentiation of carbachol-evoked Ca(2+) signals by PTH. 7 SQ22536 or DDA, inhibitors of adenylyl cyclase, inhibited PTH-evoked cyclic AMP formation and IBMX, an inhibitor of cyclic nucleotide phosphodiesterase, increased the amount of cyclic AMP detected after stimulation by PTH. None of these drugs affected the potentiation of Ca(2+) signals by maximal or submaximal concentrations of PTH. 8 We conclude that PTH potentiates the Ca(2+) release evoked by receptors that stimulate InsP(3) formation by sensitizing InsP(3) receptors through a cyclic AMP-independent mechanism.

Isoproterenol activates extracellular signal-regulated protein kinases in cardiomyocytes through calcineurin

BACKGROUND

Extracellular signal-regulated kinases (ERKs) and calcineurin have been reported to play important roles in the development of cardiac hypertrophy. We examined here the relation between calcineurin and ERKs in cardiomyocytes.

METHODS AND RESULTS

Isoproterenol activated ERKs in cultured cardiomyocytes of neonatal rats, and the activation was abolished by chelation of extracellular Ca(2+) with EGTA, blockade of L-type Ca(2+) channels with nifedipine, or depletion of intracellular Ca(2+) stores with thapsigargin. Isoproterenol-induced activation of ERKs was also significantly suppressed by calcineurin inhibitors in cultured cardiomyocytes as well as in the hearts of mice. Isoproterenol failed to activate ERKs in either the cultured cardiomyocytes or the hearts of mice that overexpress the dominant negative mutant of calcineurin. Isoproterenol elevated intracellular Ca(2+) levels at both systolic and diastolic phases and dose-dependently activated calcineurin. Inhibition of calcineurin also attenuated isoproterenol-stimulated phosphorylation of Src, Shc, and Raf-1 kinase. The immunocytochemistry revealed that calcineurin was localized in the Z band, and isoproterenol induced translocation of calcineurin and ERKs into the nucleus.

CONCLUSIONS

Calcineurin, which is activated by marked elevation of intracellular Ca(2+) levels by the Ca(2+)-induced Ca(2+) release mechanism, regulates isoproterenol-induced activation of ERKs in cardiomyocytes.

Coincident signalling between the Gi/Go-coupled delta-opioid receptor and the Gq-coupled m3 muscarinic receptor at the level of intracellular free calcium in SH-SY5Y cells

In SH-SY5Y cells, activation of delta-opioid receptors with [D-Pen(2,5)]-enkephalin (DPDPE; 1 microM) did not alter the intracellular free Ca(2+) concentration [Ca(2+)](i). However, when DPDPE was applied during concomitant Gq-coupled m3 muscarinic receptor stimulation by carbachol or oxotremorine-M, it produced an elevation of [Ca(2+)](i). The DPDPE-evoked increase in [Ca(2+)](i) was abolished when the carbachol-sensitive intracellular Ca(2+) store was emptied. There was a marked difference between the concentration-response relationship for the elevation of [Ca(2+)](i) by carbachol (EC(50) 13 microM, Hill slope 1) and the concentration-response relationship for carbachol's permissive action in revealing the delta-opioid receptor-mediated elevation of [Ca(2+)] (EC(50) 0.7 mM; Hill slope 1.8). Sequestration of free G protein beta gamma dimers by transient transfection of cells with a beta gamma binding protein (residues 495-689 of the C terminal tail of G protein-coupled receptor kinase 2) reduced the ability of delta opioid receptor activation to elevate [Ca(2+)](i). However, DPDPE did not elevate either basal or oxotremorine-M-evoked inositol phosphate production indicating that delta-opioid receptor activation did not stimulate phospholipase C. Furthermore, delta-opioid receptor activation did not result in the reversal of muscarinic receptor desensitization, membrane hyperpolarization or stimulation of sphingosine kinase. There was no coincident signalling between the delta-opioid receptor and the lysophosphatidic acid receptor which couples to elevation of [Ca(2+)](i) in SH-SY5Y cells by a PLC-independent mechanism. In SH-SY5Y cells the coincident signalling between the endogenously expressed delta-opioid and m3 muscarinic receptors appears to occur in the receptor activation-Ca(2+) release signalling pathway at a step after the activation of phospholipase C.

The role of GTP-binding protein activity in fast central synaptic transmission

Guanosine 5'-triphosphate (GTP)-binding proteins (G proteins) are involved in exocytosis, endocytosis, and recycling of vesicles in yeast and mammalian secretory cells. However, little is known about their contribution to fast synaptic transmission. We loaded guanine nucleotide analogs directly into a giant nerve terminal in rat brainstem slices. Inhibition of G-protein activity had no effect on basal synaptic transmission, but augmented synaptic depression and significantly slowed recovery from depression. A nonhydrolyzable GTP analog blocked recovery of transmission from activity-dependent depression. Neither effect was accompanied by a change in presynaptic calcium currents. Thus, G proteins contribute to fast synaptic transmission by refilling synaptic vesicles depleted after massive exocytosis.

Group 1 mGlu receptors elevate [Ca2+]i in rat cultured cortical type 2 astrocytes: [Ca2+]i synergy with adenosine A1 receptors

Brain macroglia are known to express a diverse array of neurotransmitter receptors whose signal transduction pathways may be subject to heteroreceptor 'cross-talk'. In the current study we have examined group 1 mGlu receptor-evoked [Ca2+]i signalling, and possible heteroreceptor cross-talk, in cultured type 2 astrocytes. The selective group 1 metabotropic glutamate (mGlu) receptor agonist (S)-3,5-dihydroxyphenylglycine (DHPG) elevated [Ca2+]i (EC50 = 1.7 +/- 0.6 microM); an effect reversed by the selective mGlu receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine (IC50 = 52.7 +/- 8.7 microM). DHPG-evoked [Ca2+]i responses were abolished by (1) thapsigargin (100 nM), implicating the involvement of internal Ca2+ stores in group 1 mGlu [Ca2+]i responses and (2) the removal of extracellular Ca2+. When applied alone, the selective adenosine A1 receptor agonist, N6-cyclopentyladenosine (CPA, 100 nM) failed to influence [Ca2+]i. However, in the presence of 1 microM DHPG, CPA potently (EC50 = 12.3 +/- 1.9 nM) increased [Ca2+]i responses. In the presence of 100 nM CPA, the efficacy of DHPG was doubled without any significant change in the DHPG EC50 value. This effect was reversed by either the selective adenosine A1 receptor antagonist, 8-cyclopentyltheophylline (IC50 = 50.3 +/- 19.9 nM) or overnight incubation with Pertussis toxin (100 ng/ml). We conclude that (1) type 2 astrocytes contain group 1 mGlu receptors coupled to [Ca2+]i signalling and (2) co-activation of adenosine A1 receptors enhances group 1 mGlu-evoked [Ca2+]i responses in these cells via a Gi/o G protein-mediated mechanism.

Effect of the G-protein, G alpha(i2), and G alpha(i3) subunit knockdown on bradykinin-induced signal transduction in rat-1 cells

The bradykinin (BK) B2 receptor (BKB2R) has been shown to interact with the G alpha(q) subunit family. However, it has remained unclear whether this receptor also interacts with the G alpha(i) subunit family. To further resolve this issue, two antisense expression plasmids were generated. In these, the 5'-untranslated regions of rat G alpha(i2) and G alpha(i3) cDNAs were used as specific antisense templates. The plasmids were transfected into Rat-1 cells, which expressed a stably transfected rat BKB2R cDNA and bound BK with a Kd of approximately 3 nM. In these cells, the transfected BKB2R was fully linked to inositol phosphate production, arachidonic acid (ARA) release, and Ca2+ flux. A number of cell lines, each a G alpha(i2) or G alpha(i3) knockdown, were isolated. Of these, two cell lines were chosen for study. One, designated 2-E3, displayed over a 70% decrease in the expression of G alpha(i2) without a change in the expression of G alpha(i3) or G alpha(q). Another, 3-G9, exhibited over a 70% decrease of G alpha(i3) protein without a change in G alpha(i2) or G alpha(q) expression. Knockdown of either G alpha(i2) or G alpha(i3) protein production did not affect the binding of bradykinin. In the G alpha(i2)-depleted 2-E3 cells, BK induced ARA release was reduced by more than 60%. Interestingly, the production of total inositol phosphate in response to BK was also reduced by approximately 35%. However, G alpha(i2) knockdown had no significant effect on BK-induced Ca2+ influx. In the G alpha(i3)-depleted 3-G9 cells, BK-induced ARA release was decreased by over 50%. In this case [Ca2+]i increase in response to BK was reduced by over 50%. This knockdown also resulted in reduced BK-activated total inositol phosphate production. Moreover, cAMP augmented the BK-induced ARA release. Depletions of G alpha(i2) and G alpha(i3) further enhanced this cAMP-dependent BK induction of ARA release. Taken together, these results delineate direct interaction of the BKB2R with both G alpha(i2) and G alpha(i3) subunits in addition to the heretofore described interaction of BKB2R with the G alpha(q) subunit family.