Rapid and persistent desensitization of m3 muscarinic acetylcholine receptor-stimulated phospholipase D. Concomitant sensitization of phospholipase C

cAMP guided his way: a life for G protein-mediated signal transduction and molecular pharmacology-tribute to Karl H. Jakobs

Karl H. Jakobs, former editor-in-chief of Naunyn-Schmiedeberg's Archives of Pharmacology and renowned molecular pharmacologist, passed away in April 2018. In this article, his scientific achievements regarding G protein-mediated signal transduction and regulation of canonical pathways are summarized. Particularly, the discovery of inhibitory G proteins for adenylyl cyclase, methods for the analysis of receptor-G protein interactions, GTP supply by nucleoside diphosphate kinases, mechanisms in phospholipase C and phospholipase D activity regulation, as well as the development of the concept of sphingosine-1-phosphate as extra- and intracellular messenger will presented. His seminal scientific and methodological contributions are put in a general and timely perspective to display and honor his outstanding input to the current knowledge in molecular pharmacology.

Crosstalk between mAChRM3 and β2AR, via acetylcholine PI3/PKC/PBEP1/Raf-1 MEK1/2/ERK1/2 pathway activation, in human bronchial epithelial cells after long-term cigarette smoke exposure

BACKGROUND

Cigarette smoke extract (CSE) affects the expression of non-neuronal components of cholinergic system in bronchial epithelial cells and, as PEBP1/Raf-mediated MAPK1/2 and ERK1/2 pathway, promotes inflammation and oxidative stress.

AIMS

We studied whether Acetylcholine (ACh) is involved in the mechanism of crosstalk between mAChRM3 and β2Adrenergic receptors (β2AR) promoting, via PI3/PKC/PBEP1/Raf/MEK1/2/ERK1/2 activation, β2AR desensitization, inflammation and, oxidative stress in a bronchial epithelial cell line (16HBE) after long-term exposure to cigarette smoke extract (LECSE).

METHODS

We evaluated mAChRM3 and Choline Acetyltransferase (ChAT) expression, ACh production, PEBP1, ERk1/2, and β2AR phosphorylation, as well as NOX-4, ROS production and IL-8 release in 16HBE after LECSE. The inhibitory activity of Hemicholinium (HCh-3) (a potent choline uptake blocker), LY294002 (a highly selective inhibitor of PI3 kinase), Tiotropium (Spiriva®) (anticholinergic drug) and Olodaterol (β2AR agonist), were tested in 16HBE after LECSE.

RESULTS

mAChRM3, ChAT, ACh activity, pPEBP1, pβ2AR, pERK1/2, ROS, NOX-4 and IL-8 increased after LECSE in 16HBE LECSE compared to untreated cells. HCh-3 and LY294002 (alone or in combination) as well as Tiotropium (Spiriva®) or Olodaterol (alone or in combination) all reduced the levels of pPEBP1, pβ2AR, pERK1/2, ROS, NOX-4, and IL-8 in 16HBE LECSE compared to untreated cells.

CONCLUSIONS

LECSE promotes ACh production which enhances PI3/PKC/PEBP1/Raf-ERK1/2 pathway activation, heterologous β2AR desensitization, as well as release of inflammatory and oxidative mediators in bronchial epithelial cells. The use of anticholinergic drugs and long-acting β2-agonists, alone or in combination may be dampen these inflammatory mechanisms when used in combination in some epithelial cell types.

Multiple mechanistically distinct modes of endocannabinoid mobilization at central amygdala glutamatergic synapses

The central amygdala (CeA) is a key structure at the limbic-motor interface regulating stress responses and emotional learning. Endocannabinoid (eCB) signaling is heavily implicated in the regulation of stress-response physiology and emotional learning processes; however, the role of eCBs in the modulation of synaptic efficacy in the CeA is not well understood. Here we describe the subcellular localization of CB1 cannabinoid receptors and eCB synthetic machinery at glutamatergic synapses in the CeA and find that CeA neurons exhibit multiple mechanistically and temporally distinct modes of postsynaptic eCB mobilization. These data identify a prominent role for eCBs in the modulation of excitatory drive to CeA neurons and provide insight into the mechanisms by which eCB signaling and exogenous cannabinoids could regulate stress responses and emotional learning.

Signal transduction underlying the control of urinary bladder smooth muscle tone by muscarinic receptors and beta-adrenoceptors

The normal physiological contraction of the urinary bladder, which is required for voiding, is predominantly mediated by muscarinic receptors, primarily the M₃ subtype, with the M₂ subtype providing a secondary backup role. Bladder relaxation, which is required for urine storage, is mediated by β-adrenoceptors, in most species involving a strong β₃-component. An excessive stimulation of contraction or a reduced relaxation of the detrusor smooth muscle during the storage phase of the micturition cycle may contribute to bladder dysfunction known as the overactive bladder. Therefore, interference with the signal transduction of these receptors may be a viable approach to develop drugs for the treatment of overactive bladder. The prototypical signaling pathway of M₃ receptors is activation of phospholipase C (PLC), and this pathway is also activated in the bladder. Nevertheless, PLC apparently contributes only in a very minor way to bladder contraction. Rather, muscarinic-receptor-mediated bladder contraction involves voltage-operated Ca²⁺ channels and Rho kinase. The prototypical signaling pathway of β-adrenoceptors is an activation of adenylyl cyclase with the subsequent formation of cAMP. Nevertheless, cAMP apparently contributes in a minor way only to β-adrenoceptor-mediated bladder relaxation. BK_Ca channels may play a greater role in β-adrenoceptor-mediated bladder relaxation. We conclude that apart from muscarinic receptor antagonists and β-adrenoceptor agonists, inhibitors of Rho kinase and activators of BK_Ca channels may have potential to treat an overactive bladder.

Direct stimulation of receptor-controlled phospholipase D1 by phospho-cofilin

The activity state of cofilin, which controls actin dynamics, is driven by a phosphorylation-dephosphorylation cycle. Phosphorylation of cofilin by LIM-kinases results in its inactivation, a process supported by 14-3-3zeta and reversed by dephosphorylation by slingshot phosphatases. Here we report on a novel cellular function for the phosphorylation-dephosphorylation cycle of cofilin. We demonstrate that muscarinic receptor-mediated stimulation of phospholipase D1 (PLD1) is controlled by LIM-kinase, slingshot phosphatase as well as 14-3-3zeta, and requires phosphorylatable cofilin. Cofilin directly and specifically interacts with PLD1 and upon phosphorylation by LIM-kinase1, stimulates PLD1 activity, an effect mimicked by phosphorylation-mimic cofilin mutants. The interaction of cofilin with PLD1 is under receptor control and encompasses a PLD1-specific fragment (aa 585-712). Expression of this fragment suppresses receptor-induced cofilin-PLD1 interaction as well as PLD stimulation and actin stress fiber formation. These data indicate that till now designated inactive phospho-cofilin exhibits an active cellular function, and suggest that phospho-cofilin by its stimulatory effect on PLD1 may control a large variety of cellular functions.

Pasteurella multocida Toxin-induced Activation of RhoA Is Mediated via Two Families of Gα Proteins, Gαq and Gα12/13

Pasteurella multocida toxin (PMT) is a potent mitogen, which is known to activate phospholipase Cbeta by stimulating the alpha-subunit of the heterotrimeric G protein G(q). PMT also activates RhoA and RhoA-dependent pathways. Using YM-254890, a specific inhibitor of G(q/11), we studied whether activation of RhoA involves G proteins other than G(q/11). YM-254890 inhibited PMT or muscarinic M3-receptor-mediated stimulation of phospholipase Cbeta at similar concentrations in HEK293m3 cells. In these cells, PMT-induced RhoA activation and enhancement of RhoA-dependent luciferase activity were partially inhibited by YM-254890. In Galpha(q/11)-deficient fibroblasts, PMT induced activation of RhoA, increase in RhoA-dependent luciferase activity, and increase in ERK phosphorylation. None of these effects were influenced by YM-254890. However, RhoA activation by PMT was inhibited by RGS2, RGS16, lscRGS, and dominant negative G(13)(GA), indicating involvement of Galpha(12/13) in the PMT effect on RhoA. In Galpha(12/13) gene-deficient cells, PMT-induced stimulation of RhoA, luciferase activity, and ERK phosphorylation were blocked by YM-254890, indicating the involvement of G(q). Infection with a virus harboring the gene of Galpha(13) reconstituted the increase in RhoA-dependent luciferase activity by PMT even in the presence of YM-254890. The data show that YM-254890 is able to block PMT activation of Galpha(q) and indicate that, in addition to Galpha(q), the Galpha(12/13) G proteins are targets of PMT.

Single Cell Analysis and Temporal Profiling of Agonist-mediated Inositol 1,4,5-Trisphosphate, Ca2+, Diacylglycerol, and Protein Kinase C Signaling using Fluorescent Biosensors

The magnitude and temporal nature of intracellular signaling cascades can now be visualized directly in single cells by the use of protein domains tagged with enhanced green fluorescent protein (eGFP). In this study, signaling downstream of G protein-coupled receptor-mediated phospholipase C (PLC) activation has been investigated in a cell line coexpressing recombinant M(3) muscarinic acetylcholine and alpha(1B) -adrenergic receptors. Confocal measurements of changes in inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)), using the pleckstrin homology domain of PLCdelta1 tagged to eGFP (eGFP-PH(PLCdelta)), and 1,2-diacylglycerol (DAG), using the C1 domain of protein kinase Cgamma (PKCgamma) (eGFP-C1(2)-PKCgamma), demonstrated clear translocation responses to methacholine and noradrenaline. Single cell EC(50) values calculated for each agonist indicated that responses to downstream signaling targets (Ca(2+) mobilization and PKC activation) were approximately 10-fold lower compared with respective Ins(1,4,5)P(3) and DAG EC(50) values. Examining the temporal profile of second messenger responses to sub-EC(50) concentrations of noradrenaline revealed oscillatory Ins(1,4,5)P(3), DAG, and Ca(2+) responses. Oscillatory recruitments of conventional (PKCbetaII) and novel (PKCepsilon) PKC isoenzymes were also observed which were synchronous with the Ca(2+) response measured simultaneously in the same cell. However, oscillatory PKC activity (as determined by translocation of eGFP-tagged myristoylated alanine-rich C kinase substrate protein) required oscillatory DAG production. We suggest a model that uses regenerative Ca(2+) release via Ins(1,4,5)P(3) receptors to initiate oscillatory second messenger production through a positive feedback effect on PLC. By acting on various components of the PLC signaling pathway the frequency-encoded Ca(2+) response is able to maintain signal specificity at a level downstream of PKC activation.

Sustained diacylglycerol accumulation resulting from prolonged G protein-coupled receptor agonist-induced phosphoinositide breakdown in hepatocytes

Studies in various cells have led to the idea that agonist-stimulated diacylglycerol (DAG) generation results from an early, transient phospholipase C (PLC)-catalyzed phosphoinositide breakdown, while a more sustained elevation of DAG originates from phosphatidylcholine (PC). We have examined this issue further, using cultured rat hepatocytes, and report here that various G protein-coupled receptor (GPCR) agonists, including vasopressin (VP), angiotensin II (Ang.II), prostaglandin F2alpha, and norepinephrine (NE), may give rise to a prolonged phosphoinositide hydrolysis. Preincubation of hepatocytes with 1-butanol to prevent conversion of phosphatidic acid (PA) did not affect the agonist-induced DAG accumulation, suggesting that phospholipase D-mediated breakdown of PC was not involved. In contrast, the GPCR agonists induced phosphoinositide turnover, assessed by accumulation of inositol phosphates, that was sustained for up to 18 h, even under conditions where PLC was partially desensitized. Pretreatment of hepatocytes with wortmannin, to inhibit synthesis of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PIP2), prevented agonist-induced inositol phosphate and DAG accumulation. Upon VP stimulation the level of PIP) declined, but only transiently, while increases in inositol 1,4,5-trisphosphate (InsP3) and DAG mass were sustained, suggesting that efficient resynthesis of PIP2 allowed sustained PLC activity. This was confirmed when cells were pretreated with wortmannin to prevent resynthesis of PIP2. Furthermore, metabolism of InsP3 was rapid, compared to that of DAG, with a more than 20-fold difference in half-life. Thus, rapid metabolism of InsP3 and efficient resynthesis of PIP2 may account for the larger amount of DAG generated and the more sustained time course, compared to InsP3. The results suggest that DAG accumulation that is sustained for many hours in response to VP, Ang.II, NE, and prostaglandin F2alpha in hepatocytes is mainly due to phosphoinositide breakdown.

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.

Regulation of muscarinic acetylcholine receptor signaling

Multiple mechanisms regulate the signaling of the five members of the family of the guanine nucleotide binding protein (G protein)-coupled muscarinic acetylcholine (ACh) receptors (mAChRs). Following activation by classical or allosteric agonists, mAChRs can be phosphorylated by a variety of receptor kinases and second messenger-regulated kinases. The phosphorylated mAChR subtypes can interact with beta-arrestin and presumably other adaptor proteins as well. As a result, the various mAChR signaling pathways may be differentially altered, leading to short-term or long-term desensitization of a particular signaling pathway, receptor-mediated activation of the mitogen-activated protein kinase pathway downstream of mAChR phosphorylation, as well as long-term potentiation of mAChR-mediated phospholipase C stimulation. Agonist activation of mAChRs may also induce receptor internalization and down-regulation, which proceed in a highly regulated manner, depending on receptor subtype and cell type. In this review, our current understanding of the complex regulatory processes that underlie signaling of mAChR is summarized.

Stimulation of phospholipase C-epsilon by the M3 muscarinic acetylcholine receptor mediated by cyclic AMP and the GTPase Rap2B

Stimulation of phospholipase C (PLC) by G(q)-coupled receptors such as the M(3) muscarinic acetylcholine receptor (mAChR) is caused by direct activation of PLC-beta enzymes by Galpha(q) proteins. We have recently shown that G(s)-coupled receptors can stimulate PLC-epsilon, apparently via formation of cyclic AMP and activation of the Ras-related GTPase Rap2B. Here we report that PLC stimulation by the M(3) mAChR expressed in HEK-293 cells also involves, in part, similar mechanisms. M(3) mAChR-mediated PLC stimulation and [Ca(2+)](i) increase were reduced by 2',5'-dideoxyadenosine (dd-Ado), a direct adenylyl cyclase inhibitor. On the other hand, overexpression of Galpha(s) or Epac1, a cyclic AMP-regulated guanine nucleotide exchange factor for Rap GTPases, enhanced M(3) mAChR-mediated PLC stimulation. Inactivation of Ras-related GTPases with clostridial toxins suppressed the M(3) mAChR responses. The inhibitory toxin effects were mimicked by expression of inactive Rap2B, but not of other inactive GTPases (Rac1, Ras, RalA, Rap1A, and Rap2A). Activation of the M(3) mAChR induced GTP loading of Rap2B, an effect strongly enhanced by overexpression of Galpha(s) and inhibited by dd-Ado. Overexpression of PLC-epsilon and PLC-beta1, but not PLC-gamma1 or PLC-delta1, enhanced M(3) mAChR-mediated PLC stimulation and [Ca(2+)](i) increase. In contrast, expression of a catalytically inactive PLC-epsilon mutant reduced PLC stimulation by the M(3) mAChR and abrogated the potentiating effect of Galpha(s). In conclusion, our findings suggest that PLC stimulation by the M(3) mAChR is a composite action of PLC-beta1 stimulation by Galpha(q) and stimulation of PLC-epsilon apparently mediated by G(s)-dependent cyclic AMP formation and subsequent activation of Rap2B.

Generation of choline for acetylcholine synthesis by phospholipase D isoforms

BACKGROUND

In cholinergic neurons, the hydrolysis of phosphatidylcholine (PC) by a phospholipase D (PLD)-type enzyme generates some of the precursor choline used for the synthesis of the neurotransmitter acetylcholine (ACh). We sought to determine the molecular identity of the relevant PLD using murine basal forebrain cholinergic SN56 cells in which the expression and activity of the two PLD isoforms, PLD1 and PLD2, were experimentally modified. ACh levels were examined in cells incubated in a choline-free medium, to ensure that their ACh was synthesized entirely from intracellular choline.

RESULTS

PLD2, but not PLD1, mRNA and protein were detected in these cells and endogenous PLD activity and ACh synthesis were stimulated by phorbol 12-myristate 13-acetate (PMA). Introduction of a PLD2 antisense oligonucleotide into the cells reduced PLD2 mRNA and protein expression by approximately 30%. The PLD2 antisense oligomer similarly reduced basal- and PMA-stimulated PLD activity and ACh levels. Overexpression of mouse PLD2 by transient transfection increased basal- (by 74%) and PMA-stimulated (by 3.2-fold) PLD activity. Moreover, PLD2 transfection increased ACh levels by 26% in the absence of PMA and by 2.1-fold in the presence of PMA. Overexpression of human PLD1 by transient transfection increased PLD activity by 4.6-fold and ACh synthesis by 2.3-fold in the presence of PMA as compared to controls.

CONCLUSIONS

These data identify PLD2 as the endogenous enzyme that hydrolyzes PC to generate choline for ACh synthesis in cholinergic cells, and indicate that in a model system choline generated by PLD1 may also be used for this purpose.

1,25-Dihydroxyvitamin D(3), phospholipase D and protein kinase C in keratinocyte differentiation

1,25-Dihydroxyvitamin D(3), thought to be a physiological regulator of epidermal keratinocyte growth and differentiation, also elicits the complete differentiative program in vitro, with expression of various genes/proteins characteristic of both early and late differentiation. 1,25-Dihydroxyvitamin D(3) functions by interacting with an intracellular receptor that binds to DNA at vitamin D response elements (VDRE) thereby affecting transcription. 1,25-Dihydroxyvitamin D(3) has been demonstrated to alter the expression of several enzymes involved in signal transduction, and presumably this is the mechanism through which the hormone regulates differentiation. It has recently been shown that 1,25-dihydroxyvitamin D(3) specifically increases the expression/activity of phospholipase D-1, an enzyme that hydrolyzes phospholipids to generate lipid messengers, such as diacylglycerol (DAG). DAG, in turn, is known to activate several members of the protein kinase C (PKC) family. It has been proposed that this signaling pathway mediates late differentiation events in epidermal keratinocytes. In this article the data supporting a role for PKC and phospholipase D in keratinocyte differentiation, as well as in the pathogenesis of skin diseases, are reviewed and a model is proposed for the signaling pathways that regulate this process upon exposure to 1,25-dihydroxyvitamin D(3).

G protein-coupled receptor-induced sensitization of phospholipase C stimulation by receptor tyrosine kinases

Activation of stably expressed M(2) and M(3) muscarinic acetylcholine receptors (mAChRs) as well as of endogenously expressed lysophosphatidic acid and purinergic receptors in HEK-293 cells can induce a long lasting potentiation of phospholipase C (PLC) stimulation by these and other G protein-coupled receptors (GPCRs). Here, we report that GPCRs can induce an up-regulation of PLC stimulation by receptor tyrosine kinases (RTKs) as well and provide essential mechanistic characteristics of this sensitization process. Pretreatment of HEK-293 cells for 2 min with carbachol, a mAChR agonist, lysophosphatidic acid, or ATP, followed by agonist washout, strongly increased (by 2-3-fold) maximal PLC stimulation (measured >/=40 min later) by epidermal growth factor and platelet-derived growth factor, but not insulin, and largely enhanced PLC sensitivity to these RTK agonists. The up-regulation of RTK-induced PLC stimulation was cycloheximide-insensitive and was observed for up to approximately 90 min after removal of the GPCR agonist. Sensitization of receptor-induced PLC stimulation caused by prior M(2) mAChR activation was fully prevented by pertussis toxin and strongly reduced by expression of Gbetagamma scavengers. Furthermore, inhibition of conventional protein kinase C (PKC) isoenzymes and chelation of intracellular Ca(2+) suppressed the sensitization process, while overexpression of PKC-alpha, but not PKC-betaI, further enhanced the M(2) mAChR-induced sensitization of PLC stimulation. None of these treatments affected acute PLC stimulation by either GPCR or RTK agonists. Taken together, short term activation of GPCRs can induce a strong and long lasting sensitization of PLC stimulation by RTKs, a process apparently involving G(i)-derived Gbetagammas as well as increases in intracellular Ca(2+) and activation of a PKC isoenzyme, most likely PKC-alpha.

Phospholipase C, protein kinase C, Ca(2+)/calmodulin-dependent protein kinase II, and tyrosine phosphorylation are involved in carbachol-induced phospholipase D activation in Chinese hamster ovary cells expressing muscarinic acetylcholine receptor of Caenorhabditis elegans

Recently, we have isolated a cDNA encoding a muscarinic acetylcholine receptor (mAChR) from Caenorhabditis elegans. To investigate the regulation of phospholipase D (PLD) signaling via a muscarinic receptor, we generated stable transfected Chinese hamster ovary (CHO) cells that overexpress the mAChR of C. elegans (CHO-GAR-3). Carbachol (CCh) induced inositol phosphate formation and a significantly higher Ca(2+) elevation and stimulated PLD activity through the mAChR; this was insensitive to pertussis toxin, but its activity was abolished by the phospholipase C (PLC) inhibitor U73122. Western blot analysis revealed several apparent tyrosine-phosphorylated protein bands after CCh treatment. The CCh-induced PLD activation and tyrosine phosphorylation were significantly reduced by the protein kinase C (PKC) inhibitor calphostin C and down-regulation of PKC and the tyrosine kinase inhibitor genistein. Moreover, the Ca(2+)-calmodulin-dependent protein kinase II (CaM kinase II) inhibitor KN62, in addition to chelation of extracellular or intracellular Ca(2+) by EGTA and BAPTA/AM, abolished CCh-induced PLD activation and protein tyrosine phosphorylation. Taken together, these results suggest that the PLC/PKC-PLD pathway and the CaM kinase II/tyrosine kinase-PLD pathway are involved in the activation of PLD through mAChRs of C. elegans.

Regulation of peripheral cannabinoid receptor CB2 phosphorylation by the inverse agonist SR 144528. Implications for receptor biological responses

We recently demonstrated that the selective cannabinoid receptor antagonist SR 144528 acts as an inverse agonist that blocks constitutive mitogen-activated protein kinase activity coupled to the spontaneous autoactivated peripheral cannabinoid receptor (CB2) in the Chinese hamster ovary cell line stably transfected with human CB2. In the present report, we studied the effect of SR 144528 on CB2 phosphorylation. The CB2 phosphorylation status was monitored by immunodetection using an antibody specific to the COOH-terminal CB2 which can discriminate between phosphorylated and non-phosphorylated CB2 isoforms at serine 352. We first showed that CB2 is constitutively active, phosphorylated, and internalized at the basal level. By blocking autoactivated receptors, inverse agonist SR 144528 treatment completely inhibited this phosphorylation state, leading to an up-regulated CB2 receptor level at the cell surface, and enhanced cannabinoid agonist sensitivity for mitogen-activated protein kinase activation of Chinese hamster ovary-CB2 cells. After acute agonist treatment, serine 352 was extensively phosphorylated and maintained in this phosphorylated state for more than 8 h after agonist treatment. The cellular responses to CP-55,940 were concomitantly abolished. Surprisingly, CP-55,940-induced CB2 phosphorylation was reversed by SR 144528, paradoxically leading to a non-phosphorylated CB2 which could then be fully activated by CP-55,940. The process of CP-55,940-induced receptor phosphorylation followed by SR 144528-induced receptor dephosphorylation kept recurring many times on the same cells, indicating that the agonist switches the system off but the inverse agonist switches the system back on. Finally, we showed that autophosphorylation and CP-55, 940-induced serine 352 CB2 phosphorylation involve an acidotropic GRK kinase, which does not use Gibetagamma. In contrast, SR 144528-induced CB2 dephosphorylation was found to involve an okadaic acid and calyculin A-sensitive type 2A phosphatase.

Activation of muscarinic receptors in porcine airway smooth muscle elicits a transient increase in phospholipase D activity

Phospholipase D (PLD) is a phosphodiesterase that catalyses hydrolysis of phosphatidylcholine to produce phosphatidic acid and choline. In the presence of ethanol, PLD also catalyses the formation of phosphatidylethanol, which is a unique characteristic of this enzyme. Muscarinic receptor-induced changes in the activity of PLD were investigated in porcine tracheal smooth muscle by measuring the formation of [3H]phosphatidic acid ([3H]PA) and [3H]phosphatidylethanol ([3H]PEth) after labeling the muscle strips with [3H]palmitic acid. The cholinergic receptor agonist acetylcholine (Ach) significantly but transiently increased formation of both [3H]PA and [3H]PEth in a concentration-dependent manner (>105-400% vs. controls in the presence of 10(-6) to 10(-4) M Ach) when pretreated with 100 mM ethanol. The Ach receptor-mediated increase in PLD activity was inhibited by atropine (10(-6) M), indicating that activation of PLD occurred via muscarinic receptors. Activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate (PMA) increased PLD activity that was effectively blocked by the PKC inhibitors calphostin C (10(-8) to 10(-6) M) and GFX (10(-8) to 10(-6) M). Ach-induced increases in PLD activity were also significantly, but incompletely, inhibited by both GFX and calphostin C. From the present data, we conclude that in tracheal smooth muscle, muscarinic acetylcholine receptor-induced PLD activation is transient in nature and coupled to these receptors via PKC. However, PKC activation is not solely responsible for Ach-induced activation of PLD in porcine tracheal smooth muscle.

Involvement of protein kinase C and protein kinase A in the muscarinic receptor signalling pathways mediating phospholipase C activation, arachidonic acid release and calcium mobilisation

The involvement of protein kinase C (PKC) and protein kinase A (PKA) in cholinergic signalling in CHO cells expressing the M3 subtype of the muscarinic acetylcholine receptor was examined. Muscarinic signalling was assessed by measuring carbachol-induced activation of phospholipase C (PLC), arachidonic acid release, and calcium mobilisation. Carbachol activation of PLC was not altered by inhibition of PKC with chelerythrine chloride, bisindolylmaleimide or chronic treatment with phorbol myristate acetate (PMA). Activation of PKC by acute treatment with PMA was similarly without effect. In contrast, inhibition of PKC blocked carbachol stimulation of arachidonic acid release. Likewise, PKC inhibition resulted in a decreased ability of carbachol to mobilise calcium, whereas PKC activation potentiated calcium mobilisation. Inhibition of PKA with H89 or Rp-cAMP did not alter the ability of carbachol to activate PLC. Similarly, PKA activation with Sp-cAMP or forskolin had no effect on PLC stimulation by carbachol. Carbachol-mediated release of arachidonic acid was decreased by H89 but only slightly increased by forskolin. Forskolin also increased calcium mobilisation by carbachol. These results suggest a function for PKC and PKA in M3 stimulation of arachidonic acid release and calcium mobilisation but not in PLC activation.

Muscarinic receptor activation of arachidonate-mediated Ca2+ entry in HEK293 cells is independent of phospholipase C

Receptor-enhanced entry of Ca2+ in non-excitable cells is generally ascribed to a capacitative mechanism in which the activation of the entry pathway is specifically dependent on the emptying of agonist-sensitive intracellular Ca2+ stores. Although such entry can be clearly demonstrated under conditions of maximal or near-maximal stimulation, it is uncertain whether such a mechanism can operate during the oscillatory [Ca2+]i signals that are frequently seen following stimulation with low concentrations of agonists. In this study, we report that the stimulation of human m3 muscarinic receptors stably transfected into HEK293 cells results in the appearance of a novel arachidonate-mediated Ca2+ entry pathway. We show that the generation of arachidonic acid and the activation of this pathway are specifically associated with stimulation at the low agonist concentrations that typically give rise to oscillatory [Ca2+]i signals. At such agonist concentrations, however, the generation of arachidonic acid is independent of the simultaneous activation of the phospholipase C-inositol 1,4,5-trisphosphate pathway. We further show that the arachidonate-mediated Ca2+ entry demonstrates characteristics that distinguish it from the corresponding capacitative pathway in the same cells and therefore is likely to represent an entirely distinct pathway that is specifically responsible for the receptor-enhanced entry of Ca2+ during [Ca2+]i oscillations.

Cellular mechanisms underlying two muscarinic receptor-mediated depolarizing responses in relay cells of the rat lateral geniculate nucleus

We used the whole-cell recording technique in an in vitro preparation to examine the electrophysiological actions of the muscarinic receptors on relay cells in the rat lateral geniculate nucleus. Drop application of the muscarinic agonist acetyl-beta-methylcholine resulted in a slow depolarization that persisted for several minutes. The response was insensitive to the nicotinic antagonist hexamethonium, but was blocked by atropine, a muscarinic antagonist. The response was also insensitive to blockade of synaptic transmission by tetrodotoxin, indicating a direct muscarinic effect. The muscarinic depolarization consisted of two components that were somewhat separated in time. The early portion of the muscarinic response was mediated by a large inward current with little change in input resistance, while the later portion was mediated by a small inward current associated with a large increase in input resistance. Pharmacological agents were used to distinguish the two components. Drop application of McN-A-343, an ml receptor agonist, could only mimic the later component of the muscarinic response. This was supported by the result that the later component was blocked by low concentrations of pirenzepine. These data suggest that the ml receptor only mediates the late component of the muscarinic response, while the early component is mainly mediated by the m3 receptor. The idea that both ml and m3 receptors were involved in the muscarinic depolarization was further supported by voltage-clamp analysis. This revealed that activation of the ml receptor was associated with a decrease in an inward potassium current, IKleak, while activation of the m3 receptor was likely associated with both a decrease in IKleak and an increase in the hyperpolarization-activated cation current Ih. In summary, our data suggest that muscarinic responses in geniculate relay cells result from the activation of two receptors, which modulate IKleak and Ih. Given the fact that the ascending aminergic systems also depolarize geniculate relay cells via two receptors acting on IKleak and Ih, we concluded that ascending activating systems use common mechanisms to enact the depolarizing form of arousal in relay neurons.

Neuromedin B activates phospholipase D through both PKC-dependent and PKC-independent mechanisms

The actions of neuromedin B (NMB), a recently discovered mammalian bombesin-related peptide, are mediated by interacting with a distinct receptor; however, little is known about its cellular basis of action. Recent studies show activation of phospholipase D (PLD) is an important transduction cascade for a number of GI hormones, especially for stimulation of growth and protein sorting. The purpose of the present study was to determine whether activation of the NMB receptor causes activation of PLD and to explore whether this activation was coupled to PLC activation. Rat C6 glioblastoma cells (C6 cells), which contain a low density of native NMB receptors and BALB 3T3 cells stably transfected with rat NMB receptors, were used. NMB caused a 3-fold increase in C6 cells and an 11-fold increase in rNMB-R transfected cells in PLD activity. Increases in PLD activity were rapid and NMB was 100-fold more potent than gastrin-releasing peptide (GRP). NMB caused a half-maximal increase in [Ca2+]i at 0.2 nM, in [3H]IP and PLD at 1 nM, and half-maximal receptor occupation at 1.2 nM. TPA increased PLD dose-dependently with a half-maximal effect at 60 nM. The calcium ionophore A23187 (1 microM) alone did not increase PLD activity but potentiated the effect of TPA. The Ca2+-ATPase inhibitor, thapsigargin, did not affect NMB- or TPA-stimulated PLD activities, although it blocked completely the NMB-induced increase in [Ca2+]i. The PKC inhibitor GF109203X completely abolished TPA-induced PLD activity, however, it only inhibited NMB-induced PLD activity by 20%. The combination of thapsigargin and GF109203X had the same effect as GF109203X alone. These data indicate that NMB receptor activation is coupled to both PLC and PLD. In contrast to a number of other phospholipase C-coupled receptors, NMB receptor stimulated changes in [Ca2+]i do not contribute to PLD activation. Both PKC-dependent and PKC-independent mechanisms are involved in the NMB-stimulated PLD activation with the PKC-independent pathway predominating.

Adaptations to chronic agonist exposure of mu-opioid receptor-expressing Chinese hamster ovary cells

To investigate cellular adaptation responses induced by chronic agonist treatment of the mu-opioid receptor, Chinese hamster ovary (CHO) cells were stably transfected with the rat mu-opioid receptor cDNA. Chronic treatment with agonists selective for the mu-opioid receptor, [D-Ala2, N-MePhe4, Gy-ol5]enkephalin (DAMGO), morphine and fentanyl, time- and dose-dependently induced down-regulation of the mu-opioid receptor. The down-regulation was not significantly affected by pretreatment with pertussis toxin, but was completely blocked by treatment with hypertonic sucrose, suggesting that receptor internalization mediated by clathrin-coated vesicles is an essential step in the mu-opioid receptor down-regulation. On the other hand, forskolin-stimulated cyclic AMP formation was increased by chronic DAMGO treatment, which was inhibited by pertussis toxin pretreatment. These results indicate that two adaptation responses induced by chronic agonist treatment of the mu-opioid receptor-expressing CHO cells, down-regulation of the mu-opioid receptor and supersensitization of adenylate cyclase, are mediated by distinct mechanisms.

Prolonged activation of phospholipase D in Chinese hamster ovary cells expressing platelet-activating-factor receptor lacking cytoplasmic C-terminal tail

The mechanism and role of phospholipase D (PLD) activation by platelet-activating factor (PAF) were examined with Chinese hamster ovary cells stably expressing wild-type PAF receptor (WT-H cells) and truncated PAF receptor lacking the C-terminal cytoplasmic tail (D-H cells). Treatment of D-H cells with PAF resulted in the rapid formation of Ins(1,4,5)P3, which was followed by a sustained phase for more than 10 min. In these cells, PAF-induced PLD activation lasted for more than 20 min. In contrast, PLD activation in WT-H cells was transient. PAF stimulation caused the biphasic formation of 1,2-diacylglycerol (DG) in both types of cell. The first phase was rapid and transient, coinciding with the Ins(1,4,5)P3 peak. The second sustained phase of DG formation was attenuated by butanol, which produces phosphatidylbutanol at the expense of phosphatidic acid (PA) by transphosphatidylation activity of PLD, and by propranolol, a selective inhibitor for PA phosphohydrolase catalysing the conversion of PA into DG. The DG level returned nearly to basal at 20 min after PAF stimulation in WT-H cells, whereas in D-H cells the elevated DG level was sustained for more than 20 min. The profile of translocation of protein kinase Calpha (PKCalpha) to membrane was similar to that of DG formation. In WT-H cells, PKCalpha was transiently associated with membranes and then returned to the cytosol. However, in D-H cells PKCalpha was rapidly translocated to and remained in membranes for more than 20 min. Butanol suppressed this sustained translocation of PKCalpha. Furthermore the mRNA levels of c-fos and c-jun by PAF in WT-H cells were much lower than those in D-H cells. Propranolol and butanol at concentrations that inhibited the formation of DG suppressed the PAF-induced mRNA expression of c-fos and c-jun. Taken together, the prolonged PLD activation in D-H cells confirmed a primary role for phospholipase C/PKC in PLD activation by PAF. Furthermore the results obtained here suggest that sustained PLD activation in turn leads to chronic activation and membrane translocation of PKCalpha, which might play an important role in the expression of c-fos and c-jun.

Differential effects of the M1-M5 muscarinic acetylcholine receptor subtypes on intracellular calcium and on the incorporation of choline into membrane lipids in genetically modified Chinese hamster ovary cell lines

We compared responses of Chinese hamster ovary (CHO) cell lines stably transfected with human genes for the M1-M5 muscarinic receptor subtypes to several stimuli. While ATP brought about similar increases in the concentration of intracellular Ca2+ ions ([Ca2+]i) in the cell lines expressing all individual receptor subtypes, carbachol acted with much higher potency and efficacy on the cells expressing the M1, M3, and M5 receptor subtypes than on those expressing the M2 and M4 subtypes. The maximum [Ca2+]i responses to ATP corresponded to 41-75% of the maximum responses to carbachol in the cells expressing the M1, M3, and M5 receptor subtypes. The responses to ATP were strongly suppressed (> 75% decrease) by a preliminary administration of a maximally active concentration of carbachol in these three cell lines, whereas the responses to carbachol were less sensitive to the preliminary administration of a maximally active concentration of ATP (< 25% decrease). It appears likely that carbachol and ATP release Ca2+ ions from identical intracellular stores. Tetradecanoylphorbol acetate (TPA) strongly inhibited the responses of [Ca2+]i to both carbachol and ATP and enhanced the incorporation of [14C] choline into lipids in all five CHO cell lines investigated. On the other hand, the incorporation of [14C] choline into lipids was diminished by carbachol in the cell line expressing the M3 receptor subtype and unchanged in the other cell lines. This effect of carbachol was not dependent on the presence of extracellular Ca2+ ions and was not affected by TPA, which diminished the response of [Ca2+]i to muscarinic stimulation. It is suggested that it was due to muscarinic receptor-mediated activation of phospholipase D.

Regulation of phospholipase C and D activities by small molecular weight G proteins and muscarinic receptors

The role of small molecular weight guanine nucleotide-binding proteins (G proteins) of the Rho family in muscarinic acetylcholine receptor (mAChR) signaling to phospholipase C (PLC) and phospholipase D (PLD) was studied in human embryonic kidney (HEK) cells, stably expressing the human m3 receptor subtype. Evidence for the involvement of Rho proteins in m3 mAChR signaling to both phospholipases is based on findings obtained with Clostridium (C.) difficile toxin B and C. botulinum C3 exoenzyme, both of which specifically, although by different mechanisms, inactivate Rho family G proteins. Toxin B potently inhibited both the mAChR-stimulated PLC and PLD activities in intact cells as well as the stimulation of both phospholipases by the stable GTP analog GTPgammaS in permeabilized cells, the latter effect being mimicked by C3 exoenzyme. In contrast, PLC and PLD activities, measured in the presence of exogenous phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], a substrate and cofactor for PLC and PLD, respectively, were not altered. These data suggested that the Rho-inactivating toxins inhibit stimulation of PLC and PLD by reducing the cellular level of PtdIns(4,5)P2, which was indeed found with both toxin B and C3 exoenzyme. In agreement with a crucial role of cellular PtdIns(4,5)P2 supply for PLC signaling, we observed that short-term agonist (carbachol) treatment of HEK cells caused a long-lasting increase in PtdIns(4,5)P2 level, accompanied by a potentiation of receptor- and G protein-stimulated inositol phosphate formation. Finally, studies with tyrosine kinase and tyrosine phosphatase inhibitors strongly suggest that PtdIns(4,5)P2 synthesis and mAChR-stimulated PLD activity in HEK cells apparently also involve a tyrosine phosphorylation-dependent mechanism(s). Thus, m3 mAChR signaling to PLC and PLD in HEK cells requires the concerted action of various intracellular components, most notably the complex regulation of PtdIns(4,5)P2 synthesis.

Restoration of Clostridium difficile toxin-B-inhibited phospholipase D by phosphatidylinositol 4,5-bisphosphate

Receptor signalling to phospholipase D (PLD) in human embryonic kidney (HEK) cells stably expressing the m3 muscarinic acetylcholine receptor apparently involves Rho proteins. Since phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] has been recognized as an essential cofactor for PLD activity and since activated Rho proteins have been reported to stimulate the synthesis of PtdIns(4,5)P2, we studied whether in HEK cells PLD activity is regulated by PtdIns(4,5)P2 and, in particular, whether PtdIns(4,5)P2 can restore PLD activity inhibited by Clostridium difficile toxin B, which inactivates Rho proteins. Addition of MgATP to permeabilized HEK cells increased basal PLD activity and potentiated PLD stimulation by the stable GTP analogue, guanosine 5'-[gamma-thio]triphosphate (GTP[S]), concomitant with a large increase in PtdIns(4,5)P2. On the other hand, neomycin, which binds to PtdIns(4,5)P2, inhibited basal and GTP[S]-stimulated PLD activities. Addition of PtdIns(4,5)P2 increased PLD activity in HEK cell membranes by 2-3-fold, whereas various other phospholipids were ineffective. Prior treatment of HEK cells with toxin B reduced the level of PtdIns(4,5)P2, measured either in intact cells or in membrane preparations, by about 40%. In membranes of toxin-B-treated cells, basal and GTP[S]-stimulated PLD activities were reduced, when measured with exogenous phosphatidylcholine as enzyme substrate. Inclusion of PtdIns(4,5)P2 with phosphatidylcholine in the substrate vesicles or addition of PtdIns(4,5)P2 fully restored basal and GTP[S]-stimulated PLD activities in membranes of toxin-B-treated cells. In conclusion, the data indicate that PtdIns(4,5)P2 is an essential cofactor for PLD activity in HEK cells and that inhibition of PLD activity by the Rho-inactivating toxin B is apparently caused by depletion of the PLD cofactor, PtdIns(4,5)P2.

A role for Rho in receptor- and G protein-stimulated phospholipase C. Reduction in phosphatidylinositol 4,5-bisphosphate by Clostridium difficile toxin B

Receptors coupled to heterotrimeric guanine nucleotide-binding proteins (G proteins) activate phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2)-hydrolyzing phospholipase C (PLC) enzymes by activated alpha of free beta gamma subunits of the relevant G proteins. To study whether low molecular weight G proteins of the Rho family are involved in receptor signaling to PLC, we examined the effect of Clostridium difficile toxin B, which glucosylates and thereby inactivates Rho proteins, on the regulation of PLC activity in human embryonic kidney (HEK) cells stably expressing the m3 muscarinic acetylcholine receptor (mAChR) subtype. Toxin B treatment of HEK cells did not affect basal PLC activity, but potently and efficiently inhibited mAChR-stimulated inositol phosphate formation. PLC activation by the endogenously expressed thrombin receptor and by the direct G protein activators, A1F-4 and guanosine 5'-[gamma-thio]triphosphate (GTP gamma S), studied in intact and permeabilized cells, respectively, were also inhibited by toxin B treatment. C3 exoenzyme, which ADP-ribosylates Rho proteins, mimicked the inhibitory effect of toxin B on GTP gamma S-stimulated PLC activity. Finally both toxin B and C3 exoenzyme significantly reduced, by 40 to 50%, the total level of PtdIns(4,5)P2 in HEK cells, without affecting the levels of phosphatidylinositol and phosphatidylinositol 4-phosphate. Accordingly, When PLC activity was measured with exogenous PtdIns(4,5)P2 as enzyme substrate, Ca(2+)- as well as GTP gamma S- or A1F-4-stimulated PLC activities were not altered by prior toxin B treatment. In conclusion, evidence is provided that toxin B and C3 exoenzyme, apparently by inactivating Rho proteins, inhibit G protein-coupled receptor signalling to PLC, most likely by reducing the cellular substrate supply.

Inhibition of receptor signaling to phospholipase D by Clostridium difficile toxin B. Role of Rho proteins

Rho proteins have been reported to activate phospholipase D (PLD) in in vitro preparations. To examine the role of Rho proteins in receptor signaling to PLD, we studied the effect of Clostridium difficile toxin B, which glucosylates Rho proteins, on the regulation of PLD activity in human embryonic kidney (HEK) cells stably expressing the m3 muscarinic acetylcholine receptor (mAChR). Toxin B treatment of HEK cells potently and efficiently blocked mAChR-stimulated PLD. In contrast, basal and phorbol ester-stimulated PLD activities were not or only slightly reduced. Cytochalasin B and Clostridium botulinum C2 toxin, mimicking the effect of toxin B on the actin cytoskeleton but without involving Rho proteins, had no effect on mAChR-stimulated PLD. Toxin B did not alter cell surface mAChR number and mAChR-stimulated binding of (guanosine 5'-O-(thio)triphosphate (GTP gamma S) to G proteins. In addition to mAChR-stimulated PLD, toxin B treatment also inhibited PLD activation by the direct G protein activators, AlF4- and GTP gamma S, studied in intact and permeabilized cells, respectively. Finally, C. botulinum C3 exoenzyme, which ADP-ribosylates Rho proteins, mimicked the inhibitory effect of toxin B on GTP gamma S-stimulated PLD activity. In conclusion, the data presented indicate that toxin B potently and selectively interferes with receptor coupling mechanisms to PLD, and furthermore suggest an essential role for Rho proteins in receptor signaling to PLD.