Activation of phospholipase C in SH-SY5Y neuroblastoma cells by potassium-induced calcium entry

Measurement of inositol(1,4,5)trisphosphate using a stereospecific radioreceptor mass assay

Inositol(1,4,5)trisphosphate [Ins(1,4,5)P(3)] is an important second messenger that activates its cognate Ins(1,4,5)P(3) receptor to release Ca(2+) from intracellular stores. The assay described in this chapter uses the Ins(1,4,5)P(3) receptor (essentially as a binding protein) to measure the biologically active trisphosphate (specifically from other trisphosphates). The binding protein (Ins(1,4,5)P3 receptor) is prepared from Bovine adrenal glands and this is mixed with [(3)H]-labeled and unlabeled (generated from biological samples or standards) Ins(1,4,5)P(3). Using the same principles as for radioimmunoassay/ELISA the mass of Ins(1,4,5)P(3) in biological samples can be estimated.

Structural basis for calcium and phosphatidylserine regulation of phospholipase C δ1

Many membrane-associated enzymes, including those of the phospholipase C (PLC) superfamily, are regulated by specific interactions with lipids. Previously, we have shown that the C2 domain of PLC δ1 is required for phosphatidylserine (PS)-dependent enzyme activation and that activation requires the presence of Ca(2+). To identify the site of interaction and the role of Ca(2+) in the activation mechanism, we mutagenized three highly conserved Ca(2+) binding residues (Asp-653, Asp-706, and Asp-708) to Gly in the C2 domain of PLC δ1. The PS-dependent Ca(2+) binding affinities of the mutant enzymes D653G, D706G, and D708G were reduced by 1 order of magnitude, and the maximal level of Ca(2+) binding was reduced to half of that of the native enzyme. The level of Ca(2+)-dependent PS binding was also reduced in the mutant enzymes. Under basal conditions, the Ca(2+) dependence and the maximal level of hydrolysis of phosphatidylinositol 4,5-bisphosphate were not altered in the mutants. However, the Ca(2+)-dependent PS stimulation was severely defective. PS reduces the K(m) of the native enzyme almost 20-fold, but far less for the mutants. Replacing Asp-653, Asp-706, and Asp-708 simultaneously with glycine in the C2 domain of PLC δ1 leads to a complete and selective loss of the stimulation and binding by PS. These results show that D653, D706, and D708 are required for Ca(2+) binding in the C2 domain and demonstrate a mechanism by which C2 domains can mediate regulation of enzyme activity by specific lipid ligands.

Mu-opioid receptor-mediated phosphorylation of IkappaB kinase in human neuroblastoma SH-SY5Y cells

Opioid receptors are involved in regulating neuronal survival. Here we demonstrate that activation of the mu-opioid receptor in human neuroblastoma SH-SY5Y cells led to the phosphorylations of IkappaB kinase (IKK) and p65, denoting the stimulation of the nuclear factor-kappaB (NFkappaB) transcription factor. This response was mediated through pertussis toxin-sensitive G proteins. The mu-opioid-induced IKK phosphorylation required extracellular signal-regulated protein kinase, phosphatidylinositol 3-kinase and c-Src. Moreover, c-Jun N-terminal kinase and calmodulin-dependent kinase II also participated in the IKK activation, despite the lack of involvement of phospholipase Cbeta and protein kinase C. These data suggest that the mu-opioid receptor is capable of simulating NFkappaB signaling via the phosphorylation of IKK and p65 in human neuroblastoma SH-SY5Y cells.

Cortically restricted production of IP3 leads to propagation of the fertilization Ca2+ wave along the cell surface in a model of the Xenopus egg

The fertilization Ca2+ wave in Xenopus laevis is a single, large wave of elevated free cytosolic Ca2+ concentration that emanates from the point of sperm-egg fusion and traverses the entire diameter of the egg. This phenomenon appears to involve an increase in inositol-1,4,5-trisphosphate (IP3) resulting from interaction of the sperm and egg, which then results in the activation of the endoplasmic reticulum Ca2+ release machinery. We have proposed models based on a static elevated distribution of IP3, and dynamic [IP3], however, these models have suggested that the fertilization wave passes through the center of the egg. Complementing these earlier models, we propose a more detailed model of the fertilization Ca2+ wave in Xenopus eggs to explore the hypothesis that IP3 is produced only at or near the plasma membrane. In this case, we find that the wave propagates primarily through the cortex of the egg, and that Ca2+ -induced production of IP3 at the plasma membrane allows IP3 to propagate in advance of the wave. Our model includes Ca2+ -dependent production of IP3 at the plasma membrane and IP3 degradation. Simulations in 1 dimension and axi-symmetric 3 dimensions illustrate the basic features of the wave.

The cytosolic II-III loop of Cav2.3 provides an essential determinant for the phorbol ester-mediated stimulation of E-type Ca2+ channel activity

There is growing evidence that E-type voltage dependent Ca(2+) channels (Ca(v)2.3) are involved in triggering and controlling pivotal cellular processes like neurosecretion and long-term potentiation. The mechanism underlying a novel Ca(2+) dependent stimulation of E-type Ca(2+) channels was investigated in the context of the recent finding that influx of Ca(2+) through other voltage dependent Ca(2+) channels is necessary and sufficient to directly activate protein kinase C (PKC). With Ba(2+) as charge carrier through Ca(v)2.3 channel alpha(1) subunits expressed in HEK-293 cells, activation of PKC by low concentrations of phorbol ester augmented peak I(Ba) by approximately 60%. In addition, the non-inactivating fraction of I(Ba) was increased by more than three-fold and recovery from short-term inactivation was accelerated. The effect of phorbol ester on I(Ba) was inhibited by application of the specific PKC inhibitor bisindolylmaleimide I. With Ca(2+) as charge carrier, application of phorbol ester did not change the activity of Ca(v)2.3 currents but they were modified by the PKC inhibitor bisindolylmaleimide I. These results suggest that with Ca(2+) as charge carrier the incoming Ca(2+) can activate PKC, thereby augmenting Ca(2+) influx into the cytosol. No modulation of Ca(v)2.3 channels by PKC was observed when an arginine rich region in the II-III loop of Ca(v)2.3 was eliminated. Receptor independent stimulation of PKC and its interaction with Ca(v)2.3 channels therefore represents an important positive feedback mechanism to decode electrical signals into a variety of cellular functions.

Control of the Na+/Ca2+ exchanger 3 promoter by cyclic adenosine monophosphate and Ca2+ in differentiating neurons

The human gene for member 3 of solute carrier family 8 (SLC8A3), encoding the Na+/Ca2+ exchanger isoform 3 (NCX3), was identified on chromosome 14q24.2. The minimal promoter region was predicted 250 bp upstream of exon 1. This was confirmed by luciferase reporter assays of pGL3-promoter constructs in transfected SH-SY5Y cells. The promoter activity was monitored during the differentiation of this cell line elicited by the sequential treatment with retinoic acid and brain-derived neurotrophic factor (BDNF). The activity was induced by cyclic AMP (cAMP) via the CRE (cAMP response element) and was stimulated by retinoic acid. The increase of intracellular Ca2+ induced by the partial depolarization of the plasma membrane with KCl down-regulated both the basal and the cAMP-stimulated transcription. The down-regulation of the latter may be mediated by the phosphorylation of the CRE-binding protein by a calmodulin-dependent kinase (CaMKII). The exposure of cells to BDNF after treatment with retinoic acid rapidly induced promoter activity during the initial five hours and phosphorylation of CRE-binding protein during the first two hours. The promoter activity was further enhanced by cAMP, but became insensitive to Ca2+. In BDNF-stimulated cells cAMP elevation caused the preferential phosphorylation of ATF1 instead of that of CRE-binding protein.

Inhibition of depolarization-induced [3H]noradrenaline release from SH-SY5Y human neuroblastoma cells by some second-generation H(1) receptor antagonists through blockade of store-operated Ca(2+) channels (SOCs)

In the present study, the effect of the blockade of membrane calcium channels activated by intracellular Ca(2+) store depletion on basal and depolarization-induced [3H]norepinephrine ([3H]NE) release from SH-SY5Y human neuroblastoma cells was examined. The second-generation H(1) receptor blockers astemizole, terfenadine, and loratadine, as well as the first-generation compound hydroxyzine, inhibited [3H]NE release induced by high extracellular K(+) concentration ([K(+)](e)) depolarization in a concentration-dependent manner (the IC(50)s were 2.3, 1.7, 4.8, and 9.4 microM, respectively). In contrast, the more hydrophilic second-generation H(1) receptor blocker cetirizine was completely ineffective (0.1-30 microM). The inhibition of high [K(+)](e)-induced [3H]NE release by H(1) receptor blockers seems to be related to their ability to inhibit Ca(2+) channels activated by Ca(i)(2+) store depletion (SOCs). In fact, astemizole, terfenadine, loratadine, and hydroxyzine, but not cetirizine, displayed a dose-dependent inhibitory action on the increase in intracellular Ca(2+) concentrations ([Ca(2+)](i)) obtained with extracellular Ca(2+) reintroduction after Ca(i)(2+) store depletion with thapsigargin (1 microM), an inhibitor of the sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA) pump. The rank order of potency for SOC inhibition by these compounds closely correlated with their inhibitory properties on depolarization-induced [3H]NE release from SH-SY5Y human neuroblastoma cells. Nimodipine (1 microM) plus omega-conotoxin (100 nM) did not interfere with the present model for SOC activation. In addition, the inhibition of depolarization-induced [3H]NE release does not seem to be attributable to the blockade of the K(+) currents carried by the K(+) channels encoded by the human Ether-a-Gogo Related Gene (I(HERG)) by these antihistamines. In fact, whole-cell voltage-clamp experiments revealed that the IC(50) for astemizole-induced hERG blockade is about 300-fold lower than that for the inhibition of high K(+)-induced [3H]NE release. Furthermore, current-clamp experiments in SH-SY5Y cells showed that concentrations of astemizole (3 microM) which were effective in preventing depolarization-induced [3H]NE release were unable to interfere with the cell membrane potential under depolarizing conditions (100 mM [K(+)](e)), suggesting that hERG K(+) channels do not contribute to membrane potential control during exposure to elevated [K(+)](e). Collectively, the results of the present study suggest that, in SH-SY5Y human neuroblastoma cells, the inhibition of SOCs by some second-generation antihistamines can prevent depolarization-induced neurotransmitter release.

Influence of muscarinic agonists and tyrosine kinase inhibitors on L-type Ca(2+)Channels in human and bovine trabecular meshwork cells

Trabecularmeshwork (TM), a smooth muscle-like tissue with contractile properties, is involved in the regulation of aqueous humor outflow. However, little is known about the regulation of Ca(2+)influx in trabecular meshwork cells. We investigated the influence of acetylcholine and tyrosine kinases on Ca(2+)conductances of bovine TM (BTM) and human TM (HTM) cells using the perforated-patch configuration of the patch-clamp technique and measurements of intracellular free Ca(2+)([Ca(2+)](i)). Depolarization of the cells in the presence of 10 m m Ba(2+)or Ca(2+)led to an activation of inward currents at potentials positive to -30 mV with characteristics typical of L-type Ca(2+)currents: when using 10 m m Ba(2+), maximal inward current and inactivation time constant (tau) increased; the L-type Ca(2+)channel blocker nifedipine (1 microm) reduced and the L-type Ca(2+)channel agonist BayK8644 (5 microm) enhanced maximal inward current. Acetylcholine (100 microm) and carbachol (1 microm) led to an increase in inward Ba(2+)current whereas application of the tyrosine kinase inhibitors genistein (50 microm) and lavendustin A (20 microm) resulted in a decrease in inward current. The application of daidzein (10 microm), an inactive analog of genistein had no effect. Depolarization of the cells with 135 m m K(+)or direct stimulation of L-type channels by application of BayK 8644 led to an increase in [Ca(2+)](i). Carbachol (1 microm) induced an increase in [Ca(2+)](i)which was decreased by application of the tyrosine kinase inhibitor genistein (50 microm). We conclude that HTM and BTM cells express voltage-dependent L-type Ca(2+)channels that influence intracellular Ca(2+)concentration and thus may modulate TM contractility. The activity of L-type Ca(2+)currents is influenced by muscarinic agonists and tyrosine kinases.

The activation of nitric oxide synthase by copper ion is mediated by intracellular Ca2+ mobilization in human pulmonary arterial endothelial cells

The aim of the study was to elucidate the vasodilatory mechanism due to Cu2+ by assessing nitric oxide (NO) production as determined by NOx (NO, NO2-, and NO3-) that is released from human pulmonary arterial endothelial cell (HPAEC) monolayers using a NO chemiluminescence analyzer, and also to assess Ca2+ movement using 45Ca and fura 2 in HPAEC. Cu2+ (10(-6)-10(-4) M) significantly increased NO production in a dose-dependent manner when extracellular Ca2+ was present. 45Ca influx into the adherent cells was dose-dependently enhanced by Cu(2+) (10(-6)-10(-4) M), but not by Mn(2+), Zn(2+) or Fe(2+). [Ca2+]i, measured by monitoring the fluorescence changes of fura 2, was significantly elevated in the presence of Cu2+. The increase in [Ca2+]i induced by Cu2+ was inhibited by either diethyldithiocarbamate (DDC) or the depletion of extracellular Ca2+. The dihydropyridine receptor agonist, BayK8644, significantly attenuated the Cu2+-induced increase in [Ca2+]i in a dose dependent manner and nitrendipine or nifedipine, the dihydropyridine receptor antagonists, dose-dependently inhibited a Cu2+-induced increase in [Ca2+]i. These results suggest that Cu2+ activates eNOS through the mechanism of [Ca2+]i elevation due to Ca2+ influx into HPAEC and that the Cu2+-induced [Ca2+]i elevation in HPAEC is likely due to activation of the dihydropyridine-like receptors.

Etomidate inhibits [3H]noradrenaline release from SH-SY5Y human neuroblastoma cells

We have examined the effects of the intravenous anaesthetic induction agent etomidate on K+ and carbachol evoked [3H]noradrenaline ([3H]NA) release and the associated increase in [Ca2+]i in SH-SY5Y human neuroblastoma cells in a attempt to study potential anaesthetic target site(s). Preincubation with etomidate produced a dose-dependent inhibition of both K+ and carbachol evoked [3H]NA release with estimated IC50 values of 88 and 69 microM, respectively. Only K+ stimulated increase in [Ca2+]i was inhibited by etomidate preincubation with an IC50 of 146 microM. Acute addition of etomidate after K+ challenge also inhibited the increase in [Ca2+]i with an IC50 of 99 microM. In addition etomidate displaced the binding of [3H]PN200-110 to L-type voltage sensitive Ca2+ channels with a Ki of 48 microM. As K+ but not carbachol evoked [3H]NA release is extracellular Ca2+ dependent and was inhibited by etomidate these data coupled with the PN200-110 displacement studies suggest that etomidate may interact with L-type voltage sensitive Ca2+ channels. The inhibition of carbachol evoked release without affecting the associated increase in [Ca2+]i suggests that etomidate may exert additional effects at either the muscarinic receptor or the secretory machinery in these cells.

Protein kinase C and transmitter release

1. Protein kinase C (PKC) is an important second messenger-activated enzyme. In noradrenergic nerves it appears to be tonically activated by diacylglycerol (DAG) to facilitate transmitter release and the steps in this involve activation of phospholipase C, generation of DAG and activation of PKC. It is suggested that the subsequent facilitation of transmitter release is due to the phosphorylation of proteins involved in the release process distal to Ca2+ entry, presumably those involved in vesicle dynamics. 2. There are differences between central noradrenergic neurons and sympathetic nerves. In central neurons PKC appears to be tonically active and its inhibition results in a decrease in noradrenaline release under most, if not all, conditions. 3. In sympathetic nerves PKC inhibitors only decrease transmitter release during high-frequency stimulation and not during low-frequency stimulation. At high frequency there is a gradual increase in the effect of PKC inhibitors on transmitter release during the first 15 s of a stimulation train. It is suggested that this is due to a progressive rise in intracellular Ca2+ and a consequent activation of PKC. 4. Activation of PKC by phorbol esters produces a large enhancement in action potential-evoked noradrenaline release in both the central nervous system and in peripheral tissues. The structural requirements of the phorbol esters for maximal effect suggest that the phorbol esters must access the interior of the nerve terminal to activate PKC and the neural membrane acts as a barrier for highly lipophilic phorbol esters, thereby reducing their activity. Activation of PKC represents one of the most powerful ways to enhance transmitter release and may have therapeutic potential.

Noradrenaline release and the effect of endogenous activation of the phospholipase C/protein kinase C signalling pathway in rat atria

1. It has been proposed that protein kinase C (PKC) in sympathetic nerves is activated during action-potential evoked release of noradrenaline and helps maintain transmitter output. We studied this phenomenon further in rat atria radiolabelled with [3H]-noradrenaline. 2. Noradrenaline release was elevated by continuous electrical stimulation of the atria for 10 min at either 5 or 10 Hz. Two inhibitors of PKC, polymyxin B (21 microM) and Ro 318220 (3 microM), markedly inhibited the release of noradrenaline but only at the higher stimulation frequency. 3. Further experiments were conducted with 10 Hz stimulation but for shorter train durations. In this case polymyxin B inhibited noradrenaline release during a 10 or 15 s train of impulses but not during a 5 s train. This suggests that PKC effects are induced during the stimulation train by some process. 4. The diacylglycerol kinase inhibitor R59949 (10 microM), which prevents the breakdown of diacylglycerol, enhanced noradrenaline release elicited by stimulation at 10 Hz for 10 or 15 s. This effect was not seen if polymyxin B was present and suggests that diacylglycerol is the endogenous activator of PKC. 5. The source of the diacylglycerol may be through phospholipase C pathways, since the phospholipase C inhibitor U73122 (3 microM) inhibited noradrenaline release at 10 Hz for 10 s and the effect was not seen if polymyxin B was also present. 6. It is unlikely that phospholipase D is the source of diacylglycerol. Although the phospholipase D inhibitor wortmannin (1 microM) inhibited noradrenaline release, this effect was still observed in the presence of polymyxin B. Furthermore ethanol, which inhibits diacylglycerol formation by phospholipase D, had no effect on noradrenaline release. 7. We therefore suggest that during a train of high frequency pulses phospholipase C is activated and this results in the production of diacylglycerol which in turn activates PKC. This enables the neurones to maintain transmitter release at a high level.

The effects of recombinant rat mu-opioid receptor activation in CHO cells on phospholipase C, [Ca2+]i and adenylyl cyclase

1. The rat mu-opioid receptor has recently been cloned yet its second messenger coupling remains unclear. The endogenous mu-opioid receptor in SH-SY5Y cells couples to phospholipase C (PLC), increases [Ca2+]i and inhibits adenylyl cyclase (AC). We have examined the effects of mu-opioid agonists on inositol(1,4,5)trisphosphate (Ins(1,4,5)P3), [Ca2+]i and adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation in Chinese hamster ovarian (CHO) cells transfected with the cloned mu-opioid receptor. 2. Opioid receptor binding was assessed with [3H]-diprenorphine ([3H]-DPN) as a radiolabel. Ins(1,4,5)P3 and cyclic AMP were measured by specific radioreceptor assays. [Ca2+]i was measured fluorimetrically with Fura-2. 3. Scatchard analysis of [3H]-DPN binding revealed that the Bmax varied between passages. Fentanyl (10 pM 1 microM) dose-dependently displaced [3H]-DPN, yielding a curve which had a Hill slope of less than unity (0.6 +/- 0.1), and was best fit to a two site model, with pK1 values (% of sites) of 9.97 +/- 0.4 (27 +/- 4.8%) and 7.68 +/- 0.07 (73 +/- 4.8%). In the presence of GppNHp (100 microM) and Na+ (100 mM), the curve was shifted to the right and became steeper (Hill slope = 0.9 +/- 0.1) with a pK1 value of 6.76 +/- 0.04. 4. Fentanyl (0.1 nM-1 microM) had no effect on basal, but dose-dependently inhibited forskolin (1 microM)-stimulated, cyclic AMP formation (pIC50 -7.42 +/- 0.23), in a pertussis toxin (PTX; 100 ng ml-1 for 24 h)-sensitive and naloxone-reversible manner (K1 = 1.7 nM). Morphine (1 microM) and [D-Ala2, MePhe4, gly(ol)5]-enkephalin (DAMGO, 1 microM) also inhibited forskolin (1 microM)-stimulated cyclic AMP formation, whilst [D-Pen2, D-Pen5], enkephalin (DPDPE, 1 microM) did not. 5. Fentanyl (0.1 nM-10 microM) caused a naloxone (1 microM)-reversible, dose-dependent stimulation of Ins(1,4,5)P3 formation, with a pEC50 of 7.95 +/- 0.15 (n-5), PTX (100 ng ml-1 for 24 h) abolished, whilst Ni2 (2.5 mM) inhibited (by 52%), the fentanyl-induced Ins(1,4,5)P3 response. Morphine (1 microM) and DAMGO (1 microM), but not DPDPE (1 microM), also stimulated Ins(1,4,5)P3 formation. Fentanyl (1 microM) also caused an increase in [Ca2+]i (80 +/- 16.4 nM, n-6), reaching a maximum at 26.8 +/- 2.5 s. The increase in [Ca2+]i remained elevated until sampling ended (200 s) and was essentially abolished by the addition of naloxone (1 microM). Pre-incubation with naloxone (1 microM, 3 min) completely abolished fentanyl-induced increases in [Ca2+]i. 6. In conclusion, the cloned mu-opioid receptor when expressed in CHO cells stimulates PLC and inhibits AC, both effects being mediated by a PTX-sensitive G-protein. In addition, the receptor couples to an increase in [Ca2+]i. These findings are consistent with the previously described effector-second messenger coupling of the endogenous mu-opioid receptor.

A comparative study of L-type voltage sensitive Ca2+ channels in rat brain regions and cultured neuronal cells

Radioligand binding studies using the L-type voltage sensitive Ca2+ channel (VSCC) antagonist (+)-[3H]PN200-110 revealed the following rank order channel density in rat brain and cultured neuronal cell homogenates: striatum > or = cerebrocortex > > cerebellum = brainstem > SH-SY5Y cell line > NG108-15 cell line > 1321N1 cell line > PC12 cell line. There were no significant differences in the equilibrium dissociation constant, Kd for (+)-[3H]PN200-110 or pK50 for nifedipine. K+ depolarization in SH-SY5Y cells and NG108-15 cells evoked a biphasic and monophasic increase in [Ca2+]i. The L-type Ca2+ channel antagonist nifedipine (1 microM) produced a 66 and 87% inhibition of the K(+)-evoked rise in the peak and plateau phase [Ca2+]i in SH-SY5Y cells and abolished the monophasic response in NG108-15 cells. The L-channel activator S(-)Bay K 8644 (1 microM) enhanced the K(+)-evoked increase in [Ca2+]i in both cell lines. These data demonstrate a comparatively low density of L-VSCC in undifferentiated SH-SY5Y cells, NG108-15 cells, 1321N1 cells and PC12 cells that are functionally active in at least SH-SY5Y cells and NG108-15 cells.