Phosphorylation of inositol 1,4,5-trisphosphate receptors by cAMP-dependent protein kinase. Type I, II, and III receptors are differentially susceptible to phosphorylation and are phosphorylated in intact cells

Sparks and puffs in oligodendrocyte progenitors: cross talk between ryanodine receptors and inositol trisphosphate receptors

Investigating how calcium release from the endoplasmic reticulum (ER) is triggered and coordinated is crucial to our understanding of how oligodendrocyte progenitor cells (OPs) develop into myelinating cells. Sparks and puffs represent highly localized Ca(2+) release from the ER through ryanodine receptors (RyRs) and inositol trisphosphate receptors (IP(3)Rs), respectively. To study whether sparks or puffs trigger Ca(2+) waves in OPs, we performed rapid high-resolution line scan recordings in fluo-4-loaded OP processes. We found spontaneous and evoked sparks and puffs, and we have identified functional cross talk between IP(3)Rs and RyRs. Local events evoked using the IP(3)-linked agonist methacholine (MeCh) showed significantly different morphology compared with events evoked using the caffeine analog 3,7-dimethyl-1-propargylxanthine (DMPX). Pretreatment with MeCh potentiated DMPX-evoked events, whereas inhibition of RyRs potentiated events evoked by low concentrations of MeCh. Furthermore, activation of IP(3)Rs but not RyRs was critical for Ca(2+) wave initiation. Using immunocytochemistry, we show OPs express the specific Ca(2+) release channel subtypes RyR3 and IP(3)R2 in patches along OP processes. RyRs are coexpressed with IP(3)Rs in some patches, but IP(3)Rs are also found alone. This differential distribution pattern may underlie the differences in local and global Ca(2+) signals mediated by these two receptors. Thus, in OPs, interactions between IP(3)Rs and RyRs determine the spatial and temporal characteristics of calcium signaling, from microdomains to intracellular waves.

Cross talk between cyclic nucleotides and polyphosphoinositide hydrolysis, protein kinases, and contraction in smooth muscle

This article provides an update of a minireview published in 1996 (Abdel-Latif AA. Proc Soc Exp Biol Med 211:163-177, 1996), the purpose of which was to examine in nonvascular smooth muscle the biochemical and functional cross talk between the sympathetic nervous system, which governs the formation of cAMP and muscle relaxation, and the parasympathetic nervous system, which governs the generation of IP3 and diacylglycerol, from the polyphosphoinositides, Ca2+ mobilization, and contraction. This review examines further evidence, both from nonvascular and vascular smooth muscle, for cross talk between the cyclic nucleotides, cAMP and cGMP via their respective protein kinases, and the Ca2+-dependent- and Ca2+-independent-signaling pathways involved in agonist-induced contraction. These include the IP3-Ca2+-CaM- myosin light chain kinase (MLCK) pathway and the Ca2+-independent pathways, including protein kinase C-, MAP kinase-, and Rho-kinase. In addition, MLC phosphorylation and contraction can also be increased by a decrease in myosin phosphatase activity. A summary of the cross talk between the cyclic nucleotides and these signaling pathways was presented. In smooth muscle, there are several targets for cyclic nucleotide inhibition and consequent relaxation, including the receptor, G proteins, phospholipase C-beta1-4 isoforms, IP3 receptor, Ca2+ mobilization, MLCK, MAP kinase, Rho-kinase, and myosin phosphatase. While significant progress has been made in the past four years on this cross talk, the precise mechanisms underlying the biochemical basis for the cyclic nucleotide inhibition of Ca2+ mobilization and consequently muscle contraction remain to be established. Although it is well established that second-messenger cross talk plays an important role in smooth muscle relaxation, the many sources which exist in smooth muscle for Ca2+ mobilization, coupled with the multiple signaling pathways involved in agonist-induced contraction, contribute appreciably to the difficulties found by many investigators in identifying the targets for cyclic nucleotide inhibition and consequent relaxation. Better methodology and more novel interdisciplinary approaches are required for elucidating the mechanism(s) of cAMP- and cGMP-inhibition of smooth muscle contraction.

Investigation of the roles of Ca2+ and InsP3 diffusion in the coordination of Ca2+ signals between connected hepatocytes

Glycogenolytic agonists induce coordinated Ca2+ oscillations in multicellular rat hepatocyte systems as well as in the intact liver. The coordination of intercellular Ca2+ signals requires functional gap-junction coupling. The mechanisms ensuring this coordination are not precisely known. We investigated possible roles of Ca2+ or inositol 1,4,5-trisphosphate (InsP3) as a coordinating messengers for Ca2+ spiking among connected hepatocytes. Application of ionomycin or of supra-maximal concentrations of agonists show that Ca2+ does not significantly diffuse between connected hepatocytes, although gap junctions ensure the passage of small signaling molecules, as demonstrated by FRAP experiments. By contrast, coordination of Ca2+ spiking among connected hepatocytes can be favored by a rise in the level of InsP3, via the increase of agonist concentrations, or by a shift in the affinity of InsP3 receptor for InsP3. In the same line, coordination cannot be achieved if the InsP3 is rapidly metabolized by InsP3-phosphatase in one cell of the multiplet. These results demonstrate that even if small amounts of Ca2+ diffuse across gap junctions, they most probably do not play a significant role in inducing a coordinated Ca2+ signal among connected hepatocytes. By contrast, coordination of Ca2+ oscillations is fully dependent on the diffusion of InsP3 between neighboring cells.

Differential neuronal localizations and dynamics of phosphorylated and unphosphorylated type 1 inositol 1,4,5-trisphosphate receptors

Type 1 inositol 1,4,5-trisphosphate receptors are phosphorylated by cyclic-AMP-dependent protein kinase A at serines 1589 and 1755, with serine 1755 phosphorylation greatly predominating in the brain. Inositol 1,4,5-trisphosphate receptor protein kinase A phosphorylation augments Ca(2+) release. To assess type 1 protein kinase A phosphorylation dynamics in the intact organism, we developed antibodies selective for either serine 1755 phosphorylated or unphosphorylated species. Immunohistochemical studies reveal marked variation in localization. For example, in the hippocampus the phosphorylated type 1 inositol 1,4,5-trisphosphate receptor is restricted to CA1, while the unphosphorylated receptor occurs ubiquitously in CA1-CA3 and dentate gyrus granule cells. Throughout the brain the phosphorylated type 1 inositol 1,4,5-trisphosphate receptor is selectively enriched in dendrites, while the unphosphorylated receptor predominates in cell bodies. Focal cerebral ischemia in rats and humans is associated with dephosphorylation of type 1 inositol 1,4,5-trisphosphate receptors, and glutamatergic excitation of cerebellar Purkinje cells mediated by ibogaine elicits dephosphorylation of type 1 inositol 1,4,5-trisphosphate receptors that precedes evidence of excitotoxic neuronal degeneration. We have demonstrated striking variations in regional and subcellular distribution of inositol 1,4,5-trisphosphate receptor phosphorylation that may influence normal physiological intracellular Ca(2+) signaling in rat and human brain. We have further shown that the subcellular distribution of inositol 1,4,5-trisphosphate receptor phosphorylation in neurons is regulated by excitatory neurotransmission, as well as excitotoxic insult and neuronal ischemia-reperfusion. Phosphorylation dynamics of type 1 inositol 1,4,5-trisphosphate receptors may modulate intracellular Ca(2+) release and influence the cellular response to neurotoxic insults.

Proteasomal activation mediates down-regulation of inositol 1,4,5-trisphosphate receptor and calcium mobilization in rat pancreatic islets

Inositol 1,4,5-trisphosphate receptor (IP3R) protein levels in isolated rat pancreatic islets were investigated in response to carbachol (CCh) and sulfated cholecystokinin 26-33 amide stimulation. Within 2 h, CCh reduced IP3R-I protein levels by 22% and IP3R-II and -III levels to 65% or more below basal. Sulfated cholecystokinin 26-33 amide decreased the levels of IP3R-I, -II, and -III by 34%, 60%, and 66% below basal, respectively. The effect of CCh was concentration- and time-dependent, with a persistent decline in IP3R levels for up to 6 h after the onset of stimulation. CCh-pretreated islets also showed an inhibition of glucose-stimulated insulin secretion. Proteasome inhibition completely blocked the down-regulatory effects of CCh on IP3Rs and significantly increased the insulin secretory response to glucose stimulation in the presence of CCH: Islet stimulation by glucose, alpha-ketoisocaproic acid, and tolbutamide completely protected IP3Rs against the down-regulatory effects of CCH: 2-deoxyglucose and 3-O-methyl glucose failed to affect CCh-induced IP3R down-regulation. The protective effects of glucose on IP3R down-regulation were completely inhibited by the Ca(2+) channel-blocking agent nimodipine. Intracellular Ca(2+) ([Ca(2+)](i)) levels in Fura-2 (fluorescent Ca(2+) indicator)-loaded islets, in the absence of extracellular Ca(2+), increased in response to glucose stimulation; but in islets pretreated with CCh, glucose did not increase [Ca(2+)](i) above basal levels. However, in islets pretreated with CCh and the proteasomal inhibitor MG-132 (carbobenzoxyl-leucinyl-leucinyl-leucinyl-H), the glucose-stimulated increase in [Ca(2+)](i) was significantly higher than the change observed for glucose-stimulated [Ca(2+)](i) in the absence of MG-132. The results suggest that muscarinic receptor stimulation modulates IP3R protein levels in islets through a proteasomal activation pathway, and that down-regulation of IP3Rs has a profound effect on Ca(2+) mobilization in islets that may relate to insulin secretory responsiveness.

Interactions between the inositol 1,4,5-trisphosphate and cyclic AMP signaling pathways regulate larval molting in Drosophila

Larval molting in Drosophila, as in other insects, is initiated by the coordinated release of the steroid hormone ecdysone, in response to neural signals, at precise stages during development. In this study we have analyzed, using genetic and molecular methods, the roles played by two major signaling pathways in the regulation of larval molting in Drosophila. Previous studies have shown that mutants for the inositol 1,4,5-trisphosphate receptor gene (itpr) are larval lethals. In addition they exhibit delays in molting that can be rescued by exogenous feeding of 20-hydroxyecdysone. Here we show that mutants for adenylate cyclase (rut) synergize, during larval molting, with itpr mutant alleles, indicating that both cAMP and InsP(3) signaling pathways function in this process. The two pathways act in parallel to affect molting, as judged by phenotypes obtained through expression of dominant negative and dominant active forms of protein kinase A (PKA) in tissues that normally express the InsP(3) receptor. Furthermore, our studies predict the existence of feedback inhibition through protein kinase A on the InsP(3) receptor by increased levels of 20-hydroxyecdysone.

cAMP-dependent protein kinase induces cAMP-response element-binding protein phosphorylation via an intracellular calcium release/ERK-dependent pathway in striatal neurons

Activation of the cAMP-dependent protein kinase A (PKA) pathway may induce cAMP-response element-binding protein (CREB) phosphorylation either directly or via cross-talk mechanisms with other signal transduction pathways. In this study, we have investigated in striatal primary cultures the mechanism by which activation of the cAMP/PKA-dependent pathway leads to CREB phosphorylation via the extracellular signal-regulated kinase (ERK)-dependent pathway. We have found that PKA-induced CREB phosphorylation and CREB-dependent transcription are mediated by calcium (Ca(2+)) release from intracellular stores and are blocked by inhibitors of the protein kinase C and ERK pathways. This mechanism appears to be mediated by the small G-protein Rap1, whose activation appears to be primed by PKA-induced Ca(2+) release but not further induced by direct or indirect PKA- or protein kinase C-dependent phosphorylation. These results suggest that, in striatal neurons, intracellular Ca(2+) release, Rap1, and ERK pathway play a crucial role in the PKA-induced CREB phosphorylation and CREB-dependent transcription.

Presence of soluble and particulate guanylyl cyclase in the same hippocampal astrocytes

The localisation of particulate and soluble guanylyl cyclase was studied in hippocampal astrocytes. Counting the colocalisation of cGMP immunoreactivity with the astrocytic marker glial fibrillary acidic protein after stimulation of brain slices with sodium nitroprusside (0.1 mM) or atrial natriuretic peptide (100 nM), we were able to show that at least 67% of the hippocampal astrocytes contained both guanylyl cyclase isoforms. In addition, it was shown that a large number of atrial natriuretic peptide, brain-derived natriuretic peptide or sodium nitroprusside responsive cells contain the beta1-subunit of the soluble guanylyl cyclase. The results show that, in at least a subset of hippocampal astrocytes, soluble and particulate guanylyl cyclases are simultaneously present in the same cells.

Targeted phosphorylation of inositol 1,4,5-trisphosphate receptors selectively inhibits localized Ca2+ release and shapes oscillatory Ca2+ signals

The current study provides biochemical and functional evidence that the targeting of protein kinase A (PKA) to sites of localized Ca(2+) release confers rapid, specific phosphoregulation of Ca(2+) signaling in pancreatic acinar cells. Regulatory control of Ca(2+) release by PKA-dependent phosphorylation of inositol 1,4, 5-trisphosphate (InsP(3)) receptors was investigated by monitoring Ca(2+) dynamics in pancreatic acinar cells evoked by the flash photolysis of caged InsP(3) prior to and following PKA activation. Ca(2+) dynamics were imaged with high temporal resolution by digital imaging and electrophysiological methods. The whole cell patch clamp technique was used to introduce caged compounds and to record the activity of a Ca(2+)-activated Cl(-) current. Photolysis of low concentrations of caged InsP(3) evoked Cl(-) currents that were inhibited by treatment with dibutryl-cAMP or forskolin. In contrast, PKA activators had no significant inhibitory effect on the activation of Cl(-) current evoked by uncaging Ca(2+) or by the photolytic release of higher concentrations of InsP(3). Treatment with Rp-adenosine-3',5'-cyclic monophoshorothioate, a selective inhibitor of PKA, or with Ht31, a peptide known to disrupt the targeting of PKA, largely abolished forskolin-induced inhibition of Ca(2+) release. Further evidence for the targeting of PKA to the sites of Ca(2+) mobilization was revealed using immunocytochemical methods demonstrating that the R(IIbeta) subunit of PKA was localized to the apical regions of acinar cells and co-immunoprecipitated with the type III but not the type I or type II InsP(3) receptors. Finally, we demonstrate that the pattern of signaling evoked by acetylcholine can be converted to one that is more "CCK-like" by raising cAMP levels. Our data provide a simple mechanism by which distinct oscillatory Ca(2+) patterns can be shaped.

Association of deficient type II protein kinase A activity with aberrant nuclear translocation of the RII beta subunit in systemic lupus erythematosus T lymphocytes

Systemic lupus erythematosus (SLE) is an autoimmune disorder of indeterminate etiology characterized by abnormal T cell signal transduction and altered T cell effector functions. We have previously observed a profound deficiency of total protein kinase A (PKA) phosphotransferase activity in SLE T cells. Here we examined whether reduced total PKA activity in SLE T cells is in part the result of deficient type II PKA (PKA-II) isozyme activity. The mean PKA-II activity in SLE T cells was 61% of normal control T cells. The prevalence of deficient PKA-II activity in 35 SLE subjects was 37%. Deficient isozyme activity was persistent over time and was unrelated to SLE disease activity. Reduced PKA-II activity was associated with spontaneous dissociation of the cytosolic RIIbeta2C2 holoenzyme and translocation of the regulatory (RIIbeta) subunit from the cytosol to the nucleus. Confocal immunofluorescence microscopy revealed that the RIIbeta subunit was present in approximately 60% of SLE T cell nuclei compared with only 2-3% of normal and disease controls. Quantification of nuclear RIIbeta subunit protein content by immunoprecipitation and immunoblotting demonstrated a 54% increase over normal T cell nuclei. Moreover, the RIIbeta subunit was retained in SLE T cell nuclei, failed to relocate to the cytosol, and was associated with a persistent deficiency of PKA-II activity. In conclusion, we describe a novel mechanism of deficient PKA-II isozyme activity due to aberrant nuclear translocation of the RIIbeta subunit and its retention in the nucleus in SLE T cells. Deficient PKA-II activity may contribute to impaired signaling in SLE T cells.

Distinct localization and function of1,4,5IP3 receptor subtypes and the1,3,4,5IP4 receptor GAP1IP4BP in highly purified human platelet membranes

Platelet activation is associated with an increase of cytosolic Ca++ levels. The 1,4,5IP3receptors [1,4,5IP3R] are known to mediate Ca++ release from intracellular stores of many cell types. Currently there are at least 3 distinct subtypes of1,4,5IP3R—type I, type II, and type III—with suggestions of distinct roles in Ca++ elevation. Specific receptors for 1,3,4,5IP4 belonging to the GAP1 family have also been described though their involvement with Ca++ regulation is controversial. In this study we report that platelets contain all 3 subtypes of1,4,5IP3R but in different amounts. Type I and type II receptors are predominant. In studies using highly purified platelet plasma (PM) and intracellular membranes (IM) we report a distinct localization of these receptors. The PM fractions were found to contain the type III 1,4,5IP3R and GAP1IP4BP in contrast to IM, which contained type I1,4,5IP3R. The type II receptor exhibited a dual distribution. In studies examining the labeling of surface proteins with biotin in intact platelets only the type III1,4,5IP3R was significantly labeled. Immunogold studies of ultracryosections of human platelets showed significantly more labeling of the PM with the type III receptor antibodies than with type I receptor antibodies. Ca++ flux studies were carried out with the PM to demonstrate in vitro function of inositol phosphate receptors. Ca++ release activities were present with both 1,4,5IP3 and1,3,4,5IP4 (EC50 = 1.3 and 0.8 μmol/L, respectively). Discrimination of the Ca++-releasing activities was demonstrated with cyclic adenosine monophosphate (cAMP)-dependent protein kinase (cAMP-PK) specifically inhibiting 1,4,5IP3 but not1,3,4,5IP4-induced Ca++ flux. In experiments with both PM and intact platelets, the1,4,5IP3Rs but not GAP1IP4BP were found to be substrates of cAMP-PK and cGMP-PK. Thus the Ca++ flux property of1,3,4,5IP4 is insensitive to cAMP-PK. These studies suggest distinct roles for the1,4,5IP3R subtypes in Ca++movements, with the type III receptor and GAP1IP4BPassociated with cation entry in human platelets and the type I receptor involved with Ca++ release from intracellular stores.

Distinct localization and function of1,4,5IP3 receptor subtypes and the1,3,4,5IP4 receptor GAP1IP4BP in highly purified human platelet membranes

Platelet activation is associated with an increase of cytosolic Ca++ levels. The 1,4,5IP3receptors [1,4,5IP3R] are known to mediate Ca++ release from intracellular stores of many cell types. Currently there are at least 3 distinct subtypes of1,4,5IP3R—type I, type II, and type III—with suggestions of distinct roles in Ca++ elevation. Specific receptors for 1,3,4,5IP4 belonging to the GAP1 family have also been described though their involvement with Ca++ regulation is controversial. In this study we report that platelets contain all 3 subtypes of1,4,5IP3R but in different amounts. Type I and type II receptors are predominant. In studies using highly purified platelet plasma (PM) and intracellular membranes (IM) we report a distinct localization of these receptors. The PM fractions were found to contain the type III 1,4,5IP3R and GAP1IP4BP in contrast to IM, which contained type I1,4,5IP3R. The type II receptor exhibited a dual distribution. In studies examining the labeling of surface proteins with biotin in intact platelets only the type III1,4,5IP3R was significantly labeled. Immunogold studies of ultracryosections of human platelets showed significantly more labeling of the PM with the type III receptor antibodies than with type I receptor antibodies. Ca++ flux studies were carried out with the PM to demonstrate in vitro function of inositol phosphate receptors. Ca++ release activities were present with both 1,4,5IP3 and1,3,4,5IP4 (EC50 = 1.3 and 0.8 μmol/L, respectively). Discrimination of the Ca++-releasing activities was demonstrated with cyclic adenosine monophosphate (cAMP)-dependent protein kinase (cAMP-PK) specifically inhibiting 1,4,5IP3 but not1,3,4,5IP4-induced Ca++ flux. In experiments with both PM and intact platelets, the1,4,5IP3Rs but not GAP1IP4BP were found to be substrates of cAMP-PK and cGMP-PK. Thus the Ca++ flux property of1,3,4,5IP4 is insensitive to cAMP-PK. These studies suggest distinct roles for the1,4,5IP3R subtypes in Ca++movements, with the type III receptor and GAP1IP4BPassociated with cation entry in human platelets and the type I receptor involved with Ca++ release from intracellular stores.

cAMP-Dependent potentiation of the Ca(2+)-activated release of the anionic fluorescent dye, calcein, from rat parotid acinar cells

A recent study indicates that elevation of [Ca(2+)](i) enhances the release of calcein, an anionic fluorescent dye, from isolated exocrine acinar cells, so cytoplasmic calcein is useful for monitoring the secretion of organic anions. In this study, we investigated the effect of cAMP on the calcein release evoked by elevation of [Ca(2+)](i). Isoproterenol, forskolin and dibutyryl cyclic AMP (dbcAMP) did not induce the release of calcein from isolated parotid acinar cells, but they potentiated the carbachol-induced release of calcein. Although cytoplasmic calcein is released through an increase in [Ca(2+)](i), isoproterenol potentiated the carbachol-induced release of calcein without affecting the increase in [Ca(2+)](i) evoked by a high concentration of carbachol (10(-6) M). Charybdotoxin, a K(+) channel blocker, inhibited both the carbachol-induced release and the potentiation by isoproterenol. However, the calcein permeation pathways mediating the carbachol-induced release and the isoproterenol-potentiated release exhibited distinct sensitivities to anion channel blockers. Our results indicate that the calcein release induced by carbachol is potentiated through an increase in intracellular levels of cAMP. Although both the Ca(2+)-activated release and the cAMP-potentiated release may be coupled to Ca(2+)-activated K(+) efflux, increases in both [Ca(2+)](i) and [cAMP](i) may activate the calcein conduction pathway which is not activated by an increase in [Ca(2+)](i) alone.

Mapping of IP(3)-mediated Ca(2+) signals in single human neuroblastoma SH-SY5Y cells: cell volume shaping the Ca(2+) signal

Fast confocal laser-scanning microscopy was used to study spatiotemporal properties of IP(3)-mediated Ca(2+) release signals in human SH-SY5Y neuroblastoma cells. [Ca(2+)](i) increases were not affected by ryanodine (30 microgM) or caffeine (10 mM) and largely insensitive to removal of external Ca(2+), indicating predominance of IP(3)-induced Ca(2+) release. Ca(2+) signals evoked by high concentration (10 microM) of the muscarinic agonist carbachol appeared as self-propagating waves initiating in cell processes. At low carbachol concentrations (500 nM) Ca(2+) changes in most cells displayed striking spatiotemporal heterogeneity. The Ca(2+) response in the cell body was delayed and had a smaller amplitude and a slower rise time than that in processes. Ca(2+) changes in processes either occurred in a homogeneous manner throughout the whole process or were sometimes confined to hot spots. Regional differences in surface-to-volume ratio appear to be critical clues that determine the spatiotemporal pattern of intracellular Ca(2+) release signals.

IP(3)-Independent release of Ca(2+) from intracellular stores: A novel mechanism for transduction of bitter stimuli

A variety of substances with different chemical structures elicits a bitter taste. Several different transduction mechanisms underlie detection of bitter tastants; however, these have been described in detail for only a few compounds. In addition, most studies have focused on mammalian taste cells, of which only a small subset is responsive to any particular bitter compound. In contrast, approximately 80% of the taste cells in the mudpuppy, Necturus maculosus, are bitter-responsive. In this study, we used Ca(2+) imaging and giga-seal whole cell recording to compare the transduction of dextromethorphan (DEX), a bitter antitussive, with transduction of the well-studied bitter compound denatonium. Bath perfusion of DEX (2.5 mM) increased the intracellular Ca(2+) level in most taste cells. The DEX-induced Ca(2+) increase was inhibited by thapsigargin, an inhibitor of Ca(2+) transport into intracellular stores, but not by U73122, an inhibitor of phospholipase C, or by ryanodine, an inhibitor of ryanodine-sensitive Ca(2+) stores. Increasing intracellular cAMP levels with a cell-permeant cAMP analogue and a phosphodiesterase inhibitor enhanced the DEX-induced Ca(2+) increase, which was inhibited partially by H89, a protein kinase A inhibitor. Electrophysiological measurements showed that DEX depolarized the membrane potential and inhibited voltage-gated Na(+) and K(+) currents in the presence of GDP-beta-S, a blocker of G-protein activation. DEX also inhibited voltage-gated Ca(2+) channels. We suggest that DEX, like quinine, depolarizes taste cells by block of voltage-gated K channels, which are localized to the apical membrane in mudpuppy. In addition, DEX causes release of Ca(2+) from intracellular stores by a phospholipase C-independent mechanism. We speculate that the membrane-permeant DEX may enter taste cells and interact directly with Ca(2+) stores. Comparing transduction of DEX with that of denatonium, both compounds release Ca(2+) from intracellular stores. However, denatonium requires activation of phospholipase C, and the mechanism results in a hyperpolarization rather than a depolarization of the membrane potential. These data support the hypothesis that single taste receptor cells can use multiple mechanisms for transducing the same bitter compound.

Expression of PKA inhibitor (PKI) gene abolishes cAMP-mediated protection to endothelial barrier dysfunction

We investigated the hypothesis that cAMP-dependent protein kinase (PKA) protects against endothelial barrier dysfunction in response to proinflammatory mediators. An E1-, E3-, replication-deficient adenovirus (Ad) vector was constructed containing the complete sequence of PKA inhibitor (PKI) gene (AdPKI). Infection of human microvascular endothelial cells (HMEC) with AdPKI resulted in overexpression of PKI. Treatment with 0.5 microM thrombin increased transendothelial albumin clearance rate (0.012 +/- 0.003 and 0.035 +/- 0.005 microl/min for control and thrombin, respectively); the increase was prevented with forskolin + 3-isobutyl-1-methylxanthine (F + I) treatment. Overexpression of PKI resulted in abrogation of the F + I-induced inhibition of the permeability increase. However, with HMEC infected with ultraviolet-inactivated AdPKI, the F + I-induced inhibition was present. Also, F + I treatment of HMEC transfected with reporter plasmid containing the cAMP response element-directed transcription of the luciferase gene resulted in an almost threefold increase in luciferase activity. Overexpression of PKI inhibited this induction of luciferase activity. The results show that Ad-mediated overexpression of PKI in endothelial cells abrogated the cAMP-mediated protection against increased endothelial permeability, providing direct evidence that cAMP-dependent protein kinase promotes endothelial barrier function.

Interactions between Ca2+ and cAMP in ecdysteroid secretion from the prothoracic glands of Bombyx mori

The interaction between Ca2+ and cAMP in the mediation of ecdysteroid secretion from prothoracic glands (PGs) of Bombyx mori was investigated in vitro. Omission of Ca2+ from the PGs' incubation medium decreased basal ecdysteroid secretion from day 3 until day 6. On day 6, the ability of forskolin or 3-isobutyl-1-methylxanthine (IBMX) to stimulate ecdysteroid secretion was affected by the omission of Ca2+ from the medium. The cAMP agonist Sp-adenosine 3',5'-cyclic monophosphothioate (Sp-cAMPS) and the cAMP analogue dibutyryl cyclic AMP (dbcAMP) stimulated ecdysteroid secretion even in the absence of Ca2+ from the medium. The Sp-cAMPS-stimulated ecdysteroid secretion was inhibited by the cAMP antagonist Rp-adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS) and the L-type Ca2+ channel blocker verapamil. Both the Ca2+ ionophore A23187 and the L-type Ca2+ channel agonist S(-) Bay K 8644 could stimulate ecdysteroid secretion. The A23187-induced ecdysteroid secretion was partially inhibited by Rp-cAMPS. The combined results indicate that Ca2+ and cAMP signaling pathways can cooperatively, as well as independently, stimulate ecdysteroid secretion from the PGs.

Crosstalk between protein kinase A and growth factor receptor signaling pathways in arterial smooth muscle

Crosstalk between the cyclic AMP-dependent protein kinase (PKA) and growth factor receptor signaling is one of many emerging concepts of crosstalk in signal transduction. Understanding of PKA crosstalk may have important implications for studies of crosstalk between other, less well known, signaling pathways. This review focuses on PKA crosstalk in arterial smooth muscle. Proliferation and migration of arterial smooth muscle cells (SMCs) contribute to the thickening of the blood vessel wall that occurs in many types of cardiovascular disease. PKA potently inhibits SMC proliferation by antagonizing the major mitogenic signaling pathways induced by growth factors in SMCs. PKA also inhibits growth factor-induced SMC migration. An intricate crosstalk between PKA and the mitogen-activated protein kinase (MAPK/ERK) pathway, the p70 S6 kinase pathway and cyclin-dependent kinases has been described. Further, PKA regulates expression of growth regulatory molecules. The result of PKA activation in SMCs is the potent inhibition of cell cycle traverse and SMC migration. In this review, we discuss recent advances in our understanding of the crosstalk between PKA and signaling pathways induced by growth factor receptors in SMCs, and where relevant, in other cell types in which interesting examples of PKA crosstalk have been described.

Sequencing and exon mapping of the inositol 1,4,5-trisphosphate receptor cDNA from Drosophila embryos suggests the presence of differentially regulated forms of RNA and protein

A single gene appears to code for the inositol 1,4,5-trisphosphate receptor (itpr) in Drosophila melanogaster, as compared to three known genes in mammals. Expression of the itpr gene in Drosophila occurs in a wide range of tissues and developmental stages, suggesting its requirement during diverse cellular and physiological processes. A head cDNA for the Drosophila IP3R has previously been cloned and sequenced. Here we present and analyse the sequence of cDNAs encoding the complete IP3R, obtained from embryonic stages. The embryonic cDNA is 10525bp long and is a splice variant of the head cDNA. It differs from the latter in three main respects. It has longer 5' and 3' untranslated regions, two potential casein kinase II sites are missing in the embryo form and it contains an alternate exon which results in the replacement of three residues (VHF) in the head form by five residues (GVGHSV) in the embryo form. The significance of these changes is discussed. An exon-intron map of the gene derived from sequencing of intron-containing genomic fragments is also presented. The gene has a total of 11 introns, of which more than half are clustered in a region of the modulatory domain of the IP3R.

Agonist-dependent phosphorylation of the inositol 1,4,5-trisphosphate receptor: A possible mechanism for agonist-specific calcium oscillations in pancreatic acinar cells

The properties of inositol 1,4,5-trisphosphate (IP3)-dependent intracellular calcium oscillations in pancreatic acinar cells depend crucially on the agonist used to stimulate them. Acetylcholine or carbachol (CCh) cause high-frequency (10-12-s period) calcium oscillations that are superimposed on a raised baseline, while cholecystokinin (CCK) causes long-period (>100-s period) baseline spiking. We show that physiological concentrations of CCK induce rapid phosphorylation of the IP3 receptor, which is not true of physiological concentrations of CCh. Based on this and other experimental data, we construct a mathematical model of agonist-specific intracellular calcium oscillations in pancreatic acinar cells. Model simulations agree with previous experimental work on the rates of activation and inactivation of the IP3 receptor by calcium (DuFour, J.-F., I.M. Arias, and T.J. Turner. 1997. J. Biol. Chem. 272:2675-2681), and reproduce both short-period, raised baseline oscillations, and long-period baseline spiking. The steady state open probability curve of the model IP3 receptor is an increasing function of calcium concentration, as found for type-III IP3 receptors by Hagar et al. (Hagar, R.E., A.D. Burgstahler, M.H. Nathanson, and B.E. Ehrlich. 1998. Nature. 396:81-84). We use the model to predict the effect of the removal of external calcium, and this prediction is confirmed experimentally. We also predict that, for type-III IP3 receptors, the steady state open probability curve will shift to lower calcium concentrations as the background IP3 concentration increases. We conclude that the differences between CCh- and CCK-induced calcium oscillations in pancreatic acinar cells can be explained by two principal mechanisms: (a) CCK causes more phosphorylation of the IP3 receptor than does CCh, and the phosphorylated receptor cannot pass calcium current; and (b) the rate of calcium ATPase pumping and the rate of calcium influx from the outside the cell are greater in the presence of CCh than in the presence of CCK.

Isoproterenol potentiates alpha-adrenergic and muscarinic receptor-mediated Ca2+ response in rat parotid cells

The effects of the cAMP pathway on the Ca2+ response elicited by phospholipase C-coupled receptor stimulations were studied in rat parotid cells. Although 1 microM isoproterenol (Iso) itself had no effect on the cytosolic Ca2+ concentration, the pretreatment with Iso potentiated Ca2+ responses evoked by phenylephrine. The potentiating effect of Iso was attributed to a shifting of the concentration-response curves of phenylephrine to the left and an increase in the maximal response. Half-maximal potentiation occurred at 3 nM Iso. Iso also potentiated the Ca2+ response elicited by carbachol. The potentiating effect of Iso was mimicked by forskolin (10 microM) and dibutyryl adenosine 3',5'-cyclic monophosphate (2 mM) and was blocked by 10 microM H-89. Iso potentiated the phenylephrine-induced Ca2+ response in the absence of extracellular Ca2+, but Iso did not increase the inositol trisphosphate (IP3) production induced by phenylephrine. These results suggest that the potentiation of the Ca2+ response can be attributed to a sensitization of IP3 receptors by cAMP-dependent protein kinase.

Phosphorylation of the inositol 1,4,5-trisphosphate receptor by cyclic nucleotide-dependent kinases in vitro and in rat cerebellar slices in situ

We have examined cyclic nucleotide-regulated phosphorylation of the neuronal type I inositol 1,4,5-trisphosphate (IP3) receptor immunopurified from rat cerebellar membranes in vitro and in rat cerebellar slices in situ. The isolated IP3 receptor protein was phosphorylated by both cAMP- and cGMP-dependent protein kinases on two distinct sites as determined by thermolytic phosphopeptide mapping, phosphopeptide 1, representing Ser-1589, and phosphopeptide 2, representing Ser-1756 in the rat protein (Ferris, C. D., Cameron, A. M., Bredt, D. S., Huganir, R. L., and Snyder, S. H. (1991) Biochem. Biophys. Res. Commun. 175, 192-198). Phosphopeptide maps show that cAMP-dependent protein kinase (PKA) labeled both sites with the same time course and same stoichiometry, whereas cGMP-dependent protein kinase (PKG) phosphorylated Ser-1756 with a higher velocity and a higher stoichiometry than Ser-1589. Synthetic decapeptides corresponding to the two phosphorylation sites (peptide 1, AARRDSVLAA (Ser-1589), and peptide 2, SGRRESLTSF (Ser-1756)) were used to determine kinetic constants for the phosphorylation by PKG and PKA, and the catalytic efficiencies were in agreement with the results obtained by in vitro phosphorylation of the intact protein. In cerebellar slices prelabeled with [32P]orthophosphate, activation of endogenous kinases by incubation in the presence of cAMP/cGMP analogues and specific inhibitors of PKG and PKA induced in both cases a 3-fold increase in phosphorylation of the IP3 receptor. Thermolytic phosphopeptide mapping of in situ labeled IP3 receptor by PKA showed labeling on the same sites (Ser-1589 and Ser-1756) as in vitro. In contrast to the findings in vitro, PKG preferentially phosphorylated Ser-1589 in situ. Because both PKG and the IP3 receptor are specifically enriched in cerebellar Purkinje cells, PKG may be an important IP3 receptor regulator in vivo.

Inhibition of receptor-mediated calcium responses by corticotrophin-releasing hormone in the CATH.a cell line

A region of the brain believed to be important in the CNS response to stress is the locus coeruleus, the predominant site of noradrenergic cell bodies. Corticotrophin releasing hormone (CRH) is the primary hypothalamic releasing hormone responsible for the activation of the pituitary-adrenal axis in response to stress and, in this study, we employed a locus coeruleus-like cell line, CATH.a, to investigate the modulation of receptor signalling pathways by CRH. Pituitary adenylyl cyclase-activating polypeptide (PACAP) (10 nM), vasoactive intestinal peptide (VIP) (1 microM) and carbachol (1 mM) produced transient increases in intracellular [Ca2+]. The inhibition of the carbachol (1 mM) response by CRH was concentration-dependent (EC50 = 154 +/- 1.8 nM). Calcium responses to sub-maximally effective concentrations of PACAP (5 nM), VIP (400 nM) and carbachol (1 mM) were abolished by prior exposure to CRH (1 microM). At the concentrations employed, CRH and VIP both substantially increased intracellular [3H]-cyclic AMP accumulation. The adenylyl cyclase activator forskolin (10 microM) was also effective at eliminating the agonist-induced calcium responses. Incubation with the cell permeant cyclic AMP analogue dibutyryl cyclic AMP (dbcAMP) (1 mM), an activator of protein kinase A (PKA), for 12 min prior to agonist exposure similarly abolished the intracellular calcium response to carbachol. Carbachol increased [3H]-inositol phosphate ([3H]-IP) accumulation to a maximum of 2.4 +/- 0.11-fold basal (EC50 = 6.75 +/- 0.26 microM). PACAP produced a much greater accumulation (19.9 +/- 2.1 fold basal; EC50 = 24 nM). In the presence of forskolin (10 microM), neither carbachol- nor PACAP-induced [3H]-IP accumulation was significantly different from in its absence. These results demonstrate that CRH inhibits receptor-mediated intracellular calcium responses in a locus coeruleus-like cell line possibly via activation of PKA. This modulation could be important in controlling neuronal function in vivo in stressful situations in which the levels of CRH are increased in the locus coeruleus.

The versatility of inositol phosphates as cellular signals

Cells from across the phylogenetic spectrum contain a variety of inositol phosphates. Many different functions have been ascribed to this group of compounds. However, it is remarkable how frequently several of these different inositol phosphates have been linked to various aspects of signal transduction. Therefore, this review assesses the evidence that inositol phosphates have evolved into a versatile family of second messengers.

Inositol trisphosphate receptors: Ca2+-modulated intracellular Ca2+ channels

The three subtypes of inositol trisphosphate (InsP3) receptor expressed in mammalian cells are each capable of forming intracellular Ca2+ channels that are regulated by both InsP3 and cytosolic Ca2+. The InsP3 receptors of many, though perhaps not all, tissues are biphasically regulated by cytosolic Ca2+: a rapid stimulation of the receptors by modest increases in Ca2+ concentration is followed by a slower inhibition at higher Ca2+ concentrations. Despite the widespread occurrence of this form of regulation and the belief that it is an important element of the mechanisms responsible for the complex Ca2+ signals evoked by physiological stimuli, the underlying mechanisms are not understood. Both accessory proteins and Ca2+-binding sites on InsP3 receptors themselves have been proposed to mediate the effects of cytosolic Ca2+ on InsP3 receptor function, but the evidence is equivocal. The effects of cytosolic Ca2+ on InsP3 binding and channel opening, and the possible means whereby the effects are mediated are discussed in this review.

New developments in the molecular pharmacology of the myo-inositol 1,4,5-trisphosphate receptor

Receptor-mediated activation of phospholipase C to generate inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] is a ubiquitous signalling pathway in mammalian systems. A family of three IP3 receptor subtype monomers form functional tetramers, which act as effectors for Ins(1,4,5)P3, providing a ligand-gated channel that allows Ca2+ ions to move between cellular compartments. As IP3 receptors are located principally, although not exclusively, in the endoplasmic reticular membrane, Ins(1,4,5)P3 is considered to be a second messenger that mobilizes Ca2+ from intracellular stores. Ca2+ store mobilization by Ins(1,4,5)P3 can be shown to contribute to a variety of physiological and pathophysiological phenomena, and therefore the IP3 receptor represents a novel, potential pharmacological target. In this article, Rob Wilcox and colleagues review recent developments in IP3 receptor pharmacology, with particular emphasis on ligand molecular recognition by this receptor-channel complex. The potential for designing non-inositol phosphate-based agonists and antagonists is also discussed.

PACAP activates calcium influx-dependent and -independent pathways to couple met-enkephalin secretion and biosynthesis in chromaffin cells

Pituitary adenylate cyclase activating polypeptide-27 (PACAP-27) caused a dose-dependent increase in met-enkephalin secretion and increased production of met-enkephalin peptide and proenkephalin A (PEnk) mRNA in bovine chromaffin cells, at concentrations as low as 300 pM. PACAP-38 was less potent than PACAP-27, but had similar effects. Vasoactive intestinal polypeptide (VIP) (1-100 nM) was without appreciable effect on either enkephalin secretion or biosynthesis, implicating PACAP type I receptors in PACAP-stimulated enkephalin secretion and synthesis. PACAP type I receptors can activate adenylate cyclase and stimulate phospholipase C through heterotrimeric G protein interactions, leading to increased intracellular cyclic AMP (cAMP), inositol triphosphate (IP3)-mediated calcium mobilization, and calcium- and diacylglycerol (DAG)-mediated protein kinase C (PKC) activation. Enkephalin secretion evoked by 10-100 nM PACAP-27 was not inhibited by 1 microM (-)-202-791, an L-type specific dihydropyridine calcium channel blocker, but was inhibited 65-80% by the arylalkylamine calcium channel blocker D600. Forty mM potassium-evoked secretion was inhibited > 90% by both D600 and (-)-202-791, 25 microM forskolin-induced secretion was blocked < 50% by D600 and was unaffected by (-)-202-791, and 100 nM phorbol myristate acetate (PMA)-induced secretion was unaffected by either D600 or (-)-202-791. Enkephalin biosynthesis was increased by 10 nM PACAP-27, as measured by increased met-enkephalin pentapeptide content and PEnk A mRNA levels. PACAP-, forskolin-, and PMA-stimulated enkephalin synthesis were not blocked by D600 or (-)-202-791. Elevated potassium-induced enkephalin biosynthesis upregulation was completely blocked by either D600 or (-)-202-791 at the same concentrations. PACAP acting through type I PACAP receptors couples calcium influx-dependent enkephalin secretion and calcium influx-independent enkephalin biosynthesis in chromaffin cells. Restriction of the effects of enhanced calcium influx to stimulation of secretion, but not of biosynthesis, is unique to PACAP. By contrast, potassium-induced enkephalin biosynthesis upregulation is completely calcium influx dependent, specifically via calcium influx through L-type calcium channels. We propose that subpopulations of voltage-dependent calcium channels are differentially linked to intracellular signal transduction pathways that control neuropeptide gene expression and secretion in chromaffin cells.