Calcium modulation of inositol 1,4,5-trisphosphate-induced calcium release from neuroblastoma x glioma hybrid (NG108-15) microsomes

Zn2+ is essential for Ca2+ oscillations in mouse eggs

Changes in the intracellular concentration of free calcium (Ca2+) underpin egg activation and initiation of development in animals and plants. In mammals, the Ca2+ release is periodical, known as Ca2+ oscillations, and mediated by the type 1 inositol 1,4,5-trisphosphate receptor (IP3R1). Another divalent cation, zinc (Zn2+), increases exponentially during oocyte maturation and is vital for meiotic transitions, arrests, and polyspermy prevention. It is unknown if these pivotal cations interplay during fertilization. Here, using mouse eggs, we showed that basal concentrations of labile Zn2+ are indispensable for sperm-initiated Ca2+ oscillations because Zn2+-deficient conditions induced by cell-permeable chelators abrogated Ca2+ responses evoked by fertilization and other physiological and pharmacological agonists. We also found that chemically or genetically generated eggs with lower levels of labile Zn2+ displayed reduced IP3R1 sensitivity and diminished ER Ca2+ leak despite the stable content of the stores and IP3R1 mass. Resupplying Zn2+ restarted Ca2+ oscillations, but excessive Zn2+ prevented and terminated them, hindering IP3R1 responsiveness. The findings suggest that a window of Zn2+ concentrations is required for Ca2+ responses and IP3R1 function in eggs, ensuring optimal response to fertilization and egg activation.

Zn2+ is Essential for Ca2+ Oscillations in Mouse Eggs

Changes in the intracellular concentration of free calcium (Ca2+) underpin egg activation and initiation of development in animals and plants. In mammals, the Ca2+ release is periodical, known as Ca2+ oscillations, and mediated by the type 1 inositol 1,4,5-trisphosphate receptor (IP3R1). Another divalent cation, zinc (Zn2+), increases exponentially during oocyte maturation and is vital for meiotic transitions, arrests, and polyspermy prevention. It is unknown if these pivotal cations interplay during fertilization. Here, using mouse eggs, we showed that basal concentrations of labile Zn2+ are indispensable for sperm-initiated Ca2+ oscillations because Zn2+-deficient conditions induced by cell-permeable chelators abrogated Ca2+ responses evoked by fertilization and other physiological and pharmacological agonists. We also found that chemically- or genetically generated eggs with lower levels of labile Zn2+ displayed reduced IP3R1 sensitivity and diminished ER Ca2+ leak despite the stable content of the stores and IP3R1 mass. Resupplying Zn2+ restarted Ca2+ oscillations, but excessive Zn2+ prevented and terminated them, hindering IP3R1 responsiveness. The findings suggest that a window of Zn2+ concentrations is required for Ca2+ responses and IP3R1 function in eggs, ensuring optimal response to fertilization and egg activation.

Functional determination of calcium-binding sites required for the activation of inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate receptors (IP3Rs) initiate a diverse array of physiological responses by carefully orchestrating intracellular calcium (Ca2+) signals in response to various external cues. Notably, IP3R channel activity is determined by several obligatory factors, including IP3, Ca2+, and ATP. The critical basic amino acid residues in the N-terminal IP3-binding core (IBC) region that facilitate IP3 binding are well characterized. In contrast, the residues conferring regulation by Ca2+ have yet to be ascertained. Using comparative structural analysis of Ca2+-binding sites identified in two main families of intracellular Ca2+-release channels, ryanodine receptors (RyRs) and IP3Rs, we identified putative acidic residues coordinating Ca2+ in the cytosolic calcium sensor region in IP3Rs. We determined the consequences of substituting putative Ca2+ binding, acidic residues in IP3R family members. We show that the agonist-induced Ca2+ release, single-channel open probability (P0), and Ca2+ sensitivities are markedly altered when the negative charge on the conserved acidic side chain residues is neutralized. Remarkably, neutralizing the negatively charged side chain on two of the residues individually in the putative Ca2+-binding pocket shifted the Ca2+ required to activate IP3R to higher concentrations, indicating that these residues likely are a component of the Ca2+ activation site in IP3R. Taken together, our findings indicate that Ca2+ binding to a well-conserved activation site is a common underlying mechanism resulting in increased channel activity shared by IP3Rs and RyRs.

The ER-mitochondria Ca2+ signaling in cancer progression: Fueling the monster

Cancer is a leading cause of death worldwide. All major tumor suppressors and oncogenes are now recognized to have fundamental connections with metabolic pathways. A hallmark feature of cancer cells is a reprogramming of their metabolism even when nutrients are available. Increasing evidence indicates that most cancer cells rely on mitochondrial metabolism to sustain their energetic and biosynthetic demands. Mitochondria are functionally and physically coupled to the endoplasmic reticulum (ER), the major calcium (Ca²⁺) storage organelle in mammalian cells, through special domains known as mitochondria-ER contact sites (MERCS). In this domain, the release of Ca²⁺ from the ER is mainly regulated by inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs), a family of Ca²⁺ release channels activated by the ligand IP3. IP3R mediated Ca²⁺ release is transferred to mitochondria through the mitochondrial Ca²⁺ uniporter (MCU). Once in the mitochondrial matrix, Ca²⁺ activates several proteins that stimulate mitochondrial performance. The role of IP3R and MCU in cancer, as well as the other proteins that enable the Ca²⁺ communication between these two organelles is just beginning to be understood. Here, we describe the function of the main players of the ER mitochondrial Ca²⁺ communication and discuss how this particular signal may contribute to the rise and development of cancer traits.

Structural basis for the regulation of inositol trisphosphate receptors by Ca2+ and IP3

Inositol trisphosphate receptors (IP₃R) are ubiquitous Ca²⁺-permeable channels that mediate release of Ca²⁺ from the endoplasmic reticulum to regulate numerous processes including cell division, cell death, differentiation and fertilization. IP₃R is activated by both IP₃ and its permeant ion Ca²⁺. At high concentrations, however, Ca²⁺ inhibits activity ensuring precise spatiotemporal control over intracellular Ca²⁺. Despite extensive characterization of IP₃R, the mechanisms by which these molecules control channel gating have remained elusive. Here, we present structures of full-length human type 3 IP₃R in ligand-bound and ligand-free states. Multiple IP₃-bound structures demonstrate that the large cytoplasmic domain provides a platform for propagation of long-range conformational changes to the ion conduction gate. Structures in the presence of Ca²⁺ reveal two Ca²⁺ binding sites that induce the disruption of numerous interactions between subunits, thereby inhibiting IP₃R. These structures thus begin to provide a mechanistic basis for understanding the regulation of IP₃R.

Cytoplasmic calcium oscillations and store-operated calcium influx

Intracellular calcium oscillations have fascinated scientists for decades. They provide an important cellular signal which, unlike most signalling mechanisms, is digitally encoded. While it is generally agreed that oscillations most frequently arise from cyclical release and re-uptake of intracellularly stored calcium, it is becoming increasingly clear that influx of calcium across the plasma membrane also plays a critical role in their maintenance and even in delivering their signal to the correct cellular locus. In this review we will discuss the role played by Ca(2+) entry mechanisms in Ca(2+) oscillations, and approaches to understanding the molecular nature of this Ca(2+) entry pathway.

Identification of Li+ binding sites and the effect of Li+ treatment on phospholipid composition in human neuroblastoma cells: a 7Li and 31P NMR study

Li(+) binding in subcellular fractions of human neuroblastoma SH-SY 5 Y cells was investigated using (7)Li NMR spin-lattice (T(1)) and spin-spin (T(2)) relaxation measurements, as the T(1)/T(2) ratio is a sensitive parameter of Li(+) binding. The majority of Li(+) binding occurred in the plasma membrane, microsomes, and nuclear membrane fractions as demonstrated by the Li(+) binding constants and the values of the T(1)/T(2) ratios, which were drastically larger than those observed in the cytosol, nuclei, and mitochondria. We also investigated by (31)P NMR spectroscopy the effects of chronic Li(+) treatment for 4--6 weeks on the phospholipid composition of the plasma membrane and the cell homogenate and found that the levels of phosphatidylinositol and phosphatidylserine were significantly increased and decreased, respectively, in both fractions. From these observations, we propose that Li(+) binding occurs predominantly to membrane domains, and that chronic Li(+) treatment alters the phospholipid composition at these membrane sites. These findings support those from clinical studies that have indicated that Li(+) treatment of bipolar patients results in irregularities in Li(+) binding and phospholipid metabolism. Implications of our observations on putative mechanisms of Li(+) action, including the cell membrane abnormality, the inositol depletion and the G-protein hypotheses, are discussed.

Effects of the new ethacrynic acid derivative SA9000 on intraocular pressure in cats and monkeys

To evaluate the pharmacological characteristics of the new ethacrynic acid (ECA) derivative SA9000, we examined its ocular hypotensive effects in cats and cynomolgus monkeys, its corneal toxicity in rabbits, and its binding affinities for forty-three receptors, ion channels, and second messenger systems. A 20 microl injection into the anterior chamber of eye (intracameral injection) of 0.1 mM SA9000 significantly reduced intraocular pressure (IOP) 3.8 mmHg in cats. A 10 microl intracameral injection of 1 mM SA9000 significantly reduced IOP 7 mmHg in living monkeys without evidence of in vivo (or in vitro) toxicity. The ocular hypotensive effect of SA9000 in monkeys was greater than that of ECA. The morphology of corneal endothelial and epithelial cells in rabbit eyes after intracameral injection of SA9000 was observed using electron microphotography. SA9000 at 2 mM did not induce any abnormalities, indicating that it has no corneal toxicity at a concentration higher than the minimum needed for an ocular hypotensive effect (1 mM). SA9000 at 0.01 mM showed negligible binding affinity for, or inhibition of, forty-three different receptors, ion channel proteins, and second messenger systems. These findings indicate that SA9000 has the potential to be both effective and safe as an ocular hypotensive drug, although the mechanism of action remains unclear.

PKA induces Ca2+ release and enhances ciliary beat frequency in a Ca2+-dependent and -independent manner

The intent of this work was to evaluate the role of cAMP in regulation of ciliary activity in frog mucociliary epithelium and to examine the possibility of cross talk between the cAMP- and Ca2+-dependent pathways in that regulation. Forskolin and dibutyryl cAMP induced strong transient intracellular Ca2+ concentration ([Ca2+]i) elevation and strong ciliary beat frequency enhancement with prolonged stabilization at an elevated plateau. The response was not affected by reduction of extracellular Ca2+ concentration. The elevation in [Ca2+]i was canceled by pretreatment with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM, thapsigargin, and a phospholipase C inhibitor, U-73122. Under those experimental conditions, forskolin raised the beat frequency to a moderately elevated plateau, whereas the initial strong rise in frequency was completely abolished. All effects were canceled by H-89, a selective protein kinase A (PKA) inhibitor. The results suggest a dual role for PKA in ciliary regulation. PKA releases Ca2+ from intracellular stores, strongly activating ciliary beating, and, concurrently, produces moderate prolonged enhancement of the beat frequency by a Ca2+-independent mechanism.

Regulation of protein kinase C betaII by its C2 domain

The C2 domain serves as a membrane-targeting module in a diverse group of proteins that includes the conventional protein kinase Cs. This work examines the mechanism by which the C2 domain targets protein kinase C to membranes. Molecular modeling identified two highly-charged surfaces on the C2 domain of protein kinase C betaIotaIota: the Ca2+ binding site which contains five aspartates and a basic face positioned behind the Ca2+ site that contains seven lysine residues. Both surfaces were mutated to assess their role in Ca2+-dependent membrane binding. Surprisingly, removal of four positive charges on the basic face had no effect on protein kinase C's lipid or Ca2+ sensitivity, revealing that the basic face does not provide determinants involved in lipid binding, nor is it positioned close enough to the membrane to enhance nonspecific recruitment by its electropositive face. In contrast, replacement of two negative charges with two positive charges in the Ca2+ binding site decreased protein kinase C's affinity both for Ca2+ and for anionic lipids by several orders of magnitude. The dramatic reduction in electronegative potential resulting from this mutation did not increase protein kinase C's affinity for acidic membranes in the absence of Ca2+, revealing that simple charge neutralization does not account for how Ca2+ increases protein kinase C's affinity for anionic membranes. Our data suggest that (1) the membrane interaction surface of the C2 domain is localized to the Ca2+-binding site, with the positive face positioned away from the membrane, and (2) the Ca2+ site does not serve as a simple electrostatic switch.

A putative phosphatidylserine binding motif is not involved in the lipid regulation of protein kinase C

Protein kinase C is specifically regulated by diacylglycerol and the amino phospholipid, phosphatidylserine. The molecular basis for the phosphatidylserine specificity was recently proposed to arise from the presence of a putative phosphatidylserine binding motif, FXFXLKXXXKXR, localized in the C2 domain of protein kinase C (Igarashi, K., Kaneda, M., Yamaji, A., Saido, T. C., Kikkawa, U., Ono, U., Inoue, K., and Umeda, M. (1995) J. Biol. Chem. 270, 29075-29078). To determine whether this motif mediates the interaction of protein kinase C with phosphatidylserine, the carboxyl-terminal basic residues were mutated to Ala in protein kinase C betaII (K236A and R238A), and the phosphatidylserine regulation of the mutant enzyme was examined. Membrane binding and activity measurements revealed that the phosphatidylserine regulation for the mutant protein was indistinguishable from that of wild-type protein kinase C. Specifically, neither the apparent membrane affinity for phosphatidylserine-containing membranes in the presence or absence of diacylglycerol nor the phosphatidylserine-dependence for activation was affected by removal of the conserved basic residues at the carboxyl terminus of the consensus sequence. In addition, a synthetic peptide corresponding to the amino terminus of the consensus sequence (FTFNVK) had no effect on the concentration of phosphatidylserine resulting in half-maximal activation of protein kinase C. These results reveal that the carboxyl-terminal basic residues in the consensus motif FXFXLKXXXKXR are not responsible for the phosphatidylserine selectivity of protein kinase C and that, furthermore, the region of the C2 domain containing this motif is not involved in the membrane binding of protein kinase C.

Na(+)-K(+)-ATPase in frog esophagus mucociliary cell membranes: inhibition by protein kinase C activation

We examined protein kinase C (PKC)-dependent regulation of Na(+)-K(+)-ATPase in frog mucociliary cells. Activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) or 1,2-dioctanoyl-sn-glycerol (diC8) either in intact cells or isolated membranes resulted in a specific inhibition of Na(+)-K(+)-ATPase activity by approximately 25-45%. The inhibitory effects in membranes exhibited time dependence and dose dependence [half-maximal inhibition concentration (IC50) = 0.5 +/- 0.1 nM and 2.4 +/- 0.2 microM, respectively, for TPA and diC8] and were not influenced by Ca2+. Analysis of the ouabain inhibition pattern revealed the presence of two Na(+)-K(+)-ATPase isoforms with IC50 values for cardiac glycoside of 2.6 +/- 0.8 nM and 409 +/- 65 nM, respectively. Most importantly, the isoform possessing a higher affinity for ouabain was almost completely inhibited by TPA, whereas its counterpart was hardly sensitive to the PKC activator. The results suggest that, in frog mucociliary cells, PKC regulates Na(+)-K(+)-ATPase and that this action is related to the specific Na(+)-K(+)-ATPase isoform.

Ca2+ differentially regulates conventional protein kinase Cs' membrane interaction and activation

The regulation of conventional protein kinase Cs by Ca2+ was examined by determining how this cation affects the enzyme's 1) membrane binding and catalytic function and 2) conformation. In the first part, we show that significantly lower concentrations of Ca2+ are required to effect half-maximal membrane binding than to half-maximally activate the enzyme. The disparity between binding and activation kinetics is most striking for protein kinase C betaII, where the concentration of Ca2+ promoting half-maximal membrane binding is approximately 40-fold higher than the apparent Km for Ca2+ for activation. In addition, the Ca2+ requirement for activation of protein kinase C betaII is an order of magnitude greater than that for the alternatively spliced protein kinase C betaI; these isozymes differ only in 50 amino acids at the carboxyl terminus, revealing that residues in the carboxyl terminus influence the enzyme's Ca2+ regulation. In the second part, we use proteases as conformational probes to show that Ca2+dependent membrane binding and Ca2+-dependent activation involve two distinct sets of structural changes in protein kinase C betaII. Three separate domains spanning the entire protein participate in these conformational changes, suggesting significant interdomain interactions. A highly localized hinge motion between the regulatory and catalytic halves of the protein accompanies membrane binding; release of the carboxyl terminus accompanies the low affinity membrane binding mediated by concentrations of Ca2+ too low to promote catalysis; and exposure of the amino-terminal pseudosubstrate and masking of the carboxyl terminus accompany catalysis. In summary, these data reveal that structural determinants unique to each isozyme of protein kinase C dictate the enzyme's Ca2+-dependent affinity for acidic membranes and show that, surprisingly, some of these determinants are in the carboxyl terminus of the enzyme, distal from the Ca2+-binding site in the amino-terminal regulatory domain.

Contribution of protein kinase C to the phosphorylation of rhodopsin in intact retinas

Similar to other G protein-coupled receptors, the visual receptor, rhodopsin, is phosphorylated by both a substrate-regulated kinase, rhodopsin kinase, and a second messenger-regulated kinase, protein kinase C. In the present study, the extent of involvement of protein kinase C in the light-dependent phosphorylation of rhodopsin in intact retinas was assessed using a specific activator (phorbol ester) and specific inhibitor (calphostin C) of protein kinase C. Kinetic analysis of rhodopsin phosphorylation following different illumination conditions revealed that hyperactivation of protein kinase C with phorbol ester resulted in a relative increase in rhodopsin phosphorylation that peaked 10-15 min after the onset of illumination. Following this period, the rate of rhodopsin dephosphorylation was increased in the phorbol ester-treated retinas, so that by about 30 min the amount of phosphorylation was similar to that in control retinas. Treatment of retinas with calphostin C, a potent regulatory domain-directed inhibitor of protein kinase C, resulted in an approximately 50% reduction in the light-dependent phosphorylation of rhodopsin. This inhibitor had no effect on the activity of rhodopsin kinase in vitro. Last, we show that frog rhodopsin is phosphorylated in vitro by protein kinase C from frog rod outer segments, indicating that this kinase could directly modulate rhodopsin in vivo. In conclusion, the present results reveal that the kinetics of rhodopsin phosphorylation/dephosphorylation differ markedly, depending on whether protein kinase C or rhodopsin kinase activity dominates, and that, under the conditions studied, protein kinase C contributes to approximately half of the phosphorylation of rhodopsin in intact frog retinas.

Protein kinase C induced calcium influx and sustained enhancement of ciliary beating by extracellular ATP

The major purpose of this work was to determine protein kinase C (PKC) influence on ciliary beat frequency (CBF) and to assess participation of PKC in purinergic ciliary stimulation. The experiments were performed by simultaneous measurement of [Ca2+]i and CBF on tissue culture of frog esophagus epithelium. The PKC activators TPA and DiC8 produced significant elevation of [Ca2+]i and strong frequency enhancement. The calcium elevation was inhibited by lowering the extracellular calcium level, or by La3+, but was unaffected by verapamil and the phospholipase C inhibitor U-73122, suggesting that Ca2+ influx was via non-voltage-operated calcium channels. The inhibition of [Ca2+]i elevation resulted in corresponding inhibition of CBF enhancement. The effect of TPA was blocked by the selective PKC inhibitors chelerythrine, calphostin C, and GF109203X, and by the enzyme downregulation. The downregulation of PKC, or the enzyme inhibitors did not affect the immediate response to extracellular ATP but caused rapid decay of initially stimulated [Ca2+]i and CBF to the basal level. These results suggest that PKC produces CBF enhancement via activation of calcium influx through non-voltage-operated calcium channels. This calcium influx seems to be responsible for the duration of ciliary stimulation produced by the extracellular ATP.

Inositol 1,4,5-trisphosphate-induced Ca2+ release is regulated by cytosolic Ca2+ in intact skeletal muscle

Microinjection of inositol 1,4,5-trisphosphate (InsP3) into intact skeletal muscle fibers isolated from frogs (Rana temporaria) increased resting cytosolic Ca2+ concentration ([Ca2+]i) as measured by double-barreled Ca2+-selective microelectrodes. In contrast, microinjection of inositol 1-phosphate, inositol 1,4-biphosphate, and inositol 1,4,5,6-tetrakisphosphate did not induce changes in [Ca2+]i. Incubation in low-Ca2+ solution, or in the presence of L-type Ca2+ channel blockers did not affect InsP3-induced release of cytosolic Ca2+. Neither ruthenium red, a blocker of ryanodine receptor Ca2+-release channels, nor cytosolic Mg2+, a known inhibitor of the Ca2+-induced Ca2+-release process, modified the InsP3-induced release of cytosolic Ca2+. However, heparin, a blocker of InsP3 receptors, inhibited InsP3-induced release of cytosolic Ca2+. Also, pretreatment with dantrolene or azumulene, two inhibitors of cytosolic Ca2+ release, reduced [Ca2+]i, and prevented InsP3 from inducing release of cytosolic Ca2+. Incubation in caffeine or lengthening of the muscle increased [Ca2+]i and enhanced the ability of InsP3 to induce release of cytosolic Ca2+. These results indicate that InsP3, at physiological concentrations, induces Ca2+ release in intact muscle fibers, and suggest that the InsP3-induced Ca2+ release is regulated by [Ca2+]i. A Ca2+-dependent effect of InsP3 on cytosolic Ca2+ release could be of importance under physiological or pathophysiological conditions associated with alterations in cytosolic Ca2+ homeostasis.

Inositol trisphosphate-dependent and -independent Ca2+ mobilization pathways at the vacuolar membrane of Candida albicans

Vacuolar membrane vesicles were isolated from Candida albicans protoplasts, and marker enzyme assays were employed to identify the membranes as vacuolar in origin. The mechanisms of Ca2+ uptake and Ca2+ release at the vacuolar membrane were investigated. Ca2+ accumulation by vacuolar membrane vesicles can be generated via H+/Ca2+ antiport. The inside-acid pH is in turn generated by a vacuolar-type H(+)-ATPase, as demonstrated by the sensitivity of Ca2+ uptake to ionophores and the vacuolar H(+)-ATPase inhibitor bafilomycin A1. Vacuolar membrane vesicles exhibit two Ca2+ release pathways: one induced by inositol 1,4,5-trisphosphate (InsP3) and the other by inside-positive voltage. These two pathways are distinct with respect to the amount of Ca2+ released, the nature of response to successive stimuli, and their respective pharmacological profiles. The InsP3-gated pathway exhibits a K0.5 for InsP3 of 2.4 microM but is not activated by inositol 4,5-bisphosphate or inositol 1,3,4,5-tetrakisphosphate at concentrations up to 50 microM. Ca2+ release by InsP3 is blocked partially by low molecular weight heparin. Ca2+ released by the voltage-sensitive pathway occurs at membrane potentials estimated to be over a physiological range from 0 to 80 mV. The voltage-sensitive Ca2+ release pathway can be blocked by lanthanide ions and organic channel blockers such as ruthenium red and verapamil. Furthermore, the voltage-sensitive Ca2+ release pathway exhibits Ca(2+)-induced Ca2+ release. These findings are discussed in relation to the mechanism of Ca(2+)-mediated cellular signaling in C. albicans and other fungi.

Kinetics and localization of the phosphorylation of rhodopsin by protein kinase C

Protein kinase C isolated from retina catalyzes the stoichiometric phosphorylation of bovine rhodopsin. Enzymological studies using receptor in rod outer segment membranes stripped of peripheral proteins reveal that the phosphorylation is independent of receptor conformation or liganded state; the half-time for phosphorylation of unbleached (dark-adapted) rhodopsin, bleached (light-activated) rhodopsin, and opsin (chromophore removed) is the same. The phosphorylation by protein kinase C is Ca2+ and lipid regulated; the Km for Ca2+ decreases with increasing concentrations of membrane, consistent with known properties of Ca(2+)-regulated protein kinase Cs. The Km for ATP is 27 microM, with an optimal concentration for MgCl2 of approximately 1 mM. The phosphorylation of rhodopsin by protein kinase C is inhibited by the protein kinase C-selective inhibitor sangivamycin. Proteolysis by Asp-N reveals that all the protein kinase C phosphorylation sites are on the carboxyl terminus of the receptor. Cleavage with trypsin indicates that Ser338, the primary phosphorylation site of rhodopsin kinase, is not phosphorylated significantly; rather, the primary phosphorylation site of protein kinase C is on the membrane proximal half of the carboxyl terminus. The protein kinase C-catalyzed phosphorylation of rhodopsin is analogous to the ligand-independent phosphorylation of other G protein-coupled receptors that is catalyzed by second messenger-regulated kinases.

Dual calcium ion regulation of calcineurin by calmodulin and calcineurin B

The dependence of calcineurin on Ca2+ for activity is the result of the concerted action of calmodulin, which increases the turnover rate of the enzyme and modulates its response to Ca2+ transients, and of calcineurin B, which decreases the Km of the enzyme for its substrate. The calmodulin-stimulated protein phosphatase calcineurin is under the control of two functionally distinct, but structurally similar, Ca(2+)-regulated proteins, calmodulin and calcineurin B. The Ca(2+)-dependent activation of calcineurin by calmodulin is highly cooperative (Hill coefficient of 2.8-3), and the concentration of Ca2+ needed for half-maximum activation decreases from 1.3 to 0.6 microM when the concentration of calmodulin is increased from 0.03 to 20 microM. Conversely, the affinity of calmodulin for Ca2+ is increased by more than 2 orders of magnitude in the presence of a peptide corresponding to the calmodulin-binding domain of calcineurin A. Calmodulin increases the Vmax without changing the Km value of the enzyme. Unlike calmodulin, calcineurin B interacts with calcineurin A in the presence of EGTA, and Ca2+ binding to calcineurin B stimulates native calcineurin up to only 10% of the maximum activity achieved with calmodulin. The Ca(2+)-dependent activation of a proteolyzed derivative of calcineurin, calcineurin-45, which lacks the regulatory domain, was used to study the role of calcineurin B. Removal of the regulatory domain increases the Vmax of calcineurin, as does binding of calmodulin, but it also increases the affinity of calcineurin for Ca2+. Ca2+ binding to calcineurin B decreases the Km value of calcineurin without changing its Vmax.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of heat shock on intracellular calcium mobilization in neuroblastoma x glioma hybrid cells

The effect of heat shock on agonist-stimulated intracellular Ca2+ mobilization and the expression of heat shock protein 72 (hsp72) in neuroblastoma x glioma hybrid cells (NG 108-15 cells) were examined. Hsp72 was expressed at 6 h after heat shock (42.5 degrees C, 2 h), reached a maximum at 12 h, and decreased thereafter. Bradykinin-induced [Ca2+]i rise was attenuated to 28% of control by heat shock at 2 h after heat shock, and reversion to the control level was seen 12 h later. When the cells were treated with quercetin or antisense oligodeoxyribonucleotide against hsp72 cDNA, the synthesis of hsp72 was not induced by heat shock, whereas bradykinin-induced [Ca2+]i rise was abolished and the [Ca2+]i rise was not restored. Recovery from this stressed condition was evident when cells were stimulated by the Ca(2+)-ATPase inhibitor thapsigargin, even in the presence of either quercetin or antisense oligodeoxyribonucleotide. Inositol 1,4,5-trisphosphate (IP3) production was not altered by heat shock at 12 h after heat shock, whereas IP3 receptor binding activity was reduced to 45.3%. In the presence of quercetin or antisense oligodeoxyribonucleotide, IP3 receptor binding activity decreased and reached 27.2% of the control 12 h after heat shock. Our working thesis is that heat shock transiently suppresses the IP3-mediated intracellular Ca2+ signal transduction system and that hsp72 is involved in the recovery of bradykinin-induced [Ca2+]i rise.

The inositol 1,4,5-trisphosphate receptor: kinetic properties and regulation

Inositol 1,4,5-triphosphate (InsP3) is a second messenger responsible for the mobilization of intracellular Ca2+ after receptor-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate. InsP3 binds to a specific receptor located on the membrane of an intracellular compartment and opens a Ca2+ channel causing the cytosolic Ca2+ concentration to increase. Measurement of radiolabelled InsP3 binding and InsP3-induced Ca2+ release in parallel experiments indicated that the liver InsP3 receptor exists in two main states: an active state (A) and an inactive one (I). The "I" form of the receptor is found in the presence of high Ca2+ concentrations (above 1 microM). The binding properties of the "A" and the "I" states of the receptor have been characterized by analysing a membrane fraction enriched in InsP3 receptors. The inactive "I" state displays a high affinity (Kd = 2 nM) and slow rates of association and dissociation. The active state "A" of the receptor displays complex kinetic properties. The rate of association and the rate of dissociation of labelled InsP3 are rapid phenomena probably involving several components. The apparent Kd for the InsP3 binding is about 40 nM in a low Ca2+ medium. The affinity of the "A" state of the receptor is increased by Ca2+ (at concentrations lower than 0.5 microM) and by thiol reagents. The increase of the affinity of the receptor is due to a decrease of the dissociation rate constants. This lowers the threshold such that Ca2+ is released at lower concentrations of InsP3. These data indicate that the binding of InsP3 to its receptor is a complex phenomenon involving the transition among several states.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuromedin-B receptor transfected BALB/3T3 cells: signal transduction and effects of ectopic receptor expression on cell growth

Bombesin-like peptides including neuromedin B have been proposed as autocrine or paracrine growth factors for carcinomas. To examine signal transduction and regulation of cell growth by NMB, transfectants were created with the rat NMB receptor (NMB-R) gene in BALB/3T3 cells which do not express an endogenous bombesin peptide receptor. The resultant cell line, NMB-8, expresses 800,000 NMB binding sites/cell. Addition of NMB has a biphasic effect on [3H]thymidine ([3H]dT) incorporation in confluent and quiescent cells: up to 10 nM of NMB causes a 1.5-3-fold stimulation of [3H]dT incorporation, but at greater than 10 nM there is inhibition of [3H]dT incorporation, and at 100 nM of NMB there is inhibition of cell growth. NMB causes protracted increases in intracellular Ca2+, and pertussis toxin (PT)-insensitive phosphatidylinositol (PI) turnover. NMB-mediated increase in membrane phospholipase-C activity is stimulated by guanosine 5'-O-(3-thiotriphosphate). Arachidonate release is also activated by NMB in a PT-insensitive manner. Brief exposure to 12-tetradecanoylphorbol 13-acetate inhibits NMB-mediated PI turnover but not arachidonate release. Thus, in NMB-8 cells, distinct mechanisms govern NMB-mediated phospholipase-C activation and arachidonate release. Also, neuromedin-B is potentially a bifunctional regulator of cell growth.

Regulation of inositol 1,4,5-trisphosphate receptors in rat basophilic leukemia cells. I. Multiple conformational states of the receptor in a microsomal preparation

A detailed characterization of the inositol 1,4,5-trisphosphate (IP3) receptor in rat basophilic leukemia (RBL) cells, a neoplastic mast cell line, has been possible through the growth of solid RBL cell tumors which provide a rich source of IP3 receptor. Equilibrium binding studies show a 1.6 +/- 0.1 pmol/mg of protein maximal binding capacity for [3H]IP3 at optimal Ca2+ (10 microM). The specificity of the RBL cell IP3 receptor towards phosphoinositides, ATP and heparin parallels those previously described with excitable and nonexcitable tissues. [3H]IP3 binding is slightly enhanced from < 1 nM to 10 microM Ca2+ and inhibited by > 10 microM Ca2+. Kinetic and equilibrium studies provide evidence for at least two classes or conformational states of binding sites with pico- and nanomolar affinities. At nM concentrations of IP3, neither binding to the IP3 receptor nor IP3-induced Ca2+ efflux from permeabilized cells demonstrates cooperativity. In contrast, at pM concentrations, IP3 binding kinetics deviate from simple mass action suggesting a complex interaction among binding sites for IP3 on the receptor-channel oligomer. The mechanisms that regulate [3H]IP3 binding in RBL cells are unique when compared to what has been reported in other cells.

Dissection of bradykinin-evoked responses by buffering intracellular Ca2+ in neuroblastoma x glioma hybrid NG108-15 cells

Signal transduction pathways from bradykinin (BK) receptors were investigated in NG108-15 neuroblastoma x glioma hybrid cells by buffering the intracellular calcium (Ca2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a Ca2+ chelator. BK increased inositol-1,4,5-trisphosphate (Ins(1, 4,5)P3) formation at the same rate in the control and in BAPTA-acetoxy methyl ester (AM)-treated NG108-15 cells. However, a transient increase of intracellular Ca2+ concentrations in response to BK was significantly suppressed in Ca(2+)-buffered hybrid cells. Accordingly the BK-induced outward current was inhibited in BAPTA-AM-treated hybrid cells, while the subsequent inward current associated with a fall in membrane conductance was apparently increased. The initial phase of acetylcholine release from NG108-15 cells in response to BK was markedly inhibited in BAPTA-AM-treated coculture dishes when detected as miniature end-plate potentials of myotubes, though the late phase of acetylcholine secretion was observed. These results indicate that BK induces two distinct responses in NG108-15 cells: Ins(1,4,5)P3-dependent intracellular Ca2+ rise-sensitive and -insensitive components.

Calcium ion homeostasis in smooth muscle

Ca2+ plays an important role in the regulation of smooth-muscle contraction. In this review, we will focus on the various Ca(2+)-transport processes that contribute to the cytosolic Ca2+ concentration. Mainly the functional aspects will be covered. The smooth-muscle inositol 1,4,5-trisphosphate receptor and ryanodine receptor will be extensively discussed. Smooth-muscle contraction also depends on extracellular Ca2+ and both voltage- and Ca(2+)-release-activated plasma-membrane Ca2+ channels will be reviewed. We will finally discuss some functional properties of the Ca2+ pumps that remove Ca2+ from the cytoplasm and of the Ca2+ regulation of the nucleus.

Calcium as a coagonist of inositol 1,4,5-trisphosphate-induced calcium release

Inositol 1,4,5-trisphosphate (IP3)-induced calcium release from intracellular stores is a regulator of cytosolic-free calcium levels. The subsecond kinetics and regulation of IP3-induced calcium-45 release from synaptosome-derived microsomal vesicles were resolved by rapid superfusion. Extravesicular calcium acted as a coagonist, potentiating the transient IP3-induced release of calcium-45. Thus, rapid elevation of cytosolic calcium levels may trigger IP3-induced calcium release in vivo. Extravesicular calcium also produced a more slowly developing, reversible inhibition of IP3-induced calcium-45 release. Sequential positive and negative feedback regulation by calcium of IP3-induced calcium release may contribute to transients and oscillations of cytosolic-free calcium in vivo.

Uptake characteristics of the InsP3-sensitive and -insensitive Ca2+ pools in porcine aortic smooth-muscle cells: different Ca2+ sensitivity of the Ca2(+)-uptake mechanism

We have investigated the Ca2(+)-uptake characteristics of the InsP3-sensitive and -insensitive non-mitochondrial Ca2+ pools in permeabilized cultured porcine aortic smooth-muscle cells. The InsP3-sensitive Ca2+ pool, which was also GTP sensitive, had a high Ca2+ affinity and was highly oxalate permeable. The InsP3-insensitive Ca2+ store, which was also GTP insensitive, had a much lower Ca2+ affinity and presented a low oxalate permeability. The loading of both pools decreased at high free [Ca2+], although these cells did not have a Ca2(+)-induced Ca2+ release mechanism. This decreased loading of the InsP3-sensitive Ca2+ pool at higher free [Ca2+] must be taken into consideration when investigating a possible Ca2(+)-inhibition of the InsP3-induced Ca2+ release. Part of the Ca2+ uptake into the InsP3-insensitive Ca2+ pool was not affected by the Ca2(+)-pump inhibitors vanadate, thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone.

Organization of intracellular calcium signals generated by inositol lipid-dependent hormones

Recent studies at the single cell level have demonstrated hitherto unsuspected complexities in the organization of intracellular Ca2+ homeostasis in both the temporal and spatial domains. Activation of receptors coupled to the phosphoinositide signalling system has been shown to generate [Ca2+]i oscillations in many cell types. These oscillations display diverse patterns, with variations in oscillation amplitude, latency and frequency which are often tissue and/or agonist dose specific. Furthermore, increases in [Ca2+]i can either occur uniformly or originate from a specific region and propagate throughout the cell in the form of a Ca2+ wave. The significance and underlying mechanisms responsible for these phenomena are discussed.

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.

Intracellular calcium pools in neuroblastoma x glioma hybrid NG108-15 cells

The intracellular nonmitochondrial calcium pools of saponin-permeabilized NG108-15 cells were characterized using inositol 1,4,5-trisphosphate (IP3) and GTP. IP3 or GTP alone induced release of 47 and 68%, respectively, of the calcium that was releasable by A23187. GTP induced release of a further 24% of the calcium after IP3 treatment, whereas IP3 induced release of a further 11% of the calcium after GTP treatment. Guanosine 5'-O-(3-thio)triphosphate had little effect on IP3-induced calcium release but completely inhibited GTP-induced calcium release. In contrast, heparin inhibited the action of IP3 but not that of GTP. The results imply the existence of at least three nonmitochondrial pools: (a) 31% is releasable by IP3 and GTP, (b) 11% is releasable by IP3 alone, and (c) 24% is releasable by GTP alone. GTP enhanced calcium uptake in the presence of oxalate with an EC50 of 0.6 microM and stimulated calcium release in the absence of oxalate with an EC50 of 0.32 microM. The similar EC50 values for these dual effects of GTP on calcium movement suggest that GTP exerts its dual action by the same mechanism.

Expressed cerebellar-type inositol 1,4,5-trisphosphate receptor, P400, has calcium release activity in a fibroblast L cell line

P400, inositol 1,4,5-trisphosphate receptor (InsP3-R), is a key protein to understanding the mechanisms of inositol 1,4,5-trisphosphate (InsP3)-mediated Ca2+ mobilization. We obtained the cerebellar-type P400/InsP3-R cDNA and generated an L cell transfectant (L15) that produces cDNA-derived P400/InsP3-R. In membranes, this protein displays high affinity, specificity, and capacity for InsP3, as does the cerebellar P400/InsP3-R. InsP3 can also induce greater 45Ca2+ release from the membrane vesicles of L15 cells than from those of control L cells. These results provide direct evidence that the cDNA-derived P400/InsP3-R protein is actually involved in physiological Ca2+ mobilization, through binding to InsP3 molecules in the same manner as the cerebellar P400/InsP3-R.

Feedback inhibition by calcium limits the release of calcium by inositol trisphosphate in Limulus ventral photoreceptors

Injection of inositol 1,4,5 trisphosphate (InsP3) into Limulus ventral photoreceptors elevates the concentration of intracellular calcium ions and as a consequence depolarizes the photoreceptor. This InsP3-induced elevation can be inhibited by a prior injection of calcium or InsP3 delivered 1 s earlier. Recovery from this inhibition has a half-time of between 1.5 and 5 s at 20 degrees C. Calcium released by InsP3 therefore inhibits further release of calcium from InsP3-sensitive calcium stores. This feedback inhibition may protect the calcium stores from depletion during prolonged bright illumination. Feedback inhibition, rather than periodic depletion of calcium stores, may also underlie the oscillatory bursts of InsP3-induced calcium release that have been observed in many cell types.

Release of intracellularly stored Ca2+ by inositol 1,4,5-trisphosphate--an overview

1. Inositol 1,4,5-trisphosphate (I(1,4,5)P3) releases Ca2+ from ATP-dependent Ca2+ stores in permeabilized cells and in microsomal fractions. 2. Various factors affect the amount of Ca2+ released by I(1,4,5)P3. 3. The molecular mechanism involved in the I(1,4,5)P3-induced Ca2+ release is now being investigated and I(1,4,5)P3-specific receptors and/or specific release channels are being given special attention. 4. While the I(1,4,5)P3-sensitive Ca2+ stores are presumed to locate at the endoplasmic reticulum, the relation between the I(1,4,5)P3- and the agonist-sensitive Ca2+ stores remains to be elucidated.

Evidence for a GTP-dependent increase in membrane permeability for calcium in NG108-15 microsomes

The effect of GTP on Ca2+ uptake and release was studied in a microsomal fraction isolated from neuroblastoma x glioma hybrid NG108-15 cells. GTP did not alter the ATP-dependent initial uptake of Ca2+ but markedly enhanced the efflux of Ca2+ from microsomes. GTP-dependent Ca2+ release requires the presence of millimolar concentration of Mg2+. The effect of GTP was not mimicked by other nucleotides and was competitively blocked by the thiophosphate analogue of GTP, GTP gamma S but not by the non-hydrolyzable nucleotide GMP-PNP. Addition of an inhibiting concentration of GTP gamma S after completion of GTP-induced calcium release did not result in a re-uptake of Ca2+, showing the irreversibility of the releasing effect of GTP. Our data are consistent with the hypothesis of Ca2+-dependent GTP-induced opening of a channel responsible for vectorial transport of Ca2+ ions from one intracellular compartment to another. A model is proposed suggesting that the GTP-binding protein is a GTP-specific diacylglycerol kinase.

Evidence for an alteration in the microsomal Ca2+ release mechanism in senescent rat parotid acinar cells

Previously, we have shown that Ca2+ mobilization following an alpha 1-adrenergic receptor stimulus is reduced in parotid acinar cells from senescent rats as a result of an altered ability of inositol 1,4,5-trisphosphate (IP3) to induce Ca2+ release from a non-mitochondrial, intracellular Ca2+ store (Ishikawa, Y., et al. Biochim. Biophys. Acta 968, 203-210). We have used this model to examine the IP3-induced Ca2+ release mechanism in these cells. 45Ca2+ efflux, after exposure to (-) epinephrine, from cells of young adult (3-6 months) rats was approx. 2-fold that observed from cells from older animals (approx. 24 months) either in the presence or absence of extracellular Ca2+. Similarly, cytosolic Ca2+ levels were greater in cells of young adult rats under these same incubation conditions. However, microsomal membrane preparations, from both age groups displayed similar IP3 binding sites (Kd approximately 90 nM, Bmax approximately 850 fmol/mg protein) and ATP-dependent Ca2+ transport ability (approx. 8 nmol/mg protein.min -1). These data suggest that there is an alteration in the IP3-induced Ca2+ release mechanism in microsomal membranes of parotid glands from senescent rats which may account for the decreased Ca2+ release seen after agonist stimulation of this tissue.

GTP causes calcium release from a plant microsomal fraction

Studies on a variety of animal cell types have revealed a GTP-specific calcium-releasing mechanism in a non-mitochondrial, microsomal fraction. Here we report that GTP also induces rapid release of calcium from a zucchini (Cucurbita pepo L.) hypocotyl microsomal fraction. Maximal release occurs at 50 microM, and half-maximal release at 8 microM GTP. GTP is highly specific in its effect, and may not be replaced by UTP, ATP, CTP, TTP, GMP, or by non-hydrolysable analogues of GTP. Reuptake of calcium after release does not normally occur; however, this may be induced by non-hydrolysable GTP analogues. Calcium release is also blocked by prior treatment with these analogues.