D-myo-inositol 1,3,4,5-tetrakisphosphate releases Ca2+ from crude microsomes and enriched vesicular plasma membranes, but not from intracellular stores of permeabilized T-lymphocytes and monocytes

A mathematical model of T lymphocyte calcium dynamics derived from single transmembrane protein properties

Fate decision processes of T lymphocytes are crucial for health and disease. Whether a T lymphocyte is activated, divides, gets anergic, or initiates apoptosis depends on extracellular triggers and intracellular signaling. Free cytosolic calcium dynamics plays an important role in this context. The relative contributions of store-derived calcium entry and calcium entry from extracellular space to T lymphocyte activation are still a matter of debate. Here we develop a quantitative mathematical model of T lymphocyte calcium dynamics in order to establish a tool which allows to disentangle cause-effect relationships between ion fluxes and observed calcium time courses. The model is based on single transmembrane protein characteristics which have been determined in independent experiments. This reduces the number of unknown parameters in the model to a minimum and ensures the predictive power of the model. Simulation results are subsequently used for an analysis of whole cell calcium dynamics measured under various experimental conditions. The model accounts for a variety of these conditions, which supports the suitability of the modeling approach. The simulation results suggest a model in which calcium dynamics dominantly relies on the opening of channels in calcium stores while calcium entry through calcium-release activated channels (CRAC) is more associated with the maintenance of the T lymphocyte calcium levels and prevents the cell from calcium depletion. Our findings indicate that CRAC guarantees a long-term stable calcium level which is required for cell survival and sustained calcium enhancement.

Adenine Dinucleotide Second Messengers and T-lymphocyte Calcium Signaling

Calcium signaling is a universal signal transduction mechanism in animal and plant cells. In mammalian T-lymphocytes calcium signaling is essential for activation and re-activation and thus important for a functional immune response. Since many years it has been known that both calcium release from intracellular stores and calcium entry via plasma membrane calcium channels are involved in shaping spatio-temporal calcium signals. Second messengers derived from the adenine dinucleotides NAD and NADP have been implicated in T cell calcium signaling. Nicotinic acid adenine dinucleotide phosphate (NAADP) acts as a very early second messenger upon T cell receptor/CD3 engagement, while cyclic ADP-ribose (cADPR) is mainly involved in sustained partial depletion of the endoplasmic reticulum by stimulating calcium release via ryanodine receptors. Finally, adenosine diphosphoribose (ADPR) a breakdown product of both NAD and cADPR activates a plasma membrane cation channel termed TRPM2 thereby facilitating calcium (and sodium) entry into T cells. Receptor-mediated formation, metabolism, and mode of action of these novel second messengers in T-lymphocytes will be reviewed.

Aberrant cyclization affords a C-6 modified cyclic adenosine 5'-diphosphoribose analogue with biological activity in Jurkat T cells

Two nicotinamide adenine dinucleotide (NAD(+)) analogues modified at the 6 position of the purine ring were synthesized, and their substrate properties toward Aplysia californica ADP-ribosyl cyclase were investigated. 6-N-Methyl NAD(+) (6-N-methyl nicotinamide adenosine 5'-dinucleotide 10) hydrolyzes to give the linear 6-N-methyl ADPR (adenosine 5'-diphosphoribose, 11), whereas 6-thio NHD(+) (nicotinamide 6-mercaptopurine 5'-dinucleotide, 17) generates a cyclic dinucleotide. Surprisingly, NMR correlation spectra confirm this compound to be the N1 cyclic product 6-thio N1-cIDPR (6-thio cyclic inosine 5'-diphosphoribose, 3), although the corresponding 6-oxo analogue is well-known to cyclize at N7. In Jurkat T cells, unlike the parent cyclic inosine 5'-diphosphoribose N1-cIDPR 2, 6-thio N1-cIDPR antagonizes both cADPR- and N1-cIDPR-induced Ca(2+) release but possesses weak agonist activity at higher concentration. 3 is thus identified as the first C-6 modified cADPR (cyclic adenosine 5'-diphosphoribose) analogue antagonist; it represents the first example of a fluorescent N1-cyclized cADPR analogue and is a new pharmacological tool for intervention in the cADPR pathway of cellular signaling.

Transient tyrosine phosphorylation of human ryanodine receptor upon T cell stimulation

The ryanodine receptor of Jurkat T lymphocytes was phosphorylated on tyrosine residues upon stimulation of the cells via the T cell receptor/CD3 complex. The tyrosine phosphorylation was transient, reaching a maximum at 2 min, and rapidly declined thereafter. In co-immunoprecipitates of the ryanodine receptor, the tyrosine kinases p56(lck) and p59(fyn) were detected. However, only p59(fyn) associated with the ryanodine receptor in a stimulation-dependent fashion. Both tyrosine kinases, recombinantly expressed as glutathione S-transferase (GST) fusion proteins, phosphorylated the immunoprecipitated ryanodine receptor in vitro. In permeabilized Jurkat T cells, GST-p59(fyn), but not GST-p56(lck), GST-Grb2, or GST alone, significantly and concentration-dependently enhanced Ca(2+) release by cyclic ADP-ribose. The tyrosine kinase inhibitor PP2 specifically blocked the effect of GST-p59(fyn). This indicates that intracellular Ca(2+) release via ryanodine receptors may be modulated by tyrosine phosphorylation during T cell activation.

Inositol phosphates from barley low-phytate grain mutants analysed by metal-dye detection HPLC and NMR

Inositol phosphates from barley low-phytate grain mutants and their parent variety were analysed by metal-dye detection HPLC and NMR. Compound assignment was carried out by comparison of retention times using a chemical hydrolysate of phytate [Ins(1,2,3,4,5,6)P(6)] as a reference. Co-inciding retention times indicated the presence of phytate, D/L-Ins(1,2,3,4,5)P(5), Ins(1,2,3,4,6)P(5), D/L-(1,2,4,5,6)P(5), D/L-(1,2,3,4)P(4), D/L-Ins(1,2,5,6)P(4) and D/L-Ins(1,4,5,6)P(4) in PLP1B mutants as well as the parent variety. In grain extracts from mutant lines PLP1A, PLP2A and PLP3A unusual accumulations of D/L-Ins(1,3,4,5)P(4) were observed whereas phytate and the above-mentioned inositol phosphates were present in relatively small amounts. Assignment of D/L-Ins(1,3,4,5)P(4) was corroborated by precise co-chromatography with a commercial Ins(1,3,4,5)P(4) standard and by NMR spectroscopy. Analysis of inositol phosphates during grain development revealed accumulation of phytate and D/L-Ins(1,3,4,5)P(4), which suggested the tetrakisphosphate compound to be an intermediate of phytate synthesis. This assumption was strengthened further by phytate degradation assays showing that D/L-Ins(1,3,4,5)P(4) did not belong to the spectrum of degradation products generated by endogenous phytase activity. Metabolic scenarios leading to accumulation of D/L-Ins(1,3,4,5)P(4) in barley low-phytate mutants are discussed.

Potential mechanism for bradykinin-activated and inositol tetrakisphosphate-dependent Ca2+ influx by Ras and GAP1 in fibroblast cells

Here we propose a molecular model for bradykinin receptor-operated and second messenger (inositol-1,3,4,5-tetrakisphosphate)-evoked Ca2+ influx and its potentiation by oncogenic Ras, which is not store-depletion-induced, so-called capacitative, Ca2+ influx. The principal idea for this hypothesis stems from observation that two bradykinin B2 receptor-activated signal pathways, protein tyrosine phosphorylation and formation of inositol tetrakisphosphate, merge during the Ca2+ influx process and that GTPase activating-protein 1 (GAP 1) is inositol tetrakisphosphate binding protein.

Mechanisms involved in the cellular calcium homeostasis in vascular smooth muscle: calcium pumps

The regulation of cytosolic Ca2+ homeostasis is essential for cells, and particularly for vascular smooth muscle cells. In this regulation, there is a participation of different factors and mechanisms situated at different levels in the cell, among them Ca2+ pumps play an important role. Thus, Ca2+ pump, to extrude Ca2+; Na+/Ca2+ exchanger; and different Ca2+ channels for Ca2+ entry are placed in the plasma membrane. In addition, the inner and outer surfaces of the plasmalemma possess the ability to bind Ca2+ that can be released by different agonists. The sarcoplasmic reticulum has an active role in this Ca2+ regulation; its membrane has a Ca2+ pump that facilitates luminal Ca2+ accumulation, thus reducing the cytosolic free Ca2+ concentration. This pump can be inhibited by different agents. Physiologically, its activity is regulated by the protein phospholamban; thus, when it is in its unphosphorylated state such a Ca2+ pump is inhibited. The sarcoplasmic reticulum membrane also possesses receptors for 1,4,5-inositol trisphosphate and ryanodine, which upon activation facilitates Ca2+ release from this store. The sarcoplasmic reticulum and the plasmalemma form the superficial buffer barrier that is considered as an effective barrier for Ca2+ influx. The cytosol possesses different proteins and several inorganic compounds with a Ca2+ buffering capacity. The hypothesis of capacitative Ca2+ entry into smooth muscle across the plasma membrane after intracellular store depletion and its mechanisms of inhibition and activation is also commented.

Antisense knock out of the inositol 1,3,4,5-tetrakisphosphate receptor GAP1(IP4BP) in the human erythroleukemia cell line leads to the appearance of intermediate conductance K(Ca) channels that hyperpolarize the membrane and enhance calcium influx

To study the role of the inositol 1,3,4,5-trisphosphate-binding protein GAP1(IP4BP) in store-operated Ca2+ entry, we established a human erythroleukemia (HEL) cell line in which the expression of GAP1(IP4BP) was substantially reduced by transfection with a vector containing antisense DNA under control of a Rous Sarcoma virus promoter and the Escherichia coli LacI repressor (AS-HEL cells). Control cells were transfected with vector lacking antisense DNA (V-HEL cells). GAP1(IP4BP) protein, which is a member of the GTPase-activating protein (GAP1) family, was reduced by 85% in AS-HEL cells and was further reduced by 96% by treatment with isopropylthio-beta-D- galactoside to relieve LacI repression. The loss of GAP1(IP4BP) was associated with both a membrane hyperpolarization and a substantially increased Ca2+ entry induced by thrombin or thapsigargin. The activation of intermediate conductance Ca2+-activated K+ channels in AS-HEL cells (not seen in V-HEL cells) was responsible for the membrane hyperpolarization and the enhanced Ca2+ entry, and both were blocked by charybdotoxin. Stimulated V-HEL cells did not hyperpolarize and basal Ca2+ influx was unaffected by charybdotoxin. In V-HEL cells hyperpolarized by removal of extracellular K+, the thapsigargin-stimulated Ca2+ influx was increased. Expression of mRNA for the human Ca2+-activated intermediate conductance channel KCa4 was equivalent in both AS-HEL and V-HEL cells, suggesting that the specific appearance of calcium-activated potassium current (IK(Ca)) in AS-HEL cells was possibly due to modulation of preexisting channels. Our results demonstrate that GAP1(IP4BP), likely working through a signaling pathway dependent on a small GTP-binding protein, can regulate the function of K(Ca) channels that produce a hyperpolarizing current that substantially enhances the magnitude and time course of Ca2+ entry subsequent to the release of internal Ca2+ stores.

1-(5-phospho-beta-D-ribosyl)2'-phosphoadenosine 5'-phosphate cyclic anhydride induced Ca2+ release in human T-cell lines

1-(5-Phospho-beta-D-ribosyl)2'-phosphoadenosine 5'-phosphate cyclic anhydride [2'-phospho-cyclic ADP-ribose, cAdo(2')P(5')PP-Rib] was prepared enzymatically from NADP+ using ADP-ribosyl-cyclase from Aplysia californica. The product was purified by HPLC and characterized by NMR and mass spectroscopy, by conversion to 1-(5-phospho-beta-D-ribosyl)adenosine 5'-phosphate cyclic anhydride (cADP-Rib) by alkaline phosphatase and by resistance to snake venom phosphodiesterase. cAdo-(2')P(5')PP-Rib dose-dependently released Ca2+ from an intracellular, non-endoplasmic reticular Ca2+ pool of permeabilized Jurkat and HPB. ALL T-lymphocytes. In contrast, the closely related compounds 1-(5-phospho-beta-D-ribosyl)3'phosphoadenosine 5'-phosphate cyclic anhydride and 1-(5-phospho-beta-D-ribosyl)cyclic 2',3'-phosphoadenosine 5'-phosphate cyclic anhydride did not induce Ca2+-release from permeabilized T cells. The Ca2+ pool sensitive to cAdo(2')P(5')PP-Rib partially overlapped with the Ca2+ pool sensitive to cADP-Rib recently described in T cells [Guse, A. H., da Silva, C. P., Emmrich, F., Ashamu, G. A., Potter, B. V. L. & Mayr, G. W. (1995) Characterization of cyclic adenosine diphosphate-ribose-induced Ca2+-release in T-lymphocyte cell lines, J. Immunol. 155, 3353-3359]. Control experiments suggest that the results were neither due to Ca2+ contaminations in the cADP-Rib preparation nor to catabolism of cAdo(2')P(5')PP-Rib to cADP-Rib.

Ca2+ entry induced by cyclic ADP-ribose in intact T-lymphocytes

Cyclic ADP-ribose (cADPr) is a potent Ca2+-mobilizing natural compound (Lee, H. C., Walseth, T. F., Bratt, G. T., Hayes, R. N., and Clapper, D. L. (1989) J. Biol. Chem. 264, 1608-1615) which has been shown to release Ca2+ from an intracellular store of permeabilized T-lymphocytes (Guse, A. H., Silva, C. P., Emmrich, F., Ashamu, G., Potter, B. V. L., and Mayr, G. W. (1995) J. Immunol. 155, 3353-3359). Microinjection of cADPr into intact single T lymphocytes dose dependently induced repetitive but irregular Ca2+ spikes which were almost completely dependent on the presence of extracellular Ca2+. The Ca2+ spikes induced by cADPr could be blocked either by co-injection of cADPr with the specific antagonist 8-NH2-cADPr, by omission of Ca2+ from the medium, or by superfusion of the cells with Zn2+ or SK-F 96365. Ratiometric digital Ca2+ imaging revealed that single Ca2+ spikes were initiated at several sites ("hot spots") close to the plasma membrane. These hot spots then rapidly formed a circular zone of high Ca2+ concentration below the plasma membrane which subsequently propagated like a closing optical diaphragm into the center of the cell. Taken together these data indicate a role for cADPr in Ca2+ entry in T-lymphocytes.

Lysophosphatidic acid-sensitive intracellular Ca2+ store does not regulate Ca2+ entry at plasma membrane in Jurkat human T-cells

In Jurkat T cells, the anti-CD3 antibody OKT3 and thapsigargin (TG) elevated the cytoplasmic free Ca2+ concentration ([Ca2+]i), after which it decreased to a sustained, elevated level. In contrast, lysophosphatidic acid (LPA) increased [Ca2+]i only briefly and transiently, after which it declined to the resting level of [Ca2+]i even in the presence of extracellular Ca2+. OKT3 increased Ins(1,4,5)P3 formation but neither LPA nor TG did. In the absence of extracellular Ca2+, the addition of OKT3 did not affect an elevation of [Ca2+]i induced by the subsequent addition of LPA and vice versa. In permeabilized Jurkat cells, the addition of Ins(1,4,5)P3 released Ca2+; this was inhibited by heparin, whereas LPA released Ca2+ even in the presence of heparin. cADP-ribose released Ca2+; this was additive with LPA-induced Ca2+ release and vice versa in permeabilized Jurkat cells. LPA did not stimulate Ca2+ entry and 45Ca2+ uptake but OKT3 and TG did. LPA, OKT3 and TG did not affect the sustained elevation of [Ca2+]i induced by ionomycin. The present results suggest that at least three kinds of intracellular Ca2+ stores, which are Ins(1,4,5)P3,-cADP-ribose- and LPA-sensitive, exist in Jurkat T cells, and that the LPA-sensitive intracellular Ca2+ store does not regulate Ca2+ entry at the plasma membrane.

Regulation of cADP-ribose-induced Ca2+ release by Mg2+ and inorganic phosphate

cADP-ribose (cADPr) has recently been shown to release Ca2+ from an intracellular store of permeabilized T lymphocyte cell lines (Guse, A. H., da Silva, C. P., Emmrich, F., Ashamu, G. A., Potter, B. V. L., and Mayr, G. W. (1995) J. Immunol. 155, 3353-3359). Using permeabilized Jurkat and HPB. ALL T lymphocytes, the effects of varying concentrations of inorganic phosphate and Mg2+ on cADPr-induced Ca2+ release were investigated. cADPr-induced Ca2+ release was dependent on the concentration of inorganic phosphate, showing very low Ca2+ release activity between 0.5 and 2 mM inorganic phosphate. At 4 to 5 mM inorganic phosphate, the cADPr-induced Ca2+ release was much more pronounced, reaching maximal values at 10 mM inorganic phosphate. The underlying mechanism for this stimulatory effect was an increased loading of the cADPr-sensitive Ca2+ store, which was demonstrated by enhanced resequestration of Ca2+ selectively into the cADPr-sensitive Ca2+ store. The free Mg2+ concentration also influenced cADPr-induced Ca2+ release in permeabilized cells: at 0 and 8.58 mM the release was nearly completely abolished, whereas at 1.06 mM maximal Ca2+ release by cADPr was observed. High performance liquid chromatographic analysis of exogenously added cADPr revealed that the catabolism of cADPr at varying Mg2+ and Pi concentrations had only minor relevance for the modulatory effects observed. To correlate the effects of inorganic phosphate and Mg2+ on cADPr-induced Ca2+ release observed in the permeabilized cell preparations, measurements of these ions in intact Jurkat T lymphocytes were carried out. Intact Jurkat T cells stimulated via the T cell receptor middle dotCD3 complex did not respond with significant elevation of the free intracellular Mg2+ concentration. In contrast, stimulation via the T cell receptor middle dotCD3 complex resulted in an increase in the intracellular inorganic phosphate concentration. These data indicate a role for the intracellular inorganic phosphate concentration in the regulation of cADPr-mediated Ca2+ release in T lymphocytes.

T cell receptor-mediated Ca2+ signaling: release and influx are independent events linked to different Ca2+ entry pathways in the plasma membrane

In this study, we showed that cross-linking CD3 molecules on the T cell surface resulted in Ca2+ release from the intracellular stores followed by a sustained Ca2+ influx. Inhibition of release with TMB-8 did not block the influx. However, inhibition of phospholipase C activity suppressed both Ca2+ release and influx. Once activated, the influx pathway remained open in the absence of further hydrolysis of PIP2. Thapsigargin, a microsomal Ca(2+)-ATPase inhibitor, stimulated Ca2+ entry into the cells by a mechanism other than emptying Ca2+ stores. In addition, Ca2+ entry into the Ca(2+)-depleted cells was stimulated by low basal level of cytosolic Ca2+, not by the emptying of intracellular Ca2+ stores. Both the Ca2+ release and influx were dependent on high and low concentrations of extracellular Ca2+. At low concentrations, Mn2+ entered the cell through the Ca2+ influx pathway and quenched the sustained phase of fluorescence; whereas, at higher Mn2+ concentration both the transient and the sustained phases of fluorescence were quenched. Moreover, Ca2+ release was inhibited by low concentrations of Ni2+, La3+, and EGTA, while Ca2+ influx was inhibited by high concentrations. Thus, in T cells Ca2+ influx occurs independently of IP3-dependent Ca2+ release. However, some other PIP2 hydrolysis-dependent event was involved in prolonged activation of Ca2+ influx. Extracellular Ca2+ influenced Ca2+ release and influx through the action of two plasma membrane Ca2+ entry pathways with different pharmacological and biochemical properties.

Phosphatidylinositol hydrolysis and an increase in Ca2+ concentration in the signal-transduction process triggered by murine Fc gamma RIII are not required for protein kinase C translocation

Murine class III receptors for IgG (mFc gamma RIII) are composed of an IgG-binding alpha chain associated with a gamma subunit dimer. These receptors have been shown to trigger the release of serotonin and tumor necrosis factor-alpha [Daëron, M., Latour, S., Hückel, C., Bonnerot, C. & Fridman, W. H. (1992) Immunobiology 185, 159-174], and are involved in endocytosis and phagocytosis [Daëron, M., Malbec, O., Bonnerot, C., Latour, S., Segal, D. M. & Fridman, W. H. (1994) J. Immunol. 152, 783-792]. Using a transfection model where the cDNA encoding mFc gamma RIII was stably transfected into the rat basophilic leukemia cell line RBL-2H3, we found that the functional efficiency of mFc gamma RIII is correlated with its ability to increase the intracellular Ca2+ concentration and to stimulate inositol phosphate metabolism. The deletion of intracellular sequences of the alpha subunit did not alter the ability of mFc gamma RIII to trigger the Ca2+ and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] response. After substitution of the intracellular domain of mFc gamma RIII for that of mFc gamma RIII gamma, but not that of mFc gamma RIII alpha, the chimeric receptor was also able to trigger Ca2+ and PtdIns(4.5)P2 responses. In contrast, all transfected receptors induced protein kinase C translocation. Furthermore, dimerization of the receptor was sufficient for the initiation of this protein kinase C translocation while a further crosslinking was necessary for the induction of the Ca2+ and PtdIns(4,5)P2 responses. Protein kinase C translocation therefore can be dissociated from Ca2+ mobilization, PtdIns(4,5)P2 turnover and mast cell secretory responses induced by murine Fc gamma RIII.

Inositol 1,3,4,5-tetrakisphosphate-gated channels interact with inositol 1,4,5-trisphosphate-gated channels in olfactory receptor neurons

Inositol 1,4,5-trisphosphate [InsP3(1,4,5)] is a major second messenger regulating Ca2+ signaling in excitable and nonexcitable cells. InsP3(1,4,5) is extensively metabolized through a network of phosphorylation and dephosphorylation steps to products with potential second messenger function. Inositol 1,3,4,5-tetrakisphosphate [InsP4(1,3,4,5)], the direct metabolite of InsP3(1,4,5), has also been associated with Ca2+ signaling, but whether InsP4(1,3,4,5) acts in combination with InsP3(1,4,5) or whether it regulates Ca2+ signaling directly and independently is unclear, particularly in neurons. We report that olfactory receptor neurons in the lobster (Panulirus argus) express an InsP4(1,3,4,5) receptor in the plasma membrane that is a functional channel. The channel differs in conductance, kinetics, and voltage sensitivity from two plasma membrane InsP3(1,4,5)-gated channels previously reported in these neurons. In close spatial proximity, the InsP4(1,3,4,5)-and InsP3(1,4,5)-gated channels interact reciprocally to alter the channels' open probabilities in what may be a novel mechanism for regulating Ca2+ entry in neurons.

Ca2+ release and Ca2+ entry induced by rapid cytosolic alkalinization in Jurkat T-lymphocytes

4-Aminopyridine (4-AP), a compound usually known as a K(+)-channel inhibitor, induced rapid cytosolic alkalinization from pH 7.15 to pH 7.4, and subsequently Ca2+ mobilization in the T-lymphocyte cell line Jurkat. Other weak bases, such as NH4Cl or triethanolamine, induced a smaller and/or slower increase in cytosolic pH, resulting in a lower or no detectable Ca2+ signal. In the presence of extracellular Ca2+, 4-AP mediated a rapid and sustained increase in the free cytosolic Ca2+ concentration similar to that obtained by T-cell receptor-mediated stimulation. In the absence of extracellular Ca2+, 4-AP transiently released Ca2+ from an intracellular store that is most likely identical with the agonist- and Ins(1,4,5)P3-sensitive Ca2+ pool of Jurkat T-cells. As possible mechanisms for Ca2+ release from this particular pool as induced by 4-AP we examined (i) formation of Ins(1,4,5)P3 and (ii) sensitization of the Ins(1,4,5)P3-receptor/Ca(2+)release system by increasing intracellular pH. Although 4-AP did not induce formation of inositol polyphosphates, as demonstrated by h.p.l.c. analysis, in permeabilized cells the dose-response curve for Ins(1,4,5)P3 was shifted to the left by changing the intracellular pH from 7.2 to 7.4. This indicated that sensitization of the Ins(1,4,5)P3-receptor/Ca(2+)-release system was responsible for the effects of 4-AP seen in intact cells. In conclusion, 4-AP appears a novel tool for depletion of the agonist-sensitive Ca2+ pool of T-cells without simultaneous formation of Ins(1,4,5)P3, thereby inducing capacitative Ca2+ entry in these cells.

Calcium release-activated calcium influx in cultured human mesangial cells

Ca2+ influx is a major component of the response of cultured human mesangial cells (HMC) to vasoconstrictors. Activators of phospholipase C such as angiotensin II (Ang II) release Ca2+ from intracellular stores and enhance Ca2+ influx, which in turn is modulated by Na+/Ca2+ exchange. By microfluorometry we studied the mechanisms of Ca2+ entry in resting and stimulated fura-2-loaded monolayers or single HMC. Addition of 1 to 10 mM extracellular Ca2+ to cells equilibrated in Ca(2+)-free media resulted in a rapid, persistent elevation of free cytosolic Ca2+ ([Ca2+]i), from 52 +/- 5 to 113 +/- 18 and 226 +/- 37 nM, respectively. Ca2+ influx was blocked by lanthanum or chelation with EGTA, while it was only partially inhibited by voltage-operated Ca2+ channel (VOC) blockers, such as nifedipine or verapamil. The rise of [Ca2+]i at high external [Ca2+] was not due to a Ca(2+)-sensing mechanism with release of intracellular stored Ca2+, since it was prolonged, and it was not seen in cells maintained in normal 1.25 mM [Ca2+] media. Moreover, it was not abolished by prior depletion of Ca2+ stores with 0.5 microM thapsigargin or 5 microM ionomycin in Ca(2+)-free media, which transiently increased [Ca2+]i (to 281 +/- 39 and 380 +/- 51 nM, respectively). On the contrary, both agents markedly potentiated Ca2+ influx upon addition of 1 to 10 mM [Ca2+]e, (to a maximum of 686 +/- 111 and 633 +/- 150 nM, P < 0.05 vs. control).(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium release activity and metabolism of inositol 1,4,5-trisphosphate in T cells. Modulation by novel inositol 1,4,5-trisphosphate 5-phosphatase inhibitors

Stimulation of the T cell antigen receptor/CD3 complex is followed by phospholipase C activation, phosphoinositol lipid metabolism and ultimately by a rapid rise in both myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] as well as cytosolic free calcium concentration. A 5-phosphatase plays a pivotal role in the subsequent metabolism of Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Synthetic routes have been developed which have enabled the synthesis of both natural and unnatural inositol phosphates and this approach has yielded several compounds which have been shown to act as inhibitors of Ins(1,4,5)P3 5-phosphatase. These compounds offer considerable potential for investigation of the complex metabolism and function of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in T cell activation and proliferation. We now report the time course and temperature sensitivity of Ins(1,4,5)P3-induced 45Ca2+ release in the permeabilised leukaemic T cell line Jurkat. Furthermore, we demonstrate that the metabolism of Ins(1,4,5)P3 in the presence of two novel 5-phosphatase inhibitors, namely L-myo-inositol 1,4,5-trisphosphorothioate [L-Ins(1,4,5)PS3] and myo-inositol 1,3,5-trisphosphorothioate [Ins(1,3,5)PS3], can be inhibited with concomitant elevation of the heparin-sensitive Ins(1,4,5)P3-induced release of 45Ca2+. These novel 5-phosphatase inhibitors provide a starting point for development of cell-permeable analogues which may be able to modulate cell function in intact cells and may be used as manipulative tools with which to elucidate the function of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 with respect to T cell activation.

Myo-inositol 1,3,4,5-tetrakisphosphate can independently mobilise intracellular calcium, via the inositol 1,4,5-trisphosphate receptor: studies with myo-inositol 1,4,5-trisphosphate-3-phosphorothioate and myo-inositol hexakisphosphate

Myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] acts as a full agonist for Ca2+ release in saponin-permeabilised SH-SY5Y neuroblastoma cells. Studies were conducted in the presence of myo-inositol hexakisphosphate (InsP6, 10 microM), to inhibit the Ins(1,3,4,5)P(4)-3-phosphatase catalysed back conversion of Ins(1,3,4,5)P4 to Ins(1,4,5)P3. HPLC analysis confirmed that Ins(1,3,4,5)P4 releases the entire content of Ins(1,4,5)P3-sensitive intracellular Ca2+ stores, independent of 3-phosphatase activity. Further we utilised racemic myo-inositol 1,4,5-trisphosphate-3-phosphorothioate [DL-Ins(1,3,4,5)P(4)-3S], a novel intrinsically Ins(1,3,4,5)P(4)-3-phosphatase resistant Ins(1,3,4,5)P4 analogue. DL-Ins(1,3,4,5)P(4)-3S specifically displaced [3H]Ins(1,4,5)P3 from bovine adrenal cortex Ins(1,4,5)P3 binding sites (IC50 = 889 nM, compared to Ins(1,4,5)P3, IC50 = 4.4 nM and Ins(1,3,4,5)P4, IC50 = 152 nM). DL-Ins(1,3,4,5)P(4)-3S was a full agonist for Ca2+ release (EC50 = 4.7 microM), being 90- and 2-fold less potent than Ins(1,4,5)P3 and Ins(1,3,4,5)P4 (with InsP6), respectively. DL-Ins(1,3,4,5)P(4)-3S will be an important tool for identification of potentially exclusive Ins(1,3,4,5)P4 second messenger functions, since its resistance to 3-phosphatase action precludes the inconvenient artefact of steady state Ins(1,4,5)P3 generation.

Stereoselectivity of Ins(1,3,4,5)P4 recognition sites: implications for the mechanism of the Ins(1,3,4,5)P4-induced Ca2+ mobilization

Ins(1,3,4,5)P4 was able to mobilize the entire Ins(1,4,5)P3-sensitive intracellular Ca2+ store in saponin-permeabilized SH-SY5Y human neuroblastoma cells in a concentration-dependent manner, yielding an EC50 value of 2.05 +/- 0.45 microM, compared with 0.14 +/- 0.03 microM for Ins(1,4,5)P3. However, L-Ins(1,3,4,5)P4 [= D-Ins(1,3,5,6)P4] failed to cause mobilization of intracellular Ca2+ at concentrations up to 100 microM. Binding studies using pig cerebellar membranes as a source of both Ins(1,4,5)P3/Ins(1,3,4,5)P4-specific binding sites have revealed a marked contrast in their stereospecificity requirements. Ins(1,4,5)P3-receptors from pig cerebella exhibited stringent stereospecificity, L-Ins(1,4,5)P3 and L-Ins(1,3,4,5)P4 were > 1000-fold weaker, whereas Ins(1,3,4,5)P4 (IC50 762 +/- 15 nM) was only about 40-fold weaker than D-Ins(1,4,5)P3 (IC50 20.7 +/- 9.7 nM) at displacing specific [3H]Ins(1,4,5)P3 binding from an apparently homogeneous Ins(1,4,5)P3 receptor population. In contrast, the Ins(1,3,4,5)P4-binding site exhibited poor stereoselectivity. Ins(1,3,4,5)P4 produced a biphasic displacement of specific [32P]Ins(1,3,4,5)P4 binding, with two-site analysis revealing KD values for high- and low-affinity sites of 2.1 +/- 0.5 nM and 918 +/- 161 nM respectively. L-Ins(1,3,4,5)P4 also produced a biphasic displacement of specific [32P]Ins(1,3,4,5)P4 binding which was less than 10-fold weaker than with D-Ins(1,3,4,5)P4 (IC50 values for the high- and low-affinity sites of 17.2 +/- 3.7 nM and 3010 +/- 542 nM respectively). Therefore, although L-Ins(1,3,4,5)P4 appears to be a high-affinity Ins(1,3,4,5)P4-binding-site ligand in pig cerebellum, it is a very weak agonist at the Ca(2+)-mobilizing receptors of permeabilized SH-SY5Y cells. We suggest that the ability of D-Ins(1,3,4,5)P4 to access intracellular Ca2+ stores may derive from specific interaction with the Ins(1,4,5)P3- and not the Ins(1,3,4,5)P4-receptor population.

Novel inositol containing phospholipids and phosphates: their synthesis and possible new roles in cellular signalling

Details of the widely employed PtdIns(4,5)P2 hydrolysis receptor-stimulated signalling pathway continue to be elucidated rapidly. However, it has recently become apparent that numerous other inositol lipids and phosphates are widespread and are likely to have important cellular functions. In this review, we focus particularly on three rapidly progressing areas: the synthesis and possible functions of 3-phosphorylated inositol lipids, particularly phosphatidylinositol 3,4,5-trisphosphate; the roles of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in coordinating intracellular Ca2+ mobilization and Ca2+ influx in stimulated cells; and the metabolism and possible functions of other inositol polyphosphates and of inositol polyphosphate pyrophosphates.

Intracellular Ca2+ pools in Jurkat T-lymphocytes

Jurkat T-lymphocytes comprise at least four intracellular Ca2+ pools. Pool I was agonist-sensitive and contained 23 +/- 8% (n = 18) of the total Ca(2+)-storage capacity, as shown in intact cells in the presence of EGTA. The time courses of the agonist-induced formation of Ins(1,4,5)P3 and of the Ca2+ release from pool I were nearly superimposable, indicating that the agonist-sensitive pool I is emptied by Ins(1,4,5)P3. Likewise, in permeabilized cells, the size of the Ins(1,4,5)P3-sensitive Ca2+ pool I was 27 +/- 11% (n = 14). Pool II contained 26 +/- 5% (n = 9) of intracellularly stored Ca2+ and was liberated by thapsigargin, an inhibitor of the endoplasmic-reticulum (ER) Ca(2+)-ATPase. Addition of thapsigargin before addition of agonist abolished the agonist-induced Ca2+ release in both intact and permeabilized cells, indicating that pool I is a subcompartment of the ER Ca2+ pool. The content of this ER Ca2+ pool (pools I and II) amounted to 51 +/- 15% (n = 9) in intact cells and 49 +/- 16% (n = 16) in permeabilized cells. Caffeine released Ca2+ even when the ER pool (pools I and II) was emptied by previous addition of thapsigargin, indicating the presence of a third pool independent of pools I and II. Pool III contained 23 +/- 6% (n = 8) in intact cells, but 41 +/- 8% (n = 5) in permeabilized cells. The remaining intracellularly stored Ca2+ was released by addition of the Ca2+ ionophore ionomycin. This fourth pool contained 27 +/- 8% (n = 9) in intact cells, but less than 10% in permeabilized cells. The size of pool III was increased when pools I and II were emptied before addition of caffeine, whereas the size of pool IV was decreased under such conditions. In conclusion, this first comprehensive description of intracellular Ca2+ pools in Jurkat T-lymphocytes demonstrates the presence of four different Ca2+ pools, provides estimates of their sizes and describes relationships between each other. Release of Ca2+ from pool I [Ins(1,4,5)P3-sensitive] has previously been shown to play a major role in T-cell activation, whereas the physiological role of pools II-IV remains to be established.