Ca2+ release induced by cyclic ADP-ribose

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.

Stimulation of ADP-ribosyl cyclase activity of the cell surface antigen CD38 by zinc ions resulting from inhibition of its NAD+ glycohydrolase activity

The lymphocyte cell surface antigen, CD38, which has an amino acid sequence similar to Aplysia ADP-ribosyl cyclase, catalyzes not only the hydrolysis of NAD+ and 1-(5-phospho-beta-D-ribosyl) adenosine 5'-phosphate cyclic anhydride (cyclic ADP-ribose) but also the formation of cyclic ADP-ribose from NAD+. To characterize the bifunctional enzyme properties, we produced the recombinant CD38 fused with a maltose-binding protein (MBP-CD38). Zinc ions stimulated the ADP-ribosyl cyclase activity of MBP-CD38, but inversely inhibited its NAD+ glycohydrolase activity which was approximately 100-fold dominant to the cyclase activity in the absence of Zn2+. Such dual effects of Zn2+ were also observed in the native membrane-bound CD38 of HL-60 cells which had been caused to differentiate by retinoic acid. Zinc ions inhibited the NAD+ glycohydrolase reaction catalyzed by MBP-CD38 in an uncompetitive manner, whereas they enhanced the ADP-ribosyl cyclase reaction without affecting the Km value for NAD+. There was an increase in the fluorescence intensity of a hydrophobic fluorescent probe, 8-anilino-1-naphthalenesulfonate, in the presence of MBP-CD38. The fluorescence increase was further enhanced by the addition of Zn2+ with a shift in the maximum emission wavelength from 484 nm to 470 nm, suggesting that Zn2+ caused conformational changes of MBP-CD38. These results indicate that Zn2+ directly interacts with CD38 to stimulate its ADP-ribosyl cyclase with inhibition of its NAD+ glycohydrolase, probably due to prevention of the access of water molecule to an intermediate of the enzymesubstrate complex.

Nicotinate-adenine dinucleotide phosphate-induced Ca(2+)-release does not behave as a Ca(2+)-induced Ca(2+)-release system

We investigated the dependence of nicotinate-adenine dinucleotide phosphate (NAADP)-induced Ca2+ release from intracellular stores of sea urchin egg homogenates, upon extravesicular Ca2+. In contrast to the Ca2+ release induced inositol 1',4',5'-triphosphate (IP3) or cyclic ADP-ribose (cADPR), the Ca2+ release induced by NAADP was completely independent of the free extravesicular Ca2+ over a wide range of concentrations (0-0.1 mM). The Ca2+ release triggered by either cADPR or IP3 was biphasically modulated by extravesicular Ca2+, and the Ca2+ release by these agents was abolished when the extravesicular Ca2+ was removed by chelation with 2 mM EGTA. On the other hand, NAADP-triggered Ca2+ release was not influenced by EGTA. These data indicate that while both cADPR and IP3 systems behave as functional Ca(2+)-induced Ca2+ release mechanisms, NAADP activates a Ca2+ release mechanism which is independent of the presence of extravesicular Ca2+. Therefore, the NAADP-sensitive Ca2+ release mechanisms may have a unique regulatory impact upon intracellular Ca2+ homoeostasis.

Nicotinic acid-adenine dinucleotide phosphate mobilizes Ca2+ from a thapsigargin-insensitive pool

Nicotinic acid-adenine dinucleotide phosphate (NAADP) is a novel intracellular Ca2+ releasing agent recently described in sea-urchin eggs and egg homogenates. Ca2+ release by NAADP is independent of that induced by either inositol trisphosphate (InsP3) or cyclic adenosine dinucleotide phosphate (cADPR). We now report that in sea urchin egg homogenates, NAADP releases Ca2+ from a Ca2+ pool that is distinct from those that are sensitive to InsP3 and cADPR. This organelle has distinct Ca2+ uptake characteristics: it is insensitive to thapsigargin and cyclopiazoic acid, but maintenance of the pool shows some requirement for ATP. Although the different Ca2+ pools have different characteristics, there appears to be some degree of overlap or cross-talk between the NAADP- and cADPR/InsP3-sensitive Ca2+ pools. Ca(2+)-induced Ca2+ release is unlikely to account for the apparent overlap between stores, since NAADP-induced Ca2+ release, in contrast with that stimulated by cADPR, is not potentiated by bivalent cations.

Expression and function of ryanodine receptors in nonexcitable cells

We have used reverse transcriptase-polymerase chain reaction to investigate the expression of ryanodine receptors in several excitable and nonexcitable cell types. Consistent with previous reports, we detected ryanodine receptor expression in brain, heart, and skeletal muscle. In addition, we detected ryanodine receptor expression in various other excitable cells including PC 12 and A7r5 cells. Several muscle cell lines (BC3H1, C2C12, L6, and Sol8) weakly expressed ryanodine receptor when undifferentiated but strongly expressed type 1 and type 3 ryanodine receptor isoforms when differentiated into a muscle phenotype. Only 2 (HeLa and LLC-PK1 cells) out of 11 nonexcitable cell types examined expressed ryanodine receptors. Expression of ryanodine receptors at the protein level in these cells was confirmed using [3H]ryanodine binding. We also investigated the function of ryanodine receptors in Ca2+ signaling in HeLa cells using single-cell Fura-2 imaging. Neither caffeine nor ryanodine caused a detectable elevation of cytoplasmic Ca2+ in single HeLa cells. However, ryanodine caused a significant decrease in the amplitude of Ca 2+ signals evoked by repetitive stimulation with ATP. These studies show that ryanodine receptors are expressed in some nonexcitable cell types and furthermore suggest that the ryanodine receptors may be involved in a subtle regulation of intracellular Ca2+ responses.

Nitric oxide-induced mobilization of intracellular calcium via the cyclic ADP-ribose signaling pathway

Cyclic adenosine diphosphate ribose (cADPR) is a potent endogenous calcium-mobilizing agent synthesized from beta-NAD+ by ADP-ribosyl cyclases in sea urchin eggs and in several mammalian cells (Galione, A., and White, A. (1994) Trends Cell Biol. 4, 431 436). Pharmacological studies suggest that cADPR is an endogenous modulator of Ca2+-induced Ca2+ release mediated by ryanodine-sensitive Ca2+ release channels. An unresolved question is whether cADPR can act as a Ca2+-mobilizing intracellular messenger. We show that exogenous application of nitric oxide (NO) mobilizes Ca2+ from intracellular stores in intact sea urchin eggs and that it releases Ca2+ and elevates cADPR levels in egg homogenates. 8-Amino-cADPR, a selective competitive antagonist of cADPR-mediated Ca2+ release, and nicotinamide, an inhibitor of ADP-ribosyl cyclase, inhibit the Ca2+-mobilizing actions of NO, while, heparin, a competitive antagonist of the inositol 1,4,5-trisphosphate receptor, did not affect NO-induced Ca2+ release. Since the Ca2+-mobilizing effects of NO can be mimicked by cGMP, are inhibited by the cGMP-dependent-protein kinase inhibitor, Rp-8-pCPT-cGMPS, and in egg homogenates show a requirement for the guanylyl cyclase substrate, GTP, we suggest a novel action of NO in mobilizing intracellular calcium from microsomal stores via a signaling pathway involving cGMP and cADPR. These results suggest that cADPR has the capacity to act as a Ca2+-mobilizing intracellular messenger.

Calcium and cell cycle control in early embryos

Over the past few years, we have witnessed a burgeoning series of papers addressing the role of calcium signalling in cell cycle control. In this review I will attempt to bring together all the diverse threads and discuss new concepts that have arisen from the most recent data. Because the major part of the data concerns mitosis/meiosis entry and exit, I have focused on these areas. I will jointly refer to meiotic and mitotic phases of the cell cycle as M-phase because these phases are highly comparable. Studies of the cell cycle involve a huge range of species, from plants to humans. I will, however, restrict this review to the work performed in early embryos. I apologise in advance to contributors to this field whose names I do not mention because they do not work on embryos.

Specific modulation of cyclic ADP-ribose-induced Ca2+ release by polyamines

Cyclic ADP-ribose (cADPR) is a potent mediator of Ca2+ mobilization from intracellular stores in sea urchin eggs. However, the regulation of the cADPR-induced Ca2+ release system is not yet fully elucidated. We now report that spermine and related polyamines, in physiological concentrations, were able to inhibit the Ca2+ release induced by cADPR in sea urchin egg homogenate bioassays, as measured using the Ca2+ indicator fluo 3, but had no effect on the Ca2+ release induced by D-myo-inositol 1,4,5-trisphosphate (IP3) or by nicotinate adenine dinucleotide phosphate (NAADP). Spermine was a more potent inhibitor of the cADPR-induced Ca2+ release than spermidine and putrescine. Spermine inhibited not only the release induced by cADPR but also the Ca2+ release induced by caffeine and ryanodine. Finally, pretreatment of the sea urchin egg homogenates with caffeine or Sr2+ and Ca2+ prevented the inhibitory effect of spermine on cADPR-induced Ca2+ release. We propose that polyamines, which are present in millimolar concentrations in fertilized eggs, are specific inhibitors of the ryanodine channel and perhaps may serve as endogenous regulators of the cADPR-induced Ca2+ release system.

Different calcium-dependent pathways control fertilisation-triggered glycoside release and the cortical contraction in ascidian eggs

Fertilisation of ascidian eggs induces the rapid release of a cell surface N-acetylglycosaminidase that blocks sperm binding to vitelline coat sperm receptors resulting in a block to polyspermy. Fertilisation also triggers a large contraction of the egg (thus stimulating ooplasmic segregation) that is completed within 5 min of insemination. In eggs of the ascidian Phallusia mammillata, glycosidase release and cortical contractions are blocked by BAPTA-AM [bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethyl)-ester], a cell-permeant calcium chelator, indicating that both processes are probably dependent on a rise in intracellular calcium levels. Both glycosidase release and the cortical contraction are induced by treatment of the egg with the protein synthesis inhibitor emetine, while only the glycosidase release is induced by isoproterenol, carbachol or acetylcholine. Previous work with ryanodine demonstrated that ryanodine also caused glycosidase release but not the cortical contraction. Inversely, activation by ionomycin in calcium-free sea water causes cortical contractions but not glycosidase release. Thus the two processes can be activated independently. Dextran-coupled (10 kDa) calcium green-1 injected eggs show an increase in intracellular calcium 30-40 s before the cortical contraction is triggered by fertilisation or ionomycin-induced activation. This confirms previous findings that the cortical contraction is a consequence of the activation calcium wave triggered by the sperm. The extracellular calcium requirement for the glycosidase release suggests that calcium influx may be more important for this phase of egg activation. Thus activation of ascidian eggs appears to involve two independent pathways involving calcium.

Neuronal Ca2+ stores: activation and function

The intracellular concentration of free Ca2+ ([Ca2+]i) displays complex fluctuations in response to a variety of stimuli, and acts as a pluripotent signal for many neuronal functions. It is well established that various 'metabotropic' neurotransmitter receptors can mediate the mobilization of Ca2+ stores via actions of inositol-polyphosphate second messengers, and more recent evidence suggests that 'ionotropic' receptor-mediated Ca2+ signals in neurones might also involve release of Ca2+ from intracellular stores. These two mechanisms of release of Ca2+ enable considerable temporal and spatial complexity of increases in the [Ca2+]i via multiple interactions at the level of intracellular-receptor activation. The complexity of Ca2+ signalling that is elicited via these interconnecting pathways might underlie mechanisms that are central to information transfer and integration within neuronal compartments.