Redundant mechanisms of calcium-induced calcium release underlying calcium waves during fertilization of sea urchin eggs

The two intracellular Ca2+ release channels, ryanodine receptor and inositol 1,4,5-trisphosphate receptor, play different roles during fertilization in ascidians

Fertilization in the ascidians triggers an activation wave of calcium release followed by intracellular calcium oscillations synchronous with periodic membrane potential excursions during the completion of the meiotic cell cycle. Fertilization also causes a fast decrease in the egg plasma membrane depolarization-activated calcium current and a large increase in capacitance thought to represent membrane addition to the egg surface. We have analyzed the temporal and causal relationships between these changes in the eggs of Phallusia mammillata using whole-cell patch-clamp recording while simultaneously imaging calcium with fura-2 dextran. We have defined the role of ryanodine receptor (RyR) and InsP3 receptor (InsP3R) during fertilization and meiosis by looking at the effects of InsP3, cyclic ADP ribose (cADPR), and ryanodine in perfused oocytes. We show that InsP3 (10 microM perfused through the patch pipette) is able to trigger sustained oscillations in intracellular calcium concentration in unfertilized oocytes, resembling those recorded in fertilized egg completing meiosis. In addition the sustained oscillations resulting from InsP3 perfusion in unfertilized oocytes are sufficient to cause the emission of both polar bodies. In contrast, ryanodine or cADPR never trigger detectable calcium signal in perfused oocytes. Instead, nanomolar concentrations of ryanodine or cADPR cause a capacitance change, implying a net insertion of membrane to the oocyte surface, and trigger a fast decrease in the depolarization-activated calcium current. Both changes are similar to the changes in conductance and capacitance naturally observed following fertilization. These effects, although not associated with measurable calcium signals, are abolished by coperfusion of the calcium chelator BAPTA. In contrast to ryanodine or cADPR, sustained perfusion of the oocyte with nanomolar concentrations of InsP3 causes no capacitance change and a slow and moderate decrease in calcium current. Our observations on inseminated patch-clamped eggs further indicate that membrane insertion, which starts 15-20 sec after the onset of the membrane conductance change at fertilization, can be altered by interfering with the RyR. Our results imply that, in ascidians, as in some mammals, RyR and InsP3R play distinct roles during fertilization.

Effect of a cytosolic Ca2+ concentration ramp on InsP3-induced Ca2+ release in A7r5 smooth-muscle cells and in EBTr cells from tracheal mucosa

Inositol trisphosphate (InsP3) mediated Ca2+ release is modulated in a complex way by Ca2+. It is not known how the InsP3 receptor responds to a slowly increasing cytosolic [Ca2+]. Two different cell lines (A7r5 smooth-muscle cells and EBTr cells from tracheal mucosa) were investigated. We have now stimulated the Ca2+ stores of the permeabilized cells with a near-threshold [InsP3] and then increased the cytosolic [Ca2+] from 5 nM to 3 microM in 55 steps each lasting 6 s. The rate of InsP3-induced Ca2+ release abruptly increased around 100 nM Ca2+ and reached a maximum at 300 nM Ca2+, above which the Ca2+ release became smaller. The stimulatory effect of cytosolic Ca2+ was much less than that induced by elevating the [InsP3]. The time course of activation by Ca2+ in permeabilized cells resembles the fast InsP3-induced Ca2+ release following the pacemaker [Ca2+] rise in the agonist-stimulated intact cell.

Developmental expression of the inositol 1,4,5-trisphosphate receptor and localization of inositol 1,4,5-trisphosphate during early embryogenesis in Xenopus laevis

To study the role of inositol 1,4,5-trisphosphate (IP3) receptors during early embryogenesis in Xenopus, we examined the temporal-spatial localization of Xenopus IP3 receptor (XIP3R). XIP3R protein is enriched in the animal hemisphere of early cleavage stage embryos and becomes localized in the ectoderm and involuted mesoderm in gastrula stage embryos. Up to tailbud stages, expression of XIP3R is observed in the mesodermal tissues and in most subregions of the central nervous system. A quantitative analysis of endogenous IP3 mass during normal early embryogenesis revealed an increase in IP3 mass first observed at early gastrula stage of 10.5 with an enrichment in the ectoderm throughout the gastrula stages, implying a potential role during gastrulation.

Cyclic ADP-ribose-gated Ca2+ release in sea urchin eggs requires an elevated

Cyclic ADP-ribose (cADPr) has been shown to release intracellular Ca2+ from sea urchin eggs and a variety of vertebrate cell types, although its mechanism of action remains elusive. We employed the caged version of cADPr to study the [Ca2+] transient kinetics in intact sea urchin eggs for insights into how cADPr gates Ca2+ release. Ca2+ release triggered by photolytic production of cADPr was initially slow, with an effective delay of several hundred milliseconds before the onset of a rapid Ca2+ release phase. In contrast, Ca2+ release induced by photolysis of caged inositol 1,4,5-trisphosphate was immediate in onset and roughly an order of magnitude faster. The delay before cADPr-induced Ca2+ release was eliminated when the [Ca2+] was step-elevated coincident with the photoliberation of cADPr and greatly prolonged in the presence of exogenous Ca2+ buffers. Thus, the slow onset of Ca2+ release does not reflect an intrinsically slow rate by which cADPr gates release channels. Rather, a [Ca2+] rise from resting levels is needed to achieve more than minimal cADPr activity. Full release of Ca2+ by cADPr in intact sea urchin eggs requires a positive Ca2+ feedback.

Effects of thimerosal on the transient kinetics of inositol 1,4,5-trisphosphate-induced Ca2+ release from cerebellar microsomes

Thimerosal, a thiol-reactive reagent, has been shown to increase the cytosolic Ca2+ concentration in a variety of cells by sensitizing inositol 1,4,5-trisphosphate (InsP3) receptors. Thimerosal can have both sensitizing (at concentrations of <2 microM) and inhibitory (at concentrations of >2 microM) effects on InsP3-induced Ca2+ release (IICR) from cerebellar microsomes. Transient kinetic studies were performed by employing a fluorimetric stopped-flow approach using fluo-3. IICR was found to be a bi-exponential process with a fast and a slow component. At a maximal InsP3 concentration (20 microM), the fast phase had a rate constant of 0.9 s-1 and the slow phase had a rate constant of 0.4 s-1. The amplitudes of the two phases were 60% and 40% respectively. When the rate constants for the two phases were plotted as Hill plots, the processes were found to be non-co-operative in both cases (Hill coefficient of 1.0), thus arguing for a simple mechanism linking InsP3 binding to channel opening. At a submaximal InsP3 concentration (0.2 microM), where the sensitizing effects of thimerosal are most pronounced, thimerosal increased the rate constants of both phases in a sigmoidal fashion, with a Hill coefficient of 4.0, suggesting that several cysteine residues (up to four) need to be modified in order for maximum sensitization to occur. The rate constants remained elevated even at thimerosal concentrations that inhibited IICR. The amplitude or extent of Ca2+ release was, however, elevated to a much greater extent in the slow phase, suggesting that the two phases respond differently. At maximal InsP3 concentrations, thimerosal has no effect upon the rate constants but inhibits the amplitude of Ca2+ release.

Effects of 1-methyladenine on nuclear Ca2+ transients and meiosis resumption in starfish oocytes are mimicked by the nuclear injection of inositol 1,4,5-trisphosphate and cADP-ribose

The treatment of prophase-arrested starfish oocytes with the hormone 1-methyladenine (1-MA) induces the elevation of Ca2+ in both the cytoplasm and the germinal vesicle (nucleus), and is followed by the resumption of meiosis. The injection of the modulators of the intracellular Ca2+ channels inositol 1,4,5-trisphosphate (InsP3) or cyclic adenosine diphosphate ribose (cADPr) into the germinal vesicle promoted meiosis resumption in the absence of 1-MA in about 50% of the oocytes. Caged InsP3 or caged cADPr were injected into the nuclei of oocytes together with the Ca2+ indicator calcium green dextran; their photoreleasing elicited nuclear calcium spikes which, in the case of cADPr, had repetitive behaviour. The spikes were abolished by the nuclear injection of antagonists or antibodies to the InsP3 or cADPr-sensitive Ca2+ channels. cADPr-modulated channels were localized on the membranes of the nuclear envelope using specific antibodies conjugated with IgG-gold complexes.

Disruption of the IP3 receptor gene of Drosophila affects larval metamorphosis and ecdysone release

BACKGROUND

The inositol 1,4,5-trisphosphate (IP3) receptor is an intracellular calcium channel that couples cell membrane receptors, via the second messenger IP3, to calcium signal transduction pathways within many types of cells. IP3 receptor function has been implicated in development, but the physiological processes affected by its function have yet to be elucidated. In order to identify these processes, we generated mutants in the IP3 receptor gene (itpr) of Drosophila and studied their phenotype during development.

RESULTS

All itpr mutant alleles were lethal. Lethality occurred primarily during the larval stages and was preceded by delayed moulting. Insect moulting occurs in response to the periodic release of the steroid hormone ecdysone which, in Drosophila, is synthesized and secreted by the ring gland. The observation of delayed moulting in the mutants, coupled with the expression of the IP3 receptor in the larval ring gland led us to examine the effect of the itpr alleles on ecdysone levels. On feeding ecdysone to mutant larvae, a partial rescue of the itpr phenotype was observed. In order to assess ecdysone levels at all larval stages, we examined transcripts of an ecdysone-inducible gene, E74; these transcripts were downregulated in larvae expressing each of the itpr alleles.

CONCLUSIONS

Our data show that disruption of the Drosophila IP3 receptor gene leads to lowered levels of ecdysone. Synthesis and release of ecdysone from the ring gland is thought to occur in response to a neurosecretory peptide hormone secreted by the brain. We propose that this peptide hormone requires an IP3 signalling pathway for ecdysone synthesis and release in Drosophila and other insects. This signal transduction mechanism which links neuropeptide hormones to steroid hormone secretion might be evolutionarily conserved.

7-Deaza-8-bromo-cyclic ADP-ribose, the first membrane-permeant, hydrolysis-resistant cyclic ADP-ribose antagonist

Cyclic ADP-ribose (cADPR) is a putative second messenger that has been demonstrated to mobilize Ca2+ in many cell types. Its postulated role as the endogenous regulator of ryanodine-sensitive Ca2+ release channels has been greatly supported by the advent and use of specific cADPR receptor antagonists such as 8-NH2-cADPR (Walseth, T. F., and Lee, H. C. (1993) Biochim. Biophys. Acta 1178, 235-242). However, investigations of the role of cADPR in physiological responses, such as fertilization, stimulus-secretion coupling, and excitation-contraction coupling, have been hindered by the susceptibility of cADPR receptor antagonists to hydrolysis and the need to introduce these molecules into cells by microinjection or patch clamp techniques. We have recently reported on the discovery of a poorly hydrolyzable analogue of cADPR, 7-deaza-cADPR (Bailey, V. C., Sethi, J. K., Fortt, S. M., Galione, A., and Potter, B. V. L. (1997) Chem. Biol. 4, 41-51) but this, like cADPR, is an agonist of ryanodine-sensitive Ca2+ release channels. We therefore explored the possibility of combining antagonistic activity with that of hydrolytic resistance and now report on the biological properties of the first hydrolysis-resistant cADPR receptor antagonist, 7-deaza-8-bromo-cADPR. In addition this compound has the advantage of being membrane-permeable. Together these properties make this hybrid molecule the most powerful tool to date for studying cADPR-mediated Ca2+ signaling in intact cells.

Intracellular injections of a soluble sperm factor trigger calcium oscillations and meiotic maturation in unfertilized oocytes of a marine worm

How sperm trigger activating calcium transients in eggs remains a central, unresolved question in fertilization biology. To determine if a soluble sperm factor can generate a fertilization-like calcium response in the absence of sperm-egg binding, aqueous extracts of sperm from the nemertean worm Cerebratulus lacteus were mixed with Ca2+-sensitive fluorescent dyes and injected into unfertilized, metaphase-I-arrested oocytes. Based on confocal imaging analyses, unfertilized oocytes that had been injected with sperm extract routinely produced oscillating Ca2+ waves and resumed meiotic maturation in a manner that closely resembled normal fertilization. Calcium oscillations and maturation were typically lacking in control oocytes that had been (i) injected with buffer alone or with buffer containing added calcium, (ii) given external treatments of the sperm factor, or (iii) injected with extracts made from cells other than sperm. Boiling or protease treatment essentially abolished the potency of the sperm extract, and nonboiled extracts retained full activity in >10-kDa fractions, but not in <10-kDa fractions. Collectively, such findings suggest that the sperm of C. lacteus possess a soluble protein that can bypass oolemmal surface receptors to act within the ooplasm as a trigger of repetitive Ca2+ waves and meiotic maturation. Results obtained in this study are discussed with respect to the minimum amount of extract needed for egg activation and whether the oscillogenic substance is sufficiently concentrated in a single sperm to play a biological role during fertilization.

Genetic evidence for involvement of type 1, type 2 and type 3 inositol 1,4,5-trisphosphate receptors in signal transduction through the B-cell antigen receptor

Stimulation of B-cell antigen receptor (BCR) induces a rapid increase in cytoplasmic free calcium due to its release from intracellular stores and influx from the extracellular environment. Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ligand-gated channels that release intracellular calcium stores in response to the second messenger, inositol 1,4,5-trisphosphate. Most hematopoietic cells, including B cells, express at least two of the three different types of IP3R. We demonstrate here that B cells in which a single type of IP3R has been deleted still mobilize calcium in response to BCR stimulation, whereas this calcium mobilization is abrogated in B cells lacking all three types of IP3R. Calcium mobilization by a transfected G protein-coupled receptor (muscarinic M1 receptor) was also abolished in only triple-deficient cells. Capacitative Ca2+ entry, stimulated by thapsigargin, remains unaffected by loss of all three types of IP3R. These data establish that IP3Rs are essential and functionally redundant mediators for both BCR- and muscarinic receptor-induced calcium mobilization, but not for thapsigargin-induced Ca2+ influx. We further show that the BCR-induced apoptosis is significantly inhibited by loss of all three types of IP3R, suggesting an important role for Ca2+ in the process of apoptosis.

Regulated exocytosis and sequential construction of the extracellular matrix surrounding the sea urchin zygote

After fertilization most eggs become surrounded by a complex extracellular matrix. This study examines those matrix assembly processes that are triggered by fertilization of the sea urchin egg. The study uses antibodies that identify five different storage compartments in the egg. These compartments release their protein contents in a highly regulated fashion to assemble and modify the extraembryonic layers. The exocytosis sequence begins with a fertilization wave that progresses from the site of sperm entry and elevates the fertilization envelope above a water-filled perivitelline space. The immediate surface of the zygote then becomes covered by a newly secreted hyaline layer. Prior to fertilization some of the antigens are localized to cortical granules. Others are found in "basal laminar vesicles" that are released in a wave beginning at about 30 sec, or roughly at the same time as cortical granule exocytosis. The remaining antigens are exocytosed with a rather precise timing, but with a delay of several to tens of minutes relative to the first wave of exocytosis. "Apical vesicles," so named because antigens from this class are preferentially exocytosed toward the apical cell surface of polarized cells, include antigens that are exocytosed beginning at about 5 min postfertilization. The fourth compartment, named "echinonectin vesicles" release echinonectin, a protein that is deposited to the inner side of the hyaline layer. Surface staining of echinonectin is first detected about 10-15 min following sperm contact. Finally, maternal cadherin, which is stored in yet a fifth distinct compartment, is not detected on the surface until at least 30 min following fertilization. The data are also consistent with the notion that the tightly regulated timing of exocytosis contributes to the ordered assembly of the hyaline layer and elevation of the fertilization envelope. Finally, two of the vesicle classes continue to exocytose after the cells become polarized. In polarized cells apical and basal laminar antigens are trafficked toward opposite sides of the same cell after passing through the same trans-Golgi network-like compartment.

A novel signalling mechanism for generating Ca2+ oscillations at fertilization in mammals

At fertilization in mammals the sperm activates the egg by triggering a series of oscillations in the intracellular free Ca2+ concentration. The precise sequence of events that occur between sperm-egg contact and the increases in intracellular Ca2+ remains unknown. Here, we discuss recent evidence supporting the hypothesis that a cytosolic sperm protein enters the egg after gamete membrane fusion and triggers Ca2+ oscillations from within the egg cytoplasm. Biochemical studies suggest that there exists a novel sperm protein, named oscillin, that specifically comigrates with Ca2+ oscillation-inducing activity. Oscillin has been immunolocalised to the region of the sperm that first fuses with the egg. The concept of a specific protein that triggers Ca2+ oscillations may have wider physiological significance since sperm oscillin can induce Ca2+ oscillations in somatic cells, such as neurons and hepatocytes. Unravelling the novel signalling system involved in mammalian fertilization may help reveal some fundamental molecular mechanisms responsible for triggering cytoplasmic Ca2+ oscillations.

Lithium blocks cell cycle transitions in the first cell cycles of sea urchin embryos, an effect rescued by myo-inositol

Lithium is a classical inhibitor of the phosphoinositide pathway and is teratogenic. We report the effects of lithium on the first cell cycles of sea urchin (Lytechinus pictus) embryos. Embryos cultured in 400 mM lithium chloride sea water showed marked delay to the cell cycle and a tendency to arrest prior to nuclear envelope breakdown, at metaphase and at cytokinesis. After removal of lithium, the block was reversed and embryos developed to form normal late blastulae. The lithium-induced block was also reversed by myo- but not epi-inositol, indicating that lithium was acting via the phosphoinositide pathway. Lithium microinjection before fertilization caused arrest prior to nuclear envelope breakdown at much lower concentrations (3-5 mM). Co-injection of myo-inositol prevented the block. Microinjection of 1–2 mM lithium led to block at the cleavage stage. This was also reversed by coinjection of myo-inositol. Embryos blocked by lithium microinjection proceeded rapidly into mitosis after photolysis of caged inositol 1,4,5-trisphosphate. These data demonstrate that a patent phosphoinositide signalling pathway is essential for the proper timing of cell cycle transitions and offer a possible explanation for lithium's teratogenic effects.

Studies on the mechanism for Cai-transients in sea urchin zygotes caused by refertilization and external application of sperm extract

Sea urchin zygotes can be refertilized when they are deprived of the fertilization membrane and the hyaline layer. We have earlier reported that a transient increase of the intracellular Ca2+ concentration (Cai-transient) is induced in zygotes refertilized by sperm or treated with a sperm extract (spex) (M. Osawa et al., 1994, Dev. Biol. 166, 268-276). We investigated quantitative characteristics of the Cai-transient induced by sperm and spex, using a Ca2+ indicator, Indo-1. When sperm or spex was applied to zygotes, the peak value of the Cai-transient was 1.16 or 0.69 microM, respectively. Although these values were lower than the peak value of 1.95 microM measured during normal fertilization, the entire time courses of the three types of Cai-transients were similar. The Cai-transients during fertilization is known to be caused both by the IP3-induced Ca2+ release (IICR) and by a mechanism independent of IICR. The Cai-transients during refertilization and fertilization were not inhibited by an IP3 receptor inhibitor, heparin or by a G-protein inhibitor, GDPbetaS. However, heparin delayed the time courses of both Cai-transients. These results suggest that there may be two signal transduction pathways operating during refertilization, one dependent and the other independent of IICR. By contrast, both heparin and GDPbetaS inhibited the spex-induced Cai-transient. The IP3 content in spex-treated zygotes increased, and the spex-induced Cai-transient occurred even in the absence of external Ca2+. Cai-transient was not observed when spex was injected into zygotes. These data suggest that spex induces IICR in zygotes by activating certain cell surface receptors coupled to G-proteins.

Developmental expression of the inositol 1,4,5-trisphosphate receptor and structural changes in the endoplasmic reticulum during oogenesis and meiotic maturation of Xenopus laevis

To study the development of the calcium release mechanism, we examined the temporal and spatial expression of inositol 1,4,5-trisphosphate receptor (IP3R) during the oogenesis and meiotic maturation of Xenopus laevis. Observation of a series of fixed samples by immunofluoresence microscopy revealed a relocalization of Xenopus IP3R (XIP3R)-positive structures during meiotic maturation. We visualized the endoplasmic reticulum (ER) using ER-sensitive dye, DiI, by observation under confocal laser scanning microscopy. Time-lapse visualization of the living oocytes revealed that while the ER of immature fully grown oocytes underwent relatively little movement, the ER of maturing oocytes and mature eggs moved rapidly. A possible role for the increase of ER mobility in the dynamic redistribution of XIP3R during oocyte maturation is also discussed.

Caged nicotinic acid adenine dinucleotide phosphate. Synthesis and use

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a metabolite of NADP with Ca2+ mobilizing activity. The Ca2+ release mechanism activated by NAADP as well as the Ca2+ stores that it acts on are different from those activated by either cyclic ADP-ribose or inositol 1,4,5-trisphosphate (IP3) (Lee, H. C., and Aarhus, R. (1995) J. Biol. Chem. 270, 2152-2157). In order to demonstrate unambiguously that NAADP can mobilize Ca2+ stores in live cells, a caged analog was synthesized by reacting NAADP with 1-(2-nitrophenyl)diazoethane. Anion exchange high pressure liquid chromatography (HPLC) was used to purify one particular caged form from the mixture of products. Phosphate analyses following specific enzymatic cleavage indicate that the caging group is on the 2'-phosphate. This is confirmed by 31P NMR spectroscopy, showing that the 2'-phosphate of the caged compound exhibits an altered chemical shift of -2.6 ppm as compared with 2.3 ppm determined for the 2'-phosphate of NAADP. Caged NAADP had no Ca2+ releasing activity at a concentration as high as 1 micro;M when tested on sea urchin egg microsomes. After photolysis, it released Ca2+, was effective in nanomolar range, and was indistinguishable from authentic NAADP. The regeneration of NAADP after photolysis was also confirmed by HPLC analyses. The analog is particularly susceptible to UV and can be efficiently photolyzed using a spectrofluorimeter. To demonstrate its utility in live cells, caged NAADP was microinjected into sea urchin eggs. Photolysis effectively regenerated NAADP and activated Ca2+ oscillations in the eggs. Removal of external Ca2+ did not prevent the Ca2+ oscillations but only delayed the second Ca2+ peak by about 45 s, indicating that the oscillations are due to release from internal stores and not caused by Ca2+ influx. A mechanism based on sensitization of the Ca2+ release by Ca2+ loading is proposed to account for the Ca2+ oscillation observed.

Cyclic ADP-ribose signaling in sea urchin gametes: metabolism in spermatozoa

The molecular mechanism that initiates Ca2+ signaling in sea urchin egg fertilization has not yet been clarified. To determine whether sea urchin sperm may generate and possibly supply cyclic ADP-ribose (cADPR) as a Ca2+-releasing factor in the course of sea urchin egg fertilization, we determined cADPR content and the capacity for cADPR synthesis in sea urchin sperm. cADPR content was determined using the sea urchin egg homogenate Ca2+-release bioassay combined with high-performance liquid chromatography (HPLC). We found that sperm homogenates synthesized cADPR from beta-NAD but did not synthesize cADPR when alpha-NAD was the substrate. The identity of cADPR generated by sperm homogenates was verified by HPLC analysis, use of specific Ca2+-release antagonists, and homologous desensitization of the sea urchin egg homogenate Ca2+-release bioassay. The ambient content of cADPR was approximately 0.3 nmol cADPR/g wet wt sea urchin sperm. Our results show that sperm can synthesize cADPR and that they contain cADPR levels comparable to other tissues.

A ryanodine-sensitive calcium store in ascidian eggs monitored by whole-cell patch-clamp recordings

Using whole cell patch clamp recordings on unfertilized eggs of the ascidian Ciona intestinalis, we are able to detect ryanodine receptors within the oocytes. Our approach is based on measurements of the voltage-activated inward calcium currents. Two types of Ca2+ currents have been described on the oocyte membrane of Ciona: a low threshold slowly activating current, and a high threshold faster one. We show here that caffeine induces a decrease in the intensity of the Ca2+ currents, when applied either externally or internally from the mouth of a patch pipette. Caffeine application mimics fertilization which transiently decreases the high threshold Ca2+ current density during density during the first meiotic cycle. Ryanodine (> 1 nM) has an effect similar to caffeine. This partial decrease in Ca2+ current density elicited by caffeine or ryanodine is prevented by intracellular application of the calcium chelator BAPTA, then imputable to calcium release. In summary, the depolarization-induced Ca2+ current intensity allows monitoring of an intracellular calcium store which is sensitive to low concentrations of ryanodine in Ciona oocytes. Further identification of a ryanodine receptor was obtained by immunological staining with antibodies against mammalian skeletal muscle ryanodine receptor. Ryanodine receptors were asymmetrically localized in the cortex of Ciona eggs. We discuss the methodological relevance of our patch-clamp approach, in connection with the possible biological role of such a ryanodine receptor in the early stages of development.

Thiol oxidation by 2,2'-dithiodipyridine causes a reversible increase in cytoplasmic free Ca2+ concentration in pancreatic beta-cells. Role for inositol 1,4,5-trisphosphate-sensitive Ca2+ stores

2,2'-Dithiodipyridine (2,2'-DTDP), a reactive disulphide that mobilizes Ca2+ from ryanodine-sensitive Ca2+ stores in muscle, induced a biphasic increase in cytoplasmic free Ca2+ concentration ([Ca2+]i) in pancreatic beta-cells loaded with fura 2. This increase consisted of an early transient followed by a second, slower, rise. The [Ca2+]i transient was dependent on extracellular Ca2+ and disappeared on treatment with nimodipine. The reactive disulphide caused plasma membrane depolarization, as studied by the perforated-patch configuration of the patch-clamp technique. Hence membrane depolarization and opening of the L-type voltage-gated Ca2+ channels were responsible for the first transient in [Ca2+]i. The second slower increase in [Ca2+]i was prolonged but readily reversed by the disulphide-reducing agent 1,4-dithiothreitol. This increase in [Ca2+]i was not decreased by nimodipine or by omission of extracellular Ca2+, but was eliminated when the Ins(1,4,5)P3-sensitive Ca2+ pool was first depleted by carbachol. Ryanodine or its beta-alanyl analogue did not release Ca2+ from intracellular stores, and a high concentration of ryanodine did not inhibit Ca2+ release by 2,2'-DTDP. The disulphide compound suppressed glucose metabolism and decreased the mitochondrial inner-membrane potential. We conclude that thiol oxidation by 2,2'-DTDP affects Ca2+ homeostasis in beta-cells by multiple mechanisms. However, unlike the situation in muscle, in beta-cells 2,2'-DTDP releases Ca2+ from intracellular pools by mechanisms that do not involve activation of ryanodine receptors. Instead, in these cells the Ins(1,4,5)P3-sensitive intracellular Ca2+ store comprises an alternative target for the Ca(2+)-mobilizing action of the reactive disulphide compound.

Extracellular Ca2+ entry and Ca2+ release from inositol 1,4,5-trisphosphate-sensitive stores function at fertilization in oocytes of the marine bivalve Mytilus edulis

An oocyte of the marine bivalve Mytilus edulis, which is arrested at metaphase I, reinitiates meiosis at fertilization. The fertilized oocyte shows increases in intracellular Ca2+ ([Ca2+]i) comprising three different phases: an initial large [Ca2+]i transient, a subsequent low but sustained [Ca2+]i elevation, and repetitive small [Ca2+]i transients. In this study, we have investigated the sources and mechanisms of the sperm-induced [Ca2+]i increases. Application of methoxyverapamil (D-600), an inhibitor of voltage-dependent Ca2+ influx, suppressed the initial [Ca2+]i transient but did not affect the following two phases of [Ca2+]i changes. Injection of heparin, an antagonist of the inositol 1,4,5-trisphosphate (IP3) receptor, inhibited the later two phases without much affecting the initial transient. Combined application of D-600 and heparin almost completely abolished the three phases of the sperm-induced [Ca2+]i changes. Furthermore, Ca2+ influx caused by seawater containing excess K+ was blocked by D-600 but not by heparin, and IP3-induced Ca2+ release caused by photolysis of injected ‘caged’ derivatives of IP3 was blocked by heparin but not by D-600. These results strongly suggest that two types of Ca2+ mobilization systems, the extracellular Ca2+ entry responsible for an initial [Ca2+]i transient and the IP3 receptor-mediated Ca2+ release responsible for the following two phases of [Ca2+]i changes, function at fertilization of Mytilus oocytes.

Nicotinamide inhibits cyclic ADP-ribose-mediated calcium signalling in sea urchin eggs

Cyclic ADP ribose (cADPR) is a potent Ca(2+)-releasing agent, and putative second messenger, the endogenous levels of which are tightly regulated by synthetic (ADP-ribosyl cyclases) and degradative (cADPR hydrolase) enzymes. These enzymes have been characterized in a number of mammalian and invertebrate tissues and their activities are often found on a single polypeptide. beta-NAD+, cGMP and nitric oxide (NO) have been reported to mobilize Ca2+ in the sea urchin egg via the cADPR-mediated pathway. We now report that in sea urchin egg homogenates, nicotinamide inhibits the Ca(2+)-mobilizing action of beta-NAD+, cGMP and NO, but has no effect on cADPR-induced Ca2+ release. Moreover, nicotinamide inhibits cGMP-induced regenerative Ca2+ waves in the intact sea urchin egg. By successfully separating the cADPR-metabolizing machinery from that which releases Ca2+, we have shown that nicotinamide inhibits cADPR-mediated Ca2+ signalling at the level of cADPR generation. Importantly, nicotinamide had no effect upon the hydrolysis of cADPR, and its selective action on cyclase activity was supported by its inhibition of purified Aplysia ADP-ribosyl cyclase, which does not exhibit detectable hydrolytic activity. The action of nicotinamide in blocking Ca2+ release by beta-NAD+, cGMP and NO strongly suggests that these agents act as modulators of cADPR synthesis rather than to sensitize calcium release channels to cADPR.

Adenine nucleotide diphosphates: emerging second messengers acting via intracellular Ca2+ release

Release of Ca2+ from intracellular stores is a widespread mechanism in regulation of cell function. Two hitherto unknown adenine diphosphonucleotides were recently identified, which trigger Ca2+ release from intracellular stores via channels that are distinct from the well-known receptor/channel controlled by inositol 1,4,5,-trisphosphate (IP3): cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). Here we review synthesis of cADPR from beta-NAD, its hydrolysis to adenosine diphosphoribose (noncyclic) by cADPR glycohydrolase, as well as our knowledge about the metabolism of NAADP. The Ca2+ release triggered by cADPR, NAADP, or IP3 can be distinguished by the action of inhibitors and by desensitization studies. Evidence now emerges that cADPR synthesis from beta-NAD can be stimulated, at least in some cell types by all-trans-retinoic acid as a first messenger. We then review the properties of cADPR and NAADP as potential second messengers in the intracrine regulation of cell functions. Although their exact role in signaling sequences is not yet known, cADPR and NAADP are likely to play important intracellular regulatory functions, as extensively documented for the process of egg fertilization.

The inositol 1,4,5-trisphosphate-gated Ca2+ channel: effect of the protein thiol reagent thimerosal on channel activity

The solubilized partially purified Ins(1,4,5)P3-sensitive Ca2+ channel from rat cerebellum has been reconstituted into planar lipid bilayer membranes by the 'tip-dip' method [Ehrlich (1992) Methods Enzymol. 207, 463-471] allowing low noise current records. Single-channel events have been recorded. In the presence of 10 microM Ins(1,4,5)P3, 50 microM ATP, and 0.2 microM Ca2+ the Ins(1,4,5)P3 receptor channel opens to a conductance level of 53 pS. In the presence of 100 microM thimerosal (TMS), a sulphydryl-oxidizing agent, three subconductance levels (60 pS, 80 pS and 120 pS) were observed. More than one population of mean open times was found, both in the absence and presence of TMS, although TMS affected the length of the open time by decreasing the short opening significantly from 4.05 ms to 2.78 ms and increasing the longer open time from 27.8 ms to 94.8 ms. The results indicate that TMS enhances Ins(1,4,5)P3-induced Ca2+ release by both altering the open times of the channel significantly and causing a shift to higher subconductance levels.

Transient release of calcium from inositol 1,4,5-trisphosphate-specific stores regulates mouse preimplantation development

Inositol 1,4,5-trisphosphate can regulate growth and differentiation by modulating the release of intracellular Ca2+ in a variety of cellular systems, and it is involved in oocyte activation. Recent studies suggest that mammalian preimplantation development may also be regulated by the release of Ca2+ from intracellular stores. The rate of cavitation and cell division was accelerated after a transient elevation of intracellular Ca2+ levels was induced in morulae by exposure to ethanol or ionomycin. Embryos exposed to BAPTA-AM, a chelator of intracellular Ca2+, exhibited a brief dose-dependent reduction in basal Ca2+ levels, a temporal inhibition of ionophore-induced Ca2+ signalling and a subsequent delay in blastocoel formation. BAPTA-AM at 0.5 microM did not significantly alter the basal intracellular calcium level, but chelated Ca2+ that was released after ethanol exposure and thereby attenuated the ethanol-induced acceleration of cavitation. BAPTA-AM also inhibited cell division to the 16-cell stage in a dose-dependent manner, which correlated with the inhibition of cavitation. Thimerosal and inositol 1,4,5-trisphosphate significantly elevated the intracellular Ca2+ concentration in mouse morula-stage embryos, providing evidence for the existence of inositol 1,4,5-trisphosphate-sensitive Ca2+ stores. Although caffeine failed to release intracellular Ca2+, ryanodine induced a small biphasic release of Ca2+, suggesting that ryanodine-sensitive Ca2+ stores may also exist in mouse embryos. Morulae exposed to the calmodulin inhibitor W-7 exhibited a dose-dependent delay in blastocoel formation. A 4 hour exposure to 10 microM W-7 did not significantly alter cavitation, but attenuated the ionophore-induced stimulation of blastocoel formation. This finding suggests that the developmental effects produced through Ca2+ signalling are mediated by calmodulin. Our results demonstrate that Ca2+ release in mouse morulae occurs predominantly through the inositol 1,4,5-trisphosphate receptor, and that alteration of intracellular Ca2+ levels can accelerate or delay embryonic growth and differentiation, providing a mechanistic link between the regulation of oocyte and embryonic development.

Unique inactivation properties of NAADP-sensitive Ca2+ release

Ca2+ mobilization from intracellular stores constitutes an important mechanism for generating cytoplasmic Ca2+ signals. Inositol trisphosphate (InsP3) and ryanodine receptors are the two families of intracellular Ca2+ release channels that have been identified, which may be regulated by separate intracellular messengers, InsP3, and cyclic adenosine 5'-diphosphate ribose, respectively. A third molecule, nicotinic acid adenine dinucleotide phosphate (NAADP), has recently been recognized as a potent Ca2+ releasing agent in sea urchin eggs and microsomes. We now report that non-releasing concentrations of NAADP fully and irreversibly inactivate the NAADP-sensitive Ca2+ release mechanism. This phenomenon occurred both in intact sea urchin eggs and in homogenates and is not shared by either InsP3 or cyclic adenosine 5'-diphosphate ribose. The novel properties of this Ca2+ release mechanism, giving a one-shot Ca2+ release, may be suited to irreversible cellular events.

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.

Isolation of a full-length cDNA encoding calreticulin from a PCR library of in vitro zygotes of maize

A full-size cDNA clone (1614 bp) encoding calreticulin was isolated from a PCR-based cDNA library of maize in vitro zygotes. Calreticulin is a major Ca2+ storage protein located mainly in the lumen of the endoplasmic reticulum but also in the nucleus and/or cytoplasm of some cells. A differential screening between cDNA libraries originating from 104 in vitro zygotes (18 h after in vitro fertilization) and 128 unfertilized egg cells was performed to isolate newly expressed genes or genes expressed more abundantly after fertilization. The expression of the isolated cDNA clone is enhanced after fertilization and strongly correlated to cell division. Sequence comparison to a shorter maize calreticulin cDNA isolated from a PCR based cDNA library construction from a few plant cells [12]. It is further shown that calreticulins in maize are probably transcribed from a small gene family differentially expressed in abundance in diverse tissues. The deduced amino acid sequence encodes an acidic protein (pI 4.17) of 48 kDa sharing 77-92% and 50-54% homology to other plant and animal calreticulins, respectively. The described calreticulin gene represents to our knowledge the first cDNA clone isolated from a RT/PCR cDNA library originating from only a few plant cells and is the first gene isolated from zygotes of higher plants.

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.

Ca2+ release triggered by nicotinate adenine dinucleotide phosphate in intact sea urchin eggs

Nicotinate adenine dinucleotide phosphate (NAADP) was recently identified [Lee and Aarhus (1995) J. Biol. Chem. 270, 2152-2157; Chini, Beers and Dousa (1995) J. Biol. Chem. 270, 3116-3223] as a potent Ca(2+)-releasing agent in sea urchin egg homogenates. NAADP triggered Ca2+ release by a mechanism that was distinct from inositol 1,4,5-trisphosphate (InsP3)- and cyclic ADP-ribose (cADPR)-induced Ca2+ release. When NAADP was microinjected into intact sea urchin eggs it induced a dose-dependent increase in cytoplasmic free Ca2+ which was independent of the extracellular [Ca2+]. The Ca2+ waves elicited by microinjections of NAADP originated at the site of injection and swept across the cytosol. As previously found in sea urchin egg homogenates, NAADP-induced Ca2+ release in intact eggs was not blocked by heparin or by prior desensitization to InsP3 or cADPR. Thio-NADP, a specific inhibitor of the NAADP-induced Ca2+ release in sea urchin homogenates [Chini, Beers and Dousa (1995) J. Biol. Chem. 270, 3116-3223] blocked NAADP (but not InsP3 or cADPR) injection-induced Ca2+ release in intact sea urchin eggs. Finally, fertilization of sea urchin eggs abrogated subsequent NAADP-induced Ca2+ release, suggesting that the NAADP-sensitive Ca2+ pool may participate in the fertilization response. This study demonstrates that NAADP acts as a selective Ca(2+)-releasing agonist in intact cells.

Quantal responses to inositol 1,4,5-trisphosphate are not a consequence of Ca2+ regulation of inositol 1,4,5-trisphosphate receptors

Submaximal concentrations of inositol 1,4,5-trisphosphate (InsP3) rapidly release only a fraction of the InsP3-sensitive intracellular Ca2+ stores, despite the ability of further increases in InsP3 concentration to evoke further Ca2+ release. The mechanisms underlying such quantal Ca2+ mobilization are not understood, but have been proposed to involve regulatory effects of cytosolic Ca2+ on InsP3 receptors. By examining complete concentration-effect relationships for InsP3-stimulated 45Ca2+ efflux from the intracellular stores of permeabilized hepatocytes, we demonstrate that, at 37 degrees C, responses to InsP3 are quantal in Ca(2+)-free media heavily buffered with either EGTA or BAPTA [1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid]. Lower concentrations of InsP3 were used to examine the kinetics of Ca2+ mobilization at 2 degrees C, because at the lower temperature the stores were more sensitive to InsP3: the concentration of InsP3 causing half-maximal Ca2+ release (EC50) after a 30 s incubation decreased from 281 +/- 37 nM at 37 degrees C to 68 +/- 3 nM at 2 degrees C. At 2 degrees C, the EC50 for InsP3-stimulated Ca2+ mobilization decreased as the duration of exposure to InsP3 was increased: the EC50 was 68 +/- 3 nM after 30 s, and 29 +/- 2 nM after 420 s. InsP3-stimulated Ca2+ mobilization is therefore non-quantal at 2 degrees C: InsP3 concentration determines the rate, but not the extent, of Ca2+ release. By initiating quantal responses to InsP3 at 37 degrees C and then simultaneously diluting and chilling cells to 2 degrees C, we demonstrated that the changes that underlie quantal responses do not rapidly reverse at 2 degrees C. At both 37 degrees C and 2 degrees C, modest increases in cytosolic Ca2+ increased the sensitivity of the stores to InsP3, whereas further increases were inhibitory; both Ca2+ effects persisted after prior removal of ATP. We conclude that the effects of Ca2+ on InsP3 receptors are unlikely either to be enzyme-mediated or to underlie the quantal pattern of Ca2+ release evoked by InsP3.

Agonist-stimulated cyclic ADP ribose. Endogenous modulator of Ca(2+)-induced Ca2+ release in intestinal longitudinal muscle

We have previously shown that agonist-induced Ca2+ mobilization in intestinal longitudinal muscle is mediated by ryanodine-sensitive, inositol 1,4,5-trisphosphate-insensitive sacroplasmic Ca2+ channels. Ca2+ release via these channels is triggered by agonist-stimulated Ca2+ influx and results in Ca(2+)-induced Ca2+ release. The present study examined whether cyclic ADP-ribose (cADPR) is synthesized in response to stimulation of longitudinal muscle by agonists and modulates the activity of Ca2+ release channels. Cyclic ADPR bound with high affinity to dispersed longitudinal muscle cells (IC50 1.9nM) and induced Ca2+ release (EC50 3.8 nM), increase in [Ca2+]i (EC50 2.0 nM), and contraction (EC50 1.1 nM); cADPR had no effect on circular muscle cells. The effects of cADPR were blocked by ruthenium red, dantrolene, and the specific antagonist, 8-amino-cADPR, and were augmented by caffeine but not affected by heparin. The binding of cADPR and its ability to stimulate Ca2+ release were dependent on the concentration of Ca2+. Cyclic ADPR was capable of stimulating Ca2+ release at subthreshold Ca2+ concentrations (25-100 nM) and of enhancing Ca(2+)-induced Ca2+ release. Longitudinal muscle extracts incubated with beta-NAD+ produced a time-dependent increase in Ca(2+)-mobilizing activity identified as authentic cADPR by blockade of Ca2+ release with 8-amino-cADPR and ruthenium red. Ca2+ mobilizing activity was increased by cholecystokinin octapeptide (CCK-8) in a concentration-dependent fashion. The increase induced by CCK-8 was suppressed by the CCK-A antagonist, L364,718, nifedipine, and guanyl-5'-yl thiophosphate. The study shows that ADP-ribosyl cyclase can be stimulated by agonists and that cADPR can act as an endogenous modulator of Ca(2+)-induced Ca2+ release.

The existence of inositol 1,4,5-trisphosphate and ryanodine receptors in mature bovine oocytes

Intracellular Ca2+ (Ca2+i) transients during fertilization are critical to the activation of eggs in all species studied. Activation of both the inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and ryanodine receptor (RYR) are responsible for the calcium oscillations during fertilization in sea urchin eggs. Using in vitro matured bovine oocytes loaded with Fura-2 AM ester as Ca2+i indicator, we addressed whether IP3Rs and RYRs coexist in mammalian eggs. Our results indicate that microinjection of 50–250 nM IP3 or 10–20 mM caffeine, 100–200 microM ryanodine and 4–8 microM cyclic ADP-ribose all induced Ca2+i release. The Ca2+i release induced by 250 nM IP3 could only be inhibited by prior injection of 1 mg/ml heparin which was overcome by continuous injection of IP3 to 1 microM. Prior injection of either 50 microM ruthenium red, 50 microM procaine or 1 % vehicle medium (VM) did not affect the Ca2+i release induced by IP3. Prior injection of heparin or VM did not affect the Ca2+i release induced by 10–20 mM caffeine or 200 microM ryanodine, but prior injection of 50 microM ruthenium red or procaine completely inhibited the effect of 10–20 mM caffeine. In addition, continuous injection of caffeine up to 40 mM overcame the inhibitory effect of ruthenium red or procaine. The same 50 microM concentration of ruthenium red or procaine only partially blocked the effect of 200 microM ryanodine, but 200 microM ruthenium red or procaine completely blocked the effect of 200 microM ryanodine.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium influx, fertilisation potential and egg activation in Fucus serratus

Fertilisation in the marine alga Fucus serratus is accompanied by increased influx of Ca2+ from the external medium. The onset of this increase, monitored with the Mn2+ fluorescence quench technique, corresponded with the depolarisation phase of the fertilisation potential. External Ca2+ was necessary for the onset of the fertilisation potential and the early activation events, including cell wall exocytosis. Removal of Ca2+ from, or addition of Sr2+ to, the external medium during the fertilisation potential reduced the magnitude of the depolarisation and prolonged its duration. While fertilisation potentials could not be elicited in the presence of 0.1 mM Ca2+, addition of Ba2+ in the presence of 0.1 mM Ca2+ allowed normal fertilisation potential and egg activation. Microinjection of ryanodine or cyclic guanosine 5' -monophosphate (cGMP) did not induce cytoplasmic Ca2+ elevation or egg activation. Inositol 1,4,5-triphosphate [Ins(1,4,5)P3] produced a transient elevation of cytoplasmic Ca2+, monitored using ratio photometry, but did not cause cell wall exocytosis except at the site of microinjection. The results demonstrate an essential role for Ca2+ influx during Fucus egg activation. The relative importance of influx and intracellular Ca2+ release in Fucus egg activation is discussed.

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.

Regulation of mouse egg activation: presence of ryanodine receptors and effects of microinjected ryanodine and cyclic ADP ribose on uninseminated and inseminated eggs

Sperm-induced activation of mammalian eggs is associated with a transient increase in Ca2+ concentrations thought to be derived from inositol 1,4,5-trisphosphate-sensitive and -insensitive intracellular stores. Whereas the importance of inositol 1,4,5-trisphosphate-sensitive Ca2+ stores has been evaluated, the identity and role of inositol 1,4,5-trisphosphate-insensitive stores are poorly understood. To explore the role of the ryanodine-sensitive Ca2+ store, we first used reverse transcription-polymerase chain reaction to identify transcripts of the ryanodine receptor in eggs and determined that transcripts for the type 2 and 3 receptor were present. Immunoprecipitation of radioiodinated egg extracts with an antibody that recognizes both type 2 and 3 receptors detected specifically a band of Mr = 520,000. Immunolocalization of the receptor(s) using laser-scanning confocal microscopy revealed that the receptor(s) was uniformly distributed in the cortex of the germinal vesicle-intact oocyte, but became asymmetrically localized to the cortex in a region apposed to the meiotic spindle in the metaphase II-arrested egg; this asymmetrical localization developed by metaphase I. The role of the ryanodine receptor in mouse egg activation was examined by determining the effects of microinjected ryanodine or cyclic ADP ribose on endpoints of egg activation in either uninseminated or inseminated eggs. Ryanodine induced the conversion of the zona pellucida glycoprotein ZP2 to its postfertilization form ZP2f in a biphasic concentration-dependent manner; nanomolar concentrations stimulated this conversion, whereas micromolar concentrations had no stimulatory effect. Cyclic ADP ribose also promoted the ZP2 conversion, but with a hyperbolic concentration dependence. Neither of these compounds induced cell cycle resumption. Inhibiting the inositol 1,4,5-trisphosphate-sensitive Ca2+ store did not inhibit the ryanodine-induced ZP2 conversion and, reciprocally, inhibiting the ryanodine-sensitive Ca2+ store did not inhibit the inositol 1,4,5-trisphosphate-induced ZP2 conversion. Last, treatment of eggs under conditions that would block the release of Ca2+ from the ryanodine-sensitive store had no effect on any event of egg activation following fertilization. Results of these experiments suggest that although ryanodine receptors are present and functional, release of Ca2+ from this store is not essential for sperm-induced egg activation.

The structure of the Aplysia kurodai gene encoding ADP-ribosyl cyclase, a second-messenger enzyme

The complete nucleotide (nt) sequences of the cDNA and gene encoding the marine mollusk Aplysia kurodai (Ak) ADP-ribosyl cyclase (ADRC) which synthesizes cyclic ADP-ribose (cADP-ribose), a second messenger for Ca2+ mobilization from endoplasmic reticulum, were determined. Ak ADRC consists of 258 amino acids (aa) (29 kDa). It shares 86% aa sequence homology with that from A. californica, and 31-32% homology with the human, rat and mouse cluster of differentiation 38 (CD38) that has both ADRC and cADP-ribose hydrolase activities. The Ak ADRC-encoding gene (ADRC) spans approx. 7 kb and contains eight exons and seven introns. The transcription start point (tsp) determined by primer extension analysis and S1 mapping is 28 bp downstream from the TATA box. This gene is expressed specifically in the ovotestis, although the mammalian CD38-encoding gene is expressed in many kinds of tissues and cells. The 5'-flanking region contains several consensus sequences responsible for the germ-cell-specific expression of the mouse zona pellucida 3 (ZP3) and Drosophila melanogaster chorion genes. The existence of the consensus sequences located at nt -1649, -1161, -234 and -90 may account for the ovotestis-specific expression of the Ak ADRC gene.

Beakers versus breakers: how fertilisation in the laboratory differs from fertilisation in nature

The fertilisation of free-spawning invertebrates, mainly sea urchins, has been studied extensively during the last hundred years. However, results obtained from in vitro experiments do not always reflect what happens in the real world. Organisms in their natural habitats have a complex set of challenges, cues and behaviours to contend with during fertilisation and early development, factors that are normally not considered in the laboratory setting. This review examines recent work on fertilisation ecology and discusses the relevance of these results to the findings gleaned from laboratory research. Emphasis is placed on stresses associated with fertilisation in situ, and how responses to environmental stresses (such as from turbulence, oxidative stress, ultraviolet radiation and pathogens) might affect the fertilisation process.

Calcium signaling in neurons: molecular mechanisms and cellular consequences

Neuronal activity can lead to marked increases in the concentration of cytosolic calcium, which then functions as a second messenger that mediates a wide range of cellular responses. Calcium binds to calmodulin and stimulates the activity of a variety of enzymes, including calcium-calmodulin kinases and calcium-sensitive adenylate cyclases. These enzymes transduce the calcium signal and effect short-term biological responses, such as the modification of synaptic proteins and long-lasting neuronal responses that require changes in gene expression. Recent studies of calcium signal-transduction mechanisms have revealed that, depending on the route of entry into a neuron, calcium differentially affects processes that are central to the development and plasticity of the nervous system, including activity-dependent cell survival, modulation of synaptic strength, and calcium-mediated cell death.