Fertilization Membrane Formation in Sea Urchin Eggs Induced by Drugs Known to Cause Ca2+Release from Isolated Sarcoplasmic Reticulum. (sea urchin egg/fertilization membrane/ryanodine/micronazole/procaine)

Comparative biology of calcium signaling during fertilization and egg activation in animals

During animal fertilizations, each oocyte or egg must produce a proper intracellular calcium signal for development to proceed normally. As a supplement to recent synopses of fertilization-induced calcium responses in mammals, this paper reviews the spatiotemporal properties of calcium signaling during fertilization and egg activation in marine invertebrates and compares these patterns with what has been reported for other animals. Based on the current database, fertilization causes most oocytes or eggs to generate multiple wavelike calcium oscillations that arise at least in part from the release of internal calcium stores sensitive to inositol 1,4,5-trisphosphate (IP3). Such calcium waves are modulated by upstream pathways involving oolemmal receptors and/or soluble sperm factors and in turn regulate calcium-sensitive targets required for subsequent development. Both "protostome" animals (e.g., mollusks, annelids, and arthropods) and "deuterostomes" (e.g., echinoderms and chordates) display fertilization-induced calcium waves, IP3-mediated calcium signaling, and the ability to use a combination of external calcium influx and internal calcium release. Such findings fail to support the dichotomy in calcium signaling modes that had previously been proposed for protostomes vs deuterostomes and instead suggest that various features of fertilization-induced calcium signals are widely shared throughout the animal kingdom.

Probable participation of phospholipase A2 reaction in the process of fertilization-induced activation of sea urchin eggs

In sea urchin eggs activated by sperm, A23187 or melittin, BPB (4-bromophenacyl bromide, a phospholipase A2 inhibitor) blocked fertilization envelope formation and transient CN(-)-insensitive respiration in a concentration-dependent manner. BPB had virtually no effect on the increase in [Ca2+]i (cytosolic Ca2+ level), the activity of phosphorylase a and the rate of protein synthesis, as well as acid production and augmentation of CN(-)-sensitive respiration. BPB also inhibited fertilization envelope formation and augmentation of CN(-)-insensitive respiration induced by melittin. Melittin, known to be an activator of phospholipase A2, induced the envelope formation, acid production, augmentation of CN(-)-insensitive and sensitive respiration, but did not cause any increase in [Ca2+]i, the phosphorylase a activity and the rate of protein synthesis. An activation of phospholipase A2 induced by Ca2+ or melittin seems to result in cortical vesicle discharge and production of fatty acids, which are to be utilized in CN(-)-insensitive lipid peroxidase reactions. Activation of other examined cell functions in eggs activated by sperm or A23187, probably results from Ca(2+)-triggered sequential reactions other than Ca(2+)-caused activation of phospholipase A2.

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.

Ca2+ release induced by cyclic ADP-ribose

Cyclic ADP-ribose (cADPR) is the most potent Ca(2+)-mobilizing agent known. It has been found in many different cell types, where it is synthesized from its precursor NAD(+) by ADP-ribosyl cyclases. cADPR binds to Ca(2+) channels in the endoplasmic reticulum membrane to activate a Ca(2+)-release mechanism. This release is itself potentiated by elevated cytoplasmic Ca(2+) concentrations. Thus, cADPR may function as an endogenous regulator of Ca(2+)-induced Ca(2+) release, and there is excitement that it may also function as a Ca(2+)-mobilizing second messenger.

Calcium control of metamorphosis in polychaete larvae

The importance of Ca2+ in the control of metamorphosis of a marine invertebrate larva was investigated. An excess of [Ca2+] in the external medium induced metamorphosis of Phragmatopoma californica (polychaete) larvae in a concentration-dependent manner. This effect is specific for calcium, and not simply the result of osmotic changes, as an excess of Mg2+ did not induce metamorphosis. Consistent with this finding, the calcium ionophore, A23187, also induced metamorphosis in a concentration-dependent manner. Paradoxically, however, the aromatic compounds diltiazem, verapamil, D600, and nifedipine, known to block Ca2+ channels in other systems, also induced metamorphosis. When exposed to diltiazem for only 20 h and subsequently washed free of this compound, 95% of the larvae metamorphosed and developed normally. Previous studies have demonstrated that the induction of metamorphosis in Phragmatopoma californica is controlled by chemosensory recognition of an exogenous morphogen and mediated by an excitatory pathway that involves adenyl cyclase and cyclic AMP. Because cellular excitation and cyclic AMP-dependent signal transduction generally involve the participation of calcium ion, the most parsimonious explanation for the results reported here include (1) direct control of the morphogenetic pathway by calcium ion, and (2) complexities of the calcium regulation of this process, or a functional similarity between the structurally related aromatic effectors tested and the natural inducer of metamorphosis.

The Xenopus IP3 receptor: structure, function, and localization in oocytes and eggs

To study the role of the IP3 receptor (IP3R) upon egg activation, cDNA clones encoding IP3R expressed in the Xenopus oocytes were isolated. By analyses of the primary structure and functional expression of the cDNA, Xenopus IP3R (XIP3R) was shown to have an IP3-binding domain and a putative Ca2+ channel region. Immunocytochemical studies revealed polarized distribution of XIP3R in the cytoplasm of the animal hemisphere in a well-organized endoplasmic reticulum-like structure and intensive localization in the perinuclear region of stage VI immature oocytes. In ovulated unfertilized eggs, XIP3R was densely enriched in the cortical region of both hemispheres in addition to its polarized localization. After fertilization, XIP3R showed a drastic change in its distribution in the cortical region. These results imply the predominant role of the XIP3R in both the formation and propagation of Ca2+ waves at fertilization.

Synergistic release of calcium in sea urchin eggs by caffeine and ryanodine

A transient rise in intracellular Ca2+ during fertilization is necessary for activation of the quiescent sea urchin egg. Several mechanisms contribute to the rise in Ca2+ including influx across the egg plasma membrane and release from intracellular stores. The egg contains both IP3-sensitive and -insensitive Ca2+ release mechanisms and in this study we have used single-cell spectrofluorimetry to examine the effects of caffeine and ryanodine on Ca2+ release in eggs preloaded with fura 2. Caffeine induced a small Ca2+ release that was insensitive to heparin or ruthenium red. Ca2+ liberation by caffeine could be augmented by prior treatment with thapsigargin, an inhibitor of endoplasmic reticulum Ca2+ ATPase. Variable Ca2+ releases were observed in response to microinjection of ryanodine. The action of ryanodine appeared to be enhanced by prior injection of heparin and partially inhibited by ruthenium red. The release of Ca2+ by caffeine or ryanodine was generally insufficient to trigger cortical granule exocytosis, thus these eggs could be fertilized and a second Ca2+ release during fertilization was measured. Unlike the caffeine- and ryanodine-sensitive Ca(2+)-induced Ca2+ release mechanism in somatic cells, the graded responses in eggs suggested this caffeine- and ryanodine-sensitive release mechanism is not sensitive to sudden changes in Ca2+. Thus we could examine the combined actions of caffeine and ryanodine on Ca2+ release, which were synergistic. Caffeine treatment of ryanodine-injected eggs or ryanodine injection of caffeine-treated eggs stimulated a Ca2+ release significantly larger than the release by either drug independently. The experiments presented here suggest that sea urchin eggs liberate Ca2+ in response to caffeine and ryanodine; however, the regulation of this release differs from that described for caffeine- and ryanodine-sensitive Ca(2+)-induced Ca2+ release of somatic cells.

Calcium signaling at fertilization

Calcium is well established as a second messenger in a diverse array of cell activities. Changes in intracellular Ca2+ activities range from localized releases to complex oscillations, which may encode specific cellular signals. The full variety of calcium responses is observed during the fertilization of different animal oocytes and eggs. Current research has focused on the cellular mechanisms that generate these Ca(2+)-activity changes.

Ca(2+)-induced Ca2+ release in sea urchin egg homogenates: modulation by cyclic ADP-ribose

Calcium-induced calcium release (CICR) may function widely in calcium-mediated cell signaling, but has been most thoroughly characterized in muscle cells. In a homogenate of sea urchin eggs, which display transients in the intracellular free calcium concentration ([Ca2+]i) during fertilization and anaphase, addition of Ca2+ triggered CICR. Ca2+ release was also induced by the CICR modulators ryanodine and caffeine. Responses to both Ca2+ and CICR modulators (but not Ca2+ release mediated by inositol 1,4,5-trisphosphate) were inhibited by procaine and ruthenium red, inhibitors of CICR. Intact eggs also displayed transients of [Ca2+]i when microinjected with ryanodine. Cyclic ADP-ribose, a metabolite with potent Ca(2+)-releasing properties, appears to act by way of the CICR mechanism and may thus be an endogenous modulator of CICR. A CICR mechanism is present in these nonmuscle cells as is assumed in various models of intracellular Ca2+ wave propagation.