Na+-H+ antiport during fertilization of the sea urchin egg is blocked by W-7 but is insensitive to K252a and H-7

A functional genomic and proteomic perspective of sea urchin calcium signaling and egg activation

The sea urchin egg has a rich history of contributions to our understanding of fundamental questions of egg activation at fertilization. Within seconds of sperm-egg interaction, calcium is released from the egg endoplasmic reticulum, launching the zygote into the mitotic cell cycle and the developmental program. The sequence of the Strongylocentrotus purpuratus genome offers unique opportunities to apply functional genomic and proteomic approaches to investigate the repertoire and regulation of Ca(2+) signaling and homeostasis modules present in the egg and zygote. The sea urchin "calcium toolkit" as predicted by the genome is described. Emphasis is on the Ca(2+) signaling modules operating during egg activation, but the Ca(2+) signaling repertoire has ramifications for later developmental events and adult physiology as well. Presented here are the mechanisms that control the initial release of Ca(2+) at fertilization and additional signaling components predicted by the genome and found to be expressed and operating in eggs at fertilization. The initial release of Ca(2+) serves to coordinate egg activation, which is largely a phenomenon of post-translational modifications, especially dynamic protein phosphorylation. Functional proteomics can now be used to identify the phosphoproteome in general and specific kinase targets in particular. This approach is described along with findings to date. Key outstanding questions regarding the activation of the developmental program are framed in the context of what has been learned from the genome and how this knowledge can be applied to functional studies.

Zinc and 17beta-estradiol induce modifications in Na+/H+ exchanger and pyruvate kinase activity through protein kinase C in isolated mantle/gonad cells of Mytilus galloprovincialis

We investigated the transduction pathway mediated by Zn and 17beta-estradiol in isolated mantle/gonad cells of the mussel Mytilus galloprovincialis. Both the essential metal Zn, and the estrogen 17beta-estradiol, caused an increase in intracellular pH (pHi) of isolated mantle/gonad cells of the mussel M. galloprovincialis, thus indicating the activation of the Na+/H+ exchanger (NHE). The observed effect was inhibited by EIPA (20 nM), a specific NHE inhibitor, thus verifying NHE activation. Protein kinase C (PKC) also seemed to play an activating role in zinc and 17beta-estradiol effects on NHE and PK activity. In addition, the glycolytic enzyme pyruvate kinase (PK) was increased after zinc, while it was decreased after 17beta-estradiol treatment. It is noteworthy that, both the latter effects were reversed in the presence of EIPA, indicating the involvement of NHE in the signaling mechanism. cAMP seems to participate in the signaling mechanism induced by Zn but not to that induced by 17beta-estradiol. The potential implication of the heavy metal and 17beta-estradiol on the reproductive activity of the marine animals is discussed.

Calcium-mediated inactivation of the MAP kinase pathway in sea urchin eggs at fertilization

We have evaluated the regulation of a 43-kDa MAP kinase in sea urchin eggs. Both MAP kinase and MEK (MAP kinase kinase) are phosphorylated and active in unfertilized eggs while both are dephosphorylated and inactivated after fertilization, although with distinct kinetics. Reactivation of MEK or the 43-kDa MAP kinase prior to or during the first cell division was not detected. Confocal immunolocalization microscopy revealed that phosphorylated (active) MAP kinase is present primarily in the nucleus of the unfertilized egg, with some of the phosphorylated form in the cytoplasm as well. Incubation of unfertilized eggs in the MEK inhibitor U0126 (0.5 microM) resulted in the inactivation of MEK and MAP kinase within 30 min. Incubation in low concentrations of U0126 (sufficient to inactivate MEK and MAP kinase) after fertilization had no effect on progression through the embryonic cell cycle. Microinjection of active mammalian MAP kinase phosphatase (MKP-3) resulted in inactivation of MAP kinase in unfertilized eggs, as did addition of MKP-3 to lysates of unfertilized eggs. Incubation of unfertilized eggs in the Ca(2+) ionophore A23187 led to inactivation of MEK and MAP kinase with the same kinetics as observed with sperm-induced egg activation. This suggests that calcium may be deactivating MEK and/or activating a MAP kinase-directed phosphatase. A cell-free system was used to evaluate the activation of phosphatase separately from MEK inactivation. Unfertilized egg lysates were treated with U0126 to inactivate MEK and then Ca(2+) was added. This resulted in increased MAP kinase phosphatase activity. Therefore, MAP kinase inactivation at fertilization in sea urchin eggs likely is the result of a combination of MEK inactivation and phosphatase activation that are directly or indirectly responsive to Ca(2+).

Changes in the activities of protein phosphatase type 1 and type 2A in sea urchin embryos during early development

In the eggs and embryos of sea urchins, the activity of protein phosphatase type 2A (PP2A) increased during the developmental period between fertilization and the morula stage, decreased after the prehatching blastula stage and increased again after hatching. The PP2A activity changed keeping pace with alteration to the activities of cAMP-dependent protein kinase (A kinase), Ca2+/calmodulin-dependent protein kinase (CaM kinase) and casein kinase. Probably, PP2A contributes to the quick turning off of cellular signals because of protein phosphorylation. The activity of protein phosphatase type 1 (PP1) was not detectable up to the morula stage and appreciably increased thereafter. In the isolated nucleus fraction, specific activities of PP1 and PP2A were higher than in whole embryos at all stages in early development. Exponential increase in the number of nuclei because of egg cleavage probably makes PP1 activity detectable in whole embryos after the morula stage. In isolated nuclei, the activities of PP1 and PP2A appreciably decreased after hatching, whereas the activities of A kinase, Ca2+/phospholipid-dependent protein kinase (C kinase) and CaM kinase, as well as casein kinase, became higher. In nuclei, cellular signals caused by protein phosphorylation after hatching do not seem to be turned off by these protein kinases so quickly as before hatching. The PP1 and PP2A in nuclei also seem to contribute to the elimination of signal noise.

Evidence that Src-type tyrosine kinase activity is necessary for initiation of calcium release at fertilization in sea urchin eggs

The initiation of Ca(2+) release from internal stores in the egg is a hallmark of egg activation. In sea urchins, PLCgamma activity is necessary for the production of IP(3), which leads to the initial rise in Ca(2+). To examine the possible function of a tyrosine kinase in activating PLCgamma at fertilization, sea urchin eggs were treated with the specific Src kinase inhibitor PP1 or microinjected with recombinant Src-family SH2-domain proteins, which act as dominant interfering inhibitors of Src-family kinase function. Both modes of inhibiting Src-family kinases resulted in a specific and dose-dependent delay in the onset of Ca(2+) release from the endoplasmic reticulum at fertilization. The rise in cytoplasmic pH at fertilization also was inhibited by microinjection of Src-family SH2-domain proteins. Further, an antibody directed against Src-type kinases recognized a protein of ca. M(r) 57K that was enriched in the membrane fraction of eggs. The kinase activity of this protein was stimulated rapidly and transiently at fertilization, as measured by autophosphorylation and by phosphorylation of an exogenous substrate. Together, these data indicate that a Src-type tyrosine kinase is necessary for the initiation of Ca(2+) release from the egg ER at fertilization and identify a Src-type p57 protein as a candidate in the signaling pathway leading to this Ca(2+) release.

The roles of changes in NADPH and pH during fertilization and artificial activation of the sea urchin egg

Incubating unfertilized sea urchin eggs in weak bases activates nuclear centering, DNA synthesis, and chromosome cycles. These effects were initially attributed to raising the intracellular pH (pH(i)), but later experiments indicated that these weak bases also lead to increases in reduced pyridine nucleotides. These findings raised the question whether the activation of the nucleus was due to increased pH(i) or to increased NAD(P)H or possibly other effects. This report attempts to clarify how ammonia activates eggs by independently altering NADPH and pH(i). To increase the pH(i), unfertilized eggs were injected with zwitterionic buffers. This stimulated pronuclear centering, DNA synthesis, and nuclear envelope breakdown; there appeared to be a threshold corresponding to the fertilized pH(i). However, like incubation in ammonia, injection of base also increased NAD(P)H. The NAD(P)H rise caused by directly raising the pH(i) occurred in the presence of intracellular calcium chelators, indicating that calcium is not required. Increasing NAD(P)H alone did not activate nuclear centering, DNA synthesis, or nuclear envelope breakdown. Although these experiments cannot eliminate a role for the NADPH increase in initiating events leading to nuclear centering and entry into mitosis, they provide additional and strong evidence that increasing the pH(i) may be a primary signal.

Identification of PLCgamma-dependent and -independent events during fertilization of sea urchin eggs

At fertilization, sea urchin eggs undergo a series of activation events, including a Ca2+ action potential, Ca2+ release from the endoplasmic reticulum, an increase in intracellular pH, sperm pronuclear formation, MAP kinase dephosphorylation, and DNA synthesis. To examine which of these events might be initiated by activation of phospholipase Cgamma (PLCgamma), which produces the second messengers inositol trisphosphate (IP3) and diacylglycerol, we used recombinant SH2 domains of PLCgamma as specific inhibitors. Sea urchin eggs were co-injected with a GST fusion protein composed of the two tandem SH2 domains of bovine PLCgamma and (1) Ca2+ green dextran to monitor intracellular free Ca2+, (2) BCECF dextran to monitor intracellular pH, (3) Oregon Green dUTP to monitor DNA synthesis, or (4) fluorescein 70-kDa dextran to monitor nuclear envelope formation. Microinjection of the tandem SH2 domains of PLCgamma produced a concentration-dependent inhibition of Ca2+ release and also inhibited cortical granule exocytosis, cytoplasmic alkalinization, MAP kinase dephosphorylation, DNA synthesis, and cleavage after fertilization. However, the Ca2+ action potential, sperm entry, and sperm pronuclear formation were not prevented by injection of the PLCgammaSH2 domain protein. Microinjection of a control protein, the tandem SH2 domains of the phosphatase SHP2, had no effect on Ca2+ release, cortical granule exocytosis, DNA synthesis, or cleavage. Specificity of the inhibitory action of the PLCgammaSH2 domains was further indicated by the finding that microinjection of PLCgammaSH2 domains that had been point mutated at a critical arginine did not inhibit Ca release at fertilization. Additionally, Ca2+ release in response to microinjection of IP3, cholera toxin, cADP ribose, or cGMP was not inhibited by the PLCgammaSH2 fusion protein. These results indicate that PLCgamma plays a key role in several fertilization events in sea urchin eggs, including Ca2+ release and DNA synthesis, but that the action potential, sperm entry, and male pronuclear formation can occur in the absence of PLCgamma activation or Ca2+ increase.

Differential effect of pH upon cyclic-ADP-ribose and nicotinate-adenine dinucleotide phosphate-induced Ca2+ release systems

We investigated the pH dependence and the effects of thimerosal and dithiothreitol (DTT) upon the Ca2+ release induced by cADP-ribose (cADPR) and nicotinate-adenine dinucleotide phosphate (NAADP) in sea urchin egg homogenates. Both Ca2+ release triggered by cADPR and the binding of [3H]cADPR to sea urchin egg homogenates were decreased by alkalization of the assay media from pH 7.2 to 8.9. In contrast, NAADP-triggered Ca2+ release was not influenced by changes in pH. The Ca2+ release induced by cADPR was potentiated by thimerosal and inhibited by DTT, but neither thimerosal nor DTT had any effect upon the Ca2+ release induced by NAADP. We conclude that cADPR-sensitive Ca2+-release mechanisms are dependent on pH of the assay media and are sensitive to thiol group modification. On the other hand, these functional properties are not shared by NAADP-regulated Ca2+ channels.

U73122 blocked the cGMP-induced calcium release in sea urchin eggs

U73122, a phospholipase C inhibitor, dose dependently blocks the cGMP-induced Ca2+ release in sea urchin eggs and homogenates. U73122 inhibition was prevented by cotreatment with dithiothreitol (DTT), but DTT is ineffective when eggs or homogenates were pretreated with U73122. U73122 action is different from the other sulfhydryl reagents, thimerosal and N-ethylmaleimide, which cause Ca2+ release in egg homogenates at high concentration, but at lower concentration have no significant effect on cGMP-induced Ca2+ release. Histone, a reported NAD glycohydrolase (NADase) activator, was found to induce Ca2+ release in egg homogenates via the same pathway as the cGMP response, since histone-induced Ca2+ release is blocked by Rp-8-pCPT-cGMPS, a cGMP-dependent protein kinase (PKG) inhibitor, and nicotinamide, a NADase inhibitor. Histone-induced Ca2+ release is similarly blocked by U73122. The aminosteroid U73122 does not inhibit cADPR-induced Ca2+ release, which is significantly reduced by PKG inhibitors. Furthermore, U73122 has no significant effect on phorbol 12-myristate 13-acetate induced-cytoplasmic alkalinization in intact eggs, which depends on protein kinase C activity. These results suggest that U73122 does not act as a general serine-threonine protein kinase inhibitor, and the aminosteroid inhibition of the cGMP-induced Ca2+ release may interfere with ADP ribosyl cyclase activity.

Calcium-calmodulin modulation of the activity of Na+/H+ exchanger in human platelets

This study aimed at establishing the role of calmodulin in regulating pHi of human platelets under acid loads and in stimulated states. The response of human platelets to thrombin was an initial drop of pHi followed by a recovery with a significant increase above the pre-stimulation level in control experiments and a recovery to initial values in platelets maintained in the presence of 19 mmol/l TFP (trifluoperazine = 2 trifluoromethyl-10 [3'-(1 methyl-4-piperazinyl) propyl] phenothiazine). The change in pHi after 8 min was 0.130 +/- 0.030 in the control and 0.001 +/- 0.011 pH units in TFP (P < 0.05). The initial velocity of recovery from an acid load was reduced to 56.7 +/- 6% of the control (n = 6, P < 0.05) with 50 mumol/l W7 (N-(6 aminohexyl)-5-chloro-l-naphthalene sulphonamide), and to 29.7 +/- 4.3% of the control (n = 8, P < 0.05) with 19 mumol/l TFP. The initial velocity of recovery was significantly greater in recalcified platelets than in the preparations kept in the nominal absence of extracellular calcium (1.08 +/- 0.12 vs 0.66 +/- 0.12 pH units per min, P < 0.05). Lower concentration of TFP had an inhibitory effect only in the presence of calcium. The velocities of recovery reached similar values at higher TFP concentration. The significant interaction between Ca2+ and TFP concentrations indicates that the Ca-calmodulin complex, rather than an unspecified direct action of TFP, is responsible for the modulation of the Na+/H+ exchanger. These findings indicate that calcium-calmodulin participates in both the recovery of pH after an acid load and the increase of pHi in stimulated states of human platelets.

Intracellular pH and calcium in frog early distal tubule: effects of transport inhibitors

1. The K+ channels of the apical membrane of the diluting segment (early distal tubule, EDT) of the frog are involved in the regulation of transepithelial NaCl transport. These channels are sensitive to pHi and intracellular Ca2+ (Ca2+i). Inhibition of transport by furosemide (frusemide) results in a compensatory increase in K+ channel activity. The aims of the present study were to determine whether pHi or Ca2+i were altered by furosemide, and to identify the means by which such changes were brought about. 2. Experiments were performed using single, microperfused EDT segments. Measurements of pHi and Ca2+i were made using the intracellular fluorescent probes, 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) and fura-2, respectively. 3. Furosemide increased pHi and Ca2+i. The intracellular alkalinization was the result of an alkaline shift in the set-point of the basolateral Na(+)-H+ exchanger. This response was dependent upon the increase in Ca2+i. 4. The increase in Ca2+i produced by furosemide was due to the release of Ca2+ from intracellular stores. Depletion of these stores, by 2,5-di-t-butylhydroquinone (TBQ) and caffeine, prevented the furosemide-induced changes in Ca2+ and pH. 5. Furosemide-induced activation of Na(+)-H+ exchange was prevented by the calmodulin antagonist, W-7. 6. Thus furosemide elicits a rise in Ca2+i which, via calmodulin, results in activation of Na(+)-H+ exchange. The resulting intracellular alkalinization would be expected to increase channel activity.

Cytoplasm calcium-binding proteins of germ cells and embryos of the sea urchin

Synchronous, demonstrative, easily reproducible fertilization with the following embryonic development makes the process in the sea urchin extremely attractive for studying many biological enigmas. In particular, germ and embryonic cells of the sea urchin present a wide opportunity for investigating different associated phenomena launched by an increase in concentration of Ca2+ in cells ([Ca2+]i). Ca2+ ions participate in the activation of diverse processes of respiration and sperm motility (Shapiro et al., 1990; Brokaw, 1991), chemotaxis of spermatozoa to components of the egg jelly (Ward et al., 1985), acrosomal reaction (Trimmer et al., 1986; Shapiro et al., 1990), cortical reaction, formation of the fertilization membrane (Sasaki, 1984; Sardet and Chang, 1987), cellular division in the embryo (Poenie et al., 1985; Silver, 1986; Whitaker and Patel, 1990), their adhesion (McClay and Matranga, 1986), differentiation and formation of spicules (Mitsunaga et al., 1988) and metamorphosis (Carpenter et al., 1984). The present review combines information on the function of calcium-binding proteins and their targets, calmodulin regulation of NAD-kinase, exocytosis of cortical granules, Ca(2+)- and calmodulin-dependent protein phosphatase, Ca(2+)-dependent protein phosphorylation, regulation of ion-exchanger in the germ and embryonic cells as well as Ca(2+)- and calmodulin control of sperm motility in sea urchins.

Cyclic ADP ribose activation of the ryanodine receptor is mediated by calmodulin

Cyclic ADP-ribose (cADPR) is a newly identified nucleotide which can release calcium from a variety of cells, suggesting it is a messenger for mobilizing internal Ca2+ stores. Its cyclic structure has now been confirmed by X-ray crystallography. Available results are consistent with it being a modulator of Ca(2+)-induced Ca2+ release. Here we report that sea urchin egg microsomes purified by Percoll gradients lose sensitivity to cADPR, but the response can be restored by a soluble protein in the supernatant. Purification and characterization of the protein indicate that it is calmodulin. It appears to be sensitizing the Ca2+ release mechanism because caffeine and strontium, agonists of Ca(2+)-induced Ca2+ release, can also mimic calmodulin in conferring cADPR-sensitivity. Although evidence indicates that cADPR may be an activator of the ryanodine receptor, present results point to the importance of accessory proteins such as calmodulin in modulating its activity.

Intracellular pH regulation in the early embryo

Intracellular pH (pHi) regulation is a homeostatic function of all cells. Additionally, the plasma membrane-based transporters controlling pHi are involved in growth factor activation, cell proliferation and salt transport--all processes active in early embryos. pHi regulation in the early embryos of many species exhibits unique features: in mouse preimplantation embryos, mechanisms for correcting excess acid apparently are inactive, while excess base is removed by the mechanism common in differentiated cells. Additionally, unlike differentiated cells, mouse preimplantation embryos are highly permeable to H+ until the blastocyst stage, where the epithelial cells surrounding the embryo are impermeable. In several non-mammalian species, of which the best-studied is sea urchin, cytoplasmic alkalinization at fertilization is necessary for development of the embryo, and elevated pHi must be maintained during early development. Thus, pHi regulatory mechanisms appear to be important for early embryo development in many species.

Calcium signals in growth factor signal transduction

There is a substantial amount of information which has been obtained concerning the effects of growth factors on [Ca2+]i in proliferating cells. A number of different mitogens are known to induce elevations in [Ca2+]i and some characterization of the Ca2+ response to different classes of mitogens has been obtained. In addition, much is known about whether the Ca2+ response to a particular growth factor occurs as the result of an influx of external Ca2+ or a mobilization of internal Ca2+ stores. In addition, a considerable amount of information is available on the mechanism by which the Ins(1,4,5)P3-sensitive internal Ca2+ store takes up and releases Ca2+. However, there is still a large deficiency in our information concerning other Ca2+ stores in proliferating cells as well as in our knowledge of the mechanisms for regulating Ca2+ entry pathways. Much more data addressing these issues exists for other types of agonist-stimulated cells, and we have discussed much of it in this review article. While the wealth of data in nonproliferating cells provides some indications of what mechanisms might be involved in the growth factor-induced changes in [Ca2+]i, it is clear that much work must be done in proliferating cells to fully understand how external factors such as growth factors control [Ca2+]i. In addition, much work remains to be done in identifying the mechanisms for the internal control of [Ca2+]i as cells move through the cell cycle and in identifying the role that these changes in [Ca2+]i may play throughout the cell cycle.

Intracellular calcium-mediated activation of hepatic Na+/H+ exchange by arginine vasopressin and phenylephrine

The effect of Ca++ mobilizing agonists arginine vasopressin and phenylephrine on Na+/H+ exchange was studied in freshly isolated hepatocytes and isolated perfused rat livers. The activity of Na+/H+ exchange was determined from the rate of H+ efflux, 22Na uptake and pHi recovery. Arginine vasopressin and phenylephrine stimulated H+ efflux and 22Na uptake in isolated rat hepatocytes and increased the rate of pHi recovery from acid-loaded hepatocytes. These effects were inhibited by amiloride. Arginine vasopressin- and phenylephrine-induced increases in H+ efflux were also dependent on extracellular Na+. Arginine vasopressin- and phenylephrine-induced increases in intracellular Ca++ concentration, H+ efflux, 22Na uptake and intracellular pH recovery were decreased in hepatocytes preloaded with the Ca(++)-buffering agent [bis-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid] (MAPTA). Na+/H+ exchange-dependent intracellular pH recovery from cytosolic acidification was stimulated by thapsigargin, which increases intracellular calcium concentration by inhibiting endoplasmic reticulum Ca++ ATPase. Arginine vasopressin- and phenylephrine-induced increases in intracellular pH recovery were not dependent on extracellular Ca++ and were inhibited by calmidazolium, a calmodulin inhibitor. Arginine vasopressin and phenylephrine also increased H+ efflux in the absence but not in the presence of amiloride in perfused rat livers without affecting biliary HCO3- excretion. These results indicate that arginine vasopressin and phenylephrine activate Na+/H+ exchange in rat hepatocytes, an effect mediated in part by intracellular Ca++ and calmodulin kinase. Furthermore, sinusoidal Na+/H+ exchange does not appear to be involved in biliary HCO3- excretion.

Protein kinase C activates the respiratory burst of fertilization, but not cortical granule exocytosis, in ionophore-stimulated sea urchin eggs

In a "respiratory burst", fertilized sea urchin eggs consume oxygen to produce H2O2 as an extracellular oxidant to crosslink their protective surface envelopes. The egg generates H2O2 via a NADPH-specific oxidase that requires protein kinase C for activation. To further study the physiological regulation of the respiratory burst and cortical granule exocytosis, we have measured azide-insensitive oxygen uptake and fertilization envelope assembly in ionophore-stimulated eggs. Procaine, trifluoperazine, staurosporine, and H-7, which inhibit protein kinase C by different mechanisms, suppressed egg oxygen consumption without affecting fertilization envelope assembly. In contrast, both exocytosis and oxygen uptake were blocked in N-ethylmaleimide-treated eggs. When the eggs were stimulated with ionophore in Na-free artificial seawater, which prevents the increase in pHi, oxidase activity was inhibited. This effect was reversed by elevation of cytoplasmic pH with the membrane-permeant base NH4Cl. We conclude that protein kinase C was not involved in the events downstream from the ionophore-dependent elevation of Ca2+ which induced cortical granule exocytosis. However, the respiratory burst was inhibited despite the increase in Ca2+ that triggered exocytosis. The likely target for inhibition of the burst was protein kinase C. Cytoplasmic alkalinization was necessary for optimal rates of H2O2 synthesis, further implicating pHi as a regulator of the egg oxidase.

A synthetic peptide of the pseudosubstrate domain of protein kinase C blocks cytoplasmic alkalinization during activation of the sea urchin egg

Multiple second messenger pathways have been proposed for transduction of the sperm-egg fusion event during fertilization of sea urchin eggs. Cytoplasmic alkalinization due to increased Na(+)-H+ antiport has been causally linked to many of the metabolic events during fertilization. Two possible second messenger pathways coupling sperm-egg fusion and antiporter activity are activation of protein kinase C (PKC) and Ca2(+)-calmodulin kinase. A selective inhibitor of PKC is PKC(19-36), a synthetic peptide of the pseudosubstrate domain of the kinase. Injection of PKC(19-36) into unfertilized sea urchin eggs blocked cytoplasmic alkalinization during activation by phorbol 12-myristate 13-acetate, a PKC agonist. The rise in pH during fertilization was partially blocked by PKC(19-36), which suggested that multiple pathways regulate the antiporter during fertilization. The use of fluorescein chromophores to measure intracellular pH in sea urchin eggs is also discussed.

Protein kinase C acts downstream of calcium at entry into the first mitotic interphase of Xenopus laevis

Transit into interphase of the first mitotic cell cycle in amphibian eggs is a process referred to as activation and is accompanied by an increase in intracellular free calcium [( Ca2+]i), which may be transduced into cytoplasmic events characteristic of interphase by protein kinase C (PKC). To investigate the respective roles of [Ca2+]i and PKC in Xenopus laevis egg activation, the calcium signal was blocked by microinjection of the calcium chelator BAPTA, or the activity of PKC was blocked by PKC inhibitors sphingosine or H7. Eggs were then challenged for activation by treatment with either calcium ionophore A23187 or the PKC activator PMA. BAPTA prevented cortical contraction, cortical granule exocytosis, and cleavage furrow formation in eggs challenged with A23187 but not with PMA. In contrast, sphingosine and H7 inhibited cortical granule exocytosis, cortical contraction, and cleavage furrow formation in eggs challenged with either A23187 or PMA. Measurement of egg [Ca2+]i with calcium-sensitive electrodes demonstrated that PMA treatment does not increase egg [Ca2+]i in BAPTA-injected eggs. Further, PMA does not increase [Ca2+]i in eggs that have not been injected with BAPTA. These results show that PKC acts downstream of the [Ca2+]i increase to induce cytoplasmic events of the first Xenopus mitotic cell cycle.