Calmodulin in starfish oocytes. I. Calmodulin antagonists inhibit meiosis reinitiation

The role of calcium in the cell cycle: facts and hypotheses

The regulation of cell cycle progression is a complex process which involves kinase cascades, protease action, production of second messengers and other operations. Increasing evidence now compellingly suggests that changes in the intracellular Ca2+ concentration may also have a crucial role. Ca2+ transients occur at the awakening from quiescence, at the G/S transition, during S-phase, and at the exit from mitosis. They may lead to the activation of Ca2+ binding proteins like S-100, but the key decoder of the Ca2+ signals in the cycle is calmodulin. Activation of calmodulin leads to the stimulation of protein kinases, i.e., CaM-kinase II, and of the CaM-dependent protein phosphatase calcineurin. Ample evidence now indicates the G/S transition, the progression from G2 to M, and the metaphase/anaphase transition as specific points of intervention of CaM-kinase II. Another attractive possibility for the role of Ca2+ in the cycle is through the activation of the Ca(2+)-dependent protease calpain: other proteases (e.g., the proteasome) have been suggested to be responsible for the degradation of some of cyclins, which is essential to the progression of the cycle. One of the cyclins, however, (D1) is instead degraded by calpain, which has been shown to promote both mitosis and meiosis when injected into somatic cells or oocytes.

Association of calmodulin with nuclear structures in starfish oocytes and its role in the resumption of meiosis

The resumption of meiosis in prophase-arrested starfish oocytes is induced by the hormone 1-methyladenine, which has been shown previously to induce a calcium transient in the nucleus which at this stage is called the germinal vesicle. This transient precedes the breakdown of the germinal vesicle (GVBD). Experiments were performed to establish whether nuclear calmodulin (CaM) was involved in the progression of the meiotic cycle. CaM antagonists, antibodies, and an inhibitory peptide corresponding to the CaM-binding domain of myosin-light-chain kinase have been injected into the nucleus of prophase-arrested starfish oocytes. The antagonists failed to affect the final response to 1-methyladenine, i.e. GVBD, although two antagonists delayed it, whereas the peptide inhibitor and the antibodies completely inhibited it. The antibodies suppressed the nuclear Ca2+ spikes that were shown by previous work to be induced by the photoreleasing of caged adenosine 3',5'-(cyclic)diphosphate ribose in the germinal vesicle. Immunofluorescence staining of isolated starfish oocyte nuclei with CaM antibodies showed CaM in the envelope and in the nucleolus. Immunogold labelling of oocytes revealed aggregates of CaM and of a 36-kDa protein, of the heterogeneous ribonucleoprotein particles (hnRNP), in electron-dense hnRNP in the nuclear matrix. 1-Methyladenine induced the disappearance of these hnRNP from the nucleoplasm and the translocation of CaM and the 36-kDa protein previously associated with them to the cytoplasm, prior to the breakdown of the nuclear envelope.

Protein kinase C modulators influence meiosis kinetics but not fertilizability of mouse oocytes

The role of protein kinase C (PKC) in the successive steps of mouse oocyte meiotic process was investigated. We have used either OAG, an analog of diacylglycerol, or mezerein, a nonphorbol ester diterpene, less tumor promoting than phorbol esters, as PKC activators, and staurosporine as PKC inhibitor. Cumulus-free oocytes were cultured in minimum essential medium with each of these PKC modulators and maturation stages were screened every two hours until the end of the process. Both PKC activators prevented GVBD at each tested dose for 4 hr (OAG) and 8 hr (mezerein), and decreased the frequencies of PB oocytes. The inhibitory effects of both activators were dose dependent and reversible. The addition of OAG to the culture medium after GVBD occurrence (i.e., after 4 hrs) did not affect PB extrusion whereas similar addition of mezerein significantly decreased the frequency of PB oocytes. Inhibition of PKC by staurosporine accelerated GVBD and increased the frequency of PB extrusion. When staurosporine was added after GVBD, PB extrusion occurred earlier but PB oocyte frequency was not increased. Fertilizability was not affected when oocyte maturation occurred in the presence of any of these substances despite the delay in maturation process. These results clearly indicate that the PKC pathway is involved in mouse oocyte meiotic process: activation of the enzyme would arrest meiotic process whereas its inhibition would participate in meiosis induction.

An increase of intracellular free Ca2+ is essential for spontaneous meiotic resumption by mouse oocytes

The involvement of calcium ions in the mechanism of meiotic resumption has been studied in mouse oocytes made resistant to the lethal effects of calcium-free medium (CFM) by zona pellucida removal (De Felici et al., '89). We show here that such oocytes undergo meiotic resumption in CFM (as evaluated by germinal vesicle breakdown, GVBD) at a rate comparable to that shown by oocytes cultured in medium containing 1.7 mM Ca2+. The addition to CFM of 50 u M Quin2/AM (a membrane permeable, high affinity Ca2+ chelator) totally prevents GVBD, while purported antagonists of Ca2+ release from intracellular stores, such as 150 uM 8-(N,N-diethylamino)octyl-3-4-5 trimethoxybenzoate (TMB-8) or 300 uM chlortetracycline, only cause a slight meiotic delay. On the other hand, if the oocytes are pre-incubated for 30 min in CFM supplemented with 100 uM TBM-8 plus 0.2 mM dibutyryl-cyclic AMP (dbcAMP, a reversible inhibitor of GVBD), and then cultured in the same medium, without dbcAMP, a sustained inhibition of meiotic maturation is obtained. Our observations suggest that an increase in intracellular free Ca2+ is essential for meiotic resumption by mouse oocytes; in the experimental absence of external Ca2+, release of the cation from internal stores is sufficient to allow meiotic resumption.

Sources of calcium and the involvement of calmodulin during steroidogenesis and oocyte maturation in follicles of Rana pipiens

In the amphibian ovarian follicle, progesterone production is thought to induce maturation of the enclosed oocyte. Intracellular mechanisms regulating these events in the somatic and germ cells are incompletely understood. However, calcium appears to play a role in the production and action of progesterone. Experiments using calcium antagonists were carried out to delineate the role of extra- and intracellular calcium during in vitro stimulation of follicular steroidogenesis and oocyte maturation. Calcium-free medium, verapamil, and La3+ were used to block Ca2+ influx and inhibited follicular progesterone accumulation in response to frog pituitary homogenate (FPH) or exogenous cAMP + IBMX. Progesterone accumulation was not impaired under identical conditions when pregnenolone was added to cultured follicles. TMB-8, an inhibitor of intracellular Ca2+ mobilization, partially inhibited progesterone levels stimulated by FPH at low doses but not higher doses of the inhibitor. However, TMB-8 inhibited FPH-induced oocyte germinal vesicle breakdown (GVBD) in a dose-dependent manner, as well as maturation due to exogenous progesterone or La3+. Calmodulin antagonists, W-7, R24571, and trifluoperazine, were used to assess the involvement of calmodulin in the responses of these two cell types. All three antagonists inhibited progesterone accumulation induced by FPH with the apparent order of potency being R24571 greater than W-7 greater than TFP. W-7 inhibited cAMP-induced progesterone elevation, but had no effect on conversion of pregnenolone to progesterone. Of these three calmodulin antagonists, only R24571 exhibited a dramatic ability to inhibit GVBD induced by exogenous progesterone and was associated with morphologic alterations in the oocytes. These data suggest that Ca2+, acting through calmodulin at some specific step(s) distal to cAMP elevation and prior to pregnenolone formation, is involved in FPH-induced progesterone accumulation, apparently with the participation of both extracellular and intracellular pools of Ca2+. In the oocyte, mobilization of Ca2+ from intracellular stores appears to be of primary importance to maturation while extracellular Ca2+ is not. These data provide further evidence that Ca2+ mediates the hormonally provoked responses in both cell types in the intact follicle, but that the source of Ca2+ may differ. Using intact follicles it seems apparent that exploiting this difference with selective inhibitors provides a means for differential modulation and functional uncoupling of these cells with regard to steroidogenesis and steroid action.

Action of calcium-binding proteins in egg maturation and fertilisation

There is considerable evidence that calcium acts as a primary trigger for egg maturation and fertilisation in diverse phyla. Calcium regulation has been demonstrated or suggested for numerous specific events in fertilisation, including: sperm motility, the acrosome reaction, sperm-egg binding and fusion, metabolic activation of the egg, etc. However, very little is known concerning the mechanisms whereby calcium exerts its effects. Some calcium-regulated events are mediated through calmodulin and others are likely to be as well. Additionally, protein kinase C has recently been implicated in some processes related to egg maturation and activation, although the evidence presented thus far has been indirect. Other pathways dependent upon calcium but not involving either CaM or PKC have also been identified. Much more research will be required before the multiple involvement of calcium-binding proteins in egg maturation and fertilisation are clarified.

Calcium effects on progesterone accumulation and oocyte maturation in cultured follicles of Rana pipiens

Pituitary homogenates (FPH) provoke a cascade of responses in the amphibian ovarian follicle, culminating in progesterone biosynthesis and oocyte maturation (GVBD). Calcium may play an important role as an intracellular second messenger in regulating these physiological responses. Experiments were carried out on cultured, isolated follicles of Rana pipiens to assess the effects of varying extracellular calcium on follicular progesterone accumulation and oocyte maturation. In hormonally unstimulated follicles, an increase in extracellular Ca2+ alone produced a significant increase in progesterone in methanol extracts of follicles after 4 hours of culture, and in some cases also provoked oocyte maturation assessed after 24 hours of culture. In no case did elevated Ca2+ alone stimulate maximal progesterone accumulation as compared with FPH-stimulated follicles, although the time-course of accumulation was similar. The calcium ionophore, A-23187, similarly increased progesterone accumulation in a dose-dependent manner when introduced in amphibian Ringer's (1.35 mM Ca2+), but inhibited progesterone elevation caused by increasing calcium concentrations in the culture media and FPH stimulation. Depleting free calcium from the culture medium with graded doses of the chelator EGTA decreased FPH-induced progesterone accumulation and inhibited FPH- and progesterone-induced GVBD. The calcium channel blocker, verapamil, also inhibited FPH-induced progesterone accumulation and GVDB in a dose-dependent manner, while having no effect on progesterone-induced meiotic resumption. These data strongly implicate intracellular calcium levels regulating progesterone production by ovarian follicle cells and subsequent oocyte maturation.

The releasing action of calcium upon cyclic AMP-dependent meiotic arrest in hamster oocytes

The effect of increasing cytoplasmic calcium on cyclic adenosine monophosphate (cAMP)-dependent meiotic arrest (%GV where GV is germinal vesicle) in hamster oocytes was investigated. The hypotheses tested were that calcium is required for the spontaneous maturation of hamster oocytes, elevation of calcium in the oocyte-cumulus complex can antagonize cAMP-dependent meiotic arrest, and the intraoocyte level of cAMP remains unchanged, but heterologous metabolic coupling decreases, concomitant with calcium-stimulation of germinal vesicle breakdown (GVBD). Levels of cAMP were elevated by culturing cells in the presence of dibutyryl cAMP (dbcAMP), isobutylmethylxanthine (IBMX) or forskolin and intracellular levels of calcium were manipulated by altering the CaCl2 concentration in the medium and/or by utilizing EGTA or A23187. Intracellular cAMP was determined by RIA, functional metabolic coupling was assessed by determination of the fraction of radiolabeled uridine marker transferred from the cumulus mass to the oocyte, and meiotic stage was determined cytogenetically. Compared with the proportion of oocytes that underwent meiotic maturation in control medium containing 1.53 mM CaCl2, that of cumulus-free (denuded) oocytes was unaffected by culture in the absence of added CaCl2, while that of cumulus-enclosed (intact) oocytes was significantly decreased (%GV = 59.5 +/- 4.8 and 4.2 +/- 0.9 in 0 and 1.53 mM CaCl2, respectively, P less than 0.001, where GV is germinal vesicle). EGTA prevented, in a dose-dependent manner, the spontaneous maturation of denuded oocytes that occurred in 0 mM CaCl2 (ID50 = 0.05 mM, where ID50 is the dose of EGTA that inhibited GVBD in 50% cultured oocytes). In contrast, compared with the control, less than 1 mM EGTA failed to increase the %GV of intact oocytes, although 5 mM EGTA significantly increased meiotic arrest. The %GVBD of oocytes cultured in medium containing 0 mM CaCl2 was dose-dependent on A23187 for both intact oocytes (ID50 = 3.0 microM) and for denuded oocytes cultured in the presence of 0.5 mM EGTA (ID50 = 2.7 microM). Elevated extracellular calcium significantly antagonized dbcAMP-maintained meiotic arrest in both types of oocyte and the %GV was significantly correlated with the pH of the medium [(r) = -0.78 and -0.60 for intact and denuded oocytes, respectively, P less than 0.001 in both cases]. Both CaCl2 and A23187 induced dose-dependent antagonistic effects on forskolin-maintained meiotic arrest in intact oocytes but neither antagonism was accompanied by significant dose-dependent decreases in either the intraoocyte content of cAMP or the extent of heterologous metabolic coupling.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein phosphorylation and oocyte maturation. II. Inhibition of starfish oocyte maturation by intracellular microinjection of protein phosphatases 1 and 2A and alkaline phosphatase

Oocyte maturation (meiosis re-initiation) in starfish is induced by the natural hormone 1-methyladenine (1-MeAde). Following hormonal stimulation of the oocyte, an intracellular Maturation Promoting Factor (MPF) appears in the cytoplasm which triggers nuclear envelope breakdown and maturation divisions. Microinjection of pure preparations of the catalytic subunits of protein phosphatases 1 and 2A inhibits 1-MeAde-induced maturation in a dose-dependent manner. Calmodulin-dependent protein phosphatase 2B is inefficient. Maturation induced by mimetics of 1-MeAde, such as dithiothreitol (DTT), methylglyoxal-bis(guanylhydrazone) (MGBG), 8-hydroxyeicosatetraenoic acid (8 HETE) and arachidonic acid (AA) is also inhibited by these protein phosphatases. In all cases inhibition can be reversed by increasing the concentration of 1-Me-Ade or of mimetic. Alkaline phosphatase also inhibits maturation in a dose-dependent way and in a reversible manner. Microinjection of protein phosphatase is still effective when preformed long after the end of the hormone-dependent period, and can even be effective a few minutes before the breakdown of the nuclear envelope. No detectable MPF activity is found in 1-MeAde-treated phosphatase-injected oocytes. However, microinjection of phosphatase 2A simultaneously with MPF (obtained from 1-MeAde-treated donors) does not result in inhibition. These results constitute direct evidence for the necessity of an elevated level of phosphorylated proteins for MPF activity and maturation. The mode of action of 1-MeAde in inducing starfish oocyte maturation is discussed in relation to protein phosphorylation.

Changes in calmodulin level after fertilization and during first cleavage in the egg of the urodelan amphibian Pleurodeles waltlii

We report three significant calmodulin rises related to Pleurodeles waltlii egg fertilization and following developmental events. These elevations are correlated to the major obvious Ca2+-dependent events: Na+-H+ exchange, activation of NAD kinase, triggering of cortical reaction, resumption of meiotic division II, initiation of DNA synthesis and regulation of cell division. Therefore, it is suggested that alterations in calmodulin level in fertilized egg may be part of the Ca2+-dependent regulatory mechanisms which turn on metabolisms, initiate development and govern cell cleavages.

Arachidonic acid, 12- and 15-hydroxyeicosatetraenoic acids, eicosapentaenoic acid, and phospholipase A2 induce starfish oocyte maturation

In starfish oocyte maturation (meiosis reinitiation) is induced by the natural hormone 1-methyladenine (1-Me-Ade). This paper shows that arachidonic acid (AA) induces oocyte maturation at concentrations above 0.5 microM. This maturation shares many characteristics with 1-MeAde-induced maturation: same kinetics, same required contact time, same stimulations of protein phosphorylation and sodium influx. Although calcium facilitates the AA-induced but not the 1-MeAde-induced maturation, AA, like 1-MeAde, does not stimulate the uptake of calcium. Calcium does not facilitate the uptake of AA by oocytes. Out of 36 different fatty acids (saturated and unsaturated), only eicosatetraenoic (AA) and eicosapentaenoic acids were found to mimic 1-MeAde. Calcium-dependent phospholipases A2 from bee venom and Naja venom also induce maturation (0.1-1 unit/ml) when added externally to the oocytes. Phospholipase A2 inhibitors (quinacrine, bromophenacylbromide) block maturation; inhibition is reversed by increasing the 1-MeAde concentration and only occurs during the hormone-dependent period. AA is usually metabolized through oxidation by cyclooxygenase or lipoxygenase. Cyclooxygenase inhibitors (acetylsalicylic acid, indomethacin, tolazoline) do not block maturation; prostaglandins E2, D2, F2 alpha, I2, and thromboxane B2 do not induce meiosis reinitiation. On the other hand, lipoxygenase inhibitors (quercetin, butylated hydroxytoluene, and eicosatetraynoic acid) block 1-MeAde-induced maturation; although leukotrienes (A4, B4, C4, D4, E4) have no effects on oocytes, two other lipoxygenase products, 12- and 15-hydroxyeicosatetraenoic acids (and their corresponding hydroperoxy-) induce oocyte maturation (around 1 microM). The possible mode of action of the fatty acids inducing oocyte maturation is discussed.

Calcium transients during early development in single starfish (Asterias forbesi) oocytes

Maturation and fertilization of the starfish oocyte are putative calcium-dependent events. We have investigated the spatial distribution and temporal dynamics of this calcium dependence in single oocytes of Asterias forbesi. We used the calcium photoprotein, aequorin, in conjunction with a microscope-photomultiplier and microscope-image intensifier. Surprisingly, in contrast to earlier work with Marasthenias glacialis, there is no detectable increase in intracellular-free calcium in the oocyte of A. forbesi in response to the maturation hormone 1-methyl adenine. During fertilization of the same, matured, A. forbesi oocyte there is a large increase in intracellular-free calcium. The calcium concentration increases to approximately 1 microM at the point of insemination and the region of elevated free calcium expands across the oocyte in approximately 20 s (17-19 degrees C). After the entire oocyte reaches an elevated concentration of free calcium, the concentration decreases uniformly throughout the oocyte over the next several minutes.

Participation of calcium and calmodulin in the formation of acetylcholine receptor clusters

The formation of acetylcholine receptor (AChR) clusters can be experimentally induced in cultured Xenopus myotomal muscle cells by positive polypeptide-coated latex beads (Peng, H.B., P.-C. Cheng, and P.W. Luther, 1981, Nature [Lond.], 292:831-834). This provides a simple procedure for studying the cellular process of AChR clustering. In this study, the involvement of calcium and calmodulin in this process was examined. A deprivation in extracellular calcium by calcium-free medium or by the addition of calcium antagonists such as divalent cations Co2+ and Ni2+ (1-5 mM) or organic compounds verapamil and D-600 (0.1-0.5 mM) suppressed the formation of AChR clusters induced by the latex beads in a largely reversible manner. Antagonists against calmodulin, including trifluoperazine (1-5 microM) and the naphthalene sulfonamide W-7 (20 microM), also suppressed AChR clustering. However, the effect of W-7 was much weaker than that of trifluoperazine (TFP). Although the formation of AChR clusters is inhibited by these drugs, the stability of the existent clusters is relatively insensitive to them. These data suggest that the clustering of AChR involves a Ca2+ and calmodulin-activated process. Immunofluorescence studies using an antibody against calmodulin indicate that calmodulin is diffusely distributed in the cytoplasm in addition to its localization at the I-bands. Thus I propose that a local rise in intracellular calcium caused by a locally applied stimulus, exemplified here by the polypeptide-coated latex beads, may trigger the formation of AChR clusters. Furthermore, the cellular machinery for this process may involve calmodulin and is diffusely distributed in the muscle cell.

Calmodulin in starfish oocytes. II. Trypsin treatment suppresses the trifluoperazine-sensitive step

The 1-methyladenine-induced oocyte maturation in starfish is reversibly inhibited by the anticalmodulin drug, trifluoperazine (TFP). However, when oocytes are exposed for 10 min to trypsin, they lose their sensitivity to TFP. Trypsin does not alter the length of the hormone-dependent period (1-methyladenine minimal contact time) or the 1-methyladenine concentration requirements. Trypsin-treated oocytes remain sensitive to other maturation inhibitors such as procaine, theophylline, caffeine, and D-600. Trypsin exposure modifies the protein pattern composition of the oocyte cortex (breakdown of a 140-kDa protein). TFP binding site localization was studied using fluorescence microscopy: in addition to a general diffuse fluorescence, staining is localized to probably acidic granules located in the cortex. Results are discussed in relation to calmodulin and plasma membrane calmodulin-dependent enzyme involvement in the stimulation of starfish oocyte maturation.

Possible involvement of calmodulin in maturation and activation of Chaetopterus eggs

We report the isolation of calmodulin from oocytes of Chaetopterus pergamentaceus. The identification of this protein is based on (1) activation of beef heart cAMP phosphodiesterase, (2) heat stability, (3) sensitivity to chlorpromazine, and (4) electrophoretic mobility identical to that of porcine brain calmodulin after sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the presence of either Ca2+ or EGTA. We treated oocytes with chlorpromazine and W-7 to investigate the involvement of calmodulin in meiosis initiation and egg activation. Very low concentrations of chlorpromazine inhibited germinal vesicle breakdown (GVBD). This effect was shown to be dependent upon bright indirect light, since the drug was much less effective at GVBD inhibition under conditions of very low illumination. Higher concentrations of chlorpromazine and W-7 (100 microM) inhibited GVBD and activated eggs with intact germinal vesicles as determined by fertilization envelope formation and the onset of ameboid activity. Neither egg activation nor inhibition of calmodulin stimulation of phosphodiesterase activity in vitro was affected by light. These results are consistent with a role for calmodulin in egg activation and GVBD, but suggest that chlorpromazine in bright light may prevent GVBD by some mechanism other than calmodulin inhibition.

Morphological changes during maturation of starfish oocytes: surface ultrastructure and cortical actin

The cell surface and extracellular investments of oocytes of the starfish Pisaster ochraceus are analyzed by Nomarski differential interference contrast microscopy and by scanning electron microscopy. The investing coats include a thin sheet of follicle cells, a jelly coat, and a vitelline layer; their morphologies are described. Methods are outlined for systematically removing them without altering the behavior of the oocyte so that the cell surface can be examined directly. The topography of denuded oocytes changes dramatically when they are treated with the maturation-inducing hormone, 1-methyladenine. The major topographical change is the early and transient formation of prominent surface spikes. These structures arise due to the rapid, reversible polymerization of actin into stout bundles. Polymerization and subsequent depolymerization of cortical actin is monitored by epifluorescence microscopy of oocytes stained with NBD-phallacidin, a stain which is specific for polymerized actin. Based on scanning electron microscopy, spikes apparently utilize preexisting plasma membrane of microvilli, and plasma membrane is apparently lost when spikes collapse. Long after microvilli are eliminated due to spike formation, the number of microvilli is somewhat restored, especially around the animal pole where the polar body forms. A chronology of events observed during oocyte maturation is discussed with reference to the possible mechanisms and implications of polymerization and depolymerization of cortical actin.

Lithium inhibits amplification or action of the maturation-promoting factor (MPF) in meiotic maturation of starfish oocytes

Microinjection of LiCl reversibly inhibits hormone-induced meiotic maturation of starfish oocytes. Microinjection of NaCl (even in ouabain-treated oocytes) or KCl, or external application of LiCl have no such effect. Blockade of meiotic maturation by Li+ occurs even when microinjection is performed after the hormone dependent period has ended, that is the period during which the hormone must be present in the medium in order that meiosis can take place. Li+ microinjection prevents oocytes from meiosis reinitiation following transfer of cytoplasm taken from maturing oocytes, which contain a maturation-promoting factor (MPF). Cytoplasm taken from Li+-injected and hormone-treated oocytes does not trigger meiosis reinitiation when transferred in control immature oocytes. Intracellular pH does not change following LiCl microinjection. Simultaneous microinjection of either K+, Na+, or EGTA does not prevent Li+-dependent inhibition in oocytes.

The effects of quercetin on meiosis initiation in clam and starfish oocytes

We have treated Spisula and Asterias oocytes with quercetin to determine the effects of this drug on germinal vesicle breakdown (GVBD). Quercetin (100-200 microM) reversibly inhibited GVBD when induced by excess KCl or ionophore A23187. Kinetic studies showed that quercetin blocked an early event in GVBD. Lower concentrations of quercetin (10-20 microM) blocked fertilization. However, quercetin sensitized the oocytes to initiation of GVBD by excess ions which do not normally trigger GVBD. Quercetin (100-200 microM) also blocked 1-methyladenine-induced GVBD in the starfish. In subthreshold concentrations of the hormone or in its absence, lower concentrations (20-40 microM) stimulated GVBD. The results support the hypothesis that quercetin exerts its effects on meiosis initiation through its effects on calcium sequestration.

Activation of calmodulin-dependent NAD+ kinase by trypsin

Sea urchin egg NAD+ kinase (ATP:NAD+ 2'-phosphotransferase, EC 2.7.1.23), a calmodulin-dependent enzyme, can be activated by a moderate treatment with trypsin in a similar fashion to calmodulin. Stimulation by trypsin is dependent on its concentration (half-maximal dose: 1.5 microgram/ml) but independent of the presence of calcium. This suggests that limited proteolysis is able to activate NAD+ kinase as described for several other calmodulin-activated enzymes and that these enzymes may interact with calmodulin in a similar way.