Role of calcium and cyclic nucleotides during meiosis reinitiation in starfish oocytes

Increase of cAMP upon release from prophase arrest in surf clam oocytes

Surf clam (Spisula solidissima) oocytes are spawned at the prophase I stage of meiosis, and they remain arrested at this stage until fertilization. Full oocyte meiosis reinitiation, first evidenced by germinal vesicle breakdown (GVBD), may be induced by artificial activators mimicking sperm, such as high K+ or serotonin. Previous reports indicated that treatments thought to increase the level of oocyte cAMP inhibited sperm- or serotonin-induced, but not KCl-induced, GVBD in clam oocytes. These observations extend the well known requirement for a drop in occyte cAMP levels in mammalian, amphibian or starfish oocytes and support the view that such a drop is universally important throughout the animal kingdom. We have re-examined the cAMP dependency of GVBD in clam oocytes and found that various treatments that raise oocyte cAMP levels did not, surprisingly, affect either KCl- or serotonin-induced GVBD. Such treatments, however, inhibited GVBD upon insemination of the oocytes, but this was due to the failure of sperm to fuse/penetrate the oocytes; thus, it was not an inhibition of oocyte activation as such. Direct measurements of oocyte cAMP levels after activation by serotonin, KCl or sperm showed that, contrary to expectations, there is a rise in cAMP levels before GVBD. Using SQ22536, an adenylyl cyclase inhibitor, the increase in oocyte cAMP level was partly prevented and GVBD proceeded, but with a significant retardation, indicating that the normal cAMP rise facilitates GVBD. Our work sheds light on the diversity of upstream pathways leading to activation of MPF and provides a unique model whereby the onset of meiosis reinitiation is associated with an increase, not a decrease, in oocyte cAMP levels.

5-HT causes an increase in cAMP that stimulates, rather than inhibits, oocyte maturation in marine nemertean worms

In the nemertean worms Cerebratulus lacteus and Micrura alaskensis, 5-HT (=5-hydroxytryptamine, or serotonin) causes prophase-arrested oocytes to mature and complete germinal vesicle breakdown (GVBD). To identify the intracellular pathway that mediates 5-HT stimulation, follicle-free oocytes of nemerteans were assessed for GVBD rates in the presence or absence of 5-HT after being treated with various modulators of cAMP, a well known transducer of 5-HT signaling and an important regulator of hormone-induced maturation in general. Unlike in many animals where high levels of intra-oocytic cAMP block maturation, treatment of follicle-free nemertean oocytes with agents that elevate cAMP (8-bromo-cAMP, forskolin or inhibitors of phosphodiesterases) triggered GVBD in the absence of added 5-HT. Similarly, 5-HT caused a substantial cAMP increase prior to GVBD in nemertean oocytes that had been pre-injected with a cAMP fluorosensor. Such a rise in cAMP seemed to involve G-protein-mediated signaling and protein kinase A (PKA) stimulation, based on the inhibition of 5-HT-induced GVBD by specific antagonists of these transduction steps. Although the downstream targets of activated PKA remain unknown, neither the synthesis of new proteins nor the activation of MAPKs (mitogen-activated protein kinases) appeared to be required for GVBD after 5-HT stimulation. Alternatively, pre-incubation in roscovitine, an inhibitor of maturation-promoting factor (MPF), prevented GVBD, indicating that maturing oocytes eventually need to elevate their MPF levels, as has been documented for other animals. Collectively, this study demonstrates for the first time that 5-HT can cause immature oocytes to undergo an increase in cAMP that stimulates, rather than inhibits, meiotic maturation. The possible relationship between such a form of oocyte maturation and that observed in other animals is discussed.

Involvement of cAMP in initiating maturation of the brittle-star Amphipholis kochii oocytes: induction of oocyte maturation by inhibitors of cyclic nucleotide phosphodiesterase and activators of adenylate cyclase

The maturation of brittle-star (Amphipholis kochii) oocytes, i.e., the reinitiation of meiosis accompanied by germinal vesicle breakdown (GVBD) and the acquisition of fertilizability, was induced by acid (pH 3.0) seawater containing 10 mM cAMP. Oocyte maturation was also induced by seawater of normal pH (pH 8.0) that contained either an inhibitor of cyclic nucleotide phosphodiesterase (25 mM theophylline, 25 mM caffeine) or an activator of adenylate cyclase (100 microM forskolin, 0.6 microM cholera toxin). Experiments in which the oocytes were treated with forskolin or theophylline for various periods of time demonstrated that there was a positive correlation between the oocyte cAMP level measured by radioimmunoassay and the extent of GVBD induced in each treatment: both increased as the treatment period became longer and about a threefold increase in cAMP level induced 50% GVBD. These results indicate that an increase in cAMP level initiates maturation of the brittle-star oocytes.

Starfish oocyte maturation: 1-methyladenine triggers a drop of cAMP concentration related to the hormone-dependent period

Oocyte maturation (meiosis reinitiation) in starfish is induced by the natural hormone 1-methyladenine (1-MeAde). Oocytes of Evasterias troschelii contain 0.43 pmole cyclic AMP/mg protein and 0.47 pmole cyclic GMP/mg protein. Upon stimulation by 1-MeAde the oocytes undergo a moderate (10-30%) decrease in their cAMP concentration. The concentration of cGMP remains unaltered. Oocytes treated with forskolin, an activator of adenylate cyclase, increase their cAMP concentration over 35-fold, up to 16 pmole cAMP/mg protein. When stimulated by 1-MeAde these forskolin-pretreated oocytes undergo a major (50-70%) decrease in their cAMP concentration. A similar decrease is triggered by mimetics of 1-MeAde, such as dithiothreitol, arachidonic acid (AA), and 8-hydroxyeicosatetraenoic acid (8-HETE), but not by adenine which is inactive. 1-MeAde-stimulated oocytes of Pisaster ochraceus also undergo a decrease in cAMP content, the size of which is increased by forskolin. Although a decrease in cAMP begins at sub-threshold 1-MeAde concentrations, the maximal decrease occurs at the same concentration of 1-MeAde needed for maturation induction and a further 1000-fold increase of the 1-MeAde concentration has no further effect. Upon removal of 1-MeAde, the cAMP concentration immediately increases to its original level. Sequential addition and removal of 1-MeAde triggers a sequential decrease and increase of the cAMP concentration, illustrating the continuous requirement for 1-MeAde for eliciting the decrease. Successive additions of 1-MeAde, however, do not trigger further decreases of the cAMP concentration. The temperature dependences of the cAMP concentration decrease and of the hormone-dependent period (HDP; the time of contact with 1-MeAde required for induction of maturation) are closely related. Forskolin, which increases the cAMP concentration, also increases the duration of the HDP (2.5-fold), delays the time course of protein phosphorylation burst and germinal vesicle breakdown, and inhibits AA- and 8-HETE-induced maturation. We conclude that 1-MeAde triggers a drop in cAMP concentration, which is tightly associated with the hormone-dependent period of oocyte maturation.

Do calcium and calmodulin trigger maturation in amphibian oocytes?

In contrast to earlier reports (J. L. Maller and E. G. Krebs, 1980, Curr. Top. Cell. Regul. 16, 271-311; M. Moreau, J. P. Vilian, and P. Guerrier, 1980, Dev. Biol. 78, 201-214; W. J. Wasserman and L. D. Smith, 1981, J. Cell Biol. 89, 389-394; D. Huchon, R. Ozon, E. H. Fischer, and J. G. Demaille, 1981, Mol. Cell. Endocrinol. 22, 211-222) calmodulin preparations isolated from Xenopus laevis ovaries or obtained commercially rarely induced maturation upon microinjection into individual oocytes. Calmodulin injections did result in significant cases of maturation when oocytes were first pretreated (primed) with calcium-free (EGTA) OR-2 and then injected in regular OR-2 medium. However, under these conditions the injected buffer solution alone was sometimes found to induce maturation. Under more optimal priming conditions, cases were found where as high as 100% of the oocytes matured simply by returning them to regular OR-2 medium. To determine which divalent cations could be involved in the priming effects of EGTA pretreatment we repeated the earlier ionophore work of W. J. Wasserman and Y. Masui (1975, J. Exp. Zool. 193, 369-375), looking not just at calcium and magnesium but other divalent cations as well. Several divalent cations (10 mM) were found to induce germinal vesicle breakdown with the following tentative order of efficacy, Co2+ greater than or equal to Zn2+ greater than or equal to Mn2+ greater than Ca2+ greater than Mg2+ greater than Ba2+, regardless of whether or not ionophore A23187 was present. These results, along with other reports in the literature, are discussed with respect to the theory that a rise in free calcium and calmodulin is involved in triggering oocyte maturation; we conclude that neither is involved.

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.

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.

Meiotic resumption and intracellular cAMP levels in mouse oocytes treated with compounds which act on cAMP metabolism

We have studied the effect of agents known to stimulate adenylate cyclase on spontaneous meiotic resumption in vitro by mouse oocytes. We have found that cholera toxin (CT) (up to 1 microM) and prostaglandin E1 (PGE1) (up to 160 microM) are not able to prevent meiotic resumption, but a clear dose-dependent delay in meiosis resumption was observed during the first 3 h of incubation in medium containing CT or PGE1. The effect became clearer when a small concentration of isobutylmethylxanthine (MIX) (1 microM) was added to the medium. We have also measured the cAMP content of the oocyte: the basal content is 2.1 fmol; this value drops to 0.9 fmol during a 2-h culture period. The decrease is partially prevented if CT is present in the incubation medium, while total cAMP content increases to 3.1 fmol in the presence of 1 mM MIX. The results suggest that isolated mouse oocytes contain toxin and prostaglandin sensitive adenylate cyclase and also an active phosphodiesterase system.

Is calcium the second messenger of 1-methyladenine in meiosis reinitiation of starfish oocytes?

Microinjection of EGTA into prophase-blocked oocytes does not inhibit hormone-induced meiosis reinitiation, although it prevents oocyte activation by fertilization, by ionophore A23187, or by subsequent microinjection of otherwise efficient Ca2+ buffers. In contrast microinjection of Ca2+ buffers inhibits 1-methyladenine-induced meiosis reinitiation. Oocytes can be released from Ca2+ inhibition by raising hormone concentration or by the subsequent transfer of cytoplasm taken from maturing oocytes. Ca2+-microinjected oocytes remain inhibited up to 1 h after microinjection, although free Ca2+ concentration comes back to its resting value less than 30 sec after microinjection. Cyanide, which decreases ATP content and depresses Ca2+-pumping activity, reversibly inhibits 1-methyladenine-induced meiosis reinitiation. These results do not support the hypothesis that Ca2+ is the second messenger of the hormone in meiosis reinitiation of starfish oocytes, although they support the view that elimination of Ca2+ from some component of the oocyte cortex (perhaps the plasma membrane) might be a compulsory event for transduction of the hormonal message.

Changes in cGMP levels on meiosis reinitiation of starfish oocytes

An intracellular level of cGMP (but not cAMP) transiently decreased the reinitiation of oocyte maturation in the starfish, Asterias amurensis. Exogenously applied cGMP inhibited hormone-induced maturation. Methylxanthines inhibited oocyte maturation by suppressing the decrease in cGMP levels. These results suggest that a decrease in cGMP levels is a prerequisite for meiosis reinitiation of starfish oocytes.