Properties of 1-methyladenine receptors in starfish oocyte membranes: involvement of pertussis toxin-sensitive GTP-binding protein in the receptor-mediated signal transduction

Selective dephosphorylation by PP2A-B55 directs the meiosis I-meiosis II transition in oocytes

Meiosis is a specialized cell cycle that requires sequential changes to the cell division machinery to facilitate changing functions. To define the mechanisms that enable the oocyte-to-embryo transition, we performed time-course proteomics in synchronized sea star oocytes from prophase I through the first embryonic cleavage. Although we found that protein levels were broadly stable, our analysis reveals that dynamic waves of phosphorylation underlie each meiotic stage. We found that the phosphatase PP2A-B55 is reactivated at the meiosis I/meiosis II (MI/MII) transition, resulting in the preferential dephosphorylation of threonine residues. Selective dephosphorylation is critical for directing the MI/MII transition as altering PP2A-B55 substrate preferences disrupts key cell cycle events after MI. In addition, threonine to serine substitution of a conserved phosphorylation site in the substrate INCENP prevents its relocalization at anaphase I. Thus, through its inherent phospho-threonine preference, PP2A-B55 imposes specific phosphoregulated behaviors that distinguish the two meiotic divisions.

Fluorescence Measurement and Calibration of Intracellular pH in Starfish Oocytes

Oocyte maturation is a process wherein an oocyte arrested at prophase of meiosis I resumes meiosis to become a fertilizable egg. In starfish ovaries, a hormone released from follicle cells activates the oocytes, resulting in an increase in their intracellular pH (pH_i), which is required for spindle assembly. Herein, we describe a protocol for pH_i measurement in living oocytes microinjected with the pH-sensitive dye BCECF. For in vivo BCECF calibration, we treated oocytes with artificial seawater containing CH₃COONH₄ to clamp pH_i, injected pH-standard solutions, and converted the BCECF fluorescence intensity ratios to pH_i values. Of note, if the actual pH_i is higher or lower than the known pH of injected standard solutions, the BCECF fluorescence intensity ratio will decrease or increase, respectively. On the other hand, the pH of the injected solution displaying no change in fluorescence intensity should be considered the actual pH_i. These methods for pH_i calibration and clamping are simple and reproducible.

Oocyte Maturation in Starfish

Oocyte maturation is a process that occurs in the ovaries, where an immature oocyte resumes meiosis to attain competence for normal fertilization after ovulation/spawning. In starfish, the hormone 1-methyladenine binds to an unidentified receptor on the plasma membrane of oocytes, inducing a conformational change in the heterotrimeric GTP-binding protein α-subunit (Gα), so that the α-subunit binds GTP in exchange of GDP on the plasma membrane. The GTP-binding protein βγ-subunit (Gβγ) is released from Gα, and the released Gβγ activates phosphatidylinositol-3 kinase (PI3K), followed by the target of rapamycin kinase complex2 (TORC2) and 3-phosphoinositide-dependent protein kinase 1 (PDK1)-dependent phosphorylation of serum- and glucocorticoid-regulated kinase (SGK) of ovarian oocytes. Thereafter, SGK activates Na⁺/H⁺ exchanger (NHE) to increase the intracellular pH (pHi) from ~6.7 to ~6.9. Moreover, SGK phosphorylates Cdc25 and Myt1, thereby inducing the de-phosphorylation and activation of cyclin B–Cdk1, causing germinal vesicle breakdown (GVBD). Both pHi increase and GVBD are required for spindle assembly at metaphase I, followed by MI arrest at pHi 6.9 until spawning. Due to MI arrest or SGK-dependent pHi control, spawned oocytes can be fertilized normally

1-Methyladenine: a starfish oocyte maturation-inducing substance

1-Methyladenine (1-MeAde) in starfish was the first compound to be identified as an oocyte maturation-inducing substance (MIS) among invertebrates in 1969 by Kanatani and co-workers. In starfish, the ripe ovary contains a huge number of fully grown oocytes of almost equal size. Each oocyte possesses a large nucleus (germinal vesicle, GV), which is arrested in late prophase of the first maturation division. The oocyte is surrounded by a single follicle layer. Such immature oocytes are not fertilisable. Resumption of meiosis in immature oocytes can be induced by 1-MeAde, and the mature oocytes thus become fertilisable (Kanatani et al., 1969; Kanatani, 1985). 1-MeAde is produced by ovarian follicle cells upon stimulation with a gonad-stimulating substance (GSS) released from the radial nerves (Fig. 1).

It has been demonstrated that GSS is a peptide hormone (Kanatani et al., 1971). The action of GSS on 1-MeAde production in follicle cells appears to be mediated by its receptor, G-proteins and adenylyl cyclase (Mita & Nagahama, 1991). These findings suggest that a G-protein coupled (seven transmembrane type) receptor is involved in GSS signal transduction, similarly to the pituitary-gonadal axis in vertebrates.

Thus, using degenerate probes derived from consensus sequences of the mammalian glycoprotein hormone (GTH and TSH) receptors, cDNA was cloned from mRNA of ovaries of Asterina pectinifera. The cDNA showed striking structural homology with members of the glycoprotein hormone receptor family in the transmembrane region, and contained a very large extracellular region. Expression was observed in isolated ovarian follicle cells. Thus, it seems likely that the glycoprotein hormone receptor (GTHR) family gene is related to GSS receptor in ovarian follicle cells. The phylogenic relatedness of the starfish GTHR was also assessed in relation to other vertebrate GTHRs. The analysis showed that the starfish gene diverged before differentiation of the gonadotropin (LH and FSH) and TSH receptors in vertebrates.

Starfish Apaf-1 activates effector caspase-3/9 upon apoptosis of aged eggs

Caspase-3-related DEVDase activity is initiated upon apoptosis in unfertilized starfish eggs. In this study, we cloned a starfish procaspase-3 corresponding to mammalian effector caspase containing a CARD that is similar to the amino terminal CARD of mammalian capsase-9, and we named it procaspase-3/9. Recombinant procaspase-3/9 expressed at 15 °C was cleaved to form active caspase-3/9 which has DEVDase activity. Microinjection of the active caspase-3/9 into starfish oocytes/eggs induced apoptosis. An antibody against the recombinant protein recognized endogenous procaspase-3/9 in starfish oocytes, which was cleaved upon apoptosis in aged unfertilized eggs. These results indicate that caspase-3/9 is an effector caspase in starfish. To verify the mechanism of caspase-3/9 activation, we cloned starfish Apaf-1 containing a CARD, a NOD, and 11 WD40 repeat regions, and we named it sfApaf-1. Recombinant sfApaf-1 CARD interacts with recombinant caspase-3/9 CARD and with endogenous procaspase-3/9 in cell-free preparations made from starfish oocytes, causing the formation of active caspase-3/9. When the cell-free preparation without mitochondria was incubated with inactive recombinant procaspase-3/9 expressed at 37 °C, DEVDase activity increased and apoptosome-like complexes were formed in the high molecular weight fractions containing both sfApaf-1 and cleaved caspase-3/9. These results suggest that sfApaf-1 activation is not dependent on cytochrome c.

Affinity chromatography of a binder of 1-methyladenine, the maturation-inducing hormone for starfish oocytes

Starfish oocytes are arrested at the prophase stage of the first meiotic division in the ovary. They resume meiosis by the stimulus of 1-methyladenine (1-MeAde), the maturation-inducing hormone for starfish oocytes. Putative 1-MeAde receptors have been suggested to be present on the oocyte surface, but not yet been characterized biochemically. As reported recently (T. Toraya, T. Kida, A. Kuyama, S. Matsuda, S. Tanaka, Y. Komatsu, T. Tsurukai, Biochem. Biophys. Res. Commun. 485 (2017) 41-46), it became possible to detect unknown 1-MeAde binders of starfish oocytes by immunophotoaffinity labeling, i.e., photoaffinity labeling combined with immunochemical detection. We designed and synthesized water-soluble and insoluble polymer-bound 1-MeAde derivatives. A water-soluble polymer-bound 1-MeAde derivative, in which 1-MeAde is bound to dextran through an N⁶-substituent, triggered the germinal-vesicle breakdown toward follicle-free oocytes, dejellied oocytes, and denuded oocytes. This is consistent with the idea that putative 1-MeAde receptors are located on the cell surface of starfish oocytes. A water-insoluble polymer-bound 1-MeAde derivative, in which 1-MeAde is bound to Sepharose 4B through an N⁶-substituent, served as an effective affinity adsorbent for the partial purification of a 1-MeAde binder with M_r of 47.5 K that might be a possible candidate of the maturation-inducing hormone receptors of starfish oocytes.

Immunophotoaffinity labeling of binders of 1-methyladenine, the oocyte maturation-inducing hormone of starfish

Starfish oocytes are arrested at the prophase stage of the first meiotic division in the ovary and resume meiosis by the stimulus of 1-methyladenine (1-MeAde), the oocyte maturation-inducing hormone of starfish. Putative 1-MeAde receptors on the oocyte surface have been suggested, but not yet been biochemically characterized. Immunophotoaffinity labeling, i.e., photoaffinity labeling combined with immunochemical detection, was attempted to detect unknown 1-MeAde binders including putative maturation-inducing hormone receptors in starfish oocytes. When the oocyte crude membrane fraction or its Triton X-100/EDTA extract was incubated with N⁶-[6-(5-azido-2-nitrobenzoyl)aminohexyl]carboxamidomethyl-1-methyladenine and then photo-irradiated, followed by western blotting with antibody that was raised against a 1-MeAde hapten, a single band with M_r of 47.5 K was detected. The band was lost when extract was heated at 100 °C. A similar 47.5 K band was detected in the crude membrane fraction of testis as well. Upon labeling with whole cells, this band was detected in immature and maturing oocytes, but only faintly in mature oocytes. As judged from these results, this 1-MeAde binder might be a possible candidate of the starfish maturation-inducing hormone receptors.

Hormonal stimulation of starfish oocytes induces partial degradation of the 3' termini of cyclin B mRNAs with oligo(U) tails, followed by poly(A) elongation

In yeast, plant, and mammalian somatic cells, short poly(A) tails on mRNAs are subject to uridylation, which mediates mRNA decay. Although mRNA uridylation has never been reported in animal oocytes, maternal mRNAs with short poly(A) tails are believed to be translationally repressed. In this study, we found that 96% of cyclin B mRNAs with short poly(A) tails were uridylated in starfish oocytes. Hormonal stimulation induced poly(A) elongation of cyclin B mRNA, and 62% of long adenine repeats did not contain uridine residues. To determine whether uridylated short poly(A) tails destabilize cyclin B mRNA, we developed a method for producing RNAs with the strict 3' terminal sequences of cyclin B, with or without oligo(U) tails. When we injected these synthetic RNAs into starfish oocytes prior to hormonal stimulation, we found that uridylated RNAs were as stable as nonuridylated RNAs. Following hormonal stimulation, the 3' termini of short poly(A) tails of synthesized RNAs containing oligo(U) tails were trimmed, and their poly(A) tails were subsequently elongated. These results indicate that uridylation of short poly(A) tails in cyclin B mRNA of starfish oocytes does not mediate mRNA decay; instead, hormonal stimulation induces partial degradation of uridylated short poly(A) tails in the 3'-5' direction, followed by poly(A) elongation. Oligo(U) tails may be involved in translational inactivation of mRNAs.

Immunophotoaffinity labeling of the binding proteins for 1-methyladenine, an oocyte maturation-inducing hormone of starfish

Starfish oocytes are naturally arrested at the prophase stage of the first meiotic division and resume meiosis in response to 1-methyladenine (1-MeAde), the oocyte maturation-inducing hormone of starfish. Putative receptors for 1-MeAde have not yet been characterized biochemically, although the specific binding of 1-MeAde to the isolated cortices of starfish oocytes was reported so far. Based on the structure-activity relationship of 1-MeAde analogs, we have designed a photoaffinity labeling reagent. The photoaffinity labeling of oocyte membrane fractions, followed by immunoblotting analysis with anti-1-MeAde antibody, results in the detection of an almost single protein band. This 1-MeAde-binding protein might be a possible candidate of the maturation-inducing hormone receptor of starfish.

Guanine nucleotides in the meiotic maturation of starfish oocytes: regulation of the actin cytoskeleton and of Ca(2+) signaling

Background

Starfish oocytes are arrested at the first prophase of meiosis until they are stimulated by 1-methyladenine (1-MA). The two most immediate responses to the maturation-inducing hormone are the quick release of intracellular Ca²⁺ and the accelerated changes of the actin cytoskeleton in the cortex. Compared with the later events of oocyte maturation such as germinal vesicle breakdown, the molecular mechanisms underlying the early events involving Ca²⁺ signaling and actin changes are poorly understood. Herein, we have studied the roles of G-proteins in the early stage of meiotic maturation.

Methodology/Principal Findings

By microinjecting starfish oocytes with nonhydrolyzable nucleotides that stabilize either active (GTPγS) or inactive (GDPβS) forms of G-proteins, we have demonstrated that: i) GTPγS induces Ca²⁺ release that mimics the effect of 1-MA; ii) GDPβS completely blocks 1-MA-induced Ca²⁺; iii) GDPβS has little effect on the amplitude of the Ca²⁺ peak, but significantly expedites the initial Ca²⁺ waves induced by InsP₃ photoactivation, iv) GDPβS induces unexpectedly striking modification of the cortical actin networks, suggesting a link between the cytoskeletal change and the modulation of the Ca²⁺ release kinetics; v) alteration of cortical actin networks with jasplakinolide, GDPβS, or actinase E, all led to significant changes of 1-MA-induced Ca²⁺ signaling.

Conclusions/Significance

Taken together, these results indicate that G-proteins are implicated in the early events of meiotic maturation and support our previous proposal that the dynamic change of the actin cytoskeleton may play a regulatory role in modulating intracellular Ca²⁺ release.

Incapacity of response to disulfide-reducing agent in Triton X-100-treated oocytes of starfish, Asterina pectinifera

Resumption of meiosis in starfish oocytes is induced by the natural maturation-inducing hormone, 1-methyladenine (1-MeAde). Oocyte maturation is also induced by the disulfide-reducing agent, dithiothreitol (DTT). Previous studies have shown that 1-MeAde controls meiosis by interacting with its receptors, which are located exclusively on oocyte plasma membrane. However, little is known about the mechanism of oocyte maturation induced by DTT. Thus, this study examined whether DTT interacts with 1-MeAde receptors to induce oocyte maturation. When oocytes were treated with Triton X-100, they failed to respond to 1-MeAde and DTT. Although the Triton X-100-treated oocytes recovered the capacity to respond to 1-MeAde during incubation in seawater, they remained unresponsive to DTT during seawater incubations. These results suggest that DTT does not interact with 1-MeAde receptors to induce oocyte maturation in starfish. It is possible that a protein essential for mediating DTT-induced maturation is eliminated from the oocytes surface following Triton X-100 treatment.

Stops and starts in mammalian oocytes: recent advances in understanding the regulation of meiotic arrest and oocyte maturation

Mammalian oocytes grow and undergo meiosis within ovarian follicles. Oocytes are arrested at the first meiotic prophase, held in meiotic arrest by the surrounding follicle cells until a surge of LH from the pituitary stimulates the immature oocyte to resume meiosis. Meiotic arrest depends on a high level of cAMP within the oocyte. This cAMP is generated by the oocyte, through the stimulation of the G(s) G-protein by the G-protein-coupled receptor, GPR3. Stimulation of meiotic maturation by LH occurs via its action on the surrounding somatic cells rather than on the oocyte itself. LH induces the expression of epidermal growth factor-like proteins in the mural granulosa cells that act on the cumulus cells to trigger oocyte maturation. The signaling pathway between the cumulus cells and the oocyte, however, remains unknown. This review focuses on recent studies highlighting the importance of the oocyte in producing cAMP to maintain arrest, and discusses possible targets at the level of the oocyte on which LH could act to stimulate meiotic resumption.

Activation of a pertussis toxin-sensitive, inhibitory G-protein is necessary for steroid-mediated oocyte maturation in spotted seatrout

Oocyte maturation (OM) is initiated in lower vertebrates and echinoderms when maturation-inducing substances (MIS) bind oocyte membrane receptors. This study tested the hypothesis that activation of a G(i) protein is necessary for MIS-mediated OM in spotted seatrout. Addition of MIS significantly decreased adenylyl cyclase activity in a steroid specific, pertussis toxin (PTX)-sensitive manner in oocyte membranes and microinjection of PTX into oocytes inhibited MIS-induced OM, suggesting the steroid activates a G(i) protein. MIS significantly increased [(35)S]GTPgammaS binding to ovarian membranes, confirming that MIS receptor binding activates a G-protein, and immunoprecipitation studies showed the increased [(35)S]GTPgammaS binding was associated with Galpha(i1-3) proteins. Radioligand binding studies in ovarian membranes using GTPgammaS and PTX demonstrated that the MIS binds a receptor coupled to a PTX-sensitive G-protein. This study provides the first direct evidence in a vertebrate model that MIS-induced activation of a G(i) protein is necessary for OM. These results support a mechanism of MIS action involving binding to a novel, G-protein coupled receptor and activation of an inhibitory G-protein, the most comprehensive and plausible model of MIS initiation of OM proposed to date.

Degradation of Polyubiquitinated Cyclin B Is Blocked by the MAPK Pathway at the Metaphase I Arrest in Starfish Oocytes

In the starfish ovary, maturing oocytes stimulated by 1-methyladenine undergo synchronous germinal vesicle breakdown and then arrest in metaphase of the first meiotic division (metaphase I). Immediately after spawning, an increase of intracellular pH (pH(i)) from approximately 7.0 to approximately 7.3 is induced by Na(+)/H(+) antiporter in oocytes, and meiosis reinitiation occurs. Here we show that an endogenous substrate of the proteasome, polyubiquitinated cyclin B, was stable at pH 7.0, whereas it was degraded at pH 7.3. When the MAPK pathway was blocked by MEK inhibitor U0126, degradation of polyubiquitinated cyclin B occurred even at pH 7.0 without an increase of the peptidase activity of the proteasome. These results indicate that the proteasome activity at pH 7.0 is sufficient for degradation of polyubiquitinated cyclin B and that the MAPK pathway blocks the degradation of polyubiquitinated cyclin B in the maturing oocytes in the ovary. Immediately after spawning, the increase in pH(i) mediated by Na(+)/H(+) antiporter cancels the inhibitory effects of the MAPK pathway, resulting in the degradation of polyubiquitinated cyclin B and the release of the arrest. Thus, the key step of metaphase I arrest in starfish oocytes occurs after the polyubiqutination of cyclin B but before cyclin B proteolysis by the proteasome.

Inhibitory Effect of a SALMFamide Neuropeptide on Secretion of Gonad-Stimulating Substance from Radial Nerves in the Starfish Asterina pectinifera

In starfish, the peptide hormone gonad-stimulating substance (GSS) secreted from nervous tissue stimulates oocyte maturation to induce 1-methyladenine (1-MeAde) production by ovarian follicle cells. The SALMFamide family is also known to an echinoderm neuropeptide. The present study examined effect of SALMFamide 1 (S1) on oocyte maturation of starfish Asterina pectinifera. Unlike GSS, S1 did not induce spawning in starfish ovary. In contrast, S1 was found to inhibit GSS secretion from radial nerves by treatment with high K+ concentration. Fifty percent inhibition was obtained by 0.1 mM S1. S1 did not have any effect on GSS- and 1-MeAde-induced oocyte maturation. Following incubation with a S1 antibody and subsequently with rhodamine-conjugated second antibody, neural networks were observed in ovaries. The networks were restricted mainly to their surface with little evidence of immunoreactivity inside the basement membranes. This indicates that neural networks are distributed in the ovarian wall. The result further suggests that S1 plays a role in oocyte maturation to regulate GSS secretion from the nervous system.

Metaphase I arrest of starfish oocytes induced via the MAP kinase pathway is released by an increase of intracellular pH

Reinitiation of meiosis in oocytes usually occurs as a two-step process during which release from the prophase block is followed by an arrest in metaphase of the first or second meiotic division [metaphase I (MI) or metaphase II (MII)]. The mechanism of MI arrest in meiosis is poorly understood, although it is a widely observed phenomenon in invertebrates. The blockage of fully grown starfish oocytes in prophase of meiosis I is released by the hormone 1-methyladenine. It has been believed that meiosis of starfish oocytes proceeds completely without MI or MII arrest, even when fertilization does not occur. Here we show that MI arrest of starfish oocytes occurs in the ovary after germinal vesicle breakdown. This arrest is maintained both by the Mos/MEK/MAP kinase pathway and the blockage of an increase of intracellular pH in the ovary before spawning. Immediately after spawning into seawater, activation of Na+/H+ antiporters via a heterotrimeric G protein coupling to a 1-methyladenine receptor in the oocyte leads to an intracellular pH increase that can overcome the MI arrest even in the presence of active MAP kinase.

A receptor linked to a Gi-family G-protein functions in initiating oocyte maturation in starfish but not frogs

The stimulation of oocyte maturation by 1-methyladenine in starfish, and by a steroid in frogs, has been proposed to involve G-protein-coupled receptors. To examine whether activation of receptors linked to G(i) or G(z) was sufficient to cause oocyte maturation, we expressed mammalian G(i)- and G(z)-linked receptors in starfish and frog oocytes. Application of the corresponding agonists caused meiosis to resume in the starfish but not the frog oocytes. We confirmed that the receptors were effectively expressed in the frog oocytes by using a chimeric G-protein, G(qi), that converts input from G(i)- and G(z)-linked receptors to a G(q) output and results in a contraction of the oocyte's pigment. These results argue against G(i) or G(z) functioning to cause maturation in frog oocytes. Consistently, maturation-inducing steroids did not cause pigment contraction in frog oocytes expressing G(qi), and G(z) protein was not detectable in frog oocytes. For starfish oocytes, however, our results support the conclusion that G(i) functions in 1-methyladenine signaling and suggest the possibility of using frog oocyte pigment contraction as an assay to identify the 1-methyladenine receptor. To test this concept, we coexpressed G(qi) and a starfish adenosine receptor in frog oocytes and showed that applying adenosine caused pigment contraction.

Interaction of N1-substituted adenines with 1-methyladenine receptors of starfish oocytes in induction of maturation

Starfish oocytes are arrested naturally in the late G(2) phase of the first meiotic division. In response to the natural maturation-inducing hormone, 1-methyladenine (1-MA), oocytes undergo reinitiation of meiosis with germinal vesicle breakdown. We tested 10 newly synthesized N1-substituted adenines that are 1-MA analogues to analyze the interaction between 1-MA and its stereo-specific receptors on the oocyte plasma membranes of the starfish Asterina pectinifera. Among these analogues, 1-(beta-naphthylmethyl)adenine, 1-aminoadenine and 1-(p-nitrobenzyl)adenine played agonistic roles in the induction of oocyte maturation. 1-(o-Nitrobenzyl)adenine, 1-(m-nitrobenzyl)adenine, 1-phenethyladenine and 1-(p-nitrophenethyl)adenine had antagonist effects on 1-MA-induced oocyte maturation. These agonists and antagonists behaved competitively in the binding of [3H]1-MA to receptors in oocyte cortices. In contrast, 1-(alpha-naphthylmethyl)adenine, 1-(2,4-dinitrobenzyl)adenine and 1-(p-methoxybenzyl)adenine had no effects on oocyte maturation. Our results suggest that regional-specific sterical structures at the N1-site of adenine are important in the interaction between 1-MA and its receptors in oocytes. In addition, a negative charge at the N1-site of adenine is required for binding with the receptors.

Fertilization blocks apoptosis of starfish eggs by inactivation of the MAP kinase pathway

Fully grown starfish oocytes are arrested at prophase of meiosis I. The hormonal stimulation of 1-methyladenine (1-MA) induces meiosis reinitiation and germinal vesicle breakdown (GVBD). Optimal development occurs when maturing oocytes are fertilized between GVBD and first polar body emission. In the absence of sperm, oocytes complete both meiotic divisions to yield haploid interphase-arrested eggs. We now report that spontaneous and synchronous activation of caspase-3 in starfish eggs occurs 9-12 h after 1-MA stimulation. Then, caspase-dependent membrane blebbing and egg fragmentation occur, indicating that mature eggs undergo apoptosis if not fertilized. Activation of caspase-3 and induction of apoptosis are blocked both by a MEK inhibitor and by emetine treatment which inhibits MEK kinase (Mos) synthesis. Conversely, when recombinant GST-Mos is injected into the emetine-treated eggs, apoptosis is induced. These results indicate that persistent activation of the Mos/MEK/MAP kinase cascade gives the death-activating signal in starfish eggs. Fertilization inactivates the MAP kinase pathway and suppresses apoptosis, followed by normal development.

Regulation of Xenopus oocyte meiosis arrest by G protein betagamma subunits

BACKGROUND

Progesterone induces the resumption of meiosis (maturation) in Xenopus oocytes through a nongenomic mechanism involving inhibition of an oocyte adenylyl cyclase and reduction of intracellular cAMP. However, progesterone action in Xenopus oocytes is not blocked by pertussis toxin, and this finding indicates that the inhibition of the oocyte adenylyl cyclase is not mediated by the alpha subunits of classical G(i)-type G proteins.

RESULTS

To investigate the possibility that G protein betagamma subunits, rather than alpha subunits, play a key role in regulating oocyte maturation, we have employed two structurally distinct G protein betagamma scavengers (G(t)alpha and betaARK-C(CAAX)) to sequester free Gbetagamma dimers. We demonstrated that the injection of mRNA encoding either of these Gbetagamma scavengers induced oocyte maturation. The Gbetagamma scavengers bound an endogenous, membrane-associated Gbeta subunit, indistinguishable from Xenopus Gbeta1 derived from mRNA injection. The injection of Xenopus Gbeta1 mRNA, together with bovine Ggamma2 mRNA, elevated oocyte cAMP levels and inhibited progesterone-induced oocyte maturation.

CONCLUSION

An endogenous G protein betagamma dimer, likely including Xenopus Gbeta1, is responsible for maintaining oocyte meiosis arrest. Resumption of meiosis is induced by Gbetagamma scavengers in vitro or, naturally, by progesterone via a mechanism that suppresses the release of Gbetagamma.

G protein beta gamma subunits inhibit nongenomic progesterone-induced signaling and maturation in Xenopus laevis oocytes. Evidence for a release of inhibition mechanism for cell cycle progression

Progesterone-induced maturation of Xenopus oocytes is a well known example of nongenomic signaling by steroids; however, little is known about the early signaling events involved in this process. Previous work has suggested that G proteins and G protein-coupled receptors may be involved in progesterone-mediated oocyte maturation as well as in other nongenomic steroid-induced signaling events. To investigate the role of G proteins in nongenomic signaling by progesterone, the effects of modulating Galpha and Gbetagamma levels in Xenopus oocytes on progesterone-induced signaling and maturation were examined. Our results demonstrate that Gbetagamma subunits, rather than Galpha, are the principal mediators of progesterone action in this system. We show that overexpression of Gbetagamma inhibits both progesterone-induced maturation and activation of the MAPK pathway, whereas sequestration of endogenous Gbetagamma subunits enhances progesterone-mediated signaling and maturation. These data are consistent with a model whereby endogenous free Xenopus Gbetagamma subunits constitutively inhibit oocyte maturation. Progesterone may induce maturation by antagonizing this inhibition and therefore allowing cell cycle progression to occur. These studies offer new insight into the early signaling events mediated by progesterone and may be useful in characterizing and identifying the membrane progesterone receptor in oocytes.

Protein kinase C-related kinase 2 phosphorylates the protein synthesis initiation factor eIF4E in starfish oocytes

Phosphorylation of eIF4E is required for protein synthesis during starfish oocyte maturation. The activity of protein kinase C-related kinase 2 (PRK2) increases prior to the phosphorylation of eIF4E (G. Stapleton et al., 1998, Dev. Biol. 193, 34-46). We investigate here whether eIF4E is activated by PRK2. A 3.5-kb eIF4E clone isolated from starfish cDNA is 57% identical to human eIF4E and contains the putative phosphorylation site serine-209. The serine-209 environment (SKTGS(209)MAKSRF) is similar to the consensus sequence of the phosphorylation site of protein kinase C and related kinases. A starfish eIF4E fusion protein (GST-4E) was phosphorylated in vitro by PRK2 in the presence of 1,2-diolyl-sn-glycerol 3-phosphate. In contrast, replacing the GST-4E serine-209 with an alanine significantly reduced this phosphorylation. Analysis by two-dimensional phosphopeptide mapping reveals a major phosphopeptide in trypsin-digested GST-4E, but not in its serine-209 mutant. Importantly, this major phosphopeptide in GST-4E corresponds to a major phosphopeptide of eIF4E isolated from (32)P-labeled oocytes. Thus, PRK2 may regulate translation initiation during oocyte maturation by phosphorylating the serine-209 residue of eIF4E in starfish. We also demonstrate that high levels of cAMP inhibit the activation of PRK2, eIF4E, and the eIF4E binding protein during starfish oocyte maturation, while PI3 kinase activates these proteins.

G-protein betagamma subunit-dependent phosphorylation of 62-kDa protein in the early signaling pathway of starfish oocyte maturation induced by 1-methyladenine

In starfish oocytes, maturation is induced by a hormone, 1-methyladenine (1-MA), that binds to the receptors exposed to the outer surface of the plasma membrane. The signal of 1-MA stimulates the heterotrimeric G protein, resulting in dissociation of the betagamma subunit of G protein (Gbetagamma) from a pertussis toxin-sensitive Gi-type alpha subunit. To investigate the targets for Gbetagamma, we analyzed 1-MA- or Gbetagamma-dependent phosphorylation using in vivo and in vitro systems. A 62-kDa protein was phosphorylated immediately after 1-MA treatment in intact oocytes. In the cell-free preparations, the 62-kDa protein was also phosphorylated on serine residue(s) immediately after addition of 1-MA or Gbetagamma. The Gbetagamma-dependent phosphorylation of the 62-kDa protein was inhibited by wortmannin or LY294002, which are mechanistically different inhibitors of phosphatidylinositol 3-kinase (PI3K). LY294002 also inhibited Gbetagamma- as well as 1-MA-induced maturation of oocytes. Taken together, these results indicate that the 62-kDa protein functions downstream of Gbetagamma and PI3K in the early signaling pathway of 1-MA-induced starfish oocyte maturation. The phosphorylation of the 62-kDa protein may be required for the activation of maturation-promoting factor.

Induction of germinal vesicle breakdown in a cell-free preparation from starfish oocytes

Incubation of isolated germinal vesicles in the homogenate from maturing starfish oocytes resulted in synchronous germinal vesicle breakdown (GVBD), and chromosome condensation and gathering within 30 min. GVBD in this cell-free system required aerobic conditions. The endogenous ATP-generation system was preserved in the homogenate and effective under aerobic conditions, and thus exogenous ATP was not added to the homogenate. Injection of the homogenate into immature starfish oocytes induced meiotic maturation without 1-methyladenine, indicating high activity of maturation-promoting factor (MPF) in the homogenate. MPF activity in the homogenate was stable for 2 h at room temperature, while it disappeared within 1 h in the supernatant prepared by centrifugation of the homogenate. This disappearance of MPF activity is regulated by cyclin B destruction, similar to that seen in vivo.

Components of the signaling pathway linking the 1-methyladenine receptor to MPF activation and maturation in starfish oocytes

Starfish oocytes are arrested at the G2/M-phase border of meiosis I. Exposure to their natural mitogen, 1-methyladenine (1-MA), leads to the activation of MPF and MAP kinase, resumption of the meiotic cell cycle, and fertilization competency. The 1-MA receptor has not yet been identified, but it is known to be linked functionally to a pertussis toxin-sensitive G-protein. G beta gamma appears to be the major effector of the 1-MA receptor, since injection of G beta gamma, but not activated G alpha i, leads to the activation of MPF, entry into meiosis, and oocyte maturation. The components that connect G beta gamma to MPF and MAP kinase activation in oocytes are unknown. In mammalian cells, a novel phosphatidylinositol 3-kinase, PI-3 kinase-gamma, links G beta gamma to the MAP kinase activation pathway. Here we show that PI-3 kinase is required for starfish oocyte maturation. LY294002 and wortmannin, inhibitors of PI-3 kinase, block MPF and MAP kinase activation and entry into meiosis. Inhibition by LY294002 is reversible and limited to the hormone-dependent period. Neither inhibitor, however, blocks the earliest hormone-induced event, formation of actin spikes at the cell membrane. By contrast, pertussis toxin blocks both actin spiking and later events, arguing that PI-3 kinase functions downstream of G beta gamma. Finally, we show that unlike the well-studied case in Xenopus oocytes, where MAP kinase is an essential component of the MPF activation pathway, MAP kinase is not required for either MPF activation or subsequent oocyte maturation in starfish. Instead, its major role appears to be suppression of DNA synthesis in unfertilized, haploid eggs.

G-protein-mediated signal transduction for meiosis reinitiation in starfish oocyte

Starfish oocyte maturation is induced by 1-methyladenine. There were apparently two forms of 1-methyladenine receptor affected by GTP gamma S. Pertussis toxin ADP-ribosylated the 39-kDa alpha subunit of a G protein and inhibited maturation. Also, the G protein was ADP-ribosylated by cholera toxin only when 1-methyladenine was added. The purified G protein had an heterotrimeric structure consisting of 39 kDa alpha, 37 kDa beta, and 8 kDa gamma subunits. The deduced amino acid sequence of the cDNA of starfish G alpha was 89% identical to mammalian Gi-1 alpha. The purified starfish beta gamma-subunits induced maturation when they were microinjected into oocytes.

Stimulation of Xenopus oocyte maturation by inhibition of the G-protein alpha S subunit, a component of the plasma membrane and yolk platelet membranes

Oocytes of Xenopus laevis undergo maturation when injected with an affinity-purified antibody against the COOH-terminal decapeptide of the alpha subunit of the G-protein Gs, an antibody that inhibits Gs activity. Germinal vesicle breakdown, chromosome condensation, and polar body formation occur, with a time course similar to that for oocytes treated with progesterone. The alpha S antibody-injected oocytes also acquire the ability to be activated by sperm. Coinjection of the catalytic subunit of cAMP-dependent protein kinase, or incubation with cycloheximide, inhibits maturation in response to injection of the alpha S antibody; these experiments show that the alpha S antibody acts at an early point in the pathway leading to oocyte maturation, before formation of maturation promoting factor, and like progesterone, its action requires protein synthesis. Immunogold electron microscopy shows that alpha S is present in the yolk platelet membranes as well as the plasma membrane. These results support the hypothesis that progesterone acts by inhibiting alpha S, and suggest that the target of progesterone could include yolk platelet membranes as well as the plasma membrane.

Induction of starfish oocyte maturation by the beta gamma subunit of starfish G protein and possible existence of the subsequent effector in cytoplasm

beta gamma subunits of G proteins were purified from starfish oocytes, and their role in the induction of oocyte maturation by 1-methyladenine was investigated. When injected into starfish oocytes, the purified beta gamma subunit of the starfish G protein induced germinal vesicle breakdown (GVBD) faster than that of bovine brain G protein. Injection of the starfish beta gamma into cytoplasm near the germinal vesicle (GV) induced GVBD earlier than when injected into the GV or the cytoplasm near the plasma membrane. Fluorescent-labeled beta gamma was retained in the injected area even after GVBD. Injected beta gamma also induced the formation of maturation-promoting factor as well as an increase of histone H1 kinase activity. These results suggest that beta gamma dissociates from alpha-subunit by the stimulation of 1-methyladenine and interacts with a cytoplasmic effector, which results in formation of active cdc2 kinase.

Oocyte maturation in starfish is mediated by the beta gamma-subunit complex of a G-protein

The stimulation of meiotic maturation of starfish oocytes by the hormone 1-methyladenine is mimicked by injection of beta gamma subunits of G-proteins from either retina or brain. Conversely, the hormone response is inhibited by injection of the GDP-bound forms of alpha i1 or alpha t subunits, or by injection of phosducin; all of these proteins should bind free beta gamma. alpha-subunit forms with reduced affinity for beta gamma (alpha i1 or alpha t bound to hydrolysis-resistant GTP analogs, or alpha i1-GMPPCP treated with trypsin to remove the amino terminus of the protein) are less effective inhibitors of 1-methyladenine action. These results indicate that the beta gamma subunit of a G-protein mediates 1-methyladenine stimulation of oocyte maturation.