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

Hormonal symphony: The dynamic duo of IGF and EGF in gonadotropin-induced fish ovarian development and egg maturation

Fish oocyte maturation (FOM) is a critical biological process that occurs before ovulation and is influenced by gonadotropins, particularly luteinizing hormone (LH). The release of LH stimulates the ovarian follicle to produce a maturation-inducing hormone (MIH), specifically 17α, 20β-dihydroxy-4-pregnen-3-one (17α, 20β-DP), which initiates the formation of maturation-promoting factor (MPF) through the activation of cyclin B and cdc2 kinase. Insulin-like growth factor I (IGF-I) significantly regulates ovarian functions, including steroidogenesis, by activating its membrane receptors and the tyrosine kinase pathway. IGF-I influences oocyte maturation directly via the PI3 kinase pathway, independent of steroid hormones. Additionally, epidermal growth factor (EGF) promotes cell growth and differentiation by binding to its receptor (EGFR). It is implicated in mediating human chorionic gonadotropin (hCG)-induced DNA synthesis in ovarian follicles while suppressing apoptosis. The presence of EGF in follicle cells and oocytes, along with its higher expression in oocytes, suggests it may act as a paracrine signal regulating somatic cell activity. Recent studies indicate that the activin system in follicle cells could be a target for EGF activity. The EGFR signaling pathway enhances gonadotropin-induced steroidogenesis and governs the transition of oocyte maturation stages, essential for successful fertilization. This review synthesizes current research on the roles of gonadotropins, IGFs, and EGFs in fish oocyte maturation and ovarian steroid production.

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

A Genome-Wide Screen for Wortmannin-Resistant Mutants in Schizosaccharomyces pombe: The Phosphorylation-Impaired Mutants Are Resistant to Signaling Defect

Complex human diseases such as metabolic disorders, cancer, neurodegenerative diseases, and mitochondrial dysfunctions arise from the biochemical or genetic defects in various cellular processes. Therefore, it is important to understand which metabolic processes are affected by which cellular impairment. Because genome-wide screening of mutant collections (haploid/diploid deletion library) provides important clues for the understanding of conserved biological processes and for finding potential target genes, we screened the haploid mutant collection of Schizosaccharomyces pombe with wortmannin that inhibits phosphatidylinositol-3-kinase signaling. Using genome-wide screening, we determined that 52 mutants were resistant to this chemical. When 52 genes that are deleted in these mutants were grouped in 41 different biological processes, we found that 37 of them have human orthologues and 4 genes were associated with human metabolic disorders. In addition, when we examined the pathways in which these 52 genes function, we determined that 9 genes were related to phosphorylation process. These results might provide new insights for better understanding of certain human diseases.

SGK phosphorylates Cdc25 and Myt1 to trigger cyclin B-Cdk1 activation at the meiotic G2/M transition

The kinase cyclin B-Cdk1 complex is a master regulator of M-phase in both mitosis and meiosis. At the G2/M transition, cyclin B-Cdk1 activation is initiated by a trigger that reverses the balance of activities between Cdc25 and Wee1/Myt1 and is further accelerated by autoregulatory loops. In somatic cell mitosis, this trigger was recently proposed to be the cyclin A-Cdk1/Plk1 axis. However, in the oocyte meiotic G2/M transition, in which hormonal stimuli induce cyclin B-Cdk1 activation, cyclin A-Cdk1 is nonessential and hence the trigger remains elusive. Here, we show that SGK directly phosphorylates Cdc25 and Myt1 to trigger cyclin B-Cdk1 activation in starfish oocytes. Upon hormonal stimulation of the meiotic G2/M transition, SGK is activated by cooperation between the Gβγ-PI3K pathway and an unidentified pathway downstream of Gβγ, called the atypical Gβγ pathway. These findings identify the trigger in oocyte meiosis and provide insights into the role and activation of SGK.

SGK regulates pH increase and cyclin B-Cdk1 activation to resume meiosis in starfish ovarian oocytes

Tight regulation of intracellular pH (pH_i) is essential for biological processes. Fully grown oocytes, having a large nucleus called the germinal vesicle, arrest at meiotic prophase I. Upon hormonal stimulus, oocytes resume meiosis to become fertilizable. At this time, the pH_i increases via Na⁺/H⁺ exchanger activity, although the regulation and function of this change remain obscure. Here, we show that in starfish oocytes, serum- and glucocorticoid-regulated kinase (SGK) is activated via PI3K/TORC2/PDK1 signaling after hormonal stimulus and that SGK is required for this pH_i increase and cyclin B-Cdk1 activation. When we clamped the pH_i at 6.7, corresponding to the pH_i of unstimulated ovarian oocytes, hormonal stimulation induced cyclin B-Cdk1 activation; thereafter, oocytes failed in actin-dependent chromosome transport and spindle assembly after germinal vesicle breakdown. Thus, this SGK-dependent pH_i increase is likely a prerequisite for these events in ovarian oocytes. We propose a model that SGK drives meiotic resumption via concomitant regulation of the pH_i and cell cycle machinery.

Relative importance of phosphatidylinositol-3 kinase (PI3K)/Akt and mitogen-activated protein kinase (MAPK3/1) signaling during maturational steroid-induced meiotic G2-M1 transition in zebrafish oocytes

Participation and relative importance of phosphatidylinositol-3 kinase (PI3K) and mitogen-activated protein kinase (MAPK) signalling, either alone or in combination, have been investigated during 17α,20β-dihydroxy-4-pregnen-3-one (DHP)-induced meiotic G2-M1 transition in denuded zebrafish oocyte. Results demonstrate that concomitant with rapid phosphorylation (activation) of Akt (Ser473) and MAPK (ERK1/2) at as early as 15 min of incubation, DHP stimulation promotes enhanced an GVBD response and histone H1 kinase activation between 1 and 5 h in full-grown oocytes in vitro. While p-Akt reaches its peak at 60 to 90 min and undergoes downregulation to the basal level by 240 min, ERK1/2 phosphorylation (activation) increases gradually until 120 min and remains high thereafter. Although, priming with MEK1/2 inhibitor U0126 is without effect, PI3K inhibitors, wortmannin or LY294002, delay the GVBD response significantly (P < 0.001) until 3 h but not at 5 h of incubation. Interestingly, blocking PI3K and MEK function together could abrogate steroid-induced oocyte maturation at all time points tested. While DHP stimulation promotes phospho-PKA catalytic (p-PKAc) dephosphorylation (inactivation) between 30-120 min of incubation, simultaneous inhibition of PI3K and MEK1/2 kinases abrogates DHP action. Conversely, elevated intra-oocyte cAMP, through priming with either adenylyl cyclase (AC) activator forskolin (FK) or dibutyryl cAMP (db-cAMP), abrogates steroid-induced Akt and ERK1/2 phosphorylation. Taken together, these results suggest that DHP-induced Akt and ERK activation precedes the onset of meiosis (GVBD response) in a cAMP-sensitive manner and PI3K/Akt and MEK/MAPK pathways together have a pivotal influence in the downregulation of PKA and resumption of meiotic maturation in zebrafish oocytes in vitro.

Two new competing pathways establish the threshold for cyclin-B-Cdk1 activation at the meiotic G2/M transition

Extracellular ligands control biological phenomena. Cells distinguish physiological stimuli from weak noise stimuli by establishing a ligand-concentration threshold. Hormonal control of the meiotic G2/M transition in oocytes is essential for reproduction. However, the mechanism for threshold establishment is unclear. In starfish oocytes, maturation-inducing hormones activate the PI3K–Akt pathway through the G_βγ complex of heterotrimeric G-proteins. Akt directly phosphorylates both Cdc25 phosphatase and Myt1 kinase, resulting in activation of cyclin-B–Cdk1, which then induces meiotic G2/M transition. Here, we show that cyclin-B–Cdk1 is partially activated after subthreshold hormonal stimuli, but this triggers negative feedback, resulting in dephosphorylation of Akt sites on Cdc25 and Myt1, thereby canceling the signal. We also identified phosphatase activity towards Akt substrates that exists independent of stimuli. In contrast to these negative regulatory activities, an atypical G_βγ-dependent pathway enhances PI3K–Akt-dependent phosphorylation. Based on these findings, we propose a model for threshold establishment in which hormonal dose-dependent competition between these new pathways establishes a threshold; the atypical G_βγ-pathway becomes predominant over Cdk-dependent negative feedback when the stimulus exceeds this threshold. Our findings provide a regulatory connection between cell cycle and signal transduction machineries.

Summary: Ligand–dose thresholds control ligand-dependent responses. To establish the hormonal threshold for driving meiosis, a stimulus-dependent positive regulatory pathway competes against negative feedback from cell cycle machinery.

Releasing prophase arrest in zebrafish oocyte: synergism between maturational steroid and Igf1

Binding of 17β-estradiol (E2) to novel G-protein coupled receptor, Gper1, promotes intra-oocyte adenylyl cyclase activity and transactivates epidermal growth factor receptor to ensure prophase-I arrest. Although involvement of either membrane progestin receptor (mPR) or Igf system has been implicated in regulation of meiosis resumption, possibility of concurrent activation and potential synergism between 17α,20β-dihydroxy-4-pregnen-3-one (DHP)- and Igf-mediated signalling cascades in alleviating E2 inhibition of oocyte maturation (OM) has not been investigated. Here using zebrafish (Danio rerio) defolliculated oocytes, we examined the effect of DHP and Igf1, either alone or in combination, in presence or absence of E2, on OM in vitro. While priming of denuded oocytes with E2 blocked spontaneous maturation, co-treatment with DHP (3 nM) and Igf1 (10 nM), but not alone, reversed E2 inhibition and promoted a robust increase in germinal vesicle breakdown (GVBD). Although stimulation with either Igf1 or DHP promoted Akt phosphorylation, pharmacological inhibition of PI3K/Akt signalling prevented Igf1-induced GVBD but delayed DHP action till 4-5 h of incubation. Moreover, high intra-oocyte cAMP attenuates both DHP and Igf1-mediated OM and co-stimulation with DHP and Igf1 could effectively reverse E2 action on PKA phosphorylation. Interestingly, data from in vivo studies reveal that heightened expression of igf1, igf3 transcripts in intact follicles corresponded well with elevated phosphorylation of Igf1r and Akt, mPRa immunoreactivity, PKA inhibition and accelerated GVBD response just prior to ovulation. This indicates potential synergism between maturational steroid and Igf1 which might have physiological relevance in overcoming E2 inhibition of meiosis resumption in zebrafish oocytes.

Regulation of recombinant human insulin-induced maturational events in Clarias batrachus (L.) oocytes in vitro

Regulation of insulin-mediated resumption of meiotic maturation in catfish oocytes was investigated. Insulin stimulation of post-vitellogenic oocytes promotes the synthesis of cyclin B, histone H1 kinase activation and a germinal vesicle breakdown (GVBD) response in a dose-dependent and duration-dependent manner. The PI3K inhibitor wortmannin abrogates recombinant human (rh)-insulin action on histone H1 kinase activation and meiotic G2–M1 transition in denuded and follicle-enclosed oocytes in vitro. While the translational inhibitor cycloheximide attenuates rh-insulin action, priming with transcriptional blocker actinomycin D prevents insulin-stimulated maturational response appreciably, albeit in low amounts. Compared with rh-insulin, human chorionic gonadotrophin (hCG) stimulation of follicle-enclosed oocytes in vitro triggers a sharp increase in 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DHP) secreted in the incubation medium at 12 h. Interestingly, the insulin, but not the hCG-induced, maturational response shows less susceptibility to steroidogenesis inhibitors, trilostane or dl-aminoglutethimide. In addition, priming with phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) or cell-permeable dbcAMP or adenylyl cyclase activator forskolin reverses the action of insulin on meiotic G2–M1 transition. Conversely, the adenylyl cyclase inhibitor, SQ 22536, or PKA inhibitor H89 promotes the resumption of meiosis alone and further potentiates the GVBD response in the presence of rh-insulin. Furthermore, insulin-mediated meiotic maturation involves the down-regulation of endogenous protein kinase A (PKA) activity in a manner sensitive to PI3K activation, suggesting potential involvement of a cross-talk between cAMP/PKA and insulin-mediated signalling cascade in catfish oocytes in vitro. Taken together, these results suggest that rh-insulin regulation of the maturational response in C. batrachus oocytes involves down-regulation of PKA, synthesis of cyclin B, and histone H1 kinase activation and demonstrates reduced sensitivity to steroidogenesis and transcriptional inhibition.

High cAMP attenuation of insulin-stimulated meiotic G2-M1 transition in zebrafish oocytes: interaction between the cAMP-dependent protein kinase (PKA) and the MAPK3/1 pathways

High intra-cellular cyclic nucleotide (cAMP) ensures prophase-I arrest and prevent steroid-induced meiotic G2-M1 transition in full-grown oocytes; however, relatively less information is available for cAMP regulation of growth factor-stimulated signalling events in the oocyte model. Here using zebrafish oocytes, we show that priming with dibutyryl cAMP (dbcAMP) or cAMP modulators, e.g. adenylate cyclase activator, forskolin or phosphodiesterase inhibitors (IBMX/cilostamide) block insulin action on germinal vesicle breakdown (GVBD) and histone H1 kinase activation. Though high cAMP priming attenuates insulin-induced MAPK3/1 (ERK1/2) phosphorylation (activation), following 2h of insulin stimulation it fails to block MAPK activation and GVBD. Further, insulin stimulation promotes down regulation of phospho-PKAc (inactivation) and PKA inhibition by H89/PKI-(6-22)-amide overcomes negative regulation by cAMP and induces GVBD and MAPK activation. Moreover, MEK1/2 inhibitor U0126 has no influence on H89-induced GVBD; however, it delays GVBD response in insulin-stimulated oocytes. MAPK activation by okadaic acid (OA) promotes GVBD; however, high dbcAMP abrogates OA action suggesting cross-talk between cAMP/PKA and MAPK-mediated signalling pathways may contribute significantly in maturing zebrafish oocyte.

Steroid-induced oocyte maturation in Indian shad Tenualosa ilisha (Hamilton, 1822) is dependent on phosphatidylinositol 3 kinase but not MAP kinase activation

Fully grown fish and amphibian oocytes exposed to a maturation-inducing steroid (MIS) activates multiple signal transduction pathways, leading to formation and activation of maturation-promoting factor (MPF) and induction of germinal vesicle breakdown (GVBD). The present study was to investigate if phosphatidylinositol 3 kinase (PI3 kinase) and mitogen-activated protein kinase (MAP kinase) activation are required for naturally occurring MIS, 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P)-induced cdc2 activation and oocyte maturation (OM) in Tenualosa ilisha. We observed that 17,20β-P-induced OM was significantly inhibited by PI3 kinase inhibitors Wortmannin and LY29400. 17,20 β-P was shown to activate PI3 kinase maximally at 90 min and cdc2 kinase at 16 h of treatment. Relative involvement of PI3 kinase, MAP kinase and cdc2 kinase in 17,20β-P-induced OM was examined. MAP kinase was rapidly phosphorylated and activated (60-120 min) after MIS treatment and this response preceded the activation of cdc2 kinase by several hours. A selective inhibitor of MAP kinase (MEK), PD98059, sufficiently blocked the phosphorylation and activation of MAP kinase. Inhibition of MAP kinase activity using PD98059 however, had no effect on MIS-induced cdc2 kinase activation and GVBD. These results demonstrate that activation of the PI3 kinase is required for 17,20β-P-induced cdc2 kinase activation and OM in T. ilisha. MAP kinase although was activated in response to 17,20β-P and PI3 kinase activation, it is not necessary for cdc2 activation and OM in this species.

Participation of PI3-kinase/Akt signalling in insulin stimulation of p34cdc2 activation in zebrafish oocyte: phosphodiesterase 3 as a potential downstream target

Exposure of fully grown oocytes to growth factors (insulin/IGFs) initiates various signalling cascades that culminate to final stages of oocyte maturation. Regulation of signalling pathways during growth factor-induced meiosis resumption in fish is not well characterized. Here we studied the participation of PI3K/Akt signalling pathway during recombinant human insulin (rh-insulin)-induced meiotic maturation in zebrafish (Danio rerio) oocytes. Priming of defolliculated oocytes in vitro with rh-insulin promotes germinal vesicle breakdown (GVBD) in a dose- and time-dependent manner, an effect sensitive to translation but not transcription inhibition. More than 80% of the oocytes underwent GVBD due to 0.8IU/ml rh-insulin within 10h of incubation and the kinetics of p34cdc2 kinase activation corresponded well with GVBD data. PI3K inhibitors, wortmannin and LY294002 blocked insulin, but not 17α, 20β-DHP-induced GVBD. Immunoblot analyses of oocyte extract revealed that phospho-PI3K (p85α) was up regulated within 30-60 min of insulin stimulation followed by phospho-Akt (Ser473) at 60-120 min. Though PI3K/Akt phosphorylation was largely unaffected, pre-incubation with phosphodiesterase (PDE) inhibitors, IBMX and cilostamide, but not rolipram completely blocked rh-insulin-induced p34cdc2 activation and GVBD. These results suggest that PDE3 may be one potential downstream target to PI3K/Akt signalling necessary for rh-insulin-induced GVBD in zebrafish.

A brief summary of neuroendocrine regulation of reproduction in sea stars

Over than fifty years starfishes have been widely used as model for studying the mechanisms of cell cycle regulation, oocyte maturation and fertilization. Besides, significant work has been done to investigate the role of nervous system in the control of reproduction and spawning in these animals. Nowadays, sea stars represent one of the most thoroughly studied model for hormonal regulation of reproduction among invertebrates. However, while the general picture of neuroendocrine control of asteroid reproduction can be drawn easily, our knowledge concerning the details of this process still has some gaps. Filling these gaps is essential for studying the diversity of hormonal mechanisms involved in regulation of animal reproduction. The present paper aims to briefly summarize current data on hormonal regulation of reproduction in sea stars and to highlight existing gaps in our knowledge on the details of this process.

Inhibition of PI3K/Akt pathway impairs G2/M transition of cell cycle in late developing progenitors of the avian embryo retina

PI3K/Akt is an important pathway implicated in the proliferation and survival of cells in the CNS. Here we investigated the participation of the PI3K/Akt signal pathway in cell cycle of developing retinal progenitors. Immunofluorescence assays performed in cultures of chick embryo retinal cells and intact tissues revealed the presence of phosphorylated Akt and 4E-BP1 in cells with typical mitotic profiles. Blockade of PI3K activity with the chemical inhibitor LY 294002 (LY) in retinal explants blocked the progression of proliferating cells through G2/M transition, indicated by an expressive increase in the number of cells labeled for phosphorylated histone H3 in the ventricular margin of the retina. No significant level of cell death could be detected at this region. Retinal explants treated with LY for 24 h also showed a significant decrease in the expression of phospho-Akt, phospho-GSK-3 and the hyperphosphorylated form of 4E-BP1. Although no change in the expression of cyclin B1 was detected, a significant decrease in CDK1 expression was noticed after 24 h of LY treatment both in retinal explants and monolayer cultures. Our results suggest that PI3K/Akt is an active pathway during proliferation of retinal progenitors and its activity appears to be required for proper CDK1 expression levels and mitosis progression of these cells.

A primer on meiotic resumption in starfish oocytes: the proposed signaling pathway triggered by maturation-inducing hormone

This short review updates the maturation-inducing hormonal signaling in starfish oocytes. In this system, the activation of cyclin B-Cdc2 kinase (Cdk1) that leads to meiotic resumption does not require new protein synthesis. The key intracellular mediator after hormonal stimulation by 1-methyladenine is the protein kinase Akt/PKB, which in turn directly downregulates Myt1 and upregulates Cdc25 toward the activation of cyclin B-Cdc2. Mitotic kinases including Aurora, Plk1 and Greatwall are activated downstream of cyclin B-Cdc2. The starfish oocyte thus provides a simple model system for the study of meiotic resumption.

Turn motif phosphorylation negatively regulates activation loop phosphorylation in Akt

Akt, also known as protein kinase B, has a central role in various signaling pathways that regulate cellular processes such as metabolism, proliferation and survival. On stimulation, phosphorylation of the activation loop (A-loop) and hydrophobic motif (HM) of Akt by the kinase phosphoinositide-dependent kinase 1 (PDK1) and the mammalian target of rapamycin complex 2 (mTORC2), respectively, results in Akt activation. A well-conserved threonine in the turn motif (TM) is also constitutively phosphorylated by mTORC2 and contributes to the stability of Akt. The role of TM phosphorylation in HM and A-loop phosphorylation has not been sufficiently evaluated. Using starfish oocytes as a model system, this study provides the first evidence that TM phosphorylation has a negative role in A-loop phosphorylation. In this system, the maturation-inducing hormone, 1-methyladenine, stimulates Akt to reinitiate meiosis through activation of cyclin B-Cdc2. The phosphorylation status of Akt was monitored via introduction of exogenous human Akt (hAkt) in starfish oocytes. TM and HM phosphorylation was inhibited by microinjection of an anti-starfish TOR antibody, but not by rapamycin treatment, suggesting that both phosphorylation events depend on TORC2, as reported in mammalian cells. A single or double alanine substitution at each of three phosphorylation residues revealed that TM phosphorylation renders Akt susceptible to dephosphorylation on the A-loop. When A-loop phosphatase was inhibited by okadaic acid (OA), TM phosphorylation still reduced A-loop phosphorylation, suggesting that the effect is caused at least partially through reduction of sensitivity to PDK1. Negative regulation by TM phosphorylation was also observed in constitutively active Akt and was functionally reflected in meiosis resumption. By contrast, HM phosphorylation enhanced A-loop phosphorylation and achieved full activation of Akt via a mechanism at least partially independent of TM phosphorylation. These observations provide new insight into the mechanism controlling Akt phosphorylation in the cell.

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.

Signaling pathways in ascidian oocyte maturation: the role of cyclic AMP and follicle cells in germinal vesicle breakdown

Many ascidian oocytes undergo 'spontaneous' germinal vesicle breakdown (GVBD) when transferred from the ovary to normal pH 8.2 sea water (SW); however, low pH inhibits GVBD, which can then be stimulated while remaining in the low pH SW. Oocytes of Boltenia villosa blocked from GVBD by pH 4 SW undergo GVBD in response to permeant cyclic AMP (8-bromo-cyclic AMP), phosphodiesterase inhibitors (isobutylmethylxanthine and theophylline) or the adenylyl cyclase activator forskolin. This suggests that cAMP increases during GVBD. Removal of the follicle cells or addition of a protease inhibitor inhibits GVBD in response to raised pH but not to forskolin, theophylline or 8 bromo-cAMP. Isolated follicle cells in low pH SW release protease activity in response to an increase in pH. These studies imply that the follicle cells release protease activity, which either itself stimulates an increase in oocyte cAMP level or reacts with other molecules to stimulate this process. Studies with the mitogen-activated protein (MAP) kinase inhibitors U0126 and CI 1040 suggest that MAP kinase is not involved in GVBD. The Cdc25 inhibitor NSC 95397 inhibits GVBD at 200 nm in a reversible manner.

Intracellular Ca2+ increase induces post-fertilization events via MAP kinase dephosphorylation in eggs of the hydrozoan jellyfish Cladonema pacificum

Naturally spawned eggs of the hydrozoan jellyfish Cladonema pacificum are arrested at G1-like pronuclear stage until fertilization. Fertilized eggs of Cladonema undergo a series of post-fertilization events, including loss of sperm-attracting ability, expression of adhesive materials on the egg surface, and initiation of cell cycle leading to DNA synthesis and cleavage. Here, we investigate whether these events are regulated by changes in intracellular Ca2+ concentration and mitogen-activated protein kinase (MAP kinase) activity in Cladonema eggs. We found that MAP kinase is maintained in the phosphorylated form in unfertilized eggs. Initiation of sperm-induced Ca2+ increase, which is the first sign of fertilization, was immediately followed by MAP kinase dephosphorylation within a few minutes of fertilization. The fertilized eggs typically stopped sperm attraction by an additional 5 min and became sticky around this time. They further underwent cytokinesis yielding 2-cell embryos at approximately 1 h post-fertilization, which was preceded by DNA synthesis evidenced by BrdU incorporation into the nuclei. Injection of inositol 1,4,5-trisphosphate (IP3) into unfertilized eggs, which produced a Ca2+ increase similar to that seen at fertilization, triggered MAP kinase dephosphorylation and the above post-fertilization events without insemination. Conversely, injection of BAPTA/Ca2+ into fertilized eggs at approximately 10 s after the initiation of Ca2+ increase immediately lowered the elevating Ca2+ level and inhibited the subsequent post-fertilization events. Treatment with U0126, an inhibitor of MAP kinase kinase (MEK), triggered the post-fertilization events in unfertilized eggs, where MAP kinase dephosphorylation but not Ca2+ increase was generated. Conversely, preinjection of the glutathione S-transferase (GST) fusion protein of MAP kinase kinase kinase (Mos), which maintained the phosphorylated state of MAP kinase, blocked the post-fertilization events in fertilized eggs without preventing a Ca2+ increase. These results strongly suggest that all of the three post-fertilization events, cessation of sperm attraction, expression of surface adhesion, and progression of cell cycle, lie downstream of MAP kinase dephosphorylation that is triggered by a Ca2+ increase.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

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.

Steroid-induced oocyte maturation in Atlantic croaker (Micropogonias undulatus) is dependent on activation of the phosphatidylinositol 3-kinase/Akt signal transduction pathway

Exposure of fully grown fish and amphibian oocytes to a maturation-inducing steroid (MIS) activates numerous signal transduction pathways to initiate the final stage of oocyte maturation. These events culminate in the activation of maturation-promoting factor and germinal vesicle breakdown (GVBD). In most species, exposure to MIS causes a transient decrease in oocyte cAMP levels. Whether this reduction in oocyte cAMP concentration is sufficient to induce GVBD is unclear. The current study tested the hypothesis that activation of cAMP-independent signal transduction pathways by the naturally occurring MIS, 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S), is necessary for GVBD in Atlantic croaker (Micropogonias undulatus) oocytes. Results indicate that although 20beta-S treatment of oocyte membranes significantly reduced cAMP production, incubation of follicles with the cell-permeable cAMP-dependent protein kinase (Prka) inhibitors Rp-cAMP or KT5720 did not promote GVBD in the absence of 20beta-S. Additionally, treatment of follicles with the phosphodiesterase (Pde) inhibitors Cilostamide (Pde3) or Rolipram (Pde4) significantly reduced GVBD, but they were not able to completely block it. In contrast, pharmacologic inhibition of the cAMP-independent phosphatidylinositol 3-kinase (Pik3)/Akt signal transduction pathway using the Pik3 inhibitors Wortmannin or LY294002, or the Akt inhibitor ML-9, blocked 20beta-S-induced GVBD. Finally, mitogen-activated protein kinase (Mapk1/3) activity increased after treatment with 20beta-S; however, inhibition of Mapk1/3 activity using PD98059 or U0126 had no effect on GVBD. These results demonstrate that activation of cAMP-independent signaling pathways, especially the Pik3/Akt pathway, is necessary for 20beta-S-induced GVBD in Atlantic croaker oocytes.

Several signaling pathways are involved in the control of cattle oocyte maturation

The main limit of in vitro production of domestic mammal embryos comes from the low capacity of in vitro matured oocytes to develop after fertilization. As soon as they are separated from follicular environment, oocytes spontaneously resume meiosis without completion of their terminal differentiation. Roscovitine (ROS), an inhibitor of M-phase promoting factor (MPF) kinase activity reversibly blocks the meiotic resumption in vitro. However, in cattle maturing oocytes several cellular events such as protein synthesis and phosphorylation, chromatin condensation and nuclear envelope folding escape ROS inhibition suggesting the alternative pathways in oocyte maturation. We compared the level of synthesis and phosphorylation of several protein kinases during bovine cumulus oocyte complex (COC) maturation in vitro in the presence or not of epidermal growth factor (EGF) and ROS. We showed that during the EGF-stimulated maturation, ROS neither affected the decrease of EGF receptor (EGFR) nor did inhibit totally its phosphorylation in cumulus cells and also did not totally eliminate tyrosine phosphorylation in oocytes. However, ROS did inhibit the Phosphoinositide 3-kinase (PI3) activity when oocytes mature without EGF. Accumulation of Akt/PKB (protein kinase B), JNK1/2 (jun N-terminal kinases) and Aurora-A in oocytes during maturation was not affected by ROS. However, the phosphorylation of Akt but not JNKs was diminished in ROS-treated oocytes. Thus, PI3 kinase/Akt, JNK1/2 and Aurora-A are likely to be involved in the regulation of bovine oocyte maturation and some of these pathways seem to be independent to MPF activity and meiotic resumption. This complex regulation may explain the partial meiotic arrest of ROS-treated oocytes and the accelerated maturation observed after such treatment.

Signaling pathways in ascidian oocyte maturation: Effects of various inhibitors and activators on germinal vesicle breakdown

The Ascidiacea, the invertebrate chordates, includes three orders; the Stolidobranchia is the most complex. Until the present study, the onset of oocyte maturation (germinal vesicle breakdown) had been investigated in only a single pyurid (Halocynthia roretzi), in which germinal vesicle breakdown (GVBD) begins when the oocyte contacts seawater (SW); nothing was known about internal events. This study strongly suggests the importance of protein phosphorylation in this process. Herdmania pallida (Pyuridae) functions like H. roretzi; GVBD occurs in SW. Oocytes of Cnemidocarpa irene (Styelidae) do not spontaneously undergo GVBD in SW but must be activated. Herdmania oocytes are inhibited from GVBD by pH 4 SW and subsequently activated by mastoparan (G-protein activator), A23187 (Ca2+ ionophore) or dimethylbenzanthracene (tyrosine kinase activator). This requires maturation promoting factor (MPF) activity; cyclin-dependent kinase inhibitors roscovitine and olomoucine are inhibitory. It also entails dephosphorylation as demonstrated by the ability of the phosphatase inhibitor vitamin K3 to inhibit GVBD. GVBD is also inhibited by the tyrosine kinase inhibitors tyrphostin A23 and genistein, and LY-294002, a phosphatidylinositol-3-kinase inhibitor previously shown to inhibit starfish GVBD. LY-294002 inhibits strongly when activation is by mastoparan or ionophore but not when activated by dimethylbenzanthracene (DMBA). The DMBA is hypothesized to phosphorylate a phosphatase directly or indirectly causing secondary activation, bypassing inhibition.

PDK1 is required for the hormonal signaling pathway leading to meiotic resumption in starfish oocytes

Meiotic resumption is generally under the control of an extracellular maturation-inducing hormone. It is equivalent to the G2-M phase transition in somatic cell mitosis and is regulated by cyclin B-Cdc2 kinase. However, the complete signaling pathway from the hormone to cyclin B-Cdc2 is yet unclear in any organism. A model system to analyze meiotic resumption is the starfish oocyte, in which Akt/protein kinase B (PKB) plays a key mediator in hormonal signaling that leads to cyclin B-Cdc2 activation. Here we show in starfish oocytes that when PDK1 activity is inhibited by a neutralizing antibody, maturation-inducing hormone fails to induce cyclin B-Cdc2 activation at the meiotic G2-M phase transition, even though PDK2 activity becomes detectable. These observations assign a novel role to PDK1 for a hormonal signaling intermediate toward meiotic resumption. They further support that PDK2 is a molecule distinct from PDK1 and Akt, and that PDK2 activity is not sufficient for the full activation of Akt in the absence of PDK1 activity.

Signal transduction pathways that contribute to CDK1/cyclin B activation during the first mitotic division in sea urchin embryos

In sea urchins, fertilization triggers a rapid rise in protein synthesis necessary for activation of CDK1/cyclin B, the universal cell cycle regulator. It has been shown that FRAP/mTOR is required for eIF4E release from the translational repressor 4E-BP, a process that occurs upstream of de novo cyclin B synthesis. Here, we investigate whether PI 3-kinase acts independently or upstream from FRAP/mTOR in the signal transduction pathway that links fertilization to the activation of the CDK1/cyclin B complex in sea urchin egg. We found that wortmannin, a potent inhibitor of PI 3-kinase, partially inhibited the global increase in protein synthesis triggered by fertilization. Furthermore, wortmannin treatment induced partial inhibition of cyclin B translation triggered by fertilization, in correlation with an intermediate effect of the drug on 4E-BP degradation and on the dissociation of the 4E-BP/eIF4E complex induced by fertilization. Our results presented here suggest that PI 3-kinase activity is required for completion of mitotic divisions of the sea urchin embryo. Incubation of eggs with wortmannin or microinjection of wortmannin or LY 294002 affects drastically mitotic divisions induced by fertilization. In addition, we found that wortmannin treatment inhibits dephosphorylation of the tyrosine inhibitory site of CDK1. Taken together, these data suggest that PI 3-kinase acts upstream of at least two independent targets that function in the CDK1/cyclin B activation triggered by fertilization of sea urchin oocytes. We discuss the significance of these results concerning the cascade of reactions that impinge upon the activation of the CDK1/cyclin B complex that follows sea urchin oocyte fertilization.

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.

MAP kinases regulate unfertilized egg apoptosis and fertilization suppresses death via Ca2+signaling

The default fate for eggs from many species is death by apoptosis and thus, successful fertilization depends upon suppression of the maternal death program. Little is known about the molecular triggers which activate this process or how the fertilization signal suppresses the default maternal apoptotic pathway. The MAP kinase (MAPK) family member, ERK, plays a universal and critical role in several stages of oocyte meiotic maturation, and fertilization results in ERK inactivation. In somatic cells, ERK and other MAPK family members, p38 and JNK, provide opposing signals to regulate apoptosis, however, it is not known whether MAPKs play a regulatory role in egg apoptosis, nor whether suppression of apoptosis by fertilization is mediated by MAPK activity. Here we demonstrate that MAPKs are involved in starfish egg apoptosis and we investigate the relationship between the fertilization induced signaling pathway and MAPK activation. ERK is active in post-meiotic eggs just until apoptosis onset and then p38, JNK and a third kinase are activated, and remain active through execution. Sequential activation of ERK and p38 is necessary for apoptosis, and newly synthesized proteins are required both upstream of ERK and downstream of p38 for activation of the full apoptotic program. Fertilization causes a dramatic rise in intracellular Ca2+, and we report that Ca2+ provides a necessary and sufficient pro-survival signal. The Ca2+ pathway following fertilization of both young and aged eggs causes ERK to be rapidly inactivated, but fertilization cannot rescue aged eggs from death, indicating that ERK inactivation is not sufficient to suppress apoptosis.

Germinal vesicle breakdown is not fully dependent on MAPK activation in maturing oocytes of marine nemertean worms

Previously, it has been shown that oocytes of marine nemertean worms resume meiosis and undergo germinal vesicle breakdown (GVBD) following treatment with either natural seawater (NSW), or the neurohormone serotonin (5-hydroxytryptamine or 5-HT). In this investigation of the nemerteans Cerebratulus lacteus and Cerebratulus sp., immunoblots and kinase assays were used to compare the roles of two regulatory kinases: mitogen-activated protein kinase (MAPK) and Cdc2/cyclin B (referred to as maturation promoting factor or MPF). Based on such analyses, an ERK (extracellular signal regulated kinase) type of MAPK was found to be activated concurrently with Cdc2/cyclin B during NSW- and 5-HT-induced maturation. MAPK activation occurred prior to GVBD and seemed to be controlled primarily by phosphorylation rather than de novo protein synthesis. Inhibition of MAPK signaling by U0126 was capable of delaying but not permanently blocking Cdc2/cyclin B activation and GVBD in 5-HT treated oocytes and subsets of NSW-treated oocytes. Collectively such data indicated that GVBD is not fully dependent on MAPK activation, since Cdc2/cyclin B can apparently be activated by MAPK-independent mechanism(s) in maturing nemertean oocytes.

Induction of apoptosis in starfish eggs requires spontaneous inactivation of MAPK (extracellular signal-regulated kinase) followed by activation of p38MAPK

Mitogen-activated protein kinase (MAPK) (extracellular signal-regulated kinase) prevents DNA replication and parthenogenesis in maturing oocytes. After the meiotic cell cycle in starfish eggs, MAPK activity is maintained until fertilization. When eggs are fertilized, inactivation of MAPK occurs, allowing development to proceed. Without fertilization, highly synchronous apoptosis of starfish eggs starts 10 h after germinal vesicle breakdown, which varies according to season and individual animals. For induction of the apoptosis, MAPK should be activated for a definite period, called the MAPK-dependent period, during which eggs develop competence to die, although the exact duration of the period was unclear. In this study, we show that the duration of the MAPK-dependent period was approximately 8 h. Membrane blebbing occurred approximately 2 h after the MAPK-dependent period. Surprisingly, when MAPK was inhibited by U0126 after the MAPK-dependent period, activation of caspase-3 occurred earlier than in the control eggs. Thus, inactivation of MAPK is a prerequisite for apoptosis. Also, even in the absence of the inhibitor, MAPK was inactivated spontaneously when eggs began to bleb, indicating that inactivation of MAPK after the MAPK-dependent period acts upstream of caspase-3. Inactivation of MAPK also resulted in the activation of p38MAPK, which may contribute to apoptotic body formation.

Cell-cycle control during meiotic maturation

The meiotic cell cycle, which is comprised of two consecutive M-phases, is crucial for the production of haploid germ cells. Although both mitotic and meiotic M-phases share cyclin-B-Cdc2/CDK1 as a key controller, there are meiosis-specific modulations in the regulation of cyclin-B-Cdc2. Recent insights indicate that a common pattern in these modulations can be found by considering the particular activities of mitogen-activated protein kinase (MAPK) during meiosis. The G(2)-phase arrest of meiosis I is released via specific, MAPK-independent signalling that leads to cyclin-B-Cdc2 activation; thereafter, however, the meiotic process is under the control of interplay between MAPK and cyclin-B-Cdc2.

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.

Mos is not required for the initiation of meiotic maturation in Xenopus oocytes

In Xenopus oocytes, the c-mos proto-oncogene product has been proposed to act downstream of progesterone to control the entry into meiosis I, the transition from meiosis I to meiosis II, which is characterized by the absence of S phase, and the metaphase II arrest seen prior to fertilization. Here, we report that inhibition of Mos synthesis by morpholino antisense oligonucleotides does not prevent the progesterone-induced initiation of Xenopus oocyte meiotic maturation, as previously thought. Mos-depleted oocytes complete meiosis I but fail to arrest at metaphase II, entering a series of embryonic-like cell cycles accompanied by oscillations of Cdc2 activity and DNA replication. We propose that the unique and conserved role of Mos is to prevent mitotic cell cycles of the female gamete until the fertilization in Xenopus, starfish and mouse oocytes.

Nonequivalence of maternal centrosomes/centrioles in starfish oocytes: selective casting-off of reproductive centrioles into polar bodies

It is believed that in most animals only the paternal centrosome provides the division poles for mitosis in zygotes. This paternal inheritance of the centrosomes depends on the selective loss of the maternal centrosome. In order to understand the mechanism of centrosome inheritance, the behavior of all maternal centrosomes/centrioles was investigated throughout the meiotic and mitotic cycles by using starfish eggs that had polar body (PB) formation suppressed. In starfish oocytes, the centrioles do not duplicate during meiosis II. Hence, each centrosome of the meiosis II spindle has only one centriole, whereas in meiosis I, each has a pair of centrioles. When two pairs of meiosis I centrioles were retained in the cytoplasm of oocytes by complete suppression of PB extrusion, they separated into four single centrioles in meiosis II. However, after completion of the meiotic process, only two of the four single centrioles were found in addition to the pronucleus. When the two single centrioles of a meiosis II spindle were retained in the oocyte cytoplasm by suppressing the extrusion of the second PB, only one centriole was found with the pronucleus after the completion of the meiotic process. When these PB-suppressed eggs were artificially activated to drive the mitotic cycles, all the surviving single centrioles duplicated repeatedly to form pairs of centrioles, which could organize mitotic spindles. These results indicate that the maternal centrioles are not equivalent in their intrinsic stability and reproductive capacity. The centrosomes with the reproductive centrioles are selectively cast off into the PBs, resulting in the mature egg inheriting a nonreproductive centriole, which would degrade shortly after the completion of meiosis.

Role of meiosis activating sterols, MAS, in induced oocyte maturation

Meiosis of follicle enclosed oocytes is maintained in the prophase of the first meiotic division and oocytes do not spontaneously resume meiosis during oocyte growth and follicle development. Arrest of the meiotic process is most likely secured by the presence of follicular purines, e.g. hypoxanthine, which maintain high levels of cAMP in the oocyte and which also in vitro prevent oocytes from resuming meiosis. Only in response to the mid-cycle surge of gonadotropins will oocytes of preovulatory follicles overcome the meiosis arresting effect of hypoxanthine and resume meiosis proceeding to the metaphase of the second meiotic division. Morphologically, resumption of meiosis is observed by the disappearance of the oocyte's nuclear membrane (germinal vesicle), a process called germinal vesicle breakdown (GVB). The molecular mechanism down-stream to receptor activation by which the mid-cycle surge of gonadotropins induces oocytes to resume meiosis is, however, only partly understood. The oocyte itself lacks gonadotropin receptors and its action is mediated through the attached cumulus cells. In vitro it has been shown that FSH induces synthesis of a signal in the cumulus cells, which overcomes the meiosis arresting effect of hypoxanthine. We have shown that a group of sterols, meiosis activating sterols (MAS), induces oocyte maturation in vitro even in oocytes depleted of cumulus cells. MAS were identified as intermediates in the cholesterol biosynthesis between lanosterol and cholesterol. The two best characterized members of the MAS family are FF-MAS purified from human follicular fluid (4,4-dimethyl-5alpha-cholest-8,14,24-triene-3beta-ol) and T-MAS purified from bull testicular tissue (4,4-dimethyl-5alpha-cholest-8,24-diene-3beta-ol). The synthesis, quantification, localization and tissue-accumulation of MAS are reviewed. Several publications have documented the pharmacological effect of MAS in different species, including oocytes from mouse, rat and human. Conflicting results obtained by the use of sterol synthesis inhibitors, which prevent MAS-accumulation, are also discussed. Whether FSH actually uses MAS as a signal transduction molecule for inducing oocyte maturation and the mechanism by which MAS induce resumption of meiosis is currently unknown, but data to support that MAS is part of the FSH induced signal transduction pathway are presented.

Akt inhibits Myt1 in the signalling pathway that leads to meiotic G2/M-phase transition

In eukaryotes, entry into M-phase of the cell cycle is induced by activation of cyclin B-Cdc2 kinase. At G2-phase, the activity of its inactivator, a member of the Wee1 family of protein kinases, exceeds that of its activator, Cdc25C phosphatase. However, at M-phase entry the situation is reversed, such that the activity of Cdc25C exceeds that of the Wee1 family. The mechanism of this reversal is unclear. Here we show that in oocytes from the starfish Asterina pectinifera, the kinase Akt (or protein kinase B (PKB)) phosphorylates and downregulates Myt1, a member of the Wee1 family. This switches the balance of regulator activities and causes the initial activation of cyclin B-Cdc2 at the meiotic G2/M-phase transition. These findings identify Myt1 as a new target of Akt, and demonstrate that Akt functions as an M-phase initiator.

Phosphoinositide 3-kinase-gamma induces Xenopus oocyte maturation via lipid kinase activity

Type-I phosphoinositide 3-kinases (PI3Ks) were characterized as a group of intracellular signalling proteins expressing both protein and lipid kinase activities. Recent studies implicate PI3Ks as mediators of oocyte maturation, but the molecular mechanisms are poorly defined. Here we used the Xenopus oocyte expression system as a model to investigate a possible contribution of the gamma-isoform of PI3K (PI3Kgamma) in the different pathways leading to cell-cycle progression by monitoring the time course of germinal vesicle breakdown (GVBD). Expression of a constitutive active PI3Kgamma (PI3Kgamma-CAAX) induced GVBD and increased the levels of phosphorylated Akt/protein kinase B and mitogen-activated protein kinase (MAPK). Furthermore, PI3Kgamma-CAAX accelerated progesterone-induced GVBD, but had no effect on GVBD induced by insulin. The effects of PI3Kgamma-CAAX could be suppressed by pre-incubation of the oocytes with LY294002, PD98059 or roscovitine, inhibitors of PI3K, MEK (MAPK/extracellular-signal-regulated protein kinase kinase) and cdc2/cyclin B kinase, respectively. Mutants of PI3Kgamma-CAAX, in which either lipid kinase or both lipid and protein kinase activities were altered or eliminated, did not induce significant GVBD. Our data demonstrate that expression of PI3Kgamma in Xenopus oocytes accelerates their progesterone-induced maturation and that lipid kinase activity is required to induce this effect.

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.

Postmeiotic unfertilized starfish eggs die by apoptosis

Fertilization of starfish eggs during meiosis results in rapid progression to embryogenesis as soon as meiosis II is completed. Unfertilized eggs complete meiosis and arrest in postmeiotic interphase for an, until now, indeterminate time. If they remain unfertilized, the mature postmeiotic eggs ultimately die. The aim of this study is to characterize the mechanism of death in postmeiotic unfertilized starfish eggs. We report that, in two species of starfish, in the absence of fertilization, postmeiotic interphase arrest persists for 16-20 h, after which time the cells synchronously and rapidly die. Dying eggs extrude membrane blebs, undergo cytoplasmic contraction and darkening, and fragment into vesicles in a manner reminiscent of apoptotic cells. The DNA of dying eggs is condensed, fragmented, and labeled by the TUNEL assay. Taken together, these data suggest that the default fate of postmeiotic starfish eggs, like their mammalian counterparts, is death by apoptosis. We further report that the onset and execution of apoptosis in this system is dependent on ongoing protein synthesis and is inhibited by a rise in intracellular Ca(2+), an essential component of the fertilization signaling pathway. We propose starfish eggs as a useful model to study developmentally regulated apoptosis.

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+).

Phosphatidylinositol 3-kinase in cumulus cells and oocytes is responsible for activation of oocyte mitogen-activated protein kinase during meiotic progression beyond the meiosis I stage in pigs

The roles of phosphatidylinositol 3-kinase (PI 3-kinase) during meiotic progression beyond the meiosis I (MI) stage in porcine oocytes were investigated. PI 3-kinase exists in cumulus cells and oocytes, and the PI 3-kinase inhibitor, LY294002, suppressed the activation of mitogen-activated protein (MAP) kinase in denuded oocytes during the beginning of the treatment. However, in denuded oocytes cultured with LY294002, the MAP kinase activity steadily increased, and at 48 h of cultivation MAP kinase activity, p34(cdc2) kinase activity, and proportion of oocytes that had reached the meiosis II (MII) stage were at a similar level to those of oocytes cultured without LY294002. In contrast, LY294002 almost completely inhibited the activation of MAP kinase, p34(cdc2) kinase activity, and meiotic progression to the MII stage in oocytes surrounded with cumulus cells throughout the treatment. Treating cumulus oocyte complexes (COCs) with LY294002 produced a significant decrease in the phosphorylation of connexin-43, a gap junctional protein, in cumulus cells compared with that in COCs cultured without LY294002. These results indicate that PI 3-kinase activity in cumulus cells contributes to the activation of MAP kinase and p34(cdc2) kinase, and to meiotic progression beyond the MI stage. Moreover, gap junctional communications between cumulus cells and oocytes may be closed by phosphorylation of connexin-43 through PI 3-kinase activation in cumulus cells, leading to the activation of MAP kinase in porcine oocytes.

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.