Translational regulation of the gradual increase in histone H1 kinase activity in maturing mouse oocytes

Translational activation of maternally derived mRNAs in oocytes and early embryos and the role of embryonic poly(A) binding protein (EPAB)

Transcription ceases upon stimulation of oocyte maturation and gene expression during oocyte maturation, fertilization, and early cleavage relies on translational activation of maternally derived mRNAs. Two key mechanisms that mediate translation of mRNAs in oocytes have been described in detail: cytoplasmic polyadenylation-dependent and -independent. Both of these mechanisms utilize specific protein complexes that interact with cis-acting sequences located on 3'-untranslated region (3'-UTR), and both involve embryonic poly(A) binding protein (EPAB), the predominant poly(A) binding protein during early development. While mechanistic details of these pathways have primarily been elucidated using the Xenopus model, their roles are conserved in mammals and targeted disruption of key regulators in mouse results in female infertility. Here, we provide a detailed account of the molecular mechanisms involved in translational activation during oocyte and early embryo development, and the role of EPAB in this process.

Cyclin B2 is required for progression through meiosis in mouse oocytes

Cyclins associate with cyclin-dependent serine/threonine kinase 1 (CDK1) to generate the M phase-promoting factor (MPF) activity essential for progression through mitosis and meiosis. Although cyclin B1 (CCNB1) is required for embryo development, previous studies concluded that CCNB2 is dispensable for cell cycle progression. Given previous findings of high Ccnb2 mRNA translation rates in prophase-arrested oocytes, we re-evaluated the role of this cyclin during meiosis. Ccnb2-/- oocytes underwent delayed germinal vesicle breakdown and showed defects during the metaphase-to-anaphase transition. This defective maturation was associated with compromised Ccnb1 and Moloney sarcoma oncogene (Mos) mRNA translation, delayed spindle assembly and increased errors in chromosome segregation. Given these defects, a significant percentage of oocytes failed to complete meiosis I because the spindle assembly checkpoint remained active and anaphase-promoting complex/cyclosome function was inhibited. In vivo, CCNB2 depletion caused ovulation of immature oocytes, premature ovarian failure, and compromised female fecundity. These findings demonstrate that CCNB2 is required to assemble sufficient pre-MPF for timely meiosis re-entry and progression. Although endogenous cyclins cannot compensate, overexpression of CCNB1/2 rescues the meiotic phenotypes, indicating similar molecular properties but divergent modes of regulation of these cyclins.

Mysteries in embryonic development: How can errors arise so frequently at the beginning of mammalian life?

Chromosome segregation errors occur frequently during female meiosis but also in the first mitoses of mammalian preimplantation development. Such errors can lead to aneuploidy, spontaneous abortions, and birth defects. Some of the mechanisms underlying these errors in meiosis have been deciphered but which mechanisms could cause chromosome missegregation in the first embryonic cleavage divisions is mostly a “mystery”. In this article, we describe the starting conditions and challenges of these preimplantation divisions, which might impair faithful chromosome segregation. We also highlight the pending research to provide detailed insight into the mechanisms and regulation of preimplantation mitoses.

Starting a new life is a challenging business. This Essay explores the changes at the oocyte-to-embryo transition to highlight the circumstances under which the very first and decisive — but ‘mysteriously’ error-prone — mitotic divisions occur.

Increased Expression of Maturation Promoting Factor Components Speeds Up Meiosis in Oocytes from Aged Females

The rate of chromosome segregation errors that emerge during meiosis I in the mammalian female germ line are known to increase with maternal age; however, little is known about the underlying molecular mechanism. The objective of this study was to analyze meiotic progression of mouse oocytes in relation to maternal age. Using the mouse as a model system, we analyzed the timing of nuclear envelope breakdown and the morphology of the nuclear lamina of oocytes obtained from young (2 months old) and aged females (12 months old). Oocytes obtained from older females display a significantly faster progression through meiosis I compared to the ones obtained from younger females. Furthermore, in oocytes from aged females, lamin A/C structures exhibit rapid phosphorylation and dissociation. Additionally, we also found an increased abundance of MPF components and increased translation of factors controlling translational activity in the oocytes of aged females. In conclusion, the elevated MPF activity observed in aged female oocytes affects precocious meiotic processes that can multifactorially contribute to chromosomal errors in meiosis I.

Molecular Mechanisms of Prophase I Meiotic Arrest Maintenance and Meiotic Resumption in Mammalian Oocytes

Mechanisms of meiotic prophase I arrest maintenance (germinal vesicle [GV] stage) and meiotic resumption (germinal vesicle breakdown [GVBD] stage) in mammalian oocytes seem to be very complicated. These processes are regulated via multiple molecular cascades at transcriptional, translational, and posttranslational levels, and many of them are interrelated. There are many molecular cascades of meiosis maintaining and meiotic resumption in oocyte which are orchestrated by multiple molecules produced by pituitary gland and follicular cells. Furthermore, many of these molecular cascades are duplicated, thus ensuring the stability of the entire system. Understanding mechanisms of oocyte maturation is essential to assess the oocyte status, develop effective protocols of oocyte in vitro maturation, and design novel contraceptive drugs. Mechanisms of meiotic arrest maintenance at prophase I and meiotic resumption in mammalian oocytes are covered in the present article.

High-Sensitivity and High-Resolution In Situ Hybridization of Coding and Long Non-coding RNAs in Vertebrate Ovaries and Testes

Background

Subcellular localization of coding and non-coding RNAs has emerged as major regulatory mechanisms of gene expression in various cell types and many organisms. However, techniques that enable detection of the subcellular distribution of these RNAs with high sensitivity and high resolution remain limited, particularly in vertebrate adult tissues and organs. In this study, we examined the expression and localization of mRNAs encoding Pou5f1/Oct4, Mos, Cyclin B1 and Deleted in Azoospermia-like (Dazl) in zebrafish and mouse ovaries by combining tyramide signal amplification (TSA)-based in situ hybridization with paraffin sections which can preserve cell morphology of tissues and organs at subcellular levels. In addition, the distribution of a long non-coding RNA (lncRNA), lncRNA-HSVIII, in mouse testes was examined by the same method.

Results

The mRNAs encoding Mos, Cyclin B1 and Dazl were found to assemble into distinct granules that were distributed in different subcellular regions of zebrafish and mouse oocytes, suggesting conserved and specific regulations of these mRNAs. The lncRNA-HSVIII was first detected in the nucleus of spermatocytes at prophase I of the meiotic cell cycle and was then found in the cytoplasm of round spermatids, revealing expression patterns of lncRNA during germ cell development. Collectively, the in situ hybridization method demonstrated in this study achieved the detection and comparison of precise distribution patterns of coding and non-coding RNAs at subcellular levels in single cells of adult tissues and organs.

Conclusions

This high-sensitivity and high-resolution in situ hybridization is applicable to many vertebrate species and to various tissues and organs and will be useful for studies on the subcellular regulation of gene expression at the level of RNA localization.

Electronic supplementary material

The online version of this article (10.1186/s12575-018-0071-z) contains supplementary material, which is available to authorized users.

Activity of MPF and expression of its related genes in mouse MI oocytes exposed to cadmium

Research has revealed that cadmium can disrupt ovarian function; however, few reports have focused on MI oocytes meiotic progression, especially the activity of maturation promoting factor (MPF) and its related genes (Cdk1, Ccnb1, and Cdc25b) expression. In this study, GV oocytes cultured in vitro for 0, 6, and 9 hours with five groups (control and doses of 0.05, 0.5, 2.5, and 5 μM Cd). At the same dose of cadmium but different exposure time: compared with 0h, Periodic changes in MPF activity were changed and continuously increased over time. The mRNA and protein expression of each MPF-related gene in different cadmium dose groups were changed compared with that of 0h. At the same exposure time but different dose of cadmium: compared with control group, MPF activity, mRNA and protein expressions of each MPF-related gene in all the cadmium exposure groups were increased at 9h after exposure. Cadmium maintains the high MPF activity in mouse MI oocytes during its meiotic process and disturbs the periodic change of MPF activity; meanwhile, cadmium exposure promotes the syntheses of MPF-related gene, which may be one of the molecular mechanisms for the maintenance of high MPF activity, and ultimately prevents the meiotic progression in oocytes.

The Translation of Cyclin B1 and B2 is Differentially Regulated during Mouse Oocyte Reentry into the Meiotic Cell Cycle

Control of protein turnover is critical for meiotic progression. Using RiboTag immunoprecipitation, RNA binding protein immunoprecipitation, and luciferase reporter assay, we investigated how rates of mRNA translation, protein synthesis and degradation contribute to the steady state level of Cyclin B1 and B2 in mouse oocytes. Ribosome loading onto Ccnb1 and Mos mRNAs increases during cell cycle reentry, well after germinal vesicle breakdown (GVBD). This is followed by the translation of reporters containing 3′ untranslated region of Mos or Ccnb1 and the accumulation of Mos and Cyclin B1 proteins. Conversely, ribosome loading onto Ccnb2 mRNA and Cyclin B2 protein level undergo minimal changes during meiotic reentry. Degradation rates of Cyclin B1 or B2 protein at the GV stage are comparable. The translational activation of Mos and Ccnb1, but not Ccnb2, mRNAs is dependent on the RNA binding protein CPEB1. Inhibition of Cdk1 activity, but not Aurora A kinase activity, prevents the translation of Mos or Ccnb1 reporters, suggesting that MPF is required for their translation in mouse oocytes. Conversely, Ccnb2 translation is insensitive to Cdk1 inhibition. Thus, the poised state that allows rapid meiotic reentry in mouse GV oocytes may be determined by the differential translational control of two Cyclins.

Cytoplasmic polyadenylation in mammalian oocyte maturation

Oocyte developmental competence is the ability of the mature oocyte to be fertilized and subsequently drive early embryo development. Developmental competence is acquired by completion of oocyte maturation, a process that includes nuclear (meiotic) and cytoplasmic (molecular) changes. Given that maturing oocytes are transcriptionally quiescent (as are early embryos), they depend on post-transcriptional regulation of stored transcripts for protein synthesis, which is largely mediated by translational repression and deadenylation of transcripts within the cytoplasm, followed by recruitment of specific transcripts in a spatiotemporal manner for translation during oocyte maturation and early development. Motifs within the 3' untranslated region (UTR) of messenger RNA (mRNA) are thought to mediate repression and downstream activation by their association with binding partners that form dynamic protein complexes that elicit differing effects on translation depending on cell stage and interacting proteins. The cytoplasmic polyadenylation (CP) element, Pumilio binding element, and hexanucleotide polyadenylation signal are among the best understood motifs involved in CP, and translational regulation of stored transcripts as their binding partners have been relatively well-characterized. Knowledge of CP in mammalian oocytes is discussed as well as novel approaches that can be used to enhance our understanding of the functional and contributing features to transcript CP and translational regulation during mammalian oocyte maturation. WIREs RNA 2016, 7:71-89. doi: 10.1002/wrna.1316 For further resources related to this article, please visit the WIREs website.

α-endosulfine (ENSA) regulates exit from prophase I arrest in mouse oocytes

Mammalian oocytes in ovarian follicles are arrested in meiosis at prophase I. This arrest is maintained until ovulation, upon which the oocyte exits from this arrest, progresses through meiosis I and to metaphase of meiosis II. The progression from prophase I to metaphase II, known as meiotic maturation, is mediated by signals that coordinate these transitions in the life of the oocyte. ENSA (α-endosulfine) and ARPP19 (cAMP-regulated phosphoprotein-19) have emerged as regulators of M-phase, with function in inhibition of protein phosphatase 2A (PP2A) activity. Inhibition of PP2A maintains the phosphorylated state of CDK1 substrates, thus allowing progression into and/or maintenance of an M-phase state. We show here ENSA in mouse oocytes plays a key role in the progression from prophase I arrest into M-phase of meiosis I. The majority of ENSA-deficient oocytes fail to exit from prophase I arrest. This function of ENSA in oocytes is dependent on PP2A, and specifically on the regulatory subunit PPP2R2D (also known as B55δ). Treatment of ENSA-deficient oocytes with Okadaic acid to inhibit PP2A rescues the defect in meiotic progression, with Okadaic acid-treated, ENSA-deficient oocytes being able to exit from prophase I arrest. Similarly, oocytes deficient in both ENSA and PPP2R2D are able to exit from prophase I arrest to an extent similar to wild-type oocytes. These data are evidence of a role for ENSA in regulating meiotic maturation in mammalian oocytes, and also have potential relevance to human oocyte biology, as mouse and human have genes encoding both Arpp19 and Ensa.

Cyclin B1 mRNA translation is temporally controlled through formation and disassembly of RNA granules

Temporal control of messenger RNA (mRNA) translation is an important mechanism for regulating cellular, neuronal, and developmental processes. However, mechanisms that coordinate timing of translational activation remain largely unresolved. Full-grown oocytes arrest meiosis at prophase I and deposit dormant mRNAs. Of these, translational control of cyclin B1 mRNA in response to maturation-inducing hormone is important for normal progression of oocyte maturation, through which oocytes acquire fertility. In this study, we found that dormant cyclin B1 mRNA forms granules in the cytoplasm of zebrafish and mouse oocytes. Real-time imaging of translation revealed that the granules disassemble at the time of translational activation during maturation. Formation of cyclin B1 RNA granules requires binding of the mRNA to Pumilio1 protein and depends on actin filaments. Disruption of cyclin B1 RNA granules accelerated the timing of their translational activation after induction of maturation, whereas stabilization hindered translational activation. Thus, our results suggest that RNA granule formation is critical for the regulation of timing of translational activation.

Increased CDK1 activity determines the timing of kinetochore-microtubule attachments in meiosis I

The slow increase in CDK1 activity in meiosis I acts as a timing mechanism to allow stable kinetochore–microtubule attachments only after bipolar spindle formation, thus preventing attachment errors.

Chromosome segregation during cell division depends on stable attachment of kinetochores to spindle microtubules. Mitotic spindle formation and kinetochore–microtubule (K-MT) capture typically occur within minutes of nuclear envelope breakdown. In contrast, during meiosis I in mouse oocytes, formation of the acentrosomal bipolar spindle takes 3–4 h, and stabilization of K-MT attachments is delayed an additional 3–4 h. The mechanism responsible for this delay, which likely prevents stabilization of erroneous attachments during spindle formation, is unknown. Here we show that during meiosis I, attachments are regulated by CDK1 activity, which gradually increases through prometaphase and metaphase I. Partial reduction of CDK1 activity delayed formation of stable attachments, whereas a premature increase in CDK1 activity led to precocious formation of stable attachments and eventually lagging chromosomes at anaphase I. These results indicate that the slow increase in CDK1 activity in meiosis I acts as a timing mechanism to allow stable K-MT attachments only after bipolar spindle formation, thus preventing attachment errors.

Embryonic poly(A)-binding protein (EPAB) is required for oocyte maturation and female fertility in mice

Gene expression during oocyte maturation and early embryogenesis up to zygotic genome activation requires translational activation of maternally-derived mRNAs. EPAB [embryonic poly(A)-binding protein] is the predominant poly(A)-binding protein during this period in Xenopus, mouse and human. In Xenopus oocytes, ePAB stabilizes maternal mRNAs and promotes their translation. To assess the role of EPAB in mammalian reproduction, we generated Epab-knockout mice. Although Epab(-/-) males and Epab(+/-) of both sexes were fertile, Epab(-/-) female mice were infertile, and could not generate embryos or mature oocytes in vivo or in vitro. Epab(-/-) oocytes failed to achieve translational activation of maternally-stored mRNAs upon stimulation of oocyte maturation, including Ccnb1 (cyclin B1) and Dazl (deleted in azoospermia-like) mRNAs. Microinjection of Epab mRNA into Epab(-/-) germinal vesicle stage oocytes did not rescue maturation, suggesting that EPAB is also required for earlier stages of oogenesis. In addition, late antral follicles in the ovaries of Epab(-/-) mice exhibited impaired cumulus expansion, and a 8-fold decrease in ovulation, associated with a significant down-regulation of mRNAs encoding the EGF (epidermal growth factor)-like growth factors Areg (amphiregulin), Ereg (epiregulin) and Btc (betacellulin), and their downstream regulators, Ptgs2 (prostaglandin synthase 2), Has2 (hyaluronan synthase 2) and Tnfaip6 (tumour necrosis factor α-induced protein 6). The findings from the present study indicate that EPAB is necessary for oogenesis, folliculogenesis and female fertility in mice.

Cyclin B1 turnover and the mechanism causing insensitivity of fully grown mouse oocytes to cycloheximide inhibition of meiotic resumption

Cyclin B1 turnover and the insensitivity of fully-grown mouse oocytes to cycloheximide (CHX) inhibition of germinal vesicle breakdown (GVBD) were examined by assaying GVBD and cyclin B1 levels after treatment of oocytes with various combinations of eCG and CHX. Whereas over 95% of oocytes underwent GVBD after culture for 24 h with CHX alone, only 10% did so after culture with CHX + eCG (P < 0.05). In addition, preculture with eCG alone had no effect, but preculture with eCG + CHX prevented GVBD during a second culture with CHX alone. Therefore, we inferred that eCG delayed GVBD long enough for CHX inhibition of protein synthesis to allow cyclin B1 to decrease below a threshold where GVBD became dependent upon its de novo synthesis. However, western blot revealed no cyclin B1 synthesis, but cyclin B1 degradation, as long as GVs were maintained intact with eCG. Regarding the function of CHX in preculture without protein synthesis to block subsequent GVBD, whereas eCG delayed GVBD for only 3 h, CHX had an ongoing effect that further postponed GVBD, thus allowing cyclin B1 to decrease below the threshold. When oocytes precultured with eCG + CHX were further cultured without eCG and CHX, cyclin B1 first decreased but then, because of the ongoing effects of CHX, increased to a level sufficient to induce GVBD. The content of P34Cdc2 was not altered under any of the culture conditions (P > 0.05). We concluded that insensitivity of mouse germinal vesicle (GV) oocytes to CHX was due to the presence of sufficient cyclin B1, and that cyclin B1 level in such oocytes was maintained by an equilibrium between synthesis and degradation.

Cyclin B in mouse oocytes and embryos: importance for human reproduction and aneuploidy

Oocyte maturation and early embryo development require precise coordination between cell cycle progression and the developmental programme. Cyclin B plays a major role in this process: its accumulation and degradation is critical for driving the cell cycle through activation and inactivation of the major cell cycle kinase, CDK1. CDK1 activation is required for M-phase entry whereas its inactivation leads to exit from M-phase. The tempo of oocyte meiotic and embryonic mitotic divisions is set by the rate of cyclin B accumulation and the timing of its destruction. By controlling when cyclin B destruction is triggered and by co-ordinating this with the completion of chromosome alignment, the spindle assembly checkpoint (SAC) is a critical quality control system important for averting aneuploidy and for building in the flexibility required to better integrate cell cycle progression with development. In this review we focus on cyclin B metabolism in mouse oocytes and embryos and illustrate how the cell cycle-powered clock (in fact cyclin B-powered clock) controls oocyte maturation and early embryo development, thereby providing important insight into human reproduction and potential causes of Down syndrome.

Gas6 downregulation impaired cytoplasmic maturation and pronuclear formation independent to the MPF activity

Previously, we found that the growth arrest-specific gene 6 (Gas6) is more highly expressed in germinal vesicle (GV) oocytes than in metaphase II (MII) oocytes using annealing control primer (ACP)-PCR technology. The current study was undertaken to investigate the role of Gas6 in oocyte maturation and fertilization using RNA interference (RNAi). Interestingly, despite the specific and marked decrease in Gas6 mRNA and protein expression in GVs after Gas6 RNAi, nuclear maturation including spindle structures and chromosome segregation was not affected. The only discernible effect induced by Gas6 RNAi was a change in maturation promoting factor (MPF) activity. After parthenogenetic activation, Gas6 RNAi-treated oocytes at the MII stage had not developed further and arrested at MII (90.0%). After stimulation with Sr(2+), Gas6-silenced MII oocytes had markedly reduced Ca(2+) oscillation and exhibited no exocytosis of cortical granules. In these oocytes, sperm penetration occurred during fertilization but not pronucleus (PN) formation. By roscovitine and colcemid treatment, we found that the Gas6 knockdown affected cytoplasmic maturation directly, independent to the changed MPF activity. These results strongly suggest that 1) the Gas6 signaling itself is important to the cytoplasmic maturation, but not nuclear maturation, and 2) the decreased Gas6 expression and decreased MPF activity separately or mutually influence sperm head decondensation and PN formation.

c-erbB2 and c-myb induce mouse oocyte maturation involving activation of maturation promoting factor

Proto-oncogenes are involved in cell growth, proliferation, and differentiation. In the present study, we investigated the roles and mediating pathways of proto-oncogenes c-erbB(2) and c-myb in mouse oocyte maturation by RT-PCR, real-time quantitative PCR, western blot, and recombinant proto-oncogene protein microinjection. Results showed that both c-erbB(2) and c-myb antisense oligodeoxynucleotides (c-erbB(2) ASODN and c-myb ASODN) inhibited germinal vesicle breakdown and the first polar body extrusion in a dose-dependent manner. However, microinjection of recombinant c-erbB(2) or c-myb protein into germinal vesicle stage oocytes stimulated oocyte meiotic maturation. In addition, the expression of c-erbB(2) and c-myb mRNA was detected in oocytes; and c-erbB(2) ASODN and c-myb ASODN inhibited c-erbB(2) mRNA and c-myb mRNA expression, respectively. Maturation promoting factor (MPF) inhibitor roscovitine did not affect the expression of c-erbB(2) mRNA and c-myb mRNA, but blocked the effects of recombinant c-erbB(2) and c-myb protein-induced oocyte maturation. Further, cyclin B1 protein expression in oocytes was remarkably inhibited by c-erbB(2) ASODN, c-myb ASODN, and roscovitine. Nonsense tat ODN had no effect on the expression of c-erbB(2), c-myb, and cyclin B1. These results suggest that c-erbB(2) and c-myb may induce oocyte maturation through mediating a pathway involving the activation of MPF.

Protein kinases and protein phosphatases that regulate meiotic maturation in mouse oocytes

Oocytes arrest at prophase of meiosis I (MI) and in vivo do not resume meiosis until they receive ovulatory cues. Meiotic resumption entails two rounds of chromosome segregation without an intervening round of DNA replication and an arrest at metaphase of meiosis II (MII); fertilization triggers exit from MII and entry into interphase. During meiotic resumption, there is a burst of protein phosphorylation and dephosphorylation that dramatically changes during the course of oocyte meiotic maturation. Many of these phosphorylation and dephosphorylation events are key to regulating meiotic cell cycle arrest and/or progression, chromosome dynamics, and meiotic spindle assembly and disassembly. This review, which is subdivided into sections based upon meiotic cell cycle stages, focuses on the major protein kinases and phosphatases that have defined requirements during meiosis in mouse oocytes and, when possible, connects these regulatory pathways.

Dcp1-bodies in mouse oocytes

Processing bodies (P-bodies) are cytoplasmic granules involved in the storage and degradation of mRNAs. In somatic cells, their formation involves miRNA-mediated mRNA silencing. Many P-body protein components are also found in germ cell granules, such as in mammalian spermatocytes. In fully grown mammalian oocytes, where changes in gene expression depend entirely on translational control, RNA granules have not as yet been characterized. Here we show the presence of P-body-like foci in mouse oocytes, as revealed by the presence of Dcp1a and the colocalization of RNA-associated protein 55 (RAP55) and the DEAD box RNA helicase Rck/p54, two proteins associated with P-bodies and translational control. These P-body-like structures have been called Dcp1-bodies and in meiotically arrested primary oocytes, two types can be distinguished based on their size. They also have different protein partners and sensitivities to the depletion of endogenous siRNA/miRNA and translational inhibitors. However, both type progressively disappear during in vitro meiotic maturation and are virtually absent in metaphase II-arrested secondary oocytes. Moreover, this disassembly of hDcp1a-bodies is concomitant with the posttranslational modification of EGFP-hDcp1a.

Strain difference in the timing of meiosis resumption in mouse oocytes: involvement of a cytoplasmic factor(s) acting presumably upstream of the dephosphorylation of p34cdc2kinase

Oocytes from eight inbred strains of mice were screened for the timing of germinal vesicle breakdown (GVB)in vitro. This characteristic varied between strains, reaching most extreme values in oocytes from AKR and BALB/c mice (3.1 and 1.6h after release from dibutyryl cAMP block, respectively;p<0.0001). The difference between AKR and BALB/c mice was confirmed in experiments in which GVB was inducedin vivoby stimulation with exogenous gonadotrophins. Analysis of the rate of GVB in hybrids obtained after fusion of nuclear and cytoplamic fragments of oocytes from both strains suggests that the factor responsible for the difference between AKR and BALB/c mice is located in the cytoplasm of the proghase oocytes. Finally, in oocytes from both strains stimulated to resume meiotic maturation with okadaic acid, an inhibitor of protein phosphatases types 1 and 2A the rate of GVB was the same (2.2h and 2.3h for AKR and BALB/c, respectively;p= 0.48). This suggests that the difference between strains is not related to the amount or quality of the pre-MPF (Maturation Promoting Factor) stored in the prophase oocyte, but to the factor(s) acting upstream of the dephosphorylation ofp34cdc2. kinase in the pathway leading to pre-MPF activation.

Completion of Meiosis I of preovulatory oocytes and facilitation of preimplantation embryo development by glial cell line-derived neurotrophic factor

Optimal maturation of oocytes and successful development of preimplantation embryos is essential for reproduction. We performed DNA microarray analyses of ovarian transcripts and identified glial cell line-derived neurotrophic factor (GDNF) secreted by cumulus, granulosa, and theca cells as an ovarian factor stimulated by the preovulatory LH/hCG surge. Treatment of cumulus-oocyte complexes with GDNF enhanced first polar body extrusion with increase in cyclin B1 synthesis and the GDNF actions are likely mediated by its receptor GDNF family receptor-alpha1 (GFRA1) and a co-receptor ret proto-oncogene (Ret), both expressed in oocytes. However, treatment with GDNF did not affect germinal vesicle breakdown and cytoplasmic maturation of oocytes. During the preimplantation stages, GDNF was expressed in pregnant oviducts and uteri, whereas GFRA1 and Ret were expressed in embryos throughout early development with an increase after the early blastocyst stage. In blastocysts, both GDNF and GFRA1 were exclusively localized in trophectoderm cells, whereas Ret was detected in both cell lineages. Treatment with GDNF promoted the development of two-cell-stage embryos into blastocysts showing increased cell proliferation and decreased apoptosis mainly in trophectoderm cells. Our findings suggest potential paracrine roles of GDNF in the promotion of completion of meiosis I and the development of early embryos.

Activation of maturation promoting factor in Bufo arenarum oocytes: injection of mature cytoplasm and germinal vesicle contents

Although progesterone is the established maturation inducer in amphibians, Bufo arenarum oocytes obtained during the reproductive period (spring-summer) resume meiosis with no need of an exogenous hormonal stimulus if deprived of their enveloping follicle cells, a phenomenon called spontaneous maturation. In this species it is possible to obtain oocytes competent and incompetent to undergo spontaneous maturation according to the seasonal period in which animals are captured. Reinitiation of meiosis is regulated by maturation promoting factor (MPF), a complex of the cyclin-dependent kinase p34cdc2 and cyclin B. Although the function and molecule of MPF are common among species, the formation and activation mechanisms of MPF differ according to species. This study was undertaken to evaluate the presence of pre-MPF in Bufo arenarum oocytes incompetent to mature spontaneously and the effect of the injection of mature cytoplasm or germinal vesicle contents on the resumption of meiosis. The results of our treatment of Bufo arenarum immature oocytes incompetent to mature spontaneously with sodium metavanadate (NaVO3) and dexamethasone (DEX) indicates that these oocytes have a pre-MPF, which activates and induces germinal vesicle breakdown (GVBD) by dephosphorylation on Thr-14/Tyr-15 by cdc25 phosphatase and without cyclin B synthesis. The injection of cytoplasm containing active MPF is sufficient to activate an amplification loop that requires the activation of cdc25 and protein kinase C, the decrease in cAMP levels, and is independent of protein synthesis. However, the injection of germinal vesicle content also induces GVBD in the immature receptor oocyte, a process dependent on protein synthesis but not on cdc25 phosphatase or PKC activity.

Germinal vesicle material drives meiotic cell cycle of mouse oocyte through the 3′UTR-dependent control of cyclin B1 synthesis

We compared the profile of histone H1 kinase activity, reflecting Maturation Promoting Factor (MPF) activity in oocytes bisected at the germinal vesicle (GV) stage and allowed to mature as separate oocyte halves in vitro. Whereas the oocyte halves containing the nucleus exhibited the same profile of increased kinase activity as that typical for intact oocytes, the anuclear halves revealed strong inhibition of the increase in this activity soon after germinal vesicle breakdown (GVBD). In contrast, the profile of MAP kinase activity did not differ significantly between anuclear and nucleus-containing oocyte halves throughout maturation. Of the two MPF components, CDK1 and cyclin B1, the amount of the latter was significantly reduced in anuclear halves, a reduction due to low-level synthesis and not to enhanced degradation. Expression of three reporter luciferase RNAs constructed, respectively, to contain cyclin B1-specific 3'UTR, the globin-specific 3'UTR, or no 3'UTR sequence was enhanced in nuclear halves, with significantly greater enhancement for the construct containing cyclin B1-specific 3'UTR as compared to the two other RNAs. We conclude that the profile of activity of MPF during mouse oocyte maturation is controlled by an unknown GV-associated factor(s) acting via 3'UTR-dependent control of cyclin B1 synthesis. These results require the revision of the hitherto prevailing view that the control of MPF activity during mouse oocyte maturation is independent of GV-derived material.

PKB/AKT is involved in resumption of meiosis in mouse oocytes

BACKGROUND INFORMATION

In fully grown mouse oocytes, a decrease in cAMP concentration precedes and is linked to CDK1 (cyclin-dependent kinase 1) activation. The molecular mechanism for this coupling, however, is not defined. PKB (protein kinase B, also called AKT) is implicated in CDK1 activation in lower species. During resumption of meiosis in starfish oocytes, MYT1, a negative regulator of CDK1, is phosphorylated by PKB in an inhibitory manner. It can imply that PKB is also involved in CDK1 activation in mammalian oocytes.

RESULTS

We monitored activation of PKB and CDK1 during maturation of mouse oocytes. PKB phosphorylation and activation preceded GVBD (germinal vesicle breakdown) in oocytes maturing either in vitro or in vivo. Activation was transient and PKB activity was markedly reduced when virtually all of the oocytes had undergone GVBD. PKB activation was independent of CDK1 activity, because although butyrolactone I prevented CDK1 activation and GVBD, PKB was nevertheless transiently phosphorylated and activated. LY-294002, an inhibitor of phosphoinositide 3-kinase-PKB signalling, suppressed activation of PKB and CDK1 as well as resumption of meiosis. OA (okadaic acid)-sensitive phosphatases are involved in PKB-activity regulation, because OA induced PKB hyperphosphorylation. During resumption of meiosis, PKB phosphorylated on Ser(473) is associated with nuclear membrane and centrosome, whereas PKB phosphorylated on Thr(308) is localized on centrosome only.

CONCLUSIONS

The results of the present paper indicate that PKB is involved in CDK1 activation and resumption of meiosis in mouse oocytes. The presence of phosphorylated PKB on centrosome at the time of GVBD suggests its important role for an initial CDK1 activation.

Cytoskeleton and cell cycle control during meiotic maturation of the mouse oocyte: integrating time and space

During meiotic maturation of mammalian oocytes, two successive divisions occur without an intermediate phase of DNA replication, so that haploid gametes are produced. Moreover, these two divisions are asymmetric, to ensure that most of the maternal stores are retained within the oocyte. This leads to the formation of daughter cells with different sizes: the large oocyte and the small polar bodies. All these events are dependent upon the dynamic changes in the organization of the oocyte cytoskeleton (microtubules and microfilaments) and are highly regulated in time and space. We review here the current knowledge of the interplay between the cytoskeleton and the cell cycle machinery in mouse oocytes, with an emphasis on the two major activities that control meiotic maturation in vertebrates, MPF (Maturation promoting factor) and CSF (Cytostatic factor).

The dynamics of cyclin B1 distribution during meiosis I in mouse oocytes

Cdk1-cyclin B1 kinase activity drives oocytes through meiotic maturation. It is regulated by the phosphorylation status of cdk1 and by its spatial organisation. Here we used a cyclin B1-green fluorescent protein (GFP) fusion protein to examine the dynamics of cdk1-cyclin B1 distribution during meiosis I (MI) in living mouse oocytes. Microinjection of cyclin B1-GFP accelerated germinal vesicle breakdown (GVBD) and, as previously described, overrides cAMP-mediated meiotic arrest. GVBD was pre-empted by a translocation of cyclin B1-GFP from the cytoplasm to the germinal vesicle (GV). After nuclear accumulation, cyclin B1-GFP localised to the chromatin. The localisation of cyclin B1-GFP is governed by nuclear import and export. In GV intact oocytes, cyclin export was demonstrated by showing that cyclin B1-GFP injected into the GV is exported to the cytoplasm while a similar size dextran is retained. Import was revealed by the finding that cyclin B1-GFP accumulated in the GV when export was inhibited using leptomycin B. These studies show that GVBD in mouse oocytes is sensitive to cyclin B1 abundance and that the changes in distribution of cyclin B1 contribute to progression through MI.

Endogenous regulators of protein phosphatase-1 during mouse oocyte development and meiosis

Reversible phosphorylation, involving protein kinases and phosphatases (PP), is important in regulating oocyte meiosis. Okadaic acid (OA) inhibition of PP1 and/or PP2A stimulates oocyte germinal vesicle breakdown (GVB). In oocytes, PP1 is localized in the cytoplasm and nucleus, yet endogenous regulation of oocyte PP1 has not been investigated. The objectives of the study were to identify intra-oocyte mechanisms regulating PP1 during acquisition of OA-sensitive meiotic competence and meiotic resumption. Immunohistochemical studies revealed that GVB-incompetent oocytes contained equivalent cytoplasmic and nuclear PP1. Upon development of OA-sensitive meiotic competence, PP1 displayed differential intracellular localization with significantly greater nuclear staining with distinct nucleolar rimming compared with cytoplasmic staining. Germinal vesicle-intact oocytes contained neither nuclear inhibitor of PP1, nor PP1 cytoplasmic inhibitor-1 transcripts or proteins. Reverse transcription-PCR with PP1 cytoplasmic inhibitor-2 (I2) primers and oocyte RNA amplified a predicted 330-bp product with the identical sequence to mouse liver I2. Oocytes contained a heat-stable PP1 inhibitor with biochemical properties of I2. Phosphorylation of PP1 at Thr320 by cyclin dependent kinase-1 (CDK1) causes PP1 inactivation. Germinal vesicle-intact oocytes did not contain phospho-Thr320-PP1. Upon GVB, PP1 became phosphorylated at Thr320 and this phosphorylation did not occur if GVB was blocked with the CDK1 inhibitor, roscovitine (ROSC). Inhibition of oocyte GVB with ROSC was reversible and coincided with PP1 phosphorylation at Thr320. Increased oocyte staining of nuclear PP1 compared with cytoplasmic staining at a chronological stage when oocytes gain meiotic competence, and phosphorylation and inhibition of PP1 by CDK1 at or around GVB appear to be important mechanisms in regulating oocyte PP1 activity and meiosis. In addition, these studies provide further support for PP1 being the OA-sensitive PP important in the regulation of the acquisition of meiotic competence, nuclear events during meiotic arrest, and GVB.

Analysis of the roles of cyclin B1 and cyclin B2 in porcine oocyte maturation by inhibiting synthesis with antisense RNA injection

The function of cyclin B1 (CB1) and cyclin B2 (CB2) during porcine oocyte maturation was investigated by injecting oocytes with their antisense RNAs (asRNAs). At first, protein levels of both cyclin Bs were examined by immunoblotting, revealing that immature oocytes had only CB2, at a level comparable to 1/20 to 1/40 of that detected in first metaphase oocytes. Both cyclin B syntheses were started around germinal vesicle breakdown (GVBD); CB1 and CB2 peaked at the second metaphase and first metaphase, respectively. We obtained a porcine CB2 cDNA fragment, which was 88% homologous with human CB2, by reverse-transcriptase polymerase chain reaction (RT-PCR) using total RNAs of immature porcine oocytes and a primer set of human CB2. Specific asRNAs of CB1 and CB2 were prepared in vitro. Then one, the other, or both were injected into the cytoplasm of immature oocytes. CB1 asRNA inhibited CB1 synthesis specifically; the injected oocytes underwent first meiosis normally but could not arrest at the second meiotic metaphase. CB2 asRNA inhibited CB2 synthesis specifically, but had almost no effect on the maturation of injected oocytes. When both CB1 and CB2 asRNAs were injected, synthesis of both cyclin Bs was inhibited, and GVBD was significantly suppressed but occurred slowly. These results suggest that CB1 is the principal molecule for regulation in mammalian oocyte maturation, whereas CB2 has only an accessory role. They also show that in porcine oocytes, cyclin B synthesis is not necessary for GVBD induction itself, but synthesis of at least one cyclin B, CB1 or CB2, is necessary for GVBD induction in a normal time course.

Temporal distribution of CDK4, CDK6, D-type cyclins, and p27 in developing mouse oocytes

Various molecular interactions not operating in other cell types are most likely required for mammalian oocytes to develop into fully competent eggs. This study seeks to initiate analyses of the potential oocyte-specific functions of regulators of G1/S progression-CDK4, CDK6, D-type cyclins, and p27-by first determining their expression patterns in growing and maturing mouse oocytes and in mouse embryos early after fertilization. Western blot and immunofluorescence analyses on isolated oocytes were employed to evaluate both their levels and their localization. The data show that 1). mouse oocytes contain significant amounts of all studied regulators; 2). their amounts and localization undergo dramatic changes as the oocytes grow, meiotically mature, and transit into embryogenesis; and 3). some regulators (CDK4, CDK6, cyclin D2, and p27) appear in unusual, most likely posttranslationally modified, forms. These data distinguish G1/S regulators as the potential players in molecular processes that are important for oocytes to function normally.

G2/M transition of pig oocytes: How do oocytes initiate maturation?

In the ovary, mammalian oocytes resume meiosis and mature to the second metaphase when they are stimulated with gonadotrophins. Similarly, oocytes can mature in vitro when they are liberated from ovarian follicles and cultured under appropriate conditions. Early in the process of maturation, oocytes undergo dramatic but well-ordered changes at the G2/M transition in the cell cycle including: (i) chromosome condensation; (ii) nucleolus disassembly; (iii) germinal vesicle breakdown (GVBD); and (iv) spindle formation in the first metaphase (MI-spindle). These events have been thought to be induced by MPF (maturation-promoting factor or M-phase promoting factor), now known as Cdc2 kinase or Cdk1 kinase, which consists of a catalytic subunit, Cdc2, and a cyclin B regulatory subunit. In fact, nuclear lamins are phosphorylated by Cdc2 kinase, and nuclear membrane breakdown occurs concomitantly with the activation of Cdc2 kinase in the M-phase of both somatic cells and oocytes. Based on the classical and recent studies of the pig oocyte, however, the chromosomes start to condense and the nucleolus disassembles before full activation of Cdc2 kinase, and the MI-spindle is formed after activation of both Cdc2 kinase and MAP kinase; another kinase known to become activated during oocyte maturation. These findings suggest that chromosome condensation and nucleolus disassembly in oocytes are induced by either some kinase(s) other than Cdc2 kinase and MAP kinase or some phosphatase(s). The accumulation of new results regarding the molecular nature of oocyte maturation is important for improving the reproductive technologies in domestic animals as well as in humans. (Reprod Med Biol 2003; 2: 91-99).

Translational and post-translational modifications in meiosis of the mammalian oocyte

The fully-grown oocyte is transcriptionally inactive. Therefore, translational and post-translational modifications furnish the control mechanism of key components governing meiosis. Regulation by protein synthesis provides an irreversible unidirectional mechanism for an extended period that can be restricted by a complementary degradation of the same protein. Both processes utilize tight measures to ensure precise expression at the right time in the right place. Rapid modifications such as phosphorylation and dephosphorylation supply reversible means to regulate protein action. Information regarding these extremely exciting issues is being accumulated recently in an exponential rate. However, the vast majority of these data is generated from studies conducted on Xenopus oocytes. We fully agree with Andrew Murray's statement that "The modern trend of promoting research on a small number of 'model' organisms will eventually deprive us of the opportunity to study interesting biology" [Cell 92 (1992) 157]. Thus, despite of the enormous technical difficulties resulting from the limited availability of biological material we extended our interest to mammalian model systems. Our review will attend to certain examples of such modifications in the regulatory pathway of meiosis in mammalian oocytes.

Regulation of spindle formation by active mitogen-activated protein kinase and protein phosphatase 2A during mouse oocyte meiosis

Mitogen-activated protein kinase (MAPK) and protein phosphatase 2A (PP2A) regulate oocyte meiosis, yet little is known regarding their mechanisms of action. This study addressed the functional importance of active MAPK and PP2A in regulating oocyte meiosis. Experiments were conducted to identify MAPK activation, PP2A activity, intracellular enzyme trafficking, and ultrastructural associations during meiosis. Questions of requisite kinase and/or phosphatase activity and chromatin condensation, microtubule polymerization, and spindle formation were addressed. At the protein level, MAPK and PP2A were present in constant amounts throughout the first meiotic division. Both MAPK and PP2A were activated following germinal vesicle breakdown (GVBD) in conjunction with metaphase I development. Immunocytochemical studies confirmed the absence of active MAPK in germinal vesicle-intact (GVI) and GVBD oocytes. At metaphase I and during the metaphase I/metaphase II transition, activated MAPK colocalized with microtubules, poles, and plates of meiotic spindles. Protein phosphatase 2A was dispersed evenly throughout the GVI oocyte cytoplasm. Throughout the metaphase I/metaphase II transition, PP2A colocalized with microtubules of meiotic spindles. Both active MAPK and PP2A associated with in vitro-polymerized microtubules, suggesting that active MAPK and PP2A locally regulate spindle formation. Inhibition of MAPK activation resulted in compromised microtubule polymerization, no spindle formation, and loosely condensed chromosomes. Treatment with okadaic acid (OA) or calyculin-A (CL-A), which inhibits oocyte cytoplasmic PP2A, caused an absence of microtubule polymerization and spindles, even though MAPK activity was increased under these treatment conditions. Thus, active MAPK is required, but is not sufficient, for normal meiotic spindle formation and chromosome condensation. In addition, the oocyte OA/CL-A-sensitive PP, presumably PP2A, is essential for microtubule polymerization and meiotic spindle formation.

Germ cell differentiation and synaptonemal complex formation are disrupted in CPEB knockout mice

CPEB is a sequence-specific RNA binding protein that regulates translation during vertebrate oocyte maturation. Adult female CPEB knockout mice contained vestigial ovaries that were devoid of oocytes; ovaries from mid-gestation embryos contained oocytes that were arrested at the pachytene stage. Male CPEB null mice also contained germ cells arrested at pachytene. The germ cells from the knockout mice harbored fragmented chromatin, suggesting a possible defect in homologous chromosome adhesion or synapsis. Two CPE-containing synaptonemal complex protein mRNAs, which interact with CPEB in vitro and in vivo, contained shortened poly(A) tails and mostly failed to sediment with polysomes in the null mice. Synaptonemal complexes were not detected in these animals. CPEB therefore controls germ cell differentiation by regulating the formation of the synaptonemal complex.

CPEB phosphorylation and cytoplasmic polyadenylation are catalyzed by the kinase IAK1/Eg2 in maturing mouse oocytes

In both vertebrates and invertebrates, the expression of several maternal mRNAs is regulated by cytoplasmic polyadenylation. In Xenopus oocytes, where most of the biochemical details of this process have been examined, polyadenylation is controlled by CPEB, a sequence-specific RNA binding protein. The activity of CPEB, which is to recruit cleavage and polyadenylation specificity factor (CPSF) and poly(A) polymerase (PAP) into an active cytoplasmic polyadenylation complex, is controlled by Eg2-catalyzed phosphorylation. Soon after CPEB phosphorylation and resulting polyadenylation take place, the interaction between maskin, a CPEB-associated factor, and eIF4E, the cap-binding protein, is destroyed, which results in the recruitment of mRNA into polysomes. Polyadenylation also occurs in maturing mouse oocytes, although the biochemical events that govern the reaction in these cells are not known. In this study, we have examined the phosphorylation of CPEB and have assessed the necessity of this protein for polyadenylation in maturing mouse oocytes. Immunohistochemistry has revealed that all the factors that control polyadenylation and translation in Xenopus oocytes (CPEB, CPSF, PAP, maskin, and IAK1, the murine homologue of Eg2) are also present in the cytoplasm of mouse oocytes. After the induction of maturation, a kinase is activated that phosphorylates CPEB on a critical regulatory residue, an event that is essential for CPEB activity. A peptide that competitively inhibits the activity of IAK1/Eg2 blocks the progression of meiosis in injected oocytes. Finally, a CPEB protein that acts as a dominant negative mutation because it cannot be phosphorylated by IAK1/Eg2, prevents cytoplasmic polyadenylation. These data indicate that cytoplasmic polyadenylation in mouse oocytes is mediated by IAK1/Eg2-catalyzed phosphorylation of CPEB.

Biochemical identification of Xenopus Pumilio as a sequence-specific cyclin B1 mRNA-binding protein that physically interacts with a Nanos homolog, Xcat-2, and a cytoplasmic polyadenylation element-binding protein

Translational activation of dormant cyclin B1 mRNA stored in oocytes is a prerequisite for the initiation or promotion of oocyte maturation in many vertebrates. Using a monoclonal antibody against the domain highly homologous to that of Drosophila Pumilio, we have shown for the first time in any vertebrate that a homolog of Pumilio is expressed in Xenopus oocytes. This 137-kDa protein binds to the region including the sequence UGUA at nucleotides 1335-1338 in the 3'-untranslated region of cyclin B1 mRNA, which is close to but does not overlap the cytoplasmic polyadenylation elements (CPEs). Physical in vitro association of Xenopus Pumilio with a Xenopus homolog of Nanos (Xcat-2) was demonstrated by a protein pull-down assay. The results of immunoprecipitation experiments showed in vivo interaction between Xenopus Pumilio and CPE-binding protein (CPEB), a key regulator of translational repression and activation of mRNAs stored in oocytes. This evidence provides a new insight into the mechanism of translational regulation through the 3'-end of mRNA during oocyte maturation. These results also suggest the generality of the function of Pumilio as a translational regulator of dormant mRNAs in both invertebrates and vertebrates.

Meiotic maturation of the mouse oocyte requires an equilibrium between cyclin B synthesis and degradation

Among the proteins whose synthesis and/or degradation is necessary for a proper progression through meiotic maturation, cyclin B appears to be one of the most important. Here, we attempted to modulate the level of cyclin B1 and B2 synthesis during meiotic maturation of the mouse oocyte. We used cyclin B1 or B2 mRNAs with poly(A) tails of different sizes and cyclin B1 or B2 antisense RNAs. Oocytes microinjected with cyclin B1 mRNA showed two phenotypes: most were blocked in MI, while the others extruded the first polar body in advance when compared to controls. Moreover, these effects were correlated with the length of the poly(A) tail. Thus it seems that the rate of cyclin B1 translation controls the timing of the first meiotic M phase and the transition to anaphase I. Moreover, overexpression of cyclin B1 or B2 was able to bypass the dbcAMP-induced germinal vesicle block, but only the cyclin B1 mRNA-microinjected oocytes did not extrude their first polar body. Oocytes injected with the cyclin B1 antisense progressed through the first meiotic M phase but extruded the first polar body in advance and were unable to enter metaphase II. This suggested that inhibition of cyclin B1 synthesis only took place at the end of the first meiotic M phase, most likely because the cyclin B1 mRNA was protected. The injection of cyclin B2 antisense RNA had no effect. The life observation of the synthesis and degradation of a cyclin B1-GFP chimera during meiotic maturation of the mouse oocyte demonstrated that degradation can only occur during a given period of time once it has started. Taken together, our data demonstrate that the rates of cyclin B synthesis and degradation determine the timing of the major events taking place during meiotic maturation of the mouse oocyte.

Acquisition of meiotic competence in mouse oocytes: absolute amounts of p34(cdc2), cyclin B1, cdc25C, and wee1 in meiotically incompetent and competent oocytes

M-Phase promoting factor (MPF) is a complex of p34(cdc2) and cyclin B. Results of previous studies in which relative mass amounts of these cell cycle regulators were determined suggested that the accumulation of p34(cdc2), rather than cyclin B, could be a limiting factor in the acquisition of meiotic competence in mouse oocytes. Nevertheless, in the absence of measurements of the absolute amount of these components of MPF, it is possible that the molar amount of p34(cdc2) is in excess to that of cyclin B, i.e., the accumulation of p34(cdc2) is not a limiting factor. We report measurements of the absolute mass of p34(cdc2) and cyclin B1, as well as the two proximal regulators of MPF, namely cdc25C and wee1, in meiotically incompetent and competent mouse oocytes. We find that the numbers of molecules of p34(cdc2), cyclin B1, cdc25C, and wee1 in meiotically incompetent oocytes are 1.4 x 10(6), 11.3 x 10(6), 24.6 x 10(6), 15. 6 x 10(6), respectively, and in meiotically competent oocytes the numbers are 14.3 x 10(6), 95.5 x 10(6), 80.0 x 10(6), 40.1 x 10(6), respectively. Thus, the concentration of cyclin B1 is always in excess to that of p34(cdc2), and this is consistent with the hypothesis that the accumulation of p34(cdc2) plays a role in the acquisition of meiotic competence. Last, the concentration of cdc25C is greater than that of wee1 and the concentration of each is greater than that of p34(cdc2) in both meiotically incompetent and competent oocytes.

A role for the MEK-MAPK pathway in okadaic acid-induced meiotic resumption of incompetent growing mouse oocytes

Fully grown competent mouse oocytes spontaneously resume meiosis in vitro when released from their follicular environment, in contrast to growing incompetent oocytes, which remain blocked in prophase I. The cell cycle regulators, maturation promoting factor (MPF; [p34(cdc2)/cyclin B kinase]) and mitogen-activated protein (MAP) kinases (p42(MAPK) and p44(MAPK)), are implicated in meiotic competence acquisition. Incompetent oocytes contain levels of p42(MAPK), p44(MAPK), and cyclin B proteins that are comparable to those in competent oocytes, but their level of p34(cdc2) is markedly lower. Okadaic acid (OA), an inhibitor of phosphatases 1 and 2A, induces meiotic resumption of incompetent oocytes. The kinetics and the percentage of germinal vesicle breakdown depends on whether or not oocytes have been cultured before OA treatment. We show that the fast kinetics and the high percentage of germinal vesicle breakdown induced by OA following 2 days in culture is neither the result of an accumulation of p34(cdc2) protein, nor to the activation of MPF in incompetent oocytes, but rather by the premature activation of MAP kinases. Indeed, a specific inhibitor of MAPK kinase (MEK) activity, PD98059, inhibits activation of MAP kinases and meiotic resumption. Altogether, these results indicate that the MEK-MAPK pathway is implicated in OA-induced meiotic resumption of incompetent mouse oocytes, and that the MEK-MAPK pathway can induce meiotic resumption in the absence of MPF activation.

Molecular mechanisms of the initiation of oocyte maturation: general and species-specific aspects

Stimulated by maturation-inducing hormone secreted from follicle cells surrounding the oocytes, fully-grown oocytes mature and become fertilisable. During maturation, immature oocytes resume meiosis arrested at the first prophase and proceed to the first or second metaphase at which they are naturally inseminated. Paying special attention to general and species-specific aspects, we summarise the mechanisms regulating the initial phase of oocyte maturation, from the reception of hormonal signals on the oocyte surface to activation of the maturation-promoting factor in the cytoplasm, in amphibians, fishes, mammals and marine invertebrates.

The control of cyclin B1 mRNA translation during mouse oocyte maturation

In maturing mouse oocytes, protein synthesis is required for meiotic maturation subsequent to germinal vesicle breakdown (GVBD). While the number of different proteins that must be synthesized for this progression to occur is unknown, at least one of them appears to be cyclin B1, the regulatory subunit of M-phase-promoting factor. Here, we investigate the mechanism of cyclin B1 mRNA translational control during mouse oocyte maturation. We show that the U-rich cytoplasmic polyadenylation element (CPE), a cis element in the 3' UTR of cyclin B1 mRNA, mediates translational repression in GV-stage oocytes. The CPE is also necessary for cytoplasmic polyadenylation, which stimulates translation during oocyte maturation. The injection of oocytes with a cyclin B1 antisense RNA, which probably precludes the binding of a factor to the CPE, delays cytoplasmic polyadenylation as well as the transition from GVBD to metaphase II. CPEB, which interacts with the cyclin B1 CPE and is present throughout meiotic maturation, becomes phosphorylated at metaphase I. These data indicate that CPEB is involved in both the repression and the stimulation of cyclin B1 mRNA and suggest that the phosphorylation of this protein could be involved in regulating its activity.

Cyclin synthesis controls the progression of meiotic maturation in mouse oocytes

To study the mechanisms involved in the progression of meiotic maturation in the mouse, we used oocytes from two strains of mice, CBA/Kw and KE, which differ greatly in the rate at which they undergo meiotic maturation. CBA/Kw oocytes extrude the first polar body about 7 hours after breakdown of the germinal vesicle (GVBD), whilst the oocytes from KE mice take approximately 3–4 hours longer. In both strains, the kinetics of spindle formation are comparable. While the kinetics of MAP kinase activity are very similar in both strains (although slightly faster in CBA/Kw), the rise of cdc2 kinase activity is very rapid in CBA/Kw oocytes and slow and diphasic in KE oocytes. When protein synthesis is inhibited, the activity of the cdc2 kinase starts to rise but arrests shortly after GVBD with a slightly higher level in CBA/Kw oocytes, which may correspond to the presence of a larger pool of cyclin B1 in prophase CBA/Kw oocytes. After GVBD, the rate of cyclin B1 synthesis is higher in CBA/Kw than in KE oocytes, whilst the overall level of protein synthesis and the amount of messenger RNA coding for cyclin B1 are identical in oocytes from both strains. The injection of cyclin B1 messenger RNA in KE oocytes increased the H1 kinase activity and sped up first polar body extrusion. Finally, analysis of the rate of maturation in hybrids obtained after fusion of nuclear and cytoplasmic fragments of oocytes from both strains suggests that both the germinal vesicle and the cytoplasm contain factor(s) influencing the length of the first meiotic M phase. These results demonstrate that the rate of cyclin B1 synthesis controls the length of the first meiotic M phase and that a nuclear factor able to speed up cyclin B synthesis is present in CBA/Kw oocytes.

Characterization of protein phosphatases in mouse oocytes

Okadaic acid (OA) enhances the resumption of meiosis in mouse oocytes, indicating that serine/threonine protein phosphatase-1 (PP1) and/or PP2A is involved. However, specific identification of PP1 and/or PP2A in mouse oocytes has not been reported. Here we demonstrate that fully grown germinal vesicle-intact (GVI) mouse oocytes contain mRNA corresponding to two isotypes of PP1, PP1alpha and PP1gamma. In addition, the transcript for PP2A was also present. At the protein level only PP1alpha and PP2A were recognized in fully grown GVI oocytes by Western blot analysis. Neither of the PP1gamma spliced variant proteins, PP1gamma1 and PP1gamma2, was detectable. Immunohistochemical analysis of ovarian tissue from gonadotropin-stimulated adult mice resulted in subcellular localization of both PP1alpha and PP2A, but not PP1gamma, in oocytes from all stages of folliculogenesis. In primordial oocytes, PP1alpha and PP2A were present in the cytoplasm. In more advanced stages of oogenesis, PP1alpha, although still present in the cytoplasm, was highly concentrated in the nucleus, whereas PP2A was predominantly cytoplasmic with a distinct reduction in the nuclear area. Both PP1alpha and PP2A were immunodetectable in oocytes during the prepubertal period. Eleven-day-old mouse oocytes, considered OA-insensitive and germinal vesicle breakdown (GVB)-incompetent, displayed both PP1alpha and PP2A predominantly in the cytoplasm. By 15 days of age mouse oocytes, which are beginning to acquire OA sensitivity and GVB competence, showed a relocation of PP1alpha into the nucleoplasm while PP2A remained predominantly cytoplasmic. This is the first specific identification of PP1alpha and PP2A in mouse oocytes. The differential localization of PP1alpha and PP2A, in addition to the relocation of PP1alpha during the acquisition of meiotic competence, suggests that these PPs have distinct regulatory roles during the resumption of meiosis.

SPIN, a substrate in the MAP kinase pathway in mouse oocytes

The newly cloned gene Spin encodes a 30-kDa protein, a well-defined abundant molecule found in mouse oocytes and early embryos. This protein SPIN undergoes metaphase-specific phosphorylation and binds to the spindle. To understand the role of SPIN in oocyte meiosis, oocytes were treated with drugs that affect the cell cycle by activating or inactivating specific kinases. The posttranslational modification of SPIN in the treated oocytes was then investigated by one- and two-dimensional gel electrophoresis. Modification of SPIN is inhibited by treatment with 6-dimethylaminopurine (DMAP), suggesting that SPIN is phosphorylated by a serine-threonine kinase. Furthermore, SPIN from cycloheximide-treated oocytes that lack detectable MAP kinase activity is only partially phosphorylated, indicating that SPIN may be phosphorylated by the MOS/MAP kinase pathway. To confirm this observation, SPIN was analyzed in Mos-null mutant mice lacking MAP kinase activity. Normal posttranslational modification of SPIN did not occur in Mos-null mutant oocytes. In addition, there is reduced association of SPIN with the metaphase I spindle in Mos-null mutant oocytes, as determined by immunohistochemical analysis. These findings suggest that SPIN is a substrate in the MOS/ MAP kinase pathway and further that this phosphorylation of SPIN may be essential for its interaction with the spindle.

Interplay between CDC2 kinase and MAP kinase pathway during maturation of mammalian oocytes

Two principal kinases, p34cdc2 kinase and MAP kinase play a pivotal role in maturation of mammalian oocytes. In the porcine and bovine oocytes both kinases are activated around the time of germinal vesicle breakdown (GVBD). Butyrolactone I (BL I), a specific inhibitor of cdk kinases, prevents effectively and reversibly resumption of meiosis in the porcine and bovine oocytes. Neither p34cdc2 kinase nor MAP kinase are activated in oocytes inhibited in the GV stage. The bovine oocytes maintained for 48 h in the medium supplemented with BL I, progress subsequently to metaphase II in 91%, their cumuli expand optimally and after in vitro fertilization they possess two pronuclei. When the cdc2 kinase is blocked in the porcine oocytes by BL I, MAP kinase, activated by okadaic acid treatment, is able to substitute cdc2 kinase and induce GVBD. The histone H1 kinase activity sharply decreases in the metaphase II oocytes treated by BL I and one or two female pronuclei are formed. These data indicate that BL I is a useful tool either for the two step in vitro culture of mammalian oocytes or for their activation in nuclear transfer experiments.

Analysis of the mechanism(s) of metaphase I arrest in strain LT mouse oocytes: participation of MOS

Oocytes of almost all vertebrates become arrested at metaphase II to await fertilization. Arrest is achieved with the participation of a protein complex known as cytostatic factor (CSF) that stabilizes histone H1 kinase activity. MOS and mitogen-activated protein kinase (MAPK) are important components of CSF. Strain LT/Sv mice, and strains related to LT/Sv, produce a high percentage of atypical oocytes that are arrested at metaphase I when normal oocytes have progressed to metaphase II. The potential role of MOS in metaphase I arrest was investigated using strain LT/Sv and LT-related recombinant inbred strains, LTXBO and CX8-4. MOS and MAPK are produced and functional in maturing LT oocytes. Two experimental paradigms were used to reduce or delete MOS in LT oocytes and assess effects on metaphase I arrest. First, sense and antisense Mos oligonucleotides were microinjected into metaphase I-arrested oocytes. Antisense, but not sense, Mos oligonucleotides promoted the activation of metaphase I-arrested oocytes. Second, mice carrying a Mos null mutation were crossed with LT mice, the null mutation was backcrossed three times to LT mice, and Mos(+/−) N3 mice were intercrossed to produce Mos(−/−), Mos(+/−) and Mos(+/+) N3F1 mice. Oocytes of all three Mos genotypes of N3F1 mice sustained meiotic arrest for 17 hours indicating that metaphase I arrest is not initiated by a MOS-dependent mechanism. However, unlike Mos(+/+) and Mos(+/−) CX8-4 N3F1 oocytes, metaphase I arrest of Mos(−/−) CX8-4 N3F1 oocytes was not sustained after 17 hours and became reversed gradually. These results, like the antisense Mos oligonucleotide microinjection experiments, suggest that MOS participates in sustaining metaphase I arrest in LT oocytes.

On cyclins, oocytes, and eggs

Oocyte and egg are suitable model systems for studying cell division since meiotic maturation resembles a G2/M transition and early embryonic divisions are precisely timed and occur without zygotic transcription. The analysis of oocytes and eggs from different species provides the opportunity to understand the roles of proteins that the critical to the progression and maintenance of the cell cycle. Among them, cyclins are certainly worthy of investigation. Mitotic cyclins (cyclins A and B) are clearly implicated in meiosis and early embryonic cell cycles. More recent studies have revealed that G1-type cyclins (cyclins E and D) could also play a role in both processes and cyclin H has been suggesed to participate to CAK activity (cdc2-activating kinase) in oocytes. The study of cyclins in oocytes and eggs clearly offer insights into their roles during the cell cycle.

Introduction of cyclin B induces activation of the maturation-promoting factor and breakdown of germinal vesicle in growing zebrafish oocytes unresponsive to the maturation-inducing hormone

When treated with 17alpha,20beta-dihydroxy-4-pregnen-3-one (17alpha,20beta-DP), a natural maturation-inducing hormone in fishes, fully grown zebrafish oocytes are induced to mature via the activation of the maturation-promoting factor (MPF), which consists of cdc2 (a catalytic subunit) and cyclin B (a regulatory subunit). In contrast, 17alpha,20beta-DP is unable to induce growing (previtellogenic and vitellogenic) oocytes to mature. To know the reason growing oocytes fail to mature upon 17alpha,20beta-DP treatment, we investigated changes in the components of machinery responsible for MPF activation during zebrafish oogenesis. Immunoblotting experiments using monoclonal antibodies against cdc2, cyclin B, and cdk7 (an activator of cdc2) have revealed that the concentrations of cdc2 and cdk7 are almost constant during oogenesis. Cyclin B was present in mature oocytes but absent in growing and fully grown immature oocytes. These results, which are identical to those in goldfish, strongly suggest that cyclin B is synthesized from stored (masked) mRNA after 17alpha,20beta-DP stimulation and that its binding to the preexisting cdc2 allows cdk7 to activate MPF. Microinjection of cyclin B protein induced MPF activation and germinal vesicle breakdown in growing oocytes, as well as in fully grown oocytes, indicating that cdk7 present in growing oocytes is already active. Northern blot analysis revealed the presence of cyclin B mRNA in both previtellogenic and fully grown oocytes. These results indicate that, as in fully grown oocytes, growing oocytes are already equipped with the catalytic subunit of MPF (cdc2) and its activator (cdk7) and that the appearance of the regulatory subunit of MPF (cyclin B) is sufficient for initiating maturation. Therefore, the unresponsiveness of growing oocytes to 17alpha,20beta-DP is attributable to a deficiency in the processes leading to cyclin B synthesis, which include 17alpha,20beta-DP reception on the oocyte surface, subsequent signal transduction pathways, and unmasking the stored cyclin B mRNA.