Analysis of the mechanism(s) of metaphase I arrest in maturing mouse oocytes

The signal that stimulates mammalian embryo development

Embryo development is stimulated by calcium (Ca2+) signals that are generated in the egg cytoplasm by the fertilizing sperm. Eggs are formed via oogenesis. They go through a cell division known as meiosis, during which their diploid chromosome number is halved and new genetic combinations are created by crossing over. During formation the eggs also acquire cellular components that are necessary to produce the Ca2+ signal and also, to support development of the newly formed embryo. Ionized calcium is a universal second messenger used by cells in a plethora of biological processes and the eggs develop a "toolkit", a set of molecules needed for signaling. Meiosis stops twice and these arrests are controlled by a complex interaction of regulatory proteins. The first meiotic arrest lasts until after puberty, when a luteinizing hormone surge stimulates meiotic resumption. The cell cycle proceeds to stop again in the middle of the second meiotic division, right before ovulation. The union of the female and male gametes takes place in the oviduct. Following gamete fusion, the sperm triggers the release of Ca2+ from the egg's intracellular stores which in mammals is followed by repetitive Ca2+ spikes known as Ca2+ oscillations in the cytosol that last for several hours. Downstream sensor proteins help decoding the signal and stimulate other molecules whose actions are required for proper development including those that help to prevent the fusion of additional sperm cells to the egg and those that assist in the release from the second meiotic arrest, completion of meiosis and entering the first mitotic cell division. Here I review the major steps of egg formation, discuss the signaling toolkit that is essential to generate the Ca2+ signal and describe the steps of the signal transduction mechanism that activates the egg's developmental program and turns it into an embryo.

Elevated chorionic gonadotropic hormone in transgenic mice induces parthenogenetic activation and ovarian teratomas

Both male and female reproductive functions are impacted by altered gonadotrophin secretion and action, which may also influence the development of endocrine tumours. To ascertain if chronic hypersecretion of human chorionic gonadotropin (hCG) contributes to the development of gonadal tumours, double transgenic (TG) mice that overexpress hCGα- and β-subunits were analysed. By the age of two months, ovarian tumours with characteristics of teratomas developed with 100% penetrance. Teratomas were also seen in wild-type ovaries orthotopically transplanted into TG mice, demonstrating an endocrine/paracrine mechanism for the hCG-induced ovarian tumorigenesis. Both in vitro and in vivo experiments showed oocyte parthenogenetic activation in TG females. In addition, ovaries showed reduced ovulatory gene expression, inhibited ERK1/2 phosphorylation, and impaired cumulus cell expansion. Hence, persistently high endocrine hCG activity causes parthenogenetic activation and development of ovarian teratomas, along with altered follicle development and impaired ERK1/2 signalling, offering a novel mechanism associated with the molecular pathogenesis of ovarian teratomas.

Loss of UBE2S causes meiosis I arrest with normal spindle assembly checkpoint dynamics in mouse oocytes

The timely degradation of proteins that regulate the cell cycle is essential for oocyte maturation. Oocytes are equipped to degrade proteins via the ubiquitin-proteasome system. In meiosis, anaphase promoting complex/cyclosome (APC/C), an E3 ubiquitin-ligase, is responsible for the degradation of proteins. Ubiquitin-conjugating enzyme E2 S (UBE2S), an E2 ubiquitin-conjugating enzyme, delivers ubiquitin to APC/C. APC/C has been extensively studied, but the functions of UBE2S in oocyte maturation and mouse fertility are not clear. In this study, we used Ube2s knockout mice to explore the role of UBE2S in mouse oocytes. Ube2s-deleted oocytes were characterized by meiosis I arrest with normal spindle assembly and spindle assembly checkpoint dynamics. However, the absence of UBE2S affected the activity of APC/C. Cyclin B1 and securin are two substrates of APC/C, and their levels were consistently high, resulting in the failure of homologous chromosome separation. Unexpectedly, the oocytes arrested in meiosis I could be fertilized and the embryos could become implanted normally, but died before embryonic day 10.5. In conclusion, our findings reveal an indispensable regulatory role of UBE2S in mouse oocyte meiosis and female fertility.

A rare germline BMP15 missense mutation causes hereditary ovarian immature teratoma in human

Ovarian immature teratomas (OITs) are malignant tumors originating from the ovarian germ cells that mainly occur during the first 30 y of a female's life. Early age of onset strongly suggests the presence of susceptibility gene mutations for the disease yet to be discovered. Whole exon sequencing was used to screen pathogenic mutations from pedigrees with OITs. A rare missense germline mutation (C262T) in the first exon of the BMP15 gene was identified. In silico calculation suggested that the mutation could impair the formation of mature peptides. In vitro experiments on cell lines confirmed that the mutation caused an 84.7% reduction in the secretion of mature BMP15. Clinical samples from OIT patients also showed a similar pattern of decrease in the BMP15 expression. In the transgenic mouse model, the spontaneous parthenogenetic activation significantly increased in oocytes carrying the T allele. Remarkably, a mouse carrying the T allele developed the phenotype of OIT. Oocyte-specific RNA sequencing revealed that abnormal activation of the H-Ras/MAPK pathway might contribute to the development of OIT. BMP15 was identified as a pathogenic gene for OIT which improved our understanding of the etiology of OIT and provided a potential biomarker for genetic screening of this disorder.

Molecular determinants of the meiotic arrests in mammalian oocytes at different stages of maturation

Mammalian oocytes undergo two rounds of developmental arrest during maturation: at the diplotene of the first meiotic prophase and metaphase of the second meiosis. These arrests are strictly regulated by follicular cells temporally producing the secondary messengers, cAMP and cGMP, and other factors to regulate maturation promoting factor (composed of cyclin B1 and cyclin-dependent kinase 1) levels in the oocytes. Out of these normally appearing developmental arrests, permanent arrests may occur in the oocytes at germinal vesicle (GV), metaphase I (MI), or metaphase II (MII) stage. This issue may arise from absence or altered expression of the oocyte-related genes playing key roles in nuclear and cytoplasmic maturation. Additionally, the assisted reproductive technology (ART) applications such as ovarian stimulation and in vitro culture conditions both of which harbor various types of chemical agents may contribute to forming the permanent arrests. In this review, the molecular determinants of developmental and permanent arrests occurring in the mammalian oocytes are comprehensively evaluated in the light of current knowledge. As number of permanently arrested oocytes at different stages is increasing in ART centers, potential approaches for inducing permanent arrests to obtain competent oocytes are discussed.

Unexpected repeat immature oocyte response after IVF stimulation: a case report

The purpose of ovarian stimulation in IVF is to recover mature oocytes at metaphase II stage which are capable of fertilization either when mixed with sperm or after ICSI. However, there have been instances when even after controlled ovarian stimulation (COS) and correct administration of human chorionic gonadotrophin (hCG) trigger for final oocyte maturation, the oocytes were found to be arrested at germinal vesicle (GV) or metaphase I (MI) stage. Similar dilemma is faced in cases of empty follicle syndrome (either genuine or due to inadequate response), however, in this condition, there is no retrieval of oocytes despite presence of mature looking follicles. We present an interesting case where despite presence of normally growing follicles and documentation of correct response to trigger and rise in estradiol levels, two subsequent IVF cycles; one triggered with recombinant hCG and second with GnRH agonist, hCG failed to yield mature oocytes. Both cycles yielded expected number of oocytes but all at immature MI stage even after dual trigger.

Regulation of 4E-BP1 activity in the mammalian oocyte

Fully grown mammalian oocytes utilize transcripts synthetized and stored during earlier development. RNA localization followed by a local translation is a mechanism responsible for the regulation of spatial and temporal gene expression. Here we show that the mouse oocyte contains 3 forms of cap-dependent translational repressor expressed on the mRNA level: 4E-BP1, 4E-BP2 and 4E-BP3. However, only 4E-BP1 is present as a protein in oocytes, it becomes inactivated by phosphorylation after nuclear envelope breakdown and as such it promotes cap-dependent translation after NEBD. Phosphorylation of 4E-BP1 can be seen in the oocytes after resumption of meiosis but it is not detected in the surrounding cumulus cells, indicating that 4E-BP1 promotes translation at a specific cell cycle stage. Our immunofluorescence analyses of 4E-BP1 in oocytes during meiosis I showed an even localization of global 4E-BP1, as well as of its 4E-BP1 (Thr37/46) phosphorylated form. On the other hand, 4E-BP1 phosphorylated on Ser65 is localized at the spindle poles, and 4E-BP1 phosphorylated on Thr70 localizes on the spindle. We further show that the main positive regulators of 4E-BP1 phosphorylation after NEBD are mTOR and CDK1 kinases, but not PLK1 kinase. CDK1 exerts its activity toward 4E-BP1 phosphorylation via phosphorylation and activation of mTOR. Moreover, both CDK1 and phosphorylated mTOR co-localize with 4E-BP1 phosphorylated on Thr70 on the spindle at the onset of meiotic resumption. Expression of the dominant negative 4E-BP1 mutant adversely affects translation and results in spindle abnormality. Taken together, our results show that the phosphorylation of 4E-BP1 promotes translation at the onset of meiosis to support the spindle assembly and suggest an important role of CDK1 and mTOR kinases in this process. We also show that the mTOR regulatory pathway is present in human oocytes and is likely to function in a similar way as in mouse oocytes.

From Meiosis to Mitosis: The Astonishing Flexibility of Cell Division Mechanisms in Early Mammalian Development

The execution of female meiosis and the establishment of the zygote is arguably the most critical stage of mammalian development. The egg can be arrested in the prophase of meiosis I for decades, and when it is activated, the spindle is assembled de novo. This spindle must function with the highest of fidelity and yet its assembly is unusually achieved in the absence of conventional centrosomes and with minimal influence of chromatin. Moreover, its dramatic asymmetric positioning is achieved through remarkable properties of the actin cytoskeleton to ensure elimination of the polar bodies. The second meiotic arrest marks a uniquely prolonged metaphase eventually interrupted by egg activation at fertilization to complete meiosis and mark a period of preparation of the male and female pronuclear genomes not only for their entry into the mitotic cleavage divisions but also for the imminent prospect of their zygotic expression.

Appropriate expression of Ube2C and Ube2S controls the progression of the first meiotic division

Timely degradation of protein regulators of the cell cycle is essential for the completion of cell division. This degradation is promoted by the E3 anaphase-promoting complex/cyclosome (APC/C) and mediated by the E2 ubiquitin-conjugating enzymes (Ube2s). Unlike the ample information gathered regarding the meiotic E3 APC/C, the E2s participating in this cell division have never been studied. We identified Ube2C, -S, and -D3 as the E2 enzymes that regulate APC/C activity during meiosis of mouse oocytes. Their depletion reduces the levels of the first meiotic cytokinesis by 50%, and their overexpression doubles and accelerates its completion (50% as compared with 4% at 11 h). We also demonstrated that these E2s take part in ensuring appropriate spindle formation. It is noteworthy that high levels of Ube2C bring about the resumption of the first meiotic division, regardless of the formation of the spindle, overriding the spindle assembly checkpoint. Thus, alongside their canonical function in protein degradation, Ube2C and -S also control the extrusion of the first polar body. Overall, our study characterizes new regulators and unveils the novel roles they play during the meiotic division. These findings shed light on faithful chromosome segregation in oocytes and may contribute to better understanding of aneuploidy and its consequent genetic malformations.

Dual effects of hydrogen sulfide donor on meiosis and cumulus expansion of porcine cumulus-oocyte complexes

Hydrogen sulfide (H₂S) has been revealed to be a signal molecule with second messenger action in the somatic cells of many tissues, including the reproductive tract. The aim of this study was to address how exogenous H₂S acts on the meiotic maturation of porcine oocytes, including key maturation factors such as MPF and MAPK, and cumulus expansion intensity of cumulus-oocyte complexes. We observed that the H₂S donor, Na₂S, accelerated oocyte in vitro maturation in a dose-dependent manner, following an increase of MPF activity around germinal vesicle breakdown. Concurrently, the H₂S donor affected cumulus expansion, monitored by hyaluronic acid production. Our results suggest that the H₂S donor influences oocyte maturation and thus also participates in the regulation of cumulus expansion. The exogenous H₂S donor apparently affects key signal pathways of oocyte maturation and cumulus expansion, resulting in faster oocyte maturation with little need of cumulus expansion.

Suppression of chemically induced and spontaneous mouse oocyte activation by AMP-activated protein kinase

Oocyte activation is an important process triggered by fertilization that initiates embryonic development. However, parthenogenetic activation can occur either spontaneously or with chemical treatments. The LT/Sv mouse strain is genetically predisposed to spontaneous activation. LT oocytes have a cell cycle defect and are ovulated at the metaphase I stage instead of metaphase II. A thorough understanding of the female meiosis defects in this strain remains elusive. We have reported that AMP-activated protein kinase (PRKA) has an important role in stimulating meiotic resumption and promoting completion of meiosis I while suppressing premature parthenogenetic activation. Here we show that early activation of PRKA during the oocyte maturation period blocked chemically induced activation in B6SJL oocytes and spontaneous activation in LT/SvEiJ oocytes. This inhibitory effect was associated with high levels of MAPK1/3 activity. Furthermore, stimulation of PRKA partially rescued the meiotic defects of LT/Sv mouse oocytes in concert with correction of abnormal spindle pole localization of PRKA and loss of prolonged spindle assembly checkpoint activity. Altogether, these results confirm a role for PRKA in helping sustain the MII arrest in mature oocytes and suggest that dysfunctional PRKA contributes to meiotic defects in LT/SvEiJ oocytes.

The control of meiotic maturation in mammalian oocytes

Mammalian oocytes spend the majority of their lives in a dormant state, residing in primordial follicles. This arrest, most analogous to the G2 stage of the mitotic cell cycle division, is only broken in the hours preceding ovulation, when a hormonal rise induces meiotic resumption and entry into the first meiotic division. At a molecular level, this event is triggered by CDK1 activity, and here, we examine how CDK1 is suppressed during meiotic arrest and raised for oocyte maturation. We focus on signaling: intercellular signaling between the oocyte and the somatic cells of the follicle, and spatial signaling involving the anaphase-promoting complex (APC) within the oocyte. Meiotic arrest is achieved through APC(FZR1)-mediated cyclin B1 degradation. Once meiotic resumption resumes, CDK1 levels rise, but its activity eventually needs to be suppressed for completion of the first meiotic division. This is achieved by APC(CDC20), whose activity is critically regulated by the spindle assembly checkpoint, and which induces both a loss in CDK1 activity as well as the cohesive ties holding chromosomes together.

From ubiquitin-proteasomal degradation to CDK1 inactivation: requirements for the first polar body extrusion in mouse oocytes

Completion of the first meiotic division, manifested by extrusion of the first polar body (PBI), depends on proteasomal degradation of cyclin B1 and securin and the subsequent respective CDK1 inactivation and chromosome segregation. We aimed at identifying the polyubiquitin signal that mediates proteasomal action and at a better characterization of the role of CDK1 inactivation at this stage of meiosis. Microinjections of mutated ubiquitin proteins into mouse oocytes revealed that interference with lysine-11 polyubiquitin chains abrogated chromosome segregation and reduced PBI extrusion by 63% as compared to WT ubiquitin-injected controls. Inactivation of CDK1 in oocytes arrested at first metaphase by a proteasome inhibitor fully rescued PBI extrusion. However, removal of CDK1 inhibition failed to allow progression to the second metaphase, rather, inducing PBI reengulfment in 62% of the oocytes. Inhibition of either PLK1 or MEK1/2 during the first anaphase changed spindle dimensions. The PLK1 inhibitor also blocked PBI emission and prevented RhoA translocation. Our results identified lysine-11 rather than the canonic lysine-48 ubiquitin chains as the degradation signal in oocytes resuming meiosis, further disclosing that CDK1 inactivation is necessary and sufficient for PBI emission. This information significantly contributes to our understanding of faulty chromosome segregation that may lead to aneuploidy.

Role of the inositol polyphosphate-4-phosphatase type II Inpp4b in the generation of ovarian teratomas

Teratomas are a unique class of tumors composed of ecto-, meso- and endodermal tissues, all foreign to the site of origin. In humans, the most common teratoma is the ovarian teratoma. Not much is known about the molecular and genetic etiologies of these tumors. Female carriers of the Tgkd transgene are highly susceptible to developing teratomas. Ovaries of Tgkd/+ hemizygous female mice exhibit defects in luteinization, with numerous corpora lutea, some of which contain central trapped, fully-grown oocytes. Genetically, Tgkd teratomas originate from mature oocytes that have completed meiosis I, suggesting that Tgkd teratomas originate from these trapped oocytes. The insertion of Tgkd 3' of the Inpp4b gene is associated with decreased expression of Inpp4b and changes in intracellular PI3 Kinase/AKT signaling in follicular granulosa cells. Because Inpp4b is not expressed in fully-grown wild-type or Tgkd oocytes, these findings suggest that hyperactivation of the PI3K/AKT pathway caused by the decrease in INPP4B in granulosa cells promotes an ovarian environment defective in folliculogenesis and conducive to teratoma formation.

Nitric oxide and meiotic competence of porcine oocytes

Reproductive biotechnology such as in vitro fertilization, the creation of transgenic animals or cloning by nuclear transfer depends on the use of fully grown, meiotically competent oocytes capable of completing meiotic maturation by reaching the stage of metaphase II. However, there exists only a limited quantity of these oocytes in the ovaries of females. In view of their limited number, growing oocytes without meiotic competence represent a possible source. The mechanisms controlling the acquisition of meiotic competence, however, are still not completely clear. A gas with a short half-life, nitric oxide (NO), produced by NO-synthase (NOS) enzyme can fulfill a regulatory role in this period. The objective of this study was to ascertain the role of NO in the growth phase of pig oocytes and its influence on the acquisition of meiotic competence with the help of NOS inhibitors, NO donors and their combinations. We demonstrated that the selective competitive iNOS inhibitor aminoguanidine and also the non-selective NOS inhibitor l-NAME block meiotic maturation of oocytes with partial or even full meiotic competence at the very beginning. NOS inhibitors influence even competent oocytes in the first stage of meiotic metaphase. However, blockage is less effective than at the beginning of meiotic maturation. The number of parthenogenetically activated competent oocytes greatly increased in a pure medium after inhibitor reversion. A large quantity of NO externally added to the in vitro cultivation environment disrupts the viability of oocytes. The effectiveness of the inhibitor can be reversed in oocytes by an NO donor in a very low concentration. However, the donor is not capable of pushing the oocytes farther than beyond the first stage of meiotic metaphase. The experiments confirmed the connection of NO with the growth period and the acquisition of meiotic competence. However, it is evident from the experiments that NO is not the only stimulus controlling the growth period.

Spindle assembly checkpoint-related meiotic defect in oocytes from LT/Sv mice has cytoplasmic origin and diminishes in older females

The spindle assembly checkpoint (SAC) ensures proper segregation of chromosomes by delaying anaphase onset until all kinetochores are properly attached to the spindle microtubules. Oocytes from the mouse strain LT/Sv arrest at the first meiotic metaphase (MI) due to, as reported recently, enormously prolonged activity of the SAC. We compared the dynamics of cyclin B1-GFP degradation, the process which is a measure of the SAC activity, in chromosomal and achromosomal halves of LT/Sv oocytes. In chromosome-containing oocyte halves arrested at MI, cyclin B1-GFP was not degraded indicating active SAC. However, in the halves lacking chromosomes, which is a condition precluding the SAC function, degradation always occurred confirming that MI arrest in LT/Sv oocytes is SAC dependent. Transferring the germinal vesicle (GV) from LT/Sv oocytes into the enucleated oocytes from wild-type mice resulted in the progression through meiosis one, indicating that a SAC-activating defect in LT/Sv oocytes is cytoplasmic, yet can be rescued by foreign cytoplasm. These results may help to define the etiology of the human infertility related to the oocyte MI arrest, indicating the involvement of the SAC as likely candidate, and point to GV transfer as the possible therapy. Finally, we found that majority of oocytes isolated from old LT/Sv mice complete the first meiosis. Reciprocal transfers of the GV between the oocytes from young and old LT/Sv females suggest that the factor(s) responsible for the reversal of the phenotype in oocytes from old mice is located both in the GV and in the cytoplasm.

Nucleoli from growing oocytes inhibit the maturation of enucleolated, full-grown oocytes in the pig

In mammals, the nucleolus of full-grown oocyte is essential for embryonic development but not for oocyte maturation. In our study, the role of the growing oocyte nucleolus in oocyte maturation was examined by nucleolus removal and/or transfer into previously enucleolated, growing (around 100 µm in diameter) or full-grown (120 µm) pig oocytes. In the first experiment, the nucleoli were aspirated from growing oocytes whose nucleoli had been compacted by actinomycin D treatment, and the enucleolated oocytes were matured in vitro. Most of non-treated or actinomycin D-treated oocytes did not undergo germinal vesicle breakdown (GVBD; 13% and 12%, respectively). However, the GVBD rate of enucleolated, growing oocytes significantly increased to 46%. The low GVBD rate of enucleolated, growing oocytes was restored again by the re-injection of nucleoli from growing oocytes (23%), but not when nucleoli from full-grown oocytes were re-injected into enucleolated, growing oocytes (49%). When enucleolated, full-grown oocytes were injected with nucleoli from growing or full-grown oocytes, the nucleolus in the germinal vesicle was reassembled (73% and 60%, respectively). After maturation, the enucleolated, full-grown oocytes injected with nucleoli from full-grown oocytes matured to metaphase II (56%), whereas injection with growing-oocyte nucleoli reduced this maturation to 21%. These results suggest that the growing-oocyte nucleolus is involved in the oocyte's meiotic arrest, and that the full-grown oocyte nucleolus has lost the ability.

Oocyte maturation failure: a syndrome of bad eggs

To show that disruption of meiotic competence results in cell cycle arrest, and the production of immature oocytes that are not capable of fertilization. Through an extensive review of animal studies and clinical case reports, we define the syndrome of oocyte maturation failure as a distinct oocyte disorder, present a classification system based on clinical parameters, and discuss the potential molecular origins for the disease.

Portrait of an oocyte: our obscure origin

Oocytes play a pivotal role in the cycle of human life. As we discuss here, after emerging from germline stem cells in the fetus, they grow in a follicular niche in which development is harmonized for timely ovulation and hormone secretion after puberty. Most human oocytes have poor developmental competence and are peculiarly vulnerable to chromosomal malsegregation, especially as women pass the optimal years of fertility and may begin to turn to assisted reproductive technologies (ARTs) and egg donation. Research needs to focus on the molecular factors involved and the environmental niche required for optimal development of oocytes, with the aim of increasing their numbers and quality for ARTs, since these are the factors that so often limit human fertility.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.

Proteasomal activity is required to initiate and to sustain translational activation of messenger RNA encoding the stem-loop-binding protein during meiotic maturation in mice

Developmentally regulated translation plays a key role in controlling gene expression during oogenesis. In particular, numerous mRNA species are translationally repressed in growing oocytes and become translationally activated during meiotic maturation. While many studies have focused on a U-rich sequence, termed the cytoplasmic polyadenylation element (CPE), located in the 3'-untranslated region (UTR) and the CPE-binding protein (CPEB) 1, multiple mechanisms likely contribute to translational control in oocytes. The stem-loop-binding protein (SLBP) is expressed in growing oocytes, where it is required for the accumulation of nonpolyadenylated histone mRNAs, and then accumulates substantially during meiotic maturation. We report that, in immature oocytes, Slbp mRNA carries a short poly(A) tail, and is weakly translated, and that a CPE-like sequence in the 3'-UTR is required to maintain this low activity. During maturation, Slbp mRNA becomes polyadenylated and translationally activated. Unexpectedly, proteasomal activity is required both to initiate and to sustain translational activation. This proteasomal activity is not required for the polyadenylation of Slbp mRNA during early maturation; however, it is required for a subsequent deadenylation of the mRNA that occurs during late maturation. Moreover, although CPEB1 is degraded during maturation, inhibiting its degradation by blocking mitogen-activated protein kinase 1/3 activity does not prevent the accumulation of SLBP, indicating that CPEB1 is not the protein whose degradation is required for translational activation of Slbp mRNA. These results identify a new role for proteasomal activity in initiating and sustaining translational activation during meiotic maturation.

Gene expression during the oocyte-to-embryo transition in mammals

The seminal question in modern developmental biology is the origins of new life arising from the unification of sperm and egg. The roots of this question begin from 19th to 20th century embryologists studying fertilization and embryogenesis. Although the revolution of molecular biology has yielded significant insight into the complexity of this process, the overall orchestration of genes, molecules, and cells is still not fully formed. Early mammalian development, specifically the oocyte-to-embryo transition, is essentially under "maternal command" from factors deposited in the cytoplasm during oocyte growth, independent of de novo transcription from the nascent embryo. Many of the advances in understanding this developmental period occurred in tandem with application of new methods and techniques from molecular biology, from protein electrophoresis to sequencing and assemblies of whole genomes. From this bed of knowledge, it appears that precise control of mRNA translation is a key regulator coordinating the molecular and cellular events occurring during oocyte-to-embryo transition. Notably, oocyte transcriptomes share, yet retain some uniqueness, common genetic motifs among all chordates. The common genetic motifs typically define fundamental processes critical for cellular maintenance, whereas the unique genetic features may be a source of variation and a substrate for sexual selection, genetic drift, or gene flow. One purpose for this complex interplay among genes, proteins, and cells may allow for evolution to transform and act upon the underlying processes, at molecular, structural and organismal levels, to increase diversity, which is the ultimate goal of sexual reproduction.

Spindle assembly checkpoint-related failure perturbs early embryonic divisions and reduces reproductive performance of LT/Sv mice

The phenotype of the LT/Sv strain of mice is manifested by abnormalities in oocyte meiotic cell-cycle, spontaneous parthenogenetic activation, teratomas formation, and frequent occurrence of embryonic triploidy. These abnormalities lead to the low rate of reproductive success. Recently, metaphase I arrest of LT/Sv oocytes has been attributed to the inability to timely inactivate the spindle assembly checkpoint (SAC). As differences in meiotic and mitotic SAC functioning were described, it remains obscure whether this abnormality is limited to the meiosis or also impinges on the mitotic divisions of LT/Sv embryos. Here, we show that a failure to inactivate SAC affects mitoses during preimplantation development of LT/Sv embryos. This is manifested by the prolonged localization of MAD2L1 on kinetochores of mitotic chromosomes and abnormally lengthened early embryonic M-phases. Moreover, LT/Sv embryos exhibit elevated frequency of abnormal chromosome separation during the first mitotic division. These abnormalities participate in severe impairment of preimplantation development and significantly decrease the reproductive success of this strain of mice. Thus, the common meiosis and mitosis SAC-related failure participates in a complex LT/Sv phenotype.

Metaphase I arrest in LT/Sv mouse oocytes involves the spindle assembly checkpoint

During meiotic maturation, the majority of oocytes from LT/Sv mice arrest at metaphase I. However, anaphase may be induced through parthenogenetic activation. If this happens within the ovary, it often results in the development of ovarian teratomas. Here, we show that the induction of first meiotic anaphase in LT/Sv oocytes results in incorrect chromosome segregation. In search of the molecular basis of this complex phenotype, we analyzed the localization/destruction of cohesins, as well as the function of the components of the spindle assembly checkpoint (SAC). Both localization and removal of meiotic cohesin REC8 from chromosomes are unperturbed. In contrast, there is prolonged localization of SAC proteins BUB1 and MAD2L1 (MAD2) at the metaphase I kinetochores in mutant oocytes compared with the wild-type. Interfering with BUB1 function through expression of a dominant-negative mutant protein resulted in the increase of the number of LT/Sv oocytes completing the first meiosis, which indicates SAC involvement in metaphase I arrest. These data show for the first time that there is a direct link between the SAC function and the heritable meiotic incompetence of a mammalian oocyte.

Successful delivery after the transfer of embryos obtained from a cohort of incompletely in vivo matured oocytes at retrieval time

OBJECTIVE

To report a case of successful delivery of a healthy baby after intracytoplasmic sperm injection (ICSI) in a patient with no mature oocytes at the time of oocyte retrieval.

DESIGN

Case report.

SETTING

Department of Reproductive Medicine.

PATIENT

After a controlled ovarian hyperstimulation cycle, a total of seven immature oocytes were collected.

INTERVENTION(S)

Medical management.

MAIN OUTCOME MEASURE(S)

The timing of polar body extrusion was checked every 2 hours, and ICSI was performed as soon as the first polar body was extruded.

RESULT(S)

Following incubation in culture medium, five oocytes reached the metaphase II stage within 8-8.5 hours. Three oocytes were fertilized after ICSI, and two of three cleaved embryos were transferred on day 3. The embryo transfer was followed by a single pregnancy and the delivery of a healthy baby.

CONCLUSION

This case report demonstrates that embryos obtained from in vitro matured oocytes retain the developmental competence for full-term. It illustrates the importance of regularly monitoring the polar body extrusion when all collected oocytes are immature.

Meiotic maturation of incompetent prepubertal sheep oocytes is induced by paracrine factor(s) released by gonadotropin-stimulated oocyte-cumulus cell complexes and involves mitogen-activated protein kinase activation

In this study, sheep oocyte-cumulus cell complexes (OCC) derived from medium (M) antral follicles (M-OCC) were in vitro matured alone or in coculture with OCC derived from small (S) antral follicles (S-OCC) to investigate the contribution of cumulus cells (CC) and oocytes to the process of oocyte meiotic maturation and cumulus expansion (CE). Experiments were conducted with or without gonadotropins (FSH/LH). Regardless of culture conditions, about 12% of S-oocytes reached the metaphase II stage, and S-CC showed a low degree of CE. In contrast, both maturational processes were significantly stimulated by gonadotropins in M-OCC. However, about 48% of S-oocytes progressed to metaphase II, and S-CC expanded after coculture with gonadotropin-stimulated M-OCC and M-CC but not with mural granulosa cells. Both maturational processes were inhibited when S-OCC were cocultured with M-denuded oocytes, or when S-denuded oocytes were cocultured with M-CC. The capacity of these paracrine factor(s) to activate the MAPK pathway in somatic and germ cells of S-complexes was investigated. It was found that MAPK kinase/MAPK phosphorylation levels in M-OCC but not in S-OCC were significantly increased by gonadotropins, first in CC and later in the oocytes. Kinase phosphorylations were activated only in S-oocytes cocultured with M-OCC or M-CC. These results demonstrate that soluble factors specifically produced by M-CC are capable to induce meiotic maturation and CE in S-complexes by acting via CC. These factors can induce MAPK activation only in S-oocytes, whose meiotic arrest could be due to the inability of surrounding CC to respond to gonadotropin stimulation.

Prometaphase APCcdh1 activity prevents non-disjunction in mammalian oocytes

The first female meiotic division (meiosis I, MI) is uniquely prone to chromosome segregation errors through non-disjunction, resulting in trisomies and early pregnancy loss. Here, we show a fundamental difference in the control of mammalian meiosis that may underlie such susceptibility. It involves a reversal in the well-established timing of activation of the anaphase-promoting complex (APC) by its co-activators cdc20 and cdh1. APC(cdh1) was active first, during prometaphase I, and was needed in order to allow homologue congression, as loss of cdh1 speeded up MI, leading to premature chromosome segregation and a non-disjunction phenotype. APC(cdh1) targeted cdc20 for degradation, but did not target securin or cyclin B1. These were degraded later in MI through APC(cdc20), making cdc20 re-synthesis essential for successful meiotic progression. The switch from APC(cdh1) to APC(cdc20) activity was controlled by increasing CDK1 and cdh1 loss. These findings demonstrate a fundamentally different mechanism of control for the first meiotic division in mammalian oocytes that is not observed in meioses of other species.

Essential CDK1-inhibitory role for separase during meiosis I in vertebrate oocytes

Separase not only triggers anaphase of meiosis I by proteolytic cleavage of cohesin on chromosome arms, but in vitro vertebrate separase also acts as a direct inhibitor of cyclin-dependent kinase 1 (Cdk1) on liberation from the inhibitory protein, securin. Blocking separase-Cdk1 complex formation by microinjection of anti-separase antibodies prevents polar-body extrusion in vertebrate oocytes. Importantly, proper meiotic maturation is rescued by chemical inhibition of Cdk1 or expression of Cdk1-binding separase fragments lacking cohesin-cleaving activity.

Mad2 is required for inhibiting securin and cyclin B degradation following spindle depolymerisation in meiosis I mouse oocytes

Mad2 is a pivotal component of the spindle assembly checkpoint (SAC) which inhibits anaphase promoting complex/cyclo-some (APC/C) activity by sequestering Cdc20 thereby regulating the destruction of securin and cyclin B. During mitosis, spindle depolymerisation induces a robust Mad2-dependent arrest due to inhibition of securin and cyclin B destruction. In contrast to mitosis, the molecular details underpinning the meiosis I arrest experienced by mouse oocytes exposed to spindle depolymerisation remain incompletely characterised. Notably, the role of Mad2 and the fate of the anaphase-marker, securin, are unexplored. As shown previously, we find that spindle depolymerisation by nocodazole inhibits first polar body extrusion (PBE) and stabilises cyclin B and cyclin-dependent kinase 1 activity in mouse oocytes. Here we show that stabilisation of cyclin B in nocodazole can be sustained for several hours and is associated with stabilisation of securin. These effects are SAC-mediated as, in oocytes depleted of the majority of Mad2 by morpholino antisense, securin and cyclin B are destabilised and 15% of oocytes undergo PBE. This reflects premature APC/C activation as a mutant form of cyclin B lacking its APC/C degradation signal is stable in Mad2-depleted oocytes. Moreover, homologues do not disjoin during the prolonged meiosis I arrest (> 18 h) induced by nocodaozole indicating that a non-cleavage mechanism is insufficient on its own for resolution of arm cohesion in mammalian oocytes. In conclusion, when all kinetochores lack attachment and tension, mouse oocytes mount a robust Mad2-dependent meiosis I arrest which inhibits the destruction of securin and cyclin B.

Contribution of the oocyte to embryo quality

The ability of a bovine embryo to develop to the blastocyst stage, to implant and to generate a healthy offspring is not a simple process. To clarify the importance of the contribution of the oocyte to the embryo quality, it is important to define more precisely the different types of competence expressed by oocytes. The ability to resume meiosis, to cleave upon fertilization to develop into a blastocyst, to induce pregnancy and to generate an healthy offspring are all separate events and succeeding in the first events does not ensure the success of subsequent ones. Furthermore, these events are associated with the three types of maturation processes observed in the oocyte: meiotic, cytoplasmic and molecular. These abilities vary also upon the type of follicle the oocytes is removed from. Larger or slow-growing follicles have been shown to foster better eggs than small or actively growing follicles. Hormonal stimulation can also affect oocyte competence with the nature of the effect (positive or negative) depending on timing and dose. This complex situation requires better definition of the contribution of each factor affecting the oocyte competence and the resulting embryo quality.

Genetic influences on ovulation of primary oocytes in LT/Sv strain mice

A high proportion of LT/Sv strain oocytes arrest in meiotic metaphase I (MI) and are ovulated as diploid primary oocytes rather than haploid secondary oocytes. (Mus musculus castaneus x LT/SvKau)F1 x LT/SvKau backcross females were analysed for the proportion of oocytes that arrested in MI and typed by PCR for a panel of microsatellite DNA sequences (simple sequence repeat polymorphisms) that differed between strain LT/SvKau and M. m. castaneus. This provided a whole genome scan of 86 genetic markers distributed over all 19 autosomes and the X chromosome, and revealed genetic linkage of the MI arrest phenotype to markers on chromosomes 1 and 9. Identification of these two chromosomal regions should facilitate the identification of genes involved in mammalian oocyte maturation and the control of meiosis.

Fertilization and InsP3-induced Ca2+ release stimulate a persistent increase in the rate of degradation of cyclin B1 specifically in mature mouse oocytes

Vertebrate oocytes proceed through meiosis I before undergoing a cytostatic factor (CSF)-mediated arrest at metaphase of meiosis II. Exit from MII arrest is stimulated by a sperm-induced increase in intracellular Ca2+. This increase in Ca2+ results in the destruction of cyclin B1, the regulatory subunit of cdk1 that leads to inactivation of maturation promoting factor (MPF) and egg activation. Progression through meiosis I also involves cyclin B1 destruction, but it is not known whether Ca2+ can activate the destruction machinery during MI. We have investigated Ca2+ -induced cyclin destruction in MI and MII by using a cyclin B1-GFP fusion protein and measurement of intracellular Ca2+. We find no evidence for a role for Ca2+ in MI since oocytes progress through MI in the absence of detectable Ca2+ transients. Furthermore, Ca2+ increases induced by photorelease of InsP3 stimulate a persistent destruction of cyclin B1-GFP in MII but not MI stage oocytes. In addition to a steady decrease in cyclin B1-GFP fluorescence, the increase in Ca2+ stimulated a transient decrease in fluorescence in both MI and MII stage oocytes. Similar transient decreases in fluorescence imposed on a more persistent fluorescence decrease were detected in cyclin-GFP-injected eggs undergoing fertilization-induced Ca2+ oscillations. The transient decreases in fluorescence were not a result of cyclin B1 destruction since transients persisted in the presence of a proteasome inhibitor and were detected in controls injected with eGFP and in untreated oocytes. We conclude that increases in cytosolic Ca2+ induce transient changes in autofluorescence and that the pattern of cyclin B1 degradation at fertilization is not stepwise but exponential. Furthermore, this Ca2+ -induced increase in degradation of cyclin B1 requires factors specific to mature oocytes, and that to overcome arrest at MII, Ca2+ acts to release the CSF-mediated brake on cyclin B1 destruction.

Protein kinase C activity regulates the onset of anaphase I in mouse oocytes

The metaphase-to-anaphase I transition is a key step in the completion of meiosis I. In mouse oocytes, competence to exit metaphase I (MI) is developmentally regulated and typically not acquired until the preovulatory stage. The possible role of protein kinase C (PKC) in regulating this critical transition was assessed in both normal oocytes isolated from small antral follicles (18-day-old B6SJLF1 mice), which have not yet developed the capacity to progress to metaphase II (MII), and also oocytes defective in their ability to exit MI despite development to the preovulatory stage (24-day-old CX8 recombinant inbred strains). In both systems, transient suppression of endogenous PKC activity by treatment with a PKC-specific inhibitor, bisindolylmaleimide I (BIM), promoted the onset of anaphase I in a dose-dependent manner, while activation of PKC with the phorbol ester TPA blocked progression to MII. Following a 2-h incubation with BIM, the majority of oocytes progressed to, and arrested at, MII. The resulting MII oocytes were fertilizable in vitro, showing similar cleavage and blastocyst development rates between BIM treated and untreated controls. Transferred embryos resulted in the development of pups to term in both groups. These data demonstrate that PKC plays an important role in regulating the onset of anaphase I in mouse oocytes. Moreover, it is concluded that oocytes isolated from small antral follicles become blocked at MI due to a PKC-mediated signal, suggesting that acquisition of competence to complete meiosis I involves, in part, the control of PKC activity. Similarly, failure to regulate PKC activity at the preovulatory stage likely promotes arrest at MI.

Induction and activation of meiosis and subsequent parthenogenetic development of growing pig oocytes using calcium ionophore A23187

The pig ovary contains a large number of growing oocytes, which do not mature in vitro and cannot be readily used in various biotechnologies. This study was conducted to determine the possibility of inducing meiotic maturation in growing pig oocytes with an internal diameter of 110 microm, which had developed partial meiotic competence. Most of these oocytes spontaneously stopped maturation at the metaphase I stage (68%); a limited number proceeded to the metaphase II stage (26%). Treatment with calcium ionophore A23187 (50 microM for 5 or 10 min) after 24h in vitro culture overcame the block at the metaphase I stage, and treated growing pig oocytes matured to the metaphase II stage (66%). Oocytes in which maturation had been induced by calcium ionophore were again treated with calcium ionophore. Up to 58% of the treated oocytes were activated. Parthenogenetic development in oocytes treated with ionophore for meiosis induction and activation was very limited. The portion which reached morula stage did not exceed 8% and at most 3% developed to the blastocyst stage.

Molecular control of the oocyte to embryo transition

The elucidation of the molecular control of the initiation of mammalian embryogenesis is possible now that the transcriptomes of the full-grown oocyte and two-cell stage embryo have been prepared and analysed. Functional annotation of the transcriptomes using gene ontology vocabularies, allows comparison of the oocyte and two-cell stage embryo between themselves, and with all known mouse genes in the Mouse Genome Database. Using this methodology one can outline the general distinguishing features of the oocyte and the two-cell stage embryo. This, when combined with oocyte-specific targeted deletion of genes, allows us to dissect the molecular networks at play as the differentiated oocyte and sperm transit into blastomeres with unlimited developmental potential.

Golgi apparatus dynamics during mouse oocyte in vitro maturation: effect of the membrane trafficking inhibitor brefeldin A

We have studied Golgi apparatus dynamics during mouse oocyte in vitro maturation, employing both live imaging with the fluorescent lipid BODIPY-ceramide and immunocytochemistry using several specific markers (beta-COP, giantin, and TGN38). In germinal vesicle oocytes the Golgi consisted of a series of structures, possibly cisternal stacks, dispersed in the ooplasm, but slightly more concentrated in the interior than at the cortex. A similar pattern was detected in rhesus monkey germinal vesicle oocytes. These "mini-Golgis" were functionally active because they were reversibly disrupted by the membrane trafficking inhibitor brefeldin A. However, the drug had no visible effect if the oocytes had been previously microinjected with GTP-gamma-S. During in vitro maturation the large Golgi apparatus structures fragmented at germinal vesicle breakdown, and dispersed homogenously throughout the ooplasm, remaining in a fragmented state in metaphase-II oocytes. Similarly to what has been reported using protein synthesis inhibitors, the presence of brefeldin A blocked maturation at the germinal vesicle breakdown stage before the assembly of the metaphase-I spindle. These results suggest that progression of murine oocyte maturation may require functional membrane trafficking.

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.

Inhibition of phosphatidylinositol 3-kinase or mitogen-activated protein kinase kinase leads to suppression of p34(cdc2) kinase activity and meiotic progression beyond the meiosis I stage in porcine oocytes surrounded with cumulus cells

In this study, the effects of U0126 that inhibits the activity of mitogen-activated protein (MAP) kinase kinase (MEK), and LY294002, which is a phosphatidylinositol (PI) 3-kinase inhibitor, on meiotic progression beyond the metaphase I (MI) stage in porcine oocytes were examined. Cumulus-oocyte complexes (COCs) were cultured for 22 h with 50 microM LY294002 or 10 microM U0126 following cultivation for the initial 22 h. MAP kinase activity in oocytes cultured with LY294002 or U0126 was significantly lower than that in control oocytes cultured for up to 44 h. U0126 and LY294002 significantly decreased p34(cdc2) kinase activity and the proportion of oocytes reaching the MII stage compared to those in control oocytes. Oocytes denuded after COCs had been cultured for 22 h were cultured further for 22 h with U0126 or LY294002. In the denuded oocytes, U0126 suppressed MAP kinase activity, p34(cdc2) kinase activity, and meiotic progression to the MII stage; however, LY294002 did not significantly affect the activity of these kinases and meiotic progression. These results suggest that increasing MAP kinase activity in oocytes via the PI 3-kinase signaling pathway in cumulus cells is involved in the stimulation of maturation promoting factor, leading to meiotic progression beyond the MI to MII stage in porcine oocytes.

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.

Evidence that protein kinase C (PKC) participates in the meiosis I to meiosis II transition in mouse oocytes

Oocytes from LTXBO mice exhibit a delayed entry into anaphase I and frequently enter interphase after the first meiotic division. This unique oocyte model was used to test the hypothesis that protein kinase C (PKC) may regulate the meiosis I-to-meiosis II transition. PKC activity was detected in LTXBO oocytes at prophase I and increased with meiotic maturation, with the highest (P < 0.05) activity observed at late metaphase I (MI). Treatment of late MI-stage oocytes with the PKC inhibitor, bisindolylmaleimide I (BIM), transiently reduced (P < 0.05) M-phase-promoting factor (MPF) activity and promoted (P < 0.05) progression to metaphase II (MII), while mitogen-activated protein kinase (MAPK) activity remained elevated during the MI-to-MII transition. Confocal microscopy analysis of LTXBO oocytes during this transition showed PKC-delta associated with the meiotic spindle and then with the chromosomes at MII. Inhibition of PKC activity also prevented untimely entry into interphase, but only when PKC activity was reduced in oocytes before the progression to MII and thus indicates that the transition into interphase is directly associated with the delayed triggering of anaphase I. Moreover, the defect(s) that initiate activation occur upstream of MAPK, as suppression of PKC activity failed to prevent activation by Mos(tm1Ev)/ Mos(tm1Ev) LTXBO oocytes expressing no detectable MAPK activity. In summary, PKC participates in the regulatory mechanisms that delay entry into anaphase I in LTXBO oocytes, and the disruption promotes untimely entry into interphase. Thus, loss of regulatory control over PKC activity during oocyte maturation disrupts the critical MI-to-MII transition, leading to a precocious exit from meiosis.

Differential gene expression in fully-grown oocytes between gynogenetic and gonochoristic crucian carps

Silver crucian carp (Carassius auratus gibelio) is a unique triploid bisexual species that can reproduce by gynogenesis. As all other gynogenetic animals, it keeps its chromosome integrity by inhibiting the first meiosis division (no extrusion of the first pole body). To understand the molecular events governing this reproduction mode, suppression subtractive hybridization was used to identify the genes differentially expressed in fully-grown oocytes of the gynogenetic and gonochoristic crucian carp (gyno-carp and gono-carp). From two specific subtractive cDNA libraries, the clones screened out by dot blots and virtual Northern blots were chosen to clone full-length cDNA by RACE. Four differentially expressed genes were obtained. Two are novel genes and are expressed specifically in the oocytes. The gyno-carp stores much more mRNA of cyclin A2, a new member of the fish A-type cyclin gene, in its fully-grown oocyte than in the gono-carp. The last gene is histone H2A. The histone H2As of these two closely related crucian carps are quite different in the C-terminus. Preliminary characterization of the four genes has been analyzed by nucleotide and deduced amino acid sequence and Northern analysis.

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.

Effects of wortmannin on the kinetics of GVBD and the activities of the maturation-promoting factor and mitogen-activated protein kinase during bovine oocyte maturation in vitro

The present study was conducted with the objective of examining the effect of wortmanin, a specific PI 3-kinase inhibitor, on the kinetic of GVBD, and on the activities of the maturation-promoting factor (MPF) and mitogen-activated protein (MAP) kinase during bovine oocyte maturation. The time sequence for GVBD was not different between oocytes cultured with or without wortmannin. Most of the cultured oocytes were at the filamentous bivalents stage after 4 h of culture. Six hours after the start of culture, most of the oocytes possessed germinal vesicles with condensed bivalent, and by 10 h of culture nearly all of the cultured oocytes underwent GVBD. A gradual increase in MPF activity until 12 h of culture was observed in the presence and absence of wortmannin. A sharp decrease in MPF activity in oocytes cultured without wortmannin treatment was recorded at 14 h of culture. Thereafter, MPF regained activity, reaching a maximum level at 20 to 24 h of culture. For oocytes cultured with wortmannin, no decline in the activity of MPF was observed during the interval from 12 to 24 h of culture. For these oocytes the MPF activity remained nearly stable during this transition until the end of incubation. The presence of wortmannin in the maturation medium did not alter MAP kinase activity. Taken together, these observations indicate that inhibition of PI 3-kinase does not modulate the time sequence of GVBD or the pattern of MAP kinase activity in bovine oocytes. However, PI 3-kinase might be one of the molecules that regulate the sharp reduction in the activity of MPF during the MI/MII transition.

Analysis of the mechanism(s) of metaphase I-arrest in strain LT mouse oocytes: delay in the acquisition of competence to undergo the metaphase I/anaphase transition

Fully grown oocytes of most laboratory mice progress without interruption from the germinal vesicle (GV) stage to metaphase II, where meiosis is arrested until fertilization. In contrast, many oocytes of strain LT mice arrest precociously at metaphase I and often undergo subsequent spontaneous parthenogenetic activation. Cytostatic factor (CSF), which prevents the degradation of cyclin B and maintains high maturation-promoting factor (MPF) activity, is required for maintenance of metaphase I-arrest in LT oocytes, similar to its requirement for maintaining metaphase II-arrest in normal oocytes. However, CSF does not instigate metaphase I-arrest since a temporary metaphase I-arrest occurs in MOS-null LT oocytes. This paper addresses the mechanism(s) that may instigate metaphase I-arrest and tests the hypothesis that there may be one or more defects in LT oocytes that delay their acquisition of competence to trigger the cascade of processes that normally drive entry into and progression through anaphase I. To test this hypothesis, MPF activity was artificially abrogated by treating oocytes with a general protein kinase inhibitor, 6-DMAP, at various times during the progression of meiosis I. This allowed a comparison of the time at which LT and normal oocytes become competent to undergo the metaphase I/anaphase transition even if oocytes were arrested at metaphase I when 6-DMAP-treatment was begun. There were no differences between LT and control oocytes in the kinetics of MPF suppression by 6-DMAP. However, it was found that LT oocytes do not acquire competence to undergo the metaphase I/anaphase transition in response to 6-DMAP until 50-60 min after normal oocytes. A similar delay was observed in strain CX8-4 oocytes, which also have a high incidence of metaphase I-arrest, but not in strain CX8-11 oocytes, which exhibit a low incidence of metaphase I-arrest. MOS-null LT oocytes also exhibit a delay in acquisition of competence to undergo the metaphase I/anaphase transition. Thus, a delay in competence to undergo the metaphase I/anaphase transition in response to 6-DMAP-treatment correlates with metaphase I-arrest. It is therefore hypothesized that the observed delay in acquisition of competence to enter anaphase I may instigate the sustained metaphase I-arrest in LT oocytes by allowing CSF activity to rise to a level that prevents cyclin B degradation and maintains high MPF activity before anaphase can be initiated by normal triggering mechanisms.

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.