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SummaryAurora-A kinase (AURKA), a member of the serine/threonine protein kinase family, is involved in multiple steps of mitotic progression. It regulates centrosome maturation, mitotic spindle formation, and cytokinesis. While studied extensively in somatic cells, little information is known about AURKA in the early cleavage mouse embryo with respect to acentrosomal spindle assembly. In vitro experiments in which AURKA was inactivated with specific inhibitor MLN8237 during the early stages of embryogenesis documented gradual arrest in the cleavage ability of the mouse embryo. In the AURKA-inhibited 1-cell embryos, spindle formation and anaphase onset were delayed and chromosome segregation was defective. AURKA inhibition increased apoptosis during early embryonic development. In conclusion these data suggest that AURKA is essential for the correct chromosome segregation in the first mitosis as a prerequisite for normal later development after first cleavage.

SUMMARY Polo-like kinase 1 (PLK1), a member of the serine/threonine protein kinases family, is involved in multiple steps of mitotic progression. It regulates centrosome maturation, mitotic spindle formation, and cytokinesis. While studied extensively in somatic cells, little is known about PLK1 activities in the mammalian preimplantation embryo. We examined the role of PLK1 in the one-cell mouse embryo. Western blotting showed that the PLK1 protein content increased significantly during the S-phase of the one-cell stage and declined during the first mitotic division. Activation of PLK1 preceded nuclear envelope breakdown (NEBD) in both pronuclei at the entry to first embryo mitosis. Immunofluorescence revealed the presence of phosphorylated, active PLK1 (pThr210-PLK1) in both male and female pronuclei, and in the microtubule-organizing centers (MTOCs) shortly before NEBD. During the first mitotic metaphase, pThr210-PLK1 accumulated at the spindle poles and was also associated with condensed chromosomes. Inhibition of PLK1 activity with a specific PLK1 inhibitor, BI 2536, at the one-cell stage induced the formation of a bipolar spindle that displayed disordered microtubular arrangements and dislocated, condensed chromosomes. Although such embryos entered mitosis, they did not complete mitosis and arrested at metaphase. Time-lapse recording revealed progressive misalignment of condensed chromosomes during first mitotic metaphase. These data indicate that PLK1 activity is not essential for entry into first mitosis, but is required for the events leading up to metaphase-anaphase transition in the one-cell mouse embryo. Mol. Reprod. Dev. 80: ?–?, 2013. © 2013 Wiley Periodicals, Inc.

SummaryThe cell-cycle regulators that control meiotic divisions also regulate the events that accompany the oocyte-to-zygote transition. Thus, the meiotic machinery functions as an internal pacemaker that propels the oocyte toward embryogenesis. The preimplantation embryo expresses a number of receptors that are important for initial activity of the phosphatidylinositol 3-kinase–protein kinase B (PI3K-Akt/PKB) pathway. The complete PI3K-Akt/PKB-CDK1 cascade is implicated as a key regulator of a number of cellular functions. Selective inhibition of protein kinase B (Akt/PKB) with inhibitor SH6 and cyclin-dependent kinase 1 (CDK1) with inhibitor roscovitine arrest development of the 1-cell preimplantation mouse embryo before entry into the first mitosis. The pronuclei of these inhibited embryos migrate to one another, but do not progress to pronuclei envelope breakdown and pronuclear fusion running immediately before the onset of mitosis. SH6-treated 1-cell mouse embryos showed a high occurrence of apoptosis features (nuclear fragmentation, positive terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL), active caspase-3 in both cytoplasm and nucleoplasm). In the Akt/PKB-inhibited embryos, the active phosphorylated form Ser473Akt/PKB was not detected in pronuclear areas when compared with inhibitor-free controls. Although CDK1-inhibited 1-cell embryos also failed to enter into the first mitosis, the presence of apoptotic cell death features was not observed. In the roscovitine-treated embryos, Ser473Akt/PKB was detected in the pronuclei independently of CDK1 activity. We conclude that Akt/PKB plays an important role during entry of the 1-cell mouse embryo into the first mitosis, and probably functions as a relay in the cell-cycle stage. We assume that Akt/PKB is the primary target responsible for mediating anti-apoptotic signals in the 1-cell mouse embryo.

Aurora kinase A (AURKA), which is a centrosome-localized serine/threonine kinase crucial for cell cycle control, is critically involved in centrosome maturation and spindle assembly in somatic cells. Active T288 phosphorylated AURKA localizes to the centrosome in the late G(2) and also spreads to the minus ends of mitotic spindle microtubules. AURKA activates centrosomal CDC25B and recruits cyclin B1 to centrosomes. We report here functions for AURKA in meiotic maturation of mouse oocytes, which is a model system to study the G(2) to M transition. Whereas AURKA is present throughout the entire GV-stage oocyte with a clear accumulation on microtubule organizing centers (MTOC), active AURKA becomes entirely localized to MTOCs shortly before germinal vesicle breakdown. In contrast to somatic cells in which active AURKA is present at the centrosomes and minus ends of microtubules, active AURKA is mainly located on MTOCs at metaphase I (MI) in oocytes. Inhibitor studies using Roscovitine (CDK1 inhibitor), LY-294002 (PI3K inhibitor) and SH-6 (PKB inhibitor) reveal that activation of AURKA localized on MTOCs is independent on PI3K-PKB and CDK1 signaling pathways and MOTC amplification is observed in roscovitine- and SH-6-treated oocytes that fail to undergo nuclear envelope breakdown. Moreover, microinjection of Aurka mRNA into GV-stage oocytes cultured in 3-isobutyl-1-methyl xanthine (IBMX)-containing medium to prevent maturation also results in MOTC amplification in the absence of CDK1 activation. Overexpression of AURKA also leads to formation of an abnormal MI spindle, whereas RNAi-mediated reduction of AURKA interferes with resumption of meiosis and spindle assembly. Results of these experiments indicate that AURKA is a critical MTOC-associated component involved in resumption of meiosis, MTOC multiplication, proper spindle formation and the metaphase I-metaphase II transition.

Because low levels of DNA double strand breaks (DSBs) appear not to activate the ATM-mediated prophase I checkpoint in full-grown oocytes, there may exist mechanisms to protect chromosome integrity during meiotic maturation. Using live imaging we demonstrate that low levels of DSBs induced by the radiomimetic drug Neocarzinostatin (NCS) increase the incidence of chromosome fragments and lagging chromosomes but do not lead to APC/C activation and anaphase onset delay. The number of DSBs, represented by γH2AX foci, significantly decreases between prophase I and metaphase II in both control and NCS-treated oocytes. Transient treatment with NCS increases >2-fold the number of DSBs in prophase I oocytes, but less than 30% of these oocytes enter anaphase with segregation errors. MRE11, but not ATM, is essential to detect DSBs in prophase I and is involved in H2AX phosphorylation during metaphase I. Inhibiting MRE11 by mirin during meiotic maturation results in anaphase bridges and also increases the number of γH2AX foci in metaphase II. Compromised DNA integrity in mirin-treated oocytes indicates a role for MRE11 in chromosome integrity during meiotic maturation.

Nuclear DNA helicase II (NDH II) is the bovine homolog of human RNA helicase A. The aim of this study was to compare NDH II localization between somatic cells (bovine embryonal fibroblasts) and female germ cells (oocytes), with the main focus on the dynamic changes in the redistribution of NDH II during the growth phase of the bovine oocytes. The fine granular staining of NDH II was spread in the whole nucleoplasm of fibroblasts, excluding the reticulated nucleoli. In contrast, the large reticulated nucleoli of the growing oocytes isolated from early antral follicles exhibited strong positivity for NDH II together with the immunostaining signals of upstream binding factor (UBF) and RNA polymerase I subunit (PAF53), documenting the high synthetic activity of these nucleoli. At the time of termination of oocyte growth, NDH II was preferentially located at the nucleolar periphery together with proteins of fibrillar centres. In fully grown oocytes, NDH II was still present in the thin periphery shell around the compact nucleolar core. The semiquantitative RT-PCR revealed that the average signal of NDH II mRNA in fully grown oocytes was only at 40% level in comparison with growing oocytes. Western blot analysis further confirmed that a 140 kD NDH II protein was abundant in growing oocytes, while the signal was substantially weaker in fully grown oocytes. The significant decrease in NDH II gene expression and in NDH II mRNA translation correlates with a termination of the oocyte growth. Altogether, the results demonstrate that NDH II expression parallels the activity of ribosomal RNA biosynthesis in the bovine growing oocytes.

The aim of this study was to describe the dynamic changes in the localization of the key nucleolar protein markers, fibrillarin, B23/nucleophosmin, C23/nucleolin, protein Nopp140, during the final stages of bovine oocyte growth. All these proteins were present in the large reticulated nucleoli of oocytes from the small-size category follicles ( 3-mm follicles), all studied nucleolar proteins were detected in the small compact nucleoli. The cap structure, attached to the compact nucleolus surface, was positive for UBF and PAF53 (subunit of RNA polymerase I). The UBF-positive cap showed a close structural association with p130. It is concluded that, during the process of oocyte nucleolus compaction, UBF and PAF53, proteins involved in the rDNA transcription, are segregated from fibrillarin and Nopp140, proteins essential for early steps of pre-rRNA processing. The observed changes may reflect the transition from pre-rRNA synthesis to pre-rRNA processing as an analysis of the relative abundance of the developmentally important gene transcripts confirmed. In addition, discovered structural association between UBF and p130 suggests a role for pocket proteins in ribosomal gene silencing in mammalian oocytes.

Apoptosis may occur in early embryos in which the execution of essential developmental events has failed. Thus the initiation of the apoptotic mechanism may be related to activation of the embryonic genome. In this way, developmentally incompetent cells or whole embryos are eliminated. It is likely that some link exists between failed resumption of rRNA synthesis and the incidence of apoptosis in cleaving embryos. In this context, decreased developmental potential in cleaving nucleotransferred embryos is consistent with cell loss, and very likely due to programmed cell death. The effects of apoptosis inducers on cleaving embryos have not been characterised in comparable detail to that in the case of somatic cells. Early embryos provide a very good model for study of these processes because of the specificity of rRNA transcription resumption after fertilization. In our experiments three apoptosis inducers (staurosporin 10 mM, actinomycin D 0.05 mg/ml and camptothecin 0.1 mg/ml) were used in a culture medium for 15 h at the 4-cell stage (day 2) of mouse embryos, followed by further development in a pure culture medium until fixation on days 3, 4 and 5. In staurosporin-induced embryos, light microscopy immunostaining of nucleolar proteins (fibrillarin, Nopp140, protein B23) did not reveal changes in nucleolar morphology on day 3. On days 4 and 5, more compact (roundish) nucleoli (in comparison with controls) were observed. The embryos treated with camptothecin displayed a similar staining pattern to those with staurosporin at each day. In actinomycin-D-treated embryos, marked changes in nucleolar appearance were visible as early as day 3. These changes in nucleolar morphology consisted of loss of the reticulation appearance and fragmentation of nucleoli. In addition to nucleolar changes, significantly decreased cell proliferation was observed. The induced embryos did not reach the blastocyst stage. The number of blastomeres was decreased, and staining with Hoechst 33342 revealed a significant percentage of apoptotic nuclei (condensed/fragmented nuclei) from day 4.

Polo-like kinase 1 (PLK1) orchestrates multiple events of cell division. Although PLK1 function has been intensively studied in centriole-containing and rapidly cycling somatic cells, much less is known about its function in the meiotic divisions of mammalian oocytes, which arrest for a long period of time in prophase before meiotic resumption and lack centrioles for spindle assembly. Here, using specific small molecule inhibition combined with live mouse oocyte imaging, we comprehensively characterize meiotic PLK1’s functions. We show that PLK1 becomes activated at meiotic resumption on microtubule organizing centers (MTOCs) and later at kinetochores. PLK1 is required for efficient meiotic resumption by promoting nuclear envelope breakdown. PLK1 is also needed to recruit centrosomal proteins to acentriolar MTOCs to promote normal spindle formation, as well as for stable kinetochore-microtubule attachment. Consequently, PLK1 inhibition leads to metaphase I arrest with misaligned chromosomes activating the spindle assembly checkpoint (SAC). Unlike in mitosis, the metaphase I arrest is not bypassed by the inactivation of the SAC. We show that PLK1 is required for the full activation of the anaphase promoting complex/cyclosome (APC/C) by promoting the degradation of the APC/C inhibitor EMI1 and is therefore essential for entry into anaphase I. Moreover, our data suggest that PLK1 is required for proper chromosome segregation and the maintenance of chromosome condensation during the meiosis I-II transition, independently of the APC/C. Thus, our results define the meiotic roles of PLK1 in oocytes and reveal interesting differential requirements of PLK1 between mitosis and oocyte meiosis in mammals.

Stathmin is a 19 kDa cytosolic phosphoprotein, proposed to act as a relay integrating diverse intracellular signaling pathways involved in regulation of cell proliferation, differentiation, and function. To gain further information about its significance during early development, we analyzed stathmin expression and subcellular localization in mouse oocytes and preimplantation embryos. RT-PCR analysis revealed a low expression of stathmin mRNA in unfertilized oocytes and a higher expression at the blastocyst stage. A fine cytoplasmic punctuate fluorescent immunoreactive stathmin pattern was detected in the oocyte, while it evolved toward an increasingly speckled pattern in the two-cell and later four- to eight-cell embryo, with even larger speckles at the morula stage. In blastocysts, stathmin immunoreactivity was fine and intense in inner cell mass cells, whereas it was low and variable in trophectodermal cells. Electron microscopic analysis allowed visualization with more detail of two types of stathmin immunolocalization: small clusters in the cytoplasm of oocytes and blastocyst cells, together with loosely arranged clusters around the outer membrane of cytoplasmic vesicles, corresponding to the immunofluorescent speckles in embryos until the morula stage. In conclusion, it appears from our results that maternal stathmin is accumulated in the oocyte and is relocalized within the oocyte and early preimplantation embryonic cell cytoplasm to interact with specific cytoplasmic membrane formations. Probably newly synthesized, embryonic stathmin is expressed in the blastocyst, where it is localized more uniformly in the cytoplasm mostly of inner cell mass (ICM) cells. These expression and localization patterns are probably related to the particular roles of stathmin at the successive steps of oocyte maturation and early embryonic development. They further support the proposed physiologic importance of stathmin in essential biologic regulation.

The structure-function relationships of the nucleolar substructures were studied in preimplantation rabbit embryos, where nucleologenesis is extending over the first four cell cycles and may not be synchronous in each blastomere. Immunocytochemical methods using light and electron microscopy were applied for protein and RNA localization as well as nick translation and terminal deoxynucleotidyl transferase techniques for DNA detection. DNA was gradually associated with the periphery of the compact nucleolar precursor bodies (NPBs) but was never found inside NPBs at the four-cell stage. In 16-cell embryos, some NPBs displayed a reticulated periphery forming the branching network of the dense fibrillar component (DFC) surrounding the "residual body" (remnant of NPB) in the process of activation. At the 32-cell stage, fully reticulated nucleoli were observed in each blastomere. DNA was then associated with the DFC of reticulated nucleoli. RNA was first detected at the 16 cell-stage in close contact with the DFC as well as inside the "residual body" which was not immunolabeled with the DNA antibodies used. When observed by light microscopy, fibrillarin, nucleolin, and protein B23 displayed a changing distribution pattern during nucleologenesis. At early stages (up to the 16-cell stage), small dot- and spot-like structures were distributed within the whole nuclei. In 16-cell embryos, these proteins started to accumulate in an irregular thin layer around the NPBs in the process of activation. The reorganization process described may be in relation with the redistribution of chromatin and nuclear/nucleolar matrix components during the activation of rDNA transcription localized in the NPB shell. In conclusion, nucleologenesis is only achieved at the fourth cell cycle in the cleaving rabbit embryo at the corresponding time when the first detectable nucleolus-associated RNA is detectable. Our results show a good correlation between the establishment of structure and function.

Aurora kinase A (AURKA) is an important mitotic kinase involved in the G2/M transition, centrosome maturation and separation, and spindle formation in somatic cells. We used transgenic models that specifically overexpress in mouse oocytes either wild-type (WT-AURKA) or a catalytically inactive (kinase-dead) (KD-AURKA) AURKA to gain new insights regarding the role of AURKA during oocyte maturation. AURKA activation occurs shortly after hCG administration that initiates maturation in vivo. Although AURKA activity is increased in WT-AURKA oocytes, resumption of meiosis is not observed in the absence of hCG administration. Control oocytes contain one to three microtubule organizing centers (MTOCs; centrosome equivalent) at prophase I. At the time of germinal vesicle breakdown (GVBD), the first visible marker of resumption of meiosis, the MTOC number increases. In WT-AURKA oocytes, the increase in MTOC number occurs prematurely but transiently without GVBD, whereas the increase in MTOC number does not occur in control and KD-AURKA oocytes. AURKA activation is biphasic with the initial activation not requiring CDC25B-CDK1 activity, whereas full activation, which is essential for the increase in MTOCs number, depends on CDK1 activity. AURKA activity also influences spindle length and regulates, independent of its protein kinase activity, the amount of MTOC associated with gamma-tubulin. Both WT-AURKA and KD-AURKA transgenic mice have normal fertility during first 6 mo of life. These results suggest that although AURKA is not a trigger kinase for G2/M transition in mouse oocytes, it regulates MTOC number and spindle length, and, independent of its protein kinase activity, gamma-tubulin recruitment to MTOCs.

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 Ser473 is associated with nuclear membrane and centrosome, whereas PKB phosphorylated on Thr308 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.

Insulin-like growth factor I (IGF-I) has been shown to promote mammalian early embryo development. Increased cell division or decreased cell death have been proposed as two main possible mechanisms in its effect. Here we examine the nature of this promoting effect in a model situation. Camptothecin (0.01 microg/ml) and actimomycin D (0.005 microg/ml) were used to induce apoptosis. Four-cell mouse embryos were cultured in vitro to blastocyst stage in the temporary (15 h) presence or absence of apoptotic inductors and in the permanent presence or absence of IGF-I (100 ng/ml). Embryos were assessed by morphological triple staining (Hoechst 33342, propidium iodide, Calcein AM) and comet assay on Day 5, 120 h after administration of hCG. The number of nuclei, the blastocyst formation, the proportion of embryos containing fragmented DNA and the percentage of apoptotic and secondary necrotic nuclei were assessed. Both inductors of apoptosis significantly increased the percentage of apoptotic and secondary necrotic cells and reduced total cell counts (camptothecin, P0.001; actinomycin D, P0.001). When IGF-I was added to the culture medium in the presence of an apoptosis inductor, apoptosis incidence was significantly decreased (P0.001). The addition of IGF-I into control samples also decreased the percentage of apoptotic and secondary necrotic cells. In contrast, IGF-I addition had no significant influence on embryo development (P0.05). Our data suggest a primary role for IGF-I as an apoptotic survival factor in mouse preimplantation embryos in specific conditions.

The occurrence of apoptosis in mouse preimplantation embryos was analyzed using DNA staining (Hoechst 33342, PI) for the visualization of nuclear changes and by the comet assay, a single-cell gel electrophoresis assay, modified for the analysis of blastocysts. Mouse preimplantation embryos isolated 56 h after superovulation were cultured in vitro for 64 h. Apoptosis was induced by treatment with camptothecin and actinomycin D during the first 15 h of culture. After culture in vitro, a number of embryos were stained and analyzed using morphological criteria. The remaining embryos were examined using the comet assay for the detection of DNA fragmentation. The proportion of damaged embryos in experimental groups, in comparison to controls, was dependent on the dose of apoptosis inductor. At high doses (camptothecin, microg/ml and actinomycin D, 0.05 microg/ml) over 90% (chi-square test, P

The aim of the present experiment was to analyze the chronology of pronucleus development and DNA synthesis, as well as the ultrastructure of intranuclear bodies, in bovine zygotes produced in vitro. Bovine oocytes were matured and fertilized in vitro, and sperm penetration and pronucleus development were examined. DNA synthesis was investigated by sequential incubation with [3H]- and [14C]thymidine followed by autoradiography on semithin sections. Ultrathin sections for transmission electron microscopy were prepared from the same zygotes. Sperm penetration was noted for the first time at 4 hr after in vitro insemination and reached a maximum at 6 hr. Pronucleus formation was initiated at 4 hr, and up to at least 11 hr the maternal pronucleus was more developed than its paternal counterpart. DNA synthesis was initiated at 14–15 hr, and the S-phase lasted for 8–10 hr. The most prominent ultrastructural entities of the pronuclei were the nucleolus precursor bodies (NPBs). During the S- and G2-phases, the NPBs spatially associated with clusters of interchromatin-like granules. The two components were firmly attached to each other by an electron-dense reticulum. During the late G2-phase, the NPBs were apparently detached from the interchromatin-like granules and the electron-dense reticulum again. The interaction between the intranuclear bodies and granules appears to be comparable with the situation previously described for in vivo-produced bovine zygotes (J Laurincik et al., Mol Reprod Dev 43:62–69, 1996), except for the lack of vacuolization of the NPBs during the S-phase in vitro. Mol. Reprod. Dev. 50:192–199, 1998. © 1998 Wiley-Liss, Inc.

We have studied the effect of moderately impaired maternal insulin secretion on oocyte chromosomal constitution, fertilization and zygote DNA synthesis. Female mice were injected with a single dose of streptozotocin (65 mg/kg) 14 days before fertilization/ovulation. Zygotes/oocytes were recovered from control and subdiabetic mice on day 1 of pregnancy. Compared with control animals, subdiabetic females showed a significant difference in the proportion of zygotes/oocytes. The subdiabetic mothers had a lower percentage of zygotes and a higher percentage of unfertilized and degenerated oocytes in comparison with control animals. The investigation of [3H]thymidine incorporation did not show any influence of the maternal subdiabetes on the initial zygote DNA synthesis. An analysis of the ovulated oocytes at the metaphase II stage isolated from subdiabetic mice did not reveal increased chromosomal anomalies in comparison with the controls. Control and subdiabetic mothers had a similar percentage of oocytes with a normal haploid set of chromosomes, and the incidence of aneuploidy/diploidy did not differ significantly. These observations suggest that insulin changes in subdiabetic mothers had a deleterious influence on oocyte fertilization in mice, but apparently they did not have any effect on the nuclear events.

The distribution of U1, U2, U4, U5 and U6 small nuclear ribonucleoproteins (snRNPs) and interchromatin granules (IGs) was studied by electron-microscopic immunocytochemistry (EMI) in early 2-cell mouse embryos at the onset of embryonal transcription. The localization of these antigen structures was evaluated with respect to nucleoplasmic ribonucleoprotein (RPN) regions consisting of interchromatin and perichromatin areas. SnRNP structures of maternal origin (labelled with anti-Sm antibody) were widely distributed throughout the nucleoplasm. Specifically labelled IGs were detected by gold particle clusters distributed in the interchromatin regions of the nucleoplasm. Both immunodetections were negative in nucleolar precursor bodies (NPBs). In addition, the labelling of condensed chromatin blocks with anti-DNA antibody showed heterochromatin topology at this developmental stage. Small condensed chromatin blocks were distributed throughout the nucleus and also appeared in association with the NPB rim. The observed status quo represents a transient state of nuclear structure rearrangement.

CDK1 is a pivotal regulator of resumption of meiosis and meiotic maturation of oocytes. CDC25A/B/C are dual-specificity phosphatases and activate cyclin-dependent kinases (CDKs). Although CDC25C is not essential for either mitotic or meiotic cell cycle regulation, CDC25B is essential for CDK1 activation during resumption of meiosis. Cdc25a −/− mice are embryonic lethal and therefore a role for CDC25A in meiosis is unknown. We report that activation of CDK1 results in a maturation-associated decrease in the amount of CDC25A protein, but not Cdc25a mRNA, such that little CDC25A is present by metaphase I. In addition, expression of exogenous CDC25A overcomes cAMP-mediated maintenance of meiotic arrest. Microinjection of Gfp-Cdc25a and Gpf-Cdc25b mRNAs constructs reveals that CDC25A is exclusively localized to the nucleus prior to nuclear envelope breakdown (NEBD). In contrast, CDC25B localizes to cytoplasm in GV-intact oocytes and translocates to the nucleus shortly before NEBD. Over-expressing GFP-CDC25A, which compensates for the normal maturation-associated decrease in CDC25A, blocks meiotic maturation at MI. This MI block is characterized by defects in chromosome congression and spindle formation and a transient reduction in both CDK1 and MAPK activities. Lastly, RNAi-mediated reduction of CDC25A results in fewer oocytes resuming meiosis and reaching MII. These data demonstrate that CDC25A behaves differently during female meiosis than during mitosis, and moreover, that CDC25A has a function in resumption of meiosis, MI spindle formation and the MI–MII transition. Thus, both CDC25A and CDC25B are critical for meiotic maturation of oocytes.

Immunolocalization of cyclin/PCNA (proliferating cell nuclear antigen) was performed with monoclonal antibody using immunogold methods on ultrathin cryosections of fertilized mouse eggs. Immunolabeling in pronuclei was checked 20, 22, 24 and 26 h after HCG injection. A relation between onset of pronuclei migration (early S-phase) and appearance of colloidal particle clusters was found. Afterwards, (mid S-phase) the increase of labelling and the localization of cyclin/PCNA were found throughout the pronuclei, except in the nucleolar bodies. Lower labelling appeared at the time of close reciprocal pronuclei contact (late S-phase). It is concluded that bulk and distribution of cyclin/PCNA in pronuclei is closely related to the progression of first interphase after fertilization.