Polar Body Emission Research Articles

KAT8 functions in redox homeostasis and mitochondrial dynamics during mouse oocyte meiosis progression.

As a histone acetyltransferase, lysine acetyltransferase 8 (KAT8) participates in diverse biological processes. However, the effect of KAT8 on oocyte maturation in mice remains unclear. In this study, we found that mouse oocytes overexpressing Kat8-OE induced maturation failure manifested reduced rates of GVBD and first polar body emission. In addition, immunostaining results revealed that Kat8 overexpressing oocytes showed inappropriate mitochondrial distribution patterns, overproduction of reactive oxygen species (ROS), accumulation of phosphorylated γH2AX, hyperacetylation of α-tubulin, and severely disrupted spindle/chromosome organization. Moreover, we revealed that Kat8 overexpression induced a decline in SOD1 proteins and KAT8's interaction with SOD1 in mouse ovaries via immunoprecipitation. Western blotting data confirmed that Kat8-OE induced downregulation of SOD1 expression, which is a key factor for the decline of oocyte quality in advanced maternal age. Also, the injection of Myc-Sod1 cRNA could partially rescue maternal age-induced meiotic defects in oocytes. In conclusion, our data demonstrated that high level of KAT8 inhibited SOD1 activity, which in turn induced defects of mitochondrial dynamics, imbalance of redox homeostasis, and spindle/chromosome disorganization during mouse oocyte maturation.

A localized calcium transient and polar body abscission

ABSTRACT Polar body emission is a special form of cytokinesis in oocyte meiosis that ensures the correct number of chromosomes in reproduction-competent eggs. The molecular mechanism of the last step, polar body abscission, is poorly understood. While it has been proposed that Ca2+ signaling plays important roles in embryonic cytokinesis, to date transient increases in intracellular free Ca2+ have been difficult to document in oocyte meiosis except for the global Ca2+ wave induced by sperm at fertilization. Here, we find that microinjection of the calcium chelator dibromo-BAPTA inhibits polar body abscission in Xenopus laevis oocytes. Using a novel, microtubule-targeted ratio-metric calcium sensor, we detected a calcium transient that is focused at the contractile ring-associated plasma membrane and which occurred after anaphase and constriction of the contractile ring but prior to abscission. This calcium transient was confirmed by mobile calcium probes. Further, the Ca2+-sensitive protein kinase Cβ C2 domain transiently translocated to the contractile ring-associated membrane simultaneously with the calcium transient. Collectively, these results demonstrate that a calcium transient, apparently originating at the contractile ring-associated plasma membrane, promotes polar body abscission.

Analysis of A-kinase Anchoring Protein 13 (AKAP13) in Peri-Ovulatory Oocyte Maturation [A328

INTRODUCTION: Arrest of oocytes at prophase I of meiosis is crucial for fertility and is maintained by Protein Kinase A (PKA). PKA activity is regulated by A-kinase anchoring proteins (AKAPs) that bind the regulatory subunit of PKA to control enzymatic activity. AKAP13 is expressed in oocytes and granulosa cells of humans and mice. Female mice haploinsufficient for Akap13 showed reduced litter sizes. To determine if AKAP13 in oocytes contributes to female reproductive function, we used an oocyte-specific conditional knockout (cKO) to delete Akap13 in the final stages of oocyte growth. METHODS: Oocytes were collected from mice harboring an oocyte-specific conditional knockout of Akap13 mice, with Cre recombinase expression driven by the Zp3 promoter (Zp3-Cre+, Akap13flox/flox). Akap13 DNA and mRNA were assessed by polymerase chain reaction (PCR) and quantitative PCR (qPCR), respectively. Controls were oocytes harvested from Akap13+/flox mice lacking Cre. Oocytes from control or cKO mice were examined for morphology, meiotic resumption, spindle organization, and spindle positioning at metaphase II. RESULTS: Cre-mediated excision of Akap13 DNA in oocytes was confirmed using PCR. Oocyte mRNA levels of Akap13 in cKO mice were reduced by 90% compared to controls. Live-cell imaging revealed no differences in germinal vessel breakdown, polar body emission, or completion of meiosis I in cKO oocytes as compared to controls. Staining of fixed oocytes revealed that metaphase II spindles and chromosome alignment appeared normal in in vitro-matured cKO oocytes. CONCLUSION: These studies suggest that impairment of female fertility in Akap13 haploinsufficient mice is not caused by impairment of oocyte meiotic maturation.

TMEM150B is dispensable for oocyte maturation and female fertility in mouse

Premature ovarian insufficiency (POI) refers to severe decline of ovary function in females which usually leads to infertility. It has been reported that the TMEM150B gene is mostly associated with age at natural menopause, early menopause and POI, but its role in female reproduction remains unknown. In this study, we found Tmem150b was highly expressed in mouse oocytes, but its deletion had no obvious effect on meiotic maturation of oocytes indicated by first polar body emission and spindle morphology. There were also no obvious differences in follicle development and corpus luteum formation between knockout and wild type mice. Finally, knockout of Tmem150b did not affect female fertility and sexual hormone levels. In summary, our results suggest that TMEM150B is not essential for female fertility in mice.

Resveratrol improves in vitro maturation of oocytes in aged mice and humans

To evaluate the effects of resveratrol on oocyte maturation in aged mice and humans. Experimental laboratory study. University-based reproductive medicine center. A total of 64 women 38-45years of age undergoing intracytoplasmic sperm injection (ICSI) and 48-52-week-old female C57BL/6J mice. Invitro culture in the presence of three different concentrations of resveratrol (0.1, 1.0, and 10μm) or dimethylsulfoxide. Parameters of oocyte nuclear maturation, fertilization, immunofluorescence intensity of mitochondria, and normal morphology of spindle and chromosome of oocytes undergoing invitro maturation (IVM) in aged mice and humans; blastocyst formation and levels of SRIT1, CAT, SOD1, and GPX4 gene expressions in aged mice. Resveratrol at 1.0μm significantly increased first polar body emission rate in oocytes derived from aged mice and humans, and an increased percentage of fertilization and blastocyst formation was observed in aged mice. In addition, immunofluorescence intensity of mitochondria and normal morphology of spindle and chromosome of oocytes undergoing IVM were notably improved compared with control samples in aged mice and human. Furthermore, the use of resveratrol exhibited enhanced expression patterns of SRIT1, CAT, SOD1, and GPX4 in aged mice. Resveratrol induced oocyte maturation and blastocyst formation in aged mice, and improved oocyte maturation and quality was examined in aged humans. In conclusion, 1.0μm resveratrol was the appropriate concentration in IVM medium.

Sodium fluoride disturbs DNA methylation of NNAT and declines oocyte quality by impairing glucose transport in porcine oocytes.

Sodium fluoride (NaF) is used as a medicine to prevent tooth decay; however, excessive NaF could cause a pathological damage to the health. Recent studies showed that NaF impaired mouse oocyte maturation, included of abnormal spindle configuration, actin cap formation, cortical granule-free domain formation, and the following development after fertilization. However, few studies used large animals as models to study the toxicology of NaF on oocytes maturation. We proposed a hypothesis that NaF would affect the nuclear and cytoplasmic maturation of porcine oocytes and DNA methylation pattern of imprinted genes in oocytes. Our results showed that NaF affected cumulus expansion, polar body emission, spindle morphology, cortical granule distribution, early apoptosis, and the following development after parthenogenetic activation during porcine oocyte maturation. Moreover, NaF increased the DNA methylation of NNAT and decreased its expression, which disturbed the glucose transport in oocytes. These results suggest that NaF impairs the porcine oocytes maturation epigenetically, which provides a new toxicological mechanism of NaF on the oocyte maturation. Environ. Mol. Mutagen. 59:223-233, 2018. © 2017 Wiley Periodicals, Inc.

Plk1 is essential for proper chromosome segregation during meiosis I/meiosis II transition in pig oocytes

BackgroundPolo-like kinase 1 (Plk1), as a characteristic regulator in meiosis, organizes multiple biological events of cell division. Although Plk1 has been implicated in various functions in somatic cell mitotic processes, considerably less is known regarding its function during the transition from metaphase I (MI) to metaphase II (MII) stage in oocyte meiotic progression.MethodsIn this study, the possible role of Plk1 during the MI-to-MII stage transition in pig oocytes was addressed. Initially, the spatiotemporal expression and subcellular localization pattern of Plk1 were revealed in pig oocytes from MI to MII stage using indirect immunofluorescence and confocal microscopy imaging techniques combined with western blot analyses. Moreover, a highly selective Plk1 inhibitor, GSK461364, was used to determine the potential role of Plk1 during this MI-to-MII transition progression.ResultsUpon expression, Plk1 exhibited a specific dynamic intracellular localization, and co-localization of Plk1 with α-tubulin was revealed in the meiotic spindle of pig oocyte during the transition from MI to MII stage. GSK461364 treatment significantly blocked the first polar body (pbI) emission in a dose-dependent manner and resulted in a failure of meiotic maturation, with a larger percentage of the GSK461364-treated oocytes arresting in the anaphase-telophase I (ATI) stage. Further subcellular structure examination results showed that inhibition of Plk1 with GSK461364 had no visible effect on spindle assembly but caused a significantly higher proportion of the treated oocytes to have obvious defects in homologous chromosome segregation at ATI stage.ConclusionsThus, these results indicate that Plk1 plays an essential role during the meiosis I/meiosis II transition in porcine oocytes, and the regulation is associated with Plk1’s effects on homologous chromosome segregation in the ATI stage.

Treatment of allicin improves maturation of immature oocytes and subsequent developmental ability of preimplantation embryos.

Allicin (AL) regulates the cellular redox, proliferation, viability, and cell cycle of different cells against extracellular-derived stress. This study investigated the effects of allicin treatment on porcine oocyte maturation and developmental competence. Porcine oocytes were cultured in medium supplemented with 0 (control), 0.01, 0.1, 1, 10 or 100 μM AL, respectively, during in vitro maturation (IVM). The rate of polar body emission was higher in the 0.1 AL-treated group (74.5% ± 2.3%) than in the control (68.0% ± 2.6%) (P < 0.1). After parthenogenetic activation, the rates of cleavage and blastocyst formation were significantly higher in the 0.1 AL-treated group than in the control (P < 0.05). The reactive oxygen species level at metaphase II did not significantly differ among all groups. In matured oocytes, the expression of both BAK and CASP3, and BIRC5 was significantly lower and higher, respectively, in the 0.1 AL-treated group than in the control. Similarly, the expression of BMP15 and CCNB1, and the activity of phospho-p44/42 mitogen-activated protein kinase (MAPK), significantly increased. These results indicate that supplementation of oocyte maturation medium with allicin during IVM improves the maturation of oocytes and the subsequent developmental competence of porcine oocytes.

Human exhaled air can efficiently support in vitro maturation of porcine oocytes and subsequent early embryonic development.

Air phase is an indispensable environmental factor affecting oocyte maturation and early embryo development. Human exhaled air was previously proved to be a reliable and inexpensive atmosphere that sustains normal early development of mouse and bovine embryos. However, whether human exhaled air can support in vitro maturation (IVM) of porcine oocytes is not yet known. To evaluate the feasibility of maturing oocytes in human exhaled air, we examined oocyte morphology, BMP15 expression, nuclear and cytoplasmic maturation. We found that cumulus expansion status, expression levels of BMP15 important for cumulus expansion and the rate of first polar body emission were similar among human exhaled air, 5% O2 or 20% O2 in air after IVM of 44 h. Furthermore, the percentage of metaphase II (MII) oocytes showing normal cortical and sub-membranous localization of cortical granules and diffused mitochondrial distribution patterns is comparable among groups. The cleavage, blastocyst rate and total cell number were not apparently different for parthenogenetic activated and somatic cloned embryos derived from MII oocytes matured in three air phases, suggesting oocytes matured in human exhaled air obtain normal developmental competence. Taken together, human exhaled air can efficiently support in vitro maturation of porcine oocytes and subsequent early embryonic development.

Eccentric position of the germinal vesicle and cortical flow during oocyte maturation specify the animal-vegetal axis of ascidian embryos

The animal-vegetal (A-V) axis is already set in unfertilized eggs. It plays crucial roles in coordinating germ-layer formation. However, how the A-V axis is set has not been well studied. In ascidians, unfertilized eggs are already polarized along the axis in terms of cellular components. This polarization occurs during oocyte maturation. Oocytes within the gonad have the germinal vesicle (GV) close to the future animal pole. When the GVs of full-grown oocytes were experimentally translocated to the opposite pole by centrifugal force, every aspect that designates A-V polarity was reversed in the eggs and embryos. This was confirmed by examining the cortical allocation of the meiotic spindle, the position of the polar body emission, the polarized distribution of mitochondria and postplasmic/PEM mRNA, the direction of the cortical flow during oocyte maturation, the cleavage pattern and germ-layer formation during embryogenesis. Therefore, the eccentric position of the GV triggers subsequent polarizing events and establishes the A-V axis in eggs and embryos. We emphasize important roles of the cortical flow. This is the first report in which the A-V axis was experimentally and completely reversed in animal oocytes before fertilization.

Inhibition of Rac1 GTPase activity affects porcine oocyte maturation and early embryo development

Mammalian oocyte asymmetric division relies on the eccentric positioning of the spindle, resulting in the polar body formation. Small signaling G protein Rac1 is a member of GTPases, which regulates a diverse array of cellular events, including the control of cell growth, cytoskeletal reorganization, and the activation of protein kinases. However, effects of Rac1 on the porcine oocyte maturation and early embryo development are not fully understood. In present study we investigated the role of Rac1 in oocyte maturation and embryo cleavage. We first found that Rac1 localized at the cortex of the porcine oocytes, and disrupting the Rac1 activities by treating with NSC 23766 led to the failure of polar body emission. In addition, a majority of treated oocytes exhibited abnormal spindle morphology, indicating that Rac1 may involve into porcine oocyte spindle formation. This might be due to the regulation of Rac1 on MAPK, since p-MAPK expression decreased after NSC 23766 treatments. Moreover, we found that the position of most meiotic spindles in treated oocytes were away from the cortex, indicating the roles of Rac1 on meiotic spindle positioning. Our results also showed that inhibition of Rac1 activity caused the failure of early embryo development. Therefore, our study showed the critical roles of Rac1 GTPase on porcine oocyte maturation and early embryo cleavage.

Spindle function in Xenopus oocytes involves possible nanodomain calcium signaling.

Intracellular calcium transients are a universal phenomenon at fertilization and are required for egg activation, but the exact role of Ca2+ in second-polar-body emission remains unknown. On the other hand, similar calcium transients have not been demonstrated during oocyte maturation, and yet, manipulating intracellular calcium levels interferes with first-polar-body emission in mice and frogs. To determine the precise role of calcium signaling in polar body formation, we used live-cell imaging coupled with temporally precise intracellular calcium buffering. We found that BAPTA-based calcium chelators cause immediate depolymerization of spindle microtubules in meiosis I and meiosis II. Surprisingly, EGTA at similar or higher intracellular concentrations had no effect on spindle function or polar body emission. Using two calcium probes containing permutated GFP and the calcium sensor calmodulin (Lck-GCaMP3 and GCaMP3), we demonstrated enrichment of the probes at the spindle but failed to detect calcium increase during oocyte maturation at the spindle or elsewhere. Finally, endogenous calmodulin was found to colocalize with spindle microtubules throughout all stages of meiosis. Our results-most important, the different sensitivities of the spindle to BAPTA and EGTA-suggest that meiotic spindle function in frog oocytes requires highly localized, or nanodomain, calcium signaling.

166 THE EFFECT OF TEMPERATURE DURING STORAGE OF IN VITRO-MATURED BOVINE OOCYTES IN A HEPES-BUFFERED MEDIUM ON DEVELOPMENTAL COMPETENCE

The objective of this study was to clarify the effect of the temperature during liquid storage in in vitro matured (IVM) bovine oocytes. IVM bovine oocytes were stored in Eppendorf tube containing 1 mL HEPES TCM-199 supplemented with 10% (v/v) new born calf serum at different temperatures (4°C, 15°C, 25°C, and 38.5°C) for 20 h. The developmental rates of stored and not stored (control) oocytes to the blastocyst stage, cell numbers in resultant blastocysts, and fertilization normality were evaluated after in vitro fertilization and in vitro culture. The ATP content, reduced glutathione (GSH) content, and apoptosis rates in oocytes were also determined in stored and control groups. At least 3 replicates were conducted for each experiment. The data were analysed by 1-way ANOVA followed by post hoc Fisher’s protected least significantly difference test. Percentage data were transformed to arc-sine before analysis. All of the storage groups (4, 15, 25, and 38.5°C groups, respectively) showed significantly lower blastocyst developmental rates (8.5, 14.9, 19.3, and 24.5%, respectively) compared with the control group (39.8%; P &lt; 0.05). Within the storage groups, the 25°C and the 38.5°C groups exhibited the greatest rate of blastocyst formation. In contrast, the total cell number of the 38.5°C group was significantly lower than that of control group (P &lt; 0.05), whereas that of the 25°C group was similar with the control group. The frequency of normal emission of the second polar body (2PB) was significantly greater in the control group compared with the storage groups (P &lt; 0.05). The 2PB emission rate was significantly lower in the 38.5°C group compared with the 4°C group (P &lt; 0.05) but not different from those of the 15°C and 25°C storage groups. The percentage of male pronuclear formation in the control group was significantly higher than those in the stored groups (P &lt; 0.05) except for the 25°C group. During storage at 4°C, the ATP content was significantly decreased compared with the control group (1.3 v. 1.7 pmol; P &lt; 0.05); however, in the 25°C and 38.5°C groups, the ATP content (2.0 and 1.9 pmol, respectively) was significantly higher than that in the control group (1.7 pmol; P &lt; 0.05); whereas the 15°C group showed the same ATP level compared with the control group. Storage of oocytes for 20 h reduced the GSH content compared with the control group without storage (P &lt; 0.05); however, there were no significant differences among storage groups. Annexin-V staining revealed increased incidences of early apoptotic oocytes in the 4°C and 15°C groups (P &lt; 0.05) compared with other groups. In conclusion, based on the embryo developmental competence of stored oocytes and quality of resultant blastocysts, 25°C was determined as the most suitable temperature for temporal storage of matured bovine oocytes. The study was supported by the NARO Institute of Livestock and Grassland Science, Japan (N32G4126), and the Royal Golden Jubilee-PhD scholarship (2.B.TS/53/F.2).

The SUMO Protease SENP3 Orchestrates G2-M Transition and Spindle Assembly in Mouse Oocytes.

Oocyte meiosis is a transcription quiescence process and the cell-cycle progression is coordinated by multiple post-translational modifications, including SUMOylation. SENP3 an important deSUMOylation protease has been intensively studied in ribosome biogenesis and oxidative stress. However, the roles of SENP3 in cell-cycle regulation remain enigmatic, particularly for oocyte meiotic maturation. Here, we found that SENP3 co-localized with spindles during oocyte meiosis and silencing of SENP3 severely compromised the M phase entry (germinal vesicle breakdown, GVBD) and first polar body extrusion (PBI). The failure in polar body extrusion was due to the dysfunction of γ-tubulin that caused defective spindle morphogenesis. SENP3 depletion led to mislocalization and a substantial loss of Aurora A (an essential protein for MTOCs localization and spindle dynamics) while irregularly dispersed distribution of Bora (a binding partner and activator of Aurora A) in cytoplasm instead of concentrating at spindles. The SUMO-2/3 but not SUMO-1 conjugates were globally decreased by SENP3 RNAi. Additionally, the spindle assembly checkpoint remained functional upon SENP3 RNAi. Our findings renew the picture of SENP3 function by exploring its role in meiosis resumption, spindle assembly and following polar body emission during mouse oocyte meiotic maturation, which is potentially due to its proteolytic activity that facilitate SUMO-2/3 maturation.

Zinc depletion activates porcine metaphase II oocytes independently of the protein kinase C pathway.

Zinc is an important trace element that regulates several biological functions. This study investigated the role of zinc in the metaphase (M) II arrest of porcine oocytes. N, N, N', N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), a Zn(2+) chelator, was used to deplete free zinc from porcine MII stage oocytes. TPEN treatment significantly (P < 0.01) reduced the zinc content in the cytoplasm. The percentages of oocytes in which second polar body emission and pronuclear formation occurred increased as the concentration of TPEN increased (P < 0.01), and reached 93.64 ± 5.53% and 90.61 ± 9.10%, respectively, following treatment with 10 μM TPEN. Zinc depletion also resulted in cortical granule release and spindle depolarization. Maturation-promoting factor activity, as assessed by examining p34(cdc2) activity, decreased (P < 0.05) following zinc depletion. Following TPEN treatment, embryos developed to the 4-cell stage but failed to reach the blastocyst stage. Zinc release is a common event in protein kinase C (PKC) activation. Therefore, we examined the impact of zinc depletion on phosphorylation of PKC substrates. Phosphorylation of PKC substrates was reduced (P < 0.05) in zinc-depleted oocytes, and this was rescued by phorbol 12-myristate 13-acetate (PMA) treatment. However, treatment of oocytes with both PMA and TPEN did not affect pronuclear formation or second polar body emission. These data are inconsistent with the hypothesis that oocyte activation caused by zinc depletion is mediated by the PKC pathway. This study shows that zinc has a novel role in maintaining MII arrest in porcine oocytes, but this is not mediated by the PKC pathway.

Effects of postmortem interval on mouse ovary oocyte survival and maturation.

To study the time- and temperature-dependent survival of ovarian oocytes collected from postmortem carcass, ICR mice were killed and placed for different periods (0, 1, 2, 4, 6, 8 and 10 h) at different temperatures (25°C, 4°C and 37°C). After preservation, oocyte morphology, germinal vesicle (GV) oocyte number, oocyte meiotic maturation percentage, mitochondrial distribution and intracellular glutathione (GSH) level were evaluated. The results showed no surviving oocytes could be collected by 2h, 6h, and 12 h after carcass preservation at 37°C, 25°C and 4°C, respectively. The number of collected GV oocytes in the ovary deceased as the preservation time lasted at the same temperature. Meanwhile at the same point in time, the ratio of germinal vesicle breakdown (GVBD) and the first polar body emission (PBE) gradually reduced as preservation temperature increased. In addition, the percentage of abnormal mitochondrial distribution in the preserved oocytes was obviously higher than that in the control oocytes, while GSH level was not altered in collected oocytes. Unexpectedly, neither chromosome arrangement nor spindle organization was affected as long as the oocytes from preserved carcasses could complete maturation. These data are helpful for proper use of ovary oocytes from postmortem carcass of valuable individuals.

Effects of DNA damage and short-term spindle disruption on oocyte meiotic maturation

DNA damage has recently been shown to inhibit or delay germinal vesicle breakdown (GVBD) in mouse oocytes, but once meiosis resumes, DNA-damaged oocytes are able to extrude the first polar body. In this study, using porcine oocytes, we showed that DNA damage did not affect GVBD, but inhibited the final stages of maturation, as indicated by failure of polar body emission. Unlike mitotic cells in which chromosome mis-segregation causes DNA double-strand breaks, meiotic mouse oocytes did not show increased DNA damage after disruption of chromosome attachment to spindle microtubules. Nocodazole-treated oocytes did not display increased DNA damage signals that were marked by γH2A.X signal strength, but reformed spindles and underwent maturation, although aneuploidy increased after extended nocodazole treatment. By using the mouse for parthenogenetic activation studies, we showed that early cleavage stage embryos derived from parthenogenetic activation of nocodazole-treated oocytes displayed normal activation rate and normal γH2A.X signal strength, indicating that no additional DNA damage occured. Our results suggest that DNA damage inhibits porcine oocyte maturation, while nocodazole-induced dissociation between chromosomes and microtubules does not lead to increased DNA damage either in mouse meiotic oocytes or in porcine oocytes.

CDC20 downregulation impairs spindle morphology and causes reduced first polar body emission during bovine oocyte maturation

The cell division cycle protein 20 (CDC20) is an essential regulator of cell division, encoded by the CDC20 gene. However, the role of CDC20 in bovine oocyte maturation is unknown. In this study, CDC20 morpholino antisense oligonucleotides (MOs) were microinjected into the cytoplasm of bovine oocytes to block the translation of CDC20 mRNA. CDC20 downregulation significantly reduced the rate of first polar body emission (PB1). Further analysis indicated that oocytes treated with CDC20 MO arrested before or at meiotic stage I with abnormal spindles. To further confirm the functions of CDC20 during oocyte meiotic division, CDC20 MOs were microinjected into oocytes together with a supplementary PB1. The results showed that newly synthesized CDC20 was not necessary at the meiosis II-to-anaphase II transition. Our data suggest that CDC20 is required for spindle assembly, chromosomal segregation, and PB1 extrusion during bovine oocyte maturation.

Specific deletion ofCdc42does not affect meiotic spindle organization/migration and homologous chromosome segregation but disrupts polarity establishment and cytokinesis in mouse oocytes

Mammalian oocyte maturation is distinguished by highly asymmetric meiotic divisions during which a haploid female gamete is produced and almost all the cytoplasm is maintained in the egg for embryo development. Actin-dependent meiosis I spindle positioning to the cortex induces the formation of a polarized actin cap and oocyte polarity, and it determines asymmetric divisions resulting in two polar bodies. Here we investigate the functions of Cdc42 in oocyte meiotic maturation by oocyte-specific deletion of Cdc42 through Cre-loxP conditional knockout technology. We find that Cdc42 deletion causes female infertility in mice. Cdc42 deletion has little effect on meiotic spindle organization and migration to the cortex but inhibits polar body emission, although homologous chromosome segregation occurs. The failure of cytokinesis is due to the loss of polarized Arp2/3 accumulation and actin cap formation; thus the defective contract ring. In addition, we correlate active Cdc42 dynamics with its function during polar body emission and find a relationship between Cdc42 and polarity, as well as polar body emission, in mouse oocytes.

Inactivation of mitogen‐activated protein kinase is neither necessary nor sufficient for the onset of pronuclear formation in mouse oocytes

Mammalian oocytes are arrested at metaphase II due to high MAP kinase activity. After fertilization, oocytes resume meiosis, leading to female chromosome segregation, polar body emission and pronuclear (PN) formation. Previous biochemical studies showed that MAP kinase activity remained high for several hours after fertilization and began to decrease in parallel with PN formation. It has been thought that MAP kinase activity is incompatible with PN formation, and its inactivation is required for the initiation of PN formation in mammalian oocytes. In this study, we revisited this hypothesis by examining MAP kinase activity and PN formation in individual mouse oocytes using cytological analysis. We showed that MAP kinase activity in oocytes could be evaluated using phospho-ERK1/2 immunofluorescent staining. Co-immunofluorescent staining of phospho-ERK1/2 and nuclear pore components showed that PN formation preceded MAP kinase inactivation and could be initiated while MAP kinase activity was still high. Moreover, artificial inactivation of MAP kinase or its downstream target, ribosomal S6 kinase, accelerated but did not rapidly induce PN formation. Our results show that although the MAP kinase pathway negatively regulates PN formation, its inactivation is neither necessary nor sufficient for PN formation. These results suggest the involvement of other essential factor(s) in this process.