Cover Image,Volume 234, Number 10, October 2019

Cytoskeleton which includes microtubule and actin filaments plays important roles during mammalian oocyte maturation. In the present study, we showed that protein kinase C mu (PKC mu) was one potential key molecule which affected cytoskeleton dynamics in mouse oocytes. Our results showed that PKC mu expressed and localized at the poles of the spindle during oocyte maturation, and PKC mu expression reduced in the oocytes from 6-month-old mice or 24 hr in vitro culture. We knocked down the expression of PKC mu in oocytes using morpholino injection to explore the relationship between PKC mu and subcellular structure defects. The loss of PKC mu reduced oocyte maturation competence, showing with decreased polar body extrusion rate and increased rate of symmetric division. Further analysis indicated that PKC mu decrease caused the spindle organization defects, and this could be confirmed by the decreased tubulin acetylation level. Moreover, we found that PKC mu affected the phosphorylation level of cofilin for actin assembly, which further affected cytoplasmic actin distribution and spindle positioning. In summary, our data indicated that PKC mu is one key factor for oocyte maturation through its roles on the spindle organization and actin filament distribution.     

Daam1 regulates fascin for actin assembly in mouse oocyte meiosis

As a formin protein, Daam1 (Dishevelled-associated activator of morphogenesis 1) is reported to regulate series of cell processes like endocytosis, cell morphology and migration via its effects on actin assembly in mitosis. However, whether Daam1 plays roles in female meiosis remains uncertain. In this study, we investigated the expression and functions of Daam1 during mouse oocyte meiosis. Our results indicated that Daam1 localized at the cortex of oocytes, which was similar with actin filaments. After Daam1 morpholino (MO) microinjection, the expression of Daam1 significantly decreased, which resulted in the failure of oocyte polar body extrusion. These results might be due to the defects of actin assembly, since the decreased fluorescence intensity of actin filaments in oocyte cortex and cytoplasm were observed. However, Daam1 knockdown seemed not to affect the meiotic spindle movement. In addition, we found that fascin might be the down effector of Daam1, since the protein expression of fascin decreased after Daam1 knockdown. Thus, our data suggested that Daam1 affected actin assembly during oocyte meiotic division via the regulation of fascin expression.     

RhoA-mediated FMNL1 regulates GM130 for actin assembly and phosphorylates MAPK for spindle formation in mouse oocyte meiosis

Formin-like 1 (FMNL1) is a member of Formin family proteins which are the actin nucleators. Although FMNL1 activities have been shown to be essential for cell adhesion, cytokinesis, cell polarization and migration in mitosis, the functional roles of mammalian FMNL1 during oocyte meiosis remain uncertain. In this study, we investigated the functions of FMNL1 in mouse oocytes using specific morpholino (MO) microinjection and live cell imaging. Immunofluorescent staining showed that in addition to its cytoplasmic distribution, FMNL1 was primarily localized at the spindle poles after germinal vesicle breakdown (GVBD). FMNL1 knockdown caused the low rate of polar body extrusion and resulted in large polar bodies. Time-lapse microscopic and immunofluorescence intensity analysis indicated that this might be due to the aberrant actin expression levels. Cortical polarity was disrupted as shown by a loss of actin cap and cortical granule free domain (CGFD) formation, which was confirmed by a failure of meiotic spindle positioning. And this might be the reason for the large polar body formation. Spindle formation was also disrupted, which might be due to the abnormal localization of p-MAPK. These results indicated that FMNL1 affected both actin dynamics and spindle formation for the oocyte polar body extrusion. Moreover, FMNL1 depletion resulted in aberrant localization and expression patterns of a cis-Golgi marker protein, GM130. Finally, we found that the small GTPase RhoA might be the upstream regulator of FMNL1. Taken together, our data indicate that FMNL1 is required for spindle organization and actin assembly through a RhoA-FMNL1-GM130 pathway during mouse oocyte meiosis.Key words: actin, FMNL1, golgi, polar body extrusion, spindle organization     

RhoA-mediated MLC2 regulates actin dynamics for cytokinesis in meiosis

During oocyte meiosis, the bipolar spindle forms in the central cytoplasm and then migrates to the cortex. Subsequently, the oocyte extrudes the polar body through two successive asymmetric divisions, which are regulated primarily by actin filaments. Myosin light chain2 (MLC2) phosphorylation plays pivotal roles in smooth muscle contraction, stress fiber formation, cell motility and cytokinesis. However, whether MLC2 phosphorylation participates in the oocyte polarization and asymmetric division has not been clarified. The present study investigated the expression and functions of MLC2 during mouse oocyte meiosis. Our result showed that p-MLC2 was localized in the oocyte cortex, with a thickened cap above the chromosomes. Meanwhile, p-MLC2 was also localized in the poles of spindle. Disruption of MLC2 activity by MLC2 knock down (K_D) caused the failure of polar body extrusion. Immunofluorescent staining showed that a large proportion of oocytes arrested in telophase stage and failed to undergo cytokinesis after culturing for 12 hours. In the meantime, actin filament staining at oocyte membrane and cytoplasm were reduced in MLC2 K_D oocytes. Finally, we found that the phosphorylation of MLC2 protein levels was decreased after disruption of RhoA activity. Above all, our data indicated that the RhoA-mediated MLC2 regulates the actin organization for cytokinesis during mouse oocyte maturation.     

Small GTPase RhoA regulates cytoskeleton dynamics during porcine oocyte maturation and early embryo development

Mammalian oocyte maturation is distinguished by asymmetric division that is regulated primarily by cytoskeleton, including microtubules and microfilaments. Small Rho GTPase RhoA is a key regulator of cytoskeletal organization which regulates cell polarity, migration, and division. In this study, we investigated the roles of RhoA in mammalian oocyte meiosis and early embryo cleavage. (1) Disrupting RhoA activity or knock down the expression of RhoA caused the failure of polar body emission. This may have been due to decreased actin assembly and subsequent spindle migration defects. The involvement of RhoA in this process may have been though its regulation of actin nucleators ROCK, p-Cofilin, and ARP2 expression. (2) In addition, spindle morphology was also disrupted and p-MAPK expression decreased in RhoA inhibited or RhoA KD oocytes, which indicated that RhoA also regulated MAPK phosphorylation for spindle formation. (3) Porcine embryo development was also suppressed by inhibiting RhoA activity. Two nuclei were observed in one blastomere, and actin expression was reduced, which indicated that RhoA regulated actin-based cytokinesis of porcine embryo. Thus, our results demonstrated indispensable roles for RhoA in regulating porcine oocyte meiosis and cleavage during early embryo development.     

Checkpoint kinase 1 is essential for meiotic cell cycle regulation in mouse oocytes

Checkpoint kinase 1 (Chk1) plays key roles in all currently defined cell cycle checkpoints, but its functions in mouse oocyte meiosis remain unclear. In this study, we report the expression, localization and functions of Chk1 in mouse oocyte meiosis. Chk1 was expressed from germinal vesicle (GV) to metaphase II (MII) stages and localized to the spindle from pro-metaphase I (pro-MI) to MII stages in mouse oocytes. Chk1 depletion facilitated the G₂/M transition while Chk1 overexpression inhibited the G₂/M transition as indicated by germinal vesicle breakdown (GVBD), through regulation of Cdh1 and Cyclin B1. Chk1 depletion did not affect meiotic cell cycle progression after GVBD, but its overexpression after GVBD activated the spindle assembly checkpoint and prevented homologous chromosome segregation, thus arresting oocytes at pro-MI or metaphase I (MI) stages. These results suggest that Chk1 is indispensable for prophase I arrest and functions in G₂/M checkpoint regulation in meiotic oocytes. Moreover, Chk1 overexpression affects meiotic spindle assembly checkpoint regulation and thus chromosome segregation.     

Non-muscle tropomyosin (Tpm3) is crucial for asymmetric cell division and maintenance of cortical integrity in mouse oocytes

Tropomyosins are actin-binding cytoskeletal proteins that play a pivotal role in regulating the function of actin filaments in muscle and non-muscle cells; however, the roles of non-muscle tropomyosins in mouse oocytes are unknown. This study investigated the expression and functions of non-muscle tropomyosin (Tpm3) during meiotic maturation of mouse oocytes. Tpm3 mRNA was detected at all developmental stages in mouse oocytes. Tpm3 protein was localized at the cortex during the germinal vesicle and germinal vesicle breakdown stages. However, the overall fluorescence intensity of Tpm3 immunostaining was markedly decreased in metaphase II oocytes. Knockdown of Tpm3 impaired asymmetric division of oocytes and spindle migration, considerably reduced the amount of cortical actin, and caused membrane blebbing during cytokinesis. Expression of a constitutively active cofilin mutant and Tpm3 overexpression confirmed that Tpm3 protects cortical actin from depolymerization by cofilin. The data indicate that Tpm3 plays crucial roles in maintaining cortical actin integrity and asymmetric cell division during oocyte maturation, and that dynamic regulation of cortical actin by Tpm3 is critical to ensure proper polar body protrusion.     

CDC6 regulates both G2/M transition and metaphase-to-anaphase transition during the first meiosis of mouse oocytes

Cell division cycle protein, CDC6, is essential for the initiation of DNA replication. CDC6 was recently shown to inhibit the microtubule-organizing activity of the centrosome. Here, we show that CDC6 is localized to the spindle from pro-metaphase I (MI) to MII stages of oocytes, and it plays important roles at two critical steps of oocyte meiotic maturation. CDC6 depletion facilitated the G2/M transition (germinal vesicle breakdown [GVBD]) through regulation of Cdh1 and cyclin B1 expression and CDK1 (CDC2) phosphorylation in a GVBD-inhibiting culture system containing milrinone. Furthermore, GVBD was significantly decreased after knockdown of cyclin B1 in CDC6-depleted oocytes, indicating that the effect of CDC6 loss on GVBD stimulation was mediated, at least in part, by raising cyclin B1. Knockdown of CDC6 also caused abnormal localization of γ-tubulin, resulting in defective spindles, misaligned chromosomes, cyclin B1 accumulation, and spindle assembly checkpoint (SAC) activation, leading to significant pro-MI/MI arrest and PB1 extrusion failure. These phenotypes were also confirmed by time-lapse live cell imaging analysis. The results indicate that CDC6 is indispensable for maintaining G2 arrest of meiosis and functions in G2/M checkpoint regulation in mouse oocytes. Moreover, CDC6 is also a key player regulating meiotic spindle assembly and metaphase-to-anaphase transition in meiotic oocytes.     

Kif18a regulates Sirt2-mediated tubulin acetylation for spindle organization during mouse oocyte meiosis

Background

During oocyte meiosis, the cytoskeleton dynamics, especially spindle organization, are critical for chromosome congression and segregation. However, the roles of the kinesin superfamily in this process are still largely unknown.

Results

In the present study, Kif18a, a member of the kinesin-8 family, regulated spindle organization through its effects on tubulin acetylation in mouse oocyte meiosis. Our results showed that Kif18a is expressed and mainly localized in the spindle region. Knock down of Kif18a caused the failure of first polar body extrusion, dramatically affecting spindle organization and resulting in severe chromosome misalignment. Further analysis showed that the disruption of Kif18a caused an increase in acetylated tubulin level, which might be the reason for the spindle organization defects after Kif18a knock down in oocyte meiosis, and the decreased expression of deacetylase Sirt2 was found after Kif18a knock down. Moreover, microinjections of tubulin K40R mRNA, which could induce tubulin deacetylation, protected the oocytes from the effects of Kif18a downregulation, resulting in normal spindle morphology in Kif18a-knock down oocytes.

Conclusions

Taken together, our results showed that Kif18a affected Sirt2-mediated tubulin acetylation level for spindle organization during mouse oocyte meiosis. Our results not only revealed the critical effect of Kif18a on microtubule stability, but also extended our understanding of kinesin activity in meiosis.

     

Dynein promotes porcine oocyte meiotic progression by maintaining cytoskeletal structures and cortical granule arrangement

Cytoplasmic dynein is a family of cytoskeletal motor proteins that move towards the minus-end of the microtubules to perform functions in a variety of mitotic processes such as cargo transport, organelle positioning, chromosome movement and centrosome assembly. However, its specific roles during mammalian oocyte meiosis have not been fully defined. Herein, we investigated the critical events during porcine oocyte meiotic maturation after inhibition of dynein by Ciliobrevin D treatment. We found that oocyte meiotic progression was arrested when inhibited of dynein by showing the poor expansion of cumulus cells and decreased rate of polar body extrusion. Meanwhile, the spindle assembly and chromosome alignment were disrupted, accompanied by the reduced level of acetylated α-tubulin, indicative of weakened microtubule stability. Defective actin polymerization on the plasma membrane was also observed in dynein-inhibited oocytes. In addition, inhibition of dynein caused the abnormal distribution of cortical granules and precocious exocytosis of ovastacin, a cortical granule component, which predicts that ZP2, the sperm binding site in the zona pellucida, might be prematurely cleaved in the unfertilized dynein-inhibited oocytes, potentially leading to the fertilization failure. Collectively, our findings reveal that dynein plays a part in porcine oocyte meiotic progression by regulating the cytoskeleton dynamics including microtubule stability, spindle assembly, chromosome alignment and actin polymerization. We also find that dynein mediates the normal cortical granule distribution and exocytosis timing of ovastacin in unfertilized eggs which are the essential for the successful fertilization.     

Cdc42 protein acts upstream of IQGAP1 and regulates cytokinesis in mouse oocytes and embryos

Cdc42 and Rac1 Rho family GTPases, and their interacting protein IQGAP1 are the key regulators of cell polarity. We examined the role of Cdc42 and IQGAP1 in establishing the polarity of mouse oocyte and regulation of meiotic and mitotic divisions. We showed that Cdc42 was localized on the microtubules of meiotic and mitotic spindle and in the cortex of mouse oocytes and cleaving embryos. IQGAP1 was present in the cytoplasm and cortex of growing and fully-grown oocytes. During maturation it disappeared from the cortex and during meiotic and mitotic cytokinesis it concentrated in the contractile ring. Toxin B inhibition of the binding activity of Cdc42 changed the localization of IQGAP1, inhibited emission of the first polar body, and caused disappearance of the cortical actin without affecting the migration of meiotic spindle. This indicates, that in maturing oocytes accumulation of cortical actin is not indispensable for spindle migration. In zygotes treated with toxin B actin cytoskeleton was rearranged and the first and/or subsequent cytokinesis were inhibited. Our results indicate that Cdc42 acts upstream of IQGAP1 and is involved in regulation of cytokinesis in mouse oocytes and cleaving embryos, rather than in establishing the polarity of the oocyte.     

Chk2 Regulates Cell Cycle Progression during Mouse Oocyte Maturation and Early Embryo Development

As a tumor suppressor homologue during mitosis, Chk2 is involved in replication checkpoints, DNA repair, and cell cycle arrest, although its functions during mouse oocyte meiosis and early embryo development remain uncertain. We investigated the functions of Chk2 during mouse oocyte maturation and early embryo development. Chk2 exhibited a dynamic localization pattern; Chk2 expression was restricted to germinal vesicles at the germinal vesicle (GV) stage, was associated with centromeres at pro-metaphase I (Pro-MI), and localized to spindle poles at metaphase I (MI). Disrupting Chk2 activity resulted in cell cycle progression defects. First, inhibitor-treated oocytes were arrested at the GV stage and failed to undergo germinal vesicle breakdown (GVBD); this could be rescued after Chk2 inhibition release. Second, Chk2 inhibition after oocyte GVBD caused MI arrest. Third, the first cleavage of early embryo development was disrupted by Chk2 inhibition. Additionally, in inhibitor-treated oocytes, checkpoint protein Bub3 expression was consistently localized at centromeres at the MI stage, which indicated that the spindle assembly checkpoint (SAC) was activated. Moreover, disrupting Chk2 activity in oocytes caused severe chromosome misalignments and spindle disruption. In inhibitor-treated oocytes, centrosome protein γ-tubulin and Polo-like kinase 1 (Plk1) were dissociated from spindle poles. These results indicated that Chk2 regulated cell cycle progression and spindle assembly during mouse oocyte maturation and early embryo development.     

The SUMO pathway functions in mouse oocyte maturation

Sumoylation is an important post-translational modification in which SUMO (small ubiquitin-related modifier) proteins are bonded covalently to their substrates. Studies on the roles of sumoylation in cell cycle regulation have been emerging in both mitosis from yeast to mammals and meiosis in budding yeast, but the functions of sumoylation in mammalian meiosis, especially in oocyte meiotic maturation are not well known. Here, we examined the localization and expression of SUMO-1 and SUMO-2/3, the two basic proteins in the sumoylation pathway and investigated their roles through over-expression of Senp2 during mouse oocyte maturation. Immunofluorescent staining revealed differential patterns of SUMO-1 and SUMO-2/3 localization: SUMO-1 was localized to the spindle poles in prometaphase I, MI and MII stages, around the separating homologues in anaphase I and telophase I stages of first meiosis, while SUMO-2/3 was mainly concentrated near centromeres during mouse oocyte maturation. Immunoblot analysis uncovered the different expression profiles of SUMO-1 and SUMO-2/3 modified proteins during mouse oocyte maturation. Over-expression of Senp2, a SUMO-specific isopeptidase, caused changes of SUMO-modified proteins and led to defects in MII spindle organization in mature eggs. These results suggest that the SUMO pathway may play an indispensable role during mouse oocyte meiotic maturation.     

The SUMO pathway functions in mouse oocyte maturation

Sumoylation is an important posttranslational modification in which SUMO (small ubiquitin-related modifier) proteins are bonded covalently to their substrates. Studies on the roles of sumoylation in cell cycle regulation have been emerging in both mitosis from yeast to mammals and meiosis in budding yeast, but the functions of sumoylation in mammalian meiosis, especially in oocyte meiotic maturation are not well known. Here, we examined the localization and expression of SUMO-1 and SUMO-2/3, the two basic proteins in the sumoylation pathway and investigated their roles through overexpression of Senp2 during mouse oocyte maturation. Immunofluorescent staining revealed differential patterns of SUMO-1 and SUMO-2/3 localization: SUMO-1 was localized to the spindle poles in prometaphase I, MI and MII stages, around the separating homologues in anaphase I and telophase I stages of first meiosis, while SUMO-2/3 was mainly concentrated near centromeres during mouse oocyte maturation. Immunoblot analysis uncovered the different expression profiles of SUMO-1 and SUMO-2/3 modified proteins during mouse oocyte maturation. Overexpression of Senp2, a SUMO-specific isopeptidase, caused changes of SUMO-modified proteins and led to defects in MII spindle organization in mature eggs. These results suggest that the SUMO pathway may play an indispensable role during mouse oocyte meiotic maturation.Key words: sumoylation, mouse oocyte maturation, overexpression, Senp2, MII spindle     

Rotation of meiotic spindle is controlled by microfilaments in mouse oocytes

The completion of meiosis requires the spatial and temporal coordination of cytokinesis and karyokinesis. During meiotic maturation, many events, such as formation, location, and rotation of the meiotic spindle as well as chromosomal movement, polar body extrusion, and pronuclear migration, are dependent on regulation of the cytoskeleton system. To study functions of microfilaments in meiosis, we induced metaphase II (MII) mouse oocytes to resume meiosis by in vitro fertilization or parthenogenetic activation, and we treated such oocytes with cytochalasin B (CB). The changes of the meiotic spindle, as visualized in preparations stained for beta-tubulin and chromatin, were observed by fluorescent confocal microscopy. The meiotic spindle of MII oocytes was observed to be parallel to the plasmalemma. After meiosis had resumed, the spindle rotated to the vertical position so that the second polar body could be extruded into the perivitelline space. When meiosis resumed and oocytes were treated with 10 micro g/ml of CB, the spindle rotation was inhibited. Consequently, the oocyte formed an extra pronucleus instead of extruding a second polar body. These results indicate that spindle rotation is essential for polar body extrusion; it is the microfilaments that play a crucial role in regulating rotation of the meiotic spindle.     

Small Molecule Inhibitor of Formin Homology 2 Domains (SMIFH2) Reveals the Roles of the Formin Family of Proteins in Spindle Assembly and Asymmetric Division in Mouse Oocytes

Dynamic actin reorganization is the main driving force for spindle migration and asymmetric cell division in mammalian oocytes. It has been reported that various actin nucleators including Formin-2 are involved in the polarization of the spindle and in asymmetric cell division. In mammals, the formin family is comprised of 15 proteins. However, their individual roles in spindle migration and/or asymmetric division have not been elucidated yet. In this study, we employed a newly developed inhibitor for formin family proteins, small molecule inhibitor of formin homology 2 domains (SMIFH2), to assess the functions of the formin family in mouse oocyte maturation. Treatment with SMIFH2 during in vitro maturation of mouse oocytes inhibited maturation by decreasing cytoplasmic and cortical actin levels. In addition, treatment with SMIFH2, especially at higher concentrations (500 μM), impaired the proper formation of meiotic spindles, indicating that formins play a role in meiotic spindle formation. Knockdown of the mDia2 formins caused a similar decrease in oocyte maturation and abnormal spindle morphology, mimicking the phenotype of SMIFH2-treated cells. Collectively, these results suggested that besides Formin-2, the other proteins of the formin, including mDia family play a role in asymmetric division and meiotic spindle formation in mammalian oocytes.     

Nuf2 is required for chromosome segregation during mouse oocyte meiotic maturation

Nuf2 plays an important role in kinetochore-microtubule attachment and thus is involved in regulation of the spindle assembly checkpoint in mitosis. In this study, we examined the localization and function of Nuf2 during mouse oocyte meiotic maturation. Myc₆-Nuf2 mRNA injection and immunofluorescent staining showed that Nuf2 localized to kinetochores from germinal vesicle breakdown to metaphase I stages, while it disappeared from the kinetochores at the anaphase I stage, but relocated to kinetochores at the MII stage. Overexpression of Nuf2 caused defective spindles, misaligned chromosomes, and activated spindle assembly checkpoint, and thus inhibited chromosome segregation and metaphase-anaphase transition in oocyte meiosis. Conversely, precocious polar body extrusion was observed in the presence of misaligned chromosomes and abnormal spindle formation in Nuf2 knock-down oocytes, causing aneuploidy. Our data suggest that Nuf2 is a critical regulator of meiotic cell cycle progression in mammalian oocytes.     

The Microtubule-Associated Protein ASPM Regulates Spindle Assembly and Meiotic Progression in Mouse Oocytes

The microtubule-associated protein ASPM (abnormal spindle-like microcephaly-associated) plays an important role in spindle organization and cell division in mitosis and meiosis in lower animals, but its function in mouse oocyte meiosis has not been investigated. In this study, we characterized the localization and expression dynamics of ASPM during mouse oocyte meiotic maturation and analyzed the effects of the downregulation of ASPM expression on meiotic spindle assembly and meiotic progression. Immunofluorescence analysis showed that ASPM localized to the entire spindle at metaphase I (MI) and metaphase II (MII), colocalizing with the spindle microtubule protein acetylated tubulin (Ac-tubulin). In taxol-treated oocytes, ASPM colocalized with Ac-tubulin on the excessively polymerized microtubule fibers of enlarged spindles and the numerous asters in the cytoplasm. Nocodazole treatment induced the gradual disassembly of microtubule fibers, during which ASPM remained colocalized with the dynamic Ac-tubulin. The downregulation of ASPM expression by a gene-specific morpholino resulted in an abnormal meiotic spindle and inhibited meiotic progression; most of the treated oocytes were blocked in the MI stage with elongated meiotic spindles. Furthermore, coimmunoprecipitation combined with mass spectrometry and western blot analysis revealed that ASPM interacted with calmodulin in MI oocytes and that these proteins colocalized at the spindle. Our results provide strong evidence that ASPM plays a critical role in meiotic spindle assembly and meiotic progression in mouse oocytes.     

Septin2 is modified by SUMOylation and required for chromosome congression in mouse oocytes

In mitosis, centrosomes nucleate microtubules that capture the sister kinetochores of each chromosome to facilitate chromosome congression. In contrast, during meiosis chromosome congression on the acentrosomal spindle is driven primarily by movement of chromosomes along laterally associated microtubule bundles. Previous studies have indicated that septin2 is required for chromosome congression and cytokinesis in mitosis, we therefore asked whether perturbation of septin2 would impair chromosome congression and cytokinesis in meiosis. We have investigated its expression, localization and function during mouse oocyte meiotic maturation. Septin2 was modified by SUMO-1 and its levels remained constant from GVBD to metaphase II stages. Septin2 was localized along the entire spindle at metaphase and at the midbody in cytokinesis. Disruption of septins function with an inhibitor and siRNA caused failure of the metaphase I /anaphase I transition and chromosome misalignment but inhibition of septins after the metaphase I stage did not affect cytokinesis. BubR1, a core component of the spindle checkpoint, was labeled on misaligned chromosomes and on chromosomes aligned at the metaphase plate in inhibitor-treated oocytes that were arrested in prometaphase I/metaphase I, suggesting activation of the spindle assembly checkpoint. Taken together, our results demonstrate that septin2 plays an important role in chromosome congression and meiotic cell cycle progression but not cytokinesis in mouse oocytes.