Intra-oocyte localization of MAD2 and its relationship with kinetochores, microtubules, and chromosomes in rat oocytes during meiosis

Exploring Plant Meiosis: Insights from the Kinetochore Perspective

The central player for chromosome segregation in both mitosis and meiosis is the macromolecular kinetochore structure, which is assembled by >100 structural and regulatory proteins on centromere DNA. Kinetochores play a crucial role in cell division by connecting chromosomal DNA and microtubule polymers. This connection helps in the proper segregation and alignment of chromosomes. Additionally, kinetochores can act as a signaling hub, regulating the start of anaphase through the spindle assembly checkpoint, and controlling the movement of chromosomes during anaphase. However, the role of various kinetochore proteins in plant meiosis has only been recently elucidated, and these proteins differ in their functionality from those found in animals. In this review, our current knowledge of the functioning of plant kinetochore proteins in meiosis will be summarized. In addition, the functional similarities and differences of core kinetochore proteins in meiosis between plants and other species are discussed, and the potential applications of manipulating certain kinetochore genes in meiosis for breeding purposes are explored.

Mad2 is dispensable for accurate chromosome segregation but becomes essential when oocytes are subjected to environmental stress

Accurate chromosome segregation, monitored by the spindle assembly checkpoint (SAC), is crucial for the production of euploid cells. Previous in vitro studies by us and others showed that Mad2, a core member of the SAC, performs a checkpoint function in oocyte meiosis. Here, through an oocyte-specific knockout approach in mouse, we reconfirmed that Mad2-deficient oocytes exhibit an accelerated metaphase-to-anaphase transition caused by premature degradation of securin and cyclin B1 and subsequent activation of separase in meiosis I. However, it was surprising that the knockout mice were completely fertile and the resulting oocytes were euploid. In the absence of Mad2, other SAC proteins, including BubR1, Bub3 and Mad1, were normally recruited to the kinetochores, which likely explains the balanced chromosome separation. Further studies showed that the chromosome separation in Mad2-null oocytes was particularly sensitive to environmental changes and, when matured in vitro, showed chromosome misalignment, lagging chromosomes, and aneuploidy with premature separation of sister chromatids, which was exacerbated at a lower temperature. We reveal for the first time that Mad2 is dispensable for proper chromosome segregation but acts to mitigate environmental stress in meiotic oocytes.

Maternal genetic polymorphisms in the major mitotic checkpoint genes MAD1L1 and MAD2L1 associated with the risk of survival in abnormal chromosomal fetuses

Background: The genetic etiology of fetal chromosome abnormalities remains unknown, which brings about an enormous burden for patients, families, and society. The spindle assembly checkpoint (SAC) controls the normal procedure of chromosome disjunction and may take part in the process.

Objective: The aim of this study was to explore the association between polymorphisms of MAD1L1 rs1801368 and MAD2L1 rs1283639804, involved in SAC and fetal chromosome abnormalities.

Methods: The case–control study collected 563 cases and 813 health controls to test the genotypes of MAD1L1 rs1801368 and MAD2L1 rs1283639804 polymorphisms by polymerase chain reaction–restrictive fragment length polymorphism methods (PCR-RFLP).

Results:MAD1L1 rs1801368 polymorphism was associated with fetal chromosome abnormalities alone or combined to lower homocysteine (HCY) levels (alone: dominant: OR: 1.75, 95%CI: 1.19–2.57, and p = 0.005; CT vs. CC: OR = 0.73, 95%CI: 0.57–0.94, and p = 0.016; lower HCY: C vs. T: OR = 0.74, 95%CI: 0.57–0.95, and p = 0.02; dominant: OR = 1.75, 95%CI: 0.79–1.92, and p = 0.005). No significant differences were found in other genetic models or subgroups (p > 0.05, respectively). MAD2L1 rs1283639804 polymorphism revealed a sole genotype in the studied population. HCY is significantly associated with fetal chromosome abnormalities in younger groups (OR: 1.78, 95%CI: 1.28–2.47, and p = 0.001).

Conclusion: The results implied that the polymorphism of MAD1L1 rs1801368 may become the susceptibility factor to fetal chromosome abnormalities alone or combined to lower HCY levels but not to MAD2L1 rs1283639804 polymorphism. In addition, HCY significantly affects fetal chromosomal abnormalities in younger women.

Resveratrol Hinders Postovulatory Aging by Modulating Oxidative Stress in Porcine Oocytes

Postovulatory aging of the mammalian oocytes causes deterioration of oocytes through several factors including oxidative stress. Keeping that in mind, we aimed to investigate the potential of a well-known antioxidant, resveratrol (RV), to evaluate the adverse effects of postovulatory aging in porcine oocytes. After in vitro maturation (IVM), a group of (25–30) oocytes (in three replicates) were exposed to 0, 1, 2, and 4 μmol/L of RV, respectively. The results revealed that the first polar body (PB1) extrusion rate of the oocytes significantly increased when the RV concentration reached up to 2 μmol/L (p < 0.05). Considering optimum RV concentration of 2 μmol/L, the potential of RV was evaluated in oocytes aged for 24 and 48 h. We used fluorescence microscopy to detect the relative level of reactive oxygen species (ROS), while GHS contents were measured through the enzymatic method. Our results revealed that aged groups (24 h and 48 h) treated with RV (2 μmol/L) showed higher (p < 0.05) ROS fluorescence intensity than the control group, but lower (p < 0.05) than untreated aged groups. The GSH content in untreated aged groups (24 h and 48 h) was lower (p < 0.05) than RV-treated groups, but both groups showed higher levels than the control. Similarly, the relative expression of the genes involved in antioxidant activity (CAT, GPXGSH-Px, and SOD1) in RV-treated groups was lower (p < 0.05) as compared to the control group but higher than that of untreated aged groups. Moreover, the relative mRNA expression of caspase-3 and Bax in RV-treated groups was higher (p < 0.05) than the control group but lower than untreated groups. Furthermore, the expression of Bcl-2 in the RV-treated group was significantly lower than control but higher than untreated aged groups. Taken together, our findings revealed that the RV can increase the expression of antioxidant genes by decreasing the level of ROS, and its potent antiapoptotic effects resisted against the decline in mitochondrial membrane potential in aged oocytes.

Meiotic Instability Generates a Pathological Condition in Mammalian Ovum

Maintenance of metaphase-II (M-II) arrest in ovum is required to present itself as a right gamete for successful fertilization in mammals. Surprisingly, instability of meiotic cell cycle results in spontaneous exit from M-II arrest, chromosomal scattering and incomplete extrusion of second polar body (PB-II) without forming pronuclei so called abortive spontaneous ovum activation (SOA). It remains unclear what causes meiotic instability in freshly ovulated ovum that results in abortive SOA. We propose the involvement of various signal molecules such as reactive oxygen species (ROS), cyclic 3',5' adenosine monophosphate (cAMP) and calcium (Ca²⁺) in the induction of meiotic instability and thereby abortive SOA. These signal molecules through their downstream pathways modulate phosphorylation status and activity of cyclin dependent kinase (cdk1) as well as cyclin B1 level. Changes in phosphorylation status of cdk1 and its activity, dissociation and degradation of cyclin B1 destabilize maturation promoting factor (MPF). The premature MPF destabilization and defects in other cell cycle regulators possibly cause meiotic instability in ovum soon after ovulation. The meiotic instability results in a pathological condition of abortive SOA and deteriorates ovum quality. These ova are unfit for fertilization and limit reproductive outcome in several mammalian species including human. Therefore, global attention is required to identify the underlying causes in greater details in order to address the problem of meiotic instability in ova of several mammalian species icluding human. Moreover, these activated ova may be used to create parthenogenetic embryonic stem cell lines in vitro for the use in regenerative medicine.Graphical abstract.

Cell division cycle 23 is required for mouse oocyte meiotic maturation

Precise regulation of chromosome segregation during oocyte meiosis is of vital importance to mammalian reproduction. Anaphase promoting complex/cyclosome (APC/C) is reported to play an important role in metaphase-to-anaphase transition. Here we report that cell division cycle 23 (Cdc23, also known as APC8) plays a critical role in regulating the oocyte chromosome separation. Cdc23 localized on the meiotic spindle, and microinjection of Cdc23 siRNA caused decreased ratios of metaphase-to-anaphase transition. Loss of Cdc23 resulted in abnormal spindles, misaligned chromosomes, errors of homologous chromosome segregation, and production of aneuploid oocytes. Further study showed that inactivation of spindle assembly checkpoint and degradation of Cyclin B1 and securin were disturbed after Cdc23 knockdown. Furthermore, we found that inhibiting spindle assembly checkpoint protein Msp1 partly rescued the decreased polar body extrusion and reduced the accumulation of securin in Cdc23 knockdown oocytes. Taken together, our data demonstrate that Cdc23 is required for the chromosome segregation through regulating the spindle assembly checkpoint activity, and cyclin B1 and securin degradation in meiotic mouse oocytes.

Destabilization of spindle assembly checkpoint causes aneuploidy during meiosis II in murine post-ovulatory aged oocytes

Mammalian oocyte quality degrades over time after ovulation in vitro, which can cause fatal defects such as chromosomal aneuploidy. As various oocyte manipulations employed in assisted reproductive technology are time consuming, post-ovulatory aging is a serious problem to overcome in reproductive medicine or ova research. In this study, we investigated the effects of postovulatory aging on the incidence of chromosome aneuploidy during meiosis II, with a focus on the expression of functional proteins from the spindle assembly checkpoint (SAC). Chromosome analysis was used to assess the rate of aneuploidy in in vitro aged oocytes, or in early embryos derived from aged oocytes. Immunofluorescent staining was used to detect the localization of MAD2, which is a SAC signal that monitors the correct segregation of sister chromatids. Immunoblotting was used to quantify cohesin subunits, which are adhesion factors connecting sister chromatids at the metaphase II (MII) centromere. It was shown that post-ovulatory oocyte aging inhibits MAD2 localization to the sister kinetochore. Furthermore, oocyte aging prevented cohesin subunits from being maintained or degraded at the appropriate time. These data suggest that the destabilization of SAC signaling causes sister chromatid segregation errors in MII oocytes, and consequently increases the incidence of aneuploidy in early embryos. Our findings have provided distinct evidence that the post-ovulatory aging of oocytes might also be a risk factor for aneuploidy, irrespective of maternal age.

Rad9a is involved in chromatin decondensation and post-zygotic embryo development in mice

Zygotic chromatin undergoes extensive reprogramming immediately after fertilization. It is generally accepted that maternal factors control this process. However, little is known about the underlying mechanisms. Here we report that maternal RAD9A, a key protein in DNA damage response pathway, is involved in post-zygotic embryo development, via a mouse model with conditional depletion of Rad9a alleles in oocytes of primordial follicles. Post-zygotic losses originate from delayed zygotic chromatin decondensation after depletion of maternal RAD9A. Pronucleus formation and DNA replication of most mutant zygotes are therefore deferred, which subsequently trigger the G2/M checkpoint and arrest development of most mutant zygotes. Delayed zygotic chromatin decondensation could also lead to increased reabsorption of post-implantation mutant embryos. In addition, our data indicate that delayed zygotic chromatin decondensation may be attributed to deferred epigenetic modification of histone in paternal chromatin after fertilization, as fertilization and resumption of secondary meiosis in mutant oocytes were both normal. More interestingly, most mutant oocytes could not support development beyond one-cell stage after parthenogenetic activation. Therefore, RAD9A may also play an important role in maternal chromatin reprogramming. In summary, our data reveal an important role of RAD9A in zygotic chromatin reprogramming and female fertility.

Oocyte-specific deletion of Gsα induces oxidative stress and deteriorates oocyte quality in mice

The stimulatory heterotrimeric G_s protein alpha subunit (G_sα) is a ubiquitous guanine nucleotide-binding protein that regulates the intracellular cAMP signaling pathway and consequently participates in a wide range of biological events. In the reproductive system, despite G_sα being associated with oocyte meiotic arrest in vitro, the exact role of G_sα in female fertility in vivo remains largely unknown. Here, we generated oocyte-specific G_sα knockout mice by using the Cre/LoxP system. We observed that the deletion of G_sα caused complete female infertility. Exclusion of post-implantation abnormalities, oogenesis, fertilization, and early embryo development was subsequently monitored; meiosis in G_sα-deficient oocytes precociously resumed in only 43% of antral follicles from mutant mice, indicating that alteration of meiotic pause was not the key factor in infertility. Ovulation process and number were normal, but the rate of morphological abnormal oocytes was apparently increased; spindle organization, fertilization, and early embryo development were impaired. Furthermore, the level of ROS (reactive oxygen species) and the mitochondrial aggregation increased, and antioxidant glutathione (GSH) content, ATP level, mtDNA copy number, and mitochondrial membrane potential decreased in G_sα-deficient oocytes. GV oocytes from mutant mice showed early-stage apoptosis. Meanwhile, the G_sα knockout-induced decline in oocyte quality and low developmental potential was partially rescued by antioxidant supplementation. To sum up, our results are the first to reveal that the profile of G_sα oocyte-specific deletion caused female infertility in vivo, and oxidative stress plays an important role in this event.

Sirt2-BubR1 acetylation pathway mediates the effects of advanced maternal age on oocyte quality

The level of Sirt2 protein is reduced in oocytes from aged mice, while exogenous expression of Sirt2 could ameliorate the maternal age-associated meiotic defects. To date, the underlying mechanism remains unclear. Here, we confirmed that specific depletion of Sirt2 disrupts maturational progression and spindle/chromosome organization in mouse oocytes, with compromised kinetochore-microtubule attachments. Candidate screening revealed that acetylation state of lysine 243 on BubR1 (BubR1-K243, an integral part of the spindle assembly checkpoint complex) functions during oocyte meiosis, and acetylation-mimetic mutant BubR1-K243Q results in the very similar phenotypes as Sirt2-knockdown oocytes. Furthermore, we found that nonacetylatable-mimetic mutant BubR1-K243R partly prevents the meiotic deficits in oocytes depleted of Sirt2. Importantly, BubR1-K243R overexpression in oocytes derived from aged mice markedly suppresses spindle/chromosome anomalies and thereupon lowers the incidence of aneuploid eggs. In sum, our data suggest that Sirt2-dependent BubR1 deacetylation involves in the regulation of meiotic apparatus in normal oocytes and mediates the effects of advanced maternal age on oocyte quality.

Zfp207 is a Bub3 binding protein regulating meiotic chromosome alignment in mouse oocytes

Zinc finger proteins are a massive, diverse family of proteins that serve a wide variety of biological functions. However, the roles of them during meiosis are not yet clearly defined. Here, we report that Zfp207 localizes at the kinetochores during mouse oocyte meiotic maturation. Depletion of Zfp207 leads to a significantly higher proportion of impaired spindle organization and misaligned chromosomes in oocytes. This is coupled with the defective kinetochore-microtubule attachments, and resultantly increasing incidence of aneuploid metaphase II eggs. The precocious polar body extrusion and escape of metaphase I arrest induced by nocodazole treatment in Zfp207-depleted oocytes indicates that Zfp207 is essential for activation of SAC (Spindle Assembly Checkpoint) activity. Notably, we find that Zfp207 binds to Bub3 to form a complex and maintains its protein level in oocytes, and that overexpression of Bub3 is able to partially rescue the occurrence of aneuploid eggs in Zfp207-depleted oocytes. Collectively, we identify Zfp207 as a novel Bub3 binding protein in oocytes which plays an important role in controlling meiotic chromosome alignment and SAC function.

Geminin deletion in pre-meiotic DNA replication stage causes spermatogenesis defect and infertility

Geminin plays a critical role in cell cycle regulation by regulating DNA replication and serves as a transcriptional molecular switch that directs cell fate decisions. Spermatogonia lacking Geminin disappear during the initial wave of mitotic proliferation, while geminin is not required for meiotic progression of spermatocytes. It is unclear whether geminin plays a role in pre-meiotic DNA replication in later-stage spermatogonia and their subsequent differentiation. Here, we selectively disrupted Geminin in the male germline using the Stra8-Cre/loxP conditional knockout system. Geminin-deficient mice showed atrophic testes and infertility, concomitant with impaired spermatogenesis and reduced sperm motility. The number of undifferentiated spermatogonia and spermatocytes was significantly reduced; the pachytene stage was impaired most severely. Expression of cell proliferation-associated genes was reduced in Gmnn^fl/Δ; Stra8-Cre testes compared to in controls. Increased DNA damage, decreased Cdt1, and increased phosphorylation of Chk1/Chk2 were observed in Geminin-deficient germ cells. These results suggest that geminin plays important roles in pre-meiotic DNA replication and subsequent spermatogenesis.

Rad9a is required for spermatogonia differentiation in mice

Spermatogenesis in testes requires precise spermatogonia differentiation. Spermatocytes lacking the Rad9a gene are arrested in pachytene prophase, implying a possible role for RAD9A in spermatogonia differentiation. However, numerous RAD9A-positive pachytene spermatocytes are still observed in mouse testes following Rad9a excision using the Stra8-Cre system, and it is unclear whether Rad9a deletion in spermatogonia interrupts differentiation. Here, we generated a mouse model in which Rad9a was specifically deleted in spermatogonial stem cells (SSCs) using Cre recombinase expression driven by the germ cell-specific Vasa promoter. Adult Rad9a-null male mice were infertile as a result of completely blocked spermatogonia differentiation. No early spermatocytes were detected in mutant testicular cords of 9-day-old mice. Mutant spermatogonia were prone to apoptosis, although proliferation rates were unaffected. Rad9a deletion also resulted in malformation of seminiferous tubules, in which cells assembled irregularly into clusters, and malformation led to testicular cord disruption. Our findings suggest that Rad9a is indispensable for spermatogonia differentiation and testicular development in mice.

Involvement of Cyclin-Dependent Kinase 1 during Postovulatory Aging-Mediated Abortive Spontaneous Egg Activation in Rat Eggs Cultured In Vitro

Freshly ovulated rat eggs do not remain arrested at metaphase II (MII) and undergo exit from MII arrest with initiation of extrusion of the second polar body (PBII), a characteristic feature of abortive spontaneous egg activation (SEA). The biochemical and molecular changes during postovulatory aging-mediated abortive SEA remain poorly understood. We investigated the morphological, cellular, and molecular changes during postovulatory aging-mediated abortive SEA in eggs cultured in vitro. Our results suggest that postovulatory egg aging in vitro induced initiation of PBII extrusion in a time-dependent manner. Postovulatory aging increased Wee1 kinase and Thr-14/Tyr-15 phosphorylated cyclin-dependent kinase 1 (Cdk1) levels, whereas Thr-161 phosphorylated Cdk1 and cyclin B1 levels were significantly decreased in eggs cultured in vitro. The early mitotic inhibitor 2 (Emi2) level was significantly reduced, but anaphase promoting complex/cyclosome (APC/C) and mitotic arrest deficient protein (MAD2) levels were increased initially and then reduced during a later period of in vitro culture. These results suggest that an increased Wee1 kinase level modulated the specific phosphorylation status of Cdk1, increased Cdk1 activity, and decreased the cyclin B1 level. Furthermore, the decreased Emi2 level was associated with an increased level of APC/C and decreased level of cyclin B1, which resulted in maturation promoting factor (MPF) destabilization and finally led to postovulatory aging-mediated abortive SEA in rat eggs cultured in vitro.

Geminin deletion in mouse oocytes results in impaired embryo development and reduced fertility

Geminin is an important regulator of DNA replication and cell differentiation, but its role in female reproduction remains uncertain. Maternal geminin does not regulate oocyte meiotic maturation but does control accurate DNA replication. Geminin deletion in oocytes results in impaired embryo development and reduced fertility.

Geminin controls proper centrosome duplication, cell division, and differentiation. We investigated the function of geminin in oogenesis, fertilization, and early embryo development by deleting the geminin gene in oocytes from the primordial follicle stage. Oocyte-specific disruption of geminin results in low fertility in mice. Even though there was no evident anomaly of oogenesis, oocyte meiotic maturation, natural ovulation, or fertilization, early embryo development and implantation were impaired. The fertilized eggs derived from mutant mice showed developmental delay, and many were blocked at the late zygote stage. Cdt1 protein was decreased, whereas Chk1 and H2AX phosphorylation was increased, in fertilized eggs after geminin depletion. Our results suggest that disruption of maternal geminin may decrease Cdt1 expression and cause DNA rereplication, which then activates the cell cycle checkpoint and DNA damage repair and thus impairs early embryo development.

The Dynamics and Regulatory Mechanism of Pronuclear H3k9me2 Asymmetry in Mouse Zygotes

H3K9 methylation is an important histone modification that is correlated with gene transcription repression. The asymmetric H3K9 dimethylation (H3K9me2) pattern between paternal and maternal genomes is generated soon after fertilization. In the present study, we carefully determined the dynamics of H3K9me2 changes in mouse zygotes, and investigated the regulatory mechanisms. The results indicated that histone methyltransferase G9a, but not GLP, was involved in the regulation of asymmetric H3K9me2, and G9a was the methyltransferase that induced the appearance of H3K9me2 in the male pronucleus of the zygote treated with cycloheximide. We found that there were two distinct mechanisms that regulate H3K9me2 in the male pronucleus. Before 8 h of in vitro fertilization (IVF), a mechanism exists that inhibits the association of G9a with the H3K9 sites. After 10 h of IVF the inhibition of G9a activity depends on yet unknown novel protein(s) synthesis. The two mechanisms of transfer take place between 8-10 h of IVF, and the novel protein failed to inhibit G9a activity in time, resulting in the appearance of a low level de novo H3K9me2 in the male pronucleus.

Sirt6 depletion causes spindle defects and chromosome misalignment during meiosis of mouse oocyte

Sirt6, a member of the sirtuin family of NAD-dependent protein deacetylases, has been implicated in multiple biological processes. However, the roles of Sirt6 in meiosis have not been addressed. In the present study, by employing knockdown analysis in mouse oocytes, we evaluated the effects of Sirt6 on meiotic apparatus. We found that specific depletion of Sirt6 results in disruption of spindle morphology and chromosome alignment in oocytes. Consistent with this observation, incidence of aneuploidy is also markedly increased in Sirt6-depleted oocytes. Furthermore, confocal scanning showed that kinetochore-microtubule interaction, an important mechanism controlling chromosome segregation, is severely impaired in metaphase oocytes following Sirt6 knockdown. Unexpectedly, we discovered that Sirt6 modulates the acetylation status of histone H4K16 as their knockdown specifically induces the hyperacetylation of H4K16 in oocytes, which may be associated with the defective phenotypes described above via altering kinetochore function. Altogether, our data reveal a novel function of Sirt6 during oocyte meiosis and indicate a pathway regulating meiotic apparatus.

Comparison of the effects of BPA and BPAF on oocyte spindle assembly and polar body release in mice

Bisphenol AF (BPAF), a homolog of bisphenol A (BPA), is a widely used environmental chemical that has adverse effects on reproduction. The aim of this study was to analyse the effects of BPA and BPAF exposure on oocyte maturation in vitro. Oocytes were cultured in the presence of BPA or BPAF (2, 20, 50 or 100 μg/ml) for 18 h. At concentrations of 50 and 100 μg/ml, BPA and BPAF inhibited oocyte maturation, with BPAF treatment causing a sharp decrease in the number of oocytes reaching maturity. Oocytes were exposed to BPA or BPAF at 2 μg/ml and cultured for different durations (6, 9, 12, 15 or 18 h). Both BPAF and BPA caused a cell cycle delay under these conditions. Oocytes cultured in the presence of BPA or BPAF (50 μg/ml) for 21 h were tested for the localization of α-tubulin and MAD2 using immunofluorescence. High concentrations of BPAF induced cell cycle arrest through the activation of the spindle assembly checkpoint. After 12 h of culture in BPAF (50 μg/ml), oocytes were transferred to control medium for 9 h. Only 63.3% oocytes treated in this manner progressed to metaphase II (MII). Oocytes exposed to high doses of BPA experienced a cell cycle delay, but managed to progress to MII when the culture period was prolonged. In addition, MAD2 was localized in the cytoplasm of these oocytes. In conclusion, both BPAF and BPA exposure affected oocyte maturation, however BPAF and BPA have differential effects on SAC activity.

Impairment of preimplantation porcine embryo development by histone demethylase KDM5B knockdown through disturbance of bivalent H3K4me3-H3K27me3 modifications

KDM5B (JARID1B/PLU1) is a H3K4me2/3 histone demethylase that is implicated in cancer development and proliferation and is also indispensable for embryonic stem cell self-renewal, cell fate, and murine embryonic development. However, little is known about the role of KDM5B during preimplantation embryo development. Here we show that KDM5B is critical to porcine preimplantation development. KDM5B was found to be expressed in a stage-specific manner, consistent with demethylation of H3K4me3, with the highest expression being observed from the 4-cell to the blastocyst stages. Knockdown of KDM5B by morpholino antisense oligonucleotides injection impaired porcine embryo development to the blastocyst stage. The impairment of embryo development might be caused by increased expression of H3K4me3 at the 4-cell and blastocyst stages, which disturbs the balance of bivalent H3K4me3-H3K27me3 modifications at the blastocyst stage. Decreased abundance of H3K27me3 at blastocyst stage activates multiple members of homeobox genes (HOX), which need to be silenced for faithful embryo development. Additionally, the histone demethylase KDM6A was found to be upregulated by knockdown of KDM5B, which indicated it was responsible for the decreased abundance of H3K27me3 at the blastocyst stage. The transcriptional levels of Ten-Eleven Translocation gene family members (TET1, TET2, and TET3) are found to be increased by knockdown of KDM5B, which indicates cross talk between histone modifications and DNA methylation. The studies above indicate that KDM5B is required for porcine embryo development through regulating the balance of bivalent H3K4me3-H3K27me3 modifications.

Scaffold subunit Aalpha of PP2A is essential for female meiosis and fertility in mice

Ppp2r1a encodes the scaffold subunit Aalpha of protein phosphatase 2A (PP2A), which is an important and ubiquitously expressed serine threonine phosphatase family and plays a critical role in many fundamental cellular processes. To identify the physiological role of PP2A in female germ cell meiosis, we selectively disrupted Ppp2r1a expression in oocytes by using the Cre-Loxp conditional knockout system. Here we report for the first time that oocyte-specific deletion of Ppp2r1a led to severe female subfertility without affecting follicle survival, growth, and ovulation. PP2A-Aalpha was essential for regulating oocyte meiotic maturation because depletion of PP2A-Aalpha facilitated germinal vesicle breakdown, causing elongation of the MII spindle and precocious separation of sister chromatids. The resulting eggs had high risk of aneuploidy, though they could be fertilized, leading to defective embryonic development and thus subfertility. Our findings provide strong evidence that PP2A-Aalpha within the oocyte plays an indispensable role in oocyte meiotic maturation, though it is dispensable for folliculogenesis in the mouse ovary.

Rab5a is required for spindle length control and kinetochore-microtubule attachment during meiosis in oocytes

Rab GTPases are highly conserved components of vesicle trafficking pathways. Rab5, as a master regulator of endocytic trafficking, has been shown to function in membrane tethering and docking. However, the function of Rab5 in meiosis has not been addressed. Here, we report elongated spindles and misaligned chromosomes, with kinetochore-microtubule misattachments, on specific depletion of Rab5a in mouse oocytes. Moreover, the localization and levels of centromere protein F (CENPF), a component of the nuclear matrix, are severely reduced at kinetochores in metaphase oocytes following Rab5a knockdown. Consistent with this finding, nuclear lamina disassembly in the transition from prophase arrest to meiosis I is also impaired in Rab5a-depleted oocytes. Notably, oocyte-specific ablation of CENPF phenocopies the meiotic defects resulting from Rab5a knockdown. In summary, our data support a model where Rab5a-positive vesicles, likely through interaction with nuclear lamina, modulate CENPF localization and levels at centromeres, consequently ensuring proper spindle length and kinetochore-microtubule attachment in meiotic oocytes.

Survivin is essential for fertile egg production and female fertility in mice

Survivin is the smallest member of the inhibitor of apoptosis protein (IAP) family and acts as a bifunctional protein involved in mitosis regulation and apoptosis inhibition. To identify the physiological role of Survivin in female reproduction, we selectively disrupted Survivin expression in oocytes and granulosa cells (GCs), two major cell types in the ovary, by two different Cre-Loxp conditional knockout systems, and found that both led to defective female fertility. Survivin deletion in oocytes did not affect oocyte growth, viability and ovulation, but caused tetraploid egg production and thus female infertility. Further exploration revealed that Survivin was essential for regulating proper meiotic spindle organization, spindle assembly checkpoint activity, timely metaphase-to-anaphase transition and cytokinesis. Mutant mice with Survivin depleted in GCs showed reduced ovulation and subfertility, caused by defective follicular growth, increased follicular atresia and impaired luteinization. These findings suggest that Survivin has an important role in regulating folliculogenesis and oogenesis in the adult mouse ovary.

Ex vivo expansion of functional human UCB-HSCs/HPCs by coculture with AFT024-hkirre cells

Kiaa1867 (human Kirre, hKirre) has a critical role in brain development and/or maintenance of the glomerular slit diaphragm in kidneys. Murine homolog of this gene, mKirre expressed in OP9 and AFT024 cells could support hematopoietic stem cells/hematopoietic progenitor cells (HSC/HPC) expansion in vitro. HKirre is also expressed in human FBMOB-hTERT cell line and fetal liver fibroblast-like cells but its function has remained unclear. In this paper, we cloned a hKirre gene from human fetal liver fibroblast-like cells and established a stably overexpressing hKirre-AFT024 cell line. Resultant cells could promote self-renewal and ex vivo expansion of HSCs/HPCs significantly higher than AFT024-control cells transformed with mock plasmid. The Expanded human umbilical cord blood (hUCB) CD34⁺ cells retained the capacity of multipotent differentiation as long as 8 weeks and successfully repopulated the bone marrow of sublethally irradiated NOD/SCID mice, which demonstrated the expansion of long-term primitive transplantable HSCs/HPCs. Importantly, hkirre could upregulate the expressions of Wnt-5A, BMP4, and SDF-1 and downregulate TGF-β with other hematopoietic growth factors. By SDS-PAGE and Western Blot analysis, a ~89 kDa protein in total lysate of AFT024-hKirre was identified. Supernatants from AFT024-hkirre could also support CD34⁺CD38⁻ cells expansion. These results demonstrated that the AFT024-hKirre cells have the ability to efficiently expand HSCs/HPCs.

Sirt2 functions in spindle organization and chromosome alignment in mouse oocyte meiosis

Sirtuins have been widely reported to be involved in multiple biological processes; however, their function in oocyte meiosis has not been. Here, by confocal scanning and quantitative analysis, we show that specific depletion of Sirt2 in mouse oocytes results in spindle defects and chromosome disorganization (35.5±8.7 vs. 9.6±3.8% control; P<0.05), with impaired microtubule-kinetochore interaction. Moreover, knockdown and overexpression experiments reveal that Sirt2 modulates the acetylation status of histone H4K16 and α-tubulin in oocytes, which may in part mediate the defective phenotypes described above by influencing microtubule dynamics and kinetochore function. Finally, we find lower Sirt2 protein level in oocytes from aged mice by immunoblotting and that maternal age-associated meiotic defects can be ameliorated through overexpression of Sirt2 (33.2±5.1% old vs.12.7±5.2% old+Sirt2; P<0.05), providing support for the hypothesis that decreased Sirt2 is one of a number of factors contributing to oocyte age-dependent deficits. In summary, our data indicate a role for Sirt2 during oocyte meiosis and uncover a striking beneficial effect of increased Sirt2 expression on aged oocytes.

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.

Rat eggs cannot wait: Spontaneous exit from meiotic metaphase-II arrest

Mammalian eggs await fertilisation while arrested at the second metaphase stage of meiotic division. A network of signalling pathways enables the establishment and maintenance of this metaphase-II arrest. In the absence of fertilisation, mammalian eggs can spontaneously exit metaphase II when parthenogenetically stimulated, or sometimes without any obvious stimulation. Ovulated rat eggs abortively release from metaphase-II arrest once removed from egg donors. Spontaneously activated rat eggs extrude the second polar body and proceed to the so-called metaphase III-'like' stage, with clumps of condensed chromatin scattered in the egg cytoplasm. It is still unclear what makes rat eggs susceptible to spontaneous activation; however, a vague picture of the signalling pathways involved in the process of spontaneous activation is beginning to emerge. Such cell cycle instability is one of the major reasons why it is more difficult to establish nuclear transfer in the rat. This review examines the known predisposing factors and biochemical mechanisms involved in spontaneous activation. The strategies used to prevent spontaneous metaphase-II release in rat eggs will also be discussed.

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.

Bub3 is a spindle assembly checkpoint protein regulating chromosome segregation during mouse oocyte meiosis

In mitosis, the spindle assembly checkpoint (SAC) prevents anaphase onset until all chromosomes have been attached to the spindle microtubules and aligned correctly at the equatorial metaphase plate. The major checkpoint proteins in mitosis consist of mitotic arrest-deficient (Mad)1–3, budding uninhibited by benzimidazole (Bub)1, Bub3, and monopolar spindle 1(Mps1). During meiosis, for the formation of a haploid gamete, two consecutive rounds of chromosome segregation occur with only one round of DNA replication. To pull homologous chromosomes to opposite spindle poles during meiosis I, both sister kinetochores of a homologue must face toward the same pole which is very different from mitosis and meiosis II. As a core member of checkpoint proteins, the individual role of Bub3 in mammalian oocyte meiosis is unclear. In this study, using overexpression and RNA interference (RNAi) approaches, we analyzed the role of Bub3 in mouse oocyte meiosis. Our data showed that overexpressed Bub3 inhibited meiotic metaphase-anaphase transition by preventing homologous chromosome and sister chromatid segregations in meiosis I and II, respectively. Misaligned chromosomes, abnormal polar body and double polar bodies were observed in Bub3 knock-down oocytes, causing aneuploidy. Furthermore, through cold treatment combined with Bub3 overexpression, we found that overexpressed Bub3 affected the attachments of microtubules and kinetochores during metaphase-anaphase transition. We propose that as a member of SAC, Bub3 is required for regulation of both meiosis I and II, and is potentially involved in kinetochore-microtubule attachment in mammalian oocytes.

Effects of heat stress during in vitro maturation on cytoplasmic versus nuclear components of mouse oocytes

The objectives of this study were to investigate the effect of heat stress during in vitro maturation on the developmental potential of mouse oocytes and to determine whether the deleterious effect was on the nuclear or cytoplasmic component. While rates of oocyte nuclear maturation (development to the metaphase II stage) did not differ from 37 to 40 degrees C, rates for blastocyst formation decreased significantly as maturation temperature increased from 38.5 to 39 degrees C. Chromosome spindle exchange showed that while blastocyst formation did not differ when spindles matured in vivo or in vitro at 37, 40 or 40.7 degrees C were transplanted into in vivo matured cytoplasts, no blastocyst formation was observed when in vivo spindles were transferred into the 40 degrees C cytoplasts. While oocytes reconstructed between 37 degrees C ooplasts and 37 or 40 degrees C karyoplasts developed into 4-cell embryos at a similar rate, no oocytes reconstituted between 40 degrees C ooplasts and 37 degrees C spindles developed to the 4-cell stage. Immunofluorescence microscopy revealed impaired migration of cortical granules and mitochondria in oocytes matured at 40 degrees C compared with oocytes matured at 37 degrees C. A decreased glutathione/GSSG ratio was also observed in oocytes matured at 40 degrees C. While spindle assembling was normal and no MAD2 was activated in oocytes matured at 37 or 40 degrees C, spindle assembling was affected and MAD2 was activated in some of the oocytes matured at 40.7 degrees C. It is concluded that 1) oocyte cytoplasmic maturation is more susceptible to heat stress than nuclear maturation, and 2) cytoplasmic rather than nuclear components determine the pre-implantation developmental capacity of an oocyte.

Faulty spindle checkpoint and cohesion protein activities predispose oocytes to premature chromosome separation and aneuploidy

Aneuploidy accounts for a major proportion of human reproductive failures, mental and physical anomalies, and neoplasms. To heighten our understanding of normal and abnormal chromosome segregation, additional information is needed about the underlying molecular mechanisms of chromosome segregation. Although many hypotheses have been proposed for the etiology of human aneuploidy, there has not been general acceptance of any specific hypothesis. Moreover, it is important to recognize that many potential mechanisms exist whereby chromosome missegregation may occur. One area for investigating aneuploidy centers on the biochemical changes that take place during oocyte maturation. In this regard, recent results have shown that faulty mRNA of spindle-assembly checkpoint proteins and chromosome cohesion proteins may lead to aneuploidy. Also, postovulatory and in vitro aging of mouse oocytes has been shown to lead to decreased levels of Mad2 transcripts and elevated frequencies of premature centromere separation. The intent of this review is to highlight the major events surrounding chromosome segregation and to present the published results that support the premise that faulty chromosome cohesion proteins and spindle checkpoint proteins compromise accurate chromosome segregation.

BRCA1 is required for meiotic spindle assembly and spindle assembly checkpoint activation in mouse oocytes

BRCA1 as a tumor suppressor has been widely investigated in mitosis, but its functions in meiosis are unclear. In the present study, we examined the expression, localization, and function of BRCA1 during mouse oocyte meiotic maturation. We found that expression level of BRCA1 was increased progressively from germinal vesicle to metaphase I stage, and then remained stable until metaphase II stage. Immunofluorescent analysis showed that BRCA1 was localized to the spindle poles at metaphase I and metaphase II stages, colocalizing with centrosomal protein gamma-tubulin. Taxol treatment resulted in the presence of BRCA1 onto the spindle microtubule fibers, whereas nocodazole treatment induced the localization of BRCA1 onto the chromosomes. Depletion of BRCA1 by both antibody injection and siRNA injection caused severely impaired spindles and misaligned chromosomes. Furthermore, BRCA1-depleted oocytes could not arrest at the metaphase I in the presence of low-dose nocodazole, suggesting that the spindle checkpoint is defective. Also, in BRCA1-depleted oocytes, gamma-tubulin dissociated from spindle poles and MAD2L1 failed to rebind to the kinetochores when exposed to nocodazole at metaphase I stage. Collectively, these data indicate that BRCA1 regulates not only meiotic spindle assembly, but also spindle assembly checkpoint, implying a link between BRCA1 deficiency and aneuploid embryos.

Overcoming MIII arrest from spontaneous activation in cultured rat oocytes

The rat oocyte spontaneously activates under a wide variety of conditions. This process progresses to MIII arrest that is not responsive to parthenogenetic activation and development. Insofar as activation involves extrusion of the second polar body (PBII), we set out to determine if preventing this step by inhibiting microfilaments would change the course of spontaneous activation (SA). In particular, how long does the effect of SA persist while retaining reversibility of PBII extrusion once inhibitors are removed? We wanted to determine if the eggs would be responsive to parthenogenetic activation and capable of resuming development once a permanent inhibition is achieved. We set out to determine whether SA would depend on the ovular age of oocytes. Inhibiting of PBII extrusion was achieved by affecting microtubules with demecolcine or nocodazole or actin filaments with cytochalasin B (CB) and cytochalasin D (CD). We found that all oocytes undergo SA and progression to MIII; however, the rapidity of spontaneous activation is a function of the ovular age of the oocyte. The resumption of the meiosis period changes dramatically from 20 to 180 min with decreasing ovular age. We established that suppression of PB formation can be effectively achieved in oocytes of younger ovular age, and that inhibition of PB extrusion became irreversible after 3.5 h of treatment. We established that drug-treated oocytes could undergo subsequent reactivation and in vitro development to blastocysts. The rate of in vitro development of cytochalasin-treated group was comparable to parthenogenetic controls, while nocodazole and demecolcine produced oocytes that developed at lower frequencies. Thus, the application of the microfilament inhibiting drugs helps to overcome the negative effect of SA that results in MIII arrest. Here we also show optimized parthenogenetic stimulation that resulted in development to the blastocyst stage at frequency comparable to development of fertilized embryos.

Defective cohesin is associated with age-dependent misaligned chromosomes in oocytes

Aneuploidy often results from chromosome misalignment at metaphases. Oocytes from senescence-accelerated mice (SAM) exhibit increased chromosome misalignment with age, which originates from nuclear factors. This work sought to further characterize the underlying defects of chromosome misalignments. Using immunofluorescence microscopy with specific antibodies, several specific components associated with spindles or chromosomes, including centrosomes, centromeres and cohesin complex were examined. No obvious differences were found in the distribution of centrosome focus at the spindle pole of oocytes from young and aged SAM, regardless of chromosome alignments, although cytoplasmic centrosome foci were significantly reduced in aged SAM (P < 0.0001). Oocytes from both young and aged SAM exhibited centromere-associated protein-E (CENP-E) at centromeres of all chromosomes, including misaligned chromosomes from aged SAM, demonstrating that CENP-E did not contribute to chromosome misalignments. Notably, both meiotic cohesin proteins located between sister chromatids, REC8 (recombinant 8), STAG3 (stromal antigen 3) and SMC1beta, were remarkably reduced in oocytes from aged SAM. Further, degradation of the cohesin was even more obvious in SAM than in hybrid F1 mice with age, which may explain why SAM are vulnerable to aneuploidy. This natural ageing mouse model shows that defective cohesin coincides with increased incidence of chromosome misalignment and precocious separations of sister chromatids.

Meiosis in oocytes: predisposition to aneuploidy and its increased incidence with age

Mammalian oocytes begin meiosis in the fetal ovary, but only complete it when fertilized in the adult reproductive tract. This review examines the cell biology of this protracted process: from entry of primordial germ cells into meiosis to conception. The defining feature of meiosis is two consecutive cell divisions (meiosis I and II) and two cell cycle arrests: at the germinal vesicle (GV), dictyate stage of prophase I and at metaphase II. These arrests are spanned by three key events, the focus of this review: (i) passage from mitosis to GV arrest during fetal life, regulated by retinoic acid; (ii) passage through meiosis I and (iii) completion of meiosis II following fertilization, both meiotic divisions being regulated by cyclin-dependent kinase (CDK1) activity. Meiosis I in human oocytes is associated with an age-related high rate of chromosomal mis-segregation, such as trisomy 21 (Down's syndrome), resulting in aneuploid conceptuses. Although aneuploidy is likely to be multifactorial, oocytes from older women may be predisposed to be becoming aneuploid as a consequence of an age-long decline in the cohesive ties holding chromosomes together. Such loss goes undetected by the oocyte during meiosis I either because its ability to respond and block division also deteriorates with age, or as a consequence of being inherently unable to respond to the types of segregation defects induced by cohesion loss.

Exposure of mouse oocytes to bisphenol A causes meiotic arrest but not aneuploidy

Mouse oocytes isolated from large antral follicles were exposed to a wide range of concentrations of bisphenol A (BPA) during maturation in vitro (50 ng/ml to 10 microg/ml BPA in medium). Exposure to high concentrations of BPA (10 microg/ml) affected spindle formation, distribution of pericentriolar material and chromosome alignment on the spindle (termed congression failure), and caused a significant meiotic arrest. However, BPA did not increase hyperploidy at meiosis II at any tested concentration. Some but not all meiosis I arrested oocytes had MAD2-positive foci at centromeres of chromosomes in bivalents, suggesting that they had failed to pass the spindle checkpoint control. In a second set of experiments prepubertal mice were exposed sub-chronically for 7 days to low BPA by daily oral administration, followed by in vitro maturation of the denuded oocytes to metaphase II in the absence of BPA, as this treatment protocol was previously reported to induce chromosome congression failure and therefore suspected to cause aneuploidy in oocytes. The sub-chronic exposure subtly affected spindle morphology and oocyte maturation. However, as with the exposure in vitro, there was no evidence that low BPA doses increased hyperploidy at meiosis II. In conclusion, the data suggest that mouse oocytes from mice respond to BPA-induced disturbances in spindle formation by induction of meiotic arrest. This response might result from an effective checkpoint mechanism preventing the occurrence of chromosome malsegregation and aneuploidy. Low chronic BPA exposure in vivo as such does not appear to pose a risk for induction of errors in chromosome segregation at first meiosis in mouse oocytes. Additional factors besides BPA may have caused the high rate of congression failure and the temporary increase in hyperploidy in mouse metaphase II oocytes reported previously.

Spindle formation, chromosome segregation and the spindle checkpoint in mammalian oocytes and susceptibility to meiotic error

The spindle assembly checkpoint (SAC) monitors attachment to microtubules and tension on chromosomes in mitosis and meiosis. It represents a surveillance mechanism that halts cells in M-phase in the presence of unattached chromosomes, associated with accumulation of checkpoint components, in particular, Mad2, at the kinetochores. A complex between the anaphase promoting factor/cylosome (APC/C), its accessory protein Cdc20 and proteins of the SAC renders APC/C inactive, usually until all chromosomes are properly assembled at the spindle equator (chromosome congression) and under tension from spindle fibres. Upon release from the SAC the APC/C can target proteins like cyclin B and securin for degradation by the proteasome. Securin degradation causes activation of separase proteolytic enzyme, and in mitosis cleavage of cohesin proteins at the centromeres and arms of sister chromatids. In meiosis I only the cohesin proteins at the sister chromatid arms are cleaved. This requires meiosis specific components and tight regulation by kinase and phosphatase activities. There is no S-phase between meiotic divisions. Second meiosis resembles mitosis. Mammalian oocytes arrest constitutively at metaphase II in presence of aligned chromosomes, which is due to the activity of the cytostatic factor (CSF). The SAC has been identified in spermatogenesis and oogenesis, but gender-differences may contribute to sex-specific differential responses to aneugens. The age-related reduction in expression of components of the SAC in mammalian oocytes may act synergistically with spindle and other cell organelles' dysfunction, and a partial loss of cohesion between sister chromatids to predispose oocytes to errors in chromosome segregation. This might affect dose-response to aneugens. In view of the tendency to have children at advanced maternal ages it appears relevant to pursue studies on consequences of ageing on the susceptibility of human oocytes to the induction of meiotic error by aneugens and establish models to assess risks to human health by environmental exposures.

Cytoplasmic dynein participates in meiotic checkpoint inactivation in mouse oocytes by transporting cytoplasmic mitotic arrest-deficient (Mad) proteins from kinetochores to spindle poles

The present study was designed to investigate the localization and function of cytoplasmic dynein (dynein) during mouse oocyte meiosis and its relationship with two major spindle checkpoint proteins, mitotic arrest-deficient (Mad) 1 and Mad2. Oocytes at various stages during the first meiosis were fixed and immunostained for dynein, Mad1, Mad2, kinetochores, microtubules, and chromosomes. Some oocytes were treated with nocodazole before examination. Anti-dynein antibody was injected into the oocytes at germinal vesicle (GV) stage before the examination of its effects on meiotic progression or Mad1 and Mad2 localization. Results showed that dynein was present in the oocytes at various stages from GV to metaphase II and the locations of Mad1 and Mad2 were associated with dynein’s movement. Both Mad1 and Mad2 had two existing states: one existed in the cytoplasm (cytoplasmic Mad1 or cytoplasmic Mad2), which did not bind to kinetochores, while the other bound to kinetochores (kinetochore Mad1 or kinetochore Mad2). The equilibrium between the two states varied during meiosis and/or in response to the changes of the connection between microtubules and kinetochores. Cytoplasmic Mad1 and Mad2 recruited to chromosomes when the connection between microtubules and chromosomes was destroyed. Inhibition of dynein interferes with cytoplasmic Mad1 and Mad2 transportation from chromosomes to spindle poles, thus inhibits checkpoint silence and delays anaphase onset. These results indicate that dynein may play a role in spindle checkpoint inactivation.

RNA Interference as a tool to study the function of MAD2 in mouse oocyte meiotic maturation

Spindle checkpoint proteins control entry into anaphase and chromosome segregation. As a member of spindle checkpoint proteins, MAD2 takes a central role in the regulation of anaphase onset and genome integrity. Here, we used MAD2 siRNA transfection approach to study MAD2 functions during mouse oocyte meiotic maturation in vitro. Real-time PCR and laser scanning confocal microscopy showed that we successfully downregulated MAD2 transcript and protein expression. We further demonstrated that MAD2 downregulation resulted in a shortened duration of meiosis I and meiotic spindle abnormality, suggesting the function of MAD2 in mouse oocyte meiotic maturation. We also showed that MAD2 interference to some extent decreased GVBD rate, but increased apoptosis in mouse oocytes. In conclusion, our study shows that siRNA transfection is an effective tool to study MAD2 functions, and our results provide further evidence for the role of MAD2 as a spindle checkpoint protein in mouse oocytes.

2-methoxyestradiol induces spindle aberrations, chromosome congression failure, and nondisjunction in mouse oocytes

2-Methoxyestradiol (2-ME) is a metabolite of 17beta-estradiol and a natural component of follicular fluid. Local concentrations of 2-ME may be increased by exposure to environmental pollutants that activate the expression of enzymes in the metabolic pathway from 17beta-estradiol to 2-ME. It has been suspected that this may have adverse effects on spindle formation in maturing oocytes, which would affect embryo quality. To study the dose-response patterns, we exposed denuded mouse oocytes to 2-ME during in vitro maturation. Meiotic progression, spindle morphology, centrosome integrity, and chromosome congression were examined by immunofluorescence and noninvasive polarizing microscopy (PolScope). Chromosomal constituents were assessed after spreading and C-banding. 2-ME sustained MAD2L1 expression at the centromeres and increased the number of meiosis I-blocked oocytes in a dose-dependent manner. 2-ME also caused dramatic dose-dependent increases in the hyperploidy of metaphase II oocytes. Some of these meiosis II oocytes contained anaphase I-like chromosomes, which suggests that high concentrations of the catecholestradiol interfere with the physical separation of chromosomes. Noninvasive PolScope analysis and tubulin immunofluorescence revealed that perturbations in spindle organization, which resulted in severe disturbances of the chromosome alignment at the spindle equator (congression failure), were caused by 2-ME at meiosis I and II. Pericentrin-positive centrosomes failed to align at the spindle poles, and multipolar spindles and prominent arrays of cytoplasmic microtubule asters were induced in 2-ME-exposed metaphase II oocytes. In conclusion, a micromolar level of 2-ME is aneugenic for mammalian oocytes. Therefore, exposure to 2-ME and conditions that increase the intrinsic local concentration of 2-ME in the ovary may affect fertility and increase risks for chromosomal aberrations in the oocyte and embryo.

The First Mitosis of the Mouse Embryo Is Prolonged by Transitional Metaphase Arrest1

The first mitosis of the mouse embryo is almost twice as long as the second. The mechanism of the prolongation of the first mitosis remains unknown, and it is not clear whether prometaphase or metaphase or both are prolonged. Prometaphase is characterized by dynamic chromosome movements and spindle assembly checkpoint activity, which prevents anaphase until establishment of stable kinetochore-microtubule connections. The end of prometaphase is correlated with checkpoint inactivation and disappearance of MAD2L1 (MAD2) and RSN (CLIP-170) proteins from kinetochores. Spindle assembly checkpoint operates during the early mouse mitoses, but it is not clear whether it influences their duration. Here, we determine the length of prometaphases and metaphases during the first two embryonic mitoses by time-lapse video recording of chromosomes and by immunolocalization of MAD2L1 and RSN proteins. We show that the duration of the two prometaphases does not differ and that MAD2L1 and RSN disappear from kinetochores very early during each mitosis. The first metaphase is significantly longer than the second one. Therefore, the prolongation of the first embryonic mitosis is due to a prolonged metaphase, and the spindle assembly checkpoint cannot be involved in this process. We show also that MAD2L1 staining disappears gradually from kinetochores of oocytes arrested at metaphase of the second meiotic division. This shows a striking similarity between the first embryonic mitosis and metaphase arrest in oocytes. We postulate that the first embryonic mitosis is prolonged by a transient metaphase arrest that is independent of the spindle assembly checkpoint and is similar to metaphase II arrest. The molecular mechanism of this transient arrest remains to be elucidated.

Meiotic spindle checkpoints for assessment of aneuploid oocytes

The spindle assembly checkpoint suspends cell cycle progression if improperly aligned chromosomes are detected at metaphase. Evolutionarily conserved kinetochore-associated proteins are believed to be key elements of this regulatory pathway. A breakdown in checkpoint function could bring about genomic instability, which may be responsible for the prevalence of aneuploidy in oocytes of older women. Maternal aging remains the overwhelming factor in the etiology of human aneuploidy in assisted reproduction. Defects in cell cycle checkpoint genes may play a role in its development. The existence of such monitoring mechanisms in oocytes has long been controversial. Studies providing evidence in support of and against their existence are reviewed.

Interactions of the meiotic spindle with mitotic chromosomes in GV mouse oocytes

During mitosis, a spindle checkpoint detects chromosome misalignment and halts the cell cycle progression. In meiosis of female germ cells, however, it is debatable whether such a checkpoint is present. This research employed a unique model in the mouse, mitotic chromosomes transferred to meiotic cytoplasts to investigate whether a meiotic oocyte's microtubule apparatus can effectively separate mitotic metaphase chromosomes, and whether a spindle checkpoint exists during its division. The intact germinal vesicle (GV) oocytes, enucleated GV cytoplasts, and enucleated GV cytoplasts at 15 h in-vitro maturation were transferred with a metaphase fibroblast cell. When mitotic chromosomes were transferred into enucleated or intact mouse GV oocytes, the first bipolar meiotic spindles were established and the reconstructed oocytes were able to extrude polar bodies. However, none of the reconstructed oocytes showed complete and accurate alignment of chromosomes, except the enucleated GV cytoplasts reconstructed after maturation. The spindle formation and polar body extrusion suggest that the first meiotic spindle was functional, and the chromosome misalignment did not prevent the onset of anaphase. The data indicate that a spindle checkpoint, providing surveillance of misaligned chromosomes, was overridden or compromised by the incompatibility between somatic chromosomes and meiotic spindles during the first meiotic division.

Brd4 is required for recovery from antimicrotubule drug-induced mitotic arrest: preservation of acetylated chromatin

The mammalian bromodomain protein Brd4 interacts with mitotic chromosomes by binding to acetylated histone H3 and H4 and is thought to play a role in epigenetic memory. Mitotic cells are susceptible to antimicrotubule drugs. These drugs activate multiple response pathways and arrest cells at mitosis. We found that Brd4 was rapidly released from chromosomes upon treatment with antimicrotubule drugs, including the reversible agent nocodazole. Yet, when nocodazole was withdrawn, Brd4 was reloaded onto chromosomes, and cells proceeded to complete cell division. However, cells in which a Brd4 allele was disrupted (Brd4+/-), and expressing only half of the normal Brd4 levels, were defective in reloading Brd4 onto chromosomes. Consequently, Brd4+/- cells were impaired in their ability to recover from nocodazole-induced mitotic arrest: a large fraction of +/- cells failed to reach anaphase after drug withdrawal, and those that entered anaphase showed an increased frequency of abnormal chromosomal segregation. The reloading defect observed in Brd4+/- cells coincided with selective hypoacetylation of lysine residues on H3 and H4. The histone deacetylase inhibitor trichostatin A increased global histone acetylation and perturbed nocodazole-induced Brd4 unloading. Brd4 plays an integral part in a cellular response to drug-induced mitotic stress by preserving a properly acetylated chromatin status.

Post-ovulatory aging of mouse oocytes leads to decreased MAD2 transcripts and increased frequencies of premature centromere separation and anaphase

Numerous cytological and biochemical alterations occur as mammalian oocytes age post-ovulation. Some of these changes can predispose cells to aneuploidy. The objective of this study was to test the hypothesis that the level of MAD2 spindle assembly checkpoint (SAC) transcripts decrease as mouse oocytes age post-ovulation and that this decrease was associated with chromosome missegregation. Female Institute of Cancer Research (ICR) mice were superovulated and oocytes collected at 14 h, 19 h and 24 h post-HCG for cytogenetic and quantitative real-time rapid cycle fluorescent RT-PCR analyses. Premature centromere separation (PCS) is now generally recognized as a predisposition to aneuploidy. The data showed that the frequencies of PCS-incomplete (PCS-I) did not significantly (P > 0.05) increase with time post-ovulation; whereas the proportions of oocytes displaying PCS-complete (PCS-C) and premature anaphase (PA) were significantly (P < 0.01) greater at 19 h and 24 h post-HCG, respectively. The higher frequencies of PCS-C and PA found at 19 h and 24 h coincided with decreased levels of MAD2 transcripts at these same times. Although the decline in MAD 2 transcripts with oocyte aging represents only one of many potential mechanisms responsible for aneuploidy, a compromised SAC appears to have a role in the unfavourable reproductive outcome associated with post-ovulatory aged oocytes.

Localization of Mitotic Arrest Deficient 1 (MAD1) in Mouse Oocytes During the First Meiosis and Its Functions as a Spindle Checkpoint Protein1

The present study was designed to investigate the localization of mitotic arrest deficient 1 (MAD1) in mouse oocytes during meiotic maturation and its relationship with kinetochores, chromosomes, and microtubules. Oocytes at various stages during the first meiosis were fixed and immunostained for MAD1, kinetochores, microtubules, and chromosomes. The stained oocytes were examined by confocal microscopy. Some oocytes were treated with nocodazole or Taxol before examination. The anti-MAD1 antibody was injected into the oocytes at the germinal vesicle (GV) stage for examination of chromosome alignment and spindle formation. It was found that MAD1 was present in the oocytes from the GV to prometaphase I stages around the nuclei. When the oocytes reached the metaphase I (M-I) to metaphase II (M-II) stages, MAD1 was mainly localized at the spindle poles. However, MAD1 relocated to the vicinity of the chromosomes when spindles were disassembled by nocodazole or cooling, and the relocated MAD1 moved back to the spindle poles during spindle recovery. Taxol treatment did not affect the MAD1 localization. Although anti-MAD1 antibody injection did not affect nuclear maturation, significantly higher proportions of injected oocytes had misaligned chromosomes when the oocytes reached the M-I to M-II stages. The results of the present study indicate that MAD1 is present in mouse oocytes at all stages during the first meiosis and that it participates in spindle checkpoint during meiosis. However, MAD1 could not check misaligned chromosomes during spindle recovery after the spindles were destroyed by drug or cooling, which caused some chromosomes to scatter in the oocytes.