Multiple Duties for Spindle Assembly Checkpoint Kinases in Meiosis

Heating up meiosis - Chromosome recombination and segregation under high temperatures

Heat stress is one of the major constraints to plant growth and fertility. During the current climate crisis, heat waves have increased dramatically, and even more extreme conditions are predicted for the near future, considerably affecting ecosystems and seriously threatening world food security. Although heat is very well known to affect especially reproductive structures, little is known about how heat interferes with reproduction in comparison to somatic cells and tissues. Recently, the effect of heat on meiosis as a central process in sexual reproduction has been analyzed in molecular and cytological depth. Notably, these studies are not only important for applied research by laying the foundation for breeding heat-resilient crops, but also for fundamental research, revealing general regulatory mechanisms of recombination and chromosome segregation control.

Hijacking monopolar spindle 1 (MPS1) for various cancer types by small molecular inhibitors: Deep insights from a decade of research and patents

Monopolar spindle 1 (MPS1) has garnered significant attention due to its pivotal role in regulating the cell cycle. Anomalous expression and hyperactivation of MPS1 have been associated with the onset and advancement of diverse cancers, positioning it as a promising target for therapeutic interventions. This review focuses on MPS1 small molecule inhibitors from the past decade, exploring design strategies, structure-activity relationships (SAR), safety considerations, and clinical performance. Notably, we propose prospects for MPS1 degraders based on proteolysis targeting chimeras (PROTACs), as well as reversible covalent bonding as innovative MPS1 inhibitor design strategies. The objective is to provide valuable information for future development and novel perspectives on potential MPS1 inhibitors.

Kaempferol from Alpinia officinarum hance induces G2/M cell cycle arrest in hepatocellular carcinoma cells by regulating the ATM/CHEK2/KNL1 pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Alpinia officinarum Hance (A. officinarum), a perennial herb known for its medicinal properties, has been used to treat various ailments, such as stomach pain, abdominal pain, emesis, and digestive system cancers. A. officinarum is extensively cultivated in the Qiongzhong and Baisha regions of Hainan, and it holds substantial therapeutic value for the local Li people of Hainan. Kaempferol, a flavonoid derived from A. officinarum, has demonstrated anticancer properties in various experimental and biological studies. Nevertheless, the precise mechanisms through which it exerts its anti-hepatocellular carcinoma (HCC) effects remain to be comprehensively delineated.

AIM OF THE STUDY

This investigation aims to elucidate the anti-HCC effects of kaempferol derived from A. officinarum and to delve into its underlying mechanistic pathways.

MATERIALS AND METHODS

Using ultra-high performance liquid chromatography-mass spectrometry/mass spectrometry (UPLC-MS/MS) to identify active compounds in A. officinarum. HCCLM3 and Huh7 cells were used to study the anti-HCC effect of kaempferol from A. officinarum. The cytotoxicity and proliferation of kaempferol and A. officinarum were measured using CCK-8 and EDU staining. Wound-healing assays and three-dimensional tumor spheroid models were further used to evaluate migration and the anti-HCC activity of kaempferol. The cell cycle and apoptosis were evaluated by flow cytometry. Western blot and qRT-PCR were used to detect the expression of proteins and genes associated with the cell cycle checkpoints. Finally, bioinformatics was used to analyze the relationship between the differential expression of core targets in the ATM/CHEK2/KNL1 pathway and a poor prognosis in clinical HCC samples.

RESULTS

UPLC-MS/MS was employed to detect five active compounds in A. officinarum, such as kaempferol. The CCK-8 and EDU assays showed that kaempferol and A. officinarum significantly inhibited the proliferation of HCC cells. A wound-healing assay revealed that kaempferol remarkably inhibited the migration of HCC cells. Kaempferol significantly suppressed the growth of tumor spheroids. In addition, kaempferol markedly induced G2/M arrest and promoted apoptosis of HCC cells. Mechanically, kaempferol significantly reduced the protein and mRNA expression levels of ATM, CHEK2, CDC25C, CDK1, CCNB1, MPS1, KNL1, and Bub1. Additionally, the combination of kaempferol and the ATM inhibitor KU55933 had a more significant anti-HCC effect. The results of bioinformatics showed that ATM, CHEK2, CDC25C, CDK1, and KNL1 were highly expressed in patients with HCC and cancer tissues, indicating that these genes have certain value in the clinical diagnosis of HCC.

CONCLUSIONS

Collectively, our results revealed that kaempferol from A. officinarum inhibits the cell cycle by regulating the ATM/CHEK2/KNL1 pathway in HCC cells. In summary, our research presents an innovative supplementary strategy for HCC treatment.

Vinorelbine administration impedes the timely progression of meiotic maturation and induces aneuploidy in mouse oocytes

Vinorelbine is a commonly used drug to treat various malignancies, such as breast cancer, non-small cell lung cancer, and metastatic pleural mesothelioma. Its side effects include severe neutropenia, local phlebitis, gastrointestinal reactions, and neurotoxicity. In view of the scarcity of research on vinorelbine's reproductive toxicity, this study evaluated the impact of vinorelbine ditartrate, a commonly used form of vinorelbine, on oocyte maturation in vitro. Our investigation revealed that vinorelbine ditartrate had no effect on oocyte meiotic resumption. However, it did reduce the rate of first polar body extrusion, suggesting that it could significantly impede the meiotic maturation of oocytes. Vinorelbine ditartrate exposure was found to disturb the regular spindle assembly and chromosome alignment, leading to the continuous activation of the spindle assembly checkpoint (SAC) and a delayed activation of the anaphase-promoting complex/cyclosome (APC/C), ultimately causing aneuploidy in oocytes. Consequently, the administration of vinorelbine is likely to result in oocyte aneuploidy, which can be helpful in providing a drug reference and fertility guidance in a clinical context.

Vincristine exposure impairs mouse oocyte quality by inducing spindle defects and early apoptosis

Vincristine (VCR) is a microtubule-destabilizing chemotherapeutic agent commonly administered for the treatment of cancers in patients, which can induce severe side effects including neurotoxicity. In context of the effects on female fertility, ovarian toxicity has been found in patients and mice model after VCR exposure. However, the influence of VCR exposure on oocyte quality has not been elucidated. We established VCR exposure in vitro and in vivo model. The results indicated in vitro VCR exposure contributed to failure of oocyte maturation through inducing defects in spindle assembly, activation of SAC, oxidative stress, mitochondrial dysfunction, and early apoptosis, which were confirmed by using in vivo exposure model. Moreover, in vivo VCR exposure caused aneuploidy, reduced oocyte-sperm binding ability, and the number of cortical granules in mouse oocyte cortex. Taken together, this study demonstrated that VCR could cause meiotic arrest and poor quality of mouse oocyte.

Kinesin KIFC3 is essential for microtubule stability and cytokinesis in oocyte meiosis

KIFC3 is a member of Kinesin-14 family motor proteins, which play a variety of roles such as centrosome cohesion, cytokinesis, vesicles transportation and cell proliferation in mitosis. Here, we investigated the functional roles of KIFC3 in meiosis. Our findings demonstrated that KIFC3 exhibited expression and localization at centromeres during metaphase I, followed by translocation to the midbody at telophase I throughout mouse oocyte meiosis. Disruption of KIFC3 activity resulted in defective polar body extrusion. We observed aberrant meiotic spindles and misaligned chromosomes, accompanied by the loss of kinetochore-microtubule attachment, which might be due to the failed recruitment of BubR1/Bub3. Coimmunoprecipitation data revealed that KIFC3 plays a crucial role in maintaining the acetylated tubulin level mediated by Sirt2, thereby influencing microtubule stability. Additionally, our findings demonstrated an interaction between KIFC3 and PRC1 in regulating midbody formation during telophase I, which is involved in cytokinesis regulation. Collectively, these results underscore the essential contribution of KIFC3 to spindle assembly and cytokinesis during mouse oocyte meiosis.

Age-dependent loss of cohesion protection in human oocytes

Aneuploid human eggs (oocytes) are a major cause of infertility, miscarriage, and chromosomal disorders. Such aneuploidies increase greatly as women age, with defective linkages between sister chromatids (cohesion) in meiosis as a common cause. We found that loss of a specific pool of the cohesin protector protein, shugoshin 2 (SGO2), may contribute to this phenomenon. Our data indicate that SGO2 preserves sister chromatid cohesion in meiosis by protecting a "cohesin bridge" between sister chromatids. In human oocytes, SGO2 localizes to both sub-centromere cups and the pericentromeric bridge, which spans the sister chromatid junction. SGO2 normally colocalizes with cohesin; however, in meiosis II oocytes from older women, SGO2 is frequently lost from the pericentromeric bridge and sister chromatid cohesion is weakened. MPS1 and BUB1 kinase activities maintain SGO2 at sub-centromeres and the pericentromeric bridge. Removal of SGO2 throughout meiosis I by MPS1 inhibition reduces cohesion protection, increasing the incidence of single chromatids at meiosis II. Therefore, SGO2 deficiency in human oocytes can exacerbate the effects of maternal age by rendering residual cohesin at pericentromeres vulnerable to loss in anaphase I. Our data show that impaired SGO2 localization weakens cohesion integrity and may contribute to the increased incidence of aneuploidy observed in human oocytes with advanced maternal age.

C-Type Natriuretic Peptide Pre-Treatment Improves Maturation Rate of Goat Oocytes by Maintaining Transzonal Projections, Spindle Morphology, and Mitochondrial Function

C-type natriuretic peptide (CNP) is a peptide molecule naturally found in follicles and can be used to extend meiotic resumption and enhance the potential for oocytes to develop. However, the mechanism by which CNP improves goat oocyte quality remains unclear. In this study, cumulus-oocyte complexes (COCs) from goats were pre-treated with CNP prior to IVM, and the results showed that pre-treatment with CNP enhanced goat oocyte maturation. First, we discovered that CNP maintained communication between cumulus cells and oocytes by regulating the transzonal projections (TZPs). We then found that CNP treatment reduced abnormal spindle formation and increased the expression of genes associated with spindle assembly and the spindle assembly checkpoint. Moreover, further analysis showed that oocytes exhibited better antioxidant ability in the CNP treatment group, which mainly manifested in higher glutathione (GSH) and lower reactive oxygen species (ROS) concentrations. Enhanced mitochondrial activity was signified via the augmented expression of mitochondrial oxidative metabolism and fusion and fission-related genes, thus diminishing the apoptosis of the oocytes. Overall, these results provide novel insights into the potential mechanism by which CNP treatment before IVM can improve oocyte quality.

Aurora B kinase erases monopolar microtubule-kinetochore arrays at the meiosis I-II transition

During meiosis, faithful chromosome segregation requires monopolar spindle microtubule-kinetochore arrays in MI to segregate homologous chromosomes, but bipolar in MII to segregate sister chromatids. Using fission yeasts, we found that the universal Aurora B kinase localizes to kinetochores in metaphase I and in the mid-spindle during anaphase I, as in mitosis; but in the absence of an intervening S phase, the importin α Imp1 propitiates its release from the spindle midzone to re-localize at kinetochores during meiotic interkinesis. We show that "error-correction" activity of kinetochore re-localized Aurora B becomes essential to erase monopolar arrangements from anaphase I, a prerequisite to satisfy the spindle assembly checkpoint (SAC) and to generate proper bipolar arrays at the onset of MII. This microtubule-kinetochore resetting activity of Aurora B at the MI-MII transition is required to prevent chromosome missegregation in meiosis II, a type of error often associated with birth defects and infertility in humans.

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.

Carbendazim exposure inhibits mouse oocytes meiotic maturation in vitro by destroying spindle assembly

Successful fertilization and early embryonic development heavily depend on the quality of the oocytes. Carbendazim (CBZ), a broad-spectrum fungicide, is widely available in the environment and has adverse effects on organisms. The present study focused on exploring the potential reproductive toxicity of CBZ exposure by investigating its effects on the maturation of mouse oocytes. The results demonstrated that although no disruptions were observed in the G2/M stage transition for meiosis resumption, CBZ did hinder the polar body extrusion (PBE) occurring during oocyte maturation. Cell cycle distribution analysis revealed that CBZ exposure interfered with the meiotic process, causing oocytes to be arrested at the metaphase I (MI) stage. The subsequent investigation highlighted that CBZ exposure impeded the spindle assembly and chromosomal alignment, which was linked to a decline in the level of p-MAPK. Additionally, CBZ exposure adversely affected the kinetochore-microtubule (K-MT) attachment, leading to the persistent activation of the spindle-assembly checkpoint (SAC). The study further noticed a substantial rise in the acetylation of α-tubulin and a reduction in spindle microtubule stability in CBZ-treated oocytes. In addition, the distribution pattern of estrogen receptor alpha (ERα) was altered in oocytes treated with CBZ, with abnormal aggregation on the spindles. CBZ exposure also resulted in altered histone modifications. A notable finding from this research was that the meiotic maturation of some oocytes remained unaffected even after CBZ treatment. However, during the ensuing metaphase II (MII) stage, these oocytes displayed anomalies in their spindle morphology and chromosome arrangement and diminished ability to bind to the sperm. The observations made in this study underscore the potential for CBZ to disrupt the meiotic maturation of oocytes, leading to a decline in the overall quality of oocytes.

BUB-1 and CENP-C recruit PLK-1 to control chromosome alignment and segregation during meiosis I in C. elegans oocytes

Phosphorylation is a key post-translational modification that is utilised in many biological processes for the rapid and reversible regulation of protein localisation and activity. Polo-like kinase 1 (PLK-1) is essential for both mitotic and meiotic cell divisions, with key functions being conserved in eukaryotes. The roles and regulation of PLK-1 during mitosis have been well characterised. However, the discrete roles and regulation of PLK-1 during meiosis have remained obscure. Here, we used Caenorhabditis elegans oocytes to show that PLK-1 plays distinct roles in meiotic spindle assembly and/or stability, chromosome alignment and segregation, and polar body extrusion during meiosis I. Furthermore, by a combination of live imaging and biochemical analysis we identified the chromosomal recruitment mechanisms of PLK-1 during C. elegans oocyte meiosis. The spindle assembly checkpoint kinase BUB-1 directly recruits PLK-1 to the kinetochore and midbivalent while the chromosome arm population of PLK-1 depends on a direct interaction with the centromeric-associated protein CENP-CHCP-4. We found that perturbing both BUB-1 and CENP-CHCP-4 recruitment of PLK-1 leads to severe meiotic defects, resulting in highly aneuploid oocytes. Overall, our results shed light on the roles played by PLK-1 during oocyte meiosis and provide a mechanistic understanding of PLK-1 targeting to meiotic chromosomes.

The m6A reader PRRC2A is essential for meiosis I completion during spermatogenesis

N6-methyladenosine (m6A) and its reader proteins YTHDC1, YTHDC2, and YTHDF2 have been shown to exert essential functions during spermatogenesis. However, much remains unknown about m6A regulation mechanisms and the functions of specific readers during the meiotic cell cycle. Here, we show that the m6A reader Proline rich coiled-coil 2A (PRRC2A) is essential for male fertility. Germ cell-specific knockout of Prrc2a causes XY asynapsis and impaired meiotic sex chromosome inactivation in late-prophase spermatocytes. Moreover, PRRC2A-null spermatocytes exhibit delayed metaphase entry, chromosome misalignment, and spindle disorganization at metaphase I and are finally arrested at this stage. Sequencing data reveal that PRRC2A decreases the RNA abundance or improves the translation efficiency of targeting transcripts. Specifically, PRRC2A recognizes spermatogonia-specific transcripts and downregulates their RNA abundance to maintain the spermatocyte expression pattern during the meiosis prophase. For genes involved in meiotic cell division, PRRC2A improves the translation efficiency of their transcripts. Further, co-immunoprecipitation data show that PRRC2A interacts with several proteins regulating mRNA metabolism or translation (YBX1, YBX2, PABPC1, FXR1, and EIF4G3). Our study reveals post-transcriptional functions of PRRC2A and demonstrates its critical role in the completion of meiosis I in spermatogenesis.

No Chance to Survive: Mo-CBP3-PepII Synthetic Peptide Acts on Cryptococcus neoformans by Multiple Mechanisms of Action

Multidrug-resistant Cryptococcus neoformans is an encapsulated yeast causing a high mortality rate in immunocompromised patients. Recently, the synthetic peptide Mo-CBP₃-PepII emerged as a potent anticryptococcal molecule with an MIC₅₀ at low concentration. Here, the mechanisms of action of Mo-CBP₃-PepII were deeply analyzed to provide new information about how it led C. neoformans cells to death. Light and fluorescence microscopies, analysis of enzymatic activities, and proteomic analysis were employed to understand the effect of Mo-CBP₃-PepII on C. neoformans cells. Light and fluorescence microscopies revealed Mo-CBP₃-PepII induced the accumulation of anion superoxide and hydrogen peroxide in C. neoformans cells, in addition to a reduction in the activity of superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT) in the cells treated with Mo-CBP₃-PepII. In the presence of ascorbic acid (AsA), no reactive oxygen species (ROS) were detected, and Mo-CBP₃-PepII lost the inhibitory activity against C. neoformans. However, Mo-CBP₃-PepII inhibited the activity of lactate dehydrogenase (LDH) ergosterol biosynthesis and induced the decoupling of cytochrome c (Cyt c) from the mitochondrial membrane. Proteomic analysis revealed a reduction in the abundance of proteins related to energetic metabolism, DNA and RNA metabolism, pathogenicity, protein metabolism, cytoskeleton, and cell wall organization and division. Our findings indicated that Mo-CBP₃-PepII might have multiple mechanisms of action against C. neoformans cells, mitigating the development of resistance and thus being a potent molecule to be employed in the production of new drugs against C. neoformans infections.

Separase Control and Cohesin Cleavage in Oocytes: Should I Stay or Should I Go?

The key to gametogenesis is the proper execution of a specialized form of cell division named meiosis. Prior to the meiotic divisions, the recombination of maternal and paternal chromosomes creates new genetic combinations necessary for fitness and adaptation to an ever-changing environment. Two rounds of chromosome segregation -meiosis I and II- have to take place without intermediate S-phase and lead to the creation of haploid gametes harboring only half of the genetic material. Importantly, the segregation patterns of the two divisions are fundamentally different and require adaptation of the mitotic cell cycle machinery to the specificities of meiosis. Separase, the enzyme that cleaves Rec8, a subunit of the cohesin complex constituting the physical connection between sister chromatids, has to be activated twice: once in meiosis I and immediately afterwards, in meiosis II. Rec8 is cleaved on chromosome arms in meiosis I and in the centromere region in meiosis II. This step-wise cohesin removal is essential to generate gametes of the correct ploidy and thus, embryo viability. Hence, separase control and Rec8 cleavage must be perfectly controlled in time and space. Focusing on mammalian oocytes, this review lays out what we know and what we still ignore about this fascinating mechanism.

Octocrylene exposure impairs mouse oocyte quality by inducing spindle defects and mitochondria dysfunction

One of organic ultraviolet (UV) filters, Octocrylene (OCL), is mainly used in various cosmetic products, which is being frequently detected in soil, sediment, aquatic systems and food chain. There is evidence confirmed the reproductive toxicity of OCL in Japanese medaka. However, less was known about the effects of OCL exposure on oocyte quality. Here, we investigated the impacts of OCL on mouse oocyte maturation and quality by exposing oocytes to OCL in vitro at concentrations of 8, 22, 30, 40 and 50nM. The results showed that OCL markedly reduced mouse oocyte germinal vesicle breakdown (GVBD) at 50nM and polar body extrusion (PBE) rates at 40 and 50nM. OCL exposure further disrupted spindle assembly and chromosome alignment, finally inducing aneuploid. Mitochondrial function was also damaged by OCL exposure, leading to ROS overproduction and apoptosis in oocytes. Moreover, OCL treatment impaired the distribution of cortical granules and sperm binding ability of oocytes. In summary, these data demonstrated that OCL could disturb the oocyte meiotic maturation and reduce oocyte quality.

Nivalenol affects Cyclin B1 level and activates SAC for cell cycle progression in mouse oocyte meiosis

OBJECTIVES

Nivalenol (NIV) is a secondary metabolite of type B trichothecene mycotoxin produced by Fusarium genera, which is widely found in contaminated food and crops such as corn, wheat and peanuts. NIV is reported to have hepatotoxicity, immunotoxicity, genotoxicity, and reproductive toxicity. Previous studies indicate that NIV disturbs mammalian oocyte maturation. Here, we reported that delayed cell cycle progression might be the reason for oocyte maturation defect caused by NIV exposure.

METHODS AND RESULTS

We set up a NIV exposure model and showed that NIV did not affect G2/M transition for meiosis resumption, but disrupted the polar body extrusion of oocytes. Further analysis revealed that oocytes were arrested at metaphase I, which might be due to the lower expression of Cyclin B1 after NIV exposure. After cold treatment, the microtubules were disassembled in the NIV-exposed oocytes, indicating that NIV disrupted microtubule stability. Moreover, NIV affected the attachment between kinetochore and microtubules, which further induced the activation of MAD2/BUBR1 at the kinetochores, suggesting that spindle assemble checkpoint (SAC) was continuously activated during oocyte meiotic maturation.

CONCLUSIONS

Taken together, our study demonstrated that exposure to NIV affected Cyclin B1 expression and activated microtubule stability-dependent SAC to ultimately disturb cell cycle progression in mouse oocyte meiosis.

Pterostilbene Alleviates Chlorpyrifos-Induced Damage During Porcine Oocyte Maturation

Chlorpyrifos (CPF), a widely used organophosphate pesticide, is reported to severely impair mammalian reproductive system. Pterostilbene (PTS), an effective free radical scavenger, is considered as beneficial for mammalian reproduction. However, the toxicity of CPF on oocyte maturation and whether PTS can eliminate the detrimental effect of CPF on oocytes remain unclear. Here, porcine oocytes were applied to investigate the potential effect and possible mechanism of CPF and PTS during oocyte maturation. This work demonstrated that CPF significantly delayed the meiotic progression and decreased the polar body extrusion by disturbing spindle assembly and chromosome alignment and causing DNA damage in oocytes (p < 0.05). And, CPF significantly impaired oocyte cytoplasmic maturation by inducing the high level of reactive oxygen species and decreasing glutathione content (p < 0.05). Moreover, CPF significantly triggered embryo apoptosis and reduced the blastocyst rate and cell number following parthenogenetic activation (p < 0.05). Whereas CPF-exposed oocytes were treated with PTS, these defects caused by CPF were obviously rescued, and oocyte maturation and subsequent embryonic development were also significantly ameliorated (p < 0.05). In conclusion, these results revealed that CPF exerted the toxic effect on porcine oocytes, while PTS effectively alleviated CPF-induced damage on oocytes. This work provides a potential strategy to protect oocyte maturation in mammalian species.

Osteoarthritis Affects Mammalian Oogenesis: Effects of Collagenase-Induced Osteoarthritis on Oocyte Cytoskeleton in a Mouse Model

Known as a degenerative joint disorder of advanced age affecting predominantly females, osteoarthritis can develop in younger and actively working people because of activities involving loading and injuries of joints. Collagenase-induced osteoarthritis (CIOA) in a mouse model allowed us to investigate for the first time its effects on key cytoskeletal structures (meiotic spindles and actin distribution) of ovulated mouse oocytes. Their meiotic spindles, actin caps, and chromatin were analyzed by immunofluorescence. A total of 193 oocytes from mice with CIOA and 209 from control animals were obtained, almost all in metaphase I (M I) or metaphase II (MII). The maturation rate was lower in CIOA (26.42% M II) than in controls (55.50% M II). CIOA oocytes had significantly larger spindles (average 37 μm versus 25 μm in controls, p < 0.001), with a proportion of large spindles more than 64% in CIOA versus up to 15% in controls (p < 0.001). Meiotic spindles were wider in 68.35% M I and 54.90% M II of CIOA oocytes (mean 18.04 μm M I and 17.34 μm M II versus controls: 11.64 μm M I and 12.64 μm M II), and their poles were approximately two times broader (mean 6.9 μm) in CIOA than in controls (3.6 μm). CIOA oocytes often contained disoriented microtubules. Actin cap was visible in over 91% of controls and less than 20% of CIOA oocytes. Many CIOA oocytes without an actin cap had a nonpolarized thick peripheral actin ring (61.87% of M I and 52.94% of M II). Chromosome alignment was normal in more than 82% in both groups. In conclusion, CIOA affects the cytoskeleton of ovulated mouse oocytes—meiotic spindles are longer and wider, their poles are broader and with disorganized fibers, and the actin cap is replaced by a broad nonpolarized ring. Nevertheless, meiotic spindles were successfully formed in CIOA oocytes and, even when abnormal, allowed correct alignment of chromosomes.

Separase cleaves the kinetochore protein Meikin at the meiosis I/II transition

To generate haploid gametes, germ cells undergo two consecutive meiotic divisions requiring key changes to the cell division machinery. Here, we demonstrate that the protease separase rewires key cell division processes at the meiosis I/II transition by cleaving the meiosis-specific protein Meikin. Separase proteolysis does not inactivate Meikin but instead alters its function to create a distinct activity state. Full-length Meikin and the C-terminal Meikin separase cleavage product both localize to kinetochores, bind to Plk1 kinase, and promote Rec8 cleavage, but our results reveal distinct roles for these proteins in controlling meiosis. Mutations that prevent Meikin cleavage or that conditionally inactivate Meikin at anaphase I result in defective meiosis II chromosome alignment in mouse oocytes. Finally, as oocytes exit meiosis, C-Meikin is eliminated by APC/C-mediated degradation prior to the first mitotic division. Thus, multiple regulatory events irreversibly modulate Meikin activity during successive meiotic divisions to rewire the cell division machinery at two distinct transitions.

Melatonin Promotes In Vitro Maturation of Vitrified-Warmed Mouse Germinal Vesicle Oocytes, Potentially by Reducing Oxidative Stress through the Nrf2 Pathway

Simple Summary

Cryopreservation of oocytes can cause high oxidative stress, reduce the quality of vitrified-warmed oocytes, and seriously hinder the application of oocyte cryopreservation technology in production and medicine. In this work, we found for the first time that melatonin can exert antioxidant effects through receptors and regulate the Nrf2 antioxidant pathway to respond to oxidative stress of vitrified-warmed oocytes, thereby improving both oocyte quality and the potential for subsequent development. The results illustrated the molecular mechanism of melatonin’s antioxidant effect in vitrified-warmed oocytes and provided a theoretical basis for the application of melatonin in the cryopreservation of oocytes. These findings are of great significance for the further application of oocyte cryopreservation technology to production and assisted reproduction in the future.

Abstract

Previously it was reported that melatonin could mitigate oxidative stress caused by oocyte cryopreservation; however, the underlying molecular mechanisms which cause this remain unclear. The objective was to explore whether melatonin could reduce oxidative stress during in vitro maturation of vitrified-warmed mouse germinal vesicle (GV) oocytes through the Nrf2 signaling pathway or its receptors. During in vitro maturation of vitrified-warmed mouse GV oocytes, there were decreases (p < 0.05) in the development rates of metaphase I (MI) oocytes and metaphase II (MII) and spindle morphology grades; increases (p < 0.05) in the reactive oxygen species (ROS) levels; and decreases (p < 0.05) in expressions of Nrf2 signaling pathway-related genes (Nrf2, SOD1) and proteins (Nrf2, HO-1). However, adding 10⁻⁷ mol/L melatonin to both the warming solution and maturation solutions improved (p < 0.05) these indicators. When the Nrf2 protein was specifically inhibited by Brusatol, melatonin did not increase development rates, spindle morphology grades, genes, or protein expressions, nor did it reduce vitrification-induced intracellular oxidative stress in GV oocytes during in vitro maturation. In addition, when melatonin receptors were inhibited by luzindole, the ability of melatonin to scavenge intracellular ROS was decreased, and the expressions of genes (Nrf2, SOD1) and proteins (Nrf2, HO-1) were not restored to control levels. Therefore, we concluded that 10⁻⁷ mol/L melatonin acted on the Nrf2 signaling pathway through its receptors to regulate the expression of genes (Nrf2, SOD1) and proteins (Nrf2, HO-1), and mitigate intracellular oxidative stress, thereby enhancing in vitro development of vitrified-warmed mouse GV oocytes.

Perfluorodecanoic acid induces meiotic defects and deterioration of mice oocytes in vitro

Perfluorodecanoic acid (PFDA) is a member of the perfluoroalkyl substances, which are toxic to organic functions. Recently, it has been found in follicular fluid, seriously interfering with reproduction. Follicular fluid provides the oocyte with necessary resources during the process of oocytes maturation. However, the effects of PFDA on the oocyte need investigation. Our study evaluated the impacts of PFDA on the meiosis and development potential of mouse oocytes by exposing oocytes to PFDA in vitro at 350, 400, and 450 μM concentrations. The results showed that exposure to PFDA resulted in the first meiotic prophase arrest by obstructing the function of the maturation-promoting factor. It also induced the dysfunction of the spindle assembly checkpoint, expedited the progression of the first meiotic process, and increased the risk of aneuploidy. The oocytes treated with PFDA had a broken cytoskeleton which also contributed to meiotic maturation failure. Besides, PFDA exposure caused mitochondria defections, increased the reactive oxygen species level in oocytes, and consequently induced oocyte apoptosis. Moreover, PFDA produced epigenetic modifications in oocytes and increased the frequency of mature oocytes with declined development potential. In summary, our data indicated that PFDA disturbs the meiotic process and induces oocyte quality deterioration.

Hexestrol Deteriorates Oocyte Quality via Perturbation of Mitochondrial Dynamics and Function

Hexestrol (HES) is a synthetic non-steroidal estrogen that was widely used illegally to boost the growth rate in livestock production and aquaculture. HES can also be transferred to humans from treated animals and the environment. HES has been shown to have an adverse effect on ovarian function and oogenesis, but the potential mechanism has not been clearly defined. To understand the potential mechanisms regarding how HES affect female ovarian function, we assessed oocyte quality by examining the critical events during oocyte maturation. We found that HES has an adverse effect on oocyte quality, indicated by the decreased capacity of oocyte maturation and early embryo development competency. Specifically, HES-exposed oocytes exhibited aberrant microtubule nucleation and spindle assembly, resulting in meiotic arrest. In addition, HES exposure disrupted mitochondrial distribution and the balance of mitochondrial fission and fusion, leading to aberrant mitochondrial membrane potential and accumulation of reactive oxygen species. Lastly, we found that HES exposure can increase cytosolic Ca²⁺ levels and induce DNA damage and early apoptosis. In summary, these results demonstrate that mitochondrial dysfunction and perturbation of normal mitochondrial fission and fusion dynamics could be major causes of reduced oocyte quality after HES exposure.

The essential roles of Mps1 in spermatogenesis and fertility in mice

Monopolar spindle 1 (MPS1), which plays a critical role in somatic mitosis, has also been revealed to be essential for meiosis I in oocytes. Spermatogenesis is an important process involving successive mitosis and meiosis, but the function of MPS1 in spermatogenesis remains unclear. Here, we generated Mps1 conditional knockout mice and found that Ddx4-cre-driven loss of Mps1 in male mice resulted in depletion of undifferentiated spermatogonial cells and subsequently of differentiated spermatogonia and spermatocytes. In addition, Stra8-cre-driven ablation of Mps1 in male mice led to germ cell loss and fertility reduction. Spermatocytes lacking Mps1 have blocked at the zygotene-to-pachytene transition in the prophase of meiosis I, which may be due to decreased H2B ubiquitination level mediated by MDM2. And the expression of many meiotic genes was decreased, while that of apoptotic genes was increased. Moreover, we also detected increased apoptosis in spermatocytes with Mps1 knockout, which may have been the reason why germ cells were lost. Taken together, our findings indicate that MPS1 is required for mitosis of gonocytes and spermatogonia, differentiation of undifferentiated spermatogonia, and progression of meiosis I in spermatocytes.

Longevity pathways are associated with human ovarian ageing

STUDY QUESTION

Are genes known to be involved in somatic cell ageing, particularly related to longevity pathways, associated with the accelerated ageing process of the ovary?

SUMMARY ANSWER

Growth, metabolism, and cell-cycle progression-related pathways that are involved in somatic cell ageing are also associated with ovarian ageing.

WHAT IS KNOWN ALREADY

Ovarian ageing is characterized by a gradual decline in ovarian follicle quantity, a decline in oocyte quality, and lower chances of pregnancy. Genetic pathways modulating the rate of somatic cell ageing have been researched intensively. Ovarian ageing does not follow the same timeline as somatic cell ageing, as signs of ovarian ageing occur at a younger female age, while the somatic cells are still relatively young. It is not known whether the generally recognized somatic cell longevity genes also play a role during ovarian ageing. Looking at somatic cell longevity genes can lead to new hypotheses and possible treatment options for subfertility caused by ovarian ageing.

STUDY DESIGN, SIZE, DURATION

In this observational study, we analysed a dataset of individual gene expression profiles of 38 germinal vesicle (GV) oocytes from 38 women aged between 25 and 43 years. We correlated female age (calendar age in years) and biological age (factors known to be associated with ovarian ageing such as dosage of FSH needed for ovarian hyperstimulation, and antral follicle count (AFC)) with gene expression signatures of longevity pathways.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Transcripts of 38 GV oocytes were used for individual gene expression analysis. R version 3.5.1 was used to process and analyse data. The GeneAge database (build 19) was used to obtain mouse ageing-related genes. Human to mouse orthologues were obtained using the R package biomaRt. Correlations and significance between gene expression data and age were tested for using Pearson's product moment correlation coefficient using ranked expression data. Distributions were compared with an ANOVA, and the Tukey Honest Significant Difference method was used to control for the Type I error rate across multiple comparisons.

MAIN RESULTS AND THE ROLE OF CHANCE

Of the 136 genes in the GeneAge database, the expression of 15 anti-longevity genes identified in oocytes showed a positive correlation with female calendar age and FSH dosage administered during ICSI treatment, and a negative correlation with AFC. Expression of 32 pro-longevity genes was negatively correlated with calendar age and FSH dosage, and positively correlated with AFC. In general, anti- and pro-longevity genes changed in opposing directions with advancing maternal age in oocytes. Notably, the anti-longevity genes include many ‘growth’-related genes involved in the mechanistic target of rapamycin (mTOR) Complex 1 pathway, such as EIF5A2, EIF3H, EIF4E, and mTOR. The pro-longevity genes include many cell-cycle progression-related genes involved in DNA damage repair (e.g. XRCC6, ERCC2, and MSH2) or cell-cycle checkpoint regulation genes (e.g. ATM, BRCA1, TP53, TP63, TP73, and BUB1B).

LIMITATIONS, REASONS FOR CAUTION

Using mature oocytes instead of GV-stage oocytes discarded from ICSI treatments may provide different results. No correction for multiple testing was carried out on individual genes because a small set of longevity-related genes was selected a priori for the analysis. The global trend was corrected for multiple testing and remained significant. This work was an observational study and, as no additional experimental work was performed, the associations described do not directly demonstrate the involvement of such genes in oocyte ageing.

WIDER IMPLICATIONS OF THE FINDINGS

Growth, metabolism, and cell-cycle progression-related pathways that are known to be involved in somatic cell ageing were associated with ovarian ageing. If experimental data are obtained to support these associations, we suggest that interventions known to modulate these processes could benefit women suffering from ovarian ageing.

STUDY FUNDING/COMPETING INTEREST(S)

G.E.J. is supported by a VENI grant from ZonMw (https://www.zonmw.nl). Work in the Houtkooper group is financially supported by an ERC Starting grant (No. 638290), a VIDI grant from ZonMw (No. 91715305), and the Velux Stiftung (No. 1063). M.G. declares several research and educational grants from Guerbet, Merck and Ferring (all location VUmc), outside the scope of the submitted work. The other authors report no competing interest

TRIAL REGISTRATION NUMBER

N/A.

Inhibition of CDK4/6 kinases causes production of aneuploid oocytes by inactivating the spindle assembly checkpoint and accelerating first meiotic progression

Cyclin D-CDK4/6 complex mediates the transition from the G1 to S phase in mammalian somatic cells. Meiotic oocytes pass through the G2/M transition and complete the first meiosis to reach maturation at the metaphase of meiosis II without intervening S phase, while Cyclin D-CDK4/6 complex is found to express during meiotic progression. Whether Cyclin D-CDK4/6 complex regulates meiotic cell cycle progression is not known. Here, we found its different role in oocyte meiosis: Cyclin D-CDK4/6 complex served as a regulator of spindle assembly checkpoint (SAC) to prevent aneuploidy in meiosis I. Inhibition of CDK4/6 kinases disrupted spindle assembly, chromosome alignment and kinetochore-microtubule attachments, but unexpectedly accelerated meiotic progression by inactivating SAC, consequently resulting in production of aneuploid oocytes. Further studies showed that the MPF activity decrease before first polar body extrusion was accelerated probably by inactivation of the SAC to promote ubiquitin-mediated cyclin B1 degradation. Taken together, these data reveal a novel role of Cyclin D-CDK4/6 complex in mediating control of the SAC in female meiosis I.

Origins and mechanisms leading to aneuploidy in human eggs

The gain or loss of a chromosome-or aneuploidy-acts as one of the major triggers for infertility and pregnancy loss in humans. These chromosomal abnormalities affect more than 40% of eggs in women at both ends of the age spectrum, that is, young girls as well as women of advancing maternal age. Recent studies in human oocytes and embryos using genomics, cytogenetics, and in silico modeling all provide new insight into the rates and potential genetic and cellular factors associated with aneuploidy at varying stages of development. Here, we review recent studies that are shedding light on potential molecular mechanisms of chromosome missegregation in oocytes and embryos across the entire female reproductive life span.

Kinetochore individualization in meiosis I is required for centromeric cohesin removal in meiosis II

Partitioning of the genome in meiosis occurs through two highly specialized cell divisions, named meiosis I and meiosis II. Step-wise cohesin removal is required for chromosome segregation in meiosis I, and sister chromatid segregation in meiosis II. In meiosis I, mono-oriented sister kinetochores appear as fused together when examined by high-resolution confocal microscopy, whereas they are clearly separated in meiosis II, when attachments are bipolar. It has been proposed that bipolar tension applied by the spindle is responsible for the physical separation of sister kinetochores, removal of cohesin protection, and chromatid separation in meiosis II. We show here that this is not the case, and initial separation of sister kinetochores occurs already in anaphase I independently of bipolar spindle forces applied on sister kinetochores, in mouse oocytes. This kinetochore individualization depends on separase cleavage activity. Crucially, without kinetochore individualization in meiosis I, bivalents when present in meiosis II oocytes separate into chromosomes and not sister chromatids. This shows that whether centromeric cohesin is removed or not is determined by the kinetochore structure prior to meiosis II.

A visual atlas of meiotic protein dynamics in living fission yeast

Meiosis is a carefully choreographed dynamic process that re-purposes proteins from somatic/vegetative cell division, as well as meiosis-specific factors, to carry out the differentiation and recombination pathway common to sexually reproducing eukaryotes. Studies of individual proteins from a variety of different experimental protocols can make it difficult to compare details between them. Using a consistent protocol in otherwise wild-type fission yeast cells, this report provides an atlas of dynamic protein behaviour of representative proteins at different stages during normal zygotic meiosis in fission yeast. This establishes common landmarks to facilitate comparison of different proteins and shows that initiation of S phase likely occurs prior to nuclear fusion/karyogamy.

The mitotic checkpoint is a targetable vulnerability of carboplatin-resistant triple negative breast cancers

Triple-negative breast cancer (TNBC) is the most aggressive breast cancer subtype, lacking effective therapy. Many TNBCs show remarkable response to carboplatin-based chemotherapy, but often develop resistance over time. With increasing use of carboplatin in the clinic, there is a pressing need to identify vulnerabilities of carboplatin-resistant tumors. In this study, we generated carboplatin-resistant TNBC MDA-MB-468 cell line and patient derived TNBC xenograft models. Mass spectrometry-based proteome profiling demonstrated that carboplatin resistance in TNBC is linked to drastic metabolism rewiring and upregulation of anti-oxidative response that supports cell replication by maintaining low levels of DNA damage in the presence of carboplatin. Carboplatin-resistant cells also exhibited dysregulation of the mitotic checkpoint. A kinome shRNA screen revealed that carboplatin-resistant cells are vulnerable to the depletion of the mitotic checkpoint regulators, whereas the checkpoint kinases CHEK1 and WEE1 are indispensable for the survival of carboplatin-resistant cells in the presence of carboplatin. We confirmed that pharmacological inhibition of CHEK1 by prexasertib in the presence of carboplatin is well tolerated by mice and suppresses the growth of carboplatin-resistant TNBC xenografts. Thus, abrogation of the mitotic checkpoint by CHEK1 inhibition re-sensitizes carboplatin-resistant TNBCs to carboplatin and represents a potential strategy for the treatment of carboplatin-resistant TNBCs.

Structural and functional characterization of the Spo11 core complex

Spo11, which makes DNA double-strand breaks (DSBs) essential for meiotic recombination, has long been recalcitrant to biochemical study. We provide molecular analysis of S. cerevisiae Spo11 purified with partners Rec102, Rec104 and Ski8. Rec102 and Rec104 jointly resemble the B subunit of archaeal Topoisomerase VI, with Rec104 occupying a position similar to the Top6B GHKL-type ATPase domain. Unexpectedly, the Spo11 complex is monomeric (1:1:1:1 stoichiometry), consistent with dimerization controlling DSB formation. Reconstitution of DNA binding reveals topoisomerase-like preferences for duplex-duplex junctions and bent DNA. Spo11 also binds noncovalently but with high affinity to DNA ends mimicking cleavage products, suggesting a mechanism to cap DSB ends. Mutations that reduce DNA binding in vitro attenuate DSB formation, alter DSB processing, and reshape the DSB landscape in vivo. Our data reveal structural and functional similarities between the Spo11 core complex and Topo VI, but also highlight differences reflecting their distinct biological roles.

Differential requirement for Bub1 and Bub3 in regulation of meiotic versus mitotic chromosome segregation

Accurate chromosome segregation depends on the proper attachment of kinetochores to spindle microtubules before anaphase onset. The Ipl1/Aurora B kinase corrects improper attachments by phosphorylating kinetochore components and so releasing aberrant kinetochore–microtubule interactions. The localization of Ipl1 to kinetochores in budding yeast depends upon multiple pathways, including the Bub1–Bub3 pathway. We show here that in meiosis, Bub3 is crucial for correction of attachment errors. Depletion of Bub3 results in reduced levels of kinetochore-localized Ipl1 and concomitant massive chromosome missegregation caused by incorrect chromosome–spindle attachments. Depletion of Bub3 also results in shorter metaphase I and metaphase II due to premature localization of protein phosphatase 1 (PP1) to kinetochores, which antagonizes Ipl1-mediated phosphorylation. We propose a new role for the Bub1–Bub3 pathway in maintaining the balance between kinetochore localization of Ipl1 and PP1, a balance that is essential for accurate meiotic chromosome segregation and timely anaphase onset.

Chromosome-autonomous feedback down-regulates meiotic DNA break competence upon synaptonemal complex formation

The number of DNA double-strand breaks (DSBs) initiating meiotic recombination is elevated in Saccharomyces cerevisiae mutants that are globally defective in forming crossovers and synaptonemal complex (SC), a protein scaffold juxtaposing homologous chromosomes. These mutants thus appear to lack a negative feedback loop that inhibits DSB formation when homologs engage one another. This feedback is predicted to be chromosome autonomous, but this has not been tested. Moreover, what chromosomal process is recognized as "homolog engagement" remains unclear. To address these questions, we evaluated effects of homolog engagement defects restricted to small portions of the genome using karyotypically abnormal yeast strains with a homeologous chromosome V pair, monosomic V, or trisomy XV. We found that homolog engagement-defective chromosomes incurred more DSBs, concomitant with prolonged retention of the DSB-promoting protein Rec114, while the rest of the genome remained unaffected. SC-deficient, crossover-proficient mutants ecm11 and gmc2 experienced increased DSB numbers diagnostic of homolog engagement defects. These findings support the hypothesis that SC formation provokes DSB protein dissociation, leading in turn to loss of a DSB competent state. Our findings show that DSB number is regulated in a chromosome-autonomous fashion and provide insight into how homeostatic DSB controls respond to aneuploidy during meiosis.

ASB7 Is a Novel Regulator of Cytoskeletal Organization During Oocyte Maturation

Ankyrin repeat and SOCS box (ASB) family members have a C-terminal SOCS box and an N-terminal ankyrin-related sequence of variable repeats. To date, the roles of ASB family members remain largely unknown. In the present study, by employing knockdown analysis, we investigated the effects of ASB7 on mouse oocyte meiosis. We show that specific depletion of ASB7 disrupts maturational progression and meiotic apparatus. In particular, abnormal spindle, misaligned chromosomes, and loss of cortical actin cap are frequently observed in ASB7-abated oocytes. Consistent with this observation, incidence of aneuploidy is increased in these oocytes. Meanwhile, confocal scanning reveals that loss of ASB7 impairs kinetochore–microtubule interaction and provokes the spindle assembly checkpoint during oocyte meiosis. Furthermore, we find a significant reduction of ASB7 protein in oocytes from aged mice. Importantly, increasing ASB7 expression is capable of partially rescuing the maternal age-induced meiotic defects in oocytes. Together, our data identify ASB7 as a novel player in regulating cytoskeletal organization and discover the potential effects of ASB7 on quality control of aging oocytes.

Meiosis I Kinase Regulators: Conserved Orchestrators of Reductional Chromosome Segregation

Research over the last two decades has identified a group of meiosis-specific proteins, consisting of budding yeast Spo13, fission yeast Moa1, mouse MEIKIN, and Drosophila Mtrm, with essential functions in meiotic chromosome segregation. These proteins, which we call meiosis I kinase regulators (MOKIRs), mediate two major adaptations to the meiotic cell cycle to allow the generation of haploid gametes from diploid mother cells. Firstly, they promote the segregation of homologous chromosomes in meiosis I (reductional division) by ensuring that sister kinetochores face towards the same pole (mono-orientation). Secondly, they safeguard the timely separation of sister chromatids in meiosis II (equational division) by counteracting the premature removal of pericentromeric cohesin, and thus prevent the formation of aneuploid gametes. Although MOKIRs bear no obvious sequence similarity, they appear to play functionally conserved roles in regulating meiotic kinases. Here, the known functions of MOKIRs are reviewed and their possible mechanisms of action are discussed. Also see the video abstract here https://youtu.be/tLE9KL89bwk.

The Error-Prone Kinetochore-Microtubule Attachments During Meiosis I in Vitrified Oocytes

Oocytes vitrification is frequently applied in assisted reproductive technologies. However, chromosomes segregation was error-prone during meiosis maturation of vitrified oocytes. The fidelity of chromosomes segregation depends on the correct kinetochore-microtubule attachments (KT-MTs). In meiosis I, the Aurora B/C would not spatially separate from the attachment sites upon bivalents stretched. Oocytes lack a mechanism for coordinating bivalent stretching and Aurora B/C inhibition in meiosis I. Thus, the KT-MTs are unstable in oocytes. In this study, we firstly found the incorrect KT-MTs were markedly increased in vitrified oocytes. The Aurora B/C activity in vitrified oocytes was significantly increased when the bivalents were stretched. This Aurora B/C activity could not induce a SAC response, as the SAC protein Mad2 was significantly decreased during MI stage in vitrified oocytes. Thus, the KT-MTs in vitrified oocytes were error-prone. This study, for the first time, revealed the mechanism of the incorrect KT-MTs occurred in vitrified oocytes and provided a theoretical basis for further improvement of oocytes vitrification.

Mechanisms of oocyte aneuploidy associated with advanced maternal age

It is well established that maternal age is associated with a rapid decline in the production of healthy and high-quality oocytes resulting in reduced fertility in women older than 35 years of age. In particular, chromosome segregation errors during meiotic divisions are increasingly common and lead to the production of oocytes with an incorrect number of chromosomes, a condition known as aneuploidy. When an aneuploid oocyte is fertilized by a sperm it gives rise to an aneuploid embryo that, except in rare situations, will result in a spontaneous abortion. As females advance in age, they are at higher risk of infertility, miscarriage, or having a pregnancy affected by congenital birth defects such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Turner syndrome (monosomy X). Here, we review the potential molecular mechanisms associated with increased chromosome segregation errors during meiosis as a function of maternal age. Our review shows that multiple exogenous and endogenous factors contribute to the age-related increase in oocyte aneuploidy. Specifically, the weight of evidence indicates that recombination failure, cohesin deterioration, spindle assembly checkpoint (SAC) disregulation, abnormalities in post-translational modification of histones and tubulin, and mitochondrial dysfunction are the leading causes of oocyte aneuploidy associated with maternal aging. There is also growing evidence that dietary and other bioactive interventions may mitigate the effect of maternal aging on oocyte quality and oocyte aneuploidy, thereby improving fertility outcomes. Maternal age is a major concern for aneuploidy and genetic disorders in the offspring in the context of an increasing proportion of mothers having children at increasingly older ages. A better understanding of the mechanisms associated with maternal aging leading to aneuploidy and of intervention strategies that may mitigate these detrimental effects and reduce its occurrence are essential for preventing abnormal reproductive outcomes in the human population.

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.

New Solutions to Old Problems: Molecular Mechanisms of Meiotic Crossover Control

During scientific investigations, the explanation of remarkably interesting phenomena must often await development of new methods or accrual of new observations that in retrospect can lead to lucid answers to the initial problem. A case in point is the control of genetic recombination during meiosis, which leads to crossovers between chromosomes critical for production of healthy offspring. Crossovers must be properly placed along meiotic chromosomes for their accurate segregation. Here, we review observations on two aspects of meiotic crossover control - crossover interference and repression of crossovers near centromeres, both observed more than 85 years ago. Only recently have relatively simple molecular mechanisms for these phenomena become clear through advances in both methods and understanding the molecular basis of meiotic recombination.

The cohesin release factor Wapl interacts with Bub3 to govern SAC activity in female meiosis I

During mitotic prophase, cohesins are removed from chromosome arms by Wapl to ensure faithful sister chromatid separation. However, during female meiosis I, the resolution of chiasmata requires the proteolytic cleavage of cohesin subunit Rec8 along chromosome arms by Separase to separate homologs, and thus the role of Wapl remained unknown. Here, we report that Wapl functions as a regulator of spindle assembly checkpoint (SAC) to prevent aneuploidy in meiosis I. Depletion of Wapl accelerates meiotic progression, inactivates SAC, and causes meiotic defects such as aberrant spindle/chromosome structure and incorrect kinetochore-microtubule (K-MT) attachment, consequently leading to aneuploid eggs. Notably, we identify Bub3 as a binding partner of Wapl by immunoprecipitation and mass spectrometry analysis. We further determine that Wapl controls the SAC activity by maintaining Bub3 protein level and document that exogenous Bub3 restores the normal meiosis in Wapl-depleted oocytes. Together, our findings uncover unique, noncanonical roles for Wapl in mediating control of the SAC in female meiosis I.

A PP2A-B56-Centered View on Metaphase-to-Anaphase Transition in Mouse Oocyte Meiosis I

Meiosis is required to reduce to haploid the diploid genome content of a cell, generating gametes-oocytes and sperm-with the correct number of chromosomes. To achieve this goal, two specialized cell divisions without intermediate S-phase are executed in a time-controlled manner. In mammalian female meiosis, these divisions are error-prone. Human oocytes have an exceptionally high error rate that further increases with age, with significant consequences for human fertility. To understand why errors in chromosome segregation occur at such high rates in oocytes, it is essential to understand the molecular players at work controlling these divisions. In this review, we look at the interplay of kinase and phosphatase activities at the transition from metaphase-to-anaphase for correct segregation of chromosomes. We focus on the activity of PP2A-B56, a key phosphatase for anaphase onset in both mitosis and meiosis. We start by introducing multiple roles PP2A-B56 occupies for progression through mitosis, before laying out whether or not the same principles may apply to the first meiotic division in oocytes, and describing the known meiosis-specific roles of PP2A-B56 and discrepancies with mitotic cell cycle regulation.

OLA1 is responsible for normal spindle assembly and SAC activation in mouse oocytes

Background

OLA1 is a member of the GTPase protein family; unlike other members, it possess both GTPase and ATPase activities, and can bind and hydrolyze ATP more efficiently than GTP. OLA1 participates in cell proliferation, oxidative response, protein synthesis and tumorigenesis. However, whether OLA1 is also required for oocyte meiosis is still unknown.

Methods

In this study, the localization, expression, and functions of OLA1 in the mouse oocyte meiosis were examined. Immunofluorescent and confocal microscopy were used to explore the location pattern of OLA1 in the mouse oocyte. Moreover, nocodazole treatment was used to confirm the spindle-like location of OLA1 during mouse meiosis. Western blot was used to explore the expression pattern of OLA1 in the mouse oocyte. Microinjection of siRNA was used to explore the OLA1 functions in the mouse oocyte meiosis. In addition, chromosome spreading was used to investigate the spindle assembly checkpoint (SAC) activity.

Results

Immunofluorescent staining showed that OLA1 evenly distributed in the cytoplasm at germinal vesicle (GV) stage. After meiosis resumption (GVBD), OLA1 co-localized with spindles, which was further identified by nocodazole treatment experiments. Knockdown of OLA1 impaired the germinal vesicle breakdown progression and finally resulted in a lower polar body extrusion rate. Immunofluorescence analysis indicated that knockdown of OLA1 led to abnormal spindle assembly, which was evidenced by multipolar spindles in OLA1-RNAi-oocytes. After 6 h post-GVBD in culture, an increased proportion of oocyte which has precociously entered into anaphase/telephase I (A/TI) was observed in OLA1-knockdown oocytes, suggesting that loss of OLA1 resulted in the premature segregation of homologous chromosomes. In addition, the chromosome spread analysis suggested that OLA1 knockdown induced premature anaphase onset was due to the precocious inactivation of SAC. Taken together, we concluded that OLA1 plays important role in GVBD, spindle assembly and SAC activation maintenance in oocyte meiosis.

Meiotic arrest occurs most frequently at metaphase and is often incomplete in azoospermic men

OBJECTIVE

To establish which meiotic checkpoints are activated in males with severe spermatogenic impairment to improve phenotypic characterization of meiotic defects.

DESIGN

Retrospective observational study.

SETTING

University medical center research laboratory and andrology clinic.

PATIENT(S)

Forty-eight patients with confirmed spermatogenic impairment (Johnsen scores 3-6) and 15 controls (Johnsen score 10).

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Quantitative assessment of immunofluorescent analyses of specific markers to determine meiotic entry, chromosome pairing, progression of DNA double-strand break repair, crossover formation, formation of meiotic metaphases, metaphase arrest, and spermatid formation, resulting in a novel classification of human meiotic arrest types.

RESULT(S)

Complete metaphase arrest was observed most frequently (27%), and the patients with the highest frequency of apoptotic metaphases also displayed a reduction in crossover number. Incomplete metaphase arrest was observed in 17% of the patients. Only four patients (8%) displayed a failure to complete meiotic chromosome pairing leading to pachytene arrest. Two new types of meiotic arrest were defined: premetaphase and postmetaphase arrest (15% and 13%, respectively).

CONCLUSION(S)

Meiotic arrest in men occurs most frequently at meiotic metaphase. This arrest can be incomplete, resulting in low numbers of spermatids, and often occurs in association with reduced crossover frequency. The phenotyping approach described here provides mechanistic insights to help identify candidate infertility genes and to assess genotype-phenotype correlations in individual cases.

ProTAME Arrest in Mammalian Oocytes and Embryos Does Not Require Spindle Assembly Checkpoint Activity

In both mitosis and meiosis, metaphase to anaphase transition requires the activity of a ubiquitin ligase known as anaphase promoting complex/cyclosome (APC/C). The activation of APC/C in metaphase is under the control of the checkpoint mechanism, called the spindle assembly checkpoint (SAC), which monitors the correct attachment of all kinetochores to the spindle. It has been shown previously in somatic cells that exposure to a small molecule inhibitor, prodrug tosyl-l-arginine methyl ester (proTAME), resulted in cell cycle arrest in metaphase, with low APC/C activity. Interestingly, some reports have also suggested that the activity of SAC is required for this arrest. We focused on the characterization of proTAME inhibition of cell cycle progression in mammalian oocytes and embryos. Our results show that mammalian oocytes and early cleavage embryos show dose-dependent metaphase arrest after exposure to proTAME. However, in comparison to the somatic cells, we show here that the proTAME-induced arrest in these cells does not require SAC activity. Our results revealed important differences between mammalian oocytes and early embryos and somatic cells in their requirements of SAC for APC/C inhibition. In comparison to the somatic cells, oocytes and embryos show much higher frequency of aneuploidy. Our results are therefore important for understanding chromosome segregation control mechanisms, which might contribute to the premature termination of development or severe developmental and mental disorders of newborns.

Melatonin Improves In Vitro Development of Vitrified-Warmed Mouse Germinal Vesicle Oocytes Potentially via Modulation of Spindle Assembly Checkpoint-Related Genes

The present study aimed to investigate the effect of melatonin (MT) supplementation on in vitro maturation of vitrified mouse germinal vesicle (GV) oocytes. The fresh oocytes were randomly divided into three groups: untreated (control), or vitrified by open-pulled straw method without (vitrification group) or with MT supplementation (vitrification + MT group). After warming, oocytes were cultured in vitro, then the reactive oxygen species (ROS) and glutathione (GSH) levels, mitochondrial membrane potential, ATP levels, spindle morphology, mRNA expression of spindle assembly checkpoint (SAC)-related genes (Mps1, BubR1, Mad1, Mad2), and their subsequent developmental potential in vitro were evaluated. The results showed that vitrification/warming procedures significantly decreased the percentage of GV oocytes developed to metaphase II (MII) stage, the mitochondrial membrane potential, ATP content, and GSH levels, remarkably increased the ROS levels, and significantly impaired the spindle morphology. The expressions of SAC-related genes were also altered in vitrified oocytes. However, when 10-7 mol/L MT was administered during the whole length of the experiment, the percentage of GV oocytes matured to MII stage was significantly increased, and the other indicators were also significantly improved and almost recovered to the normal levels relative to the control. Thus, we speculate that MT might regulate the mitochondrial membrane potential, ATP content, ROS, GSH, and expression of SAC-related genes, potentially increasing the in vitro maturation of vitrified-warmed mouse GV oocytes.

Postovulatory ageing modifies sperm-induced Ca2+ oscillations in mouse oocytes through a conditions-dependent, multi-pathway mechanism

Postovulatory ageing of mammalian oocytes occurs between their ovulation and fertilization and has been shown to decrease their developmental capabilities. Aged oocytes display numerous abnormalities, including altered Ca²⁺ signalling. Fertilization-induced Ca²⁺ oscillations are essential for activation of the embryonic development, therefore maintaining proper Ca²⁺ homeostasis is crucial for the oocyte quality. In the present paper, we show that the mechanism underlying age-dependent alterations in the pattern of sperm-triggered Ca²⁺ oscillations is more complex and multifaceted than previously believed. Using time-lapse imaging accompanied by immunostaining and molecular analyses, we found that postovulatory ageing affects the amount of Ca²⁺ stored in the cell, expression of Ca²⁺ pump SERCA2, amount of available ATP and distribution of endoplasmic reticulum and mitochondria in a manner often strongly depending on ageing conditions (in vitro vs. in vivo). Importantly, those changes do not have to be caused by oxidative stress, usually linked with the ageing process, as they occur even if the amount of reactive oxygen species remains low. Instead, our results suggest that aberrations in Ca²⁺ signalling may be a synergistic result of ageing-related alterations of the cell cycle, cytoskeleton, and mitochondrial functionality.

Camptothecin induces mitotic arrest through Mad2-Cdc20 complex by activating the JNK-mediated Sp1 pathway

Camptothecin (CPT) is a popular therapeutic agent that targets topoisomerase I. Our findings demonstrated that CPT-induced microtubule polymerization results in markedly increased histone H3 phosphorylation. CPT also enhanced interactions between the mitotic checkpoint proteins, Mad2 and Cdc20, and thereby increased mitotic arrest. Transient knockdown of Mad2 completely restored cell cycle progression from CPT-induced mitotic arrest, while simultaneously reduced cyclin B1 and Cdk1 expression. Moreover, we found that c-Jun N-terminal kinase (JNK) acts upstream of Sp1, which upregulates p21-mediated mitotic arrest in response to CPT; furthermore, knockdown of p21 restored cell cycle progression, while inhibition of Cdks completely restored cell cycle progression from CPT-induced mitotic arrest. We hypothesized that, during mitotic arrest in response to CPT, cell survival signaling blocks apoptosis, thereby enhancing mitotic arrest. As expected, a caspase-9 inhibitor, z-LEHD-FMK, and an autophagy inhibitor, 3-methyladenine (3 MA), significantly diminished CPT-induced mitotic arrest. On the other hand, when Mad2 was depleted, z-LEHD-FMK and 3 MA markedly increased apoptosis, and restored cell cycle progression. Taken together, these results suggest that CPT decodes the action of topoisomerase I-mediated tubulin targeting drugs, leading to mitotic arrest by upregulating Mad2 through the JNK-mediated Sp1 pathway and autophagy formation from tubulin polymerization.

Chemically induced aneuploidy in germ cells. Part II of the report of the 2017 IWGT workgroup on assessing the risk of aneugens for carcinogenesis and hereditary diseases

As part of the 7th International Workshops on Genotoxicity Testing held in Tokyo, Japan in November 2017, a workgroup of experts reviewed and assessed the risk of aneugens for human health. The present manuscript is one of three manuscripts from the workgroup and reports on the unanimous consensus reached on the evidence for aneugens affecting germ cells, their mechanisms of action and role in hereditary diseases. There are 24 chemicals with strong or sufficient evidence for germ cell aneugenicity providing robust support for the ability of chemicals to induce germ cell aneuploidy. Interference with microtubule dynamics or inhibition of topoisomerase II function are clear characteristics of germ cell aneugens. Although there are mechanisms of chromosome segregation that are unique to germ cells, there is currently no evidence for germ cell-specific aneugens. However, the available data are heavily skewed toward chemicals that are aneugenic in somatic cells. Development of high-throughput screening assays in suitable animal models for exploring additional targets for aneuploidy induction, such as meiosis-specific proteins, and to prioritize chemicals for the potential to be germ cell aneugens is encouraged. Evidence in animal models support that: oocytes are more sensitive than spermatocytes and somatic cells to aneugens; exposure to aneugens leads to aneuploid conceptuses; and, the frequencies of aneuploidy are similar in germ cells and zygotes. Although aneuploidy in germ cells is a significant cause of infertility and pregnancy loss in humans, there is currently limited evidence that aneugens induce hereditary diseases in human populations because the great majority of aneuploid conceptuses die in utero. Overall, the present work underscores the importance of protecting the human population from exposure to chemicals that can induce aneuploidy in germ cells that, in contrast to carcinogenicity, is directly linked to an adverse outcome.

Insights into Resistance Mechanisms of Inhibitors to Mps1 C604Y Mutation via a Comprehensive Molecular Modeling Study

Mono-polar spindle 1 (Mps1/TTK) represents a protein kinase reported to be vital for cell division processes and is generally regarded as an attractive target for the treatment of hepatocellular carcinoma, breast carcinoma, and colon cancer. However, the C604Y mutation has been linked to acquired resistance. Recently, three potential small-molecule inhibitors of Mps1 (i.e., reversine, NMS-P715, and its derivative Cpd-5) were reported for the C604Y mutation that exhibit significant resistance to NMS-P715 and Cpd-5, but retain affinity for reversine. In this study, classical molecular dynamic (MD) simulations, accelerated MD (aMD) simulations, and umbrella sampling (US) simulations were performed to illustrate the resistance mechanisms of inhibitors to Mps1. The classical MD simulations combined with free energy calculations revealed that reversine features similar binding affinity characteristics to both Mps1^WT and Mps1^C604Y, but both NMS-P715 and Cpd-5 feature much higher binding affinities to Mps1^WT than to Mps1^C604Y. The major variations were shown to be controlled by electrostatic energy and the conformational change of A-loop-induced entropy increased. The large conformational changes of Mps1^C604Y bound to NMS-P715 and Cpd-5 were also observed in aMD simulations. The US simulation results further suggest that reversine and Cpd-5 both exhibit similar dissociation processes from both Mps1^WT and Mps1^C604Y, but Cpd-5 and NMS-P715 were found to dissociate more easily from Mps1^C604Y than from Mps1^WT, thus a reduced residence time was responsible for the inhibitors resistance to the C604Y mutation. The physical principles provided by the present study may provide important clues for the discovery and rational design of novel inhibitors to combat the C604Y mutation of Mps1.