Distinct classes of lagging chromosome underpin age-related oocyte aneuploidy in mouse

Chromosome architecture and low cohesion bias acrocentric chromosomes towards aneuploidy during mammalian meiosis

Aneuploidy in eggs is a leading cause of miscarriages or viable developmental syndromes. Aneuploidy rates differ between individual chromosomes. For instance, chromosome 21 frequently missegregates, resulting in Down Syndrome. What causes chromosome-specific aneuploidy in meiosis is unclear. Chromosome 21 belongs to the class of acrocentric chromosomes, whose centromeres are located close to the chromosome end, resulting in one long and one short chromosome arm. We demonstrate that acrocentric chromosomes are generally more often aneuploid than metacentric chromosomes in porcine eggs. Kinetochores of acrocentric chromosomes are often partially covered by the short chromosome arm during meiosis I in human and porcine oocytes and orient less efficiently toward the spindle poles. These partially covered kinetochores are more likely to be incorrectly attached to the spindle. Additionally, sister chromatids of acrocentric chromosomes are held together by lower levels of cohesin, making them more vulnerable to age-dependent cohesin loss. Chromosome architecture and low cohesion therefore bias acrocentric chromosomes toward aneuploidy during mammalian meiosis.

Hallmarks of female reproductive aging in physiologic aging mice

The female reproductive axis is one of the first organ systems to age, which has consequences for fertility and overall health. Here, we provide a comprehensive overview of the biological process of female reproductive aging across reproductive organs, tissues and cells based on research with widely used physiologic aging mouse models, and describe the mechanisms that underpin these phenotypes. Overall, aging is associated with dysregulation of the hypothalamic-pituitary-ovarian axis, perturbations of the ovarian stroma, reduced egg quantity and quality, and altered uterine morphology and function that contributes to reduced capacity for fertilization and impaired embryo development. Ultimately, these age-related phenotypes contribute to altered pregnancy outcomes and adverse consequences in offspring. Conserved mechanisms of aging, as well as those unique to the reproductive system, underlie these phenotypes. The knowledge of such mechanisms will lead to development of therapeutics to extend female reproductive longevity and support endocrine function and overall health.

Increased DNA damage in full-grown oocytes is correlated with diminished autophagy activation

Unlike mild DNA damage exposure, DNA damage repair (DDR) is reported to be ineffective in full-grown mammalian oocytes exposed to moderate or severe DNA damage. The underlying mechanisms of this weakened DDR are unknown. Here, we show that moderate DNA damage in full-grown oocytes leads to aneuploidy. Our data reveal that DNA-damaged oocytes have an altered, closed, chromatin state, and suggest that the failure to repair damaged DNA could be due to the inability of DDR proteins to access damaged loci. Our data also demonstrate that, unlike somatic cells, mouse and porcine oocytes fail to activate autophagy in response to DNA double-strand break-inducing treatment, which we suggest may be the cause of the altered chromatin conformation and inefficient DDR. Importantly, autophagy activity is further reduced in maternally aged oocytes (which harbor severe DNA damage), and its induction is correlated with reduced DNA damage in maternally aged oocytes. Our findings provide evidence that reduced autophagy activation contributes to weakened DDR in oocytes, especially in those from aged females, offering new possibilities to improve assisted reproductive therapy in women with compromised oocyte quality.

Rejuvenation of aged oocyte through exposure to young follicular microenvironment

Reproductive aging is a major cause of fertility decline, attributed to decreased oocyte quantity and developmental potential. A possible cause is aging of the surrounding follicular somatic cells that support oocyte growth and development by providing nutrients and regulatory factors. Here, by creating chimeric follicles, whereby an oocyte from one follicle was transplanted into and cultured within another follicle whose native oocyte was removed, we show that young oocytes cultured in aged follicles exhibited impeded meiotic maturation and developmental potential, whereas aged oocytes cultured within young follicles were significantly improved in rates of maturation, blastocyst formation and live birth after in vitro fertilization and embryo implantation. This rejuvenation of aged oocytes was associated with enhanced interaction with somatic cells, transcriptomic and metabolomic remodeling, improved mitochondrial function and higher fidelity of meiotic chromosome segregation. These findings provide the basis for a future follicular somatic cell-based therapy to treat female infertility.

Oocyte and embryo culture under oil profoundly alters effective concentrations of small molecule inhibitors

Culture of oocytes and embryos in media under oil is a cornerstone of fertility treatment, and extensively employed in experimental investigation of early mammalian development. It has been noted anecdotally by some that certain small molecule inhibitors might lose activity in oil-covered culture systems, presumably by drug partitioning into the oil. Here we took a pseudo-pharmacological approach to appraise this formally using mouse oocytes and embryos. Using different culture dish designs with defined media:oil volume ratios, we show that the EC50 of the widely employed microtubule poison nocodazole shifts as a function of the media:oil ratio, such that nocodazole concentrations that prevent cell division in oil-free culture fail to in oil-covered media drops. Relatively subtle changes in culture dish design lead to measurable changes in EC50. This effect is not specific to one type of culture oil, and can be readily observed both in oocyte and embryo culture experiments. We subsequently applied a similar approach to a small panel of widely employed cell cycle-related inhibitors, finding that most lose activity in standard oil-covered oocyte/embryo culture systems. Our data suggest that loss of small molecule activity in oil-covered oocyte and embryo culture is a widespread phenomenon with potentially far-reaching implications for data reproducibility, and we recommend avoiding oil-covered culture for experiments employing inhibitors/drugs wherever possible.

Chromosome Division in Early Embryos-Is Everything under Control? And Is the Cell Size Important?

Chromosome segregation in female germ cells and early embryonic blastomeres is known to be highly prone to errors. The resulting aneuploidy is therefore the most frequent cause of termination of early development and embryo loss in mammals. And in specific cases, when the aneuploidy is actually compatible with embryonic and fetal development, it leads to severe developmental disorders. The main surveillance mechanism, which is essential for the fidelity of chromosome segregation, is the Spindle Assembly Checkpoint (SAC). And although all eukaryotic cells carry genes required for SAC, it is not clear whether this pathway is active in all cell types, including blastomeres of early embryos. In this review, we will summarize and discuss the recent progress in our understanding of the mechanisms controlling chromosome segregation and how they might work in embryos and mammalian embryos in particular. Our conclusion from the current literature is that the early mammalian embryos show limited capabilities to react to chromosome segregation defects, which might, at least partially, explain the widespread problem of aneuploidy during the early development in mammals.

Aneuploidy in oocytes from women of advanced maternal age: analysis of the causal meiotic errors and impact on embryo development

STUDY QUESTION

In oocytes of advanced maternal age (AMA) women, what are the mechanisms leading to aneuploidy and what is the association of aneuploidy with embryo development?

SUMMARY ANSWER

Known chromosome segregation errors such as precocious separation of sister chromatids explained 90.4% of abnormal chromosome copy numbers in polar bodies (PBs), underlying impaired embryo development.

WHAT IS KNOWN ALREADY

Meiotic chromosomal aneuploidies in oocytes correlate with AMA (>35 years) and can affect over half of oocytes in this age group. This underlies the rationale for PB biopsy as a form of early preimplantation genetic testing for aneuploidy (PGT-A), as performed in the 'ESHRE STudy into the Evaluation of oocyte Euploidy by Microarray analysis' (ESTEEM) randomized controlled trial (RCT). So far, chromosome analysis of oocytes and PBs has shown that precocious separation of sister chromatids (PSSC), Meiosis II (MII) non-disjunction (ND), and reverse segregation (RS) are the main mechanisms leading to aneuploidy in oocytes.

STUDY DESIGN, SIZE, DURATION

Data were sourced from the ESTEEM study, a multicentre RCT from seven European centres to assess the clinical utility of PGT-A on PBs using array comparative genomic hybridization (aCGH) in patients of AMA (36-40 years). This included data on the chromosome complement in PB pairs (PGT-A group), and on embryo morphology in a subset of embryos, up to Day 6 post-insemination, from both the intervention (PB biopsy and PGT-A) and control groups.

PARTICIPANTS/MATERIALS, SETTING, METHODS

ESTEEM recruited 396 AMA patients: 205 in the intervention group and 191 in the control group. Complete genetic data from 693 PB pairs were analysed. Additionally, the morphology from 1034 embryos generated from fertilized oocytes (two pronuclei) in the PB biopsy group and 1082 in the control group were used for statistical analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

Overall, 461/693 PB pairs showed abnormal segregation in 1162/10 810 chromosomes. The main observed abnormal segregations were compatible with PSSC in Meiosis I (MI) (n = 568/1162; 48.9%), ND of chromatids in MII or RS (n = 417/1162; 35.9%), and less frequently ND in MI (n = 65/1162; 5.6%). For 112 chromosomes (112/1162; 9.6%), we observed a chromosome copy number in the first PB (PB1) and second PB (PB2) that is not explained by any of the known mechanisms causing aneuploidy in oocytes. We observed that embryos in the PGT-A arm of the RCT did not have a significantly different morphology between 2 and 6 days post-insemination compared to the control group, indicating that PB biopsy did not affect embryo quality. Following age-adjusted multilevel mixed-effect ordinal logistic regression models performed for each embryo evaluation day, aneuploidy was associated with a decrease in embryo quality on Day 3 (adjusted odds ratio (aOR) 0.62, 95% CI 0.43-0.90), Day 4 (aOR 0.15, 95% CI 0.06-0.39), and Day 5 (aOR 0.28, 95% CI 0.14-0.58).

LIMITATIONS, REASON FOR CAUTION

RS cannot be distinguished from normal segregation or MII ND using aCGH. The observed segregations were based on the detected copy number of PB1 and PB2 only and were not confirmed by the analysis of embryos. The embryo morphology assessment was static and single observer.

WIDER IMPLICATIONS OF THE FINDINGS

Our finding of frequent unexplained chromosome copy numbers in PBs indicates that our knowledge of the mechanisms causing aneuploidy in oocytes is incomplete. It challenges the dogma that aneuploidy in oocytes is exclusively caused by mis-segregation of chromosomes during MI and MII.

STUDY FUNDING/COMPETING INTEREST(S)

Data were mined from a study funded by ESHRE. Illumina provided microarrays and other consumables necessary for aCGH testing of PBs. None of the authors have competing interests.

TRIAL REGISTRATION NUMBER

Data were mined from the ESTEEM study (ClinicalTrials.gov Identifier NCT01532284).

Insulin regulates gap junction intercellular communication in porcine granulosa cells through modulation of connexin43 protein expression

Gap junction intercellular communication (GJIC) among granulosa cells plays an important role in folliculogenesis, and it is temporal-spatially regulated during follicular development. Connexin (Cx) proteins predominantly form the basal structure of gap junctions in granulosa cells. In our study, immunohistochemical analysis revealed that Cx43 is the most widely expressed connexin in porcine follicles, especially among the large antral follicles. With application of insulin on porcine granulosa cells, we found that insulin significantly facilitated the protein level of Cx43, not mRNA level. This process is dependent on the phosphorylated activities of AKT and Erk since selective AKT and Erk inhibitors, LY294002 and U0126, respectively, hampered the potential of insulin to up-regulate Cx43 protein expression. As a consequence, the insulin-enhanced Cx43-couple GJIC activity in porcine granulosa cells was corresponding attenuated by the administration of LY294002 and U0126. Our findings provide a new insight into the molecular mechanisms by which insulin mediates cell-cell communication in porcine granulosa cells and sheds light on nutrition-reproduction interactions.

Making a good egg: human oocyte health, aging, and in vitro development

Mammalian eggs (oocytes) are formed during fetal life and establish associations with somatic cells to form primordial follicles that create a store of germ cells (the primordial pool). The size of this pool is influenced by key events during the formation of germ cells and by factors that influence the subsequent activation of follicle growth. These regulatory pathways must ensure that the reserve of oocytes within primordial follicles in humans lasts for up to 50 years, yet only approximately 0.1% will ever be ovulated with the rest undergoing degeneration. This review outlines the mechanisms and regulatory pathways that govern the processes of oocyte and follicle formation and later growth, within the ovarian stroma, through to ovulation with particular reference to human oocytes/follicles. In addition, the effects of aging on female reproductive capacity through changes in oocyte number and quality are emphasized, with both the cellular mechanisms and clinical implications discussed. Finally, the details of current developments in culture systems that support all stages of follicle growth to generate mature oocytes in vitro and emerging prospects for making new oocytes from stem cells are outlined.

Multi-Omics Analysis Reveals Translational Landscapes and Regulations in Mouse and Human Oocyte Aging

Abnormal resumption of meiosis and decreased oocyte quality are hallmarks of maternal aging. Transcriptional silencing makes translational control an urgent task during meiosis resumption in maternal aging. However, insights into aging-related translational characteristics and underlying mechanisms are limited. Here, using multi-omics analysis of oocytes, it is found that translatomics during aging is related to changes in the proteome and reveals decreased translational efficiency with aging phenotypes in mouse oocytes. Translational efficiency decrease is associated with the N6-methyladenosine (m6A) modification of transcripts. It is further clarified that m6A reader YTHDF3 is significantly decreased in aged oocytes, inhibiting oocyte meiotic maturation. YTHDF3 intervention perturbs the translatome of oocytes and suppress the translational efficiency of aging-associated maternal factors, such as Hells, to affect the oocyte maturation. Moreover, the translational landscape is profiled in human oocyte aging, and the similar translational changes of epigenetic modifications regulators between human and mice oocyte aging are observed. In particular, due to the translational silence of YTHDF3 in human oocytes, translation activity is not associated with m6A modification, but alternative splicing factor SRSF6. Together, the findings profile the specific translational landscapes during oocyte aging in mice and humans, and uncover non-conservative regulators on translation control in meiosis resumption and maternal aging.

Twenty years of merotelic kinetochore attachments: a historical perspective

Micronuclei, small DNA-containing structures separate from the main nucleus, were used for decades as an indicator of genotoxic damage. Micronuclei containing whole chromosomes were considered a biomarker of aneuploidy and were believed to form, upon mitotic exit, from chromosomes that lagged behind in anaphase as all other chromosomes segregated to the poles of the mitotic spindle. However, the mechanism responsible for inducing anaphase lagging chromosomes remained unknown until just over twenty years ago. Here, I summarize what preceded and what followed this discovery, highlighting some of the open questions and opportunities for future investigation.

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.

Biallelic variants in MAD2L1BP (p31comet) cause female infertility characterized by oocyte maturation arrest

Human oocyte maturation arrest represents one of the severe conditions for female patients with primary infertility. However, the genetic factors underlying this human disease remain largely unknown. The spindle assembly checkpoint (SAC) is an intricate surveillance mechanism that ensures accurate segregation of chromosomes throughout cell cycles. Once the kinetochores of chromosomes are correctly attached to bipolar spindles and the SAC is satisfied, the MAD2L1BP, best known as p31comet, binds mitosis arrest deficient 2 (MAD2) and recruits the AAA+-ATPase TRIP13 to disassemble the mitotic checkpoint complex (MCC), leading to the cell-cycle progression. In this study, by whole-exome sequencing (WES), we identified homozygous and compound heterozygous MAD2L1BP variants in three families with female patients diagnosed with primary infertility owing to oocyte metaphase I (MI) arrest. Functional studies revealed that the protein variants resulting from the C-terminal truncation of MAD2L1BP lost their binding ability to MAD2. cRNA microinjection of full-length or truncated MAD2L1BP uncovered their discordant roles in driving the extrusion of polar body 1 (PB1) in mouse oocytes. Furthermore, the patient's oocytes carrying the mutated MAD2L1BP resumed polar body extrusion (PBE) when rescued by microinjection of full-length MAD2L1BP cRNAs. Together, our studies identified and characterized novel biallelic variants in MAD2L1BP responsible for human oocyte maturation arrest at MI, and thus prompted new therapeutic avenues for curing female primary infertility.

Imaging the subcellular viscoelastic properties of mouse oocytes

In recent years, cellular biomechanical properties have been investigated as an alternative to morphological assessments for oocyte selection in reproductive science. Despite the high relevance of cell viscoelasticity characterization, the reconstruction of spatially distributed viscoelastic parameter images in such materials remains a major challenge. Here, a framework for mapping viscoelasticity at the subcellular scale is proposed and applied to live mouse oocytes. The strategy relies on the principles of optical microelastography for imaging in combination with the overlapping subzone nonlinear inversion technique for complex-valued shear modulus reconstruction. The three-dimensional nature of the viscoelasticity equations was accommodated by applying an oocyte geometry-based 3D mechanical motion model to the measured wave field. Five domains-nucleolus, nucleus, cytoplasm, perivitelline space, and zona pellucida-could be visually differentiated in both oocyte storage and loss modulus maps, and statistically significant differences were observed between most of these domains in either property reconstruction. The method proposed herein presents excellent potential for biomechanical-based monitoring of oocyte health and complex transformations across lifespan. It also shows appreciable latitude for generalization to cells of arbitrary shape using conventional microscopy equipment.

Morphokinetic parameters of mouse oocyte meiotic maturation and cumulus expansion are not affected by reproductive age or ploidy status

INTRODUCTION

Morphokinetic analysis using a closed time-lapse monitoring system (EmbryoScope + ™) provides quantitative metrics of meiotic progression and cumulus expansion. The goal of this study was to use a physiologic aging mouse model, in which egg aneuploidy levels increase, to determine whether there are age-dependent differences in morphokinetic parameters of oocyte maturation.

METHODS

Denuded oocytes and intact cumulus-oocyte complexes (COCs) were isolated from reproductively young and old mice and in vitro matured in the EmbryoScope + ™. Morphokinetic parameters of meiotic progression and cumulus expansion were evaluated, compared between reproductively young and old mice, and correlated with egg ploidy status.

RESULTS

Oocytes from reproductively old mice were smaller than young counterparts in terms of GV area (446.42 ± 4.15 vs. 416.79 ± 5.24 µm2, p < 0.0001) and oocyte area (4195.71 ± 33.10 vs. 4081.62 ± 41.04 µm2, p < 0.05). In addition, the aneuploidy incidence was higher in eggs with advanced reproductive age (24-27% vs. 8-9%, p < 0.05). There were no differences in the morphokinetic parameters of oocyte maturation between oocytes from reproductively young and old mice with respect to time to germinal vesicle breakdown (GVBD) (1.03 ± 0.03 vs. 1.01 ± 0.04 h), polar body extrusion (PBE) (8.56 ± 0.11 vs. 8.52 ± 0.15 h), duration of meiosis I (7.58 ± 0.10 vs. 7.48 ± 0.11 h), and kinetics of cumulus expansion (0.093 ± 0.002 vs. 0.089 ± 0.003 µm/min). All morphokinetic parameters of oocyte maturation were similar between euploid and aneuploid eggs irrespective of age.

CONCLUSION

There is no association between age or ploidy and the morphokinetics of mouse oocyte in vitro maturation (IVM). Future studies are needed to evaluate whether there is an association between morphokinetic dynamics of mouse IVM and embryo developmental competence.

Ru360 protects against vitrification-induced oocyte meiotic defects by restoring mitochondrial function

Oocyte vitrification has been widely application in female fertility preservation. Recent studies found that vitrification of immature (germinal vesicle stage, GV) oocytes increased the risk of aneuploidy during meiotic maturation; however, the underlying mechanisms and the strategies to prevent this defect remain unexplored. In this study, we found that vitrification of GV oocytes decreased the first polarbody extrusion rate (90.51 ± 1.04% vs. 63.89 ± 1.39%, p < 0.05) and increased the aneuploid rate (2.50% vs. 20.00%, p < 0.05), accompanied with a series of defects during meiotic maturation, including aberrant spindle morphology, chromosome misalignment, incorrect Kinetochore-Microtubule attachments (KT-MTs) and weakened spindle assembly checkpoint protein complex (SAC) function. We also found that vitrification disrupted mitochondrial function by increasing mitochondrial Ca²⁺ levels. Importantly, inhibition of mitochondrial Ca²⁺ entry by 1 μM Ru360 significantly restored mitochondrial function and rescued the meiotic defects, indicating that the increase of mitochondrial Ca²⁺, at least, was a cause of meiotic defects in vitrified oocytes. These results shed light on the molecular mechanisms of oocyte vitrification-induced adverse effects of meiotic maturation and provided a potential strategy to improve oocyte cryopreservation protocols further.

Actin limits egg aneuploidies associated with female reproductive aging

Aging-related centromeric cohesion loss underlies premature separation of sister chromatids and egg aneuploidy in reproductively older females. Here, we show that F-actin maintains chromatid association after cohesion deterioration in aged eggs. F-actin disruption in aged mouse eggs exacerbated untimely dissociation of sister chromatids, while its removal in young eggs induced extensive chromatid separation events generally only seen in advanced reproductive ages. In young eggs containing experimentally reduced cohesion, F-actin removal accelerated premature splitting and scattering of sister chromatids in a microtubule dynamics-dependent manner, suggesting that actin counteracts chromatid-pulling spindle forces. Consistently, F-actin stabilization restricted scattering of unpaired chromatids generated by complete degradation of centromeric cohesion proteins. We conclude that actin mitigates egg aneuploidies arising from age-related cohesion depletion by limiting microtubule-driven separation and dispersion of sister chromatids. This is supported by our finding that spindle-associated F-actin structures are disrupted in eggs of reproductively older females.

Aneuploidy in mammalian oocytes and the impact of maternal ageing

During fertilization, the egg and the sperm are supposed to contribute precisely one copy of each chromosome to the embryo. However, human eggs frequently contain an incorrect number of chromosomes - a condition termed aneuploidy, which is much more prevalent in eggs than in either sperm or in most somatic cells. In turn, aneuploidy in eggs is a leading cause of infertility, miscarriage and congenital syndromes. Aneuploidy arises as a consequence of aberrant meiosis during egg development from its progenitor cell, the oocyte. In human oocytes, chromosomes often segregate incorrectly. Chromosome segregation errors increase in women from their mid-thirties, leading to even higher levels of aneuploidy in eggs from women of advanced maternal age, ultimately causing age-related infertility. Here, we cover the two main areas that contribute to aneuploidy: (1) factors that influence the fidelity of chromosome segregation in eggs of women from all ages and (2) factors that change in response to reproductive ageing. Recent discoveries reveal new error-causing pathways and present a framework for therapeutic strategies to extend the span of female fertility.

Overexpression of Tfap2a in Mouse Oocytes Impaired Spindle and Chromosome Organization

Transcription factor AP-2-alpha (Tfap2a) is an important sequence-specific DNA-binding protein that can regulate the transcription of multiple genes by collaborating with inducible viral and cellular enhancer elements. In this experiment, the expression, localization, and functions of Tfap2a were investigated in mouse oocytes during maturation. Overexpression via microinjection of Myc-Tfap2a mRNA into the ooplasm, immunofluorescence, and immunoblotting were used to study the role of Tfap2a in mouse oocyte meiosis. According to our results, Tfap2a plays a vital role in mouse oocyte maturation. Levels of Tfap2a in GV oocytes of mice suffering from type 2 diabetes increased considerably. Tfap2a was distributed in both the ooplasm and nucleoplasm, and its level gradually increased as meiosis resumption progressed. The overexpression of Tfap2a loosened the chromatin, accelerated germinal vesicle breakdown (GVBD), and blocked the first polar body extrusion 14 h after maturation in vitro. The width of the metaphase plate at metaphase I stage increased, and the spindle and chromosome organization at metaphase II stage were disrupted in the oocytes by overexpressed Tfap2a. Furthermore, Tfap2a overexpression dramatically boosted the expression of p300 in mouse GV oocytes. Additionally, the levels of pan histone lysine acetylation (Pan Kac), histone H4 lysine 12 acetylation (H4K12ac), and H4 lysine 16 acetylation (H4K16ac), as well as pan histone lysine lactylation (Pan Kla), histone H3 lysine18 lactylation (H3K18la), and H4 lysine12 lactylation (H4K12la), were all increased in GV oocytes after Tfap2a overexpression. Collectively, Tfap2a overexpression upregulated p300, increased the levels of histone acetylation and lactylation, impeded spindle assembly and chromosome alignment, and ultimately hindered mouse oocyte meiosis.

Oocyte Development and Quality in Young and Old Mice following Exposure to Atrazine

BACKGROUND

Egg development has unique features that render it vulnerable to environmental perturbation. The herbicide atrazine is an endocrine disruptor shown to have detrimental effects on reproduction across several vertebrate species.

OBJECTIVES

This study was designed to determine whether exposure to low levels of atrazine impairs meiosis in female mammals, using a mouse model; in particular, the study's researchers sought to determine whether and how the fidelity of oocyte chromosome segregation may be affected and whether aging-related aneuploidy is exacerbated.

METHODS

Female C57BL/6J mice were exposed to two levels of atrazine in drinking water: The higher level equaled aqueous saturation, and the lower level corresponded to detected environmental contamination. To model developmental exposure, atrazine was ingested by pregnant females at 0.5 d post coitum and continued until pups were weaned at 21 d postpartum. For adult exposure, 2-month-old females ingested atrazine for 3 months. Following exposure, various indicators of oocyte development and quality were determined, including: a) chromosome synapsis and crossing over in fetal oocytes using immunofluorescence staining of prophase-I chromosome preparations; b) sizes of follicle pools in sectioned ovaries; c) efficiencies of in vitro fertilization and early embryogenesis; d) chromosome alignment and segregation in cultured oocytes; e) chromosomal errors in metaphase-I and -II (MI and MII) preparations; and f) sister-chromatid cohesion via immunofluorescence intensity of cohesin subunit REC8 on MI-chromosome preparations, and measurement of interkinetochore distances in MII preparations.

RESULTS

Mice exposed to atrazine during development showed slightly higher levels of defects in chromosome synapsis, but sizes of initial follicle pools were indistinguishable from controls. However, although more eggs were ovulated, oocyte quality was lower. At the chromosome level, frequencies of spindle misalignment and numerical and structural abnormalities were greater at both meiotic divisions. In vitro fertilization was less efficient, and there were more apoptotic cells in blastocysts derived from eggs of atrazine-exposed females. Similar levels of chromosomal defects were seen in oocytes following both developmental and adult exposure regimens, suggesting quiescent primordial follicles may be a consequential target of atrazine. An important finding was that defects were observed long after exposure was terminated. Moreover, chromosomally abnormal eggs were very frequent in older mice, implying that atrazine exposure during development exacerbates effects of maternal aging on oocyte quality. Indeed, analogous to the effects of maternal age, weaker cohesion between sister chromatids was observed in oocytes from atrazine-exposed animals.

CONCLUSION

Low-level atrazine exposure caused persistent changes to the female mammalian germline in mice, with potential consequences for reproductive lifespan and congenital disease. https://doi.org/10.1289/EHP11343.

Quantitative morphokinetic parameters identify novel dynamics of oocyte meiotic maturation and cumulus expansion†

Meiotic maturation and cumulus expansion are essential for the generation of a developmentally competent gamete, and both processes can be recapitulated in vitro. We used a closed time-lapse incubator (EmbryoScope+™) to establish morphokinetic parameters of meiotic progression and cumulus expansion in mice and correlated these outcomes with egg ploidy. The average time to germinal vesicle breakdown (GVBD), time to first polar body extrusion (PBE), and duration of meiosis I were 0.91 ± 0.01, 8.82 ± 0.06, and 7.93 ± 0.06 h, respectively. The overall rate of cumulus layer expansion was 0.091 ± 0.002 μm/min, and the velocity of expansion peaked during the first 8 h of in vitro maturation (IVM) and then slowed. IVM of oocytes exposed to Nocodazole, a microtubule disrupting agent, and cumulus oocyte complexes (COCs) to 4-methylumbelliferone, a hyaluronan synthesis inhibitor, resulted in a dose-dependent perturbation of morphokinetics, thereby validating the system. The incidence of euploidy following IVM was >90% for both denuded oocytes and intact COCs. No differences were observed between euploid and aneuploid eggs with respect to time to GVBD (0.90 ± 0.22 vs. 0.97 ± 0.19 h), time to PBE (8.89 ± 0.98 vs. 9.10 ± 1.42 h), duration of meiosis I (8.01 ± 0.91 vs. 8.13 ± 1.38 h), and overall rate and kinetics of cumulus expansion (0.089 ± 0.02 vs 0.088 ± 0.03 μm/min) (P > 0.05). These morphokinetic parameters provide novel quantitative and non-invasive metrics for the evaluation of meiotic maturation and cumulus expansion and will enable screening compounds that modulate these processes.

Cytoskeletal form and function in mammalian oocytes and zygotes

The actin and microtubule cytoskeletons of mammalian oocytes and zygotes exist in distinct forms at various subcellular locations. This enables each cytoskeletal system to perform vastly different functions in time and space within the same cell. In recent years, key discovery enabling tools including light-sensitive microscopy assays have helped to illuminate cytoskeletal form and function in female reproductive cell biology. New findings include unexpected participation of F-actin in oocyte chromosome segregation, oocyte specific modes of spindle self-organization as well as existence of nuclear actin polymers whose functions are only starting to emerge. Functional actin-microtubule interactions have also been identified as an important feature that supports mammalian embryo development. Other advances have revealed reproductive age-related changes in chromosome structure and dynamics that predispose mammalian eggs to aneuploidy.

Chromosome Segregation in the Oocyte: What Goes Wrong during Aging

Human female fertility and reproductive lifespan decrease significantly with age, resulting in an extended post-reproductive period. The central dogma in human female reproduction contains two important aspects. One is the pool of oocytes in the human ovary (the ovarian reserve; approximately 10⁶ at birth), which diminishes throughout life until menopause around the age of 50 (approximately 10³ oocytes) in women. The second is the quality of oocytes, including the correctness of meiotic divisions, among other factors. Notably, the increased rate of sub- and infertility, aneuploidy, miscarriages, and birth defects are associated with advanced maternal age, especially in women above 35 years of age. This postponement is also relevant for human evolution; decades ago, the female aging-related fertility drop was not as important as it is today because women were having their children at a younger age. Spindle assembly is crucial for chromosome segregation during each cell division and oocyte maturation, making it an important event for euploidy. Consequently, aberrations in this segregation process, especially during the first meiotic division in human eggs, can lead to implantation failure or spontaneous abortion. Today, human reproductive medicine is also facing a high prevalence of aneuploidy, even in young females. However, the shift in the reproductive phase of humans and the strong increase in errors make the problem much more dramatic at later stages of the female reproductive phase. Aneuploidy in human eggs could be the result of the non-disjunction of entire chromosomes or sister chromatids during oocyte meiosis, but partial or segmental aneuploidies are also relevant. In this review, we intend to describe the relevance of the spindle apparatus during oocyte maturation for proper chromosome segregation in the context of maternal aging and the female reproductive lifespan.

Mechanism of spindle pole organization and instability in human oocytes

Human oocytes are prone to assembling meiotic spindles with unstable poles, which can favor aneuploidy in human eggs. The underlying causes of spindle instability are unknown. We found that NUMA (nuclear mitotic apparatus protein)-mediated clustering of microtubule minus ends focused the spindle poles in human, bovine, and porcine oocytes and in mouse oocytes depleted of acentriolar microtubule-organizing centers (aMTOCs). However, unlike human oocytes, bovine, porcine, and aMTOC-free mouse oocytes have stable spindles. We identified the molecular motor KIFC1 (kinesin superfamily protein C1) as a spindle-stabilizing protein that is deficient in human oocytes. Depletion of KIFC1 recapitulated spindle instability in bovine and aMTOC-free mouse oocytes, and the introduction of exogenous KIFC1 rescued spindle instability in human oocytes. Thus, the deficiency of KIFC1 contributes to spindle instability in human oocytes.