Recent insights into spindle function in mammalian oocytes and early embryos

Isoform-resolved transcriptome of the human preimplantation embryo

Human preimplantation development involves extensive remodeling of RNA expression and splicing. However, its transcriptome has been compiled using short-read sequencing data, which fails to capture most full-length mRNAs. Here, we generate an isoform-resolved transcriptome of early human development by performing long- and short-read RNA sequencing on 73 embryos spanning the zygote to blastocyst stages. We identify 110,212 unannotated isoforms transcribed from known genes, including highly conserved protein-coding loci and key developmental regulators. We further identify 17,964 isoforms from 5,239 unannotated genes, which are largely non-coding, primate-specific, and highly associated with transposable elements. These isoforms are widely supported by the integration of published multi-omics datasets, including single-cell 8CLC and blastoid studies. Alternative splicing and gene co-expression network analyses further reveal that embryonic genome activation is associated with splicing disruption and transient upregulation of gene modules. Together, these findings show that the human embryo transcriptome is far more complex than currently known, and will act as a valuable resource to empower future studies exploring development.

Exploring the Impact of Controlled Ovarian Stimulation and Non-Invasive Oocyte Assessment in ART Treatments

Invasive and noninvasive features are normally applied to select developmentally competent oocytes and embryos that can increase the take-home baby rates in assisted reproductive technology. The noninvasive approach mainly applied to determine oocyte and embryo competence has been, since the early days of IVF, the morphological evaluation of the mature cumulus-oocyte complex at the time of pickup, first polar body, zona pellucida thickness, perivitelline space and cytoplasm appearance. Morphological evaluation of oocyte quality is one of the options used to predict successful fertilization, early embryo development, uterine implantation and the capacity of an embryo to generate a healthy pregnancy to term. Thus, this paper aims to provide an analytical revision of the current literature relating to the correlation between ovarian stimulation procedures and oocyte/embryo quality. In detail, several aspects of oocyte quality such as morphological features, oocyte competence and its surrounding environment will be discussed. In addition, the main noninvasive features as well as novel approaches to biomechanical parameters of oocytes that might be correlated with the competence of embryos to produce a healthy pregnancy and live birth will be illustrated.

Fisetin Delays Postovulatory Oocyte Aging by Regulating Oxidative Stress and Mitochondrial Function through Sirt1 Pathway

The quality of oocytes determines the development potential of an embryo and is dependent on their timely fertilization after ovulation. Postovulatory oocyte aging is an inevitable factor during some assisted reproduction technology procedures, which results in poor fertilization rates and impairs embryo development. We found that fisetin, a bioactive flavonol contained in fruits and vegetables, delayed postovulatory oocyte aging in mice. Fisetin improved the development of aged oocytes after fertilization and inhibited the Sirt1 reduction in aged oocytes. Fisetin increased the GSH level and Sod2 transcription level to inhibit ROS accumulation in aged oocytes. Meanwhile, fisetin attenuated aging-induced spindle abnormalities, mitochondrial dysfunction, and apoptosis. At the molecular level, fisetin decreased aging-induced aberrant expression of H3K9me3. In addition, fisetin increased the expression levels of the mitochondrial transcription factor Tfam and the mitochondrial genes Co2 and Atp8 by upregulating Sirt1 in aged oocytes. Finally, inhibition of Sirt1 reversed the anti-aging effects of fisetin. Taken together, fisetin delayed postovulatory oocyte aging by upregulating Sirt1.

Xrcc5/Ku80 is required for the repair of DNA damage in fully grown meiotically arrested mammalian oocytes

Mammalian oocytes spend most of their life in a unique state of cell cycle arrest at meiotic prophase I, during which time they are exposed to countless DNA-damaging events. Recent studies have shown that DNA double-strand break repair occurs predominantly via the homologous recombination (HR) pathway in small non-growing meiotically arrested oocytes (primordial follicle stage). However, the DNA repair mechanisms employed by fully grown meiotically arrested oocytes (GV-stage) have not been studied in detail. Here we established a conditional knockout mouse model to explore the role of Ku80, a critical component of the nonhomologous end joining (NHEJ) pathway, in the repair of DNA damage in GV oocytes. GV oocytes lacking Ku80 failed to repair etoposide-induced DNA damage, even when only low levels of damage were sustained. This indicates Ku80 is needed to resolve DSBs and that HR cannot compensate for a compromised NHEJ pathway in fully-grown oocytes. When higher levels of DNA damage were induced, a severe delay in M-phase entry was observed in oocytes lacking XRCC5 compared to wild-type oocytes, suggesting that Ku80-dependent repair of DNA damage is important for the timely release of oocytes from prophase I and resumption of meiosis. Ku80 was also found to be critical for chromosome integrity during meiotic maturation following etoposide exposure. These data demonstrate that Ku80, and NHEJ, are vital for quality control in mammalian GV stage oocytes and reveal that DNA repair pathway choice differs in meiotically arrested oocytes according to growth status.

Exposure of mouse oocytes to N,N-dimethylformamide impairs mitochondrial functions and reduces oocyte quality

N,N-dimethylformamide (DMF) is a widely-used solvent for the synthesis of synthetic fibers such as polyacrylonitrile fiber, and can also be used to make medicine. Although this organic solvent has multipurpose applications, its biological toxicity cannot be ignored and its impact on mammalian reproduction remains largely unexplored. Our study found that DMF exposure inhibited oocyte maturation and fertilization ability. Transcriptomic analysis indicated that DMF exposure changed the expression of genes and transposable elements in oocytes. Subcellular structure examination found that DMF exposure caused mitochondrial dysfunction, abnormal aggregation of mitochondria and decreased mitochondrial membrane potential in mouse oocytes. Its exposure also caused abnormal distribution of Golgi apparatus and endoplasmic reticulum which formed large number of clusters. In addition, oxidative stress occurs in oocytes exposed to DMF, which was manifested by an increase in the level of reactive oxygen species. We found that DMF exposure induced disordered spindle and chromosomes abnormality. Meanwhile, we examined various histone modification levels in oocytes exposed to DMF and found that DMF exposure reduced H3K9me3, H3K9ac, H3K27ac, and H4K16ac levels in mouse oocytes. Moreover, DMF-treated oocytes failed to form pronuclei after fusion with normal sperm. Collectively, DMF exposure caused mitochondrial damage, oxidative stress, spindle assembly and chromosome arrangement disorder, leading to oocyte maturation arrest and fertilization failure.

Toxic effects of 4-methylimidazole on the maturation and fertilization of mouse oocytes

Individuals of all ages, including children and teenagers, consume 4-methylimidazole (4-MI) in their food. 4-MI is a caramel-colored waste product that has previously been linked to human carcinogenesis and has shown possible signs of reproductive toxicity. This study aimed to determine whether 4-MI is harmful to oocytes during meiosis and fertilization. Female mice were intragastrically administered 0, 50, or 100 mg/kg body weight of 4-MI daily for 10 days. We found that 4-MI affects the quality of oocytes by affecting their meiotic ability and fertility potential. Specifically, 4-MI rendered the meiotic spindles and chromosomes less stable, which halted oocyte maturation and resulted in aneuploidy. 4-MI also slowed the decrease in the levels of cortical granules and their component ovastacin; consequently, sperms could not be bound and fertilization could not occur. We also found that mitochondrial dysfunction was associated with oocytes deterioration. This led to reactive oxygen species accumulation and cell death. Altogether, our findings reveal that the poor condition of oocytes subjected to 4-MI is primarily attributable to mitochondrial malfunction and redox alterations.

DNA damage in preimplantation embryos and gametes: specification, clinical relevance and repair strategies

BACKGROUND

DNA damage is a hazard that affects all cells of the body. DNA-damage repair (DDR) mechanisms are in place to repair damage and restore cellular function, as are other damage-induced processes such as apoptosis, autophagy and senescence. The resilience of germ cells and embryos in response to DNA damage is less well studied compared with other cell types. Given that recent studies have described links between embryonic handling techniques and an increased likelihood of disease in post-natal life, an update is needed to summarize the sources of DNA damage in embryos and their capacity to repair it. In addition, numerous recent publications have detailed novel techniques for detecting and repairing DNA damage in embryos. This information is of interest to medical or scientific personnel who wish to obtain undamaged embryos for use in offspring generation by ART.

OBJECTIVE AND RATIONALE

This review aims to thoroughly discuss sources of DNA damage in male and female gametes and preimplantation embryos. Special consideration is given to current knowledge and limits in DNA damage detection and screening strategies. Finally, obstacles and future perspectives in clinical diagnosis and treatment (repair) of DNA damaged embryos are discussed.

SEARCH METHODS

Using PubMed and Google Scholar until May 2021, a comprehensive search for peer-reviewed original English-language articles was carried out using keywords relevant to the topic with no limits placed on time. Keywords included ‘DNA damage repair’, ‘gametes’, ‘sperm’, ‘oocyte’, ‘zygote’, ‘blastocyst’ and ‘embryo’. References from retrieved articles were also used to obtain additional articles. Literature on the sources and consequences of DNA damage on germ cells and embryos was also searched. Additional papers cited by primary references were included. Results from our own studies were included where relevant.

OUTCOMES

DNA damage in gametes and embryos can differ greatly based on the source and severity. This damage affects the development of the embryo and can lead to long-term health effects on offspring. DDR mechanisms can repair damage to a certain extent, but the factors that play a role in this process are numerous and altogether not well characterized. In this review, we describe the multifactorial origin of DNA damage in male and female gametes and in the embryo, and suggest screening strategies for the selection of healthy gametes and embryos. Furthermore, possible therapeutic solutions to decrease the frequency of DNA damaged gametes and embryos and eventually to repair DNA and increase mitochondrial quality in embryos before their implantation is discussed.

WIDER IMPLICATIONS

Understanding DNA damage in gametes and embryos is essential for the improvement of techniques that could enhance embryo implantation and pregnancy success. While our knowledge about DNA damage factors and regulatory mechanisms in cells has advanced greatly, the number of feasible practical techniques to avoid or repair damaged embryos remains scarce. Our intention is therefore to focus on strategies to obtain embryos with as little DNA damage as possible, which will impact reproductive biology research with particular significance for reproductive clinicians and embryologists.

CENP-V is required for proper chromosome segregation through interaction with spindle microtubules in mouse oocytes

Proper chromosome segregation is essential to avoid aneuploidy, yet this process fails with increasing age in mammalian oocytes. Here we report a role for the scarcely described protein CENP-V in oocyte spindle formation and chromosome segregation. We show that depending on the oocyte maturation state, CENP-V localizes to centromeres, to microtubule organizing centers, and to spindle microtubules. We find that Cenp-V-/- oocytes feature severe deficiencies, including metaphase I arrest, strongly reduced polar body extrusion, increased numbers of mis-aligned chromosomes and aneuploidy, multipolar spindles, unfocused spindle poles and loss of kinetochore spindle fibres. We also show that CENP-V protein binds, diffuses along, and bundles microtubules in vitro. The spindle assembly checkpoint arrests about half of metaphase I Cenp-V-/- oocytes from young adults only. This finding suggests checkpoint weakening in ageing oocytes, which mature despite carrying mis-aligned chromosomes. Thus, CENP-V is a microtubule bundling protein crucial to faithful oocyte meiosis, and Cenp-V-/- oocytes reveal age-dependent weakening of the spindle assembly checkpoint.

Inferring simple but precise quantitative models of human oocyte and early embryo development

Macroscopic, phenomenological models are useful as concise framings of our understandings in fields from statistical physics to finance to biology. Constructing a phenomenological model for development would provide a framework for understanding the complicated, regulatory nature of oogenesis and embryogenesis. Here, we use a data-driven approach to infer quantitative, precise models of human oocyte maturation and pre-implantation embryo development, by analysing clinical in-vitro fertilization (IVF) data on 7399 IVF cycles resulting in 57 827 embryos. Surprisingly, we find that both oocyte maturation and early embryo development are quantitatively described by simple models with minimal interactions. This simplicity suggests that oogenesis and embryogenesis are composed of modular processes that are relatively siloed from one another. In particular, our analysis provides strong evidence that (i) pre-antral follicles produce anti-Müllerian hormone independently of effects from other follicles, (ii) oocytes mature to metaphase-II independently of the woman's age, her BMI and other factors, (iii) early embryo development is memoryless for the variables assessed here, in that the probability of an embryo transitioning from its current developmental stage to the next is independent of its previous stage. Our results both provide insight into the fundamentals of oogenesis and embryogenesis and have implications for the clinical IVF.

Perfluorononanoic acid impedes mouse oocyte maturation by inducing mitochondrial dysfunction and oxidative stress

Perfluorononanoic acid (PFNA), a member of PFAS, is frequently detected in human blood and tissues, even in follicular fluid of women. The exposure of PFNA, but not PFOA and PFOS, is positively correlated with miscarriage and increased time to pregnancy. Toxicological studies indicated that PFNA exposure is associated with immunotoxicity, hepatotoxicity, developmental toxicity, and reproductive toxicity in animals. However, there is little information regarding the toxic effects of PFNA on oocyte maturation. In this study, we investigated the toxic effects of PFNA exposure on mouse oocyte maturation in vitro. Our results showed that 600 μM PFNA significantly inhibited germinal vesicle breakdown (GVBD) and polar body extrusion (PBE) in mouse oocytes. Our further study revealed that PFNA induced abnormal metaphase I (MI) spindle assembly, evidenced by malformed spindles and mislocalization of p-ERK1/2 in PFNA-treated oocytes. We also found that PFNA induced abnormal mitochondrial distribution and increased mitochondrial membrane potential. Consequently, PFNA increased reactive oxygen species (ROS) levels, leading to oxidative stress, DNA damage, and eventually early-stage apoptosis in oocytes. In addition, after 14 h culture, PFNA disrupted the formation of metaphase II (MII) spindle in most PFNA-treated oocytes with polar bodies. Collectively, our results indicate that PFNA interferes with oocyte maturation in vitro via disrupting spindle assembly, damaging mitochondrial functions, and inducing oxidative stress, DNA damage, and early-stage apoptosis.

Seasonal Effect on Developmental Competence, Oxidative Status and Tubulin Assessment of Prepubertal Ovine Oocyte

Simple Summary

Oocytes obtained from the ovaries of slaughtered prepubertal ewes can be incorporated into an in vitro embryo production system. The collection of this material is possible at two different times of the year, spring and autumn. The first period is linked to the natural reproductive cycle of the sheep. The second, on the other hand, is linked to the manipulation of the reproductive seasonality which allows the oestrus cycle to be controlled artificially. The analysis highlighted that the collection season influences oocytes quality from prepubertal donors in terms of improved energetic and oxidative status, microtubular organization, and developmental competence in oocytes recovered in spring. Data obtained underline that oocytes seem to be genetically and evolutionarily programmed to give their best in spring, this being the most favorable period for newborns.

Abstract

The reproductive seasonality of domestic animals is often manipulated in order to have more reproductive periods for commercial purposes related to the production of milk and meat. It is scientifically proven that such an alteration of the reproductive activity in sheep entails a deterioration in oocyte quality, leading to an inability to generate embryos. Since oocytes obtained from prepubertal ewes can be incorporated into an in vitro embryo production system and considering that their quality is crucial to the success of in vitro procedures, the aim of this work was to investigate the effect of seasons on the quality of prepubertal ovine oocytes collected in autumn and spring. Ovaries were collected from a local slaughterhouse from 30–40-day-old suckling lambs during both seasons. Following 24 h of in vitro maturation, oocytes developmental competence, reactive oxygen species (ROS) intracellular levels, and mitochondrial activity were evaluated, and a tubulin assessment was performed. The results on embryo production, as a percentage of first divisions and number of blastocysts obtained, were significantly higher in oocytes collected in the spring. Mitochondrial activity in oocytes was higher, and ROS production significantly lower, in spring than in autumn. Tubulin PTMs (tyrosinated and acetylated α-tubulin) showed a higher immunoreactivity in oocytes collected in spring compared with autumn sampling. Our data showed that seasons may affect the developmental competence, energetic status, and tubulin assessment of oocytes recovered from prepubertal ewes. Therefore, special care should be taken when choosing the period of the year for prepuberal ovine oocytes collection aimed at in vitro embryo reproduction programs.

Application of the modified handmade cloning technique to pigs

Although somatic cell nuclear transfer (SCNT) is frequently employed to produce cloned animals in laboratories, this technique is expensive and inefficient. Therefore, the handmade cloning (HMC) technique has been suggested to simplify and advance the cloning process, however, HMC wastes many oocytes and leads to mitochondrial heteroplasmy. To solve these problems, we propose a modified handmade cloning (mHMC) technique that uses simple laboratory equipment, i.e., a Pasteur pipette and an alcohol lamp, applying it to porcine embryo cloning. To validate the application of mHMC to pig cloning, embryos produced through SCNT and mHMC are compared using multiple methods, such as enucleation efficiency, oxidative stress, embryo developmental competence, and gene expression. The results show no significant differences between techniques except in the enucleation efficiency. The 8-cell and 16-cell embryo developmental competence and Oct4 expression levels exhibit significant differences. However, the blastocyst rate is not significantly different between mHMC and SCNT. This study verifies that cloned embryos derived from the two techniques exhibit similar generation and developmental competence. Thus, we suggest that mHMC could replace SCNT for simpler and cheaper porcine cloning.

Stable kinetochore-microtubule attachments restrict MTOC position and spindle elongation in oocytes

In mouse oocytes, acentriolar MTOCs functionally replace centrosomes and act as microtubule nucleation sites. Microtubules nucleated from MTOCs initially assemble into an unorganized ball‐like structure, which then transforms into a bipolar spindle carrying MTOCs at its poles, a process called spindle bipolarization. In mouse oocytes, spindle bipolarization is promoted by kinetochores but the mechanism by which kinetochore–microtubule attachments contribute to spindle bipolarity remains unclear. This study demonstrates that the stability of kinetochore–microtubule attachment is essential for confining MTOC positions at the spindle poles and for limiting spindle elongation. MTOC sorting is gradual and continues even in the metaphase spindle. When stable kinetochore–microtubule attachments are disrupted, the spindle is unable to restrict MTOCs at its poles and fails to terminate its elongation. Stable kinetochore fibers are directly connected to MTOCs and to the spindle poles. These findings suggest a role for stable kinetochore–microtubule attachments in fine‐tuning acentrosomal spindle bipolarity.

The Impact of Embryo Storage Time on Pregnancy and Perinatal Outcomes and the Time Limit of Vitrification: A Retrospective Cohort Study

BACKGROUND

The technique of embryo cryopreservation has been increasingly applied in clinical settings. However, there has been a concern about the safety and efficacy of long-term freezing of embryos. Therefore, the aim of this study was to evaluate whether storage time of vitrification had any effects on pregnancy as well as perinatal outcomes, further, to explore the appropriate time limit of vitrification.

METHODS

The study included women who underwent at least one frozen-thawed cycle with single embryo transfer between January 1^st, 2016 and September 30^th, 2019. Patients were assigned into 3 groups according to the storage time (<3 months, 3-12 months and >12 months) to evaluate the impact of embryo storage time on pregnancy and perinatal outcomes. To further investigate the time limit of vitrification, propensity score matching was used to compare the primary outcomes of patients with storage time of 1-3 years, 3-5 years, and >5 years to those stored for ≤1 year.

RESULTS

A total of 9806 frozen-thawed embryo transfer cycles were included in our study. After adjustment for confounding variables, no significant differences were found in pregnancy outcomes among groups. However, postponement of transfer increased the risks of large for gestational age and placenta previa. In addition, after propensity score matching, 171 cycles with storage time >5 years were matched with those ≤1 year, both the clinical pregnancy rate and live birth rate decreased significantly when the storage time exceeded 5 years.

CONCLUSIONS

The duration of vitrification did not significantly affect the pregnancy outcomes within 5 years period. However, the clinical pregnancy rate and live birth rate both decreased significantly when the duration of vitrification exceeded 5 years. It is worth noting that the conclusion was drawn from a small sample study after propensity score matching and should be treated with caution. In addition, the cycles were from different time periods, which could have an impact on the results.

Iodoacetic acid disrupts mouse oocyte maturation by inducing oxidative stress and spindle abnormalities

Disinfection by-products (DBPs) are compounds produced during the water disinfection process. Iodoacetic acid (IAA) is one of the unregulated DBPs in drinking water, with potent cytotoxicity and genotoxicity in animals. However, whether IAA has toxic effects on oocyte maturation remains unclear. Here, we show that IAA exposure resulted in metaphase I (MI) arrest and polar-body-extrusion failure in mouse oocytes, indicating that IAA had adverse effects on mouse oocyte maturation in vitro. Particularly, IAA treatment caused abnormal spindle assembly and chromosome misalignment. Previous studies reported that IAA is a known inducer of oxidative stress in non-germline cells. Correspondingly, we found that IAA exposure increased the reactive oxygen species (ROS) levels in oocytes in a dose-dependent manner, indicating IAA exposure could induce oxidative stress in oocytes. Simultaneously, DNA damage was also elevated in the nuclei of these IAA-exposed mouse oocytes, evidenced by increased γ-H2AX focus number. In addition, the un-arrested oocytes entered metaphase II (MII) with severe defects in spindle morphologies and chromosome alignment after 14-h IAA treatment. An antioxidant, N-acetyl-L-cysteine (NAC), reduced the elevated ROS level and restored the meiotic maturation in the IAA-exposed oocytes, which indicates that IAA-induced maturation failure in oocytes was mainly mediated by oxidative stress. Collectively, our results indicate that IAA exposure interfered with mouse oocyte maturation by elevating ROS levels, disrupting spindle assembly, inducing DNA damage, and causing MI arrest.

The toxic effects and possible mechanisms of decabromodiphenyl ethane on mouse oocyte

Decabromodiphenyl ethane (DBDPE), a widely used new brominated flame retardant, is added into flammable materials to achieve fire retardation. As it is continuously detected in the environment, it has become an emerging environmental pollutant. However, the effects of DBDPE exposure on oocyte maturation and its underlying mechanisms remain unknown. This study found that DBDPE exposure inhibited the rate of germinal vesicle breakdown (GVBD), first polar body extrusion (PBE) and fertilization of mouse oocytes. After 14 h of exposure to DBDPE, metaphase II (MII) oocytes showed that the hardness of zona pellucida (ZP) markedly increased and that the spindle morphology was abnormal. Moreover, DBDPE exposure induced abnormal mitochondrial distribution, mitochondrial dysfunction, and ATP deficiency. Simultaneously, DBDPE exposure down-regulated the expression of antioxidant-related genes (Sod2, Gpx1) and increased the level of reactive oxygen species (ROS) in oocytes. The results of immunofluorescence and qRT-PCR revealed that autophagy occurred in DBDPE-treated oocytes with high expression of autophagy-related protein (LC3) and genes (Lc3, Beclin1). Meanwhile, DBDPE significantly up-regulated the protein (Bax) and mRNA (Bax, Caspase3) levels of pro-apoptosis genes. However, the protein and mRNA expression of anti-apoptosis genes Bcl-2 was dramatically down-regulated in DBDPE-exposed oocytes. Collectively, DBDPE exposure impaired mitochondrial function, causing oxidative damage, autophagy and apoptosis in oocytes.

Effects of age on pregnancy outcomes in patients with simple tubal factor infertility receiving frozen-thawed embryo transfer

This study was to retrospectively analyze the effect of the age of embryos transfer and oocyte retrieval on the clinical pregnancy outcome in patients with simple tubal factor infertility (TFI) who received frozen-thawed embryo transfer. Patients (n = 3619) with simple TFI who underwent in vitro fertilization (IVF)/ intracytoplasmic sperm injection (ICSI) frozen-thawed embryo transfer at our hospital were enrolled. Univariate logistic regression analysis, categorical multivariate logistic regression analysis, curve fitting and threshold effect analysis were performed. Age of embryo transfer was a significant (P < 0.05) independent risk factor affecting the clinical pregnancy, live birth, and miscarriage rates. The Clinical pregnancy outcome declined significantly after the age of 34 years. After limiting the female oocyte retrieval age to ≤ 34 years, no significant change was detected in the clinical pregnancy, live birth, or miscarriage rate with increase of transplantation age. In conclusion, in patients with simple TFI undergoing IVF/ICSI frozen-thawed embryo transfer, age is a significant independent risk factor affecting the clinical pregnancy, live birth, and miscarriage rate. Aging of oocytes has a greater impact on the clinical pregnancy in women with simple TFI than the aging of the body. Patients with TFI can freeze embryos in advance to preserve fertility.

Cdk1 negatively regulates the spindle localization of Prc1 in mouse oocytes

Chromosome segregation requires the formation of a bipolar spindle. The timely bipolarization of the acentrosomal spindle during meiosis I in mouse oocytes depends on the antiparallel microtubule crosslinker Prc1. How Prc1 is regulated in oocytes remains poorly understood. In this study, we show that the kinase Cdk1 negatively regulates the spindle localization of Prc1 in mouse oocytes. The acute inhibition of Cdk1 activity led to excessive localization of Prc1 at the spindle and kinetochores, whereas the overactivation of Cdk1 had opposite effects. The overexpression of Prc1 carrying mutations at Cdk1-mediated phosphorylation sites increased its localization to the spindle, accelerated spindle bipolarization and caused spindle-checkpoint-dependent arrest at metaphase I. Overactivation of Cdk1 delayed spindle bipolarization, which was reversed by the overexpression of a phospho-mutant form but not the wild-type form of Prc1. These results suggest that Cdk1-mediated phosphorylation negatively regulates Prc1 localization to ensure the timely bipolarization of the acentrosomal spindle during meiosis I in mammalian oocytes.

Hfm1 participates in Golgi-associated spindle assembly and division in mouse oocyte meiosis

HFM1 (helicase for meiosis 1) is widely recognized as an ATP-dependent DNA helicase and is expressed mainly in germ-line cells. HFM1 is a candidate gene of premature ovarian failure (POF), hence it is also known as POF9. However, the roles of HFM1 in mammalian oocytes remain uncertain. To investigate the functions of HFM1, we established a conditional knockout (cKO) mouse model. Specific knockout of Hfm1 in mouse oocytes from the primordial follicle stage resulted in depletion of ovarian follicular reserve and subfertility of mice. In particular, abnormal spindle, misaligned chromosomes, loss of cortical actin cap, and failing polar body extrusion were readily observed in Hfm1-cKO oocytes. Further studies indicated that in addition to its cytoplasmic distribution, Hfm1 accumulated at the spindle poles, colocalized with the Golgi marker protein, GM130. Generally, GM130 signals overlapped with p-Mapk at the two spindle poles to regulate meiotic spindle assembly and asymmetric division. In this research, centrosome associated proteins, such as GM130 and p-Mapk, detached from the spindle poles in Hfm1-cKO oocytes. In conclusion, our data suggest that Hfm1 participates in Golgi-associated spindle assembly and division in mouse oocyte meiosis. These findings provide clues for pathogenesis of POF.

Prc1-rich kinetochores are required for error-free acentrosomal spindle bipolarization during meiosis I in mouse oocytes

Acentrosomal meiosis in oocytes represents a gametogenic challenge, requiring spindle bipolarization without predefined bipolar cues. While much is known about the structures that promote acentrosomal microtubule nucleation, less is known about the structures that mediate spindle bipolarization in mammalian oocytes. Here, we show that in mouse oocytes, kinetochores are required for spindle bipolarization in meiosis I. This process is promoted by oocyte-specific, microtubule-independent enrichment of the antiparallel microtubule crosslinker Prc1 at kinetochores via the Ndc80 complex. In contrast, in meiosis II, cytoplasm that contains upregulated factors including Prc1 supports kinetochore-independent pathways for spindle bipolarization. The kinetochore-dependent mode of spindle bipolarization is required for meiosis I to prevent chromosome segregation errors. Human oocytes, where spindle bipolarization is reportedly error prone, exhibit no detectable kinetochore enrichment of Prc1. This study reveals an oocyte-specific function of kinetochores in acentrosomal spindle bipolarization in mice, and provides insights into the error-prone nature of human oocytes.

Oocyte meiosis must achieve spindle bipolarization without predefined spatial cues. Yoshida et al. demonstrate that spindle bipolarization during meiosis I in mouse oocytes requires kinetochores to prevent chromosome segregation errors, a phenomenon that does not occur in error-prone human oocytes.

The toxic effects of Fluorene-9-bisphenol on porcine oocyte in vitro maturation

Fluorene-9-bisphenol (9,9-bis(4-hydroxyphenyl)-fluorene [BHPF]) is a bisphenol A (BPA) substitute used in the production of "BPA-free" plastics, now has been identified is harmful to living organisms. Our previous study showed that BHPF impaired mouse denuded oocyte in vitro maturation. However, there is a question that whether BHPF is still able to affect oocyte maturation in the presence of dense cumulus cells. In the present study, we checked the toxic effects of BHPF on porcine oocyte maturation which is derived from COCs in vitro culture. Our results showed that BHPF (50 μM) inhibited the expansion of cumulus cells, led to a significant decrease in polar body extrusion (PBE). Importantly, BHPF resulted in abnormal spindle assembly, ATP level decrease, reactive oxygen species (ROS) accumulation and early apoptosis in porcine oocytes, which are all negative to oocyte maturation. Furthermore, BHPF also declined porcine oocyte quality by disturbing the cortical granules (CGs) distribution. In conclusion, our study showed that BHPF still inhibited oocyte maturation even in the presence of cumulus cells leading to abnormal spindle assembly, ATP decrease, increased ROS level, early apoptosis, and disturbed CGs distribution in porcine oocytes, and also indicates that BHPF has a wide range toxic effects on oocyte in different species.

METTL3-mediated m6A is required for murine oocyte maturation and maternal-to-zygotic transition

N⁶-methyladenosine (m⁶A) is the most prevalent epigenetic modification of messenger RNA (mRNA) in higher eukaryotes; this modification is mainly catalyzed by a methyltransferase complex including methyltransferase-like 3 (METTL3) as a key factor. Although m⁶A modification has been proven to play an essential role in diverse biological processes, our knowledge of Mettl3 is still limited because Mettl3 mutations are lethal to embryos in both mammals and plants. In this study, we knocked down Mettl3 by microinjection of its specific short interfering RNAs (siRNAs) or morpholino into fully grown germinal vesicle (GV) oocytes. As a result, we demonstrated that knocking down Mettl3 in female germ cells severely inhibited oocyte maturation by decreasing mRNA translation efficiency and led to defects in the maternal-to-zygotic transition, probably due to its interference in disrupting mRNA degradation. The discovery from this study suggests that the reversible m⁶A modification has vital functions in mammalian oocyte maturation and pre-implantation embryonic development processes.

Segregating Chromosomes in the Mammalian Oocyte

Chromosome segregation errors in human oocytes lead to aneuploid embryos that cause infertility and birth defects. Here we provide an overview of the chromosome-segregation process in the mammalian oocyte, highlighting mechanistic differences between oocytes and somatic cells that render oocytes so prone to segregation error. These differences include the extremely large size of the oocyte cytoplasm, the unique geometry of meiosis-I chromosomes, idiosyncratic function of the spindle assembly checkpoint, and dramatically altered oocyte cell-cycle control and spindle assembly, as compared to typical somatic cells. We summarise recent work suggesting that aging leads to a further deterioration in fidelity of chromosome segregation by impacting multiple components of the chromosome-segregation machinery. In addition, we compare and contrast recent results from mouse and human oocytes, which exhibit overlapping defects to differing extents. We conclude that the striking propensity of the oocyte to mis-segregate chromosomes reflects the unique challenges faced by the spindle in a highly unusual cellular environment.

Deletion of maternal UHRF1 severely reduces mouse oocyte quality and causes developmental defects in preimplantation embryos

The ubiquitin-like, containing PHD and RING finger domains, 1 (UHRF1) protein recognizes DNA methylation and histone modification and plays a critical role in epigenetic regulation. Recently, UHRF1 was shown to have a role in DNA methylation in oocytes and early embryos. Here, we reveal that maternal UHRF1 determines the quality of mouse oocytes. We generated oocyte-specific Uhrf1-knockout mice and found that females were sterile, and few maternal UHRF1-null embryos developed into blastocysts. The UHRF1-null oocytes had an increased incidence of aneuploidy and DNA damage. In addition to defective DNA methylation, histone modification was affected during oogenesis, with UHRF1-null germinal vesicle and metaphase II-stage oocytes exhibiting reduced global histone H3 lysine 9 dimethylation levels and elevated acetylation of histone H4 lysine 12. Taken together, our results suggest that UHRF1 plays an important role in determining oocyte quality and affects epigenetic regulation of oocyte maturation as a maternal protein, which is crucial for embryo developmental potential. Further exploration of the biologic function and underlying mechanisms of maternal UHRF1 will enhance our understanding of the maternal control of the oocyte and early embryonic development.-Cao, Y., Li, M., Liu, F., Ni, X., Wang, S., Zhang, H., Sui, X., Huo, R. Deletion of maternal UHRF1 severely reduces mouse oocyte quality and causes developmental defects in preimplantation embryos.

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

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

Separation and Loss of Centrioles From Primordidal Germ Cells To Mature Oocytes In The Mouse

Oocytes, including from mammals, lack centrioles, but neither the mechanism by which mature eggs lose their centrioles nor the exact stage at which centrioles are destroyed during oogenesis is known. To answer questions raised by centriole disappearance during oogenesis, using a transgenic mouse expressing GFP-centrin-2 (GFP CETN2), we traced their presence from e11.5 primordial germ cells (PGCs) through oogenesis and their ultimate dissolution in mature oocytes. We show tightly coupled CETN2 doublets in PGCs, oogonia, and pre-pubertal oocytes. Beginning with follicular recruitment of incompetent germinal vesicle (GV) oocytes, through full oocyte maturation, the CETN2 doublets separate within the pericentriolar material (PCM) and a rise in single CETN2 pairs is identified, mostly at meiotic metaphase-I and -II spindle poles. Partial CETN2 foci dissolution occurs even as other centriole markers, like Cep135, a protein necessary for centriole duplication, are maintained at the PCM. Furthermore, live imaging demonstrates that the link between the two centrioles breaks as meiosis resumes and that centriole association with the PCM is progressively lost. Microtubule inhibition shows that centriole dissolution is uncoupled from microtubule dynamics. Thus, centriole doublets, present in early G2-arrested meiotic prophase oocytes, begin partial reduction during follicular recruitment and meiotic resumption, later than previously thought.

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

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

Intrinsically Defective Microtubule Dynamics Contribute to Age-Related Chromosome Segregation Errors in Mouse Oocyte Meiosis-I

Chromosome segregation errors in mammalian oocytes compromise development and are particularly prevalent in older females, but the aging-related cellular changes that promote segregation errors remain unclear [1, 2]. Aging causes a loss of meiotic chromosome cohesion, which can explain premature disjunction of sister chromatids [3-7], but why intact sister pairs should missegregate in meiosis-I (termed non-disjunction) remains unknown. Here, we show that oocytes from naturally aged mice exhibit substantially altered spindle microtubule dynamics, resulting in transiently multipolar spindles that predispose the oocytes to kinetochore-microtubule attachment defects and missegregation of intact sister chromatid pairs. Using classical micromanipulation approaches, including reciprocally transferring nuclei between young and aged oocytes, we show that altered microtubule dynamics are not attributable to age-related chromatin changes. We therefore report that altered microtubule dynamics is a novel primary lesion contributing to age-related oocyte segregation errors. We propose that, whereas cohesion loss can explain premature sister separation, classical non-disjunction is instead explained by altered microtubule dynamics, leading to aberrant spindle assembly.

Abce1 orchestrates M-phase entry and cytoskeleton architecture in mouse oocyte

ATP-binding cassette E1 (ABCE1) is a member of the ATP-binding cassette transporters and essential for diverse biological events regulating abroad range of biological functions including viral infection, cell proliferation, anti-apoptosis, translation initiation and ribosome biogenesis. Here, we discovered that Abce1 also plays indispensable roles in mouse oocyte meiotic progression. In the present study, we examined the expression, localization, and function of Abce1 during mouse oocyte meiotic maturation. Immunostaining and confocal microscopy identified that Abce1 localized as small dots in nucleus in germinal vesicle stage. After germinal vesicle breakdown, it dispersedly localized around the whole spindle apparatus. During the anaphase and telophase stages, Abce1 was just like a cap to localize around the two pole region of spindle but not the midbody and chromosome. Knockdown of Abce1 by specific siRNA injection delayed the resumption of meiosis (GVBD) and affected the extrusion of first polar body. Moreover, the process of spindle assembly and chromosome alignment were severely impaired. Abce1-RNAi led to the dissociation of γ-tubulin and p-MAPK from spindle poles, thus caused mounts of spindle morphology abnormities and chromosome alignment defects, leading to high incidence of aneuploidy. Our findings refresh the knowledge of Abce1 function by exploring its role in oocyte meiotic resumption, spindle assembly and chromosome alignment.

Tubulin posttranslational modifications in in vitro matured prepubertal and adult ovine oocytes

Microtubules (MTs), polymers of alpha/beta-tubulin heterodimers, are involved in crucial functions in eukaryotic cells. MTs physiology can be influenced by a variety of post-translational modifications (PTMs), including tyrosination, detyrosination, delta 2 modification, acetylation, polyglutamylation, polyglycylation. In mammalian oocytes, MTs are essential for meiosis, regulating the formation of meiotic spindle and chromosomes movements. Considering that the patterns of tubulin PTMs (tyrosination, detyrosination, acetylation, polyglutamylation and delta 2 modification) have not been investigated in ovine oocytes, this study has been designed to investigate their presence and quantification in in vitro matured (IVM) adult and prepubertal ovine oocytes. Oocytes from adult and lamb Sarda ewes, regularly slaughtered at the local abattoir, were in vitro matured, fixed, and processed by indirect immunofluorescence and confocal microscopy analyses at metaphase II stage. Our results revealed a well detectable signal for total, tyrosinated and acetylated α-tubulin in meiotic spindle of both sheep and lamb oocytes. On the other hand, no immunopositivity were appreciable for detyrosinated, polyglutamylated, and delta 2 tubulin in meiotic spindle of both sheep and lamb oocytes. As regard the tyrosinated and the acetylated α-tubulin PTMs, through the quantification of the fluorescence intensity, we did not find significant differences in their expression in meiotic spindle of sheep, while in lamb the acetylated tubulin levels were predominant in comparison with tyrosinated. Our results in addition to investigating for the first time the different tubulin PTMs in the spindle organization of ovine oocytes, showed a different microtubule pattern between adult and prepubertal oocytes. The microtubule cytoskeleton survey may thus suggest further cues to better understand skill-related problems in in the acquisition of oocyte competence.

The mutual benefits of research in wild animal species and human-assisted reproduction

Studying the reproductive biology of wild animal species produces knowledge beneficial to their management and conservation. However, wild species also share intriguing similarities in reproductive biology with humans, thereby offering alternative models for better understanding the etiology of infertility and developing innovative treatments. The purpose of this review is to raise awareness in different scientific communities about intriguing connections between wild animals and humans regarding infertility syndromes or improvement of fertility preservation. The objectives are to (1) highlight commonalities between wild species and human fertility, (2) demonstrate that research in wild species-assisted reproductive technologies can greatly enhance success in human reproductive medicine, and (3) recognize that human fertility preservation is highly inspiring and relevant to wild species conservation. In addition to having similar biological traits in some wild species and humans, the fact of sharing the same natural environment and the common needs for more options in fertility preservation are strong incentives to build more bridges that will eventually benefit both animal conservation and human reproductive medicine.

Poly(ADP-ribose) mediates asymmetric division of mouse oocyte

Before fertilization, mammalian oocyte undergoes an asymmetric division which depends on eccentric positioning of the spindle at the oocyte cortex to form a polar body and an egg. Since the centriole is absent and, as a result, the polar array microtubules are not fully developed in oocytes, microtubules have seldom been considered as required for eccentric positioning of the spindle, while actin-related forces have instead been proposed to be primarily responsible for this process. However, the existing models are largely conflicting and the underlying mechanism of asymmetric division is still elusive. Here we show that poly(ADP-ribose) (PAR) is enriched at mouse oocyte cortical area throughout meiosis. Specific removal of cortical PAR results in an ectopic spindle and a failure of asymmetric division. During spindle migration, when the spindle deviates from the center of oocyte by a pushing force of cytoplasmic actin, the short polar array microtubules emanating from the juxtacortical spindle pole extend to the cortex and penetrate into cortical PAR, docking and stabilizing the spindle at the cortex which facilitates the asymmetric division. This process depends on the affinity between PAR and microtubule-associated proteins such as Spindly, which contributes to a physical link for cortical PAR and the spindle. Notably, fusing a PAR-binding domain to end-binding protein 3, a plus-end tracking protein at the polar array microtubules, restores the asymmetric division of oocytes with Spindly knockdown. Thus, our work demonstrates a comprehensive mechanism for oocyte spindle positioning and asymmetric division.

Quantitative Microinjection of Morpholino Antisense Oligonucleotides into Mouse Oocytes to Examine Gene Function in Meiosis-I

Specific protein depletion is a powerful approach for assessing individual gene function in cellular processes, and has been extensively employed in recent years in mammalian oocyte meiosis-I. Conditional knockout mice and RNA interference (RNAi) methods such as siRNA or dsRNA microinjection are among several approaches to have been applied in this system over the past decade. RNAi by microinjection of Morpholino antisense Oligonucleotides (MO), in particular, has proven highly popular and tractable in many studies, since MOs have high specificity of interaction, low cell toxicity, and are more stable than other microinjected RNAi molecules. Here, we describe a method of MO microinjection into the mouse germinal vesicle-stage (GV) oocyte followed by a simple immunofluorescence approach for examination of gene function in meiosis-I.

Regulation of chromosome segregation in oocytes and the cellular basis for female meiotic errors

BACKGROUND

Meiotic chromosome segregation in human oocytes is notoriously error-prone, especially with ageing. Such errors markedly reduce the reproductive chances of increasing numbers of women embarking on pregnancy later in life. However, understanding the basis for these errors is hampered by limited access to human oocytes.

OBJECTIVE AND RATIONALE

Important new discoveries have arisen from molecular analyses of human female recombination and aneuploidy along with high-resolution analyses of human oocyte maturation and mouse models. Here, we review these findings to provide a contemporary picture of the key players choreographing chromosome segregation in mammalian oocytes and the cellular basis for errors.

SEARCH METHODS

A search of PubMed was conducted using keywords including meiosis, oocytes, recombination, cohesion, cohesin complex, chromosome segregation, kinetochores, spindle, aneuploidy, meiotic cell cycle, spindle assembly checkpoint, anaphase-promoting complex, DNA damage, telomeres, mitochondria, female ageing and female fertility. We extracted papers focusing on mouse and human oocytes that best aligned with the themes of this review and that reported transformative and novel discoveries.

OUTCOMES

Meiosis incorporates two sequential rounds of chromosome segregation executed by a spindle whose component microtubules bind chromosomes via kinetochores. Cohesion mediated by the cohesin complex holds chromosomes together and should be resolved at the appropriate time, in a specific step-wise manner and in conjunction with meiotically programmed kinetochore behaviour. In women, the stage is set for meiotic error even before birth when female-specific crossover maturation inefficiency leads to the formation of at-risk recombination patterns. In adult life, multiple co-conspiring factors interact with at-risk crossovers to increase the likelihood of mis-segregation. Available evidence support that these factors include, but are not limited to, cohesion deterioration, uncoordinated sister kinetochore behaviour, erroneous microtubule attachments, spindle instability and structural chromosomal defects that impact centromeres and telomeres. Data from mice indicate that cohesin and centromere-specific histones are long-lived proteins in oocytes. Since these proteins are pivotal for chromosome segregation, but lack any obvious renewal pathway, their deterioration with age provides an appealing explanation for at least some of the problems in older oocytes.

WIDER IMPLICATIONS

Research in the mouse model has identified a number of candidate genes and pathways that are important for chromosome segregation in this species. However, many of these have not yet been investigated in human oocytes so it is uncertain at this stage to what extent they apply to women. The challenge for the future involves applying emerging knowledge of female meiotic molecular regulation towards improving clinical fertility management.

CoQ10 increases mitochondrial mass and polarization, ATP and Oct4 potency levels, and bovine oocyte MII during IVM while decreasing AMPK activity and oocyte death

PURPOSE

We tested whether mitochondrial electron transport chain electron carrier coenzyme Q10 (CoQ10) increases ATP during bovine IVM and increases %M2 oocytes, mitochondrial polarization/mass, and Oct4, and decreases pAMPK and oocyte death.

METHODS

Bovine oocytes were aspirated from ovaries and cultured in IVM media for 24 h with 0, 20, 40, or 60 μM CoQ10. Oocytes were assayed for ATP by luciferase-based luminescence. Oocyte micrographs were quantitated for Oct4, pAMPK (i.e., activity), polarization by JC1 staining, and mitochondrial mass by MitoTracker Green staining.

RESULTS

CoQ10 at 40 μM was optimal. Oocytes at 40 μM enabled 1.9-fold more ATP than 0 μM CoQ10. There was 4.3-fold less oocyte death, 1.7-fold more mitochondrial charge polarization, and 3.1-fold more mitochondrial mass at 40 μM than at 0 μM CoQ10. Increased ATP was associated with 2.2-fold lower AMPK thr172P activation and 2.1-fold higher nuclear Oct4 stemness/potency protein at 40 μM than at 0 μM CoQ10. CoQ10 is hydrophobic, and at all doses, 50% was lost from media into oil by ~ 12 h. Replenishing CoQ10 at 12 h did not significantly diminish dead oocytes.

CONCLUSIONS

The data suggest that CoQ10 improves mitochondrial function in IVM where unwanted stress, higher AMPK activity, and Oct4 potency loss are induced.

ALADIN is required for the production of fertile mouse oocytes

The nucleoporin ALADIN is required for multiple stages of oocyte maturation and for supporting embryonic divisions postfertilization.

Asymmetric cell divisions depend on the precise placement of the spindle apparatus. In mammalian oocytes, spindles assemble close to the cell’s center, but chromosome segregation takes place at the cell periphery where half of the chromosomes are expelled into small, nondeveloping polar bodies at anaphase. By dividing so asymmetrically, most of the cytoplasmic content within the oocyte is preserved, which is critical for successful fertilization and early development. Recently we determined that the nucleoporin ALADIN participates in spindle assembly in somatic cells, and we have also shown that female mice homozygously null for ALADIN are sterile. In this study we show that this protein is involved in specific meiotic stages, including meiotic resumption, spindle assembly, and spindle positioning. In the absence of ALADIN, polar body extrusion is compromised due to problems in spindle orientation and anchoring at the first meiotic anaphase. ALADIN null oocytes that mature far enough to be fertilized in vitro are unable to support embryonic development beyond the two-cell stage. Overall, we find that ALADIN is critical for oocyte maturation and appears to be far more essential for this process than for somatic cell divisions.

Oocyte polarized light microscopy, assay of specific follicular fluid metabolites, and gene expression in cumulus cells as different approaches to predict fertilization efficiency after ICSI

BACKGROUND

The complex relationship between oocyte morphology, specific follicular fluid metabolites, gene expression in cumulus granulosa cells, and oocyte competence toward fertilization and embryo development still needs further clarification.

METHODS

Forty-six oocytes retrieved from the largest pre-ovulatory follicle of patients undergoing intra-cytoplasmic sperm injection (ICSI) were considered assessing: (a) oocyte morphological characteristics at polarized light microscopy (PLM), (b) specific follicular fluid (FF) metabolites previously suggested to influence oocyte competence (AMH, markers of redox status and of cytotoxicity), (c) transcription of AMH and AMH type II receptor genes in cumulus cells. Data were analyzed using mono-parametric tests and multivariable logistic analysis in order to correlate morphological and biochemical data with fertilization.

RESULTS

Comparing normally fertilized oocytes (n = 29, F group) with unfertilized (n = 17, nF group) we observed that: (a) the meiotic spindle area and major axis were significantly higher in nF group and in fertilized oocytes undergoing an early embryo development arrest; (b) AMH level in FF was comparable in F and nF groups; (c) the FF of nF group contained significantly higher levels of cytotoxicity (lactate dehydrogenase) and oxidative stress (Cu,Zn-superoxide dismutase, catalase, 4-hydroxynonenal-protein conjugates) markers; (d) cumulus cells of nF group showed significantly higher AMH receptor type II gene expression.

CONCLUSIONS

Taken together, these observations suggest that an excessive cytotoxicity level can alter AMH signal transduction within cumulus cells, in turn leading to partial inhibition of aromatase activity, altered cytoplasmic maturation and increased oxidative stress, factors able to impair oocyte fertilization competence and embryo growth.

Effect of women's age on embryo morphology, cleavage rate and competence-A multicenter cohort study

This multicenter cohort study on embryo assessment and outcome data from 11,744 IVF/ICSI cycles with 104,830 oocytes and 42,074 embryos, presents the effect of women’s age on oocyte, zygote, embryo morphology and cleavage parameters, as well as cycle outcome measures corrected for confounding factors as center, partner’s age and referral diagnosis. Cycle outcome data confirmed the well-known effect of women’s age. Oocyte nuclear maturation and proportion of 2 pro-nuclear (2PN) zygotes were not affected by age, while a significant increase in 3PN zygotes was observed in both IVF and ICSI (p<0.0001) with increasing age. Maternal age had no effect on cleavage parameters or on the morphology of the embryo day 2 post insemination. Interestingly, initial hCG value after single embryo transfer followed by ongoing pregnancy was increased with age in both IVF (p = 0.007) and ICSI (p = 0.001) cycles. For the first time, we show that a woman’s age does impose a significant footprint on early embryo morphological development (3PN). In addition, the developmentally competent embryos were associated with increased initial hCG values as the age of the women increased. Further studies are needed to elucidate, if this increase in initial hCG value with advancing maternal age is connected to the embryo or the uterus.

Maternal control of early embryogenesis in mammals

Oocyte quality is a critical factor limiting the efficiency of assisted reproductive technologies (ART) and pregnancy success in farm animals and humans. ART success is diminished with increased maternal age, suggesting a close link between poor oocyte quality and ovarian aging. However, the regulation of oocyte quality remains poorly understood. Oocyte quality is functionally linked to ART success because the maternal-to-embryonic transition (MET) is dependent on stored maternal factors, which are accumulated in oocytes during oocyte development and growth. The MET consists of critical developmental processes, including maternal RNA depletion and embryonic genome activation. In recent years, key maternal proteins encoded by maternal-effect genes have been determined, primarily using genetically modified mouse models. These proteins are implicated in various aspects of early embryonic development, including maternal mRNA degradation, epigenetic reprogramming, signal transduction, protein translation and initiation of embryonic genome activation. Species differences exist in the number of cell divisions encompassing the MET and maternal-effect genes controlling this developmental window. Perturbations of maternal control, some of which are associated with ovarian aging, result in decreased oocyte quality.

Predictive value of spindle retardance in embryo implantation rate

PURPOSE

We evaluated the relationship between meiotic spindle characteristics and in vitro fertilization cycle outcome.

METHODS

Five hundred sixty-nine oocytes from 86 in vitro fertilization cycles were analyzed for fertilization and subsequent implantation rates. Oocytes were assessed for maturation status. The oocytes and embryos were cultured in sequential and nonsequential media (G Series, Vitrolife, Sweden) and incubated in 6% CO₂, 5% O₂ at 37 °C. Two hours following oocyte decumulation (38-39 h post-hCG/GnRH administration) and prior to microinjection, the structure of the meiotic spindle was assessed using the Oosight Imaging System (CRI, UK).

RESULTS

Four hundred fifty-six oocytes (80.5%) had a visible meiotic spindle, 82 (14.7%) had no meiotic spindle, and 31 (5.5%) were in telophase I. Oocytes exhibiting a meiotic spindle had a significantly higher fertilization rate and a lower rate of abnormal fertilization. Implantation data were obtained for 195 of the embryos transferred. The implantation rate for embryos derived from oocytes with a meiotic spindle was 32.9%, while in embryos originating from oocytes without a meiotic spindle and oocytes in telophase, this value dropped significantly (8.8 and 0%, respectively). To determine the correlation between retardance values and implantation rate for each oocyte, we established four groups, finding a range of retardance values with significantly higher implantation rates (27.5, 21, 29.3, and 53.8%, respectively).

CONCLUSION

Meiotic spindle imaging may be a valuable tool for prediction of oocyte quality, and retardance values of meiotic spindles, together with classical morphological classification, can be useful to select embryos with a higher implantation potential.

Polar body transfer restores the developmental potential of oocytes to blastocyst stage in a case of repeated embryo fragmentation

PURPOSE

We aimed to determine the developmental potential of human reconstructed oocytes after polar body genome transfer (PBT) and to report the case of a woman with multiple cycles of severe embryo fragmentation.

METHODS

Fresh and cryopreserved first polar bodies (PB1s) were transferred to enucleated metaphase II oocytes (PB1T), while fresh PB2s were removed from fertilized oocytes and used instead of the female pronucleus in donor zygotes. Reconstructed oocytes underwent intracytoplasmic sperm injection (ICSI) and were cultured to blastocyst. Biopsied trophectoderm cells of PBT-derived blastocysts were screened for chromosomes by next-generation sequencing (NGS). Then, cryopreserved PB1T was carried out in one woman with a history of several cycles of extensive embryo fragmentation, and the blastocysts derived from PB1T were screened for aneuploidy but not transferred to the patient.

RESULTS

There were no significant differences in the rates of normal fertilization and blastocyst formation between fresh and cryopreserved PB1T and control oocytes. Of the three fresh and three cryopreserved PB1T-derived blastocysts, two and one blastocysts exhibited normal diploidy respectively. In contrast, 17 PB2 transfers yielded 16 two pronuclei (2PN) zygotes with one normal and one small-sized pronucleus each and no blastocyst formation. In the female patient, 18 oocytes were inseminated by ICSI in the fourth cycle and the PB1s were biopsied. Although the embryos developed from the patient's own oocytes showed severe fragmentation, the oocytes reconstructed after PB1T produced three chromosomally normal blastocysts.

CONCLUSIONS

Normal blastocysts can develop from human reconstructed oocytes after PB1T. The application of the first PB transfers may be beneficial to patients with a history of poor embryo development and excessive fragmentation.

Live-Cell Imaging of Meiotic Spindle and Chromosome Dynamics in Maize (Zea mays)

Live-cell imaging is a powerful tool that allows investigators to directly observe the dynamics of cellular processes. Live imaging has proven particularly useful in studying mitotic and meiotic chromosome segregation, where the assembly of spindles and movement of chromosomes can be quantified in ways not possible with fixed cells. This protocol describes how to image live meiosis in the agriculturally important plant, maize. The creation of fluorescently tagged tubulin allows visualization of maize spindles, and nucleic acid dyestain chromosomes. This protocol describes all steps required for live imaging, including how to grow plants, screen for relevant genotypes, harvest meiotic cells, and collect live movies of meiosis. While this protocol was developed for imaging fluorescently tagged tubulin, it can be easily modified to observe the meiotic dynamics of any fluorescently labeled protein of interest. © 2016 by John Wiley & Sons, Inc.

PLK1 regulates spindle formation kinetics and APC/C activation in mouse zygote

Polo-like kinase 1 (PLK1) is involved in essential events of cell cycle including mitosis in which it participates in centrosomal microtubule nucleation, spindle bipolarity establishment and cytokinesis. Although PLK1 function has been studied in cycling cancer cells, only limited data are known about its role in the first mitosis of mammalian zygotes. During the 1-cell stage of mouse embryo development, the acentriolar spindle is formed and the shift from acentriolar to centrosomal spindle formation progresses gradually throughout the preimplantation stage, thus providing a unique possibility to study acentriolar spindle formation. We have shown previously that PLK1 activity is not essential for entry into first mitosis, but is required for correct spindle formation and anaphase onset in 1-cell mouse embryos. In the present study, we extend this knowledge by employing quantitative confocal live cell imaging to determine spindle formation kinetics in the absence of PLK1 activity and answer the question whether metaphase arrest at PLK1-inhibited embryos is associated with low anaphase-promoting complex/cyclosome (APC/C) activity and consequently high securin level. We have shown that inhibition of PLK1 activity induces a delay in onset of acentriolar spindle formation during first mitosis. Although these PLK1-inhibited 1-cell embryos were finally able to form a bipolar spindle, not all chromosomes were aligned at the metaphase equator. PLK1-inhibited embryos were arrested in metaphase without any sign of APC/C activation with high securin levels. Our results document that PLK1 controls the onset of spindle assembly and spindle formation, and is essential for APC/C activation before anaphase onset in mouse zygotes.

Involvement of Rab6a in organelle rearrangement and cytoskeletal organization during mouse oocyte maturation

Rab GTPases have been reported to define the identity and transport routes of vesicles. Rab6 is one of the most extensively studied Rab proteins involved in regulating organelle trafficking and integrity maintenance. However, to date, the function of Rab6 in mammalian oocytes has not been addressed. Here we report severe disorganization of endoplasmic reticulum upon specific knockdown of Rab6a in mouse oocytes. In line with this finding, intracellular Ca²⁺ stores are accordingly reduced in Rab6a-depleted oocytes. Furthermore, in these oocytes, we observe the absence of cortical granule free domain, which is a kind of special organelle in matured oocytes and its exocytosis is calcium dependent. On the other hand, following Rab6a knockdown, the prominent defects of cytoskeletal structures are detected during oocyte meiosis. In particular, the majority of Rab6a-depleted oocytes fail to form the actin cap, and the frequency of spindle defects and chromosome misalignment is significantly elevated. In summary, our data reveal that Rab6a not only participates in modulating the organization of oocyte organelles, but also is a novel regulator of meiotic apparatus in mammalian oocytes.

A Balance between Nuclear and Cytoplasmic Volumes Controls Spindle Length

Proper assembly of the spindle apparatus is crucially important for faithful chromosome segregation during anaphase. Thanks to the effort over the last decades, we have very detailed information about many events leading to spindle assembly and chromosome segregation, however we still do not understand certain aspects, including, for example, spindle length control. When tight regulation of spindle size is lost, chromosome segregation errors emerge. Currently, there are several hypotheses trying to explain the molecular mechanism of spindle length control. The number of kinetochores, activity of molecular rulers, intracellular gradients, cell size, limiting spindle components, and the balance of the spindle forces seem to contribute to spindle size regulation, however some of these mechanisms are likely specific to a particular cell type. In search for a general regulatory mechanism, in our study we focused on the role of cell size and nuclear to cytoplasmic ratio in this process. To this end, we used relatively large cells isolated from 2-cell mouse embryos. Our results showed that the spindle size upper limit is not reached in these cells and suggest that accurate control of spindle length requires balanced ratio between nuclear and cytoplasmic volumes.

Peroxynitrite deteriorates oocyte quality through disassembly of microtubule organizing centers

Previous theoretical studies have suggested that utilization of 3-D imaging to acquire morphologic parameters of meiotic spindles may be useful in infertility related procedures as an assessment of oocyte quality. However, our results show that treatment of oocytes with increasing concentrations of peroxynitrite (ONOO(-)) caused a dramatic alteration in spindle shape in which morphologic parameters are not measurable or are uninformative in terms of oocyte quality. Metaphase II mouse oocytes (n=520) were treated with increasing concentrations of ONOO(-), after which all oocytes were fixed and subjected to indirect immunofluorescence. Oocyte quality was assessed by alterations in the microtubule-organizing center (MTOC), pericentrin location, microtubule morphology, and chromosomal alignment. In untreated oocytes, pericentrin is primarily assembled utilizing the acentrosomal MTOC, which appears as a condensation at both spindle poles. The spindle has a symmetrical pointed barrel shape, assembled around the chromosomal plate at the spindle equator. Oocytes treated with low concentrations of ONOO(-) (<2.5 μM) showed shortening of the spindle apparatus, while pericentrin scatters from a tight condensation to a dispersed cluster around each spindle pole. At higher ONOO(-) concentrations (>2.5μM) the central attachments between microtubules are strained and bend or unevenly break, and the MTOC proteins are further dispersed or undetectable. Peroxynitrite mediated MTOC damage, which deranges the chromosomal scaffold at the time of assembly and separation, caused the deterioration in oocyte quality. These results provide a link between reactive oxygen species and poor reproductive outcomes and elucidate the underlying etiology, which could be used as a superior biomarker for oocyte quality compared to existing assessment tools.

Oocyte Meiotic Spindle Assembly and Function

Gametogenesis in animal oocytes reduces the diploid genome content of germline precursors to a haploid state in gametes by discarding ¾ of the duplicated chromosomes through a sequence of two meiotic cell divisions called meiosis I and II. The assembly of the microtubule-based spindle structure that mediates this reduction in genome content remains poorly understood compared to our knowledge of mitotic spindle assembly and function. In this review, we consider the diversity of oocyte meiotic spindle assembly and structure across animal phylogeny, review recent advances in our understanding of how animal oocytes assemble spindles in the absence of the centriole-based microtubule-organizing centers that dominate mitotic spindle assembly, and discuss different models for how chromosomes are captured and moved to achieve chromosome segregation during oocyte meiotic cell division.

Ca(2+) dynamics in oocytes from naturally-aged mice

The ability of human metaphase-II arrested eggs to activate following fertilisation declines with advancing maternal age. Egg activation is triggered by repetitive increases in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the ooplasm as a result of sperm-egg fusion. We therefore hypothesised that eggs from older females feature a reduced ability to mount appropriate Ca²⁺ responses at fertilisation. To test this hypothesis we performed the first examination of Ca²⁺ dynamics in eggs from young and naturally-aged mice. Strikingly, we find that Ca²⁺ stores and resting [Ca²⁺]_i are unchanged with age. Although eggs from aged mice feature a reduced ability to replenish intracellular Ca²⁺ stores following depletion, this difference had no effect on the duration, number, or amplitude of Ca²⁺ oscillations following intracytoplasmic sperm injection or expression of phospholipase C zeta. In contrast, we describe a substantial reduction in the frequency and duration of oscillations in aged eggs upon parthenogenetic activation with SrCl₂. We conclude that the ability to mount and respond to an appropriate Ca²⁺ signal at fertilisation is largely unchanged by advancing maternal age, but subtle changes in Ca²⁺ handling occur that may have more substantial impacts upon commonly used means of parthenogenetic activation.