The regulation of maturation promoting factor during prophase I arrest and meiotic entry in mammalian oocytes

Oocyte competence develops: nuclear maturation synchronously with cytoplasm maturation

Human oocyte maturation is a lengthy process that takes place over the course of which oocytes gain the inherent ability to support the next developmental stages in a progressive manner. This process includes intricate and distinct events related to nuclear and cytoplasmic maturation. Nuclear maturation includes mostly chromosome segregation, whereas rearrangement of organelles, storage of mRNAs and transcription factors occur during cytoplasmic maturation.Human oocyte maturation, both in vivo and in vitro, occurs through a process that is not yet fully understood. However, it is believed that the second messenger, cyclic adenosine monophosphate (cAMP), plays a pivotal role in the upkeep of the meiotic blocking of the human oocyte. Relatively high levels of cAMP in the human oocyte are required to maintain meiosis blocked, whereas lower levels of cAMP in the oocyte enable meiosis to resume. Oocyte cAMP concentration is controlled by a balance between adenylate cyclase and phosphodiesterases, the enzymes responsible for cAMP generation and breakdown.In addition to nuclear maturation, the female gamete requires a number of complicated structural and biochemical modifications in the cytoplasmic compartment to be able to fertilize normally. According to ultrastructural studies, during the transition from the germinal vesicle stage to metaphase II (MII), several organelles reorganize their positions. The cytoskeletal microfilaments and microtubules found in the cytoplasm facilitate these movements and regulate chromosomal segregation.The aim of this review is to focus on the nuclear and cytoplasmic maturation by investigating the changes that take place in the process of oocytes being competent for development.

The global phosphorylation landscape of mouse oocytes during meiotic maturation

Phosphorylation is a key post-translational modification regulating protein function and biological outcomes. However, the phosphorylation dynamics orchestrating mammalian oocyte development remains poorly understood. In the present study, we apply high-resolution mass spectrometry-based phosphoproteomics to obtain the first global in vivo quantification of mouse oocyte phosphorylation. Of more than 8000 phosphosites, 75% significantly oscillate and 64% exhibit marked upregulation during meiotic maturation, indicative of the dominant regulatory role. Moreover, we identify numerous novel phosphosites on oocyte proteins and a few highly conserved phosphosites in oocytes from different species. Through functional perturbations, we demonstrate that phosphorylation status of specific sites participates in modulating critical events including metabolism, translation, and RNA processing during meiosis. Finally, we combine inhibitor screening and enzyme-substrate network prediction to discover previously unexplored kinases and phosphatases that are essential for oocyte maturation. In sum, our data define landscape of the oocyte phosphoproteome, enabling in-depth mechanistic insights into developmental control of germ cells.

Identification of a novel splicing variant of thyroid hormone receptor interaction protein 13 (TRIP13) in female infertility characterized by oocyte maturation arrest

PURPOSE

As a cause of primary female infertility, oocyte maturation arrest (OMA) is characterized by failure to obtain mature oocytes due to abnormal meiosis. We aimed to identify pathogenic variants in two sisters with OMA phenotype from a non-consanguineous family.

METHODS

Whole-exome sequencing and Sanger sequencing were conducted to identify and validate the disease-causing gene variant. Additionally, we performed a minigene assay, quantitative reverse transcription PCR, and Western blotting to assess the effects of the variant.

RESULTS

We identified a novel homozygous splicing variant (c.1021-11T>C) in TRIP13, which followed a recessive inheritance pattern. Minigene assay showed that the variant could disrupt the integrity of TRIP13 mRNA, as evidenced by the production of an alternative transcript with intron10 intermediate retention of 79 bp. Compared to normal controls, the expression of TRIP13 mRNA and abundance of TRIP13 protein were also significantly decreased in Epstein-Barr virus-immortalized lymphoblastoid cells derived from affected individuals.

CONCLUSION

Our findings confirm the contribution of genetic factors to OMA and expand the mutation spectrum of TRIP13 in female infertility.

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

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

Loss of UBE2S causes meiosis I arrest with normal spindle assembly checkpoint dynamics in mouse oocytes

The timely degradation of proteins that regulate the cell cycle is essential for oocyte maturation. Oocytes are equipped to degrade proteins via the ubiquitin-proteasome system. In meiosis, anaphase promoting complex/cyclosome (APC/C), an E3 ubiquitin-ligase, is responsible for the degradation of proteins. Ubiquitin-conjugating enzyme E2 S (UBE2S), an E2 ubiquitin-conjugating enzyme, delivers ubiquitin to APC/C. APC/C has been extensively studied, but the functions of UBE2S in oocyte maturation and mouse fertility are not clear. In this study, we used Ube2s knockout mice to explore the role of UBE2S in mouse oocytes. Ube2s-deleted oocytes were characterized by meiosis I arrest with normal spindle assembly and spindle assembly checkpoint dynamics. However, the absence of UBE2S affected the activity of APC/C. Cyclin B1 and securin are two substrates of APC/C, and their levels were consistently high, resulting in the failure of homologous chromosome separation. Unexpectedly, the oocytes arrested in meiosis I could be fertilized and the embryos could become implanted normally, but died before embryonic day 10.5. In conclusion, our findings reveal an indispensable regulatory role of UBE2S in mouse oocyte meiosis and female fertility.

PAK1-Dependent Regulation of Microtubule Organization and Spindle Migration Is Essential for the Metaphase I-Metaphase II Transition in Porcine Oocytes

P21-activated kinase 1 (PAK1) is a critical downstream target that mediates the effect of small Rho GTPase on the regulation of cytoskeletal kinetics, cell proliferation, and cell migration. PAK1 has been identified as a crucial regulator of spindle assembly during the first meiotic division; however, its roles during the metaphase I (MI) to metaphase II (MII) transition in oocytes remain unclear. In the present study, the potential function of PAK1 in regulating microtubule organization and spindle positioning during the MI-MII transition was addressed in porcine oocytes. The results showed that activated PAK1 was co-localized with α-tubulin, and its expression was increased from the MI to MII stage (p < 0.001). However, inhibiting PAK1 activity with an inhibitor targeting PAK1 activation-3 (IPA-3) at the MI stage decreased the first polar body (PB1) extrusion rate (p < 0.05), with most oocytes arrested at the anaphase-telophase (ATI) stage. IPA-3-treated oocytes displayed a decrease in actin distribution in the plasma membrane (p < 0.001) and an increase in the rate of defects in MII spindle reassembly with abnormal spindle positioning (p < 0.001). Nevertheless, these adverse effects of IPA-3 on oocytes were reversed when the disulfide bond between PAK1 and IPA-3 was reduced by dithiothreitol (DTT). Co-immunoprecipitation revealed that PAK1 could recruit activated Aurora A and transform acidic coiled-coil 3 (TACC3) to regulate spindle assembly and interact with LIM kinase 1 (LIMK1) to facilitate actin filament-mediated spindle migration. Together, PAK1 is essential for microtubule organization and spindle migration during the MI-MII transition in porcine oocytes, which is associated with the activity of p-Aurora A, p-TACC3 and p-LIMK1.

LSM14B coordinates protein component expression in the P-body and controls oocyte maturation

The generation of mature and healthy oocytes is the most critical event in the entire female reproductive process, and the mechanisms regulating this process remain to be studied. Here, we demonstrate that Smith-like (LSM) family member 14B (LSM14B) regulates oocyte maturation, and the loss of LSM14B in mouse ovaries leads to abnormal oocyte MII arrest and female infertility. Next, we find the aberrant transcriptional activation, indicated by abnormal non-surrounded nucleolus and surrounded nucleolus oocyte proportions, and abnormal chromosome assembly and segregation in Lsm14b-deficient mouse oocytes. The global transcriptome analysis suggests that many transcripts involved in cytoplasmic processing body (P-body) function are altered in Lsm14b-deficient mouse oocytes. Deletion of Lsm14b results in the expression and/or localization changes of P-body components (such as LSM14A, DCP1A, and 4E-T). Notably, DDX6, a key component of the P-body, is downregulated and accumulates in the nuclei in Lsm14b-deficient mouse oocytes. Taken together, our data suggest that LSM14B links mouse oocyte maturation to female fertility through the regulation of the P-body.

ALKBH5 controls the meiosis-coupled mRNA clearance in oocytes by removing the N 6-methyladenosine methylation

N6-methyladenosine (m6A) maintains maternal RNA stability in oocytes. One regulator of m6A, ALKBH5, reverses m6A deposition and is essential in RNA metabolism. However, the specific role of ALKBH5 in oocyte maturation remains elusive. Here, we show that Alkbh5 depletion causes a wide range of defects in oocyte meiosis and results in female infertility. Temporal profiling of the maternal transcriptomes revealed striking RNA accumulation in Alkbh5-/- oocytes during meiotic maturation. Analysis of m6A dynamics demonstrated that ALKBH5-mediated m6A demethylation ensures the timely degradation of maternal RNAs, which is severely disrupted following Alkbh5-/- depletion. A distinct subset of transcripts with persistent m6A peaks are recognized by the m6A reader IGF2BP2 and thus remain stabilized, resulting in impaired RNA clearance. Additionally, reducing IGF2BP2 in Alkbh5-depleted oocytes partially rescued these defects. Overall, this work identifies ALKBH5 as a key determinant of oocyte quality and unveil the facilitating role of ALKBH5-mediated m6A removal in maternal RNA decay.

The Role of Cyclin-Dependent Kinases (CDK) 4/6 in the Ovarian Tissue and the Possible Effects of Their Exogenous Inhibition

The combination of cyclin-dependent kinase (CDK) 4/6 inhibitors with endocrine therapy is the standard treatment for patients with HR+/HER2- advanced breast cancer. Recently, this combination has also entered the early setting as an adjuvant treatment in patients with HR+/HER2- disease at a high risk of disease recurrence following (neo)adjuvant chemotherapy. Despite their current use in clinical practice, limited data on the potential gonadotoxicity of CDK4/6 inhibitors are available. Hence, fully informed treatment decision making by premenopausal patients concerned about the potential development of premature ovarian insufficiency and infertility with the proposed therapy remains difficult. The cell cycle progression of granulosa and cumulus cells is a critical process for ovarian function, especially for ensuring proper follicular growth and acquiring competence. Due to the pharmacological properties of CDK4/6 inhibitors, there could be a potentially negative impact on ovarian function and fertility in women of reproductive age. This review aims to summarize the role of the cyclin D-CDK4 and CDK6 complexes in the ovary and the potential impact of CDK4/6 inhibition on its physiological processes.

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.

Impact of Ovarian Factor Mediums on the Apoptotic Gene Expression and Embryo Quality Derived From Vitrified Immature Human Oocytes

Background

Condition mediums have a potential role in oocyte development. In this study, we evaluated the effects of different mediums on the developmental potential of vitrified immature human oocyte after IVM and parthenogenesis by ionomycin.

Methods

Immature oocytes were collected from 184 women after vitrification/thawing and maturation, in three types of IVM mediums separately. Finally, 151 IVM MΙΙ oocytes were obtained and randomly divided into six groups and underwent the following intervention. Fresh and vitrified-thawing MΙΙ oocytes were activated after IVM in three conditioned mediums by ionomycin. Mediums included 1) Minimum Essential Medium Alpha (α-MEM) (as control medium), 2) α-MEM supplemented with supernatants of Mesenchyme bone marrow (B.M), 3) α-MEM with ovarian growth factors (O.F). Then, scoring of parthenote embryos was undertaken in accordance with pertinent morphological properties. Moreover, the expression of Bax and Bcl2 were determined in the parthenote embryos.

Result

Percentage of the degenerated oocyte, 2-4 cells, 4-8 cells, and 16 cells, was different in the experimental groups. Also, cytoplasmic maturation and blastocyst formation rates were significantly different (p < 0.05) between the control and the other mediums. The highest mRNA expression levels of Bcl2 and Bax genes in parthenotes were observed in the fIVM O.F and vIVM α-MEM mediums, respectively. vIVM, α-MEM and fIVM O.F showed the lowest expression of Bcl2 and Bax genes, respectively.

Conclusion

Our findings indicate that the O.F. medium had more potent effects on oocyte growth and cytoplasmic maturation up to the blastocyst stage with the highest expression level of the BCL2 gene and the lowest relative amount of the BAX gene in this medium. The results of the present study have been verified only for parthenogenetically activated embryos, and any positive effect of the environment on the egg/embryo fertilized with sperm requires more extensive studies.

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.

The combination of rolipram and cilostamide improved the developmental competence of cloned porcine embryos

In vitro maturation of porcine oocytes is characterized by asynchronous cytoplasmic and nuclear maturation, leading to less competent oocytes supporting embryo development. The purpose of this study was to evaluate the combined effect of rolipram and cilostamide as cyclic Adenine monophosphate (cAMP) modulators to find the maximum cAMP levels that temporarily arrest meiosis. We determined the optimal time to maintain functional gap junction communication during pre-in vitro maturation to be four hours. Oocyte competence was evaluated by the level of glutathione, reactive oxygen species, meiotic progression, and gene expression. We evaluated embryonic developmental competence after parthenogenetic activation and somatic cell nuclear transfer. The combined treatment group showed significantly higher glutathione and lower reactive oxygen species levels and a higher maturation rate than the control and single treatment groups. Cleavage and blastocyst formation rates in parthenogenetic activation and somatic cell nuclear transfer embryos were higher in two-phase in vitro maturation than in the other groups. The relative levels of BMP15and GDF9 expression were increased in two-phase in vitro maturation. Somatic cell nuclear transfer blastocysts from two-phase in vitro maturation oocytes showed a lower level of expression of apoptotic genes than the control, indicating better pre-implantation developmental competence. The combination of rolipram and cilostamide resulted in optimal synchrony of cytoplasmic and nuclear maturation in porcine in vitro matured oocytes and there by enhanced the developmental competence of pre-implantation embryos.

Gonadotropins as pharmacological agents in assisted reproductive technology and polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a complex endocrinopathy associated with subfertility/infertility and pregnancy complications. Most PCOS women opt for assisted reproductive technologies (ART) for successful conception; however, optimization of the relative doses of the gonadotropins [follicle-stimulating hormone (FSH), luteinizing hormone (LH)/human chorionic gonadotropin (hCG)] for appropriate steroidogenesis, without causing ovarian hyperstimulatory syndrome (OHSS), is challenging. Embryonic factors probably do not contribute to pregnancy loss in PCOS women, albeit hormonal imbalance impairs the metabolic microenvironment critical for oocyte maturation and endometrial receptivity. Certain clinical studies have confirmed the role of metabolic corrections in increasing the rate of pregnancy in PCOS women. This review focuses on the impact of untimely high LHCGR and/or LH levels on oocyte/embryo quality, pregnancy outcomes in ART, and exploring LHCGR as a potential drug target in PCOS women.

Ste20-like kinase activity promotes meiotic resumption and spindle microtubule stability in mouse oocytes

Ste20-like kinase (SLK) is involved in cell proliferation and migration in somatic cells. This study aims to explore SLK expression and function in mouse oocyte meiosis. Western blot, immunofluorescence, Co-immunoprecipitation, drug treatment, cRNA construct and in vitro transcription, microinjection of morpholino oilgo (MO) and cRNA were performed in oocytes. High and stable protein expression of SLK was detected in mouse oocyte meiosis, with dynamic distribution in the nucleus, chromosomes and spindle apparatus. SLK phosphorylation emerges around meiotic resumption and reaches a peak during metaphase I (MI) and metaphase II. SLK knockdown with MO or expression of kinase-dead SLK K63R dramatically delays meiotic resumption due to sequentially suppressed phosphorylation of Polo-like kinase 1 (Plk1) and cell division cycle 25C (CDC25C) and dephosphorylation of cyclin-dependent kinase 1 (CDK1). SLK depletion promotes ubiquitination-mediated degradation of paxillin, an antagonist to α-tubulin deacetylation, and thus destroys spindle assembly and chromosome alignment; these phenotypes can be substantially rescued by exogenous expression of SLK kinase active fragment. Additionally, exogenous SLK effectively promotes meiotic progression and spindle assembly in aging oocytes with reduced SLK. Collectively, this study reveals SLK is required for meiotic resumption and spindle assembly in mouse oocyte meiosis.

Cilostamide, a phosphodiesterase 3A inhibitor, sustains meiotic arrest of rat oocytes by modulating cyclic adenosine monophosphate level and the key regulators of maturation promoting factor

Cilostamide, a phosphodiesterase 3A (Pde3A) inhibitor, is known to increase intraoocyte cyclic adenosine monophosphate (cAMP) level which is involved in sustaining meiotic arrest of the oocytes. To explore the mechanisms involved in the cilostamide-mediated meiotic arrest of the oocytes, the present study describes the effects of cilostamide on cAMP level and related factors involved in maturation of the oocytes at its different meiotic stages; diplotene, metaphase I (MI) and metaphase II (MII). The oocytes from these three stages were collected from rat ovary and incubated with 10 µM cilostamide for 3 h in CO₂ incubator. The levels of cAMP, cyclic guanosine monophosphate (cGMP) and the key players of maintaining meiotic arrest during oocyte maturation; Emi2, Apc, Cyclin B1, and Cdk1, were analyzed in diplotene, MI and MII stages. Pde3A was found to be expressed at all three stages but with the lowest level in MI oocyte. As compared to the control sets, the cAMP concentration was found to be highest in MII whereas cGMP was highest in the diplotene stage of cilostamide-treated group. The treated group showed declined reactive oxygen species level as compared with the control counterparts. Relatively increased levels of the Emi2, Cyclin B1, and phosphorylated thr161 of Cdk1 versus declined levels of phosphorylated thr14/tyr15 of Cdk1 in diplotene and MII stage oocytes are known to be involved in maintaining meiotic arrest and all these factors were found to undergo similar pattern of change due to the treatment with cilostamide. The findings thus suggest that cilostamide treatment promotes meiotic arrest by Pde3A inhibition led increase of both cAMP and cGMP level vis-a-vis modulation of the related regulatory factors such as Emi2, CyclinB1, and phosphorylated status of Cdk1 in diplotene and MII stage oocytes. Such a mechanism of meiotic arrest could allow the oocyte to prepare itself for meiotic maturation and thereby to improve oocyte quality.

Prolyl Isomerase, Pin1, Controls Meiotic Progression in Mouse Oocytes

During meiotic maturation, accurate progression of meiosis is ensured by multiple protein kinases and by signal transduction pathways they are involved in. However, the mechanisms regulating the functions of phosphorylated proteins are unclear. Herein, we investigated the role of Pin1, a peptidyl-prolyl cis-trans isomerase family member that regulates protein functions by altering the structure of the peptide bond of proline in phosphorylated proteins in meiosis. First, we analyzed changes in the expression of Pin1 during meiotic maturation and found that although its levels were constant, its localization was dynamic in different stages of meiosis. Furthermore, we confirmed that the spindle rotates near the cortex when Pin1 is inhibited by juglone during meiotic maturation, resulting in an error in the extrusion of the first polar body. In Pin1^-/- mice, frequent polar body extrusion errors were observed in ovulation, providing insights into the mechanism underlying the errors in the extrusion of the polar body. Although multiple factors and mechanisms might be involved, Pin1 functions in meiosis progression via actin- and microtubule-associated phosphorylated protein targets. Our results show that functional regulation of Pin1 is indispensable in oocyte production and should be considered while developing oocyte culture technologies for reproductive medicine and animal breeding.

Transient inhibition of CDK2 activity prevents oocyte meiosis I completion and egg activation in mouse

Mammalian oocytes are arrested at the diplotene stage of prophase I during fetal or postnatal development. It was reported that cyclin-dependent kinases (CDK1) was the sole CDK to drive the resumption of meiosis and CDK2 was dispensable for meiosis progression in mouse oocytes according to the conditional knockout studies. However, a recent study showed that CDK2 activity is essential for meiotic division and gametogenesis by means of gene-directed mutagenesis, which avoids the compensatory activation of other CDKs. Taken the compensatory effect between CDKs after gene knockout, the physiological function of CDK2 activity in oocyte maturation remains unclear. To address this issue, we applied a specific small-molecule inhibitor to restrain CDK2 activity transiently during oocyte meiotic maturation. Surprisingly, transient inhibition of CDK2 activity severely prevented the meiosis I completion although the meiotic resumption was not affected. Then we found that CDK2 activity was required for establishment of normal spindle and chromosome dynamics. Notably, CDK2 inhibition interrupted the anaphase-promoting complex/cyclosome (APC/C)-dependent degradation pathway by maintaining the activation of spindle assembly checkpoint (SAC). Interestingly, CDK2 inhibition prevented the egg activation as well. Overall, our data demonstrate that CDK2 kinase activity is required for proper dynamics of spindle and chromosomes, whose disturbance induces the continuous SAC activation and subsequent inactivation of APC/C activity in oocyte meiosis.

Neurotransmitters, neuropeptides and calcium in oocyte maturation and early development

A primary reason behind the high level of complexity we embody as multicellular organisms is a highly complex intracellular and intercellular communication system. As a result, the activities of multiple cell types and tissues can be modulated resulting in a specific physiological function. One of the key players in this communication process is extracellular signaling molecules that can act in autocrine, paracrine, and endocrine fashion to regulate distinct physiological responses. Neurotransmitters and neuropeptides are signaling molecules that renders long-range communication possible. In normal conditions, neurotransmitters are involved in normal responses such as development and normal physiological aspects; however, the dysregulation of neurotransmitters mediated signaling has been associated with several pathologies such as neurodegenerative, neurological, psychiatric disorders, and other pathologies. One of the interesting topics that is not yet fully explored is the connection between neuronal signaling and physiological changes during oocyte maturation and fertilization. Knowing the importance of Ca²⁺ signaling in these reproductive processes, our objective in this review is to highlight the link between the neuronal signals and the intracellular changes in calcium during oocyte maturation and embryogenesis. Calcium (Ca²⁺) is a ubiquitous intracellular mediator involved in various cellular functions such as releasing neurotransmitters from neurons, contraction of muscle cells, fertilization, and cell differentiation and morphogenesis. The multiple roles played by this ion in mediating signals can be primarily explained by its spatiotemporal dynamics that are kept tightly checked by mechanisms that control its entry through plasma membrane and its storage on intracellular stores. Given the large electrochemical gradient of the ion across the plasma membrane and intracellular stores, signals that can modulate Ca²⁺ entry channels or Ca²⁺ receptors in the stores will cause Ca²⁺ to be elevated in the cytosol and consequently activating downstream Ca²⁺-responsive proteins resulting in specific cellular responses. This review aims to provide an overview of the reported neurotransmitters and neuropeptides that participate in early stages of development and their association with Ca²⁺ signaling.

Meiosis resumption in human primordial germ cells from induced pluripotent stem cells by in vitro activation and reconstruction of ovarian nests

Background

The differentiation of human induced pluripotent stem cells (iPSCs) into oocytes, which involves the transformation from mitosis to meiosis, has been a hotspot of biological research for many years and represents a desirable experimental model and potential strategy for treating infertility. At present, studies have shown that most cells stagnate in the oogonium stage after differentiation into primordial germ cells (PGCs) from human iPSCs.

Methods

iPSCs carrying a SYCP3-mkate2 knock-in reporter were generated by the CRISPR/Cas9 strategy to monitor meiosis status during induced differentiation from iPSCs into oocytes. These induced PGCs/oogonia were activated by small molecules from the Wnt signaling pathway and then cocultured with reconstructed human ovarian nests in vivo for further development.

Results

First, human PGCs and oogonia were efficiently induced from iPSCs. Second, induced dormant PGCs resumed meiosis and then differentiated into primary oocytes through the in vitro activation of the Wnt signaling pathway. Finally, a new coculture system involving the reconstruction of ovarian nests in vitro could facilitate the differentiation of oocytes.

Conclusions

Human PGCs/oogonia induced from iPSCs can be activated and used to resume meiosis by molecules of the Wnt signaling pathway. The coculture of activated PGCs and reconstruction of ovarian nests facilitated differentiation into primary oocytes and the generation of haploid human oocytes in vivo. These findings established a new strategy for germline competence in primary oocytes and provided a keystone for human gametogenesis in vitro and in vivo.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03019-3.

Histone Demethylase AMX-1 Regulates Fertility in a p53/CEP-1 Dependent Manner

Histone methylation shapes the epigenetic configuration and adjusts multiple fundamental nuclear processes, including transcription, cell cycle control and DNA repair. The absence of histone demethylase LSD1/SPR-5 leads to progressive fertility defects as well as a reduction in brood size. Similarly, C. elegans LSD2 homolog AMX-1 has been implicated in regulating H3K4me2 and maintaining interstrand crosslinks (ICL) susceptibility. However, the mechanisms of how lack of AMX-1 induces sterility have not been addressed so far. This study investigated the histone demethylase AMX-1 in C. elegans and uncovered how amx-1 contributes to sterility in a p53/CEP-1 dependent manner. We show that while sterility in spr-5 mutants exhibited progressive over generations, amx-1 mutants displayed non-transgenerational fertility defects. Also, amx-1 mutants exhibited a reduced number of sperms and produced low brood size (LBS) or sterile worms that retain neither sperms nor germline nuclei, suggesting that fertility defects originated from germline development failure. Surprisingly, sterility exhibited in amx-1 was mediated by p53/CEP-1 function. Consistent with this result, upregulation of Piwi expression in amx-1 mutants suggested that AMX-1 is essential for germline development by regulating Piwi gene expressions. We propose that AMX-1 is required for proper Piwi expression and transposon silencing in a p53/CEP-1 dependent manner; thus, the absence of AMX-1 expression leads to defective meiotic development and sterility. This study elucidates how LSD2/AMX-1 contributes to sterility, therefore, expanding the boundaries of histone demethylase function.

The oocyte spindle midzone pauses Cdk1 inactivation during fertilization to enable male pronuclear formation and embryo development

Inactivation of cyclin-dependent kinase 1 (Cdk1), controlled by cyclin B1 proteolysis, orders events during mitotic exit. Here, we used a FRET biosensor to study Cdk1 activity while simultaneously monitoring anaphase II and pronuclear (PN) formation in live mouse eggs throughout fertilization. We find that Cdk1 inactivation occurs over two phases separated by a 3-h pause, the first induces anaphase II and the second induces PN formation. Although both phases require the inhibitory Cdk1 kinase Wee1B, only the first involves cyclin B1 proteolysis. Enforcing the 3-h pause is critical for providing the delay required for male PN formation and is mediated by spindle midzone-dependent sequestration of Wee1B between the first and second phases. Thus, unlike continuous Cdk1 inactivation driven by cyclin B1 proteolysis during mitotic exit, MII oocytes engineer a physiologically important pause during fertilization involving two different pathways to inactivate Cdk1, only the first of which requires proteolysis.

Oocyte maturation abnormalities - A systematic review of the evidence and mechanisms in a rare but difficult to manage fertility pheneomina

A small proportion of infertile women experience repeated oocyte maturation abnormalities (OMAS). OMAS include degenerated and dysmorphic oocytes, empty follicle syndrome, oocyte maturation arrest (OMA), resistant ovary syndrome and maturation defects due to primary ovarian insufficiency. Genetic factors play an important role in OMAS but still need specifications. This review documents the spectrum of OMAS and to evaluate the multiple subtypes classified as OMAS. In this review, readers will be able to understand the oocyte maturation mechanism, gene expression and their regulation that lead to different subtypes of OMAs, and it will discuss the animal and human studies related to OMAS and lastly the treatment options for OMAs. Literature searches using PubMed, MEDLINE, Embase, National Institute for Health and Care Excellence were performed to identify articles written in English focusing on Oocyte Maturation Abnormalities by looking for the following relevant keywords. A search was made with the specified keywords and included books and documents, clinical trials, animal studies, human studies, meta-analysis, randomized controlled trials, reviews, systematic reviews and options written in english. The search detected 3,953 sources published from 1961 to 2021. After title and abstract screening for study type, duplicates and relevancy, 2,914 studies were excluded. The remaining 1,039 records were assessed for eligibility by full-text reading and 886 records were then excluded. Two hundred and twenty seven full-text articles and 0 book chapters from the database were selected for inclusion. Overall, 227 articles, one unpublished and one abstract paper were included in this final review. In this review study, OMAS were classified and extensively evaluatedand possible treatment options under the light of current information, present literature and ongoing studies. Either genetic studies or in vitro maturation studies that will be handled in the future will lead more informations to be reached and may make it possible to obtain pregnancies.

Effects of Delta-9 Tetrahydrocannabinol (THC) on Oocyte Competence and Early Embryonic Development

Recent changes in legal status and public perception of cannabis have contributed to an increase use amongst women of reproductive age. Concurrently, there is inadequate evidence-based knowledge to guide clinical practice regarding cannabis and its effects on fertility and early embryonic development. This study aimed to evaluate the effects of the primary psychoactive component of cannabis, delta-9 tetrahydrocannabinol (THC), during oocyte maturation, and its impact on the developing embryo. Bovine oocytes were matured in vitro for 24 h under clinically relevant doses of THC mimicking plasma levels achieved after therapeutic (0.032 μM) and recreational (0.32 and 3.2 μM) cannabis use. THC-treated oocytes were assessed for development and quality parameters at both the oocyte and embryo level. Characteristics of oocytes treated with cannabinoid receptor antagonists were also assessed. Oocytes treated with 0.32 and 3.2 μM THC, were significantly less likely to reach metaphase II (p < 0.01) and consequently had lower cleavage rates at day 2 post-fertilization (p < 0.0001). Treatment with cannabinoid receptor antagonists restored this effect (p < 0.05). Oocytes that did reach MII showed no differences in spindle morphology. Oocytes treated with 0.032 μM THC had significantly lower connexin mRNA (p < 0.05) (correlated with decreased quality), but this was not confirmed at the protein level. At the blastocyst stage there were no significant differences in developmental rates or the proportion of trophectoderm to inner cell mass cells between the control and treatment groups. These blastocysts, however, displayed an increased level of apoptosis in the 0.32 and 3.2 μM groups (p < 0.0001). Our findings suggest a possible disruptive effect of cannabis on oocyte maturation and early embryonic development.

GVBD rate is an independent predictor for pregnancy in ICSI patients with surplus immature oocytes

INTRODUCTION

It was reported that there were still up to 30% immature retrieved oocyte at germinal vesicle (GV) or metaphase I (MI) stage. Whether the spontaneous maturity competency of immature oocytes associated to the clinical outcome of in vitro fertilization (IVF) cycles remains unclear and unexplored. This study aimed to investigate how the oocyte developmental parameters in in vitro maturation (IVM) affect clinical outcomes of intracytoplasmic sperm injection (ICSI) cycles.

METHODS

This retrospective cohort study included couples undergoing ICSI in a university-affiliated hospital. Surplus immature oocytes during ICSI were collected and cultured in vitro. The numbers of germinal vesicle (GV) oocytes undergoing GV breakdown (GVBD) and polar body 1 extrusion within 24 h culture were recorded. The main outcome measurements were demographic baselines and oocyte developmental parameters in IVM associated with pregnancy outcomes.

RESULTS

A total of 191 couples were included with an overall GVBD rate of 63.7% (327/513) and oocyte maturation rate of 46.8% (240/513). 53.4% (102/191) of them had embryos transferred freshly, which originated from metaphase II oocytes that matured spontaneously in vivo, and 60.8% (62/102) got pregnant. Among factors with a P-value < 0.2 in univariate logistic regression analyses of pregnancy correlation, GVBD rate (OR 3.220, 95% CI 1.060-9.782, P=0.039) and progesterone level on human chorionic gonadotropin (HCG) day (OR 0.231, 95% CI 0.056-0.949, P=0.042) remained significant in the multivariate model. The area under the curve (AUC) of the predictive nomogram was 0.729 (95% CI 0.632-0.826) with an acceptable calibration. Moreover, decision curve analyses illustrated the superior overall net benefit of models that included the GVBD rate in clinical decisions within a wide range of threshold probabilities.

CONCLUSION

In conclusion, GVBD rate and progesterone level on HCG day may be associated with pregnancy outcomes in infertile couples during the regular ICSI procedure. An elevated GVBD rate within 24 h may greatly increase the likelihood of pregnancy in infertile couples during ICSI. This preliminary study may optimize clinical pregnancy prediction, which provides support in decision-making in clinical practice.

Oocyte Competence Biomarkers Associated With Oocyte Maturation: A Review

Oocyte developmental competence is one of the determining factors that influence the outcomes of an IVF cycle regarding the ability of a female gamete to reach maturation, be fertilized, and uphold an embryonic development up until the blastocyst stage. The current approach of assessing the competency of an oocyte is confined to an ambiguous and subjective oocyte morphological evaluation. Over the years, a myriad of biomarkers in the cumulus-oocyte-complex has been identified that could potentially function as molecular predictors for IVF program prognosis. This review aims to describe the predictive significance of several cumulus-oocyte complex (COC) biomarkers in evaluating oocyte developmental competence. A total of eight acclaimed cumulus biomarkers are examined in the study. RT-PCR and microarray analysis were extensively used to assess the significance of these biomarkers in foreseeing oocyte developmental competence. Notably, these biomarkers regulate vital processes associated with oocyte maturation and were found to be differentially expressed in COC encapsulating oocytes of different maturity. The biomarkers were reviewed according to the respective oocyte maturation events namely: nuclear maturation, apoptosis, and extracellular matrix remodeling, and steroid metabolism. Although substantial in vitro evidence was presented to justify the potential use of cumulus biomarkers in predicting oocyte competency and IVF outcomes, the feasibility of assessing these biomarkers as an add-on prognostic procedure in IVF is still restricted due to study challenges.

A Novel Assisted Oocyte Activation Method Improves Fertilization in Patients With Recurrent Fertilization Failure

Total fertilization failure (TFF) occurs in 1–3% of total intracytoplasmic sperm injection (ICSI) cycles and can reoccur in subsequent cycles. Despite the high success rate with the application of assisted oocyte activation (AOA), there is still a small number of couples who cannot obtain fertilized eggs after conventional calcium (Ca²⁺) ionophores-based ICSI-AOA. Six couples experiencing repeated TFF or low fertilization (<10%) after ICSI and conventional ICSI-AOA were enrolled in this study. Compared with the regular ICSI group and the conventional ICSI-AOA group, the new AOA method, a combination of cycloheximide (CHX) and ionomycin, can significantly increase the fertilization rate from less than 10 up to approximately 50% in most cases. The normal distribution of sperm-related oocyte activation factor phospholipase C zeta (PLCζ1) in the sperms of the cases indicated the absence of an aberrant Ca²⁺ signaling activation. The results of the whole-embryo aneuploidies analysis indicated that oocytes receiving the novel AOA treatment had the potential to develop into blastocysts with normal karyotypes. Our data demonstrated that CHX combined with ionomycin was able to effectively improve the fertilization rate in the majority of patients suffering from TFF. This novel AOA method had a potential therapeutic effect on those couples experiencing TFF, even after conventional AOA, which may surmount the severe fertilization deficiencies in patients with a repeated low fertilization or TFF.

Melatonin Protects Against Mdivi-1-Induced Abnormal Spindle Assembly and Mitochondrial Superoxide Production During Porcine Oocyte Maturation

Mitochondrial division inhibitor 1 (Mdivi-1) reportedly provides a close connection between oocyte maturation and mitochondrial function in pigs. N-acetyl-5-methoxy-tryptamine (melatonin) is known to be a representative antioxidant with the ability to rehabilitate meiotic maturation of porcine oocytes. However, the ability of melatonin to recover Mdivi-1-mediated disruption of spindle formation during meiotic maturation of porcine oocytes during in vitro maturation (IVM) has not been studied. Here, we first investigated changes in mitochondrial length, such as fragmentation and elongation form, in mature porcine oocytes during IVM. Mature oocytes require appropriate mitochondrial fission for porcine oocyte maturation. We identified a dose-dependent reduction in meiotic maturation in porcine oocytes following Mdivi-1 treatment (50, 75, and 100 μM). We also confirmed changes in mitochondrial fission protein levels [dynamin-related protein 1 phosphorylation at serine 616 (pDRP1-Ser616) and dynamin-related protein 1 (DRP1)], mitochondrial membrane potential, and ATP production in 75 μM Mdivi-1-treated oocytes. As expected, Mdivi-1 significantly reduced mitochondrial function and DRP1 protein levels and increased spindle abnormalities in porcine oocytes. In addition, we confirmed that melatonin restores abnormal spindle assembly and reduces meiotic maturation rates by Mdivi-1 during porcine oocyte maturation. Interestingly, the expression levels of genes that reduce DNA damage and improve tubulin formation were enhanced during porcine meiotic maturation. Taken together, these results suggest that melatonin has direct beneficial effects on meiotic maturation through tubulin formation factors during porcine oocyte maturation.

Oocyte meiosis-coupled poly(A) polymerase α phosphorylation and activation trigger maternal mRNA translation in mice

Mammalian oocyte maturation is driven by strictly regulated polyadenylation and translational activation of maternal mRNA stored in the cytoplasm. However, the poly(A) polymerase (PAP) that directly mediates cytoplasmic polyadenylation in mammalian oocytes has not been determined. In this study, we identified PAPα as the elusive enzyme that catalyzes cytoplasmic mRNA polyadenylation implicated in mouse oocyte maturation. PAPα was mainly localized in the germinal vesicle (GV) of fully grown oocytes but was distributed to the ooplasm after GV breakdown. Inhibition of PAPα activity impaired cytoplasmic polyadenylation and translation of maternal transcripts, thus blocking meiotic cell cycle progression. Once an oocyte resumes meiosis, activated CDK1 and ERK1/2 cooperatively mediate the phosphorylation of three serine residues of PAPα, 537, 545 and 558, thereby leading to increased activity. This mechanism is responsible for translational activation of transcripts lacking cytoplasmic polyadenylation elements in their 3′-untranslated region (3′-UTR). In turn, activated PAPα stimulated polyadenylation and translation of the mRNA encoding its own (Papola) through a positive feedback circuit. ERK1/2 promoted Papola mRNA translation in a 3′-UTR polyadenylation signal-dependent manner. Through these mechanisms, PAPα activity and levels were significantly amplified, improving the levels of global mRNA polyadenylation and translation, thus, benefiting meiotic cell cycle progression.

Echinoderm Microtubule Associated Protein Like 1 Is Indispensable for Oocyte Spindle Assembly and Meiotic Progression in Mice

Completion of the first meiosis is an essential prerequisite for producing a functionally normal egg for fertilization and embryogenesis, but the precise mechanisms governing oocyte meiotic progression remains largely unclear. Here, we report that echinoderm microtubule associated protein (EMAP) like 1 (EML1), a member of the conserved EMAP family proteins, plays a crucial role in the control of oocyte meiotic progression in the mouse. Female mice carrying an ENU-induced nonsense mutation (c.1956T > A; p.Tyr652∗) of Eml1 are infertile, and the majority of their ovulated oocytes contain abnormal spindles and misaligned chromosomes. In accordance with the mutant oocyte phenotype, we find that EML1 is colocalized with spindle microtubules during the process of normal oocyte meiotic maturation, and knockdown (KD) of EML1 by specific morpholinos in the fully grown oocytes (FGOs) disrupts the integrity of spindles, and delays meiotic progression. Moreover, EML1-KD oocytes fail to progress to metaphase II (MII) stage after extrusion of the first polar body, but enter into interphase and form a pronucleus containing decondensed chromatins. Further analysis shows that EML1-KD impairs the recruitment of γ-tubulin and pericentrin to the spindle poles, as well as the attachment of kinetochores to microtubules and the proper inactivation of spindle assembly checkpoint at metaphase I (MI). The loss of EML1 also compromises the activation of maturation promoting factor around the time of oocyte resumption and completion of the first meiosis, which, when corrected by WEE1/2 inhibitor PD166285, efficiently rescues the phenotype of oocyte delay of meiotic resumption and inability of reaching MII. Through IP- mass spectrometry analysis, we identified that EML1 interacts with nuclear distribution gene C (NUDC), a critical mitotic regulator in somatic cells, and EML1-KD disrupts the specific localization of NUDC at oocyte spindles. Taken together, these data suggest that EML1 regulates acentrosomal spindle formation and the progression of meiosis to MII in mammalian oocytes, which is likely mediated by distinct mechanisms.

Genetic factors as potential molecular markers of human oocyte and embryo quality

Successful human reproduction requires gamete maturation, fertilization, and early embryonic development. Human oocyte maturation includes nuclear and cytoplasmic maturation, and abnormalities in the process will lead to infertility and recurrent failure of IVF/ICSI attempts. In addition, the quality of oocytes/embryos in the clinic can only be determined by morphological markers, and there is currently a lack of molecular markers for determining oocyte quality. As the number of patients undergoing IVF/ICSI has increased, many patients have been identified with recurrent IVF/ICSI failure. However, the genetic basis behind this phenotype remains largely unknown. In recent years, a few mutant genes have been identified by us and others, which provide potential molecular markers for determining the quality of oocytes/embryos. In this review, we outline the genetic determinants of abnormalities in the processes of oocyte maturation, fertilization, and early embryonic development. Currently, 16 genes (PATL2, TUBB8, TRIP13, ZP1, ZP2, ZP3, PANX1, TLE6, WEE2, CDC20, BTG4, PADI6, NLRP2, NLRP5, KHDC3L, and REC114) have been reported to be the causes of oocyte maturation arrest, fertilization failure, embryonic arrest, and preimplantation embryonic lethality. These abnormalities mainly have Mendelian inheritance patterns, including both dominant inheritance and recessive inheritance, although in some cases de novo mutations have also appeared. In this review, we will introduce the effects of each gene in the specific processes of human early reproduction and will summarize all known variants in these genes and their corresponding phenotypes. Variants in some genes have specific effects on certain steps in the early human reproductive processes, while other variants result in a spectrum of phenotypes. These variants and genetic markers will lay the foundation for individualized genetic counseling and potential treatments for patients and will be the target for precision treatments in reproductive medicine.

Senataxin: A New Guardian of the Female Germline Important for Delaying Ovarian Aging

Early decline in ovarian function known as premature ovarian aging (POA) occurs in around 10% of women and is characterized by a markedly reduced ovarian reserve. Premature ovarian insufficiency (POI) affects ~1% of women and refers to the severe end of the POA spectrum in which, accelerated ovarian aging leads to menopause before 40 years of age. Ovarian reserve refers to the total number of follicle-enclosed oocytes within both ovaries. Oocyte DNA integrity is a critical determinant of ovarian reserve since damage to DNA of oocytes within primordial-stage follicles triggers follicular apoptosis leading to accelerated follicle depletion. Despite the high prevalence of POA, very little is known regarding its genetic causation. Another little-investigated aspect of oocyte DNA damage involves low-grade damage that escapes apoptosis at the primordial follicle stage and persists throughout oocyte growth and later follicle development. Senataxin (SETX) is an RNA/DNA helicase involved in repair of oxidative stress-induced DNA damage and is well-known for its roles in preventing neurodegenerative disease. Recent findings uncover an important role for SETX in protecting oocyte DNA integrity against aging-induced increases in oxidative stress. Significantly, this newly identified SETX-mediated regulation of oocyte DNA integrity is critical for preventing POA and early-onset female infertility by preventing premature depletion of the ovarian follicular pool and reducing the burden of low-grade DNA damage both in primordial and fully-grown oocytes.

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

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

Mechanisms of Oocyte Maturation and Related Epigenetic Regulation

Meiosis is the basis of sexual reproduction. In female mammals, meiosis of oocytes starts before birth and sustains at the dictyate stage of meiotic prophase I before gonadotropins-induced ovulation happens. Once meiosis gets started, the oocytes undergo the leptotene, zygotene, and pachytene stages, and then arrest at the dictyate stage. During each estrus cycle in mammals, or menstrual cycle in humans, a small portion of oocytes within preovulatory follicles may resume meiosis. It is crucial for females to supply high quality mature oocytes for sustaining fertility, which is generally achieved by fine-tuning oocyte meiotic arrest and resumption progression. Anything that disturbs the process may result in failure of oogenesis and seriously affect both the fertility and the health of females. Therefore, uncovering the regulatory network of oocyte meiosis progression illuminates not only how the foundations of mammalian reproduction are laid, but how mis-regulation of these steps result in infertility. In order to provide an overview of the recently uncovered cellular and molecular mechanism during oocyte maturation, especially epigenetic modification, the progress of the regulatory network of oocyte meiosis progression including meiosis arrest and meiosis resumption induced by gonadotropins is summarized. Then, advances in the epigenetic aspects, such as histone acetylation, phosphorylation, methylation, glycosylation, ubiquitination, and SUMOylation related to the quality of oocyte maturation are reviewed.

Meiosis Resumption of Immature Human Oocytes following Treatment with Calcium Ionophore In Vitro

Objective:

In vitro maturation (IVM) of human oocytes is used to induce meiosis progression in immature retrieved oocytes. Calcium (Ca2+) has a central role in oocyte physiology. Passage through meiosis phase to another phase is controlled by increasing intracellular Ca2+. Therefore, the current research was conducted to evaluate the role of calcium ionophore (CI) on human oocyte IVM.

Materials and Methods:

In this clinical trial study, immature human oocytes were obtained from 216 intracytoplasmic sperm injection (ICSI) cycles. After ovarian stimulation, germinal vesicle (GV) stage oocytes were collected and categorized into two groups: with and without 10 µM CI treatment. Next, oocyte nuclear maturation was assessed after 24–28 hours of culture. Real-time reverse transcription polymerase chain reaction (RT-PCR) was used to assess the transcript profile of several oocyte maturation-related genes (MAPK3, CCNB1, CDK1, and cyclin D1 [CCND1]) and apoptotic-related genes (BCL-2, BAX, and Caspase-3). Oocyte glutathione (GSH) and reactive oxygen species (ROS) levels were assessed using Cell Tracker Blue and 2’,7’-dichlorodihydrofluorescein diacetate (H2DCFDA) fluorescent dye staining. Oocyte spindle configuration and chromosome alignment were analysed by immunocytochemistry.

Results:

The metaphase II (MII) oocyte rate was higher in CI‐treated oocytes (73.53%) compared to the control (67.43%) group, but this difference was not statistically significant (P=0.13). The mRNA expression profile of oocyte maturation-related genes (MAPK3, CCNB1, CDK1, and CCND1) (P<0.05) and the anti-apoptotic BCL-2 gene was remarkably up-regulated after treatment with CI (P=0.001). The pro-apoptotic BAX and Caspase-3 relative expression levels did not change significantly. The CI‐treated oocyte cytoplasm had significantly higher GSH and lower ROS (P<0.05). There was no statistically significant difference in meiotic spindle assembly and chromosome alignment between CI treatment and the control group oocytes.

Conclusion:

The finding of the current study supports the role of CI in meiosis resumption of human oocytes. (Registration Number: IRCT20140707018381N4)

The effect of copper supplementation on in vitro maturation of porcine cumulus-oocyte complexes and subsequent developmental competence after parthenogenetic activation

Copper (Cu) ions have redox activity and act as cofactors of enzymes related to respiration, radical detoxification, and iron metabolism. In this study, we aimed to examine the effects of copper (II) chloride dihydrate (CuCl2·2H2O) on porcine oocytes during in vitro maturation (IVM) and subsequent embryonic development following parthenogenetic activation (PA). Nuclear maturation, intracellular glutathione (GSH) and reactive oxygen species (ROS) levels, cumulus expansion, the mRNA expression levels of various genes, and developmental competence were analyzed. During IVM, the maturation medium was supplemented with various concentrations of Cu (0, 0.7, 1.4, and 2.8 μg/mL). After 42 h of IVM, Cu supplementation significantly increased the number of oocytes in the metaphase II stage. Further, the 1.4 μg/mL Cu group showed significantly higher intracellular GSH levels than the control group. However, Cu supplementation increased intracellular ROS levels regardless of their concentration. Additionally, the mRNA levels of Has-2, the cumulus cell expansion-related gene, were higher in all the Cu-treated groups than in the control group. The cumulus cell expansion index was higher in the 0.7 and 1.4 μg/mL Cu groups than in the other groups. In the 0.7 μg/mL Cu group, the mRNA expression levels of PCNA, Zar1, and NPM2, which are related to developmental competence, were significantly higher than those in the control group. Moreover, increased levels of Sod1 transcript, correlated with the antioxidative response, were observed in the 0.7 and 1.4 μg/mL Cu groups. The apoptosis rate in Cu-treated cumulus cells and oocytes was decreased compared to that in the corresponding control groups. Upon evaluation of subsequent embryonic development after PA, the 0.7 μg/mL Cu group showed significantly improved cleavage and blastocyst formation rate compared to the control group. In conclusion, our results suggest that Cu supplementation at appropriate concentrations in IVM medium improves porcine oocyte maturation and the subsequent embryonic potential of PA embryos by reducing oxidative stress and apoptosis.

Oocytes mount a noncanonical DNA damage response involving APC-Cdh1-mediated proteolysis

In mitotic cells, DNA damage induces temporary G2 arrest via inhibitory Cdk1 phosphorylation. In contrast, fully grown G2-stage oocytes readily enter M phase immediately following chemical induction of DNA damage in vitro, indicating that the canonical immediate-response G2/M DNA damage response (DDR) may be deficient. Senataxin (Setx) is involved in RNA/DNA processing and maintaining genome integrity. Here we find that mouse oocytes deleted of Setx accumulate DNA damage when exposed to oxidative stress in vitro and during aging in vivo, after which, surprisingly, they undergo G2 arrest. Moreover, fully grown wild-type oocytes undergo G2 arrest after chemotherapy-induced in vitro damage if an overnight delay is imposed following damage induction. Unexpectedly, this slow-evolving DDR is not mediated by inhibitory Cdk1 phosphorylation but by APC-Cdh1–mediated proteolysis of the Cdk1 activator, cyclin B1, secondary to increased Cdc14B-dependent APC-Cdh1 activation and reduced Emi1-dependent inhibition. Thus, oocytes are unable to respond immediately to DNA damage, but instead mount a G2/M DDR that evolves slowly and involves a phosphorylation-independent proteolytic pathway.

Particulate Matter Exposure During Oocyte Maturation: Cell Cycle Arrest, ROS Generation, and Early Apoptosis in Mice

Particulate matter (PM) is a general atmospheric pollutant released into the air by an anthropogenic and naturally derived mixture of substances. Current studies indicate that fine dust can result in different health defects, including endothelial dysfunction, asthma, lung cancer, cardiovascular diseases, uterine leiomyoma, deterioration in sperm quality, and overall birth impairment. However, the most prominent effects of PM₁₀ (diameter < 10 μM) exposure on the female reproductive system, especially with respect to oocyte maturation, remain unclear. In the present study, maturing mouse oocytes were treated with PM₁₀ and the phenotypes of the resulting toxic effects were investigated. Exposure to PM₁₀ led to impairment of maturation capacity by inducing cell cycle arrest and blocking normal polar body extrusion during in vitro maturation and activation of fertilization of mouse oocytes. Additionally, defects in tubulin formation and DNA alignment were observed in PM₁₀-treated oocytes during metaphase I to anaphase/telophase I transition. Moreover, PM₁₀ induced reactive oxygen species generation, mitochondrial dysfunction, DNA damage, and early apoptosis. Taken together, these results indicate that PM₁₀ exposure leads to a decline in oocyte quality and affects the subsequent embryonic development potential of mammalian oocytes.

Data mining combined with experiments to validate CEP55 as a prognostic biomarker in colorectal cancer

INTRODUCTION

Colorectal cancer (CRC) is a common tumor with high morbidity and mortality. Current specific diagnosis regarding CRC remains complicated and costly, and specific diagnostic biomarkers are lacking.

METHODS

To find potential diagnostic and prognostic biomarkers for CRC, we screened and analyzed many CRC sequencing data by The Cancer Genome Atlas Program and Gene Expression Omnibus, and validated that CEP55 may be a potential diagnostic biomarker for CRC by molecular cytological experiments and immunohistochemistry, among others.

RESULTS

We found that CEP55 is upregulated in CRC tissues and tumor cells and can promote CRC proliferation and metastasis by activating the p53/p21 axis and that CEP55 mutations in tumor patients result in worse overall survival and disease-free survival time. Besides, we also found that genes, such as CDK1, CCNB1, NEK2, KIF14, CDCA5, and RFC3 were upregulated in tumors, and their mutations would affect the prognosis of CRC patients, but these results await for more experimental evidence.

CONCLUSION

Our study validates CEP55 as a potential diagnostic and prognostic biomarker for CRC, and we also provide multiple genes and potential molecular mechanisms that may serve as diagnostic and prognostic markers for CRC.

The effect of vitrification after warming on the expressions of p38, CDK1, and cyclin B in immature goat oocytes followed by in vitro maturation

Background and Aim

The combination of vitrification techniques and in vitro maturation can reduce oocyte competence. Mitogen-activated protein kinase and maturation-promoting factor are significant in oocyte meiotic maturation regulation. This study aimed to analyze vitrification's effect, after warming followed by in vitro maturation, on the expressions of protein 38 (p38), cyclin-dependent kinase 1 (CDK1), and cyclin B and oocyte maturation level.

Materials and Methods

Immature goat oocytes were soaked in vitrification and warming solutions. The procedure was followed by in vitro maturation and in vitro maturation without post-warming vitrification as a control. These oocytes, along with their cumulus, were vitrified using hemistraw in liquid nitrogen. Oocyte maturation was carried out in a maturation medium that was added with 10 μg/mL of FSH, 10 μg/mL of LH, and 1 μg/mL E₂ for 22 h. The expressions of p38, CDK1, and cyclin B were observed using immunocytochemical methods, which were assessed semiquantitatively according to the modified Remmele method. The oocyte maturation level was observed using the aceto-orcein staining method based on the achievement of chromosomes up to the metaphase II stage and/or the formation of the polar body I.

Results

p38 expression in vitrified oocytes after warming, followed by in vitro maturation, increased insignificantly (p≥0.05), with the acquisition of 3.91±2.69 and 2.69±0.50 in the control oocytes. CDK1 expression in vitrified oocytes decreased significantly (p≤0.05) after warming, followed by in vitro maturation, with the acquisition of 2.73±1.24 and 7.27±4.39 in the control oocytes. Cyclin B expression in vitrified oocytes decreased insignificantly (p≥0.05) after warming, followed by in vitro maturation, with the acquisition of 3.09±1.4 and 4.18±2.61 in the control oocytes. The proportion of vitrified oocyte maturation levels after warming, followed by in vitro maturation, decreased significantly (p≤0.05), with the acquisition of 45.45% and 77.27% in the control oocytes.

Conclusion

This study concluded that vitrification after warming resulted in an insignificant increase in p38 expression, a significant decrease in CDK1 expression, an insignificant decrease in cyclin B expression, and a significant reduction in oocyte maturation levels.

Functions of cyclins and CDKs in mammalian gametogenesis†

Cyclins and cyclin-dependent kinases (CDKs) are key regulators of the cell cycle. Most of our understanding of their functions has been obtained from studies in single-cell organisms and mitotically proliferating cultured cells. In mammals, there are more than 20 cyclins and 20 CDKs. Although genetic ablation studies in mice have shown that most of these factors are dispensable for viability and fertility, uncovering their functional redundancy, CCNA2, CCNB1, and CDK1 are essential for embryonic development. Cyclin/CDK complexes are known to regulate both mitotic and meiotic cell cycles. While some mechanisms are common to both types of cell divisions, meiosis has unique characteristics and requirements. During meiosis, DNA replication is followed by two successive rounds of cell division. In addition, mammalian germ cells experience a prolonged prophase I in males or a long period of arrest in prophase I in females. Therefore, cyclins and CDKs may have functions in meiosis distinct from their mitotic functions and indeed, meiosis-specific cyclins, CCNA1 and CCNB3, have been identified. Here, we describe recent advances in the field of cyclins and CDKs with a focus on meiosis and early embryogenesis.

Bi-allelic Missense Pathogenic Variants in TRIP13 Cause Female Infertility Characterized by Oocyte Maturation Arrest

Normal oocyte meiosis is a prerequisite for successful human reproduction, and abnormalities in the process will result in infertility. In 2016, we identified mutations in TUBB8 as responsible for human oocyte meiotic arrest. However, the underlying genetic factors for most affected individuals remain unknown. TRIP13, encoding an AAA-ATPase, is a key component of the spindle assembly checkpoint, and recurrent homozygous nonsense variants and a splicing variant in TRIP13 are reported to cause Wilms tumors in children. In this study, we identified homozygous and compound heterozygous missense pathogenic variants in TRIP13 responsible for female infertility mainly characterized by oocyte meiotic arrest in five individuals from four independent families. Individuals from three families suffered from oocyte maturation arrest, whereas the individual from the fourth family had abnormal zygote cleavage. All displayed only the infertility phenotype without Wilms tumors or any other abnormalities. In vitro and in vivo studies showed that the identified variants reduced the protein abundance of TRIP13 and caused its downstream molecule, HORMAD2, to accumulate in HeLa cells and in proband-derived lymphoblastoid cells. The chromosome mis-segregation assay showed that variants did not have any effects on mitosis. Injecting TRIP13 cRNA into oocytes from one affected individual was able to rescue the phenotype, which has implications for future therapeutic treatments. This study reports pathogenic variants in TRIP13 responsible for oocyte meiotic arrest, and it highlights the pivotal but different roles of TRIP13 in meiosis and mitosis. These findings also indicate that different dosage effects of mutant TRIP13 might result in two distinct human diseases.

DPAGT1-Mediated Protein N-Glycosylation Is Indispensable for Oocyte and Follicle Development in Mice

Post-translational modification of proteins by N-linked glycosylation is crucial for many life processes. However, the exact contribution of N-glycosylation to mammalian female reproduction remains largely undefined. Here, DPAGT1, the enzyme that catalyzes the first step of protein N-glycosylation, is identified to be indispensable for oocyte development in mice. Dpagt1 missense mutation (c. 497A>G; p. Asp166Gly) causes female subfertility without grossly affecting other functions. Mutant females ovulate fewer eggs owing to defective development of growing follicles. Mutant oocytes have a thin and fragile zona pellucida (ZP) due to the reduction in glycosylation of ZP proteins, and display poor developmental competence after fertilization in vitro. Moreover, completion of the first meiosis is accelerated in mutant oocytes, which is coincident with the elevation of aneuploidy. Mechanistically, transcriptomic analysis reveals the downregulation of a number of transcripts essential for oocyte meiotic progression and preimplantation development (e.g., Pttgt1, Esco2, Orc6, and Npm2) in mutant oocytes, which could account for the defects observed. Furthermore, conditional knockout of Dpagt1 in oocytes recapitulates the phenotypes observed in Dpagt1 mutant females, and causes complete infertility. Taken together, these data indicate that protein N-glycosylation in oocytes is essential for female fertility in mammals by specific control of oocyte development.

Newly Identified Regulators of Ovarian Folliculogenesis and Ovulation

Each follicle represents the basic functional unit of the ovary. From its very initial stage of development, the follicle consists of an oocyte surrounded by somatic cells. The oocyte grows and matures to become fertilizable and the somatic cells proliferate and differentiate into the major suppliers of steroid sex hormones as well as generators of other local regulators. The process by which a follicle forms, proceeds through several growing stages, develops to eventually release the mature oocyte, and turns into a corpus luteum (CL) is known as “folliculogenesis”. The task of this review is to define the different stages of folliculogenesis culminating at ovulation and CL formation, and to summarize the most recent information regarding the newly identified factors that regulate the specific stages of this highly intricated process. This information comprises of either novel regulators involved in ovarian biology, such as Ube2i, Phoenixin/GPR73, C1QTNF, and α-SNAP, or recently identified members of signaling pathways previously reported in this context, namely PKB/Akt, HIPPO, and Notch.

The CRL4-DCAF13 ubiquitin E3 ligase supports oocyte meiotic resumption by targeting PTEN degradation

Cullin ring-finger ubiquitin ligase 4 (CRL4) has multiple functions in the maintenance of oocyte survival and meiotic cell cycle progression. DCAF13, a novel CRL4 adaptor, is essential for oocyte development. But the mechanisms by which CRL4-DCAF13 supports meiotic maturation remained unclear. In this study, we demonstrated that DCAF13 stimulates the meiotic resumption-coupled activation of protein synthesis in oocytes, partially by maintaining the activity of PI3K signaling pathway. CRL4-DCAF13 targets the polyubiquitination and degradation of PTEN, a lipid phosphatase that inhibits PI3K pathway as well as oocyte growth and maturation. Dcaf13 knockout in oocytes caused decreased CDK1 activity and impaired meiotic cell cycle progression and chromosome condensation defects. As a result, chromosomes fail to be aligned at the spindle equatorial plate, the spindle assembly checkpoint is activated, and most Dcaf13 null oocytes are arrested at the prometaphase I. The DCAF13-dependent PTEN degradation mechanism fits in as a missing link between CRL4 ubiquitin E3 ligase and PI3K pathway, both of which are crucial for translational activation during oocyte GV-MII transition.

The 5.8S pre-rRNA maturation factor, M-phase phosphoprotein 6, is a female fertility factor required for oocyte quality and meiosis

OBJECTIVES

M-phase phosphoprotein 6 (MPP6) is important for 5.8S pre-rRNA maturation in somatic cells and was screened as a female fertility factor. However, whether MPP6 functions in oocyte meiosis and fertility is not yet known. We aimed to address this.

MATERIALS AND METHODS

Mouse oocytes with surrounded nucleus (SN) or non-surrounded nucleus (NSN) were used for all experiments. Peptide nanoparticle-mediated antibody transfection was used to deplete MPP6. Immunofluorescence staining, immunohistochemistry and live tracker staining were used to examine MPP6 localization and characterize phenotypes after control or MPP6 depletion. High-fidelity PCR and fluorescence in situ hybridization (FISH) were used to examine the localization and level of 5.8S rRNAs. Western blot was used to examine the protein level. MPP6-EGFP mRNA microinjection was used to do the rescue.

RESULTS

MPP6 was enriched within ovaries and oocytes. MPP6 depletion significantly impeded oocyte meiosis. MPP6 depletion increased 5.8S pre-rRNA. The mRNA levels of MPP6 and 5.8S rRNA decreased within ageing oocytes, and MPP6 mRNA injection partially increased 5.8S rRNA maturation and improved oocyte quality.

CONCLUSIONS

MPP6 is required for 5.8S rRNA maturation, meiosis and quality control in mouse oocytes, and MPP6 level might be a marker for oocyte quality.

Luteinizing Hormone Action in Human Oocyte Maturation and Quality: Signaling Pathways, Regulation, and Clinical Impact

The ovarian follicle luteinizing hormone (LH) signaling molecules that regulate oocyte meiotic maturation have recently been identified. The LH signal reduces preovulatory follicle cyclic nucleotide levels which releases oocytes from the first meiotic arrest. In the ovarian follicle, the LH signal reduces cyclic nucleotide levels via the CNP/NPR2 system, the EGF/EGF receptor network, and follicle/oocyte gap junctions. In the oocyte, reduced cyclic nucleotide levels activate the maturation promoting factor (MPF). The activated MPF induces chromosome segregation and completion of the first and second meiotic divisions. The purpose of this paper is to present an overview of the current understanding of human LH signaling regulation of oocyte meiotic maturation by identifying and integrating the human studies on this topic. We found 89 human studies in the literature that identified 24 LH follicle/oocyte signaling proteins. These studies show that human oocyte meiotic maturation is regulated by the same proteins that regulate animal oocyte meiotic maturation. We also found that these LH signaling pathway molecules regulate human oocyte quality and subsequent embryo quality. Remarkably, in vitro maturation (IVM) prematuration culture (PMC) protocols that manipulate the LH signaling pathway improve human oocyte quality of cultured human oocytes. This knowledge has improved clinical human IVM efficiency which may become a routine alternative ART for some infertile patients.

The regulation of the follicular synchronization and sensitivity of rats with PCOS by AMH during prolonged pituitary downregulation

The modified prolonged gonadotropin-releasing hormone agonist (GnRH-a) protocol lessens the ovarian hyperstimulation syndrome (OHSS) effect and improves the clinical pregnancy rate of women with polycystic ovary syndrome (PCOS) compared with the standard long GnRH-a protocol. However, the molecular basis of this process needs to be elucidated. Sprague Dawley (SD) female rats with letrozole-induced PCOS were divided into GnRH-a and blank groups. Rats in the GnRH-a group were given triptorelin for 11 days, whereas those in blank group were given an equal volume of 0.9% NaCl. Meanwhile, the changes in estrus cycle, hormonal profile, ovary index, ovarian histopathology and body weight were measured. The expressions of anti-mullerian hormone (AMH), type II receptor of AMH (AMHRII), and FSH receptor (FSHR) were taken as the indicators of follicular sensitivity. Changes of follicular counting and differences in antral follicle diameter at each stage were evaluated. The number of follicles from primordial to antral stages increased during downregulation and the differences in antral follicle diameter were reduced in the GnRH-a group, whereas no significant difference was found in the blank group. The results of Western blotting and ELISA indicated that the level of AMH in ovarian total protein and serum had a similar dynamic change in the GnRH-a group. The results of immunohistochemistry showed that follicular AMH, AMHRII, and FSHR significantly decreased in the GnRH-a group. Prolonged GnRH-a protocol can improve synchronization and sensitivity of follicular development by balancing the expressions of AMH, AMHRII, and FSHR among follicles at all levels, thereby achieving better therapeutic effect.

The cyclin B2/CDK1 complex inhibits separase activity in mouse oocyte meiosis I

Chromosome segregation is driven by separase, activity of which is inhibited by binding to securin and cyclin B1/CDK1. In meiosis, premature separase activity will induce aneuploidy or abolish chromosome segregation owing to the untimely destruction of cohesin. Recently, we have proved that cyclin B2 can compensate for cyclin B1 in CDK1 activation for the oocyte meiosis G2/M transition. In the present study, we identify an interaction between cyclin B2/CDK1 and separase in mouse oocytes. We find that cyclin B2 degradation is required for separase activation during the metaphase I-anaphase I transition because the presence of stable cyclin B2 leads to failure of homologous chromosome separation and to metaphase I arrest, especially in the simultaneous absence of securin and cyclin B1. Moreover, non-phosphorylatable separase rescues the separation of homologous chromosomes in stable cyclin B2-arrested cyclin B1-null oocytes. Our results indicate that cyclin B2/CDK1 is also responsible for separase inhibition via inhibitory phosphorylation to regulate chromosome separation in oocyte meiosis, which may not occur in other cell types.

YWHA (14-3-3) protein isoforms and their interactions with CDC25B phosphatase in mouse oogenesis and oocyte maturation

Background

Immature mammalian oocytes are held arrested at prophase I of meiosis by an inhibitory phosphorylation of cyclin-dependent kinase 1 (CDK1). Release from this meiotic arrest and germinal vesicle breakdown is dependent on dephosphorylation of CDK1 by the protein, cell cycle division 25B (CDC25B). Evidence suggests that phosphorylated CDC25B is bound to YWHA (14-3-3) proteins in the cytoplasm of immature oocytes and is thus maintained in an inactive form. The importance of YWHA in meiosis demands additional studies.

Results

Messenger RNA for multiple isoforms of the YWHA protein family was detected in mouse oocytes and eggs. All seven mammalian YWHA isoforms previously reported to be expressed in mouse oocytes, were found to interact with CDC25B as evidenced by in situ proximity ligation assays. Interaction of YWHAH with CDC25B was indicated by Förster Resonance Energy Transfer (FRET) microscopy. Intracytoplasmic microinjection of oocytes with R18, a known, synthetic, non-isoform-specific, YWHA-blocking peptide promoted germinal vesicle breakdown. This suggests that inhibiting the interactions between YWHA proteins and their binding partners releases the oocyte from meiotic arrest. Microinjection of isoform-specific, translation-blocking morpholino oligonucleotides to knockdown or downregulate YWHA protein synthesis in oocytes suggested a role for a specific YWHA isoform in maintaining the meiotic arrest. More definitively however, and in contrast to the knockdown experiments, oocyte-specific and global deletion of two isoforms of YWHA, YWHAH (14-3-3 eta) or YWHAE (14-3-3 epsilon) indicated that the complete absence of either or both isoforms does not alter oocyte development and release from the meiotic prophase I arrest.

Conclusions

Multiple isoforms of the YWHA protein are expressed in mouse oocytes and eggs and interact with the cell cycle protein CDC25B, but YWHAH and YWHAE isoforms are not essential for normal mouse oocyte maturation, fertilization and early embryonic development.