Maternal SMC2 is essential for embryonic development via participating chromosome condensation in mice

During the oocyte growth, maturation and zygote development, chromatin structure keeps changing to regulate different nuclear activities. Here, we reported the role of SMC2, a core component of condensin complex, in oocyte and embryo development. Oocyte-specific conditional knockout of SMC2 caused female infertility. In the absence of SMC2, oocyte meiotic maturation and ovulation occurred normally, but chromosome condensation showed defects and DNA damages were accumulated in oocytes. The pronuclei were abnormally organized and micronuclei were frequently observed in fertilized eggs, their activity was impaired, and embryo development was arrested at the one-cell stage, suggesting that maternal SMC2 is essential for embryonic development.

SRSF2 is required for mRNA splicing during spermatogenesis

BACKGROUND

RNA splicing plays significant roles in fundamental biological activities. However, our knowledge about the roles of alternative splicing and underlying mechanisms during spermatogenesis is limited.

RESULTS

Here, we report that Serine/arginine-rich splicing factor 2 (SRSF2), also known as SC35, plays critical roles in alternative splicing and male reproduction. Male germ cell-specific deletion of Srsf2 by Stra8-Cre caused complete infertility and defective spermatogenesis. Further analyses revealed that deletion of Srsf2 disrupted differentiation and meiosis initiation of spermatogonia. Mechanistically, by combining RNA-seq data with LACE-seq data, we showed that SRSF2 regulatory networks play critical roles in several major events including reproductive development, spermatogenesis, meiotic cell cycle, synapse organization, DNA recombination, chromosome segregation, and male sex differentiation. Furthermore, SRSF2 affected expression and alternative splicing of Stra8, Stag3 and Atr encoding critical factors for spermatogenesis in a direct manner.

CONCLUSIONS

Taken together, our results demonstrate that SRSF2 has important functions in spermatogenesis and male fertility by regulating alternative splicing.

Gut microbiota-bile acid-vitamin D axis plays an important role in determining oocyte quality and embryonic development

OBJECTIVE

To reveal whether gut microbiota and their metabolites are correlated with oocyte quality decline caused by circadian rhythm disruption, and to search possible approaches for improving oocyte quality.

DESIGN

A mouse model exposed to continuous light was established. The oocyte quality, embryonic development, microbial metabolites and gut microbiota were analyzed. Intragastric administration of microbial metabolites was conducted to confirm the relationship between gut microbiota and oocyte quality and embryonic development.

RESULTS

Firstly, we found that oocyte quality and embryonic development decreased in mice exposed to continuous light. Through metabolomics profiling and 16S rDNA-seq, we found that the intestinal absorption capacity of vitamin D was decreased due to significant decrease of bile acids such as lithocholic acid (LCA), which was significantly associated with increased abundance of Turicibacter. Subsequently, the concentrations of anti-Mullerian hormone (AMH) hormone in blood and melatonin in follicular fluid were reduced, which is the main reason for the decline of oocyte quality and early embryonic development, and this was rescued by injection of vitamin D3 (VD3). Secondly, melatonin rescued oocyte quality and embryonic development by increasing the concentration of lithocholic acid and reducing the concentration of oxidative stress metabolites in the intestine. Thirdly, we found six metabolites that could rescue oocyte quality and early embryonic development, among which LCA of 30 mg/kg and NorDCA of 15 mg/kg had the best rescue effect.

CONCLUSION

These findings confirm the link between ovarian function and gut microbiota regulation by microbial metabolites and have potential value for improving ovary function.

SRSF1-mediated alternative splicing is required for spermatogenesis

Alternative splicing (AS) plays significant roles in a multitude of fundamental biological activities. AS is prevalent in the testis, but the regulations of AS in spermatogenesis is only little explored. Here, we report that Serine/arginine-rich splicing factor 1 (SRSF1) plays critical roles in alternative splicing and male reproduction. Male germ cell-specific deletion of Srsf1 led to complete infertility by affecting spermatogenesis. Mechanistically, by combining RNA-seq data with LACE-seq data, we showed that SRSF1 affected the AS of Stra8 in a direct manner and Dazl, Dmc1, Mre11a, Syce2 and Rif1 in an indirect manner. Our findings demonstrate that SRSF1 has crucial functions in spermatogenesis and male fertility by regulating alternative splicing.

NLRP14 Safeguards Calcium Homeostasis via Regulating the K27 Ubiquitination of Nclx in Oocyte-to-Embryo Transition

Sperm-induced Ca2+ rise is critical for driving oocyte activation and subsequent embryonic development, but little is known about how lasting Ca2+ oscillations are regulated. Here it is shown that NLRP14, a maternal effect factor, is essential for keeping Ca2+ oscillations and early embryonic development. Few embryos lacking maternal NLRP14 can develop beyond the 2-cell stage. The impaired developmental potential of Nlrp14-deficient oocytes is mainly caused by disrupted cytoplasmic function and calcium homeostasis due to altered mitochondrial distribution, morphology, and activity since the calcium oscillations and development of Nlrp14-deficient oocytes can be rescued by substitution of whole cytoplasm by spindle transfer. Proteomics analysis reveal that cytoplasmic UHRF1 (ubiquitin-like, containing PHD and RING finger domains 1) is significantly decreased in Nlrp14-deficient oocytes, and Uhrf1-deficient oocytes also show disrupted calcium homeostasis and developmental arrest. Strikingly, it is found that the mitochondrial Na+ /Ca2+ exchanger (NCLX) encoded by Slc8b1 is significantly decreased in the Nlrp14mNull oocyte. Mechanistically, NLRP14 interacts with the NCLX intrinsically disordered regions (IDRs) domain and maintain its stability by regulating the K27-linked ubiquitination. Thus, the study reveals NLRP14 as a crucial player in calcium homeostasis that is important for early embryonic development.

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

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

Crosstalk between m6A and coding/non-coding RNA in cancer and detection methods of m6A modification residues

N6-methyladenosine (m6A) is one of the most common and well-known internal RNA modifications that occur on mRNAs or ncRNAs. It affects various aspects of RNA metabolism, including splicing, stability, translocation, and translation. An abundance of evidence demonstrates that m6A plays a crucial role in various pathological and biological processes, especially in tumorigenesis and tumor progression. In this article, we introduce the potential functions of m6A regulators, including "writers" that install m6A marks, "erasers" that demethylate m6A, and "readers" that determine the fate of m6A-modified targets. We have conducted a review on the molecular functions of m6A, focusing on both coding and noncoding RNAs. Additionally, we have compiled an overview of the effects noncoding RNAs have on m6A regulators and explored the dual roles of m6A in the development and advancement of cancer. Our review also includes a detailed summary of the most advanced databases for m6A, state-of-the-art experimental and sequencing detection methods, and machine learning-based computational predictors for identifying m6A sites.

Insights into the adverse effects of prepubertal chronic ethanol exposure on adult female reproduction

Heavy drinking in women is known to adversely affect pregnancy and fertility. However, pregnancy is a complex process, and the adverse effects of ethanol on pregnancy does not mean that ethanol will have adverse effects on all stages from gamete to fetal formation. Similarly, the adverse effects of ethanol before and after adolescence cannot be generalized. To focus on the effects of prepubertal ethanol on female reproductive ability, we established a mouse model of prepubertal ethanol exposure by changing drinking water to 20% v/v ethanol. Some routine detections were performed on the model mice, and details such as mating, fertility, reproductive organ and fetal weights were recorded day by day after discontinuation of ethanol exposure. Prepubertal ethanol exposure resulted in decreased ovarian weight and significantly reduced oocyte maturation and ovulation after sexual maturation, however, normal morphology oocytes with discharged polar body showed normal chromosomes and spindle morphology. Strikingly, oocytes with normal morphology from ethanol exposed mice showed reduced fertilization rate, but once fertilized they had the ability to develop to blastocysts. RNA-seq analysis showed that the gene expression of the ethanol exposed oocytes with normal morphology had been altered. These results show the adverse effects of prepubertal alcohol exposure on adult female reproductive health.

SQSTM1 facilitates oocyte maturation through inducing ACLY degradation mediated by selective autophagy

Selective autophagy specifically eliminates certain intracellular substrates through the autophagy pathway. Organelles and aggregation-prone proteins can be degraded through the autophagy receptor protein SQSTM1/p62, which renders them a promising therapeutic approach against infertility. He et al. demonstrate that blocking of autophagy in cumulus granulosa cells can directly attenuate citrate levels and in turn affect oocyte maturation quality. Further findings show that SQSTM1 connects K63-polyubiquitinated ACLY (ATP citrate lyase) during the process of selective autophagic degradation, which further compromises the homeostasis of citrate. Therefore, the quality of oocyte meiotic maturation can be evaluated by the levels of selective autophagy in cumulus granulosa cells.

Mitochondrial E3 ubiquitin ligase MARCH5 is required for mouse oocyte meiotic maturation†

As the most abundant organelles in oocytes, mitochondria play an important role in maintaining oocyte quality. Here, we report that March5, encoding a mitochondrial ubiquitin ligase that promotes mitochondrial elongation, plays a critical role in mouse oocyte meiotic maturation via regulating mitochondrial function. The subcellular localization of MARCH5 was similar to the mitochondrial distribution during mouse oocyte meiotic progression. Knockdown of March5 caused decreased ratios of the first polar body extrusion. March5-siRNA injection resulted in oocyte mitochondrial dysfunctions, manifested by increased reactive oxygen species, decreased ATP content as well as decreased mitochondrial membrane potential, leading to reduced ability of spindle formation and an increased ratio of kinetochore-microtubule detachment. Further study showed that the continuous activation of the spindle assembly checkpoint and the failure of Cyclin B1 degradation caused MI arrest and first polar body (PB1) extrusion failure in March5 knockdown oocytes. Taken together, our results demonstrated that March5 plays an essential role in mouse oocyte meiotic maturation, possibly via regulation of mitochondrial function and/or ubiquitination of microtubule dynamics- or cell cycle-regulating proteins.

Maternal exposure to 4-vinylcyclohexene diepoxide during pregnancy induces subfertility and birth defects of offspring in mice

4-vinylcyclohexene diepoxide (VCD), widely used in industry, is a hazardous compound that can cause premature ovarian failure, but whether maternal VCD exposure affects the health and reproduction of offspring is unknown. Here we focused on the effects of VCD on fertility and physical health of F1 and F2 offspring in mice. The pregnant mice were injected intraperitoneally with different dosages of VCD once every day from 6.5 to 18.5 days post-coitus (dpc). We showed that maternal exposure to VCD during pregnancy significantly reduced the litter size and ovarian reserve, while increasing microtia occurrences of F1 mice. The cytospread staining showed a significant inhibition of meiotic prophase I progression from the zygotene stage to the pachytene stage. Mechanistically, the expression level of DNA damage marker (γ-H2AX) and BAX/BCL2 ratios were significantly increased, and RAD51 and DMC1 were extensively recruited to DNA double strand breaks sites in the oocytes of offspring from VCD-exposed mothers. Overall, our results provide solid evidence showing that maternal exposure to VCD during pregnancy has intergenerational deleterious effects on the offspring.

Septin 9 controls CCNB1 stabilization via APC/CCDC20 during meiotic metaphase I/anaphase I transition in mouse oocytes

The anaphase promoting complex/cyclosome (APC/C) and its cofactors CDH1 and CDC20 regulate the accumulation/degradation of CCNB1 during mouse oocyte meiotic maturation. Generally, the CCNB1 degradation mediated by APC/C^CDC20 activity is essential for the transition from metaphase to anaphase. Here, by using siRNA and mRNA microinjection, as well as time-lapse live imaging, we showed that Septin 9, which mediates the binding of septins to microtubules, is critical for oocyte meiotic cell cycle progression. The oocytes were arrested at the MI stage and the connection between chromosome kinetochores and spindle microtubules was disrupted after Septin 9 depletion. As it is well known that spindle assembly checkpoint (SAC) is an important regulator of the MI-AI transition, we thus detected the SAC activity and the expression of CDC20 and CCNB1 which were the downstream proteins of SAC during this critical period. The signals of Mad1 and BubR1 still remained on the kinetochores of chromosomes in Septin 9 siRNA oocytes at 9.5 h of in vitro culture when most control oocytes entered anaphase I. The expression of CCNB1 did not decrease and the expression of CDC20 did not increase at 9.5 h in Septin 9 siRNA oocytes. Microinjection of mRNA encoding Septin 9 or CDC20 could partially rescue MI arrest caused by Septin 9 siRNA. These results suggest that Septin 9 is required for meiotic MI-AI transition by regulating the kinetochore-microtubule connection and SAC protein localization on kinetochores, whose effects are transmitted to APC/C^CDC20 activity and CCNB1 degradation in mouse oocytes.

Miro1 regulates mitochondrial homeostasis and meiotic resumption of mouse oocyte

Miro1, a mitochondrial Rho GTPase1, is a kind of mitochondrial outer membrane protein involved in the regulation of mitochondrial anterograde transport and its subcellular distribution. Mitochondria influence reproductive processes of mammals in some aspects. Mitochondria are important for oocyte maturation, fertilization and embryonic development. The purpose of this study was to evaluate whether Miro1 regulates mouse oocyte maturation by altering mitochondrial homeostasis. We showed that Miro1 was expressed in mouse oocyte at different maturation stages. Miro1 mainly distributed in the cytoplasm and around the spindle during oocyte maturation. Small interference RNA-mediated Miro1 depletion caused significantly abnormal distribution of mitochondria and endoplasmic reticulum as well as mitochondrial dysfunction, resulting in severely impaired germinal vesicle breakdown (GVBD) of mouse oocytes. For those oocytes which went through GVBD in the Miro1-depleted group, part of them were inhibited in meiotic prophase I stage with abnormal chromosome arrangement and scattered spindle length. Our results suggest that Miro1 is essential for maintaining the maturation potential of mouse oocyte.

Maternal EHMT2 is essential for homologous chromosome segregation by regulating Cyclin B3 transcription in oocyte meiosis

During oocyte growth, various epigenetic modifications are gradually established, accompanied by accumulation of large amounts of mRNAs and proteins. However, little is known about the relationship between epigenetic modifications and meiotic progression. Here, by using Gdf9-Cre to achieve oocyte-specific ablation of Ehmt2 (Euchromatic-Histone-Lysine-Methyltransferase 2) from the primordial follicle stage, we found that female mutant mice were infertile. Oocyte-specific knockout of Ehmt2 caused failure of homologous chromosome separation independent of persistently activated SAC during the first meiosis. Further studies revealed that lacking maternal Ehmt2 affected the transcriptional level of Ccnb3, while microinjection of exogenous Ccnb3 mRNA could partly rescue the failure of homologous chromosome segregation. Of particular importance was that EHMT2 regulated ccnb3 transcriptions by regulating CTCF binding near ccnb3 gene body in genome in oocytes. In addition, the mRNA level of Ccnb3 significantly decreased in the follicles microinjected with Ctcf siRNA. Therefore, our findings highlight the novel function of maternal EHMT2 on the metaphase I-to-anaphase I transition in mouse oocytes: regulating the transcription of Ccnb3.

OTSSP167 leads to follicular dysplasia and negatively affects oocyte quality in mice

OTSSP167 is an anti-tumor drug significantly inhibiting tumor growth in xenotransplantation studies using mouse breast, lung, prostate, and pancreatic cancer cell lines. Its phase I clinical trial has been completed, indicating its great potential for future treatment of solid tumors. However, its drug-related adverse effects on reproductive systems have not yet been reported. In this study, we evaluated the effects of OTSSP167 on reproduction of female mice by determining oocyte quality and follicular development. We selected four-week-old female ICR mice for a 21-day intraperitoneal injection of OTSSP167 at a dose of 5mg/kg/d. We found that OTSSP167 could block the meiotic process of oocytes, leading to a decrease in oocyte maturation and ovulated oocyte numbers, as well as a decrease in the quality of oocytes. The results showed that OTSSP167 treatment caused disordered spindle assembly, decreased mitochondria membrane potential, and increased accumulation of reactive oxygen species in oocytes. Further investigation showed that OTSSP167 induced DNA double-strand breaks, as indicated by increased levels of γH2AX in oocytes of primordial follicles and granulosa cells of growing follicles, which induced follicular atresia and decreased the numbers of follicles at various growing stages. Our study suggests that OTSSP167 treatment may have serious effects on the ovary and consequences for female cancer patients, providing strong evidence for the necessity of protecting female fertility in clinical OTSSP167 trials.

PPP4C facilitates homologous recombination DNA repair by dephosphorylating PLK1 during early embryo development

Mammalian early embryo cells have complex DNA repair mechanisms to maintain genomic integrity, and homologous recombination (HR) plays the main role in response to double-strand DNA breaks (DSBs) in these cells. Polo-like kinase 1 (PLK1) participates in the HR process and its overexpression has been shown to occur in a variety of human cancers. Nevertheless, the regulatory mechanism of PLK1 remains poorly understood, especially during the S and G2 phase. Here, we show that protein phosphatase 4 catalytic subunit (PPP4C) deletion causes severe female subfertility due to accumulation of DNA damage in oocytes and early embryos. PPP4C dephosphorylated PLK1 at the S137 site, negatively regulating its activity in the DSB response in early embryonic cells. Depletion of PPP4C induced sustained activity of PLK1 when cells exhibited DNA lesions that inhibited CHK2 and upregulated the activation of CDK1, resulting in inefficient loading of the essential HR factor RAD51. On the other hand, when inhibiting PLK1 in the S phase, DNA end resection was restricted. These results demonstrate that PPP4C orchestrates the switch between high-PLK1 and low-PLK1 periods, which couple the checkpoint to HR.

Epitalon protects against post-ovulatory aging-related damage of mouse oocytes in vitro

The developmental potential of oocytes decreases with time after ovulation in vivo or in vitro. Epitalon is a synthetic short peptide made of four amino acids (alanine, glutamic acid, aspartic acid, and glycine), based on a natural peptide called epithalamion extracted from the pineal gland. It is a potent antioxidant, comparable to melatonin, that may confer longevity benefits. The current study aims to test the protective effects of Epitalon on the quality of post-ovulatory aging oocytes. Epitalon at 0.1mM was added to the culture medium, and the quality of oocytes was evaluated at 6h, 12h, and 24h of culture. We found that 0.1mM Epitalon reduced intracellular reactive oxygen species. Epitalon treatment significantly decreased frequency of spindle defects and abnormal distribution of cortical granules during aging for 12h and 24h, while increased mitochondrial membrane potential and DNA copy number of mitochondria, thus decreasing apoptosis of oocytes by 24h of in vitro aging. Our results suggest that Epitalon can delay the aging process of oocytes in vitro via modulating mitochondrial activity and ROS levels.

Single cell RNA sequencing techniques and applications in research of ovary development and related diseases

The ovary is a highly organized composite of germ cells and various types of somatic cells, whose communications dictate ovary development to generate functional oocytes. The differences between individual cells might have profound effects on ovary functions. Single cell RNA sequencing techniques are promising approaches to explore the cell type composition of organisms, the dynamics of transcriptomes, the regulatory network between genes and the signaling pathways between cell types at the single cell resolution. In this review, we provide an overview of the currently available single cell RNA sequencing techniques including Smart-seq2 and Drop-seq, as well as their applications in biological and clinical research to provide a better understanding on the molecular mechanisms underlying ovary development and associated diseases.

Effects of various calcium transporters on mitochondrial Ca2+ changes and oocyte maturation

Ca2+ participates in many important cellular processes, but the underlying mechanisms are still poorly understood, especially during oocyte maturation. First, we confirmed that calcium in the culture medium was essential for oocyte maturation. Next, various inhibitors of Ca2+ channels were applied to investigate their roles in mitochondrial Ca2+ changes and oocyte maturation. Our results showed that Trmp7, Orai, T-type Ca2+ channels and Na+ /Ca2+ exchanger complex (NCLX) were important for oocyte maturation. Trmp7 inhibition delayed germinal vesicle breakdown. Orai and NCLX inhibition significantly weakened the distribution of mitochondrial Ca2+ around the nucleus compared to the Ctrl group. Interestingly, even T-type Ca2+ channels-specific inhibitor Mibefradil blocked germinal vesicle breakdown; mitochondrial Ca2+ surrounding the nucleus still was maintained at a high level without spindle formation. Two calcium transporter inhibitors, Thapsigargin and Ruthenium Red, which have been confirmed to inhibit oocyte activation, did not significantly affect oocyte maturation. Increasing the knowledge of calcium transport may provide a basis to build on for improving oocyte in vitro maturation in human assisted reproduction clinics.

High-throughput sequencing reveals landscapes of female germ cell development

Female germ cell development is a highly complex process that includes meiosis initiation, oocyte growth recruitment, oocyte meiosis retardation and resumption and final meiotic maturation. A series of coordinated molecular signaling factors ensure successful oogenesis. The recent rapid development of high-throughput sequencing technologies allows for the dynamic omics in female germ cells, which is essential for further understanding the regulatory mechanisms of molecular events comprehensively. In this review, we summarize the current literature of multi-omics sequenced by epigenome-, transcriptome- and proteome-associated technologies, which provide valuable information for understanding the regulation of key events during female germ cell development.

Septin 4 controls CCNB1 stabilization via APC/CCDC20 during meiotic G2/M transition in mouse oocytes

In mammals, oocytes are arrested at G2/prophase for a long time, which is called germinal vesicle (GV) arrest. After puberty, fully-grown oocytes are stimulated by a gonadotropin surge to resume meiosis as indicated by GV breakdown (GVBD). CCNB1 is accumulated to a threshold level to trigger the activation of maturation promoting factor (MPF), inducing the G2/M transition. It is generally recognized that the anaphase-promoting complex/cyclosome (APC/C) and its cofactor CDH1 (also known as FZR1) regulates the accumulation/degradation of CCNB1. Here, by using small interfering RNA (siRNA) and messenger RNA (mRNA) microinjection, immunofluorescence and confocal microscopy, immunoprecipitation, time-lapse live imaging, and immunoblotting analysis, we showed that Septin 4 regulates the G2/M transition by regulating the accumulation of CCNB1 via APC/C^CDC20 . Depletion of Septin 4 caused GV arrest by reducing CCNB1 accumulation. Unexpectedly, the expression level of CDC20 was higher in Septin 4 siRNA-injected oocytes than in control oocytes, but there was no significant change in the expression level of CDH1. Importantly, the reduced GVBD after Septin 4 depletion could be rescued not only by over-expressing CCNB1 but also could be partially rescued by depleting CDC20. Taken together, our results demonstrate that Septin 4 may play a critical role in meiotic G2/M transition by indirect regulation of CCNB1 stabilization in mouse 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.

Nuclear and cytoplasmic quality of oocytes derived from serum-free culture of secondary follicles in vitro

In vitro culture of follicles is a promising technology to generate large quantities of mature oocytes and it could offer a novel option of assisted reproductive technologies. Here we described a 2-dimensional follicular serum-free culture system with 3-dimensional effect that can make secondary follicles develop into antral follicles (78.52%), generating developmentally mature oocytes in vitro (66.45%). The oocytes in this serum-free system completed the first meiosis; spindle assembly and chromosome congression in most oocytes matured from follicular culture were normal. However, these oocytes showed significantly lower activation and embryonic development rates, and their ability to produce Ca²⁺ oscillations was also lower in response to parthenogenetic activation, after which a 2-cell embryonic developmental block occurred. Oocytes matured from follicular culture displayed increased abnormal mitochondrial distribution and increased reactive oxygen species levels when compared to in vivo matured oocytes. These data are important for understanding the reasons for reduced developmental potential of oocytes matured from follicular culture, and for further improving the cultivation system.

Single-cell RNA sequencing reveals species-specific time spans of cell cycle transitions in early oogenesis

Oogenesis is a highly regulated process and its basic cellular events are evolutionarily conserved. However, the time spans of oogenesis differ substantially among species. To explore these interspecies differences in oogenesis, we performed single-cell RNA-sequencing on mouse and monkey female germ cells and downloaded the single-cell RNA-sequencing data of human female germ cells. The cell cycle analyses indicate that the period and extent of cell cycle transitions are significantly different between the species. Moreover, hierarchical clustering of critical cell cycle genes and the interacting network of cell cycle regulators also exhibit distinguished patterns across species. We propose that differences in the regulation of cell cycle transitions may underlie female germ cell developmental allochrony between species. A better understanding of the cell cycle transition machinery will provide new insights into the interspecies differences in female germ cell developmental time spans.

Oligoasthenoteratospermia and sperm tail bending in PPP4C-deficient mice

Protein phosphatase 4 (PPP4) is a protein phosphatase that, although highly expressed in the testis, currently has an unclear physiological role in this tissue. Here, we show that deletion of PPP4 catalytic subunit gene Ppp4c in the mouse causes male-specific infertility. Loss of PPP4C, when assessed by light microscopy, did not obviously affect many aspects of the morphology of spermatogenesis, including acrosome formation, nuclear condensation and elongation, mitochondrial sheaths arrangement and '9 + 2' flagellar structure assembly. However, the PPP4C mutant had sperm tail bending defects (head-bent-back), low sperm count, poor sperm motility and had cytoplasmic remnants attached to the middle piece of the tail. The cytoplasmic remnants were further investigated by transmission electron microscopy to reveal that a defect in cytoplasm removal appeared to play a significant role in the observed spermiogenesis failure and resulting male infertility. A lack of PPP4 during spermatogenesis causes defects that are reminiscent of oligoasthenoteratospermia (OAT), which is a common cause of male infertility in humans. Like the lack of functional PPP4 in the mouse model, OAT is characterized by abnormal sperm morphology, low sperm count and poor sperm motility. Although the causes of OAT are probably heterogeneous, including mutation of various genes and environmentally induced defects, the detailed molecular mechanism(s) has remained unclear. Our discovery that the PPP4C-deficient mouse model shares features with human OAT might offer a useful model for further studies of this currently poorly understood disorder.

Single-cell RNA sequencing reveals regulation of fetal ovary development in the monkey (Macaca fascicularis)

Germ cells are vital for reproduction and heredity. However, the mechanisms underlying female germ cell development in primates, especially in late embryonic stages, remain elusive. Here, we performed single-cell RNA sequencing of 12,471 cells from whole fetal ovaries, and explored the communications between germ cells and niche cells. We depicted the two waves of oogenesis at single-cell resolution and demonstrated that progenitor theca cells exhibit similar characteristics to Leydig cells in fetal monkey ovaries. Notably, we found that ZGLP1 displays differentially expressed patterns between mouse and monkey, which is not overlapped with NANOG in monkey germ cells, suggesting its role in meiosis entry but not in activating oogenic program in primates. Furthermore, the majority of germ cell clusters that sharply express PRDM9 and SPO11 might undergo apoptosis after cyst breakdown, leading to germ cell attrition. Overall, our work provides new insights into the molecular and cellular basis of primate fetal ovary development at single-cell resolution.

PRC2 and EHMT1 regulate H3K27me2 and H3K27me3 establishment across the zygote genome

The formation of zygote is the beginning of mammalian life, and dynamic epigenetic modifications are essential for mammalian normal development. H3K27 di-methylation (H3K27me2) and H3K27 tri-methylation (H3K27me3) are marks of facultative heterochromatin which maintains transcriptional repression established during early development in many eukaryotes. However, the mechanism underlying establishment and regulation of epigenetic asymmetry in the zygote remains obscure. Here we show that maternal EZH2 is required for the establishment of H3K27me3 in mouse zygotes. However, combined immunostaining with ULI-NChIP-seq (ultra-low-input micrococcal nuclease-based native ChIP-seq) shows that EZH1 could partially safeguard the role of EZH2 in the formation of H3K27me2. Meanwhile, we identify that EHMT1 is involved in the establishment of H3K27me2, and that H3K27me2 might be an essential prerequisite for the following de novo H3K27me3 modification on the male pronucleus. In this work, we clarify the establishment and regulatory mechanisms of H3K27me2 and H3K27me3 in mouse zygotes.

Why is oocyte aneuploidy increased with maternal aging?

One of the main causes of pregnancy failure and fetus abortion is oocyte aneuploidy, which is increased with maternal aging. Numerous possible causes of oocyte aneuploidy in aged women have been proposed, including cross-over formation defect, cohesin loss, spindle deformation, spindle assembly checkpoint malfunction, microtubule-kinetochore attachment failure, kinetochore mis-orientation, mitochondria dysfunction-induced increases in reactive oxygen species, protein over-acetylation, and DNA damage. However, it still needs to be answered if these aneuploidization factors have inherent relations, and how to prevent chromosome aneuploidy in aged oocytes. Epidemiologically, oocyte aneuploidy has been found to be weakly associated with higher homocysteine concentrations, obesity, ionizing radiation and even seasonality. In this review, we summarize the research progress and present an integrated view of oocyte aneuploidization.

Mitochondrial Ca2 + Is Related to Mitochondrial Activity and Dynamic Events in Mouse Oocytes

Mitochondrial energy insufficiency is strongly associated with oocyte activation disorders. Ca²⁺, especially that in the mitochondrial matrix, plays a pivotal role in mitochondrial energy supplementation, but the underlying mechanisms are still only poorly understood. An encoded mitochondrial matrix Ca²⁺ probe (Mt-GCaMP6s) was introduced to observe mitochondrial Ca²⁺ ([Ca²⁺]_m) dynamic changes during oocyte maturation and activation. We found that active mitochondria surrounding the nucleus showed a higher [Ca²⁺]_m than those distributed in the cortex during oocyte maturation. During oocyte partheno-activation, the patterns of Ca²⁺ dynamic changes were synchronous in the cytoplasm and mitochondria. Such higher concentration of mitochondrial matrix Ca²⁺ was closely related to the distribution of mitochondrial calcium uptake (MICU) protein. We further showed that higher [Ca²⁺]_m mitochondria around the chromosomes in oocytes might have a potential role in stimulating mitochondrial energy for calmodulin-responsive oocyte spindle formation, while synchronizing Ca²⁺ functions in the cytoplasm and nuclear area are important for oocyte activation.

Regulation of [Ca2+]i oscillations and mitochondrial activity by various calcium transporters in mouse oocytes

Oocyte activation inefficiency is one of the reasons for female infertility and Ca²⁺ functions play a critical role in the regulation of oocyte activation. We used various inhibitors of Ca²⁺ channels located on the membrane, including sarcoplasmic/ endoplasmic reticulum Ca²⁺ATPases (SERCAs, the main Ca²⁺ pumps which decrease the intracellular Ca²⁺ level by refilling Ca²⁺ into the sarcoplasmic reticulum), transient receptor potential (TRP) ion channel subfamily member 7 (TRPM7, a Ca²⁺/Mg²⁺-permeable non-selective cation channel), T-type Ca²⁺ channels and calcium channel Orai1, to investigate their roles in [Ca²⁺]_i oscillation patterns and mitochondrial membrane potential during oocyte activation by real-time recording. Our results showed that SERCAs, TRPM7 and T-type Ca²⁺ channels were important for initiation and maintenance of [Ca²⁺]_i oscillations, which was required for mitochondrial membrane potential elevation during oocyte activation, as well as oocyte cytoskeleton stability and subsequent embryo development. Increasing the knowledge of calcium transport may provide a theoretical basis for improving oocyte activation in human assisted reproduction clinics.

Single-cell RNA sequencing reveals the landscape of early female germ cell development

Meiosis initiation is a crucial step for the production of haploid gametes, which occurs from anterior to posterior in fetal ovaries. The asynchrony of the transition from mitosis to meiosis results in heterogeneity in the female germ cell populations, which limits the studies of meiosis initiation and progression at a higher resolution level. To dissect the process of meiosis initiation, we investigated the transcriptional profiles of 19 363 single germ cells collected from E12.5, E14.5, and E16.5 mouse fetal ovaries. Clustering analysis identified seven groups and defined dozens of corresponding transcription factors, providing a global view of cellular differentiation from primordial germ cells toward meiocytes. Furthermore, we explored the dynamics of gene expression within the developmental trajectory with special focus on the critical state of meiosis. We found that meiosis initiation occurs as early as E12.5 and the cluster of oogonia_4 is the critical state between mitosis and meiosis. Our data provide key insights into the transcriptome features of peri-meiotic female germ cells, which offers new information not only on meiosis initiation and progression but also on screening pathogenic mutations in meiosis-associated diseases.

RNA-Seq transcriptome reveals different molecular responses during human and mouse oocyte maturation and fertilization

BACKGROUND

Female infertility is a worldwide concern and the etiology of infertility has not been thoroughly demonstrated. Although the mouse is a good model system to perform functional studies, the differences between mouse and human also need to be considered. The objective of this study is to elucidate the different molecular mechanisms underlying oocyte maturation and fertilization between human and mouse.

RESULTS

A comparative transcriptome analysis was performed to identify the differentially expressed genes and associated biological processes between human and mouse oocytes. In total, 8513 common genes, as well as 15,165 and 6126 uniquely expressed genes were detected in human and mouse MII oocytes, respectively. Additionally, the ratios of non-homologous genes in human and mouse MII oocytes were 37 and 8%, respectively. Functional categorization analysis of the human MII non-homologous genes revealed that cAMP-mediated signaling, sister chromatid cohesin, and cell recognition were the major enriched biological processes. Interestingly, we couldn't detect any GO categories in mouse non-homologous genes.

CONCLUSIONS

This study demonstrates that human and mouse oocytes exhibit significant differences in gene expression profiles during oocyte maturation, which probably deciphers the differential molecular responses to oocyte maturation and fertilization. The significant differences between human and mouse oocytes limit the generalizations from mouse to human oocyte maturation. Knowledge about the limitations of animal models is crucial when exploring a complex process such as human oocyte maturation and fertilization.

Effects of mitochondria-associated Ca2+ transporters suppression on oocyte activation

Oocyte activation deficiency leads to female infertility. [Ca²⁺ ]_i oscillations are required for mitochondrial energy supplement transition from the resting to the excited state, but the underlying mechanisms are still very little known. Three mitochondrial Ca²⁺ channels, Mitochondria Calcium Uniporter (MCU), Na⁺ /Ca²⁺ Exchanger (NCLX) and Voltage-dependent Ca²⁺ Channel (VDAC), were deactivated by inhibitors RU360, CGP37157 and Erastin, respectively. Both Erastin and CGP37157 inhibited mitochondrial activity significantly while attenuating [Ca²⁺ ]_i and [Ca²⁺ ]_m oscillations, which caused developmental block of pronuclear formation. Thus, NCLX and VDAC are two mitochondria-associated Ca²⁺ transporter proteins regulating oocyte activation, which may be used as potential targets to treat female infertility. SIGNIFICANCE OF THE STUDY: NCLX and VDAC are two mitochondria-associated Ca²⁺ transporter proteins regulating oocyte activation.

DNA methylation establishment of CpG islands near maternally imprinted genes on chromosome 7 during mouse oocyte growth

The genome methylation is globally erased in early fetal germ cells, and it is gradually re-established during gametogenesis. The expression of some imprinted genes is regulated by the methylation status of CpG islands, while the exact time of DNA methylation establishment near maternal imprinted genes during oocyte growth is not well known. Here, growing oocytes were divided into three groups based on follicle diameters including the S-group (60-100 μm), M-group (100-140 μm), and L-group (140-180 μm). The fully grown germinal vesicle (GV)-stage and metaphase II (M2)-stage mature oocytes were also collected. These oocytes were used for single-cell bisulfite sequencing to detect the methylation status of CpG islands near imprinted genes on chromosome 7. The results showed that the CpG islands near Ndn, Magel2, Mkrn3, Peg12, and Igf2 were completely unmethylated, but those of Peg3, Snrpn, and Kcnq1ot1 were hypermethylated in MII-stage oocytes. The methylation of CpG islands near different maternal imprinted genes occurred asynchronously, being completed in later-stage growing oocytes, fully grown GV oocytes, and mature MII-stage oocytes, respectively. These results show that CpG islands near some maternally imprinted genes are not necessarily methylated, and that the establishment of methylation of other maternally imprinted genes is completed at different stages of oocyte growth, providing a novel understanding of the establishment of maternally imprinted genes in oocytes.

Cell division cycle 23 is required for mouse oocyte meiotic maturation

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

Mechanistic insights into the reduced developmental capacity of in vitro matured oocytes and importance of cumulus cells in oocyte quality determination

In vitro maturation of oocytes is a promising assisted reproductive technology (ART) for infertility treatment, although it is still not a routine technique for human ART due to reduced embryonic development. The aim of the present study was to clarify the possible reasons for reduced capacity of in vitro matured oocytes. Our results showed that the oocytes matured in vitro displayed increased abnormal mitochondrial distribution, reduced mitochondrial membrane potential, and increased reactive oxygen species levels when compared to in vivo matured oocytes. These results were not different in oocytes matured in vitro with or without cumulus cells. Notably, in vitro matured oocytes displayed increased mitochondrial DNA numbers probably due to functional compensation. In vitro matured oocytes showed significantly lower activation and embryonic development rates, and their ability to produce Ca²⁺ oscillations was much lower in response to parthenogenetic activation, especially in oocytes matured in vitro without cumulus cells with nearly half of them failing to produce calcium waves upon strontium chloride stimulation. These data are important for understanding the reasons for reduced developmental potential of in vitro matured oocytes and the importance of cumulus cells for oocyte quality.

CENP-W regulates kinetochore-microtubule attachment and meiotic progression of mouse oocytes

Oocyte meiotic maturation failure and unfaithful chromosome segregation are major causes for female infertility. Here, we showed that CENP-W, a relatively novel member of the kinetochore protein family, was expressed in mouse oocytes from the germinal vesicle (GV) to metaphase II (MII) stages. Confocal microscopy revealed that CENP-W was localized in the germinal vesicle in the GV stage, and then became concentrated on kinetochores during oocyte maturation. Knockdown of CENP-W by specific siRNA injection in vitro caused kinetochore-microtubule detachment, resulting in severely defective spindles and misaligned chromosomes, leading to metaphase I arrest and failure of first polar body (PB1) extrusion. Correspondingly, spindle assembly checkpoint (SAC) activation was observed in CENP-W knockdown oocytes even after 10h of culture. Our results suggest that CENP-W acts as a kinetochore protein, which takes part in kinetochore-microtubule attachment, thus mediating the progression of oocyte meiotic maturation.

Deletion of BAF250a affects oocyte epigenetic modifications and embryonic development

BRG1-associated factor 250a (BAF250a) is a component of the SWI/SNF adenosine triphosphate-dependent chromatin remodeling complex, which has been shown to control chromatin structure and transcription. BAF250a was reported to be a key component of the gene regulatory machinery in embryonic stem cells controlling self-renewal, differentiation, and cell lineage decisions. Here we constructed Baf250a^F/F ;Gdf9-cre (Baf250a^CKO ) mice to specifically delete BAF250a in oocytes to investigate the role of maternal BAF250a in female germ cells and embryo development. Our results showed that BAF250a deletion did not affect folliculogenesis, ovulation, and fertilization, but it caused late embryonic death. RNA sequencing analysis showed that the expression of genes involved in cell proliferation and differentiation, tissue morphogenesis, histone modification, and nucleosome remodeling were perturbed in Baf250a^CKO MII oocytes. We showed that covalent histone modifications such as H3K27me3 and H3K27ac were also significantly affected in oocytes, which may reduce oocyte quality and lead to birth defects. In addition, the DNA methylation level of Igf2r, Snrpn, and Peg3 differentially methylated regions was decreased in Baf250a^CKO oocytes. Quantitative real-time polymerase chain reaction analysis showed that the relative messenger RNA (mRNA) expression levels of Igf2r and Snrpn were significantly increased. The mRNA expression level of Dnmt1, Dnmt3a, Dnmt3l, and Uhrf1 was decreased, and the protein expression in these genes was also reduced, which might be the cause for impaired imprinting establishment. In conclusion, our results demonstrate that BAF250a plays an important role in oocyte transcription regulation, epigenetic modifications, and embryo development.

CENP-T regulates both the G2/M transition and anaphase entry by acting through CDH1 in meiotic oocytes

Oocyte meiotic maturation failure is one of the major causes for female infertility. Meiotic resumption (the G2/M transition) and progression through metaphase I (MI) are two critical stages of oocyte meiotic maturation. Here, we report that centromere protein T (CENP-T), an internal kinetochore protein, plays a critical role in meiotic resumption of mouse oocytes. Depletion of CENP-T by siRNA injection increased the CDH1 (also known as FZR1) level, resulting in increased activity of the anaphase-promoting complex (APC)-CDH1 complex, and further leading to decreased levels of the cyclin protein CCNB1, attenuated maturation-promoting factor (MPF) activity, and finally severely compromised meiotic resumption. The impaired meiotic resumption caused by CENP-T depletion could be rescued by overexpression of exogenous CCNB1 or knockdown of endogenous CDH1. Overexpression of exogenous CENP-T resulted in decreased CDH1 levels, which accelerated the progression of G2/M transition, and accelerated meiotic cell cycle progression after germinal vesicle breakdown (GVBD). Unexpectedly, spindle organization after GVBD was not affected by the overexpression, but the distribution of chromosomes was affected. Our findings reveal a novel role for CENP-T in regulating meiotic progression by acting through CDH1.

CDC6 regulates both G2/M transition and metaphase-to-anaphase transition during the first meiosis of mouse oocytes

Cell division cycle protein, CDC6, is essential for the initiation of DNA replication. CDC6 was recently shown to inhibit the microtubule-organizing activity of the centrosome. Here, we show that CDC6 is localized to the spindle from pro-metaphase I (MI) to MII stages of oocytes, and it plays important roles at two critical steps of oocyte meiotic maturation. CDC6 depletion facilitated the G2/M transition (germinal vesicle breakdown [GVBD]) through regulation of Cdh1 and cyclin B1 expression and CDK1 (CDC2) phosphorylation in a GVBD-inhibiting culture system containing milrinone. Furthermore, GVBD was significantly decreased after knockdown of cyclin B1 in CDC6-depleted oocytes, indicating that the effect of CDC6 loss on GVBD stimulation was mediated, at least in part, by raising cyclin B1. Knockdown of CDC6 also caused abnormal localization of γ-tubulin, resulting in defective spindles, misaligned chromosomes, cyclin B1 accumulation, and spindle assembly checkpoint (SAC) activation, leading to significant pro-MI/MI arrest and PB1 extrusion failure. These phenotypes were also confirmed by time-lapse live cell imaging analysis. The results indicate that CDC6 is indispensable for maintaining G2 arrest of meiosis and functions in G2/M checkpoint regulation in mouse oocytes. Moreover, CDC6 is also a key player regulating meiotic spindle assembly and metaphase-to-anaphase transition in meiotic oocytes.

Type 1 diabetes affects zona pellucida and genome methylation in oocytes and granulosa cells

Diabetes affects oocyte nuclear and cytoplasmic quality. In this study, we generated a type 1 diabetes (T1D) mouse model by STZ injection to study the effects of T1D on zona pellucida and genomic DNA methylation of oocytes and granulosa cells. T1D mice showed fewer ovulated oocytes, reduced ovarian reserve, disrupted estrus cycle, and significantly ruptured zona pellucida in 2-cell in vivo embryos compared to controls. Notably, diabetic oocytes displayed thinner zona pellucida and treatment of oocytes with high concentration glucose reduced the zona pellucida thickness. Differential methylation genes in oocytes and granulosa cells were analyzed by methylation sequencing. These genes were significantly enriched in GO terms by GO analysis, and these GO terms were involved in multiple aspects of growth and development. Most notably, the abnormal methylation genes in oocytes may be related to oocyte zona pellucida changes in diabetic mice. These findings provide novel basic data for further understanding and elucidating dysgenesis and epigenetic changes in type 1 diabetes mellitus.

Resveratrol delays postovulatory aging of mouse oocytes through activating mitophagy

Resveratrol (3,5,4'-trihydroxystilbene, RSV) is a natural potential anti-aging polyphenolic compound frequently used as a nutritional supplement against several diseases. However, the underlying mechanisms by which resveratrol regulates postovulatory aging of oocytes are still insufficiently known. In this study, we found that resveratrol could delay postovulatory aging and improve developmental competence of oocytes through activating selective mitophagy in the mouse. Resveratrol could maintain spindle morphology but it disturbed cortical granule (CG) distribution during oocyte aging. This might be due to upregulated mitophagy, since blocking mitophagy by cyclosporin A (CsA) treatment affected oocyte quality by damaging mitochondrial function and it decreased embryonic development. In addition, we also observed an involvement of FoxO3a in regulating mitophagy in aging oocytes following resveratrol treatment. Taken together, our results provide evidence that mitophagy induced by resveratrol is a potential mechanism to protect against postovulatory oocyte aging.