Oocyte-derived microvilli control female fertility by optimizing ovarian follicle selection in mice

Is preovulatory follicle selection influenced by the production of oocyte-secreted factors?

The mammalian ovary ovulates only a fraction of the oocytes it produces, as more than 99% are discarded during development. Females devote a large amount of energy to pregnancy, lactation and subsequent parental care, hence there is strong imperative to produce highly competitive offspring. It would be evolutionarily advantageous if the mammalian ovary had developed a method to detect which developing ovarian follicles contain good-quality oocytes, and preferentially select them for ovulation. No such mechanism has been clearly identified to date. Oocyte-secreted factors (OSFs) such as BMP15 and GDF9, represent one form of communication from oocyte to follicle somatic cells. Herein we discuss the hypothesis that OSFs can increase the growth rate of ovarian follicles, which provides the follicle with a greater ability to compete for follicle dominance and selection for ovulation. Some limited evidence suggests that oocytes with higher OSF secretion produce higher quality embryos but further investigation is needed to firmly link the two concepts of OSFs providing an indication of oocyte quality, and OSFs increasing the chances of follicle selection for ovulation.

Human oocyte quality and reproductive health

Declining female fertility is a health issue that has received broad global attention. Oocyte quality is the key limiting factor of female fertility, and key processes affecting oocyte quality involve the secretion of and response to hormones, ovarian function, oogenesis, oocyte maturation, and meiosis. However, compared with other species, the research and understanding of human oocyte quality and human reproductive health is limited. This review highlights our current understanding of the physiological factors and pathological factors related to human oocyte quality and discusses potential treatments. In terms of physiology, we discuss the regulation of the hypothalamic-pituitary-gonadal axis, granulosa cells, key subcellular structures, maternal mRNA homeostasis, the extracellular matrix, the maternal microenvironment, and multi-omics resources related to human oocyte quality. In terms of pathology, we review hypothalamic-pituitary-gonadal defects, ovarian dysfunction (including premature ovarian insufficiency and polycystic ovary syndrome), human oocyte development defects, and aging. In terms of the pathological aspects of human oocyte development and quality defects, nearly half of the reported pathogenic genes are involved in meiosis, while the remainder are involved in maternal mRNA regulation, the subcortical maternal complex, zona pellucida formation, ion channels, protein transport, and mitochondrial function. Furthermore, we outline the emerging scientific prospects and challenges for future explorations of the biological mechanisms behind infertility and the development of clinical treatments. This review seeks to deepen our understanding of the mechanisms regulating human oocyte quality and to provide novel insights into clinical female infertility characterized by defects in oocyte quality and oocyte development.

The idiosyncrasies of oocytes

Animal oocytes face extreme challenges. They remain dormant in the body for long periods of time. To support offspring development and health, they need to store genetic material and maternal factors stably and at the same time manage cellular damage in a reliable manner. Recent studies have provided new insights on how oocytes cope with such challenges. This review discusses the many unusual or idiosyncratic nature of oocytes and how understanding oocyte biology can help us address issues of reproduction and intergenerational inheritance.

Immunohistochemical assessment of ERM proteins (ezrin, radixin, moesin) in the ovaries of different species

The ezrin/radixin/moesin proteins play a central role in cross-linking plasma membrane proteins with the actin cytoskeleton. Despite intensive ERM protein research in many tissues and pathologies, little is known about these proteins in healthy tissues of reproductive organs. Therefore, we examined ezrin, phosphorylated ezrin/radixin/moesin (pan-pERM), radixin, and moesin distribution at the cellular level by means of immunohistochemistry in ovaries of the following animal species: mouse, dog, cat, sheep, pig, horse, and cynomolgus monkey. Ezrin was expressed in oocytes, ovarian surface, granulosa cells and corpus luteum. A characteristic, predominantly membranous pan-pERM staining pattern was observed in ovarian surface epithelium, oocyte, granulosa cells and corpus luteum. Moesin immunoreactivity was present in all ovarian structures with a prominent signal in endothelial cells of blood vessels. Oocytes, granulosa cells and corpus luteum revealed mainly nuclear radixin staining. Staining pattern and subcellular localization (membranous, cytoplasmic, nuclear) varied between different animal species and between particular ERM proteins as well. This data may help gain new insights into the physiological function of ERM proteins in biological events in the female reproductive system.

Ovarian Endometrioma Disrupts Oocyte-Cumulus Communication and Mitochondrial Function, With Melatonin Mitigating the Effects

Ovarian endometrioma (OEM), a particularly severe form of endometriosis, is an oestrogen-dependent condition often associated with pain and infertility. The mechanisms by which OEM impairs fertility, particularly through its direct impact on oocyte-cumulus cell (CC) communication and related pathways, remain poorly understood. This study investigates the impact of OEM on oocyte-CC communication and explores melatonin's therapeutic potential. We used a mouse model of OEM and employed ovarian transcriptome and gene set enrichment analyses to identify disrupted gene pathways, alongside phalloidin staining for cytoskeletal analysis, gap junction coupling analysis for intercellular communication, and mitochondrial function assessments for cellular metabolism. Our results showed that OEM significantly impairs steroidogenesis and cumulus cell function, leading to increased apoptosis, disrupted transzonal projections (TZPs), and impaired antioxidant transfer to oocytes. This culminates in oxidative stress, mitochondrial dysfunction, and compromised ATP production. OEM oocytes also exhibited severe abnormalities, including DNA damage, maturation defects, spindle assembly disruptions, and increased aneuploidy. This study identifies disrupted TZPs as a key pathological feature in OEM and highlights melatonin's potential to restore intercellular communication, mitigate oxidative damage, and improve reproductive outcomes.

Transcriptional insights on the incomplete cytoplasmic maturation and developmental potential of oocytes cultured without granulosa cells in mice

BACKGROUND

Oocyte maturation is crucial for female fertility and embryonic development, encompassing nuclear and cytoplasmic maturation. Supportive cells of follicles, such as granulosa cells, are essential for oocyte growth and maturation. Oocytes can achieve nuclear maturation without granulosa cells during in vitro maturation (IVM). However, there is still a higher chance of incomplete cytoplasmic maturation for these oocytes with mature nuclei compared with oocytes cultured with granulosa cells. Oocytes with incomplete cytoplasmic maturation have lower fertilization rates and developmental potential than mature ones, although underlying mechanisms are poorly understood. Identifying key genes and signaling pathways associated with oocyte cytoplasmic maturation can help further elucidate the maturing process of oocytes and understand the impact of immature oocytes on embryonic development, throwing insights into the strategy to improve the success rate of assisted reproductive technologies.

RESULTS

Our study investigated murine oocytes maturing with and without granulosa cells. IVM without granulosa cells yielded oocytes with lower nuclear maturation rates than IVM with granulosa cells and in vivo maturation (IVO). Even though oocytes could achieve nuclear maturation without granulosa cells, they showed incomplete cytoplasmic maturation featuring higher levels of reactive oxygen species, lower mitochondrial density, and higher proportions of cells with abnormal distributions of cortical granules. Of note, oocytes with immature and mature cytoplasm had distinct transcriptional profiles. In the immature oocytes, we observed a deficient mRNA restoration of genes in crucial regulatory pathways of cellular growth and division, potentially affecting embryonic development. Differentially expressed genes (DEGs) between immature and mature oocytes were identified to be highly expressed in different pre-implantation stages, such as the MII oocyte, the 8-cell stage, and the ICM stage. Identified DEGs were enriched in key regulatory pathways of fertilization and embryonic development, such as energy and metabolic pathways. These observations indicated that the impeded development potential of oocytes with immature cytoplasm might be the result of abnormal gene expressions during oocyte maturation.

CONCLUSIONS

We show that granulosa cells are important for both nuclear and cytoplasmic maturation of oocytes. Abnormal gene expression in oocytes with incomplete cytoplasmic maturation may be associated with potential defects in fertilization and embryonic development.

The GPER is an important factor through which somatic cells regulate oocyte maternal mRNA translation and developmental competence

The G protein-coupled estrogen receptor (GPER) plays a crucial role in various biological processes, but its regulation of oocyte meiosis remains unclear. In this study, we generated a Gper1 knockout in growing oocytes using Zp3-Cre, revealing that GPER is essential for oocyte maturation and embryo development. RNA-seq analysis indicated that GPER deficiency significantly altered the oocyte transcriptome and disrupted mRNA translation. Immunoprecipitation mass spectrometry revealed that GPER directly interacts with HSP90 and modulates the ERK1/2 and PI3K-AKT signaling pathways, which are vital for enhancing maternal mRNA translation and developmental potential. We also found that cumulus cell-derived GPER-positive vesicles and delivered to oocytes through a RAB11A-dependent pathway. RAB11A facilitates GPER recycling, preventing its degradation in late endosomes and promoting its plasma membrane localization. Moreover, epidermal growth factor (EGF) improves GPER expression in cumulus cells by upregulating RAB11A, thereby enhancing the exocytosis of recycling vesicles. Knockdown of Rab11a severely reduced GPER-positive vesicles in oocytes, impairing spindle morphogenesis and meiosis. Our findings highlight the critical role of somatic cell signals in regulating maternal mRNA translation and oocyte quality for embryonic development.

Exocyst complex component 1 (Exoc1) loss in dormant oocyte disrupts c-KIT and growth differentiation factor (GDF9) subcellular localization and causes female infertility in mice

A limited number of female germ cells support reproduction in many mammals. The follicle, composed of oocytes and supporting granulosa cells, forms the basis of oogenesis. Crosstalk between oocytes and granulosa cells is essential for the formation, dormancy, re-awakening, and maturation of oocytes. The oocyte expresses c-KIT and growth differentiation factor-9 (GDF-9), which are major factors in this crosstalk. The downstream signalling pathways of c-KIT and GDF-9 have been well-documented; however, their intra-oocyte trafficking pathway remains unclear. Our study reveals that the exocyst complex, a heterotetrameric protein complex important for tethering in vesicular transport, is important for proper intra-oocyte trafficking of c-KIT and GDF9 in mice. We found that depletion of oocyte-specific EXOC1, a component of the exocyst complex, impaired oocyte re-awakening and cyst breakdown, and inhibited granulosa cell proliferation during follicle growth. The c-KIT receptor is localised on the oocyte plasma membrane. The oocyte-specific Kit conditional knockout mice were reported to exhibit impaired oocyte re-awakening and reduced oocyte cyst breakdown. GDF9 is a protein secreted extracellularly in the oocyte. Previous studies have shown that Gdf9 knockout mice impaired proliferation and granulosa cell multilayering in growing follicles. We found that both c-KIT and GDF9 abnormally stuck in the EXOC1-depleted oocyte cytoplasm. These abnormal phenotypes were also observed in oocytes depleted of exocyst complex members EXOC3 and EXOC7. These results clearly show that the exocyst complex is essential for proper intra-oocyte trafficking of c-KIT and GDF9. Inhibition of this complex causes complete loss of female fertility in mice. Our findings build a platform for research related to trafficking mechanisms of vital crosstalk factors for oogenesis.

Cellular and molecular regulations of oocyte selection and activation in mammals

Oocytes, a uniquely pivotal cell population, play a central role in species continuity. In mammals, oogenesis involves distinct processes characterized by sequential rounds of selection, arrest, and activation to produce a limited number of mature eggs, fitting their high-survival yet high-cost fertility. During the embryonic phase, oocytes undergo intensive selection via cytoplasmic and organelle enrichment, accompanied by the onset and arrest of meiosis, thereby establishing primordial follicles (PFs) as a finite reproductive reserve. Subsequently, the majority of primary oocytes enter a dormant state and are gradually recruited through a process termed follicle activation, essential for maintaining orderly fertility. Following activation, oocytes undergo rapid growth, experiencing cycles of arrest and activation regulated by endocrine and paracrine signals, ultimately forming fertilizable eggs. Over the past two decades, advancements in genetically modified animal models, high-resolution imaging, and omics technologies have significantly enhanced our understanding of the cellular and molecular mechanisms that govern mammalian oogenesis. These advances offer profound insights into the regulatory mechanisms of mammalian reproduction and associated female infertility disorders. In this chapter, we provide an overview of current knowledge in mammalian oogenesis, with a particular emphasis on oocyte selection and activation in vivo.

Application of uniconazole in improving the high-throughput genetic transformation efficiency in maize

Agrobacterium-mediated genetic transformation is the most effective and widely used delivery system for candidate genes and genome editors in maize, which is an important crop with the largest planting area and the highest yield. Here, we used gibberellin synthesis inhibitor, uniconazole, to enhance the stem strength of regenerated plantlets resulting in a significantly increase from 11.6 % to 18.2 % in the percentage of regenerated plantlets, and the transformation frequency was also improved from 9.4 % to 15.6 % in the test experiments. The physiological condition of immature embryo is greatly affected by ear source, season and insect pests, while it can cause significant fluctuations in the transformation frequency. Our optimization works at the differentiation subculture stage, avoiding the impact on the physiological condition of immature embryo. So, it can be applicated to high-throughput genetic transformation in different seasons and different ear sources throughout the year. The productive experiment results indicated that the annual average transformation frequency significantly improved from 2.76 % to 7.14 % (approximately 2.6 folds improvement), and the tissue culture cycle was shortened from 115 days to 106 days by using optimized system. Our optimized genetic transformation system opens avenues for maize improvement based on transgenic and genome editing technology.

Phosphorylated ERM regulates meiotic maturation in mouse oocytes

The cytoskeleton of mammal oocytes provides structural support to the plasma membrane and contributes to critical cellular dynamic processes such as nuclear positioning, germinal vesicle breakdown, spindle orientation, chromosome segregation, polar body extrusion, and transmembrane signaling pathways. The ERM family (ezrin, radixin and moesin) well known as membrane-cytoskeletal crosslinkers play a crucial role in organizing plasma membrane domains through their capacity to interact with transmembrane proteins and the underlying cytoskeleton. Recent works mainly focused on the structural analysis of the ERM family members and their binding partners, together with multiple functions in cell mitosis, have significantly advanced our understanding of the importance of membrane-cytoskeletal interactions. In the present study, we documented that p-ERM was expressed and localized at cortical and nucleus during mouse oocyte meiosis. p-ERM and microfilaments were colocalized from GV to MII during mouse oocyte maturation. After being treated with cytochalasin B (CB), the F-actin was disassembled. Meanwhile, p-ERM exhibited a diffuse cytoplasmic distribution and no special staining was detected in either the oocyte membrane or condensed chromosomes. p-ERM depletion by trim-away caused the meiotic procedure arrest with a significantly lower polar body extrusion rate. Collectively, these data demonstrate that the subcellular distribution of p-ERM is correlated with microfilaments. Meanwhile, the p-ERM contributes to the first polar extrusion but does not regulate the microfilament assembly.

Decoding molecular features of bovine oocyte fate during antral follicle growth via single-cell multi-omics analysis†

Antral follicle size is a useful predictive marker of the competency of enclosed oocytes for yielding an embryo following in vitro maturation and fertilization. However, the molecular mechanisms underpinning oocyte developmental potential during bovine antral follicle growth are still unclear. Here, we used a modified single-cell multi-omics approach to analyze the transcriptome, DNA methylome, and chromatin accessibility in parallel for oocytes and cumulus cells collected from bovine antral follicles of different sizes. Transcriptome profiling identified three types of oocytes (small, medium, and large) that underwent different developmental trajectories, with large oocytes exhibiting the largest average follicle size and characteristics resembling metaphase-II oocytes. Differential expression analysis and real-time polymerase chain reaction assay showed that most replication-dependent histone genes were highly expressed in large oocytes. The joint analysis of multi-omics data revealed that the transcription of 20 differentially expressed genes in large oocytes was associated with both DNA methylation and chromatin accessibility. In addition, oocyte-cumulus interaction analysis showed that inflammation, DNA damage, and p53 signaling pathways were active in small oocytes, which had the smallest average follicle sizes. We further confirmed that p53 pathway inhibition in the in vitro maturation experiments using oocytes obtained from small antral follicles could improve the quality of oocytes and increased the blastocyte rate after in vitro fertilization and culture. Our work provides new insights into the intricate orchestration of bovine oocyte fate determination during antral folliculogenesis, which is instrumental for optimizing in vitro maturation techniques to optimize oocyte quality.

In pre-clinical study fetal hypoxia caused autophagy and mitochondrial impairment in ovary granulosa cells mitigated by melatonin supplement

INTRODUCTION

Fetal hypoxia has long-term effects on postnatal reproductive functions and the mitochondrial impairments of ovarian granulosa cells may be one of the causes. Melatonin applied to mitigate mitochondrial dysfunction and autophagy in mammalian cells has been reported. However, the potential mechanisms by which fetal hypoxia damages reproductive function in neonatal female mice and the melatonin effects on this problem remain unclear.

OBJECTIVES

This research aimed to explore the mechanism that fetal hypoxia damages reproductive function in neonatal female mice and attempt to improve the reproductive function by treating with melatonin in vivo and in vitro.

METHODS

We established a fetal hypoxia model and confirmed that fetal hypoxia affects ovarian function by inducing GC excessive autophagy. Transcriptomic analysis, gene interference, cell immunofluorescence, immunohistochemistry and western blot were conducted to explore and verify the underlying mechanisms in mice GCs and KGN cells. Finally, melatonin treatment was executed on hypoxia-treated mice GCs and KGN cells and melatonin injection to fetal-hypoxia-treated mice to determine its effect.

RESULTS

The results of in vitro experiments found that fetal hypoxia led to mitochondrial dysfunction in ovarian GCs causing autophagic cell death. And the PI3K/Akt/FoxO pathway mediated the occurrence of this process by transcriptome analysis of ovarian GCs from normal and fetal hypoxia mice, which was further verified in mice GCs and KGN cells. Additionally, melatonin administration prevented autophagic injuries and mitochondrial impairments in hypoxia-treated mice GCs and KGN cells. Meanwhile, in vivo experiments by melatonin injection ameliorated oxidative stress of ovary in fetal-hypoxia-treated mice and improved their low fertility.

CONCLUSION

Our data found that fetal hypoxia causes ovarian GCs excessive autophagy leading to low fertility in neonatal female mice and mitigated by melatonin. These results provide a potential therapy for hypoxic stress-related reproductive disorders.

Oocyte-specific deletion of eukaryotic translation initiation factor 5 causes apoptosis of mouse oocytes within the early-growing follicles by mitochondrial fission defect-reactive oxygen species-DNA damage

BACKGROUND

Mutations in several translation initiation factors are closely associated with premature ovarian insufficiency (POI), but the underlying pathogenesis remains largely unknown.

METHODS AND RESULTS

We generated eukaryotic translation initiation factor 5 (Eif5) conditional knockout mice aiming to investigate the function of eIF5 during oocyte growth and follicle development. Here, we demonstrated that Eif5 deletion in mouse primordial and growing oocytes both resulted in the apoptosis of oocytes within the early-growing follicles. Further studies revealed that Eif5 deletion in oocytes downregulated the levels of mitochondrial fission-related proteins (p-DRP1, FIS1, MFF and MTFR) and upregulated the levels of the integrated stress response-related proteins (AARS1, SHMT2 and SLC7A1) and genes (Atf4, Ddit3 and Fgf21). Consistent with this, Eif5 deletion in oocytes resulted in mitochondrial dysfunction characterized by elongated form, aggregated distribution beneath the oocyte membrane, decreased adenosine triphosphate content and mtDNA copy numbers, and excessive accumulation of reactive oxygen species (ROS) and mitochondrial superoxide. Meanwhile, Eif5 deletion in oocytes led to a significant increase in the levels of DNA damage response proteins (γH2AX, p-CHK2 and p-p53) and proapoptotic proteins (PUMA and BAX), as well as a significant decrease in the levels of anti-apoptotic protein BCL-xL.

CONCLUSION

These findings indicate that Eif5 deletion in mouse oocytes results in the apoptosis of oocytes within the early-growing follicles via mitochondrial fission defects, excessive ROS accumulation and DNA damage. This study provides new insights into pathogenesis, genetic diagnosis and potential therapeutic targets for POI.

KEY POINTS

Eif5 deletion in oocytes leads to arrest in oocyte growth and follicle development. Eif5 deletion in oocytes impairs the translation of mitochondrial fission-related proteins, followed by mitochondrial dysfunction. Depletion of Eif5 causes oocyte apoptosis via ROS accumulation and DNA damage response pathway.

SMAD4 promotes somatic-germline contact during murine oocyte growth

Development of the mammalian oocyte requires physical contact with the surrounding granulosa cells of the follicle, which provide it with essential nutrients and regulatory signals. This contact is achieved through specialized filopodia, termed transzonal projections (TZPs), that extend from the granulosa cells to the oocyte surface. Transforming growth factor (TGFβ) family ligands produced by the oocyte increase the number of TZPs, but how they do so is unknown. Using an inducible Cre recombinase strategy together with expression of green fluorescent protein to verify Cre activity in individual cells, we examined the effect of depleting the canonical TGFβ mediator, SMAD4, in mouse granulosa cells. We observed a 20-50% decrease in the total number of TZPs in SMAD4-depleted granulosa cell-oocyte complexes, and a 50% decrease in the number of newly generated TZPs when the granulosa cells were reaggregated with wild-type oocytes. Three-dimensional image analysis revealed that TZPs of SMAD4-depleted cells were longer than controls and more frequently oriented towards the oocyte. Strikingly, the transmembrane proteins, N-cadherin and Notch2, were reduced by 50% in SMAD4-depleted cells. SMAD4 may thus modulate a network of cell adhesion proteins that stabilize the attachment of TZPs to the oocyte, thereby amplifying signalling between the two cell types.

CRL4DCAF13 E3 ubiquitin ligase targets MeCP2 for degradation to prevent DNA hypermethylation and ensure normal transcription in growing oocytes

The DNA methylation is gradually acquired during oogenesis, a process sustained by successful follicle development. However, the functional roles of methyl-CpG-binding protein 2 (MeCP2), an epigenetic regulator displaying specifical binding with methylated DNA, remains unknown in oogenesis. In this study, we found MeCP2 protein was highly expressed in primordial and primary follicle, but was almost undetectable in secondary follicles. However, in aged ovary, MeCP2 protein is significantly increased in both oocyte and granulosa cells. Overexpression of MeCP2 in growing oocyte caused transcription dysregulation, DNA hypermethylation, and genome instability, ultimately leading to follicle growth arrest and apoptosis. MeCP2 is targeted by DCAF13, a substrate recognition adaptor of the Cullin 4-RING (CRL4) E3 ligase, and polyubiquitinated for degradation in both cells and oocytes. Dcaf13-null oocyte exhibited an accumulation of MeCP2 protein, and the partial rescue of follicle growth arrest induced by Dcaf13 deletion was observed following MeCP2 knockdown. The RNA-seq results revealed that large amounts of genes were regulated by the DCAF13-MeCP2 axis in growing oocytes. Our study demonstrated that CRL4DCAF13 E3 ubiquitin ligase targets MeCP2 for degradation to ensure normal DNA methylome and transcription in growing oocytes. Moreover, in aged ovarian follicles, deceased DCAF13 and DDB1 protein were observed, indicating a potential novel mechanism that regulates ovary aging.

Antioxidant sericin averts the disruption of oocyte-follicular cell communication triggered by oxidative stress

Antioxidants are free radical scavengers that increase oocyte quality and improve female fertility by suppressing oxidative stress. However, the related mechanisms remain unclear. The present study was designed to examine whether a reduction of oxidative stress from using the antioxidant sericin led to expanded cumulus cell (CC)-oocyte communication and oocyte developmental acquisition in a bovine model. We found that cumulus-oocyte complexes (COCs) matured in the presence of sericin showed a significantly increased oocyte meiotic maturation rate (P < 0.01) and accelerated subsequent blastocyst formation, as more blastocysts were found at the hatched stage (P < 0.05) compared to that in the control group. In contrast to the control group, sericin suppressed H2O2 levels in COCs, resulting in a markedly enhanced CC-oocyte gap junction communication index and number of transzonal projections, which were preserved until 18 h of oocyte maturation. These findings indicate that sericin reduces disruption of oocyte-follicular cell communication induced by oxidative stress. Sericin consistently increased intra-oocyte glutathione (GSH) levels and reduced oocyte H2O2 levels (P < 0.05), both of which were ablated when GSH synthesis was inhibited by buthionine sulfoximide (an inhibitor of GSH synthesis). Furthermore, the inhibition of GSH synthesis counteracted the positive effects of sericin on subsequent embryo developmental competence (P < 0.01). Intra-oocyte GSH levels were positively associated with blastocyst development and quality. These outcomes demonstrate new perspectives for the improvement of oocyte quality in assisted reproductive technology and may contribute to developing treatment strategies for infertility and cancer.

Polycomb repressive complex 1 modulates granulosa cell proliferation in early folliculogenesis to support female reproduction

Rationale: Premature ovarian insufficiency (POI) is an accelerated reduction in ovarian function inducing infertility. Folliculogenesis defects have been reported to trigger POI as a consequence of ovulation failure. However, the underlying mechanisms remain unclear due to the genetic complexity and heterogeneity of POI. Methods: We used whole genome sequencing (WGS), conditional knockout mouse models combined with laser capture microdissection (LCM), and RNA/ChIP sequencing to analyze the crucial roles of polycomb repressive complex 1 (PRC1) in clinical POI and mammalian folliculogenesis. Results: A deletion mutation of MEL18, the key component of PRC1, was identified in a 17-year-old patient. However, deleting Mel18 in granulosa cells (GCs) did not induce infertility until its homolog, Bmi1, was deleted simultaneously. Double deficiency of BMI1/MEL18 eliminated PRC1 catalytic activity, upregulating cyclin-dependent kinase inhibitors (CDKIs) and thus blocking GC proliferation during primary-to-secondary follicle transition. This defect led to damaged intercellular crosstalk, eventually resulting in gonadotropin response failure and infertility. Conclusions: Our findings highlighted the pivotal role of PRC1 as an epigenetic regulator of gene transcription networks in GC proliferation during early folliculogenesis. In the future, a better understanding of molecular details of PRC1 structural and functional abnormalities may contribute to POI diagnosis and therapeutic options.

Ovarian follicle size or growth rate can both be determinants of ovulatory follicle selection in mice†

The endocrinology regulating ovulation of the desired number of oocytes in the ovarian cycle is well described, particularly in mono-ovulatory species. Less is known about the characteristics that make one follicle suitable for ovulation while most other follicles die by atresia. Bromodeoxyuridine (BrdU) injection was used to characterize granulosa cell proliferation rates in developing ovarian follicles in the estrous cycle of mice. This methodology allowed identification of follicle diameters of secondary (80-130 μm), follicle-stimulating hormone (FSH)-sensitive (130-170 μm), FSH-dependent (170-350 μm), and preovulatory (>350 μm) follicles. Few preovulatory-sized follicles were present in the ovaries of mice at estrus, the beginning of the cycle. Progressive increases were seen at metestrus and diestrus, when full accumulation of the preovulatory cohort (~10 follicles) occurred. BrdU pulse-chase studies determined granulosa cell proliferation rates in the 24-48 h before the follicle reached the preovulatory stage. This showed that slow-growing follicles were not able to survive to the preovulatory stage. Mathematical modeling of follicle growth rates determined that the largest follicles at the beginning of the cycle had the greatest chance of becoming preovulatory. However, smaller follicles could enter the preovulatory follicle pool if low numbers of large antral follicles were present at the beginning of the cycle. In this instance, rapidly growing follicles had a clear selection advantage. The developing follicle pool displays heterogeneity in granulosa cell proliferation rates, even among follicles at the same stage of development. This parameter appears to influence whether a follicle can ovulate or become atretic.

Microvilli regulate the release modes of alpha-tectorin to organize the domain-specific matrix architecture of the tectorial membrane

The tectorial membrane (TM) is an apical extracellular matrix (ECM) in the cochlea essential for auditory transduction. The TM exhibits highly ordered domain-specific architecture. Alpha-tectorin/TECTA is a glycosylphosphatidylinositol (GPI)-anchored ECM protein essential for TM organization. Here, we identified that TECTA is released by distinct modes: proteolytic shedding by TMPRSS2 and GPI-anchor-dependent release from the microvillus tip. In the medial/limbal domain, proteolytically shed TECTA forms dense fibers. In the lateral/body domain produced by the supporting cells displaying dense microvilli, the proteolytic shedding restricts TECTA to the microvillus tip and compartmentalizes the collagen-binding site. The tip-localized TECTA, in turn, is released in a GPI-anchor-dependent manner to form collagen-crosslinking fibers, required for maintaining the spacing and parallel organization of collagen fibrils. Overall, we showed that distinct release modes of TECTA determine the domain-specific organization pattern, and the microvillus coordinates the release modes along its membrane to organize the higher-order ECM architecture.

BCAS2 regulates oocyte meiotic prophase I by participating in mRNA alternative splicing

Oocyte meiotic prophase I (MI) is an important event in female reproduction. Breast cancer amplified sequence 2 (BCAS2) is a component of the spliceosome. Previous reports have shown that BCAS2 is critical in male germ cell meiosis, oocyte development, and early embryo genome integrity. However, the role of BCAS2 in oocyte meiosis has not been reported. We used Stra8-GFPCre mice to knock out Bcas2 in oocytes during the pachytene phase. The results of fertility tests showed that Bcas2 conditional knockout (cKO) in oocytes results in infertility in female mice. Morphological analysis showed that the number of primordial follicles in the ovaries of 2-month-old (M) mice was significantly reduced and that follicle development was blocked. Further analysis showed that the number of primordial follicles decreased and that follicle development was slowed in 7-day postpartum (dpp) ovaries. Moreover, primordial follicles undergo apoptosis, and DNA damage cannot be repaired in primary follicle oocytes. Meiosis was abnormal; some oocytes could not reach the diplotene stage, and more oocytes could not develop to the dictyotene stage. Alternative splicing (AS) analysis revealed abnormal AS of deleted in azoospermia like (Dazl) and diaphanous related formin 2 (Diaph2) oogenesis-related genes in cKO mouse ovaries, and the process of AS was involved by CDC5L and PRP19.

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

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

The stromal microenvironment and ovarian aging: mechanisms and therapeutic opportunities

For decades, most studies of ovarian aging have focused on its functional units, known as follicles, which include oocytes and granulosa cells. However, in the ovarian stroma, there are a variety of somatic components that bridge the gap between general aging and ovarian senescence. Physiologically, general cell types, microvascular structures, extracellular matrix, and intercellular molecules affect folliculogenesis and corpus luteum physiology alongside the ovarian cycle. As a result of damage caused by age-related metabolite accumulation and external insults, the microenvironment of stromal cells is progressively remodeled, thus inevitably perturbing ovarian physiology. With the established platforms for follicle cryopreservation and in vitro maturation and the development of organoid research, it is desirable to develop strategies to improve the microenvironment of the follicle by targeting the perifollicular environment. In this review, we summarize the role of stromal components in ovarian aging, describing their age-related alterations and associated effects. Moreover, we list some potential techniques that may mitigate ovarian aging based on their effect on the stromal microenvironment.

Analyzing the cellular and molecular atlas of ovarian mesenchymal cells provides a strategy against female reproductive aging

Ovarian mesenchymal cells (oMCs) constitute a distinct microenvironment that supports folliculogenesis under physiological conditions. Supplementation of exogenous non-ovarian mesenchymal-related cells has been reported to be an efficient approach to improve ovarian functions. However, the development and cellular and molecular characteristics of endogenous oMCs remain largely unexplored. In this study, we surveyed the single-cell transcriptomic landscape to dissect the cellular and molecular changes associated with the aging of oMCs in mice. Our results showed that the oMCs were composed of five ovarian differentiated MC (odMC) populations and one ovarian mesenchymal progenitor (oMP) cell population. These cells could differentiate into various odMCs via an oMP-derived route to construct the ovarian stroma structures. Comparative analysis revealed that ovarian aging was associated with decreased quantity of oMP cells and reduced quality of odMCs. Based on the findings of bioinformatics analysis, we designed different strategies involving supplementation with young oMCs to examine their effects on female fertility and health. Our functional investigations revealed that oMCs supplementation prior to ovarian senescence was the optimal method to improve female fertility and extend the reproductive lifespan of aged females in the long-term.

Exomap1 mouse: A transgenic model for in vivo studies of exosome biology

Exosomes are small extracellular vesicles (sEVs) of ~30-150 nm in diameter that are enriched in exosome marker proteins and play important roles in health and disease. To address large unanswered questions regarding exosome biology in vivo, we created the Exomap1 transgenic mouse, which in response to Cre recombinase expresses the most highly enriched exosomal marker protein known, human CD81, fused to mNeonGreen (HsCD81mNG), and prior to Cre expresses a mitochondrial red fluorescent protein. Validation of the exomap1 mouse with eight distinct Cre drivers demonstrated that HsCD81mNG was expressed only in response to Cre, that murine cells correctly localized HsCD81mNG to the plasma membrane, and that this led to the secretion of HsCD81mNG in EVs that had the size (~70-80 nm), topology, and composition of exosomes. Furthermore, cell type-specific activation of the exomap1 transgene allowed us to use quantitative single molecule localization microscopy to calculate the cell type-specific contribution to biofluid exosome populations. Specifically, we show that neurons contribute ~1% to plasma and cerebrospinal fluid exosome populations whereas hepatocytes contribute ~15% to plasma exosome populations, numbers that reflect the known vascular permeabilities of brain and liver. These observations validate the use of Exomap1 mouse models for in vivo studies of exosome biology.

Intracellular fraction of zona pellucida protein 3 is required for the oocyte-to-embryo transition in mice

In oocyte biology, the zona pellucida has long been known to operate three extracellular functions downstream of the secretory pathway, namely, encasing the oocytes in ovarian follicles, mediating sperm-oocyte interaction, and preventing premature embryo contact with oviductal epithelium. The present study uncovers a fourth function that is fundamentally distinct from the other three, being critical for embryonic cell survival in mice. Intriguingly, the three proteins of the mouse zona pellucida (ZP1, ZP2, ZP3) were found abundantly present also inside the embryo 4 days after fertilization, as shown by mass spectrometry, immunoblotting, and immunofluorescence. Contrary to current understanding of the roles of ZP proteins, ZP3 was associated more with the cytoskeleton than with secretory vesicles in the subcortical region of metaphase II oocytes and zygotes, and was excluded from regions of cell-cell contact in cleavage-stage embryos. Trim-away-mediated knockdown of ZP3 in fertilized oocytes hampered the first zygotic cleavage, while ZP3 overexpression supported blastocyst formation. Transcriptome analysis of ZP3-knockdown embryos pointed at defects of cytoplasmic translation in the context of embryonic genome activation. This conclusion was supported by reduced protein synthesis in the ZP3-knockdown and by the lack of cleavage arrest when Trim-away was postponed from the one-cell to the late two-cell stage. These data place constraints on the notion that zona proteins only operate in the extracellular space, revealing also a role during the oocyte-to-embryo transition. Ultimately, these data recruit ZP3 into the family of maternal factors that contribute to developmental competence of mouse oocytes.

A heterozygous ZP2 mutation causes zona pellucida defects and female infertility in mouse and human

The zona pellucida (ZP) is an extracellular glycoprotein matrix surrounding mammalian oocytes. Recently, numerous mutations in genes encoding ZP proteins have been shown to be possibly related to oocyte abnormality and female infertility; few reports have confirmed the functions of these mutations in living animal models. Here, we identified a novel heterozygous missense mutation (NM_001376231.1:c.1616C>T, p.Thr539Met) in ZP2 from a primary infertile female. We showed that the mutation reduced ZP2 expression and impeded ZP2 secretion in cell lines. Furthermore, we constructed the mouse model with the mutation (Zp2T541M) using CRISPR-Cas9. Zp2WT/T541M female mice had normal fertility though generated oocytes with the thin ZP, whereas Zp2T541M female mice were completely infertile due to degeneration of oocytes without ZP. Additionally, ZP deletion impaired folliculogenesis and caused female infertility in Zp2T541M mice. Our study not only expands the spectrum of ZP2 mutation sites but also, more importantly, increases the understanding of pathogenic mechanisms of ZP2 mutations.

A plant cytorhabdovirus modulates locomotor activity of insect vectors to enhance virus transmission

Transmission of many plant viruses relies on phloem-feeding insect vectors. However, how plant viruses directly modulate insect behavior is largely unknown. Barley yellow striate mosaic virus (BYSMV) is transmitted by the small brown planthopper (SBPH, Laodelphax striatellus). Here, we show that BYSMV infects the central nervous system (CNS) of SBPHs, induces insect hyperactivity, and prolongs phloem feeding duration. The BYSMV accessory protein P6 interacts with the COP9 signalosome subunit 5 (LsCSN5) of SBPHs and suppresses LsCSN5-regulated de-neddylation from the Cullin 1 (CUL1), hereby inhibiting CUL1-based E3 ligases-mediated degradation of the circadian clock protein Timeless (TIM). Thus, virus infection or knockdown of LsCSN5 compromises TIM oscillation and induces high insect locomotor activity for transmission. Additionally, expression of BYSMV P6 in the CNS of transgenic Drosophila melanogaster disturbs circadian rhythm and induces high locomotor activity. Together, our results suggest the molecular mechanisms whereby BYSMV modulates locomotor activity of insect vectors for transmission.

Integrated bioinformatics analysis elucidates granulosa cell whole-transcriptome landscape of PCOS in China

BACKGROUND

Polycystic ovary syndrome (PCOS) is a common reproductive, neuroendocrine, and metabolic disorder in women of reproductive age that affects up to 5-10% of women of reproductive age. The aetiology of follicle development arrest and critical issues regarding the abnormal follicular development in PCOS remain unclear. The present study aims to systematically evaluate granulosa cell whole-transcriptome sequencing data to gain more insights into the transcriptomic landscape and molecular mechanism of PCOS in China.

METHODS

In the present study, the microarray datasets GSE138518, GSE168404, GSE193123, GSE138572, GSE95728, and GSE145296 were downloaded from the Gene Expression Omnibus (GEO) database. Subsequently, differential expression analysis was performed on the PCOS and control groups, followed by functional interaction prediction analysis to investigate gene-regulatory circuits in PCOS. Finally, hub genes and their associated ncRNAs were validated by qPCR in human-luteinized granulosa (hGL) cells and were correlated with the clinical characteristics of the patients.

RESULTS

A total of 200 differentially expressed mRNAs, 3 differentially expressed miRNAs, 52 differentially expressed lncRNAs, and 66 differentially expressed circRNAs were found in PCOS samples compared with controls. GO and KEGG enrichment analyses indicated that the DEGs were mostly enriched in phospholipid metabolic processes, steroid biosynthesis and inflammation related pathways. In addition, the upregulated miRNA hsa-miR-205-5p was significantly enriched in the ceRNA network, and two hub genes, MVD and PNPLA3, were regulated by hsa-miR-205-5p, which means that hsa-miR-205-5p may play a fundamental role in the pathogenesis of PCOS. We also found that MVD and PNPLA3 were related to metabolic processes and ovarian steroidogenesis, which may be the cause of the follicle development arrest in PCOS patients.

CONCLUSIONS

In summary, we systematically constructed a ceRNA network depicting the interactions between the ncRNAs and the hub genes in PCOS and control subjects and correlated the hub genes with the clinical characteristics of the patients, which provides valuable insights into the granulosa cell whole-transcriptome landscape of PCOS in China.

Filopodia-like protrusions of adjacent somatic cells shape the developmental potential of oocytes

The oocyte must grow and mature before fertilization, thanks to a close dialogue with the somatic cells that surround it. Part of this communication is through filopodia-like protrusions, called transzonal projections (TZPs), sent by the somatic cells to the oocyte membrane. To investigate the contribution of TZPs to oocyte quality, we impaired their structure by generating a full knockout mouse of the TZP structural component myosin-X (MYO10). Using spinning disk and super-resolution microscopy combined with a machine-learning approach to phenotype oocyte morphology, we show that the lack of Myo10 decreases TZP density during oocyte growth. Reduction in TZPs does not prevent oocyte growth but impairs oocyte-matrix integrity. Importantly, we reveal by transcriptomic analysis that gene expression is altered in TZP-deprived oocytes and that oocyte maturation and subsequent early embryonic development are partially affected, effectively reducing mouse fertility. We propose that TZPs play a role in the structural integrity of the germline-somatic complex, which is essential for regulating gene expression in the oocyte and thus its developmental potential.

Exomap1 mouse: a transgenic model for in vivo studies of exosome biology

Exosomes are small extracellular vesicles (sEVs) of ~30-150 nm in diameter that have the same topology as the cell, are enriched in selected exosome cargo proteins, and play important roles in health and disease. To address large unanswered questions regarding exosome biology in vivo, we created the exomap1 transgenic mouse model. In response to Cre recombinase, exomap1 mice express HsCD81mNG, a fusion protein between human CD81, the most highly enriched exosome protein yet described, and the bright green fluorescent protein mNeonGreen. As expected, cell type-specific expression of Cre induced the cell type-specific expression of HsCD81mNG in diverse cell types, correctly localized HsCD81mNG to the plasma membrane, and selectively loaded HsCD81mNG into secreted vesicles that have the size (~80 nm), topology (outside out), and content (presence of mouse exosome markers) of exosomes. Furthermore, mouse cells expressing HsCD81mNG released HsCD81mNG-marked exosomes into blood and other biofluids. Using high-resolution, single-exosome analysis by quantitative single molecule localization microscopy, we show here that that hepatocytes contribute ~15% of the blood exosome population whereas neurons contribute <1% of blood exosomes. These estimates of cell type-specific contributions to blood EV population are consistent with the porosity of liver sinusoidal endothelial cells to particles of ~50-300 nm in diameter, as well as with the impermeability of blood-brain and blood-neuron barriers to particles >5 nm in size. Taken together, these results establish the exomap1 mouse as a useful tool for in vivo studies of exosome biology, and for mapping cell type-specific contributions to biofluid exosome populations. In addition, our data confirm that CD81 is a highly-specific marker for exosomes and is not enriched in the larger microvesicle class of EVs.

From in vivo to in vitro: exploring the key molecular and cellular aspects of human female gametogenesis

Human oogenesis is a highly complex and not yet fully understood process due to ethical and technological barriers that limit studies in the field. In this context, replicating female gametogenesis in vitro would not only provide a solution for some infertility problems, but also be an excellent study model to better understand the biological mechanisms that determine the formation of the female germline. In this review, we explore the main cellular and molecular aspects involved in human oogenesis and folliculogenesis in vivo, from the specification of primordial germ cells (PGCs) to the formation of the mature oocyte. We also sought to describe the important bidirectional relationship between the germ cell and the follicular somatic cells. Finally, we address the main advances and different methodologies used in the search for obtaining cells of the female germline in vitro.

The protective effect of resveratrol on largemouth bass (Micropterus salmoides) during out-of-season spawning

In aquaculture production, out-of-season spawning is beneficial to solve the seasonal shortage of fry that are normally produced once annually by species such as largemouth bass (Micropterus salmoides), thereby implementing year-round fry production. Maintaining low temperature over a period of several months can delay largemouth bass ovarian development, but it can cause severe stress to their reproductive function, leading to decreased fertility during out-of-season spawning. Feeding with antioxidants is one of the most effective methods to alleviate the negative effects of low temperature stress. Therefore, the purpose of this study is to: (a) evaluate the changes in oocyte morphology, antioxidant capacity, reproductive hormone-related index, cell apoptosis and autophagy during the out-of-season spawning of largemouth bass, and (b) to investigate the protective effect of the antioxidant resveratrol on this fish during out-of-season spawning from May through August. The study was divided into two groups (three replicates per group, 2000 fish per replicate): control group (Control) (exposure to water temperature of 12-17 °C) and resveratrol supplementation group (Res) (exposure to water temperature of 12-17 °C and fed with 200 mg/kg resveratrol). The results show that: (1) The serum hormones LH and E2 increased first and then remained unchanged, and the ovarian section showed that the ovary remained in stage IV. (2) In the process of off-season reproduction, a large number of follicles experienced follicular atresia, accompanied by endoplasmic reticulum expansion, nuclear chromatin condensation and mitochondrial swelling, which was relieved after feeding resveratrol. (3) Resveratrol decreased the ovarian ROS content and improved the activities of CAT and other antioxidant enzymes in the ovary and liver to some extent. (4) Resveratrol reduced the level of pro-apoptotic (Bax, Caspase3, Caspase8, Caspase9) and autophagy-related components (LC3-B, Beclin-1) while increasing the transcription level of anti-apoptotic (Bcl-2) factors. These findings suggest that resveratrol alleviates some adverse effects of largemouth bass during out-of-season spawning to some extent and provide a model for efficient and high-quality out-of-season spawning.

Quantitative proteomic analysis uncovers protein-expression profiles during gonadotropin-dependent folliculogenesis in mice†

Ovarian follicle is the basic functional unit of female reproduction, and is composed of oocyte and surrounding granulosa cells. In mammals, folliculogenesis strictly rely on gonadotropin regulations to determine the ovulation and the quality of eggs. However, the dynamic changes of protein-expressing profiles in follicles at different developmental stages remain largely unknown. By performing mass-spectrometry-based quantitative proteomic analysis of mouse follicles, we provide a proteomic database (~3000 proteins) that covers three key stages of gonadotropin-dependent folliculogenesis. By combining bioinformatics analysis with in situ expression validation, we showed that our proteomic data well reflected physiological changes during folliculogenesis, which provided potential to predict unknown regulators of folliculogenesis. Additionally, by using the oocyte structural protein zona pellucida protein 2 as the internal control, we showed the possibility of our database to predict the expression dynamics of oocyte-expressing proteins during folliculogenesis. Taken together, we provide a high-coverage proteomic database to study protein-expression dynamics during gonadotropin-dependent folliculogenesis in mammals.

A New Fluorescent Probe Tool: ERNathG

β-d-Glucuronidase (GUS) plays a pivotal role in both clinical treatment assessment and environmental monitoring. Existing tools for GUS detection suffer from (1) poor continuity due to a gap between the optimal pH of the probes and the enzyme and (2) diffusion from the detection site due to lack of an anchoring structure. Here we report a novel GUS pH-matching and endoplasmic reticulum-anchoring strategy for GUS recognition. The new fluorescent probe tool was termed ERNathG, which was designed and synthesized with β-d-glucuronic acid as the GUS-specific recognition site and 4-hydroxy-1,8-naphthalimide as a fluorescence reporting group, with a p-toluene sulfonyl as an anchoring group. This probe enabled the continuous and anchored detection of GUS without pH-adjustment for the related assessment of common cancer cell lines and gut bacteria. The probe's properties are far superior to those of commonly used commercial molecules.

Morphological and Chemical Investigation of Ovarian Structures in a Bovine Model by Contrast-Enhanced X-ray Imaging and Microscopy

An improved understanding of an ovary's structures is highly desirable to support advances in folliculogenesis knowledge and reproductive medicine, with particular attention to fertility preservation options for prepubertal girls with malignant tumors. Although currently the golden standard for structural analysis is provided by combining histological sections, staining, and visible 2D microscopic inspection, synchrotron radiation phase-contrast microtomography is becoming a new challenge for three-dimensional studies at micrometric resolution. To this aim, the proper use of contrast agents can improve the visualization of internal structures in ovary tissues, which normally present a low radiopacity. In this study, we report a comparison of four staining protocols, based on iodine or tungsten containing agents, applied to bovine ovarian tissues fixed in Bouin's solution. The microtomography (microCT) analyses at two synchrotron facilities under different set-ups were performed at different energies in order to maximize the image contrast. While tungsten-based agents allow large structures to be well identified, Iodine ones better highlight smaller features, especially when acquired above the K-edge energy of the specific metal. Further scans performed at lower energy where the setup was optimized for overall quality and sensitivity from phase-contrast still provided highly resolved visualization of follicular and intrafollicular structures at different maturation stages, independent of the staining protocol. The analyses were complemented by X-ray Fluorescence mapping on 2D sections, showing that the tungsten-based agent has a higher penetration in this type of tissues.

Simulated microgravity reduces quality of ovarian follicles and oocytes by disrupting communications of follicle cells

Ovarian follicles are the fundamental structures that support oocyte development, and communications between oocytes and follicle somatic cells are crucial for oogenesis. However, it is unknown that whether exposure to microgravity influences cellular communications and ovarian follicle development, which might be harmful for female fertility. By 3D culturing of ovarian follicles under simulated microgravity (SMG) conditions in a rotating cell culture system, we found that SMG treatment did not affect the survival or general growth of follicles but decreased the quality of cultured follicles released oocytes. Ultrastructure detections by high-resolution imaging showed that the development of cellular communicating structures, including granulosa cell transzonal projections and oocyte microvilli, were markedly disrupted. These abnormalities caused chaotic polarity of granulosa cells (GCs) and a decrease in oocyte-secreted factors, such as Growth Differentiation Factor 9 (GDF9), which led to decreased quality of oocytes in these follicles. Therefore, the quality of oocytes was dramatically improved by the supplementations of GDF9 and NADPH-oxidase inhibitor apocynin. Together, our results suggest that exposure to simulated microgravity impairs the ultrastructure of ovarian follicles. Such impairment may affect female fertility in space environment.

Intercellular communication in the cumulus-oocyte complex during folliculogenesis: A review

During folliculogenesis, the oocyte and surrounding cumulus cells form an ensemble called the cumulus-oocyte complex (COC). Due to their interdependence, research on the COC has been a hot issue in the past few decades. A growing body of literature has revealed that intercellular communication is critical in determining oocyte quality and ovulation. This review provides an update on the current knowledge of COC intercellular communication, morphology, and functions. Transzonal projections (TZPs) and gap junctions are the most described structures of the COC. They provide basic metabolic and nutrient support, and abundant molecules for signaling pathways and regulations. Oocyte-secreted factors (OSFs) such as growth differentiation factor 9 and bone morphogenetic protein 15 have been linked with follicular homeostasis, suggesting that the communications are bidirectional. Using advanced techniques, new evidence has highlighted the existence of other structures that participate in intercellular communication. Extracellular vesicles can carry transcripts and signaling molecules. Microvilli on the oocyte can induce the formation of TZPs and secrete OSFs. Cell membrane fusion between the oocyte and cumulus cells can lead to sharing of cytoplasm, in a way making the COC a true whole. These findings give us new insights into related reproductive diseases like polycystic ovary syndrome and primary ovarian insufficiency and how to improve the outcomes of assisted reproduction.

Transzonal projections: Essential structures mediating intercellular communication in the mammalian ovarian follicle

The development of germ cells relies on contact and communication with neighboring somatic cells that provide metabolic support and regulatory signals. In females, contact is achieved through thin cytoplasmic processes that project from follicle cells surrounding the oocyte, extend through an extracellular matrix (ECM) that lies between them, and reach its surface. In mammals, the ECM is termed the zona pellucida and the follicular cell processes are termed transzonal projections (TZPs). TZPs become detectable when the zona pellucida is laid down during early folliculogenesis and subsequently increase in number as oocyte growth progresses. They then rapidly disappear at the time of ovulation, permanently breaking germ-soma contact. Here we review the life cycle and functions of the TZPs. We begin with an overview of the morphology and cytoskeletal structure of TZPs, in the context of actin- and tubulin-based cytoplasmic processes in other cell types. Next, we review the roles played by TZPs in mediating progression through successive stages of oocyte development. We then discuss two mechanisms that may generate TZPs-stretching at pre-existing points of granulosa cell-oocyte contact and elaboration of new processes that push through the zona pellucida-as well as gene products implicated in their formation or function. Finally, we describe the signaling pathways that cause TZPs to be retracted in response to signals that also trigger meiotic maturation and ovulation of the oocyte. The principles and mechanisms that govern TZP behavior may be relevant to understanding communication between physically separated cells in other physiological contexts.

Asynchronous embryonic germ cell development leads to a heterogeneity of postnatal ovarian follicle activation and may influence the timing of puberty onset in mice

Background

Ovarian follicles, which are the basic units of female reproduction, are composed of oocytes and surrounding somatic (pre) granulosa cells (GCs). A recent study revealed that signaling in somatic preGCs controlled the activation (initial recruitment) of follicles in the adult ovaries, but it is also known that there are two waves of follicle with age-related heterogeneity in their developmental dynamics in mammals. Although this heterogeneity was proposed to be crucial for female reproduction, our understanding of how it arises and its significance is still elusive.

Results

In the current study, by deleting the key secreted factor KIT ligand from preGCs and analyzing the follicle cell developmental dynamics, we revealed distinct patterns of activation and growth associated with the two waves of follicles in mouse ovary. Our results confirmed that activation of adult wave follicles is initiated by somatic preGCs and dependent on the KIT ligand. By contrast, activation of first wave follicles, which are awakened from germ cells before follicle formation, can occur in the absence of preGC-secreted KIT ligand in postnatal ovaries and appears to be oocyte-initiated. We also found that the asynchronous activity of phosphatidylinositol 3 kinases (PI3K) signaling and meiotic process in embryonic germ cells lead to the follicle heterogeneity in postnatal ovaries. In addition, we supplied evidence that the time sequence of embryonic germ cell development and its related first wave follicle growth are correlated to the time of puberty onset in females.

Conclusion

Taken together, our study provides evidence that asynchronous development of embryonic oocytes leads to the heterogeneity of postnatal ovarian follicle activation and development, and affects the timing of onset of puberty in females.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01318-y.

MTOR-mediated interaction between the oocyte and granulosa cells regulates the development and function of both compartments in mice

Coordinated development of the germline and the somatic compartments within a follicle is an essential prerequisite for creating a functionally normal oocyte. Bi-directional communication between the oocyte and the granulosa cells enables the frequent interchange of metabolites and signals that support the development and functions of both compartments. Mechanistic target of Rapamycin (MTOR), a conserved serine/threonine kinase and a widely recognized integrator of signals and pathways key for cellular metabolism, proliferation, and differentiation, is emerging as a major player that regulates many factes of oocyte and follicle development. Here, we summarized our recent observations on the role of oocyte- and granulosa cell-expressed MTOR in the control of the oocyte's and granulosa cell's own development, as well as the development of one another, and provided new data that further strengthen the role of cumulus cell-expressed MTOR in synchronizing oocyte and follicle development. Inhibition of MTOR induced oocyte meiotic resumption in cultured large antral follicles, as well as cumulus expansion and the expression of cumulus expansion-related transcripts in cumulus-oocyte complexes in vitro. In vivo, the activity of MTOR in cumulus cells was diminished remarkablely by 4 h after hCG administration. These results thus suggest that activation of MTOR in cumulus cells contributes to the maintenance of oocyte meiotic arrest before the LH surge. Based on the observations made by us here and previously, we propose that MTOR is an essential mediator of the bi-directional communication between the oocyte and granulosa cells that regulates the development and function of both compartments.

MYO10 promotes transzonal projection (TZP)-dependent germ line-somatic contact during mammalian folliculogenesis

Granulosa cells of growing ovarian follicles elaborate filopodia-like structures termed transzonal projections (TZPs) that supply the enclosed oocyte with factors essential for its development. Little is known, however, of the mechanisms underlying the generation of TZPs. We show in mouse and human that filopodia, defined by an actin backbone, emerge from granulosa cells in early-stage primary follicles and that actin-rich TZPs become detectable as soon as a space corresponding to the zona pellucida appears. mRNA encoding Myosin10 (MYO10), a motor protein that accumulates at the base and tips of filopodia and has been implicated in their initiation and elongation, is present in granulosa cells and oocytes of growing follicles. MYO10 protein accumulates in foci located mainly between the oocyte and innermost layer of granulosa cells, where it co-localizes with actin. In both mouse and human, the number of MYO10 foci increases as oocytes grow, corresponding to the increase in the number of actin-TZPs. RNAi-mediated depletion of MYO10 in cultured mouse granulosa cell-oocyte complexes is associated with a 52% reduction in the number of MYO10 foci and a 28% reduction in the number of actin-TZPs. Moreover, incubation of cumulus-oocyte complexes in the presence of epidermal growth factor, which triggers a 93% reduction in the number of actin-TZPs, is associated with a 55% reduction in the number of MYO10 foci. These results suggest that granulosa cells possess an ability to elaborate filopodia, which when directed towards the oocyte become actin-TZPs, and that MYO10 increases the efficiency of formation or maintenance of actin-TZPs.

Female fertility and the zona pellucida

Fertility in female mammals, including mice and humans, is dependent on the presence of a zona pellucida (ZP) around growing oocytes and unfertilized eggs. A ZP is required to stabilize contacts between oocyte microvilli and follicle cell projections that traverse the ZP to form gap junctions that support the health of growing oocytes and developing follicles. In the absence of a ZP, due to inactivation or mutation of genes encoding ZP proteins, there is a loss of contacts between growing oocytes and neighboring follicle cells and a concomitant reduction in the production of ovulated eggs that results in female infertility.

Effects of Androgen Excess-Related Metabolic Disturbances on Granulosa Cell Function and Follicular Development

Polycystic ovary syndrome (PCOS) is a common reproductive endocrine disease in women of reproductive age. Ovarian dysfunction including abnormal steroid hormone synthesis and follicular arrest play a vital role in PCOS pathogenesis. Hyperandrogenemia is one of the important characteristics of PCOS. However, the mechanism of regulation and interaction between hyperandrogenism and ovulation abnormalities are not clear. To investigate androgen-related metabolic state in granulosa cells of PCOS patients, we identified the transcriptome characteristics of PCOS granulosa cells by RNA-seq. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of differentially expressed genes (DEGs) revealed that genes enriched in lipid metabolism pathway, fatty acid biosynthetic process and ovarian steroidogenesis pathway were abnormally expressed in PCOS granulosa cells in comparison with that in control. There are close interactions among these three pathways as identified by analysis of the protein-protein interaction (PPI) network of DEGs. Furthermore, in vitro mouse follicle culture system was established to explore the effect of high androgen and its related metabolic dysfunction on follicular growth and ovulation. RT-qPCR results showed that follicles cultured with dehydroepiandrosterone (DHEA) exhibited decreased expression levels of cumulus expansion-related genes (Has2, Ptx3, Tnfaip6 and Adamts1) and oocyte maturation-related genes (Gdf9 and Bmp15), which may be caused by impaired steroid hormone synthesis and lipid metabolism, thus inhibited follicular development and ovulation. Furthermore, the inhibition effect of DHEA on follicle development and ovulation was ameliorated by flutamide, an androgen receptor (AR) antagonist, suggesting the involvement of AR signaling. In summary, our study offers new insights into understanding the role of androgen excess induced granulosa cell metabolic disorder in ovarian dysfunction of PCOS patients.

Oocyte quality following in vitro follicle development†

In vitro follicle development (IVFD) is an adequate model to obtain basic knowledge of folliculogenesis and provides a tool for ovarian toxicity screening. IVFD yielding competent oocytes may also offer an option for fertility and species preservation. To promote follicle growth and oocyte maturation in vitro, various culture systems are utilized for IVFD in rodents, domestic animals, wild animals, nonhuman primates, and humans. Follicle culture conditions have been improved by optimizing gonadotropin levels, regulatory factors, nutrient supplements, oxygen concentration, and culture matrices. This review summarizes quality assessment of oocytes generated from in vitro-developed antral follicles from the preantral stage, including oocyte epigenetic and genetic profile, cytoplasmic and nuclear maturation, preimplantation embryonic development following in vitro fertilization, as well as pregnancy and live offspring after embryo transfer. The limitations of oocyte quality evaluation following IVFD and the gaps in our knowledge of IVFD to support proper oocyte development are also discussed. The information may advance our understanding of the requirements for IVFD, with a goal of producing competent oocytes with genetic integrity to sustain embryonic development resulting in healthy offspring.

The cell biology of fertilization: Gamete attachment and fusion

Fertilization is defined as the union of two gametes. During fertilization, sperm and egg fuse to form a diploid zygote to initiate prenatal development. In mammals, fertilization involves multiple ordered steps, including the acrosome reaction, zona pellucida penetration, sperm-egg attachment, and membrane fusion. Given the success of in vitro fertilization, one would think that the mechanisms of fertilization are understood; however, the precise details for many of the steps in fertilization remain a mystery. Recent studies using genetic knockout mouse models and structural biology are providing valuable insight into the molecular basis of sperm-egg attachment and fusion. Here, we review the cell biology of fertilization, specifically summarizing data from recent structural and functional studies that provide insights into the interactions involved in human gamete attachment and fusion.