Assessment of active translation in cumulus-enclosed and denuded oocytes during standard in vitro maturation and early embryo development

STUDY QUESTION

Which actively translated maternal transcripts are differentially regulated between clinically relevant in vitro and in vivo maturation (IVM) conditions in mouse oocytes and zygotes?

SUMMARY ANSWER

Our findings uncovered significant differences in the global transcriptome as well as alterations in the translation of specific transcripts encoding components of energy production, cell cycle regulation, and protein synthesis in oocytes and RNA metabolism in zygotes.

WHAT IS KNOWN ALREADY

Properly regulated translation of stored maternal transcripts is a crucial factor for successful development of oocytes and early embryos, particularly due to the transcriptionally silent phase of meiosis.

STUDY DESIGN, SIZE, DURATION

This is a basic science study utilizing an ICR mouse model, best suited for studying in vivo maturation. In the treatment group, fully grown germinal vesicle oocytes from stimulated ovaries were in vitro matured to the metaphase II (MII) stage either as denuded without gonadotropins (IVM DO), or as cumulus-oocyte complexes (IVM COC) in the presence of 0.075 IU/ml recombinant FSH (rFSH) and 0.075 IU/ml recombinant hCG (rhCG). To account for changes in developmental competence, IVM COC from non-stimulated ovaries (IVM COC-) were included. In vivo matured MII oocytes (IVO) from stimulated ovaries were used as a control after ovulation triggering with rhCG. To simulate standard IVM conditions, we supplemented media with amino acids, vitamins, and bovine serum albumin. Accordingly, in vitro pronuclear zygotes (IMZ) were generated by IVF from IVM DO, and were compared to in vivo pronuclear zygotes (IVZ). All experiments were performed in quadruplicates with samples collected for both polyribosome fractionation and total transcriptome analysis. Samples were collected over three consecutive months.

PARTICIPANTS/MATERIALS, SETTING, METHODS

All ICR mice were bred under legal permission for animal experimentation (no. MZE-24154/2021-18134) obtained from the Ministry of Agriculture of the Czech Republic. Actively translated (polyribosome occupied) maternal transcripts were detected in in vitro and in vivo matured mouse oocytes and zygotes by density gradient ultracentrifugation, followed by RNA isolation and high-throughput RNA sequencing. Bioinformatic analysis was performed and subsequent data validation was done by western blotting, radioactive isotope, and mitotracker dye labelling.

MAIN RESULTS AND THE ROLE OF CHANCE

Gene expression analysis of acquired polysome-derived high-throughput RNA sequencing data revealed significant changes (RPKM ≥ 0.2; P ≤ 0.005) in translation between in vitro and in vivo matured oocytes and respectively produced pronuclear zygotes. Surprisingly, the comparison between IVM DO and IVM COC RNA-seq data of both fractionated and total transcriptome showed very few transcripts with more than a 2-fold difference. Data validation by radioactive isotope labelling revealed a decrease in global translation bof20% in IVM DO and COC samples in comparison to IVO samples. Moreover, IVM conditions compromised oocyte energy metabolism, which was demonstrated by both changes in polysome recruitment of each of 13 mt-protein-coding transcripts as well as by validation using mitotracker red staining.

LARGE SCALE DATA

The data discussed in this publication have been deposited in NCBI's Gene Expression Omnibus and are accessible through GEO Series accession number GSE241633 (https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE241633).

LIMITATIONS, REASONS FOR CAUTION

It is extremely complicated to achieve in vivo consistency in animal model systems such as porcine or bovine. To achieve a high reproducibility of in vivo stimulations, the ICR mouse model was selected. However, careful interpretation of our findings with regard to assisted reproductive techniques has to be made by taking into consideration intra-species differences between the mouse model and humans. Also, the sole effect of the cumulus cells' contribution could not be adequately addressed by comparing IVM COC and IVM DO, because the IVM DO were matured without gonadotropin supplementation.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings confirmed the inferiority of standard IVM technology compared with the in vivo approach. It also pointed at compromised biological processes employed in the critical translational regulation of in vitro matured MII oocytes and pronuclear zygotes. By highlighting the importance of proper translational regulation during in vitro oocyte maturation, this study should prompt further clinical investigations in the context of translation.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the Czech Grant Agency (22-27301S), Charles University Grant Agency (372621), Ministry of Education, Youth and Sports (EXCELLENCE CZ.02.1.01/0.0/0.0/15_003/0000460 OP RDE), and Institutional Research Concept RVO67985904. No competing interest is declared.

Global crotonylome identifies EP300-regulated ANXA2 crotonylation in cumulus cells as a regulator of oocyte maturation

Lysine crotonylation (Kcr), a newly discovered post-translational modification, played a crucial role in physiology and disease progression. However, the roles of crotonylation in oocyte meiotic resumption remain elusive. As abnormal cumulus cell development will cause oocyte maturation arrest and female infertility, we report that cumulus cells surrounding human meiotic arrested oocytes showed significantly lower crotonylation, which was associated with decreased EP300 expression and blocked cumulus cell expansion. In cultured human cumulus cells, exogenous crotonylation or EP300 activator promoted cell proliferation and reduced cell apoptosis, whereas EP300 knockdown induced the opposite effect. Transcriptome profiling analysis in human cumulus cells indicated that functions of crotonylation were associated with activation of epidermal growth factor receptor (EGFR) pathway. Importantly, we characterized the Kcr proteomics landscape in cumulus cells by LC-MS/MS analysis, and identified that annexin A2 (ANXA2) was crotonylated in cumulus cells in an EP300-dependent manner. Crotonylation of ANXA2 enhanced the ANXA2-EGFR binding, and then activated the EGFR pathway to affect cumulus cell proliferation and apoptosis. Using mouse oocytes IVM model and EP300 knockout mice, we further confirmed that crotonylation alteration in cumulus cells affected the oocyte maturation. Together, our results indicated that EP300-mediated crotonylation is important for cumulus cells functions and oocyte maturation.

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.

Neurotrophin-4 promotes in vitro development and maturation of human secondary follicles yielding metaphase II oocytes and successful blastocyst formation

STUDY QUESTION

Does a matrix-free culture system supplemented with neurotrophic factor 4 (NT4) improve human in vitro follicular development and meiotic maturation, ultimately resulting in fertilizable oocytes?

SUMMARY ANSWER

NT4 supplementation of in vitro culture significantly enhances the growth, steroid hormone production, and maturity potential of human secondary follicles derived from fresh ovarian medulla (from post- and pre-pubertal patients), thereby yielding fertilizable oocytes.

WHAT IS KNOWN ALREADY

Reconstituting folliculogenesis in vitro is of paramount importance in the realms of fertility preservation, reproductive biology research, and reproductive toxicity assessments. However, the efficiency of in vitro culture systems remains suboptimal, as the attainment of fertilizable oocytes from in vitro growth (IVG) of human follicles remains unachieved, with the data being particularly scant regarding follicles from prepubertal girls. We have previously found that mouse oocytes from secondary follicles derived from IVG are deficient in neuroendocrine regulation. NT4 and its corresponding receptor have been identified in human follicles. Significantly, the addition of NT4 during the IVG process markedly enhances both follicle growth and oocyte maturation rates in mice.

STUDY DESIGN SIZE DURATION

Fresh medulla tissue obtained during tissue preparation for ovarian tissue cryopreservation (OTC) were collected from 10 patients aged from 6 to 21 years old, all of whom had undergone unilateral oophorectomy as a means of fertility preservation. Isolated secondary follicles were individually cultured in vitro with or without NT4 in a matrix-free system.

PARTICIPANTS/MATERIALS SETTING METHODS

Secondary follicles, extracted via enzymatic digestion and mechanical disruption from each patient, were randomly allocated to either a control group or an NT4-supplemented group (100 ng/ml), followed by individual culture on an ultra-low attachment plate. Follicle growth and viability were assessed by microscopy. Levels of anti-Müllerian hormone (AMH), estradiol, and progesterone in the medium were quantified. An oocyte-specific marker was identified using confocal fluorescence microscopy following DEAD box polypeptide 4 (DDX4) staining. The competence of individual oocytes for maturation and fertilization were assessed after IVM and ICSI with donated sperm samples.

MAIN RESULTS AND THE ROLE OF CHANCE

Overall, isolated follicles from both groups survived up to 6 weeks with increasing diameters over the duration (P < 0.05), reaching terminal diameters of almost 1 mm with confirmed steroidogenesis and expression of oocyte marker (DDX4), and producing morphologically normal MII oocytes. When compared with the control group, the NT4 group had a similar initial follicular diameter (206 ± 61.3 vs 184 ± 93.4 μm) but exhibited a significant increase in follicular diameter from the ninth day of culture onwards (P < 0.05). From Week 3, estradiol and progesterone production were significantly increased in the NT4 group, while no significant difference was observed in AMH production between groups. The proportion of 'fast-growth' follicles in the NT4 group was significantly higher than that in the control group (13/23 vs 6/24, P < 0.05). An increased efficiency of MII oocyte maturation per live follicle in the NT4 group was also observed (control group vs NT4 group, 4/24 vs 10/23, P < 0.05). It is noteworthy that an MII oocyte obtained from the control group exhibited abnormal fertilization after ICSI. In contrast, an MII oocyte acquired from the NT4 group progressed to the blastocyst stage and showed potential for transfer.

LARGE SCALE DATA

N/A.

LIMITATIONS REASONS FOR CAUTION

The cohort examined in this study was all patients diagnosed with beta-thalassemia major. Whether this culture system is effective for patients with other diseases remains unknown. Since the chosen dose of NT4 was established based on dose finding in mice, the optimal dose for use in a human IVG system needs further confirmation. The oocytes and embryos procured from this study have not been quantified for ploidy status or epigenetic signatures.

WIDER IMPLICATIONS OF THE FINDINGS

Fresh medulla tissue obtained during tissue preparation for OTC may serve as a precious source of fertilizable oocytes for female fertility preservation, even for pre-pubertal girls, without the threat of tumour reintroduction. After further characterization and optimization of the system, this culture system holds the potential to provide a powerful future research tool, for the comprehensive exploration of human follicular development mechanisms and for conducting reproductive toxicity evaluations.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by the National Key R&D Program of China (grant number 2022YFC2703000) and National Natural Science Foundation of China (grant numbers 82271651 and 81871214). The medium used in human follicle in vitro culture in this study has been applied for a national invention patent in China (No. 202211330660.7). The inventors of the patent, in order, are: Y.G., C.F., and X.L.

Oocyte vitrification for fertility preservation is an evolving practice requiring a new mindset: societal, technical, clinical, and basic science-driven evolutions

Infertility is a condition with profound social implications. Indeed, it is not surprising that evolutions in both medicine and society affect the way in vitro fertilization is practiced. The keywords in modern medicine are the four principles, which implicitly involve a constant update of our knowledge and our technologies to fulfill the "prediction" and "personalization" tasks, and a continuous reshaping of our mindset in view of all relevant societal changes to fulfill the "prevention" and "participation" tasks. A worldwide aging population whose life priorities are changing requires that we invest in fertility education, spreading actionable information to allow women and men to make meaningful reproductive choices. Fertility preservation for both medical and nonmedical reasons is still very much overlooked in many countries worldwide, demanding a comprehensive update of our approach, starting from academia and in vitro fertilization laboratories, passing through medical offices, and reaching out to social media. Reproduction medicine should evolve from being a clinical practice to treat a condition to being a holistic approach to guarantee patients' reproductive health and well-being. Oocyte vitrification for fertility preservation is the perfect use case for this transition. This tool is acquiring a new identity to comply with novel indications and social needs, persisting technical challenges, brand-new clinical technologies, and novel revolutions coming from academia. This "views and reviews" piece aims at outlining the advancement of oocyte vitrification from all these tightly connected perspectives.

A fresh start for IVM: capacitating the oocyte for development using pre-IVM

BACKGROUND

While oocyte IVM is practiced sporadically it has not achieved widespread clinical practice globally. However, recently there have been some seminal advances in our understanding of basic aspects of oocyte biology and ovulation from animal studies that have led to novel approaches to IVM. A significant recent advance in IVM technology is the use of biphasic IVM approaches. These involve the collection of immature oocytes from small antral follicles from minimally stimulated patients/animals (without hCG-priming) and an ∼24 h pre-culture of oocytes in an advanced culture system ('pre-IVM') prior to IVM, followed by routine IVF procedures. If safe and efficacious, this novel procedure may stand to make a significant impact on human ART practices.

OBJECTIVE AND RATIONALE

The objectives of this review are to examine the major scientific advances in ovarian biology with a unique focus on the development of pre-IVM methodologies, to provide an insight into biphasic IVM procedures, and to report on outcomes from animal and clinical human data, including safety data. The potential future impact of biphasic IVM on ART practice is discussed.

SEARCH METHODS

Peer review original and review articles were selected from PubMed and Web of Science searches for this narrative review. Searches were performed using the following keywords: oocyte IVM, pre-IVM, biphasic IVM, CAPA-IVM, hCG-triggered/primed IVM, natural cycle IVF/M, ex-vivo IVM, OTO-IVM, oocyte maturation, meiotic competence, oocyte developmental competence, oocyte capacitation, follicle size, cumulus cell (CC), granulosa cell, COC, gap-junction communication, trans-zonal process, cAMP and IVM, cGMP and IVM, CNP and IVM, EGF-like peptide and IVM, minimal stimulation ART, PCOS.

OUTCOMES

Minimizing gonadotrophin use means IVM oocytes will be collected from small antral (pre-dominant) follicles containing oocytes that are still developing. Standard IVM yields suboptimal clinical outcomes using such oocytes, whereas pre-IVM aims to continue the oocyte's development ex vivo, prior to IVM. Pre-IVM achieves this by eliciting profound cellular changes in the oocyte's CCs, which continue to meet the oocyte's developmental needs during the pre-IVM phase. The literature contains 25 years of animal research on various pre-IVM and biphasic IVM procedures, which serves as a large knowledge base for new approaches to human IVM. A pre-IVM procedure based on c-type natriuretic peptide (named 'capacitation-IVM' (CAPA-IVM)) has undergone pre-clinical human safety and efficacy trials and its adoption into clinical practice resulted in healthy live birth rates not different from conventional IVF.

WIDER IMPLICATIONS

Over many decades, improvements in clinical IVM have been gradual and incremental but there has likely been a turning of the tide in the past few years, with landmark discoveries in animal oocyte biology finally making their way into clinical practice leading to improved outcomes for patients. Demonstration of favorable clinical results with CAPA-IVM, as the first clinically tested biphasic IVM system, has led to renewed interest in IVM as an alternative, low-intervention, low-cost, safe, patient-friendly ART approach, and especially for patients with PCOS. The same new approach is being used as part of fertility preservation in patients with cancer and holds promise for social oocyte freezing.

Transcriptome analyses reveal transcriptional profiles of horse oocytes before and after in vitro maturation

Oocyte in vitro maturation is necessary for the study and application of animal-assisted reproduction technology in animal reproduction and breeding. The comprehensive transcriptional profile of equine oocyte maturated in vitro has not been fully mined yet, which makes many key transcriptional events still unidentified. Here, Smart-seq2 was performed to analyse the gene expression pattern and the underlying regulatory mechanism of horse germinal vesicle (GV) and in vitro metaphase II (MII) oocytes. The results showed that 6402 genes (2640 up-regulated and 3762 down-regulated in MII samples compared to GV) and 4021 lncRNA transcripts (1210 up-regulated and 2811 down-regulated in MII samples compared to GV) were differentially expressed in GV and MII oocytes. Further, GO and KEGG analysis found that differentially expressed mRNAs and lncRNAs were mainly enriched in the pathways related to energy and lipid metabolism. In addition, LGALS3 was found a key gene in mediating the regulation of oocyte meiosis recovery and fertilization ability. This study provides novel knowledge about gene expression and energy metabolism during equine oocyte maturation and a reference for the further study and application of assisted reproductive technology in horse reproduction and breeding.

Developmental Hurdles That Can Compromise Pregnancy during the First Month of Gestation in Cattle

Several key developmental events are associated with early embryonic pregnancy losses in beef and dairy cows. These developmental problems are observed at a greater frequency in pregnancies generated from in-vitro-produced bovine embryos. This review describes critical problems that arise during oocyte maturation, fertilization, early embryonic development, compaction and blastulation, embryonic cell lineage specification, elongation, gastrulation, and placentation. Additionally, discussed are potential remediation strategies, but unfortunately, corrective actions are not available for several of the problems being discussed. Further research is needed to produce bovine embryos that have a greater likelihood of surviving to term.

Combination of CNP, MT and FLI during IVM Significantly Improved the Quality and Development Abilities of Bovine Oocytes and IVF-Derived Embryos

Oocyte maturation is a critical step in the completion of female gametogenesis in the ovary; thus, for subsequent fertilization and embryogenesis. Vitrification of embryo also has been shown to be closely associated with oocyte maturation. To improve the quality and developmental potential of bovine oocytes derived from in vitro maturation (IVM), Pre-IVM with C-type natriuretic peptide (CNP), melatonin (MT) and in combination, IGF1, FGF2, LIF (FLI) were supplemented in the IVM medium. In this current study, we cultured bovine oocytes in Pre-IVM with CNP for 6 h before transferring them to the IVM medium supplemented with MT and FLI. The developmental potential of bovine oocytes was then investigated by measuring the reactive oxygen species (ROS), the intracellular glutathione (GSH) and ATP levels, the transzonal projections (TZP), the mitochondrial membrane potential (ΔΨm), cacline-AM, and the expression of related genes (cumulus cells (CCs), oocytes, blastocysts). The results revealed that oocytes treated with a combination of CNP, MT, and FLI had dramatically improved the percentage of oocytes developed to blastocyst, ATP content, GSH levels, TZP intensity, the ΔΨm, cacline-AM fluorescence intensity, and considerably reduced ROS levels of oocytes. Furthermore, the survival rate and the hatched rate after vitrification of the CNP+MT+FLI group were significantly higher than those other groups. Thus, we speculated that CNP+MT+FLI increases the IVM of bovine oocytes. In conclusion, our findings deepen our understanding and provide new perspectives on targeting the combination of CNP, MT and FLI to enhance the quality and developmental potential of bovine oocytes.

Progress toward species-tailored prematuration approaches in carnivores

In the past four decades, the bovine model has been highly informative and inspiring to assisted reproductive technologies (ART) in other species. Most of the recent advances in ART have come from studies in cattle, particularly those unveiling the importance of several processes that must be recapitulated in vitro to ensure the proper development of the oocyte. The maintenance of structural and functional communications between the cumulus cells and the oocyte and a well-orchestrated chromatin remodeling with the gradual silencing of transcriptional activity represent essential processes for the progressive acquisition of oocyte developmental competence. These markers are now considered the milestones of physiological approaches to increase the efficiency of reproductive technologies. Different in vitro approaches have been proposed. In particular, the so-called "pre-IVM" or "prematuration" is a culture step performed before in vitro maturation (IVM) to support the completion of the oocyte differentiation process. Although these attempts only partially improved the embryo quality and yield, they currently represent a proof of principle that oocytes retrieved from an ovary or an ovarian batch shouldn't be treated as a whole and that tailored approaches can be developed for culturing competent oocytes in several species, including humans. An advancement in ART's efficiency would be desirable in carnivores, where the success is still limited. Since the progress in reproductive medicine has often come from comparative studies, this review highlights aspects that have been critical in other species and how they may be extended to carnivores.

Optical imaging detects metabolic signatures associated with oocyte quality

Oocyte developmental potential is intimately linked to metabolism. Existing approaches to measure metabolism in the cumulus oocyte complex (COC) do not provide information on the separate cumulus and oocyte compartments. Development of an assay that achieves this may lead to an accurate diagnostic for oocyte quality. Optical imaging of the autofluorescent cofactors NAD(P)H and FAD provides a spatially resolved indicator of metabolism via the optical redox ratio ($\mathrm{FAD}/\left[\mathrm{NAD}\left(\mathrm{P}\right)\mathrm{H}+\mathrm{FAD}\right]$). This may provide an assessment of oocyte quality. Here, we determined whether the optical redox ratio is a robust methodology for measuring metabolism in the cumulus and oocyte compartments compared with oxygen consumption in the whole COC. We also determined whether optical imaging could detect metabolic differences associated with poor oocyte quality (etomoxir-treated). We used confocal microscopy to measure NAD(P)H and FAD, and extracellular flux to measure oxygen consumption. We found that the optical redox ratio was an accurate reflection of metabolism in the oocyte compartment when compared with oxygen consumption (whole COC). Etomoxir-treated COCs showed significantly lower levels of NAD(P)H and FAD compared to control. While confocal imaging demonstrated the premise, we validated this approach using hyperspectral imaging, which is clinically compatible due to its low energy dose. This confirmed lower NAD(P)H and FAD in etomoxir-treated COCs. When comparing imaged vs non-imaged COCs, subsequent preimplantation development and post-transfer viability were comparable. Collectively, these results demonstrate that label-free optical imaging of metabolic cofactors is a safe and sensitive assay for measuring metabolism and has potential to assess oocyte developmental competence.

Immune Regulation in Polycystic Ovary Syndrome

A polycystic ovarian syndrome (PCOS) is the most common endocrine disorder affecting females . Furthermore, it is a heterogeneous disease with a variety of etiologies and outcomes. Patients frequently complain about infertility, irregular menstruation, acne, seborrheic dermatitis, hirsutism, and obesity. PCOS can be caused by hypothalamic-pituitary-ovarian axis dysfunction, heredity, or metabolic abnormalities. PCOS is characterized by chronic low-level inflammation, which includes an imbalance in pro-inflammatory factor secretion, endothelial cell dysfunction, and leukocytosis. PCOS is also distinguished by hormonal and immune dysregulation. During PCOS, immune cells and immune regulatory molecules play critical roles in maintaining metabolic homeostasis and regulating immune responses. Because of oligo/anovulation, patients with PCOS have low progesterone levels. Therefore, low progesterone levels in PCOS overstimulate the immune system, causing it to produce more estrogen, which leads to a variety of autoantibodies. This review aims to summarize the immune regulation involved in the pathogenesis of PCOS and pave the way for the development of better PCOS treatment options in the near future.

Factors Influencing the In Vitro Maturation (IVM) of Human Oocyte

In vitro maturation (IVM) of oocytes is a promising assisted reproductive technology (ART) deemed as a simple and safe procedure. It is mainly used in patients with impaired oocyte maturation and in fertility preservation for women facing the risk of losing fertility. However, to date, it is still not widely used in clinical practice because of its underperformance. The influencing factors, such as biphasic IVM system, culture medium, and the supplementation, have a marked effect on the outcomes of oocyte IVM. However, the role of different culture media, supplements, and follicular priming regimens in oocyte IVM have yet to be fully clarified and deserve further investigation.

Nurturing the egg: the essential connection between cumulus cells and the oocyte

The determinants of oocyte quality remain uncertain. Under suitable conditions, which have yet to be defined, the gamete grows and acquires the competence to resume meiosis, be fertilised and undergo embryonic development at least beyond genome activation, after which the blastomere is autonomous enough to adapt to the specificity of its environment. This review describes the central role played by the oocyte in reproductive success and how communication between cumulus cells and the oocyte are essential to proper oogenesis and the quality of the resulting gamete. While most attempts to improve oocyte quality have been directed at gonadotrophin-based systemic endocrine signalling, it is proposed that parallel control of fertility may act locally within ovarian follicles through intimate cooperation between somatic cells and the oocyte via the network of transzonal projections. This intercellular communication may prove to be more sensitive to environmental conditions than systemic endocrine signalling, which is essential for many non-reproductive tissues.

cAMP Modulators before In Vitro Maturation Decrease DNA Damage and Boost Developmental Potential of Sheep Oocytes

Simple Summary

Oocyte in vitro maturation has massive potential for the generation of great numbers of embryos for research and for the application of assisted reproductive technologies, such as in vitro embryo production. However, the developmental ability of in vitro matured oocytes is lower than those matured in vivo. Here, incubating the oocytes with cAMP modulating agents for two hours before in vitro maturation decreased oocyte DNA damage and increased the number of embryos generated after in vitro fertilization. The present findings could help to develop new methods to improve the quality and developmental potential of in vitro matured oocytes.

Abstract

To date, the underlying mechanisms by which cAMP modulators act during in vitro maturation to improve oocyte developmental competence are poorly understood. Here, we sought to fill this knowledge gap by evaluating the use of phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) and adenylyl cyclase activator forskolin during a culture period of 2 h before in vitro maturation (pre-IVM) on the nuclear and cytoplasmic maturation features in essential organelles, cumulus cells activity, and in vitro developmental potential of sheep oocytes. Results showed that pre-IVM treatment significantly decreased (p < 0.05) the DNA damage of mature oocytes (pre-IVM = 2.08% ± 3.51% vs. control = 20.58% ± 3.51%) and increased (p ≤ 0.05) expanded blastocyst rates compared to the control (from the total of oocytes: pre-IVM = 23.89% ± 1.47% vs. control = 18.22% ± 1.47%, and from the cleaved embryos: pre-IVM = 45.16% ± 1.73% vs. control = 32.88% ± 1.73%). Considering that oocytes are highly vulnerable to the accumulation of DNA damage because of exposure to in vitro culture conditions, our results suggest that the modulation of intra-oocyte cAMP levels with forskolin and IBMX before IVM might afford oocytes a more effective DNA repair mechanism to overcome damage obstacles and ultimately improve developmental competence. This previously unappreciated action of cAMP modulators could help to develop improved methods for assisted reproduction technologies in animal and clinical research.

Simulated physiological oocyte maturation (SPOM) improves developmental competence of in vitro produced goat embryos

The effect of simulated physiological oocyte maturation on the developmental competence, reactive oxygen species production and apoptosis rate of in vitro produced goat embryos were studied in the present experiment. Oocytes and spermatozoa were recovered from ovaries and epididymis, respectively, procured from a local small animal abattoir. The oocytes aspirated from the ovaries were allocated into two groups, control (subjected to routine in vitro maturation, fertilization and culture) and simulated physiological oocyte maturation (SPOM) group (subjected to prematuration, followed by routine in vitro maturation, fertilization and culture). The SPOM group showed a significantly (p < 0.05) higher maturation and blastocyst rates (90.60 ± 0.46% and 29.09 ± 2.59%, respectively) as compared to the control group (85.29 ± 0.98% and 24.09 ± 1.08%). The intensity of reactive oxygen species of the embryos in the control group (14.98 ± 0.83 pixels/embryo) was significantly (p < 0.05) higher than the SPOM group (9.60 ± 0.76 pixels/embryo). The apoptosis rate was also significantly (p < 0.05) higher in the embryos of the control group (9.18 ± 1.07%) as compared to the SPOM group (5.71 ± 0.90%). In conclusion, the simulated physiological oocyte maturation system significantly increases the developmental competence of the oocytes and decreases the intensity of reactive oxygen species and embryonic apoptosis in abattoir derived goat embryos.

Establishment of lncRNA-mRNA network in bovine oocyte between germinal vesicle and metaphase II stage

Long non-coding RNA (lncRNA), a type of non-protein coding transcripts with lengths exceeding 200 nucleotides, is reported to be widely involved in many cellular and developmental processes. However, few roles of lncRNA in oocyte development have been defined. In this study, to uncover the effect of lncRNA during oocyte maturation, bovine germinal vesicle (GV) and in vitro matured metaphase II (MII) oocytes underwent RNA sequencing. Results revealed a wealth of candidate lncRNAs, which might participate in the biological processes of stage-specific oocytes. Furthermore, their trans- and cis-regulatory effects were investigated in-depth by using bioinformatic software. Functional enrichment analysis of target genes showed that these lncRNAs were likely involved in the regulation of many key signaling pathways during bovine oocyte maturation from GV to MII stage, as well as multiple lncRNA-mRNA networks. One novel lncRNA (MSTRG.19140) was particularly interesting, as it appeared to mediate the regulation of oocyte meiotic resumption, progesterone-mediated oocyte maturation, and cell cycle. Therefore, this study enhanced insights into the regulation of molecular mechanisms of bovine oocyte maturation from a lncRNA-mRNA network perspective.

Recreating the Follicular Environment: A Customized Approach for In Vitro Culture of Bovine Oocytes Based on the Origin and Differentiation State

The mammalian ovary is a large source of oocytes organized into follicles at various stages of folliculogenesis. However, only a limited number of them can be used for in vitro embryo production (IVEP), while most have yet to complete growth and development to attain full meiotic and embryonic developmental competence. While the in vitro growth of primordial follicles in the ovarian cortex has the potential to produce mature oocytes, it is still at an experimental stage. The population of early antral follicles (EAFs), instead, may represent a reserve of oocytes close to completing the growth phase, which might be more easily exploited in vitro and could increase the number of female gametes dedicated to IVEP.Here we present in vitro culture strategies that have been developed utilizing physiological parameters to support the specific needs of oocytes at distinct stages of differentiation, in order to expand the source of female gametes for IVEP by maximizing the attainment of fertilizable oocytes. Furthermore, these culture systems provide powerful tools to dissect the molecular processes that direct the final differentiation of the mammalian oocyte.

Molecular characteristics of oocytes and somatic cells of follicles at different sizes that influence in vitro oocyte maturation and embryo production

During the last 10 to 15 yr, in vitro research to predict antral follicle growth and oocyte maturation has delivered interesting advances in the knowledge of processes regulating follicle growth and developmental competence of oocytes. This review discusses the contribution of cumulus and mural granulosa cells in the process of oocyte maturation and cumulus expansion in cumulus-oocyte complexes (COCs) from follicles of different sizes and shows that differences in gene expression in oocytes, granulosa, and theca cells of small and large follicles impact the success of in vitro blastocyst development. In addition, the molecular mechanisms by which COC metabolism and antioxidant defense provide oocyte competence are highlighted. Furthermore, new insights and perspectives on molecular and cellular regulation of in vitro oocyte maturation are emphasized.

Metabolic co-dependence of the oocyte and cumulus cells: essential role in determining oocyte developmental competence

BACKGROUND

Within the antral follicle, the oocyte is reliant on metabolic support from its surrounding somatic cells. Metabolism plays a critical role in oocyte developmental competence (oocyte quality). In the last decade, there has been significant progress in understanding the metabolism of the cumulus-oocyte complex (COC) during its final stages of growth and maturation in the follicle. Certain metabolic conditions (e.g. obesity) or ART (e.g. IVM) perturb COC metabolism, providing insights into metabolic regulation of oocyte quality.

OBJECTIVE AND RATIONALE

This review provides an update on the progress made in our understanding of COC metabolism, and the metabolic conditions that influence both meiotic and developmental competence of the oocyte.

SEARCH METHODS

The PubMed database was used to search for peer-reviewed original and review articles. Searches were performed adopting the main terms 'oocyte metabolism', 'cumulus cell metabolism', 'oocyte maturation', 'oocyte mitochondria', 'oocyte metabolism', 'oocyte developmental competence' and 'oocyte IVM'.

OUTCOMES

Metabolism is a major determinant of oocyte quality. Glucose is an essential requirement for both meiotic and cytoplasmic maturation of the COC. Glucose is the driver of cumulus cell metabolism and is essential for energy production, extracellular matrix formation and supply of pyruvate to the oocyte for ATP production. Mitochondria are the primary source of ATP production within the oocyte. Recent advances in real-time live cell imaging reveal dynamic fluctuations in ATP demand throughout oocyte maturation. Cumulus cells have been shown to play a central role in maintaining adequate oocyte ATP levels by providing metabolic support through gap junctional communication. New insights have highlighted the importance of oocyte lipid metabolism for oocyte oxidative phosphorylation for ATP production, meiotic progression and developmental competence. Within the last decade, several new strategies for improving the developmental competence of oocytes undergoing IVM have emerged, including modulation of cyclic nucleotides, the addition of precursors for the antioxidant glutathione or endogenous maturation mediators such as epidermal growth factor-like peptides and growth differentiation factor 9/bone morphogenetic protein 15. These IVM additives positively alter COC metabolic endpoints commonly associated with oocyte competence. There remain significant challenges in the study of COC metabolism. Owing to the paucity in non-invasive or in situ techniques to assess metabolism, most work to date has used in vitro or ex vivo models. Additionally, the difficulty of measuring oocyte and cumulus cell metabolism separately while still in a complex has led to the frequent use of denuded oocytes, the results from which should be interpreted with caution since the oocyte and cumulus cell compartments are metabolically interdependent, and oocytes do not naturally exist in a naked state until after fertilization. There are emerging tools, including live fluorescence imaging and photonics probes, which may provide ways to measure the dynamic nature of metabolism in a single oocyte, potentially while in situ.

WIDER IMPLICATIONS

There is an association between oocyte metabolism and oocyte developmental competence. Advancing our understanding of basic cellular and biochemical mechanisms regulating oocyte metabolism may identify new avenues to augment oocyte quality and assess developmental potential in assisted reproduction.

Prokineticin 1 is a new biomarker of human oocyte competence: expression and hormonal regulation throughout late folliculogenesis

CONTEXT

Prokineticin 1 (PROK1) quantification in global follicular fluid (FF) has been recently reported as a predictive biomarker of in vitro fertilization (IVF) outcome. It is now necessary to evaluate its clinical usefulness in individual follicles.

OBJECTIVES

To evaluate the clinical value of PROK1 secretion in individual FF to predict oocyte competence. To determine the impact of follicular size, oocyte maturity, and gonadotropin treatments on PROK1 secretion.

DESIGN AND SETTING

Prospective cohort study from May 2015 to May 2017 at the University Hospital of Grenoble.

PATIENTS

A total of 69 infertile couples underwent IVF.

INTERVENTION(S)

Collection of 298 individual FF from 44 women undergoing IVF; 52 individual cumulus cell (CC) samples and 15 CC primary cultures from 25 women undergoing IVF-intracytoplasmic sperm injection (ICSI).

MAIN OUTCOME MEASURE(S)

Oocyte competence was defined as the ability to sustain embryo development to the blastocyst stage. Follicular size was measured by 2D-sonography. PROK1 concentration was quantified by ELISA assay.

RESULTS

PROK1 concentration was correlated to follicular size (r = 0.85, P = 2.2 × 10-16). Normalized PROK1 concentration in FF was predictive of subsequent oocyte competence (AUROC curve = 0.76 [95% CI, 0.69-0.83]; P = 1.7 × 10-9), irrespectively of day-2 embryo morphokinetic parameters. The expression and secretion of PROK1 were increased in FF and CC of mature oocytes (P < 0.01). Follicle Stimulating Hormone and hCG up-regulated PROK1 secretion in CC primary cultures (P < 0.01; P < 0.05), probably through the cAMP pathway (P < 0.01).

CONCLUSIONS

PROK1 quantification in individual FF could constitute a new predictive biomarker of oocyte competence in addition with embryo morphokinetic parameters.

TRIAL REGISTRATION NUMBER

none.

A prematuration approach to equine IVM: considering cumulus morphology, seasonality, follicle of origin, gap junction coupling and large-scale chromatin configuration in the germinal vesicle

Several studies report that a two-step culture where mammalian oocytes are first kept under meiosis-arresting conditions (prematuration) followed by IVM is beneficial to embryo development. The most promising results were obtained by stratifying the oocyte population using morphological criteria and allocating them to different culture conditions to best meet their metabolic needs. In this study, horse oocytes were characterised to identify subpopulations that may benefit from prematuration. We investigated gap-junction (GJ) coupling, large-scale chromatin configuration and meiotic competence in compact and expanded cumulus-oocyte complexes (COCs) according to follicle size (<1, 1-2, >2cm) and season. Then we tested the effect of cilostamide-based prematuration in compact COCs collected from follicles <1 and 1-2cm in diameter on embryo development. Meiotic competence was not affected by prematuration, whereas COCs from follicles 1-2cm in diameter yielded embryos with a higher number of cells per blastocyst than oocytes that underwent direct IVM (P<0.01, unpaired Mann-Whitney test), suggesting improved developmental competence. Oocytes collected from follicles <1cm in diameter were not affected by prematuration. This study represents an extensive characterisation of the functional properties of immature horse oocytes and is the first report of the effects of cilostamide-based prematuration in horse oocyte IVM on embryo development.

Pannexin 1 inhibition delays maturation and improves development of Bos taurus oocytes

Background

Intercellular exchange between the oocyte and its surrounding cells within the follicular environment is critical for oocyte maturation and subsequent development. In vertebrates this exchange is facilitated through gap junctions formed by connexin membrane proteins. Another family of membrane proteins called pannexins are able to form single membrane channels that allow cellular exchanges with the extracellular environment. The most ubiquitously expressed and studied member, pannexin 1 (PANX1), has yet to be described thoroughly in female reproductive tissues or functionally studied during oocyte maturation. Here, we look into the expression of pannexin 1 in bovine cumulus-oocyte complexes (COCs), as well as, its potential role in oocyte maturation and development.

Results

We show that pannexin 1 is expressed in bovine COCs and that the expression of PANX1 was significantly lower in COCs isolated from large antral follicles (> 5 mm) compared to those isolated from small antral follicles (< 2 mm). Supporting this we also found lower expression of PANX1 in oocytes with higher developmental potential when compared to oocytes with lower developmental potential. We further found that PANX1 channel inhibition during in vitro maturation resulted in temporarily delayed meiotic maturation and improved in vitro developmental outcomes while decreasing intercellular reactive oxygen species.

Conclusions

These data suggests PANX1 is differentially expressed at a critical stage of follicular development when oocytes are acquiring developmental competence, and may play a role in the timing of oocyte maturation.

Association between mild stimulated IVF/M cycle and early embryo arrest in sub fertile women with/without PCOS

BACKGROUND

The in vitro maturation (IVM) technique has physical and financial benefits, but a lower efficiency and outcome that is still unclear whether it is related to polycystic ovary syndrome (PCOS) itself or the IVM procedure. In this study, we analyzed the clinical and laboratory outcomes of an optimized IVM protocol in patients with and without PCOS. We also discussed the possible reasons for early embryo arrest in the IVM cycle.

METHODS

This prospective study involved 58 PCOS patients and 56 non-PCOS patients who underwent mild stimulated IVF combined IVM (IVF/M) cycles. The clinical and laboratory outcomes were compared between the two groups. Also, metaphase II (MII) oocytes were obtained after IVM from the two groups, and in vivo MII oocytes randomly collected from IVF patients were examined for mitochondrial function using a laser scanning confocal microscope (LSCM). The aneuploidy rate for arrested cleavage embryos from IVM and IVF oocytes were screened using Next Generation Sequencing (NGS).

RESULTS

Mildly stimulated IVF/M resulted in cumulative clinical pregnancy and implantation rates (40.2, 28.7% in the PCOS group vs. 41.9, 36% in the non-PCOS group), respectively. The blastocyst formation rates were comparable (28% vs. 28.2%) in PCOS and non-PCOS groups, respectively. Using LSCM, there was a significant decrease in the mitochondrial membrane potential of IVM oocytes compared with the control IVF oocytes (P < 0.001), but no significant difference between the PCOS and non-PCOS groups. The NGS showed that the aneuploidy rates were comparable (75, 75, and 66.6%) in IVM-PCOS, IVM-non-PCOS, and control IVF arrested embryos, respectively.

CONCLUSIONS

The mildly stimulated IVF/M protocol produced acceptable clinical outcomes in PCOS and non-PCOS patients. IVM itself rather than the PCOS condition adversely affected the embryo development through its effect on mitochondrial function, which appeared to be a possible cause for the embryo arrest in the IVM cycles rather than chromosomal aneuploidy.

The Phosphodiesterase Inhibitor, Isobutyl-1-Methylxanthine Prevents the Sudden Drop in Cyclic Adenosine Monophosphate Concentration and Modulates Glucose Metabolism of Equine Cumulus-Oocyte Complexes Matured in Vitro

Spontaneous nuclear maturation of mammalian oocytes can occur when physically removed from the ovarian follicle during in vitro oocyte maturation (IVM), largely because of a decrease in cyclic adenosine monophosphate (cAMP) concentration. Modulation of oocyte cAMP during IVM by using phosphodiesterase inhibitors has been shown to maintain elevated oocyte cAMP concentrations and control meiotic resumption of bovine and ovine oocytes. This study determined the effect of inclusion of isobutyl-1-methylxanthine (IBMX) during collection and the first 12 hours of incubation of equine oocytes on cAMP concentration and glucose metabolism of cumulus-oocyte complexes (COCs). Abattoir-derived COCs were collected in aspiration medium with (Asp-IBMX) or without (Asp) IBMX. Cumulus-oocyte complexes were then incubated for 12 hours in IVM medium with (Mat-IBMX) or without (Mat) IBMX, followed by additional 24 hours in Mat medium. The cAMP concentration, glucose consumption, lactate production, and metaphase II rates of the COCs were assessed. Cumulus-oocyte complexes aspirated into Asp-IBMX (62.2 ± 2.6 fmol per COC) medium had higher cAMP concentration than Asp (31.8 ± 2.8 fmol per COC) control group (P < .05). Likewise, at 12 hours of IVM, Mat-IBMX group (33.2 ± 2.1 fmol per COC) had higher cAMP concentration than the Mat group (7.68 ± 0.5 fmol per COC; P < .05). Glucose consumption and lactate production were lower during the first 12 hours of incubation in COCs cultured in Mat-IBMX (P < .05). Isobutyl-1-methylxanthine prevented the rapid drop in cAMP concentration and altered metabolism of glucose by the COC. Preventing the sudden drop in cAMP prevents the premature nuclear maturation of in vitro-matured oocytes causing poor developmental competence.

Meiotic block with roscovitine improves competence of porcine oocytes by fine-tuning activities of different cyclin-dependent kinases

Successful use of oocytes from small follicles (SFs) is of great importance for animal embryo production and human in vitro fertilization with reduced hormone-related side effects. How in vitro meiotic arrest maintenance (MAM) increases the competence of oocytes is not clear. In this study, pig oocytes recovered from SF of 1-2 mm and medium-follicles (MF) of 3-6 mm in diameter from abattoir ovaries were treated by various MAM treatments to improve their competence. The results showed that 25 µM roscovitine or 1 mM db-cAMP efficiently blocked germinal vesicle breakdown in both SF and MF oocytes suggesting a similar cyclin-dependent kinase (CDK) 1 level between the two oocyte groups. MAM with 15- and 25-µM roscovitine alone or with 1-mM db-cAMP improved competence of SF and MF oocytes, respectively, with a promoted chromatin configuration transition from surrounded nucleoli (SN) to re-decondensation (RDC) pattern that supported substantial gene transcription. However, MAM with db-cAMP alone or with higher concentrations of roscovitine did not improve oocyte competence, could not support an SN-to-RDC transition, and/or evoked a premature chromatin condensation (PMC) that suppressed gene transcription. Both CDK2 and CDK5 contents were higher (p < .05) in MF than in SF oocytes. It is concluded that the competence of pig oocytes, particularly that of SF oocytes can be improved by MAM using a proper roscovitine concentration that promotes gene transcription by inhibiting CDK5 while letting CDK2 off to prevent PMC.

Effects of cyclic adenosine monophosphate modulators on maturation and quality of vitrified-warmed germinal vesicle stage mouse oocytes

BACKGROUND

It is still one of the unresolved issues if germinal vesicle stage (GV) oocytes can be successfully cryopreserved for fertility preservation and matured in vitro without damage after warming. Several studies have reported that the addition of cyclic adenosine monophosphate (cAMP) modulators to in vitro maturation (IVM) media improved the developmental potency of mature oocytes though vitrification itself provokes cAMP depletion. We evaluated whether the addition of cAMP modulators after GV oocytes retrieval before vitrification enhances maturation and developmental capability after warming of GV oocytes.

METHODS

Retrieved GV oocytes of mice were divided into cumulus-oocyte complexes (COCs) and denuded oocytes (DOs). Then, GV oocytes were cultured with or without dibutyryl-cAMP (dbcAMP, cAMP analog) and 3-isobutyl-l-methylxanthine (phosphodiesterase inhibitor) during the pre-vitrification period for 30 min.

RESULTS

One hour after warming, the ratio of oocytes that stayed in the intact GV stage was significantly higher in groups treated with cAMP modulators. After 18 h of IVM, the percentage of maturation was significantly higher in the COC group treated with dbcAMP. The expression of F-actin, which is involved in meiotic spindle migration and chromosomal translocation, is likewise increased in this group. However, there was no difference in chromosome and spindle organization integrity or developmental competence between the MII oocytes of all groups.

CONCLUSIONS

Increasing the intracellular cAMP level before vitrification of the GV oocytes maintained the cell cycle arrest, and this process may facilitate oocyte maturation after IVM by preventing cryodamage and synchronizing maturation between nuclear and cytoplasmic components. The role of cumulus cells seems to be essential for this mechanism.

Combined use of dbcAMP and IBMX minimizes the damage induced by a long-term artificial meiotic arrest in mouse germinal vesicle oocytes

Phosphodiesterase (PDE)-mediated reduction of cyclic adenosine monophosphate (cAMP) activity can initiate germinal vesicle (GV) breakdown in mammalian oocytes. It is crucial to maintain oocytes at the GV stage for a long period to analyze meiotic resumption in vitro. Meiotic resumption can be reversibly inhibited in isolated oocytes by cAMP modulator forskolin, cAMP analog dibutyryl cAMP (dbcAMP), or PDE inhibitors, milrinone (Mil), Cilostazol (CLZ), and 3-isobutyl-1-methylxanthine (IBMX). However, these chemicals negatively affect oocyte development and maturation when used independently. Here, we used ICR mice to develop a model that could maintain GV-stage arrest with minimal toxic effects on subsequent oocyte and embryonic development. We identified optimal concentrations of forskolin, dbcAMP, Mil, CLZ, IBMX, and their combinations for inhibiting oocyte meiotic resumption. Adverse effects were assessed according to subsequent development potential, including meiotic resumption after washout, first polar body extrusion, early apoptosis, double-strand DNA breaks, mitochondrial distribution, adenosine triphosphate levels, and embryonic development. Incubation with a combination of 50.0 μM dbcAMP and 10.0 μM IBMX efficiently inhibited meiotic resumption in GV-stage oocytes, with low toxicity on subsequent oocyte maturation and embryonic development. This work proposes a novel method with reduced toxicity to effectively arrest and maintain mouse oocytes at the GV stage.

Participation of the adenosine salvage pathway and cyclic AMP modulation in oocyte energy metabolism

A follicular spike in cyclic AMP (cAMP) and its subsequent degradation to AMP promotes oocyte maturation and ovulation. In vitro matured (IVM) oocytes do not receive the cAMP increase that occurs in vivo, and artificial elevation of cAMP in IVM cumulus-oocyte complexes improves oocyte developmental potential. This study examined whether mouse oocytes can use the cAMP degradation product AMP to generate ATP via the adenosine salvage pathway, and examined whether pharmacological elevation of cAMP in IVM cumulus-oocyte complexes alters ATP levels. Oocytes cultured with isotopic ¹³C₅-AMP dose-dependently produced ¹³C₅-ATP, however total cellular ATP remained constant. Pharmacological elevation of cAMP using forskolin and IBMX prior to IVM decreased oocyte ATP and ATP:ADP ratio, and promoted activity of the energy regulator AMPK. Conversely, cumulus cells exhibited higher ATP and no change in AMPK. Culture of oocytes without their cumulus cells or inhibition of their gap-junctional communication yielded lower oocyte ¹³C₅-ATP, indicating that cumulus cells facilitate ATP production via the adenosine salvage pathway. In conclusion, this study demonstrates that mouse oocytes can generate ATP from AMP via the adenosine salvage pathway, and cAMP elevation alters adenine nucleotide metabolism and may provide AMP for energy production via the adenosine salvage pathway during the energetically demanding process of meiotic maturation.

Capacitation IVM improves cumulus function and oocyte quality in minimally stimulated mice

PURPOSE

Oocyte in vitro maturation (IVM) is a patient-friendly reproductive technology but lower success rates than IVF have limited its uptake. Capacitation-IVM (CAPA-IVM) is an innovative new IVM system currently undergoing clinical evaluation. This study aimed to determine temporal effects of the pre-IVM phase of CAPA-IVM on cumulus function and oocyte developmental competence in mildly-stimulated mice.

METHODS

Immature cumulus oocyte complexes (COCs) derived from mildly stimulated (23 h PMSG) 28-day-old mice underwent pre-IVM for 0-24 h in medium containing c-type natriuretic peptide (CNP), E₂, FSH and insulin, prior to IVM (CAPA-IVM). The effect of pre-IVM duration on cumulus cell function and embryo development post-CAPA-IVM/IVF was assessed.

RESULTS

Day 6 blastocyst rate increased incrementally with increasing pre-IVM duration: 40.6 ± 2.0%, 45.8 ± 1.2%, 52.2 ± 3.5%, 53.3 ± 5.9%, and 59.9 ± 2.5% for 0, 2, 6, 12, and 24 h pre-IVM, respectively (P < 0.01). DNA content/COC, a measure of cumulus cell proliferation, was significantly higher with 24 h pre-IVM group compared to 0, 2, or 6 h pre-IVM (P < 0.001). Pre-IVM for 24 h significantly increased cumulus expansion and mRNA expression of matrix genes Has2 and Tnfaip6 and Areg relative to no pre-IVM control (P < 0.01). Cumulus-oocyte gap-junctional communication (GJC) was maintained throughout 24 h pre-IVM (P < 0.0001), and GJC loss was slowed during the subsequent IVM phase, whilst meiotic resumption was accelerated (P < 0.05). Pre-IVM increased COC ATP and ADP content (P < 0.05), but not AMP, ATP/ADP, and energy charge.

CONCLUSION

The pre-IVM phase of CAPA-IVM improves the quality of IVM oocytes in a temporally dependent manner and significantly influences cumulus cell function including increased cell proliferation, cumulus expansion, and prolonged cumulus-oocyte GJC.

Treatment with cyclic adenosine monophosphate modulators prior to in vitro maturation alters the lipid composition and transcript profile of bovine cumulus-oocyte complexes and blastocysts

Mammalian oocytes resume meiosis spontaneously after removal from the ovarian follicle. We tested the effects of a 2-h prematuration treatment (Pre-IVM) with forskolin (FSK) and 3-isobutyl-1-methylxanthine (IBMX) in bovine cumulus-oocyte complexes (COCs) on the lipid content of oocytes and blastocysts, on the membrane lipid composition of blastocysts and on the transcriptional profiling of cumulus cells and blastocysts in a high-throughput platform. Embryonic development rates to the morula (mean 56.1%) or blastocyst (mean 26.3%) stages were unaffected by treatment. Lipid content was not affected after Pre-IVM, but was increased after IVM in treated oocytes. Conversely, the lipid content was reduced in Pre-IVM blastocysts. Pre-IVM COCs generated blastocysts containing blastomeres with more unsaturated lipids in their membranes. Pre-IVM also altered the relative abundance of 31 gene transcripts after 2h and 16 transcripts after 24h in cumulus cells, while seven transcripts were altered in blastocysts. Our results suggest that the Pre-IVM treatment affected the lipid composition and transcriptional profiles of COCs and blastocysts. Therefore, Pre-IVM with FSK and IBMX could be used either to prevent spontaneous meiotic resumption during IVM or to modulate lipid composition in the membrane and cytoplasm of blastocysts, potentially improving bovine embryos.

Mitofusin1 in oocyte is essential for female fertility

Mitofusins (Mfn) are the important regulators of mitochondrial organization in mammalian cells; however, their roles during oocyte development remain unknown. In the present study, we generated mice with oocyte-specific knockout of Mfn1 or Mfn2 (Mfn1^fl/fl;Zp3-Cre or Mfn2^fl/fl;Zp3-Cre). We report that deletion of Mfn1, but not Mfn2, in oocytes leads to female mice sterility, associated with the defective folliculogenesis and impaired oocyte quality. In specific, follicles are arrested at secondary stage in Mfn1^fl/fl;Zp3-Cre mice, accompanying with the reduced proliferation of granulosa cells. Moreover, alterations of mitochondrial structure and distribution pattern are readily observed in Mfn1-null oocytes. Consistent with this, mitochondrial activity and function are severely disrupted in oocytes from Mfn1^fl/fl;Zp3-Cre mice. In addition, the differentially expressed genes in Mfn1-deleted oocytes are also identified by whole-transcriptome sequencing. In sum, these results demonstrate that Mfn1-modulated mitochondrial function is essential for oocyte development and folliculogenesis, providing a novel mechanism determining female fertility.

Evaluation of the simulated physiological oocyte maturation (SPOM) system on F1 Gyr × Holstein oocytes and embryos

The present study aimed to evaluate the effect of the simulated physiological oocyte maturation (SPOM) system on F1 Gyr × Holstein oocytes and embryos by evaluating the meiotic arrest, embryo production rates, total number of cells and lipid score. Three experiments were conducted and the following three experimental groups were formed according to in vitro maturation (IVM) treatments: CONTROL 1 (TCM 199 medium without FBS), CONTROL 2 (commercial medium) and SPOM (TCM 199 medium with forskolin and 3-isobutyl-1-methylxanthine (IBMX) in pre-IVM and extended IVM with cilostamide). In the first experiment (ovum pick-up), a significant (P &lt; 0.05) reduction in the percentage of matured oocytes in SPOM group was observed. In the second (slaughterhouse ovaries) and third (ovum pickup) experiments, the cleavage and blastocyst rates were reduced (P &lt; 0.05) in the SPOM group. There was no significant (P &gt; 0.05) difference in total number of cells among the groups. No difference (P &gt; 0.05) was found on lipid score among the groups at Day 7 of development, in both Experiments 2 and 3. At Day 9 (Experiment 2), only the CONTROL 2 showed a significant increase (P &gt; 0.05) compared with the other treatments. It was concluded that under our conditions, the SPOM system was efficient in prolonging meiotic arrest on Gyr × Holstein oocytes, offering the oocytes in vitro conditions more similar to those found in vivo; however, it adversely affected embryo production rates and promoted no beneficial effect on the total number of cells and the lipid score.

Simulated physiological oocyte maturation has side effects on bovine oocytes and embryos

PURPOSE

Oocyte maturation is a complex process involving nuclear and cytoplasmic modulations, during which oocytes acquire their ability to become fertilized and support embryonic development. The oocyte is apparently "primed" for maturation during its development in the dominant follicle. As bovine oocytes immediately resume meiosis when cultured, it was hypothesized that delaying resumption of meiosis with cyclic nucleotide modulators before in vitro maturation (IVM) would allow the oocytes to acquire improved developmental competence.

METHODS

We tested the Simulated Physiological Oocyte Maturation (SPOM) system that uses forskolin and 3-isobutyl-1-methylxanthine for 2 h prior to IVM against two different systems of conventional IVM (Con-IVM). We evaluated the ultrastructure of matured oocytes and blastocysts and also assessed the expression of 96 genes related to embryo quality in the blastocysts.

RESULTS

In summary, the SPOM system resulted in lower blastocyst rates than both Con-IVM systems (30 ± 9.1 vs. 35 ± 8.7; 29 ± 2.6 vs. 38 ± 2.8). Mature SPOM oocytes had significantly increased volume and number of vesicles, reduced volume and surface density of large smooth endoplasmic reticulum clusters, and lower number of mitochondria than Con-IVM oocytes. SPOM blastocysts showed only subtle differences with parallel undulations of adjacent trophectoderm plasma membranes and peripherally localized ribosomes in cells of the inner cell mass compared with Con-IVM blastocysts. SPOM blastocysts, however, displayed significant downregulation of genes related to embryonic developmental potential when compared to Con-IVM blastocysts.

CONCLUSIONS

Our results show that the use of the current version of the SPOM system may have adverse effects on oocytes and blastocysts calling for optimized protocols for improving oocyte competence.

Chronic restraint stress disturbs meiotic resumption through APC/C-mediated cyclin B1 excessive degradation in mouse oocytes

Psychological stress, which exerts detrimental effects on human reproduction, may compromise the meiotic competence of oocytes. Meiotic resumption, germinal vesicle breakdown (GVBD), is the first milestone to confer meiotic competence to oocytes. In the practice of assisted reproductive technology (ART), the timing for GVBD is associated with the rates of cleavage and blastocyst formation. However, whether chronic stress compromises oocyte competence by influencing GVBD and the underlying mechanisms are unclear. In the present study, a chronic restraint stress (CRS) mouse model was used to investigate the effects of stress on oocyte meiotic resumption, as well as the mechanisms. Following a 4-week chronic restraint stress in female mice, the percentage of abnormal bipolar spindles increased and indicated compromised oocyte competence in the CRS group. Furthermore, we identified a decreased percentage of GVBD and prolonged time of GVBD in the CRS mouse oocytes compared with the control group. CRS simultaneously reduced the expression of cyclin B1 (CCNB1), which represents a regulatory subunit of M-phase/mature promoting factor (MPF). However, MG132, an inhibitor of anaphase-promoting complex/cyclosome (APC/C), could rescue the prolonged time of GVBD and increase the expression level of CCNB1 of oocytes from the CRS mice. Collectively, our results demonstrated that stress disturbed meiotic resumption through APC/C-mediated CCNB1 degradation, thus providing a novel understanding for stress-related oocyte quality decline; moreover, it may provide a non-invasive approach to select high-quality gametes and novel targets for molecular therapy to treat stress-related female infertility.

An improved IVM method for cumulus-oocyte complexes from small follicles in polycystic ovary syndrome patients enhances oocyte competence and embryo yield

STUDY QUESTION

Are meiotic and developmental competence of human oocytes from small (2-8 mm) antral follicles improved by applying an optimized IVM method involving a prematuration step in presence of C-Type Natriuretic Peptide (CNP) followed by a maturation step in presence of FSH and Amphiregulin (AREG)?

SUMMARY ANSWER

A strategy involving prematuration culture (PMC) in the presence of CNP followed by IVM using FSH + AREG increases oocyte maturation potential leading to a higher availability of Day 3 embryos and good-quality blastocysts for single embryo transfer.

WHAT IS KNOWN ALREADY

IVM is a minimal-stimulation ART with reduced hormone-related side effects and risks for the patients, but the approach is not widely used because of an efficiency gap compared to conventional ART. In vitro systems that enhance synchronization of nuclear and cytoplasmic maturation before the meiotic trigger are crucial to optimize human IVM systems. However, previous PMC attempts have failed in sustaining cumulus-oocyte connections throughout the culture period, which prohibited a normal cumulus-oocyte communication and precluded an adequate response by the cumulus-oocyte complex (COC) to the meiotic trigger.

STUDY DESIGN, SIZE, DURATION

A first prospective study involved sibling oocytes from a group of 15 patients with polycystic ovary syndrome (PCOS) to evaluate effects of a new IVM culture method on oocyte nuclear maturation and their downstream developmental competence. A second prospective study in an additional series of 15 women with polycystic ovaries characterized and fine-tuned the culture conditions.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Fifteen women with PCOS (according to Rotterdam criteria) underwent IVM treatment after 3-5 days of highly purified human menopausal gonadotropin (HP-hMG) stimulation and no human chorionic gonadotropin (hCG) trigger before oocyte retrieval. A first study was designed with sibling oocytes to prospectively evaluate the impact of an IVM culture method: 24 h PMC with CNP + 30 h IVM with FSH and AREG, on embryo yield, in comparison to the standard (30 h) IVM clinical protocol (Group I, n = 15). A second prospective study was performed in 15 women with polycystic ovaries, to characterize and optimize the PMC conditions (Group II, n = 15). The latter study involved the evaluation of oocyte meiotic arrest, the preservation of cumulus-oocyte transzonal projections (TZPs), the patterns of oocyte chromatin configuration and cumulus cells apoptosis following the 24 and 46 h PMC. Furthermore, oocyte developmental potential following PMC (24 and 46 h) + IVM was also evaluated. The first 20 good-quality blastocysts from PMC followed by IVM were analysed by next generation sequencing to evaluate their aneuploidy rate.

MAIN RESULTS AND THE ROLE OF CHANCE

PMC in presence of CNP followed by IVM using FSH and AREG increased the meiotic maturation rate per COC to 70%, which is significantly higher than routine standard IVM (49%; P ≤ 0.001). Hence, with the new system the proportion of COCs yielding transferable Day 3 embryos and good-quality blastocysts increased compared to routine standard IVM (from 23 to 43%; P ≤ 0.001 and from 8 to 18%; P ≤ 0.01, respectively). CNP was able to prevent meiosis resumption for up to 46 h. After PMC, COCs had preserved cumulus-oocyte TZPs. The blastocysts obtained after PMC + IVM did not show increased aneuploidy rates as compared to blastocysts from conventional ART.

LIMITATIONS REASONS FOR CAUTION

The novel IVM approach in PCOS patients was tested in oocytes derived from small antral follicles which have an intrinsically low developmental potential. Validation of the system would be required for COCs from different (larger) follicular sizes, which may involve further adjustment of PMC conditions. Furthermore, considering that this is a novel strategy in human IVM treatment, its global efficiency needs to be confirmed in large prospective randomized controlled trials. The further application in infertile patients without PCOS, e.g. cancer patients, remains to be evaluated.

WIDER IMPLICATIONS OF THE FINDINGS

The findings of this pilot study suggest that the efficiency gap between IVM and conventional IVF can be reduced by fine-tuning of the culture methods. This novel strategy opens new perspectives for safe and patient-friendly ART in patients with PCOS.

STUDY FUNDING/COMPETING INTEREST(S)

IVM research at the Vrije Universiteit Brussel has been supported by grants from: the Institute for the Promotion of Innovation by Science and Technology in Flanders (Agentschap voor Innovatie door Wetenschap en Technologie-IWT, project 110680); the Fund for Research Flanders (Fonds Wetenschappelijk Onderzoek-Vlaanderen-FWO, project G.0343.13), the Belgian Foundation Against Cancer (HOPE project, Dossier C69). The authors have no conflicts of interest.

Effects of pre-maturational culture duration on developmental competence of bovine small-sized oocytes

We investigated the effects of pre-maturational (pre-IVM) culture on the developmental competence of small-sized bovine oocytes (110 and < 115 µm). Oocytes were cultured with 3-isobutyl-1-methylxanthine (IBMX) for 0, 5, or 10 h and subjected to in vitro maturation, fertilization, and culture. The cleavage rate (73%) of small-sized oocytes with 5 h pre-IVM was higher than those with 0 and 10 h pre-IVM (61 and 62%, respectively). The blastocyst rate (16%) of embryos derived from small-sized oocytes with 5 h pre-IVM was higher than those with 0 and 10 h pre-IVM (9 and 8%, respectively). In addition, small-sized oocytes with 5 h pre-IVM had a higher mean cell number in blastocysts (134.1 ± 34.8) than those with 0 and 10 h pre-IVM (100.2 ± 17.2 and 107.8 ± 23.7, respectively). In conclusion, the pre-IVM of small-sized oocytes with IBMX for 5 h improved the developmental competence of bovine oocytes, as well as the quality of blastocysts.

A pre-in vitro maturation medium containing cumulus oocyte complex ligand-receptor signaling molecules maintains meiotic arrest, supports the cumulus oocyte complex and improves oocyte developmental competence

STUDY QUESTION

Can a pre-in vitro maturation (pre-IVM) medium containing signaling molecules rather than chemical/pharmaceutical agents, sustain meiotic arrest and improve developmental competence of in vitro matured oocytes in CF1 outbred mice?

SUMMARY ANSWER

A short 2 h period of pre-IVM prevents spontaneous meiotic resumption, improves mitochondria activity in subsequently matured oocytes, and increases developmental competence, pregnancy rate and implantation of resulting embryos.

WHAT IS KNOWN ALREADY

Spontaneous resumption of meiosis in vitro is detrimental for oocyte developmental competence. Pre-IVM systems that prevent spontaneous meiotic resumption with chemical/pharmaceutical agents are a promising approach to improving IVM oocyte competence; however, the success of these methods has proven to be inconsistent.

STUDY DESIGN, SIZE, DURATION

This study consisted of a series of experiments using cumulus oocyte complexes (COC) derived from outbred mice following ovarian stimulation. The study was designed to examine if a novel, ligand/receptor-based pre-IVM treatment could sustain meiotic arrest in vitro and improve oocyte developmental competence, compared to control IVM. Two pre-IVM durations (2 h and 24 h) were evaluated, and the effect of the mitochondrial stimulator PQQ during 24 h pre-IVM was studied.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Murine (outbred CF1) immature COC were cultured in vitro in the presence of C-type natriuretic peptide (CNP) (30 nM), estradiol (100 nM), FSH (1 × 10-4 IU/ml) and bone morphogenic protein 15 (BMP15) (100 ng/ml) for 2 h or 24 h prior to IVM. Meiotic status during pre-IVM and IVM was analyzed using orcein staining, and functionality of gap junction communication was confirmed using the functional gap junction inhibitor carbenoxolone (CBX). Oocytes exposed to pre-IVM treatment were compared to control oocytes collected on the same day from the same females and undergoing standard IVM. Developmental competence and embryo viability was assessed by oocyte mitochondrial activity and ATP concentration, in vitro embryo development following IVF and in vitro culture, blastocyst cell number and allocation, embryo morphokinetics, and embryo transfer. Differences were determined to be significant when P < 0.05.

MAIN RESULTS AND THE ROLE OF CHANCE

Both a short (2 h) and long (24 h) pre-IVM period successfully prevented spontaneous resumption of meiosis. Moreover, gap junctions remained open during the pre-IVM period, as shown by the resumption of meiosis (95.9 ± 2.1%) in the presence of CBX during pre-IVM. A 2 h pre-IVM treatment improved blastocyst development after 96 h of culture per cleaved embryo compared to control (71.9 ± 7.4% versus 53.3 ± 6.2%, respectively), whereas a longer 24 h pre-IVM had no effect on development. A short 2 h period of pre-IVM increased mitochondrial activity in mature oocytes. On the contrary, mitochondrial activity was reduced in mature oocytes following 24 h of arrest and IVM. Treatment of arrested COC with pyrroloquinoline quinone (PQQ) during the 24 h pre-IVM period successfully maintained mitochondrial activity equal to control. However, PQQ was not able to improve blastocyst development compared to pre-IVM 24 h without PQQ. Moreover, ATP concentration in mature oocytes following pre-IVM and/or IVM, did not differ between treatments. A 2 h pre-IVM period prior to IVM improved pregnancy rate following transfer to recipient females. Implantation was also improved after transfer of embryos derived from oocytes arrested for either 2 h or 24 h prior to IVM, compared to control IVM derived embryos (41.9 ± 9%, 37.2 ± 9.5% and 17.2 ± 8.3%, respectively), although fetal development did not differ.

LIMITATIONS, REASONS FOR CAUTION

Slower meiotic resumption and enhanced mitochondrial activity likely contribute to improved developmental competence of oocytes exposed to pre-IVM for 2 h, but further experiments are required to identify specific mechanisms. Maintaining oocytes in meiotic arrest for 24 h with this approach could be a potential window to improve oocyte quality. However, an initial attempt to utilize this period of arrest to manipulate quality with PQQ, a mitochondrial stimulator, did not improve oocyte competence.

WIDER IMPLICATIONS OF THE FINDINGS

IVM could be an attractive clinical alternative to conventional IVF, with reduced time, cost and reliance on high doses of exogenous hormones to stimulate follicle growth, thus eliminating ovarian hyperstimulation syndrome (OHSS). Currently IVM is not widely used as it results in reduced embryo development and lower pregnancy outcomes compared to embryos produced from in vivo matured oocytes. Our approach to IVM, incorporating a ligand/receptor pre-IVM period, could improve human oocyte quality following IVM leading to routine adoption of this patient friendly technology. In addition, our methodology of pre-IVM containing signaling molecules rather than chemical/pharmaceutical agents may prove to be more consistent at improving oocyte quality than those focusing only on cAMP modulation with pharmacological agents. Finally, a reliable method of maintaining oocytes in meiotic arrest in vitro provides a novel window of opportunity in which the oocyte may be manipulated to address specific physiological deficiencies prior to meiotic resumption.

LARGE SCALE DATA

N/A.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by the Colorado Center for Reproductive Medicine (CCRM, Lone Tree, Colorado USA). We declare no conflict of interest.

Effect of pre-in vitro maturation with cAMP modulators on the acquisition of oocyte developmental competence in cattle

The administration of follicle-stimulating hormone (FSH) prior to oocyte retrieval improves oocyte developmental competence. During bovine embryo production in vitro, however, oocytes are typically derived from FSH-unprimed animals. In the current study, we examined the effect of pre-in vitro maturation (IVM) with cAMP modulators, also known as the second messengers of FSH, on the developmental competence of oocytes derived from small antral follicles (2–4 mm) of FSH-unprimed animals. Pre-IVM with N6,2ʹ-O-dibutyryladenosine 3′,5′-cyclicmonophosphate (dbcAMP) and 3-isobutyl-1-methylxanthine (IBMX) for 2 h improved the blastocyst formation in oocytes stimulated by FSH or amphiregulin (AREG). Furthermore, pre-IVM enhanced the expression of the FSH- or AREG-stimulated extracellular matrix-related genes HAS2, TNFAIP6, and PTGS2, and epidermal growth factor (EGF)-like peptide-related genes AREG and EREG. Additionally, pre-IVM with dbcAMP and IBMX enhanced the expression of EGFR, and also increased and prolonged cumulus cell-oocyte gap junctional communication. The improved oocyte development observed using the pre-IVM protocol was ablated by an EGF receptor phosphorylation inhibitor. These results indicate that pre-IVM with cAMP modulators could contribute to the acquisition of developmental competence by bovine oocytes from small antral follicles through the modulation of EGF receptor signaling and oocyte-cumulus/cumulus-cumulus gap junctional communication.

Meiotic arrest with roscovitine and follicular fluid improves cytoplasmic maturation of porcine oocytes by promoting chromatin de-condensation and gene transcription

The developmental capacity of in vitro matured oocytes is inferior to that of the in vivo matured ones due to insufficient cytoplasmic maturation. Although great efforts were made to accomplish better cytoplasmic maturation by meiotic arrest maintenance (MAM) before in vitro maturation (IVM), limited progress has been achieved in various species. This study showed that MAM of porcine oocytes was better achieved with roscovitine than with dibutyryl cyclic adenosine monophosphate (db-cAMP) or 3-isobutyl-1-methylxanthine. Oocyte developmental competence after IVM was significantly improved following MAM in 199 + FF medium (TCM-199 containing 10% porcine follicular fluid and 25 µM roscovitine) to a level even higher than that in control oocytes matured without pre-MAM. Observations on other markers further confirmed the positive effects of MAM in 199 + FF on oocyte cytoplasmic maturation. During MAM culture in 199 + FF, re-decondensation (RDC) of condensed chromatin occurred, and transcription of genes beneficial to cytoplasmic maturation was evident in some of the oocytes with surrounded nucleoli (SN). However, MAM with db-cAMP neither induced RDC nor improved oocyte developmental potential. Together, the results suggest that MAM in the presence of FF and roscovitine improved the developmental competence of porcine oocytes by promoting a pre-GVBD chromatin de-condensation and expression of beneficial genes.

Transporting cumulus complexes using novel meiotic arresting conditions permits maintenance of oocyte developmental competence

PURPOSE

The aim of this study is to evaluate the effect of a novel bovine cumulus oocyte complex (COC) shipping media designed to arrest meiotic resumption during transport on meiotic arrest, as well as meiotic resumption, subsequent embryonic development, and embryo quality.

METHODS

Bovine cumulus oocyte complexes were transported overnight from the collection facility to the laboratory. COCs were placed in control in vitro maturation (IVM) or in shipping arrest medium (SAM) containing multiple meiotic inhibitors, and then shipped to our laboratory. Upon arrival, meiotic status was assessed, control COCs were inseminated, and arrested COCs were matured and inseminated the next day. Embryonic development and quality were analyzed.

RESULTS

When bovine COC arrived at the laboratory after overnight shipment (21 h) in SAM, the majority of oocytes remained at the GV stage (75.6 ± 2.9% GV). Arrested oocytes successfully resumed and completed meiosis during IVM after removal from SAM (96.8 ± 0.5% metaphase II compared to control 88.3 ± 5.0%). Moreover, the development of blastocysts per COC was not different from control (22.3 ± 2.4% for control and 18.7 ± 2.1% for SAM), nor was any difference detected in blastocyst quality as determined by cell number and allocation.

CONCLUSIONS

Our study demonstrates that a physiological system incorporating cyclic adenosine monophosphate and cyclic guanosine monophosphate modulators can be used to maintain meiotic arrest followed by successful nuclear maturation and pre-implantation embryo development equal to control IVM-derived embryos. Our results offer promising insights for the development of pre-IVM media that may improve oocyte developmental competence in vitro.

New approaches regarding the in vitro maturation of oocytes: manipulating cyclic nucleotides and their partners in crime

Several discoveries have been described recently (5-10 years) about the biology of ovarian follicles (oocyte, cumulus cells and granulosa cells), including new aspects of cellular communication, the control of oocyte maturation and the acquisition of oocyte competence for fertilization and further embryo development. These advances are nourishing assisted reproduction techniques (ART) with new possibilities, in which novel culture systems are being developed and tested to improve embryo yield and quality. This mini-review aims to describe how the recent knowledge on the physiological aspects of mammalian oocyte is reflecting as original or revisited approaches into the context of embryo production. These new insights include recent findings on the mechanisms that control oocyte maturation, especially modulating intraoocyte levels of cyclic nucleotides during in vitro maturation using endogenous or exogenous agents. In this mini-review we also discuss the positive and negative effects of these manipulations on the outcoming embryo.

Oocyte maturation and quality: role of cyclic nucleotides

The cyclic nucleotides, cAMP and cGMP, are the key molecules controlling mammalian oocyte meiosis. Their roles in oocyte biology have been at the forefront of oocyte research for decades, and many of the long-standing controversies in relation to the regulation of oocyte meiotic maturation are now resolved. It is now clear that the follicle prevents meiotic resumption through the actions of natriuretic peptides and cGMP - inhibiting the hydrolysis of intra-oocyte cAMP - and that the pre-ovulatory gonadotrophin surge reverses these processes. The gonadotrophin surge also leads to a transient spike in cAMP in the somatic compartment of the follicle. Research over the past two decades has conclusively demonstrated that this surge in cAMP is important for the subsequent developmental capacity of the oocyte. This is important, as oocyte in vitro maturation (IVM) systems practised clinically do not recapitulate this cAMP surge in vitro, possibly accounting for the lower efficiency of IVM compared with clinical IVF. This review particularly focuses on this latter aspect - the role of cAMP/cGMP in the regulation of oocyte quality. We conclude that clinical practice of IVM should reflect this new understanding of the role of cyclic nucleotides, thereby creating a new generation of ART and fertility treatment options.

Multiple Mechanisms Cooperate to Constitutively Exclude the Transcriptional Co-Activator YAP from the Nucleus During Murine Oogenesis

Reproduction depends on the generation of healthy oocytes. Improving therapeutic strategies to prolong or rescue fertility depends on identifying the inter- and intracellular mechanisms that direct oocyte development under physiological conditions. Growth and proliferation of multiple cell types is regulated by the Hippo signaling pathway, whose chief effectors are the transcriptional co-activator YAP and its paralogue WWTR1. To resolve conflicting results concerning the potential role of Hippo in mammalian oocyte development, we systematically investigated the expression and localization of YAP in mouse oocytes. We report that that YAP is expressed in the germ cells beginning as early as Embryonic Day 15.5 and subsequently throughout pre- and postnatal oocyte development. However, YAP is restricted to the cytoplasm at all stages. YAP is phosphorylated at serine-112 in growing and fully grown oocytes, identifying a likely mechanistic basis for its nuclear exclusion, and becomes dephosphorylated at this site during meiotic maturation. Phosphorylation at serine-112 is regulated by a mechanism dependent on cyclic AMP and protein kinase A, which is known to be active in oocytes prior to maturation. Growing oocytes also contain a subpopulation of YAP, likely dephosphorylated, that is able enter the oocyte nucleus, but it is not retained there, implying that oocytes lack the cofactors required to retain YAP in the nucleus. Thus, although YAP is expressed throughout oocyte development, phosphorylation-dependent and -independent mechanisms cooperate to ensure that it does not accumulate in the nucleus. We conclude that nuclear YAP does not play a significant physiological role during oocyte development in mammals.

Cyclic AMP Affects Oocyte Maturation and Embryo Development in Prepubertal and Adult Cattle

High cAMP levels during in vitro maturation (IVM) have been related to improved blastocyst yields. Here, we employed the cAMP/cGMP modulators, forskolin, IBMX, and cilostamide, during IVM to unravel the role of high cAMP in early embryonic development produced from prepubertal and adult bovine oocytes. Oocytes were collected via transvaginal aspiration and randomly assigned to three experimental groups: TCM24 (24h IVM/control), cAMP30 (2h pre-IVM (forskolin-IBMX), 30h IVM-cilostamide), and DMSO30 (Dimethyl Sulfoxide/vehicle control). After IVM, oocytes were fertilized in vitro and zygotes were cultured in vitro to blastocysts. Meiotic progression, cAMP levels, mRNA abundance of selected genes and DNA methylation were evaluated in oocytes. Blastocysts were used for gene expression or DNA methylation analyses. Blastocysts from the cAMP30 groups were transferred to recipients. The cAMP elevation delayed meiotic progression, but developmental rates were not increased. In immature oocytes, mRNA abundance of PRKACA was higher for cAMP30 protocol and no differences were found for PDE3A, SMAD2, ZAR1, PRDX1 and SLC2A8. EGR1 gene was up-regulated in prepubertal cAMP30 immature oocytes and down-regulated in blastocysts from all in vitro treatments. A similar gene expression profile was observed for DNMT3b, BCL2L1, PRDX1 and SLC2A8 in blastocysts. Satellite DNA methylation profiles were different between prepubertal and adult oocytes and blastocysts derived from the TCM24 and DMSO30 groups. Blastocysts obtained from prepubertal and adult oocytes in the cAMP30 treatment displayed normal methylation profiles and produced offspring. These data indicate that cAMP regulates IVM in prepubertal and adult oocytes in a similar manner, with impact on the establishment of epigenetic marks and acquisition of full developmental competency.

Extending prematuration with cAMP modulators enhances the cumulus contribution to oocyte antioxidant defence and oocyte quality via gap junctions

STUDY QUESTION

Can bovine oocyte antioxidant defence and oocyte quality be improved by extending the duration of pre-in vitro maturation (IVM) with cyclic adenosine mono-phosphate (cAMP) modulators?

SUMMARY ANSWER

Lengthening the duration of cAMP-modulated pre-IVM elevates intra-oocyte reduced glutathione (GSH) content and reduces hydrogen peroxide (H2O2) via increased cumulus cell-oocyte gap-junctional communication (GJC), associated with an improvement in subsequent embryo development and quality.

WHAT IS KNOWN ALREADY

Oocytes are susceptible to oxidative stress and the oocyte's most important antioxidant glutathione is supplied, at least in part, by cumulus cells. A temporary inhibition of spontaneous meiotic resumption in oocytes can be achieved by preventing a fall in cAMP, and cyclic AMP-modulated pre-IVM maintains cumulus-oocyte GJC and improves subsequent embryo development.

STUDY DESIGN, SIZE, DURATION

This study consisted of a series of 10 experiments using bovine oocytes in vitro, each with multiple replicates. A range of pre-IVM durations were examined as the key study treatments which were compared with a control. The study was designed to examine if one of the oocyte's major antioxidant defences can be enhanced by pre-IVM with cAMP modulators, and to examine the contribution of cumulus-oocyte GJC on these processes.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Immature bovine cumulus-oocyte complexes were treated in vitro without (control) or with the cAMP modulators; 100 µM forskolin (FSK) and 500 µM 3-isobutyl-1-methyxanthine (IBMX), for 0, 2, 4 or 6 h (pre-IVM phase) prior to IVM. Oocyte developmental competence was assessed by embryo development and quality post-IVM/IVF. Cumulus-oocyte GJC, intra-oocyte GSH and H2O2 were quantified at various time points during pre-IVM and IVM, in the presence and the absence of functional inhibitors: carbenoxolone (CBX) to block GJC and buthionine sulfoximide (BSO) to inhibit glutathione synthesis.

MAIN RESULTS AND THE ROLE OF CHANCE

Pre-IVM with FSK + IBMX increased subsequent blastocyst formation rate and quality compared with standard IVM (P < 0.05), regardless of pre-IVM duration. The final blastocyst yields (proportion of blastocysts/immature oocyte) were 26.3% for the control, compared with 39.2, 35.2 and 34.2%, for the 2, 4 and 6 h pre-IVM FSK + IBMX treatments, respectively. In contrast to standard IVM (control), pre-IVM with cAMP modulators maintained open gap junctions between cumulus cells and oocytes for the duration (6 h) of pre-IVM examined, and persisted for a further 8 h in the IVM phase. Cyclic AMP-modulated pre-IVM increased intra-oocyte GSH levels at the completion of both pre-IVM and IVM, in a pre-IVM duration-dependent manner (P < 0.05), which was ablated when GJC was blocked using CBX (P < 0.05). By 4 h of pre-IVM treatment with cAMP modulators, oocyte H2O2 levels were reduced compared the control (P < 0.05), although this beneficial effect was lost when oocytes were co-treated with BSO. Inhibiting glutathione synthesis with BSO during pre-IVM ablated any positive benefits of cAMP-mediated pre-IVM on oocyte developmental competence (P < 0.01).

LIMITATIONS, REASONS FOR CAUTION

It is unclear if the improvement in oocyte antioxidant defence and developmental competence reported here is due to direct transfer of total and/or reduced glutathione from cumulus cells to the oocyte via gap junctions, or whether a GSH synthesis signal and/or amino acid substrates are supplied to the oocyte via gap junctions. Embryo transfer experiments are required to determine if the cAMP-mediated improvement in blastocyst rates leads to improved live birth rates.

WIDER IMPLICATIONS OF THE FINDINGS

IVM offers significant benefits to infertile and cancer patients and has the potential to significantly alter ART practice, if IVM efficiency in embryo production could be improved closer to that of conventional IVF (using ovarian hyperstimulation). Pre-IVM with cAMP modulators is a simple and reliable means to improve IVM outcomes.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by grants and fellowships from the National Health and Medical Research Council of Australia (1007551, 627007, 1008137, 1023210) and by scholarships from the Chinese Scholarship Council (CSC) awarded to H.J.L. and the Japanese Society for the Promotion of Science Postdoctoral Fellowship for Research Abroad awarded to S.S. The Fluoview FV10i confocal microscope was purchased as part of the Sensing Technologies for Advanced Reproductive Research (STARR) facility, funded by the South Australian Premier's Science and Research Fund. We acknowledge partial support from the Australian Research Council Centre of Excellence for Nanoscale BioPhotonics (CE140100003). We declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.