Reduced oxygen concentration during in vitro oocyte maturation alters global DNA methylation in the maternal pronucleus of subsequent zygotes in cattle

Factors Influencing the Maturation and Developmental Competence of Yak (Bos grunniens) Oocytes In Vitro

The yak (Bos grunniens) is a unique breed living on the Qinghai-Tibet Plateau and its surrounding areas, providing locals with a variety of vital means of living and production. However, the yak has poor sexual maturity and low fertility. High-quality mature oocytes are the basis of animal breeding technology. Recently, in vitro culturing of oocytes and embryo engineering technology have been applied to yak breeding. However, compared to those observed in vivo, the maturation rate and developmental capacity of in vitro oocytes are still low, which severely limits the application of in vitro fertilization and embryo production in yaks. This review summarizes the endogenous and exogenous factors affecting the in vitro maturation (IVM) and developmental ability of yak oocytes reported in recent years and provides a theoretical basis for obtaining high-quality oocytes for in vitro fertilization and embryo production in yaks.

Unraveling the Consequences of Oxygen Imbalance on Early Embryo Development: Exploring Mitigation Strategies

Although well-established and adopted by commercial laboratories, the in vitro embryo production system still requires refinements to achieve its highest efficiency. Early embryonic development is a dynamic event, demanding suitable conditions to provide a high number of embryos with quality and competence. The first step to obtaining an optimized in vitro environment is to know the embryonic metabolism and energy request throughout the different stages of development. Oxygen plays a crucial role in several key biological processes necessary to sustain and complete embryonic development. Nonetheless, there is still controversy regarding the optimal in vitro atmospheric concentrations during culture. Herein, we discuss the impact of oxygen tension on the viability of in vitro-produced embryos during early development. The importance of oxygen tension is addressed as its roles regarding essential embryonic traits, including embryo production rates, embryonic cell viability, gene expression profile, epigenetic regulation, and post-cryopreservation survival. Finally, we highlight the damage caused by in vitro unbalanced oxygen tensions and strategies to mitigate the harmful effects.

RNA-sequencing reveals genes linked with oocyte developmental potential in bovine cumulus cells

Cumulus cells provide an interesting biological material to perform analyses to understand the molecular clues determining oocyte competence. The objective of this study was to analyze the transcriptional differences between cumulus cells from oocytes exhibiting different developmental potentials following individual in vitro embryo production by RNA-seq. Cumulus cells were allocated into three groups according to the developmental potential of the oocyte following fertilization: (1) oocytes developing to blastocysts (Bl+), (2) oocytes cleaving but arresting development before the blastocyst stage (Bl-), and (3) oocytes not cleaving (Cl-). RNAseq was performed on 4 (Cl-) or 5 samples (Bl+ and Bl-) of cumulus cells pooled from 10 cumulus-oocyte complexes per group. A total of 49, 50, and 18 differentially expressed genes (DEGs) were detected in the comparisons Bl+ versus Bl-, Bl+ versus Cl- and Bl- versus Cl-, respectively, showing a fold change greater than 1.5 at an adjusted p value <0.05. Focussing on DEGs in cumulus cells from Bl+ group, 10 DEGs were common to both comparisons (10/49 from Bl+ vs. Bl-, 10/50 from Bl+ vs. Cl-). These DEGs correspond to 6 upregulated genes (HBE1, ITGA1, PAPPA, AKAP12, ITGA5, and SLC1A4), and 4 downregulated genes (GSTA1, PSMB8, FMOD, and SFRP4) in Bl+ compared to the other groups, from which 7 were validated by quantitative PCR (HBE1, ITGA1, PAPPA, AKAP12, ITGA5, PSMB8 and SFRP4). These genes are involved in critical biological functions such as integrin-mediated cell adhesion, oxygen availability, IGF and Wnt signaling or PKA pathway, highlighting specific biological processes altered in incompetent in vitro maturation oocytes.

Supplementation with asiatic acid during in vitro maturation improves porcine oocyte developmental competence by regulating oxidative stress

Asiatic acid is a natural triterpene found in Centella asiatica that acts as an effective free radical scavenger. Our previous research showed that asiatic acid delayed porcine oocyte ageing in vitro and improved preimplantation embryo development competence in vitro; however, the protective effects of asiatic acid against oxidative stress in porcine oocyte maturation are still unclear. Here, we investigated the effects of asiatic acid on porcine oocyte in vitro maturation (IVM) and subsequent embryonic development competence after parthenogenetic activation (PA) and in vitro fertilization (IVF). The results of the present research showed that 10 μM asiatic acid supplementation did not affect the expansion of cumulus cells or polar body extrusion of porcine oocytes, while asiatic acid application significantly increased the subsequent blastocyst formation rate and quality of porcine PA and IVF embryos. Hydrogen peroxide (H₂O₂) is a reactive oxygen species (ROS) that induces oxidative stress in porcine oocytes. As expected, asiatic acid supplementation not only decreased intracellular ROS levels but also attenuated H₂O₂-induced intracellular ROS generation. Further analysis revealed that asiatic acid supplementation enhanced intracellular glutathione production, mitochondrial membrane potential, and ATP generation at the end of IVM. In summary, our results reveal that asiatic acid supplementation exerts beneficial effects on porcine oocytes by regulating oxidative stress during the IVM process and could act as a potential antioxidant in porcine oocytes matured in vitro production systems.

Oxidative stress in oocyte aging and female reproduction

In a healthy body, reactive oxygen species (ROS) and antioxidants remain balanced. When the balance is broken toward an overabundance of ROS, oxidative stress appears and may lead to oocyte aging. Oocyte aging is mainly reflected as the gradual decrease of oocyte quantity and quality. Here, we aim to review the relationship between oxidative stress and oocyte aging. First, we introduced that the defective mitochondria, the age-related ovarian aging, the repeated ovulation, and the high-oxygen environment were the ovarian sources of ROS in vivo and in vitro. And we also introduced other sources of ROS accumulation in ovaries, such as overweight and unhealthy lifestyles. Then, we figured that oxidative stress may act as the "initiator" for oocyte aging and reproductive pathology, which specifically causes follicular abnormally atresia, abnormal meiosis, lower fertilization rate, delayed embryonic development, and reproductive disease, including polycystic ovary syndrome and ovary endometriosis cyst. Finally, we discussed current strategies for delaying oocyte aging. We introduced three autophagy antioxidant pathways like Beclin-VPS34-Atg14, adenosine 5'-monophosphate (AMP)-activated protein kinase/mammalian target of rapamycin (AMPK/mTOR), and p62-Keap1-Nrf2. And we also describe the different antioxidants used to combat oocyte aging. In addition, the hypoxic (5% O₂ ) culture environment for oocytes avoiding oxidative stress in vitro. So, this review not only contribute to our general understanding of oxidative stress and oocyte aging but also lay the foundations for the therapies to treat premature ovarian failure and oocyte aging in women.

Effect of oxygen tension and antioxidants on the developmental competence of buffalo oocytes cultured in vitro

Aim:

Oxidative stress (OS) is one of the major disruptors of oocyte developmental competence, which appears due to the imbalance between the production and neutralization of reactive oxygen species (ROS).

Materials and Methods:

In Experiment 1, buffalo oocytes were in vitro matured, fertilized, and cultured at 38.5°C under 5% CO₂ + 20% O₂ in standard CO₂ incubator (OS) or under 5% O₂ + 5% CO₂ + 90% N₂ (Multi-gas incubator, low O₂). In Experiment 2, buffalo cumulus oocytes complexes (COCs) were matured in Basic maturation medium (BMM) composed of TCM199+ 10% FCS+ 10 µg/ml FSH+ 50 µg/ml gentamicin (control group) or in BMM supplemented with 50 μM ascorbic acid (ascorbic acid group) or 3.0 mM glutathione (glutathione group) or 10^-5 M melatonin (melatonin group) and cultured at 38.5°C under 20% O₂ for 24 h. Matured buffalo oocytes in control, ascorbic acid, or melatonin groups were fertilized and zygotes were cultured for 8 days under the same conditions.

Results:

In both experiments, maturation, cleavage, and blastocyst rates were recorded. Results showed that culture of buffalo oocytes under low O₂ (5% O₂) significantly increased maturation, cleavage, and blastocyst rates (p<0.05). Meanwhile, under 20% O₂, addition of 10^-5 M melatonin or 50 μM ascorbic acid to in vitro maturation (IVM) medium significantly improved cumulus cell expansion, nuclear maturation rates of buffalo oocytes (p<0.05), and increased cleavage and blastocyst rates (p<0.05).

Conclusion:

About 5% O₂ is the optimum condition for in vitro production of buffalo embryos, and addition of 10^-5 M melatonin to IVM medium for oocytes cultured under 20% O₂ could alleviate the adverse effect of high oxygen tension and increased embryo yield.

DNA methylation studies in cattle

Investigation of the role of epigenetics in cattle breeding is gaining importance. DNA methylation represents an epigenetic modification which is essential for genomic stability and maintenance of development. Recently, DNA methylation research in cattle has intensified. The studies focus on the definition of methylomes in various organs and tissues in relation to the expression of genes underlying economically important traits, and explore methylome changes under developmental, environmental, disease, and diet influences. The investigations further characterize the methylation patterns of gametes in connection with their quality, and study methylome alterations in the developing naturally or assisted produced zygotes, embryos, and fetuses, considering their viability. A wide array of technologies developed for accurate and precise analysis of DNA methylation patterns is employed for both single-gene and genome-wide studies. Overall, the research is directed towards the identification of single methylation markers or their combinations which may be useful in the selection and breeding of animals to ensure cattle improvement.

Effects of oxygen concentrations on developmental competence and transcriptomic profile of yak oocytes

Oxygen concentration influences oocyte quality and subsequent embryo development, but it remains unclear whether oxygen concentrations affect the developmental competence and transcriptomic profile of yak oocytes. In this study, we investigated the effects of different oxygen concentrations (5% versus 20%) on the developmental competence, reactive oxygen species (ROS) levels, glutathione (GSH) content, and transcriptomic profile of yak oocytes. The results showed that a low oxygen concentration significantly increased the maturation rate of yak oocytes (81.2 ± 2.2% vs 75.9 ± 1.3%) and the blastocyst quality of yak in vitro fertilized embryos. Analysis of ROS and GSH showed that a low oxygen concentration reduced ROS levels and increased the content of GSH (75.05 ± 7.1 ng/oocyte vs 50.63 ± 5.6 ng/oocyte). Furthermore, transcriptomic analysis identified 120 differentially expressed genes (DEGs) between the two groups of oocytes. Gene enrichment analysis of the DEGs indicated multiple cellular processes, including oxidative phosphorylation, transcription regulation, mitochondrial regulation, oestrogen signalling pathway, HIF-1 signalling pathway, TNF signalling pathway, were involved in the response to oxygen concentration alterations. Taken together, these results indicated that a low oxygen concentration improved the developmental competence of yak oocytes.

The bovine cumulus proteome is influenced by maturation condition and maturational competence of the oocyte

In vitro maturation (IVM) of oocytes has still a negative impact on the developmental competence of oocytes. Therefore, this study analysed the cumulus proteome of individual cumulus-oocyte complexes (COCs) with and without maturational competence, matured under in vivo or in vitro conditions (n = 5 per group). A novel, ultrasensitive mass spectrometry (MS) based protein profiling approach, using label-free quantification, was applied. The detected cumulus proteome included 2226 quantifiable proteins and was highly influenced by the maturation condition (479 differentially expressed proteins) as well as maturational competence of the corresponding oocyte (424 differentially expressed proteins). Enrichment analysis showed an overrepresentation of the complement and coagulation cascades (CCC), ECM-receptor interaction and steroid biosynthesis in cumulus of COCs that matured successfully under in vivo conditions. Verification of the origin of CCC proteins was achieved through detection of C3 secretion into the maturation medium, with significantly increasing concentrations from 12 (48.4 ng/ml) to 24 hours (68 ng/ml: p < 0.001). In relation, concentrations in follicular fluid, reflecting the in vivo situation, were >100x higher. In summary, this study identified important pathways that are impaired in IVM cumulus, as well as potential markers of the maturational competence of oocytes.

Melatonin Enhances the Development of Porcine Cloned Embryos by Improving DNA Methylation Reprogramming

Incomplete DNA methylation reprogramming in cloned embryos leads to poor cloning efficiency. Melatonin has been proven to improve the development of cloned embryos, however, the role of melatonin during somatic cell nuclear transfer remains unclear. This work demonstrated that 10^-7 M melatonin significantly enhanced the developmental progress, reduced the arrested rate before zygotic genome activation, and upregulated the blastocyst rate of cloned embryos. Melatonin also promoted the pseudo-pronucleus formation, increased blastocyst cell number, and reduced embryo apoptosis through upregulating the expression of antiapoptosis factors while downregulating the transcription of proapoptosis genes. Further study displayed that DNA methylation reprogramming related genes were greatly improved in cloned embryos when treated with melatonin; then, melatonin effectively promoted genomic DNA demethylation and DNA remethylation, DNA demethylation of pluripotency related gene Oct4, DNA methylation maintenance of imprinted gene H19/Igf2, and DNA remethylation of tissue-specific gene Thy1 in cloned embryos. Thus, zygotic genome activation related gene Eif1a, pluripotency related genes Oct4, Nanog, and Sox2, imprinted genes Igf2 and H19, and blastocyst quality related genes Cdx2 and ATP1b1 were remarkably upregulated, and tissue-specific genes Thy1 and Col5a2 were considerably silenced. In conclusion, melatonin enhanced the development of cloned embryos by ameliorating DNA methylation reprogramming. This work reveals that melatonin can regulate DNA methylation reprogramming and provides a novel insight to improve cloning efficiency.