Factors affecting oocyte and embryo transcriptomes

In vitro production of meiotically competent oocytes from early antral follicles in sheep

The potential of using long in vitro culture (LIVC) of cumulus-oocyte complexes (COCs) from early antral follicles (EAFs) as an assisted reproductive technology in cattle has shown promising results. This study explored the feasibility of applying this technology to sheep as seasonal breeding animals. Ovaries from sheep were collected during both the breeding and non-breeding seasons. COCs were isolated from EAFs (350-450 μm) and cultured in TCM199 medium supplemented with 0.15 μg/mL Zn sulfate, 10-4IU/mL FSH, 10 ng/mL estradiol, 50 ng/mL testosterone, 50 ng/mL progesterone, and 5 μM Cilostamide. After five days of LIVC, the COCs were submitted to an in vitro maturation procedure. The results indicate successful in vitro development of COCs, evidenced by a significant increase in oocyte diameter (p < 0.000) and the preservation of gap junction communication between oocyte and cumulus cells. The gradual uncoupling was accompanied by a progressive chromatin transition from the non-surrounded nucleolus (NSN) to the surrounded nucleolus (SN) (p < 0.000), coupled with a gradual decrease in global transcriptional activity and an increase in oocyte meiotic competence (p < 0.000). Maintenance of oocyte-cumulus investment architecture, viability, and metaphase II capability was significantly higher in COCs collected during the breeding season (p < 0.000), suggesting higher quality than those obtained during the non-breeding season. In conclusion, our study confirms LIVC feasibility in sheep, emphasizing increased effectiveness during the breeding season in isolating higher-quality COCs from EAFs. These findings can influence improving the LIVC system in mammals with seasonal reproduction.

From follicle to blastocyst: microRNA-34c from follicular fluid-derived extracellular vesicles modulates blastocyst quality

BACKGROUND

Within the follicular fluid, extracellular vesicles (EVs) guide oocyte growth through their cargo microRNAs (miRNAs). Here, we investigated the role of EVs and their cargo miRNAs by linking the miRNAs found in EVs, derived from the fluid of an individual follicle, to the ability of its oocyte to become a blastocyst (competent) or not (non-competent).

METHODS

Bovine antral follicles were dissected, categorized as small (2-4 mm) or large (5-8 mm) and the corresponding oocytes were subjected to individual maturation, fertilization and embryo culture to the blastocyst stage. Follicular fluid was pooled in 4 groups (4 replicates) based on follicle size and competence of the corresponding oocyte to produce a blastocyst. Follicular fluid-derived EVs were isolated, characterized, and subjected to miRNA-sequencing (Illumina Miseq) to assess differential expression (DE) in the 4 groups. Functional validation of the effect of miR-34c on embryo development was performed by supplementation of mimics and inhibitors during in vitro maturation (IVM).

RESULTS

We identified 16 DE miRNAs linked to oocyte competence when follicular size was not considered. Within the large and small follicles, 46 DE miRNAs were driving blastocyst formation in each group. Comparison of EVs from competent small and large follicles revealed 90 DE miRNAs. Cell regulation, cell differentiation, cell cycle, and metabolic process regulation were the most enriched pathways targeted by the DE miRNAs from competent oocytes. We identified bta-miR-34c as the most abundant in follicular fluid containing competent oocytes. Supplementation of miR-34c mimic and inhibitor during IVM did not affect embryo development. However, blastocyst quality, as evidenced by higher cell numbers, was significantly improved following oocyte IVM in the presence of miR-34c mimics, while miR-34c inhibitors resulted in the opposite effect.

CONCLUSION

This study demonstrates the regulatory effect of miRNAs from follicular fluid-derived EVs on oocyte competence acquisition, providing a further basis for understanding the significance of miRNAs in oocyte maturation and embryonic development. Up-regulation of miR-34c in EVs from follicular fluid containing competent oocytes and the positive impact of miR-34c mimics added during IVM on the resulting blastocysts indicate its pivotal role in oocyte competence.

PADI6: What we know about the elusive fifth member of the peptidyl arginine deiminase family

Peptidyl arginine deiminase 6 (PADI6) is a maternal factor that is vital for early embryonic development. Deletion and mutations of its encoding gene in female mice or women lead to early embryonic developmental arrest, female infertility, maternal imprinting defects and hyperproliferation of the trophoblast. PADI6 is the fifth and least well-characterized member of the peptidyl arginine deiminases (PADIs), which catalyse the post-translational conversion of arginine to citrulline. It is less conserved than the other PADIs, and currently has no reported catalytic activity. While there are many suggested functions of PADI6 in the early mouse embryo, including in embryonic genome activation, cytoplasmic lattice formation, maternal mRNA and ribosome regulation, and organelle distribution, the molecular mechanisms of its function remain unknown. In this review, we discuss what is known about the function of PADI6 and highlight key outstanding questions that must be answered if we are to understand the crucial role it plays in early embryo development and female fertility. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

Use of new recombinant proteins for ovarian stimulation in ruminants

Currently, gonadotropin products (follicle stimulating hormone, FSH, and luteinizing hormone, LH) used in animal reproduction are produced by extraction and purification from abattoir-derived pituitary glands. This method, relying on animal-derived materials, carries the potential risk of hormone contamination and pathogen transmission. Additionally, chorionic gonadotropins are extracted from the blood of pregnant mares (equine chorionic gonadotropin; eCG) or the urine of pregnant women (human chorionic gonadotropin; hCG). However, recent advancements have introduced recombinant gonadotropins for assisted animal reproduction therapies. The traditional use of FSH for superovulation has limitations, including labor requirements and variability in superovulation response, affecting the success of in vivo (SOV) and in vitro (OPU/IVEP) embryo production. FSH treatment for superstimulation before OPU can promote the growth of a homogenous follicular population and the recovery of competent oocytes suitable for IVEP procedures. At present, a single injection of a preparation of long-acting bovine recombinant FSH (rFSH) produced similar superovulation responses resulting in the production of good-quality in vivo and in vitro embryos. Furthermore, the treatment with eCG at FTAI protocol has demonstrated its efficacy in promoting follicular growth, ovulation, and P/AI, mainly in heifers and anestrous cows. Currently, treatment with recombinant glycoproteins with eCG-like activity (r-eCG) have shown promising results in increasing follicular growth, ovulation, and P/AI in cows submitted to P4/E2 -based protocols. Bovine somatotropin (bST) is a naturally occurring hormone found in cows. Recombinant bovine somatotropin (rbST), produced through genetic engineering techniques, has shown potential in enhancing reproductive outcomes in ruminants. Treatment with rbST has been found to improve P/IA, increase donor embryo production, and enhance P/ET in recipients. The use of recombinant hormones allows to produce non-animal-derived products, offering several advantages in assisted reproductive technologies for ruminants. This advancement opens up new possibilities for improving reproductive efficiency and success rates in the field of animal reproduction.

Comparing the effects of a commercial and a prototype vitrification medium on meiotic spindle morphology and survival rate of mouse oocytes

OBJECTIVE

To compare oocyte survival and meiotic spindle normality between vitrified-warmed oocytes in a mouse embryo assay using Tvitri-4 or Ingámed vitrification media.

METHODS

C57BL/6 female mice aged 8-12 weeks were submitted to superovulation with pregnant mare's serum gonadotropin and human chorionic gonadotropin (hCG) for obtaining of in vivo matured oocytes. The oocytes were randomly distributed into one of the following three groups: CTR - control (fresh oocytes); ING - oocytes vitrified-warmed in a standard commercial kit supplied by Ingámed, and T4 - oocytes vitrified-warmed in the novel prototype Tvitri-4 medium. After warming and recovery culture, oocytes were assessed with respect to survival rate (SR) and both meiotic spindle morphology and chromosome alignment of each oocyte fixed in the sagittal position after immunostaining and analysis by confocal microscopy.

RESULTS

A total of 354 mature oocytes were vitrified in ING (n=178) and T4 (n=176), out of which 299 (85%) survived after warming. Oocyte survival rates were not statistically different (p=0.08) between ING (145/178=81.5%) and T4 (154/176=87.5%). Regarding meiotic normality, there were no significant changes in the proportion of oocytes with normal meiotic spindle morphology and chromosome structure between ING (52,2%) and T4 (63.4%) after warming (RR: 0.95, 95% CI: 0.92-1.607). When the meiotic normality was assessed using the CTR group as a reference in the analysis of relative risk, no significant differences were observed between T4 (63.4%) and CTR (70.5%) (RR: 0.95, 95% CI: 0.72-1.12). On the other hand, the percentage of oocytes retaining normal meiotic spindle morphology and chromosome configuration in ING (52.2%) was lower than in the CTR group (RR: 0.95, 95% CI: 0.57-0.97).

CONCLUSIONS

The novel prototype Tvitri-4 medium was efficient in preserve survival rate and meiotic spindle normality of oocytes and, with further verification, may be able to replace commercially available media in future clinical applications.

Characteristics of miRNAs Present in Bovine Sperm and Associations With Differences in Fertility

Small non-coding RNAs have been linked to different phenotypes in bovine sperm, however attempts to identify sperm-borne molecular biomarkers of male fertility have thus far failed to identify a robust profile of expressed miRNAs related to fertility. We hypothesized that some differences in bull fertility may be reflected in the levels of different miRNAs in sperm. To explore such differences in fertility that are not due to differences in visible metrics of sperm quality, we employed Next Generation Sequencing to compare the miRNA populations in Bos taurus sperm from bulls with comparable motility and morphology but varying Sire Conception Rates. We identified the most abundant miRNAs in both populations (miRs -34b-3p; -100-5p; -191-5p; -30d-4p; -21-5p) and evaluated differences in the overall levels and specific patterns of isomiR expression. We also explored correlations between specific pairs of miRNAs in each population and identified 10 distinct pairs of miRNAs that were positively correlated in bulls with higher fertility and negatively correlated in comparatively less fertile individuals. Furthermore, 8 additional miRNA pairs demonstrated the opposite trend; negatively correlated in high fertility animals and positively correlated in less fertile bulls. Finally, we performed pathway analysis to identify potential roles of miRNAs present in bull sperm in the regulation of specific genes that impact spermatogenesis and embryo development. Together, these results present a comprehensive picture of the bovine sperm miRNAome that suggests multiple potential roles in fertility.

Pathology of hyperandrogenemia in the oocyte of polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is the most common ovulatory disorder in the world and is associated with multiple adverse outcomes. The phenotype is widely varied, with several pathologies contributing to the spectrum of the disease including insulin resistance, obesity and hyperandrogenemia. Of these, the role of hyperandrogenemia and the mechanism by which it causes dysfunction remains poorly understood. Early studies have shown that androgens may affect the metabolic pathways of a cell, and this may pose hazards at the level of the mitochondria. As mitochondria are strictly maternally inherited, this would provide an exciting explanation not only to the pathophysiology of PCOS as a disease, but also to the inheritance pattern. This review seeks to summarize what is known about PCOS and associated adverse outcomes with focus on the role of hyperandrogenemia and specific emphasis on the oocyte.

Toxicidade in vitro da Niedenzuella (Tetrapterys) multiglandulosa em embriões bovinos

Resumo Niedenzuella (Tetrapterys) multiglandulosa, uma videira encontrada no Brasil, tem sido correlacionada a surtos de intoxicações em bovinos e búfalos, gerando perdas econômicas relacionadas à morte por insuficiência cardíaca, aborto, natimorto e mortalidade neonatal. O objetivo deste estudo foi examinar o potencial embriotóxico do extrato vegetal aquoso em embriões bovinos in vitro. O estudo in vitro foi realizado em cinco repetições de cultura de embriões bovinos distribuídos em dois grupos: controle, meio de cultura de embriões in vitro sem o extrato aquoso da planta; grupo tratado, com adição de 2,7mg / mL de extrato vegetal aquoso (10%) à cultura do embrião no sexto dia de cultivo. A taxa de clivagem foi avaliada no dia 2 da cultura de células. Viabilidade, eclodibilidade e subdesenvolvimento de blastocistos no sétimo, oitavo e nono dia (D7, D8 e D9, respectivamente) de cultura foram avaliados em microscópio estereoscópico. No dia 7, os blastocistos foram submetidos ao ensaio TUNEL para determinar o índice apoptótico. Observamos redução significativa da produção de blastocisto / número de embriões clivados (60,6% vs 41,5%); redução da produção de blastocistos / número total de oócitos bovinos maturados (35,1% vs 21,3%); e taxas de eclosão embrionária (38,0% vs 10,0%). No entanto, nenhum efeito foi observado na taxa de apoptose. Em conclusão, o extrato aquoso das folhas de N. multiglandulosa reduz a viabilidade do embrião bovino in vitro, sugerindo possíveis efeitos prejudiciais no desenvolvimento embrionário.

In vitro toxicity of Niedenzuella (Tetrapterys) multiglandulosa on bovine embryos

Abstract Niedenzuella (Tetrapterys) multiglandulosa, a vine plant found in Brazil, has been correlated to outbreaks of poisoning in cattle and buffaloes, generating economic losses related to the death due to heart failure, miscarriage, abortion, stillbirth, and neonatal mortality. The aim of this study was to examine the embryotoxic potential of the aqueous plant extract on in vitro bovine embryos. In vitro study was performed in five replicates of bovine embryo culture assigned in two groups: control, in vitro embryo culture medium without the aqueous plant extract; treated group, with addition of 2.7mg/mL of aqueous plant extract (10%) to the embryo culture on the sixth day of culture. Cleavage rate was evaluated at day 2 of the cell culture. Viability, hatchability and underdevelopment of blastocysts on the seventh, eighth, and ninth days (D7, D8, and D9, respectively) of culture were assessed under stereoscopic microscope. On day 7, blastocysts were submitted to TUNEL assay to determine apoptotic index. In vitro exposure of bovine embryos to of N. multiglandulosa resulted in reduced embryo development and survival, evaluated by dark cytoplasm indicating poor morphology and poor quality with marked reduction of hatchability. We observed a significant reduction of blastocyst production/number of cleaved embryos (60.6% vs 41.5%); reduction of blastocysts production/total number of matured bovine oocytes (35.1% vs 21.3%); and embryonic hatching rates (38.0% vs 10.0%). However, no effects were observed on the apoptotic rate. In conclusion, aqueous extract of N. multiglandulosa leaves reduces bovine embryo viability in vitro, suggesting possible detrimental effects on embryo development.

Sex steroid dynamics and mRNA transcript profiles of growth- and development-related genes during embryogenesis following induced follicular maturation in European eel

Hormones and mRNA transcripts of maternal origin deposited in the egg may affect early embryonic development in oviparous species. These hormones include steroids, such as estradiol-17β (E2), testosterone (T), 11-ketotestosterone (11-kt), 17α,20ß-dihydroxy-4-pregnen-3-one (DHP), and cortisol, which also play an important role in fish reproduction. In European eel, Anguilla anguilla, which does not reproduce naturally in captivity, vitellogenesis in female broodstock is commonly induced by administration of salmon or carp pituitary extract (PE) as an exogenous source of gonadotropins, while follicular maturation is stimulated by a priming dose of PE followed by provision of DHP as a maturation inducing hormone. In this regard, the main purpose of the present study was to evaluate effects of induced follicular maturation on reproductive success in European eel, focusing on maternal transfer and dynamics of steroids and mRNA transcripts of growth- and development-related genes throughout embryogenesis. The results showed that maternal blood plasma concentrations of E2, T and DHP were reflected in the unfertilized eggs. Moreover, a negative relationship between concentrations of E2 and DHP in eggs and embryos and quality parameters measured as fertilization success, cleavage abnormalities, embryonic survival, and hatch success was found. Concomitant mRNA transcript abundance analysis including genes involved in stress response (hsp70, hsp90), somatotropic axis (gh, igf1, igf2a, igf2b), lipid (cpt1a, cpt1b, pigf5) and thyroid metabolism (dio1, dio2, dio3, thrαb, thrβa, thrβb) varied among unfertilized egg batches. For the majority of genes, mRNA abundance increased during the maternal-to-zygotic transition in connection to activation of the transcription of the embryos own genome. mRNA abundance of dio1, cpt1a and cpt1b throughout embryogenesis was related to embryonic developmental competence. Notably, mRNA abundance of dio3 was positively associated with E2 concentrations, while the mRNA abundance of thrαb was negatively related to T concentrations in the unfertilized eggs, which may suggest an interaction between the thyroid and steroid hormone systems. Altogether, maternal plasma concentrations of E2 and DHP were reflected in the eggs, with high concentrations of these steroids in the eggs being negatively associated with embryonic developmental competence. Additionally, high transcript levels of two of the investigated genes (dio1, cpt1b) were positively associated with embryonic developmental competence. This study reveals maternal transfer of steroids and mRNA transcripts to the eggs, which may be significant contributors to the variability in embryonic survival observed in European eel captive reproduction.

Hyperandrogenemia alters mitochondrial structure and function in the oocytes of obese mouse with polycystic ovary syndrome

Capsule

Hyperandrogenemia in an obese PCOS mouse model results in altered glucose/insulin metabolism and mitochondrial structure and function in the oocytes, in part explaining adverse outcomes and inheritance patterns seen in PCOS.

Objective

To study the oocyte quality by means of mitochondrial structure and function in a well-established classic PCOS mouse model.

Design

Animal study using an obese PCOS mouse model compared with control.

Setting

Animal research facility in a tertiary care university hospital setting.

Animals

C57/B6J mice.

Intervention

Three week old mice had subdermal implants of DHT controlled release pellet or placebo for 90 days.

Main Outcome Measures

The mouse model was validated by performing glucose tolerance test, HbA1c levels, body weight and estrous cycle analyses. Oocytes were subsequently isolated and were used to investigate mitochondrial membrane potential, oxidative stress, lipid peroxidation, ATP production, mtDNA copy number, transcript abundance, histology and electron microscopy.

Results

Results showed glucose intolerance and hyperinsulinemia along with dysregulated estrus cycle. Analysis of the oocytes demonstrated impaired inner mitochondrial membrane function, increased ATP production and mtDNA copy number, altered RNA transcript abundance and aberrant ovarian histology. Electron microscopy of the oocytes showed severely impaired mitochondrial ultrastructure.

Conclusion

The obese PCOS mouse model shows a decreased oocyte quality related to impaired mitochondrial function.

The role of apoptosis in cryopreserved animal oocytes and embryos

Cryopreservation of both gametes and embryos, both for storage and for the preservation of their developmental capacity is a critical aspect of assisted reproductive technology. The survival of reproductive material following cryopreservation protocols is not only vital to clinical applications in the human in vitro fertilisation clinic, but is also important in the in vitro production of livestock embryos. The ability to routinely cryopreserve oocytes and embryos of livestock species has the potential to improve animal welfare, reduce environmental impact, and reduce the associated costs for breeding companies through the reduction of live animal transportation. Unfortunately, frozen oocytes and embryos are regularly documented to contain a higher proportion of apoptotic cells compared to their non-frozen counterparts, with freezing procedures thought to trigger apoptotic pathways of cell death. Comparisons between frozen and non-frozen samples also show changes in the gene expression of apoptotic factors such as Bcl-2 and Bax in response to cryopreservation. Apoptotic inhibition has the potential to improve cryosurvival, and how to achieve this is subject to debate. Here, we review how exposure to low temperatures during cryopreservation may be responsible for the abnormal activation of apoptotic pathways in mammalian oocytes and embryos, and discuss the ways in which they can be influenced to improve cryopreservation protocols, particularly in agriculturally important species.

Nutritional Status Impacts Epigenetic Regulation in Early Embryo Development: A Scoping Review

With the increasing maternal age and the use of assisted reproductive technology in various countries worldwide, the influence of epigenetic modification on embryonic development is increasingly notable and prominent. Epigenetic modification disorders caused by various nutritional imbalance would cause embryonic development abnormalities and even have an indelible impact on health in adulthood. In this scoping review, we summarize the main epigenetic modifications in mammals and the synergies among different epigenetic modifications, especially DNA methylation, histone acetylation, and histone methylation. We performed an in-depth analysis of the regulation of various epigenetic modifications on mammals from zygote formation to cleavage stage and blastocyst stage, and reviewed the modifications of key sites and their potential molecular mechanisms. In addition, we discuss the effects of nutrition (protein, lipids, and one-carbon metabolism) on epigenetic modification in embryos and emphasize the importance of various nutrients in embryonic development and epigenetics during pregnancy. Failures in epigenetic regulation have been implicated in mammalian and human early embryo loss and disease. With the use of reproductive technologies, it is becoming even more important to establish developmentally competent embryos. Therefore, it is essential to evaluate the extent to which embryos are sensitive to these epigenetic modifications and nutrition status. Understanding the epigenetic regulation of early embryo development will help us make better use of reproductive technologies and nutrition regulation to improve reproductive health in mammals.

Electroporation-Mediated Genome Editing of Livestock Zygotes

The introduction of genome editing reagents into mammalian zygotes has traditionally been accomplished by cytoplasmic or pronuclear microinjection. This time-consuming procedure requires expensive equipment and a high level of skill. Electroporation of zygotes offers a simplified and more streamlined approach to transfect mammalian zygotes. There are a number of studies examining the parameters used in electroporation of mouse and rat zygotes. Here, we review the electroporation conditions, timing, and success rates that have been reported for mice and rats, in addition to the few reports about livestock zygotes, specifically pigs and cattle. The introduction of editing reagents at, or soon after, fertilization can help reduce the rate of mosaicism, the presence of two of more genotypes in the cells of an individual; as can the introduction of nuclease proteins rather than mRNA encoding nucleases. Mosaicism is particularly problematic in large livestock species with long generation intervals as it can take years to obtain non-mosaic, homozygous offspring through breeding. Gene knockouts accomplished via the non-homologous end joining pathway have been more widely reported and successfully accomplished using electroporation than have gene knock-ins. Delivering large DNA plasmids into the zygote is hindered by the zona pellucida (ZP), and the majority of gene knock-ins accomplished by electroporation have been using short single stranded DNA (ssDNA) repair templates, typically less than 1 kb. The most promising approach to deliver larger donor repair templates of up to 4.9 kb along with genome editing reagents into zygotes, without using cytoplasmic injection, is to use recombinant adeno-associated viruses (rAAVs) in combination with electroporation. However, similar to other methods used to deliver clustered regularly interspaced palindromic repeat (CRISPR) genome-editing reagents, this approach is also associated with high levels of mosaicism. Recent developments complementing germline ablated individuals with edited germline-competent cells offer an approach to avoid mosaicism in the germline of genome edited founder lines. Even with electroporation-mediated delivery of genome editing reagents to mammalian zygotes, there remain additional chokepoints in the genome editing pipeline that currently hinder the scalable production of non-mosaic genome edited livestock.

Pregnancy and Calving Rates of Cloned Dromedary Camels Produced by Conventional and Handmade Cloning Techniques and In Vitro and In Vivo Matured Oocytes

Despite practical implication of cloning in camelids, its broad application has been hampered by technical and biological problems. Method of somatic cell nuclear transfer (SCNT) and oocyte competence are the principal technical and biological factors, respectively, that determine the cloning efficiency. This study, therefore, investigated differential contributions of two SCNT methods [modified handmade cloning (mHMC) vs. conventional (cNT)] and two recipient oocyte sources [abattoir-derived (Vitro) vs. FSH-stimulated (Vivo)] on the efficiency of dromedary camel cloning. The mHMC method supported similar rates of fusion, cleavage, and total blastocyst development, compared to conventional NT (cNT) (94, 89.1, and 68.5% vs. 78.9, 92, and 73.5%, respectively) when Vivo oocytes are used. However, using Vitro oocytes, mHMC supported significantly higher rates for these criteria, except for the cleavage, compared to cNT (95.5, 76.2, 25.2% vs. 75.3, 76.7, and 13.9%, respectively). A total of seven clones were born from mHMC/Vitro (four calves), mHMC/Vivo (one calf), cNT/Vitro (one calf), and mHMC/Vivo&Vivo (one calf)-derived embryos with overall efficiencies of 31.9, 26.6, 20, and 30% for initial pregnancy, 10.6, 6.6, 7.5, and 5% for development to term, and 8.5, 6.6, 2.5, 5% for development to weaving, respectively. To conclude, the quality of recipient oocyte greatly impacts cloning efficiency in vitro with no apparent carrying over effect on cloning efficiency in vivo, but the efficiency of SCNT method may compensate for the initial poor oocyte competence during in vitro and in vivo development of cloned embryos. The introduced mHMC could be a superior alternative to conventional method for simple, fast, and efficient production of cloned offspring in camelids.

Inhibition of germinal vesicle breakdown using IBMX increases microRNA-21 in the porcine oocyte

BACKGROUND

Germinal vesicle breakdown (GVBD) occurs during oocyte meiotic maturation, a period when transcriptional processes are virtually inactive. Thus, the maturing oocyte is reliant on processes such as post-transcriptional gene regulation (PTGR) to regulate the mRNA and protein repertoire. MicroRNA (miRNA) are a class of functional small RNA that target mRNA to affect their abundance and translational efficiency. Of particular importance is miRNA-21 (MIR21) due to its role in regulating programmed cell death 4 (PDCD4). The objective of this study was to characterize the abundance and regulation of MIR21 in relation to GVBD.

METHODS

Oocytes were collected from aspirated porcine tertiary follicles. Relative abundance of mature MIR21 was quantified at 0, 8, 16, 24, 32, and 42 h of in vitro (IVM) with or without treatment with 3-isobutyl-1-methylxanthine (IBMX).

RESULTS

IBMX increased abundance of MIR21 at 24 h approximately 30-fold compared to control oocytes (P < 0.05), and the induced increase in MIR21 abundance at 24 h was concomitant with premature depletion of PDCD4 protein abundance. To characterize the effect of artificially increasing MIR21 on oocyte competence without inhibiting GVBD, a MIR21 mimic, scrambled microRNA negative control, or nuclease free water was micro-injected into denuded oocytes at 21 h of IVM. The maturation rate of oocytes injected with synthetic MIR21 (63.0 ± 7.5%) was higher than oocytes injected with negative controls (P < 0.05).

CONCLUSIONS

Inhibition of nuclear meiotic maturation via IBMX significantly increased MIR21 and decreased its target, PDCD4. Injection of a MIR21 mimic increased oocyte maturation rate. Our results indicate MIR21 is active and important during meiotic maturation of the oocyte.

Prenatal androgen induced lean PCOS impairs mitochondria and mRNA profiles in oocytes

Polycystic ovary syndrome (PCOS) is the most common ovulatory defect in women. Although most PCOS patients are obese, a subset of PCOS women are lean but show similar risks for adverse fertility outcomes. A lean PCOS mouse model was created using prenatal androgen administration. This developmentally programmed mouse model was used for this study. Our objective was to investigate if mitochondrial structure and functions were compromised in oocytes obtained from lean PCOS mouse. The lean PCOS mouse model was validated by performing glucose tolerance test, HbA1c levels, body weight and estrous cycle analyses. Oocytes were isolated and were used to investigate inner mitochondrial membrane potential, oxidative stress, lipid peroxidation, ATP production, mtDNA copy number, transcript abundance, histology and electron microscopy. Our results demonstrate that lean PCOS mice has similar weight to that of the controls but exhibited glucose intolerance and hyperinsulinemia along with dysregulated estrus cycle. Analysis of their oocytes show impaired inner mitochondrial membrane function, elevated reactive oxygen species (ROS), increased RNA transcript abundance and aberrant ovarian histology. Electron microscopy of the oocytes showed impaired mitochondrial ultrastructure. In conclusion, the lean PCOS mouse model shows a decreased oocyte quality related to impaired mitochondrial ultrastructure and function.

The blueprint of RNA storages relative to oocyte developmental competence in cattle (Bos taurus)

From the time oocytes leave quiescence, there are constant microenvironmental influences contributing to development, thus acquiring developmental competence is not a simple, linear phenomenon. During folliculogenesis, oocytes experience many morphological and cytological changes that contribute toward the acquisition of developmental competence, a process defined by an oocyte’s ability to progress through folliculogenesis, be fertilized, undergo cleavage, and develop into an embryo. Many factors, such as ovarian follicle size, cow age, and the morphology of the cumulus–oocyte complex, have been extensively investigated to understand this process. In parallel to aiding in the understanding of oocyte biology, these features have been used to characterize an oocyte’s ability to achieve competence. In addition, oocytes undergo intense gene transcription and protein translation to accumulate the maternal stores. When the oocyte is fully grown, most genes are transcriptionally inactive, and the chromatin is densely compacted. More recently, RNA profiling has been used to further define the transcriptional parameters that are associated with oocyte development. Here, focusing on cattle, we provide an overview of the experimental models commonly used to understand the underlying biology related to oocyte developmental competence. We compiled public data and showed that cattle oocytes can express over 15 000 protein-coding genes, suggesting a complex transcriptome landscape. Surprisingly, less than 2% of the expressed genes have been linked to developmental competence. The identification of the gene products that contribute to oocyte development, and understanding their biological function, are a vital component of our quest toward defining oocyte developmental competence at the molecular level.

Lipid characterization of in vitro-produced bovine embryos with distinct kinetics of development

Human embryo studies have proposed the use of additional morphological evaluations related to the moment of the first cell divisions as relevant to embryo viability. Nevertheless, there are still not enough data available related to morphokinetic analysis and its relationship with lipid composition in embryos. Therefore, the aim of this study was to address the lipid profile of bovine embryos with different developmental kinetics: fast (four or more cells) and slow (two or three cells) at 40 h post-insemination (hpi), at three time points of in vitro culture (40, 112 and 186 hpi) and compare these to profiles of in vivo embryos. The lipid profiles of embryos were analyzed by matrix-assisted laser desorption ionization mass spectrometry, which mainly detected pools of membrane lipids such as phosphatidylcholine and sphingomyelin. In addition to their structural function, these lipid classes have an important role in cell signalling, particularly regarding events such as stress and pregnancy. Different patterns of lipids in the fast and slow groups were revealed in all the analyzed stages. Also, differences between in vitro embryos were more pronounced at 112 hpi, a critical moment due to embryonic genome activation. At the blastocyst stage, in vitro-produced embryos, despite the kinetics, had a closer lipid profile when compared with in vivo blastocysts. In conclusion, the kinetics of development had a greater effect on the membrane lipid profiles throughout the embryo culture, especially at the 8-16-cell stage. The in vitro environment affects lipid composition and may compromise cell signalling and function in blastocysts.

Superstimulation prior to the ovum pick-up improves the in vitro embryo production in nulliparous, primiparous and multiparous buffalo (Bubalus bubalis) donors

The aim of this study was to evaluate the ovarian follicular population, the oocyte yield and the in vitro embryo production (IVEP) of nulliparous (NU), primiparous (PR) and multiparous (MU) buffalo donors submitted to the superstimulation with FSH prior to the ovum pick-up (OPU). A total of 54 buffalo donors (18 NU, 15 PR and 21MU) received an intravaginal progesterone device (1.0 g) plus estradiol benzoate [2.0 mg, intramuscular (im)] at random stage of the estrous cycle (Day 0) during the breeding season (autumn and winter). Buffaloes from different categories were then randomly allocated to one of two groups (Control or FSH), in a cross-over experimental design. Buffalo donors in the Control group received no further treatment, whereas buffalo donors in the FSH group received a total dosage of 200 mg im of FSH on Days 4 and 5, in four decreasing doses 12 h apart (57, 57, 43 and 43 mg). On Day 7, the progesterone device was removed and the OPU procedure was performed in both groups. The same semen was used across all replicates and donor category. Data were analyzed by the GLIMMIX procedure of SAS 9.4^®. There was no interaction between FSH treatment and animal category for all analyzed variables. Furthermore, no differences between animal category (P = 0.73) and FSH treatment (P = 0.53) were observed regarding the total follicles aspirated. However, the FSH treatment increased (P < 0.001) the proportion of large (>10 mm; FSH = 16.2% and Control = 2.0%) and medium-sized follicles (6-10 mm; FSH = 36.3% and Control = 6.1%) available for the OPU procedure. The total of recovered oocytes was greater in NU than in MU, and PR were similar to NU and MU (P = 0.05). No effect of FSH treatment was observed (P = 0.85) for this variable. Buffalo donors treated with FSH had a greater viable oocytes rate (P = 0.03), blastocyst rate (P = 0.03) and embryo yield per OPU-IVEP session (P = 0.07), however, no category effects were observed for these variables. These results provided evidence that superstimulation with FSH increased the proportion of large and medium-sized follicles available for the OPU procedure. Consequently, the FSH treatment enhanced the proportion of viable oocytes for culture and resulted in greater blastocyst rates and embryo yield per OPU-IVEP session in all buffalo donors categories.

Genetic merit for fertility traits in Holstein cows: VI. Oocyte developmental competence and embryo development

The hypothesis of this study was that cows with good genetic merit for fertility traits (Fert+) would produce oocytes and embryos of greater quality than cows with poor genetic merit for fertility traits (Fert-) and that mRNA expression of candidate genes would reflect the observed differences in quality. The aim of the study, therefore, was to determine the effect of genetic merit for fertility traits on morphological classification and mRNA abundance of key genes in immature oocytes and cumulus cells following ovum pick-up and in embryos following superovulation, artificial insemination (AI), and uterine flushing. In experiment 1, 17 Fert+ and 11 Fert- cows, ranging from 54 to 84 d in milk, were submitted to ovum pick-up on 4 occasions during a 2-wk period. Recovered cumulus-oocyte complexes (COC) were morphologically graded. Oocytes and cumulus cells were separated, and mRNA abundance of genes associated with oocyte developmental competence was measured. There was no effect of genotype on the distribution of COC grades or on the mRNA abundance of the candidate genes in grade 1 COC. In experiment 2, 20 Fert+ and 19 Fert- cows, ranging from 71 to 189 d in milk, were submitted to superovulation and AI. The uteri of cows that responded to the superovulation protocol (17 Fert+ and 16 Fert- cows) were nonsurgically flushed 7 d postovulation. Recovered embryos were morphologically graded, and mRNA abundance of genes associated with embryo development was measured in grade 1 blastocysts. The response to the superovulation protocol was assessed by counting the number of codominant follicles on the day of AI, which was similar for both genotypes (22.0 ± 9.7 and 19.8 ± 8.2 for Fert+ and Fert- cows, respectively). There was no effect of genotype on the proportion of transferable embryos recovered or on the mRNA abundance of the candidate genes tested in the grade 1 blastocysts. Of the total embryos classified as blastocysts, however, the Fert+ cows tended to have a greater proportion of grade 1 blastocysts compared with Fert- cows (90% vs. 64%, respectively). In conclusion, genetic merit for fertility traits had a no effect on mRNA abundance of the candidate genes that were examined in immature oocytes and cumulus cells and in embryos recovered after superovulation. The observed differences in morphological blastocyst quality following superovulation would suggest that the superior reproductive performance of Fert+ cows could arise during the later stages of embryo development from d 7 until maternal recognition of pregnancy.

Proteomic analysis of germinal vesicles in the domestic cat model reveals candidate nuclear proteins involved in oocyte competence acquisition

STUDY QUESTION

Do nuclear proteins in the germinal vesicle (GV) contribute to oocyte competence acquisition during folliculogenesis?

SUMMARY ANSWER

Proteomic analysis of GVs identified candidate proteins for oocyte competence acquisition, including a key RNA processing protein-heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNPA2B1).

WHAT IS KNOWN ALREADY

The domestic cat GV, which is physiologically similar to the human GV, gains the intrinsic ability to resume meiosis and support early embryo development during the pre-antral-to-antral follicle transition. However, little is known about nuclear proteins that contribute to this developmental process.

STUDY DESIGN SIZE, DURATION

GVs were enriched from pre-antral (incompetent) and antral (competent) follicles from 802 cat ovaries. Protein lysates were subjected to quantitative proteomic analysis to identify differentially expressed proteins in GVs from the two follicular categories.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Two biological replicates (from independent pools of ovaries) of pre-antral versus antral samples were labeled by tandem mass tags and then assessed by liquid chromatography-tandem mass spectrometry. Proteomic data were analyzed according to gene ontology and a protein-protein interaction network. Immunofluorescent staining and protein inhibition assays were used for validation.

MAIN RESULTS AND THE ROLE OF CHANCE

A total of 174 nuclear proteins was identified, with 54 being up-regulated and 22 down-regulated (≥1.5-fold) after antrum formation. Functional protein analysis through gene ontology over-representation tests revealed that changes in molecular network within the GVs during this transitional phase were related to chromatin reorganization, gene transcription, and maternal RNA processing and storage. Protein inhibition assays verified that hnRNPA2B1, a key nuclear protein identified, was required for oocyte meiotic maturation and subsequent blastocyst formation.

LARGE SCALE DATA

Data are available via ProteomeXchange with identifier PXD007211.

LIMITATIONS REASONS FOR CAUTION

Proteins identified by proteomic comparison may (i) be involved in processes other than competence acquisition during the pre-antral-to-antral transition or (ii) be co-expressed in other macrostructures besides the GV. Expressional and functional validations should be performed for candidate proteins before downstream application.

WIDER IMPLICATIONS OF THE FINDINGS

Collective results generated a blueprint to better understand the molecular mechanisms involved in GV competence acquisition and identified potential nuclear competence markers for human fertility preservation.

STUDY FUNDING AND COMPETING INTEREST(S)

Funded by the National Center for Research Resources (R01 RR026064), a component of the National Institutes of Health (NIH) and currently by the Office of Research Infrastructure Programs/Office of the Director (R01 OD010948). The authors declare that there is no conflict of interest.

Retinoic acid improves maturation rate and upregulates the expression of antioxidant-related genes in in vitro matured buffalo (Bubalus bubalis) oocytes

Retinoic acid, vitamin A metabolite, plays a role in oocyte development and maturation in different ways including gene expression alteration and/or prohibiting oxidative stress. The objective of this study was to examine the effect of 9-cis-retinoic acid (9-cisRA) on the quality and maturation rate of buffalo oocytes. Cumulus-oocyte complexes (COCs, n = 460) were collected from ovaries of slaughtered buffalos. Varying concentrations of 9-cisRA (0, 5, 50, and 200 nM) were added to the maturation medium, and the following parameters were analyzed: (i) maturation and cleavage rates, (ii) mitochondrial activity and reactive oxygen species (ROS) levels, (iii) expression level of antioxidant-related genes (PRDX1, SOD1, CAT, HOMX1, and GPX4) using RT-qPCR. Maturation rate was significantly improved in 5 nM 9-cisRA oocyte group (95.8%, P < .05) compared to control and other treatment groups (86.7% in control group). The same oocyte group exhibited significantly higher mitochondrial membrane potential activity and lower ROS accumulation level compared to other treatment groups. Antioxidant-related genes were up-regulated in oocytes matured with 5 or 50 nM 9-cisRA compared to control and 200 nM 9-cisRA groups. In contrast, 200 nM of 9-cisRA showed a clear down-regulation for antioxidant-related genes except for PRDX1. In conclusion, supplementation of 9-cisRA with a lower concentration (5 nM) to the buffalo oocytes maturation media promotes maturation rate through a protection mechanism that maintains adequate levels of antioxidant-related transcripts and improves mitochondrial activity. However, 9-cisRA has no significant effect on the cleavage rate of the treated oocytes.

The consequences of porcine IVM medium supplementation with follicular fluid become reflected in embryo quality, yield and gene expression patterns

Oocyte and embryo developmental competence are shaped by multiple extrinsic and intrinsic factors. One of the most extensive research areas in the last decade is the regulation of lipid metabolism in oocytes and embryos of different species. We hypothesized that differences in developmental competence of oocytes and embryos between prepubertal and cyclic gilts may arise due to distinct fatty acid profiles in follicular fluid. We found that supplementation of oocyte maturation media with follicular fluid from prepubertal pigs affected quality and development of embryos from prepubertal pigs while embryos of cyclic pigs were not affected. PLIN2, SCD and ACACA transcripts involved in lipid metabolism were upregulated in embryos originating from oocytes of prepubertal pigs matured with autologous follicular fluid. The surface occupied by lipid droplets tend to increase in oocytes matured with follicular fluid from prepubertal pigs regardless oocyte origin. The change into follicular fluid of cyclic pigs increased the efficiency of embryo culture and improved quality, while gene expression was similar to embryos obtained from cyclic gilts. We assume that the follicular fluids of prepubertal and cyclic pigs influenced the quality of oocytes and embryos obtained from prepubertal pigs which are more susceptible to suboptimal in vitro culture conditions.

Comparative aspects of early lineage specification events in mammalian embryos - insights from reverse genetics studies

Within the mammalian class, formation of the blastocyst is morphologically highly conserved among different species. The molecular and cellular events during preimplantation embryo development have been studied extensively in the mouse as model organism, because multiple genetically defined strains and a plethora of reverse genetics tools are available to dissect specific gene functions and regulatory networks. However, major differences in preimplantation developmental kinetics, implantation, and placentation exist among mammalians, and recent studies in species other than mouse showed, that even regulatory mechanisms of the first lineage differentiation events and maintenance of pluripotency are not always conserved. Here, we focus on the first and the second lineage segregation in mouse and bovine embryos, when the first differentiated cell types emerge. We outline their common features and differences in the regulation of these essential events during embryonic development with a glance at further species. In addition, we show how new reverse genetics strategies aid the study of regulatory circuits in embryos of domestic species, enhancing our overall understanding of mammalian preimplantation development.

Transcriptional profiles of crossbred embryos derived from yak oocytes in vitro fertilized with cattle sperm

During mammalian pre-implantation embryonic development, dramatic and orchestrated changes occur in gene transcription. Pregnancy rates were low when yak females were crossbred with cattle breeds, but few studies exist to describe the unique molecular network regulation behind the pre-implantation development of these embryos. We determined the transcriptomes of crossbred embryos derived from yak oocytes in vitro fertilized with Jersey sperm using Illumina RNA-seq for the first time in this study. Embryos were sampled at the 2-, 4-, and 8-cell, morula and blastocyst stages. The results showed that in total, 291.9 million short reads were generated from the five libraries of 2-, 4-, and 8-cell, morula and blastocyst stages, with 276.2 million high-quality reads selected for further analysis. Eighty to 91% of the clean reads were aligned against the yak reference genome. A total of 19,072 transcripts were identified in five libraries, of which 7,785 transcripts were co-expressed in each stage and 2,013 transcripts were stage-specific. When a |log₂ ratio| ≥1 and q-value ≤ 0.05 were set as thresholds for identifying differentially expressed genes (DEGs), we detected a total of 3,690 to 10,298 DEGs between any two consecutive stages. Based on the results of GO and KEGG enrichment, some of these DEGs potentially play an important role in regulating pre-implantation development, but they are most likely stage-specific. There were 2,960, 7,287, 6,420, 7,724 and 10,417 DEGs in 2-, 4-, 8-cell, morula and blastocyst stages between the crossbred embryos and purebred embryos of the yak, respectively, leading to a large difference in GO terms and pathways. In conclusion, we sequenced transcriptomes of in vitro-produced crossbred embryos of yak and cattle during pre-implantation and provided comprehensive examinations of gene activities. These will be helpful for development of assisted reproductive technology and better understanding the early maternal-fetal or maternal-embryonic dialog in inter-species crossbreeding.

Characterization of transcriptional complexity during pre-implantation development of the yak (Bos grunniens) using RNA-Seq

The objective of this study was to investigate the mechanism that regulates pre-implantation development of the yak (Bos grunniens). We determined the transcriptomes of in vitro-produced yak embryos at two-cell, four-cell, eight-cell stages, and morula and blastocyst using the Illumina RNA-seq for the first time. We obtained 47.36-50.86 million clean reads for each stage, of which, 85.65%-90.02% reads were covered in the reference genome. A total of 17,368 genes were expressed during the two-cell stage to blastocyst of the yak, of which 7,236 genes were co-expressed at all stages, whereas 10,132 genes were stage-specific expression. Transcripts from 9,827 to 14,893 different genes were detected in various developmental stages. When |log₂ ratio| ≥ 1 and q-value <0.05 were set as thresholds for identifying differentially expressed genes (DEGs), we detected a total of 6,922-10,555 DEGs between any two consecutive stages. The GO distributions of these DEGs were classified into three categories: biological processes (23 terms), cellular components (22 terms) and molecular functions (22 terms). Pathway analysis revealed 310 pathways of the DEGs that were operative in early pre-implantation yak development, of which 32 were the significantly enriched pathways. In conclusion, this is the first report to investigate the mechanism that regulates yak embryonic development using high-throughput sequencing, which provides a comprehensive framework of transcriptome landscapes of yak pre-implantation embryos.

Single-cell RNA sequencing reveals developmental heterogeneity of blastomeres during major genome activation in bovine embryos

Embryonic development is initially controlled by maternal RNAs and proteins stored in the oocyte, until gene products gradually generated by the embryo itself take over. Major embryonic genome activation (EGA) in bovine embryos occurs at the eight- to 16-cell stage. Morphological observations, such as size of blastomeres and distribution of microvilli, suggested heterogeneity among individual cells already at this developmental stage. To address cell heterogeneity on the transcriptome level, we performed single-cell RNA sequencing of 161 blastomeres from 14 in vitro produced bovine embryos at Day 2 (n = 6) and Day 3 (n = 8) post fertilization. Complementary DNA libraries were prepared using the Single-Cell RNA-Barcoding and Sequencing protocol and sequenced. Non-supervised clustering of single-cell transcriptome profiles identified six clusters with specific sets of genes. Most embryos were comprised of cells from at least two different clusters. Sorting cells according to their transcriptome profiles resulted in a non-branched pseudo-time line, arguing against major lineage inclination events at this developmental stage. In summary, our study revealed heterogeneity of transcriptome profiles among single cells in bovine Day 2 and Day 3 embryos, suggesting asynchronous blastomere development during the phase of major EGA.

Optimal doses of EGF and GDNF act as biological response modifiers to improve porcine oocyte maturation and quality

It is well documented that both epidermal growth factor (EGF) and glial cell line-derived neurotrophic factor (GDNF) are critical for porcine oocyte maturation, however, little information is known about their mechanism of action in vitro. To gain insight into the mechanisms of action of the optimum doses of EGF and GDNF on porcine oocyte maturation, porcine cumulus–oocyte complexes (COCs) were matured in defined porcine oocyte medium supplemented with EGF, GDNF or a combination of both at varying concentrations (0–100 ng/ml) for 44 h. Nuclear and cytoplasmic maturation were determined in terms of nuclear stage after DNA staining with Hoechst and cortical granule distribution after lectin labeling, respectively. Mature oocytes were subsequently collected for gene expression analysis or subjected to in vitro fertilization and cultured for 7 days. The results showed that EGF and/or GDNF, when administered in a certain dose (50 ng/μl) to the maturation medium, not only effectively improved the synchronization of nuclear and cytoplasmic maturation processes within the oocyte, but enhanced expression of their corresponding receptors in mature oocytes (P < 0.05). Moreover, supplementation with an optimal combination of EGF + GDNF resulted in elevation of TFAM transcripts as well as a decrease of caspase-3 transcripts compared with the other studied groups (P < 0.05). Collectively, our results indicate that treatment of porcine oocytes with specific-dose combinations of EGF and GDNF stimulates oocyte quality and competence by transcriptional modulation of genes involved in oocyte survival and competence.

Sperm-carried RNAs play critical roles in mouse embryonic development

Recently, numerous studies have reported that the mature sperm contains both coding and non-coding RNAs and the sperm delivers some RNAs to the oocyte at fertilization. However, the functions of the RNAs carried to the oocyte by sperm at fertilization in embryonic development remains a mystery. In this study, the mature spermatozoa were treated with lysolecithin, pronase and RNases (RNase A and RNase H) to remove the sperm-carried RNAs, and then injected into normal mature oocyte. The results showed that after the treatment, the content of the sperm RNAs was decreased by about 90%. The blastocyst formation rate and the live birth rate of the embryos from intracytoplasmic sperm injection (ICSI) using the treated sperm were significantly decreased (P<0.01), while these effects were partially rescued by injecting total wide-type sperm RNAs. The reproductive capacity of offspring (F0) in sperm-treated group was similar with that in control group (P>0.05), but the body weight of F1 mice from sperm-treated group was lower than that in control group after two weeks of birth (P<0.05). These results demonstrated that the sperm-carried RNAs have important roles in embryonic development.

Characteristics of candidate genes associated with embryonic development in the cow: Evidence for a role for WBP1 in development to the blastocyst stage

The goal was to gain understanding of how 12 genes containing SNP previously related to embryo competence to become a blastocyst (BRINP3, C1QB, HSPA1L, IRF9, MON1B, PARM1, PCCB, PMM2, SLC18A2, TBC1D24, TTLL3 and WBP1) participate in embryonic development. Gene expression was evaluated in matured oocytes and embryos. BRINP3 and C1QB were not detected at any stage. For most other genes, transcript abundance declined as the embryo developed to the blastocyst stage. Exceptions were for PARM1 and WBP1, where steady-state mRNA increased at the 9-16 cell stage. The SNP in WBP1 caused large differences in the predicted three-dimensional structure of the protein while the SNP in PARM1 caused smaller changes. The mutation in WBP1 causes an amino acid substitution located close to a P-P-X-Y motif involved in protein-protein interactions. Moreover, the observation that the reference allele varies between mammalian species indicates that the locus has not been conserved during mammalian evolution. Knockdown of mRNA for WBP1 decreased the percent of putative zygotes becoming blastocysts and reduced the number of trophectoderm cells and immunoreactive CDX2 in the resulting blastocysts. WBP1 is an important gene for embryonic development in the cow. Further research to identify how the SNP in WBP1 affects processes leading to differentiation of the embryo into TE and ICM lineages is warranted.

Peptidylarginine deiminase 1-catalyzed histone citrullination is essential for early embryo development

Peptidylarginine deiminase (PADI) enzymes are increasingly being associated with the regulation of chromatin structure and gene activity via histone citrullination. As one of the PADI family members, PADI1 has been mainly reported to be expressed in the epidermis and uterus, where the protein in keratinocytes is thought to promote differentiation by citrullinating filament proteins. However, the roles of PADI1 in preimplantation development have not been addressed. Using a PADI1-specific inhibitor and Padi1-morpholino knockdown, we found that citrullination of histone tails at H4R3 and H3R2/8/17 were markedly reduced in the 2- and 4-cell embryos. Consistent with this observation, early embryo development was also arrested at the 4-cell stage upon depletion of PADI1 or inhibition of PADI1 enzyme activity. Additionally, by employing 5-ethynyl uridine (EU) incorporation analysis, ablation of PADI1 function led to a dramatic decrease in overall transcriptional activity, correlating well with the reduced levels of phosphorylation of RNA Pol II at Ser2 observed at 2- or 4-cell stage of embryos under Padi1 knockdown or inhibiting PADI1. Thus, our data reveal a novel function of PADI1 during early embryo development transitions by catalyzing histone tail citrullination, which facilitates early embryo genome transactivation.

Transcriptome activity of isochores during preimplantation process in human and mouse

This work investigates the role of isochores during preimplantation process. Using RNA-seq data from human and mouse preimplantation stages, we created the spatio-temporal transcriptional profiles of the isochores during preimplantation. We found that from early to late stages, GC-rich isochores increase their expression while GC-poor ones decrease it. Network analysis revealed that modules with few coexpressed isochores are GC-poorer than medium-large ones, characterized by an opposite expression as preimplantation advances, decreasing and increasing respectively. Our results reveal a functional contribution of the isochores, supporting the presence of structural-functional interactions during maturation and early-embryonic development.

Gamete activation: basic knowledge and clinical applications

Background

The first clues to the process of gamete activation date back to nearly 60 years ago. The mutual activation of gametes is a crucial event during fertilization. In the testis and ovaries, spermatozoa and oocytes are in a state of meiotic and metabolic quiescence and require reciprocal signals in order to undergo functional changes that lead to competence for fertilization. First, the oocyte activates sperm by triggering motility, chemoattraction, binding and the acrosome reaction, culminating with the fusion of the two plasma membranes. At the end of this cascade of events, collectively known as sperm capacitation, sperm-induced oocyte activation occurs, generating electrical, morphological and metabolic modifications in the oocyte.

Objective and rationale

The aim of this review is to provide the current state of knowledge regarding the entire process of gamete activation in selected specific animal models that have contributed to our understanding of fertilization in mammals, including humans. Here we describe in detail the reciprocal induction of the two activation processes, the molecules involved and the mechanisms of cell interaction and signal transduction that ultimately result in successful embryo development and creation of a new individual.

Search methods

We carried out a literature survey with no restrictions on publication date (from the early 1950s to March 2016) using PubMed/Medline, Google Scholar and Web of Knowledge by utilizing common keywords applied in the field of fertilization and embryo development. We also screened the complete list of references published in the most recent research articles and relevant reviews published in English (both animal and human studies) on the topics investigated.

Outcomes

Literature on the principal animal models demonstrates that gamete activation is a pre-requisite for successful fertilization, and is a process common to all species studied to date. We provide a detailed description of the dramatic changes in gamete morphology and behavior, the regulatory molecules triggering gamete activation and the intracellular ions and second messengers involved in active metabolic pathways in different species. Recent scientific advances suggest that artificial gamete activation may represent a novel technique to improve human IVF outcomes, but this approach requires caution.

Wider implications

Although controversial, manipulation of gamete activation represents a promising tool for ameliorating the fertilization rate in assisted reproductive technologies. A better knowledge of mechanisms that transform the quiescent oocyte into a pluripotent cell may also provide new insights for the clinical use of stem cells.

The Principal Forces of Oocyte Polarity Are Evolutionary Conserved but May Not Affect the Contribution of the First Two Blastomeres to the Blastocyst Development in Mammals

Oocyte polarity and embryonic patterning are well-established features of development in lower species. Whether a similar form of pre-patterning exists in mammals is currently under hot debate in mice. This study investigated this issue for the first time in ovine as a large mammal model. Microsurgical trisection of unfertilized MII-oocytes revealed that cortical cytoplasm around spindle (S) contained significant amounts of total maternal mRNAs and proteins compared to matched cytoplast hemispheres that were located either near (NS) or far (FS) -to-spindle. RT-qPCR provided striking examples of maternal mRNA localized to subcellular substructures S (NPM2, GMNN, H19, PCAF, DNMT3A, DNMT1, and STELLA), NS (SOX2, NANOG, POU5F1, and TET1), and FS (GCN) of MII oocyte. Immunoblotting revealed that specific maternal proteins DNMT3A and NANOG were asymmetrically enriched in MII-spindle-half of the oocytes. Topological analysis of sperm entry point (SEP) revealed that sperm preferentially entered via the MII-spindle-half of the oocytes. Even though, the topological position of first cleavage plane with regard to SEP was quite stochastic. Spatial comparison of lipid content revealed symmetrical distribution of lipids between 2-cell blastomeres. Lineage tracing using Dil, a fluorescent dye, revealed that while the progeny of leading blastomere of 2-cell embryos contributed to more cells in the developed blastocysts compared to lagging counterpart, the contributions of leading and lagging blastomeres to the embryonic-abembryonic parts of the developed blastocysts were almost unbiased. And finally, separated sister blastomeres of 2-cell embryos had an overall similar probability to arrest at any stage before the blastocyst (2-cell, 4-cell, 8-cell, and morula) or to achieve the blastocyst stage. It was concluded that the localization of maternal mRNAs and proteins at the spindle are evolutionarily conserved between mammals unfertilized ovine oocyte could be considered polar with respect to the spatial regionalization of maternal transcripts and proteins. Even though, the principal forces of this definitive oocyte polarity may not persist during embryonic cleavages.

Lipidome signatures in early bovine embryo development

Mammalian preimplantation embryonic development is a complex, conserved, and well-orchestrated process involving dynamic molecular and structural changes. Understanding membrane lipid profile fluctuation during this crucial period is fundamental to address mechanisms governing embryogenesis. Therefore, the aim of the present work was to perform a comprehensive assessment of stage-specific lipid profiles during early bovine embryonic development and associate with the mRNA abundance of lipid metabolism-related genes (ACSL3, ELOVL5, and ELOVL6) and with the amount of cytoplasmic lipid droplets. Immature oocytes were recovered from slaughterhouse-derived ovaries, two-cell embryos, and eight- to 16-cell embryos, morula, and blastocysts that were in vitro produced under different environmental conditions. Lipid droplets content and mRNA transcript levels for ACSL3, ELOVL5, and ELOVL6, monitored by lipid staining and quantitative polymerase chain reaction, respectively, increased at morula followed by a decrease at blastocyst stage. Relative mRNA abundance changes of ACSL3 were closely related to cytoplasmic lipid droplet accumulation. Characteristic dynamic changes of phospholipid profiles were observed during early embryo development and related to unsaturation level, acyl chain length, and class composition. ELOVL5 and ELOVL6 mRNA levels were suggestive of overexpression of membrane phospholipids containing elongated fatty acids with 16, 18, and 20 carbons. In addition, putative biomarkers of key events of embryogenesis, embryo lipid accumulation, and elongation were identified. This study provides a comprehensive description of stage-specific lipidome signatures and proposes a mechanism to explain its potential relationship with the fluctuation of both cytoplasmic lipid droplets content and mRNA levels of lipid metabolism-related genes during early bovine embryo development.

Detection of genes associated with developmental competence of bovine oocytes

The developmental competence of oocytes is acquired progressively during folliculogenesis and is linked to follicular size. It has been documented that oocytes originating from larger follicles exhibit a greater ability to develop to the blastocyst stage. The differences in cytoplasmic factors such as mRNA transcripts could explain the differences in oocyte developmental potential. We used bovine oligonucleotide microarrays to characterize differences between the gene expression profiles of germinal vesicle stage (GV) oocytes with greater developmental competence from medium follicles (MF) and those with less developmental competence from small follicles (SF). After normalizing the microarray data, our analysis found differences in the level of 60 transcripts (≥1.4 fold), corresponding to 49 upregulated and 11 downregulated transcripts in MF oocytes compared to SF oocytes. The gene expression data were classified according to gene ontology, the majority of the genes were associated with the regulation of transcription, translation, the cell cycle, and mitochondrial activity. A subset of 16 selected genes was validated for GV oocytes by quantitative real-time RT-PCR; significant differences (P˂0.01) were found in the level of TAF1A, MTRF1L, ATP5C1, UBL5 and MAP3K13 between the MF and SF oocytes. After maturation the transcript level remained stable for ATP5F1, BRD7, and UBL5 in both oocyte categories. The transcript level of another 13 genes substantially dropped in the MF and/or SF oocytes. It can be concluded that the developmental competence of bovine oocytes and embryos may be a quantitative trait dependent on small changes in the transcription profiles of many genes.

The ART of selecting the best embryo: A review of early embryonic mortality and bovine embryo viability assessment methods

Animal reproductive biotechnology is continually evolving. Significant advances have been made in our understanding of early embryonic mortality and embryo development in domestic animals, which has improved the selection and success of in vitro technologies. Yet our knowledge is still relatively limited such that identifying a single embryo with the highest chance of survival and development for transfer remains challenging. While invasive methods such as embryo biopsy can provide useful information regarding the genetic status of the embryos, morphological assessment remains the most common evaluation. A recent shift, however, favors alternative, adjunct approaches for non-invasive assessment of an embryo's viability and developmental potential. Various analytical techniques have facilitated the evaluation of cellular health through the metabolome, the assessment of end products of cellular metabolism, or by analyzing spent media for small RNAs. This review discusses the application of noninvasive approaches for ascertaining the health and viability of in vitro-produced bovine embryos. A comparative analysis of noninvasive techniques for embryo assessment currently being investigated in cattle and humans is also discussed.

Genes and Conditions Controlling Mammalian Pre- and Post-implantation Embryo Development

Embryo quality during the in vitro developmental period is of great clinical importance. Experimental genetic studies during this period have demonstrated the association between specific gene expression profiles and the production of healthy blastocysts. Although the quality of the oocyte may play a major role in embryo development, it has been well established that the post - fertilization period also has an important and crucial role in the determination of blastocyst quality. A variety of genes (such as OCT, SOX2, NANOG) and their related signaling pathways as well as transcription molecules (such as TGF-β, BMP) have been implicated in the pre- and post-implantation period. Furthermore, DNA methylation has been lately characterized as an epigenetic mark since it is one of the most important processes involved in the maintenance of genome stability. Physiological embryo development appears to depend upon the correct DNA methylation pattern. Due to the fact that soon after fertilization the zygote undergoes several morphogenetic and developmental events including activation of embryonic genome through the transition of the maternal genome, a diverse gene expression pattern may lead to clinically important conditions, such as apoptosis or the production of a chromosomically abnormal embryo. The present review focused on genes and their role during pre-implantation embryo development, giving emphasis on the various parameters that may alter gene expression or DNA methylation patterns. The pre-implantation embryos derived from in vitro culture systems (in vitro fertilization) and the possible effects on gene expression after the prolonged culture conditions are also discussed.

A mRNA landscape of bovine embryos after standard and MAPK-inhibited culture conditions: a comparative analysis

Background

Genes and signalling pathways involved in pluripotency have been studied extensively in mouse and human pre-implantation embryos and embryonic stem (ES) cells. The unsuccessful attempts to generate ES cell lines from other species including cattle suggests that other genes and pathways are involved in maintaining pluripotency in these species. To investigate which genes are involved in bovine pluripotency, expression profiles were generated from morula, blastocyst, trophectoderm and inner cell mass (ICM) samples using microarray analysis. As MAPK inhibition can increase the NANOG/GATA6 ratio in the inner cell mass, additionally blastocysts were cultured in the presence of a MAPK inhibitor and changes in gene expression in the inner cell mass were analysed.

Results

Between morula and blastocyst 3,774 genes were differentially expressed and the largest differences were found in blastocyst up-regulated genes. Gene ontology (GO) analysis shows lipid metabolic process as the term most enriched with genes expressed at higher levels in blastocysts. Genes with higher expression levels in morulae were enriched in the RNA processing GO term. Of the 497 differentially expressed genes comparing ICM and TE, the expression of NANOG, SOX2 and POU5F1 was increased in the ICM confirming their evolutionary preserved role in pluripotency. Several genes implicated to be involved in differentiation or fate determination were also expressed at higher levels in the ICM. Genes expressed at higher levels in the ICM were enriched in the RNA splicing and regulation of gene expression GO term. Although NANOG expression was elevated upon MAPK inhibition, SOX2 and POU5F1 expression showed little increase. Expression of other genes in the MAPK pathway including DUSP4 and SPRY4, or influenced by MAPK inhibition such as IFNT, was down-regulated.

Conclusion

The data obtained from the microarray studies provide further insight in gene expression during bovine embryonic development. They show an expression profile in pluripotent cells that indicates a pluripotent, epiblast-like state. The inability to culture ICM cells as stem cells in the presence of an inhibitor of MAPK activity together with the reported data indicates that MAPK inhibition alone is not sufficient to maintain a pluripotent character in bovine cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1448-x) contains supplementary material, which is available to authorized users.

Cytoplasmic, rather than nuclear-DNA, insufficiencies as the major cause of poor competence of vitrified oocytes

To unravel the differential contributions of nuclear-DNA and cytoplasm to the poor 'competence' of oocytes after cryopreservation, reciprocal exchange of metaphase II-spindle chromosomal complex (karyoplast) between vitrified and fresh oocytes was carried out in an ovine animal model. Karyoplast exchange per se was accomplished with high efficiency and in-vitro development of oocytes reconstituted with fresh-karyoplast and vitrified-cytoplast (FK/VC) showed no improvement over VK/VC and control-vitrification oocytes. Blastocyst development of oocytes that were reconstituted with vitrified-karyoplast and fresh-cytoplast (VK/FC) approached that of fresh-controls, however, and was significantly higher than FK/VC, VK/VC, and control-vitrification (all P ≤ 0.05). These results point toward 'cytoplasmic insufficiencies' as the main cause of poor 'competence' of matured oocytes after vitrification.

Usefulness of bovine and porcine IVM/IVF models for reproductive toxicology

Women presenting fertility problems are often helped by Assisted Reproductive Techniques (ART), such as in vitro fertilization (IVF) programs. However, in many cases the etiology of the in/subfertility remains unknown even after treatment. Although several aspects should be considered when assisting a woman with problems to conceive, a survey on the patients' exposure to contaminants would help to understand the cause of the fertility problem, as well as to follow the patient properly during IVF. Daily exposure to toxic compounds, mainly environmental and dietary ones, may result in reproductive impairment. For instance, because affects oocyte developmental competence. Many of these compounds, natural or synthetic, are endocrine disruptors or endocrine active substances that may impair reproduction. To understand the risks and the mechanism of action of such chemicals in human cells, the use of proper in vitro models is essential. The present review proposes the bovine and porcine models to evaluate toxic compounds on oocyte maturation, fertilization and embryo production in vitro. Moreover, we discuss here the species-specific differences when mice, bovine and porcine are used as models for human.

MicroRNA-34 family expression in bovine gametes and preimplantation embryos

BACKGROUND

Oocyte fertilization and successful embryo implantation are key events marking the onset of pregnancy. In sexually reproducing organisms, embryogenesis begins with the fusion of two haploid gametes, each of which has undergone progressive stages of maturation. In the final stages of oocyte maturation, minimal transcriptional activity is present and regulation of gene expression occurs primarily at the post-transcriptional level. MicroRNAs (miRNA) are potent effectors of post-transcriptional gene silencing and recent evidence demonstrates that the miR-34 family of miRNA are involved in both spermatogenesis and early events of embryogenesis.

METHODS

The profile of miR-34 miRNAs has not been characterized in gametes or embryos of Bos taurus. We therefore used quantitative reverse transcription PCR (qRT-PCR) to examine this family of miRNAs: miR-34a, -34b and -34c as well as their precursors in bovine gametes and in vitro produced embryos. Oocytes were aspirated from antral follicles of bovine ovaries, and sperm cells were isolated from semen samples of 10 bulls with unknown fertility status. Immature and in vitro matured oocytes, as well as cleaved embryos, were collected in pools. Gametes, embryos and ovarian and testis tissues were purified for RNA.

RESULTS

All members of the miR-34 family are present in bovine spermatozoa, while only miR-34a and -34c are present in oocytes and cleaved (2-cell) embryos. Mir-34c demonstrates variation among different bulls and is consistently expressed throughout oocyte maturation and in the embryo. The primary transcript of the miR-34b/c bicistron is abundant in the testes and present in ovarian tissue but undetectable in oocytes and in mature spermatozoa.

CONCLUSIONS

The combination of these findings suggest that miR-34 miRNAs may be required in developing bovine gametes of both sexes, as well as in embryos, and that primary miR-34b/c processing takes place before the completion of gametogenesis. Individual variation in sperm miR-34 family abundance may offer potential as a biomarker of male bovine fertility.