Individual bovine in vitro embryo production and cumulus cell transcriptomic analysis to distinguish cumulus-oocyte complexes with high or low developmental potential

RNA sequencing and gene co-expression network of in vitro matured oocytes and blastocysts of buffalo

In reproductive technologies, uncovering the molecular aspects of oocyte and embryo competence under different conditions is crucial for refining protocols and enhancing efficiency. RNA-seq generates high-throughput data and provides transcriptomes that can undergo additional computational analyses. This study presented the transcriptomic profiles of in vitro matured oocytes and blastocysts produced in vitro from buffalo crossbred (Bubalus bubalis), coupled with gene co-expression and module preservation analysis. Cumulus Oophorus Complexes, obtained from slaughterhouse-derived ovaries, were subjected to in vitro maturation to yield metaphase II oocytes (616) or followed in vitro fertilization and culture to yield blastocysts for sequencing (526). Oocyte maturation (72%, ±3.34 sd) and embryo development (21.3%, ±4.18 sd) rates were obtained from three in vitro embryo production routines following standard protocols. Sequencing of 410 metaphase II oocytes and 70 hatched blastocysts (grade 1 and 2) identified a total of 13,976 genes, with 62% being ubiquitously expressed (8,649). Among them, the differentially expressed genes (4,153) and the strongly variable genes with the higher expression (fold-change above 11) were highlighted in oocytes (BMP15, UCHL1, WEE1, NLRPs, KPNA7, ZP2, and ZP4) and blastocysts (APOA1, KRT18, ANXA2, S100A14, SLC34A2, PRSS8 and ANXA2) as representative indicators of molecular quality. Additionally, genes exclusively found in oocytes (224) and blastocysts (2,200) with specific biological functions were identified. Gene co-expression network and module preservation analysis revealed strong preservation of functional modules related to exosome components, steroid metabolism, cell proliferation, and morphogenesis. However, cell cycle and amino acid transport modules exhibited weak preservation, which may reflect differences in embryo development kinetics and the activation of cell signaling pathways between buffalo and bovine. This comprehensive transcriptomic profile serves as a valuable resource for assessing the molecular quality of buffalo oocytes and embryos in future in vitro embryo production assays.

Biochemical profiling of the follicular environment to predict oocyte competence in cattle

To identify markers of oocyte competence, we compared the biochemical characteristics of fluid and cells from follicles containing oocytes with different capacities to form an embryo. Follicles (5-6 mm) were dissected, and follicular fluid (FF), granulosa cells (GC), cumulus cells (CC) from immature and mature cumulus-oocyte-complexes (COC) were individually collected. The oocytes were matured, fertilized, and cultured individually until day 8 (D8) of development. On D8, the samples were grouped according to embryo production into those that gave rise to blastocysts (EMB) and those that did not reach the blastocyst stage (NEMB). In CCs from immature and mature COCs and GCs, expression of CASP3, SERPINE2, VCAN, LUM, FSHR, EGFR, PGR, and GHR genes was quantified. Cell-free DNA (cfDNA), progesterone, and estradiol concentrations in the FF were determined. Data were analyzed by Mann-Whitney U test (GraphPad Prism 9). GHR was highly expressed in immature CCs from the EMB group, whereas CASP3 was highly expressed in mature CCs from the NEMB group (P<0.05). During maturation, the expression of CASP3 and GHR genes increased only in the NEMB group. ART2 cfDNA was highly detected in FF of the NEMB compared to the EMB group. Progesterone concentration was similar between the groups, whereas estradiol concentration was higher (P<0.05) in the EMB than in the NEMB group. It was concluded that a higher level of GHR transcripts in immature CCs, lower CASP3 expression in CCs from matured COCs, lower levels of ART2, and higher estradiol concentrations in FF may indicate oocytes with greater potential for development.

A L-glutamine binding protein modified MNM structured optical fiber biosensor based on surface plasmon resonance sensing for detection of L-glutamine metabolism in vitro embryo culture

A surface plasmon resonance (SPR) optical fiber sensor with multimode-coreless-multimode (MNM) structure was developed, which modified by L-glutamine-binding protein (QBP) for detection of L-glutamine (Gln). The QBP was immobilized on the surface of gold films by chemical cross-linking and exhibited a binding affinity for L-glutamine. The conformation of QBP can be changed from the "open" to the "closed", which led to a red-shift of the SPR peak when QBP bounded to L-glutamine. There was a good linear correlation between is a dependence of the SPR peak on and the concentration of L-glutamine concentration in the range 10-100 μM, with a sensitivity of 10.797nm/log10[Gln] for L-glutamine in the in vitro embryo culture (IVC) medium environment, and the limit of detection (LOD) is 1.187 μM. This QBP-modified MNM structure optical fiber SPR sensor provides a new idea for the developmental potential assessment of embryos in the process of in vitro embryo culture.

Cumulus Cell Transcriptome after Cumulus-Oocyte Complex Exposure to Nanomolar Cadmium in an In Vitro Animal Model of Prepubertal and Adult Age

Cadmium (Cd), a highly toxic pollutant, impairs oocyte fertilization, through oxidative damage on cumulus cells (CCs). This study analysed the transcriptomic profile of CCs of cumulus-oocyte complexes (COCs) from adult and prepubertal sheep, exposed to Cd nanomolar concentration during in vitro maturation. In both age-groups, CCs of matured oocytes underwent RNA-seq, data analysis and validation. Differentially expressed genes (DEGs) were identified in adult (n = 99 DEGs) and prepubertal (n = 18 DEGs) CCs upon Cd exposure. Transcriptomes of adult CCs clustered separately between Cd-exposed and control samples, whereas prepubertal ones did not as observed by Principal Component Analysis. The transcriptomic signature of Cd-induced CC toxicity was identified by gene annotation and literature search. Genes associated with previous studies on ovarian functions and/or Cd effects were confirmed and new genes were identified, thus implementing the knowledge on their involvement in such processes. Enrichment and validation analysis showed that, in adult CCs, Cd acted as endocrine disruptor on DEGs involved in hormone biosynthesis, cumulus expansion, regulation of cell signalling, growth and differentiation and oocyte maturation, whereas in prepubertal CCs, Cd affected DEGs involved in CC development and viability and CC-oocyte communications. In conclusion, these DEGs could be used as valuable non-invasive biomarkers for oocyte competence.

The human cumulus cell transcriptome provides poor predictive value for embryo transfer outcome

RESEARCH QUESTION

Is the transcriptome of cumulus cells a good predictor of the embryo's developmental competence?

DESIGN

Cumulus cells were collected from donor oocytes and their transcriptome was analysed by RNA sequencing analysis at >30 × 10⁶ reads in samples grouped according to the developmental potential of their enclosed oocyte: not able to develop to the blastocyst stage (Bl-), able to develop to the blastocyst stage but failing to establish a pregnancy (P-), or able to develop to the blastocyst stage and to establish a clinical pregnancy (P+).

RESULTS

The cumulus cell trancriptome was largely independent of the developmental potential as, using a false dscovery rate-adjusted P-value of <0.05, only 10, 11 and 5 genes were differentially expressed for the comparisons P+ versus P-, P+ versus Bl-, and P- versus Bl-, respectively, out of a total of 17,469 genes expressed. Between the differentially expressed genes, those showing little overlap between samples from different groups were CHAC1, up-regulated in the P- and P+ groups compared with the Bl- group, and CENPE, CD93, PECAM1 and HSPA1B, which showed the opposite expression pattern. Focusing on the pregnancy potential, only EPN3 was consistently downregulated in the P+ compared with the P- and Bl- groups.

CONCLUSIONS

The cumulus cell transcriptome is largely unrelated to the establishment of clinical pregnancy following embryo transfer, although the expression level of a subset of genes in cumulus cells may indicate the ability to develop to the blastocyst stage.

Maternal metabolic health and fertility: we should not only care about but also for the oocyte!

Metabolic disorders due to obesity and unhealthy lifestyle directly alter the oocyte's microenvironment and impact oocyte quality. Oxidative stress and mitochondrial dysfunction play key roles in the pathogenesis. Acute effects on the fully grown oocytes are evident, but early follicular stages are also sensitive to metabolic stress leading to a long-term impact on follicular cells and oocytes. Improving the preconception health is therefore of capital importance but research in animal models has demonstrated that oocyte quality is not fully recovered. In the in vitro fertilisation clinic, maternal metabolic disorders are linked with disappointing assisted reproductive technology results. Embryos derived from metabolically compromised oocytes exhibit persistently high intracellular stress levels due to weak cellular homeostatic mechanisms. The assisted reproductive technology procedures themselves form an extra burden for these defective embryos. Minimising cellular stress during culture using mitochondrial-targeted therapy could rescue compromised embryos in a bovine model. However, translating such applications to human in vitro fertilisation clinics is not simple. It is crucial to consider the sensitive epigenetic programming during early development. Research in humans and relevant animal models should result in preconception care interventions and in vitro strategies not only aiming at improving fertility but also safeguarding offspring health.

Using Cumulus Cell Biopsy as a Non-Invasive Tool to Access the Quality of Bovine Oocytes: How Informative Are They?

The present study aimed to determine whether cumulus cells (CC) biopsy, acquired before or after in vitro maturation (IVM), presents similar gene expression pattern and if would compromises oocyte quality. First, immature cumulus oocyte complexes (COCs) were distributed: (1) maturated in groups (control); (2) individually maturated, but not biopsied; (3) subjected to CC biopsy before maturation and individually matured; (4) individually matured and submitted to CC biopsy after maturation; (5) individually matured and CC biopsied before and after maturation. Secondly, candidate genes, described as potential markers of COCs quality, were quantified by RT-qPCR in CCs before and after IVM. After in vitro fertilization (IVF), zygotes were tracked and sorted regarding their developmental potential: fully developed to embryo, cleaved and arrested, and not-cleaved. The COC’s biopsy negatively affects embryo development (p < 0.05), blastocyst cell number (p < 0.05), and apoptotic cell ratio (p < 0.05), both before and after IVM. The PTGS2, LUM, ALCAM, FSHR, PGR, SERPINE2, HAS2, and PDRX3 genes were differentially expressed (p < 0.05) on matured CCs. Only PGR gene (p = 0.04) was under-expressed on matured CCs on Not-Cleaved group. The SERPINE2 gene was overexpressed (p = 0.01) in the Cleaved group on immature CCs. In summary, none of the selected gene studies can accurately predict COC’s fate after fertilization.

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.

Protein source in maturation media affects gene expression in cumulus cells and embryo development in cattle

We aimed to evaluate if protein source (PS) alterations during IVM affect embryo sex/development and gene expression profile in cumulus cells (CCs). Bovine oocytes were matured and cultured in the presence of FBS or BSA. Then, the PS effect during IVM on gene expression (GPC4, VCAN, GHR, PTGS2, and ALCAM) was determined. CC biopsy was removed before and after IVM treatments. After fertilization and cultured, CCs were grouped according to their fate into CCs from immature COCs, CCs from COCs that did or did not result in embryos (according to PS). Results showed that when the culture was performed in FBS presence, blastocyst rate was higher (p < 0.05) than BSA. However, when embryos were cultured with BSA, no effect (p > 0.05) of PS during IVM was observed. PS used during IVM did not affect embryos sex (p > 0.05) but changed VCAN, GHR, PTGS2, and ALCAM genes expression. No differences (p > 0.05) were observed between immature and mature CCs groups in gene expression, regardless of their fate. Only the GHR gene was related to embryo production but just with FBS on IVM. In conclusion, PS can affect embryo development when using the serum on IVM and IVC, influences CCs gene expression, and has to be considered when studying oocyte quality markers.

Metabolic exchanges between the oocyte and its environment: focus on lipids

Finely regulated fatty acid (FA) metabolism within ovarian follicles is crucial to follicular development and influences the quality of the enclosed oocyte, which relies on the surrounding intra-follicular environment for its growth and maturation. A growing number of studies have examined the association between the lipid composition of follicular compartments and oocyte quality. In this review, we focus on lipids, their possible exchanges between compartments within the ovarian follicle and their involvement in different pathways during oocyte final growth and maturation. Lipidomics provides a detailed snapshot of the global lipid profiles and identified lipids, clearly discriminating the cells or fluid from follicles at distinct physiological stages. Follicular fluid appears as a main mediator of lipid exchanges between follicular somatic cells and the oocyte, through vesicle-mediated and non-vesicular transport of esterified and free FA. A variety of expression data allowed the identification of common and cell-type-specific actors of lipid metabolism in theca cells, granulosa cells, cumulus cells and oocytes, including key regulators of FA uptake, FA transport, lipid transformation, lipoprotein synthesis and protein palmitoylation. They act in harmony to accompany follicular development, and maintain intra-follicular homeostasis to allow the oocyte to accumulate energy and membrane lipids for subsequent meiotic divisions and first embryo cleavages.

Accompaniment of Time-Lapse Parameters and Cumulus Cell RNA-Sequencing in Embryo Evaluation

The aim of this study was to investigate the use of time-lapse morphokinetic parameters and cumulus cells transcriptomic profile to achieve a more accurate and non-invasive method in embryo evaluation. Two hundred embryos from 20 couples were evaluated based on morphokinetic characteristics using time-lapse. Embryos were divided into the high-quality, moderate-quality, and bad-quality groups. Non-fertilized oocytes were considered as the fourth group. T5 (time to five cells), S2 (time from three to four cells), and CC2 (time from two to three cells) were recorded. Also, the cumulus cells of the respective oocytes were divided into high-quality, moderate-quality, bad-quality, and non-fertilized groups based on the grading of the embryos. Then their transcriptomic profiles were analyzed by RNA-sequencing. Finally, the correlation between differentially expressed genes and embryo time-lapse parameters was investigated. T5 was the only timing that showed a statistically significant difference between high-quality group and other groups. RNA-sequencing results showed that 37 genes were downregulated and 106 genes were upregulated in moderate, bad-quality, and non-fertilized groups compared to high-quality group (q value < 0.05). These genes were involved in the main biological processes such as cell cycle, DNA repair, cell signaling and communication, transcription, and cell metabolism. Embryos graded in different groups showed different transcriptomic profiles in the related cumulus cells. Therefore, it seems that embryo selection using the combination of cytokinetics and cumulus cells gene expression can improve the accuracy of the embryo selection and pregnancy rate in ART clinics.

Oocyte Morphometric Assessment and Gene Expression Profiling of Oocytes and Cumulus Cells as Biomarkers of Oocyte Competence in Sheep

Oocyte quality is crucial for subsequent embryo development and so it is a major challenge in assisted reproductive technologies. The aim of the present work was to evaluate the morphometric parameters of oocytes (experiment 1) and the relative gene expression of oocytes and cumulus cells (CCs) (experiment 2) as biomarkers of oocyte quality after individually culturing them (one oocyte or embryo/drop). In experiment 1, individually matured oocytes were measured and classified into small, intermediate, and large oocytes after a cluster analysis, based on total diameter (with zona pellucida, ZP), oocyte diameter (without ZP), and ZP thickness. These oocytes were individually fertilized in vitro and cultured. The embryo development was evaluated up to the blastocyst stage. According to the total diameter, oocyte diameter, and ZP thickness, the blastocyst rate decreased in the small oocytes group (3.1 ± 3.1, 14.1 ± 9.4, and 26.7 ± 3.9, respectively) compared to the intermediate (29.4 ± 5.2, 30.5 ± 10.1, and 28.6 ± 9.6, respectively) and large oocytes groups (54.2 ± 13.5, 44.4 ± 3.9, and 67.6 ± 12.4, respectively). In addition, the probability of reaching the blastocyst stage was positively related to the total diameter (p < 0.001), oocyte diameter (p < 0.05), and ZP thickness (p < 0.001). Furthermore, the relative gene expression of BAX, BCL2, GDF9, and GJA1 was lower in oocytes classified as large. In experiment 2, the mRNA transcript relative abundance pattern of genes in CCs was evaluated according to oocyte total diameter and developmental stage reached. CCs from oocytes classified as large and oocytes capable of developing to the blastocyst stage had a lower relative expression of BAX, STAR, and PTGS2, while a higher expression of HAS2 and SDC2 transcript was observed for those oocytes. In conclusion, oocyte morphometric parameters and gene expression analysis in oocytes and CCs provide methods for the identification of the most competent oocytes for assisted reproductive technologies in sheep.

Transcriptome profile of goat folliculogenesis reveals the interaction of oocyte and granulosa cell in correlation with different fertility population

To understand the molecular and genetic mechanisms related to the litter size in one species of two different populations (high litter size and low litter size), we performed RNA-seq for the oocytes and granulosa cells (GCs) at different developmental stages of follicle, and identified the interaction of genes from both sides of follicle (oocyte and GCs) and the ligand-receptor pairs from these two sides. Our data were very comprehensive to uncover the difference between these two populations regarding the folliculogenesis. First, we identified a set of potential genes in oocyte and GCs as the marker genes which can be used to determine the goat fertility capability and ovarian reserve ability. The data showed that GRHPR, GPR84, CYB5A and ERAL1 were highly expressed in oocyte while JUNB, SCN2A, MEGE8, ZEB2, EGR1and PRRC2A were highly expressed in GCs. We found more functional genes were expressed in oocytes and GCs in high fertility group (HL) than that in low fertility group (LL). We uncovered that ligand-receptor pairs in Notch signaling pathway and transforming growth factor-β (TGF-β) superfamily pathways played important roles in goat folliculogenesis for the different fertility population. Moreover, we discovered that the correlations of the gene expression in oocytes and GCs at different stages in the two populations HL and LL were different, too. All the data reflected the gene expression landscape in oocytes and GCs which was correlated well with the fertility capability.

The Involvement of Lumican in Human Ovulatory Processes

Based on a previous global transcriptome sequencing project, we hypothesized that Lumican (LUM) might play a role in ovulatory processes. We sought to determine LUM gene expression under various conditions in human preovulatory follicles. The in vitro expression of LUM mRNA in mural (MGCs) and cumulus (CGCs) granulosa cells was characterized using quantitative real-time polymerase chain reaction (qRT-PCR). Immunohistochemical staining was used to identify human LUM expression in follicles at different developmental stages. Cell signaling studies were performed by treating human MGCs with human chorionic gonadotropin (hCG) and both, different stimulators and inhibitors to determine their effect on LUM expression by using qRT-PCR. Cell confluence studies were carried out to study the correlation between LUM expression and follicle cell proliferation. Follicular MGCs and CGCs of women undergoing in vitro fertilization (IVF) procedures due to endometriosis were analyzed for differences in LUM expression patterns by qRT-PCR. LUM mRNA expression was significantly higher in MGCs as compared to CGCs. In CGCs, LUM mRNA was higher in mature metaphase II (MII) oocytes than in germinal vesicle (GV) and metaphase I (MI) oocytes. LUM expression was significantly upregulated in response to hCG in cultured MGCs. Immunohistochemistry of human ovaries revealed LUM was mostly present in MGCs of large preovulatory and postovulatory follicles and absent from primordial follicles. Using pharmacological activators and inhibitors, we demonstrated that LUM induction by luteinizing hormone (LH)/hCG is carried through the mitogen-activated protein kinase (MEK) pathway. LUM expression was induced in high-density cell cultures in a confluence-dependent manner. MGCs from follicles of subjects with endometriosis exhibited reduced mRNA transcription levels compared to control subjects. Our study confirms that LUM is a newly discovered ovulatory gene. LUM might play an important role during the preovulatory period up until ovulation as well as in endometriosis infertility. A better understanding of LUM's role might provide potential new treatment paradigms for some types of female infertility.

Effects of Pre-ovulatory Follicular Fluid of Repeat Breeder Dairy Cows on Bovine Fertility Transcriptomic Markers and Oocytes Maturation and Fertilization Capacity

The current study aimed to determine the effects of the preovulatory follicular fluid (FF) of normal heifer (NH) and repeat breeder cows with subclinical endometritis (SCE) or without (nSCE) on oocyte maturation (Experiment 1) and fertilization rates (Experiment 2). Moreover, the pattern of gene expression of cumulus oocyte-complexes was evaluated in Experiment 1. In Experiment 1, nuclear maturation in the nSCE group was higher, compared to that in the SCE group (P = 0.05). In addition, the oocyte nuclear maturation in the normal heifer was significantly higher, in comparison to that of SCE groups (P < 0.05). Furthermore, the mean percentage of normal oocyte fertilization was higher in the nSCE group, compared to that in the SCE group (P < 0.05). The expressions of growth differentiation factor, GDF9; steroidogenic acute regulatory, StAR and follicle-stimulating hormone receptor, FSHr in the NH group were significantly higher, compared to those in SCE and nSCE groups (P < 0.05). Moreover, the expressions of all genes in the nSCE group were not significant, in comparison to those in the SCE group (P > 0.05). The supplementation of oocyte maturation medium with FF from pre-ovulatory follicles of repeat breeder cows resulted in less oocyte maturation and cumulus cell expansion. In conclusion, the lower fertility in RB cows could be ascribed to the lower oocyte maturation rate and less expression of GDF9, StAR, and FSHr in the cumulus-oocyte complexes.

Follicular fluid during individual oocyte maturation enhances cumulus expansion and improves embryo development and quality in a dose-specific manner

We evaluated the effect of supplementation of different concentrations of bovine follicular fluid (FF) during in vitro maturation (IVM) on oocyte development and blastocyst quality in group and individual culture conditions. To do so, in vitro maturation medium (TCM-199 with 20 ng/mL epidermal growth factor and 50 μg/mL gentamycin) was supplemented with 0 (control), 1, 5, or 10% of FF. Follicular fluid was collected from slaughterhouse-derived ovaries, selecting follicles between 12 and 20 mm in diameter. Oocytes were either produced in groups or individually matured, fertilized, and cultured to the blastocyst stage, allowing for separate follow-up of each oocyte. Development (cleavage and blastocyst rates) among experimental groups were fitted in mixed-effects models, and blastocyst quality parameters (assessed via differential apoptotic staining) were evaluated in mixed linear regression models. We also assessed the cumulus expansion (prior and after maturation) for individual culture conditions, and their difference was fitted in mixed linear regression models. The FF was collected from two batches, with an estradiol/progesterone ratio higher than 1. The FF batch did not affect the development or blastocyst quality in group or individual culture conditions (P > 0.05). In group culture, development was similar among experimental groups (P > 0.05). Five or 10% of FF supplementation improved (P ˂ 0.05) aspects of blastocyst quality such as total cell numbers (TCN), trophectoderm (TE), inner cell mass (ICM), and ICM/TCN and apoptotic cells/TCN ratio in comparison to control. In the individual culture system, 5% FF supplementation increased (P ˂ 0.05) day 8 blastocyst rate (33 ± 3.4% (LSM ± SE)) in comparison to control (20 ± 2.7%) and 1% FF supplementation (19 ± 2.6%) but it was not different (P > 0.05) from 10% FF supplementation (28 ± 3.4%). Five percent of FF supplementation resulted in greater TCN, ICM, and ICM/TCN than control (P ˂ 0.05). It also resulted in a greater expansion of cumulus cell investment than the other groups (P ˂ 0.05), with a 3-fold increase compared to control. In conclusion, 5% of FF supplementation during IVM improved the cumulus expansion and the blastocyst development and quality in an individual culture system. However, FF supplementation during maturation in a group culture system did not increase development, but it modestly improved some embryo quality aspects when 5 or 10% of FF was added.

Treatment of in vitro-Matured Bovine Oocytes With Tauroursodeoxycholic Acid Modulates the Oxidative Stress Signaling Pathway

In several species, oocyte and embryo competence are improved by the addition of endoplasmic reticulum (ER) stress inhibitors to in vitro maturation (IVM) medium and/or in vitro culture (IVC) medium. This study aimed to evaluate the effects of three concentrations of tauroursodeoxycholic acid (TUDCA; 50, 200, and 1,000 μM), a chemical chaperone for relieving ER stress, during IVM of bovine cumulus-oocyte complexes (COCs) for 24 h. Treated oocytes were analyzed for nuclear maturation, reactive oxygen species (ROS) production, mitochondrial activity, and abundance of target transcripts. In addition, the number of pronuclei in oocytes was evaluated after 18-20 h of insemination, and the rates of blastocyst and hatched blastocyst formation were evaluated after 7 and 8/9 days of culture, respectively. We further evaluated the transcript abundance of embryonic quality markers. Our findings showed that supplementation of IVM medium with 200 μM of TUDCA decreased ROS production and increased abundance of transcripts related to antioxidant activity in oocytes (CAT, GPX1, and HMOX1) and embryos (GPX1 and PRDX3). Interestingly, high concentration of TUDCA (1,000 μM) was toxic to oocytes, reducing the nuclear maturation rate, decreasing mitochondrial activity, and increasing the abundance of ER stress (HSPA5) and cellular apoptosis (CASP3 and CD40) related transcripts. The results of this study suggest that treatment with 200 μM of TUDCA is associated with a greater resistance to oxidative stress and indirectly with ER stress relief in bovine oocytes.

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

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

Senescence and Apoptosis During in vitro Embryo Development in a Bovine Model

According to the World Health Organization, infertility affects up to 14% of couples under reproductive age, leading to an exponential rise in the use of assisted reproduction as a route for conceiving a baby. In the same way, thousands of embryos are produced in cattle and other farm animals annually, leading to increased numbers of individuals born. All reproductive manipulations entail deviations of natural phenotypes and genotypes, with in vitro embryo technologies perhaps showing the biggest effects, although these alterations are still emerging. Most of these indications have been provided by animal models, in particular the bovine species, due to its similarities to human early embryo development. Oocytes and embryos are highly sensitive to environmental stress in vivo and in vitro. Thus, during in vitro culture, a number of stressful conditions affect embryonic quality and viability, inducing subfertility and/or long-term consequences that may reach the offspring. A high proportion of the embryos produced in vitro are arrested at a species-specific stage of development during the first cell divisions. These arrested embryos do not show signs of programmed cell death during early cleavage stages. Instead, defective in vitro produced embryos would enter a permanent cell cycle arrest compatible with cellular senescence, in which they show active metabolism and high reactive oxygen species levels. Later in development, mainly during the morula and blastocyst stages, apoptosis would mediate the elimination of certain cells, accomplishing both a physiological role in to balancing cell proliferation and death, and a pathological role preventing the transmission of damaged cells with an altered genome. The latter would acquire relevant importance in in vitro produced embryos that are submitted to stressful environmental stimuli. In this article, we review the mechanisms mediating apoptosis and senescence during early embryo development, with a focus on in vitro produced bovine embryos. Additionally, we shed light on the protective role of senescence and apoptosis to ensure that unhealthy cells and early embryos do not progress in development, avoiding long-term detrimental effects.

Oocyte Selection for In Vitro Embryo Production in Bovine Species: Noninvasive Approaches for New Challenges of Oocyte Competence

Simple Summary

The efficiency of producing embryos using in vitro technologies in cattle species remains lower when compared to mice, indicating that the proportion of female gametes that fail to develop after in vitro manipulation is considerably large. Considering that the intrinsic quality of the oocyte is one of the main factors affecting embryo production, the precise identification of noninvasive markers that predict oocyte competence is of major interest. The aim of this review was to explore the current literature on different noninvasive markers associated with oocyte quality in the bovine model. Apart from some controversial findings, the presence of cycle-related structures in ovaries, a follicle size between 6 and 10 mm, a large slightly expanded investment without dark areas, large oocyte diameter (>120 microns), dark cytoplasm, and the presence of a round and smooth first polar body have been associated with better embryonic development. In addition, the combination of oocyte and zygote selection, spindle imaging, and the anti-Stokes Raman scattering microscopy together with studies decoding molecular cues in oocyte maturation have the potential to further optimize the identification of oocytes with better developmental competence for in vitro technologies in livestock species.

Abstract

The efficiency of producing embryos using in vitro technologies in livestock species rarely exceeds the 30–40% threshold, indicating that the proportion of oocytes that fail to develop after in vitro fertilization and culture is considerably large. Considering that the intrinsic quality of the oocyte is one of the main factors affecting blastocyst yield, the precise identification of noninvasive cellular or molecular markers that predict oocyte competence is of major interest to research and practical applications. The aim of this review was to explore the current literature on different noninvasive markers associated with oocyte quality in the bovine model. Apart from some controversial findings, the presence of cycle-related structures in ovaries, a follicle size between 6 and 10 mm, large number of surrounding cumulus cells, slightly expanded investment without dark areas, large oocyte diameter (>120 microns), dark cytoplasm, and the presence of a round and smooth first polar body have been associated with better competence. In addition, the combination of oocyte and zygote selection via brilliant cresyl blue (BCB) test, spindle imaging, and the anti-Stokes Raman scattering microscopy together with studies decoding molecular cues in oocyte maturation have the potential to further optimize the identification of oocytes with better developmental competence for in-vitro-derived technologies in livestock species.

Hypoxia-inducible factor (HIF1alpha) inhibition modulates cumulus cell function and affects bovine oocyte maturation in vitro†

Various metabolic and hormonal factors expressed in cumulus cells are positively correlated with the in vitro maturation (IVM) of oocytes. However, the role of hypoxia sensing both during maturation of cumulus–oocyte complexes (COCs) as well as during the resumption of meiosis remains uncertain. HIF1alpha plays major roles in cellular responses to hypoxia, and here we investigated its role during bovine COC maturation by assessing the expression of related genes in cumulus cells. COCs were divided into the following groups: immature (control), in vitro matured (IVM/control), or matured in the presence of a blocker of HIF1alpha activity (echinomycin, IVM/E). We found an inhibition of cumulus cell expansion in IVM/E, compared with the IVM/control. Transcript levels of several factors (n = 13) were assessed in cumulus cells. Decreased expression of HAS2, TNFAIP6, TMSB4, TMSB10, GATM, GLUT1, CX43, COX2, PTGES, and STAR was found in IVM/E (P < 0.05). Additionally, decreased protein levels were detected for STAR, HAS2, and PCNA (P < 0.05), while activated-Caspase 3 remained unaffected in IVM/E. Progesterone output decreased in IVM/E. The application of PX-478, another blocker of HIF1alpha expression, yielded identical results. Negative effects of HIF1alpha suppression were further observed in the significantly decreased oocyte maturation and blastocyst rates from COCs matured with echinomycin (P < 0.05) or PX-478 (P < 0.05). These results support the importance of HIF1alpha for COC maturation and subsequent embryo development. HIF1alpha is a multidirectional factor controlling intercellular communication within COCs, steroidogenic activity, and oocyte development rates, and exerting effects on blastocyst rates.

Bovine oocyte maturation: acquisition of developmental competence

Although millions of oocytes are formed during embryo and fetal development in the cow, only a small fraction of these will form a developmentally competent oocyte and be fertilised. Development to competence relies on an intimate contact between the oocyte and the surrounding somatic cells in ovarian follicles, via both direct cell-cell contact and paracrine signalling. An important aspect of oocyte maturation is the segregation of homologous chromosomes and subsequently sister chromatids to form a haploid oocyte. Furthermore, the cytoplasm needs to be prepared for the formation of pronuclei and nuclear reprogramming to form a totipotent zygote. Conditions such as high levels of fatty acids or oxidative stress constrain the developmental competence of oocytes, and a better insight into these processes may help improve in vitro and in vivo oocyte maturation success. In addition, identification of the developmentally competent oocyte is useful for the efficiency of (artificial) reproduction.

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.

Contributions of RNA-seq to improve in vitro embryo production (IVP)

In Vitro Embryo Production (IVP) is widely used to improve the reproductive efficiency of livestock animals, however increasing the embryo development rates and pregnancy outcomes is still a challenge for some species. Thus, the lack of biological knowledge hinders developing specie-specific IVP protocols. Therefore, the contributions of RNA-seq to generate relevant biological knowledge and improve the efficiency of IVP in livestock animals are reviewed herein.

The use of adenosine to inhibit oocyte meiotic resumption in Bos taurus during pre-IVM and its potential to improve oocyte competence

One of the major challenges of artificial reproductive technologies is to develop new methods for producing greater numbers of embryos. An oocyte fosters the ability to develop into an embryo before oocyte meiotic resumption. The aim of the present study was to assess the effect of adenosine (ADO), a purine nucleoside found in follicular fluid, on the inhibition of oocyte meiotic resumption and the production of blastocysts. The results showed the efficacy of ADO to inhibit oocyte meiotic resumption. The use of ADO (3 mM) during a pre-in vitro maturation (pre-IVM) culture period of 6 h resulted in a significant increase (p < 0.05) of blastocysts compared to control conditions with no pre-IVM culture period. No effect on the percentage of cleavage was observed. The effect of adenosine on blastocyst yield was time- and concentration-dependent with an optimum effect at 3 mM for 6 h. Supplementing the ADO pre-IVM culture medium with estradiol, follicle-stimulating hormone, progesterone, epidermal growth factor, insulin-like growth factor-2 or reelin did not improve the blastocyst yield. Transcriptional analyses of ADO-treated cumulus cells revealed that NRP1, RELN, MAN1A1, THRA and GATM were up-regulated. Finally, bioinformatic analysis identified mitochondrial function as the top canonical pathway affected by ADO. This opens up new opportunities for further investigations.

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

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

Identification of differential abundances of mRNA transcript in cumulus cells and CCND1 associated with yak oocyte developmental competence

The development of an accurate and noninvasive preselection process for competent oocytes is essential to achieve a highly efficient in vitro production (IVP) of embryos. Cumulus cells (CCs) have important functions in oocyte growth, development, maturation, and fertilization. It, therefore, is important to know if the quality of oocytes can be ascertained by assessment of gene expression of the surrounding CCs or not. The aim of this study was to identify differentially expressed genes in yak CCs from oocytes with varying developmental competences as possible biomarkers for distinguishing oocyte competence. The isolated CCs were pooled into immature and mature groups in accordance with the maturation outcome of oocytes. A total of 9516 genes were differentially expressed in the two CC categories (P <  0.05). With a minimum change of 2.5-fold, 45 up-regulated and 79 down-regulated genes were observed in CCs belonging to the mature group compared with those in the immature group (P <  0.01). These genes were primarily enriched for the cell cycle, meiosis, cell signaling, metabolism, and apoptosis. The selected candidate genes (CCND1, BMP15, GDF9, H19, KLF4, GPC1, SYCP3, and CTSB) were validated using quantitative real-time polymerase chain reaction (RT-qPCR) and there were expression patterns similar to those detected with transcriptome analysis. The CCs from fertilized oocytes arrested at the 2-cell (2-cell group), or 8-cell (8-cell group) stages or that developed into blastocysts (the blastocyst group) had a 1.5-, 1.8-, and 2.3-fold increase, respectively, in mRNA relative abundance of CCND1 compared with CCs from unfertilized oocytes (P <  0.05). The results with the RT-qPCR analysis confirmed that the relative abundance of CCND1 mRNA in CCs was associated with oocyte developmental competence. In conclusion, RNA-Seq is useful in extracting transcriptomes and selecting markers associated with oocyte developmental competence. Furthermore, the expression of the CCND1 gene in yak CCs can be used to preselect oocytes for IVP efficiency.

Analysis of the equine "cumulome" reveals major metabolic aberrations after maturation in vitro

Background

Maturation of oocytes under in vitro conditions (IVM) results in impaired developmental competence compared to oocytes matured in vivo. As oocytes are closely coupled to their cumulus complex, elucidating aberrations in cumulus metabolism in vitro is important to bridge the gap towards more physiological maturation conditions. The aim of this study was to analyze the equine “cumulome” in a novel combination of proteomic (nano-HPLC MS/MS) and metabolomic (UPLC-nanoESI-MS) profiling of single cumulus complexes of metaphase II oocytes matured either in vivo (n = 8) or in vitro (n = 7).

Results

A total of 1811 quantifiable proteins and 906 metabolic compounds were identified. The proteome contained 216 differentially expressed proteins (p ≤ 0.05; FC ≥ 2; 95 decreased and 121 increased in vitro), and the metabolome contained 108 metabolites with significantly different abundance (p ≤ 0.05; FC ≥ 2; 24 decreased and 84 increased in vitro). The in vitro “cumulome” was summarized in the following 10 metabolic groups (containing 78 proteins and 21 metabolites): (1) oxygen supply, (2) glucose metabolism, (3) fatty acid metabolism, (4) oxidative phosphorylation, (5) amino acid metabolism, (6) purine and pyrimidine metabolism, (7) steroid metabolism, (8) extracellular matrix, (9) complement cascade and (10) coagulation cascade. The KEGG pathway “complement and coagulation cascades” (ID4610; n = 21) was significantly overrepresented after in vitro maturation. The findings indicate that the in vitro condition especially affects central metabolism and extracellular matrix composition. Important candidates for the metabolic group oxygen supply were underrepresented after maturation in vitro. Additionally, a shift towards glycolysis was detected in glucose metabolism. Therefore, under in vitro conditions, cumulus cells seem to preferentially consume excess available glucose to meet their energy requirements. Proteins involved in biosynthetic processes for fatty acids, cholesterol, amino acids, and purines exhibited higher abundances after maturation in vitro.

Conclusion

This study revealed the marked impact of maturation conditions on the “cumulome” of individual cumulus oocyte complexes. Under the studied in vitro milieu, cumulus cells seem to compensate for a lack of important substrates by shifting to aerobic glycolysis. These findings will help to adapt culture media towards more physiological conditions for oocyte maturation.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5836-5) contains supplementary material, which is available to authorized users.

Effect of BMP15 and/or AMH during in vitro maturation of oocytes from involuntarily culled dairy cows

The high metabolic activity to which the dairy cattle are exposed to maintain milk production altered steroid metabolism that affects reproductive physiology and reduce oocyte competence. Our aims were (a) to characterize the competence of immature oocytes collected from dairy cattle based on the expression of genes in cumulus cells (CCs) and (b) to improve oocyte competence to support preimplantation embryo development by the supplementation of maturation medium with bone morphogenetic protein 15 (BMP15) and/or anti-mullerian hormone (AMH). Oocyte donors were identified at the moment of ovary collection and grouped by involuntarily culled dairy cows (Holstein breed) or beef cattle. The embryo development speed to blastocyst of the cull dairy cattle versus beef cattle (control group) was lower. Besides, <10% of oocytes (with CC biopsies) derived from dairy cattle were able to develop to the blastocyst stage. In addition, a higher level of expression and a positive correlation were observed in the expression of most of the genes evaluated (LUM, KRT18, KRT8, CLIC3, BMPR1B, and SLC38A3) in the cumulus-oocyte complexes that produced blastocysts versus those which did not develop correctly (arrested development). Further, use of BMP15 in the maturation of oocytes from dairy cattle seems to increase competence, modulating the expression of OCT4, SOX2, CDX2, GATA6, and TP1 in resulting blastocysts.

Cumulus-oocyte interactions and programmed cell death in bovine embryos produced in vitro

This study investigated the interactions between cumulus cells (CCs) and oocytes and programmed cell death in bovine cumulus-oocyte complexes (COCs) with different morphological characteristics. DNA fragmentation was assessed in CCs at 0 and 24 h of maturation, as well as parthenogenetic developmental competence on the 9th day post-activation, blastocyst quality and BCL-2 and BAX transcript levels in matured CCs. Most immature oocytes in the COC-A group (full cumulus and several compact layers) were in the initial germinal vesicle (iGV) stage, exhibiting minimal or no DNA damage. In contrast, after follicle removal, the COCB (partial cumulus and one or two cell layers) and C (expanded cumulus) groups presented in more advanced GV stages and exhibited DNA fragmentation. After maturation, significant increases in fragmented nuclei were noted in COCC and COCB groups. Embryos resulting from the COC-A developed more rapidly and had increased competence compared to embryos resulting from groups COCB and COCC. The COCB group exhibited the highest BAX protein levels and a reduced BCL-2/BAX protein ratio. The results show a negative correlation between nuclear fragmentation and embryonic development potential in COCs with different morphologies. In addition, a low BCL-2/BAX protein ratio might be associated with an increase in nuclear fragmentation in CCs.

Connexin 43 coupling in bovine cumulus cells, during the follicular growth phase, and its relationship to in vitro embryo outcomes

Gap junctional coupling between cumulus cells is required for oocytes to reach developmental competence. Multiple connexins, which form these gap junctions, have been found within the ovarian follicles of several species including bovine. The aim of this study was to determine the role of connexin 43 (CX43) and its relationship to embryo development, after in vitro fertilization (IVF). Cumulus-oocyte complexes (COCs) were obtained from abattoir sourced, mixed breed, bovine ovaries. COCs were isolated from follicles ranging from 2 to 5 mm in size, representing the preselected follicle pool. Immediately after isolation, two cumulus cell biopsies were collected and stored for analysis pending determination of developmental outcomes. Using in vitro procedures, COCs were individually matured, fertilized, and cultured to the blastocyst stage. Biopsies were grouped as originating from COCs that arrested at the two-cell stage (low developmental competence [LDC]) or having developed to the late morula/blastocyst stage (high developmental competence [HDC]), after IVF and embryo culture. The expression level of CX43 was found to be significantly higher in cumulus cells from COCs that had an HDC when compared with those that had an LDC. Moreover, the gap junctional intercellular coupling rate was significantly higher in cumulus from COCs deemed to have an HDC. Significantly higher expression of the cumulus health markers luteinizing hormone receptor and cytochrome p450 19A1 was found in the cumulus originating from oocytes with HDC, suggesting that this system may provide a mechanism for noninvasively testing for oocyte health in preselected bovine follicles.

Functional signaling and gene regulatory networks between the oocyte and the surrounding cumulus cells

Background

The maturation and successful acquisition of developmental competence by an oocyte, the female gamete, during folliculogenesis is highly dependent on molecular interactions with somatic cells. Most of the cellular interactions identified, thus far, are modulated by growth factors, ions or metabolites. We hypothesized that this interaction is also modulated at the transcriptional level, which leads to the formation of gene regulatory networks between the oocyte and cumulus cells. We tested this hypothesis by analyzing transcriptome data from single oocytes and the surrounding cumulus cells collected from antral follicles employing an analytical framework to determine interdependencies at the transcript level.

Results

We overlapped our transcriptome data with putative protein-protein interactions and identified hundreds of ligand-receptor pairs that can transduce paracrine signaling between an oocyte and cumulus cells. We determined that 499 ligand-encoding genes expressed in oocytes and cumulus cells are functionally associated with transcription regulation (FDR < 0.05). Ligand-encoding genes with specific expression in oocytes or cumulus cells were enriched for biological functions that are likely associated with the coordinated formation of transzonal projections from cumulus cells that reach the oocyte’s membrane. Thousands of gene pairs exhibit significant linear co-expression (absolute correlation > 0.85, FDR < 1.8 × 10^− 5) patterns between oocytes and cumulus cells. Hundreds of co-expressing genes showed clustering patterns associated with biological functions (FDR < 0.5) necessary for a coordinated function between the oocyte and cumulus cells during folliculogenesis (i.e. regulation of transcription, translation, apoptosis, cell differentiation and transport).

Conclusion

Our analyses revealed a complex and functional gene regulatory circuit between the oocyte and surrounding cumulus cells. The regulatory profile of each cumulus-oocyte complex is likely associated with the oocytes’ developmental potential to derive an embryo.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4738-2) contains supplementary material, which is available to authorized users.

CUMULUS CELL GENES AS POTENTIAL BIOMARKERS OF OOCYTE AND EMBRYO DEVELOPMENTAL COMPETENCE

Bogomoletz Institute of Physiology NASU, Ukraine, Kiev. The selection of embryos with high implantation potential is the most important task in assisted reproductive technology. Today, this selection is based on subjective morphological criteria such as growth rate, early cleavage, the degree of fragmentation, blastocyst formation. However, the morphological assessment alone does not accurately predict oocyte/early stage embryo competence. Thus, the development of an objective, accurate, fast and affordable tests to determine oocyte quality and embryo viability could increase the chance of a successful pregnancy and reduce the number of embryos to transfer. The advent of new technologies, the so-called OMIKS, has allowed to identify novel biomarkers that can be used in cycle of in vitro fertilization (IVF) for oocyte and / or embryo selection. During folliculogenesis oocyte plays a dominant role in regulation of cumulus (CC) and granulosa cell (GC) functions, and it is consequently believed that functions of GC and CC indirectly reflect oocyte’s competence. Cell functions and active cell processes are regulated through gene expression therefore, gene expression analysis in GC and/or CC could provide a non-invasive method for identification of the most competent oocytes and embryos. In cumulus cells, genes have been identified that characterize the oocyte ability to undergo meiotic maturation, successful fertilization and early embryonic development. Among them cyclooxygenase 2, gremlin 1 and hyaluronan synthase-2, which play an important roles during oocyte development, ovulation and fertilization. This article reviews the recent data regarding these genes as potential biomarkers for selection of oocytes and embryos in the IVF program.

Nutrition and maternal metabolic health in relation to oocyte and embryo quality: critical views on what we learned from the dairy cow model

Although fragmented and sometimes inconsistent, the proof of a vital link between the importance of the physiological status of the mother and her subsequent reproductive success is building up. High-yielding dairy cows are suffering from a substantial decline in fertility outcome over past decades. For many years, this decrease in reproductive output has correctly been considered multifactorial, with factors including farm management, feed ratios, breed and genetics and, last, but not least, ever-rising milk production. Because the problem is complex and requires a multidisciplinary approach, it is hard to formulate straightforward conclusions leading to improvements on the 'work floor'. However, based on remarkable similarities on the preimplantation reproductive side between cattle and humans, there is a growing tendency to consider the dairy cow's negative energy balance and accompanying fat mobilisation as an interesting model to study the impact of maternal metabolic disorders on human fertility and, more specifically, on oocyte and preimplantation embryo quality. Considering the mutual interest of human and animal scientists studying common reproductive problems, this review has several aims. First, we briefly introduce the 'dairy cow case' by describing the state of the art of research into metabolic imbalances and their possible effects on dairy cow reproduction. Second, we try to define relevant in vitro models that can clarify certain mechanisms by which aberrant metabolite levels may influence embryonic health. We report on recent advances in the assessment of embryo metabolism and meantime critically elaborate on advantages and major limitations of in vitro models used so far. Finally, we discuss hurdles to be overcome to successfully translate the scientific data to the field.

MicroRNA Expression in Bovine Cumulus Cells in Relation to Oocyte Quality

Cumulus cells play an essential role during oocyte maturation and the acquisition of fertilizability and developmental competence. Micro(mi)RNAs can post-transcriptionally regulate mRNA expression, and we hypothesized that miRNA profiles in cumulus cells could serve as an indicator of oocyte quality. Cumulus cell biopsies from cumulus-oocyte-complexes that either yielded a blastocyst or failed to cleave after exposure to sperm cells were analyzed for miRNA expression. On average, 332 miRNA species with more than 10 reads and 240 miRNA species with more than 50 reads were identified in cumulus cells; this included nine previously undescribed microRNAs. The most highly expressed miRNAs in cumulus cells were miR-21, members of the let-7 family and miR-155. However, no repeatable differences in miRNA expression between the cumulus cells from oocytes that became blastocysts versus those from non-cleaved oocytes were identified. Further examination of individual cumulus cell samples showed a wide variability in miRNA expression level. We therefore conclude that miRNA expression in cumulus cells cannot be used as an oocyte quality marker.

Comparative analysis of granulosa cell gene expression in association with oocyte competence in FSH-stimulated Holstein cows

Ovarian stimulation with exogenous FSH followed by FSH withdrawal or ‘coasting’ is an effective means of increasing the number of oocytes obtainable for the in vitro production of cattle embryos. However, the quality of the oocytes thus obtained varies considerably from one cow to the next. The aim of the present study was to gain a better understanding of the follicular conditions associated with low oocyte developmental competence. Granulosa cells from 94 Holstein cows in a commercial embryo production facility were collected following ovarian stimulation and coasting. Microarray analysis showed 120 genes expressed with a differential of at least 1.5 when comparing donors of mostly competent with donors of mostly incompetent oocytes. Using ingenuity pathway analysis, we revealed the main biological functions and potential upstream regulators that distinguish donors of mostly incompetent oocytes. These are involved in cell proliferation, apoptosis, lipid metabolism, retinol availability and insulin signalling. In summary, we demonstrated that differences in follicle maturity at collection could explain differences in oocyte competence associated with individual animals. We also revealed deficiencies in lipid metabolism and retinol signalling in granulosa cells from donors of mostly incompetent oocytes.

The Consequences of Maternal-Embryonic Cross Talk During the Periconception Period on Subsequent Embryonic Development

The periconception period comprises the final maturation of sperm and the processes of fertilization and early embryonic development, which take place in the oviduct. The final goal of these important events is to lead to establishment of pregnancy leading to the birth of healthy offspring. Studies in rodents and domestic animals have demonstrated that environmental conditions experienced during early development affect critical aspects of future growth, metabolism, gene expression, and physiology. Similarly, in vitro culture of embryos can be associated with changes in fetal growth, gene expression and regulation, and postnatal behavior.In the oviduct, the cross talk between the mother and gametes/embryo begins after ovulation, between the oocyte and the female reproductive tract, and continues with the sperm and the early embryo after successful fertilization. These signals are mainly the result of direct interaction of gametes and embryos with oviductal and endometrial cells, influencing the microenvironment at the specific location. Identifying and understanding the mechanisms involved in this cross talk during the critical period of early reproductive events leading to pregnancy establishment could potentially lead to improvements in current in vitro embryo production systems in domestic mammals and humans. In this review, we discuss current knowledge of the short- and long-term consequences of in vitro embryo production on embryo development.

Identification of molecular markers for oocyte competence in bovine cumulus cells

Cumulus cells (CCs) have an important role during oocyte growth, competence acquisition, maturation, ovulation and fertilization. In an attempt to isolate potential biomarkers for bovine in vitro fertilization, we identified genes differentially expressed in bovine CCs from oocytes with different competence statuses, through microarray analysis. The model of follicle size, in which competent cumulus-oocyte complexes (COCs) were recovered from bigger follicles (≥8.0 mm in diameter) and less competent ones from smaller follicles (1-3 mm), was used. We identified 4178 genes that were differentially expressed (P < 0.05) in the two categories of CCs. The list was further enriched, through the use of a 2.5-fold change in gene expression as a cutoff value, to include 143 up-regulated and 80 down-regulated genes in CCs of competent COCs compared to incompetent COCs. These genes were screened according to their cellular roles, most of which were related to cell cycle, DNA repair, energy metabolism, metabolism of amino acids, cell signaling, meiosis, ovulation and inflammation. Three candidate genes up-regulated (FGF11, IGFBP4, SPRY1) and three down-regulated (ARHGAP22, COL18A1 and GPC4) in CCs from COCs of big follicles (≥8.1 mm) were selected for qPCR analysis. The selected genes showed the same expression patterns by qPCR and microarray analysis. These genes may be potential genetic markers that predict oocyte competence in in vitro fertilization routines.

Differences in cumulus cell gene expression indicate the benefit of a pre-maturation step to improve in-vitro bovine embryo production

STUDY QUESTION

Does the gene expression profile of cumulus cells (CC) accompanying oocytes with different degrees of chromatin compaction within the germinal vesicle (GV) reflect the oocyte's quality and response in culture during in-vitro embryo production (IVP).

SUMMARY ANSWER

The transcriptomic profile of the CC is related to oocyte competence, setting the stage for the development of customized pre-maturation strategies to improve IVP.

WHAT IS KNOWN ALREADY

Oocytes complete the acquisition of their competence during antral follicle development. During this period, the chromatin configuration within the GV changes dynamically and is indicative of oocyte's developmental potential. The interactions between somatic and germ cells modulate chromatin morphology and function and are critical for acquisition of oocyte competence.

STUDY DESIGN, SIZE, DURATION

Bovine cumulus-oocyte complexes (COC) were isolated from 0.5 to 6 mm antral follicles. Surrounding CC were separated from the oocyte and classified as GV0, GV1, GV2 and GV3 according to the degree of the oocyte's chromatin compaction.

PARTICIPANTS/MATERIALS, SETTING, METHOD

RNA extracted from CC of each group was amplified and hybridized on a bovine embryo-specific 44 K Agilent slide. The CC_GV1, CC_GV2 and CC_GV3 classes were each hybridized against the CC_GV0 class, representing an early oocyte differentiation stage with poor development competence. The data were normalized and fold changes of the differentially expressed genes were determined. Microarray data were validated using quantitative RT-PCR on selected targets. Microarray data were further analyzed through: (i) between-group analysis (BGA), which classifies the samples according to their transcriptomic profiles; (ii) cluster analysis according to the expression profile of each gene; and (iii) Ingenuity Pathway Analysis (IPA) to study gene regulation patterns and predicted functions. Furthermore, CC of each GV group were cultured and apoptotic cells were assessed after 3 h by caspase analysis. Finally, based on the analysis of CC transcriptomic profiles and the relationship between morphological features of the COC and the oocyte chromatin configuration, a customized, stage-dependent oocyte pre-maturation (pre-IVM) system was used to improve oocyte developmental potential before IVM. For this, the blastocyst rate and quality were assessed after in-vitro maturation and fertilization of pre-matured oocytes.

MAIN RESULTS AND THE ROLE OF CHANCE

Overall, quantitative RT-PCR results of a subset of five selected genes were consistent with the microarray data. Clustering analysis generated 16 clusters representing the main profiles of transcription modulation. Of the 5571 significantly differentially expressed probes, the majority (25.49%) best fitted with cluster #6 (downregulation between CC_GV0 and CC_GV1 and stable low levels in successive groups). IPA identified the most relevant functions associated with each cluster. Genes included in cluster #1 were mostly related to biological processes such as 'cell cycle' and 'cell death and survival', whereas genes included in cluster #5 were mostly related to 'gene expression'. Interestingly, 'lipid metabolism' was the most significant function identified in clusters #6, #9 and #12. IPA of gene lists obtained from each contrast (i.e., CC_GV0 vs. CC_GV1; CC_GV0 vs. CC_GV2; CC_GV0 vs. CC_GV3) revealed that the main affected function in each contrast was 'cell death and survival'. Importantly, apoptosis was predicted to be inhibited in CC_GV1 and CC_GV2, but activated in CC_GV3. Caspase analysis indicated that a low percentage of CC_GV0 was prone to undergo apoptosis but apoptosis increased significantly in CC from oocytes with condensed chromatin, reaching a peak in CC_GV3 (P < 0.05). Finally, the tailored oocyte pre-maturation strategy, based on morphological features of the COC and the oocyte chromatin configuration, demonstrated that pre-IVM improved the developmental capability of oocytes at early stages of differentiation (GV1-enriched COC) but was detrimental for oocytes at more advanced stages of development (GV2 and GV3-enriched COC).

LARGE SCALE DATA

The data are available through the GEO series accession number GSE79886.

LIMITATIONS, REASONS FOR CAUTION

This study was conducted with bovine samples. Whether or not the results are applicable to human oocytes requests further elucidation. Embryo transfer experiments are required to determine whether the improvement in blastocyst rates in the tailored system leads to increased live birth rates.

WIDER IMPLICATIONS OF THE FINDINGS

The identification of multiple non-invasive biomarkers predictive of oocyte quality can greatly strengthen the pre-IVM approach aimed to improve IVM outcomes. These results have potentially important implications in treating human infertility and in developing breeding schemes for domestic mammals.

STUDY FUNDING/COMPETING INTERESTS

This work was supported in part by NSERC Strategic Network EmbryoGENE, Canada and in part by CIG-Marie Curie Actions-Reintegration Grants within the EU 7FP (n. 303640, 'Pro-Ovum'). The authors declare no potential conflict of interest.

FSH stimulation of anestrous cats improves oocyte quality and development of parthenogenetic embryos

In the domestic cat, the efficiency of in vitro embryo production systems is negatively affected during the nonbreeding season. The objective of this research was to evaluate the effect of FSH stimulation in anestrous cats, on quality of cumulus-oocyte complexes (COCs) and in vitro developmental competence after parthenogenetic activation. To accomplish this purpose, anestrous cats were grouped into: (1) FSH treated (serial doses of 5 mg of porcine FSH each, every 24 hours, for 4 days) and (2) untreated control. The COCs were classified morphologically and a proportion of grade I and II COCs was used for expression analysis of FSHR, LHCGR, EGFR, PTGS2, EGR1, GDF9, and GATM by RT-qPCR. In addition, another proportion of grade I and II COCs was matured in vitro and used for parthenogenetic activation. After 8 days in culture, blastocyst and hatching blastocyst rates were assessed, and the expression of OCT4, SOX2, NANOG, CDX2, and GATA6 was evaluated. The COCs in the FSH group had an enhanced quality, a higher expression of LHCGR and a lower expression of GATM than did COCs from the control group (P < 0.05). Furthermore, embryos in the FSH group had increased blastocyst and hatching blastocyst rates, and those embryos had a higher expression of OCT4 and GATA than their counterparts from the control group (P < 0.05). In conclusion, ovarian stimulation of anestrous cats with FSH improved quality and increased the expression of LHCGR in COCs. The enhanced in vitro developmental competence, after parthenogenetic activation of oocytes from FSH-treated cats, coincided with an increased expression of OCT4 and GATA6 in blastocysts and hatching blastocysts.

Molecular markers for oocyte competence in bovine cumulus cells

This study aimed to quantify the expression of candidate genes in cumulus cells (CCs) from cumulus-oocyte complexes (COCs) with high and low potential for in vitro development up to the blastocyst stage. First, the effects of individual culture and biopsy on embryo development were evaluated. Individuals cultured using the well of the well system were compared with individuals cultured in 20 μL droplets (microdroplets) and those cultured in groups (control). Blastocyst rates were lower for the individual culture systems (P < 0.05; well of the well = 17.9%, n = 95; microdrop = 26.3%, n = 95) than for the control group (45.0%, n = 209). Second, the effects of biopsy on embryo production were compared between the control and microdroplet cultures, and no effects (P > 0.05) were observed for either group. Finally, the expression profiles of glypican 4 (GPC4), IGF4-binding protein, follicle-stimulating hormonereceptor, growth hormone receptor, epidermal growth factor receptor, fibroblast growth factor 11, solute carrier family 2 member 1, solute carrier family 2 member 3,sprouty homolog 1, versican, and keratin protein 8 in CCs obtained by biopsy were quantified by real-time polymerase chain reaction. Cumulus cells were categorized on the basis of the fates of the COCs: expanded blastocyst, cleaved and arrested, and uncleaved. The GPC4 gene was overexpressed (P = 0.007) in CCs from oocytes that formed embryos compared with those that produced cleaved and arrested embryos. We concluded that individual culture reduced blastocyst production; however, biopsy did not affect embryo development. The profile of GPC4 expression can be used as a marker to distinguish COCs with potential for embryo development from those with limited developmental potential.

Transcriptomic analysis of cyclic AMP response in bovine cumulus cells

Acquisition of oocyte developmental competence needs to be understood to improve clinical outcomes of assisted reproduction. The stimulation of cumulus cell concentration of cyclic adenosine 3'5'-monophosphate (cAMP) by pharmacological agents during in vitro maturation (IVM) participates in improvement of oocyte quality. However, precise coordination and downstream targets of cAMP signaling in cumulus cells are largely unknown. We have previously demonstrated better embryo development after cAMP stimulation for first 6 h during IVM. Using this model, we investigated cAMP signaling in cumulus cells through in vitro culture of cumulus-oocyte complexes (COCs) in the presence of cAMP raising agents: forskolin, IBMX, and dipyridamole (here called FID treatment). Transcriptomic analysis of cumulus cells indicated that FID-induced differentially expressed transcripts were implicated in cumulus expansion, steroidogenesis, cell metabolism, and oocyte competence. Functional genomic analysis revealed that protein kinase-A (PKA), extracellular signal regulated kinases (ERK1/2), and calcium (Ca(2+)) pathways as key regulators of FID signaling. Inhibition of PKA (H89) in FID-supplemented COCs or substitution of FID with calcium ionophore (A23187) demonstrated that FID activated primarily the PKA pathway which inhibited ERK1/2 phosphorylation and was upstream of calcium signaling. Furthermore, inhibition of ERK1/2 phosphorylation by FID supported a regulation by dual specific phosphatase (DUSP1) via PKA. Our findings imply that cAMP (FID) regulates cell metabolism, steroidogenesis, intracellular signaling and cumulus expansion through PKA which modulates these functions through optimization of ERK1/2 phosphorylation and coordination of calcium signaling. These findings have implications for development of new strategies for improving oocyte in vitro maturation leading to better developmental competence.