The development of a complex technology such as in vitro fertilization (IVF) requires years of experimentation, sometimes comparing several species to learn how to create the right in vitro environment for oocytes, spermatozoa and early embryos. At the same time, individual species characteristics such as gamete physiology and gamete interaction are recently evolved traits and must be analysed within the context of each species. In the last 40 years since the birth of Louise Brown, IVF techniques progressed and are now used in multiple domestic and non-domestic animal species around the world. This does not mean that the technology is completely matured or satisfactory; a number of problems remain to be solved and several procedures still need to be optimized. The development of IVF in cattle is particularly interesting since agriculture practices permitted the commercial development of the procedure and it is now used at a scale comparable to human IVF (millions of newborns). The genomic selection of young animals or even embryos combined with sexing and freezing technologies is driving a new era of IVF in the dairy sector. The time has come for a retrospective analysis of the success and pitfalls of the last 40 years of bovine IVF and for the description of the challenges to overcome in the years to come.

The aim of this study was to investigate the effect of short-term treatment (first 2 or 6 h) with recombinant human follicle-stimulating hormone (r-hFSH) during in vitro maturation (IVM) on the developmental competence of bovine oocytes. The roles of protein kinase A (PKA) and protein kinase C (PKC) (possibly involved in FSH response), were investigated using activators (Sp-cAMPS, PMA) or inhibitors (Rp-cAMPS, sphingosine) of these two protein kinases, respectively. The developmental competence of bovine oocytes was measured by the rate of blastocyst formation after in vitro fertilization (IVF). Our results showed that when cumulus–oocyte complexes (COCs) were cultured with r-hFSH for the first 6 h, a highly significant (P < 0.0001) improvement is seen in blastocyst development rate as a proportion of oocytes in culture compared with those matured with r-hFSH for the first 2 or 24 h. A transient exposure (6 h) to the highest dose (100 μM) of forskolin (an activator of adenylate cyclase) increased (P < 0.05) the rate of blastocyst formation. But the PKA inhibitors (Rp-cAMPS) did not affect the stimulatory effects of r-hFSH on the blastocyst yield. However, stimulation of PKC by low doses of PMA (0.1–0.5 μM) during short-term treatment, enhanced (P < 0.0001) the developmental capacity of oocytes, while sphingosine (a specific inhibitor of PKC) inhibited (P < 0.05) the stimulatory effects of r-hFSH on the rate of blastocyst formation. Our results indicate that although the developmental capacity of bovine oocytes in vitro can be modulated by both the PKA, and the PKC pathways, the activation of PKC during short-term treatment can mimic the effect of r-hFSH on the cytoplasmic maturation in bovine oocytes in vitro.

Recent genomic studies have shed light on the impact of in vitro culture (IVC) on embryonic homeostasis and the differential gene expression profiles associated with lower developmental competence. Consistently, the embryonic stress responses to IVC conditions correlate with transcriptomic changes in pathways related to energetic metabolism, extracellular matrix remodelling and inflammatory signalling. These changes appear to result from a developmental adaptation that enhances a Warburg-like effect known to occur naturally during blastulation. First discovered in cancer cells, the Warburg effect (increased glycolysis under aerobic conditions) is thought to result from mitochondrial dysfunction. In the case of IVC embryos, culture conditions may interfere with mitochondrial maturation and oxidative phosphorylation, forcing cells to rely on glycolysis in order to maintain energetic homeostasis. While beneficial in the short term, such adaptations may lead to epigenetic changes with potential long-term effects on implantation, foetal growth and post-natal health. We conclude that lessening the detrimental effects of IVC on mitochondrial activity would lead to significantly improved embryo quality.

Distinct epigenetic modification events regulate gene expression and chromatin structure during the period between the immature oocyte and the blastocyst. Throughout this developmental period, important methylation fluctuations occur on genomic DNA and histones. Finding single orcombinations offactors, which are at work during this period is essential to understand the entire epigenetic process. With this in mind, we assessed the precise temporal expression profile, during preimplantation embryo development, of 15 key regulators involved in RNA, DNA or histone methylation, chromatin modification or silencing and transcription regulation. To achieve this, real-time RT-PCR was used to quantify the mRNA levels of ATF7IP, DMAP1, EHMT1, EHMT2, HELLS, JARID1A, JARID1B, JMJD1A, JMJD2A, LSD1, MeCP2, METTL3, PRMT2, PRMT5 and RCOR2, in the oocyte and throughout in vitro bovine embryo development. Our results demonstrate that all the 15 key regulators were present to different degrees in the developmental stages tested, and they can be divided into three different groups depending on their respective mRNA profile.

Cleavage-stage bovine embryos are transcriptionally quiescent until they reach the 8- to 16-cell stage, and thus rely on the reserves provided by the stored maternal mRNAs and proteins found in the oocytes to achieve their first cell divisions. The objective of this study was to characterize the expression and localization of the transcriptional and translational regulators, Y box binding protein 2 (YBX2), TATA box-binding protein (TBP), and activating transcription factor 2 (ATF2), during bovine early embryo development. Germinal vesicle (GV)- and metaphase II (MII)-stage oocytes, as well as 2-, 4-, 8-, 16-cell-stage embryos, morula, and blastocysts, produced in vitro were analyzed for temporal and spatial protein expression. Using Q-PCR, ATF2 mRNA expression was shown to remain constant from the GV-stage oocyte to the four-cell embryo, and then decreased through to the blastocyst stage. By contrast, the protein levels of ATF2 remained constant throughout embryo development and were found in both the cytoplasm and the nucleus. Both TBP and YBX2 showed opposite protein expression patterns, as YBX2 protein levels decreased throughout development, while TBP levels increased through to the blastocyst stage. Immunolocalization studies revealed that TBP protein was localized in the nucleus of 8- to 16-cell-stage embryos, whereas the translational regulator YBX2 was exclusively cytoplasmic and disappeared from the 16-cell stage onward. This study shows that YBX2, TBP, and ATF2 are differentially regulated through embryo development, and provides insight into the molecular events occurring during the activation of the bovine genome during embryo development in vitro.

The use of younger gamete donors in dairy cattle genetic selection programs significantly accelerates genetic gains by decreasing the interval between generations. Ovarian stimulation (OS) and the practice of follicle-stimulating hormone (FSH) withdrawal, also known as coasting, are intensively used in pre-pubertal heifers without detrimental effects on subsequent reproductive performance but generally with lower embryo yields. However, recent data from embryo transfer programs showed similar embryo yields in younger and sexually mature animals but with a significant difference in the coasting period. The aim of the present study was to identify a set of granulosa cell biomarkers capable of distinguishing optimal follicle differentiation from late differentiation and atresia in order to assess the differences in coasting dynamics between pre- and post-pubertal donors. We integrated transcriptomic data sets from a public depository and used vote counting meta-analysis in order to elucidate the molecular changes occurring in granulosa cells during late follicle differentiation and atresia. The meta-analysis revealed the gene expression associated with follicle demise, and most importantly, identified potential biomarkers of that status in bovine granulosa cells. The comparison of the expression of six biomarkers between pre- and post-pubertal donors revealed that younger donors had more signs of atresia after the same period of coasting. We found different follicular dynamics following coasting in younger donors. It is possible that younger donors are less capable to sustain follicular survival most likely due to insufficient luteinizing hormone signaling. In summary, the pre-pubertal status influences follicular dynamics and reduces the oocyte developmental competence curve following OS and FSH withdrawal in heifers.

The LH surge induces a multitude of events that are essential for ovulation and corpus luteum formation. The transcriptional responses to the LH surge of preovulatory granulosa cells (GCs) are complex and still poorly understood. In this study, a genome-wide bovine oligo array was used to determine how the gene expression profile of GCs is modulated by the LH surge. GCs from three different stages were used to assess the short- and long-term effects of this hormone on follicle differentiation: 1) 2 h before induction of the LH surge, 2) 6 h and 3) 22 h after the LH surge. The results obtained were a list of differentially expressed transcripts for each GC group. To provide a comprehensive understanding of the processes at play, biological annotations were used to reveal the different functions of transcripts, confirming that the LH surge acts in a temporal manner. The pre-LH group is involved in typical tasks such as cell division, development, and proliferation, while the early response to the LH surge included features such as response to stimulus, vascularization, and lipid synthesis, which are indicative of cells preparing for ovulation. The late response of GCs revealed terms associated with protein localization and intracellular transport, corresponding to the future secretion task that will be required for the transformation of GCs into corpus luteum. Overall, results described in this study provide new insights into the different transcriptional steps that GCs go through during ovulation and before luteinization.

Abstract

Cumulus cells (CCs) are essential for oocytes to reach full development competency and become fertilized. Many major functional properties of CCs are triggered by gonadotropins and governed by the oocyte. Consequently, cumulus may reflect oocyte quality and is often used for oocyte selection. The most visible function of CCs is their ability for rapid extracellular matrix expansion after the LH surge. Although unexplained, LH induces the final maturation and improves oocyte quality. To study the LH signaling and gene expression cascade patterns close to the germinal vesicle breakdown, bovine CCs collected at 2 h before and 6 h after the LH surge were hybridized to a custom-made microarray to better understand the LH genomic action and find differentially expressed genes associated with the LH-induced oocyte final maturation. Functional genomic analysis of the 141 overexpressed and 161 underexpressed clones was performed according to their molecular functions, gene networks, and cell compartments. Following real-time PCR validation of our gene lists, some interesting pathways associated with the LH genomic action on CCs and their possible roles in oocyte final maturation, ovulation, and fertilization are discussed. A list of early potential markers of oocyte competency in vivo and in vitro is thereafter suggested. These early biomarkers are a preamble to understand the LH molecular pathways that trigger the final oocyte competence acquisition process in bovine.

In the case of in vitro embryonic production, it is known that not all oocytes detain the developmental capacity to form an embryo. This capacity appears to be acquired through completion of folliculogenesis, during which the oocyte and follicular cells influence their respective destinies. The differentiation status of granulosa cells (GCs) could therefore offer an indicator of oocyte quality. The aim of this study was to compare mRNA transcript abundance in GCs associated with oocytes that subsequently reach or not the blastocyst stage. GCs were collected from cattle following an ovarian stimulation protocol that did or did not include the administration of LH. GCs were classified according to the developmental stage achieved by the associated oocytes. Transcript abundance was measured by microarray. Follicles (n=189) obtained from cows before and after the LH surge were essentially similar and the rates of oocytes reaching the blastocyst stage were not significantly different (52 vs 41%), but blastocyst quality was significantly better in the post-LH-surge group. In GCs from the pre-LH-surge group and associated with developmentally competent oocytes, 18 overexpressed and 22 underexpressed transcripts were found, including novel uncharacterized transcripts, whereas no differentially expressed transcripts were associated with developmentally different oocytes in the post-LH-surge group. The novel transcriptomic response associated with LH appeared to mask the difference. Based on oocyte developmental competence, the period prior to the LH surge appears best suited for studying competence-associated mRNA transcripts in bovine follicle cells.

One of the challenges in mammalian reproduction is to understand the basic physiology of oocyte quality. It is believed that the follicle status is linked to developmental competence of the enclosed oocyte. To explore the link between follicles and competence in cows, previous research at our laboratory has developed an ovarian stimulation protocol that increases and then decreases oocyte quality according to the timing of oocyte recovery post-FSH withdrawal (coasting). Using this protocol, we have obtained the granulosa cells associated with oocytes of different qualities at selected times of coasting. Transcriptome analysis was done with Embryogene microarray slides and validation was performed by real-time PCR. Results show that the major changes in gene expression occurred from 20 to 44 h of coasting, when oocyte quality increases. Secondly, among upregulated genes (20–44 h), 25% were extracellular molecules, highlighting potential granulosa signaling cascades. Principal component analysis identified two patterns: one resembling the competence profile and another associated with follicle growth and atresia. Additionally, three major functional changes were identified: i) the end of follicle growth (BMPR1B, IGF2, and RELN), involving interactions with the extracellular matrix (TFPI2); angiogenesis (NRP1), including early hypoxia, and potentially oxidative stress (GFPT2, TF, and VNN1) and ii) apoptosis (KCNJ8) followed by iii) inflammation (ANKRD1). This unique window of analysis indicates a progressive hypoxia during coasting mixed with an increase in apoptosis and inflammation. Potential signaling pathways leading to competence have been identified and will require downstream testing. This preliminary analysis supports the potential role of the follicular differentiation in oocyte quality both during competence increase and decrease phases.