Comparison of gene expression and mitochondria number between bovine blastocysts obtained <i>in vitro</i> and <i>in vivo</i>

Current status of the intrafollicular transfer of immature oocytes (IFIOT) in cattle: A review

Intrafollicular Transfer of Immature Oocytes (IFIOT) has emerged as an alternative to the currently used systems for bovine embryo production. This technique associates the rapid multiplication of bovine females under a completely in vivo culture condition, eliminating the need for superstimulatory hormones in the in vivo system (IVD) and the costly laboratory setup required for in vitro embryo production (IVP). Despite being a promising technique, the results obtained to date have been unsatisfactory for commercial use. Only approximately 10 % -12 % of viable embryos are recovered from the total number of injected oocytes, which limits their use in genetic improvement programs. IFIOT problems can occur in any of the steps involved; therefore, each step must be carefully examined to identify those that have the most negative impact on the final embryo recovery. This review summarizes the different studies conducted using the IFIOT to provide a comprehensive analysis of the main factors that can influence the effectiveness of this technique.

miRNAs in Follicular and Oviductal Fluids Support Global DNA Demethylation in Early-Stage Embryos

Global methylation levels differ in in vitro- and in vivo-developed embryos. Follicular fluid (FF) contains extracellular vesicles (EVs) containing miRNAs that affect embryonic development. Here, we examined our hypothesis that components in FF affect global DNA methylation and embryonic development. Oocytes and FF were collected from bovine ovaries. Treatment of zygotes with a low concentration of FF induced global DNA demethylation, improved embryonic development, and reduced DNMT1/3A levels. We show that embryos take up EVs containing labeled miRNA secreted from granulosa cells and the treatment of zygotes with EVs derived from FF reduces global DNA methylation in embryos. Furthermore, the methylation levels of in vitro-developed blastocysts were higher than those of in their vivo counterparts. Based on small RNA-sequencing and in silico analysis, we predicted miR-29b, -199a-3p, and -148a to target DNMTs and to induce DNA demethylation, thereby improving embryonic development. Moreover, among FF from 30 cows, FF with a high content of these miRNAs demethylated more DNA in the embryos than FF with a lower miRNA content. Thus, miRNAs in FF play a role in early embryonic development.

The mitochondrial respiration signature of the bovine blastocyst reflects both environmental conditions of development as well as embryo quality

The major limitation of the widespread use of IVP derived embryos is their consistent deficiencies in vitality when compared with their ex vivo derived counterparts. Although embryo metabolism is considered a useful metric of embryo quality, research connecting mitochondrial function with the developmental capacity of embryos is still lacking. Therefore, the aim of the present study was to analyse bovine embryo respiration signatures in relation to developmental capacity. This was achieved by taking advantage of two generally accepted metrics for developmental capacity: (I) environmental conditions during development (vivo vs. vitro) and (II) developmental kinetics (day 7 vs. day 8 blastocysts). Our study showed that the developmental environment affected total embryo oxygen consumption while different morphokinetics illustrating the embryo qualities correlate with maximal mitochondrial respiration, mitochondrial spare capacity, ATP-linked respiration as well as efficiency of ATP generation. This respiration fingerprint for high embryo quality is reflected by relatively lower lipid contents and relatively higher ROS contents. In summary, the results of the present study extend the existing knowledge on the relationship between bovine embryo quality and the signature of mitochondrial respiration by considering contrasting developmental environments as well as different embryo morphokinetics.

Review: Overview of the transcriptomic landscape in bovine blastocysts and elongated conceptuses driving developmental competence

In cattle, pregnancy loss due to early embryonic mortality is a major concern that significantly impacts reproductive efficiency. Given the economic importance of cattle in livestock productivity, much research has been carried out to comprehend the regulatory mechanisms underlying this early embryo loss. Thus, understanding the molecular principles behind the reciprocal communication between the maternal uterus and the developing conceptus is paramount. Measurement of mRNA expression through a variety of techniques is widely used to unravel the complex and dynamic interaction between these two players. Development of high-throughput technologies, such as microarrays and RNA sequencing, have allowed global quantification of the full range of expressed mRNA, or transcriptome, of a biological sample. Therefore, numerous investigators have applied one or the other method to study the bovine embryo transcriptome at different developmental checkpoints and under different conditions. The goal of this article was to review studies involving the use of high-throughput techniques to study the transcriptome of the bovine embryo from the blastocyst (∼day 7) to the elongating conceptus stage (∼days 13-16) in terms of developmental capacity and the impact of procedures for in vitro embryo production. Furthermore, the differentially expressed genes reported by each study and enriched pathways were compared to determine common terms. The studies described here highlight differences in the transcriptome (i) between blastocysts with divergent ability to sustain a pregnancy, (ii) between age-matched elongated conceptuses with divergent developmental fates, and (iii) between blastocysts and elongated conceptuses produced in vitro or in vivo. Comparison between these works, supported by other studies involving transcriptomic data integration presented at the end of this review, highlights the involvement of pathways related to energy metabolism in embryonic competence, which may be altered because of the procedures involved in the in vitro production of embryos.

The proteomic analysis of bovine embryos developed in vivo or in vitro reveals the contribution of the maternal environment to early embryo

BACKGROUND

Despite many improvements with in vitro culture systems, the quality and developmental ability of mammalian embryos produced in vitro are still lower than their in vivo counterparts. Though previous studies have evidenced differences in gene expression between in vivo- and in vitro-derived bovine embryos, there is no comparison at the protein expression level.

RESULTS

A total of 38 pools of grade-1 quality bovine embryos at the 4-6 cell, 8-12 cell, morula, compact morula, and blastocyst stages developed either in vivo or in vitro were analyzed by nano-liquid chromatography coupled with label-free quantitative mass spectrometry, allowing for the identification of 3,028 proteins. Multivariate analysis of quantified proteins showed a clear separation of embryo pools according to their in vivo or in vitro origin at all stages. Three clusters of differentially abundant proteins (DAPs) were evidenced according to embryo origin, including 463 proteins more abundant in vivo than in vitro across development and 314 and 222 proteins more abundant in vitro than in vivo before and after the morula stage, respectively. The functional analysis of proteins found more abundant in vivo showed an enrichment in carbohydrate metabolism and cytoplasmic cellular components. Proteins found more abundant in vitro before the morula stage were mostly localized in mitochondrial matrix and involved in ATP-dependent activity, while those overabundant after the morula stage were mostly localized in the ribonucleoprotein complex and involved in protein synthesis. Oviductin and other oviductal proteins, previously shown to interact with early embryos, were among the most overabundant proteins after in vivo development.

CONCLUSIONS

The maternal environment led to higher degradation of mitochondrial proteins at early developmental stages, lower abundance of proteins involved in protein synthesis at the time of embryonic genome activation, and a global upregulation of carbohydrate metabolic pathways compared to in vitro production. Furthermore, embryos developed in vivo internalized large amounts of oviductin and other proteins probably originated in the oviduct as soon as the 4-6 cell stage. These data provide new insight into the molecular contribution of the mother to the developmental ability of early embryos and will help design better in vitro culture systems.

miR-17-5p in bovine oviductal fluid affects embryo development

This study identified microRNAs (miRNAs) in bovine oviductal fluids (OFs) and examined the effect of miR-17-5p in OFs on embryonic development to the blastocyst stage. Small RNA-seq of extracellular vesicles of OFs revealed 242 miRNAs. Additionally, analyzing expressions of randomly selected OF-miRNAs with RT-qPCR in the culture medium of oviductal epithelial cells indicated that the abundance of miRNAs in OFs increased during the luteal phase. miR-17-5p mimic-treated eight-cell-stage zona pellucida-free embryos showed improved embryonic development to the blastocyst stage. The effect of the miR-17-5p mimic was confirmed using a dual-luciferase assay and immunostaining. In addition, RNA-seq of the miR-17-5p mimic- or control-treated embryos revealed differentially expressed genes (DEGs), suggesting possible pathways that overlapped with the in silico-predicted pathways for miR-17-5p targeting genes. Furthermore, ingenuity pathway analysis of DEG predicted miR-17 to be a significant upstream regulator. Our results suggest that miR-17-5p in OFs regulates embryonic development in bovines.

Free fatty acid treatment of mouse preimplantation embryos demonstrates contrasting effects of palmitic acid and oleic acid on autophagy

Treatment of mouse preimplantation embryos with elevated palmitic acid (PA) reduces blastocyst development, while co-treatment with PA and oleic acid (OA) together rescues blastocyst development to control frequencies. To understand the mechanistic effects of PA and OA treatment on early mouse embryos, we investigated the effects of PA and OA, alone and in combination, on autophagy during preimplantation development in vitro. We hypothesized that PA would alter autophagic processes and that OA co-treatment would restore control levels of autophagy. Two-cell stage mouse embryos were placed into culture medium supplemented with 100 μM PA, 250 μM OA, 100 μM PA and 250 μM OA, or KSOMaa medium alone (control) for 18 - 48 h. The results demonstrated that OA co-treatment slowed developmental progression after 30 h of co-treatment but restored control blastocyst frequencies by 48 h. PA treatment elevated LC3-II puncta and p62 levels per cell while OA co-treatment returned to control levels of autophagy by 48 h. Autophagic mechanisms are altered by non-esterified fatty acid (NEFA) treatments during mouse preimplantation development in vitro, where PA elevates autophagosome formation and reduces autophagosome degradation levels, while co-treatment with OA reversed these PA-effects. Autophagosome-lysosome co-localization only differed between PA and OA alone treatment groups. These findings advance our understanding of the effects of free fatty acid exposure on preimplantation development, and they uncover principles that may underlie the associations between elevated fatty acid levels and overall declines in reproductive fertility.

Optimised CO2-containing medium for in vitro culture and transportation of mouse preimplantation embryos without CO2 incubator

Conventional in vitro culture and manipulation of mouse embryos require a CO₂ incubator, which not only increases the cost of performing experiments but also hampers the transport of embryos to the other laboratories. In this study, we established and tested a new CO₂ incubator-free embryo culture system and transported embryos using this system. Using an Anaero pouch, which is a CO₂ gas-generating agent, to increase the CO₂ partial pressure of CZB medium to 4%–5%, 2-cell embryos were cultured to the blastocyst stage in a sealed tube without a CO₂ incubator at 37°C. Further, the developmental rate to blastocyst and full-term development after embryo transfer were comparable with those of usual culture method using a CO₂ incubator (blastocyst rate: 97% versus 95%, respectively; offspring rate: 30% versus 35%, respectively). Furthermore, using a thermal bottle, embryos were reliably cultured using this system for up to 2 days at room temperature, and live offspring were obtained from embryos transported in this simple and very low-cost manner without reducing the offspring rate (thermal bottle: 26.2% versus CO₂ incubator: 34.3%). This study demonstrates that CO₂ incubators are not essential for embryo culture and transportation and that this system provides a useful, low-cost alternative for mouse embryo culture and manipulation.

Transcriptome of D14 in vivo x in vitro bovine embryos: is there any difference?

It is well-established that in vitro culture affects quality, gene expression, and epigenetic processes in bovine embryos and that trophectoderm cells are the most susceptible to abnormalities. These changes have been reported as the main factors responsible for losses observed after transfer of in vitro-produced embryos. The present study aimed to investigate the effect of an in vitro system on bovine embryo transcriptional profiles on D14 of development. Two groups were used-one with embryos produced in vitro until D7 (day 7; VT group) and another with embryos produced in vivo by hormonal stimulation, with embryos collected on D7 (VV group). D7 embryos at similar developmental stages from both treatments were transferred to recipient uteri and recollected on D14. From D14 embryos of both treatments, trophoblast samples were removed by biopsy for sexing and transcriptome analyses. Embryos were sexed by polymerase chain reaction (PCR), and only males were used for RNA sequencing. In total, 29,005 transcripts were expressed, from which 900 were differentially expressed, but only 29 genes were significantly differentially expressed. In addition, 20 genes were found uniquely for VV and 27 for VT. These findings suggested that although the uterine environment minimized transcriptional differences, it was not able to make trophoblasts from the in vitro embryos similar to the in vivo ones. The few genes exhibiting differences are in control of important events that may be responsible for embryonic losses occurring during the first period of gestation.

Electrospray mass spectrometry analysis of blastocoel fluid as a potential tool for bovine embryo selection

PURPOSE

The aim of this study was to analyze the metabolic profiles of blastocoel fluid (BF) obtained from bovine embryos produced in vivo and in vitro.

METHODS

Expanded blastocysts (20/group) that were in vitro and in vivo derived at day 7 were used. BF was collected and analyzed under direct infusion conditions using a microTOF-Q^® mass spectrometer with electrospray ionization and a mass range of 50-650 m/z.

RESULTS

The spectrometry showed an evident difference in the metabolic profiles of BF from in vivo and in vitro produced embryos. These differences were very consistent between the samples of each group suggesting that embryo fluids can be used to identify the origin of the embryo. Ions 453.15 m/z, 437.18 m/z, and 398.06 m/z were identified as biomarkers for the embryo's origin with 100% sensitivity and specificity. Although it was not possible to unveil the molecular identity of the differential ions, the resulting spectrometric profiles provide a phenotype capable of differentiating embryos and hence constitute a potential parameter for embryo selection.

CONCLUSION

To the best of our knowledge, our results showed, for the first time, an evident difference between the spectrometric profiles of the BF from bovine embryos produced in vivo and in vitro.

Mitochondrial reactive oxygen species regulate mitochondrial biogenesis in porcine embryos

The number of mitochondria in blastocysts is a potential marker of embryo quality. However, the molecular mechanisms governing the mitochondrial number in embryos are unclear. This study was conducted to investigate the effect of reduced mitochondrial reactive oxygen species (ROS) levels on mitochondrial biogenesis in porcine embryos. Oocytes were collected from gilt ovaries and activated to generate over 4 cell-stage embryos at day 2 after activation. These embryos were cultured in media containing either 0.1 μM MitoTEMPOL (MitoT), 0.5 μM Mitoquinol (MitoQ), or vehicle (ethanol) for 5 days to determine the rate of development to the blastocyst stage. The mitochondrial number in blastocysts was evaluated by real-time polymerase chain reaction (PCR). Five days after activation, the embryos (early morula stage) were subjected to immunostaining to determine the expression levels of NRF2 in the nucleus. In addition, the expression levels of PGC1α and TFAM in the embryos were examined by reverse transcription PCR. One day of incubation with the antioxidants reduced the ROS content in the embryos but did not affect the rate of development to the blastocyst stage. Blastocysts developed in medium containing MitoT had lower mitochondrial DNA copy numbers and ATP content, whereas MitoQ showed similar but insignificantly trends. Treatment of embryos with either MitoT or MitoQ decreased the expression levels of NRF2 in the nucleus and levels of PGC1α and TFAM. These findings indicate that reductions in mitochondrial ROS levels are associated with low mitochondrial biogenesis in embryos.

Transcription profiles of oocytes during maturation and embryos during preimplantation development in vivo in the goat

RNA sequencing performed on goat matured oocytes and preimplantation embryos generated invivo enabled us to define the transcriptome for goat preimplantation embryo development. The largest proportion of changes in gene expression in goat was found at the 16-cell stage, not as previously defined at the 8-cell stage, and is later than in other mammalian species. In all, 6482 genes were identified to be significantly differentially expressed across all consecutive developmental stage comparisons, and the important signalling pathways involved in each development transition were determined. In addition, we identified genes that appear to be transcribed only at a specific stage of development. Using weighted gene coexpression network analysis, we found nine stage-specific modules of coexpressed genes that represent the corresponding stage of development. Furthermore, we identified conserved key members (or hub genes) of the goat transcriptional networks. Their association with other embryo genes suggests that they may have important regulatory roles in embryo development. Our cross-mammalian species transcriptomic comparisons demonstrate both conserved and goat-specific features of preimplantation development.

Xanthan gum and locust bean gum substrate improves bovine embryo development

One of the major difference between the in vivo and in vitro embryonic environments is the stiffness of the culture substrate. Xanthan gum (XG) and locust bean gum (LBG) are natural materials that are safe, inexpensive and easy to handle. In this study, we investigated the effects of using a polysaccharide culture substrate made from 1% XG and 1% LBG (XG-LBG gel) on bovine embryonic development. Oocytes collected from bovine ovaries were subjected to maturation, and fertilization to generate embryos at an early developmental stage (>4 cell stage). Cleaved embryos were further cultured in a well of 96-well cell culture plate coated with or without XG-LBG gel for 5 days. While the developmental rate up to the blastocyst stage did not differ between the two culture systems (control, 38.0 vs. gel, 38.6%), blastocysts developed on the XG-LBG gel produced significantly high cell numbers and ATP content. Embryos cultured on XG-LBG gels for 24 hr had high expression levels of F-actin and a highly even distribution of E-cadherin. In addition, embryos developed on XG-LBG gel demonstrated increased translocation of YAP to the nucleus and increased connective tissue growth factor (CTGF) protein levels (downstream of Hippo signalling). These findings suggest that soft culture substrates improve embryonic development by enhancing mechanotransduction, including YAP-CTGF signalling.