142 Nulliparity alters gene expression in inner cell mass and trophoblast of equine blastocysts in old mares

An increased incidence in early embryo loss has been observed in aged mares. Moreover, the first foal born to a mare is lighter than her subsequent foals, with reported impaired placental function at term. Because trophoblast function may be affected from the embryo stage, the aim of this project was to determine the effect of parity in aged mares on gene expression in Day-8.5 embryos. Middle-aged (13.5±2.2 years) nulliparous (never foaled) (ON) or multiparous (1.8±1.6 foals) (OM) Saddlebred, non-nursing mares were inseminated with the semen of one unique stallion. At 8 days post-ovulation (10 days post-hCG), embryos were recovered by uterine flushing and bisected to obtain samples of pure (trophectoderm, TE) or inner cell mass enriched (ICM) trophoblast. Paired end, non-oriented RNA sequencing was performed with Illumina (NextSEqn 500) on 5 and 6 TE and ICM collected from ON and OM, respectively. Differential expression was analysed with DESEqn 2. Embryo size was included in the model and a P<0.05 cutoff was used after false discovery rate correction. Gene set enrichment analysis (GSEA) was performed using the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) databases. Out of the 13 007 and 12 706 genes expressed in ICM and TE, respectively, only 8 in ICM and 6 in TE were differentially expressed, with 2 genes in common. Nevertheless, 19 gene sets were enriched and 6 depleted in the ICM of ON, whereas 2 gene sets were enriched and 8 depleted in the TE of ON compared with OM. Gene sets involved in ribosomal activity and structure, proteasome, integral component of plasma membrane, and immune response were enriched in ICM from ON embryos, and gene sets linked to sphingolipid metabolism, nucleosome, and constituents of the extracellular matrix (ECM) were depleted. In TE from ON mares, enriched gene sets were involved with ribosomes and depleted gene sets were linked to extracellular matrix, focal adhesion, myosin complex, and sequence-specific DNA binding. Overall, 1 enriched (linked to extracellular matrix) and 1 depleted gene set (involved in ribosomal structure) were common to ICM and TE. Thus, embryos from aged nulliparous mares seem to have higher protein turnover and higher immune response compared with those of OM, whereas the depletion of gene sets associated with extracellular matrix and membrane may indicate differences in cellular organisation into lineages. More work is ongoing to study effects on subsequent development.

51 Bovine embryos with distinct early morphokinetic pathways present different post-embryonic genome activation transcriptomic patterns and different cryotolerance

We recently developed and validated a methodology based on early morphokinetics to predict 6 categories of bovine embryos, including 4 categories of blastocysts: early hatching blastocysts (EHB), hatching blastocysts (HB), low hatching blastocysts (LHB), and arrhythmic blastocysts (AB) (Reis et al. 2018 Anim. Reprod. 15, 601). We hypothesised that different early morphokinetic pathways could (1) be accompanied by different biological patterns (EGA, etc.) and (2) influence interaction with environmental constraints (cryopreservation, etc.). The objective of this study was (1) to investigate whether transcriptomic differences between the predicted EHB, HB, LHB, or AB could be observed immediately after the fourth embryonic cycle (just after EGA); (2) to assess cryotolerance of these morphokinetic categories. Time lapses were produced during 4 days post invitro insemination (dpi) (Reis et al. 2018). Study 1: 128 embryos having finished the fourth embryonic cycle were individually dry frozen at 4.7 dpi, classified as EHB, HB, LHB, or AB, and pooled into four batches of eight embryos/category (total 16 samples). Total RNA was extracted, amplified using the SMARTseq V4 ultralow input kit (Clontech), libraries were prepared using the Nextera XT Illumina library preparation kit, and sequenced (paired-end 50-34bp) on an Illumina NextSEqn 500 instrument. Identification of differentially expressed genes was achieved using Limma package (R). The P-values were adjusted using the Benjamini and Hochberg false discovery rate (Saenz-de-Juano et al. 2014). Fold change >2 or <0.5 and adjusted P<0.05 were considered significant. Enrichment analysis was performed using the Web server enrichR. Study 2: 90 embryos showing signs of expansion (diameter >135µm and −2µm at zona pellucida) were slow frozen at 6.2 or 7 dpi, classified as EHB (n=8), HB (n=29), LHB (n=32), or AB (n=21), and further thawed and cultured invitro for 72h. Cryotolerance was evaluated at 24 and 72h post-thawing. Statistical analysis was performed using chi-square or Fisher test. Study 1: the AB category presented 258, 699, and 899 upregulated genes and 40, 62, and 60 downregulated in comparison to LHB, HB, and EHB, respectively. The functional comparison of the two extremes (EHB vs. AB), at 4.7 dpi, showed that AB embryos present intense transcription regulation activity. The monocarboxylic acid binding pathway was upregulated in EHB. Study 2: the post-thawing invitro survival was similar between EHB, HB, LHB, and AB (50%, 34.5%, 31.3%, and 42.9%, respectively). The hatching/alive ratio in EHB category tended to be lower than AB (1/4 vs. 8/9; P=0.052). Frozen/thawed EHB and HB presented reduced hatching rates, whereas LHB and AB significantly increased the hatching potential after thawing compared with their not-frozen counterparts of the learning database (25% vs. 100%; 50% vs. 100%; 70% vs. 11%; 88.9% vs. 18%, respectively). Morphokinetic prediction highlighted a new classification of bovine embryos with different biological features. This could contribute to improve practice on IVF embryo transfer taking into account these differences.

Progressive methylation of POU5F1 regulatory regions during blastocyst development

The POU5F1 gene encodes one of the 'core' transcription factors necessary to establish and maintain pluripotency in mammals. Its function depends on its precise level of expression, so its transcription has to be tightly regulated. To date, few conserved functional elements have been identified in its 5' regulatory region: a distal and a proximal enhancer, and a minimal promoter, epigenetic modifications of which interfere with POU5F1 expression and function in in vitro-derived cell lines. Also, its permanent inactivation in differentiated cells depends on de novo methylation of its promoter. However, little is known about the epigenetic regulation of POU5F1 expression in the embryo itself. We used the rabbit blastocyst as a model to analyze the methylation dynamics of the POU5F1 5' upstream region, relative to its regulated expression in different compartments of the blastocyst over a 2-day period of development. We evidenced progressive methylation of the 5' regulatory region and the first exon accompanying differentiation and the gradual repression of POU5F1 Methylation started in the early trophectoderm before complete transcriptional inactivation. Interestingly, the distal enhancer, which is known to be active in naïve pluripotent cells only, retained a very low level of methylation in primed pluripotent epiblasts and remained less methylated in differentiated compartments than the proximal enhancer. This detailed study identified CpGs with the greatest variations in methylation, as well as groups of CpGs showing a highly correlated behavior, during differentiation. Moreover, our findings evidenced few CpGs with very specific behavior during this period of development.

Different co-culture systems have the same impact on bovine embryo transcriptome

During the last few years, several co-culture systems using either BOEC or VERO feeder cells have been developed to improve bovine embryo development and these systems give better results at high oxygen concentration (20%). In parallel, the SOF medium, used at 5% O2, has been developed to mimic the oviduct fluid. Since 2010s, the SOF medium has become popular in improving bovine embryo development and authors have started to associate this medium to co-culture systems. Nevertheless, little is known about the putative benefit of this association on early development. To address this question, we have compared embryo transcriptomes in four different culture conditions: SOF with BOEC or VERO at 20% O2, and SOF without feeders at 5% or 20% O2. Embryos have been analyzed at 16-cell and blastocyst stages. Co-culture systems did not improve the developmental rate when compared to 5% O2. Direct comparison of the two co-culture systems failed to highlight major differences in embryo transcriptome at both developmental stages. Both feeder cell types appear to regulate the same cytokines and growth factors pathways, and thus to influence embryo physiology in the same way. In blastocysts, when compared to culture in SOF at 5% O2, BOEC or VERO seems to reduce cell survival and differentiation by, at least, negatively regulating STAT3 and STAT5 pathways. Collectively, in SOF medium both blastocysts rate and embryo transcriptome suggest no influence of feeder origin on bovine early development and no beneficial impact of co-culture systems when compared to 5% O2.

120 MATERNAL EXPOSURE TO DIESEL ENGINE EXHAUST DURING PREGNANCY AFFECTS EARLY EMBRYO DEVELOPMENT IN A RABBIT MODEL

Airborne pollution has been associated with various adverse effects on human reproductive health, especially intrauterine growth retardation and early pregnancy loss. However, few studies have analysed its effect on early development. Diesel exhaust particles (DEP) have been shown to alter blastocyst formation when diluted in embryo culture medium (2010 Toxicol Sci. 117, 200–208), but no data are available concerning the effect of maternal inhalation of diesel exhaust on early embryo development. Our study has been designed to answer this question using rabbit as a model and DEP doses mimicking daily exposure to traffic in large European cities. New Zealand female rabbits were superovulated by means of 5 subcutaneous administration of pFSH for 3 days before mating, followed 10 to 12 h later by an intravenous administration of 30 IU of hCG at the time of mating (natural mating). Dams were exposed to a representative air pollution mixture; that is, diluted diesel engine exhaust (1 mg m–3; N = 14) or clean air (N = 12), for 1 h every morning and afternoon, from Day 3 to Day 6 post-coitum (dpc). At 6 dpc, in vivo-developed embryos were collected from uteri perfused with PBS and counted; their diameter was measured on pictures using ImageJ software (NIH, Bethesda, MD, USA). Another group of female rabbits was exposed to the same inhalation conditions from 3 to 27 dpc without superovulation treatment. Measures by ultrasound were performed on these dams at 7 dpc. Data were analysed by Mann-Whitney test and ANOVA, including dams as cofactor. At 6 dpc, number of embryos per dams was higher in exposed group compared with control (P < 0.05). In contrast, embryo diameter was significantly lower in the DEP exposed group compared with the clean air exposed group (P < 0.01). Gene expression analysis is being performed in these embryos. At 7 dpc, ultrasound measurements evidenced a decrease in embryo diameter, perimeter, and volume in the exposed group compared with control (P < 0.01, P < 0.01, and P < 0.01, respectively). These data indicate that repeated exposure to airborne pollution even for daily short periods affects early development. Consequences of maternal DEP exposure on feto-placental development are under investigation.

Hyperlipidic hypercholesterolemic diet in prepubertal rabbits affects gene expression in the embryo, restricts fetal growth and increases offspring susceptibility to obesity

Maternal hypercholesterolemia has been shown to lead to fetal intra-uterine growth retardation (IUGR) in rabbits. The effects of a long term maternal hyperlipidemic and hypercholesterolemic diet on embryo, fetal and post-natal development, have not been addressed so far. Rabbit does were fed either a hypercholesterolemic (0.2%) hyperlipidic (8%) (HH) or a control (C) diet from 10 weeks of age. Sixteen does (N = 8 HH and N = 8 C) were euthanized at 18 weeks to assess the effect of the diet on dams before mating. Embryos from 18 females (N = 9 HH and N = 9 C) were collected from the oviducts at the 16-20 cell stage (embryonic genome activation stage) for gene expression analysis (micro array and quantitative RT-PCR). Thirty females (N = 16 HH and N = 14 C) were mated naturally and fetal growth was monitored by ultrasound. Six of them (N = 4 HH and N = 2 C) were euthanized at D28 of gestation to collect fetuses and placentas. Finally, the remaining 24 does delivered at term and litters were cross fostered and equilibrated in number to create 4 groups according to the biological dam and the foster dam (C-C, C-HH, HH-C, HH-HH). Growth was monitored until weaning. A subset of 26 offspring from the 4 groups was fed the control diet until 25 weeks of age and then fed the HH diet for three weeks. All does had similar growth rates and bodyweight. Transcriptomic analyses evidenced an overexpression of Adipophilin in HH embryos at the stage of embryonic genome activation. This was confirmed by quantitative RT-PCR. During pregnancy, IUGR was observed from D9 by ultrasound and subsequently, fetal weight at 28 days, birthweight and fat deposition in newborn offspring were significantly decreased in HH (P < 0.05). After weaning, there was no significant difference for weight between HH-HH and HH-C offspring and both groups became significantly heavier (P < 0.0001) than C-C and C-HH offspring. During the 3 weeks when offspring were fed the HH diet, the differences in feed intake were no longer significant between groups but the differences in body weight remained. At post-mortem, offspring from HH does had significantly more abdominal and inter-scapular fat than offspring from C does (P < 0.05). These data illustrate the importance of maternal nutrition before and during gestation in the establishment and control of the growth trajectory of the conceptus and in the onset of disease in adult life.

Retrotransposon expression as a defining event of genome reprogramming in fertilized and cloned bovine embryos

Genome reprogramming is the ability of a nucleus to modify its epigenetic characteristics and gene expression pattern when placed in a new environment. Low efficiency of mammalian cloning is attributed to the incomplete and aberrant nature of genome reprogramming after somatic cell nuclear transfer (SCNT) in oocytes. To date, the aspects of genome reprogramming critical for full-term development after SCNT remain poorly understood. To identify the key elements of this process, changes in gene expression during maternal-to-embryonic transition in normal bovine embryos and changes in gene expression between donor cells and SCNT embryos were compared using a new cDNA array dedicated to embryonic genome transcriptional activation in the bovine. Three groups of transcripts were mostly affected during somatic reprogramming: endogenous terminal repeat (LTR) retrotransposons and mitochondrial transcripts were up-regulated, while genes encoding ribosomal proteins were downregulated. These unexpected data demonstrate specific categories of transcripts most sensitive to somatic reprogramming and likely affecting viability of SCNT embryos. Importantly, massive transcriptional activation of LTR retrotransposons resulted in similar levels of their transcripts in SCNT and fertilized embryos. Taken together, these results open a new avenue in the quest to understand nuclear reprogramming driven by oocyte cytoplasm.

Revealing the dynamics of gene expression during embryonic genome activation and first differentiation in the rabbit embryo with a dedicated array screening

Early mammalian development is characterized by extensive changes in nuclear functions that result from epigenetic modifications of the newly formed embryonic genome. While the first embryonic cells are totipotent, this status spans only a few cell cycles. At the blastocyst stage, the embryo already contains differentiated trophectoderm cells and pluripotent inner cell mass cells. Concomitantly, the embryonic genome becomes progressively transcriptionally active. During this unique period of development, the gene expression pattern has been mainly characterized in the mouse, in which embryonic genome activation (EGA) spans a single cell cycle after abrupt epigenetic modifications. To further characterize this period, we chose to analyze it in the rabbit, in which, as in most mammals, EGA is more progressive and occurs closer to the first cell differentiation events. In this species, for which no transcriptomic arrays were available, we focused on genes expressed at EGA and first differentiation and established a 2,000-gene dedicated cDNA array. Screening this with pre-EGA, early post-EGA, and blastocyst embryos divided genes into seven clusters of expression according to their regulation during this period and revealed their dynamics of expression during EGA and first differentiation. Our results point to transient properties of embryo transcriptome at EGA, due not only to the transition between maternal and embryonic transcripts but also to the transient expression of a subset of embryonic genes whose functions remained largely uncharacterized. They also provide a first view of the functional consequences of the changes in gene expression program.