Alteration of the embryonic microenvironment and sex-specific responses of the preimplantation embryo related to a maternal high-fat diet in the rabbit model

The maternal metabolic environment can be detrimental to the health of the offspring. In a previous work, we showed that maternal high-fat (HH) feeding in rabbit induced sex-dependent metabolic adaptation in the fetus and led to metabolic syndrome in adult offspring. As early development representing a critical window of susceptibility, in the present work we aimed to explore the effects of the HH diet on the oocyte, preimplantation embryo and its microenvironment. In oocytes from females on HH diet, transcriptomic analysis revealed a weak modification in the content of transcripts mainly involved in meiosis and translational control. The effect of maternal HH diet on the embryonic microenvironment was investigated by identifying the metabolite composition of uterine and embryonic fluids collected in vivo by biomicroscopy. Metabolomic analysis revealed differences in the HH uterine fluid surrounding the embryo, with increased pyruvate concentration. Within the blastocoelic fluid, metabolomic profiles showed decreased glucose and alanine concentrations. In addition, the blastocyst transcriptome showed under-expression of genes and pathways involved in lipid, glucose and amino acid transport and metabolism, most pronounced in female embryos. This work demonstrates that the maternal HH diet disrupts the in vivo composition of the embryonic microenvironment, where the presence of nutrients is increased. In contrast to this nutrient-rich environment, the embryo presents a decrease in nutrient sensing and metabolism suggesting a potential protective process. In addition, this work identifies a very early sex-specific response to the maternal HH diet, from the blastocyst stage.

Multigenerational Effects of a Complex Human-Relevant Exposure during Folliculogenesis and Preimplantation Embryo Development: The FEDEXPO Study

Animal toxicological studies often fail to mimic the complexity of the human exposome, associating low doses, combined molecules and long-term exposure. Since the reproductive potential of a woman begins in the fetal ovary, the literature regarding the disruption of its reproductive health by environmental toxicants remains limited. Studies draw attention to follicle development, a major determinant for the quality of the oocyte, and the preimplantation embryo, as both of them are targets for epigenetic reprogramming. The "Folliculogenesis and Embryo Development EXPOsure to a mixture of toxicants: evaluation in the rabbit model" (FEDEXPO) project emerged from consideration of these limitations and aims to evaluate in the rabbit model the impacts of an exposure to a mixture of known and suspected endocrine disrupting chemicals (EDCs) during two specific windows, including folliculogenesis and preimplantation embryo development. The mixture combines eight environmental toxicants, namely perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), dichlorodiphenyldichloroethylene (DDE), hexachlorobenzene (HCB), β-hexachlorocyclohexane (β-HCH), 2,2'4,4'-tetrabromodiphenyl ether (BDE-47), di(2-ethylhexyl) phthalate (DEHP) and bisphenol S (BPS), at relevant exposure levels for reproductive-aged women based on biomonitoring data. The project will be organized in order to assess the consequences of this exposure on the ovarian function of the directly exposed F0 females and monitor the development and health of the F1 offspring from the preimplantation stage. Emphasis will be made on the reproductive health of the offspring. Lastly, this multigenerational study will also tackle potential mechanisms for the inheritance of health disruption via the oocyte or the preimplantation embryo.

Major transcriptomic, epigenetic and metabolic changes underlie the pluripotency continuum in rabbit preimplantation embryos

Despite the growing interest in the rabbit model for developmental and stem cell biology, the characterization of embryos at the molecular level is still poorly documented. We conducted a transcriptome analysis of rabbit preimplantation embryos from E2.7 (morula stage) to E6.6 (early primitive streak stage) using bulk and single-cell RNA-sequencing. In parallel, we studied oxidative phosphorylation and glycolysis, and analysed active and repressive epigenetic modifications during blastocyst formation and expansion. We generated a transcriptomic, epigenetic and metabolic map of the pluripotency continuum in rabbit preimplantation embryos, and identified novel markers of naive pluripotency that might be instrumental for deriving naive pluripotent stem cell lines. Although the rabbit is evolutionarily closer to mice than to primates, we found that the transcriptome of rabbit epiblast cells shares common features with those of humans and non-human primates.

Maternal age affects equine day 8 embryo gene expression both in trophoblast and inner cell mass

BACKGROUND

Breeding a mare until she is not fertile or even until her death is common in equine industry but the fertility decreases as the mare age increases. Embryo loss due to reduced embryo quality is partly accountable for this observation. Here, the effect of mare's age on blastocysts' gene expression was explored. Day 8 post-ovulation embryos were collected from multiparous young (YM, 6-year-old, N = 5) and older (OM, > 10-year-old, N = 6) non-nursing Saddlebred mares, inseminated with the semen of one stallion. Pure or inner cell mass (ICM) enriched trophoblast, obtained by embryo bisection, were RNA sequenced. Deconvolution algorithm was used to discriminate gene expression in the ICM from that in the trophoblast. Differential expression was analyzed with embryo sex and diameter as cofactors. Functional annotation and classification of differentially expressed genes and gene set enrichment analysis were also performed.

RESULTS

Maternal aging did not affect embryo recovery rate, embryo diameter nor total RNA quantity. In both compartments, the expression of genes involved in mitochondria and protein metabolism were disturbed by maternal age, although more genes were affected in the ICM. Mitosis, signaling and adhesion pathways and embryo development were decreased in the ICM of embryos from old mares. In trophoblast, ion movement pathways were affected.

CONCLUSIONS

This is the first study showing that maternal age affects gene expression in the equine blastocyst, demonstrating significant effects as early as 10 years of age. These perturbations may affect further embryo development and contribute to decreased fertility due to aging.

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.

Effects of first-generation in utero exposure to diesel engine exhaust on second-generation placental function, fatty acid profiles and foetal metabolism in rabbits: preliminary results

Atmospheric pollution has major health effects on directly exposed subjects but intergenerational consequences are poorly characterized. We previously reported that diesel engine exhaust (DE) could lead to structural changes in the placenta of in utero exposed rabbits (first generation, F1). The effects of maternal exposure to DE were further studied on second-generation (F2) rabbits. Pregnant F0 females were exposed to filtered, diluted DE (1 mg/m³, median particle diameter: 69 nm) or clean filtered air (controls) for 2 h/day, 5 days/week by nose-only exposure during days 3–27 post-conception (dpc). Adult female offspring (F1) were mated to control males: F1 tissues and F2 foeto-placental units were collected at 28 dpc and placental structure and gene expression (microarray) analysed. Fatty acid profiles were determined in foetal and maternal plasma, maternal liver and placenta. In F1, compared to controls, hepatic neutral lipid contents were increased in exposed animals without change in the blood biochemistry. In F2, the placental lipid contents were higher, with higher monounsaturated fatty acids and reduced pro-inflammatory arachidonic acid (AA), without placental structural changes. Conversely, the proportion of anti-inflammatory n-3 polyunsaturated fatty acids in F2 plasma was increased while that of AA was decreased. Gene set enrichment analyses (GSEA) of F2 placenta transcriptomic data identified that the proteasome complex and ubiquitin pathways genes were over-represented and ion channel function and inflammation pathways genes were under-represented in exposed animals. These preliminary results demonstrate that diesel engine exhaust exposure and in utero indirect exposure should be considered as a programming factor within the context of the DOHaD (Developmental Origins of Health and Disease) with a probable intergenerational transmission.

Differentiation of derived rabbit trophoblast stem cells under fluid shear stress to mimic the trophoblastic barrier

BACKGROUND

The placenta controls exchanges between the mother and the fetus and therefore fetal development and growth. The maternal environment can lead to disturbance of placental functions, with consequences on the health of the offspring. Since the rabbit placenta is very close to that of humans, rabbit models can provide biomedical data to study human placental function. Yet, to limit the use of animal experiments and to investigate the mechanistic aspects of placental function, we developed a new cell culture model in which rabbit trophoblast cells are differentiated from rabbit trophoblast stem cells.

METHODS

Rabbit trophoblast stems cells were derived from blastocysts and differentiated onto a collagen gel and in the presence of a flow of culture medium to mimic maternal blood flow. Transcriptome analysis was performed on the stem and differentiated cells.

RESULTS

Our culture model allows the differentiation of trophoblast stem cells. In particular, the fluid shear stress enhances microvilli formation on the differentiated cell surface, lipid droplets formation and fusion of cytotrophoblasts into syncytiotrophoblasts. In addition, the transcriptome analysis confirms the early trophoblast identity of the derived stem cells and reveals upregulation of signaling pathways involved in trophoblast differentiation.

CONCLUSION

Thereby, the culture model allows mimicking the in vivo conditions in which maternal blood flow exerts a shear stress on trophoblast cells that influences their phenotype.

GENERAL SIGNIFICANCE

Our culture model can be used to study the differentiation of trophoblast stem cells into cytotrophoblasts and syncytiotrophoblasts, as well as the trophoblast function in physiological and pathological conditions.

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.

Docosahexaenoic acid mechanisms of action on the bovine oocyte-cumulus complex

Background

Supplementation of bovine oocyte-cumulus complexes during in vitro maturation (IVM) with 1 μM of docosahexaenoic acid (DHA), C22:6 n-3 polyunsaturated fatty acid, was reported to improve in vitro embryo development. The objective of this paper was to decipher the mechanisms of DHA action.

Results

Transcriptomic analysis of 1 μM DHA-treated and control cumulus cells after 4 h IVM showed no significant difference in gene expression. MALDI-TOF mass spectrometry analysis of lipid profiles in DHA-treated and control oocytes and cumulus cells after IVM showed variations of only 3 out of 700 molecular species in oocytes and 7 out of 698 species in cumulus cells (p < 0.01).

We showed expression of free fatty acid receptor FFAR4 in both oocytes and cumulus cells, this receptor is known to be activated by binding to DHA. FFAR4 protein was localized close to the cellular membrane by immunofluorescence. Functional studies demonstrated that supplementation with FFAR4 agonist TUG-891 (1 μM or 5 μM) during IVM led to an increased blastocyst rate (39.5% ± 4.1%, 41.3% ± 4.1%), similar to DHA 1 μM treatment (39.2% ± 4.1%) as compared to control (25.2% ± 3.6%).

FFAR4 activation via TUG-891 led to beneficial effect on oocyte developmental competence and might explain in part similar effects of DHA.

Conclusions

In conclusion, we suggested that low dose of DHA (1 μM) during IVM might activate regulatory mechanisms without evident effect on gene expression and lipid content in oocyte-cumulus complexes, likely through signaling pathways which need to be elucidated in further studies.

Electronic supplementary material

The online version of this article (10.1186/s13048-017-0370-z) contains supplementary material, which is available to authorized users.

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.

Reprogramming of rabbit induced pluripotent stem cells toward epiblast and chimeric competency using Krüppel-like factors

Rabbit induced pluripotent stem cells (rbiPSCs) possess the characteristic features of primed pluripotency as defined in rodents and primates. In the present study, we reprogrammed rbiPSCs using human Krüppel-like factors (KLFs) 2 and 4 and cultured them in a medium supplemented with fetal calf serum and leukemia inhibitory factor. These cells (designated rbEKA) were propagated by enzymatic dissociation for at least 30 passages, during which they maintained a normal karyotype. This new culturing protocol resulted in transcriptional and epigenetic reconfiguration, as substantiated by the expression of transcription factors and the presence of histone modifications associated with naïve pluripotency. Furthermore, microarray analysis of rbiPSCs, rbEKA cells, rabbit ICM cells, and rabbit epiblast showed that the global gene expression profile of the reprogrammed rbiPSCs was more similar to that of rabbit ICM and epiblast cells. Injection of rbEKA cells into 8-cell stage rabbit embryos resulted in extensive colonization of ICM in 9% early-blastocysts (E3.5), epiblast in 10% mid-blastocysts (E4.5), and embryonic disk in 1.4% pre-gastrulae (E6). Thus, these results indicate that KLF2 and KLF4 triggered the conversion of rbiPSCs into epiblast-like, embryo colonization-competent PSCs. Our results highlight some of the requirements to achieve bona fide chimeric competency.

A Panel of Embryonic Stem Cell Lines Reveals the Variety and Dynamic of Pluripotent States in Rabbits

Conventional rabbit embryonic stem cell (ESC) lines are derived from the inner cell mass (ICM) of pre-implantation embryos using methods and culture conditions that are established for primate ESCs. In this study, we explored the capacity of the rabbit ICM to give rise to ESC lines using conditions similar to those utilized to generate naive ESCs in mice. On single-cell dissociation and culture in fibroblast growth factor 2 (FGF2)-free, serum-supplemented medium, rabbit ICMs gave rise to ESC lines lacking the DNA-damage checkpoint in the G1 phase like mouse ESCs, and with a pluripotency gene expression profile closer to the rabbit ICM/epiblast profiles. These cell lines can be converted to FGF2-dependent ESCs after culture in conventional conditions. They can also colonize the rabbit pre-implantation embryo. These results indicate that rabbit epiblast cells can be coaxed toward different types of pluripotent stem cells and reveal the dynamics of pluripotent states in rabbit ESCs.

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.

Alteration of DNA demethylation dynamics by in vitro culture conditions in rabbit pre-implantation embryos

Alterations to DNA methylation have been attributed to in vitro culture and may affect normal embryo development. We chose to analyze DNA methylation reprogramming in the rabbit which, of the species with delayed transcriptional activation of the embryonic genome, allows easy comparisons between in vivo-developed (IVD) and in vitro-cultured (IVC) embryos. In this species, variations in DNA methylation had not previously been quantified, even in IVD embryos. IVD and IVC embryos were recovered at the 2, 4, 8 and 16-cell, morula and blastocyst stages. Immunostaining for 5-methyl-cytidine and normalization of the quantity of methylated DNA vs. the total DNA content were then performed. Our quantitative results evidenced DNA demethylation during pre-implantation development in both IVD and IVC embryos, but with different kinetics. Demethylation occurred earlier in vitro than in vivo between the 2 and 8-cell stages in IVC embryos, reaching its lowest level, while it only started at the 4-cell stage and ended at the 16-cell stage in IVD embryos. We also showed that an absence of serum from the culture medium significantly altered the degree of DNA demethylation. Finally, at the blastocyst stage, ICM was more methylated than the trophectoderm in all cases. Despite a morphological delay observed in in vitro cultured blastocysts, the difference in DNA methylation between ICM and trophectoderm cells appeared at the same time post-fertilization in IVD and IVC embryos, which may reflect another difference in the dynamics of DNA methylation during blastocyst formation. Our data thus clearly establish an effect of embryonic environment on DNA methylation reprogramming during pre-implantation development in a non-rodent species.

Dynamics of DNA methylation levels in maternal and paternal rabbit genomes after fertilization

The reprogramming of DNA methylation in early embryos has been considered to be essential for the reprogramming of differentiated parental genomes to totipotency, the transcription of embryonic genome activation (EGA) and subsequent development. However, its degree appears to differ as a function of species and it may be altered by the in vitro environment. While the rabbit is a pertinent model for species with a delayed EGA because both in vivo and in vitro developed embryos are easily available, the status of DNA methylation levels in both parental genomes after fertilization remains controversial. In order to generate precise data on the DNA methylation status in rabbit zygotes, we first of all defined five pronuclear (PN) stages during the first cell cycle and then classified in vivo and in vitro developed rabbit zygotes according to these PN stages. Using this classification we precisely quantified both methylated DNA and the total DNA content during the one cell stage. The quantification of methylated DNA, normalized for the total DNA content, showed that both pronuclei displayed distinct patterns of DNA methylation reprogramming. In the maternal pronucleus (MP) the methylation level remained constant throughout the one cell stage, thanks to maintenance methylation during the S phase. Conversely, in the paternal pronucleus (PP) partial demethylation occurred before replication, probably as a result of active DNA demethylation, while maintenance methylation subsequently occurred during the S phase. Interestingly, we showed that PP DNA methylation reprogramming was partially altered by the in vitro environment. Taken together, our approach evidenced that rabbit is one of the species displaying partial DNA demethylation in the PP, and for the first time demonstrated maintenance methylation activity in both pronuclei during the first S phase.

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.