Trophectoderm lineage determination in cattle

NOTCH signaling pathway is required for bovine early embryonic development†

The NOTCH signaling pathway plays an important role in regulating various biological processes, including lineage specification and apoptosis. Multiple components of the NOTCH pathway have been identified in mammalian preimplantation embryos. However, the precise role of the NOTCH pathway in early embryonic development is poorly understood, especially in large animals. Here, we show that the expression of genes encoding key transcripts of the NOTCH pathway is dynamic throughout early embryonic development. We also confirm the presence of active NOTCH1 and RBPJ. By using pharmacological and RNA interference tools, we demonstrate that the NOTCH pathway is required for the proper development of bovine early embryos. This functional consequence could be partly attributed to the major transcriptional mediator, Recombination Signal Binding Protein For Immunoglobulin Kappa J Region (RBPJ), whose deficiency also compromised the embryo quality. Indeed, both NOTCH1 and RBPJ knockdown cause a significant increase of histone H3 serine 10 phosphorylation (pH3S10, a mitosis marker) positive blastomeres, suggesting a cell cycle arrest at mitosis. Importantly, RNA sequencing analyses reveal that either NOTCH1 or RBPJ depletion triggers a reduction in H1FOO that encodes the oocyte-specific linker histone H1 variant. Interestingly, depleting H1FOO results in detrimental effects on the developmental competence of early embryos, similar with NOTCH1 inhibition. Overall, our results reveal a crucial role for NOTCH pathway in regulating bovine preimplantation development, likely by controlling cell proliferation and maintaining H1FOO expression.

The landscape of accessible chromatin in bovine oocytes and early embryos

Chromatin reorganization governs the regulation of gene expression during preimplantation development. However, the landscape of chromatin dynamics in this period has not been explored in bovine. In this study, we constructed a genome-wide map of accessible chromatin in bovine oocytes and early embryos using an improved assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) which revealed unique features of the accessible chromatin during bovine early embryo development. We found that chromatin accessibility is low in oocytes and 2-/4-cell embryos, followed by a significant increase in embryos during major embryonic genome activation (EGA), and peaked in elongating day 14 embryos. Genome-wide characteristics of open chromatin showed that ATAC-seq signals in both transcription start sites (TSS) and transcription end sites (TES) were strong. Additionally, the distal ATAC-seq peaks were enriched in repeat elements in a type-specific and stage-specific manner. We further unveiled a series of transcription factor (TF) motifs with distinct variation of enrichment from distal ATAC-seq peaks. By integrated analysis of chromatin accessibility with transcriptomes and DNA methylomes in bovine early embryos, we showed that promoter accessibility was positively correlated with gene expression, especially during major EGA, and was strongly correlated to DNA methylation and CpG density. Finally, we identified the critical chromatin signatures and TFs that differ between in vivo and in vitro derived blastocysts, which provides insights to the potential mechanisms leading to low quality of embryos produced in vitro. Together, this comprehensive analysis revealed critical features of chromatin landscape and epigenetic reprogramming during bovine preimplantation embryo development.

Induced pluripotent stem cells from farm animals

The development of the induced pluripotent stem cells (iPSCs) technology has revolutionized the world on the establishment of pluripotent stem cells (PSCs) across a great variety of animal species. Generation of iPSCs from domesticated animals would provide unrestricted cell resources for the study of embryonic development and cell differentiation of these species, for screening and establishing desired traits for sustainable agricultural production, and as veterinary and preclinical therapeutic tools for animal and human diseases. Induced PSCs from domesticated animals thus harbor enormous scientific, economical, and societal values. Although much progress has been made toward the generation of PSCs from these species, major obstacles remain precluding the exclamation of the establishment of bona fide iPSCs. The most prominent of them remain the inability of these cells to silence exogenous reprogramming factors, the obvious reliance on exogenous factors for their self-renewal, and the restricted development potential in vivo. In this review, we summarize the history and current progress in domestic farm animal iPSC generation, with a focus on swine, ruminants (cattle, ovine, and caprine), horses, and avian species (quails and chickens). We also discuss the problems associated with the farm animal iPSCs and potential future directions toward the complete reprogramming of somatic cells from farm animals.

Single-cell analysis of nonhuman primate preimplantation development in comparison to humans and mice

BACKGROUND

Genetic programs underlying preimplantation development and early lineage segregation are highly conserved across mammals. It has been suggested that nonhuman primates would be better model organisms for human embryogenesis, but a limited number of studies have investigated the monkey preimplantation development. In this study, we collect single cells from cynomolgus monkey preimplantation embryos for transcriptome profiling and compare with single-cell RNA-seq data derived from human and mouse embryos.

RESULTS

By weighted gene-coexpression network analysis, we found that cynomolgus gene networks have greater conservation with human embryos including a greater number of conserved hub genes than that of mouse embryos. Consistently, we found that early ICM/TE lineage-segregating genes in monkeys exhibit greater similarity with human when compared to mouse, so are the genes in signaling pathways such as LRP1 and TCF7 involving in WNT pathway. Last, we tested the role of one conserved pre-EGA hub gene, SIN3A, using a morpholino knockdown of maternal RNA transcripts in monkey embryos followed by single-cell RNA-seq. We found that SIN3A knockdown disrupts the gene-silencing program during the embryonic genome activation transition and results in developmental delay of cynomolgus embryos.

CONCLUSION

Taken together, our study provided new insight into evolutionarily conserved and divergent transcriptome dynamics during mammalian preimplantation development.

Interleukin-6 promotes primitive endoderm development in bovine blastocysts

Background

Interleukin-6 (IL6) was recently identified as an embryotrophic factor in bovine embryos, where it acts primarily to mediate inner cell mass (ICM) size. This work explored whether IL6 affects epiblast (EPI) and primitive endoderm (PE) development, the two embryonic lineages generated from the ICM after its formation. Nuclear markers for EPI (NANOG) and PE (GATA6) were used to differentiate the two cell types.

Results

Increases (P < 0.05) in total ICM cell numbers and PE cell numbers were detected in bovine blastocysts at day 8 and 9 post-fertilization after exposure to 100 ng/ml recombinant bovine IL6. Also, IL6 increased (P < 0.05) the number of undifferentiated ICM cells (cells containing both PE and EPI markers). The effects of IL6 on EPI cell numbers were inconsistent. Studies were also completed to explore the importance of Janus kinase 2 (JAK2)-dependent signaling in bovine PE cells. Definitive activation of STAT3, a downstream target for JAK2, was observed in PE cells. Also, pharmacological inhibition of JAK2 decreased (P < 0.05) PE cell numbers.

Conclusions

To conclude, IL6 manipulates ICM development after EPI/PE cell fates are established. The PE cells are the target for IL6, where a JAK-dependent signal is used to regulate PE numbers.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12861-020-00235-z.

Cell differentiation events in pre-implantation mouse and bovine embryos

Early mammal embryogenesis starts with oocyte fertilization, giving rise to the zygote. The events that the newly formed zygote surpasses are crucial to the embryo developmental success. Shortly after activation of its genome, cells of the embryo segregate into the inner cell mass (ICM) or the trophectoderm (TE). The first will give rise to the embryo while the latter will become the placenta. This first segregation involves cellular and molecular processes that include cell polarity linked to intracellular pathway activation, which will regulate the transcription of trophectoderm-related genes. Then, cells of the ICM undergo the second event of mammalian cell differentiation, which consists of the separation between epiblast (EPI) and hypoblast or primitive endoderm (PrE). This second segregation involves paracrine signaling, leading to differential expression of key genes that will dictate the fate of the cell. Although these processes are described in detail in the mouse, recent studies suggest that the bovine embryo could also be an interesting model for early development, since there are differences to the mouse and similarities with early human embryogenesis. In this review, we gathered the main data available in the literature upon bovine and mouse early development events, suggesting that both models should be analyzed and studied in a complementary way, to better model early events occurring in human development.

Technical note: improving the efficiency of generating bovine extraembryonic endoderm cells

The formation of extraembryonic endoderm (XEN) occurs early in embryonic development. The cell types that develop from the XEN remain poorly studied in ruminant species because of the lack of suitable cell culture model systems. The goal of this work was to establish a protocol for producing XEN cell cultures from bovine blastocysts. Previous work identified fibroblast growth factor 2 (FGF2) as a facilitator of bovine XEN development. Further refinements in culture conditions studied here included exposure to 20% fetal bovine serum and FGF2 replenishment. These modifications yielded an endoderm outgrowth formation incidence of 81.6% ± 5.5% compared with 33.3% ± 5.5% in bovine serum albumin (BSA)-supplemented controls. These cells resembled XEN when examined morphologically and contained XEN transcripts (GATA binding protein 4 [GATA4] and GATA binding protein 6 [GATA6]) as well as transcripts present in visceral (BCL2 interacting protein 1 [BNIP1] and vascular endothelial growth factor A [VEGFA]) and parietal (C-X-C motif chemokine receptor 4 [CXCR4], thrombomodulin [THBD], and hematopoietically expressed homeobox [HHEX]) XEN. Two XEN cell lines were maintained for prolonged culture. Both lines continued to proliferate for approximately 6 wk before becoming senescent. These cultures maintained an XEN-like state and continued to express GATA4 and GATA6 until senescence. An increase in the abundance of visceral and parietal XEN transcripts was observed with continued culture, suggesting that these cells either undergo spontaneous differentiation or retain the ability to form various XEN cell types. Stocks of cultured cells exposed to a freeze-thaw procedure possessed similar phenotypic and genotypic behaviors as nonfrozen cells. To conclude, a procedure for efficient production of primary bovine XEN cell cultures was developed. This new protocol may assist researchers in exploring this overlooked cell type for its roles in nutrient supply during embryogenesis.

Initiation of a conserved trophectoderm program in human, cow and mouse embryos

Current understandings of cell specification in early mammalian pre-implantation development are based mainly on mouse studies. The first lineage differentiation event occurs at the morula stage, with outer cells initiating a trophectoderm (TE) placental progenitor program. The inner cell mass arises from inner cells during subsequent developmental stages and comprises precursor cells of the embryo proper and yolk sac1. Recent gene-expression analyses suggest that the mechanisms that regulate early lineage specification in the mouse may differ in other mammals, including human2-5 and cow6. Here we show the evolutionary conservation of a molecular cascade that initiates TE segregation in human, cow and mouse embryos. At the morula stage, outer cells acquire an apical-basal cell polarity, with expression of atypical protein kinase C (aPKC) at the contact-free domain, nuclear expression of Hippo signalling pathway effectors and restricted expression of TE-associated factors such as GATA3, which suggests initiation of a TE program. Furthermore, we demonstrate that inhibition of aPKC by small-molecule pharmacological modulation or Trim-Away protein depletion impairs TE initiation at the morula stage. Our comparative embryology analysis provides insights into early lineage specification and suggests that a similar mechanism initiates a TE program in human, cow and mouse embryos.

Embryonic disc formation following post-hatching bovine embryo development in vitro

Failures during conceptus elongation are a major cause of pregnancy losses in ungulates, exerting a relevant economic impact on farming. The developmental events occurring during this period are poorly understood, mainly because this process cannot be recapitulated in vitro. Previous studies have established an in vitro post-hatching development (PHD) system that supports bovine embryo development beyond the blastocyst stage, based on agarose gel tunnels and serum- and glucose-enriched medium. Unfortunately, under this system embryonic disc formation is not achieved and embryos show notorious signs of apoptosis and necrosis. The objective of this study has been to develop an in vitro system able to support embryonic disc formation. We first compared post-hatching development inside agarose tunnels or free-floating over an agarose-coated dish in serum- and glucose-enriched medium (PHD medium). Culture inside agarose tunnels shaped embryo morphology by physical constriction, but it restricted embryo growth and did not provide any significant advantage in terms of development of hypoblast and epiblast lineages. In contrast to PHD medium, a chemically defined and enriched medium (N2B27) supported complete hypoblast migration and epiblast survival in vitro, even in the absence of agarose coating. Cells expressing the pluripotency marker SOX2 were observed in ~56% of the embryos and ~25% developed embryonic disc-like structures formed by SOX2+ cells. In summary, here we provide a culture system that supports trophectoderm proliferation, hypoblast migration and epiblast survival after the blastocyst stage.

Identification of the Lineage Markers and Inhibition of DAB2 in In Vitro Fertilized Porcine Embryos

Specification of embryonic lineages is an important question in the field of early development. Numerous studies analyzed the expression patterns of the candidate transcripts and proteins in humans and mice and clearly determined the markers of each lineage. To overcome the limitations of human and mouse embryos, the expression of the marker transcripts in each cell has been investigated using in vivo embryos in pigs. In vitro produced embryos are more accessible, can be rapidly processed with low cost. Therefore, we analyzed the characteristics of lineage markers and the effects of the DAB2 gene (trophectoderm marker) in in vitro fertilized porcine embryos. We investigated the expression levels of the marker genes during embryonic stages and distribution of the marker proteins was assayed in day 7 blastocysts. Then, the shRNA vectors were injected into the fertilized embryos and the differences in the marker transcripts were analyzed. Marker transcripts showed diverse patterns of expression, and each embryonic lineage could be identified with localization of marker proteins. In DAB2-shRNA vectors injected embryos, HNF4A and PDGFRA were upregulated. DAB2 protein level was lower in shRNA-injected embryos without significant differences. Our results will contribute to understanding of the mechanisms of embryonic lineage specification in pigs.

The biochemistry surrounding bovine conceptus elongation†

Conceptus elongation is a fundamental developmental event coinciding with a period of significant pregnancy loss in cattle. The process has yet to be recapitulated in vitro, whereas in vivo it is directly driven by uterine secretions and indirectly influenced by systemic progesterone. To better understand the environment facilitating this critical reproductive phenomenon, we interrogated the biochemical composition of uterine luminal fluid from heifers with high vs physiological circulating progesterone on days 12-14 of the estrous cycle-the window of conceptus elongation-initiation-by high-throughput untargeted ultrahigh-performance liquid chromatography tandem mass spectroscopy. A total of 233 biochemicals were identified, clustering within 8 superpathways [amino acids (33.9%), lipids (32.2%), carbohydrates (8.6%), nucleotides (8.2%), xenobiotics (6.4%), cofactors and vitamins (5.2%), energy substrates (4.7%), and peptides (0.9%)] and spanning 66 metabolic subpathways. Lipids dominated total progesterone (39.1%) and day (57.1%) effects; however, amino acids (48.5%) and nucleotides (14.8%) accounted for most day by progesterone interactions. Corresponding pathways over-represented in response to day and progesterone include (i) methionine, cysteine, s-adenosylmethionine, and taurine (9.3%); (ii) phospholipid (7.4%); and (iii) (hypo)xanthine and inosine purine metabolism (5.6%). Moreover, under physiological conditions, the uterine lumen undergoes a metabolic shift after day 12, and progesterone supplementation increases total uterine luminal biochemical abundance at a linear rate of 0.41-fold day-1-resulting in a difference (P ≤ 0.0001) by day 14. This global metabolic analysis of uterine fluid during the initiation of conceptus elongation offers new insights into the biochemistry of maternal-embryo communication, with implications for improving ruminant fertility.

Characterisation of bovine embryos following prolonged culture in embryonic stem cell medium containing leukaemia inhibitory factor

In order to help elucidate the process of epiblast and trophoblast cell differentiation in bovine embryos invitro, we attempted to develop a suitable culture medium to allow extended embryo culture. Day 7 bovine blastocysts developed in conventional medium were cultured further in embryonic stem cell medium with or without leukaemia inhibitory factor (LIF) until Day 23. At Day 14, the expression of octamer-binding transcription factor 3/4 (OCT3/4) and VIMENTIN was significantly higher in embryos cultured with than without LIF, but embryonic disc formation was not observed. Although expression of SRY (sex determining region Y)-box 17 (SOX17) mRNA was significantly lower in Day 14 embryos cultured with and without LIF than in invivo embryos, hypoblast cells formed just inside the trophoblast cells of the invitro-cultured embryos. On Day 23, expression of placental lactogen (PL) and prolactin-related protein 1 (PRP1) was not affected by LIF in invitro-cultured embryos, levels of both genes were significantly lower in the invitro than invivo embryos. Similar to invivo embryos, binucleate cell clusters seen in Day 23invitro-cultured embryos were composed of PL-negative and -positive cells. These results suggest that our culture system partially reproduced the differentiation process of trophoblast cells invivo.

Follistatin treatment modifies DNA methylation of the CDX2 gene in bovine preimplantation embryos

CDX2 plays a crucial role in the formation and maintenance of the trophectoderm epithelium in preimplantation embryos. Follistatin supplementation during the first 72 hr of in vitro culture triggers a significant increase in blastocyst rates, CDX2 expression, and trophectoderm cell numbers. However, the underlying epigenetic mechanisms by which follistatin upregulates CDX2 expression are not known. Here, we investigated whether stimulatory effects of follistatin are linked to alterations in DNA methylation within key regulatory regions of the CDX2 gene. In vitro-fertilized (IVF) zygotes were cultured with or without 10 ng/ml of recombinant human follistatin for 72 hr, then cultured without follistatin until Day 7. The bisulfite-sequencing analysis revealed differential methylation (DM) at specific CpG sites within the CDX2 promoter and intron 1 following follistatin treatment. These DM CpG sites include five hypomethylated sites at positions -1384, -1283, -297, -163, and -23, and four hypermethylated sites at positions -1501, -250, -243, and +20 in the promoter region. There were five hypomethylated sites at positions +3060, +3105, +3219, +3270, and +3545 in intron 1. Analysis of transcription factor binding sites using MatInspector combined with a literature search revealed a potential association between differentially methylated CpG sites and putative binding sites for key transcription factors involved in regulating CDX2 expression. The hypomethylated sites are putative binding sites for FXR, STAF, OCT1, KLF, AP2 family, and P53 protein, whereas the hypermethylated sites are putative binding sites for NRSF. Collectively, our results suggest that follistatin may increase CDX2 expression in early bovine embryos, at least in part, by modulating DNA methylation at key regulatory regions.

Common principles of early mammalian embryo self-organisation

Pre-implantation mammalian development unites extreme plasticity with a robust outcome: the formation of a blastocyst, an organised multi-layered structure ready for implantation. The process of blastocyst formation is one of the best-known examples of self-organisation. The first three cell lineages in mammalian development specify and arrange themselves during the morphogenic process based on cell-cell interactions. Despite decades of research, the unifying principles driving early mammalian development are still not fully defined. Here, we discuss the role of physical forces, and molecular and cellular mechanisms, in driving self-organisation and lineage formation that are shared between eutherian mammals.

On the enigmatic disappearance of Rauber's layer

The polar trophoblast overlays the epiblast in eutherian mammals and, depending on the species, has one of two different fates. It either remains a single-layered, thinning epithelium called "Rauber's layer," which soon disintegrates, or, alternatively, it keeps proliferating, contributing heavily to the population of differentiating, invasive trophoblast cells and, at least in mice, to the induction of gastrulation. While loss of the persistent polar trophoblast in mice leads to reduced induction of gastrulation, we show here that prevention of the loss of the polar trophoblast in cattle results in ectopic domains of the gastrulation marker, BRACHYURY This phenotype, and increased epiblast proliferation, arose when Rauber's layer was maintained for a day longer by countering apoptosis through BCL2 overexpression. This suggests that the disappearance of Rauber's layer is a necessity, presumably to avoid excessive signaling interactions between this layer and the subjacent epiblast. We note that, in all species in which the polar trophoblast persists, including humans and mice, ectopic polar trophoblast signaling is prevented via epiblast cavitation which leads to the (pro)amniotic cavity, whose function is to distance the central epiblast from such signaling interactions.

Gene expression patterns of in vivo-derived sheep blastocysts is more affected by vitrification than slow freezing technique

Transfer of fresh sheep embryos frequently results in higher pregnancy rate compared to cryopreserved ones, possibly due to a failure in the communication between the cryopreserved embryo and the endometrium during pre-implantation and pregnancy establishment. Thus, this study assessed the effect of sheep embryo cryopreservation (slow freezing or vitrification) on embryo survival rate and expression of genes related to trophectoderm differentiation (CDX2), pluripotency maintenance (NANOG), cell proliferation (TGFB1), mitochondrial activity (NRF1) and apoptosis (BAX and BCL2). Superovulation (n = 32 ewes) was performed and embryos were transcervically collected. One hundred good quality (Grade I and II) embryos were allocated into three groups: fresh embryos (CTL; n = 15), slow freezing (SF; n = 42) or vitrification (VT; n = 43). After thawing/warming, three pools of five blastocysts per group were used for RT-qPCR; the remaining 55 embryos were cultured in vitro in SOFaa medium at 38.5 °C and 5% CO₂ (SF: n = 27 and VT: n = 28). Survival rate of SF and VT were, respectively, 29.6% (8/27) and 14.2% (4/28) at 24 h; and 48.1% (13/27) and 32.1% (9/28) at 48 h (P > 0.05). Only CDX2 was affected (up-regulated, P < 0.05) in both groups compared to CTL. The BAX transcript was upregulated in VT, compared to SF group. The VT increased (P < 0.05) the expression of all genes, except for NANOG and NRF1, when compared to the CTL. In conclusion, although in vitro survival was similar between techniques, VT led to increased changes in blastocyst gene expression compared to CTL and SF.

Roles of OCT4 in pathways of embryonic development and cancer progression

Somatic cells may be reprogrammed to pluripotent state by ectopic expression of certain transcription factors; namely, OCT4, SOX2, KLF4 and c-MYC. However, the molecular and cellular mechanisms are not adequately understood, especially for human embryonic development. Studies during the last five years implicated importance of OCT4 in human zygotic genome activation (ZGA), patterns of OCT4 protein folding and role of specialized sequences in binding to DNA for modulation of gene expression during development. Epigenetic modulation of OCT4 gene and post translational modifications of OCT4 protein activity in the context of multiple cancers are important issues. A consensus is emerging that chromatin organization and epigenetic landscape play crucial roles for the interactions of transcription factors, including OCT4 with the promoters and/or regulatory sequences of genes associated with human embryonic development (ZGA through lineage specification) and that when the epigenome niche is deregulated OCT4 helps in cancer progression, and how OCT4 silencing in somatic cells of adult organisms may impact ageing.

Biochemical characterization of progesterone-induced alterations in bovine uterine fluid amino acid and carbohydrate composition during the conceptus elongation window†

Pregnancy establishment in cattle is contingent on conceptus elongation-a fundamental developmental event coinciding with the time during which most pregnancies fail. Elongation in vivo is directly driven by uterine secretions, indirectly influenced by systemic progesterone concentrations, and has yet to be recapitulated in vitro. To better understand the microenvironment evolved to facilitate this phenomenon, the amino acid and carbohydrate composition of uterine fluid was interrogated using high-throughput metabolomics on days 12, 13, and 14 of the estrous cycle from heifers with normal and high circulating progesterone. A total of 99 biochemicals (79 amino acids and 20 carbohydrates) were consistently identified, of which 31 showed a day by progesterone interaction. Fructose and mannitol/sorbitol did not exhibit a day by progesterone interaction, but displayed the greatest individual fluctuations (P ≤ 0.05) with respective fold increases of 18.39 and 28.53 in high vs normal progesterone heifers on day 12, and increases by 10.70-fold and 14.85-fold in the uterine fluid of normal progesterone animals on day 14 vs day 12. Moreover, enrichment analyses revealed that the phenylalanine, glutathione, polyamine, and arginine metabolic pathways were among the most affected by day and progesterone. In conclusion, progesterone had a largely stabilizing effect on amino acid flux, and identified biochemicals of likely importance to conceptus elongation initiation include arginine, fructose, glutamate, and mannitol/sorbitol.

WNT signalling supported by MEK/ERK inhibition is essential to maintain pluripotency in bovine preimplantation embryo

Capturing stable embryonic stem cell (ESC) lines from domesticated animals still remains one of the challenges of non-rodent embryology. The stake is high, as stable ESCs derived from species such as cattle present high economic and scientific value. Understanding of the processes leading to the embryonic lineage segregation is crucial to provide species-orientated molecular environment capable of supporting self-renewal and pluripotency. Therefore, the aim of this study was to validate the action of the two core regulatory pathways (WNT and MEK/ERK) during bovine embryo development. In vitro produced bovine embryos were obtained in the presence of inhibitors (i), which enable activation of the WNT pathway (via GSK3i, CHIR99021) and suppression of MEK signalling by PD0325901 in the 2i system and PD184325 and SU5402 in the 3i system. We have followed the changes in the distribution of the key lineage specific markers both at the transcript and protein level. Our results showed that WNT signalling promotes the expression of key inner cell mass (ICM) specific markers in bovine embryos, regardless of the MEK/ERK inhibitor cocktail used. MEK/ERK downregulation is crucial to maintain OCT4 and NANOG expression within the ICM and to prevent their exclusion from the trophectoderm (TE). At the same time, the classical TE marker (CDX2) was downregulated at the mRNA and protein level. As a follow up for the observed pluripotency stimulating effect of the inhibitors, we have tested the potential of the 2i and the 3i culture conditions (supported by LIF) to derive primary bovine ESC lines. As a result, we propose a model in which all of the primary signalling pathways determining embryonic cell fate are active in bovine embryos, yet the requirement for pluripotency maintenance in cattle may differ from the described standards. WNT activation leads to the formation (and stabilisation of the ICM) and MEK/ERK signalling is maintained at low levels. Unlike in the mouse, GATA6 is expressed in both ICM and TE. MEK/ERK signalling affects HP formation in cattle, but this process is activated at the post-blastocyst stage. With regard to self-renewal, 2i is preferable, as 3i also blocks the FGF receptor, what may prevent PI3K signalling, important for pluripotency and self-renewal.

Effects of abundances of OCT-4 mRNA transcript on goat pre-implantation embryonic development

Unlike in mice, the function of pluripotent markers in early embryonic development of domestic animals remains to be elucidated and this may account for the failure to establish embryonic stem cell lines for these species. To study the functions of the OCT-4 protein which has important actions in maintenance of pluripotent and self-renewal processes during early embryonic development, there was induced reduction in relative abundance of OCT-4 mRNA transcript during goat early embryonic development by using RNA interference techniques. The injection of OCT-4 siRNA into goat IVF presumptive zygotes resulted in a decrease in the relative abundance of OCT-4 mRNA transcript; however, there was development of these embryos to the blastocyst stage at the same rate as there was in the control group. The blastocysts from the treated groups had a similar number of TE, ICM, and total cells compared to those from the control group. Although there was a greater relative abundance of NANOG, REX1, and CDX2 mRNA transcript in the embryos injected with siRNA at the 8-16 cell stage, the relative transcript abundances were similar for the control and treatment groups at the blastocyst stage. The relative abundance of SOX2 mRNA transcript was similar for the treatment and control group. It, therefore, is concluded that inhibition of abundances of OCT-4 mRNA transcript to about 20 % of that of the untreated control group did not affect blastocyst formation rate in goats. The functions of OCT-4 in maintaining ICM and TE integrity, however, remains to be assessed.

Untangling early embryo development using single cell genomics

The zygote undergoes five cell divisions prior to the first signs of lineage segregation. Blastocyst formation requires segregation of the trophectoderm, needed for implantation, and the inner cell mass, which differentiate towards the major lineages of the fetus. This process is broadly conserved in mammals, however, in recent years investigations using high throughput single cell transcriptomics have provided new insights on the gene regulatory networks and epigenetic mechanisms controlling these processes in different species, highlighting novel unique evolutionary adaptations. Although analysis of single cell datasets is inherently challenging due to stochastic gene expression in single cells, continuous development of novel computational tools have contributed to improving the quality of these datasets. Single cell -omics provides detailed information on discrete cellular states, and when combined with spatial transcriptomics it can inform on the relationship between cellular compartments and fate determination. This technology has recently been used to shed new light into the progression of lineage segregation, establishment of pluripotency, epigenetic regulation and signalling pathways participating in mammalian pre-gastrulation development. The adoption of these new technologies for generating high-resolution maps of embryogenesis will readily translate into biotechnological applications that will have significant impact in livestock production.

NANOG is required to form the epiblast and maintain pluripotency in the bovine embryo

During preimplantation development, the embryo undergoes two consecutive lineages specifications. The first cell fate decision determines which cells give rise to the trophectoderm (TE) and the inner cell mass (ICM). Subsequently, the ICM differentiates into hypoblast and epiblast, the latter giving rise to the embryo proper. The transcription factors that govern these cell fate decisions have been extensively studied in the mouse, but are still poorly understood in other mammalian species. In the present study, the role of NANOG in the formation of the epiblast and maintenance of pluripotency in the bovine embryo was investigated. Using a CRISPR-Cas9 approach, guide RNAs were designed to target exon 2, resulting in a functional deletion of bovine NANOG at the zygote stage. Disruption of NANOG resulted in the embryos that form a blastocoel and an ICM composed of hypoblast cells. Furthermore, NANOG-null embryos showed lower expression of epiblast cell markers SOX2 and HA2AFZ, and hypoblast marker GATA6; without affecting the expression of TE markers CDX2 and KRT8. Results indicate that NANOG, has no apparent role in segregation or maintenance of the TE, but it is required to derive and maintain the pluripotent epiblast and during the second lineage commitment in the bovine embryo.

Trophectoderm regeneration to support full-term development in the inner cell mass isolated from bovine blastocyst

Which comes first: tissue structure or cell differentiation? Although different cell types establish distinct structures delineating the inside and outside of an embryo, they progressively become specified by the blastocyst stage, when two types of cell lineages are formed: the inner cell mass (ICM) and the trophectoderm (TE). This inside-outside aspect can be experimentally converted by the isolation of the ICM from a blastocyst, leading to a posteriori externalization of the blastomeres composing the outermost layer of the ICM. Here, we investigated the totipotency of isolated mouse and bovine ICMs to determine whether they are competent for TE regeneration. Surprisingly, a calf was generated from the bovine isolated ICM with re-formed blastocoel (re-iICM), but no mouse re-iICMs developed to term. To further explore the cause of difference in developmental competency between the mouse and bovine re-iICMs, we investigated the SOX17 protein expression that is a representative molecular marker of primitive endoderm. The localization pattern of SOX17 was totally different between mouse and bovine embryos. Particularly, the ectopic SOX17 localization in the TE might be associated with lethality of mouse re-iICMs. Meanwhile, transcriptome sequencing revealed that some of the bovine re-iICMs showed transcriptional patterns of TE-specific genes similar to those of whole blastocysts. Our findings suggest that TE regeneration competency is maintained longer in bovine ICMs than in mouse ICMs and provide evidence that the ICM/TE cell fate decision is influenced by structural determinants, including positional information of each blastomere in mammalian embryos.

Regulation of Cell Fate Decisions in Early Mammalian Embryos

Early embryogenesis is characterized by the segregation of cell lineages that fulfill critical roles in the establishment of pregnancy and development of the fetus. The formation of the blastocyst marks the emergence of extraembryonic precursors, needed for implantation, and of pluripotent cells, which differentiate toward the major lineages of the adult organism. The coordinated emergence of these cell types shows that these processes are broadly conserved in mammals. However, developmental heterochrony and changes in gene regulatory networks highlight unique evolutionary adaptations that may explain the diversity in placentation and in the mechanisms controlling pluripotency in mammals. The incorporation of new technologies, including single-cell omics, imaging, and gene editing, is instrumental for comparative embryology. Broadening the knowledge of mammalian embryology will provide new insights into the mechanisms driving evolution and development. This knowledge can be readily translated into biomedical and biotechnological applications in humans and livestock, respectively.

New tools for cell reprogramming and conversion: Possible applications to livestock

Somatic cell nuclear transfer and iPS are both forms of radical cell reprogramming able to transform a fully differentiated cell type into a totipotent or pluripotent cell. Both processes, however, are hampered by low efficiency and, in the case of iPS, the application to livestock species is uncertain.

Epigenetic manipulation has recently emerged as an efficient and robust alternative method for cell reprogramming. It is based upon the use of small molecules that are able to modify the levels of DNA methylation with 5-azacitidyne as one of the most widely used. Among a number of advantages, it includes the fact that it can be applied to domestic species including pig, dog and cat.

Treated cells undergo a widespread demethylation which is followed by a renewed methylation pattern induced by specific chemical stimuli that lead to the desired phenotype. A detailed study of the mechanisms of epigenetic manipulation revealed that cell plasticity is achieved through the combined action of a reduced DNA methyl transferase activity with an active demethylation driven by the TET protein family. Surprisingly the same combination of molecular processes leads to the transformation of fibroblasts into iPS and regulate the epigenetic changes that take place during early development and, hence, during reprogramming following SCNT.

Finally, it has recently emerged that mechanic stimuli in the form of a 3D cell rearrangement can significantly enhance the efficiency of epigenetic reprogramming as well as of maintenance of pluripotency. Interestingly these mechanic stimuli act on the same mechanisms both in epigenetic cell conversion with 5-Aza-CR and in iPS.

We suggest that the balanced combination of epigenetic erasing, 3D cell rearrangement and chemical induction can go a long way to obtain ad hoc cell types that can fully exploit the current exiting development brought by gene editing and animal cloning in livestock production.

Regulation of present and future development by maternal regulatory signals acting on the embryo during the morula to blastocyst transition - insights from the cow

The preimplantation embryo has a remarkable ability to execute its developmental program using regulatory information inherent within itself. Nonetheless, the uterine environment is rich in cell signaling molecules termed embryokines that act on the embryo during the morula-to-blastocyst transition, promoting blastocyst formation and programming the embryo for subsequent developmental events. Programming can not only affect developmental processes important for continuance of development in utero but also affect characteristics of the offspring during postnatal life. Given the importance of embryokines for regulation of embryonic development, it is likely that some causes of infertility involve aberrant secretion of embryokines by the uterus. Embryokines found to regulate development of the bovine embryo include insulin-like growth factor 1, colony stimulating factor 2 (CSF2), and dickkopf WNT signaling pathway inhibitor 1. Embryo responses to CSF2 exhibit sexual dimorphism, suggesting that sex-specific programming of postnatal function is caused by maternal signals acting on the embryo during the preimplantation period that regulate male embryos differently than female embryos.

Importance of prostate androgen-regulated mucin-like protein 1 in development of the bovine blastocyst

Background

Prostate androgen-regulated mucin-like protein 1 (PARM1) is a pro-proliferative and anti-apoptotic glycoprotein involved in the endoplasmic reticulum (ER) stress response. A single nucleotide polymorphism in the coding region of PARM1 has been associated with competence of bovine embryos to develop to the blastocyst stage. Here we tested the importance of PARM1 for development by evaluating consequences of reducing PARM1 mRNA abundance on embryonic development and differentiation, gene expression and resistance to ER stress.

Results

Knockdown of PARM1 using an anti-PARM1 GapmeR did not affect competence of embryos to develop into blastocysts but decreased the number of trophectoderm (TE) cells in the blastocyst and tended to increase the number of cells in the blastocyst inner cell mass (ICM). Treatment of embryos with anti-PARM1 GapmeR affected expression of 4 and 3 of 90 genes evaluated at the compact-morula and blastocyst stage of development at days 5.5 and 7.5 after fertilization, respectively. In morulae, treatment increased expression of DAB2, INADL, and STAT3 and decreased expression of CCR2. At the blastocyst stage, knockdown of PARM1 increased expression of PECAM and TEAD4 and decreased expression of CCR7. The potential role of PARM1 in ER stress response was determined by evaluating effects of knockdown of PARM1 on development of embryos after exposure to heat shock or tunicamycin and on expression of ATF6, DDIT3 and EIF2AK3 at the compact morula and blastocyst stages. Both heat shock and tunicamycin reduced the percent of embryos becoming a blastocyst but response was unaffected by PARM1 knockdown. Similarly, there was no effect of knockdown on steady-state amounts of ATF6, DDIT3 or EIF2AK3.

Conclusion

PARM1 participates in formation of TE and ICM cells in early embryonic development but there is no evidence for the role of PARM1 in the ER stress response.

Electronic supplementary material

The online version of this article (10.1186/s12861-019-0195-7) contains supplementary material, which is available to authorized users.

Mild hypothermia promotes the viability of in vitro-produced bovine blastocysts and their transcriptional expression of the cold-inducible transcription factor Rbm3 during in vitro culture

In this study, we evaluated the effects of holding in vitro-produced bovine blastocysts under mild hypothermia (33°C or 35°C), by examining viability and hatching rates of day 7 blastocysts (day 0: in vitro fertilization) cultured for 6 days and transcriptional expression of cold-inducible transcription factors Cirp and Rbm3, implicated in mild hypothermia-induced cellular protection against various types of stress. In the normothermic control (38.5°C), viability of the embryos decreased rapidly after day 10, and most samples were degenerated on day 13. However, mild hypothermia, particularly at 33°C, resulted in maintenance of high embryonic survival rates until day 13 (77.1% on day 13) and significant increases in transcriptional expression of Rbm3 in day 11 embryos compared with those at 38.5°C. Thus, our results suggested that upregulation of Rbm3 may occur in response to mild hypothermia in many bovine embryos, providing insights into the effects of mild hypothermia on embryo quality.

Role of yes-associated protein 1, angiomotin, and mitogen-activated kinase kinase 1/2 in development of the bovine blastocyst

The morula-stage embryo is transformed into a blastocyst composed of epiblast, hypoblast, and trophectoderm (TE) through mechanisms that, in the mouse, involve the Hippo signaling and mitogen-activated kinase (MAPK) pathways. Using the cow as an additional model, we tested the hypotheses that TE and hypoblast differentiation were regulated by the Hippo pathway regulators, yes-associated protein 1 (YAP1) and angiomotin (AMOT), and MAPK kinase 1/2 (MAPK1/2). The presence of YAP1 and CDX2 in the nucleus and cytoplasm of MII oocytes and embryos was evaluated by immunofluorescence labeling. For both molecules, localization changed from cytoplasmic to nuclear as development advanced. Inhibition of YAP1 activity, either by verteporfin or a YAP1 targeting GapmeR, reduced the percent of zygotes that became blastocysts, the proportion of blastocysts that hatched and numbers of CDX2+ cells in blastocysts. Moreover, the YAP1-targeting GapmeR altered expression of 15 of 91 genes examined in the day 7.5 blastocyst. Treatment of embryos with an AMOT targeting GapmeR did not affect blastocyst development or hatching but altered expression of 16 of 91 genes examined at day 7.5 and reduced the number of CDX2+ nuclei and YAP1+ nuclei in blastocysts at day 8.5 of development. Inhibition of MAPK1/2 with PD0325901 did not affect blastocyst development but increased the number of epiblast cells. Results indicate a role for YAP1 and AMOT in function of TE in the bovine blastocyst. YAP1 can also affect function of the epiblast and hypoblast, and MAPK signaling is important for inner cell mass differentiation by reducing epiblast numbers.

Physiological profile of undifferentiated bovine blastocyst-derived trophoblasts

Trophectoderm of blastocysts mediate early events in fetal-maternal communication, enabling implantation and establishment of a functional placenta. Inadequate or impaired developmental events linked to trophoblasts directly impact early embryo survival and successful implantation during a crucial period that corresponds with high incidence of pregnancy losses in dairy cows. As yet, the molecular basis of bovine trophectoderm development and signaling towards initiation of implantation remains poorly understood. In this study, we developed methods for culturing undifferentiated bovine blastocyst-derived trophoblasts and used both transcriptomics and proteomics in early colonies to categorize and elucidate their functional characteristics. A total of 9270 transcripts and 1418 proteins were identified and analyzed based on absolute abundance. We profiled an extensive list of growth factors, cytokines and other relevant factors that can effectively influence paracrine communication in the uterine microenvironment. Functional categorization and analysis revealed novel information on structural organization, extracellular matrix composition, cell junction and adhesion components, transcription networks, and metabolic preferences. Our data showcase the fundamental physiology of bovine trophectoderm and indicate hallmarks of the self-renewing undifferentiated state akin to trophoblast stem cells described in other species. Functional features uncovered are essential for understanding early events in bovine pregnancy towards initiation of implantation.

Progesterone alters the bovine uterine fluid lipidome during the period of elongation

Successful bovine pregnancy establishment hinges on conceptus elongation, a key reproductive phenomenon coinciding with the period during which most pregnancies fail. Elongation is yet to be recapitulated in vitro, whereas in vivo it is directly driven by uterine secretions and indirectly influenced by prior circulating progesterone levels. To better understand the microenvironment evolved to facilitate this fundamental developmental event, uterine fluid was recovered on Days 12–14 of the oestrous cycle – the window of conceptus elongation initiation – from cycling heifers supplemented, or not, with progesterone. Subsequent lipidomic profiling of uterine luminal fluid by advanced high-throughput metabolomics revealed the consistent presence of 75 metabolites, of which 47% were intricately linked to membrane biogenesis, and with seven displaying a day by progesterone interaction (P ≤ 0.05). Four metabolic pathways were correspondingly enriched according to day and P4 – i.e. comprised metabolites whose concentrations differed between groups (normal vs high P4) at different times (Days 12 vs 13 vs 14). These were inositol, phospholipid, glycerolipid and primary bile acid metabolism. Moreover, P4 elevated total uterine luminal fluid lipid content on Day 14 (P < 0.0001) relative to all other comparisons. The data combined suggest that maternal lipid supply during the elongation-initiation window is primarily geared towards conceptus membrane biogenesis. In summary, progesterone supplementation alters the lipidomic profile of bovine uterine fluid during the period of conceptus elongation initiation.

Dynamics of The Expression of Pluripotency and Lineage Specific Genes in The Pre and Peri-Implantation Goat Embryo

Objective

Two critical points of early development are the first and second lineage segregations, which are regulated by a wide spectrum of molecular and cellular factors. Gene regulatory networks, are one of the important components which handle inner cell mass (ICM) and trophectoderm (TE) fates and the pluripotency status across different mammalian species. Considering the importance of goats in agriculture and biotechnology, this study set out to investigate the dynamics of expression of the core pluripotency markers at the mRNA and protein levels.

Materials and Methods

In this experimental study, the expression pattern of three pluripotency markers (Oct4, Nanog and Sox2) and the linage specific markers (Rex1, Gata4 and Cdx2) were quantitatively assessed in in vitro matured (MII) oocytes and embryos at three distinctive stages: 8-16 cell stage, day-7 (D7) blastocysts and D14 blastocysts. Moreover, expression of Nanog, Oct4, Sox2 proteins, and their localization in the goat blastocyst was observed through immunocytochemistry.

Results

Relative levels of mRNA transcripts for Nanog and Sox2 in D3 (8-16 cell) embryos were significantly higher than D7 blastocysts and mature oocytes, while Oct4 was only significantly higher than D7 blastocysts. However, the expression pattern of Rex1, as an epiblast linage marker, decreased from the oocyte to the D14 stage. The expression pattern of Gata4 and Cdx2, as extra embryonic linage markers, also showed a similar trend from oocyte to D3 while their expressions were up-regulated in D14 blastocysts.

Conclusion

Reduction in Nanog, Oct4, Sox2 mRNA transcription and a late increase in extra embryonic linage markers suggests that the developmental program of linage differentiation is retarded in goat embryos compared to previously reported data on mice and humans. This is likely related to late the implantation in goats.

Stem cells on regenerative and reproductive science in domestic animals

Stem cells are undifferentiated and self-renewable cells that present new possibilities for both regenerative medicine and the understanding of early mammalian development. Adult multipotent stem cells are already widely used worldwide in human and veterinary medicine, and their therapeutic signalling, particularly with respect to immunomodulation, and their trophic properties have been intensively studied. The derivation of embryonic stem cells (ESCs) from domestic species, however, has been challenging, and the poor results do not reflect the successes obtained in mouse and human experiments. More recently, the generation of induced pluripotent stem cells (iPSCs) via the forced expression of specific transcription factors has been demonstrated in domestic species and has introduced new potentials in regenerative medicine and reproductive science based upon the ability of these cells to differentiate into a variety of cells types in vitro. For example, iPSCs have been differentiated into primordial germ-like cells (PGC-like cells, PGCLs) and functional gametes in mice. The possibility of using iPSCs from domestic species for this purpose would contribute significantly to reproductive technologies, offering unprecedented opportunities to restore fertility, to preserve endangered species and to generate transgenic animals for biomedical applications. Therefore, this review aims to provide an updated overview of adult multipotent stem cells and to discuss new possibilities introduced by the generation of iPSCs in domestic animals, highlighting the possibility of generating gametes in vitro via PGCL induction.

Specification and formation of the neural crest: Perspectives on lineage segregation

The neural crest is a fascinating embryonic population unique to vertebrates that is endowed with remarkable differentiation capacity. Thought to originate from ectodermal tissue, neural crest cells generate neurons and glia of the peripheral nervous system, and melanocytes throughout the body. However, the neural crest also generates many ectomesenchymal derivatives in the cranial region, including cell types considered to be of mesodermal origin such as cartilage, bone, and adipose tissue. These ectomesenchymal derivatives play a critical role in the formation of the vertebrate head, and are thought to be a key attribute at the center of vertebrate evolution and diversity. Further, aberrant neural crest cell development and differentiation is the root cause of many human pathologies, including cancers, rare syndromes, and birth malformations. In this review, we discuss the current findings of neural crest cell ontogeny, and consider tissue, cell, and molecular contributions toward neural crest formation. We further provide current perspectives into the molecular network involved during the segregation of the neural crest lineage.

Reflections on Model Organisms in Evolutionary Developmental Biology

This chapter reflects on and makes explicit the distinctiveness of reasoning practices associated with model organisms in the context of evolutionary developmental research. Model organisms in evo-devo instantiate a unique synthesis of model systems strategies from developmental biology and comparative strategies from evolutionary biology that negotiate a tension between developmental conservation and evolutionary change to address scientific questions about the evolution of development and the developmental basis of evolutionary change. We review different categories of model systems that have been advanced to understand practices found in the life sciences in order to comprehend how evo-devo model organisms instantiate this synthesis in the context of three examples: the starlet sea anemone and the evolution of bilateral symmetry, leeches and the origins of segmentation in bilaterians, and the corn snake to understand major evolutionary change in axial and appendicular morphology.

Early specification and development of rabbit neural crest cells

The phenomenal migratory and differentiation capacity of neural crest cells has been well established across model organisms. While the earliest stages of neural crest development have been investigated in non-mammalian model systems such as Xenopus and Aves, the early specification of this cell population has not been evaluated in mammalian embryos, of which the murine model is the most prevalent. Towards a more comprehensive understanding of mammalian neural crest formation and human comparative studies, we have used the rabbit as a mammalian system for the study of early neural crest specification and development. We examine the expression profile of well-characterized neural crest markers in rabbit embryos across developmental time from early gastrula to later neurula stages, and provide a comparison to markers of migratory neural crest in the chick. Importantly, we apply explant specification assays to address the pivotal question of mammalian neural crest ontogeny, and provide the first evidence that a specified population of neural crest cells exists in the rabbit gastrula prior to the overt expression of neural crest markers. Finally, we demonstrate that FGF signaling is necessary for early rabbit neural crest formation, as SU5402 treatment strongly represses neural crest marker expression in explant assays. This study pioneers the rabbit as a model for neural crest development, and provides the first demonstration of mammalian neural crest specification and the requirement of FGF signaling in this process.

CDX2 is essential for human IVF early embryonic development

Human in vitro fertilization (IVF) embryos usually have developmental block at 4-8 cell stages, in which only 30%-50% percent of IVF embryos develop to blastocyst stage. This is the main cause for low efficiency and repeated treatment failure in human fertility therapy. Transcription factor CDX2 is one of the key genes in human early embryonic development, which decides the differentiation from blastomere to trophoblastic lineage. However, the relationship between the developmental block of human IVF embryos and CDX2 is poorly understood. In this study, we showed that CDX2 is crucial for human IVF embryonic development. q-PCR analysis confirmed that the level of CDX2 in developmental block embryos was dramatically decreased. Immunofluorescence analysis further demonstrated that CDX2 protein level in developmental block embryos was significantly decreased compared with that in control embryos. We also addressed whether re-expression of CDX2 could rescue developmental block defects in human IVF developmental block embryos. As expected, these developmental block defects could be partially rescued by overexpression of CDX2 protein. In conclusion, these findings suggest that CDX2 plays an important role in the process of human early embryonic development.

Integration of molecules to construct the processes of conceptus implantation to the maternal endometrium

During the peri-implantation period, ruminant conceptuses go through rapid elongation, followed by their attachment to the uterine endometrial epithelial cells, during which interferon-tau (IFNT), a trophectodermal cytokine required for the process of maternal recognition of pregnancy, is expressed in a temporal and spatial manner. On day 22 (day 0 = day of estrus), 2 to 3 d after the initiation of bovine conceptus attachment to the uterine epithelium, when IFNT production begins to subside, the expression of molecules related to epithelial-mesenchymal transition, zinc finger E-box binding homeobox 1, snail family transcriptional repressor 2, N-cadherin, and vimentin was found in the trophectoderm. Through the use of in vitro coculture system with bovine trophoblast CT-1 and endometrial epithelial cells, a series of experiments have been conducted to elucidate mechanisms associated with the regulation of IFNT gene transcription and conceptus implantation, including epithelial-mesenchymal transition processes. Expression of IFNT, both up- and downregulation, during the peri-implantation period is tightly controlled. Cytokines and cell adhesion molecules such as epidermal growth factor, basic fibroblast growth factor, transforming growth factor beta, activin A, L-selectin-podocalyxin, and vascular cell adhesion molecule 1-integrin α4 expressed in utero all contribute to the initiation of epithelial-mesenchymal transition in the trophectoderm. These results indicate that conceptus implantation to the uterine endometrium proceeds while elongated conceptuses and endometria express cell adhesion molecules and their receptors, and the trophectoderm experiences epithelial-mesenchymal transition. Data accumulated suggest that while the conceptus and the endometrial epithelium adhere, trophectodermal cells must gain more flexibility for binucleate and possibly trinucleate cell formation during the peri-implantation period, and that understanding and constructing the conditions throughout implantation processes is key to improving ruminants' fertility.

Analyzing bovine OCT4 and NANOG enhancer activity in pluripotent stem cells using fluorescent protein reporters

Green fluorescent protein (GFP) reporters controlled by the regulatory region of OCT4 and NANOG—two master regulators for pluripotency are widely used in studies of pluripotent stem cell establishment and embryo development. Alongside the challenge in establishing bovine pluripotent stem cells, the application of bovine-specific gene reporters has rarely been explored.

Using lentivirus-based GFP reporter, we investigated the upstream regulatory regions of bovine OCT4 and NANOG. These reporters show activity in both naïve- and primed-state pluripotency when infected into mouse and human embryonic stem cells (ESCs), respectively. Consistent with what is found in humans and mice, the bovine OCT4-distal enhancer (bOCT4-DE) but not the proximal enhancer (bOCT4-PE) region is preferentially activated in naïve-state pluripotency. Furthermore, the bOCT4-DE region is silenced upon conversion of naive-state ESCs into primed-state epiblast stem cells (EpiSCs). Co-infection of mouse fibroblasts with the reprograming factors for induced pluripotent stem cell (iPSC) induction leads to the generation of GFP positive colonies, demonstrating that these GFP reporters can serve as live indicators for induced pluripotent cell establishment. We further proved that the bovine OCT4 distal enhancer is active in bovine blastocysts. We established the lentiviral-based fluorescent reporters controlled by bovine OCT4 and NANOG enhancer sequences. These reporter constructs show activity in naïve- and primed-pluripotent states. These reporters may serve as versatile tools for bovine ESC/iPSC generation and identification, as well as for developmental studies of bovine embryos.

Comparative aspects of early lineage specification events in mammalian embryos - insights from reverse genetics studies

Within the mammalian class, formation of the blastocyst is morphologically highly conserved among different species. The molecular and cellular events during preimplantation embryo development have been studied extensively in the mouse as model organism, because multiple genetically defined strains and a plethora of reverse genetics tools are available to dissect specific gene functions and regulatory networks. However, major differences in preimplantation developmental kinetics, implantation, and placentation exist among mammalians, and recent studies in species other than mouse showed, that even regulatory mechanisms of the first lineage differentiation events and maintenance of pluripotency are not always conserved. Here, we focus on the first and the second lineage segregation in mouse and bovine embryos, when the first differentiated cell types emerge. We outline their common features and differences in the regulation of these essential events during embryonic development with a glance at further species. In addition, we show how new reverse genetics strategies aid the study of regulatory circuits in embryos of domestic species, enhancing our overall understanding of mammalian preimplantation development.

Directions and applications of CRISPR technology in livestock research

The ablation (KO) or targeted insertion (KI) of specific genes or sequences has been essential to test their roles on a particular biological process. Unfortunately, such genome modifications have been largely limited to the mouse model, as the only way to achieve targeted mutagenesis in other mammals required from somatic cell nuclear transfer, a time- and resource-consuming technique. This difficulty has left research in livestock species largely devoided of KO and targeted KI models, crucial tools to uncover the molecular roots of any physiological or pathological process. Luckily, the eruption of site-specific endonucleases, and particularly CRISPR technology, has empowered farm animal scientists to consider projects that could not develop before. In this sense, the availability of genome modification in livestock species is meant to change the way research is performed on many fields, switching from descriptive and correlational approaches to experimental research. In this review we will provide some guidance about how the genome can be edited by CRISPR and the possible strategies to achieve KO or KI, paying special attention to an initially overlooked phenomenon: mosaicism. Mosaicism is produced when the zygote´s genome edition occurs after its DNA has replicated, and is characterized by the presence of more than two alleles in the same individual, an undesirable outcome when attempting direct KO generation. Finally, the possible applications on different fields of livestock research, such as reproduction or infectious diseases are discussed.

Building Principles for Constructing a Mammalian Blastocyst Embryo

The self-organisation of a fertilised egg to form a blastocyst structure, which consists of three distinct cell lineages (trophoblast, epiblast and hypoblast) arranged around an off-centre cavity, is unique to mammals. While the starting point (the zygote) and endpoint (the blastocyst) are similar in all mammals, the intervening events have diverged. This review examines and compares the descriptive and functional data surrounding embryonic gene activation, symmetry-breaking, first and second lineage establishment, and fate commitment in a wide range of mammalian orders. The exquisite detail known from mouse embryogenesis, embryonic stem cell studies and the wealth of recent single cell transcriptomic experiments are used to highlight the building principles underlying early mammalian embryonic development.

Temporal expression of pluripotency-associated transcription factors in sheep and cattle preimplantation embryos

SummaryPluripotency-associated transcription factors (PATFs) modulate gene expression during early mammalian embryogenesis. Despite a strong understanding of PATFs during mouse embryogenesis, limited progress has been made in ruminants. This work aimed to describe the temporal expression of eight PATFs during both sheep and cattle preimplantation development. Transcript availability of PATFs was evaluated by reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) in eggs, cleavage-stage embryos, morulae, and blastocysts. Transcripts of five genes were detected in all developmental stages of both species (KLF5, OCT4, RONIN, ZFP281, and ZFX). Furthermore, CMYC was detected in all cattle samples but was found from cleavage-stage onwards in sheep. In contrast, NR0B1 was detected in all sheep samples but was not detected in cattle morulae. GLIS1 displayed the most significant variation in temporal expression between species, as this PATF was only detected in cattle eggs and sheep cleavage-stage embryos and blastocysts. In silico analysis suggested that cattle and sheep PATFs share similar size, isometric point and molecular weight. A phenetic analysis showed two patterns of PATF clustering between cattle and sheep, among several mammalian species. In conclusion, the temporal expression of pluripotency-associated transcription factors differs between sheep and cattle, suggesting species-specific regulation during preimplantation development.

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.

Reciprocal regulation of TEAD4 and CCN2 for the trophectoderm development of the bovine blastocyst

The first segregation at the blastocyst stage is the symmetry-breaking event to characterize two cell components; namely, inner cell mass (ICM) and trophectoderm (TE). TEA domain transcription factor 4 (TEAD4) is a well-known regulator to determine TE properties of blastomeres in rodent models. However, the roles of bovine TEAD4 in blastocyst development have been unclear. We here aimed to clarify the mechanisms underlining TE characterization by TEAD4 in bovine blastocysts. We first found that theTEAD4mRNA expression level was greater in TE than in ICM, which was further supported by TEAD4 immunofluorescent staining. Subsequently, we examined the expression patterns of TE-expressed genes;CDX2,GATA2andCCN2, in theTEAD4-knockdown (KD) blastocysts. These expression levels significantly decreased in theTEAD4KD blastocysts compared with controls. Of these downregulated genes, theCCN2expression level decreased the most. We further analyzed the expression levels of TE-expressed genes;CDX2,GATA2andTEAD4in theCCN2KD blastocysts. Strikingly, theCCN2KD blastocysts showed the downregulation ofCDX2,GATA2andTEAD4. Furthermore, the ratio of TE-to-ICM cell numbers in theCCN2KD blastocysts significantly decreased compared to controls. To our knowledge, this is the first study showing the regulation ofCCN2expression thoroughTEAD4in mammalian embryos. Not only that, this study also provides evidence that reciprocal regulation ofTEAD4andCCN2is required for TE development with appropriate gene expression in bovine blastocysts.

Embryonic POU5F1 is Required for Expanded Bovine Blastocyst Formation

POU5F1 is a transcription factor and master regulator of cell pluripotency with indispensable roles in early embryo development and cell lineage specification. The role of embryonic POU5F1 in blastocyst formation and cell lineage specification differs between mammalian species but remains completely unknown in cattle. The CRISPR/Cas9 system was utilized for targeted disruption of the POU5F1 gene by direct injection into zygotes. Disruption of the bovine POU5F1 locus prevented blastocyst formation and was associated with embryonic arrest at the morula stage. POU5F1 knockout morulas developed at a similar rate as control embryos and presented a similar number of blastomeres by day 5 of development. Initiation of SOX2 expression by day 5 of development was not affected by lack of POU5F1. On the other hand, CDX2 expression was aberrant in embryos lacking POU5F1. Notably, the phenotype observed in bovine POU5F1 knockout embryos reveals conserved functions associated with loss of human embryonic POU5F1 that differ from Pou5f1- null mice. The similarity observed in transcriptional regulation of early embryo development between cattle and humans combined with highly efficient gene editing techniques make the bovine a valuable model for human embryo biology with expanded applications in agriculture and assisted reproductive technologies.

Bovine lineage specification revealed by single-cell gene expression analysis from zygote to blastocyst

Preimplantation embryos undergo zygotic genome activation and lineage specification resulting in three distinct cell types in the late blastocyst. The molecular mechanisms underlying this progress are largely unknown in bovines. Here, we sought to analyze an extensive set of regulators at the single-cell level to define the events involved in the development of the bovine blastocyst. Using a quantitative microfluidics approach in single cells, we analyzed mRNA levels of 96 genes known to function in early embryonic development and maintenance of stem cell pluripotency in parallel in 384 individual cells from bovine preimplantation embryos. The developmental transitions can be distinguished by distinctive gene expression profiles and we identified NOTCH1, expressed in early developmental stages, while T-box 3 (TBX3) and fibroblast growth factor receptor 4 (FGFR4), expressed in late developmental stages. Three lineages can be segregated in bovine expanded blastocysts based on the expression patterns of lineage-specific genes such as disabled homolog 2 (DAB2), caudal type homeobox 2 (CDX2), ATPase H+/K+ transporting non-gastric alpha2 subunit (ATP12A), keratin 8 (KRT8), and transcription factor AP-2 alpha (TFAP2A) for trophectoderm; GATA binding protein 6 (GATA6) and goosecoid homeobox (GSC) for primitive endoderm; and Nanog homeobox (NANOG), teratocarcinoma-derived growth factor 1 (TDGF1), and PR/SET domain 14 (PRDM14) for epiblast. Moreover, some lineage-specific genes were coexpressed in blastomeres from the morula. The commitment to trophectoderm and inner cell mass lineages in bovines occurs later than in the mouse, and KRT8 might be an earlier marker for bovine trophectoderm cells. We determined that TDGF1 and PRDM14 might play pivotal roles in the primitive endoderm and epiblast specification of bovine blastocysts. Our results shed light on early cell fate determination in bovine preimplantation embryos and offer theoretical support for deriving bovine embryonic stem cells.

Asynchronous CDX2 expression and polarization of porcine trophoblast cells reflects a species-specific trophoderm lineage determination progress model

Upregulation of Cdx2 expression in outer cells is a key event responsible for cell lineage segregation between the inner cell mass and the trophoderm (TE) in mouse morula-stage embryos. In TE cells, polarization can regulate Hippo and Rho-associated kinase (Rho-ROCK) signaling to induce the nuclear location of YAP, which has been demonstrated to further induce the expression of Cdx2. However, we found that CDX2 expression could not be detected in the outer cells of porcine morula-stage embryos but only in some TE cells at the early blastocyst stage. The biological significance and the regulation mechanism of this species-specific CDX2 expression pattern have still not been determined. We show here that an asynchronous CDX2 expression pattern exists in porcine TE cells during the development of the blastocyst. We demonstrate that CDX2 expression in porcine TE cells depends on the nuclear localization of YAP and polarization of the embryo through Y27632 treatment. We found that the polarization process in the morula to the late blastocyst stage porcine embryos was asynchronous, which was revealed by the apical localization of phosphorylated EZRIN staining. Artificially enhancing the number of polarized blastomeres by culturing the separated blastomeres of four-cell stage porcine embryos resulted in increased CDX2-positive cell numbers. These results indicate that the mechanism of CDX2 expression regulation is conserved, but the polarization progress is not conserved between the pig and the mouse, and results in a species-specific trophoblast determination progress model.