Gata3 regulates trophoblast development downstream of Tead4 and in parallel to Cdx2

Hippo Signaling in Embryogenesis and Development

Hippo pathway components are structurally and functionally conserved and are notable for their role in controlling organ size. More diverse functions of the Hippo pathway have been recognized, including development, tissue homeostasis, wound healing and regeneration, immunity, and tumorigenesis. During embryogenesis, different signaling pathways are repeatedly and cooperatively activated, leading to differential gene expression in specific developmental contexts. In this article, we present an overview on the regulation and function of the Hippo pathway in mammalian early development. We introduce the Hippo pathway components and major upstream signals that act through this pathway to influence embryogenesis. We also discuss the roles of Hippo pathway in tissue specification and organ development during organogenesis.

MED20 is essential for early embryogenesis and regulates NANOG expression

Mediator is an evolutionarily conserved multi-subunit complex, bridging transcriptional activators and repressors to the general RNA polymerase II (Pol II) initiation machinery. Though the Mediator complex is crucial for the transcription of almost all Pol II promoters in eukaryotic organisms, the phenotypes of individual Mediator subunit mutants are each distinct. Here, we report for the first time, the essential role of subunit MED20 in early mammalian embryo development. Although Med20 mutant mouse embryos exhibit normal morphology at E3.5 blastocyst stage, they cannot be recovered at early post-gastrulation stages. Outgrowth assays show that mutant blastocysts cannot hatch from the zona pellucida, indicating impaired blastocyst function. Assessments of cell death and cell lineage specification reveal that apoptosis, inner cell mass, trophectoderm and primitive endoderm markers are normal in mutant blastocysts. However, the epiblast marker NANOG is ectopically expressed in the trophectoderm of Med20 mutants, indicative of defects in trophoblast specification. These results suggest that MED20 specifically, and the Mediator complex in general, are essential for the earliest steps of mammalian development and cell lineage specification.

TEAD4 ensures postimplantation development by promoting trophoblast self-renewal: An implication in early human pregnancy loss

Early pregnancy loss affects ∼15% of all implantation-confirmed human conceptions. However, evolutionarily conserved molecular mechanisms that regulate self-renewal of trophoblast progenitors and their association with early pregnancy loss are poorly understood. Here, we provide evidence that transcription factor TEAD4 ensures survival of postimplantation mouse and human embryos by controlling self-renewal and stemness of trophoblast progenitors within the placenta primordium. In an early postimplantation mouse embryo, TEAD4 is selectively expressed in trophoblast stem cell-like progenitor cells (TSPCs), and loss of Tead4 in postimplantation mouse TSPCs impairs their self-renewal, leading to embryonic lethality before embryonic day 9.0, a developmental stage equivalent to the first trimester of human gestation. Both TEAD4 and its cofactor, yes-associated protein 1 (YAP1), are specifically expressed in cytotrophoblast (CTB) progenitors of a first-trimester human placenta. We also show that a subset of unexplained recurrent pregnancy losses (idiopathic RPLs) is associated with impaired TEAD4 expression in CTB progenitors. Furthermore, by establishing idiopathic RPL patient-specific human trophoblast stem cells (RPL-TSCs), we show that loss of TEAD4 is associated with defective self-renewal in RPL-TSCs and rescue of TEAD4 expression restores their self-renewal ability. Unbiased genomics studies revealed that TEAD4 directly regulates expression of key cell cycle genes in both mouse and human TSCs and establishes a conserved transcriptional program. Our findings show that TEAD4, an effector of the Hippo signaling pathway, is essential for the establishment of pregnancy in a postimplantation mammalian embryo and indicate that impairment of the Hippo signaling pathway could be a molecular cause for early human pregnancy loss.

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.

Mechanisms of human embryo development: from cell fate to tissue shape and back

Gene regulatory networks and tissue morphogenetic events drive the emergence of shape and function: the pillars of embryo development. Although model systems offer a window into the molecular biology of cell fate and tissue shape, mechanistic studies of our own development have so far been technically and ethically challenging. However, recent technical developments provide the tools to describe, manipulate and mimic human embryos in a dish, thus opening a new avenue to exploring human development. Here, I discuss the evidence that supports a role for the crosstalk between cell fate and tissue shape during early human embryogenesis. This is a critical developmental period, when the body plan is laid out and many pregnancies fail. Dissecting the basic mechanisms that coordinate cell fate and tissue shape will generate an integrated understanding of early embryogenesis and new strategies for therapeutic intervention in early pregnancy loss.

Esrrb plays important roles in maintaining self-renewal of trophoblast stem cells (TSCs) and reprogramming somatic cells to induced TSCs

Trophoblast stem cells (TSCs), which can be derived from the trophoectoderm of a blastocyst, have the ability to sustain self-renewal and differentiate into various placental trophoblast cell types. Meanwhile, essential insights into the molecular mechanisms controlling the placental development can be gained by using TSCs as the cell model. Esrrb is a transcription factor that has been shown to play pivotal roles in both embryonic stem cell (ESC) and TSC, but the precise mechanism whereby Esrrb regulates TSC-specific transcriptome during differentiation and reprogramming is still largely unknown. In the present study, we elucidate the function of Esrrb in self-renewal and differentiation of TSCs, as well as during the induced TSC (iTSC) reprogramming. We demonstrate that the precise level of Esrrb is critical for stem state maintenance and further trophoblast differentiation of TSCs, as ectopically expressed Esrrb can partially block the rapid differentiation of TSCs in the absence of fibroblast growth factor 4. However, Esrrb depletion results in downregulation of certain key TSC-specific transcription factors, consequently causing a rapid differentiation of TSCs and these Esrrb-deficient TSCs lose the ability of hemorrhagic lesion formation in vivo. This function of Esrrb is exerted by directly binding and activating a core set of TSC-specific target genes including Cdx2, Eomes, Sox2, Fgfr4, and Bmp4. Furthermore, we show that Esrrb overexpression can facilitate the MEF-to-iTSC conversion. Moreover, Esrrb can substitute for Eomes to generate GEsTM-iTSCs. Thus, our findings provide a better understanding of the molecular mechanism of Esrrb in maintaining TSC self-renewal and during iTSC reprogramming.

Zfp281 Shapes the Transcriptome of Trophoblast Stem Cells and Is Essential for Placental Development

Placental development is a key event in mammalian reproduction and embryogenesis. However, the molecular basis underlying placental development is not fully understood. Here, we conduct a forward genetic screen to identify regulators for extraembryonic development and identify Zfp281 as a key factor. Zfp281 overexpression in mouse embryonic stem cells facilitates the induction of trophoblast stem-like cells. Zfp281 is preferentially expressed in the undifferentiated trophoblast stem cell population in an FGF-dependent manner, and disruption of Zfp281 in mice causes severe defects in early placental development. Consistently, Zfp281-depleted trophoblast stem cells exhibit defects in maintaining the transcriptome and differentiation capacity. Mechanistically, Zfp281 interacts with MLL or COMPASS subunits and occupies the promoters of its target genes. Importantly, ZNF281, the human ortholog of this factor, is required to stabilize the undifferentiated status of human trophoblast stem cells. Thus, we identify Zfp281 as a conserved factor for the maintenance of trophoblast stem cell plasticity.

Trophoblast lineage specification in the mammalian preimplantation embryo

BACKGROUND

The establishment of the trophectoderm (TE) and the inner cell mass (ICM) is the first cell lineage segregation that occurs in mammalian preimplantation development. TE will contribute to the placenta while ICM cells give rise to the epiblast (EPI) and primitive endoderm (PrE). There are two historical models for TE/ICM segregation: the positional (inside-outside) model and the polarity model, but both models alone cannot explain the mechanism of TE/ICM segregation.

METHODS

This article discusses a current possible model based on recent studies including the finding through live-cell imaging of the expression patterns of caudal type homeobox 2 (Cdx2), a key transcription factor of TE differentiation in the mouse embryo.

RESULTS

It was observed that a part of outer Cdx2-expressing blastomeres was internalized at the around 20- to 30-cell stage, downregulates Cdx2, ceases TE differentiation, and participates in ICM lineages.

CONCLUSION

The early blastomere, which starts differentiation toward the TE cell fate, still has plasticity and can change its fate. Differentiation potency of all blastomeres until approximately the 32-cell stage is presumably not irreversibly restricted even if they show heterogeneity in their epigenetic modifications or gene expression patterns.

N6-methyladenine in DNA antagonizes SATB1 in early development

The recent discovery of N6-methyladenine (N6-mA) in mammalian genomes suggests that it may serve as an epigenetic regulatory mechanism1. However, the biological role of N6-mA and the molecular pathways that exert its function remain unclear. Here we show that N6-mA has a key role in changing the epigenetic landscape during cell fate transitions in early development. We found that N6-mA is upregulated during the development of mouse trophoblast stem cells, specifically at regions of stress-induced DNA double helix destabilization (SIDD)2-4. Regions of SIDD are conducive to topological stress-induced unpairing of the double helix and have critical roles in organizing large-scale chromatin structures3,5,6. We show that the presence of N6-mA reduces the in vitro interactions by more than 500-fold between SIDD and SATB1, a crucial chromatin organizer that interacts with SIDD regions. Deposition of N6-mA also antagonizes SATB1 function in vivo by preventing its binding to chromatin. Concordantly, N6-mA functions at the boundaries between euchromatin and heterochromatin to restrict the spread of euchromatin. Repression of SIDD-SATB1 interactions mediated by N6-mA is essential for gene regulation during trophoblast development in cell culture models and in vivo. Overall, our findings demonstrate an unexpected molecular mechanism for N6-mA function via SATB1, and reveal connections between DNA modification, DNA secondary structures and large chromatin domains in early embryonic development.

Epigenetic regulation of mouse preimplantation embryo development

After fertilization, mouse embryos go through preimplantation development to give rise to blastocyst. Two key molecular events, zygotic genome activation (ZGA) and the first cell lineage specification, are essential for the process. Recent advances in low-input epigenomics profiling techniques allow the analysis of these events at a molecular level, which revealed a critical role of epigenetic and chromatin reprogramming in ZGA and the first cell lineage specification. Additionally, the establishment of an in vitro embryonic stem cell (ESC) to two-cell embryo-like conversion system have also contributed to the molecular understanding of preimplantation development. In this review, we summarize recent advances in epigenetic regulation of mouse preimplantation development, point out the remaining questions, and propose strategies to tackle these questions.

The role of RHOA signaling in trophectoderm cell-fate decision in cattle

Mammalian blastocysts are composed of two distinct cell lineages, namely the inner cell mass (ICM) and trophectoderm (TE). TE cells that give rise to the embryonic placenta are marked by an exclusive expression of the key determinant transcription factor, CDX2. Although Hippo signaling pathway is known to be responsible for this TE-specific expression of CDX2, the upstream regulator of this pathway in mammalian embryos is still controversial. In the present study, the involvement of the small molecular G protein, RHOA, in TE cell-fate decision in cattle was investigated. Inhibition of RHOA by the specific inhibitor, C3 transferase (C3), severely impaired the blastocyst formation. Further, C3 treatment significantly decreased the number of blastomeres with nuclearized YAP1, the prominent effector of Hippo pathway. An artificial isolation of ICM cells from blastocysts followed by the continuing culture to regenerate TE cells was conducted and showed that TE re-emergence from the isolated ICM is governed by Hippo pathway and suppressed by C3 treatment like that observed in developing embryos. Finally, the long-term exposure to C3 suggests the presence of alternative regulators of CDX2 expression other than RHOA signaling because there were still CDX2-positive cells after C3 treatment. These results demonstrated that RHOA signaling plays a significant role in TE cell-fate decision by regulating Hippo pathway in cattle.

The enigmatic morula: mechanisms of development, cell fate determination, self-correction and implications for ART

BACKGROUND

Assisted reproduction technology offers the opportunity to observe the very early stages of human development. However, due to practical constraints, for decades morphological examination of embryo development has been undertaken at a few isolated time points at the stages of fertilisation (Day 1), cleavage (Day 2-3) and blastocyst (Day 5-6). Rather surprisingly, the morula stage (Day 3-4) has been so far neglected, despite its involvement in crucial cellular processes and developmental decisions.

OBJECTIVE AND RATIONALE

The objective of this review is to collate novel and unsuspected insights into developmental processes occurring during formation of the morula, highlighting the key importance of this stage for a better understanding of preimplantation development and an improvement of ART.

SEARCH METHODS

PubMed was used to search the MEDLINE database for peer-reviewed English-language original articles and reviews concerning the morula stage in mammals. Searches were performed by adopting 'embryo', 'morula', 'compaction', 'cell fate' and 'IVF/assisted reproduction' as main terms, in association with other keywords expressing concepts relevant to the subject (e.g. cell polarity). The most relevant publications, i.e. those concerning major phenomena occurring during formation of the morula in established experimental models and the human species, were assessed and discussed critically.

OUTCOMES

Novel live cell imaging technologies and cell biology studies have extended our understanding of morula formation as a key stage for the development of the blastocyst and determination of the inner cell mass (ICM) and the trophectoderm (TE). Cellular processes, such as dynamic formation of filopodia and cytoskeleton-mediated zippering cell-to-cell interactions, intervene to allow cell compaction (a geometrical requisite essential for development) and formation of the blastocoel, respectively. At the same time, differential orientation of cleavage planes, cell polarity and cortical tensile forces interact and cooperate to position blastomeres either internally or externally, thereby influencing their cellular fate. Recent time lapse microscopy (TLM) observations also suggest that in the human the process of compaction may represent an important checkpoint for embryo viability, through which chromosomally abnormal blastomeres are sensed and eliminated by the embryo.

WIDER IMPLICATIONS

In clinical embryology, the morula stage has been always perceived as a 'black box' in the continuum of preimplantation development. This has dictated its virtual exclusion from mainstream ART procedures. Recent findings described in this review indicate that the morula, and the associated process of compaction, as a crucial stage not only for the formation of the blastocyst, but also for the health of the conceptus. This understanding may open new avenues for innovative approaches to embryo manipulation, assessment and treatment.

Deciphering a distinct regulatory network of TEAD4, CDX2 and GATA3 in humans for trophoblast transition from embryonic stem cells

The placenta is an essential organ for the fetus, but its regulatory mechanism for formation of functional trophoblast lineage remains elusive in humans. Although widely known in mice, TEAD4 and its downstream targets CDX2 and GATA3 have not been determined in human models. In this work, we used a human model of trophoblast transition from BAP (BMP4, A83-01 and PD173074)-treated human embryonic stem cells (hESCs) and performed multiple gain- and loss-of-function tests of TEAD4, CDX2 or GATA3 to study their roles during this process. Although hESCs with TEAD4 deletion maintain pluripotency, their trophoblast transition potentials are attenuated. This impaired trophoblast transition could be rescued by separately overexpressing TEAD4, CDX2 or GATA3. Furthermore, trophoblast transition from hESCs is also attenuated by knockout of CDX2 but remains unaffected with deletion of GATA3. However, CDX2-overexpressed hESCs maintain pluripotency, whereas overexpression of GATA3 in hESCs leads to spontaneous differentiation including trophoblast lineage. In brief, our findings using a human model of trophoblast transition from BAP-treated hESCs reveal transcription roles of TEAD4, CDX2 and GATA in humans that are different from those in mice. We hope that this evidence can aid in understanding the distinct transcriptional network regulating trophoblast development in humans.

Induction and maintenance of specific multipotent progenitor stem cells synergistically mediated by Activin A and BMP4 signaling

We recently reported that epiblast stem cells (EpiSCs)-like cells could be derived from preimplantation embryos (named as AFSCs). Here, we established AFSCs from pre-implantation embryos of multiple mouse strains and showed that unlike EpiSCs, the derivation efficiency of AFSCs was affected by the genetic background. We then used AFSCs lines to dissect the roles of Activin A (Act A) and basic fibroblast growth factor and reported that Act A alone was capable of maintaining self-renewal but not developmental potential in vivo. Finally, we established a novel experimental system, in which AFSCs were efficiently converted to multipotent progenitor stem cells using Act A and bone morphogenetic protein 4 (named as ABSCs). Importantly, these ABSCs contributed to neural mesodermal progenitors and lateral plate mesoderm in postimplantation chimeras. Taken together, our study established a robust experimental system for the generation of specific multipotent progenitor stem cells that was self-renewable and capable of contributing to embryonic and extra-embryonic tissues.

Role of Wnt Signaling During In-Vitro Bovine Blastocyst Development and Maturation in Synergism with PPARδ Signaling

Wnt/β-catenin signaling plays vital role in the regulation of cellular proliferation, migration, stem cells cell renewal and genetic stability. This pathway is crucial during the early developmental process; however, the distinct role of Wnt/β-catenin signaling during pre-implantation period of bovine embryonic development is obscure. Here, we evaluated the critical role of Wnt/β-catenin pathway in the regulation of bovine blastocyst (BL) development and hatching. 6 bromoindurbin-3’oxime (6-Bio) was used to stimulate the Wnt signaling. Treatment with 6-Bio induced the expression of peroxisome proliferator-activated receptor-delta (PPARδ). Interestingly, the PPARδ co-localized with β-catenin and form a complex with TCF/LEF transcription factor. This complex potentiated the expression of several Wnt directed genes, which regulate early embryonic development. Inhibition of PPARδ with selective inhibitor 4-chloro-N-(2-{[5-trifluoromethyl]-2-pyridyl]sulfonyl}ethyl)benzamide (Gsk3787) severely perturbed the BL formation and hatching. The addition of Wnt agonist successfully rescued the BL formation and hatching ability. Importantly, the activation of PPARδ expression by Wnt stimulation enhanced cell proliferation and fatty acid oxidation (FAO) metabolism to improve BL development and hatching. In conclusion, our study provides the evidence that Wnt induced PPARδ expression co-localizes with β-catenin and is a likely candidate of canonical Wnt pathway for the regulation of bovine embryonic development.

Single-cell chromatin accessibility maps reveal regulatory programs driving early mouse organogenesis

During mouse embryonic development, pluripotent cells rapidly divide and diversify, yet the regulatory programs that define the cell repertoire for each organ remain ill-defined. To delineate comprehensive chromatin landscapes during early organogenesis, we mapped chromatin accessibility in 19,453 single nuclei from mouse embryos at 8.25 days post-fertilization. Identification of cell-type-specific regions of open chromatin pinpointed two TAL1-bound endothelial enhancers, which we validated using transgenic mouse assays. Integrated gene expression and transcription factor motif enrichment analyses highlighted cell-type-specific transcriptional regulators. Subsequent in vivo experiments in zebrafish revealed a role for the ETS factor FEV in endothelial identity downstream of ETV2 (Etsrp in zebrafish). Concerted in vivo validation experiments in mouse and zebrafish thus illustrate how single-cell open chromatin maps, representative of a mammalian embryo, provide access to the regulatory blueprint for mammalian organogenesis.

Specification of the First Mammalian Cell Lineages In Vivo and In Vitro

Our understanding of how the first mammalian cell lineages arise has been shaped largely by studies of the preimplantation mouse embryo. Painstaking work over many decades has begun to reveal how a single totipotent cell is transformed into a multilayered structure representing the foundations of the body plan. Here, we review how the first lineage decision is initiated by epigenetic regulation but consolidated by the integration of morphological features and transcription factor activity. The establishment of pluripotent and multipotent stem cell lines has enabled deeper analysis of molecular and epigenetic regulation of cell fate decisions. The capability to assemble these stem cells into artificial embryos is an exciting new avenue of research that offers a long-awaited window into cell fate specification in the human embryo. Together, these approaches are poised to profoundly increase our understanding of how the first lineage decisions are made during mammalian embryonic development.

Glycolysis-Independent Glucose Metabolism Distinguishes TE from ICM Fate during Mammalian Embryogenesis

The mouse embryo undergoes compaction at the 8-cell stage, and its transition to 16 cells generates polarity such that the outer apical cells are trophectoderm (TE) precursors and the inner cell mass (ICM) gives rise to the embryo. Here, we report that this first cell fate specification event is controlled by glucose. Glucose does not fuel mitochondrial ATP generation, and glycolysis is dispensable for blastocyst formation. Furthermore, glucose does not help synthesize amino acids, fatty acids, and nucleobases. Instead, glucose metabolized by the hexosamine biosynthetic pathway (HBP) allows nuclear localization of YAP1. In addition, glucose-dependent nucleotide synthesis by the pentose phosphate pathway (PPP), along with sphingolipid (S1P) signaling, activates mTOR and allows translation of Tfap2c. YAP1, TEAD4, and TFAP2C interact to form a complex that controls TE-specific gene transcription. Glucose signaling has no role in ICM specification, and this process of developmental metabolism specifically controls TE cell fate.

Exploring trophoblast-specific Tead4 enhancers through chromatin conformation capture assays followed by functional screening

Tead4 is critical for blastocyst development and trophoblast differentiation. We assayed long-range chromosomal interactions on the Tead4 promoter in mouse embryonic stem (ES) cells and trophoblast stem (TS) cells. Using luciferase reporter assays with ES and TS cells for 34 candidate enhancer regions, we identified five genomic fragments that increased Tead4 promoter activity in a TS-specific manner. The five loci consisted of three intra- and two inter-chromosomal loci relative to Tead4 on chromosome 6. We established five mouse lines with one of the five enhancer elements deleted and evaluated the effect of each deletion on Tead4 expression in blastocysts. By quantitative RT-PCR, we measured a 42% decrease in Tead4 expression in the blastocysts with a homozygous deletion with a 1.5 kb genomic interval on chromosome 19 (n = 14) than in wild-type blastocysts. By conducting RNA-seq analysis, we confirmed the trans effect of this enhancer deletion on Tead4 without significant cis effects on its neighbor genes at least within a 1.7 Mb distance. Our results demonstrated that the genomic interval on chromosome 19 is required for the appropriate level of Tead4 expression in blastocysts and suggested that an inter-chromosomal enhancer-promoter interaction may be the underlying mechanism.

ROCK and RHO Playlist for Preimplantation Development: Streaming to HIPPO Pathway and Apicobasal Polarity in the First Cell Differentiation

In placental mammalian development, the first cell differentiation produces two distinct lineages that emerge according to their position within the embryo: the trophectoderm (TE, placenta precursor) differentiates in the surface, while the inner cell mass (ICM, fetal body precursor) forms inside. Here, we discuss how such position-dependent lineage specifications are regulated by the RHOA subfamily of small GTPases and RHO-associated coiled-coil kinases (ROCK). Recent studies in mouse show that activities of RHO/ROCK are required to promote TE differentiation and to concomitantly suppress ICM formation. RHO/ROCK operate through the HIPPO signaling pathway, whose cell position-specific modulation is central to establishing unique gene expression profiles that confer cell fate. In particular, activities of RHO/ROCK are essential in outside cells to promote nuclear localization of transcriptional co-activators YAP/TAZ, the downstream effectors of HIPPO signaling. Nuclear localization of YAP/TAZ depends on the formation of apicobasal polarity in outside cells, which requires activities of RHO/ROCK. We propose models of how RHO/ROCK regulate lineage specification and lay out challenges for future investigations to deepen our understanding of the roles of RHO/ROCK in preimplantation development. Finally, as RHO/ROCK may be inhibited by certain pharmacological agents, we discuss their potential impact on human preimplantation development in relation to fertility preservation in women.

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.

Choice of factors and medium impinge on success of ESC to TSC conversion

INTRODUCTION

The first lineage separation in mammalian development occurs when totipotent cells of the zygote give rise to the inner cell mass and the trophectoderm. The lineages are strictly separated by an epigenetic barrier. In vitro derivatives of these lineages embryonic stem cells (ESC) and trophoblast stem cells (TSC) are used to study the requirements needed to overcome the barrier in ESC to TSC conversion approaches.

METHODS

Different combinations of TSC transcription factors were induced in ESC for three days. Cells were kept in TS medium with fetal bovine serum (FBS) or the chemically defined TX medium. Obtained cells were analysed for OCT4 levels, TSC surface marker levels, expression of TSC markers and methylation status of Elf5, Oct4 and Nanog promoters. Further, long-term culture stability and in vitro and in vivo differentiation was tested.

RESULTS

Overexpression of Gata3, Eomes, Tfap2c, Ets2 and Cdx2 in ESC resulted in induction of TSC fate. Overexpression of Cdx2 or four factors (Gata3, Eomes, Tfap2c and Ets2) resulted in complete conversion only when cells were cultured in TX medium. The obtained induced TSC (iTSC) display characteristics of bona fide TSC in terms of marker expression and promoter methylation patterns. The generated converted cells were shown to display self-renewal and to be capable to differentiate into TSC derivatives in vitro and in vivo.

CONCLUSION

Gata3, Eomes, Tfap2c, Ets2 and Cdx2 overexpression in ESC resulted in stable iTSC fate independent of culture conditions. For four factors or Cdx2 alone, TX medium is required for complete TSC conversion.

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.

Global transcriptome analysis of different stages of preimplantation embryo development in river buffalo

Background

Water buffalo (Bubalus bubalis) are divided into river buffalo and swamp buffalo subspecies and are essential livestock for agriculture and the local economy. Studies on buffalo reproduction have primarily focused on optimal fertility and embryonic mortality. There is currently limited knowledge on buffalo embryonic development, especially during the preimplantation period. Assembly of the river buffalo genome offers a reference for omics studies and facilitates transcriptomic analysis of preimplantation embryo development (PED).

Methods

We revealed transcriptomic profile of four stages (2-cell, 8-cell, Morula and Blastocyst) of PED via RNA-seq (Illumina HiSeq4000). Each stage comprised three biological replicates. The data were analyzed according to the basic RNA-seq analysis process. Ingenuity analysis of cell lineage control, especially transcription factor (TF) regulatory networks, was also performed.

Results

A total of 21,519 expressed genes and 67,298 transcripts were predicted from approximately 81.94 Gb of raw data. Analysis of transcriptome-wide expression, gene coexpression networks, and differentially expressed genes (DEGs) allowed for the characterization of gene-specific expression levels and relationships for each stage. The expression patterns of TFs, such as POU5F1, TEAD4, CDX4 and GATAs, were elucidated across diverse time series; most TF expression levels were increased during the blastocyst stage, during which time cell differentiation is initiated. All of these TFs were involved in the composition of the regulatory networks that precisely specify cell fate. These findings offer a deeper understanding of PED at the transcriptional level in the river buffalo.

Development of the human placenta

The placenta is essential for normal in utero development in mammals. In humans, defective placental formation underpins common pregnancy disorders such as pre-eclampsia and fetal growth restriction. The great variation in placental types across mammals means that animal models have been of limited use in understanding human placental development. However, new tools for studying human placental development, including 3D organoids, stem cell culture systems and single cell RNA sequencing, have brought new insights into this field. Here, we review the morphological, molecular and functional aspects of human placental formation, with a focus on the defining cell of the placenta - the trophoblast.

The first cell fate decision in pre-implantation mouse embryos

Fertilization happens when sperm and oocytes meet, which is a complicated process involving many important types of biological activation. Beginning in the 2-cell stage, an important event referred to as zygotic genome activation (ZGA) occurs, which governs the subsequent development of the embryo. In ZGA, multiple epigenetic modifications are involved and critical for pre-implantation development. These changes occur after ZGA, resulting in blastomeres segregate into two different lineages. Some blastomeres develop into the inner cell mass (ICM), and others develop into the trophectoderm (TE), which is considered the first cell fate decision. How this process is initiated and the exact molecular mechanisms involved are fascinating questions that remain to be answered. In this review, we introduce some possible developmental models of the first cell fate decision and discuss the signalling pathways and transcriptional networks regulating this process.

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.

A framework for TRIM21-mediated protein depletion in early mouse embryos: recapitulation of Tead4 null phenotype over three days

Background

While DNA and RNA methods are routine to disrupt the expression of specific genes, complete understanding of developmental processes requires also protein methods, because: oocytes and early embryos accumulate proteins and these are not directly affected by DNA and RNA methods. When proteins in the oocyte encounter a specific antibody and the TRIpartite Motiv-containing 21 (TRIM21) ubiquitin-protein ligase, they can be committed to degradation in the proteasome, producing a transient functional knock-out that reveals the role of the protein. However, there are doubts about whether this targeted proteolysis could be successfully used to study mammalian development, because duration of the transient effect is unknown, and also because amounts of reagents delivered must be adequate in relation to the amount of target protein, which is unknown, too.

Results

We show that the mouse egg contains up to 1E-02 picomoles/protein, as estimated by mass spectrometry using the intensity-based absolute quantification (iBAQ) algorithm. However, the egg can only accommodate ≈1E-04 picomoles of antibody or TRIM21 without incurring toxic effects. Within this framework, we demonstrate that TRIM21-mediated protein depletion efficiently disrupts the embryonic process of trophectoderm formation, which critically depends on the TEA domain family member 4 (Tead4) gene. TEAD4 depletion starting at the 1-cell stage lasts for 3 days prior to a return of gene and protein expression to baseline. This time period is long enough to result in a phenotype entirely consistent with that of the published null mutation and RNA interference studies: significant underexpression of trophectodermal genes Cdx2 and Gata3 and strongly impaired ability of embryos to cavitate and implant in the uterus. Omics data are available via ProteomeXchange (PXD012613) and GEO (GSE124844).

Conclusions

TRIM21-mediated protein depletion can be an effective means to disrupt gene function in mouse development, provided the target gene is chosen carefully and the method is tuned accurately. The knowledge gathered in this study provides the basic know-how (prerequisites, requirements, limitations) to expedite the protein depletion of other genes besides Tead4.

Mechanisms of early placental development in mouse and humans

The importance of the placenta in supporting mammalian development has long been recognized, but our knowledge of the molecular, genetic and epigenetic requirements that underpin normal placentation has remained remarkably under-appreciated. Both the in vivo mouse model and in vitro-derived murine trophoblast stem cells have been invaluable research tools for gaining insights into these aspects of placental development and function, with recent studies starting to reshape our view of how a unique epigenetic environment contributes to trophoblast differentiation and placenta formation. These advances, together with recent successes in deriving human trophoblast stem cells, open up new and exciting prospects in basic and clinical settings that will help deepen our understanding of placental development and associated disorders of pregnancy.

Mapping cell migrations and fates in a gastruloid model to the human primitive streak

Although fate maps of early embryos exist for nearly all model organisms, a fate map of the gastrulating human embryo remains elusive. Here, we use human gastruloids to piece together a rudimentary fate map for the human primitive streak (PS). This is possible because differing levels of BMP, WNT and NODAL lead to self-organization of gastruloids into homogenous subpopulations of endoderm and mesoderm, and comparative analysis of these gastruloids, together with the fate map of the mouse embryo, allows the organization of these subpopulations along an anterior-posterior axis. We also developed a novel cell tracking technique that detected robust fate-dependent cell migrations in our gastruloids comparable with those found in the mouse embryo. Taken together, our fate map and recording of cell migrations provides a first coarse view of what the human PS may resemble in vivo.

Novel key roles for structural maintenance of chromosome flexible domain containing 1 (Smchd1) during preimplantation mouse development

Structural maintenance of chromosome flexible domain containing 1 (Smchd1) is a chromatin regulatory gene for which mutations are associated with facioscapulohumeral muscular dystrophy and arhinia. The contribution of oocyte- and zygote-expressed SMCHD1 to early development was examined in mice ( Mus musculus) using a small interfering RNA knockdown approach. Smchd1 knockdown compromised long-term embryo viability, with reduced embryo nuclear volumes at the morula stage, reduced blastocyst cell number, formation and hatching, and reduced viability to term. RNA sequencing analysis of Smchd1 knockdown morulae revealed aberrant increases in expression of a small number of trophectoderm (TE)-related genes and reduced expression of cell proliferation genes, including S-phase kinase-associated protein 2 ( Skp2). Smchd1 expression was elevated in embryos deficient for Caudal-type homeobox transcription factor 2 ( Cdx2, a key regulator of TE specification), indicating that Smchd1 is normally repressed by CDX2. These results indicate that Smchd1 plays an important role in the preimplantation embryo, regulating early gene expression and contributing to long-term embryo viability. These results extend the known functions of SMCHD1 to the preimplantation period and highlight important function for maternally expressed Smchd1 messenger RNA and protein.

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.

Characterisation of Osteopontin in an In Vitro Model of Embryo Implantation

At the onset of pregnancy, embryo implantation is initiated by interactions between the endometrial epithelium and the outer trophectoderm cells of the blastocyst. Osteopontin (OPN) is expressed in the endometrium and is implicated in attachment and signalling roles at the embryo-epithelium interface. We have characterised OPN in the human endometrial epithelial Ishikawa cell line using three different monoclonal antibodies, revealing at least nine distinct molecular weight forms and a novel secretory pathway localisation in the apical domain induced by cell organisation into a confluent epithelial layer. Mouse blastocysts co-cultured with Ishikawa cell layers served to model embryo apposition, attachment and initial invasion at implantation. Exogenous OPN attenuated initial, weak embryo attachment to Ishikawa cells but did not affect the attainment of stable attachment. Notably, exogenous OPN inhibited embryonic invasion of the underlying cell layer, and this corresponded with altered expression of transcription factors associated with differentiation from trophectoderm (Gata2) to invasive trophoblast giant cells (Hand1). These data demonstrate the complexity of endometrial OPN forms and suggest that OPN regulates embryonic invasion at implantation by signalling to the trophectoderm.

Effect of EZH2 knockdown on preimplantation development of porcine parthenogenetic embryos

The EZH2 protein endows the polycomb repressive complex 2 (PRC2) with histone lysine methyltransferase activity that is associated with transcriptional repression. Recent investigations have documented crucial roles for EZH2 in mediating X-inactivation, stem cell pluripotency and cancer metastasis. However, there is little evidence demonstrating the maternal effect of EZH2 on porcine preimplantation development. Here, we took parthenogenetic activation embryos to eliminate the confounding paternal influence. We showed that the dynamic expression of EZH2 during early development was accompanied by changes in H3K27me3 levels. Depletion of EZH2 in MII oocytes by small interfering RNA not only impaired embryonic development at the blastocyst stage (P < 0.05), but also disrupted the equilibrium of H3K4me3 and H3K27me3 in the embryo. Interestingly, the expression of TET1, a member of Ten-Eleven Translocation gene family for converting 5-methylcytosine (5 mC) to 5-hydroxymethylcytosine (5hmC), was decreased after EZH2 knockdown, in contrast to the increase of the other two members, TET2 and TET3 (P < 0.05). These results indicate a correlation between histone methylation and DNA methylation, and between EZH2 and TET1. Along with the downregulation of TET1, the expression of the pluripotency gene NANOG was decreased (P < 0.05), which is consistent with a previous finding in mouse ES cells. Meanwhile, the abundance of OCT4 and SOX2 were also down-regulated. Moreover, EZH2 knockdown reduced the capacity of cells in the blastocysts to resist apoptosis. Taken together, our data suggest that EZH2 is integral to the developmental program of porcine parthenogenetic embryos and exerts its function by regulating pluripotency, differentiation and apoptosis.

Transitions in cell potency during early mouse development are driven by Notch

The Notch signalling pathway plays fundamental roles in diverse developmental processes in metazoans, where it is important in driving cell fate and directing differentiation of various cell types. However, we still have limited knowledge about the role of Notch in early preimplantation stages of mammalian development, or how it interacts with other signalling pathways active at these stages such as Hippo. By using genetic and pharmacological tools in vivo, together with image analysis of single embryos and pluripotent cell culture, we have found that Notch is active from the 4-cell stage. Transcriptomic analysis in single morula identified novel Notch targets, such as early naïve pluripotency markers or transcriptional repressors such as TLE4. Our results reveal a previously undescribed role for Notch in driving transitions during the gradual loss of potency that takes place in the early mouse embryo prior to the first lineage decisions.

We start life as a single cell, which immediately begins to divide to form an embryo that will eventually contain all the different kinds of cells found in the adult body. During the first few rounds of cell division, embryonic cells can become any type of adult cells, but also form the placenta, the organ that sustains the embryo while in the womb. As cells keep on dividing, they lose this ability, called potency, and they take on more specific and inflexible roles.

The first choice embryonic cells must make is whether to become part of the placenta or part of the future body. These types of decisions are controlled by molecular cascades known as signalling pathways, which relay information from the cells surface to its control centre. There, specific genes get turned on or off in response to an outside signal.

Previous research showed that two signalling pathways, Hippo and Notch, help separate placenta cells from those that will form the rest of the body. However, it was not known whether the two pathways worked independently, or if they were overlapping. Menchero et al. therefore wanted to find out when exactly the Notch pathway started to be active, and examine how it helped cells to either become the placenta or part of the future body.

Experiments with developing mouse embryos showed that the Notch pathway was activated after the very first two cell divisions, when the embryo consists of only four cells. Genetic manipulations combined with drug treatments that changed the activity of the Notch pathway confirmed that Notch and Hippo acted independently at this stage. Further, larger-scale analysis of gene activity in these embryos also revealed that Notch signalling was working in a previously unknown way: it turned off the genes that maintain potency, pushing the cells to become more specialised.

Ultimately, identifying this new mode of action for the Notch pathway in the early embryo may help to understand how the signalling cascade acts in other types of processes. This knowledge could be useful, for example, to push embryonic cells grown in the laboratory towards a desired fate.

Dysregulation of the Hippo pathway signaling in aging and cancer

Human beings are facing emerging degenerative and cancer diseases, in large part, as a consequence of increased life expectancy. In the near future, researchers will have to put even more effort into fighting these new challenges, one of which will be prevention of cancer while continuing to improve the aging process through this increased life expectancy. In the last few decades, relevance of the Hippo pathway on cancer has become an important study since it is a major regulator of organ size control and proliferation. However, its deregulation can induce tumors throughout the body by regulating cell proliferation, disrupting cell polarity, releasing YAP and TAZ from the Scribble complexes and facilitating survival gene expression via activation of TEAD transcription factors. This pathway is also involved in some of the most important mechanisms that control the aging processes, such as the AMP-activated protein kinase and sirtuin pathways, along with autophagy and oxidative stress response/antioxidant defense. This could be the link between two tightly connected processes that could open a broader range of targeted molecular therapies to fight aging and cancer. Therefore, available knowledge of the processes involved in the Hippo pathway during aging and cancer must necessarily be well understood.

Optimal-Transport Analysis of Single-Cell Gene Expression Identifies Developmental Trajectories in Reprogramming

Understanding the molecular programs that guide differentiation during development is a major challenge. Here, we introduce Waddington-OT, an approach for studying developmental time courses to infer ancestor-descendant fates and model the regulatory programs that underlie them. We apply the method to reconstruct the landscape of reprogramming from 315,000 single-cell RNA sequencing (scRNA-seq) profiles, collected at half-day intervals across 18 days. The results reveal a wider range of developmental programs than previously characterized. Cells gradually adopt either a terminal stromal state or a mesenchymal-to-epithelial transition state. The latter gives rise to populations related to pluripotent, extra-embryonic, and neural cells, with each harboring multiple finer subpopulations. The analysis predicts transcription factors and paracrine signals that affect fates and experiments validate that the TF Obox6 and the cytokine GDF9 enhance reprogramming efficiency. Our approach sheds light on the process and outcome of reprogramming and provides a framework applicable to diverse temporal processes in biology.

RHOA activity in expanding blastocysts is essential to regulate HIPPO-YAP signaling and to maintain the trophectoderm-specific gene expression program in a ROCK/actin filament-independent manner

STUDY QUESTION

What molecular signals are required to maintain the functional trophectoderm (TE) during blastocyst expansion of the late stage of preimplantation development?

SUMMARY ANSWER

The activity of ras homology family member A (RHOA) GTPases is necessary to retain the expanded blastocyst cavity and also to sustain the gene expression program specific to TE.

WHAT IS KNOWN ALREADY

At the early stages of preimplantation development, the precursor of the TE lineage is generated through the molecular signals that integrate RHOA, RHO-associated coiled-coil containing protein kinase (ROCK), the apicobasal cell polarity, and the HIPPO-Yes-associated protein (YAP) signaling pathway. By contrast, molecular mechanisms regulating the maintenance of the TE characteristics at the later stage, which is crucial for blastocyst hatching and implantation, are scarcely understood.

STUDY DESIGN, SIZE, DURATION

Expanding mouse blastocysts, obtained from crosses of the F1 (C57BL6 × DBA/2) strain, were exposed to chemical agents that interfere with RHOA, ROCK, or the actin cytoskeleton for up to 8 h, and effects on the blastocyst cavity, HIPPO-YAP signaling, and cell lineage-specific gene expression profiles were examined.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Mouse embryos at the embryonic stage E3.5 (expanding blastocysts) and E4.5 (fully expanded blastocysts) were treated with RHOA inhibitor (C3 exoenzyme), ROCK inhibitor (Y27632), or actin filament disruptors (cytochalasin B and latrunculin A). The integrity of the blastocyst cavity was evaluated based on the gross morphology. Effects on HIPPO-YAP signaling were assessed based on the presence of nuclearized YAP protein by immunofluorescence staining and the expression of YAP/TEA domain family member (TEAD) target genes by quantitative RT-PCR (qRT-PCR). The impact of these disruptors on cell lineages was evaluated based on expression of the TE-specific and inner cell mass-specific marker genes by qRT-PCR. The integrity of the apicobasal cell polarity was assessed by localization of protein kinase C zeta (PRKCZ; apical) and scribbled planar cell polarity (SCRIB; basal) proteins by immunofluorescence staining. For comparisons, cultured cell lines, NIH/3T3 (mouse fibroblast) and P19C5 (mouse embryonal carcinoma), were also treated with RHOA inhibitor, ROCK inhibitor, and actin filament disruptors for up to 8 h, and effects on HIPPO-YAP signaling were assessed based on expression of YAP/TEAD target genes by qRT-PCR. Each experiment was repeated using three independent batches of embryos (n = 40-80 per batch) or cell collections. Statistical analyses of data were performed, using one-way ANOVA and two-sample t-test.

MAIN RESULTS AND THE ROLE OF CHANCE

Inhibition of RHOA deflated the cavity, diminished nuclear YAP (P < 0.01), and down-regulated the YAP/TEAD target and TE-specific marker genes in both E3.5 and E4.5 blastocysts (P < 0.05), indicating that the maintenance of the key TE characteristics is dependent on RHOA activity. However, inhibition of ROCK or disruption of actin filament only deflated the blastocyst cavity, but did not alter HIPPO-YAP signaling or lineage-specific gene expressions, suggesting that the action of RHOA to sustain the TE-specific gene expression program is not mediated by ROCK or the actomyosin cytoskeleton. By contrast, ROCK inhibitor and actin filament disruptors diminished YAP/TEAD target gene expressions in cultured cells to a greater extent than RHOA inhibitor, implicating that the regulation of HIPPO-YAP signaling in expanding blastocysts is distinctly different from that in the cell lines. Furthermore, the apicobasal cell polarity proteins in the expanding blastocyst were mislocalized by ROCK inhibition but not by RHOA inhibition, indicating that cell polarity is not linked to regulation of HIPPO-YAP signaling. Taken together, our study suggests that RHOA activity is essential to maintain the TE lineage in the expanding blastocyst and it regulates HIPPO-YAP signaling and the lineage-specific gene expression program through mechanisms that are independent of ROCK or actomyosin cytoskeleton.

LARGE-SCALE DATA

Not applicable.

LIMITATIONS, REASONS FOR CAUTION

This study was conducted using one species, the mouse. Direct translation of the experiments and findings to human fertility preservation and ART requires further investigations.

WIDER IMPLICATIONS OF THE FINDINGS

The elucidation of the mechanisms of TE formation is highly pertinent to fertility preservation in women. Our findings may raise awareness among providers of ART that the TE is sensitive to disturbance even in the late stage of blastocyst expansion and that rational approaches should be devised to avoid conditions that may impair the TE and its function.

STUDY FUNDING/COMPETING INTEREST(S)

This study was funded by grants from the Ingeborg v.F. McKee Fund of the Hawaii Community Foundation (16ADVC-78882 to V.B.A.), and the National Institutes of Health (P20 GM103457 and R03 HD088839 to V.B.A.). The authors have no conflict of interest to declare.

Visualizing HIPPO Signaling Components in Mouse Early Embryonic Development

The HIPPO signaling pathway plays an early and essential role in mammalian embryogenesis. The earliest known roles for HIPPO signaling during mouse development include segregating fetal and extraembryonic lineages and establishing the pluripotent progenitors of embryonic stem (ES) cells. In the mouse early embryo, HIPPO signaling responds to multiple cell biological inputs, including cell polarization, cytoskeleton, and cell environment, to influence gene expression and the first cell fate decisions in development. Methods to monitor and manipulate HIPPO signaling in the mouse early embryo are fundamental to discovering mechanisms regulating pluripotency in vivo, but properties of the early embryo, such as small cell number and spherical architecture, pose unique challenges for signaling pathway analysis. Here, we share approaches for visualizing HIPPO signaling in mouse early embryos. In addition, these methods can be applied to visualize HIPPO signaling in other spherical or cystic structures comprised of relatively few cells, such as organoids, or for the examination of other signaling pathways in these contexts.

Quantitative Real-Time PCR to Measure YAP/TAZ Activity in Human Cells

Transcription coactivators Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ, also known as WWTR1) are homologs of the Drosophila Yorkie (Yki) protein and are major downstream effectors of the evolutionarily conserved Hippo pathway. YAP/TAZ play critical roles in regulation of cell proliferation, apoptosis, and stemness, thus mediate functions of the Hippo pathway in organ size control and tumorigenesis. The Hippo pathway inhibits YAP/TAZ through phosphorylation, which leads to YAP/TAZ cytoplasmic retention and degradation. Dephosphorylated and nuclear-localized YAP/TAZ bind to transcription factors, especially the TEAD family proteins, thus transactivate the expression of specific genes. Therefore, measuring the expression level of YAP/TAZ target genes is a critical approach to assess Hippo pathway activity. Through gene expression profiling in different tissues and cells using techniques such as microarray and RNA-seq, many target genes of YAP/TAZ have been identified. Some of these genes were confirmed to be direct YAP/TAZ targets by chromatin immunoprecipitation (ChIP)-PCR or ChIP-seq. These works made it possible to quickly determine YAP/TAZ activity by measuring the mRNA levels of several YAP/TAZ target genes, such as CTGF, CYR61, and miR-130a by quantitative real-time PCR (qPCR). In this chapter, we demonstrate the use of qPCR to measure YAP/TAZ activity in MCF10A cells.

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.

Genetic Control of Early Cell Lineages in the Mammalian Embryo

Establishing the different lineages of the early mammalian embryo takes place over several days and several rounds of cell divisions from the fertilized egg. The resulting blastocyst contains the pluripotent cells of the epiblast, from which embryonic stem cells can be derived, as well as the extraembryonic lineages required for a mammalian embryo to survive in the uterine environment. The dynamics of the cellular and genetic interactions controlling the initiation and maintenance of these lineages in the mouse embryo are increasingly well understood through application of the tools of single-cell genomics, gene editing, and in vivo imaging. Exploring the similarities and differences between mouse and human development will be essential for translation of these findings into new insights into human biology, derivation of stem cells, and improvements in fertility treatments.

Divergent wiring of repressive and active chromatin interactions between mouse embryonic and trophoblast lineages

The establishment of the embryonic and trophoblast lineages is a developmental decision underpinned by dramatic differences in the epigenetic landscape of the two compartments. However, it remains unknown how epigenetic information and transcription factor networks map to the 3D arrangement of the genome, which in turn may mediate transcriptional divergence between the two cell lineages. Here, we perform promoter capture Hi-C experiments in mouse trophoblast (TSC) and embryonic (ESC) stem cells to understand how chromatin conformation relates to cell-specific transcriptional programmes. We find that key TSC genes that are kept repressed in ESCs exhibit interactions between H3K27me3-marked regions in ESCs that depend on Polycomb repressive complex 1. Interactions that are prominent in TSCs are enriched for enhancer-gene contacts involving key TSC transcription factors, as well as TET1, which helps to maintain the expression of TSC-relevant genes. Our work shows that the first developmental cell fate decision results in distinct chromatin conformation patterns establishing lineage-specific contexts involving both repressive and active interactions.

Regulation of energy metabolism during early mammalian development: TEAD4 controls mitochondrial transcription

Early mammalian development is crucially dependent on the establishment of oxidative energy metabolism within the trophectoderm (TE) lineage. Unlike the inner cell mass, TE cells enhance ATP production via mitochondrial oxidative phosphorylation (OXPHOS) and this metabolic preference is essential for blastocyst maturation. However, molecular mechanisms that regulate establishment of oxidative energy metabolism in TE cells are incompletely understood. Here, we show that conserved transcription factor TEAD4, which is essential for pre-implantation mammalian development, regulates this process by promoting mitochondrial transcription. In developing mouse TE and TE-derived trophoblast stem cells (TSCs), TEAD4 localizes to mitochondria, binds to mitochondrial DNA (mtDNA) and facilitates its transcription by recruiting mitochondrial RNA polymerase (POLRMT). Loss of TEAD4 impairs recruitment of POLRMT, resulting in reduced expression of mtDNA-encoded electron transport chain components, thereby inhibiting oxidative energy metabolism. Our studies identify a novel TEAD4-dependent molecular mechanism that regulates energy metabolism in the TE lineage to ensure mammalian development.

Reprogramming of Extraembryonic Trophoblast Stem Cells into Embryonic Pluripotent State by Fusion with Embryonic Stem Cells

Pluripotential reprogramming has been examined using various technologies, including nuclear transfer, cell fusion, and direct reprogramming. Many studies have used differentiated cells for reprogramming experiments, and nearly all type of somatic cells can acquire pluripotency. However, within the embryo, other cells types are present in addition to somatic cells. The blastocyst stage embryo consists of two main types of cells, inner cell mass and trophectoderm (TE). TE cells are the first differentiated form of the totipotent zygote and differ from epiblast cells. Thus, we examined whether extraembryonic cells can be reprogrammed using a cell-cell fusion method. Trophoblast stem cells (TSCs), which can be obtained from the TE, are known to acquire pluripotency by transcription factor Oct4 overexpression or somatic cell nuclear transfer. In this study, we demonstrated that TSCs can acquire pluripotent properties by cell fusion with embryonic stem cells (ESCs). TSC-ESC hybrids reactivated Oct4-GFP and displayed self-renewal properties. They expressed the pluripotency markers Oct4 and Nanog, whereas the expression of Cdx2 and Tead4, trophoblast lineage markers, was diminished. Moreover, these cells developed into three germ layers similarly to other pluripotent stem cells. RNA-seq analysis showed that global gene expression patterns of TSC-ESC hybrids are more similar to ESCs than TSCs. Thus, we demonstrated that TSCs successfully complete reprogramming and acquire pluripotency by cell fusion-induced reprogramming.

Transcription coactivator Cited1 acts as an inducer of trophoblast-like state from mouse embryonic stem cells through the activation of BMP signaling

Trophoblast lineages, precursors of the placenta, are essential for post-implantation embryo survival. However, the regulatory network of trophoblast development remains incompletely understood. Here, we report that Cited1, a transcription coactivator, is a robust inducer for trophoblast-like state from mouse embryonic stem cells (ESCs). Depletion of Cited1 in ESCs compromises the trophoblast lineage specification induced by BMP signaling. In contrast, overexpression of Cited1 in ESCs induces a trophoblast-like state with elevated expression of trophoblast marker genes in vitro and generation of trophoblastic tumors in vivo. Furthermore, global transcriptome profile analysis indicates that ectopic Cited1 activates a trophoblast-like transcriptional program in ESCs. Mechanistically, Cited1 interacts with Bmpr2 and Smad4 to activate the Cited1–Bmpr2–Smad1/5/8 axis in the cytoplasm and Cited1–Smad4–p300 complexes in the nucleus, respectively. Collectively, our results show that Cited1 plays an important role in regulating trophoblast lineage specification through activating the BMP signaling pathway.

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.

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.