Loss of the extraembryonic ectoderm in Elf5 mutants leads to defects in embryonic patterning

The involvement of Elf5 in regulating keratinocyte proliferation and differentiation processes in skin

Skin and hair development is regulated by multitude of programs of activation and silencing of gene expression to maintain normal skin and hair follicle (HF) development, homeostasis, and cycling. Here, we have identified E74-like factor 5 (Elf5) transcription factor, as a novel regulator of keratinocyte proliferation and differentiation processes in skin. Expression analysis has revealed that Elf5 expression was localised and elevated in stem/progenitor cell populations of both the epidermis (basal and suprabasal) and in HF bulge and hair germ stem cell (SCs) compartments during skin and hair development and cycling. Expressional and functional analysis using RT-qPCR, western blot and colony forming assays, revealed that Elf5 plays an important role in regulating keratinocyte proliferation and differentiation processes as well as potentially determining cell fate by regulating the stem/progenitor cell populations in skin and HFs. These data will provide a platform for pharmacological manipulation of Elf5 in skin, leading to advancements in many areas of research, including stem cell, regenerative medicine, and ageing.

Transcriptomic effects of alginate hydrogel applied to the production of bovine embryos

In vitro-produced blastocysts are exposed to different stimuli when compared with in vivo ones. This includes the culture of in vitro embryos in a sturdy petri-dish, while in vivo embryos develop in a soft and dynamic structure. Here we hypothesized that a softer environment could differently modulate the in vitro produced embryos. To that aim, presumptive zygotes were produced by in vitro fertilization and divided into three groups: 1) Cultured in a regular Petri dish - Control (CON); 2) Cultured on top of an alginate hydrogel surface (TOP); 3) Encapsulated inside an alginate hydrogel sphere (ENC) and cultured. We observed a decrease in blastocyst rate in TOP and ENC compared with the CON. Profiling of 383 bovine miRNAs, we found 3 miRNAs involved in cell proliferation being differently modulated by the TOP and ENC groups (miR-1246; miR-1260b, and miR-541). Analyzing global levels of DNA methylation and hydroxymethylation, we observed increased levels of the two marks in the TOP group when compared with the CON and ENC systems. RNA sequencing (RNA-seq) analysis carried out using blastocysts showed alterations in several developmentally important genes among the three groups. In summary, our results indicate that in vitro embryo production was possible to achieve up to the blastocyst stage. However, with the experimental conditions used herein, the alginate hydrogels adversely affected the embryo development, which were paralleled by epigenetic and transcriptomic changes.

SLC25A1 regulates placental development to ensure embryonic heart morphogenesis

22q11.2 deletion syndrome (22q11.2DS) is the most common chromosomal microdeletion syndrome. Congenital heart defects are prevalent in 22q11.2DS but the etiology is still poorly understood. In this study, we aimed to gain mechanistic insights into the heart defects that result from 22q11.2 deletion, with a focus on Slc25a1, which is located in the deletion segment. Whereas global knockout of Slc25a1 in mice produced a variety of heart malformations, cardiac deletion of Slc25a1 had little effect on heart development. We then found that trophoblast-specific Slc25a1 deletion recapitulated heart anomalies in the global knockout mice. Further study identified SLC25A1 as a regulator of trophoblast and placental development through modulation of histone H3K27 acetylation at the promoters and enhancers of key genes involved in trophoblast differentiation. Finally, administration of recombinant human pregnancy-specific glycoprotein 1 (PSG1), a trophoblast-derived secretory glycoprotein, partially corrected placental and embryonic heart defects. This study defines the role of SLC25A1 in heart development by regulating placental development, and provides new insights to understand the etiology of 22q11.2DS.

Temporal BMP4 effects on mouse embryonic and extraembryonic development

The developing placenta, which in mice originates through the extraembryonic ectoderm (ExE), is essential for mammalian embryonic development. Yet unbiased characterization of the differentiation dynamics of the ExE and its interactions with the embryo proper remains incomplete. Here we develop a temporal single-cell model of mouse gastrulation that maps continuous and parallel differentiation in embryonic and extraembryonic lineages. This is matched with a three-way perturbation approach to target signalling from the embryo proper, the ExE alone, or both. We show that ExE specification involves early spatial and transcriptional bifurcation of uncommitted ectoplacental cone cells and chorion progenitors. Early BMP4 signalling from chorion progenitors is required for proper differentiation of uncommitted ectoplacental cone cells and later for their specification towards trophoblast giant cells. We also find biphasic regulation by BMP4 in the embryo. The early ExE-originating BMP4 signal is necessary for proper mesoendoderm bifurcation and for allantois and primordial germ cell specification. However, commencing at embryonic day 7.5, embryo-derived BMP4 restricts the primordial germ cell pool size by favouring differentiation of their extraembryonic mesoderm precursors towards an allantois fate. ExE and embryonic tissues are therefore entangled in time, space and signalling axes, highlighting the importance of their integrated understanding and modelling in vivo and in vitro.

Gastrulation-stage gene expression in Nipbl+/- mouse embryos foreshadows the development of syndromic birth defects

In animal models, Nipbl deficiency phenocopies gene expression changes and birth defects seen in Cornelia de Lange syndrome, the most common cause of which is Nipbl haploinsufficiency. Previous studies in Nipbl+/- mice suggested that heart development is abnormal as soon as cardiogenic tissue is formed. To investigate this, we performed single-cell RNA sequencing on wild-type and Nipbl+/- mouse embryos at gastrulation and early cardiac crescent stages. Nipbl+/- embryos had fewer mesoderm cells than wild-type and altered proportions of mesodermal cell subpopulations. These findings were associated with underexpression of genes implicated in driving specific mesodermal lineages. In addition, Nanog was found to be overexpressed in all germ layers, and many gene expression changes observed in Nipbl+/- embryos could be attributed to Nanog overexpression. These findings establish a link between Nipbl deficiency, Nanog overexpression, and gene expression dysregulation/lineage misallocation, which ultimately manifest as birth defects in Nipbl+/- animals and Cornelia de Lange syndrome.

Murine trophoblast organoids as a model for trophoblast development and CRISPR-Cas9 screening

The placenta becomes one of the most diversified organs during placental mammal radiation. The main in vitro model for studying mouse trophoblast development is the 2D differentiation model of trophoblast stem cells, which is highly skewed to certain lineages and thus hampers systematic screens. Here, we established culture conditions for the establishment, maintenance, and differentiation of murine trophoblast organoids. Murine trophoblast organoids under the maintenance condition contain stem cell-like populations, whereas differentiated organoids possess various trophoblasts resembling placental ones in vivo. Ablation of Nubpl or Gcm1 in trophoblast organoids recapitulated their deficiency phenotypes in vivo, suggesting that those organoids are valid in vitro models for trophoblast development. Importantly, we performed an efficient CRISPR-Cas9 screening in mouse trophoblast organoids using a focused sgRNA (single guide RNA) library targeting G protein-coupled receptors. Together, our results establish an organoid model to investigate mouse trophoblast development and a practicable approach to performing forward screening in trophoblast lineages.

Zika virus targets human trophoblast stem cells and prevents syncytialization in placental trophoblast organoids

Zika virus (ZIKV) infection during pregnancy threatens pregnancy and fetal health. However, the infectivity and pathological effects of ZIKV on placental trophoblast progenitor cells in early human embryos remain largely unknown. Here, using human trophoblast stem cells (hTSCs), we demonstrated that hTSCs were permissive to ZIKV infection, and resistance to ZIKV increased with hTSC differentiation. Combining gene knockout and transcriptome analysis, we demonstrated that the intrinsic expression of AXL and TIM-1, and the absence of potent interferon (IFN)-stimulated genes (ISGs) and IFNs contributed to the high sensitivity of hTSCs to ZIKV. Furthermore, using our newly developed hTSC-derived trophoblast organoid (hTSC-organoid), we demonstrated that ZIKV infection disrupted the structure of mature hTSC-organoids and inhibited syncytialization. Single-cell RNA sequencing (scRNA-seq) further demonstrated that ZIKV infection of hTSC-organoids disrupted the stemness of hTSCs and the proliferation of cytotrophoblast cells (CTBs) and probably led to a preeclampsia (PE) phenotype. Overall, our results clearly demonstrate that hTSCs represent the major target cells of ZIKV, and a reduced syncytialization may result from ZIKV infection of early developing placenta. These findings deepen our understanding of the characteristics and consequences of ZIKV infection of hTSCs in early human embryos.

A New Approach to Generate Gastruloids to Develop Anterior Neural Tissues

Embryonic development is a complex process integrating cell fate decisions and morphogenesis in a spatiotemporally controlled manner. Previous studies with model organisms laid the foundation of our knowledge on post-implantation development; however, studying mammalian embryos at this stage is a difficult and laborious process. Early attempts to recapitulate mammalian development in vitro begun with embryoid bodies (EBs), in which aggregates of mouse embryonic stem cells (mESCs) were shown to differentiate into spatially arranged germ layers. A more revised version of EBs, gastruloids, improved the germ layer differentiation efficiency and demonstrated cell fate patterning on multiple axes. However, gastruloids lack anterior neural progenitors that give rise to brain tissues in the embryo. Here, we report a novel culture protocol to coax mESCs into post-implantation epiblast-like (EPI) aggregates in high throughput on bioengineered microwell arrays. We show that upon inhibition of the Wnt signaling pathway, EPI aggregates establish an extended axial patterning, leading to co-derivation of anterior neural progenitors and posterior tissues. Our approach is amenable to large-scale studies aimed at identifying novel regulators of gastrulation and anterior neural development that is currently out of reach with existing embryoid models. This work should contribute to the advancement of the nascent field of synthetic embryology, opening up exciting perspectives for various applications of pluripotent stem cells in disease modeling and tissue engineering. Key features A new gastruloid culture system to model post-implantation mouse embryonic development in vitro High-throughput formation of epiblast-like aggregates on hydrogel microwells Builds upon conventional gastruloid cultures and provides insight into the role of Wnt signaling for the formation of anterior neural tissues Graphical overview.

RBBP4 is an epigenetic barrier for the induced transition of pluripotent stem cells into totipotent 2C-like cells

Cellular totipotency is critical for whole-organism generation, yet how totipotency is established remains poorly illustrated. Abundant transposable elements (TEs) are activated in totipotent cells, which is critical for embryonic totipotency. Here, we show that the histone chaperone RBBP4, but not its homolog RBBP7, is indispensable for maintaining the identity of mouse embryonic stem cells (mESCs). Auxin-induced degradation of RBBP4, but not RBBP7, reprograms mESCs to the totipotent 2C-like cells. Also, loss of RBBP4 enhances transition from mESCs to trophoblast cells. Mechanistically, RBBP4 binds to the endogenous retroviruses (ERVs) and functions as an upstream regulator by recruiting G9a to deposit H3K9me2 on ERVL elements, and recruiting KAP1 to deposit H3K9me3 on ERV1/ERVK elements, respectively. Moreover, RBBP4 facilitates the maintenance of nucleosome occupancy at the ERVK and ERVL sites within heterochromatin regions through the chromatin remodeler CHD4. RBBP4 depletion leads to the loss of the heterochromatin marks and activation of TEs and 2C genes. Together, our findings illustrate that RBBP4 is required for heterochromatin assembly and is a critical barrier for inducing cell fate transition from pluripotency to totipotency.

Time-aligned hourglass gastrulation models in rabbit and mouse

The hourglass model describes the convergence of species within the same phylum to a similar body plan during development; however, the molecular mechanisms underlying this phenomenon in mammals remain poorly described. Here, we compare rabbit and mouse time-resolved differentiation trajectories to revisit this model at single-cell resolution. We modeled gastrulation dynamics using hundreds of embryos sampled between gestation days 6.0 and 8.5 and compared the species using a framework for time-resolved single-cell differentiation-flows analysis. We find convergence toward similar cell-state compositions at E7.5, supported by the quantitatively conserved expression of 76 transcription factors, despite divergence in surrounding trophoblast and hypoblast signaling. However, we observed noticeable changes in specification timing of some lineages and divergence of primordial germ cell programs, which in the rabbit do not activate mesoderm genes. Comparative analysis of temporal differentiation models provides a basis for studying the evolution of gastrulation dynamics across mammals.

Establishment of the fetal-maternal interface: developmental events in human implantation and placentation

Early pregnancy is a complex and well-orchestrated differentiation process that involves all the cellular elements of the fetal-maternal interface. Aberrant trophoblast-decidual interactions can lead to miscarriage and disorders that occur later in pregnancy, including preeclampsia, intrauterine fetal growth restriction, and preterm labor. A great deal of research on the regulation of implantation and placentation has been performed in a wide range of species. However, there is significant species variation regarding trophoblast differentiation as well as decidual-specific gene expression and regulation. Most of the relevant information has been obtained from studies using mouse models. A comprehensive understanding of the physiology and pathology of human implantation and placentation has only recently been obtained because of emerging advanced technologies. With the derivation of human trophoblast stem cells, 3D-organoid cultures, and single-cell analyses of differentiated cells, cell type-specific transcript profiles and functions were generated, and each exhibited a unique signature. Additionally, through integrative transcriptomic information, researchers can uncover the cellular dysfunction of embryonic and placental cells in peri-implantation embryos and the early pathological placenta. In fact, the clinical utility of fetal-maternal cellular trafficking has been applied for the noninvasive prenatal diagnosis of aneuploidies and the prediction of pregnancy complications. Furthermore, recent studies have proposed a viable path toward the development of therapeutic strategies targeting placenta-enriched molecules for placental dysfunction and diseases.

The Fgf/Erf/NCoR1/2 repressive axis controls trophoblast cell fate

Placental development relies on coordinated cell fate decisions governed by signalling inputs. However, little is known about how signalling cues are transformed into repressive mechanisms triggering lineage-specific transcriptional signatures. Here, we demonstrate that upon inhibition of the Fgf/Erk pathway in mouse trophoblast stem cells (TSCs), the Ets2 repressor factor (Erf) interacts with the Nuclear Receptor Co-Repressor Complex 1 and 2 (NCoR1/2) and recruits it to key trophoblast genes. Genetic ablation of Erf or Tbl1x (a component of the NCoR1/2 complex) abrogates the Erf/NCoR1/2 interaction. This leads to mis-expression of Erf/NCoR1/2 target genes, resulting in a TSC differentiation defect. Mechanistically, Erf regulates expression of these genes by recruiting the NCoR1/2 complex and decommissioning their H3K27ac-dependent enhancers. Our findings uncover how the Fgf/Erf/NCoR1/2 repressive axis governs cell fate and placental development, providing a paradigm for Fgf-mediated transcriptional control.

MLL3/MLL4 methyltransferase activities control early embryonic development and embryonic stem cell differentiation in a lineage-selective manner

H3K4me1 methyltransferases MLL3 (KMT2C) and MLL4 (KMT2D) are critical for enhancer activation, cell differentiation and development. However, roles of MLL3/4 enzymatic activities and MLL3/4-mediated enhancer H3K4me1 in these processes remain unclear. Here we report that constitutive elimination of both MLL3 and MLL4 enzymatic activities prevents initiation of gastrulation and leads to early embryonic lethality in mice. However, selective elimination of MLL3/4 enzymatic activities in embryonic, but not extraembryonic, lineages leaves gastrulation largely intact. Consistent with this, embryonic stem cells (ESCs) lacking MLL3/4 enzymatic activities can differentiate toward the three embryonic germ layers but show aberrant differentiation to extraembryonic endoderm (ExEn) and trophectoderm. The failure in ExEn differentiation can be attributed to markedly reduced enhancer-binding of the lineage-determining transcription factor GATA6. Furthermore, we show that MLL3/4-catalyzed H3K4me1 is largely dispensable for enhancer activation during ESC differentiation. Together, our findings suggest a lineage-selective, but enhancer activation-independent, role of MLL3/4 methyltransferase activities in early embryonic development and ESC differentiation.

Efficient generation of ETX embryoids that recapitulate the entire window of murine egg cylinder development

The murine embryonic-trophoblast-extra-embryonic endoderm (ETX) model is an integrated stem cell-based model to study early postimplantation development. It is based on the self-assembly potential of embryonic, trophoblast, and hypoblast/primitive/visceral endoderm-type stem cell lines (ESC, TSC, and XEN, respectively) to arrange into postimplantation egg cylinder-like embryoids. Here, we provide an optimized method for reliable and efficient generation of ETX embryoids that develop into late gastrulation in static culture conditions. It is based on transgenic Gata6-overproducing ESCs and modified assembly and culture conditions. Using this method, up to 43% of assembled ETX embryoids exhibited a correct spatial distribution of the three stem cell derivatives at day 4 of culture. Of those, 40% progressed into ETX embryoids that both transcriptionally and morphologically faithfully mimicked in vivo postimplantation mouse development between E5.5 and E7.5. The ETX model system offers the opportunity to study the murine postimplantation egg cylinder stages and could serve as a source of various cell lineage precursors.

A Uterus-Inspired Niche Drives Blastocyst Development to the Early Organogenesis

The fundamental physical features such as the mechanical properties and microstructures of the uterus need to be considered when building in vitro culture platforms to mimic the uterus for embryo implantation and further development but have long been neglected. Here, a uterus-inspired niche (UN) constructed by grafting collagen gels onto polydimethylsiloxane based on a systematic investigation of a series of parameters (varying concentrations and thicknesses of collagen gel) is established to intrinsically specify and simulate the mechanics and microstructures of the mouse uterus. This brand-new and unique system is robust in supporting embryo invasion, as evidenced by the special interaction between the embryos and the UN system and successfully promoting E3.5 embryo development into the early organogenesis stage. This platform serves as a powerful tool for developmental biology and tissue engineering.

Primary specification of blastocyst trophectoderm by scRNA-seq: New insights into embryo implantation

Mechanisms of implantation such as determination of the attachment pole, fetal-maternal communication, and underlying causes of implantation failure are largely unexplored. Here, we performed single-cell RNA sequencing on peri-implantation embryos from both humans and mice to explore trophectoderm (TE) development and embryo-endometrium cross-talk. We found that the transcriptomes of polar and mural TE diverged after embryos hatched from the zona pellucida in both species, with polar TE being more mature than mural TE. The implantation poles show similarities in cell cycle activities, as well as in expression of genes critical for implantation and placentation. Embryos that either fail to attach in vitro or fail to implant in vivo show abnormalities in pathways related to energy production, protein metabolism, and 18S ribosomal RNA m⁶A methylation. These findings uncover the gene expression characteristics of humans and mice TE differentiation during the peri-implantation period and provide new insights into embryo implantation.

Epiblast inducers capture mouse trophectoderm stem cells in vitro and pattern blastoids for implantation in utero

The embryo instructs the allocation of cell states to spatially regulate functions. In the blastocyst, patterning of trophoblast (TR) cells ensures successful implantation and placental development. Here, we defined an optimal set of molecules secreted by the epiblast (inducers) that captures in vitro stable, highly self-renewing mouse trophectoderm stem cells (TESCs) resembling the blastocyst stage. When exposed to suboptimal inducers, these stem cells fluctuate to form interconvertible subpopulations with reduced self-renewal and facilitated differentiation, resembling peri-implantation cells, known as TR stem cells (TSCs). TESCs have enhanced capacity to form blastoids that implant more efficiently in utero due to inducers maintaining not only local TR proliferation and self-renewal, but also WNT6/7B secretion that stimulates uterine decidualization. Overall, the epiblast maintains sustained growth and decidualization potential of abutting TR cells, while, as known, distancing imposed by the blastocyst cavity differentiates TR cells for uterus adhesion, thus patterning the essential functions of implantation.

Revisiting Epithelial Carcinogenesis

The origin of cancer remains one of the most important enigmas in modern biology. This paper presents a hypothesis for the origin of carcinomas in which cellular aging and inflammation enable the recovery of cellular plasticity, which may ultimately result in cancer. The hypothesis describes carcinogenesis as the result of the dedifferentiation undergone by epithelial cells in hyperplasia due to replicative senescence towards a mesenchymal cell state with potentially cancerous behavior. In support of this hypothesis, the molecular, cellular, and histopathological evidence was critically reviewed and reinterpreted when necessary to postulate a plausible generic series of mechanisms for the origin and progression of carcinomas. In addition, the implications of this theoretical framework for the current strategies of cancer treatment are discussed considering recent evidence of the molecular events underlying the epigenetic switches involved in the resistance of breast carcinomas. The hypothesis also proposes an epigenetic landscape for their progression and a potential mechanism for restraining the degree of dedifferentiation and malignant behavior. In addition, the manuscript revisits the gradual degeneration of the nonalcoholic fatty liver disease to propose an integrative generalized mechanistic explanation for the involution and carcinogenesis of tissues associated with aging. The presented hypothesis might serve to understand and structure new findings into a more encompassing view of the genesis of degenerative diseases and may inspire novel approaches for their study and therapy.

Mice lacking DCAF2 in placenta die at the gastrulation stage

UV-damaged DNA-binding protein 1 (DDB1) and cullin 4-associated factor 2 (DCAF2, also known as DTL or CDT2) is an evolutionarily highly conserved substrate recognition factor in the cullin 4 RING E3 ubiquitin ligase (CRL4) complex. This complex degrades multiple DNA replication and cell cycle-associated proteins to maintain genome stability. To clarify the function of DCAF2 in vivo, we used Cre recombinase driven by the Elf5 promoter to generate knockout mouse model that was specifically deleted Dcaf2 in the trophoblast lineage (Elf5-Cre; Dcaf2^fl/fl, Dcaf2 cKO). Here, we show that mice with the genotype Elf5-Cre; Dcaf2^fl/+ are normal and fertile. However, after mating of Elf5-Cre; Dcaf2^fl/+ mice with Dcaf2^fl/fl, no Dcaf2 cKO pups were born. Timed pregnancy studies have shown that Dcaf2 cKO mice developed abnormally on embryonic day 5.5 and died at gastrulation stage. It is worth noting that the extraembryonic ectoderm of Dcaf2 cKO mice is severely reduced or missing and leading to embryonic death. We also proved that stronger DNA damage accumulated in the trophoblastic cells of Dcaf2 cKO mice at E8.5. In addition, higher expression of Caspase-3 was found in the embryonic and trophoblastic cells of these cKO mice. In general, our research shows that the placental DCAF2 is crucial to the formation of gastrula.

Derivation of functional trophoblast stem cells from primed human pluripotent stem cells

Trophoblast stem cells (TSCs) have recently been derived from human embryos and early-first-trimester placenta; however, aside from ethical challenges, the unknown disease potential of these cells limits their scientific utility. We have previously established a bone morphogetic protein 4 (BMP4)-based two-step protocol for differentiation of primed human pluripotent stem cells (hPSCs) into functional trophoblasts; however, those trophoblasts could not be maintained in a self-renewing TSC-like state. Here, we use the first step from this protocol, followed by a switch to newly developed TSC medium, to derive bona fide TSCs. We show that these cells resemble placenta- and naive hPSC-derived TSCs, based on their transcriptome as well as their in vitro and in vivo differentiation potential. We conclude that primed hPSCs can be used to generate functional TSCs through a simple protocol, which can be applied to a widely available set of existing hPSCs, including induced pluripotent stem cells, derived from patients with known birth outcomes.

Nutrient Transporter Gene Expression in the Early Conceptus-Implications From Two Mouse Models of Diabetic Pregnancy

Maternal diabetes in early pregnancy increases the risk for birth defects in the offspring, particularly heart, and neural tube defects. While elevated glucose levels are characteristic for diabetic pregnancies, these are also accompanied by hyperlipidemia, indicating altered nutrient availability. We therefore investigated whether changes in the expression of nutrient transporters at the conception site or in the early post-implantation embryo could account for increased birth defect incidence at later developmental stages. Focusing on glucose and fatty acid transporters, we measured their expression by RT-PCR in the spontaneously diabetic non-obese mouse strain NOD, and in pregnant FVB/N mouse strain dams with Streptozotocin-induced diabetes. Sites of expression in the deciduum, extra-embryonic, and embryonic tissues were determined by RNAscope in situ hybridization. While maternal diabetes had no apparent effects on levels or cellular profiles of expression, we detected striking cell-type specificity of particular nutrient transporters. For examples, Slc2a2/Glut2 expression was restricted to the endodermal cells of the visceral yolk sac, while Slc2a1/Glut1 expression was limited to the mesodermal compartment; Slc27a4/Fatp4 and Slc27a3/Fatp3 also exhibited reciprocally exclusive expression in the endodermal and mesodermal compartments of the yolk sac, respectively. These findings not only highlight the significance of nutrient transporters in the intrauterine environment, but also raise important implications for the etiology of birth defects in diabetic pregnancies, and for strategies aimed at reducing birth defects risk by nutrient supplementation.

Bioengineered embryoids mimic post-implantation development in vitro

The difficulty of studying post-implantation development in mammals has sparked a flurry of activity to develop in vitro models, termed embryoids, based on self-organizing pluripotent stem cells. Previous approaches to derive embryoids either lack the physiological morphology and signaling interactions, or are unconducive to model post-gastrulation development. Here, we report a bioengineering-inspired approach aimed at addressing this gap. We employ a high-throughput cell aggregation approach to simultaneously coax mouse embryonic stem cells into hundreds of uniform epiblast-like aggregates in a solid matrix-free manner. When co-cultured with mouse trophoblast stem cell aggregates, the resulting hybrid structures initiate gastrulation-like events and undergo axial morphogenesis to yield structures, termed EpiTS embryoids, with a pronounced anterior development, including brain-like regions. We identify the presence of an epithelium in EPI aggregates as the major determinant for the axial morphogenesis and anterior development seen in EpiTS embryoids. Our results demonstrate the potential of EpiTS embryoids to study peri-gastrulation development in vitro.

EHF is essential for epidermal and colonic epithelial homeostasis, and suppresses Apc-initiated colonic tumorigenesis

Ets homologous factor (EHF) is a member of the epithelial-specific Ets (ESE) family of transcription factors. To investigate its role in development and epithelial homeostasis, we generated a series of novel mouse strains in which the Ets DNA-binding domain of Ehf was deleted in all tissues (Ehf-/-) or specifically in the gut epithelium. Ehf-/- mice were born at the expected Mendelian ratio, but showed reduced body weight gain, and developed a series of pathologies requiring most Ehf-/- mice to reach an ethical endpoint before reaching 1 year of age. These included papillomas in the facial skin, abscesses in the preputial glands (males) or vulvae (females), and corneal ulcers. Ehf-/-mice also displayed increased susceptibility to experimentally induced colitis, which was confirmed in intestinal-specific Ehf knockout mice. Gut-specific Ehf deletion also impaired goblet cell differentiation, induced extensive transcriptional reprogramming in the colonic epithelium and enhanced Apc-initiated adenoma development. The Ets DNA-binding domain of EHF is therefore essential for postnatal homeostasis of the epidermis and colonic epithelium, and its loss promotes colonic tumour development.

Integrating High-Throughput Approaches and in vitro Human Trophoblast Models to Decipher Mechanisms Underlying Early Human Placenta Development

The placenta is a temporary but pivotal organ for human pregnancy. It consists of multiple specialized trophoblast cell types originating from the trophectoderm of the blastocyst stage of the embryo. While impaired trophoblast differentiation results in pregnancy disorders affecting both mother and fetus, the molecular mechanisms underlying early human placenta development have been poorly understood, partially due to the limited access to developing human placentas and the lack of suitable human in vitro trophoblast models. Recent success in establishing human trophoblast stem cells and other human in vitro trophoblast models with their differentiation protocols into more specialized cell types, such as syncytiotrophoblast and extravillous trophoblast, has provided a tremendous opportunity to understand early human placenta development. Unfortunately, while high-throughput research methods and omics tools have addressed numerous molecular-level questions in various research fields, these tools have not been widely applied to the above-mentioned human trophoblast models. This review aims to provide an overview of various omics approaches that can be utilized in the study of human in vitro placenta models by exemplifying some important lessons obtained from omics studies of mouse model systems and introducing recently available human in vitro trophoblast model systems. We also highlight some key unknown questions that might be addressed by such techniques. Integrating high-throughput omics approaches and human in vitro model systems will facilitate our understanding of molecular-level regulatory mechanisms underlying early human placenta development as well as placenta-associated complications.

SUV39H2 controls trophoblast stem cell fate

BACKGROUND

The placenta is formed by the coordinated expansion and differentiation of trophoblast stem (TS) cells along a multi-lineage pathway. Dynamic regulation of histone 3 lysine 9 (H3K9) methylation is pivotal to cell differentiation for many cell lineages, but little is known about its involvement in trophoblast cell development.

METHODS

Expression of H3K9 methyltransferases was surveyed in rat TS cells maintained in the stem state and following differentiation. The role of suppressor of variegation 3-9 homolog 2 (SUV39H2) in the regulation of trophoblast cell lineage development was investigated using a loss-of-function approach in rat TS cells and ex vivo cultured rat blastocysts.

RESULTS

Among the twelve-known H3K9 methyltransferases, only SUV39H2 exhibited robust differential expression in stem versus differentiated TS cells. SUV39H2 transcript and protein expression were high in the stem state and declined as TS cells differentiated. Disruption of SUV39H2 expression in TS cells led to an arrest in TS cell proliferation and activation of trophoblast cell differentiation. SUV39H2 regulated H3K9 methylation status at loci exhibiting differentiation-dependent gene expression. Analyses of SUV39H2 on ex vivo rat blastocyst development supported its role in regulating TS cell expansion and differentiation. We further identified SUV39H2 as a downstream target of caudal type homeobox 2, a master regulator of trophoblast lineage development.

CONCLUSIONS

Our findings indicate that SUV39H2 contributes to the maintenance of TS cells and restrains trophoblast cell differentiation.

GENERAL SIGNIFICANCE

SUV39H2 serves as a contributor to the epigenetic regulation of hemochorial placental development.

Maintenance of mouse trophoblast stem cells in KSR-based medium allows conventional 3D culture

Mouse trophoblast stem cells (TSCs) can differentiate into trophoblast cells, which constitute the placenta. Under conventional culture conditions, in a medium supplemented with 20% fetal bovine serum (FBS), fibroblast growth factor 4 (FGF4), and heparin and in the presence of mouse embryonic fibroblast cells (MEFs) as feeder cells, TSCs maintain their undifferentiated, proliferative status. MEFs can be replaced by a 70% MEF-conditioned medium (MEF-CM) or by TGF-ß/activin A. To find out if KnockOut^TM Serum Replacement (KSR) can replace FBS for TSC maintenance, we cultured mouse TSCs in KSR-based, FBS-free medium and investigated their proliferation capacity, stemness, and differentiation potential. The results indicated that fibronectin, vitronectin, or laminin coating was necessary for adhesion of TSCs under KSR-based conditions but not for their survival or proliferation. While the presence of FGF4, heparin, and activin A was not sufficient to support the proliferation of TSCs, the addition of a pan-retinoic acid receptor inverse agonist and a ROCK-inhibitor yielded a proliferation rate comparable to that obtained under the conventional FBS-based conditions. TSCs cultured under the KSR-based conditions had a gene expression and DNA methylation profile characteristic of TSCs and exhibited a differentiation potential. Moreover, under KSR-based conditions, we could obtain a suspension culture of TSCs using extracellular matrix (ECM) coating-free dishes. Thus, we have established here, KSR-based culture conditions for the maintenance of TSCs, which should be useful for future studies.

OCT4 induces embryonic pluripotency via STAT3 signaling and metabolic mechanisms

OCT4 is a fundamental component of the molecular circuitry governing pluripotency in vivo and in vitro. To determine how OCT4 establishes and protects the pluripotent lineage in the embryo, we used comparative single-cell transcriptomics and quantitative immunofluorescence on control and OCT4 null blastocyst inner cell masses at two developmental stages. Surprisingly, activation of most pluripotency-associated transcription factors in the early mouse embryo occurs independently of OCT4, with the exception of the JAK/STAT signaling machinery. Concurrently, OCT4 null inner cell masses ectopically activate a subset of trophectoderm-associated genes. Inspection of metabolic pathways implicates the regulation of rate-limiting glycolytic enzymes by OCT4, consistent with a role in sustaining glycolysis. Furthermore, up-regulation of the lysosomal pathway was specifically detected in OCT4 null embryos. This finding implicates a requirement for OCT4 in the production of normal trophectoderm. Collectively, our findings uncover regulation of cellular metabolism and biophysical properties as mechanisms by which OCT4 instructs pluripotency.

Transcript assembly improves expression quantification of transposable elements in single-cell RNA-seq data

Transposable elements (TEs) are an integral part of the host transcriptome. TE-containing noncoding RNAs (ncRNAs) show considerable tissue specificity and play important roles during development, including stem cell maintenance and cell differentiation. Recent advances in single-cell RNA-seq (scRNA-seq) revolutionized cell type-specific gene expression analysis. However, effective scRNA-seq quantification tools tailored for TEs are lacking, limiting our ability to dissect TE expression dynamics at single-cell resolution. To address this issue, we established a TE expression quantification pipeline that is compatible with scRNA-seq data generated across multiple technology platforms. We constructed TE-containing ncRNA references using bulk RNA-seq data and showed that quantifying TE expression at the transcript level effectively reduces noise. As proof of principle, we applied this strategy to mouse embryonic stem cells and successfully captured the expression profile of endogenous retroviruses in single cells. We further expanded our analysis to scRNA-seq data from early stages of mouse embryogenesis. Our results illustrated the dynamic TE expression at preimplantation stages and revealed 146 TE-containing ncRNA transcripts with substantial tissue specificity during gastrulation and early organogenesis.

Unique features and emerging in vitro models of human placental development

Background

The placenta is an essential organ for the normal development of mammalian fetuses. Most of our knowledge on the molecular mechanisms of placental development has come from the analyses of mice, especially histopathological examination of knockout mice. Choriocarcinoma and immortalized cell lines have also been used for basic research on the human placenta. However, these cells are quite different from normal trophoblast cells.

Methods

In this review, we first provide an overview of mouse and human placental development with particular focus on the differences in the anatomy, transcription factor networks, and epigenetic characteristics between these species. Next, we discuss pregnancy complications associated with abnormal placentation. Finally, we introduce emerging in vitro models to study the human placenta, including human trophoblast stem (TS) cells, trophoblast and endometrium organoids, and artificial embryos.

Main findings

The placental structure and development differ greatly between humans and mice. The recent establishment of human TS cells and trophoblast and endometrial organoids enhances our understanding of the mechanisms underlying human placental development.

Conclusion

These in vitro models will greatly advance our understanding of human placental development and potentially contribute to the elucidation of the causes of infertility and other pregnancy complications.

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.

Modeling human trophoblast, the placental epithelium at the maternal fetal interface

Appropriate human trophoblast lineage specification and differentiation is crucial for the establishment of normal placentation and maintenance of pregnancy. However, due to the lack of proper modeling systems, the molecular mechanisms of these processes are still largely unknown. Much of the early studies in this area have been based on animal models and tumor-derived trophoblast cell lines, both of which are suboptimal for modeling this unique human organ. Recent advances in regenerative and stem cell biology methods have led to development of novel in vitro model systems for studying human trophoblast. These include derivation of human embryonic and induced pluripotent stem cells and establishment of methods for the differentiation of these cells into trophoblast, as well as the more recent derivation of human trophoblast stem cells. In addition, advances in culture conditions, from traditional two-dimensional monolayer culture to 3D culturing systems, have led to development of trophoblast organoid and placenta-on-a-chip model, enabling us to study human trophoblast function in context of more physiologically accurate environment. In this review, we will discuss these various model systems, with a focus on human trophoblast, and their ability to help elucidate the key mechanisms underlying placental development and function. This review focuses on model systems of human trophoblast differentiation, including advantages and limitations of stem cell-based culture, trophoblast organoid, and organ-on-a-chip methods and their applications in understanding placental development and disease.

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.

Atypical protein kinase C iota (PKCλ/ι) ensures mammalian development by establishing the maternal-fetal exchange interface

In utero mammalian development relies on the establishment of the maternal-fetal exchange interface, which ensures transportation of nutrients and gases between the mother and the fetus. This exchange interface is established via development of multinucleated syncytiotrophoblast cells (SynTs) during placentation. In mice, SynTs develop via differentiation of the trophoblast stem cell-like progenitor cells (TSPCs) of the placenta primordium, and in humans, SynTs are developed via differentiation of villous cytotrophoblast (CTB) progenitors. Despite the critical need in pregnancy progression, conserved signaling mechanisms that ensure SynT development are poorly understood. Herein, we show that atypical protein kinase C iota (PKCλ/ι) plays an essential role in establishing the SynT differentiation program in trophoblast progenitors. Loss of PKCλ/ι in the mouse TSPCs abrogates SynT development, leading to embryonic death at approximately embryonic day 9.0 (E9.0). We also show that PKCλ/ι-mediated priming of trophoblast progenitors for SynT differentiation is a conserved event during human placentation. PKCλ/ι is selectively expressed in the first-trimester CTBs of a developing human placenta. Furthermore, loss of PKCλ/ι in CTB-derived human trophoblast stem cells (human TSCs) impairs their SynT differentiation potential both in vitro and after transplantation in immunocompromised mice. Our mechanistic analyses indicate that PKCλ/ι signaling maintains expression of GCM1, GATA2, and PPARγ, which are key transcription factors to instigate SynT differentiation programs in both mouse and human trophoblast progenitors. Our study uncovers a conserved molecular mechanism, in which PKCλ/ι signaling regulates establishment of the maternal-fetal exchange surface by promoting trophoblast progenitor-to-SynT transition during placentation.

Esrrb function is required for proper primordial germ cell development in presomite stage mouse embryos

Estrogen related receptor beta (Esrrb) is an orphan nuclear receptor that is required for self-renewal and pluripotency in mouse embryonic stem (ES) cells. However, in the early post-implantation mouse embryo, Esrrb is specifically expressed in the extraembryonic ectoderm (ExE) and plays a crucial role in trophoblast development. Previous studies showed that Esrrb is also required to maintain trophoblast stem (TS) cells, the in vitro stem cell model of the early trophoblast lineage. In order to identify regulatory targets of Esrrb in vivo, we performed microarray analysis of Esrrb-null versus wild-type post-implantation ExE, and identified 30 genes down-regulated in Esrrb-mutants. Among them is Bmp4, which is produced by the ExE and known to be critical for primordial germ cell (PGC) specification in vivo. We further identified an enhancer region bound by Esrrb at the Bmp4 locus by performing Esrrb ChIP-seq and luciferase reporter assay using TS cells. Finally, we established a knockout mouse line in which the enhancer region was deleted using CRISPR/Cas9 technology. Both Esrrb-null embryos and enhancer knockout embryos expressed lower levels of Bmp4 in the ExE, and had reduced numbers of PGCs. These results suggested that Esrrb functions as an upstream factor of Bmp4 in the ExE, regulating proper PGC development in mice.

Super-enhancer-guided mapping of regulatory networks controlling mouse trophoblast stem cells

Trophectoderm (TE) lineage development is pivotal for proper implantation, placentation, and healthy pregnancy. However, only a few TE-specific transcription factors (TFs) have been systematically characterized, hindering our understanding of the process. To elucidate regulatory mechanisms underlying TE development, here we map super-enhancers (SEs) in trophoblast stem cells (TSCs) as a model. We find both prominent TE-specific master TFs (Cdx2, Gata3, and Tead4), and >150 TFs that had not been previously implicated in TE lineage, that are SE-associated. Mapping targets of 27 SE-predicted TFs reveals a highly intertwined transcriptional regulatory circuitry. Intriguingly, SE-predicted TFs show 4 distinct expression patterns with dynamic alterations of their targets during TSC differentiation. Furthermore, depletion of a subset of TFs results in dysregulation of the markers for specialized cell types in placenta, suggesting a role during TE differentiation. Collectively, we characterize an expanded TE-specific regulatory network, providing a framework for understanding TE lineage development and placentation.

Trophectoderm lineage development is essential for implantation, placentation, and healthy pregnancy. Here the authors map super-enhancers (SEs) in trophoblast stem cells and find both TE-specific master regulators and 150 previous uncharacterised transcription factors that are SE-associated, providing insight into trophectoderm-specific regulatory networks.

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.

Human placenta and trophoblast development: key molecular mechanisms and model systems

Abnormal placentation is considered as an underlying cause of various pregnancy complications such as miscarriage, preeclampsia and intrauterine growth restriction, the latter increasing the risk for the development of severe disorders in later life such as cardiovascular disease and type 2 diabetes. Despite their importance, the molecular mechanisms governing human placental formation and trophoblast cell lineage specification and differentiation have been poorly unravelled, mostly due to the lack of appropriate cellular model systems. However, over the past few years major progress has been made by establishing self-renewing human trophoblast stem cells and 3-dimensional organoids from human blastocysts and early placental tissues opening the path for detailed molecular investigations. Herein, we summarize the present knowledge about human placental development, its stem cells, progenitors and differentiated cell types in the trophoblast epithelium and the villous core. Anatomy of the early placenta, current model systems, and critical key regulatory factors and signalling cascades governing placentation will be elucidated. In this context, we will discuss the role of the developmental pathways Wingless and Notch, controlling trophoblast stemness/differentiation and formation of invasive trophoblast progenitors, respectively.

Hemochorial placentation: development, function, and adaptations

Placentation is a reproductive adaptation that permits fetal growth and development within the protected confines of the female reproductive tract. Through this important role, the placenta also determines postnatal health and susceptibility to disease. The hemochorial placenta is a prominent feature in primate and rodent development. This manuscript provides an overview of the basics of hemochorial placental development and function, provides perspectives on major discoveries that have shaped placental research, and thoughts on strategies for future investigation.

Embryonic Cul4b is important for epiblast growth and location of primitive streak layer cells

Cul4b-null (Cul4bΔ/Y) mice undergo growth arrest and degeneration during the early embryonic stages and die at E9.5. The pathogenic causes of this lethality remain incompletely characterized. However, it has been hypothesized that the loss of Cul4b function in extraembryonic tissues plays a key role. In this study, we investigated possible causes of death for Cul4b-null embryos, particularly in regard to the role of embryonic Cul4b. First, we show that the loss of embryonic Cul4b affects the growth of the inner cell mass in vitro and delays epiblast development during the gastrulation period at E6.5~E7.5 in vivo, as highlighted by the absence of the epiblastic transcription factor Brachyury from E6.5~E7.5. Additionally, at E7.5, strong and laterally expanded expression of Eomes and Fgf8 signaling was detected. Sectioning of these embryos showed disorganized primitive streak layer cells. Second, we observed that Mash2-expressing cells were present in the extraembryonic tissues of Cul4b-deficient embryos at E6.5 but were absent at E7.5. In addition, the loss of Cul4b resulted in decreased expression of cyclin proteins, which are required for the cell cycle transition from G1 to S. Taken together, these observations suggest that the embryonic expression of Cul4b is important for epiblast growth during E6.5~E7.5, and the loss of Cul4b results in either delayed growth of the epiblast or defective localization of primitive streak layer cells. As a result, the signaling activity mediated by the epiblast for subsequent ectoplacental cone development is affected, with the potential to induce growth retardation and lethality in Cul4bΔ/Y embryos.

The Differentiation Potency of Trophoblast Stem Cells from Mouse Androgenetic Embryos

In mice, trophoblast stem (TS) cells are derived from the polar trophectoderm of blastocysts. TS cells cultured in the presence of fibroblast growth factor 4 (Fgf4) are in an undifferentiated state and express undifferentiated marker genes such as Cdx2. After removing Fgf4 from the culture medium, TS cells drastically reduce the expression of undifferentiated marker genes, stop cell proliferation, and differentiate into all trophoblast cell subtypes. To clarify the roles of the parental genomes in placentation, we previously established TS cells from androgenetic embryos (AGTS cells). AGTS cells are in the undifferentiated state when cultured with Fgf4 and express undifferentiated marker genes. After removing Fgf4, AGTS cells differentiate into trophoblast giant cells (TGCs), but not into spongiotrophoblast cells, and some of the AGTS cells continue to proliferate. In this study, we investigated the differentiation potency of AGTS cells by analyzing the expression of undifferentiated marker genes and all trophoblast cell subtype-specific genes. After removing Fgf4, some undifferentiated marker genes (Cdx2, Eomes and Elf5) continued to be expressed. Interestingly, TGCs differentiated from AGTS cells also expressed Cdx2, but not Prl3d1. Moreover, the expression of Gcm1 and Synb was induced after the differentiation, indicating that AGTS cells preferentially differentiated into labyrinth progenitor cells. Cdx2 knockdown resulted in increased Prl3d1 expression, suggesting that Fgf4-independent Cdx2 expression inhibited the functional TGCs. Moreover, Fgf4-independent Cdx2 expression was activated by Gab1, one of the paternally expressed imprinted genes via the mitogen-activated protein kinase kinase (MEK)-extracellular signal regulated protein kinase (ERK) pathway. These results suggested that the paternal genome activates the MEK-ERK pathway without the Fgf4 signal, accelerates the differentiation into labyrinth progenitor cells and controls the function of TGCs.

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.

ELF3, ELF5, EHF and SPDEF Transcription Factors in Tissue Homeostasis and Cancer

The epithelium-specific ETS (ESE) transcription factors (ELF3, ELF5, EHF and SPDEF) are defined by their highly conserved ETS DNA binding domain and predominant epithelial-specific expression profile. ESE transcription factors maintain normal cell homeostasis and differentiation of a number of epithelial tissues, and their genetic alteration and deregulated expression has been linked to the progression of several epithelial cancers. Herein we review the normal function of the ESE transcription factors, the mechanisms by which they are dysregulated in cancers, and the current evidence for their role in cancer progression. Finally, we discuss potential therapeutic strategies for targeting or reactivating these factors as a novel means of cancer treatment.

Amniotic ectoderm expansion in mouse occurs via distinct modes and requires SMAD5-mediated signalling

Upon gastrulation, the mammalian conceptus transforms rapidly from a simple bilayer into a multilayered embryo enveloped by its extra-embryonic membranes. Impaired development of the amnion, the innermost membrane, causes major malformations. To clarify the origin of the mouse amnion, we used single-cell labelling and clonal analysis. We identified four clone types with distinct clonal growth patterns in amniotic ectoderm. Two main types have progenitors in extreme proximal-anterior epiblast. Early descendants initiate and expand amniotic ectoderm posteriorly, while descendants of cells remaining anteriorly later expand amniotic ectoderm from its anterior side. Amniogenesis is abnormal in embryos deficient in the bone morphogenetic protein (BMP) signalling effector SMAD5, with delayed closure of the proamniotic canal, and aberrant amnion and folding morphogenesis. Transcriptomics of individual Smad5 mutant amnions isolated before visible malformations and tetraploid chimera analysis revealed two amnion defect sets. We attribute them to impairment of progenitors of the two main cell populations in amniotic ectoderm and to compromised cuboidal-to-squamous transition of anterior amniotic ectoderm. In both cases, SMAD5 is crucial for expanding amniotic ectoderm rapidly into a stretchable squamous sheet to accommodate exocoelom expansion, axial growth and folding morphogenesis.

Comparative analysis of mouse and human placentae across gestation reveals species-specific regulators of placental development

An increasing body of evidence points to significant spatio-temporal differences in early placental development between mouse and human, but a detailed comparison of placentae in these two species is missing. We set out to compare placentae from both species across gestation, with a focus on trophoblast progenitor markers. We found that CDX2 and ELF5, but not EOMES, are expressed in early post-implantation trophoblast subpopulations in both species. Genome-wide expression profiling of mouse and human placentae revealed clusters of genes with distinct co-expression patterns across gestation. Overall, there was a closer fit between patterns observed in the placentae when the inter-species comparison was restricted to human placentae through gestational week 16 (thus, excluding full-term samples), suggesting that the developmental timeline in mouse runs parallel to the first half of human placental development. In addition, we identified VGLL1 as a human-specific marker of proliferative cytotrophoblast, where it is co-expressed with the transcription factor TEAD4. As TEAD4 is involved in trophectoderm specification in the mouse, we posit a regulatory role for VGLL1 in early events during human placental development.

Capturing and Interconverting Embryonic Cell Fates in a Dish

Cells of the early embryo are totipotent because they will differentiate to produce the fetus and its surrounding extraembryonic tissues. By contrast, embryonic stem (ES) cells are considered to be merely pluripotent because they lack the ability to efficiently produce extraembryonic cell types. The relatively limited developmental potential of ES cells can be explained by the observation that ES cells are derived from the embryo after its cells have already begun to specialize and lose totipotency. Meanwhile, at the time that pluripotent ES cell progenitors are specified, so are the multipotent progenitors of two extraembryonic stem cell types: trophoblast stem (TS) cells and extraembryonic endoderm stem (XEN) cells. Notably, all three embryo-derived stem cell types are capable of either self-renewing or differentiating in a lineage-appropriate manner. These three types of embryo-derived stem cell serve as paradigms for defining the genes and pathways that define and maintain unique stem cell identities. Remarkably, some of the mechanisms that maintain the specific developmental potential of each stem cell line do so by preventing conversion to another stem cell fate. This chapter highlights noteworthy studies that have identified the genes and pathways that normally limit the interconversion of stem cell identities.

GATA2/3-TFAP2A/C transcription factor network couples human pluripotent stem cell differentiation to trophectoderm with repression of pluripotency

To elucidate the molecular basis of BMP4-induced differentiation of human pluripotent stem cells (PSCs) toward progeny with trophectoderm characteristics, we produced transcriptome, epigenome H3K4me3, H3K27me3, and CpG methylation maps of trophoblast progenitors, purified using the surface marker APA. We combined them with the temporally resolved transcriptome of the preprogenitor phase and of single APA+ cells. This revealed a circuit of bivalent TFAP2A, TFAP2C, GATA2, and GATA3 transcription factors, coined collectively the "trophectoderm four" (TEtra), which are also present in human trophectoderm in vivo. At the onset of differentiation, the TEtra factors occupy multiple sites in epigenetically inactive placental genes and in OCT4 Functional manipulation of GATA3 and TFAP2A indicated that they directly couple trophoblast-specific gene induction with suppression of pluripotency. In accordance, knocking down GATA3 in primate embryos resulted in a failure to form trophectoderm. The discovery of the TEtra circuit indicates how trophectoderm commitment is regulated in human embryogenesis.

Epigenetic restriction of extraembryonic lineages mirrors the somatic transition to cancer

In mammals, the canonical somatic DNA methylation landscape is established upon specification of the embryo proper and subsequently disrupted within many cancer types. However, the underlying mechanisms that direct this genome-scale transformation remain elusive, with no clear model for its systematic acquisition or potential developmental utility. Here, we analysed global remethylation from the mouse preimplantation embryo into the early epiblast and extraembryonic ectoderm. We show that these two states acquire highly divergent genomic distributions with substantial disruption of bimodal, CpG density-dependent methylation in the placental progenitor. The extraembryonic epigenome includes specific de novo methylation at hundreds of embryonically protected CpG island promoters, particularly those that are associated with key developmental regulators and are orthologously methylated across most human cancer types. Our data suggest that the evolutionary innovation of extraembryonic tissues may have required co-option of DNA methylation-based suppression as an alternative to regulation by Polycomb-group proteins, which coordinate embryonic germ-layer formation in response to extraembryonic cues. Moreover, we establish that this decision is made deterministically, downstream of promiscuously used-and frequently oncogenic-signalling pathways, via a novel combination of epigenetic cofactors. Methylation of developmental gene promoters during tumorigenesis may therefore reflect the misappropriation of an innate trajectory and the spontaneous reacquisition of a latent, developmentally encoded epigenetic landscape.

From the stem of the placental tree: trophoblast stem cells and their progeny

Trophoblast stem cells (TSCs) retain the capacity to self-renew indefinitely and harbour the potential to differentiate into all trophoblast subtypes of the placenta. Recent studies have shown how signalling cascades integrate with transcription factor circuits to govern the fine balance between TSC self-renewal and differentiation. In addition, breakthroughs in reprogramming strategies have enabled the generation of TSCs from fibroblasts, opening up exciting new avenues that may allow the isolation of this stem cell type from other species, notably humans. Here, we review these recent advances in light of their importance for understanding placental pathologies and developing personalised medicine approaches for pregnancy complications.