ELF5-enforced transcriptional networks define an epigenetically regulated trophoblast stem cell compartment in the human placenta

Complete and unidirectional conversion of human embryonic stem cells to trophoblast by BMP4

Human ES cells (hESC) exposed to bone morphogenic protein 4 (BMP4) in the absence of FGF2 have become widely used for studying trophoblast development, but the soundness of this model has been challenged by others, who concluded that differentiation was primarily toward mesoderm rather than trophoblast. Here we confirm that hESC grown under the standard conditions on a medium conditioned by mouse embryonic fibroblasts in the presence of BMP4 and absence of FGF2 on a Matrigel substratum rapidly convert to an epithelium that is largely KRT7(+) within 48 h, with minimal expression of mesoderm markers, including T (Brachyury). Instead, they begin to express a series of trophoblast markers, including HLA-G, demonstrate invasive properties that are independent of the continued presence of BMP4 in the medium, and, over time, produce extensive amounts of human chorionic gonadotropin, progesterone, placental growth factor, and placental lactogen. This process of differentiation is not dependent on conditioning of the medium by mouse embryonic fibroblasts and is accelerated in the presence of inhibitors of Activin and FGF2 signaling, which at day 2 provide colonies that are entirely KRT7(+) and in which the majority of cells are transiently CDX2(+). Colonies grown on two chemically defined media, including the one in which BMP4 was reported to drive mesoderm formation, also differentiate at least partially to trophoblast in response to BMP4. The experiments demonstrate that the in vitro BMP4/hESC model is valid for studying the emergence and differentiation of trophoblasts.

Analysis of human embryos from zygote to blastocyst reveals distinct gene expression patterns relative to the mouse

Early mammalian embryogenesis is controlled by mechanisms governing the balance between pluripotency and differentiation. The expression of early lineage-specific genes can vary significantly between species, with implications for developmental control and stem cell derivation. However, the mechanisms involved in patterning the human embryo are still unclear. We analyzed the appearance and localization of lineage-specific transcription factors in staged preimplantation human embryos from the zygote until the blastocyst. We observed that the pluripotency-associated transcription factor OCT4 was initially expressed in 8-cell embryos at 3 days post-fertilization (dpf), and restricted to the inner cell mass (ICM) in 128-256 cell blastocysts (6dpf), approximately 2 days later than the mouse. The trophectoderm (TE)-associated transcription factor CDX2 was upregulated in 5dpf blastocysts and initially coincident with OCT4, indicating a lag in CDX2 initiation in the TE lineage, relative to the mouse. Once established, the TE expressed intracellular and cell-surface proteins cytokeratin-7 (CK7) and fibroblast growth factor receptor-1 (FGFR1), which are thought to be specific to post-implantation human trophoblast progenitor cells. The primitive endoderm (PE)-associated transcription factor SOX17 was initially heterogeneously expressed in the ICM where it co-localized with a sub-set of OCT4 expressing cells at 4-5dpf. SOX17 was progressively restricted to the PE adjacent to the blastocoel cavity together with the transcription factor GATA6 by 6dpf. We observed low levels of Laminin expression in the human PE, though this basement membrane component is thought to play an important role in mouse PE cell sorting, suggesting divergence in differentiation mechanisms between species. Additionally, while stem cell lines representing the three distinct cell types that comprise a mouse blastocyst have been established, the identity of cell types that emerge during early human embryonic stem cell derivation is unclear. We observed that derivation from plating intact human blastocysts resulted predominantly in the outgrowth of TE-like cells, which impairs human embryonic stem cell derivation. Altogether, our findings provide important insight into developmental patterning of preimplantation human embryos with potential consequences for stem cell derivation.

Review: Trophoblast differentiation from human embryonic stem cells

The human embryo is not a feasible experimental system for the detailed study of implantation and early placentation, so surrogate systems have been sought for investigating the determination of the trophectoderm lineage, its differentiation into trophoblasts of the early implantation site, and subsequently the morphogenesis of the definitive placenta. An alternative to the use of embryos for studying early placental development was revealed by work with human embryonic stem cells (hESC), demonstrating BMP2/4-stimulated trophoblast differentiation, and spontaneous formation from embryoid bodies (EBs). These cells display a trophoblastic transcriptome, as well as a placental protein and steroid hormone secretory profile, and invasive and chemotactic behavior resembling human placental trophoblasts. With EB-derived trophoblasts, two-dimensional and three-dimensional paradigms and other modifications of the culture environment, including extracellular matrix and aggregation with placental fibroblasts, impact on trophoblast differentiation. Recent studies have questioned the identity of the trophoblasts directed by BMP treatment of hESC, and careful attention to culture conditions is needed to interpret different results among research groups. Although the precise placental counterpart of the hESC-derived trophoblast remains unclear, hESC-derived trophoblasts remain an intriguing platform for modeling early implantation.

Embryonic stem cell-derived trophoblast differentiation: a comparative review of the biology, function, and signaling mechanisms

The development of the placenta is imperative for successful pregnancy establishment, yet the earliest differentiation events of the blastocyst-derived trophectoderm that forms the placenta remain difficult to study in humans. Human embryonic stem cells (hESC) display a unique ability to form trophoblast cells when induced to differentiate either by the addition of exogenous BMP4 or by the formation of cellular aggregates called embryoid bodies. While mouse trophoblast stem cells (TSC) have been isolated from blastocyst outgrowths, mouse ESC do not spontaneously differentiate into trophoblast cells. In this review, we focus on addressing the similarities and differences between mouse TSC differentiation and hESC-derived trophoblast differentiation. We discuss the functional and mechanistic diversity that is found in different species models. Of central importance are the unique signaling events that trigger downstream gene expression that create specific cellular fate decisions. We support the idea that we must understand the nuances that hESC differentiation models display so that investigators can choose the appropriate model system to fit experimental needs.

Homeobox genes and down-stream transcription factor PPARγ in normal and pathological human placental development

The placenta provides critical transport functions between the maternal and fetal circulations during intrauterine development. Formation of this interface is controlled by nuclear transcription factors including homeobox genes. Here we summarize current knowledge regarding the expression and function of homeobox genes in the placenta. We also describe the identification of target transcription factors including PPARγ, biological pathways regulated by homeobox genes and their role in placental development. The role of the nuclear receptor PPARγ, ligands and target genes in human placental development is also discussed. A better understanding of these pathways will improve our knowledge of placental cell biology and has the potential to reveal new molecular targets for the early detection and diagnosis of pregnancy complications including human fetal growth restriction.

5-Aza-dC treatment induces mesenchymal-to-epithelial transition in 1st trimester trophoblast cell line HTR8/SVneo

Placental trophoblast invasion involves a cellular transition from epithelial to mesenchymal phenotype. Cytotrophoblasts undergo epithelial to mesenchymal transition (EMT) when differentiating into extravillous trophoblasts and gaining the capacity of invasion. In this research, we investigated the role of DNA methylation in trophoblasts during this EMT. First, using BeWo and HTR8/SVneo cell lines as models of cytotrophoblasts and extravillous trophoblasts, respectively, we analyzed the gene expression and DNA methylation status of the known epithelial marker genes, E-Cadherin and Cytokeratin7. We found that, in HTR8/SVneo cells, both genes were silenced and their promoters hypermethylated, as compared with the high-level gene expression and promoter hypomethylation observed in BeWo cells. This result suggests that dynamic DNA methylation of epithelial marker genes plays a critical role in the trophoblast EMT process. To verify these results, we treated HTR8/SVneo cells with 5-aza-dC, a known inhibitor of DNA methyltransferase, for three days. Five-Aza-dC treatment significantly increased the expression of epithelial marker genes and slightly decreased the expression of mesenchymal genes, as detected by qRT-PCR, immunocytochemistry and Western blot. Furthermore, 5-aza-dC treated HTR8/SVneo cells changed their morphology from mesenchymal into epithelial phenotype, indicating that 5-aza-dC induced mesenchymal to epithelial transition. Lastly, we examined the effect of 5-aza-dC on trophoblast migration and invasion capacity. We applied 5-aza-dC to HTR8/SVneo cells in trans-well cell migration and invasion assays and found that 5-aza-dC treatment decreased trophoblast migration and invasion capacity. In conclusion, DNA methylation of epithelial marker genes represents a molecular mechanism for the process of trophoblast EMT.

Human trophoblast progenitors: where do they reside?

In humans, very little is known about the factors that regulate trophoblast (TB) specification, expansion of the initial TB population, and formation of the cytotrophoblast (CTB) populations that populate the chorionic villi. The absence of human trophoblast progenitor cell (hTPC) lines that can be propagated in vitro has been a limiting factor. Because attempts to derive TB stem cells from the trophectoderm of the human blastocyst have so far failed, investigators use alternative systems as cell culture models including TBs derived from human embryonic stem cells (hESCs), immortalized CTBs, and cell lines established from TB tumors. Additionally, the characteristics of mature TBs have been extensively studied using primary cultures of CTBs and explants of placental chorionic villi. However, none of these models can be used to study TB progenitor self-renewal and differentiation. Furthermore, the propagation of human TB progenitors from villous CTBs (vCTBs) has not been achieved. The downregulation of key markers of cell cycle progression in vCTBs by the end of the first trimester of pregnancy may indicate that these cells are not a source of human TB progenitors later in pregnancy. In contrast, mesenchymal cells of the villi and chorion continue to proliferate until the end of pregnancy. We recently reported isolation of continuously self-renewing hTPCs from chorionic mesenchyme and showed that they differentiated into the mature TB cell types of the villi, evidence that they can function as TB progenitors. This new cell culture model enables a molecular analysis of the seminal steps in human TB differentiation that have yet to be studied in humans. In turn, this information can be used to trace the origins of pregnancy complications that are associated with faulty TB growth and differentiation.

Trophoblast differentiation of human embryonic stem cells

Molecular mechanisms regulating human trophoblast differentiation remain poorly understood due to difficulties in obtaining primary tissues from very early developmental stages in humans. Therefore, the use of human embryonic stem cells (hESCs) as a source for generating trophoblast tissues is of significant interest. Trophoblast-like cells have been obtained through treatment of hESCs with bone morphogenetic protein (BMP) or inhibitors of activin/nodal/transforming growth factor-β signaling, or through protocols involving formation of embryoid bodies (EBs); however, there is controversy over whether hESC-derived cells are indeed analogous to true trophoblasts found in vivo. In this review, we provide an overview of previously described efforts to obtain trophoblasts from hESCs. We also discuss the merits and limitations of hESCs as a source of trophoblast derivatives.

Roles of CDX2 and EOMES in human induced trophoblast progenitor cells

Abnormal trophoblast lineage proliferation and differentiation in early pregnancy have been associated with the pathogenesis of placenta diseases of pregnancy. However, there is still a gap in understanding the molecular mechanisms of early placental development due to the limited primary trophoblast cultures and fidelity of immortalized trophoblast lines. Trophoblasts stem (TS) cells, an in vitro model of trophectoderm that can differentiate into syncytiotrophoblasts and extravillous trophoblasts, can be an attractive tool for early pregnancy research. TS cells are well established in mouse but not in humans due to insufficient knowledge of which trophoblast lineage-specific transcription factors are involved in human trophectoderm (TE) proliferation and differentiation. Here, we applied induced pluripotent stem cell technique to investigate the human trophoblast lineage-specific transcription factors. We established human induced trophoblast progenitor (iTP) cells by direct reprogramming the fibroblasts with a pool of mouse trophoblast lineage-specific transcription factors consisting of CDX2, EOMES, and ELF5. The human iTP cells exhibit epithelial morphology and can be maintained in vitro for more than 2 months. Gene expression profile of these cells was tightly clustered with human trophectoderm but not with human neuron progenitor cells, mesenchymal stem cells, or endoderm cells. These cells are capable of differentiating into cells with an invasive capacity, suggesting extravillous trophoblasts. They also form multi-nucleated cells which secrete human chorionic gonadotropin and estradiol, consistent with a syncytiotrophoblast phenotype. Our results provide the evidence that transcription factors CDX2 and EOMES may play critical roles in human iTP cell generation.

ELF5 suppresses estrogen sensitivity and underpins the acquisition of antiestrogen resistance in luminal breast cancer

The transcription factor ELF5 is responsible for gene expression patterning underlying molecular subtypes of breast cancer and may mediate acquired resistance to anti-estrogen therapy.

We have previously shown that during pregnancy the E-twenty-six (ETS) transcription factor ELF5 directs the differentiation of mammary progenitor cells toward the estrogen receptor (ER)-negative and milk producing cell lineage, raising the possibility that ELF5 may suppress the estrogen sensitivity of breast cancers. To test this we constructed inducible models of ELF5 expression in ER positive luminal breast cancer cells and interrogated them using transcript profiling and chromatin immunoprecipitation of DNA followed by DNA sequencing (ChIP-Seq). ELF5 suppressed ER and FOXA1 expression and broadly suppressed ER-driven patterns of gene expression including sets of genes distinguishing the luminal molecular subtype. Direct transcriptional targets of ELF5, which included FOXA1, EGFR, and MYC, accurately classified a large cohort of breast cancers into their intrinsic molecular subtypes, predicted ER status with high precision, and defined groups with differential prognosis. Knockdown of ELF5 in basal breast cancer cell lines suppressed basal patterns of gene expression and produced a shift in molecular subtype toward the claudin-low and normal-like groups. Luminal breast cancer cells that acquired resistance to the antiestrogen Tamoxifen showed greatly elevated levels of ELF5 and its transcriptional signature, and became dependent on ELF5 for proliferation, compared to the parental cells. Thus ELF5 provides a key transcriptional determinant of breast cancer molecular subtype by suppression of estrogen sensitivity in luminal breast cancer cells and promotion of basal characteristics in basal breast cancer cells, an action that may be utilised to acquire antiestrogen resistance.

The molecular subtypes of breast cancer are distinguished by their intrinsic patterns of gene expression and can be used to group patients with different prognoses and treatment options. Although molecular subtyping tests are currently under evaluation, some of them are already in use to better tailor therapy for patients; however, the molecular events that are responsible for these different patterns of gene expression in breast cancer are largely undefined. The elucidation of their mechanistic basis would improve our understanding of the disease process and enhance the chances of developing better predictive and prognostic markers, new therapies, and interventions to overcome resistance to existing therapies. Here, we show that the transcription factor ELF5 is responsible for much of the patterning of gene expression that distinguishes the breast cancer subtypes. Additionally, our data suggest that ELF5 may also be involved in the development of resistance to therapies designed to stop estrogen stimulation of breast cancer. These effects of ELF5 appear to represent a partial carryover into breast cancer of its normal role in the mammary gland, where it is responsible for the development of milk-producing structures during pregnancy.

Regulatory pathways controlling the endovascular invasive trophoblast cell lineage

Hemochorial placentation is characterized by trophoblast-directed uterine spiral artery remodeling. The rat and human both possess hemochorial placentation and exhibit remarkable similarities regarding the depth of trophoblast invasion and the extent of uterine vascular modification. In vitro and in vivo research methodologies have been established using the rat as an animal model to investigate the extravillous/invasive trophoblast lineage. With these research approaches, two signaling pathways controlling the differentiation and invasion of the trophoblast cell lineage have been identified: i) hypoxia/hypoxia inducible factor and ii) phosphatidylinositol 3-kinase/AKT/Fos like antigen 1. Dissection of these pathways has facilitated identification of fundamental regulators of the invasive trophoblast cell lineage.

Model systems for studying trophoblast differentiation from human pluripotent stem cells

This review focuses on a now well-established model for generating cells of the trophoblast (TB) lineage by treating human embryonic stem cells (ESC) and induced pluripotent stem cells (iPSC) with the growth factor BMP4. We first discuss the opposing roles of FGF2 and BMP4 in directing TB formation and the need to exclude the former from the growth medium to minimize the co-induction of mesoderm and endoderm. Under these conditions, there is up-regulation of several transcription factors implicated in TB lineage emergence within 3 h of BMP4 exposure and, over a period of days and especially under a high O(2) gas atmosphere, gradual appearance of cell types carrying markers for more differentiated TB cell types, including extravillous TB and syncytioTB. We describe the potential value of including low molecular weight pharmaceutical agents that block activin A (INHBA) and FGF2 signaling to support BMP4-directed differentiation. We contend that the weight of available evidence supports the contention that BMP4 converts human ESC and iPSC of the so-called epiblast type unidirectionally to TB. We also consider the argument that BMP4 treatment of human ESC in the absence of exogenous FGF2 leads only to the emergence of mesoderm derivatives to be seriously flawed. Instead, we propose that, when signaling networks supporting pluripotency ESC or iPSC become unsustainable and when specification towards extra-embryonic mesoderm and endoderm are rendered inoperative, TB emerges as a major default state to pluripotency.

Distinct lineage specification roles for NANOG, OCT4, and SOX2 in human embryonic stem cells

Nanog, Oct4, and Sox2 are the core regulators of mouse (m)ESC pluripotency. Although their basic importance in human (h)ESCs has been demonstrated, the mechanistic functions are not well defined. Here, we identify general and cell-line-specific requirements for NANOG, OCT4, and SOX2 in hESCs. We show that OCT4 regulates, and interacts with, the BMP4 pathway to specify four developmental fates. High levels of OCT4 enable self-renewal in the absence of BMP4 but specify mesendoderm in the presence of BMP4. Low levels of OCT4 induce embryonic ectoderm differentiation in the absence of BMP4 but specify extraembryonic lineages in the presence of BMP4. NANOG represses embryonic ectoderm differentiation but has little effect on other lineages, whereas SOX2 and SOX3 are redundant and repress mesendoderm differentiation. Thus, instead of being panrepressors of differentiation, each factor controls specific cell fates. Our study revises the view of how self-renewal is orchestrated in hESCs.

Health during pregnancy and beyond: Fetal trophoblast cells as chief co-ordinators of intrauterine growth and reproductive success

Abstract Differentiation of extra-embryonic tissues and organs, notably the placenta, is vital for embryonic development and growth throughout gestation, starting from a few days after fertilization when the trophoblast cell lineage arises until parturition. In utero metabolic programming events may even extend the impact of placental function well into adulthood as they may predispose the offspring to common pathologies such as diabetes and cardiovascular disease. This review summarizes key steps that lead up to formation of a functional placenta. It highlights recent insights that have advanced our view of how early trophoblast expansion is achieved and how sufficient maternal blood supply to the developing fetus is secured. Exciting cumulative data have revealed the importance of a close cross-talk between the embryo proper and extra-embryonic trophoblast cells that involves extracellular matrix components in the establishment of a stem cell-like niche and proliferation compartment. Remarkably, placental function also relies on beneficial interactions between trophoblast cells and maternal immune cells at the implantation site. Our growing knowledge of the molecular mechanisms involved in trophoblast differentiation and function will help to devise informed approaches aimed at deciphering how placentation is controlled in humans as an essential process for reproductive success and long-term health.

Elf5, hormones and cell fate

Recent elucidation of the stem and progenitor cell hierarchies that operate during normal tissue and organ development has provided a foundation for the development of new insights into the disease process. These hierarchies are established by genetic mechanisms, which specify and determine cell fate and act as cell-clade gatekeepers, upon which all multicellular organisms depend for viability. Perturbation of this gatekeeper function characterizes developmentally based diseases, such as cancer. Here, the emerging gatekeeper and master regulatory roles of the ETS transcription factor Elf5 in several diverse developmental scenarios is reviewed, and how this function intersects with hormonal and growth factor mediated regulation of these processes is shown.

A placenta for life

The chorioallantoic placenta is the defining organ of eutherians that has enabled prolonged intrauterine gestation. As such, normal placental development and function are essential for mammalian reproductive success. Reflecting the key role of this organ in providing nutrients to the embryo, the characteristic cell type that forms substantial parts of the placenta is called 'trophoblast' (from Greek trephein 'to feed' and blastos 'germinator'). However, in addition to regulating nutrient supply, the placenta also exerts a number of other pivotal functions that highlight the importance of normal trophoblast differentiation for a successful pregnancy. In this guest symposium, 'Trophoblast Development', several contributors summarize insights gained from recent studies in the mouse that have advanced our understanding of trophoblast biology. This includes how the earliest trophoblast cells are set aside to expand in a stem- or progenitor-cell compartment under tight genetic and epigenetic control and how subsequent differentiation into the various placental cell types is controlled to ensure normal placentation. The relevance of these contributions range from early developmental cell fate decisions, stem cell biology and placental development for healthy pregnancy to the impact of placental failures on long-term health, with important clinical implications for assisted reproductive technology procedures and pregnancy-associated complications.

BRACHYURY and CDX2 mediate BMP-induced differentiation of human and mouse pluripotent stem cells into embryonic and extraembryonic lineages

BMP is thought to induce hESC differentiation toward multiple lineages including mesoderm and trophoblast. The BMP-induced trophoblast phenotype is a long-standing paradox in stem cell biology. Here we readdressed BMP function in hESCs and mouse epiblast-derived cells. We found that BMP4 cooperates with FGF2 (via ERK) to induce mesoderm and to inhibit endoderm differentiation. These conditions induced cells with high levels of BRACHYURY (BRA) that coexpressed CDX2. BRA was necessary for and preceded CDX2 expression; both genes were essential for expression not only of mesodermal genes but also of trophoblast-associated genes. Maximal expression of the latter was seen in the absence of FGF but these cells coexpressed mesodermal genes and moreover they differed in cell surface and epigenetic properties from placental trophoblast. We conclude that BMP induces human and mouse pluripotent stem cells primarily to form mesoderm, rather than trophoblast, acting through BRA and CDX2.

The role of transcription factor Tcfap2c/TFAP2C in trophectoderm development

In recent years, knowledge regarding the genetic and epigenetic programmes governing specification, maintenance and differentiation of the extraembryonic lineage has advanced substantially. Establishment and analysis of mice deficient in genes implicated in trophoblast lineage and the option to generate and manipulate murine stem cell lines from the inner cell mass and the trophectoderm in vitro represent major advances. The activating enhancer binding protein 2 (AP2) family of transcription factors is expressed during mammalian development and in certain malignant diseases. This article summarizes the data regarding expression and function of murine Tcfap2 and human TFAP2 in extraembryonic development and differentiation. It also presents a model integrating Tcfap2c into the framework of trophoblast development and highlights the requirement of Tcfap2c to maintain trophoblast stem cells. With regard to human trophoblast cell-lineage restriction, the role of TFAP2C in lineage specification and maintenance is speculated upon. Furthermore, an overview of target genes of AP2 in mouse and human affecting placenta development and function is provided and the evidence suggesting that defects in regulating TFAP2 members might contribute to placental defects is discussed.

Trophoblast development

This review summarises current knowledge about the specification, commitment and maintenance of the trophoblast lineage in mice and cattle. Results from gene expression studies, in vivo loss-of-function models and in vitro systems using trophoblast and embryonic stem cells have been assimilated into a model seeking to explain trophoblast ontogeny via gene regulatory networks. While trophoblast differentiation is quite distinct between cattle and mice, as would be expected from their different modes of implantation, recent studies have demonstrated that differences arise much earlier during trophoblast development.

Rat placentation: an experimental model for investigating the hemochorial maternal-fetal interface

The rat possesses hemochorial placentation with deep intrauterine trophoblast cell invasion and trophoblast-directed uterine spiral artery remodeling; features shared with human placentation. Recognition of these similarities spurred the establishment of in vitro and in vivo research methods using the rat as an animal model to address mechanistic questions regarding development of the hemochorial placenta. The purpose of this review is to provide the requisite background to help move the rat to the forefront in placentation research.

Epigenetic features of the mouse trophoblast

Trophoblast cells are required for the growth and survival of the fetus during pregnancy, and failure to maintain appropriate trophoblast regulation is associated with placental insufficiencies and intrauterine growth restriction. Development of the trophoblast lineage is mediated by interactions between genetic and epigenetic factors. This review will focus on new insights that have been gained from analysis of mouse models into the epigenetic mechanisms that are required for the early establishment of the trophoblast lineage and for the development of specialized cell types of the fetal placenta. In particular, the importance of DNA methylation, 5-hydroxymethylcytosine and histone modifications in orchestrating trophoblast gene expression and functional outcome will be discussed. These insights are beginning to be extended towards human studies and initial results suggest that the causes and consequences of a variety of placental pathologies are related to epigenetic processes. Furthermore, the epigenetic landscape that regulates trophoblast cells seems to be particularly vulnerable to perturbation during development. This has major implications for diet and other environmental factors during pregnancy.

Establishment of human trophoblast progenitor cell lines from the chorion

Placental trophoblasts are key determinants of in utero development. Mouse trophoblast (TB) stem cells, which were first derived over a decade ago, are a powerful cell culture model for studying their self-renewal or differentiation. Our attempts to isolate an equivalent population from the trophectoderm of human blastocysts generated colonies that quickly differentiated in vitro. This finding suggested that the human placenta has another progenitor niche. Here, we show that the chorion is one such site. Initially, we immunolocalized pluripotency factors and TB fate determinants in the early gestation placenta, amnion, and chorion. Immunoreactive cells were numerous in the chorion. We isolated these cells and plated them in medium containing fibroblast growth factor which is required for human embryonic stem cell self-renewal, and an inhibitor of activin/nodal signaling. Colonies of polarized cells with a limited lifespan emerged. Trypsin dissociation yielded continuously self-replicating monolayers. Colonies and monolayers formed the two major human TB lineages-multinucleate syncytiotrophoblasts and invasive cytotrophoblasts (CTBs). Transcriptional profiling experiments revealed the factors associated with the self-renewal or differentiation of human chorionic TB progenitor cells (TBPCs). They included imprinted genes, NR2F1/2, HMGA2, and adhesion molecules that were required for TBPC differentiation. Together, the results of these experiments suggested that the chorion is one source of epithelial CTB progenitors. These findings explain why CTBs of fully formed chorionic villi have a modest mitotic index and identify the chorionic mesoderm as a niche for TBPCs that support placental growth.

Isolation and characterization of human trophoblast side-population (SP) cells in primary villous cytotrophoblasts and HTR-8/SVneo cell line

Recently, numerous studies have identified that immature cell populations including stem cells and progenitor cells can be found among “side-population” (SP) cells. Although SP cells isolated from some adult tissues have been reported elsewhere, isolation and characterization of human trophoblast SP remained to be reported. In this study, HTR-8/SVneo cells and human primary villous cytotrophoblasts (vCTBs) were stained with Hoechst 33342 and SP and non-SP (NSP) fractions were isolated using a cell sorter. A small population of SP cells was identified in HTR-8/SVneo cells and in vCTBs. SP cells expressed several vCTB-specific markers and failed to express syncytiotrophoblast (STB) or extravillous cytotrophopblast (EVT)-specific differentiation markers. SP cells formed colonies and proliferated on mouse embryonic fibroblast (MEF) feeder cells or in MEF conditioned medium supplemented with heparin/FGF2, and they also showed long-term repopulating property. SP cells could differentiate into both STB and EVT cell lineages and expressed several differentiation markers. Microarray analysis revealed that IL7R and IL1R2 were exclusively expressed in SP cells and not in NSP cells. vCTB cells sorted as positive for both IL7R and IL1R2 failed to express trophoblast differentiation markers and spontaneously differentiated into both STB and EVT in basal medium. These features shown by the SP cells suggested that IL7R and IL1R2 are available as markers to detect the SP cells and that vCTB progenitor cells and trophoblast stem cells were involved in the SP cell population.

Calcium signaling in placenta

The placenta sustains the developing fetus throughout gestation and its major functions include nutrition, gas and waste exchange via a variety of passive or active mechanisms. Up to 30 g of calcium (Ca(2+)) actively crosses the trophoblast layer during human pregnancy. The Ca(2+) ion not only plays an important role for skeletal development but is also an essential second messenger. This review is intended to highlight the implications of Ca(2+) signaling during reproduction and specifically placentation. Initially, a Ca(2+) wave induces fertilization of the oocyte. The intracellular Ca(2+) concentration is key for the blastocyst implantation, proper placental development and function. Current knowledge of many proteins involved in placental Ca(2+) regulation and their function in pathologic conditions is largely limited. Recent studies, however, point to alterations in Ca(2+) homeostasis in placental pathologies such as pre-eclampsia (PE) and intrauterine growth restriction (IUGR). A broader understanding of the role of Ca(2+) signaling during human reproduction may offer insight into impaired pregnancy outcomes.

FGF4-dependent stem cells derived from rat blastocysts differentiate along the trophoblast lineage

Differentiated trophoblast cell lineages arise from trophoblast stem (TS) cells. To date such a stem cell population has only been established in the mouse. The objective of this investigation was to establish TS cell populations from rat blastocysts. Blastocysts were cultured individually on a feeder layer of rat embryonic fibroblasts (REFs) in fibroblast growth factor-4 (FGF4) and heparin supplemented culture medium. Once cell colonies were established REF feeder layers could be replaced with REF conditioned medium. The blastocyst-derived cell lines, in either proliferative or differentiated states, did not express genes indicative of ICM-derived tissues. In the proliferative state the cells expressed established stem cell-associated markers of TS cells. Cells ceased proliferation and differentiated when FGF4, heparin, and REF conditioned medium were removed. Differentiation was characterized by a decline of stem cell-associated marker gene expression, the appearance of large polyploid cells (trophoblast giant cells), and the expression of trophoblast differentiation-associated genes. Collectively, the data indicate that the rat blastocyst-derived cell lines not only possess many features characteristic of mouse TS cells but also possess some distinct properties. These rat TS cell lines represent valuable new in vitro models for analyses of mechanisms controlling TS cell renewal and differentiation.

Human trophoblast in trisomy 21: a model for cell-cell fusion dynamic investigation

Trophoblastic cell fusion is one essential step of the human trophoblast differentiation leading to formation of the syncytiotrophoblast, site of the numerous placental functions. This process is multifactorial and finely regulated. Using the physiological model of primary culture of trophoblastic cells isolated from human placenta, we have identified different membrane proteins directly involved in trophoblastic cell fusion: connexin 43, ZO-1 and recently syncytins. These fusogenic membrane retroviral envelop glycoproteins: syncytin-1 (encoded by the HERV-W gene) and syncytin-2 (encoded by the FRD gene) and their receptors are major factors involved in human placental development. Disturbances of syncytiotrophoblast formation are observed in trisomy 21-affected placentas. Overexpression of the copper/zinc superoxide dismutase (SOD-1), encoded by chromosome 21 as well as an abnormal hCG signaling are implicated in the defect of syncytiotrophoblast formation. This abnormal trophoblast fusion and differentiation in trisomy 21-affected placenta is reversible in vitro by different ways.

Mitochondrial inhibition during preimplantation embryogenesis shifts the transcriptional profile of fetal mouse brain

Environmental stress results in perturbations to mitochondrial function in the preimplantation embryo and hinders subsequent embryo and possibly offspring development. Global gene expression in fetal mouse brain was investigated following targeted mitochondrial inhibition by amino-oxyacetate (AOA) from the 2-cell to the blastocyst stage. Blastocysts were transferred to pseudopregnant recipients and RNA extracted from Day 18 fetal brains for microarray interrogation. Exposure to 5 μM AOA during preimplantation embryo development induced differential expression of 166 genes (>1.25 fold) in the fetal brain, relative to control medium-cultured embryos. Altered expression pathways included carbohydrate metabolism, neurological development, cellular proliferation and death, DNA replication, recombination and repair. Of 28 genes exhibiting the greatest change in expression, qPCR confirmed that 16 were significantly altered. Targeted qPCR assessment of a further 20 genes associated with methylation, acetylation and mitochondrial dysfunction revealed that three were significantly altered (Immp1l, Nars2, Sat2) and Dmap1 exhibited a sex-specific response to AOA exposure. Only 2/48 genes had significantly altered expression by qPCR (Nola3, Timm8b) in fetal brains exposed to 50 μM AOA embryo culture, excluding an AOA dose-dependent response. It was concluded that perturbation of mitochondrial function induced by 5 μM AOA during preimplantation embryo development alters gene expression in the neonatal brain in a manner that suggests that proper brain development may be compromised.

Characterization of invasive trophoblasts generated from human embryonic stem cells

BACKGROUND

Abnormal human embryo implantation leads to poor foetal development and miscarriage, or pre-eclampsia. Ethical and practical considerations concerning implantation limit its investigation, and it is often difficult to extrapolate findings in laboratory animals when implantation processes show diverse species differences. Therefore, it is important to develop new in vitro models to study the earliest events of human implantation. The aim of this study was to derive trophoblast cell lines from human embryonic stem cells (hESCs) by a robust protocol and co-culture of these cells with an established endometrial cell culture system to validate a model of trophoblast invasion at implantation.

METHODS

Derivation of trophoblast cell lines from hESC lines was established by spontaneous and induced differentiation of embryoid bodies and by initial measurement of hCGβ secretion by enzyme-linked immunosorbent assay and their phenotype investigated using gene- and protein-expression markers. Vesicles formed from an aggregating trophoblast were co-cultured with decidualized human endometrial stromal cells in hypoxic (2% oxygen) and normoxic (20% oxygen) environments.

RESULTS

Derived villous cytotrophoblast cell (CTB) lines further differentiated to invasive, extra-villous CTBs. Eventually, cells lost their proliferative capacity, with some lines acquiring karyotypic changes, such as a gain in the X chromosome. Cell-invasion assays confirmed that the extra-villous CTBs were invasive and erosion of the endometrial stromal layer occurred, particularly under hypoxic conditions in vitro.

CONCLUSIONS

Trophoblast cell lines derived from hESCs differentiate and adapt in vitro and can be used as a model to study the mechanisms of early trophoblast invasion.

Trophoblast stem cells

Trophoblast stem cells (TSC) are the precursors of the differentiated cells of the placenta. In the mouse, TSC can be derived from outgrowths of either blastocyst polar trophectoderm (TE) or extraembryonic ectoderm (ExE), which originates from polar TE after implantation. The mouse TSC niche appears to be located within the ExE adjacent to the epiblast, on which it depends for essential growth factors, but whether this cellular architecture is the same in other species remains to be determined. Mouse TSC self-renewal can be sustained by culture on mitotically inactivated feeder cells, which provide one or more factors related to the NODAL pathway, and a medium supplemented with FGF4, heparin, and fetal bovine serum. Repression of the gene network that maintains pluripotency and emergence of the transcription factor pathways that specify a trophoblast (TR) fate enables TSC derivation in vitro and placental formation in vivo. Disrupting the pluripotent network of embryonic stem cells (ESC) causes them to default to a TR ground state. Pluripotent cells that have acquired sublethal chromosomal alterations may be sequestered into TR for similar reasons. The transition from ESC to TSC, which appears to be unidirectional, reveals important aspects of initial fate decisions in mice. TSC have yet to be derived from domestic species in which remarkable TR growth precedes embryogenesis. Recent derivation of TSC from blastocysts of the rhesus monkey suggests that isolation of the human equivalents may be possible and will reveal the extent to which mechanisms uncovered by using animal models are true in our own species.

Genome-wide expression profile of first trimester villous and extravillous human trophoblast cells

We have examined the transcriptional changes associated with differentiation from villous to extravillous trophoblast using a whole genome microarray. Villous trophoblast (VT) is in contact with maternal blood and mediates nutrient exchange whereas extravillous trophoblast (EVT) invades the decidua and remodels uterine arteries. Using highly purified first trimester trophoblast we identified over 3000 transcripts that are differentially expressed. Many of these transcripts represent novel functions and pathways that show co-ordinated up-regulation in VT or EVT. In addition we identify new players in established functions such as migration, immune modulation and cytokine or angiogenic factor secretion by EVT. The transition from VT to EVT is also characterised by alterations in transcription factors such as STAT4 and IRF9, which may co-ordinate these changes. Transcripts encoding several members of the immunoglobulin-superfamily, which are normally expressed on leukocytes, were highly transcribed in EVT but not expressed as protein, indicating specific control of translation in EVT. Interactions of trophoblast with decidual leukocytes are involved in regulating EVT invasion. We show that decidual T-cells, macrophages and NK cells express the inhibitory collagen receptor LAIR-1 and that EVT secrete LAIR-2, which can block this interaction. This represents a new mechanism by which EVT can modulate leukocyte function in the decidua. Since LAIR-2 is detectable in the urine of pregnant, but not non-pregnant women, trophoblast-derived LAIR-2 may also have systemic effects during pregnancy.

Regulation of early trophoblast differentiation - lessons from the mouse

The earliest stages of trophoblast differentiation are of tremendous importance to mediate implantation and to lay the anatomical foundations for normal placental development and function throughout gestation. Yet our molecular insights into these early developmental processes in humans have been limited by the inaccessibility of material and the unavailability of trophoblast cell lines that fully recapitulate the behaviour of early placental trophoblast. In this review we highlight recent advances that have come from the study of distinct stem cell types representative of the embryonic and extraembryonic lineages in the mouse, and from the study of mouse mutants. These models have revealed the presence of intricate transcriptional networks that are set up by signalling pathways, translating extracellular growth factor and cell positional information into distinct lineage-specific transcriptional programmes. The trophoblast specificity of these networks is ensured by epigenetic mechanisms including DNA methylation and histone modifications that complement each other to define trophoblast cell fate and differentiation. Despite the anatomical differences between mouse and human placentas, it seems that important aspects of early trophoblast specification are conserved between both species. Thus we may be able to build on our insights from the mouse to better understand early trophoblast differentiation in the human conceptus which is important for improving assisted reproductive technologies and may enable us in the future to derive human trophoblast stem cell lines. These advances will facilitate the investigation of genetic, epigenetic and environmental influences on early trophoblast differentiation in normal as well as in pathological conditions.