Regulation of early trophoblast differentiation - lessons from the mouse

KAT8-mediated H4K16ac is essential for sustaining trophoblast self-renewal and proliferation via regulating CDX2

Abnormal trophoblast self-renewal and differentiation during early gestation is the major cause of miscarriage, yet the underlying regulatory mechanisms remain elusive. Here, we show that trophoblast specific deletion of Kat8, a MYST family histone acetyltransferase, leads to extraembryonic ectoderm abnormalities and embryonic lethality. Employing RNA-seq and CUT&Tag analyses on trophoblast stem cells (TSCs), we further discover that KAT8 regulates the transcriptional activation of the trophoblast stemness marker, CDX2, via acetylating H4K16. Remarkably, CDX2 overexpression partially rescues the defects arising from Kat8 knockout. Moreover, increasing H4K16ac via using deacetylase SIRT1 inhibitor, EX527, restores CDX2 levels and promoted placental development. Clinical analysis shows reduced KAT8, CDX2 and H4K16ac expression are associated with recurrent pregnancy loss (RPL). Trophoblast organoids derived from these patients exhibit impaired TSC self-renewal and growth, which are significantly ameliorated with EX527 treatment. These findings suggest the therapeutic potential of targeting the KAT8-H4K16ac-CDX2 axis for mitigating RPL, shedding light on early gestational abnormalities.

Directed Differentiation of Human Pluripotent Stem Cells to Cytotrophoblast and Syncytiotrophoblast

Human pluripotent stem cells (hPSCs) form an ideal system to study the formation of placental cells, from an undifferentiated human embryonic stem cell state. The conventional human in vitro model systems to study the human placenta cannot be employed for understanding placental dysfunctions or the development of specialized placental cell types. Hence, human PSCs make an ideal model system to study human placental development and disorders. Here, we describe an efficient and validated protocol to reproducibly study the formation of human cytotrophoblasts (CTBs) and syncytiotrophoblast (STBs) from undifferentiated hPSCs. CTBs are the trophoblast stem cells that can differentiate into specialized placental cell types such as STBs. The multinucleated STB plays vital role in the exchange of nutrients and gases across the placenta and secretes several hormones during pregnancy, such as human chorionic gonadotropin β (hCGβ). Here we describe two methods of seeding the hPSCs: chemical (clumps method) and enzymatic methods (single cells) to differentiate them to CTB and STB, activating BMP (B) signaling and inhibiting ACTIVIN/NODAL and FGF signaling pathways (2i), thus naming our protocol as "B2i" (Sudheer et al., Stem Cells Dev 21:2987-3000, 2012). This protocol forms the perfect model system for understanding in vitro placentation, modeling diseases arising from abnormal placentation that cause complications such as miscarriage, preeclampsia or intrauterine growth restriction (IUGR), and drug discovery for placental disorders.

Trophoblast Differentiation: Mechanisms and Implications for Pregnancy Complications

Placental development is a tightly controlled event, in which cell expansion from the trophectoderm occurs in a spatiotemporal manner. Proper trophoblast differentiation is crucial to the vitality of this gestational organ. Obstructions to its development can lead to pregnancy complications, such as preeclampsia, fetal growth restriction, and preterm birth, posing severe health risks to both the mother and offspring. Currently, the only known treatment strategy for these complications is delivery, making it an important area of research. The aim of this review was to summarize the known information on the development and mechanistic regulation of trophoblast differentiation and highlight the similarities in these processes between the human and mouse placenta. Additionally, the known biomarkers for each cell type were compiled to aid in the analysis of sequencing technologies.

Similarities and differences in placental development between humans and cynomolgus monkeys

Background

The placenta is an extraembryonic organ, which is essential to maintain a normal pregnancy. However, placental development in humans is poorly understood because of technical and ethical reasons.

Methods

We analyzed the anatomical localization of each trophoblastic subtype in the cynomolgus monkey placenta by immunohistochemistry in the early second trimester. Histological differences among the mouse, cynomolgus monkey, and human placenta were compared. The PubMed database was used to search for studies on placentation in rodents and primates.

Main findings

The anatomical structures and subtypes of the placenta in cynomolgus monkeys are highly similar to those in humans, with the exception of fewer interstitial extravillous trophoblasts in cynomolgus monkeys.

Conclusion

The cynomolgus monkey appears to be a good animal model to investigate human placentation.

Vaginal Exposure to Candida albicans During Early Gestation Results in Adverse Pregnancy Outcomes via Inhibiting Placental Development

Vulvovaginal candidiasis (VVC) is considered the second most common cause of vaginitis after bacterial vaginosis and the most common lower genital tract infection during pregnancy. Candida albicans (C. albicans), an opportunistic pathogen, is the major species causing VVC. Recently, increasing researches have shown that lower reproductive tract infection during pregnancy can lead to various adverse pregnancy outcomes. However, the underlying mechanisms are not fully understood. Hence, we successfully established a mouse model of vaginal C. albicans infection and characterized the adverse pregnancy outcomes. C. albicans infection strikingly increased abortion rate and decreased litter size. Further analysis of placental development demonstrated that placental structure was abnormal, including that the area of spongiotrophoblast (Spo) and labyrinth (Lab) was reduced, and the formation of placental vessel was decreased in Lab zone. Accordingly, the expression of marker genes during placental development was downregulated. Collectively, the above findings revealed that vaginal C. albicans infection during pregnancy can inhibit placental development and ultimately lead to adverse pregnancy outcomes. This study enhances our comprehension of the effect of VVC on pregnancy, and placental dysplasia as a feasible orientation to explore VVC during pregnancy.

Mammalian Placentation: A Tribute to E.C. Amoroso's Contributions to Placenta Development

Establishment of viviparity in mammals evolved through not only the long-term retainment of the fetus within the maternal uterus but differentiation and expansion of cell layers to form functional membranes to exchange O₂/CO₂ and nutrients between the placenta and maternal circulations. Development of a fetal placental vascular circulation to interact with the maternal uterus is critical to the survival of all species. However, the fascination with the mammalian placenta is the robust variation in types, form, attachment, invasiveness, structure, cell differentiation, endocrine function, and regulation of the maternal immune system. Despite the obvious role of the placenta to support fetal development, mammals have evolved multiple strategies to give live birth at term. The placenta and the maternal-fetal interface during pregnancy can be quite simple to very complex. Professor E.C. Amoroso contributed greatly to the study of comparative placentation in animals. His paper "Placentation" in Marshall's Physiology of Reproduction published in 1952 remains the standard for comparative placental anatomy today. The present volume on "Mammalian Placentation" brings together current reviews for leading experts to diversity of placentation in a number of mammalian species. Chapters will discuss viviparity, blastocyst formation, and placentation in the cow, pig, horse, mouse, dog, primate, human, elephant, and marsupials.

Ultra-High-Resolution 3D Optical Coherence Tomography Reveals Inner Structures of Human Placenta-Derived Trophoblast Organoids

OBJECTIVE

3D optical coherence tomography (OCT) is used for analyses of human placenta organoids in situ without sample preparation.

METHODS

The trophoblast organoids analyzed were derived from primary human trophoblast. In this study a custom made ultra-high-resolution spectral domain OCT system with uniform spatial and axial resolution of 2.48 μm in organoid tissue was used. The obtained OCT results align to differentiation status tested via quantitative polymerase chain reaction, Western blot analyses, immunohistochemistry, and immunofluorescence of histological sections.

RESULTS

3D OCT enables a more detailed placenta organoid monitoring compared to brightfield microscopy. Inner architecture with light scattering "bridges" surrounding cavities were visualized and quantified in situ for the first time. The formation of these bridges and cavities is congruent to differentiated trophoblast organoids having developed syncytiotrophoblasts.

CONCLUSION

Using 3D OCT in living placenta organoids is a fast tool to assess the differentiation status and resolve internal structures in situ, which is not possible with standard live cell imaging modality.

SIGNIFICANCE

Only recently human placenta-derived organoids were established, allowing to have a highly reproducible and stable in vitro model to investigate not only developmental but also physiological and pathophysiological processes during early pregnancy. To our knowledge, this work is the first to analyze living human placenta organoids using 3D OCT. Thereby, the rapid and especially non-endpoint OCT qualitative analyses align to the differentiation stage of organoids, which will aid future advancement in this field.

Capturing human trophoblast development with naive pluripotent stem cells in vitro

Trophoblasts are extraembryonic cells that are essential for maintaining pregnancy. Human trophoblasts arise from the morula as trophectoderm (TE), which, after implantation, differentiates into cytotrophoblasts (CTs), syncytiotrophoblasts (STs), and extravillous trophoblasts (EVTs), composing the placenta. Here we show that naïve, but not primed, human pluripotent stem cells (PSCs) recapitulate trophoblast development. Naive PSC-derived TE and CTs (nCTs) recreated human and monkey TE-to-CT transition. nCTs self-renewed as CT stem cells and had the characteristics of proliferating villous CTs and CTs in the cell column of the first trimester. Notably, although primed PSCs differentiated into trophoblast-like cells (BMP4, A83-01, and PD173074 [BAP]-treated primed PSCs [pBAPs]), pBAPs were distinct from nCTs and human placenta-derived CT stem cells, exhibiting properties consistent with the amnion. Our findings establish an authentic paradigm for human trophoblast development, demonstrating the invaluable properties of naive human PSCs. Our system provides a platform to study the molecular mechanisms underlying trophoblast development and related diseases.

Chromatin Regulation in Development: Current Understanding and Approaches

The regulation of mammalian stem cell fate during differentiation is complex and can be delineated across many levels. At the chromatin level, the replacement of histone variants by chromatin-modifying proteins, enrichment of specific active and repressive histone modifications, long-range gene interactions, and topological changes all play crucial roles in the determination of cell fate. These processes control regulatory elements of critical transcriptional factors, thereby establishing the networks unique to different cell fates and initiate waves of distinctive transcription events. Due to the technical challenges posed by previous methods, it was difficult to decipher the mechanism of cell fate determination at early embryogenesis through chromatin regulation. Recently, single-cell approaches have revolutionised the field of developmental biology, allowing unprecedented insights into chromatin structure and interactions in early lineage segregation events during differentiation. Here, we review the recent technological advancements and how they have furthered our understanding of chromatin regulation during early differentiation events.

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.

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

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

Establishment of macaque trophoblast stem cell lines derived from cynomolgus monkey blastocysts

The placenta forms a maternal-fetal junction that supports many physiological functions such as the supply of nutrition and exchange of gases and wastes. Establishing an in vitro culture model of human and non-human primate trophoblast stem/progenitor cells is important for investigating the process of early placental development and trophoblast differentiation. In this study, we have established five trophoblast stem cell (TSC) lines from cynomolgus monkey blastocysts, named macTSC #1-5. Fibroblast growth factor 4 (FGF4) enhanced proliferation of macTSCs, while other exogenous factors were not required to maintain their undifferentiated state. macTSCs showed a trophoblastic gene expression profile and trophoblast-like DNA methylation status and also exhibited differentiation capacity towards invasive trophoblast cells and multinucleated syncytia. In a xenogeneic chimera assay, these stem cells contributed to trophectoderm (TE) development in the chimeric blastocysts. macTSC are the first primate trophoblast cell lines whose proliferation is promoted by FGF4. These cell lines provide a valuable in vitro culture model to analyze the similarities and differences in placental development between human and non-human primates.

Multipotency of mouse trophoblast stem cells

Background

In a number of disease processes, the body is unable to repair injured tissue, promoting the need to develop strategies for tissue repair and regeneration, including the use of cellular therapeutics. Trophoblast stem cells (TSCs) are considered putative stem cells as they differentiate into other subtypes of trophoblast cells. To identify cells for future therapeutic strategies, we investigated whether TSCs have properties of stem/progenitor cells including self-renewal and the capacity to differentiate into parenchymal cells of fetal organs, in vitro and in vivo.

Methods

TSCs were isolated using anti-CD117 micro-beads, from embryonic day 18.5 placentas. In vitro, CD117⁺ TSCs were cultured, at a limiting dilution in growth medium for the development of multicellular clones and in specialized medium for differentiation into lung epithelial cells, cardiomyocytes, and retinal photoreceptor cells. CD117⁺ TSCs were also injected in utero into lung, heart, and the sub-retinal space of embryonic day 13.5 fetuses, and the organs were harvested for histological assessment after a natural delivery.

Results

We first identified CD117⁺ cells within the labyrinth zone and chorionic basal plate of murine placentas in late pregnancy, embryonic day 18.5. CD117⁺ TSCs formed multicellular clones that remained positive for CD117 in vitro, consistent with self-renewal properties. The clonal cells demonstrated multipotency, capable of differentiating into lung epithelial cells (endoderm), cardiomyocytes (mesoderm), and retinal photoreceptor cells (ectoderm). Finally, injection of CD117⁺ TSCs in utero into lungs, hearts, and the sub-retinal spaces of fetuses resulted in their engraftment on day 1 after birth, and the CD117⁺ TSCs differentiated into lung alveolar epithelial cells, heart cardiomyocytes, and retina photoreceptor cells, corresponding with the organs in which they were injected.

Conclusions

Our findings demonstrate that CD117⁺ TSCs have the properties of stem cells including clonogenicity, self-renewal, and multipotency. In utero administration of CD117⁺ TSCs engraft and differentiate into resident cells of the lung, heart, and retina during mouse development.

Electronic supplementary material

The online version of this article (10.1186/s13287-020-1567-4) contains supplementary material, which is available to authorized users.

Tri-ortho-cresyl phosphate (TOCP)-induced reproductive toxicity involved in placental apoptosis, autophagy and oxidative stress in pregnant mice

Tri-ortho-cresyl phosphate (TOCP) has been widely used as plasticizers, and reported causing reproductive toxicity in mammals. However, little is known about the toxic effect on the placenta. In this study, dams were orally administered different doses of TOCP to explore the effect of TOCP on placental development. Results showed that TOCP exposure significantly reduced numbers of implanted embryo, caused atrophy and collapse of ectoplacental cone, and decreased total areas of placenta and numbers of PCNA-positive cells. Expression levels of placental development genes were prominently downregulated in the TOCP-treated groups. Moreover, TOCP administration induced placental apoptosis and autophagy by upregulating P53, Bax, Beclin-1, ratio of LC3 II/LC3 I and Atg5 and downregulating Bcl-2 protein. In addition, TOCP exposure markedly inhibited activities of catalase and superoxide dismutase and increased the production of H₂ O₂ and malondialdehyde. Collectively, these findings suggest that apoptosis, autophagy and oxidative stress may be involved in the TOCP-induced reproductive toxicity.

Aberrant expression of imprinted lncRNA MEG8 causes trophoblast dysfunction and abortion

Long noncoding RNAs (lncRNAs) are a group of noncoding RNAs whose nucleotides are longer than 200 bp. Previous studies have shown that they play an important regulatory role in many developmental processes and biological pathways. However, the contributions of lncRNAs to placental development are largely unknown. Here, our study aimed to investigate the lncRNA expression signatures in placental development by performing a microarray lncRNA screen. Placental samples were obtained from pregnant C57BL/6 female mice at three key developmental time points (embryonic day E7.5, E13.5, and E19.5). Microarrays were used to analyze the differential expression of lncRNAs during placental development. In addition to the genomic imprinting region and the dynamic DNA methylation status during placental development, we screened imprinted lncRNAs whose expression was controlled by DNA methylation during placental development. We found that the imprinted lncRNA Rian may play an important role during placental development. Its homologous sequence lncRNA MEG8 (RIAN) was abnormally highly expressed in human spontaneous abortion villi. Upregulation of MEG8 expression in trophoblast cell lines decreased cell proliferation and invasion, whereas downregulation of MEG8 expression had the opposite effect. Furthermore, DNA methylation results showed that the methylation of the MEG8 promoter region was increased in spontaneous abortion villi. There was dynamic spatiotemporal expression of imprinted lncRNAs during placental development. The imprinted lncRNA MEG8 is involved in the regulation of early trophoblast cell function. Promoter methylation abnormalities can cause trophoblastic cell defects, which may be one of the factors that occurs in early unexplained spontaneous abortion.

Isolation and functional characterization of a novel endogenous inverse agonist of estrogen related receptors (ERRs) from human pregnancy urine

Estrogen-receptor related receptors (ERRs) which consists of ERRα, ERRβ and ERRγ belong to the orphan nuclear receptor subfamily 3, group B (NR3B) subfamily, and are constitutively active. ERRs have been shown to actively modulate estrogenic responses, and to play an essential role in pregnancy, and are implicated in breast cancer progression. Despite intensive efforts, no endogenous ligand other than the ubiquitous sterol, cholesterol which binds ERRα, has been identified for ERRs so far. The discovery of ligands that bind these orphan receptors will allow the manipulation of this pathway and may lead to novel strategies for the treatment of cancer and other diseases. We previously reported the identification of a novel endogenous estradienolone-like steroid (ED) that is strongly bound to sex hormone binding globulin, in pregnant women. Our recent results show that ED acts as an inverse agonist of ERRα and ERRγ by directly interacting with these receptors, and inhibiting their transcriptional activity. We also demonstrate that ED inhibits the growth of both estrogen receptor-positive (MCF-7) and estrogen receptor-negative (MDA-MB-231) breast cancer cells in a dose dependent manner, while of displaying a little effect on normal epithelial breast cells. Furthermore, the anti-mitogenic effect of ED in breast cancer cells is ERRα-dependent. These data suggest that ED-ERR interaction may represent a novel physiologically relevant hormone response pathway in the human. The finding that ED inhibits both ER negative and ER positive breast cancer cell growth may have important implications in pathophysiology breast cancer.

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.

Insulinotropic nucleobindin-2/nesfatin-1 is dynamically expressed in the haemochorial mouse and human placenta

The placenta is the physiological bridge between mother and fetus and has life-sustaining functions during pregnancy, including metabolic regulation, fetal protection and hormone secretion. Nucleobindin-2 (NUCB2) is a calcium- and DNA-binding protein and precursor of nesfatin-1, a signalling peptide with multiple functions, including regulation of energy homeostasis and glucose transport. These are also key functions of the placenta, yet NUCB2/nesfatin-1 expression has never been comprehensively studied in this organ. In the present study, mouse placental samples from Embryonic Day (E) 7.5 to E17.5 and human chorionic villi from the first and second trimester, as well as term pregnancy, were analysed for NUCB2/nesfatin-1 expression by immunohistochemistry with an antiserum that recognised both NUCB2 and nesfatin-1. From E7.5 to E9.5, NUCB2/nesfatin-1 was expressed in the ectoplacental cone, then parietal trophoblast giant cells and early spongiotrophoblast. At E10.5–12.5, NUCB2/nesfatin-1 expression became detectable in the developing labyrinth. From E12.5 and onwards, NUCB2/nesfatin-1 was expressed in the glycogen trophoblast cells, as well as highly expressed in syncytiotrophoblast, sinusoidal trophoblast giant cells and fetal capillary endothelial cells of the labyrinth. In all trimesters of human pregnancy, NUCB2/nesfatin-1 was highly expressed in syncytiotrophoblast. In addition, there was a significant increase in NUCB2 expression in human primary trophoblast cells induced to syncytialise. Thus, the haemochorial mammalian placenta is a novel source of NUCB2/nesfatin-1 and likely a site of its action, with potential roles in glucose homeostasis and/or nutrient sensing.

Suppression of Fgf2 by ETS2 repressor factor (ERF) is required for chorionic trophoblast differentiation

ETS2 repressor factor (ERF) is a ubiquitous transcriptional repressor regulated by Extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation. Homozygous deletion of Erf in mice blocks chorionic trophoblast differentiation, resulting in the failure of chorioallantoic fusion and subsequent embryo death. Fibroblast growth factor (FGF) signaling is important for proper trophoblast stem cell (TSC) differentiation and development of the hemochorial placenta. Lack of Fgf2 promotes TSC differentiation, while FGF4 or FGF2 is required for murine TSC maintenance. Here, we show that low in vivo Fgf2 mRNA abundance occurs in patches of placental chorion cells and ex vivo in TSCs. This expression is repressed via direct interaction of ERF with the Fgf2 transcription unit is increased in the absence of ERF, and is decreased in the presence of an ERF mutant resistant to ERK phosphorylation. Thus, FGF2 inhibition by ERF appears to be necessary for proper chorionic TSC differentiation, and may account for the block of chorionic trophoblast differentiation in Erf-knockout animals. The differentiation of ERF-overexpressing TSC lines also suggests that ERF may have an FGF2-independent effect during the commitment towards syncytiotrophoblasts.

What Is Trophoblast? A Combination of Criteria Define Human First-Trimester Trophoblast

Controversy surrounds reports describing the derivation of human trophoblast cells from placentas and embryonic stem cells (ESC), partly due to the difficulty in identifying markers that define cells as belonging to the trophoblast lineage. We have selected criteria that are characteristic of primary first-trimester trophoblast: a set of protein markers, HLA class I profile, methylation of ELF5, and expression of microRNAs (miRNAs) from the chromosome 19 miRNA cluster (C19MC). We tested these criteria on cells previously reported to show some phenotypic characteristics of trophoblast: bone morphogenetic protein (BMP)-treated human ESC and 2102Ep, an embryonal carcinoma cell line. Both cell types only show some, but not all, of the four trophoblast criteria. Thus, BMP-treated human ESC have not fully differentiated to trophoblast. Our study identifies a robust panel, including both protein and non-protein-coding markers that, in combination, can be used to reliably define cells as characteristic of early trophoblast.

Stem cell insights into human trophoblast lineage differentiation

BACKGROUND

The human placenta is vital for fetal development, yet little is understood about how it forms successfully to ensure a healthy pregnancy or why this process is inadequate in 1 in 10 pregnancies, leading to miscarriage, intrauterine growth restriction or preeclampsia. Trophoblasts are placenta-specific epithelial cells that maximize nutrient exchange. All trophoblast lineages are thought to arise from a population of trophoblast stem cells (TSCs). However, whilst the isolation of murine TSC has led to an explosion in understanding murine placentation, the isolation of an analogous human TSC has proved more difficult. Consequently, alternative methods of studying human trophoblast lineage development have been employed, including human embryonic stem cells (hESCs), induced pluripotent stem cells (iPS) and transformed cell lines; but what do these proxy models tell us about what is happening during early placental development?

OBJECTIVE AND RATIONALE

In this systematic review, we evaluate current approaches to understanding human trophoblast lineage development in order to collate and refine these models and inform future approaches aimed at establishing human TSC lines.

SEARCH METHODS

To ensure all relevant articles were analysed, an unfiltered search of Pubmed, Embase, Scopus and Web of Science was conducted for 25 key terms on the 13th May 2016. In total, 47 313 articles were retrieved and manually filtered based on non-human, non-English, non-full text, non-original article and off-topic subject matter. This resulted in a total of 71 articles deemed relevant for review in this article.

OUTCOMES

Candidate human TSC populations have been identified in, and isolated from, both the chorionic membrane and villous tissue of the placenta, but further investigation is required to validate these as 'true' human TSCs. Isolating human TSCs from blastocyst trophectoderm has not been successful in humans as it was in mice, although recently the first reported TSC line (USFB6) was isolated from an eight-cell morula. In lieu of human TSC lines, trophoblast-like cells have been induced to differentiate from hESCs and iPS. However, differentiation in these model systems is difficult to control, culture conditions employed are highly variable, and the extent to which they accurately convey the biology of 'true' human TSCs remains unclear, particularly as a consensus has not been met among the scientific community regarding which characteristics a human TSC must possess.

WIDER IMPLICATIONS

Human TSC models have the potential to revolutionize our understanding of trophoblast differentiation, allowing us to make significant gains in understanding the underlying pathology of pregnancy disorders and to test potential therapeutic interventions on cell function in vitro. In order to do this, a collaborative effort is required to establish the criteria that define a human TSC to confirm the presence of human TSCs in both primary isolates and to determine how accurately trophoblast-like cells derived from current model systems reflect trophoblast from primary tissue. The in vitro systems currently used to model early trophoblast lineage formation have provided insights into early human placental formation but it is unclear whether these trophoblast-like cells are truly representative of primary human trophoblast. Consequently, continued refinement of current models, and standardization of culture protocols is essential to aid our ability to identify, isolate and propagate 'true' human TSCs from primary tissue.

Human pluripotent stem cells as a model of trophoblast differentiation in both normal development and disease

Trophoblast is the primary epithelial cell type in the placenta, a transient organ required for proper fetal growth and development. Different trophoblast subtypes are responsible for gas/nutrient exchange (syncytiotrophoblasts, STBs) and invasion and maternal vascular remodeling (extravillous trophoblasts, EVTs). Studies of early human placental development are severely hampered by the lack of a representative trophoblast stem cell (TSC) model with the capacity for self-renewal and the ability to differentiate into both STBs and EVTs. Primary cytotrophoblasts (CTBs) isolated from early-gestation (6-8 wk) human placentas are bipotential, a phenotype that is lost with increasing gestational age. We have identified a CDX2(+)/p63(+) CTB subpopulation in the early postimplantation human placenta that is significantly reduced later in gestation. We describe a reproducible protocol, using defined medium containing bone morphogenetic protein 4 by which human pluripotent stem cells (hPSCs) can be differentiated into CDX2(+)/p63(+) CTB stem-like cells. These cells can be replated and further differentiated into STB- and EVT-like cells, based on marker expression, hormone secretion, and invasive ability. As in primary CTBs, differentiation of hPSC-derived CTBs in low oxygen leads to reduced human chorionic gonadotropin secretion and STB-associated gene expression, instead promoting differentiation into HLA-G(+) EVTs in an hypoxia-inducible, factor-dependent manner. To validate further the utility of hPSC-derived CTBs, we demonstrated that differentiation of trisomy 21 (T21) hPSCs recapitulates the delayed CTB maturation and blunted STB differentiation seen in T21 placentae. Collectively, our data suggest that hPSCs are a valuable model of human placental development, enabling us to recapitulate processes that result in both normal and diseased pregnancies.

Isolation and characterisation of a novel trophoblast side-population from first trimester placentae

The placenta is responsible for all nutrient and gas exchange between mother and baby during pregnancy. The differentiation of specialised placental epithelial cells called trophoblasts is essential for placental function, but we understand little about how these populations arise. Mouse trophoblast stem cells have allowed us to understand many of the factors that regulate murine trophoblast lineage development, but the human placenta is anatomically very different from the mouse, and it is imperative to isolate a human trophoblast stem cell to understand human placental development. Here we have developed a novel methodology to isolate a Hoechst side-population of trophoblasts from early gestation placentae and compared their transcriptome to differentiated trophoblast populations (cytotrophoblasts and extravillous trophoblasts) using microarray technology. Side-population trophoblasts clustered as a transcriptomically distinct population but were more closely related to cytotrophoblasts than extravillous trophoblasts. Side-population trophoblasts up-regulated a number of genes characteristic of trophectoderm and murine trophoblast stem cells in comparison to cytotrophoblasts or extravillous trophoblasts and could be distinguished from both of these more mature populations by a unique set of 22 up-regulated genes, which were enriched for morphogenesis and organ development and the regulation of growth functions. Cells expressing two of these genes (LAMA2 and COL6A3) were distributed throughout the cytotrophoblast layer at the trophoblast/mesenchymal interface. Comparisons to previously published trophoblast progenitor populations suggest that the side-population trophoblasts isolated in this work are a novel human trophoblast population. Future work will determine whether these cells exhibit functional progenitor/stem cell attributes.

Label-free and non-invasive monitoring of porcine trophoblast derived cells: differentiation in serum and serum-free media

Traditional approaches to characterize stem cell differentiation are time-consuming, lengthy and invasive. Here, Raman microspectroscopy (RM) and atomic force microscopy (AFM) - both considered as non-invasive techniques - are applied to detect the biochemical and biophysical properties of trophoblast derived stem-like cells incubated up to 10 days under conditions designed to induce differentiation. Significant biochemical and biophysical differences between control cells and differentiated cells were observed. Quantitative real time PCR was also applied to analyze gene expression. The relationship between cell differentiation and associated cellular biochemical and biomechanical changes were discussed. Monitoring trophoblast cells differentiation.

Psg22 expression in mouse trophoblast giant cells is associated with gene inversion and co-expression of antisense long non-coding RNAs

Pregnancy-specific glycoproteins (PSGs) are secreted carcinoembryonic antigen (CEA)-related cell adhesion molecules-related members of the immunoglobulin superfamily and are encoded by multigene families in species with haemochorial placentation. PSGs may be the most abundant trophoblast-derived proteins in human maternal blood in late pregnancy and there is evidence that dysregulation of PSG expression is associated with gestational pathology. PSGs are produced by syncytiotrophoblast in the human placenta and by trophoblast giant cells (TGCs) and spongiotrophoblast in rodents, and are implicated in immune regulation, angiogenesis and regulation of platelet function. PSGs are encoded by 17 genes in the mouse and ten genes in the human. While functions appear to be conserved, the typical protein domain organisation differs between species. We analysed the evolution of the mouse Psg genomic locus structure and report inversion of the Psg22 gene within the locus. Psg22 is the most abundant Psg transcript detected in the first half of mouse pregnancy and we identified antisense long non-coding RNA (lncRNA) transcripts adjacent to Psg22 associated with an active local chromatin conformation. This suggests that an epigenetic regulatory mechanism may underpin high Psg22 expression relative to the other Psg gene family members in TGCs.

Transient SNAIL1 expression is necessary for metastatic competence in breast cancer

SNAIL1 has been suggested to regulate breast cancer metastasis based on analyses of human breast tumor transcriptomes and experiments using cancer cell lines and xenografts. However, in vivo genetic experimental support for a role for SNAIL1 in breast cancer metastasis that develops in an immunocompetent tumor microenvironment has not been determined. To address this question, we created a genetic SNAIL1 model by coupling an endogenous SNAIL1 reporter with an inducible SNAIL1 transgene. Using multiple genetic models of breast cancer, we demonstrated that endogenous SNAIL1 expression was restricted to primary tumors that ultimately disseminate. SNAIL1 gene deletion either during the premalignant phase or after primary tumors have reached a palpable size blunted metastasis, indicating that late metastasis was the main driver of metastasis and that this was dependent on SNAIL1. Importantly, SNAIL1 expression during breast cancer metastasis was transient and forced transient, but not continuous. SNAIL1 expression in breast tumors was sufficient to increase metastasis.

EZH2 is essential for development of mouse preimplantation embryos

Enhancer of zeste homologue 2 (Ezh2) is essential for the development of the early mouse preimplantation embryo. Loss of Ezh2 results in embryonic lethality in mice. Ezh2-deficient embryos display impaired outgrowth potential, defective establishment of Ezh2-null embryonic stem (ES) cells and adherence and differentiation of the trophoblast layer into giant cells. We investigated if Ezh2 controls the fate of embryos at an earlier stage by treating with cycloheximide (CHX) or microinjecting short interfering RNA (siRNA) to restrict embryonic Ezh2 expression during preimplantation. CHX inhibited de novo EZH2 protein synthesis in zygotes, suggesting that EZH2 requires de novo synthesis during post-fertilisation stages. We found that loss of Ezh2 at the pronuclear stage caused severe growth retardation and reduced blastocyst formation. Expression of the pluripotency-associated markers Oct4, Sox2 and Nanog were significantly decreased in embryos that had been injected with Ezh2 siRNA. In addition, Ezh2 loss induced upregulated expression of genes related to the differentiation of germ layers, including Gata6, Hoxb1 and Hand1. Finally, apoptosis was increased in the blastocyst embryos with Ezh2 knockdown. Modification of histone H3-Lysine 27 de-methylation and tri-methylation (H3K27me2/3) was strongly reduced in Ezh2 siRNA embryos. We conclude that Ezh2 is essential for early preimplantation embryo development through the regulation of epigenetic modification and apoptosis.

DNA methyltransferases and TETs in the regulation of differentiation and invasiveness of extra-villous trophoblasts

Specialized cell types of trophoblast cells form the placenta in which each cell type has particular properties of proliferation and invasion. The placenta sustains the growth of the fetus throughout pregnancy and any aberrant trophoblast differentiation or invasion potentially affects the future health of the child and adult. Recently, the field of epigenetics has been applied to understand differentiation of trophoblast lineages and embryonic stem cells (ESC), from fertilization of the oocyte onward. Each trophoblast cell-type has a distinctive epigenetic profile and we will concentrate on the epigenetic mechanism of DNA methyltransferases and TETs that regulate DNA methylation. Environmental factors affecting the mother potentially regulate the DNA methyltransferases in trophoblasts, and so do steroid hormones, cell cycle regulators, such as p53, and cytokines, especially interlukin-1β. There are interesting questions of why trophoblast genomes are globally hypomethylated yet specific genes can be suppressed by hypermethylation (in general, tumor suppressor genes, such as E-cadherin) and how invasive cell-types are liable to have condensed chromatin, as in metastatic cancer cells. Future work will attempt to understand the interactive nature of all epigenetic mechanisms together and their effect on the complex biological system of trophoblast differentiation and invasion in normal as well as pathological conditions.

Retroviruses facilitate the rapid evolution of the mammalian placenta

The mammalian placenta exhibits elevated expression of endogenous retroviruses (ERVs), but the evolutionary significance of this feature remains unclear. I propose that ERV-mediated regulatory evolution was, and continues to be, an important mechanism underlying the evolution of placental development. Many recent studies have focused on the co-option of ERV-derived genes for specific functional adaptations in the placenta. However, the co-option of ERV-derived regulatory elements could potentially lead to the incorporation of entire gene regulatory networks, which, I argue, would facilitate relatively rapid developmental evolution of the placenta. I suggest a model in which an ancient retroviral infection led to the establishment of the ancestral placental developmental gene network through the co-option of ERV-derived regulatory elements. Consequently, placental development would require elevated tolerance to ERV activity. This in turn would expose a continuous stream of novel ERV mutations that may have catalyzed the developmental diversification of the mammalian placenta.

Keratin 5-Cre-driven excision of nonmuscle myosin IIA in early embryo trophectoderm leads to placenta defects and embryonic lethality

In studies initially focused on roles of nonmuscle myosin IIA (NMIIA) in the developing mouse epidermis, we have discovered that a previously described cytokeratin 5 (K5)-Cre gene construct is expressed in early embryo development. Mice carrying floxed alleles of the nonmuscle myosin II heavy chain gene (NMHC IIA(flox/flox)) were crossed with the K5-Cre line. The progeny of newborn pups did not show a Mendelian genotype distribution, suggesting embryonic lethality. Analysis of post-implantation conceptuses from embryonic day (E)9.5 to E13.5 revealed poorly developed embryos and defective placentas, with significantly reduced labyrinth surface area and blood vessel vascularization. These results suggested the novel possibility that the bovine K5 promoter-driven Cre-recombinase was active early in trophoblast-lineage cells that give rise to the placenta. To test this possibility, K5-Cre transgenic mice were crossed with the mT/mG reporter mouse in which activation of GFP expression indicates Cre transgene expression. We observed activation of K5-Cre-driven GFP expression in the ectoplacental cone, in the extraembryonic ectoderm, and in trophoblast giant cells in the E6.5 embryo. In addition, we observed GFP expression at E11.5 to E13.5 in both the labyrinth of the placenta and the yolk sac. NMIIA expression was detected in these same cell types in normal embryos, as well as in E13.5 yolk sac and labyrinth. These findings taken together suggest that NMHC IIA may play critical roles in the early trophoblast-derived ectoplacental cone and extraembryonic ectoderm, as well as in the yolk sac and labyrinth tissues that form later. Our findings are consistent with phenotypes of constitutive NMIIA knockout mice made earlier, that displayed labyrinth and yolk sac-specific defects, but our findings extend those observations by suggesting possible NMIIA roles in trophoblast lineages as well. These results furthermore demonstrate that K5-Cre gene constructs, previously reported to be activated starting at approximately E12.5 in the forming epidermis, may be widely useful as drivers for activation of cre/lox based gene excision in early embryo extraembronic trophoblast tissues as well.

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.

Morphometric analysis of the placenta in the New World mouse Necromys lasiurus (Rodentia, Cricetidae): a comparison of placental development in cricetids and murids

BACKGROUND

Stereology is an established method to extrapolate three-dimensional quantities from two-dimensional images. It was applied to placentation in the mouse, but not yet for other rodents. Herein, we provide the first study on quantitative placental development in a sigmodontine rodent species with relatively similar gestational time. Placental structure was also compared to the mouse, in order to evaluate similarities and differences in developmental patterns at the end of gestation.

METHODS

Fetal and placental tissues of Necromys lasiurus were collected and weighed at 3 different stages of gestation (early, mid and late gestation) for placental stereology. The total and relative volumes of placenta and of its main layers were investigated. Volume fractions of labyrinth components were quantified by the One Stop method in 31 placentae collected from different individuals, using the Mercator software. Data generated at the end of gestation from N. lasiurus placentae were compared to those of Mus musculus domesticus obtained at the same stage.

RESULTS

A significant increase in the total absolute volumes of the placenta and its main layers occurred from early to mid-gestation, followed by a reduction near term, with the labyrinth layer becoming the most prominent area. Moreover, at the end of gestation, the total volume of the mouse placenta was significantly increased compared to that of N. lasiurus although the proportions of the labyrinth layer and junctional zones were similar. Analysis of the volume fractions of the components in the labyrinth indicated a significant increase in fetal vessels and sinusoidal giant cells, a decrease in labyrinthine trophoblast whereas the proportion of maternal blood space remained stable in the course of gestation. On the other hand, in the mouse, volume fractions of fetal vessels and sinusoidal giant cells decreased whereas the volume fraction of labyrinthine trophoblast increased compared to N. lasiurus placenta.

CONCLUSIONS

Placental development differed between N. lasiurus and M. musculus domesticus. In particular, the low placental efficiency in N. lasiurus seemed to induce morphological optimization of fetomaternal exchanges. In conclusion, despite similar structural aspects of placentation in these species, the quantitative dynamics showed important differences.

The dysfunctional placenta epigenome: causes and consequences

The placenta is a fetal–maternal endocrine organ responsible for ensuring proper fetal development throughout pregnancy. Adverse insults to the intrauterine environment often lead to expression level changes in placental genes, many of which are epigenetically regulated by DNA methylation, histone modifications and ncRNA interference. These epigenetic alterations may cause placental dysfunction, resulting in offspring of low birthweight owing to adverse pregnancy complications such as intrauterine growth restriction. Numerous epidemiological studies have shown a strong correlation between low birthweight and increased risk of developing metabolic diseases and neurological imbalances in adulthood, and in subsequent generations, indicating that epigenetic regulation of gene expression can be propagated stably with long-term effects on health. This article provides an overview of the various environmental factors capable of inducing detrimental changes to the placental epigenome, as well as the corresponding mechanisms that prime the offspring for onset of disease later in life.

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.

Environmental regulation of placental phenotype: implications for fetal growth

Environmental conditions during pregnancy determine birthweight, neonatal viability and adult phenotype in human and other animals. In part, these effects may be mediated by the placenta, the principal source of nutrients for fetal development. However, little is known about the environmental regulation of placental phenotype. Generally, placental weight is reduced during suboptimal conditions like maternal malnutrition or hypoxaemia but compensatory adaptations can occur in placental nutrient transport capacity to help maintain fetal growth. In vivo studies show that transplacental glucose and amino acid transfer adapt to the prevailing conditions induced by manipulating maternal calorie intake, dietary composition and hormone exposure. These adaptations are due to changes in placental morphology, metabolism and/or abundance of specific nutrient transporters. This review examines environmental programming of placental phenotype with particular emphasis on placental nutrient transport capacity and its implications for fetal growth, mainly in rodents. It also considers the systemic, cellular and molecular mechanisms involved in signalling environmental cues to the placenta. Ultimately, the ability of the placenta to balance the competing interests of mother and fetus in resource allocation may determine not only the success of pregnancy in producing viable neonates but also the long-term health of the offspring.

In vitro culture of mouse embryos reduces differential gene expression between inner cell mass and trophectoderm

Differences in gene expression and imprinting have been reported, comparing in vivo versus in vitro generated preimplantation embryos. Furthermore, mouse studies have shown that placenta development is altered following in vitro culture. However, the molecular mechanisms underlying these findings are unknown. We therefore isolated trophectoderm (TE) and inner cell mass (ICM) cells from in vivo and in vitro fertilization (IVF) embryos and evaluated their transcriptome using microarrays. We found that the transcriptomes of in vitro produced ICM and TE cells showed remarkably few differences compared to ICM and TE cells of in vivo generated embryos. In vitro fertilization embryos showed a reduced number of TE cells compared to in vivo embryos. In addition, TE of IVF embryos showed significant downregulation of solute transporter genes and of genes involved in placenta formation (Eomesodermin, Socs3) or implantation (Hbegf). In summary, IVF and embryo culture significantly affects the transcriptome of ICM and TE cells.

Dissecting the first transcriptional divergence during human embryonic development

The trophoblast cell lineage is specified early at the blastocyst stage, leading to the emergence of the trophectoderm and the pluripotent cells of the inner cell mass. Using a double mRNA amplification technique and a comparison with transcriptome data on pluripotent stem cells, placenta, germinal and adult tissues, we report here some essential molecular features of the human mural trophectoderm. In addition to genes known for their role in placenta (CGA, PGF, ALPPL2 and ABCG2), human trophectoderm also strongly expressed Laminins, such as LAMA1, and the GAGE Cancer/Testis genes. The very high level of ABCG2 expression in trophectoderm, 7.9-fold higher than in placenta, suggests a major role of this gene in shielding the very early embryo from xenobiotics. Several genes, including CCKBR and DNMT3L, were specifically up-regulated only in trophectoderm, indicating that the trophoblast cell lineage shares with the germinal lineage a transient burst of DNMT3L expression. A trophectoderm core transcriptional regulatory circuitry formed by 13 tightly interconnected transcription factors (CEBPA, GATA2, GATA3, GCM1, KLF5, MAFK, MSX2, MXD1, PPARD, PPARG, PPP1R13L, TFAP2C and TP63), was found to be induced in trophectoderm and maintained in placenta. The induction of this network could be recapitulated in an in vitro trophoblast differentiation model.

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.

The placental imprintome and imprinted gene function in the trophoblast glycogen cell lineage

Imprinted genes represent a unique class of autosomal genes expressed from only one of the parental alleles during development. The choice of the expressed allele is not random but rather is determined by the parental origin of the allele. Consequently, the mouse genome contains more than 100 genes expressed preferentially or exclusively from the maternally or the paternally inherited allele. Current research efforts are focused on understanding the molecular mechanism of this epigenetic phenomenon as well as the biological functions of the genes under its regulation. Both theoretical considerations and experimental results support a role for genomic imprinting in the regulation of embryonic growth and placental biology. In this review, recent efforts to establish the complete set of genes showing imprinted expression in the mouse placenta are first discussed. Then, the evidence suggesting that imprinted genes might be implicated in the emergence, maintenance and function of trophoblast glycogen cells is presented. Although the origin and functions of this trophoblast cell lineage are currently unknown, the analysis of mutations in imprinted genes in the mouse are providing new insights into these issues. The implications of this work for placental pathologies in human are also discussed.

RNA-Seq analysis uncovers transcriptomic variations between morphologically similar in vivo- and in vitro-derived bovine blastocysts

Background

A valuable tool for both research and industry, in vitro fertilization (IVF) has applications range from gamete selection and preservation of traits to cloning. Although IVF has achieved worldwide use, with approximately 339,685 bovine embryos transferred in 2010 alone, there are still continuing difficulties with efficiency. It is rare to have more than 40% of fertilized in vitro cattle oocytes reach blastocyst stage by day 8 of culture, and pregnancy rates are reported as less than 45% for in vitro produced embryos. To investigate potential influences in-vitro fertilization (IVF) has on embryonic development, this study compares in vivo- and in vitro-derived bovine blastocysts at a similar stage and quality grade (expanded, excellent quality) to determine the degree of transcriptomic variation beyond morphology using RNA-Seq.

Results

A total of 26,906,451 and 38,184,547 fragments were sequenced for in vitro and in vivo embryo pools, respectively. We detected expression for a total of 17,634 genes, with 793 genes showing differential expression between the two embryo populations with false discovery rate (FDR) < 0.05. There were also 395 novel transcribed units found, of which 45 were differentially expressed (FDR < 0.05). In addition, 4,800 genes showed evidence of alternative splicing, with 873 genes displaying differential alternative splicing between the two pools (FDR < 0.05). Using GO enrichment analysis, multiple biological pathways were found to be significantly enriched for differentially expressed genes (FDR < 0.01), including cholesterol and sterol synthesis, system development, and cell differentiation.

Conclusions

Thus, our results support that IVF may influence at the transcriptomic level and that morphology is limited in full characterization of bovine preimplantation embryos.

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.

Human placentation from nidation to 5 weeks of gestation. Part II: Tools to model the crucial first days

Human pregnancy is unusual with respect to monthly spontaneous decidualisation as well as the degree of placental invasion and interaction with the decidualised endometrial stroma. This review covers in vivo animal models and in vitro cell culture models that have been used to study the earliest stages of human implantation and placentation from nidation to 5 weeks of gestation. The field has expanded rapidly in recent years due to the generation of human embryonic stem cell lines and the ability of some scientists to culture human blastocysts. These models have enabled researchers to begin to elucidate the interactions involved in human blastocyst apposition, adhesion and implantation. However, we still understand very little about the differentiation processes involved in the formation of the placenta. Continued improvements to current models, including the potential isolation of a human trophoblast stem cell, will significantly enhance our ability to define the molecular and structural events occurring during human implantation and early placental development.

SATB homeobox proteins regulate trophoblast stem cell renewal and differentiation

The morphogenesis of the hemochorial placenta is dependent upon the precise expansion and differentiation of trophoblast stem (TS) cells. SATB homeobox 1 (SATB1) and SATB2 are related proteins that have been implicated as regulators of some stem cell populations. SATB1 is highly expressed in TS cells, which prompted an investigation of SATB1 and the related SATB2 as regulators of TS cells. SATB1 and SATB2 were highly expressed in rat TS cells maintained in the stem state and rapidly declined following induction of differentiation. SATB proteins were also present within the rat placenta during early stages of its morphogenesis and disappeared as gestation advanced. Silencing Satb1 or Satb2 expression decreased TS cell self-renewal and increased differentiation, whereas ectopic expression of SATB proteins promoted TS cell expansion and blunted differentiation. Eomes, a key transcriptional regulator of TS cells, was identified as a target for SATB proteins. SATB knockdown decreased Eomes transcript levels and promoter activity, whereas SATB ectopic expression increased Eomes transcript levels and promoter activity. Electrophoretic mobility shift assay as well as chromatin immunoprecipitation analyses demonstrated that SATB proteins physically associate with a regulatory site within the Eomes promoter. We conclude that SATB proteins promote TS cell renewal and inhibit differentiation. These actions are mediated in part by regulating the expression of the TS cell stem-associated transcription factor, EOMES.

Placentation in Sigmodontinae: a rodent taxon native to South America

BACKGROUND

Sigmodontinae, known as "New World rats and mice," is a large subfamily of Cricetidae for which we herein provide the first comprehensive investigation of the placenta.

METHODS

Placentas of various gestational ages ranging from early pregnancy to near term were obtained for five genera, i.e. Necromys, Euryoryzomys, Cerradomys, Hylaeamys, and Oligoryzomys. They were investigated by means of histology, immunohistochemistry, a proliferation marker, DBA-lectin staining and transmission electron microscopy.

RESULTS

The chorioallantoic placenta was organized in a labyrinthine zone, spongy zone and decidua and an inverted yolk sac persisted until term. The chorioallantoic placenta was hemotrichorial. The interhemal barrier comprised fetal capillary endothelium and three layers of trophoblast, an outermost, cellular layer and two syncytial ones, with interspersed trophoblast giant cells (TGC). In addition, accumulations of TGC occurred below Reichert's membrane. The junctional zone contained syncytial trophoblast, proliferative cellular trophoblast, glycogen cells and TGC that were situated near to the maternal blood channels. In three of the genera, TGC were also accumulated in distinct areas at the placental periphery. PAS-positive glycogen cells derived from the junctional zone invaded the decidua. Abundant maternal uNK cells with positive response to PAS, vimentin and DBA-lectin were found in the decidua. The visceral yolk sac was completely inverted and villous.

CONCLUSION

The general aspect of the fetal membranes in Sigmodontinae resembled that found in other cricetid rodents. Compared to murid rodents there were larger numbers of giant cells and in some genera these were seen to congregate at the periphery of the placental disk. Glycogen cells were found to invade the decidua but we did not identify trophoblast in the walls of the deeper decidual arteries. In contrast these vessels were surrounded by large numbers of uNK cells. This survey of wild-trapped specimens from five genera is a useful starting point for the study of placentation in an important subfamily of South American rodents. We note, however, that some of these rodents can be captive bred and recommend that future studies focus on the study of time dated pregnancies.

The role of DNA methylation in mammalian development

DNA methylation is involved in a number of important processes such as maintaining genome stability, silencing of retrotransposons, co-ordinating mono-alleleic expression of parentally imprinted genes and ensuring transcriptional repression of genes on the inactive X chromosome. Further, correct DNA methylation patterns are necessary for normal development and lineage commitment. DNA methylation provides a stable and heritable epigenetic mark. However, it can be removed, either actively or passively, during periods of reprogramming in primordial germ cells and preimplantation embryos. By combining immunofluorescence data with recent insights from genome-wide studies utilizing techniques such as Bisulphite-seq and MeDIP-ChIP, a clearer picture of the dynamic patterns of DNA methylation throughout gametogenesis, preimplantation development and early lineage commitment is beginning to emerge. The continuing use of these next-generation technologies to elucidate genome-wide methylation patterns in a variety of cellular contexts will further understanding of how this epigenetic mark contributes to lineage commitment, differentiation and pluripotency and, ultimately, to human health and disease.

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.

Sox2 is essential for formation of trophectoderm in the preimplantation embryo

Background

In preimplantation mammalian development the transcription factor Sox2 (SRY-related HMG-box gene 2) forms a complex with Oct4 and functions in maintenance of self-renewal of the pluripotent inner cell mass (ICM). Previously it was shown that Sox2−/− embryos die soon after implantation. However, maternal Sox2 transcripts may mask an earlier phenotype. We investigated whether Sox2 is involved in controlling cell fate decisions at an earlier stage.

Methods and Findings

We addressed the question of an earlier role for Sox2 using RNAi, which removes both maternal and embryonic Sox2 mRNA present during the preimplantation period. By depleting both maternal and embryonic Sox2 mRNA at the 2-cell stage and monitoring embryo development in vitro we show that, in the absence of Sox2, embryos arrest at the morula stage and fail to form trophectoderm (TE) or cavitate. Following knock-down of Sox2 via three different short interfering RNA (siRNA) constructs in 2-cell stage mouse embryos, we have shown that the majority of embryos (76%) arrest at the morula stage or slightly earlier and only 18.7–21% form blastocysts compared to 76.2–83% in control groups. In Sox2 siRNA-treated embryos expression of pluripotency associated markers Oct4 and Nanog remained unaffected, whereas TE associated markers Tead4, Yap, Cdx2, Eomes, Fgfr2, as well as Fgf4, were downregulated in the absence of Sox2. Apoptosis was also increased in Sox2 knock-down embryos. Rescue experiments using cell-permeant Sox2 protein resulted in increased blastocyst formation from 18.7% to 62.6% and restoration of Sox2, Oct4, Cdx2 and Yap protein levels in the rescued Sox2-siRNA blastocysts.

Conclusion and Significance

We conclude that the first essential function of Sox2 in the preimplantation mouse embryo is to facilitate establishment of the trophectoderm lineage. Our findings provide a novel insight into the first differentiation event within the preimplantation embryo, namely the segregation of the ICM and TE lineages.