Different epigenetic states define syncytiotrophoblast and cytotrophoblast nuclei in the trophoblast of the human placenta

Dynamic and distinct histone modifications facilitate human trophoblast lineage differentiation

The placenta serves as an essential organ for fetal growth throughout pregnancy. Histone modification is a crucial regulatory mechanism involved in numerous biological processes and development. Nevertheless, there remains a significant gap in our understanding regarding the epigenetic regulations that influence trophoblast lineage differentiation, a fundamental aspect of placental development. Here, through comprehensive mapping of H3K4me3, H3K27me3, H3K9me3, and H3K27ac loci during the differentiation of trophoblast stem cells (TSCs) into syncytiotrophoblasts (STs) and extravillous trophoblasts (EVTs), we reveal dynamic reconfiguration in H3K4me3 and H3K27ac patterns that establish an epigenetic landscape conducive to proper trophoblast lineage differentiation. We observe that broad H3K4me3 domains are associated with trophoblast lineage-specific gene expression. Unlike embryonic stem cells, TSCs lack robust bivalent domains. Notably, the repression of ST- and EVT-active genes in TSCs is primarily attributed to the weak H3K4me3 signal rather than bivalent domains. We also unveil the inactivation of TSC enhancers precedes the activation of ST enhancers during ST formation. Our results provide a comprehensive global map of diverse histone modifications, elucidating the dynamic histone modifications during trophoblast lineage differentiation.

Single-nucleus multi-omic profiling of human placental syncytiotrophoblasts identifies cellular trajectories during pregnancy

The human placenta has a vital role in ensuring a successful pregnancy. Despite the growing body of knowledge about its cellular compositions and functions, there has been limited research on the heterogeneity of the billions of nuclei within the syncytiotrophoblast (STB), a multinucleated entity primarily responsible for placental function. Here we conducted integrated single-nucleus RNA sequencing and single-nucleus ATAC sequencing analyses of human placentas from early and late pregnancy. Our findings demonstrate the dynamic heterogeneity and developmental trajectories of STB nuclei and their correspondence with human trophoblast stem cell (hTSC)-derived STB. Furthermore, we identified transcription factors associated with diverse STB nuclear lineages through their gene regulatory networks and experimentally confirmed their function in hTSC and trophoblast organoid-derived STBs. Together, our data provide insights into the heterogeneity of human STB and represent a valuable resource for interpreting associated pregnancy complications.

In Silico and In Vitro Mapping of Receptor-Type Protein Tyrosine Phosphatase Receptor Type D in Health and Disease: Implications for Asprosin Signalling in Endometrial Cancer and Neuroblastoma

BACKGROUND

Protein Tyrosine Phosphatase Receptor Type D (PTPRD) is involved in the regulation of cell growth, differentiation, and oncogenic transformation, as well as in brain development. PTPRD also mediates the effects of asprosin, which is a glucogenic hormone/adipokine derived following the cleavage of the C-terminal of fibrillin 1. Since the asprosin circulating levels are elevated in certain cancers, research is now focused on the potential role of this adipokine and its receptors in cancer. As such, in this study, we investigated the expression of PTPRD in endometrial cancer (EC) and the placenta, as well as in glioblastoma (GBM).

METHODS

An array of in silico tools, in vitro models, tissue microarrays (TMAs), and liquid biopsies were employed to determine the gene and protein expression of PTPRD in healthy tissues/organs and in patients with EC and GBM.

RESULTS

PTPRD exhibits high expression in the occipital lobe, parietal lobe, globus pallidus, ventral thalamus, and white matter, whereas in the human placenta, it is primarily localised around the tertiary villi. PTPRD is significantly upregulated at the mRNA and protein levels in patients with EC and GBM compared to healthy controls. In patients with EC, PTPRD is significantly downregulated with obesity, whilst it is also expressed in the peripheral leukocytes. The EC TMAs revealed abundant PTPRD expression in both low- and high-grade tumours. Asprosin treatment upregulated the expression of PTPRD only in syncytialised placental cells.

CONCLUSIONS

Our data indicate that PTPRD may have potential as a biomarker for malignancies such as EC and GBM, further implicating asprosin as a potential metabolic regulator in these cancers. Future studies are needed to explore the potential molecular mechanisms/signalling pathways that link PTPRD and asprosin in cancer.

The Role of DNMT1 and C/EBPα in the Regulation of CYP11A1 Expression During Syncytialization of Human Placental Trophoblasts

Progesterone synthesized in the placenta is essential for pregnancy maintenance. CYP11A1 is a key enzyme in progesterone synthesis, and its expression increases greatly during trophoblast syncytialization. However, the underlying mechanism remains elusive. Here, we demonstrated that passive demethylation of CYP11A1 promoter accounted for the upregulation of CYP11A1 expression during syncytialization with the participation of the transcription factor C/EBPα. We found that the methylation rate of a CpG locus in the CYP11A1 promoter was significantly reduced along with decreased DNA methyltransferase 1 (DNMT1) expression and its enrichment at the CYP11A1 promoter during syncytialization. DNMT1 overexpression not only increased the methylation of this CpG locus in the CYP11A1 promoter, but also decreased CYP11A1 expression and progesterone production. In silico analysis disclosed multiple C/EBPα binding sites in both CYP11A1 and DNMT1 promoters. C/EBPα expression and its enrichments at both the DNMT1 and CYP11A1 promoters were significantly increased during syncytialization. Knocking-down C/EBPα expression increased DNMT1 while it decreased CYP11A1 expression during syncytialization. Conclusively, C/EBPα plays a dual role in the regulation of CYP11A1 during syncytialization. C/EBPα not only drives CYP11A1 expression directly, but also indirectly through downregulation of DNMT1, which leads to decreased methylation in the CpG locus of the CYP11A1 promoter, resulting in increased progesterone production during syncytialization.

The role of DNA hydroxymethylation and TET enzymes in placental development and pregnancy outcome

The placenta is a temporary organ that is essential for supporting mammalian embryo and fetal development. Understanding the molecular mechanisms underlying trophoblast differentiation and placental function may contribute to improving the diagnosis and treatment of obstetric complications. Epigenetics plays a significant role in the regulation of gene expression, particularly at imprinted genes, which are fundamental in the control of placental development. The Ten-Eleven-Translocation enzymes are part of the epigenetic machinery, converting 5-methylcytosine (5mC) into 5-hydroxymethylcytosine (5hmC). DNA hydroxymethylation is thought to act as an intermediate in the DNA demethylation mechanism and potentially be a stable and functionally relevant epigenetic mark on its own. The role of DNA hydroxymethylation during differentiation and development of the placenta is not fully understood but increasing knowledge in this field will help to evaluate its potential role in pregnancy complications. This review focuses on DNA hydroxymethylation and its epigenetic regulators in human and mouse placental development and function. Additionally, we address 5hmC in the context of genomic imprinting mechanism and in pregnancy complications, such as intrauterine growth restriction, preeclampsia and pregnancy loss. The cumulative findings show that DNA hydroxymethylation might be important for the control of gene expression in the placenta and suggest a dynamic role in the differentiation of trophoblast cell types during gestation.

Paternal obesity induces epigenetic aberrations and gene expression changes in placenta and fetus

Paternal epigenome regulates placental and fetal growth. However, the effect of paternal obesity on placenta and its subsequent effect on the fetus via sperm remains unknown. We previously discovered abnormal methylation of imprinted genes involved in placental and fetal development in the spermatozoa of obese rats. In the present study, elaborate epigenetic characterization of sperm, placenta, and fetus was performed. For 16 weeks, male rats were fed either control or a high-fat diet. Following mating studies, sperm, placenta, and fetal tissue were collected. Significant changes were observed in placental weights, morphology, and cell populations. Methylation status of imprinted genes-Igf2, Peg3, Cdkn1c, and Gnas in spermatozoa, correlated with their expression in the placenta and fetus. Placental DNA methylating enzymes and 5-methylCytosine levels increased. Furthermore, in spermatozoa, DNA methylation of a few genes involved in pathways associated with placental endocrine function-gonadotropin-releasing hormone, prolactin, estrogen, and vascular endothelial growth factor, correlated with their expression in placenta and fetus. Changes in histone-modifying enzymes were also observed in the placenta. Histone marks H3K4me3, H3K9me3, and H4ac were downregulated, while H3K27me3 and H3ac were upregulated in placentas derived from obese male rats. This study shows that obesity-related changes in sperm methylome translate into abnormal expression in the F1-placenta fathered by the obese male, presumably affecting placental and fetal development.

EZH2 enhances proliferation and migration of trophoblast cell lines by blocking GADD45A-mediated p38/MAPK signaling pathway

Impaired activity of the trophoblasts is a major contributor to the progression of pregnancy pathologies including preeclampsia (PE). This research probed the function of enhancer of zeste homolog 2 (EZH2) in activity of trophoblast cells and its correlation with growth arrest and DNA damage inducible alpha (GADD45A). EZH2 was predicted to be downregulated in placental tissues in PE according to a gene chip analysis, and reduced expression of EZH2 was detected in the placental tissues of patients with PE. Overexpression of EZH2 augmented proliferation and invasiveness of two trophoblast cell lines HTR-8/SVneo and JEG3 cells. EZH2 catalyzed trimethylation of lysine 27 on histone 3 (H3K27me3) in GADD45A promoter to suppress its transcription. GADD45A silencing increased the activity of the trophoblast cell lines and inactivated the p38/mitogen-activated protein kinase (MAPK) signaling pathway. Rescue experiments confirmed that either inhibition of GADD45A or p38 restored the proliferation, migration, and invasiveness of the trophoblast cell lines suppressed by EZH2 silencing. In conclusion, this work suggests that EZH2 enhances activity of trophoblast cell lines by suppressing GADD45A-mediated p38/MAPK signaling pathway.

Epigenetic regulation of epithelial to mesenchymal transition: a trophoblast perspective

Epigenetic changes alter expression of genes at both pre- and post-transcriptional levels without changing their DNA sequence. Accumulating evidence suggests that such changes can modify cellular behaviour and characteristics required during development and in response to various extracellular stimuli. Trophoblast cells develop from the outermost trophectoderm layer of the blastocyst and undergo many phenotypic changes as the placenta develops. One such phenotypic change is differentiation of the epithelial natured cytotrophoblasts into the mesenchymal natured extravillous trophoblasts. The extravillous trophoblasts are primarily responsible for invading into the maternal decidua and thus establishing connection with the maternal spiral arteries. Any dysregulation of this process can have adverse effects on the pregnancy outcome. Hence, tight regulation of this epithelial-mesenchymal transition is critical for successful pregnancy. This review summarizes the recent research on the epigenetic regulation of the epithelial-mesenchymal transition occurring in the trophoblast cells during placental development. The functional significance of chemical modifications of DNA and histone, which regulate transcription, as well as non-coding RNAs, which control gene expression post-transcriptionally, is discussed in relation to trophoblast biology.

Elevated placental histone H3K4 methylation via upregulated histone methyltransferases SETD1A and SMYD3 in preeclampsia and its possible involvement in hypoxia-induced pathophysiological process

INTRODUCTION

Disturbance in placental epigenetic regulation contributes to the pathogenesis of preeclampsia (PE). Although aberrant placental DNA methylation status in PE has been thoroughly studied, the role of histone modifications, including histone methylation, in PE remains unclear. Moreover, no study has ever reported the association between PE and placental histone methylation status by focusing on histone methyltransferases. The present study aimed to investigate the possible involvement of placental epigenetic regulation by histone methylation via histone methyltransferases in the pathophysiology of PE.

METHODS

Placental mRNA expression of histone methyltransferases was examined using quantitative RT-PCR. Protein expression of histone methyltransferases and histone methylation status in placentas and trophoblast cell lines were assessed by immunoblotting and immunohistochemistry.

RESULTS

Expression profile of histone methyltransferases in the placentas using quantitative RT-PCR revealed that the mRNA expression levels of histone 3 lysine 4 (H3K4) methyltransferases, SETD1A and SMYD3, were significantly increased in placentas from PE patients. Immunoblotting and immunohistochemistry revealed that not only protein expression levels of SETD1A and SMYD3, but also H3K4 methylation status was increased in the trophoblasts from PE placentas. In vitro studies using HTR-8/SV-neo and BeWo cells showed that hypoxia induced the expression levels of SETD1A and SMYD3, and subsequently enhanced H3K4 methylation. Furthermore, the overexpression of SETD1A and SMYD3 in HTR-8/SV-neo cells enhanced H3K4 methylation in response to hypoxia.

DISCUSSION

Our study results suggest that placental epigenetic alteration by enhanced histone H3K4 methylation through upregulated SETD1A and SMYD3 might play a role in the pathophysiological process of PE associated with hypoxia.

Placental DNA methylation signatures of maternal smoking during pregnancy and potential impacts on fetal growth

Maternal smoking during pregnancy (MSDP) contributes to poor birth outcomes, in part through disrupted placental functions, which may be reflected in the placental epigenome. Here we present a meta-analysis of the associations between MSDP and placental DNA methylation (DNAm) and between DNAm and birth outcomes within the Pregnancy And Childhood Epigenetics (PACE) consortium (N = 1700, 344 with MSDP). We identify 443 CpGs that are associated with MSDP, of which 142 associated with birth outcomes, 40 associated with gene expression, and 13 CpGs are associated with all three. Only two CpGs have consistent associations from a prior meta-analysis of cord blood DNAm, demonstrating substantial tissue-specific responses to MSDP. The placental MSDP-associated CpGs are enriched for environmental response genes, growth-factor signaling, and inflammation, which play important roles in placental function. We demonstrate links between placental DNAm, MSDP and poor birth outcomes, which may better inform the mechanisms through which MSDP impacts placental function and fetal growth.

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

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

Depletion of CTCF disrupts PSG gene expression in the human trophoblast cell line Swan 71

Pregnancy‐specific glycoproteins (PSGs) are fetal proteins secreted by the placenta during pregnancy. The PSG level in maternal serum is an indicator of risk for pregnancy complications. However, little is known about the molecular mechanisms underlying PSG gene expression. Recently, the importance of epigenetic regulation of placental genes has been emphasized in the study of developmental defects and placental disease. In this study, the role of the CCCTC‐binding factor (CTCF) in regulation of PSG expression was investigated to better understand the epigenetic regulatory mechanisms of the PSG genes. Inhibition of CTCF expression disturbed transcription of several PSG genes: PSG1, PSG2, PSG4, PSG5, PSG8, and PSG9 were upregulated and PSG6 and PSG11 were downregulated. These transcriptional changes were correlated with decreased CTCF binding and changes in histone modification at the PSG promoters. Our data demonstrate that CTCF is a potential mediator in the regulation of PSG gene expression.

Cell-specific characterization of the placental methylome

BACKGROUND

DNA methylation (DNAm) profiling has emerged as a powerful tool for characterizing the placental methylome. However, previous studies have focused primarily on whole placental tissue, which is a mixture of epigenetically distinct cell populations. Here, we present the first methylome-wide analysis of first trimester (n = 9) and term (n = 19) human placental samples of four cell populations: trophoblasts, Hofbauer cells, endothelial cells, and stromal cells, using the Illumina EPIC methylation array, which quantifies DNAm at > 850,000 CpGs.

RESULTS

The most distinct DNAm profiles were those of placental trophoblasts, which are central to many pregnancy-essential functions, and Hofbauer cells, which are a rare fetal-derived macrophage population. Cell-specific DNAm occurs at functionally-relevant genes, including genes associated with placental development and preeclampsia. Known placental-specific methylation marks, such as those associated with genomic imprinting, repetitive element hypomethylation, and placental partially methylated domains, were found to be more pronounced in trophoblasts and often absent in Hofbauer cells. Lastly, we characterize the cell composition and cell-specific DNAm dynamics across gestation.

CONCLUSIONS

Our results provide a comprehensive analysis of DNAm in human placental cell types from first trimester and term pregnancies. This data will serve as a useful DNAm reference for future placental studies, and we provide access to this data via download from GEO (GSE159526), through interactive exploration from the web browser ( https://robinsonlab.shinyapps.io/Placental_Methylome_Browser/ ), and through the R package planet, which allows estimation of cell composition directly from placental DNAm data.

Placentation in the Human and Higher Primates

Placentation in humans is precocious and highly invasive compared to other mammals. Implantation is interstitial, with the conceptus becoming completely embedded within the endometrium towards the end of the second week post-fertilization. Villi initially form over the entire surface of the chorionic sac, stimulated by histotrophic secretions from the endometrial glands. The secondary yolk sac never makes contact with the chorion, and a choriovitelline placenta is never established. However, recent morphological and transcriptomic analyses suggest that the yolk sac plays an important role in the uptake of nutrients from the coelomic fluid. Measurements performed in vivo demonstrate that early development takes place in a physiological, low-oxygen environment that protects against teratogenic free radicals and maintains stem cells in a multipotent state. The maternal arterial circulation to the placenta is only fully established around 10-12 weeks of gestation. By then, villi have regressed over the superficial, abembryonic pole, leaving the definitive discoid placenta, which is of the villous, hemochorial type. Remodeling of the maternal spiral arteries is essential to ensure a high-volume but low-velocity inflow into the mature placenta. Extravillous trophoblast cells migrate from anchoring villi and surround the arteries. Their interactions with maternal immune cells release cytokines and proteases that are key to remodeling, and a successful pregnancy.

Epigenetic regulation of folate receptor-α (FOLR1) in human placenta of preterm newborns

INTRODUCTION

Folates are essential nutrients for fetal development and pregnancy outcomes; they are transported to the fetus during gestation through specific folate transporters located in the placenta. In preterm newborns, we previously showed a lower placental mRNA expression of FOLR1 along with higher folate and lower vitamin B12 cord blood levels. Thereby we aimed to explore FOLR1 methylation in placentas of preterm newborns and hypothesized an increased FOLR1 methylation associated with cord blood folates and vitamin B12 concentrations.

METHODS

FOLR1 methylation and mRNA were determined by methylation sensitive - high resolution melting (MS-HRM) and by real-time PCR respectively, in two placental sides of placental tissues: maternal (basal, BP) and fetal plates (chorionic, CP) of moderate preterm infants (32-36 gestational age) and term birth (37-41 gestational weeks). Folates and vitamin B12 were determined by electrochemiluminescence in umbilical cord blood samples from term and preterm newborns.

RESULTS

We found that in preterm newborns, FOLR1 mRNA was lower in both plates of placenta compared with term newborns (p < 0,05) and was negatively associated with methylation of FOLR1 in CP. Preterm newborns presented higher folate and lower vitB12 concentrations in cord blood which correlated with increased placental FOLR1 methylation.

DISCUSSION

In preterm newborns, placental FOLR1 expression is regulated by epigenetic mechanisms and presumably by maternal concentrations of folate and vitamin B12.

Reactive oxygen species from mitochondria impacts trophoblast fusion and the production of endocrine hormones by syncytiotrophoblasts

The placenta, a tissue that is metabolically active and rich in mitochondria, forms a critical interface between the mother and developing fetus. Oxidative stress within this tissue, derived from the dysregulation of reactive oxygen species (ROS), has been linked to a number of adverse fetal outcomes. While such outcomes have been associated with mitochondrial dysfunction, the causal role of mitochondrial dysfunction and mitochondrially generated ROS in altering the process of placentation remains unclear. In this study, mitochondrial complex I activity was attenuated using 10 nM rotenone to induce cellular oxidative stress by increasing mitochondrial ROS production in the BeWo choriocarcinoma cell line. Increased mitochondrial ROS resulted in a significant decrease in the transcripts which encode for proteins associated with fusion (GCM1, ERVW-1, and ERVFRD-1) resulting in a 5-fold decrease in the percentage of BeWo fusion. This outcome was associated with increased indicators of mitochondrial fragmentation, as determined by decreased expression of MFN2 and OPA1 along with an increase in a marker of mitochondrial fission (DRP1). Importantly, increased mitochondrial ROS also resulted in a 5.0-fold reduction of human placental lactogen (PL) and a 4.4-fold reduction of insulin like growth factor 2 (IGF2) transcripts; hormones which play an important role in regulating fetal growth. The pre-treatment of rotenone-exposed cells with 5 mM N-acetyl cysteine (NAC) resulted in the prevention of these ROS mediated changes in BeWo function and supports a central role for mitochondrial ROS signaling in the maintenance and function of the materno-fetal interface.

Altered expression of epigenetic regulators and imprinted genes in human placenta and fetal tissues from second trimester spontaneous pregnancy losses

Epigenetic mechanisms such as genomic imprinting have a fundamental role in embryo and fetal development. Hence, we here studied expression levels of epigenetic modifiers and imprinted genes in cases of ididopathic spontaneous abortion (SA). Thirty-five placental samples and 35 matched fetal tissues from second trimester SA were analysed; including 16 controls (placental and fetal infections as the known cause of spontaneous abortion) and 19 idiopathic SA cases. Transcript levels of epigenetic regulators and imprinted genes were measured by qRT-PCR and methylation at imprinted genes was studied by bisulfite genomic sequencing and MS-MLPA. Global DNA hydroxymethylation (5-hmC) levels were measured by an ELISA-based assay. We observed an upregulation of TET2 and TET3 in placental samples from idiopathic SA cases; however, no significant difference in global 5-hmC levels was observed. On the contrary, in fetal tissues, TET3 was markedly downregulated in idiopathic SA, showing an opposite trend to that observed in placental tissue. IGF2 and CDKN1C were upregulated and MEST downregulated in placentas from idiopathic SA cases; concordantly, IGF2 was also upregulated in fetal tissues from idiopathic SA cases. Although not reaching statistical significance, an increase in methylation levels of MEST, KvDMR1 and H19 DMRs was observed in idiopathic SA cases, concordantly with the observed changes in expression. Our study reveals, for the first time, deregulation of epigenetic modifiers and imprinted genes in both placental and fetal tissues from idiopathic SA cases in the second trimester of pregnancy, indicating a critical role during pregnancy.

The significance of the placental genome and methylome in fetal and maternal health

The placenta is a crucial organ for supporting a healthy pregnancy, and defective development or function of the placenta is implicated in a number of complications of pregnancy that affect both maternal and fetal health, including maternal preeclampsia, fetal growth restriction, and spontaneous preterm birth. In this review, we highlight the role of the placental genome in mediating fetal and maternal health by discussing the impact of a variety of genetic alterations, from large whole-chromosome aneuploidies to single-nucleotide variants, on placental development and function. We also discuss the placental methylome in relation to its potential applications for refining diagnosis, predicting pathology, and identifying genetic variants with potential functional significance. We conclude that understanding the influence of the placental genome on common placental-mediated pathologies is critical to improving perinatal health outcomes.

Mechanisms of early placental development in mouse and humans

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

Placental mitochondria and reactive oxygen species in the physiology and pathophysiology of pregnancy

Mitochondria are central to cell function. The placenta forms the interface between maternal and fetal systems, and placental mitochondria have critical roles in maintaining pregnancy. The placenta is unusual in having two adjacent cell layers (cytotrophoblasts and the syncytiotrophoblast) with vastly different mitochondria that have distinct functions in health and disease. Mitochondria both produce the majority of reactive oxygen species (ROS), and are sensitive to ROS. ROS are important in allowing cells to sense their environment through mitochondrial-centred signalling, and this signalling also helps cells/tissues adapt to changing environments. However, excessive ROS are damaging, and increased ROS levels are associated with pregnancy complications, including the important disorders preeclampsia and gestational diabetes mellitus. Here we review the function of placental mitochondria in healthy pregnancy, and also in pregnancy complications. Placental mitochondria are critical to cell function, and mitochondrial damage is a feature of pregnancy complications. However, the responsiveness of mitochondria to ROS signalling may be central to placental adaptations that mitigate damage, and placental mitochondria are an attractive target for the development of therapeutics to improve pregnancy outcomes.

The Role of Epigenetics in Placental Development and the Etiology of Preeclampsia

In this review, we comprehensively present the function of epigenetic regulations in normal placental development as well as in a prominent disease of placental origin, preeclampsia (PE). We describe current progress concerning the impact of DNA methylation, non-coding RNA (with a special emphasis on long non-coding RNA (lncRNA) and microRNA (miRNA)) and more marginally histone post-translational modifications, in the processes leading to normal and abnormal placental function. We also explore the potential use of epigenetic marks circulating in the maternal blood flow as putative biomarkers able to prognosticate the onset of PE, as well as classifying it according to its severity. The correlation between epigenetic marks and impacts on gene expression is systematically evaluated for the different epigenetic marks analyzed.

Characterization of 5-methylcytosine and 5-hydroxymethylcytosine in human placenta cell types across gestation

The placenta is an important organ in pregnancy, however, very little is understood about placental development at a molecular level. This includes the role of epigenetic mechanisms and how they change throughout gestation. DNA methylation studies in this organ are complicated by the different cell types that make up the placenta, each with their own unique transcriptome and epigenome. Placental dysfunction is often associated with pregnancy complications such as preeclampsia (PE). Aberrant DNA methylation in the placenta has been identified in pregnancy complications. We used immunohistochemistry (IHC) and immunofluorescence (IF) to localize 5-methylcytosine (5-mC) and 5-hydroxymethylcytosine (5-hmC) in placenta tissue from first and second trimester as well as uncomplicated term and PE samples. IHC analysis of whole placental tissues showed 5-mC increased across gestation. When cytotrophoblasts (CTB) and syncytiotrophoblasts (STB) were isolated and assessed using IF, both 5-mC and 5-hmC increased in term CTBs compared to first/second-trimester samples. Staining intensity of 5-hmC was higher in first/second trimester STBs compared to CTBs (P = 0.0011). Finally, IHC staining of term tissue from PE and uncomplicated pregnancies revealed higher 5-mC staining intensity in placentas from PE pregnancies (P = 0.028). Our study has shown increased 5-mC and 5-hmC staining intensities across gestation and differed between two trophoblast populations. Differences in DNA methylation profiles between placental cell types may be indicative of different functions and requires further study to elucidate what changes accompany placental pathologies.

Genome-Wide Identification of Histone Modifications Involved in Placental Development in Pigs

Development of placental folds is a critical event affecting placental function in pigs because it can increase surface area for improvement in capillary density as gestation advances. However, the molecular mechanisms of the event are not well defined. Histone modifications have important roles in gene regulation. To investigate their effects on regulation of genes controlling porcine placental development, RNA-seq and ChIP-seq of porcine placental tissues from gestational days 50 (establishment stage of placental folds) and 95 (expanding stage of placental folds) were carried out in this study. The differentially expressed genes were identified and of which the down- and up-regulated genes are related to endoplasmic reticulum (ER) stress and angiogenesis, respectively. In addition, we mapped the genome-wide profiles of histone H3 lysine 4 trimethylation (H3K4me3) and histone H3 lysine 27 acetylation (H3K27ac), which are associated with transcriptional activation. A number of differential modification regions between the 2 gestational stages were identified and majority of them are those with increased signals of H3K4me3 (14,576 out of 16,931). Furthermore, we observed that the increase of H3K4me3 is significantly correlated with the elevated expression levels of the neighboring genes, and notably, these genes were enriched in pathways related to blood vessel formation and microvascular permeability. Taken together, the findings suggest important roles of histone modifications on placental remolding in response to developmental changes.

Analysis of the Epigenome in Multiplex Pre-eclampsia Families Identifies SORD, DGKI, and ICA1 as Novel Candidate Risk Genes

Pre-eclampsia is a serious heritable disorder that affects 5-8% of pregnancies worldwide. While classical genetic studies have identified several susceptibility genes they do not fully explain the heritability of pre-eclampsia. An additional contribution to risk can be quantified by examining the epigenome, in particular the methylome, which is a representation of interactions between environmental and genetic influences on the phenotype. Current array-based epigenetic studies only examine 2-5% of the methylome. Here, we used whole-genome bisulfite sequencing (WGBS) to determine the entire methylome of 13 individuals from two multiplex pre-eclampsia families, comprising one woman with eclampsia, six women with pre-eclampsia, four women with uncomplicated normotensive pregnancies and two male relatives. The analysis of WGBS profiles using two bioinformatics platforms, BSmooth and Bismark, revealed 18,909 differentially methylated CpGs and 4157 differentially methylated regions (DMRs) concordant in females. The methylation patterns support the involvement of previously reported candidate genes, including COL4A1, SLC2A4, PER3, FLT1, GPI, LCT, DDAH1, TGFB3, DLX5, and LRP1B. Statistical analysis of DMRs revealed three novel genes significantly correlated with pre-eclampsia: sorbitol dehydrogenase (SORD, p = 9.98 × 10^-6), diacylglycerol kinase iota (DGKI, p = 2.52 × 10^-5), and islet cell autoantigen 1 (ICA1, 7.54 × 10^-3), demonstrating the potential of WGBS in families for elucidating the role of epigenome in pre-eclampsia and other complex diseases.

The Attenuation of Trophoblast Invasion Caused by the Downregulation of EZH2 Is Involved in the Pathogenesis of Human Recurrent Miscarriage

Recurrent miscarriage (RM) is currently defined as two or more losses of a clinically established intrauterine pregnancy. Despite years of research, RM continues to be a clinically frustrating challenge for patients and physicians, and its etiology remains poorly understood. Accumulating evidence has suggested that epigenetic modifications are involved in early embryogenesis, and defects in epigenetic patterning contribute to the development of RM. Here, we studied the role of enhancer of zeste homolog 2 (EZH2) in the pathogenesis of RM and found that the EZH2 expression was significantly decreased in the villi from women with RM compared with that in control villi. EZH2 promoted the invasion of trophoblast cells. Moreover, EZH2 could promote epithelial-mesenchymal transition by epigenetically silencing CDX1. Both chromatin immunoprecipitation (ChIP)-PCR and dual-luciferase report assays demonstrated that EZH2 repressed CDX1 transcription via direct binding to its promoter region and then trimethylating Histone3-Lysine27. Furthermore, we discovered that progesterone, which is used extensively in the treatment of miscarriage and RM, increased the expression of EZH2 via the extracellular signaling-regulated kinase (ERK1/2) pathway. These findings revealed that EZH2 may regulate trophoblast invasion as an epigenetic factor, suggesting that EZH2 might be a potential therapeutic target for RM.

Dynamic expression of TET1, TET2, and TET3 dioxygenases in mouse and human placentas throughout gestation

INTRODUCTION

Throughout pregnancy, the placenta dynamically changes as trophoblast progenitors differentiate into mature trophoblast cell subtypes. This process is in part controlled by epigenetic regulation of DNA methylation leading to the inactivation of 'progenitor cell' genes and the activation of 'differentiation' genes. TET methylcytosine dioxygenases convert 5-methylcytosine (5-mC) to 5-hydroxymethylcytosine (5-hmC) during DNA demethylation events. Here, we determine the spatiotemporal expression of TET1, TET2, and TET3 in specific trophoblast cell populations of mouse and human placentas throughout gestation, and consider their role in trophoblast cell differentiation and function.

METHODS

In situ hybridization analysis was conducted to localize Tet1, Tet2, and Tet3 mRNA at key stages of mouse placental development. The distribution of 5-mC and 5-hmC in these samples was also evaluated. In comparison, expression patterns of TET1, TET2, and TET3 protein in human placentas were determined in first trimester and term pregnancies.

RESULTS

In mouse, Tet1-3 mRNA was widely expressed in trophoblast cell populations from embryonic (E) day 8.5 to E12.5 including in progenitor and differentiated cells. However, expression became restricted to specific trophoblast giant cell subtypes by late gestation (E14.5 to E18.5). This coincided with cellular changes in 5-mC and 5-hmC levels. In human, cell columns, extravillous trophoblast and syncytiotrophoblast expressed TET1-3 whereas only TET3 was expressed in villus cytotrophoblast cells in first trimester and term placentas.

DISCUSSION

Altogether, our data suggest that TET enzymes may play a dynamic role in the regulation of transcriptional activity of trophoblast progenitors and differentiated cell subtypes in mouse and human placentas.

Placental Origins of Chronic Disease

Epidemiological evidence links an individual's susceptibility to chronic disease in adult life to events during their intrauterine phase of development. Biologically this should not be unexpected, for organ systems are at their most plastic when progenitor cells are proliferating and differentiating. Influences operating at this time can permanently affect their structure and functional capacity, and the activity of enzyme systems and endocrine axes. It is now appreciated that such effects lay the foundations for a diverse array of diseases that become manifest many years later, often in response to secondary environmental stressors. Fetal development is underpinned by the placenta, the organ that forms the interface between the fetus and its mother. All nutrients and oxygen reaching the fetus must pass through this organ. The placenta also has major endocrine functions, orchestrating maternal adaptations to pregnancy and mobilizing resources for fetal use. In addition, it acts as a selective barrier, creating a protective milieu by minimizing exposure of the fetus to maternal hormones, such as glucocorticoids, xenobiotics, pathogens, and parasites. The placenta shows a remarkable capacity to adapt to adverse environmental cues and lessen their impact on the fetus. However, if placental function is impaired, or its capacity to adapt is exceeded, then fetal development may be compromised. Here, we explore the complex relationships between the placental phenotype and developmental programming of chronic disease in the offspring. Ensuring optimal placentation offers a new approach to the prevention of disorders such as cardiovascular disease, diabetes, and obesity, which are reaching epidemic proportions.

A repressive role of enhancer of zeste homolog 2 in 11β-hydroxysteroid dehydrogenase type 2 expression in the human placenta

The expression of 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), which acts as a placental glucocorticoid barrier, is silenced in cytotrophoblasts but substantially up-regulated during syncytialization. However, the repressive mechanism of 11β-HSD2 expression before syncytialization and how this repression is lifted during syncytialization remain mostly unresolved. Here we found that enhancer of zeste homolog 2 (EZH2) accounts for the silence of 11β-HSD2 expression via trimethylation of histone H3 lysine 27 at the promoter of the 11β-HSD2 gene. Further studies revealed that, upon syncytialization, human chorionic gonadotropin reduced the phosphorylation of retinoblastoma protein (pRB) via activation of the cAMP/PKA pathway, which sequesters E2F transcription factor 1 (E2F1), the transcription factor for EZH2 expression. As a result of inactivation of the pRB-E2F1-EZH2 pathway, the repressive marker trimethylation of histone H3 lysine 27 at the 11β-HSD2 promoter is removed, which leads to the robust expression of 11β-HSD2 during syncytialization.

Variable DAXX gene methylation is a common feature of placental trophoblast differentiation, preeclampsia, and response to hypoxia

Placental functioning relies on the appropriate differentiation of progenitor villous cytotrophoblasts (CTBs) into extravillous cytotrophoblasts (EVCTs), including invasive EVCTs, and the multinucleated syncytiotrophoblast (ST) layer. This is accompanied by a general move away from a proliferative, immature phenotype. Genome-scale expression studies have provided valuable insight into genes that are associated with the shift to both an invasive EVCT and ST phenotype, whereas genome-scale DNA methylation analysis has shown that differentiation to ST involves widespread methylation shifts, which are counteracted by low oxygen. In the current study, we sought to identify DNA methylation variation that is associated with transition from CTB to ST in vitro and from a noninvasive to invasive EVCT phenotype after culture on Matrigel. Of the several hundred differentially methylated regions that were identified in each comparison, the majority showed a loss of methylation with differentiation. This included a large differentially methylated region (DMR) in the gene body of death domain-associated protein 6 (DAXX ), which lost methylation during both CTB syncytialization to ST and EVCT differentiation to invasive EVCT. Comparison to publicly available methylation array data identified the same DMR as among the most consistently differentially methylated genes in placental samples from preeclampsia pregnancies. Of interest, in vitro culture of CTB or ST in low oxygen increases methylation in the same region, which correlates with delayed differentiation. Analysis of combined epigenomics signatures confirmed DAXX DMR as a likely regulatory element, and direct gene expression analysis identified a positive association between methylation at this site and DAXX expression levels. The widespread dynamic nature of DAXX methylation in association with trophoblast differentiation and placenta-associated pathologies is consistent with an important role for this gene in proper placental development and function.-Novakovic, B., Evain-Brion, D., Murthi, P., Fournier, T., Saffery, R. Variable DAXX gene methylation is a common feature of placental trophoblast differentiation, preeclampsia, and response to hypoxia.

MTA3 Regulates Extravillous Trophoblast Invasion Through NuRD Complex

Extravillous trophoblast (EVT) invasion is required for remodeling uterine tertiary arteries and placenta development during pregnancy. Compromised EVT invasion may contribute to the pathology of placenta-related diseases. Metastasis -associated protein 3 (MTA3) is one of the subunits of nucleosome remodeling and deacetylation (NuRD) complex that represses transcription in a histone deacetylase-dependent manner. MTA3 is reported to be down-regulated in preeclamptic placentas, suggesting its potential role in EVT invasion. Here, we investigate the role of MTA3 in EVT invasion by studying its molecular mechanisms in EVT cells. First, we confirmed MTA3 expression in the EVT cells in human placenta using immunohistochemistry. We then used lentivirus-mediated MTA3 short hairpin RNA (shRNA) to knock down MTA3 expression in EVT-derived HTR8/SVneo cells and found higher invasion capacity in MTA3 knockdown cells. Using quantitative real-time PCR, we showed higher expression of invasion-related genes matrix metalloproteinase 2 (MMP2), matrix metalloproteinase 9 (MMP9), and transcription factor Snail in MTA3 knockdown compared with control cells. Co-immunoprecipitation-Western blot assay showed the protein-protein interaction of histone deacetylase 1 (HDAC1), a subunit of NuRD, with MTA3 in HTR8/SVneo cells. Co-immunoprecipitation-Mass spectrometry assay further identified 71 proteins interacting with MTA3, including NuRD subunits, heterochromatin proteins, epigenetics modifiers and transcription factors. This result not only indicated the involvement of NuRD complex in MTA3’s function, but also demonstrated the complicated multiple co-players in MTA3 and NuRD complex mediated transcription repression in EVT. In summary, our data demonstrates that MTA3 regulates EVT invasion and related gene expression via NuRD complex in EVT.

Recent progress towards understanding the role of DNA methylation in human placental development

Epigenetic modifications, and particularly DNA methylation, have been studied in many tissues, both healthy and diseased, and across numerous developmental stages. The placenta is the only organ that has a transient life of 9 months and undergoes rapid growth and dynamic structural and functional changes across gestation. Additionally, the placenta is unique because although developing within the mother, its genome is identical to that of the foetus. Given these distinctive characteristics, it is not surprising that the epigenetic landscape affecting placental gene expression may be different to that in other healthy tissues. However, the role of epigenetic modifications, and particularly DNA methylation, in placental development remains largely unknown. Of particular interest is the fact that the placenta is the most hypomethylated human tissue and is characterized by the presence of large partially methylated domains (PMDs) containing silenced genes. Moreover, how and why the placenta is hypomethylated and what role DNA methylation plays in regulating placental gene expression across gestation are poorly understood. We review genome-wide DNA methylation studies in the human placenta and highlight that the different cell types that make up the placenta have very different DNA methylation profiles. Summarizing studies on DNA methylation in the placenta and its relationship with pregnancy complications are difficult due to the limited number of studies available for comparison. To understand the key steps in placental development and hence what may be perturbed in pregnancy complications requires large-scale genome-wide DNA methylation studies coupled with transcriptome analyses.

Potential role of epigenetic mechanisms in regulation of trophoblast differentiation, migration, and invasion in the human placenta

The proper establishment and organogenesis of the placenta is crucial for intrauterine fetal growth and development. Endometrial invasion by the extravillous trophoblast cells, as well as formation of the syncytiotrophoblast (STB), are of vital importance for placental function. Trophoblast migration and invasion is often compared to tumor metastasis, which uses many of the same molecular mechanisms. However, unlike cancer cells, both initiation and the extent of trophoblast invasion are tightly regulated by feto-maternal cross-talk, which when perturbed, results in a wide range of abnormalities. Multiple factors control the trophoblast, including cytokines and hormones, which are subject to transcriptional regulatory networks. The relevance of epigenetics in transcriptional regulation of trophoblast differentiation and invasion, as well as in the onset of placenta-related pregnancy disorders, became recognized decades ago. Although, there has been tremendous progress in uncovering the molecular foundation of placental development, there is still much to be learned about the epigenetic machinery, and its role in trophoblast differentiation and invasion. This review will provide an overview of the epigenetic control of trophoblast differentiation and invasion. It will also highlight the major epigenetic mechanisms involved in pregnancy complications related to placental deficiencies.

Transcriptomic and epigenomic landscapes during cell fusion in BeWo trophoblast cells

INTRODUCTION

Syncytialization is a process essential to the genesis and vitality of the decisive maternal-fetal interface, the syncytiotrophoblast. While the role of specific genes important in syncytial fusion is appreciated, an integrated global analysis of syncytialization is absent.

METHODS

We leveraged a variety of approaches (RNA-seq, genome-scale DNA methylation and ChIP-seq) to assemble a genome-wide transcriptomic and epigenomic view of syncytialization in BeWo cells.

RESULTS

RNA-seq analysis of expression profiles revealed alterations in ∼3000 genes over the 3 day time-course of forskolin, including identification of several previously unrecognized genes to be involved in syncytialization. These genes were enriched for cell differentiation, morphogenesis, blood vessel and placental labyrinth development and steroid hormone response. Genome-scale DNA methylation via reduced representation bisulfite sequencing (RRBS) showed altered methylation of a number of CpGs associated with cell differentiation and commitment. Finally, genome-wide localization of seven key histone marks encompassing permissive (H3K4me3, H3K9ac, H3K27ac), enhancer (H3K4me1), elongation (H3K36me3) and repressive (H3K27me3, H3K9me3) states was performed via ChiP-seq. These analyses clearly revealed that syncytialization was associated with a gain in transcriptionally permissive/active marks (H3K4me3, K9ac, K27ac and K36me3) among genes that are either constitutive or upregulated in syncytialization.

DISCUSSION

Overall, these results provide a novel resource to elucidate the underlying epigenetic mechanisms coordinating transcriptional changes associated with syncytialization in BeWo cells.