Notch1 controls development of the extravillous trophoblast lineage in the human placenta

A quantitative image analysis platform for assessing trophoblast differentiation

Immunofluorescence microscopy is extensively used in characterization of trophoblast differentiation in vitro. However, such data is primarily used to confirm the presence of protein markers or qualitatively compare levels of protein markers across experimental conditions. Imaging data, when processed and analyzed appropriately can provide quantitative and spatial information, and provide biological insight. Towards this end, here we present MATroph, an open-source MATLAB-based computational tool to process images generated by immunofluorescent microscopy. MATroph automatically executes a series of image processing operations, including the classification of red, blue, and green channels from images, background extraction, morphological operations, and image filtering. From the isolated blue channels corresponding to nuclear staining, this tool generates numerical values for cell number. Additionally, relative levels and spatial location of proteins are obtained by mapping red and green channel pixels to blue pixels by assigning minimum pixel distance between the blue and other color objects. Thus, this tool provides information about intracellular protein accumulation areas. Additionally, this tool can also classify cells as single cells or part of colonies, and extract information on protein levels for each; this is particularly useful for quantitative studies on extravillous trophoblast maturation. We provide a user-guide to analyze the relative levels of markers relevant to human trophoblast stem cell self-renewal and differentiation. Importantly, MATroph is composed of a simple MATLAB algorithm, and its implementation requires minimal expertise in programming.

METTL3 shapes m6A epitranscriptomic landscape for successful human placentation

Methyltransferase-like 3 (METTL3), the catalytic enzyme of methyltransferase complex for m6A methylation of RNA, is essential for mammalian development. However, the importance of METTL3 in human placentation remains largely unexplored. Here, we show that a fine balance of METTL3 function in trophoblast cells is essential for successful human placentation. Both loss-of and gain-in METTL3 functions are associated with adverse human pregnancies. A subset of recurrent pregnancy losses and preterm pregnancies are often associated with loss of METTL3 expression in trophoblast progenitors. In contrast, METTL3 is induced in pregnancies associated with fetal growth restriction (FGR). Our loss of function analyses showed that METTL3 is essential for the maintenance of human TSC self-renewal and their differentiation to extravillous trophoblast cells (EVTs). In contrast, loss of METTL3 in human TSCs promotes syncytiotrophoblast (STB) development. Global analyses of RNA m6A modification and METTL3-RNA interaction in human TSCs showed that METTL3 regulates m6A modifications on the mRNA molecules of critical trophoblast regulators, including GATA2, GATA3, TEAD1, TEAD4, WWTR1, YAP1, TFAP2C and ASCL2, and loss of METTL3 leads to depletion of mRNA molecules of these critical regulators. Importantly, conditional deletion of Mettl3 in trophoblast progenitors of an early post-implantation mouse embryo also leads to arrested self-renewal. Hence, our findings indicate that METLL3 is a conserved epitranscriptomic governor in trophoblast progenitors and ensures successful placentation by regulating their self-renewal and dictating their differentiation fate.

Identification of HTRA4 as a Transcriptional Target of p63 in Trophoblast

The placenta plays a crucial role in pregnancy success. ΔNp63α (p63), a transcription factor from the TP53 family, is highly expressed in villous cytotrophoblasts (CTBs), the epithelial stem cells of the human placenta, and is involved in CTB maintenance and differentiation. We examined the mechanisms of action of p63 by identifying its downstream targets. Gene expression changes were evaluated following overexpression and knockdown of p63 in the JEG3 choriocarcinoma cell line, using microarray-based RNA profiling. High-temperature requirement A4 (HTRA4), a placenta-specific serine protease involved in trophoblast differentiation and altered in preeclampsia, was identified as a gene reciprocally regulated by p63, and its expression was characterized in primary human placental tissues by RNA-sequencing and in situ hybridization. Potential p63 DNA-binding motifs were identified in the HTRA4 promoter, and p63 occupancy at some of these sites was confirmed using chromatin immunoprecipitation, followed by quantitative PCR in both JEG3 and trophoblast stem cells. These data begin to identify members of the transcriptional network downstream of p63, thus laying the groundwork for probing mechanisms by which this important transcription factor regulates trophoblast stemness and differentiation.

Suppression of GATA3 promotes epithelial-mesenchymal transition and simultaneous cellular senescence in human extravillous trophoblasts

The regulatory mechanism of the transcription factor GATA3 in the differentiation and maturation process of extravillous trophoblasts (EVT) in early pregnancy placenta, as well as its relevance to the occurrence of pregnancy disorders, remains poorly understood. This study leveraged single-cell RNA sequencing data from placental organoid models and placental tissue to explore the dynamic changes in GATA3 expression during EVT maturation. The expression pattern exhibited an initial upregulation followed by subsequent downregulation, with aberrant GATA3 localization observed in cases of recurrent miscarriage (RM). By identifying global targets regulated by GATA3 in primary placental EVT cells, JEG3, and HTR8/SVneo cell lines, this study offered insights into its regulatory mechanisms across different EVT cell models. Shared regulatory targets among these cell types and activation of trophoblast cell marker genes emphasized the importance of GATA3 in EVT differentiation and maturation. Knockdown of GATA3 in JEG3 cells led to repression of GATA3-induced epithelial-mesenchymal transition (EMT), as evidenced by changes in marker gene expression levels and enhanced migration ability. Additionally, interference with GATA3 accelerated cellular senescence, as indicated by reduced proliferation rates and increased activity levels for senescence-associated β-galactosidase enzyme, along with elevated expression levels for senescence-associated genes. This study provides comprehensive insights into the dual role of GATA3 in regulating EMT and cellular senescence during EVT differentiation, shedding light on the dynamic changes in GATA3 expression in normal and pathological placental conditions.

Notch Signaling: An Emerging Paradigm in the Pathogenesis of Reproductive Disorders and Diverse Pathological Conditions

The highly conserved Notch pathway, a pillar of juxtacrine signaling, orchestrates intricate intercellular communication, governing diverse developmental and homeostatic processes through a tightly regulated cascade of proteolytic cleavages. This pathway, culminating in the migration of the Notch intracellular domain (NICD) to the nucleus and the subsequent activation of downstream target genes, exerts a profound influence on a plethora of molecular processes, including cell cycle progression, lineage specification, cell-cell adhesion, and fate determination. Accumulating evidence underscores the pivotal role of Notch dysregulation, encompassing both gain and loss-of-function mutations, in the pathogenesis of numerous human diseases. This review delves deep into the multifaceted roles of Notch signaling in cellular dynamics, encompassing proliferation, differentiation, polarity maintenance, epithelial-mesenchymal transition (EMT), tissue regeneration/remodeling, and its intricate interplay with other signaling pathways. We then focus on the emerging landscape of Notch aberrations in gynecological pathologies predisposing individuals to infertility. By highlighting the exquisite conservation of Notch signaling in Drosophila and its power as a model organism, we pave the way for further dissection of disease mechanisms and potential therapeutic interventions through targeted modulation of this master regulatory pathway.

Immunological regulation and the role of autophagy in preeclampsia

Autophagy is a bulk degradation system that maintains cellular homeostasis by producing energy and/or recycling excess proteins. During early placentation, extravillous trophoblasts invade the decidua and uterine myometrium, facing maternal immune cells, which participate in the immune suppression of paternal and fetal antigens. Regulatory T cells will likely increase in response to a specific antigen before and during early pregnancy. Insufficient expansion of antigen-specific Treg cells, which possess the same T cell receptor, is associated with the pathophysiology of preeclampsia, suggesting sterile systemic inflammation. Autophagy is involved in reducing inflammation through the degradation of inflammasomes and in the differentiation and function of regulatory T cells. Autophagy dysregulation induces protein aggregation in trophoblasts, resulting in placental dysfunction. In this review, we discuss the role of regulatory T cells in normal pregnancies. In addition, we discuss the association between autophagy and regulatory T cells in the development of preeclampsia based on reports on the role of autophagy in autoimmune diseases.

The GATA transcriptional program dictates cell fate equilibrium to establish the maternal-fetal exchange interface and fetal development

The placenta establishes a maternal-fetal exchange interface to transport nutrients and gases between the mother and the fetus. Establishment of this exchange interface relies on the development of multinucleated syncytiotrophoblasts (SynT) from trophoblast progenitors, and defect in SynT development often leads to pregnancy failure and impaired embryonic development. Here, we show that mouse embryos with conditional deletion of transcription factors GATA2 and GATA3 in labyrinth trophoblast progenitors (LaTPs) have underdeveloped placenta and die by ~embryonic day 9.5. Single-cell RNA sequencing analysis revealed excessive accumulation of multipotent LaTPs upon conditional deletion of GATA factors. The GATA factor-deleted multipotent progenitors were unable to differentiate into matured SynTs. We also show that the GATA factor-mediated priming of trophoblast progenitors for SynT differentiation is a conserved event during human placentation. Loss of either GATA2 or GATA3 in cytotrophoblast-derived human trophoblast stem cells (human TSCs) drastically inhibits SynT differentiation potential. Identification of GATA2 and GATA3 target genes along with comparative bioinformatics analyses revealed that GATA factors directly regulate hundreds of common genes in human TSCs, including genes that are essential for SynT development and implicated in preeclampsia and fetal growth retardation. Thus, our study uncovers a conserved molecular mechanism, in which coordinated function of GATA2 and GATA3 promotes trophoblast progenitor-to-SynT commitment, ensuring establishment of the maternal-fetal exchange interface.

YAP-mediated trophoblast dysfunction: the common pathway underlying pregnancy complications

Yes-associated protein (YAP) is a pivotal regulator in cellular proliferation, survival, differentiation, and migration, with significant roles in embryonic development, tissue repair, and tumorigenesis. At the maternal-fetal interface, emerging evidence underscores the importance of precisely regulated YAP activity in ensuring successful pregnancy initiation and progression. However, despite the established association between YAP dysregulation and adverse pregnancy outcomes, insights into the impact of aberrant YAP levels in fetal-derived, particularly trophoblast cells, and the ensuing dysfunction at the maternal-fetal interface remain limited. This review comprehensively examines YAP expression and its regulatory mechanisms in trophoblast cells throughout pregnancy. We emphasize its integral role in placental development and maternal-fetal interactions and delve into the correlations between YAP dysregulation and pregnancy complications. A nuanced understanding of YAP's functions during pregnancy could illuminate intricate molecular mechanisms and pave the way for innovative prevention and treatment strategies for pregnancy complications. Video Abstract.

Gene-network based analysis of human placental trophoblast subtypes identifies critical genes as potential targets of therapeutic drugs

During early pregnancy, extravillous trophoblasts (EVTs) play a crucial role in modifying the maternal uterine environment. Failures in EVT lineage formation and differentiation can lead to pregnancy complications such as preeclampsia, fetal growth restriction, and pregnancy loss. Despite recent advances, our knowledge on molecular and external factors that control and affect EVT development remains incomplete. Using trophoblast organoid in vitro models, we recently discovered that coordinated manipulation of the transforming growth factor beta (TGFβ) signaling is essential for EVT development. To further investigate gene networks involved in EVT function and development, we performed weighted gene co-expression network analysis (WGCNA) on our RNA-Seq data. We identified 10 modules with a median module membership of over 0.8 and sizes ranging from 1005 (M1) to 72 (M27) network genes associated with TGFβ activation status or in vitro culturing, the latter being indicative for yet undiscovered factors that shape the EVT phenotypes. Lastly, we hypothesized that certain therapeutic drugs might unintentionally interfere with placentation by affecting EVT-specific gene expression. We used the STRING database to map correlations and the Drug-Gene Interaction database to identify drug targets. Our comprehensive dataset of drug-gene interactions provides insights into potential risks associated with certain drugs in early gestation.

NOTCH3 signalling controls human trophoblast stem cell expansion and differentiation

Failures in growth and differentiation of the early human placenta are associated with severe pregnancy disorders such as pre-eclampsia and fetal growth restriction. However, regulatory mechanisms controlling development of placental epithelial cells, the trophoblasts, remain poorly elucidated. Using trophoblast stem cells (TSCs), trophoblast organoids (TB-ORGs) and primary cytotrophoblasts (CTBs) of early pregnancy, we herein show that autocrine NOTCH3 signalling controls human placental expansion and differentiation. The NOTCH3 receptor was specifically expressed in proliferative CTB progenitors and its active form, the nuclear NOTCH3 intracellular domain (NOTCH3-ICD), interacted with the transcriptional co-activator mastermind-like 1 (MAML1). Doxycycline-inducible expression of dominant-negative MAML1 in TSC lines provoked cell fusion and upregulation of genes specific for multinucleated syncytiotrophoblasts, which are the differentiated hormone-producing cells of the placenta. However, progenitor expansion and markers of trophoblast stemness and proliferation were suppressed. Accordingly, inhibition of NOTCH3 signalling diminished growth of TB-ORGs, whereas overexpression of NOTCH3-ICD in primary CTBs and TSCs showed opposite effects. In conclusion, the data suggest that canonical NOTCH3 signalling plays a key role in human placental development by promoting self-renewal of CTB progenitors.

Exploring maternal-fetal interface with in vitro placental and trophoblastic models

The placenta, being a temporary organ, plays a crucial role in facilitating the exchange of nutrients and gases between the mother and the fetus during pregnancy. Any abnormalities in the development of this vital organ not only lead to various pregnancy-related disorders that can result in fetal injury or death, but also have long-term effects on maternal health. In vitro models have been employed to study the physiological features and molecular regulatory mechanisms of placental development, aiming to gain a detailed understanding of the pathogenesis of pregnancy-related diseases. Among these models, trophoblast stem cell culture and organoids show great promise. In this review, we provide a comprehensive overview of the current mature trophoblast stem cell models and emerging organoid models, while also discussing other models in a systematic manner. We believe that this knowledge will be valuable in guiding further exploration of the complex maternal-fetal interface.

Single-cell characterization of self-renewing primary trophoblast organoids as modeling of EVT differentiation and interactions with decidual natural killer cells

BACKGROUND

Extravillous trophoblast cell (EVT) differentiation and its communication with maternal decidua especially the leading immune cell type natural killer (NK) cell are critical events for placentation. However, appropriate in vitro modelling system and regulatory programs of these two events are still lacking. Recent trophoblast organoid (TO) has advanced the molecular and mechanistic research in placentation. Here, we firstly generated the self-renewing TO from human placental villous and differentiated it into EVTs (EVT-TO) for investigating the differentiation events. We then co-cultured EVT-TO with freshly isolated decidual NKs for further study of cell communication. TO modelling of EVT differentiation as well as EVT interaction with dNK might cast new aspect for placentation research.

RESULTS

Single-cell RNA sequencing (scRNA-seq) was applied for comprehensive characterization and molecular exploration of TOs modelling of EVT differentiation and interaction with dNKs. Multiple distinct trophoblast states and dNK subpopulations were identified, representing CTB, STB, EVT, dNK1/2/3 and dNKp. Lineage trajectory and Seurat mapping analysis identified the close resemblance of TO and EVT-TO with the human placenta characteristic. Transcription factors regulatory network analysis revealed the cell-type specific essential TFs for controlling EVT differentiation. CellphoneDB analysis predicted the ligand-receptor complexes in dNK-EVT-TO co-cultures, which relate to cytokines, immunomodulation and angiogenesis. EVT was known to affect the immune properties of dNK. Our study found out that on the other way around, dNKs could exert effects on EVT causing expression changes which are functionally important.

CONCLUSION

Our study documented a single-cell atlas for TO and its applications on EVT differentiation and communications with dNKs, and thus provide methodology and novel research cues for future study of human placentation.

A comprehensive review of human trophoblast fusion models: recent developments and challenges

As an essential component of the maternal-fetal interface, the placental syncytiotrophoblast layer contributes to a successful pregnancy by secreting hormones necessary for pregnancy, transporting nutrients, mediating gas exchange, balancing immune tolerance, and resisting pathogen infection. Notably, the deficiency in mononuclear trophoblast cells fusing into multinucleated syncytiotrophoblast has been linked to adverse pregnancy outcomes, such as preeclampsia, fetal growth restriction, preterm birth, and stillbirth. Despite the availability of many models for the study of trophoblast fusion, there exists a notable disparity from the ideal model, limiting the deeper exploration into the placental development. Here, we reviewed the existing models employed for the investigation of human trophoblast fusion from several aspects, including the development history, latest progress, advantages, disadvantages, scope of application, and challenges. The literature searched covers the monolayer cell lines, primary human trophoblast, placental explants, human trophoblast stem cells, human pluripotent stem cells, three-dimensional cell spheres, organoids, and placenta-on-a-chip from 1938 to 2023. These diverse models have significantly enhanced our comprehension of placental development regulation and the underlying mechanisms of placental-related disorders. Through this review, our objective is to provide readers with a thorough understanding of the existing trophoblast fusion models, making it easier to select most suitable models to address specific experimental requirements or scientific inquiries. Establishment and application of the existing human placental trophoblast fusion models.

EP300 facilitates human trophoblast stem cell differentiation

Early placenta development involves cytotrophoblast differentiation into extravillous trophoblast (EVT) and syncytiotrophoblast (STB). Defective trophoblast development and function may result in severe pregnancy complications, including fetal growth restriction and pre-eclampsia. The incidence of these complications is increased in pregnancies of fetuses affected by Rubinstein-Taybi syndrome, a developmental disorder predominantly caused by heterozygous mutations in CREB-binding protein (CREBBP) or E1A-binding protein p300 (EP300). Although the acetyltransferases CREBBP and EP300 are paralogs with many overlapping functions, the increased incidence of pregnancy complications is specific for EP300 mutations. We hypothesized that these complications have their origin in early placentation and that EP300 is involved in that process. Therefore, we investigated the role of EP300 and CREBBP in trophoblast differentiation, using human trophoblast stem cells (TSCs) and trophoblast organoids. We found that pharmacological CREBBP/EP300 inhibition blocks differentiation of TSCs into both EVT and STB lineages, and results in an expansion of TSC-like cells under differentiation-inducing conditions. Specific targeting by RNA interference or CRISPR/Cas9-mediated mutagenesis demonstrated that knockdown of EP300 but not CREBBP, inhibits trophoblast differentiation, consistent with the complications seen in Rubinstein-Taybi syndrome pregnancies. By transcriptome sequencing, we identified transforming growth factor alpha (TGFA, encoding TGF-α) as being strongly upregulated upon EP300 knockdown. Moreover, supplementing differentiation medium with TGF-α, which is a ligand for the epidermal growth factor receptor (EGFR), likewise affected trophoblast differentiation and resulted in increased TSC-like cell proliferation. These findings suggest that EP300 facilitates trophoblast differentiation by interfering with at least EGFR signaling, pointing towards a crucial role for EP300 in early human placentation.

Placental microRNAs relate to early childhood growth trajectories

BACKGROUND

Poor placental function is a common cause of intrauterine growth restriction, which in turn is associated with increased risks of adverse health outcomes. Our prior work suggests that birthweight and childhood obesity-associated genetic variants functionally impact placental function and that placental microRNA are associated with birthweight. To address the influence of the placenta beyond birth, we assessed the relationship between placental microRNAs and early childhood growth.

METHODS

Using the SITAR package, we generated two parameters that describe individual weight trajectories of children (0-5 years) in the New Hampshire Birth Cohort Study (NHBCS, n = 238). Using negative binomial generalized linear models, we identified placental microRNAs that relate to growth parameters (FDR < 0.1), while accounting for sex, gestational age at birth, and maternal parity.

RESULTS

Genes targeted by the six growth trajectory-associated microRNAs are enriched (FDR < 0.05) in growth factor signaling (TGF/beta: miR-876; EGF/R: miR-155, Let-7c; FGF/R: miR-155; IGF/R: Let-7c, miR-155), calmodulin signaling (miR-216a), and NOTCH signaling (miR-629).

CONCLUSIONS

Growth-trajectory microRNAs target pathways affecting placental proliferation, differentiation and function. Our results suggest a role for microRNAs in regulating placental cellular dynamics and supports the Developmental Origins of Health and Disease hypothesis that fetal environment can have impacts beyond birth.

IMPACT

We found that growth trajectory associated placenta microRNAs target genes involved in signaling pathways central to the formation, maintenance and function of placenta; suggesting that placental cellular dynamics remain critical to infant growth to term and are under the control of microRNAs. Our results contribute to the existing body of research suggesting that the placenta plays a key role in programming health in the offspring. This is the first study to relate molecular patterns in placenta, specifically microRNAs, to early childhood growth trajectory.

An miRNA-mRNA integrative analysis in human placentas and mice: role of the Smad2/miR-155-5p axis in the development of fetal growth restriction

Introduction: Functional disorder of the placenta is the principal cause of fetal growth restriction (FGR), usually cured with suitable clinical treatment and good nursing. However, some FGR mothers still give birth to small for gestational age (SGA) babies after treatment. The ineffectiveness of treatment in such a group of patients confused physicians of obstetrics and gynecology. Methods: In this study, we performed a microRNA-messenger RNA integrative analysis of gene expression profiles obtained from Gene Expression Omnibus. Differentially expressed genes were screened and checked using quantitative polymerase chain reaction. Target genes of significantly changed microRNA were screened and enriched for Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses. Function of the obtained microRNA-messenger RNA was evaluated using HTR-8/SVneo trophoblast cells, human umbilical vein endothelial cells, and heterozygote male mice. Result: MiR-155-5p was upregulated (p = 0.001, fold-change = 2.275) in fetal-side placentals. Among the hub genes identified as key targets for miR-155-5p in fetal reprogramming, Smad2 was downregulated (p = 0.002, fold change = 0.426) and negatively correlated with miR-155-5p expression levels (r = -0.471, p < 1.0 E - 04) in fetal-side placental tissues. The miR-155-5p mimic blocks Smad2 expression and suppresses villous trophoblast cell and endothelial cell function (proliferation, migration, and invasion), indicating a close relationship with placental development. Luciferase assays further confirmed the targeting of miR-155-5p to Smad2. Furthermore, Smad2^+/- heterozygote male mice were born small with low body weight (p = 0.0281) and fat composition (p = 0.013) in the fourth week post-natal. Discussion: We provide the first evidence of the role of the Smad2/miR-155-5p axis in the placental pathologies of FGR. Our findings elucidate the pathogenesis of FGR and provide new therapeutic targets.

Laminin switches terminal differentiation fate of human trophoblast stem cells under chemically defined culture conditions

Human trophoblast stem cells (hTSCs) have emerged as a powerful tool to model early placental development in vitro. Analogous to the epithelial cytotrophoblast in the placenta, hTSCs can differentiate into cells of the extravillous trophoblast (EVT) lineage or the multinucleate syncytiotrophoblast (STB). Here we present a chemically defined culture system for STB and EVT differentiation of hTSCs. Notably, in contrast to current approaches, we neither utilize forskolin for STB formation nor transforming growth factor-beta (TGFβ) inhibitors or a passage step for EVT differentiation. Strikingly, the presence of a single additional extracellular cue-laminin-111-switched the terminal differentiation of hTSCs from STB to the EVT lineage under these conditions. In the absence of laminin-111, STB formation occurred, with cell fusion comparable to that obtained with differentiation mediated by forskolin; however, in the presence of laminin-111, hTSCs differentiated to the EVT lineage. Protein expression of nuclear hypoxia-inducible factors (HIF1α and HIF2α) was upregulated during EVT differentiation mediated by laminin-111 exposure. A heterogeneous mixture of Notch1+ EVTs in colonies and HLA-G+ single-cell EVTs were obtained without a passage step, reminiscent of heterogeneity in vivo. Further analysis showed that inhibition of TGFβ signaling affected both STB and EVT differentiation mediated by laminin-111 exposure. TGFβ inhibition during EVT differentiation resulted in decreased HLA-G expression and increased Notch1 expression. On the other hand, TGFβ inhibition prevented STB formation. The chemically defined culture system for hTSC differentiation established herein facilitates quantitative analysis of heterogeneity that arises during hTSC differentiation and will enable mechanistic studies in vitro.

Spatial multiomics map of trophoblast development in early pregnancy

The relationship between the human placenta-the extraembryonic organ made by the fetus, and the decidua-the mucosal layer of the uterus, is essential to nurture and protect the fetus during pregnancy. Extravillous trophoblast cells (EVTs) derived from placental villi infiltrate the decidua, transforming the maternal arteries into high-conductance vessels1. Defects in trophoblast invasion and arterial transformation established during early pregnancy underlie common pregnancy disorders such as pre-eclampsia2. Here we have generated a spatially resolved multiomics single-cell atlas of the entire human maternal-fetal interface including the myometrium, which enables us to resolve the full trajectory of trophoblast differentiation. We have used this cellular map to infer the possible transcription factors mediating EVT invasion and show that they are preserved in in vitro models of EVT differentiation from primary trophoblast organoids3,4 and trophoblast stem cells5. We define the transcriptomes of the final cell states of trophoblast invasion: placental bed giant cells (fused multinucleated EVTs) and endovascular EVTs (which form plugs inside the maternal arteries). We predict the cell-cell communication events contributing to trophoblast invasion and placental bed giant cell formation, and model the dual role of interstitial EVTs and endovascular EVTs in mediating arterial transformation during early pregnancy. Together, our data provide a comprehensive analysis of postimplantation trophoblast differentiation that can be used to inform the design of experimental models of the human placenta in early pregnancy.

The human placenta shapes the phenotype of decidual macrophages

During human pregnancy, placenta-derived extravillous trophoblasts (EVTs) invade the decidua and communicate with maternal immune cells. The decidua distinguishes into basalis (decB) and parietalis (decP). The latter remains unaffected by EVT invasion. By defining a specific gating strategy, we report the accumulation of macrophages in decB. We describe a decidua basalis-associated macrophage (decBAM) population with a differential transcriptome and secretome compared with decidua parietalis-associated macrophages (decPAMs). decBAMs are CD11c^hi and efficient inducers of Tregs, proliferate in situ, and secrete high levels of CXCL1, CXCL5, M-CSF, and IL-10. In contrast, decPAMs exert a dendritic cell-like, motile phenotype characterized by induced expression of HLA class II molecules, enhanced phagocytosis, and the ability to activate T cells. Strikingly, EVT-conditioned media convert decPAMs into a decBAM phenotype. These findings assign distinct macrophage phenotypes to decidual areas depending on placentation and further highlight a critical role for EVTs in the induction of decB-associated macrophage polarization.

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.

MicroRNAs: key regulators of the trophoblast function in pregnancy disorders

The placenta is essential for a successful pregnancy and healthy intrauterine development in mammals. During human pregnancy, the growth and development of the placenta are inseparable from the rapid proliferation, invasion, and migration of trophoblast cells. Previous reports have shown that the occurrence of many pregnancy disorders may be closely related to the dysfunction of trophoblasts. However, the function regulation of human trophoblast cells in the placenta is poorly understood. Therefore, studying the factors that regulate the function of trophoblast cells is necessary. MicroRNAs (miRNAs) are small, non-coding, single-stranded RNA molecules. Increasing evidence suggests that miRNAs play a crucial role in regulating trophoblast functions. This review outlines the role of miRNAs in regulating the function of trophoblast cells and several common signaling pathways related to miRNA regulation in pregnancy disorders.

Transcriptomic mapping of the metzincin landscape in human trophoblasts

The metzincin family of metalloproteases coordinates tissue developmental processes through regulation of growth factor availability, receptor signaling, and cell-cell/cell-matrix adhesion. While roles for select metzincins in controlling trophoblast functions in human placental development have been described, a comprehensive understanding of metzincin dynamics during trophoblast differentiation is lacking. To address this knowledge gap, single cell transcriptomic datasets derived from first trimester chorionic villi and decidua were used to decipher metzincin expression profiles and kinetics in diverse cell types within the utero-placental interface. Further, specific protease-substrate interactions within progenitor trophoblasts were examined to better define the progenitor niche. Within the uterine-placental compartment, 43 metzincin proteases were expressed across 15 cell-type clusters. Metzincin subgroups expressed in placental trophoblasts, placental mesenchymal cells, uterine stromal, and immune cells included multiple matrix metalloproteases (MMPs), a disintegrin and metalloproteases (ADAMs), a disintegrin and metalloproteases with thrombospondin repeats (ADAMTSs), pappalysins, and astacins. Within the trophoblast compartment, eight distinct trophoblasts states were identified: four cytotrophoblast (CTB), one syncytiotrophoblast precursor (SCTp), two column CTB (cCTB), and one extravillous trophoblast (EVT). Within these states 7 MMP, 8 ADAM, 4 ADAMTS, 2 pappalysin, and 3 astacin proteases were expressed. Cell trajectory modeling shows that expression of most (19/24) metzincins increase during EVT differentiation, though expression of select metalloproteases increase along the villous pathway. Eleven metzincins (ADAM10, -17, MMP14, -15, -19, -23B, ADAMTS1, -6, -19, TLL-1, -2) showed enrichment within CTB progenitors, and analysis of metzincin-substrate interactions identified ∼150 substrates and binding partners, including FBN2 as an ADAMTS6-specific substrate. Together, this work characterizes the metzincin landscape in human first trimester trophoblasts and establishes insight into the roles specific proteases perform within distinct trophoblast niches and across trophoblast differentiation. This resource serves as a guide for future investigations into the roles of metzincin proteases in human placental development.

Single-Cell Immunobiology of the Maternal-Fetal Interface

Pregnancy success requires constant dialogue between the mother and developing conceptus. Such crosstalk is facilitated through complex interactions between maternal and fetal cells at distinct tissue sites, collectively termed the "maternal-fetal interface." The emergence of single-cell technologies has enabled a deeper understanding of the unique processes taking place at the maternal-fetal interface as well as the discovery of novel pathways and immune and nonimmune cell types. Single-cell approaches have also been applied to decipher the cellular dynamics throughout pregnancy, in parturition, and in obstetrical syndromes such as recurrent spontaneous abortion, preeclampsia, and preterm labor. Furthermore, single-cell technologies have been used during the recent COVID-19 pandemic to evaluate placental viral cell entry and the impact of SARS-CoV-2 infection on maternal and fetal immunity. In this brief review, we summarize the current knowledge of cellular immunobiology in pregnancy and its complications that has been generated through single-cell investigations of the maternal-fetal interface.

Placental Therapeutic Targets and Nanodelivery Systems

Pregnancy complications affect millions of women each year. Some of these diseases have high morbidity and mortality such as preeclampsia. At present, there is no safe and effective treatment for pregnancy complications, so it is still a difficult clinical problem. As many pregnancy complications are closely related to placental dysplasia, placenta-specific therapy, as an important method, is expected to be a safe, effective, and specific therapeutic strategy. This review explains in detail the placenta physiological structure, characteristics, and action mechanism of some biomolecules and signaling pathways that play roles in normal development and disorders of the development of the placenta, and how to use these biomolecules as therapeutic targets when the placenta disorder causes disease, combining the latest progress in the field of nanodelivery systems, so as to lay a foundation for the development of placenta-specific therapy of pregnancy complications.

Hippo signaling cofactor, WWTR1, at the crossroads of human trophoblast progenitor self-renewal and differentiation

Healthy progression of human pregnancy relies on cytotrophoblast (CTB) progenitor self-renewal and its differentiation toward multinucleated syncytiotrophoblasts (STBs) and invasive extravillous trophoblasts (EVTs). However, the underlying molecular mechanisms that fine-tune CTB self-renewal or direct its differentiation toward STBs or EVTs during human placentation are poorly defined. Here, we show that Hippo signaling cofactor WW domain containing transcription regulator 1 (WWTR1) is a master regulator of trophoblast fate choice during human placentation. Using human trophoblast stem cells (human TSCs), primary CTBs, and human placental explants, we demonstrate that WWTR1 promotes self-renewal in human CTBs and is essential for their differentiation to EVTs. In contrast, WWTR1 prevents induction of the STB fate in undifferentiated CTBs. Our single-cell RNA sequencing analyses in first-trimester human placenta, along with mechanistic analyses in human TSCs revealed that WWTR1 fine-tunes trophoblast fate by directly regulating WNT signaling components. Importantly, our analyses of placentae from pathological pregnancies show that extreme preterm births (gestational time ≤28 wk) are often associated with loss of WWTR1 expression in CTBs. In summary, our findings establish the critical importance of WWTR1 at the crossroads of human trophoblast progenitor self-renewal versus differentiation. It plays positive instructive roles in promoting CTB self-renewal and EVT differentiation and safeguards undifferentiated CTBs from attaining the STB fate.

Inhibition of YAP/TAZ-TEAD activity induces cytotrophoblast differentiation into syncytiotrophoblast in human trophoblast

During placentation, placental cytotrophoblast (CT) cells differentiate into syncytiotrophoblast (ST) cells and extravillous trophoblast (EVT) cells. In the placenta, the expression of various genes is regulated by the Hippo pathway through a transcription complex, Yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ)-TEA domain transcription factor (TEAD) (YAP/TAZ-TEAD) activity. YAP/TAZ-TEAD activity is controlled by multiple factors and signaling, such as cyclic AMP (cAMP) signaling. cAMP signaling is believed to be involved in the regulation of trophoblast function but is not yet fully understood. Here we showed that YAP/TAZ-TEAD expressions and their activities were altered by cAMP stimulation in BeWo cells, a human choriocarcinoma cell line. The repression of YAP/TAZ-TEAD activity induced the expression of ST-specific genes without cAMP stimulation, and transduction of constitutively active YAP, i.e., YAP-5SA, resulted in the repression of 8Br-cAMP-induced expressions of ST-specific genes in a TEAD-dependent manner. We also investigated the role of YAP/TAZ-TEAD in maintaining CT cells and their differentiation into ST and EVT cells using human trophoblast stem (TS) cells. YAP/TAZ-TEAD activity was involved in maintaining the stemness of TS cells. Induction or repression of YAP/TAZ-TEAD activity resulted in marked changes in the expression of ST-specific genes. Using primary CT cells, which spontaneously differentiate into ST-like cells, the effects of YAP-5SA transduction were investigated, and the expression of ST-specific genes was found to be repressed. These results indicate that the inhibition of YAP/TAZ-TEAD activity, with or without cAMP stimulation, is essential for the differentiation of CT cells into ST cells.

Human Chorionic Villous Differentiation and Placental Development

In humans, the placenta provides the only fetomaternal connection and is essential for establishing a pregnancy as well as fetal well-being. Additionally, it allows maternal physiological adaptation and embryonic immunological acceptance, support, and nutrition. The placenta is derived from extra-embryonic tissues that develop rapidly and dynamically in the first weeks of pregnancy. It is primarily composed of trophoblasts that differentiate into villi, stromal cells, macrophages, and fetal endothelial cells (FEC). Placental differentiation may be closely related to perinatal diseases, including fetal growth retardation (FGR) and hypertensive disorders of pregnancy (HDP), and miscarriage. There are limited findings regarding human chorionic villous differentiation and placental development because conducting in vivo studies is extremely difficult. Placental tissue varies widely among species. Thus, experimental animal findings are difficult to apply to humans. Early villous differentiation is difficult to study due to the small tissue size; however, a detailed analysis can potentially elucidate perinatal disease causes or help develop novel therapies. Artificial induction of early villous differentiation using human embryonic stem (ES) cells/induced pluripotent stem (iPS) cells was attempted, producing normally differentiated villi that can be used for interventional/invasive research. Here, we summarized and correlated early villous differentiation findings and discussed clinical diseases.

Transforming growth factor-β signaling governs the differentiation program of extravillous trophoblasts in the developing human placenta

Abnormal placentation has been noticed in a variety of pregnancy complications such as miscarriage, early-onset preeclampsia, and fetal growth restriction. Defects in the developmental program of extravillous trophoblasts (EVTs), migrating from placental anchoring villi into the maternal decidua and its vessels, is thought to be an underlying cause. Yet, key regulatory mechanisms controlling commitment and differentiation of the invasive trophoblast lineage remain largely elusive. Herein, comparative gene expression analyses of HLA-G-purified EVTs, isolated from donor-matched placenta, decidua, and trophoblast organoids (TB-ORGs), revealed biological processes and signaling pathways governing EVT development. In particular, bioinformatics analyses and manipulations in different versatile trophoblast cell models unraveled transforming growth factor-β (TGF-β) signaling as a crucial pathway driving differentiation of placental EVTs into decidual EVTs, the latter showing enrichment of a secretory gene signature. Removal of Wingless signaling and subsequent activation of the TGF-β pathway were required for the formation of human leukocyte antigen-G⁺ (HLA-G⁺) EVTs in TB-ORGs that resemble in situ EVTs at the level of global gene expression. Accordingly, TGF-β-treated EVTs secreted enzymes, such as DAO and PAPPA2, which were predominantly expressed by decidual EVTs. Their genes were controlled by EVT-specific induction and genomic binding of the TGF-β downstream effector SMAD3. In summary, TGF-β signaling plays a key role in human placental development governing the differentiation program of EVTs.

Modelling human placental villous development: designing cultures that reflect anatomy

The use of in vitro tools to study trophoblast differentiation and function is essential to improve understanding of normal and abnormal placental development. The relative accessibility of human placentae enables the use of primary trophoblasts and placental explants in a range of in vitro systems. Recent advances in stem cell models, three-dimensional organoid cultures, and organ-on-a-chip systems have further shed light on the complex microenvironment and cell-cell crosstalk involved in placental development. However, understanding each model's strengths and limitations, and which in vivo aspects of human placentation in vitro data acquired does, or does not, accurately reflect, is key to interpret findings appropriately. To help researchers use and design anatomically accurate culture models, this review both outlines our current understanding of placental development, and critically considers the range of established and emerging culture models used to study this, with a focus on those derived from primary tissue.

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

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

Transcription factor networks in trophoblast development

The placenta sustains embryonic development and is critical for a successful pregnancy outcome. It provides the site of exchange between the mother and the embryo, has immunological functions and is a vital endocrine organ. To perform these diverse roles, the placenta comprises highly specialized trophoblast cell types, including syncytiotrophoblast and extravillous trophoblast. The coordinated actions of transcription factors (TFs) regulate their emergence during development, subsequent specialization, and identity. These TFs integrate diverse signaling cues, form TF networks, associate with chromatin remodeling and modifying factors, and collectively determine the cell type-specific characteristics. Here, we summarize the general properties of TFs, provide an overview of TFs involved in the development and function of the human trophoblast, and address similarities and differences to their murine orthologs. In addition, we discuss how the recent establishment of human in vitro models combined with -omics approaches propel our knowledge and transform the human trophoblast field.

TEAD4 as an Oncogene and a Mitochondrial Modulator

TEAD4 (TEA Domain Transcription Factor 4) is well recognized as the DNA-anchor protein of YAP transcription complex, which is modulated by Hippo, a highly conserved pathway in Metazoa that controls organ size through regulating cell proliferation and apoptosis. To acquire full transcriptional activity, TEAD4 requires co-activator, YAP (Yes-associated protein) or its homolog TAZ (transcriptional coactivator with PDZ-binding motif) the signaling hub that relays the extracellular stimuli to the transcription of target genes. Growing evidence suggests that TEAD4 also exerts its function in a YAP-independent manner through other signal pathways. Although TEAD4 plays an essential role in determining that differentiation fate of the blastocyst, it also promotes tumorigenesis by enhancing metastasis, cancer stemness, and drug resistance. Upregulation of TEAD4 has been reported in several cancers, including colon cancer, gastric cancer, breast cancer, and prostate cancer and serves as a valuable prognostic marker. Recent studies show that TEAD4, but not other members of the TEAD family, engages in regulating mitochondrial dynamics and cell metabolism by modulating the expression of mitochondrial- and nuclear-encoded electron transport chain genes. TEAD4’s functions including oncogenic activities are tightly controlled by its subcellular localization. As a predominantly nuclear protein, its cytoplasmic translocation is triggered by several signals, such as osmotic stress, cell confluency, and arginine availability. Intriguingly, TEAD4 is also localized in mitochondria, although the translocation mechanism remains unclear. In this report, we describe the current understanding of TEAD4 as an oncogene, epigenetic regulator and mitochondrial modulator. The contributing mechanisms will be discussed.

WNT and NOTCH signaling in human trophoblast development and differentiation

Correct development of the human placenta and its differentiated epithelial cells, syncytial trophoblasts (STBs) and extravillous trophoblasts (EVTs), is crucial for a successful pregnancy outcome. STBs develop by cell fusion of mononuclear cytotrophoblasts (CTBs) in placental floating villi, whereas migratory EVTs originate from specialized villi anchoring to the maternal decidua. Defects in trophoblast differentiation have been associated with severe pregnancy disorders such as early-onset preeclampsia and fetal growth restriction. However, the evolutionary pathways underlying normal and adverse placentation are poorly understood. Herein, we discuss Wingless (WNT) and NOTCH signaling, two pathways that play pivotal roles in human placenta and trophoblast development. Whereas WNT is necessary for expansion of trophoblast progenitors and stem cells, NOTCH1 is required for proliferation and survival of EVT precursors. Differentiation of the latter is orchestrated by a switch in NOTCH receptor expression as well as by changes in WNT ligands and their downstream effectors.

Cell trajectory modeling identifies a primitive trophoblast state defined by BCAM enrichment

In early placental development, progenitor cytotrophoblasts (CTB) differentiate along one of two cellular trajectories: the villous or extravillous pathways. CTB committed to the villous pathway fuse with neighboring CTB to form the outer multinucleated syncytiotrophoblast (SCT), whereas CTB committed to the extravillous pathway differentiate into invasive extravillous trophoblasts (EVT). Unfortunately, little is known about the processes controlling human CTB progenitor maintenance and differentiation. To address this, we established a single cell RNA sequencing (scRNA-seq) dataset from first trimester placentas to identify cell states important in trophoblast progenitor establishment, renewal and differentiation. Multiple distinct trophoblast states were identified, representing progenitor CTB, column CTB, SCT precursors and EVT. Lineage trajectory analysis identified a progenitor origin that was reproduced in human trophoblast stem cell organoids. Heightened expression of basal cell adhesion molecule (BCAM) defined this primitive state, where BCAM enrichment or gene silencing resulted in enhanced or diminished organoid growth, respectively. Together, this work describes at high-resolution trophoblast heterogeneity within the first trimester, resolves gene networks within human CTB progenitors and identifies BCAM as a primitive progenitor marker and possible regulator.

Joan Hunt Senior award lecture: New tools to shed light on the 'black box' of pregnancy

Correct establishment of the placenta is critical to the success of a pregnancy, but many of the key events take place during or shortly after implantation and are inaccessible for study. This inaccessibility, coupled with the lack of a suitable preclinical animal model, means that knowledge of human early placental development and function is extremely limited. Hence, the first trimester is often referred to as the 'black box' of pregnancy. However, recent advances in the derivation of trophoblast stem cells and organoid cultures of the trophoblast and endometrium are opening new opportunities for basic and translational research, providing for the first time cells that faithfully replicate their tissue of origin and proliferate and differentiate in culture in a stable and reproducible manner. These cells are valuable new tools for investigating cell-lineage differentiation and maternal-fetal interactions, but become even more powerful when combined with advances in bioengineering, microfabrication and microfluidic technologies. Assembloids of the endometrium comprising various cell types as model systems to investigate events at implantation, and placentas-on-a-chip for the study of nutrient transfer or drug screening are just two examples. This is a rapidly advancing field that may usher in more personalised approaches to infertility and pregnancy complications. Many of the developments are still at the proof-of-principle phase, but with continued refinement they are likely to shed important light on events that are fundamental to our reproduction as individuals and as a species, yet for ethical reasons are hidden from view.

The role of fascin in carcinogenesis and embryo implantation

The cytoskeleton, with its actin bundling proteins, plays crucial roles in a host of cellular function, such as cancer metastasis, antigen presentation and trophoblast migration and invasion, as a result of cytoskeletal remodeling. A key player in cytoskeletal remodeling is fascin. Upregulation of fascin induces the transition of epithelial phenotypes to mesenchymal phenotypes through complex interaction with transcription factors. Fascin expression also regulates mitochondrial F-actin to promote oxidative phosphorylation (OXPHOS) in some cancer cells. Trophoblast cells, on the other hand, exhibit similar physiological functions, involving the upregulation of genes crucial for its migration and invasion. Owing to the similar tumor-like characteristics among cancer and trophoblats, we review recent studies on fascin in relation to cancer and trophoblast cell biology; and based on existing evidence, link fascin to the establishment of the maternal-fetal interface.

From stem cells to spiral arteries: A journey through early placental development

Early placental development lays the foundation of a healthy pregnancy, and numerous tightly regulated processes must occur for the placenta to meet the increasing nutrient and oxygen exchange requirements of the growing fetus later in gestation. Inadequacies in early placental development can result in disorders such as fetal growth restriction that do not present clinically until the second half of gestation. Indeed, growth restricted placentae exhibit impaired placental development and function, including reduced overall placental size, decreased branching of villi and the blood vessels within them, altered trophoblast function, and impaired uterine vascular remodelling, which together combine to reduce placental exchange capacity. This review explores the importance of early placental development across multiple anatomical aspects of placentation, from the stem cells and lineage hierarchies from which villous core cells and trophoblasts arise, through extravillous trophoblast invasion and spiral artery remodelling, and finally remodelling of the larger uterine vessels.

MiR-222-3p Inhibits Trophoblast Cell Migration and Alleviates Preeclampsia in Rats Through Inhibiting HDAC6 and Notch1 Signaling

MiR-222-3p was found to be upregulated in plasma of patients with severe preeclampsia (PE). However, its role in PE progression remains elusive. This study aimed to explore the underlying role and mechanism of miR-222-3p in PE progression. Herein, we verified that miR-222-3p was upregulated and HDAC6 mRNA was downregulated in placentas of PE patients compared with normal pregnant controls as measured by RT-qPCR. And miR-222-3p expression was negatively correlated with HDAC6 mRNA expression in PE patients. HTR8/SVneo trophoblast cells were transfected with miR-222-3p mimic or miR-222-3p inhibitor, and we found that MiR-222-3p overexpression inhibited proliferation, migration, and matrix metalloproteinase (MMP)-2 and MMP-9 levels in HTR-8/SVneo cells, while miR-222-3p silencing showed the opposite results. Online bioinformatics analysis and dual-luciferase reporter assay confirmed that HDAC6 was a target of miR-222-3p. HDAC6 overexpression promoted HTR-8/SVneo cell proliferation and migration, while HDAC6 knockdown suppressed cell proliferation and migration. Moreover, HDAC6 overexpression and Notch1 signaling activation both reversed the inhibitory effects of miR-222-3p on trophoblast cell proliferation and migration. Additionally, treatment with miR-222-3p inhibitor attenuated blood pressure and fetal detrimental changes in PE rats. Collectively, our findings suggested that MiR-222-3p inhibited HDAC6 expression and blocked the Notch1 signaling, thus suppressing trophoblast cell proliferation and migration and attenuating blood pressure and fetal detrimental changes in PE rats, which is expected to become a therapeutic target for PE.

Characterization of primary models of human trophoblast

Two recently developed models, trophoblast organoids and trophoblast stem cells (TSCs), are useful tools to further the understanding of human placental development. Both differentiate from villous cytotrophoblast (VCT) to either extravillous trophoblast (EVT) or syncytiotrophoblast (SCT). Here, we compare the transcriptomes and miRNA profiles of these models to identify which trophoblast they resemble in vivo. Our findings indicate that TSCs do not readily undergo SCT differentiation and closely resemble cells at the base of the cell columns from where EVT derives. In contrast, organoids are similar to VCT and undergo spontaneous SCT differentiation. A defining feature of human trophoblast is that VCT and SCT are human leukocyte antigen (HLA) null, whereas EVT expresses HLA-C, -G and -E molecules. We find that trophoblast organoids retain these in vivo characteristics. In contrast, TSCs express classical HLA-A and HLA-B molecules, and maintain their expression after EVT differentiation, with upregulation of HLA-G. Furthermore, HLA expression in TSCs differs when grown in 3D rather than in 2D, suggesting that mechanical cues are important. Our results can be used to select the most suitable model for the study of trophoblast development, function and pathology.

Summary: Characterization of trophoblast organoids and trophoblast stem cells as exciting models of human placentation enables the selection of the most suitable system to address specific research questions.

MSX2 safeguards syncytiotrophoblast fate of human trophoblast stem cells

Multiple placental pathologies are associated with failures in trophoblast differentiation, yet the underlying transcriptional regulation is poorly understood. Here, we discovered msh homeobox 2 (MSX2) as a key transcriptional regulator of trophoblast identity using the human trophoblast stem cell model. Depletion of MSX2 resulted in activation of the syncytiotrophoblast transcriptional program, while forced expression of MSX2 blocked it. We demonstrated that a large proportion of the affected genes were directly bound and regulated by MSX2 and identified components of the SWItch/Sucrose nonfermentable (SWI/SNF) complex as strong MSX2 interactors and target gene cobinders. MSX2 cooperated specifically with the SWI/SNF canonical BAF (cBAF) subcomplex and cooccupied, together with H3K27ac, a number of differentiation genes. Increased H3K27ac and cBAF occupancy upon MSX2 depletion imply that MSX2 prevents premature syncytiotrophoblast differentiation. Our findings established MSX2 as a repressor of the syncytiotrophoblast lineage and demonstrated its pivotal role in cell fate decisions that govern human placental development and disease.

Abnormal Cullin1 neddylation-mediated p21 accumulation participates in the pathogenesis of recurrent spontaneous abortion by regulating trophoblast cell proliferation and differentiation

The study explores the role of neddylation in early trophoblast development and its alteration during the pathogenesis of recurrent spontaneous abortion (RSA). Immunofluorescence and western blot were conducted to evaluate the expression pattern of NEDD8 protein in the first-trimester placentas of healthy control and RSA patients. Neddylated-cullins, especially neddylated-cullin1, were downregulated and their substrate, p21, was accumulated in RSA samples. NEDD8 cytoplasmic recruitment was observed in extravillous trophoblast (EVT) progenitors of RSA placentas. Consistent with the results of clinical samples, neddylation inhibition using MLN4924 in trophoblast cell lines caused obvious p21 accumulation and free NEDD8 cytoplasmic recruitment. Further in vitro study demonstrated neddylation inhibition attenuated proliferation of Jeg-3 cells via p21 accumulation. Moreover, when trophoblast stem (TS) cells derived from first-trimester placentas were cultured for differentiation analyses. MLN4924 impaired the differentiation of TS cells towards EVTs by downregulating HLA-G and GATA3. p21 knockdown could partly rescue MLN4924-suppressed HLA-G and GATA3 expression. In conclusion, cullin1 neddylation-mediated p21 degradation is required for trophoblast proliferation and can affect trophoblast plasticity by affecting HLA-G and GATA3 expression. The results provide insights into the pathological mechanism of RSA and the biological regulation of trophoblast development.

Implications for preeclampsia: hypoxia-induced Notch promotes trophoblast migration

In the first trimester of human pregnancy, low oxygen tension or hypoxia is essential for proper placentation and placenta function. Low oxygen levels and activation of signaling pathways have been implicated as critical mediators in the promotion of trophoblast differentiation, migration, and invasion with inappropriate changes in oxygen tension and aberrant Notch signaling both individually reported as causative to abnormal placentation. Despite crosstalk between hypoxia and Notch signaling in multiple cell types, the relationship between hypoxia and Notch in first trimester trophoblast function is not understood. To determine how a low oxygen environment impacts Notch signaling and cellular motility, we utilized the human first trimester trophoblast cell line, HTR-8/SVneo. Gene set enrichment and ontology analyses identified pathways involved in angiogenesis, Notch and cellular migration as upregulated in HTR-8/SVneo cells exposed to hypoxic conditions. DAPT, a γ-secretase inhibitor that inhibits Notch activation, was used to interrogate the crosstalk between Notch and hypoxia pathways in HTR-8/SVneo cells. We found that hypoxia requires Notch activation to mediate HTR-8/SVneo cell migration, but not invasion. To determine if our in vitro findings were associated with preeclampsia, we analyzed the second trimester chorionic villous sampling (CVS) samples and third trimester placentas. We found a significant decrease in expression of migration and invasion genes in CVS from preeclamptic pregnancies and significantly lower levels of JAG1 in placentas from pregnancies with early-onset preeclampsia with severe features. Our data support a role for Notch in mediating hypoxia-induced trophoblast migration, which may contribute to preeclampsia development.

Paradigms for investigating invasive trophoblast cell development and contributions to uterine spiral artery remodeling

Uterine spiral arteries are extensively remodeled during placentation to ensure sufficient delivery of maternal blood to the developing fetus. Uterine spiral arterial remodeling is complex, as cells originating from both mother and developing conceptus interact at the maternal interface to regulate the extracellular matrix remodeling and vasculature restructuring necessary for successful placentation. Despite this complexity, one mechanism critical to spiral artery remodeling is trophoblast cell invasion into the maternal compartment. Invasive trophoblast cells include both interstitial and endovascular populations that exhibit spatiotemporal differences in uterine invasion, including proximity to uterine spiral arteries. Interstitial trophoblast cells invade the uterine parenchyma where they are interspersed among stromal cells. Endovascular trophoblast cells infiltrate uterine spiral arteries, replace endothelial cells, adopt a pseudo-endothelial cell phenotype, and engineer vessel remodeling. Impaired trophoblast cell invasion and, consequently, insufficient uterine spiral arterial remodeling can lead to the development of pregnancy disorders, such as preeclampsia, intrauterine growth restriction, and premature birth. This review provides insights into invasive trophoblast cells and their function during normal placentation as well as in settings of disease.

Modeling human peri-implantation placental development and function†

It is very difficult to gain a better understanding of the events in human pregnancy that occur during and just after implantation because such pregnancies are not yet clinically detectable. Animal models of human placentation are inadequate. In vitro models that utilize immortalized cell lines and cells derived from trophoblast cancers have multiple limitations. Primary cell and tissue cultures often have limited lifespans and cannot be obtained from the peri-implantation period. We present here two contemporary models of human peri-implantation placental development: extended blastocyst culture and stem-cell derived trophoblast culture. We discuss current research efforts that employ these models and how such models might be used in the future to study the "black box" stage of human pregnancy.

Organoids as Novel Models for Embryo Implantation Study

In the last decade, organoids have become emerging novel models for biomedical research. Organoids are small, self-organized three-dimensional (3D) tissue cultures derived from stem cells that mimic certain tissues or organs. In reproductive medicine, researchers have generated numerous organoids including blastoid (blastocyst organoid), endometrial organoid, and trophoblast organoid. These organdies provide useful models for studying the embryo implantation mechanism through observation of cell differentiation, gene expression, and epigenetic profiles at the implantation stage. As in vitro tissue models, organoids could be coupled with many other frontier technologies such as gene editing and genomic sequencing. However, the main drawback of organoids is that they do not fully mimic their counterparts in vivo tissues. Furthermore, there is a consensus of research ethics on organoids that may limit the types of studies that scientists perform with. Nevertheless, all discoveries and efforts surrounding organoids still greatly benefit therapy development for reproductive clinics.

Molecular and immunological developments in placentas

Cytotrophoblasts differentiate in two directions during early placentation: syncytiotrophoblasts (STBs) and extravillous trophoblasts (EVTs). STBs face maternal immune cells in placentas, and EVTs, which invade the decidua and uterine myometrium, face the cells in the uterus. This situation, in which trophoblasts come into contact with maternal immune cells, is known as the maternal-fetal interface. Despite fetuses and fetus-derived trophoblast cells being of the semi-allogeneic conceptus, fetuses and placentas are not rejected by the maternal immune system because of maternal-fetal tolerance. The acquired tolerance develops during normal placentation, resulting in normal fetal development in humans. In this review, we introduce placental development from the viewpoint of molecular biology. In addition, we discuss how the disruption of placental development could lead to complications in pregnancy, such as hypertensive disorder of pregnancy, fetal growth restriction, or miscarriage.

LncRNA H19-Derived miR-675-5p Accelerates the Invasion of Extravillous Trophoblast Cells by Inhibiting GATA2 and Subsequently Activating Matrix Metalloproteinases

The invasion of extravillous trophoblast (EVT) cells into the maternal decidua, which plays a crucial role in the establishment of a successful pregnancy, is highly orchestrated by a complex array of regulatory mechanisms. Non-coding RNAs (ncRNAs) that fine-tune gene expression at epigenetic, transcriptional, and post-transcriptional levels are involved in the regulatory mechanisms of EVT cell invasion. However, little is known about the characteristic features of EVT-associated ncRNAs. To elucidate the gene expression profiles of both coding and non-coding transcripts (i.e., mRNAs, long non-coding RNAs (lncRNAs), and microRNAs (miRNAs)) expressed in EVT cells, we performed RNA sequencing analysis of EVT cells isolated from first-trimester placentae. RNA sequencing analysis demonstrated that the lncRNA H19 and its derived miRNA miR-675-5p were enriched in EVT cells. Although miR-675-5p acts as a placental/trophoblast growth suppressor, there is little information on the involvement of miR-675-5p in trophoblast cell invasion. Next, we evaluated a possible role of miR-675-5p in EVT cell invasion using the EVT cell lines HTR-8/SVneo and HChEpC1b; overexpression of miR-675-5p significantly promoted the invasion of both EVT cell lines. The transcription factor gene GATA2 was shown to be a target of miR-675-5p; moreover, small interfering RNA-mediated GATA2 knockdown significantly promoted cell invasion. Furthermore, we identified MMP13 and MMP14 as downstream effectors of miR-675-5p/GATA2-dependent EVT cell invasion. These findings suggest that miR-675-5p-mediated GATA2 inhibition accelerates EVT cell invasion by upregulating matrix metalloproteinases.

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.

Adaptations of the human placenta to hypoxia: opportunities for interventions in fetal growth restriction

BACKGROUND

The placenta is the functional interface between the mother and the fetus during pregnancy, and a critical determinant of fetal growth and life-long health. In the first trimester, it develops under a low-oxygen environment, which is essential for the conceptus who has little defense against reactive oxygen species produced during oxidative metabolism. However, failure of invasive trophoblasts to sufficiently remodel uterine arteries toward dilated vessels by the end of the first trimester can lead to reduced/intermittent blood flow, persistent hypoxia and oxidative stress in the placenta with consequences for fetal growth. Fetal growth restriction (FGR) is observed in ∼10% of pregnancies and is frequently seen in association with other pregnancy complications, such as preeclampsia (PE). FGR is one of the main challenges for obstetricians and pediatricians, as smaller fetuses have greater perinatal risks of morbidity and mortality and postnatal risks of neurodevelopmental and cardio-metabolic disorders.

OBJECTIVE AND RATIONALE

The aim of this review was to examine the importance of placental responses to changing oxygen environments during abnormal pregnancy in terms of cellular, molecular and functional changes in order to highlight new therapeutic pathways, and to pinpoint approaches aimed at enhancing oxygen supply and/or mitigating oxidative stress in the placenta as a mean of optimizing fetal growth.

SEARCH METHODS

An extensive online search of peer-reviewed articles using PubMed was performed with combinations of search terms including pregnancy, placenta, trophoblast, oxygen, hypoxia, high altitude, FGR and PE (last updated in May 2020).

OUTCOMES

Trophoblast differentiation and placental establishment are governed by oxygen availability/hypoxia in early pregnancy. The placental response to late gestational hypoxia includes changes in syncytialization, mitochondrial functions, endoplasmic reticulum stress, hormone production, nutrient handling and angiogenic factor secretion. The nature of these changes depends on the extent of hypoxia, with some responses appearing adaptive and others appearing detrimental to the placental support of fetal growth. Emerging approaches that aim to increase placental oxygen supply and/or reduce the impacts of excessive oxidative stress are promising for their potential to prevent/treat FGR.

WIDER IMPLICATIONS

There are many risks and challenges of intervening during pregnancy that must be considered. The establishment of human trophoblast stem cell lines and organoids will allow further mechanistic studies of the effects of hypoxia and may lead to advanced screening of drugs for use in pregnancies complicated by placental insufficiency/hypoxia. Since no treatments are currently available, a better understanding of placental adaptations to hypoxia would help to develop therapies or repurpose drugs to optimize placental function and fetal growth, with life-long benefits to human health.