Review: Trophoblast differentiation from human embryonic stem cells

Exerkine apelin reverses obesity-associated placental dysfunction by accelerating mitochondrial biogenesis in mice

Maternal exercise (ME) protects against adverse effects of maternal obesity (MO) on fetal development. As a cytokine stimulated by exercise, apelin (APN) is elevated due to ME, but its roles in mediating the effects of ME on placental development remain to be defined. Two studies were conducted. In the first study, 18 female mice were assigned to control (CON), obesogenic diet (OB), or OB with exercise (OB/Ex) groups (n = 6); in the second study, the same number of female mice were assigned to three groups; CON with PBS injection (CD/PBS), OB/PBS, or OB with apelin injection (OB/APN). In the exercise study, daily treadmill exercise during pregnancy significantly elevated the expression of PR domain 16 (PRDM16; P < 0.001), which correlated with enhanced oxidative metabolism and mitochondrial biogenesis in the placenta (P < 0.05). More importantly, these changes were partially mirrored in the apelin study. Apelin administration upregulated PRDM16 protein level (P < 0.001), mitochondrial biogenesis (P < 0.05), placental nutrient transporter expression (P < 0.001), and placental vascularization (P < 0.01), which were impaired due to MO (P < 0.05). In summary, MO impairs oxidative phosphorylation in the placenta, which is improved by ME; apelin administration partially mimics the beneficial effects of exercise on improving placental function, which prevents placental dysfunction due to MO.NEW & NOTEWORTHY Maternal exercise prevents metabolic disorders of mothers and offspring induced by high-fat diet. Exercise intervention enhances PRDM16 activation, oxidative metabolism, and vascularization of placenta, which are inhibited due to maternal obesity. Similar to maternal exercise, apelin administration improves placental function of obese dams.

Hypothesis: human trophectoderm biopsy downregulates the expression of the placental growth factor gene

Preeclampsia (PE) and intrauterine growth retardation (IUGR) are the results of defective placentation associated with the downregulation of different genes in the human trophoblast including the Placental Growth Factor (PGF). TrophEctoderm (TE) biopsy is increasingly performed for Pre-implantation Genetic Testing of Aneuploidies and it involves the traumatical removal of an unpredictable number of mural TE cells from the human blastocyst. We observed strikingly similar obstetrical and neonatal complications in pregnancies where the placenta bears PGF downmodulation or a TE biopsy has been done. In both groups, the risk of PE, IUGR, congenital cardiac ventricular septal defects, caesarean section, sex ratio in favour of males and preterm birth is significantly increased compared to controls. Given the high degree of correlation, the observation may not be a casual one. We postulate herein that the TE biopsy may induce persistent dysregulation of different genes in the placenta including PGF. The mechanism proposed is the disruption of tight junctions caused by the TE biopsy.

Aquaporins during pregnancy

Aquaporins (AQPs) are water channels proteins that facilitate water flux across cell membranes in response to osmotic gradients. Despite of the differences in the mammalian placentas, the conserved combination of AQPs expressed in placental and fetal membranes throughout gestation suggests that these proteins may be important in the regulation of fetal water homeostasis. Thus, AQPs may regulate the amniotic fluid volume and participate in the trans-placental transfer of water. Apart from their classical roles, recent studies have revealed that placental AQPs may also cooperate in cellular processes such as the migration and the apoptosis of the trophoblasts. Aquaglyceroporins can also participate in the energy metabolism and in the urea elimination across the placenta. Many factors including oxygen, hormones, acid-basis homeostasis, maternal dietary status, interaction with other transport proteins and osmotic stress are proposed to regulate their expression and function during gestation and alterations result in pathological pregnancies.

Genomic Profiling of BDE-47 Effects on Human Placental Cytotrophoblasts

Despite gradual legislative efforts to phase out flame retardants (FRs) from the marketplace, polybrominated diphenyl ethers (PBDEs) are still widely detected in human maternal and fetal tissues, eg, placenta, due to their continued global application in consumer goods and inherent biological persistence. Recent studies in rodents and human placental cell lines suggest that PBDEs directly cause placental toxicity. During pregnancy, trophoblasts play key roles in uterine invasion, vascular remodeling, and anchoring of the placenta-fetal unit to the mother. Thus, to study the potential consequences of PBDE exposures on human placental development, we used an in vitro model: primary villous cytotrophoblasts (CTBs). Following exposures, the endpoints that were evaluated included cytotoxicity, function (migration, invasion), the transcriptome, and the methylome. In a concentration-dependent manner, common PBDE congeners, BDE-47 and -99, significantly reduced cell viability and increased death. Upon exposures to sub-cytotoxic concentrations (≤ 5 µM), we observed BDE-47 accumulation in CTBs with limited evidence of metabolism. At a functional level, BDE-47 hindered the ability of CTBs to migrate and invade. Transcriptomic analyses of BDE-47 effects suggested concentration-dependent changes in gene expression, involving stress pathways, eg, inflammation and lipid/cholesterol metabolism as well as processes underlying trophoblast fate, eg, differentiation, migration, and vascular morphogenesis. In parallel assessments, BDE-47 induced low-level global increases in methylation of CpG islands, including a subset that were proximal to genes with roles in cell adhesion/migration. Thus, using a primary human CTB model, we showed that PBDEs induced alterations at cellular and molecular levels, which could adversely impact placental development.

Human Endometrial Extracellular Vesicles Functionally Prepare Human Trophectoderm Model for Implantation: Understanding Bidirectional Maternal-Embryo Communication

Embryo implantation into maternal endometrium is critical for initiation and establishment of pregnancy, requiring developmental synchrony between endometrium and blastocyst. However, factors regulating human endometrial-embryo cross talk and facilitate implantation remain largely unknown. Extracellular vesicles (EVs) are emerging as important mediators of this process. Here, a trophectoderm spheroid-based in vitro model mimicking the pre-implantation human embryo is used to recapitulate important functional aspects of blastocyst implantation. Functionally, human endometrial EVs, derived from hormonally treated cells synchronous with implantation, are readily internalized by trophectoderm cells, regulating adhesive and invasive capacity of human trophectoderm spheroids. To gain molecular insights into mechanisms underpinning endometrial EV-mediated enhancement of implantation, quantitative proteomics reveal critical alterations in trophectoderm cellular adhesion networks (cell adhesion molecule binding, cell-cell adhesion mediator activity, and cell adherens junctions) and metabolic and gene expression networks, and the soluble secretome from human trophectodermal spheroids. Importantly, transfer of endometrial EV cargo proteins to trophectoderm to mediate changes in trophectoderm function is demonstrated. This is highlighted by correlation among endometrial EVs, the trophectodermal proteome following EV uptake, and EV-mediated trophectodermal cellular proteome, important for implantation. This work provides an understanding into molecular mechanisms of endometrial EV-mediated regulation of human trophectoderm functions-fundamental in understanding human endometrium-embryo signaling during implantation.

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

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

Human trophoblast stem cell self-renewal and differentiation: Role of decorin

The origin and regulation of stem cells sustaining trophoblast renewal in the human placenta remain unclear. Decorin, a leucine-rich proteoglycan restrains trophoblast proliferation, migration/invasiveness and endovascular differentiation, and local decorin overproduction is associated with preeclampsia (PE). Here, we tested the role of decorin in human trophoblast stem cell self-renewal and differentiation, using two models: an immortalized first trimester trophoblast cell line HTR-8/SVneo (HTR) and freshly isolated primary trophoblast (p-trophoblast) from early first trimester (6-9 weeks) placentas. Self-renewal capacity was measured by spheroid forming ability of single cells on ultra-low attachment plates for multiple generations. Markers of embryonic stem (ES) cells, trophoblast stem (TS) cells and trophoblast were used to identify stem cell hierarchy. Differentiation markers for syncytial and extravillous (EVT) pathways were employed to identify differentiated cells. Bewo cells were additionally used to explore DCN effects on syncytialization. Results reveal that the incidence of spheroid forming stem-like cells was 13-15% in HTR and 0.1-0.4%, in early first trimester p-trophoblast, including a stem cell hierarchy of two populations of ES and TS-like cells. DCN restrained ES cell self-renewal, promoted ES to TS transition and maintenance of TS cell stem-ness, but inhibited TS cell differentiation into both syncytial and EVT pathways.

The many faces of Pluripotency: in vitro adaptations of a continuum of in vivo states

Pluripotency defines the propensity of a cell to differentiate into, and generate, all somatic, as well as germ cells. The epiblast of the early mammalian embryo is the founder population of all germ layer derivatives and thus represents the bona fide in vivo pluripotent cell population. The so-called pluripotent state spans several days of development and is lost during gastrulation as epiblast cells make fate decisions towards a mesoderm, endoderm or ectoderm identity. It is now widely recognized that the features of the pluripotent population evolve as development proceeds from the pre- to post-implantation period, marked by distinct transcriptional and epigenetic signatures. During this period of time epiblast cells mature through a continuum of pluripotent states with unique properties. Aspects of this pluripotent continuum can be captured in vitro in the form of stable pluripotent stem cell types. In this review we discuss the continuum of pluripotency existing within the mammalian embryo, using the mouse as a model, and the cognate stem cell types that can be derived and propagated in vitro. Furthermore, we speculate on embryonic stage-specific characteristics that could be utilized to identify novel, developmentally relevant, pluripotent states.

Tissue organization alters gene expression in equine induced trophectoderm cells

Rapid morphological and gene expression changes occur during the early formation of a mammalian blastocyst. Critical to successful retention of the blastocyst and pregnancy is a functional trophectoderm (TE) that supplies the developing embryo with paracrine factors and hormones. The contribution of TE conformational changes to gene expression was examined in equine induced trophoblast (iTr) cells. Equine iTr cells were cultured as monolayers or in suspension to form spheres. The spheres are hollow and structurally reminiscent of native equine blastocysts. Total RNA was isolated from iTr monolayers and spheres and analyzed by RNA sequencing. An average of 32.2 and 31million aligned reads were analyzed for the spheres and monolayers, respectively. Forty-four genes were unique to monolayers and 45 genes were expressed only in spheres. Conformation did not affect expression of CDX2, POU5F1, TEAD4, ETS2, ELF3, GATA2 or TFAP2A, the core gene network of native TE. Bioinformatic analysis was used to identify classes of genes differentially expressed in response to changes in tissue shape. In both iTr spheres and monolayers, the majority of the differentially expressed genes were associated with binding activity in cellular, developmental and metabolic processes. Inherent to protein:protein interactions, several receptor-ligand families were identified in iTr cells with enrichment of genes coding for PI3-kinase and MAPK signaling intermediates. Our results provide evidence for ligand initiated kinase signaling pathways that underlie early trophectoderm structural changes.

Human trophoblast stem cells: Real or not real?

Abnormal trophoblast differentiation is the root cause of many placenta-based pregnancy complications, including preeclampsia and fetal growth restriction. Human trophoblast differentiation is difficult to study due to the lack of a stem cell model. Such a multipotent "trophoblast stem" (TS) cell, with the ability to differentiate into all trophoblast subtypes, has been derived from mouse blastocysts, but attempts to derive similar human cells have failed. We consider here several possibilities for the TS cell niche in the human placenta. Aside from discussion of such a niche in the pre-implantation blastocyst, we discuss evidence for these TS cells residing in the post-implantation villous cytotrophoblast layer, or even in the non-trophoblast portions, of the human placenta. It is our hope that recognition of the niche would lead to successful derivation and in vitro establishment of such cells, which could then be disseminated widely to the placental biology community for advancing the field. Availability of self-renewing human TS cells, whose gene expression and environment could be manipulated, will provide a platform, not just for the study of pathophysiology of placental disease, but also for the discovery of diagnostic biomarkers and therapeutic targets for common pregnancy complications.

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

Development of the human placenta and its different epithelial trophoblasts is crucial for a successful pregnancy. Besides fusing into a multinuclear syncytium, the exchange surface between mother and fetus, progenitors develop into extravillous trophoblasts invading the maternal uterus and its spiral arteries. Migration into these vessels promotes remodelling and, as a consequence, adaption of blood flow to the fetal-placental unit. Defects in remodelling and trophoblast differentiation are associated with severe gestational diseases, such as preeclampsia. However, mechanisms controlling human trophoblast development are largely unknown. Herein, we show that Notch1 is one such critical regulator, programming primary trophoblasts into progenitors of the invasive differentiation pathway. At the 12th wk of gestation, Notch1 is exclusively detected in precursors of the extravillous trophoblast lineage, forming cell columns anchored to the uterine stroma. At the 6th wk, Notch1 is additionally expressed in clusters of villous trophoblasts underlying the syncytium, suggesting that the receptor initiates the invasive differentiation program in distal regions of the developing placental epithelium. Manipulation of Notch1 in primary trophoblast models demonstrated that the receptor promotes proliferation and survival of extravillous trophoblast progenitors. Notch1 intracellular domain induced genes associated with stemness of cell columns, myc and VE-cadherin, in Notch1^- fusogenic precursors, and bound to the myc promoter and enhancer region at RBPJκ cognate sequences. In contrast, Notch1 repressed syncytialization and expression of TEAD4 and p63, two regulators controlling self-renewal of villous cytotrophoblasts. Our results revealed Notch1 as a key factor promoting development of progenitors of the extravillous trophoblast lineage in the human placenta.

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

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

Placental-derived stem cells: Culture, differentiation and challenges

Stem cell therapy is a promising approach to clinical healing in several diseases. A great variety of tissues (bone marrow, adipose tissue, and placenta) are potentially sources of stem cells. Placenta-derived stem cells (p-SCs) are in between embryonic and mesenchymal stem cells, sharing characteristics with both, such as non-carcinogenic status and property to differentiate in all embryonic germ layers. Moreover, their use is not ethically restricted as fetal membranes are considered medical waste after birth. In this context, the present review will be focused on the biological properties, culture and potential cell therapy uses of placental-derived stem cells. Immunophenotype characterization, mainly for surface marker expression, and basic principles of p-SC isolation and culture (mechanical separation or enzymatic digestion of the tissues, the most used culture media, cell plating conditions) will be presented. In addition, some preclinical studies that were performed in different medical areas will be cited, focusing on neurological, liver, pancreatic, heart, muscle, pulmonary, and bone diseases and also in tissue engineering field. Finally, some challenges for stem cell therapy applications will be highlighted. The understanding of the mechanisms involved in the p-SCs differentiation and the achievement of pure cell populations (after differentiation) are key points that must be clarified before bringing the preclinical studies, performed at the bench, to the medical practice.