How trophoblasts fuse: an in-depth look into placental syncytiotrophoblast formation

TFEB safeguards trophoblast syncytialization in humans and mice

Nutrient sensing and adaptation in the placenta are essential for pregnancy viability and proper fetal growth. Our recent study demonstrated that the placenta adapts to nutrient insufficiency through mechanistic target of rapamycin (mTOR) inhibition-mediated trophoblast differentiation toward syncytiotrophoblasts (STBs), a highly specialized multinucleated trophoblast subtype mediating extensive maternal-fetal interactions. However, the underlying mechanism remains elusive. Here, we unravel the indispensable role of the mTORC1 downstream transcriptional factor TFEB in STB formation both in vitro and in vivo. TFEB deficiency significantly impaired STB differentiation in human trophoblasts and placenta organoids. Consistently, systemic or trophoblast-specific deletion of Tfeb compromised STB formation and placental vascular construction, leading to severe embryonic lethality. Mechanistically, TFEB conferred direct transcriptional activation of the fusogen ERVFRD-1 in human trophoblasts and thereby promoted STB formation, independent of its canonical function as a master regulator of the autophagy-lysosomal pathway. Moreover, we demonstrated that TFEB directed the trophoblast syncytialization response driven by mTOR complex 1 (mTORC1) signaling. TFEB expression positively correlated with the reinforced trophoblast syncytialization in human fetal growth-restricted placentas exhibiting suppressed mTORC1 activity. Our findings substantiate that the TFEB-fusogen axis ensures proper STB formation during placenta development and under nutrient stress, shedding light on TFEB as a mechanistic link between nutrient-sensing machinery and trophoblast differentiation.

Regulation of placental amino acid transport in health and disease

Abnormal fetal growth, i.e., intrauterine growth restriction (IUGR) or fetal growth restriction (FGR) and fetal overgrowth, is associated with increased perinatal morbidity and mortality and is strongly linked to the development of metabolic and cardiovascular disease in childhood and later in life. Emerging evidence suggests that changes in placental amino acid transport may contribute to abnormal fetal growth. This review is focused on amino acid transport in the human placenta, however, relevant animal models will be discussed to add mechanistic insights. At least 25 distinct amino acid transporters with different characteristics and substrate preferences have been identified in the human placenta. Of these, System A, transporting neutral nonessential amino acids, and System L, mediating the transport of essential amino acids, have been studied in some detail. Importantly, decreased placental Systems A and L transporter activity is strongly associated with IUGR and increased placental activity of these two amino acid transporters has been linked to fetal overgrowth in human pregnancy. An array of factors in the maternal circulation, including insulin, IGF-1, and adiponectin, and placental signaling pathways such as mTOR, have been identified as key regulators of placental Systems A and L. Studies using trophoblast-specific gene targeting in mice have provided compelling evidence that changes in placental Systems A and L are mechanistically linked to altered fetal growth. It is possible that targeting specific placental amino acid transporters or their upstream regulators represents a novel intervention to alleviate the short- and long-term consequences of abnormal fetal growth in the future.

Fetus Exposure to Drugs and Chemicals: A Holistic Overview on the Assessment of Their Transport and Metabolism across the Human Placental Barrier

BACKGROUND

The placenta exerts a crucial role in fetus growth and development during gestation, protecting the fetus from maternal drugs and chemical exposure. However, diverse drugs and chemicals (xenobiotics) can penetrate the maternal placental barrier, leading to deleterious, adverse effects concerning fetus health. Moreover, placental enzymes can metabolize drugs and chemicals into more toxic compounds for the fetus. Thus, evaluating the molecular mechanisms through which drugs and chemicals transfer and undergo metabolism across the placental barrier is of vital importance. In this aspect, this comprehensive literature review aims to provide a holistic approach by critically summarizing and scrutinizing the potential molecular processes and mechanisms governing drugs and chemical transfer and metabolism across the placental barrier, which may lead to fetotoxicity effects, as well as analyzing the currently available experimental methodologies used to assess xenobiotics placental transfer and metabolism.

METHODS

A comprehensive and in-depth literature review was conducted in the most accurate scientific databases such as PubMed, Scopus, and Web of Science by using relevant and effective keywords related to xenobiotic placental transfer and metabolism, retrieving 8830 published articles until 5 February 2024. After applying several strict exclusion and inclusion criteria, a final number of 148 relevant published articles were included.

RESULTS

During pregnancy, several drugs and chemicals can be transferred from the mother to the fetus across the placental barrier by either passive diffusion or through placental transporters, resulting in fetus exposure and potential fetotoxicity effects. Some drugs and chemicals also appear to be metabolized across the placental barrier, leading to more toxic products for both the mother and the fetus. At present, there is increasing research development of diverse experimental methodologies to determine the potential molecular processes and mechanisms of drug and chemical placental transfer and metabolism. All the currently available methodologies have specific strengths and limitations, highlighting the strong demand to utilize an efficient combination of them to obtain reliable evidence concerning drug and chemical transfer and metabolism across the placental barrier. To derive the most consistent and safe evidence, in vitro studies, ex vivo perfusion methods, and in vivo animal and human studies can be applied together with the final aim to minimize potential fetotoxicity effects.

CONCLUSIONS

Research is being increasingly carried out to obtain an accurate and safe evaluation of drug and chemical transport and metabolism across the placental barrier, applying a combination of advanced techniques to avoid potential fetotoxic effects. The improvement of the currently available techniques and the development of novel experimental protocols and methodologies are of major importance to protect both the mother and the fetus from xenobiotic exposure, as well as to minimize potential fetotoxicity effects.

Directed biomechanical compressive forces enhance fusion efficiency in model placental trophoblast cultures

The syncytiotrophoblast is a multinucleated structure that arises from fusion of mononucleated cytotrophoblasts, to sheath the placental villi and regulate transport across the maternal-fetal interface. Here, we ask whether the dynamic mechanical forces that must arise during villous development might influence fusion, and explore this question using in vitro choriocarcinoma trophoblast models. We demonstrate that mechanical stress patterns arise around sites of localized fusion in cell monolayers, in patterns that match computational predictions of villous morphogenesis. We then externally apply these mechanical stress patterns to cell monolayers and demonstrate that equibiaxial compressive stresses (but not uniaxial or equibiaxial tensile stresses) enhance expression of the syndecan-1 and loss of E-cadherin as markers of fusion. These findings suggest that the mechanical stresses that contribute towards sculpting the placental villi may also impact fusion in the developing tissue. We then extend this concept towards 3D cultures and demonstrate that fusion can be enhanced by applying low isometric compressive stresses to spheroid models, even in the absence of an inducing agent. These results indicate that mechanical stimulation is a potent activator of cellular fusion, suggesting novel avenues to improve experimental reproductive modelling, placental tissue engineering, and understanding disorders of pregnancy development.

Engineered 3D Hydrogel Matrices to Modulate Trophoblast Stem Cell-Derived Placental Organoid Phenotype

Placental organoid models are a promising platform to study human placental development and function. Organoid systems typically use naturally derived hydrogel extracellular matrices (ECM), resulting in batch-to-batch variability that limits experimental reproducibility. As an alternative, synthetic ECM-mimicking hydrogel matrices offer greater consistency and control over environmental cues. Here, we generated trophoblast stem cell-derived placental organoids using poly(ethylene glycol) (PEG) hydrogels with tunable degradability and placenta-derived ECM cues to evaluate trophoblast differentiation relative to Matrigel and two-dimensional (2D) culture controls. Our data demonstrate that PEG hydrogels support trophoblast viability and metabolic function comparable to gold standard Matrigel. Additionally, phenotypic characterization via proteomic analysis revealed that PEG and Matrigel matrices drive syncytiotrophoblast and extravillous trophoblast-dominant placental organoid phenotypes, respectively. Further, three-dimensional (3D) environments promoted greater integrin expression and ECM production than 2D culture. This study demonstrates that engineered 3D culture environments can be used to reliably generate placental organoids and guide trophoblast differentiation.

Associations Between Prenatal Vitamin D and Placental Gene Expression

Background

Vitamin D is a hormone regulating gene transcription. Prenatal vitamin D has been linked to immune and vascular function in the placenta, a key organ of pregnancy. To date, studies of vitamin D and placental gene expression have focused on a limited number of candidate genes. Transcriptome-wide RNA sequencing can provide a more complete representation of the placental effects of vitamin D.

Objective

We investigated the association between prenatal vitamin D levels and placental gene expression in a large, prospective pregnancy cohort.

Methods

Participants were recruited in Shelby County, Tennessee in the Conditions Affecting Neurocognitive Development and Learning in Early childhood (CANDLE) study. Vitamin D level (plasma total 25-hydroxyvitatmin D, [25(OH)D]) was measured at mid-pregnancy (16-28 weeks' gestation) and delivery. Placenta samples were collected at birth. RNA was isolated and sequenced. We identified differentially expressed genes (DEGs) using adjusted linear regression models. We also conducted weighted gene co-expression network analysis (WGCNA).

Results

The median 25(OH)D of participants was 21.8 ng/mL at mid-pregnancy (N=774, IQR: 15.4-26.5 ng/mL) and 23.6 ng/mL at delivery (N=753, IQR: 16.8-29.1 ng/mL). Placental expression of 25 DEGs was associated with 25(OH)D at mid-pregnancy, but no DEG was associated with 25(OH)D at delivery. DEGs were related to energy metabolism, cytoskeletal function, and RNA transcription. Using WGCNA, we identified 2 gene modules whose expression was associated with 25(OH)D at mid-pregnancy and 1 module associated with 25(OH)D at delivery. These modules were enriched for genes related to mitochondrial and cytoskeletal function, and were regulated by transcription factors including ARNT2, BHLHE40, FOSL2, JUND, and NFKB1.

Conclusions

Our results indicate that 25(OH)D during mid-pregnancy, but not at delivery, is associated with placental gene expression at birth. Future research is needed to investigate a potential role of vitamin D in programming placental mitochondrial metabolism, intracellular transport, and transcriptional regulation during pregnancy.

Placenta: an old organ with new functions

The transition from oviparity to viviparity and the establishment of feto-maternal communications introduced the placenta as the major anatomical site to provide nutrients, gases, and hormones to the developing fetus. The placenta has endocrine functions, orchestrates maternal adaptations to pregnancy at different periods of pregnancy, and acts as a selective barrier to minimize exposure of developing fetus to xenobiotics, pathogens, and parasites. Despite the fact that this ancient organ is central for establishment of a normal pregnancy in eutherians, the placenta remains one of the least studied organs. The first step of pregnancy, embryo implantation, is finely regulated by the trophoectoderm, the precursor of all trophoblast cells. There is a bidirectional communication between placenta and endometrium leading to decidualization, a critical step for maintenance of pregnancy. There are three-direction interactions between the placenta, maternal immune cells, and the endometrium for adaptation of endometrial immune system to the allogeneic fetus. While 65% of all systemically expressed human proteins have been found in the placenta tissues, it expresses numerous placenta-specific proteins, whose expression are dramatically changed in gestational diseases and could serve as biomarkers for early detection of gestational diseases. Surprisingly, placentation and carcinogenesis exhibit numerous shared features in metabolism and cell behavior, proteins and molecular signatures, signaling pathways, and tissue microenvironment, which proposes the concept of "cancer as ectopic trophoblastic cells". By extensive researches in this novel field, a handful of cancer biomarkers has been discovered. This review paper, which has been inspired in part by our extensive experiences during the past couple of years, highlights new aspects of placental functions with emphasis on its immunomodulatory role in establishment of a successful pregnancy and on a potential link between placentation and carcinogenesis.

Engineering placental trophoblast fusion: A potential role for biomechanics in syncytialization

The process by which placental trophoblasts fuse to form the syncytiotrophoblast around the chorionic villi is not fully understood. Mechanical features of the in vivo and in vitro culture environments have recently emerged as having the potential to influence fusion efficiency, and considering these mechanical cues may ultimately allow predictive control of trophoblast syncytialization. Here, we review recent studies that suggest that biomechanical factors such as shear stress, tissue stiffness, and dimensionally-related stresses affect villous trophoblast fusion efficiency. We then discuss how these stimuli might arise in vivo and how they can be incorporated in cultures to study and enhance villous trophoblast fusion. We believe that this mechanical paradigm will provide novel insight into manipulating the syncytialization process to better engineer improved models, understand disease progression, and ultimately develop novel therapeutic strategies.

Current research and evidence gaps on placental development in iron deficiency anemia

Studying the effects of maternal iron deficiency anemia (IDA) is complex owing to its diverse causes, each independently impacting the placenta and fetus. Simple treatment with iron supplements does not always resolve the anemia. Therefore, delving into how IDA alters placental development at a molecular level is crucial to further optimize treatment. This review addresses the effects of IDA on placental structures and functions, including changes in oxygen levels, blood vessels, and the immune system. Profound understanding of physiological characteristics and regulatory mechanisms of placental development is key to explain the mechanisms of abnormal placental development in pregnancy-associated disorders. In turn, future strategies for the prevention and treatment of pregnancy complications involving the placenta can be devised. These studies are significant for improving human reproductive health, enhancing sociodemographic qualities, and even lifelong wellbeing, a focal point in future placental research.

Shear stress in the intervillous space promotes syncytial formation of iPS cells-derived trophoblasts†

The intervillous space of human placenta is filled with maternal blood, and villous trophoblasts are constantly exposed to the shear stress generated by maternal blood pressure and flow throughout the entire gestation period. However, the effects of shear stress on villous trophoblasts and their biological significance remain unknown. Here, using our recently established naïve human pluripotent stem cells-derived cytotrophoblast stem cells (nCTs) and a device that can apply arbitrary shear stress to cells, we investigated the impact of shear stress on early-stage trophoblasts. After 72 h of exposure to 10 dyn/cm2 shear stress, nCTs became fused and multinuclear, and mRNA expression of the syncytiotrophoblast (ST) markers, such as glial cell missing 1, endogenous retrovirus group W member 1 envelope, chorionic gonadotropin subunit beta 3, syndecan 1, pregnancy specific beta-1-glycoprotein 3, placental growth factor, and solute carrier family 2 member 1 were significantly upregulated compared to static conditions. Immunohistochemistry showed that shear stress increased fusion index, human chorionic gonadotropin secretion, and human placental lactogen secretion. Increased microvilli formation on the surface of nCTs under flow conditions was detected using scanning electron microscopy. Intracellular cyclic adenosine monophosphate significantly increased under flow conditions. Moreover, transcriptome analysis of nCTs subjected to shear stress revealed that shear stress upregulated ST-specific genes and downregulated CT-specific genes. Collectively, these findings indicate that shear stress promotes the differentiation of nCTs into ST.

Somatic Cell Fusion in Host Defense and Adaptation

Evidence of fusion of somatic cells has been noted in health and in disease for more than a century. The most obvious but uncertain hallmark has been the presence of multiple nuclei in cells. Although multinucleated cells are found in normal and diseased tissues, the benefit or harm of such cells can be difficult to elucidate. Still more difficult however is the identification of mononuclear cells previously formed by fusion of somatic cells with one or more nuclei disposed. The later process can introduce mutations that promote viral diversification, cancer, and tissue senescence. Less obvious the potential benefits of cell fusion. Recent work in cell biology, immunology, and genomic analysis however makes it possible to postulate benefits and potentially arrive at novel therapeutic agents and approaches that replicate or enhance these benefits.

Trophoblast Syncytialization: A Metabolic Crossroads

During placentation, villous cytotrophoblast (CTB) stem cells proliferate and fuse, giving rise to the multinucleated syncytiotrophoblast (STB), which represents the terminally differentiated villous layer as well as the maternal-fetal interface. The syncytiotrophoblast is at the forefront of nutrient, gas, and waste exchange while also harboring essential endocrine functions to support pregnancy and fetal development. Considering that mitochondrial dynamics and respiration have been implicated in stem cell fate decisions of several cell types and that the placenta is a mitochondria-rich organ, we will highlight the role of mitochondria in facilitating trophoblast differentiation and maintaining trophoblast function. We discuss both the process of syncytialization and the distinct metabolic characteristics associated with CTB and STB sub-lineages prior to and during syncytialization. As mitochondrial respiration is tightly coupled to redox homeostasis, we emphasize the adaptations of mitochondrial respiration to the hypoxic placental environment. Furthermore, we highlight the critical role of mitochondria in conferring the steroidogenic potential of the STB following differentiation. Ultimately, mitochondrial function and morphological changes centrally regulate respiration and influence trophoblast fate decisions through the production of reactive oxygen species (ROS), whose levels modulate the transcriptional activation or suppression of pluripotency or commitment genes.

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.

Molecular compartmentalization in a syncytium: restricted mobility of proteins within the sea urchin skeletogenic mesenchyme

Multinucleated cells, or syncytia, are found in diverse taxa. Their biological function is often associated with the compartmentalization of biochemical or cellular activities within the syncytium. How such compartments are generated and maintained is poorly understood. The sea urchin embryonic skeleton is secreted by a syncytium, and local patterns of skeletal growth are associated with distinct sub-domains of gene expression within the syncytium. For such molecular compartments to be maintained and to control local patterns of skeletal growth: (1) the mobility of TFs must be restricted to produce stable differences in the transcriptional states of nuclei within the syncytium; and (2) the mobility of biomineralization proteins must also be restricted to produce regional differences in skeletal growth. To test these predictions, we expressed fluorescently tagged forms of transcription factors and biomineralization proteins in sub-domains of the skeletogenic syncytium. We found that both classes of proteins have restricted mobility within the syncytium and identified motifs that limit their mobility. Our findings have general implications for understanding the functional and molecular compartmentalization of syncytia.

Wound-Induced Syncytia Outpace Mononucleate Neighbors during Drosophila Wound Repair

All organisms have evolved to respond to injury. Cell behaviors like proliferation, migration, and invasion replace missing cells and close wounds. However, the role of other wound-induced cell behaviors is not understood, including the formation of syncytia (multinucleated cells). Wound-induced epithelial syncytia were first reported around puncture wounds in post-mitotic Drosophila epidermal tissues, but have more recently been reported in mitotically competent tissues such as the Drosophila pupal epidermis and zebrafish epicardium. The presence of wound-induced syncytia in mitotically active tissues suggests that syncytia offer adaptive benefits, but it is unknown what those benefits are. Here, we use in vivo live imaging to analyze wound-induced syncytia in mitotically competent Drosophila pupae. We find that almost half the epithelial cells near a wound fuse to form large syncytia. These syncytia use several routes to speed wound repair: they outpace diploid cells to complete wound closure; they reduce cell intercalation during wound closure; and they pool the resources of their component cells to concentrate them toward the wound. In addition to wound healing, these properties of syncytia are likely to contribute to their roles in development and pathology.

Exploring the Role of Mediterranean and Westernized Diets and Their Main Nutrients in the Modulation of Oxidative Stress in the Placenta: A Narrative Review

Oxidative stress is a major cellular event that occurs in the placenta, fulfilling critical physiological roles in non-pathological pregnancies. However, exacerbated oxidative stress is a pivotal feature of different obstetric complications, like pre-eclampsia, fetal growth restriction, and other diseases. Compelling evidence supports the relevant role of diet during pregnancy, with pleiotropic consequences for maternal well-being. The present review aims to examine the complex background between oxidative stress and placental development and function in physiological conditions, also intending to understand the relationship between different dietary patterns and the human placenta, particularly how this could influence oxidative stress processes. The effects of Westernized diets (WDs) and high-fat diets (HFDs) rich in ultra-processed foods and different additives are compared with healthy patterns such as a Mediterranean diet (MedDiet) abundant in omega 3 polyunsaturated fatty acids, monounsaturated fatty acids, polyphenols, dietary fiber, and vitamins. Although multiple studies have focused on the role of specific nutrients, mostly in animal models and in vitro, further observational and intervention studies focusing on the placental structure and function in women with different dietary patterns should be conducted to understand the precise influence of diet on this organ.

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.

Physiological and pathological evidence of O-GlcNAcylation regulation during pregnancy related process

O-GlcNAcylation is a dynamic and reversible post-translational modification (PTM) controlled by the enzymes O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Changes in its expression lead to a breakdown in cellular homeostasis, which is linked to several pathological processes. Placentation and embryonic development are periods of high cell activity, and imbalances in cell signaling pathways can result in infertility, miscarriage, or pregnancy complications. O-GlcNAcylation is involved in cellular processes such as genome maintenance, epigenetic regulation, protein synthesis/degradation, metabolic pathways, signaling pathways, apoptosis, and stress response. Trophoblastic differentiation/invasion and placental vasculogenesis, as well as zygote viability and embryonic neuronal development, are all dependent on O-GlcNAcylation. This PTM is required for pluripotency, which is a required condition for embryonic development. Further, this pathway is a nutritional sensor and cell stress marker, which is primarily measured by the OGT enzyme and its product, protein O-GlcNAcylation. Yet, this post-translational modification is enrolled in metabolic and cardiovascular adaptations during pregnancy. Finally, evidence of how O-GlcNAc impacts pregnancy during pathological conditions such as hyperglycemia, gestational diabetes, hypertension, and stress disorders are reviewed. Considering this scenario, progress in understanding the role of O- GlcNAcylation in pregnancy is required.

Endogenous retrovirus-derived enhancers confer the transcriptional regulation of human trophoblast syncytialization

Endogenous retroviruses (ERVs) have been proposed as a driving force for the evolution of the mammalian placenta, however, the contribution of ERVs to placental development and the underlying regulatory mechanism remain largely elusive. A key process of placental development is the formation of multinucleated syncytiotrophoblasts (STBs) in direct contact with maternal blood, through which constitutes the maternal-fetal interface critical for nutrient allocation, hormone production and immunological modulation during pregnancy. We delineate that ERVs profoundly rewire the transcriptional program of trophoblast syncytialization. Here, we first determined the dynamic landscape of bivalent ERV-derived enhancers with dual occupancy of H3K27ac and H3K9me3 in human trophoblast stem cells (hTSCs). We further demonstrated that enhancers overlapping several ERV families tend to exhibit increased H3K27ac and reduced H3K9me3 occupancy in STBs relative to hTSCs. Particularly, bivalent enhancers derived from the Simiiformes-specific MER50 transposons were linked to a cluster of genes important for STB formation. Importantly, deletions of MER50 elements adjacent to several STB genes, including MFSD2A and TNFAIP2, significantly attenuated their expression concomitant to compromised syncytium formation. Together, we propose that ERV-derived enhancers, MER50 specifically, fine-tune the transcriptional networks accounting for human trophoblast syncytialization, which sheds light on a novel ERV-mediated regulatory mechanism underlying placental development.

Metformin impairs trophoblast metabolism and differentiation in a dose-dependent manner

Metformin is a widely prescribed medication whose mechanism of action is not completely defined and whose role in gestational diabetes management remains controversial. In addition to increasing the risk of fetal growth abnormalities and preeclampsia, gestational diabetes is associated with abnormalities in placental development including impairments in trophoblast differentiation. Given that metformin impacts cellular differentiation events in other systems, we assessed metformin's impact on trophoblast metabolism and differentiation. Using established cell culture models of trophoblast differentiation, oxygen consumption rates and relative metabolite abundance were determined following 200 µM (therapeutic range) and 2000 µM (supra-therapeutic range) metformin treatment using Seahorse and mass-spectrometry approaches. While no differences in oxygen consumption rates or relative metabolite abundance were detected between vehicle and 200 µM metformin-treated cells, 2000 µM metformin impaired oxidative metabolism and increased the abundance of lactate and TCA cycle intermediates, α-ketoglutarate, succinate, and malate. Examining differentiation, treatment with 2000 μM, but not 200 µM metformin, impaired HCG production and expression of multiple trophoblast differentiation markers. Overall, this work suggests that supra-therapeutic concentrations of metformin impair trophoblast metabolism and differentiation whereas metformin concentrations in the therapeutic range do not strongly impact these processes.

Mitochondrial citrate metabolism and efflux regulate BeWo differentiation

Cytotrophoblasts fuse to form and renew syncytiotrophoblasts necessary to maintain placental health throughout gestation. During cytotrophoblast to syncytiotrophoblast differentiation, cells undergo regulated metabolic and transcriptional reprogramming. Mitochondria play a critical role in differentiation events in cellular systems, thus we hypothesized that mitochondrial metabolism played a central role in trophoblast differentiation. In this work, we employed static and stable isotope tracing untargeted metabolomics methods along with gene expression and histone acetylation studies in an established BeWo cell culture model of trophoblast differentiation. Differentiation was associated with increased abundance of the TCA cycle intermediates citrate and α-ketoglutarate. Citrate was preferentially exported from mitochondria in the undifferentiated state but was retained to a larger extent within mitochondria upon differentiation. Correspondingly, differentiation was associated with decreased expression of the mitochondrial citrate transporter (CIC). CRISPR/Cas9 disruption of the mitochondrial citrate carrier showed that CIC is required for biochemical differentiation of trophoblasts. Loss of CIC resulted in broad alterations in gene expression and histone acetylation. These gene expression changes were partially rescued through acetate supplementation. Taken together, these results highlight a central role for mitochondrial citrate metabolism in orchestrating histone acetylation and gene expression during trophoblast differentiation.

Roles of TGF-β1 in Viral Infection during Pregnancy: Research Update and Perspectives

Transforming growth factor-beta 1 (TGF-β1) is a pleiotropic growth factor playing various roles in the human body including cell growth and development. More functions of TGF-β1 have been discovered, especially its roles in viral infection. TGF-β1 is abundant at the maternal-fetal interface during pregnancy and plays an important function in immune tolerance, an essential key factor for pregnancy success. It plays some critical roles in viral infection in pregnancy, such as its effects on the infection and replication of human cytomegalovirus in syncytiotrophoblasts. Interestingly, its role in the enhancement of Zika virus (ZIKV) infection and replication in first-trimester trophoblasts has recently been reported. The above up-to-date findings have opened one of the promising approaches to studying the mechanisms of viral infection during pregnancy with links to corresponding congenital syndromes. In this article, we review our current and recent advances in understanding the roles of TGF-β1 in viral infection. Our discussion focuses on viral infection during pregnancy, especially in the first trimester. We highlight the mutual roles of viral infection and TGF-β1 in specific contexts and possible functions of the Smad pathway in viral infection, with a special note on ZIKV infection. In addition, we discuss promising approaches to performing further studies on this topic.

Metformin impairs trophoblast metabolism and differentiation in dose dependent manner

Metformin is a widely prescribed medication whose mechanism of action is not completely defined and whose role in gestational diabetes management remains controversial. In addition to increasing risks of fetal growth abnormalities and preeclampsia, gestational diabetes is associated with abnormalities in placental development including impairments in trophoblast differentiation. Given that metformin impacts cellular differentiation events in other systems, we assessed metformin's impact on trophoblast metabolism and differentiation. Using established cell culture models of trophoblast differentiation, oxygen consumption rates and relative metabolite abundance were determined following 200 μM (therapeutic range) and 2000 μM (supra-therapeutic range) metformin treatment using Seahorse and mass-spectrometry approaches. While no differences in oxygen consumption rates or relative metabolite abundance were detected between vehicle and 200 μM metformin treated cells, 2000 μM metformin impaired oxidative metabolism and increased abundance of lactate and TCA cycle intermediates, α-ketoglutarate, succinate, and malate. Examining differentiation, treatment with 2000 μM, but not 200 μM metformin, impaired HCG production and expression of multiple trophoblast differentiation markers. Overall, this work suggests that supra-therapeutic concentrations of metformin impairs trophoblast metabolism and differentiation whereas metformin concentrations in the therapeutic range do not strongly impact these processes.

Mitochondrial citrate metabolism and efflux regulates trophoblast differentiation

Cytotrophoblasts fuse to form and renew syncytiotrophoblasts necessary to maintain placental health throughout gestation. During cytotrophoblast to syncytiotrophoblast differentiation, cells undergo regulated metabolic and transcriptional reprogramming. Mitochondria play a critical role in differentiation events in cellular systems, thus we hypothesized that mitochondrial metabolism played a central role in trophoblast differentiation. In this work, we employed static and stable isotope tracing untargeted metabolomics methods along with gene expression and histone acetylation studies in an established cell culture model of trophoblast differentiation. Trophoblast differentiation was associated with increased abundance of the TCA cycle intermediates citrate and α-ketoglutarate. Citrate was preferentially exported from mitochondria in the undifferentiated state but was retained to a larger extent within mitochondria upon differentiation. Correspondingly, differentiation was associated with decreased expression of the mitochondrial citrate transporter (CIC). CRISPR/Cas9 disruption of the mitochondrial citrate carrier showed that CIC is required for biochemical differentiation of trophoblasts. Loss of CIC resulted in broad alterations in gene expression and histone acetylation. These gene expression changes were partially rescued through acetate supplementation. Taken together, these results highlight a central role for mitochondrial citrate metabolism in orchestrating histone acetylation and gene expression during trophoblast differentiation.

Infectious and environmental placental insults: from underlying biological pathways to diagnostics and treatments

Because the placenta is bathed in maternal blood, it is exposed to infectious agents and chemicals that may be present in the mother's circulation. Such exposures, which do not necessarily equate with transmission to the fetus, may primarily cause placental injury, thereby impairing placental function. Recent research has improved our understanding of the mechanisms by which some infectious agents are transmitted to the fetus, as well as the mechanisms underlying their impact on fetal outcomes. However, less is known about the impact of placental infection on placental structure and function, or the mechanisms underlying infection-driven placental pathogenesis. Moreover, recent studies indicate that noninfectious environmental agents accumulate in the placenta, but their impacts on placental function and fetal outcomes are unknown. Critically, diagnosing placental insults during pregnancy is very difficult and currently, this is possible only through postpartum placental examination. Here, with emphasis on humans, we discuss what is known about the impact of infectious and chemical agents on placental physiology and function, particularly in the absence of maternal-fetal transmission, and highlight knowledge gaps with potential implications for diagnosis and intervention against placental pathologies.