Blimp1/Prdm1 governs terminal differentiation of endovascular trophoblast giant cells and defines multipotent progenitors in the developing placenta

Chemerin facilitates placental trophoblast invasion and spiral artery remodeling through the pentose phosphate pathway

AIMS

The invasion of trophoblasts and remodeling of spiral arteries are the requisite processes for successful placentation. A defect of trophoblast invasion is closely associated with pregnancy complications, including miscarriage and preeclampsia. In this study, we investigated the function of chemerin in trophoblast invasion and artery remodeling and explored the underlying mechanism in this process.

MAIN METHODS

Immunostaining was performed to examine chemerin expression in different days of mouse placenta and early stage of human placenta. Chemerin KO and LPS-treated mice, with exogenous chemerin peptide, were used to evaluate trophoblast giant cells (TGC) invasion, artery remodeling, and NK cell infiltration. Chemerin KO and LPS-treated decidua on E8.5 were conducted in metabolites file and measured related enzymes' expression. Chemerin's function was further examined by human trophoblast HTR-8 cell migration and the enzymes expression in the pentose phosphate pathway.

KEY FINDINGS

Chemerin has high expression in mouse-invasive TGC and human extra-villous trophoblast cells. Deficiency of chemerin and LPS treatment in pregnant mice impaired placental TGC invasion, spiral artery remodeling, and NK cell infiltration in decidua, which mainly attributed to the downregulation of metabolites and G6PD and RPIA expression in pentose phosphate pathway (PPP). Chemerin activated the PPP to accelerate HTR-8 cell migration. Exogenous chemerin administration remarkably attenuated the defect of TGC invading and artery remodeling in LPS-treated mice, and promoted NK infiltration and maternal blood perfusion.

SIGNIFICANCE

This study described the indispensable role of chemerin in trophoblast invasion and arterial remodeling, and suggested its potential application in pregnancy complications miscarriage and preeclampsia.

The zinc-finger transcription factor Blimp1/Prdm1 is required for uterine remodelling and repair in the mouse

The zinc finger transcription factor Blimp1/PRDM1 regulates gene expression in diverse cell types. Its activity controls the maternal decidual response at early post-implantation stages of development. The present experiments demonstrate surprisingly that Blimp1 activity in the uterus is required for tissue remodelling at sites of embryonic failure. Moreover Blimp1 mutant females are refractory to RU486 induced decidual shedding. RNA-seq together with immunostaining experiments strongly suggest that the failure to up-regulate expression of the matrix metalloprotease Mmp10 in combination with insufficient suppression of BMP signalling, likely explain Blimp1-dependent phenotypic changes. In the post-partum uterus Blimp1 together with Mmp10 are highly upregulated at sites of tissue repair following placental detachment. Conditional Blimp1 removal significantly impairs the re-epithelization process and severely impacts involution of the endometrium and luminal epithelium. Overall these results identify Blimp1 as a master regulator of uterine tissue remodelling and repair.

Activated PRDM1-CREBBP contributes to preeclampsia by regulating apoptosis and invasion of the human trophoblast cells

Preeclampsia (PE) is a multifactorial disorder of pregnancy, characterized by new-onset gestational hypertension. High-throughput mRNA sequencing (RNA-seq) was performed to analyze the gene expression patterns in placentas from patients with early-onset PE (EOPE). PR domain zinc-finger protein 1 (PRDM1) expression increased in the chorionic villi and placental basal plate from patients with PE and nitro-l-arginine methyl ester (L-NAME)-treated rats. Inhibition of PRDM1 enhanced trophoblast/extravillous trophoblast (EVT) cell invasion/migration and reduced apoptosis under hypoxia/reoxygenation (H/R) conditions. RNA-seq data indicated that the expression of CREB-binding protein (CREBBP), a transcriptional coactivator, was upregulated in preeclamptic placentas and showed a positive correlation with that of PRDM1. Genetic and pharmacological inhibition of CREBBP exhibited anti-apoptotic and pro-invasive roles. H/R stimulation upregulated CREBBP expression and augmented the binding of PRDM1 to CREBBP's promoter. CREBBP was further validated as a direct downstream target of PRDM1. Collectively, our work reveals an involvement of the activated PRDM1-CREBBP axis in PE-associated trophoblast dysfunction.

A spatiotemporal transcriptomic atlas of mouse placentation

The placenta, a temporary but essential organ for gestational support, undergoes intricate morphological and functional transformations throughout gestation. However, the spatiotemporal patterns of gene expression underlying placentation remain poorly understood. Utilizing Stereo-seq, we constructed a Mouse Placentation Spatiotemporal Transcriptomic Atlas (MPSTA) spanning from embryonic day (E) 7.5 to E14.5, which includes the transcriptomes of large trophoblast cells that were not captured in previous single-cell atlases. We defined four distinct strata of the ectoplacental cone, an early heterogeneous trophectoderm structure, and elucidated the spatial trajectory of trophoblast differentiation during early postimplantation stages before E9.5. Focusing on the labyrinth region, the interface of nutrient exchange in the mouse placenta, our spatiotemporal ligand-receptor interaction analysis unveiled pivotal modulators essential for trophoblast development and placental angiogenesis. We also found that paternally expressed genes are exclusively enriched in the placenta rather than in the decidual regions, including a cluster of genes enriched in endothelial cells that may function in placental angiogenesis. At the invasion front, we identified interface-specific transcription factor regulons, such as Atf3, Jun, Junb, Stat6, Mxd1, Maff, Fos, and Irf7, involved in gestational maintenance. Additionally, we revealed that maternal high-fat diet exposure preferentially affects this interface, exacerbating inflammatory responses and disrupting angiogenic homeostasis. Collectively, our findings furnish a comprehensive, spatially resolved atlas that offers valuable insights and benchmarks for future explorations into placental morphogenesis and pathology.

IL-27/Blimp-1 axis regulates the differentiation and function of Tim-3+ Tregs during early pregnancy

Decidual regulatory T cells (Tregs) are essential for successful pregnancy outcome. A subset of Tregs, T cell immunoglobulin and mucin domain-containing protein 3-positive regulatory T cells (TregsTim-3+), plays a central role in the acceptance of the fetus during early stages of normal pregnancy. The molecular mechanism regulating the differentiation and function of TregsTim-3+ is unknown. Here, we investigated the role of the transcription factor B lymphocyte-induced maturation protein 1 (Blimp-1) on decidual TregTim-3+ differentiation. We demonstrated that Blimp-1 enhanced the coexpression of negative costimulatory molecules (Tim-3, T cell immunoreceptor with Ig and ITIM domains, and programmed cell death protein 1) on Tregs and improved their immunosuppressive functions, including increased IL-10 secretion, suppression of effector T cell proliferation, and promotion of macrophage polarization toward the M2 phenotype. Furthermore, we showed that IL-27 regulated the expression of Tim-3 and Blimp-1 through the STAT1 signaling pathway and that transfer of TregsBlimp-1+ into an abortion-prone mouse model effectively reduced embryo absorption rate. We postulated that abnormalities in the IL-27/Blimp-1 axis might be associated with recurrent pregnancy loss (RPL). These findings provided insights for developing more efficient immunotherapies for women with RPL.

Conditionally mutant animal model for investigating the invasive trophoblast cell lineage

Placental development involves coordinated expansion and differentiation of trophoblast cell lineages possessing specialized functions. Among the differentiated trophoblast cell lineages are invasive trophoblast cells, which exit the placenta and invade the uterus, where they restructure the uterine parenchyma and facilitate remodeling of uterine spiral arteries. The rat exhibits deep intrauterine trophoblast cell invasion, a feature shared with human placentation, and is also amenable to gene manipulation using genome-editing techniques. In this investigation, we generated a conditional rat model targeting the invasive trophoblast cell lineage. Prolactin family 7, subfamily b, member 1 (Prl7b1) is uniquely and abundantly expressed in the rat invasive trophoblast cell lineage. Disruption of Prl7b1 did not adversely affect placental development. We demonstrated that the Prl7b1 locus could be effectively used to drive the expression of Cre recombinase in invasive trophoblast cells. Our rat model represents a new tool for investigating candidate genes contributing to the regulation of invasive trophoblast cells and their roles in trophoblast-guided uterine spiral artery remodeling.

CRISPR screening in human trophoblast stem cells reveals both shared and distinct aspects of human and mouse placental development

The placenta serves as the interface between the mother and fetus, facilitating the exchange of gases and nutrients between their separate blood circulation systems. Trophoblasts in the placenta play a central role in this process. Our current understanding of mammalian trophoblast development relies largely on mouse models. However, given the diversification of mammalian placentas, findings from the mouse placenta cannot be readily extrapolated to other mammalian species, including humans. To fill this knowledge gap, we performed CRISPR knockout screening in human trophoblast stem cells (hTSCs). We targeted genes essential for mouse placental development and identified more than 100 genes as critical regulators in both human hTSCs and mouse placentas. Among them, we further characterized in detail two transcription factors, DLX3 and GCM1, and revealed their essential roles in hTSC differentiation. Moreover, a gene function-based comparison between human and mouse trophoblast subtypes suggests that their relationship may differ significantly from previous assumptions based on tissue localization or cellular function. Notably, our data reveal that hTSCs may not be analogous to mouse TSCs or the extraembryonic ectoderm (ExE) in which in vivo TSCs reside. Instead, hTSCs may be analogous to progenitor cells in the mouse ectoplacental cone and chorion. This finding is consistent with the absence of ExE-like structures during human placental development. Our data not only deepen our understanding of human trophoblast development but also facilitate cross-species comparison of mammalian placentas.

Murine trophoblast organoids as a model for trophoblast development and CRISPR-Cas9 screening

The placenta becomes one of the most diversified organs during placental mammal radiation. The main in vitro model for studying mouse trophoblast development is the 2D differentiation model of trophoblast stem cells, which is highly skewed to certain lineages and thus hampers systematic screens. Here, we established culture conditions for the establishment, maintenance, and differentiation of murine trophoblast organoids. Murine trophoblast organoids under the maintenance condition contain stem cell-like populations, whereas differentiated organoids possess various trophoblasts resembling placental ones in vivo. Ablation of Nubpl or Gcm1 in trophoblast organoids recapitulated their deficiency phenotypes in vivo, suggesting that those organoids are valid in vitro models for trophoblast development. Importantly, we performed an efficient CRISPR-Cas9 screening in mouse trophoblast organoids using a focused sgRNA (single guide RNA) library targeting G protein-coupled receptors. Together, our results establish an organoid model to investigate mouse trophoblast development and a practicable approach to performing forward screening in trophoblast lineages.

Acquisition of neural fate by combination of BMP blockade and chromatin modification

Neural induction is a process where naive cells are converted into committed cells with neural characteristics, and it occurs at the earliest step during embryogenesis. Although the signaling molecules and chromatin remodeling for neural induction have been identified, the mutual relationships between these molecules are yet to be fully understood. By taking advantage of the neural differentiation system of mouse embryonic stem (ES) cells, we discovered that the BMP signal regulates the expression of several polycomb repressor complex (PRC) component genes. We particularly focused on Polyhomeotic Homolog 1 (Phc1) and established Phc1-knockout (Phc1-KO) ES cells. We found that Phc1-KO failed to acquire the neural fate, and the cells remained in pluripotent or primitive non-neural states. Chromatin accessibility analysis suggests that Phc1 is essential for chromatin packing. Aberrant upregulation of the BMP signal was confirmed in the Phc1 homozygotic mutant embryos. Taken together, Phc1 is required for neural differentiation through epigenetic modification.

Trophoblast Differentiation: Mechanisms and Implications for Pregnancy Complications

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

CONDITIONALLY MUTANT ANIMAL MODEL FOR INVESTIGATING THE INVASIVE TROPHOBLAST CELL LINEAGE

Placental development involves coordinated expansion and differentiation of trophoblast cell lineages possessing specialized functions. Among the differentiated trophoblast cell lineages are invasive trophoblast cells, which exit the placenta and invade into the uterus where they restructure the uterine parenchyma and facilitate remodeling of uterine spiral arteries. The rat exhibits deep intrauterine trophoblast cell invasion, a feature shared with human placentation, and is also amenable to gene manipulation using genome editing techniques. In this investigation, we generated a conditional rat model targeting the invasive trophoblast cell lineage. Prolactin family 7, subfamily b, member 1 ( Prl7b1 ) is uniquely and abundantly expressed in the rat invasive trophoblast cell lineage. Disruption of Prl7b1 did not adversely affect placental development. We demonstrated that the Prl7b1 locus could be effectively used to drive the expression of Cre recombinase in invasive trophoblast cells. Our rat model represents a new tool for investigating candidate genes contributing to the regulation of invasive trophoblast cells and their contributions to trophoblast-guided uterine spiral artery remodeling.

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

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

Identifying novel regulators of placental development using time-series transcriptome data

The placenta serves as a connection between the mother and the fetus during pregnancy, providing the fetus with oxygen, nutrients, and growth hormones. However, the regulatory mechanisms and dynamic gene interaction networks underlying early placental development are understudied. Here, we generated RNA-sequencing data from mouse fetal placenta at embryonic days 7.5, 8.5, and 9.5 to identify genes with timepoint-specific expression, then inferred gene interaction networks to analyze highly connected network modules. We determined that timepoint-specific gene network modules were associated with distinct developmental processes, and with similar expression profiles to specific human placental cell populations. From each module, we identified hub genes and their direct neighboring genes, which were predicted to govern placental functions. We confirmed that four novel candidate regulators identified through our analyses regulate cell migration in the HTR-8/SVneo cell line. Overall, we predicted several novel regulators of placental development expressed in specific placental cell types using network analysis of bulk RNA-sequencing data. Our findings and analysis approaches will be valuable for future studies investigating the transcriptional landscape of early development.

Possible transfer of lncRNA H19-derived miRNA miR-675-3p to adjacent H19-non-expressing trophoblast cells in near-term mouse placenta

LncRNA H19 serves as a regulatory RNA in mouse placental development. However, there is little information available on the in situ expression of H19 in the late-gestation mouse placenta. In this study, we performed quantitative polymerase chain reaction (qPCR) and in situ hybridization (ISH) analyses of lncRNA H19 and its exon 1-derived miRNA miR-675-3p to identify cell types expressing these non-coding RNAs in the mouse placenta during mid-to-late gestation. By qPCR analysis, we confirmed that H19 was highly expressed during mid-to-late gestation (E10.5-E18.5) and that H19-derived miRNA miR-675-3p was remarkably upregulated in the E18.5 placenta. ISH analysis revealed trophoblast cell type-specific expression of lncRNA H19 and miR-675-3p during later stages of gestation. In the junctional zone and decidua of late-gestation placenta, H19 was expressed in trophoblast giant cells and glycogen trophoblast cells; however, H19 was absent in spongiotrophoblast cells. In the labyrinth and chorionic plate, H19 was present in sinusoidal mononuclear trophoblast giant cells, fetal vascular endothelial cells, and basal chorionic trophoblast cells, but not in syncytiotrophoblasts. As expected, these lncRNA H19-expressing cells exhibited miR-675-3p in the E18.5 placenta. Intriguingly, miR-675-3p was also present in H19-negative spongiotrophoblast cells and syncytiotrophoblasts, implying the possible transfer of miR-675-3p from H19-exprssing cells to adjacent H19-non-expressing trophoblast cells. These findings suggest that the mouse placenta expresses lncRNA H19 in a trophoblast cell type-specific fashion during later stages of gestation.

Improving the secretory capacity of CHO producer cells: The effect of controlled Blimp1 expression, a master transcription factor for plasma cells

With the advent of novel therapeutic proteins with complex structures, cellular bottlenecks in secretory pathways have hampered the high-yield production of difficult-to-express (DTE) proteins in CHO cells. To mitigate their limited secretory capacity, recombinant CHO (rCHO) cells were engineered to express Blimp1, a master regulator orchestrating B cell differentiation into professional secretory plasma cells, using the streamlined CRISPR/Cas9-based recombinase-mediated cassette exchange landing pad platform. The expression of Blimp1α or Blimp1β in rCHO cells producing DTE recombinant human bone morphogenetic protein-4 (rhBMP-4) increased specific rhBMP-4 productivity (q_rhBMP-4). However, since Blimp1α expression suppressed cell growth more significantly than Blimp1β expression, only Blimp1β expression enhanced rhBMP-4 yield. In serum-free suspension culture, Blimp1β expression significantly increased the rhBMP-4 concentration (>3-fold) and q_rhBMP-4 (>4-fold) without significant increase in hBMP-4 transcript levels. In addition, Blimp1β expression facilitated mature rhBMP-4 secretion by active proteolytic cleavage in the secretory pathway. Transcriptomic profiling (RNA-seq) revealed global changes in gene expression patterns that promote protein processing in secretory organelles. In-depth integrative analysis of the current RNA-seq data, public epigenome/RNA-seq data, and in silico analysis identified 45 potential key regulators of Blimp1 that are consistently up- or down-regulated in Blimp1β expressing rCHO cells and plasma cells. Blimp1β expression also enhanced the production of easy-to-express monoclonal antibodies (mAbs) and modulated the expression of key regulators in rCHO cells producing mAb. Taken together, the results show that controlled expression of Blimp1β improves the production capacity of rCHO cells by regulating secretory machinery and suggest new opportunities for engineering promising targets that are resting in CHO cells.

Cannabinoid and planar cell polarity signaling converges to direct placentation

Directed trophoblast migration toward the maternal mesometrial pole is critical for placentation and pregnancy success. Trophoblasts replace maternal arterial endothelial cells to increase blood supply to the placenta. Inferior trophoblast invasion results in pregnancy complications including preeclampsia, intrauterine growth restriction, miscarriage, and preterm delivery. The maternal chemotactic factors that direct trophoblast migration and the mechanism by which trophoblasts respond to these factors are not clearly understood. Here, we show that invasive trophoblasts deficient in Vangl2, a core planar cell polarity (PCP) component, fail to invade in maternal decidua, and this deficiency results in middle-gestational fetal demise. Previously, we have shown that tightly regulated endocannabinoids via G protein-coupled cannabinoid receptor CB1 are critical to the invasion of trophoblasts called spiral artery trophoblast giant cells (SpA-TGCs). We find that CB1 directly interacts with VANGL2. Trophoblast stem cells devoid of Cnr1 and/or Vangl2 show compromised cell migration. To study roles of VANGL2 and CB1 in trophoblast invasion in vivo, we conditionally deleted Cnr1 (coding CB1) and Vangl2 in progenitors of SpA-TGCs using trophoblast-specific protein alpha (Tpbpa)-Cre. We observed that signaling mediated by VANGL2 and CB1 restrains trophoblasts from random migration by keeping small GTPases quiescent. Our results show that organized PCP in trophoblasts is indispensable for their directed movement and that CB1 exerts its function by direct interaction with membrane proteins other than its canonical G protein-coupled receptor role.

VEGF Maintains Maternal Vascular Space Homeostasis in the Mouse Placenta through Modulation of Trophoblast Giant Cell Functions

Vascular endothelial growth factor (VEGF) is an angiogenic growth factor that acts primarily on endothelial cells, but numerous studies suggest that VEGF also acts on non-endothelial cells, including trophoblast cells. Inhibition of VEGF signaling by excess production of the endogenous soluble VEGF receptor sFlt1 in trophoblast cells has been implicated in several pregnancy complications. Our previous studies and other reports have shown that VEGF directly regulates placental vascular development and functions and that excess VEGF production adversely affects placental vascular development. Trophoblast giant cells (TGCs) line the maternal side of the placental vasculature in mice and function like endothelial cells. In this study, we specifically examined the effect of excess VEGF signaling on TGC development associated with defective placental vascular development using two mouse models an endometrial VEGF overexpression model and a placenta-specific sFlt1 knockdown model. Placentas of endometrial VEGF-overexpressing dams at embryonic days (E) 11.5 and 14.5 showed dramatic enlargement of the venous maternal spaces in junctional zones. The size and number of the parietal TGCs that line these venous spaces in the placenta were also significantly increased. Although junctional zone venous blood spaces from control and VEGF-overexpressing dams were not markedly different in size at E17.5, the number and size of P-TGCs were both significantly increased in the placentas from VEGF-overexpressing dams. In sFlt1 knockdown placentas, however, there was a significant increase in the size of the sinusoidal TGC-lined, alkaline phosphatase-positive maternal blood spaces in the labyrinth. These results suggest that VEGF signaling plays an important role in maintaining the homeostasis of the maternal vascular space in the mouse placenta through modulation of TGC development and differentiation, similar to the effect of VEGF on endothelial cells in other vascular beds.

Molecular fingerprint of female bovine embryos produced in vitro with high competence to establish and maintain pregnancy†

The objective was to identify the transcriptomic profile of in vitro-derived embryos with high competence to establish and maintain gestation. Embryos produced with X-sorted sperm were cultured from day 5 to day 7 in serum-free medium containing 10 ng/ml recombinant bovine colony-stimulating factor 2 (CSF2) or vehicle. The CSF2 was administered because this molecule can increase blastocyst competence for survival after embryo transfer. Blastocysts were harvested on day 7 of culture and manually bisected. One demi-embryo from a single blastocyst was transferred into a synchronized recipient and the other half was used for RNA-seq analysis. Using P < 0.01 and a fold change >2-fold or <0.5 fold as cutoffs, there were 617 differentially expressed genes (DEG) between embryos that survived to day 30 of gestation vs those that did not, 470 DEG between embryos that survived to day 60 and those that did not, 432 DEG between embryos that maintained pregnancy from day 30 to day 60 vs those where pregnancy failed after day 30, and 635 DEG regulated by CSF2. Pathways and ontologies in which DEG were overrepresented included many related to cellular responses to stress and cell survival. It was concluded that gene expression in the blastocyst is different between embryos that are competent to establish and maintain pregnancy vs those that are not. The relationship between expression of genes related to cell stress and subsequent embryonic survival probably reflects cellular perturbations caused by embryonic development taking place in the artificial environment associated with cell culture.

STOX1 deficiency is associated with renin-mediated gestational hypertension and placental defects

The pathogenesis of preeclampsia and other hypertensive disorders of pregnancy remains poorly defined despite the substantial burden of maternal and neonatal morbidity associated with these conditions. In particular, the role of genetic variants as determinants of disease susceptibility is understudied. Storkhead-box protein 1 (STOX1) was first identified as a preeclampsia risk gene through family-based genetic linkage studies in which loss-of-function variants were proposed to underlie increased preeclampsia susceptibility. We generated a genetic Stox1 loss-of-function mouse model (Stox1 KO) to evaluate whether STOX1 regulates blood pressure in pregnancy. Pregnant Stox1-KO mice developed gestational hypertension evidenced by a significant increase in blood pressure compared with WT by E17.5. While severe renal, placental, or fetal growth abnormalities were not observed, the Stox1-KO phenotype was associated with placental vascular and extracellular matrix abnormalities. Mechanistically, we found that gestational hypertension in Stox1-KO mice resulted from activation of the uteroplacental renin-angiotensin system. This mechanism was supported by showing that treatment of pregnant Stox1-KO mice with an angiotensin II receptor blocker rescued the phenotype. Our study demonstrates the utility of genetic mouse models for uncovering links between genetic variants and effector pathways implicated in the pathogenesis of hypertensive disorders of pregnancy.

New era of trophoblast research: integrating morphological and molecular approaches

Many pregnancy complications are the result of dysfunction in the placenta. The pathogenic mechanisms of placenta-mediated pregnancy complications, however, are unclear. Abnormal placental development in these conditions begins in the first trimester, but no symptoms are observed during this period. To elucidate effective preventative treatments, understanding the differentiation and development of human placenta is crucial. This review elucidates the uniqueness of the human placenta in early development from the aspect of structural characteristics and molecular markers. We summarise the morphogenesis of human placenta based on human specimens and then compile molecular markers that have been clarified by immunostaining and RNA-sequencing data across species. Relevant studies were identified using the PubMed database and Google Scholar search engines up to March 2020. All articles were independently screened for eligibility by the authors based on titles and abstracts. In particular, the authors carefully examined literature on human placentation. This review integrates the development of human placentation from morphological approaches in comparison with other species and provides new insights into trophoblast molecular markers. The morphological features of human early placentation are described in Carnegie stages (CS), from CS3 (floating blastocyst) to CS9 (emerging point of tertiary villi). Molecular markers are described for each type of trophoblast involved in human placental development. We summarise the character of human trophoblast cell lines and explain how long-term culture system of human cytotrophoblast, both monolayer and spheroid, established in recent studies allows for the generation of human trophoblast cell lines. Due to differences in developmental features among species, it is desirable to understand early placentation in humans. In addition, reliable molecular markers that reflect normal human trophoblast are needed to advance trophoblast research. In the clinical setting, these markers can be valuable means for morphologically and functionally assessing placenta-mediated pregnancy complications and provide early prediction and management of these diseases.

Emerging Roles of PRDM Factors in Stem Cells and Neuronal System: Cofactor Dependent Regulation of PRDM3/16 and FOG1/2 (Novel PRDM Factors)

PRDI-BF1 (positive regulatory domain I-binding factor 1) and RIZ1 (retinoblastoma protein-interacting zinc finger gene 1) (PR) homologous domain containing (PRDM) transcription factors are expressed in neuronal and stem cell systems, and they exert multiple functions in a spatiotemporal manner. Therefore, it is believed that PRDM factors cooperate with a number of protein partners to regulate a critical set of genes required for maintenance of stem cell self-renewal and differentiation through genetic and epigenetic mechanisms. In this review, we summarize recent findings about the expression of PRDM factors and function in stem cell and neuronal systems with a focus on cofactor-dependent regulation of PRDM3/16 and FOG1/2. We put special attention on summarizing the effects of the PRDM proteins interaction with chromatin modulators (NuRD complex and CtBPs) on the stem cell characteristic and neuronal differentiation. Although PRDM factors are known to possess intrinsic enzyme activity, our literature analysis suggests that cofactor-dependent regulation of PRDM3/16 and FOG1/2 is also one of the important mechanisms to orchestrate bidirectional target gene regulation. Therefore, determining stem cell and neuronal-specific cofactors will help better understanding of PRDM3/16 and FOG1/2-controlled stem cell maintenance and neuronal differentiation. Finally, we discuss the clinical aspect of these PRDM factors in different diseases including cancer. Overall, this review will help further sharpen our knowledge of the function of the PRDM3/16 and FOG1/2 with hopes to open new research fields related to these factors in stem cell biology and neuroscience.

Mononuclear phagocyte regulation by the transcription factor Blimp-1 in health and disease

B lymphocyte‐induced maturation protein‐1 (Blimp‐1), the transcription factor encoded by the gene Prdm1, plays a number of crucial roles in the adaptive immune system, which result in the maintenance of key effector functions of B‐ and T‐cells. Emerging clinical data, as well as mechanistic evidence from mouse studies, have additionally identified critical functions of Blimp‐1 in the maintenance of immune homeostasis by the mononuclear phagocyte (MNP) system. Blimp‐1 regulation of gene expression affects various aspects of MNP biology, including developmental programmes such as fate decisions of monocytes entering peripheral tissue, and functional programmes such as activation, antigen presentation and secretion of soluble inflammatory mediators. The highly tissue‐, subset‐ and state‐specific regulation of Blimp‐1 expression in MNPs suggests that Blimp‐1 is a dynamic regulator of immune activation, integrating environmental cues to fine‐tune the function of innate cells. In this review, we will discuss the current knowledge regarding Blimp‐1 regulation and function in macrophages and dendritic cells.

The transcriptional repressor Blimp1/PRDM1 regulates the maternal decidual response in mice

The transcriptional repressor Blimp1 controls cell fate decisions in the developing embryo and adult tissues. Here we describe Blimp1 expression and functional requirements within maternal uterine tissues during pregnancy. Expression is robustly up-regulated at early post-implantation stages in the primary decidual zone (PDZ) surrounding the embryo. Conditional inactivation results in defective formation of the PDZ barrier and abnormal trophectoderm invasion. RNA-Seq analysis demonstrates down-regulated expression of genes involved in cell adhesion and markers of decidualisation. In contrast, genes controlling immune responses including IFNγ are up-regulated. ChIP-Seq experiments identify candidate targets unique to the decidua as well as those shared across diverse cell types including a highly conserved peak at the Csf-1 gene promoter. Interestingly Blimp1 inactivation results in up-regulated Csf1 expression and macrophage recruitment into maternal decidual tissues. These results identify Blimp1 as a critical regulator of tissue remodelling and maternal tolerance during early stages of pregnancy.

Evolution of male pregnancy associated with remodeling of canonical vertebrate immunity in seahorses and pipefishes

Among vertebrates, pregnancy has evolved more than 150 times independently. A fundamental problem for pregnancy to evolve is inadvertent rejection of the embryo when being recognized as foreign tissue by the vertebrate’s adaptive immune system. We show that the unique evolution of male pregnancy in pipefishes and seahorses coincided with a genomic modification of one arm of the adaptive immune system. Our findings indicate a trade-off between immunological tolerance and embryo rejection to accompanying the emergence of male pregnancy. That syngnathids survive in an ocean of microbes despite their drastically modified immune defense suggests an unexpected immunological flexibility. Our results may improve the understanding of immune-deficiency diseases and call for a reassessment of vertebrate immunity.

A fundamental problem for the evolution of pregnancy, the most specialized form of parental investment among vertebrates, is the rejection of the nonself-embryo. Mammals achieve immunological tolerance by down-regulating both major histocompatibility complex pathways (MHC I and II). Although pregnancy has evolved multiple times independently among vertebrates, knowledge of associated immune system adjustments is restricted to mammals. All of them (except monotremata) display full internal pregnancy, making evolutionary reconstructions within the class mammalia meaningless. Here, we study the seahorse and pipefish family (syngnathids) that have evolved male pregnancy across a gradient from external oviparity to internal gestation. We assess how immunological tolerance is achieved by reconstruction of the immune gene repertoire in a comprehensive sample of 12 seahorse and pipefish genomes along the “male pregnancy” gradient together with expression patterns of key immune and pregnancy genes in reproductive tissues. We found that the evolution of pregnancy coincided with a modification of the adaptive immune system. Divergent genomic rearrangements of the MHC II pathway among fully pregnant species were identified in both genera of the syngnathids: The pipefishes (Syngnathus) displayed loss of several genes of the MHC II pathway while seahorses (Hippocampus) featured a highly divergent invariant chain (CD74). Our findings suggest that a trade-off between immunological tolerance and embryo rejection accompanied the evolution of unique male pregnancy. That pipefishes survive in an ocean of microbes without one arm of the adaptive immune defense suggests a high degree of immunological flexibility among vertebrates, which may advance our understanding of immune-deficiency diseases.

Super-enhancer-guided mapping of regulatory networks controlling mouse trophoblast stem cells

Trophectoderm (TE) lineage development is pivotal for proper implantation, placentation, and healthy pregnancy. However, only a few TE-specific transcription factors (TFs) have been systematically characterized, hindering our understanding of the process. To elucidate regulatory mechanisms underlying TE development, here we map super-enhancers (SEs) in trophoblast stem cells (TSCs) as a model. We find both prominent TE-specific master TFs (Cdx2, Gata3, and Tead4), and >150 TFs that had not been previously implicated in TE lineage, that are SE-associated. Mapping targets of 27 SE-predicted TFs reveals a highly intertwined transcriptional regulatory circuitry. Intriguingly, SE-predicted TFs show 4 distinct expression patterns with dynamic alterations of their targets during TSC differentiation. Furthermore, depletion of a subset of TFs results in dysregulation of the markers for specialized cell types in placenta, suggesting a role during TE differentiation. Collectively, we characterize an expanded TE-specific regulatory network, providing a framework for understanding TE lineage development and placentation.

Trophectoderm lineage development is essential for implantation, placentation, and healthy pregnancy. Here the authors map super-enhancers (SEs) in trophoblast stem cells and find both TE-specific master regulators and 150 previous uncharacterised transcription factors that are SE-associated, providing insight into trophectoderm-specific regulatory networks.

Hemochorial placentation: development, function, and adaptations

Placentation is a reproductive adaptation that permits fetal growth and development within the protected confines of the female reproductive tract. Through this important role, the placenta also determines postnatal health and susceptibility to disease. The hemochorial placenta is a prominent feature in primate and rodent development. This manuscript provides an overview of the basics of hemochorial placental development and function, provides perspectives on major discoveries that have shaped placental research, and thoughts on strategies for future investigation.

Spatiotemporal coordination of trophoblast and allantoic Rbpj signaling directs normal placental morphogenesis

The placenta, responsible for the nutrient and gas exchange between the mother and fetus, is pivotal for successful pregnancy. It has been shown that Rbpj, the core transcriptional mediator of Notch signaling pathway, is required for normal placentation in mice. However, it remains largely unclear how Rbpj signaling in different placental compartments coordinates with other important regulators to ensure normal placental morphogenesis. In this study, we found that systemic deletion of Rbpj led to abnormal chorioallantoic morphogenesis and defective trophoblast differentiation in the ectoplacental cone (EPC). Employing mouse models with selective deletion of Rbpj in the allantois versus trophoblast, combining tetraploid aggregation assay, we demonstrated that allantois-expressed Rbpj is essential for chorioallantoic attachment and subsequent invagination of allantoic blood vessels into the chorionic ectoderm. Further studies uncovered that allantoic Rbpj regulates chorioallantoic fusion and morphogenesis via targeting Vcam1 in a Notch-dependent manner. Meanwhile, we also revealed that trophoblast-expressed Rbpj in EPC facilitates Mash2’s transcriptional activity, promoting the specification of Tpbpα-positive trophoblasts, which differentiate into trophoblast subtypes responsible for interstitial and endovascular invasion at the later stage of placental development. Collectively, our study further shed light on the molecular network governing placental development and functions, highlighting the necessity of a spatiotemporal coordination of Rbpj signaling for normal placental morphogenesis.

Genetic basis for primordial germ cells specification in mouse and human: Conserved and divergent roles of PRDM and SOX transcription factors

Primordial germ cells (PGCs) are embryonic precursors of sperm and egg that pass on genetic and epigenetic information from one generation to the next. In mammals, they are induced from a subset of cells in peri-implantation epiblast by BMP signaling from the surrounding tissues. PGCs then initiate a unique developmental program that involves comprehensive epigenetic resetting and repression of somatic genes. This is orchestrated by a set of signaling molecules and transcription factors that promote germ cell identity. Here we review significant findings on mammalian PGC biology, in particular, the genetic basis for PGC specification in mice and human, which has revealed an evolutionary divergence between the two species. We discuss the importance and potential basis for these differences and focus on several examples to illustrate the conserved and divergent roles of critical transcription factors in mouse and human germline.

Genetic dissection of Nodal and Bmp signalling requirements during primordial germ cell development in mouse

The essential roles played by Nodal and Bmp signalling during early mouse development have been extensively documented. Here we use conditional deletion strategies to investigate functional contributions made by Nodal, Bmp and Smad downstream effectors during primordial germ cell (PGC) development. We demonstrate that Nodal and its target gene Eomes provide early instructions during formation of the PGC lineage. We discover that Smad2 inactivation in the visceral endoderm results in increased numbers of PGCs due to an expansion of the PGC niche. Smad1 is required for specification, whereas in contrast Smad4 controls the maintenance and migration of PGCs. Additionally we find that beside Blimp1, down-regulated phospho-Smad159 levels also distinguishes PGCs from their somatic neighbours so that emerging PGCs become refractory to Bmp signalling that otherwise promotes mesodermal development in the posterior epiblast. Thus balanced Nodal/Bmp signalling cues regulate germ cell versus somatic cell fate decisions in the early posterior epiblast.

Transcription factor ASCL2 is required for development of the glycogen trophoblast cell lineage

The basic helix-loop-helix (bHLH) transcription factor ASCL2 plays essential roles in diploid multipotent trophoblast progenitors, intestinal stem cells, follicular T-helper cells, as well as during epidermal development and myogenesis. During early development, Ascl2 expression is regulated by genomic imprinting and only the maternally inherited allele is transcriptionally active in trophoblast. The paternal allele-specific silencing of Ascl2 requires expression of the long non-coding RNA Kcnq1ot1 in cis and the deposition of repressive histone marks. Here we show that Del^7AI, a 280-kb deletion allele neighboring Ascl2, interferes with this process in cis and leads to a partial loss of silencing at Ascl2. Genetic rescue experiments show that the low level of Ascl2 expression from the paternal Del^7AI allele can rescue the embryonic lethality associated with maternally inherited Ascl2 mutations, in a level-dependent manner. Despite their ability to support development to term, the rescued placentae have a pronounced phenotype characterized by severe hypoplasia of the junctional zone, expansion of the parietal trophoblast giant cell layer, and complete absence of invasive glycogen trophoblast cells. Transcriptome analysis of ectoplacental cones at E7.5 and differentiation assays of Ascl2 mutant trophoblast stem cells show that ASCL2 is required for the emergence or early maintenance of glycogen trophoblast cells during development. Our work identifies a new cis-acting mutation interfering with Kcnq1ot1 silencing function and establishes a novel critical developmental role for the transcription factor ASCL2.

By controlling precise networks of target genes, transcription factors play important roles in cell fate determination during development. The Ascl2 gene codes for a transcription factor essential for the maintenance of progenitor cell populations able to differentiate into specialized cell types in the intestine and in the extra-embryonic trophoblast lineage. The trophoblast is an essential component of the placenta, an organ required for development of the embryo in placental mammals. Ascl2 belongs to a group of unusual genes, called imprinted genes, which are expressed from only a single parental copy. Ascl2 is only expressed from the maternally inherited copy in the trophoblast, the paternal copy being kept silent. Here, we describe an engineered deletion neighboring Ascl2 that interferes with the complete silencing of the paternal copy of the gene. We show that the low amount of ASCL2 produced from this deletion can rescue the embryonic lethality associated with non-functional maternal copies of Ascl2. Although the rescued embryos can often survive to term, their placenta is highly disorganized and lacks members of a specific cell lineage, the trophoblast glycogen cells. By analyzing the transcriptional profile of mutant trophoblast progenitors in vivo and of differentiated trophoblast stem cells, we show that ASCL2 plays a very early role in the formation of this cell lineage.

Blimp-1/PRDM1 is a critical regulator of Type III Interferon responses in mammary epithelial cells

The transcriptional repressor Blimp-1 originally cloned as a silencer of type I interferon (IFN)-β gene expression controls cell fate decisions in multiple tissue contexts. Conditional inactivation in the mammary gland was recently shown to disrupt epithelial cell architecture. Here we report that Blimp-1 regulates expression of viral defense, IFN signaling and MHC class I pathways, and directly targets the transcriptional activator Stat1. Blimp-1 functional loss in 3D cultures of mammary epithelial cells (MECs) results in accumulation of dsRNA and expression of type III IFN-λ. Cultures treated with IFN lambda similarly display defective lumen formation. These results demonstrate that type III IFN-λ profoundly influences the behavior of MECs and identify Blimp-1 as a critical regulator of IFN signaling cascades.

Sirtuin1 is required for proper trophoblast differentiation and placental development in mice

INTRODUCTION

Placental insufficiency, arising from abnormal trophoblast differentiation and function, is a major cause of fetal growth restriction. Sirtuin-1 (Sirt1) is a ubiquitously-expressed NAD-dependent protein deacetylase which plays a key role in numerous cellular processes, including cellular differentiation and metabolism. Though Sirt1 has been widely studied, its role in placentation and trophoblast differentiation is unclear.

METHOD

Sirt1-heterozygous mice were mated and evaluated at various points during embryogenesis. In situ hybridization and immunohistochemistry were used to further characterize the placental phenotype of Sirt1-null mice. Wild-type (WT) and Sirt1-null mouse trophoblast stem cell (TSC) lines were derived from e3.5 littermate blastocysts. These cells were then evaluated at various points following differentiation. Differentiation was evaluated by expression of lineage specific markers using qPCR and flow cytometry, as well as Matrigel invasion assays. Global gene expression changes were evaluated using microarray-based RNA profiling; changes in specific pathways were validated using qPCR and western blot.

RESULTS

In the absence of Sirt1, both embryos and placentas were small, with placentas showing abnormalities in both the labyrinthine layer and junctional zone. Sirt1-null TSCs exhibited an altered phenotype in both undifferentiated and differentiated states, phenotypes which corresponded to changes in pathways relevant to both TSC maintenance and differentiation. Specifically, Sirt1-null TSC showed blunted differentiation, and appeared to be suspended in an Epcam^high trophoblast progenitor state.

DISCUSSION

Our results suggest that Sirt1 is required for proper TSC differentiation and placental development.

Long-lived unipotent Blimp1-positive luminal stem cells drive mammary gland organogenesis throughout adult life

The hierarchical relationships between various stem and progenitor cell subpopulations driving mammary gland morphogenesis and homoeostasis are poorly understood. Conditional inactivation experiments previously demonstrated that expression of the zinc finger transcriptional repressor Blimp1/PRDM1 is essential for the establishment of epithelial cell polarity and functional maturation of alveolar cells. Here we exploit a Prdm1.CreERT2-LacZ reporter allele for lineage tracing experiments. Blimp1 expression marks a rare subpopulation of unipotent luminal stem cells that initially appear in the embryonic mammary gland at around E17.5 coincident with the segregation of the luminal and basal compartments. Fate mapping at multiple time points in combination with whole-mount confocal imaging revealed these long-lived unipotent luminal stem cells survive consecutive involutions and retain their identity throughout adult life. Blimp1⁺ luminal stem cells give rise to Blimp1⁻ progeny that are invariably Elf5⁺ERα⁻PR⁻. Thus, Blimp1 expression defines a mammary stem cell subpopulation with unique functional characteristics.

The role of stem/progenitor cell populations in mammary gland morphogenesis is not well understood. Here, the authors show that a transcriptional repressor, Blimp1, is expressed in a rare luminal stem cell population, which contribute to duct formation, and survive multiple rounds of pregnancy and involution.

The eutheria-specific miR-290 cluster modulates placental growth and maternal-fetal transport

The vertebrate-specific ESCC microRNA family arises from two genetic loci in mammals: miR-290/miR-371 and miR-302. The miR-302 locus is found broadly among vertebrates, whereas the miR-290/miR-371 locus is unique to eutheria, suggesting a role in placental development. Here, we evaluate that role. A knock-in reporter for the mouse miR-290 cluster is expressed throughout the embryo until gastrulation, when it becomes specifically expressed in extraembryonic tissues and the germline. In the placenta, expression is limited to the trophoblast lineage, where it remains highly expressed until birth. Deletion of the miR-290 cluster gene (Mirc5) results in reduced trophoblast progenitor cell proliferation and a reduced DNA content in endoreduplicating trophoblast giant cells. The resulting placenta is reduced in size. In addition, the vascular labyrinth is disorganized, with thickening of the maternal-fetal blood barrier and an associated reduction in diffusion. Multiple mRNA targets of the miR-290 cluster microRNAs are upregulated. These data uncover a crucial function for the miR-290 cluster in the regulation of a network of genes required for placental development, suggesting a central role for these microRNAs in the evolution of placental mammals.

Genetic redundancy of GATA factors in the extraembryonic trophoblast lineage ensures the progression of preimplantation and postimplantation mammalian development

GATA transcription factors are implicated in establishing cell fate during mammalian development. In early mammalian embryos, GATA3 is selectively expressed in the extraembryonic trophoblast lineage and regulates gene expression to promote trophoblast fate. However, trophoblast-specific GATA3 function is dispensable for early mammalian development. Here, using dual conditional knockout mice, we show that genetic redundancy of Gata3 with paralog Gata2 in trophoblast progenitors ensures the successful progression of both pre- and postimplantation mammalian development. Stage-specific gene deletion in trophoblasts reveals that loss of both GATA genes, but not either alone, leads to embryonic lethality prior to the onset of their expression within the embryo proper. Using ChIP-seq and RNA-seq analyses, we define the global targets of GATA2/GATA3 and show that they directly regulate a large number of common genes to orchestrate stem versus differentiated trophoblast fate. In trophoblast progenitors, GATA factors directly regulate BMP4, Nodal and Wnt signaling components that promote embryonic-extraembryonic signaling cross-talk, which is essential for the development of the embryo proper. Our study provides genetic evidence that impairment of trophoblast-specific GATA2/GATA3 function could lead to early pregnancy failure.

Summary: During trophoblast development in mice, GATA2 and GATA3 act synergistically by directly regulating a large number of common genes, and together are important to ensure trophoblast lineage progression.

From the stem of the placental tree: trophoblast stem cells and their progeny

Trophoblast stem cells (TSCs) retain the capacity to self-renew indefinitely and harbour the potential to differentiate into all trophoblast subtypes of the placenta. Recent studies have shown how signalling cascades integrate with transcription factor circuits to govern the fine balance between TSC self-renewal and differentiation. In addition, breakthroughs in reprogramming strategies have enabled the generation of TSCs from fibroblasts, opening up exciting new avenues that may allow the isolation of this stem cell type from other species, notably humans. Here, we review these recent advances in light of their importance for understanding placental pathologies and developing personalised medicine approaches for pregnancy complications.

The transcriptional repressor Blimp1 is expressed in rare luminal progenitors and is essential for mammary gland development

Mammary gland morphogenesis depends on a tight balance between cell proliferation, differentiation and apoptosis, to create a defined functional hierarchy within the epithelia. The limited availability of stem cell/progenitor markers has made it challenging to decipher lineage relationships. Here, we identify a rare subset of luminal progenitors that express the zinc finger transcriptional repressor Blimp1, and demonstrate that this subset of highly clonogenic luminal progenitors is required for mammary gland development. Conditional inactivation experiments using K14-Cre and WAPi-Cre deleter strains revealed essential functions at multiple developmental stages. Thus, Blimp1 regulates proliferation, apoptosis and alveolar cell maturation during puberty and pregnancy. Loss of Blimp1 disrupts epithelial architecture and lumen formation both in vivo and in three-dimensional (3D) primary cell cultures. Collectively, these results demonstrate that Blimp1 is required to maintain a highly proliferative luminal subset necessary for mammary gland development and homeostasis.

Highlighted article: In the mouse mammary gland, Blimp1 marks a rare progenitor population, and is required for cell proliferation and polarity as well as efficient milk production.

Mapping the chromatin landscape and Blimp1 transcriptional targets that regulate trophoblast differentiation

Trophoblast stem cells (TSCs) give rise to specialized cell types within the placenta. However, the regulatory mechanisms that guide trophoblast cell fate decisions during placenta development remain ill defined. Here we exploited ATAC-seq and transcriptional profiling strategies to describe dynamic changes in gene expression and chromatin accessibility during TSC differentiation. We detect significantly increased chromatin accessibility at key genes upregulated as TSCs exit from the stem cell state. However, downregulated gene expression is not simply due to the loss of chromatin accessibility in proximal regions. Additionally, transcriptional targets recognized by the zinc finger transcriptional repressor Prdm1/Blimp1, an essential regulator of placenta development, were identified in ChIP-seq experiments. Comparisons with previously reported ChIP-seq datasets for primordial germ cell-like cells and E18.5 small intestine, combined with functional annotation analysis revealed that Blimp1 has broadly shared as well as cell type-specific functional activities unique to the trophoblast lineage. Importantly, Blimp1 not only silences TSC gene expression but also prevents aberrant activation of divergent developmental programmes. Overall the present study provides new insights into the chromatin landscape and Blimp1-dependent regulatory networks governing trophoblast gene expression.

Sca-1 identifies a trophoblast population with multipotent potential in the mid-gestation mouse placenta

Trophoblast stem (TS) cells in the mouse derive from the polar trophectoderm of the blastocyst and persist through early gestation (to E8.5) to support placental development. Further development and growth is proposed to rely on layer-restricted progenitor cells. Stem cell antigen (Sca) -1 is a member of the Ly6 gene family and a known marker of stem cells in both hematopoietic and non-hematopoietic mouse tissues. Having identified that Sca-1 mRNA was highly expressed in mouse TS cells in culture, we found that it was also expressed in a subset of trophoblast within the chorion and labyrinth layer of the mouse placenta. Isolation and in vitro culture of Sca-1⁺ trophoblast cells from both differentiated TS cell cultures and dissected mouse placentae resulted in proliferating colonies that expressed known markers of TS cells. Furthermore, these cells could be stimulated to differentiate and expressed markers of both junctional zone and labyrinth trophoblast subtypes in a manner comparable to established mouse TS cell lines. Our results suggest that we have identified a subpopulation of TS cell-like cells that persist in the mid- to late- gestation mouse placenta as well as a cell surface protein that can be used to identify and isolate these cells.

GATA factors: Master regulators of gene expression in trophoblast progenitors

Mammalian reproduction is critically dependent on trophoblast cells, which ensure embryo implantation and placentation. Development of trophoblast cell lineages is a multi-step process and relies upon proper spatial and temporal gene expression, which is regulated by multiple transcription factors. However, most of the transcription factors that are implicated in trophoblast development regulate gene expression at a specific developmental stage or in a specific trophoblast subtype. In contrast, recent studies from our group and other laboratories indicate that conserved GATA family of transcription factors, GATA2 and GATA3, are important to regulate gene expression at multiple stages of trophoblast development. Furthermore, our conditional gene deletion studies revealed that functional redundancy of GATA2 and GATA3 ensures both self-renewal of trophoblast stem and progenitor cells and their differentiation to trophoblast cells of a matured placenta. Together these findings indicate that GATA2/GATA3 are the master orchestrators of gene expression in trophoblast cells and they fine tune gene regulatory network to establish distinct trophoblast cell types during placentation.

Review: Sexual dimorphism in the formation, function and adaptation of the placenta

Exposure of the embryo or fetus to perturbations in utero can result in intrauterine growth restriction, a primary risk factor for the development of adult disease. However, despite similar exposures, males and females often have altered disease susceptibility or progression from different stages of life. Fetal growth is largely mediated by the placenta, which, like the fetus is genetically XX or XY. The placenta and its associated trophoblast lineages originate from the trophectoderm (TE) of the early embryo. Rodent models (rat, mouse, spiny mouse), have been used extensively to examine placenta development and these have demonstrated the growth trajectory of the placenta in females is generally slower compared to males, and also shows altered adaptive responses to stressful environments. These placental adaptations are likely to depend on the type of stressor, duration, severity and the window of exposure during development. Here we describe the divergent developmental pathways between the male and female placenta contributing to altered differentiation of the TE derived trophoblast subtypes, placental growth, and formation of the placental architecture. Our focus is primarily genetic or environmental perturbations in rodent models which show altered placental responsiveness between sexes. We suggest that perturbations during early placental development may have greater impact on viability and growth of the female fetus whilst those occurring later in gestation may preferentially affect the male fetus. This may be of great relevance to human pregnancies which result from assisted reproductive technologies or complications such as pre-eclampsia and diabetes.

PRDM1/BLIMP1 is widely distributed to the nascent fetal-placental interface in the mouse gastrula

BACKGROUND

PRDM1 is a transcriptional repressor that contributes to primordial germ cell (PGC) development. During early gastrulation, epiblast-derived PRDM1 is thought to be restricted to a lineage-segregated germ line in the allantois. However, given recent findings that PGCs overlap an allantoic progenitor pool that contributes widely to the fetal-umbilical interface, posterior PRDM1 may also contribute to soma.

RESULTS

Within the posterior mouse gastrula (early streak, 12-s stages, embryonic days ∼6.75-9.0), PRDM1 localized to all tissues containing putative PGCs; however, PRDM1 was also found in all three primary germ layers, their derivatives, and two presumptive growth centers, the allantoic core domain and ventral ectodermal ridge. While PRDM1 and STELLA colocalized predominantly within the hindgut, where putative PGCs reside, other colocalizing cells were found in non-PGC sites. Additional PRDM1 and STELLA cells were found independent of each other throughout the posterior region, including the hindgut. The Prdm1-Cre-driven reporter supported PRDM1 localization in the majority of sites; however, some Prdm1 descendants were found in sites independent of PRDM1 protein, including allantoic mesothelium and hindgut endoderm.

CONCLUSIONS

Posterior PRDM1 contributes more broadly to the developing fetal-maternal connection than previously recognized, and PRDM1 and STELLA, while overlapping in putative PGCs, also co-localize in several other tissues. Developmental Dynamics 246:50-71, 2017. © 2016 Wiley Periodicals, Inc.

Increased dosage of the imprinted Ascl2 gene restrains two key endocrine lineages of the mouse Placenta

Imprinted genes are expressed primarily from one parental allele by virtue of a germ line epigenetic process. Achaete-scute complex homolog 2 (Ascl2 aka Mash2) is a maternally expressed imprinted gene that plays a key role in placental and intestinal development. Loss-of-function of Ascl2 results in an expansion of the parietal trophoblast giant cell (P-TGC) lineage, an almost complete loss of Trophoblast specific protein alpha (Tpbpa) positive cells in the ectoplacental cone and embryonic failure by E10.5. Tpbpa expression marks the progenitors of some P-TGCs, two additional trophoblast giant cell lineages (spiral artery and canal), the spongiotrophoblast and the glycogen cell lineage. Using a transgenic model, here we show that elevated expression of Ascl2 reduced the number of P-TGC cells by 40%. Elevated Ascl2 also resulted in a marked loss of the spongiotrophoblast and a substantial mislocalisation of glycogen cells into the labyrinth. In addition, Ascl2-Tg placenta contained considerably more placental glycogen than wild type. Glycogen cells are normally located within the junctional zone in close contact with spongiotrophoblast cells, before migrating through the P-TGC layer into the maternal decidua late in gestation where their stores of glycogen are released. The failure of glycogen cells to release their stores of glycogen may explain both the inappropriate accumulation of glycogen and fetal growth restriction observed late in gestation in this model. In addition, using in a genetic cross we provide evidence that Ascl2 requires the activity of a second maternally expressed imprinted gene, Pleckstrin homology-like domain, family a, member 2 (Phlda2) to limit the expansion of the spongiotrophoblast. This "belts and braces" approach demonstrates the importance of genomic imprinting in regulating the size of the placental endocrine compartment for optimal placental development and fetal growth.

Single-cell RNA-seq reveals cell type-specific transcriptional signatures at the maternal-foetal interface during pregnancy

Growth and survival of the mammalian embryo within the uterine environment depends on the placenta, a highly complex vascularized organ comprised of both maternal and foetal tissues. Recent experiments demonstrate that the zinc finger transcriptional repressor Prdm1/Blimp1 is essential for specification of spiral artery trophoblast giant cells (SpA-TGCs) that invade and remodel maternal blood vessels. To learn more about functional contributions made by Blimp1+ cell lineages here we perform the first single-cell RNA-seq analysis of the placenta. Cell types of both foetal and maternal origin are profiled. Comparisons with microarray datasets from mutant placenta and in vitro differentiated trophoblast stem cells allow us to identify Blimp1-dependent transcripts enriched in SpA-TGCs. Our experiments provide new insights into the functionally distinct cell types present at the maternal–foetal interface and advance our knowledge of dynamic gene expression patterns controlling placental morphogenesis and vascular mimicry.

The zinc finger transcriptional repressor Prdm1/Blimp1 is essential for remodelling maternal blood vessels in a subset of trophoblast cells. Here, the authors perform single-cell RNA-seq analysis on this Blimp1+ lineage, identifying functionally distinct cell types present at the maternal–foetal interface.

Developmental Competence for Primordial Germ Cell Fate

During mammalian embryonic development, the trophectoderm and primitive endoderm give rise to extraembryonic tissues, while the epiblast differentiates into all somatic lineages and the germline. Remarkably, only a few classes of signaling pathways induce the differentiation of these progenitor cells into diverse lineages. Accordingly, the functional outcome of a particular signal depends on the developmental competence of the target cells. Thus, developmental competence can be defined as the ability of a cell to integrate intrinsic and extrinsic cues to execute a specific developmental program toward a specific cell fate. Downstream of signaling, there is the combinatorial activity of transcription factors and their cofactors, which is modulated by the chromatin state of the target cells. Here, we discuss the concept of developmental competence, and the factors that regulate this state with reference to the specification of mammalian primordial germ cells.

Lhx1 functions together with Otx2, Foxa2, and Ldb1 to govern anterior mesendoderm, node, and midline development

Costello et al. demonstrate that Smad4/Eomes-dependent Lhx1 expression in the epiblast marks the entire definitive endoderm lineage, the anterior mesendoderm, and midline progenitors. In proteomic experiments, they characterize a complex comprised of Lhx1, Otx2, and Foxa2 as well as the chromatin-looping protein Ldb1.

Gene regulatory networks controlling functional activities of spatially and temporally distinct endodermal cell populations in the early mouse embryo remain ill defined. The T-box transcription factor Eomes, acting downstream from Nodal/Smad signals, directly activates the LIM domain homeobox transcription factor Lhx1 in the visceral endoderm. Here we demonstrate Smad4/Eomes-dependent Lhx1 expression in the epiblast marks the entire definitive endoderm lineage, the anterior mesendoderm, and midline progenitors. Conditional inactivation of Lhx1 disrupts anterior definitive endoderm development and impedes node and midline morphogenesis in part due to severe disturbances in visceral endoderm displacement. Transcriptional profiling and ChIP-seq (chromatin immunoprecipitation [ChIP] followed by high-throughput sequencing) experiments identified Lhx1 target genes, including numerous anterior definitive endoderm markers and components of the Wnt signaling pathway. Interestingly, Lhx1-binding sites were enriched at enhancers, including the Nodal-proximal epiblast enhancer element and enhancer regions controlling Otx2 and Foxa2 expression. Moreover, in proteomic experiments, we characterized a complex comprised of Lhx1, Otx2, and Foxa2 as well as the chromatin-looping protein Ldb1. These partnerships cooperatively regulate development of the anterior mesendoderm, node, and midline cell populations responsible for establishment of the left–right body axis and head formation.

Placenta: the forgotten organ

The placenta sits at the interface between the maternal and fetal vascular beds where it mediates nutrient and waste exchange to enable in utero existence. Placental cells (trophoblasts) accomplish this via invading and remodeling the uterine vasculature. Amazingly, despite being of fetal origin, trophoblasts do not trigger a significant maternal immune response. Additionally, they maintain a highly reliable hemostasis in this extremely vascular interface. Decades of research into how the placenta differentiates itself from embryonic tissues to accomplish these and other feats have revealed a previously unappreciated level of complexity with respect to the placenta's cellular composition. Additionally, novel insights with respect to roles played by the placenta in guiding fetal development and metabolism have sparked a renewed interest in understanding the interrelationship between fetal and placental well-being. Here, we present an overview of emerging research in placental biology that highlights these themes and the importance of the placenta to fetal and adult health.

Cyclin E1 plays a key role in balancing between totipotency and differentiation in human embryonic cells

STUDY HYPOTHESIS

We aimed to investigate if Cyclin E1 (CCNE1) plays a role in human embryogenesis, in particular during the early developmental stages characterized by a short cell cycle.

STUDY FINDING

CCNE1 is expressed in plenipotent human embryonic cells and plays a critical role during hESC derivation via the naïve state and, potentially, normal embryo development.

WHAT IS KNOWN ALREADY

A short cell cycle due to a truncated G1 phase has been associated with the high developmental capacity of embryonic cells. CCNE1 is a critical G1/S transition regulator. CCNE1 overexpression can cause shortening of the cell cycle and it is constitutively expressed in mouse embryonic stem cells and cancer cells.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

We investigated expression of CCNE1 in human preimplantation embryo development and embryonic stem cells (hESC). Functional studies included CCNE1 overexpression in hESC and CCNE1 downregulation in the outgrowths formed by plated human blastocysts. Analysis was performed by immunocytochemistry and quantitative real-time PCR. Mann-Whitney statistical test was applied.

MAIN RESULTS AND THE ROLE OF CHANCE

The CCNE1 protein was ubiquitously and constitutively expressed in the plenipotent cells of the embryo from the 4-cell stage up to and including the full blastocyst. During blastocyst expansion, CCNE1 was downregulated in the trophectoderm (TE) cells. CCNE1 shortly co-localized with NANOG in the inner cell mass (ICM) of expanding blastocysts, mimicking the situation in naïve hESC. In the ICM of expanded blastocysts, which corresponds with primed hESC, CCNE1 defined a subpopulation of cells different from NANOG/POU5F1-expressing pluripotent epiblast (EPI) cells and GATA4/SOX17-expressing primitive endoderm (PrE) cells. This CCNE1-positive cell population was associated with visceral endoderm based on transthyretin expression and marked the third cell lineage within the ICM, besides EPI and PrE, which had never been described before. We also investigated the role of CCNE1 by plating expanded blastocysts for hESC derivation. As a result, all the cells including TE cells re-gained CCNE1 and, consequently, NANOG expression, resembling the phenotype of naïve hESC. The inhibition of CCNE1 expression with siRNA blocked proliferation and caused degeneration of those plated cells.

LIMITATIONS, REASONS FOR CAUTION

The study is based on a limited number of good-quality human embryos donated to research.

WIDER IMPLICATIONS OF THE FINDINGS

Our study sheds light on the processes underlying the high developmental potential of early human embryonic cells. The CCNE1-positive plenipotent cell type corresponds with a phenotype that enables early human embryos to recover after fragmentation, cryodamage or (single cell) biopsy on day 3 for preimplantation genetic diagnosis. Knowledge on the expression and function of genes responsible for this flexibility will help us to better understand the undifferentiated state in stem cell biology and might enable us to improve technologies in assisted reproduction.

LARGE SCALE DATA

NA STUDY FUNDING AND COMPETING INTERESTS: This research is supported by grants from the Fund for Scientific Research - Flanders (FWO-Vlaanderen), the Methusalem (METH) of the VUB and Scientific Research Fond Willy Gepts of UZ Brussel. There are no competing interests.