Identification of committed placental stem cell lines for studies of differentiation

Human Embryogenesis: A Comparative Perspective

Understanding human embryology has historically relied on comparative approaches using mammalian model organisms. With the advent of low-input methods to investigate genetic and epigenetic mechanisms and efficient techniques to assess gene function, we can now study the human embryo directly. These advances have transformed the investigation of early embryogenesis in nonrodent species, thereby providing a broader understanding of conserved and divergent mechanisms. Here, we present an overview of the major events in human preimplantation development and place them in the context of mammalian evolution by comparing these events in other eutherian and metatherian species. We describe the advances of studies on postimplantation development and discuss stem cell models that mimic postimplantation embryos. A comparative perspective highlights the importance of analyzing different organisms with molecular characterization and functional studies to reveal the principles of early development. This growing field has a fundamental impact in regenerative medicine and raises important ethical considerations.

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.

AMPK Knockdown in Placental Labyrinthine Progenitor Cells Results in Restriction of Critical Energy Resources and Terminal Differentiation Failure

Placental abnormalities can cause Pregnancy-Associated Disorders, including preeclampsia, intrauterine growth restriction, and placental insufficiency, resulting in complications for both the mother and fetus. Trophoblast cells within the labyrinthine layer of the placenta facilitate the exchange of nutrients, gases, and waste between mother and fetus; therefore, the development of this cell layer is critical for fetal development. As trophoblast cells differentiate, it is assumed their metabolism changes with their energy requirements. We hypothesize that proper regulation of trophoblast metabolism is a key component of normal placental development; therefore, we examined the role of AMP-activated kinase (AMPK, PRKAA1/2), a sensor of cellular energy status. Our previous studies have shown that AMPK knockdown alters both trophoblast differentiation and nutrient transport. In this study, AMPKα1/2 shRNA was used to investigate the metabolic effects of AMPK knockdown on SM10 placental labyrinthine progenitor cells before and after differentiation. Extracellular flux analysis confirmed that AMPK knockdown was sufficient to reduce trophoblast glycolysis, mitochondrial respiration, and ATP coupling efficiency. A reduction in AMPK in differentiated trophoblasts also resulted in increased mitochondrial volume. These data indicate that a reduction in AMPK disrupts cellular metabolism in both progenitors and differentiated placental trophoblasts. This disruption correlates to abortive trophoblast differentiation that may contribute to the development of Pregnancy-Associated Disorders.

Id2 Mediates Differentiation of Labyrinthine Placental Progenitor Cell Line, SM10

The placenta is an organ that is formed transiently during pregnancy, and appropriate placental development is necessary for fetal survival and growth. Proper differentiation of the labyrinthine layer of the placenta is especially crucial, as it establishes the fetal-maternal interface that is involved in physiological exchange processes. Although previous studies have indicated the importance of inhibitor of differentiation/inhibitor of DNA binding-2 (Id2) helix-loop-helix transcriptional regulator in mediating cell differentiation, the ability of Id2 to regulate differentiation toward the labyrinthine (transport) lineage of the placenta has yet to be determined. In the current study, we have generated labyrinthine trophoblast progenitor cells with increased (SM10-Id2) or decreased (SM10-Id2-shRNA) Id2 expression and determined the effect on TGF-β-induced differentiation. Our Id2 overexpression and knockdown analyses indicate that Id2 mediates TGF-β-induced morphological differentiation of labyrinthine trophoblast cells, as Id2 overexpression prevents differentiation and Id2 knockdown results in differentiation. Thus, our data indicate that Id2 is an important molecular mediator of labyrinthine trophoblast differentiation. An understanding of the regulators of trophoblast progenitor differentiation toward the labyrinthine lineage may offer insights into events governing pregnancy-associated disorders, such as placental insufficiency, fetal growth restriction, and preeclampsia.

Ggnbp2 Is Essential for Pregnancy Success via Regulation of Mouse Trophoblast Stem Cell Proliferation and Differentiation

The Ggnbp2 null mutant embryos died in utero between Embryonic Days 13.5 to 15.5 with dysmorphic placentae, characterized by excessive nonvascular cell nests consisting of proliferative trophoblastic tissue and abundant trophoblast stem cells (TSCs) in the labyrinth. Lethality of Ggnbp2 null embryos was caused by insufficient placental perfusion as a result of remarkable decreases in both fetal and maternal blood vessels in the labyrinth. These defects were accompanied by a significant elevation of c-Met expression and phosphorylation and its downstream effector Stat3 activation. Knockdown of Ggnbp2 in wild-type TSCs in vitro provoked the proliferation but delayed the differentiation with an upregulation of c-Met expression and an enhanced phosphorylation of c-Met and Stat3. In contrast, overexpression of Ggnbp2 in wild-type TSCs exhibited completely opposite effects compared to knockdown TSCs. These results suggest that loss of GGNBP2 in the placenta aberrantly overactivates c-Met-Stat3 signaling, alters TSC proliferation and differentiation, and ultimately compromises the structure of placental vascular labyrinth. Our studies for the first time demonstrate that GGNBP2 is an essential factor for pregnancy success acting through the maintenance of a balance of TSC proliferation and differentiation during placental development.

AMPK and Placental Progenitor Cells

AMPK is important in numerous physiological systems but plays a vital role in embryonic and placental development. The placenta is a unique organ that is the essential lifeline between the mother and baby during pregnancy and gestation. During placental development, oxygen concentrations are very low until cells differentiate to establish the appropriate lineages that take on new functions required for placental and embryonic survival. Balancing the oxygen regulatory environment with the demands for energy and need to maintain metabolism during this process places AMPK at the center of maintaining placental cellular homeostasis as it integrates and responds to numerous complex stimuli. AMPK plays a critical role in sensing metabolic and energy changes. Once activated, it turns on pathways that produce energy and shuts down catabolic processes. AMPK coordinates cell growth, differentiation, and nutrient transport to maintain cell survival. Appropriate regulation of AMPK is essential for normal placental and embryonic development, and its dysregulation may lead to pregnancy-associated disorders such as intrauterine growth restriction, placental insufficiency, or preeclampsia.

AMPK knockdown in placental trophoblast cells results in altered morphology and function

The placenta is a transient organ that develops upon the initiation of pregnancy and is essential for embryonic development and fetal survival. The rodent placenta consists of distinct lineages and includes cell types that are analogous to those that make up the human placenta. Trophoblast cells within the labyrinth layer, which lies closest to the fetus, fuse and come in contact with maternal blood, thus facilitating nutrient and waste exchange between the mother and the baby. Abnormalities of the placenta may occur as a result of cellular stress and have been associated with pregnancy-associated disorders: such as preeclampsia, intrauterine growth restriction, and placental insufficiency. Cellular stress has also been shown to alter proliferation and differentiation rates of trophoblast cells. This stress response is important for cell survival and ensures continued placental functionality. AMP-activated protein kinase is an important sensor of cellular metabolism and stress. To study the role of AMPK in the trophoblast cells, we used RNA interference to simultaneously knockdown levels of both the AMPK alpha isoforms, AMPKα1 and AMPKα2. SM10 trophoblast progenitor cells were transduced with AMPKα1/2 shRNA and stable clones were established to analyze the effects of AMPK knockdown on important cellular functions. Our results indicate that a reduction in AMPK levels causes alterations in cell morphology, growth rate, and nutrient transport, thus identifying an important role for AMPK in the regulation of placental trophoblast differentiation.

Adrenomedullin promotes rat trophoblast stem cell differentiation

Accumulating data suggest that adrenomedullin (ADM) regulates the trophoblast cell growth, migration, and invasion. However, the effect of ADM on trophoblast differentiation is poorly understood. In this study, we hypothesized that ADM promotes the differentiation of trophoblast stem cells (TSCs) into trophoblast giant cells (TGCs). Using rat TSCs, Rcho-1 cells, we investigated the effect of ADM on TSC differentiation into TGCs in differentiation or stem cell media, respectively, and explored the effect of ADM on the mechanistic target of rapamycin (MTOR) signaling in trophoblast cell differentiation. The results include: 1) in the presence of differentiation medium, 10⁻⁷ M ADM, but not lower doses, elevated (P < 0.05) Prl3b1/Esrrb (i.e., the ratio of mRNA levels) by 1.7-fold compared to that in control; 2) the supplementation of ADM antagonist, regardless of the concentration of ADM, reduced (P < 0.05) Prl3b1/Esrrb by 2-fold, compared to control group, while the supplementation of CGRP antagonist, regardless of the concentration of ADM, did not change Prl3b1/Esrrb; 3) in the presence of stem cell medium, ADM did not alter the expression of TSC and TGC marker genes, however, the ratio of Prl3b1/Esrrb was reduced (P < 0.05) by ADM antagonist compared to that in control; and 4) ADM increased (P < 0.05) phosphorylated MTOR proteins and the ratio of phosphorylated to total MTOR proteins by 2.0- and 1.7-fold, respectively. The results indicate that ADM promotes but does not induce the differentiation of TSCs to TGCs in a dose-dependent manner and MTOR signaling may play a role in this process.

Prenatal administration of dexamethasone during early pregnancy negatively affects placental development and function in mice

Prenatal treatment of dexamethasone, a synthetic stress hormone, leads to low birth weight and affects adult pathophysiology. Because fetal growth and survival are critically dependent on successful placental development, we aimed to investigate the effects of prenatal dexamethasone exposure on placental growth and function, particularly focusing on issues surrounding the time of stress exposure in a developmental context. Dexamethasone was administered at a dosage of 1 mg/kg BW (DEX1) or 10 mg/kg BW (DEX10) intraperitoneally at gestational d 7.5, 8.5, and 9.5 in pregnant mice. Placentas were then dissected at gestational d 11.5 and 18.5. Placental size and weight were reduced at d 11.5 in a dose-dependent manner (P = 0.11 for saline vs. DEX1 and P < 0.001 for DEX1 vs. DEX10 in size; P = 0.34 for saline vs. DEX1 and P < 0.01 for DEX1 vs. DEX10 in weight). In contrast, a considerable heterogeneity was shown at d 18.5, especially in DEX10-treated mice. Some placentas were small and malformed whereas some were enlarged with structural abnormalities in spongiotrophoblasts and labyrinth layers. Although placental overgrowth under such condition seemed to compromise fetal demand for nutrient supply, disorganized cell structure with reduced fetal vasculature observed in large placentas suggests that prenatal stress exposure during the early gestational period negatively affects placental development and efficiency.

Hypoxia activates constitutive luciferase reporter constructs

Hypoxia has been identified as a contributing factor in the pathophysiology of several diseases and oxygen regulation is important during stem cell development, particularly in early embryogenesis. One aspect that has emerged is the role of hypoxia-inducible factors, or HIFs in regulating the effect of hypoxia. Studies in our laboratory sought to examine the hypoxic regulation of HIF activity in placental trophoblast cells, through the use of dual-reporter luciferase assays. Our study demonstrates that hypoxic conditions cause a significant increase in the level of constitutive luciferase reporter activity. We also show that this induction is not a cell type or species-specific phenomenon and provides an alternative method for normalizing transfection efficiency in luciferase assays under hypoxic conditions. Our results suggest that in studies dealing with hypoxic conditions, caution should be used when interpreting measurements of transcriptional activity by traditional dual-reporter assays.

New insights for Ets2 function in trophoblast using lentivirus-mediated gene knockdown in trophoblast stem cells

Mouse trophoblast stem (TS) cells represent a unique in vitro system that provides an unlimited supply of TS cells for the study of trophoblast differentiation and TS cell self-renewal. Although the mouse transcription factor Ets2 is required for TS cell self-renewal, its role in this and in TS cell differentiation has not been explored fully, partly due to the early lethality of Ets2 null mice. To address this, we developed a novel lentivirus-based system that resulted in efficient Ets2 knockdown in the overwhelming majority of TS cells. This system enables functional studies in TS cells, especially for genes required for TS cell self-renewal because TS cell derivation using gene-knockout embryos for such genes depends on TS cell self-renewal. Using morphological/morphometric criteria and gene expression analysis, we show that the requirement for Ets2 in self-renewal of TS cells cultured in 'stem cell medium' (SCM) involves maintenance of the expression of genes that inhibit TS cell differentiation in SCM, such as Cdx2 and Esrrb, and preservation of the undifferentiated TS cell morphology. During TS cell differentiation caused by Cdx2/Esrrb downregulation, due to either Ets2 knockdown in SCM or culture in differentiation medium (DM), Ets2 is also required for the promotion of trophoblast giant cell (TGC) and junctional zone trophoblast (JZT) differentiation. This TGC differentiation involves Ets2-dependent expression of Hand1, a gene required for the differentiation of all TGC types. This study uncovers new roles for Ets2 in TS cell self-renewal and differentiation and demonstrates the usefulness of this lentivirus system for gene function studies in TS cells.

Hypoxia-inducible factor 1alpha (HIF1A) mediates distinct steps of rat trophoblast differentiation in gradient oxygen

Defective differentiation of invasive, placental trophoblast cells has been associated with several pregnancy-related disorders. This study examines the molecular, functional, and morphological differentiation of lineage-specific, trophoblast giant cells under a gradient of oxygen concentrations. Low oxygen (3%) inhibited differentiation, but this inhibition was relieved in a stepwise fashion with increasing levels of oxygen. The oxygen-sensitive hypoxia-inducible factor 1alpha (HIF1A) is a major transcriptional regulator of the cellular response to low oxygen, and increased HIF1A protein levels and activity corresponded with the maintenance of the stem cell-like state and inhibition of trophoblast differentiation in low oxygen. Furthermore, constitutive expression of an oxygen-insensitive, active form of HIF1A protein mimicked the effects of low oxygen, inhibiting the differentiation of trophoblast giant cells. This study is the first to delineate the stepwise effects of oxygen on the activation of the trophoblast giant cell differentiation process and establishes a new paradigm from which to investigate trophoblast differentiation. In addition, this is the first reported study to demonstrate that constitutive HIF1A activity mediates oxygen's inhibition of differentiation. These results suggest that a dysregulation of HIF1A could contribute to impaired placental development.

Collagen IV Induces Trophoectoderm Differentiation of Mouse Embryonic Stem Cells

The earliest segregation of lineages in the developing embryo is the commitment of cells to the inner cell mass or the trophoectoderm in preimplantation blastocysts. The exogenous signals that control commitment to a particular cell lineage are poorly understood; however, it has been suggested that extracellular "niche" and extracellular matrix, in particular, play an important role in determining the developmental fate of stem cells. Collagen IV (ColIV) has been reported to direct embryonic stem (ES) cell differentiation to mesodermal lineages in both mouse and human ES cells. To define the effects of ColIV on ES cell differentiation and to identify the resulting heterogeneous cell types, we performed microarray analyses and determined global gene expression. We observed that ColIV induced the expression of mesodermal genes specific to hematopoietic, endothelial, and smooth muscle cells and, surprisingly, also a panel of trophoectoderm-restricted markers. This effect was specific to collagen IV, as no trophoblast differentiation was seen on collagen I, laminin, or fibronectin. Stimulation with basic fibroblast growth factor (FGF) or FGF4 increased the number of trophoectodermal cells. These cells were isolated under clonal conditions and successfully differentiated into a variety of trophoblast derivatives. Interestingly, differentiation of ES cells to trophoblastic lineages was only seen in ES cell lines maintained on embryonic feeder layers and was caudal-type homeobox protein 2 (Cdx2)-dependent, consistent with Cdx2's postulated role in trophoectoderm commitment. Our data suggest that, given the appropriate extracellular stimuli, mouse embryonic stem cells can differentiate into trophoectoderm. Disclosure of potential conflicts of interest is found at the end of this article.

Transforming growth factor-beta induces differentiation of the labyrinthine trophoblast stem cell line SM10

The mammalian placenta consists of different trophoblast cell types that assist in the variety of functions required for the maintenance of pregnancy. In rodents, labyrinthine trophoblasts of the placenta are especially important, because they are capable of differentiating into fused labyrinthine cells, which form the feto-maternal exchange surface. Even though the molecular signals triggering labyrinthine trophoblast differentiation are poorly understood, transforming growth factor-beta (TGF-beta) has been shown to be present in the placental environment and alter trophoblast development. In this study, we investigated the effects of TGF-beta on the differentiation of the labyrinthine trophoblast stem cell lines SM10 and HRP-1. RT-PCR analyses demonstrated that while the molecular expression of labyrinthine-specific lineage markers (Esx1, Tfeb, and Tec) was maintained in TGF-beta-treated SM10 and HRP-1 cells, TGF-beta induced the down-regulation of trophoblast stem cell markers Id2 and Cdx2. In contrast, TGF-beta induced the expression of a marker of differentiated labyrinthine trophoblasts, Gcm1, only in the SM10 cell line. Furthermore, we demonstrated an increased glucose uptake in the TGF-beta-treated SM10 cells, indicative of functional differentiation. Finally, cell fusion in TGF-beta-treated SM10 and HRP-1 cells was investigated by western blotting analysis of placental alkaline phosphatase and cadherin-11 and by microscopic analyses of cell morphology using green fluorescent protein (GFP) and rhodamine phalloidin staining. The western blotting and morphological analyses indicate TGF-beta-induced cell fusion and morphological differentiation in the SM10 cell line. The SM10 cell line will provide a new and unique model for detailed analysis of TGF-beta-induced molecular events associated with labyrinthine trophoblast differentiation and function.

Hypoxia inhibits differentiation of lineage-specific Rcho-1 trophoblast giant cells

Defects in placental development lead to pregnancies at risk for miscarriage and intrauterine growth retardation and are associated with preeclampsia, a leading cause of maternal death and premature birth. In preeclampsia, impaired placental formation has been associated with alterations in a specific trophoblast lineage, the invasive trophoblast cells. In this study, an RT-PCR Trophoblast Gene Expression Profile previously developed by our laboratory was utilized to examine the lineage-specific gene expression of the rat Rcho-1 trophoblast cell line. Our results demonstrated that Rcho-1 cells represent an isolated, trophoblast population committed to the giant cell lineage. RT-PCR analysis revealed that undifferentiated Rcho-1 cells expressed trophoblast stem cell marker, Id2, and trophoblast giant cell markers. On differentiation, Rcho-1 cells downregulated Id2 and upregulated Csh1, a marker of the trophoblast giant cell lineage. Neither undifferentiated nor differentiated Rcho-1 cells expressed spongiotrophoblast marker Tpbpa or labyrinthine markers Esx1 and Tec. Differentiating Rcho-1 cells in hypoxia did not alter the expression of lineage-specific markers; however, hypoxia did inhibit the downregulation of the trophoblast stem cell marker Id2. Differentiation in hypoxia also blocked the induction of CSH1 protein. In addition, hypoxia inhibited stress fiber formation and abolished the induction of palladin, a protein associated with stress fiber formation and focal adhesions. Thus, Rcho-1 cells can be maintained as a proliferative, lineage-specific cell line that is committed to the trophoblast giant cell lineage on differentiation in both normoxic and hypoxic conditions; however, hypoxia does inhibit aspects of trophoblast giant cell differentiation at the molecular, morphological, and functional levels.