Id-2 regulates critical aspects of human cytotrophoblast differentiation, invasion and migration

Identification of differentially expressed genes in the uterine endometrium on day 12 of the estrous cycle and pregnancy in pigs

Maternal recognition of pregnancy in pigs occurs approximately on Day (D) 12 of pregnancy and is critical for embryo implantation. The presence of the conceptus in the uterine lumen during this period changes uterine endometrial function to prepare for attachment of the conceptus to the endometrial epithelial cells and maintain luteal function in the ovary. Although much is known about endometrial gene expression, the genes expressed in the uterine endometria and the cellular and molecular mechanisms of those gene products during the period of implantation and maternal recognition of pregnancy in pigs are still not completely defined. To better understand the interactions between the maternal uterus and conceptus during the implantation process, we searched genes differentially expressed in the endometria on D12 of pregnancy compared to those on D12 of the estrous cycle. A new reverse transcription-polymerase chain reaction (RT-PCR)-based method that involves annealing control primers (ACPs) was employed. Using 120 ACPs, we sequenced 12 differentially expressed genes (DEGs) and identified those genes using the Basic Local Alignment Search Tool (BLAST). Northern blot hybridization analysis confirmed the differential expression of those DEGs in the uterine endometrium. In situ hybridization analysis determined the cell-type specific expression of the DEGs in the uterine endometrium. Further analysis of the DEGs found in this study will provide insights into the cellular and molecular basis of maternal and fetal interactions during the period of maternal recognition of pregnancy in the pig. Mol. Reprod. Dev. 76: 75-84, 2009. (c) 2008 Wiley-Liss, Inc.

Persistent hypomethylation in the promoter of nucleosomal binding protein 1 (Nsbp1) correlates with overexpression of Nsbp1 in mouse uteri neonatally exposed to diethylstilbestrol or genistein

Neonatal exposure of CD-1 mice to diethylstilbestrol (DES) or genistein (GEN) induces uterine adenocarcinoma in aging animals. Uterine carcinogenesis in this model is ovarian dependent because its evolution is blocked by prepubertal ovariectomy. This study seeks to discover novel uterine genes whose expression is altered by such early endocrine disruption via an epigenetic mechanism. Neonatal mice were treated with 1 or 1000 microg/kg DES, 50 mg/kg GEN, or oil (control) on d 1-5. One group of treated mice was killed before puberty on d 19. Others were ovariectomized or left intact, and killed at 6 and 18 months of age. Methylation-sensitive restriction fingerprinting was performed to identify differentially methylated sequences associated with neonatal exposure to DES/GEN. Among 14 candidates, nucleosomal binding protein 1 (Nsbp1), the gene for a nucleosome-core-particle binding protein, was selected for further study because of its central role in chromatin remodeling. In uteri of immature control mice, Nsbp1 promoter CpG island (CGI) was minimally methylated. Once control mice reached puberty, the Nsbp1 CGI became hypermethylated, and gene expression declined further. In contrast, in neonatal DES/GEN-treated mice, the Nsbp1 CGI stayed anomalously hypomethylated, and the gene exhibited persistent overexpression throughout life. However, if neonatal DES/GEN-treated mice were ovariectomized before puberty, the CGI remained minimally to moderately methylated, and gene expression was subdued except in the group treated with 1000 microg/kg DES. Thus, the life reprogramming of uterine Nsbp1 expression by neonatal DES/GEN exposure appears to be mediated by an epigenetic mechanism that interacts with ovarian hormones in adulthood.

Cytomegalovirus infection in the human placenta: maternal immunity and developmentally regulated receptors on trophoblasts converge

During human pregnancy, CMV infects the uterine-placental interface with varied outcomes from fetal intrauterine growth restriction to permanent birth defects, depending on the level of maternal immunity and gestational age. Virus spreads from infected uterine blood vessels, amplifies by replicating in decidual cells, and disseminates to the placenta in immune complexes. Cytotrophoblasts--epithelial cells of the placenta--differentiate along two distinct pathways. In the first, cells fuse into syncytiotrophoblasts covering the surface of chorionic villi that transport substances from the maternal to fetal bloodstream. In the second, cells invade the uterine interstitium and blood vessels, remodel the vasculature and form anchoring villi. CMV initiates replication in cytotrophoblast progenitor cells of floating villi, whereas syncytiotrophoblasts are spared. This extraordinary pattern of focal infection in underlying cells hinges on virion receptors being upregulated as villous cytotrophoblasts begin to differentiate. Expression of developmentally regulated receptors could explain viral replication in spatially distinct maternal and fetal compartments. Reduced invasiveness of infected cells could impair remodeling of the uterine vasculature, restrict maternal blood flow and access of the fetus to nutrients causing intrauterine growth restriction.

Hypermethylation of RASSF1A in human and rhesus placentas

The pseudomalignant nature of the placenta prompted us to search for tumor suppressor gene hypermethylation, a phenomenon widely reported in cancer, in the human placenta. Nine tumor suppressor genes were studied. Hypermethylation of the Ras association domain family 1 A (RASSF1A) gene was found in human placentas from all three trimesters of pregnancy but was absent in other fetal tissues. Hypermethylation of Rassf1 was similarly observed in placentas from the rhesus monkey but not the mouse. An inverse relationship between RASSF1A promoter methylation and gene expression was demonstrated by bisulfite sequencing of microdissected placental cells and immunohistochemical staining of placental tissue sections using an anti-RASSF1A antibody. Treatment of choriocarcinoma cell lines, JAR and JEG3, by 5-aza-2'-deoxycytidine and trichostatin A led to reduction in RASSF1A methylation but increased expression. These observations extend the analogy between the primate placenta and malignant tumors to the epigenetic level.

Developmental regulation of human cytomegalovirus receptors in cytotrophoblasts correlates with distinct replication sites in the placenta

Cytomegalovirus (CMV), the major viral cause of congenital disease, infects the uterus and developing placenta and spreads to the fetus throughout gestation. Virus replicates in invasive cytotrophoblasts in the decidua, and maternal immunoglobulin G (IgG)-CMV virion complexes, which are transcytosed by the neonatal Fc receptor across syncytiotrophoblasts, infect underlying cytotrophoblasts in chorionic villi. Immunity is central to protection of the placenta-fetal unit: infection can occur when IgG has a low neutralizing titer. Here we used immunohistochemical and function-blocking methods to correlate infection in the placenta with expression of potential CMV receptors in situ and in vitro. In placental villi, syncytiotrophoblasts express the virion receptor epidermal growth factor receptor (EGFR) but lack integrin coreceptors, and virion uptake occurs without replication. Focal infection can occur when transcytosed virions reach EGFR-expressing cytotrophoblasts that selectively initiate expression of alphaV integrin. In cell columns, proximal cytotrophoblasts lack receptors and distal cells express integrins alpha1beta1 and alphaVbeta3, enabling virion attachment. In the decidua, invasive cytotrophoblasts expressing coreceptors upregulate EGFR, thereby dramatically increasing susceptibility to infection. Our findings indicate that virion interactions with cytotrophoblasts expressing receptors in the placenta (i) change as the cells differentiate and (ii) correlate with spatially distinct sites of CMV replication in maternal and fetal compartments.

Control of human trophoblast function

The trophoblast, i.e. the peripheral part of the human conceptus, exerts a crucial role in implantation and placentation. Both processes properly occur as a consequence of an intimate dialogue between fetal and maternal tissues, fulfilled by membrane ligands and receptors, as well as by hormone and local factor release. During blastocyst implantation, generation of distinct trophoblast cell types begins, namely the villous and the extravillous trophoblast, the former of which is devoted to fetal-maternal exchanges and the latter binds the placental body to the uterine wall. Physiological placentation is characterized by the invasion of the uterine spiral arteries by extravillous trophoblast cells arising from anchoring villi. Due to this invasion, the arterial structure is replaced by amorphous fibrinoid material and endovascular trophoblastic cells. This transformation establishes a low-resistance, high-capacity perfusion system from the radial arteries to the intervillous space, in which the villous tree is embedded. The physiology of pregnancy depends upon the orderly progress of structural and functional changes of villous and extravillous trophoblast, whereas a derangement of such processes can lead to different types of complications of varying degrees of gravity, including possible pregnancy loss and maternal life-threatening diseases. In this review we describe the mechanisms which regulate trophoblast differentiation, proliferation, migration and invasiveness, and the alterations in these mechanisms which lead to pathological conditions. Furthermore, based on the growing evidence that proper inflammatory changes and oxidative balance are needed for successful gestation, we explain the mechanisms by which agents able to influence such processes may be useful in the prevention and treatment of pregnancy disorders.

Proteomic analysis of hypoxia-induced responses in the syncytialization of human placental cell line BeWo

Syncytiotrophoblast formation is affected by a number of pathological conditions and suppressed syncytiotrophoblast formation due to hypoxia may play a role in the pathogenesis of preeclampsia. However, the molecular basis of hypoxia-inhibited trophoblast syncytialization is poorly understood. To determine the effect of hypoxia on trophoblast syncytialization, a proteomic analysis was performed in the human cytotrophoblast cell line BeWo using two-dimensional electrophoresis and MALDI-TOF-TOF-MS. Hypoxia induced marked inhibition of BeWo cell fusion and differentiation. The proteomic profiling was established under hypoxia in BeWo cell syncytialization. The results showed that twenty proteins were significantly up-or down-regulated under hypoxia, compared with cells under normoxia. In response to hypoxia, three antioxidants, peroxiredoxin 1, peroxiredoxin 2 and 1-Cys peroxiredoxin, were down-regulated, two proteins involved in glycolysis pathway (malate dehydrogenase and enolase) were up-regulated. The expression of two members of the annexin family (annexin A2 and annexin A5) increased. We also found a decreased expression of 14-3-3 tau protein in hypoxia treated cells. Proteins implied in protein degradation and folding were also identified. The expression of two cytoskeleton components (keratin 1 and beta-actin) was found to be down-regulated. In addition, galectin-3 was up-regulated. These proteins have been implicated in regulating cellular oxidative stress, glycolysis, signal transduction, protein folding and degradation, cell mobility and cytoskeletal structure formation. Western blot analysis revealed that the levels of peroxiredoxin 1 and 14-3-3 tau decreased, whereas the levels of annexin A5 and annexin A2 increased in BeWo cells under hypoxia. These findings provided new insights into the molecular mechanisms in mediating cellular response to hypoxia in trophoblast syncytialization.

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.

Identification of committed placental stem cell lines for studies of differentiation

Trophoblasts provide a model to investigate fundamental mechanisms of stem cell differentiation, but the availability of trophoblast stem cell lines is limited. Here we report the development of an RT-PCR-based lineage-specific profile as a method to identify the lineages of placental trophoblast cells routinely and specifically. This profiling method was used to analyze the mouse SM10 and rat HRP-1 cell lines, isolated from a region of the placental labyrinth, but of previously unidentified lineage. Using this profile, the expression of trophoblast stem cell markers was detected in the SM10 and HRP-1 cells. In contrast, no expression of a marker of differentiated labyrinthine trophoblast was detected. Additionally, both cell lines expressed labyrinthine trophoblast-specific genes and did not express lineage-specific markers of spongiotrophoblasts or trophoblast giant cells. Our results suggest that SM10 and HRP-1 cell lines are trophoblast stem cell-like cell lines that can be maintained in undifferentiated but committed state in cell culture. These cell lines express labyrinthine-specific genes and are committed to differentiate solely into functional labyrinthine trophoblasts. Our profiling method provides a new technique to identify stem cells and their lineage-specific differentiation. This method additionally indicates that SM10 and HRP-1 cell lines provide new systems for future studies of stem cell differentiation, allowing investigation of basic mechanisms of differentiation, which may provide insights into the biophysics of development of a specialized system. This method should also prove to be useful for identification of other stem cell lines and examination of lineage-specific commitment.

Defective implantation and placentation: laying the blueprint for pregnancy complications

Normal implantation and placentation is critical for pregnancy success. Many pregnancy-related complications that present late in gestation (such as pre-eclampsia and preterm labour) appear to have their origins early in pregnancy with abnormalities in implantation and placental development. Implantation is characterized by invasion of the maternal tissues of the uterus by fetal trophoblast, and the degree to which trophoblast invades these tissues appears to be a major determinant of pregnancy outcome. Excessive invasion can lead to abnormally firm attachment of the placenta to the myometrium (placenta accreta) with increased maternal and perinatal morbidity. Inadequate invasion, specifically restricted endovascular invasion, has been implicated in the pathophysiology of such conditions as pre-eclampsia (gestational proteinuric hypertension), preterm premature rupture of membranes, preterm labour, and intrauterine growth restriction. The molecular and cellular mechanisms responsible for implantation remain enigmatic. This review will include an overview of implantation followed by a discussion of a number of molecular mechanisms implicated in defective implantation and placentation including the role of decidual prostaglandins and haemorrhage in regulating trophoblast invasion. An improved understanding of the molecular mechanisms responsible for abnormal implantation and placentation will likely improve clinicians' abilities to treat disorders that occur along this continuum, including infertility, recurrent pregnancy loss, pre-eclampsia, and preterm birth.

Initial differentiation of blastomeres in 4-cell human embryos and its significance for early embryogenesis and implantation

This brief review is devoted to the nature of early blastomere differentiation in human 4-cell embryos and its consequences for embryonic development. Precursor cells of inner cell mass, germline, and trophectoderm may be formed at this stage, the clearest evidence being available for trophectoderm. The sites of these precursor cells in the embryo could be ascertained using markers for animal and vegetal poles, observing specific cleavage planes, and assessing gene and protein expression. This opens new opportunities for studying 4-cell embryos and removing or replacing specific cells. Knowledge of the properties of individual blastomeres should help in improving assisted human reproduction, performing preimplantation genetic diagnosis, and perhaps establishing specific stem cell lines. Special attention is paid to well-characterized trophectoderm, the trophectoderm stem cell, and possible new forms of clinical application.

Id helix-loop-helix proteins are differentially expressed in gestational trophoblastic disease

AIMS

To assess the expression of Id proteins in trophoblastic tissues and to correlate this with clinical parameters, proliferative and apoptotic indices as well as to related oncogene expression.

METHODS AND RESULTS

Immunohistochemistry for Id1, Id2, Id3 and Id4 was performed on 83 trophoblastic tissues including 17 normal first-trimester placentas, seven term placentas, 47 hydatidiform moles (HM), and 12 spontaneous miscarriages. The four Id proteins were predominantly expressed in the villous and implantation site intermediate trophoblast. Expression of Id1 in HM was significantly higher than that in normal placenta (P = 0.0006) and spontaneous miscarriage (P = 0.0001) but did not correlate with subsequent development of gestational trophoblastic neoplasia (GTN). Id1 expression correlated with the proliferation index as assessed by MCM7 (P = 0.003) and Ki67 (P = 0.017) and with the apoptotic activity assessed by TUNEL (P = 0.001) and M30 CytoDeath antibody (P = 0.013). Moreover, the expression of Id1 correlated with the expression of p53 (P = 0.004), p21(WAF1) (/CIP1) (P = 0.003) but not with p16 (P = 0.107).

CONCLUSIONS

Id proteins may play a role in the regulation of proliferative and apoptotic activity in trophoblastic tissue and are potentially useful in differentiating molar and non-molar gestation, but are not helpful in predicting GTN.

Cell cycle in mouse development

Mice likely represent the most-studied mammalian organism, except for humans. Genetic engineering in embryonic stem cells has allowed derivation of mouse strains lacking particular cell cycle proteins. Analyses of these mutant mice, and cells derived from them, facilitated the studies of the functions of cell cycle apparatus at the organismal and cellular levels. In this review, we give some background about the cell cycle progression during mouse development. We next discuss some insights about in vivo functions of the cell cycle proteins, gleaned from mouse knockout experiments. Our text is meant to provide examples of the recent experiments, rather than to supply an extensive and complete list.

PL74, a novel member of the transforming growth factor-beta superfamily, is overexpressed in preeclampsia and causes apoptosis in trophoblast cells

PL74, a novel member of the TGFbeta superfamily that has highest expression in placenta, is a multifunctional peptide that can induce differentiation, inhibit inflammatory stimulation of TNFalpha, and execute apoptosis after p53 overexpression and cytotoxic injury. To study its expression and function in placenta and preeclampsia, we first determined mRNA expression in nine normal and 10 preeclamptic placentas. PL74 mRNA was overexpressed by 57.3% in preeclampsia. Transfection of PL74 into term cytotrophoblasts resulted in increased apoptosis by terminal uridine deoxynucleotidyl nick end labeling labeling (control, 2.8 +/- 0.5%; PL74, 19.1 +/- 0.2%; P < 0.005). Addition of PL74 protein to HTR8/SVneo extravillous cytotrophoblast cells showed a dose-response (0-100 ng/ml) inhibition of [3H]thymidine uptake and increase in apoptosis shown by terminal uridine deoxynucleotidyl nick end labeling and histone-associated DNA fragment ELISA (control, 0.11 +/- 0.01 absorbance units; PL74, 0.21 +/- 0.01; P < 0.01). PL74 did not alter cytotrophoblast invasion using a Matrigel in vitro invasion assay. Cytokine regulation of PL74 mRNA expression in term cytotrophoblasts showed that epidermal growth factor and IFNgamma increased PL74 expression, but TGFbeta and TNFalpha had no effect. Transfection of antisense PL74 into term cytotrophoblast cells resulted in an inhibition of spontaneous differentiation at 2 and 24 h of culture (control vector, 30.8 +/- 3.1% and 26.4 +/- 1.2%; antisense PL74, 17.6 +/- 1.8%and 12.6 +/- 1.4% syncytial units, at 2 and 24 h respectively; P < 0.01). We conclude that PL74 is overexpressed in preeclampsia and may thus promote apoptosis of cytotrophoblasts at the expense of differentiation. PL74 secretion is induced by IFNgamma and may play a role in abnormal placental responses in preeclampsia.

The human cytotrophoblastic cell, a mononuclear chameleon

The human placenta represents an abundant; easily accessible and unlimited study material (at birth a human placenta provides about 500 g of trophoblast). Cytotrophoblastic cells (CTB) are one constituent of the human placenta and represent epithelial cells with fascinating properties: They are able to fuse to form syncytia, can behave like immotile polarized epithelial cells, can phenocopy stromal fibroblasts or endothelial cells or undergo a mesenchymal-like transformation that converts them into non proliferative and highly invasive cells. Like a chameleon, CTB are thus able to adapt to their immediate environment by phenocopying their neighbor cells. This review describes the different routes that CTB follow during their differentiation pathways, the regulation of these at the molecular level, it gives also an overview of the pathologies associated with faulty pathways and describes the usual phenotypic markers used to identify the different CTB subsets. This review is intended to stimulate investigators not acquainted with the field of placental biology to use CTB as a model to study important biological functions in vitro, such as cell fusion, cell invasion and cell transformation.

Molecular and morphologic analyses of expression of ESX1L in different stages of human placental development

The mRNA expression of the ESX1L gene was analyzed by RT-PCR and in situ hybridization in human normal cytogenetically placentas, of different gestational ages. Our RT-PCR analysis showed that ESX1L mRNA is expressed from 5 weeks of gestation until term, suggesting a role not only in trophoblast differentiation but also in the maintenance of the villi and microvasculature. We also observed, by in situ hybridization, that ESX1L mRNA is expressed by cytotrophoblast from chorionic plate, syncytiotrophoblast and stromal cells of all terminal, intermediate and stem villi of term placentas. ESX1L mRNA expression was more pronounced in trophoblast cells of terminal villi than in intermediate and stem villi. In conclusion, ESX1L is expressed during all stages of placental development and is localized to sparse areas of trophoblast in terminal villi in association with cytotrophoblastic cells.

Trophoblast stem cells differentiate in vitro into invasive trophoblast giant cells

Trophoblast cells are characterized by an invasive behavior into the surrounding uterine tissue. In rodents, an early peri-/endovascular type of invasion exerted by trophoblast giant cells can be distinguished from a late interstitial type carried out by glycogen trophoblast cells. Analysis of the molecular mechanisms of trophoblast invasion has been hampered, however, by the complex temporal and spatial patterns of invasion. We utilized trophoblast stem (TS) cell lines to study trophoblast invasion in vitro and to establish a model that facilitates investigation of this process on the molecular level. Our results showed that trophoblast giant cells that differentiate from TS cells in vitro are capable of penetrating a reconstituted basement membrane matrix. Consequently, invasion rates were increased in various giant cell differentiation-promoting conditions. We also derived TS cell lines that are homozygous for a mutation of the Hand1 transcription factor. The Hand1-/- TS cells showed reduced levels of giant cell differentiation and exhibited an approximately 50% decrease in invasion rates. In summary, trophoblast giant cells that differentiate from TS cells in vitro recapitulate the invasive capacity of normal trophoblast cells in vivo. The TS cell system is a valuable tool to identify and quantitatively study regulators of trophoblast invasion.

Id2 is a primary partner for the E2-2 basic helix-loop-helix transcription factor in the human placenta

We screened a term placental cDNA library by the yeast two-hybrid approach with Id2, a negative regulator of basic helix-loop-helix (bHLH) factors. Of the clones obtained, approximately one-third were the E2-2 bHLH transcription factor. Id2 and E2-2 were shown to interact in direct two-hybrid assays in yeast cells, as well as immunoprecipitation assays in mammalian cells. Immunohistochemical analysis demonstrated co-localization of both Id2 and E2-2 in placental trophoblasts. Co-transfection of JEG-3 cells with E2-2 and Id2, and a luciferase reporter construct under the control of the human chorionic gonadotropin alpha-subunit promoter revealed that E2-2 had a negative effect on CGalpha-subunit transcription, which could be relieved by overexpression of Id2. The library was in turn rescreened with E2-2, and Id2 and Id1 were essentially the only clones obtained. We conclude that Id2 is a primary binding partner for the bHLH transcription factor E2-2 in the human placenta.

The Hand1, Stra13 and Gcm1 transcription factors override FGF signaling to promote terminal differentiation of trophoblast stem cells

The trophoblast cell lineage is an interesting model system because it is composed of a limited number of cell types that are spatially patterned. Trophoblast stem (TS) cells reside within a layer called the chorion and either remain as stem cells or differentiate into spongiotrophoblast (SpT), trophoblast giant (TG) cells or syncytiotrophoblast cells (SynT) of the labyrinth. Maintenance of the TS phenotype is dependent on stimulation by FGF4, whereas differentiation and/or maintenance of the differentiated derivatives are dependent on key transcription factors: Mash2 for SpT, Hand1 for TG cells and Gcm1 for SynT cells. TS cells proliferate and retain their stem cell phenotype in culture in response to FGF4 and an additional factor(s) that can be provided by conditioned medium from embryonic fibroblast feeder cells (CM). To understand the functions of Hand1, Mash2 and Gcm1 at a cellular level, we tested the effects of their ectopic and over-expression on the ability of TS cells to either continue to proliferate or differentiate into their alternative fates. Expression of Mash2 alone had no effects on TS cell differentiation. However, Mash2-transfected cells continued to divide longer after withdrawal of FGF/CM. Hand1 promoted TGC differentiation, even in the continued presence of FGF4/CM. Stra13, another bHLH factor gene that is expressed in TG cells, also induced TG differentiation. Gcm1 induced a rapid arrest of TS proliferation but, in contrast to Hand1 and Stra13, blocked TG cell differentiation. Although Gcm1 was not sufficient to promote SynT formation, expression of an antisense Gcm1 transcript blocked SynT differentiation. These data suggest that Mash2 functions to promote transient FGF4-independent amplification of trophoblast cells that are progressing towards the SpT and TG cell phenotype. By contrast, Hand1 and Stra13 promote cell cycle exit and restrict cells towards the TG fate, whereas Gcm1 promotes cell cycle exit and restriction towards the SynT fate.

Critical role for transcription factor AP-2alpha in human trophoblast differentiation

To examine whether AP-2alpha is a critical component of the genetic program that directs human trophoblast differentiation, we used DNA microarray analyses to characterize the effects of a dominant-negative form of the AP-2 protein upon in vitro differentiating cytotrophoblast cells. Human cytotrophoblast cells (>95% pure) were cultured for 3 days in the presence of control medium or medium containing an adenovirus that expresses a dominant-negative mutant of AP-2 (Ad2.AP-2D/N) or an adenovirus lacking the AP-2 mutant gene (Ad.WT). DNA microarray analyses using Affymetrix human U95Av2 GeneChips were performed on RNA extracted from the three groups of cells immediately prior to and after 3 days of cell culture. Cells infected with Ad2.AP-2D/N or Ad2.WT underwent morphological differentiation similar to that of uninfected cells, with greater than 90% of the cells in each group fusing to form multinucleated syncytiotrophoblast cells. However, Ad2.AP-2D/N markedly inhibited the induction or repression of many genes that were regulated in the noninfected and Ad2.WT-infected cells during differentiation. Eighteen of the 25 most induced genes and 17 of the 20 most repressed genes during differentiation were AP-2 dependent, with the majority of these related to extracellular organization, cellular communication, and signal transduction. Taken together, these findings strongly suggest that AP-2 plays a critical role for both the induction and repression of genes that comprise postsyncytialization gene expression programs of trophoblast differentiation and maturation. AP-2, however, is not required for the fusion of cytotrophoblast cells to form a syncytium or the expression of syncytin.

New roles for the RB tumor suppressor protein

For a gene whose existence was first postulated in 1971, was cloned in 1986 and whose functions have been extensively characterized ever since, you might be inclined to think there was not much new to report regarding the retinoblastoma tumor suppressor gene (RB)--but you would be wrong to make such an assumption. RB is still piquing our interest with several activities defined over the past year that reveal new and exciting roles for this key tumor suppressor gene. These functions include regulation of senescence through specific gene silencing mechanisms, control of developmental processes in extra-embryonic tissues, maintaining tissue homeostasis and determining survival responses to chemotherapy.

Human cytotrophoblasts promote endothelial survival and vascular remodeling through secretion of Ang2, PlGF, and VEGF-C

Cytotrophoblasts are specialized epithelial cells of the human placenta that differentiate to acquire tumor-like properties that allow them to invade the uterus. Concurrently, they develop endothelial-like characteristics. This transformation allows cytotrophoblasts to replace the maternal cells that line uterine vessels, thereby diverting maternal blood to the placenta. Previously, we showed that invading cytotrophoblasts secrete VEGF-C and PlGF, factors that regulate their acquisition of an endothelial-like phenotype. Here, we examined the cells' expression of angiopoietin ligands and their Tie receptors. The data show that cytotrophoblasts predominantly expressed Ang2. We also studied the paracrine functions of Ang2 and the VEGFs by culturing uterine microvascular endothelial cells in cytotrophoblast-conditioned medium, which supported their growth. Removal of VEGF-C, PlGF, and/or Ang2 from the medium caused a marked reduction in cell number due to massive apoptosis. We also assayed the angiogenic potential of cytotrophoblast-derived factors in the chick chorioallantoic membrane assay. The results showed that they stimulated angiogenesis to a level comparable to that of basic FGF. These data suggest that invasive human cytotrophoblasts use an unusual repertoire of factors to influence the angiogenic state of maternal blood vessels and that this cross talk plays an important part in the endovascular component of uterine invasion.

Regulatory transcription factors controlling function and differentiation of human trophoblast--a review

In transgenic mice, homozygous mutations of trophoblast-specific transcription factors such as Hand1, Mash-2, I-mfa or GCM1 revealed their key regulatory roles in induction, maintenance or differentiation of distinct placental trophoblast subpopulations in vivo. Descriptive studies have shown that several of these factors are also expressed in the human placenta, suggesting that the molecular mechanisms governing trophoblast differentiation could be similar in mice and men. While an increasing number of putative developmental regulators are being identified in the human placenta, little information is available regarding whether the particular factors play an essential role in trophoblast differentiation processes such as formation of anchoring villi, placental bed invasion or syncytialization. However, expression of abundant trophoblast-specific products such as hormones can be regarded as a hallmark of differentiation, suggesting that the factors controlling their transcription could also be involved in the developmental processes of the placenta. Indeed, studies in different model systems revealed that the human homologues of murine trophoblast-specific transcriptional regulators interact with the promoter regions of typical placental genes such as aromatase P450 (CYP19), chorionic gonadotrophin (CG) or placental lactogen (PL). Additionally, the unique combination of more broadly distributed transcription factors of the Sp or Ap-2 protein family in a particular trophoblast cell type is required to govern mRNA expression in a differentiation-dependent manner. Here, we will summarize our present knowledge on these individual transcription factors that are involved in human trophoblast function and differentiation.

Genes, development and evolution of the placenta

Through studies of transgenic and mutant mice, it is possible to describe molecular pathways that control the development of all major trophoblast cell subtypes and structures of the placenta. For example, the proliferation of trophoblast stem cells is dependent on FGF signalling and downstream transcription factors Cdx2, Eomes and Err2. Several bHLH transcription factors regulate the progression from trophoblast stem cells to spongiotrophoblast and to trophoblast giant cells (Id1/2, Mash2, Hand1, Stra13). Intercellular actions critical for maintaining stable precursor cell populations are dependent on the gap junction protein Cx31 and the growth factor Nodal. Differentiation towards syncytiotrophoblast as well as the initiation of chorioallantoic (villous) morphogenesis is regulated by the Gcm1 transcription factor, and subsequent labyrinth development is dependent on Wnt, HGF and FGF signalling. These insights suggest that most of the genes that evolved to regulate placental development are either identical to ones used in other organ systems (e.g., FGF and epithelial branching morphogenesis), were co-opted to take on new functions (e.g., AP-2gamma, Dlx3, Hand1), or arose via gene duplication to take on a specialized placental function (e.g., Gcm1, Mash2). Many of the human orthologues of these critical genes show restricted expression patterns that are consistent with a conserved function. Such information is aiding the comparison of the human and mouse placenta. In addition, the prospect of a conserved function clearly suggests potential mechanisms for explaining complications of human placental development.

Gene induction and categorical reprogramming during in vitro human endometrial fibroblast decidualization

Gene induction and categorical reprogramming during in vitro human endometrial fibroblast decidualization. Physiol Genomics 7: 135-148, 2001. First published September 21, 2001; 10.1152/physiolgenomics.00061.2001.-Human decidual fibroblasts undergo a differentiative commitment to the acquisition of endocrine, metabolic, and structural cell functions in a process known as decidualization. Decidualization is critical for embryo implantation and placental function. We characterized gene expression pattern kinetics during decidual fibroblast differentiation by microarray analysis. Of 6,918 genes analyzed, 121 genes were induced by more than twofold, 110 were downregulated, and 50 showed biphasic behavior. Dynamically regulated genes were could be fit into nine K-means algorithm-based kinetic pattern groups, and by biologic classification, into five categories: cell and tissue function, cell and tissue structure, regulation of gene expression, expressed sequence tag (EST), and "function unknown." Reprogramming of genes within specific functional groups and gene families was a prominent feature that consisted of simultaneous induction and downregulation of a set of genes with related function. We previously observed a conceptually similar process during fetal trophoblast differentiation, in which the same phenomena applied to different genes. Of the 569 dynamically regulated genes regulated by either model, only 81 of these were in common. These results suggest that reprogramming of gene expression within focused functional categories represents a fundamental aspect of cellular differentiation.

Expression of ID genes in differentiated elements of human male germ cell tumors

The ID genes are members of a family of genes that encode helix-loop-helix (HLH)-containing proteins. The Id proteins, unlike other HLH proteins, lack an adjacent DNA binding domain and hence act as dominant negative regulators of HLH transcription factors that have been implicated in control of cellular differentiation. Although the role of Id genes in murine development has been documented, their roles in human embryogenesis remain unknown. In this study, human male germ cell tumors (GCTs) were used as a model for examining the expression of the ID genes in various histologies that are reflective of different temporal phases of human development. In seminomas, little or no expression of IDI, ID2, and ID3 was detected, consistent with the uncommitted germ cell-like phenotype of this tumor histology. Likewise, GCTs with histologies reflective of extraembryonic and embryonic patterns of differentiation exhibited patterns of expression of the three ID genes often similar to those noted during murine development. It was also evident, as revealed by ID expression patterns, that despite the overall aberrant spatial differentiation patterns displayed by these tumors, some tissue-tissue interactions reminiscent of those observed during normal embryogenesis are retained. Thus, adult male GCTs offer a unique system in which the role of genes such as the IDs can be studied in human embryogenesis.

Expression of the aromatase cytochrome P450 encoding gene in cattle and sheep

During this report the tissue-specific expression and promoter usage of the aromatase cytochrome P450 encoding gene, Cyp19, are compared between cattle and sheep. In addition, data will be presented on the identification of cis-acting regulatory sequences located in the bovine placenta-specific promoter 1.1. In cattle and sheep Cyp19 is mainly expressed in the foetal placental layer and ovarian granulosa cells but also in other organs as brain or testis. Differently spliced transcripts of Cyp19 which include an invariable coding region but a variable 5'-untranslated region could be detected in tissues of both species. However, in contrast to ovary and brain which express homologous transcript variants, different transcripts are present in placentae suggesting that also different placenta-specific promoter regions are active in cattle and sheep. The analysis of the chromatin structure of the main placental promoter 1.1 in different bovine tissues revealed that hypomethylation and the occurrence of DNaseI hypersensitive sites (HS) within this region are associated with promoter activity. Active regulatory elements were identified in reporter gene studies in JEG-3 choriocarcinoma cells. The co-localisation of an E-box element within one of the placenta-specific HS suggests that this element is important for Cyp19 expression in the bovine placenta.

A new panoptic stain for developmental biology--the mouse placenta paradigm

A panoptic histological stain, PHTA, is introduced for routine use in developmental biology. The protocol is based on three dyes, hematoxylin and, after tannic acid treatment, phloxine B and azure B. It was optimized for differential staining of extracellular matrix, cytoplasm and chromatin. The method is quick, inexpensive and well-reproducible. The mouse placenta is used here to demonstrate the excellent suitability of PHTA for routine morphology.

Plasma membrane-associated pY397FAK is a marker of cytotrophoblast invasion in vivo and in vitro

During human pregnancy specialized placental cells of fetal origin, termed cytotrophoblasts, invade the uterus and its blood vessels. This tumor-like process anchors the conceptus to the mother and diverts the flow of uterine blood to the placenta. Previously, we showed that the expression of molecules with important functional roles, including a number of extracellular matrix integrin receptors, is precisely modulated during cytotrophoblast invasion in situ. Here we exploited this observation to study the role of the focal adhesion kinase (FAK), which transduces signals from the extracellular matrix and recruits additional signaling proteins to focal adhesions. Immunolocalization studies on tissue sections showed that FAK is expressed by cytotrophoblasts in all stages of differentiation. Because extracellular matrix-induced integrin clustering results in FAK (auto)phosphorylation on tyrosine 397 (Y397FAK), we also localized this form of the molecule. Immunolocalization experiments detected Y397FAK in a subset of cytotrophoblasts near the surface of the uterine wall. To assess the functional relevance of this observation, we used an adenovirus strategy to inhibit cytotrophoblast expression of FAK as the cells differentiated along the invasive pathway in vitro. Compared to control cells transduced with a wild-type virus, cytotrophoblasts that expressed antisense FAK exhibited a striking reduction in their ability to invade an extracellular matrix substrate. When cytotrophoblast differentiation was compromised (hypoxia in vitro, preeclampsia in vivo), Y397FAK levels associated with the plasma membrane were strikingly lower, although total FAK levels did not change. Together our results suggest that (auto)phosphorylation of Y397 on FAK is a critical component of the signaling pathway that mediates cytotrophoblast migration/invasion.

Human cytotrophoblast expression of the von Hippel-Lindau protein is downregulated during uterine invasion in situ and upregulated by hypoxia in vitro

The von Hippel-Lindau tumor-suppressor protein (pVHL) regulates the stability of HIF1 alpha and HIF2 alpha and thus is pivotal in cellular responses to changes in oxygen tension. Paradoxically, human cytotrophoblasts proliferate under hypoxic conditions comparable to those measured in the early gestation placenta (2% O(2)), but differentiate into tumorlike invasive cells under well-oxygenated conditions such as those found in the uterus. We sought to explain this phenomenon in terms of pVHL expression. In situ, pVHL immunolocalized to villous cytotrophoblast stem cells, and expression was enhanced at sites of cell column initiation; in both of these relatively hypoxic locations, cytoplasmic staining for HIF2 alpha was also detected. As cytotrophoblasts attached to and invaded the uterus, which results in their increased exposure to oxygen, pVHL staining was abruptly downregulated concordant with localization of HIF2 alpha to the nucleus. In vitro, hypoxia (2% O(2)) upregulated cytotrophoblast pVHL expression together with HIF2 alpha, which localized to the cytoplasm; culture under well-oxygenated conditions greatly reduced levels of both molecules. These results, together with the placental defects previously observed in VHL(-/-) mice, suggest that pVHL is a component of the mechanism that transduces local differences in oxygen tension at the maternal-fetal interface to changes in the biological behavior of cytotrophoblasts. Furthermore, these data provide the first example of oxygen-dependent changes in pVHL abundance.

Genes governing placental development

The placenta is essential for fetal growth because it promotes the delivery of nutrients and oxygen from the maternal circulation. In mice, many gene mutations disrupt formation of the placenta, with specific effects at different times and on different components. Studies of these mutations are beginning to provide insights into both the molecular pathways required for formation of different placental substructures and the nature of intercellular interactions, between trophoblast, mesenchymal and vascular components, that regulate placental development. Conserved gene expression patterns in humans should enable the elucidation of the molecular basis of human placental dysfunction.

Key regulatory transcription factors involved in placental trophoblast development--a review

Specification of the trophoblast cell lineage comprising the outermost epithelial cell layer of the blastocyst occurs early in development and is a prerequisite for implantation of the embryo and subsequent formation of the placenta, a multifunctional organ which is indispensable for the proper development of the fetus. Trophoblast stem cells of the placenta give rise to distinct highly differentiated trophoblast subtypes which build the functional units of the organ. These specialized cells assure anchorage of the embryo to the mother, establishing a vascular connection transporting nutrients and gases and expression of hormones that are required for the successful progression of pregnancy. Developmental processes of the trophoblast occur in a spatially and temporally highly organized manner. Despite these facts, little is known on the key regulatory factors which commit and differentiate trophoblast cells in humans. Recent studies in mice, however, provided evidence that various cell-type specific transcription factors play crucial roles in the developmental programme of the trophoblast. In this review we will focus on the function of these major regulatory factors in murine trophoblast/placental development and discuss the potential role of their homologues in the human system.

Genes regulating embryonic and fetal survival

Embryonic mortality in both farm animals and humans occurs most frequently during the first few weeks after conception. It can be attributed to abnormalities in the earliest developmental processes during embryogenesis that include implantation, maternal recognition of pregnancy, and formation of the placenta and cardiovascular system. The molecular mechanisms that are essential for all of these early processes are being elucidated at a rapid pace using transgenic and gene knockout approaches in mice. Two important general conclusions have emerged from this work. First, placental defects can occur by a number of different molecular mechanisms and can result from defects in the development or function of its trophoblast, mesenchymal or vascular components. Second, placental and cardiovascular functions are intimately linked. Cells of the placenta, for example, produce hormones that have profound effects on maternal and fetal cardiac and vascular function. In addition, development of the two is linked mechanistically through the use of some genes that are essential for development of both. Understanding the molecular basis of these processes should help to address the major limits to the success of embryo transfer, IVF and embryo cloning practices in livestock species.

Placental cell fates are regulated in vivo by HIF-mediated hypoxia responses

Placental development is profoundly influenced by oxygen (O2) tension. Human cytotrophoblasts proliferate in vitro under low O2 conditions but differentiate at higher O2 levels, mimicking the developmental transition they undergo as they invade the placental bed to establish the maternal–fetal circulation in vivo. Hypoxia-inducible factor-1 (HIF-1), consisting of HIF-1α and ARNT subunits, activates many genes involved in the cellular and organismal response to O2deprivation. Analysis of Arnt−/− placentas reveals an aberrant cellular architecture due to altered cell fate determination of Arnt−/− trophoblasts. Specifically, Arnt−/− placentas show greatly reduced labyrinthine and spongiotrophoblast layers, and increased numbers of giant cells. We further show that hypoxia promotes the in vitro differentiation of trophoblast stem cells into spongiotrophoblasts as opposed to giant cells. Our results clearly establish that O2 levels regulate cell fate determination in vivo and that HIF is essential for mammalian placentation. The unique placental phenotype of Arnt−/− animals also provides an important tool for studying the disease of preeclampsia. Interestingly, aggregation of Arnt−/− embryonic stem (ES) cells with tetraploid wild-type embryos rescues their placental defects; however, these embryos still die from yolk sac vascular and cardiac defects.

Regulation of a multigenic invasion programme by the transcription factor, AP-1: re-expression of a down-regulated gene, TSC-36, inhibits invasion

The transcription factor AP-1 (activator protein-1) is required for transformation by many oncogenes, which function upstream of it in the growth factor-ras signal transduction pathway. Previously, we proposed that one role of AP-1 in transformation is to regulate the expression of a multigenic invasion programme. As a test of this proposal we sought to identify AP-1 regulated genes based upon their differential expression in 208F rat fibroblasts transformed by FBR-v-fos (FBR), and to determine if they functioned in the invasion programme. Subtracted cDNA libraries specific for up- or down-regulated genes in FBRs compared to 208Fs were constructed and analysed. Northern analysis revealed that the cDNAs in both libraries represented differentially expressed genes. Nucleic acid sequence analysis of randomly selected cDNA clones from each library coupled with searches of nucleic acid and amino acid sequence databases determined that many of the cDNAs represented proteins that function in various aspects of the invasion process. Functional analysis of one the down-regulated genes, TSC-36/follistatin-related protein (TSC-36/Frp), which has not previously been associated with invasion, demonstrated that its expression in FBRs inhibited in vitro invasion. These results support the proposal that AP-1 in transformed cells regulates a multigenic invasion programme.

Placental expression and chromosomal localization of the human Gcm 1 gene

Although gcm was first recognized for its role in specifying glial cell fate in Drosophila melanogaster, its mammalian counterparts are expressed predominantly in non-neural tissues. Here we demonstrate expression of the mouse and human GCM 1 proteins in placenta. We have prepared a highly specific antibody that recognizes the GCM 1 protein and have used it to assess the temporal and spatial expression profile of the protein. In both mouse and human placenta, the protein is associated with cells that are involved with exchange between maternal and fetal blood supplies: the labyrinthine cells of the mouse placenta and the syncytio- and cytotrophoblasts of the human placenta. Using the full-length hGcm 1 cDNA as a probe, we have mapped the gene on human chromosome 6p12 by fluorescent in situ hybridization.