Trophoblast differentiation in vitro: establishment and characterisation of a serum-free culture model for murine secondary trophoblast giant cells

The trophoblast giant cells of cricetid rodents

Giant cells are a prominent feature of placentation in cricetid rodents. Once thought to be maternal in origin, they are now known to be trophoblast giant cells (TGCs). The large size of cricetid TGCs and their nuclei reflects a high degree of polyploidy. While some TGCs are found at fixed locations, others migrate throughout the placenta and deep into the uterus where they sometimes survive postpartum. Herein, we review the distribution of TGCs in the placenta of cricetids, including our own data from the New World subfamily Sigmodontinae, and attempt a comparison between the TGCs of cricetid and murid rodents. In both families, parietal TGCs are found in the parietal yolk sac and as a layer between the junctional zone and decidua. In cricetids alone, large numbers of TGCs, likely from the same lineage, accumulate at the edge of the placental disk. Common to murids and cricetids is a haemotrichorial placental barrier where the maternal-facing layer consists of cytotrophoblasts characterized as sinusoidal TGCs. The maternal channels of the labyrinth are supplied by trophoblast-lined canals. Whereas in the mouse these are lined largely by canal TGCs, in cricetids canal TGCs are interspersed with syncytiotrophoblast. Transformation of the uterine spiral arteries occurs in both murids and cricetids and spiral artery TGCs line segments of the arteries that have lost their endothelium and smooth muscle. Since polyploidization of TGCs can amplify selective genomic regions required for specific functions, we argue that the TGCs of cricetids deserve further study and suggest avenues for future research.

Transcriptional Analysis by Nascent RNA FISH of In Vivo Trophoblast Giant Cells or In Vitro Short-term Cultures of Ectoplacental Cone Explants

The placenta derives from one extra-embryonic lineage, the trophectoderm. In the peri-implantation murine blastocyst, mural trophectoderm cells differentiate into primary trophoblast giant cells (TGCs) while the polar trophectoderm overlying the inner cell mass continues to proliferate later differentiating into secondary TGCs. TGCs play a key role in developing placenta and are essential for a successful pregnancy. Investigation of transcriptional regulation of specific genes during post-implantation development can give insights into TGCs development. Cells of the ectoplacental cone (EPC) from embryos at 7-7.5 days of gestation (E7-7.5), derived from the polar trophectoderm, differentiate into secondary TGCs1. TGCs can be studied in situ, on cryostat sections of embryos at E7 although the number of TGCs is very low at this stage. An alternative means of analyzing secondary TGCs is to use short-term cultures of individual EPCs from E7 embryos. We propose a technique to investigate the transcriptional status of genes of interest both in vivo and in vitro at the single-cell level using fluorescent in situ hybridization (RNA FISH) to visualize nascent transcripts. This technique provides a direct readout of gene expression and enables assessment of the chromosomal status of TGCs, which are large endoreplicating cells. Indeed, a key feature of terminal differentiation of TGCs is that they exit the cell cycle and undergo multiple rounds of endoreplication.This approach can be applied to detect expression of any gene expressed from autosomes and/or sex chromosomes and can provide important information into developmental mechanisms as well as placental diseases.

Maternal nutrition modifies trophoblast giant cell phenotype and fetal growth in mice

Mammalian placentation is dependent upon the action of trophoblast cells at the time of implantation. Appropriate fetal growth, regulated by maternal nutrition and nutrient transport across the placenta, is a critical factor for adult offspring long-term health. We have demonstrated that a mouse maternal low-protein diet (LPD) fed exclusively during preimplantation development (Emb-LPD) increases offspring growth but programmes adult cardiovascular and metabolic disease. In this study, we investigate the impact of maternal nutrition on post-implantation trophoblast phenotype and fetal growth. Ectoplacental cone explants were isolated at day 8 of gestation from female mice fed either normal protein diet (NPD: 18% casein), LPD (9% casein) or Emb-LPD and cultured in vitro. We observed enhanced spreading and cell division within proliferative and secondary trophoblast giant cells (TGCs) emerging from explants isolated from LPD-fed females when compared with NPD and Emb-LPD explants after 24 and 48 h. Moreover, both LPD and Emb-LPD explants showed substantial expansion of TGC area during 24–48 h, not observed in NPD. No difference in invasive capacity was observed between treatments using Matrigel transwell migration assays. At day 17 of gestation, LPD- and Emb-LPD-fed conceptuses displayed smaller placentas and larger fetuses respectively, resulting in increased fetal:placental ratios in both groups compared with NPD conceptuses. Analysis of placental and yolk sac nutrient signalling within the mammalian target of rapamycin complex 1 pathway revealed similar levels of total and phosphorylated downstream targets across groups. These data demonstrate that early post-implantation embryos modify trophoblast phenotype to regulate fetal growth under conditions of poor maternal nutrition.

Unusual chromatin status and organization of the inactive X chromosome in murine trophoblast giant cells

Mammalian X-chromosome inactivation (XCI) enables dosage compensation between XX females and XY males. It is an essential process and its absence in XX individuals results in early lethality due primarily to extra-embryonic defects. This sensitivity to X-linked gene dosage in extra-embryonic tissues is difficult to reconcile with the reported tendency of escape from XCI in these tissues. The precise transcriptional status of the inactive X chromosome in different lineages has mainly been examined using transgenes or in in vitro differentiated stem cells and the degree to which endogenous X-linked genes are silenced in embryonic and extra-embryonic lineages during early postimplantation stages is unclear. Here we investigate the precise temporal and lineage-specific X-inactivation status of several genes in postimplantation mouse embryos. We find stable gene silencing in most lineages, with significant levels of escape from XCI mainly in one extra-embryonic cell type: trophoblast giant cells (TGCs). To investigate the basis of this epigenetic instability, we examined the chromatin structure and organization of the inactive X chromosome in TGCs obtained from ectoplacental cone explants. We find that the Xist RNA-coated X chromosome has a highly unusual chromatin content in TGCs, presenting both heterochromatic marks such as H3K27me3 and euchromatic marks such as histone H4 acetylation and H3K4 methylation. Strikingly, Xist RNA does not form an overt silent nuclear compartment or Cot1 hole in these cells. This unusual combination of silent and active features is likely to reflect, and might underlie, the partial activity of the X chromosome in TGCs.

Bovine trophoblastic cell differentiation and binucleation involves enhanced endogenous retrovirus element expression

BACKGROUND

Endogenous retrovirus (ERV) envelope (env) genes are involved in the differentiation of trophoblastic cells in humans and mice. However, there is limited information about their roles in ruminant trophoblastic cells. Thus, we attempted to explore the possible roles of ERV elements in the binucleation of bovine trophoblastic cells using in vitro bovine trophoblastic (BT) cell lines.

METHODS

In this study, blastocysts and elongated embryos were obtained from Japanese Black cows, and endometrial and fetal membrane tissues were collected from day 17 to 37 of gestation. The gene expression levels of four ERV elements, bERVE (bovine endogenous retrovirus envelope element-like transcript) -A, bERVE-B, BERV (bovine endogenous retrovirus) -K1 env, and BERV-K2 env, were analyzed in the fetal and endometrial tissue and cultured BT cell lines using quantitative RT-PCR. On-Matrigel gel and on-collagen gel culturing were used to induce binucleate cell (BNC) formation in the BT cell lines. How the culture conditions affected the expression of BNC-specific genes and ERV elements was examined by quantitative RT-PCR and immunocytochemistry.

RESULTS

bERVE-A, bERVE-B, BERV-K1 env, and BERV-K2 env were expressed in almost all BT cell lines; however, only bERVE-A and BERV-K1 env were detected in trophoblastic tissues during the peri-implantation period. In the on-Matrigel cultures, the expression levels of BNC-specific genes and molecules were enhanced in the BT cells. The expression levels of bERVE-A and BERV-K1 env were also increased in the BT cells during on-Matrigel culturing. The BT cell expression levels of these ERV elements were consistent with those of BNC-specific genes during on-Matrigel culturing (P < 0.01).

CONCLUSIONS

These results suggest that bERVE-A and BERV-K1 env are involved in the expression of BNC-specific genes and the progression of bovine trophoblastic cell binucleation, as their expression levels increased during periods of increased BNC-specific molecule expression, which is strongly suggestive of the development of BNC from mononucleate trophoblastic cells. The on-Matrigel culture system is a convenient in vitro tool for studying bovine trophoblastic cell lineages.

An imprinted GFP insertion reveals long-range epigenetic regulation in embryonic lineages

Imprinted genes are often grouped in clusters at defined chromosomal locations. Long-range regulatory effects are implicated in the control of imprinting and these could be co-opted in the emergence of novel imprinted genes during evolution. We present a detailed analysis of a novel imprinted GFP mouse line. Tel7KI is a new insertion allele near the Ins2 locus within a cluster of imprinted genes on distal mouse Chr7. The GFP reporter becomes regulated by the host domain in two notable fashions. First, transcription of GFP is imprinted and active exclusively from the maternally inherited allele in the embryo. Second, the expressed maternal allele is subject to position effects reflecting a distinct pattern of expression. The GFP reporter acquires silencing DNA methylation marks on the paternal allele after fertilization. This imprinting is not acquired in the placenta, where GFP is active from both parental alleles, demonstrating key epigenetic differences between embryonic and extraembryonic lineages. Our analysis shows that imprinted clusters can provide environments conducive to the acquisition of imprinting upon novel inserted transcriptional units. The Tel7KI line offers new powerful avenues to explore both genetic and environmental factors implicated in the acquisition and maintenance of imprinted transcription in mammals.

Genes and signals regulating murine trophoblast cell development

A fundamental step in embryonic development is cell differentiation whereby highly specialised cell types are developed from a single undifferentiated, fertilised egg. One of the earliest lineages to form in the mammalian conceptus is the trophoblast, which contributes exclusively to the extraembryonic structures that form the placenta. Trophoblast giant cells (TGCs) in the rodent placenta form the outermost layer of the extraembryonic compartment, establish direct contact with maternal cells, and produce a number of pregnancy-specific cytokine hormones. Giant cells differentiate from proliferative trophoblasts as they exit the cell cycle and enter a genome-amplifying endocycle. Normal differentiation of secondary TGCs is a critical step toward the formation of the placenta and normal embryonic development. Trophoblast development is also of particular interest to the developmental biologist and immunobiologist, as these cells constitute the immediate cellular boundary between the embryonic and maternal tissues. Abnormalities in the development of secondary TGCs results in severe malfunction of the placenta. Herein we review new information that has been accumulated recently regarding the molecular and cellular regulation of trophoblast and placenta development. In particular, we discuss the molecular aspects of murine TGC differentiation. We also focus on the role of growth and transcription factors in TGC development.

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.

PTHrP induces changes in cell cytoskeleton and E-cadherin and regulates Eph/Ephrin kinases and RhoGTPases in murine secondary trophoblast cells

The differentiation of murine trophoblast giant cells (TGCs) is well characterised at the molecular level and, to some extent, the cellular level. Currently, there is a rudimentary understanding about factors regulating the cellular differentiation of secondary TGCs. Using day 8.5 p.c.-ectoplacental cone (EPC) explant in serum-free culture, we have found parathyroid hormone-related protein (PTHrP) to regulate cellular changes during TGC differentiation. PTHrP greatly stimulated the formation and organisation of actin stress fibres and actin expression in trophoblast outgrowth. This coincided with changing cell shape into a flattened/fibroblastic morphology, suppression of E-cadherin expression, and increased cell spreading in culture. PTHrP also increased the nuclear staining of beta-catenin and, similar to activator protein-2gamma (AP-2gamma), showed microtubule-dependent nuclear localisation in vitro. These cellular and behavioural changes correlated with changes in the expression of RhoGTPases and in both expression and phosphorylation of Eph/Ephrin kinases. The effects of PTHrP on trophoblast cellular differentiation were abolished after blocking its action. In conclusion, PTHrP provides an excellent example of the extrinsic factors that, through their network of activities, plays an important role in cellular differentiation of secondary TGCs.

PTHrP promotes murine secondary trophoblast giant cell differentiation through induction of endocycle, upregulation of giant-cell-promoting transcription factors and suppression of other trophoblast cell types

The murine trophoblast cell lineage represents an intriguing experimental cell model as it is composed of four trophoblast stem (TS)-derived cell types: trophoblast giant cells (TGCs), spongiotrophoblast, syncytotrophoblast, and glycogen trophoblast cells. To investigate the role of parathyroid hormone-related protein (PTHrP) in TGC differentiation, we analyzed the effect of exogenous PTHrP on secondary TGCs of day 8.5 p.c. ectoplacental cone explant culture. Secondary TGCs expressed PTHrP and PTHR1 receptor in vivo and in vitro. TGCs treated with PTHrP had reduced proliferation and decreased apoptosis starting from day 2 in culture, and enhanced properties of giant cell differentiation: increased DNA synthesis, number of cells with giant nuclei and expression of placental lactogen-II (PL-II). The induction of TGC formation by PTHrP correlated with downregulation of cyclin B1 and mSNA expression, but upregulation of cyclin D1, thus allowing mitotic-endocycle transition. Moreover, PTHrP treatment influenced TGC differentiation by inducing the expression of transcription factors known to stimulate giant cell formation: Stra13 and AP-2gamma, and inhibiting the formation of other trophoblast cell types by suppressing trophoblast progenitors and spongiotrophoblast-promoting factors, Eomes, Mash-2, and mSNA. Taken together with the spatial and temporal patterns of TGC formation and PTHrP synthesis in vivo, these findings indicate an important role for PTHrP in the differentiation of secondary TGCs during placentation.