Polycomb repressive complexes 1 and 2 are each essential for maintenance of X inactivation in extra-embryonic lineages.

In female mammals, one of the two X chromosomes becomes inactivated during development by X-chromosome inactivation (XCI). Although Polycomb repressive complex (PRC) 1 and PRC2 have both been implicated in gene silencing, their exact roles in XCI during in vivo development have remained elusive. To this end, we have studied mouse embryos lacking either PRC1 or PRC2. Here we demonstrate that the loss of either PRC has a substantial impact on maintenance of gene silencing on the inactive X chromosome (Xi) in extra-embryonic tissues, with overlapping yet different genes affected, indicating potentially independent roles of the two complexes. Importantly, a lack of PRC1 does not affect PRC2/H3K27me3 accumulation and a lack of PRC2 does not impact PRC1/H2AK119ub1 accumulation on the Xi. Thus PRC1 and PRC2 contribute independently to the maintenance of XCI in early post-implantation extra-embryonic lineages, revealing that both Polycomb complexes can be directly involved and differently deployed in XCI.

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.

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.

Allantois and placenta as developmental sources of hematopoietic stem cells.

While the aortic region, the para-aortic splanchnopleura/aorta-gonads-mesonephros (P-Sp/AGM) is currently considered as the source of definitive hematopoietic stem cells during development, the mouse placenta has been found to generate large numbers of these cells and to remain functional in this respect for a longer period than the P-Sp/AGM. The fetal component, which derives from the fused allantois and chorion, is responsible for this activity. We and others have shown that the pre-fusion allantois (before the stage of 6 pairs of somites) is able to yield clonogenic progenitors, provided that it is pre-cultured in toto before it is dissociated into single cells and seeded in semi-solid medium. Thus placental hematopoiesis can be concluded to derive from intrinsic precursors. It is similar in this regard to the yolk sac which both produces hematopoietic progenitors and supports their multiplication and differentiation. Hematopoietic activity, detected by in vitro colony assays, has also been recently uncovered in the human placenta. According to the data available, this newly identified source probably provides a large number of HSC during development and must play a foremost role in founding the definitive hematopoietic system.

Neuropilin 1 and CD25 co-regulation during early murine thymic differentiation.

Neuropilin 1 (NP1) is a receptor for both semaphorin and vascular endothelial growth factor expressed by subpopulations of neuronal and endothelial cells. In the immune system, NP1 is present on dendritic and regulatory T cells. Here, we show that NP1 is expressed in the murine thymus, starting on day 12.5 of gestation. In the adult, NP1 is mainly expressed by CD4(-)CD8(-) double negative cells, CD4+CD8+ double positive cells, and CD4+CD25+ regulatory T cells but barely detected in single CD4+ and CD8+ positive thymocytes. Within the CD4(-)CD8(-)CD3(-) (triple-negative, TN) immature cells, NP1 expression starts in TN3 (CD44(-)CD25+) and increases in TN4 (CD44(-)CD25(-)) cells. In order to study the role of NP1 in thymocyte differentiation, we generated mice in which the np1 gene is selectively disrupted in the T-cell lineage. The mutant mice display normal thymocyte, peripheral, conventional and CD4+CD25+Foxp3+ regulatory T-cell populations. However, we observe a down-regulation of the CD25 expression between the TN3 and TN4 stages that is (i) correlated to increased expression of NP1 in control mice and (ii) altered in mutant mice, suggesting that NP1 is co-regulated with CD25 during early immature thymocyte differentiation.

Hematopoietic potential of the pre-fusion allantois.

We previously showed that the fetal component of the placenta has a vigorous hematopoietic activity. Whether this organ is an environmental niche where hematopoietic stem cells (HSC) proliferate and become committed to various lineages, or whether it is also a site for HSC emergence, was left open. This issue can be addressed only if the components that will give rise to the placenta are tested prior to vascularization. The fetal part of the placenta forms through the fusion of the allantois and the chorionic plate around the stage of 7 somite pairs. The allantois, a mesodermal rudiment that provides fetal blood vessels to the placenta, was retrieved before fusion. We found in this rudiment expression of CD41, a known marker of early embryonic hematopoietic progenitors. c-Kit encoding a progenitor specific receptor was also expressed. Significantly, as early as the 1-2 somite stage, the allantois yielded erythroid, myeloid and multipotent clonogenic progenitors, when pre-cultured in toto prior to seeding in a semisolid medium. These results provide evidence that the allantois has hematopoietic potential per se. Whether this potential also involves the ability to produce HSC is still to be determined.

Regulatory T cells in the establishment and maintenance of self-tolerance: role of the thymic epithelium.

The thymus constitutes the microenvironment for T lymphocyte differentiation and acquisition of self-tolerance. Aiming to specify the contributions of the two essential parts of the thymus, namely hemopoietic and epithelial, we have devised experimental models in birds and mice. Chimeric thymuses, xenogeneic in birds and allogeneic in mice, were constructed early in development. In both models we could demonstrate a critical role of the epithelial component of the thymic stroma in induction and maintenance of self-tolerance. These experiments showed that suppression mechanisms are also implicated in these events, strongly suggesting the existence of regulatory T cells in both models. Before these experiments the control of self-tolerance was usually attributed to suppressive cells. However, as the cell phenotypes were not identified, the role of these cells was disregarded. Numerous studies since our investigations argue in favour of regulatory mechanisms. The work we initiated several years ago represents a contribution to our understanding of the two linked and opposite aspects of immune-responded control, namely self-tolerance and autoimmunity.

(alpha)IIb Integrin, a novel marker for hemopoietic progenitor cells.

Integrin (alpha)IIb(beta)3 (abbreviated as (alpha)IIb), also known as GPIIb-IIIa or CD41/CD61, is a cell adhesion molecule expressed on cells belonging to the megakaryocytic lineage. Aiming to identify new markers of hemopoietic progenitor cells (HPC), we undertook a developmental study of this molecule since it remains controversial if this integrin is expressed by various progenitors. We reported the expression pattern of two integrins, in both of which the beta3 chain is present, respectively associated with alphaV and alpha IIb in the chick embryo. While at E3.5, the earliest time at which these integrins can be detected, (alpha)V(beta)3 becomes expressed by endothelial cells in the aorta (and only in the aorta), (alpha)IIb(beta)3 becomes detected in the well-defined intra-aortic clusters made up of HPC. The latter were found to be multilineage progenitors when sorted for (alpha)IIb expression and analyzed by means of clonogenic assays. In mice also, (alpha)IIb is expressed in the intra-embryonic site of HPC generation, the intra-arterial clusters in the embryo proper, as well as in sites where HPC migrate. Finally we provided the first evidence in two species that multipotent HPC expressing (alpha)IIb are able to differentiate not only into cells of the erythroid and myeloid lineages but also into lymphocytes. These cell populations actually coexpress (alpha)IIb and c-Kit. These data establish (alpha)IIb as a novel marker for HPC, which appears at very early stages in the embryo. Capitalizing on this finding, other investigators confirmed it and suggested that (alpha)IIb plays a role in regulating hematopoietic development.

Expression of alphaVbeta3 integrin in the chick embryo aortic endothelium.

The integrin chain alphaV, expressed in association with beta3, by cells of the megakaryocytic/thrombocytic and endothelial lineages is thought to play an important role in angiogenesis. alphaVbeta3 expression by endothelial cells is not constitutive but induced by various stimuli in avian and human models. Here the developmental pattern of alphaVbeta3 expression was analysed in the chick embryo by immunocytochemistry, using a specific monoclonal antibody. On day 2 of development alphaVbeta3 expression was restricted to rare cells in the blood stream, in the embryo proper and in the yolk sac blood islands. AlphaVbeta3 expression by endothelial cells became detectable on day 3 and was restricted to the dorsal aorta. Interestingly it was absent from the intra-aortic hemopoietic clusters (E3.5) which, as we have showed previously, express the alphaIIbbeta3 integrin and display progenitor potentialities. However the endothelium underlying intra-embryonic hemopoietic clusters expressed this integrin. In contrast E6-7 para-aortic hemopoietic foci contained numerous alphaVbeta3 positive cells. Both alphaVbeta3 and alphaIIbbeta3 were expressed in these latter hemopoietic sites, while alphaVbeta3 was still selectively expressed by the aortic endothelium until E6. Thereafter, at E7 the pulmonary artery also expressed it. Since alphaIIbbeta3 is expressed by avian and murine multilineage hemopoietic progenitors, we then studied the hemopoietic potentialities of alphaVbeta3/alphaIIbeta3 double positive cells from embryonic bone marrow differentiating in vitro in erythro-myeloid conditions. Thrombocytic, erythroid and myeloid progenitor potentialities were found within the cell population expressing both beta3 integrins.

AlphaIIb integrin expression during development of the murine hemopoietic system.

Integrin alphaIIb is a cell adhesion molecule expressed in association with beta3 by cells of the megakaryocytic lineage, from committed progenitors to platelets. While it is clear that lymphohemopoietic cells differentiating along other lineages do not express this molecule, it has been questioned whether mammalian hemopoietic stem cells (HSC) and various progenitor cells express it. In this study, we detected alphaIIb expression in midgestation embryo in sites of HSC generation, such as the yolk sac blood islands and the hemopoietic clusters lining the walls of the major arteries, and in sites of HSC migration, such as the fetal liver. Since c-Kit, which plays an essential role in the early stages of hemopoiesis, is expressed by HSC, we studied the expression of the alphaIIb antigen in the c-Kit-positive population from fetal liver and adult bone marrow differentiating in vitro and in vivo into erythromyeloid and lymphocyte lineages. Erythroid and myeloid progenitor activities were found in vitro in the c-Kit(+)alphaIIb(+) cell populations from both origins. On the other hand, a T cell developmental potential has never been considered for c-Kit(+)alphaIIb(+) progenitors, except in the avian model. Using organ cultures of embryonic thymus followed by grafting into athymic nude recipients, we demonstrate herein that populations from murine fetal liver and adult bone marrow contain T lymphocyte progenitors. Migration and maturation of T cells occurred, as shown by the development of both CD4(+)CD8- and CD4-CD8(+) peripheral T cells. Multilineage differentiation, including the B lymphoid lineage, of c-Kit(+)alphaIIb(+) progenitor cells was also shown in vivo in an assay using lethally irradiated congenic recipients. Taken together, these data demonstrate that murine c-Kit(+)alphaIIb(+) progenitor cells have several lineage potentialities since erythroid, myeloid, and lymphoid lineages can be generated.