Epigenetic control of normal placental (and foetal) development and stem cell potency
The most potent stem cells are derived from the early embryo, or after reprogramming of adult cells to reflect such an early developmental stage. Importantly, the differentiative capacity of stem cells is dictated by events that occur during these early stages of development and that impose strict fate barriers onto cells of the early embryo and stem cells derived from them. Thus, only a few days after fertilization, at the blastocyst stage, an irreversible developmental decision is made that separates cells of the future placenta (so-called trophoblast cells) from those that will give rise to the embryo (Fig. 1). The emergence of trophoblast cells as the earliest differentiated cell type after fertilization reflects their imminent importance for embryo implantation and nutrition throughout intrauterine development.
Figure 1. (Click to enlarge)
The first definitive cell lineage decision occurs at the blastocyst stage between the inner cell mass (ICM) and the outer trophectoderm (TE). The TE establishes the trophoblast cell lineage that exclusively contributes to formation of the placenta. Distinct types of stem cells can be derived from both cell lineages, embryonic stem (ES) cells from the ICM and trophoblast stem (TS) cell from the TE. These stem cells cannot normally inter-convert or "transdifferentiate", such that their developmental potential is restricted to the embryo and placenta, respectively. This cell fate fixation is achieved by an epigenetic barrier that sets up lineage-specific transcriptional circuits.
Epigenetic lineage barriers
We are interested to understand how these very tight lineage barriers are established, and how they may be overcome to widen the developmental potency of stem cells. The basis of cell fate commitment lies in the establishment of a cellular memory that is achieved by epigenetic modifications and defines fate restriction of cell lineages and their derived stem cells. We are exploring epigenetic modifications that contribute to the stable maintenance of cell lineage identity with focus on DNA methylation and poly(ADP-ribosyl)ation on the candidate level as well as in global epigenomic and proteomic approaches.
Transcriptional networks and their regulation that determine self-renewal and differentiation within the trophoblast lineage
The first cell lineages give rise to distinct types of stem cells, most notably embryonic (ES) and trophoblast (TS) stem cells (Fig. 1). Stem cell self-renewal depends on a variety of transcription factors, and we are exploring the transcriptional networks within the trophoblast lineage genome-wide using state-of-the-art high-throughput sequencing approaches, and link these with the stem cell type-specific methylomes to delineate their epigenetic regulation (Fig. 2). Together with Dr Simon Cook at the Babraham Institute, we are also investigating signaling pathways that lead to the setting up of the distinct transcriptional networks, and their epigenetic regulation. Further, we are collaborating to extrapolate these insights into the human situation to gain insights into the regulation of earliest developmental processes that ensure normal progression of pregnancy. These approaches will advance our knowledge of early development and reproduction, as well as control of stem cell potency and differentiation with impact on their application in regenerative medicine.
Figure 2. (Click to enlarge)
Transcription factor networks that determine the establishment of the trophoblast lineage in early development and that dictate self-renewal and differentiation of trophoblast stem (TS) cells in vitro. Immunostaining shows the co-expression of two key factors, Tcfap2c (red) and Eomes (green) in a TS cell colony within a partially differentiated trophoblast population.
Establishment of a functional placenta
Trophoblast (stem) cell differentiation gives rise to a variety of highly specialized placental cell types. One of them, called trophoblast giant cells in mice, starts to penetrate deeply into the maternal uterus where they make contact to maternal arteries (Fig. 3). This invasive process is essential for the feto-maternal circulatory interface of the placenta to develop. It is through this close interaction of fetal and maternal blood circulations that nutrition of the embryo is ensured.
Figure 3. (Click to enlarge)
(A) An early mouse embryo (E7.5-E8.5) within its implantation site in the uterus, stained for a giant cell marker gene. Giant cells (GCs, stained in dark purple) line the entire implantation site and invade into the surrounding maternal uterine tissue in the area indicated by the dashed box.
(B) Close-up view of the invasion zone. Arrows indicate the direction of giant cell invasion.
(C) Trophoblast tissue cultured in vitro and stained for a microtubule marker (red) and the nucleus (blue). Note the size of the giant cell (arrow) in comparison to surrounding cells (arrowheads).
Another area of interest of our laboratory is to elucidate developmental cues that determine differentiation of the trophoblast cell lineage and its maintenance thereafter. We focus in particular on factors governing the specialized and strictly regulated process of trophoblast giant cell differentiation and invasion. Gene expression in these cells is fine-tuned and depends highly on depth of invasion. We are investigating the function of some of these genes, in particular two proteases, for their role in trophoblast giant cell-mediated remodeling of the maternal vasculature and the resulting effect on fetal growth.
As trophoblast cells are essential for connecting the embryo to the maternal blood circulation and thus to the supply of nutrients and gases, it is clear that this process is very sensitive to deregulations, including by epigenome disruptors, that have enormous impact on fetal development. By understanding, modeling and being able to manipulate key differentiation pathways we will learn about the mechanisms that ensure a normal progression of pregnancy, healthy babies and mothers as well as health during later adult life.
Updated 23 August, 2011