ELF5-controlled transcriptional networks define stem cell potency and differentiation in the trophoblast compartment

Lay summary (4000 chrs) Stem cells are cells that have the potential to reproduce themselves ('self-renew') as well as to form various terminally differentiated cell types of the body. For this reason stem cells are of high interest for biomedical research and regenerative medicine. We are particularly interested in stem cells that contribute exclusively to tissues of the placenta (so-called trophoblast stem cells). These stem cells are of enormous value to study developmental processes at the earliest stages of pregnancy where biological material is not available from humans. In the longer term they may also prove to be valuable for the treatment of pregnancy disorders many of which are based on a defective function of the placenta, i.e. the organ that mediates all nutrition and oxygen supply to the growing baby. A key question is how the expansion and then differentiation of this particular stem cell type is regulated to ensure the formation of a fully functional placenta. The importance of this question is further reinforced by the fact that residual trophoblast stem cells pose a tremendous risk of aggressive tumour formation in the uterus of the mother. We have identified that this mechanism may be self-controlled by a specific molecule that regulates the activity of placental genes, the transcription factor ELF5. This project investigates the molecular basis how ELF5 promotes both, trophoblast stem cell self-renewal as well as differentiation into specific placental cell types. This molecular 'switch' function between promoting the expansion of a specific stem cell population followed by triggering their terminal differentiation is of fundamental importance for a healthy pregnancy (for both mother and baby). It may also prove to be translatable into other cell and organ systems where this molecule is present and that are particularly susceptible to an imbalance between proliferation and differentiation, such as the breast with its susceptibility to develop cancer. Hence understanding the mechanism of ELF5 function may establish a paradigm for molecular pathways that ensure normal reproduction as well as health throughout adult life and ageing.

Trophoblast stem (TS) cells represent the developmental counterpart to embryonic stem (ES) cells, capable of differentiating into all trophoblast cell types of the placenta. Proper expansion followed by differentiation of the TS cell compartment in vivo is essential for normal development and reproduction. We have recently discovered the transcription factor ELF5 as a master regulator with dual function in the trophoblast lineage. ELF5 is critical for TS cell self-renewal by reinforcing a transcription factor circuit with CDX2 and EOMES. However, sustained ELF5 expression triggers exit from the stem cell niche and commitment to differentiate. This control mechanism may represent an intrinsic limitation to (stem) cell hyperproliferation in the healthy organism. In the current proposal we will investigate how this 'molecular switch' function of ELF5 governs self-renewal and differentiation in the trophoblast compartment. We will determine the transcriptional networks established by the core CDX2/EOMES/ELF5 circuit in TS cells. By integrating these data with the pluripotency networks present in ES cells, as well as with other genetic and epigenetic profiling datasets, we will gain critical insights into the shared and the distinct hallmarks that define the stem cell state in TS versus ES cells, enabling us to correlate genetic and epigenetic profiles with stem cell potency. Further, we will elucidate the mechanistic basis of the differentiation-promoting function of ELF5 by assessing the importance of absolute ELF5 levels, niche environment-dependent ELF5 protein complex components and post-translational modifications that may fine-tune their function. Our data will provide fundamental insights into stem cell biology, differentiation control, the use of stem cells in regenerative medicine, reproduction and development, key areas to ensure fetal and maternal health as well as healthy ageing.

This project will have substantial impact on many levels, it will benefit the immediate and the wider research community, the clinical areas of obstetrics & gynaecology and child health, it will advance our understanding of requirements for healthy ageing, open up new avenues for regenerative medicine as well as systems biology and biological modelling approaches, and through all this will benefit the public as a whole. The research areas directly affected by our work, nationally and internationally, include stem cell biology, developmental biology and reproductive medicine, obstetrics & gynaecology, cancer biology, systems biology and mathematical modelling, biochemistry as well as cell biology. The results will advance our understanding, and refine modelling, of stem cell behaviour and differentiation potential which is crucial for the therapeutic use in regenerative medicine. They will enable the generation of comprehensive integrated models of genome and chromatin regulation that determine the developmental potency of a (stem) cell which will benefit a wide research community. Our insights will have direct impact on our understanding of mechanisms that underlie ageing and the maintenance of health, and thereby on improving the quality of life. The immediate impact of this study is on advancing our understanding of early development, placentation and healthy pregnancy. Through fetal programming processes, these areas are also of major importance for health in adult life. In the wider field of Obstetrics & Gynaecology they will lead to a better understanding of causes, and in the longer term improved treatments, of pregnancy complications. They will also lead towards refined approaches for the derivation of human trophoblast stem cells whose establishment has not been successful to date. Such stem cells would be most beneficial for research as well as for therapeutic approaches as they would allow, in an unprecedented manner, to study developmental processes early after fertilization where human material is not available. In the longer term they may also prove most valuable for the treatment of common placenta-based pregnancy complications. The wider scientific community will benefit from oral and poster presentations of the ongoing advances of the project at national and international conferences. The results of this work will be published in peer-reviewed open access journals or deposited in open access databases. The integrated genome-wide transcriptional network data will be made available on open-access servers which will enable us and other researchers to expand the dataset in an interactive manner. Potential opportunities for commercial exploitation are provided if the results discover novel mechanisms of stem cell self-renewal and differentiation, maintenance of developmental potency and triggers to avoid stem cell-associated tumourigenic and teraogenic risks. The lay audience will benefit from presentations, press interviews and/or public lectures, and through the social impact of the results of the study. We further actively engage in training the next generation and fostering scientific enthusiasm through visits at schools and School's Day Projects at the Babraham Institute. The project involves in-depth training of the staff involved which will make them highly employable in the academic and commercial research sector, in scientific communication and on scientific/stem cell research advisory boards. The project falls centrally into BBSRC's priority area on stem cell biology. It also has direct impact on the strategic priority area on 'Ageing research: lifelong health and wellbeing', a) by its relevance for normal pregnancy because fetal under-nutrition, e.g. due to a malfunctional placenta, increases the likelihood of cardiovascular disease and diabetes in later life; and b) by its relevance to understand the self-limitation of stem cell proliferation to prevent tumour formation.