Mechanisms regulating the timing of developmental events in the early mouse embryo

An unanswered fundamental question in biology is what ensures that individuals of a particular species are all roughly the same size at birth. This highly reproducible rate of embryonic development is also clinically relevant as it is essential for the health of the foetus: retarded growth is an important cause of foetal and neonatal morbidity and mortality; over-growth is an important contributor to congenital malformations. In spite of the importance of understanding how embryo growth is regulated, we still know very little about this process. In this proposal we aim to tackle this problem. We will perform our studies using the mouse embryo as our model system as these embryos are amenable to manipulations that would not be ethical in humans. We will focus on analysing the period just after the embryo has implanted into the uterus, as during this period it has been shown that undersized embryos display catch-up growth and oversized embryos reduce in size until they are the same size as normal embryos. By analysing how undersized and oversized embryos scale to the size of normal ones we anticipate that we will gain understanding on the processes that ensure that all embryo grow at a similar rate. Specifically, we will do three things:

1) We will establish how undersized and oversized embryos adjust their growth rate by studying if it is by modulating the rate at which their cells divide and/or the rate at which their cells die.

2) We will determine what signals are exchanged between cells to promote the growth of undersized embryos and restrict the growth of oversized embryos. For this we will exploit our preliminary observations that indicate that a pathway that regulates growth by integrating nutrient or growth factors is important. To test the importance of this pathway we will manipulate how the cells of the embryo respond to these inputs to establish which of them drives the catch-up growth of undersized embryos and the slowing down in growth of oversized ones. We will also study how these inputs they act differently in embryos that are not the right size.

3) During the period that oversized and undersized embryos adjust their size they start to form all the different types of cells that will form the different organs. For this reason we will study how the mechanisms that ensure the embryos are the proper size affect the formation of the different cell types of the embryo.

Given the importance of embryo size for a normal pregnancy, we expect that the above described studies will help us develop methods to identify and treat embryos with abnormal growth rates early on in pregnancy.

A fundamental question in biology is what ensures that individuals of a particular species are all roughly the same size at birth. In spite of the importance of this question, there is relatively little known about it. Here we will tackle this unknown by studying the mechanisms that regulate embryo size during early post-implantation mouse development. For this we will compare normal embryos to double-sized embryos, that reduce in size at the time of gastrulation, and to half-sized embryos, that catch up in growth just after gastrulation. As part of this comparison we will do three things:
First, we will establish the relative importance of changes in proliferation and cell death for the scaling of over- and under-sized embryos.
Second, we will study the pathways that regulate early embryo size. This will be done by focussing on mTOR signalling, a nutrient and growth factor sensing pathway revealed by our preliminary data. As part of this analysis, we will study how mTOR is activated during embryo scaling, what inputs determine its activity and how these inputs ensure that embryo size is appropriate for the developmental stage at which the embryo is at. This candidate pathway approach will be complemented by an unbiassed search for novel pathways involved in embryo scaling, performed by comparing the single cell transcriptome of normal embryos to that of over- and under-sized embryos.
Finally, we will study how the mechanism that regulate embryo size impact the patterning and morphogenesis of the embryo. For this, we will compare the timing of gastrulation of over-sized and under-sized embryos to that of normal embryos. Using these embryos we will also study how embryo size affects the signals that induce gastrulation.
Together, we anticipate that the experiments described will provide invaluable information regarding how embryo size is regulated during early mammalian development.

Our studies will help uncover the mechanisms that regulate the size of mammalian embryos. Retarded growth is an important cause of foetal and neonatal morbidity and mortality and over-growth is an important contributor to congenital malformations. Our work will therefore help understand some of the underlying reasons for these phenotypes.

Our studies will also be important for the regenerative medicine field. The large promise that the use pluripotent stem cells hold for cells therapy has carried with it a large degree of interest both from the academic and medical communities. Recent advanced have included the development of "organs in a dish" (or organoids) from these pluripotent stem cells. These organoids can be used to study human development in a functional way, for disease modelling and drug screening if generated from patient-derived induced pluripotent stem cells, and in the longer term can potentially be used as an alternative to organ transplant. However, increased knowledge about how these organoids form in a robust and reproducible way is vital ensure their scientific relevance as well as to allow their safe use in regenerative medicine. Therefore, the results will also be relevant to translational research.

The beneficiaries of this work can be classed into the following groups of people:
1) In the short term the primary beneficiaries are those with the following research interests:
- Researchers interested in understanding embryo growth regulation
- Clinicians interested in understanding the causes of human embryo growth retardation and congenital abnormalities caused by overgrowths.
- Those interested in the use organoids for regenerative medicine.

2) The commercial sector and clinicians will benefit from insight into how to develop tools that are more sensitive, more reliable and less invasive than those already available to diagnose and treat growth retardation and congenital malformations.

3) This project will train early-career researchers in emerging methodologies in embryology and stem cell biology, contributing to their career development, as well as producing individuals capable of carrying out future research in the biomedical sciences. At a time when industry is moving in the direction of interdisciplinary research, such individuals will be highly sought-after not only in academia but also in the commercial sector

4) Teachers and students interested in reproductive biology will benefit from knowledge about the fate of aneuploid cells during development and how they affect normal embryogenesis.

5) A final beneficiary of our work will be the lay public. By highlighting the medical relevance of understanding developmental processes and their potential impact on pregnancy outcome, our work will provide factual input to and therefore benefit the public discussion about the importance of basic research for human health.

The results of this research will be conveyed to other researchers through the publication of findings in peer-reviewed journals, by reporting unpublished work at conferences and through personal communication with other scientists. Though the results will primarily be disseminated through scientific journals, we will liaise with dedicated Media Teams at Imperial College and Oxford University to issue a press release when appropriate. We take seriously the responsibility of scientists to engage with the lay public, to raise awareness among them of the results of publicly funded research, to openly debate ethical issues relating to our research so that public opinion may be formed in an informed manner and to take the excitement of our research to the children of today, who will be the scientists of tomorrow. For these reasons we engage through our Universities but also as individuals in activities aimed at the public dissemination of science.