Mechanisms of Activin/Nodal signalling in pluripotency and differentiation of human embryonic stem cells

Stem cells are remarkable in having the capacity to undergo specialisation into distinct tissue types, such as blood or skin. In addition, many stem cells are capable of self-renewal during cell division, so that they not only generate a specialised cell but also generate another stem cell. Our bodies are thereby able to retain their tissue-specific stem cells (such as those responsible for generating blood or skin) for our entire lifetimes. Such adult stem cells are generally thought to generate only a few types of cells, related to the specific tissue that they are derived from and are responsible for maintaining. Another type of stem cell can be derived from early stages of human development, namely from surplus embryos that arise in the treatment of human infertility using in vitro fertilisation. Such spare embryos, donated under informed consent by patients who have completed their therapies, are capable of generating embryonic stem cells when grown in petri dish. These stem cells are particularly interesting because they appear to have the ability to specialise into all of the tissue types in the entire body.

The current proposal is to investigate the molecular details of how human embryonic stem cells maintain their pluripotency, and how the biochemical information that is responsible for starting their journey on the path to specialisation actually works.

In order to achieve the therapeutic potential of human embryonic stem cells, it will be necessary to scale up their growth in pristine conditions, and to generate significant quantities of clinically useful cell types. These goals will require an understanding of the mechanisms of pluripotency and differentiation in these cells. Current information indicates that human embryonic stem cells differ from their mouse counterparts in this regard. We therefore propose to investigate how the growth factors Activin and Nodal and their Smad signalling pathway act first to maintain pluripotency and then to induce differentiation into the precursors of mesoderm and endoderm.

Our hypothesis is that a distinct set of Smad binding partners are involved in maintaining pluripotency, and our first objective is to identify and functionally evaluate them. We also propose studies of the forkhead transcription factor, FoxH1, to determine how it functions during mesoderm and endoderm differentiation. Our third objective is to identify the downstream target genes through which these Smad binding partners exert their effects. These studies should provide essential insights into how Activin, Nodal and related growth factors are able to have varied developmental roles and should reveal how they can be used to generate homogeneous populations of either pluripotent or differentiated human cells for therapeutic use.