Comparative transcriptional control of establishment, maintenance and collapse of naive pluripotency in rodents and primates in vivo

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The value of stem cell lines that can multiply in culture and retain the capacity to become any tissue type in the body is immense, both from the point of view of generating animal models for research, and to produce patient-specific human tissue for drug screening or cell replacement therapy. Stem cells derived from the founder tissue of the foetus in the early mouse embryo, known as pluripotent embryonic stem (ES) cells, have been proven to generate any type of adult tissue, including the germ cells, by observing their contribution to animals produced after integrating ES cells into a host embryo. Human ES cells, however, have characteristics distinct from mouse ES cells; this is evident in their molecular attributes, morphology and culture requirements. In fact, human ES cells resemble much more closely the cells found in more mature mouse embryos that have already implanted in the uterus and are considered to be 'primed' for differentiation. These cells are harder to grow and are more restricted in their differentiation repertoire compared with mouse ES cells. In the interests of therapeutic relevance, it would be highly desirable to understand how to capture cells from human embryos that are more akin to mouse ES cells. Although attempts have been made to produce such cells, either by manipulating the culture conditions in which they are grown, or by means of 'reprogramming' strategies that can revert adult cell types to a more embryonic condition, it has not been possible to capture human cells that can thrive in such a state. We believe that the only way to overcome this obstacle is to understand how these founder embryonic cells are formed in mouse embryos, from which true ES cell lines can be readily obtained. The ultimate goal is to use this knowledge as a blueprint to probe the development of early human embryos (or non-human primate embryos as a model for humans) and exploit alternative pathways to capture similar pluripotent cells from primates. In addition to discovering how to derive the ideal type of stem cells from human embryos, this knowledge will be used to generate useful cells from adult tissues and patient samples. In this project we plan to expand on our previous work to produce a detailed molecular portrait of mouse embryos before, during and after the stage in animal development when pluripotent cells are naturally produced. We will make use of genetic modification strategies to explore the importance of master control genes in this process. In addition, we can add highly specific chemicals to the culture medium that activate or suppress different cellular behaviours to reveal how these control genes are turned on or off. We have already begun to identify molecular differences between rodent (mouse) and primate (marmoset) embryos. We will extend this concept and then test how marmoset embryos respond to external signals that we predict may be beneficial to development. We will use this information to target specific pathways that could be enhanced or inhibited to allow pluripotent cells to be captured, thereby providing the essential starting point to create useful tissues for drug screening and, ultimately, cell replacement therapy.