Principal Investigator Richard Young
Human development begins with a single fertilized cell and leads to an organism with 1013 cells with over 200 different cell types. Each cell type has a unique gene expression program that is controlled by a subset of the approximately 2000 protein regulators encoded in the genome together with an unknown number of RNA regulators. We have initiated mapping of human regulatory circuitry by focusing our efforts on embryonic stem cells and several differentiated cell types where key signaling pathways, transcription factors and chromatin regulators necessary for establishment and maintenance of cell state have been identified. We have augmented our study of human cells with similar studies in murine cells, where genetic manipulation is more feasible.
Embryonic stem cells are a pluripotent, self-renewing cell type that give rise to differentiated cells during embryogenesis. How pluripotency and self-renewal are regulated is poorly understood, but the transcription factors Oct4 and Nanog, which are expressed exclusively during preimplantation development and in ES cells, have been shown to be critical for these key features of ES cells. We discovered that Oct4 and Nanog associate with active genes involved in cellular proliferation and silent genes encoding most developmental transcription factors. We postulate that Oct4 and Nanog are required for pluripotency because they repress genes encoding transcription factors that are key to differentiation and development. This may explain why genetic perturbation of these “pluripotency regulators” causes ES cells to begin differentiating.
Transcription factors and chromatin regulators contribute to the transcriptional regulatory circuitry responsible for pluripotency and self-renewal in embryonic stem cells. Polycomb group proteins are chromatin regulators that are essential for early development in metazoans but their contributions to human development are not well understood. We have found that Polycomb Repressive Complexes (PRCs) occupy large portions of the genes encoding developmental regulators that are also bound by Oct4 and Nanog. Thus, one role for PRCs in ES cells is to contribute to repression of genes encoding developmental transcription factors which, if expressed, would cause ES cells to differentiate.
While the outlines of the core transcriptional regulatory circuitry of ES cells are emerging from these and related studies, we are also beginning to map the core circuitry of T lymphocytes, muscle and neural cells. Each of these differentiated cell types has several “master regulators”, identified in previous studies, whose target genes have yet to be identified, thus providing a logical starting point for mapping the transcriptional regulatory circuitry of these cells.