Human embryonic stem (ES) cells are a remarkable cell type that are derived from a group of cells called the inner cell mass (ICM) of a very early stage embryo (about 100 cells in total) obtained from in vitro fertilization program. Human ES cells can be expanded in culture in an undifferentiated state (self-renewal) without limit while retaining the capacity to differentiate into nearly any type of cell. Human ES cells offer an important renewable resource for future cell replacement therapies for many diseases such as Parkinson’s disease, spinal cord injury etc. However, before the full potential of human ES cells can be exploited in the clinic, we need to understand more about human ES cells so we can control their fate towards either self-renewal or towards differentiation into a specific cell type required for cell replacement therapy. Currently it is a problem just to grow human ES cells, let alone to understand how human ES cells make their choice between self-renewal and differentiation. In contrast, several signaling pathways which are important for mouse ES cell self-renewal have been identified, and as a result of this, it is possible to grow mouse ES cells in a fully defined condition. However, these pathways seem to be not operating in human ES cells. This would argue that human ES cells are very different from mouse ES cells, and that understanding of human ES cells may not benefit from the research of mouse ES cells. However, we have recently made striking discoveries on mouse ES cells. We found that for mouse ES cell self-renewal does not require any added growth factors or cytokines but only the elimination of signals that induce differentiation. These new findings provide us with a new prospective to understand human ES cells. Through understanding some of the basic mechanisms involved in human ES cell maintenance, we should be able to develop a more efficient and better method to grow human ES cells, which is clearly important if these cells are to be used clinically.
Human embryonic stem (ES) cells can be maintained indefinitely while retaining the ability to make any type of human tissue. In the future, human ES cells may hold the key to replacing cells lost in many devastating diseases such as Parkinsons and diabetes. But for human ES cells to be of use clinically, they will first have to be multiplied in very large numbers. Scientists must, therefore, learn how to control the growth of stem cells in the laboratory. When a human ES cell divides it can either produce identical copies of itself (self-renewal) or it can produce other more specialised cell types, such as nerve or muscle cells. Understanding how a stem cell makes this choice between self-renewal and differentiation is the central challenge in stem cell research. This proposal is intended to apply the knowledge we obtained from extensive research on mouse ES cells for the better understanding how human ES cells make their decision whether to self-renewal or to differentiate. The direct benefit from this proposal research will be the development of more efficient culture conditions for the growth of human ES cells, which is a critical step leading to the clinical application of human ES cell-derived cells.
Human ES cells are routinely grown on feeders with medium containing serum or serum replacements supplemented with bFGF. Although progress has been made in improving culture conditions, the pathways involved in the maintenance of human ES cell self-renewal remain largely unknown. The main purpose of this project was to decipher the requirements for sustaining human ES cell self-renewal and to understand the molecular basis of these requirements. So far we have made the following findings: 1. Sustained activation of STAT3 supports mouse ES cell self-renewal in the absence of feeders ,whereas activation of STAT3 induces differentiation of human ES cells and epiblast-derived stem cells (EpiSCs); 2. Self-renewal of mouse/rat ES cells, but not human ES cells or EpiSCs, is sustained by inhibition of glycogen synthase kinase-3 (GSK3) and mitogen activated protein kinase (MAPK); 3. Activation of integrin pathway enhances self-renewal of human ES cells, but not mouse cells; 4. bFGF supports human ES cell self-renewal through an Erk1/2-dependent pathway. Our findings suggest that the requirements for sustaining self-renewal of human and rodent ES cells are fundamentally different and that human ES cells are most likely analogous to rodent EpiSCs. We are currently generating and characterizing human cells that resemble mouse/rat ES cells. Understanding the basic mechanisms involved in human ES cell maintenance will eventually lead us to develop better methods for the growth of human ES cells, which is clearly important if these cells are to be used clinically.