Epigenetic Barriers to Cell Fate Reprogramming

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During embryonic development, cells become increasingly committed to a certain cell fate. They rarely, if ever, change to another type. For example, as skin cell do not naturally change to, or give rise to a brain cell. However, this stable commitment of a cell can be reverted by certain experimental procedures, as for example when the nucleus of a specialised cell is transplanted to an enucleated egg. During this reprogramming, the gene expression pattern of the differentiated cell can be changed to that of an embryonic cell. This process is of interest because identifying how reprogramming takes place can help us to understand how cells maintain their identity. This is important because many pathological conditions arise from loss of cell identity. Second, the embryonic stem cells that can be obtained by reprogramming a specialised cell of one kind, e.g. a skin cell, can then be made to produce healthy specialized cells of an other kind, e.g. brain cells. This has potential application in cell-replacement therapy, where these cells can compensate the loss of irreversibly damaged or defective cells. It might be possible to derive replacement heart, neuronal or pancreatic cells from another cell type of the same individual, thereby avoiding the need for immunosuppression treatments.Despite its enormous potential, the usefulness of this approach is limited by its low efficiency, as only a small number of cells can be reprogrammed. The reason why adult body cells like skin do not respond to nuclear reprogramming is likely related to an inherent resistance of one kind of cell to change to another kind. Indeed, cells produced by transplanting the nucleus of a specialised cell to an egg often continue to show characteristics, such as expression of genes, of the specialised cell they were derived from. For example, a reprogrammed skin cell continues to express skin genes and fails to fully activate genes of the new cell type. This suggests that during specialisation, cells have acquired memory of their cellular identity, which prevents unwanted changes in cell type during normal development or during reprogramming.The aim of our project is to elucidate the phenomenon of cellular memory. We use nuclear transplantation of specialised cell nuclei to Xenopus eggs as a model system to understand this process. In stark contrast to human eggs, Xenopus eggs are readily available in large quantities and easy to manipulate. Since the mechanisms underlying cellular memory are most likely conserved between vertebrates, and given the ethical concerns of using human material for this kind of research, Xenopus is the ideal model system to perform this study. The project consists of three stages. First we will investigate how many embryos produced by nuclear transfer show a strong memory of the cell type they were derived from. We will identify the genes that show cellular memory after nuclear transplantation. Second, we will investigate the molecules associated with cellular memory genes, since these molecules likely inhibit a change in cell fate during normal development and during nuclear reprogramming. Lastly, we propose to change the memory-conferring factors in specialised cells before transplanting their nuclei to eggs in order to increase the efficiency of new cell type generation. In this way, we will understand which modifications of nuclei can improve the generation of high quality embryonic cells that can be used for cell replacement. Furthermore, we will gain a better insight into the mechanisms important for cellular memory and the stability of cell differentiation during normal development and disease.