Epigenetic factors influencing resistance to nuclear reprogramming.
Bottom Line: Transcription factors, chromatin modifications, and noncoding RNAs can increase the efficiency of reprogramming.However, the success of nuclear reprogramming is limited by epigenetic mechanisms that stabilise the state of gene expression in somatic cells and thereby resist efficient reprogramming.We see this as a step towards understanding the mechanisms by which nuclear reprogramming takes place.
Affiliation: Wellcome Trust/Cancer Research UK Gurdon Institute, The Henry Wellcome Building of Cancer and Developmental Biology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QN, UK.Show MeSH
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Mentions: Different systems have been used to reprogram cells (Figure 1). These include nuclear transfer to eggs and oocytes, cell fusion and overexpression of transcription factors. The nucleus of a specialised cell can be reprogrammed by somatic cell nuclear transfer (SCNT) to an enucleated egg (also called metaphase II oocyte; [1–3] but see also ). In this case, a somatic cell nucleus is reprogrammed by the egg to behave like the nucleus of an embryonic cell, and cells of the resulting embryo are pluripotent and able to differentiate into many, and sometimes all, cell types unrelated to the original donor nucleus (Figure 1a). The transcriptional state of somatic cell nuclei can also be reprogrammed by nuclear transfer to Xenopus meiotic prophase I oocytes (Figure 1b) . Another route is to fuse two cells from different origins in such a way that the two nuclei of different cell types occupy the same cytoplasm; such fused cells form heterokaryons and cell hybrids (Figure 1c) [6–10]. In heterokaryons, the nuclei remain as separate entities within a common cytoplasm for a few days . In proliferating cell hybrids, progression through the cell cycle causes the nuclei to fuse and give rise to synkaryons, which we do not discuss here. In heterokaryons, the nucleus of one donor cell is induced to express genes characteristic of the other donor cell, thereby providing an opportunity to investigate the mechanism of reprogramming. The cells fused can be of different species or differentiation state. For example, mouse ES cells can be fused to human fibroblasts . Pluripotency can be induced in somatic cells by overexpression of a few transcription factors, originally Oct4, Sox2 (both of which are required for pluripotency), Klf4 and c-Myc (Figure 1d) . The induced pluripotent stem (iPS) cells obtained have been well reviewed by others [12,13]. However, regardless of the system used, the proportion of nuclei or cells that are reprogrammed to new cell types is always low. This shows the resistance of somatic cells to reprogramming and reflects the stability of the differentiated state. Here, we concentrate on the epigenetic factors that promote or restrict the success or efficiency of nuclear reprogramming.
Affiliation: Wellcome Trust/Cancer Research UK Gurdon Institute, The Henry Wellcome Building of Cancer and Developmental Biology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QN, UK.