Efficiencies and mechanisms of nuclear reprogramming

Abstract

The differentiated state of somatic cells is highly stable, but it can be experimentally reversed. The resulting cells can then be redirected into many different pathways. Nuclear reprogramming has been achieved by nuclear transfer to eggs, cell fusion, and overexpression of transcription factors. The mechanisms of nuclear reprogramming are not understood, but some insight into them is provided by comparing the efficiencies of different reprogramming strategies. Here, we compare these efficiencies by describing the frequency and rapidity with which reprogramming is induced and by the proportion of cells and level of expression in which reprogramming is achieved. We comment on the mechanisms that lead to successful somatic-cell reprogramming and on those that resist in helping to maintain the differentiated state of somatic cells.

Figure 1. Nuclear reprogramming strategies

A. Nuclear reprogramming by nuclear transfer involves the transplantation of a somatic cell nucleus into an enucleated egg (metaphase II oocyte), which can lead, in some cases, to the production of fertile adults, genetically identical to the donor nucleus. B. Direct transplantation of multiple mammalian somatic cell nuclei into the nucleus of a Xenopus prophase oocyte I does not lead to new cells, but induces fast and direct transcriptional reprogramming of previously silenced genes. C. Reprogramming of somatic cells by cell fusion produces heterokaryons and cell hybrids, possessing many molecular characteristics of pluripotency when pluripotent cells are fused to somatic ones. D. Overexpression of transcription factors can induce a lineage switch. E. Ground state pluripotency can be induced in somatic cells by overexpression of a few factors such as Oct4, Sox2, KLF4 and c-Myc.

Figure 2. Nuclear reprogramming efficiencies judged by different criteria

The left part of the figure shows the reprogramming steps of each system. The graphs on the right indicate the efficiency (%) of reprogramming as judged by different criteria. The origin of donor nuclei is indicated. A. Efficiency judged by the proportion of nuclear transfer Xenopus embryos obtained from somatic cell nuclear transfers to egg (metaphase II oocytes) that reach a particular stage of development. * Includes results from serial nuclear transfer experiments. B. Efficiency judged by the proportion of mouse embryos obtained by somatic cell nuclear transfers to mouse eggs (MII oocyte) that reach a particular stage of development. C. Efficiency judged by the level of transcriptional reactivation of pluripotency genes after nuclear transfer to Xenopus oocytes. D. Reprogramming efficiency after cell fusion, as judged by the number of colonies obtained or the extent of gene reactivation. E. Induced pluripotency: efficiency judged by the number of colonies obtained from starting cells by using various factor delivery approaches. Numbers in parentheses indicate normalized efficiencies (see text).