Nuclear reprogramming and epigenetic rejuvenation.

ABSTRACT

William Harvey’s motto Ex ovo omnia (‘All from the egg’) on the frontispiece of his treatise On the generation of living creatures (1651) was well chosen and extraordinarily prescient. Centuries later, the egg was shown to have a striking capacity for bringing forth life – life produced by experimental manipulation, ‘animal cloning’, outside the normal physiology of fertilization (Gurdon and Byrne 2005). Cloning through somatic cell nuclear transfer (SCNT) showed that the restriction of developmental potential during cellular differentiation is the result of epigenetic changes in gene expression rather than through loss of DNA – although certain lineages, such as B- and T-cells, are known to undergo programmed DNA rearrangements (Hochedlinger and Jaenisch 2002). It is the reversal of these epigenetic changes during ‘reprogramming’ of the specialized adult nucleus within the reconstructed embryo that results in its re-acquisition of developmental potential and the consequent recapitulation of development, ultimately giving rise to a cloned newborn. A commonly held defi nition is that nuclear reprogramming by SCNT is the process by which a specialized nucleus reacquires developmental potential (Singh 1999). However, nuclear reprogramming is much more than this. It is a manylayered process. Intimately associated with developmental reprogramming of the specialized adult nucleus to an earlier, embryonic, totipotent state is age reprogramming; the ageing ‘clock’ of the transferred nucleus is reset back to zero; an old cell can give rise to newborn clone (Wilmut et al. 1997). This begs the question of whether age reprogramming can be separated from developmental reprogramming. Being able to reprogramme the ageing clock in isolation, while maintaining the differentiated state of a cell, would essentially mean that the cell is made young again rejuvenated. Clearly, should this be achieved, the consequences would be profound (fi gure 1). Nuclear reprogramming observed in ‘classical’ animal cloning (where adult cells are reprogrammed to an embryonic state after SCNT) has recently been recapitulated in vitro by the generation of embryonic-like induced pluripotent stem cells (iPS cells; Takashi and Yamanaka 2006). Induction of iPS cells allows the process of epigenetic rejuvenation of adult cells to embryonic cells, as seen after SCNT, to be studied in a well-defi ned system (Surani and McClaren 2006). However, certain features of the rejuvenation seen in classical cloning are likely to differ from that seen in iPS cell generation. For one, telomeres, whose shortening is seen as a key characteristic of ageing cells, are ‘rejuvenated’ by telomerase during iPS cell induction from old somatic cells (Marion et al. 2009). This mechanism is unlikely to be the major mechanism for ‘rejuvenating’ telomeres in eggs after SCNT. During the early cleavage divisions, telomeres are lengthened by a telomere sister-chromatid exchange recombination mechanism that is peculiar to this stage of development and is under the sole control of the maternal cytoplasm, as it is unaffected by the absence of a paternal chromosomal complement (Liu et al. 2009). Thus, the mechanism(s) and pathway(s) of rejuvenation that result from SCNT, which have yet to be uncovered, are likely to be different from those operating during the generation of iPS cells. Notwithstanding these differences, SCNT or introduction of ‘reprogramming factors’ into somatic cells both appear to direct developmental reprogramming and age reprogramming seamlessly age reprogramming does not take place without de-differentiation into embryonic cells (developmental reprogramming). But can these intimately associated aspects of nuclear reprogramming be disentangled? While it seems hardly possible, recent work indicates that age reprogramming might indeed be separable from developmental reprogramming. Differentiation of myelomonocytic progenitors into macrophages involves an exit from the cell cycle.