See Attachment
OVERCOME CELLULAR SENESCENCE
During reprogramming, fibroblasts not only become pluripotent, they also become immortal. Fibroblasts proliferate a finite period of time before entering into senescence. In contrast, ES cells and iPS cells do not experience such a limitation. Immortalization requires that at least two barriers be overcome: cellular senescence and telomere shortening [for reviews, Drayton and Peters, 2002; Herbig and Sedivy, 2006]. Rb and p53 are the key senescence-inducing factors. Interestingly, in ES cells, the Rb pathway is constitutively inactivated due to hyperphosphorylation [Savatier et al., 1994], while certain aspects of p53 function are compromised [Qin et al., 2007]. Although activity of the Rb and p53 pathways have not been examined in iPS cells, a similar cell cycle profile to ES cells suggests that both of pathways are also down-regulated. However, escaping from cellular senescence does not automatically ensure immortality. Cells can enter a so-called replication crisis state where they undergo apoptosis if their telomere erodes below a critical length [Wright and Shay, 1992; Ducray et al., 1999]. To avoid telomere shortening, the activity of telomerase must be up-regulated. Myc directly up-regulates the transcription of TERT, the gene encoding the enzymatic subunit of the telomerase [Wu et al., 1999]. However, it is unclear if elevated telomerase activity in iPS cells is due to ectopic expression of c-Myc and how much the resulting change of telomerase activity contributes to reprogramming. It is also unclear how the four factors find ways to inactivate Rb and p53 and to what extent this contributes to reprogramming, where fibroblasts that are defective in Rb or p53 pathways may be informative resource to test this hypothesis. It is also worth stressing that immortalization by reprogramming is different from transformation. iPS cells are immortal in their undifferentiated state. Upon differentiation, both the Rb and p53 pathways become fully functional again through yet unknown mechanisms.
The successful isolation of iPS cells from fibroblasts has brought a new era of stem cell biology, one providing the opportunity for researchers to understand the nature of pluripotency. It will be necessary to develop novel methods to create iPS cells without modification. The next a few years will see new reprogramming factors and pathways identified, not to mention more studies designated to reveal the molecular mechanisms of reprogramming. Understanding these mechanism may in turn guide us to the development of novel methods to generate the next generation of iPS cells, ones with direct clinical utility.
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