ES cells are likely to have developed this form of metabolism as an adaptation to the hypoxic in vivo environment of the early embryo[69]. Interestingly, various groups have shown that iPS cell purchase GW 4064 reprogramming is enhanced by hypoxia[70,71], likely due to the acceleration of this metabolic shift. MATURATION Tanabe et al[72] have recently identified the maturation stage of iPS cell reprogramming as being a major bottleneck in the process, which is likely to account for the

low efficiency of the process generally. They demonstrate that LIN28, but not NANOG, shp53 or CYCLIN D1, promotes maturation of iPS cells. During maturation, epigenetic changes occur allowing expression of the first pluripotency-associated genes[40]. These genes include Fbxo15, Sall4, Oct4, Nanog and Esrrb. Interestingly, Esrrb has been shown to be sufficient to reprogram MEFs in collaboration with Sox2 and Oct4[73]. LIF/STAT3 signalling is required for the maturation phase of mouse iPS cell reprogramming[74]. Interestingly, pre-iPS cell colony formation has been observed in the absence of LIF, however, beyond day 6 of reprogramming these colonies detach. This is likely due to the requirement that cells undergoing the reprogramming process have for LIF signalling to maintain cMyc expression[75].

In addition, Tang et al[74] demonstrate that LIF/STAT3 activation induces earlier formation of an increased number of pre-iPS cell colonies. Mechanistically, this group demonstrate that LIF/STAT3 signalling is required for demethylation of pluripotency-associated gene promoters. Specifically, STAT3 signalling was shown to directly block the action of the DNA methyltransferase DNMT1 and Histone deacetylases 2, 3 and 8. Wnt signalling also enhances

the maturation phase of mouse somatic cell reprogramming whereby exogenous stimulation of the pathway using Wnt3a between days 6 and 9 after induction of reprogramming enhances the formation of Nanog positive colonies[76]. Various groups have suggested that expression of Nanog is necessary for cells to advance from the maturation phase to the stabilisation stage[39,77] and thus, Samavarchi et al[36] suggest that Nanog expression alone is responsible for mediating the transition from pre-iPS cells to stably reprogrammed cells. This group demonstrate that removal of the reprogramming Batimastat factors from mouse iPS cells at day 9 after induction of reprogramming did not induce phenotypic reversion. Other groups, however, have reported different time points for the stabilisation stage, including day 11[78,79] and day 16[80], suggesting that this can vary depending on discrete protocols and culture variations. It is clear that there remains substantial information to be learned regarding this critical intermediary step but NANOG appears to play a pivotal role in iPS cell maturation. STABILISATION Only around 1% of cells that initiate reprogramming make it to the stabilisation stage[72].