The transition of EpiSCs to an ES-like state provides an additional approach to reveal the molecular requirements for attaining pre-implantation pluripotency. Overexpression of find more Nanog together with a change in culture conditions can drive reprogramming of EpiSC to ES-like cells [4 and 6]. This conversion is accompanied by acquisition of an ES cell gene expression profile and is marked by reactivation of the inactive X chromosome in female lines
[6]. Although similar reprogramming capacities have been reported for other TFs including Esrrb [33••], Klfs [24 and 49], Nr5a2 [50], Stat3 [51] and, surprisingly, the germ cell marker Prdm14 [52•], the relative efficiency with which most of these factors reprogramme EpiSCs with respect to one another remains unresolved. Similarly
to Esrrb, Klf4 and Klf5, Prdm14 is also a transcriptional target of Nanog ([33••] and Figure 2), and its ability to reprogramme EpiSC underscores the overlap in characteristics between migratory PGCs and ES cells. Nanog was previously shown to be required for conversion of EpiSC to ES cells [6]. However, overexpression of the Nanog target Esrrb bypasses this requirement [33••], raising the possibility that the action of Nanog during reprogramming may be accounted for by Esrrb. Testing this notion by attempting to reprogramming Esrrb-null EpiSCs with Nanog should resolve this issue. Notably, RG7204 while Esrrb requires 5′Azacytidine to complete reprogramming of Nanog−/− pre-iPS, reprogramming of Nanog−/− EpiSC is induced efficiently by Esrrb alone. Possibly EpiSCs have a closer methylation profile to ES cells than pre-iPS cells. In this regard, Nanog, Oct4 and Sox2 are expressed in EpiSCs and their promoters are unmethylated, while the pre-implantation markers Rex-1, Stella and Fbxo15 have methylated promoters in a fraction of the EpiSC population [ 9••,
25 and 26]. This difference between the reprogramming of Nanog−/− pre-iPS Acyl CoA dehydrogenase and Nanog−/− EpiSCs highlights the dual activity that Nanog exerts during reprogramming, with only the transcriptional upregulation of silent target genes, and not the reversion of methylation marks being required for EpiSC reprogramming. In contrast to human ES cells and EpiSC [2 and 51], mouse ES cells self-renew in response to LIF. Nanog was isolated on the basis of its ability to confer LIF independent self-renewal of mouse ES cells [8], an activity now shown to require the Nanog target gene Esrrb [33••]. Both Esrrb and the additional direct Nanog target gene Klf4 [33••], can confer LIF independence upon mouse ES cells [8, 18 and 40•], though to varying degrees [33••]. It will be illuminating to determine more fully the epistatic relationship between TFs required to confer LIF independent self-renewal. Klf4 is also elevated in response to LIF [18] suggesting that the Nanog and the LIF-activated cascades may converge on a similar set of target genes to impose the pre-implantation PGRN configuration [53].