Intriguingly, hyper-activation of LIF signalling can even override the programmes induced by buy Vorinostat Activin and FGF in EpiSC, to promote the generation of chimaera-competent ES-like cells [55••]. In contrast, enforced Nanog expression in EpiSC lines cannot drive reprogramming without removal of Activin/FGF [6]. To determine whether extrinsic signals are dominant over intrinsic

determinants in dictating pluripotent states, it will be important to test the ability of LIF hyper-activation to reprogramme Nanog−/− EpiSC and whether Nanog overexpression can reprogramme EpiSC cell lines cultured in N2B27/Activin/FGF supplemented with LIF. As mentioned above, human ES cells can be established from pre-implantation embryos under conditions used to establish mouse post-implantation (not pre-implantation) pluripotent cell lines [2 and 3]. This raises the question of whether an equivalent of the mouse pre-implantation pluripotent state exists in humans. Attempts have been made to generate human ES cells that possess desirable traits of mouse ES cells such as clonogenicity [56, 57, 58 and 59•]. LIF-dependent human ES cells were obtained using Oct4/Sox2/Nanog/lin28 www.selleckchem.com/products/PD-0332991.html [56 and 60] or using Nanog alongside Oct4/Sox2/Klf4/myc [57]. LIF-dependent human cells express pre-implantation markers,

though to varying degrees [57, 58, 59• and 60]. Studies using Oct4/Sox2/Klf4/myc without Nanog found that conversion of human ES cells to a LIF-dependent CYTH4 state was possible either in the presence of a compounds that boost Klf4 expression [58] or by including an Nr5a2 transgene [59•]: both of these TFs can reprogramme EpiSCs [24 and 50]. With one notable exception [59•], these cells remain dependent on continued transgene expression [57 and 58] or signal modulators [56 and 60]. Perhaps, in these latter cases self-renewal of converted human ES cells is not robustly sustained because LIF signalling cannot sufficiently activate the pre-implantation PGRN which requires further

reinforcement from additional TFs. Nanog is crucial in driving establishment of pluripotency during specification of the pre-implantation epiblast and for maintenance of the specified PGC population later in development. By combining in vivo studies with results generated by in vitro reprogramming of Nanog−/− somatic cells, and the identification of Nanog transcriptional targets, at least two aspects of Nanog activity have emerged: first, Nanog regulates expression of other pre-implantation TFs, and thus stands close to the top of the transcriptional hierarchy governing the pre-implantation PGRN; second, Nanog interacts with a number of epigenetic factors [ 48•• and 61], targeting them to chromatin and possibly initiating reversion of repressive marks at silent genes during establishment of pluripotency ( Figure 3).