A naive state of pluripotency

A naive state of pluripotency is functionally defined by the capacity of mouse ESCs to participate in germ layer differentiation and generate germline competent chimeras following their incorporation into host blastocysts (Nichols and Smith, 2009). The naïve cells seem to mimic, although do not completely match, the transcriptome of the pluripotent epiblast of the late blastocyst. Studies at both molecular and functional levels suggest that the epiblast of the E3.75 to E4.5 Phenformin Supplier is likely to be the founder tissue of ESCs (Boroviak et al., 2014; Brook and Gardner, 1997). The success in isolating ESCs from the epiblast of the permissive 129 mouse strain declines precipitously between E5 and E6 regardless of the culture conditions (LIF/Serum or 2i/LIF) used for derivation (Boroviak et al., 2014; Gardner and Brook, 1997). Gene knockout studies have demonstrated that none of the components of the LIF signaling pathway, i.e., LIF receptor, signal transducer gp130, and STAT3, are required for pre-implantation embryo development (Li et al., 1995; Nakashima et al., 1999; Takeda et al., 1997; Ware et al., 1995; Yoshida et al., 1996). Activation of LIF signaling is, however, required for blastocyst survival during diapause (Nichols et al., 2001), a physiological process that momentarily prevents implantation to delay pregnancies in mice. LIF signaling prevents apoptosis in the epiblast until implantation takes place and the epiblast resumes cell division. In sharp contrast to the in vivo situation, LIF signaling stimulates the cell cycle of ESCs in LIF/serum culture (Coronado et al., 2013). Therefore, the activation of LIF signaling may be a cellular response to the drive to cell immortalization in vitro.
Modulation of pluripotency reveals a progressive poising for lineage differentiation Several studies have reported the conversion of ESCs from the naive state to other states of pluripotency (Fig. 1A). ESCs cultured in a medium conditioned by the human HepG2 hepatocarcinoma cells (known as MEDII) convert to a morphologically distinct population, the early primitive ectoderm-like (EPL) cells (Rathjen et al., 1999). EPL cells could differentiate into derivatives of the three germ layers in vitro, indicating that they may retain pluripotency. However, EPL cells have lost chimera-forming ability (Rathjen et al., 1999). The establishment of EPL cells is accompanied by changes in gene expression pattern such as the down-regulation of the early epiblast markers Gbx2 and Rex1 and the up-regulation of the late epiblast marker Fgf5, suggesting that the EPL cells resemble the epiblast population of the post-implantation mouse embryo (Pelton et al., 2002; Rathjen et al., 1999). We currently lack a complete transcriptome characterization to benchmark EPL cells against an epiblast from the implanting blastocyst stage to the gastrula stage. Moreover, whether EPL cells can be derived directly from the epiblast of a postimplantation embryo under the MEDII condition is presently not known. ESCs can be converted into the cells that are similar to the epiblast stem cells (EpiSCs) that are derived directly from the epiblast of postimplantation embryo. This is accomplished by culturing ESCs as small colonies in a chemically defined culture medium supplemented with knockout serum replacement factors (KOSR), FGF2 and Activin A (Guo et al., 2009). The transition is accompanied by gain of FGF2/ERK and Activin A/Smad signaling dependency, down-regulation of early epiblast markers and up-regulation of late epiblast markers (Osteil et al., 2016a). Like the EPL cells, the converted cells retain the capacity to differentiate into derivatives of the three germ layers in vitro but lose chimeric competency, suggesting that ESC-derived EpiSCs may be developmentally similar to the epiblast population of the post-implantation mouse embryo. EpiSCs can be derived directly from the epiblast of E6 to E8 post-implantation mouse embryos. These cells display a global gene expression profile similar to that of the epiblast of the post-implantation embryo, but distinct from that of the ESCs (Brons et al., 2007; Kojima et al., 2013; Tesar et al., 2007). The characteristic features of EpiSCs have led to the notion of a primed state of pluripotency, which is presumably closer to the commitment of lineage differentiation (Tesar, 2016). Atop transcriptomic reconfiguration, the shift from naive to primed pluripotency is accompanied by genome-wide hyper-methylation, enhanced activity of DNA methylation, ATP-dependent chromatin modification and nucleosome remodeling (Ficz et al., 2013; Habibi et al., 2013; Leitch et al., 2013), differential expression of miRNA clusters (Jouneau et al., 2012) and selective enhancer activity (Factor et al., 2012), and a transition from oxidative phosphorylation to glycolysis for energy production (Zhou et al., 2012).