Interestingly KLF and TFCP L

Interestingly, KLF2 and TFCP2L1 are also involved in LIF/STAT3-mediated ESC self-renewal. KLF2, which is functionally redundant with KLF4, a known target of LIF/STAT3 signaling, can maintain mESCs in an undifferentiated state in the presence of serum (Hall et al., 2009). TFCP2L1, a key target of LIF/STAT3 signaling pathway, can also largely substitute the effect of LIF on ESC self-renewal (Martello et al., 2013; Ye et al., 2013). Notably, both KLF2 and TFCP2L1 can partially replace the function of PD on promoting ESC self-renewal (Ye et al., 2013; Yeo et al., 2014). However, when overexpressed alone, neither of them could maintain ESC self-renewal under serum-free condition in the absence of LIF and 2i (Figure 2D). It would be of great interest to explore how KLF2 and TFCP2L1 interact with each other in maintaining pluripotency. Another important effect of KLF2 and TFCP2L1 we observed is to reprogram EpiSCs back to naive state ESCs (Figures 4D–4I). Both factors are highly expressed in mouse ESCs but barely detectable in EpiSCs (Hall et al., 2009; Ye et al., 2013). Previous studies have demonstrated that forced expression of each factor alone could reprogram EpiSCs back to ESCs when combined with 2i/LIF (Hall et al., 2009; Ye et al., 2013). In our study, we found that combined expression of Klf2 and Tfcp2l1 can convert EpiSCs back to ESCs in the absence of 2i (Figures 4G–4I), which is consistent with their function in mESC maintenance (Figure 2D). However, the underlying molecular mechanism of such a phenomenon needs to be further investigated. KLF2 and TFCP2L1 have been defined as two of the essential transcription factors for the induction of naive state pluripotency. Forced expression of Klf2 in combination with another pluripotency factor Nanog is sufficient to induce naive state pluripotency in human ESCs, which share many defining features with mouse EpiSCs (Takashima et al., 2014). Additionally, depletion of Tfcp2l1 in naive state human pluripotent stem Digoxigenin-11-dUTP collapses the naive pluripotent state (Takashima et al., 2014). Tfcp2l1 is highly expressed in the inner cell mass of human blastocysts but significantly downregulated during derivation of human ESCs (O’Leary et al., 2012), which suggests that TFCP2L1 might also play a role in establishing naive state pluripotency in human. Therefore, our study provides an expanded understanding of ESC self-renewal mechanism, a progress that might be critical in developing novel culture conditions for the derivation and maintenance of authentic ESCs from species other than rodents.
Experimental Procedures
Author Contributions
Acknowledgments
Introduction Transcription start sites (TSSs) of developmentally regulated genes are frequently marked by overlapping domains of active (H3K4me3) and repressive (H3K27me3) histone marks. These “bivalent” domains are generally believed to be stable in self-renewing pluripotent stem cells (PSCs) and serve to establish a “poised” transcriptional state (Bernstein et al., 2006; Mikkelsen et al., 2007). During lineage specification, the bivalent state is resolved, allowing developmental genes to be activated or more stably repressed, depending on the lineage being specified. The molecular mechanisms underpinning these epigenetic changes are poorly understood but are ultimately regulated through the concerted action of histone methyl-transferases (HMTs) and histone de-methylases (Voigt et al., 2013). In PSCs, H3K4me3 is established through the activity of trxG complexes containing MLL or SET enzymes (Bledau et al., 2014; Denissov et al., 2014; Hu et al., 2013), while the PRC2 complex establishes domains of H3K27 trimethylation (Boyer et al., 2006; Lee et al., 2006). JARID1 is thought to be important for the erasure of H3K4me3 (Christensen et al., 2007), while de-methylation of H3K27me3 is controlled by the activity of JMJD3 and UTX complexes (Agger et al., 2007). Although significant effort has been placed on understanding the biochemical role of these HMT complexes, only limited information is available on how this network of epigenetic modifiers is controlled in the pluripotent state and how they poise cells during the initial stages of differentiation.