In this process, Esrrb forms a complex with Oct4 and Sox2, and synergistically upregulates the expression of pluripotency genes in MEFs. Remarkably, with the help of other Yamanaka factors (Klf4/Sox2/c-Myc), another orphan nuclear receptor, Nr5a2, was able to substitute for Oct4 in iPSC generation . In addition, Nr5a2 could greatly enhance iPSC reprogramming in conjunction with activation of another nuclear receptor, RARa/g . The finding that the RARa agonist (CD437) and RARg agonist (AM580) dramatically increased reprogramming efficiency further supports the notion that nuclear receptors play important roles in regulating
somatic cell reprogramming. Many studies demonstrated that small-molecule epigenetic modifiers could significantly influence reprogramming process and even substitute for certain reprogramming AZD6244 purchase transcription factors (Figure 2). BIX01294, an inhibitor of G9a histone methyltransferase (HMTase),
was shown to enable reprogramming of neural precursor cells or fibroblasts transduced with only two TFs, Oct4 and Klf4 . Besides well-known HDAC inhibitors (e.g. VPA, NaB) that have been demonstrated to facilitate reprogramming in various contexts [40 and 41], Parnate, an inhibitor of histone demethylase LSD1, was shown to enhance iPSC reprogramming as well . Interestingly, the well-known antioxidant compound vitamin C was recently shown to enhance reprogramming by modulating the activity of the histone demethylases Jhdm1a/1b . These findings highlight the dynamic changes of histone PTC124 order modifications in reprogramming. Recent mechanistic studies of iPSC reprogramming further illustrated how epigenetic changes are orchestrated in the early and late stages of reprogramming. Koche et al. showed that activated chromatin marks (e.g. H3K4 methylation) were targeted to promoters of pluripotency and developmentally regulated genes (e.g. Fgf4 and Lin28) before transcriptional
activation during the early phase of iPSC reprogramming [ 43]. It was also reported that two epigenetic factors, Parp1 and Tet2, were recruited to pluripotency loci (e.g. Nanog and Esrrb) and established early epigenetic marks by converting 5-methylcytosine (5mC) into 5-hydroxymethylcytosine (5hmC) during reprogramming [ 44]. Interestingly, it was reported that along with Klf4, Sox2, and c-Myc, another GNA12 Tet family protein Tet1 could enable somatic cell reprogramming in the absence of the key transcription factor Oct4 or nuclear receptors Esrrb and Nr5a2 [ 45], highlighting the important role of DNA demethylation (through hydroxymethylation) in reprogramming. Furthermore, other specific histone modifications were identified to occur in reprogramming. For example, inhibition of the H3K79 histone methyltransferase DOT1L (e.g. by a small molecule inhibitor) and the H3K9 methyltransferase Setdb1 (e.g. by RNAi) was shown to enhance iPSC generation [ 46 and 47].