2. Embryonic stem cells
2.4.3. WNT/Β‐CATENIN
2.4.3. WNT/Β‐CATENIN
Besides LIF and BMP4 signaling, the canonical Wnt/β‐catenin pathway is also involved in maintaining the pluripotent state of both mouse and human ESC [48]. As can be seen in Figures 2 and 3, upon binding of the Wnt protein on the surface receptor Frizzled (Fzd), the glycogen‐synthase kinase‐3 (GSK3) gets inhibited, leading to a reduced degradation and, thus, increased levels of β‐catenin [43]. After translocation to the nucleus, β‐catenin acts together with members of the Tcf transcription factors to form complexes that mediate gene expression. As shown in Figure 3, Tcf3 represses the pluripotency genes Oct4, Nanog and Sox2. β‐catenin blocks the suppressive function of Tcf3 [49, 50]. Subsequently, the target genes Oct4, Nanog and Sox2 are activated [48]. Berge et al. demonstrated the
dependency of mESC for Wnt signals, in contrast to EpiSCs [51]. β‐catenin does not only operate with Tcf3, but promotes pluripotency together with Tcf1 [50].
Figure 3. Wnt/βcatenin, E‐cadherin and LIF pathways in mESC are involved in the maintenance of pluripotency in mESC. The correlation between the Wnt pathway and the Tcf3 and Tcf1 proteins are illustrated. After mobilization of βcatenin into the nucleus, it complexes with Tcf3. By these means Tcf3’s suppressive function on pluripotency genes like Oct4 is abolished and pluripotency is maintained. In a complex with Tcf1, βcatenin activates pluripotency associated genes. (Taken from [49]).
2.5. Transcription factors involved in pluripotency
Three transcription factors have been shown to play a major role in maintaining the pluripotent state of both the inner cell mass and embryonic stem cells: Nanog, Oct3/4 and Sox2.
2.5.1. NANOG is a transcription factor that is found in both human and mouse pluripotent stem cells [52]. While Nanog is necessary for the formation of germ cells [52], it does not
reported a trend towards differentiation of cells upon transient downregulation of Nanog, but not commitment to differentiation. Furthermore, Nanog seems indispensable for the
formation of germ cells since Nanog‐/‐ cells cannot reach the genital ridge during development [53]. Nanog can preserve pluripotency in mESC cells in the absence of LIF, and by these means LIF/Stat3 signaling is circumvented [52]. The sequence that Nanog binds to has been proposed but it is still a controversial matter [33].
Loh et al. point out the importance of Nanog as a key regulator of pluripotency by controlling Oct4 and Sox2 expression levels. Indeed, physiological levels of Oct4 and Sox2 prevent differentiation, while overexpression of Oct4 triggers differentiation [54]. Both Nanog and Oct4 have Esrrb and Rif1 as common targets. These have been shown to be important for conserving pluripotency in mESC [55].
2.5.2. OCT4 is member of the octamer transcription factor class. As the class name indicates, Oct4 binds to a DNA site composted of the eight base pairs ATGCAAAT [56, 57].
Oct4 is member of the POU (Pit, Oct and Unc) transcription factor class that interacts with DNA through two DNA binding domains [33], one low and one high affinity binding domain [58].
To uncover the role of Oct3/4 in ESC, Niwa et al. regulated the expression levels of Oct3/4 via a tetracycline‐regulated transactivator and a transactivator‐responsive Oct3/4 transgene in Oct3/4 null mESC [54]. They show that ESC do not keep their pluripotent state but differentiate into trophoectoderm upon downregulation of Oct3/4, regardless of the presence or absence of LIF. Since an overexpression of Oct3/4 triggers differentiation as well, Niwa et al. concluded that a controlled Oct3/4 level within ES cells is necessary to keep
the stem cell phenotype. They defined the differentiation threshold at 50% of the endogenous Oct3/4 expression levels.
Loh et al. determined the targets of Oct4 and Nanog in the whole mESC genome using, among other methods, paired‐end ditag technology in combination with chromatin immunoprecipitation (ChIP) [55]. Between the human and the mouse genome, Oct4 and Nanog targets did not result to overlap much, as this accounted for 9% for Oct4 and 13% for Nanog, with ~1000 binding sites for Oct4 and ~3000 for Nanog. Nevertheless, Loh et al.
demonstrated that Oct4 and Nanog shared some important targets. Downstream genes of Nanog and Oct binding targets were shown to be involved in maintaining pluripotency.
Nanog is not the only transcription factor binding at the same genomic location as Oct4. In fact, Oct4 was proposed as “anchor point” for the further binding of other transcription factors [38]. Furthermore, Oct4 and other factors seem to autoregulate their own expression levels [38]. Enhancer regions of Oct4 and Nanog are bound by additional transcriptions factors.
2.5.3 SOX2 belongs to a family of proteins that contains a “high mobility group (HMG) box DNA binding domain box”. Sox2 was shown to bind in minor grooves of DNA at defined binding sites [33]. Loh et al. demonstrated that Sox2 binding sites overlap to a great extends with Oct4 sites, indicating that Sox2 and Oct4 act together to mediate their target genes.
2.6. The chromatin state in ESC
Transcription factors are not the only key actors for conserving the pluripotent state of ESC:
non‐coding‐ and microRNA as well as the chromatin status of ESC [38] also play an important role.
In ESC chromatin is organized to a higher degree as euchromatin in comparison to differentiated cells. While cells commit to different lineages during differentiation, they reorganize their chromatin structure and more compact heterochromatin is found [59, 60].
Chromatin structure is influenced by the methylation status of DNA, as well as the modifications of histones. In addition, ATP‐dependent enzymes remodeling chromatin have been shown to influence the chromatin status in ESC [60].
2.7. The metabolism in ESC
Knowledge has been gained on the genetic and epigenetic status of pluripotent stem cells.
Chromatin status as well as DNA methylation pattern change extensively during the transit from pluripotent to differentiated cells.
Besides the genetic and epigenetic status being important to keep the pluripotent characteristics within a ESC, the group of Banerjee pointed out the importance of metabolism in the control of ESC proliferation and differentiation [61]. They used the chemical component Carbonyl Cyanide m‐Chlorophenylhydrazone to knock down mitochondrial function. While the cell cycle phase was not affected, the proliferation rate of mESC was slowed down in comparison to untreated mESC, indicating the importance of mitochondria on ESC proliferation, also during differentiation. Indeed, the expression
profiling after seven days of treatment revealed significant differences in genes related to development and differentiation. Further investigation needs to be done to understand the role of mitochondria within proliferating and differentiating ESC. However, these results underline the importance of grasping the impact of mESC metabolism regulation in pluripotency and differentiation conditions.