Of the somatic cells included

Of the somatic g proteins included in our study, MEFs revealed the most complex requirements for signaling pathway modulation and allowed highly efficient iPSC formation only in the presence of all three compounds. In contrast, hepatoblast reprogramming was greatly facilitated by TGF-β inhibition alone, whereas GMPs required only Wnt activation—either by GSK3β inhibition or by enforced expression of Ctnnb1—to rapidly reactivate pluripotency loci and to enter a pluripotent state. These cell-type-specific requirements are reflected in the relative strength of TGF-β and Wnt signaling in the starting cell populations, suggesting a way to prospectively identify somatic cell types particularly amenable to factor-mediated reprogramming. The observation that AA, a cofactor of chromatin-modifying enzymes, did not significantly enhance iPSC formation from blood progenitor cells indicates that an epigenetic state favorable for reprogramming might preexist in these cells. However, we cannot rule out that AA modulates GMP reprogramming in a way not measured by our assays. We observed marked differences between fibroblasts and somatic progenitor cells with respect to the synchronicity of reprogramming, defined as the percentage of cells within an emerging colony that expresses ESC-specific genes. Thus, whereas GMPs and hepatoblasts rapidly gave rise to colonies containing predominantly cells that had reactivated endogenous pluripotency loci, we only infrequently observed nascent MEF-iPSCs with these characteristics (see the model in Figure 4F). This supports the notion that specific molecular features “prime” progenitor cells for efficient reprogramming. Indeed, reducing MAP kinase signaling and elevating the levels of the histone demethylase KDM2B—two intrinsic features of GMPs we identified—facilitated the synchronous reactivation of pluripotency loci in MEFs. This is in agreement with the ability of KDM2B to activate genes during early phases of iPSC formation (Liang et al., 2012) and suggests that rapid removal of epigenetic barriers by this enzyme might be involved in the remarkable reprogramming response of GMPs. It will be interesting to study how OKSM factors, chromatin modulators, and CTNNB1, an interaction partner of pluripotency factors (Kelly et al., 2011), cooperate to achieve rapid iPSC formation. Our observation that GMPs can readily acquire pluripotency upon OKSM expression is reminiscent of a recent report that describes privileged reprogramming properties of a fast cycling subset within this progenitor cell population (Guo et al., 2014). Although our results do not exclude a role of fast cell-cycle transition in synchronous GMP reprogramming, they imply that the molecular mechanisms underlying this phenomenon are complex and involve additional cellular features. The synchronous reactivation of pluripotency loci in almost all GMP-derived iPSC colonies suggests that 3c conditions override any heterogeneity that might exist within the GMP pool. The high colony-formation efficiency of close to 100% in less than a week, the rapid and homogeneous reactivation of core pluripotency loci, and the virtual absence of nonreprogrammed cells upon OKSM expression in blood progenitors in the presence of 3c resemble reprogramming kinetics reported after MBD3 ablation in somatic cells (Rais et al., 2013). This suggests that at least some adult cell types can achieve so-called nonstochastic or deterministic reprogramming upon more subtle experimental modulation than the genetic interference with essential endogenous genes (see model in Figure 4F). Collectively, our results define cell-type-specific requirements for highly efficient and synchronous iPSC formation from different somatic cells by combined modulation of signaling pathways and chromatin modifiers. This provides a refined framework for the further exploration of the mechanisms underlying the erasure of the somatic state and the induction of pluripotency.