The mouse intestinal epithelium is

The mouse intestinal epithelium is renewed through intestinal stem cells (ISCs) every 3–4 days (Barker, 2014). The “stem cell zone” model identifying crypt purchase AICAR columnar (CBC) cells at the base of the intestinal crypt (Cheng and Leblond, 1974) and “+4 stem cell” model identifying a ring of cells above the CBC cells are two current models of ISCs (Buczacki et al., 2013). Leucine-rich G-protein-coupled receptor 5 (LGR5) marks active CBC stem cells (Barker et al., 2007), while B-cell-specific Moloney murine leukemia virus integration site 1 (BMI1) marks the +4 position quiescent ISCs (Sangiorgi and Capecchi, 2008). BMI1 is a component of the polycomb repressor complex, which functions in gene silencing (Luis et al., 2012). Other markers for the quiescent ISCs at the +4 position have also been identified (Montgomery et al., 2011; Powell et al., 2012; Takeda et al., 2011). Functional studies have demonstrated that BMI1+ ISCs contribute to the regenerative response to ionizing radiation injury while LGR5+ ISCs are susceptible and unable to lineage trace post-radiation injury (Yan et al., 2012). Radiation injury to the gut is a common complication in patients undergoing cancer radiotherapy. Radiation causes extensive DNA damage followed by apoptosis, particularly in the actively proliferating cells of the gut, leading to gastrointestinal (GI) syndrome (Anno et al., 1989). Thus studying mechanisms by which the intestinal epithelial cells regenerate in response to radiation is important for developing strategies to minimize injury resulting from ionizing irradiation. The zinc-finger transcription factor Krüppel-like factor 4 (KLF4) is expressed in the intestinal epithelium, promotes cellular differentiation, and contributes to epithelial homeostasis (Ghaleb et al., 2011). KLF4 also exhibits an anti-apoptotic activity to γ-irradiation in vitro by inhibiting the p53-mediated apoptosis (Ghaleb et al., 2007a). In vivo, we demonstrated that deletion of Klf4 from the intestinal epithelium has a detrimental effect on the survival of mice following total body irradiation (Talmasov et al., 2014). In the short term, after irradiation KLF4 suppresses both apoptosis and proliferation and assists in DNA damage repair (Talmasov et al., 2014). In contrast, during the regenerative phase following irradiation, KLF4 plays an opposite role by promoting crypt cell survival and proliferation (Talmasov et al., 2014). KLF4 therefore exerts a context-dependent activity in the gut epithelium in response to ionizing irradiation. This context-dependent nature of KLF4\'s transcriptional activity has previously been observed in vitro (Rowland and Peeper, 2006). To reconcile the dichotomy between KLF4\'s cytostatic nature at homeostasis and pro-proliferative activity during regeneration subsequent to irradiation, we determined the contribution of KLF4 in controlling the fate of BMI1+ ISCs by lineage tracing. Our results indicate that KLF4 is a critical factor that determines the proliferative potential of BMI1+ stem cell-derived lineage.
Results and Discussion
Experimental Procedures
Author Contributions
Acknowledgments This research was supported by grants from the NIH (CA084197 and DK052230) to V.W.Y. C.-K.K. was supported by a T32 Medical Scientist Training Program grant (GM008444). We thank Dr. Jie Yang and Ivan Crnosija for support with statistical analysis.
Introduction Induced pluripotent stem cells (iPSCs) can be generated from somatic cells by introducing Oct4, Sox2, Klf4, and c-Myc (Takahashi and Yamanaka, 2006). This can be achieved by reprogramming the transcription network and epigenetic signature of the parental somatic cells. iPSCs have several benefits for basic research, drug innovation, and regeneration therapy. However, recent studies have reported genetic and epigenetic variations with iPSCs (Hussein et al., 2011; Kim et al., 2010; Polo et al., 2010; Ruiz et al., 2012; Stadtfeld et al., 2010; Taapken et al., 2011), which influences gene expression and could lead to functional diversity within iPSC replicates (Liang and Zhang, 2013). In fact, several studies have reported the heterogeneous differentiation potential among generated iPSC clones compared with those of embryonic stem cells (ESCs) (Feng et al., 2010; Hu et al., 2010; Narsinh et al., 2011). It is important that every iPSC clone shows high quality without variation for basic research and clinical purposes. Many attempts have been made to solve these problems by various methods (Gafni et al., 2013), but not all iPSCs exhibit quality as high as that of ESCs.