Introduction The pluripotency promoting role

Introduction The pluripotency-promoting role of the reprogramming factors OCT4, SOX2, KLF4, and MYC (OSKM) is widely appreciated. However, these reprogramming factors also promote expression of non-pluripotency genes. For example, OCT4 (Pou5f1) directly promotes expression of genes important for mouse primitive endoderm (Aksoy et al., 2013; Frum et al., 2013; Le Bin et al., 2014), an extraembryonic lineage present in the blastocyst, SOX2 indirectly promotes expression of primitive endoderm genes in the mouse KU-57788 (Wicklow et al., 2014), KLF4 may regulate expression of primitive endoderm genes in the mouse blastocyst (Morgani and Brickman, 2015), and MYC regulates endodermal genes in fibroblasts and embryonic stem cells (ESCs) (Neri et al., 2012; Smith et al., 2010). These observations raise the possibility that OSKM induce expression of endodermal genes in somatic cells. In support of this idea, several groups have reported that endodermal genes, such as Gata6, Gata4, and Sox17, are upregulated in protocols used to reprogram fibroblasts to induced pluripotent stem cells (iPSCs) (Hou et al., 2013; Serrano et al., 2013; Zhao et al., 2015). However, there is no consensus as to whether endodermal gene expression promotes or antagonizes the acquisition of pluripotency. GATA4 and GATA6 can reportedly substitute for OCT4 to produce iPSCs (Shu et al., 2013, 2015), arguing that endodermal genes promote acquisition of pluripotency. Consistent with this, endodermal genes are reportedly expressed by cells as they become pluripotent during chemical reprogramming (Hou et al., 2013; Zhao et al., 2015). By contrast, other evidence suggests that endodermal genes oppose pluripotency during reprogramming. For example, Gata4 interferes with the acquisition of pluripotency during OSKM reprogramming (Serrano et al., 2013), Gata6 is expressed in some partially reprogrammed cells (Mikkelsen et al., 2008), which are thought to be trapped in a state between differentiated and pluripotent (Meissner et al., 2007), and Gata6 knockdown led to increased expression of Nanog in these cells (Mikkelsen et al., 2008). Thus, endodermal genes have been described as indicators of incomplete reprogramming. Here, we show that OSKM drive cells along two distinct and parallel pathways, one pluripotent and one endodermal.
Results and Discussion
Experimental Procedures
Author Contributions
Acknowledgments We thank Jason Knott, Beronda Montgomery, David Arnosti, and Monique Floer for discussion. This work was supported by California Institute for Regenerative Medicine grant TG2-01157 to A.P., NIH grants R03 HD077112 to K.W., R01 HD075093 to K.L., R01 GM104009 to A.R., MSU AgBioResearch, and Michigan State University.
Introduction Human mesenchymal stromal cells (MSCs; also known as mesenchymal stem cells) have gained considerable attention for their promising potential in cell-mediated therapy. MSCs are progenitor cells whose capacity to differentiate into osteoblasts and modulate the immune response makes them instrumental in bone regenerative medicine (Kode et al., 2009; Stappenbeck and Miyoshi, 2009; Alvarez et al., 2015; Deng et al., 2015). Clinical trials have already demonstrated that implantation of MSCs is an effective and safe treatment modality in defect repair (Bianco et al., 2013; Quarto et al., 2001; Wang et al., 2012). Although MSCs used in cell therapies are mostly isolated from adult bone marrow, proliferation and differentiation capacity of these MSCs from bone marrow (BMSCs) has been shown to decline as the donor patient ages (Kern et al., 2006; Quarto et al., 2001). Due to these shortcomings of BMSCs, human embryonic stem cells (hESCs), which have the potential to provide an unlimited supply of MSCs, are potential alternative sources for MSCs (Choo and Lim, 2011; Thomson, 1998). hESC-derived MSCs are similar to BMSCs biologically and functionally, but with higher osteogenic potential and proliferation rates as well as less immunogenicity (Li et al., 2013). Three methods have been developed to promote the differentiation of hESCs into MSCs: (1) the formation of three-dimensional embryonic bodies, (2) the culture of hESCs on stromal cells, and (3) the culture of hESCs as monolayers (Arpornmaeklong et al., 2009; Barberi et al., 2005; Villa-Diaz et al., 2012; Karp et al., 2006). However, the molecular mechanism by which hESCs commit to MSC fate remains elusive.