Atractyloside Dipotassium Salt In mESC cultures cells take o

In mESC cultures, Atractyloside Dipotassium Salt take on a mesendodermal fate before being committed to either mesoderm or DE (Tada et al., 2005). Activin is used as a surrogate for nodal as they both activate Smad2/3 signaling via activation of the Alk4 receptor (Schier, 2003). In the mesendoderm population, high concentrations of nodal/activin-signaling induce anterior streak and DE cells while BMP4 or low concentrations of nodal/activin induce posterior streak and mesoderm (Kubo et al., 2004; Hansson et al., 2009; Willems and Leyns, 2008). The PS genes T, Mix-like 1 (Mixl1), and Gsc are expressed in this population in response to increasing concentrations of activin. High activin levels further induce the DE markers Sox17, Cadherin1 (Cdh1 (E-cadherin)), and Forkhead box a2 (Foxa2). In contrast, BMP4 induces T, Mixl1, and the mesodermal marker Fetal like kinase 1 (Flk1; VEGFR2/Kdr; (Gadue et al., 2005)). During mESC differentiation, T-expressing cells give rise to both meso- and endodermal derivatives (Kubo et al., 2004) and we have previously shown that a T-GFP reporter cell line (T; (Fehling et al., 2003)) is preferentially activated by BMP4 or by a low concentration of activin (Hansson et al., 2009). Fibroblast growth factor (Fgf) signaling is required for the differentiation of mesendoderm and DE to occur properly in mES cells (Hansson et al., 2009; Willems and Leyns, 2008; Funa et al., 2008; Morrison et al., 2008). The Fgf family of proteins consists of 22 members named Fgf1–23 (Fgf15 is the mouse ortholog of human Fgf19). They activate one or more of four receptor tyrosine kinases, the Fgf receptors (FGFR)1–4. FGFR1–3 have two splice variants in their Ig-like domain III, the FGFR(III)b or FGFR(III)c isoforms (FGFRb or FGFRc hereafter; (Itoh and Ornitz, 2004; Ornitz and Itoh, 2001)). Fgfs are involved in many functions in the developing embryo such as germ layer formation, cell proliferation, and cell migration (Ornitz and Itoh, 2001). In early mouse development, Fgf signaling is necessary for the migration of epiblast cells through the PS (Ciruna et al., 1997; Guo and Li, 2007). The loss of Fgf4 is lethal at Embryonic Days (E)4–5, due to the inability of epiblast cells to undergo epithelial-to-mesenchymal transition and migrate through the PS (Feldman et al., 1995). FGFR1−/− mice also die at gastrulation and both Fgf4 and FGFR1 are expressed in the ICM and PS (Yamaguchi et al., 1994; Deng et al., 1994). Fgf4 is expressed in pluripotent mES cells and has been shown to be necessary for differentiation into ectoderm and mesoderm lineages, suggesting a crucial role of Fgf4 in the initiation of differentiation (Kunath et al., 2007). Nevertheless, previous work has shown that Fgf4−/− cells can differentiate in vitro, albeit at a low frequency, and give rise to tumors consisting of a wide range of differentiated cell types in vivo (Wilder et al., 1997). Here we show that ES cells deficient for Fgf4, a potential FGFRc isoform-activating ligand, were able to differentiate to definitive endoderm cells at levels comparable to wt and Fgf4+/− cells. Furthermore, we extend our previous finding that Fgf signaling is necessary for DE formation (Hansson et al., 2009), and investigate the effects of activating different FGFR isoforms on mesendoderm and DE differentiation. We demonstrate that FGFRc isoforms are up-regulated in both Sox17+ and Sox17– populations emerging after activin treatment. By means of reporter cell lines and immunocytochemistry we find that Fgfs which preferentially activate FGFRc isoforms augment the expression of PS and mesendoderm markers T and Gsc and selectively expand an EpCAM+Sox17− population. In contrast, Fgfs activating FGFRb isoforms have no effect on the expression of these markers nor on the expansion of EpCAM+ cells. Fgfs activating the FGFRc isoforms show the highest mitogenic effects early in the differentiation period, while proliferation rates are reduced later in the culture period as expected from increased contact inhibition at higher cell densities. We conclude that FGFRc isoforms promote mesendoderm but not DE formation and that Fgf4 signaling is dispensable for induction of DE in mES cells.