Fgfs activating only FGFRb isotypes had no effect on

Fgfs activating only FGFRb isotypes had no effect on the number of EpCAM+Sox17-GFPHi buy Atractyloside Dipotassium Salt developing in response to activin or on the number of EdU-incorporating cells. Since FGFR inhibitors reduce the numbers of Sox17-GFPHi cells (Hansson et al., 2009) we suspect that endogenous Fgf signaling is sufficient and therefore the further addition of b-isoform-specific factors has no effect. A putative endogenous Fgf may be Fgf3, which is expressed in the PS during gastrulation and which only activates FGFRb isoforms (Ornitz et al., 1996; Zhang et al., 2006; Wilkinson et al., 1988). Alternatively, induction of EpCAM+Sox17-GFPHi cells may rely on precise levels of FGFRc signaling and increasing this signaling by addition of exogenous Fgf2 may be detrimental to formation of EpCAM+Sox17-GFPHi cells. The FGFR1b and -2b expression we observe may render the cells competent to respond to signals occurring later during organogenesis. Optimal induction of DE may be supported by a combination of factors, with initial activation of FGFRc isoforms during early differentiation to promote mesendoderm formation followed by a second step where activation of FGFRb isoforms maintains definitive endoderm.
Materials and methods
Acknowledgments We thank Drs. G. Keller, S. Nishikawa, and S. J. Morrison for the T-GFP, Gsc-GFP, and Sox17-GFP cell lines, respectively, and A. Rizzino and T. Kunath for Fgf4+/− and Fgf4−/− cell lines. We thank Søren Refsgaard Lindskog for excellent technical assistance. This work was made possible by funding from the Juvenile Diabetes Research Foundation (to P.S.).
Introduction The elevated blood glucose that characterizes diabetes mellitus results from the loss of insulin-producing β-cells from the islets of Langerhans in the pancreas (type 1 diabetes mellitus) or from a relative deficiency of insulin production in a setting of reduced insulin sensitivity (type 2 diabetes mellitus). Transplantation of cadaveric pancreata or the β-cell-containing islets thereof offers the only cure for patients dependent on exogenous insulin (Shapiro et al., 2000). However, immune-mediated damage to the transplanted β-cells and insufficient revascularization (Lau and Carlsson, 2009) leading to nutrient and oxygen-deprivation, especially inside large islets, limits islet survival. As a consequence, the long-term benefits of these grafts are limited; most recipients remain insulin-independent for less than two years (Ryan et al., 2005). Besides, current numbers of donor organs cannot provide enough material for all patients. The potential of human embryonic stem cells (hESCs) to differentiate into any somatic cell type, including glucose-responsive insulin-producing cells (Kroon et al., 2008), offers a possible solution to the shortage of transplantable cells. Unfortunately, current in vitro differentiation protocols do not generate homogeneous populations of functional β-cells (for review, see Van Hoof et al., 2009a). Furthermore, these protocols rely on consecutive exposures of hESCs to various factors, either in an adherent monolayer format or after an embryoid body-formation step, which is associated with spontaneous differentiation into various undesired cell types. In addition, the size of end-stage clusters varies greatly, and is difficult to control. The sensitivity of β-cells to external glucose levels, and their responsive insulin release, is highly dependent on cell-cell contact. Notably, clusters comprising multiple β-cells have a greater glucose-stimulated insulin release than single β-cells (Meda et al., 1990; Jaques et al., 2008). Furthermore, β-cells secrete more insulin when in three-dimensional aggregates than in monolayers (Brereton et al., 2006). In addition, nutrient deprivation is observed in transplanted islets with diameters exceeding 100μm (Lehmann et al., 2007). Therefore, hESC-derived β-cells will most likely function optimally after transplantation when clustered in multicellular structures that are approximately 100μm in diameter, a size that is large enough for efficient glucose-responsive insulin secretion, yet small enough to prevent nutrient starvation of cells residing in the core.