1999). signaling. These results provide evidence for at least two distinct FGF-dependent signal transduction pathways: a Sprouty-insensitive Ras/MAPK pathway required for the transcription of most mesodermal genes, and a Sprouty-sensitive pathway required for coordination of cellular morphogenesis. have implicated a number of signaling pathways in the inductive events leading to the formation and patterning of the mesoderm. Members of the fibroblast growth factor (FGF) family of secreted polypeptides have the ability to induce mesoderm in na?ve ectodermal tissue, a capacity shared with TGF–type proteins (Kimelman and Griffin Rabbit Polyclonal to MCL1 2000). Moreover, the expression of a dominant negative form of the FGF receptor (dnFGFR) at the time of mesoderm induction completely blocks this process in vivo (Amaya et al. 1991; 1993). FGF receptor signaling is also thought to be involved in the subsequent maintenance of the mesodermal tissue as the expression of dnFGFR after the initial induction again results in the loss of mesodermal markers (Kroll and Amaya 1996). This maintenance function is a result of an autocrine loop, which involves the activation of transcription, which reinforces the mesodermal fate of the induced tissue (Isaacs et al. 1994; Schulte-Merker and Smith 1995). Once mesoderm is formed, gastrulation can proceed through the orchestrated movement of the three BMS-708163 (Avagacestat) germ layers, to produce the anteriorCposterior (A-P) axis of the embryo. These cell movements involve both involution and convergent extension. Convergent extension is characterized by the polarization of the mesodermal cells and their mediolateral intercalation to produce a pronounced elongation of the A-P axis (Keller 1991; Keller et al. 1992). The mechanisms by which convergent extension and gastrulation as a whole are coordinated remain poorly understood, although recent evidence has implicated signaling through a noncanonical Wnt pathway (Djiane et al. 2000; Heisenberg et al. 2000; Tada and Smith 2000; Wallingford et al. 2000). FGFs constitute a BMS-708163 (Avagacestat) second signal transduction pathway, which has also been implicated in the processes of gastrulation morphogenesis (for review, see Rossant et al. 1997). embryos overexpressing the dnFGFR fail to undergo gastrulation; however, the absence of mesoderm in these embryos has made it BMS-708163 (Avagacestat) impossible to determine whether FGF signaling has a direct role in morphogenesis, or if the observed gastrulation defects are secondary to the failure of mesoderm formation. FGFs constitute a family of peptide growth factors, which with binding to their receptors (FGFR), induce receptor dimerization and autophosphorylation of a number of intracellular tyrosine residues (Mohammadi et al. 1996). These BMS-708163 (Avagacestat) phosphorylated tyrosines then serve as docking sites for Src homology 2 (SH2) domain-containing proteins such as phospholipase C- (PLC-), which binds specifically to phosphotyrosine 766 (Y766) and stimulates the phosphatidylinositol hydrolysis and the mobilization of Ca2+ in the cell (Mohammadi et al. 1992; Peters et al. 1992). Another consequence of receptor autophosphorylation is to activate the GTPase Ras, setting off a cascade of kinases including Raf, MEK, and finally MAPK, which ultimately results in processes such as the induction of gene expression (Fambrough et al. 1999). FGF-mediated mesoderm induction in expression and mesoderm formation. In contrast, the binding of PLC- to Y766 is not essential for this process (Muslin et al. 1994). Receptor tyrosine kinase (RTK) signaling is regulated at a number of levels including ligand availability, phosphatase activity, and Ras inhibitory proteins such as Ras-GAP. Another inhibitor of FGF signaling, termed Sprouty (Spry), has been identified in (Hacohen et al. 1998). Spry plays a role in the development of the apical branching pattern of airways, a process known to require FGF. mutations lead to the growth of multiple fine branches from the stalks of the primary branches (Hacohen et al. 1998), a phenotype similar to that observed when FGF signaling is hyperactive, suggesting that Spry inhibits FGFR activity. Moreover, the FGF pathway induces the expression of epidermal growth factor receptor (EGFr) and Torso (Casci et al. 1999). These predominantly genetic BMS-708163 (Avagacestat) studies also suggested that instead of acting extracellularly, Spry acts intracellularly to inhibit the Ras/MAPK pathway (Casci et al. 1999; Reich et al. 1999). Several proposals as to the precise position at which Spry impinges on the FGF pathway have been suggested ranging from a receptor proximal mode of regulation through interactions with Ras, to regulating Raf or molecules further downstream (Casci et al. 1999;.