The receptors of the FGFs (FGFRs) form a subfamily of cell surface receptor tyrosine kinases (RTKs) that includes four receptors in vertebrates (FGFR1 to 4), two in Drosophila (heartless and breathless), and one in C. elegans (egl-15). They are single spanning transmembrane proteins, with an extracellular

domain that binds to FGF ligands, heparan sulfate proteoglycans, and cell adhesion molecules, and an intracellular domain that harbours the tyrosine kinase activity of the receptor and interacts with intracellular substrates and signal transduction molecules ( Böttcher and Niehrs, 2005) ( Dorsomorphin in vivo Figure 1). FGFRs exist in multiple isoforms, in particular isoforms b and c that are generated by tissue-specific alternative splicing events and have very different FGF-binding specificities ( Zhang et al., 2006; Figure 1). However, the specificities of FGF ligand-receptor interactions have been established in a cell culture assay, and since these interactions are strongly influenced by cofactors such as HSPGs, they may differ substantially in an in vivo context. Binding of FGFs to FGFRs triggers receptor dimerization and tyrosine kinase activation, resulting in autophosphorylation of the intracellular domain of the receptor

NSC 683864 cell line and recruitment and assembly of signaling complexes. Multiple pathways have been shown to operate downstream of FGFRs (Figure 2). Briefly, the MAPK/Erk signaling cascade is the pathway most commonly employed by FGFRs and results in stimulation of the expression and/or activation of various transcription factors that act as effectors of the pathway, including Ets proteins, AP1, GATA proteins, c-myc, and CREB ( Yordy and Muise-Helmericks, 2000), and in the induction of multiple feedback inhibitors including Sef, MKP3, and Sproutys ( Figure 2; see below). The MAPK/Erk pathway is particularly important in mediating the

proliferative activity of FGFs. Activation of a second pathway, the PLCγ/Ca2+ pathway, has been implicated in the stimulation of neurite outgrowth by FGF2 ( Doherty and Walsh, Metformin datasheet 1996). The PI3 kinase/Akt pathway mediates some of the activities of FGFs in other tissues but there is little evidence for its role in neural development in vivo downstream of FGFRs. An additional transduction pathway involving the docking proteins FRS2 α and β and the small GTPases Rnd1 and RhoA has been shown to mediate the effect of FGF signaling on cytoskeletal rearrangements and neurite outgrowth in PC12 cells ( Harada et al., 2005). FGF signaling is regulated at multiple levels, resulting in a tight control of its level, its spread, and its timing. Some of the mechanisms involved are specific to FGF signaling while others regulate RTK signaling in general.