Fibroblast growth factors and their receptors in the central nervous system

Fibroblast growth factors (FGFs) and their receptors constitute an elaborate signaling system that participates in many developmental and repair processes of virtually all mammalian tissues. Among the 23 FGF members, ten have been identified in the brain. Four FGF receptors (FGFRs), receptor tyrosine kinases, are known so far. Ligand binding of these receptors greatly depends on the presence of heparan sulfate proteoglycans, which act as low affinity FGFRs. Ligand binding specificity of FGFRs depends on the third extracellular Ig-like domain, which is subject to alternative splicing. Activation of FGFRs triggers several intracellular signaling cascades. These include phosphorylation of src and PLCgamma leading finally to activation of PKC, as well as activation of Crk and Shc. SNT/FRS2 serves as an alternative link of FGFRs to the activation of PKC and, in addition, activates the Ras signaling cascade. In the CNS, FGFs are widely expressed; FGF-2 is predominantly synthesized by astrocytes, whereas other FGF family members, e.g., FGF-5, FGF-8, and FGF-9, are primarily synthesized by neurons. During CNS development FGFs play important roles in neurogenesis, axon growth, and differentiation. In addition, FGFs are major determinants of neuronal survival both during development and during adulthood. Adult neurogenesis depends greatly on FGF-2. Finally, FGF-1 and FGF-2 seem to be involved in the regulation of synaptic plasticity and processes attributed to learning and memory.