IDENTIFICATION OF THE CYTOPLASMIC REGIONS OF FIBROBLAST GROWTH-FACTOR(FGF) RECEPTOR-1 WHICH PLAY IMPORTANT ROLES IN INDUCTION OF NEURITE OUTGROWTH IN PC12 CELLS BY FGF-1

Fibroblast growth factor 1 (FGF-1) induces neurite outgrowth in PC12 c ells. Recently, we have shown that the FGF receptor 1 (FGFR-1) is much more potent than FGFR-3 in induction of neurite outgrowth. To identif y the cytoplasmic regions of FGFR-1 that are responsible for the induc tion of neurite outgrowth in PC12 cells, we took advantage of this dif ference and prepared receptor chimeras containing different regions of the FGFR-1 introduced into the FGFR-3 protein. The chimeric receptors were introduced into FGF-nonresponsive variant PC12 cells (fnr-PC12 c ells), and their ability to mediate FGF-stimulated neurite outgrowth o f the cells was assessed. The juxtamembrane (JM) and carboxy-terminal (COOH) regions of FGFR-1 were identified as conferring robust and mode rate abilities, respectively, for induction of neurite outgrowth to FG FR-3. Analysis of FGF-stimulated activation of signal transduction rev ealed that the JM region of FGPR-1 conferred strong and sustained tyro sine phosphorylation of several cellular proteins and activation of MA P kinase. The SNT/FRS2 protein was demonstrated to be one of the cellu lar substrates preferentially phosphorylated by chimeras containing th e JM domain of FGFR-1. SNT/FRS2 links FGF signaling to the MAP kinase pathway. Thus, the ability of FGFR-1 JM domain chimeras to induce stro ng sustained phosphorylation of this protein would explain the ability of these chimeras to activate MAP kinase and hence neurite outgrowth. The role of the COOH region of FGFR-1 in induction of neurite outgrow th involved the tyrosine residue at amino acid position 764, a site re quired for phospholipase C gamma binding and activation, whereas the J M region functioned primarily through a non-phosphotyrosine-dependent mechanism. In contrast, assessment of the chimeras in the pre-B lympho id fell line BaF3 for FGF-l-induced mitogenesis revealed that the JM r egion did not play a role in this cell type. These data indicate that FGFR signaling can be regulated at the level of intracellular interact ions and that signaling pathways for neurite outgrowth and mitogenesis use different regions of the FGFR.