ANALYSIS OF MUTANT PLATELET-DERIVED GROWTH-FACTOR RECEPTORS EXPRESSEDIN PC12 CELLS IDENTIFIES SIGNALS GOVERNING SODIUM-CHANNEL INDUCTION DURING NEURONAL DIFFERENTIATION

The mechanisms governing neuronal differentiation, including the signa ls underlying the induction of voltage-dependent sodium (Na+) channel expression by neurotrophic factors, which occurs independent of Ras ac tivity, are not well understood. Therefore, Na+ channel induction was analyzed in sublines of PC12 cells stably expressing platelet-derived growth factor (PDGF) beta receptors with mutations that eliminate acti vation of specific signaling molecules. Mutations eliminating activati on of phosphatidylinositol 3-kinase (PI3K), phospholipase C-gamma (PLC (gamma)), the GTPase-activating protein (GAP), and Syp phosphatase fai led to diminish the inducation of type II Na+ channel alpha-subunit mR NA and functional Na+ channel expression by PDGF, as determined by RNa se protection assays and whole-cell patch clamp recording. However, mu tation of juxtamembrane tyrosines that bind members of the Src family of kinases upon receptor activation inhibited the induction of functio nal Na+ channels while leaving the induction of type II alpha-subunit mRNA intact. Mutation of juxtamembrane tyrosines in combination with m utations eliminating activation of PI3K, PLC gamma, GAP, and Syp aboli shed the induction of type II alpha-subunit mRNA, suggesting that at L east partially redundant signaling mechanisms mediate this induction. The differential effects of the receptor mutations on Na+ channel expr ession did not reflect global changes in receptor signaling capabiliti es, as in all of the mutant receptors analyzed, the induction of c-fos and transin mRNAs still occurred. The results reveal an important rol e for the Src family in the induction of Na+ channel expression and hi ghlight the multiplicity and combinatorial nature of the signaling mec hanisms governing neuronal differentiation.