Signaling pathways for growth-factor- and cytokine -triggered induction of neuronal genes

Establishment of neuronal phenotype from precursor cells requires the coordinated expression of a host of neuronal genes. Among these events, NGF-triggered induction of the PN1 sodium channel gene provides a unique mechanism by which neurons can acquire long-term changes in response to brief exposure to neurotrophic factors.;In this dissertation, the signaling mechanisms for neuromodulators, such as NGF, IL-6, and IFN-gamma, in mediating triggered induction of neuronal gene expression was investigated in PC12 cells. The studies herein suggest that the PLC-gamma binding site (Tyr-785) of the NGF receptor, Trk A, can mediate NGF induction of PN1 gene expression, whereas mutation of the Shc binding site (Tyr-490) does not substantially alter the magnitude or kinetics of NGF induction of the PN1 gene. These findings, together with my results on the kinetics of PLC-gamma pathway stimulation, suggest that a prolonged activation of PLC-gamma initiated by a pulsatile treatment of cells with NGF for 2 min may mediate the long-term induction of PN1 gene expression at 5 hr after the removal of NGF. PLC-gamma signaling generates two intracellular messengers, DAG which is an activator of PKC and IP3 which mediates a rise in intracellular calcium concentration. The role of intracellular calcium was found to be crucial for NGF induced PN1 gene expression, while the effect of PK-C may be dispensable.;I have also found that, similar to NGF and IFN-gamma, treatment of PC12 cells with a neuropoietic cytokine IL-6 resulted in induced PN1 gene expression in PC12 cells. An analysis of STAT signaling pathways indicated that they play an important role in cytokine mediated PN1 gene expression. Prolonged activation of STAT1 in response to brief exposure to IFN-gamma for 5 min may mediate the long-term induction of PN1 gene expression at 7 hrs after the removal of IFN-gamma. Obligatory requirement of STAT3 signaling in IL-6 triggered induction of PN1 gene expression has also been demonstrated. Treatment with IFN-gamma as well as with NGF or H-IL6 also resulted in induced peripherin gene expression. Furthermore, peripherin gene expression could be also triggered by brief treatment with IFN-gamma through the activation of STAT1, in a similar manner as with the induction of the PN1.;The studies in this dissertation have suggested a molecular model in which the prolonged activation of specific signaling pathways mediate the long-term induction of neuronal gene expression in response to a brief exposure to neuromodulators that may be present only transiently during the development of the nervous system.