Absence of additional NGF stimulation, indicating the ability of FGFR1 to trigger the mobilization of “primed” intermediaries necessary for neurite formation. Considering all of these results together, we conclude that nuclear FGFR1 signaling is required and sufficient for NGF-induced de novo and regenerative neuritogenesis. Another frequently used event marking NGF-induced PC12 differentiation is the up-regulation of TH, the rate-limiting enzyme in catecholamine synthesis. The effect exerted at the level of gene transcription is mediated through the proximal th gene promoter as a result of binding diverse ssTF that interact with co-activator CBP and its 
partner FGFR1. FGFR1 had no effect on th-Luc expression in the absence of NGF stimulation. Changes in low basal th-Luc expression observed in FGFR1 transfected cells did not attain statistical significance. In contrast, co-transfection of FGFR1 produced a TWS119 moa statistically significant 2-fold increase in th-Luc promoter activity. FGFR1- and NGF-induced increases were not additive suggesting a common mechanism of stimulation. In conclusion, NGF-induced nuclear accumulation of FGFR1 is both necessary and sufficient to up-regulate th gene promoter activity. NGF has been shown to set in motion and provide chronic regulation of diverse actions that include many features of the neuronal phenotype, such as gene reprogramming and neuritogenesis. This process is controlled by activating RSK and Nur77, which are also central features of the INFS mechanism. Our results demonstrate that INFS contributes unique and previously unknown requirements to support specific end-points of the NGF mechanism, including gene activation and neurite outgrowth. These results help to explain how NGF signal transduction activates Nur-dependent gene activities which underlie neuronal differentiation. A central and essential functional feature of INFS is the nuclear accumulation of FGFR1. The present study verifies that the nuclear accumulation of FGFR1 constitutes a common response to NGF in both neural crest derived rat PC12 and human neuroblastoma cells. In live cells FLIP studies demonstrate nuclear and cytoplasmic FGFR1 are in kinetically distinct, yet connected cellular compartments. NGF promotes FGFR1 nuclear accumulation by reducing FGFR1 nuclear export, adding to other established mechanisms of FGFR1 nuclear accumulation: generation of cytosolic, rapidly diffusing FGFR1 facilitated by FGFR1 binding proteins, RSK1 and NLS-containing 23 kDa FGF-2, importin-b-mediated nuclear transfer and regulation of intranuclear FGFR1 mobility. Thus, there are several potential regulatory mechanisms through which NGF influences both the nuclear import and export of FGFR1 and FGFR1-dependent gene regulation. Fig. 6A summarizes the juxtaposition of NGF signaling and INFS activation. NGF stimulates the MAP/ERK pathway resulting in increased RSK1 activity, which is known to promote FGFR1 release from cytoplasmic pre-Golgi membranes and generate high BEZ235 PI3K inhibitor mobility cytosolic receptor that accumulates in the cell nucleus. Nuclear accumulation of FGFR1 may be facilitated by its NLScontaining ligand FGF-2, suggested by NGF upregulation of fgf-2 mRNA. RSK1 binding to FGFR1 in the nucleus decreases FGFR1 mobility, further promoting the nuclear accumulation of FGFR1. Nuclear accumulation of FGFR1 correlates with FGFR1-Nur binding, in which Nurs restrict the intranuclear movement of FGFR1. Thus, the FGFR1 interaction with RSK1 and Nurs may underlie the NGF inhibition of FGFR1 nuclear export, shown by our FLIP experiments, and the NGF-induced nuclear accumulation of FGFR1. Finally, the FGFR1-Nur cooperation at the Nur-targeted DNA sites transduces gene activation by NGF. Together these observations offer mechanistic support for INFS mediated action of NGF.