We have shown that basic fibroblast growth factor (FGF-2) promotes the survival of axotomized retinal ganglion cells (RGCs) in the adult frog, Rana pipiens. FGF-2 treatment to the optic nerve after axotomy increases the synthesis of BDNF and TrkB mRNAs in RGCs. Here we investigate whether FGF-2 application has long term effects on BDNF and TrkB expression that could account for its survival effect on RGCs.

BDNF and TrkB immunoreactive RGCs were counted 6 weeks after axotomy, in retinas with FGF-2- or PBS-treated nerves. Western blot analysis was performed to measure changes in these proteins. In situ hybridization and real time PCR were used to assess the changes in expression of BDNF and TrkB mRNAs over this long-term survival period. To prevent the upregulation of BDNF in RGCs, siRNA was applied to the optic nerve after axotomy. A significant increase in BDNF-immunopositive RGCs was observed 6 weeks after axotomy and treatment with FGF-2, compared to PBS controls. The number of TrkB-immunoreactive RGCs was not significantly altered. Real time PCR and in situ hybridization showed firstly that FGF-2 treatment of the nerve caused a long-term upregulation of BDNF mRNA in the RGCs and, secondly, that BDNF siRNA blocked this upregulation. RGC survival at six weeks was also significantly reduced by BDNF siRNA. In conclusion, the long-term upregulation of BDNF by axonally-applied FGF-2 clearly plays a physiologically important role in the survival and regenerative capabilities of frog RGCs. This work was supported by NIH grants GM008224 and G12RR-03051.