Abstract
Abstract: :
Purpose: We have previously reported that application of basic fibroblast growth factor (FGF–2) rescues a significant number of retinal ganglion cells (RGCs) from death after axotomy. In the present study, we investigate whether FGF–2 application to the frog optic nerve after injury affects transcription of the neurotrophin BDNF and its receptor TrkB in the injured RGCs, and whether this expression is regulated by the activation of the ERK signaling cascade and the downstream transcription factor CREB. Methods: Frog optic nerves were cut and treated with FGF–2 or PBS. After 24 hours or 1 week the FGF–2– and PBS–treated retinas were dissected and RNA was extracted for real–time RT–PCR to measure BDNF and TrkB expression. BDNF expression was also examined in the retinas with in situ hybridization. To determine the involvement of the ERK signaling cascade in regulating BDNF and TrkB expression, a MEK inhibitor was applied to the optic nerve before axotomy and FGF–2 application. The activation of the signaling proteins ERK and CREB in the retina was examined by immunohistochemistry and Western blot with antibodies that recognize the phosphorylated forms of these proteins. Results: FGF–2 application to the injured optic nerve increased BDNF and TrkB mRNA levels significantly at 24hrs and 1wk after axotomy, compared to mRNA levels of the axotomized, untreated retinas. This increase of BDNF mRNA expression after FGF–2 application was localized in the injured RGCs. The increase in BDNF mRNA in the FGF–2 treated retinas was blocked significantly by pretreatment of the optic nerve with the MEK inhibitor PD0958. However, MEK inhibitor only produced a moderate but significant decrease in TrkB expression. pERK and pCREB were increased in the RGCs 1 wk after axotomy and FGF–2 application, this activation being decreased significantly by the MEK inhibitor. Conclusion: FGF–2 promotes the survival of injured RGCs of the frog in part through the up–regulation of BDNF and TrkB transcription. Our results suggest that FGF–2 activates the expression of these proteins via the ERK signaling cascade and the transcription factor CREB.
Keywords: ganglion cells • neuroprotection • growth factors/growth factor receptors