Purpose: Brain-derived neurotrophic factor (BDNF) has a potential neuroprotective effect on axotomized retinal ganglion cells (RGCs); however, the mechanism, in regard to intracellular signaling, of BDNF-induced neuroprotection of RGCs is largely unknown. Intracellular signaling was investigated, by using axotomized RGCs and the relative contribution of the two major downstream signaling routes of TrkB determined--that is, mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3 kinase (PI3K)-Akt routes, mediated by BDNF.
Methods: Neuroprotective effects of BDNF were determined by quantifying the surviving RGCs after axotomy, by retrograde labeling. The MAPK and Akt levels were determined by Western blot analysis and activity assays. Quantification of the relative contribution of the two signaling pathways was performed by use of specific inhibitors for MAPK and PI3K (i.e., U0126 and LY294002, respectively).
Results: Intravitreous administration of BDNF had the most profound neuroprotective effects on axotomized RGCs among the neurotrophins. Burst phosphorylation of MAPK and Akt was induced by BDNF within 1 hour and was sustained over 2 weeks in the whole retina. Immunohistochemistry revealed that phosphorylated MAPK was detected in the RGCs and retinal Müller cells, and Akt was in the RGCs. BDNF-induced phosphorylation of MAPK and Akt was suppressed by their specific inhibitors. Moreover, administration of U0126 and LY294002 decreased significantly, but only partially, the neuroprotective effect of BDNF on the axotomized RGCs.
Conclusions: BDNF-mediated signaling involves activation of both MAPK and Akt on the axotomized adult rat retina, and the collaboration of both MAPK and PI3K-Akt pathways seems to be necessary in neuroprotective signaling in axotomized RGCs.