Neuroprotective mechanism of glial cell line-derived neurotrophic factor in mesencephalic neurons

Glial cell line-derived neurotrophic factor (GDNF) provides neuroprotection, but its neuroprotective mechanism has not been resolved. We investigated the neuroprotective mechanism of GDNF using primary culture of the rat mesencephalon. Bleomycin sulfate (BLM) and L-buthionine-[S,R]-sulfoximine (BSO) caused apoptosis in both dopaminergic and nondopaminergic neurons, as revealed by the presence of chromatin condensation, and positive staining by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling (TUNEL). GDNF preincubation blocked the neurotoxicity and reduced the number of the TUNEL-positive cells caused by BLM and BSO exposure. In contrast, GDNF did not provide neuroprotection against glutamate toxicity, which was not accompanied by these apoptotic features. The neuroprotection was mediated by phosphatidylinositol 3-kinase, an effector downstream from c-Ret, because it was blocked by LY294002. GDNF pretreatment caused up-regulation of Bcl-2 and Bcl-x. Furthermore, GDNF suppressed oxygen radical accumulation caused by BLM. Apoptosis induced by BLM and BSO was blocked by a caspase-3 inhibitor. Caspase-3 activity was elevated by BLM and suppressed by GDNF pretreatment. These findings indicate that GDNF has no effect on necrosis but exerts protection against apoptosis by activation of phosphatidylinositol 3-kinase and the subsequent up-regulation of Bcl-2 and Bcl-x, which suppresses accumulation of oxygen radicals followed by caspase-3 activation.