NEUROTROPHINS RESCUE CEREBELLAR GRANULE NEURONS FROM OXIDATIVE STRESS-MEDIATED APOPTOTIC DEATH - SELECTIVE INVOLVEMENT OF PHOSPHATIDYLINOSITOL 3-KINASE AND THE MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAY

Cerebellar granule neurons maintained in medium containing serum and 2 5 mM K+ reliably undergo an apoptotic death when switched to serum-fre e medium with 5 mM K+. New mRNA and protein synthesis and formation of reactive oxygen intermediates are required steps in K+ deprivation-in duced apoptosis of these neurons. Here we show that neurotrophins, mem bers of the nerve growth factor gene family, protect from K+/serum dep rivation-induced apoptotic death of cerebellar granule neurons in a te mporally distinct manner. Switching granule neurons, on day in vitro ( DIV) 4, 10, 20, 30, or 40, from high-K+ to low-K+/serum-free medium de creased viability by >50% when measured after 30 h. Treatment of low-K + granule neurons at DIV 4 with nerve growth factor, brain-derived neu rotrophic factor (BDNF), neurotrophin-3, or neurotrophin-4/5 (NT-4/5) demonstrated concentration-dependent (1-100 ng/ml) protective effects only for BDNF and NT-4/5. Between DIV 10 and 20, K+-deprived granule n eurons showed decreasing sensitivity to BDNF and no response to NT-4/5 . Cerebellar granule neuron death induced by K+ withdrawal at DIV 30 a nd 40 was blocked only by neurotrophin-3. BDNF and NT-4/5 also circumv ented glutamate-induced oxidative death in DIV 1-2 granule neurons. Gr anule neuron death caused by K+ withdrawal or glutamate-triggered oxid ative stress was, moreover, limited by free radical scavengers like me latonin. Neurotrophin-protective effects, but not those of antioxidant s, were blocked by selective inhibitors of phosphatidylinositol 3-kina se or the mitogen-activated protein kinase pathway, depending on the n ature of the oxidant stress. These observations indicate that the surv ival-promoting effects of neurotrophins for central neurons, whose cel lular antioxidant defenses are challenged, require activation of disti nct signal transduction pathways.