Astrocytic phospholipase A2 contributes to neuronal glutamate toxicity

The role of astrocytes in glutamate toxicity has been controversial. Here, we show that astrocytes in neuron-astrocyte co-cultures increased neuronal sensitivity to chronic glutamate exposure but not to acute exposure. Enhanced neuronal toxicity by chronic exposure was dependent on astrocyte cell numbers. A reduced generation of extracellular H2O2 induced by glutamate was observed in co-cultures. Further, neuronal glutamate toxicity was not suppressed by NADPH oxidase (Nox) inhibitors, catalase or Nox4 knockdown in co-cultures, whereas these compounds effectively reduced the toxicity in pure neuron cultures. Instead, the intracellular scavenger of reactive oxygen species, N-acetylcysteine (NAC), reduced neuronal cytotoxicity in co-cultures, whereas catalase worked in pure neuron cultures. Lipoxygenase (LOX) inhibitors attenuated neuronal glutamate toxicity in co-cultures but not in pure neuron cultures. Neuronal 5-LOX activity was increased only in co-cultures, whereas 12-LOX activity was increased in both types of cultures. The cyclooxygenase (COX) inhibitors, indomethacin and NS-398, and the phospholipase A2 (PLA2) inhibitors, LY311727 and MAFP, more effectively reduced neuronal glutamate toxicity in co-cultures than in pure neuron cultures. However, in co-cultures, pre-treating neurons and astrocytes with the same inhibitors generated opposite results. COX inhibitors suppressed neuronal glutamate toxicity in pre-treated neurons rather than astrocytes, whereas PLA2 inhibitors reduced the toxicity in pre-treated astrocytes rather than neurons. Gene-specific knockdown of PLA2 confirmed these results. Knockdown of cPLA2α and/or sPLA2-V in astrocytes rather than in neurons more effectively reduced glutamate toxicity in co-cultures. These findings suggest that astrocytic PLA2 activity increases neuronal sensitivity to chronic glutamate exposure in neuron-astrocyte co-cultures.