Potential role for astroglial D-amino acid oxidase in extracellular D-serine metabolism and cytotoxicity

D-Amino acid oxidase (DAO) is a flavoenzyme that catalyzes the oxidation Of D-amino acids. In the brain, gene expression of DAO is detected in astrocytes. Among the possible substrates of DAO in ViVO, D-serine is proposed to be a neuromodulator of the N-methyl-D-aspartate (NMIDA) receptor. In a search for the physiological role of DAO in the brain, we investigated the metabolism of extracellular D-serine in glial cells. Here we show that after D-serine treatment, rat primary type-1 astrocytes exhibited increased cell death. In order to enhance the enzyme activity of DAO in cells, we established stable rat C6 glial cells overexpressing mouse DAO designated as C6/DAO. Treatment with a high dose of D-serine led to the production of hydrogen peroxide (11202) followed by apoptosis in C6/DAO cells. Among the amino acids tested, D-serine specifically exhibited a significant cell death-inducing effect. DAO inhibitors, i.e., sodium benzoate and chlorpromazine, partially prevented the death of C6/DAO cells treated with D-serine, indicating the involvement of DAO activity in D-serine metabolism. Overall, we consider that extracellular D-serine can gain access to intracellular DAO, being metabolized to produce H2O2. These results support the proposal that astroglial DAO plays an important role in metabolizing a neuromodulator, D-serine.