Mitochondrial dysfunction in SOD1^G93A-bearing astrocytes promotes motor neuron degeneration: Prevention by mitochondrial-targeted antioxidants

Mitochondrial dysfunction and oxidative stress contribute to motor neuron degeneration in amyotrophic lateral sclerosis (ALS). Recent reports indicate that astrocytes expressing the mutations of superoxide dismutase-1 (SOD1) may contribute to motor neuron injury in ALS. Here,we provide evidence that mitochondrial dysfunction in SOD1^G93A rat astrocytes causes astrocytes to induce apoptosis of motor neurons. Mitochondria from SOD1 ^G93A rat astrocytes displayed a defective respiratory function, including decreased oxygen consumption, lack of ADP-dependent respiratory control, and decreased membrane potential. Protein 3-nitrotyrosine was detected immunochemically in mitochondrial proteins from SOD1^G93A astrocytes, suggesting that mitochondrial defects were associated with nitroxidative damage. Furthermore, superoxide radical formation in mitochondria was increased in SOD1^G93A astrocytes. Similar defects were found in mitochondria isolated from the spinal cord of SOD1^G93A rats, and pretreatment of animals with the spin trap 5,5-dimethyl-1-pyrroline N-oxide restored mitochondrial function, forming adducts with mitochondrial proteins in vivo. As shown previously, SOD1^G93A astrocytes induced death of motor neurons in cocultures, compared with nontransgenic ones. This behavior was recapitulated when nontransgenic astrocytes were treated with mitochondrial inhibitors. Remarkably, motor neuron loss was prevented by preincubation of SOD1 ^G93A astrocytes with antioxidants and nitric oxide synthase inhibitors. In particular, low concentrations (∼10 nM) of two mitochondrial-targeted antioxidants, ubiquinone and carboxy-proxyl nitroxide, each covalently coupled to a triphenylphosphonium cation (Mito-Q and Mito-CP, respectively), prevented mitochondrial dysfunction, reduced superoxide production in SOD1^G93A astrocytes, and restored motor neuron survival. Together, our results indicate that mitochondrial dysfunction in astrocytes critically influences motor neuron survival and support the potential pharmacological utility of mitochondrial-targeted antioxidants in ALS treatment.