Overexpression of SOD1 protects vulnerable motor neurons after spinal cord injury by attenuating mitochondrial cytochrome c release

Defective Cu,Zn-superoxide dismutase (SOD1) is responsible for some types of amyotrophic lateral sclerosis, and ventral horn motor neurons (VMN) have been shown to die through a mitochondria-dependent apoptotic pathway after chronic exposure to high levels of reactive oxygen species (ROS). VMN are also selectively vulnerable to mild spinal cord injury (SCI); however, the involvement of SOD1, ROS, and apoptosis in their death has not been clarified. Mild compression SCI was induced in SOD1-overexpressing transgenic rats and wild-type littermates. Superoxide production, mitochondrial release of cytochrome c, and activation of caspase-9 were examined, and apoptotic DNA injury was also characterized. In the wild-type animals, increased superoxide production, mitochondrial release of cytochrome c, and cleaved caspase-9 were observed exclusively in VMN after SCI. Subsequently, a majority of VMN (75%) selectively underwent delayed apoptotic cell death. Transgenic animals showed less superoxide production, mitochondrial cytochrome c release, and caspase-9 activation, resulting in death of only 45% of the VMN. These results suggest that the ROS-initiated mitochondrial signaling pathway possibly plays a pivotal role in apoptotic VMN death after SCI and that increased levels of SOD1 in VMN reduce oxidative stress, thereby attenuating the activation of the pathway and delayed cell death.