Subunit-destabilizing mutations in Drosophila copper/zinc superoxide dismutase: neuropathology and a model of dimer dysequilibrium.

Mutations in Cu/Zn superoxide dismutase (SOD), a hallmark of familial amyotrophic lateral sclerosis (FALS) in humans, are shown here to confer striking neuropathology in Drosophila. Heterozygotes with one wild-type and one deleted SOD allele retain the expected 50% of normal activity for this dimeric enzyme. However, heterozygotes with one wild-type and one missense SOD allele show lesser SOD activities, ranging from 37% for a heterozygote carrying a missense mutation predicted from structural models to destabilize the dimer interface, to an average of 13% for several heterozygotes carrying missense mutations predicted to destabilize the subunit fold. Genetic and biochemical evidence suggests a model of dimer dysequilibrium whereby SOD activity in missense heterozygotes is reduced through entrapment of wild-type subunits into unstable or enzymatically inactive heterodimers. This dramatic impairment of the activity of wild-type subunits in vivo has implications for our understanding of FALS and for possible therapeutic strategies.

Apoptosis and necrosis: two distinct events induced, respectively, by mild and intense insults with N-methyl-D-aspartate or nitric oxide/superoxide in cortical cell cultures.

N-Methyl-D-aspartate (NMDA) receptor-mediated neurotoxicity may depend, in part, on the generation of nitric oxide (NO.) and superoxide anion (O2.-), which react to form peroxynitrite (OONO-). This form of neurotoxicity is thought to contribute to a final common pathway of injury in a wide variety of acute and chronic neurologic disorders, including focal ischemia, trauma, epilepsy, Huntington disease, Alzheimer disease, amyotrophic lateral scelerosis, AIDS dementia, and other neurodegenerative diseases. Here, we report that exposure of cortical neurons to relatively short durations or low concentrations of NMDA, S-nitrosocysteine, or 3-morpholinosydnonimine, which generate low levels of peroxynitrite, induces a delayed form of neurotoxicity predominated by apoptotic features. Pretreatment with superoxide dismutase and catalase to scavenge O2.- partially prevents the apoptotic process triggered by S-nitrosocysteine or 3-morpholinosydnonimine. In contrast, intense exposure to high concentrations of NMDA or peroxynitrite induces necrotic cell damage characterized by acute swelling and lysis, which cannot be ameliorated by superoxide dismutase and catalase. Thus, depending on the intensity of the initial insult, NMDA or nitric oxide/superoxide can result in either apoptotic or necrotic neuronal cell damage.