Stabilization of mutant and zinc-deficient SOD dimers by copper,zinc SOD enhances Fas-mediated motor neuron death

Familial cases of amyotrophic lateral sclerosis (ALS) can be caused by mutations on the gene for superoxide dismutase I (SOD1); these mutations occur all along the protein and produce structural and folding defects that lead to a decreased affinity for zinc. SOD1 that lacks zinc is sufficient to kill motor neurons. Here, we show that the zinc-deficient SOD1-treated motor neurons die via a Fas-mediated mechanism involving MAPKs as well as the production of nitric oxide, superoxide and peroxynitrite.;In PC12 cells, peroxynitrite activates p38 and JNK MAPKs. Distinct from the MAPK pathway, addition of peroxynitrite also leads to the inactivation of the Akt pathway. Tyrosine-containing peptides attenuate the effect of peroxynitrite while having no effect on PC12 cells treated with hydrogen peroxide, indicating peroxynitrite-induced tyrosine nitration as a precursor to PC12 cell death in this condition.;We also show that normal motor neurons treated with zinc-deficient SOD1 in the presence of tyrosine-containing peptides are protected. Both inhibition of nitric oxide and scavenging of superoxide are protective in normal motor neurons treated with zinc-deficient SOD1, further indicating a role for peroxynitrite in zinc-deficient SOD1-induced motor neuron death.;Interestingly, the addition of mutant or wild-type Cu,Zn SOD to SOD1 G93A rat motor neurons in vitro led to an enhanced pathology dependent on the catalytic copper of SOD1 as well as superoxide production. Indeed, the addition of Cu,Zn SOD1 to normal motor neurons treated with zinc-deficient SOD1 also led to an increase in the cell death observed when compared to motor neurons treated with zinc-deficient SOD alone. Motor neurons treated with zinc-deficient plus Cu,Zn SOD1 also die through the Fas pathway, MAPK activation, superoxide and nitric oxide formation. We hypothesize that Cu,Zn SOD1 stabilizes the mutant or zinc-deficient SOD and inhibits its aggregation, which leads to the increased ability for the mutant or zinc-deficient SOD to perform its aberrant oxidative chemistry.