CuZn superoxide dismutase (SOD1) accumulates in vacuolated mitochondria in transgenic mice expressing amyotrophic lateral sclerosis-linked SOD1 mutations.

Cytosolic Cu/Zn superoxide dismutase (SOD1) is a ubiquitous small cytosolic metalloenzyme, which catalyses the conversion of superoxide anion to hydrogen peroxide. Mutations in the SOD1 gene cause a familial form of amyotrophic lateral sclerosis (fALS). The mechanism by which mutant SOD1s cause the degeneration of motor neurons is not understood. Transgenic mice expressing multiple copies of fALS-mutant SOD1s develop an ALS-like motor neuron disease. Vacuolar degeneration of mitochondria has been identified as the main pathological feature associated with motor neuron death and paralysis in several lines of fALS-SOD1 mice. Using confocal and electron microscopy we show that mutant SOD1 is present at a high concentration in vacuolated mitochondria, where it colocalises with cytochrome c. Mutant SOD1 is also present in mildly swollen mitochondria prior to the appearance of vacuoles, suggesting that the leakage or translocation of mutant human SOD1 in mitochondria may be the primary event triggering their further degeneration. Vacuolated mitochondria containing SOD1 also occur in transgenic mice expressing a high concentration of wildtype human SOD1. In sum, our data suggest that both fALS-mutant and wild-type SOD1 may cross the mitochondrial outer membrane, and by doing so induce the degeneration of these mitochondria.