Background: Amyotrophic lateral sclerosis (ALS) is an age-dependent neurodegenerative disease that causes motor neuron degeneration, paralysis and death. Mutations in Cu, Zn superoxide dismutase (SOD1) are one cause for the familial form of this disease. Transgenic mice expressing mutant SOD1 develop age-dependent motor neuron degeneration, skeletal muscle weakness, paralysis and death similar to humans. The mechanism whereby mutant SOD1 induces motor neuron degeneration is not understood but widespread mitochondrial vacuolation has been observed during early phases of motor neuron degeneration. How this vacuolation develops is not clear, but could involve autophagic vacuolation, mitochondrial permeability transition (MPT) or uncharacterized mechanisms. To determine which of these possibilities are true, we examined the vacuolar patterns in detail in transgenic mice expressing mutant SOD1G93A.
Results: Vacuolar patterns revealed by electron microscopy (EM) suggest that vacuoles originate from the expansion of the mitochondrial intermembrane space and extension of the outer mitochondrial membrane. Immunofluorescence microscopy and immuno-gold electron microscopy reveal that vacuoles are bounded by SOD1 and mitochondrial outer membrane markers, but the inner mitochondrial membrane marker is located in focal areas inside the vacuoles. Small vacuoles contain cytochrome c while large vacuoles are porous and lack cytochrome c. Vacuoles lack lysosomal signal but contain abundant peroxisomes and SOD1 aggregates.
Conclusion: These findings demonstrate that mutant SOD1, possibly by toxicity associated with its aggregation, causes mitochondrial degeneration by inducing extension and leakage of the outer mitochondrial membrane, and expansion of the intermembrane space. This could release the pro-cell death molecules normally residing in the intermembrane space and initiate motor neuron degeneration. This Mitochondrial Vacuolation by Intermembrane Space Expansion (MVISE) fits neither MPT nor autophagic vacuolation mechanisms, and thus, is a previously uncharacterized mechanism of mitochondrial degeneration in mammalian CNS.