The C. elegans Opa1 Homologue EAT-3 Is Essential for Resistance to Free Radicals

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Abstract
The C. elegans eat-3 gene encodes a mitochondrial dynamin family member homologous to Opa1 in humans and Mgm1 in yeast. We find that mutations in the C. elegans eat-3 locus cause mitochondria to fragment in agreement with the mutant phenotypes observed in yeast and mammalian cells. Electron microscopy shows that the matrices of fragmented mitochondria in eat-3 mutants are divided by inner membrane septae, suggestive of a specific defect in fusion of the mitochondrial inner membrane. In addition, we find that C. elegans eat-3 mutant animals are smaller, grow slower, and have smaller broodsizes than C. elegans mutants with defects in other mitochondrial fission and fusion proteins. Although mammalian Opa1 is antiapoptotic, mutations in the canonical C. elegans cell death genes ced-3 and ced-4 do not suppress the slow growth and small broodsize phenotypes of eat-3 mutants. Instead, the phenotypes of eat-3 mutants are consistent with defects in oxidative phosphorylation. Moreover, eat-3 mutants are hypersensitive to paraquat, which promotes damage by free radicals, and they are sensitive to loss of the mitochondrial superoxide dismutase sod-2. We conclude that free radicals contribute to the pathology of C. elegans eat-3 mutants. Dominant Optic Atrophy is a progressive eye disease caused by degeneration of retinal ganglion cells. The most prevalent form of DOA is caused by mutations in the Opa1 protein. This protein is required for fusion between mitochondria, it has an anti-apoptotic function, and it is required for mitochondrial DNA segregation. It has, nevertheless, been difficult to understand why mutations in Opa1 specifically affect retinal ganglion cells. We used rhe nematode C. elegans as a model to study the underlying causes of Opa1 pathologies. C. elegans Opa1 is encoded by the eat-3 gene. Mutants are sluggish, grow slowly, remain small, and have small broodsizes. These phenotypes are not suppressed by mutations in cell death genes, suggesting that apoptosis does not contribute to eat-3 pathogenesis. Instead, eat-3 mutants are hypersensitive to paraquat, which promotes damage by free radicals, and they are sensitive to loss of the mitochondrial superoxide dismutase sod-2, which is needed to eliminate free radicals from the mitochondrial matrix. Moreover, eat-3 mutants overexpress SOD-2, most likely compensating for increased free radical production. These results show that C. elegans EAT-3 is important for resistance to free radicals and they raise the possibility that free radicals contribute to DOA in humans.