Causative role of oxidative stress in a Drosophila model of Friedreich ataxia

Abstract

Friedreich ataxia (FA), the most common form of hereditary ataxia, is caused by a deficit in the mitochondrial protein frataxin. While several hypotheses have been suggested, frataxin function is not well understood. Oxidative stress has been suggested to play a role in the pathophysiology of FA, but this view has been recently questioned, and its link to frataxin is unclear. Here, we report the use of RNA interference (RNAi) to suppress the Drosophila frataxin gene (fh) expression. This model system parallels the situation in FA patients, namely a moderate systemic reduction of frataxin levels compatible with normal embryonic development. Under these conditions, fh-RNAi flies showed a shortened life span, reduced climbing abilities, and enhanced sensitivity to oxidative stress. Under hyperoxia, fh-RNAi flies also showed a dramatic reduction of aconitase activity that seriously impairs the mitochondrial respiration while the activities of succinate dehydrogenase, respiratory complex I and II, and indirectly complex III and IV are normal. Remarkably, frataxin overexpression also induced the oxidative-mediated inactivation of mitochondrial aconitase. This work demonstrates, for the first time, the essential function of frataxin in protecting aconitase from oxidative stress-dependent inactivation in a multicellular organism. Moreover our data support an important role of oxidative stress in the progression of FA and suggest a tissue-dependent sensitivity to frataxin imbalance. We propose that in FA, the oxidative mediated inactivation of aconitase, which occurs normally during the aging process, is enhanced due to the lack of frataxin.

Hydrogen peroxide scavenging rescues frataxin deficiency in a Drosophila model of Friedreich's ataxia

“…In an earlier study with our Drosophila FRDA model (17), we showed that widespread RNAi-mediated DFH deficiency with the ubiquitous Gal4 driver, daG 32 , caused delayed metamorphosis, impaired eclosion, and severe early mortality of the few surviving adults, and that overexpression of SOD1, SOD2, or CAT by the daG 32 -Gal4 driver did not improve the capacity of DFH-deficient larvae to develop into adults. In the present work, we have focused DFH deficiency on the adult PNS, which spares preadult effects, leaving reduced adult life span as the predominant phenotype.…”

“…That overexpression of the H 2 O 2 -scavenging enzyme, CAT, restores the short life span of DFH-deficient Drosophila strongly implicates H 2 O 2 as a critical element in the mechanism responsible for this early adult mortality phenotype. Although CAT provides robust restoration of life span in DFH-deficient flies, the disruption of iron metabolism arising from DFH deficiency should also occur in the mitochondrial matrix where DFH also resides (17,32) and where mitochondrial superoxide emanating from the respiratory chain is reduced to H 2 O 2 by SOD2 (33). In other words, all of the reactants for the generation of hydroxyl radical via Fenton chemistry should be present in the mitochondrial matrix of DFH-deficient flies, and enhanced scavenging of H 2 O 2 in the mitochondrial matrix should equal or surpass that in the cytoplasm.…”

“…17 could be at least partially caused by H 2 O 2 -mediated oxidative damage to iron cofactors. The exquisite sensitivity of the mACON Fe-S cluster to frataxin deficiency has been well established (17,23,32,46). Thus, the increased activity of ACON by H 2 O 2 scavenging may foreshadow discovery of proa b Fig.…”

“…We propose that these apparently disparate observations share a common mechanistic origin. Resistance of DFH-deficient adults to rescue by augmentation with SOD1 or SOD2 argues that superoxide is not a component of the pathophysiological mechanism of DFH deficiency, which means that although DFH deficiency may suppress overall respiratory chain activity through the impaired synthesis and restoration of essential Fe-S clusters (17,32), such suppressed respiratory chain activity is likely not accompanied by an increased flux of superoxide. If it were, augmentation with SOD2 and/or SOD1 would have restored life span of DFHdeficient adults.…”

Frataxin Depletion in Yeast Triggers Up-regulation of Iron Transport Systems before Affecting Iron-Sulfur Enzyme Activities

“…When these activities were restored, the activities of several iron-sulfur-containing enzymes were also recovered, suggesting that ironsulfur deficiency in ⌬yfh1 yeasts is due to the lack of SOD activities. In frataxin-deficient Drosophila melanogaster, aconitase deficiency is observed only under hyperoxia conditions (19), whereas H 2 O 2 scavenging restores aconitase activity (20). All of these observations suggest that lack of iron-sulfur enzymatic activities in frataxin-deficient cells could be a consequence of oxidative stress conditions generated by iron accumulation.…”