In vivo analysis of the mitochondrial protein frataxin in Drosophila melanogaster

Friedreich’s ataxia is the most common recessive ataxia in the Caucasian population. It is caused by a defect in the expression of the mitochondrial protein frataxin and leads to neurodegeneration. The main role of frataxin lies in iron-sulphur cluster biosynthesis and loss of the protein results in mitochondrial iron accumulation and diminished energy production within the cell. Although mitochondrial function is severely affected, little is known about the influence of frataxin deficiency on mitochondrial dynamics and homeostasis. In this work, I utilized previously established Drosophila melanogaster RNAi models of Friedreich’s ataxia. To unravel the effects of frataxin depletion on the mitochondrial network, I focused on glia and muscle tissue, since these RNAi lines are not very efficient in neurons. I could show that mitochondrial morphology is disturbed, in agreement with the compromised membrane potential and energy production. Through the use of autophagy markers like p62, GFP-Atg8a and the newly established mtRosella, I performed a detailed analysis of mitochondrial degradation in frataxin deficient flies. I could demonstrate that damaged mitochondria are successfully targeted for degradation via mitophagy and promoting autophagy slightly improves Friedreich’s ataxia phenotypes. To analyse potential genetic interactions between frataxin and genes involved in mitochondrial homeostasis, I carried out a forward genetic screen. This screen identified knockdown of Marf, the Drosophila homologue of Mitofusin, as a suppressor of Friedreich’s ataxia phenotypes in glia. Since Marf has been described to play a pivotal role in mitochondrial fusion, mitophagy and the interface between mitochondria and the ER, I examined how Marf silencing affects all three pathways in a frataxin deficient background. Interestingly, I could show that loss of frataxin triggers the ER stress response and identified ER stress as a key factor in the disease. Whereas mitochondrial morphology and degradation were unaffected by Marf silencing, it suppressed ER stress in frataxin-deficient cells and this was sufficient to improve locomotion, brain degeneration and lipid dyshomeostasis. Furthermore, application of the chemical TUDCA, a known attenuator of ER stress, was also able to partially rescue Friedreich’s ataxia phenotypes. These results suggest that the protection mediated by Marf knockdown is linked to its role in mitochondrial-ER contacts instead of mitochondrial fusion or degradation. Furthermore, ER stress appears to play a crucial role in the aetiology of the disease