To investigate if frataxin function and dysfunction is associated with mitochondrial dynamics in FRDA.

Friedreich ataxia (FRDA), the most common inherited ataxia,  is a life shortening autosomal recessive neurodegenerative disease caused by a genetic deficiency of frataxin, a mitochondrial protein crucial for Fe/S cluster formation and bioenergetics. Mitochondrial dysfunction has thus been suggested in FRDA. Mitochondrial dynamics play crucial roles in bioenergetics, metabolism, cell division and apoptosis through coordinated cycles of fusion and fission of individual mitochondrial organelles or interconnected networks in cells. However, the pathophysiological role of frataxin in association with mitochondrial dynamics in FRDA remains largely unknown.

The morphology of the mitochondrial network in primary fibroblasts derived from FRDA patients and healthy individuals were examined using immunofluorescence to endogenous mitochondrial marker GRP75 and transfected mito-GFP. Live time-lapse imaging was performed using cotransfection of mito-DsRed2 and mito-photoactivable GFP into fibroblasts to monitor mitochondrial fusion/fission under confocal microscopy. Co-immunoprecipitation, double immunocytochemical staining and protein-overlay assays were performed to examine the association of frataxin with mitochondrial fission protein and phospholipids.

The mitochondrial network is excessively fragmented and fails to fuse together in FRDA patient fibroblasts. Knockdown of frataxin significantly increases fragmentation of mitochondrial network in control fibroblasts, whereas overexpression of frataxin decreases fragmentation in patient fibroblasts. Furthermore, co-immunoprecipitation and protein-lipid overlay assays show that frataxin interacts with mitochondrial fission protein Drp1 and phosphatidic acid, the core components that coordinate mitochondrial fusion/fission balance. Moreover, frataxin knockdown significantly increases the levels and the clusters of active form of phospho-Drp1 (Ser616) at the fission sites of mitochondrial network in human fibroblasts.