Pharmacologic Reversal Of Drp1-Dependent Mitochondrial Fragmentation, A Homeostatic Mechanism In Friedreich Ataxia

Joseph Edward Johnson, University of Pennsylvania


Friedreich ataxia is an autosomal recessive neurodegenerative disease that presents during adolescence that causes dysmetria, dysarthria, and ataxia; as the disease progresses, patients develop cardiomyopathy, which is the typical cause of death. This disease, currently without an FDA-approved therapy, is characterized by deficiency in frataxin, a small mitochondrial protein that is highly conserved across species. GAA trinucleotide repeat expansions in the first intron of the FXN gene decrease transcription of this gene, resulting in lower frataxin protein levels. Frataxin participates in iron-sulfur cluster biosynthesis, iron chaperoning, and in maintaining iron homeostasis; thus, frataxin deficiency impairs the functions of proteins that are dependent upon iron-sulfur cluster prosthetic groups. Aconitase and Complex II of the electron transport chain, for example, rely upon iron-sulfur clusters for their roles in ATP synthesis. Therefore, Friedreich ataxia patients also exhibit decreased ATP levels in addition to other mitochondrial impairments. Investigating the effects of the disease and potential therapeutics on mitochondrial functioning is therefore of critical importance to develop a viable therapeutic strategy for this disease. Alterations to mitochondrial morphology are associated with impairments to mitochondrial functioning in numerous neurodegenerative diseases; investigating the morphological changes that occur through mitochondrial fission is therefore critical to understanding the impact of this disease on the mitochondria. Mitochondrial fission is executed by Drp1, a GTPase, so utilizing the specific Drp1 inhibitor P110 provides an invaluable tool for studying morphological changes. Additionally, mitochondrial morphology is greatly affected by cardiolipin, a four-tailed phospholipid that anchors the electron transport chain. The electrostatic interactions that are critical to cardiolipin’s functions are disrupted by oxidative stress, so the cardiolipin-stabilizing compound SS-31 may provide bioenergetic benefits to Friedreich ataxia patients. This dissertation examines the effects of Friedreich ataxia on mitochondrial morphology and function. It establishes that frataxin deficiency increases mitochondrial fragmentation that is dependent upon Drp1 activity. Furthermore, it demonstrates that SS-31 may also reverse fragmentation in this disease and that this compound operates on the same pathway as P110. Finally, it provides direction for future studies regarding mitochondrial-based therapeutic strategies for FRDA.