Deficient mitochondrial respiration impairs sirtuin activity in dorsal root ganglia in Friedreich Ataxia mouse and cell models
Friedreich ataxia (FA) is a rare, recessive neuro-cardiodegenerative disease caused by deficiency of the mitochondrial protein frataxin. Mitochondrial dysfunction, a reduction in the activity of iron-sulfur enzymes, iron accumulation, and increased oxidative stress have been described. However, the mechanisms causing such cellular disturbances in mammals are not completely understood. Dorsal root ganglion (DRG) sensory neurons are among the cellular types most affected in the early stages of this disease. We have previously demonstrated that frataxin depletion in primary cultures of DRG neurons results in calcium dysregulation, neurite degeneration and apoptotic cell death. However, its effect on mitochondrial function remains to be elucidated. In the present study, we found that in primary cultures of DRG neurons as well as in DRGs from the FXNI151F mouse model, frataxin deficiency resulted in lower activity and levels of the electron transport complexes, mainly complexes I and II. As a consequence, the NAD+/NADH ratio was reduced and SirT3, a mitochondrial NAD+-dependent deacetylase, was impaired. We identified alpha tubulin as the major acetylated protein from DRG homogenates whose levels were increased in FXNI151F mice compared to WT mice. Mitochondrial superoxide dismutase (SOD2), a SirT3 substrate, displayed increased acetylation in frataxin-deficient DRG neurons. Since SOD2 acetylation inactivates the enzyme, and higher levels of mitochondrial superoxide anion were detected, oxidative stress markers were analyzed. Elevated levels of hydroxynonenal bound to proteins and mitochondrial Fe2+ accumulation were detected when frataxin decreased. Honokiol, a SirT3 activator, restores mitochondrial respiration. Altogether, these results provide the molecular bases to understand mitochondria dysfunction in sensory neurons which have greater susceptibility to frataxin deficiency compared to other tissues.