Posttranslational regulation of mitochondrial frataxin and identification of compounds that increase frataxin levels in Friedreich's ataxia

Frataxin Loss Promotes Angiotensin II-Induced Endothelial-to-Mesenchymal Transition

BACKGROUND

The metabolic flexibility of endothelial cells is linked to their phenotypic plasticity. Frataxin is critical in determining the iron metabolism and fate of endothelial cells. This study aimed to investigate frataxin-mediated metabolic remodeling during the endothelial-to-mesenchymal transition (EndoMT).

METHODS AND RESULTS

Endothelial cell-specific frataxin knockout and frataxin mutation mice were subjected to angiotensin II to induce hypertension. EndoMT and cardiac fibrosis were assessed using histological and protein expression analyses. Fatty acid oxidation (FAO) in microvascular endothelial cells was measured using a Seahorse XF96 analyzer. We showed that inhibition of FAO accompanies angiotensin II-induced EndoMT. Frataxin knockout mice promote EndoMT, associated with increased cardiac fibrosis following angiotensin II infusion. Angiotensin II reduces frataxin expression, which leads to mitochondrial iron overload and subsequent carbonylation of sirtuin 3. In turn, carbonylated sirtuin 3 contributes to the acetylated frataxin at lysine 189, making it more prone to degradation. The frataxin/sirtuin 3 feedback loop reduces hydroxyl-CoA dehydrogenase α subunit-mediated FAO. Additionally, silymarin is a scavenger of free radicals, restoring angiotensin II-induced reduction of FAO activity and sirtuin 3 and frataxin expression, improving EndoMT both in vitro and in vivo. Furthermore, frataxin mutation mice showed suppressed EndoMT and improved cardiac fibrosis.

CONCLUSIONS

The frataxin/sirtuin 3 feedback loop has the potential to attenuate angiotensin II-induced EndoMT by improving FAO.

The Regulation of the Disease-Causing Gene FXN

Friedreich's ataxia (FRDA) is a progressive neurodegenerative disease caused in almost all patients by expanded guanine-adenine-adenine (GAA) trinucleotide repeats within intron 1 of the FXN gene. This results in a relative deficiency of frataxin, a small nucleus-encoded mitochondrial protein crucial for iron-sulfur cluster biogenesis. Currently, there is only one medication, omaveloxolone, available for FRDA patients, and it is limited to patients 16 years of age and older. This necessitates the development of new medications. Frataxin restoration is one of the main strategies in potential treatment options as it addresses the root cause of the disease. Comprehending the control of frataxin at the transcriptional, post-transcriptional, and post-translational stages could offer potential therapeutic approaches for addressing the illness. This review aims to provide a general overview of the regulation of frataxin and its implications for a possible therapeutic treatment of FRDA.

A peptide derived from TID1S rescues frataxin deficiency and mitochondrial defects in FRDA cellular models

Friedreich's ataxia (FRDA), the most common recessive inherited ataxia, results from homozygous guanine-adenine-adenine (GAA) repeat expansions in intron 1 of the FXN gene, which leads to the deficiency of frataxin, a mitochondrial protein essential for iron-sulphur cluster synthesis. The study of frataxin protein regulation might yield new approaches for FRDA treatment. Here, we report tumorous imaginal disc 1 (TID1), a mitochondrial J-protein cochaperone, as a binding partner of frataxin that negatively controls frataxin protein levels. TID1 interacts with frataxin both in vivo in mouse cortex and in vitro in cortical neurons. Acute and subacute depletion of frataxin using RNA interference markedly increases TID1 protein levels in multiple cell types. In addition, TID1 overexpression significantly increases frataxin precursor but decreases intermediate and mature frataxin levels in HEK293 cells. In primary cultured human skin fibroblasts, overexpression of TID1S results in decreased levels of mature frataxin and increased fragmentation of mitochondria. This effect is mediated by the last 6 amino acids of TID1S as a peptide made from this sequence rescues frataxin deficiency and mitochondrial defects in FRDA patient-derived cells. Our findings show that TID1 negatively modulates frataxin levels, and thereby suggests a novel therapeutic target for treating FRDA.

Protective effect of FXN overexpression on ferroptosis in L-Glu-induced SH-SY5Y cells

BACKGROUND

Alzheimer's disease (AD) is a complex, multifactorial neurodegenerative disease. However, the pathogenesis remains unclear. Recently, an increasing number of studies have demonstrated that ferroptosis is a new type of iron-dependent programmed cell death, contributes to the death of nerve cells in AD. By controlling iron homeostasis and mitochondrial function, the particular protein called frataxin (FXN), which is situated in the mitochondrial matrix, is a critical regulator of ferroptosis disease. It is encoded by the nuclear gene FXN. Here, we identified a novel underlying mechanism through which ferroptosis mediated by FXN contributes to AD.

METHODS

Human neuroblastoma cells (SH-SY5Y) were injured by L-glutamate (L-Glu). Overexpression of FXN by lentiviral transfection. In each experimental group, we assessed the ultrastructure of the mitochondria, the presence of iron and intracellular Fe2 + , the levels of reactive oxygen species, the mitochondrial membrane potential (MMP), and lipid peroxidation. Quantification was done for malondialdehyde (MDA) and reduced glutathione (GSH), as well as reactive oxygen species (ROS). Western blot and cellular immunofluorescence assays were used to detect the expression of xCT and GPX4 proteins which in System Xc-/GPX4 pathway, and the protein expressions of ACSL4 and TfR1 were investigated by Western blot.

RESULTS

The present work showed: (1) The expression of FXN was reduced in the L-Glu group; (2) Compared with the Control group, MMP was reduced in the L-Glu group, and mitochondria were observed to shrink and cristae were deformed, reduced or disappeared by transmission electron microscopy, and after FXN overexpression and ferrostatin-1 (Fer-1) (10 μmol/L) intervened, MMP was increased and mitochondrial morphology was significantly improved, suggesting that mitochondrial function was impaired in the L-Glu group, and overexpression of FXN could improve the manifestation of mitochondrial function impairment. (3) In the L-Glu group, ROS, MDA, iron ion concentration and Fe2+ levels were increased, GSH was decreased. Elevated expression of ACSL4 and TfR1, important regulatory proteins of ferroptosis, was detected by Western blot, and the expression of xCT and GPX4 in the System Xc-/GPX4 pathway was reduced by Western blot and cellular immunofluorescence. However, the above results were reversed when FXN overexpression and Fer-1 intervened.

CONCLUSION

To conclude, our research demonstrates that an elevated expression of FXN effectively demonstrates a robust neuroprotective effect against oxidative damage induced by L-Glu. Moreover, it mitigates mitochondrial dysfunction and lipid metabolic dysregulation associated with ferroptosis. FXN overexpression holds promise in potential therapeutic strategies for AD by inhibiting ferroptosis in nerve cells and fostering their protection.