Skeletal Muscle Involvement in Friedreich Ataxia.

Friedreich Ataxia (FRDA) is an inherited neuromuscular disorder triggered by a deficit of the mitochondrial protein frataxin. At a cellular level, frataxin deficiency results in insufficient iron-sulfur cluster biosynthesis and impaired mitochondrial function and adenosine triphosphate production. The main clinical manifestation is a progressive balance and coordination disorder which depends on the involvement of peripheral and central sensory pathways as well as of the cerebellum. Besides the neurological involvement, FRDA affects also the striated muscles. The most prominent manifestation is a hypertrophic cardiomyopathy, which also represents the major determinant of premature mortality. Moreover, FRDA displays skeletal muscle involvement, which contributes to the weakness and marked fatigue evident throughout the course of the disease. Herein, we review skeletal muscle findings in FRDA generated by functional imaging, histology, as well as multiomics techniques in both disease models and in patients. Altogether, these findings corroborate a disease phenotype in skeletal muscle and support the notion of progressive mitochondrial damage as a driver of disease progression in FRDA. Furthermore, we highlight the relevance of skeletal muscle investigations in the development of biomarkers for early-phase trials and future therapeutic strategies in FRDA.

Deciphering the ferroptosis pathways in dorsal root ganglia of Friedreich ataxia models. The role of LKB1/AMPK, KEAP1, and GSK3ß in the impairment of the NRF2 response.

Friedreich ataxia (FA) is a rare neurodegenerative disease caused by decreased levels of the mitochondrial protein frataxin. Frataxin has been related in iron homeostasis, energy metabolism, and oxidative stress. Ferroptosis has recently been shown to be involved in FA cellular degeneration; however, its role in dorsal root ganglion (DRG) sensory neurons, the cells that are affected the most and the earliest, is mostly unknown. In this study, we used primary cultures of frataxin-deficient DRG neurons as well as DRG from the FXNI151F mouse model to study ferroptosis and its regulatory pathways. A lack of frataxin induced upregulation of transferrin receptor 1 and decreased ferritin and mitochondrial iron accumulation, a source of oxidative stress. However, there was impaired activation of NRF2, a key transcription factor involved in the antioxidant response pathway. Decreased total and nuclear NRF2 explains the downregulation of both SLC7A11 (a member of the system Xc, which transports cystine required for glutathione synthesis) and glutathione peroxidase 4, responsible for increased lipid peroxidation, the main markers of ferroptosis. Such dysregulation could be due to the increase in KEAP1 and the activation of GSK3ß, which promote cytosolic localization and degradation of NRF2. Moreover, there was a deficiency in the LKB1/AMPK pathway, which would also impair NRF2 activity. AMPK acts as a positive regulator of NRF2 and it is activated by the upstream kinase LKB1. The levels of LKB1 were reduced when frataxin decreased, in agreement with reduced pAMPK (Thr172), the active form of AMPK. SIRT1, a known activator of LKB1, was also reduced when frataxin decreased. MT-6378, an AMPK activator, restored NRF2 levels, increased GPX4 levels and reduced lipid peroxidation. In conclusion, this study demonstrated that frataxin deficiency in DRG neurons disrupts iron homeostasis and the intricate regulation of molecular pathways affecting NRF2 activation and the cellular response to oxidative stress, leading to ferroptosis.

Ferrostatin-1 specifically targets mitochondrial iron-sulfur clusters and aconitase to improve cardiac function in Sirtuin 3 cardiomyocyte knockout mice.

AIMS: Ferroptosis is a form of iron-regulated cell death implicated in ischemic heart disease. Our previous study revealed that Sirtuin 3 (SIRT3) is associated with ferroptosis and cardiac fibrosis. In this study, we tested whether the knockout of SIRT3 in cardiomyocytes (SIRT3cKO) promotes mitochondrial ferroptosis and whether the blockade of ferroptosis would ameliorate mitochondrial dysfunction. METHODS AND RESULTS: Mitochondrial and cytosolic fractions were isolated from the ventricles of mice. Cytosolic and mitochondrial ferroptosis were analyzed by comparison to SIRT3loxp mice. An echocardiography study showed that SIRT3cKO mice developed heart failure as evidenced by a reduction of EF% and FS% compared to SIRT3loxp mice. Comparison of mitochondrial and cytosolic fractions of SIRT3cKO and SIRT3loxp mice revealed that, upon loss of SIRT3, mitochondrial, but not cytosolic, total lysine acetylation was significantly increased. Similarly, acetylated p53 was significantly upregulated only in the mitochondria. These data demonstrate that SIRT3 is the primary mitochondrial deacetylase. Most importantly, loss of SIRT3 resulted in significant reductions of frataxin, aconitase, and glutathione peroxidase 4 (GPX4) in the mitochondria. This was accompanied by a significant increase in levels of mitochondrial 4-hydroxynonenal. Treatment of SIRT3cKO mice with the ferroptosis inhibitor ferrostatin-1 (Fer-1) for 14 days significantly improved preexisting heart failure. Mechanistically, Fer-1 treatment significantly increased GPX4 and aconitase expression/activity, increased mitochondrial iron­sulfur clusters, and improved mitochondrial membrane potential and Complex IV activity. CONCLUSIONS: Inhibition of ferroptosis ameliorated cardiac dysfunction by specifically targeting mitochondrial aconitase and iron­sulfur clusters. Blockade of mitochondrial ferroptosis may be a novel therapeutic target for mitochondrial cardiomyopathies.

Expression and processing of mature human frataxin after gene therapy in mice.

Friedreich's ataxia is a degenerative and progressive multisystem disorder caused by mutations in the highly conserved frataxin (FXN) gene that results in FXN protein deficiency and mitochondrial dysfunction. While gene therapy approaches are promising, consistent induction of therapeutic FXN protein expression that is sub-toxic has proven challenging, and numerous therapeutic approaches are being tested in animal models. FXN (hFXN in humans, mFXN in mice) is proteolytically modified in mitochondria to produce mature FXN. However, unlike endogenous hFXN, endogenous mFXN is further processed into N-terminally truncated, extra-mitochondrial mFXN forms of unknown function. This study assessed mature exogenous hFXN expression levels in the heart and liver of C57Bl/6 mice 7-10 months after intravenous administration of a recombinant adeno-associated virus encoding hFXN (AAVrh.10hFXN) and examined the potential for hFXN truncation in mice. AAVrh.10hFXN induced dose-dependent expression of hFXN in the heart and liver. Interestingly, hFXN was processed into truncated forms, but found at lower levels than mature hFXN. However, the truncations were at different positions than mFXN. AAVrh.10hFXN induced mature hFXN expression in mouse heart and liver at levels that approximated endogenous mFXN levels. These results suggest that AAVrh.10hFXN can likely induce expression of therapeutic levels of mature hFXN in mice.

Mechanism and structural dynamics of sulfur transfer during de novo [2Fe-2S] cluster assembly on ISCU2.

Maturation of iron-sulfur proteins in eukaryotes is initiated in mitochondria by the core iron-sulfur cluster assembly (ISC) complex, consisting of the cysteine desulfurase sub-complex NFS1-ISD11-ACP1, the scaffold protein ISCU2, the electron donor ferredoxin FDX2, and frataxin, a protein dysfunctional in Friedreich's ataxia. The core ISC complex synthesizes [2Fe-2S] clusters de novo from Fe and a persulfide (SSH) bound at conserved cluster assembly site residues. Here, we elucidate the poorly understood Fe-dependent mechanism of persulfide transfer from cysteine desulfurase NFS1 to ISCU2. High-resolution cryo-EM structures obtained from anaerobically prepared samples provide snapshots that both visualize different stages of persulfide transfer from Cys381NFS1 to Cys138ISCU2 and clarify the molecular role of frataxin in optimally positioning assembly site residues for fast sulfur transfer. Biochemical analyses assign ISCU2 residues essential for sulfur transfer, and reveal that Cys138ISCU2 rapidly receives the persulfide without a detectable intermediate. Mössbauer spectroscopy assessing the Fe coordination of various sulfur transfer intermediates shows a dynamic equilibrium between pre- and post-sulfur-transfer states shifted by frataxin. Collectively, our study defines crucial mechanistic stages of physiological [2Fe-2S] cluster assembly and clarifies frataxin's molecular role in this fundamental process.

MRPS16 promotes lung adenocarcinoma growth via the PI3K/AKT/Frataxin signalling axis.

Although MRPS16 is involved in cancer development, its mechanisms in developing LAUD remain unclear. Herein, qRT-PCR, WB and IHC were utilized for evaluating MRPS16 expression levels, while functional assays besides animal experiments were performed to measure MRPS16 effect on LAUD progression. Using WB, the MRPS16 effect on PI3K/AKT/Frataxin signalling pathway was tested. According to our study, MRPS16 was upregulated in LAUD and was correlated to the advanced TNM stage as well as poor clinical outcomes, which represent an independent prognostic factor. Based on functional assays, MRPS16 is involved in promoting LAUD growth, migration and invasion, which was validated further in subsequent analyses through PI3K/AKT/Frataxin pathway activation. Moreover, MRPS16-knockdown-mediated Frataxin overexpression was shown to restore the reduction in tumour cells proliferation, migration and invasion. Our results revealed that MRPS16 caused an aggressive phenotype to LAUD and was a poor prognosticator; thus, targeting MRPS16 may be effectual in LAUD treatment.

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.

A kinetic model of iron trafficking in growing Saccharomyces cerevisiae cells; applying mathematical methods to minimize the problem of sparse data and generate viable autoregulatory mechanisms.

Iron is an essential transition metal for all eukaryotic cells, and its trafficking throughout the cell is highly regulated. However, the overall cellular mechanism of regulation is poorly understood despite knowing many of the molecular players involved. Here, an ordinary-differential-equations (ODE) based kinetic model of iron trafficking within a growing yeast cell was developed that included autoregulation. The 9-reaction 8-component in-silico cell model was solved under both steady-state and time-dependent dynamical conditions. The ODE for each component included a dilution term due to cell growth. Conserved rate relationships were obtained from the null space of the stoichiometric matrix, and the reduced-row-echelon-form was used to distinguish independent from dependent rates. Independent rates were determined from experimentally estimated component concentrations, cell growth rates, and the literature. Simple rate-law expressions were assumed, allowing rate-constants for each reaction to be estimated. Continuous Heaviside logistical functions were used to regulate rate-constants. These functions acted like valves, opening or closing depending on component "sensor" concentrations. Two cellular regulatory mechanisms were selected from 134,217,728 possibilities using a novel approach involving 6 mathematically-defined filters. Three cellular states were analyzed including healthy wild-type cells, iron-deficient wild-type cells, and a frataxin-deficient strain of cells characterizing the disease Friedreich's Ataxia. The model was stable toward limited perturbations, as determined by the eigenvalues of Jacobian matrices. Autoregulation allowed healthy cells to transition to the diseased state when triggered by a mutation in frataxin, and to the iron-deficient state when cells are placed in iron-deficient growth medium. The in-silico phenotypes observed during these transitions were similar to those observed experimentally. The model also predicted the observed effects of hypoxia on the diseased condition. A similar approach could be used to solve ODE-based kinetic models associated with other biochemical processes operating within growing cells.

Quantification of human mature frataxin protein expression in nonhuman primate hearts after gene therapy.

Deficiency in human mature frataxin (hFXN-M) protein is responsible for the devastating neurodegenerative and cardiodegenerative disease of Friedreich's ataxia (FRDA). It results primarily through epigenetic silencing of the FXN gene by GAA triplet repeats on intron 1 of both alleles. GAA repeat lengths are most commonly between 600 and 1200 but can reach 1700. A subset of approximately 3% of FRDA patients have GAA repeats on one allele and a mutation on the other. FRDA patients die most commonly in their 30s from heart disease. Therefore, increasing expression of heart hFXN-M using gene therapy offers a way to prevent early mortality in FRDA. We used rhesus macaque monkeys to test the pharmacology of an adeno-associated virus (AAV)hu68.CB7.hFXN therapy. The advantage of using non-human primates for hFXN-M gene therapy studies is that hFXN-M and monkey FXN-M (mFXN-M) are 98.5% identical, which limits potential immunologic side-effects. However, this presented a formidable bioanalytical challenge in quantification of proteins with almost identical sequences. This could be overcome by the development of a species-specific quantitative mass spectrometry-based method, which has revealed for the first time, robust transgene-specific human protein expression in monkey heart tissue. The dose response is non-linear resulting in a ten-fold increase in monkey heart hFXN-M protein expression with only a three-fold increase in dose of the vector.

SIRT3 Regulates Clearance of Apoptotic Cardiomyocytes by Deacetylating Frataxin.

BACKGROUND: Efferocytosis is an activity of macrophages that is pivotal for the resolution of inflammation in hypertension. The precise mechanism by which macrophages coordinate efferocytosis and internalize apoptotic cardiomyocytes remains unknown. The aim of this study was to determine whether SIRT3 (sirtuin-3) is required for both apoptotic cardiomyocyte engulfment and anti-inflammatory responses during efferocytosis. METHODS: We generated myeloid SIRT3 knockout mice and FXN (frataxin) knock-in mice carrying an acetylation-defective lysine to arginine K189R mutation (FXNK189R). The mice were given Ang II (angiotensin II) infusion for 7 days. We analyzed cardiac macrophages' mitochondrial iron levels, efferocytosis activity, and phenotype both in vivo and in vitro. RESULTS: We showed that SIRT3 deficiency exacerbated Ang II-induced downregulation of the efferocytosis receptor MerTK (c-Mer tyrosine kinase) and proinflammatory cytokine production, accompanied by disrupted mitochondrial iron homeostasis in cardiac macrophages. Quantitative acetylome analysis revealed that SIRT3 deacetylated FXN at lysine 189. Ang II attenuated SIRT3 activity and enhanced the acetylation level of FXNK189. Acetylated FXN further reduced the synthesis of ISCs (iron-sulfur clusters), resulting in mitochondrial iron accumulation. Phagocytic internalization of apoptotic cardiomyocytes increased myoglobin content, and derived iron ions promoted mitochondrial iron overload and lipid peroxidation. An iron chelator deferoxamine improved the levels of MerTK and efferocytosis, thereby attenuating proinflammatory macrophage activation. FXNK189R mice showed improved macrophage efferocytosis, reduced cardiac inflammation, and suppressed cardiac fibrosis. CONCLUSIONS: The SIRT3-FXN axis has the potential to resolve cardiac inflammation by increasing macrophage efferocytosis and anti-inflammatory activities.

Frataxin inhibits the sensitivity of the myocardium to ferroptosis by regulating iron homeostasis.

RATIONALE: Myocardial ischemia/reperfusion (I/R) injury is characterized by cell death via various cellular mechanisms upon reperfusion. As a new type of cell death, ferroptosis provides new opportunities to reduce myocardial cell death. Ferroptosis is known to be more active during reperfusion than ischemia. However, the mechanisms regulating ferroptosis during ischemia and reperfusion remain largely unknown. METHODS: The contribution of ferroptosis in ischemic and reperfused myocardium were detected by administered of Fer-1, a ferroptosis inhibitor to C57BL/6 mice, followed by left anterior descending (LAD) ligation surgery. Ferroptosis was evaluated by measurement of cell viability, ptgs2 mRNA level, iron production, malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) levels. H9C2 cells were exposed to hypoxia/reoxygenation to mimic in vivo I/R. We used LC-MS/MS to identify potential E3 ligases that interacted with frataxin in heart tissue. Cardiac-specific overexpression of frataxin in whole heart was achieved by intracardiac injection of frataxin, carried by adeno-associated virus serotype 9 (AAV9) containing cardiac troponin T (cTnT) promoter. RESULTS: We showed that regulators of iron metabolism, especially iron regulatory protein activity, were increased in the ischemic myocardium or hypoxia cardiomyocytes. In addition, we found that frataxin, which is involved in iron metabolism, is differentially expressed in the ischemic and reperfused myocardium and involved in the regulation of cardiomyocytes ferroptosis. Furthermore, we identified an E3 ligase, NHL repeat-containing 1 (NHLRC1), that mediates frataxin ubiquitination degradation. Cardiac-specific overexpression of frataxin ameliorated myocardial I/R injury through ferroptosis inhibition. CONCLUSIONS: Through a multi-level study from molecule to animal model, these findings uncover the key role of frataxin in inhibiting cardiomyocyte ferroptosis and provide new strategies and perspectives for the treatment of myocardial I/R injury.

Drosophila melanogaster frataxin: protein crystal and predicted solution structure with identification of the iron-binding regions.

Friedreich's ataxia (FRDA) is a hereditary cardiodegenerative and neurodegenerative disease that affects 1 in 50 000 Americans. FRDA arises from either a cellular inability to produce sufficient quantities or the production of a nonfunctional form of the protein frataxin, a key molecule associated with mitochondrial iron-sulfur cluster biosynthesis. Within the mitochondrial iron-sulfur cluster (ISC) assembly pathway, frataxin serves as an allosteric regulator for cysteine desulfurase, the enzyme that provides sulfur for [2Fe-2S] cluster assembly. Frataxin is a known iron-binding protein and is also linked to the delivery of ferrous ions to the scaffold protein, the ISC molecule responsible for the direct assembly of [2Fe-2S] clusters. The goal of this report is to provide structural details of the Drosophila melanogaster frataxin ortholog (Dfh), using both X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy, in order to provide the foundational insight needed to understand the structure-function correlation of the protein. Additionally, NMR iron(II) titrations were used to provide metal contacts on the protein to better understand how it binds iron and aids its delivery to the ISC scaffold protein. Here, the structural and functional similarities of Dfh to its orthologs are also outlined. Structural data show that bacterial, yeast, human and Drosophila frataxins are structurally similar, apart from a structured C-terminus in Dfh that is likely to aid in protein stability. The iron-binding location on helix 1 and strand 1 of Dfh is also conserved across orthologs.

Iron-frataxin involved in the protective effect of quercetin against alcohol-induced liver mitochondrial dysfunction.

Emerging evidence supports the beneficial effect of quercetin on liver mitochondrial disorders. However, the molecular mechanism by which quercetin protects mitochondria is limited, especially in alcoholic liver disease. In this study, C57BL/6N mice were fed with Lieber De Carli liquid diet (28% ethanol-derived calories) for 12 weeks plus a single binge ethanol and intervened with quercetin (100 mg/kg.bw). Moreover, HepG2CYP2E1+/+ were stimulated with ethanol (100 mM) and quercetin (50 µM) to investigate the effects of mitochondrial protein frataxin. The results indicated that quercetin alleviated alcohol-induced histopathological changes and mitochondrial functional disorders in mice livers. Consistent with increased PINK1, Parkin, Bnip3 and LC3II as well as decreased p62, TOM20 and VDAC1 expression, the inhibition of mitophagy by ethanol was blocked by quercetin. Additionally, quercetin improved the imbalance of iron metabolism-related proteins expression in alcohol-fed mice livers. Compared with ethanol-treated Lv-empty HepG2CYP2E1+/+ cells, frataxin deficiency further exacerbated the inhibition of mitochondrial function. Conversely, restoration of frataxin expression ameliorated the effect of ethanol. Furthermore, frataxin deficiency reduced the protective effects of quercetin on mitochondria disordered by ethanol. Attentively, ferric ammonium citrate (FAC) and deferiprone decreased or increased frataxin expression in HepG2CYP2E1+/+, respectively. Notably, we further found FAC reversed the increasing effect of quercetin on frataxin expression. Ultimately, silencing NCOA4 attenuated the inhibition of quercetin on LDH release and mitochondrial membrane potential increase, and similar results were observed by adding FAC. Collectively, these findings demonstrated quercetin increased frataxin expression through regulating iron level, thereby mitigating ethanol-induced mitochondrial dysfunction.

Evolution of an Iron-Detoxifying Protein: Eukaryotic and Rickettsia Frataxins Contain a Conserved Site Which Is Not Present in Their Bacterial Homologues.

Friedreich's ataxia is a neurodegenerative disease caused by mutations in the frataxin gene. Frataxin homologues, including bacterial CyaY proteins, can be found in most species and play a fundamental role in mitochondrial iron homeostasis, either promoting iron assembly into metaloproteins or contributing to iron detoxification. While several lines of evidence suggest that eukaryotic frataxins are more effective than bacterial ones in iron detoxification, the residues involved in this gain of function are unknown. In this work, we analyze conservation of amino acid sequence and protein structure among frataxins and CyaY proteins to identify four highly conserved residue clusters and group them into potential functional clusters. Clusters 1, 2, and 4 are present in eukaryotic frataxins and bacterial CyaY proteins. Cluster 3, containing two serines, a tyrosine, and a glutamate, is only present in eukaryotic frataxins and on CyaY proteins from the Rickettsia genus. Residues from cluster 3 are blocking a small cavity of about 40 Å present in E. coli's CyaY. The function of this cluster is unknown, but we hypothesize that its tyrosine may contribute to prevent formation of reactive oxygen species during iron detoxification. This cluster provides an example of gain of function during evolution in a protein involved in iron homeostasis, as our results suggests that Cluster 3 was present in the endosymbiont ancestor of mitochondria and was conserved in eukaryotic frataxins.

Yeast cells depleted of the frataxin homolog Yfh1 redistribute cellular iron: Studies using Mössbauer spectroscopy and mathematical modeling.

The neurodegenerative disease Friedreich's ataxia arises from a deficiency of frataxin, a protein that promotes iron-sulfur cluster (ISC) assembly in mitochondria. Here, primarily using Mössbauer spectroscopy, we investigated the iron content of a yeast strain in which expression of yeast frataxin homolog 1 (Yfh1), oxygenation conditions, iron concentrations, and metabolic modes were varied. We found that aerobic fermenting Yfh1-depleted cells grew slowly and accumulated FeIII nanoparticles, unlike WT cells. Under hypoxic conditions, the same mutant cells grew at rates similar to WT cells, had similar iron content, and were dominated by FeII rather than FeIII nanoparticles. Furthermore, mitochondria from mutant hypoxic cells contained approximately the same levels of ISCs as WT cells, confirming that Yfh1 is not required for ISC assembly. These cells also did not accumulate excessive iron, indicating that iron accumulation into yfh1-deficient mitochondria is stimulated by O2. In addition, in aerobic WT cells, we found that vacuoles stored FeIII, whereas under hypoxic fermenting conditions, vacuolar iron was reduced to FeII. Under respiring conditions, vacuoles of Yfh1-deficient cells contained FeIII, and nanoparticles accumulated only under aerobic conditions. Taken together, these results informed a mathematical model of iron trafficking and regulation in cells that could semiquantitatively simulate the Yfh1-deficiency phenotype. Simulations suggested partially independent regulation in which cellular iron import is regulated by ISC activity in mitochondria, mitochondrial iron import is regulated by a mitochondrial FeII pool, and vacuolar iron import is regulated by cytosolic FeII and mitochondrial ISC activity.

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

Friedreich's ataxia (FRDA) is a degenerative disease caused by a decrease in the mitochondrial protein frataxin (Fxn), which is involved in iron-sulfur cluster (ISC) synthesis. Diminutions in Fxn result in decreased ISC synthesis, increased mitochondrial iron accumulation, and impaired mitochondrial function. Here, we show that conditions that result in increased mitochondrial reactive oxygen species in yeast or mammalian cell culture give rise to increased turnover of Fxn but not of other ISC synthesis proteins. We demonstrate that the mitochondrial Lon protease is involved in Fxn degradation and that iron export through the mitochondrial metal transporter Mmt1 protects yeast Fxn from degradation. We also determined that when FRDA fibroblasts were grown in media containing elevated iron, mitochondrial reactive oxygen species increased and Fxn decreased compared to WT fibroblasts. Furthermore, we screened a library of FDA-approved compounds and identified 38 compounds that increased yeast Fxn levels, including the azole bifonazole, antiparasitic fipronil, antitumor compound dibenzoylmethane, antihypertensive 4-hydroxychalcone, and a nonspecific anion channel inhibitor 4,4-diisothiocyanostilbene-2,2-sulfonic acid. We show that top hits 4-hydroxychalcone and dibenzoylmethane increased mRNA levels of transcription factor nuclear factor erythroid 2-related factor 2 in FRDA patient-derived fibroblasts, as well as downstream antioxidant targets thioredoxin, glutathione reductase, and superoxide dismutase 2. Taken together, these findings reveal that FRDA progression may be in part due to oxidant-mediated decreases in Fxn and that some approved compounds may be effective in increasing mitochondrial Fxn in FRDA, delaying disease progression.

Modelling Protein Plasticity: The Example of Frataxin and Its Variants.

Frataxin (FXN) is a protein involved in storage and delivery of iron in the mitochondria. Single-point mutations in the FXN gene lead to reduced production of functional frataxin, with the consequent dyshomeostasis of iron. FXN variants are at the basis of neurological impairment (the Friedreich's ataxia) and several types of cancer. By using altruistic metadynamics in conjunction with the maximal constrained entropy principle, we estimate the change of free energy in the protein unfolding of frataxin and of some of its pathological mutants. The sampled configurations highlight differences between the wild-type and mutated sequences in the stability of the folded state. In partial agreement with thermodynamic experiments, where most of the analyzed variants are characterized by lower thermal stability compared to wild type, the D104G variant is found with a stability comparable to the wild-type sequence and a lower water-accessible surface area. These observations, obtained with the new approach we propose in our work, point to a functional switch, affected by single-point mutations, of frataxin from iron storage to iron release. The method is suitable to investigate wide structural changes in proteins in general, after a proper tuning of the chosen collective variable used to perform the transition.

During FeS cluster biogenesis, ferredoxin and frataxin use overlapping binding sites on yeast cysteine desulfurase Nfs1.

In mitochondria, cysteine desulfurase (Nfs1) plays a central role in the biosynthesis of iron-sulfur (FeS) clusters, cofactors critical for activity of many cellular proteins. Nfs1 functions both as a sulfur donor for cluster assembly and as a binding platform for other proteins functioning in the process. These include not only the dedicated scaffold protein (Isu1) on which FeS clusters are synthesized but also accessory FeS cluster biogenesis proteins frataxin (Yfh1) and ferredoxin (Yah1). Yfh1 has been shown to activate cysteine desulfurase enzymatic activity, whereas Yah1 supplies electrons for the persulfide reduction. While Yfh1 interaction with Nfs1 is well understood, the Yah1-Nfs1 interaction is not. Here, based on the results of biochemical experiments involving purified WT and variant proteins, we report that in Saccharomyces cerevisiae, Yah1 and Yfh1 share an evolutionary conserved interaction site on Nfs1. Consistent with this notion, Yah1 and Yfh1 can each displace the other from Nfs1 but are inefficient competitors when a variant with an altered interaction site is used. Thus, the binding mode of Yah1 and Yfh1 interacting with Nfs1 in mitochondria of S. cerevisiae resembles the mutually exclusive binding of ferredoxin and frataxin with cysteine desulfurase reported for the bacterial FeS cluster assembly system. Our findings are consistent with the generally accepted scenario that the mitochondrial FeS cluster assembly system was inherited from bacterial ancestors of mitochondria.

Heme Oxygenase-1 (HMOX-1) and inhibitor of differentiation proteins (ID1, ID3) are key response mechanisms against iron-overload in pancreatic ß-cells.

Iron overload promotes the generation of reactive oxygen species (ROS). Pancreatic ß-cells can counter oxidative stress through multiple anti-oxidant responses. Herein, RNA-sequencing was used to describe the expression profile of iron regulatory genes in human islets with or without diabetes. Functional experiments including siRNA silencing, qPCR, western blotting, cell viability, ELISA and RNA-sequencing were performed as means of identifying the genetic signature of the protective response following iron overload-induced stress in human islets and INS-1. FTH1 and FTL genes were highly expressed in human islets and INS-1 cells, while hepcidin (HAMP) was low. FXN, DMT1 and FTHL1 genes were differentially expressed in diabetic islets compared to control. Silencing of Hamp in INS-1 cells impaired insulin secretion and influenced the expression of ß-cell key genes. RNA-sequencing analysis in iron overloaded INS-1 cells identified Id1 and Id3 as the top down-regulated genes, while Hmox1 was the top upregulated. Expression of ID1, ID3 and HMOX1 was validated at the protein level in INS-1 cells and human islets. Differentially expressed genes (DEGs) were enriched for TGF-ß, regulating stem cells, ferroptosis, and HIF-1 signaling. Hmox1-silenced cells treated with FAC elevated the expression of Id1 and Id3 expression than untreated cells. Our findings suggest that HMOX1, ID1 and ID3 define the response mechanism against iron-overload-induced stress in ß-cells.