Friedreich Ataxia (FRDA) is an autosomal recessive degenerative disorder. It is caused by an abnormal expansion of GAA trinucleotide repeats in the first intron of the gene encoding frataxin. Since rates of cell division have been linked to oxidative stress, we have examined several parameters of oxidative stress in a FRDA primary fibroblast cell line that had a dramatically different growth rate. In the FRDA fibroblasts, the high level of reactive oxygen species (ROS) indicated elevated oxidative stress. The elevated glutathione peroxidase (Gpx) activity in the ROS defense system may represent an adaptive response to the high oxidative stress. The increased mitochondrial membrane potential (MMP) likely contributed to increased oxidant production, which could be contributed by elevated ROS. This increased oxidant production might be responsible for increased rate of progression through the cell cycle.<p> Furthermore, the elevated oxidative stress is also associated with progressive neural pathology of FRDA. In FRDA, pathology is first seen in the dorsal root ganglia and the dorsal columns of the spinal cord. Due to the abnormal metal distribution seen in the FRDA spinal cord and medulla, we hypothesized that metal binding proteins were abnormally distributed in FRDA. In our FRDA samples, we observed the well established histopathology of FRDA and examined the distribution of some metal binding proteins (frataxin, ferritin and metallothionein) through immunohistochemistry. Our results showed demyelination and loss of axons in the degeneration areas of the two FRDA cases. In addition, we found that the metal binding proteins were abnormally distributed in the FRDA spinal cord and the medulla. The abnormal distributions of the metal binding proteins were characterized by low expressions of iron binding proteins, especially frataxin and cytosolic ferritin, and undetectable expression of the copper and zinc binding protein, metallothionein. In summary, the rapid cell growth is a feature of FRDA fibroblast cell lines. We also tested Gpx activity, measured oxidant levels and determined the MMP in a FRDA primary fibroblast cell line that had a dramatically fast growth rate. The FRDA histopathology studies showed the metal binding proteins including frataxin, ferritin and metallothionein were abnormally distributed in the spinal cord and the medulla.

Friedreich Ataxia (FRDA) is an autosomal recessive degenerative disorder. It is caused by an abnormal expansion of GAA trinucleotide repeats in the first intron of the gene encoding frataxin. Since rates of cell division have been linked to oxidative stress, we have examined several parameters of oxidative stress in a FRDA primary fibroblast cell line that had a dramatically different growth rate. In the FRDA fibroblasts, the high level of reactive oxygen species (ROS) indicated elevated oxidative stress. The elevated glutathione peroxidase (Gpx) activity in the ROS defense system may represent an adaptive response to the high oxidative stress. The increased mitochondrial membrane potential (MMP) likely contributed to increased oxidant production, which could be contributed by elevated ROS. This increased oxidant production might be responsible for increased rate of progression through the cell cycle.<p> Furthermore, the elevated oxidative stress is also associated with progressive neural pathology of FRDA. In FRDA, pathology is first seen in the dorsal root ganglia and the dorsal columns of the spinal cord. Due to the abnormal metal distribution seen in the FRDA spinal cord and medulla, we hypothesized that metal binding proteins were abnormally distributed in FRDA. In our FRDA samples, we observed the well established histopathology of FRDA and examined the distribution of some metal binding proteins (frataxin, ferritin and metallothionein) through immunohistochemistry. Our results showed demyelination and loss of axons in the degeneration areas of the two FRDA cases. In addition, we found that the metal binding proteins were abnormally distributed in the FRDA spinal cord and the medulla. The abnormal distributions of the metal binding proteins were characterized by low expressions of iron binding proteins, especially frataxin and cytosolic ferritin, and undetectable expression of the copper and zinc binding protein, metallothionein. In summary, the rapid cell growth is a feature of FRDA fibroblast cell lines. We also tested Gpx activity, measured oxidant levels and determined the MMP in a FRDA primary fibroblast cell line that had a dramatically fast growth rate. The FRDA histopathology studies showed the metal binding proteins including frataxin, ferritin and metallothionein were abnormally distributed in the spinal cord and the medulla.

Friedreich ataxia (FRDA) is an inherited autosomal recessive neurodegenerative disorder caused by a GAA trinucleotide repeat expansion mutation within the first intron of the FXN gene. Normal individuals have 5 to 30 GAA repeats, whereas affected individuals have from approximately 70 to more than 1,000 GAA triplets. In addition to progressive neurological disability, FRDA is associated with cardiomyopathy and an increased risk of diabetes mellitus. Currently there is no effective therapy for FRDA and this is perhaps due to the lack of an effective system to test potential drugs. Therefore, the main aim of this thesis is to develop a novel cell culture system, to aid in rapid drug screening for FRDA. Firstly, I have demonstrated the establishment of novel cell culture systems, including primary fibroblasts, neural stem cells (NSC) and splenocytes, from FRDA YAC transgenic mouse models (YG8 and YG22). Then, I have shown the differentiation of NSCs into neurons, oligodendrocytes and astrocytes. The presence of these cells was confirmed by using cell specific immunofluorescence assays. I have also shown that both YG8 and YG22 rescue mice have less tolerance to hydrogen peroxide induced oxidative stress than WT mice, as similarly seen in FRDA patient fibroblasts. Recent findings indicate that FRDA is associated with heterochromatin-mediated silencing of the FXN gene accompanied by histone changes, flanking the GAA repeats. This suggested potential therapeutic use of compounds which can reduce the methylation and increase the acetylation of histone proteins. Therefore, using human and mouse primary fibroblast cell lines I have investigated the efficacy and tolerability of various DNA demethylating agents, GAA interacting compounds and class III histone deacetylase (HDAC) inhibitors. Although DNA demethylating agents showed increased FXN expression, no correlation between the level of DNA methylation and FXN expression was identified. Nevertheless, the use of GAA interacting compounds, particularly DB221, and the HDAC inhibitor, nicotinamide, have shown encouraging results, provoking us to use such compounds in future long-term in vivo studies. In addition, I have also investigated the long-term efficacy of two benzamide-type HDAC inhibitors, RGFA 136 and RGFP 109, on the FRDA YAC transgenic mice. No overt toxicity was identified with either drug, indicating a safe administration of these compounds. Both compounds produced improved functional analysis together with significantly reduced DRG neurodegeneration. However, neither of these compounds was shown to significantly increase the FXN mRNA expression. Nevertheless, elevated levels of frataxin protein in the brain tissues were obtained with RGFP 109, suggesting that RGFP 109 is capable of crossing the blood-brain barrier. I have also found increased levels of global acetylated H3 and H4 histone proteins in brain tissues, along with significant increase in aconitase enzyme activity, particularly with RGFP 109 treatments. Overall, these results support future clinical trial development with such compounds.

Thesis (M.Sc.)--Memorial University of Newfoundland, 2004. / Bibliography: leaves 79-97.

Thesis (Ph. D.)--University of Oklahoma. / Includes bibliographical references.

Friedreich ataxia (FRDA) is an autosomal recessive neurodegenerative disorder, caused by a GAA repeat expansion mutation within intron 1 of the FXN gene, resulting in reduced level of frataxin protein. Normal individuals have 5 to 40 GAA repeat sequences, whereas affected individuals have approximately 70 to more than 1000 GAA triplets. Frataxin is a mitochondrial protein involved in iron-sulphur cluster and heme biosynthesis. The reduction in frataxin expression leads to oxidative stress, mitochondrial iron accumulation and consequential cell death with the primary sites of neurons of the dorsal root ganglia and the dentate nucleus of the cerebellum. FRDA, which is the most common inherited ataxia, affecting 1:50,000 Caucasians, is characterised by neurodegeneration, cardiomyopathy, diabetes mellitus and skeletal deformities. To investigate FRDA molecular disease mechanisms and therapy, several human FXN YAC transgenic mouse models have been established: Y47R, containing normal-sized (GAA)9 repeats; YG8R and YG22R, which initially contained expanded GAA repeats of 90-190 units and 190 units, respectively, but which have subsequently been bred to now contain expanded GAA repeats of 120-220 units and 170-260 units, respectively, and YG8sR (YG8R with a small GAA band) that was recently generated from YG8R breeding. To determine the FXN transgene copy number in the enhanced GAA repeat expansion-based FRDA mouse lines, a TaqMan qPCR assay was developed. The results demonstrated that the YG22R and Y47R lines had a single copy of the FXN transgene while the YG8R line had two copies. The YG8s lines showed less than one copy of the target gene, suggesting potential deletion of the FXN gene. Single integration sites of all transgenes were confirmed by fluorescence in situ hybridisation (FISH) analysis of metaphase and interphase chromosomes. However, in the YG8s line, at least 25% of the YG8s cells had no signals, while the remaining cells showed one signal corresponding to the transgenic FXN gene. In addition, the analysis of FXN exons in YG8s rescue mice by PCR confirmed the presence of all FXN exons in these lines, suggesting the incidence of somatic mosaicism in these lines. Extended functional analysis was carried out on these mice from 4 to 12 months of age. Coordination ability of YG8R, YG8sR and YG22R ‘FRDA-like’ mice, together with Y47R and C57BL6/J wild-type control mice, was assessed using accelerating rotarod analysis. The results indicated a progressive decrease in the motor coordination of YG8R, YG22R and YG8sR mice compared to Y47R or C57BL6/J controls. Locomotor activity was also assessed using an open field beam-breaker apparatus followed by four additional functional analyses including beam-walk, hang wire, grip strength and foot print tests. The results indicated significant functional deficits in the FRDA mouse models. Glucose and insulin tolerance tests were also conducted in the FRDA mouse models, indicating glucose intolerance and insulin hypersensitivity in the aforementioned lines. To investigate the correlation between the FRDA-like pathological phenotype and frataxin deficiency in the FRDA mouse models, frataxin mRNA and protein levels as well as somatic GAA repeat instability were examined. The results indicated that somatic GAA repeats increased in the cerebellum and brain of YG22R, YG8R and YG8sR mice, together with significantly reduced levels of FXN mRNA and protein in the liver of YG8R and YG22R compared to Y47R. However, YG8sR lines showed a significant decrease in FXN mRNA in all of the examined tissues compared to Y47R human FXN and C57BL6/J mouse Fxn mRNA. Protein expression levels were also considerably reduced in all the tissues of YG8sR mice compared to Y47R. Subsequently, the telomere length of human and mouse FRDA and control fibroblasts was assessed using qPCR and Q-FISH. The results indicated that the FRDA cells had chromosomes with relatively longer telomeric repeats in comparison to the controls. FRDA cells were screened for expression of telomerase activity using the TRAP assay and a quantitative assay for hTERT mRNA expression using TaqMan qRT-PCR. The results indicated that telomerase activity was not present in the FRDA cells. To investigate whether FRDA cells maintained their telomeres by ALT associated PML bodies (APBs), co-localisation of PML bodies with telomeres was assessed in these cells using combined immunofluorescence to PML and Q-FISH for telomere detection. The results demonstrated that the FRDA cells had significantly higher co-localised PML foci with telomeric DNA compared to the normal cells. Moreover, telomere sister chromatid exchange (T-SCE) frequencies were analysed in the human FRDA cell lines using chromosome orientation FISH (CO-FISH). The results indicated a significant increase in T-SCE levels of the FRDA cell lines relative to the controls. Furthermore, growth curve and population doubling analysis of the human FRDA and control fibroblasts was carried out. The results showed that the FRDA fibroblast cell cultures underwent growth arrest with higher cumulative population doubling compared to the controls. Though, further analysis of telomere length at different passage numbers revealed that the FRDA cells lost telomeres faster than the controls. Finally, the telomere dysfunction-induced foci (TIF) assay was performed to detect DNA damage in the human FRDA fibroblast cells using an antibody against DNA damage marker γ-H2AX and a synthetic PNA probe for telomeres. The frequency of γ-H2AX foci was significantly higher in the FRDA cells compared to the controls. Similarly, the FRDA cells had greater frequencies of TIFs in comparison to the controls, suggesting induced telomere dysfunction in the FRDA cells.

The objectives of my PhD were to identify the spatial and the temporal dynamics of the brain areas involved in tactile change detection as well as the neural mechanisms responsible for the processing of tactile change detection. To that aim, three specific MEG studies were performed; each of them is addressing specific research aims.The first study investigated the spatiotemporal dynamics of the multilevel cortical processing of tactile change detection in human healthy subjects. This study disclosed a hierarchical organization from unimodal early tactile change detection at secondary somatosensory cortex to multi modal complex processing at bilateral temporo-parietal junctions, posterior parietal cortex and supplementary motor areas. The second study aimed at discriminating between debated neural mechanisms responsible for the genesis of the somatosensory mismatch negativity (sMMN). To do so, we manipulated the predictability of the deviant stimuli and the response to omissions in different kind of oddballs, the response to deviant stimuli paired with standards and occurring alone. We found out that mechanisms for early tactile change detection reflected by the sMMN were better explained by the predictive coding theory compared to the adaptation and adjustment theories. Finally we sought to characterize the alterations in early cortical tactile change detection in Friedreich Ataxia (FRDA); a neurological disorder characterized by somatosensory and cerebellar pathways degeneration. The aim of this work was to study the role of the cerebellum in the genesis of sMMN and its potential selectivity for somatosensory change detection compared to auditory. This study demonstrated that, in FRDA, both tactile and auditory pathways are affected at the level of primary sensory neurons and dorsal root/spiral ganglia in a genetically determined. By contrasts, early cortical sensory change detection in FRDA was impaired only in the tactile modality in line with the sMMN impairment described in patients with acquired cerebellar lesions or during cerebellar inhibition by trans cranial magnetic stimulation. These data brought novel empirical evidence supporting the contribution of spinocerebellar tracts in sMMN genesis at cSII cortex.In conclusion, this PhD contributed to identify the network responsible for tactile change detection that involves cuneocerebellar spinocerebellar tract and cSII cortex as somatosensory specific areas and TPJ, SMA & PPC as multimodal brain areas. We further provided evidence that early change detection mechanisms at SII cortex fall under the predictive coding framework and that change detection is hierarchically organized with inputs from low level areas for genesis of an adequate generative model of our environment and conscious representation of our body. / Doctorat en Sciences médicales (Médecine) / info:eu-repo/semantics/nonPublished

L’ataxie de Friedreich (AF) est une maladie mitochondriale caractérisée par une ataxie sensitive et spinocérébelleuse, une cardiomyopathie et du diabète, pour laquelle il n’existe pas de traitement. L’AF résulte de niveaux réduits de frataxine (FXN), une protéine mitochondriale impliquée dans la biosynthèse des centres Fe-S. La neurophysiopathologie précise de la maladie n’est pas identifiée et malgré d’intenses progrès ces dernières années, il n’existait pas de bon modèle pour développer des approches thérapeutiques visant à stopper ou réverser l’atteinte sensitive de l’AF. Nous avons donc généré un nouveau modèle murin qui récapitule l’ataxie sensitive et la neuropathie associée au déficit en FXN. Plusieurs mécanismes moléculaires affectés en absence de FXN dans les neurones proprioceptifs, primairement affectés dans l’AF, ont été identifiés. Nous avons également démontré l’efficacité d’une approche de thérapie génique, basée sur l’utilisation de vecteur adéno-associés (AAV) exprimant la FXN humaine, pour réverser la neuropathie, établissant la preuve de concept du potentiel d’une telle approche pour l’atteinte sensitive de l’AF. / Friedreich ataxia (FA) is a rare mitochondrial disease characterized by sensory and spinocerebellar ataxia, hypertrophic cardiomyopathy, and diabetes, for which there is no treatment. FA is caused by reduced levels of frataxin (FXN), an essential mitochondrial protein involved in the biosynthesis of Fe-S clusters. To date, FA precise neuropathophysiology is not identified and despite significant progresses in recent the years, there was no good model to develop therapeutic approaches in order to stop or reverse the sensory ataxia associated to the FA. Thus, we have generated a new neuronal mouse model that recapitulates the sensory ataxia and the neuropathy associated to FXN deficiency. Several molecular mechanisms dysregulated in the absence of FXN in the proprioceptive neurons, primarily affected in FA, were identified. Furthermore, we have demonstrated the efficacy of a gene therapy (GT) approach, based on the delivery of adeno-associated vectors (AAV) expressing the human FXN, to reverse the sensitive neuropathy, thus establishing the preclinical proof of concept for the potential of GT in treating FA sensitive neuropathy.

Les neurodégénérescences avec accumulation de fer dans le cerveau (Neurodegeneration with Brain Iron Accumulation, NBIA) sont des maladies neurodégénératives progressives, génétiquement hétérogènes. On connait actuellement 11 gènes de ces maladies mais pour la plupart d’entre eux leur lien avec l’accumulation en fer est encore incompris. Ce travail de thèse présente deux nouveaux gènes de NBIA identifiés par séquençage d’exome dans deux familles indépendantes. Le premier gène, REPS1, est impliqué dans le recyclage de l’endosome. Les fibroblastes de patients sont caractérisés par une accumulation de fer qui est corrigée par l’expression de l’ADNc de REPS1 dans ces cellules. Le deuxième gène, CRAT, code une carnitine acétyltransferase et le déficit de β-oxydation détecté dans les fibroblastes du patient a été corrigé par l’expression de l’ADNc CRAT normal. Le rôle de REPS1 dans le recyclage de l’endosome a mis sur la voie du mécanisme physiopathologique des NBIA. En effet, les fibroblastes des patients REPS1 et CRAT mais aussi d’autres patients avec des mutations d’autres gènes connus de NBIA (PANK2, PLA2G6, FA2H, C19ORF12) ont une accumulation massive en fer et une anomalie de recyclage du récepteur à la transferrine (TfR1). TfR1 permet l’entrée du fer par endocytose et son expression est régulée par le contenu en fer des cellules. La seule régulation connue de l’homéostasie du fer se fait au niveau post-transcriptionnel par le système IRP/IRE qui est fonctionnel dans les fibroblastes NBIA alors que la protéine TfR1 s’accumule. Cette accumulation de fer montre ainsi qu’il existe une régulation post-traductionnelle, jusqu’ici inconnue, et qui n’est pas fonctionnelle dans les NBIA. Nous avons pu montrer que cette régulation se faisait par une palmitoylation du TfR1, déficitaire dans les NBIA, mais restaurée par l’artesunate. Ainsi quel que soit le gène muté, tous les NBIA résultent d’une anomalie de recyclage du TfR1 permettant de les définir comme des maladies du trafic intracellulaire. La deuxième partie de la thèse s’intéresse au mécanisme physiopathologique de l’ataxie de Friedreich (FRDA) caractérisée elle aussi par une accumulation de fer dans le cerveau. FRDA est due à des expansions de triplets dans le premier intron du gène FXN conduisant à l’extinction de FXN et de PIP5K1B situé en amont. L’étude de modèles cellulaires dans lesquels le gène FXN et/ou PIP5K1B ont été éteints par siRNA et de fibroblastes de patients a permis de mettre en évidence une anomalie de l’homéostasie du fer qui rappelle celle observée dans les NBIA. L’ensemble de ces résultats a permis de comprendre le mécanisme physiopathologique des NBIA, de mettre à jour une régulation encore inconnue de l’homéostasie du fer mais aussi d’envisager une voie de traitement des NBIA. / Neurodegeneration with brain iron accumulation (NBIA) encompasses a group of rare neurogenerative disorders with different clinical and molecular features, underlined by progressive extrapyramidal dysfunction and iron accumulation in the brain. To date, mutations in 11 genes are currently known. Nevertheless for most of them their link with iron accumulation is still misunderstood. This work presents two novel NBIA genes identified by exome sequencing in two independent families. The first gene, REPS1, is involved in endosome recycling. Patient’s fibroblasts are characterized by iron overload corrected by wild-type REPS1 cDNA overexpression. The second gene, CRAT, encodes a carnitine acetyltransferase and a β-oxidation deficit in patient’s fibroblasts has been fixed by overexpression of wild-type CRAT cDNA. The function of REPS1 in endosome recycling put on the path of the NBIA pathophysiological mechanism. Indeed, fibroblasts of REPS1 patients but also from other patients mutated in various NBIA genes (CRAT, PANK2, PLA2G6, FA2H, C19ORF12) present massive iron accumulation and abnormal transferrin receptor (TfR1) recycling. TfR1 allows iron uptake by endocytosis and its expression is regulated by the iron cellular status. The only known regulation of iron homeostasis occurs at the posttranscriptional level by the IRE/IRP system which is functional in NBIA fibroblasts whereas TfR1 protein accumulates. This iron accumulation highlights a yet unknown posttranslational regulation which is not functional in NBIA. We have been able to demonstrate that this regulation occurs via TfR1 palmitoylation, which is defective in NBIA, but restored by artesunate. Hence, whatever the disease gene, all NBIA gave rise to abnormal TfR1 recycling which allows defining NBIA as intracellular trafficking disease. The second part of the thesis focused on the pathophysiological mechanism of the Friedreich ataxia (FRDA) also characterized by brain iron overload . FRDA is related to triplets expansions in the first intron of FXN gene leading to the extinction of FXN and PIP5K1B upstream gene. Studying cellular models knocked down for FXN and/or PIP5K1B by siRNA and patients’ fibroblasts of patients allowed to detect abnormal iron homeostasis reminiscent of NBIA. All these results allowed to decipher the NBIA pathophysiological mechanism, to highlight a yet unknown iron homeostasis regulation and to open possible ways towards therapeutic drugs for NBIA.

L’ataxie de Friedreich est une maladie neurodégénérative sévère causée par un défaut d’expression de la frataxine (FXN), une petite protéine mitochondriale impliquée dans la biogenèse des centres fer-soufre (Fe/S), des groupement prosthétiques aux fonctions cellulaires essentielles. Chez les mammifères, il a été montré que la frataxine stimule la synthèse in vitro de centres Fe/S sur la protéine d’échaffaudage ISCU, grâce à l’augmentation de la production d’ions sulfures par le complexe NFS1-ISD11-ISCU. Cependant, le mécanisme par lequel la frataxine active la biogenèse des centres Fe/S n’a pas encore été défini. Nous avons étudié les effets de FXN sur les cinétiques de formation et de réduction des persulfures, des intermédiaires clés de la production d’ions sulfures, générés par la cystéiene désulfurase NFS1, à l’aide d’un test de détection des persulfures basé sur l’utilisation de composés synthétiques peptide-maléimide et de la spectrométrie de masse. Nous avons montré que FXN active deux réactions très similaires : la réduction du persulfure de NFS1 par des réducteurs à thiols comme le DTT, la L-cystéine et le glutathion et le transfert de soufre de NFS1 vers ISCU, conduisant à l’accumulation de persulfure sur la cystéine C104 d’ISCU. Nous avons constaté que la vitesse de réduction du persulfure d’ISCU par les thiols n’est pas affectée en présence de FXN et que ce persulfure est réduit plus lentement que celui de NFS1. Nous avons corrélé l’activation par FXN de la réduction du persulfure de NFS1 par les thiols à une stimulation de l’assemblage d’un centre Fe/S sur ISCU. Dans nos conditions expérimentales, l’atome de soufre du persulfure d’ISCU n’est pas incorporé dans le centre Fe/S synthétisé, mais nos résultats ne permettent pas d’exclure que ce persulfure puisse être réduit par une réductase dédiée, encore non identifiée. L’ensemble de nos données indiquent que le rôle de la frataxine est de contrôler la réduction du persulfure de NFS1, en augmentant les vitesses de transfert de soufre vers ISCU et de réduction du persulfure de NFS1 par les thiols. / Friedreich ataxia is a severe neurodegenerative disease caused by reduced expression of frataxin (FXN), a small mitochondrial protein involved in iron-sulfur (Fe/S) cluster biogenesis which are prostetic groups with essential cellular functions. It has been shown in vitro that mammalian FXN activates Fe/S cluster synthesis on the scaffold protein ISCU, by rising up suflide ion production by NFS1-ISD11-ISCU complex. However, the mechanism by which frataxin stimulates Fe/S cluster biogenesis has not been yet defined. We have studied the effect of FXN on the kinetics of formation and reduction of persulfides that are key intermediates of sulfide ion production generated by NFS1, using mass spectrometry and a new detection assay for persulfide based on gel-mobility shift following alkylation by maleimide-peptide compounds. We demonstrate that frataxin activates two similar reactions : sulfur transfer from cysteine desulfurase NFS1 to ISCU leading to accumulation of a persulfide on ISCUcysteine C104 and reduction of NFS1 persulfide by thiol reducers such as DTT, L-cysteine and glutathion. We have observed that FXN does not stimulate the rate of ISCU persulfide reduction by thiols and that this persulfide is reduced much more slowly than NFS1 persulfide. We have then correlated the reduction of NFS1 persulfide with Fe/S cluster assembly. Under our experimental conditions, the sulfur from ISCU persulfide is not incorporated into the Fe/S cluster. However, we cannot exclude that an as yet not identfiied reductase could reduces ISCU persulfide and trigger Fe/S cluster assembly. Overall, our data point to a regulatory function of FXN as an enhancer of persulfide reduction, stimulating the rates of sulfur transfer to ISCU and NFS1 persulfide.