Supramolecular assemblies of human frataxin are formed via subunit-subunit interactions mediated by a non-conserved amino-terminal region

Simultaneous Quantification of Mitochondrial Mature Frataxin and Extra-Mitochondrial Frataxin Isoform E in Friedreich’s Ataxia Blood

Friedreich’s ataxia (FRDA) is an autosomal recessive disease caused by an intronic guanine-adenine-adenine (GAA) triplet expansion in the frataxin (FXN) gene, which leads to reduced expression of full-length frataxin (1–210) also known as isoform 1. Full-length frataxin has a mitochondrial targeting sequence, which facilitates its translocation into mitochondria where it is processed through cleavage at G⁴¹-L⁴² and K⁸⁰-S⁸¹ by mitochondrial processing (MPP) to release mitochondrial mature frataxin (81–210). Alternative splicing of FXN also leads to expression of N-terminally acetylated extra-mitochondrial frataxin (76–210) named isoform E because it was discovered in erythrocytes. Frataxin isoforms are undetectable in serum or plasma, and originally whole blood could not be used as a biomarker in brief therapeutic trials because it is present in erythrocytes, which have a half-life of 115-days and so frataxin levels would remain unaltered. Therefore, an assay was developed for analyzing frataxin in platelets, which have a half-life of only 10-days. However, our discovery that isoform E is only present in erythrocytes, whereas, mature frataxin is present primarily in short-lived peripheral blood mononuclear cells (PBMCs), granulocytes, and platelets, meant that both proteins could be quantified in whole blood samples. We now report a quantitative assay for frataxin proteoforms in whole blood from healthy controls and FRDA patients. The assay is based on stable isotope dilution coupled with immunoprecipitation (IP) and two-dimensional-nano-ultrahigh performance liquid chromatography/parallel reaction monitoring/high resolution mass spectrometry (2D-nano-UHPLC-PRM/HRMS). The lower limit of quantification was 0.5 ng/mL for each proteoform and the assays had 100% sensitivity and specificity for discriminating between healthy controls (n = 11) and FRDA cases (N = 100 in year-1, N = 22 in year-2,3). The mean levels of mature frataxin in whole blood from healthy controls and homozygous FRDA patients were significantly different (p < 0.0001) at 7.5 ± 1.5 ng/mL and 2.1 ± 1.2 ng/mL, respectively. The mean levels of isoform E in whole blood from healthy controls and homozygous FRDA patients were significantly different (p < 0.0001) at 26.8 ± 4.1 ng/mL and 4.7 ± 3.3 ng/mL, respectively. The mean levels of total frataxin in whole blood from healthy controls and homozygous FRDA patients were significantly different (p < 0.0001) at 34.2 ± 4.3 ng/mL and 6.8 ± 4.0 ng/mL, respectively. The assay will make it possible to rigorously monitor the natural history of the disease and explore the potential role of isoform E in etiology of the disease. It will also facilitate the assessment of therapeutic interventions (including gene therapy approaches) that attempt to increase frataxin protein expression as a treatment for this devastating disease.

Recent Advances in the Elucidation of Frataxin Biochemical Function Open Novel Perspectives for the Treatment of Friedreich’s Ataxia

Friedreich’s ataxia (FRDA) is the most prevalent autosomic recessive ataxia and is associated with a severe cardiac hypertrophy and less frequently diabetes. It is caused by mutations in the gene encoding frataxin (FXN), a small mitochondrial protein. The primary consequence is a defective expression of FXN, with basal protein levels decreased by 70–98%, which foremost affects the cerebellum, dorsal root ganglia, heart and liver. FXN is a mitochondrial protein involved in iron metabolism but its exact function has remained elusive and highly debated since its discovery. At the cellular level, FRDA is characterized by a general deficit in the biosynthesis of iron-sulfur (Fe-S) clusters and heme, iron accumulation and deposition in mitochondria, and sensitivity to oxidative stress. Based on these phenotypes and the proposed ability of FXN to bind iron, a role as an iron storage protein providing iron for Fe-S cluster and heme biosynthesis was initially proposed. However, this model was challenged by several other studies and it is now widely accepted that FXN functions primarily in Fe-S cluster biosynthesis, with iron accumulation, heme deficiency and oxidative stress sensitivity appearing later on as secondary defects. Nonetheless, the biochemical function of FXN in Fe-S cluster biosynthesis is still debated. Several roles have been proposed for FXN: iron chaperone, gate-keeper of detrimental Fe-S cluster biosynthesis, sulfide production stimulator and sulfur transfer accelerator. A picture is now emerging which points toward a unique function of FXN as an accelerator of a key step of sulfur transfer between two components of the Fe-S cluster biosynthetic complex. These findings should foster the development of new strategies for the treatment of FRDA. We will review here the latest discoveries on the biochemical function of frataxin and the implication for a potential therapeutic treatment of FRDA.

Mechanism of Iron–Sulfur Cluster Assembly: In the Intimacy of Iron and Sulfur Encounter

Iron–sulfur (Fe–S) clusters are protein cofactors of a multitude of enzymes performing essential biological functions. Specialized multi-protein machineries present in all types of organisms support their biosynthesis. These machineries encompass a scaffold protein on which Fe–S clusters are assembled and a cysteine desulfurase that provides sulfur in the form of a persulfide. The sulfide ions are produced by reductive cleavage of the persulfide, which involves specific reductase systems. Several other components are required for Fe–S biosynthesis, including frataxin, a key protein of controversial function and accessory components for insertion of Fe–S clusters in client proteins. Fe–S cluster biosynthesis is thought to rely on concerted and carefully orchestrated processes. However, the elucidation of the mechanisms of their assembly has remained a challenging task due to the biochemical versatility of iron and sulfur and the relative instability of Fe–S clusters. Nonetheless, significant progresses have been achieved in the past years, using biochemical, spectroscopic and structural approaches with reconstituted system in vitro. In this paper, we review the most recent advances on the mechanism of assembly for the founding member of the Fe–S cluster family, the [2Fe2S] cluster that is the building block of all other Fe–S clusters. The aim is to provide a survey of the mechanisms of iron and sulfur insertion in the scaffold proteins by examining how these processes are coordinated, how sulfide is produced and how the dinuclear [2Fe2S] cluster is formed, keeping in mind the question of the physiological relevance of the reconstituted systems. We also cover the latest outcomes on the functional role of the controversial frataxin protein in Fe–S cluster biosynthesis.

Characterization of a new N-terminally acetylated extra-mitochondrial isoform of frataxin in human erythrocytes

Frataxin is a highly conserved protein encoded by the frataxin (FXN) gene. The full-length 210-amino acid form of protein frataxin (1-210; isoform A) expressed in the cytosol of cells rapidly translocates to the mitochondria, where it is converted to the mature form (81-210) by mitochondrial processing peptidase. Mature frataxin (81-210) is a critically important protein because it facilitates the assembly of mitochondrial iron-sulfur cluster protein complexes such as aconitase, lipoate synthase, and succinate dehydrogenases. Decreased expression of frataxin protein is responsible for the devastating rare genetic disease of Friedreich's ataxia. The mitochondrial form of frataxin has long been thought to be present in erythrocytes even though paradoxically, erythrocytes lack mitochondria. We have discovered that erythrocyte frataxin is in fact a novel isoform of frataxin (isoform E) with 135-amino acids and an N-terminally acetylated methionine residue. There is three times as much isoform E in erythrocytes (20.9 ± 6.4 ng/mL) from the whole blood of healthy volunteers (n = 10) when compared with the mature mitochondrial frataxin present in other blood cells (7.1 ± 1.0 ng/mL). Isoform E lacks a mitochondrial targeting sequence and so is distributed to both cytosol and the nucleus when expressed in cultured cells. When extra-mitochondrial frataxin isoform E is expressed in HEK 293 cells, it is converted to a shorter isoform identical to the mature frataxin found in mitochondria, which raises the possibility that it is involved in disease etiology. The ability to specifically quantify extra-mitochondrial and mitochondrial isoforms of frataxin in whole blood will make it possible to readily follow the natural history of diseases such as Friedreich's ataxia and monitor the efficacy of therapeutic interventions.

Copper redox chemistry of plant frataxins

The presence of a conserved cysteine residue in the C-terminal amino acid sequences of plant frataxins differentiates these frataxins from those of other kingdoms and may be key in frataxin assembly and function. We report a full study on the ability of Arabidopsis (AtFH) and Zea mays (ZmFH-1 and ZmFH-2) frataxins to assemble into disulfide-bridged dimers by copper-driven oxidation and to revert to monomers by chemical reduction. We monitored the redox assembly-disassembly process by electrospray ionization mass spectrometry, electrophoresis, UV-Vis spectroscopy, and fluorescence measurements. We conclude that plant frataxins AtFH, ZmFH-1 and ZmFH-2 are oxidized by Cu2+ and exhibit redox cysteine monomer - cystine dimer interexchange. Interestingly, the tendency to interconvert is not the same for each protein. Through yeast phenotypic rescue experiments, we show that plant frataxins are important for plant survival under conditions of excess copper, indicating that these proteins might be involved in copper metabolism.

Plant Frataxin in Metal Metabolism

Frataxin is a highly conserved protein from prokaryotes to eukaryotes. Several functions related to iron metabolism have been postulated for this protein, including Fe-S cluster and heme synthesis, response to oxidative damage and oxidative phosphorylation. In plants, the presence of one or two isoforms of this protein with dual localization in mitochondria and chloroplasts has been reported. Frataxin deficiency affects iron metabolism in both organelles, leading to an impairment of mitochondrial respiration, and chlorophyll and photosynthetic electron transport deficiency in chloroplasts. In addition, plant frataxins can react with Cu²⁺ ions and dimerize, which causes the reduction of free Cu ions. This could provide an additional defense mechanism against the oxidation of Fe-S groups by Cu ions. While there is a consensus on the involvement of frataxin in iron homeostasis in most organisms, the interaction of plant frataxins with Cu ions, the presence of different isoforms, and/or the localization in two plant organelles suggest that this protein might have additional functions in vegetal tissues.

Iron-induced oligomerization of human FXN81-210 and bacterial CyaY frataxin and the effect of iron chelators

Patients suffering from the progressive neurodegenerative disease Friedreich's ataxia have reduced expression levels of the protein frataxin. Three major isoforms of human frataxin have been identified, FXN42-210, FXN56-210 and FXN81-210, of which FXN81-210 is considered to be the mature form. Both long forms, FXN42-210 and FXN56-210, have been shown to spontaneously form oligomeric particles stabilized by the extended N-terminal sequence. The short variant FXN81-210, on other hand, has only been observed in the monomeric state. However, a highly homologous E. coli frataxin CyaY, which also lacks an N-terminal extension, has been shown to oligomerize in the presence of iron. To explore the mechanisms of stabilization of short variant frataxin oligomers we compare here the effect of iron on the oligomerization of CyaY and FXN81-210. Using dynamic light scattering, small-angle X-ray scattering, electron microscopy (EM) and cross linking mass spectrometry (MS), we show that at aerobic conditions in the presence of iron both FXN81-210 and CyaY form oligomers. However, while CyaY oligomers are stable over time, FXN81-210 oligomers are unstable and dissociate into monomers after about 24 h. EM and MS studies suggest that within the oligomers FXN81-210 and CyaY monomers are packed in a head-to-tail fashion in ring-shaped structures with potential iron-binding sites located at the interface between monomers. The higher stability of CyaY oligomers can be explained by a higher number of acidic residues at the interface between monomers, which may result in a more stable iron binding. We also show that CyaY oligomers may be dissociated by ferric iron chelators deferiprone and DFO, as well as by the ferrous iron chelator BIPY. Surprisingly, deferiprone and DFO stimulate FXN81-210 oligomerization, while BIPY does not show any effect on oligomerization in this case. The results suggest that FXN81-210 oligomerization is primarily driven by ferric iron, while both ferric and ferrous iron participate in CyaY oligomer stabilization. Analysis of the amino acid sequences of bacterial and eukaryotic frataxins suggests that variations in the position of the acidic residues in helix 1, β-strand 1 and the loop between them may control the mode of frataxin oligomerization.

SAXS and stability studies of iron-induced oligomers of bacterial frataxin CyaY

Frataxin is a highly conserved protein found in both prokaryotes and eukaryotes. It is involved in several central functions in cells, which include iron delivery to biochemical processes, such as heme synthesis, assembly of iron-sulfur clusters (ISC), storage of surplus iron in conditions of iron overload, and repair of ISC in aconitase. Frataxin from different organisms has been shown to undergo iron-dependent oligomerization. At least two different classes of oligomers, with different modes of oligomer packing and stabilization, have been identified. Here, we continue our efforts to explore the factors that control the oligomerization of frataxin from different organisms, and focus on E. coli frataxin CyaY. Using small-angle X-ray scattering (SAXS), we show that higher iron-to-protein ratios lead to larger oligomeric species, and that oligomerization proceeds in a linear fashion as a results of iron oxidation. Native mass spectrometry and online size-exclusion chromatography combined with SAXS show that a dimer is the most common form of CyaY in the presence of iron at atmospheric conditions. Modeling of the dimer using the SAXS data confirms the earlier proposed head-to-tail packing arrangement of monomers. This packing mode brings several conserved acidic residues into close proximity to each other, creating an environment for metal ion binding and possibly even mineralization. Together with negative-stain electron microscopy, the experiments also show that trimers, tetramers, pentamers, and presumably higher-order oligomers may exist in solution. Nano-differential scanning fluorimetry shows that the oligomers have limited stability and may easily dissociate at elevated temperatures. The factors affecting the possible oligomerization mode are discussed.

Architecture of the Human Mitochondrial Iron-Sulfur Cluster Assembly Machinery

Fe-S clusters, essential cofactors needed for the activity of many different enzymes, are assembled by conserved protein machineries inside bacteria and mitochondria. As the architecture of the human machinery remains undefined, we co-expressed in Escherichia coli the following four proteins involved in the initial step of Fe-S cluster synthesis: FXN42-210 (iron donor); [NFS1]·[ISD11] (sulfur donor); and ISCU (scaffold upon which new clusters are assembled). We purified a stable, active complex consisting of all four proteins with 1:1:1:1 stoichiometry. Using negative staining transmission EM and single particle analysis, we obtained a three-dimensional model of the complex with ∼14 Å resolution. Molecular dynamics flexible fitting of protein structures docked into the EM map of the model revealed a [FXN42-210]24·[NFS1]24·[ISD11]24·[ISCU]24 complex, consistent with the measured 1:1:1:1 stoichiometry of its four components. The complex structure fulfills distance constraints obtained from chemical cross-linking of the complex at multiple recurring interfaces, involving hydrogen bonds, salt bridges, or hydrophobic interactions between conserved residues. The complex consists of a central roughly cubic [FXN42-210]24·[ISCU]24 sub-complex with one symmetric ISCU trimer bound on top of one symmetric FXN42-210 trimer at each of its eight vertices. Binding of 12 [NFS1]2·[ISD11]2 sub-complexes to the surface results in a globular macromolecule with a diameter of ∼15 nm and creates 24 Fe-S cluster assembly centers. The organization of each center recapitulates a previously proposed conserved mechanism for sulfur donation from NFS1 to ISCU and reveals, for the first time, a path for iron donation from FXN42-210 to ISCU.

The Structure of the Complex between Yeast Frataxin and Ferrochelatase: CHARACTERIZATION AND PRE-STEADY STATE REACTION OF FERROUS IRON DELIVERY AND HEME SYNTHESIS

Frataxin is a mitochondrial iron-binding protein involved in iron storage, detoxification, and delivery for iron sulfur-cluster assembly and heme biosynthesis. The ability of frataxin from different organisms to populate multiple oligomeric states in the presence of metal ions, e.g. Fe(2+) and Co(2+), led to the suggestion that different oligomers contribute to the functions of frataxin. Here we report on the complex between yeast frataxin and ferrochelatase, the terminal enzyme of heme biosynthesis. Protein-protein docking and cross-linking in combination with mass spectroscopic analysis and single-particle reconstruction from negatively stained electron microscopic images were used to verify the Yfh1-ferrochelatase interactions. The model of the complex indicates that at the 2:1 Fe(2+)-to-protein ratio, when Yfh1 populates a trimeric state, there are two interaction interfaces between frataxin and the ferrochelatase dimer. Each interaction site involves one ferrochelatase monomer and one frataxin trimer, with conserved polar and charged amino acids of the two proteins positioned at hydrogen-bonding distances from each other. One of the subunits of the Yfh1 trimer interacts extensively with one subunit of the ferrochelatase dimer, contributing to the stability of the complex, whereas another trimer subunit is positioned for Fe(2+) delivery. Single-turnover stopped-flow kinetics experiments demonstrate that increased rates of heme production result from monomers, dimers, and trimers, indicating that these forms are most efficient in delivering Fe(2+) to ferrochelatase and sustaining porphyrin metalation. Furthermore, they support the proposal that frataxin-mediated delivery of this potentially toxic substrate overcomes formation of reactive oxygen species.

Frataxin expression in reticulocytes of non-splenectomized and splenectomized patients with HbE-β-thalassaemia

OBJECTIVES

A previous report revealed that thalassaemic patients with transfusional iron overload developed oxidative stress. The aims of this study were to investigate the FXN mRNA levels in the reticulocytes of patients with HbE-beta-thalassaemia who were treated with regular transfusions, to compare the results with those from normal controls and to evaluate the relationships of the levels of FXN mRNA with malondialdehyde (MDA) and iron parameters in these patients.

DESIGN AND METHODS

The levels of FXN mRNA in the reticulocytes of patients (30 non-splenectomized and 30 splenectomized) and 30 normal individuals were assessed by RT-PCR. The levels of MDA and the transferrin saturations (TSs) were analysed with thiobarbituric acid-reactive substance assay and spectrometry, respectively. The level of ferritin was determined by ELISA.

RESULTS

The levels of FXN mRNA, MDA, ferritin, and TS in the patients were significantly higher than those in the controls. The levels of FXN mRNA, MDA, and ferritin in the non-splenectomized and splenectomized patients were significantly different, but the levels of TS in these two patient groups were comparable. The relative FXN expression in the patients was found to be correlated with the levels of MDA and ferritin but not correlated with TS.

CONCLUSIONS

The elevation of FXN expression in the reticulocytes of these patients seems to be linked to oxidative stress and iron status.

Dysregulation of cellular iron metabolism in Friedreich ataxia: from primary iron-sulfur cluster deficit to mitochondrial iron accumulation

Friedreich ataxia (FRDA) is the most common recessive ataxia in the Caucasian population and is characterized by a mixed spinocerebellar and sensory ataxia frequently associating cardiomyopathy. The disease results from decreased expression of the FXN gene coding for the mitochondrial protein frataxin. Early histological and biochemical study of the pathophysiology in patient's samples revealed that dysregulation of iron metabolism is a key feature of the disease, mainly characterized by mitochondrial iron accumulation and by decreased activity of iron-sulfur cluster enzymes. In the recent past years, considerable progress in understanding the function of frataxin has been provided through cellular and biochemical approaches, pointing to the primary role of frataxin in iron-sulfur cluster biogenesis. However, why and how the impact of frataxin deficiency on this essential biosynthetic pathway leads to mitochondrial iron accumulation is still poorly understood. Herein, we review data on both the primary function of frataxin and the nature of the iron metabolism dysregulation in FRDA. To date, the pathophysiological implication of the mitochondrial iron overload in FRDA remains to be clarified.

The role of the N-terminal tail for the oligomerization, folding and stability of human frataxin

The N-terminal stretch of human frataxin (hFXN) intermediate (residues 42–80) is not conserved throughout evolution and, under defined experimental conditions, behaves as a random-coil. Overexpression of hFXN56–210 in Escherichia coli yields a multimer, whereas the mature form of hFXN (hFXN81–210) is monomeric. Thus, cumulative experimental evidence points to the N-terminal moiety as an essential element for the assembly of a high molecular weight oligomer. The secondary structure propensity of peptide 56–81, the moiety putatively responsible for promoting protein–protein interactions, was also studied. Depending on the environment (TFE or SDS), this peptide adopts α-helical or β-strand structure. In this context, we explored the conformation and stability of hFXN56–210. The biophysical characterization by fluorescence, CD and SEC-FPLC shows that subunits are well folded, sharing similar stability to hFXN90–210. However, controlled proteolysis indicates that the N-terminal stretch is labile in the context of the multimer, whereas the FXN domain (residues 81–210) remains strongly resistant. In addition, guanidine hydrochloride at low concentration disrupts intermolecular interactions, shifting the ensemble toward the monomeric form. The conformational plasticity of the N-terminal tail might impart on hFXN the ability to act as a recognition signal as well as an oligomerization trigger. Understanding the fine-tuning of these activities and their resulting balance will bear direct relevance for ultimately comprehending hFXN function.

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hFXN56–210 is well-folded and shares similar stability to hFXN90–210.

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The oligomeric form of hFXN56–210 can be disassembled and reassembled in vitro.

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Proteolysis leads to the oligomer disassembly: subunits are abridged to hFXN81–210.

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Isolated peptide hFXN56–81 acquires structure in TFE and SDS solutions.

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The N-terminal tail is structurally malleable and triggers oligomerization.

The molecular basis of iron-induced oligomerization of frataxin and the role of the ferroxidation reaction in oligomerization

The role of the mitochondrial protein frataxin in iron storage and detoxification, iron delivery to iron-sulfur cluster biosynthesis, heme biosynthesis, and aconitase repair has been extensively studied during the last decade. However, still no general consensus exists on the details of the mechanism of frataxin function and oligomerization. Here, using small-angle x-ray scattering and x-ray crystallography, we describe the solution structure of the oligomers formed during the iron-dependent assembly of yeast (Yfh1) and Escherichia coli (CyaY) frataxin. At an iron-to-protein ratio of 2, the initially monomeric Yfh1 is converted to a trimeric form in solution. The trimer in turn serves as the assembly unit for higher order oligomers induced at higher iron-to-protein ratios. The x-ray crystallographic structure obtained from iron-soaked crystals demonstrates that iron binds at the trimer-trimer interaction sites, presumably contributing to oligomer stabilization. For the ferroxidation-deficient D79A/D82A variant of Yfh1, iron-dependent oligomerization may still take place, although >50% of the protein is found in the monomeric state at the highest iron-to-protein ratio used. This demonstrates that the ferroxidation reaction controls frataxin assembly and presumably the iron chaperone function of frataxin and its interactions with target proteins. For E. coli CyaY, the assembly unit of higher order oligomers is a tetramer, which could be an effect of the much shorter N-terminal region of this protein. The results show that understanding of the mechanistic features of frataxin function requires detailed knowledge of the interplay between the ferroxidation reaction, iron-induced oligomerization, and the structure of oligomers formed during assembly.

Heterotrifunctional chemical cross-linking mass spectrometry confirms physical interaction between human frataxin and ISU

The progressive neurodegenerative disease Friedreich's ataxia is caused by a decreased level of expression of frataxin, a putative iron chaperone. Frataxin is thought to transiently interact with ISU, the scaffold protein onto which iron-sulfur clusters are assembled, to deliver ferrous iron. Photoreactive heterotrifunctional chemical cross-linking confirmed the interaction between frataxin and ISU in the presence of iron and validated that transient interactions can be covalently trapped with this method. Because frataxin may participate in transient interactions with other mitochondrial proteins, this cross-linking approach may reveal new protein partners and pathways in which it interacts and help deduce direct, downstream consequences of its deficiency.

HSC20 interacts with frataxin and is involved in iron-sulfur cluster biogenesis and iron homeostasis

Friedreich's ataxia is a neurodegenerative disorder caused by mutations in the frataxin gene that produces a predominantly mitochondrial protein whose primary function appears to be mitochondrial iron-sulfur cluster (ISC) biosynthesis. Previously we demonstrated that frataxin interacts with multiple components of the mammalian ISC assembly machinery. Here we demonstrate that frataxin interacts with the mammalian mitochondrial chaperone HSC20. We show that this interaction is iron-dependent. We also show that like frataxin, HSC20 interacts with multiple proteins involved in ISC biogenesis including the ISCU/Nfs1 ISC biogenesis complex and the GRP75 ISC chaperone. Furthermore, knockdown of HSC20 caused functional defects in activity of mitochondrial ISC-containing enzymes and also defects in ISC protein expression. Alterations up or down of frataxin expression caused compensatory changes in HSC20 expression inversely, as expected of two cooperating proteins operating in the same pathway and suggesting a potential therapeutic strategy for the disease. Knockdown of HSC20 altered cytosolic and mitochondrial iron pools and increased the expression of transferrin receptor 1 and iron regulatory protein 2 consistent with decreased iron bioavailability. These results indicate that HSC20 interacts with frataxin structurally and functionally and is important for ISC biogenesis and iron homeostasis in mammals. Furthermore, they suggest that HSC20 may act late in the ISC pathway as a chaperone in ISC delivery to apoproteins and that HSC20 should be included in multi-protein complex studies of mammalian ISC biogenesis.

Oligomerization propensity and flexibility of yeast frataxin studied by X-ray crystallography and small-angle X-ray scattering

Frataxin is a mitochondrial protein with a central role in iron homeostasis. Defects in frataxin function lead to Friedreich's ataxia, a progressive neurodegenerative disease with childhood onset. The function of frataxin has been shown to be closely associated with its ability to form oligomeric species; however, the factors controlling oligomerization and the types of oligomers present in solution are a matter of debate. Using small-angle X-ray scattering, we found that Co(2+), glycerol, and a single amino acid substitution at the N-terminus, Y73A, facilitate oligomerization of yeast frataxin, resulting in a dynamic equilibrium between monomers, dimers, trimers, hexamers, and higher-order oligomers. Using X-ray crystallography, we found that Co(2+) binds inside the channel at the 3-fold axis of the trimer, which suggests that the metal has an oligomer-stabilizing role. The results reveal the types of oligomers present in solution and support our earlier suggestions that the trimer is the main building block of yeast frataxin oligomers. They also indicate that different mechanisms may control oligomer stability and oligomerization in vivo.

Mitochondrial mayhem: the mitochondrion as a modulator of iron metabolism and its role in disease

The mitochondrion plays vital roles in various aspects of cellular metabolism, ranging from energy transduction and apoptosis to the synthesis of important molecules such as heme. Mitochondria are also centrally involved in iron metabolism, as exemplified by disruptions in mitochondrial proteins that lead to perturbations in whole-cell iron processing. Recent investigations have identified a host of mitochondrial proteins (e.g., mitochondrial ferritin; mitoferrins 1 and 2; ABCBs 6, 7, and 10; and frataxin) that may play roles in the homeostasis of mitochondrial iron. These mitochondrial proteins appear to participate in one or more processes of iron storage, iron uptake, and heme and iron-sulfur cluster synthesis. In this review, we present and critically discuss the evidence suggesting that the mitochondrion may contribute to the regulation of whole-cell iron metabolism. Further, human diseases that arise from a dysregulation of these mitochondrial molecules reveal the ability of the mitochondrion to communicate with cytosolic iron metabolism to coordinate whole-cell iron processing and to fulfill the high demands of this organelle for iron. This review highlights new advances in understanding iron metabolism in terms of novel molecular players and diseases associated with its dysregulation.

Friedreich's ataxia: past, present and future

Friedreich's ataxia (FRDA) is an autosomal recessive inherited disorder characterized by progressive gait and limb ataxia, dysarthria, areflexia, loss of vibratory and position sense, and a progressive motor weakness of central origin. Additional features include hypertrophic cardiomyopathy and diabetes. Large GAA repeat expansions in the first intron of the FXN gene are the most common mutation underlying FRDA. Patients show severely reduced levels of a FXN-encoded mitochondrial protein called frataxin. Frataxin deficiency is associated with abnormalities of iron metabolism: decreased iron-sulfur cluster (ISC) biogenesis, accumulation of iron in mitochondria and depletion in the cytosol, enhanced cellular iron uptake. Some models have also shown reduced heme synthesis. Evidence for oxidative stress has been reported. Respiratory chain dysfunction aggravates oxidative stress by increasing leakage of electrons and the formation of superoxide. In vitro studies have demonstrated that Frataxin deficient cells not only generate more free radicals, but also show a reduced capacity to mobilize antioxidant defenses. The search for experimental drugs increasing the amount of frataxin is a very active and timely area of investigation. In cellular and in animal model systems, the replacement of frataxin function seems to alleviate the symptoms or even completely reverse the phenotype. Therefore, drugs increasing the amount of frataxin are attractive candidates for novel therapies. This review will discuss recent findings on FRDA pathogenesis, frataxin function, new treatments, as well as recent animal and cellular models. Controversial aspects are also discussed.

The Heart in Friedreich's Ataxia: Basic Findings and Clinical Implications

Friedreich's Ataxia is the most common inherited ataxia in man. It is a mitochondrial disease caused by severely reduced expression of the iron binding protein, frataxin. A large GAA triplet expansion in the human FRDA gene encoding this protein inhibits expression of this gene. It is inherited in an autosomal recessive pattern and typically diagnosed in childhood. The primary symptoms include severe and progressive neuropathy, and a hypertrophic cardiomyopathy that may cause death. The cardiomyopathy is difficult to treat and is frequently associated with arrhythmias, heart failure, and intolerance of cardiovascular stress, such as surgeries. Innovative approaches to therapy, such as histone deacetylase inhibitors, and enzyme replacement with cell penetrant peptide fusion proteins, hold promise for this and other similar mitochondrial disorders. This review will focus on the basic findings of this disease, and the cardiomyopathy associated with its diagnosis.

Normal and Friedreich ataxia cells express different isoforms of frataxin with complementary roles in iron-sulfur cluster assembly

Friedreich ataxia (FRDA) is an autosomal recessive degenerative disease caused by insufficient expression of frataxin (FXN), a mitochondrial iron-binding protein required for Fe-S cluster assembly. The development of treatments to increase FXN levels in FRDA requires elucidation of the steps involved in the biogenesis of functional FXN. The FXN mRNA is translated to a precursor polypeptide that is transported to the mitochondrial matrix and processed to at least two forms, FXN(42-210) and FXN(81-210). Previous reports suggested that FXN(42-210) is a transient processing intermediate, whereas FXN(81-210) represents the mature protein. However, we find that both FXN(42-210) and FXN(81-210) are present in control cell lines and tissues at steady-state, and that FXN(42-210) is consistently more depleted than FXN(81-210) in samples from FRDA patients. Moreover, FXN(42-210) and FXN(81-210) have strikingly different biochemical properties. A shorter N terminus correlates with monomeric configuration, labile iron binding, and dynamic contacts with components of the Fe-S cluster biosynthetic machinery, i.e. the sulfur donor complex NFS1·ISD11 and the scaffold ISCU. Conversely, a longer N terminus correlates with the ability to oligomerize, store iron, and form stable contacts with NFS1·ISD11 and ISCU. Monomeric FXN(81-210) donates Fe(2+) for Fe-S cluster assembly on ISCU, whereas oligomeric FXN(42-210) donates either Fe(2+) or Fe(3+). These functionally distinct FXN isoforms seem capable to ensure incremental rates of Fe-S cluster synthesis from different mitochondrial iron pools. We suggest that the levels of both isoforms are relevant to FRDA pathophysiology and that the FXN(81-210)/FXN(42-210) molar ratio should provide a useful parameter to optimize FXN augmentation and replacement therapies.

Friedreich ataxia: molecular mechanisms, redox considerations, and therapeutic opportunities

Mitochondrial dysfunction and oxidative damage are at the origin of numerous neurodegenerative diseases like Friedreich ataxia and Alzheimer and Parkinson diseases. Friedreich ataxia (FRDA) is the most common hereditary ataxia, with one individual affected in 50,000. This disease is characterized by progressive degeneration of the central and peripheral nervous systems, cardiomyopathy, and increased incidence of diabetes mellitus. FRDA is caused by a dynamic mutation, a GAA trinucleotide repeat expansion, in the first intron of the FXN gene. Fewer than 5% of the patients are heterozygous and carry point mutations in the other allele. The molecular consequences of the GAA triplet expansion is transcription silencing and reduced expression of the encoded mitochondrial protein, frataxin. The precise cellular role of frataxin is not known; however, it is clear now that several mitochondrial functions are not performed correctly in patient cells. The affected functions include respiration, iron-sulfur cluster assembly, iron homeostasis, and maintenance of the redox status. This review highlights the molecular mechanisms that underlie the disease phenotypes and the different hypothesis about the function of frataxin. In addition, we present an overview of the most recent therapeutic approaches for this severe disease that actually has no efficient treatment.

Mapping brain metals to evaluate therapies for neurodegenerative disease

The brain is rich in metals and has a high metabolic rate, making it acutely vulnerable to the toxic effects of endogenously produced free radicals. The abundant metals, iron and copper, transfer single electrons as they cycle between their reduced (Fe(2+) , Cu(1+) ) and oxidized (Fe(3+) , Cu(2+) ) states making them powerful catalysts of reactive oxygen species (ROS) production. Even redox inert zinc, if present in excess, can trigger ROS production indirectly by altering mitochondrial function. While metal chelators seem to improve the clinical outcome of several neurodegenerative diseases, their mechanisms of action remain obscure and the effects of long-term use are largely unknown. Most chelators are not specific to a single metal and could alter the distribution of multiple metals in the brain, leading to unexpected consequences over the long-term. We show here how X-ray fluorescence will be a valuable tool to examine the effect of chelators on the distribution and amount of metals in the brain.

Iron-sulfur proteins in health and disease

Iron-sulfur (Fe/S) proteins are a class of ubiquitous components that assist in vital and diverse biochemical tasks in virtually every living cell. These tasks include respiration, iron homeostasis and gene expression. The past decade has led to the discovery of novel Fe/S proteins and insights into how their Fe/S cofactors are formed and incorporated into apoproteins. This review summarizes our current knowledge of mammalian Fe/S proteins, diseases related to deficiencies in these proteins and on disorders stemming from their defective biogenesis. Understanding both the physiological functions of Fe/S proteins and how Fe/S clusters are formed will undoubtedly enhance our ability to identify and treat known disorders of Fe/S cluster biogenesis and to recognize hitherto undescribed Fe/S cluster-related diseases.

Understanding the molecular mechanisms of Friedreich's ataxia to develop therapeutic approaches

Friedreich's ataxia (FRDA) is a neurodegenerative disease caused by reduced expression of the mitochondrial protein frataxin. The physiopathological consequences of frataxin deficiency are a severe disruption of iron-sulfur cluster biosynthesis, mitochondrial iron overload coupled to cellular iron dysregulation and an increased sensitivity to oxidative stress. Frataxin is a highly conserved protein, which has been suggested to participate in a variety of different roles associated with cellular iron homeostasis. The present review discusses recent advances that have made crucial contributions in understanding the molecular mechanisms underlying FRDA and in advancements toward potential novel therapeutic approaches. Owing to space constraints, this review will focus on the most commonly accepted and solid molecular and biochemical studies concerning the function of frataxin and the physiopathology of the disease. We invite the reader to read the following reviews to have a more exhaustive overview of the field [Pandolfo, M. and Pastore, A. (2009) The pathogenesis of Friedreich ataxia and the structure and function of frataxin. J. Neurol., 256 (Suppl. 1), 9-17; Gottesfeld, J.M. (2007) Small molecules affecting transcription in Friedreich ataxia. Pharmacol. Ther., 116, 236-248; Pandolfo, M. (2008) Drug insight: antioxidant therapy in inherited ataxias. Nat. Clin. Pract. Neurol., 4, 86-96; Puccio, H. (2009) Multicellular models of Friedreich ataxia. J. Neurol., 256 (Suppl. 1), 18-24].

The N-terminus of mature human frataxin is intrinsically unfolded

Frataxin is a highly conserved nuclear-encoded mitochondrial protein whose deficiency is the primary cause of Friedreich's ataxia, an autosomal recessive neurodegenerative disease. The frataxin structure comprises a well-characterized globular domain that is present in all species and is preceded in eukaryotes by a non-conserved N-terminal tail that contains the mitochondrial import signal. Little is known about the structure and dynamic properties of the N-terminal tail. Here, we show that this region is flexible and intrinsically unfolded in human frataxin. It does not alter the iron-binding or self-aggregation properties of the globular domain. It is therefore very unlikely that this region could be important for the conserved functions of the protein.

Friedreich's Ataxia: from the (GAA)n repeat mediated silencing to new promising molecules for therapy

Friedreich's ataxia (FRDA) is a neurodegenerative disease due to a pathological expansion of a GAA triplet repeat in the first intron of the FXN gene encoding for the mitochondrial protein frataxin. The expansion is responsible for most cases of FRDA through the formation of a nonusual B-DNA structure and heterochromatin conformation that determine a direct transcriptional silencing and the subsequent reduction in frataxin expression. Among other functions, frataxin is an iron chaperone central for the assembly of iron-sulfur clusters in mitochondria; its reduction is associated with iron accumulation in mitochondria, increased cellular sensitivity to oxidative stress and cell damage. There is, nowadays, no effective therapy for FRDA and current therapeutic strategies mainly act to slow down the consequences of frataxin deficiency. Therefore, drugs that are able to increase the amount of frataxin are excellent candidates for a rational approach to FRDA therapy. Recently, several drugs have been assessed for their ability to increase the amount of cellular frataxin, including human recombinant erythropoietin, histone deacetylase inhibitors, and the PPAR-gamma agonists.

The in vivo mitochondrial two-step maturation of human frataxin

Deficiency in the nuclear-encoded mitochondrial protein frataxin causes Friedreich ataxia (FRDA), a progressive neurodegenerative disorder associating spinocerebellar ataxia and cardiomyopathy. Although the exact function of frataxin is still a matter of debate, it is widely accepted that frataxin is a mitochondrial iron chaperone involved in iron-sulfur cluster and heme biosynthesis. Frataxin is synthesized as a precursor polypeptide, directed to the mitochondrial matrix where it is proteolytically cleaved by the mitochondrial processing peptidase to the mature form via a processing intermediate. The mature form was initially reported to be encoded by amino acids 56-210 (m(56)-FXN). However, two independent reports have challenged these studies describing two different forms encoded by amino acids 78-210 (m(78)-FXN) and 81-210 (m(81)-FXN). Here, we provide evidence that mature human frataxin corresponds to m(81)-FXN, and can rescue the lethal phenotype of fibroblasts completely deleted for frataxin. Furthermore, our data demonstrate that the migration profile of frataxin depends on the experimental conditions, a behavior which most likely contributed to the confusion concerning the endogenous mature frataxin. Interestingly, we show that m(56)-FXN and m(78)-FXN can be generated when the normal maturation process of frataxin is impaired, although the physiological relevance is not clear. Furthermore, we determine that the d-FXN form, previously reported to be a degradation product, corresponds to m(78)-FXN. Finally, we demonstrate that all frataxin isoforms are generated and localized within the mitochondria. The clear identification of the N-terminus of mature FXN is an important step for designing therapeutic approaches for FRDA based on frataxin replacement.

Assembly of the iron-binding protein frataxin in Saccharomyces cerevisiae responds to dynamic changes in mitochondrial iron influx and stress level

Defects in frataxin result in Friedreich ataxia, a genetic disease characterized by early onset of neurodegeneration, cardiomyopathy, and diabetes. Frataxin is a conserved mitochondrial protein that controls iron needed for iron-sulfur cluster assembly and heme synthesis and also detoxifies excess iron. Studies in vitro have shown that either monomeric or oligomeric frataxin delivers iron to other proteins, whereas ferritin-like frataxin particles convert redox-active iron to an inert mineral. We have investigated how these different forms of frataxin are regulated in vivo. In Saccharomyces cerevisiae, only monomeric yeast frataxin (Yfh1) was detected in unstressed cells when mitochondrial iron uptake was maintained at a steady, low nanomolar level. Increments in mitochondrial iron uptake induced stepwise assembly of Yfh1 species ranging from trimer to > or = 24-mer, independent of interactions between Yfh1 and its major iron-binding partners, Isu1/Nfs1 or aconitase. The rate-limiting step in Yfh1 assembly was a structural transition that preceded conversion of monomer to trimer. This step was induced, independently or synergistically, by mitochondrial iron increments, overexpression of wild type Yfh1 monomer, mutations that stabilize Yfh1 trimer, or heat stress. Faster assembly kinetics correlated with reduced oxidative damage and higher levels of aconitase activity, respiratory capacity, and cell survival. However, deregulation of Yfh1 assembly resulted in Yfh1 aggregation, aconitase sequestration, and mitochondrial DNA depletion. The data suggest that Yfh1 assembly responds to dynamic changes in mitochondrial iron uptake or stress exposure in a highly controlled fashion and that this may enable frataxin to simultaneously promote respiratory function and stress tolerance.

Lateral-flow immunoassay for the frataxin protein in Friedreich's ataxia patients and carriers

Friedreich's Ataxia (FA) is an inherited neurodegenerative disease caused by reduction in levels of the mitochondrial protein frataxin. Currently there are no simple, reliable methods to accurately measure the concentrations of frataxin protein. We designed a lateral-flow immunoassay that quantifies frataxin protein levels in a variety of sample materials. Using recombinant frataxin we evaluated the accuracy and reproducibility of the assay. The assay measured recombinant human frataxin concentrations between 40 and 4000 pg/test or approximately 0.1-10 nM of sample. The intra and inter-assay error was <10% throughout the working range. To evaluate clinical utility of the assay we used genetically defined lymphoblastoid cells derived from FA patients, FA carriers and controls. Mean frataxin concentrations in FA patients and carriers were significantly different from controls and from one another (p=0.0001, p=0.003, p=0.005, respectively) with levels, on average, 29% (patients) and 64% (carriers) of the control group. As predicted, we observed an inverse relationship between GAA repeat number and frataxin protein concentrations within the FA patient cohort. The lateral flow immunoassay provides a simple, accurate and reproducible method to quantify frataxin protein in whole cell and tissue extracts, including primary samples obtained by non-invasive means, such as cheek swabs and whole blood. The assay is a novel tool for FA research that may facilitate improved diagnostic and prognostic evaluation of FA patients and could also be used to evaluate efficacy of therapies designed to cure FA by increasing frataxin protein levels.

Ancestral roles of eukaryotic frataxin: mitochondrial frataxin function and heterologous expression of hydrogenosomal Trichomonas homologues in trypanosomes

Frataxin is a small conserved mitochondrial protein; in humans, mutations affecting frataxin expression or function result in Friedreich's ataxia. Much of the current understanding of frataxin function comes from informative studies with yeast models, but considerable debates remain with regard to the primary functions of this ubiquitous protein. We exploit the tractable reverse genetics of Trypanosoma brucei in order to specifically consider the importance of frataxin in an early branching lineage. Using inducible RNAi, we show that frataxin is essential in T. brucei and that its loss results in reduced activity of the marker Fe-S cluster-containing enzyme aconitase in both the mitochondrion and cytosol. Activities of mitochondrial succinate dehydrogenase and fumarase also decreased, but the concentration of reactive oxygen species increased. Trypanosomes lacking frataxin also exhibited a low mitochondrial membrane potential and reduced oxygen consumption. Crucially, however, iron did not accumulate in frataxin-depleted mitochondria, and as T. brucei frataxin does not form large complexes, it suggests that it plays no role in iron storage. Interestingly, RNAi phenotypes were ameliorated by expression of frataxin homologues from hydrogenosomes of another divergent protist Trichomonas vaginalis. Collectively, the data suggest trypanosome frataxin functions primarily only in Fe-S cluster biogenesis and protection from reactive oxygen species.

Structural basis of the iron storage function of frataxin from single-particle reconstruction of the iron-loaded oligomer

The mitochondrial protein frataxin plays a central role in mitochondrial iron homeostasis, and frataxin deficiency is responsible for Friedreich ataxia, a neurodegenerative and cardiac disease that affects 1 in 40000 children. Here we present a single-particle reconstruction from cryoelectron microscopic images of iron-loaded 24-subunit oligomeric frataxin particles at 13 and 17 A resolution. Computer-aided classification of particle images showed heterogeneity in particle size, which was hypothesized to result from gradual accumulation of iron within the core structure. Thus, two reconstructions were created from two classes of particles with iron cores of different sizes. The reconstructions show the iron core of frataxin for the first time. Compared to the previous reconstruction of iron-free particles from negatively stained images, the higher resolution of the present reconstruction allowed a more reliable analysis of the overall three-dimensional structure of the 24-meric assembly. This was done after docking the X-ray structure of the frataxin trimer into the EM reconstruction. The structure revealed a close proximity of the suggested ferroxidation sites of different monomers to the site proposed to serve in iron nucleation and mineralization. The model also assigns a new role to the N-terminal helix of frataxin in controlling the channel at the 4-fold axis of the 24-subunit oligomer. The reconstructions show that, together with some common features, frataxin has several unique features which distinguish it from ferritin. These include the overall organization of the oligomers, the way they are stabilized, and the mechanisms of iron core nucleation.

HDAC inhibitors correct frataxin deficiency in a Friedreich ataxia mouse model

BACKGROUND

Friedreich ataxia, an autosomal recessive neurodegenerative and cardiac disease, is caused by abnormally low levels of frataxin, an essential mitochondrial protein. All Friedreich ataxia patients carry a GAATTC repeat expansion in the first intron of the frataxin gene, either in the homozygous state or in compound heterozygosity with other loss-of-function mutations. The GAA expansion inhibits frataxin expression through a heterochromatin-mediated repression mechanism. Histone modifications that are characteristic of silenced genes in heterochromatic regions occur at expanded alleles in cells from Friedreich ataxia patients, including increased trimethylation of histone H3 at lysine 9 and hypoacetylation of histones H3 and H4.

METHODOLOGY/PRINCIPAL FINDINGS

By chromatin immunoprecipitation, we detected the same heterochromatin marks in homozygous mice carrying a (GAA)(230) repeat in the first intron of the mouse frataxin gene (KIKI mice). These animals have decreased frataxin levels and, by microarray analysis, show significant gene expression changes in several tissues. We treated KIKI mice with a novel histone deacetylase inhibitor, compound 106, which substantially increases frataxin mRNA levels in cells from Friedreich ataxia individuals. Treatment increased histone H3 and H4 acetylation in chromatin near the GAA repeat and restored wild-type frataxin levels in the nervous system and heart, as determined by quantitative RT-PCR and semiquantitative western blot analysis. No toxicity was observed. Furthermore, most of the differentially expressed genes in KIKI mice reverted towards wild-type levels.

CONCLUSIONS/SIGNIFICANCE

Lack of acute toxicity, normalization of frataxin levels and of the transcription profile changes resulting from frataxin deficiency provide strong support to a possible efficacy of this or related compounds in reverting the pathological process in Friedreich ataxia, a so far incurable neurodegenerative disease.

Advancements in the pathophysiology of Friedreich's Ataxia and new prospects for treatments

On November 9-12, 2006, the Friedreich's Ataxia Research Alliance (FARA) and the National Institutes of Health (NIH) hosted the Third International Friedreich's Ataxia (FRDA) Scientific Conference at the NIH in Bethesda, Maryland, highlighting the exciting research leading now to a variety of clinical trials that show promise of effective treatments for this devastating disorder. Nearly 150 leading FRDA scientists from around the world discussed their new insights and findings. The presence of six pharmaceutical and biotechnology companies underscored the importance of the public-private partnership that has grown in the past years. Some of these companies are already involved in advancing promising drug compounds into clinical trials, while others are eager to help take newer discoveries through drug development and into subsequent clinical trials. National Institute of Neurological Disorders and Stroke (NINDS) Director Dr. Story Landis noted in her opening remarks for the conference that there was a "palpable sense of energy, excitement, and enthusiasm" over the scientific progress made since the FRDA gene was discovered over 10 years ago.

Effects of the deletion of the Escherichia coli frataxin homologue CyaY on the respiratory NADH:ubiquinone oxidoreductase

Background

Frataxin is discussed as involved in the biogenesis of iron-sulfur clusters. Recently it was discovered that a frataxin homologue is a structural component of the respiratory NADH:ubiquinone oxidoreductase (complex I) in Thermus thermophilus. It was not clear whether frataxin is in general a component of complex I from bacteria. The Escherichia coli homologue of frataxin is coined CyaY.

Results

We report that complex I is completely assembled to a stable and active enzyme complex equipped with all known iron-sulfur clusters in a cyaY mutant of E. coli. However, the amount of complex I is reduced by one third compared to the parental strain. Western blot analysis and live cell imaging of CyaY engineered with a GFP demonstrated that CyaY is located in the cytoplasm and not attached to the membrane as to be expected if it were a component of complex I.

Conclusion

CyaY plays a non-essential role in the assembly of complex I in E. coli. It is not a structural component but may transiently interact with the complex.

In vivo maturation of human frataxin

The defective expression of frataxin causes the hereditary neurodegenerative disorder Friedreich's ataxia (FRDA). Human frataxin is synthesized as a 210 amino acid precursor protein, which needs proteolytic processing into mitochondria to be converted into the functional mature form. In vitro processing of human frataxin was previously described to yield a 155 amino acid mature form, corresponding to residues 56-210 (frataxin(56-210)). Here, we studied the maturation of frataxin by in vivo overexpression in human cells. Our data show that the main form of mature frataxin is generated by a proteolytic cleavage between Lys80 and Ser81, yielding a 130 amino acid protein (frataxin(81-210)). This maturation product corresponds to the endogenous frataxin detected in human heart, peripheral blood lymphocytes or dermal fibroblasts. Moreover, we demonstrate that frataxin(81-210) is biologically functional, as it rescues aconitase defects in frataxin-deficient cells derived from FRDA patients. Importantly, our data indicate that frataxin(56-210) can be produced in vivo when the primary 80-81 maturation site is unavailable, suggesting the existence of proteolytic mechanisms that can actively control the size of the mature product, with possible functional implications.

The chemical form of mitochondrial iron in Friedreich’s ataxia

Friedreich's ataxia (FRDA) results from cellular damage caused by a deficiency in the mitochondrial matrix protein frataxin. To address the effect of frataxin deficiency on mitochondrial iron chemistry, the heavy mitochondrial fraction (HMF) was isolated from primary fibroblasts from FRDA affected and unaffected individuals. X-ray absorption spectroscopy was used to characterize the chemical form of iron. Near K-edge spectra were fitted with a series of model iron compounds to determine the proportion of each iron species. Most of the iron in both affected and unaffected fibroblasts was ferrihydrite. The iron K-edge from unaffected HMFs were best fitted with poorly organized ferrihydrite modeled by frataxin whereas HMFs from affected cells were best fitted with highly organized ferrihydrite modeled by ferritin. Both had several minor iron species but these did not differ consistently with disease. Since the iron K-edge spectra of ferritin and frataxin are very similar, we present additional evidence for the presence of ferritin-bound iron in HMF. The predominant ferritin subunit in HMFs from affected cells resembled mitochondrial ferritin (MtFt) in size and antigenicity. Western blotting of native gels showed that HMF from affected cells had 3-fold more holoferritin containing stainable iron. We conclude that most of the iron in fibroblast HMF from both affected and unaffected cells is ferrihydrite but only FRDA affected cells mineralize significant iron in mitochondrial ferritin.

N-terminal iron-mediated self-cleavage of human frataxin: regulation of iron binding and complex formation with target proteins

Frataxin is an iron-binding mitochondrial matrix protein that has been shown to mediate iron delivery during iron-sulfur cluster and heme biosynthesis. Mitochondrial processing peptidase (MPP) yields a form of human frataxin corresponding to residues 56-210. However, structural and functional studies have focused on a core structure that results from an ill-defined cleavage event at the N-terminus. Herein we show that the N-terminus of MPP-processed frataxin shows a unique high-affinity iron site and that this iron center appears to mediate a self-cleavage reaction. Moreover, the N-terminus appears to block previously defined iron-binding sites located on the carboxylate-rich surface defined by the helix (alpha1) and the beta-sheet (beta1), most likely through electrostatic contact with the carboxylate-rich surface on the core protein, as well as inhibiting iron-promoted binding of the iron-sulfur cluster assembly scaffold partner protein, ISU. The physiological significance of iron-mediated release of the N-terminal residues from this anionic surface is discussed.

Mitochondrial frataxin interacts with ISD11 of the NFS1/ISCU complex and multiple mitochondrial chaperones

The neurodegenerative disorder Friedreich's ataxia (FRDA) is caused by mutations in frataxin, a mitochondrial protein whose function remains controversial. Using co-immunoprecipitation and mass spectrometry we identified multiple interactors of mitochondrial frataxin in mammalian cells. One interactor was mortalin/GRP75, a homolog of the yeast ssq1 chaperone that integrates iron-sulfur clusters into imported mitochondrial proteins. Another interactor was ISD11, recently identified as a component of the eukaryotic complex Nfs1/ISCU, an essential component of iron-sulfur cluster biogenesis. Interactions between frataxin and ISD11, and frataxin and GRP75 were confirmed by co-immunoprecipitation experiments in both directions. Immunofluorescence analysis demonstrated that ISD11 co-localized with both frataxin and with mitochondria. The point mutations I154F and W155R in frataxin cause FRDA and are clustered to one surface of the protein, and these mutations decrease the interaction of frataxin with ISD11. The frataxin/ISD11 interaction was also decreased by the chelator EDTA, and was increased by supplementation with nickel but not other metal ions. Nickel supplementation rescued the defective interaction of mutant frataxin I154F and W155R with ISD11. Upon ISD11 depletion by siRNA in HEK293T cells, the amount of the Nfs1/ISCU protein complex declined, as did the activity of the iron-sulfur cluster enzyme aconitase, while the cellular iron content was increased, as seen in tissues from FRDA patients. Furthermore, ISD11 mRNA levels were decreased in FRDA patient cells. These data suggest that frataxin binds the iron-sulfur biogenesis Nfs1/ISCU complex through ISD11, that the interaction is nickel-dependent, and that multiple consequences of frataxin deficiency are duplicated by ISD11 deficiency.

Distinct iron binding property of two putative iron donors for the iron-sulfur cluster assembly: IscA and the bacterial frataxin ortholog CyaY under physiological and oxidative stress conditions

Frataxin, a small mitochondrial protein linked to the neurodegenerative disease Friedreich ataxia, has recently been proposed as an iron donor for the iron-sulfur cluster assembly. An analogous function has also been attributed to IscA, a key member of the iron-sulfur cluster assembly machinery found in bacteria, yeast, and humans. Here we have compared the iron binding property of IscA and the frataxin ortholog CyaY from Escherichia coli under physiological and oxidative stress conditions. In the presence of the thioredoxin reductase system, which emulates the intracellular redox potential, CyaY fails to bind any iron even at a 10-fold excess of iron in the incubation solution. Under the same physiologically relevant conditions, IscA efficiently recruits iron and transfers the iron for the iron-sulfur cluster assembly in a proposed scaffold IscU. In the presence of hydrogen peroxide, however, IscA completely loses its iron binding activity, whereas CyaY becomes a competent iron-binding protein and attenuates the iron-mediated production of hydroxyl free radicals. Hydrogen peroxide appears to oxidize the iron binding thiol groups in IscA, thus blocking the iron binding in the protein. Once the oxidized thiol groups in IscA are re-reduced with the thioredoxin reductase system, the iron binding activity of IscA is fully restored. On the other hand, hydrogen peroxide has no effect on the iron binding carboxyl groups in CyaY, allowing the protein to bind iron under oxidative stress conditions. The results suggest that IscA is capable of recruiting intracellular iron for the iron-sulfur cluster assembly under normal physiological conditions, whereas CyaY may serve as an iron chaperon to sequester redox active free iron and alleviate cellular oxidative damage under oxidative stress conditions.

The structures of frataxin oligomers reveal the mechanism for the delivery and detoxification of iron

Defects in the mitochondrial protein frataxin are responsible for Friedreich ataxia, a neurodegenerative and cardiac disease that affects 1:40,000 children. Here, we present the crystal structures of the iron-free and iron-loaded frataxin trimers, and a single-particle electron microscopy reconstruction of a 24 subunit oligomer. The structures reveal fundamental aspects of the frataxin mechanism. The trimer has a central channel in which one atom of iron binds. Two conformations of the channel with different metal-binding affinities suggest that a gating mechanism controls whether the bound iron is delivered to other proteins or transferred to detoxification sites. The trimer constitutes the basic structural unit of the 24 subunit oligomer. The architecture of this oligomer and several features of the trimer structure demonstrate striking similarities to the iron-storage protein ferritin. The data reveal how stepwise assembly provides frataxin with the structural flexibility to perform two functions: metal delivery and detoxification.

The structure and function of frataxin

Frataxin, a highly conserved protein found in prokaryotes and eukaryotes, is required for efficient regulation of cellular iron homeostasis. Humans with a frataxin deficiency have the cardio- and neurodegenerative disorder Friedreich's ataxia, commonly resulting from a GAA trinucleotide repeat expansion in the frataxin gene. While frataxin's specific function remains a point of controversy, the general consensus is that the protein assists in controlling cellular iron homeostasis by directly binding iron. This review focuses on the structural and biochemical aspects of iron binding by the frataxin orthologs and outlines molecular attributes that may help explain the protein's role in different cellular pathways.

A pool of extramitochondrial frataxin that promotes cell survival

Frataxin is a mitochondrial protein involved in iron metabolism. Defective expression of frataxin causes Friedreich ataxia (FA), an inherited degenerative syndrome characterized by ataxia, cardiomyopathy, and high incidence of diabetes. Here we report that frataxin-deficient cells are more prone to undergo stress-induced mitochondrial damage and apoptosis, while the overexpression of frataxin confers protection to a variety of cell types. Moreover, we reveal the existence of an extramitochondrial pool of frataxin, which can efficiently prevent mitochondrial damage and apoptosis in different cellular systems. Remarkably, extramitochondrial frataxin can fully replace mitochondrial frataxin in promoting survival of FA cells.

Iron-sulfur cluster biosynthesis: characterization of Escherichia coli CYaY as an iron donor for the assembly of [2Fe-2S] clusters in the scaffold IscU

The biogenesis of iron-sulfur [Fe-S] clusters requires the coordinated delivery of both iron and sulfide. Sulfide is provided by cysteine desulfurases that use L-cysteine as sulfur source. So far, the physiological iron donor has not been clearly identified. CyaY, the bacterial ortholog of frataxin, an iron binding protein thought to be involved in iron-sulfur cluster formation in eukaryotes, is a good candidate because it was shown to bind iron. Nevertheless, no functional in vitro studies showing an involvement of CyaY in [Fe-S] cluster biosynthesis have been reported so far. In this paper we demonstrate for the first time a specific interaction between CyaY and IscS, a cysteine desulfurase participating in iron-sulfur cluster assembly. Analysis of the iron-loaded CyaY protein demonstrated a strong binding of Fe(3+) and a weak binding of Fe(2+) by CyaY. Biochemical analysis showed that the CyaY-Fe(3+) protein corresponds to a mixture of monomer, intermediate forms (dimer-pentamers), and oligomers with the intermediate one corresponding to the only stable and soluble iron-containing form of CyaY. Using spectroscopic methods, this form was further demonstrated to be functional in vitro as an iron donor during [Fe-S] cluster assembly on the scaffold protein IscU in the presence of IscS and cysteine. All of these results point toward a link between CyaY and [Fe-S] cluster biosynthesis, and a possible mechanism for the process is discussed.

Frataxin, iron-sulfur clusters, heme, ROS, and aging

A deficiency in mitochondrial frataxin causes an increased generation of mitochondrial reactive oxygen species (ROS), which may contribute to the cell degenerative features of Friedreich's ataxia. In this work the authors demonstrate mitochondrial iron-sulfur cluster (ISC) defects and mitochondrial heme defects, and suggest how both may contribute to increased mitochondrial ROS in lymphoblasts from human patients. Mutant cells are deficient in the ISC-requiring mitochondrial enzymes aconitase and succinate dehydrogenase, but not in the non-ISC mitochondrial enzyme citrate synthase; also, the mitochondrial iron-sulfur scaffold protein IscU2 co-immunoprecipitates with frataxin in vivo. Presumably as a consequence of the iron-sulfur cluster defect, cytochrome c heme is deficient in mutants, as well as heme-dependent Complex IV. Mitochondrial superoxide is elevated in mutants, which may be a consequence of cytochrome c deficiency. Hydrogen peroxide, glutathione peroxidase activity, and oxidized glutathione (GSSG) are each elevated in mutants, consistent with activation of the glutathione peroxidase pathway. Mutant status blunted the effects of Complex III and IV inhibitors, but not a Complex I inhibitor, on superoxide production. This suggests that heme defects late in the electron transport chain of mutants are responsible for increased mutant superoxide. The impact of ISC and heme defects on ROS production with age are discussed.

Mitochondrial iron detoxification is a primary function of frataxin that limits oxidative damage and preserves cell longevity

Friedreich ataxia is a severe autosomal-recessive disease characterized by neurodegeneration, cardiomyopathy and diabetes, resulting from reduced synthesis of the mitochondrial protein frataxin. Although frataxin is ubiquitously expressed, frataxin deficiency leads to a selective loss of dorsal root ganglia neurons, cardiomyocytes and pancreatic beta cells. How frataxin normally promotes survival of these particular cells is the subject of intense debate. The predominant view is that frataxin sustains mitochondrial energy production and other cellular functions by providing iron for heme synthesis and iron-sulfur cluster (ISC) assembly and repair. We have proposed that frataxin not only promotes the biogenesis of iron-containing enzymes, but also detoxifies surplus iron thereby affording a critical anti-oxidant mechanism. These two functions have been difficult to tease apart, however, and the physiologic role of iron detoxification by frataxin has not yet been demonstrated in vivo. Here, we describe mutations that specifically impair the ferroxidation or mineralization activity of yeast frataxin, which are necessary for iron detoxification but do not affect the iron chaperone function of the protein. These mutations increase the sensitivity of yeast cells to oxidative stress, shortening chronological life span and precluding survival in the absence of the anti-oxidant enzyme superoxide dismutase. Thus, the role of frataxin is not limited to promoting ISC assembly or heme synthesis. Iron detoxification is another function of frataxin relevant to anti-oxidant defense and cell longevity that could play a critical role in the metabolically demanding environment of non-dividing neuronal, cardiac and pancreatic beta cells.

Frataxin deficiency alters heme pathway transcripts and decreases mitochondrial heme metabolites in mammalian cells

Deficiency of the frataxin mRNA alters the transcriptome, triggering neuro- and cardiodegeneration in Friedreich's ataxia. We microarrayed murine frataxin-deficient heart tissue, liver tissue and cardiocytes and observed a transcript down-regulation to up-regulation ratio of nearly 2:1 with a mitochondrial localization of transcriptional changes. Combining all mouse and human microarray data for frataxin-deficient cells and tissues, the most consistently decreased transcripts were mitochondrial coproporphyrinogen oxidase (CPOX) of the heme pathway and mature T-cell proliferation 1, a homolog of yeast COX23, which is thought to function as a mitochondrial metallochaperone. Quantitative RT-PCR studies confirmed the significant down-regulation of Isu1, CPOX and ferrochelatase at 10 weeks in mouse hearts. We observed that mutant cells were resistant to aminolevulinate-dependent toxicity, as expected if the heme pathway was inhibited. Consistent with this, we observed increased cellular protoporphyrin IX levels, reduced mitochondrial heme a and heme c levels and reduced activity of cytochrome oxidase, suggesting a defect between protoporphyrin IX and heme a. Fe-chelatase activities were similar in mutants and controls, whereas Zn-chelatase activities were slightly elevated in mutants, supporting the idea of an altered metal-specificity of ferrochelatase. These results suggest that frataxin deficiency causes defects late in the heme pathway. As ataxic symptoms occur in other diseases of heme deficiency, the heme defect we observe in frataxin-deficient cells could be primary to the pathophysiological process.

Formation of protein-coated iron minerals

The ability of iron to cycle between Fe(2+) and Fe(3+) forms has led to the evolution, in different forms, of several iron-containing protein cofactors that are essential for a wide variety of cellular processes, to the extent that virtually all cells require iron for survival and prosperity. The redox properties of iron, however, also mean that this metal is potentially highly toxic and this, coupled with the extreme insolubility of Fe(3+), presents the cell with the significant problem of how to maintain this essential metal in a safe and bioavailable form. This has been overcome through the evolution of proteins capable of reversibly storing iron in the form of a Fe(3+) mineral. For several decades the ferritins have been synonymous with the function of iron storage. Within this family are subfamilies of mammalian, plant and bacterial ferritins which are all composed of 24 subunits assembled to form an essentially spherical protein with a central cavity in which the mineral is laid down. In the past few years it has become clear that other proteins, belonging to the family of DNA-binding proteins from starved cells (the Dps family), which are oligomers of 12 subunits, and to the frataxin family, which may contain up to 48 subunits, are also able to lay down a Fe(3+) mineral core. Here we present an overview of the formation of protein-coated iron minerals, with particular emphasis on the structures of the protein coats and the mechanisms by which they promote core formation. We show on the one hand that significant mechanistic similarities exist between structurally dissimilar proteins, while on the other that relatively small structural differences between otherwise similar proteins result in quite dramatic mechanistic differences.