Iron-sulfur protein biogenesis in eukaryotes: components and mechanisms

Iron regulation by hepatocytes and free radicals

Iron is an essential metallic microelement for life. However, iron overload is toxic. The liver serves an important role as a storehouse for iron in the body. About 20–25 mg of iron is required each day for hemoglobin synthesis. To maintain iron homeostasis, transferrin and transferrin receptors are primarily involved in the uptake of iron into hepatocytes, ferritin in its storage, and ferroportin in its export. Moreover, hepcidin controls ferroportin and plays a central role in the iron metabolism. Excess “free” reactive iron produces damaging free radicals via Fenton or Harber-Weiss reactions. Produced free radicals attack cellular proteins, lipids and nucleic acid. Several detoxification system and anti-oxidant defense mechanisms exist to prevent cellular damage by free radicals. Excessive free radicals can lead to hepatocellular damage, liver fibrosis, and hepatocarcinogenesis.

Key players and their role during mitochondrial iron-sulfur cluster biosynthesis

Iron-sulfur clusters are multifaceted iron-containing cofactors coordinated and utilized by numerous proteins in nearly all biological systems. Fe-S-cluster-containing proteins help direct pathways essential for cell viability and participate in biological applications ranging from nucleotide biosynthesis and stability, protein translation, enzyme catalysis, and mitochondrial metabolism. Fe-S-containing proteins function by utilizing the unique electronic and chemical properties inherent in the Fe containing cofactor. Fe-S clusters are constructed of inorganic iron and sulfide arranged in a distinct caged structural makeup ranging from [Fe(2) -S(2) ], [Fe(3) -S(4) ], [Fe(4) -S(4) ], up to [Fe(8) -S(8) ] clusters. In eukaryotes, cluster activity is controlled in part at the assembly level and the major pathway for cluster production exists within the mitochondria. Recent insight into the pathway of mitochondrial cluster assembly has come from new in vivo and in vitro reports that provided direct insight into how all protein partners within the assembly pathway interact. However, we are only just beginning to understand the role of each protein within this complex pageant that is mitochondrial Fe-S cluster assembly. In this report we present results, using the yeast model for mitochondrial assembly, to describe the molecular details of how important proteins in the pathway coordinate for cluster assembly.

Evolution of photosynthesis

Energy conversion of sunlight by photosynthetic organisms has changed Earth and life on it. Photosynthesis arose early in Earth's history, and the earliest forms of photosynthetic life were almost certainly anoxygenic (non-oxygen evolving). The invention of oxygenic photosynthesis and the subsequent rise of atmospheric oxygen approximately 2.4 billion years ago revolutionized the energetic and enzymatic fundamentals of life. The repercussions of this revolution are manifested in novel biosynthetic pathways of photosynthetic cofactors and the modification of electron carriers, pigments, and existing and alternative modes of photosynthetic carbon fixation. The evolutionary history of photosynthetic organisms is further complicated by lateral gene transfer that involved photosynthetic components as well as by endosymbiotic events. An expanding wealth of genetic information, together with biochemical, biophysical, and physiological data, reveals a mosaic of photosynthetic features. In combination, these data provide an increasingly robust framework to formulate and evaluate hypotheses concerning the origin and evolution of photosynthesis.

Mechanistic study of mitochondria-dependent programmed cell death induced by aluminium phytotoxicity using fluorescence techniques

Recent studies have suggested that aluminium (Al) induces programmed cell death (PCD) in plants. To investigate possible mechanisms, fluorescence techniques were used to monitor the behaviour of mitochondria in vivo, as well as the activation of caspase-3-like activity during protoplast PCD induced by Al. A quick burst of mitochondrial reactive oxygen species (ROS) was detected in Al-treated protoplasts. The mitochondrial swelling and mitochondrial transmembrane potential (MTP) loss occurred prior to cell death. Pre-incubation with ascorbic acid (AsA, antioxidant molecule) retarded mitochondrial swelling and MTP loss. The real-time detection of caspase-3-like activation was achieved by measuring the degree of fluorescence resonance energy transfer (FRET). At 30 min after exposure to Al, caspase-3-like protease activation, indicated by the decrease in the FRET ratio, occurred, taking about 1 h to reach completion in single living protoplasts. The mitochondrial permeability transition pore (MPTP) inhibitor, cyclosporine (CsA) gave significant protection against MTP loss and subsequent caspase-3-like activation. Our data also showed that Al-induced mitochondrial ROS possibly originated from complex I and III damage in the respiratory chain through the interaction between Al and iron-sulphur (Fe-S) protein. Alternative oxidase (AOX), the unique respiratory terminal oxidase in plants, was demonstrated to play protective roles in Al-induced protoplast death. Our results showed that mitochondrial swelling and MTP loss, as well as the generation of mitochondrial ROS play important roles in Al-induced caspase-3-like activation and PCD, which provided new insight into the signalling cascades that modulate Al phytotoxicity mechanism.

The value of Arabidopsis research in understanding human disease states

Although Arabidopsis thaliana is traditionally viewed as the key model organism for plant biology it is becoming increasingly clear that Arabidopsis represents an invaluable tool in our efforts to understand molecular mechanisms that underpin human disease states. A comparison of the annotated Arabidopsis thaliana and human genome sequences reveals that a high percentage of genes implicated in human diseases are also present in Arabidopsis. Although Arabidopsis and humans diverged 1.6 billion years ago recent studies have demonstrated remarkable conservation of protein function and cellular processes between these seemingly distant species. In particular, cellular processes associated with neurodegenerative disorders, such as Alzheimer's and Parkinson's disease, and the neurological disorder Friedreich Ataxia have been dissected using Arabidopsis.

Candida albicans Hap43 is a repressor induced under low-iron conditions and is essential for iron-responsive transcriptional regulation and virulence

Candida albicans is an opportunistic fungal pathogen that exists as normal flora in healthy human bodies but causes life-threatening infections in immunocompromised patients. In addition to innate and adaptive immunities, hosts also resist microbial infections by developing a mechanism of "natural resistance" that maintains a low level of free iron to restrict the growth of invading pathogens. C. albicans must overcome this iron-deprived environment to cause infections. There are three types of iron-responsive transcriptional regulators in fungi; Aft1/Aft2 activators in yeast, GATA-type repressors in many fungi, and HapX/Php4 in Schizosaccharomyces pombe and Aspergillus species. In this study, we characterized the iron-responsive regulator Hap43, which is the C. albicans homolog of HapX/Php4 and is repressed by the GATA-type repressor Sfu1 under iron-sufficient conditions. We provide evidence that Hap43 is essential for the growth of C. albicans under low-iron conditions and for C. albicans virulence in a mouse model of infection. Hap43 was not required for iron acquisition under low-iron conditions. Instead, it was responsible for repression of genes that encode iron-dependent proteins involved in mitochondrial respiration and iron-sulfur cluster assembly. We also demonstrated that Hap43 executes its function by becoming a transcriptional repressor and accumulating in the nucleus in response to iron deprivation. Finally, we found a connection between Hap43 and the global corepressor Tup1 in low-iron-induced flavinogenesis. Taken together, our data suggest a complex interplay among Hap43, Sfu1, and Tup1 to coordinately regulate iron acquisition, iron utilization, and other iron-responsive metabolic activities.

Redox regulation of SCO protein function: controlling copper at a mitochondrial crossroad

Reversible changes in the redox state of cysteine residues represent an important mechanism with which to regulate protein function. In mitochondria, such redox reactions modulate the localization or activity of a group of proteins, most of which function in poorly defined pathways with essential roles in copper delivery to cytochrome c oxidase (COX) during holoenzyme biogenesis. To date, a total of 8 soluble (COX17, COX19, COX23, PET191, CMC1-4) and 3 integral membrane (COX11, SCO1, SCO2) accessory proteins with cysteine-containing domains that reside within the mitochondrial intermembrane space (IMS) have been identified in yeast, all of which have human orthologues. Compelling evidence from studies of COX17, SCO1, and SCO2 argues that regulation of the redox state of their cysteines is integral to their metallochaperone function. Redox also appears to be crucial to the regulation of a SCO-dependent, mitochondrial signaling pathway that modulates the rate of copper efflux from the cell. Here, I review our understanding of redox-dependent modulation of copper delivery to COX and IMS-localized copper-zinc superoxide dismutase (SOD1) during the maturation of each enzyme, and discuss how this in turn may serve to functionally couple mitochondrial copper handling pathways with those localized elsewhere in the cell to regulate cellular copper homeostasis.

DockAnalyse: an application for the analysis of protein-protein interactions

BACKGROUND

Is it possible to identify what the best solution of a docking program is? The usual answer to this question is the highest score solution, but interactions between proteins are dynamic processes, and many times the interaction regions are wide enough to permit protein-protein interactions with different orientations and/or interaction energies. In some cases, as in a multimeric protein complex, several interaction regions are possible among the monomers. These dynamic processes involve interactions with surface displacements between the proteins to finally achieve the functional configuration of the protein complex. Consequently, there is not a static and single solution for the interaction between proteins, but there are several important configurations that also have to be analyzed.

RESULTS

To extract those representative solutions from the docking output datafile, we have developed an unsupervised and automatic clustering application, named DockAnalyse. This application is based on the already existing DBscan clustering method, which searches for continuities among the clusters generated by the docking output data representation. The DBscan clustering method is very robust and, moreover, solves some of the inconsistency problems of the classical clustering methods like, for example, the treatment of outliers and the dependence of the previously defined number of clusters.

CONCLUSIONS

DockAnalyse makes the interpretation of the docking solutions through graphical and visual representations easier by guiding the user to find the representative solutions. We have applied our new approach to analyze several protein interactions and model the dynamic protein interaction behavior of a protein complex. DockAnalyse might also be used to describe interaction regions between proteins and, therefore, guide future flexible dockings. The application (implemented in the R package) is accessible.

Characterization of the human HSC20, an unusual DnaJ type III protein, involved in iron-sulfur cluster biogenesis

The importance of mitochondrial iron-sulfur cluster (ISC) biogenesis for human health has been well established, but the roles of some components of this critical pathway still remain uncharacterized in mammals. Among them is human heat shock cognate protein 20 (hHSC20), the putative human homolog of the specialized DnaJ type co-chaperones, which are crucial for bacterial and fungal ISC assembly. Here, we show that the human HSC20 protein can complement for its counterpart in yeast, Jac1p, and interacts with its proposed human partners, hISCU and hHSPA9. hHSC20 is expressed in various human tissues and localizes mainly to the mitochondria in HeLa cells. However, small amounts were also detected extra-mitochondrially. RNA interference-mediated depletion of hHSC20 specifically reduced the activities of both mitochondrial and cytosolic ISC-containing enzymes. The recovery of inactivated ISC enzymes was markedly delayed after an oxidative insult of hHSC20-deficient cells. Conversely, overexpression of hHSC20 substantially protected cells from oxidative insults. These results imply that hHSC20 is an integral component of the human ISC biosynthetic machinery that is particularly important in the assembly of ISCs under conditions of oxidative stress. A cysteine-rich N-terminal domain, which clearly distinguishes hHSC20 from the specialized DnaJ type III proteins of fungi and most bacteria, was found to be important for the integrity and function of the human co-chaperone.

The emerging role of iron dyshomeostasis in the mitochondrial decay of aging

Recent studies show that cellular and mitochondrial iron increases with age. Iron overload, especially in mitochondria, increases the availability of redox-active iron, which may be a causal factor in the extensive age-related biomolecular oxidative damage observed in aged organisms. Such damage is thought to play a major role in the pathogenesis of iron overload diseases and age-related pathologies. Indeed, recent findings of the beneficial effects of iron manipulation in life extension in Caenorhabditis elegans, Drosophila and transgenic mice have sparked a renewed interest in the potential role of iron in longevity. A substantial research effort now focuses on developing and testing safe pharmacologic interventions to combat iron dyshomeostasis in aging, acute injuries and in iron overload disorders.

Low aerobic mitochondrial energy metabolism in poorly- or undifferentiated neuroblastoma

BACKGROUND

Succinate dehydrogenase (SDH) has been associated with carcinogenesis in pheochromocytoma and paraganglioma. In the present study we investigated components of the oxidative phosphorylation system in human neuroblastoma tissue samples.

METHODS

Spectrophotometric measurements, immunohistochemical analysis and Western blot analysis were used to characterize the aerobic mitochondrial energy metabolism in neuroblastomas (NB).

RESULTS

Compared to mitochondrial citrate synthase, SDH activity was severely reduced in NB (n = 14) versus kidney tissue. However no pathogenic mutations could be identified in any of the four subunits of SDH. Furthermore, no genetic alterations could be identified in the two novel SDH assembly factors SDHAF1 and SDH5. Alterations in genes encoding nfs-1, frataxin and isd-11 that could lead to a diminished SDH activity have not been detected in NB.

CONCLUSION

Because downregulation of other complexes of the oxidative phosphorylation system was also observed, a more generalized reduction of mitochondrial respiration seems to be present in neuroblastoma in contrast to the single enzyme defect found in hereditary pheochromocytomas.

Adenosyl radical: reagent and catalyst in enzyme reactions

Adenosine is undoubtedly an ancient biological molecule that is a component of many enzyme cofactors: ATP, FADH, NAD(P)H, and coenzyme A, to name but a few, and, of course, of RNA. Here we present an overview of the role of adenosine in its most reactive form: as an organic radical formed either by homolytic cleavage of adenosylcobalamin (coenzyme B(12), AdoCbl) or by single-electron reduction of S-adenosylmethionine (AdoMet) complexed to an iron-sulfur cluster. Although many of the enzymes we discuss are newly discovered, adenosine's role as a radical cofactor most likely arose very early in evolution, before the advent of photosynthesis and the production of molecular oxygen, which rapidly inactivates many radical enzymes. AdoCbl-dependent enzymes appear to be confined to a rather narrow repertoire of rearrangement reactions involving 1,2-hydrogen atom migrations; nevertheless, mechanistic insights gained from studying these enzymes have proved extremely valuable in understanding how enzymes generate and control highly reactive free radical intermediates. In contrast, there has been a recent explosion in the number of radical-AdoMet enzymes discovered that catalyze a remarkably wide range of chemically challenging reactions; here there is much still to learn about their mechanisms. Although all the radical-AdoMet enzymes so far characterized come from anaerobically growing microbes and are very oxygen sensitive, there is tantalizing evidence that some of these enzymes might be active in aerobic organisms including humans.

The essentials of protein import in the degenerate mitochondrion of Entamoeba histolytica

Several essential biochemical processes are situated in mitochondria. The metabolic transformation of mitochondria in distinct lineages of eukaryotes created proteomes ranging from thousands of proteins to what appear to be a much simpler scenario. In the case of Entamoeba histolytica, tiny mitochondria known as mitosomes have undergone extreme reduction. Only recently a single complete metabolic pathway of sulfate activation has been identified in these organelles. The E. histolytica mitosomes do not produce ATP needed for the sulfate activation pathway and for three molecular chaperones, Cpn60, Cpn10 and mtHsp70. The already characterized ADP/ATP carrier would thus be essential to provide cytosolic ATP for these processes, but how the equilibrium of inorganic phosphate could be maintained was unknown. Finally, how the mitosomal proteins are translocated to the mitosomes had remained unclear. We used a hidden Markov model (HMM) based search of the E. histolytica genome sequence to discover candidate (i) mitosomal phosphate carrier complementing the activity of the ADP/ATP carrier and (ii) membrane-located components of the protein import machinery that includes the outer membrane translocation channel Tom40 and membrane assembly protein Sam50. Using in vitro and in vivo systems we show that E. histolytica contains a minimalist set up of the core import components in order to accommodate a handful of mitosomal proteins. The anaerobic and parasitic lifestyle of E. histolytica has produced one of the simplest known mitochondrial compartments of all eukaryotes. Comparisons with mitochondria of another amoeba, Dictystelium discoideum, emphasize just how dramatic the reduction of the protein import apparatus was after the loss of archetypal mitochondrial functions in the mitosomes of E. histolytica.

All eukaryotic organisms have mitochondria, organelles cordoned by a double membrane, which are descendants of an ancestral bacterial endosymbiont. Nowadays, mitochondria are fully integrated into the context of diverse cellular processes and serve in providing energy, iron-containing prosthetic groups and some of the cellular building blocks like lipids and amino acids. In multi-cellular organisms, mitochondria play an additional vital role in cell signaling pathways and programmed cell death. In some unicellular eukaryotes which inhabit oxygen poor environments, intriguing mitochondrial adaptations have taken place resulting in the creation of specialized compartments known as mitosomes and hydrogenosomes. Several important human pathogens like Entamoeba histolytica, Giardia intestinalis, Trichomonas vaginalis and microsporidia contain these organelles and in many cases the function and biogenesis of these organelles remain unknown. In this paper, we investigated the protein import pathways into the mitosomes of E. histolytica, which represent one of the simplest mitochondria-related compartment discovered yet. In accordance with the limited organellar proteome, we show that only core components of mitochondria-related protein import machines are present in E. histolytica to serve for the import of a small set of substrate proteins.

Chloroplast HCF101 is a scaffold protein for [4Fe-4S] cluster assembly

Oxygen-evolving chloroplasts possess their own iron-sulfur cluster assembly proteins including members of the SUF (sulfur mobilization) and the NFU family. Recently, the chloroplast protein HCF101 (high chlorophyll fluorescence 101) has been shown to be essential for the accumulation of the membrane complex Photosystem I and the soluble ferredoxin-thioredoxin reductases, both containing [4Fe-4S] clusters. The protein belongs to the FSC-NTPase ([4Fe-4S]-cluster-containing P-loop NTPase) superfamily, several members of which play a crucial role in Fe/S cluster biosynthesis. Although the C-terminal ISC-binding site, conserved in other members of the FSC-NTPase family, is not present in chloroplast HCF101 homologues using Mössbauer and EPR spectroscopy, we provide evidence that HCF101 binds a [4Fe-4S] cluster. 55Fe incorporation studies of mitochondrially targeted HCF101 in Saccharomyces cerevisiae confirmed the assembly of an Fe/S cluster in HCF101 in an Nfs1-dependent manner. Site-directed mutagenesis identified three HCF101-specific cysteine residues required for assembly and/or stability of the cluster. We further demonstrate that the reconstituted cluster is transiently bound and can be transferred from HCF101 to a [4Fe-4S] apoprotein. Together, our findings suggest that HCF101 may serve as a chloroplast scaffold protein that specifically assembles [4Fe-4S] clusters and transfers them to the chloroplast membrane and soluble target proteins.

Biophysical characterization of the iron in mitochondria from Atm1p-depleted Saccharomyces cerevisiae

Atm1p is an ABC transporter localized in the mitochondrial inner membrane; it functions to export an unknown species into the cytosol and is involved in cellular iron metabolism. Depletion or deletion of Atm1p causes Fe accumulation in mitochondria and a defect in cytosolic Fe/S cluster assembly but reportedly not a defect in mitochondrial Fe/S cluster assembly. In this study the nature of the accumulated Fe was examined using Mossbauer spectroscopy, EPR, electronic absorption spectroscopy, X-ray absorption spectroscopy, and electron microscopy. The Fe that accumulated in aerobically grown cells was in the form of iron(III) phosphate nanoparticles similar to that which accumulates in yeast frataxin Yfh1p-deleted or yeast ferredoxin Yah1p-depleted cells. Relative to WT mitochondria, Fe/S cluster and heme levels in Atm1p-depleted mitochondria from aerobic cells were significantly diminished. Atm1p depletion also caused a buildup of nonheme Fe(II) ions in the mitochondria and an increase in oxidative damage. Atm1p-depleted mitochondria isolated from anaerobically grown cells exhibited WT levels of Fe/S clusters and hemes, and they did not hyperaccumulate Fe. Atm1p-depleted cells lacked Leu1p activity, regardless of whether they were grown aerobically or anaerobically. These results indicate that Atm1p does not participate in mitochondrial Fe/S cluster assembly and that the species exported by Atm1p is required for cytosolic Fe/S cluster assembly. The Fe/S cluster defect and the Fe-accumulation phenotype, resulting from the depletion of Atm1p in aerobic cells (but not in anaerobic cells), may be secondary effects that are observed only when cells are exposed to oxygen during growth. Reactive oxygen species generated under these conditions might degrade iron-sulfur clusters and lower heme levels in the organelle.

A G-protein editor gates coenzyme B12 loading and is corrupted in methylmalonic aciduria

The mechanism by which docking fidelity is achieved for the multitude of cofactor-dependent enzymes is poorly understood. In this study, we demonstrate that delivery of coenzyme B(12) or 5'-deoxyadenosylcobalamin by adenosyltransferase to methylmalonyl-CoA mutase is gated by a small G protein, MeaB. While the GTP-binding energy is needed for the editing function; that is, to discriminate between active and inactive cofactor forms, the chemical energy of GTP hydrolysis is required for gating cofactor transfer. The G protein chaperone also exerts its editing function during turnover by using the binding energy of GTP to elicit release of inactive cofactor that is occasionally formed during the catalytic cycle of MCM. The physiological relevance of this mechanism is demonstrated by a patient mutation in methylmalonyl-CoA mutase that does not impair the activity of this enzyme per se but corrupts both the fidelity of the cofactor-loading process and the ejection of inactive cofactor that forms occasionally during catalysis. Consequently, cofactor in the incorrect oxidation state gains access to the mutase active site and is not released if generated during catalysis, leading, respectively, to assembly and accumulation of inactive enzyme and resulting in methylmalonic aciduria.

Dual localized AtHscB involved in iron sulfur protein biogenesis in Arabidopsis

Background

Iron-sulfur clusters are ubiquitous structures which act as prosthetic groups for numerous proteins involved in several fundamental biological processes including respiration and photosynthesis. Although simple in structure both the assembly and insertion of clusters into apoproteins requires complex biochemical pathways involving a diverse set of proteins. In yeast, the J-type chaperone Jac1 plays a key role in the biogenesis of iron sulfur clusters in mitochondria.

Methodology/Principal Findings

In this study we demonstrate that AtHscB from Arabidopsis can rescue the Jac1 yeast knockout mutant suggesting a role for AtHscB in iron sulfur protein biogenesis in plants. In contrast to mitochondrial Jac1, AtHscB localizes to both mitochondria and the cytosol. AtHscB interacts with AtIscU1, an Isu-like scaffold protein involved in iron-sulfur cluster biogenesis, and through this interaction AtIscU1 is most probably retained in the cytosol. The chaperone AtHscA can functionally complement the yeast Ssq1knockout mutant and its ATPase activity is enhanced by AtHscB and AtIscU1. Interestingly, AtHscA is also localized in both mitochondria and the cytosol. Furthermore, AtHscB is highly expressed in anthers and trichomes and an AtHscB T-DNA insertion mutant shows reduced seed set, a waxless phenotype and inappropriate trichome development as well as dramatically reduced activities of the iron-sulfur enzymes aconitase and succinate dehydrogenase.

Conclusions

Our data suggest that AtHscB together with AtHscA and AtIscU1 plays an important role in the biogenesis of iron-sulfur proteins in both mitochondria and the cytosol.

Mammalian iron transport

Iron is essential for basic cellular processes but is toxic when present in excess. Consequently, iron transport into and out of cells is tightly regulated. Most iron is delivered to cells bound to plasma transferrin via a process that involves transferrin receptor 1, divalent metal-ion transporter 1 and several other proteins. Non-transferrin-bound iron can also be taken up efficiently by cells, although the mechanism is poorly understood. Cells can divest themselves of iron via the iron export protein ferroportin in conjunction with an iron oxidase. The linking of an oxidoreductase to a membrane permease is a common theme in membrane iron transport. At the systemic level, iron transport is regulated by the liver-derived peptide hepcidin which acts on ferroportin to control iron release to the plasma.

Analysis of iron-sulfur protein maturation in eukaryotes

Iron-sulfur (Fe/S) proteins play crucial roles in living cells by participating in enzyme catalysis, electron transfer and the regulation of gene expression. The biosynthesis of the inorganic Fe/S centers and their insertion into apoproteins require complex cellular machinery located in the mitochondria (Fe/S cluster (ISC) assembly machinery systems) and cytosol (cytosolic Fe/S protein assembly (CIA) systems). Functional defects in Fe/S proteins or their biogenesis components lead to human diseases underscoring the functional importance of these inorganic cofactors for life. In this protocol, we describe currently available methods to follow the activity and de novo biogenesis of Fe/S proteins in eukaryotic cells. The assay systems are useful to follow Fe/S protein maturation in different cellular compartments, identify novel Fe/S proteins and their biogenesis factors, investigate the molecular mechanisms underlying the maturation process in vivo and analyze the effects of genetic mutations in Fe/S protein-related genes. Comprehensive analysis of one biogenesis component or target Fe/S protein takes about 10 d.

Overexpression of mitochondrial ferritin sensitizes cells to oxidative stress via an iron-mediated mechanism

Mitochondrial ferritin (MtFt) is a newly identified H-ferritin-like protein expressed only in mitochondria. Previous studies have shown that its overexpression markedly affects intracellular iron homeostasis and rescues defects caused by frataxin deficiency. To assess how MtFt exerts its function under oxidative stress conditions, MtFt overexpressing cells were treated with tert-butyl-hydroperoxide (tBHP), and the effects of MtFt expression on cell survival and iron homeostasis were examined. We found that MtFt expression was associated with decreased mitochondrial metabolic activity and reduced glutathione levels as well as a concomitant increase in reactive oxygen species levels and apoptosis. Moreover, mechanistic studies demonstrated that tBHP treatment led to a prolonged decrease in cytosolic ferritins levels in MtFt-expressing cells, while ferritin levels recovered to basal levels in control counterparts. tBHP treatment also resulted in elevated transferrin receptors, followed by more iron acquisition in MtFt expressing cells. The high molecular weight desferrioxamine, targeting to lysosomes, as well as the hydrophobic iron chelator salicylaldehyde isonicotinoyl hydrazone significantly attenuated tBHP-induced cell damage. In conclusion, the current study indicates that both the newly acquired iron from the extracellular environment and internal iron redistribution from ferritin degradation may be responsible for the increased sensitivity to oxidative stress in MtFt-expressing cells.

Overexpression of Drosophila mitoferrin in l(2)mbn cells results in dysregulation of Fer1HCH expression

Mrs3p and Mrs4p (Mrs3/4p) are yeast mitochondrial iron carrier proteins that play important roles in ISC (iron-sulphur cluster) and haem biosynthesis. At low iron conditions, mitochondrial and cytoplasmic ISC protein maturation is correlated with MRS3/4 expression. Zebrafish mitoferrin1 (mfrn1), one of two MRS3/4 orthologues, is essential for erythropoiesis, but little is known about the ubiquitously expressed paralogue mfrn2. In the present study we identified a single mitoferrin gene (dmfrn) in the genome of Drosophila melanogaster, which is probably an orthologue of mfrn2. Overexpression of dmfrn in the Drosophila l(2)mbn cell line (mbn-dmfrn) resulted in decreased binding between IRP-1A (iron regulatory protein 1A) and stem-loop RNA structures referred to as IREs (iron responsive elements). mbn-dmfrn cell lines also had increased cytoplasmic aconitase activity and slightly decreased iron content. In contrast, iron loading results in decreased IRP-1A–IRE binding, but increased cellular iron content, in experimental mbn-dmfrn and control cell lines. Iron loading also increases cytoplasmic aconitase activity in all cell lines, but with slightly higher activity observed in mbn-dmfrn cells. From this we concluded that dmfrn overexpression stimulates cytoplasmic ISC protein maturation, as has been reported for MRS3/4 overexpression. Compared with control cell lines, mbn-dmfrn cells had higher Fer1HCH (ferritin 1 heavy chain homologue) transcript and protein levels. RNA interference of the putative Drosophila orthologue of human ABCB7, a mitochondrial transporter involved in cytoplasmic ISC protein maturation, restored Fer1HCH transcript levels of iron-treated mbn-dmfrn cells to those of control cells grown in normal medium. These results suggest that dmfrn overexpression in l(2)mbn cells causes an ‘overestimation’ of the cellular iron content, and that regulation of Fer1HCH transcript abundance probably depends on cytoplasmic ISC protein maturation.

Mitochondrial proteomics in experimental autoimmune uveitis oxidative stress

PURPOSE

Photoreceptor mitochondrial oxidative stress is the initial pathologic event in experimental autoimmune uveitis. In this study, the authors determined alterations in retinal mitochondrial protein levels in response to oxidative stress during the early phase of experimental autoimmune uveitis (EAU).

METHODS

Retinal mitochondrial fractions during early EAU were prepared and subjected to two-dimensional difference in gel electrophoresis (2D-DIGE). Protein spots showing differential expression were excised and subjected to matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS) for peptide identification. Levels of these proteins were also confirmed by Western blot analysis. mRNA expression of these proteins was confirmed by real-time PCR. TUNEL staining was performed to detect apoptosis.

RESULTS

2D-DIGE analysis revealed differential expression of 13 proteins. Ten proteins were overexpressed, including manganese-SOD, alphaA crystallin, beta crystallin, and four proteins were downregulated, including adenosine triphosphate (ATP) synthase, malate dehydrogenase, and calretinin. Increased levels of alphaA crystallin, betaB2 crystallin, MnSOD, and aconitase and decreased levels of ATP synthase were confirmed by Western blot analysis. qPCR also confirmed the increased expression of alphaA crystallin, betaB2 crystallin, MnSOD, and Hsp70. Apoptosis was absent during this phase.

CONCLUSIONS

The presence of mitochondrial-specific oxidative stress-related proteins in the early EAU retina along with the downregulation of ATP synthase provides early evidence of stress-related retinal damage. The presence of high levels of alphaA and betaB2 crystallin in the mitochondria may prevent cell death during early EAU.

Iron-based redox switches in biology

By virtue of its unique electrochemical properties, iron makes an ideal redox active cofactor for many biologic processes. In addition to its important role in respiration, central metabolism, nitrogen fixation, and photosynthesis, iron also is used as a sensor of cellular redox status. Iron-based sensors incorporate Fe-S clusters, heme, and mononuclear iron sites to act as switches to control protein activity in response to changes in cellular redox balance. Here we provide an overview of iron-based redox sensor proteins, in both prokaryotes and eukaryotes, that have been characterized at the biochemical level. Although this review emphasizes redox sensors containing Fe-S clusters, proteins that use heme or novel iron sites also are discussed.

A general map of iron metabolism and tissue-specific subnetworks

Iron is required for survival of mammalian cells. Recently, understanding of iron metabolism and trafficking has increased dramatically, revealing a complex, interacting network largely unknown just a few years ago. This provides an excellent model for systems biology development and analysis. The first step in such an analysis is the construction of a structural network of iron metabolism, which we present here. This network was created using CellDesigner version 3.5.2 and includes reactions occurring in mammalian cells of numerous tissue types. The iron metabolic network contains 151 chemical species and 107 reactions and transport steps. Starting from this general model, we construct iron networks for specific tissues and cells that are fundamental to maintaining body iron homeostasis. We include subnetworks for cells of the intestine and liver, tissues important in iron uptake and storage, respectively, as well as the reticulocyte and macrophage, key cells in iron utilization and recycling. The addition of kinetic information to our structural network will permit the simulation of iron metabolism in different tissues as well as in health and disease.

Small RNA-induced differential degradation of the polycistronic mRNA iscRSUA

Most polycistronic genes are expressed in a single transcript, in which each cistron produces a fixed amount of protein. In this report, we show the first example of differential degradation of a polycistronic gene induced by a small regulatory RNA (sRNA). Our data show that the iron-responsive sRNA, RyhB, binds to the second cistron of the polycistronic mRNA, iscRSUA, which encodes the necessary machinery for biosynthesis of Fe-S clusters, and promotes the cleavage of the downstream iscSUA transcript. This cleavage gives rise to the remaining 5'-section of the transcript encoding IscR, a transcriptional regulator responsible for activation and repression of several genes depending on the cellular Fe-S level. Our data indicate that the iscR transcript is stable and that translation is active. The stability of the iscR transcript depends on a 111-nucleotide long non-translated RNA section located between iscR and iscS, which forms a strong repetitive extragenic palindromic secondary structure and may protect against ribonucleases degradation. This novel regulation shows how sRNAs and mRNA structures can work together to modulate the transcriptional response to a specific stress.

A proteomic survey of Chlamydomonas reinhardtii mitochondria sheds new light on the metabolic plasticity of the organelle and on the nature of the alpha-proteobacterial mitochondrial ancestor

Mitochondria play a key role in the life and death of eukaryotic cells, yet the full spectrum of mitochondrial functions is far from being fully understood, especially in photosynthetic organisms. To advance our understanding of mitochondrial functions in a photosynthetic cell, an extensive proteomic survey of Percoll-purified mitochondria from the metabolically versatile, hydrogen-producing green alga Chlamydomonas reinhardtii was performed. Different fractions of purified mitochondria from Chlamydomonas cells grown under aerobic conditions were analyzed by nano-liquid chromatography-electrospray ionization-mass spectrometry after protein separation on sodium dodecyl sulfate polyacrylamide gel electrophoresis or on blue-native polyacrylamide gel electrophoresis. Of the 496 nonredundant proteins identified, 149 are known or predicted to reside in other cellular compartments and were thus excluded from the molecular and evolutionary analyses of the Chlamydomonas proteome. The mitochondrial proteome of the photosynthetic alga reveals important lineage-specific differences with other mitochondrial proteomes, reflecting the high metabolic diversity of the organelle. Some mitochondrial metabolic pathways in Chlamydomonas appear to combine typical mitochondrial enzymes and bacterial-type ones, whereas others are unknown among mitochondriate eukaryotes. The comparison of the Chlamydomonas proteins to their identifiable homologs predicted from 354 sequenced genomes indicated that Arabidopsis is the most closely related nonalgal eukaryote. Furthermore, this phylogenomic analysis shows that free-living alpha-proteobacteria from the metabolically versatile orders Rhizobiales and Rhodobacterales better reflect the gene content of the ancestor of the chlorophyte mitochondria than parasitic alpha-proteobacteria with reduced and specialized genomes.

Iron-sulfur cluster biosynthesis

Iron-sulfur (Fe-S) clusters are present in more than 200 different types of enzymes or proteins and constitute one of the most ancient, ubiquitous and structurally diverse classes of biological prosthetic groups. Hence the process of Fe-S cluster biosynthesis is essential to almost all forms of life and is remarkably conserved in prokaryotic and eukaryotic organisms. Three distinct types of Fe-S cluster assembly machinery have been established in bacteria, termed the NIF, ISC and SUF systems, and, in each case, the overall mechanism involves cysteine desulfurase-mediated assembly of transient clusters on scaffold proteins and subsequent transfer of pre-formed clusters to apo proteins. A molecular level understanding of the complex processes of Fe-S cluster assembly and transfer is now beginning to emerge from the combination of in vivo and in vitro approaches. The present review highlights recent developments in understanding the mechanism of Fe-S cluster assembly and transfer involving the ubiquitous U-type scaffold proteins and the potential roles of accessory proteins such as Nfu proteins and monothiol glutaredoxins in the assembly, storage or transfer of Fe-S clusters.

Molecular basis of inherited microcytic anemia due to defects in iron acquisition or heme synthesis

Microcytic anemia is the most commonly encountered anemia in general medical practice. Nutritional iron deficiency and beta thalassemia trait are the primary causes in pediatrics, whereas bleeding disorders and anemia of chronic disease are common in adulthood. Microcytic hypochromic anemia can result from a defect in globin genes, in heme synthesis, in iron availability or in iron acquisition by the erythroid precursors. These microcytic anemia can be sideroblastic or not, a trait which reflects the implications of different gene abnormalities. Iron is a trace element that may act as a redox component and therefore is integral to vital biological processes that require the transfer of electrons as in oxygen transport, oxidative phosphorylation, DNA biosynthesis and xenobiotic metabolism. However, it can also be pro-oxidant and to avoid its toxicity, iron metabolism is strictly controlled and failure of these control systems could induce iron overload or iron deficient anemia. During the past few years, several new discoveries mostly arising from human patients or mouse models have highlighted the implication of iron metabolism components in hereditary microcytic anemia, from intestinal absorption to its final inclusion into heme. In this paper we will review the new information available on the iron acquisition pathway by developing erythrocytes and its regulation, and we will consider only inherited microcytosis due to heme synthesis or to iron metabolism defects. This information could be useful in the diagnosis and classification of these microcytic anemias.

The ABC-transporter AtmA is involved in nickel and cobalt resistance of Cupriavidus metallidurans strain CH34

Cupriavidus metallidurans CH34 genome contains an ortholog of Atm1p named AtmA (Rmet_0391, YP_582546). In Saccharomyces cerevisiae, the ABC-type transport system Atm1p is involved in export of iron-sulfur clusters from mitochondria into the cytoplasm for assembly of cytoplasmic iron-sulfur containing proteins. An atmA mutant of C. metallidurans was sensitive to nickel and cobalt but not iron cations. AtmA increased also resistance to these cations in Escherichia coli strains that carry deletions of the genes for other nickel and cobalt transport systems. In C. metallidurans, atmA expression was not significantly induced by nickel and cobalt, but repressed by zinc. AtmA was purified as a 70 kDa protein after expression in E. coli. ATPase activity of AtmA was stimulated by nickel and cobalt.

Friedreich ataxia: an update on animal models, frataxin function and therapies

Friedreich ataxia (FRDA) is an autosomal recessive progressively debilitating degenerative disease that principally affects the nervous system and the heart. Although FRDA is considered a rare disease, is the most common inherited ataxia. It is caused by loss-of-function mutations in the FXN gene, mainly an expanded GAA triplet repeat in the intron 1. The genetic defect results in the reduction of frataxin levels, a protein targeted to the mitochondria. Frataxin deficiency leads to mitochondrial dysfunction, oxidative damage and iron accumulation. Studies of the yeast and animal models of the disease have led to propose several different roles for frataxin. Animal models have also been important for dissecting the steps of pathogenesis in FRDA and they are essential for the development of effective therapies. Currently, antioxidant and iron chelation therapies are under evaluation in clinical trials. Gene reactivation, gene therapy and protein replacement strategies for FRDA are promising approaches. This review focuses on the current models developed for FRDA, the different roles proposed for frataxin and the progress of potential treatment strategies for the disease.

Iron-sulfur cluster biogenesis systems and their crosstalk

The biogenesis of iron-sulfur clusters ([Fe-S]) plays a very important role in many essential functions of life. Several [Fe-S] biogenesis systems have been discovered, such as the NIF (nitrogen fixation), SUF (mobilisation of sulfur) and ISC (iron-sulfur cluster) systems in bacteria, and the ISC-like and CIA (cytosolic iron-sulfur protein assembly) systems in yeast. Experimental evidence has revealed that SUF and ISC in bacteria communicate with each other partly through IscR to coordinate the utilisation of iron and cysteine. The ISC-like system in yeast is localised to the mitochondria, while the ISC-dependent CIA system is localised to the cytosol; this suggests a possible role for the ISC mitochondrial export machinery in mediating crosstalk between the two systems. Based on genetic analysis, the model plant Arabidopsis thaliana contains three [Fe-S] biogenesis systems similar to SUF, ISC and CIA named AtSUF, AtISC and AtCIA. Possible communication between these three systems has been proposed.

The import and function of diatom and plant frataxins in the mitochondrion of Trypanosoma brucei

Frataxin is a conserved mitochondrial protein, almost universally present in prokaryotes and eukaryotes, where it is implicated in Fe-S cluster assembly and several other processes. Here we show that frataxins from the diatom Thalassiosira pseudonana and the plant Arabidopsis thaliana are efficiently targeted and processed in the mitochondrion of the evolutionary distant excavate kinetoplastid flagellate Trypanosoma brucei. Moreover, both heterologous frataxins are able to rescue a lethal deficiency for T. brucei frataxin.

Reduction of ethanol-derived acetaldehyde induced motivational properties by L-cysteine

BACKGROUND

Experimental evidences suggest that acetaldehyde (ACD) contributes to the positive motivational properties of ethanol (EtOH) as assessed by the place conditioning paradigm; indeed, we found that by reducing ACD production and/or by using ACD-sequestrating agents, EtOH is deprived from its motivational properties. Thiol products, such as the amino acid cysteine, are known to be effective ACD-sequestering agents. Cysteine is able to covalently bind ACD thereby forming a stable, nontoxic 2-methyl-thiazolidine-4-carboxylic acid compound. Thus, we treated rats with l-cysteine before intragastric administration of EtOH or ACD.

METHODS

Male Wistar rats were pretreated intraperitoneally with saline or l-cysteine (10, 20, or 30 mg/kg), before intragastric administration of saline, EtOH (1 g/kg), or ACD (20 mg/kg). The specificity of l-cysteine effect was addressed using morphine-induced conditioned place preference (cpp) (2.5 mg/kg, i.p.).

RESULTS

l-cysteine dose-dependently prevented both EtOH and ACD-induced cpp but did not interfere with morphine-induced cpp, suggesting that l-cysteine specifically modulates the motivational properties of EtOH.

CONCLUSION

The present results further underscore the role of EtOH-derived ACD in EtOH-induced motivational properties. l-cysteine, by binding EtOH-derived ACD, would deprive it of its rewarding properties and reduce its abuse liability.

Molecular Chaperones HscA/Ssq1 and HscB/Jac1 and Their Roles in Iron-Sulfur Protein Maturation

Genetic and biochemical studies have led to the identification of several cellular pathways for the biosynthesis of iron-sulfur proteins in different organisms. The most broadly distributed and highly conserved system involves an Hsp70 chaperone and J-protein co-chaperone system that interacts with a scaffold-like protein involved in [FeS]-cluster preassembly. Specialized forms of Hsp70 and their co-chaperones have evolved in bacteria (HscA, HscB) and in certain fungi (Ssq1, Jac1), whereas most eukaryotes employ a multifunctional mitochondrial Hsp70 (mtHsp70) together with a specialized co-chaperone homologous to HscB/Jac1. HscA and Ssq1 have been shown to specifically bind to a conserved sequence present in the [FeS]-scaffold protein designated IscU in bacteria and Isu in fungi, and the crystal structure of a complex of a peptide containing the IscU recognition region bound to the HscA substrate binding domain has been determined. The interaction of IscU/Isu with HscA/Ssq1 is regulated by HscB/Jac1 which bind the scaffold protein to assist delivery to the chaperone and stabilize the chaperone-scaffold complex by enhancing chaperone ATPase activity. The crystal structure of HscB reveals that the N-terminal J-domain involved in regulation of HscA ATPase activity is similar to other J-proteins, whereas the C-terminal domain is unique and appears to mediate specific interactions with IscU. At the present time the exact function(s) of chaperone-[FeS]-scaffold interactions in iron-sulfur protein biosynthesis remain(s) to be established. In vivo and in vitro studies of yeast Ssq1 and Jac1 indicate that the chaperones are not required for [FeS]-cluster assembly on Isu. Recent in vitro studies using bacterial HscA, HscB and IscU have shown that the chaperones destabilize the IscU[FeS] complex and facilitate cluster delivery to an acceptor apo-protein consistent with a role in regulating cluster release and transfer. Additional genetic and biochemical studies are needed to extend these findings to mtHsp70 activities in higher eukaryotes.