Characterization of mitochondrial ferritin in Drosophila

Prolonged pre-incubation increases the susceptibility of Galleria mellonella larvae to bacterial and fungal infection

Galleria mellonella larvae are widely used for assessing the virulence of microbial pathogens and for measuring the in vivo activity of antimicrobial agents and produce results comparable to those that can be obtained using mammals. The aim of the work described here was to ascertain the effect of pre-incubation at 15°C for 1, 3, 6 or 10 weeks on the susceptibility of larvae to infection with Candida albicans and Staphylococcus aureus. Larvae infected with C. albicans after 1 week pre-incubation at 15°C showed 73.3 ± 3.3% survival at 24 hours post-infection while those infected after 10 weeks pre-incubation showed 30 ± 3.3% survival (P < 0.01). Larvae infected with S. aureus after 1 week pre-incubation showed 65.5 ± 3.3% survival after 24 hours while those infected after 10 weeks pre-incubation showed 13.3 ± 3.3% (P < 0.001). Analysis of the haemocyte density in larvae pre-incubated for 3-10 weeks showed a reduction in haemocytes over time but a proportionate increase in the density of granular haemocytes in the population as determined by FACS analysis. Proteomic analysis revealed decreased abundance of proteins associated with metabolic pathways (e.g. malate dehydrogenase, fructose-1,6-bisphosphatase, glyceraldehyde-3-phosphate dehydrogenase) and prophenoloxidase. G. mellonella larvae are a useful in vivo model system but the duration of the pre-incubation stage significantly affects their susceptibility to microbial pathogens possibly as a result of altered metabolism.

Transcriptome profiling of the testis reveals genes involved in spermatogenesis and marker discovery in the oriental fruit fly, Bactrocera dorsalis

The testis is a highly specialized tissue that plays a vital role in ensuring fertility by producing spermatozoa, which are transferred to the female during mating. Spermatogenesis is a complex process, resulting in the production of mature sperm, and involves significant structural and biochemical changes in the seminiferous epithelium of the adult testis. The identification of genes involved in spermatogenesis of Bactrocera dorsalis (Hendel) is critical for a better understanding of its reproductive development. In this study, we constructed a cDNA library of testes from male B. dorsalis adults at different ages, and performed de novo transcriptome sequencing to produce a comprehensive transcript data set, using Illumina sequencing technology. The analysis yielded 52 016 732 clean reads, including a total of 4.65 Gb of nucleotides. These reads were assembled into 47 677 contigs (average 443 bp) and then clustered into 30 516 unigenes (average 756 bp). Based on BLAST hits with known proteins in different databases, 20 921 unigenes were annotated with a cut-off E-value of 10(-5). The transcriptome sequences were further annotated using the Clusters of Orthologous Groups, Gene Orthology and the Kyoto Encyclopedia of Genes and Genomes databases. Functional genes involved in spermatogenesis were analysed, including cell cycle proteins, metalloproteins, actin, and ubiquitin and antihyperthermia proteins. Several testis-specific genes were also identified. The transcripts database will help us to understand the molecular mechanisms underlying spermatogenesis in B. dorsalis. Furthermore, 2913 simple sequence repeats and 151 431 single nucleotide polymorphisms were identified, which will be useful for investigating the genetic diversity of B. dorsalis in the future.

Iron metabolism in hard ticks (Acari: Ixodidae): the antidote to their toxic diet

Ticks are notorious parasitic arthropods, known for their completely host-blood-dependent lifestyle. Hard ticks (Acari: Ixodidae) feed on their hosts for several days and can ingest blood more than a hundred times their unfed weight. Their blood-feeding habit facilitates the transmission of various pathogens. It is remarkable how hard ticks cope with the toxic nature of their blood meal, which contains several molecules that can promote oxidative stress including iron. While it is required in several physiological processes, high amounts of iron can be dangerous because iron can also participate in the formation of free radicals that may cause cellular damage and death. Here we review the current knowledge on heme and inorganic iron metabolism in hard ticks and compare it with that in vertebrates and other arthropods. We briefly discuss the studies on heme transport, storage and detoxification, and the transport and storage of inorganic iron, with emphasis on the functions of tick ferritins. This review points out other aspects of tick iron metabolism that warrant further investigation, as compared to mammals and other arthropods. Further understanding of this physiological process may help in formulating new control strategies for tick infestation and the spread of tick-borne diseases.

Conserved metallomics in two insect families evolving separately for a hundred million years

Μetal cofactors are required for enzymatic catalysis and structural stability of many proteins. Physiological metal requirements underpin the evolution of cellular and systemic regulatory mechanisms for metal uptake, storage and excretion. Considering the role of metal biology in animal evolution, this paper asks whether metal content is conserved between different fruit flies. A similar metal homeostasis was previously observed in Drosophilidae flies cultivated on the same larval medium. Each species accumulated in the order of 200 µg iron and zinc and approximately ten-fold less manganese and copper per gram dry weight of the adult insect. In this paper, data on the metal content in fourteen species of Tephritidae, which are major agricultural pests worldwide, are presented. These fruit flies can be polyphagous (e.g., Ceratitis capitata) or strictly monophagous (e.g., Bactrocera oleae) or oligophagous (e.g., Anastrepha grandis) and were maintained in the laboratory on five distinct diets based on olive oil, carrot, wheat bran, zucchini and molasses, respectively. The data indicate that overall metal content and distribution between the Tephritidae and Drosophilidae species was similar. Reduced metal concentration was observed in B. oleae. Feeding the polyphagous C. capitata with the diet of B. oleae resulted in a significant quantitative reduction of all metals. Thus, dietary components affect metal content in some Tephritidae. Nevertheless, although the evidence suggests some fruit fly species evolved preferences in the use or storage of particular metals, no metal concentration varied in order of magnitude between these two families of Diptera that evolved independently for over 100 million years.

Molecular profile and functional characterization of the ferritin H subunit from rock bream (Oplegnathus fasciatus), revealing its putative role in host antioxidant and immune defense

Ferritins are iron binding proteins made out of 24 subunits, involved in iron homeostasis and metabolism in cellular environments. Here, we sought to identify and functionally characterize a one type of subunits of ferritin (ferritin H-like subunit) from rock bream (Oplegnathus fasciatus; RbFerH). The complete coding sequence of RbFerH was 531 bp in length, encoding a 177-amino acid protein with a predicted molecular mass of 20.8 kDa. The deduced protein structure possessed the domain architecture characteristic of known ferritin H subunits, including metal ligands for iron binding, a ferroxidase center, and two iron-binding region signatures. As expected, the 5' untranslated region of the RbFerH cDNA sequence contained a putative iron response element region, a characteristic regulatory element involved in its translation. The RbFerH gene comprised 5 exons and 4 introns spanning a 4195 bp region. Overexpressed recombinant RbFerH protein demonstrated prominent Fe(II) ion depriving activity, bacteriostatic properties, and protective effects against oxidative double-stranded DNA damage. Using quantitative polymerase chain reaction (qPCR), we found that RbFerH was expressed ubiquitously in the majority of physiologically important tissues in rock bream. A greater abundance of the mRNA transcripts were detected in blood and liver tissues. Upon administering different microbial pathogens and pathogen-derived mitogens, RbFerH transcription was markedly elevated in the blood of rock bream. Taken together, our findings suggest that RbFerH acts as a potent iron sequestrator in rock bream and may actively participate in antimicrobial as well as antioxidative defense.

Mice lacking mitochondrial ferritin are more sensitive to doxorubicin-mediated cardiotoxicity

Mitochondrial ferritin is a functional ferritin that localizes in the mitochondria. It is expressed in the testis, heart, brain, and cells with active respiratory activity. Its overexpression in cultured cells protected against oxidative damage and reduced cytosolic iron availability. However, no overt phenotype was described in mice with inactivation of the FtMt gene. Here, we used the doxorubicin model of cardiac injury in a novel strain of FtMt-null mice to investigate the antioxidant role of FtMt. These mice did not show any evident phenotype, but after acute treatment to doxorubicin, they showed enhanced mortality and altered heart morphology with fibril disorganization and severe mitochondrial damage. Signs of mitochondrial damage were present also in mock-treated FtMt(-/-) mice. The hearts of saline- and doxorubicin-treated FtMt(-/-) mice had higher thiobarbituric acid reactive substance levels, heme oxygenase 1 expression, and protein oxidation, but did not differ from FtMt(+/+) in the cardiac damage marker B-type natriuretic peptide (BNP), ATP levels, and apoptosis. However, the autophagy marker LC3 was activated. The results show that the absence of FtMt, which is highly expressed in the heart, increases the sensitivity of heart mitochondria to the toxicity of doxorubicin. This study represents the first in vivo evidence of the antioxidant role of FtMt.

KEY MESSAGE

Mitochondrial ferritin (FtMt) expressed in the heart has a protective antioxidant role. Acute treatment with doxorubicin caused the death of all FtMt(-/-) and only of 60 % FtMt(+/+) mice. The hearts of FtMt(-/-) mice showed fibril disorganization and mitochondrial damage. Markers of oxidative damage and autophagy were increased in FtMt(-/-) hearts. This is the first in vivo evidence of the antioxidant role of FtMt.

Characterization and recombinant protein expression of ferritin light chain homologue in the silkworm, Bombyx mori

The silkworm genome encodes three iron storage proteins or ferritins, Fer1HCH, Fer2LCH, and Fer3HCH. Probing our EST library constructed from 1-day-old silkworm eggs revealed only Fer2LCH mRNA, which encoded for a protein with a predicted putative N-glycosylation site. Developmental and tissue expression analyses during embryogenesis revealed that Fer2LCH mRNA was abundant from 6 h to 6 days after oviposition. Transcriptional expression of Fer2LCH during the postembryonic stage is also high in the larval fat body and mid-gut, and then is upregulated in all pupal tissues tested. We found that Fer2LCH mRNA contains an iron-responsive element, suggesting this ferritin subunit is subject to translational control. Although ferritin expression has been shown to increase following immune challenge in other insects, the levels of Fer2LCH mRNA were not significantly induced following viral or bacterial infection of Bombyx mori. Using a baculovirus expression system we expressed recombinant BmFer2LCH protein, which was detectable in the cytoplasmic fraction, likely in a compartment of the secretory pathway, and was shown to undergo posttranslational modifications including N-glycosylation. In particular, rBmFer2LCH carbohydrate chains were composed of mannose and GlcNAc. We suggest that Fer2LCH is important for iron homeostasis and maintaining normal organ function in silkworms.

Two kinds of ferritin protect ixodid ticks from iron overload and consequent oxidative stress

Ticks are obligate hematophagous parasites that have successfully developed counteractive means against their hosts' immune and hemostatic mechanisms, but their ability to cope with potentially toxic molecules in the blood remains unclear. Iron is important in various physiological processes but can be toxic to living cells when in excess. We previously reported that the hard tick Haemaphysalis longicornis has an intracellular (HlFER1) and a secretory (HlFER2) ferritin, and both are crucial in successful blood feeding and reproduction. Ferritin gene silencing by RNA interference caused reduced feeding capacity, low body weight and high mortality after blood meal, decreased fecundity and morphological abnormalities in the midgut cells. Similar findings were also previously reported after silencing of ferritin genes in another hard tick, Ixodes ricinus. Here we demonstrated the role of ferritin in protecting the hard ticks from oxidative stress. Evaluation of oxidative stress in Hlfer-silenced ticks was performed after blood feeding or injection of ferric ammonium citrate (FAC) through detection of the lipid peroxidation product, malondialdehyde (MDA) and protein oxidation product, protein carbonyl. FAC injection in Hlfer-silenced ticks resulted in high mortality. Higher levels of MDA and protein carbonyl were detected in Hlfer-silenced ticks compared to Luciferase-injected (control) ticks both after blood feeding and FAC injection. Ferric iron accumulation demonstrated by increased staining on native HlFER was observed from 72 h after iron injection in both the whole tick and the midgut. Furthermore, weak iron staining was observed after Hlfer knockdown. Taken together, these results show that tick ferritins are crucial antioxidant molecules that protect the hard tick from iron-mediated oxidative stress during blood feeding.

An insight into the transcriptome of the digestive tract of the bloodsucking bug, Rhodnius prolixus

The bloodsucking hemipteran Rhodnius prolixus is a vector of Chagas' disease, which affects 7-8 million people today in Latin America. In contrast to other hematophagous insects, the triatomine gut is compartmentalized into three segments that perform different functions during blood digestion. Here we report analysis of transcriptomes for each of the segments using pyrosequencing technology. Comparison of transcript frequency in digestive libraries with a whole-body library was used to evaluate expression levels. All classes of digestive enzymes were highly expressed, with a predominance of cysteine and aspartic proteinases, the latter showing a significant expansion through gene duplication. Although no protein digestion is known to occur in the anterior midgut (AM), protease transcripts were found, suggesting secretion as pro-enzymes, being possibly activated in the posterior midgut (PM). As expected, genes related to cytoskeleton, protein synthesis apparatus, protein traffic, and secretion were abundantly transcribed. Despite the absence of a chitinous peritrophic membrane in hemipterans - which have instead a lipidic perimicrovillar membrane lining over midgut epithelia - several gut-specific peritrophin transcripts were found, suggesting that these proteins perform functions other than being a structural component of the peritrophic membrane. Among immunity-related transcripts, while lysozymes and lectins were the most highly expressed, several genes belonging to the Toll pathway - found at low levels in the gut of most insects - were identified, contrasting with a low abundance of transcripts from IMD and STAT pathways. Analysis of transcripts related to lipid metabolism indicates that lipids play multiple roles, being a major energy source, a substrate for perimicrovillar membrane formation, and a source for hydrocarbons possibly to produce the wax layer of the hindgut. Transcripts related to amino acid metabolism showed an unanticipated priority for degradation of tyrosine, phenylalanine, and tryptophan. Analysis of transcripts related to signaling pathways suggested a role for MAP kinases, GTPases, and LKBP1/AMP kinases related to control of cell shape and polarity, possibly in connection with regulation of cell survival, response of pathogens and nutrients. Together, our findings present a new view of the triatomine digestive apparatus and will help us understand trypanosome interaction and allow insights into hemipteran metabolic adaptations to a blood-based diet.

Iron absorption in Drosophila melanogaster

The way in which Drosophila melanogaster acquires iron from the diet remains poorly understood despite iron absorption being of vital significance for larval growth. To describe the process of organismal iron absorption, consideration needs to be given to cellular iron import, storage, export and how intestinal epithelial cells sense and respond to iron availability. Here we review studies on the Divalent Metal Transporter-1 homolog Malvolio (iron import), the recent discovery that Multicopper Oxidase-1 has ferroxidase activity (iron export) and the role of ferritin in the process of iron acquisition (iron storage). We also describe what is known about iron regulation in insect cells. We then draw upon knowledge from mammalian iron homeostasis to identify candidate genes in flies. Questions arise from the lack of conservation in Drosophila for key mammalian players, such as ferroportin, hepcidin and all the components of the hemochromatosis-related pathway. Drosophila and other insects also lack erythropoiesis. Thus, systemic iron regulation is likely to be conveyed by different signaling pathways and tissue requirements. The significance of regulating intestinal iron uptake is inferred from reports linking Drosophila developmental, immune, heat-shock and behavioral responses to iron sequestration.

Aconitase causes iron toxicity in Drosophila pink1 mutants

The PTEN-induced kinase 1 (PINK1) is a mitochondrial kinase, and pink1 mutations cause early onset Parkinson's disease (PD) in humans. Loss of pink1 in Drosophila leads to defects in mitochondrial function, and genetic data suggest that another PD-related gene product, Parkin, acts with pink1 to regulate the clearance of dysfunctional mitochondria (mitophagy). Consequently, pink1 mutants show an accumulation of morphologically abnormal mitochondria, but it is unclear if other factors are involved in pink1 function in vivo and contribute to the mitochondrial morphological defects seen in specific cell types in pink1 mutants. To explore the molecular mechanisms of pink1 function, we performed a genetic modifier screen in Drosophila and identified aconitase (acon) as a dominant suppressor of pink1. Acon localizes to mitochondria and harbors a labile iron-sulfur [4Fe-4S] cluster that can scavenge superoxide to release hydrogen peroxide and iron that combine to produce hydroxyl radicals. Using Acon enzymatic mutants, and expression of mitoferritin that scavenges free iron, we show that [4Fe-4S] cluster inactivation, as a result of increased superoxide in pink1 mutants, results in oxidative stress and mitochondrial swelling. We show that [4Fe-4S] inactivation acts downstream of pink1 in a pathway that affects mitochondrial morphology, but acts independently of parkin. Thus our data indicate that superoxide-dependent [4Fe-4S] inactivation defines a potential pathogenic cascade that acts independent of mitophagy and links iron toxicity to mitochondrial failure in a PD–relevant model.

In this work we provide mechanistic insight linking together two of the earliest observations in Parkinson's disease: the excessive build-up of iron in diseased substantia nigra neurons and mitochondrial dysfunction particularly increased reactive oxygen species production at the level of Complex I. We identify aconitase mutants as strong genetic suppressors of Parkinson-related pink1 mutant phenotypes, both at the organismal and at the cellular/mitochondrial level. We show that the mitochondrial dysfunction in pink1 mutants that includes Complex I dysfunction results in superoxide-dependent inactivation of the Aconitase iron-sulfur cluster, leading to the release of iron and peroxide that combine to produce hydroxyl radicals and mitochondrial failure. Consequently, scavenging free iron using expression of mitoferritin or decreasing the levels of aconitase both rescue pink1 mutants; while increased wild-type Aconitase, but not a mutant that does not harbor an iron-sulfur cluster, results in severe mitochondrial defects. Given that reduced electron transport chain activity, increased oxidative stress, and natural iron build-up in the substantia nigra are common factors in sporadic and familial forms of Parkinson's disease, we believe that oxidative inactivation of Aconitase may represent an important pathogenic cascade underlying neuronal dysfunction in Parkinson's disease.

Genes for iron metabolism influence circadian rhythms in Drosophila melanogaster

Haem has been previously implicated in the function of the circadian clock, but whether iron homeostasis is integrated with circadian rhythms is unknown. Here we describe an RNA interference (RNAi) screen using clock neurons of Drosophila melanogaster. RNAi is targeted to iron metabolism genes, including those involved in haem biosynthesis and degradation. The results indicate that Ferritin 2 Light Chain Homologue (Fer2LCH) is required for the circadian activity of flies kept in constant darkness. Oscillations of the core components in the molecular clock, PER and TIM, were also disrupted following Fer2LCH silencing. Other genes with a putative function in circadian biology include Transferrin-3, CG1358 (which has homology to the FLVCR haem export protein) and five genes implicated in iron-sulfur cluster biosynthesis: the Drosophila homologues of IscS (CG12264), IscU (CG9836), IscA1 (CG8198), Iba57 (CG8043) and Nubp2 (CG4858). Therefore, Drosophila genes involved in iron metabolism are required for a functional biological clock.

Multicopper oxidase-1 is a ferroxidase essential for iron homeostasis in Drosophila melanogaster

Multicopper ferroxidases catalyze the oxidation of ferrous iron to ferric iron. In yeast and algae, they participate in cellular uptake of iron; in mammals, they facilitate cellular efflux. The mechanisms of iron metabolism in insects are still poorly understood, and insect multicopper ferroxidases have not been identified. In this paper, we present evidence that Drosophila melanogaster multicopper oxidase-1 (MCO1) is a functional ferroxidase. We identified candidate iron-binding residues in the MCO1 sequence and found that purified recombinant MCO1 oxidizes ferrous iron. An association between MCO1 function and iron homeostasis was confirmed by two observations: RNAi-mediated knockdown of MCO1 resulted in decreased iron accumulation in midguts and whole insects, and weak knockdown increased the longevity of flies fed a toxic concentration of iron. Strong knockdown of MCO1 resulted in pupal lethality, indicating that MCO1 is an essential gene. Immunohistochemistry experiments demonstrated that MCO1 is located on the basal surfaces of the digestive system and Malpighian tubules. We propose that MCO1 oxidizes ferrous iron in the hemolymph and that the resulting ferric iron is bound by transferrin or melanotransferrin, leading to iron storage, iron withholding from pathogens, regulation of oxidative stress, and/or epithelial maturation. These proposed functions are distinct from those of other known ferroxidases. Given that MCO1 orthologues are present in all insect genomes analyzed to date, this discovery is an important step toward understanding iron metabolism in insects.

Three ferritin subunits involved in immune defense from bay scallop Argopecten irradians

Ferritin is a ubiquitous protein that plays an important role in iron storage and iron-withholding strategy of innate immunity. In this study, three genes encoding different ferritin subunits were cloned from bay scallop Argopecten irradians (AiFer1, AiFer2 and AiFer3) by rapid amplification of cDNA ends (RACE) approaches based on the known ESTs. The open reading frames of the three ferritins are of 516 bp, 522 bp and 519 bp, encoding 171,173 and 172 amino acids, respectively. All the AiFers contain a putative Iron Regulatory Element (IRE) in their 5'-untranslated regions. The deduced amino acid sequences of AiFers possess both the ferroxidase center of mammalian H ferritin and the iron nucleation site of mammalian L ferritin. Gene structure study revealed two distinct structured genes encoding a ferritin subunit (AiFer3). Quantitative real-time PCR analysis indicated the significant up-regulation of AiFers in hemocytes after challenged with Listonella anguillarum, though the magnitudes of AiFer1 and AiFer2 were much higher than that of AiFer3. Taken together, these results suggest that AiFers are likely to play roles in both iron storage and innate immune defense against microbial infections.

Multifunctional ferritin cage nanostructures for fluorescence and MR imaging of tumor cells

Bionanoparticles and nanostructures have attracted increasing interest as versatile and promising tools in many applications including biosensing and bioimaging. In this study, to image and detect tumor cells, ferritin cage-based multifunctional hybrid nanostructures were constructed that: (i) displayed both the green fluorescent protein and an Arg-Gly-Asp peptide on the exterior surface of the ferritin cages; and (ii) incorporated ferrimagnetic iron oxide nanoparticles into the ferritin interior cavity. The overall architecture of ferritin cages did not change after being integrated with fusion proteins and ferrimagnetic iron oxide nanoparticles. These multifunctional nanostructures were successfully used as a fluorescent imaging probe and an MRI contrast agent for specifically probing and imaging α(v)β(3) integrin upregulated tumor cells. The work provides a promising strategy for tumor cell detection by simultaneous fluorescence and MR imaging.

Translocation of heme oxygenase-1 to mitochondria is a novel cytoprotective mechanism against non-steroidal anti-inflammatory drug-induced mitochondrial oxidative stress, apoptosis, and gastric mucosal injury

The mechanism of action of heme oxygenase-1 (HO-1) in mitochondrial oxidative stress (MOS)-mediated apoptotic tissue injury was investigated. MOS-mediated gastric mucosal apoptosis and injury were introduced in rat by indomethacin, a non-steroidal anti-inflammatory drug. Here, we report that HO-1 was not only induced but also translocated to mitochondria during gastric mucosal injury to favor repair mechanisms. Furthermore, mitochondrial translocation of HO-1 resulted in the prevention of MOS and mitochondrial pathology as evident from the restoration of the complex I-driven mitochondrial respiratory control ratio and transmembrane potential. Mitochondrial translocation of HO-1 also resulted in time-dependent inhibition of apoptosis. We searched for the plausible mechanisms responsible for HO-1 induction and mitochondrial localization. Free heme, the substrate for HO-1, was increased inside mitochondria during gastric injury, and mitochondrial entry of HO-1 decreased intramitochondrial free heme content, suggesting that a purpose of mitochondrial translocation of HO-1 is to detoxify accumulated heme. Heme may activate nuclear translocation of NF-E2-related factor 2 to induce HO-1 through reactive oxygen species generation. Electrophoretic mobility shift assay and chromatin immunoprecipitation studies indicated nuclear translocation of NF-E2-related factor 2 and its binding to HO-1 promoter to induce HO-1 expression during gastric injury. Inhibition of HO-1 by zinc protoporphyrin aggravated the mucosal injury and delayed healing. Zinc protoporphyrin further reduced the respiratory control ratio and transmembrane potential and enhanced MOS and apoptosis. In contrast, induction of HO-1 by cobalt protoporphyrin reduced MOS, corrected mitochondrial dysfunctions, and prevented apoptosis and gastric injury. Thus, induction and mitochondrial localization of HO-1 are a novel cytoprotective mechanism against MOS-mediated apoptotic tissue injury.

Ferritin overexpression in Drosophila glia leads to iron deposition in the optic lobes and late-onset behavioral defects

Cellular and organismal iron storage depends on the function of the ferritin protein complex in insects and mammals alike. In the central nervous system of insects, the distribution and relevance of ferritin remain unclear, though ferritin has been implicated in Drosophila models of Alzheimers' and Parkinsons' disease and in Aluminum-induced neurodegeneration. Here we show that transgene-derived expression of ferritin subunits in glial cells of Drosophila melanogaster causes a late-onset behavioral decline, characterized by loss of circadian rhythms in constant darkness and impairment of elicited locomotor responses. Anatomical analysis of the affected brains revealed crystalline inclusions of iron-loaded ferritin in a subpopulation of glial cells but not significant neurodegeneration. Although transgene-induced glial ferritin expression was well tolerated throughout development and in young flies, it turned disadvantageous at older age. The flies we characterize in this report contribute to the study of ferritin in the Drosophila brain and can be used to assess the contribution of glial iron metabolism in neurodegenerative models of disease.

In vivo analysis of proteomes and interactomes using Parallel Affinity Capture (iPAC) coupled to mass spectrometry

Affinity purification coupled to mass spectrometry provides a reliable method for identifying proteins and their binding partners. In this study we have used Drosophila melanogaster proteins triple tagged with Flag, Strep II, and Yellow fluorescent protein in vivo within affinity pull-down experiments and isolated these proteins in their native complexes from embryos. We describe a pipeline for determining interactomes by Parallel Affinity Capture (iPAC) and show its use by identifying partners of several protein baits with a range of sizes and subcellular locations. This purification protocol employs the different tags in parallel and involves detailed comparison of resulting mass spectrometry data sets, ensuring the interaction lists achieved are of high confidence. We show that this approach identifies known interactors of bait proteins as well as novel interaction partners by comparing data achieved with published interaction data sets. The high confidence in vivo protein data sets presented here add new data to the currently incomplete D. melanogaster interactome. Additionally we report contaminant proteins that are persistent with affinity purifications irrespective of the tagged bait.

Iron depletion in the intestines of Malvolio mutant flies does not occur in the absence of a multicopper oxidase

Malvolio (Mvl) encodes the sole Drosophila melanogaster homologue of divalent metal transporter-1 (DMT1). The Drosophila transporter has been implicated in iron, manganese and copper cellular import. Indeed, the extent of metal specificity for this family of transporters is still under investigation in many eukaryotic species. Here, we revisit metal accumulation in Mvl mutants raised under normal and metal-supplemented diets. We found iron deficiency in Mvl mutant flies, whereas whole body copper and manganese concentrations remained unaltered. Iron supplementation restored total body iron concentrations in Mvl mutants, but without replenishing iron stores in the middle midgut, suggesting a role for Mvl in systemic iron trafficking, in addition to a role in intestinal iron absorption. Interestingly, dietary copper sulphate supplementation further exacerbated the iron deficiency. We investigated whether dietary copper affected iron storage through the function of an insect multicopper oxidase (MCO), because the mammalian MCO ceruloplasmin is known to regulate iron storage in the liver. We identified a Drosophila MCO mutant that suppressed aspects of the Mvl mutant phenotype and most notably Mvl, MCO3 double mutants showed normal intestinal iron storage. Therefore, MCO3 may encode an insect ferroxidase. Intriguingly, MCO3 mutants had a mild accumulation of copper, which was suppressed in Mvl mutants, revealing a reciprocal genetic interaction between the two genes.

Cytosolic and mitochondrial ferritins in the regulation of cellular iron homeostasis and oxidative damage

BACKGROUND

Ferritin structure is designed to maintain large amounts of iron in a compact and bioavailable form in solution. All ferritins induce fast Fe(II) oxidation in a reaction catalyzed by a ferroxidase center that consumes Fe(II) and peroxides, the reagents that produce toxic free radicals in the Fenton reaction, and thus have anti-oxidant effects. Cytosolic ferritins are composed of the H- and L-chains, whose expression are regulated by iron at a post-transcriptional level and by oxidative stress at a transcriptional level. The regulation of mitochondrial ferritin expression is presently unclear.

SCOPE OF REVIEW

The scope of the review is to update recent progress regarding the role of ferritins in the regulation of cellular iron and in the response to oxidative stress with particular attention paid to the new roles described for cytosolic ferritins, to genetic disorders caused by mutations of the ferritin L-chain, and new findings on mitochondrial ferritin.

MAJOR CONCLUSIONS

The new data on the adult conditional knockout (KO) mice for the H-chain and on the hereditary ferritinopathies with mutations that reduce ferritin functionality strongly indicate that the major role of ferritins is to protect from the oxidative damage caused by iron deregulation. In addition, the study of mitochondrial ferritin, which is not iron-regulated, indicates that it participates in the protection against oxidative damage, particularly in cells with high oxidative activity.

GENERAL SIGNIFICANCE

Ferritins have a central role in the protection against oxidative damage, but they are also involved in non-iron-dependent processes.

Insect ferritins: Typical or atypical?

Insects transmit millions of cases of disease each year, and cost millions of dollars in agricultural losses. The control of insect-borne diseases is vital for numerous developing countries, and the management of agricultural insect pests is a very serious business for developed countries. Control methods should target insect-specific traits in order to avoid non-target effects, especially in mammals. Since insect cells have had a billion years of evolutionary divergence from those of vertebrates, they differ in many ways that might be promising for the insect control field-especially, in iron metabolism because current studies have indicated that significant differences exist between insect and mammalian systems. Insect iron metabolism differs from that of vertebrates in the following respects. Insect ferritins have a heavier mass than mammalian ferritins. Unlike their mammalian counterparts, the insect ferritin subunits are often glycosylated and are synthesized with a signal peptide. The crystal structure of insect ferritin also shows a tetrahedral symmetry consisting of 12 heavy chain and 12 light chain subunits in contrast to that of mammalian ferritin that exhibits an octahedral symmetry made of 24 heavy chain and 24 light chain subunits. Insect ferritins associate primarily with the vacuolar system and serve as iron transporters-quite the opposite of the mammalian ferritins, which are mainly cytoplasmic and serve as iron storage proteins. This review will discuss these differences.

Drosophila mitoferrin is essential for male fertility: evidence for a role of mitochondrial iron metabolism during spermatogenesis

Background

Mammals and Drosophila melanogaster share some striking similarities in spermatogenesis. Mitochondria in spermatids undergo dramatic morphological changes and syncytial spermatids are stripped from their cytoplasm and then individually wrapped by single membranes in an individualization process. In mammalian and fruit fly testis, components of the mitochondrial iron metabolism are expressed, but so far their function during spermatogenesis is unknown. Here we investigate the role of Drosophila mitoferrin (dmfrn), which is a mitochondrial carrier protein with an established role in the mitochondrial iron metabolism, during spermatogenesis.

Results

We found that P-element insertions into the 5'-untranslated region of the dmfrn gene cause recessive male sterility, which was rescued by a fluorescently tagged transgenic dmfrn genomic construct (dmfrn^venus). Testes of mutant homozygous dmfrn^SH115 flies were either small with unorganized content or contained some partially elongated spermatids, or testes were of normal size but lacked mature sperm. Testis squashes indicated that spermatid elongation was defective and electron micrographs showed mitochondrial defects in elongated spermatids and indicated failed individualization. Using a LacZ reporter and the dmfrn^venus transgene, we found that dmfrn expression in testes was highest in spermatids, coinciding with the stages that showed defects in the mutants. Dmfrn-venus protein accumulated in mitochondrial derivatives of spermatids, where it remained until most of it was stripped off during individualization and disposed of in waste bags. Male sterility in flies with the hypomorph alleles dmfrn^BG00456 and dmfrn^EY01302 over the deletion Df(3R)ED6277 was increased by dietary iron chelation and suppressed by iron supplementation of the food, while male sterility of dmfrn^SH115/Df(3R)ED6277 flies was not affected by food iron levels.

Conclusions

In this work, we show that mutations in the Drosophila mitoferrin gene result in male sterility caused by developmental defects. From the sensitivity of the hypomorph mutants to low food iron levels we conclude that mitochondrial iron is essential for spermatogenesis. This is the first time that a link between the mitochondrial iron metabolism and spermatogenesis has been shown. Furthermore, due to the similar expression patterns of some mitochondrial iron metabolism genes in Drosophila and mammals, it is likely that our results are applicable for mammals as well.

Iron testes: sperm mitochondria as a context for dissecting iron metabolism

A recent paper in BMC Developmental Biology reports that a mitochondrial iron importer is required for Drosophila male fertility and normal mitochondrial shaping in spermatids. This suggests that mitochondrial morphogenesis during insect spermatogenesis may be a useful new context in which to study iron metabolism.

See research article http://www.biomedcentral.com/1471-213X/10/68

Zinc accumulation in heterozygous mutants of fumble, the pantothenate kinase homologue of Drosophila

Coenzyme A (CoA) functions in the intracellular trafficking of acetyl groups. In humans, mutations in the pantothenate kinase-2 gene, which encodes a key enzyme in CoA biosynthesis, are associated with neurodegeneration and premature death. Diagnosis is based on iron accumulation in the globus pallidus observed by magnetic resonance imaging. We investigated the elemental composition of the fumble mutant, a model of the human disease. Surprisingly, flies carrying a fumble loss-of-function allele had a three-fold increase in total zinc levels per dry weight when compared to control strains, but no change in total iron, copper or manganese levels. Accordingly, zinc supplementation had an adverse impact on the development of fumble mutant larvae, but zinc chelation failed to protect.

Wolbachia interferes with ferritin expression and iron metabolism in insects

Wolbachia is an intracellular bacterium generally described as being a facultative reproductive parasite. However, Wolbachia is necessary for oogenesis completion in the wasp Asobara tabida. This dependence has evolved recently as a result of interference with apoptosis during oogenesis. Through comparative transcriptomics between symbiotic and aposymbiotic individuals, we observed a differential expression of ferritin, which forms a complex involved in iron storage. Iron is an essential element that is in limited supply in the cell. However, it is also a highly toxic precursor of Reactive Oxygen Species (ROS). Ferritin has also been shown to play a key role in host-pathogen interactions. Measuring ferritin by quantitative RT-PCR, we confirmed that ferritin was upregulated in aposymbiotic compared to symbiotic individuals. Manipulating the iron content in the diet, we showed that iron overload markedly affected wasp development and induced apoptotic processes during oogenesis in A. tabida, suggesting that the regulation of iron homeostasis may also be related to the obligate dependence of the wasp. Finally, we demonstrated that iron metabolism is influenced by the presence of Wolbachia not only in the obligate mutualism with A. tabida, but also in facultative parasitism involving Drosophila simulans and in Aedes aegypti cells. In these latter cases, the expression of Wolbachia bacterioferritin was also increased in the presence of iron, showing that Wolbachia responds to the concentration of iron. Our results indicate that Wolbachia may generally interfere with iron metabolism. The high affinity of Wolbachia for iron might be due to physiological requirement of the bacterium, but it could also be what allows the symbiont to persist in the organism by reducing the labile iron concentration, thus protecting the cell from oxidative stress and apoptosis. These findings also reinforce the idea that pathogenic, parasitic and mutualistic intracellular bacteria all use the same molecular mechanisms to survive and replicate within host cells. By impacting the general physiology of the host, the presence of a symbiont may select for host compensatory mechanisms, which extends the possible consequences of persistent endosymbiont on the evolution of their hosts.

A disruption in iron-sulfur center biogenesis via inhibition of mitochondrial dithiol glutaredoxin 2 may contribute to mitochondrial and cellular iron dysregulation in mammalian glutathione-depleted dopaminergic cells: implications for Parkinson's disease

Parkinson's disease (PD) is characterized by early glutathione depletion in the substantia nigra (SN). Among its various functions in the cell, glutathione acts as a substrate for the mitochondrial enzyme glutaredoxin 2 (Grx2). Grx2 is involved in glutathionylation of protein cysteine sulfhydryl residues in the mitochondria. Although monothiol glutathione-dependent oxidoreductases (Grxs) have previously been demonstrated to be involved in iron-sulfur (Fe-S) center biogenesis, including that in yeast, here we report data suggesting the involvement of mitochondrial Grx2, a dithiol Grx, in iron-sulfur biogenesis in a mammalian dopaminergic cell line. Given that mitochondrial dysfunction and increased cellular iron levels are two important hallmarks of PD, this suggests a novel potential mechanism by which glutathione depletion may affect these processes in dopaminergic neurons. We report that depletion of glutathione as substrate results in a dose-dependent Grx2 inhibition and decreased iron incorporation into a mitochondrial complex I (CI) and aconitase (m-aconitase). Mitochondrial Grx2 inhibition through siRNA results in a corresponding decrease in CI and m-aconitase activities. It also results in significant increases in iron-regulatory protein (IRP) binding, likely as a consequence of conversion of Fe-S-containing cellular aconitase to its non-Fe-S-containing IRP1 form. This is accompanied by increased transferrin receptor, decreased ferritin, and subsequent increases in mitochondrial iron levels. This suggests that glutathione depletion may affect important pathologic cellular events associated with PD through its effects on Grx2 activity and mitochondrial Fe-S biogenesis.

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.

Ferritins: a family of molecules for iron storage, antioxidation and more

Ferritins are characterized by highly conserved three-dimensional structures similar to spherical shells, designed to accommodate large amounts of iron in a safe, soluble and bioavailable form. They can have different architectures with 12 or 24 equivalent or non-equivalent subunits, all surrounding a large cavity. All ferritins readily interact with Fe(II) to induce its oxidation and deposition in the cavity in a mineral form, in a reaction that is catalyzed by a ferroxidase center. This is an anti-oxidant activity that consumes Fe(II) and peroxides, the reagents that produce toxic free radicals in the Fenton reaction. The mechanism of ferritin iron incorporation has been characterized in detail, while that of iron release and recycling has been less thoroughly studied. Generally ferritin expression is regulated by iron and by oxidative damage, and in vertebrates it has a central role in the control of cellular iron homeostasis. Ferritin is mostly cytosolic but is found also in mammalian mitochondria and nuclei, in plant plastids and is secreted in insects. In vertebrates the cytosolic ferritins are composed of H and L subunit types and their assembly in a tissues specific ratio that permits flexibility to adapt to cell needs. The H-ferritin can translocate to the nuclei in some cell types to protect DNA from iron toxicity, or can be actively secreted, accomplishing various functions. The mitochondrial ferritin is found in mammals, it has a restricted tissue distribution and it seems to protect the mitochondria from iron toxicity and oxidative damage. The various functions attributed to the cytosolic, nuclear, secretory and mitochondrial ferritins are discussed.

Homing in on iron homeostasis in plants

Iron is essential for plants but is not readily accessible and is also potentially toxic. As plants are a major dietary source of iron worldwide, understanding plant iron homeostasis is pivotal for improving not only crop yields but also human nutrition. Although iron acquisition from the environment is well characterized, the transporters and reductases involved in plant organellar iron transport and some of the transcription factors that regulate iron uptake have only recently been discovered. Here, we discuss newly characterized molecular players, focusing on Arabidopsis. Localization of iron to the right compartment and accessibility of iron stores are proving crucial for maintaining proper iron homeostasis and will need to be considered in biofortification efforts currently underway.

Evidence for metabolic provisioning by a common invertebrate endosymbiont, Wolbachia pipientis, during periods of nutritional stress

Wolbachia are ubiquitous inherited endosymbionts of invertebrates that invade host populations by modifying host reproductive systems. However, some strains lack the ability to impose reproductive modification and yet are still capable of successfully invading host populations. To explain this paradox, theory predicts that such strains should provide a fitness benefit, but to date none has been detected. Recently completed genome sequences of different Wolbachia strains show that these bacteria may have the genetic machinery to influence iron utilization of hosts. Here we show that Wolbachia infection can confer a positive fecundity benefit for Drosophila melanogaster reared on iron-restricted or -overloaded diets. Furthermore, iron levels measured from field-collected flies indicated that nutritional conditions in the field were overall comparable to those of flies reared in the laboratory on restricted diets. These data suggest that Wolbachia may play a previously unrecognized role as nutritional mutualists in insects.

Wolbachia are bacteria that infect millions of insect species worldwide. Wolbachia aren't infectious, but are maternally inherited symbionts passed from mother to offspring. To infect a host population, Wolbachia behave as reproductive parasites and alter the host reproductive system in a manner that increases infected female reproductive success. Some strains of Wolbachia, however, cannot manipulate their host's reproductive systems—yet they can successfully infect insect populations. How is this possible? Here we show that a Wolbachia strain that naturally infects Drosophila melanogaster, and induces very low levels of reproductive parasitism, can also act as a nutritional mutualist. When D. melanogaster flies were reared on normal diets, we observed no cost or benefit associated with the Wolbachia infection. But, if we reared flies on diets containing either very low or high amounts of iron, Wolbachia-infected flies produced more eggs than uninfected flies. As wild-caught flies contain low amounts of iron, our results suggest that flies in the wild should benefit from their Wolbachia symbiont.

Iron loaded ferritin secretion and inhibition by CI-976 in Aedes aegypti larval cells

Ferritin is a multimer of 24 subunits of heavy and light chains. In mammals, iron taken into cells is stored in ferritin or incorporated into iron-containing proteins. Very little ferritin is found circulating in mammalian serum; most is retained in the cytoplasm. Female mosquitoes, such as Aedes aegypti (yellow fever mosquito, Diptera), require a blood meal for oogenesis. Mosquitoes receive a potentially toxic level of iron in the blood meal which must be processed and stored. We demonstrate by (59)Fe pulse-chase experiments that cultured A. aegypti larval CCL-125 cells take up iron from culture media and store it in ferritin found mainly in the membrane fraction and secrete iron-loaded ferritin. We observe that in these larval cells ferritin co-localizes with ceramide-containing membranes in the absence of iron. With iron treatment, ferritin is found associated with ceramide-containing membranes as well as in cytoplasmic non-ceramide vesicles. Treatment of CCL-125 cells with iron and CI-976, an inhibitor of lysophospholipid acyl transferases, disrupts ferritin secretion with a concomitant decrease in cell viability. Interfering with ferritin secretion may limit the ability of mosquitoes to adjust to the high iron load of the blood meal and decrease iron delivery to the ovaries reducing egg numbers.

Mitochondrial ferritin limits oxidative damage regulating mitochondrial iron availability: hypothesis for a protective role in Friedreich ataxia

Mitochondrial ferritin (FtMt) is a nuclear-encoded iron-sequestering protein that specifically localizes in mitochondria. In mice it is highly expressed in cells characterized by high-energy consumption, while is undetectable in iron storage tissues like liver and spleen. FtMt expression in mammalian cells was shown to cause a shift of iron from cytosol to mitochondria, and in yeast it rescued the defects associated with frataxin deficiency. To study the role of FtMt in oxidative damage, we analyzed the effect of its expression in HeLa cells after incubation with H(2)O(2) and Antimycin A, and after a long-term growth in glucose-free media that enhances mitochondrial respiratory activity. FtMt reduced the level of reactive oxygen species (ROS), increased the level of adenosine 5'triphosphate and the activity of mitochondrial Fe-S enzymes, and had a positive effect on cell viability. Furthermore, FtMt expression reduces the size of cytosolic and mitochondrial labile iron pools. In cells grown in glucose-free media, FtMt level was reduced owing to faster degradation rate, however it still protected the activity of mitochondrial Fe-S enzymes without affecting the cytosolic iron status. In addition, FtMt expression in fibroblasts from Friedreich ataxia (FRDA) patients prevented the formation of ROS and partially rescued the impaired activity of mitochondrial Fe-S enzymes, caused by frataxin deficiency. These results indicate that the primary function of FtMt involves the control of ROS formation through the regulation of mitochondrial iron availability. They are consistent with the expression pattern of FtMt observed in mouse tissues, suggesting a FtMt protective role in cells characterized by defective iron homeostasis and respiration, such as in FRDA.

Molecular characterization of iron binding proteins, transferrin and ferritin heavy chain subunit, from the bumblebee Bombus ignitus

Transferrin and ferritin are iron-binding proteins involved in transport and storage of iron as part of iron metabolism. Here, we describe the cDNA cloning and characterization of transferrin (Bi-Tf) and the ferritin heavy chain subunit (Bi-FerHCH), from the bumblebee Bombus ignitus. Bi-Tf cDNA spans 2340 bp and encodes a protein of 706 amino acids and Bi-FerHCH cDNA spans 1393 bp and encodes a protein of 217 amino acids. Comparative analysis revealed that Bi-Tf appears to have residues comprising iron-binding sites in the N-terminal lobe, and Bi-FerHCH contains a 5'UTR iron-responsive element and seven conserved amino acid residues associated with a ferroxidase center. The Bi-Tf and Bi-FerHCH cDNAs were expressed as 79 kDa and 27 kDa polypeptides, respectively, in baculovirus-infected insect Sf9 cells. Northern blot analysis revealed that Bi-Tf exhibits fat body-specific expression and Bi-FerHCH shows ubiquitous expression. The expression profiles of the Bi-Tf and Bi-FerHCH in the fat body of B. ignitus worker bees revealed that Bi-Tf and Bi-FerHCH are differentially induced in a time-dependent manner in a single insect by wounding, bacterial challenge, and iron overload.

Intracellular iron transport and storage: from molecular mechanisms to health implications

Maintenance of proper "labile iron" levels is a critical component in preserving homeostasis. Iron is a vital element that is a constituent of a number of important macromolecules, including those involved in energy production, respiration, DNA synthesis, and metabolism; however, excess "labile iron" is potentially detrimental to the cell or organism or both because of its propensity to participate in oxidation-reduction reactions that generate harmful free radicals. Because of this dual nature, elaborate systems tightly control the concentration of available iron. Perturbation of normal physiologic iron concentrations may be both a cause and a consequence of cellular damage and disease states. This review highlights the molecular mechanisms responsible for regulation of iron absorption, transport, and storage through the roles of key regulatory proteins, including ferroportin, hepcidin, ferritin, and frataxin. In addition, we present an overview of the relation between iron regulation and oxidative stress and we discuss the role of functional iron overload in the pathogenesis of hemochromatosis, neurodegeneration, and inflammation.

Hydrogen peroxide scavenging rescues frataxin deficiency in a Drosophila model of Friedreich's ataxia

Friedreich's ataxia (FRDA) is a neurodegenerative disorder arising from a deficit of the mitochondrial iron chaperone, frataxin. Evidence primarily from yeast and mammalian cells is consistent with the hypothesis that a toxic hydroxyl radical generated from hydrogen peroxide (H2O2) via iron-catalyzed Fenton chemistry at least partially underlies the pathology associated with this disease. However, no whole-organism studies have been presented that directly test this hypothesis. We recently developed a Drosophila model that recapitulates the principal hallmarks of FRDA [Anderson PR, Kirby K, Hilliker A, Phillips JP (2005) Hum Mol Genet 14:3397-3405]. Using the Drosophila FRDA model, we now report that ectopic expression of enzymes that scavenge H2O2 suppresses the deleterious phenotypes associated with frataxin deficiency. In contrast, genetic augmentation with enzymes that scavenge superoxide is without effect. Augmentation of endogenous catalase restores the activity of the reactive oxygen species (ROS)-sensitive mitochondrial enzyme, aconitase and enhances resistance to H2O2 exposure, both of which are diminished by frataxin deficiency. Collectively, these data argue that H2O2 is an important pathogenic substrate underlying the phenotypes arising from frataxin deficiency in Drosophila and that interventions that reduce this specific ROS can effectively ameliorate these phenotypes. The therapeutic implications of these findings are clear and we believe warrant immediate clinical investigation.