A mutation in a mitochondrial ABC transporter results in mitochondrial dysfunction through oxidative damage of mitochondrial DNA

We have isolated a Saccharomyces cerevisiae mutant that shows an increased tendency to form cytoplasmic petites (respiration-deficient ρ⁻ or ρ⁰ mutants) in response to treatment of cells growing on a solid medium with the DNA-damaging agent methyl methanesulfonate or ultraviolet light. The mutation in this strain, atm1-1, was found to cause a single amino acid substitution in ATM1, a nuclear gene that encodes the mitochondrial ATP-binding cassette (ABC) transporter. When the mutant cells were grown in liquid glucose medium, they accumulated free iron within the mitochondria and at the same time gave rise to spontaneous cytoplasmic petite mutants, as seen previously in cells carrying a mutation in a gene homologous to the human gene responsible for Friedreich's ataxia. Analysis of the effects of free iron and malonic acid (an inhibitor of oxidative respiration in mitochondria) on the incidence of petites among the mutant cells indicated that spontaneous induction of petites was a consequence of oxidative stress rather than a direct effect of either a defect in the ATM1 gene or the accumulation of free iron. We observed an increase in the incidence of strand breaks in the mitochondrial DNA of the atm1-1 mutant cells. Furthermore, we found that rates of induction of petites and accumulation of strand breaks in mitochondrial DNA were enhanced in the atm1-1 mutant by the introduction of another mutation, mhr1-1, which results in a deficiency in mitochondrial DNA repair. These observations indicate that spontaneous induction of petites in the atm1-1 mutant is a consequence of oxidative damage to mitochondrial DNA mediated by enhanced accumulation of mitochondrial iron. Received: 26 March 1999 / Accepted: 29 June 1999     

Mitochondrial functioning of constitutive iron uptake mutations in <Emphasis Type="Italic">Cryptococcus neoformans </Emphasis>

Randomly obtained, constitutive plasma membrane ferric reductase/ferrous uptake mutants of Cryptococcus neoformans were mapped to four distinct loci by meiotic analysis. One of those loci, FRR1 , was previously found homologous to MRS3 and MRS4 of Saccharomyces cerevisiae , which determine proteins involved in mitochondrial transport of iron. We were able to complement, clone, sequence and thereby identify two of the three remaining constitutive uptake loci. FRR3 was found to be homologous to ISU1 and ISU2 of S. cerevisiae, which form mitochondrial iron–sulfur complexes; FRR4 was found to be homologous to YFH1, the yeast frataxin homologue, which also participates in iron–sulfur cluster biogenesis. Because of the constitutive iron uptake seen in these mutants, mitochondria appear to have a central role in the cellular iron economy; moreover, as judged by our mutational statistics, the genetic machinery for mitochondrial iron accumulation may be more complex than that of the cytoplasm.     

<Emphasis Type="Italic">Drosophila mitoferrin</Emphasis>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.     

Mitochondrial quality control mediated by PINK1 and PRKN: links to iron metabolism and tumor immunity

Mitochondrial quality control is an essential process required to maintain cellular homeostasis and functions. Mutations of PINK1 and PRKN/PARK2 contribute to the risk of Parkinson disease. Our recent findings indicate that depletion of Pink1 and Prkn promotes pancreatic tumorigenesis in KRAS-driven engineered mouse models. Mechanistically, PINK1- and PRKN-mediated autophagic degradation of mitochondrial iron importers (e.g., SLC25A37 and SLC25A28) suppresses pancreatic tumor growth by attenuating mitochondrial iron accumulation, inflammasome activation, HMGB1 release, and subsequent immune checkpoint expression. Consequently, pharmacological or genetic inhibition of mitochondrial iron-dependent signals prolongs animal survival and reverses pancreatic tumor phenotype in vivo. Thus, PINK1- and PRKN-mediated immunometabolism provides new insights into the tumor microenvironment and could be a suitable target for new pancreatic cancer treatments.     

The vacuolar H+-ATPase of Saccharomyces cerevisiae is required for efficient copper detoxification, mitochondrial function, and iron metabolism

Mutations in the GEF2 gene of the yeast Saccharomyces cerevisiae have pleiotropic effects. The gef2 mutants display a petite phenotype. These cells grow slowly on several different carbon sources utilized exclusively or primarily by respiration. This phenotype is suppressed by adding large amounts of iron to the growth medium. A defect in mitochondrial function may be the cause of the petite phenotype: the rate of oxygen consumption by intact gef2 cells and by mitochondrial fractions isolated from gef2 mutants was reduced 60%–75% relative to wild type. Cytochrome levels were unaffected in gef2 mutants, indicating that heme accumulation is not significantly altered in these strains. The gef2 mutants were also more sensitive than wild type to growth inhibition by several divalent cations including Cu. We found that the cup5 mutation, causing Cu sensitivity, is allelic to gef2 mutations. The GEF2 gene was isolated, sequenced, and found to be identical to VMA3, the gene encoding the vacuolar H ⁺-ATPase proteolipid subunit. These genetic and biochemical analyses demonstrate that the vacuolar H ⁺-ATPase plays a previously unknown role in Cu detoxification, mitochondrial function, and iron metabolism.     

The GEF1 gene of Saccharomyces cerevisiae encodes an integral membrane protein; mutations in which have effects on respiration and iron-limited growth

We have isolated a new class of respiration-defective, i.e petite, mutants of the yeast Saccharomyces cerevisiae. Mutations in the GEF1 gene cause cells to grow slowly on rich media containing carbon sources utilized by respiration. This phenotype is suppressed by adding high concentrations of iron to the growth medium. Gef1 ^– mutants also fail to grow on a fermentable carbon source, glucose, when iron is reduced to low concentrations in the medium, suggesting that the GEF1 gene is required for efficient metabolism of iron during growth on fermentable as well as respired carbon sources. However, activity of the iron uptake system appears to be unaffected in gef1 ^– mutants. Fe(II) transporter activity and regulation is normal in gef1 ^– mutants. Fe(III) reductase induction during iron-limited growth is disrupted, but this appears to be a secondary effect of growth rate alterations. The wild-type GEF1 gene was cloned and sequenced; it encodes a protein of 779 amino acids, 13 possible transmembrane domains, and significant similarity to chloride channel proteins from fish and mammals, suggesting that GEF1 encodes an integral membrane protein. A gef1 ^– deletion mutation generated in vitro and introduced into wild-type haploid strains by gene transplacement was not lethal. Oxygen consumption by intact gef1 ^– cells and by mitochondrial fractions isolated from gef1 ^– mutants was reduced 25–50% relative to wild type, indicating that mitochondrial function is defective in these mutants. We suggest that GEF1 encodes a transport protein that is involved in intracellular iron metabolism.     

Mitochondrial calcium uniporter blocker effectively prevents brain mitochondrial dysfunction caused by iron overload

AimsAlthough iron overload induces oxidative stress and brain mitochondrial dysfunction, and is associated with neurodegenerative diseases, brain mitochondrial iron uptake has not been investigated. We determined the role of mitochondrial calcium uniporter (MCU) in brain mitochondria as a major route for iron entry. We hypothesized that iron overload causes brain mitochondrial dysfunction, and that the MCU blocker prevents iron entry into mitochondria, thus attenuating mitochondrial dysfunction.Main methodsIsolated brain mitochondria from male Wistar rats were used. Iron (Fe^2 + and Fe^3 +) at 0–286 μM were applied onto mitochondria at various incubation times (5–30 min), and the mitochondrial function was determined. Effects of MCU blocker (Ru-360) and iron chelator were studied.Key findingsBoth Fe^2 + and Fe^3 + entered brain mitochondria and caused mitochondrial swelling in a dose- and time-dependent manner, and caused mitochondrial depolarization and increased ROS production. However, Fe^2 + caused more severe mitochondrial dysfunction than Fe^3 +. Although all drugs attenuated mitochondrial dysfunction caused by iron overload, only an MCU blocker could completely prevent ROS production and mitochondrial depolarization.SignificanceOur findings indicated that iron overload caused brain mitochondrial dysfunction, and that an MCU blocker effectively prevented this impairment, suggesting that MCU could be the major portal for brain mitochondrial iron uptake.     

The many faces of mitophagy

Failure to maintain mitochondrial integrity is linked to age‐related conditions, such as neurodegeneration. Two genes linked to Parkinson's disease, PINK1 and Parkin, play a key role in targeting the degradation of dysfunctional mitochondria (mitophagy). However, the mechanisms regulating the PINK1/Parkin pathway and other processes that impinge on mitochondrial turnover are poorly understood. Two articles in EMBO reports, by the Przedborski and Ganley groups 1 2 , shed light on a new role for processed, cytoplasmic PINK1, and show that depletion of cellular iron levels stimulates PINK1/Parkin‐independent mitophagy.     

Regulation of Mitochondrial Iron Import through Differential Turnover of Mitoferrin 1 and Mitoferrin 2

Mitoferrin 1 and mitoferrin 2 are homologous members of the mitochondrial solute carrier family. Mitoferrin 1 is required for mitochondrial iron delivery in developing erythrocytes. Here we show that mitoferrin 1 and mitoferrin 2 contribute to mitochondrial iron delivery in a variety of cells. Reductions in mitoferrin 1 and/or mitoferrin 2 levels by RNA interference result in decreased mitochondrial iron accumulation, heme synthesis, and iron-sulfur cluster synthesis. The ectopic expression of mitoferrin 1 in nonerythroid cells silenced for mitoferrin 2 or the expression of mitoferrin 2 in cells silenced for mitoferrin 1 restored heme synthesis to “baseline” levels. The ectopic expression of mitoferrin 2, however, did not support hemoglobinization in erythroid cells deficient in mitoferrin 1. Mitoferrin 2 could not restore heme synthesis in developing erythroid cells because of an inability of the protein to accumulate in mitochondria. The half-life of mitoferrin 1 was increased in developing erythroid cells, while the half-life of mitoferrin 2 did not change. These results suggest that mitochondrial iron accumulation is tightly regulated and that controlling mitoferrin levels within the mitochondrial membrane provides a mechanism to regulate mitochondrial iron levels.Iron is a required element for all eukaryotes, but iron can be toxic at high concentrations. Consequently, the cellular acquisition of iron is highly regulated, as is the concentration of free iron in biological fluids. The regulation of iron concentration is extended to cellular organelles that either store or utilize iron. Mitochondria utilize iron for the synthesis of heme and iron-sulfur (Fe-S) clusters. These prosthetic groups are used within the mitochondria and are exported for use by cytosolic and nuclear proteins. The mechanisms that regulate mitochondrial iron levels are not known, although it is clear that mitochondrial iron levels must be regulated. For example, the loss of function mutations in genes that encode enzymes required for Fe-S cluster synthesis or the Atm1 transporter that exports Fe-S clusters, results in excessive mitochondrial iron accumulation in yeast and humans (for a review, see reference 11).The mechanisms that regulate mitochondrial iron pools are not well defined. Mitochondrial iron pools might be regulated at the level of import. Mitoferrin 1 (Mfrn1) has been shown to be required for mitochondrial iron import in developing erythroid cells. A mutation in zebrafish Mfrn1 (frascati) or the deletion of mouse Mfrn1 leads to defects in hemoglobinization due to a deficit in mitochondrial iron uptake (17). The phenotype of frascati zebrafish is restricted to developing red blood cells; other cell types showed no evidence of a mitochondrial iron phenotype. Mfrn1 has a paralogue, Mfrn2, and both genes have homologues MRS3 and MRS4 in Saccharomyces cerevisiae. Yeast with deletions of MRS3 and MRS4 grows poorly under low iron conditions due to impaired mitochondrial iron acquisition (5, 10, 13, 23). In yeast, the expression of Mfrn1 or Mfrn2 in Δmrs3 Δmrs4 cells can correct the poor growth under low iron conditions. The expression of either mouse or zebrafish Mfrn1 as a transgene in frascati zebrafish corrected the hemoglobin deficiency in cells, but the expression of Mfrn2 did not (17). These observations raise three questions. (i) What is the role of Mfrn2 in mitochondrial iron metabolism? (ii) Is iron transport into mitochondria regulated? (iii) If Mfrn2 transports iron into the mitochondria of vertebrate cells, why doesn''t Mfrn2 rescue the mitochondrial defect in Mfrn1-deficient zebrafish?Here, we show that Mfrn1 and Mfrn2 can transport iron into the mammalian mitochondria of nonerythroid cells. The ectopic expression of either Mfrn1 or Mfrn2 can restore mitochondrial iron transport in cells silenced for Mfrn2 and -1, respectively, but ectopic expression has little effect on increasing mitochondrial iron levels above the baseline values. Mitochondrial iron levels do not increase over the baseline because the levels of Mfrns are regulated posttranslationally. Mfrn1 accumulates in the mitochondria of developing red blood cells as a result of an increased protein half-life. In contrast, Mfrn2 does not accumulate in developing red blood cells or other cells, as the half-life of Mfrn2 protein remains constant.     

Cellular iron homeostasis mediated by the Mrs4–Ccc1–Smf3 pathway is essential for mitochondrial function,morphogenesis and virulence in Candida albicans

Iron bioavailability is crucial for mitochondrial metabolism and biosynthesis. Dysregulation of cellular iron homeostasis affects multiple aspects of mitochondrial physiology and cellular processes. However, the intracellular iron trafficking pathway in Candida albicans remains unclear. In this study, we characterized the Mrs4–Ccc1–Smf3 pathway, and demonstrated its important role in maintaining cellular iron levels. Double deletion of vacuolar iron exporter SMF3 and mitochondrial iron transporter MRS4 further elevated cellular iron levels in comparison with the single MRS4 deletion. However, deletion of vacuolar iron importer CCC1 in the mrs4?/? mutant restored cellular iron homeostasis to normal wild-type levels, and also normalized most of the defective phenotypes in response to various environmental stresses. Our results also suggested that both Mrs4 and Ccc1 contributed to the maintenance of mitochondrial function. The mrs4?/? and mrs4?/?smf3?/? mutants exhibited an obvious decrease in aconitase activities and mitochondrial membrane potential, whereas deletion of CCC1 in the mrs4?/? mutant effectively rescued these defects. Furthermore, we also found that the Mrs4–Ccc1–Smf3 pathway was indispensable for cell-wall stability, antifungal drug tolerance, filamentous growth and virulence, supporting the novel viewpoint that mitochondria might be the promising target for better antifungal therapies. Interestingly, the addition of exogenous iron failed to rescue the defects on non-fermentable carbon sources or hyphae-inducing medium, indicating that the defects in mitochondrial respiration and filamentous development might result from the disturbance of cellular iron homeostasis rather than environmental iron deprivation. Taken together, our results propose the Mrs4–Ccc1–Smf3 pathway as a potentially attractive target for antifungal drug development.     

Ceruloplasmin Protects Against Rotenone-Induced Oxidative Stress and Neurotoxicity

To clarify the neuroprotective property of ceruloplasmin and the pathogenesis of aceruloplasminemia, we generated ceruloplasmin-deficient (CP ^−/−) mice on the C57BL/10 genetic background and further treated them with a mitochondrial complex I inhibitor, rotenone. There was no iron accumulation in the brains of CP ^−/− mice at least up to 60 weeks of age. Without rotenone treatment, CP ^−/− mice showed slight motor dysfunction compared with CP ^+/+ mice, but there were no detectable differences in the levels of oxidative stress markers between these two groups. A low dose of rotenone did not affect the mitochondrial complex I activity in our mice, however, it caused a significant change in motor behavior, neuropathology, or the levels of oxidative stress markers in CP ^−/− mice, but not in CP ^+/+ mice. Our data support that ceruloplasmin protects against rotenone-induced oxidative stress and neurotoxicity, probably through its antioxidant properties independently of its function of iron metabolism.     

Bacterial‐type oxygen detoxification and iron‐sulfur cluster assembly in amoebal relict mitochondria

The assembly of vital reactive iron‐sulfur (Fe‐S) cofactors in eukaryotes is mediated by proteins inherited from the original mitochondrial endosymbiont. Uniquely among eukaryotes, however, Entamoeba and Mastigamoeba lack such mitochondrial‐type Fe‐S cluster assembly proteins and possess instead an analogous bacterial‐type system acquired by lateral gene transfer. Here we demonstrate, using immunomicroscopy and biochemical methods, that beyond their predicted cytosolic distribution the bacterial‐type Fe‐S cluster assembly proteins NifS and NifU have been recruited to function within the relict mitochondrial organelles (mitosomes) of Entamoeba histolytica. Both Nif proteins are 10‐fold more concentrated within mitosomes compared with their cytosolic distribution suggesting that active Fe‐S protein maturation occurs in these organelles. Quantitative immunoelectron microscopy showed that amoebal mitosomes are minute but highly abundant cellular structures that occupy up to 2% of the total cell volume. In addition, protein colocalization studies allowed identification of the amoebal hydroperoxide detoxification enzyme rubrerythrin as a mitosomal protein. This protein contains functional Fe‐S centres and exhibits peroxidase activity in vitro. Our findings demonstrate the role of analogous protein replacement in mitochondrial organelle evolution and suggest that the relict mitochondrial organelles of Entamoeba are important sites of metabolic activity that function in Fe‐S protein‐mediated oxygen detoxification.     

Mutation in the abcb7 gene causes abnormal iron and fatty acid metabolism in developing medaka fish

The medaka fish (Oryzias latipes) is an emerging model organism for which a variety of unique developmental mutants have now been generated. Our recent mutagenesis screening of the medaka isolated a unique mutant that develops a fatty liver at larval stages. Positional cloning identified the responsible gene as medaka abcb7. Abcb7, a mitochondrial ABC (ATP binding cassette) half-transporter, has been implicated in iron metabolism. Recently, human Abcb7 was found to be mutated in X-linked sideroblastic anemia with cerebellar ataxia (XLSA/A). The homozygous medaka mutant exhibits abnormal iron metabolism in erythrocytes and accumulation of lipid in the liver. Microarray and in situ hybridization analyses demonstrated that the expression of genes involved in iron and lipid metabolisms are both affected in the mutant liver, suggesting novel roles of Abcb7 in the development of physiologically functional liver. The medaka abcb7 mutant thus could provide insights into the pathogenesis of XLSA/A as well as the normal function of the gene.     

An ultrastructural assessment of mitochondria in the gastric parietal cell with the high iron cliamine method

Summary The ultrastructural densification of mitochondria by the high iron diamine (HID) method has been investigated by staining gastric parietal cells with each component of the HID reagent, alone or in combination, or with an otherwise modified HID solution. The effect of certain chemical treatments prior to staining has also been assessed. These tests provided evidence for at east three cytochemically distinguishable constituents in the mitochondrial matrix. In addition, the results from these tests and observations obtained with a newly introduced diaminobenzidine (DAB)-FeCl₃ staining reagent indicated that the probable mode of action of the HID method in imparting density to mitochondrial matrix entails combination between a complex of iron with polymerized or aggregatedm- andp-diamine and some matrix constituent. Assessment of the cytochemical staining in varied fixation condition revealed that the lucent population of mitochondria recently described in HID-stained parietal cells reflects a failure to stain with the method because of inadequate fixation.The possible nature of the HID-reactive substance in the mitochondrial matrix is discussed in the light of these observations and other cytochemical reactivities.