Microcytic anemia, erythropoietic protoporphyria, and neurodegeneration in mice with targeted deletion of iron-regulatory protein 2

Cellular and mitochondrial iron homeostasis in vertebrates

Iron plays an essential role in cellular metabolism and biological processes. However, due to its intrinsic redox activity, free iron is a potentially toxic molecule in cellular biochemistry. Thus, organisms have developed sophisticated ways to import, sequester, and utilize iron. The transferrin cycle is a well-studied iron uptake pathway that is important for most vertebrate cells. Circulating iron can also be imported into cells by mechanisms that are independent of transferrin. Once imported into erythroid cells, iron is predominantly consumed by the mitochondria for the biosynthesis of heme and iron sulfur clusters. This review focuses on canonical transferrin-mediated and the newly discovered, non-transferrin mediated iron uptake pathways, as well as, mitochondrial iron homeostasis in higher eukaryotes. This article is part of a Special Issue entitled: Cell Biology of Metals.

Structural and molecular characterization of iron-sensing hemerythrin-like domain within F-box and leucine-rich repeat protein 5 (FBXL5)

Mammalian cells maintain iron homeostasis by sensing changes in bioavailable iron levels and promoting adaptive responses. FBXL5 is a subunit of an E3 ubiquitin ligase complex that mediates the stability of iron regulatory protein 2, an important posttranscriptional regulator of several genes involved in iron metabolism. The stability of FBXL5 is regulated in an iron- and oxygen-responsive manner, contingent upon the presence of its N-terminal domain. Here we present the atomic structure of the FBXL5 N terminus, a hemerythrin-like α-helical bundle fold not previously observed in mammalian proteins. The core of this domain employs an unusual assortment of amino acids necessary for the assembly and sensing properties of its diiron center. These regulatory features govern the accessibility of a mapped sequence required for proteasomal degradation of FBXL5. Detailed molecular and structural characterization of the ligand-responsive hemerythrin domain provides insights into the mechanisms by which FBXL5 serves as a unique mammalian metabolic sensor.

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

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

Knockout mouse models of iron homeostasis

Murine models have made valuable contributions to our understanding of iron metabolism. Investigation of mice with inherited forms of anemia has led to the discovery of novel proteins involved in iron homeostasis. A growing number of murine models are being developed to investigate mitochondrial iron metabolism. Mouse strains are available for the major forms of hereditary hemochromatosis. Findings in murine models support the concept that the pathogenesis of nearly all forms of hereditary hemochromatosis involves inappropriately low expression of hepcidin. The availability of mice with floxed iron-related genes allows the study of the in vivo consequences of cell-selective deletion of these genes.

The Escherichia coli protein YfeX functions as a porphyrinogen oxidase, not a heme dechelatase

The protein YfeX from Escherichia coli has been proposed to be essential for the process of iron removal from heme by carrying out a dechelation of heme without cleavage of the porphyrin macrocycle. Since this proposed reaction is unique and would represent the first instance of the biological dechelation of heme, we undertook to characterize YfeX. Our data reveal that YfeX effectively decolorizes the dyes alizarin red and Cibacron blue F3GA and has peroxidase activity with pyrogallal but not guiacol. YfeX oxidizes protoporphyrinogen to protoporphyrin in vitro. However, we were unable to detect any dechelation of heme to free porphyrin with purified YfeX or in cellular extracts of E. coli overexpressing YfeX. Additionally, Vibrio fischeri, an organism that can utilize heme as an iron source when grown under iron limitation, is able to grow with heme as the sole source of iron when its YfeX homolog is absent. Plasmid-driven expression of YfeX in V. fischeri grown with heme did not result in accumulation of protoporphyrin. We propose that YfeX is a typical dye-decolorizing peroxidase (or DyP) and not a dechelatase. The protoporphyrin reported to accumulate when YfeX is overexpressed in E. coli likely arises from the intracellular oxidation of endogenously synthesized protoporphyrinogen and not from dechelation of exogenously supplied heme. Bioinformatic analysis of bacterial YfeX homologs does not identify any connection with iron acquisition but does suggest links to anaerobic-growth-related respiratory pathways. Additionally, some genes encoding homologs of YfeX have tight association with genes encoding a bacterial cytoplasmic encapsulating protein.

IMPORTANCE

Acquisition of iron from the host during infection is a limiting factor for growth and survival of pathogens. Host heme is the major source of iron in infections, and pathogenic bacteria have evolved complex mechanisms to acquire heme and abstract the iron from heme. Recently Létoffé et al. (Proc. Natl. Acad. Sci. U.S.A. 106:11719-11724, 2009) reported that the protein YfeX from E. coli is able to dechelate heme to remove iron and leave an intact tetrapyrrole. This is totally unlike any other described biological system for iron removal from heme and, thus, would represent a dramatically new feature with potentially profound implications for our understanding of bacterial pathogenesis. Given that this reaction has no precedent in biological systems, we characterized YfeX and a related protein. Our data clearly demonstrate that YfeX is not a dechelatase as reported but is a peroxidase that oxidizes endogenous porphyrinogens to porphyrins.

Iron insufficiency compromises motor neurons and their mitochondrial function in Irp2-null mice

Genetic ablation of Iron Regulatory Protein 2 (Irp2, Ireb2), which post-transcriptionally regulates iron metabolism genes, causes a gait disorder in mice that progresses to hind-limb paralysis. Here we have demonstrated that misregulation of iron metabolism from loss of Irp2 causes lower motor neuronal degeneration with significant spinal cord axonopathy. Mitochondria in the lumbar spinal cord showed significantly decreased Complex I and II activities, and abnormal morphology. Lower motor neurons appeared to be the most adversely affected neurons, and we show that functional iron starvation due to misregulation of iron import and storage proteins, including transferrin receptor 1 and ferritin, may have a causal role in disease. We demonstrated that two therapeutic approaches were beneficial for motor neuron survival. First, we activated a homologous protein, IRP1, by oral Tempol treatment and found that axons were partially spared from degeneration. Secondly, we genetically decreased expression of the iron storage protein, ferritin, to diminish functional iron starvation. These data suggest that functional iron deficiency may constitute a previously unrecognized molecular basis for degeneration of motor neurons in mice.

Iron regulatory protein-1 and -2: transcriptome-wide definition of binding mRNAs and shaping of the cellular proteome by iron regulatory proteins

Iron regulatory proteins (IRPs) 1 and 2 are RNA-binding proteins that control cellular iron metabolism by binding to conserved RNA motifs called iron-responsive elements (IREs). The currently known IRP-binding mRNAs encode proteins involved in iron uptake, storage, and release as well as heme synthesis. To systematically define the IRE/IRP regulatory network on a transcriptome-wide scale, IRP1/IRE and IRP2/IRE messenger ribonucleoprotein complexes were immunoselected, and the mRNA composition was determined using microarrays. We identify 35 novel mRNAs that bind both IRP1 and IRP2, and we also report for the first time cellular mRNAs with exclusive specificity for IRP1 or IRP2. To further explore cellular iron metabolism at a system-wide level, we undertook proteomic analysis by pulsed stable isotope labeling by amino acids in cell culture in an iron-modulated mouse hepatic cell line and in bone marrow-derived macrophages from IRP1- and IRP2-deficient mice. This work investigates cellular iron metabolism in unprecedented depth and defines a wide network of mRNAs and proteins with iron-dependent regulation, IRP-dependent regulation, or both.

The FBXL5-IRP2 axis is integral to control of iron metabolism in vivo

Iron-dependent degradation of iron-regulatory protein 2 (IRP2) is a key event for maintenance of an appropriate intracellular concentration of iron. Although FBXL5 (F box and leucine-rich repeat protein 5) is thought to mediate this degradation, the role of FBXL5 in the control of iron homeostasis in vivo has been poorly understood. We have now found that mice deficient in FBXL5 died in utero, associated with excessive iron accumulation. This embryonic mortality was prevented by additional ablation of IRP2, suggesting that impaired IRP2 degradation is primarily responsible for the death of Fbxl5(-)(/-) mice. We also found that liver-specific deletion of Fbxl5 resulted in deregulation of both hepatic and systemic iron homeostasis, leading to the development of steatohepatitis. The liver-specific mutant mice died with acute liver failure when fed a high-iron diet. Thus, our results uncover a major role for FBXL5 in ensuring an appropriate supply of iron to cells.

β-thalassemia: a model for elucidating the dynamic regulation of ineffective erythropoiesis and iron metabolism

β-thalassemia is a disease characterized by anemia and is associated with ineffective erythropoiesis and iron dysregulation resulting in iron overload. The peptide hormone hepcidin regulates iron metabolism, and insufficient hepcidin synthesis is responsible for iron overload in minimally transfused patients with this disease. Understanding the crosstalk between erythropoiesis and iron metabolism is an area of active investigation in which patients with and models of β-thalassemia have provided significant insight. The dependence of erythropoiesis on iron presupposes that iron demand for hemoglobin synthesis is involved in the regulation of iron metabolism. Major advances have been made in understanding iron availability for erythropoiesis and its dysregulation in β-thalassemia. In this review, we describe the clinical characteristics and current therapeutic standard in β-thalassemia, explore the definition of ineffective erythropoiesis, and discuss its role in hepcidin regulation. In preclinical experiments using interventions such as transferrin, hepcidin agonists, and JAK2 inhibitors, we provide evidence of potential new treatment alternatives that elucidate mechanisms by which expanded or ineffective erythropoiesis may regulate iron supply, distribution, and utilization in diseases such as β-thalassemia.

Abnormal mitoferrin-1 expression in patients with erythropoietic protoporphyria

OBJECTIVE

Most patients with erythropoietic protoporphyria have deficient ferrochelatase (FECH) activity due to changes in FECH DNA. We evaluated seven patients with erythropoietic protoporphyria phenotype in whom abnormalities of FECH DNA were not found by conventional analysis. The major focus was mitoferrin-1 (MFRN1), the mitochondrial transporter of Fe used for heme formation by FECH and for 2Fe2S cluster synthesis, which is critical to FECH activity/stability.

MATERIALS AND METHODS

Four patients had a deletion in ALAS2 that causes enzyme gain-of-function, resulting in increased formation of protoporphyrin; one had a heterozygous major deletion in FECH DNA. All had an abnormal transcript of MFRN1 in messenger RNA extracted from blood leukocytes and/or liver tissue. The abnormal transcript contained an insert of intron 2 that had a stop codon. The consequences of abnormal MFRN1 expression were examined using zebrafish and yeast MFRN-deficient strains and cultured lymphoblasts from the patients.

RESULTS

Abnormal human MFRN1 complementary DNA showed loss-of-function in zebrafish and yeast mutants, whereas normal human MFRN1 complementary DNA rescued both. Using cultured lymphoblasts, quantitative reverse transcription polymerase chain reaction showed increased formation of abnormal transcript that was accompanied by decreased formation of normal transcript and reduced FECH activity in patients compared to normal lines. A positive correlation coefficient (0.75) was found between FECH activity and normal MFRN1 messenger RNA in lymphoblasts. However, no obvious cause for increased formation of abnormal transcript was identified in MFRN1 exons and splice junctions.

CONCLUSIONS

Abnormal MFRN1 expression can contribute to erythropoietic protoporphyria phenotype in some patients, probably by causing a reduction in FECH activity.

Regulation of cellular iron metabolism

Iron is an essential but potentially hazardous biometal. Mammalian cells require sufficient amounts of iron to satisfy metabolic needs or to accomplish specialized functions. Iron is delivered to tissues by circulating transferrin, a transporter that captures iron released into the plasma mainly from intestinal enterocytes or reticuloendothelial macrophages. The binding of iron-laden transferrin to the cell-surface transferrin receptor 1 results in endocytosis and uptake of the metal cargo. Internalized iron is transported to mitochondria for the synthesis of haem or iron–sulfur clusters, which are integral parts of several metalloproteins, and excess iron is stored and detoxified in cytosolic ferritin. Iron metabolism is controlled at different levels and by diverse mechanisms. The present review summarizes basic concepts of iron transport, use and storage and focuses on the IRE (iron-responsive element)/IRP (iron-regulatory protein) system, a well known post-transcriptional regulatory circuit that not only maintains iron homoeostasis in various cell types, but also contributes to systemic iron balance.

Targeted deletion of the mouse Mitoferrin1 gene: from anemia to protoporphyria

Mitoferrin1 is 1 of 2 homologous mitochondrial iron transporters and is required for mitochondrial iron delivery in developing erythroid cells. We show that total deletion of Mfrn1 in embryos leads to embryonic lethality. Selective deletion of Mfrn1 in adult hematopoietic tissues leads to severe anemia because of a deficit in erythroblast formation. Deletion of Mfrn1 in hepatocytes has no phenotype or biochemical effect under normal conditions. In the presence of increased porphyrin synthesis, however, deletion of Mfrn1 in hepatocytes results in a decreased ability to convert protoporphyrin IX into heme, leading to protoporphyria, cholestasis, and bridging cirrhosis. Our results show that the activity of mitoferrin1 is required to manage an increase in heme synthesis. The data also show that alterations in heme synthesis within hepatocytes can lead to protoporphyria and hepatotoxicity.

Iron regulatory proteins: from molecular mechanisms to drug development

Eukaryotic cells require iron for survival but, as an excess of poorly liganded iron can lead to the catalytic production of toxic radicals that can damage cell structures, regulatory mechanisms have been developed to maintain appropriate cell and body iron levels. The interactions of iron responsive elements (IREs) with iron regulatory proteins (IRPs) coordinately regulate the expression of the genes involved in iron uptake, use, storage, and export at the post-transcriptional level, and represent the main regulatory network controlling cell iron homeostasis. IRP1 and IRP2 are similar (but not identical) proteins with partially overlapping and complementary functions, and control cell iron metabolism by binding to IREs (i.e., conserved RNA stem-loops located in the untranslated regions of a dozen mRNAs directly or indirectly related to iron metabolism). The discovery of the presence of IREs in a number of other mRNAs has extended our knowledge of the influence of the IRE/IRP regulatory network to new metabolic pathways, and it has been recently learned that an increasing number of agents and physiopathological conditions impinge on the IRE/IRP system. This review focuses on recent findings concerning the IRP-mediated regulation of iron homeostasis, its alterations in disease, and new research directions to be explored in the near future.

Erythropoiesis and iron sulfur cluster biogenesis

Erythropoiesis in animals is a synchronized process of erythroid cell differentiation that depends on successful acquisition of iron. Heme synthesis depends on iron through its dependence on iron sulfur (Fe-S) cluster biogenesis. Here, we review the relationship between Fe-S biogenesis and heme synthesis in erythropoiesis, with emphasis on the proteins, GLRX5, ABCB7, ISCA, and C1orf69. These Fe-S biosynthesis proteins are highly expressed in erythroid tissues, and deficiency of each of these proteins has been shown to cause anemia in zebrafish model. GLRX5 is involved in the production and ABCB7 in the export of an unknown factor that may function as a gauge of mitochondrial iron status, which may indirectly modulate activity of iron regulatory proteins (IRPs). ALAS2, the enzyme catalyzing the first step in heme synthesis, is translationally controlled by IRPs. GLRX5 may also provide Fe-S cofactor for ferrochelatase, the last enzyme in heme synthesis. ISCA and C1orf69 are thought to assemble Fe-S clusters for mitochondrial aconitase and for lipoate synthase, the enzyme producing lipoate for pyruvate dehydrogenase complex (PDC). PDC and aconitase are involved in the production of succinyl-CoA, a substrate for heme biosynthesis. Thus, many steps of heme synthesis depend on Fe-S cluster assembly.

Two to tango: regulation of Mammalian iron metabolism

Disruptions in iron homeostasis from both iron deficiency and overload account for some of the most common human diseases. Iron metabolism is balanced by two regulatory systems, one that functions systemically and relies on the hormone hepcidin and the iron exporter ferroportin, and another that predominantly controls cellular iron metabolism through iron-regulatory proteins that bind iron-responsive elements in regulated messenger RNAs. We describe how the two distinct systems function and how they "tango" together in a coordinated manner. We also highlight some of the current questions in mammalian iron metabolism and discuss therapeutic opportunities arising from a better understanding of the underlying biological principles.

Selective translational control of the Alzheimer amyloid precursor protein transcript by iron regulatory protein-1

Iron influx increases the translation of the Alzheimer amyloid precursor protein (APP) via an iron-responsive element (IRE) RNA stem loop in its 5'-untranslated region. Equal modulated interaction of the iron regulatory proteins (IRP1 and IRP2) with canonical IREs controls iron-dependent translation of the ferritin subunits. However, our immunoprecipitation RT-PCR and RNA binding experiments demonstrated that IRP1, but not IRP2, selectively bound the APP IRE in human neural cells. This selective IRP1 interaction pattern was evident in human brain and blood tissue from normal and Alzheimer disease patients. We computer-predicted an optimal novel RNA stem loop structure for the human, rhesus monkey, and mouse APP IREs with reference to the canonical ferritin IREs but also the IREs encoded by erythroid heme biosynthetic aminolevulinate synthase and Hif-2α mRNAs, which preferentially bind IRP1. Selective 2'-hydroxyl acylation analyzed by primer extension analysis was consistent with a 13-base single-stranded terminal loop and a conserved GC-rich stem. Biotinylated RNA probes deleted of the conserved CAGA motif in the terminal loop did not bind to IRP1 relative to wild type probes and could no longer base pair to form a predicted AGA triloop. An AGU pseudo-triloop is key for IRP1 binding to the canonical ferritin IREs. RNA probes encoding the APP IRE stem loop exhibited the same high affinity binding to rhIRP1 as occurs for the H-ferritin IRE (35 pm). Intracellular iron chelation increased binding of IRP1 to the APP IRE, decreasing intracellular APP expression in SH-SY5Y cells. Functionally, shRNA knockdown of IRP1 caused increased expression of neural APP consistent with IRP1-APP IRE-driven translation.

Crosstalk between Iron Metabolism and Erythropoiesis

Iron metabolism and erythropoiesis are inextricably linked. The majority of iron extracted from circulation daily is used for hemoglobin synthesis. In the last 15 years, major advances have been made in understanding the pathways regulating iron metabolism. Hepcidin is a key regulator of iron absorption and recycling and is itself regulated by erythropoiesis. While several viable candidates have been proposed, elucidating the “erythroid regulator” of hepcidin continues to generate significant experimental activity in the field. Although the mechanism responsible for sensing iron demand for erythropoiesis is still incompletely understood, evaluating diseases in which disordered erythropoiesis and/or iron metabolism are showcased has resulted in a more robust appreciation of potential candidates coordinated erythroid iron demand with regulators of iron supply. We present data drawn from four different conditions—iron deficiency, congenital hypotransferrinemia, beta-thalassemia, and hereditary hemochromatosis—both in human and non-human models of disease, together suggesting that erythroid iron demand exerts a stronger influence on circulating iron supply than systemic iron stores. Greater understanding of the interplay between the key factors involved in the regulation of iron metabolism and erythropoiesis will help develop more effective therapies for disorders of iron overload, iron deficiency, and hemoglobin synthesis.

Iron and porphyrin trafficking in heme biogenesis

Iron is an essential element for diverse biological functions. In mammals, the majority of iron is enclosed within a single prosthetic group: heme. In metazoans, heme is synthesized via a highly conserved and coordinated pathway within the mitochondria. However, iron is acquired from the environment and subsequently assimilated into various cellular pathways, including heme synthesis. Both iron and heme are toxic but essential cofactors. How is iron transported from the extracellular milieu to the mitochondria? How are heme and heme intermediates coordinated with iron transport? Although recent studies have answered some questions, several pieces of this intriguing puzzle remain unsolved.

Tumorigenic properties of iron regulatory protein 2 (IRP2) mediated by its specific 73-amino acids insert

Iron regulatory proteins, IRP1 and IRP2, bind to mRNAs harboring iron responsive elements and control their expression. IRPs may also perform additional functions. Thus, IRP1 exhibited apparent tumor suppressor properties in a tumor xenograft model. Here we examined the effects of IRP2 in a similar setting. Human H1299 lung cancer cells or clones engineered for tetracycline-inducible expression of wild type IRP2, or the deletion mutant IRP2_Δ73 (lacking a specific insert of 73 amino acids), were injected subcutaneously into nude mice. The induction of IRP2 profoundly stimulated the growth of tumor xenografts, and this response was blunted by addition of tetracycline in the drinking water of the animals, to turnoff the IRP2 transgene. Interestingly, IRP2_Δ73 failed to promote tumor growth above control levels. As expected, xenografts expressing the IRP2 transgene exhibited high levels of transferrin receptor 1 (TfR1); however, the expression of other known IRP targets was not affected. Moreover, these xenografts manifested increased c-MYC levels and ERK1/2 phosphorylation. A microarray analysis identified distinct gene expression patterns between control and tumors containing IRP2 or IRP1 transgenes. By contrast, gene expression profiles of control and IRP2_Δ73-related tumors were more similar, consistently with their growth phenotype. Collectively, these data demonstrate an apparent pro-oncogenic activity of IRP2 that depends on its specific 73 amino acids insert, and provide further evidence for a link between IRPs and cancer biology.

Glutaredoxin 5 deficiency causes sideroblastic anemia by specifically impairing heme biosynthesis and depleting cytosolic iron in human erythroblasts

Glutaredoxin 5 (GLRX5) deficiency has previously been identified as a cause of anemia in a zebrafish model and of sideroblastic anemia in a human patient. Here we report that GLRX5 is essential for iron-sulfur cluster biosynthesis and the maintenance of normal mitochondrial and cytosolic iron homeostasis in human cells. GLRX5, a mitochondrial protein that is highly expressed in erythroid cells, can homodimerize and assemble [2Fe-2S] in vitro. In GLRX5-deficient cells, [Fe-S] cluster biosynthesis was impaired, the iron-responsive element-binding (IRE-binding) activity of iron regulatory protein 1 (IRP1) was activated, and increased IRP2 levels, indicative of relative cytosolic iron depletion, were observed together with mitochondrial iron overload. Rescue of patient fibroblasts with the WT GLRX5 gene by transfection or viral transduction reversed a slow growth phenotype, reversed the mitochondrial iron overload, and increased aconitase activity. Decreased aminolevulinate delta, synthase 2 (ALAS2) levels attributable to IRP-mediated translational repression were observed in erythroid cells in which GLRX5 expression had been downregulated using siRNA along with marked reduction in ferrochelatase levels and increased ferroportin expression. Erythroblasts express both IRP-repressible ALAS2 and non-IRP-repressible ferroportin 1b. The unique combination of IRP targets likely accounts for the tissue-specific phenotype of human GLRX5 deficiency.

Iron homeostasis and its interaction with prolyl hydroxylases

The ability of iron to accept or donate electrons, coupled with the ability of oxygen to act as an electron acceptor, renders both elements essential to normal cellular biology. However, these same chemical properties allow free iron in solution to generate toxic free radicals, particularly in combination with oxygen. Thus, closely interwoven homeostatic mechanisms have evolved to regulate both iron and oxygen concentrations at the systemic and the cellular levels. Systemically, iron levels are regulated through hepcidin-mediated uptake of iron in the duodenum, whereas intracellular free-iron levels are controlled through iron-regulatory proteins (IRPs). Cardiorespiratory changes increase systemic oxygen delivery, whereas at a cellular level, many responses to altered oxygen levels are coordinated by hypoxia-inducible factor (HIF). However, the mechanisms of iron homeostasis also are regulated by oxygen availability, with alterations in both hepcidin and IRP activity. In addition, many genes involved in iron homeostasis are direct targets of HIF. Furthermore, HIF activation is modulated by intracellular iron, through regulation of hydroxylase activity, which requires iron as a cofactor. In addition, HIF-2alpha translation is controlled by IRP activity, providing another level of interdependence between iron and oxygen homeostasis.

Molecular genetic approaches to understanding the roles and regulation of iron in brain health and disease

Iron is essential in the brain, yet too much iron can be toxic. Tight regulation of iron in the brain may involve intrinsic mechanisms that control internal homeostasis independent of systemic iron status. Iron abnormalities occur in various neurological disorders, usually with symptoms or neuropathology associated with movement impairment or behavioral disturbances rather than cognitive impairment or dementia. Consistent with this, polymorphisms in the HFE gene, associated with the iron overload disorder hemochromatosis, show stronger associations with the movement disorder amyotrophic lateral sclerosis (motor neuron disease) than with cognitive impairment. Such associations may arise because certain brain regions involved in movement or executive control are particularly iron-rich, notably the basal ganglia, and may be highly reliant on iron. Various mechanisms, including iron redistribution causing functional iron deficiency, lysosomal and mitochondrial abnormalities or oxidative damage, could underlie iron-related neuropathogenesis. Clarifying how iron contributes causatively to neurodegeneration may improve treatment options in a range of neurodegenerative disorders. This review considers how modern molecular genetic approaches can be applied to resolve the complex molecular systems and pathways by which brain iron homeostasis is regulated and the molecular changes that occur with iron dyshomeostasis and neuropathogenesis.

Posttranslational stability of the heme biosynthetic enzyme ferrochelatase is dependent on iron availability and intact iron-sulfur cluster assembly machinery

Mammalian ferrochelatase, the terminal enzyme in the heme biosynthetic pathway, possesses an iron-sulfur [2Fe-2S] cluster that does not participate in catalysis. We investigated ferrochelatase expression in iron-deficient erythropoietic tissues of mice lacking iron regulatory protein 2, in iron-deficient murine erythroleukemia cells, and in human patients with ISCU myopathy. Ferrochelatase activity and protein levels were dramatically decreased in Irp2(-/-) spleens, whereas ferrochelatase mRNA levels were increased, demonstrating posttranscriptional regulation of ferrochelatase in vivo. Translation of ferrochelatase mRNA was unchanged in iron-depleted murine erythroleukemia cells, and the stability of mature ferrochelatase protein was also unaffected. However, the stability of newly formed ferrochelatase protein was dramatically decreased during iron deficiency. Ferrochelatase was also severely depleted in muscle biopsies and cultured myoblasts from patients with ISCU myopathy, a disease caused by deficiency of a scaffold protein required for Fe-S cluster assembly. Together, these data suggest that decreased Fe-S cluster availability because of cellular iron depletion or impaired Fe-S cluster assembly causes reduced maturation and stabilization of apo-ferrochelatase, providing a direct link between Fe-S biogenesis and completion of heme biosynthesis. We propose that decreased heme biosynthesis resulting from impaired Fe-S cluster assembly can contribute to the pathogenesis of diseases caused by defective Fe-S cluster biogenesis.

Mapping of Char10, a novel malaria susceptibility locus on mouse chromosome 9

Resistance to blood-stage malaria in AcB55 and AcB61 is caused by a loss of function mutation in pyruvate kinase (Pklr(I90N)). Likewise, pyruvate kinase (PK) deficiency in humans is protective against Plasmodium replication in vitro. We identified a third AcB strain, AcB62 that also carries the Pklr(I90N) mutation. However, AcB62 mice were susceptible to P.chabaudi infection and showed high levels of parasite replication (54-62% peak parasitemia). AcB62 mice showed the hallmarks of PK deficiency-associated anemia similar to AcB55/61 with reticulocytosis, splenic red pulp expansion, tissue iron overload, and increased expression of iron metabolism proteins. This suggests that malaria susceptibility in AcB62 is not because of absence of PK deficiency-associated pathophysiology. To map novel genetic factors affecting malaria susceptibility in AcB62, we generated an informative F2 population using AcB62 (Pklr(I90N)) and CBA-Pk(slc) (Pklr(G338D)) as progenitors and identified a novel locus on chromosome 9 (Char10; LOD=7.24) that controls peak parasitemia. A weaker linkage to the Pklr region of chromosome 3 (LOD=3.7) was also detected, a finding that may reflect the segregation of the two defective Pklr alleles. AcB62 alleles at both loci are associated with higher peak parasitemia. These results identify Char10 as a novel locus modulating severity of malaria in the context of PK deficiency.

Discovery of genes essential for heme biosynthesis through large-scale gene expression analysis

Heme biosynthesis consists of a series of eight enzymatic reactions that originate in mitochondria and continue in the cytosol before returning to mitochondria. Although these core enzymes are well studied, additional mitochondrial transporters and regulatory factors are predicted to be required. To discover such unknown components, we utilized a large-scale computational screen to identify mitochondrial proteins whose transcripts consistently coexpress with the core machinery of heme biosynthesis. We identified SLC25A39, SLC22A4, and TMEM14C, which are putative mitochondrial transporters, as well as C1orf69 and ISCA1, which are iron-sulfur cluster proteins. Targeted knockdowns of all five genes in zebrafish resulted in profound anemia without impacting erythroid lineage specification. Moreover, silencing of Slc25a39 in murine erythroleukemia cells impaired iron incorporation into protoporphyrin IX, and vertebrate Slc25a39 complemented an iron homeostasis defect in the orthologous yeast mtm1Delta deletion mutant. Our results advance the molecular understanding of heme biosynthesis and offer promising candidate genes for inherited anemias.

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.

Cysteine oxidation regulates the RNA-binding activity of iron regulatory protein 2

Iron regulatory protein 2 (IRP2) is an RNA-binding protein that regulates the posttranscriptional expression of proteins required for iron homeostasis such as ferritin and transferrin receptor 1. IRP2 RNA-binding activity is primarily regulated by iron-mediated proteasomal degradation, but studies have suggested that IRP2 RNA binding is also regulated by thiol oxidation. We generated a model of IRP2 bound to RNA and found that two cysteines (C512 and C516) are predicted to lie in the RNA-binding cleft. Site-directed mutagenesis and thiol modification show that, while IRP2 C512 and C516 do not directly interact with RNA, both cysteines are located within the RNA-binding cleft and must be unmodified/reduced for IRP2-RNA interactions. Oxidative stress induced by cellular glucose deprivation reduces the RNA-binding activity of IRP2 but not IRP2-C512S or IRP2-C516S, consistent with the formation of a disulfide bond between IRP2 C512 and C516 during oxidative stress. Decreased IRP2 RNA binding is correlated with reduced transferrin receptor 1 mRNA abundance. These studies provide insight into the structural basis for IRP2-RNA interactions and reveal an iron-independent mechanism for regulating iron homeostasis through the redox regulation of IRP2 cysteines.

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 power plant of the cell is also a smithy: the emerging role of mitochondria in cellular iron homeostasis

Iron is required for a barrage of essential biochemical functions in virtually every species of life. Perturbation of the availability or utilization of iron in these functions or disruption of other components along iron-requiring pathways can not only lead to cellular/organismal insufficiency of respective biochemical end-products but also result in a broad derangement of iron homeostasis. This is largely because of the elaborate regulatory mechanisms that connect cellular iron utilization with uptake and distribution. Such mechanisms are necessitated by the 'double-edged' nature of the metal, whose very property as a useful biological catalyst also makes it able to generate highly toxic compounds. Since the majority of iron is dispatched onto a functional course by mitochondria-localized pathways, these organelles are in an ideal position within the cellular iron anabolic pathways to be a central site for regulation of iron homeostasis. The goal of this article is to provide an overview of how mitochondria acquire and use iron and examine the ramifications of disturbances in these processes on overall cellular iron homeostasis.

Cell-autonomous and systemic context-dependent functions of iron regulatory protein 2 in mammalian iron metabolism

Mice with total and constitutive iron regulatory protein 2 (IRP2) deficiency exhibit microcytosis and altered body iron distribution with duodenal and hepatic iron loading and decreased iron levels in splenic macrophages. To explore cell-autonomous and systemic context-dependent functions of IRP2 and to assess the systemic consequences of local IRP2 deficiency, we applied Cre/Lox technology to specifically ablate IRP2 in enterocytes, hepatocytes, or macrophages, respectively. This study reveals that the hepatic and duodenal manifestations of systemic IRP2 deficiency are largely explained by cell-autonomous functions of IRP2. By contrast, IRP2-deficient macrophages from otherwise IRP2-sufficient mice do not display the abnormalities of macrophages from systemically IRP2-deficient animals, suggesting that these result from IRP2 disruption in other cell type(s). Mice with enterocyte-, hepatocyte-, or macrophage-specific IRP2 deficiency display normal red blood cell and plasma iron parameters, supporting the notion that the microcytosis in IRP2-deficient mice likely reflects an intrinsic defect in hematopoiesis. This work defines the respective roles of IRP2 in the determination of critical body iron parameters such as organ iron loading and erythropoiesis.

Systemic iron homeostasis and the iron-responsive element/iron-regulatory protein (IRE/IRP) regulatory network

The regulation and maintenance of systemic iron homeostasis is critical to human health. Iron overload and deficiency diseases belong to the most common nutrition-related pathologies across the globe. It is now well appreciated that the hormonal hepcidin/ferroportin system plays an important regulatory role for systemic iron metabolism. We review recent data that uncover the importance of the cellular iron-responsive element/iron-regulatory protein (IRE/IRP) regulatory network in systemic iron homeostasis. We also discuss how the IRE/IRP regulatory system communicates with the hepcidin/ferroportin system to connect the control networks for systemic and cellular iron balance.

Aconitate hydratase of mammals under oxidative stress

Data on the structure, functions, regulation of activity, and expression of cytosolic and mitochondrial aconitate hydratase isoenzymes of mammals are reviewed. The role of aconitate hydratase and structurally similar iron-regulatory protein in maintenance of homeostasis of cell iron is described. Information on modifications of the aconitate hydratase molecule and changes in expression under oxidative stress is generalized. The role of aconitate hydratase in the pathogenesis of some diseases is considered.

C-terminal deletions in the ALAS2 gene lead to gain of function and cause X-linked dominant protoporphyria without anemia or iron overload

All reported mutations in ALAS2, which encodes the rate-regulating enzyme of erythroid heme biosynthesis, cause X-linked sideroblastic anemia. We describe eight families with ALAS2 deletions, either c.1706-1709 delAGTG (p.E569GfsX24) or c.1699-1700 delAT (p.M567EfsX2), resulting in frameshifts that lead to replacement or deletion of the 19-20 C-terminal residues of the enzyme. Prokaryotic expression studies show that both mutations markedly increase ALAS2 activity. These gain-of-function mutations cause a previously unrecognized form of porphyria, X-linked dominant protoporphyria, characterized biochemically by a high proportion of zinc-protoporphyrin in erythrocytes, in which a mismatch between protoporphyrin production and the heme requirement of differentiating erythroid cells leads to overproduction of protoporphyrin in amounts sufficient to cause photosensitivity and liver disease.

Stat5 regulates cellular iron uptake of erythroid cells via IRP-2 and TfR-1

Erythropoiesis strictly depends on signal transduction through the erythropoietin receptor (EpoR)-Janus kinase 2 (Jak2)-signal transducer and activator of transcription 5 (Stat5) axis, regulating proliferation, differentiation, and survival. The exact role of the transcription factor Stat5 in erythropoiesis remained puzzling, however, since the first Stat5-deficient mice carried a hypomorphic Stat5 allele, impeding full phenotypical analysis. Using mice completely lacking Stat5--displaying early lethality--we demonstrate that these animals suffer from microcytic anemia due to reduced expression of the antiapoptotic proteins Bcl-x(L) and Mcl-1 followed by enhanced apoptosis. Moreover, transferrin receptor-1 (TfR-1) cell surface levels on erythroid cells were decreased more than 2-fold on erythroid cells of Stat5(-/-) animals. This reduction could be attributed to reduced transcription of TfR-1 mRNA and iron regulatory protein 2 (IRP-2), the major translational regulator of TfR-1 mRNA stability in erythroid cells. Both genes were demonstrated to be direct transcriptional targets of Stat5. This establishes an unexpected mechanistic link between EpoR/Jak/Stat signaling and iron metabolism, processes absolutely essential for erythropoiesis and life.

Tempol-mediated activation of latent iron regulatory protein activity prevents symptoms of neurodegenerative disease in IRP2 knockout mice

In mammals, two homologous cytosolic regulatory proteins, iron regulatory protein 1 (also known as IRP1 and Aco1) and iron regulatory protein 2 (also known as IRP2 and Ireb2), sense cytosolic iron levels and posttranscriptionally regulate iron metabolism genes, including transferrin receptor 1 (TfR1) and ferritin H and L subunits, by binding to iron-responsive elements (IREs) within target transcripts. Mice that lack IRP2 develop microcytic anemia and neurodegeneration associated with functional cellular iron depletion caused by low TfR1 and high ferritin expression. IRP1 knockout (IRP1(-/-)) animals do not significantly misregulate iron metabolism, partly because IRP1 is an iron-sulfur protein that functions mainly as a cytosolic aconitase in mammalian tissues and IRP2 activity increases to compensate for loss of the IRE binding form of IRP1. The neurodegenerative disease of IRP2(-/-) animals progresses slowly as the animals age. In this study, we fed IRP2(-/-) mice a diet supplemented with a stable nitroxide, Tempol, and showed that the progression of neuromuscular impairment was markedly attenuated. In cell lines derived from IRP2(-/-) animals, and in the cerebellum, brainstem, and forebrain of animals maintained on the Tempol diet, IRP1 was converted from a cytosolic aconitase to an IRE binding protein that stabilized the TfR1 transcript and repressed ferritin synthesis. We suggest that Tempol protected IRP2(-/-) mice by disassembling the cytosolic iron-sulfur cluster of IRP1 and activating IRE binding activity, which stabilized the TfR1 transcript, repressed ferritin synthesis, and partially restored normal cellular iron homeostasis in the brain.

Iron-sulfur cluster biogenesis and human disease

Iron-sulfur (Fe-S) clusters are essential for numerous biological processes, including mitochondrial respiratory chain activity and various other enzymatic and regulatory functions. Human Fe-S cluster assembly proteins are frequently encoded by single genes, and inherited defects in some of these genes cause disease. Recently, the spectrum of diseases attributable to abnormal Fe-S cluster biogenesis has extended beyond Friedreich ataxia to include a sideroblastic anemia with deficiency of glutaredoxin 5 and a myopathy associated with a deficiency of a Fe-S cluster assembly scaffold protein, ISCU. Mutations within other mammalian Fe-S cluster assembly genes could be causative for human diseases that manifest distinctive combinations of tissue-specific impairments. Thus, defects in the iron-sulfur cluster biogenesis pathway could underlie many human diseases.

Recent advances in the understanding of inherited sideroblastic anaemia

Sideroblastic anaemia includes a heterogeneous group of rare conditions, characterized by decreased haem synthesis and mitochondrial iron overload, which are diagnosed by the presence of ringed sideroblasts in the bone marrow aspirate. The most frequent form is X-linked sideroblastic anaemia, caused by mutations of delta-aminolevulinic acid synthase 2 (ALAS2), the enzyme that catalyses the first and regulatory step of haem synthesis in erythroid precursors and is post-transcriptionally controlled by the iron regulatory proteins. Impaired haem production causes variable degrees of anaemia and mitochondrial iron accumulation as ringed sideroblasts. The heterogeneity and complexity of sideroblastic anaemia is explained by an increasing number of recognized molecular defects. New forms have been recognized as being linked to the deficient function of mitochondrial proteins involved in iron-sulphur cluster biogenesis, such as ABCB7 and GLRX5, which are extremely rare but represent important biological models. Local mitochondrial iron overload is present in all sideroblastic anaemias, whereas systemic iron overload occurs only in the forms because of primary or secondary deficiency of ALAS2.

Iron-independent phosphorylation of iron regulatory protein 2 regulates ferritin during the cell cycle

Iron regulatory protein 2 (IRP2) is a key iron sensor that post-transcriptionally regulates mammalian iron homeostasis by binding to iron-responsive elements (IREs) in mRNAs that encode proteins involved in iron metabolism (e.g. ferritin and transferrin receptor 1). During iron deficiency, IRP2 binds IREs to regulate mRNA translation or stability, whereas during iron sufficiency IRP2 is degraded by the proteasome. Here, we identify an iron-independent IRP2 phosphorylation site that is regulated by the cell cycle. IRP2 Ser-157 is phosphorylated by Cdk1/cyclin B1 during G(2)/M and is dephosphorylated during mitotic exit by the phosphatase Cdc14A. Ser-157 phosphorylation during G(2)/M reduces IRP2 RNA-binding activity and increases ferritin synthesis, whereas Ser-157 dephosphorylation during mitotic exit restores IRP2 RNA-binding activity and represses ferritin synthesis. These data show that reversible phosphorylation of IRP2 during G(2)/M has a role in modulating the iron-independent expression of ferritin and other IRE-containing mRNAs during the cell cycle.

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.

Human iron regulatory protein 2 is easily cleaved in its specific domain: consequences for the haem binding properties of the protein

Mammalian IRPs (iron regulatory proteins), IRP1 and IRP2, are cytosolic RNA-binding proteins that post-transcriptionally control the mRNA of proteins involved in storage, transport, and utilization of iron. In iron-replete cells, IRP2 undergoes degradation by the ubiquitin/proteasome pathway. Binding of haem to a 73aa-Domain (73-amino-acid domain) that is unique in IRP2 has been previously proposed as the initial iron-sensing mechanism. It is shown here that recombinant IRP2 and the 73aa-Domain are sensitive to proteolysis at the same site. NMR results suggest that the isolated 73aa-Domain is not structured. Iron-independent cleavage of IRP2 within the 73aa-Domain also occurs in lung cancer (H1299) cells. Haem interacts with a cysteine residue only in truncated forms of the 73aa-Domain, as shown by a series of complementary physicochemical approaches, including NMR, EPR and UV-visible absorption spectroscopy. In contrast, the cofactor is not ligated by the same residue in the full-length peptide or intact IRP2, although non-specific interaction occurs between these molecular forms and haem. Therefore it is unlikely that the iron-dependent degradation of IRP2 is mediated by haem binding to the intact 73aa-Domain, since the sequence resembling an HRM (haem-regulatory motif) in the 73aa-Domain does not provide an axial ligand of the cofactor unless this domain is cleaved.

Iron regulatory proteins are essential for intestinal function and control key iron absorption molecules in the duodenum

Iron regulatory proteins (IRPs) orchestrate the posttranscriptional regulation of critical iron metabolism proteins at the cellular level. Redundancy between IRP1 and IRP2 associated with embryonic lethality of doubly IRP-deficient mice has precluded the study of IRP function in vivo. Here we use Cre/Lox technology to generate viable organisms lacking IRP expression in a single tissue, the intestine. Mice lacking intestinal IRP expression develop intestinal malabsorption and dehydration postnatally and die within 4 weeks of birth. We demonstrate that IRPs control the expression of divalent metal transporter 1 (DMT1) mRNA and protein, a limiting intestinal iron importer. IRPs are also shown to be critically important to secure physiological levels of the basolateral iron exporter ferroportin. IRPs are thus essential for intestinal function and organismal survival and coordinate the synthesis of key iron metabolism proteins in the duodenum.

Iron-regulatory proteins: molecular biology and pathophysiological implications

Iron is required for key cellular functions, and there is a strong link between iron metabolism and important metabolic processes, such as cell growth, apoptosis and inflammation. Diseases that are directly or indirectly related to iron metabolism represent major health problems. Iron-regulatory proteins (IRPs) 1 and 2 are key controllers of vertebrate iron metabolism and post-transcriptionally regulate expression of the major iron homeostasis genes. Here we discuss how dysregulation of the IRP system can result from both iron-related and unrelated effectors and explain how this can have important pathological consequences in several human disorders.

Gene Dosage Analysis Identifies Large Deletions of the FECH Gene in 10% of Families with Erythropoietic Protoporphyria

Erythropoietic protoporphyria (EPP) is an inherited cutaneous porphyria characterized by partial deficiency of ferrochelatase (FECH), accumulation of protoporphyrin IX in erythrocytes, skin, and liver, and acute photosensitivity. Genetic counseling in EPP requires identification of FECH mutations, but current sequencing-based procedures fail to detect mutations in about one in six families. We have used gene dosage analysis by quantitative PCR to identify large deletions of the FECH gene in 19 (58%) of 33 unrelated UK patients with EPP in whom mutations could not be detected by sequencing. Seven deletions were identified, six of which were previously unreported. Breakpoints were identified for six deletions (c.1-7887-IVS1+2425insTTCA; c.1-9629-IVS1+2437; IVS2-1987-IVS4+352del; c.768-IVS7+244del; IVS7+2784-IVS9+108del; IVS6+2350-TGA+95del). Five breakpoints were in intronic repeat sequences (AluSc, AluSq, AluSx, L1MC4). The remaining deletion (Del Ex3-4) is likely to be a large insertion-deletion. Combining quantitative PCR with routine sequencing increased the sensitivity of mutation detection in 189 unrelated UK patients with EPP from 83% (95% CI: 76-87%) to 93% (CI: 88-96%) (P=0.003). Our findings show that large deletions of the FECH gene are an important cause of EPP. Gene dosage analysis should be incorporated into routine procedures for mutation detection in EPP.