Translational regulation in vivo of the Drosophila melanogaster mRNA encoding succinate dehydrogenase iron protein via iron responsive elements

Shotgun and TMT-Labeled Proteomic Analysis of the Ovarian Proteins of an Insect Vector, Aedes aegypti (Diptera: Culicidae)

Aedes aegypti [Linnaeus in Hasselquist; yellow fever mosquito] transmits several viruses that infect millions of people each year, including Zika, dengue, yellow fever, chikungunya, and West Nile. Pathogen transmission occurs during blood feeding. Only the females blood feed as they require a bloodmeal for oogenesis; in the bloodmeal, holo-transferrin and hemoglobin provide the females with a high iron load. We are interested in the effects of the bloodmeal on the expression of iron-associated proteins in oogenesis. Previous data showed that following digestion of a bloodmeal, ovarian iron concentrations doubles by 72 hr. We have used shotgun proteomics to identify proteins expressed in Ae. aegypti ovaries at two oogenesis developmental stages following blood feeding, and tandem mass tag-labeling proteomics to quantify proteins expressed at one stage following feeding of a controlled iron diet. Our findings provide the first report of mosquito ovarian protein expression in early and late oogenesis. We identify proteins differentially expressed in the two oogenesis development stages. We establish that metal-associated proteins play an important role in Ae. aegypti oogenesis and we identify new candidate proteins that might be involved in mosquito iron metabolism. Finally, this work identified a unique second ferritin light chain subunit, the first reported in any species. The shotgun proteomic data are available via ProteomeXchange with identifier PXD005893, while the tandem mass tag-labeled proteomic data are available with identifier PXD028242.

Conservation in the Iron Responsive Element Family

Iron responsive elements (IREs) are mRNA stem-loop targets for translational control by the two iron regulatory proteins IRP1 and IRP2. They are found in the untranslated regions (UTRs) of genes that code for proteins involved in iron metabolism. There are ten “classic” IRE types that define the conserved secondary and tertiary structure elements necessary for proper IRP binding, and there are 83 published “IRE-like” sequences, most of which depart from the established IRE model. Here are structurally-guided discussions regarding the essential features of an IRE and what is important for IRE family membership.

A versatile toolkit for CRISPR-Cas13-based RNA manipulation in Drosophila

Advances in CRISPR technology have immensely improved our ability to manipulate nucleic acids, and the recent discovery of the RNA-targeting endonuclease Cas13 adds even further functionality. Here, we show that Cas13 works efficiently in Drosophila, both ex vivo and in vivo. We test 44 different Cas13 variants to identify enzymes with the best overall performance and show that Cas13 could target endogenous Drosophila transcripts in vivo with high efficiency and specificity. We also develop Cas13 applications to edit mRNAs and target mitochondrial transcripts. Our vector collection represents a versatile tool collection to manipulate gene expression at the post-transcriptional level.

Methods for Studying Iron Regulatory Protein 1: An Important Protein in Human Iron Metabolism

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) are two cytosolic proteins that maintain cellular iron homeostasis by regulating the expression of genes involved in iron metabolism. IRPs respond to cellular iron deficiency by binding to iron-responsive elements (IREs) found in the mRNAs of iron metabolism transcripts, enhancing iron import, and reducing iron storage, utilization, and export. IRP1, a bifunctional protein, exists in equilibrium between a [Fe₄S₄] cluster containing cytosolic aconitase, and an apoprotein that binds to IREs. At high cellular iron levels, this equilibrium is shifted more toward iron-sulfur cluster containing aconitase, whereas IRP2 undergoes proteasomal degradation by an E3 ubiquitin ligase complex that contains an F-box protein, FBXL5. Irp1^-/- mice develop polycythemia and pulmonary hypertension, whereas Irp2^-/- mice develop microcytic anemia and progressive neurodegeneration, indicating that Irp1 has important functions in the erythropoietic and pulmonary systems, and Irp2 has essential roles in supporting erythropoiesis and nervous system functions. Mice lacking both Irp1 and Irp2 die during embryogenesis, suggesting that functions of Irp1 and Irp2 are redundant. In this review, we will focus on the methods for studying IRP1 activities and function in cells and animals.

Iron Status Is Associated with Performance on Executive Functioning Tasks in Nonanemic Young Women

BACKGROUND

Iron deficiency (ID) is prevalent, particularly among women of reproductive age (WRA). How mild ID without anemia relates to cognition is poorly understood. Executive functioning (EF) has emerged as potentially being affected by mild ID in WRA.

OBJECTIVE

We sought to examine how iron markers relate to performance on EF tasks in nonanemic WRA.

METHODS

Participants included 127 females aged 18-35 y. Hematological indicators included hemoglobin, RBC distribution width, transferrin saturation (TSAT), ferritin, transferrin receptor (TfR), and total body iron (TBI). EF was assessed using 5 tasks. Associations between EF outcomes and iron status were examined using continuous iron predictors and group comparisons.

RESULTS

Better iron status was associated with better attention [faster reaction time (RT) with lower TfR (P = 0.028) and higher TSAT (P = 0.013)], inhibitory control [lower RT variability with higher TSAT (P = 0.042) and planning ability (faster planning time and a smaller planning time increase with increasing difficulty with higher ferritin; P = 0.010)]. No associations with iron status were found for several EF outcomes, possibly due to performance ceilings. Paradoxically, worse performance on a working memory task was related to better iron status, which may reflect hippocampal-frontal interference [lower capacity with lower TfR (P = 0.034) and higher TBI (P = 0.043) and a larger accuracy change with increasing difficulty with higher TBI (P = 0.016)]. Longer RTs on a working memory task were observed among those with positive TBI (iron surplus; P = 0.021) and <2 abnormal iron markers (P = 0.013) compared with those with negative TBI (iron deficit) and ≥2 abnormal markers, respectively.

CONCLUSIONS

These findings suggest cognitive ramifications of mild ID in otherwise healthy WRA and have implications for daily well-being. Future investigators should explore how brain system interactions change according to iron availability.

Iron regulatory proteins and their role in controlling iron metabolism

Cellular iron homeostasis is regulated by post-transcriptional feedback mechanisms, which control the expression of proteins involved in iron uptake, release and storage. Two cytoplasmic proteins with mRNA-binding properties, iron regulatory proteins 1 and 2 (IRP1 and IRP2) play a central role in this regulation. Foremost, IRPs regulate ferritin H and ferritin L translation and thus iron storage, as well as transferrin receptor 1 (TfR1) mRNA stability, thereby adjusting receptor expression and iron uptake via receptor-mediated endocytosis of iron-loaded transferrin. In addition splice variants of iron transporters for import and export at the plasma-membrane, divalent metal transporter 1 (DMT1) and ferroportin are regulated by IRPs. These mechanisms have probably evolved to maintain the cytoplasmic labile iron pool (LIP) at an appropriate level. In certain tissues, the regulation exerted by IRPs influences iron homeostasis and utilization of the entire organism. In intestine, the control of ferritin expression limits intestinal iron absorption and, thus, whole body iron levels. In bone marrow, erythroid heme biosynthesis is coordinated with iron availability through IRP-mediated translational control of erythroid 5-aminolevulinate synthase mRNA. Moreover, the translational control of HIF2α mRNA in kidney by IRP1 coordinates erythropoietin synthesis with iron and oxygen supply. Besides IRPs, body iron absorption is negatively regulated by hepcidin. This peptide hormone, synthesized and secreted by the liver in response to high serum iron, downregulates ferroportin at the protein level and thereby limits iron absorption from the diet. Hepcidin will not be discussed in further detail here.

Involvement of the iron regulatory protein from Eisenia andrei earthworms in the regulation of cellular iron homeostasis

Iron homeostasis in cells is regulated by iron regulatory proteins (IRPs) that exist in different organisms. IRPs are cytosolic proteins that bind to iron-responsive elements (IREs) of the 5′- or 3′-untranslated regions (UTR) of mRNAs that encode many proteins involved in iron metabolism. In this study, we have cloned and described a new regulatory protein belonging to the family of IRPs from the earthworm Eisenia andrei (EaIRP). The earthworm IRE site in 5′-UTR of ferritin mRNA most likely folds into a secondary structure that differs from the conventional IRE structures of ferritin due to the absence of a typically unpaired cytosine that participates in protein binding. Prepared recombinant EaIRP and proteins from mammalian liver extracts are able to bind both mammalian and Eisenia IRE structures of ferritin mRNA, although the affinity of the rEaIRP/Eisenia IRE structure is rather low. This result suggests the possible contribution of a conventional IRE structure. When IRP is supplemented with a Fe-S cluster, it can function as a cytosolic aconitase. Cellular cytosolic and mitochondrial fractions, as well as recombinant EaIRP, exhibit aconitase activity that can be abolished by the action of oxygen radicals. The highest expression of EaIRP was detected in parts of the digestive tract. We can assume that earthworms may possess an IRE/IRP regulatory network as a potential mechanism for maintaining cellular iron homeostasis, although the aconitase function of EaIRP is most likely more relevant.

The physiological functions of iron regulatory proteins in iron homeostasis - an update

Iron regulatory proteins (IRPs) regulate the expression of genes involved in iron metabolism by binding to RNA stem-loop structures known as iron responsive elements (IREs) in target mRNAs. IRP binding inhibits the translation of mRNAs that contain an IRE in the 5′untranslated region of the transcripts, and increases the stability of mRNAs that contain IREs in the 3′untranslated region of transcripts. By these mechanisms, IRPs increase cellular iron absorption and decrease storage and export of iron to maintain an optimal intracellular iron balance. There are two members of the mammalian IRP protein family, IRP1 and IRP2, and they have redundant functions as evidenced by the embryonic lethality of the mice that completely lack IRP expression (Irp1^-/-/Irp2^-/- mice), which contrasts with the fact that Irp1^-/- and Irp2^-/- mice are viable. In addition, Irp2^-/- mice also display neurodegenerative symptoms and microcytic hypochromic anemia, suggesting that IRP2 function predominates in the nervous system and erythropoietic homeostasis. Though the physiological significance of IRP1 had been unclear since Irp1^-/- animals were first assessed in the early 1990s, recent studies indicate that IRP1 plays an essential function in orchestrating the balance between erythropoiesis and bodily iron homeostasis. Additionally, Irp1^-/- mice develop pulmonary hypertension, and they experience sudden death when maintained on an iron-deficient diet, indicating that IRP1 has a critical role in the pulmonary and cardiovascular systems. This review summarizes recent progress that has been made in understanding the physiological roles of IRP1 and IRP2, and further discusses the implications for clinical research on patients with idiopathic polycythemia, pulmonary hypertension, and neurodegeneration.

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.

Mitochondrial ferritin in the substantia nigra in restless legs syndrome

Restless legs syndrome (RLS) is a neurological disorder that is thought to involve decreased iron availability in the brain. Iron is required for oxidative metabolism and plays a critical role in redox reactions in mitochondria. The recent discovery of mitochondrial ferritin (FtMt) provided the opportunity to identify a potential correlation between iron and mitochondrial function in RLS. Human substantia nigra (SN) and putamen autopsy samples from 8 RLS cases and 8 controls were analyzed. Mitochondrial ferritin levels in RLS SN tissue homogenate samples assessed by immunoblots had more FtMt than control samples (p < 0.01), whereas there were no significant differences in FtMt in the putamen samples. By immunohistochemistry, neuromelanin-containing neurons in the SN were the predominant cell type expressing FtMt. Staining in neurons in RLS samples was consistently greater than that in controls. Cytochrome c oxidase staining, which reflects numbers of mitochondria, showed a similar staining pattern to that of FtMt, whereas there was less immunostaining in the RLS cases for cytosolic H-ferritin. These results suggest that increased numbers of mitochondria in neurons in RLS and increased FtMt might contribute to insufficient cytosolic iron levels in RLS SN neurons; they are consistent with the hypothesis that energy insufficiency in these neurons may be involved in the pathogenesis of RLS.

Differential translational regulation of IRE-containing mRNAs in Drosophila melanogaster by endogenous IRP and a constitutive human IRP1 mutant

Insects, like vertebrates, express iron regulatory proteins (IRPs) that may regulate proteins in cellular iron storage and energy metabolism. Two mRNAs, an unspliced form of ferritin H mRNA and succinate dehydrogenase subunit b (SDHb) mRNA, are known to comprise an iron responsive element (IRE) in their 5'-untranslated region making them susceptible to translational repression by IRPs at low iron levels. We have investigated the effect of wild-type human IRP1 (hIRP1) and the constitutively active mutant hIRP1-S437 in transgenic Drosophila melanogaster. Endogenous Drosophila IRE-binding activity was readily detected in gel retardation assays. However, translational repression assessed by polysome gradients was only visible for unspliced IRE-containing ferritin H mRNA, but not for SDHb mRNA. Upon expression of exogenous hIRP1-S437 both mRNAs were strongly repressed. This correlated with a diminished survival rate of adult flies with hIRP1 and complete lethality with hIRP1-S437. We conclude that constitutive IRP1 expression is deleterious to fly survival, probably due to the essential function of SDHb or proteins encoded by yet unidentified target mRNAs.

Evolution of the iron-responsive element

An RNA hairpin structure referred to as the iron-responsive element (IRE) and iron regulatory proteins (IRPs) are key players in the control of iron metabolism in animal cells. They regulate translation initiation or mRNA stability, and the IRE is found in a variety of mRNAs, such as those encoding ferritin, transferrin receptor (Tfr), erythroid aminolevulinic acid synthase (eALAS), mitochondrial aconitase (mACO), ferroportin, and divalent metal transporter 1 (DMT1). We have studied the evolution of the IRE by considering all mRNAs previously known to be associated with this structure and by computationally examining its occurrence in a large variety of eukaryotic organisms. More than 100 novel sequences together with approximately 50 IREs that were previously reported resulted in a comprehensive view of the phylogenetic distribution of this element. A comparison of the different mRNAs shows that the IREs of eALAS and mACO are found in chordates, those of ferroportin and Tfr1 are found in vertebrates, and the IRE of DMT1 is confined to mammals. In contrast, the IRE of ferritin occurs in a majority of metazoa including lower metazoa such as sponges and Nematostella (sea anemone). These findings suggest that the ferritin IRE represents the ancestral version of this type of translational control and that during the evolution of higher animals the IRE structure was adopted by other genes. On the basis of primary sequence comparison between different organisms, we suggest that some of these IREs developed by "convergent evolution" through stepwise changes in sequence, rather than by recombination events.

Of two cytosolic aconitases expressed in Drosophila, only one functions as an iron-regulatory protein

In mammalian cells, iron homeostasis is largely regulated by post-transcriptional control of gene expression through the binding of iron-regulatory proteins (IRP1 and IRP2) to iron-responsive elements (IREs) contained in the untranslated regions of target mRNAs. IRP2 is the dominant iron sensor in mammalian cells under normoxia, but IRP1 is the more ancient protein in evolutionary terms and has an additional function as a cytosolic aconitase. The Caenorhabditis elegans genome does not contain an IRP2 homolog or identifiable IREs; its IRP1 homolog has aconitase activity but does not bind to mammalian IREs. The Drosophila genome offers an evolutionary intermediate containing two IRP1-like proteins (IRP-1A and IRP-1B) and target genes with IREs. Here, we used purified recombinant IRP-1A and IRP-1B from Drosophila melanogaster and showed that only IRP-1A can bind to IREs, although both proteins possess aconitase activity. These results were also corroborated in whole-fly homogenates from transgenic flies that overexpress IRP-1A and IRP-1B in their fat bodies. Ubiquitous and muscle-specific overexpression of IRP-1A, but not of IRP-1B, resulted in pre-adult lethality, underscoring the importance of the biochemical difference between the two proteins. Domain-swap experiments showed that multiple amino acid substitutions scattered throughout the IRP1 domains are synergistically required for conferring IRE binding activity. Our data suggest that as a first step during the evolution of the IRP/IRE system, the ancient cytosolic aconitase was duplicated in insects with one variant acquiring IRE-specific binding.

Secreted ferritin: mosquito defense against iron overload?

The yellow fever mosquito, Aedes aegypti, must blood feed in order to complete her life cycle. The blood meal provides a high level of iron that is required for egg development. We are interested in developing control strategies that interfere with this process. We show that A. aegypti larval cells synthesize and secrete ferritin in response to iron exposure. Cytoplasmic ferritin is maximal at low levels of iron, consists of both the light chain (LCH) and heavy chain (HCH) subunits and reflects cytoplasmic iron levels. Secreted ferritin increases in direct linear relationship to iron dose and consists primarily of HCH subunits. Although the messages for both subunits increase with iron treatment, our data indicate that mosquito HCH synthesis could be partially controlled at the translational level as well. Importantly, we show that exposure of mosquito cells to iron at low concentrations increases cytoplasmic iron, while higher iron levels results in a decline in cytoplasmic iron levels indicating that excess iron is removed from mosquito cells. Our work indicates that HCH synthesis and ferritin secretion are key factors in the response of mosquito cells to iron exposure and could be the primary mechanisms that allow these insects to defend against an intracellular iron overload.

Secretion of ferritin by iron-laden macrophages and influence of lipoproteins

Increasing evidence supports a role of cellular iron in the initiation and development of atherosclerosis. We and others reported earlier that iron-laden macrophages are associated with LDL oxidation, angiogenesis, nitric oxide production and apoptosis in atherosclerotic processes. Here we have further studied perturbed iron metabolism in macrophages, their interaction with lipoproteins and the origin of iron accumulation in human atheroma. In both early and advanced human atheroma lesions, hemoglobin and ferritin accumulation correlated with the macrophage-rich areas. Iron uptake into macrophages, via transferrin receptors or scavenger receptor-mediated erythrophagocytosis, increased cellular iron and accelerated ferritin synthesis at both mRNA and protein levels. The binding activity of iron regulatory proteins was enhanced by desferrioxamine (DFO) and decreased by hemin and iron compounds. Iron-laden macrophages exocytosed both iron and ferritin into the culture medium. Exposure to oxidized low-density lipoprotein (oxLDL, >or=50 microg/mL) resulted in <20% apoptosis of iron-laden human macrophages, but cells remained impermeable after a 24 h period and an increased excretion of ferritin could be observed by immunostaining techniques. Exposure to high-density lipoprotein (HDL) significantly decreased ferritin excretion from these cells. We conclude: (i) erythrophagocytosis and hemoglobin catabolism by macrophages contribute to ferritin accumulation in human atherosclerotic lesions and; (ii) iron uptake into macrophages leads to increased synthesis and secretion of ferritin; (iii) oxidized LDL and HDL have different effects on these processes.

The regulatory effect of heme on erythroid aminolevulinate synthase in natural erythroid cells

A major enzymatic pathway in erythroid cells is the eight-step formation of heme, starting with the erythroid isoform of aminolevulinate synthase (eALAS). We studied the regulation of eALAS synthesis by heme in natural erythroid cells. Erythroid cells from mouse blood or bone marrow were incubated with different concentrations of heme and labelled with [35S]methionine. This was followed by immunoprecipitation of eALAS proteins. Northern blot analysis was done on mRNA isolated from bone marrow. Incubation with heme (5-100 muM) was shown to clearly inhibit eALAS synthesis in erythroid cells of bone marrow. This inhibitory effect of heme could also be observed in peripheral blood cells at higher concentrations while the preform of eALAS was rather increased. However, at lower concentrations of heme (1-10 microM), eALAS synthesis increased. Northern blot studies argued the inhibitory effect was at the posttranscriptional level. Our results suggest that the net effect of murine eALAS regulation by heme varies with the degree of erythroid differentiation. Heme formation seems to be more tightly controlled in the bone marrow (nucleated) cells in order to prevent oxidative cell damage, compared to more differentiated erythroid cells.

Complex II defect via down-regulation of iron-sulfur subunit induces mitochondrial dysfunction and cell cycle delay in iron chelation-induced senescence-associated growth arrest

Mitochondria play a pivotal role as an ATP generator in aerobically growing cells, and their defects have long been implicated in the cellular aging process, although its detailed underlying mechanisms remain unclear. Recently, we found that, in the cellular senescent process of Chang cells induced by desferroxamine mesylate, an iron chelator, a significant decrease of intracellular ATP level was accompanied by decline in complex II activity, which preceded acquisition of the senescent phenotype. In the present study, we investigated the mechanism of how the mitochondrial ATP productivity was damaged by iron chelation and how complex II defect was involved in the senescent arrest. The ATP loss was irreversible and accompanied by sustained collapse of mitochondrial membrane potential (Delta psi m), but the ATP loss itself did not seem to be essential in progression to the senescent arrest. The Delta psi m disruption was due to decreased mitochondrial respiration, which was primarily associated with the defective complex II activity. Furthermore, we found that the declined activity of complex II was mainly due to down-regulation of protein expression of the iron-sulfur subunit, which was associated with the irreversibility of the arrest. Finally, we demonstrated that specific inhibition of complex II with 2-thenoyltrifluoroacetone induced overall delay of the cell cycle, suggesting that the delayed arrest by desferroxamine mesylate might be in part due to inhibition of complex II activity. Taken together, our results suggest that complex II might be considered as one of the primary factors to regulate mitochondrial respiratory function by responding to the cellular iron level, thereby influencing cellular growth.

Cloning and molecular characterization of two mosquito iron regulatory proteins

Iron regulatory proteins (IRPs) control the synthesis of various proteins at the translational level by binding to iron responsive elements (IREs) in the mRNAs. Iron, infection, and stress can alter IRP/IRE binding activity. Insect messenger RNAs for ferritin and succinate dehydrogenase subunit b have IREs that are active translational control sites. We have cloned and sequenced cDNAs encoding proteins from the IRP1 family for the mosquitoes, Aedes aegypti and Anopheles gambiae. Both deduced amino acid sequences show substantial similarity to human IRP1 and Drosophila IRP1A and IRP1B, and all of the residues thought to be involved in aconitase activity and iron-sulfur cluster formation are conserved. Recombinant A. aegypti IRP1 binds to transcripts of the IREs of mosquito or human ferritin subunit mRNAs. No significant change in A. gambiae IRP1 messenger RNA could be detected during the various developmental stages of the life cycle, following iron loading by blood feeding, or after bacterial or parasitic infections. These data suggest that there is no change in gene transcription. Furthermore, bacterial challenge of A. gambiae cells did not change IRP1 protein levels. In contrast, IRP1 binding activity for the IRE was elevated following immune induction. These data show that changes in IRP1/IRE binding activity occur as part of the insect immune response.

Iron metabolism in insects

Like other organisms, insects must balance two properties of ionic iron, that of an essential nutrient and a potent toxin. Iron must be acquired to provide catalysis for oxidative metabolism, but it must be controlled to avoid destructive oxidative reactions. Insects have evolved distinctive forms of the serum iron transport protein, transferrin, and the storage protein, ferritin. These proteins may serve different functions in insects than they do in other organisms. A form of translational control of protein synthesis by iron in insects is similar to that of vertebrates. The Drosophila melanogaster genome contains many genes that may encode other proteins involved in iron metabolism.

Repression of Manduca sexta ferritin synthesis by IRP1/IRE interaction

Mammalian ferritin subunit synthesis is controlled at the translational level by the iron regulatory protein 1 (IRP1)/iron responsive element (IRE) interaction. Insect haemolymph ferritin subunit messages have an IRE in the 5'-untranslated region (UTR). We have shown that recombinant M. sexta IRP1 represses the in vitro translation of both the heavy and light chain ferritin subunits from this species without altering transcription. Deletion of either the 5'-UTR or the IRE from the mRNA abolishes IRP1 repression. Our studies indicated that the translational control of ferritin synthesis by IRP/IRE interaction could occur in insects in a manner similar to that of mammals. To our knowledge, this is the first report of the control of insect ferritin synthesis by IRP1/IRE interaction. Furthermore, this is the first indication that the synthesis of a secreted ferritin subunit can also be controlled in this manner.

Manduca sexta IRP1: molecular characterization and in vivo response to iron

Manduca sexta IRP1 was cloned and sequenced. The deduced amino acid sequence of Manduca IRP1 shows high similarity to other IRP1 proteins. The Cys residues required as ligands for the iron sulfur cluster, as well as all residues necessary for aconitase activity are conserved in the insect protein. Purified recombinant Manduca IRP1 binds specifically to transcripts of the iron responsive element (IRE) of Manduca or human ferritin subunit mRNA. Binding activity of the recombinant protein was not influenced by the presence of beta-mercaptoethanol. However, IRP/IRE binding activity of cytoplasmic extracts from fat body was decreased by reducing agents in a dose-responsive manner. Fat body IRP1/IRE binding activity was reduced for Manduca sexta larvae injected with low doses of iron, while IRP1 mRNA and protein levels remained stable. At higher iron doses, binding activity increased and stabilized. Hemolymph ferritin levels showed an inverse relationship to IRP1/IRE binding activity. These data suggest that the Manduca IRP1 is likely involved in translational control of ferritin synthesis in a manner similar to that found in vertebrates. However, factors other than iron can influence IRP/IRE interaction and hemolymph ferritin levels in insects.

A novel endoproteolytic processing activity in mitochondria of erythroid cells and the role in heme synthesis

The erythroid isoform of aminolevulinate synthase (eALAS) protein is a major control point in erythroid heme synthesis and hemoglobin formation. Erythroid cells were extracted from mouse blood and bone marrow and metabolically labeled with 35S-methionine. This was followed by immunoprecipitation of eALAS protein products. The results show that the N-terminus of the expected full-length 59-kd form of the eALAS protein is truncated in bone marrow erythroid cells by approximately 7 kd. More differentiated erythroid cells in the peripheral blood exhibit very little of this protein truncation. Erythroid cells from the bone marrow were isolated using monoclonal antibody TER-119 and were shown to contain a unique endoprotease activity that could cleave the eALAS protein to the shorter form in vitro. With or without the mitochondrial signal sequence, the eALAS protein could serve as a substrate for the cleavage. This cleavage renders a functional eALAS protein and only removes a domain of unclear function, which has previously been reported to vary in size as a result of alternative RNA splicing. The protease activity was enriched from the membranes of mitochondria from bone marrow cells and was shown to be different from mitochondrial processing peptidase, medullasin, and other known proteases. Apart from the mitochondrial processing peptidase that cleaves the import signal sequence, this is the first description of a mitochondrially located site-specific processing protease activity.

A novel endoproteolytic processing activity in mitochondria of erythroid cells and the role in heme synthesis

The erythroid isoform of aminolevulinate synthase (eALAS) protein is a major control point in erythroid heme synthesis and hemoglobin formation. Erythroid cells were extracted from mouse blood and bone marrow and metabolically labeled with 35S-methionine. This was followed by immunoprecipitation of eALAS protein products. The results show that the N-terminus of the expected full-length 59-kd form of the eALAS protein is truncated in bone marrow erythroid cells by approximately 7 kd. More differentiated erythroid cells in the peripheral blood exhibit very little of this protein truncation. Erythroid cells from the bone marrow were isolated using monoclonal antibody TER-119 and were shown to contain a unique endoprotease activity that could cleave the eALAS protein to the shorter form in vitro. With or without the mitochondrial signal sequence, the eALAS protein could serve as a substrate for the cleavage. This cleavage renders a functional eALAS protein and only removes a domain of unclear function, which has previously been reported to vary in size as a result of alternative RNA splicing. The protease activity was enriched from the membranes of mitochondria from bone marrow cells and was shown to be different from mitochondrial processing peptidase, medullasin, and other known proteases. Apart from the mitochondrial processing peptidase that cleaves the import signal sequence, this is the first description of a mitochondrially located site-specific processing protease activity.

Iron-dependent changes in cellular energy metabolism: influence on citric acid cycle and oxidative phosphorylation

Iron modulates the expression of the critical citric acid cycle enzyme aconitase via a translational mechanism involving iron regulatory proteins. Thus, the present study was undertaken to investigate the consequences of iron perturbation on citric acid cycle activity, oxidative phosphorylation and mitochondrial respiration in the human cell line K-562. In agreement with previous data iron increases the activity of mitochondrial aconitase while it is reduced upon addition of the iron chelator desferrioxamine (DFO). Interestingly, iron also positively affects three other citric acid cycle enzymes, namely citrate synthase, isocitric dehydrogenase, and succinate dehydrogenase, while DFO decreases the activity of these enzymes. Consequently, iron supplementation results in increased formation of reducing equivalents (NADH) by the citric acid cycle, and thus in increased mitochondrial oxygen consumption and ATP formation via oxidative phosphorylation as shown herein. This in turn leads to downregulation of glucose utilization. In contrast, all these metabolic pathways are reduced upon iron depletion, and thus glycolysis and lactate formation are significantly increased in order to compensate for the decrease in ATP production via oxidative phosphorylation in the presence of DFO. Our results point to a complex interaction between iron homeostasis, oxygen supply and cellular energy metabolism in human cells.

Regulation of genes of iron metabolism by the iron-response proteins

Iron is an essential nutrient, yet excess iron can be toxic to cells. The uptake of iron by mammalian cells is post-transcriptionally regulated by the interaction of iron-response proteins (IRP1 and IRP2) with iron-response elements (IREs) found in the mRNAs of genes of iron metabolism, such as ferritin, the transferrin receptor, erythroid aminolevulinic acid synthase, and mitochondrial aconitase. The IRPs are RNA binding proteins that bind to the IRE (found in the mRNAs of the regulated genes) in an iron- dependent manner. Binding of IRPs to the IREs leads to changes in the expression of the regulated genes and subsequent changes in the uptake, utilization, or storage of intracellular iron. Recent work has demonstrated that the binding of the IRPs to the IREs can also be modulated by changes in the redox state or oxidative stress level of the cell. These findings provide an important link between iron metabolism and states of oxidative stress.

Comparison of the beluga whale (Delphinapterus leucas) expressed genes for 5-aminolevulinate synthase with those in other vertebrates

The cDNA and inferred amino acid sequences were determined for beluga whale (Delphinapterus leucas) erythroid (E) and housekeeping (H) forms of 5-aminolevulinate synthase (ALS), and they were compared with known sequences for five other vertebrates with particular attention to regulatory features. The cDNAs for whale ALS-E and -H encode, respectively, proteins of 582 and 640 amino acids. Sequence alignments suggest that the whale ALS-H, like those for rat and chicken, has an N-terminal mitochondrial targeting sequence of 56 amino acids. There is a high degree of amino acid conservation between the beluga whale proteins and those of other vertebrates, including regulatory elements and functional residues that have been defined in other ALSs. Both whale proteins contain three heme regulatory motifs suggesting that mitochondrial uptake may be regulated by heme. The ALS-E mRNA contains an iron responsive element in its 5'-untranslated region indicating that its expression may be post-transcriptionally regulated by cellular iron. This extensive structural similarity and the presence of the same regulatory elements found in other ALSs indicate that regulation of ALS in beluga whale is similar to that in other vertebrates.

Molecular cloning of mouse glycolate oxidase. High evolutionary conservation and presence of an iron-responsive element-like sequence in the mRNA

Iron regulatory proteins (IRPs) control the synthesis of several proteins in iron metabolism by binding to iron-responsive elements (IREs), a hairpin structure in the untranslated region (UTR) of corresponding mRNAs. Binding of IRPs to IREs in the 5' UTR inhibits translation of ferritin heavy and light chain, erythroid aminolevulinic acid synthase, mitochondrial aconitase, and Drosophila succinate dehydrogenase b, whereas IRP binding to IREs in the 3' UTR of transferrin receptor mRNA prolongs mRNA half-life. To identify new targets of IRPs, we devised a method to enrich IRE-containing mRNAs by using recombinant IRP-1 as an affinity matrix. A cDNA library established from enriched mRNA was screened by an RNA-protein band shift assay. This revealed a novel IRE-like sequence in the 3' UTR of a liver-specific mouse mRNA. The newly identified cDNA codes for a protein with high homology to plant glycolate oxidase (GOX). Recombinant protein expressed in bacteria displayed enzymatic GOX activity. Therefore, this cDNA represents the first vertebrate GOX homologue. The IRE-like sequence in mouse GOX exhibited strong binding to IRPs at room temperature. However, it differs from functional IREs by a mismatch in the middle of its upper stem and did not confer iron-dependent regulation in cells.

Control of translation initiation in animals

Regulation of translation initiation is a central control point in animal cells. We review our current understanding of the mechanisms of regulation, drawing particularly on examples in which the biological consequences of the regulation are clear. Specific mRNAs can be controlled via sequences in their 5' and 3' untranslated regions (UTRs) and by alterations in the translation machinery. The 5'UTR sequence can determine which initiation pathway is used to bring the ribosome to the initiation codon, how efficiently initiation occurs, and which initiation site is selected. 5'UTR-mediated control can also be accomplished via sequence-specific mRNA-binding proteins. Sequences in the 3' untranslated region and the poly(A) tail can have dramatic effects on initiation frequency, with particularly profound effects in oogenesis and early development. The mechanism by which 3'UTRs and poly(A) regulate initiation may involve contacts between proteins bound to these regions and the basal translation apparatus. mRNA localization signals in the 3'UTR can also dramatically influence translational activation and repression. Modulations of the initiation machinery, including phosphorylation of initiation factors and their regulated association with other proteins, can regulate both specific mRNAs and overall translation rates and thereby affect cell growth and phenotype.

An atypical iron-responsive element (IRE) within crayfish ferritin mRNA and an iron regulatory protein 1 (IRP1)-like protein from crayfish hepatopancreas

A putative crayfish iron-responsive element (IRE) is present in the 5'-untranslated region of the crayfish ferritin mRNA. The putative crayfish IRE is in a cap-proximal position and shares most of the structural features of the consensus IRE, but the RNA stem-loop structure contains a bulge of a guanine instead of a cytosine at the expected position, so far thought to be a hallmark of IREs. By using an electromobility shift assay this IRE was shown to specifically bind purified recombinant human iron regulatory protein 1 (IRP1) as well as a factor(s) present in a homogenate of crayfish hepatopancreas, likely to be a crayfish IRP1 homologue. With mutations in the crayfish IRE, the affinity of IRP to IRE was drastically decreased. A cDNA encoding an IRP1-like protein was cloned from the hepatopancreas of crayfish. This protein has sequence similarities to IRP, and contains all the active-site residues of aconitase, two putative RNA-binding regions and a putative contact site between RNA and IRP. These results show that a crayfish IRE, lacking the bulged C, can bind IRP1 in vitro and that an IRP1-like protein present in crayfish hepatopancreas may have both aconitase and RNA-binding activities.

Drosophila ferritin mRNA: alternative RNA splicing regulates the presence of the iron-responsive element

Several mRNAs encoding the same ferritin subunit of Drosophila melanogaster were identified. Alternative RNA splicing and utilisation of different polyadenylation sites were found to generate the transcripts. The alternative RNA splicing results in ferritin transcripts with four unique 5' untranslated regions. Only one of them contains an iron-responsive element. The iron-responsive element was found to bind in vitro specifically to human recombinant iron regulatory protein 1. Furthermore, the ferritin subunit mRNAs are differentially expressed during development. Our data provides the first molecular evidence that the presence of iron-responsive element in a ferritin mRNA is regulated by alternative RNA splicing.

Molecular genetics of succinate:quinone oxidoreductase in eukaryotes

Succinate:quinone oxidoreductase is a membrane-associated complex in mitochondria, often referred to as complex II, based on the fractionation scheme developed by Y. Hatefi and colleagues. It consists of four peptides, two of which are integral membrane proteins (15 and 12-13 kDa, respectively) and two others that are peripheral membrane proteins, i.e., a flavoprotein (Fp, 70 kDa) and an iron-protein (Ip, 27 kDa). The mature, functional complex contains a cytochrome in association with the membrane proteins, a flavin linked covalently to the largest peptide, and three iron-sulfur clusters in the 27-kDa subunit. The present review touches only briefly on the biochemical and biophysical properties of this complex. Instead, the focus is on the molecular-genetic studies that have become possible since the first genes from eukaryotes were cloned in 1989. The evolutionary conservation of the amino acid sequence of both the Fp and the Ip peptides has facilitated the cloning of these genes from a large variety of eukaryotic organisms by PCR-based methods. The review addresses questions related to the regulation of the expression of these genes, with an emphasis on mammals and yeast, for which most of the information is available. Four different genes have to be co-ordinately regulated. Transcriptional as well as posttranscriptional regulatory mechanisms have been observed in diverse organisms. Intriguing observations have been made in studies of this enzyme during the life cycle of organisms existing alternately under aerobic and anaerobic conditions. Naturally occurring or induced mutations in these genes have shed light on several questions related to the assembly of this complex, and on the relationship between structure and function. Four different peptides are imported into the mitochondria. They have to be modified, folded, and assembled. The stage is set for the exploration of highly specific changes introduced by site-directed mutagenesis. Until recently the genes were believed to be exclusively nuclear in all eukaryotes, but exceptions have since been found. This finding has relevance in the discussion of the evolution of mitochondria from prokaryotes. A highly conserved set of genes is found in prokaryotes, and some informative comparisons on gene organization and expression in prokaryotes and eukaryotes have been included.

Iron and gene expression: molecular mechanisms regulating cellular iron homeostasis

In recent years, specific post-transcriptional mechanisms in the cytoplasm of vertebrate cells have been elucidated that directly affect the stability and translation of mRNAs coding for central proteins in iron metabolism. This review shall focus primarily on these mechanisms. Other levels of control, either affecting gene transcription and/ or related to the function of iron-capturing substances and transmembrane transport, are also likely to exist and to influence the iron balance and utilization. They are, however, much less clear.

Translational regulation of mRNAs with distinct IRE sequences by iron regulatory proteins 1 and 2

Iron regulatory proteins 1 and 2 (IRP-1, IRP-2) interact with iron-responsive elements (IREs) present in the 5'- or 3'-untranslated regions (UTR) of several mRNAs coding for proteins in iron metabolism. Whereas binding of IRP-1 and -2 to an IRE in the 5'-UTR inhibits mRNA translation in vitro, it has remained unknown whether either endogenous protein is sufficient to control translation in mammalian cells. We analyzed this question by taking advantage of published mutant IREs that are exclusively recognized by either IRP-1 or IRP-2 in vitro. These IREs were inserted into the 5'-UTR of a human growth hormone reporter mRNA, and translational regulation was measured in stably transfected mouse L cells. Cells cultured in iron-rich or -depleted medium were labeled with [35S]methionine, and secreted growth hormone was immunoprecipitated. IREs with loop sequence specific for IRP-1 (UAGUAC), IRP-2 (CCGAGC), or both proteins (GAGUCG and the wild-type CAGUGC sequence) all mediated translational regulation, in contrast to a control sequence (GCUCCG) that binds neither IRP-1 nor IRP-2. Control experiments excluded IRP-1 binding to the IRP-2-specific sequence in vivo. The present data demonstrate that IRP-1 and IRP-2 can independently function as translational repressors in living cells.

Redox modulation of iron regulatory proteins by peroxynitrite

Expression of several proteins of higher eukaryotes is post-transcriptionally regulated by interaction of iron-responsive elements (IREs) on their mRNAs and iron regulatory proteins (IRP1 and IRP2). IRP1 is a redox-sensitive iron-sulfur protein whose regulatory activity is modulated by iron depletion, synthesis of nitric oxide, or oxidative stress. IRP2 is closely related to IRP1, but it does not possess a [Fe-S] cluster. IRP2 is also regulated by intracellular iron level, but it is assumed that regulation is achieved by accelerated turn-over. In this report, the effect of peroxynitrite, a strong oxidant produced when nitric oxide and O-2 are biosynthesized simultaneously, on the RNA binding activity of IRP1 and IRP2 was investigated in vitro. Macrophage cytosolic extracts were exposed directly to a bolus addition of peroxynitrite or to SIN-1, which releases a continuous flux of peroxynitrite. Under these two experimental conditions, IRP1 lost its aconitase activity but did not gain increased capacity to bind IRE. However, addition of low amounts of the disulfide-reducing agent 2-ME during the binding assay revealed formation of a complex between IRP1 and IRE. Substrates of aconitase, which bind to the cluster of IRP1, prevented this effect, pointing to the [Fe-S] cluster as the target of peroxynitrite. Moreover, single mutation of the redox active Cys437 precluded oxidation of human recombinant IRP1 by SIN-1. Collectively, these results imply that peroxynitrite predisposes IRP1 to bind IREs under a suitable reducing environment. It is assumed that in addition to disrupting the cluster peroxynitrite also promotes disulfide bridge(s) between proximal cysteine residues in the vicinity of the IRE-binding domain, in particular Cys437. When exposed to peroxynitrite, IRP2 lost its spontaneous IRE binding activity, which was restored by further exposure to 2-mercaptoethanol, thus showing that peroxynitrite can also regulate IRP2 by a post-translational event.

Isolation and properties of Drosophila melanogaster ferritin--molecular cloning of a cDNA that encodes one subunit, and localization of the gene on the third chromosome

Ferritin was purified from iron-fed Drosophila melanogaster extracts by centrifugation in a gradient of potassium bromide. On polyacrylamide gel electrophoresis, the product showed two protein bands corresponding to the ferritin monomer and dimer. Electrophoresis following dissociation with SDS and 2-mercaptoethanol revealed three strong bands of approximately 25, 26, and 28 kDa. N-terminal amino acid sequences were identical for the 25-kDa and 26-kDa subunits, but different for the 28-kDa subunit. Conserved ferritin PCR primers were used to amplify a 360-bp cDNA product, which was used to isolate a clone from a D. melanogaster cDNA library that contained the complete coding sequence for a ferritin subunit. Additional 5' sequence obtained by the RACE method revealed the presence of a putative iron regulatory element. The PCR product was also used to locate the position of the ferritin subunit gene at region 99F on the right arm of the third chromosome. The deduced amino acid sequence of the D. melanogaster ferritin subunit contained a signal sequence and resembled most closely ferritin of the mosquito Aedes aegypti. The evolution of ferritin sequences is discussed.

Comparative nutrition of iron and copper

The suggestion from nutritional studies with mammals of a link between iron and copper metabolism has been reinforced by recent investigations with yeast cells. Iron must be in the reduced ferrous (FeII) state for uptake by yeast cells, and reoxidation to ferric (FeIII) by a copper oxidase is part of the transport process. Thus, yeast cells deficient in copper are unable to absorb iron. In an analogous way, animals deficient in copper appear to be unable to move FeII out of cells, probably because it cannot be oxidized to FeIII. Invertebrate animals use copper and iron in ways very similar to vertebrates, with some notable exceptions. In the cases where vertebrates and invertebrates are similar, the latter may be useful models for vertebrate metabolism. In cases where they differ (e.g. predominance of serum ferritin in insects, oxygen transport by a copper protein in many arthropods, central importance of phenoloxidase, a copper enzyme in arthropods), the differences may represent processes that are exaggerated in invertebrates and thus more amenable to study in these organisms. On the other hand, they may represent processes unique to invertebrates, thus providing novel information on species diversity.

Iron regulatory proteins 1 and 2

Iron uptake and storage in mammalian cells is at least partly regulated at a post-transcriptional level by the iron regulatory proteins (IRP-1 and IRP-2). These cytoplasmic regulators share 79% similarity in protein sequence and bind tightly to conserved mRNA stem-loops, named iron-responsive elements (IREs). The IRP:IRE interaction underlies the regulation of translation and stability of several mRNAs central to iron metabolism. The question of why the cell requires two such closely related regulatory proteins may be resolved as we learn more about the expression and regulation of these proteins. It is evident so far that, despite similarities, the IRPs differ in several important respects. They are coordinately regulated by cellular iron, but whereas IRP-1 is inactivated by high iron levels, IRP-2 is rapidly degraded. Further differences arise in their expression and RNA-binding specificity. The two proteins each recognise a large repertoire of IRE-like sequences, including a small group of exclusive RNA targets. These findings hint that IRP-1 and IRP-2 may bind preferentially to certain mRNAs in vivo, possibly extending their known functions beyond the regulation of intracellular iron homeostasis.