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

Two ferritin genes (MdFerH and MdFerL) are involved in iron homeostasis, antioxidation and immune defense in housefly Musca domestica

Ferritin is a ubiquitous multi-subunit iron storage protein, made up of heavy chain and light chain subunits. In recent years, invertebrate ferritins have emerged as an important, yet largely underappreciated, component of host defense and antioxidant system. Here, two alternatively spliced transcripts encoding for a unique ferritin heavy chain homolog (MdFerH), and a transcript encoding for a light chain homolog (MdFerL) are cloned and characterized from Musca domestica. Comparing with MdFerH1, a fragment is absent at the 5' untranslated region of MdFerH2, where a putative iron response element is present. Amino acid sequence analysis shows that MdFerH possesses a strictly conserved ferroxidase site, while MdFerL has a putative atypical active center. Tissue distribution analysis indicates that MdFers are enriched expressed in gut. When the larvae receive diverse stimulations, including challenge by bacteria, exposure to excess Fe²⁺, doxorubicin or ultraviolet, the expression of MdFers is positively up-regulated in different degrees and different temporal patterns, indicating their potential roles in oxidative stress. The two mRNA isoforms of MdFerH appear to be differentially expressed in different tissues, but seem to show the similar expression patterns under diverse stress conditions. Further investigation reveals that silencing MdFers can alter the redox homeostasis, leading elevated mortalities of larvae following bacterial infection. Inspiringly, recombinant MdFerL produced in Pichia pastoris shows significant iron-chelating activity in vitro. These results suggest a pivotal role of ferritins from housefly in iron homeostasis, antibacterial immunity and redox balance.

Impact of Autophagy and Aging on Iron Load and Ferritin in Drosophila Brain

Biometals such as iron, copper, potassium, and zinc are essential regulatory elements of several biological processes. The homeostasis of biometals is often affected in age-related pathologies. Notably, impaired iron metabolism has been linked to several neurodegenerative disorders. Autophagy, an intracellular degradative process dependent on the lysosomes, is involved in the regulation of ferritin and iron levels. Impaired autophagy has been associated with normal pathological aging, and neurodegeneration. Non-mammalian model organisms such as Drosophila have proven to be appropriate for the investigation of age-related pathologies. Here, we show that ferritin is expressed in adult Drosophila brain and that iron and holoferritin accumulate with aging. At whole-brain level we found no direct relationship between the accumulation of holoferritin and a deficit in autophagy in aged Drosophila brain. However, synchrotron X-ray spectromicroscopy revealed an additional spectral feature in the iron-richest region of autophagy-deficient fly brains, consistent with iron-sulfur. This potentially arises from iron-sulfur clusters associated with altered mitochondrial iron homeostasis.

The response of ferritin to LPS and acute phase of Pseudomonas infection

Plasma ferritin is an important extracellular iron storage molecule, whose concentration increases drastically in cancer and infection. During infection, the pathogen usurps host iron for its survival and pathogenicity; hence, maintenance of the plasma ferritin level during infection is a crucial host defence mechanism. In this study, the horseshoe crab plasma ferritin complex was purified, characterized, and its involvement in innate immune defence was investigated. The plasma ferritin appears as a 21-kDa subunit on SDS-PAGE. Full-length ferritin-H cDNAs (CrFer-H1 and CrFer-H2) were cloned. Analysis of the 5′ UTR indicates the existence of a functional iron-response element, suggesting that both the CrFer-H genes may be post-transcriptionally regulated. Northern analysis shows that the CrFer-H is ubiquitously expressed. Within 3 h of lipopolysaccharide challenge, the gene is up-regulated by > 12-fold. In contrast, iron-loading did not result in any significant change. When challenged with Pseudomonas aeruginosa , the plasma ferritin disappeared between 6—48 h and re-appeared thereafter, suggesting that during infection, ferritin may be concealed intracellularly as it withholds iron from the invading pathogen. Taken together, these results provide insights into the importance of plasma ferritin as an evolutionarily conserved molecule for the iron-withholding strategy of innate immunity.

Identification and characterization of a Macrobrachium nipponense ferritin subunit regulated by iron ion and pathogen challenge

Ferritin, a major iron storage protein in most living organisms, plays a crucial role in iron metabolism. In this study, the ferritin subunit MnFer was identified in the oriental river prawn (Macrobrachium nipponense) and functionally characterized. The full-length cDNA of MnFer is 999 bp in size with a 122-bp 5'-untranslated region (UTR), a 364-bp 3'-UTR and a 513-bp open reading frame that encodes a protein possessing 171 amino acids and a deduced molecular weight of 19.40 kDa. Prawn ferritin transcripts are expressed in muscle, heart, hepatopancreas, gill, hemocytes, ovary and testis. Quantitative real-time PCR revealed that the abundance of ferritin transcript was highest in the hepatopancreas followed by muscle. Ferritin transcript expression in muscle increased six-fold 3 h after the injection of iron. In the gill, a four-fold increase in ferritin transcript expression was detected 3 h post-injection; the expression remained elevated for 48 h. Heart ferritin mRNA expression increased up to seven-fold at 24 h post-injection. No significant difference was found in the hepatopancreas. The iron binding capacity of recombinant ferritin protein was also demonstrated in this study. In hemocyte experiments, the transcriptional expression of MnFer showed the strongest response to Aeromonas hydrophila. As a whole, our study suggested that the ferritin from M. nipponense may play critical roles in cellular and organismic iron homeostasis along with in innate immune defense.

Identification and functional characterization of a novel ferritin subunit from the tropical sea cucumber, Stichopus monotuberculatus

Ferritin is one of the major non-harm iron storage proteins that found in most cell types of animals, plants and microorganisms. In this study, a ferritin subunit named StmFer was identified from the sea cucumber (Stichopus monotuberculatus) and characterized functionally. The full-length cDNA of StmFer is 1184 bp in size with a 5'-untranslated region (UTR) of 131 bp, a 3'-UTR of 531 bp and an open reading frame of 522 bp that encoding a protein of 173 amino acids with a deduced molecular weight of 19.95 kDa. StmFer possesses both the ferroxidase center of vertebrate ferritin heavy subunit and iron nucleation sites of vertebrate ferritin light subunit. For the gene structure, StmFer contains only three exons separated by two introns. Higher levels of mRNA expression were noticed in intestine and coelomocyte of S. monotuberculatus by northern blot analysis. In in vitro experiments performed in coelomocytes, transcriptional expression of StmFer showed the strongest response to polyriboinosinic polyribocytidylic acid [Poly (I:C)] (9.08 fold up-regulation), followed by lipopolysaccharides (LPS), ferrous chloride (FeCl2) and inactivated bacteria (Vibrio alginolyticus) (7.84, 7.41 and 4.90 fold up-regulation, respectively) after 3 h post-challenge. In addition, the anti-oxidation activity and iron binding capacity of recombinant ferritin protein were demonstrated in this study. As a whole, our study suggested that the ferritin from sea cucumber may play critical roles not only in the cellular and organismic iron homeostasis, but also in the innate immune defense.

Alternative splicing contributes to the coordinated regulation of ferritin subunit levels in Bactrocera dorsalis (Hendel)

A constant ratio of ferritin heavy chain homolog (HCH) and light chain homolog (LCH) subunits seems to be required to compose the ferritin heteropolymer protein in insects. However, the mechanism by which insect LCH genes regulate protein levels remains unclear. We report that alternative promoters and alternative splicing contribute to maintaining a constant ratio of the two subunits, BdFer1HCH and BdFer2LCH (ferritin 1 HCH and ferritin 2 LCH), in Bactrocera dorsalis, a notorious quarantine pest. The genes BdFer1HCH and BdFer2LCH were identified with a series of potential transcription factor binding sites and were shown to be clustered within the genome in a "head to head" fashion. Thus, we unearthed a potential post-transcriptional mechanism to regulate the levels of LCH subunits, and confirmed that the expressions of BdFer1HCH and BdFer2LCH were induced by 20-hydroecdysone, iron overload, and immune challenge.

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.

Identification and characterization of a Tegillarca granosa ferritin regulated by iron ion exposure and thermal stress

Ferritin, a conserved iron storage protein of most living organisms, plays a crucial role in iron metabolism. In this study, the ferritin gene from Tegillarca granosa (denoted as TgFER) was identified by expressed sequence tag (EST) and PCR approaches. The full-length cDNA of TgFER was of 895bp, consisting of a 5'-UTR of 163 bp with a putative iron regulatory element (IRE), a 3'-UTR of 213 bp, and a complete open reading frame of 519 bp encoding a polypeptide with 172 amino acid residues. The predicted molecular mass of deduced amino acid of TgFER was 20.00 kDa and the theoretical pI was 4.89. The deduced amino acid of TgFER shared high identities to ferritin from abalone, oyster, clam and human. The tissue distribution of TgFER in the tissues of mantle, foot, gill, haemocytes and hepatopancreas was examined by quantitative real-time PCR (q-PCR) and mRNA transcripts of TgFER were found to be dominately expressed in haemocytes, hepatopancreas and gill and weakly in foot and mantle. The temporal expression of TgFER in haemocytes or hepatopancreases after challenged by metals ion (FeCl₂ and FeCl₃) exposure and thermal stress were also analyzed with q-PCR. The diverse expression patterns of TgFER were detected depending upon the types of stimulators and tissues. The ployconal antibodies generated from the recombinant product of TgFER could be specifically identified not only the recombinant product, but also the native protein from haemocytes. All these results strongly suggested that TgFER was involved in the iron metabolism and thermal stress regulation in T. granosa.

The role of iron in the proliferation of Drosophila l(2) mbn cells

Iron is essential for life and is needed for cell proliferation and cell cycle progression. Iron deprivation results first in cell cycle arrest and then in apoptosis. The Drosophila tumorous larval hemocyte cell line l(2) mbn was used to study the sensitivity and cellular response to iron deprivation through the chelator desferrioxamine (DFO). At a concentration of 10 μM DFO or more the proliferation was inhibited reversibly, while the amount of dead cells did not increase. FACS analysis showed that the cell cycle was arrested in G1/S-phase and the transcript level of cycE was decreased to less than 50% of control cells. These results show that iron chelation in this insect tumorous cell line causes a specific and coordinated cell cycle arrest.

Wolbachia interferes with ferritin expression and iron metabolism in insects

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

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

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

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.

Sampling Daphnia's expressed genes: preservation, expansion and invention of crustacean genes with reference to insect genomes

Background

Functional and comparative studies of insect genomes have shed light on the complement of genes, which in part, account for shared morphologies, developmental programs and life-histories. Contrasting the gene inventories of insects to those of the nematodes provides insight into the genomic changes responsible for their diversification. However, nematodes have weak relationships to insects, as each belongs to separate animal phyla. A better outgroup to distinguish lineage specific novelties would include other members of Arthropoda. For example, crustaceans are close allies to the insects (together forming Pancrustacea) and their fascinating aquatic lifestyle provides an important comparison for understanding the genetic basis of adaptations to life on land versus life in water.

Results

This study reports on the first characterization of cDNA libraries and sequences for the model crustacean Daphnia pulex. We analyzed 1,546 ESTs of which 1,414 represent approximately 787 nuclear genes, by measuring their sequence similarities with insect and nematode proteomes. The provisional annotation of genes is supported by expression data from microarray studies described in companion papers. Loci expected to be shared between crustaceans and insects because of their mutual biological features are identified, including genes for reproduction, regulation and cellular processes. We identify genes that are likely derived within Pancrustacea or lost within the nematodes. Moreover, lineage specific gene family expansions are identified, which suggest certain biological demands associated with their ecological setting. In particular, up to seven distinct ferritin loci are found in Daphnia compared to three in most insects. Finally, a substantial fraction of the sampled gene transcripts shares no sequence similarity with those from other arthropods. Genes functioning during development and reproduction are comparatively well conserved between crustaceans and insects. By contrast, genes that were responsive to environmental conditions (metal stress) and not sex-biased included the greatest proportion of genes with no matches to insect proteomes.

Conclusion

This study along with associated microarray experiments are the initial steps in a coordinated effort by the Daphnia Genomics Consortium to build the necessary genomic platform needed to discover genes that account for the phenotypic diversity within the genus and to gain new insights into crustacean biology. This effort will soon include the first crustacean genome sequence.

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.

Homeostatic mechanisms for iron storage revealed by genetic manipulations and live imaging of Drosophila ferritin

Ferritin is a symmetric, 24-subunit iron-storage complex assembled of H and L chains. It is found in bacteria, plants, and animals and in two classes of mutations in the human L-chain gene, resulting in hereditary hyperferritinemia cataract syndrome or in neuroferritinopathy. Here, we examined systemic and cellular ferritin regulation and trafficking in the model organism Drosophila melanogaster. We showed that ferritin H and L transcripts are coexpressed during embryogenesis and that both subunits are essential for embryonic development. Ferritin overexpression impaired the survival of iron-deprived flies. In vivo expression of GFP-tagged holoferritin confirmed that iron-loaded ferritin molecules traffic through the Golgi organelle and are secreted into hemolymph. A constant ratio of ferritin H and L subunits, secured via tight post-transcriptional regulation, is characteristic of the secreted ferritin in flies. Differential cellular expression, conserved post-transcriptional regulation via the iron regulatory element, and distinct subcellular localization of the ferritin subunits prior to the assembly of holoferritin are all important steps mediating iron homeostasis. Our study revealed both conserved features and insect-specific adaptations of ferritin nanocages and provides novel imaging possibilities for their in vivo characterization.

Molecular characterization of iron binding proteins from Glossina morsitans morsitans (Diptera: Glossinidae)

The regulation of iron is critical for maintaining homeostasis in the tsetse fly (Diptera: Glossinidae), in which both adult sexes are strict blood feeders. We have characterized the cDNAs for two putative iron-binding proteins (IBPs) involved in transport and storage; transferrin (GmmTsf1) and ferritin from Glossina morsitans morsitans. GmmTsf1 transcripts are detected in the female fat body and in adult reproductive tissues, and only in the adult developmental stage in a bloodmeal independent manner. In contrast, the ferritin heavy chain (GmmFer1HCH) and light chain (GmmFer2LCH) transcripts are expressed ubiquitously, suggesting a more general role for these proteins in iron transport and storage. Protein domain predictions for each IBP suggest both the conservation and loss of several motifs present in their vertebrate homologues. In concert with many other described insect transferrins (Tfs), putative secreted GmmTsf1 maintains 3 of the 5 residues necessary for iron-binding in the N-terminal lobe, but exhibits a loss of this iron-binding ability in the C-terminal lobe as well as a loss of large sequence blocks. Both putative GmmFer1HCH and GmmFer2LCH proteins have signal peptides, similar to other insect ferritins. GmmFer2LCH has lost the 5'UTR iron-responsive element (IRE) and, thus, translation is no longer regulated by cellular iron levels. On the other hand, GmmFer1HCH maintains both the conserved ferroxidase center and the 5'UTR IRE; however, transcript variants suggest a more extensive regulatory mechanism for this subunit.

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.

Sequencing and characterization of a cDNA encoding a ferritin subunit of Colorado potato beetle, Leptinotarsa decemlineata

A differentially expressed cDNA fragment (P311) from Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), was identified by restriction fragment differential display-polymerase chain reaction (RFDD-PCR) technique, and showed a strong similarity to ferritin heavy chain subunits of other organisms. Based on P311, we constructed specific primers and obtained a 840-bp cDNA fragment spanning the open reading frame of CPB ferritin subunit using the rapid amplification of cDNA ends (RACE) technique. The sequence encodes 213 amino acid residues, including a 19 amino acid signal peptide. The sequence has a conserved cysteine in the N-terminus and has the seven conserved residues that comprise the ferroxidase center, which is the feature of heavy chain ferritins of vertebrates. The CPB ferritin subunit has high amino acid sequence identity with the Apriona germari (69.3%), Galleria mellonela (54.5%), Manduca sexta (54.0%), Drosophila melanogaster (53.2%), Calpodes ethlius (51.4%), and Nilaparvata lugens (47.6%) but lower identity with the Anopheles gambiae (38.7%) and Aedes aegypti (37.8%). Using Northern blot analysis, the subunit mRNA was identified from fat body and midgut of 4th instars with much higher mRNA levels found in midgut than that in fat body (2.5-fold). Nevertheless, only the levels of mRNA in fat body was induced by dexamethasone (1.5-fold).

Molecular cloning, expression and characterization of cDNAs encoding the ferritin subunits from the beetle, Apriona germari

Insect secreted ferritins are composed of subunits, which resemble heavy and light chains of vertebrate cytosolic ferritins. We describe here the cloning, expression and characterization of cDNAs encoding the ferritin heavy-chain homologue (HCH) and light-chain homologue (LCH) from the mulberry longicorn beetle, Apriona germari (Coleoptera, Cerambycidae). The A. germari ferritin LCH and HCH cDNA sequences were comprised of 672 and 636 bp encoding 224 and 212 amino acid residues, respectively. The A. germari ferritin HCH subunit contained the conserved motifs for the ferroxidase center typical of vertebrate ferritin heavy chains and the iron-responsive element (IRE) sequence with a predicted stem-loop structure was present in the 5'-untranslated region (UTR) of ferritin HCH mRNA. However, the A. germari ferritin LCH subunit had no IRE at its 5'-UTR and ferroxidase center residues. Phylogenetic analysis confirmed the deduced protein sequences of A. germari ferritin HCH and LCH being divided into two types, G type (LCH) and S type (HCH). Southern blot analysis suggested the possible presence of each A. germari ferritin subunit gene as a single copy and Northern blot analysis confirmed a higher expression pattern in midgut than fat body. The cDNAs encoding the A. germari ferritin subunits were expressed as approximately 30 kDa (LCH) and 26 kDa (HCH) polypeptides in baculovirus-infected insect cells. Western blot analysis and iron staining assay confirmed that A. germari ferritin has a native molecular mass of approximately 680 kDa.

Compartment-specific protection of iron-sulfur proteins by superoxide dismutase

Iron and oxygen are essential but potentially toxic constituents of most organisms, and their transport is meticulously regulated both at the cellular and systemic levels. Compartmentalization may be a homeostatic mechanism for isolating these biological reactants in cells. To investigate this hypothesis, we have undertaken a genetic analysis of the interaction between iron and oxygen metabolism in Drosophila. We show that Drosophila iron regulatory protein-1 (IRP1) registers cytosolic iron and oxidative stress through its labile iron sulfur cluster by switching between cytosolic aconitase and RNA-binding functions. IRP1 is strongly activated by silencing and genetic mutation of the cytosolic superoxide dismutase (Sod1), but is unaffected by silencing of mitochondrial Sod2. Conversely, mitochondrial aconitase activity is relatively insensitive to loss of Sod1 function, but drops dramatically if Sod2 activity is impaired. This strongly suggests that the mitochondrial boundary limits the range of superoxide reactivity in vivo. We also find that exposure of adults to paraquat converts cytosolic aconitase to IRP1 but has no affect on mitochondrial aconitase, indicating that paraquat generates superoxide in the cytosol but not in mitochondria. Accordingly, we find that transgene-mediated overexpression of Sod2 neither enhances paraquat resistance in Sod1+ flies nor compensates for lack of SOD1 activity in Sod1-null mutants. We conclude that in vivo, superoxide is confined to the subcellular compartment in which it is formed, and that the mitochondrial and cytosolic SODs provide independent protection to compartment-specific protein iron-sulfur clusters against attack by superoxide generated under oxidative stress within those compartments.

Extrathymic TCR Gene Rearrangement in Human Small Intestine: Identification of New Splice Forms of Recombination Activating Gene-1 mRNA with Selective Tissue Expression

Two new 5'-untranslated region (5'UTR) exons were identified in the human gene for the lymphocyte-specific endonuclease recombination activating gene-1 (RAG1) required for the somatic recombination yielding functional Ag receptors. These 5'UTR exons were used in three different splice forms by jejunal lymphocytes of the T cell lineage. RAG1 mRNA containing the previously described 5'UTR exon was not expressed in these cells. Conversely, one of the new 5'UTR exons was not expressed in thymus. The new RAG1 mRNA splice forms were all expressed in immature T cells (CD2(+)CD7(+)CD3(-)). This cell population also expressed high levels of mRNA for the pre-T alpha-chain. In situ hybridization demonstrated jejunal cells expressing the new splice forms of RAG1 mRNA, both intraepithelially and in lamina propria. Pre-T alpha-chain mRNA-expressing cells were detected at the same sites. These results strongly suggest ongoing TCR gene rearrangement in human small intestinal mucosa, yielding T cells specially adapted for this environment. This seems to be achieved by two parallel processes, extrathymic T cell development and peripheral Ag-driven TCR editing.

Structured RNA upstream of insect cap distal iron responsive elements enhances iron regulatory protein-mediated control of translation

Iron regulatory protein (IRP) blocks ribosomal assembly by binding to an iron responsive element (IRE) located proximal (<60 nts) to the mRNA cap, thereby repressing translation. Constructs with IREs located 60-100 nts from the cap permit ribosomal assembly but the ribosomes pause at IRE/IRP complexes resulting in partial repression of translation. However, insect ferritin mRNAs have cap-distal IREs located 90-156 nts from the cap. Because iron can be toxic, it seems unlikely that insects would be unable to fully regulate ferritin synthesis at the level of translation. Calpodes ferritin consists of two subunits, S and G. In vitro translation of Calpodes ferritin and IRP1 from fat body mRNA yields only G subunits suggesting that IRP1 more efficiently represses translation of the S subunit than the G. When repression is removed by the addition of IRE competitor RNA, the synthesis of both subunits is greatly increased. S and G ferritin mRNAs have identical IREs in similar far cap-distal positions. While both ferritin mRNAs are predicted to have stem-loops between the IRE and the RNA cap, in general insect S mRNAs have more cap-proximal RNA structure than G mRNAs. Therefore, we examined the effect of upstream secondary structure on ribosomal assembly onto S ferritin mRNA constructs using sucrose gradient analysis of translation initiation complexes. We found no evidence for ribosomal assembly on wild type Calpodes S ferritin mRNA in the presence of IRP1 while constructs lacking the wild type secondary structure showed ribosomal pausing. Constructs with wild type secondary structure preceded by an unstructured upstream leader assemble ribosomes in the presence or absence of IRP1. Sequence and RNA folding analyses of other insect ferritins with cap-distal IREs failed to identify any common sequences or IRE-like structures that might bind to IRP1 with lower affinity or to another RNA binding protein. We propose that stem-loops upstream from the IRE act like pleats that shorten the effective distance between the IRE and cap and allow full translational repression by IRP1. In this way some cap-distal IREs may function like cap-proximal ones.

Cloning and expression of 32 kDa ferritin from Galleria mellonella

We have sequenced a cDNA clone encoding 32-kDa ferritin subunit in the Wax Moth, Galleria mellonella. The 32-kDa ferritin subunit cDNA was obtained from PCR using identical primer designed from highly conserved regions of insect ferritins. RACE PCR was used to obtain the complete protein coding sequence. The 32-kDa ferritin subunit encoded a 232 amino acid polypeptide, containing a 19 leader peptide. The iron-responsive element (IRE) sequence with a predicted stem-loop structure was present in the 5'-untranslated region of the wax moth 32-kDa ferritin subunit mRNA. The 32-kDa sequence alignment had 78 and 69% identity with Manduca sexta and Calpodes ethlius (G), respectively. The G. mellonella ferritin subunits showed minimal identity with each other (19%). The glycosylation site (Asn-X-Ser/Thr) was found in the 32-kDa subunit but not in the 26-kDa subunit. Northern blot analysis showed that the mRNA expression of the 32-kDa ferritin was detected in the fat body and midgut. The fat body expression increased after 6 h and the mRNA in midgut dramatically increased about 3-fold the expression level at 12 h after iron feeding. Western blot revealed that a protein level of the 32-kDa subunit is abundant in midgut after 12 and 24 h iron feeding.

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.

Insects, oxygen, and iron

Konrad Bloch developed an interest in insects because they are unable to make sterols, and in yeast because these cells need oxygen to make sterols and unsaturated fatty acids. Insects, like all other organisms, must deal with the toxic effects of oxygen in the presence of iron, which itself is a vital nutrient. They do so by making proteins with high affinity for ferric or ferrous ions. Two such proteins are transferrins and ferritins. Insects produce both of these proteins, but use them in different ways from most other organisms. Insect transferrins appear to be involved in innate immunity, perhaps by sequestering ferric ions to prevent pathogens and parasites from utilizing them. Insect ferritins, unlike those of any other group of organisms, are exported into the extracellular space (hemolymph). They may be involved in iron transport and/or protection against iron overload in the diet.

Evolution of the acute phase response: iron release by echinoderm (Asterias forbesi) coelomocytes, and cloning of an echinoderm ferritin molecule

That the plasma concentration of certain divalent cations change during an inflammatory insult provides a major host defense response in vertebrate animals. This study was designed to investigate the involvement of iron sequestration in invertebrate immune responses. A ferritin molecule was cloned from an echinoderm coelomocyte cDNA library. The amino acid sequence showed sequence homology with vertebrate ferritin. The cDNA contained a conserved iron responsive element sequence. Studies showed that stimulated coelomocytes released iron into in vitro culture supernatants. The amount of iron in the supernatants decreased over time when the amebocytes were stimulated with LPS or PMA. Coelomocytes increased expression of ferritin mRNA after stimulation. In vertebrates, cytokines can cause changes in iron levels in macrophages. Similarly, echinoderm macrokines produced decreases in iron levels in coelomocyte supernatant fluids. These results suggest that echinoderm ferritin is an acute phase protein and suggest that sequestration of iron is an ancient host defense response in animals.

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.

Enteric bacteria counteract lipopolysaccharide induction of antimicrobial peptide genes

The humoral immunity of Drosophila involves the production of antimicrobial peptides, which are induced by evolutionary conserved microbial molecules, like LPS. By using Drosophila mbn-2 cells, we found that live bacteria, including E. coli, Salmonella typhimurium, Erwinia carotovora, and Pseudomonas aeruginosa, prevented LPS from inducing antimicrobial peptide genes, while Micrococcus luteus and Streptococcus equi did not. The inhibitory effect was seen at bacterial levels from 20 per mbn-2 cell, while antimicrobial peptides were induced at lower bacterial concentrations (< or =2 bacteria per cell) also in the absence of added LPS. Gel shift experiment suggests that the inhibitory effect is upstream or at the level of the activation of the transcription factor Relish, a member of the NF-kappaB/Rel family. The bacteria have to be in physical contact with the cells, but not phagocytosed, to prevent LPS induction. Interestingly, the inhibiting mechanism is, at least for E. coli, independent of the type III secretion system, indicating that the inhibitory mechanism is unrelated to the one earlier described for YopJ from Yersinia.

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.

Cloning and expression of a ferritin subunit for Galleria mellonella

Ferritin was purified from iron-fed Galleria mellonella hemolymph by ultra centrifugation and FPLC (Superose 6). SDS-PAGE revealed three bands of 26, 30, and 32 kDa. The ferritin 26 kDa subunit cDNA was obtained from RT-PCR using primer designed from N-terminal sequence analysis. 5'-RACE was used to obtain the complete protein coding sequence. The sequence encodes a 211 amino acid polypeptide including a 20 amino acid leader peptide. An IRE (iron-responsive element) sequence with a predicted stem-loop structure was present in the 5'-UTR of ferritin mRNA. Sequence alignment has a sequence identity with Calpodes ethlius (S)(74%), Drosophila melanogaster (50%), and Aedes aegypti (39%). Northern blot analysis indicated that there were 1.5- and 1.75-fold increases in the expression of ferritin mRNA after iron-fed fat body and midgut, respectively. Also, we confirmed that the ferritin mRNA is not expressed in adult ovary and testis. Arch.

Nitric oxide up-regulates ferritin mRNA level in snail neurons

We cloned and sequenced the ferric ion-binding protein, ferritin, from the nervous system of the pulmonate snail, Helix pomatia. Helix H-ferritin cDNA contains a 519-bp open reading frame (ORF) and predicts an iron-responsive element (IRE) at the 5'-untranslated region (5'-UTR) of the ferritin mRNA. The deduced amino acid sequence revealed 86% similarity with Lymnaea stagnalis ferritin and about 70% similarity with vertebrate H-ferritin. While secreted ferritin isoforms contain a signalling sequence at their N-terminal end, Helix ferritin does not contain this sorting signal indicating that it is restricted to the cytoplasm. The amino acid ligands at positions Glu25, Tyr30, Glu59, Glu60, His63, Glu105 and Gln139 indicate an active ferroxidase site in Helix ferritin. In situ hybridization visualized ferritin mRNA in neuronal cell bodies but not in the neuropil. In contrast, ferritin-immunoreactive protein was localized in cell bodies and neurites. We further demonstrate that the NO donors S-nitroso-N-acetylpenicillamine (SNAP), or hydroxylamine (HA), increase the intracellular ferritin mRNA level by about 55%. In conclusion, our findings show that Helix neurons express an intracellular H-ferritin isoform and suggest that iron and NO metabolism are coupled.

Iron regulatory proteins and the molecular control of mammalian iron metabolism

Mammalian iron homeostasis is maintained through the concerted action of sensory and regulatory networks that modulate the expression of proteins of iron metabolism at the transcriptional and/or post-transcriptional levels. Regulation of gene transcription provides critical developmental, cell cycle, and cell-type-specific controls on iron metabolism. Post-transcriptional control through the action of iron regulatory protein 1 (IRP1) and IRP2 coordinate the use of messenger RNA-encoding proteins that are involved in the uptake, storage, and use of iron in all cells of the body. IRPs may also provide a link between iron availability and cellular citrate use. Multiple factors, including iron, nitric oxide, oxidative stress, phosphorylation, and hypoxia/reoxygenation, influence IRP function. Recent evidence indicates that there is diversity in the function of the IRP system with respect to the response of specific IRPs to the same effector, as well as the selectivity with which IRPs modulate the use of specific messenger RNA.

Organization of the ferritin genes in Drosophila melanogaster

The organization of two closely clustered genes, Fer1HCH and Fer2LCH, encoding the heavy-chain homolog (HCH) and the light-chain homolog (LCH) subunits of Drosophila melanogaster ferritin are reported here. The 5019-bp sequence of the cluster was assembled from genomic fragments obtained by polymerase chain reaction (PCR) amplification of genomic DNA and from sequences obtained from the Berkeley Drosophila Genome Project (BDGP) (http://www.fruitfly.org). These genes, located at position 99F1, have different exon-intron structures (Fer1HCH has three introns and Fer2LCH has two introns) and are divergently transcribed. Computer analysis of the possibly shared promoter regions revealed the presence of putative metal regulatory elements (MREs), a finding consistent with the upregulation of these genes by iron, and putative NF-kappaB-like binding sites. The structure of two other invertebrate ferritin genes, from the nematode Caenorhabditis elegans (located on chromosomes I and V), was also analyzed. Both nematode genes have two introns, lack iron-responsive elements (IREs), and encode ferritin subunits similar to vertebrate H chains. These findings, along with comparisons of ferritin genes from invertebrates, vertebrates, and plants, suggest that the specialization of ferritin H and L type chains, the complex exon-intron organization of plant and vertebrate genes, and the use of the IRE/iron regulatory protein (IRP) mechanism for regulation of ferritin synthesis are recent evolutionary acquisitions.

Secreted ferritin subunits are of two kinds in insects molecular cloning of cDNAs encoding two major subunits of secreted ferritin from Calpodes ethlius

In insects, holoferritin is easily visible in the vacuolar system of tissues that filter the hemolymph and, at least in Lepidoptera, is abundant in the hemolymph. Sequences reported for insect secreted ferritins from Lepidoptera and Diptera have high sequence diversity. We examined the nature of this diversity for the first time by analyzing sequences of cDNAs encoding two ferritin subunits from one species, Calpodes ethlius (Lepidoptera, Hesperiidae). We found that insect secreted ferritin subunits are of two types with little resemblance to each other. Ferritin was isolated from iron loaded hemolymph of C. ethlius fifth instar larvae by differential centrifugation. The N-terminal amino acid sequences for the nonglycosylated subunit with Mr 24,000 (S) and the largest glycosylated subunit with Mr 31,000 (G) were determined. The N-termini of the two subunits were different and were used to construct degenerate PCR primers. The same cDNA products were amplified from cDNA libraries from the midgut which secretes holoferritin and from the fat body which secretes iron-poor apoferritin. The G subunit most closely resembles the glycosylated ferritin subunit from Manduca sexta and the S subunit resembles the Drosophila small subunit. The S and G subunits from Calpodes were dissimilar and distinct from the cytosolic ferritins of vertebrates and invertebrates. Additional sequences were obtained by 5' and 3' RACE from separate fat body and midgut RACE libraries. cDNAs encoding both subunits had a consensus iron responsive element (IRE) in a conserved cap-distal location of their 5' UTR. An integrin-binding RGD motif found in the G subunit and conserved in Manduca may facilitate iron uptake through a calreticulin (mobilferrin)/integrin pathway. Calpodes and other insect ferritins have conserved cysteine residues to which fatty acids can be linked. Dynamic acylation of ferritin may slow but not prevent its passage out of the ER.

Fe-S proteins in sensing and regulatory functions

In the past five to ten years, it has become increasingly apparent that the function of Fe-S clusters is not limited to electron transfer, a function implicit in their discovery. We now know that the vulnerability of these structures to oxidative destruction is used by nature in sensing O2, iron, and possibly also nitric oxide. Changes in the oxidation state of Fe-S clusters can also serve as a reversible switch.

Iron availability dramatically alters the distribution of ferritin subunit messages in Drosophila melanogaster

Insect ferritins have subunits homologous to the heavy and light chains of vertebrate ferritins. Cloning and sequence of the heavy chain homologue (HCH) of Drosophila melanogaster ferritin subunit have been reported earlier. When Northern blots of D. melanogaster RNA were probed with a cDNA for this HCH, three bands were observed. It was shown that these represented at least four classes of mRNA of various lengths. The polymorphism results from alternative splicing of an intron in the 5' untranslated region (UTR) that contains the iron-responsive element (IRE) and from two alternative polyadenylation sites in the 3' UTR. This has also been reported by others [Lind, M. I., Ekengren, S., Melefors, O. & Söderhäll, K. (1998) FEBS Lett. 436, 476-482]. By hybridizing Northern blots with specific probes, it has been shown that the relative proportions of the messages vary with the life stage and especially with iron supplementation of the diet. Iron significantly increases the amount of ferritin HCH messages and dramatically shifts the balance toward those messages that lack an IRE and/or have a short 3' UTR. In the larvae this change takes place in the gut, but not in the fat body. We speculate that this dramatic shift in message distribution may result from an effect of iron on the rate of transcription or message degradation, or from an effect on the splicing process itself. Synthesis of ferritin HCH subunit mRNAs that lack an IRE may be important under conditions of iron overload.

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.