Iron transport by Nramp2/DMT1: pH regulation of transport by 2 histidines in transmembrane domain 6

Identification of a divalent metal transporter required for cellular iron metabolism in malaria parasites

Plasmodium falciparum malaria parasites invade and multiply inside red blood cells (RBCs), the most iron-rich compartment in humans. Like all cells, P. falciparum requires nutritional iron to support essential metabolic pathways, but the critical mechanisms of iron acquisition and trafficking during RBC infection have remained obscure. Parasites internalize and liberate massive amounts of heme during large-scale digestion of RBC hemoglobin within an acidic food vacuole (FV) but lack a heme oxygenase to release porphyrin-bound iron. Although most FV heme is sequestered into inert hemozoin crystals, prior studies indicate that trace heme escapes biomineralization and is susceptible to non-enzymatic degradation within the oxidizing FV environment to release labile iron. Parasites retain a homolog of divalent metal transporter 1 (DMT1), a known mammalian iron transporter, but its role in P. falciparum iron acquisition has not been tested. Our phylogenetic studies indicate that P. falciparum DMT1 (PfDMT1) retains conserved molecular features critical for metal transport. We localized this protein to the FV membrane and defined its orientation in an export-competent topology. Conditional knockdown of PfDMT1 expression is lethal to parasites, which display broad cellular defects in iron-dependent functions, including impaired apicoplast biogenesis and mitochondrial polarization. Parasites are selectively rescued from partial PfDMT1 knockdown by supplementation with exogenous iron, but not other metals. These results support a cellular paradigm whereby PfDMT1 is the molecular gatekeeper to essential iron acquisition by blood-stage malaria parasites and suggest that therapeutic targeting of PfDMT1 may be a potent antimalarial strategy.

Study on peptide-peptide interactions between transmembrane domains of Slc11a1 in model membranes

In this study, we determined the interaction between TM4 and TM2/TM3 domain of Solute carrier family 11 member 1 (Slc11a1) by circular dichroism (CD) and fluorescence spectrum. The results indicated that, the cation transport process was likely to be accomplished by the collaboration of multiple TM domains rather than by TM4 domain alone. Therefore, this finding suggested possible transportation theory and be helpful to elucidate the mechanism of Slc11a1 in cation transport process.

Molecular Mechanism of Nramp-Family Transition Metal Transport

The Natural resistance-associated macrophage protein (Nramp) family of transition metal transporters enables uptake and trafficking of essential micronutrients that all organisms must acquire to survive. Two decades after Nramps were identified as proton-driven, voltage-dependent secondary transporters, multiple Nramp crystal structures have begun to illustrate the fine details of the transport process and provide a new framework for understanding a wealth of preexisting biochemical data. Here we review the relevant literature pertaining to Nramps' biological roles and especially their conserved molecular mechanism, including our updated understanding of conformational change, metal binding and transport, substrate selectivity, proton transport, proton-metal coupling, and voltage dependence. We ultimately describe how the Nramp family has adapted the LeuT fold common to many secondary transporters to provide selective transition-metal transport with a mechanism that deviates from the canonical model of symport.

Identification and functional characterization of natural resistance-associated macrophage protein 2 from sea cucumber Apostichopus japonicus

As a member of natural resistance-associated macrophage protein (Nramp) family, Nramp2 conservatively exists in the cell membrane across species and is essential for normal iron homeostasis in an H⁺-dependent manner. Withholding available iron represents an important host defense strategy. However, the function of Nramp2 in response to invading pathogens is largely unknown in invertebrates. In this study, a unique echinoderm Nramp2 was identified from sea cucumber Apostichopus japonicus (designated as AjNramp2). The cDNA sequence of AjNramp2 was 2360 bp, with a putative open reading frame of 1713 bp, encoding a typical Nramp domain containing protein with 570 amino acid residues. Structural analysis revealed that AjNramp2 consisted of highly conserved helix regions similar with the human Nramp2. Spatial expression analysis revealed that AjNramp2 was ubiquitously expressed in all examined tissues, with the highest level found in the intestine. Immunohistochemistry assay showed that AjNramp2 was mainly located in the cellular membrane in coelomocytes. Vibrio splendidus challenge and lipopolysaccharide (LPS) stimulation could significantly promote the expression of AjNramp2, which was consistent with the cellular iron level in coelomocytes. Moreover, when the expression of AjNramp2 was knocked down by siRNA-AjNramp2, the cellular iron level was coordinately decreased in coelomocytes under LPS stimulation. Taken together, results indicated that AjNramp2 serves as an iron transport receptor to withhold available iron and may contribute to the nutritional immunity defense system of sea cucumber.

Analysis of Genetic Variation in the Bovine SLC11A1 Gene, Its Influence on the Expression of NRAMP1 and Potential Association With Resistance to Bovine Tuberculosis

Bovine tuberculosis (bTB), caused by Mycobacterium bovis, is a chronic zoonotic disease where host genetics is thought to contribute to susceptibility or resistance. One of the genes implicated is the SLC11A1 gene, that encodes for the natural resistance-associated macrophage protein 1 (NRAMP1). The aim of this study was to identify SLC11A1 polymorphisms and to investigate any resulting functional differences in NRAMP1 expression that might be correlated with resistance/susceptibility to M. bovis infection. Sequencing of the SLC11A1 gene in cDNA isolated from Brown Swiss, Holstein Friesian, and Sahiwal cattle identified five single nucleotide polymorphisms (SNPs) in the coding region, but only one of these (SNP4, c.1066C>G, rs109453173) was present in all three cattle breeds and therefore warranted further investigation. Additionally, variations of 10, 11, and 12 GT repeats were identified in a microsatellite (MS1) in the SLC11A1 3′UTR. Measurement of NRAMP1 expression in bovine macrophages by ELISA showed no differences between cells generated from the different breeds. Furthermore, variations in the length of the MS1 microsatellite did not impact on NRAMP1 protein expression as analyzed by luciferase reporter assay. However, further analysis of the ELISA data identified that the presence of the alternative G allele at SNP4 was associated with increased expression of NRAMP1 in bovine macrophages. Since NRAMP1 has been shown to influence the survival of intracellular pathogens such as M. bovis through the sequestering of iron, it is possible that cattle expressing the alternative G allele might have an increased resistance to bTB through increased NRAMP1 expression in their macrophages.

Bioinformatics and Transcriptional Study of the Nramp Gene in the Extreme Acidophile Acidithiobacillus ferrooxidans Strain DC

The family of Nramp (natural resistance-associated macrophage protein) metal ion transporter functions in diverse organisms from bacteria to humans. Acidithiobacillus ferrooxidans (At. ferrooxidans) is a Gram-negative bacterium that lives at pH 2 in high concentrations of soluble ferrous ion (600 mM). The AFE_2126 protein of At. ferrooxidans of the Dachang Copper Mine (DC) was analyzed by bioinformatics software or online tools, showing that it was highly homologous to the Nramp family, and its subcellular localization was predicted to locate in the cytoplasmic membrane. Transcriptional study revealed that AFE_2126 was expressed by Fe2+-limiting conditions in At. ferrooxidans DC. It can be concluded that the AFE_2126 protein may function in ferrous ion transport into the cells. Based on the ΔpH of the cytoplasmic membrane between the periplasm (pH 3.5) and the cytoplasm (pH 6.5), it can be concluded that Fe2+ is transported in the direction identical to that of the H+ gradient. This study indirectly confirmed that the function of Nramp in At. ferrooxidans DC can transport divalent iron ions.

Transmembrane helix 6b links proton and metal release pathways and drives conformational change in an Nramp-family transition metal transporter

The natural resistance-associated macrophage protein (Nramp) family encompasses transition metal and proton cotransporters that are present in many organisms from bacteria to humans. Recent structures of Deinococcus radiodurans Nramp (DraNramp) in multiple conformations revealed the intramolecular rearrangements required for alternating access of the metal-binding site to the external or cytosolic environment. Here, using recombinant proteins and metal transport and cysteine accessibility assays, we demonstrate that two parallel cytoplasm-accessible networks of conserved hydrophilic residues in DraNramp, one lining the wide intracellular vestibule for metal release and the other forming a narrow proton transport pathway, are essential for metal transport. We further show that mutagenic or posttranslational modifications of transmembrane helix (TM) 6b, which structurally links these two pathways, impede normal conformational cycling and metal transport. TM6b contains two highly conserved histidines, His²³² and His²³⁷ We found that different mutagenic perturbations of His²³², just below the metal-binding site along the proton exit route, differentially affect DraNramp's conformational state, suggesting that His²³² serves as a pivot point for conformational changes. In contrast, any replacement of His²³⁷, lining the metal exit route, locked the transporter in a transport-inactive outward-closed state. We conclude that these two histidines, and TM6b more broadly, help trigger the bulk rearrangement of DraNramp to the inward-open state upon metal binding and facilitate return of the empty transporter to an outward-open state upon metal release.

Unique structural features in an Nramp metal transporter impart substrate-specific proton cotransport and a kinetic bias to favor import

Natural resistance-associated macrophage protein (Nramp) transporters enable uptake of essential transition metal micronutrients in numerous biological contexts. These proteins are believed to function as secondary transporters that harness the electrochemical energy of proton gradients by "coupling" proton and metal transport. Here we use the Deinococcus radiodurans (Dra) Nramp homologue, for which we have determined crystal structures in multiple conformations, to investigate mechanistic details of metal and proton transport. We untangle the proton-metal coupling behavior of DraNramp into two distinct phenomena: ΔpH stimulation of metal transport rates and metal stimulation of proton transport. Surprisingly, metal type influences substrate stoichiometry, leading to manganese-proton cotransport but cadmium uniport, while proton uniport also occurs. Additionally, a physiological negative membrane potential is required for high-affinity metal uptake. To begin to understand how Nramp's structure imparts these properties, we target a conserved salt-bridge network that forms a proton-transport pathway from the metal-binding site to the cytosol. Mutations to this network diminish voltage and ΔpH dependence of metal transport rates, alter substrate selectivity, perturb or eliminate metal-stimulated proton transport, and erode the directional bias favoring outward-to-inward metal transport under physiological-like conditions. Thus, this unique salt-bridge network may help Nramp-family transporters maximize metal uptake and reduce deleterious back-transport of acquired metals. We provide a new mechanistic model for Nramp proton-metal cotransport and propose that functional advantages may arise from deviations from the traditional model of symport.

Mutation at Different Sites of Metal Transporter Gene OsNramp5 Affects Cd Accumulation and Related Agronomic Traits in Rice (Oryza sativa L.)

OsNramp5 is a key gene involved in the control of the uptake of Cd, Mn, and other metal ions by rice root cells. The functional deficiency of this gene can significantly reduce the accumulation of Cd in rice grains, but the effects of its mutation on agronomic traits such as yield and quality have not been investigated comprehensively yet. In the present study, three Huanghuazhan-based OsNramp5 mutants [LCH1 (Low Cadmium Huanghuazhan 1), LCH2 (Low Cadmium Huanghuazhan 2), and LCH3 (Low Cadmium Huanghuazhan 3)] were obtained using clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) technology. The mutation-type analysis showed that LCH1, LCH2, and LCH3 encoded defective OsNramp5 protein sequences containing only 76aa, 176aa, and 266aa, respectively. The determination of metal content and the statistics of related agronomic traits revealed that the functionally deficient OsNramp5 not only significantly reduced the accumulation of Cd in the grains of the mutants but also affected rice yield and quality. However, with the decrease of OsNramp5 mutation degree, its effects on chlorenchyma Mn accumulation, yield, and quality were also diminished. Additionally, we also found that the increase in the concentration of Mn in the soil restored the phenotype of the declined yield and quality due to the functional deficiency of OsNramp5. Our findings provide novel insights into and new materials for breeding rice varieties with low Cd accumulation and excellent agronomic traits under severe Cd pollution environment.

Allelic Variation of NtNramp5 Associated with Cultivar Variation in Cadmium Accumulation in Tobacco

Tobacco (Nicotiana tabacum) is a cadmium (Cd) accumulator, and smoking is a major source of Cd exposure. In the present study, we identified two tobacco cultivars with contrasting phenotypes of Cd and manganese (Mn) accumulation in both hydroponic and soil pot experiments. Physiological experiments showed that the two cultivars differed in Cd uptake, but not in Cd translocation from roots to shoots. A homolog of OsNramp5 (natural resistance-associated macrophage protein 5), NtNramp5, was isolated from both cultivars. There was no significant difference in the expression level of NtNramp5 in the roots between the two cultivars. Sequence analysis revealed that the low Cd/Mn-accumulating cultivar possesses an NtNramp5 allele with a predicted mutation for early translation termination, resulting in a truncated protein missing 104 amino acids in the C-terminus of the full-length NtNramp5 found in the high Cd/Mn-accumulating cultivar. Both proteins were found to be localized to the plasma membrane. Heterologous expression of the two alleles of NtNramp5 in yeast showed that the full-length protein had transport activities for both Mn and Cd, whereas the truncated protein had no transport activity for Mn and a weak transport activity for Cd. These results suggest that NtNramp5 is a transporter for Mn and Cd, and the allelic variation in the coding region of NtNramp5 probaby explains the cultivar difference in Cd and Mn accumulation.

Tumor-initiating cells of breast and prostate origin show alterations in the expression of genes related to iron metabolism

The importance of iron in the growth and progression of tumors has been widely documented. In this report, we show that tumor-initiating cells (TICs), represented by spheres derived from the MCF7 cell line, exhibit higher intracellular labile iron pool, mitochondrial iron accumulation and are more susceptible to iron chelation. TICs also show activation of the IRP/IRE system, leading to higher iron uptake and decrease in iron storage, suggesting that level of properly assembled cytosolic iron-sulfur clusters (FeS) is reduced. This finding is confirmed by lower enzymatic activity of aconitase and FeS cluster biogenesis enzymes, as well as lower levels of reduced glutathione, implying reduced FeS clusters synthesis/utilization in TICs. Importantly, we have identified specific gene signature related to iron metabolism consisting of genes regulating iron uptake, mitochondrial FeS cluster biogenesis and hypoxic response (ABCB10, ACO1, CYBRD1, EPAS1, GLRX5, HEPH, HFE, IREB2, QSOX1 and TFRC). Principal component analysis based on this signature is able to distinguish TICs from cancer cells in vitro and also Leukemia-initiating cells (LICs) from non-LICs in the mouse model of acute promyelocytic leukemia (APL). Majority of the described changes were also recapitulated in an alternative model represented by MCF7 cells resistant to tamoxifen (TAMR) that exhibit features of TICs. Our findings point to the critical importance of redox balance and iron metabolism-related genes and proteins in the context of cancer and TICs that could be potentially used for cancer diagnostics or therapy.

Structural and mechanistic basis of proton-coupled metal ion transport in the SLC11/NRAMP family

Secondary active transporters of the SLC11/NRAMP family catalyse the uptake of iron and manganese into cells. These proteins are highly conserved across all kingdoms of life and thus likely share a common transport mechanism. Here we describe the structural and functional properties of the prokaryotic SLC11 transporter EcoDMT. Its crystal structure reveals a previously unknown outward-facing state of the protein family. In proteoliposomes EcoDMT mediates proton-coupled uptake of manganese at low micromolar concentrations. Mutants of residues in the transition-metal ion-binding site severely affect transport, whereas a mutation of a conserved histidine located near this site results in metal ion transport that appears uncoupled to proton transport. Combined with previous results, our study defines the conformational changes underlying transition-metal ion transport in the SLC11 family and it provides molecular insight to its coupling to protons.

Cellular uptake of transition metal ions is mediated by members of the SLC11/NRAMP family. Here the authors determine the structural and functional properties of EcoDMT, a bacterial SLC11 transporter, gathering molecular insight into its transport mechanism and proton coupling process.

Conserved methionine dictates substrate preference in Nramp-family divalent metal transporters

Natural resistance-associated macrophage protein (Nramp) family transporters catalyze uptake of essential divalent transition metals like iron and manganese. To discriminate against abundant competitors, the Nramp metal-binding site should favor softer transition metals, which interact either covalently or ionically with coordinating molecules, over hard calcium and magnesium, which interact mainly ionically. The metal-binding site contains an unusual, but conserved, methionine, and its sulfur coordinates transition metal substrates, suggesting a vital role in their transport. Using a bacterial Nramp model system, we show that, surprisingly, this conserved methionine is dispensable for transport of the physiological manganese substrate and similar divalents iron and cobalt, with several small amino acid replacements still enabling robust uptake. Moreover, the methionine sulfur's presence makes the toxic metal cadmium a preferred substrate. However, a methionine-to-alanine substitution enables transport of calcium and magnesium. Thus, the putative evolutionary pressure to maintain the Nramp metal-binding methionine likely exists because it-more effectively than any other amino acid-increases selectivity for low-abundance transition metal transport in the presence of high-abundance divalents like calcium and magnesium.

Metal ion binding of the third and fourth domains of Slc11a1 in a model membrane

In this paper, differential scanning calorimetric (DSC) experiments have shown that the ability of third and fourth transmembrane domains of Slc11a1 to perturb DMPC model membranes is affected by metal ions.

Dictyostelium Nramp1, which is structurally and functionally similar to mammalian DMT1 transporter, mediates phagosomal iron efflux

The Nramp (Slc11) protein family is widespread in bacteria and eukaryotes, and mediates transport of divalent metals across cellular membranes. The social amoeba Dictyostelium discoideum has two Nramp proteins. Nramp1, like its mammalian ortholog (SLC11A1), is recruited to phagosomal and macropinosomal membranes, and confers resistance to pathogenic bacteria. Nramp2 is located exclusively in the contractile vacuole membrane and controls, synergistically with Nramp1, iron homeostasis. It has long been debated whether mammalian Nramp1 mediates iron import or export from phagosomes. By selectively loading the iron-chelating fluorochrome calcein in macropinosomes, we show that Dictyostelium Nramp1 mediates iron efflux from macropinosomes in vivo. To gain insight in ion selectivity and the transport mechanism, the proteins were expressed in Xenopus oocytes. Using a novel assay with calcein, and electrophysiological and radiochemical assays, we show that Nramp1, similar to rat DMT1 (also known as SLC11A2), transports Fe(2+) and manganese, not Fe(3+) or copper. Metal ion transport is electrogenic and proton dependent. By contrast, Nramp2 transports only Fe(2+) in a non-electrogenic and proton-independent way. These differences reflect evolutionary divergence of the prototypical Nramp2 protein sequence compared to the archetypical Nramp1 and DMT1 proteins.

Identification of mutations allowing Natural Resistance Associated Macrophage Proteins (NRAMP) to discriminate against cadmium

Each essential transition metal plays a specific role in metabolic processes and has to be selectively transported. Living organisms need to discriminate between essential and non-essential metals such as cadmium (Cd(2+) ), which is highly toxic. However, transporters of the natural resistance-associated macrophage protein (NRAMP) family, which are involved in metal uptake and homeostasis, generally display poor selectivity towards divalent metal cations. In the present study we used a unique combination of yeast-based selection, electrophysiology on Xenopus oocytes and plant phenotyping to identify and characterize mutations that allow plant and mammalian NRAMP transporters to discriminate between their metal substrates. We took advantage of the increased Cd(2+) sensitivity of yeast expressing AtNRAMP4 to select mutations that decrease Cd(2+) sensitivity while maintaining the ability of AtNRAMP4 to transport Fe(2+) in a population of randomly mutagenized AtNRAMP4 cDNAs. The selection identified mutations in three residues. Among the selected mutations, several affect Zn(2+) transport, whereas only one, E401K, impairs Mn(2+) transport by AtNRAMP4. Introduction of the mutation F413I, located in a highly conserved domain, into the mammalian DMT1 transporter indicated that the importance of this residue in metal selectivity is conserved among NRAMP transporters from plant and animal kingdoms. Analyses of overexpressing plants showed that AtNRAMP4 affects the accumulation of metals in roots. Interestingly, the mutations selectively modify Cd(2+) and Zn(2+) accumulation without affecting Fe transport mediated by NRAMP4 in planta. This knowledge may be applicable for limiting Cd(2+) transport by other NRAMP transporters from animals or plants.

Divalent metal transporter 1 (DMT1) in the brain: implications for a role in iron transport at the blood-brain barrier, and neuronal and glial pathology

Iron is required in a variety of essential processes in the body. In this review, we focus on iron transport in the brain and the role of the divalent metal transporter 1 (DMT1) vital for iron uptake in most cells. DMT1 locates to cellular membranes and endosomal membranes, where it is a key player in non-transferrin bound iron uptake and transferrin-bound iron uptake, respectively. Four isoforms of DMT1 exist, and their respective characteristics involve a complex cell-specific regulatory machinery all controlling iron transport across these membranes. This complexity reflects the fine balance required in iron homeostasis, as this metal is indispensable in many cell functions but highly toxic when appearing in excess. DMT1 expression in the brain is prominent in neurons. Of serious dispute is the expression of DMT1 in non-neuronal cells. Recent studies imply that DMT1 does exist in endosomes of brain capillary endothelial cells denoting the blood-brain barrier. This supports existing evidence that iron uptake at the BBB occurs by means of transferrin-receptor mediated endocytosis followed by detachment of iron from transferrin inside the acidic compartment of the endosome and DMT1-mediated pumping iron into the cytosol. The subsequent iron transport across the abluminal membrane into the brain likely occurs by ferroportin. The virtual absent expression of transferrin receptors and DMT1 in glial cells, i.e., astrocytes, microglia and oligodendrocytes, suggest that the steady state uptake of iron in glia is much lower than in neurons and/or other mechanisms for iron uptake in these cell types prevail.

Involvement of Histidine Residue His382 in pH Regulation of MCT4 Activity

Monocarboxylate transporter 4 (MCT4) is a pH-dependent bi-directional lactate transporter. Transport of lactate via MCT4 is increased by extracellular acidification. We investigated the critical histidine residue involved in pH regulation of MCT4 function. Transport of lactate via MCT4 was measured by using a Xenopus laevis oocyte expression system. MCT4-mediated lactate transport was inhibited by Zn2+ in a pH physiological condition but not in an acidic condition. The histidine modifier DEPC (diethyl pyrocarbonate) reduced MCT4 activity but did not completely inactivate MCT4. After treatment with DEPC, pH regulation of MCT4 function was completely knocked out. Inhibitory effects of DEPC were reversed by hydroxylamine and suppressed in the presence of excess lactate and Zn2+. Therefore, we performed an experiment in which the extracellular histidine residue was replaced with alanine. Consequently, the pH regulation of MCT4-H382A function was also knocked out. Our findings demonstrate that the histidine residue His382 in the extracellular loop of the transporter is essential for pH regulation of MCT4-mediated substrate transport activity.

Conserved histidine of metal transporter AtNRAMP1 is crucial for optimal plant growth under manganese deficiency at chilling temperatures

Manganese (Mn) is an essential nutrient required for plant growth, in particular in the process of photosynthesis. Plant performance is influenced by various environmental stresses including contrasting temperatures, light or nutrient deficiencies. The molecular responses of plants exposed to such stress factors in combination are largely unknown. Screening of 108 Arabidopsis thaliana (Arabidopsis) accessions for reduced photosynthetic performance at chilling temperatures was performed and one accession (Hog) was isolated. Using genetic and molecular approaches, the molecular basis of this particular response to temperature (G × E interaction) was identified. Hog showed an induction of a severe leaf chlorosis and impaired growth after transfer to lower temperatures. We demonstrated that this response was dependent on the nutrient content of the soil. Genetic mapping and complementation identified NRAMP1 as the causal gene. Chlorotic phenotype was associated with a histidine to tyrosine (H239Y) substitution in the allele of Hog NRAMP1. This led to lethality when Hog seedlings were directly grown at 4°C. Chemical complementation and hydroponic culture experiments showed that Mn deficiency was the major cause of this G × E interaction. For the first time, the NRAMP-specific highly conserved histidine was shown to be crucial for plant performance.

Pathophysiology of the Belgrade rat

The Belgrade rat is an animal model of divalent metal transporter 1 (DMT1) deficiency. This strain originates from an X-irradiation experiment first reported in 1966. Since then, the Belgrade rat’s pathophysiology has helped to reveal the importance of iron balance and the role of DMT1. This review discusses our current understanding of iron transport homeostasis and summarizes molecular details of DMT1 function. We describe how studies of the Belgrade rat have revealed key roles for DMT1 in iron distribution to red blood cells as well as duodenal iron absorption. The Belgrade rat’s pathology has extended our knowledge of hepatic iron handling, pulmonary and olfactory iron transport as well as brain iron uptake and renal iron handling. For example, relationships between iron and manganese metabolism have been discerned since both are essential metals transported by DMT1. Pathophysiologic features of the Belgrade rat provide us with a unique and interesting animal model to understand iron homeostasis.

Penetration of three transmembrane segments of Slc11a1 in lipid bilayers

Slc11a1 is a divalent metal cation transporter with 12 putative transmembrane domains (TM) and plays a role in host defense. In present work, we investigated the secondary structure and topology of the peptides associated to Slc11a1-TM2, TM3 and TM4 (wildtype peptides and function-relating mutants) in the phospholipid vesicles (DMPC, DMPG and their mixtures) using circular dichroism, fluorescence spectroscopy and differential scanning calorimetry. We found that TM3 is obviously different in secondary structure and topology from TM2 to TM4 in the lipid membranes. The peptide TM3 is less structured and embedded in the lipid membranes less deeply than TM2 and TM4 at pH 5.5 and 7. The insertion position of TM3 in the lipid membranes is adjusted by pH, more deeply at more acidic pH environment, whereas the locations of TM2 and TM4 in the lipid membranes are less changed with pH. The E139A substitution of TM3 significantly impairs the pH dependence of the buried depth of TM3 and causes a pronounced increase in helicity in all DMPG-containing lipid vesicles at pH 5.5 and 7 and in DMPC at pH 4. In contrast, TM2 and TM4 are similar in topology. The G169D mutation has little effect on the topological arrangement of TM4 in membranes. The property of headgroups of the phospholipids has an effect on the secondary structure and topology of the peptides. All peptides could be structured with more helicity and embedded more deeply in DMPG-containing lipid vesicles than in DMPC membrane at pH 5.5 and 7.

Characterization of chicken natural resistance-associated macrophage protein encoding genes (Nramp1 and Nramp2) and association with salmonellosis resistance

Natural resistance-associated macrophage protein 1 and 2 encoding genes (Nramp1 and Nramp2) are related to many diseases. We cloned the cDNA of chicken Nramp1 and Nramp2 genes, characterized their expression and polymorphisms, and investigated the association of some SNPs with resistance to salmonellosis. The Nramp1 cDNA was 1746 bp long and the Nramp2 cDNA was 1938 bp long. These cDNAs are similar to previously reported cDNAs, varying by two and one amino acids, respectively. The chicken Nramp1 gene expressed predominantly in liver, thymus and spleen in both females and males. The Nramp2 gene expressed in almost all tissues, but predominantly in breast muscle, leg muscle, cerebrum, cerebellum, lung, kidney, and heart in both females and males. We identified 45 SNPs and 2 indels in the chicken Nramp1 gene; three of 13 SNPs in the exons were missense mutations (Arg223Gln, Ala273Glu and Arg497Gln). Association analysis indicated that A24101991G is significantly associated with chicken salmonellosis resistance. These results will be useful for functional investigation of chicken Nramp1 and Nramp2 genes.

Subcellular localization of iron and heme metabolism related proteins at early stages of erythrophagocytosis

Background

Senescent red blood cells (RBC) are recognized, phagocytosed and cleared by tissue macrophages. During this erythrophagocytosis (EP), RBC are engulfed and processed in special compartments called erythrophagosomes. We previously described that following EP, heme is rapidly degraded through the catabolic activity of heme oxygenase (HO). Extracted heme iron is then either exported or stored by macrophages. However, the cellular localization of the early steps of heme processing and iron extraction during EP remains to be clearly defined.

Methodology/Principal Findings

We took advantage of our previously described cellular model of EP, using bone marrow-derived macrophages (BMDM). The subcellular localization of both inducible and constitutive isoforms of HO (HO-1 and HO-2), of the divalent metal transporters (Nramp1, Nramp2/DMT1, Fpn), and of the recently identified heme transporter HRG-1, was followed by fluorescence and electron microscopy during the earliest steps of EP. We also looked at some ER [calnexin, glucose-6-phosphatase (G6Pase) activity] and lysosomes (Lamp1) markers during EP. In both quiescent and LPS-activated BMDM, Nramp1 and Lamp1 were shown to be strong markers of the erythrophagolysosomal membrane. HRG-1 was also recruited to the erythrophagosome. Furthermore, we observed calnexin labeling and G6Pase activity at the erythrophagosomal membrane, indicating the contribution of ER in this phagocytosis model. In contrast, Nramp2/DMT1, Fpn, HO-1 and HO-2 were not detected at the membrane of erythrophagosomes.

Conclusions/Significance

Our study highlights the subcellular localization of various heme- and iron-related proteins during early steps of EP, thereby suggesting a model for heme catabolism occurring outside the phagosome, with heme likely being transported into the cytosol through HRG1. The precise function of Nramp1 at the phagosomal membrane in this model remains to be determined.

DMT1 (IRE) expression in intestinal and erythroid cells is regulated by peripheral benzodiazepine receptor-associated protein 7

The divalent metal transporter 1 (DMT1) is essential for cellular uptake of iron, mediating iron absorption across the duodenal brush border membrane. We have previously shown that with iron feeding DMT1 in the brush border membrane undergoes endocytosis into the subapical compartment of enterocytes. To understand the mechanisms of iron-induced endocytosis of DMT1, we used the yeast two-hybrid system to find proteins that interact with DMT1 and isolated from a rat duodenal cDNA library a protein that interacts specifically with the IRE containing isoform of DMT1 {DMT1 [iron-responsive element (IRE)]}. The protein (Genbank AY336075) is 97.5% identical with peripheral benzodiazepine receptor-associated protein 7 (PAP7), a protein that interacts with the peripheral benzodiazepine receptor. PAP7 is ubiquitously expressed in the rat and in multiple cell lines with consensus sequences including a nuclear localization signal and a Golgi dynamic domain. PAP7, expressed on the brush border of rat duodenum, copurified with DMT1 in brush border membrane vesicles, and following iron feeding, was internalized in parallel with the internalization of DMT1. To determine if PAP7 plays a role in cellular iron metabolism, we downregulated PAP7 expression in K562 cells with small interfering RNA. Following the decrease in PAP7 protein, DMT1 (IRE) protein but not mRNA was significantly downregulated but without effect on DMT1 (non-IRE), transferin (Tf)R1, or ferritin expression. Lowered levels of PAP7 resulted also in decreased cell proliferation and G(1) cell cycle arrest. These data are consistent with PAP7 interacting with DMT1 (IRE) and regulating DMT1 (IRE) expression in K562 cells by modulating expression of DMT1 (IRE) protein.

H(+)-coupled divalent metal-ion transporter-1: functional properties, physiological roles and therapeutics

Divalent metal-ion transporter-1 (DMT1) is a widely expressed, iron-preferring membrane transport protein. Animal models establish that DMT1 plays indispensable roles in intestinal nonheme-iron absorption and iron acquisition by erythroid precursor cells. Rare mutations in human DMT1 result in severe microcytic-hypochromic anemia. When we express DMT1 in RNA-injected Xenopus oocytes, we observe rheogenic Fe(2+) transport that is driven by the proton electrochemical potential gradient. In that same preparation, DMT1 also transports cadmium and manganese but not copper. Whether manganese metabolism relies upon DMT1 remains unclear but DMT1 contributes to the effects of overexposure to cadmium and manganese in some tissues. There exist at least four DMT1 isoforms that arise from variant transcription of the SLC11A2 gene. Whereas these isoforms display identical functional properties, N- and C-terminal variations contain cues that direct the cell-specific targeting of DMT1 isoforms to discrete subcellular compartments (plasma membrane, endosomes, and lysosomes). An iron-responsive element (IRE) in the mRNA 3'-untranslated region permits the regulation of some isoforms by iron status, and additional mechanisms by which DMT1 is regulated are emerging. Natural-resistance-associated macrophage protein-1 (NRAMP1)-the only other member of the mammalian SLC11 gene family-contributes to antimicrobial function by extruding from the phagolysosome divalent metal ions (e.g. Mn(2+)) that may be essential cofactors for bacteria-derived enzymes or required for bacterial growth. The principal or only intestinal nonheme-iron transporter, DMT1 is a validated therapeutic target in hereditary hemochromatosis (HHC) and other iron-overload disorders.

Low doses of cadmium chloride and methallothionein-1-bound cadmium display different accumulation kinetics and induce different genes in cells of the human nephron

Background/Aims

The present study was conducted to investigate the renal tubular handling of inorganic cadmium (Cd²⁺) by exposing primary human tubular cell cultures to physiologically relevant doses of cadmium chloride (CdCl₂). Furthermore, the cellular accumulation of Cd²⁺ was compared to that of metallothionein-1-bound Cd (Cd7MT-1). Finally, this study aimed to investigate the effect of the accumulation of Cd (both Cd²⁺ and Cd7MT-1) in renal cells on the expression of genes relevant to nephrotoxic processes.

Methods

Cd concentration was measured using atomic absorption spectrometry. mRNA expression was evaluated by quantitative real-time RT-PCR.

Results

Cd²⁺ accumulated into human tubular cells in a concentration- and time-dependent way. Furthermore, cellular accumulation of Cd²⁺ was different from the cellular accumulation of Cd7MT-1, indicative for different uptake routes. Finally, mRNA expression of the genes encoding the anti-oxidative proteins metallothionein-1 (MT-1) and heme-oxygenase-1 (HO-1) as well as the pro-apoptotic Bcl-2-associated X protein (Bax) were upregulated by CdCl₂ and not by Cd7MT1.

Conclusion

In the presence of physiologically relevant Cd concentrations, tubular accumulation of the element in its inorganic form is different from that of Cd7MT-1. Furthermore, the tubular accumulation of inorganic Cd induces mRNA expression of genes of which the protein products may play a role in Cd-associated renal toxicity.

The natural resistance-associated macrophage protein from the protozoan parasite Perkinsus marinus mediates iron uptake

Microbial pathogens succeed in acquiring essential metals such as iron and manganese despite their limited availability because of the host's immune response. The eukaryotic natural resistance-associated macrophage proteins mediate uptake of divalent metals and, during infection, may compete directly for metal acquisition with the pathogens' transporters. In this study, we characterize the Nramp gene family of Perkinsus marinus, an intracellular parasite of the eastern oyster, and through yeast complementation, we demonstrate for the first time for a protozoan parasite that Nramp imports environmental Fe. Three PmNramp isogenes differ in their exon-intron structures and encode transcripts that display a trans splicing leader at the 5' end. The protein sequences share conserved properties predicted for the Nramp/Solute carrier 11 (Slc11) family, such as 12-transmembrane segment (TMS) topology (N- and C-termini cytoplasmic) and preferential conservation of four TMS predicted to form a pseudosymmetric proton/metal symport pathway. Yeast fet3fet4 mutant complementation assays showed iron transport activity for PmNramp1 and a fusion chimera of the PmNramp3 hydrophobic core and PmNramp1 N- and C-termini. PmNramp1 site-directed mutagenesis demonstrated that Slc11 invariant and predicted pseudosymmetric motifs (TMS1 Asp-Pro-Gly and TMS6 Met-Pro-His) are key for transport function. PmNramp1 TMS1 mutants D76E, G78A, and D76E/G78A prevented membrane protein expression, while TMS6 M250A, H252Y, and M250A/H252Y specifically abrogated Fe uptake; the TMS6 H252Y mutation also correlates with divergence from Nramp specificity for divalent metals.

Folding and assembly of TMD 6-related segments of DMT 1 in trifluoroethanol aqueous solution

Divalent metal-ion transporter 1 (DMT1) belongs to a large class of metal-ion transporters that drive the translocation of a wide range of divalent metal substrates across membranes toward the cytosol with couple of protons. Two highly conserved histidines in the sixth transmembrane domain (TMD6) are essential for metal transport activity in DMT1. In the present study, we determine the high-resolution structures of three 25-residue peptides, corresponding to TMD6 of the wildtype DMT1 (the segment 255-279) and its H267A and H272A mutants, in 30% TFE-d(2) aqueous solution by the combined use of circular dichroism (CD) and NMR spectroscopies. The wildtype peptide forms an 'α-helix-extended segment-α-helix' structure with two helices spanning over Gly258-Ala262 and Met265-Lys277 linked by a hinge at residues Val263-Ile264. The H267A mutation reduces the hinge to one residue (Ile264), while the H272A mutation extends the flexible region of the central part from Val263 to His267. Diffusion-ordered spectroscopy (DOSY) study demonstrates that all the peptides are self-assembly as trimer in 30% TFE-d(2) aqueous solution. The H272A substitution decreases the intermolecular interaction whereas the H267A substitution may enhance the intermolecular interaction. The specific structure of the discontinuous helix and the self-assembly feature of DMT1-TMD6 may be crucial for its biological function. The changes in conformation and intermolecular interaction induced by histidine substitution may be correlated with the deficiency of DMT1 in metal-ion permeation.

A novel vanadium transporter of the Nramp family expressed at the vacuole of vanadium-accumulating cells of the ascidian Ascidia sydneiensis samea

BACKGROUND

Vanadium is an essential transition metal in biological systems. Several key proteins related to vanadium accumulation and its physiological function have been isolated, but no vanadium ion transporter has yet been identified.

METHODS

We identified and cloned a member of the Nramp/DCT family of membrane metal transporters (AsNramp) from the ascidian Ascidia sydneiensis samea, which can accumulate extremely high levels of vanadium in the vacuoles of a type of blood cell called signet ring cells (also called vanadocytes). We performed immunological and biochemical experiments to examine its expression and transport function.

RESULTS

Western blotting analysis showed that AsNramp was localized at the vacuolar membrane of vanadocytes. Using the Xenopus oocyte expression system, we showed that AsNramp transported VO(2+) into the oocyte as pH-dependent manner above pH 6, while no significant activity was observed below pH 6. Kinetic parameters (K(m) and V(max)) of AsNramp-mediated VO(2+) transport at pH 8.5 were 90nM and 9.1pmol/oocyte/h, respectively. A rat homolog, DCT1, did not transport VO(2+) under the same conditions. Excess Fe(2+), Cu(2+), Mn(2+), or Zn(2+) inhibited the transport of VO(2+). AsNramp was revealed to be a novel VO(2+)/H(+) antiporter, and we propose that AsNramp mediates vanadium accumulation coupled with the electrochemical gradient generated by vacuolar H(+)-ATPase in vanadocytes.

GENERAL SIGNIFICANCE

This is the first report of identification and functional analysis on a membrane transporter for vanadium ions.

Structure and metal ion binding of the first transmembrane domain of DMT1

DMT1 is an integral membrane protein with 12 putative transmembrane domains. As a divalent metal ion transporter, it plays an important role in metal ion homeostasis from bacteria to human. Loss-function mutations at the conserved motif DPGN located within the first transmembrane domain (TMD1) of DMT1 indicate the significance of TMD1 in the biological function of the protein. In the present work, we study the structure, topology and metal ion binding of DMT1-TMD1 peptide by nuclear magnetic resonance using sodium dodecyl sulfate and dodecylphosphocholine micelles as membrane mimics. We find that the peptide forms an α-helix-extended segment-α-helix configuration in which the motif DPGN locates at the central flexible region. The N-terminal part of the peptide is deeply embedded in micelles, while the motif section and the C-terminal part are close to the surface of micelles. The peptide can bind to Mn2+ and Co2+ ions by the side chains of the negatively charged residues in the motif section and the C-terminal part of TMD1. The crucial role of the central flexible region and the C-terminal part of TMD1 in metal ion capture is confirmed by the binding of the N-terminal part truncated TMD1 to metal ions.

Evidence that highly conserved residues of transmembrane segment 6 of Escherichia coli MntH are important for transport activity

Nramp (natural resistance-associated macrophage protein) family members have been characterized in mammals, yeast, and bacteria as divalent metal ion/H(+) symporters. In previous work, a bioinformatic approach was used for the identification of residues that are conserved within the Nramp family [Haemig, H. A., and Brooker, R. J. (2004) J. Membr. Biol. 201 (2), 97-107]. On the basis of site-directed mutagenesis of highly conserved negatively charged residues, a model was proposed for the metal binding site of the Escherichia coli homologue, MntH. In this study, we have focused on the highly conserved residues, including two histidines, of transmembrane segment 6 (TMS-6). Multiple mutants were made at the eight conserved sites (i.e., Gly-205, Ala-206, Met-209, Pro-210, His-211, Leu-215, His-216, and Ser-217) in TMS-6 of E. coli MntH. Double mutants involving His-211 and His-216 were also created. The results indicate the side chain volume of these residues is critically important for function. In most cases, only substitutions that are closest in side chain volume still permit transport. In addition, the K(m) for metal binding is largely unaffected by mutations in TMS-6, whereas V(max) values were decreased in all mutants characterized kinetically. Thus, these residues do not appear to play a role in metal binding. Instead, they may comprise an important face on TMS-6 that is critical for protein conformational changes during transport. Also, in contrast to other studies, our data do not strongly indicate that the conserved histidine residues play a role in the pH regulation of metal transport.

Identification of an "alpha-helix-extended segment-alpha-helix" conformation of the sixth transmembrane domain in DMT1

DMT1 (divalent metal ion transporter 1) is one member of a family of proton-coupled transporters that facilitate the cellular absorption of divalent metal ions. A pair of mutation-sensitive and highly conserved histidines in the sixth transmembrane domain (TM6) of DMT1 was found to be important for proton-metal ion cotransport. In the present work, we investigate the structures and locations of the peptides from TM6 of DMT1 and its H267A and H272A mutants in SDS micelles by CD and NMR methods. The circular dichroism studies show that the alpha-helix is a predominant conformation for the wildtype peptide and H267A mutant in SDS micelles, whereas the helicity is evidently decreased for H272A mutant. The pH value has little effect on the alpha-helical contents of the three peptides. The NMR studies indicate that the wildtype peptide in SDS micelles forms an "alpha-helix-extended segment-alpha-helix" structure in which the His267 locates near the central part of the extended segment, while the His272 is involved in the alpha-helical folding. Both histidines are buried in SDS micelles as evidenced by their pK(a) values. The structure of the wildtype peptide is evidently changed by the mutations of H267A and H272A. The H267A mutant forms an ordered structure consisting of an alpha-helix from the C-terminus to the central part and continuous turns in the residual part. The extended structure in the central part of the wildtype peptide is abolished by H267A mutation. The H272A mutation mainly induces unfolding of the short helix in the N-terminal side, while the short helix in the C-terminal side and unordered conformation in the central part remain. All the three peptides are embedded in SDS micelles, and the H267A mutant is inserted more deeply due to increasing hydrophobicity in the central part of the peptide. The specific "alpha-helix-extended segment-alpha-helix" structure of TM6 may have an important implication for the binding of the transporter to H(+) and metal ions and the conformation change induced by the mutations of two highly conserved histidines may be correlated to the deficiency of the transport activity of DMT1.

Transmembrane topology of the mammalian Slc11a2 iron transporter

The mammalian Slc11a1 and Slc11a2 proteins define a large family of secondary metal transporters. Slc11a1 and Slc11a2 function as pH-dependent divalent cation transporters that play a critical role in host defenses against infections and in Fe²⁺ homeostasis, respectively. The position and polarity of individual transmembrane domains (TMD) of Slc11a2 were studied by an epitope tagging method based on the insertion of small antigenic hemagglutinin A (HA) peptides (YPYDVPDYAS) in predicted intra- or extracellular loops of the protein. The tagged proteins were expressed in transfected LLC-PK1 kidney cells and tested for transport activity, and the polarity of inserted tags with respect to the plasma membrane was determined by immunofluorescence in intact and permeabilized cells. HA epitope tags were inserted at positions 1, 98, 131, 175, 201, 243, 284, 344, 403, 432, 468, 504, and 561. Insertions at positions 98, 131, 175, 403, and 432 abrogated metal transport by Slc11a2, while insertions at positions 1, 201, 243, 284, 344, 468, 504, and 561 resulted in functional proteins. Topology mapping in functional HA-tagged Slc11a2 proteins indicated that the N-terminus (1), as well as loops delineated by TMD4−5 (201), TMD6−7 (284), and TMD10−11 (468), and C-terminus (561) are intracellular, while loops separating TMD5−6 (243), TMD7−8 (344), and TMD11−12 (504) are extracellular. These results are compatible with a topology of 12 transmembrane domains, with intracellular amino and carboxy termini. Structural models constructed by homology threading support this 12TMD topology and show 2-fold structural symmetry in the arrangement of membrane helices for TM1−5 and TM6−10 (conserved Slc11 hydrophobic core).

Down-regulation of a manganese transporter in the face of metal toxicity

The yeast Smf1p Nramp manganese transporter is posttranslationally regulated by environmental manganese. Smf1p is stabilized at the cell surface with manganese starvation, but is largely degraded in the vacuole with physiological manganese through a mechanism involving the Rsp5p adaptor complex Bsd2p/Tre1p/Tre2p. We now describe an additional level of Smf1p regulation that occurs with toxicity from manganese, but not other essential metals. This regulation is largely Smf1p-specific. As with physiological manganese, toxic manganese triggers vacuolar degradation of Smf1p by trafficking through the multivesicular body. However, regulation by toxic manganese does not involve Bsd2p/Tre1p/Tre2p. Toxic manganese triggers both endocytosis of cell surface Smf1p and vacuolar targeting of intracellular Smf1p through the exocytic pathway. Notably, the kinetics of vacuolar targeting for Smf1p are relatively slow with toxic manganese and require prolonged exposures to the metal. Down-regulation of Smf1p by toxic manganese does not require transport activity of Smf1p, whereas such transport activity is needed for Smf1p regulation by manganese starvation. Furthermore, the responses to manganese starvation and manganese toxicity involve separate cellular compartments. We provide evidence that manganese starvation is sensed within the lumen of the secretory pathway, whereas manganese toxicity is sensed within an extra-Golgi/cytosolic compartment of the cell.

Role of the glutamate 185 residue in proton translocation mediated by the proton-coupled folate transporter SLC46A1

The proton-coupled folate transporter (PCFT) SLC46A1 mediates uphill folate transport into enterocytes in proximal small intestine coupled to the inwardly directed proton gradient. Hereditary folate malabsorption is due to loss-of-function mutations in the PCFT gene. This study addresses the functional role of conserved charged amino acid residues within PCFT transmembrane domains with a detailed analysis of the PCFT E185 residue. D156A-, E185A-, E232A-, R148A-, and R376A-PCFT mutants lost function at pH 5.5, as assessed by transient transfection in folate transport-deficient HeLa cells. At pH 7.4, function was preserved only for E185A-PCFT. Loss of function for E185A-PCFT at pH 5.5 was due to an eightfold decrease in the [(3)H]methotrexate (MTX) influx V(max); the MTX influx K(t) was identical to that of wild-type (WT)-PCFT (1.5 microM). Consistent with the intrinsic functionality of E185A-PCFT, [(3)H]MTX influx at pH 5.5 or 7.4 was trans-stimulated in cells preloaded with nonlabeled MTX or 5-formyltetrahydrofolate. Replacement of E185 with Leu, Cys, His, or Gln resulted in a phenotype similar to E185A-PCFT. However, there was greater preservation of activity (approximately 38% of WT) for the similarly charged E185D-PCFT at pH 5.5. All E185 substitution mutants were biotin accessible at the plasma membrane at a level comparable to WT-PCFT. These observations suggest that the E185 residue plays an important role in the coupled flows of protons and folate mediated by PCFT. Coupling appears to have a profound effect on the maximum rate of transport, consistent with augmentation of a rate-limiting step in the PCFT transport cycle.

The functional roles of the His247 and His281 residues in folate and proton translocation mediated by the human proton-coupled folate transporter SLC46A1

This report addresses the functional role of His residues in the proton-coupled folate transporter (PCFT; SLC46A1), which mediates intestinal folate absorption. Of ten His residues, only H247A and H281A mutations altered function. The folic acid influx Kt at pH 5.5 for H247A was downward arrow 8.4-fold. Although wild type (WT)-PCFT Ki values varied among the folates, Ki values were much lower and comparable for H247-A, -R, -Q, or -E mutants. Homology modeling localized His247 to the large loop separating transmembrane domains 6 and 7 at the cytoplasmic entrance of the translocation pathway in hydrogen-bond distance to Ser172. The folic acid influx Kt for S172A-PCFT was decreased similar to H247A. His281 faces the extracellular region in the seventh transmembrane domain. H281A-PCFT results in loss-of-function due to approximately 12-fold upward arrow in the folic acid influx Kt. When the pH was decreased from 5.5 to 4.5, the WT-PCFT folic acid influx Kt was unchanged, but the Kt decreased 4-fold for H281A. In electrophysiological studies in Xenopus oocytes, both WT-PCFT- and H281A-PCFT-mediated folic acid uptake produced current and acidification, and both exhibited a low level of folate-independent proton transport (slippage). Slippage was markedly increased for the H247A-PCFT mutant. The data suggest that disruption of the His247 to Ser172 interaction results in a PCFT conformational alteration causing a loss of selectivity, increased substrate access to a high affinity binding pocket, and proton transport in the absence of a folate gradient. The His281 residue is not essential for proton coupling but plays an important role in PCFT protonation, which, in turn, augments folate binding to the carrier.

Regulation of the divalent metal ion transporter DMT1 and iron homeostasis by a ubiquitin-dependent mechanism involving Ndfips and WWP2

Many ion channels and transporters are regulated by ubiquitination mediated by the Nedd4 family of HECT-type ubiquitin ligases (E3s). These E3s commonly interact with substrates via their WW domains that bind to specific motifs in target proteins. However, not all potential targets of these E3s contain WW-binding motifs. Therefore, accessory proteins may mediate the interaction between Nedd4 family members and their targets. Here we report that the divalent metal ion transporter DMT1, the primary nonheme iron transporter in mammals, is regulated by ubiquitination mediated by the Nedd4 family member WWP2. DMT1 interacts with 2 WW domain-interacting proteins, Ndfip1 and Ndfip2, previously proposed to have roles in protein trafficking. This promotes DMT1 ubiquitination and degradation by WWP2. Consistent with these observations, Ndfip1−/− mice show increased DMT1 activity and a concomitant increase in hepatic iron deposition, indicating an essential function of Ndfip1 in iron homeostasis. This novel mechanism of regulating iron homeostasis suggests that Ndfips and WWP2 may contribute to diseases involving aberrant iron transport.

Phlebotomies or erythropoietin injections allow mobilization of iron stores in a mouse model mimicking intensive care anemia

OBJECTIVE

Anemia in critically ill patients is frequent and consists of chronic disease associated with blood losses. These two mechanisms have opposite effects on iron homeostasis, especially on the expression of the iron regulatory hormone hepcidin. We developed a mouse model mimicking the intensive care anemia to explore iron homeostasis.

DESIGN

Experimental study.

SETTING

University-based research laboratory.

SUBJECTS

C57BL/6 mice.

INTERVENTIONS

Mice received either a single intraperitoneal injection of lipopolysaccharide followed 1 week later by zymosan, or were subjected to repeated phlebotomies by retro-orbital punctures, or both. Several subsets of mice were analyzed over a 14-day period to describe the mouse model of intensive care anemia. Additional mice received erythropoietin injections with or without the zymosan treatment and were killed at day 5, to perform a more detailed analysis.

MEASUREMENTS AND MAIN RESULTS

We observed anemia as soon as 5 days after zymosan injection, together with increased messenger RNA (mRNA) levels for interleukin-6 and hepcidin. Phlebotomies alone fully suppressed hepcidin mRNA expression. Interestingly, in mice treated with zymosan and phlebotomies, hepcidin expression was suppressed, despite the persistent increase in interleukin-6. Stimulation of erythropoiesis by erythropoietin injections also led to a decrease in hepcidin mRNA in zymosan-treated mice. In these situations combining inflammation and erythropoiesis stimulation, there was no change in ferroportin, the membrane iron exporter, at the mRNA level, whereas ferroportin protein increased. Macrophage iron stores (assessed by histology using diaminobenzidine staining, or by quantification of nonheme iron and ferritin concentrations) were depleted in the spleen.

CONCLUSIONS

These results suggest that the erythroid factor dominates over inflammation for hepcidin regulation, and that iron could be mobilized in these situations combining inflammation and erythropoiesis stimulation.

Structure analysis of the fourth transmembrane domain of Nramp1 in model membranes

Nramp1 (natural resistance-associated macrophage protein 1) is an integral membrane protein with 12 putative transmembrane domains. As a proton-coupled divalent metal cation transporter, it is involved in defense against intracellular pathogens. Disease-causing mutation in Nramp1 occurring at glycine 169 located within the fourth transmembrane domain (TM4) suggests functional importance of this domain. In this paper, we study the three-dimensional structures of a peptide, corresponding to the TM4 of the wild-type Nramp1, in SDS micelles and 2, 2, 2-trifluoroethanol solvent using CD and NMR spectroscopies. We have found that an alpha-helix is predominantly induced in membrane-mimetic environments and the folding of the C-terminal residues is regulated by pH in SDS micelles. The peptide is embedded in SDS micelles and self-associated by coiled-coil interactions. The helix of the peptide in TFE is lengthened towards the N-terminus compared with those in SDS micelles at acidic pH and the self-association of the peptide is also observed in TFE. The fact that Mn(2+) ions are accessible to Asp-14 located in the interior of SDS micelles is found and the binding affinity is increased with increasing pH. The self-association of the peptide may provide a path by which Mn(2+) ions pass through the membrane.