A novel R416C mutation in human DMT1 (SLC11A2) displays pleiotropic effects on function and causes microcytic anemia and hepatic iron overload

Oral iron therapy: Current concepts and future prospects for improving efficacy and outcomes

Iron deficiency (ID) and iron-deficiency anaemia (IDA) are global public health concerns, most commonly afflicting children, pregnant women and women of childbearing age. Pathological outcomes of ID include delayed cognitive development in children, adverse pregnancy outcomes and decreased work capacity in adults. IDA is usually treated by oral iron supplementation, typically using iron salts (e.g. FeSO4 ); however, dosing at several-fold above the RDA may be required due to less efficient absorption. Excess enteral iron causes adverse gastrointestinal side effects, thus reducing compliance, and negatively impacts the gut microbiome. Recent research has sought to identify new iron formulations with better absorption so that lower effective dosing can be utilized. This article outlines emerging research on oral iron supplementation and focuses on molecular mechanisms by which different supplemental forms of iron are transported across the intestinal epithelium and whether these transport pathways are subject to regulation by the iron-regulatory hormone hepcidin.

The Associations between Metalloestrogens, GSTP1, and SLC11A2 Polymorphism and the Risk of Endometrial Cancer

Background: The incidence of endometrial cancer (EC) is still rising. Numerous risk factors including patient characteristics and molecular instability have been identified for EC. The presence of specific molecular markers allows specific diagnostic and prognostic approaches. Several single nucleotide polymorphisms (SNPs) have been identified to influence endometrial cancer risk. Metalloestrogens are metal ions which can mimic estrogen activity; however, their role in uterine pathologies remains unknown. This study aimed to investigate total blood trace elements levels and evaluate the distribution of selected genotypes in GSTP1 and SLC11A2 genes. Methods: This retrospective case-control analysis was carried out in peripheral blood samples of 110 women with endometrial cancer (EC; n = 21), uterine fibroma (n = 25), endometrial polyp (n = 48), and normal endometrium (n = 16). Analysis included measurement of metals and phosphor in serum, and of genetic polymorphisms in GST (rs1695) and SLC11A2 (rs224589) in DNA from white blood cells. Serum trace elements were measured using ICP-OES spectrometry. SNPs were identified using Taq Man real-time PCR genotyping assays. Results: The study confirmed higher age (OR 2.19, 95% CI 1.69–2.24), post-menopausal status (OR 1.89, 95% CI 1.36–1.94), and diabetes type 2 (OR 1.54; 95% CI 0.97–1.72) as independent risk factors for EC. We also found a high level of Cd (OR 1.49; 95% CI 1.31–1.63) and a low level of Co (OR 0.76; 95% CI 0.53–0.59) to be independent risk factors of EC. None of the tested polymorphisms of GSTP1 and SLC11A2 were associated with EC risk. However, high Cd (OR 1.21, 95% CI 1.15–1.29) and Ni (OR 1.07, 95% CI 1.05–1.18) serum levels were significantly associated with a SLC1A2 TG genotype, and high Cd levels with GSTP1 (OR 1.05, 95% CI 1.01–1.13).

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.

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.

Oral Gavage of Ginger Nanoparticle-Derived Lipid Vectors Carrying Dmt1 siRNA Blunts Iron Loading in Murine Hereditary Hemochromatosis

Nanoparticles (NPs) have been utilized to deliver drugs to the intestinal epithelium in vivo. Moreover, NPs derived from edible plants are less toxic than synthetic NPs. Here, we utilized ginger NP-derived lipid vectors (GDLVs) in a proof-of-concept investigation to test the hypothesis that inhibiting expression of divalent metal-ion transporter 1 (Dmt1) would attenuate iron loading in a mouse model of hereditary hemochromatosis (HH). Initial experiments using duodenal epithelial organ cultures from intestine-specific Dmt1 knockout (KO) (Dmt1int/int) mice in the Ussing chamber established that Dmt1 is the only active iron importer during iron-deficiency anemia. Further, when Dmt1int/int mice were crossed with mice lacking the iron-regulatory hormone, hepcidin (Hepc-/-), iron loading was abolished. Hence, intestinal Dmt1 is required for the excessive iron absorption that typifies HH. Additional experiments established a protocol to produce GDLVs carrying functional Dmt1 small interfering RNAs (siRNAs) and to target these gene delivery vehicles to the duodenal epithelium in vivo (by incorporating folic acid [FA]). When FA-GDLVs carrying Dmt1 siRNA were administered to weanling Hepc-/- mice for 16 days, intestinal Dmt1 mRNA expression was attenuated and tissue iron accumulation was blunted. Oral delivery of functional siRNAs by FA-GDLVs is a suitable therapeutic approach to mitigate iron loading in murine HH.

Iron regulatory protein (IRP)-iron responsive element (IRE) signaling pathway in human neurodegenerative diseases

The homeostasis of iron is vital to human health, and iron dyshomeostasis can lead to various disorders. Iron homeostasis is maintained by iron regulatory proteins (IRP1 and IRP2) and the iron-responsive element (IRE) signaling pathway. IRPs can bind to RNA stem-loops containing an IRE in the untranslated region (UTR) to manipulate translation of target mRNA. However, iron can bind to IRPs, leading to the dissociation of IRPs from the IRE and altered translation of target transcripts. Recently an IRE is found in the 5′-UTR of amyloid precursor protein (APP) and α-synuclein (α-Syn) transcripts. The levels of α-Syn, APP and amyloid β-peptide (Aβ) as well as protein aggregation can be down-regulated by IRPs but are up-regulated in the presence of iron accumulation. Therefore, inhibition of the IRE-modulated expression of APP and α-Syn or chelation of iron in patient’s brains has therapeutic significance to human neurodegenerative diseases. Currently, new pre-drug IRE inhibitors with therapeutic effects have been identified and are at different stages of clinical trials for human neurodegenerative diseases. Although some promising drug candidates of chemical IRE inhibitors and iron-chelating agents have been identified and are being validated in clinical trials for neurodegenerative diseases, future studies are expected to further establish the clinical efficacy and safety of IRE inhibitors and iron-chelating agents in patients with neurodegenerative diseases.

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.

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.

Iron in Eukarya

Iron is essential for the normal physiological function of all organisms. In humans it is required for a plethora of biochemical roles including the transport of oxygen in the blood and energy production in the mitochondria. However, iron is also highly cytotoxic when present at high levels as it readily participates in oxidation-reduction reactions that lead to the generation of reactive oxygen species. One unique feature of iron biology is the lack of excretory mechanisms to remove excess iron from the body. Therefore, the concerted action of several genes and proteins working together to regulate the movement of iron across cell membranes, its storage in peripheral tissues and its physiological utilization in the body is essential for maintaining iron homeostasis. Humans are exposed to iron in a number of chemical forms (haem or non-haem; ferric or ferrous). This chapter will describe how humans acquire iron from their diet; the subsequent delivery of iron to its sites of utilization and storage; and how iron is recycled from effete erythrocytes for re-use in metabolism. Mutations in a number of the genes controlling iron metabolism have been identified and study of the pathological consequences of these mutations has allowed us to gain a greater understanding of how the body senses changes in iron status and coordinates its transport, storage and utilization to maintain homeostasis.

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.

Mammalian iron transporters: families SLC11 and SLC40

This review is focused on the mammalian SLC11 and SLC40 families and their roles in iron homeostasis. The SLC11 family is composed of two members, SLC11A1 and SLC11A2. SLC11A1 is expressed in the lysosomal compartment of macrophages and in the tertiary granules of neutrophils, playing a key role in innate resistance against infection by intracellular microbes. SLC11A2 is a key player in iron metabolism and is ubiquitously expressed, most notably in the proximal duodenum, immature erythroid cells, brain, placenta and kidney. Intestinal iron absorption is mediated by SLC11A2 at the apical membrane of enterocytes, followed by basolateral exit via SLC40A1. To meet the daily requirement for iron, approximately 80% of the iron comes from the breakdown of hemoglobin following macrophage phagocytosis of senescent erythrocytes (iron recycling). Both SLC11A1 and SLC11A2 play an important role in macrophage iron recycling. SLC11A2 also transports iron into the cytosol across the membrane of endocytotic vesicles of the transferrin receptor-cycle. SLC40A1 is the sole member of the SLC40 family and is involved in the only cellular iron efflux mechanism described. SLC40A1 is highly expressed in several tissues and cells that play a critical role in body iron homeostasis. The signaling pathways that regulate SLC11A2 and SLC40A1 expression at transcriptional, post-transcriptional and post-translational levels are discussed. The roles of SLC11A2 and/or SLC40A1 in iron-associated disorders such as hemochromatosis, neurodegenerative diseases, and breast cancer are also summarized.

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.

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.

Molecular and cellular mechanisms underlying neural tube defects in the loop-tail mutant mouse

Loop-tail (Lp) mice show a very severe neural tube defect (craniorachischisis) caused by mutations in the Vangl2 gene (D255E, S464N). Mammalian Vangl1 and Vangl2 are membrane proteins that play critical roles in development such as establishment of planar cell polarity (PCP) in epithelial layers and convergent extension movements during neurogenesis and cardiogenesis. Vangl proteins are thought to assemble with other PCP proteins (Dvl, Pk) to form membrane-bound PCP signaling complexes that provide polarity information to the cell. In the present study, we show that Vangl1 is expressed exclusively at the plasma membrane of transfected MDCK cells, where it is targeted to the basolateral membrane. Experiments with an inserted exofacial HA epitope indicate that the segment delimited by the predicted transmembrane domains 1 and 2 is exposed to the extracellular milieu. Comparative studies of the Lp-associated pathogenic mutation D255E indicate that the targeting of the mutant variant at the plasma membrane is greatly reduced; the mutant variant is predominantly retained intracellularly in endoplasmic reticulum (ER) vesicles colocalizing with the ER marker calreticulin. In addition, the D255E variant shows drastically reduced stability with a half-life of approximately 2 h, compared to >9 h for its wild type counterpart and is rapidly degraded in a proteasome-dependent and MG132 sensitive pathway. These findings highlight a critical role for D255 for normal folding and processing of Vangl proteins, with highly conservative substitutions not tolerated at that site. Our study provide an experimental framework for the analysis of human VANGL mutations recently identified in familial and sporadic cases of spina bifida.

SMF-1, SMF-2 and SMF-3 DMT1 orthologues regulate and are regulated differentially by manganese levels in C. elegans

Manganese (Mn) is an essential metal that can exert toxic effects at high concentrations, eventually leading to Parkinsonism. A major transporter of Mn in mammals is the divalent-metal transporter (DMT1). We characterize here DMT1-like proteins in the nematode C. elegans, which regulate and are regulated by Mn and iron (Fe) content. We identified three new DMT1-like genes in C. elegans: smf-1, smf-2 and smf-3. All three can functionally substitute for loss of their yeast orthologues in S. cerevisiae. In the worm, deletion of smf-1 or smf-3 led to an increased Mn tolerance, while loss of smf-2 led to increased Mn sensitivity. smf mRNA levels measured by QRT-PCR were up-regulated upon low Mn and down-regulated upon high Mn exposures. Translational GFP-fusions revealed that SMF-1 and SMF-3 strongly localize to partially overlapping apical regions of the gut epithelium, suggesting a differential role for SMF-1 and SMF-3 in Mn nutritional intake. Conversely, SMF-2 was detected in the marginal pharyngeal epithelium, possibly involved in metal-sensing. Analysis of metal content upon Mn exposure in smf mutants revealed that SMF-3 is required for normal Mn uptake, while smf-1 was dispensable. Higher smf-2 mRNA levels correlated with higher Fe content, supporting a role for SMF-2 in Fe uptake. In smf-1 and smf-3 but not in smf-2 mutants, increased Mn exposure led to decreased Fe levels, suggesting that both metals compete for transport by SMF-2. Finally, SMF-3 was post-translationally and reversibly down-regulated following Mn-exposure. In sum, we unraveled a complex interplay of transcriptional and post-translational regulations of 3 DMT1-like transporters in two adjacent tissues, which regulate metal-content in C. elegans.

Mutations in the gene encoding DMT1: clinical presentation and treatment

Divalent metal transporter 1 (DMT1) is the protein that allows elemental iron entry into the duodenal cell. It is expressed ubiquitously and it also allows the iron exit from the endosomes. This protein plays a central role in iron metabolism and it is strictly regulated. Several animal models elucidate its role in physiology. Recently three patients affected with DMT1 deficiency have been described. This recessively inherited condition appears at birth with severe microcytic anemia. Serum markers could be particularly useful to establish a correct diagnosis: high serum iron, normal total iron-binding capacity (TIBC), increased saturation of transferrin (Tf), slightly elevated ferritin, and increased soluble transferrin receptor (sTfR). Increased free erythrocyte protoporphyrins (FEPs) could address the diagnosis to iron-deficient anemia. All patients appeared to respond to erythropoietin (Epo) administration. Because mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) did not change during Epo treatment, it was concluded that Epo did not improve iron utilization of the erythroblasts but likely reduced the degree or intensity of apoptosis, affecting erythropoiesis. Moreover liver iron overload was present and documented in all of the affected patients. In this review we analyze the role of DMT1 in iron metabolism and the major causes of reduction and their consequences in animal models as well in humans, and we attempt to define the correct treatment for human mutants.

Polymorphisms and mutations of human TMPRSS6 in iron deficiency anemia

Male subjects with iron deficiency from the general population were examined for polymorphisms or sporadic mutations in TMPRSS6 to identify genetic risk factors for iron deficiency anemia. Three uncommon non-synonymous polymorphisms were identified, G228D, R446W, and V795I (allele frequencies 0.0074, 0.023 and 0.0074 respectively), of which the R446W polymorphism appeared to be overrepresented in the anemic population. In addition, three children with iron refractory iron deficiency anemia, and one sibling with iron responsive iron deficiency anemia were also examined for polymorphisms or sporadic mutations in TMPRSS6. Two children (family 1) were compound heterozygotes for a L674F mutation and a previously described splicing defect predicted to cause skipping of exon 13 (IVS13+1 G>A). One child from the second family was homozygous for a deletion (497T) causing a frameshift (L166X+36) and premature termination. The sibling and mother from the second family were compound heterozygotes for the L166X mutation and the uncommon R446W polymorphism. Although in vitro expression studies demonstrated that the R446W isoform was biologically similar to wildtype Tmprss6, clinical data indicate that the R446W produces a milder disease when carried in trans with severe mutation in Tmprss6. The four children carrying mutations in TMPRSS6 all exhibited inappropriately high urinary hepcidin levels for the degree of iron deficiency.

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).

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.

Forward genetic dissection of immunity to infection in the mouse

Forward genetics is an experimental approach in which gene mapping and positional cloning are used to elucidate the molecular mechanisms underlying phenotypic differences between two individuals for a given trait. This strategy has been highly successful for the study of inbred mouse strains that show differences in innate susceptibility to bacterial, parasitic, fungal, and viral infections. Over the past 20 years, these studies have led to the identification of a number of cell populations and critical biochemical pathways and proteins that are essential for the early detection of and response to invading pathogens. Strikingly, the macrophage is the point of convergence for many of these genetic studies. This has led to the identification of diverse pathways involved in extracellular and intracellular pathogen recognition, modification of the properties and content of phagosomes, transcriptional response, and signal transduction for activation of adaptive immune mechanisms. In models of viral infections, elegant genetic studies highlighted the pivotal role of natural killer cells in the detection and destruction of infected cells.

Manganese transport in eukaryotes: the role of DMT1

Manganese (Mn) is a transition metal that is essential for normal cell growth and development, but is toxic at high concentrations. While Mn deficiency is uncommon in humans, Mn toxicity is known to be readily prevalent due to occupational overexposure in miners, smelters and possibly welders. Excessive exposure to Mn can cause Parkinson's disease-like syndrome; patients typically exhibit extrapyramidal symptoms that include tremor, rigidity and hypokinesia [Calne DB, Chu NS, Huang CC, Lu CS, Olanow W. Manganism and idiopathic parkinsonism: similarities and differences. Neurology 1994;44(9):1583-6; Dobson AW, Erikson KM, Aschner M. Manganese neurotoxicity. Ann NY Acad Sci 2004;1012:115-28]. Mn-induced motor neuron diseases have been the subjects of numerous studies; however, this review is not intended to discuss its neurotoxic potential or its role in the etiology of motor neuron disorders. Rather, it will focus on Mn uptake and transport via the orthologues of the divalent metal transporter (DMT1) and its possible implications to Mn toxicity in various categories of eukaryotic systems, such as in vitro cell lines, in vivo rodents, the fruitfly, Drosophila melanogaster, the honeybee, Apis mellifera L., the nematode, Caenorhabditis elegans and the baker's yeast, Saccharomyces cerevisiae.

Natural history of recessive inheritance of DMT1 mutations

DMT1 deficiency causes microcytic hypochromic anemia due to decreased erythroid iron utilization. Anemia is present from birth. Transferrin saturation is high and serum ferritin is mildly elevated, despite liver iron overload. DMT1 deficiency must be considered in the differential diagnosis of microcytic hypochromic anemia observed in the newborn period.

Genetic analysis of resistance to infections in mice: A/J meets C57BL/6J

Susceptibility to infectious diseases has long been known to have a genetic component in human populations. This genetic effect is often complex and difficult to study as it is further modified by environmental factors including the disease-causing pathogen itself. The laboratory mouse has proved a useful alternative to implement a genetic approach to study host defenses against infections. Our laboratory has used genetic analysis and positional cloning to characterize single and multi-gene effects regulating inter-strain differences in the susceptibility of A/J and C57BL/6J mice to infection with several bacterial and parasitic pathogens. This has led to the identification of several proteins including Nrampl (Slc11a1), Birc1e, Icsbp, C5a, and others that play critical roles in the antimicrobial defenses of macrophages against intracellular pathogens. The use of AcB/BcA recombinant congenic strains has further facilitated the characterization of single gene effects in complex traits such as susceptibility to malaria. The genetic identification of erythrocyte pyruvate kinase (Pklr) and myeloid pantetheinase enzymes (Vnn1/3) as key regulators of blood-stage parasitemia has suggested that cellular redox potential may be a key biochemical determinant of Plasmodium parasite replication. Expanding these types of studies to additional inbred strains and to emerging stocks of mutagenized mice will undoubtedly continue to unravel the molecular basis of host defense against infections.

Molecular genetic studies of DMT1 on 12q in French-Canadian restless legs syndrome patients and families

Converging evidence from clinical observations, brain imaging and pathological findings strongly indicate impaired brain iron regulation in restless legs syndrome (RLS). Animal models with mutation in (DMT1) divalent metal transporter 1 gene, an important brain iron transporter, demonstrate a similar iron deficiency profile as found in RLS brain. The human DMT1 gene, mapped to chromosome 12q near the RLS1 locus, qualifies as an excellent functional and possible positional candidate for RLS. DMT1 protein levels were assessed in lymphoblastoid cell lines from RLS patients and controls. Linkage analyses were carried out with markers flanking and within the DMT1 gene. Selected patient samples from RLS families with compatible linkage to the RLS1 locus on 12q were fully sequenced in both the coding regions and the long stretches of UTR sequences. Finally, selected sequence variants were further studied in case/control and family-based association tests. A clinical association of anemia and RLS was further confirmed in this study. There was no detectable difference in DMT1 protein levels between RLS patient lymphoblastoid cell lines and normal controls. Non-parametric linkage analyses failed to identify any significant linkage signals within the DMT1 gene region. Sequencing of selected patients did not detect any sequence variant(s) compatible with DMT1 harboring RLS causative mutation(s). Further studies did not find any association between ten SNPs, spanning the whole DMT1 gene region, and RLS affection status. Finally, two DMT1 intronic SNPs showed positive association with RLS in patients with a history of anemia, when compared to RLS patients without anemia.

Molecular mechanisms involved in intestinal iron absorption

Iron is an essential trace metal in the human diet due to its obligate role in a number of metabolic processes. In the diet, iron is present in a number of different forms, generally described as haem (from haemoglobin and myoglobin in animal tissue) and non-haem iron (including ferric oxides and salts, ferritin and lactoferrin). This review describes the molecular mechanisms that co-ordinate the absorption of iron from the diet and its release into the circulation. While many components of the iron transport pathway have been elucidated, a number of key issues still remain to be resolved. Future work in this area will provide a clearer picture regarding the transcellular flux of iron and its regulation by dietary and humoral factors.

Analysis of the rainbow trout solute carrier 11 family reveals iron import < or = pH 7.4 and a functional isoform lacking transmembrane domains 11 and 12

A Xenopus oocyte heterologous expression system was used to characterise iron transport properties of two members of the solute carrier 11 (slc11) protein family isolated from rainbow trout gills. One cDNA clone differed from the trout Slc11alpha containing an additional 52bp in the exon between transmembrane domains (TM) 10 and 11. The 52bp contained a stop codon, resulting in a novel isoform lacking the last two TM (termed slc11gamma). Slc11gamma and another isoform slc11beta, import Fe(2+) at external pHs < or = to 7.4. Trout slc11beta Fe(2+) import was more sensitive to inhibition by divalent metals. The novel vertebrate slc11gamma isoform functions without TM11 and 12.

Recent progress in structure-function analyses of Nramp proton-dependent metal-ion transporters

The natural resistance-associated macrophage protein (Nramp) homologs form a family of proton-coupled transporters that facilitate the cellular absorption of divalent metal ions (Me2+, including Mn2+, Fe2+, Co2+, and Cd2+). The Nramp, or solute carrier 11 (SLC11), family is conserved in eukaryotes and bacteria. Humans and rodents express 2 parologous genes that are associated with iron disorders and immune diseases. The NRAMP1 (SLC11A1) protein is specific to professional phagocytes and extrudes Me2+ from the phagosome to defend against ingested microbes; polymorphisms in the NRAMP1 gene are associated with various immune diseases. Several isoforms of NRAMP2 (SLC11A2, DMT1, DCT1) are expressed ubiquitously in recycling endosomes or specifically at the apical membrane of epithelial cells in intestine and kidneys, and can contribute to iron overload, whereas mutations impairing NRAMP2 function cause a form of congenital microcytic hypochromic anemia. Structure-function studies, using various experimental models, and mutagenesis approaches have begun to reveal the overall transmembrane organization of Nramp, some of the transmembrane segments (TMS) that are functionally important, and an unusual mechanism coupling Me2+ and proton H+ transport. The approaches used include functional complementation of yeast knockout strains, electrophysiology analyses in Xenopus oocytes, and transport assays that use mammalian and bacterial cells and direct and indirect measurements of SLC11 transporter properties. These complementary studies enabled the identification of TMS1 and 6 as crucial structural segments for Me2+ and H+ symport, and will help develop a deeper understanding of the Nramp transport mechanism and its contribution to Me2+ homeostasis in human health and diseases.

Defective targeting of hemojuvelin to plasma membrane is a common pathogenetic mechanism in juvenile hemochromatosis

Hemojuvelin (HJV) positively modulates the iron regulator hepcidin, and its mutations are the major cause of juvenile hemochromatosis (JH), a recessive disease leading to iron overload. Defective HJV reduces hepcidin up-regulation both in humans and in Hjv-deficient mice. To investigate the JH pathogenesis and the functional properties of human HJV we studied the biosynthesis and maturation of 6 HJV pathogenic mutants in HeLa and HepG2 cells. We show that proteolytic processing is defective in mutants F170S, W191C, and G320V, but not in G99V and C119F. Moreover, we show that mutants G99V and C119F are targeted to the cell surface, while F170S, W191C, G320V, and R326X (lacking the glycosilphosphatidylinositol [GPI] anchor) are mainly retained in the endoplasmic reticulum, although all mutants are released as soluble forms (s-HJV) in a proportion that is modulated by iron supplementation. Membrane HJV (m-HJV) is mainly composed of the cleaved protein, and its level is increased by iron in wild-type (WT) mice but not in the mutants. Altogether, the data demonstrate that the loss of HJV membrane export is central to the pathogenesis of JH, and that HJV cleavage is essential for the export. The results support a dual function for s- and m-HJV in iron deficiency and overload, respectively.

Identification of a tyrosine-based motif (YGSI) in the amino terminus of Nramp1 (Slc11a1) that is important for lysosomal targeting

In macrophages, Nramp1 (Slc11a1) is expressed in lysosomes and restricts replication of intracellular pathogens by removing divalent metals (Mn2+ and Fe2+) from the phagolysosome. Nramp2 (DMT1, Slc11a2) is expressed both at the duodenal brush border where it mediates uptake of dietary iron and ubiquitously at the plasma membrane/recycling endosomes of many cell types where it transports transferrin-associated iron across the endosomal membrane. In Nramp2, a carboxyl-terminal cytoplasmic motif ((555)YLLNT(559)) is critical for internalization and recycling of the transporter from the plasma membrane. Here we studied the subcellular trafficking properties of Nramp1 and investigated the cis-acting sequences responsible for targeting to lysosomes. For this, we constructed and studied Nramp1/Nramp2 chimeric proteins where homologous domains of each protein were exchanged. Chimeras exchanging the amino-(upstream TM1) and carboxyl-terminal (downstream TM12) cytoplasmic segments of both transporters were stably expressed in porcine LLC-PK1 kidney cells and were studied with respect to expression, maturation, stability, cell surface targeting, transport activity, and subcellular localization. An Nramp2 isoform II chimera bearing the amino terminus of Nramp1 was not expressed at the cell surface but was targeted to lysosomes. This lysosomal targeting was abolished by single alanine substitutions at Tyr15 and Ile18 of a (15)YGSI(18) motif present in the amino terminus of Nramp1. These results identify YGSI as a tyrosine-based sorting signal responsible for lysosomal targeting of Nramp1.