Ectopic expression of Nramp1 in COS-1 cells modulates iron accumulation

Intratumoral Heterogeneity and Immune Modulation in Lung Adenocarcinoma in Female Smokers and Never Smokers

SIGNIFICANCE

Single-cell analysis of healthy lung tissue and lung cancer reveals distinct tumor cell populations, including cells with differential immune modulating capacity between smokers and never smokers, which could guide future therapeutic strategies.

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.

Leishmania infection triggers hepcidin-mediated proteasomal degradation of Nramp1 to increase phagolysosomal iron availability

Natural resistance-associated macrophage protein 1 (Nramp1) was originally discovered as a genetic determinant of resistance against multiple intracellular pathogens, including Leishmania. It encodes a transmembrane protein of the phago-endosomal compartments, where it functions as an iron transporter. But the mechanism by which Nramp1 controls host-pathogen dynamics and determines final outcome of an infection is yet to be fully deciphered. Whether the expression of Nramp1 is altered in response to a pathogen attack is also unknown. To address these, Nramp1 status was examined in Leishmania major-infected murine macrophages. We observed that at 12 hrs post infection, there was drastic lowering of Nramp1 level accompanied by increased phagolysosomal iron content and enhanced intracellular parasite growth. Leishmania infection-induced Nramp1 downregulation was caused by ubiquitin-proteasome degradation pathway, which in turn was found to be mediated by the iron-regulatory peptide hormone hepcidin. Blocking of Nramp1 degradation with proteasome inhibitor or transcriptional agonist of hepcidin resulted in depletion of phagolysosomal iron pool that led to significant reduction of intracellular parasite burden. Interestingly, Nramp1 level was restored to normalcy after 30 hrs of infection with a concomitant drop in phagolysosomal iron, which is suggestive of a host counteractive response to deprive the pathogen of this essential micronutrient. Taken together, our study implicates Nramp1 as a central player in the host-pathogen battle for phagolysosomal iron. We also report Nramp1 as a novel target for hepcidin, and this 'hepcidin-Nramp1' axis may have a broader role in regulating macrophage iron homeostasis.

Functional analysis of 3'UTR polymorphisms in the caprine SLC11A1 gene and its association with the Mycobacterium avium subsp. paratuberculosis infection

The study aimed to investigate whether the genetic polymorphisms in the 3'UTR of the caprine SLC11A1 gene are functional, and to assess the role of MAP as a regulatory parameter in gene expression. To this goal we constructed plasmids expressing the Luciferase reporter gene in transient transfections of a mouse (Balb/c) macrophage cell line (RAW264.7), incorporating those polymorphisms that our previous work indicated as more prominent in terms of SLC11A1 expression and responsiveness to MAP infection. Gene expression variation was recorded on the average of the respective measurements after exposure to Mycobacterium avium subsp. paratuberculosis (MAP) combined with microbial antigens and cytokines. In silico analysis of the region under study allowed identification of one cis-acting RNA element, five putative transcriptional regulatory elements and 85 3'end microRNA binding sites. The two polymorphic regions (regions A and B) of the 3'UTR of the caprine SLC11A1 gene were recognized as regulators of its activity, at transcriptional and post-transcriptional level. The GT16 polymorphism at region A, combined with the GT8 polymorphism at region B, results in up-regulation of the SLC11A1 gene. The specific genotype was also found to be more responsive to MAP exposure at a statistically significant level.

Antimicrobial responses of teleost phagocytes and innate immune evasion strategies of intracellular bacteria

During infection, macrophage lineage cells eliminate infiltrating pathogens through a battery of antimicrobial responses, where the efficacy of these innate immune responses is pivotal to immunological outcomes. Not surprisingly, many intracellular pathogens have evolved mechanisms to overcome macrophage defenses, using these immune cells as residences and dissemination strategies. With pathogenic infections causing increasing detriments to both aquacultural and wild fish populations, it is imperative to garner greater understanding of fish phagocyte antimicrobial responses and the mechanisms by which aquatic pathogens are able to overcome these teleost macrophage barriers. Insights into the regulation of macrophage immunity of bony fish species will lend to the development of more effective aquacultural prophylaxis as well as broadening our understanding of the evolution of these immune processes. Accordingly, this review focuses on recent advances in the understanding of teleost macrophage antimicrobial responses and the strategies by which intracellular fish pathogens are able to avoid being killed by phagocytes, with a focus on Mycobacterium marinum.

In vitro expression of the SLC11A1 gene in goat monocyte-derived macrophages challenged with Mycobacterium avium subsp paratuberculosis

Johne's disease or paratuberculosis is a chronic, progressive intestinal disease of ruminants caused by Mycobacterium avium subsp. paratuberculosis (MAP). One of the genes that have been targeted with regard to resistance or sensitivity to paratuberculosis is the SLC11A1 (solute carrier family 11 member A1). Here we extend our previous work to the sequence and structure analysis of the caprine SLC11A1 gene and we assess the functional impact of the most frequent polymorphisms of the 3' UTR region of the SLC11A1 gene to its expression in goat macrophages exposed in vitro to MAP. The role of these polymorphisms in primary immune response is also investigated with connection to gene expression of two interleukins (IL), one of which pro (IL-1a), and the other anti-inflammatory (IL-10). In order to assess gene response, quantitative detection of the SLC11A1, IL-10 and IL1a mRNA was performed by real time PCR before, and at 1, 3 and 24h after exposure of primary cultures of peripheral blood monocyte-derived macrophages to MAP, collected from 54 goats of the Greek native goat breed. Sequence analysis of the 3' UTR end of the caprine SLC11A1 gene determined its full length to be 522 bases. Structure analysis confirmed the presence of two microsatellites consisted of a variable number of guanine-thymine repeats (regions A and B). The homozygous B7 genotype [B(GTn)7/7] was associated at a statistically significant level with increased expression of the SLC11A1 and IL-1α genes indicating increased in vitro responsiveness and therefore resistance of mononuclear derived macrophages to MAP infection.

Transcription factor ATF-3 regulates allele variation phenotypes of the human SLC11A1 gene

Genetic polymorphisms in the human solute carrier family 11 member 1 (SLC11A1) gene predispose to susceptibility to infectious/inflammatory diseases and cancer. Human susceptibility to these diseases exhibits allelic association with a polymorphic regulatory Z-DNA-forming microsatellite of a (GT/AC)n repeat. The carriage of different alleles may influence chromatin remodeling and accessibility by transcription factors. Of particular importance is the binding site for the Activating Protein 1 (AP-1) elements, (ATF-3 and c-Jun), adjacent to the 5' sequence of the Z-DNA-forming polymorphism. The aim of the study was to characterize the transcriptional mechanisms controlling different alleles of SLC11A1 expression by ATF-3 and c-Jun. Allele 2, [T(GT)5AC(GT)5AC(GT)10GGCAGA(G)6], and Allele 3, [T(GT)5AC(GT)5AC(GT)9GGCAGA(G)6], were subcloned into the PGL2Basic vector. Transient transfections of THP-1 cells with the constructs, in the presence or absence of pATF-3 were preformed. Luciferase expression was determined. To document the recruitment of ATF-3 and c-Jun, to the polymorphic promoter alleles in vivo, we performed ChIP assays with transient transfected THP-1 cells treated with or without lipopolyssacharides. Our data documented that ATF-3 suppresses the transcriptional activation of Allele-3, and this suppression is enhanced in the presence of lipopolyssacharides. Our findings suggest that ATF-3 and c-Jun may influence heritable variation in SLC11A1-dependent innate resistance to infection and inflammation both within and between populations.

Mycobacteria and the intraphagosomal environment: take it with a pinch of salt(s)!

Ancient protozoan phagocytes and modern professional phagocytes of metazoans, such as macrophages, employ evolutionarily conserved mechanisms to kill microbes. These mechanisms rely on microbial ingestion, followed by maturation of the phagocytic vacuole, or so-called phagosome. Phagosome maturation includes a series of fusion and fission events with the host cell endosomes and lysosomes, leading to a rapid increase of the degradative properties of the vacuole and to the destruction of the ingested microbe within a very hostile intracellular compartment, the phagolysosome. Historically, the mechanisms and weapons used by phagocytes to kill microbes have been separated into different classes. Phagosomal acidification, together with the production of reactive oxygen and nitrogen species, the selective manipulation of various ions in the phagosomal lumen, and finally the engagement of a battery of acidic hydrolases, are well-recognized players in this process. However, it is relatively recently that interconnections among these mechanisms have become apparent. In this review, we will focus on some emerging concepts about these interconnected aspects of the warfare at the host-pathogen interface, using mostly Mycobacterium tuberculosis as an example of intracellular pathogen. In particular, recent discoveries on the role of phagosomal ions and other chemicals in the control of pathogens, as well as mechanisms evolved by intracellular pathogens to circumvent or even exploit the weapons of the host cell will be discussed.

NRAMP1 polymorphism and viral factors in Sardinian multiple sclerosis patients

BACKGROUND

Multiple sclerosis (MS) is believed to be an autoimmune disease occurring in genetically predisposed individuals after an appropriate environmental exposure such as viral infections. Recent studies suggest a significant association between MS and the functional 5'-(GT)n polymorphism in the promoter region of the NRAMP1 gene. In the present study we aimed to evaluate the contribution of the allelic variation in the NRAMPI promoter to MS susceptibility and to study the role of viral infection in relation to specific NRAMP1 genotypes, in a Sardinian cohort.

METHODS

Sixty MS patients and 66 healthy individuals were genotyped, and screened for the presence of Epstein-bar virus (EBV) and JC virus (JCV) sequences.

RESULTS

Consistent with previous autoimmune disease studies, allele 3 at the functional 5'(GT)n promoter region repeat polymorphism, was significantly overrepresented among MS patients when compared to controls (p=0.02). The EBV and JCV sequences were detected in 8/60 (13.33%) and in 4/60 (6.66%) of MS patients respectively and in 5/66 (7.57%) and in 0/66 of controls.

CONCLUSION

The allelic variation in the NRAMP1 promoter may contribute to MS susceptibility in the Sardinian population. The viral sequences were not confined to a specific NRAMP1 genotype.

Evolution of differences in transport function in Slc11a family members

Slc11a1 (formerly Nramp1) is a proton/divalent cation transporter that regulates cation homeostasis in macrophages. Slc11a2 mediates divalent cation uptake via the gut and delivery into cells. The mode of action of the two transporters remains controversial. Heterologous expression in frog oocytes shows Slc11a2 is a symporter, whereas Slc11a1 is an antiporter fluxing divalent cations against the proton gradient. This explains why Slc11a2, but not Slc11a1, can complement EGTA sensitivity in smf1Delta/smf2Delta/smf3Delta yeast. However, some studies of transport in mammalian cells suggest Slc11a1 is a symporter. We now demonstrate that Slc11a1, but not Slc11a2, complements a divalent cation stress phenotype in bsd2Delta/rer1Delta yeast. This is the first description of a yeast complementation assay for Slc11a1 function. Given the prior demonstration in frog oocytes that Slc11a1 acts as an antiporter, the most plausible interpretation of the data is that Slc11a1 is rescuing bsd2Delta/rer1Delta yeast by exporting divalent cations. Chimaeras define the N terminus, and a segment of the protein core preceding transmembrane domain 9 through transmembrane domain 12, as important in rescuing the divalent cation stress phenotype. EGTA sensitivity and divalent cation stress phenotypes in yeast expressing Slc11a orthologues show that symport activity is ancestral. Molecular changes that mediate rescue of the divalent cation stress phenotype post-date frogs and co-evolved with Slc11a1 orthologues that regulate divalent cation homeostasis in macrophages and resistance to infection in chickens and mammals.

The human solute carrier family 11 member 1 protein (SLC11A1): linking infections, autoimmunity and cancer?

SLC11A1 is known to link infections, autoimmunity and cancers. A review is presented of the mechanisms by which a balance is maintained between infections caused by pathogens (viral, bacterial and protozoan; intracellular and extracellular) and disorders resulting from (acute or chronic) inflammation, and of the interactions that determine how the initial innate immune system directs subsequent acquired immune responses in human populations

Slc11a1, formerly Nramp1, is expressed in dendritic cells and influences major histocompatibility complex class II expression and antigen-presenting cell function

Solute carrier family 11 member a1 (Slc11a1; formerly Nramp1) encodes a late endosomal/lysosomal protein/divalent cation transporter that regulates iron homeostasis in macrophages. During macrophage activation, Slc11a1 has multiple pleiotropic effects on gene regulation and function, including gamma interferon-induced class II expression and antigen-presenting cell function. The wild-type allele at Slc11a1 has been associated with a bias in Th1 cell function in vivo, which is beneficial in resistance to infection against intracellular macrophage pathogens but detrimental in contributing to development of type 1 diabetes. The extent to which this depends on macrophage versus dendritic cell (DC) function is not known. Here we show that Slc11a1 is expressed in late endosomes and/or lysosomes of CD11c(+) DCs. DCs from mutant and congenic wild-type mice upregulate interleukin-12 (IL-12) and IL-10 mRNA in response to lipopolysaccharide (LPS) stimulation, but the ratio of IL-10 to IL-12 is higher in unstimulated DCs and DCs stimulated for 15 h with LPS from mutant mice than from wild-type mice. DCs from wild-type mice upregulate major histocompatibility complex class II in response to LPS more efficiently than DCs from mutant mice. Unstimulated DCs from wild-type and mutant mice present ovalbumin (OVA) peptide with an efficiency equivalent to that of an OVA-specific CD4 T-cell line, but DCs from wild-type mice are more efficient at processing and presenting OVA or Leishmania activator of cell kinase (LACK) protein to OVA- and LACK-specific T cells. These data indicate that wild-type Slc11a1 expressed in DCs may play a role both in determining resistance to infectious disease and in susceptibility to autoimmune disease such as type 1 diabetes.

Immunolocalisation of the D. melanogaster Nramp homologue Malvolio to gut and Malpighian tubules provides evidence that Malvolio and Nramp2 are orthologous

Nramp (Slc11a1) genes in mammals are associated with the transport of iron and other divalent cations; Nramp1 in macrophages involved in the innate immune response against intracellular pathogens, and Nramp2 with duodenal iron uptake and the transferrin–transferrin-receptor pathway of iron assimilation. The Drosophila melanogaster Nramp-related gene is known as Malvolio. The localisation of Malvolio protein was inferred from the enhancer trap line initially used to isolate Malvolio in a screen for mutants with defects in taste perception. Here we describe the generation of a Malvolio-reactive polyclonal antibody and apply it to evaluate Malvolio localisation during stages of D. melanogaster development,and compare the results with the localisation of the enhancer trap line identified with beta-galactosidase. All immunolocalisation studies have been confirmed to be specific with Malvolio-blocking peptides. Our results demonstrated expression within Malpighian tubules, testis, brain, the amnioserosa of embryos, the larval and adult alimentary canal. Expression within the gut was of significant interest, as mammalian Nramp2 in the gut plays a primary role in the acquisition of dietary iron. We confirm expression within the central nervous system and in cells of the haematopoietic system. By immunohistochemistry we showed that expression within cells was either punctuate, diffuse cytoplasmic or plasma membrane associated, or both. The staining within the gut indicates a degree of conservation of components for iron acquisition between flies and mammals, suggesting that a comparable mechanism has been retained during evolution.

Normoxic stabilization of hypoxia-inducible factor-1alpha by modulation of the labile iron pool in differentiating U937 macrophages: effect of natural resistance-associated macrophage protein 1

Hypoxia-inducible factor (HIF) is a transcription factor with major roles in many cellular and systemic responses to hypoxia. Activation of HIF pathways under hypoxia is mediated by suppression of the Fe(2+)- and O(2)-dependent HIF hydroxylase enzymes that normally inactivate HIFalpha subunits. Mechanisms underlying induction of HIF in normoxic conditions are less clearly understood. In human cancers, infiltrating macrophages show up-regulation of HIF and it has recently been shown that normoxic expression of HIF-1alpha is essential for macrophage function. Here, we report studies of HIF-1alpha induction following phorbol-12-myristate 13-acetate (PMA)-induced differentiation of monocytic U937 and THP1 cells. HIF-1alpha was markedly up-regulated under normoxia in this setting and this involved failure of HIF-1alpha prolyl hydroxylation despite the presence of O(2). Fluorescence measurements showed that differentiation was associated with marked reduction of the labile iron pool. Both the reduction in labile iron pool and the up-regulation of HIF-1alpha were suppressed by RNA interference-mediated down-regulation of the iron transporter natural resistance-associated macrophage protein 1. Up-regulation of HIF-1alpha following PMA-induced differentiation was also abolished by addition of Fe(2+) or ascorbate. These results indicate that physiologic changes in macrophage iron metabolism have an important effect on HIF hydroxylase pathways and suggest means by which the system could be manipulated for therapeutic benefit.

Function and mechanism of action of Dictyostelium Nramp1 (Slc11a1) in bacterial infection

Dictyostelium amoebae are professional phagocytes, which ingest bacteria as the principal source of food. We have cloned the Dictyostelium homologue of human natural resistance-associated membrane protein 1 (Nramp1) [solute carrier family 11 member 1 (Slc11a1)], an endo-lysosomal membrane protein that confers on macrophages resistance to infection by a variety of intracellular bacteria and protozoa. The Dictyostelium Nramp1 gene encodes a protein of 53 kDa with 11 putative transmembrane domains. The Nramp1 gene is transcribed during the growth-phase and downregulated to barely detectable levels upon starvation. To gain insights into their intracellular localization, we fused Nramp1 or the vatB subunit of the V-H(+)ATPase with green fluorescent protein and expressed in cells. Green fluorescent protein-vatB was inserted in membranes of all acidic compartments and the contractile vacuole network and decorated macropinosomes and phagosomes. Green fluorescent protein-Nramp1 decorated macropinosomes and phagosomes, in addition to intracellular vesicular compartments positive for endosomal SNARE protein Vti1 or vacuolin, a marker of the exocytic pathway. Nramp1 disruption generated mutants that were more permissive hosts than wild-type cells for intracellular growth of Legionella pneumophila and Micobacterium avium. Nramp1 overexpression protected cells from L. pneumophila infection. Evidence is provided that Nramp1 transports metal cations out of the phagolysosome in an ATP-dependent process and that L. pneumophila and M. avium use different mechanisms to neutralize Nramp1 activity.

Modulation of Iron Availability at the Host-Pathogen Interface in Phagocytic Cells

This review summarizes recent data on iron metabolism in macrophages, with a special emphasis on possible bacteriostatic and bactericidal consequences for intracellular pathogens. It includes the role of biological chelators and transporters in normal macrophage physiology and antimicrobial defense. Iron is an essential metal cofactor for many biochemical pathways in mammals. However, excess iron promotes the formation of cytotoxic oxygen derivatives so that systemic iron levels must be tightly regulated. The mechanism of iron recycling by macrophages including iron efflux from erythrocyte-containing phagosomes, iron release from macrophages, and entry into the transferrin (Tf) cycle remain poorly understood. Ferroportin expression in the liver, spleen, and bone marrow cells appears to be restricted to macrophages. Mutant mice bearing a conditional deletion of the ferroportin gene in macrophages show retention of iron by hepatic Kupffer cells and splenic macrophages. Hepcidin is induced by lipopolysaccharide (LPS) in mouse spleens and splenic macrophage in vitro and appears to mediate the LPS-induced down-regulation of ferroportin in the intestine and in splenic macrophages, suggesting that inflammatory agents may regulate iron metabolism through modulation of ferroportin expression. The host transporter Nramp1 may compete directly with bacterial divalent-metal transport systems for the acquisition of divalent metals within the phagosomal space. The ultimate outcome of these competing interactions influences the ability of pathogens to survive and replicate intracellularly. This seems particularly relevant to the Salmonella, Leishmania, and Mycobacterium spp., in which inactivating mutations in Nramp1 abrogate the natural resistance of macrophages to these pathogens.

RNA-binding protein HuR is required for stabilization of SLC11A1 mRNA and SLC11A1 protein expression

The solute carrier family 11 member 1 (SLC11A1, formerly NRAMP1) gene is associated with infectious and autoimmune diseases and plays an important role in macrophage activation. Human SLC11A1 mRNA contains an AU-rich element (ARE) within the 3' untranslated region; however, its role in the regulation of SLC11A1 gene expression has not been elucidated. Here we analyze the expression of SLC11A1 in human monocytes and HL-60 cells and then use HL-60 cells as a model to determine whether RNA-binding protein HuR is associated with the ARE and involved in SLC11A1 mRNA turnover. Our results demonstrate a binding of HuR to the SLC11A1 ARE in phorbol myristate acetate (PMA)-differentiated cells dramatically increased compared to that in undifferentiated cells. Interestingly, PMA-induced accumulation of cytoplasmic HuR occurs in parallel with an increase in the binding of HuR to SLC11A1 ARE and with an increase in the SLC11A1 mRNA level. This suggests that HuR's cytoplasmic localization plays an important role in the regulation of SLC11A1 expression. We also observe that down-regulation of HuR expression by RNA interference (RNAi) results in a decrease in SLC11A1 expression which can be restored by the addition of recombinant HuR protein to the RNAi-treated cells. Finally, we show that HuR overexpression in HL-60 cells significantly increases the SLC11A1 mRNA stability. Taken together, our data demonstrate that HuR is a key mediator of posttranscriptional regulation and expression of the SLC11A1 gene.

Association of functional polymorphisms of SLC11A1 with risk of esophageal cancer in the South African Colored population

Several environmental factors have been implicated in the etiology of esophageal cancer (EC). The purpose of this study was to assess the likelihood that variation in the SLC11A1 gene contributes to EC susceptibility, possibly due to its role in inflammation and iron metabolism. The regions of the gene containing potential functional polymorphisms, including the promoter region and exon 2, were investigated. The study cohort included 105 EC South African Colored patients with squamous cell carcinoma (SCC) and 110 population-matched controls, with South African Colored referring to individuals of mixed ancestry. A significantly decreased frequency of the -237C-->T promoter polymorphism was observed in the patient group with EC compared with the population-matched control group (P < 0.002, chi(2) with Yates's correction=7.87). A statistically significant disease association was also observed with allele 3 of the 5'-(GT)n promoter polymorphism (P < 0.0006, chi(2) with Yates's correction=10.16), but only in the absence of the T-allele at nucleotide position -237 following allelic stratification. Four novel variants were identified in intron 1 (IVS1-28C-->T) and exon 2 (112G-->A, 148delGACCAGCCC, 157insGACCAGCCCAG). The novel intronic polymorphism, IVS1-28C-->T, was also significantly associated with EC (P < 0.05, chi(2) with Yates's correction=2.52). We demonstrate association of genetic variation in both the promoter region and intron 1 of the SLC11A1 gene with EC susceptibility.

Incomplete glycosylation and defective intracellular targeting of mutant solute carrier family 11 member 1 (Slc11a1)

Solute carrier family 11 member 1 (Slc11a1, formerly Nramp1) is a highly glycosylated, 12 transmembrane domain protein expressed in macrophages. It resides in the membrane of late endosomes and lysosomes, where it functions as a bivalent cation transporter. Mice susceptible to infection by various intracellular pathogens including Leishmania donovani and Salmonella typhimurium carry a glycine to aspartic acid substitution at position 169 (G169D, Gly(169)-->Asp), within transmembrane domain 4 of Slc11a1. To investigate the molecular pathogenesis of infectious disease susceptibility, we compared the behaviour of heterologously and endogenously expressed wild-type and mutant Slc11a1 by immunofluorescence, immunoelectron microscopy and Western-blot analysis. We found occasional late endosome/lysosome staining of mutant protein using immunoelectron microscopy, but most of the mutant Slc11a1 was retained within the ER (endoplasmic reticulum). Using glycosylation as a marker for protein maturation in two independent heterologous expression systems, we found that most mutant Slc11a1 existed as an ER-dependent, partially glycosylated intermediate species. Correct endosomal targeting of wild-type Slc11a1 continued despite disruption of N-glycosylation sites, indicating that glycosylation did not influence folding or sorting. We propose that the G169D mutation causes localized misfolding of Slc11a1, resulting in its retention in the ER and manifestation of the loss of function phenotype.

Slc11a1-mediated resistance to Salmonella enterica serovar Typhimurium and Leishmania donovani infections does not require functional inducible nitric oxide synthase or phagocyte oxidase activity

Solute carrier family 11a member 1 (Slc11a1; formerly natural resistance-associated macrophage protein 1) encodes a late endosomal/lysosomal protein/divalent cation transporter, which regulates iron homeostasis in macrophages. During macrophage activation, Slc11a1 exerts pleiotropic effects on gene regulation and function, including generation of nitric oxide (NO) via inducible NO synthase (iNOS; encoded by Nos2A) and of reactive oxygen intermediates (ROI) via the phagocyte oxidase complex. As NO and ROI have potent antimicrobial activity in macrophages, it was assumed that their activities would contribute to Slc11a1-regulated innate resistance to Salmonella enterica serovar Typhimurium and Leishmania donovani. By intercrossing mice with gene disruptions at Nos2A and Cybb (encoding gp91phox, the heavy chain subunit of cytochrome b-245 and an essential component of phagocyte NADPH oxidase) onto equivalent Slc11a1 wild-type and mutant genetic backgrounds, we demonstrate that neither iNOS nor gp91phox activity is required for Slc11a1-mediated innate resistance to either infection. Functional gp91phox and iNOS are required to control S. enterica serovar Typhimurium in non-Slc11a1-regulated phases of infection. For L. donovani, an organ-specific requirement for iNOS to clear parasites from the spleen was observed at 50 days post-infection, but neither iNOS nor gp91phox influenced late-phase infection in the liver. This contrasted with Leishmania major infection, which caused rapid lesion growth and death in iNOS knockout mice and some exacerbation of disease with gp91phox deficiency. This highlights the adaptive differences in tissue and cellular tropisms between L. donovani and L. major and the different genes and mechanisms that regulate visceral versus cutaneous forms of the disease.

In vitro response of the striped bass natural resistance-associated macrophage protein, Nramp, to LPS and Mycobacterium marinum exposure

Mycobacteriosis in Chesapeake Bay (USA) striped bass Morone saxatilis is an ongoing disease problem with important economic implications for a large commercial and recreational fishery. Additionally, striped bass serve as a reservoir of potential mycobacterial zoonoses. Recently, we described a striped bass gene homolog of the natural resistance-associated macrophage protein family (MsNramp), which is responsible for resistance to mycobacterial infections in mice. Striped bass MsNramp is strongly induced in peritoneal exudate cells (PE) in vivo after intraperitoneal injection with Mycobacterium spp. The purpose of the present study was to investigate short-term in vitro MsNramp expression and reactive oxygen intermediate (ROI) production in primary cultures of adherent PE after exposure to bacterial lipopolysaccharide (LPS), or live- or heat-killed (HK) Mycobacterium marinum. PE expressed significantly higher levels of MsNramp at 4 and 24 h post-treatment with live and HK M. marinum. MsNramp response to LPS was dose-dependent in these cells, with maximum expression at 4 h and 20 microg/ml LPS. Treatment of PE with LPS resulted in increased intracellular superoxide anion levels, whereas treatment with live M. marinum caused a significant depression. This study is the first report of induction of a teleost Nramp in vitro by mycobacteria, and supports findings of teleost Nramp induction by LPS.

Iron, manganese, and cobalt transport by Nramp1 (Slc11a1) and Nramp2 (Slc11a2) expressed at the plasma membrane

Mutations in the Nramp1 gene (Slc11a1) cause susceptibility to infection by intracellular pathogens. The Nramp1 protein is expressed at the phagosomal membrane of macrophages and neutrophils and is a paralog of the Nramp2 (Slc11a2) iron transporter. The Nramp1 transport mechanism at the phagosomal membrane has remained controversial. An Nramp1 protein modified by insertion of a hemagglutinin epitope into the predicted TM7/8 loop was expressed at the plasma membrane of Chinese hamster ovary cells as demonstrated by immunofluorescence and surface biotinylation. Experiments in Nramp1HA transfectants using the metal-sensitive fluorophors calcein and Fura2 showed that Nramp1HA can mediate Fe2+, Mn2+, and Co2+ uptake. Similar results were obtained in transport studies using radioisotopic 55Fe2+ and 54Mn2+. Nramp1HA transport was dependent on time, temperature, and acidic pH, occurring down the proton gradient. These experiments suggest that Nramp1HA may be a more efficient transporter of Mn2+ compared to Fe2+ and a more efficient Mn2+ transporter than Nramp2HA. The membrane topology and transport properties of Nramp1HA and Nramp2HA were indistinguishable, suggesting that Nramp1 divalent-metal transport at the phagosomal membrane is mechanistically similar to that of Nramp2 at the membrane of acidified endosomes. These results clarify the mechanism by which Nramp1 contributes to phagocyte defenses against infections.

NRAMP1 polymorphisms, susceptibility and clinical features of tuberculosis

OBJECTIVES

To study whether NRAMP1 gene polymorphisms in a Japanese population affect susceptibility to tuberculosis and clinical features of tuberculosis.

METHODS

Polymerase chain reaction and restriction fragment-length polymorphism analyses were used to type NRAMP1 polymorphisms in 95 patients with pulmonary tuberculosis and 90 controls. Clinical features of patients also were investigated.

RESULTS

No association was seen between susceptibility to tuberculosis and NRAMP1 polymorphisms. Patients with the D543N A allele were significantly more likely than others to develop a cavitary lesion; by logistic regression analysis with adjustment for gender, age, and presence of diabetes, the odds ratio in patients with an A allele was 5.16 (95% confidence interval, 1.30-20.45).

CONCLUSIONS

Genetic variation in the human NRAMP1 gene may be associated with cavitation in patients with tuberculosis.

Iron transport: emerging roles in health and disease

In the theater of cellular life, iron plays an ambiguous and yet undoubted lead role. Iron is a ubiquitous core element of the earth and plays a central role in countless biochemical pathways. It is integral to the catalysis of the redox reactions of oxidative phosphorylation in the respiratory chain, and it provides a specific binding site for oxygen in the heme binding moiety of hemoglobin, which allows oxygen transport in the blood. Its biological utility depends upon its ability to readily accept or donate electrons, interconverting between its ferric (Fe3+) and ferrous (Fe2+) forms. In contrast to these beneficial features, free iron can assume a dangerous aspect catalyzing the formation of highly reactive compounds such as cytotoxic hydroxyl radicals that cause damage to the macromolecular components of cells, including DNA and proteins, and thereby cellular destruction. The handling of iron in the body must therefore be very carefully regulated. Most environmental iron is in the Fe3+ state, which is almost insoluble at neutral pH. To overcome the virtual insolubility and potential toxicity of iron, a myriad of specialized transport systems and associated proteins have evolved to mediate regulated acquisition, transport, and storage of iron in a soluble, biologically useful, non-toxic form. We are gradually beginning to understand how these proteins individually and in concert serve to maintain cellular and whole body homeostasis of this crucial yet potentially harmful metal ion. Furthermore, studies are increasingly implicating iron and its associated transport in specific pathologies of many organs. Investigation of the transport proteins and their functions is beginning to unravel the detailed mechanisms underlying the diseases associated with iron deficiency, iron overload, and other dysfunctions of iron metabolism.

Iron and carcinogenesis: from Fenton reaction to target genes

Reactive oxygen species (ROS) have been shown to be associated with a wide variety of pathological phenomena such as carcinogenesis, inflammation, radiation and reperfusion injury. Iron, the most abundant transition metal ion in our body, may work as a catalyst for the generation of ROS in pathological conditions. In the past few years, there have been great advances in the understanding of iron metabolism. These include the discoveries of iron transporters and the gene responsible for hereditary hemochromatosis. Iron overload has been shown to be associated with carcinogenesis. We recently identified the major target genes (p16(INK4A) and p15(INK4B) tumor suppressor genes, which encode cyclin-dependent kinase inhibitors) in a ferric nitrilotriacetate-induced rat renal carcinogenesis model, in which the Fenton reaction is induced in the renal proximal tubules. Allelic loss of the p16 gene occurs early in carcinogenesis and specifically at the p16 loci as compared with other tumor suppressor genes. This led to the novel concept of 'genomic sites vulnerable to the Fenton reaction'. Here, recent new findings on iron metabolism are reviewed and the concept of the vulnerable sites explored. More effort to link iron metabolism with human carcinogenesis is anticipated.

Genetic regulation of cell function in response to iron overload or chelation

Iron influences many aspects of cell function on different biochemical levels. This review considers effects mediated through iron-dependent changes in gene expression in mammalian cells. Several classes of related genes are responsive to cellular iron levels, but no clear patterns readily account for the toxicity of iron overload or the consequences of removal of iron with chelating agents. Here we group some of the genes influenced by iron status into those related to iron metabolism, oxygen and oxidative stress, energy metabolism, cell cycle regulation, and tissue fibrosis. Iron excess and chelation do not generally result in a continuous or graded transcriptional response, but indicate operation of distinct mechanisms. An emerging concept is that iron signals through generation of reactive oxygen species to activate transcription factors such as NF-kappaB, whereas iron removal mimics hypoxia, perhaps by disrupting iron-based O(2) sensors and influencing gene expression through, e.g., the hypoxia-inducible factor, HIF-1. Heme and other metalloporphyrins have other distinct mechanisms for regulating transcription. Regulation of gene expression through iron-responsive elements in mRNAs coded by several genes is one of the best understood mechanisms of translational control.

Regulation of Salmonella enterica serovar Typhimurium mntH transcription by H(2)O(2), Fe(2+), and Mn(2+)

MntH, a bacterial homolog of mammalian natural resistance associated macrophage protein 1 (Nramp1), is a primary transporter for Mn(2+) influx in Salmonella enterica serovar Typhimurium and Escherichia coli. S. enterica serovar Typhimurium MntH contributes to H(2)O(2) resistance and is important for full virulence. Consistent with its phenotype and function, mntH is regulated at the transcriptional level by both H(2)O(2) and substrate cation. We have now identified three trans-acting regulatory factors and the three corresponding cis-acting mntH promoter motifs that mediate this regulation. In the presence of hydrogen peroxide, mntH is activated by OxyR, acting through an OxyR-binding motif centered just upstream of the likely -35 RNA polymerase-binding site. In the presence of Fe(2+), mntH is repressed primarily by Fur, acting through a Fur-binding motif overlapping the -35 region. In the presence of Mn(2+), mntH is repressed primarily by the Salmonella equivalent of E. coli b0817, a distant homolog of the Bacillus subtilis manganese transport repressor, MntR, acting through an inverted-repeat motif located between the likely -10 polymerase binding site and the ribosome binding site. E. coli b0817 was recently shown to bind the identical inverted-repeat motif in the E. coli mntH promoter and hence has been renamed MntR (S. I. Patzer and K. Hantke, J. Bacteriol. 183:4806-4813, 2001). Using Deltafur, DeltamntR, and Deltafur DeltamntR mutant strains as well as mutations in the Fur- and MntR-binding motif elements, we found that Fe(2+) can also mediate repression through the Mn(2+) repressor MntR.

SitABCD is the alkaline Mn(2+) transporter of Salmonella enterica serovar Typhimurium

MntH, a bacterial homolog of the mammalian natural resistance-associated macrophage protein 1 (Nramp1), is a primary Mn(2+) transporter of Salmonella enterica serovar Typhimurium and Escherichia coli. S. enterica serovar Typhimurium MntH expression is important for full virulence; however, strains carrying an mntH deletion are only partially attenuated and display no obvious signs of Mn(2+) deficiency. We noted that promoter sequences for mntH and for the putative Fe(2+) transporter sitABCD appeared to have the same regulatory element responsive to Mn(2+) and so hypothesized that sitABCD could transport Mn(2+) with high affinity. We have now characterized transport by SitABCD in S. enterica serovar Typhimurium using (54)Mn(2+) and (55)Fe(2+) and compared its properties to those of MntH. SitABCD mediates the influx of Mn(2+) with an apparent affinity (K(a)) identical to that of MntH, 0.1 microM. It also transports Fe(2+) but with a K(a) 30 to 100 times lower, 3 to 10 microM. Inhibition of (54)Mn(2+) transport by Fe(2+) and of (55)Fe(2+) transport by Mn(2+) gave inhibition constants comparable to each cation's K(a) for influx. Since micromolar concentrations of free Fe(2+) are improbable in a biological system, we conclude that SitABCD functions physiologically as a Mn(2+) transporter. The cation inhibition profiles of SitABCD and MntH are surprisingly similar for two structurally and energetically unrelated transporters, with a Cd(2+) K(i) of approximately 1 microM and a Co(2+) K(i) of approximately 20 microM and with Ni(2+), Cu(2+), and Fe(3+) inhibiting both transporters only at concentrations of >0.1 mM. The one difference is that Zn(2+) exhibits potent inhibition of SitABCD (K(i) = 1 to 3 microM) but inhibits MntH weakly (K(i) > 50 microM). We have previously shown that MntH transports Mn(2+) most effectively under acidic conditions. In sharp contrast, SitABCD has almost no transport capacity at acid pHs and optimally transports Mn(2+) at slightly alkaline pHs. Overall, coupled with evidence that each transporter is multiply but distinctly regulated at the transcriptional level, the distinct transport properties of MntH versus SitABCD suggest that each transporter may be specialized for Mn(2+) uptake in different physiological environments.

The natural resistance-associated macrophage protein 1 Slc11a1 (formerly Nramp1) and iron metabolism in macrophages

Slc11a1 (solute carrier family 11 member 1) (formerly Nramp1) modulation of iron metabolism in macrophages plays an important role in early phase macrophage activation, and therefore host innate immunity. This review focuses on the role of Nramp1 in intramacrophage iron metabolism, with emphasis on the two prevailing mechanisms of Nramp1 modulation of iron metabolism in macrophages.

Polymorphism in the rhesus macaque (Macaca mulatta) NRAMP1 gene: lack of an allelic association to tuberculosis susceptibility

Although previous tuberculosis (TB) research has suggested that underlying genetic factors influence a host's response and ability to survive Mycobacterium infection, only recently has a gene been identified, the 'natural resistance-associated macrophage protein 1' (NRAMP1) gene, which provides a degree of natural resistance to infection by some Mycobacterium species. To date, however, the role that NRAMP1 may play in resistance to Mycobacterium infection has only been examined in mouse and man. Here, we present data generated at NRAMP1 among a group of rhesus macaques (Macaca mulatta) that were euthanized because of an outbreak of Mycobacterium tuberculosis during quarantine. Data were also generated on unrelated (and healthy) rhesus macaques in order to better determine the frequency and degree of genetic polymorphism within Macaca at the NRAMP1 locus. These data represent the first study designed to examine the role that NRAMP1 may play in TB susceptibility among rhesus macaques.

Divalent-metal transport by NRAMP proteins at the interface of host-pathogen interactions

The NRAMP family of divalent-metal transporters plays a key role in the homeostasis of iron and other metals. NRAMP2 (DMT1) acts as an iron-uptake protein in both the duodenum and in peripheral tissues. NRAMP1 functions as a divalent-metal efflux pump at the phagosomal membrane of macrophages and neutrophils, and mutations in NRAMP1 cause susceptibility to several intracellular pathogens. NRAMP homologues have been identified in bacteria and are involved in acquiring divalent metals from the extracellular environment. Interestingly, bacterial and mammalian NRAMP proteins would compete for the same essential substrates within the microenvironment of the phagosome, at the interface of host-pathogen interactions.

Nramp1 modulates iron homoeostasis in vivo and in vitro: evidence for a role in cellular iron release involving de-acidification of intracellular vesicles

Nramp1 controls responses to infection and encodes a biallelic (G169D) macrophage-restricted divalent-cation transporter. Nramp1(D169) is phenotypically null. We demonstrate Nramp1 is implicated in iron regulation in vivo. In spleen, expression is exclusive to Nramp1(G169) strains within the red pulp. By morphometric analysis, the distribution of splenic iron, following systemic overload, correlates with Nramp1 genotype. More iron is located within the red pulp in Nramp1(D169) strains, whereas in Nramp1(G169) strains iron deposits are localized within the marginal-zone metallophilic cells. Nramp1 immunoreactive protein is not present in control brain, but inducible within a hemorrhagic lesion model in Nramp1(G169) strains. Nramp1 protein expression demonstrates an inverse correlation to the presence of iron. Nramp1(G169) strains show no Perl's stain-reactive iron within the lesion. In contrast, Nramp1(D169) strains display iron-staining cells. The process of cellular iron regulation was investigated in vitro in Nramp1(G169) transfectant Raw264.7 macrophages. Greater (30-50%) iron efflux from Nramp1(G169) compared with Nramp1(D169) cells was determined. The extent of Nramp1-dependent iron-release was influenced by bafilomycin A1, and endogenous nitric oxide synthesis, both inhibitors of vacuolar-ATPase. This study demonstrates that Nramp1 regulates macrophage iron handling, and probably facilitates iron release from macrophages undergoing erythrophagocytosis in vivo.

Natural-resistance-associated macrophage protein 1 is an H+/bivalent cation antiporter

In mammals, natural-resistance-associated macrophage protein 1 (Nramp1) regulates macrophage activation and is associated with infectious and autoimmune diseases. Nramp2 is associated with anaemia. Both belong to a highly conserved eukaryote/prokaryote protein family. We used Xenopus oocytes to demonstrate that, like Nramp2, Nramp1 is a bivalent cation (Fe2+, Zn2+ and Mn2+) transporter. Strikingly, however, where Nramp2 is a symporter of H+ and metal ions, Nramp1 is a highly pH-dependent antiporter that fluxes metal ions in either direction against a proton gradient. At pH 9.0, oocytes injected with cRNA from wild-type murine Nramp1 with a glycine residue at position 169 (Nramp1(G169); P=3.22x10(-6)) and human NRAMP1 (P=3.87x10(-5)) showed significantly enhanced uptake of radiolabelled Zn2+ compared with water-injected controls. At pH 5.5, Nramp1(G169) (P=1.34x10(-13)) and NRAMP1 (P=1.09x10(-6)) oocytes showed significant efflux of Zn2+. Zn2+ transport was abolished when the proton gradient was dissipated using carbonyl cyanide p-trifluoromethoxyphenylhydrazone. Using pre-acidified oocytes, currents of 130+/-57 nA were evoked by 100 microM Zn2+ at pH 7.5, and 139+/-47 nA by 100 microM Fe2+ at pH 7.0, in Nramp1(G169) oocytes; currents of 254+/-49 nA and 242+/-26 nA were evoked, respectively, in NRAMP1 oocytes. Steady-state currents evoked by increasing concentrations of Zn2+ were saturable, with apparent affinity constants of approx. 614 nM for Nramp1(G169) and approx. 562 nM for NRAMP1 oocytes, and a curvilinear voltage dependence of transporter activity (i.e. the data points approximate to a curve that approaches a linear asymptote). In the present study we propose a new model for metal ion homoeostasis in macrophages. Under normal physiological conditions, Nramp2, localized to early endosomal membranes, delivers extracellularly acquired bivalent cations into the cytosol. Nramp1, localized to late endosomal/lysosomal membranes, delivers bivalent cations from the cytosol into this acidic compartment where they may directly affect antimicrobial activity.

Nramp1 is expressed in neurons and is associated with behavioural and immune responses to stress

The gene Nramp1 encoding the natural resistance-associated macrophage protein (Nramp1) influences susceptibility to intracellular infections and autoimmune diseases, and the humoral response to stress. Nramp1 functions as a proton/divalent cation antiporter in the membranes of late endosomes/lysosomes, regulating cytoplasmic iron levels in macrophages. The Drosophila homologue of Nramp1 is expressed in sensory neurons and macrophages, and influences taste behaviour directly through divalent cation transport. Here we demonstrate that murine Nramp1 is also expressed on neurons as well as microglial cells in the brain and influences the behavioural response to stress, hypothalamus-pituitary-adrenal (HPA) axis activation and mortality following Toxoplasma gondii infection in control and prestressed mice. We hypothesise that, although differences in HPA activation translate into differences in adrenal enlargement and basal circulating corticosterone levels, the primary influence of Nramp1 is at the level of the neuronal response to stress. These results provide new insight into the possible roles of divalent cation transporters of the Nramp gene family in regulating metal ion homeostasis in the brain and its pathological implications.

Analysis of the NRAMP1 Gene Implicated in Iron Transport: Association with Multiple Sclerosis and Age Effects

Multiple sclerosis (MS) is believed to be an autoimmune process occurring in genetically susceptible individuals after an appropriate environmental exposure. We have exploited the homogeneous Afrikaner population of European ancestry to investigate the likelihood that iron dysregulation, in association with infectious and/or autoimmune disease susceptibility, may underlie the MS phenotype in a subgroup of patients. The functional Z-DNA forming repeat polymorphism of the natural resistance-associated macrophage protein-1 (NRAMP1) gene was analyzed in 104 patients diagnosed with MS and 522 Caucasian controls. A family-based control group consisting of 32 parental alleles not transmitted to MS offspring was additionally studied to exclude the likelihood of population substructures. Statistically significant differences in allelic distribution were observed between the patient and control samples drawn from the same population (P < 0.01). Evidence is furthermore provided that alleles considered to be detrimental in relation to autoimmune disease susceptibility may be maintained in the population as a consequence of improved survival to reproductive age following infectious disease challenge. Although it remains to be determined whether the disease phenotype in MS patients with allele 5 of the NRAMP1 promoter polymorphism is directly related to dysregulation of iron or modified susceptibility to viral infection and/or autoimmunity, a combination of these processes most likely underlies the disease phenotype in these patients. In view of the emerging role of polymorphic variants in complex diseases and minimizing of possible confounding factors in this association study, we conclude that allelic variation in the NRAMP1 promoter may contribute significantly to MS susceptibility in the South African Caucasian population.

Natural resistance to intracellular infections: natural resistance-associated macrophage protein 1 (Nramp1) functions as a pH-dependent manganese transporter at the phagosomal membrane

Mutations at the natural resistance-associated macrophage protein 1 (Nramp1) locus cause susceptibility to infection with antigenically unrelated intracellular pathogens. Nramp1 codes for an integral membrane protein expressed in the lysosomal compartment of macrophages, and is recruited to the membrane of phagosomes soon after the completion of phagocytosis. To define whether Nramp1 functions as a transporter at the phagosomal membrane, a divalent cation-sensitive fluorescent probe was designed and covalently attached to a porous particle. The resulting conjugate, zymosan-FF6, was ingested by macrophages and its fluorescence emission was recorded in situ after phagocytosis, using digital imaging. Quenching of the probe by Mn(2+) was used to monitor the flux of divalent cations across the phagosomal membrane in peritoneal macrophages obtained from Nramp1-expressing (+/+) and Nramp1-deficient (-/-) macrophages. Phagosomes from Nramp1(+/+) mice extrude Mn(2+) faster than their Nramp(-/-) counterparts. The difference in the rate of transport is eliminated when acidification of the phagosomal lumen is dissipated, suggesting that divalent metal transport through Nramp1 is H(+) dependent. These studies suggest that Nramp1 contributes to defense against infection by extrusion of divalent cations from the phagosomal space. Such cations are likely essential for microbial function and their removal from the phagosomal microenvironment impairs pathogenesis, resulting in enhanced bacteriostasis or bactericidal activity.

Interferon-gamma and lipopolysaccharide regulate the expression of Nramp2 and increase the uptake of iron from low relative molecular mass complexes by macrophages

The natural resistance associated macrophage protein 2 (Nramp2) is a transporter that is involved in iron (Fe) uptake from transferrin (Tf) and low molecular mass Fe complexes. Here we describe the effect of the inflammatory mediators interferon-gamma (IFN-gamma) and lipopolysaccharide (LPS) on the expression of Nramp2 mRNA and Fe uptake by cells of the macrophage lineage. After incubation of the RAW264.7 macrophage cell line with LPS there was a sevenfold increase in the expression of the 2.3 kb Nramp2 mRNA transcript when compared with the control, but little effect on the Nramp2 3.1 kb transcript. These results indicate differential regulation of the two transcripts. Treatment with LPS resulted in an increase in 59Fe uptake from 59Fe-nitrilotriacetic acid, while transferrin receptor (TfR) mRNA levels and 59Fe uptake from 59Fe-Tf were decreased. Paradoxically, at the same time, an increase in iron regulatory protein (IRP)1 RNA-binding activity was observed. Incubation with IFN-gamma (50 U.mL-1) resulted in a marked decrease in TfR mRNA levels but had no effect on Nramp2 mRNA expression. Exposure of RAW264.7 cells to both IFN-gamma and LPS resulted in a fourfold increase in the Nramp2 2.3-kb transcript and a four to fivefold decrease in the 3.1-kb transcript when compared with the control. Furthermore, there was a decrease in TfR mRNA levels despite an increase in IRP1 RNA-binding activity and a marked increase in inducible nitric oxide synthase mRNA expression. Hence, TfR and Nramp2 mRNA expression did not appear to be regulated in a concerted manner. Similar responses to those found above for RAW264.7 cells were also observed in the J774 macrophage cell line and also for primary cultures of mouse peritoneal macrophages. These results are of interest as the TfR and Nramp2 are thought to act together during Fe uptake from Tf. This is the first report to demonstrate regulation of the Nramp2 mRNA transcripts by inflammatory mediators.

The NRAMP proteins of Salmonella typhimurium and Escherichia coli are selective manganese transporters involved in the response to reactive oxygen

NRAMPs (natural resistance-associated macrophage proteins) have been characterized in mammals as divalent transition metal transporters involved in iron metabolism and host resistance to certain pathogens. The mechanism of pathogen resistance is proposed to involve sequestration of Fe2+ and Mn2+, cofactors of both prokaryotic and eukaryotic catalases and superoxide dismutases, not only to protect the macrophage against its own generation of reactive oxygen species, but to deny the cations to the pathogen for synthesis of its protective enzymes. NRAMP homologues are also present in bacteria. We report the cloning and characterization of the single NRAMP genes in Escherichia coli and Salmonella enterica ssp. typhimurium, and the cloning of two distinct NRAMP genes from Pseudomonas aeruginosa and an internal fragment of an NRAMP gene in Burkholderia cepacia. The genes are designated mntH because the two enterobacterial NRAMPs encode H+-stimulated, highly selective manganese(II) transport systems, accounting for all Mn2+ uptake in each species under the conditions tested. For S. typhimurium MntH, the Km for 54Mn2+ ( approximately 0.1 microM) was pH independent, but maximal uptake increased as pH decreased. Monovalent cations, osmotic strength, Mg2+ and Ca2+ did not inhibit 54Mn2+ uptake. Ni2+, Cu2+ and Zn2+ inhibited uptake with Kis greater than 100 microM, Co2+ with a Ki of 20 microM and Fe2+ with a Ki that decreased from 100 microM at pH 7. 6 to 10 microM at pH 5.5. Fe3+ and Pb2+ inhibited weakly, exhibiting Kis of 50 microM, while Cd2+ was a potent inhibitor with a Ki of about 1 microM. E. coli MntH had a similar inhibition profile, except that Kis were three- to 10-fold higher. Both S. typhimurium and E. coli MntH also transport 55Fe2+ however, the Kms are equivalent to the Kis for Fe2+ inhibition of Mn2+ uptake, and are thus too high to be physiologically relevant. In both S. typhimurium and E. coli, mntH:lacZ constructs were strongly induced by hydrogen peroxide, weakly induced by EDTA and unresponsive to paraquat, consistent with the presence of Fur and OxyR binding sites in the promoters. Strains overexpressing mntH were more susceptible to growth inhibition by Mn2+ and Cd2+ than wild type, and strains lacking a functional mntH gene were more susceptible to killing by hydrogen peroxide. In S. typhimurium strain SL1344, mntH mutants showed no defect in invasion of or survival in cultured HeLa or RAW264.7 macrophage cells; however, expression of mntH:lacZ was induced severalfold by 3 h after invasion of the macrophages. S. typhimurium mntH mutants showed only a slight attenuation of virulence in BALB/c mice. Thus, the NRAMP Mn2+ transporter MntH and Mn2+ play a role in bacterial response to reactive oxygen species and possibly have a role in pathogenesis.

Emerging mechanisms for heavy metal transport in plants

Heavy metal ions such as Cu(2+), Zn(2+), Mn(2+), Fe(2+), Ni(2+) and Co(2+) are essential micronutrients for plant metabolism but when present in excess, these, and non-essential metals such as Cd(2+), Hg(2+) and Pb(2+), can become extremely toxic. Thus mechanisms must exist to satisfy the requirements of cellular metabolism but also to protect cells from toxic effects. The mechanisms deployed in the acquisition of essential heavy metal micronutrients have not been clearly defined although a number of genes have now been identified which encode potential transporters. This review concentrates on three classes of membrane transporters that have been implicated in the transport of heavy metals in a variety of organisms and could serve such a role in plants: the heavy metal (CPx-type) ATPases, the natural resistance-associated macrophage protein (Nramp) family and members of the cation diffusion facilitator (CDF) family. We aim to give an overview of the main features of these transporters in plants in terms of structure, function and regulation drawing on information from studies in a wide variety of organisms.

Attenuation of MHC Class II Expression in Macrophages Infected with Mycobacterium bovis Bacillus Calmette-Guerin Involves Class II Transactivator and Depends on the Nramp1 Gene

The natural resistance associated macrophage protein 1 (Nramp1) gene determines the ability of murine macrophages to control infection with a group of intracellular pathogens, including Salmonella typhimurium, Leishmania donovani, and Mycobacterium bovis bacillus Calmette-Guérin (BCG). The expression of the resistant allele of the Nramp1 gene in murine macrophages is associated with a more efficient expression of several macrophage activation-associated genes, including class II MHC loci. In this study, we investigated the molecular mechanisms involved in IFN-γ-induced MHC class II expression in three types of macrophages: those expressing a wild-type allele of the Nramp1 gene (B10R and 129/Mφ), those carrying a susceptible form of the Nramp1 gene (B10S), and those derived from 129-Nramp1-knockout mice (129/Nramp1-KO). Previously, we published results showing that Ia protein expression is significantly higher in the IFN-γ-induced B10R macrophages, compared with its susceptible counterpart. In this paper, we also show that the higher expression of Ia protein in B10R cells is associated with higher I-Aβ mRNA expression, which correlates with a higher level of IFN-γ-induced phosphorylation of the STAT1-α protein and subsequently with elevated expression of class II transactivator (CIITA) mRNA, compared with B10S. Furthermore, we demonstrate that the infection of macrophages with M. bovis BCG results in a down-regulation of CIITA mRNA expression and, consequently, in the inhibition of Ia induction. Therefore, our data explain, at least in part, the molecular mechanism involved in the inhibition of I-Aβ gene expression in M. bovis BCG-infected macrophages activated with IFN-γ.

Metal ion transporters in mammals: structure, function and pathological implications

Despite the importance of metal ions in several catalytic functions, there has been, until recently, little molecular information available on the mechanisms whereby metal ions are actively taken up by mammalian cells. The classical concept for iron uptake into mammalian cells has been the endocytosis of transferrin-bound Fe3+ by the transferrin receptor. Studies with hypotransferrinaemic mice revealed that in the intestine mucosal transferrin is derived from the plasma and that its presence is not required in the intestinal lumen for dietary iron absorption. This suggests that, at least in the intestine, other non-receptor-mediated uptake systems exist. The molecular identification of metal ion transporters is of great importance, in particular since an increasing number of human diseases are thought to be related to disturbances in metal ion homeostasis, including metal ion overload and deficiency disorders (i.e. anaemia, haemochromatosis, Menkes disease, Wilson's disease), and neurodegenerative diseases (i.e. Alzheimer's, Friedreich's ataxia and Parkinson's diseases). Furthermore, susceptibilities to mycobacterial infections are caused by metal ion transporter defects. The pathological implications of disturbed metal ion homeostasis confirm the vital roles these metal ions play in the catalytic function of many enzymes, in gene regulation (zinc-finger proteins), and in free radical homeostasis. Recent insights have significantly advanced our knowledge of how metal ions are taken up or released by mammalian cells. The purpose of this review is to summarize these advances and to give an overview on the growing number of mammalian metal ion transporters.

Iron acquisition by plants

In nongraminaceous plants, the FeII-transporter gene and ferric-chelate reductase gene have been cloned from Arabidopsis thaliana, whereas FeIII-reductase has not. In graminaceous monocots, the genes for mugineic acids (MAs) synthesis, nas (nicotianamine synthase) and naat (nicotianamine aminotransferase), have been cloned from barley, whereas the FeIII-MAs transporter gene is yet to be cloned. Transferrin absorption in Dunaliella has been reported, suggesting a phagocytotic (endocytotic) Fe-acquisition mechanism. Work to develop transgenic cultivars tolerant to Fe-deficiency in calcareous soils is now in progress.

High level expression of Nramp1G169 in RAW264.7 cell transfectants: analysis of intracellular iron transport

Nramp1 (natural resistance-associated macrophage protein) was positionally cloned as the defective biallelic locus in inbred mouse strains associated with uncontrolled proliferation of obligate intracellular macrophage pathogens. The causative defect was described as G169D within membrane spanning domain 4 of a transporter. The biochemical activity of Nramp1 is implied from sequence conservation with Nramp2. Nramp2 encodes a divalent cation transporter and is the carrier of a defect in models of microcytic anaemia, associated with impaired intestinal iron uptake. Iron sequestration has been proposed as an antimicrobial mechanism. Therefore, such an activity for Nramp1 is consistent with model systems. Here we showed that Nramp1 directs iron transport within the macrophage. We describe stable, high-level Nramp1G169 allele-derived polypeptide expression in Balb/c Nramp1D169 RAW264.7 cells. Transfectants express levels, comparable to those in Nramp1G169-resistant macrophages, of a 90-100x103 MW Nramp1 polypeptide. Expression of the Nramp1 polypeptide correlates with lower cellular iron loads and a reduced chelatable iron pool following challenge with iron: nitrilotriacetate. Pulse chase experiments support an enhanced iron flux in expressing cells. These data are supported using the fluorescent iron probe calcein. In Nramp1G169-expressing cells we observed an increased iron flux into the cytoplasm from a calcein-inaccessible cellular location. These data suggest Nramp1, in resting macrophage cells, mobilizes iron, from an intracellular vesicle, which is destined for cell secretion. We propose that under these conditions Nramp1 plays a role in a salvage pathway of iron recycling.

Evidence for a functional repeat polymorphism in the promoter of the human NRAMP1 gene that correlates with autoimmune versus infectious disease susceptibility

A polymorphism in the promoter of human NRAMP1 encodes a Z-DNA forming dinucleotide repeat with four alleles: (1) t(gt)5ac(gt)5ac(gt)11g; (2) t(gt)5ac(gt)5 ac(gt)10g; (3) t(gt)5ac(gt)5ac(gt)9g; and (4) t(gt)5ac(gt)9g. Alleles 1 and 4 are rare (gene frequencies ∼0.001); alleles 2 and 3 occur at gene frequencies ∼0.20−0.25 and ∼0.75− 0.80, respectively. Here, luciferase reporter gene constructs are used to show that the four alleles differ in their ability to drive gene expression. In the absence of exogenous stimuli, alleles 1, 2, and 4 are poor promoters; allele 3 drives high expression, indicating that the repeat itself has endogenous enhancer activity. All four alleles show a similar percentage enhancement of reporter gene expression in the presence of interferon-γ, consistent with the multiple interferon-γ response elements both 5′ and 3′ of the Z-DNA forming repeat. However, while the addition of bacterial lipopolysaccharide (LPS) has no effect on alleles 1 and 4, it causes significant reduction in expression driven by allele 2 and enhances expression driven by allele 3, suggesting that the juxtaposition of LPS related response elements (NFκB, AP-1, NF-IL6) may be differentially affected by the two commonly occurring alleles. These results are consistent with the hypothesis that chronic hyperactivation of macrophages associated with allele 3 is functionally linked to autoimmune disease susceptibility, while the poor level of NRAMP1 expression promoted by allele 2 contributes to infectious disease susceptibility. Conversely, allele 3 protects against infectious disease and allele 2 against autoimmune disease. Hence, alleles that are detrimental in relation to autoimmune disease susceptibility may be maintained in the population because they improve survival to reproductive age following infectious disease challenge.

Molecular insights into mechanisms of iron transport

The past 3 years have witnessed extraordinary progress in our understanding of mammalian iron transport and homeostasis. The first transmembrane iron transporter has been found. Mutations in this protein, in two animal models with iron-transport defects, have helped to define the roles of this protein in vivo. The gene defective in patients with hereditary hemochromatosis has been identified, and much has been learned about the structure and function of its gene product. Finally, our ability to make a molecular diagnosis of hereditary hemochromatosis has called attention to new iron-loading disorders, including African iron overload and juvenile hemochromatosis.

Role of iron in Nramp1-mediated inhibition of mycobacterial growth

Innate resistance to mycobacterial growth is mediated by a gene, Nramp1. We have previously reported that Nramp1 mRNA from macrophages of Mycobacterium bovis BCG-resistant (Bcgr) mice is more stable than Nramp1 mRNA from macrophages of BCG-susceptible (Bcgs) mice. Based on these observations and on reports that show that the closely related Nramp2 gene is a metal ion transporter, we evaluated the effect of iron on the growth of Mycobacterium avium within macrophages as well as on the stability of Nramp1 mRNA. The addition of iron to macrophages from Bcgs mice resulted in a stimulation of mycobacterial growth. In contrast, iron increased the capacity of macrophages from Bcgr mice to control the growth of M. avium. When we treated recombinant gamma interferon (IFN-gamma)-activated macrophages with iron, we found that iron abrogated the growth inhibitory effect of IFN-gamma-activated macrophages from Bcgs mice but that it did not affect the capacity of macrophages from Bcgr mice to control microbial growth. A more detailed examination of the effect of iron on microbial growth showed that the addition of small quantities of iron to resident macrophages from Bcgr mice stimulated antimicrobial activity within a very narrow dose range. The effect of iron on the growth inhibitory activity of macrophages from Bcgr mice was abrogated by the addition of catalase or mannitol to the culture medium. These results are consistent with an Fe(II)-mediated stimulation of the Fenton/Haber-Weiss reaction and hydroxyl radical-mediated inhibition of mycobacterial growth.

Genetic regulation of macrophage activation: understanding the function of Nramp1 (=Ity/Lsh/Bcg)

The Nramp1 gene was originally described as Ity/Lsh/Bcg, a single gene controlling resistance and susceptibility of inbred mice to a range of intramacrophage pathogens. Functional studies demonstrated that Ity/Lsh/Bcg had multiple pleiotropic effects on macrophage activation pathways, broadening interest in the gene to include its candidacy as an autoimmune disease susceptibility gene. In 1993 the gene was positionally cloned and found to encode a polytopic integral membrane protein of unknown function. Subsequent studies have localized the protein to late endosomal and lysosomal compartments, and demonstrated that it functions as an iron transporter. Precisely how this function influences macrophage activation pathways is still under investigation, but is likely to include direct effects on pathogen survival in the endosomal/lysosomal compartment as well as influences on intracellular signalling pathways and in regulating mRNA stability. Several studies now provide evidence for a role for NRAMP1 in determining human susceptibility to autoimmune (rheumatoid arthritis. juvenile rheumatoid arthritis, diabetes, Crohn's disease) and infectious (tuberculosis, leprosy) diseases. Amongst these. data are accumulating to support the hypothesis that a functional Z-DNA forming repeat polymorphism in the promoter region of human NRAMP1 contributes directly to disease susceptibility. Four alleles have been observed, alleles 1 and 4 are rare (gene frequencies approximately equal to 0.001), alleles 2 and 3 occur at gene frequencies approximately 0.25 and approximately 0.75, respectively. In the absence of exogenous stimuli, alleles 1, 2 and 4 are poor promoters of gene expression in a luciferase reporter gene system; allele 3 drives high expression. Allele 3 shows allelic association with autoimmune disease susceptibility, allele 2 with infectious disease susceptibility. Hence, balancing selection is likely to be maintaining these two alleles in human populations. Although the association of NRAMP1 with autoimmune disease susceptibility may be related to any one of the multiple pleiotropic effects associated with macrophage activation, the function of NRAMP1 as an iron transporter now prompts more interesting speculation that regulation of iron transport may contribute directly to the disease phenotype in arthritic disease. Patients suffering from rheumatoid arthritis show increased deposition of iron in the synovial membrane, which may contribute to free radical generation and local inflammation. Further analysis of NRAMP1 function will continue to be of importance in understanding the molecular basis to autoimmune and infectious disease susceptibility.