Computer-identified nuclear localization signal in exon 1A of the transporter DMT1 is essentially ineffective in nuclear targeting

Mitochondria represent another locale for the divalent metal transporter 1 (DMT1)

The divalent metal transporter (DMT1) is well known for its roles in duodenal iron absorption across the apical enterocyte membrane, in iron efflux from the endosome during transferrin-dependent cellular iron acquisition, as well as in uptake of non-transferrin bound iron in many cells. Recently, using multiple approaches, we have obtained evidence that the mitochondrial outer membrane is another subcellular locale of DMT1 expression. While iron is of vital importance for mitochondrial energy metabolism, its delivery is likely to be tightly controlled due to iron's damaging redox properties. Here we provide additional support for a role of DMT1 in mitochondrial iron acquisition by immunofluorescence colocalization with mitochondrial markers in cells and isolated mitochondria, as well as flow cytometric quantification of DMT1-positive mitochondria from an inducible expression system. Physiological consequences of mitochondrial DMT1 expression are discussed also in consideration of other DMT1 substrates, such as manganese, relevant to mitochondrial antioxidant defense.

Evidence for mitochondrial localization of divalent metal transporter 1 (DMT1)

In mammalian cells, mitochondria receive most incoming iron, yet no entry pathway for iron at the outer mitochondrial membrane (OMM) has been characterized. Our results show that the divalent metal transporter 1 (DMT1) occurs in the OMM. Immunoblots detected DMT1 in mitochondria from a pneumocyte cell model in their OMM. Using the split-ubiquitin yeast 2-hybrid system, we found that cytochrome c oxidase subunit II (COXII) and the translocase of OMM 6-kDa subunit (Tom6) homologue interact with DMT1. COXII coimmunoprecipitates with DMT1. There are 4 DMT1 isoforms that differ at the N and C termini. Using HEK293 cells that inducibly express all of the 4 ends of DMT1, we found all of them in the OMM, as detected by immunoblots after cell fractionation, and in isolated mitochondria, as detected by immunofluorescence. Immunoblot analysis of purified cell fractions from rat renal cortex confirmed and extended these results to the kidney, which expressed high levels of DMT1. Immunogold labeling detected DMT1 colocalization in mitochondria with the voltage-dependent anion-selective channel protein-1, which is expressed in the OMM. We suggest that DMT1 not only exports iron from endosomes, but also serves to import the metal into the mitochondria.

1B/(-)IRE DMT1 expression during brain ischemia contributes to cell death mediated by NF-κB/RelA acetylation at Lys310

The molecular mechanisms responsible for increasing iron and neurodegeneration in brain ischemia are an interesting area of research which could open new therapeutic approaches. Previous evidence has shown that activation of nuclear factor kappa B (NF-κB) through RelA acetylation on Lys310 is the prerequisite for p50/RelA-mediated apoptosis in cellular and animal models of brain ischemia. We hypothesized that the increase of iron through a NF-κB-regulated 1B isoform of the divalent metal transporter-1 (1B/DMT1) might contribute to post-ischemic neuronal damage. Both in mice subjected to transient middle cerebral artery occlusion (MCAO) and in neuronally differentiated SK-N-SH cells exposed to oxygen-glucose-deprivation (OGD), 1A/DMT1 was only barely expressed while the 1B/DMT1 without iron-response-element (−IRE) protein and mRNA were early up-regulated. Either OGD or over-expression of 1B/(−)IRE DMT1 isoform significantly increased iron uptake, as detected by total reflection X-ray fluorescence, and iron-dependent cell death. Iron chelation by deferoxamine treatment or (−)IRE DMT1 RNA silencing displayed significant neuroprotection against OGD which concomitantly decreased intracellular iron levels. We found evidence that 1B/(−)IRE DMT1 was a target gene for RelA activation and acetylation on Lys310 residue during ischemia. Chromatin immunoprecipitation analysis of the 1B/DMT1 promoter showed there was increased interaction with RelA and acetylation of H3 histone during OGD exposure of cortical neurons. Over-expression of wild-type RelA increased 1B/DMT1 promoter-luciferase activity, the (−)IRE DMT1 protein, as well as neuronal death. Expression of the acetylation-resistant RelA-K310R construct, which carried a mutation from lysine 310 to arginine, but not the acetyl-mimic mutant RelA-K310Q, down-regulated the 1B/DMT1 promoter, consequently offering neuroprotection. Our data showed that 1B/(−)IRE DMT1 expression and intracellular iron influx are early downstream responses to NF-κB/RelA activation and acetylation during brain ischemia and contribute to the pathogenesis of stroke-induced neuronal damage.

Ndfip1-deficient mice have impaired DMT1 regulation and iron homeostasis

The divalent metal ion transporter DMT1 is critical for nonheme iron import. We have previously shown that DMT1 is regulated in vitro by ubiquitination that is facilitated by the adaptor proteins Ndfip1 and Ndfip2. Here we report that in Ndfip1(-/-) mice fed a low- iron diet, DMT1 expression and activity in duodenal enterocytes are significant higher than in the wild-type animals. This correlates with an increase in serum iron levels and transferrin saturation. Liver and spleen iron stores were also increased in Ndfip1(-/-) mice fed a normal diet. Counterintuitive to the increase in iron uptake, Ndfip1(-/-) mice fed a low iron diet develop severe microcytic, hypochromic anemia. We demonstrate that this is due to a combination of iron deficiency and inflammatory disease in Ndfip1(-/-) mice, because Ndfip1(-/-)/Rag1(-/-) immunodeficient mice fed a low iron diet did not develop anemia and showed an iron overload phenotype. These data demonstrate that Ndfip1 is a critical mediator of DMT1 regulation in vivo, particularly under iron restricted conditions.

Functional properties of multiple isoforms of human divalent metal-ion transporter 1 (DMT1)

DMT1 (divalent metal-ion transporter 1) is a widely expressed metal-ion transporter that is vital for intestinal iron absorption and iron utilization by most cell types throughout the body, including erythroid precursors. Mutations in DMT1 cause severe microcytic anaemia in animal models. Four DMT1 isoforms that differ in their N- and C-termini arise from mRNA transcripts that vary both at their 5'-ends (starting in exon 1A or exon 1B) and at their 3'-ends giving rise to mRNAs containing (+) or lacking (-) the 3'-IRE (iron-responsive element) and resulting in altered C-terminal coding sequences. To determine whether these variations result in functional differences between isoforms, we explored the functional properties of each isoform using the voltage clamp and radiotracer assays in cRNA-injected Xenopus oocytes. 1A/IRE+-DMT1 mediated Fe2+-evoked currents that were saturable (K(0.5)(Fe) approximately 1-2 microM), temperature-dependent (Q10 approximately 2), H+-dependent (K(0.5)(H) approximately 1 muM) and voltage-dependent. 1A/IRE+-DMT1 exhibited the provisional substrate profile (ranked on currents) Cd2+, Co2+, Fe2+, Mn2+>Ni2+, V3+>>Pb2+. Zn2+ also evoked large currents; however, the zinc-evoked current was accounted for by H+ and Cl- conductances and was not associated with significant Zn2+ transport. 1B/IRE+-DMT1 exhibited the same substrate profile, Fe2+ affinity and dependence on the H+ electrochemical gradient. Each isoform mediated 55Fe2+ uptake and Fe2+-evoked currents at low extracellular pH. Whereas iron transport activity varied markedly between the four isoforms, the activity for each correlated with the density of anti-DMT1 immunostaining in the plasma membrane, and the turnover rate of the Fe2+ transport cycle did not differ between isoforms. Therefore all four isoforms of human DMT1 function as metal-ion transporters of equivalent efficiency. Our results reveal that the N- and C-terminal sequence variations among the DMT1 isoforms do not alter DMT1 functional properties. We therefore propose that these variations serve as tissue-specific signals or cues to direct DMT1 to the appropriate subcellular compartments (e.g. in erythroid cells) or the plasma membrane (e.g. in intestine).

Comparison of mammalian cell lines expressing distinct isoforms of divalent metal transporter 1 in a tetracycline-regulated fashion

DMT1 (divalent metal transporter; also known as SLC11A2, DCT1 or Nramp2) is responsible for ferrous iron uptake in the duodenum, iron exit from endosomes during the transferrin cycle and some transferrin-independent iron uptake in many cells. Four protein isoforms differ by starting in exon 1A or 2 and ending with alternative peptides encoded by mRNA that contains or lacks an IRE (iron responsive element; +/-IRE). We have compared 1A/+IRE and 2/-IRE DMT1 during regulated ectopic expression. HEK-293-F (human embryonic kidney-293-fast growing variant) cells were stably transfected with each construct expressed from a tetracycline-regulated CMV promoter. Reverse transcriptase-PCR analysis showed that construct expression responded to doxycycline. Immunofluorescence staining of cells, using antibodies specific for DMT1 isoforms, confirmed an increase in expression in the plasma membrane and cytosolic vesicles after doxycycline treatment, but with isoform specific distributions. Immunoblotting also revealed stimulation of expression. Nevertheless, both DMT1 isoforms performed similarly in assays for functional properties based on 54Mn2+ and 59Fe2+ uptake. Mn incorporation after doxycycline treatment was approximately 10-fold greater than that of untreated cells, while expression in the untreated cells was approximately 5-fold greater than in the untransfected cells. Uptake of Mn depended on addition of doxycycline, with half maximal response at approximately 1 nM doxycycline. Doxycycline-stimulated Mn and Fe uptake was linear with time for 10 min but not over longer periods. Transport exhibited a pH optimum at approximately 5.5 and dependence on incubation temperature and Mn or Fe concentration. The new cell lines should prove useful for research on metal homoeostasis, toxicological studies and efforts to identify distinctive properties of the isoforms.

Lung injury after ozone exposure is iron dependent

We tested the hypothesis that oxidative stress and biological effect after ozone (O3) exposure are dependent on changes in iron homeostasis. After O3 exposure, healthy volunteers demonstrated increased lavage concentrations of iron, transferrin, lactoferrin, and ferritin. In normal rats, alterations of iron metabolism after O3 exposure were immediate and preceded the inflammatory influx. To test for participation of this disruption in iron homeostasis in lung injury following O3 inhalation, we exposed Belgrade rats, which are functionally deficient in divalent metal transporter 1 (DMT1) as a means of iron uptake, and controls to O3. Iron homeostasis was disrupted to a greater extent and the extent of injury was greater in Belgrade rats than in control rats. Nonheme iron and ferritin concentrations were higher in human bronchial epithelial (HBE) cells exposed to O3 than in HBE cells exposed to filtered air. Aldehyde generation and IL-8 release by the HBE cells was also elevated following O3 exposure. Human embryonic kidney (HEK 293) cells with elevated expression of a DMT1 construct were exposed to filtered air and O3. With exposure to O3, elevated DMT1 expression diminished oxidative stress (i.e., aldehyde generation) and IL-8 release. We conclude that iron participates critically in the oxidative stress and biological effects after O3 exposure.

Brain capillary endothelial cells mediate iron transport into the brain by segregating iron from transferrin without the involvement of divalent metal transporter 1

Rats were studied for [(59)Fe-(125)I]transferrin uptake in total brain, and fractions containing brain capillary endothelial cells (BCECs) or neurons and glia. (59)Fe was transported through BCECs, whereas evidence of similar transport of transferrin was questionable. Intravenously injected transferrin localized to BCECs and failed to accumulate within neurons, except near the ventricles. No significant difference in [(125)I]transferrin distribution was observed between Belgrade b/b rats with a mutation in divalent metal transporter I (DMT1), and Belgrade +/b rats with regard to accumulation in vascular and postvascular compartments. (59)Fe occurred in significantly lower amounts in the postvascular compartment in Belgrade b/b rats, indicating impaired iron uptake by transferrin receptor and DMT1-expressing neurons. Immunoprecipitation with transferrin antibodies on brains from Belgrade rats revealed lower uptake of transferrin-bound (59)Fe. In postnatal (P)0 rats, less (59)Fe was transported into the postvascular compartment than at later ages, suggesting that BCECs accumulate iron at P0. Supporting this notion, an in situ perfusion technique revealed that BCECs accumulated ferrous and ferric iron only at P0. However, BCECs at P0 together with those of older age lacked DMT1. In conclusion, BCECs probably mediate iron transport into the brain by segregating iron from transferrin without involvement of DMT1.

Divalent metal transporter 1 in the kidney proximal tubule is expressed in late endosomes/lysosomal membranes: implications for renal handling of protein-metal complexes

The H+-coupled polyligand transport protein divalent metal transporter 1 (DMT1) plays a key role in mammalian iron homeostasis. It has a widespread pattern of expression including tissues associated with iron acquisition and storage. Interestingly, it is also highly expressed in the kidney, yet its function in this tissue is unknown. The aim of this study was to determine the cellular location of DMT1 in proximal tubule cells as a first step to determining the role of this protein in the kidney. To do this we performed RT-PCR and immunostaining experiments using rat kidney and the S1 proximal tubule-derived WKPT-0293 Cl.2 cell line. RT-PCR revealed that mRNAs encoding all four DMT1 splice variants were present in RNA extracted from rat kidney cortex or WKPT-0293 Cl.2 cells. Immunostaining of rat kidney cortex or WKPT-0293 Cl.2 cells showed that DMT1 protein was expressed intracellularly and was not present in the plasma membrane. Expression of DMT1 partially colocalized with the late endosomal/lysosomal proteins LAMP1 and cathepsin-L. Using immunogold labeling, DMT1 was shown to be expressed in the membranes of late endosomes/lysosomes. Uptake of Alexa Fluor 546-transferrin was only observed following application to the apical membrane of WKPT-0293 Cl.2 cells. Within these cells, Alexa Fluor 546-transferrin colocalized with DMT1. In conclusion, renal proximal tubular cells express DMT1 in the membranes of organelles, including late endosomes/lysosomes, associated with processing of apically sequestered transferrin. These findings have implications for renal iron handling and possibly for the handling of nephrotoxic metals that are also DMT1 ligands, including Cd2+.

Post-translational and transcriptional regulation of DMT1 during P19 embryonic carcinoma cell differentiation by retinoic acid

Studies were performed to determine the regulation of DMT1 (divalent metal transporter 1) during RA (retinoic acid)-induced differentiation of P19 embryonic carcinoma cells. Protein and mRNA expression for the +/-IRE (iron response element) forms of DMT1, but not the 1A isoform, were down-regulated within the first few hours upon removal of RA, at which time the cells began to differentiate. The turnover of the +/-IRE isoforms of DMT1 protein during this period was found to be dependent on both the proteasomal and lysosomal pathways. Changes in mRNA levels were shown to be regulated by nitric oxide produced by the induction of neuronal nitric oxide synthase after removal of RA. Nitric oxide functions by inhibiting NF-kappaB (nuclear factor kappaB) nuclear translocation and the subsequent binding to the putative NF-kappaB response element (at -19 to -23) within the 1B promoter. Gel-shift analysis and chromatin immunoprecipitation assay indicated that nuclear NF-kappaB is capable of binding to this response element and that binding decreases during early stages of differentiation. Luciferase reporter gene assay demonstrated that a mutation in this binding domain leads to decreased activity. These results demonstrate that during neuronal differentiation of P19 cells, there is a decrease in specific isoforms of DMT1 via both post-translational and transcriptional mechanisms.

Current knowledge of chromosome 12 susceptibility genes for late-onset Alzheimer's disease

In the last decade, it has become more apparent the important role genes play in the development of late-onset Alzheimer's disease (AD). Great efforts, involving human genome scans and candidate gene studies, have been given towards identifying susceptibility genes for AD. A number of regions on different chromosomes have been reported to demonstrate linkage for AD. Of these, findings on chromosome 12 are some of the most compelling. Worldwide genetic association studies pre-dating and subsequent to recent linkage studies have identified and focused upon a number of genes that map to the areas of reported linkage on chromosome 12, however, analyses of those genes studied to date, on the whole, remain inconclusive and ambiguous. This paper reviews studies that have provided evidence of linkage for AD on chromosome 12 and in turn discusses the work conducted to date on candidate genes that have been identified and map to the chromosome 12 regions of interest.

Hypoxia induces changes in expression of isoforms of the divalent metal transporter (DMT1) in rat pheochromocytoma (PC12) cells

Although hypoxia has been shown to increase the expression of a variety of proteins involved in iron homeostasis, including transferrin and its receptor, little is known about the effect of low oxygen on formation of isoforms of the major iron transport protein, divalent metal transporter 1, DMT1. Accordingly, we examined the effects of hypoxia on expression and subcellular distribution of the different isoforms of DMT1 in rat PC12 cells. Treatment with low oxygen modestly increased expression of protein and mRNA levels for both the +IRE and -IRE species of DMT1. In contrast, expression of the exon 1A containing species of DMT1 was greatly increased by hypoxia as indicated by Western blot and real-time RT-PCR analysis. Message levels for the 1A isoforms increased approximately 60-fold after exposure of PC12 cells to 1% oxygen for 5 h. The subcellular distribution of exon 1A isoforms of DMT1 remained consistently in the cytoplasmic milieu of the cell after hypoxic exposure, as also did the distribution of +IRE species of DMT1. The -IRE species of DMT1, however, responded to hypoxia by becoming increasingly associated with the regions adjoining the outer cellular membranes, while a portion partially colocalized with an early endosomal marker (EEA). Hypoxia also caused a significant increase in the uptake of manganese in PC12 cells. In summary, these results demonstrate that hypoxia selectively increases expression of exon 1A containing species of DMT1 with lesser increases in either the +IRE or -IRE isoforms the transporter.

Candidate gene association study of solute carrier family 11a members 1 (SLC11A1) and 2 (SLC11A2) genes in Alzheimer's disease

Divalent cations are strongly implicated in Alzheimer's disease (AD) pathogenesis, and can regulate amyloid beta-peptide aggregation. The proton-divalent cation transporters encoded by SLC11A1 (formerly NRAMP1) on chromosome 2q35, and SLC11A2 (also known as DCT1 and DMT1) on chromosome 12q13, are expressed in the brain and regulate ion homeostasis from endosomal compartments. SLC11A1 also has pleiotropic effects on pro-inflammatory responses that may be important in AD. We analyzed seven informative polymorphisms in the SLC11A1 and SLC11A2 genes encoding these divalent cation transporters in a sample of 216 late-onset AD cases and 323 age-matched controls. We found only borderline evidence (p=0.08) for an allelic association between SNP rs407135 at SLC11A2 and AD, in which the variant allele was protective (odd ratio (OR) 0.77; 95% CI 0.56-1.04) relative to the more common allele. There was no interaction with apolipoprotein E (APOE) varepsilon4, but stratification by gender showed that all of the effect of SLC11A2 was in the male patient group. No other associations with AD were observed at SLC11A1 or SLC11A2, indicating no major effect of either gene for the occurrence of AD.