Mechanisms of manganese transport in rabbit erythroid cells

Validation of blood arsenic and manganese assessment from archived clotted erythrocyte fraction in an urban cohort of mother-child dyads

Exposure to arsenic (As) and manganese (Mn) from contaminated food, drinking water and dust are linked to a host of adverse health effects. The recent discovery of unmonitored community exposures to hazardous levels of metals, as seen in the Flint Water Crisis and East Chicago, have demonstrated a need for novel biomonitoring methods utilizing samples other than whole blood. Here, we present a method utilizing clotted erythrocyte fraction samples, a blood component commonly archived in biorepositories, to predict whole blood levels of As and Mn. This method would allow for innovative retrospective assessments of environmental exposures in previously unused samples. Whole blood and clotted erythrocyte fraction samples were simultaneously collected from 84 participants in the Airborne Exposure to Semivolatile Organic Pollutants (AESOP) cohort study of mother-child dyads in East Chicago. Clotted erythrocyte fraction samples were prepared by alkaline dilution and subsequently analyzed using inductively coupled plasma-mass spectrometry. A strong linear relationship was observed between whole blood and clotted erythrocyte fraction with Pearson correlation coefficients (r, p < 0.001) of 0.74, and 0.82 for As and Mn, respectively. Modeled whole blood Mn levels predicted from clotted erythrocyte fractions evaluated at a test threshold representing the NHANES median of 9.7 μg/L, were found to have diagnostic sensitivity of 88% and specificity of 71%. Clotted erythrocyte partitioning of As was tested on a wide range of oral gavage doses using a rat model. Results from this investigation demonstrate clotted erythrocyte fraction samples are a viable alternative biological sample for retrospective public health surveillance of environmental exposure to As and Mn.

Pathophysiology of the Belgrade rat

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

Manganese in human parenteral nutrition: considerations for toxicity and biomonitoring

The iatrogenic risks associated with excessive Mn administration in parenteral nutrition (PN) patients are well documented. Hypermanganesemia and neurotoxicity are associated with the duration of Mn supplementation, Mn dosage, as well as pathological conditions, such as anemia or cholestasis. Recent PN guidelines recommend the biomonitoring of patients if they receive Mn in their PN longer than 30 days. The data in the literature are conflicting about the method for assessing Mn stores in humans as a definitive biomarker of Mn exposure or induced-neurotoxicity has yet to be identified. The biomonitoring of Mn relies on the analysis of whole blood Mn (WB Mn) levels, which are highly variable among human population and are not strictly correlated with Mn-induced neurotoxicity. Alterations in dopaminergic (DAergic) and catecholaminergic metabolism have been studied as predictive biomarkers of Mn-induced neurotoxicity. Given these limitations, this review addresses various approaches for biomonitoring Mn exposure and neurotoxic risk.

Quantitative measurement of changes in calcium channel activity in vivo utilizing dynamic manganese-enhanced MRI (dMEMRI)

The ability of manganese ions (Mn(2+)) to enter cells through calcium ion (Ca(2+)) channels has been used for depolarization dependent brain functional imaging with manganese-enhanced MRI (MEMRI). The purpose of this study was to quantify changes to Mn(2+) uptake in rat brain using a dynamic manganese-enhanced MRI (dMEMRI) scanning protocol with the Patlak and Logan graphical analysis methods. The graphical analysis was based on a three-compartment model describing the tissue and plasma concentration of Mn. Mn(2+) uptake was characterized by the total distribution volume of manganese (Mn) inside tissue (V(T)) and the unidirectional influx constant of Mn(2+) from plasma to tissue (K(i)). The measurements were performed on the anterior (APit) and posterior (PPit) parts of the pituitary gland, a region with an incomplete blood brain barrier. Modulation of Ca(2+) channel activity was performed by administration of the stimulant glutamate and the inhibitor verapamil. It was found that the APit and PPit showed different Mn(2+) uptake characteristics. While the influx of Mn(2+) into the PPit was reversible, Mn(2+) was found to be irreversibly trapped in the APit during the course of the experiment. In the PPit, an increase of Mn(2+) uptake led to an increase in V(T) (from 2.8±0.3 ml/cm(3) to 4.6±1.2 ml/cm(3)) while a decrease of Mn(2+) uptake corresponded to a decrease in V(T) (from 2.8±0.3 ml/cm(3) to 1.4±0.3 ml/cm(3)). In the APit, an increase of Mn(2+) uptake led to an increase in K(i) (from 0.034±0.009 min(-1) to 0.049±0.012 min(-1)) while a decrease of Mn(2+) uptake corresponded to a decrease in K(i) (from 0.034±0.009 min(-1) to 0.019±0.003 min(-1)). This work demonstrates that graphical analysis applied to dMEMRI data can quantitatively measure changes to Mn(2+) uptake following modulation of neural activity.

Passive transport pathways for Ca(2+) and Co(2+) in human red blood cells. (57)Co(2+) as a tracer for Ca(2+) influx

The passive transport of calcium and cobalt and their interference were studied in human red cells using (45)Ca and (57)Co as tracers. In ATP-depleted cells, with the ATP concentration reduced to about 1μM, the progress curve for (45)Ca uptake at 1mM rapidly levels off with time, consistent with a residual Ca-pump activity building up at increasing [Ca(T)](c) to reach at [Ca(T)](c) about 5μmol(lcells)(-1) a maximal pump rate that nearly countermands the passive Ca influx, resulting in a linear net uptake at a low level. In ATP-depleted cells treated with vanadate, supposed to cause Ca-pump arrest, a residual pump activity is still present at high [Ca(T)](c). Moreover, vanadate markedly increases the passive Ca(2+) influx. The residual Ca-pump activity in ATP-depleted cells is fuelled by breakdown of the large 2,3-DPG pool, rate-limited by the sustainable ATP-turnover at about 40-50μmol(lcells)(-1)h(-1). The apparent Ca(2+) affinity of the Ca-pump appears to be markedly reduced compared to fed cells. The 2,3-DPG breakdown can be prevented by inhibition of the 2,3-DPG phosphatase by tetrathionate, and under these conditions the (45)Ca uptake is markedly increased and linear with time, with the unidirectional Ca influx at 1mM Ca(2+) estimated at 50-60μmol(lcells)(-1)h(-1). The Ca influx increases with the extracellular Ca(2+) concentration with a saturating component, with K(½(Ca)) about 0.3mM, plus a non-saturating component. From (45)Ca-loaded, ATP-depleted cells the residual Ca-pump can also be detected as a vanadate- and tetrathionate-sensitive efflux. The (45)Ca efflux is markedly accelerated by external Ca(2+), both in control cells and in the presence of vanadate or tetrathionate, suggesting efflux by carrier-mediated Ca/Ca exchange. The (57)Co uptake is similar in fed cells and in ATP-depleted cells (exposed to iodoacetamide), consistent with the notion that Co(2+) is not transported by the Ca-pump. The transporter is thus neither SH-group nor ATP or phosphorylation dependent. The (57)Co uptake shows several similarities with the (45)Ca uptake in ATP-depleted cells supplemented with tetrathionate. The uptake is linear with time, and increases with the cobalt concentration with a saturating component, with J(max) about 16μmol(lcells)(-1)h(-1) and K(½(Co)) about 0.1mM, plus a non-saturating component. The (57)Co and (45)Ca uptake shows mutual inhibition, and at least the stochastic Ca(2+) influx is inhibited by Co(2+). The (57)Co and (45)Ca uptake are both insensitive to the 1,4-dihydropyridine Ca-channel blocker nifedipine, even at 100μM. The (57)Co uptake is increased at high negative membrane potentials, indicating that the uptake is at least partially electrogenic. The (57)Co influx amounts to about half the (45)Ca influx in ATP-depleted cells. It is speculated that the basal Ca(2+) and Co(2+) uptake could be mediated by a common transporter, probably with a channel-like and a carrier-mediated component, and that (57)Co could be useful as a tracer for at least the channel-like Ca(2+) entry pathway in red cells, since it is not itself transported by the Ca-pump and, moreover, is effectively buffered in the cytosol by binding to hemoglobin, without interfering with Ca(2+) buffering. The molecular identity of the putative common transporter(s) remains to be defined.

The role of transferrin receptor 1 and 2 in transferrin-bound iron uptake in human hepatoma cells

Transferrin receptor (TFR) 1 and 2 are expressed in the liver; TFR1 levels are regulated by cellular iron levels while TFR2 levels are regulated by transferrin saturation. The aims of this study were to 1) determine the relative importance of TFR1 and TFR2 in transferrin-bound iron (TBI) uptake by HuH7 human hepatoma cells and 2) characterize the role of metal-transferrin complexes in the regulation of these receptors. TFR expression was altered by 1) incubation with metal-transferrin (Tf) complexes, 2) TFR1 and TFR2 small interfering RNA knockdown, and 3) transfection with a human TFR2 plasmid. TBI uptake was measured using (59)Fe-(125)I-labeled Tf and mRNA and protein expression by real-time PCR and Western blot analysis, respectively. Fe(2)Tf, Co(2)Tf, and Mn(2)Tf increased TFR2 protein expression, indicating that the upregulation was not specifically regulated by iron-transferrin but also other metal-transferrins. In addition, Co(2)Tf and Mn(2)Tf upregulated TFR1, reduced ferritin, and increased hypoxia-inducible factor-1alpha protein expression, suggesting that TFR1 upregulation was due to a combination of iron deficiency and chemical hypoxia. TBI uptake correlated with changes in TFR1 but not TFR2 expression. TFR1 knockdown reduced iron uptake by 80% while TFR2 knockdown did not affect uptake. At 5 microM transferrin, iron uptake was not affected by combined TFR1 and TFR2 knockdown. Transfection with a hTFR2 plasmid increased TFR2 protein expression, causing a 15-20% increase in iron uptake and ferritin levels. This shows for the first time that TFR-mediated TBI uptake is mediated primarily via TFR1 but not TFR2 and that a high-capacity TFR-independent pathway exists in hepatoma cells.

Pharmacokinetics of pulmonary manganese absorption: evidence for increased susceptibility to manganese loading in iron-deficient rats

High levels of airborne manganese can be neurotoxic, yet little is known about absorption of this metal via the lungs. Intestinal manganese uptake is upregulated by iron deficiency and is thought to be mediated by divalent metal transporter 1 (DMT1), an iron-regulated factor known to play a role in dietary iron absorption. To better characterize metal absorption from the lungs to the blood and test whether iron deficiency may modify this process, the pharmacokinetics of pulmonary manganese and iron absorption by control and iron-deficient rats were compared. Levels of DMT1 expression in the lungs were determined to explore potential changes induced by iron deficiency that might alter metal absorption. The pharmacokinetic curves for intratracheally instilled (54)Mn and (59)Fe were significantly different, suggesting that pulmonary uptake of the two metals involves different mechanisms. Intratracheally instilled iron-deficient rats had significantly higher blood (54)Mn levels, whereas blood (59)Fe levels were significantly reduced compared with controls. The same trend was observed when radioisotopes were delivered by intravenous injection, indicating that iron-deficient rats have altered blood clearance of manganese. In situ analysis revealed the presence of DMT1 transcripts in airway epithelium; however, mRNA levels did not change in iron deficiency. Although lung DMT1 levels and metal absorption did not appear to be influenced by iron deficiency, the differences in blood clearance of instilled manganese identified by this study support the idea that iron status can influence the potential toxicity of this metal.

Iron and magnesium exchange via the low affinity iron transporter in rabbit erythroid cells—exchange rates and the action of valinomycin, diethylstilbestrol and protein kinase inhibitors

Evidence was presented previously that rabbit erythroid cells possess a low-affinity Fe2+ transport system which operates via the Na+/Mg2+antiport [Biochim. Biophys. Acta 1282 (1996) 163]. This was investigated further by measurements of Mg2+ efflux as well as Fe2+ uptake by the cells and by examining the inhibitory effects of valinomycin, diethylstilbestrol (DES) and protein kinase inhibitors. Mg2+ efflux and Fe2+ uptake were measured using rabbit reticulocytes and mature erythrocytes incubated in isotonic KCl or NaCl solutions. Both processes were slower in mature cells than reticulocytes. Mg2+ efflux into KCl solution was much lower than into NaCl solution but was stimulated by addition of Fe2+ to the solution. The rate of Fe2+-stimulated Mg2+ efflux closely followed that of Fe2+ uptake in a one-to-one molar ratio. Valinomycin, DES and the protein kinase inhibitors all inhibited Fe2+ uptake from KCl solution. Valinomycin also inhibited Fe2+-stimulated Mg2+ efflux into KCl solution but markedly stimulated the efflux into NaCl. Maximal inhibition of Fe2+ uptake from KCl solution required the presence of K+, Rb+ or Cs+ ions with which valinomycin forms strong complexes. The results could not be explained on the basis of changes in cell membrane potential or cell volume. By contrast, the increase in Mg2+ efflux into NaCl solution produced by valinomycin was accompanied by cell shrinkage and production of a more negative membrane potential, either of which may be responsible for the effect. The inhibition produced by the protein kinase inhibitors indicate that phosphorylation of the transporter or an associated protein by protein tyrosine kinase is probably required to activate the transporter.

Application of metallothionein null cells to investigation of cadmium transport

Metallothionein (MT) is an important factor for cadmium resistance in mammalian cells. Most Cd-resistant cell lines thus far established have shown enhanced production of MT protein. However, the presence of high concentrations of MT, which traps cellular cadmium ions efficiently, has hindered the investigation of cadmium transport in Cd-resistant cells. Utilization of MT null mice or cultured cells derived from MT null mice is a useful way to isolate and characterize non-MT factor(s) for Cd resistance. Primary cultured cells derived from embryos of MT-I/II null mouse showed an increased sensitivity to cadmium compared with control cells. Immortalization of these cells by introducing simian virus 40 large T antigen cDNA has enabled the development of Cd-resistant MT null cells. The established Cd-resistant MT null cells exhibited a reduced accumulation of cadmium due to a decreased rate of cellular cadmium uptake. Application of the multitracer technique has demonstrated that the uptake of Mn(II) was also markedly reduced in Cd-resistant MT null cells. Kinetic and competition studies on Cd(II) and Mn(II) uptake in these cells revealed that a high-affinity transport system for Mn(II) is used, at least in part, for cellular Cd(II) uptake. Thus, the utilization of MT null cells has permitted the detection of a novel cadmium transport system.

Identification of the Escherichia coli K-12 Nramp orthologue (MntH) as a selective divalent metal ion transporter

The Escherichia coli mntH (formerly yfeP) gene encodes a putative membrane protein (MntH) highly similar to members of the eukaryotic Nramp family of divalent metal ion transporters. To determine the function of E. coli MntH, a null mutant was created and MntH was overexpressed both in wild-type E. coli and in the metal-dependent mutant hflB1(Ts). At the restrictive temperature 42 degrees C, the mntH null mutation reduces the suppression of hflB1(Ts) thermosensitivity by exogenous divalent metals. Conversely, overexpression of MntH restores growth at 42 degrees C, increases suppression of the ts phenotype by Fe(II) and Ni(II) and renders hflB1(Ts) cells hypersensitive to Mn(II). Transport studies in intact cells show that MntH selectively facilitates uptake of 54Mn(II) and 55Fe(II) in a temperature-, time- and proton-dependent manner. Competition studies in uptake assays and growth inhibition experiments in hflB1(Ts) mutants together indicate that MntH is a divalent metal cation transporter of broad substrate specificity. The functional characteristics of MntH suggest that it corresponds to the previously described manganese transporter of E. coli. This study indicates that proton-dependent divalent metal ion uptake has been preserved in the Nramp family from bacteria to humans.

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.

Transport mechanisms for iron and other transition metals in rat and rabbit erythroid cells

1. Earlier studies have shown that Fe2+ transport into erythroid cells is inhibited by several transition metals (Mn2+, Zn2+, Co2+, Ni2+) and that Fe2+ transport can occur by two saturable mechanisms, one of high affinity and the other of low affinity. Also, the transport of Zn2+ and Cd2+ into erythroid cells is stimulated by NaHCO3 and NaSCN. The aim of the present investigation was to determine whether all of these transition metals can be transported by the processes described for Fe2+, Zn2+ and Cd2+ and to determine the properties of the transport processes. 2. Rabbit reticulocytes and mature erythrocytes and reticulocytes from homozygous and heterozygous Belgrade rats were incubated with radiolabelled samples of the metals under conditions known to be optimal for high- and low-affinity Fe2+ transport and for the processes mediated by NaHCO3 and NaSCN. 3. All of the metals were transported by the high- and low-affinity Fe2+ transport processes and could compete with each other for transport. The Km and Vmax values and the effects of incubation temperature and metabolic inhibitors were similar for all the metals. NaHCO3 and NaSCN increased the uptake of Zn2+ and Cd2+ but not the other metals. 4. The uptake of all of the metals by the high-affinity process was much lower in reticulocytes from homozygous Belgrade rats than in those from heterozygous animals, but there was no difference with respect to low-affinity transport. 5. It is concluded that the high- and low-affinity 'iron' transport mechanisms can also transport several other transition metals and should therefore be considered as general transition metal carriers.