Interactions between tissue uptake of lead and iron in normal and iron-deficient rats during development

Environmental lead intoxication, which frequently causes neurological disturbances, and iron deficiency are clinical problems commonly found in children. Also, iron deficiency has been shown to augment lead absorption from the intestine. Hence, there is evidence for an interaction between lead and iron metabolism which could produce changes in lead and iron uptake by the brain and other tissues. These possibilities were investigated using 15-, 21-, and 63-old rats with varying nutritional iron and lead status. Dams were fed diets containing 0 or 3% lead-acetate and 0.2% lead-acetate in the drinking water. After weaning, 0.2% lead-acetate in the drinking water became the sole source of dietary lead. Measurements were made of tissue lead and nonheme iron levels and the uptake of⁵⁹Fe after intravenous injection of transferrin-bound⁵⁹Fe. Iron deficiency was associated with increased intestinal absorption of lead as indicated by blood and kidney lead levels in rats exposed to dietary lead. However, iron deficiency did not increase lead deposition in the brain, and in all rats brain lead levels were relatively low (<0.1 μg/g). Lead concentrations in the liver were below 2 μg/g, whereas kidneys had almost 20 times this concentration. Animals with iron deficiency had lower liver iron levels and had increased brain⁵⁹Fe uptake in comparison to control rats. However, iron levels in brain and kidneys were unaffected by lead intoxication regardless of the animal's iron status.⁵⁹Fe uptake rates were also unaffected by lead, but increased rates of uptake were apparent in iron-deficient rats. Lead did increase liver iron levels in all iron-adequate rats, but iron deficiency had little effect. It is concluded that, compared with other tissues, the blood-brain barrier largely restricts lead uptake by the brain and that the uptake that does occur is unrelated to the iron status of the animal. Also, the level of lead intoxication produced in this investigation did not influence iron uptake by the brain and kidneys, but liver iron stores could be incresed if iron levels were already adequate.     

Brain iron homeostasis.

The anatomical and cellular distribution of non-haem iron, ferritin, transferrin, and the transferrin receptor have been studied in postmortem human brain and these studies, together with data on the uptake and transport of labeled iron, by the rat brain, have been used to elucidate the role of iron and other metal ions in certain neurological disorders. High levels of non-haem iron, mainly in the form of ferritin, are found in the extrapyramidal system, associated predominantly with glial cells. In contrast to non-haem iron, the density of transferrin receptors is highest in cortical and brainstem structures and appears to relate to the iron requirement of neurones for mitochondrial respiratory activity. Transferrin is synthesized within the brain by oligodendrocytes and the choroid plexus, and is present in neurones, consistent with receptor mediated uptake. The uptake of iron into the brain appears to be by a two-stage process involving initial deposition of iron in the brain capillary endothelium by serum transferrin, and subsequent transfer of iron to brain-derived transferrin and transport within the brain to sites with a high transferrin receptor density. A second, as yet unidentified mechanism, may be involved in the transfer of iron from neurones possessing transferrin receptors to sites of storage in glial cells in the extrapyramidal system. The distribution of iron and the transferrin receptor may be of relevance to iron-induced free radical formation and selective neuronal vulnerability in neurodegenerative disorders.     

Effects of Iron Deficiency on Serotonin Uptake In Vitro by Rat Brain Synaptic Vesicles

Abstract: The effects of moderate and severe degrees of iron deficiency on brain and liver nonhaem iron levels and 5-hydroxytryptamine (serotonin; 5-HT) uptake by synaptic vesicles in vitro were investigated in experimental rats. Data obtained suggested that in both moderate and severe forms of iron deficiency, 5-HT uptake by brain synaptic vesicles is decreased and is accompanied by a reduction in brain and liver nonhaem iron levels. On repletion with iron for 4 weeks, the deficient group of rats showed a normalisation of 5-HT uptake by synaptic vesicles and liver nonhaem iron content, whereas the brain nonhaem iron concentration still showed a significant deficit. The data thus suggest that changes in the uptake of 5-HT by brain synaptic vesicles that accompany iron depletion and repletion are more rapid than changes in the total nonhaem iron concentration in the brain. The observation that 5-HT uptake by brain synaptic vesicles is decreased in iron deficiency suggests a probable role for iron in 5-HT storage in rat brain.     

Iron, Transferrin, and Ferritin in the Rat Brain During Development and Aging

Abstract: Iron is a universal cofactor for mitochondrial energy generation and supports the growth and differentiation of all cell types. In the CNS, iron is a key component of systems responsible for myelination and the synthesis of several neurotransmitters. In this study the spatial and temporal pattern of iron and its regulatory proteins transferrin and ferritin are quantitatively examined in the rat CNS during the first 3 weeks of postnatal life and in adults and aged animals. The midbrain, the cerebral cortex, and the cerebellum-pons are examined independently. Iron, transferrin, and ferritin concentrations are highest in all three brain regions at birth and decrease in each region to minimum levels during the third postnatal week. The decrease in levels of iron, transferrin, and ferritin is most pronounced in the cerebellum-pons and cortex and least in the midbrain. From postnatal day 17, iron (total iron content) and ferritin levels increase throughout the lifetime of the rat. In contrast, transferrin levels remain fairly constant in each brain region after postnatal day 24. The midbrain region, which includes the iron-rich regions such as the globus pallidus, substantia nigra, and red nucleus, has the least change in iron with development, has the highest level of ferritin during development, and consistently has the highest level of transferrin at all ages. These observations are consistent with reports that iron is important for normal motor function. Transferrin did not increase after postnatal day 24 in the three brain regions examined despite increasing amounts of iron, which implies a decrease in iron mobility in the aged rats, a finding that is consistent with observations of human brain tissue. The data reported in this study demonstrate that iron acquisition and mobilization systems in the CNS are established early in development and that the overall pattern of acquisition among brain regions is similar. These data offer support and insight into established concepts that a sufficient iron supply is critical for normal neurological development.     

Dual regulation of the Arabidopsis high-affinity root iron uptake system by local and long-distance signals

Regulation of the root high-affinity iron uptake system by whole-plant signals was investigated at the molecular level in Arabidopsis, through monitoring FRO2 and IRT1 gene expression. These two genes encode the root ferric-chelate reductase and the high-affinity iron transporter, respectively, involved in the iron deficiency-induced uptake system. Recovery from iron-deficient conditions and modulation of apoplastic iron pools indicate that iron itself plays a major role in the regulation of root iron deficiency responses at the mRNA and protein levels. Split-root experiments show that the expression of IRT1 and FRO2 is controlled both by a local induction from the root iron pool and through a systemic pathway involving a shoot-borne signal, both signals being integrated to tightly control production of the root iron uptake proteins. We also show that IRT1 and FRO2 are expressed during the day and down-regulated at night and that this additional control is overruled by iron starvation, indicating that the nutritional status prevails on the diurnal regulation. Our work suggests, for the first time to our knowledge, that like in grasses, the root iron acquisition in strategy I plants may also be under diurnal regulation. On the basis of the new molecular insights provided in this study and given the strict coregulation of IRT1 and FRO2 observed, we present a model of local and long-distance regulation of the root iron uptake system in Arabidopsis.     

Iron uptake by the ichthyotoxic Chattonella marina (Raphidophyceae): impact of superoxide generation1

Significant production of superoxide, a known reductant of both inorganic and organically complexed iron(III), occurs in natural systems by both biotic and abiotic pathways. We have investigated the generation of superoxide by Chattonella marina (Subrahman.) Y. Hara et Chihara, a phytoplankton taxon known to produce high levels of this reactive oxygen species, and examined the role of superoxide in the acquisition of iron by this organism. Additionally, a generalized model for iron acquisition by C. marina has been developed, which includes three pathways of iron acquisition from organically complexed iron(III): nondissociative reductive uptake, dissociative reductive uptake, and nonreductive dissociative uptake. The model is shown to be particularly useful in ascertaining the relative importance of these various iron‐uptake pathways as a function of solution parameters including concentration and iron‐binding strength of the organic ligand and superoxide concentration. Our results suggest that superoxide can participate in the C. marina iron‐uptake process when iron is complexed to weak ligands, such as citrate, but plays only a minor role when iron is bound to a strong ligand. It thus appears that facilitation of iron acquisition is not the sole purpose of superoxide production by these organisms.     

Paralogous outer membrane proteins mediate uptake of different forms of iron and synergistically govern virulence in Francisella tularensis tularensis

Francisella tularensis subsp. tularensis is a highly infectious bacterium causing acute disease in mammalian hosts. Mechanisms for the acquisition of iron within the iron-limiting host environment are likely to be critical for survival of this intracellular pathogen. FslE (FTT0025) and FupA (FTT0918) are paralogous proteins that are predicted to form β-barrels in the outer membrane of virulent strain Schu S4 and are unique to Francisella species. Previous studies have implicated both FupA, initially identified as a virulence factor and FslE, encoded by the siderophore biosynthetic operon, in iron acquisition. Using single and double mutants, we demonstrated that these paralogs function in concert to promote growth under iron limitation. We used a (55)Fe transport assay to demonstrate that FslE is involved in siderophore-mediated ferric iron uptake, whereas FupA facilitates high affinity ferrous iron uptake. Optimal replication within J774A.1 macrophage-like cells required at least one of these uptake systems to be functional. In a mouse model of tularemia, the ΔfupA mutant was attenuated, but the ΔfslE ΔfupA mutant was significantly more attenuated, implying that the two systems of iron acquisition function synergistically to promote virulence. These studies highlight the importance of specific iron acquisition functions, particularly that of ferrous iron, for virulence of F. tularensis in the mammalian host.     

Iron uptake by glial cells

Dynamic studies of iron metabolism in brain are generally unavailable despite the fact that a number of neurologic conditions are associated with excessive accumulation of iron in central nervous tissue. Cortical non-neuronal (glial) cultures were prepared from fetal mouse brain. After 13 days the cultures were exposed to radiolabeled iron. Brisk and linear total iron uptake and ferritin iron uptake occurred over 4 hours. When methylamine or ammonium chloride was added, (both known inhibitors of transferrin iron release because of their lysosomotropic properties), total iron uptake was diminished. Further studies indicated that meth-ylamine inhibits glial cell ferritin iron incorporation. Glial cell iron transport is similar to previously reported neuronal cell iron transport (1) but glial cell iron uptake proceeds at a faster rate and is more susceptible to the inhibition of certain lysosomotropic agents. The data reinforces the likelihood that iron uptake by nervous tissues is transferrin-mediated.     

Role of porins in iron uptake by Mycobacterium smegmatis

Many bacteria rely on siderophores to extract iron from the environment. However, acquisition of iron-loaded siderophores is dependent on high-affinity uptake systems that are not produced under high-iron conditions. The fact that bacteria are able to maintain iron homeostasis in the absence of siderophores indicates that alternative iron acquisition systems exist. It has been speculated that such low-affinity uptake of iron in Gram-negative bacteria includes diffusion of iron ions or chelates across the outer membrane through porins. The outer membrane of the saprophytic Mycobacterium smegmatis contains the Msp family of porins, which enable the diffusion of small and hydrophilic solutes, such as monosaccharides, amino acids, and phosphate. However, it is unknown how cations cross the outer membrane of mycobacteria. Here, we show that the Msp porins of M. smegmatis are involved in the acquisition of soluble iron under high-iron conditions. Uptake of ferric ions by a triple porin mutant was reduced compared to wild-type (wt) M. smegmatis. An intracellular iron reporter indicated that derepression of iron-responsive genes occurs at higher iron concentrations in the porin mutant. This was consistent with the finding that the porin mutant produced more siderophores under low-iron conditions than wt M. smegmatis. In contrast, uptake of the exochelin MS, the main siderophore of M. smegmatis, was not affected by the lack of porins, indicating that a specific outer membrane siderophore receptor exists. These results provide, to our knowledge, the first experimental evidence that general porins are indeed the outer membrane conduit of low-affinity iron acquisition systems in bacteria.     

New insights into iron acquisition by cyanobacteria: an essential role for ExbB-ExbD complex in inorganic iron uptake

Cyanobacteria are globally important primary producers that have an exceptionally large iron requirement for photosynthesis. In many aquatic ecosystems, the levels of dissolved iron are so low and some of the chemical species so unreactive that growth of cyanobacteria is impaired. Pathways of iron uptake through cyanobacterial membranes are now being elucidated, but the molecular details are still largely unknown. Here we report that the non-siderophore-producing cyanobacterium Synechocystis sp. PCC 6803 contains three exbB-exbD gene clusters that are obligatorily required for growth and are involved in iron acquisition. The three exbB-exbDs are redundant, but single and double mutants have reduced rates of iron uptake compared with wild-type cells, and the triple mutant appeared to be lethal. Short-term measurements in chemically well-defined medium show that iron uptake by Synechocystis depends on inorganic iron (Fe′) concentration and ExbB-ExbD complexes are essentially required for the Fe′ transport process. Although transport of iron bound to a model siderophore, ferrioxamine B, is also reduced in the exbB-exbD mutants, the rate of uptake at similar total [Fe] is about 800-fold slower than Fe′, suggesting that hydroxamate siderophore iron uptake may be less ecologically relevant than free iron. These results provide the first evidence that ExbB-ExbD is involved in inorganic iron uptake and is an essential part of the iron acquisition pathway in cyanobacteria. The involvement of an ExbB-ExbD system for inorganic iron uptake may allow cyanobacteria to more tightly maintain iron homeostasis, particularly in variable environments where iron concentrations range from limiting to sufficient.     

Olfactory ferric and ferrous iron absorption in iron-deficient rats

The absorption of metals from the nasal cavity to the blood and the brain initiates an important route of occupational exposures leading to health risks. Divalent metal transporter-1 (DMT1) plays a significant role in the absorption of intranasally instilled manganese, but whether iron uptake would be mediated by the same pathway is unknown. In iron-deficient rats, blood (59)Fe levels after intranasal administration of the radioisotope in the ferrous form were significantly higher than those observed for iron-sufficient control rats. Similar results were obtained when ferric iron was instilled intranasally, and blood levels of (59)Fe were even greater in the iron-deficient rats compared with the amount of ferrous iron absorbed. Experiments with Belgrade (b/b) rats showed that DMT1 deficiency limited ferric iron uptake from the nasal cavity to the blood compared with +/b controls matched for iron deficiency. These results indicate that olfactory uptake of ferric iron by iron-deficient rats involves DMT1. Western blot experiments confirmed that DMT1 levels are significantly higher in iron-deficient rats compared with iron-sufficient controls in olfactory tissue. Thus the molecular mechanism of olfactory iron absorption is regulated by body iron status and involves DMT1.     

Effective regulation of iron acquisition in graminaceous plants. The role of mugineic acids as phytosiderophores

Discovery of mugineic acids as phytosiderophores has shown that some graminaceous monocotyledonous plants have a different iron acquisition strategy (strategy II) from dicotyledonous and nongraminaceous monocotyledonous plants (strategy I). The process of iron acquisition by strategy II plants can be divided into four main steps: biosynthesis, secretion, solubilization, and uptake, all of which are effectively regulated by different systems. The biosynthesis of mugineic acids is controlled by an on-off system which is operated under the control of iron demand in the plant. All mugineic acids share the same biosynthetic pathway from L-methionine to 2'-deoxymugineic acid, but the subsequent steps differ among plant species and even cultivars. The biosynthesis of mugineic acids is associated with the methionine recycling pathway. The secretion of mugineic acids shows a distinct diumal rhythm. Mugineic acids solubilize sparingly soluble inorganic iron by chelation and possess a high chelation affinity for iron, but not for other polyvalent ions such as Ca²⁺, Mg²⁺ and Al³⁺. The iron uptake process is regulated by a specific uptake system that transports the mugineic acid-Fe(III) complex as an intact molecule. This system specifically recognizes the mugineic acid-Fe(III) complexes, but not other mugineic acid-metal or synthetic chelator-Fe(III) complexes, suggesting that binding sites with strict recognition for stereostructure of the complex are located on the plasma membrane. All these regulatory systems are considered to represent an efficient strategy to acquire adequate amounts of iron and to avoid factors unfavorable for iron acquisition such as high pH, high concentrations of bicarbonate, Ca^2- and Mg²⁺, microbial degradation, and uptake of other metals that are common in calcareous soils.     

植物铁吸收、转运和调控的分子机制研究进展

铁是植物正常生命活动所必需的微量矿质元素, 铁离子的吸收、转运和利用是一个复杂的过程, 很多基因参与了这一过程。本文对近10年来发现和分离的参与植物铁吸收、转运及调控的基因研究进展进行了综述。根据最近的研究结果, 提出了植物控制铁吸收的分子调控模式(机理I)。     

Effects of chelators on iron uptake and release by the brain in the rat

The iron chelators desferrioxamine (DFO), pyridoxal isonicotinoyl hydrazone (PIH), 2,2-bipyridine, diethylenetriamine penta-acetic acid (DTPA) and 1,2 dimethyl-3-hydroxy pyrid-4-one (CP20) were analysed for their ability to change⁵⁹Fe uptake and release from the brain of 15- and 63-day rats either during or after intravenous injection of⁵⁹Fe-¹²⁵I-transferrin. DTPA was the only chelator unable to significantly reduce iron uptake into the brain of 15-day rats. This indicates that iron is not released from transferrin at the luminal surface of brain capillary endothelial cells. CP20 was able to reduce iron uptake in the brain by 85% compared to 28% with DFO. Only CP20 was able to significantly reduce brain iron uptake in 63 day rats. Once⁵⁹Fe had entered the brain no chelator used was able to mediate its release. All of the chelators except CP20 had similar effects on femur iron uptake as they did on brain uptake, suggesting similar iron uptake mechanisms. It is concluded that during the passage of transferrin-bound iron into the brain the iron is released from transferrin within endothelial cells after endocytosis of transferrin.     

植物铁吸收、转运和调控的分子机制研究进展

铁是植物正常生命活动所必需的微量矿质元素,铁离子的吸收、转运和利用是一个复杂的过程,很多基因参与了这一过程。本文对近10年来发现和分离的参与植物铁吸收、转运及调控的基因研究进展进行了综述。根据最近的研究结果,提出了植物控制铁吸收的分子调控模式(机理I)。     

Transferrin and Iron Uptake by the Brain: Effects of Altered Iron Status

Transferrin (Tf) and iron uptake by the brain were measured in rats using 59Fe-125I-Tf and 131I-albumin (to correct for the plasma content of 59Fe and 125I-Tf in the organs). The rats were aged from 15 to 63 days and were fed (a) a low-iron diet (iron-deficient) or, as control, the same diet supplemented with iron, or (b) a chow diet with added carbonyl iron (iron overload), the chow diet alone acting as its control. Iron deficiency was associated with a significant decrease and iron overload with a significant increase in brain nonheme iron concentration relative to the controls. In each dietary treatment group, the uptake of Tf and iron by the brain decreased as the rats aged from 15 to 63 days. Both Tf and iron uptake were significantly greater in the iron-deficient rats than in their controls and lower in the iron-loaded rats than in the corresponding controls. Overall, iron deficiency produced about a doubling and iron overload a halving of the uptake values compared with the controls. In contrast to that in the brain, iron uptake by the femurs did not decrease with age and there was relatively little difference between the different dietary groups. 125I-Tf uptake by the brains of the iron-deficient rats increased very rapidly after injection of the labelled proteins, within 15 min reaching a plateau level which was maintained for at least 6 h. The uptake of 59Fe, however, increased rapidly for 1 h and then more slowly, and in terms of percentage of injected dose reached much higher values than did 125I-Tf uptake.(ABSTRACT TRUNCATED AT 250 WORDS)     

Iron-responsive olfactory uptake of manganese improves motor function deficits associated with iron deficiency

Iron-responsive manganese uptake is increased in iron-deficient rats, suggesting that toxicity related to manganese exposure could be modified by iron status. To explore possible interactions, the distribution of intranasally-instilled manganese in control and iron-deficient rat brain was characterized by quantitative image analysis using T1-weighted magnetic resonance imaging (MRI). Manganese accumulation in the brain of iron-deficient rats was doubled after intranasal administration of MnCl(2) for 1- or 3-week. Enhanced manganese level was observed in specific brain regions of iron-deficient rats, including the striatum, hippocampus, and prefrontal cortex. Iron-deficient rats spent reduced time on a standard accelerating rotarod bar before falling and with lower peak speed compared to controls; unexpectedly, these measures of motor function significantly improved in iron-deficient rats intranasally-instilled with MnCl(2). Although tissue dopamine concentrations were similar in the striatum, dopamine transporter (DAT) and dopamine receptor D(1) (D1R) levels were reduced and dopamine receptor D(2) (D2R) levels were increased in manganese-instilled rats, suggesting that manganese-induced changes in post-synaptic dopaminergic signaling contribute to the compensatory effect. Enhanced olfactory manganese uptake during iron deficiency appears to be a programmed "rescue response" with beneficial influence on motor impairment due to low iron status.     

Effect of nicotine on transferrin binding and iron uptake by cultured rat placenta

The effect of nicotine on transferrin and iron transport in placental cells has been studied. Nicotine inhibits iron uptake but has little effect on the steady-state levels of transferrin. The effect is temperature and concentration dependent and is not reversible. At a concentration of 15 mM nicotine inhibited transferrin endocytosis by 40%, while iron uptake was decreased by nearly 60%. Nicotine exerted a similar effect on reticulocytes, but other amines, either tertiary or quaternary, had little or no effect on either iron uptake or steady-state intracellular transferrin levels. The results suggest that nicotine acts by blocking uptake, probably by acting as a weak base inhibiting iron release from transferrin, and inhibiting exocytosis with a resultant block of endocytosis. The concentrations required to exert an effect are too high to implicate inhibition of iron transport in the effects of smoking on pregnancy.