Nutritional aspects of manganese homeostasis

Manganese (Mn) is an essential mineral. It is present in virtually all diets at low concentrations. The principal route of intake for Mn is via food consumption, but in occupational cohorts, inhalation exposure may also occur (this subject will not be dealt with in this review). Humans maintain stable tissue levels of Mn. This is achieved via tight homeostatic control of both absorption and excretion. Nevertheless, it is well established that exposure to high oral, parenteral or ambient air concentrations of Mn can result in elevations in tissue Mn levels. Excessive Mn accumulation in the central nervous system (CNS) is an established clinical entity, referred to as manganism. It resembles idiopathic Parkinson's disease (IPD) in its clinical features, resulting in adverse neurological effects both in laboratory animals and humans. This review focuses on an area that to date has received little consideration, namely the potential exposure of parenterally fed neonates to exceedingly high Mn concentrations in parenteral nutrition solutions, potentially increasing their risk for Mn-induced adverse health sequelae. The review will consider (1) the essentiality of Mn; (2) the concentration ranges, means and variation of Mn in various foods and infant formulas; (3) the absorption, distribution, and elimination of Mn after oral exposure and (4) the factors that raise a theoretical concern that neonates receiving total parenteral nutrition (TPN) are exposed to excessive dietary Mn.     

Ethacrynic acid and furosemide alter Cl,K, and Na distribution between blood,choroid plexus,CSF, and brain

Can loop diuretics like ethacrynic acid and furosemide, when administered intravenously, significantly alter ion transport and fluid dynamics in CNS? To shed light on this unresolved issue, we tested the ability of these agents to effect redistribution of Na, K and Cl in adult rat brain. Cl penetration into various CNS regions was assessed as the volume of distribution, i.e., uptake, of³⁶Cl from blood. Ethacrynic acid and furosemide (50 mg/kg IV) reduced by 20–30% the rate of permeation of³⁶Cl across the blood-CSF barrier, and they elevated [K] and [Cl] in choroid plexus (CP) by 15–25%. The loop diuretic-induced buildup of K and Cl in CP (lateral and 4th ventricle) was likely a reflection of decreased movement of these ions across the apical membrane into CSF.³⁶Cl activity in parietal cortex and pons-medulla decreased in treatment with furosemide and ethacrynic acid, due to slowing of Cl transport across blood-brain and/or blood-CSF barriers. Our inhibitory findings in intact rats are consistent with those from previous in vitro experiments demonstrating diminution by loop diuretics of Na, K and Cl transport across isolated CP membranes.     

Manganese accumulation in the CNS and associated pathologies

Manganese (Mn) is an essential metal for life. It is a key constituent of clue enzymes in the central nervous system, contributing to antioxidant defenses, energetic metabolism, ammonia detoxification, among other important functions. Until now, Mn transport mechanisms are partially understood; however, it is known that it shares some mechanisms of transport with iron. CNS is susceptible to Mn toxicity because it possesses mechanisms that allow Mn entry and favor its accumulation. Cases of occupational Mn exposure have been extensively reported in the literature; however, there are other ways of exposure, such as long-term parental nutrition and liver failure. Manganism and hepatic encephalopathy are the most common pathologies associated with the effects of Mn exposure. Both pathologies are associated with motor and psychiatric disturbances, related in turn to mechanisms of damage such as oxidative stress and neurotransmitters alterations, the dopaminergic system being one of the most affected. Although manganism and Parkinson??s disease share some characteristics, they differ in many aspects that are discussed here. The mechanisms for Mn transport and its participation in manganism and hepatic encephalopathy are also considered in this review. It is necessary to find an effective therapeutic strategy to decrease Mn levels in exposed individuals and to treat Mn long term effects. In the case of patients with chronic liver failure it would be worthwhile to test a low-Mn diet in order to ameliorate symptoms of hepatic encephalopathy possibly related to Mn accumulation.     

Cellular localization and developmental changes of Zip8, Zip14 and transferrin receptor 1 in the inner ear of rats

Prior studies have demonstrated that the inner ear can accumulate a variety of essential and potentially toxic heavy metals including manganese, lead, cobalt and cadmium. Metal accumulation is regulated in part by the functionality and affinity of these metals for the different transport systems responsible for uptake across the blood-cochlea barrier and their subsequent uptake into the different cells within the inner ear. Transport of these metals across cell membranes occurs by many of the same transport systems which include DMT1, Zip8 and Zip14. All three metal transporters have been identified in the cochlea based on quantitative PCR analysis. Prior studies in our laboratory examined the localization and developmental changes of DMT1 in rat cochlea and since the two Zip proteins are also likely to contribute to the transport of essential and non-essential divalent cations, we performed immunolabeling experiments in postnatal day three rat pups and adult rats. For comparison, we also immunolabeled the specimens with antibody against transferrin receptor 1 (TfR1) which is important in DMT1-mediated transport of Fe and Mn. Results presented in this paper demonstrate that the cellular and subcellular distribution of both Zip8 and Zip14 within the different components of the inner ear are distinct from that of DMT1. Nuclear localization for both Zip transporters as well as TfR1 was observed. The findings also reveal that the selective distribution of the three proteins was altered during development presumably to meet the changing needs of the cells to maintain normal and functional levels of iron and other essential metals.     

On the mechanism of oleate transport across human brain microvessel endothelial cells

The blood–brain barrier formed by the brain capillary endothelial cells provides a protective barrier between the systemic blood and the extracellular environment of the CNS. As most fatty acids in the brain enter from the blood, we examined the mechanism of oleate (C18:1) transport across primary human brain microvessel endothelial cells (HBMEC). The permeability of [1-¹⁴C]oleate was determined using confluent cells grown on Transwell® inserts in both the absence or presence of bovine serum albumin in the basolateral media, and following inhibition of various fatty acid transporters. The passage of [1-¹⁴C]oleate across confluent HBMEC monolayers was significantly enhanced when fatty acid free albumin was present in the basolateral media. The presence of the non-specific fatty acid uptake inhibitor phloretin significantly decreased [1-¹⁴C]oleate uptake by HBMEC and the subsequent release of [1-¹⁴C]oleate into the basolateral medium. Knockdown of fatty acid transport protein-1 or fatty acid translocase/CD36 significantly decreased [1-¹⁴C]oleate transport across the HBMEC monolayer from either apical as well as basolateral sides. The findings indicate that a fatty acid acceptor is a requirement for oleate transport across HBMEC monolayers. In addition, transport of oleate across HBMEC is, in part, a transcellular process mediated by fatty acid transport proteins.     

Cu(II) and Zn(II) ions alter the dynamics and distribution of Mn(II) in cultured chick glial cells

Previous studies revealed that Mn(II) is accumulated in cultured glial cells to concentrations far above those present in whole brain or in culture medium. The data indicated that Mn(II) moves across the plasma membrane into the cytoplasm by facilitated diffusion or counter-ion transport with Ca(II), then into mitochondria by active transport. The fact that 1–10 M Mn(II) ions activate brain glutamine synthetase makes important the regulation of Mn(II) transport in the CNS. Since Cu(II) and Zn(II) caused significant changes in the accumulation of Mn(II) by glia, the mechanisms by which these ions alter the uptake and efflux of Mn(II) ions has been investigated systematically under chemically defined conditions. The kinetics of [⁵⁴MN]-Mn(II) uptake and efflux were determined and compared under four different sets of conditions: no adducts, Cu(II) or Zn(II) added externally, and with cells preloaded with Cu(II) or Zn(II) in the presence and absence of external added metal ions. Zn(II) ions inhibit the initial velocity of Mn(II) uptake, increase total Mn(II) accumulated, but do not alter the rate or extent Mn(II) efflux. Cu(II) ions increase both the initial velocity and the net Mn(II) accumulated by glia, with little effect on rate or extent of Mn(II) efflux. These results predict that increases in Cu(II) or Zn(II) levels may also increase the steady-state levels of Mn(II) in the cytoplasmic fraction of glial cells, which may in turn alter the activity of Mn(II)-sensitive enzymes in this cell compartment.     

Variance components of short-term biomarkers of manganese exposure in an inception cohort of welding trainees

Various biomarkers of exposure have been explored as a way to quantitatively estimate an internal dose of manganese (Mn) exposure, but given the tight regulation of Mn in the body, inter-individual variability in baseline Mn levels, and variability in timing between exposure and uptake into various biological tissues, identification of a valuable and useful biomarker for Mn exposure has been elusive. Thus, a mixed model estimating variance components using restricted maximum likelihood was used to assess the within- and between-subject variance components in whole blood, plasma, and urine (MnB, MnP, and MnU, respectively) in a group of nine newly-exposed apprentice welders, on whom baseline and subsequent longitudinal samples were taken over a three month period. In MnB, the majority of variance was found to be between subjects (94%), while in MnP and MnU the majority of variance was found to be within subjects (79% and 99%, respectively), even when controlling for timing of sample. While blood seemed to exhibit a homeostatic control of Mn, plasma and urine, with the majority of the variance within subjects, did not. Results presented here demonstrate the importance of repeat measure or longitudinal study designs when assessing biomarkers of Mn, and the spurious associations that could result from cross-sectional analyses.     

Effects of protons and HZE particles on glutamate transport in astrocytes, neurons and mixed cultures

Radiation-induced neurotoxicity is a well-characterized phenomenon. However, the underlying mechanism of this toxicity is poorly understood. In the central nervous system (CNS), excitotoxic mechanisms are implicated in many neurodegenerative disease processes. Pivotal to the excitotoxic pathway is dysfunction of glutamate signaling. We reported previously that exposure to low-LET γ radiation results in altered glutamate transport in neurons and astrocytes. In the present study, we sought to investigate the effects of various particle radiations of differing LET on glutamate transport as a measure of the neurochemical vulnerability of the CNS. NTera2-derived neurons and astrocytes isolated as pure and mixed cultures were exposed to doses of 10 cGy, 50 cGy or 2 Gy of 250 MeV protons, 290 MeV/nucleon carbon ions, or 1000 MeV/nucleon iron ions. Transporter function was assessed at 3 h, 2 days and 7days after exposure. Functional assessment of glutamate transport revealed that neurons and astrocytes respond in a reciprocal manner after exposure to particle radiation. Uptake activity in neurons increased after particle irradiation. This effect was evident as late as our last time (7 days) after exposure (P < 0.05). In astrocytes, transporter activity decreased after exposure. The decrease in uptake observed in astrocytes was evident 7 days after exposure to carbon and iron ions. Uptake in mixed cultures after exposure to all three forms of radiation revealed a muted interactive response suggestive of the individual responses of each cellular phenotype acting in opposition.     

Brain capillary endothelium and choroid plexus epithelium regulate transport of transferrin-bound and free iron into the rat brain

Iron transport into the CNS is still not completely understood. Using a brain perfusion technique in rats, we have shown a significant brain capillary uptake of circulating transferrin (Tf)-bound and free 59Fe (1 nm) at rates of 136 +/- 26 and 182 +/- 23 microL/g/min, respectively, while their respective transport rates into brain parenchyma were 1.68 +/- 0.56 and 1.52 +/- 0.48 microL/g/min. Regional Tf receptor density (Bmax) in brain endothelium determined with 125I-holo-Tf correlated well with 59Fe-Tf regional brain uptake rates reflecting significant vascular association of iron. Tf-bound and free circulating 59Fe were sequestered by the choroid plexus and transported into the CSF at low rates of 0.17 +/- 0.01 and 0.09 +/- 0.02 microL/min/g, respectively, consistent with a 10-fold brain-CSF concentration gradient for 59Fe, Tf-bound or free. We conclude that transport of circulating Tf-bound and free iron could be equally important for its delivery to the CNS. Moreover, data suggest that entry of Tf-bound and free iron into the CNS is determined by (i) its initial sequestration by brain capillaries and choroid plexus, and (ii) subsequent controlled and slow release from vascular structures into brain interstitial fluid and CSF.     

Update on Boron in Higher Plants - Uptake, Primary Translocation and Compartmentation

Abstract: This review focuses on the uptake and primary translocation of boron (B), as well as on the subcellular compartmentation of B and its role in cell walls of higher plants. B uptake occurs via passive diffusion across the lipid bilayer, facilitated transport through major intrinsic proteins (MIPs), and energy-dependent transport through a high affinity uptake system. Whereas the first two represent passive uptake systems, which are constitutively present, the latter is induced by low B supply and is able to establish a concentration gradient for B between the root symplasm and the external medium. At high B supply, a substantial retention of B can be observed at xylem loading, and passive processes are most likely responsible for that. At low B supply, another energy-dependent high affinity transport system for B seems to be induced which establishes an additional concentration gradient between root symplasm and the xylem. The possible significance of all these processes at various B supplies is discussed. The role of soluble B complexes in uptake and primary translocation of B has been evaluated, but the few data available do not allow comprehensive conclusions to be drawn. In any case, there are no indications that soluble B complexes play a major role in either uptake or primary translocation of B. The subcellular compartmentation of B still remains a matter of controversy, but it is unequivocally clear that B is present in all subcellular compartments (apoplasm, cell wall, cytosol and vacuole). The relative distribution of B between these is dependent on plant species and experimental conditions and may vary greatly. Recent results on the well-established role of B in cell walls are summarized and their physiological significance discussed.     

Diversity in ABC transporters: Type I,II and III importers

Abstract

ATP-binding cassette transporters are multi-subunit membrane pumps that transport substrates across membranes. While significant in the transport process, transporter architecture exhibits a range of diversity that we are only beginning to recognize. This divergence may provide insight into the mechanisms of substrate transport and homeostasis. Until recently, ABC importers have been classified into two types, but with the emergence of energy-coupling factor (ECF) transporters there are potentially three types of ABC importers. In this review, we summarize an expansive body of research on the three types of importers with an emphasis on the basics that underlie ABC importers, such as structure, subunit composition and mechanism.     

Regulation of hexose transport in L8 myocytes by glucose: possible sites of interaction

Previous work demonstrated that glucose controls its own transport rate in rat skeletal muscle: exposure to high glucose levels down-regulates muscle hexose transport, while glucose withdrawal results in elevated transport rates (J. Biol. Chem. 261:16827-16833, 1986). The present study investigates the mechanism of this autoregulatory system. Preincubation of L8 myocytes at 16 mM glucose reduced subsequent 2-deoxy-D-glucose (dGlc) uptake by 40% within 3 h. Cycloheximide (1 microM) mimicked the action of glucose; the effects of glucose and cycloheximide were not additive. At 50 microM, cycloheximide prevented the modulations of glucose transport induced by exposure of muscle cells to high or low glucose concentrations. Inhibition of glycosylation with tunicamycin A1 reduced the basal dGlc uptake, but did not prevent its up-regulation following glucose withdrawal. Inhibition of RNA synthesis by actinomycin D prevented the down-regulatory effect of glucose. These results indicate that continuous protein synthesis and protein glycosylation are required for the maintenance of the steady-state dGlc uptake. We suggest that glucose exerts its autoregulatory effect on hexose transport by modifying the incorporation of active glucose transporters into the plasma membrane rather than changing their rate of degradation. It is hypothesized that this effect is mediated by a non-glycosylated protein involved in the translocation or activation of glucose transporters.     

Functional biology of plant phosphate uptake at root and mycorrhiza interfaces

Phosphorus (P) is an essential plant nutrient and one of the most limiting in natural habitats as well as in agricultural production world-wide. The control of P acquisition efficiency and its subsequent uptake and translocation in vascular plants is complex. The physiological role of key cellular structures in plant P uptake and underlying molecular mechanisms are discussed in this review, with emphasis on phosphate transport across the cellular membrane at the root and arbuscular-mycorrhizal (AM) interfaces. The tools of molecular genetics have facilitated novel approaches and provided one of the major driving forces in the investigation of the basic transport mechanisms underlying plant P nutrition. Genetic engineering holds the potential to modify the system in a targeted way at the root-soil or AM symbiotic interface. Such approaches should assist in the breeding of crop plants that exhibit improved P acquisition efficiency and thus require lower inputs of P fertilizer for optimal growth. Whether engineering of P transport systems can contribute to enhanced P uptake will be discussed.     

Hepatic uptake and biliary excretion of manganese in the little skate,Leucoraja erinacea

The liver is a major organ involved in regulating whole body manganese (Mn) homeostasis; however, the mechanisms of Mn transport across the hepatocyte basolateral and canalicular membranes remain poorly defined. To gain insight into these transport steps, the present study measured hepatic uptake and biliary excretion of Mn in an evolutionarily primitive marine vertebrate, the elasmobranch Leucoraja erinacea, the little skate. Mn was rapidly removed from the recirculating perfusate of isolated perfused skate livers in a dose-dependent fashion; however, only a small fraction was released into bile (< 2% in 6 h). Mn was also rapidly taken up by freshly isolated skate hepatocytes in culture. Mn uptake was inhibited by a variety of divalent metals, but not by cesium. Analysis of the concentration-dependence of Mn uptake revealed of two components, with apparent K_m values 1.1 ± 0.1 µM and 112 ± 29 µM. The K_m value for the high-affinity component was similar to the measured skate blood Mn concentration, 1.9 ± 0.5 µM. Mn uptake was reduced by nearly half when bicarbonate was removed from the culture medium, but was unaffected by a change in pH from 6.5 to 8.5, or by substitution of Na with Li or K. Mn efflux from the hepatocytes was also rapid, and was inhibited when cells were treated with 0.5 mM 2,4-dinitrophenol to deplete ATP levels. These data indicate that skate liver has efficient mechanisms for removing Mn from the sinusoidal circulation, whereas overall biliary excretion is low and appears to be mediated in part by an ATP-sensitive mechanism.     

Adenosine uptake site heterogeneity in the mammalian CNS? Uptake inhibitors as probes and potential neuropharmaceuticals

Inhibitors of adenosine uptake or transport have been used clinically for some time in certain cardiovascular diseases. More recently, some of them have also been investigated for possible clinical use in combination with antimetabolites based on the observed heterogeneity of nucleoside transport in mammalian tumor cells. Such a heterogeneity of adenosine uptake and uptake sites has now also been suggested in the mammalian CNS. The aim of this article is, therefore, to review the present status of our knowledge of adenosine uptake in the mammalian CNS, compare it with our far more advanced knowledge of nucleoside transport in other mammalian cells and suggest direction of future research. The possible implications for the development of adenosine uptake inhibitors as adenosinergic neuropharmaceuticals will be discussed based on our knowledge of the physiological function of adenosine in the CNS.     

Assembly-dependent endocytosis and clearance of extracellular alpha-synuclein

Abnormal folding and accumulation of alpha-synuclein is implicated in several neurological disorders including Parkinson's disease. Although alpha-synuclein is a typical cytoplasmic protein, a small amount of both monomeric and aggregated forms is secreted from cells and is present in human body fluids, such as cerebrospinal fluid. Extracellular alpha-synuclein aggregates have been shown to be neurotoxic, posing a challenge to any cell exposed to them. Here, we examine the internalization of various forms of extracellular alpha-synuclein, including fibrils, oligomers, and monomer, into neuronal cells and their subsequent degradation. Internalization of fibrillar alpha-synuclein could be inhibited by low temperature or the expression of a dominant-negative mutant dynamin-1 K44A, suggesting the endocytosis-mediated internalization. The internalized fibrils moved through the endosomal pathway and were degraded in the lysosome, which ultimately resulted in the clearance of the alpha-synuclein aggregates from the culture medium. Non-fibrillar oligomeric aggregates were also internalized via endocytosis and degraded by the lysosome. In contrast to aggregate uptake, the internalization of monomeric alpha-synuclein was unaffected by cold temperature and the expression of dynamin-1 K44A, consistent with direct translocation across the plasma membrane. Internalized monomers rapidly pass the plasma membrane, escaping the cells before being degraded by the cellular proteolytic systems. These results suggest that only aggregated forms of extracellular alpha-synuclein can be cleared by cell-mediated uptake and degradation, and this might represent a mechanism of preventing neurons from exposure to potentially toxic alpha-synuclein.     

Manganese Uptake and Efflux in Cultured Rat Astrocytes

Astrocytes play a central role in manganese (Mn) regulation in the CNS. Using primary astrocyte cultures from neonatal rat brains, these studies demonstrate a specific high-affinity transport system for Mn2+. Saturation kinetics are clearly indicated by both 1/v versus 1/s plots (Km = 0.30 +/- 0.03 microM; Vmax = 0.30 +/- 0.02 nmol/mg of protein/min) and plots of v versus [s]. Several divalent cations (Co2+, Zn2+, and Pb2+) failed to inhibit the initial rate of 54Mn2+ uptake. In contrast, extracellular Ca2+ at 10 microM decreased 54Mn2+ uptake. Exchange with extracellular Mn2+ was not obligatory for the efflux of 54Mn2+ into extracellular medium because efflux occurred into Mn(2+)-free extracellular medium, but efflux of 54Mn2+ was enhanced when astrocytes were equilibrated in the presence of unlabeled Mn2+. Efflux of 54Mn2+ was biphasic with both a rapid and a slow component. Efflux was most rapid during the first 10 min of incubation, with 27.5 +/- 2.2% of 54Mn2+ transported extracellularly, and 37.2 +/- 1.2% of preloaded 54Mn2+ was retained by the astrocytes at 120 min. These studies show, for the first time, that mammalian astrocytes can transport Mn via a specific transport system.     

Histamine (re)uptake by astrocytes: an experimental and computational study

Astrocytes participate in the clearance of neurotransmitters by their uptake and subsequent enzymatic degradation. Histamine as a polar and/or protonated molecule must use a carrier to be transported across the cell membrane, although a specific histamine transporter has not been elucidated, yet. In this work we upgraded the kinetic studies of histamine uptake into neonatal rat cultured type 1 astrocytes with quantum chemical calculations of histamine pKa values in conjunction with Langevin dipoles solvation model as the first step toward microscopic simulation of transport. Our results indicate that astrocytes transport histamine by at least two carrier mediated processes, a concentration gradient dependent passive and a sodium-dependent and ATP-driven active transport. We also demonstrated that histamine protonation states depend on the polarity of the environment. In conclusion we suggest that histamine, a polar molecule at physiological pH uses at least two different mechanisms for its uptake into astrocytes –an electrodiffusion and Na⁺-dependent and ouabain sensitive active process. We emphasize relevance of knowledge of histamines protonation states at the rate limiting step of its transport for microscopic simulation that will be possible when structure of histamine transporter is known.     

Foliar water uptake: Processes,pathways, and integration into plant water budgets

Nearly all plant families, represented across most major biomes, absorb water directly through their leaves. This phenomenon is commonly referred to as foliar water uptake. Recent studies have suggested that foliar water uptake provides a significant water subsidy that can influence both plant water and carbon balance across multiple spatial and temporal scales. Despite this, our mechanistic understanding of when, where, how, and to what end water is absorbed through leaf surfaces remains limited. We first review the evidence for the biophysical conditions necessary for foliar water uptake to occur, focusing on the plant and atmospheric water potentials necessary to create a gradient for water flow. We then consider the different pathways for uptake, as well as the potential fates of the water once inside the leaf. Given that one fate of water from foliar uptake is to increase leaf water potentials and contribute to the demands of transpiration, we also provide a quantitative synthesis of observed rates of change in leaf water potential and total fluxes of water into the leaf. Finally, we identify critical research themes that should be addressed to effectively incorporate foliar water uptake into traditional frameworks of plant water movement.