Mammalian zinc transporters

Cyproterone acetate induces a cellular tolerance to cadmium in rat liver epithelial cells involving reduced cadmium accumulation

Several reports indicate that some steroids, in particular sex steroid hormones, can modify cadmium toxicity. We recently reported that cyproterone acetate (CA), a synthetic steroidal antiandrogen that is closely related in structure to progesterone, affects cadmium toxicity in mice. In the present study, we investigated the effect of CA on cadmium toxicity in a rat liver epithelial cell line (TRL 1215) in vitro. Cells were exposed to various concentrations of CA (0,1,10, or 50 microM) for 24 h and subsequently exposed to cadmium (0,50, or 100 microM; as CdCl2) for an additional 24 h. CA pretreatment resulted in a clear decrease in the sensitivity to cadmium. Additional time course study showed CA pretreatment provided protection against cadmium toxicity but only when given for 6 or more hours prior to cadmium exposure. Cellular cadmium accumulation was markedly reduced (60% decrease) in cells pretreated for 6 or more hours with CA. In the presence of protein synthesis inhibitors the protective effect of CA toward cadmium toxicity was abolished. However, in the presence of the GSH synthesis inhibitor, L-buthionine (S,R)-sulfoximide (BSO), the protective effect of CA toward cadmium toxicity remained. CA alone increased metallothionein (MT) levels 2.4-fold, while cadmium (50 microM) alone resulted in a 8.9-fold increase over control. However, cadmium-induced MT synthesis was markedly decreased by CA pretreatment probably because of reduced cadmium accumulation. Analysis of various metal transporters by bDNA signal amplification assay revealed that the ZnT-1 transporter gene, which encodes for a membrane protein associated with zinc efflux, was expressed three-fold more in CA treated cells than control. These data show that CA pretreatment provides protection against cadmium toxicity in vitro and indicate that this protection is due to a decreased accumulation of cadmium rather than through activation of MT synthesis. This decrease of cellular cadmium accumulation appears to be related to events that require protein synthesis and may be due to activation of the genes associated with zinc efflux.

The human ZIP1 transporter mediates zinc uptake in human K562 erythroleukemia cells

The ZIP superfamily of transporters plays important roles in metal ion uptake in diverse organisms. There are 12 ZIP-encoding genes in humans, and we hypothesize that many of these proteins are zinc transporters. In this study, we addressed the role of one human ZIP gene, hZIP1, in zinc transport. First, we examined (65)Zn uptake activity in K562 erythroleukemia cells overexpressing hZIP1. These cells accumulated more zinc than control cells because of increased zinc influx. Moreover, consistent with its role in zinc uptake, hZIP1 protein was localized to the plasma membrane. Our results also demonstrated that hZIP1 is responsible for the endogenous zinc uptake activity in K562 cells. hZIP1 is expressed in untransfected K562 cells, and the increase in mRNA levels found in hZIP1-overexpressing cells correlated with the increased zinc uptake activity. Furthermore, hZIP1-dependent (65)Zn uptake was biochemically indistinguishable from the endogenous activity. Finally, inhibition of endogenous hZIP1 expression with antisense oligonucleotides caused a marked decrease in endogenous (65)Zn uptake activity. The observation that hZIP1 is the major zinc transporter in K562 cells, coupled with its expression in many normal cell types, indicates that hZIP1 plays an important role in zinc uptake in human tissues.

Retrograde tracing of zinc-enriched (ZEN) neuronal somata in rat spinal cord

The zinc selenide autometallographic (ZnSeAMG) technique for tracing the retrograde axonal transport of zinc ions in zinc-enriched (ZEN) neurons was used to map the distribution of ZEN neuronal somata in rat spinal cord. After a local injection of sodium selenide into the dorsal or ventral horn, ZnSeAMG-labeled ZEN neurons appeared in Rexed's laminae V, VII and X while laminae I and II were void. A few scattered ZEN somata were observed in the remaining laminae. The labeled neurons differed in shape and size, and the relatively high level of labeled somata around the injection site suggests that many ZEN neurons have relatively short axons or boutons en passage close to the neuronal origin. Ultrastructurally, the retrogradely transported zinc selenide clusters were found in the lysosomes of ZEN somata and proximal dendrites. Electron microscopic studies also revealed two different kinds of ZEN terminals: (1) terminals with flat synaptic vesicles making symmetric synaptic contacts; and (2) terminals with round vesicles making asymmetric synaptic contacts. The present study suggests the existence of propriospinal systems of ZEN neurons comprising both segmental and intersegmental ZEN connections and having either inhibitory or excitatory ZEN terminals. The ZEN neurons seem to form a vast network of terminals located primarily in the gray matter, but also contacting dendrites radiating into the white matter. Important functions of this rather massive system of ZEN terminals can not be deduced from our present knowledge, but the systems appear to be involved in both motor and sensory functions.

Ablation of the metal ion-induced endocytosis of the prion protein by disease-associated mutation of the octarepeat region

The neurodegenerative spongiform encephalopathies, or prion diseases, are characterized by the conversion of the normal cellular form of the prion protein PrP(C) to a pathogenic form, PrP(Sc) [1]. There are four copies of an octarepeat PHGG(G/S)WGQ that specifically bind Cu(2+) ions within the N-terminal half of PrP(C) [2--4]. This has led to proposals that prion diseases may, in part, be due to abrogation of the normal cellular role of PrP(C) in copper homeostasis [5]. Here, we show that murine PrP(C) is rapidly endocytosed upon exposure of neuronal cells to physiologically relevant concentrations of Cu(2+) or Zn(2+), but not Mn(2+). Deletion of the four octarepeats or mutation of the histidine residues (H68/76 dyad) in the central two repeats abolished endocytosis, indicating that the internalization of PrP(C) is governed by metal binding to the octarepeats. Furthermore, a mutant form of PrP that contains nine additional octarepeats and is associated with familial prion disease [6] failed to undergo Cu(2+)-mediated endocytosis. For the first time, these results provide evidence that metal ions can promote the endocytosis of a mammalian prion protein in neuronal cells and that neurodegeneration associated with some prion diseases may arise from the ablation of this function due to mutation of the octarepeat region.

Zinc and the Eye

Zinc, a trace element that influences cell metabolism through a variety of mechanisms, appears to play an integral role in maintaining normal ocular function. This element is present in high concentrations in ocular tissue, particularly in retina and choroid. Zinc deficiency has been shown in a number of species to result in a variety of gross, ultrastructural and electrophysiologic ocular manifestations. The physiological functions for zinc have been studied predominantly in retina and retinal pigment epithelium where zinc is believed to interact with taurine and vitamin A. modify photoreceptor plasma membranes, regulate the light-rhodopsin reaction, modulate synaptic transmission and serve as an antioxidant. Suboptimal zinc status in North America may influence the development and progression of several chronic eye diseases. Zinc supplementation trials and epidemiological studies have produced conflicting results concerning the role of zinc in age-related macular degeneration. Additional well-controlled supplementation trials are indicated to clarify the role of zinc in this disease. Future investigations must also expand our understanding of the mechanisms by which zinc regulates ocular morphology and function.

The Zrc1 is involved in zinc transport system between vacuole and cytosol in Saccharomyces cerevisiae

The ZRC1 gene encodes a multicopy suppressor of zinc toxicity in Saccharomyces cerevisiae; however, previously we found that the expression of ZRC1 was induced when the intracellular zinc level was decreased. Zrc1 has six putative transmembrane domains and we determined that a Zrc1-GFP fusion protein was localized to the vacuolar membrane. The steady state level of intracellular zinc in a zrc1Delta mutant cultured in the zinc-abundant medium was lower than that in wild type. No distinct difference was observed in the basal activity of glyoxalase I, which is a cytosolic enzyme requiring zinc for catalytic function and is used here as a marker for cytosolic zinc-availability, between wild type and zrc1Delta mutant, although the activity was decreased much greater extent in the zrc1Delta mutant if the cells were exposed to the metal-limited medium. Similarly, the basal expression level of ZRC1-lacZ reporter gene in zrc1Delta mutant was the same as that in wild type; however, the fold of induction of ZRC1-lacZ expression in zrc1Delta mutant under the zinc-limited conditions was higher than that in the wild type. Based on these results, we present a tentative model for the function of Zrc1 as a mechanism to maintain the zinc homeostasis in yeast.

Age-dependent influence of dietary zinc restriction on short-term memory in male rats

Zinc is an essential micro-nutrient involved in numerous physiological functions. The high content of zinc in the hippocampus, coupled with the integral involvement of the hippocampus in memory, strongly implicates zinc in memory processing. The hypothesis of the current study was that dietary zinc restriction influenced short-term memory in postweaned rats, and this influence was age-dependent. Male rats (43 days to 18 months old) were divided into five experimental groups based on age, and fed zinc-adequate (zinc at 20 mg/kg as zinc chloride) or zinc-deficient (zinc less than 1-2 mg/kg) diets for a minimum of 3 weeks. Short-term memory was assessed using the distal-cue version of the Morris water maze (MWM). All rats fed the zinc-restricted diet exhibited cyclic anorexia, decreased weight gain, and significantly lower liver and femur zinc concentrations compared to age-matched controls. Further, whole brain, hippocampal, and cerebral wet weights were significantly reduced in the zinc-restricted treatment groups of all the age groups. Only zinc-restricted rats that were less than 62 days of age at the start of zinc restriction demonstrated significantly prolonged escape latencies in the water maze, indicating deficits in short-term memory. Regression analyses confirmed that the short-term memory deficits were correlated with significantly lower hippocampal and cerebral zinc concentrations compared to age-matched control and pair-fed rats. These results emphasize the significance of a critical age of influence for dietary zinc in memory processing, and the importance of considering age when studying zinc nutriture and CNS function.

Cloning, expression, and characterization of human cytosolic aminopeptidase P: a single manganese(II)-dependent enzyme

The mammalian bradykinin-degrading enzyme aminopeptidase P (AP-P; E. C. 3.4.11.9) is a metal-dependent enzyme and is a member of the peptidase clan MG. AP-P exists as membrane-bound and cytosolic forms, which represent distinct gene products. A partially truncated clone encoding the cytosolic form was obtained from a human pancreatic cDNA library and the 5' region containing the initiating Met was obtained by 5' rapid accumulation of cDNA ends (RACE). The open reading frame encodes a protein of 623 amino acids with a calculated molecular mass of 69,886 Da. The full-length cDNA with a C-terminal hexahistidine tag was expressed in Escherichia coli and COS-1 cells and migrated on SDS-PAGE with a molecular mass of 71 kDa. The expressed cytosolic AP-P hydrolyzed the X-Pro bond of bradykinin and substance P but did not hydrolyze Gly-Pro-hydroxyPro. Hydrolysis of bradykinin was inhibited by 1,10-phenanthroline and by the specific inhibitor of the membrane-bound form of mammalian AP-P, apstatin. Inductively coupled plasma atomic emission spectroscopy of AP-P expressed in E. coli revealed the presence of 1 mol of manganese/mol of protein and insignificant amounts of cobalt, iron, and zinc. The enzymatic activity of AP-P was promoted in the presence of Mn(II), and this activation was increased further by the addition of glutathione. The only other metal ion to cause slight activation of the enzyme was Co(II), with Ca(II), Cu(II), Mg(II), Ni(II), and Zn(II) all being inhibitory. Removal of the metal ion from the protein was achieved by treatment with 1,10-phenanthroline. The metal-free enzyme was reactivated by the addition of Mn(II) and, partially, by Fe(II). Neither Co(II) nor Zn(II) reactivated the metal-free enzyme. On the basis of these data we propose that human cytosolic AP-P is a single metal ion-dependent enzyme and that manganese is most likely the metal ion used in vivo.

Visualization of labile zinc and its role in apoptosis of primary airway epithelial cells and cell lines

The respiratory epithelium is vulnerable to noxious substances, resulting in the shedding of cells and decreased protection. Zinc (Zn), an antioxidant and cytoprotectant, can suppress apoptosis in a variety of cells. Here we used the novel Zn-specific fluorophore Zinquin to visualize and quantify labile intracellular Zn in respiratory epithelial cells. Zinquin fluorescence in isolated ciliated tracheobronchial epithelial cells and intact epithelium from sheep and pigs revealed an intense fluorescence in the apical and mitochondria-rich cytoplasm below the cilia. Zinquin fluorescence was quenched by the Zn chelator N,N,N', N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) and increased by the Zn ionophore pyrithione. We also assessed whether changes in intracellular labile Zn would influence susceptibility of these cells to apoptosis by hydrogen peroxide. Our results confirm that Zn deficiency enhanced hydrogen peroxide-induced caspase activation from 1.24 +/- 0.12 to 2.58 +/- 0.53 units. microg protein(-1). h(-1) (P </= 0.05); Zn supplementation suppressed these effects. These findings are consistent with the hypothesis that Zn protects upper respiratory epithelial cells and may have implications for human asthma where there is hypozincemia and epithelial damage.

The transcription factor MTF-1 mediates metal regulation of the mouse ZnT1 gene

Metal regulation of the mouse zinc transporter (ZnT)-1 gene was examined in cultured cells and in the developing conceptus. Zinc or cadmium treatment of cell lines rapidly (3 h) and dramatically (about 12-fold) induced ZnT1 mRNA levels. In cells incubated in medium supplemented with Chelex-treated fetal bovine serum, to remove metal ions, levels of ZnT1 mRNA were reduced, and induction of this message in response to zinc or cadmium was accentuated (up to 31-fold induction). Changes in ZnT1 gene expression in these experiments paralleled those of metallothionein I (MT-I). Inhibition of RNA synthesis blocked metal induction of ZnT1 and MT-I mRNAs, whereas inhibition of protein synthesis did not. Metal response element-binding transcription factor (MTF)-1 mediates metal regulation of the metallothionein I gene. In vitro DNA-binding assays demonstrated that mouse MTF-1 can bind avidly to the two metal-response element sequences found in the ZnT1 promoter. Using mouse embryo fibroblasts with homozygous deletions of the MTF-1 gene, it was shown that this transcription factor is essential for basal as well as metal (zinc and cadmium) regulation of the ZnT1 gene in these cells. In vivo, ZnT1 mRNA was abundant in the midgestation visceral yolk sac and placenta. Dietary zinc deficiency during pregnancy down-regulated ZnT1 and MT-I mRNA levels (4-5-fold and >20-fold, respectively) in the visceral yolk sac, but had little effect on these mRNAs in the placenta. Homozygous knockout of the MTF-1 gene in transgenic mice also led to a 4-6-fold reduction in ZnT1 mRNA levels and a loss of MT-I mRNA in the visceral yolk sac. These results suggest that MTF-1 mediates the response to metal ions of both the ZnT1 and the MT-I genes the visceral yolk sac. Overall, these studies suggest that MTF-1 directly coordinates the regulation of genes involved in zinc homeostasis and protection against metal toxicity.

Identification and localization of divalent metal transporter-1 (DMT-1) in term human placenta

The mechanism by which iron is transported from mother to fetus is incompletely understood. Whereas transferrin receptor (TfR) is responsible for iron uptake from maternal serum by the syncytiotrophoblast, the proteins responsible for intracytoplasmic transport and for delivery to the fetal serum remain unknown. The aim of this study was to determine whether the recently characterized endosomal membrane iron transporter, divalent metal ion transporter-1 (DMT-1), is expressed in human syncytiotrophoblast, and whether its cellular localization would support roles for cytoplasmic and placental-fetal iron transport. Six micron sections of frozen, term human placenta were assessed immunohistochemically using a polyclonal antibody to rat DMT-1 and a monoclonal antibody to human TfR. DMT-1 was found both in the cytoplasm and at the junction of the fetal (basal) membrane and fetal vessels, while TfR was localized predominantly to the maternal (apical) side of the syncytiotrophoblastic membrane. Double staining demonstrated no overlap between the two proteins on the apical membrane and minimal areas of overlap in the cytoplasm. We postulate that the syncytiotrophoblast takes up diferric transferrin from serum via TfR, subsequently incorporating the transferrin : TfR complex via endosomes. Subsequent transport of iron out of the endosome and across the basal membrane to the fetus may occur via DMT-1.

Defective organellar membrane protein trafficking in Ap3b1-deficient cells

AP-3 is a heterotetrameric protein complex involved in intracellular vesicle transport. Molecular analyses show that Ap3b1, which encodes the AP-3 (&bgr;)3A subunit, is altered in pearl mice. To provide genetic evidence that mutation of Ap3b1 is responsible for the pearl phenotype and to determine the null phenotype, the Ap3b1 gene was disrupted by homologous recombination. Mice homozygous for the resulting allele, Ap3b1(LN), or compound heterozygotes with pearl, displayed phenotypes similar to those of pearl mice, confirming that Ap3b1 is the causal gene for pearl. Moreover, pearl is likely to be a hypomorph as the Ap3b1(LN) homozygotes had a lighter coat color and accumulated fewer of the micro3 and (&dgr;)3 subunits of AP-3 than did pearl mice. Finally, immunofluorescence analysis of fibroblasts and melanocytes cultured from Ap3b1(LN) homozygotes revealed that the lysosomal membrane proteins Lamp I and Lamp II and the melanosomal membrane protein tyrosinase were mislocalized. In particular, the Lamp proteins were clustered on the cell surface. These findings strengthen the evidence for an alternate pathway via the plasma membrane for cargo normally transported to organelles by AP-3.

Expression of ZRC1 coding for suppressor of zinc toxicity is induced by zinc-starvation stress in Zap1-dependent fashion in Saccharomyces cerevisiae

The ZRC1 gene was cloned as a multicopy suppressor of zinc toxicity in Saccharomyces cerevisiae. Zrc1 belongs to CDF (cation diffusion facilitator) family. The transporters belonging to this family are thought to play an important role in metal detoxification. However, we found that cell growth of zrc1Delta mutant was lowered under the metal-limited conditions, which was restored by zinc. The Zap1 transcription factor is crucial for expression of several genes responsive to zinc-starvation stress. The expression of ZRC1 was induced in Zap1-dependent fashion when the intracellular zinc level was decreased and this induction was repressed by zinc. These results imply an important role of Zrc1 in the zinc-starvation stress.

Zn(2+): a novel ionic mediator of neural injury in brain disease

Zn(2+) is the second most prevalent trace element in the body and is present in particularly large concentrations in the mammalian brain. Although Zn(2+) is a cofactor for many enzymes in all tissues, a unique feature of brain Zn(2+) is its vesicular localization in presynaptic terminals, where its release is dependent on neural activity. Although the physiological significance of synaptic Zn(2+) release is little understood, it probably plays a modulatory role in synaptic transmission. Furthermore, several lines of evidence support the idea that, upon excessive synaptic Zn(2+) release, its accumulation in postsynaptic neurons contributes to the selective neuronal loss that is associated with certain acute conditions, including epilepsy and transient global ischaemia. More speculatively, Zn(2+) dis-homeostasis might also contribute to some degenerative conditions, including Alzheimer's disease. Further elucidation of the pathological actions of Zn(2+) in the brain should result in new therapeutic approaches to these conditions.

Prenatal iron supplements impair zinc absorption in pregnant Peruvian women

Prenatal iron supplements may adversely influence zinc absorption during pregnancy. To examine the impact of prenatal iron supplements on supplemental zinc absorption, fractional zinc absorption was measured in 47 pregnant Peruvian women during the third trimester of pregnancy (33 +/- 1 wk gestation). Of these 47 women, 30 received daily prenatal supplements from wk 10-24 of pregnancy until delivery. Supplements contained 60 mg of Fe and 250 microg of folate without [iron group (Fe), n = 16] or with [iron and zinc supplemented group (Fe + Zn), n = 14] 15 mg of Zn. The remaining 17 women [unsupplemented control group (C)] received no prenatal supplementation. Zinc concentrations were measured in plasma, urine and cord blood and percentage zinc absorption was determined following dosing with oral ((67)Zn) and intravenous ((70)Zn) stable zinc isotopes. Percentage zinc absorption was significantly lower than controls in fasting women receiving iron- containing prenatal supplements (20.5 +/- 6.4 vs. 20.2 +/- 4.6 vs. 47.0 +/- 12.6%, Fe, Fe + Zn and C groups, respectively, P: < 0.0001, n = 40). Plasma zinc concentrations were also significantly lower in the Fe group compared to the C group (8.2 +/- 2.2 vs. 9.2 +/- 2.2 vs. 10.9 +/- 1. 8 micromol/L, Fe, Fe + Zn and C groups, respectively, P: = 0.002), and cord zinc concentrations were significantly related to maternal plasma Zn levels (y = 6.383 + 0.555x, r = 0.486, P: = 0.002). The inclusion of zinc in prenatal supplements may reduce the potential for iron supplements to adversely influence zinc status in populations at risk for deficiency of both these nutrients.

Albumin facilitates zinc acquisition by endothelial cells

Albumin has long been observed to have a marked influence on the delivery of zinc to cells, but the mechanism of the interaction remains elusive. We examined whether albumin facilitates the acquisition of zinc by endothelial cells. Cultures of endothelial cells were used to analyze binding and acquisition of zinc and albumin to test this interaction. Our results indicate that albumin plays a role in facilitating the physiological delivery of zinc to endothelial cells. Albumin receptors that preferentially recognize albumin molecules carrying a zinc atom were demonstrated on the endothelial cell surface. Endocytosis is instrumental in albumin uptake, which was also consistently true of zinc uptake. Zinc and albumin were acquired by the cells in a 1:1 molar stoichiometry during the first 20 min of incubation in a medium with equimolar concentrations of zinc and albumin. The amount of albumin associated with the cells stabilized after 30 min, whereas the amount of zinc continued to increase. One possible explanation for this result is that a physiological route for zinc delivery into endothelial cells is by co-transport with albumin via receptor-mediated endocytosis.

The ZIP family of metal transporters

Members of the ZIP gene family, a novel metal transporter family first identified in plants, are capable of transporting a variety of cations, including cadmium, iron, manganese and zinc. Information on where in the plant each of the ZIP transporters functions and how each is controlled in response to nutrient availability may allow the manipulation of plant mineral status with an eye to (1) creating food crops with enhanced mineral content, and (2) developing crops that bioaccumulate or exclude toxic metals.

Integrative aspects of zinc transporters

Cells maintain zinc concentrations with relatively narrow limits. Nevertheless, physiologically relevant changes in free Zn(II) pools or changes in Zn bound to specific ligands or within vesicles may occur without a major change in total cellular zinc concentrations. The task of maintaining such levels rests in part with zinc transporter proteins. The genes for some putative zinc transporters have recently been cloned. As of this time, most have not been directly shown to transport zinc in functional studies, albeit evidence is strong that they have such a function. Zinc transporter (ZnT)-1 was identified as a rescue agent for cells maintained in very high extracellular zinc conditions; therefore, ZnT-1 has been suggested to function as an exporter. ZnT-1 is expressed in a variety of tissues, including intestine, kidney and liver. Intestinal expression is regional, being much greater in duodenum and jejunum and in villus versus crypt cells. Immunolocalization places ZnT-1 at the basolateral membrane of intestinal enterocytes and epithelial cells of the distal renal tubules. Regulation of ZnT-1 mRNA and ZnT-1 protein does not change markedly with changes in dietary zinc level except when a large single oral zinc supplement is provided. ZnT-1 is induced by transient ischemia of the forebrain. ZnT-2 and ZnT-3 may function in tissue-specific vesicular zinc transport. ZnT-4 is believed to be abundant in mammary gland and may be associated with zinc secretion into milk. A mutation of the ZnT-4 gene may account for the lethal milk (lm) syndrome. The putative zinc transporters identified thus far appear to have characteristics commensurate with functions in integrative zinc acquisition and homeostasis.

Zinc transport in the brain: routes of zinc influx and efflux in neurons

Studies of the routes of entry and exit for zinc in different tissues and cell types have shown that zinc can use several pathways of exit or entry. In neurons, known pathways include (1) presynaptic release along with glutamate when synaptic vesicles empty their contents into the synaptic cleft, (2) voltage-gated L-type Ca(2+) channels and glutamate-gated channels that provide an entry route when cells are depolarized and that mediate extracellular zinc toxicity and (3) a plasma membrane transporter potentially present in all neurons important for cellular zinc homeostasis. The least understood of these pathways, in terms of mechanism, is the transporter pathway. The kinetics of zinc uptake in cultured neurons under resting conditions are consistent with and suggest the existence of a saturable transporter in the plasma membrane. The proteins responsible for plasma membrane zinc transport have not yet been definitely identified. Likely candidates include two proteins identified by molecular cloning termed zinc transporter 1 and divalent cation transporter DCT1. Both proteins have been shown to be expressed in the brain, but only DCT1 is clearly demonstrated to be a transport protein, whereas zinc transporter 1 may only modulate zinc transport in association with as-yet-unidentified proteins. Understanding the mechanism and neuromodulation of plasma membrane zinc transport will be an important first step toward a complete understanding of neuronal zinc homeostasis.

Overview of zinc absorption and excretion in the human gastrointestinal tract

Zinc homeostasis is primarily maintained via the gastrointestinal system by the processes of absorption of exogenous zinc and gastrointestinal secretion and excretion of endogenous zinc. Although these processes modulate net absorption and the size of the readily exchangeable zinc pools, there are limits to the effectiveness of the homeostatic mechanisms of these and other systems. As a result of the interplay of the subcellular regulation of these mechanisms and host, dietary and environmental factors, zinc deficiency is not uncommon, especially on a global basis. This overview briefly reviews current understanding about the subcellular mechanisms of zinc absorption and transport. Factors recognized to affect zinc absorption at the whole body level are reviewed and include the amount and form of zinc consumed; dietary promoters, such as animal protein and low-molecular-weight organic compounds; dietary inhibitors, such as phytate and possibly iron and calcium when consumed as supplements; and physiologic states, such as pregnancy, lactation and early infancy, all of which increase the demand for absorbed zinc. The control of endogenously secreted zinc is less well understood. Available data suggest that the quantity of secreted zinc with each meal may be considerable and that efficient reabsorption is critical to the maintenance of normal zinc balance. Factors that have been proposed to interfere with the normal reabsorption of endogenous zinc include phytate and unabsorbed fat. Understanding of the dietary, physiologic, pathologic and environmental factors that may adversely affect these processes, and therefore zinc homeostasis, will be critical to preventing and treating zinc deficiency in human populations.

Importance of zinc in the central nervous system: the zinc-containing neuron

Zinc is essential to the structure and function of myriad proteins, including regulatory, structural and enzymatic. It is estimated that up to 1% of the human genome codes for zinc finger proteins. In the central nervous system, zinc has an additional role as a neurosecretory product or cofactor. In this role, zinc is highly concentrated in the synaptic vesicles of a specific contingent of neurons, called "zinc-containing" neurons. Zinc-containing neurons are a subset of glutamatergic neurons. The zinc in the vesicles probably exceeds 1 mmol/L in concentration and is only weakly coordinated with any endogenous ligand. Zinc-containing neurons are found almost exclusively in the forebrain, where in mammals they have evolved into a complex and elaborate associational network that interconnects most of the cerebral cortices and limbic structures. Indeed, one of the intriguing aspects of these neurons is that they compose somewhat of a chemospecific "private line" of the mammalian cerebral cortex. The present review outlines (1) the methods used to discover, define and describe zinc-containing neurons; (2) the neuroarchitecture and synaptology of zinc-containing neural circuits; (3) the physiology of regulated vesicular zinc release; (4) the "life cycle" and molecular biology of vesicular zinc; (5) the importance of synaptically released zinc in the normal and pathological processes of the cerebral cortex; and (6) the role of specific and nonspecific stressors in the release of zinc.

Refining the mouse chromosomal location of Cdm, the major gene associated with susceptibility to cadmium-induced testicular necrosis

Cadmium (Cd++) is a widespread environmental pollutant and classifed as an IARC 'Category I' human carcinogen. Cd++ can also cause severe renal toxicity and may be involved clinically in cardiovascular disease and osteoporosis. Genetic differences in sensitivity to cadmium toxicity have been noted in humans, whereas, among inbred mouse strains, unequivocal genetic data exist. Resistance to cadmium-induced testicular damage was reported in 1973 to be associated with a single major recessive gene, named Cdm, which has now been localized to mouse chromosome (Chr) 3. Using polymorphic microsatellite markers and semiquantitative histological parameters, we have corroborated the original 1973 data concerning mendelian inheritance and have further refined the region containing the Cdm gene from more than 24 cM to 0.64 cM (estimated 40-80 genes). We phenotyped 26 recombinant inbred lines generated from C57BL/6J (B6, resistant) and DBA/2J (D2, sensitive) inbred mice, and determined that the Cdm gene maps between microsatellite markers D3Mit110 and D3Mit255. Although toxicity to numerous heavy metals is well known, virtually no molecular mechanisms have yet been uncovered either in humans or laboratory animals. Identification and characterization of the mouse Cdm gene should enhance our understanding of heavy metal toxicity by identifying and characterizing, for the first time, a major mammalian gene responsible for susceptibility to diseases caused by heavy metal toxicity.

Dietary zinc deficiency decreases glutathione S-transferase expression in the rat olfactory epithelium

Zinc deficiency leads to olfactory and gustatory dysfunction, but little is known about the underlying molecular mechanism of this phenomenon. We examined the effect of dietary zinc deficiency on the rat olfactory epithelium. Immunoreactivities of glutathione S-transferase (GST) mu, neuron-specific enolase (NSE) and proliferating cell nuclear antigen (PCNA), and in situ hybridization of GST mu mRNA in the olfactory epithelia were examined under different dietary zinc intake conditions. Adult male rats were fed a zinc-deficient (ZD) diet (0.5 mg zinc/kg diet), whereas control rats, including pair-fed (PF) and zinc-adequate (ad libitum consumption, AL) groups, were fed a zinc-adequate diet (58 mg zinc/kg diet) for 7 wk. We also examined the effect of zinc replacement (ZR) by subsequently feeding half of the ZD group a zinc-adequate diet for 5 wk after the initial 7-wk deprivation. No significant differences in immunoreactivity for NSE in olfactory epithelial receptor cells or for PCNA in basal cells were noted among groups. Intense GST mu immunoreactivity and hybridization signals were observed in olfactory supporting cells of AL, PF and ZR groups, but very minimal or no such signal was noted in ZD rats. Our findings indicated that zinc deficiency reduces GST mu expression in the supporting cells of rat olfactory epithelia but does not affect receptor cell proliferation or maintenance.

Efficacy of exogenous oral zinc in treatment of patients with carbonic anhydrase VI deficiency

BACKGROUND

We previously described a disorder in 18 patients with decreased parotid saliva gustin/carbonic anhydrase (CA) VI secretion associated with loss of taste (hypogeusia) and smell (hyposmia) and distorted taste (dysgeusia) and smell (dysosmia). Because gustin/CAVI is a zinc-dependent enzyme we instituted a study of treatment with exogenous zinc to attempt to stimulate synthesis/secretion of gustin/CAVI and thereby attempt to correct the symptoms of this disorder.

METHODS

Fourteen of the 18 patients with this disorder completed the study. They were treated with 100 mg of exogenous zinc daily for 4 to 6 months, in an open clinical trial. Both before and after treatment, measurements were obtained of parotid saliva gustin/CAVI, parotid saliva, serum and urine zinc, taste and smell function, and, in some patients, examination of circumvallate taste buds by electron microscopy.

RESULTS

Treatment success was predicated upon significant increases in parotid saliva gustin/CAVI. This occurred in 10 of the 14 patients who were labeled responders; they also exhibited improvement in taste and smell acuity, a diminution in dysgeusia and dysosmia and increased zinc concentrations in parotid saliva, serum, and urine. Taste bud morphology returned to normal in each responder in whom it was measured. No increase in gustin/CAVI occurred in 4 patients who were labeled nonresponders; they exhibited no improvement in taste or smell acuity and no increases in parotid saliva zinc. However, serum and urine zinc increased to levels similar to those measured in the 10 responders. Two of 4 nonresponders reported diminution in dysgeusia and dysosmia. Taste bud morphology did not change from the abnormal state in the 1 nonresponder in whom it was measured.

CONCLUSIONS

Zinc treatment is effective in patients in whom this trace metal increases synthesis/secretion of gustin/CAVI and ineffective in those in whom it does not. Increased gustin/CAVI in this disorder is probably associated with zinc stimulation of the gene responsible for the synthesis/secretion of gustin/CAVI. Among nonresponders, zinc was ineffective for several possible reasons, including resistance to zinc and possible sialylation of gustin/CAVI, which may render it functionally ineffective. Results suggest the hypothesis that gustin/CAVI is a trophic factor that promotes growth and development of taste buds through its action on taste bud stem cells.

Expression of the mouse metallothionein-I and -II genes provides a reproductive advantage during maternal dietary zinc deficiency

The function of metallothionein in zinc homeostasis was examined by using mice homozygous for knockout (KO) of the metallothionein-I or -II (MT-I and MT-II) genes. Pregnant MT-I/II KO mice or control mice were fed a zinc-deficient (1 microg/g or 5 microg/g) diet or a zinc-adequate (50 microg/g) diet during specific periods of pregnancy, and the effects on morphogenesis of the embryos were determined at day 14 of pregnancy (day 1 = vaginal plug). In the homozygous MT-I/II KO, as well as in the nontransgenic control mice, severe dietary zinc deficiency (1 microg/g) beginning on day 1 of pregnancy was embryotoxic and teratogenic, and the majority of the embryos in both strains were dead by mid-gestation. However, 53% of the surviving embryos in the MT-I/II KO mice were morphologically abnormal compared to only 32% of the embryos in the control mice. In subsequent experiments, moderate dietary zinc deficiency (5 microg/g beginning on day 1 of pregnancy or 1 microg/g dietary zinc beginning on day 8 of pregnancy) exerted teratogenic, but not embryotoxic effects. Embryos in the MT-I/II KO mice were 260 to 290% as likely to develop abnormally than were embryos in the control mice fed these same diets. These results demonstrate that the expression of the MT-I and -II genes in pregnant females improves reproductive success during maternal dietary zinc deficiency.

Zinc, copper, and iron metabolism during porcine fetal development

Zinc, copper, and iron levels in maternal and fetal pig tissues and fluids were measured starting on d 30 of gestation and continuing to term (d 114) at 10-d intervals. Fetal hematocrit increased from a low of 19% on d 30 to 32% by d 50, after which it remained above 30% to term. Amniotic fluid zinc, copper, and iron all reached maximal levels by d 60 of gestation. Maternal serum zinc levels fluctuated little during gestation, but fetal serum zinc concentration was significantly elevated above maternal levels during the second trimester. Fetal serum copper levels were significantly lower than maternal values throughout gestation and this was also the case for ceruloplasmin oxidase activity. Maternal serum iron reached its lowest level by d 80 of gestation when rate of transfer of iron to the developing fetuses was high. Fetal serum iron declined throughout gestation, reaching its lowest level on d 100. In general, fetal liver concentrations of zinc, copper, and iron were higher than the corresponding maternal values throughout gestation. Distinct increases were noted for fetal hepatic zinc and copper concentrations during the second trimester of pregnancy and these were accompanied by increases in cytosolic and metallothionein-bound zinc and copper levels. Maternal hepatic iron declined during the second trimester, reaching its lowest point on d 80, indicative of the shunting of maternal iron reserves to fetal tissues. Fetal kidney metal levels did not demonstrate any distinctive developmental patterns with respect to zinc, copper, or iron concentrations, but a general accumulation of each metal was observed as gestation progressed. The results of this study highlight some of the distinct changes occurring in the metabolism of zinc, copper, and iron in both maternal and fetal tissues and fluids during gestation in the pig.

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.

Zinc transfer to the breastfed infant

Zinc is a micronutrient which is critical to normal growth and development. Zinc concentrations in human milk decline sharply during the early months post partum, regardless of maternal zinc intake. Milk zinc concentrations do not increase in response to increased maternal zinc intake if maternal zinc status is adequate. The mechanism of zinc secretion into milk is not fully understood. A mutation in the gene for a zinc transporter protein may account for abnormally low milk zinc concentrations associated with severe zinc deficiency in breastfed infants. The zinc requirements of breastfed infants are generally met with exclusive breastfeeding through 5-6 months of age, due to the favorable bioavailability of the zinc in human milk. Because of declining milk zinc concentrations and intake, zinc status in exclusively breastfed infants is likely to become marginal beyond 6 months of age, and may become suboptimal for some infants if exclusive breastfeeding continues. The choice of complementary foods is important to maintain adequate zinc status in breastfed infants after 6 months.

[Diagnosis of zinc deficiency]

Though far more common, particularly in elderly people, than was previously assumed, marginal zinc deficiency does not lead to the classical manifestations of zinc deficiency and is therefore difficult to diagnose. There is therefore a need for sensitive parameters that can reliably demonstrate even marginal zinc deficiency, as suboptimal zinc status can seriously impair human health, performance, reproductive functions, and mental and physical development. The most important criteria for the diagnosis of zinc deficiency are critically discussed. The laboratory parameters currently considered to be the most useful indicators of marginal zinc deficiency are zinc-binding capacity and serum/plasma alkaline phosphatase activity before and after zinc supplementation (zinc tolerance test!). In order to obtain a reliable assessment of a patients zinc status, a number of different diagnostic parameters should always be measured.

Zeroing in on zinc uptake in yeast and plants

Zinc is an essential micronutrient. Genes responsible for zinc uptake have now been identified from yeast and plants. These genes belong to an extended family of cation transporters called the ZIP gene family. Zinc efflux genes that belong to another transporter family, the CDF family, have also been identified in yeast and Arabidopsis. It is clear that studies in yeast can greatly aid our understanding of zinc metabolism in plants.

Regulation of caspase activation and apoptosis by cellular zinc fluxes and zinc deprivation: A review

Non-toxic agents that target intracellular signalling pathways in apoptosis may have potential therapeutic use in many diseases. One such agent is the transition metal Zn, a dietary cytoprotectant and anti-oxidant, which stimulates cell proliferation and suppresses apoptosis. Zn is maintained in discrete subcellular pools that are critical for the functional and structural integrity of cells. The present review initially describes the current state of knowledge on the cellular biology of Zn, especially the critical free or loosely bound (labile) pools of Zn, which are thought to regulate apoptosis. We then review the evidence relating Zn to apoptosis, including studies from our laboratory showing potent synergy between intracellular Zn deficiency and the short chain fatty acid butyrate in induction of caspase activation and the downstream events of apoptosis. Our studies have also reported the suppressive effects of micromolar concentrations of Zn on caspase-3 activation in cell-free models. Other key issues that will be discussed include the identification of the putative molecular targets of Zn and the evidence that systemic changes in labile Zn levels are sufficient to alter susceptibility to apoptosis and lead to physiopathological changes in the human body. Finally, we propose that labile Zn may serve as a coordinate regulator of mitosis and apoptosis to regulate tissue growth.

Considerations for determining 'optimal nutrition' for copper, zinc, manganese and molybdenum

Defining optimal dietary intakes of Cu and Zn throughout the life cycle continues to present a considerable challenge for nutrition scientists. Although the daily intake of these micronutrients is below that currently recommended for many groups, traditional biochemical indicators of nutritional status for these trace metals largely remain within the normal range. Thus, it is unclear whether the recommended daily intakes may be unnecessarily high, or if the commonly-used markers simply lack the necessary sensitivity and specificity that are required for accurately assessing Cu and Zn status. The increasing number of reports that daily supplements with these trace metals enhance the activities of selective metalloenzymes and specific cellular and organ processes further points out the need to differentiate between meeting the requirement and providing optimal nutriture. Results from recent studies suggesting that alternative molecular and functional markers possess sufficient sensitivity to better assess Cu and Zn status are discussed. Likewise, recent studies evaluating the impact of very low and excessive levels of dietary Mn and Mo on selective biochemical and metabolic indicators are reviewed.

Ionic selectivity of the Ca2+/H+ antiport in synaptic vesicles of sheep brain cortex

As we previously reported, synaptic vesicles isolated from sheep brain cortex contain a Ca2+/H+ antiport that permits Ca2+ accumulation inside the vesicles ( approximately 5 nmol/mg protein) at expenses of the pH gradient generated by the H+-pumping ATPase. We observed that the system associates Ca2+ influx to H+ release and operates with low affinity for Ca2+. In the present work, we found that Ca2+/H+ antiport mediates exchange of protons with other cations such as Zn2+ and Cd2+, suggesting that these cations and Ca2+ share the same transporter molecules to enter the intravesicular space. Zn2+ and Cd2+ induce H+ release in a concentration-dependent manner (fluorimetrically evaluated) and they inhibit the antiport-mediated Ca2+ uptake by the vesicles (isotopically measured). In contrast, large cations such as Ba2+ and Cs+ do not alter Ca2+ influx and they are unable to induce proton release from the vesicles. With respect to Sr2+, which has an intermediary size relatively to the other groups of cations, we found that it does not induce H+ liberation from the vesicles, but it has a concentration-dependent inhibitory effect on the Ca2+-induced H+ release and Ca2+ uptake by the vesicles. These results indicate that the cation selectivity of the synaptic vesicles Ca2+/H+ antiport is essentially determined by the size of the cation transported into the vesicles.

Transient peaks in zinc and metallothionein levels during differentiation of 3T3L1 cells

A novel role for zinc mediated by metallothionein (MT) is found in the process of differentiation of 3T3L1 mouse fibroblasts to adipocytes. Twenty-four hours after the stimulation of differentiation by hormones, the cells enter into a phase of synchronous proliferation. In this phase the cellular contents of zinc and metallothionein rise rapidly to fivefold and threefold levels, respectively. Simultaneously MT is translocated from the cytoplasm to the nucleus. The rise of intracellular zinc is essential for the transition from G0/G1- to S-phase of the cell cycle. Deprivation of zinc with N,N,N', N'-tetrakis[2-pyridyl]ethylenediamine, a membrane-permeable zinc chelator, inhibited hormonal induced proliferation. After the short phase of proliferation a slower stage of actual differentiation to adipocytes begins. The elevated levels of MT and zinc decline quickly to start levels, and a rapid redistribution of MT to the cytoplasm occurs. We propose that the nuclear translocation of MT mediates the transfer of zinc to nuclear factors in the mitogenic process. The redistribution of MT to the cytoplasm and the decrease of the zinc content are postulated to be required for the start of the actual differentiation.

The regulation of mineral absorption in the gastrointestinal tract

The absorption of metal ions in the mammalian single-stomached gut is fortunately highly selective, and both luminal and tissue regulation occur. Initially, assimilation of metal ions in an available form is facilitated by the intestinal secretions, chiefly soluble mucus (mucin) that retards hydrolysis of ions such as Cu, Fe and Zn. Metal ions then bind and traverse the mucosally-adherent mucus layer with an efficiency M+ > M2+ > M3+. At the mucosa Fe3+ is probably uniquely reduced to Fe2+, and all divalent cations (including Fe2+) are transported by a membrane protein (such as divalent cation transporter 1) into the cell. This minimizes absorption of toxic trivalent metals (e.g. Al3+). Intracellular metal-binding molecules (such as mobilferrin) may be present at the intracellular side of the apical membrane, anchored to a transmembrane protein such as an integrin complex. This mobilferrin would receive the metal ion from divalent cation transporter 1 and, with part of the integrin molecule, transport the metal to the cytosol for safe sequestration in a larger complex such as ferritin or 'paraferritin'. beta 2-Microglobulin and HFE (previously termed human leucocyte antigen H) may be involved in stabilizing metal mobilferrin-integrin to form this latter complex. Finally, a systemic metal-binding protein such as transferrin may enter the antiluminal (basolateral) side of the cell for binding of the sequestered metal ion and delivery to the circulation. Regulatory proteins, such as HFE, may determine the degree of ion transport from intestinal cells to the circulation. Gradients in pH and perhaps pCa or even pNa could allow the switching of ions between the different transporters throughout this mechanism.

Recently identified molecular aspects of intestinal iron absorption

Gene mapping techniques to identify gene mutations in humans and animals with phenotypic abnormalities in iron metabolism are providing important insights into the probable molecular mediators of intestinal iron absorption. Positional gene cloning in humans with hereditary hemochromatosis has identified a mutation in a novel major histocompatibility complex (MHC) gene called HFE that is likely to be involved in regulating intestinal iron absorption. In addition, recent observations based on positional cloning strategies in the mk/mk mouse and the Belgrade (b/b) rat rodent models of hypochromic, microcytic anemia have shown that the phenotypic abnormality in iron metabolism is associated with a mutation in the Nramp2 gene. Functional cloning studies in Xenopus oocytes have characterized DCT1 (Nramp2) as an iron-regulated proton-coupled divalent cation transporter. Nramp2 is likely to be the membrane transporter that functions in controlling iron entry across the apical membrane and in the export of iron out of endosomal vesicles. The observation that the expression of both HFE and Nramp2 mRNAs are reciprocally regulated by cellular iron status in Caco-2 cells, a human intestinal cell line, lends additional credence to the notion that these proteins may work in concert to regulate intestinal iron absorption.

Metallothionein knockout and transgenic mice exhibit altered intestinal processing of zinc with uniform zinc-dependent zinc transporter-1 expression

A role for metallothionein in intestinal zinc absorption has been the subject of considerable debate. If metallothionein affects zinc absorption, then those factors that induce metallothionein synthesis (e.g., heavy metals, hormones) should alter zinc absorption and homeostasis. The present studies used metallothionein transgenic mice (overexpressing) and metallothionein knockout mice (no expression of metallothionein-1 or metallothionein-2) to examine directly the effects of metallothionein on zinc absorption, independent of secondary effects that could be caused by metallothionein inducers. Zinc absorption was examined by administering a single oral zinc dose (0.5 mmol/kg) by feeding tube to metallothionein transgenic and metallothionein knockout mice and measuring the serum zinc concentration. Two hours after the dose, the serum zinc concentration was 2.3 times higher in metallothionein knockout mice than in their control strain. Conversely, the concentration was elevated only one third as much in the metallothionein transgenic mice as in their controls after the zinc dose. We found that the serum zinc concentration was inversely related to the level of metallothionein protein. The intestinal zinc content was higher in the metallothionein knockout mice, however, suggesting that metallothionein did not reduce zinc absorption by simply sequestering zinc in the mucosa. The expression of the zinc transporter ZnT-1 was directly related to the serum zinc level and was independent of the level of metallothionein. These results further support metallothionein as an important component for reducing the efficiency of zinc absorption at elevated zinc intakes.