The SLC39 family of metal ion transporters

Functional expression of a low-affinity zinc uptake transporter (FrZIP2) from pufferfish (Takifugu rubripes) in MDCK cells

Zinc is a vital micronutrient to all organisms and it is therefore very important to determine the mechanisms that regulate cellular zinc uptake. Previously, we reported on zinc uptake transporters from zebrafish (Danio rerio; DrZIP1) and Fugu pufferfish (Takifugu rubripes; FrZIP1) that facilitated cellular zinc uptake of high affinity (K(m)<0.5 microM) in both CHSE214 [chinook salmon (Oncorhynchus tshawytscha) embryonic 214] cells and Xenopus laevis oocytes. To investigate additional biochemical pathways of zinc uptake in fish, we molecularly cloned the second fish member (FrZIP2) of the SLC39 subfamily II from Fugu pufferfish gill. Functional characterization suggests that FrZIP2 stimulated zinc uptake in a temperature-, time-, concentration- and pH-dependent manner when overexpressed in MDCK cells (Madin-Darby canine kidney cells). In comparison with FrZIP1 and DrZIP1 (<0.5 microM), FrZIP2 appears to represent a low-affinity zinc uptake transporter (K(m)=13.6 microM) in pufferfish. FrZIP2 protein was selective for zinc, but it might also transport Cu2+, since 20 times excess of Cu2+ completely abolished its zinc uptake activity. The zinc uptake by FrZIP2 was stimulated in a slightly acidic medium (pH 5.5-6.5) and was completely blocked at pH 7.5 and above, suggesting that an inward H+ gradient might provide a driving force for zinc transport by FrZIP2. Furthermore, FrZIP2-mediated zinc uptake activity was slightly inhibited by 0.5 mM HCO3-, indicating that FrZIP2 may employ a different mechanism of zinc translocation from the assumed HCO3--coupled zinc transport used by human SLC39A2. The FrZIP2 gene was expressed in all the tissues studied herein, with especially high levels in the ovary and intestines. Thus FrZIP2 may be a prominent zinc uptake transporter of low affinity in many cell types of Fugu pufferfish.

Manganese

Manganese is an essential mineral that is found at low levels in virtually all diets. Manganese ingestion represents the principal route of human exposure, although inhalation also occurs, predominantly in occupational cohorts. Regardless of intake, animals generally maintain stable tissue manganese levels as a result of homeostatic mechanisms that tightly regulate the absorption and excretion of this metal. However, high-dose exposures are associated with increased tissue manganese levels, causing adverse neurological, reproductive and respiratory effects. In humans, manganese-induced neurotoxicity is associated with a motor dysfunction syndrome, commonly referred to as manganism or Parkinsonism, which is of paramount concern and is considered to be one of the most sensitive endpoints. This article focuses on the dosimetry of manganese with special focus on transport mechanisms of manganese into the CNS. It is not intended to be an exhaustive review of the manganese literature; rather it aims to provide a useful synopsis of contemporary studies from which the reader may progress to other research citations as desired. Specific emphasis is directed towards recent published literature on manganese transporters' systemic distribution of manganese upon inhalation exposure as well as the utility of magnetic resonance imaging in quantifying brain manganese distribution.

The Zn2+-transporting pathways in pancreatic beta-cells: a role for the L-type voltage-gated Ca2+ channel

In pancreatic beta-cells Zn(2+) is crucial for insulin biosynthesis and exocytosis. Despite this, little is known about mechanisms of Zn(2+) transport into beta-cells or the regulation and compartmentalization of Zn(2+) within this cell type. Evidence suggests that Zn(2+) in part enters neurons and myocytes through specific voltage-gated calcium channels (VGCC). Using a Zn(2+)-selective fluorescent dye with high affinity and quantum yield, FluoZin-3 AM and the plasma membrane potential dye DiBAC(4)(3) we applied fluorescent microscopy techniques for analysis of Zn(2+)-accumulating pathways in mouse islets, dispersed islet cells, and beta-cell lines (MIN6 and beta-TC6f7 cells). Because the stimulation of insulin secretion is associated with cell depolarization, Zn(2+) (5-10 mum) uptake was analyzed under basal (1 mm glucose) and stimulatory (10-20 mm glucose, tolbutamide, tetraethylammonium, and high K(+)) conditions. Under both basal and depolarized states, beta-cells were capable of Zn(2+) uptake, and switching from basal to depolarizing conditions resulted in a marked increase in the rate of Zn(2+) accumulation. Importantly, L-type VGCC (L-VGCC) blockers (verapamil, nitrendipine, and nifedipine) as well as nonspecific inhibitors of Ca(2+) channels, Gd(3+) and La(3+), inhibited Zn(2+) uptake in beta-cells under stimulatory conditions with little or no change in Zn(2+) accumulation under low glucose conditions. To determine the mechanism of VGCC-independent Zn(2+) uptake the expression of a number of ZIP family Zn(2+) transporter mRNAs in islets and beta-cells was investigated. In conclusion, we demonstrate for the first time that, in part, Zn(2+) transport into beta-cells takes place through the L-VGCC. Our investigation demonstrates direct Zn(2+) accumulation in insulin-secreting cells by two pathways and suggests that the rate of Zn(2+) transport across the plasma membrane is dependent upon the metabolic status of the cell.

Molecular cloning and functional characterization of a high-affinity zinc importer (DrZIP1) from zebrafish (Danio rerio)

Zinc is a vital micronutrient to all organisms and a potential toxicant to aquatic animals. It is therefore of importance to understand the mechanism of zinc regulation. In the present study, we molecularly cloned and functionally characterized a zinc transporter of the SLC39A family [commonly referred to as the ZIP (Zrt- and Irt-related protein) family] from the gill of zebrafish (Danio rerio) (DrZIP1). DrZIP1 protein was found to localize at the plasma membrane and to function as a zinc uptake transporter when being expressed in either chinook salmon (Oncorhynchus tshawytscha) embryonic 214 cells or Xenopus laevis oocytes. In comparison with pufferfish transporter proteins (FrZIP2 and FrECaC) that are known to facilitate cellular zinc uptake, DrZIP1 appears to have high affinity to bind and transport zinc, suggesting that it maybe a high-affinity zinc uptake transporter (Km < 0.5 microM) in fish. Orthologues of DrZIP1 were also identified in both freshwater and seawater pufferfish (Tetraodon nigroviridis and Takifugu rubripes), indicating that these proteins may be functionally conserved among different fish species. DrZIP1 mRNA is expressed in all the tissues examined in the present study and thus DrZIP1 may be a constitutive zinc uptake transporter in many cell types of zebrafish.

Extracellular zinc and ATP-gated P2X receptor calcium entry channels: New zinc receptors as physiological sensors and therapeutic targets

In this review, we focus on two attributes of P2X receptor channel function, one essential and one novel. First, we propose that P2X receptors are extracellular sensors as well as receptors and ion channels. In particular, the large extracellular domain (that comprises 70% of the molecular mass of the receptor channel protein) lends itself to be a cellular sensor. Moreover, its exquisite sensitivity to extracellular pH, ionic strength, and multiple ligands evokes the function of a sensor. Second, we propose that P2X receptors are extracellular zinc receptors as well as receptors for nucleotides. We provide novel data in multiple publications and illustrative data in this invited review to suggest that zinc triggers ATP-independent activation of P2X receptor channel function. In this light, P2X receptors are the cellular site of integration between autocrine and paracrine zinc signaling and autocrine and paracrine purinergic signaling. P2X receptors may sense changes in these ligands as well as in extracellular pH and ionic strength and transduce these sensations via calcium and/or sodium entry and changes in membrane potential.

Acquired cadmium resistance in metallothionein-I/II(-/-) knockout cells: role of the T-type calcium channel Cacnalpha1G in cadmium uptake

Metallothioneins (MTs) are cytoplasmic proteins that sequester certain divalent cations and are considered a primary cellular defense against the toxic transition metal cadmium (Cd(2+)). MT-I/II(-/-) knockout [MT(-/-)] cells are available and serve as an excellent tool to study non-MT-related mechanisms in metal tolerance. In the current study, Cd(2+)-resistant MT(-/-) (CdR) and CdR revertant (CdR-rev) cell lines were developed and characterized to investigate non-MT-mediated cellular protection mechanisms. Resistance to Cd(2+) was approximately 70-fold higher in CdR than the parental MT(-/-) cell line (IC(50) = 20 versus 0.3 microM, respectively) and was stable in the absence of Cd(2+) for 35 days. Accumulation of Cd(2+) by the CdR cell line was reduced by approximately 95% compared with parental cells, primarily because of a decreased Cd(2+) uptake. Cd(2+) uptake by the MT(-/-) parental cell line was independent of sodium, energy, and electrogenic potential. Uptake was saturable (K(m) = 65 nM; V(max) = 4.9 pmol/mg/min) and pH-dependent (maximal at pH 6.5-7). Potent inhibitors of Cd(2+) uptake included Zn(2+) (IC(50) = 7 microM), Mn(2+) (IC(50) = 0.4 microM), and the T-type Ca(2+) channel antagonist mibefradil (IC(50) = 5 microM), whereas other metals (including Fe(2+)) and L-type Ca(2+) channel antagonists had little effect. Immunoblot and real-time reverse transcription-polymerase chain reaction analysis indicated that the Cacnalpha(1G) T-type Ca(2+) channel was expressed at a reduced level in CdR compared with the parental MT(-/-) cell line, suggesting it is important for Cd(2+) uptake. The CdR1-rev cell line was found to have a Cd(2+) uptake and sensitivity level in between that of the CdR1 and MT(-/-) cell lines. Consistent with this was an intermediate expression of Cacnalpha(1G) in the CdR-rev cell line. These data suggest that decreased expression of Cacnalpha(1G) protects cells from Cd(2+) exposure by limiting Cd(2+) uptake.

Two major branches of anti-cadmium defense in the mouse: MTF-1/metallothioneins and glutathione

Metal-responsive transcription factor 1 (MTF-1) regulates expression of its target genes in response to various stress conditions, notably heavy metal load, via binding to metal response elements (MREs) in the respective enhancer/promoter regions. Furthermore, it serves a vital function in embryonic liver development. However, targeted deletion of Mtf1 in the liver after birth is no longer lethal. For this study, Mtf1 conditional knockout mice and control littermates were both mock- or cadmium-treated and liver-specific transcription was analyzed. Besides the well-characterized metallothionein genes, several new MTF-1 target genes with MRE motifs in the promoter region emerged. MTF-1 is required for the basal expression of selenoprotein W, muscle 1 gene (Sepw1) that encodes a glutathione-binding and putative antioxidant protein, supporting a role of MTF-1 in the oxidative stress response. Furthermore, MTF-1 mediates the cadmium-induced expression of N-myc downstream regulated gene 1 (Ndrg1), which is induced by several stress conditions and is overexpressed in many cancers. MTF-1 is also involved in the cadmium response of cysteine- and glycine-rich protein 1 gene (Csrp1), which is implicated in cytoskeletal organization. In contrast, MTF-1 represses the basal expression of Slc39a10, a putative zinc transporter. In a pathway independent of MTF-1, cadmium also induced the transcription of genes involved in the synthesis and regeneration of glutathione, a cadmium-binding antioxidant. These data provide strong evidence for two major branches of cellular anti-cadmium defense, one via MTF-1 and its target genes, notably metallothioneins, the other via glutathione, with an apparent overlap in selenoprotein W.

Evidence for pH dependent Zn2+influx in K562 erythroleukemia cells: studies using ZnAF-2F fluorescence and 65Zn2+ uptake

Using both ZnAF-2F (a Zn2+ specific fluorophore) and 65Zn2+, we determined the rate of transporter mediated Zn2+ influx (presumably mediated by the SLC39A1 gene product, protein name hZIP1) under steady state conditions and studied the effects of extracellular acidification. When K562 erythroleukemia cells were placed in Zn2+ containing buffers (1-60 microM), the initial rate of 65Zn2+ accumulation mirrored the apparent rise in free intracellular Zn2+ concentrations sensed by ZnAF-2F. Therefore, newly transported Zn2+ equilibrated with the free intracellular Zn2+ pool sensed by ZnAF-2F. A new steady state with elevated free intracellular Zn2+ was established after about 30 min. An estimate of 11 microM for the Km and 0.203 nmol/mg/s for the Vmax were obtained for Zn2+ influx. 65Zn2+ uptake and ZnAF-2F fluorescent changes were inhibited by extracellular acidification (range tested: pH 8-6, IC50 = pH 6.34). The IC50 for proton effects was close to the pKa for histidine, suggesting conserved histidine residues present in SLC39A1 play a critical role in Zn2+ influx and are involved in the pH effect.

Expression of the zinc transporter ZIP1 in osteoclasts

Zinc has been previously demonstrated to be a potent inhibitor of osteoclastogenesis and osteoclast function. The mechanisms for cellular uptake of zinc into osteoclasts have not been characterized. We have corroborated previous studies on the reduction of osteoclastogenesis in the presence of extracellular zinc. We demonstrate that osteoclasts express a ubiquitous plasma membrane zinc transporter, namely ZIP1, which was diffusely distributed throughout the cytoplasm. Following an adenoviral-mediated overexpression of ZIP1 in murine osteoclasts, ZIP1 was predominantly colocalized with actin at the sealing zone and significantly inhibited osteoclast function, as assessed by resorptive activity. Finally, overexpression of ZIP1 negatively impacted NF-kappaB binding activity, as assessed by electrophoretic mobility shift assays. In conclusion, these data both corroborate previous studies on regulation of osteoclast formation and activity by zinc and reveal the presence of a zinc uptake mechanism that exerts an important effect on osteoclast activity.

Zinc and zinc transporters in normal prostate and the pathogenesis of prostate cancer

Zinc is an essential metal for all cells. It plays a role in a wide variety of physiological and biochemical processes. In the prostate epithelial cell the accumulation of high cellular zinc is a specialized function that is necessary for these cells to carry out the major physiological functions of production and secretion of citrate. The production of citrate and its secretion into prostatic fluid is a differentiated function of the prostate epithelial cells that is apparently important for reproduction. The loss of citrate and zinc accumulation is the most consistent and persistent characteristic of prostate malignancy. This characteristic of prostate cancer indicates that the lost ability of the malignant cells to accumulate zinc and citrate is an important factor in the development and progression of malignancy. The lost ability of the epithelial cells to accumulate zinc and thus to also accumulate citrate is the result of decreased expression of specific zinc uptake transporters. The purpose of this presentation is to review the current understanding of zinc and zinc homeostasis in the prostate and the role of zinc and zinc transporters in the normal function of the prostate and the pathogenesis of prostate cancer.

ZnT-8, A Pancreatic Beta-Cell-Specific Zinc Transporter

The zinc content in the pancreatic beta cell is among the highest of the body. Zinc appears to be an important metal for insulin-secreting cells as insulin is stored inside secretory vesicles as a solid hexamer bound with two Zn(2+) ions per hexamer. Zinc is also an important component of insulin secretion mechanisms and is likely to modulate the function of neighbouring cells via paracrine/autocrine interactions. Therefore beta cells undoubtedly need very efficient and specialized transporters to accumulate sufficient amounts of zinc in secretion vesicles. We report here the discovery and the characteristics of a new zinc transporter, ZnT-8, belonging to the CDF (Cation Diffusion Facilitator) family and expressed only in pancreatic beta cells. This transporter, localized in secretion vesicles membrane, facilitates the accumulation of zinc from the cytoplasm into intracellular insulin-containing vesicles and is a major component for providing zinc to insulin maturation and/or storage processes in insulin-secreting pancreatic beta cells. We discovered mammalian orthologs (rat, mouse, chimpanzee, and dog) and found these ZnT-8 proteins very similar (98% conserved amino acids) to human ZnT-8, indicating a high conservation during evolution.

Analysis of mechanism for human γδ T cell recognition of nonpeptide antigens

Whereas human gammadelta T cells respond to nonpeptide antigens like pyrophosphomonoesters and alkyl amines in the primary reactions, only pyrophosphomonoesters provoke proliferative responses in the secondary responses. To elucidate the differences in stimulatory activity between the two groups of nonpeptide antigens, we systematically analyzed time courses of gene expressions by microarray analyses. While 253 genes were induced by stimulation with 2-methyl-3-butenyl-1-pyrophosphate (2M3B1PP), only 35 genes were detected after stimulation with isobutyl amine. Then, gammadelta T cells expressed various cytokines like XCL1-2, CCL3-4, TNF-alpha, and IFN-gamma in response to 2M3B1PP in a time-dependent manner, while transient expressions were observed in IBA during the time period. The differences in such responsiveness are likely to originate from the activation state of NFAT, which is involved in the expression of transcription factors, EGR1-3 and NR4A1-2, and might play a crucial role in effector functions of gammadelta T cells.

Zinc deficiency is associated with increased brain zinc import and LIV-1 expression and decreased ZnT-1 expression in neonatal rats

Zinc (Zn) deficiency has been associated with adverse behavioral outcomes in infants and children. However, Zn deficiency does not affect brain Zn concentration, suggesting that brain Zn homeostasis is tightly regulated. The recent identification of Zn-specific transport proteins allowed us to examine effects of low Zn intake on tissue Zn level, brain Zn uptake, and zinc transporter expression and localization in neonatal rat brain. Female rats were fed diets differing only in Zn content [7, moderately zinc deficient (ZD); 10, marginally zinc deficient (MZD); or 25 mg Zn/kg, control] and pups were killed on postnatal d 11. Plasma and brain Zn concentrations were measured, brain Zn uptake was assessed using (65)Zn, brain metallothionein-I and -III; LIV-1, zinc transporter ZnT-1, and ZnT-3 expression was measured by semiquantitative RT-PCR. LIV-1 localization in the brain was determined by immunohistochemistry; brain and hippocampi LIV-1 and ZnT-1 protein expressions were measured by Western blotting. Plasma Zn concentration was lower in MZD and ZD pups, whereas brain Zn concentration was not affected. Brain Zn uptake was higher in MZD and ZD rats compared with controls. Metallothionein-I and ZnT-1 expressions were lower and LIV-1 expression was higher in the whole brain of MZD and ZD pups. Metallothionein-III and ZnT-3 mRNA expressions were not affected. LIV-1 was localized to the plasma membrane of many brain cell types, including hippocampal pyramidal neurons and the apical membrane of the choroid plexus. Our results indicate that Zn deficiency results in alterations in Zn transporter expression, which facilitates increased brain Zn uptake and results in the conservation of brain Zn during Zn deficiency.

Interleukin-6 regulates the zinc transporter Zip14 in liver and contributes to the hypozincemia of the acute-phase response

Infection and inflammation produce systemic responses that include hypozincemia and hypoferremia. The latter involves regulation of the iron transporter ferroportin 1 by hepcidin. The mechanism of reduced plasma zinc is not known. Transcripts of the two zinc transporter gene families (ZnT and Zip) were screened for regulation in mouse liver after turpentine-induced inflammation and LPS administration. Zip14 mRNA was the transporter transcript most up-regulated by inflammation and LPS. IL-6 knockout (IL-6(-/-)) mice did not exhibit either hypozincemia or the induction of Zip14 with turpentine inflammation. However, in IL-6(-/-) mice, LPS produced a milder hypozincemic response but no Zip14 induction. Northern analysis showed Zip14 up-regulation was specific for the liver, with one major transcript. Immunohistochemistry, using an antibody to an extracellular Zip14 epitope, showed both LPS and turpentine increased abundance of Zip14 at the plasma membrane of hepatocytes. IL-6 produced increased expression of Zip14 in primary hepatocytes cultures and localization of the protein to the plasma membrane. Transfection of mZip14 cDNA into human embryonic kidney cells increased zinc uptake as measured by both a fluorescent probe for free Zn(2+) and (65)Zn accumulation, as well as by metallothionein mRNA induction, all indicating that Zip14 functions as a zinc importer. Zip14 was localized in plasma membrane of the transfected cells. These in vivo and in vitro experiments demonstrate that Zip14 expression is up-regulated through IL-6, and that this zinc transporter most likely plays a major role in the mechanism responsible for hypozincemia that accompanies the acute-phase response to inflammation and infection.

Perinatal omega-3 polyunsaturated fatty acid supply modifies brain zinc homeostasis during adulthood

Dietary omega-3 polyunsaturated fatty acid (PUFA) influences the expression of a number of genes in the brain. Zinc transporter (ZnT) 3 has been identified as a putative transporter of zinc into synaptic vesicles of neurons and is found in brain areas such as hippocampus and cortex. Neuronal zinc is involved in the formation of amyloid plaques, a major characteristic of Alzheimer's disease. The present study evaluated the influence of dietary omega-3 PUFA on the expression of the ZnT3 gene in the brains of adult male Sprague-Dawley rats. The rats were raised and/or maintained on a control (CON) diet that contained omega-3 PUFA or a diet deficient (DEF) in omega-3 PUFA. ZnT3 gene expression was analyzed by using real-time PCR, free zinc in brain tissue was determined by zinquin staining, and total zinc concentrations in plasma and cerebrospinal fluid were determined by atomic absorption spectrophotometry. Compared with CON-raised animals, DEF-raised animals had increased expression of ZnT3 in the brain that was associated with an increased level of free zinc in the hippocampus. In addition, compared with CON-raised animals, DEF-raised animals had decreased plasma zinc level. No difference in cerebrospinal fluid zinc level was observed. The results suggest that overexpression of ZnT3 due to a perinatal omega-3 PUFA deficiency caused abnormal zinc metabolism in the brain. Conceivably, the influence of dietary omega-3 PUFA on brain zinc metabolism could explain the observation made in population studies that the consumption of fish is associated with a reduced risk of dementia and Alzheimer's disease.

Zip3 plays a major role in zinc uptake into mammary epithelial cells and is regulated by prolactin

During lactation, a substantial amount of Zn2+ is transferred by the mammary gland from the maternal circulation into milk; thus secretory mammary epithelial cells must tightly regulate Zn2+ transport to ensure optimal Zn2+ transfer to the suckling neonate. To date, six Zn2+ import proteins (Zip1–6) have been identified; however, Zip3 expression is restricted to tissues with unique requirements for Zn2+, such as the mammary gland, which suggests that it may play a specialized role in this tissue. In the present study, we have used a unique mammary epithelial cell model (HC11) to characterize the role of Zip3 in mammary epithelial cell Zn2+ transport. Confocal microscopy demonstrated that Zip3 is localized to the cell surface in mammary epithelial cells and transiently relocalized to an intracellular compartment in cells with a secretory phenotype. Total 65Zn transport was higher in secreting cells, while gene silencing of Zip3 decreased 65Zn uptake into mammary epithelial cells, particularly in those with a secretory phenotype. Finally, reduced expression of Zip3 ultimately resulted in cell death, indicating that mammary epithelial cells have a unique requirement for Zip3-mediated Zn2+ import, which may reflect the unique requirement for Zn2+ of this highly specialized cell type and thus provides a physiological explanation for the restricted tissue distribution of this Zn2+ importer.

Physical interaction and functional coupling between ACDP4 and the intracellular ion chaperone COX11, an implication of the role of ACDP4 in essential metal ion transport and homeostasis

Divalent metal ions such as copper, manganese, and cobalt are essential for cell development, differentiation, function and survival. These essential metal ions are delivered into intracellular domains as cofactors for enzymes involved in neuropeptide and neurotransmitter synthesis, superoxide metabolism, and other biological functions in a target specific fashion. Altering the homeostasis of these essential metal ions is known to connect to a number of human diseases including Alzheimer disease, amyotrophic lateral sclerosis, and pain. It remains unclear how these essential metal ions are delivered to intracellular targets in mammalian cells. Here we report that rat spinal cord dorsal horn neurons express ACDP4, a member of Ancient Conserved Domain Protein family. By screening a pretransformed human fetal brain cDNA library in a yeast two-hybrid system, we have identified that ACDP4 specifically interacts with COX11, an intracellular metal ion chaperone. Ectopic expression of ACDP4 in HEK293 cells resulted in enhanced toxicity to metal ions including copper, manganese, and cobalt. The metal ion toxicity became more pronounced when ACDP4 and COX11 were co-expressed ectopically in HEK293 cells, suggesting a functional coupling between them. Our results indicate a role of ACDP4 in metal ion homeostasis and toxicity. This is the first report revealing a functional aspect of this ancient conserved domain protein family. We propose that ACDP is a family of transporter protein or chaperone proteins for delivering essential metal ions in different mammalian tissues. The expression of ACDP4 on spinal cord dorsal horn neurons may have implications in sensory neuron functions under physiological and pathological conditions.

Identification and characterization of a novel mammalian Mg2+ transporter with channel-like properties

BACKGROUND

Intracellular magnesium is abundant, highly regulated and plays an important role in biochemical functions. Despite the extensive evidence for unique mammalian Mg2+ transporters, few proteins have been biochemically identified to date that fulfill this role. We have shown that epithelial magnesium conservation is controlled, in part, by differential gene expression leading to regulation of Mg2+ transport. We used this knowledge to identify a novel gene that is regulated by magnesium.

RESULTS

Oligonucleotide microarray analysis was used to identify a novel human gene that encodes a protein involved with Mg2+-evoked transport. We have designated this magnesium transporter (MagT1) protein. MagT1 is a novel protein with no amino acid sequence identity to other known transporters. The corresponding cDNA comprises an open reading frame of 1005 base pairs encoding a protein of 335 amino acids. It possesses five putative transmembrane (TM) regions with a cleavage site, a N-glycosylation site, and a number of phosphorylation sites. Based on Northern analysis of mouse tissues, a 2.4 kilobase transcript is present in many tissues. When expressed in Xenopus laevis oocytes, MagT1 mediates saturable Mg2+ uptake with a Michaelis constant of 0.23 mM. Transport of Mg2+ by MagT1 is rheogenic, voltage-dependent, does not display any time-dependent inactivation. Transport is very specific to Mg2+ as other divalent cations did not evoke currents. Large external concentrations of some cations inhibited Mg2+ transport (Ni2+, Zn2+, Mn2+) in MagT1-expressing oocytes. Ca2+and Fe2+ were without effect. Real-time reverse transcription polymerase chain reaction and Western blot analysis using a specific antibody demonstrated that MagT1 mRNA and protein is increased by about 2.1-fold and 32%, respectively, in kidney epithelial cells cultured in low magnesium media relative to normal media and in kidney cortex of mice maintained on low magnesium diets compared to those animals consuming normal diets. Accordingly, it is apparent that an increase in mRNA levels is translated into higher protein expression.

CONCLUSION

These studies suggest that MagT1 may provide a selective and regulated pathway for Mg2+ transport in epithelial cells.

SLC39A14, a LZT protein, is induced in adipogenesis and transports zinc

During adipocyte differentiation, there is an underlying complex series of gene expressions. We have previously isolated many genes whose expression levels are quickly elevated by the addition of inducers to mouse 3T3-L1 preadipocyte cells. Here we report the isolation and characterization of SLC39A14, a member of the LZT proteins, one of the subfamilies of ZIP transporters. The expression of the SLC39A14 gene was strongly and rapidly induced at the early stages of differentiation. Moreover, it was highly restricted to the potential differentiation state of 3T3-L1 cells and the expression level was quite low in the nonadipogenic NIH-3T3 cells, indicating a dominant expression in adipocyte differentiation. The zinc uptake assay revealed that SLC39A14 functions as a zinc transporter. Taken together, these results suggest that SLC39A14 plays a role as a zinc transporter during the early stages of adipogenesis.

Identification of mouse SLC39A8 as the transporter responsible for cadmium-induced toxicity in the testis

Testicular necrosis is a sensitive endpoint for cadmium (Cd(2+), Cd) toxicity across all species tested. Resistance to Cd-induced testicular damage is a recessive trait assigned to the Cdm locus on mouse chromosome 3. We first narrowed the Cdm-gene-containing region to 880 kb. SNP analysis of this region from two sensitive and two resistant inbred strains demonstrated a 400-kb haplotype block consistent with the Cd-induced toxicity phenotype; in this region is the Slc39a8 gene encoding a member of the solute-carrier superfamily. Slc39a8 encodes SLC39A8 (ZIP8), whose homologs in plant and yeast are putative zinc transporters. We show here that ZRT-, IRT-like protein (ZIP)8 expression in cultured mouse fetal fibroblasts leads to a >10-fold increase in the rate of intracellular Cd influx and accumulation and 30-fold increase in sensitivity to Cd-induced cell death. The complete ZIP8 mRNA and intron-exon splice junctions have no nucleotide differences between two sensitive and two resistant strains of mice; by using situ hybridization, we found that ZIP8 mRNA is prominent in the vascular endothelial cells of the testis of the sensitive strains of mice but absent in these cells of resistant strains. Slc39a8 is therefore the Cdm gene, defining sensitivity to Cd toxicity specifically in vascular endothelial cells of the testis.

Characterization of a Glomus intraradices gene encoding a putative Zn transporter of the cation diffusion facilitator family

A full-length cDNA (GintZnT1) encoding a putative Zn transporter was isolated from the extraradical mycelium of Glomus intraradices. Based on its sequence analysis, GintZnT1 was classified as a member of the cation diffusion facilitator (CDF) family of heavy metal transporters. Functional analysis of GintZnT1 was performed by heterologous expression in yeast mutants defective in different CDFs. Although Zn sensitivity of the mutants was not reverted, an effect of GintZnT1 on the labile regulatory Zn pool was detected by using a Zn-regulated beta-galactosidase reporter gene. GintZnT1 expression was studied in the extraradical mycelium obtained from a symbiotic root organ culture. Gin +/- ZnT1 was up-regulated in the extraradical mycelium of G. intraradices upon short-time exposure to Zn and when the mycelia were developed in 75 microM Zn supplemented plates. These data suggest a role of GintZnT1 in Zn compartmentalization and in the protection of G. intraradices against Zn stress.

Zinc Transporters, ZnT5 and ZnT7, Are Required for the Activation of Alkaline Phosphatases, Zinc-requiring Enzymes That Are Glycosylphosphatidylinositol-anchored to the Cytoplasmic Membrane

Numerous proteins are properly folded by binding with zinc during their itinerary in the biosynthetic-secretory pathway. Several transporters have been implicated in the zinc entry into secretory compartments from cytosol, but their precise roles are poorly understood. We report here that two zinc transporters (ZnT5 and ZnT7) localized in the secretory apparatus are responsible for loading zinc to alkaline phosphatases (ALPs) that are glycosylphosphatidylinositol-anchored membrane proteins exposed to the extracellular site. Disruption of the ZnT5 gene in DT40 cells decreased the ALP activity to 45% of that in the wild-type cells. Disruption of the ZnT7 gene lowered the ALP activity only by 20%. Disruption of both genes markedly decreased the ALP activity to <5%. Overexpression of human ZnT5 or ZnT7 in DT40 cells deficient in both ZnT5 and ZnT7 genes recovered the ALP activity to the level comparable to that in the wild-type cells. The inactive ALP protein in DT40 cells deficient in both ZnT5 and ZnT7 genes was transported to cytoplasmic membrane like the active ALP protein in the wild-type cells. Thus both ZnT5 and ZnT7 contribute to the conversion of apo-ALP to holo-ALP.

The Mammalian Zip5 Protein Is a Zinc Transporter That Localizes to the Basolateral Surface of Polarized Cells

The mouse and human Zip5 proteins are members of the ZIP family of metal ion transporters. In this study, we present evidence that mouse Zip5 is a zinc uptake transporter that is specific for Zn(II) over other potential metal ion substrates. We also show that, unlike many other mammalian ZIP proteins, the endocytic removal of mZip5 from the plasma membrane is not triggered by zinc treatment. Thus, the activity of mZip5 does not appear to be down-regulated by zinc repletion. Zip5 expression is restricted to many tissues important for zinc homeostasis, including the intestine, pancreas, liver, and kidney. Zip5 is similar in sequence to the Zip4 protein, which is involved in the uptake of dietary zinc. Co-expression of Zip4 and Zip5 in the intestine led to the hypothesis that these proteins play overlapping roles in the uptake of dietary zinc across the apical membrane of intestinal enterocytes. Surprisingly, however, we found that mZip5 localizes specifically to the basolateral membrane of polarized Madin-Darby canine kidney cells. These observations suggest that Zip5 plays a novel role in polarized cells by carrying out serosal-to-mucosal zinc transport. Furthermore, given its expression in tissues important to zinc homeostasis, we propose that Zip5 plays a central role in controlling organismal zinc status.

Exchangeable zinc ions transiently accumulate in a vesicular compartment in the yeast Saccharomyces cerevisiae

The baker's yeast Saccharomyces cerevisiae was used as a model to visualize intracellular labile zinc under conditions of nutritional zinc imbalance. Zinc-specific staining was performed in yeast cells using both Zinquin fluorescence and zinc-selenium autometallography. Both techniques resulted in specific labeling of an intracellular vesicular compartment that was present in wild type cells as well as in the vacuolar Zn transporter mutants Deltazrc1 and Deltacot1. This compartment, that closely resembles mammalian zincosomes, appeared rapidly under conditions of zinc availability and was independent of endocytosis. However, persistence of the zinc loaded vesicles in nutritional zinc deficiency was dependent on the presence of functional Zrc1 and Cot1 vacuolar transporters. Overall our findings indicate that labile zinc in yeast cells enters a dynamic vesicular compartment which could represent an extremely important defence to buffer both zinc excess and deficiency.

Differential regulation of zinc efflux transporters ZnT-1, ZnT-5 and ZnT-7 gene expression by zinc levels: a real-time RT–PCR study

Intracellular zinc levels are strictly regulated by zinc channels and zinc-binding proteins to maintain cellular zinc-dependent functions. We demonstrated a correlation between extracellular zinc concentration and intracellular exchangeable zinc levels using the fluorescent zinc-specific probes zinquin and zinpyr-1. The effect of extracellular zinc status on the regulation of the two trans-Golgi network directed zinc transporters ZnT-5 and ZnT-7 was next studied by real-time RT-PCR in zinc supplemented or depleted HeLa cells. While sub-toxic extracellular zinc addition strongly induced the efflux transporter ZnT-1 gene expression, consistent with its activation by the transcription factor MTF-1, treated HeLa cells did not display any change in ZnT-5 and ZnT-7 mRNA levels compared to control cells. In contrast, zinc depletion induced by non-toxic doses of the zinc chelator TPEN (N,N,N',N' tetrakis-(2 pyridylmethyl) ethylene diamine) resulted in a up to eight-fold induction of transporters ZnT-5 and ZnT-7 mRNA levels, providing the first evidence of a transcriptional control of these two zinc efflux transporters by zinc deficiency in cultured cells.

Zinc metabolism in adolescents with Crohn's disease

Low serum zinc concentrations have been reported in Crohn's disease (CD) and overt zinc deficiency has been described, but little is known about the effect of CD on zinc metabolism in adolescents. The aim of this study was to measure zinc absorption, endogenous fecal zinc excretion, urinary zinc excretion, and zinc balance in children with stable CD and in matched controls. Subjects were 15 children, ages 8-18 y, with stable CD, and 15 healthy matched controls. Subjects were adapted to diets providing 12 mg/d elemental zinc for 2 wk, and then admitted for a 6-d metabolic study. Stable zinc isotopes were given intravenously and orally, and urine and feces collected for 6 d. Fractional zinc absorption, endogenous fecal zinc excretion, and zinc balance were calculated using established stable isotope methods. In subjects with CD, zinc absorption (10.9% +/- 6.1 versus 23.4 +/- 15.8, p = 0.008) and plasma zinc concentration (0.85 mg/dL +/- 0.15 versus 1.25 +/- 0.35, p = 0.004) were significantly reduced, compared with controls. Despite this, there were no significant differences in endogenous fecal zinc excretion (2.0 mg +/- 1.5 versus 1.5 +/- 1.5, p = 0.34) or urinary zinc excretion (0.9 mg +/- 0.7 versus 1.0 +/- 0.7, p = 0.47). Zinc balance was significantly lower in CD (-1.5 mg +/- 1.5) than in controls (+0.6 mg +/- 3.1, p < 0.0001). In conclusion, adolescents with CD have significantly reduced zinc absorption. Despite this, they were unable to reduce endogenous fecal zinc excretion to restore normal zinc balance and had a significantly worse zinc balance and lower plasma zinc concentration than controls.

Investigation of lymphocyte gene expression for use as biomarkers for zinc status in humans

A bioassay for zinc status in humans has been sought due to the importance of zinc, an essential trace metal, for many divergent functions in the human body; however, a sensitive bioassay for zinc status in humans is lacking. To address this issue, we established gene expression profiles of human lymphoblastoid cells treated with 0 or 30 micro mol/L ZnSO(4) using microarray technology. A limited number of genes were responsive to 30 micro mol/L zinc based on the analysis of Affymetrix human genome U133A GeneChips. We also examined the gene expression patterns of zinc transporters in human lymphoblastoid cells using quantitative RT-PCR analysis. ZNT1 was upregulated in lymphoblastoid cells, whereas ZIP1 was downregulated in response to the increased zinc concentrations in the culture media. To evaluate the potential applications of using both zinc transporter genes as biomarkers of zinc status, we measured the expression levels of ZIP1 and ZNT1 in the peripheral leukocytes collected from 2 different age groups of Korean women. After administration of a zinc supplement (22 mg zinc gluconate/d for 27 d), ZIP1 expression decreased by 17% (P < 0.01) and 21% (P < 0.05) in the peripheral leukocytes collected from 15 young (20-25 y) and 10 elderly (64-75 y) subjects, respectively. ZNT1 expression was not affected by taking the zinc supplement. These data suggest a potential application of ZIP1 as a biomarker of zinc status in humans.

In silico identification and expression of SLC30 family genes: an expressed sequence tag data mining strategy for the characterization of zinc transporters' tissue expression

Background

Intracellular zinc concentration and localization are strictly regulated by two main protein components, metallothioneins and membrane transporters. In mammalian cells, two membrane transporters family are involved in intracellular zinc homeostasis: the uptake transporters called SLC39 or Zip family and the efflux transporters called SLC30 or ZnT family. ZnT proteins are members of the cation diffusion facilitator (CDF) family of metal ion transporters.

Results

From genomic databanks analysis, we identified the full-length sequences of two novel SLC30 genes, SLC30A8 and SLC30A10, extending the SLC30 family to ten members. We used an expressed sequence tag (EST) data mining strategy to determine the pattern of ZnT genes expression in tissues. In silico results obtained for already studied ZnT sequences were compared to experimental data, previously published. We determined an overall good correlation with expression pattern obtained by RT-PCR or immunomethods, particularly for highly tissue specific genes.

Conclusion

The method presented herein provides a useful tool to complete gene families from sequencing programs and to produce preliminary expression data to select the proper biological samples for laboratory experimentation.