L-type Ca(2+) channel-mediated Zn(2+) toxicity and modulation by ZnT-1 in PC12 cells

Zinc, Copper, and Calcium: A Triangle in the Synapse for the Pathogenesis of Vascular-Type Senile Dementia

Zinc (Zn) and copper (Cu) are essential for normal brain functions. In particular, Zn and Cu are released to synaptic clefts during neuronal excitation. Synaptic Zn and Cu regulate neuronal excitability, maintain calcium (Ca) homeostasis, and play central roles in memory formation. However, in pathological conditions such as transient global ischemia, excess Zn is secreted to synaptic clefts, which causes neuronal death and can eventually trigger the pathogenesis of a vascular type of senile dementia. We have previously investigated the characteristics of Zn-induced neurotoxicity and have demonstrated that low concentrations of Cu can exacerbate Zn neurotoxicity. Furthermore, during our pharmacological approaches to clarify the molecular pathways of Cu-enhanced Zn-induced neurotoxicity, we have revealed the involvement of Ca homeostasis disruption. In the present review, we discuss the roles of Zn and Cu in the synapse, as well as the crosstalk between Zn, Cu, and Ca, which our study along with other recent studies suggest may underlie the pathogenesis of vascular-type senile dementia.

Distribution of Iron, Copper, Zinc and Cadmium in Glia, Their Influence on Glial Cells and Relationship with Neurodegenerative Diseases

Recent data on the distribution and influence of copper, zinc and cadmium in glial cells are summarized. This review also examines the relationship between those metals and their role in neurodegenerative diseases like Alzheimer disease, multiple sclerosis, Parkinson disease and Amyotrophic lateral sclerosis, which have become a great challenge for today's physicians. The studies suggest that among glial cells, iron has the highest concentration in oligodendrocytes, copper in astrocytes and zinc in the glia of hippocampus and cortex. Previous studies have shown neurotoxic effects of copper, iron and manganese, while zinc can have a bidirectional effect, i.e., neurotoxic but also neuroprotective effects depending on the dose and disease state. Recent data point to the association of metals with neurodegeneration through their role in the modulation of protein aggregation. Metals can accumulate in the brain with aging and may be associated with age-related diseases.

The Toxoplasma plant-like vacuolar compartment (PLVAC)

Toxoplasma gondii belongs to the phylum Apicomplexa and is an important cause of congenital disease and infection in immunocompromised patients. T. gondii shares several characteristics with plants including a nonphotosynthetic plastid termed apicoplast and a multivesicular organelle that was named the plant-like vacuole (PLV) or vacuolar compartment (VAC). The name plant-like vacuole was selected based on its resemblance in composition and function to plant vacuoles. The name VAC represents its general vacuolar characteristics. We will refer to the organelle as PLVAC in this review. New findings in recent years have revealed that the PLVAC represents the lysosomal compartment of T. gondii which has adapted peculiarities to fulfill specific Toxoplasma needs. In this review, we discuss the composition and functions of the PLVAC highlighting its roles in ion storage and homeostasis, endocytosis, exocytosis, and autophagy.

Excessive Zinc Ion Caused PC12 Cell Death Correlating with Inhibition of NOS and Increase of RAGE in Cells

Zinc ion (Zn²⁺) is an important functional factor; however, excessive Zn²⁺ can be toxic. To understand the neurotoxicity of excessive Zn²⁺ and the underlying mechanism, PC12 cells were treated with excessive Zn²⁺ and Zn²⁺ plus N, N, N', N'-Tetrakisethylenediamine (TPEN), a zinc ion chelator agent. Trypan blue and 3-(4,5-dimethyl-2- thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide, thiazolyl blue tetrazolium bromide (MTT) assays were used to test cell viability; the relative kits were used to detect the activity of NOS synthase and the content of the receptor for advanced glycation end product (RAGE) in cells. We observed that excessive zinc caused PC12 cell damage and that TPEN partially reversed cell damage caused by excessive zinc. In addition, excessive zinc decreased total nitric oxide synthase (TNOS) activity in cells, in which constitutive nitric oxide synthase (cNOS) activity was significantly reduced; however, inducible nitric oxide synthase (iNOS) activity was extremely promoted. Moreover, excessive zinc upregulated the expression of RAGE, and TPEN effectively reversed the increase in RAGE induced by excessive zinc ions. Therefore, we concluded that excessive zinc caused PC12 cell damage, correlating with the inhibition of NOS and increase of RAGE induced in cells.

Neuronal signalling of zinc: from detection and modulation to function

Zinc is an essential trace element that stabilizes protein structures and allosterically modulates a plethora of enzymes, ion channels and neurotransmitter receptors. Labile zinc (Zn²⁺) acts as an intracellular and intercellular signalling molecule in response to various stimuli, which is especially important in the central nervous system. Zincergic neurons, characterized by Zn²⁺ deposits in synaptic vesicles and presynaptic Zn²⁺ release, are found in the cortex, hippocampus, amygdala, olfactory bulb and spinal cord. To provide an overview of synaptic Zn²⁺ and intracellular Zn²⁺ signalling in neurons, the present paper summarizes the fluorescent sensors used to detect Zn²⁺ signals, the cellular mechanisms regulating the generation and buffering of Zn²⁺ signals, as well as the current perspectives on their pleiotropic effects on phosphorylation signalling, synapse formation, synaptic plasticity, as well as sensory and cognitive function.

Crosstalk of copper and zinc in the pathogenesis of vascular dementia

Copper and zinc are essential for normal brain functions. Both are localized in presynaptic vesicles and are secreted into synaptic clefts during neuronal excitation. Despite their significance, excesses of copper and zinc are neurotoxic. In particular, excess zinc after transient global ischemia plays a central role in the ischemia-induced neurodegeneration and pathogenesis of vascular type senile dementia. We previously found that sub-lethal concentrations of copper remarkably exacerbated zinc-induced neurotoxicity, and we investigated the molecular pathways of copper-enhanced zinc-induced neurotoxicity. The endoplasmic reticulum stress pathway, the stress-activated protein kinases/c-‍Jun amino-terminal kinases pathway, and mitochondrial energy production failure were revealed to be involved in the neurodegenerative processes. Regarding the upstream factors of these pathways, we focused on copper-derived reactive oxygen species and the disruption of calcium homeostasis. Because excess copper and zinc may be present in the synaptic clefts during ischemia, it is possible that secreted copper and copper-induced reactive oxygen species may enhance zinc neurotoxicity and eventually contribute to the pathogenesis of vascular type senile dementia.

Copper as a Collaborative Partner of Zinc-Induced Neurotoxicity in the Pathogenesis of Vascular Dementia

Copper is an essential trace element and possesses critical roles in various brain functions. A considerable amount of copper accumulates in the synapse and is secreted in neuronal firings in a manner similar to zinc. Synaptic copper and zinc modulate neuronal transmission and contribute to information processing. It has been established that excess zinc secreted during transient global ischemia plays central roles in ischemia-induced neuronal death and the pathogenesis of vascular dementia. We found that a low concentration of copper exacerbates zinc-induced neurotoxicity, and we have demonstrated the involvement of the endoplasmic reticulum (ER) stress pathway, the stress-activated protein kinases/c-Jun amino-terminal kinases (SAPK/JNK) signaling pathway, and copper-induced reactive oxygen species (ROS) production. On the basis of our results and other studies, we discuss the collaborative roles of copper in zinc-induced neurotoxicity in the synapse and the contribution of copper to the pathogenesis of vascular dementia.

Carnosine as a Possible Drug for Zinc-Induced Neurotoxicity and Vascular Dementia

Increasing evidence suggests that the metal homeostasis is involved in the pathogenesis of various neurodegenerative diseases including senile type of dementia such as Alzheimer’s disease, dementia with Lewy bodies, and vascular dementia. In particular, synaptic Zn²⁺ is known to play critical roles in the pathogenesis of vascular dementia. In this article, we review the molecular pathways of Zn²⁺-induced neurotoxicity based on our and numerous other findings, and demonstrated the implications of the energy production pathway, the disruption of calcium homeostasis, the production of reactive oxygen species (ROS), the endoplasmic reticulum (ER)-stress pathway, and the stress-activated protein kinases/c-Jun amino-terminal kinases (SAPK/JNK) pathway. Furthermore, we have searched for substances that protect neurons from Zn²⁺-induced neurotoxicity among various agricultural products and determined carnosine (β-alanyl histidine) as a possible therapeutic agent for vascular dementia.

The Vacuolar Zinc Transporter TgZnT Protects Toxoplasma gondii from Zinc Toxicity

Zinc (Zn2+) is the most abundant biological metal ion aside from iron and is an essential element in numerous biological systems, acting as a cofactor for a large number of enzymes and regulatory proteins. Zn2+ must be tightly regulated, as both the deficiency and overabundance of intracellular free Zn2+ are harmful to cells. Zn2+ transporters (ZnTs) play important functions in cells by reducing intracellular Zn2+ levels by transporting the ion out of the cytoplasm. We characterized a Toxoplasma gondii gene (TgGT1_251630, TgZnT), which is annotated as the only ZnT family Zn2+ transporter in T. gondii TgZnT localizes to novel vesicles that fuse with the plant-like vacuole (PLV), an endosome-like organelle. Mutant parasites lacking TgZnT exhibit reduced viability in in vitro assays. This phenotype was exacerbated by increasing zinc concentrations in the extracellular media and was rescued by media with reduced zinc. Heterologous expression of TgZnT in a Zn2+-sensitive Saccharomyces cerevisiae yeast strain partially restored growth in media with higher Zn2+ concentrations. These results suggest that TgZnT transports Zn2+ into the PLV and plays an important role in the Zn2+ tolerance of T. gondii extracellular tachyzoites.IMPORTANCEToxoplasma gondii is an intracellular pathogen of human and animals. T. gondii pathogenesis is associated with its lytic cycle, which involves invasion, replication, egress out of the host cell, and invasion of a new one. T. gondii must be able to tolerate abrupt changes in the composition of the surrounding milieu as it progresses through its lytic cycle. We report the characterization of a Zn2+ transporter of T. gondii (TgZnT) that is important for parasite growth. TgZnT restored Zn2+ tolerance in yeast mutants that were unable to grow in media with high concentrations of Zn2+ We propose that TgZnT plays a role in Zn2+ homeostasis during the T. gondii lytic cycle.

Protective effects of voltage-gated calcium channel antagonists against zinc toxicity in SN56 neuroblastoma cholinergic cells

One of the pathological site effects in excitotoxic activation is Zn²⁺ overload to postsynaptic neurons. Such an effect is considered to be equivalent to the glutamate component of excitotoxicity. Excessive uptake of Zn²⁺ by active voltage-dependent transport systems in these neurons may lead to significant neurotoxicity. The aim of this study was to investigate whether and which antagonists of the voltage gated calcium channels (VGCC) might modify this Zn²⁺-induced neurotoxicity in neuronal cells. Our data demonstrates that depolarized SN56 neuronal cells may take up large amounts of Zn²⁺ and store these in cytoplasmic and mitochondrial sub-fractions. The mitochondrial Zn²⁺ excess suppressed pyruvate uptake and oxidation. Such suppression was caused by inhibition of pyruvate dehydrogenase complex, aconitase and NADP-isocitrate dehydrogenase activities, resulting in the yielding of acetyl-CoA and ATP shortages. Moreover, incoming Zn²⁺ increased both oxidized glutathione and malondialdehyde levels, known parameters of oxidative stress. In depolarized SN56 cells, nifedipine treatment (L-type VGCC antagonist) reduced Zn²⁺ uptake and oxidative stress. The treatment applied prevented the activities of PDHC, aconitase and NADP-IDH enzymes, and also yielded the maintenance of acetyl-CoA and ATP levels. Apart from suppression of oxidative stress, N- and P/Q-type VGCCs presented a similar, but weaker protective influence. In conclusion, our data shows that in the course of excitotoxity, impairment to calcium homeostasis is tightly linked with an excessive neuronal Zn²⁺ uptake. Hence, the VGCCs types L, N and P/Q share responsibility for neuronal Zn²⁺ overload followed by significant energy-dependent neurotoxicity. Moreover, Zn²⁺ affects the target tricarboxylic acid cycle enzymes, yields acetyl-CoA and energy deficits as well.

Zinc transport and the inhibition of the L-type calcium channel are two separable functions of ZnT-1

Traditionally, proteins are considered to perform a single role, be it as an enzyme, a channel, a transporter or as a structural scaffold. However, recent studies have described moonlighting proteins that perform distinct and independent functions; for example, TRPM7 is both an ion channel and a kinase. ZnT-1 is a member of the Carrier Diffusion Facilitator family that is expressed throughout the phylogenetic tree from bacteria to humans. Since its cloning in 1995, ZnT-1 is considered a major extruder of Zn²⁺ based on its capability to protect cells against zinc toxicity. Recently, we reported that ZnT-1 inhibits the L-type calcium channel (LTCC), a major Zn²⁺ and Ca²⁺ entry pathway. Here we show that ZnT-1 is a dual-function protein by demonstrating that its abilities to exchange Zn²⁺/H⁺ and to inhibit the LTCC are independent of each other and are mediated by different parts of the protein. Specifically, mutations in the membrane-spanning helices that render ZnT-1 unable to transport zinc do not prevent it from inhibiting the LTCC. Moreover, a fragment consisting of the intracellular ZnT-1 C-terminal, which lacks all ion-transfer segments, inhibits the LTCC as efficiently as wild-type ZnT-1. Our data therefore indicates that ZnT-1 performs two structurally independent functions related to zinc homeostasis.

细胞内锌稳态调控及其在胰腺癌发生发展过程中的作用

锌参与300种以上的细胞生理过程, 包括DNA及蛋白合成、酶的活化以及细胞内信号转导过程. 同时锌也是许多酶类, 如碳酸酐酶、基质金属蛋白酶的关键组分, 而这些酶类与缺氧、血管生成、细胞增殖及肿瘤转移密切相关, 因此, 锌的获取对于恶性肿瘤的生长和进展非常重要. 细胞内锌离子的浓度变化受到ZnT1、ZIP4、金属硫蛋白及金属转录因子1等的调控, 细胞内锌稳态是多种调控机制参与下锌内流、锌外流和保留之间动态平衡的结果. 已有大量研究证实锌稳态调控失衡与胰腺癌的发生和转移有关, 因此, 锌及锌稳态的调控异常在胰腺癌的发生发展中具有重要作用.

Copper Enhances Zinc-Induced Neurotoxicity and the Endoplasmic Reticulum Stress Response in a Neuronal Model of Vascular Dementia

Zinc (Zn), an essential trace element, is secreted by synaptic vesicles during neuronal excitation and plays several critical roles in neuronal information processing. However, excess Zn ion (Zn²⁺) is neurotoxic and has a causative role in the pathogenesis of vascular dementia. Here, we investigated the molecular mechanism of Zn²⁺-induced neurotoxicity by using immortalized hypothalamic neurons (GT1-7 cells), which are more vulnerable than other neuronal cells to Zn²⁺. We examined the effects of other metal ions on the Zn²⁺-induced neurotoxicity in these cells and found that sub-lethal concentrations of copper ion (Cu²⁺) markedly exacerbated Zn²⁺-induced neurotoxicity. The co-administration of Cu²⁺ and Zn²⁺ also significantly increased the expression of genes related to the endoplasmic reticulum's stress response, including CHOP, GADD34, and ATF4. Similar to Zn²⁺, Cu²⁺ is stored in presynaptic vesicles and secreted during neuronal excitation. Thus, based on our results, we hypothesize here that Cu²⁺ interacts with Zn²⁺ in the synapse to synergistically promote neuronal death and significantly influence the pathogenesis of vascular dementia.

What can flies tell us about zinc homeostasis?

Zinc is an essential micronutrient for all organisms. For multicellular organisms, zinc uptake, storage, distribution and export are tightly regulated at both cellular and organismal levels, to cope with the multiple requirements versus the toxicity of the metal ion. During the past decade, the fruit fly Drosophila melanogaster has become an important model organism for the elucidation of metazoan zinc homeostasis. This review describes our current knowledge of various zinc transporters in Drosophila, with an emphasis on the process of dietary zinc uptake in the fly. We also discuss how Drosophila was used as a model to facilitate our understanding of the role of zinc in neurodegenerative diseases.

Zinc Modulates Nanosilver-Induced Toxicity in Primary Neuronal Cultures

Silver nanoparticles (NAg) have recently become one of the most commonly used nanomaterials. Since the ability of nanosilver to enter the brain has been confirmed, there has been a need to investigate mechanisms of its neurotoxicity. We previously showed that primary neuronal cultures treated with nanosilver undergo destabilization of calcium homeostasis via a mechanism involving glutamatergic NMDA receptors. Considering the fact that zinc interacts with these receptors, the aim of the present study was to examine the role of zinc in mechanisms of neuronal cell death in primary cultures. In cells treated with nanosilver, we noted an imbalance between extracellular and intracellular zinc levels. Thus, the influence of zinc deficiency and supplementation on nanosilver-evoked cytotoxicity was investigated by treatment with TPEN (a chelator of zinc ions), or ZnCl₂, respectively. Elimination of zinc leads to complete death of nanosilver-treated CGCs. In contrast, supplementation with ZnCl₂ increases viability of CGCs in a dose-dependent manner. Addition of zinc provided protection against the extra/intracellular calcium imbalance in a manner similar to MK-801, an antagonist of NMDA receptors. Zinc chelation by TPEN decreases the mitochondrial potential and dramatically increases the rate of production of reactive oxygen species. Our results indicate that zinc supplementation positively influences nanosilver-evoked changes in CGCs. This is presumed to be due to an inhibitory effect on NMDA-sensitive calcium channels.

Differential zinc permeation and blockade of L-type Ca2+ channel isoforms Cav1.2 and Cav1.3

Certain voltage-activated Ca2+ channels have been reported to act as potential zinc entry routes. However, it remains to be determined whether zinc can permeate individual Ca2+ channel isoforms. We expressed recombinant Ca2+ channel isoforms in Xenopus oocytes and attempted to record zinc currents from them using a two-electrode voltage clamp method. We found that, in an extracellular zinc solution, inward currents arising from zinc permeation could be recorded from Xenopus oocytes expressing L-type Cav1.2 or Cav1.3 isoforms, but not from oocytes expressing Cav2.2, Cav2.3, Cav3.1, or Cav3.2. Zinc currents through Cav1.2 and Cav1.3 were blocked by nimodipine, but enhanced by (±)Bay K8644, supporting the finding that zinc can permeate both L-type Cav1.2 and Cav1.3 channel isoforms. We also examined the blocking effects of low concentrations of zinc on Ca2+ currents through the L-type channel isoforms. Low micro-molar zinc potently blocked Ca2+ currents through Cav1.2 and Cav1.3 with different sensitivities (IC50 for Cav1.2 and Cav1.3=18.4 and 34.1 μM) and de-accelerated the activation and inactivation kinetics in a concentration-dependent manner. Notably, mild acidifications of the external zinc solution increased zinc currents through Cav1.2 and Cav1.3, with the increment level for Cav1.3 being greater than that for Cav1.2. In overall, we provide evidence that Cav1.2 and Cav1.3 isoforms are capable of potentially functioning as zinc permeation routes, through which zinc entry can be differentially augmented by mild acidifications.

Protective activity of carnosine and anserine against zinc-induced neurotoxicity: a possible treatment for vascular dementia

Carnosine (β-alanyl-L-histidine) is a small dipeptide with numerous beneficial effects, including the maintenance of the acid-base balance, antioxidant properties, chelating agent, anti-crosslinking, and anti-glycation activities. High levels of carnosine and its analogue anserine (1-methyl carnosine) are found in skeletal muscle and the brain. Zinc (Zn)-induced neurotoxicity plays a crucial role in the pathogenesis of vascular dementia (VD), and carnosine inhibits Zn-induced neuronal death. Here, the protective activity of carnosine against Zn-induced neurotoxicity and its molecular mechanisms such as cellular Zn influx and Zn-induced gene expression were investigated using immortalised hypothalamic neurons (GT1-7 cells). Carnosine and anserine protected against Zn-induced neurotoxicity not by preventing increases in intracellular Zn(2+) but by participating in the regulation of the endoplasmic reticulum (ER) stress pathway and the activity-regulated cytoskeletal protein (Arc). Accordingly, carnosine and anserine protected against neurotoxicity induced by ER-stress inducers thapsigargin and tunicamycin. Hence, carnosine and anserine are expected to have future therapeutic potential for VD and other neurodegenerative diseases.

Zinc transporter-1: a novel NMDA receptor-binding protein at the postsynaptic density

Zinc (Zn(2+) ) is believed to play a relevant role in the physiology and pathophysiology of the brain. Hence, Zn(2+) homeostasis is critical and involves different classes of molecules, including Zn(2+) transporters. The ubiquitous Zn(2+) transporter-1 (ZNT-1) is a transmembrane protein that pumps cytosolic Zn(2+) to the extracellular space, but its function in the central nervous system is not fully understood. Here, we show that ZNT-1 interacts with GluN2A-containing NMDA receptors, suggesting a role for this transporter at the excitatory glutamatergic synapse. First, we found that ZNT-1 is highly expressed at the hippocampal postsynaptic density (PSD) where NMDA receptors are enriched. Two-hybrid screening, coimmunoprecipitation experiments and clustering assay in COS-7 cells demonstrated that ZNT-1 specifically binds the GluN2A subunit of the NMDA receptor. GluN2A deletion mutants and pull-down assays indicated GluN2A(1390-1464) domain as necessary for the binding to ZNT-1. Most importantly, ZNT-1/GluN2A complex was proved to be dynamic, since it was regulated by induction of synaptic plasticity. Finally, modulation of ZNT-1 expression in hippocampal neurons determined a significant change in dendritic spine morphology, PSD-95 clusters and GluN2A surface levels, supporting the involvement of ZNT-1 in the dynamics of excitatory PSD. Zn(2+) transporter-1 (ZNT-1) pumps cytosolic Zn(2+) to the extracellular space, but its function in the central nervous system is not fully understood. We show that ZNT-1 interacts with GluN2A-containing NMDA receptors at the glutamatergic synapse. Most importantly, ZNT-1/GluN2A complex is regulated by induction of synaptic plasticity. Modulation of ZNT-1 expression in hippocampal neurons determined a shrinkage of dendritic spines and a reduction of GluN2A surface levels supporting the involvement of ZNT-1 in the dynamics of the excitatory synapse.

Zinc is released by cultured astrocytes as a gliotransmitter under hypoosmotic stress-loaded conditions and regulates microglial activity

AIM

Astrocytes contribute to the maintenance of brain homeostasis via the release of gliotransmitters such as ATP and glutamate. Here we examined whether zinc was released from astrocytes under stress-loaded conditions, and was involved in the regulation of microglial activity as a gliotransmitter.

MAIN METHODS

Hypoosmotic stress was loaded to astrocytes using balanced salt solution prepared to 214-314 mOsmol/L, and then intra- and extra-cellular zinc levels were assessed using Newport Green DCF diacetate (NG) and ICP-MS, respectively. Microglial activation by the astrocytic supernatant was assessed by their morphological changes and poly(ADP-ribose) (PAR) polymer accumulation.

KEY FINDINGS

Exposure of astrocytes to hypoosmotic buffer, increased the extracellular ATP level in osmolarity-dependent manners, indicating a load of hypoosmotic stress. In hypoosmotic stress-loaded astrocytes, there were apparent increases in the intra- and extra-cellular zinc levels. Incubation of microglia in the astrocytic conditioned medium transformed them into the activated "amoeboid" form and induced PAR formation. Administration of an extracellular zinc chelator, CaEDTA, to the astrocytic conditioned medium almost completely prevented the microglial activation. Treatment of astrocytes with an intracellular zinc chelator, TPEN, suppressed the hypoosmotic stress-increased intracellular, but not the extracellular, zinc level, and the increase in the intracellular zinc level was blocked partially by a nitric oxide synthase inhibitor, but not by CaEDTA, indicating that the mechanisms underlying the increases in the intra- and extra-cellular zinc levels might be different.

SIGNIFICANCE

These findings suggest that under hypoosmotic stress-loaded conditions, zinc is released from astrocytes and then plays a primary role in microglial activation as a gliotransmitter.

The molecular mechanisms of zinc neurotoxicity and the pathogenesis of vascular type senile dementia

Zinc (Zn) is an essential trace element that is abundantly present in the brain. Despite its importance in normal brain functions, excess Zn is neurotoxic and causes neurodegeneration following transient global ischemia and plays a crucial role in the pathogenesis of vascular-type dementia (VD). We have investigated the molecular mechanisms of Zn-induced neurotoxicity using immortalized hypothalamic neurons (GT1–7 cells) and found that carnosine (β-alanyl histidine) and histidine (His) inhibited Zn²⁺-induced neuronal death. A DNA microarray analysis revealed that the expression of several genes, including metal-related genes (metallothionein and Zn transporter 1), endoplasmic reticulum (ER)-stress related genes (GADD34, GADD45, and p8), and the calcium (Ca)-related gene Arc (activity-related cytoskeleton protein), were affected after Zn exposure. The co-existence of carnosine or His inhibited the expression of GADD34, p8, and Arc, although they did not influence the expression of the metal-related genes. Therefore, ER-stress and the disruption of Ca homeostasis may underlie the mechanisms of Zn-induced neurotoxicity, and carnosine might be a possible drug candidate for the treatment of VD.

A novel epigenetic CREB-miR-373 axis mediates ZIP4-induced pancreatic cancer growth

Changes in the intracellular levels of the essential micronutrient zinc have been implicated in multiple diseases including pancreatic cancer; however, the molecular mechanism is poorly understood. Here, we report a novel mechanism where increased zinc mediated by the zinc importer ZIP4 transcriptionally induces miR-373 in pancreatic cancer to promote tumour growth. Reporter, expression and chromatin immunoprecipitation assays demonstrate that ZIP4 activates the zinc-dependent transcription factor CREB and requires this transcription factor to increase miR-373 expression through the regulation of its promoter. miR-373 induction is necessary for efficient ZIP4-dependent enhancement of cell proliferation, invasion, and tumour growth. Further analysis of miR-373 in vivo oncogenic function reveals that it is mediated through its negative regulation of TP53INP1, LATS2 and CD44. These results define a novel ZIP4-CREB-miR-373 signalling axis promoting pancreatic cancer growth, providing mechanistic insights explaining in part how a zinc transporter functions in cancer cells and may have broader implications as inappropriate regulation of intracellular zinc levels plays an important role in many other diseases.

Zinc induced damage to kidney proximal tubular cells: studies on chemical speciation leading to a mechanism of damage

This study was carried out to investigate whether zinc can potentiate renal toxicity using monolayer cultures of kidney proximal tubular cells and if so to establish the chemical species and the mechanism involved.

METHODS

Zinc was prepared as the citrate complex at pH 7.4 in phosphate buffered saline. Monolayers of kidney proximal tubular cells under standard cell culture conditions were exposed to zinc concentrations of 0, 5 10, 20, 50 and 100 μmol/L. To assess cellular damage, thiazol blue (MTT) uptake, NAG and LDH release, DAPI staining and Tunel assay were used. Cytoprotective agents: trolox, cysteine, glutathione, ascorbic acid and sodium selenite were used to investigate if the damage was reversible.

RESULTS

Incubation of kidney cells with zinc citrate showed a dose related reduction in cell viability (p<0.005) associated with cellular uptake of zinc ions. After 24 h incubation with 100 μmol/L Zn citrate, NAG release was not significantly different compared to the control whereas LDH increased 3 fold. DAPI staining showed apoptotic bodies within the cells confirmed by Tunel assay using flow cytometry. Electron microscopy showed significant morphological changes including loss of brush border, vacuolated cytoplasm and condensed nuclei. Trolox almost completely (>85±5%) and sodium selenite partially recovered (40±4%) the viability of cells exposed to Zn but no protection was observed with other cytoprotectants, e.g. glutathione, cysteine or ascorbic acid. In conclusion zinc can induce damage to kidney cells by a mechanism dependent on zinc ions entering the cell, binding to the cell organelles and disrupting cellular processes rather than damage initiated by free radical and ROS production.

Reduced extracellular zinc levels facilitate glutamate-mediated oligodendrocyte death after trauma

Spinal cord injury results in irreversible paralysis, axonal injury, widespread oligodendrocyte death, and white matter damage. Although the mechanisms underlying these phenomena are poorly understood, previous studies from our laboratory indicate that inhibiting activation of the nuclear factor-κB transcription factor in astrocytes reduces white matter damage and improves functional recovery following spinal cord injury. In the current study, we demonstrate that activation of the nuclear factor-κB transcription factor within astrocytes results in a significant increase in oligodendrocyte death following trauma by reducing extracellular zinc levels and inducing glutamate excitotoxicity. By using an ionotropic glutamate receptor antagonist (CNQX), we show that astroglial nuclear factor-κB-mediated oligodendrocyte death is dependent on glutamate signaling despite no change in extracellular glutamate concentrations. Further analysis demonstrated a reduction in levels of extracellular zinc in astrocyte cultures with functional nuclear factor-κB signaling following trauma. Cotreatment of oligodendrocytes with glutamate and zinc showed a significant increase in oligodendrocyte toxicity under low-zinc conditions, suggesting that the presence of zinc at specific concentrations can prevent glutamate excitotoxicity. These studies demonstrate a novel role for zinc in regulating oligodendrocyte excitotoxicity and identify new therapeutic targets to prevent oligodendrocyte cell death in central nervous system trauma and disease.

D-histidine and L-histidine attenuate zinc-induced neuronal death in GT1-7 cells

Although zinc (Zn) is an essential trace element, excess Zn causes neuronal death following transient global ischemia and plays a central role in the pathogenesis of vascular-type dementia. In this study, we developed a rapid and convenient screening system for substances that prevent Zn-induced neurotoxicity by using GT1-7 cells (immortalized hypothalamic neurons), with the aim of identifying a treatment for vascular-type dementia. Among tested, we found a protective substance in the extract of round herring (Etrumeus teres), and determined its structure as l-histidine. Analysis of the structure-activity relationship by using histidine analogues revealed that both l-histidine and d-histidine exhibit the same neuroprotective activity. Furthermore, we investigated the molecular mechanisms underlying the protective effect of histidine on Zn-induced neurotoxicity using Zn imaging and gene expression analysis, and found that histidine protects against Zn-induced neurotoxicity not by inhibiting Zn chelation, thereby preventing increases in intracellular Zn(2+). Moreover, it is also suggested that endoplasmic reticulum (ER) stress and activity-regulated cytoskeleton associated protein (Arc) are implicated in Zn-induced degeneration of neurons.

Chemical coding of zinc-enriched neurons in the intramural ganglia of the porcine jejunum

Zinc ions in the synaptic vesicles of zinc-enriched neurons (ZEN) seem to have an important role in normal physiological and pathophysiological processes in target organ innervation. The factor directly responsible for the transport of zinc ions into synaptic vesicles is zinc transporter 3 (ZnT3), a member of the divalent cation zinc transporters and an excellent marker of ZEN neurons. As data concerning the existence of ZEN neurons in the small intestine is lacking, this study was designed to disclose the presence and neurochemical coding of such neurons in the porcine jejunum. Cryostat sections (10 mμ thick) of porcine jejunum were processed for routine double- and triple-immunofluorescence labeling for ZnT3 in various combinations with immunolabeling for other neurochemicals including pan-neuronal marker (PGP9.5), substance P (SP), somatostatin (SOM), vasoactive intestinal peptide (VIP), nitric oxide synthase (NOS), leu-enkephalin (LENK), vesicular acetylcholine transporter (VAChT), neuropeptide Y (NPY), galanin (GAL), and calcitonin-gene related peptide (CGRP). Immunohistochemistry revealed that approximately 39%, 49%, and 45% of all PGP9.5- positive neurons in the jejunal myenteric (MP), outer submucous (OSP), and inner submucous (ISP) plexuses, respectively, were simultaneously ZnT3⁺. The majority of ZnT3⁺ neurons in all plexuses were also VAChT-positive. Both VAChT-positive and VAChT-negative ZnT3⁺ neurons co-expressed a variety of active substances with diverse patterns of co-localization depending on the plexus studied. In the MP, the largest populations among both VAChT-positive and VAChT-negative ZnT3⁺ neurons were NOS-positive cells. In the OSP and ISP, substantial subpopulations of ZnT3⁺ neurons were VAChT-positive cells co-expressing SOM and GAL, respectively. The broad-spectrum of active substances that co-localize with the ZnT3⁺ neurons in the porcine jejunum suggests that ZnT3 takes part in the regulation of various processes in the gut, both in normal physiological and during pathophysiological processes.

Neurochemical characterization of zinc transporter 3-like immunoreactive (ZnT3(+)) neurons in the intramural ganglia of the porcine duodenum

The SLC30 family of divalent cation transporters is thought to be involved in the transport of zinc in a variety of cellular pathways. Zinc transporter 3 (ZnT3) is involved in the transport of zinc into synaptic vesicles or intracellular organelles. As the presence of ZnT3 immunoreactive neurons has recently been reported in both the central and peripheral nervous systems of the rat, the present study was aimed at disclosing the presence of a zinc-enriched neuron enteric population in the porcine duodenum to establish a preliminary insight into their neurochemical coding. Double- and triple-immunofluorescence labeling of the porcine duodenum for ZnT3 with the pan-neuronal marker (PGP 9.5), substance P, somatostatin, vasoactive intestinal peptide (VIP), nitric oxide synthase (NOS), leu-enkephalin, vesicular acetylcholine transporter (VAChT), neuropeptide Y, galanin (GAL), and calcitonin gene-related peptide were performed. Immunohistochemistry revealed that approximately 35, 43, and 48 % of all PGP9.5-postive neurons in the myenteric (MP), outer submucous (OSP), and inner submucous (ISP) plexuses, respectively, of the porcine duodenum were simultaneously ZnT3⁺. In the present study, ZnT3⁺ neurons coexpressed a broad spectrum of active substances, but co-localization patterns unique to the plexus were studied. In the ISP, all ZnT3⁺ neurons were VAChT positive, and the largest populations among these cells formed ZnT3⁺/VAChT⁺/GAL⁺ and ZnT3⁺/VAChT⁺/VIP⁺ cells. In the OSP and MP, the numbers of ZnT3⁺/VAChT⁺ neurons were two times smaller, and substantial subpopulations of ZnT3⁺ neurons in both these plexuses formed ZnT3⁺/NOS⁺ cells. The large population of ZnT3⁺ neurons in the porcine duodenum and a broad spectrum of active substances which co-localize with this peptide suggest that ZnT3 takes part in the regulation of various processes in the gut both in normal physiology and during pathological processes.

The role of zinc transporter ZIP4 in prostate carcinoma

OBJECTIVE

Normal prostate tissues have a unique function of accumulating high levels of zinc. This capability is lost during the early stage in the development of prostate malignancy. ZIP4 is an important zinc transporter. In this study, we explore the expression of ZIP4 in prostate carcinoma and invest the functional contributions of ZIP4 to cancer growth in vitro.

METHODS

ZIP4 expression was detected in 14 prostate carcinoma and 20 BPH tissues by real-time RT-PCR and western blot. To invest the biological function of ZIP4 in prostate carcinoma cells, ZIP4 stable over-expression in cells was established in prostate carcinoma cell line DU145 (DU145-ZIP4) and ZIP4 short hairpin RNA(shRNA) expression in stable cells was also established in prostate carcinoma cell line 22RV1(22RV1-shRNA). The proliferation, migration, and invasion ability of the prostate carcinoma cells were detected.

RESULTS

The expression of ZIP4 mRNA and protein are significantly down-regulated in prostate carcinoma tissues compared with that in BPH tissues. However, we found that there was no correlation between the ZIP4 expression and the pathologic grade of prostate carcinoma. In in vitro studies, over-expression of ZIP4 not only inhibits the proliferation but also inhibits the invasive ability of prostate carcinoma cell line DU145-ZIP4. At the same time, we found silencing of ZIP4 was associated with increased cell proliferation and invasion ability in 22RV1-shRNA cell line. However, both DU145-ZIP4 and 22RV1-shRNA cells showed a significant reduction on cell migration ability compared with the control.

CONCLUSION

The results indicate that ZIP4 expression is down-regulated in prostate carcinoma and it may serve as a promising biomarker for prostate carcinoma. ZIP4 has an inhibitory effect on prostate carcinoma cell proliferation and invasion. It suggests that ZIP4 may be a tumor suppressor gene and down-regulation of ZIP4 may be a critical early event in the development of prostate carcinoma.

Dietary zinc absorption: A play of Zips and ZnTs in the gut

Studies on dietary zinc absorption are of fundamental nutritional significance, owing to the ubiquity of zinc in biological processes and the severe outcomes of zinc deficiency in humans. Insights into the molecular basis of dietary zinc absorption have advanced in recent years through functional characterization of zinc transporters in cell culture, immunohistochemical studies on rodent intestine and analysis of gene knockout mice. Zinc transporters with manifested expression in enterocytes include ZnT1, ZnT2, ZnT4, ZnT5, ZnT6, ZnT7, Zip4, and Zip5. Among them, ZIP4, the gene responsible for Acrodermatitis enteropathica, an inherited human zinc deficiency, mediates dietary zinc uptake into enterocytes across the apical membrane, while ZnT1 is involved in zinc efflux from enterocytes across the basolateral membrane into circulation. The intracellular trafficking pathways for zinc retention and movement between apical and basolateral sides of the enterocytes have yet to be defined. The utilization of Drosophila model in elucidating molecular mechanisms of dietary zinc absorption is also discussed in this review.

Cytosolic zinc buffering and muffling: their role in intracellular zinc homeostasis

Our knowledge of the molecular mechanisms of intracellular homeostatic control of zinc ions is now firmly grounded on experimental findings gleaned from the study of zinc proteomes and metallomes, zinc transporters, and insights from the use of computational approaches. A cell's repertoire of zinc homeostatic molecules includes cytosolic zinc-binding proteins, transporters localized to cytoplasmic and organellar membranes, and sensors of cytoplasmic free zinc ions. Under steady state conditions, a primary function of cytosolic zinc-binding proteins is to buffer the relatively large zinc content found in most cells to a cytosolic zinc(ii) ion concentration in the picomolar range. Under non-steady state conditions, zinc-binding proteins and transporters act in concert to modulate transient changes in cytosolic zinc ion concentration in a process that is called zinc muffling. For example, if a cell is challenged by an influx of zinc ions, muffling reactions will dampen the resulting rise in cytosolic zinc ion concentration and eventually restore the cytosolic zinc ion concentration to its original value by shuttling zinc ions into subcellular stores or by removing zinc ions from the cell. In addition, muffling reactions provide a potential means to control changes in cytosolic zinc ion concentrations for purposes of cell signalling in what would otherwise be considered a buffered environment not conducive for signalling. Such intracellular zinc ion signals are known to derive from redox modifications of zinc-thiolate coordination environments, release from subcellular zinc stores, and zinc ion influx via channels. Recently, it has been discovered that metallothionein binds its seven zinc ions with different affinities. This property makes metallothionein particularly well positioned to participate in zinc buffering and muffling reactions. In addition, it is well established that metallothionein is a source of zinc ions under conditions of redox signalling. We suggest that the biological functions of transient changes in cytosolic zinc ion concentrations (presumptive zinc signals) complement those of calcium ions in both spatial and temporal dimensions.

Comparison of uptake and neuroprotective potential of seven zinc-salts

Zinc plays an important role as an antioxidant in different cells treated with various kinds of oxidative stressors. Although intracellular Zn(2+) is important in many cellular events, little is known about the cellular uptake of this trace metal and the intracellular status that is required for its optimal function. Since previous reports usually employed only one type of zinc-salt, in this work was compared cellular uptake and antioxidative potential of seven zinc-salts in order to discriminate whether different counterions and ligands may influence its function. Oxidative stress was induced by peroxide or iron in neuronal PC12 cells. We compared uptake of zinc-salts into the labile Zn(2+) pool of PC12 cells as well as their effects on the prevention of cell death, glutathione depletion, lipid peroxidation and ROS production. Zinc-salts provided better protection against oxidative stress-induced in PC12 cultures by peroxide than by iron. Preincubations with zinc-salts displayed better neuroprotection in all cases than coincubations. Zinc-histidine complex was shown to be the most potent compound. Our results indicated that protective effect of zinc is not related to its uptake into PC12 cells, what is indicated by the rather low salt concentrations required for the cell protection and by the observation that despite a superior antioxidant effect of zinc-histidine, the uptake of this salt by PC12 cells was remarkably lower in comparison with other zinc-salts. Although zinc-sulfate exerted weak neuroprotective potential, accumulation of Zn(2+) from this salt within cells was significantly higher compared to other salts. The differences in accumulation of zinc-salts were not specific and unique to PC12 cells, since similar results were obtained in rat primary hepatocytes and endothelial HUVEC cells.

Mammalian zinc transporters: nutritional and physiologic regulation

Research advances defining how zinc is transported into and out of cells and organelles have increased exponentially within the past five years. Research has progressed through application of molecular techniques including genomic analysis, cell transfection, RNA interference, kinetic analysis of ion transport, and application of cell and animal models including knockout mice. The knowledge base has increased for most of 10 members of the ZnT family and 14 members of the Zrt-, Irt-like protein (ZIP) family. Relative to the handling of dietary zinc is the involvement of ZnT1, ZIP4, and ZIP5 in intestinal zinc transport, involvement of ZIP10 and ZnT1 in renal zinc reabsorption, and the roles of ZIP5, ZnT2, and ZnT1 in pancreatic release of endogenous zinc. These events are major factors in regulation of zinc homeostasis. Other salient findings are the involvement of ZnT2 in lactation, ZIP14 in the hypozincemia of inflammation, ZIP6, ZIP7, and ZIP10 in metastatic breast cancer, and ZnT8 in insulin processing and as an autoantigen in diabetes.

Molecular basis for zinc transporter 1 action as an endogenous inhibitor of L-type calcium channels

The L-type calcium channel (LTCC) has a variety of physiological roles that are critical for the proper function of many cell types and organs. Recently, a member of the zinc-regulating family of proteins, ZnT-1, was recognized as an endogenous inhibitor of the LTCC, but its mechanism of action has not been elucidated. In the present study, using two-electrode voltage clamp recordings in Xenopus oocytes, we demonstrate that ZnT-1-mediated inhibition of the LTCC critically depends on the presence of the LTCC regulatory beta-subunit. Moreover, the ZnT-1-induced inhibition of the LTCC current is also abolished by excess levels of the beta-subunit. An interaction between ZnT-1 and the beta-subunit, as demonstrated by co-immunoprecipitation and by fluorescence resonance energy transfer, is consistent with this result. Using surface biotinylation and total internal reflection fluorescence microscopy in HEK293 cells, we show a ZnT-1-dependent decrease in the surface expression of the pore-forming alpha(1)-subunit of the LTCC. Similarly, a decrease in the surface expression of the alpha(1)-subunit is observed following up-regulation of the expression of endogenous ZnT-1 in rapidly paced cultured cardiomyocytes. We conclude that ZnT-1-mediated inhibition of the LTCC is mediated through a functional interaction of ZnT-1 with the LTCC beta-subunit and that it involves a decrease in the trafficking of the LTCC alpha(1)-subunit to the surface membrane.

Down-regulation of ZIP4 by RNA interference inhibits pancreatic cancer growth and increases the survival of nude mice with pancreatic cancer xenografts

PURPOSE

Zinc levels have been correlated with cancer risk, although the role of zinc and zinc transporters in cancer progression is largely unknown. We recently found that a zinc transporter, ZIP4, is overexpressed in pancreatic cancer. In this study, we further deciphered the role that ZIP4 plays in a pancreatic cancer mouse model by silencing ZIP4.

EXPERIMENTAL DESIGN

ZIP4 stable silencing was established in pancreatic cancer cell lines ASPC-1 (ASPC-shZIP4) and BxPC-3 (BxPC-shZIP4) by short hairpin RNA using retrovirus vectors. The stable cells were characterized in vitro and in vivo using a nude mouse xenograft model.

RESULTS

Silencing of ZIP4 was associated with decreased cell proliferation, migration, and invasion. Both ASPC-shZIP4 and BxPC-shZIP4 cells showed a significant reduction in tumor volume and weight in the s.c. model, and decreased primary tumor weight in the orthotopic model compared with the vector control cells (ASPC-shV and BxPC-shV). Silencing of ZIP4 also caused reduced incidence of tumor metastasis in the mice and downsized the tumor grade. More importantly, silencing of ZIP4 significantly increased the survival rate of nude mice with orthotopic xenografts (P = 0.005). All ASPC-shZIP4-injected mice (100%) remained alive up to 32 days after tumor implantation, whereas only 30% of the ASPC-shV mice were alive at the same time point. CyclinD1 expression was decreased in the ASPC-shZIP4 xenografts.

CONCLUSIONS

These results identify a previously uncharacterized role of ZIP4 in pancreatic cancer progression, and indicate that knocking down ZIP4 by short hairpin RNA might be a novel treatment strategy for pancreatic cancers with ZIP4 overexpression.

Dietary zinc absorption is mediated by ZnT1 in Drosophila melanogaster

Zinc is an essential nutritional factor involved in many key biological processes. However, the physiological function of zinc transporters at the organismal level is not well characterized. Early embryonic lethality of Znt1 knockout mice precludes functional analysis of the role of ZnT1 in dietary zinc absorption. Here, we report the identification and characterization of the Drosophila ZnT1 orthologue, dZnT1, for its role in Drosophila dietary zinc absorption. In cell culture, dZnT1 promoted zinc transport to reduce cytoplasmic zinc levels. Ubiquitous RNA interference of dZnT1 in Drosophila resulted in developmental arrest under restriction of dietary zinc, while dZnT1-overexpressing flies exhibited hypersensitivity to zinc. dZnT1 was prominently expressed in restricted regions of the midgut and exhibited a distribution on the basolateral membrane of the enterocytes. Gut-specific silencing of dZnT1 was sufficient to evoke lethality under zinc scarcity. Human ZnT1, but not ZnT7 or ZnT4, could rescue the zinc-acquiring defects caused by dZnT1 silencing. Taken together, our results proved that dZnT1 is a key zinc transporter in dietary zinc absorption, functioning by pumping zinc out of the enterocytes across the basolateral membrane. This study will be helpful in understanding the fundamental process of acquiring dietary zinc in higher eukaryotes.

Zinc transport by respiratory epithelial cells and interaction with iron homeostasis

Despite recurrent exposure to zinc through inhalation of ambient air pollution particles, relatively little information is known about the homeostasis of this metal in respiratory epithelial cells. We describe zinc uptake and release by respiratory epithelial cells and test the postulate that Zn(2+) transport interacts with iron homeostasis in these same cells. Zn(2+) uptake after 4 and 8 h of exposure to zinc sulfate was concentration- and time-dependent. A majority of Zn(2+) release occurred in the 4 h immediately following cell exposure to ZnSO(4). Regarding metal importers, mRNA for Zip1 and Zip2 showed no change after respiratory epithelial cell exposure to zinc while mRNA for divalent metal transporter (DMT)1 increased. Western blot assay for DMT1 protein supported an elevated expression of this transport protein following zinc exposure. RT-PCR confirmed mRNA for the metal exporters ZnT1 and ZnT4 with the former increasing after ZnSO(4). Cell concentrations of ferritin increased with zinc exposure while oxidative stress, measured as lipid peroxides, was decreased supporting an anti-oxidant function for Zn(2+). Increased DMT1 expression, following pre-incubations of respiratory epithelial cells with TNF-alpha, IFN-gamma, and endotoxin, was associated with significantly decreased intracellular zinc transport. Finally, incubations of respiratory epithelial cells with both zinc sulfate and ferric ammonium citrate resulted in elevated intracellular concentrations of both metals. We conclude that exposure to zinc increases iron uptake by respiratory epithelial cells. Elevations in cell iron can possibly affect an increased expression of DMT1 and ferritin which function to diminish oxidative stress. Comparable to other metal exposures, changes in iron homeostasis may contribute to the biological effects of zinc in specific cells and tissues.

Comparative in vitro toxicity of seven zinc-salts towards neuronal PC12 cells

Currently much attention has been given to the neurotoxicity of zinc, yet little is known about the influence of the counterions present. Therefore, we investigated the influence of different Zn(2+)-salts (concentrations range 0.05-0.3 mM) on cell viability, ATP and glutathione concentration and caspase activation in differentiated PC12 cells as a model for neuronal cells. Generally, at concentrations of 0.05 mM most Zn(2+)-salts were not cytotoxic except for zinc-citrate. At concentrations between 0.1 and 0.3 mM Zn(2+) a significant decrease in GSH and ATP levels preceded cell death induced by all salts, except of zinc-histidinate. Zinc-citrate and zinc-sulphate turned out to be the most toxic salts particularly at low concentrations. Analyses of caspase 3/7 activity showed that dependent on the concentration and the type of the salt used cell death may show more or less signs of both, necrosis and apoptosis. Interestingly, the uptake of Zn(2+) from zinc-sulphate and zinc-citrate was significantly higher than that of other salts, implicating a correlation between uptake and toxicity. In conclusion, Zn(2+)-salts could be divided into three categories with high (zinc-citrate, zinc-sulphate), moderate (zinc-orotate, zinc-acetate, zinc-chloride(,) zinc-gluconate) and low cytotoxicity (zinc-histidinate).

Stable overexpression of human metallothionein-IIA in a heart-derived cell line confers oxidative protection

Metallothionein (MT) is a metal binding protein and cardioprotective. In order to understand the molecular mechanisms underlying the role of MT in the heart, in the current study we established a stable MT-IIA over-expressing cardiac cell line, and evaluated its anti-oxidative property. Rat heart-derived H9c2 cell line was stably transfected with a vector in which the human MT-IIA gene was placed under the control of the constitutively active beta-actin promoter. The transfected cell line (H9c2MT7) exhibited similar growth kinetics and morphology. Western blotting analysis showed that H9c2MT7 had a remarkable increased MT protein level compared with the parent cell line H9c2. Addition of 25 microM ZnSO(4) had an undetectable effect on the induction of endogenous MT, but it likely stabilized the MT protein that is expressed only in H9c2MT7 cells. H9c2MT7 cells showed marked reduction in reactive oxygen species production when exposed to hydrogen peroxide or subjected to hypoxia/reoxygenation challenge evaluated by dihydroethidium staining. In addition, transfection of MT conferred cellular resistance to cadmium toxicity. In summary, we have established a stable human MT-IIA over-expressing cardiac cell line; and this cell line showed a markedly increased oxidative protection and would be useful for dissection of the mechanisms of MT in the cardiac protection.

Insights into Zn2+homeostasis in neurons from experimental and modeling studies

To understand the mechanisms of neuronal Zn2+homeostasis better, experimental data obtained from cultured cortical neurons were used to inform a series of increasingly complex computational models. Total metals (inductively coupled plasma-mass spectrometry), resting metallothionein,65Zn2+uptake and release, and intracellular free Zn2+levels using ZnAF-2F were determined before and after neurons were exposed to increased Zn2+, either with or without the addition of a Zn2+ionophore (pyrithione) or metal chelators [EDTA, clioquinol (CQ), and N, N, N′, N′-tetrakis(2-pyridylmethyl)ethylenediamine]. Three models were tested for the ability to match intracellular free Zn2+transients and total Zn2+content observed under these conditions. Only a model that incorporated a muffler with high affinity for Zn2+, trafficking Zn2+to intracellular storage sites, was able to reproduce the experimental results, both qualitatively and quantitatively. This “muffler model” estimated the resting intracellular free Zn2+concentration to be 1.07 nM. If metallothionein were to function as the exclusive cytosolic Zn2+muffler, the muffler model predicts that the cellular concentration required to match experimental data is greater than the measured resting concentration of metallothionein. Thus Zn2+buffering in resting cultured neurons requires additional high-affinity cytosolic metal binding moieties. Added CQ, as low as 1 μM, was shown to selectively increase Zn2+influx. Simulations reproduced these data by modeling CQ as an ionophore. We conclude that maintenance of neuronal Zn2+homeostasis, when challenged with Zn2+loads, relies heavily on the function of a high-affinity muffler, the characteristics of which can be effectively studied with computational models.

The increase in zinc levels and upregulation of zinc transporters are mediated by nitric oxide in the cerebral cortex after transient ischemia in the rat

The transient occlusion of cerebral arteries causes an increase in zinc levels in the brain, which is associated with a production of nitric oxide (NO). The types of zinc transporters (ZnT) involved in zinc homeostasis in the cerebral cortex after hypoxia-ischemia are not completely known. We studied the effect of the transient occlusion (10 min) of the common carotid artery (CCA) on NO-induced zinc levels, ZnT mRNA expression, and cell-death markers in the cerebral cortex-hippocampus of the rat. Nitrites, zinc, and lipoperoxidation were quantified by colorimetric methods, ZnT expression was determined by RT-PCR, caspase-3 by ELISA and immunohistochemistry, and histopathological alterations by H&E staining. After restoration of the blood flow, the basal levels of NO and zinc increased in a biphasic manner over time, but the peaks of NO levels appeared earlier (2 h and 24 h) than those of zinc (6 h and 36 h). Upregulation of ZnT1, ZnT2, and ZnT4 mRNAs was determined after 8-h postreperfusion, but ZnT3 RNA levels were unaffected. Lipoperoxidation and caspase-3 levels were also increased, and necrosis and apoptosis were present at 24 h postreperfusion. All the effects determined were prevented by l-nitro-arginine methyl ester injected 1 h before the occlusion of the CCA. Our results suggest that the upregulation of ZnT1, ZnT2, and ZnT4 was to decrease the cytosolic zinc levels caused by NO after transient occlusion of the CCA, although this was unable to lead to physiological levels of zinc and to prevent cell damage in the cerebral cortex-hippocampus of the rat.

Zip6 (LIV-1) regulates zinc uptake in neuroblastoma cells under resting but not depolarizing conditions

Impaired zinc homeostasis is implicated in many cases of brain injury and pathogenesis. While several routes of zinc influx have been identified in neurons under depolarizing conditions, zinc uptake mechanisms during resting conditions are unknown. We have previously detected Zip6 at the plasma membrane of rat neurons, suggesting a role for Zip6 in neuronal zinc uptake. Zinc uptake under resting and depolarizing membrane potentials was measured in SH-SY5Y neuroblastoma cells using 65Zn. Zinc uptake was higher under depolarizing conditions, compared with resting conditions, and could be reduced by high extracellular calcium, gadolinium, or nimodipine, which suggests that L-type calcium channels are significant routes of zinc uptake under depolarizing membrane potential. In contrast, zinc uptake under resting conditions was not affected by calcium or calcium channel antagonists. Zip6 was localized to the plasma membrane in SH-SY5Y cells, and siRNA-mediated down-regulation of Zip6 expression reduced zinc uptake during resting, but not depolarizing conditions. Zinc treatment (100 microM Zn) reduced zinc uptake under resting, but not depolarizing conditions, which was associated with lower plasma membrane-associated and total Zip6 protein abundance. These results demonstrate that Zip6 functions as a zinc import protein in neuroblastoma cells, that zinc influx during resting and depolarizing conditions occurs via distinctly different processes in these cells, and suggest that neuronal zinc uptake may be down-regulated by excess zinc levels, but only under resting conditions.