Divergent consequences arise from metallothionein overexpression in astrocytes: zinc buffering and oxidant-induced zinc release

Metallothionein 3 promotes osteoclast differentiation and survival by regulating the intracellular Zn2+ concentration and NRF2 pathway

In osteoclastogenesis, the metabolism of metal ions plays an essential role in controlling reactive oxygen species (ROS) production, mitochondrial biogenesis, and survival, and differentiation. However, the mechanism regulating metal ions during osteoclast differentiation remains unclear. The metal-binding protein metallothionein (MT) detoxifies heavy metals, maintains metal ion homeostasis, especially zinc, and manages cellular redox levels. We carried out tests using murine osteoclast precursors to examine the function of MT in osteoclastogenesis and evaluated their potential as targets for future osteoporosis treatments. MT genes were significantly upregulated upon differentiation from osteoclast precursors to mature osteoclasts in response to receptor activators of nuclear factor-κB (NF-κB) ligand (RANKL) stimulation, and MT3 expression was particularly pronounced in mature osteoclasts among MT genes. The knockdown of MT3 in osteoclast precursors demonstrated a remarkable inhibition of differentiation into mature osteoclasts. In preosteoclasts, MT3 knockdown suppressed the activity of mitogen-activated protein kinase (MAPK) and NF-κB signaling pathways upon RANKL stimulation, leading to affect cell survival through elevated cleaved Caspase 3 and poly (ADP-ribose) polymerase (PARP) levels. Additionally, ROS levels were decreased, and nuclear factor erythroid 2-related factor 2 (NRF2) (a suppressor of ROS) and the downstream antioxidant proteins, such as catalase (CAT) and heme oxygenase 1 (HO-1), were more highly expressed in the MT3 preosteoclast knockdowns. mitochondrial ROS, which is involved in mitochondrial biogenesis and the production of reactive oxygen species, were similarly decreased because cAMP response element-binding (CREB) and peroxisome proliferator-activated receptor γ coactivator 1β (PGC-1β) were less activated due to MT3 depletion. Thus, by modulating ROS through the NRF2 pathway, MT3 plays a crucial role in regulating osteoclast differentiation and survival, acting as a metabolic modulator of intracellular zinc ions.

Structural Role of Cadmium and Zinc in Metallothionein Oxidation by Hydrogen Peroxide: The Resilience of Metal-Thiolate Clusters

Oxidative stress is a state involving an imbalance of reactive oxygen species in a cell and is linked to a variety of diseases. The metal-binding protein metallothionein (MT) may play a role in protection due to its high cysteine content. Many studies have shown that oxidative stress will cause MT to both form disulfide bonds and release bound metals. However, studies on the more biologically relevant partially metalated MTs have been largely neglected. Additionally, most studies to date have used spectroscopic methods that cannot detect specific intermediate species. In this paper, we describe the oxidation and the subsequent metal displacement pathway of fully and partially metalated MTs with hydrogen peroxide. The rates of the reactions were monitored using electrospray ionization mass spectrometry (ESI-MS) techniques, which resolved and characterized the individual intermediate M_x(SH)_yMT species. The rate constants were calculated for each species formation. Through ESI-MS and circular dichroism spectroscopy, it was found that the three metals in the β-domain were the first to be released from the fully metalated MTs. The Cd(II) in the partially metalated Cd(II)-bound MTs rearranged to form a protective Cd₄MT cluster structure upon exposure to oxidation. The partially metalated Zn(II)-bound MTs oxidized at a faster rate as the Zn(II) did not rearrange in response to oxidation. Additionally, density functional theory calculations showed that the terminally bound cysteines were more negative and thus more susceptible to oxidation than the bridging cysteines. The results of this study highlight the importance of metal-thiolate structures and metal identity in MT's response to oxidation.

The role of labile Zn2+ and Zn2+-transporters in the pathophysiology of mitochondria dysfunction in cardiomyocytes

An important energy supplier of cardiomyocytes is mitochondria, similar to other mammalian cells. Studies have demonstrated that any defect in the normal processes controlled by mitochondria can lead to abnormal ROS production, thereby high oxidative stress as well as lack of ATP. Taken into consideration, the relationship between mitochondrial dysfunction and overproduction of ROS as well as the relation between increased ROS and high-level release of intracellular labile Zn²⁺, those bring into consideration the importance of the events related with those stimuli in cardiomyocytes responsible from cellular Zn²⁺-homeostasis and responsible Zn²⁺-transporters associated with the Zn²⁺-homeostasis and Zn²⁺-signaling. Zn²⁺-signaling, controlled by cellular Zn²⁺-homeostatic mechanisms, is regulated with intracellular labile Zn²⁺ levels, which are controlled, especially, with the two Zn²⁺-transporter families; ZIPs and ZnTs. Our experimental studies in mammalian cardiomyocytes and human heart tissue showed that Zn²⁺-transporters localizes to mitochondria besides sarco(endo)plasmic reticulum and Golgi under physiological condition. The protein levels as well as functions of those transporters can re-distribute under pathological conditions, therefore, they can interplay among organelles in cardiomyocytes to adjust a proper intracellular labile Zn²⁺ level. In the present review, we aimed to summarize the already known Zn²⁺-transporters localize to mitochondria and function to stabilize not only the cellular Zn²⁺ level but also cellular oxidative stress status. In conclusion, one can propose that a detailed understanding of cellular Zn²⁺-homeostasis and Zn²⁺-signaling through mitochondria may emphasize the importance of new mitochondria-targeting agents for prevention and/or therapy of cardiovascular dysfunction in humans.

The Redox Biology of Excitotoxic Processes: The NMDA Receptor, TOPA Quinone, and the Oxidative Liberation of Intracellular Zinc

This special issue of Frontiers in Neuroscience-Neurodegeneration celebrates the 50th anniversary of John Olney's seminal work introducing the concept of excitotoxicity as a mechanism for neuronal cell death. Since that time, fundamental research on the pathophysiological activation of glutamate receptors has played a central role in our understanding of excitotoxic cellular signaling pathways, leading to the discovery of many potential therapeutic targets in the treatment of acute or chronic/progressive neurodegenerative disorders. Importantly, excitotoxic signaling processes have been found repeatedly to be closely intertwined with oxidative cellular cascades. With this in mind, this review looks back at long-standing collaborative efforts by the authors linking cellular redox status and glutamate neurotoxicity, focusing first on the discovery of the redox modulatory site of the N-methyl-D-aspartate (NMDA) receptor, followed by the study of the oxidative conversion of 3,4-dihydroxyphenylalanine (DOPA) to the non-NMDA receptor agonist and neurotoxin 2,4,5-trihydroxyphenylalanine (TOPA) quinone. Finally, we summarize our work linking oxidative injury to the liberation of zinc from intracellular metal binding proteins, leading to the uncovering of a signaling mechanism connecting excitotoxicity with zinc-activated cell death-signaling cascades.

Assessing cadmium-based quantum dots effect on the gonads of the marine mussel Mytilus galloprovincialis

This study assesses the sex-specific effects induced by CdTe QDs, on the marine mussel Mytilus galloprovincialis in comparison to its dissolved counterpart. A 14 days exposure to CdTe QDs and dissolved Cd was conducted (10 μg Cd L^-1), analysing Cd accumulation, oxidative stress, biotransformation, metallothionein and oxidative damage in the gonads. Both Cd forms caused significant antioxidant alterations, whereby QDs were more pro-oxidant, leading to oxidative damage, being females more affected. Overall, biochemical impairments on gonads of M. galloprovincialis demonstrate that the reproductive toxicity induced by CdTe QDs in mussels are sex-dependent and mediated by oxidative stress and lipid peroxidation. It is crucial to acknowledge how gametes are affected by metal-based nanoparticles, such as Cd-based QDs. As well as understanding the potential changes they may undergo at the cellular level during gametogenesis, embryogenesis and larval development potentially leading to serious impacts on population sustainability and ecosystem health.

Mitochondrial Zn2+ Accumulation: A Potential Trigger of Hippocampal Ischemic Injury

Ischemic stroke is a major cause of death and disabilities worldwide, and it has been long hoped that improved understanding of relevant injury mechanisms would yield targeted neuroprotective therapies. While Ca²⁺ overload during ischemia-induced glutamate excitotoxicity has been identified as a major contributor, failures of glutamate targeted therapies to achieve desired clinical efficacy have dampened early hopes for the development of new treatments. However, additional studies examining possible contributions of Zn²⁺, a highly prevalent cation in the brain, have provided new insights that may help to rekindle the enthusiasm. In this review, we discuss both old and new findings yielding clues as to sources of the Zn²⁺ that accumulates in many forebrain neurons after ischemia, and mechanisms through which it mediates injury. Specifically, we highlight the growing evidence of important Zn²⁺ effects on mitochondria in promoting neuronal injury. A key focus has been to examine Zn²⁺ contributions to the degeneration of highly susceptible hippocampal pyramidal neurons. Recent studies provide evidence of differences in sources of Zn²⁺ and its interactions with mitochondria in CA1 versus CA3 neurons that may pertain to their differential vulnerabilities in disease. We propose that Zn²⁺-induced mitochondrial dysfunction is a critical and potentially targetable early event in the ischemic neuronal injury cascade, providing opportunities for the development of novel neuroprotective strategies to be delivered after transient ischemia.

Zn2+-induced disruption of neuronal mitochondrial function: Synergism with Ca2+, critical dependence upon cytosolic Zn2+ buffering, and contributions to neuronal injury

Excitotoxic Zn²⁺ and Ca²⁺ accumulation contributes to neuronal injury after ischemia or prolonged seizures. Synaptically released Zn²⁺ can enter postsynaptic neurons via routes including voltage sensitive Ca²⁺ channels (VSCC), and, more rapidly, through Ca²⁺ permeable AMPA channels. There are also intracellular Zn²⁺ binding proteins which can either buffer neuronal Zn²⁺ influx or release bound Zn²⁺ into the cytosol during pathologic conditions. Studies in culture highlight mitochondria as possible targets of Zn²⁺; cytosolic Zn²⁺ can enter mitochondria and induce effects including loss of mitochondrial membrane potential (ΔΨ_m), mitochondrial swelling, and reactive oxygen species (ROS) generation. While brief (5 min) neuronal depolarization (to activate VSCC) in the presence of 300 μM Zn²⁺ causes substantial delayed neurodegeneration, it only mildly impacts acute mitochondrial function, raising questions as to contributions of Zn²⁺-induced mitochondrial dysfunction to neuronal injury. Using brief high (90 mM) K⁺/Zn²⁺ exposures to mimic neuronal depolarization and extracellular Zn²⁺ accumulation as may accompany ischemia in vivo, we examined effects of disrupted cytosolic Zn²⁺ buffering and/or the presence of Ca²⁺, and made several observations: 1. Mild disruption of cytosolic Zn²⁺ buffering-while having little effects alone-markedly enhanced mitochondrial Zn²⁺ accumulation and dysfunction (including loss of ∆Ψ_m, ROS generation, swelling and respiratory inhibition) caused by relatively low (10-50 μM) Zn²⁺ with high K⁺. 2. The presence of Ca²⁺ during the Zn²⁺ exposure decreased cytosolic and mitochondrial Zn²⁺ accumulation, but markedly exacerbated the consequent dysfunction. 3. Paralleling effects on mitochondria, disruption of buffering and presence of Ca²⁺ enhanced Zn²⁺-induced neurodegeneration. 4. Zn²⁺ chelation after the high K⁺/Zn²⁺ exposure attenuated both ROS production and neurodegeneration, supporting the potential utility of delayed interventions. Taken together, these data lend credence to the idea that in pathologic states that impair cytosolic Zn²⁺ buffering, slow uptake of Zn²⁺ along with Ca²⁺ into neurons via VSCC can disrupt the mitochondria and induce neurodegeneration.

Differential Vulnerability of CA1 versus CA3 Pyramidal Neurons After Ischemia: Possible Relationship to Sources of Zn2+ Accumulation and Its Entry into and Prolonged Effects on Mitochondria

Excitotoxic mechanisms contribute to the degeneration of hippocampal pyramidal neurons after recurrent seizures and brain ischemia. However, susceptibility differs, with CA1 neurons degenerating preferentially after global ischemia and CA3 neurons after limbic seizures. Whereas most studies address contributions of excitotoxic Ca²⁺ entry, it is apparent that Zn²⁺ also contributes, reflecting accumulation in neurons either after synaptic release and entry through postsynaptic channels or upon mobilization from intracellular Zn²⁺-binding proteins such as metallothionein-III (MT-III). Using mouse hippocampal slices to study acute oxygen glucose deprivation (OGD)-triggered neurodegeneration, we found evidence for early contributions of excitotoxic Ca²⁺ and Zn²⁺ accumulation in both CA1 and CA3, as indicated by the ability of Zn²⁺ chelators or Ca²⁺ entry blockers to delay pyramidal neuronal death in both regions. However, using knock-out animals (of MT-III and vesicular Zn²⁺ transporter, ZnT3) and channel blockers revealed substantial differences in relevant Zn²⁺ sources, with critical contributions of presynaptic release and its permeation through Ca²⁺- (and Zn²⁺)-permeable AMPA channels in CA3 and Zn²⁺ mobilization from MT-III predominating in CA1. To assess the consequences of the intracellular Zn²⁺ accumulation, we used OGD exposures slightly shorter than those causing acute neuronal death; under these conditions, cytosolic Zn²⁺ rises persisted for 10-30 min after OGD, followed by recovery over ∼40-60 min. Furthermore, the recovery appeared to be accompanied by mitochondrial Zn²⁺ accumulation (via the mitochondrial Ca²⁺ uniporter MCU) in CA1 but not in CA3 neurons and was markedly diminished in MT-III knock-outs, suggesting that it depended upon Zn²⁺ mobilization from this protein.

SIGNIFICANCE STATEMENT

The basis for the differential vulnerabilities of CA1 versus CA3 pyramidal neurons is unclear. The present study of events during and after acute oxygen glucose deprivation highlights a possible important difference, with rapid synaptic entry of Ca²⁺ and Zn²⁺ contributing more in CA3, but with delayed and long-lasting accumulation of Zn²⁺ within mitochondria occurring in CA1 but not CA3 pyramidal neurons. These data may be consistent with observations of prominent mitochondrial dysfunction as a critical early event in the delayed degeneration of CA1 neurons after ischemia and support a hypothesis that mitochondrial Zn²⁺ accumulation in the early reperfusion period may be a critical and targetable upstream event in the injury cascade.

Metallothioneins 1 and 2, but not 3, are regulated by nutritional status in rat white adipose tissue

Background

Cumulating evidence underlines the role of adipose tissue metallothionein (MT) in the development of obesity and type 2 diabetes. Fasting/refeeding was shown to affect MT gene expression in the rodent liver. The influence of nutritional status on MT gene expression in white adipose tissue (WAT) is inconclusive. The aim of this study was to verify if fasting and fasting/refeeding may influence expression of MT genes in WAT of rats.

Results

Fasting resulted in a significant increase in MT1 and MT2 gene expressions in retroperitoneal, epididymal, and inguinal WAT of rats, and this effect was reversed by refeeding. Altered expressions of MT1 and MT2 genes in all main fat depots were reflected by changes in serum MT1 and MT2 levels. MT1 and MT2 messenger RNA (mRNA) levels in WAT correlated inversely with serum insulin concentration. Changes in MT1 and MT2 mRNA levels were apparently not related to total zinc concentrations and MTF1 and Zn transporter mRNA levels in WAT. Fasting or fasting/refeeding exerted no effect on the expression of MT3 gene in WAT. Addition of insulin to isolated adipocytes resulted in a significant decrease in MT1 and MT2 gene expressions. In contrast, forskolin or dibutyryl-cAMP (dB-cAMP) enhanced the expressions of MT1 and MT2 genes in isolated adipocytes. Insulin partially reversed the effect of dB-cAMP on MT1 and MT2 gene expressions.

Conclusions

This study showed that the expressions of MT1 and MT2 genes in WAT are regulated by nutritional status, and the regulation may be independent of total zinc concentration.

Electronic supplementary material

The online version of this article (doi:10.1186/s12263-016-0533-3) contains supplementary material, which is available to authorized users.

Fluorescence detection of intracellular cadmium with Leadmium Green

Leadmium Green is a commercially available, small molecule, fluorescent probe advertised as a detector of free intracellular cadmium (Cd(2+)) and lead (Pb(2+)). Leadmium Green has been used in various paradigms, such as tracking Cd(2+) sequestration in plant cells, heavy metal export in protozoa, and Pb(2+) absorption by vascular endothelial cells. However very little information is available regarding its affinity and selectivity for Cd(2+), Pb(2+), and other metals. We evaluated the in vitro selectivity of Leadmium Green using spectrofluorimetry. Consistent with manufacturer's claims, Leadmium Green was sensitive to Cd(2+) (KD ~600 nM) and also Pb(2+) (KD ~9.0 nM) in a concentration-dependent manner, and furthermore proved insensitive to Ca(2+), Co(2+), Mn(2+) and Ni(2+). Leadmium Green also responded to Zn(2+) with a KD of ~82 nM. Using fluorescence microscopy, we evaluated Leadmium Green in live mouse hippocampal HT22 cells. We demonstrated that Leadmium Green detected ionophore-mediated acute elevations of Cd(2+) or Zn(2+) in a concentration-dependent manner. However, the maximum fluorescence produced by ionophore-delivered Zn(2+) was much less than that produced by Cd(2+). When tested in a model of oxidant-induced liberation of endogenous Zn(2+), Leadmium Green responded weakly. We conclude that Leadmium Green is an effective probe for monitoring intracellular Cd(2+), particularly in models where Cd(2+) accumulates rapidly, and when concomitant fluctuations of intracellular Zn(2+) are minimal.

Metallothionein regulates intracellular zinc signaling during CD4(+) T cell activation

BACKGROUND

The ultra-low redox potential and zinc binding properties of the intracellular pool of mammalian metallothioneins (MT) suggest a role for MT in the transduction of redox signals into intracellular zinc signals. Increased expression of MT after exposure to heavy metals, oxidative stress, or inflammatory cytokines leads to an increased intracellular redox-mobilizable zinc pool that can affect downstream zinc-sensitive signaling pathways. CD4(+) T helper cells are poised to be influenced by MT transduced zinc signaling because they produce intracellular reactive oxygen species following activation through the T cell receptor and are sensitive to small changes in intracellular [Zn(2+)].

RESULTS

MT expression and intracellular [Zn(2+)] are both increased during primary activation and expansion of naïve CD4(+) T cells into the Tr1 phenotype in vitro. When Tr1 cells from wildtype mice are compared with congenic mice lacking functional Mt1 and Mt2 genes, the expression of intracellular MT is associated with a greater increase in intracellular [Zn(2+)] immediately following exposure to reactive oxygen species or upon restimulation through the T cell receptor. The release of Zn(2+) from MT is associated with a greater increase in p38 MAPK activation following restimulation and decreased p38 MAPK activation in MT knockout Tr1 cells can be rescued by increasing intracellular [Zn(2+)]. Additionally, IL-10 secretion is increased in MT knockout Tr1 cells compared with wildtype controls and this increase is prevented when the intracellular [Zn(2+)] is increased experimentally.

CONCLUSIONS

Differences in zinc signaling associated with MT expression appear to be a result of preferential oxidation of MT and concomitant release of Zn(2+). Although zinc is released from many proteins following oxidation, release is greater when the cell contains an intracellular pool of MT. By expressing MT in response to certain environmental conditions, CD4(+) T cells are able to more efficiently release intracellular zinc and regulate signaling pathways following stimulation. The link between MT expression and increased zinc signaling following activation represents an important immunomodulatory mechanism of MT and illuminates the complex role MT plays in shaping immune responses.

ZNT-1 Expression Reduction Enhances Free Zinc Accumulation in Astrocytes After Ischemic Stroke

Excess intracellular zinc has been implicated in ischemic brain cell death. We previously reported that extracellular zinc increases intracellular free zinc level only in hypoxic astrocytes but not in normoxia astrocytes. However, the underlying mechanisms remain to be elucidated. Zinc transporters ZnTs and ZIPs mediate intracellular zinc efflux and extracellular zinc influx. In the present study, we determined the effect of hypoxia/reoxygenation on ZnT-1 and ZIP-1. Hypoxia/reoxygenation did not change the ZIP-1 level in astrocytes. Remarkably, hypoxia/reoxygenation dramatically decreased ZnT-1 expression, which can be difficult to reverse by the addition of extracellular zinc, although extracellular zinc treatment significantly increased ZnT-1 level at normoxia. These results suggest that hypoxia/reoxygenation blocked zinc efflux, whereas zinc influx may be at a similar level to that in normoxia, providing a novel mechanism for intracellular free zinc accumulation after ischemic stroke.

Functional consequences of the over-expression of TRPC6 channels in HEK cells: impact on the homeostasis of zinc

The canonical transient receptor potential 6 (TRPC6) protein is a non-selective cation channel able to transport essential trace elements like iron (Fe) and zinc (Zn) through the plasma membrane. Its over-expression in HEK-293 cells causes an intracellular accumulation of Zn, indicating that it could be involved in Zn transport. This finding prompted us to better understand the role played by TRPC6 in Zn homeostasis. Experiments done using the fluorescent probe FluoZin-3 showed that HEK cells possess an intracellular pool of mobilisable Zn present in compartments sensitive to the vesicular proton pump inhibitor Baf-A, which affects endo/lysosomes. TRPC6 over-expression facilitates the basal uptake of Zn and enhances the size of the pool of Zn sensitive to Baf-A. Quantitative RT-PCR experiments showed that TRPC6 over-expression does not affect the mRNA expression of Zn transporters (ZnT-1, ZnT-5, ZnT-6, ZnT-7, ZnT-9, Zip1, Zip6, Zip7, and Zip14); however it up-regulates the mRNA expression of metallothionein-I and -II. This alters the Zn buffering capacities of the cells as illustrated by the experiments done using the Zn ionophore Na pyrithione. In addition, HEK cells over-expressing TRPC6 grow slower than their parental HEK cells. This feature can be mimicked by growing HEK cells in a culture medium supplemented with 5 μM of Zn acetate. Finally, a proteomic analysis revealed that TRPC6 up-regulates the expression of the actin-associated proteins ezrin and cofilin-1, and changes the organisation of the actin cytoskeleton without changing the cellular actin content. Altogether, these data indicate that TRPC6 is participating in the transport of Zn and influences the Zn storage and buffering capacities of the cells.

Astrocytes of the murine model for Down Syndrome Ts65Dn display reduced intracellular ionic zinc

Zinc is an essential trace element that is critical for a large number of structural proteins, enzymatic processes and transcription factors. In the brain, zinc ions are involved in synaptic transmission. The homeostasis of zinc is crucial for cell survival and function, and cells have developed a wide variety of systems to control zinc concentration. Alterations in free zinc concentration have been related with brain dysfunction. Down Syndrome individuals present alterations in free zinc concentration and in some of the proteins related with zinc homeostasis. We have analyzed the amount of free zinc and the zinc chelating protein metallothionein 3 in the astrocytes using primary cultures of the murine model Ts65Dn. We have observed a higher number of zinc positive spots in the cytoplasm of trisomic astrocytes but a decrease in the total concentration of total intracellular free zinc concentration (including the spots) respect to control astrocytes. Using FM1-43 staining, we found that the endocytic function remains unaltered. Therefore, a possible explanation for this lower concentration of free zinc could be the higher concentration of metallothionein 3 present in the cytoplasm of trisomic astrocytes. The blockade of metallothionein 3 expression using an specific siRNA induced an increase in the concentration of free zinc in basal conditions but failed to increase the uptake of zinc after incubation with zinc ions.

Proton-dependent zinc release from intracellular ligands

In cultured cortical and hippocampal neurons when intracellular pH drops from 6.6 to 6.1, yet unclear intracellular stores release micromolar amounts of Zn(2+) into the cytosol. Mitochondria, acidic organelles, and/or intracellular ligands could release this Zn(2+) . Although exposure to the protonophore FCCP precludes reloading of the mitochondria and acidic organelles with Zn(2+) , FCCP failed to compromise the ability of the intracellular stores to repeatedly release Zn(2+) . Therefore, Zn(2+) -releasing stores were not mitochondria or acidic organelles but rather intracellular Zn(2+) ligands. To test which ligands might be involved, the rate of acid-induced Zn(2+) release from complexes with cysteine, glutathione, histidine, aspartate, glutamate, glycine, and carnosine was investigated; [Zn(2+) ] was monitored in vitro using the ratiometric Zn(2+) -sensitive fluorescent probe FuraZin-1. Carnosine failed to chelate Zn(2+) but did chelate Cu(2+) ; the remaining ligands chelated Zn(2+) and upon acidification were releasing it into the medium. However, when pH was decreasing from 6.6 to 6.1, only zinc-cysteine complexes rapidly accelerated the rate of Zn(2+) release. The zinc-cysteine complexes also released Zn(2+) when a histidine-modifying agent, diethylpyrocarbonate, was applied at pH 7.2. Since the cytosolic zinc-cysteine complexes can contain micromolar amounts of Zn(2+) , these complexes may represent the stores responsible for an acid-induced intracellular Zn(2+) release. This study aimed at identifying intracellular stores which release Zn(2+) when pHi drops from 6.6 to 6.1. It was found that these stores are not mitochondria or acidic organelles, but rather intracellular Zn(2+) ligands. When the pH was decreasing from 6.6 to 6.1, only zinc-cysteine complexes showed a rapid acceleration in the rate of Zn(2+) release. Therefore, the stores responsible for an acid-induced intracellular Zn(2+) release in neurons may be the cytosolic zinc-cysteine complexes.

Enhancement of cellular antioxidant-defence preserves diastolic dysfunction via regulation of both diastolic Zn2+ and Ca2+ and prevention of RyR2-leak in hyperglycemic cardiomyocytes

We examined whether cellular antioxidant-defence enhancement preserves diastolic dysfunction via regulation of both diastolic intracellular free Zn²⁺ and Ca²⁺ levels ([Zn²⁺]_i and [Ca²⁺]_i) levels N-acetyl cysteine (NAC) treatment (4 weeks) of diabetic rats preserved altered cellular redox state and also prevented diabetes-induced tissue damage and diastolic dysfunction with marked normalizations in the resting [Zn²⁺]_i and [Ca²⁺]_i. The kinetic parameters of transient changes in Zn²⁺ and Ca²⁺ under electrical stimulation and the spatiotemporal properties of Zn²⁺ and Ca²⁺ sparks in resting cells are found to be normal in the treated diabetic group. Biochemical analysis demonstrated that the NAC treatment also antagonized hyperphosphorylation of cardiac ryanodine receptors (RyR2) and significantly restored depleted protein levels of both RyR2 and calstabin2. Incubation of cardiomyocytes with 10 µM ZnCl₂ exerted hyperphosphorylation in RyR2 as well as higher phosphorphorylations in both PKA and CaMKII in a concentration-dependent manner, similar to hyperglycemia. Our present data also showed that a subcellular oxidative stress marker, NF-κB, can be activated if the cells are exposed directly to Zn²⁺. We thus for the first time report that an enhancement of antioxidant defence in diabetics via directly targeting heart seems to prevent diastolic dysfunction due to modulation of RyR2 macromolecular-complex thereby leading to normalized [Ca²⁺]_i and [Zn²⁺]_i in cardiomyocytes.

Mechanisms of rapid reactive oxygen species generation in response to cytosolic Ca2+ or Zn2+ loads in cortical neurons

Excessive “excitotoxic” accumulation of Ca²⁺ and Zn²⁺ within neurons contributes to neurodegeneration in pathological conditions including ischemia. Putative early targets of these ions, both of which are linked to increased reactive oxygen species (ROS) generation, are mitochondria and the cytosolic enzyme, NADPH oxidase (NOX). The present study uses primary cortical neuronal cultures to examine respective contributions of mitochondria and NOX to ROS generation in response to Ca²⁺ or Zn²⁺ loading. Induction of rapid cytosolic accumulation of either Ca²⁺ (via NMDA exposure) or Zn²⁺ (via Zn²⁺/Pyrithione exposure in 0 Ca²⁺) caused sharp cytosolic rises in these ions, as well as a strong and rapid increase in ROS generation. Inhibition of NOX activation significantly reduced the Ca²⁺-induced ROS production with little effect on the Zn²⁺- triggered ROS generation. Conversely, dissipation of the mitochondrial electrochemical gradient increased the cytosolic Ca²⁺ or Zn²⁺ rises caused by these exposures, consistent with inhibition of mitochondrial uptake of these ions. However, such disruption of mitochondrial function markedly suppressed the Zn²⁺-triggered ROS, while partially attenuating the Ca²⁺-triggered ROS. Furthermore, block of the mitochondrial Ca²⁺ uniporter (MCU), through which Zn²⁺ as well as Ca²⁺ can enter the mitochondrial matrix, substantially diminished Zn²⁺ triggered ROS production, suggesting that the ROS generation occurs specifically in response to Zn²⁺ entry into mitochondria. Finally, in the presence of the sulfhydryl-oxidizing agent 2,2'-dithiodipyridine, which impairs Zn²⁺ binding to cytosolic metalloproteins, far lower Zn²⁺ exposures were able to induce mitochondrial Zn²⁺ uptake and consequent ROS generation. Thus, whereas rapid acute accumulation of Zn²⁺ and Ca²⁺ each can trigger injurious ROS generation, Zn²⁺ entry into mitochondria via the MCU may do so with particular potency. This may be of particular relevance to conditions like ischemia in which cytosolic Zn²⁺ buffering is impaired due to acidosis and oxidative stress.

ß-blocker timolol prevents arrhythmogenic Ca²⁺ release and normalizes Ca²⁺ and Zn²⁺ dyshomeostasis in hyperglycemic rat heart

Defective cardiac mechanical activity in diabetes results from alterations in intracellular Ca(2+) handling, in part, due to increased oxidative stress. Beta-blockers demonstrate marked beneficial effects in heart dysfunction with scavenging free radicals and/or acting as an antioxidant. The aim of this study was to address how β-blocker timolol-treatment of diabetic rats exerts cardioprotection. Timolol-treatment (12-week), one-week following diabetes induction, prevented diabetes-induced depressed left ventricular basal contractile activity, prolonged cellular electrical activity, and attenuated the increase in isolated-cardiomyocyte size without hyperglycemic effect. Both in vivo and in vitro timolol-treatment of diabetic cardiomyocytes prevented the altered kinetic parameters of Ca(2+) transients and reduced Ca(2+) loading of sarcoplasmic reticulum (SR), basal intracellular free Ca(2+) and Zn(2+) ([Ca(2+)]i and [Zn(2+)]i), and spatio-temporal properties of the Ca(2+) sparks, significantly. Timolol also antagonized hyperphosphorylation of cardiac ryanodine receptor (RyR2), and significantly restored depleted protein levels of both RyR2 and calstabin2. Western blot analysis demonstrated that timolol-treatment also significantly normalized depressed levels of some [Ca(2+)]i-handling regulators, such as Na(+)/Ca(2+) exchanger (NCX) and phospho-phospholamban (pPLN) to PLN ratio. Incubation of diabetic cardiomyocytes with 4-mM glutathione exerted similar beneficial effects on RyR2-macromolecular complex and basal levels of both [Ca(2+)]i and [Zn(2+)]i, increased intracellular Zn(2+) hyperphosphorylated RyR2 in a concentration-dependent manner. Timolol also led to a balanced oxidant/antioxidant level in both heart and circulation and prevented altered cellular redox state of the heart. We thus report, for the first time, that the preventing effect of timolol, directly targeting heart, seems to be associated with a normalization of macromolecular complex of RyR2 and some Ca(2+) handling regulators, and prevention of Ca(2+) leak, and thereby normalization of both [Ca(2+)]i and [Zn(2+)]i homeostasis in diabetic rat heart, at least in part by controlling the cellular redox status of hyperglycemic cardiomyocytes.

Intracellular dialysis disrupts Zn2+ dynamics and enables selective detection of Zn2+ influx in brain slice preparations

We examined the impact of intracellular dialysis on fluorescence detection of neuronal intracellular Zn(2+) accumulation. Comparison between two dialysis conditions (standard; 20 min, brief; 2 min) by standard whole-cell clamp revealed a high vulnerability of intracellular Zn(2+) buffers to intracellular dialysis. Thus, low concentrations of zinc-pyrithione generated robust responses in neurons with standard dialysis, but signals were smaller in neurons with short dialysis. Release from oxidation-sensitive Zn(2+) pools was reduced by standard dialysis, when compared with responses in neurons with brief dialysis. The dialysis effects were partly reversed by inclusion of recombinant metallothionein-3 in the dialysis solution. These findings suggested that extensive dialysis could be exploited for selective detection of transmembrane Zn(2+) influx. Different dialysis conditions were then used to probe responses to synaptic stimulation. Under standard dialysis conditions, synaptic stimuli generated significant FluoZin-3 signals in wild-type (WT) preparations, but responses were almost absent in preparations lacking vesicular Zn(2+) (ZnT3-KO). In contrast, under brief dialysis conditions, intracellular Zn(2+) transients were very similar in WT and ZnT3-KO preparations. This suggests that both intracellular release and transmembrane flux can contribute to intracellular Zn(2+) accumulation after synaptic stimulation. These results demonstrate significant confounds and potential use of intracellular dialysis to investigate intracellular Zn(2+) accumulation mechanisms.

Zinc promotes the death of hypoxic astrocytes by upregulating hypoxia-induced hypoxia-inducible factor-1alpha expression via poly(ADP-ribose) polymerase-1

AIM

Pathological release of excess zinc ions has been implicated in ischemic brain cell death. However, the underlying mechanisms remain to be elucidated. In stroke, ischemia-induced zinc release and hypoxia-inducible factor-1 (HIF-1) accumulation concurrently occur in the ischemic tissue. The present study tests the hypothesis that the presence of high intracellular zinc concentration is a major cause of modifications to PARP-1 and HIF-1α during hypoxia, which significantly contributes to cell death during ischemia.

METHODS

Primary cortical astrocytes and C8-D1A cells were exposed to different concentrations of zinc chloride. Cell death rate and protein expression of HIF-1 and Poly(ADP-ribose) polymerase (PARP)-1 were examined after 3-h hypoxic treatment.

RESULTS

Although 3-h hypoxia or 100 μM of zinc alone did not induce noticeable cytotoxicity, their combination led to a dramatic increase in astrocytic cell death in a zinc-concentration-dependent manner. Exposure of astrocytes to hypoxia for 3 h remarkably increased the levels of intracellular zinc and HIF-1α protein, which was further augmented by added exogenous zinc. Notably, HIF-1α knockdown blocked zinc-induced astrocyte death. Moreover, knockdown of PARP-1, another important protein in the response of hypoxia, attenuated the overexpression of HIF-1α and reduced the cell death rate.

CONCLUSIONS

Our studies show that zinc promotes hypoxic cell death through overexpression of the hypoxia response factor HIF-1α via the cell fate determine factor PARP-1 modification, which provides a novel mechanism for zinc-mediated ischemic brain injury.

The role of metallothionein in oxidative stress

Free radicals are chemical particles containing one or more unpaired electrons, which may be part of the molecule. They cause the molecule to become highly reactive. The free radicals are also known to play a dual role in biological systems, as they can be either beneficial or harmful for living systems. It is clear that there are numerous mechanisms participating on the protection of a cell against free radicals. In this review, our attention is paid to metallothioneins (MTs) as small, cysteine-rich and heavy metal-binding proteins, which participate in an array of protective stress responses. The mechanism of the reaction of metallothioneins with oxidants and electrophilic compounds is discussed. Numerous reports indicate that MT protects cells from exposure to oxidants and electrophiles, which react readily with sulfhydryl groups. Moreover, MT plays a key role in regulation of zinc levels and distribution in the intracellular space. The connections between zinc, MT and cancer are highlighted.

Cytosolic zinc accumulation contributes to excitotoxic oligodendroglial death

Dyshomeostasis of cytosolic Zn(2+) is a critical mediator of neuronal damage during excitotoxicity. However, the role of this cation in oligodendrocyte pathophysiology is not well understood. The current study examined the contribution of Zn(2+) deregulation to oligodendrocyte injury mediated by AMPA receptors. Oligodendrocytes loaded with the Zn(2+)-selective indicator FluoZin-3 responded to mild stimulation of AMPA receptors with fast cytosolic Zn(2+) rises that resulted from intracellular release, as they were not blocked by the extracellular Zn(2+) chelator Ca-EDTA. Pharmacological experiments suggested that AMPA-induced Zn(2+) mobilization depends on cytosolic Ca(2+) accumulation, arises from mitochondria and protein-bound pools, and is triggered by mechanisms that do not involve the generation of reactive oxygen species. Moreover, intracellular Zn(2+) rises resulting from AMPA receptor activation seem to be promoted by Ca(2+)-dependent cytosolic acidification. Addition of the cell-permeable Zn(2+) chelator TPEN significantly reduced mitochondrial membrane depolarization, reactive oxygen species production, and cell death by sub-maximal activation of AMPA receptors both in vitro and in situ, suggesting that Zn(2+) deregulation is an important mediator of oligodendrocyte excitotoxicity. These data provide evidence that strategies aimed at maintaining Zn(2+) homeostasis may be useful for the treatment of disorders in which excitotoxicity is an important trigger of oligodendroglial death.

The neurophysiology and pathology of brain zinc

Our understanding of the roles played by zinc in the physiological and pathological functioning of the brain is rapidly expanding. The increased availability of genetically modified animal models, selective zinc-sensitive fluorescent probes, and novel chelators is producing a remarkable body of exciting new data that clearly establishes this metal ion as a key modulator of intracellular and intercellular neuronal signaling. In this Mini-Symposium, we will review and discuss the most recent findings that link zinc to synaptic function as well as the injurious effects of zinc dyshomeostasis within the context of neuronal death associated with major human neurological disorders, including stroke, epilepsy, and Alzheimer's disease.

Zinc: new clues to diverse roles in brain ischemia

Cerebral ischemia is a leading cause of morbidity and mortality, reflecting the extraordinary sensitivity of the brain to a brief loss of blood flow. A significant goal has been to identify pathways of neuronal injury that are selectively activated after stroke and may be amenable to drug therapy. An important advance was made nearly 25 years ago when Ca(2+) overload was implicated as a critical link between glutamate excitotoxicity and ischemic neurodegeneration. However, early hope for effective therapies faded as glutamate-targeted trials repeatedly failed to demonstrate efficacy in humans. In a review in 2000 in this journal, we described new evidence linking a related cation, zinc (Zn(2+)), to neuronal injury, emphasizing sources and mechanisms of Zn(2+) toxicity. The current review highlights progress over the last decade, emphasizing mechanisms through which Zn(2+) ions (from multiple sources) participate together with Ca(2+) in different stages of cascades of ischemic injury.

Serum or target deprivation-induced neuronal death causes oxidative neuronal accumulation of Zn2+ and loss of NAD+

Trophic deprivation-mediated neuronal death is important during development, after acute brain or nerve trauma, and in neurodegeneration. Serum deprivation (SD) approximates trophic deprivation in vitro, and an in vivo model is provided by neuronal death in the mouse dorsal lateral geniculate nucleus (LGNd) after ablation of the visual cortex (VCA). Oxidant-induced intracellular Zn(2+) release ([Zn(2+) ](i) ) from metallothionein-3 (MT-III), mitochondria or 'protein Zn(2+) ', was implicated in trophic deprivation neurotoxicity. We have previously shown that neurotoxicity of extracellular Zn(2+) required entry, increased [Zn(2+) ](i) , and reduction of NAD(+) and ATP levels causing inhibition of glycolysis and cellular metabolism. Exogenous NAD(+) and sirtuin inhibition attenuated Zn(2+) neurotoxicity. Here we show that: (1) Zn(2+) is released intracellularly after oxidant and SD injuries, and that sensitivity to these injuries is proportional to neuronal Zn(2+) content; (2) NAD(+) loss is involved - restoration of NAD(+) using exogenous NAD(+) , pyruvate or nicotinamide attenuated these injuries, and potentiation of NAD(+) loss potentiated injury; (3) neurons from genetically modified mouse strains which reduce intracellular Zn(2+) content (MT-III knockout), reduce NAD(+) catabolism (PARP-1 knockout) or increase expression of an NAD(+) synthetic enzyme (Wld(s) ) each had attenuated SD and oxidant neurotoxicities; (4) sirtuin inhibitors attenuated and sirtuin activators potentiated these neurotoxicities; (5) visual cortex ablation (VCA) induces Zn(2+) staining and death only in ipsilateral LGNd neurons, and a 1 mg/kg Zn(2+) diet attenuated injury; and finally (6) NAD(+) synthesis and levels are involved given that LGNd neuronal death after VCA was dramatically reduced in Wld(s) animals, and by intraperitoneal pyruvate or nicotinamide. Zn(2+) toxicity is involved in serum and trophic deprivation-induced neuronal death.

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.

Regulation of water permeability through aquaporin-4

Aquaporin-4 (AQP4) is a predominant water channel protein in mammalian brains that is distributed with the highest density in the perivascular and subpial astrocyte end-feet. AQP4 is a critical component of an integrated water and potassium homeostasis. Expression and regulation of AQP4 have been studied to understand the roles of AQP4 in physiology and several pathological conditions. Indeed, AQP4 has been implicated in several neurological conditions, such as brain edema and seizure. AQP4 is dynamically regulated at different levels: channel gating, subcellular distribution, phosphorylation, protein-protein interactions and orthogonal array formation. In this review, we focus on the short-term regulation of AQP4. Phosphorylation of AQP4 is important; AQP4 is inhibited when Ser180 is phosphorylated and activated when Ser111 is phosphorylated. AQP4 is also regulated by several metal ions. These metal ions inhibit AQP4 by interacting with the Cys178 residue located in the cytoplasmic loop D, suggesting that AQP4 is regulated by intracellular signaling pathways in response to extracellular stimuli. Recently, it was demonstrated that AQP4 may be inhibited by arylsulfonamides, antiepileptic drugs and other related chemical compounds. Structural analysis of AQP4 may guide a drug design for AQP4.

Rising zinc: a significant cause of ischemic neuronal death in the CA1 region of rat hippocampus

There is a rising intracellular Zn2+ transient during neuronal ischemic hypoxia (oxygen-glucose deprivation and reoxygenation, OGD/R). The results of our recent works suggest that the OGD/R-induced Zn2+ transient can readily be mistaken for a Ca2+ transient. The aim of this study was to examine the respective functions of Zn2+ and Ca2+ in OGD/R-induced neuronal injury. We showed that [Zn2+]i accumulation was consistently met with the induction of OGD/R-induced cell injury. Ca2+ accumulation induced with high [K+] (to open voltage-gated calcium channels) or ionomycin (a Ca2+ ionophore) caused a moderate neuronal injury that was reduced significantly by the application of the Zn2+ chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN). In comparison, Zn2+ accumulation, induced with the Zn2+ ionophore pyrithione, resulted in significantly greater injury. The application of nimodipine and MK801 was shown to attenuate neuronal injury only from a mild (10 mins) OGD insult. Neuronal injury from more severe (30 mins) OGD was not mitigated by the ion channel antagonists, whereas treatment with the Zn2+ chelator TPEN did afford significant protection from cell injury. These results indicate Zn2+-mediated damage to be of greater consequence than Ca2+-mediated damage, and collectively support the suggestion that Zn2+ accumulation may be a more significant causal factor of OGD/R-induced neuronal injury.

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.

Intracellular zinc release, 12-lipoxygenase activation and MAPK dependent neuronal and oligodendroglial death

Zinc translocation from presynaptic nerve terminals to postsynaptic neurons has generally been considered the critical step leading to the accumulation of intracellular free zinc and subsequent neuronal injury. Recent evidence, however, strongly suggests that the liberation of zinc from intracellular stores upon oxidative and nitrative stimulation contributes significantly to the toxicity of this metal not only to neurons, but also to oligodendrocytes. The exact cell death signaling pathways triggered by zinc are beginning to be deciphered. In this review, we describe how the activation of 12-lipoxygenase and mitogen-activated protein kinase (MAPK) contribute to the toxicity of liberated zinc to neurons and oligodendrocytes.

Intracellular zinc release, 12-lipoxygenase activation and MAPK dependent neuronal and oligodendroglial death

Zinc translocation from presynaptic nerve terminals to postsynaptic neurons has generally been considered the critical step leading to the accumulation of intracellular free zinc and subsequent neuronal injury. Recent evidence, however, strongly suggests that the liberation of zinc from intracellular stores upon oxidative and nitrative stimulation contributes significantly to the toxicity of this metal not only to neurons, but also to oligodendrocytes. The exact cell death signaling pathways triggered by zinc are beginning to be deciphered. In this review, we describe how the activation of 12-lipoxygenase and mitogen-activated protein kinase (MAPK) contribute to the toxicity of liberated zinc to neurons and oligodendrocytes.

Zn2+ sensitivity of high- and low-voltage activated calcium channels

The essential cation zinc (Zn2+) blocks voltage-dependent calcium channels in several cell types, which exhibit different sensitivities to Zn2+. The specificity of the Zn2+ effect on voltage-dependent calcium channel subtypes has not been systematically investigated. In this study, we used a transient protein expression system to determine the Zn2+ effect on low- and high-voltage activated channels. We found that in Ba2+, the IC50 value of Zn2+ was alpha1-subunit-dependent with lowest value for CaV1.2, and highest for CaV3.1; the sensitivity of the channels to Zn2+ was approximately ranked as CaV1.2>CaV3.2>CaV2.3>CaV2.2=CaV 2.1>or=CaV3.3=CaV3.1. Although the CaV2.2 and CaV3.1 channels had similar IC50 for Zn2+ in Ba2+, the CaV2.2, but not CaV3.1 channels, had approximately 10-fold higher IC50 to Zn2+ in Ca2+. The reduced sensitivity of CaV2.2 channels to Zn2+ in Ca2+ was partially reversed by disrupting a putative EF-hand motif located external to the selectivity filter EEEE locus. Thus, our findings support the notion that the Zn2+ block, mediated by multiple mechanisms, may depend on conformational changes surrounding the alpha1 pore regions. These findings provide fundamental insights into the mechanism underlying the inhibitory effect of zinc on various Ca2+ channel subtypes.

Metallothionein redox cycle and function

The biologic function of metallothionein (MT) has been a perplexing topic ever since the discovery of this protein. Many studies have suggested that MT plays a role in the homeostasis of essential metals such as zinc and copper, detoxification of toxic metals such as cadmium, and protection against oxidative stress. However, mechanistic insights into the actions of MT have not been adequately achieved. MT contains high levels of sulfur. The mutual affinity of sulfur and transition metals makes the binding of these metals to MT thermodynamically stable. Under physiologic conditions, zinc-MT is the predominant form of the metal-binding protein. The recognition of the redox regulation of zinc release from or binding to MT provides an alternate perspective on biologic function of MT. Oxidation of the thiolate cluster by a number of mild cellular oxidants causes zinc release and formation of MT-disulfide (or thionin if all metals are released from MT, but this is unlikely to occur in vivo), which have been demonstrated in vivo. Therefore, the thermodynamic stability of zinc binding makes MT an ideal zinc reservoir in vivo, and the redox regulation of zinc mobilization enables MT function in zinc homeostasis. MT-disulfide can be reduced by glutathione in the presence of selenium catalyst, restoring the capacity of the protein to bind zinc. This MT redox cycle may play a crucial role in MT biologic function. It may link to the homeostasis of essential metals, detoxification of toxic metals and protection against oxidative stress.

Biphasic modulation of ryanodine receptors by sulfhydryl oxidation in rat ventricular myocytes

To understand better the modulation of ryanodine receptors (RyRs) during oxidative stress, the effect of 4,4'-dithiodipyridine (DTDP), a cell-permeant and thiol-reactive oxidant, on global Ca(2+) signal and spontaneous Ca(2+) sparks of rat ventricular myocytes was investigated. It was shown that a brief Ca(2+) transient was elicited by DTDP, when its concentration was raised to 100 microM DTDP. In addition a dose-dependent increase of cytoplasmic free Zn(2+) concentration was induced by DTDP. An increase of the frequency of spontaneous Ca(2+) sparks appeared at 3 microM DTDP, whereas higher concentration of DTDP caused a biphasic change of the frequency in both intact and permeabilized myocytes. Consistent with the biphasic effect, caffeine-induced Ca(2+) transients were similarly affected. Because DTDP did not reduce the free Ca(2+) concentration in the sarcoplasmic reticulum lumen, it is likely that the effects of DTDP on the frequency and caffeine-induced Ca(2+) transients are due mainly to sulfhydryl oxidation-induced activation and subsequent inactivation of RyRs. Unlike the frequency, the spatio-temporal properties of Ca(2+) sparks were not influenced by DTDP. The finding that DTDP does not affect the duration of Ca(2+) sparks is inconsistent with that the DTDP-induced increase of the open time of reconstituted RyR channels. The mechanism underlying this discrepancy, especially the possible role of the interaction between arrayed RyRs in myocytes, is discussed. This study suggests that, even if oxidative stress is mild enough not to cause intracellular Ca(2+) accumulation, it may affect signaling pathways through directly modulating the RyR or its complex and in turn changing the frequency of spontaneous Ca(2+) sparks. Thus, the functional importance of moderate oxidative stress should not be overlooked.

Selenium prevents diabetes-induced alterations in [Zn2+]iand metallothionein level of rat heart via restoration of cell redox cycle

Intracellular free zinc concentration ([Zn2+]i) is very important for cell functions, and its excessive accumulation is cytotoxic. [Zn2+]ican increase rapidly in cardiomyocytes because of mobilization of Zn2+from intracellular stores by reactive oxygen species (ROS). Moreover, ROS have been proposed to contribute to direct and/or indirect damage to cardiomyocytes in diabetes. To address these hypotheses, we investigated how elevated [Zn2+]iin cardiomyocytes could contribute to diabetes-induced alterations in intracellular free calcium concentration ([Ca2+]i). We also investigated its relationship to the changes of metallothionein (MT) level of the heart. Cardiomyocytes from normal rats loaded with fura-2 were used to fluorometrically measure resting [Zn2+]i(0.52 ± 0.06 nM) and [Ca2+]i(26.53 ± 3.67 nM). Fluorescence quenching by the heavy metal chelator N, N, N′, N′-tetrakis(2-pyridylmethyl)ethylenediamine was used to quantify [Zn2+]i. Our data showed that diabetic cardiomyocytes exhibited significantly increased [Zn2+]i(0.87 ± 0.05 nM ) and [Ca2+]i(49.66 ± 9.03 nM), decreased levels of MT and reduced glutathione, increased levels of lipid peroxidation and nitric oxide products, and decreased activities of superoxide dismutase, glutathione reductase, and glutathione peroxidase. Treatment (4 wk) of diabetic rats with sodium selenite (5 μmol·kg body wt−1·day−1) prevented these defects induced by diabetes. A comparison of present data with previously observed beneficial effects of selenium treatment on diabetes-induced contractile dysfunction of the heart can suggest that an increase in [Zn2+]imay contribute to oxidant-induced alterations of excitation-contraction coupling in diabetes. In addition, we showed that oxidative stress is involved in the etiology of diabetes-induced downregulation of heart function via depressed endogenous antioxidant defense mechanisms.

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.

Motexafin gadolinium and zinc induce oxidative stress responses and apoptosis in B-cell lymphoma lines

There is an emerging appreciation of the importance of zinc in regulating cancer cell growth and proliferation. Recently, we showed that the anticancer agent motexafin gadolinium (MGd) disrupted zinc metabolism in A549 lung cancer cells, leading, in the presence of exogenous zinc, to cell death. Here, we report the effect of MGd and exogenous zinc on intracellular levels of free zinc, oxidative stress, proliferation, and cell death in exponential phase human B-cell lymphoma and other hematologic cell lines. We find that increased levels of oxidative stress and intracellular free zinc precede and correlate with cell cycle arrest and apoptosis. To better understand the molecular basis of these cellular responses, gene expression profiling analyses were conducted on Ramos cell cultures treated with MGd and/or zinc acetate. Cultures treated with MGd or zinc acetate alone elicited transcriptional responses characterized by induction of metal response element-binding transcription factor-1 (MTF-1)-regulated and hypoxia-inducible transcription factor-1 (HIF-1)-regulated genes. Cultures cotreated with MGd and zinc acetate displayed further increases in the levels of MTF-1- and HIF-1-regulated transcripts as well as additional transcripts regulated by NF-E2-related transcription factor 2. These data provide insights into the molecular changes that accompany the disruption of intracellular zinc homeostasis and support a role for MGd in treatment of B-cell hematologic malignancies.

Metallothionein transfers zinc to mitochondrial aconitase through a direct interaction in mouse hearts

Previous studies have shown that in a cell-free system, metallothionein (MT) releases zinc when the environment becomes oxidized and the released zinc is transferred to a zinc-binding protein if such a protein is present. However, it is unknown whether and how zinc transfers from MT to other proteins in vivo. The present study was undertaken to test the hypothesis that if zinc transfer from MT to other proteins occurs in vivo, the transfer would proceed through a direct interaction between MT and a specific group of proteins. The heart extract obtained from MT-null mice was incubated with 65Zn-MT or 65ZnCl2 and the proteins receiving 65Zn were separated by blue-native PAGE (BN-PAGE) or sodium dodecyl sulfate-PAGE (SDS-PAGE), and detected by autoradiography. A unique 65Zn-binding band was observed from the 65Zn-MT-incubated, but not the 65ZnCl2-incubated preparation. The analysis using matrix assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry revealed that mitochondrial aconitase (m-aconitase) was among the proteins accepting Zn directly from Zn-MT. The m-aconitase, not the cytosolic aconitase (c-aconitase), was co-immunoprecipitated with MT. This study demonstrates that MT transfers zinc to m-aconitase through a direct interaction.

Metallothionein reduces central nervous system inflammation, neurodegeneration, and cell death following kainic acid-induced epileptic seizures

We examined metallothionein (MT)-induced neuroprotection during kainic acid (KA)-induced excitotoxicity by studying transgenic mice with MT-I overexpression (TgMT mice). KA induces epileptic seizures and hippocampal excitotoxicity, followed by inflammation and delayed brain damage. We show for the first time that even though TgMT mice were more susceptible to KA, the cerebral MT-I overexpression decreases the hippocampal inflammation and delayed neuronal degeneration and cell death as measured 3 days after KA administration. Hence, the proinflammatory responses of microglia/macrophages and lymphocytes and their expression of interleukin (IL)-1, IL-6, IL-12, tumor necrosis factor-alpha and matrix metalloproteinases (MMP-3, MMP-9) were significantly reduced in hippocampi of TgMT mice relative to wild-type mice. Also by 3 days after KA, the TgMT mice showed significantly less delayed damage, such as oxidative stress (formation of nitrotyrosine, malondialdehyde, and 8-oxoguanine), neurodegeneration (neuronal accumulation of abnormal proteins), and apoptotic cell death (judged by TUNEL and activated caspase-3). This reduced bystander damage in TgMT mice could be due to antiinflammatory and antioxidant actions of MT-I but also to direct MT-I effects on the neurons, in that significant extracellular MT presence was detected. Furthermore, MT-I overexpression stimulated astroglia and increased immunostaining of antiinflammatory IL-10, growth factors, and neurotrophins (basic fibroblastic growth factor, transforming growth factor-beta, nerve growth factor, brain-derived neurotrophic factor, glial-derived neurotrophic factor) in hippocampus. Accordingly, MT-I has different functions that likely contribute to the increased neuron survival and improved CNS condition of TgMT mice. The data presented here add new insight into MT-induced neuroprotection and indicate that MT-I therapy could be used against neurological disorders.

Fluorescence detection of redox-sensitive metals in neuronal culture: focus on iron and zinc

Detection of neurotoxic metals in the intracellular milieu has made an important contribution to the understanding of the mechanism of metal-induced neuronal injury. Fluorescent, metal-sensitive dyes have proven to be valuable in the measurement of a variety of neurotoxic cations in neurons, and these dyes have provided a number of insights into the relationships between elevations in the cytosolic free-metal concentrations and neuronal death. However, the dyes also have important limitations that can make the interpretation of dye signals difficult. In this review, the characteristics of dyes that can be used to detect both iron and zinc inside neurons, and the methods necessary to distinguish these ions from other intracellular signals, are reviewed. Also provided are examples of the use of the dyes for the redox-sensitive detection of iron and zinc. Finally, the challenges facing the use of these dyes for quantitative determination of changes in intracellular free-ion concentrations are discussed.