The neurobiology of zinc in health and disease

Heavy Metal Ions in Normal Physiology, Toxic Stress, and Cytoprotection

As a group, heavy metals include both those essential for normal biological functioning (e.g., Cu and Zn), and nonessential metals (e.g., Cd, Hg, and Pb). Both essential and nonessential metals can be present at concentrations that disturb normal biological functions, and which evoke cellular stress responses. The cellular targets for metal toxicity include tissues of the kidney, liver, heart, and the immune response and nervous systems. Intriguingly, manipulations of specific metals, their reservoirs, and the cellular stress response can have therapeutic effects on certain diseases. In this minireview, we will consider both the biological responses to stressful levels of heavy metal cations, and experimental and clinical manipulations of these cations as a means to improve human health parameters.

Zinc is a novel intracellular second messenger

Zinc is an essential trace element required for enzymatic activity and for maintaining the conformation of many transcription factors; thus, zinc homeostasis is tightly regulated. Although zinc affects several signaling molecules and may act as a neurotransmitter, it remains unknown whether zinc acts as an intracellular second messenger capable of transducing extracellular stimuli into intracellular signaling events. In this study, we report that the cross-linking of the high affinity immunoglobin E receptor (Fcɛ receptor I [FcɛRI]) induced a release of free zinc from the perinuclear area, including the endoplasmic reticulum in mast cells, a phenomenon we call the zinc wave. The zinc wave was dependent on calcium influx and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase activation. The results suggest that the zinc wave is involved in intracellular signaling events, at least in part by modulating the duration and strength of FcɛRI-mediated signaling. Collectively, our findings indicate that zinc is a novel intracellular second messenger.

The inhibition kinetics and thermodynamic changes of tyrosinase via the zinc ion

We found that Zn(2+) conspicuously inactivated tyrosinase in a mixed-type inhibition manner: the final level of residual activity was abolished at the equilibrium state with concentration of 0.25 mM Zn(2+). Changes of both K(m) and V(max) by various concentrations of Zn(2+) in Lineweaver-Burk plot were observed. To see whether Zn(2+) also induced conformational change of tyrosinase and how thermodynamical changes by ligand binding were occurred, the intrinsic fluorescence studies as well as calorimetric measurements were conducted. The results showed that the Zn(2+) binding to tyrosinase directly induced conformational change of tyrosinase, and the changes of thermodynamic parameters such as enthalpy (DeltaH), Gibbs free-energy (DeltaG), and entropy (DeltaS) were obtained as 60+/-7.0 kJ/mol, -14.54 kJ/mol and 248.53 J/(K mol), respectively. The inactivating effect of Zn(2+) on tyrosinase was completely prevented by incubation with bovine serum albumin, which has a Zn(2+) binding motif in its structure. We suggested that Zn(2+) ligand-binding affected the substrate's accessibility due to the conformational changes and thus, the complex type of inhibition has occurred with the calorimetric changes.

Fluorescence imaging study of extracellular zinc at the hippocampal mossy fiber synapse

Although synaptically released, vesicular Zn(2+) has been proposed to play a neuromodulatory or neuronal signaling role at the mossy fiber-CA3 synapse, Zn(2+) release remains controversial, especially when detected using fluorescent imaging. In the present study, we investigated synaptically released Zn(2+) at the mossy fiber (MF) synapse in rat hippocampal slices using three chemically distinct, fluorescent Zn(2+) indicators. The indicators employed for this study were cell membrane impermeable (or extracellular) Newport Green [K(DZn2+) approximatelly 1 microM] , Zinpyr-4 K(DZn2+) approximately 1 nM and FluoZin-3 K(DZn2+) approximately 15 nM, chosen, in part, for their distinct dissociation constants. Among the three indicators, FluoZin-3 was also sensitive to Ca(2+) K(DCa2+) approximately 200-300 microM which was present in the extracellular medium ([Ca(2+)](o)>2mM). Hippocampal slices loaded with either Newport Green or FluoZin-3 showed increases in fluorescence after electrical stimulation of the mossy fiber pathway. These results are consistent with previous studies suggesting the presence of synaptically released Zn(2+) in the extracellular space during neuronal activities; however, the rise in FluoZin-3 fluorescence observed was complicated by the data that the addition of exogenous Zn(2+) onto FluoZin-3 loaded slices gave little change in fluorescence. In the slices loaded with the high-affinity indicator Zinpyr-4, there was little change in fluorescence after mossy fiber activation by electrical stimulation. Further study revealed that the sensitivity of Zinpyr-4 was mitigated by saturation with Zn(2+) contamination from the slice. These data suggest that the sensitivity and selectivity of a probe may affect individual outcomes in a given experimental system.

Clioquinol attenuates zinc-dependent beta-cell death and the onset of insulitis and hyperglycemia associated with experimental type I diabetes in mice

Zinc in the pancreas is co-released with insulin from beta-cells reaching concentrations similar to those found in the vicinity of glutamatergic synapses. In the brain, the role of zinc in excitotoxic brain damage is well established. In contrast, its role in islet destruction during diabetes is poorly understood. We have studied the efficacy of zinc homeostatic proteins and an intracellular zinc chelator, clioquinol, in conferring resistance against zinc toxicity in pancreatic islets. We further assessed the ability of clioquinol to protect the islets in an experimental model of type I diabetes. Our results indicate that endogenous mechanisms for lowering [Zn]i are deficient in the insulinoma cell line, MIN6, and that permeation of Zn2+ triggered cell death. Application of the low affinity, intracellular zinc chelator, clioquinol, reduced Zn2+-induced cell death by 80%. In addition, chelation of zinc ions by clioquinol in vivo prevented onset of multiple low dose streptozotocin-induced diabetes, and reduced the insulitis and hyperglycemia associated with this model. Furthermore, the glucose tolerance test (GTT) score of multiple low dose streptozotocin (MLD-STZ) mice pretreated with clioquinol was, statistically indistinguishable from that of untreated, control mice. Taken together, our results point to the potential utility of in vivo zinc chelation as a therapeutic strategy for treatment of idiopathic type I diabetes.

The effect of deoxygenation on whole-cell conductance of red blood cells from healthy individuals and patients with sickle cell disease

Red blood cells from patients with sickle cell disease (SCD) exhibit increased electrogenic cation permeability, particularly following deoxygenation and hemoglobin (Hb) polymerisation. This cation permeability, termed P(sickle), contributes to cellular dehydration and sickling, and its inhibition remains a major goal for SCD treatment. Nevertheless, its characteristics remain poorly defined, its molecular identity is unknown, and effective inhibitors have not been established. Here, patch-clamp methodology was used to record whole-cell currents in single red blood cells from healthy individuals and patients with SCD. Oxygenated normal red blood cells had a low membrane conductance, unaffected by deoxygenation. Oxygenated HbS cells had significantly increased conductance and, on deoxygenation, showed a further rise in membrane conductance. The deoxygenation-induced pathway was variable in magnitude. It had equal permeability to Na(+) and K(+), but was less permeable to NMDG(+) and Cl(-). Conductance to Ca(2+) was also of a similar magnitude to that of monovalent cations. It was inhibited by DIDS (100 microM), Zn(2+) (100 microM), and by Gd(3+) (IC(50) of approximately 2 microM). It therefore shares some properties with P(sickle). These findings represent the first electrical recordings of single HbS cells and will facilitate progress in understanding altered red blood cell cation transport characteristics of SCD.

The zinc/thiolate redox biochemistry of metallothionein and the control of zinc ion fluctuations in cell signaling

Free zinc ions are potent effectors of proteins. Their tightly controlled fluctuations ("zinc signals") in the picomolar range of concentrations modulate cellular signaling pathways. Sulfur (cysteine) donors generate redox-active coordination environments in proteins for the redox-inert zinc ion and make it possible for redox signals to induce zinc signals. Amplitudes of zinc signals are determined by the cellular zinc buffering capacity, which itself is redox-sensitive. In part by interfering with zinc and redox buffering, reactive species, drugs, toxins, and metal ions can elicit zinc signals that initiate physiological and pathobiochemical changes or lead to cellular injury when free zinc ions are sustained at higher concentrations. These interactions establish redox-inert zinc as an important factor in redox signaling. At the center of zinc/redox signaling are the zinc/thiolate clusters of metallothionein. They can transduce zinc and redox signals and thereby attenuate or amplify these signals.

Coregulation of light neurofilament mRNA by poly(A)-binding protein and aldolase C: Implications for neurodegeneration

The multifunctional proteins aldolase C and poly (A)-binding protein (PABP) undergo competitive interactions in cells coexpressing aldolase C and NF-L. A specific in vivo interaction between aldolase C and NF-L mRNA had been localized to a 68 nt segment of the transcript spanning the translation termination signal. It is shown here that the poly (A)-binding protein (PABP) binds the body of the NF-L transcript and increases its levels of expression when an excess of PABP is transiently provided in trans. Immunoprecipitation of PABP-associated ribonucleoprotein complexes of human spinal cord pulls down the dimeric form of aldolase C suggesting that their co-regulation of NF-L expression could be linked to the oligomerization status of aldolase C. An ex vivo model of mRNA decay has assessed mechanisms whereby aldolase C and PABP control NF-L expression. This model shows that aldolase C is a zinc-activated ribonuclease that cleaves the transcript at sites closed to the end-terminal structures. Immunological and biochemical depletion of endogenous PABP increases the instability of the transcript suggesting that PABP shields the NF-L mRNA from aldolase attack. An in vitro model shows that a mutant NF-L 68, in which the 45 nt of proximal 3'-UTR is replaced with unrelated sequence, is not degraded by aldolase C. Taken together, the findings might have important consequences for understanding causal mechanisms underlying neurodegeneration.

Zn2+-dependent redox switch in the intracellular T1-T1 interface of a Kv channel

The thiol-based redox regulation of proteins plays a central role in cellular signaling. Here, we investigated the redox regulation at the Zn(2+) binding site (HX(5)CX(20)CC) in the intracellular T1-T1 inter-subunit interface of a Kv4 channel. This site undergoes conformational changes coupled to voltage-dependent gating, which may be sensitive to oxidative stress. The main results show that internally applied nitric oxide (NO) inhibits channel activity profoundly. This inhibition is reversed by reduced glutathione and suppressed by intracellular Zn(2+), and at least two Zn(2+) site cysteines are required to observe the NO-induced inhibition (Cys-110 from one subunit and Cys-132 from the neighboring subunit). Biochemical evidence suggests strongly that NO induces a disulfide bridge between Cys-110 and Cys-132 in intact cells. Finally, further mutational studies suggest that intra-subunit Zn(2+) coordination involving His-104, Cys-131, and Cys-132 protects against the formation of the inhibitory disulfide bond. We propose that the interfacial T1 Zn(2+) site of Kv4 channels acts as a Zn(2+)-dependent redox switch that may regulate the activity of neuronal and cardiac A-type K(+) currents under physiological and pathological conditions.

Zinc-mediated neuronal death is dependent on Trk activation

Zinc release is a primary mediator of neuronal death. Here we show that zinc-mediated death of neurons in vitro is dependent on nerve growth factor (NGF) stimulation and does not occur in response to exposure to leukemia inhibitory factor. NGF priming is regulated, not by the traditional neurotrophin death receptor, p75NTR, but by TrkA, in a protein- and mRNA synthesis-dependent manner. Furthermore, Trk signaling promotes raised free intracellular zinc, mediating neuronal death after extracellular application of zinc. Thus, regulators of Trk signaling provide attractive targets for future treatment of zinc-associated neurological diseases, including stroke, epilepsy and brain trauma.

Zinc fluxes and zinc transporter genes in chronic diseases

The group IIb metal zinc (Zn) is an essential dietary component that can be found in protein rich foods such as meat, seafood and legumes. Thousands of genes encoding Zn binding proteins were identified, especially after the completion of genome projects, an indication that a great number of biological processes are Zn dependent. Imbalance in Zn homeostasis was found to be associated with several chronic diseases such as asthma, diabetes and Alzheimer's disease. As it is now evident for most nutrients, body Zn status results from the interaction between diet and genotype. Zn ions cross biological membranes with the aid of specialized membrane proteins, belonging to the ZRT/IRT-related Proteins (ZIP) and zinc transporters (ZnT) families. The ZIPs are encoded by the Slc39A gene family and are responsible for uptake of the metal, ZnTs are encoded by the Slc30A genes and are involved in intracellular traffic and/or excretion. Both ZnTs and Zips exhibit unique tissue-specific expression, differential responsiveness to dietary Zn deficiency and excess, as well as to physiological stimuli via hormones and cytokines. Intracellular Zn concentration is buffered by metallothioneins (MTs), a class of cytosolic protein with high affinity for metals. Scattered information is available on the role of proteins responsible for regulating Zn fluxes in the onset and progression of chronic diseases. This paper reviews reports that link Zn transporter genes, their allelic variants and/or expression profiles in the context of specific diseases. Further investigation in this direction is very important, since Zn imbalance can result not only from insufficient dietary intake, but also from impaired activity of proteins that regulate Zn metabolism, thus contributing to multifactorial diseases.

Anxiety-like behavior of young rats after 2-week zinc deprivation

The relationship between neuronal function in the brain and neuropsychological behavior were analyzed in young rats fed a zinc-deficient diet for 1-2 weeks. Serum zinc concentration was less than 50% of that of the control. However, zinc concentration in the hippocampal perfusate measured by the in vivo microdialysis was not decreased after 2-week zinc deprivation. Timm's stain, with which histochemically reactive zinc in the presynaptic vesicle is detected, was not also attenuated in the brain. On the other hand, serum corticosterone concentration, which was determined in the morning, was markedly increased after 2-week zinc deprivation and intracellular calcium signal, which was determined by fura-2 AM, was also increased in the hippocampus. In the hippocampus in zinc deficiency, intracellular free calcium concentration may be altered prior to the decrease in zinc concentration in the extracellular fluid. When rats were subjected to the open-field test, the frequency of line crossing and the time of grooming were decreased after 2-week zinc deprivation. In the plus-maze test, the time spent in the open arms was also decreased in zinc-deficient rats, suggesting that anxiety-like behavior is increased in zinc deficiency. The present study indicates that the increase in anxiety-like behavior in zinc deficiency may be linked to the increased concentration of basal free calcium in hippocampal cells, probably due to the increase in serum corticosterone concentration.

X-ray fluorescence microprobe imaging in biology and medicine

Characteristic X-ray fluorescence is a technique that can be used to establish elemental concentrations for a large number of different chemical elements simultaneously in different locations in cell and tissue samples. Exposing the samples to an X-ray beam is the basis of X-ray fluorescence microscopy (XFM). This technique provides the excellent trace element sensitivity; and, due to the large penetration depth of hard X-rays, an opportunity to image whole cells and quantify elements on a per cell basis. Moreover, because specimens prepared for XFM do not require sectioning, they can be investigated close to their natural, hydrated state with cryogenic approaches. Until several years ago, XFM was not widely available to bio-medical communities, and rarely offered resolution better then several microns. This has changed drastically with the development of third-generation synchrotrons. Recent examples of elemental imaging of cells and tissues show the maturation of XFM imaging technique into an elegant and informative way to gain insight into cellular processes. Future developments of XFM-building of new XFM facilities with higher resolution, higher sensitivity or higher throughput will further advance studies of native elemental makeup of cells and provide the biological community including the budding area of bionanotechnology with a tool perfectly suited to monitor the distribution of metals including nanovectors and measure the results of interactions between the nanovectors and living cells and tissues.

Acrodermatitis enteropathica and an overview of zinc metabolism

Acrodermatitis enteropathica is a rare autosomal recessive disorder of zinc deficiency. The genetic defect has been mapped to 8q24 and the defective gene identified as SLC39A4, which encodes the zinc transporter Zip4. The diagnosis is made by way of clinical presentation together with histopathology and laboratory tests. Here we provide an overview of zinc metabolism and a description of inherited and acquired zinc deficiency.

Zinc induces cell death in immortalized embryonic hippocampal cells via activation of Akt-GSK-3β signaling

Zinc is an essential catalytic and structural element of many proteins and a signaling messenger that is released by neuronal activity at many central excitatory synapses. Excessive synaptic release of zinc followed by entry into vulnerable neurons contributes severe neuronal cell death. We have previously observed that zinc-induced neuronal cell death is accompanied by Akt activation in embryonic hippocampal progenitor (H19-7) cells. In the present study, we examined the role of Akt activation and its downstream signaling events during extracellular zinc-induced neuronal cell death. Treatment of H19-7 cells with 10 microM of zinc plus zinc ionophore, pyrithione, led to increased phosphorylation of Akt at Ser-473/Thr-308 and increased Akt kinase activity. Zinc-induced Akt activation was accompanied by increased Tyr-phosphorylated GSK-3beta as well as increased GSK-3beta kinase activity. Transient overexpression of a kinase-deficient Akt mutant remarkably suppressed GSK-3beta activation and cell death. Furthermore, tau phosphorylation, but not the degradation of beta-catenin, was dependent upon zinc-induced GSK-3beta activation and contributed to cell death. The current data suggest that, following exposure to zinc, the sequential activation of Akt and GSK-3beta plays an important role directing hippocampal neural precursor cell death.

Systemic pyruvate administration markedly reduces infarcts and motor deficits in rat models of transient and permanent focal cerebral ischemia

Pyruvate markedly reduces neuronal death following transient global ischemia. In the present study, we investigated the possible neuroprotective effect of pyruvate in focal ischemia. Pyruvate (62.5-250 mg/kg) treatment, regardless of whether given intraperitoneally (ip) or intravenously (iv), decreased infarct volume by more than 50% in both transient (1 h) and permanent occlusion models. The infarct-reducing effects of pyruvate were maintained 14 days (d) after MCAO. Interestingly, higher doses failed to reduce the infarct size. Pyruvate administration also reduced motor deficits. Magnetic resonance (MR) spectroscopy revealed that protective doses of pyruvate, but not the non-protective doses, were associated with a reduction in the level of lactate compared with saline controls. Diffusion-weighted MR images further confirmed infarct reduction in pyruvate-treated rats. Pyruvate is an endogenous metabolite of glycolysis, and hence is unlikely to have serious side effects. Considering its substantial neuroprotective capacity in focal cerebral ischemia, a clinical trial is warranted.

Zinspy sensors with enhanced dynamic range for imaging neuronal cell zinc uptake and mobilization

Thiophene moieties were incorporated into previously described Zinspy (ZS) fluorescent Zn(II) sensor motifs (Nolan, E. M.; Lippard, S. J. Inorg. Chem. 2004, 43, 8310-8317) to provide enhanced fluorescence properties, low-micromolar dissociation constants for Zn(II), and improved Zn(II) selectivity. Halogenation of the xanthenone and benzoate moieties of the fluorescein platform systematically modulates the excitation and emission profiles, pH-dependent fluorescence, Zn(II) affinity, and Zn(II) complexation rates, offering a general strategy for tuning multiple properties of xanthenone-based metal ion sensors. Extensive biological studies in cultured cells and primary neuronal cultures demonstrate 2-{6-hydroxy-3-oxo-4,5-bis[(pyridin-2-ylmethylthiophen-2-ylmethylamino)methyl]-3H-xanthen-9-yl}benzoic acid (ZS5) to be a versatile imaging tool for detecting Zn(II) in vivo. ZS5 localizes to the mitochondria of HeLa cells and allows visualization of glutamate-mediated Zn(II) uptake in dendrites and Zn(II) release resulting from nitrosative stress in neurons.

Highly selective 4-amino-1,8-naphthalimide based fluorescent photoinduced electron transfer (PET) chemosensors for Zn(II) under physiological pH conditions

The design and synthesis of two novel fluorescent sensors based on the photoinduced electron transfer (PET) concept, and , for the detection of zinc under competitive media is described. These sensors are based on the 4-amino-1,8-naphthalimide fluorophore, which has an absorption band centred at 450 nm and emits in the green with lambda(max) approximately 550 nm. By functionalizing the chromophore with a simple benzyl or ethyl-aryl based iminodiacetate receptor at the 4-position, both high selectivity and sensitivity were achieved for the sensing of Zn(II) over other competitive transition and Group I and II metal ions. These sensors were also shown to be pH independent, with a pKa of 2.3 being determined for , which allows these to be used in highly competitive pH media. Upon sensing of Zn(II) the fluorescence emission spectrum is 'switched on' demonstrating the suppression of PET from the receptor to the fluorophore. For , the sensing of Zn(II) was achieved with Kd = 4 nM when measured in pH 7.4 buffered solution, in the presence of 1.1 mM of EGTA.

Intracellular zinc elevation measured with a "calcium-specific" indicator during ischemia and reperfusion in rat hippocampus: a question on calcium overload

Much of our current evidence concerning of the role of calcium (Ca2+) as a second messenger comes from its interaction with fluorescent probes; however, many Ca2+ probes also have a higher affinity for another divalent cation: zinc (Zn2+). In this study, using a selective Zn2+ probe (Newport Green), we investigated the accumulation of intracellular Zn2+ transients in acute rat hippocampal slices during ischemia, simulated by oxygen and glucose deprivation (OGD). Subsequent reperfusion with glucose-containing oxygenated medium resulted in an additional increase in intracellular Zn2+. Such observations compelled us to investigate the contribution of Zn2+ to the alleged intracellular Ca2+ overload occurring in ischemia and reperfusion. Using confocal fluorescent microscopy of Calcium Green-1, a widely used Ca2+ indicator, we detected increases in fluorescence intensity during OGD and reperfusion. However, application of a Zn2+ chelator, at the peak of the fluorescence elevation (interpreted as Ca2+ overload), resulted in a significant drop in intensity, suggesting that rising Zn2+ is the primary source of the increasing Calcium Green-1 fluorescence. Finally, staining with the cell viability indicator propidium iodide revealed that Zn2+ is responsible for the ischemic neuronal cell death, because Zn2+ chelation prevented cells from sustaining ischemic damage. Current cellular models of ischemic injury center on Ca2+-mediated excitotoxicity. Our results indicate that Zn2+ elevation contributes to conventionally recognized Ca2+ overload and also suggest that the role of Ca2+ in neurotoxicity described previously using Ca2+ probes may need to be re-examined to determine whether effect previously attributed to Ca2+ could, in part, be attributable to Zn2+.

Subunit-specific modulation of T-type calcium channels by zinc

Zinc (Zn2+) functions as a signalling molecule in the nervous system and modulates many ionic channels. In this study, we have explored the effects of Zn2+ on recombinant T-type calcium channels (CaV3.1, CaV3.2 and CaV3.3). Using tsA-201 cells, we demonstrate that CaV3.2 current (IC50, 0.8 microm) is significantly more sensitive to Zn2+ than are CaV3.1 and CaV3.3 currents (IC50, 80 microm and approximately 160 microm, respectively). This inhibition of CaV3 currents is associated with a shift to more negative membrane potentials of both steady-state inactivation for CaV3.1, CaV3.2 and CaV3.3 and steady-state activation for CaV3.1 and CaV3.3 currents. We also document changes in kinetics, especially a significant slowing of the inactivation kinetics for CaV3.1 and CaV3.3, but not for CaV3.2 currents. Notably, deactivation kinetics are significantly slowed for CaV3.3 current (approximately 100-fold), but not for CaV3.1 and CaV3.2 currents. Consequently, application of Zn2+ results in a significant increase in CaV3.3 current in action potential clamp experiments, while CaV3.1 and CaV3.2 currents are significantly reduced. In neuroblastoma NG 108-15 cells, the duration of CaV3.3-mediated action potentials is increased upon Zn2+ application, indicating further that Zn2+ behaves as a CaV3.3 channel opener. These results demonstrate that Zn2+ exhibits differential modulatory effects on T-type calcium channels, which may partly explain the complex features of Zn2+ modulation of the neuronal excitability in normal and disease states.

A Startling Role for Synaptic Zinc

It has long been known that the synaptic vesicles of certain glutamatergic terminals, as well as some inhibitory terminals, are richly supplied with zinc ions, yet the functional role of this pool of zinc in synaptic transmission has remained elusive. In this issue of Neuron, Hirzel et al. provide direct in vivo evidence that endogenous zinc is required for proper functioning of neuronal circuitry in the brainstem and spinal cord. They show that knockin mice carrying a point mutation which eliminates zinc potentiation of alpha1-containing glycine receptors develop severe sensorimotor deficits characteristic of impaired glycinergic neurotransmission.

Zinc release in the lateral nucleus of the amygdala by stimulation of the entorhinal cortex

Zinc release in the lateral nucleus of the amygdala was examined using rat brain slices. The lateral and basolateral nuclei in the amygdala were evidently stained by Timm's sulfide-silver staining method. When the amygdala including both the nuclei was stimulated with 100 mM KCl by means of in vivo microdialysis, extracellular zinc concentration was increased significantly. Zinc release in the lateral nucleus of the amygdala innervated by the entorhinal cortex was next examined in brain slices double-stained with zinc and calcium indicators. Extracellular zinc signal (ZnAF-2) in the lateral nucleus was increased with intracellular calcium signal (calcium orange) during delivery of tetanic stimuli to the entorhinal cortex. Both the increases were completely inhibited by addition of 1 micro M tetrodotoxin, a sodium channel blocker. Furthermore, calcium signal in the lateral nucleus during delivery of tetanic stimuli to the entorhinal cortex was increased in the presence of 10 micro M CNQX, an AMPA/KA receptor antagonist, and this increase was facilitated by addition of 1 mM CaEDTA, a membrane-impermeable zinc chelator. The present study suggested that zinc is released in the lateral nucleus of the amygdala by depolarization of the entorhinal neurons. In the lateral nucleus, zinc released may suppress the increase in presynaptic calcium signal.

Zinc-buffering capacity of a eukaryotic cell at physiological pZn

In spite of the paramount importance of zinc in biology, dynamic aspects of cellular zinc metabolism remain poorly defined at the molecular level. Investigations with human colon cancer (HT-29) cells establish a total cellular zinc concentration of 264 microM. Remarkably, about 10% of the potential high-affinity zinc-binding sites are not occupied by zinc, resulting in a surplus of 28 muM ligands (average Kd(c) = 83 pM) that ascertain cellular zinc-buffering capacity and maintain the "free" zinc concentration in proliferating cells at picomolar levels (784 pM, pZn = 9.1). This zinc-buffering capacity allows zinc to fluctuate only with relatively small amplitudes (DeltapZn = 0.3; below 1 nM) without significantly perturbing physiological pZn. Thus, the "free" zinc concentrations in resting and differentiated HT-29 cells are 614 pM and 1.25 nM, respectively. The calculation of these "free" zinc concentrations is based on measurements at different concentrations of the fluorogenic zinc-chelating agent and extrapolation to a zero concentration of the agent. It depends on the state of the cell, its buffering capacity, and the zinc dissociation constant of the chelating agent. Zinc induction of thionein (apometallothionein) ensures a surplus of unbound ligands, increases zinc-buffering capacity and the availability of zinc (DeltapZn = 0.8), but preserves the zinc-buffering capacity of the unoccupied high-affinity zinc-binding sites, perhaps for crucial physiological functions. Jointly, metallothionein and thionein function as the major zinc buffer under conditions of increased cellular zinc.

Metals in motor neuron diseases

Degenerative processes within the nervous system are common features in disease entities such as dementia of Alzheimer type (DAT), Parkinson disease (PD), and amyotrophic lateral sclerosis (ALS). ALS is a neurodegenerative disease with unknown etiology; widespread muscle wasting and respiratory failure lead to death within a few years. Denervation can be detected with electromyography and axonal deterioration monitored by motor unit number estimates. Several suggestions about the cause of ALS have emerged but no solid theory has yet precipitated. Lead or mercury exposure has been suggested. Exposure data alone cannot support this connection. Alterations in metal kinetics may underlie the deterioration of motor function observed in patients with ALS. In this review the role of metals in motor neuron disease is discussed. Both classic studies on exposure and recent understanding of metal binding proteins are considered. Aspects of peak exposure and excretion are merged toward an understanding of metal dynamics in ALS. An overview of chemical and electrophysiological investigations is given in the context of neurodegeneration.

Quantitative genetic analysis of brain copper and zinc in BXD recombinant inbred mice

Copper and zinc are trace nutrients essential for normal brain function, yet an excess of these elements can be toxic. It is important therefore that these metals be closely regulated. We recently conducted a quantitative trait loci (QTL) analysis to identify chromosomal regions in the mouse containing possible regulatory genes. The animals came from 15 strains of the BXD/Ty recombinant inbred (RI) strain panel and the brain regions analyzed were frontal cortex, caudate-putamen, nucleus accumbens and ventral midbrain. Several QTL were identified for copper and/or zinc, most notably on chromosomes 1, 8, 16 and 17. Genetic correlational analysis also revealed associations between these metals and dopamine, cocaine responses, saccharine preference, immune response and seizure susceptibility. Notably, the QTL on chromosome 17 is also associated with seizure susceptibility and contains the histocompatibility H2 complex. This work shows that regulation of zinc and copper is under polygenic influence and is intimately related to CNS function. Future work will reveal genes underlying the QTL and how they interact with other genes and the environment. More importantly, revelation of the genetic underpinnings of copper and zinc brain homeostasis will aid our understanding of neurological diseases that are related to copper and zinc imbalance.

Oxidative stress and brain aging: is zinc the link?

Zn(2+) dyshomeostasis has been strongly linked to neuronal injury in many neurological conditions. Toxic accumulation of intracellular free Zn(2+) ([Zn(2+)](i)) may result from either flux of the cation through glutamate receptor-associated channels, voltage-sensitive calcium channels, or Zn(2+)-sensitive membrane transporters. Injurious [Zn(2+)](i) rises can also result from release of the cation from intracellular sites such as metallothioneins (MTs) and mitochondria. Chronic inflammation and oxidative stress are hallmarks of aging. Zn(2+) homeostasis is affected by oxidative stress, which is a potent trigger for detrimental Zn(2+) release from MTs. Interestingly, Zn(2+) itself is a strong inducer of oxidative stress by promoting mitochondrial and extra-mitochondrial production of reactive oxygen species. In this review, we examine how Zn(2+) dyshomeostasis and oxidative stress might act synergistically to promote aging-related neurodegeneration.

In vivo expression and functional characterization of the zinc transporter ZnT8 in glucose-induced insulin secretion

Insulin-secreting pancreatic beta cells are exceptionally rich in zinc. In these cells, zinc is required for zinc-insulin crystallization within secretory vesicles. Secreted zinc has also been proposed to be a paracrine and autocrine modulator of glucagon and insulin secretion in pancreatic alpha and beta cells, respectively. However, little is known about the molecular mechanisms underlying zinc accumulation in insulin-containing vesicles. We previously identified a pancreas-specific zinc transporter, ZnT-8, which colocalized with insulin in cultured beta cells. In this paper we studied its localization in human pancreatic islet cells, and its effect on cellular zinc content and insulin secretion. In human pancreatic islet cells, ZnT-8 was exclusively expressed in insulin-producing beta cells, and colocalized with insulin in these cells. ZnT-8 overexpression stimulated zinc accumulation and increased total intracellular zinc in insulin-secreting INS-1E cells. Furthermore, ZnT-8-overexpressing cells display enhanced glucose-stimulated insulin secretion compared with control cells, only for a high glucose challenge, i.e. >10 mM glucose. Altogether, these data strongly suggest that the zinc transporter ZnT-8 is a key protein for both zinc accumulation and regulation of insulin secretion in pancreatic beta cells.

The inhibition of mitochondrial complex I (NADH:ubiquinone oxidoreductase) by Zn2+

NADH:ubiquinone oxidoreductase (complex I) from bovine heart mitochondria is a highly complicated, membrane-bound enzyme. It is central to energy transduction, an important source of cellular reactive oxygen species, and its dysfunction is implicated in neurodegenerative and muscular diseases and in aging. Here, we describe the effects of Zn2+ on complex I to define whether complex I may contribute to mediating the pathological effects of zinc in states such as ischemia and to determine how Zn2+ can be used to probe the mechanism of complex I. Zn2+ inhibits complex I more strongly than Mg2+, Ca2+, Ba2+, and Mn2+ to Cu2+ or Cd2+. It does not inhibit NADH oxidation or intramolecular electron transfer, so it probably inhibits either proton transfer to bound quinone or proton translocation. Thus, zinc represents a new class of complex I inhibitor clearly distinct from the many ubiquinone site inhibitors. No evidence for increased superoxide production by zinc-inhibited complex I was detected. Zinc binding to complex I is mechanistically complicated. During catalysis, zinc binds slowly and progressively, but it binds rapidly and tightly to the resting state(s) of the enzyme. Reactivation of the inhibited enzyme upon the addition of EDTA is slow, and inhibition is only partially reversible. The IC50 value for the Zn2+ inhibition of complex I is high (10-50 microm, depending on the enzyme state); therefore, complex I is unlikely to be a major site for zinc inhibition of the electron transport chain. However, the slow response of complex I to a change in Zn2+ concentration may enhance any physiological consequences.

Episodic ataxia type 1 mutation F184C alters Zn2+-induced modulation of the human K+ channel Kv1.4-Kv1.1/Kvbeta1.1

Episodic ataxia type 1 (EA1) is a Shaker-like channelopathy characterized by continuous myokymia and attacks of imbalance with jerking movements of the head, arms, and legs. Although altered expression and gating properties of Kv1.1 channels underlie EA1, several disease-causing mechanisms remain poorly understood. It is likely that Kv1.1, Kv1.4, and Kvbeta1.1 subunits form heteromeric channels at hippocampal mossy fiber boutons from which Zn(2+) ions are released into the synaptic cleft in a Ca(2+)-dependent fashion. The sensitivity of this macromolecular channel complex to Zn(2+) is unknown. Here, we show that this heteromeric channel possesses a high-affinity (<10 muM) and a low-affinity (<0.5 mM) site for Zn(2+), which are likely to regulate channel availability at distinct presynaptic membranes. Furthermore, the EA1 mutation F184C, located within the S1 segment of the Kv1.1 subunit, markedly decreased the equilibrium dissociation constants for Zn(2+) binding to the high- and low-affinity sites. The functional characterization of the Zn(2+) effects on heteromeric channels harboring the F184C mutation also showed that this ion significantly 1) slowed the activation rate of the channel, 2) increased the time to reach peak current amplitude, 3) decreased the rate and amount of current undergoing N-type inactivation, and 4) slowed the repriming of the channel compared with wild-type channels. These results demonstrate that the EA1 mutation F184C will not only sensitize the homomeric Kv1.1 channel to extracellular Zn(2+), but it will also endow heteromeric channels with a higher sensitivity to this metal ion. During the vesicular release of Zn(2+), its effects will be in addition to the intrinsic gating defects caused by the mutation, which is likely to exacerbate the symptoms by impairing the integration and transmission of signals within specific brain areas.

Endothelial response to stress from exogenous Zn2+resembles that of NO-mediated nitrosative stress, and is protected by MT-1 overexpression

While nitric oxide (NO)-mediated biological interactions have been intensively studied, the underlying mechanisms of nitrosative stress with resulting pathology remain unclear. Previous studies have demonstrated that NO exposure increases free zinc ions (Zn2+) within cells. However, the resulting effects on endothelial cell survival have not been adequately resolved. Thus the purpose of this study was to investigate the role of altered zinc homeostasis on endothelial cell survival. Initially, we confirmed the previously observed significant increase in free Zn2+with a subsequent induction of apoptosis in our pulmonary artery endothelial cells (PAECs) exposed to the NO donor N-[2-aminoethyl]- N-[2-hydroxy-2-nitrosohydrazino]-1,2-ethylenediamine. However, NO has many effects upon cell function and we wanted to specifically evaluate the effects mediated by zinc. To accomplish this we utilized the direct addition of zinc chloride (ZnCl2) to PAEC. We observed that Zn2+-exposed PAECs exhibited a dose-dependent increase in superoxide (O2−·) generation that was localized to the mitochondria. Furthermore, we found Zn2+-exposed PAECs exhibited a significant reduction in mitochondrial membrane potential, loss of cardiolipin from the inner leaflet, caspase activation, and significant increases in TdT-mediated dUTP nick end labeling-positive cells. Furthermore, using an adenoviral construct for the overexpression of the Zn2+-binding protein, metallothionein-1 (MT-1), we found either MT-1 overexpression or coincubation with a Zn2+-selective chelator, N, N,N′, N′-tetrakis(2-pyridylmethyl)ethylene-diamide, in PAECs significantly protected the mitochondria from both NO and Zn2+-mediated disruption and induction of apoptosis and cell death. In summary, our results indicate that a loss of Zn2+homeostasis produces mitochondrial dysfunction, increased oxidative stress, and apoptotic cell death. We propose that regulation of Zn2+levels may represent a potential therapeutic target for disease associated with both nitrosative and oxidative stress.

Zinc coordination environments in proteins as redox sensors and signal transducers

Zinc/cysteine coordination environments in proteins are redox-active. Oxidation of the sulfur ligands mobilizes zinc, while reduction of the oxidized ligands enhances zinc binding, providing redox control over the availability of zinc ions. Some zinc proteins are redox sensors, in which zinc release is coupled to conformational changes that control varied functions such as enzymatic activity, binding interactions, and molecular chaperone activity. Whereas the released zinc ion in redox sensors has no known function, the redox signal is transduced to specific and sensitive zinc signals in redox transducers. Released zinc can bind to sites on other proteins and modulate signal transduction, generation of metabolic energy, mitochondrial function, and gene expression. The paradigm of such redox transducers is the zinc protein metallothionein, which, together with its apoprotein, thionein, functions at a central node in cellular signaling by redistributing cellular zinc, presiding over the availability of zinc, and interconverting redox and zinc signals. In this regard, the transduction of nitric oxide (NO) signals into zinc signals by metallothionein has received particular attention. It appears that redox-inert zinc has been chosen to control some aspects of cellular thiol/disulfide redox metabolism. Tight control of zinc is essential for redox homeostasis because both increases and decreases of cellular zinc elicit oxidative stress. Depending on its availability, zinc can be cytoprotective as a pro-antioxidant or cytotoxic as a pro-oxidant. Any condition with acute or chronic oxidative stress is expected to perturb zinc homeostasis.

Measuring cell viability with membrane impermeable zinc fluorescent indicator

Recent findings suggest that the accumulation of cytoplasmic zinc [Zn2+]i is a ubiquitous component in the cell death cascade. Zn2+ can be liberated from intracellular stores following oxidative stress and contribute to cell death processes. Here we show that the membrane/cell impermeable Zn2+ fluorescent indicator Newport Green (NG), which is non-toxic and impermeable to the membranes of healthy cells, can label unhealthy cells in tissue slices in a manner comparable to the traditional viability indicator propidium iodide (PI). Using confocal microscopy, we detected PI labeled nuclei colocalized with NG fluorescence. Our results indicate that cells which absorbed PI into their nuclei also allowed cell-impermeable Zn2+ dye to penetrate their plasma membranes, subsequently exhibiting cytosolic and nuclear fluorescence. As in PI staining, we observed marked increases in NG fluorescence in damaged/dead cells of tissue slices. Two other cell impermeable fluorescent Zn2+ dyes, Fluozin-3 and Zinpyr-4, also stained cytosolic Zn2+ in PI labeled cells. Our data indicates that the application of a Zn2+ fluorescent indicator is a fast, simple, non-toxic and reliable method for visualizing cell viability within in vitro tissue preparations. Accordingly, we demonstrate that intracellular accumulation of Zn2+ correlates with neuronal death.

Aldehydes release zinc from proteins. A pathway from oxidative stress/lipid peroxidation to cellular functions of zinc

Oxidative stress, lipid peroxidation, hyperglycemia-induced glycations and environmental exposures increase the cellular concentrations of aldehydes. A novel aspect of the molecular actions of aldehydes, e.g. acetaldehyde and acrolein, is their reaction with the cysteine ligands of zinc sites in proteins and concomitant zinc release. Stoichiometric amounts of acrolein release zinc from zinc-thiolate coordination sites in proteins such as metallothionein and alcohol dehydrogenase. Aldehydes also release zinc intracellularly in cultured human hepatoma (HepG2) cells and interfere with zinc-dependent signaling processes such as gene expression and phosphorylation. Thus both acetaldehyde and acrolein induce the expression of metallothionein and modulate protein tyrosine phosphatase activity in a zinc-dependent way. Since minute changes in the availability of cellular zinc have potent effects, zinc release is a mechanism of amplification that may account for many of the biological effects of aldehydes. The zinc-releasing activity of aldehydes establishes relationships among cellular zinc, the functions of endogenous and xenobiotic aldehydes, and redox stress, with implications for pathobiochemical and toxicologic mechanisms.

Targeting glycogen synthase kinase-3 as an approach to develop novel mood-stabilising medications

Historically, success in the pharmacological treatment of bipolar disorder has arisen either from serendipitous findings or from studies with drugs (antipsychotics and anticonvulsants) developed for other indications (schizophrenia and epilepsy, respectively). Lithium has been in widespread clinical use in the treatment of bipolar disorder for > 30 years. Development of lithium-mimetic compounds has the potential to result in a more specific medication, with fewer side effects and a less narrow dose range. However, novel medications based upon a known mechanism of action of this drug are yet to be developed. Increasing evidence suggests that a next-generation lithium compound may derive from knowledge of a direct target of lithium, glycogen synthase kinase-3 (GSK-3). GSK-3 is an intracellular enzyme implicated as a critical component in many neuronal signalling pathways. However, despite the large body of preclinical data discussed in this review, definitive validation of GSK-3 as therapeutically relevant target of lithium will require clinical trials with novel GSK-3 inhibitors. A number of recent reports suggest that it is possible to develop selective, small-molecule GSK-3 inhibitors.

Transient synaptic zinc-positive thalamocortical terminals in the developing barrel cortex

In rat barrel cortex, layer 4 has a transiently high density of zinc-positive terminations from postnatal day (P)9 to P12 [P.W. Land & L. Shamalla-Hannah (2002)J. Comp. Neurol., 447, 43-56]. These terminations have been proposed to originate from cortico-cortical connections, but their exact origin is unknown. To determine their sources, we injected sodium selenite into the barrel cortex of two adult rats and 32 pups, from P5 to P28. As predicted, abundant zinc-positive cortically projecting neurons were visible around the injection sites and in distant cortical areas. From P9 to P13, however, neurons retrogradely labeled by zinc selenite occurred in the thalamus, in topographically appropriate regions of the ventroposterior medial (VPM) and posterior nuclei (Po). Because there are no previous reports of zinc-positive sensory thalamocortical connections, we sought corroboration of this unexpected finding by electron microscopy. This revealed a subset of boutons in layers 4 and 1, positive for both zinc and vesicular glutamate transporter 2, a protein used by thalamocortical terminations. Finally, in an additional nine rats, we carried out in situ hybridization for zinc transporter 3 mRNA. Moderate signal was detected in VPM and Po at P10, but this disappeared by P28. In contrast, a strong signal was apparent in the anterodorsal nucleus, which projects to limbic areas, and this persisted at P28. The timing of the transient zinc-positive terminations in the sensory thalamus roughly coincides with the onset of exploratory and whisking behavior in the middle of the second postnatal week; and this suggests zinc is important for activity-related refinement of circuitry.

Zinc homeostasis in aging: two elusive faces of the same "metal"

Proteins involved in zinc homeostasis may be altered in aging. This phenomenon may lead to zinc deficiency in the peripheral blood and an accumulation of zinc bound to insoluble aggregates at the extracellular level in the brain. Therefore, it should be more correct to talk about aging as a condition associated with zinc dyshomeostasis rather than deficiency. Restoring functional zinc homeostasis in aging people is an attractive field for antiaging research, but requires further knowledge than the current state of the art.

A role for zinc in postsynaptic density asSAMbly and plasticity?

Chemical synapses are asymmetric cell junctions that mediate communication between neurons. Multidomain scaffolding proteins of the Shank family act as major organizing elements of the "postsynaptic density"--that is, the cytoskeletal protein matrix associated with the postsynaptic membrane. A recent study has shown that the C-terminal sterile alpha-motif or "SAM domain" of Shank3 (also known as ProSAP2) can form two-dimensional sheets of helical fibers. Assembly and packaging of these fibers are markedly enhanced by the presence of Zn2+ ions. Zn2+ can be released together with glutamate from synaptic vesicles and can enter the postsynaptic cell through specific ionotropic receptors. Based on these observations, we propose a new model of synaptic plasticity in which Zn2+ influx directly and instantly modulates the structure and function of the postsynaptic density.

Exocytosis of vesicular zinc reveals persistent depression of neurotransmitter release during metabotropic glutamate receptor long-term depression at the hippocampal CA3-CA1 synapse

Exocytosis can be directly measured in mammalian brain slices by fluorescence detection of vesicular zinc release. Detection of the low-level evoked zinc signal [Zn]t was first demonstrated at the zinc-rich hippocampal mossy fiber pathway and required the use of high-frequency presynaptic stimulation. Here, we show that release after individual action potentials can be reliably detected even at non-mossy fiber, zinc-poor synapses in the hippocampus, a major enhancement in the temporal resolution of the technique. Short-term facilitation of release properties of zinc-positive CA3-CA1 Schaffer collateral/commissural synapses in the stratum radiatum differ from those at mossy fibers but are similar to those measured for the EPSP [field EPSP (fEPSP)]. The N-type Ca2+ channel toxin omega-conotoxin GVIA inhibited both the [Zn]t and fEPSP equally, and the modulation of neurotransmitter release by neuropeptide Y, baclofen, and adenosine as revealed by [Zn]t closely resembles that measured for the fEPSP. A long-standing controversy in hippocampal synaptic plasticity involves the site of long-term depression (LTD) at these synapses. Using zinc release as a direct marker for exocytotic events and a surrogate marker for glutamate release, we demonstrate that persistent depression of presynaptic release occurs in the late expression of DHPG [(S)-3,5-dihydroxyphenylglycine]-induced LTD at this synapse. The ability to examine release dynamics with zinc fluorescence detection will facilitate exploration of the molecular pharmacology and plasticity of exocytosis at many CNS synapses.

Degradation of the Alzheimer Disease Amyloid β-Peptide by Metal-dependent Up-regulation of Metalloprotease Activity

Biometals play an important role in Alzheimer disease, and recent reports have described the development of potential therapeutic agents based on modulation of metal bioavailability. The metal ligand clioquinol (CQ) has shown promising results in animal models and small phase clinical trials; however, the actual mode of action in vivo has not been determined. We now report a novel effect of CQ on amyloid beta-peptide (Abeta) metabolism in cell culture. Treatment of Chinese hamster ovary cells overexpressing amyloid precursor protein with CQ and Cu(2+) or Zn(2+) resulted in an approximately 85-90% reduction of secreted Abeta-(1-40) and Abeta-(1-42) compared with untreated controls. Analogous effects were seen in amyloid precursor protein-overexpressing neuroblastoma cells. The secreted Abeta was rapidly degraded through up-regulation of matrix metalloprotease (MMP)-2 and MMP-3 after addition of CQ and Cu(2+). MMP activity was increased through activation of phosphoinositol 3-kinase and JNK. CQ and Cu(2+) also promoted phosphorylation of glycogen synthase kinase-3, and this potentiated activation of JNK and loss of Abeta-(1-40). Our findings identify an alternative mechanism of action for CQ in the reduction of Abeta deposition in the brains of CQ-treated animals and potentially in Alzheimer disease patients.

Zinc influx and physiological consequences in the beta-insulinoma cell line, Min6

In the mammalian pancreas, high concentrations of Zn(2+) are co-secreted with insulin, which may then permeate via abundant L-type Ca(2+) channels (LTCC) present on the beta-cells. Neither the mechanisms utilized by these cells to lower cytosolic Zn(2+) nor the implications of increased intracellular Zn(2+) on beta-cell survival are well understood. To address this, we employed cell imaging of Zn(2+) and Ca(2+) in the beta-insulinoma cell line, Min6. Depolarization induced an intense zinc influx that was blocked by nifedipine and verapamil, indicating that Zn(2+) permeates via the LTCC. Both Ca(2+) and Zn(2+) permeated concomitantly, yet while Ca(2+) was subsequently removed from the cytosol, Zn(2+) was retained in the cells. Fluorescent staining of vesicular Zn(2+) using ZP1 demonstrated that Zn(2+) could be slowly sequestered following a brief exposure to low concentration of Zn(2+). In contrast, cells were unable to sequester Zn(2+) following application of high concentrations, which was followed by massive cell death. Our results demonstrate homeostatic crosstalk between the plasma membrane and intracellular zinc transporters and suggest that attenuating zinc influx may enhance beta-cell survival.

Implementation of X-ray fluorescence microscopy for investigation of elemental abnormalities in amyotrophic lateral sclerosis

The abnormalities of metallochemical reactions may contribute to the pathogenesis of Amyotrophic Lateral Sclerosis (ALS). In the present work, an investigation of the elemental composition of the gray matter, nerve cells and white matter from spinal cord tissues representing three ALS cases and five non-ALS controls was performed. This was done with the use of the synchrotron microbeam X-ray fluorescence technique (micro-SRXRF). The following elements were detected in the tissue sections: P, S, Cl, K, Ca, Fe, Cu, Zn and Br. A higher accumulation of Cl, K, Ca, Zn and Br was observed in the nerve cell bodies than in the surrounding tissue. Contrary to all other elements, Zn accumulation was lower in the white matter areas than in the gray matter ones. The results of quantitative analysis showed that there were no general abnormalities in the elemental accumulation between the ALS and the control group. However, for individual ALS cases such abnormalities were observed for the nerve cells. We also demonstrated differences in the elemental accumulation between the analyzed ALS cases.

Increase in synaptic hippocampal zinc concentration following chronic but not acute zinc treatment in rats

Electroconvulsive seizures (ECS), one of the most effective treatments of depression, induce mossy fiber sprouting (when assayed by means of synaptic zinc method), and this indicates an increase in the synaptic zinc level in the hippocampus following such therapy. The aim of the present study was to investigate the influence of acute and chronic zinc hydroaspartate administration on the synaptic and total zinc level in the rat hippocampus. We used two methods of zinc determination: (1) zinc-selenium method, which images the pool of synaptic zinc, and (2) flame atomic absorption spectrometry, which assays the total concentration of zinc. Our results indicate that chronic (14 x 65 mg/kg), but not acute, zinc hydroaspartate administration intraperitoneally (i.p.) increases the pool of synaptic zinc in the majority of rat hippocampal layers (by 72-190%), except for the stratum moleculare and stratum radiatum CA, and perforant path DG. On the other hand, no changes were found in total hippocampal zinc level, measured by flame atomic absorption spectrometry. These data suggest that chronic zinc treatment increases the pool of synaptic zinc in the hippocampus, and this effect is similar to that observed following chronic ECS treatment. The measurement of zinc concentration in the whole hippocampus by the flame atomic absorption spectrometry method is not sensitive enough to detect such subtle alteration.

Neural Overexcitation and Implication of NMDA and AMPA Receptors in a Mouse Model of Temporal Lobe Epilepsy Implying Zinc Chelation

PURPOSE

Zinc chelation with diethyldithiocarbamate (DEDTC) during nondamaging kainic acid administration enhances excitotoxicity to the level of cell damage. The objective of this work was to study the developing of the lesion in this model of temporal lobe epilepsy and the implications of the different types of glutamate receptors.

METHODS

The antagonist of the N-methyl-D-aspartate (NMDA) receptor MK-801, and the antagonist of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor GYKI52466, were used concomitantly with intraperitoneal administration of kainic acid (15 mg/kg) followed by DEDTC (150 mg/kg) in mouse. The animals were killed at different times from 4 h to 7 days. Fos proteins were used as markers of cell overexcitation; heat-shock protein 72 (HSP72) as marker of cell stress.

RESULTS

Neither kainic acid nor DEDTC alone, at the doses used, led to cell loss, HSP72 expression, or permanent Fos protein induction. When combined, the hilus and cornu ammonis were damaged; principal cells in these areas coexpressed c-Fos and HSP72, with the exception of CA2; interneurons did not express HSP72 in any area. MK-801 completely abolished damage and HSP72 expression from the hippocampus. GYKI52466 blocked CA1 damage and HSP72 expression in the CA1 but not in the CA3.

CONCLUSIONS

Synaptic zinc increases the tolerance of hippocampus to overexcitation. All the areas that are fated to die are determined simultaneously; the damage in the CA1 is not an extension of the damage in the CA3. Damage of the CA3 is dependent on kainate and NMDA receptors, whereas the damage of the CA1 depends on AMPA and NMDA receptors.

Imaging synaptic zinc: promises and perils

It is well established that some excitatory nerve terminals have high concentrations of Zn(2+) in their synaptic vesicles. For some time, it has been believed that synaptic Zn(2+) is released during neurotransmission and acts as a neuromodulator. Fluorescent Zn(2+) indicators that do not penetrate membranes offer the prospect of rendering the release of Zn(2+) visible. Here, I take a critical look at fluorimetric imaging experiments devised to determine whether Zn(2+) is released and show that they are particularly susceptible to artifacts. Moreover, I will argue that recent experiments suggest that, rather than being released, Zn(2+) is presented to the extracellular space firmly coordinated to presynaptic macromolecules.

Use of a zinc fluorophore to measure labile pools of zinc in body fluids and cell-conditioned media

Here we describe a rapid and sensitive zinquin-based fluorometric assay that enables one to monitor levels of labile Zn(II) in body fluids, buffers, and cell-conditioned culture media as well as changes in these pools in disease. Labile pools of Zn(II) are free or loosely bound pools and more tightly bound but zinquin-accessible pools in contrast to the fixed pools of Zn(II) within metalloproteins. In human plasma, mean labile Zn(II) was 8.1 µM (SEM 0.53; n = 81) and constituted about 70% of the total plasma Zn(II) and >90% of human plasma albumin Zn(II). Plasma labile Zn(II) was significantly depleted after 7 days of Zn(II) deprivation in mice, despite only small changes in body weight. Labile Zn(II) concentrations were also measured in the induced sputum plugs, saliva, and urine of normal adults and were 1.30 µM (SEM 0.27; n = 73), 0.11 µM (SEM 0.11; n = 6), and 0.23 µM (SEM 0.08; n = 8), respectively. Urinary labile Zn(II) concentration was significantly increased in some patients with type II diabetes mellitus (overall mean was 0.90 µM, (SEM 0.30; n = 12). The technique may be particularly useful in assessing extracellular Zn(II) levels in diseases associated with altered Zn(II) homeostasis, identifying those subjects most in need of Zn(II) supplementation, and defining the optimum concentrations of available Zn(II) in buffers and culture media.

Concentrations of extracellular free zinc (pZn)e in the central nervous system during simple anesthetization, ischemia and reperfusion

"Free Zn2+" (rapidly exchangeable Zn2+) is stored along with glutamate in the presynaptic terminals of specific specialized (gluzinergic) cerebrocortical neurons. This synaptically releasable Zn2+ has been recognized as a potent modulator of glutamatergic transmission and as a key toxin in excitotoxic neuronal injury. Surprisingly (despite abundant work on bound zinc), neither the baseline concentration of free Zn2+ in the brain nor the presumed co-release of free Zn2+ and glutamate has ever been directly observed in the intact brain in vivo. Here, we show for the first time in dialysates of rat and rabbit brain and human CSF samples from lumbar punctures that: (i) the resting or "tonic" level of free Zn2+ signal in the extracellular fluid of the rat, rabbit and human being is approximately 19 nM (95% range: 5-25 nM). This concentration is 15,000-fold lower than the "300 microM" concentration which is often used as the "physiological" concentration of free zinc for stimulating neural tissue. (ii) During ischemia and reperfusion in the rabbit, free zinc and glutamate are (as has often been presumed) released together into the extracellular fluid. (iii) Unexpectedly, Zn2+ is also released alone (without glutamate) at a variable concentration for several hours during the reperfusion aftermath following ischemia. The source(s) of this latter prolonged release of Zn2+ is/are presumed to be non-synaptic and is/are now under investigation. We conclude that both Zn2+ and glutamate signaling occur in excitotoxicity, perhaps by two (or more) different release mechanisms.

Zinquin identifies subcellular compartmentalization of zinc in cortical neurons. Relation to the trafficking of zinc and the mitochondrial compartment

Zinquin (Zn(2+) selective fluorophore), when used to visualize intracellular Zn(2+), typically shows brightly fluorescent perinuclear endosome-like structures, presumably identifying Zn(2+) containing organelles. In this study, zinquin identified numerous and widespread sites of Zn(2+) compartmentalization in primary cultures of embryonic rat cortical neurons. Nuclear fluorescence, however, was absent. We labeled neuronal mitochondria with MitoTracker Green in the presence of zinquin and show that the fluorescent patterns of MitoTracker Green and zinquin were distinct and clearly different in both the perinuclear region and in processes. The mitochondrial compartment was much larger than the sum of the areas of zinquin fluorescence, as indicated by the small amount (<10% MitoTracker Green over zinquin) of overlap of MitoTracker Green on zinquin. Zinquin fluorescence was unaffected by carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) treatment. The zinquin fluorescent objects were generally spherical in shape with a average diameter of about 0.6 mum. Most fluorescent objects, nearly two thirds on average, appeared to be docked, but both anterograde and retrograde movements were observed by time lapse image analysis. Although some fluorescent objects moved as much as 1 mum in 5 min, typical movements were smaller, usually 0.5 mum or less. Colchicine treatment caused striking aggregation of MitoTracker Green most noticeable in the perinuclear region. Zinquin fluorescence similarly showed reduced distribution throughout the cytoplasm, suggesting that zinquin fluorescent structures were associated with microtubules. Treatment with cytochalasin D had little noticeable effect on either the pattern of zinquin and MitoTracker Green fluorescence or their coincidence. Thus, numerous Zn(2+) sequestering organelles/structures are present in perinuclear regions and processes of cultured neurons and are sometimes found coincident with mitochondria. We demonstrated real time trafficking of sequestered Zn(2+), using zinquin fluorescence, apparently associated with an endosome-like compartment or protein complexes in the cytosol.

Modulation of inhibitory glycine receptors in cultured embryonic mouse hippocampal neurons by zinc, thiol containing redox agents and carnosine

Modulation of inhibitory glycine receptors by zinc (Zn(2+)) and endogenous redox agents such as glutathione may alter inhibition in the mammalian brain. Despite the abundance of Zn(2+) in the hippocampus and its ability to modulate glycine receptors, few studies have examined Zn(2+) modulation of hippocampal glycine receptors. Whether redox agents modulate hippocampal glycine receptors also remains unknown. This study examined Zn(2+) and redox modulation of glycine receptor-mediated currents in cultured embryonic mouse hippocampal neurons using whole-cell recordings. Zn(2+) concentrations below 10 microM potentiated currents elicited by low glycine, beta-alanine, and taurine concentrations by 300-400%. Zn(2+) concentrations above 300 microM produced nearly complete inhibition. Potentiating Zn(2+) concentrations shifted the dose-response curves for the three agonists to the left and decreased the Hill coefficient for glycine and beta-alanine but not taurine. Inhibiting Zn(2+) concentrations shifted the dose-response curves for glycine and beta-alanine to the right but reduced the maximum taurine response. Histidine residues may participate in potentiation because diethyl pyrocarbonate and pH 5.4 diminished Zn(2+) enhancement of glycine currents. pH 5.4 diminished Zn(2+) block of glycine currents, but diethyl pyrocarbonate did not. These findings indicate that separate sites mediate Zn(2+) potentiation and inhibition. The redox agents glutathione, dithiothreitol, tris(2-carboxyethyl)phosphine, and 5,5'-dithiobis(2-nitrobenzoic acid) did not alter glycine currents by a redox mechanism. However, glutathione and dithiothreitol interfered with the effects of Zn(2+) on glycine currents by chelating it. Carnosine had similar effects. Thus, Zn(2+) and thiol containing redox agents that chelate Zn(2+) modulate hippocampal glycine receptors with the mechanism of Zn(2+) modulation being agonist dependent.

Synaptic release of zinc from brain slices: factors governing release, imaging, and accurate calculation of concentration

Cerebrocortical neurons that store and release zinc synaptically are widely recognized as critical in maintenance of cortical excitability and in certain forms of brain injury and disease. Through the last 20 years, this synaptic release has been observed directly or indirectly and reported in more than a score of publications from over a dozen laboratories in eight countries. However, the concentration of zinc released synaptically has not been established with final certainty. In the present work we have considered six aspects of the methods for studying release that can affect the magnitude of zinc release, the imaging of the release, and the calculated concentration of released zinc. We present original data on four of the issues and review published data on two others. We show that common errors can cause up to a 3000-fold underestimation of the concentration of released zinc. The results should help bring consistency to the study of synaptic release of zinc.