Stimulation-induced uptake and release of zinc in hippocampal slices

Molecular and biophysical basis of glutamate and trace metal modulation of voltage-gated Ca(v)2.3 calcium channels

Here, we describe a new mechanism by which glutamate (Glu) and trace metals reciprocally modulate activity of the Ca_v2.3 channel by profoundly shifting its voltage-dependent gating. We show that zinc and copper, at physiologically relevant concentrations, occupy an extracellular binding site on the surface of Ca_v2.3 and hold the threshold for activation of these channels in a depolarized voltage range. Abolishing this binding by chelation or the substitution of key amino acid residues in IS1–IS2 (H111) and IS2–IS3 (H179 and H183) loops potentiates Ca_v2.3 by shifting the voltage dependence of activation toward more negative membrane potentials. We demonstrate that copper regulates the voltage dependence of Ca_v2.3 by affecting gating charge movements. Thus, in the presence of copper, gating charges transition into the “ON” position slower, delaying activation and reducing the voltage sensitivity of the channel. Overall, our results suggest a new mechanism by which Glu and trace metals transiently modulate voltage-dependent gating of Ca_v2.3, potentially affecting synaptic transmission and plasticity in the brain.

Co-release of glutamate and GABA from single, identified mossy fibre giant boutons

Several laboratories have provided immunohistochemical, molecular biological and electrophysiological evidence that the glutamatergic granule cells of the dentate gyrus can transiently express a GABAergic phenotype during development. Electrophysiological recordings on hippocampal slices obtained during this period have shown that stimulation of the mossy fibres (MFs) provokes simultaneous monosynaptic GABA(A) and glutamate receptor-mediated responses in their target cells,which have the pharmacological and physiological characteristics of MF neurotransmission. This evidence, although strongly supporting the hypothesis that MFs co-release glutamate and GABA, is indirect, as the extracellular stimulation used in slice experiments could activate fibres other than MFs. In this study, we show that selective stimulation of single, identified MF boutons (MFBs) attached to the apical dendrites of dissociated pyramidal cells of developing rats produced synaptic currents mediated by either glutamate receptors only or by both glutamate and GABA(A) receptors. By contrast, stimulation of MFBs of adult rats produced exclusively glutamate receptor-mediated responses. All responses evoked by stimulation of MFBs underwent strong frequency-dependent potentiation and were depressed by the activation of presynaptic metabotropic glutamate receptors. On the other hand, synaptic responses evoked by stimulation of interneuronal boutons located on the soma or on the basal dendrites of the same pyramidal cells were exclusively mediated by GABA(A) receptors, underwent frequency-dependent depression and were unaffected by mGluR agonists.We here demonstrate that the simultaneous glutamatergic and GABAergic responses evoked by MF stimulation in pyramidal cells of CA3 during development have a common origin in the giant MFBs.

A highly sensitive two-photon fluorescent probe for mitochondrial zinc ions in living tissue

We report a highly sensitive two-photon probe (SZn2-Mito) which shows a 70-fold two-photon excited fluorescence enhancement in response to Zn(2+) and can selectively detect mitochondrial Zn(2+) in a rat hippocampal slice at a depth of 100-200 μm by using two-photon microscopy.

Alteration of the cerebral zinc pool in a mouse model of Alzheimer disease

Synaptic vesicle Zn is regulated by zinc transporter 3 (ZnT3) and is involved in neurotransmission and synaptic plasticity. Here, we describe extensive alterations of ZnT3-regulated Zn pools in the brains of human amyloid precursor protein-transgenic (Tg2576) mice. In contrast to wild-type littermates in which ZnT3 expression and synaptic Zn increased with age, there were age-dependent reductions in ZnT3 expression and synaptic Zn levels in the hippocampal mossy fiber area of Tg2576 mice. In these mice, a novel Zn pool and ZnT3 expression were colocalized and appeared along dystrophic neurites surrounding compact amyloid plaques that were identified by in situ blue fluorescence, congophilic birefringence, and Aβ42 immunoreactivity. Zn-specific histofluorescence and ZnT3 immunofluorescence in dystrophic neurites were also colocalized with the δ-subunit of adaptor protein complex 3, lysosome-associated membrane protein, cathepsin D, and neurofilament-containing hyperphosphorylated paired helical filaments. The synaptic vesicle marker protein synaptophysin and vesicle-associated membrane protein were not found in these neurites, suggesting a role of ZnT3 distinct from itsnormal role in synaptic Zn. ZnT3 immunoreactivity and Zn histofluorescence were also evident in activated astrocytes. These datasuggest that extensive modifications of the cerebral Zn pool, particularly synaptic Zn, may underlie neuronal dysfunction characteristic of Alzheimer disease.

Synaptic release and extracellular actions of Zn2+ limit propagation of spreading depression and related events in vitro and in vivo

Cortical spreading depression (CSD) is a consequence of a slowly propagating wave of neuronal and glial depolarization (spreading depolarization; SD). Massive release of glutamate contributes to SD propagation, and it was recently shown that Zn(2+) is also released from synaptic vesicles during SD. The present study examined consequences of extracellular Zn(2+) accumulation on the propagation of SD. SD mechanisms were studied first in murine brain slices, using focal KCl applications as stimuli and making electrical and optical recordings in hippocampal area CA1. Elevating extracellular Zn(2+) concentrations with exogenous ZnCl(2) reduced SD propagation rates. Selective chelation of endogenous Zn(2+) (using TPEN or CaEDTA) increased SD propagation rates, and these effects appeared due to chelation of Zn(2+) derived from synaptic vesicles. Thus, in tissues where synaptic Zn(2+) release was absent [knockout (KO) of vesicular Zn(2+) transporter ZnT-3], SD propagation rates were increased, and no additional increase was observed following chelation of endogenous Zn(2+) in these tissues. The role of synaptic Zn(2+) was then examined on CSD in vivo. ZnT-3 KO animals had higher susceptibility to CSD than wild-type controls as evidenced by significantly higher propagation rates and frequencies. Studies of candidate mechanisms excluded changes in neuronal excitability, presynaptic release, and GABA receptors but left open a possible contribution of N-methyl-d-aspartate (NMDA) receptor inhibition. These results suggest the extracellular accumulation of synaptically released Zn(2+) can serve as an intrinsic inhibitor to limit SD events. The inhibitory action of extracellular Zn(2+) on SD may counteract to some extent the neurotoxic effects of intracellular Zn(2+) accumulation in acute brain injury models.

Probing the efficacy of peptide-based inhibitors against acid- and zinc-promoted oligomerization of amyloid-β peptide via single-oligomer spectroscopy

One avenue for prevention and treatment of Alzheimer's disease involves inhibiting the aggregation of amyloid-β peptide (Aβ). Given the deleterious effects reported for Aβ dimers and trimers, it is important to investigate inhibition of the earliest association steps. We have employed quantized photobleaching of dye-labeled Aβ peptides to characterize four peptide-based inhibitors of fibrillogenesis and/or cytotoxicity, assessing their ability to inhibit association in the smallest oligomers (n=2-5). Inhibitors were tested at acidic pH and in the presence of zinc, conditions that may promote oligomerization in vivo. Distributions of peptide species were constructed by examining dozens of surface-tethered monomers and oligomers, one at a time. Results show that all four inhibitors shift the distribution of Aβ species toward monomers; however, efficacies vary for each compound and sample environment. Collectively, these studies highlight promising design strategies for future oligomerization inhibitors, affording insight into oligomer structures and inhibition mechanisms in two physiologically significant environments.

Ca permeable AMPA channels in diseases of the nervous system

Since the discovery and molecular characterization of Ca(2+)-permeable AMPA channels just over two decades ago, a large body of evidence has accumulated implicating contributions of these unusual glutamate activated channels to selective neurodegeneration in certain conditions, including ischemia and amyotrophic lateral sclerosis. Factors likely involved in their contributions to disease include their distinct patterns of expression in certain neuronal populations, their upregulation via various mechanisms in response to disease associated stresses, and their high permeability to Zn(2+) as well as to Ca(2+). However, full characterization of their contributions to certain diseases as well as development of therapeutics has been limited by the lack of selective and bioavailable blockers of these channels that can be employed in animals or humans. This review summarizes some of the clues that have emerged over recent years to the contributions of these channels in disease.

Modulation of neuronal signal transduction and memory formation by synaptic zinc

The physiological role of synaptic zinc has remained largely enigmatic since its initial detection in hippocampal mossy fibers over 50 years ago. The past few years have witnessed a number of studies highlighting the ability of zinc ions to regulate ion channels and intracellular signaling pathways implicated in neuroplasticity, and others that shed some light on the elusive role of synaptic zinc in learning and memory. Recent behavioral studies using knock-out mice for the synapse-specific zinc transporter ZnT-3 indicate that vesicular zinc is required for the formation of memories dependent on the hippocampus and the amygdala, two brain centers that are prominently innervated by zinc-rich fibers. A common theme emerging from this research is the activity-dependent regulation of the Erk1/2 mitogen-activated-protein kinase pathway by synaptic zinc through diverse mechanisms in neurons. Here we discuss current knowledge on how synaptic zinc may play a role in cognition through its impact on neuronal signaling.

Brain-Delivery of Zinc-Ions as Potential Treatment for Neurological Diseases: Mini Review

Homeostasis of metal ions such as Zn(2+) is essential for proper brain function. Moreover, the list of psychiatric and neurodegenerative disorders involving a dysregulation of brain Zn(2+)-levels is long and steadily growing, including Parkinson's and Alzheimer's disease as well as schizophrenia, attention deficit and hyperactivity disorder, depression, amyotrophic lateral sclerosis, Down's syndrome, multiple sclerosis, Wilson's disease and Pick's disease. Furthermore, alterations in Zn(2+)-levels are seen in transient forebrain ischemia, seizures, traumatic brain injury and alcoholism. Thus, the possibility of altering Zn(2+)-levels within the brain is emerging as a new target for the prevention and treatment of psychiatric and neurological diseases. Although the role of Zn(2+) in the brain has been extensively studied over the past decades, methods for controlled regulation and manipulation of Zn(2+) concentrations within the brain are still in their infancy. Since the use of dietary Zn(2+) supplementation and restriction has major limitations, new methods and alternative approaches are currently under investigation, such as the use of intracranial infusion of Zn(2+) chelators or nanoparticle technologies to elevate or decrease intracellular Zn(2+) levels. Therefore, this review briefly summarizes the role of Zn(2+) in psychiatric and neurodegenerative diseases and highlights key findings and impediments of brain Zn(2+)-level manipulation. Furthermore, some methods and compounds, such as metal ion chelation, redistribution and supplementation that are used to control brain Zn(2+)-levels in order to treat brain disorders are evaluated.

Dependence of the histofluorescently reactive zinc pool on zinc transporter-3 in the normal brain

In the brain, free zinc levels are under the exquisite control of a variety of zinc-regulating systems, in which zinc transporter (ZnT) proteins play a central role. ZnT3, which is prominently expressed in the brain, facilitates the concentration of free zinc in pre-synaptic vesicles. In addition to histochemical staining methods, a variety of zinc-specific fluorescence dyes has been developed to image or analyze zinc in brain tissue. In this study, we demonstrate the close correlations between histofluorescently reactive zinc and ZnT3. We examined the overlapping distribution of the zinc-specific fluorescent dye, N-(6-methoxy-8-quinolyl)-p-toluenesulfonamide (TSQ)-, and ZnT3-immunoreactive fluorescence throughout the normal brain. TSQ and ZnT3-antibody intensely stained the hippocampus, cortex and amygdala, highlighting the characteristic laminar organization of these regions by variably staining the different layers. TSQ fluorescence and ZnT3 immunoreactivity were roughly co-localized with synaptophysin along the neuropil, but were absent in the neuronal soma. However, albeit relatively faint, TSQ fluorescence was also found throughout the brains of ZnT3-knockout mice. Although these results may indicate the presence of very small cerebral free zinc pools distinct from synaptic vesicle zinc, the synaptic vesicle zinc pool is predominant, accounting for more than 95% of the entire histofluorescently reactive zinc pool in the hippocampus and cortex. Thus, the physiological activity of free zinc in the normal brain might largely depend on the pool of synaptic vesicle zinc that is determined by ZnT3.

Role of metal ions in aggregation of intrinsically disordered proteins in neurodegenerative diseases

Neurodegenerative diseases constitute a set of pathological conditions originating from the slow, irreversible, and systematic cell loss within the various regions of the brain and/or the spinal cord. Depending on the affected region, the outcomes of the neurodegeneration are very broad and diverse, ranging from the problems with movements to dementia. Some neurodegenerative diseases are associated with protein misfolding and aggregation. Many proteins that misfold in human neurodegenerative diseases are intrinsically disordered; i.e., they lack a stable tertiary and/or secondary structure under physiological conditions in vitro. These intrinsically disordered proteins (IDPs) functionally complement ordered proteins, being typically involved in regulation and signaling. There is accumulating evidence that altered metal homeostasis may be related to the progression of neurodegenerative diseases. This review examines the effects of metal ion binding on the aggregation pathways of IDPs found in neurodegenerative diseases.

Microglial zinc uptake via zinc transporters induces ATP release and the activation of microglia

Previously, we demonstrated that extracellular zinc plays a key role in transient global ischemia-induced microglial activation through sequential activation of NADPH oxidase and poly(ADP-ribose) polymerase (PARP)-1. However, it remains unclear how zinc causes the sequential activation of microglia. Here, we examined whether transporter-mediated zinc uptake is necessary for microglial activation. Administration of zinc to microglia activated them through reactive oxygen species (ROS) generation and poly(ADP-ribose) (PAR) formation, which were suppressed by intracellular zinc chelation with 25 μM TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine) or 2 μM BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester). The (65)Zn uptake by microglia was temperature- and dose-dependent, and it was blocked by metal cations, but not by L-type calcium channel blockers nifedipine and nimodipine. Expression of Zrt-Irt-like protein (ZIP)1, a plasma membrane-type zinc transporter, was detected in microglia, and nickel, a relatively sensitive substrate/inhibitor of ZIP1, showed cis- and trans-inhibitory effects on the (65)Zn uptake. Exposure of microglia to zinc increased the extracellular ATP concentration, which was suppressed by intracellular zinc chelation and inhibition of hemichannels. mRNA expression of several types of P2 receptors was detected in microglia, and periodate-oxidized ATP, a selective P2×7 receptor antagonist, attenuated the zinc-induced microglial activation via NADPH oxidase and PARP-1. Exogenous ATP and 2'(3')-O-(4-benzoyl-benzoyl) ATP also caused microglial activation through ROS generation and PAR formation. These findings demonstrate that ZIP1-mediated uptake of zinc induces ATP release and autocrine/paracrine activation of P2X(7) receptors, and then activates microglia, suggesting that zinc transporter-mediated uptake of zinc is a trigger for microglial activation via the NADPH oxidase and PARP-1 pathway. © 2011 Wiley-Liss, Inc.

Zinc effects on NMDA receptor gating kinetics

Zinc accumulates in the synaptic vesicles of certain glutamatergic forebrain neurons and modulates neuronal excitability and synaptic plasticity by multiple poorly understood mechanisms. Zinc directly inhibits NMDA-sensitive glutamate-gated channels by two separate mechanisms: high-affinity binding to N-terminal domains of GluN2A subunits reduces channel open probability, and low-affinity voltage-dependent binding to pore-lining residues blocks the channel. Insight into the high-affinity allosteric effect has been hampered by the receptor's complex gating; multiple, sometimes coupled, modulatory mechanisms; and practical difficulties in avoiding transient block by residual Mg(2+). To sidestep these challenges, we examined how nanomolar zinc concentrations changed the gating kinetics of individual block-resistant receptors. We found that block-insensitive channels had lower intrinsic open probabilities but retained high sensitivity to zinc inhibition. Binding of zinc to these receptors resulted in longer closures and shorter openings within bursts of activity but had no effect on interburst intervals. Based on kinetic modeling of these data, we conclude that zinc-bound receptors have higher energy barriers to opening and less stable open states. We tested this model for its ability to predict zinc-dependent changes in macroscopic responses and to infer the impact of nanomolar zinc concentrations on synaptic currents mediated by 2A-type NMDA receptors.

Zinc modulation of glycine receptors

Glycine receptors are widely expressed in the mammalian central nervous system, and previous studies have demonstrated that glycine receptors are modulated by endogenous zinc. Zinc is concentrated in synaptic vesicles in several brain regions but is particularly abundant in the hippocampus and olfactory bulb. In the present study, we used patch-clamp electrophysiology of rat hippocampal and olfactory bulb neurons in primary culture to examine the effects of zinc on glycine receptors. Although glycine has been reported to reach millimolar concentrations during synaptic transmission, most previous studies on the effects of zinc on glycine receptors have used relatively low concentrations of glycine. High concentrations of glycine cause receptor desensitization. Our current results extend our previous demonstration that the modulatory actions of zinc are largely prevented when co-applied with desensitizing concentrations of glycine (300 μM), suggesting that the effects of zinc are dependent on the state of the receptor. In contrast, pre-application of 300 μM zinc, prior to glycine (300 μM) application, causes a slowly developing inhibition with a slow rate of recovery, suggesting that the timing of zinc and glycine release also influences the effects of zinc. Furthermore, previous evidence suggests that synaptically released zinc can gain intracellular access, and we provide the first demonstration that low concentrations of intracellular zinc can potentiate glycine receptors. These results support the notion that zinc has complex effects on glycine receptors and multiple factors may interact to influence the efficacy of glycinergic transmission.

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.

Differential effects of Zn2+ on activation, deactivation, and inactivation kinetics in neuronal voltage-gated Na+ channels

Whole-cell, patch-clamp recordings were carried out in acutely dissociated neurons from entorhinal cortex (EC) layer II to study the effects of Zn(2+) on Na(+) current kinetics and voltage dependence. In the presence of 200 μM extracellular Cd(2+) to abolish voltage-dependent Ca(2+) currents, and 100 mM extracellular Na(+), 1 mM Zn(2+) inhibited the transient Na(+) current, I (NaT), only to a modest degree (~17% on average). A more pronounced inhibition (~36%) was induced by Zn(2+) when extracellular Na(+) was lowered to 40 mM. Zn(2+) also proved to modify I (NaT) voltage-dependent and kinetic properties in multiple ways. Zn(2+) (1 mM) shifted the voltage dependence of I (NaT) activation and that of I (NaT) onset speed in the positive direction by ~5 mV. The voltage dependence of I (NaT) steady-state inactivation and that of I (NaT) inactivation kinetics were markedly less affected by Zn(2+). By contrast, I (NaT) deactivation speed was prominently accelerated, and its voltage dependence was shifted by a significantly greater amount (~8 mV on average) than that of I (NaT) activation. In addition, the kinetics of I (NaT) recovery from inactivation were significantly slowed by Zn(2+). Zn(2+) inhibition of I (NaT) showed no signs of voltage dependence over the explored membrane-voltage window, indicating that the above effects cannot be explained by voltage dependence of Zn(2+)-induced channel-pore block. These findings suggest that the multiple, voltage-dependent state transitions that the Na(+) channel undergoes through its activation path are differentially sensitive to the gating-modifying effects of Zn(2+), thus resulting in differential modifications of the macroscopic current's activation, inactivation, and deactivation. Computer modeling provided support to this hypothesis.

Zinc binding to the Tyr402 and His402 allotypes of complement factor H: possible implications for age-related macular degeneration

The Tyr402His polymorphism of complement factor H (FH) with 20 short complement regulator (SCR) domains is associated with age-related macular degeneration (AMD). How FH contributes to disease pathology is not clear. Both FH and high concentrations of zinc are found in drusen deposits, the key feature of AMD. Heterozygous FH is inhibited by zinc, which causes FH to aggregate. Here, zinc binding to homozygous FH was studied. By analytical ultracentrifugation, large amounts of oligomers were observed with both the native Tyr402 and the AMD-risk His402 homozygous allotypes of FH and both the recombinant SCR-6/8 allotypes with Tyr/His402. X-ray scattering also showed that both FH and SCR-6/8 allotypes strongly aggregated at > 10 μM zinc. The SCR-1/5 and SCR-16/20 fragments were less likely to bind zinc. These observations were supported by bioinformatics predictions. Starting from known zinc binding sites in crystal structures, we predicted 202 putative partial surface zinc binding sites in FH, most of which were in SCR-6. Metal site prediction web servers also suggested that SCR-6 and other domains bind zinc. Predicted SCR-6/8 dimer structures showed that zinc binding sites could be formed at the protein–protein interface that would lead to daisy-chained oligomers. It was concluded that zinc binds weakly to FH at multiple surface locations, most probably within the functionally important SCR-6/8 domains, and this explains why zinc inhibits FH activity. Given the high pathophysiological levels of bioavailable zinc present in subretinal deposits, we discuss how zinc binding to FH may contribute to deposit formation and inflammation associated with AMD.

Zinc enhances long-term potentiation through P2X receptor modulation in the hippocampal CA1 region

Zn²(+) is an essential ion that is stored in and co-released from glutamatergic synapses and it modulates neurotransmitter receptors involved in long-term potentiation (LTP). However, the mechanism(s) underlying Zn²(+) -induced modulation of LTP remain(s) unclear. As the purinergic P2X receptors are relevant targets for Zn²(+) action, we have studied their role in LTP modulation by Zn²(+) in the CA1 region of rat hippocampal slices. Induction of LTP in the presence of Zn²(+) revealed a biphasic effect - 5-50 μm enhanced LTP induction, whereas 100-300 μm Zn²(+) inhibited LTP. The involvement of a purinergic mechanism is supported by the fact that application of the P2X receptor antagonists 2',3'-O-(2,4,6-trinitrophenyl) ATP (TNP-ATP) and periodate-oxidized ATP fully abolished the facilitatory effect of Zn²(+) . Notably, application of the P2X₇ receptor-specific antagonist Brilliant Blue G did not modify the Zn²(+) -dependent facilitation of LTP. Exogenous ATP also produced a biphasic effect - 0.1-1 μm ATP facilitated LTP, whereas 5-10 μm inhibited LTP. The facilitatory effect of ATP was abolished by the application of TNP-ATP and was modified in the presence of 5 μm Zn²(+) , suggesting that P2X receptors are involved in LTP induction and that Zn²(+) leads to an increase in the affinity of P2X receptors for ATP. The latter confirms our previous results from heterologous expression systems. Collectively, our results indicate that Zn²(+) at low concentrations enhances LTP by modulating P2X receptors. Although it is not yet clear which purinergic receptor subtype(s) is responsible for these effects on LTP, the data presented here suggest that P2X₄ but not P2X₇ is involved.

Cell-trappable quinoline-derivatized fluoresceins for selective and reversible biological Zn(II) detection

The synthesis and spectroscopic characterization of two new, cell-trappable fluorescent probes for Zn(II) are presented. These probes, 2-(4,5-bis(((6-(2-ethoxy-2-oxoethoxy)quinolin-8-yl)amino)methyl)-6-hydroxy-3-oxo-3H-8 xanthen-9-yl)benzoic acid (QZ2E) and 2,2'-((8,8'-(((9-(2-carboxyphenyl)-6-hydroxy-3-oxo-3H-xanthene-4,5-diyl)bis(methylene))bis(azanediyl))bis(quinoline-8,6-diyl))bis(oxy))diacetic acid (QZ2A), are poorly emissive in the off-state but exhibit dramatic increases in fluorescence upon Zn(II) binding (120 ± 10-fold for QZ2E, 30 ± 7-fold for QZ2A). This binding is selective for Zn(II) over other biologically relevant metal cations, toxic heavy metals, and most first-row transition metals and is of appropriate affinity (K(d1)(QZ2E) = 150 ± 100 μM, K(d2)(QZ2E) = 3.5 ± 0.1 mM, K(d1)(QZ2A) = 220 ± 30 μM, K(d2)(QZ2A) = 160 ± 80 μM, K(d3)(QZ2A) = 9 ± 6 μM) to reversibly bind Zn(II) at physiological levels. In live cells, QZ2E localizes to the Gogli apparatus where it can detect Zn(II). It is cell-membrane-permeable until cleavage of its ester groups by intracellular esterases produces QZ2A, a negatively charged acid form that cannot cross the cell membrane.

Roles of zinc and metallothionein-3 in oxidative stress-induced lysosomal dysfunction, cell death, and autophagy in neurons and astrocytes

Zinc dyshomeostasis has been recognized as an important mechanism for cell death in acute brain injury. An increase in the level of free or histochemically reactive zinc in astrocytes and neurons is considered one of the major causes of death of these cells in ischemia and trauma. Although zinc dyshomeostasis can lead to cell death via diverse routes, the major pathway appears to involve oxidative stress.

Recently, we found that a rise of zinc in autophagic vacuoles, including autolysosomes, is a prerequisite for lysosomal membrane permeabilization and cell death in cultured brain cells exposed to oxidative stress conditions. The source of zinc in this process is likely redox-sensitive zinc-binding proteins such as metallothioneins, which release zinc under oxidative conditions. Of the metallothioneins, metallothionein-3 is especially enriched in the central nervous system, but its physiologic role in this tissue is not well established. Like other metallothioneins, metallothionein-3 may function as metal detoxicant, but is also known to inhibit neurite outgrowth and, sometimes, promote neuronal death, likely by serving as a source of toxic zinc release. In addition, metallothionein-3 regulates lysosomal functions. In the absence of metallothionein-3, there are changes in lysosome-associated membrane protein-1 and -2, and reductions in certain lysosomal enzymes that result in decreased autophagic flux. This may have dual effects on cell survival. In acute oxidative injury, zinc dyshomeostasis and lysosomal membrane permeabilization are diminished in metallothionein-3 null cells, resulting in less cell death. But over the longer term, diminished lysosomal function may lead to the accumulation of abnormal proteins and cause cytotoxicity.

The roles of zinc and metallothionein-3 in autophagy and/or lysosomal function have just begun to be investigated. In light of evidence that autophagy and lysosomes may play significant roles in the pathogenesis of various neurological diseases, further insight into the contribution of zinc dynamics and metallothionein-3 function may help provide ways to effectively regulate these processes in brain cells.

Microglial activation in stroke: therapeutic targets

Microglial activation is an early response to brain ischemia and many other stressors. Microglia continuously monitor and respond to changes in brain homeostasis and to specific signaling molecules expressed or released by neighboring cells. These signaling molecules, including ATP, glutamate, cytokines, prostaglandins, zinc, reactive oxygen species, and HSP60, may induce microglial proliferation and migration to the sites of injury. They also induce a nonspecific innate immune response that may exacerbate acute ischemic injury. This innate immune response includes release of reactive oxygen species, cytokines, and proteases. Microglial activation requires hours to days to fully develop, and thus presents a target for therapeutic intervention with a much longer window of opportunity than acute neuroprotection. Effective agents are now available for blocking both microglial receptor activation and the microglia effector responses that drive the inflammatory response after stroke. Effective agents are also available for targeting the signal transduction mechanisms linking these events. However, the innate immune response can have beneficial as well deleterious effects on outcome after stoke, and a challenge will be to find ways to selectively suppress the deleterious effects of microglial activation after stroke without compromising neurovascular repair and remodeling.

Zinc dyshomeostasis is linked with the loss of mucolipidosis IV-associated TRPML1 ion channel

Chelatable zinc is important in brain function, and its homeostasis is maintained to prevent cytotoxic overload. However, certain pathologic events result in intracellular zinc accumulation in lysosomes and mitochondria. Abnormal lysosomes and mitochondria are common features of the human lysosomal storage disorder known as mucolipidosis IV (MLIV). MLIV is caused by the loss of TRPML1 ion channel function. MLIV cells develop large hyperacidic lysosomes, membranous vacuoles, mitochondrial fragmentation, and autophagic dysfunction. Here, we observed that RNA interference of mucolipin-1 gene (TRPML1) in HEK-293 cells mimics the MLIV cell phenotype consisting of large lysosomes and membranous vacuoles that accumulate chelatable zinc. To show that abnormal chelatable zinc levels are indeed correlated with MLIV pathology, we quantified its concentration in cultured MLIV patient fibroblast and control cells with a spectrofluorometer using N-(6-methoxy-8-quinolyl)-p-toluene sulfonamide fluorochrome. We found a significant increase of chelatable zinc levels in MLIV cells but not in control cells. Furthermore, we quantified various metal isotopes in whole brain tissue of TRPML1(-/-) null mice and wild-type littermates using inductively coupled plasma mass spectrometry and observed that the zinc-66 isotope is markedly elevated in the brain of TRPML1(-/-) mice when compared with controls. In conclusion, we show for the first time that the loss of TRPML1 function results in intracellular chelatable zinc dyshomeostasis. We propose that chelatable zinc accumulation in large lysosomes and membranous vacuoles may contribute to the pathogenesis of the disease and progressive cell degeneration in MLIV patients.

Dynamic, experience-dependent modulation of synaptic zinc within the excitatory synapses of the mouse barrel cortex

Increasing evidence suggests that synaptic zinc, found within the axon terminals of a subset of glutamatergic neurons in the cerebral cortex, is intricately involved in cortical plasticity. Using the vibrissae/barrel cortex model of cortical plasticity, we have previously shown manipulations of sensory input leads to rapid changes in synaptic zinc levels within the corresponding regions of the somatotopic map in the cortex. Here, using electron microscopy, we show how some of these changes are mediated at the synaptic level. We found that the density of zincergic synapses increased significantly in layers II/III, IV, and V. In layers IV and V, this change occurred in the absence of a significant increase in excitatory synapse density, which seems to indicate that excitatory synapses, which previously did not contain synaptic zinc, begin to newly house zinc within its synaptic vesicles. Our results show that excitatory neurons can dynamically change the phenotype of the vesicular content of their synapses in response to changes in sensory input. Given the range of modulatory effects zinc can have on neurotransmission, such a change in the complement of vesicular contents presumably allow these neurons to utilize synaptic zinc to facilitate plasticity. Thus, our results further support the role of zinc as an active participant in the processes contributing to experience-dependent cortical plasticity.

Conserved rhodopsin intradiscal structural motifs mediate stabilization: effects of zinc

Retinitis pigmentosa (RP), a neurodegenerative disorder, can arise from single point mutations in rhodopsin, leading to a cascade of protein instability, misfolding, aggregation, rod cell death, retinal degeneration, and ultimately blindness. Divalent cations, such as zinc and copper, have allosteric effects on misfolded aggregates of comparable neurodegenerative disorders including Alzheimer disease, prion diseases, and ALS. We report that two structurally conserved low-affinity zinc coordination motifs, located among a cluster of RP mutations in the intradiscal loop region, mediate dose-dependent rhodopsin destabilization. Disruption of native interactions involving histidines 100 and 195, through site-directed mutagenesis or exogenous zinc coordination, results in significant loss of receptor stability. Furthermore, chelation with EDTA stabilizes the structure of both wild-type rhodopsin and the most prevalent rhodopsin RP mutation, P(23)H. These interactions suggest that homeostatic regulation of trace metal concentrations in the rod outer segment of the retina may be important both physiologically and for an important cluster of RP mutations. Furthermore, with a growing awareness of allosteric zinc binding domains on a diverse range of GPCRs, such principles may apply to many other receptors and their associated diseases.

Chemical blocking of zinc ions in CNS increases neuronal damage following traumatic brain injury (TBI) in mice

Background

Traumatic brain injury (TBI) is one of the leading causes of disability and death among young people. Although much is already known about secondary brain damage the full range of brain tissue responses to TBI remains to be elucidated. A population of neurons located in cerebral areas associated with higher cognitive functions harbours a vesicular zinc pool co-localized with glutamate. This zinc enriched pool of synaptic vesicles has been hypothesized to take part in the injurious signalling cascade that follows pathological conditions such as seizures, ischemia and traumatic brain injury. Pathological release of excess zinc ions from pre-synaptic vesicles has been suggested to mediate cell damage/death to postsynaptic neurons.

Methodology/Principal Findings

In order to substantiate the influence of vesicular zinc ions on TBI, we designed a study in which damage and zinc movements were analysed in several different ways. Twenty-four hours after TBI ZnT3-KO mice (mice without vesicular zinc) were compared to littermate Wild Type (WT) mice (mice with vesicular zinc) with regard to histopathology. Furthermore, in order to evaluate a possible neuro-protective dimension of chemical blocking of vesicular zinc, we treated lesioned mice with either DEDTC or selenite. Our study revealed that chemical blocking of vesicular zinc ions, either by chelation with DEDTC or accumulation in zinc-selenium nanocrystals, worsened the effects on the aftermath of TBI in the WT mice by increasing the number of necrotic and apoptotic cells within the first 24 hours after TBI, when compared to those of chemically untreated WT mice.

Conclusion/Significance

ZnT3-KO mice revealed more damage after TBI compared to WT controls. Following treatment with DEDTC or selenium an increase in the number of both dead and apoptotic cells were seen in the controls within the first 24 hours after TBI while the degree of damage in the ZnT3-KO mice remained largely unchanged. Further analyses revealed that the damage development in the two mouse strains was almost identical after either zinc chelation or zinc complexion therapy.

Disulfides as redox switches: from molecular mechanisms to functional significance

The molecular mechanisms underlying thiol-based redox control are poorly defined. Disulfide bonds between Cys residues are commonly thought to confer extra rigidity and stability to their resident protein, forming a type of proteinaceous spot weld. Redox biologists have been redefining the role of disulfides over the last 30-40 years. Disulfides are now known to form in the cytosol under conditions of oxidative stress. Isomerization of extracellular disulfides is also emerging as an important regulator of protein function. The current paradigm is that the disulfide proteome consists of two subproteomes: a structural group and a redox-sensitive group. The redox-sensitive group is less stable and often associated with regions of stress in protein structures. Some characterized redox-active disulfides are the helical CXXC motif, often associated with thioredoxin-fold proteins; and forbidden disulfides, a group of metastable disulfides that disobey elucidated rules of protein stereochemistry. Here we discuss the role of redox-active disulfides as switches in proteins.

Zinc: the brain's dark horse

Zinc is a life-sustaining trace element, serving structural, catalytic, and regulatory roles in cellular biology. It is required for normal mammalian brain development and physiology, such that deficiency or excess of zinc has been shown to contribute to alterations in behavior, abnormal central nervous system development, and neurological disease. In this light, it is not surprising that zinc ions have now been shown to play a role in the neuromodulation of synaptic transmission as well as in cortical plasticity. Zinc is stored in specific synaptic vesicles by a class of glutamatergic or "gluzinergic" neurons and is released in an activity-dependent manner. Because gluzinergic neurons are found almost exclusively in the cerebral cortex and limbic structures, zinc may be critical for normal cognitive and emotional functioning. Conversely, direct evidence shows that zinc might be a relatively potent neurotoxin. Neuronal injury secondary to in vivo zinc mobilization and release occurs in several neurological disorders such as Alzheimer's disease and amyotrophic lateral sclerosis, in addition to epilepsy and ischemia. Thus, zinc homeostasis is integral to normal central nervous system functioning, and in fact its role may be underappreciated. This article provides an overview of zinc neurobiology and reviews the experimental evidence that implicates zinc signals in the pathophysiology of neuropsychiatric diseases. A greater understanding of zinc's role in the central nervous system may therefore allow for the development of therapeutic approaches where aberrant metal homeostasis is implicated in disease pathogenesis.

An overview of hypoglycemia in the critically ill

Hypoglycemia is a common and serious problem among patients with diabetes mellitus. It is also perceived as the most important obstacle to tight glucose control using intensive insulin therapy in critically ill patients. Because glucose is an obligatory metabolic fuel for the brain, hypoglycemia always represents an emergency that signals the inability of the brain to meet its energy needs. When left untreated, hypoglycemia can result in permanent brain damage and ultimately, death. In the context of critical illness that limits endogenous glucose production and increases glucose utilization, inadequate nutrition, or insufficient provision of glucose, intensive insulin therapy is the most frequent cause of hypoglycemia. Neurogenic and neuroglycopenic symptoms of hypoglycemia can remain unknown because of the underlying critical illness and sedation. Thus, close and reliable monitoring of the glycemic level is crucial in detecting hypoglycemia. In prospective randomized controlled studies comparing the effects of two glucose regimens, intensive insulin therapy aimed to reach strict glucose control (<110 mg/dl) but increased the incidence of severe hypoglycemia (<40 mg/dl) by four- to sixfold. Severe hypoglycemia is statistically associated with adverse outcomes in intensive care unit patients, although a direct causal relationship has not been demonstrated.

Role of zinc in hypoglycemia-induced neuron death

The mechanism of hypoglycemia-induced neuronal death has been extensively studied since a Swedish scientist, Roland Auer, developed a rat model of insulin-induced hypoglycemia one quarter of a century ago. Currently, mounting data derived from these studies are making it possible to propose an ever more complete mechanism of hypoglycemia-induced neuron death. Although salient differences between the two insults do exist, the pattern and progression of neuron death after acute/severe hypoglycemia have been observed to share many similarities with ischemia-induced neuron death. One important consideration is that the divalent cation zinc is a common neurotoxic factor in both conditions. For the last decade, our laboratory has offered evidence for the hypothesis that synaptic vesicular zinc release and subsequent translocation into the postsynaptic neuron are key upstream events in both neurological injuries of ischemia and hypoglycemia. The release of vesicular zinc from presynaptic terminals depends on nitric oxide production, which is initiated by glucose reperfusion after hypoglycemia. Postsynaptic intracellular influx of zinc after hypoglycemia increases reactive oxygen species production and PARP-1 activation, which eventually leads to neuronal death. Thus, hypoglycemia-induced neuronal death is not simply caused by a lack of neuronal glucose availability but rather by sequential zinc-mediated events that impinge upon the process of neuronal death. This paper focuses on the role of zinc in hypoglycemia-induced neuron death; first, the temporal events of zinc release and translocation after severe hypoglycemia; second, the role of zinc on reactive oxygen species production; and third, the role of zinc on poly(ADP-ribose) polymerase 1 activation. Here, we also speculate on possible intervention strategies to prevent hypoglycemia-induced neuronal death.

Zinc in the physiology and pathology of the CNS

The past few years have witnessed dramatic progress on all frontiers of zinc neurobiology. The recent development of powerful tools, including zinc-sensitive fluorescent probes, selective chelators and genetically modified animal models, has brought a deeper understanding of the roles of this cation as a crucial intra- and intercellular signalling ion of the CNS, and hence of the neurophysiological importance of zinc-dependent pathways and the injurious effects of zinc dyshomeostasis. The development of some innovative therapeutic strategies is aimed at controlling and preventing the damaging effects of this cation in neurological conditions such as stroke and Alzheimer's disease.

Validation of TPEN as a zinc chelator in fluorescence probing of calcium in cells with the indicator Fura-2

Fura-2 is widely used as a fluorescent probe to monitor dynamic changes in cytosolic free calcium in cells, where Ca(2+) can enter through several types of voltage-operated or ligand-gated channels. However, Fura-2 is also sensitive to other metal ions, such as zinc, which may be involved in ionic channels and receptors. There is interest, in particular, in studying the synapses between mossy fibers and CA3 pyramidal cells which contain both calcium and high quantities of free or loosely bound zinc. We have found, through fluorescence probing, that endogenous zinc inhibits mossy fiber calcium transients. However, since these results might be explained by an effect of the zinc chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) on the spectral properties of Fura-2, we have carried out a validation of the method through fluorescence excitation spectra of the complex Fura-2/calcium, and show that TPEN does not affect these spectra. This supports the idea that the observed calcium enhancement is related to a zinc inhibition of presynaptic calcium mechanisms, and confirms the use of the chelator TPEN as a general procedure for the biophysical study of Ca(II) in the presence of Zn(II) using Fura-2.

Hippocampal zinc infusion delays the development of afterdischarges and seizures in a kindling model of epilepsy

PURPOSE

Zinc occurs in high concentration in synaptic vesicles of glutamatergic terminals including hippocampal mossy fibers. This vesicular zinc can be synaptically released during neuronal activity, including seizures. Zinc inhibits certain subtypes of N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid (GABA)(A) receptors. By blocking NMDA excitation or GABA inhibition, an excess of zinc may alter the excitability of hippocampal circuits, which contribute to the development of seizures.

METHODS

Twenty-one adult Wistar rats were implanted under anesthesia with Alzet pumps releasing vehicle, 10 microM ZnCl(2) or 1,000 microM ZnCl(2), at a rate of 0.25 microl/h continuously into the hippocampal hilus for 4 weeks. Kindling was performed by daily awake commissural stimulation at 60 Hz and afterdischarges were recorded from a dentate gyrus electrode. Development of behavioral Racine seizure stages was recorded by a blinded investigator.

RESULTS

The development of behavioral Racine seizure stages was delayed only in rats infused with 1,000 microM ZnCl(2) (p < 0.02). With completion of kindling at stimulation number 20, all groups had reached the same maximum level of behavioral seizures. The expected increased progression of afterdischarge duration was inhibited by both 10 microM ZnCl(2) and 1,000 microM ZnCl(2) infusion compared to control animals (p < 0.01). At stimulation number 18, all groups had reached the same maximum duration of afterdischarges.

DISCUSSION

We conclude that excess infused zinc delayed the development of behavioral seizures in a kindling model of epilepsy. These data support the hypothesis that zinc synaptically released during seizures may alter hippocampal excitability similar to zinc infused in our experiment.

Small-molecule fluorescent sensors for investigating zinc metalloneurochemistry

The metalloneurochemistry of Zn(II) is of substantial current interest. Zinc is the second most abundant d-block metal ion in the human brain, and its distribution varies with relatively high concentrations found in the hippocampus. Brain zinc is generally divided into two types, protein-bound and loosely bound, the latter also being termed histochemically observable, chelatable, labile, or mobile zinc. The neurophysiological and neuropathological significance of mobile Zn(II) remains enigmatic. Studies of Zn(II) distribution, translocation, and function in vivo require tools for its detection. Because Zn(II) has a closed-shell d(10) configuration and no convenient spectroscopic signature, fluorescence is a well-suited method for monitoring Zn(II) in biological contexts. This Account summarizes work by our laboratory addressing the design, preparation, characterization, and use of small-molecule fluorescent sensors for imaging Zn(II) in living cells and samples of brain tissue. These sensors provide "turn-on" or ratiometric Zn(II) detection in aqueous solution at neutral pH. By making alterations to the Zn(II)-binding unit and fluorophore platform, we have devised sensors with varied photophysical and metal-binding properties. Several of these probes have been applied to image Zn(II) distribution, uptake, and mobilization in a variety of cell types, including neuronal cultures.

Brain iron deficiency and excess; cognitive impairment and neurodegeneration with involvement of striatum and hippocampus

While iron deficiency is not perceived as a life threatening disorder, it is the most prevalent nutritional abnormality in the world, and a better understanding of modes and sites of action, can help devise better treatment programs for those who suffer from it. Nowhere is this more important than in infants and children that make up the bulk of iron deficiency in society. Although the effects of iron deficiency have been extensively studied in systemic organs, until very recently little attention was paid to its effects on brain function. The studies of Oski at Johns Hopkin Medical School in 1974, demonstrating the impairment of learning in young school children with iron deficiency, prompted us to study its relevance to brain biochemistry and function in an animal model of iron deficiency. Indeed, rats made iron deficient have lowered brain iron and impaired behaviours including learning. This can become irreversible especially in newborns, even after long-term iron supplementation. We have shown that in this condition it is the brain striatal dopaminergic-opiate system which becomes defective, resulting in alterations in circadian behaviours, cognitive impairment and neurochemical changes closely associated with them. More recently we have extended these studies and have established that cognitive impairment may be closely associated with neuroanatomical damage and zinc metabolism in the hippocampus due to iron deficiency, and which may result from abnormal cholinergic function. The hippocampus is the focus of many studies today, since this brain structure has high zinc concentration and is highly involved in many forms of cognitive deficits as a consequence of cholinergic deficiency and has achieved prominence because of dementia in ageing and Alzheimer's disease. Thus, it is now apparent that cognitive impairment may not be attributed to a single neurotransmitter, but rather, alterations and interactions of several systems in different brain regions. In animal models of iron deficiency it is apparent that dopaminergic interaction with the opiate system and cholinergic neurotransmission may be defective.