hZnT8 (Slc30a8) Transgenic Mice That Overexpress the R325W Polymorph Have Reduced Islet Zn2+ and Proinsulin Levels, Increased Glucose Tolerance After a High-Fat Diet, and Altered Levels of Pancreatic Zinc Binding Proteins.

Zinc (Zn2+) is involved in both type 1 diabetes (T1DM) and type 2 diabetes (T2DM). The wild-type (WT) form of the ß-cell-specific Zn2+ transporter, ZNT8, is linked to T2DM susceptibility. ZnT8 null mice have a mild phenotype with a slight decrease in glucose tolerance, whereas patients with the ZnT8 R325W polymorphism (rs13266634) have decreased proinsulin staining and susceptibility to T2DM. We measured Zn2+, insulin, and proinsulin stainings and performed intraperitoneal glucose tolerance testing in transgenic mice overexpressing hZnT8 WT or hZnT8 R325W fed a normal or high-fat diet. The hZnT8 R325W transgenic line had lower pancreatic [Zn2+]i and proinsulin and higher insulin and glucose tolerance compared with control littermates after 10 weeks of a high-fat diet in male mice. The converse was true for the hZnT8 WT transgenic line, and dietary Zn2+ supplementation also induced glucose intolerance. Finally, pancreatic zinc binding proteins were identified by Zn2+-affinity chromatography and proteomics. Increasing pancreatic Zn2+ (hZnT8WT) induced nucleoside diphosphate kinase B, and Zn2+ reduction (hZnT8RW) induced carboxypeptidase A1. These data suggest that pancreatic Zn2+ and proinsulin levels covary but are inversely variant with insulin or glucose tolerance in the HFD model of T2DM suggesting novel therapeutic targets.

Intracellular Zn2+ accumulation enhances suppression of synaptic activity following spreading depolarization.

Spreading depolarization (SD) is a feed-forward wave that propagates slowly throughout brain tissue and recovery from SD involves substantial metabolic demand. Presynaptic Zn(2+) release and intracellular accumulation occurs with SD, and elevated intracellular Zn(2+) ([Zn(2+) ]i ) can impair cellular metabolism through multiple pathways. We tested here whether increased [Zn(2+) ]i could exacerbate the metabolic challenge of SD, induced by KCl, and delay recovery in acute murine hippocampal slices. [Zn(2+) ]i loading prior to SD, by transient ZnCl2 application with the Zn(2+) ionophore pyrithione (Zn/Pyr), delayed recovery of field excitatory post-synaptic potentials (fEPSPs) in a concentration-dependent manner, prolonged DC shifts, and significantly increased extracellular adenosine accumulation. These effects could be due to metabolic inhibition, occurring downstream of pyruvate utilization. Prolonged [Zn(2+) ]i accumulation prior to SD was required for effects on fEPSP recovery and consistent with this, endogenous synaptic Zn(2+) release during SD propagation did not delay recovery from SD. The effects of exogenous [Zn(2+) ]i loading were also lost in slices preconditioned with repetitive SDs, implying a rapid adaptation. Together, these results suggest that [Zn(2+) ]i loading prior to SD can provide significant additional challenge to brain tissue, and could contribute to deleterious effects of [Zn(2+) ]i accumulation in a range of brain injury models.

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

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

NAD(+) maintenance attenuates light induced photoreceptor degeneration.

Light-induced retinal damage (LD) occurs after surgery or sun exposure. We previously showed that zinc (Zn(2+)) accumulated in photoreceptors and RPE cells after LD but prior to cell death, and pyruvate or nicotinamide attenuated the resultant death perhaps by restoring nicotinamide adenine dinucleotide (NAD(+)) levels. We first examined the levels of NAD(+) and the efficacy of pyruvate or nicotinamide in oxidative toxicities using primary retinal cultures. We next manipulated NAD(+) levels in vivo and tested the affect on LD to photoreceptors and RPE. NAD(+) levels cycle with a 24-h rhythm in mammals, which is affected by the feeding schedule. Therefore, we tested the affect of increasing NAD(+) levels on LD by giving nicotinamide, inverting the feeding schedule, or using transgenic mice which overexpress cytoplasmic nicotinamide mononucleotide adenyl-transferase-1 (cytNMNAT1), an NAD(+) synthetic enzyme. Zn(2+) accumulation was also assessed in culture and in retinal sections. Retinas of light damaged animals were examined by OCT and plastic sectioning, and retinal NAD(+) levels were measured. Day fed, or nicotinamide treated rats showed less NAD(+) loss, and LD compared to night fed rats or untreated rats without changing the Zn(2+) staining pattern. CytNMNAT1 showed less Zn(2+) staining, NAD(+) loss, and cell death after LD. In conclusion, intense light, Zn(2+) and oxidative toxicities caused an increase in Zn(2+), NAD(+) loss, and cell death which were attenuated by NAD(+) restoration. Therefore, NAD(+) levels play a protective role in LD-induced death of photoreceptors and RPE cells.

A reduced zinc diet or zinc transporter 3 knockout attenuate light induced zinc accumulation and retinal degeneration.

Our previous study on retinal light exposure suggests the involvement of zinc (Zn(2+)) toxicity in the death of RPE and photoreceptors (LD) which could be attenuated by pyruvate and nicotinamide, perhaps through restoration of NAD(+) levels. In the present study, we examined Zn(2+) toxicity, and the effects of NAD(+) restoration in primary retinal cultures. We then reduced Zn(2+) levels in rodents by reducing Zn(2+) levels in the diet, or by genetics and measured LD. Sprague Dawley albino rats were fed 2, or 61 mg Zn(2+)/kg of diet for 3 weeks, and exposed to 18 kLux of white light for 4 h. We light exposed (70 kLux of white light for 50 h) Zn(2+) transporter 3 knockout (ZnT3-KO, no synaptic Zn(2+)), or RPE65 knockout mice (RPE65-KO, lack rhodopsin cycling), or C57/BI6/J controls and determined light damage and Zn(2+) staining. Retinal Zn(2+) staining was examined at 1 h and 4 h after light exposure. Retinas were examined after 7 d by optical coherence tomography and histology. After LD, rats fed the reduced Zn(2+) diet showed less photoreceptor Zn(2+) staining and degeneration compared to a normal Zn(2+) diet. Similarly, ZnT3-KO and RPE65-KO mice showed less Zn(2+) staining, NAD(+) loss, and RPE or photoreceptor death than C57/BI6/J control mice. Dietary or ZnT3-dependent Zn(2+) stores, and intracellular Zn(2+) release from rhodopsin recycling are suggested to be involved in light-induced retinal degeneration. These results implicate novel rhodopsin-mediated mechanisms and therapeutic targets for LD. Our companion manuscript demonstrates that pharmacologic, circadian, or genetic manipulations which maintain NAD(+) levels reduce LD.

Mitochondrial inhibitor models of Huntington's disease and Parkinson's disease induce zinc accumulation and are attenuated by inhibition of zinc neurotoxicity in vitro or in vivo.

BACKGROUND: Inhibition of mitochondrial function occurs in many neurodegenerative diseases, and inhibitors of mitochondrial complexes I and II are used to model them. The complex II inhibitor, 3-nitroproprionic acid (3-NPA), kills the striatal neurons susceptible in Huntington's disease. The complex I inhibitor N-methyl-4-phenylpyridium (MPP(+)) and 6-hydroxydopamine (6-OHDA) are used to model Parkinson's disease. Zinc (Zn(2+)) accumulates after 3-NPA, 6-OHDA and MPP(+) in situ or in vivo. OBJECTIVE: We will investigate the role of Zn(2+) neurotoxicity in 3-NPA, 6-OHDA and MPP(+). METHODS: Murine striatal/midbrain tyrosine hydroxylase positive, or near-pure cortical neuronal cultures, or animals were exposed to 3-NPA or MPP(+) and 6-OHDA with or without neuroprotective compounds. Intracellular zinc ([Zn(2+)](i)), nicotinamide adenine dinucleotide (NAD(+)), NADH, glycolytic intermediates and neurotoxicity were measured. RESULTS: We showed that compounds or genetics which restore NAD(+) and attenuate Zn(2+) neurotoxicity (pyruvate, nicotinamide, NAD(+), increased NAD(+) synthesis, sirtuin inhibition or Zn(2+) chelation) attenuated the neuronal death induced by these toxins. The increase in [Zn(2+)](i) preceded a reduction in the NAD(+)/NADH ratio that caused a reversible glycolytic inhibition. Pyruvate, nicotinamide and NAD(+) reversed the reductions in the NAD(+)/NADH ratio, glycolysis and neuronal death after challenge with 3-NPA, 6-OHDA or MPP(+), as was previously shown for exogenous Zn(2+). To test efficacy in vivo, we injected 3-NPA into the striatum of rats and systemically into mice, with or without pyruvate. We observed early striatal Zn(2+) fluorescence, and pyruvate significantly attenuated the 3-NPA-induced lesion and restored behavioral scores. CONCLUSIONS: Together, these studies suggest that Zn(2+) accumulation caused by MPP(+) and 3-NPA is a novel preventable mechanism of the resultant neurotoxicity.

Dietary zinc reduction, pyruvate supplementation, or zinc transporter 5 knockout attenuates ß-cell death in nonobese diabetic mice, islets, and insulinoma cells.

Pancreatic zinc (Zn(2+)) concentrations are linked to diabetes and pancreatic dysfunction, but Zn(2+) is also required for insulin processing and packaging. Zn(2+) released with insulin increases ß-cell pancreatic death after streptozotocin toxin exposure in vitro and in vivo. Triosephosphate accumulation, caused by NAD(+) loss and glycolytic enzyme dysfunction, occur in type-1 diabetics (T1DM) and animal models. We previously showed these mechanisms are also involved in Zn(2+) neurotoxicity and are attenuated by nicotinamide- or pyruvate-induced restoration of NAD(+) concentrations, Zn(2+) restriction, or inhibition of Sir2 proteins. We tested the hypothesis that similar Zn(2+)- and NAD(+)-mediated mechanisms are involved in ß-cell toxicity in models of ongoing T1DM using mouse insulinoma cells, islets, and nonobese diabetic (NOD) mice. Zn(2+), streptozotocin, and cytokines caused NAD(+) loss and death in insulinoma cells and islets, which were attenuated by Zn(2+) restriction, pyruvate, nicotinamide, NAD(+), and inhibitors of Sir2 proteins. We measured diabetes incidence and mortality in NOD mice and demonstrated that pyruvate supplementation, or genetic or dietary Zn(2+) reduction, attenuated these measures. T-lymphocyte infiltration, punctate Zn(2+) staining, and ß-cell loss increased with time in islets of NOD mice. Dietary Zn(2+) restriction or Zn(2+) transporter 5 knockout reduced pancreatic Zn(2+) staining and increased ß-cell mass, glucose homeostasis, and survival in NOD mice, whereas Zn(2+) supplementation had the opposite effects. Pancreatic Zn(2+) reduction or NAD(+) restoration (pyruvate or nicotinamide supplementation) are suggested as novel targets for attenuating T1DM.

Involvement of SIRT1 in Zn2+, Streptozotocin, Non-Obese Diabetic, and Cytokine-Mediated Toxicities of ß-cells.

Zn2+ toxicity is implicated in pancreatic ß-cell death that occurs secondarily to: streptozotocin exposure in vitro; and both autoimmune attack or streptozotocin in vivo models of T1DM. This is demonstrated by reduced ß-cell death or diabetic incidence in vitro or in NOD mice after treatment with Zn2+ preferring chelators, pyruvate, nicotinamide, a reduced zinc diet, sirtuin inhibitors, or zinc transporter knockout. These therapeutics are also demonstrated to be efficacious against Zn2+ neurotoxicity. AIMS: To determine if the sirtuin pathway is involved in Zn2+-, streptozotocin-, or cytokine-mediated ß-cell death in vitro, and streptozotocin-, or NOD induced T1DM in vivo. METHODS: Sensitivity of MIN6 cells expressing empty vector, sirtuin protein-1 (SIRT1) or its siRNA, to Zn2+, streptozotocin, or cytokines, and effects on NAD+ levels were determined. Covariance of manipulating SIRT1 levels with diabetic incidence was tested in vivo. RESULTS: 1) sirtuin pathway inhibition or SIRT1 knockdown attenuated Zn2+-, STZ-, and cytokine-mediated toxicity and NAD+ loss in ß-cells, 2) SIRT1 overexpression potentiated these toxicities, 3) young SIRT1 ß-cell transgenic mice have improved glucose tolerance under basal conditions, but upon aging showed increased sensitivity to streptozotocin compared to SIRT1 +/- mice, and 4) SIRT1 +/- mice in an NOD background or exposed to streptozotocin trended toward reduced diabetic incidence and mortality compared to wildtype. CONCLUSIONS: These results have implicated SIRT1-mediated NAD+ loss in Zn2+, STZ, or cytokine toxicities of MIN6, and in NOD or streptozotocin T1DM animal models. Modulation of ß-cell Zn2+ and NAD+ levels, and the sirtuin pathway could be novel therapeutic targets for T1DM.

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.

Thiamine supplementation attenuated hepatocellular carcinoma in the Atp7b mouse model of Wilson's disease.

BACKGROUND: Wilson's disease is caused by a genetic defect in P-type Cu(2+)-ATPase (Atp7b), resulting in Cu(2+) accumulation in the liver, toxicity, and hepatocellular carcinoma. Exposure of HepG2 cells, and livers of Atp7b mutant mice to toxic Cu(2+) resulted in oxidation, (KGDH) and (PDH) enzyme inhibition, and death that was attenuated by thiamine. MATERIALS AND METHODS: The effect of oral thiamine supplementation (2%) on hepatocellular carcinoma induced by Cu(2+) accumulation in the livers of Atp7b animals at 4, 6, 9, 12, 16, and 21 months was demonstrated using gross morphology and multi-nucleate analysis. RESULTS: By 16 months of age, untreated Atp7b animals became moribund, their livers were >180% the weight of controls and >75% of their liver was cancerous. At 16 months the livers of thiamine treated Atp7b mice were <130% the weight of controls and <30% cancerous, and at 21 months the mice were still active. However thiamine was ineffective in a subcutaneous xenograft model. CONCLUSION: This study suggests that thiamine may constitute a prophylactic for Wilson's disease-induced hepatocellular carcinoma.

Spreading depression and related events are significant sources of neuronal Zn2+ release and accumulation.

Spreading depression (SD) involves coordinated depolarizations of neurons and glia that propagate through the brain tissue. Repetitive SD-like events are common following human ischemic strokes, and are believed to contribute to the enlargement of infarct volume. Accumulation of Zn(2+) is also implicated in ischemic neuronal injury. Synaptic glutamate release contributes to SD propagation, and because Zn(2+) is costored with glutamate in some synaptic vesicles, we examined whether Zn(2+) is released by SD and may therefore provide a significant source of Zn(2+) in the postischemic period. Spreading depression-like events were generated in acutely prepared murine hippocampal slices by deprivation of oxygen and glucose (OGD), and Zn(2+) release was detected extracellularly by a Zn(2+)-selective indicator FluoZin-3. Deprivation of oxygen and glucose-SD produced large FluoZin-3 increases that propagated with the event, and signals were abolished in tissues from ZnT3 knockout animals lacking synaptic Zn(2+). Synaptic Zn(2+) release was also maintained with repetitive SDs generated by microinjections of KCl under normoxic conditions. Intracellular Zn(2+) accumulation in CA1 neurons, assessed using microinjection of FluoZin-3, showed significant increases following SD that was attributed to synaptic Zn(2+) release. These results suggest that Zn(2+) is released during SDs and could provide a significant source of Zn(2+) that contributes to neurodegeneration in the postischemic period.

Light-induced photoreceptor and RPE degeneration involve zinc toxicity and are attenuated by pyruvate, nicotinamide, or cyclic light.

PURPOSE: Light-induced damage can be a problem after surgery or sun exposure. Short-duration, intense light causes preferential photoreceptor death in the superior central retina of albino mice and rats and serves as a model of oxidation-induced neurodegeneration. Previous work on retinal ischemia-induced neuronal death suggests the involvement of zinc (Zn(2+)) toxicity in the death and collapse of many retinal cell layers and demonstrates the protective efficacy of pyruvate. Retinal pigment epithelial (RPE) cells were shown to be sensitive to oxidative stress, and zinc, causing loss of nicotinamide adenine dinucleotide (NAD+) and adenine triphosphate (ATP), which was prevented by pyruvate and nicotinamide. We previously showed similar results in cortical neurons exposed to oxidative stress or Zn(2+). In vivo, Zn(2+) is normally present in the inner and outer segments (associated with rhodopsin), Bruch's membrane and sclera (elastin), RPE, and the outer plexiform layer of the eye (synaptic). In this study, we examine the role of Zn(2+) in oxidative stress and light-induced damage in vitro and in vivo. METHODS: We modeled retinal toxicity in cell-culture lines derived from retinal tissue: Müller and human retinal pigment epithelial (ARPE-19) cells and a cone photoreceptor-derived line (661W). These cultures were exposed to Zn(2+) and OS, and the therapeutic efficacy of pyruvate, nicotinamide, and NAD(+) was determined. Sprague Dawley albino rats were exposed to 18 kLux of white fluorescent light for 1-4 h in the presence and absence of pyruvate, nicotinamide, lactate, and cyclic light. The intracellular free zinc concentration ([Zn(2+)](i)) and cell damage were assessed 0.5 and 7 days later, respectively. RESULTS: We show that Zn(2+) and oxidative stress results in increased [Zn(2+)](i) and that Zn(2+) therapeutic compounds (pyruvate, nicotinamide, and NAD(+)) and inhibitors of previously implicated pathways (sirtuin) are efficacious in vitro. Exposure to 18 kLux of cool white fluorescent light for 1 h induced a large increase in Zn(2+) staining 4-14 h later, particularly in the superior outer nuclear layer and RPE of dark-maintained Sprague Dawley albino rats; 4 h of light was required to induce similar damage in cyclic light-maintained rats. Photoreceptors and RPE cells died in untreated animals at 3-7 days. However, nicotinamide and pyruvate (intraperitoneal), but not lactate, attenuated this death in treated animals, as measured using optical coherence tomography and confirmed by counting photoreceptor nuclei. CONCLUSIONS: Zn(2+) plays a role in this injury, as suggested by the increased Zn(2+) staining and the efficacy of Zn(2+) therapeutics. These results suggest that cyclic light maintenance, Zn(2+) chelation, pyruvate, and nicotinamide promote RPE and photoreceptor survival after injury and could be effective for various forms of retinal neurodegeneration. These results could have immediate clinical applications in surgery- or sun exposure- induced light damage to the retina.

Zip4 (Slc39a4) expression is activated in hepatocellular carcinomas and functions to repress apoptosis, enhance cell cycle and increase migration.

BACKGROUND: The zinc transporter ZIP4 (Slc39a4) is important for proper mammalian development and is an essential gene in mice. Recent studies suggest that this gene may also play a role in pancreatic cancer. METHODS/PRINCIPAL FINDINGS: Herein, we present evidence that this essential zinc transporter is expressed in hepatocellular carcinomas. Zip4 mRNA and protein were dramatically elevated in hepatocytes in the majority of human hepatocellular carcinomas relative to noncancerous surrounding tissues, as well as in hepatocytes in hepatocellular carcinomas occurring in farnesoid X receptor-knockout mice. Interestingly, meta-analysis of microarray data in the Geo and Oncomine databases suggests that Zip4 mRNA may also be elevated in many types of cancer. Potential mechanisms of action of ZIP4 were examined in cultured cell lines. RNAi knockdown of Zip4 in mouse Hepa cells significantly increased apoptosis and modestly slowed progression from G(0)/G(1) to S phase when cells were released from hydroxyurea block into zinc-deficient medium. Cell migration assays revealed that RNAi knockdown of Zip4 in Hepa cells depressed in vitro migration whereas forced over-expression in Hepa cells and MCF-7 cells enhanced in vitro migration. CONCLUSIONS: ZIP4 may play a role in the acquisition of zinc by hepatocellular carcinomas, and potentially many different cancerous cell-types, leading to repressed apoptosis, enhanced growth rate and enhanced invasive behavior.

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

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

Decreased brain zinc availability reduces hippocampal neurogenesis in mice and rats.

In the adult brain, neurogenesis occurs in the subgranular zone of the dentate gyrus (DG), where high levels of vesicular zinc are localized in the presynaptic terminals. To determine whether zinc has a role in modulating hippocampal neurogenesis under normal or pathologic conditions, we manipulated the level of vesicular zinc experimentally. To reduce hippocampal vesicular zinc, rats were either fed a zinc-deficient diet or treated with a zinc chelator, clioquinol (CQ). The number of progenitor cells and immature neurons was decreased significantly in the DG after 6 weeks of dietary zinc deprivation. Conversely, the number of progenitor cells and immature neurons was restored after a 2-week reversal to a normal zinc-containing diet. Similarly, a 1-week treatment with the zinc chelator, CQ, reduced the number of progenitor cells. The results of our previous study showed that hypoglycemia increased hippocampal neurogenesis. This study shows that zinc chelation reduced hypoglycemia-induced progenitor cell proliferation and neurogenesis. Finally, the role of vesicular zinc on neurogenesis was further assessed in zinc transporter 3 (ZnT3) gene deleted mice. Zinc transporter 3 knockout (KO) mice had significantly fewer proliferating progenitor cells and immature neurons after hypoglycemia. Our data provide converging evidence in support of the essential role zinc has in modulating hippocampal neurogenesis.

Sequential release of nitric oxide, zinc, and superoxide in hypoglycemic neuronal death.

Oxidative stress and zinc release are both known to contribute to neuronal death after hypoglycemia; however, the cause-effect relationships between these events are not established. Here we found, using a rat model of profound hypoglycemia, that the neuronal zinc release and translocation that occur immediately after hypoglycemia are prevented by the nitric oxide synthase inhibitor 7-nitroindazole but not by overexpression of superoxide dismutase-1 (SOD-1). However, overexpression of SOD-1 prevented activation of poly(ADP-ribose) polymerase-1 (PARP-1) and neuronal death, suggesting that zinc release is upstream of superoxide production. Accordingly, zinc-induced superoxide production was blocked in neuronal cultures by the NADPH oxidase inhibitor apocynin and by genetic deficiency in the p47(phox) subunit of NADPH oxidase. A key role for the vesicular zinc pool in this process was suggested by reduced superoxide formation and neuronal death in mice deficient in zinc transporter 3. Together, these findings suggest a series of events in which nitric oxide production triggers vesicular zinc release, which in turn activates NADPH oxidase and PARP-1. This sequence may also occur in other central nervous system disorders in which zinc, nitric oxide, and oxidative stress have been linked.

Zinc neurotoxicity is dependent on intracellular NAD levels and the sirtuin pathway.

Zinc neurotoxicity has been demonstrated in ischemic, seizure, hypoglycemic, and trauma-induced neuronal death where Zn(2+) is thought to be synaptically released and taken up in neighbouring neurons, reaching toxic concentrations. We previously demonstrated that toxicity of extracellular Zn(2+) depended on entry, elevation in intracellular free Zn(2+) ([Zn(2+)](i)), a reduction in NAD(+) and ATP levels, and dysfunction of glycolysis and cellular metabolism. We suggested that PARP-1 activation alone can not explain this loss of neuronal NAD(+). NAD(+) was recently demonstrated to permeate neurons and glia, and we have now shown that exogenous NAD(+) can reduce Zn(2+) neurotoxicity, and 3-acetylpyridine, which generates inactive NAD(+), potentiated Zn(2+) neurotoxicity. Sirtinol and 2-hydroxynaphthaldehyde, inhibitors of the sirtuin pathway (SIRT proteins are NAD(+)-catabolic protein deacetylases), attenuated both acute and chronic Zn(2+) neurotoxicity. Resveratrol and fisetin (sirtuin activators) potentiated NAD(+) loss and Zn(2+) neurotoxicities. Furthermore, neuronal cultures derived from the Wld(s) mouse, which overexpress the NAD(+) synthetic enzyme nicotinamide mononucleotide adenyl transferase (NMNAT-1), had reduced sensitivity to Zn(2+) neurotoxicity. Finally, nicotinamide was demonstrated to attenuate CA1 neuronal death after 10 min of global ischemia in rat even if administered 1 h after the insult. Together with previous data, these results further implicate NAD(+) levels in Zn(2+) neurotoxicity.

Cu2+ toxicity inhibition of mitochondrial dehydrogenases in vitro and in vivo.

Wilson's disease results from mutations in the P-type Cu(2+)-ATPase causing Cu(2+) toxicity. We previously demonstrated that exposure of mixed neuronal/glial cultures to 20 microM Cu(2+) induced ATP loss and death that were attenuated by mitochondrial substrates, activators, and cofactors. Here, we show differential cellular sensitivity to Cu(2+) that was equalized to 5 microM in the presence of the copper exchanger/ionophore, disulfiram. Because Cu(2+) facilitates formation of oxygen radicals (ROS) which inhibit pyruvate dehydrogenase (PDH) and alpha-ketoglutarate dehydrogenase (KGDH), we hypothesized that their inhibition contributed to Cu(2+)-induced death. Toxic CU(2+) exposure was accompanied by early inhibition of neuronal and hepatocellular PDH and KGDH activities, followed by reduced mitochondrial transmembrane potential, DeltaPsi(M). Thiamine (1-6 mM), and dihydrolipoic acid (LA, 50 microM), required cofactors for PDH and KGDH, attenuated this enzymatic inhibition and subsequent death in all cell types. Furthermore, liver PDH and KGDH activities were reduced in the Atp7b mouse model of Wilson's disease prior to liver damage, and were partially restored by oral thiamine supplementation. These data support our hypothesis that Cu(2+)-induced ROS may inhibit PDH and KGDH resulting in neuronal and hepatocellular death. Therefore, thiamine or lipoic acid may constitute potential therapeutic agents for Wilson's disease.

Potassium attenuates zinc-induced death of cultured cortical astrocytes.

Transient global ischemia induces CA1 hippocampal neuronal death without astrocyte death, perhaps mediated in part by the toxic translocation of zinc from presynaptic terminals to postsynaptic neurons. We tested the hypothesis that cellular depolarization, which occurs in the ischemic brain due to increased extracellular potassium and energy failure, might contribute to astrocyte resistance to zinc-induced death. We previously reported that neurons in mixed cortical neuronal-astrocyte cultures were more vulnerable to a 5-15-min exposure to Zn(2+) than astrocytes in the same cultures. In the present report, we show that (1) neurons in isolation or in conjunction with astrocytes were 2-3-fold more sensitive to a 15-min nondepolarizing Zn(2+) exposure than are glia; (2) KCl-induced depolarization attenuated glial vulnerability to zinc toxicity but potentiated neuronal vulnerability to zinc toxicity; (3) Zn(2+)-induced glial death was attenuated by T-type Ca(2+) channel blockade, as well as compounds that increase NAD(+) levels; and (4) both astrocytic (65)Zn(2+) accumulation and the increase in astrocytic [Zn(2+)](i) induced by Zn(2+) exposure were also attenuated by depolarization or T-type Ca(2+) channel blockers. Zn(2+)-induced cell death in astrocytes was at least in part apoptotic, as caspase-3 was activated, and the caspase inhibitor Z-Val-Ala-Asp-fluoromethylketone partially attenuated Zn(2+)-induced death. The levels of peak [Zn(2+)](i) achieved in astrocytes during this toxic nondepolarizing Zn(2+) exposure (250 nM) were substantially greater than those achieved in neurons (40 nM). In glia, exposure to 400 microM Zn(2+) induced a 13-mV depolarization, which can activate T-type Ca(2+) channels. This Zn(2+)-induced astrocyte death, like neuronal death, was attenuated by the addition of pyruvate or niacinamide to the exposure medium.