Zinc translocation accelerates infarction after mild transient focal ischemia

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.

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

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

Erythropoietin prevents zinc accumulation and neuronal death after traumatic brain injury in rat hippocampus: in vitro and in vivo studies

Erythropoietin (Epo) has been gaining great interest for its potential neuroprotective effect in various neurological insults. However, the molecular mechanism underlying how Epo exerts the function is not clear. Recent studies have indicated that Zn(2+) may have a key role in selective cell death in excitotoxicity after injury. In the present study, we studied the effect of recombinant human Epo (rhEpo) in zinc-induced neurotoxicity both in vitro and in vivo. Exposure of cultured hippocampal neurons to 200 muM ZnC1(2) for 20 min resulted in remarkable neuronal injury, revealed by assessing neuronal morphology. By measuring mitochondrial function using MTT assay, we found that application of rhEpo (0.1 U/ml) 24 h before zinc exposure resulted in a significant increase of neuronal survival (0.6007+/-0.2280 Epo group vs 0.2333+/-0.1249 in control group; n=4, p<0.01). Furthermore, we demonstrated that administration of rhEpo (5,000 IU/kg, intraperitoneal) 30 min after traumatic brain injury (TBI) in rats dramatically protected neuronal death indicated by ZP4 staining, a new zinc-specific fluorescent sensor which has been widely used to indicate neuronal damage after excitotoxic injury (n=5/group, p<0.05). Neuronal damage was also assessed by Fluoro-Jade B (FJB) staining, a highly specific fluorescent marker for the degenerating neurons. Consistent with ZP4 staining, we found the beneficial effects of rhEpo on neuronal survival in hippocampus after TBI (n=5/group, p<0.05). Our results suggest that rhEpo can significantly reduce the pathological Zn(2+) accumulation in rat hippocampus after TBI as well as zinc-induced cell death in cultured cells, which may potentially contribute to its neuronal protection after excitotoxic brain damage.

Intracellular Zn2+ increases contribute to the progression of excitotoxic Ca2+ increases in apical dendrites of CA1 pyramidal neurons

Sustained intracellular Ca(2+) elevation is a well-established contributor to neuronal injury following excessive activation of N-methyl-d-aspartic acid (NMDA)-type glutamate receptors. Zn(2+) can also be involved in excitotoxic degeneration, but the relative contributions of these two cations to the initiation and progression of excitotoxic injury is not yet known. We previously concluded that extended NMDA exposure led to sustained Ca(2+) increases that originated in apical dendrites of CA1 neurons and then propagated slowly throughout neurons and caused rapid necrotic injury. However the fluorescent indicator used in those studies (Fura-6F) may also respond to Zn(2+), and in the present work we examine possible contributions of Zn(2+) to indicator signals and to the progression of degenerative signaling along murine CA1 dendrites. Selective chelation of Zn(2+) with N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) significantly delayed, but did not prevent the development and progression of sustained high-level Fura-6F signals from dendrites to somata. Rapid indicator loss during the Ca(2+) overload response, which corresponds to rapid neuronal injury, was also not prevented by TPEN. The relationship between cytosolic Zn(2+) and Ca(2+) levels was assessed in single CA1 neurons co-loaded with Fura-6F and the Zn(2+)-selective indicator FluoZin-3. NMDA exposure resulted in significant initial increases in FluoZin-3 increases that were prevented by TPEN, but not by extracellular Zn(2+) chelation with Ca-EDTA. Consistent with this result, Ca-EDTA did not delay the progression of Fura-6F signals during NMDA. Removal of extracellular Ca(2+) reduced, but did not prevent FluoZin-3 increases. These results suggest that sustained Ca(2+) increases indeed underlie Fura-6F signals that slowly propagate throughout neurons, and that Ca(2+) (rather than Zn(2+)) increases are ultimately responsible for neuronal injury during NMDA. However, mobilization of Zn(2+) from endogenous sources leads to significant neuronal Zn(2+) increases, that in turn contribute to mechanisms of initiation and progression of progressive Ca(2+) deregulation.

Essential role for zinc-triggered p75NTR activation in preconditioning neuroprotection

Ischemic preconditioning (PC) of the brain is a phenomenon by which mild ischemic insults render neurons resistant to subsequent strong insults. Key steps in ischemic PC of the brain include caspase-3 activation and poly(ADP-ribose) polymerase-1 (PARP-1) cleavage, but upstream events have not been clearly elucidated. We have tested whether endogenous zinc is required for ischemic PC of the brain in rats. Mild, transient zinc accumulation was observed in certain neurons after ischemic PC. Moreover, intraventricular administration of CaEDTA during ischemic PC abrogated both zinc accumulation and the protective effect against subsequent full ischemia. To elucidate the mechanism of the zinc-triggered PC (Zn PC) effect, cortical cultures were exposed to sublethal levels of zinc, and 18 h later to lethal levels of zinc or NMDA. Zn PC exhibited the characteristic features of ischemic PC, including caspase-3 activation, PARP-1 cleavage, and HSP70 induction, all of which are crucial for subsequent neuroprotection against NMDA or zinc toxicity. HSP70 induction was necessary for protection, as it halted caspase-3 activation before apoptosis. Interestingly, in both Zn PC in vitro and ischemic PC in vivo, p75(NTR) was necessary for neuroprotection. These results suggest that caspase-3 activation during ischemic PC, a necessary event for subsequent neuroprotection, may result from mild zinc accumulation and the consequent p75(NTR) activation in neurons.

The role of zinc in cerebral ischemia

Ischemic stroke is one of the most pervasive life-threatening neurological conditions for which there currently exists limited therapeutic intervention beyond prevention. As calcium-focused neuroprotective strategies have met with limited clinical success, it is imperative that alternative therapeutic targets be considered in the attempt to antagonize ischemic-mediated injury. As such, zinc, which is able to function both as a signaling mediator and neurotoxin, has been implicated in cerebral ischemia. While zinc was first purported to have a role in cerebral ischemia nearly twenty years ago, our understanding of how zinc mediates ischemic injury is still in its relative infancy. Within this review, we examine some of the studies by which zinc has exerted either neuroprotective or neurotoxic effects during global and focal cerebral ischemia.

The role of zinc in cerebral ischemia

Ischemic stroke is one of the most pervasive life-threatening neurological conditions for which there currently exists limited therapeutic intervention beyond prevention. As calcium-focused neuroprotective strategies have met with limited clinical success, it is imperative that alternative therapeutic targets be considered in the attempt to antagonize ischemic-mediated injury. As such, zinc, which is able to function both as a signaling mediator and neurotoxin, has been implicated in cerebral ischemia. While zinc was first purported to have a role in cerebral ischemia nearly twenty years ago, our understanding of how zinc mediates ischemic injury is still in its relative infancy. Within this review, we examine some of the studies by which zinc has exerted either neuroprotective or neurotoxic effects during global and focal cerebral ischemia.

Mechanism and regulation of cellular zinc transport

Zinc is an essential cofactor for the activity and folding of up to ten percent of mammalian proteins and can modulate the function of many others. Because of the pleiotropic effects of zinc on every aspect of cell physiology, deficits of cellular zinc content, resulting from zinc deficiency or excessive rise in its cellular concentration, can have catastrophic consequences and are linked to major patho-physiologies including diabetes and stroke. Thus, the concentration of cellular zinc requires establishment of discrete, active cellular gradients. The cellular distribution of zinc into organelles is precisely managed to provide the zinc concentration required by each cell compartment. The complexity of zinc homeostasis is reflected by the surprisingly large variety and number of zinc homeostatic proteins found in virtually every cell compartment. Given their ubiquity and importance, it is surprising that many aspects of the function, regulation, and crosstalk by which zinc transporters operate are poorly understood. In this mini-review, we will focus on the mechanisms and players required for generating physiologically appropriate zinc gradients across the plasma membrane and vesicular compartments. We will also highlight some of the unsolved issues regarding their role in cellular zinc homeostasis.

Mechanism and regulation of cellular zinc transport

Zinc is an essential cofactor for the activity and folding of up to ten percent of mammalian proteins and can modulate the function of many others. Because of the pleiotropic effects of zinc on every aspect of cell physiology, deficits of cellular zinc content, resulting from zinc deficiency or excessive rise in its cellular concentration, can have catastrophic consequences and are linked to major patho-physiologies including diabetes and stroke. Thus, the concentration of cellular zinc requires establishment of discrete, active cellular gradients. The cellular distribution of zinc into organelles is precisely managed to provide the zinc concentration required by each cell compartment. The complexity of zinc homeostasis is reflected by the surprisingly large variety and number of zinc homeostatic proteins found in virtually every cell compartment. Given their ubiquity and importance, it is surprising that many aspects of the function, regulation, and crosstalk by which zinc transporters operate are poorly understood. In this mini-review, we will focus on the mechanisms and players required for generating physiologically appropriate zinc gradients across the plasma membrane and vesicular compartments. We will also highlight some of the unsolved issues regarding their role in cellular zinc homeostasis.

The lack of effects of zinc and nitric oxide in initial state of pilocarpine-induced seizures

In this study we investigated whether intracerebroventricular (i.c.v.) injection of L-NAME (a nitric oxide synthase inhibitor) or CaEDTA (an extracellular zinc chelator) or the combination of the two could affect the initial phase of pilocarpine induced (2 h) seizures. Two groups of rats were used. Animals from both groups were given with i.c.v. injections of either saline (10 microl), L-NAME (150 microg/10 microl), CaEDTA (100 mM/10 microl) or L-NAME and CaEDTA. One group received pilocarpine HCl (380 mg/kg i.p.) the other served as control. Pilocarpine HCl was injected intraperitoneally 10 min later. The behavior of the animals was observed for 2h and the intensity of their seizures was scored. The rats were then sacrificed and their brains were removed and analyzed for zinc ions by using the immersion autometallography and the TSQ fluorescence staining. All the animals which received pilocarpine HCl developed seizures. Despite treatment with L-NAME and/or CaEDTA we found that the latency and the intensity of seizures were similar in both groups investigated. The distribution of stainable zinc ions and the intensity of staining in hippocampus were not affected by pilocarpine and found unchanged after L-NAME and/or CaEDTA injections in both the control animals and the pilocarpine treated animals. The data suggest that the nitric oxide system and zinc ions do not affect pilocarpine-induced seizures in their initial state.

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.

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+.

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.

Irreversible aggregation of protein synthesis machinery after focal brain ischemia

Focal brain ischemia leads to a slow type of neuronal death in the penumbra that starts several hours after ischemia and continues to mature for days. During this maturation period, blood flow, cellular ATP and ionic homeostasis are gradually recovered in the penumbral region. In striking contrast, protein synthesis is irreversibly inhibited. This study used a rat focal brain ischemia model to investigate whether or not irreversible translational inhibition is due to abnormal aggregation of translational complex components, i.e. the ribosomes and their associated nascent polypeptides, protein synthesis initiation factors and co-translational chaperones. Under electron microscopy, most rosette-shaped polyribosomes were relatively evenly distributed in the cytoplasm of sham-operated control neurons, but clumped into large abnormal aggregates in penumbral neurons subjected to 2 h of focal ischemia followed by 4 h of reperfusion. The abnormal ribosomal protein aggregation lasted until the onset of delayed neuronal death at 24-48 h of reperfusion after ischemia. Biochemical study further suggested that translational complex components, including small ribosomal subunit protein 6 (S6), large subunit protein 28 (L28), eukaryotic initiation factors 2alpha, 4E and 3eta, and co-translational chaperone heat-shock cognate protein 70 (HSC70) and co-chaperone Hdj1, were all irreversibly clumped into large abnormal protein aggregates after ischemia. Translational complex components were also highly ubiquitinated. This study clearly demonstrates that focal ischemia leads to irreversible aggregation of protein synthesis machinery that contributes to neuronal death after focal brain ischemia.

Release of vesicular Zn2+ in a rat transient middle cerebral artery occlusion model

In the brain, Zn(2+) is stored in synaptic vesicles of a subgroup of glutamatergic nerve terminals. Although it has been reported that this Zn(2+) is released upon the excitation of nerves in vitro, there has been little study of the release of Zn(2+) during ischemia in vivo. Here, using brain microdialysis, the release of vesicular Zn(2+) was investigated in vivo. When the vesicular Zn(2+) was released into the synaptic cleft by a depolarizing stimulation achieved by perfusion with Ringer's solution containing high K(+) (100mM KCl), a significant increase in the extracellular concentration of Zn(2+) could be detected by microdialysis. Then, we investigated the release of vesicular Zn(2+) in a rat transient middle cerebral artery occlusion model using microdialysis. Consequently, the extracellular Zn(2+) level in the cortex increased within 15 min of the start of occlusion and reached a peak at 30 min, which was about twice the basal level. After 30 min, it declined with time returning to the basal level 15 min after reperfusion, which was performed after 60 min of occlusion. The results suggest that vesicular Zn(2+) would be released into the synaptic cleft during brain ischemia in vivo.

Zn2+ inhibits mitochondrial movement in neurons by phosphatidylinositol 3-kinase activation

Mitochondria have been identified as targets of the neurotoxic actions of zinc, possibly through decreased mitochondrial energy production and increased reactive oxygen species accumulation. It has been hypothesized that impairment of mitochondrial trafficking may be a mechanism of neuronal injury. Here, we report that elevated intraneuronal zinc impairs mitochondrial trafficking. At concentrations just sufficient to cause injury, zinc rapidly inhibited mitochondrial movement without altering morphology. Zinc chelation initially restored movement, but the actions of zinc became insensitive to chelator in <10 min. A search for downstream signaling events revealed that inhibitors of phosphatidylinositol (PI) 3-kinase prevented this zinc effect on movement. Moreover, transient inhibition of PI 3-kinase afforded neuroprotection against zinc-mediated toxicity. These data illustrate a novel mechanism that regulates mitochondrial trafficking in neurons and also suggest that mitochondrial trafficking may be closely coupled to neuronal viability.

Calcium and Reactive Oxygen Species Mediated Zn2+-Induced Apoptosis in PC12 Cells

The release of excessive Zn(2+) from presynaptic boutons into extracellular regions contributes to neuronal apoptotic events, which result in neuronal cell death. However, the mechanisms of Zn(2+)-induced neuronal cell death are still unclear. Therefore, we investigated the dynamics of intracellular Zn(2+), calcium, and reactive oxygen species in PC12 cells. The addition of Zn(2+) produced cell death in a concentration- and time-dependent manner. (45)Ca(2+) influx occurred just after the treatment with Zn(2+), although subsequent hydroxyl radical ((*)OH) production did not begin until 3 h after Zn(2+) exposure. (*)OH production was significantly attenuated in Ca(2+)-free medium or by L-type Ca(2+) channel antagonist treatment, but it was independent of the intracellular Zn(2+) content. Dantrolene treatment had no protective effects against Zn(2+)-induced cell death. Treatment with N-acetyl-L-cysteine blocked (*)OH generation and subsequent cell death. These data indicate that Ca(2+) influx and subsequent (*)OH production are critical events in Zn(2+)-induced toxicity in PC12 cells.

Endogenous zinc in neurological diseases

The use of zinc in medicinal skin cream was mentioned in Egyptian papyri from 2000 BC (for example, the Smith Papyrus), and zinc has apparently been used fairly steadily throughout Roman and modern times (for example, as the American lotion named for its zinc ore, 'Calamine'). It is, therefore, somewhat ironic that zinc is a relatively late addition to the pantheon of signal ions in biology and medicine. However, the number of biological functions, health implications and pharmacological targets that are emerging for zinc indicate that it might turn out to be 'the calcium of the twenty-first century'. Here neurobiological roles of endogenous zinc is summarized.

Clinical pharmacology of DP-b99 in healthy volunteers: first administration to humans

AIMS

To investigate the safety, tolerability and pharmacokinetics of DP-b99 in healthy volunteers. DP-b99 is a newly developed lipophilic, cell permeable derivative of BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid), which selectively modulates the distribution of metal ions in hydrophobic milieu, and is in clinical development as a neuroprotectant for cerebral ischaemic stroke. To our knowledge no BAPTA derivative has ever been administered to man. Here we report the first human administration of DP-b99 in a phase I, two-part, double-blind, randomized placebo controlled study, with single IV doses of 0.003-1.0 mg kg(-1) day(-1) DP-b99 (part 1) or multiple ascending doses of 0.03-1.0 mg kg(-1) day(-1) DP-b99 over 4 days (part 2).

METHODS

A double-blind, dose escalating tolerability study of DP-b99 in normal (young - aged between 18 and 40 years and elderly - aged between 65 and 85 years) healthy adult male volunteers was conducted. Part 1 of the study investigated single administration of ascending intravenous doses, and part 2 examined the effects of ascending doses given repeatedly over 4 days. Twenty-four young volunteers in part 1 received single dose administrations and 26 young volunteers in part 2 received repeated ascending dose administrations of either intravenous DP-b99 or placebo. Subsequently, 10 elderly volunteers received repeated intravenous DP-b99 (1 mg kg(-1)) or placebo in part 2 over 4 days. Adverse events were identified by either subject self reporting or based upon laboratory parameters (blood chemistry, complete blood cell count, prothrombin time (PT), activated partial thromboplastin (PTT), physical examination, vital signs (blood pressure, pulse rate, respiratory rate, body temperature) and urinalysis. A comprehensive set of cardiovascular parameters was assessed as well (blood pressure, 12 lead-ECG recordings and continuous bedside cardiac monitoring for 6 h on day 1).

RESULTS

The administration of DP-b99 up to the highest dose of 1.0 mg kg(-1) was well tolerated and had an acceptable safety profile up to the highest dose of 1.0 mg kg(-1) tested in both study parts. No serious or severe adverse events were encountered. Eight mild to moderate adverse events were observed in six of the seven young subjects treated with four repeated doses of 1.0 mg kg(-1), with reversible phlebitis being the most frequently reported adverse event. The drug was tolerated better at the injection site by the elderly group compared with the younger subjects. No adverse effects were observed in cardiovascular parameters sensitive to trans-membranous calcium concentrations. The pharmacokinetic parameters were derived by noncompartmental analysis. On day 1 following administration of 1 mg kg(-1) the mean half-life of DP-b99 in young volunteers was 3.47 +/- 0.90 h and in the elderly was 2.11 +/- 0.09 h. On day 4 following the same administration of DP-b99 the mean half-life was 4.36 +/- 1.49 and 2.10 +/- 1.14 h in the young and elderly, respectively. There was higher systemic exposure in the elderly, for example C(max), had a mean 1.6-fold higher exposure on day 1 (95% CI Lower 0.90, Upper 2.74) and 2.5-fold on day 4 (95%CI 1.70, 3.68). This increase is in line with the presumed central role of hepatic blood flow in the elimination of DP-b99. No accumulation was observed after repeated dosing with 1 mg kg(-1) (mean accumulation calculated by AUC(0, 24 h) (day 4) : AUC(0, 24 h) (day 1) and was observed to be between 0.9 and 1.3 (young, elderly).

CONCLUSIONS

This study suggests that DP-b99 is well tolerated in healthy young and elderly volunteers within the dose range evaluated. Studies to investigate further the efficacy of the compound are in progress.

The neurobiology of zinc in health and disease

The use of zinc in medicinal skin cream was mentioned in Egyptian papyri from 2000 BC (for example, the Smith Papyrus), and zinc has apparently been used fairly steadily throughout Roman and modern times (for example, as the American lotion named for its zinc ore, 'Calamine'). It is, therefore, somewhat ironic that zinc is a relatively late addition to the pantheon of signal ions in biology and medicine. However, the number of biological functions, health implications and pharmacological targets that are emerging for zinc indicate that it might turn out to be 'the calcium of the twenty-first century'.

Zinc Supplementation: Neuroprotective or Neurotoxic?

While zinc is essential for normal brain function and repair, recent work has implicated this trace element in the neuronal damage and death that follow traumatic brain injury, stroke, and seizure. Therefore, the development of new zinc-based therapeutic strategies will need to consider the emerging roles of zinc in the central nervous system.

Boron dipyrromethene fluorophore based fluorescence sensor for the selective imaging of Zn(II) in living cells

A simple PET fluorescence sensor (BDA) for Zn2+ that utilizes 1,3,5,7-tetramethyl-boron dipyrromethene as a reporting group and di(2-picolyl)amine as a chelator for Zn2+ has been synthesized and characterized. BDA has an excitation (491 nm) and emission wavelength (509 nm) in the visible range. The fluorescence quantum yields of the zinc-free and zinc-bound states of BDA are 0.077 and 0.857, respectively. With a low pKa of 2.1 +/- 0.1, BDA has the advantage of less sensitivity to pH than fluorescein-based Zn2+ sensors, and the fluorescence emission of zinc-binding is pH-independent in the range of pH 3-10. Under physiological conditions, metal ions such as Na+, K+, Ca2+, Mg2+, Mn2+ and Fe2+ have little interference. The apparent dissociation constant (Kd) is 1.0 +/- 0.1 nM. Using fluorescence microscopy, the sensor is shown to be capable of imaging intracellular Zn2+ changes.

Distribution of zincergic neurons in the mouse forebrain

Synaptically released zinc is thought to play an important role in neuronal signaling by modulating excitatory and inhibitory receptors and intracellular signaling proteins. Consequently, neurons that release zinc have been implicated in synaptic plasticity underlying learning and memory as well as neuropathological processes such as epilepsy, stroke, and Alzheimer's disease. To characterize the distribution of these neurons, investigators have relied on a technique that involves the retrograde transport of zinc-selenium crystals from axonal boutons to the cell bodies of origin. However, one major problem with this method is that labeling of cell bodies is obscured by high levels of staining in synaptic boutons, particularly within forebrain structures where this staining is most intense. Here, we used a modification of the retrograde labeling method that eliminates terminal staining for zinc, thereby enabling a clear and comprehensive description of these neurons. Zincergic neurons were found in all cerebral cortical regions and were arranged in a distinct laminar pattern, restricted to layers 2/3, 5, and 6 with no labeling in layer 4. In the hippocampus, labeling was present in CA1, CA3, and the dentate gyrus but not in CA2. Labeled cell bodies were also observed in most amygdaloid nuclei, anterior olfactory nuclei, claustrum, tenia tecta, endopiriform region, lateral ventricle, lateral septum, zona incerta, superior colliculus, and periaqueductal gray. Moreover, retrograde labeling was also noted in the dorsomedial and lateral hypothalamus, regions that previously were thought to be devoid of neurons with a zincergic phenotype. Collectively these data show that zincergic neurons comprise a large population of neurons in the murine forebrain and will provide an anatomical framework for understanding the functional importance of these neurons in the mammalian brain.

Pyrrolidine dithiocarbamate and zinc inhibit proteasome-dependent proteolysis

Proteasomes play important roles in a variety of cellular processes such as cell cycle progression, signal transduction and immune responses. Proteasome activity is important in maintaining rapid turnover of short-lived proteins, as well as preventing accumulation of misfolded or damaged proteins. Alteration in ubiquitin-proteasome function may be detrimental to its crucial role in maintaining cellular homeostasis. Here, we have found that treatment of pyrrolidine dithiocarbamate (PDTC), a zinc ionophore, resulted in the accumulation of several proteasome substrates including p53 and p21 in HeLa cells. The PDTC effect was due to an extended half-life of these proteins through the mobilization of zinc. PDTC and/or zinc also increased fluorescence intensity of Ub(G76V)-GFP fusion protein that is degraded rapidly by the ubiquitin-proteasome system. Treatment of cells with zinc induced formation of ubiquitinated inclusions in the centrosome, a histological marker of proteasome inhibition. Western blotting showed zinc-induced increase in laddering bands of polyubiquitin-conjugated proteins. In vitro study, zinc inhibited the ubiquitin-independent proteasomal degradations of p21 and alpha-synuclein. These results suggest that zinc may modulate cell functions through its action on the turnover of proteins that are susceptible to proteasome-dependent proteolysis.

Characterization of extracellular accumulation of Zn2+ during ischemia and reperfusion of hippocampus slices in rat

The mammalian CNS contains an abundance of chelatable zinc that is sequestered in the vesicles of glutamatergic presynaptic terminals and co-released with glutamate. Considerable Zn(2+) is also released during cerebral ischemia and reperfusion (I/R) although the mechanism of this release has not been elucidated. We report here the real time observation of increase of the concentration of extracellular Zn(2+) ([Zn(2+)](o)), accompanied by a rapid increase of intracellular free Zn(2+)concentration, in the areas of dentate gyrus (DG), CA1 and CA3 in acute rat hippocampus slices during ischemia simulated by deprivation of oxygen and glucose (OGD) followed by reperfusion with normal artificial cerebrospinal fluid. A brief period of OGD caused a sustained increase of [Zn(2+)](o). Subsequent reperfusion with oxygenated medium containing glucose resulted in a further increase of [Zn(2+)](o). Longer periods of OGD caused greater increases of [Zn(2+)](o,) and subsequent reperfusion caused still further increases of [Zn(2+)](o,) regardless of OGD duration. The Zn(2+) chelator CaEDTA (10 mM) significantly reduced the increase of [Zn(2+)] induced by OGD and reperfusion. Significant regional differences of [Zn(2+)](o) over the areas of the DG, CA1 and CA3 were not observed during I/R. Neither sodium channel blockade by tetrodotoxin (2 microM), perfusion with nominally calcium-free medium nor anatomical disassociation of the DG, CA1 and CA3 regions from one another by lesioning affected the increase of [Zn(2+)](o). The non-specific nitric oxide synthase (NOS) inhibitor, Nomega-nitro-l-arginine methyl ester (1 mM), however, blocked the increase of [Zn(2+)](o) during ischemia and reperfusion. The data indicate the important role of NO in causing the release of Zn(2+) during I/R and suggest that NOS inhibitors may be used to reduce Zn(2+)-induced neuronal injury.

Zinc release contributes to hypoglycemia-induced neuronal death

Neurons exposed to zinc exhibit activation of poly(ADP-ribose) polymerase-1 (PARP-1), an enzyme that normally participates in DNA repair but promotes cell death when extensively activated. Endogenous, vesicular zinc in brain is released to the extracellular space under conditions causing neuronal depolarization. Here, we used a rat model of insulin-induced hypoglycemia to assess the role of zinc release in PARP-1 activation and neuronal death after severe hypoglycemia. Zinc staining with N-(6-methoxy-8-quinolyl)-para-toluenesulfonamide (TSQ) showed depletion of presynaptic vesicular zinc from hippocampal mossy fiber terminals and accumulation of weakly bound zinc in hippocampal CA1 cell bodies after severe hypoglycemia. Intracerebroventricular injection of the zinc chelator calcium ethylene-diamine tetraacetic acid (CaEDTA) blocked the zinc accumulation and significantly reduced hypoglycemia-induced neuronal death. CaEDTA also attenuated the accumulation of poly(ADP-ribose), the enzymatic product of PARP-1, in hippocampal neurons. These results suggest that zinc translocation is an intermediary step linking hypoglycemia to PARP-1 activation and neuronal death.

Wide therapeutic time window for nimesulide neuroprotection in a model of transient focal cerebral ischemia in the rat

Results from several studies indicate that cyclooxygenase-2 (COX-2) is involved in ischemic brain injury. The purpose of this study was to evaluate the neuroprotective effects of the selective COX-2 inhibitor nimesulide on cerebral infarction and neurological deficits in a standardized model of transient focal cerebral ischemia in rats. Three doses of nimesulide (3, 6 and 12 mg/kg; i.p.) or vehicle were administered immediately after stroke and additional doses were given at 6, 12, 24, 36 and 48 h after ischemia. In other set of experiments, the effect of nimesulide was studied in a situation in which its first administration was delayed for 3-24 h after ischemia. Total, cortical and subcortical infarct volumes and functional outcome (assessed by neurological deficit score and rotarod performance) were determined 3 days after ischemia. The effect of nimesulide on prostaglandin E(2) (PGE(2)) levels in the injured brain was also investigated. Nimesulide dose-dependently reduced infarct volume and improved functional recovery when compared to vehicle. Of interest is the finding that neuroprotection conferred by nimesulide (reduction of infarct size and neurological deficits and improvement of rotarod performance) was also observed when treatment was delayed until 24 h after ischemia. Further, administration of nimesulide in a delayed treatment paradigm completely abolished PGE(2) accumulation in the postischemic brain, suggesting that COX-2 inhibition is a promising therapeutic strategy for cerebral ischemia to target the late-occurring inflammatory events which amplify initial damage.

Effect of zinc in ischemic brain injury in an embolic model of stroke in rats

Zinc is prevalent in the mammalian central nervous system and its role in ischemic brain injury is still controversial. In the present study, the effect of zinc in ischemic brain injury was examined in an embolic model of stroke in rats. Furthermore, the effect of zinc in combination with bicuculline, a GABAa antagonist, was also examined in the ischemic injury. Treatment with zinc or zinc plus bicuculline increased infarct volume significantly and also worsened neurological deficits. Moreover, treatment with zinc plus bicuculline also enhanced ischemic brain edema. These results thus support the hypothesis that administration of zinc i.p. worsens the outcome of ischemic brain injury in the embolic model of stroke in rats.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Clioquinol effects on tissue chelatable zinc in mice

Recent evidence for the involvement of zinc in the formation of beta-amyloid plaques in the brain in Alzheimer's disease has led to the establishment of new therapeutic strategies for the degenerative disorder based on metal chelation. The present experiment was conducted on a membrane-permeable zinc chelator, clioquinol (CQ), that has shown potential in initial studies on a mouse model of Alzheimer's disease [1]. The degree of chelatable zinc in mice treated with CQ, delivered by two different routes, was measured using complementary protocols for identifying chelatable zinc: 6-methoxy-8-quinolyl- p-toluenesulfonamide (TSQ) histofluorescence, and selenite autometalography. Mice injected intraperitoneally with CQ showed a dramatic reduction in chelatable zinc in brain, testis, and pancreas. In contrast, mice given CQ orally showed no significant change in levels of chelatable zinc in these tissues. This suggests that CQ administered orally to patients with Alzheimer's disease should not significantly perturb chelatable zinc levels in key organs and may be used over long periods without adverse endocrinological and reproductive effects related to zinc deficiency. In contrast, CQ injected intraperitoneally may be used not only as a tool for investigating chelatable zinc pools but also in a clinical context. For example, injected CQ could be employed in situations requiring the rapid buffering of excessive chelatable zinc following ischemic episodes or brain trauma. Thus, our findings indicate that CQ has considerable potential as a versatile scientific and clinical tool used for selective modulation of zinc pools.