Contribution of Zinc-Dependent Delayed Calcium Influx via TRPC5 in Oxidative Neuronal Death and its Prevention by Novel TRPC Antagonist.

Oxidative stress is a key mediator of neuronal death in acute brain injuries, such as epilepsy, trauma, and stroke. Although it is accompanied by diverse cellular changes, increases in levels of intracellular zinc ion (Zn2+) and calcium ion (Ca2+) may play a critical causative role in oxidative neuronal death. However, the mechanistic link between Zn2+ and Ca2+ dyshomeostasis in neurons during oxidative stress is not well-understood. Here, we show that the exposure of cortical neurons to H2O2 led to a zinc-triggered calcium influx, which resulted in neuronal death. The cyclin-dependent kinase inhibitor, NU6027, inhibited H2O2-induced Ca2+ increases and subsequent cell death in cortical neurons, without affecting the early increase in Zn2+. Therefore, we attempted to identify the zinc-regulated Ca2+ pathway that was inhibited by NU6027. The expression profile in cortical neurons identified transient receptor potential cation channel 5 (TRPC5) as a candidate that is known to involve in the generation of epileptiform burst firing and epileptic neuronal death (Phelan KD et al. 2012a; Phelan KD et al. 2013b). NU6027 inhibited basal and zinc-augmented TRPC5 currents in TRPC5-overexpressing HEK293 cells. Consistently, cortical neurons from TRPC5 knockout mice were highly resistant to H2O2-induced death. Moreover, NU6027 is neuroprotective in kainate-treated epileptic rats. Our results demonstrate that TRPC5 is a novel therapeutic target against oxidative neuronal injury in prolonged seizures and that NU6027 is a potent inhibitor of TRPC5.

Correction to: Contribution of Zinc-Dependent Delayed Calcium Influx via TRPC5 in Oxidative Neuronal Death and its Prevention by Novel TRPC Antagonist.

After the publication of this work errors were noticed in Fig. 3b and 4d.

Angiotensin II potentiates zinc-induced cortical neuronal death by acting on angiotensin II type 2 receptor.

BACKGROUND: The angiotensin system has several non-vascular functions in the central nervous system. For instance, inhibition of the brain angiotensin system results in a reduction in neuronal death following acute brain injury such as ischemia and intracerebral hemorrhage, even under conditions of constant blood pressure. Since endogenous zinc has been implicated as a key mediator of ischemic neuronal death, we investigated the possibility that the angiotensin system affects the outcome of zinc-triggered neuronal death in cortical cell cultures. RESULTS: Exposure of cortical cultures containing neurons and astrocytes to 300 µM zinc for 15 min induced submaximal death in both types of cells. Interestingly, addition of angiotensin II significantly enhanced the zinc-triggered neuronal death, while leaving astrocytic cell death relatively unchanged. Both type 1 and 2 angiotensin II receptors (AT1R and AT2R, respectively) were expressed in neurons as well as astrocytes. Zinc neurotoxicity was substantially attenuated by PD123319, a specific inhibitor of AT2R, and augmented by CGP42112, a selective activator of AT2R, indicating a critical role for this receptor subtype in the augmentation of neuronal cell death.Because zinc toxicity occurs largely through oxidative stress, the levels of superoxides in zinc-treated neurons were assessed by DCF fluorescence microscopy. Combined treatment with zinc and angiotensin II substantially increased the levels of superoxides in neurons compared to those induced by zinc alone. This increase in oxidative stress by angiotensin II was completely blocked by the addition of PD123319. Finally, since zinc-induced oxidative stress may be caused by induction and/or activation of NADPH oxidase, the activation status of Rac and the level of the NADPH oxidase subunit p67phox were measured. Angiotensin II markedly increased Rac activity and the levels of p67phox in zinc-treated neurons and astrocytes in a PD123319-dependent manner. CONCLUSION: The present study shows that the angiotensin system, especially that involving AT2R, may have an oxidative injury-potentiating effect via augmentation of the activity of NADPH oxidase. Hence, blockade of angiotensin signaling cascades in the brain may prove useful in protecting against the oxidative neuronal death that is likely to occur in acute brain injury.

Clioquinol induces autophagy in cultured astrocytes and neurons by acting as a zinc ionophore.

Recent studies have demonstrated that clioquinol, an antibiotic with an anti-amyloid effect, acts as a zinc ionophore under physiological conditions. Because increases in labile zinc may induce autophagy, we examined whether clioquinol induces autophagy in cultured astrocytes in a zinc-dependent manner. Within 1h of exposure to 0.1-10 µM clioquinol, the levels of microtubule-associated protein 1 light chain 3 (LC3)-II, a marker of autophagy, began to increase in astrocytes. Confocal live-cell imaging of GFP-LC3-transfected astrocytes showed the formation of LC3(+) autophagic vacuoles (AVs), providing a further indication that clioquinol induced autophagy. Addition of 3-methyladenine or small-interfering RNA against autophagy-related gene 6 (ATG6/Beclin-1) blocked clioquinol-induced increases in LC3-II. FluoZin-3 fluorescence microscopy showed that, like the zinc ionophore pyrithione, clioquinol increased intracellular zinc levels in the cytosol and AVs in an extracellular zinc-dependent manner. Zinc chelation with N,N,N',N'-tetrakis-(2-pyridylmethyl) ethylenediamine (TPEN) reduced, and addition of zinc increased the levels of LC3-II and LC3(+) puncta, indicating that zinc influx plays a key role therein. Moreover, astrocytes and SH-SY5Y cells expressing mutant huntingtin (mHttQ74) accumulated less aggregates when treated with clioquinol, and this effect was reversed by TPEN. These results indicate that clioquinol-induced autophagy is likely to be physiologically functional. The present study demonstrates that clioquinol induces autophagy in a zinc-dependent manner and contributes to clearance of aggregated proteins in astrocytes and neurons. Hence, in addition to its metal-chelating effect in and around amyloid beta (Aß) plaques, clioquinol may contribute to the reduction of Aß loads by activating autophagy by increasing or normalizing intracellular zinc levels in brain cells.

Metallothionein-3 regulates lysosomal function in cultured astrocytes under both normal and oxidative conditions.

Cellular zinc plays a key role in lysosomal change and cell death in neurons and astrocytes under oxidative stress. Here, using astrocytes lacking metallothionein-3 (MT3), a potential source of labile zinc in the brain, we studied the role of MT3 in oxidative stress responses. H(2)O(2) induced a large increase in labile zinc in wild-type (WT) astrocytes, but stimulated only a modest rise in MT3-null astrocytes. In addition, H(2)O(2)-induced lysosomal membrane permeabilization (LMP) and cell death were comparably attenuated in MT3-null astrocytes. Expression and glycosylation of Lamp1 (lysosome-associated membrane protein 1) and Lamp2 were increased in MT3-null astrocytes, and the activities of several lysosomal enzymes were significantly reduced, indicating an effect of MT3 on lysosomal components. Consistent with lysosomal dysfunction in MT3-null cells, the level of LC3-II (microtubule-associated protein 1 light chain 3), a marker of early autophagy, was increased by oxidative stress in WT astrocytes, but not in MT3-null cells. Similar changes in Lamp1, LC3, and cathepsin-D were induced by the lysosomal inhibitors bafilomycin A1, chloroquine, and monensin, indicating that lysosomal dysfunction may lie upstream of changes observed in MT3-null astrocytes. Consistent with this idea, lysosomal accumulation of cholesterol and lipofuscin were augmented in MT3-null astrocytes. Similar to the results seen in MT3-null cells, MT3 knockdown by siRNA inhibited oxidative stress-induced increases in zinc and LMP. These results indicate that MT3 may play a key role in normal lysosomal function in cultured astrocytes.

Copper activates TrkB in cortical neurons in a metalloproteinase-dependent manner.

Copper (Cu) is an endogenous metal that is physiologically essential in the brain and that, like zinc (Zn), may be synaptically released in certain regions. Previously, we demonstrated that Zn activates TrkB in cultured cortical neurons in a metalloproteinase (MP)-dependent manner. To determine whether Cu has similar properties, we exposed cortical cultures for 15 min to various metals and performed Western blots to detect tyrosine-phosphorylated TrkB (p-Trk). Whereas Cd, Mn, Fe(II), and Fe(III) had no effect on the level of p-Trk, 10 microM of Cu in phosphate-containing (Hanks' balanced salt solution) or 10 nM in phosphate-lacking salt solution (control salt solution), increased levels of p-Trk. Cu also activated extracellular signal-regulated kinase 1/2 and Src tyrosine kinase, signaling molecules activated downstream of TrkB. Cu decreased levels of probrain-derived neurotrophic factor (pro-BDNF) in cells but increased levels of pro- and mature BDNF in the media. Addition of MP inhibitors completely blocked the Cu-induced increases in pro-BDNF and BDNF as well as TrkB activation, indicating that MP mediates most of the Cu effect. Furthermore, Cu increased the activity of matrix metalloproteinase 2 (MMP2) and MMP9 in cortical neurons. These findings indicate that, like Zn, Cu activates MPs, releases pro-BDNF from cells, and phosphorylates TrsB. Because Cu, like Zn, is released in certain brain areas with neuronal activity, metal-triggered TrkB activation may occur in both Cu- and Zn-containing synapses.

Cytosolic labile zinc: a marker for apoptosis in the developing rat brain.

Cytosolic zinc accumulation was thought to occur specifically in neuronal death (necrosis) following acute injury. However, a recent study demonstrated that zinc accumulation also occurs in adult rat neurons undergoing apoptosis following target ablation, and in vitro experiments have shown that zinc accumulation may play a causal role in various forms of apoptosis. Here, we examined whether intraneuronal zinc accumulation occurs in central neurons undergoing apoptosis during development. Embryonic and newborn Sprague-Dawley rat brains were double-stained for terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling (TUNEL) detection of apoptosis and immunohistochemical detection of stage-specific neuronal markers, such as nestin, proliferating cell nuclear antigen (PCNA), TuJ1 and neuronal nuclear specific protein (NeuN). The results revealed that apoptotic cell death occurred in neurons of diverse stages (neural stem cells, and dividing, young and adult neurons) throughout the brain during the embryonic and early postnatal periods. Further staining of brain sections with acid fuchsin or zinc-specific fluorescent dyes showed that all of the apoptotic neurons were acidophilic and contained labile zinc in their cell bodies. Cytosolic zinc accumulation was also observed in cultured cortical neurons undergoing staurosporine- or sodium nitroprusside (SNP)-induced apoptosis. In contrast, zinc chelation with CaEDTA or N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) reduced SNP-induced apoptosis but not staurosporine-induced apoptosis, indicating that cytosolic zinc accumulation does not play a causal role in all forms of apoptosis. Finally, the specific cytosolic zinc accumulation may have a practical application as a relatively simple marker for neurons undergoing developmental apoptosis.

Activation of the Trk signaling pathway by extracellular zinc. Role of metalloproteinases.

In certain brain regions, extracellular zinc concentrations can rise precipitously as intense neuronal activity releases large amounts of zinc from the nerve terminals. Although zinc release has been suggested to play a pathological role, its precise physiological effect is poorly understood. Here, we report that exposure to micromolar quantities of zinc for only a few minutes robustly and specifically activated tropomyosin-related kinase (Trk) receptors, most likely TrkB, in cultured cortical neurons. We further found that Trk activation by zinc is extracellularly mediated by activation of metalloproteinases, which release pro-BDNF from cells and convert pro-BDNF to mature BDNF. These results suggest that activity-dependent release of extracellular zinc leads to metalloproteinase activation, which plays a critically important role in Trk receptor activation at zinc-containing synapses.