Roles for H+ /K+ -ATPase and zinc transporter 3 in cAMP-mediated lysosomal acidification in bafilomycin A1-treated astrocytes.

Vacuolar ATPase (v-ATPase) is the main proton pump that acidifies vesicles such as lysosomes. Disruption in the lysosomal localization of v-ATPase leads to lysosomal dysfunction, thus contributing to the pathogenesis of lysosomal storage disorders and neurodegenerative diseases such as Alzheimer's disease. Recent studies showed that increases in cyclic AMP (cAMP) levels acidify lysosomes and consequently enhance autophagy flux. Although the upregulation of v-ATPase function may be the key mechanism underlying the cAMP-mediated lysosomal acidification, it is unknown whether a mechanism independent of v-ATPase may be contributing to this phenomenon. In the present study, we modeled v-ATPase dysfunction in brain cells by blocking lysosomal acidification in cortical astrocytes through treatment with bafilomycin A1, a selective v-ATPase inhibitor. We observed that cAMP reversed the pH changes via the activation of protein kinase A; interestingly, cAMP also increased autophagy flux even in the presence of bafilomycin A1, suggesting the presence of an alternative route of proton entry. Notably, pharmacological inhibitors and siRNAs of H+ /K+ -ATPase markedly shifted the lysosomal pH toward more alkaline values in bafilomycin A1/cAMP-treated astrocytes, suggesting that H+ /K+ -ATPase may be the alternative route of proton entry for lysosomal acidification. Furthermore, the cAMP-mediated reversal of lysosomal pH was nullified in the absence of ZnT3 that interacts with H+ /K+ -ATPase. Our results suggest that the H+ /K+ -ATPase/ZnT3 complex is recruited to lysosomes in a cAMP-dependent manner and functions as an alternative proton pump for lysosomes when the v-ATPase function is downregulated, thus providing insight into the potential development of a new class of lysosome-targeted therapeutics in neurodegenerative diseases.

Zinc transporter 3 modulates cell proliferation and neuronal differentiation in the adult hippocampus.

The subgranular zone of the dentate gyrus is a subregion of the hippocampus that has two uniquely defining features; it is one of the most active sites of adult neurogenesis as well as the location where the highest concentrations of synaptic zinc are found, the mossy fiber terminals. Therefore, we sought to investigate the idea that vesicular zinc plays a role as a modulator of hippocampal adult neurogenesis. Here, we used ZnT3-/- mice, which are depleted of synaptic-vesicle zinc, to test the effect of targeted deletion of this transporter on adult neurogenesis. We found that this manipulation reduced progenitor cell turnover as well as led to a marked defect in the maturation of newborn cells that survive in the DG toward a neuronal phenotype. We also investigated the effects of zinc (ZnCl2 ), n-acetyl cysteine (NAC), and ZnCl2 plus 2NAC (ZN) supplement on adult hippocampal neurogenesis. Compared with ZnCl2 or NAC, administration of ZN resulted in an increase in proliferation of progenitor cells and neuroblast. ZN also rescued the ZnT3 loss-associated reduction of neurogenesis via elevation of insulin-like growth factor-1 and ERK/CREB activation. Together, these findings reveal that ZnT3 plays a highly important role in maintaining adult hippocampal neurogenesis and supplementation by ZN has a beneficial effect on hippocampal neurogenesis, as well as providing a therapeutic target for enhanced neuroprotection and repair after injury as demonstrated by its ability to prevent aging-dependent cognitive decline in ZnT3-/- mice. Therefore, the present study suggests that ZnT3 and vesicular zinc are essential for adult hippocampal neurogenesis.

Role of zinc dyshomeostasis in inflammasome formation in cultured cortical cells following lipopolysaccharide or oxygen-glucose deprivation/reperfusion exposure.

Exposure of mouse mixed cortical cell cultures to lipopolysaccharide (LPS) resulted in inflammasome formation in neurons and astrocytes, as indicated by increases in the levels of NLRP3, ASC, caspase-1, and IL-1ß. LPS exposure concurrently increased the level of free zinc in the cytosol of both cell types. Addition of the membrane-permeant zinc chelator TPEN blocked the increases in the levels of NLRP3 and caspase-1 as well as the release of inflammatory cytokines, indicating a role for increased zinc in LPS-induced inflammasome formation. Oxygen-glucose deprivation (OGD), a cellular model of hypoxia, also induced inflammasome formation and zinc dyshomeostasis in cortical cells, effects that were abolished upon zinc chelation with TPEN. A similar mechanism appeared to be at work in vivo. Whereas intraperitoneal injection of LPS in mice resulted in inflammasome formation and microglial activation in the brain, it caused little induction of inflammasome formation in ZnT3-null mice, which lack synaptic zinc, suggesting a specific role for synaptic zinc in LPS-induced formation of inflammasomes in the mouse brain.

A Novel Zinc Chelator, 1H10, Ameliorates Experimental Autoimmune Encephalomyelitis by Modulating Zinc Toxicity and AMPK Activation.

Previous studies in our lab revealed that chemical zinc chelation or zinc transporter 3 (ZnT3) gene deletion suppresses the clinical features and neuropathological changes associated with experimental autoimmune encephalomyelitis (EAE). In addition, although protective functions are well documented for AMP-activated protein kinase (AMPK), paradoxically, disease-promoting effects have also been demonstrated for this enzyme. Recent studies have demonstrated that AMPK contributes to zinc-induced neurotoxicity and that 1H10, an inhibitor of AMPK, reduces zinc-induced neuronal death and protects against oxidative stress, excitotoxicity, and apoptosis. Here, we sought to evaluate the therapeutic efficacy of 1H10 against myelin oligodendrocyte glycoprotein 35-55-induced EAE. 1H10 (5 µg/kg) was intraperitoneally injected once per day for the entire experimental course. Histological evaluation was performed three weeks after the initial immunization. We found that 1H10 profoundly reduced the severity of the induced EAE and that there was a remarkable suppression of demyelination, microglial activation, and immune cell infiltration. 1H10 also remarkably inhibited EAE-associated blood-brain barrier (BBB) disruption, MMP-9 activation, and aberrant synaptic zinc patch formation. Furthermore, the present study showed that long-term treatment with 1H10 also reduced the clinical course of EAE. Therefore, the present study suggests that zinc chelation and AMPK inhibition with 1H10 may have great therapeutic potential for the treatment of multiple sclerosis.

Changes in plasma lipoxin A4, resolvins and CD59 levels after ischemic and traumatic brain injuries in rats.

Ischemic and traumatic brain injuries are the major acute central nervous system disorders that need to be adequately diagnosed and treated. To find biomarkers for these acute brain injuries, plasma levels of some specialized pro-resolving mediators (SPMs, i.e., lipoxin A4 [LXA4], resolvin [Rv] E1, RvE2, RvD1 and RvD2), CD59 and interleukin (IL)-6 were measured at 0, 6, 24, 72, and 168 h after global cerebral ischemic (GCI) and traumatic brain injuries (TBI) in rats. Plasma LXA4 levels tended to increase at 24 and 72 h after GCI. Plasma RvE1, RvE2, RvD1, and RvD2 levels showed a biphasic response to GCI; a significant decrease at 6 h with a return to the levels of the sham group at 24 h, and again a decrease at 72 h. Plasma CD59 levels increased at 6 and 24 h post-GCI, and returned to basal levels at 72 h post-GCI. For TBI, plasma LXA4 levels tended to decrease, while RvE1, RvE2, RvD1, and RvD2 showed barely significant changes. Plasma IL-6 levels were significantly increased after GCI and TBI, but with different time courses. These results show that plasma LXA4, RvE1, RvE2, RvD1, RvD2, and CD59 levels display differential responses to GCI and TBI, and need to be evaluated for their usefulness as biomarkers.

Inhibition of Drp1 Ameliorates Synaptic Depression, Aß Deposition, and Cognitive Impairment in an Alzheimer's Disease Model.

Excessive mitochondrial fission is a prominent early event and contributes to mitochondrial dysfunction, synaptic failure, and neuronal cell death in the progression of Alzheimer's disease (AD). However, it remains to be determined whether inhibition of excessive mitochondrial fission is beneficial in mammal models of AD. To determine whether dynamin-related protein 1 (Drp1), a key regulator of mitochondrial fragmentation, can be a disease-modifying therapeutic target for AD, we examined the effects of Drp1 inhibitor on mitochondrial and synaptic dysfunctions induced by oligomeric amyloid-ß (Aß) in neurons and neuropathology and cognitive functions in Aß precursor protein/presenilin 1 double-transgenic AD mice. Inhibition of Drp1 alleviates mitochondrial fragmentation, loss of mitochondrial membrane potential, reactive oxygen species production, ATP reduction, and synaptic depression in Aß-treated neurons. Furthermore, Drp1 inhibition significantly improves learning and memory and prevents mitochondrial fragmentation, lipid peroxidation, BACE1 expression, and Aß deposition in the brain in the AD model. These results provide evidence that Drp1 plays an important role in Aß-mediated and AD-related neuropathology and in cognitive decline in an AD animal model. Therefore, inhibiting excessive Drp1-mediated mitochondrial fission may be an efficient therapeutic avenue for AD.SIGNIFICANCE STATEMENT Mitochondrial fission relies on the evolutionary conserved dynamin-related protein 1 (Drp1). Drp1 activity and mitochondria fragmentation are significantly elevated in the brains of sporadic Alzheimer's disease (AD) cases. In the present study, we first demonstrated that the inhibition of Drp1 restored amyloid-ß (Aß)-mediated mitochondrial dysfunctions and synaptic depression in neurons and significantly reduced lipid peroxidation, BACE1 expression, and Aß deposition in the brain of AD mice. As a result, memory deficits in AD mice were rescued by Drp1 inhibition. These results suggest that neuropathology and combined cognitive decline can be attributed to hyperactivation of Drp1 in the pathogenesis of AD. Therefore, inhibitors of excessive mitochondrial fission, such as Drp1 inhibitors, may be a new strategy for AD.

The anti-ALS drug riluzole attenuates pericyte loss in the diabetic retinopathy of streptozotocin-treated mice.

Loss of pericytes, considered an early hallmark of diabetic retinopathy, is thought to involve abnormal activation of protein kinase C (PKC). We previously showed that the anti-amyotrophic lateral sclerosis (ALS) drug riluzole functions as a PKC inhibitor. Here, we examined the effects of riluzole on pathological changes in diabetic retinopathy. Pathological endpoints examined in vivo included the number of pericytes and integrity of retinal vessels in streptozotocin (STZ)-induced diabetic mice. In addition, PKC activation and the induction of monocyte chemotactic protein (MCP1) were assessed in diabetic mice and in human retinal pericytes exposed to advanced glycation end product (AGE) or modified low-density lipoprotein (mLDL). The diameter of retinal vessels and the number of pericytes were severely reduced, and the levels of MCP1 and PKC were increased in STZ-induced diabetic mice. Administration of riluzole reversed all of these changes. Furthermore, the increased expression of MCP1 in AGE- or mLDL-treated cultured retinal pericytes was inhibited by treatment with riluzole or the PKC inhibitor GF109203X. In silico modeling showed that riluzole fits well within the catalytic pocket of PKC. Taken together, our results demonstrate that riluzole attenuates both MCP1 induction and pericyte loss in diabetic retinopathy, likely through its direct inhibitory effect on PKC.

Heterogeneous nuclear ribonucleoprotein A1 post-transcriptionally regulates Drp1 expression in neuroblastoma cells.

Excessive mitochondrial fission is associated with the pathogenesis of neurodegenerative diseases. Dynamin-related protein 1 (Drp1) possesses specific fission activity in the mitochondria and peroxisomes. Various post-translational modifications of Drp1 are known to modulate complex mitochondrial dynamics. However, the post-transcriptional regulation of Drp1 remains poorly understood. Here, we show that the heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) regulates Drp1 expression at the post-transcriptional level. hnRNP A1 directly interacts with Drp1 mRNA at its 3'UTR region, and enhances translation potential without affecting mRNA stability. Down-regulation of hnRNP A1 induces mitochondrial elongation by reducing Drp1 expression. Moreover, depletion of hnRNP A1 suppresses 3-NP-mediated mitochondrial fission and dysfunction. In contrast, over-expression of hnRNP A1 promotes mitochondrial fragmentation by increasing Drp1 expression. Additionally, hnRNP A1 significantly exacerbates 3-NP-induced mitochondrial dysfunction and cell death in neuroblastoma cells. Interestingly, treatment with 3-NP induces subcellular translocation of hnRNP A1 from the nucleus to the cytoplasm, which accelerates the increase in Drp1 expression in hnRNP A1 over-expressing cells. Collectively, our findings suggest that hnRNP A1 controls mitochondrial dynamics by post-transcriptional regulation of Drp1.

Zinc transporter 3 (ZnT3) gene deletion reduces spinal cord white matter damage and motor deficits in a murine MOG-induced multiple sclerosis model.

The present study aimed to evaluate the role of zinc transporter 3 (ZnT3) on multiple sclerosis (MS) pathogenesis. Experimental autoimmune encephalomyelitis (EAE), a disease model of multiple sclerosis, was induced by immunization with myelin oligodendrocyte glycoprotein (MOG35-55) in female mice. Three weeks after the initial immunization, demyelination, immune cell infiltration and blood brain barrier (BBB) disruption in the spinal cord were analyzed. Clinical signs of EAE first appeared on day 11 and reached a peak level on day 19 after the initial immunization. ZnT3 gene deletion profoundly reduced the daily clinical score of EAE. The ZnT3 gene deletion-mediated inhibition of the clinical course of EAE was accompanied by suppression of inflammation and demyelination in the spinal cord. The motor deficit accompanying neuropathological changes associated with EAE were mild in ZnT3 gene deletion mice. This reduction in motor deficit was accompanied by coincident reductions in demyelination and infiltration of encephalitogenic immune cells including CD4+ T cells, CD8+ T cells, CD20+ B cells and F4/80+ microglia in the spinal cord. These results demonstrate that ZnT3 gene deletion inhibits the clinical features and neuropathological changes associated with EAE. ZnT3 gene deletion also remarkably inhibited formation of EAE-associated aberrant synaptic zinc patches, matrix metalloproteinases-9 (MMP-9) activation and BBB disruption. Therefore, amelioration of EAE-induced clinical and neuropathological changes by ZnT3 gene deletion suggests that vesicular zinc may be involved in several steps of MS pathogenesis.

Developmental endothelial locus-1 is a homeostatic factor in the central nervous system limiting neuroinflammation and demyelination.

Inflammation in the central nervous system (CNS) and disruption of its immune privilege are major contributors to the pathogenesis of multiple sclerosis (MS) and of its rodent counterpart, experimental autoimmune encephalomyelitis (EAE). We have previously identified developmental endothelial locus-1 (Del-1) as an endogenous anti-inflammatory factor, which inhibits integrin-dependent leukocyte adhesion. Here we show that Del-1 contributes to the immune privilege status of the CNS. Intriguingly, Del-1 expression decreased in chronic-active MS lesions and in the inflamed CNS in the course of EAE. Del-1-deficiency was associated with increased EAE severity, accompanied by increased demyelination and axonal loss. As compared with control mice, Del-1(-/-) mice displayed enhanced disruption of the blood-brain barrier and increased infiltration of neutrophil granulocytes in the spinal cord in the course of EAE, accompanied by elevated levels of inflammatory cytokines, including interleukin-17 (IL-17). The augmented levels of IL-17 in Del-1-deficiency derived predominantly from infiltrated CD8(+) T cells. Increased EAE severity and neutrophil infiltration because of Del-1-deficiency was reversed in mice lacking both Del-1 and IL-17 receptor, indicating a crucial role for the IL-17/neutrophil inflammatory axis in EAE pathogenesis in Del-1(-/-) mice. Strikingly, systemic administration of Del-1-Fc ameliorated clinical relapse in relapsing-remitting EAE. Therefore, Del-1 is an endogenous homeostatic factor in the CNS protecting from neuroinflammation and demyelination. Our findings provide mechanistic underpinnings for the previous implication of Del-1 as a candidate MS susceptibility gene and suggest that Del-1-centered therapeutic approaches may be beneficial in neuroinflammatory and demyelinating disorders.

Down-regulation of mortalin exacerbates Aß-mediated mitochondrial fragmentation and dysfunction.

Mitochondrial dynamics greatly influence the biogenesis and morphology of mitochondria. Mitochondria are particularly important in neurons, which have a high demand for energy. Therefore, mitochondrial dysfunction is strongly associated with neurodegenerative diseases. Until now various post-translational modifications for mitochondrial dynamic proteins and several regulatory proteins have explained complex mitochondrial dynamics. However, the precise mechanism that coordinates these complex processes remains unclear. To further understand the regulatory machinery of mitochondrial dynamics, we screened a mitochondrial siRNA library and identified mortalin as a potential regulatory protein. Both genetic and chemical inhibition of mortalin strongly induced mitochondrial fragmentation and synergistically increased Aß-mediated cytotoxicity as well as mitochondrial dysfunction. Importantly we determined that the expression of mortalin in Alzheimer disease (AD) patients and in the triple transgenic-AD mouse model was considerably decreased. In contrast, overexpression of mortalin significantly suppressed Aß-mediated mitochondrial fragmentation and cell death. Taken together, our results suggest that down-regulation of mortalin may potentiate Aß-mediated mitochondrial fragmentation and dysfunction in AD.

Angiopoietin-1 blocks neurotoxic zinc entry into cortical cells via PIP2 hydrolysis-mediated ion channel inhibition.

Excessive entry of zinc ions into the soma of neurons and glial cells results in extensive oxidative stress and necrosis of cortical cells, which underlies acute neuronal injury in cerebral ischemia and epileptic seizures. Here, we show that angiopoietin-1 (Ang1), a potent angiogenic ligand for the receptor tyrosine kinase Tie2 and integrins, inhibits the entry of zinc into primary mouse cortical cells and exerts a substantial protective effect against zinc-induced neurotoxicity. The neuroprotective effect of Ang1 was mediated by the integrin/focal adhesion kinase (FAK) signaling axis, as evidenced by the blocking effects of a pan-integrin inhibitory RGD peptide and PF-573228, a specific chemical inhibitor of FAK. Notably, blockade of zinc-permeable ion channels by Ang1 was attributable to phospholipase C-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate. Collectively, these data reveal a novel role of Ang1 in regulating the activity of zinc-permeable ion channels, and thereby protecting cortical cells against zinc-induced neurotoxicity.

Indomethacin preconditioning induces ischemic tolerance by modifying zinc availability in the brain.

Intracellular zinc overload causes neuronal injury during the course of neurological disorders, whereas mild levels of zinc are beneficial to neurons. Previous reports indicated that non-steroidal anti-inflammatory drugs, including indomethacin and aspirin, can reduce the risk of ischemic stroke. This study found that chronic pretreatment of rats with indomethacin, a non-selective cyclooxygenase inhibitor, provided tolerance to ischemic injuries in an animal model of stroke by eliciting moderate zinc elevation in neurons. Consecutive intraperitoneal injection of indomethacin (3mg/kg/day for 28 days) led to modest increases in intraneuronal zinc as well as synaptic zinc content, with no significant stimulation of neuronal death. Furthermore, indomethacin induced the expressions of intracellular zinc homeostatic and neuroprotective proteins, rendering the brain resistant against ischemic damages and improving neurological outcomes. However, administration of a zinc-chelator, N,N,N',N'-tetra(2-picolyl)ethylenediamine (TPEN; 15 mg/kg/day), immediately after indomethacin administration eliminated the beneficial actions of the drug. Therefore, indomethacin preconditioning can modulate intracellular zinc availability, contributing to ischemic tolerance in the brain after stroke.

The role of reciprocal activation of cAbl and Mst1 in the oxidative death of cultured astrocytes.

The protein kinase Mst1 (mammalian Sterile 20-like kinase 1) likely plays a role in oxidative neuronal cell death as a target of its activator, cAbl. We previously found that H2O2-induced death of astrocytes is mediated by cAbl in a metallothionein-3 (Mt3)-dependent manner. In the present study, we examined a possible role for Mst1 in the oxidative death of astrocytes. Treatment of cortical astrocytes with 170 µM H2O2 activated Mst1. Knockdown of Mst1 reduced H2O2-induced cell death, indicating that Mst1 activation contributes to astrocytic cell death. STI571, an inhibitor of cAbl, blocked induction/activation of Mst1 and H2O2-induced cell death. However, Mst1 silencing also inhibited induction/activation of cAbl, suggesting that the two kinases are regulated by a reciprocal activating mechanism. The zinc chelator TPEN blocked induction/activation of cAbl and Mst1, indicating that these phenomena are dependent on the rise of intracellular zinc. Moreover, H2O2 exposure did not increase free zinc levels in Mt3-null astrocytes, suggesting that the increased levels of free zinc were largely from Mt3. Consistent with the involvement of FoxO1/3, which may play a role in the Mst1-cell death cascade, we found an increase in the level of phosphorylated FoxO1/3 in H2O2-treated astrocytes. Moreover, inhibition of cAbl or Mst1 reversed this effect. The present results suggest the interesting possibility that cAbl and Mst1 are reciprocally activated under oxidative stress conditions in astrocytes. Both kinases appear to be regulated by changes in the levels of free zinc originating from Mt3 and contribute to oxidative cell death through a FoxO-dependent mechanism.

Neuropathogenic role of adenylate kinase-1 in Aß-mediated tau phosphorylation via AMPK and GSK3ß.

Abnormally hyperphosphorylated tau is often caused by tau kinases, such as GSK3ß and Cdk5. Such occurrence leads to neurofibrillary tangle formation and neuronal degeneration in tauopathy, including Alzheimer's disease (AD). However, little is known about the signaling cascade underlying the pathologic phosphorylation of tau by Aß(42). In this study, we show that adenylate kinase 1 (AK1) is a novel regulator of abnormal tau phosphorylation. AK1 expression is markedly increased in the brains of AD patients and AD model mice and is significantly induced by Aß(42) in the primary neurons. Ectopic expression of AK1 alone augments the pathologic phosphorylation of tau at PHF1, CP13 and AT180 epitopes and enhances the formation of tau aggregates. Inversely, downregulation of AK1 alleviates Aß(42)-induced hyperphosphorylation of tau. AK1 plays a role in Aß(42)-induced impairment of AMPK activity and GSK3ß activation in the primary neurons. Pharmacologic studies show that treatment with an AMPK inhibitor activates GSK3ß, and a GSK3ß inhibitor attenuates AK1-mediated tau phosphorylation. In a Drosophila model of human tauopathy, the retinal expression of human AK1 severely exacerbates rough eye phenotype and increases abnormal tau phosphorylation. Further, neural expression of AK1 reduces the lifespan of tau transgenic files. Taken together, these observations indicate that the neuronal expression of AK1 is induced by Aß(42) to increase abnormal tau phosphorylation via AMPK-GSK3ß and contributes to tau-mediated neurodegeneration, providing a new upstream modulator of GSK3ß in the pathologic phosphorylation of tau.

Autophagy activation and neuroprotection by progesterone in the G93A-SOD1 transgenic mouse model of amyotrophic lateral sclerosis.

Progesterone (PG) exerts neuroprotective effects under conditions such as brain ischemia, traumatic brain injury, and spinal cord injury. Previously, we reported that PG activates autophagy, a potential neuroprotective mechanism, in cortical astrocytes. In the present study, we explored the possibility that PG, by activating autophagy in spinal cord cells, protects against motoneuron degeneration in transgenic (Tg) mice expressing the human G93A-SOD1 (superoxide dismutase 1) mutant, a model of amyotrophic lateral sclerosis. PG treatment increased autophagic flux in G93A-SOD1 Tg spinal cord astrocyte cultures and mice. In addition, PG treatment reduced mutant SOD1 protein levels and motoneuronal death. Inhibition of autophagy with 3-methyladenine (3MA) reversed these PG effects, indicating that activation of autophagy contributed to the PG neuroprotection. PG effects in vivo were tested by intraperitoneally injecting male G93A-SOD1 Tg mice with different doses of PG (2, 4, or 8mg/kg) or vehicle from 70days of age until death. Measurements of motor functions using rota-rod tests showed that the onset of symptoms was not different among groups, but the progression of motor dysfunction was significantly delayed in the PG-treated group compared with the vehicle control group. The average lifespan was also prolonged in the PG-injected group. Histological examinations revealed that PG treatment substantially reduced the death of spinal motoneurons at 14weeks of age with a concomitant decrease in mutant SOD1 levels. Our results demonstrated that PG delays neurodegenerative progress in G93A-SOD1 transgenic mice, possibly through activation of autophagy in the spinal cord.

The neurophysiology and pathology of brain zinc.

Our understanding of the roles played by zinc in the physiological and pathological functioning of the brain is rapidly expanding. The increased availability of genetically modified animal models, selective zinc-sensitive fluorescent probes, and novel chelators is producing a remarkable body of exciting new data that clearly establishes this metal ion as a key modulator of intracellular and intercellular neuronal signaling. In this Mini-Symposium, we will review and discuss the most recent findings that link zinc to synaptic function as well as the injurious effects of zinc dyshomeostasis within the context of neuronal death associated with major human neurological disorders, including stroke, epilepsy, and Alzheimer's disease.

Role of zinc metallothionein-3 (ZnMt3) in epidermal growth factor (EGF)-induced c-Abl protein activation and actin polymerization in cultured astrocytes.

Recent evidence indicates that zinc plays a major role in neurochemistry. Of the many zinc-binding proteins, metallothionein-3 (Mt3) is regarded as one of the major regulators of cellular zinc in the brain. However, biological functions of Mt3 are not yet well characterized. Recently, we found that lysosomal dysfunction in metallothionein-3 (Mt3)-null astrocytes involves down-regulation of c-Abl. In this study, we investigated the role of Mt3 in c-Abl activation and actin polymerization in cultured astrocytes following treatment with epidermal growth factor (EGF). Compared with wild-type (WT) astrocytes, Mt3-null cells exhibited a substantial reduction in the activation of c-Abl upon treatment with EGF. Consistent with previous studies, activation of c-Abl by EGF induced dissociation of c-Abl from F-actin. Mt3 added to astrocytic cell lysates bound F-actin, augmented F-actin polymerization, and promoted the dissociation of c-Abl from F-actin, suggesting a possible role for Mt3 in this process. Conversely, Mt3-deficient astrocytes showed significantly reduced dissociation of c-Abl from F-actin following EGF treatment. Experiments using various peptide fragments of Mt3 showed that a fragment containing the N-terminal TCPCP motif (peptide 1) is sufficient for this effect. Removal of zinc from Mt3 or pep1 with tetrakis(2-pyridylmethyl)ethylenediamine abrogated the effect of Mt3 on the association of c-Abl and F-actin, indicating that zinc binding is necessary for this action. These results suggest that ZnMt3 in cultured astrocytes may be a normal component of c-Abl activation in EGF receptor signaling. Hence, modulation of Mt3 levels or distribution may prove to be a useful strategy for controlling cytoskeletal mobilization following EGF stimulation in brain cells.

Neuroprotection by urokinase plasminogen activator in the hippocampus.

Tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA), which are both used for thrombolytic treatment of acute ischemic stroke, are serine proteases that convert plasminogen to active plasmin. Although recent experimental evidences have raised controversy about the neurotoxic versus neuroprotective roles of tPA in acute brain injury, uPA remains unexplored in this context. In this study, we evaluated the effect of uPA on neuronal death in the hippocampus of mice after kainate-induced seizures. In the normal brain, uPA was localized to both nuclei and cytosol of neurons. Following severe kainate-induced seizures, uPA completely disappeared in degenerating neurons, whereas uPA-expressing astrocytes substantially increased, suggesting reactive astrogliosis. uPA-knockout mice were more vulnerable to kainate-induced neuronal death than wild-type mice. Consistent with this, inhibition of uPA by intracerebral injection of the uPA inhibitor UK122 increased the level of neuronal death. In contrast, prior administration of recombinant uPA significantly attenuated neuronal death. Collectively, these results indicate that uPA renders neurons resistant to kainate-induced excitotoxicity. Moreover, recombinant uPA suppressed cell death in primary cultures of hippocampal neurons exposed to H2O2, zinc, or various excitotoxins, suggesting that uPA protects against neuronal injuries mediated by the glutamate receptor, or by oxidation- or zinc-induced death signaling pathways. Considering that tPA may facilitate neurodegeneration in acute brain injury, we suggest that uPA, as a neuroprotectant, might be beneficial for the treatment of acute brain injuries such as ischemic stroke.

Zinc enhances autophagic flux and lysosomal function through transcription factor EB activation and V-ATPase assembly.

The stimulation of autophagy or lysosomes has been considered therapeutic for neurodegenerative disorders because the accumulation of misfolded proteins is commonly observed in the brains of individuals with these diseases. Although zinc is known to play critical roles in the functions of lysosomes and autophagy, the mechanism behind this regulatory relationship remains unclear. Therefore, in this study, we examined which mechanism is involved in zinc-mediated activation of autophagy and lysosome. Exposure to zinc at a sub-lethal concentration activated autophagy in a concentration-dependent manner in mRFP-GFP-LC3-expressing H4 glioma cells. Zinc also rescued the blocking of autophagic flux arrested by pharmaceutical de-acidification. Co-treatment with zinc attenuated the chloroquine (CQ)-induced increase in the number and size of mRFP-GFP-LC3 puncta in H4 cells and accumulation of p62 by CQ or ammonium chloride in both H4 and mouse cerebrocortical cultures. Zinc rapidly induced the expression of cathepsin B (CTSB) and cathepsin D (CTSD), representative lysosomal proteases in neurons, which appeared likely to be mediated by transcription factor EB (TFEB). We observed the translocation of TFEB from neurite to nucleus and the dephosphorylation of TFEB by zinc. The addition of cycloheximide, a chemical inhibitor of protein synthesis, inhibited the activity of CTSB and CTSD at 8 h after zinc exposure but not at 1 h, indicating that only late lysosomal activation was dependent on the synthesis of CTSB and CTSD proteins. At the very early time point, the activation of cathepsins was mediated by an increased assembly of V-ATPase on lysosomes and resultant lysosomal acidification. Finally, considering that P301L mutation in tau protein causes frontotemporal dementia through aggressive tau accumulation, we investigated whether zinc reduces the accumulation of protein aggregates in SK-N-BE(2)-C neuroblastoma cells expressing wild-type tau or mutant P301L-tau. Zinc markedly attenuated the levels of phosphorylated tau and total tau as well as p62 in both wild-type and mutant tau-overexpressing cells. We also observed that zinc was more effective than rapamycin at inducing TFEB-dependent CTSB and CTSD expression and V-ATPase-dependent lysosomal acidification and CTSB/CTSD activation. These results suggest that the regulation of zinc homeostasis could be a new approach for developing treatments for neurodegenerative diseases, including Alzheimer's and Parkinson's.