The expression system influences stability, maturation efficiency, and oligomeric properties of the potassium-chloride co-transporter KCC2

The neuron-specific K+/Cl- co-transporter 2, KCC2, which is critical for brain development, regulates γ-aminobutyric acid-dependent inhibitory neurotransmission. Consistent with its function, mutations in KCC2 are linked to neurodevelopmental disorders, including epilepsy, schizophrenia, and autism. KCC2 possesses 12 transmembrane spans and forms an intertwined dimer. Based on its complex architecture and function, reduced cell surface expression and/or activity have been reported when select disease-associated mutations are present in the gene encoding the protein, SLC12A5. These data suggest that KCC2 might be inherently unstable, as seen for other complex polytopic ion channels, thus making it susceptible to cellular quality control pathways that degrade misfolded proteins. To test these hypotheses, we examined KCC2 stability and/or maturation in five model systems: yeast, HEK293 cells, primary rat neurons, and rat and human brain synaptosomes. Although studies in yeast revealed that KCC2 is selected for endoplasmic reticulum-associated degradation (ERAD), experiments in HEK293 cells supported a more subtle role for ERAD in maintaining steady-state levels of KCC2. Nevertheless, this system allowed for an analysis of KCC2 glycosylation in the ER and Golgi, which serves as a read-out for transport through the secretory pathway. In turn, KCC2 was remarkably stable in primary rat neurons, suggesting that KCC2 folds efficiently in more native systems. Consistent with these data, the mature glycosylated form of KCC2 was abundant in primary rat neurons as well as in rat and human brain. Together, this work details the first insights into the influence that the cellular and membrane environments have on several fundamental KCC2 properties, acknowledges the advantages and disadvantages of each system, and helps set the stage for future experiments to assess KCC2 in a normal or disease setting.

Neurodevelopmental Consequences of Dietary Zinc Deficiency: A Status Report

Zinc is a tightly regulated trace mineral element playing critical roles in growth, immunity, neurodevelopment, and synaptic and hormonal signaling. Although severe dietary zinc deficiency is relatively uncommon in the United States, dietary zinc deficiency is a substantial public health concern in low- and middle-income countries. Zinc status may be a key determinant of neurodevelopmental processes. Indeed, limited cohort studies have shown that serum zinc is lower in people diagnosed with autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), and depression. These observations have sparked multiple studies investigating the mechanisms underlying zinc status and neurodevelopmental outcomes. Animal models of perinatal and adult dietary zinc restriction yield distinct behavioral phenotypes reminiscent of features of ASD, ADHD, and depression, including increased anxiety and immobility, repetitive behaviors, and altered social behaviors. At the cellular and molecular level, zinc has demonstrated roles in neurogenesis, regulation of cellular redox status, transcription factor trafficking, synaptogenesis, and the regulation of synaptic architecture via the Shank family of scaffolding proteins. Although mechanistic questions remain, the current evidence suggests that zinc status is important for adequate neuronal development and may be a yet overlooked factor in the pathogenesis of several psychiatric conditions. This review aims to summarize current knowledge of the role of zinc in the neurophysiology of the perinatal period, the many cellular targets of zinc in the developing brain, and the potential consequences of alterations in zinc homeostasis in early life.

Copper induces neuron-sparing, ferredoxin 1-independent astrocyte toxicity mediated by oxidative stress

Copper is an essential enzyme cofactor in oxidative metabolism, anti-oxidant defenses, and neurotransmitter synthesis. However, intracellular copper, when improperly buffered, can also lead to cell death. Given the growing interest in the use of copper in the presence of the ionophore elesclomol (CuES) for the treatment of gliomas, we investigated the effect of this compound on the surround parenchyma-namely neurons and astrocytes in vitro. Here, we show that astrocytes were highly sensitive to CuES toxicity while neurons were surprisingly resistant, a vulnerability profile that is opposite of what has been described for zinc and other toxins. Bolstering these findings, a human astrocytic cell line was similarly sensitive to CuES. Modifications of cellular metabolic pathways implicated in cuproptosis, a form of copper-regulated cell death, such as inhibition of mitochondrial respiration or knock-down of ferredoxin 1 (FDX1), did not block CuES toxicity to astrocytes. CuES toxicity was also unaffected by inhibitors of apoptosis, necrosis or ferroptosis. However, we did detect the presence of lipid peroxidation products in CuES-treated astrocytes, indicating that oxidative stress is a mediator of CuES-induced glial toxicity. Indeed, treatment with anti-oxidants mitigated CuES-induced cell death in astrocytes indicating that oxidative stress is a mediator of CuES-induced glial toxicity. Lastly, prior induction of metallothioneins 1 and 2 in astrocytes with zinc plus pyrithione was strikingly protective against CuES toxicity. As neurons express high levels of metallothioneins basally, these results may partially account for their resistance to CuES toxicity. These results demonstrate a unique toxic response to copper in glial cells which contrasts with the cell selectivity profile of zinc, another biologically relevant metal.

GluA1-Shank3 interaction decreases in response to chronic neuronal depolarization

Interactions between AMPA receptors and synaptic scaffolding proteins are key regulators of synaptic receptor density and, thereby, synapse strength. Shank3 is one such scaffolding protein with high clinical relevance, as genetic variants and deletions of this protein have been linked to autism spectrum disorder. Shank3 acts as a master regulator of the postsynaptic density of glutamatergic synapses, interacting with ionotropic and metabotropic glutamate receptors and cytoskeletal elements to modulate synaptic structure. Notably, Shank3 has been shown to interact directly with the AMPAR subunit GluA1, and Shank3 knockout animals show deficits in AMPAR-mediated synaptic transmission. In this study, we sought to characterize the stability of GluA1-Shank3 interaction in response to chronic stimuli using a highly sensitive and specific proximity ligation assay. We found that GluA1-Shank3 interactions decrease in response to prolonged neuronal depolarization induced by elevated extracellular potassium, and that this reduced interaction is blocked by NMDA receptor antagonism. These results firmly establish the close interaction of GluA1 and Shank3 in cortical neurons in vitro, and that this select interaction is subject to modulation by depolarization.

Microglial reprogramming by Hv1 antagonism protects neurons from inflammatory and glutamate toxicity

Although the precise mechanisms determining the neurotoxic or neuroprotective activation phenotypes in microglia remain poorly characterized, metabolic changes in these cells appear critical for these processes. As cellular metabolism can be tightly regulated by changes in intracellular pH, we tested whether pharmacological targeting of the microglial voltage-gated proton channel 1 (Hv1), an important regulator of intracellular pH, is critical for activated microglial reprogramming. Using a mouse microglial cell line and mouse primary microglia cultures, either alone, or co-cultured with rat cerebrocortical neurons, we characterized in detail the microglial activation profile in the absence and presence of Hv1 inhibition. We observed that activated microglia neurotoxicity was mainly attributable to the release of tumor necrosis factor alpha, reactive oxygen species, and zinc. Strikingly, pharmacological inhibition of Hv1 largely abrogated inflammatory neurotoxicity not only by reducing the production of cytotoxic mediators but also by promoting neurotrophic molecule production and restraining excessive phagocytic activity. Importantly, the Hv1-sensitive change from a pro-inflammatory to a neuroprotective phenotype was associated with metabolic reprogramming, particularly via a boost in NADH availability and a reduction in lactate. Most critically, Hv1 antagonism not only reduced inflammatory neurotoxicity but also promoted microglia-dependent neuroprotection against a separate excitotoxic injury. Our results strongly suggest that Hv1 blockers may provide an important therapeutic tool against a wide range of inflammatory neurodegenerative disorders.

Intracellular zinc signaling influences NMDA receptor function by enhancing the interaction of ZnT1 with GluN2A

Zinc, loaded into glutamate-containing presynaptic vesicles and released into the synapse in an activity-dependent manner, modulates neurotransmission through its actions on postsynaptic targets, prominently via high-affinity inhibition of GluN2A-containing NMDA receptors. Recently, we identified a postsynaptic transport mechanism that regulates endogenous zinc inhibition of NMDARs. In this new model of zinc regulation, the postsynaptic transporter ZnT1 mediates zinc inhibition of NMDARs by binding to GluN2A. Through this interaction, ZnT1, a transporter that moves zinc from the cytoplasm to the extracellular domain, generates a zinc microdomain that modulates NMDAR-mediated neurotransmission. As ZnT1 expression is transcriptionally driven by the metal-responsive transcription factor 1 (MTF-1), we found that intracellular zinc strongly drives MTF-1 in cortical neurons in vitro and increases the number of GluN2A-ZnT1 interactions, thereby enhancing tonic zinc inhibition of NMDAR-mediated currents. Importantly, this effect is absent when the interaction between GluN2A and ZnT1 is disrupted by a cell-permeable peptide. These results suggest that zinc-regulated gene expression can dynamically regulate NMDAR-mediated synaptic processes.

SNAP23 regulates KCC2 membrane insertion and activity following mZnR/GPR39 activation in hippocampal neurons

Modulation of the neuronal K⁺/Cl⁻ cotransporter 2 (KCC2) activity, which mediates Cl⁻ export, is critical to neuronal function. Here, we demonstrate that KCC2 interacts with the SNARE protein synaptosome-associated protein 23, SNAP23, an essential component of membrane insertion machinery. Using KCC2 truncated mutants, we show that KCC2 C-terminal domain is essential for membrane targeting and SNAP23-dependent upregulation of KCC2 activity triggered by activation of the Zn²⁺-sensitive receptor mZnR/GPR39 in HEK293 cells. Expression of SNAP23 phosphorylation-insensitive mutants or inhibition of its upstream activator IκB kinase (IKK) prevents mZnR/GPR39 upregulation of KCC2 activity in mouse hippocampal neurons. We further find that SNAP23 interacts with Syntaxin 1A and KCC2, and that all three proteins exhibit increased membrane insertion following mZnR/GPR39 activation in neurons. Our results elucidate a G-protein-coupled receptor-dependent pathway for regulation of KCC activity, mediated via interaction with SNARE proteins.

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Neuronal K⁺/Cl⁻ cotransporter 2 (KCC2) is regulated via interaction with SNAP23

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Zn²⁺ enhances interaction and membrane insertion of SNAP23, Syntaxin 1A, and KCC2

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Zn²⁺-dependent mZnR/GPR39 regulation of KCC2 requires SNAP23 phosphorylation

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Epithelial KCC3 regulation by ZnR/GPR39 also requires SNAP23

Imprecision in Precision Medicine: Differential Response of a Disease-Linked GluN2A Mutant to NMDA Channel Blockers

Mutations in N-methyl-d-aspartate receptors (NMDAR) subunits have been implicated in a growing number of human neurodevelopmental disorders. Previously, a de novo mutation in GRIN2A, encoding the GluN2A subunit, was identified in a patient with severe epilepsy and developmental delay. This missense mutation, which leads to GluN2A-P552R, produces significant dendrotoxicity in transfected rodent cortical neurons, as evidenced by pronounced dendritic blebbing. This injurious process can be prevented by treatment with the NMDA antagonist memantine. Given the increasing use of FDA approved NMDA antagonists to treat patients with GRIN mutations, who may have seizures refractory to traditional anti-epileptic drugs, we investigated whether additional NMDA antagonists were effective in attenuating neurotoxicity associated with GluN2A-P552R expression. Intriguingly, we found that while treatment with memantine can effectively block GluN2A-P552R-mediated dendrotoxicity, treatment with ketamine does not, despite the fact that both drugs work as open NMDAR channel blockers. Interestingly, we found that neurons expressing GluN2A-P552R were more vulnerable to an excitotoxic insult-an effect that, in this case, could be equally rescued by both memantine and ketamine. These findings suggest that GluN2A-P552R induced dendrotoxicity and increased vulnerability to excitotoxic stress are mediated through two distinct mechanisms. The differences between memantine and ketamine in halting GluN2A-P552R dendrotoxicity could not be explained by NMDA antagonist induced changes in MAP or Src kinase activation, previously shown to participate in NMDA-induced excitotoxicity. Our findings strongly suggest that not all NMDA antagonists may be of equal clinical utility in treating GRIN2A-mediated neurological disorders, despite a shared mechanism of action.

Evolutionary rate covariation identifies SLC30A9 (ZnT9) as a mitochondrial zinc transporter

Recent advances in genome sequencing have led to the identification of new ion and metabolite transporters, many of which have not been characterized. Due to the variety of subcellular localizations, cargo and transport mechanisms, such characterization is a daunting task, and predictive approaches focused on the functional context of transporters are very much needed. Here we present a case for identifying a transporter localization using evolutionary rate covariation (ERC), a computational approach based on pairwise correlations of amino acid sequence evolutionary rates across the mammalian phylogeny. As a case study, we find that poorly characterized transporter SLC30A9 (ZnT9) coevolves with several components of the mitochondrial oxidative phosphorylation chain, suggesting mitochondrial localization. We confirmed this computational finding experimentally using recombinant human SLC30A9. SLC30A9 loss caused zinc mishandling in the mitochondria, suggesting that under normal conditions it acts as a zinc exporter. We therefore propose that ERC can be used to predict the functional context of novel transporters and other poorly characterized proteins.

The Function and Regulation of Zinc in the Brain

Nearly sixty years ago Fredrich Timm developed a histochemical technique that revealed a rich reserve of free zinc in distinct regions of the brain. Subsequent electron microscopy studies in Timm- stained brain tissue found that this "labile" pool of cellular zinc was highly concentrated at synaptic boutons, hinting a possible role for the metal in synaptic transmission. Although evidence for activity-dependent synaptic release of zinc would not be reported for another twenty years, these initial findings spurred decades of research into zinc's role in neuronal function and revealed a diverse array of signaling cascades triggered or regulated by the metal. Here, we delve into our current understanding of the many roles zinc plays in the brain, from influencing neurotransmission and sensory processing, to activating both pro-survival and pro-death neuronal signaling pathways. Moreover, we detail the many mechanisms that tightly regulate cellular zinc levels, including metal binding proteins and a large array of zinc transporters.

Lessons from Recent Advances in Ischemic Stroke Management and Targeting Kv2.1 for Neuroprotection

Achieving neuroprotection in ischemic stroke patients has been a multidecade medical challenge. Numerous clinical trials were discontinued in futility and many were terminated in response to deleterious treatment effects. Recently, however, several positive reports have generated the much-needed excitement surrounding stroke therapy. In this review, we describe the clinical studies that significantly expanded the time window of eligibility for patients to receive mechanical endovascular thrombectomy. We further summarize the results available thus far for nerinetide, a promising neuroprotective agent for stroke treatment. Lastly, we reflect upon aspects of these impactful trials in our own studies targeting the Kv2.1-mediated cell death pathway in neurons for neuroprotection. We argue that recent changes in the clinical landscape should be adapted by preclinical research in order to continue progressing toward the development of efficacious neuroprotective therapies for ischemic stroke.

The Redox Biology of Excitotoxic Processes: The NMDA Receptor, TOPA Quinone, and the Oxidative Liberation of Intracellular Zinc

This special issue of Frontiers in Neuroscience-Neurodegeneration celebrates the 50th anniversary of John Olney's seminal work introducing the concept of excitotoxicity as a mechanism for neuronal cell death. Since that time, fundamental research on the pathophysiological activation of glutamate receptors has played a central role in our understanding of excitotoxic cellular signaling pathways, leading to the discovery of many potential therapeutic targets in the treatment of acute or chronic/progressive neurodegenerative disorders. Importantly, excitotoxic signaling processes have been found repeatedly to be closely intertwined with oxidative cellular cascades. With this in mind, this review looks back at long-standing collaborative efforts by the authors linking cellular redox status and glutamate neurotoxicity, focusing first on the discovery of the redox modulatory site of the N-methyl-D-aspartate (NMDA) receptor, followed by the study of the oxidative conversion of 3,4-dihydroxyphenylalanine (DOPA) to the non-NMDA receptor agonist and neurotoxin 2,4,5-trihydroxyphenylalanine (TOPA) quinone. Finally, we summarize our work linking oxidative injury to the liberation of zinc from intracellular metal binding proteins, leading to the uncovering of a signaling mechanism connecting excitotoxicity with zinc-activated cell death-signaling cascades.

Synaptic zinc inhibition of NMDA receptors depends on the association of GluN2A with the zinc transporter ZnT1

The NMDA receptor (NMDAR) is inhibited by synaptically released zinc. This inhibition is thought to be the result of zinc diffusion across the synaptic cleft and subsequent binding to the extracellular domain of the NMDAR. However, this model fails to incorporate the observed association of the highly zinc-sensitive NMDAR subunit GluN2A with the postsynaptic zinc transporter ZnT1, which moves intracellular zinc to the extracellular space. Here, we report that disruption of ZnT1-GluN2A association by a cell-permeant peptide strongly reduced NMDAR inhibition by synaptic zinc in mouse dorsal cochlear nucleus synapses. Moreover, synaptic zinc inhibition of NMDARs required postsynaptic intracellular zinc, suggesting that cytoplasmic zinc is transported by ZnT1 to the extracellular space in close proximity to the NMDAR. These results challenge a decades-old dogma on how zinc inhibits synaptic NMDARs and demonstrate that presynaptic release and a postsynaptic transporter organize zinc into distinct microdomains to modulate NMDAR neurotransmission.

Targeted disruption of Kv2.1-VAPA association provides neuroprotection against ischemic stroke in mice by declustering Kv2.1 channels

Kv2.1 channels mediate cell death-enabling loss of cytosolic potassium in neurons following plasma membrane insertion at somatodendritic clusters. Overexpression of the carboxyl terminus (CT) of the cognate channel Kv2.2 is neuroprotective by disrupting Kv2.1 surface clusters. Here, we define a seven-amino acid declustering domain within Kv2.2 CT (DP-2) and demonstrate its neuroprotective efficacy in a murine ischemia-reperfusion model. TAT-DP-2, a membrane-permeable derivative, induces Kv2.1 surface cluster dispersal, prevents post-injurious pro-apoptotic potassium current enhancement, and is neuroprotective in vitro by disrupting the association of Kv2.1 with VAPA. TAT-DP-2 also induces Kv2.1 cluster dispersal in vivo in mice, reducing infarct size and improving long-term neurological function following stroke. We suggest that TAT-DP-2 induces Kv2.1 declustering by disrupting Kv2.1-VAPA association and scaffolding sites required for the membrane insertion of Kv2.1 channels following injury. We present the first evidence of targeted disruption of Kv2.1-VAPA association as a neuroprotective strategy following brain ischemia.

Heterogeneous clinical and functional features of GRIN2D-related developmental and epileptic encephalopathy

N-methyl d-aspartate receptors are ligand-gated ionotropic receptors mediating a slow, calcium-permeable component of excitatory synaptic transmission in the CNS. Variants in genes encoding NMDAR subunits have been associated with a spectrum of neurodevelopmental disorders. Here we report six novel GRIN2D variants and one previously-described disease-associated GRIN2D variant in two patients with developmental and epileptic encephalopathy. GRIN2D encodes for the GluN2D subunit protein; the GluN2D amino acids affected by the variants in this report are located in the pre-M1 helix, transmembrane domain M3, and the intracellular carboxyl terminal domain. Functional analysis in vitro reveals that all six variants decreased receptor surface expression, which may underline some shared clinical symptoms. In addition the GluN2D(Leu670Phe), (Ala675Thr) and (Ala678Asp) substitutions confer significantly enhanced agonist potency, and/or increased channel open probability, while the GluN2D(Ser573Phe), (Ser1271Phe) and (Arg1313Trp) substitutions result in a mild increase of agonist potency, reduced sensitivity to endogenous protons, and decreased channel open probability. The GluN2D(Ser573Phe), (Ala675Thr), and (Ala678Asp) substitutions significantly decrease current amplitude, consistent with reduced surface expression. The GluN2D(Leu670Phe) variant slows current response deactivation time course and increased charge transfer. GluN2D(Ala678Asp) transfection significantly decreased cell viability of rat cultured cortical neurons. In addition, we evaluated a set of FDA-approved NMDAR channel blockers to rescue functional changes of mutant receptors. This work suggests the complexity of the pathological mechanisms of GRIN2D-mediated developmental and epileptic encephalopathy, as well as the potential benefit of precision medicine.

Defining the Kv2.1-syntaxin molecular interaction identifies a first-in-class small molecule neuroprotectant

The neuronal cell death-promoting loss of cytoplasmic K+ following injury is mediated by an increase in Kv2.1 potassium channels in the plasma membrane. This phenomenon relies on Kv2.1 binding to syntaxin 1A via 9 amino acids within the channel intrinsically disordered C terminus. Preventing this interaction with a cell and blood-brain barrier-permeant peptide is neuroprotective in an in vivo stroke model. Here a rational approach was applied to define the key molecular interactions between syntaxin and Kv2.1, some of which are shared with mammalian uncoordinated-18 (munc18). Armed with this information, we found a small molecule Kv2.1-syntaxin-binding inhibitor (cpd5) that improves cortical neuron survival by suppressing SNARE-dependent enhancement of Kv2.1-mediated currents following excitotoxic injury. We validated that cpd5 selectively displaces Kv2.1-syntaxin-binding peptides from syntaxin and, at higher concentrations, munc18, but without affecting either synaptic or neuronal intrinsic properties in brain tissue slices at neuroprotective concentrations. Collectively, our findings provide insight into the role of syntaxin in neuronal cell death and validate an important target for neuroprotection.

Molecular Neuroprotection Induced by Zinc-Dependent Expression of Hepatitis C-Derived Protein NS5A Targeting Kv2.1 Potassium Channels

We present the design of an innovative molecular neuroprotective strategy and provide proof-of-concept for its implementation, relying on the injury-mediated activation of an ectopic gene construct. As oxidative injury leads to the intracellular liberation of zinc, we hypothesize that tapping onto the zinc-activated metal regulatory element (MRE) transcription factor 1 system to drive expression of the Kv2.1-targeted hepatitis C protein NS5A (hepatitis C nonstructural protein 5A) will provide neuroprotection by preventing cell death-enabling cellular potassium loss in rat cortical neurons in vitro. Indeed, using biochemical and morphologic assays, we demonstrate rapid expression of MRE-driven products in neurons. Further, we report that MRE-driven NS5A expression, induced by a slowly evolving excitotoxic stimulus, functionally blocks injurious, enhanced Kv2.1 potassium whole-cell currents and improves neuronal viability. We suggest this form of "on-demand" neuroprotection could provide the basis for a tenable therapeutic strategy to prevent neuronal cell death in neurodegeneration.

Zn(2+) -induced Ca(2+) release via ryanodine receptors triggers calcineurin-dependent redistribution of cortical neuronal Kv2.1 K(+) channels

KEY POINTS

Increases in intracellular Zn(2+) concentrations are an early, necessary signal for the modulation of Kv2.1 K(+) channel localization and physiological function. Intracellular Zn(2+) -mediated Kv2.1 channel modulation is dependent on calcineurin, a Ca(2+) -activated phosphatase. We show that intracellular Zn(2+) induces a significant increase in ryanodine receptor-dependent cytosolic Ca(2+) transients, which leads to a calcineurin-dependent redistribution of Kv2.1 channels from pre-existing membrane clusters to diffuse localization. As such, the link between Zn(2+) and Ca(2+) signalling in this Kv2.1 modulatory pathway is established. We observe that a sublethal ischaemic preconditioning insult also leads to Kv2.1 redistribution in a ryanodine receptor-dependent fashion. We suggest that Zn(2+) may be an early and ubiquitous signalling molecule mediating Ca(2+) release from the cortical endoplasmic reticulum via ryanodine receptor activation.

ABSTRACT

Sublethal injurious stimuli in neurons induce transient increases in free intracellular Zn(2+) that are associated with regulating adaptive responses to subsequent lethal injury, including alterations in the function and localization of the delayed-rectifier potassium channel, Kv2.1. However, the link between intracellular Zn(2+) signalling and the observed changes in Kv2.1 remain undefined. In the present study, utilizing exogenous Zn(2+) treatment, along with a selective Zn(2+) ionophore, we show that transient elevations in intracellular Zn(2+) concentrations are sufficient to induce calcineurin-dependent Kv2.1 channel dispersal in rat cortical neurons in vitro, which is accompanied by a relatively small but significant hyperpolarizing shift in the voltage-gated activation kinetics of the channel. Critically, using a molecularly encoded calcium sensor, we found that the calcineurin-dependent changes in Kv2.1 probably occur as a result of Zn(2+) -induced cytosolic Ca(2+) release via activation of neuronal ryanodine receptors. Finally, we couple this mechanism with an established model for in vitro ischaemic preconditioning and show that Kv2.1 channel modulation in this process is also ryanodine receptor-sensitive. Our results strongly suggest that intracellular Zn(2+) -initiated signalling may represent an early and possibly widespread component of Ca(2+) -dependent processes in neurons.

Disruption of KV2.1 somato-dendritic clusters prevents the apoptogenic increase of potassium currents

As the predominant mediator of the delayed rectifier current, K_V2.1 is an important regulator of neuronal excitability. K_V2.1, however, also plays a well-established role in apoptotic cell death. Apoptogenic stimuli induce syntaxin-dependent trafficking of K_V2.1, resulting in an augmented delayed rectifier current that acts as a conduit for K⁺ efflux required for pro-apoptotic protease/nuclease activation. Recent evidence suggests that K_V2.1 somato-dendritic clusters regulate the formation of endoplasmic reticulum-plasma membrane junctions that function as scaffolding sites for plasma membrane trafficking of ion channels, including K_V2.1. However, it is unknown whether K_V2.1 somato-dendritic clusters are required for apoptogenic trafficking of K_V2.1. By overexpression of a protein derived from the C-terminus of the cognate channel K_V2.2 (K_V2.2CT), we induced calcineurin-independent disruption of K_V2.1 somato-dendritic clusters in rat cortical neurons, without altering the electrophysiological properties of the channel. We observed that K_V2.2CT-expressing neurons are less susceptible to oxidative stress-induced cell death. Critically, expression of K_V2.2CT effectively blocked the increased current density of the delayed rectifier current associated with oxidative injury, supporting a vital role of K_V2.1-somato-dendritic clusters in apoptogenic increases in K_V2.1-mediated currents.

GRIN2D Recurrent De Novo Dominant Mutation Causes a Severe Epileptic Encephalopathy Treatable with NMDA Receptor Channel Blockers

N-methyl-D-aspartate receptors (NMDARs) are ligand-gated cation channels that mediate excitatory synaptic transmission. Genetic mutations in multiple NMDAR subunits cause various childhood epilepsy syndromes. Here, we report a de novo recurrent heterozygous missense mutation-c.1999G>A (p.Val667Ile)-in a NMDAR gene previously unrecognized to harbor disease-causing mutations, GRIN2D, identified by exome and candidate panel sequencing in two unrelated children with epileptic encephalopathy. The resulting GluN2D p.Val667Ile exchange occurs in the M3 transmembrane domain involved in channel gating. This gain-of-function mutation increases glutamate and glycine potency by 2-fold, increases channel open probability by 6-fold, and reduces receptor sensitivity to endogenous negative modulators such as extracellular protons. Moreover, this mutation prolongs the deactivation time course after glutamate removal, which controls the synaptic time course. Transfection of cultured neurons with human GRIN2D cDNA harboring c.1999G>A leads to dendritic swelling and neuronal cell death, suggestive of excitotoxicity mediated by NMDAR over-activation. Because both individuals' seizures had proven refractory to conventional antiepileptic medications, the sensitivity of mutant NMDARs to FDA-approved NMDAR antagonists was evaluated. Based on these results, oral memantine was administered to both children, with resulting mild to moderate improvement in seizure burden and development. The older proband subsequently developed refractory status epilepticus, with dramatic electroclinical improvement upon treatment with ketamine and magnesium. Overall, these results suggest that NMDAR antagonists can be useful as adjuvant epilepsy therapy in individuals with GRIN2D gain-of-function mutations. This work further demonstrates the value of functionally evaluating a mutation, enabling mechanistic understanding and therapeutic modeling to realize precision medicine for epilepsy.

Synthesis and Evaluation of Potent KCNQ2/3-Specific Channel Activators

KQT-like subfamily (KCNQ) channels are voltage-gated, noninactivating potassium ion channels, and their down-regulation has been implicated in several hyperexcitability-related disorders, including epilepsy, neuropathic pain, and tinnitus. Activators of these channels reduce the excitability of central and peripheral neurons, and, as such, have therapeutic utility. Here, we synthetically modified several moieties of the KCNQ2-5 channel activator retigabine, an anticonvulsant approved by the U.S. Food and Drug Administration. By introducing a CF3-group at the 4-position of the benzylamine moiety, combined with a fluorine atom at the 3-position of the aniline ring, we generated Ethyl (2-amino-3-fluoro-4-((4-(trifluoromethyl)benzyl)amino)phenyl)carbamate (RL648_81), a new KCNQ2/3-specific activator that is >15 times more potent and also more selective than retigabine. We suggest that RL648_81 is a promising clinical candidate for treating or preventing neurologic disorders associated with neuronal hyperexcitability.

Regulation of neuronal pH by the metabotropic Zn(2+)-sensing Gq-coupled receptor, mZnR/GPR39

Synaptically released Zn(2+) acts as a neurotransmitter, in part, by activating the postsynaptic metabotropic Zn(2+)-sensing Gq protein-coupled receptor (mZnR/GPR39). In previous work using epithelial cells, we described crosstalk between Zn(2+) signaling and changes in intracellular pH and/or extracellular pH (pHe). As pH changes accompany neuronal activity under physiological and pathological conditions, we tested whether Zn(2+) signaling is involved in regulation of neuronal pH. Here, we report that up-regulation of a major H(+) extrusion pathway, the Na(+)/H(+) exchanger (NHE), is induced by mZnR/GPR39 activation in an extracellular-regulated kinase 1/2-dependent manner in hippocampal neurons in vitro. We also observed that changes in pHe can modulate neuronal mZnR/GPR39-dependent signaling, resulting in reduced activity at pHe 8 or 6.5. Similarly, Zn(2+)-dependent extracellular-regulated kinase 1/2 phosphorylation and up-regulation of NHE activity were absent at acidic pHe. Thus, our results suggest that when pHe is maintained within the physiological range, mZnR/GPR39 activation can up-regulate NHE-dependent recovery from intracellular acidification. During acidosis, as pHe drops, mZnR/GPR39-dependent NHE activation is inhibited, thereby attenuating further H(+) extrusion. This mechanism may serve to protect neurons from excessive decreases in pHe. Thus, mZnR/GPR39 signaling provides a homeostatic adaptive process for regulation of intracellular and extracellular pH changes in the brain. We show that the postsynaptic metabotropic Zn(2+)-sensing Gq protein-coupled receptor (mZnR/GPR39) activation induces up-regulation of a major neuronal H(+) extrusion pathway, the Na(+)/H(+) exchanger (NHE), thereby enhancing neuronal recovery from intracellular acidification. Changes in extracellular pH (pHe), however, modulate neuronal mZnR/GPR39-dependent signaling, resulting in reduced activity at pHe 8 or 6.5. This mechanism may serve to protect neurons from excessive decreases in pHe during acidosis. Hence, mZnR/GPR39 signaling provides a homeostatic adaptive process for regulation of intracellular and extracellular pH changes in the brain.

Critical role of Casein kinase 2 in hepatitis C NS5A-mediated inhibition of Kv2.1 K(+) channel function

Inhibiting injury-induced increases in outward K(+) currents is sufficient to block cell death in cortical neuronal injury models. It is now known that apoptosis is facilitated in hepatocytes by the same K(+) channel as in cortical neurons, namely, the delayed rectifier K(+) channel Kv2.1. The hepatitis C virus (HCV) protein NS5A prevents the apoptosis-enabling loss of intracellular potassium by inhibiting Kv2.1 function and thus blocking hepatocyte cell death. Critically, neurons expressing NS5A1b (from HCV genotype 1b), but not NS5A1a, can be protected from lethal injurious stimuli via a block of Kv2.1-mediated potassium currents. Here, we identify a key component unique to NS5A1b, which is necessary for restricting Kv2.1 currents and establishing neuroprotection. By comparing the sequence differences between NS5A1b and 1a we identify putative casein kinase 2 (CK2) phosphorylation regions unique to the 1b genotype. We show that selective inhibition of CK2 in cortical neurons results in loss of NS5A1b's ability to depress outward potassium currents, and, surprisingly, potentiates currents in non-NS5A-expressing cells. As such, our results suggest that NS5A1b-mediated inhibition of Kv2.1 function is critically dependent on its phosphorylation status at genotypic-specific CK2-directed residues. Importantly, inhibiting NS5A viral replicative function with the novel HCV drug Ledipasvir does not impair the ability of this protein to block Kv2.1 function. This suggests that the modulation of NS5A function by CK2 may be a component of HCV unique to the regulation of apoptosis.

Homeostatic regulation of KCC2 activity by the zinc receptor mZnR/GPR39 during seizures

The aim of this study was to investigate the role of the synaptic metabotropic zinc receptor mZnR/GPR39 in physiological adaptation to epileptic seizures. We previously demonstrated that synaptic activation of mZnR/GPR39 enhances inhibitory drive in the hippocampus by upregulating neuronal K(+)/Cl(-) co-transporter 2 (KCC2) activity. Here, we first show that mZnR/GPR39 knockout (KO) adult mice have dramatically enhanced susceptibility to seizures triggered by a single intraperitoneal injection of kainic acid, when compared to wild type (WT) littermates. Kainate also substantially enhances seizure-associated gamma oscillatory activity in juvenile mZnR/GPR39 KO hippocampal slices, a phenomenon that can be reproduced in WT tissue by extracellular Zn(2+) chelation. Importantly, kainate-induced synaptic Zn(2+) release enhances surface expression and transport activity of KCC2 in WT, but not mZnR/GPR39 KO hippocampal neurons. Kainate-dependent upregulation of KCC2 requires mZnR/GPR39 activation of the Gαq/phospholipase C/extracellular regulated kinase (ERK1/2) signaling cascade. We suggest that mZnR/GPR39-dependent upregulation of KCC2 activity provides homeostatic adaptation to an excitotoxic stimulus by increasing inhibition. As such, mZnR/GPR39 may provide a novel pharmacological target for dampening epileptic seizure activity.

Regulation of Pro-Apoptotic Phosphorylation of Kv2.1 K+ Channels

Caspase activity during apoptosis is inhibited by physiological concentrations of intracellular K⁺. To enable apoptosis in injured cortical and hippocampal neurons, cellular loss of this cation is facilitated by the insertion of Kv2.1 K⁺ channels into the plasma membrane via a Zn²⁺/CaMKII/SNARE-dependent process. Pro-apoptotic membrane insertion of Kv2.1 requires the dual phosphorylation of the channel by Src and p38 at cytoplasmic N- and C-terminal residues Y124 and S800, respectively. In this study, we investigate if these phosphorylation sites are mutually co-regulated, and whether putative N- and C-terminal interactions, possibly enabled by Kv2.1 intracellular cysteine residues C73 and C710, influence the phosphorylation process itself. Studies were performed with recombinant wild type and mutant Kv2.1 expressed in Chinese hamster ovary (CHO) cells. Using immunoprecipitated Kv2.1 protein and phospho-specific antibodies, we found that an intact Y124 is required for p38 phosphorylation of S800, and, importantly, that Src phosphorylation of Y124 facilitates the action of the p38 at the S800 residue. Moreover, the actions of Src on Kv2.1 are substantially decreased in the non-phosphorylatable S800A channel mutant. We also observed that mutations of either C73 or C710 residues decreased the p38 phosphorylation at S800 without influencing the actions of Src on tyrosine phosphorylation of Kv2.1. Surprisingly, however, apoptotic K⁺ currents were suppressed only in cells expressing the Kv2.1(C73A) mutant but not in those transfected with Kv2.1(C710A), suggesting a possible structural alteration in the C-terminal mutant that facilitates membrane insertion. These results show that intracellular N-terminal domains critically regulate phosphorylation of the C-terminal of Kv2.1, and vice versa, suggesting possible new avenues for modifying the apoptotic insertion of these channels during neurodegenerative processes.

Syntaxin-binding domain of Kv2.1 is essential for the expression of apoptotic K+ currents

Intracellular signalling cascades triggered by oxidative injury can lead to upregulation of Kv2.1 K(+) channels at the plasma membrane of dying neurons. Membrane incorporation of new channels is necessary for enhanced K(+) efflux and a consequent reduction of intracellular K(+) that facilitates apoptosis. We showed previously that the observed increase in K(+) currents is a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated process, and that the SNARE protein syntaxin binds directly to Kv2.1 channels. In the present study, we tested whether disrupting the interaction of Kv2.1 and syntaxin promoted the survival of cortical neurons following injury. Syntaxin is known to bind to Kv2.1 in a domain comprising amino acids 411-522 of the channel's cytoplasmic C terminus (C1a). Here we show that this domain is required for the apoptotic K(+) current enhancement. Moreover, expression of an isolated, Kv2.1-derived C1a peptide is sufficient to suppress the injury-induced increase in currents by interfering with Kv2.1/syntaxin binding. By subdividing the C1a peptide, we were able to localize the syntaxin binding site on Kv2.1 to the most plasma membrane-distal residues of C1a. Importantly, expression of this peptide segment in neurons prevented the apoptotic K(+) current enhancement and cell death following an oxidative insult, without greatly impairing baseline K(+) currents or normal electrical profiles of neurons. These results establish that binding of syntaxin to Kv2.1 is crucial for the manifestation of oxidant-induced apoptosis, and thereby reveal a potential new direction for therapeutic intervention in the treatment of neurodegenerative disorders.

The role of intracellular zinc release in aging, oxidative stress, and Alzheimer's disease

Brain aging is marked by structural, chemical, and genetic changes leading to cognitive decline and impaired neural functioning. Further, aging itself is also a risk factor for a number of neurodegenerative disorders, most notably Alzheimer’s disease (AD). Many of the pathological changes associated with aging and aging-related disorders have been attributed in part to increased and unregulated production of reactive oxygen species (ROS) in the brain. ROS are produced as a physiological byproduct of various cellular processes, and are normally detoxified by enzymes and antioxidants to help maintain neuronal homeostasis. However, cellular injury can cause excessive ROS production, triggering a state of oxidative stress that can lead to neuronal cell death. ROS and intracellular zinc are intimately related, as ROS production can lead to oxidation of proteins that normally bind the metal, thereby causing the liberation of zinc in cytoplasmic compartments. Similarly, not only can zinc impair mitochondrial function, leading to excess ROS production, but it can also activate a variety of extra-mitochondrial ROS-generating signaling cascades. As such, numerous accounts of oxidative neuronal injury by ROS-producing sources appear to also require zinc. We suggest that zinc deregulation is a common, perhaps ubiquitous component of injurious oxidative processes in neurons. This review summarizes current findings on zinc dyshomeostasis-driven signaling cascades in oxidative stress and age-related neurodegeneration, with a focus on AD, in order to highlight the critical role of the intracellular liberation of the metal during oxidative neuronal injury.

Voltage-gated potassium channels at the crossroads of neuronal function, ischemic tolerance, and neurodegeneration

Voltage-gated potassium (Kv) channels are widely expressed in the central and peripheral nervous system and are crucial mediators of neuronal excitability. Importantly, these channels also actively participate in cellular and molecular signaling pathways that regulate the life and death of neurons. Injury-mediated increased K(+) efflux through Kv2.1 channels promotes neuronal apoptosis, contributing to widespread neuronal loss in neurodegenerative disorders such as Alzheimer's disease and stroke. In contrast, some forms of neuronal activity can dramatically alter Kv2.1 channel phosphorylation levels and influence their localization. These changes are normally accompanied by modifications in channel voltage dependence, which may be neuroprotective within the context of ischemic injury. Kv1 and Kv7 channel dysfunction leads to neuronal hyperexcitability that critically contributes to the pathophysiology of human clinical disorders such as episodic ataxia and epilepsy. This review summarizes the neurotoxic, neuroprotective, and neuroregulatory roles of Kv channels and highlights the consequences of Kv channel dysfunction on neuronal physiology. The studies described in this review thus underscore the importance of normal Kv channel function in neurons and emphasize the therapeutic potential of targeting Kv channels in the treatment of a wide range of neurological diseases.

Glutamate transporter expression and function in a striatal neuronal model of Huntington's disease

Excitotoxicity may contribute to the pathogenesis of Huntington's disease. High affinity Na+ dependent glutamate transporters, residing in the plasma membrane, clear glutamate from the extracellular space and are the primary means of protection against excitotoxicity. Many reports suggest that Huntington's disease is associated with a decrease in the expression and function of glutamate transporters. We studied the expression and function of these transporters in a cellular model of Huntington's disease, STHdh(Q111/Q111) and STHdh(Q7/Q7) cells. We found that only GLT-1b and EAAC1 were expressed in these cell lines and only EAAC1 significantly contributed to the glutamate uptake. Surprisingly, there was an increase in Na+-dependent glutamate uptake in STHdh(Q111/Q111) cells accompanied by an increase in surface expression of EAAC1. We studied the influence of the Akt pathway on EAAC1 mediated uptake, since EAAC1 surface expression is influenced by Akt and previous studies have shown increased Akt expression in STHdh(Q111/Q111) cells. Glutamate uptake was inhibited by Akt pathway inhibitors in both the STHdh(Q7/Q7) and the STHdh(Q111/Q111) cell lines. We found no difference in Akt activation between the two cell lines under our conditions of culture. Therefore a difference in Akt activation does not seem to explain the increase in EAAC1 mediated uptake in the STHdh(Q111/Q111) cells.

Targeted single-neuron infection with rabies virus for transneuronal multisynaptic tracing

The transynaptic and retrograde spread of rabies virus make it an efficient and robust transneuronal tracer, capable of revealing connectivity patterns of multisynaptic, neuronal circuits with great detail. Current techniques begin by infecting many neurons simultaneously, from which higher-order neurons are then labeled sequentially in time. Here we report on a method that can initially infect a single neuron-of-choice, allowing for greater precision and specificity of labeled circuits.

SNARE-dependent upregulation of potassium chloride co-transporter 2 activity after metabotropic zinc receptor activation in rat cortical neurons in vitro

The major outward chloride transporter in neurons is the potassium chloride co-transporter 2 (KCC2), critical for maintaining an inhibitory reversal potential for GABA(A) receptor channels. In a recent study, we showed that Zn(2+) regulates GABA(A) reversal potentials in the hippocampus by enhancing the activity of KCC2 through an increase in its surface expression. Zn(2+) initiates this process by activating the Gq-coupled metabotropic Zn(2+) receptor/G protein-linked receptor 39 (mZnR/GPR39). Here, we first demonstrated that mZnR/GPR39 is functional in cortical neurons in culture, and then tested the hypothesis that the increase in KCC2 activity is mediated through a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent process. We established the presence of functional mZnR in rat cultured cortical neurons by loading cells with a Ca(2+) indicator and exposing cells to Zn(2+), which triggered consistent Ca(2+) responses that were blocked by the Gq antagonist YM-254890, but not by the metabotropic glutamate receptor antagonist (RS)-α-methyl-4-carboxyphenylglycine (MCPG). Importantly, Zn(2+) treatment under these conditions did not increase the intracellular concentrations of Zn(2+) itself. We then measured KCC2 activity by monitoring both the rate and relative amount of furosemide-sensitive NH(4)(+) influx through the co-transporter using an intracellular pH-sensitive fluorescent indicator. We observed that Zn(2+) pretreatment induced a Ca(2+)-dependent increase in KCC2 activity. The effects of Zn(2+) on KCC2 activity were also observed in wild-type mouse cortical neurons in culture, but not in neurons obtained from mZnR/GPR39(-/-) mice, suggesting that Zn(2+) acts through mZnR/GPR39 activation to upregulate KCC2 activity. We next transfected rat cortical neurons with a plasmid encoding botulinum toxin C1 (Botox C1), which cleaves the SNARE proteins syntaxin 1 and synaptosomal-associated protein 25 (SNAP-25). Basal KCC2 activity was similar in both transfected and non-transfected neurons. Non-transfected cells, or cells transfected with marker vector alone, showed a Zn(2+)-dependent increase in KCC2 activity. In contrast, KCC2 activity in neurons expressing Botox C1 was unchanged by Zn(2+). These results suggest that SNARE proteins are necessary for the increased activity of KCC2 after Zn(2+) stimulation of mZnR/GPR39.

Redox regulation of intracellular zinc: molecular signaling in the life and death of neurons

Zn(2+) has emerged as a major regulator of neuronal physiology, as well as an important signaling agent in neural injury. The intracellular concentration of this metal is tightly regulated through the actions of Zn(2+) transporters and the thiol-rich metal binding protein metallothionein, closely linking the redox status of the cell to cellular availability of Zn(2+). Accordingly, oxidative and nitrosative stress during ischemic injury leads to an accumulation of neuronal free Zn(2+) and the activation of several downstream cell death processes. While this Zn(2+) rise is an established signaling event in neuronal cell death, recent evidence suggests that a transient, sublethal accumulation of free Zn(2+) can also play a critical role in neuroprotective pathways activated during ischemic preconditioning. Thus, redox-sensitive proteins, like metallothioneins, may play a critical role in determining neuronal cell fate by regulating the localization and concentration of intracellular free Zn(2+).

Upregulation of KCC2 activity by zinc-mediated neurotransmission via the mZnR/GPR39 receptor

Vesicular Zn(2+) regulates postsynaptic neuronal excitability upon its corelease with glutamate. We previously demonstrated that synaptic Zn(2+) acts via a distinct metabotropic zinc-sensing receptor (mZnR) in neurons to trigger Ca(2+) responses in the hippocampus. Here, we show that physiological activation of mZnR signaling induces enhanced K(+)/Cl(-) cotransporter 2 (KCC2) activity and surface expression. As KCC2 is the major Cl(-) outward transporter in neurons, Zn(2+) also triggers a pronounced hyperpolarizing shift in the GABA(A) reversal potential. Mossy fiber stimulation-dependent upregulation of KCC2 activity is eliminated in slices from Zn(2+) transporter 3-deficient animals, which lack synaptic Zn(2+). Importantly, activity-dependent ZnR signaling and subsequent enhancement of KCC2 activity are also absent in slices from mice lacking the G-protein-coupled receptor GPR39, identifying this protein as the functional neuronal mZnR. Our work elucidates a fundamentally important role for synaptically released Zn(2+) acting as a neurotransmitter signal via activation of a mZnR to increase Cl(-) transport, thereby enhancing inhibitory tone in postsynaptic cells.

Inhibitory effects of chalcone glycosides isolated from Brassica rapa L. 'hidabeni' and their synthetic derivatives on LPS-induced NO production in microglia

Activation of microglia induces the production of various inflammatory mediators including nitric oxide (NO), leading to neurodegeneration in many central nervous system diseases. In this study, we examined the effects of chalcone glycosides isolated from Brassica rapa L. 'hidabeni' on lipopolysaccharide (LPS)-induced NO production using rat immortalized microglia HAPI cells. 4'-O-β-D-Glucopyranosyl-3',4-dimethoxychalcone (A2) inhibited LPS-induced inducible NO synthase (iNOS) expression and NO production. However, A2 did not affect nuclear factor-κB and mitogen-activated protein kinase pathways. The signal transduction and activator of transcription 1 (STAT1), which is activated via production of IFN-β by LPS, is an important transcription factor responsible for LPS-induced iNOS expression. A2 suppressed LPS-induced phosphorylation and nuclear translocation of STAT1, although it had no effects on LPS-induced IFN-β expression. These results indicate that the inhibitory effect of A2 is due to the prevention of STAT signaling. Moreover, structure-activity relationship studies on newly synthesized 'hidabeni' chalcone derivatives showed that 4'-O-β-D-glucopyranosyl-3'-methoxychalcone (A11), which has no functional groups in the B-ring, inhibits LPS-induced NO production more potently than A2.

Complex role of zinc in methamphetamine toxicity in vitro

Methamphetamine is a drug of abuse that can induce oxidative stress and neurotoxicity to dopaminergic neurons. We have previously reported that oxidative stress promotes the liberation of intracellular Zn(2+) from metal-binding proteins, which, in turn, can initiate neuronal injurious signaling processes. Here, we report that methamphetamine mobilizes Zn(2+) in catecholaminergic rat pheochromocytoma (PC12) cells, as measured by an increase in Zn(2+)-regulated gene expression driven by the metal response element transcription factor-1. Moreover, methamphetamine-liberated Zn(2+) was responsible for a pronounced enhancement in voltage-dependent K(+) currents in these cells, a process that normally accompanies Zn(2+)-dependent cell injury. Overnight exposure to methamphetamine induced PC12 cell death. This toxicity could be prevented by the cell-permeant zinc chelator N,N,N', N'-tetrakis(2-pyridylmethyl)-ethylenediamine (TPEN), and by over-expression of the Zn(2+)-binding protein metallothionein 3 (MT3), but not by tricine, an extracellular Zn(2+) chelator. The toxicity of methamphetamine to PC12 cells was enhanced by the presence of co-cultured microglia. Remarkably, under these conditions, TPEN no longer protected but, in fact, dramatically exacerbated methamphetamine toxicity, tricine again being without effect. Over-expression of MT3 in PC12 cells did not mimic these toxicity-enhancing actions of TPEN, suggesting that the chelator affected microglial function. Interestingly, P2X receptor antagonists reversed the toxicity-enhancing effect of TPEN. As such, endogenous levels of intracellular Zn(2+) may normally interfere with the activation of P2X channels in microglia. We conclude that Zn(2+) plays a significant but complex role in modulating the cellular response of PC12 cells to methamphetamine exposure in both the absence and presence of microglia.

Zinc bells rang in Jerusalem!

"Oh, Jerusalem of gold, and of light, and of bronze..." goes the popular song. But it was another metal that towered above the Jerusalem landscape during the meeting of the International Society for Zinc Biology (ISZB; http://www.iszb.org/), held at Mishkenot Sha'ananim, a whisper away from the Old City walls. More than 100 scientists gathered on 1 to 5 December 2009 to discuss their research on the biology of this metal. Zinc is a double-edged sword. Zinc supplementation accelerates wound healing and growth and promotes an effective immune response. On the other hand, zinc deficiency leads to growth retardation and impaired learning and memory function, and has been linked to mood disorders. At the cellular level, however, uncontrolled increases in zinc concentrations can lead to neuronal cell death and may be involved in neurodegenerative disorders. Through regulation of various intracellular signaling pathways, zinc can accelerate cell growth and possibly contribute to cancer. However, despite the physiological and clinical importance of this metal, research on the molecular basis of these effects is still in its infancy. The 2009 ISZB meeting provided a venue for investigators working on various zinc-related issues to share their thoughts and ideas and to promote the growth of this field.

ERK signaling leads to mitochondrial dysfunction in extracellular zinc-induced neurotoxicity

A zinc-induced signaling pathway leading to extracellular signal-regulated kinase 1/2 (ERK1/2) activation and subsequent neuronal death has been investigated. We find that an extracellular zinc application stimulates biphasic phosphorylation of ERK1/2 and p38 MAPK in rat cultured neurons. The activation of ERK1/2, but not p38, is responsible for zinc neurotoxicity as only U0126, a MEK inhibitor that blocks ERK1/2 phosphorylation, significantly protects cortical neurons from zinc exposure. Over-expression of a dominant negative Ras mutant blocks zinc-induced Elk1-dependent gene expression in neurons, indicating the involvement of Ras activation in the zinc pathway leading to ERK phosphorylation and Elk1 signaling. We also find that zinc treatment results in neuronal mitochondrial hyperpolarization. Importantly, both U0126 and bongkrekic acid, an inhibitor of the mitochondrial adenine nucleotide translocase, effectively reduce zinc-triggered mitochondrial changes. As bongkrekic acid also prevents zinc-triggered neuronal death but not ERK1/2 phosphorylation, activation of MAPK signaling precedes and is required for mitochondrial dysfunction and cell death. These results provide new insight on the mechanism of extracellular zinc-induced toxicity in which the regulation of mitochondrial function by the Ras/MEK/ERK pathway is closely associated with neuronal viability.

Zn2+ regulates Kv2.1 voltage-dependent gating and localization following ischemia

The delayed-rectifier K(+) channel Kv2.1 exists in highly phosphorylated somatodendritic clusters. Ischemia induces rapid Kv2.1 dephosphorylation and a dispersal of these clusters, accompanied by a hyperpolarizing shift in their voltage-dependent activation kinetics. Transient modulation of Kv2.1 activity and localization following ischemia is dependent on a rise in intracellular Ca(2+)and the protein phosphatase calcineurin. Here, we show that neuronal free Zn(2+)also plays a critical role in the ischemic modulation of Kv2.1. We found that sub-lethal ischemia in cultured rat cortical neurons led to characteristic hyperpolarizing shifts in K(+) current voltage dependency and pronounced dephosphorylation of Kv2.1. Zn(2+)chelation, similar to calcineurin inhibition, attenuated ischemic induced changes in K(+) channel activation kinetics. Zn(2+)chelation during ischemia also blocked Kv2.1 declustering. Surprisingly, we found that the Zn(2+)rise following ischemia occurred in spite of calcineurin inhibition. Therefore, a calcineurin-independent rise in neuronal free Zn(2+) is critical in altering Kv2.1 channel activity and localization following ischemia. The identification of Zn(2+) in mediating ischemic modulation of Kv2.1 may lead to a better understanding of cellular adaptive responses to injury.

Preimplantation genetic diagnosis for BRCA1/2--a novel clinical experience

OBJECTIVE

To describe our 2-year experience with preimplantation genetic diagnosis (PGD) for carriers of mutations in the genes BRCA1 and BRCA2, the dilemmas incurred and the lessons learned.

METHODS

We collected data on those carriers of BRCA1/2 mutations who applied for PGD counseling and who decided to proceed. We describe the PGD procedures that were conducted and their outcome.

RESULTS

Ten carriers of BRCA1/2 mutations applied for PGD counseling, seven were healthy, and three were BC survivors. Eight women needed in vitro fertilization (IVF) because of coexisting infertility. After counseling, six opted for the procedure and five of them underwent PGD for the BRCA mutation. In one of these PGD, fluorescence in situ hybridization (FISH) analysis for chromosomes 21, X and Y was also performed. Three women conceived, each in the first treatment attempt. One of them gave birth to twins, the second to a singleton and the third is currently pregnant. During the pregnancies, dilemmas concerning PGD confirmation were discussed.

CONCLUSIONS

PGD is an acceptable reproductive option for BRCA mutation carriers, especially for those who require IVF due to fertility problems. Discussion of this option should be carried out with sensitivity, taking into account the age of the woman, her health, fertility status and emotional state. Confirmatory prenatal diagnosis may not always be encouraged.

Regulation of apoptotic potassium currents by coordinated zinc-dependent signalling

Oxidant-liberated intracellular Zn(2+) regulates neuronal apoptosis via an exocytotic membrane insertion of Kv2.1-encoded ion channels, resulting in an enhancement of voltage-gated K(+) currents and a loss of intracellular K(+) that is necessary for caspase-mediated proteolysis. In the present study we show that an N-terminal tyrosine of Kv2.1 (Y124), which is a known target of Src kinase, is critical for the apoptotic current surge. Moreover, we demonstrate that Y124 works in concert with a C-terminal serine (S800) target of p38 mitogen-activated protein kinase (MAPK) to regulate Kv2.1-mediated current enhancement. While Zn(2+) was previously shown to activate p38, we show here that this metal inhibits cytoplasmic protein tyrosine phosphatase (Cyt-PTPepsilon), which specifically targets Y124. Importantly, a point mutation of Y124 to a non-phosphorylatable residue or over-expression of Cyt-PTPepsilon protects cells from injury. Kv2.1-encoded channels thus regulate neuronal survival by providing a converging input for two Zn(2+)-dependent signal transduction cascades.

Intracellular zinc inhibits KCC2 transporter activity

We found that K(+)/Cl(-) co-transporter 2 (KCC2) activity, monitored with wide-field fluorescence, was inhibited by intracellular Zn(2+), a major component of neuronal injury. Zn(2+)-mediated KCC2 inhibition produced a depolarizing shift of GABA(A) reversal potentials in rat cortical neurons. Moreover, oxygen-glucose deprivation attenuated KCC2 activity in a Zn(2+)-dependent manner. The link between Zn(2+) and KCC2 activity provides a previously unknown target for neuroprotection and may be important in activity-dependent regulation of inhibitory synaptic transmission.

NMDA potentiation by visible light in the presence of a fluorescent neurosteroid analogue

N-Methyl-D-aspartate (NMDA) receptors are widely studied because of their importance in synaptic plasticity and excitotoxic cell death. Here we report a novel method of potentiating NMDA receptors with fluorescence excited by blue (480 nm) light. In the presence of 300 nM of a (7-nitro-2,1,3-benzoxadiazol-4-yl) amino (NBD)-tagged neuroactive steroid carrier C2-NBD-(3alpha,5alpha)-3-hydroxypregnan-20-one (C2-NBD 3alpha5alphaP), responses of cultured hippocampal neurons to 10 microM NMDA were potentiated to 219.2 +/- 9.2% of the baseline response (100%) by a 30 s exposure to 480 nm light. The potentiation decayed back to baseline with a time constant of 80.6 s. Responses to 1 microM and 100 microM NMDA were potentiated to 147.9 +/- 9.6% and 174.1 +/- 15.6% of baseline, respectively, suggesting that visible-light potentiation is relatively insensitive to NMDA concentration. Peak autaptic NMDA responses were potentiated to 178.9 +/- 22.4% of baseline. Similar potentiation was seen with 10 microM NBD-lysine, suggesting that visible-light potentiation is not a steroid effect. Potentiation was also seen with a steroid analogue in which the NBD was replaced with fluorescein, suggesting that NBD is not the only fluorophore capable of supporting visible-light potentiation. UV light and redox potentiation of NMDA receptors largely occluded subsequent blue light potentiation (127.7 +/- 7.4% and 120.2 +/- 6.2% of baseline, respectively). The NR1a(C744A,C798A) mutant that is insensitive to redox and UV potentiation was also largely unaffected by visible-light potentiation (135.0 +/- 10.0% of baseline). Finally, we found that the singlet oxygen scavenger furfuryl alcohol decreased visible-light potentiation. Collectively, these data suggest that visible-light potentiation of NMDA receptors by fluorescence excitation shares mechanisms with UV and redox potentiation and may involve singlet oxygen production.

Protein kinase C regulation of neuronal zinc signaling mediates survival during preconditioning

Sub-lethal activation of cell death processes initiate pro-survival signaling cascades. As intracellular Zn(2+) liberation mediates neuronal death pathways, we tested whether a sub-lethal increase in free Zn(2+) could also trigger neuroprotection. Neuronal free Zn(2+) transiently increased following preconditioning, and was both necessary and sufficient for conferring excitotoxic tolerance. Lethal exposure to NMDA led to a delayed increase in Zn(2+) that contributed significantly to excitotoxicity in non-preconditioned neurons, but not in tolerant neurons, unless preconditioning-induced free Zn(2+) was chelated. Thus, preconditioning may trigger the expression of Zn(2+)-regulating processes, which, in turn, prevent subsequent Zn(2+)-mediated toxicity. Indeed, preconditioning increased Zn(2+)-regulated gene expression in neurons. Examination of the molecular signaling mechanism leading to this early Zn(2+) signal revealed a critical role for protein kinase C (PKC) activity, suggesting that PKC may act directly on the intracellular source of Zn(2+). We identified a conserved PKC phosphorylation site at serine-32 (S32) of metallothionein (MT) that was important in modulating Zn(2+)-regulated gene expression and conferring excitotoxic tolerance. Importantly, we observed increased PKC-induced serine phosphorylation in immunopurified MT1, but not in mutant MT1(S32A). These results indicate that neuronal Zn(2+) serves as an important, highly regulated signaling component responsible for the initiation of a neuroprotective pathway.

Assessment of cell viability in primary neuronal cultures

This unit contains five protocols for assaying cell viability in vitro using primary neuronal cultures, including a novel method for use with transfected neurons. Three of the assays are based on the principle that cell death cascades alter membrane permeability. The lactate dehydrogenase (LDH) release assay measures the amount of the cytoplasmic enzyme released into the bathing medium, while the trypan blue and propidium iodide assays measure the ability of cells to exclude dye from their cytoplasm. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay measures the mitochondrial activity of viable cells by quantifying the conversion of the tetrazolium salt to its formazan product. Finally, the fifth assay details the measurement of luciferase expression as an indication of neuronal viability within a relatively small population of transfected neurons.

Selective inhibition of mitogen-activated protein kinase phosphatases by zinc accounts for extracellular signal-regulated kinase 1/2-dependent oxidative neuronal cell death

Oxidative stress induced by glutathione depletion in the mouse HT22 neuroblastoma cell line and embryonic rat immature cortical neurons causes a delayed, sustained activation of extracellular signal-regulated kinase (ERK) 1/2, which is required for cell death. This sustained activation of ERK1/2 is mediated primarily by a selective inhibition of distinct ERK1/2-directed phosphatases either by enhanced degradation (i.e., for mitogen-activated protein kinase phosphatase-1) or as shown here by reductions in enzymatic activity (i.e., for protein phosphatase type 2A). The inhibition of ERK1/2 phosphatases in HT22 cells and immature neurons subjected to glutathione depletion results from oxidative stress because phosphatase activity is restored in cells treated with the antioxidant butylated hydroxyanisole. This leads to reduced ERK1/2 activation and neuroprotection. Furthermore, an increase in free intracellular zinc that accompanies glutathione-induced oxidative stress in HT22 cells and immature neurons contributes to selective inhibition of ERK1/2 phosphatase activity and cell death. Finally, ERK1/2 also functions to maintain elevated levels of zinc. Thus, the elevation of intracellular zinc within neurons subjected to oxidative stress can trigger a robust positive feedback loop operating through activated ERK1/2 that rapidly sets into motion a zinc-dependent pathway of cell death.