Do we need zinc to think?

The crystal structure of the regulatory domain of the human sodium-driven chloride/bicarbonate exchanger

The sodium-driven chloride/bicarbonate exchanger (NDCBE) is essential for maintaining homeostatic pH in neurons. The crystal structure at 2.8 Å resolution of the regulatory N-terminal domain of human NDCBE represents the first crystal structure of an electroneutral sodium-bicarbonate cotransporter. The crystal structure forms an equivalent dimeric interface as observed for the cytoplasmic domain of Band 3, and thus establishes that the consensus motif VTVLP is the key minimal dimerization motif. The VTVLP motif is highly conserved and likely to be the physiologically relevant interface for all other members of the SLC4 family. A novel conserved Zn²⁺-binding motif present in the N-terminal domain of NDCBE is identified and characterized in vitro. Cellular studies confirm the Zn²⁺ dependent transport of two electroneutral bicarbonate transporters, NCBE and NBCn1. The Zn²⁺ site is mapped to a cluster of histidines close to the conserved ETARWLKFEE motif and likely plays a role in the regulation of this important motif. The combined structural and bioinformatics analysis provides a model that predicts with additional confidence the physiologically relevant interface between the cytoplasmic domain and the transmembrane domain.

Review: Cav2.3 R-type Voltage-Gated Ca2+ Channels - Functional Implications in Convulsive and Non-convulsive Seizure Activity

Background:

Researchers have gained substantial insight into mechanisms of synaptic transmission, hyperexcitability, excitotoxicity and neurodegeneration within the last decades. Voltage-gated Ca²⁺ channels are of central relevance in these processes. In particular, they are key elements in the etiopathogenesis of numerous seizure types and epilepsies. Earlier studies predominantly targeted on Ca_v2.1 P/Q-type and Ca_v3.2 T-type Ca²⁺ channels relevant for absence epileptogenesis. Recent findings bring other channels entities more into focus such as the Ca_v2.3 R-type Ca²⁺ channel which exhibits an intriguing role in ictogenesis and seizure propagation. Ca_v2.3 R-type voltage gated Ca²⁺ channels (VGCC) emerged to be important factors in the pathogenesis of absence epilepsy, human juvenile myoclonic epilepsy (JME), and cellular epileptiform activity, e.g. in CA1 neurons. They also serve as potential target for various antiepileptic drugs, such as lamotrigine and topiramate.

Objective:

This review provides a summary of structure, function and pharmacology of VGCCs and their fundamental role in cellular Ca²⁺ homeostasis. We elaborate the unique modulatory properties of Ca_v2.3 R-type Ca²⁺ channels and point to recent findings in the proictogenic and proneuroapoptotic role of Ca_v2.3 R-type VGCCs in generalized convulsive tonic–clonic and complex-partial hippocampal seizures and its role in non-convulsive absence like seizure activity.

Conclusion:

Development of novel Ca_v2.3 specific modulators can be effective in the pharmacological treatment of epilepsies and other neurological disorders.

Autocrine effect of Zn²⁺ on the glucose-stimulated insulin secretion

It is well known that zinc (Zn(2+)) is required for the process of insulin biosynthesis and the maturation of insulin secretory granules in pancreatic beta (β)-cells, and that changes in Zn(2+) levels in the pancreas have been found to be associated with diabetes. Glucose-stimulation causes a rapid co-secretion of Zn(2+) and insulin with similar kinetics. However, we do not know whether Zn(2+) regulates insulin availability and secretion. Here we investigated the effect of Zn(2+) on glucose-stimulated insulin secretion (GSIS) in isolated mouse pancreatic islets. Whereas Zn(2+) alone (control) had no effect on the basal secretion of insulin, it significantly inhibited GSIS. The application of CaEDTA, by removing the secreted Zn(2+) from the extracellular milieu of the islets, resulted in significantly increased GSIS, suggesting an overall inhibitory role of secreted Zn(2+) on GSIS. The inhibitory action of Zn(2+) was mostly mediated through the activities of KATP/Ca(2+) channels. Furthermore, during brief paired-pulse glucose-stimulated Zn(2+) secretion (GSZS), Zn(2+) secretion following the second pulse was significantly attenuated, probably by the secreted endogenous Zn(2+) after the first pulse. Such an inhibition on Zn(2+) secretion following the second pulse was completely reversed by Zn(2+) chelation, suggesting a negative feedback mechanism, in which the initial glucose-stimulated Zn(2+) release inhibits subsequent Zn(2+) secretion, subsequently inhibiting insulin co-secretion as well. Taken together, these data suggest a negative feedback mechanism on GSZS and GSIS by Zn(2+) secreted from β-cells, and the co-secreted Zn(2+) may act as an autocrine inhibitory modulator.

Effects of nano and conventional zinc oxide on anxiety-like behavior in male rats

Objectives:

Current drug therapies for psychological disorders, such as anxiety, are not as effective as expected, and it has been shown that zinc supplements, such as zinc oxide (ZnO), can influence anxiety. ZnO nanoparticles (ZnO NPs) are among the most used nanomaterials produced and applied in many products.

Materials and Methods:

This study investigated the effects of ZnO NPs in comparison with conventional ZnO (cZnO) on anxiety-like behaviors. Adult male Wistar rats were divided into groups: Control (receiving saline 0.9%), ZnO NPs (5, 10, 20 mg/kg), and cZnO (5, 10, 20 mg/kg). All drugs were dispersed in saline 0.9%, and 30 minutes after intraperitoneal (i.p.) injection of drugs, elevated plus maze apparatus was used to evaluate anxiety.

Results:

ZnO NPs (5 mg/kg) and cZnO (10 and 20 mg/kg) significantly increased the percentage of time spent in open arm (open arm time % OAT) compared with the control group (P < 0.05). This indicates the anxiolytic effect of such components; in addition, ZnO NPs (20 mg/kg) reduced locomotor activity (P < 0.05). Serum zinc concentration increased by both anxiolytic dose of components (from 1.75 ± 1.07 (mg/l) in control group to 5.31 ± 0.53 (mg/l) in ZnO NPs (5 mg/kg) and 10.38 ± 0.90 (mg/l) in cZnO (10 mg/kg) groups). Also, all doses increased serum pH (from 7.3 ± 0.05 in control group to 8.1 ± 0.05 in ZnO NPs (5 mg/kg) and 8.05 ± 0.01 in cZnO (10 mg/kg) groups and kept them constant after 24 hours.

Conclusion:

Results indicate that the anxiolytic effect of ZnO NPs is much higher than its conventional form, but the introduction of ZnO NPs, as a new drug for treatment of anxiety disorder, needs further investigations.

A specific role for hippocampal mossy fiber's zinc in rapid storage of emotional memories

We investigated the specific role of zinc present in large amounts in the synaptic vesicles of mossy fibers and coreleased with glutamate in the CA3 region. In previous studies, we have shown that blockade of zinc after release has no effect on the consolidation of spatial learning, while zinc is required for the consolidation of contextual fear conditioning. Although both are hippocampo-dependent processes, fear conditioning to the context implies a strong emotional burden. To verify the hypothesis that zinc could play a specific role in enabling sustainable memorization of a single event with a strong emotional component, we used a neuropharmacological approach combining a glutamate receptor antagonist with different zinc chelators. Results show that zinc is mandatory to allow the consolidation of one-shot memory, thus being the key element allowing the hippocampus submitted to a strong emotional charge to switch from the cognitive mode to a flashbulb memory mode. Individual differences in learning abilities have been known for a long time to be totally or partially compensated by distributed learning practice. Here we show that contextual fear conditioning impairments due to zinc blockade can be efficiently reduced by distributed learning practice.

Zinc and insulin in pancreatic beta-cells

Zinc (Zn2+) is an essential element crucial for growth and development, and also plays a role in cell signaling for cellular processes like cell division and apoptosis. In the mammalian pancreas, Zn2+ is essential for the correct processing, storage, secretion, and action of insulin in beta (β)-cells. Insulin is stored inside secretory vesicles or granules, where two Zn2+ ions coordinate six insulin monomers to form the hexameric-structure on which maturated insulin crystals are based. The total Zn2+ content of the mammalian pancreas is among the highest in the body, and Zn2+ concentration reach millimolar levels in the interior of the dense-core granule. Changes in Zn2+ levels in the pancreas have been found to be associated with diabetes. Hence, the relationship between co-stored Zn2+ and insulin undoubtedly is critical to normal β-cell function. The advances in the field of Zn2+ biology over the last decade have facilitated our understanding of Zn2+ trafficking, its intracellular distribution and its storage. When exocytosis of insulin occurs, insulin granules fuse with the β-cell plasma membrane and release their contents, i.e., insulin as well as substantial amount of free Zn2+, into the extracellular space and the local circulation. Studies increasingly indicate that secreted Zn2+ has autocrine or paracrine signaling in β-cells or the neighboring cells. This review discusses the Zn2+ homeostasis in β-cells with emphasis on the potential signaling role of Zn2+ to islet biology.

Inhibitory effect of zinc on glucose-stimulated zinc/insulin secretion in an insulin-secreting β-cell line

Diminished or inappropriate secretion of insulin is associated with type II diabetes. The cellular/molecular mechanism coupled with the regulation of insulin secretion is still under intense investigation. Divalent ion zinc (Zn(2+)) is co-packaged and co-secreted with insulin and is intimately involved in the process of insulin biosynthesis and the maturation of insulin secretory granules. The study reported here investigated glucose-stimulated zinc secretion (GSZS) and the effect of zinc on glucose-stimulated insulin secretion (GSIS) in the HIT-T15 pancreatic β-cell line. Zinc secretion was measured using a newly developed fluorescent zinc imaging approach, and the insulin secretion was measured using an enzyme-linked immunosorbent assay. There was apparent granular-like zinc staining in β-cells. The application of glucose induced detectable zinc secretion or GSZS. Like GSIS, GSZS was dependent on the glucose concentration (5-20 mm) and the presence of extracellular calcium. The application of a zinc chelator enhanced GSZS. When brief paired-pulse glucose stimulations, which involve the initial glucose stimulation followed by a second round of glucose stimulation, were applied, zinc secretion or GSZS that followed the first pulse was inhibited. This inhibition was reversed by zinc chelation, suggesting a feedback mechanism on GSZS by zinc secreted from β-cells. Finally, the application of zinc (50 μm) strongly inhibited GSIS as measured by enzyme-linked immunosorbent assay. The present study suggests that insulin secretion is regulated by co-secreted zinc that may act as an autocrine inhibitory modulator.

Zinc and human disease

The vast knowledge of the physiologic functions of zinc in at least 3000 proteins and the recent recognition of fundamental regulatory functions of zinc(II) ions released from cells or within cells links this nutritionally essential metal ion to numerous diseases. However, this knowledge so far has had remarkably limited impact on diagnosing, preventing, and treating human diseases. One major roadblock is a lack of suitable biomarkers that would detect changes in cellular zinc metabolism and relate them to specific disease outcomes. It is not only the right amount of zinc in the diet that maintains health. At least as important is the proper functioning of the dozens of proteins that control cellular zinc homeostasis, regulate intracellular traffic of zinc between the cytosol and vesicles/organelles, and determine the fluctuations of signaling zinc(II) ions. Cellular zinc deficiencies or overloads, a term referring to zinc concentrations exceeding the cellular zinc buffering capacity, compromise the redox balance. Zinc supplementation may not readily remedy zinc deficiency if other factors limit the capability of a cell to control zinc. The role of zinc in human diseases requires a general understanding of the wide spectrum of functions of zinc, how zinc is controlled, how it interacts with the metabolism of other metal ions, in particular copper and iron, and how perturbation of specific zinc-dependent molecular processes causes disease and influences the progression of disease.

Modulation of neuronal signal transduction and memory formation by synaptic zinc

The physiological role of synaptic zinc has remained largely enigmatic since its initial detection in hippocampal mossy fibers over 50 years ago. The past few years have witnessed a number of studies highlighting the ability of zinc ions to regulate ion channels and intracellular signaling pathways implicated in neuroplasticity, and others that shed some light on the elusive role of synaptic zinc in learning and memory. Recent behavioral studies using knock-out mice for the synapse-specific zinc transporter ZnT-3 indicate that vesicular zinc is required for the formation of memories dependent on the hippocampus and the amygdala, two brain centers that are prominently innervated by zinc-rich fibers. A common theme emerging from this research is the activity-dependent regulation of the Erk1/2 mitogen-activated-protein kinase pathway by synaptic zinc through diverse mechanisms in neurons. Here we discuss current knowledge on how synaptic zinc may play a role in cognition through its impact on neuronal signaling.

The neuronal Kv4 channel complex

Kv4 channel complexes mediate the neuronal somatodendritic A-type K(+) current (I(SA)), which plays pivotal roles in dendritic signal integration. These complexes are composed of pore-forming voltage-gated alpha-subunits (Shal/Kv4) and at least two classes of auxiliary beta-subunits: KChIPs (K(+)-Channel-Interacting-Proteins) and DPLPs (Dipeptidyl-Peptidase-Like-Proteins). Here, we review our investigations of Kv4 gating mechanisms and functional remodeling by specific auxiliary beta-subunits. Namely, we have concluded that: (1) the Kv4 channel complex employs novel alternative mechanisms of closed-state inactivation; (2) the intracellular Zn(2+) site in the T1 domain undergoes a conformational change tightly coupled to voltage-dependent gating and is targeted by nitrosative modulation; and (3) discrete and specific interactions mediate the effects of KChIPs and DPLPs on activation, inactivation and permeation of Kv4 channels. These studies are shedding new light on the molecular bases of I(SA) function and regulation.

Contribution of extracellular negatively charged residues to ATP action and zinc modulation of rat P2X2 receptors

Two histidines are known to be essential for zinc potentiation of rat P2X2 receptors, but the chemistry of zinc coordination would suggest that other residues also participate in this zinc-binding site. There is also a second lower affinity zinc-binding site in P2X2 receptors whose constituents are unknown. To assess whether the extracellular acidic residues of the P2X2 receptor contribute to zinc potentiation or inhibition, site-directed mutagenesis was used to produce alanine substitutions at each extracellular glutamate or aspartate. Two electrode voltage clamp recordings from Xenopus oocytes indicated that 7 of the 34 mutants (D82A, E85A, E91A, E115A, D136A, D209A, and D281A) were deficient in zinc potentiation and one mutant (E84A) was deficient in zinc inhibition. Additional tests on cysteine mutants at these eight positions indicated that D136 is the only residue that is a strong candidate to be at the potentiating zinc-binding site, and that E84 is unlikely to be at the inhibitory zinc-binding site.

Evidence that the ZNT3 protein controls the total amount of elemental zinc in synaptic vesicles

The ZNT3 protein decorates the presynaptic vesicles of central neurons harboring vesicular zinc, and deletion of this protein removes staining for zinc. However, it has been unclear whether only histochemically reactive zinc is lacking or if, indeed, total elemental zinc is missing from neurons lacking the Slc30a3 gene, which encodes the ZNT3 protein. The limitations of conventional histochemical procedures have contributed to this enigma. However, a novel technique, microprobe synchrotron X-ray fluorescence, reveals that the normal 2- to 3-fold elevation of zinc concentration normally present in the hippocampal mossy fibers is absent in Slc30a3 knockout (ZNT3) mice. Thus, the ZNT3 protein evidently controls not only the "stainability" but also the actual mass of zinc in mossy-fiber synaptic vesicles. This work thus confirms the metal-transporting role of the ZNT3 protein in the brain.

Intracellular zinc elevation measured with a "calcium-specific" indicator during ischemia and reperfusion in rat hippocampus: a question on calcium overload

Much of our current evidence concerning of the role of calcium (Ca2+) as a second messenger comes from its interaction with fluorescent probes; however, many Ca2+ probes also have a higher affinity for another divalent cation: zinc (Zn2+). In this study, using a selective Zn2+ probe (Newport Green), we investigated the accumulation of intracellular Zn2+ transients in acute rat hippocampal slices during ischemia, simulated by oxygen and glucose deprivation (OGD). Subsequent reperfusion with glucose-containing oxygenated medium resulted in an additional increase in intracellular Zn2+. Such observations compelled us to investigate the contribution of Zn2+ to the alleged intracellular Ca2+ overload occurring in ischemia and reperfusion. Using confocal fluorescent microscopy of Calcium Green-1, a widely used Ca2+ indicator, we detected increases in fluorescence intensity during OGD and reperfusion. However, application of a Zn2+ chelator, at the peak of the fluorescence elevation (interpreted as Ca2+ overload), resulted in a significant drop in intensity, suggesting that rising Zn2+ is the primary source of the increasing Calcium Green-1 fluorescence. Finally, staining with the cell viability indicator propidium iodide revealed that Zn2+ is responsible for the ischemic neuronal cell death, because Zn2+ chelation prevented cells from sustaining ischemic damage. Current cellular models of ischemic injury center on Ca2+-mediated excitotoxicity. Our results indicate that Zn2+ elevation contributes to conventionally recognized Ca2+ overload and also suggest that the role of Ca2+ in neurotoxicity described previously using Ca2+ probes may need to be re-examined to determine whether effect previously attributed to Ca2+ could, in part, be attributable to Zn2+.

Role of the Menkes copper-transporting ATPase in NMDA receptor-mediated neuronal toxicity

Menkes disease, a fatal neurodegenerative disorder resulting in seizures, hypotonia, and failure to thrive, is due to inherited loss-of-function mutations in the gene encoding a copper-transporting ATPase (Atp7a) on the X chromosome. Although affected patients exhibit signs and symptoms of copper deficiency, the mechanisms resulting in neurologic disease remain unknown. We recently discovered that Atp7a is required for the production of an NMDA receptor-dependent releasable copper pool within hippocampal neurons, a finding that suggests a role for copper in activity-dependent modulation of synaptic activity. In support of this hypothesis, we now demonstrate that copper chelation exacerbates NMDA-mediated excitotoxic cell death in primary hippocampal neurons, whereas the addition of copper is specifically protective and results in a significant decrease in cytoplasmic Ca(2+) levels after NMDA receptor activation. Consistent with the known neuroprotective effect of NMDA receptor nitrosylation, we show here that this protective effect of copper depends on endogenous nitric oxide production in hippocampal neurons, demonstrating in vivo links among neuroprotection, copper metabolism, and nitrosylation. Atp7a is required for these copper-dependent effects: Hippocampal neurons isolated from newborn Mo(br) mice reveal a marked sensitivity to endogenous glutamate-mediated NMDA receptor-dependent excitotoxicity in vitro, and mild hypoxic/ischemic insult to these mice in vivo results in significantly increased caspase 3 activation and neuronal injury. Taken together, these data reveal a unique connection between copper homeostasis and NMDA receptor activity that is of broad relevance to the processes of synaptic plasticity and excitotoxic cell death.

Target-dependent use of co-released inhibitory transmitters at central synapses

Corelease of GABA and glycine by mixed neurons is a prevalent mode of inhibitory transmission in the vertebrate hindbrain. However, little is known of the functional organization of mixed inhibitory networks. Golgi cells, the main inhibitory interneurons of the cerebellar granular layer, have been shown to contain GABA and glycine. We show here that, in the vestibulocerebellum, Golgi cells contact both granule cells and unipolar brush cells, which are excitatory relay interneurons for vestibular afferences. Whereas IPSCs in granule cells are mediated by GABA(A) receptors only, Golgi cell inhibition of unipolar brush cells is dominated by glycinergic currents. We further demonstrate that a single Golgi cell can perform pure GABAergic inhibition of granule cells and pure glycinergic inhibition of unipolar brush cells. This specialization results from the differential expression of GABA(A) and glycine receptors by target cells and not from a segregation of GABA and glycine in presynaptic terminals. Thus, postsynaptic selection of coreleased fast transmitters is used in the CNS to increase the diversity of individual neuronal outputs and achieve target-specific signaling in mixed inhibitory networks.

QZ1 and QZ2: rapid, reversible quinoline-derivatized fluoresceins for sensing biological Zn(II)

QZ1, 2-[2-chloro-6-hydroxy-3-oxo-5-(quinolin-8-ylaminomethyl)-3H-xanthen-9-yl]benzoic acid, and QZ2, 2-[6-hydroxy-3-oxo-4,5-bis-(quinolin-8-ylaminomethyl)-3H-xanthen-9-yl]benzoic acid, two fluorescein-based dyes derivatized with 8-aminoquinoline, have been prepared and their photophysical, thermodynamic, and zinc-binding kinetic properties determined. Because of their low background fluorescence and highly emissive Zn(II) complexes, QZ1 and QZ2 have a large dynamic range, with approximately 42- and approximately 150-fold fluorescence enhancements upon Zn(II) coordination, respectively. These dyes have micromolar K(d) values for Zn(II) and are selective for Zn(II) over biologically relevant concentrations of the alkali and alkaline earth metals. The Zn(II) complexes also fluoresce brightly in the presence of excess Mn(II), Fe(II), Co(II), Cd(II), and Hg(II), offering improved specificity for Zn(II) over di(2-picolyl)amine-based Zn(II) sensors. Stopped-flow kinetic investigations indicate that QZ1 and QZ2 bind Zn(II) with k(on) values of (3-4) x 10(6) M(-1) s(-1), compared to (6-8) x 10(5) M(-1) s(-1) for select ZP (Zinpyr) dyes, at 4.3 degrees C. Dissociation of Zn(II) from QZ1 and QZ2 occurs with k(off) values of 150 and 160 s(-1), over 5 orders of magnitude larger than those for ZP probes, achieving reversibility on the biological (millisecond) time scale. Laser scanning confocal and two-photon microscopy studies reveal that QZ2 is cell-permeable and Zn(II)-responsive in vivo. Because of its weaker affinity for Zn(II), QZ2 responds to higher concentrations of intracellular Zn(II) than members of the ZP family, illustrating that binding affinity is an important parameter for Zn(II) detection in vivo.

Selective block of the human 2-P domain potassium channel, TASK-3, and the native leak potassium current, IKSO, by zinc

Background potassium channels control the resting membrane potential of neurones and regulate their excitability. Two-pore-domain potassium (2-PK) channels have been shown to underlie a number of such neuronal background currents. Currents through human TASK-1, TASK-2 and TASK-3 channels expressed in Xenopus oocytes were inhibited by extracellular acidification. For TASK-3, mutation of histidine 98 to aspartate or alanine considerably reduced this effect of pH. Zinc was found to be a selective blocker of TASK-3 with virtually no effect on TASK-1 or TASK-2. Zinc had an IC(50) of 19.8 microM for TASK-3, at +80 mV, with little voltage dependence associated with this inhibition. TASK-3 H98A had a much reduced sensitivity to zinc suggesting this site is important for zinc block. Surprisingly, TASK-1 also has histidine in position 98 but is insensitive to zinc block. TASK-3 and TASK-1 differ at position 70 with glutamate for TASK-3 and lysine for TASK-1. TASK-3 E70K also had a much reduced sensitivity to zinc while the corresponding reverse mutation in TASK-1, K70E, induced zinc sensitivity. A TASK-3-TASK-1 concatamer channel was comparatively zinc insensitive. For TASK-3, it is concluded that positions E70 and H98 are both critical for zinc block. The native cerebellar granule neurone (CGN) leak current, IK(SO), is sensitive to block by zinc, with current reduced to 0.58 of control values in the presence of 100 microM zinc. This suggests that TASK-3 channels underlie a major component of IK(SO). It has recently been suggested that zinc is released from inhibitory synapses onto CGNs. Therefore it is possible that inhibition of IK(SO) in cerebellar granule cells by synaptically released zinc may have important physiological consequences.

Allosteric modulation of neurotransmitter transporters at excitatory synapses

The regulation of glutamate and glycine concentrations within excitatory synapses plays an important role in maintaining a dynamic signalling process between neurones, but the failure to regulate the concentrations of these neurotransmitters has been implicated in the pathogenesis of various neurological disorders. In this review we shall discuss how glutamate and glycine transporters regulate synaptic concentrations of these neurotransmitters and how endogenous allosteric modulators influence transporter function. Whilst glutamate transport inhibitors are unlikely to be of therapeutic value because their potential to cause excitoxicity and cell death, a greater understanding of how endogenous compounds allosterically modulate glutamate transporters may provide alternate drug targets. On the other hand, there are some promising drugs that inhibit glycine transporters, which are being trialled as an alternate treatment for schizophrenia. We shall discuss how the activity of one such compound may be expected to influence excitatory neurotransmission.

Zinc sensing for cellular application

Numerous tools for Zn2+ sensing in living cells have become available in the past three years. Among them, fluorescence imaging using fluorescent sensor molecules has been the most popular approach. Some of these sensor molecules can be used to visualize Zn2+ in living cells. Some of the biological functions of Zn2+ have been clarified using these sensor molecules, especially in neuronal cells, which contain a high concentration of free Zn2+.