Endogenous zinc depresses GABAergic transmission via T-type Ca(2+) channels and broadens the time window for integration of glutamatergic inputs in dentate granule cells

TSQ Incubation Enhances Autometallographic Zinc Detection in Cultured Astrocytes

Zinc is a critical ion for a large number of cellular functions. In the central nervous system, zinc ions are involved in synaptic transmission. Therefore, zinc homeostasis is essential, and cells have developed a variety of mechanisms to control cellular zinc concentration, including the zincosome formation. Alterations of free zinc levels have been associated with brain dysfunction and are present in many illnesses and syndromes. Astrocytes are implicated in the maintenance of the neuronal milleu and brain homeostasis. In this work, we have analyzed the combination of direct (TSQ) and indirect (autometallography) zinc detection methods to increase sensitivity for studying zinc uptake by rat astrocytes in vitro. Zincosome formation was visualized with the zinc fluorochrome TSQ by light microscopy. Additionally, we improved both zinc precipitation and cellular fixation methods to preserve zinc ions and make them suitable for autometallography development. Our tests pinpointed paraformaldehyde and sodium sulfide as the more adequate methods for cellular fixation and zinc precipitation, respectively. TSQ incubation and pH of the fixative were shown to be crucial for autometallography. Using this improved method, we visualized the zinc content of zincosomes at the ultrastructural level both as silver autometallographic precipitates and as electrodense sulfide-osmium zinc precipitates.

A multimodal electrochemical approach to measure the effect of zinc on vesicular content and exocytosis in a single cell model of ischemia

Zinc ion is essential for normal brain function that modulates synaptic activity and neuronal plasticity and it is associated with memory formation. Zinc is considered to be a contributing factor to the pathogenesis of ischemia, but the association between zinc and ischemia on vesicular exocytosis is unclear. In this study, we used a combination of chemical analysis methods and a cell model of ischemia/reperfusion to investigate exocytotic release and vesicular content, as well as the effect of zinc alteration on vesicular exocytosis. Oxygen–glucose deprivation and reperfusion (OGDR) was used as an in vitro model of ischemia in a model cell line. Exocytotic release and vesicular storage of catecholamine content were increased following OGDR, resulting in a higher fraction of release during exocytosis. However, zinc eliminated these increases following OGDR and the fraction of release remained unchanged. Understanding the consequences of zinc accumulation on vesicular exocytosis at the early stage of OGDR should aid in the development of therapeutic strategies to reduce ischemic brain injury. As the fraction released has been suggested to be related to presynaptic plasticity, insights are gained towards deciphering ischemia related memory impairment.

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.

Synaptic Zinc Enhances Inhibition Mediated by Somatostatin, but not Parvalbumin, Cells in Mouse Auditory Cortex

Cortical inhibition is essential for brain activity and behavior. Yet, the mechanisms that modulate cortical inhibition and their impact on sensory processing remain less understood. Synaptically released zinc, a neuromodulator released by cortical glutamatergic synaptic vesicles, has emerged as a powerful modulator of sensory processing and behavior. Despite the puzzling finding that the vesicular zinc transporter (ZnT3) mRNA is expressed in cortical inhibitory interneurons, the actions of synaptic zinc in cortical inhibitory neurotransmission remain unknown. Using in vitro electrophysiology and optogenetics in mouse brain slices containing the layer 2/3 (L2/3) of auditory cortex, we discovered that synaptic zinc increases the quantal size of inhibitory GABAergic neurotransmission mediated by somatostatin (SOM)- but not parvalbumin (PV)-expressing neurons. Using two-photon imaging in awake mice, we showed that synaptic zinc is required for the effects of SOM- but not PV-mediated inhibition on frequency tuning of principal neurons. Thus, cell-specific zinc modulation of cortical inhibition regulates frequency tuning.

Recent progress on the role of GABAergic neurotransmission in the pathogenesis of Alzheimer's disease

Despite their possible causative role, targeting amyloidosis, tau phosphorylation, acetylcholine esterase, glutamate, oxidative stress and mitochondrial metabolism have not yet led to the development of drugs to cure Alzheimer's disease (AD). Recent preclinical and clinical reports exhibit a surge in interest in the role of GABAergic neurotransmission in the pathogenesis of AD. The interaction among GABAergic signaling, amyloid-β and acetylcholine is shown to affect the homeostasis between excitation (glutamate) and inhibition (GABA) in the brain. As a consequence, over-excitation leads to neurodegeneration (excitotoxicity) and impairment in the higher level functions. Previously, the glutamate arm of this balance received the most attention. Recent literature suggests that over-excitation is primarily mediated by dysfunctional GABA signaling and can possibly be restored by rectifying anomalous metabolism observed in the GABAergic neurons during AD. Additionally, neurogenesis and synaptogenesis have also been linked with GABAergic signaling. This association may provide a basis for the needed repair mechanism. Furthermore, several preclinical interventional studies revealed that targeting various GABA receptor subtypes holds potential in overcoming the memory deficits associated with AD. In conclusion, the recent scientific literature suggests that GABAergic signaling presents itself as a promising target for anti-AD drug development.

Parallel in vivo and in vitro transcriptomics analysis reveals calcium and zinc signalling in the brain as sensitive targets of HBCD neurotoxicity

Hexabromocyclododecane (HBCD) is a brominated flame retardant (BFR) that accumulates in humans and affects the nervous system. To elucidate the mechanisms of HBCD neurotoxicity, we used transcriptomic profiling in brains of female mice exposed through their diet to HBCD (199 mg/kg body weight per day) for 28 days and compared with those of neuronal N2A and NSC-19 cell lines exposed to 1 or 2 µM HBCD. Similar pathways and functions were affected both in vivo and in vitro, including Ca²⁺ and Zn²⁺ signalling, glutamatergic neuron activity, apoptosis, and oxidative stress. Release of cytosolic free Zn²⁺ by HBCD was confirmed in N2A cells. This Zn²⁺ release was partially quenched by the antioxidant N-acetyl cysteine indicating that, in accordance with transcriptomic analysis, free radical formation is involved in HBCD toxicity. To investigate the effects of HBCD in excitable cells, we isolated mouse hippocampal neurons and monitored Ca²⁺ signalling triggered by extracellular glutamate or zinc, which are co-released pre-synaptically to trigger postsynaptic signalling. In control cells application of zinc or glutamate triggered a rapid rise of intracellular [Ca²⁺]. Treatment of the cultures with 1 µM of HBCD was sufficient to reduce the glutamate-dependent Ca²⁺ signal by 50%. The effect of HBCD on zinc-dependent Ca²⁺ signalling was even more pronounced, resulting in the reduction of the Ca²⁺ signal with 86% inhibition at 1 µM HBCD. Our results show that low concentrations of HBCD affect neural signalling in mouse brain acting through dysregulation of Ca²⁺ and Zn²⁺ homeostasis.

Zn2+ reduction induces neuronal death with changes in voltage-gated potassium and sodium channel currents

In the present study, cultured rat primary neurons were exposed to a medium containing N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), a specific cell membrane-permeant Zn²⁺ chelator, to establish a model of free Zn²⁺ deficiency in neurons. The effects of TPEN-mediated free Zn²⁺ ion reduction on neuronal viability and on the performance of voltage-gated sodium channels (VGSCs) and potassium channels (Kvs) were assessed. Free Zn²⁺ deficiency 1) markedly reduced the neuronal survival rate, 2) reduced the peak amplitude of I_Na, 3) shifted the I_Na activation curve towards depolarization, 4) modulated the sensitivity of sodium channel voltage-dependent inactivation to a depolarization voltage, and 5) increased the time course of recovery from sodium channel inactivation. In addition, free Zn²⁺ deficiency by TPEN notably enhanced the peak amplitude of transient outward K⁺ currents (I_A) and delayed rectifier K⁺ currents (I_K), as well as caused hyperpolarization and depolarization directional shifts in their steady-state activation curves, respectively. Zn²⁺ supplementation reversed the effects induced by TPEN. Our results indicate that free Zn²⁺ deficiency causes neuronal damage and alters the dynamic characteristics of VGSC and Kv currents. Thus, neuronal injury caused by free Zn²⁺ deficiency may correlate with its modulation of the electrophysiological properties of VGSCs and Kvs.

Modelling zinc changes at the hippocampal mossy fiber synaptic cleft

Zinc, a transition metal existing in very high concentrations in the hippocampal mossy fibers from CA3 area, is assumed to be co-released with glutamate and to have a neuromodulatory role at the corresponding synapses. The synaptic action of zinc is determined both by the spatiotemporal characteristics of the zinc release process and by the kinetics of zinc binding to sites located in the cleft area, as well as by their concentrations. This work addresses total, free and complexed zinc concentration changes, in an individual synaptic cleft, following single, short and long periods of evoked zinc release. The results estimate the magnitude and time course of the concentrations of zinc complexes, assuming that the dynamics of the release processes are similar to those of glutamate. It is also considered that, for the cleft zinc concentrations used in the model (≤ 1 μM), there is no postsynaptic zinc entry. For this reason, all released zinc ends up being reuptaken in a process that is several orders of magnitude slower than that of release and has thus a much smaller amplitude. The time derivative of the total zinc concentration in the cleft is represented by the difference between two alpha functions, corresponding to the released and uptaken components. These include specific parameters that were chosen assuming zinc and glutamate co-release, with similar time courses. The peak amplitudes of free zinc in the cleft were selected based on previously reported experimental cleft zinc concentration changes evoked by single and multiple stimulation protocols. The results suggest that following a low amount of zinc release, similar to that associated with one or a few stimuli, zinc clearance is mainly mediated by zinc binding to the high-affinity sites on the NMDA receptors and to the low-affinity sites on the highly abundant GLAST glutamate transporters. In the case of higher zinc release brought about by a larger group of stimuli, most zinc binding occurs essentially to the GLAST transporters, having the corresponding zinc complex a maximum concentration that is more than one order of magnitude larger than that for the high and low affinity NMDA sites. The other zinc complexes considered in the model, namely those formed with sites on the AMPA receptors, calcium and K_ATP channels and with ATP molecules, have much smaller contributions to the synaptic zinc clearance.

Genetic alteration of the metal/redox modulation of Cav3.2 T-type calcium channel reveals its role in neuronal excitability

KEY POINTS

In this study, we describe a new knock-in (KI) mouse model that allows the study of the H191-dependent regulation of T-type Cav3.2 channels. Sensitivity to zinc, nickel and ascorbate of native Cav3.2 channels is significantly impeded in the dorsal root ganglion (DRG) neurons of this KI mouse. Importantly, we describe that this H191-dependent regulation has discrete but significant effects on the excitability properties of D-hair (down-hair) cells, a sub-population of DRG neurons in which Cav3.2 currents prominently regulate excitability. Overall, this study reveals that the native H191-dependent regulation of Cav3.2 channels plays a role in the excitability of Cav3.2-expressing neurons. This animal model will be valuable in addressing the potential in vivo roles of the trace metal and redox modulation of Cav3.2 T-type channels in a wide range of physiological and pathological conditions.

ABSTRACT

Cav3.2 channels are T-type voltage-gated calcium channels that play important roles in controlling neuronal excitability, particularly in dorsal root ganglion (DRG) neurons where they are involved in touch and pain signalling. Cav3.2 channels are modulated by low concentrations of metal ions (nickel, zinc) and redox agents, which involves the histidine 191 (H191) in the channel's extracellular IS3-IS4 loop. It is hypothesized that this metal/redox modulation would contribute to the tuning of the excitability properties of DRG neurons. However, the precise role of this H191-dependent modulation of Cav3.2 channel remains unresolved. Towards this goal, we have generated a knock-in (KI) mouse carrying the mutation H191Q in the Cav3.2 protein. Electrophysiological studies were performed on a subpopulation of DRG neurons, the D-hair cells, which express large Cav3.2 currents. We describe an impaired sensitivity to zinc, nickel and ascorbate of the T-type current in D-hair neurons from KI mice. Analysis of the action potential and low-threshold calcium spike (LTCS) properties revealed that, contrary to that observed in WT D-hair neurons, a low concentration of zinc and nickel is unable to modulate (1) the rheobase threshold current, (2) the afterdepolarization amplitude, (3) the threshold potential necessary to trigger an LTCS or (4) the LTCS amplitude in D-hair neurons from KI mice. Together, our data demonstrate that this H191-dependent metal/redox regulation of Cav3.2 channels can tune neuronal excitability. This study validates the use of this Cav3.2-H191Q mouse model for further investigations of the physiological roles thought to rely on this Cav3.2 modulation.

A possible significant role of zinc and GPR39 zinc sensing receptor in Alzheimer disease and epilepsy

Zinc the essential trace element, plays a significant role in the brain development and in the proper brain functions at every stage of life. Misbalance of zinc (Zn(2+)) ions in the central nervous system is involved in the pathogenesis of numerous neurodegenerative disorders such as Alzheimer's disease, Depression, and Epilepsy. In brain, Zn(2+) has been identified as a ligand, capable of activating and inhibiting the receptors including the NMDA-type glutamate receptors (NMDARs), GABAA receptors, nicotinic acetylcholine receptors (nAChRs), glycine receptors (glyR) and serotonin receptors (5-HT3). Recently GPR39 has been identified as a zinc-specific receptor, widely expressed in brain tissues including the frontal cortex, amygdala, and hippocampus. GPR39, when binding with Zn(2+) has shown promising therapeutic potentials. This review presents current knowledge regarding the role of GPR39 zinc sensing receptor in brain, with a focus on Alzheimer's disease and Epilepsy. Although the results are encouraging, further research is needed to clarify zinc and GPR39 role in the treatment of Alzheimer's disease and Epilepsy.

Modulation of extrasynaptic NMDA receptors by synaptic and tonic zinc

Many excitatory synapses contain high levels of mobile zinc within glutamatergic vesicles. Although synaptic zinc and glutamate are coreleased, it is controversial whether zinc diffuses away from the release site or whether it remains bound to presynaptic membranes or proteins after its release. To study zinc transmission and quantify zinc levels, we required a high-affinity rapid zinc chelator as well as an extracellular ratiometric fluorescent zinc sensor. We demonstrate that tricine, considered a preferred chelator for studying the role of synaptic zinc, is unable to efficiently prevent zinc from binding low-nanomolar zinc-binding sites, such as the high-affinity zinc-binding site found in NMDA receptors (NMDARs). Here, we used ZX1, which has a 1 nM zinc dissociation constant and second-order rate constant for binding zinc that is 200-fold higher than those for tricine and CaEDTA. We find that synaptic zinc is phasically released during action potentials. In response to short trains of presynaptic stimulation, synaptic zinc diffuses beyond the synaptic cleft where it inhibits extrasynaptic NMDARs. During higher rates of presynaptic stimulation, released glutamate activates additional extrasynaptic NMDARs that are not reached by synaptically released zinc, but which are inhibited by ambient, tonic levels of nonsynaptic zinc. By performing a ratiometric evaluation of extracellular zinc levels in the dorsal cochlear nucleus, we determined the tonic zinc levels to be low nanomolar. These results demonstrate a physiological role for endogenous synaptic as well as tonic zinc in inhibiting extrasynaptic NMDARs and thereby fine tuning neuronal excitability and signaling.

Which insights have we gained from the kindling and post-status epilepticus models?

Experimental animal epilepsy research got a big boost since the discovery that daily mild and short (seconds) tetanic stimulations in selected brain regions led to seizures with increasing duration and severity. This model that was developed by Goddard (1967) became known as the kindling model for epileptogenesis and has become a widely used model for temporal lobe epilepsy with complex partial seizures. During the late ninety-eighties the number of publications related to electrical kindling reached its maximum. However, since the kindling procedure is rather labor intensive and animals only develop spontaneous seizures (epilepsy) after hundreds of stimulations, research has shifted toward models in which the animals exhibit spontaneous seizures after a relatively short latent period. This led to post-status epilepticus (SE) models in which animals experience SE after injection of pharmacological compounds (e.g. kainate or pilocarpine) or via electrical stimulation of (limbic) brain regions. These post-SE models are the most widely used models in epilepsy research today. However, not all aspects of mesial temporal lobe epilepsy (MTLE) are reproduced and the widespread brain damage is often a caricature of the situation in the patient. Therefore, there is a need for models that can better replicate the disease. Kindling, although already a classic model, can still offer valid clues in this context. In this paper, we review different aspects of the kindling model with emphasis on experiments in the rat. Next, we review characteristic properties of the post-SE models and compare the neuropathological, electrophysiological and molecular differences between kindling and post-SE epilepsy models. Finally, we shortly discuss the advantages and disadvantages of these models.

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.

T-type channel-mediated neurotransmitter release

Besides controlling a wide variety of cell functions, T-type channels have been shown to regulate neurotransmitter release in peripheral and central synapses and neuroendocrine cells. Growing evidence over the last 10 years suggests a key role of Cav3.2 and Cav3.1 channels in controlling basal neurosecretion near resting conditions and sustained release during mild stimulations. In some cases, the contribution of low-voltage-activated (LVA) channels is not directly evident but requires either the activation of coupled presynaptic receptors, block of ion channels, or chelation of metal ions. Concerning the coupling to the secretory machinery, T-type channels appear loosely coupled to neurotransmitter and hormone release. In neurons, Cav3.2 and Cav3.1 channels mainly control the asynchronous appearance of "minis" [miniature inhibitory postsynaptic currents (mIPSCs) and miniature excitatory postsynaptic currents (mEPSCs)]. The same loose coupling is evident from membrane capacity and amperometric recordings in chromaffin cells and melanotropes where the low-threshold-driven exocytosis possesses the same linear Ca(2+) dependence of the other voltage-gated Ca(2+) channels (Cav1 and Cav2) that is strongly attenuated by slow calcium buffers. The intriguing issue is that, despite not expressing a consensus "synprint" site, Cav3.2 channels do interact with syntaxin 1A and SNAP-25 and, thus, may form nanodomains with secretory vesicles that can be regulated at low voltages. In this review, we discuss all the past and recent issues related to T-type channel-secretion coupling in neurons and neuroendocrine cells.