Histochemical localization of neurons with zinc-permeable AMPA/kainate channels in rat brain slices

Micromolar concentrations of Zn2+ depress cellular excitability through a blockade of calcium current in rat adrenal slices

The present work, using chromaffin cells in rat adrenal slices (RCCs), aims to describe what type of ionic current alterations induced by zinc underlies their effects reported on synaptic transmission. Thus, Zn²⁺ blocked calcium channels of RCCs in a time- and concentration-dependent manner with an IC₅₀ of 391 μM. This blockade was partially reversed upon washout and was greater at more depolarizing holding potentials (i.e. 32 ± 5% at -110 mV, and 43 ± 6% at -50 mV, after 5 min perfusion). In ω-toxins-sensitive calcium channels (N-, P- and Q-types), Zn²⁺caused a lower blockade of I_Ca, 33.3%, than in ω-toxins-resistant ones (L-type, 55.3%; and R-type, 90%). This compound inhibited calcium current at all test potentials and shows a shift of the I-V curve to more depolarized values of about 10 mV. The sodium current was not blocked by acute application of high Zn²⁺concentrations. Voltage-dependent potassium current was marginally affected by high Zn²⁺ concentrations showing no concentration-dependence. Nevertheless, calcium- and voltage-dependent potassium current was drastically depressed in a dose-dependent manner, with an IC₅₀ of 453 μM. This blockade was related to the prevention of Ca²⁺ influx through voltage-dependent calcium channels coupled to BK channels. Under current-clamp conditions, RCCs exhibit a resting potential of -50.7 mV, firing spontaneous APs (1-2 spikes/s) generated by the opening of Na⁺ and Ca²⁺-channels, and terminated by the activation of voltage and Ca²⁺-activated K⁺-channels (BK). We found that the blockade of these ionic currents by Zn²⁺ led to a drastic alteration of cellular excitability with a depolarization of the membrane potential, the slowdown and broadening of the APs and the severe reduction of the after hyperpolarization (AHP) which led to a decrease in the APs firing frequency. Taken together, these results point to a neurotoxic action evoked by zinc that is associated with changes to cellular excitability by blocking the ionic currents responsible for both the neurotransmitter release and the action potentials firing.

Nanoparticles and neurotoxicity: Dual response of glutamatergic receptors

Although the use of nanoparticles for neuro-diagnostic and neurotherapeutic purposes provides superior benefits than the conventional approaches, it may be potentially toxic in central nervous system. In this respect, nanotechnological research focuses on nanoneurotoxicity-nanoneurosafety concepts. Despite these efforts, nanoparticles (NPs) may cause neurotoxicity, neuroinflammation, and neurodegeneration by penetrating the brain-olfactory route and blood-brain barrier (BBB). Indeed, due to their unique structures nanomaterials can easily cross biological barriers, thus avoid drug delivery problems. Despite the advancement of nanotechnology for designing therapeutic agents, toxicity of these nanomaterials is still a concern. Activation of neurons by astrocytic glutamate is a result of NPs-mediated astrocyte-neuron crosstalk. Increased extracellular glutamate levels due to enhanced synthesis and reduced reuptake may induce neuronal damage by abnormal activation of extrasynaptic N-methyl d-aspartate receptor (NMDAR) subunits. NMDAR is the key factor that mediates the disturbances in intracellular calcium homeostasis, mitochondrial dysfunction and generation of reactive oxygen species in NPs exposed neurons. While some NPs cause neuronal death by inducing NMDARs, others may be neurotoxic through the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors or protect the neurons via blocking NMDARs. However, mechanisms of dual effects of NPs, neurotoxicity or neuroprotection are not precisely known. Some NPs present neuroprotective effect either by selectively inhibiting extrasynaptic subunit of NMDARs or by attenuating oxidative stress. NPs-related proinflammatory activation of microglia contributes to the dysfunction and cytotoxicity in neurons. Therefore, investigation of the interaction of NPs with the neuronal signaling molecules and neuronal receptors is necessary for the better understanding of the neurotoxicity or neurosafety of nanomaterials.

Trace elements during primordial plexiform network formation in human cerebral organoids

Systematic studies of micronutrients during brain formation are hindered by restrictions to animal models and adult post-mortem tissues. Recently, advances in stem cell biology have enabled recapitulation of the early stages of human telencephalon development in vitro. In the present work, we analyzed cerebral organoids derived from human pluripotent stem cells by synchrotron radiation X-ray fluorescence in order to measure biologically valuable micronutrients incorporated and distributed into the exogenously developing brain. Our findings indicate that elemental inclusion in organoids is consistent with human brain tissue and involves P, S, K, Ca, Fe and Zn. Occurrence of different concentration gradients also suggests active regulation of elemental transmembrane transport. Finally, the analysis of pairs of elements shows interesting elemental interaction patterns that change from 30 to 45 days of development, suggesting short- or long-term associations, such as storage in similar compartments or relevance for time-dependent biological processes. These findings shed light on which trace elements are important during human brain development and will support studies aimed to unravel the consequences of disrupted metal homeostasis for neurodevelopmental diseases, including those manifested in adulthood.

Zn2+-dependent Activation of the Trk Signaling Pathway Induces Phosphorylation of the Brain-enriched Tyrosine Phosphatase STEP: MOLECULAR BASIS FOR ZN2+-INDUCED ERK MAPK ACTIVATION

Excessive release of Zn(2+) in the brain is implicated in the progression of acute brain injuries. Although several signaling cascades have been reported to be involved in Zn(2+)-induced neurotoxicity, a potential contribution of tyrosine phosphatases in this process has not been well explored. Here we show that exposure to high concentrations of Zn(2+) led to a progressive increase in phosphorylation of the striatal-enriched phosphatase (STEP), a component of the excitotoxic-signaling pathway that plays a role in neuroprotection. Zn(2+)-mediated phosphorylation of STEP61 at multiple sites (hyperphosphorylation) was induced by the up-regulation of brain-derived neurotropic factor (BDNF), tropomyosin receptor kinase (Trk) signaling, and activation of cAMP-dependent PKA (protein kinase A). Mutational studies further show that differential phosphorylation of STEP61 at the PKA sites, Ser-160 and Ser-221 regulates the affinity of STEP61 toward its substrates. Consistent with these findings we also show that BDNF/Trk/PKA mediated signaling is required for Zn(2+)-induced phosphorylation of extracellular regulated kinase 2 (ERK2), a substrate of STEP that is involved in Zn(2+)-dependent neurotoxicity. The strong correlation between the temporal profile of STEP61 hyperphosphorylation and ERK2 phosphorylation indicates that loss of function of STEP61 through phosphorylation is necessary for maintaining sustained ERK2 phosphorylation. This interpretation is further supported by the findings that deletion of the STEP gene led to a rapid and sustained increase in ERK2 phosphorylation within minutes of exposure to Zn(2+). The study provides further insight into the mechanisms of regulation of STEP61 and also offers a molecular basis for the Zn(2+)-induced sustained activation of ERK2.

Relationship between Zinc (Zn (2+) ) and Glutamate Receptors in the Processes Underlying Neurodegeneration

The results from numerous studies have shown that an imbalance between particular neurotransmitters may lead to brain circuit dysfunction and development of many pathological states. The significance of glutamate pathways for the functioning of the nervous system is equivocal. On the one hand, glutamate transmission is necessary for neuroplasticity, synaptogenesis, or cell survival, but on the other hand an excessive and long-lasting increased level of glutamate in the synapse may lead to cell death. Under clinical conditions, hyperactivity of the glutamate system is associated with ischemia, epilepsy, and neurodegenerative diseases such as Alzheimer's, Huntington's, and many others. The achievement of glutamate activity in the physiological range requires efficient control by endogenous regulatory factors. Due to the fact that the free pool of ion Zn(2+) is a cotransmitter in some glutamate neurons; the role of this element in the pathophysiology of a neurodegenerative diseases has been intensively studied. There is a lot of evidence for Zn(2+) dyshomeostasis and glutamate system abnormalities in ischemic and neurodegenerative disorders. However, the precise interaction between Zn(2+) regulative function and the glutamate system is still not fully understood. This review describes the relationship between Zn(2+) and glutamate dependent signaling pathways under selected pathological central nervous system (CNS) conditions.

Zinc-permeable ion channels: effects on intracellular zinc dynamics and potential physiological/pathophysiological significance

Zinc (Zn(2+)) is one of the most important trace metals in the body. It is necessary for the normal function of a large number of protein s including enzymes and transcription factors. While extracellular fluid may contain up to micromolar Zn(2+), intracellular Zn(2+) concentration is generally maintained at a subnanomolar level; this steep gradient across the cell membrane is primarily attributable to Zn(2+) extrusion by Zn(2+) transporting systems. Interestingly, systematic investigation has revealed that activities, previously believed to be dependent on calcium (Ca(2+)), may be partially mediated by Zn(2+). This is also supported by new findings that some Ca(2+)-permeable channels such as voltage-dependent calcium channels (VDCCs), N-methyl-D-aspartate receptors (NMDA), and amino-3- hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPA-Rs) are also permeable to Zn(2+). Thus, the importance of Zn(2+) in physiological and pathophysiological processes is now more widely appreciated. In this review, we describe Zn(2+)- permeable membrane molecules, especially Zn(2+)-permeable ion channels, in intracellular Zn(2+)dynamics and Zn(2+) mediated physiology/pathophysiology.

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

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

Ion channels and zinc: mechanisms of neurotoxicity and neurodegeneration

Ionotropic glutamate receptors, such as NMDA, AMPA and kainate receptors, are ligand-gated ion channels that mediate much of the excitatory neurotransmission in the brain. Not only do these receptors bind glutamate, but they are also regulated by and facilitate the postsynaptic uptake of the trace metal zinc. This paper discusses the role of the excitotoxic influx and accumulation of zinc, the mechanisms responsible for its cytotoxicity, and a number of disorders of the central nervous system that have been linked to these neuronal ion channels and zinc toxicity including ischemic brain injury, traumatic brain injury, and epilepsy.

Zinc in the central nervous system: From molecules to behavior

The trace metal zinc is a biofactor that plays essential roles in the central nervous system across the lifespan from early neonatal brain development through the maintenance of brain function in adults. At the molecular level, zinc regulates gene expression through transcription factor activity and is responsible for the activity of dozens of key enzymes in neuronal metabolism. At the cellular level, zinc is a modulator of synaptic activity and neuronal plasticity in both development and adulthood. Given these key roles, it is not surprising that alterations in brain zinc status have been implicated in a wide array of neurological disorders including impaired brain development, neurodegenerative disorders such as Alzheimer's disease, and mood disorders including depression. Zinc has also been implicated in neuronal damage associated with traumatic brain injury, stroke, and seizure. Understanding the mechanisms that control brain zinc homeostasis is thus critical to the development of preventive and treatment strategies for these and other neurological disorders.

New silver-gold intensification method of diaminobenzidine for double-labeling immunoelectron microscopy

The available methods for double-labeling preembedding immunoelectron microscopy are highly limited because not only should the ultrastructure be preserved, but also the different antigens should be visualized by reaction end products that can be clearly distinguished in gray-scale images. In these procedures, one antigen is detected with 3,3'-diaminobenzidine (DAB) chromogen, resulting in a homogeneous deposit, whereas the other is labeled with either a gold-tagged immunoreagent, or DAB polymer, on the surface of which metallic silver is precipitated. The detection of the second antigen is usually impeded by the first, leading to false-negative results. The authors aimed to diminish this hindrance by a new silver intensification technique of DAB polymer, which converts the deposit from amorphous to granular. The method includes three major postdevelopmental steps: (1) treatment of nickel-enhanced DAB with sulfide, (2) silver deposition in the presence of hydroquinone under acidic conditions, and (3) precious metal replacement with gold thiocyanate. This new sulfide-silver-gold intensification of DAB (SSGI) allows a subsequent detection of other antigens using DAB. In conclusion, the new technique loads fine gold particles onto the DAB deposit at a very low background level, thereby allowing a reliable discernment between the elements stained for the two antigens at the ultrastructural level.

Albumin protects cultured cerebellar granule neurons against zinc neurotoxicity

In this study, we have investigated the role of albumin in zinc-induced neurotoxicity of cultured rat cerebellar granule neurons. ZnCl2 induces cerebellar granule cell death in a time- and concentration-dependent manner and albumin in serum affords a significant protection against zinc-induced neurotoxicity. We further measured intracellular zinc concentrations in the presence or absence of albumin. The results showed that exogenously added ZnCl2 induced an increase in intracellular zinc concentrations in a concentration-dependent fashion in albumin-free buffer. However, in the presence of albumin, it dramatically decreased ZnCl2-induced elevation of intracellular zinc concentrations. These results indicate that albumin protects against zinc-induced neurotoxicity by blocking extracellular zinc uptake by neurons.

Response to kainic acid injections: changes in staining for zinc, FOS, cell death and glial response in the rat forebrain

A pool of zinc is present in synaptic vesicles in a population of glutamatergic neurones. Zinc appears to modulate synaptic transmission and cause neuronal death. The status of vesicular zinc, neuronal death and glial reaction in the rat forebrain was analysed after a systemic injection of kainic acid in order to establish a model for future studies on zinc function. Rats received a systemic injection of kainic acid (10 mg/kg) and were killed 3, 6, 12, 24 and 48 h post-treatment. Timm's method and zinquin staining were used to detect zinc. Immunostaining for Fos-like proteins and staining with Fluoro-Jade B were used to detect cell reaction and degeneration, respectively. Glial fibrillary acidic protein and tomato lectin were used as glial markers. Zinquin staining for zinc rose transitorily in neuronal somata 6 h after injection (not observed at 24-48 h) in the piriform and entorhinal cortices, amygdala and hippocampus. In contrast sulphide/silver staining for zinc showed virtually no rise in cytoplasmic zinc (except in cornus ammonis field 1 of the hippocampus) 6 h after injection, and a decrease (bleaching) in some terminal fields starting 12 h after injection and increasing at 24-48 h. The areas most affected by the zinc bleaching were the olfactory bulb, piriform and entorhinal cortices, endopiriform and amygdaloid nuclei. Transitory Fos immunostaining (within neuronal nuclei) was observed between 3 and 12 h after kainate treatment in many telencephalic areas: olfactory bulb, cortex (piriform, hippocampal and neocortex) and amygdaloid nuclei. This was accompanied by changes in glial markers starting 3 h after injection. Fluoro-Jade B staining in neurones (degeneration) appeared 6 h after treatment and increased later. Degenerating areas generally coincided with those showing Fos immunoreactivity. Zinquin and sulphide/silver methods revealed various pools of zinc after kainate injection: a cytoplasmic pool and a terminal field (or vesicular) pool. Cytoplasmic zinc (zinquin) was coincident, in time and location, with cell degeneration, thus implicating zinc in cell death. This zinc may not have come from presynaptic stores, since no bleaching (sulphide/silver method) was observed 6 h after injection. Future experiments altering zinc pools (e.g. by chelating agents) may elucidate the function of zinc.