Zinc release from thapsigargin/IP3-sensitive stores in cultured cortical neurons

Cytosolic zinc mediates the cytotoxicity of thiol-reactive electrophiles in rat vascular smooth muscle cells

The aberrant increase or dysregulation of cytosolic Zn2+ concentration ([Zn2+]cyt) has been associated with cellular dysfunction and cytotoxicity. In this study, we postulated that Zn2+ mediates the cytotoxicity of thiol-reactive electrophiles. This notion was grounded on earlier research, which revealed that thiol-reactive electrophiles may disrupt Zn2+-binding motifs, consequently causing Zn2+ to be released from Zn2+-binding proteins, and leading to a surge in [Zn2+]cyt. The thiol-reactive electrophiles N-ethylmaleimide (NEM) and diamide were observed to induce an increase in [Zn2+]cyt, possibly through the impairment of Zn2+-binding motifs, and subsequent stimulation of reactive oxygen species (ROS) formation, resulting in cytotoxicity in primary cultured rat vascular smooth muscle cells. These processes were negated by the thiol donor N-acetyl-L-cysteine and the Zn2+ chelator TPEN. Similar outcomes were detected with co-treatment involving Zn2+ and Zn2+ ionophores such as pyrithione or disulfiram. Moreover, TPEN was found to inhibit cytotoxicity triggered by short-term exposure to various thiol-reactive electrophiles including hydrogen peroxide, acrylamide, acrylonitrile, diethyl maleate, iodoacetic acid, and iodoacetamide. In conclusion, our findings suggest that cytosolic Zn2+ acts as a universal mediator in the cytotoxic effects produced by thiol-reactive electrophiles.

Fluorescent Sensor Based on 1H-Pyrazolo[3,4-b]quinoline Derivative for Detecting Zn2+ Cations

The photophysical and sensory properties of the donor-acceptor pyrazoloquinoline derivative (PQPc) were investigated using absorption, steady-state, and time-resolved fluorescence measurements. The compound synthesized from commercial, readily available substrates exhibited absorptions in the UV-Vis range, with a maximum of the longwave band around 390 nm. The maximum fluorescence was around 460-480 nm, depending on the solvent. The quantum yield was between 12.87% (for n-hexane) and 0.75% (for acetonitrile) and decreased with increasing solvent polarity. The PET mechanism was implicated as the cause of fluorescence quenching. Divalent ions such as Zn2+, Pb2+, Cd2+, Ca2+, Mg2+, Co2+, Ni2+, and Cu2+ were introduced to study the fluorescent response of PQPc. A 13-times increase in fluorescence quantum yield was observed after the addition of Zn2+ ions. Detailed research was carried out for the PQPc-Zn2+ system in order to check the possibility of analytical applications of PQPc as a fluorescent sensor. A detection limit of Zn2+ was set at the value level 1.93 × 10-7 M. PQPc-Zn2+ complexes had a stoichiometry of 1:1 with a binding constant of 859 M-1. Biological studies showed that the sensor was localized in cells near the membrane and cytoplasm and may be used to detect zinc ions in eukaryotic cells.

An unconventional gatekeeper mutation sensitizes inositol hexakisphosphate kinases to an allosteric inhibitor

Inositol hexakisphosphate kinases (IP6Ks) are emerging as relevant pharmacological targets because a multitude of disease-related phenotypes has been associated with their function. While the development of potent IP6K inhibitors is gaining momentum, a pharmacological tool to distinguish the mammalian isozymes is still lacking. Here, we implemented an analog-sensitive approach for IP6Ks and performed a high-throughput screen to identify suitable lead compounds. The most promising hit, FMP-201300, exhibited high potency and selectivity toward the unique valine gatekeeper mutants of IP6K1 and IP6K2, compared to the respective wild-type (WT) kinases. Biochemical validation experiments revealed an allosteric mechanism of action that was corroborated by hydrogen deuterium exchange mass spectrometry measurements. The latter analysis suggested that displacement of the αC helix, caused by the gatekeeper mutation, facilitates the binding of FMP-201300 to an allosteric pocket adjacent to the ATP-binding site. FMP-201300 therefore serves as a valuable springboard for the further development of compounds that can selectively target the three mammalian IP6Ks; either as analog-sensitive kinase inhibitors or as an allosteric lead compound for the WT kinases.

Delineating Zinc Influx Mechanisms during Platelet Activation

Zinc (Zn2+) is released by platelets during a hemostatic response to injury. Extracellular zinc ([Zn2+]o) initiates platelet activation following influx into the platelet cytosol. However, the mechanisms that permit Zn2+ influx are unknown. Fluctuations in intracellular zinc ([Zn2+]i) were measured in fluozin-3-loaded platelets using fluorometry and flow cytometry. Platelet activation was assessed using light transmission aggregometry. The detection of phosphoproteins was performed by Western blotting. [Zn2+]o influx and subsequent platelet activation were abrogated by blocking the sodium/calcium exchanged, TRP channels, and ZIP7. Cation store depletion regulated Zn2+ influx. [Zn2+]o stimulation resulted in the phosphorylation of PKC substates, MLC, and β3 integrin. Platelet activation via GPVI or Zn2+ resulted in ZIP7 phosphorylation in a casein kinase 2-dependent manner and initiated elevations of [Zn2+]i that were sensitive to the inhibition of Orai1, ZIP7, or IP3R-mediated pathways. These data indicate that platelets detect and respond to changes in [Zn2+]o via influx into the cytosol through TRP channels and the NCX exchanger. Platelet activation results in the externalization of ZIP7, which further regulates Zn2+ influx. Increases in [Zn2+]i contribute to the activation of cation-dependent enzymes. Sensitivity of Zn2+ influx to thapsigargin indicates a store-operated pathway that we term store-operated Zn2+ entry (SOZE). These mechanisms may affect platelet behavior during thrombosis and hemostasis.

The role of Zn2+ in shaping intracellular Ca2+ dynamics in the heart

Increasing evidence suggests that Zn2+ acts as a second messenger capable of transducing extracellular stimuli into intracellular signaling events. The importance of Zn2+ as a signaling molecule in cardiovascular functioning is gaining traction. In the heart, Zn2+ plays important roles in excitation-contraction (EC) coupling, excitation-transcription coupling, and cardiac ventricular morphogenesis. Zn2+ homeostasis in cardiac tissue is tightly regulated through the action of a combination of transporters, buffers, and sensors. Zn2+ mishandling is a common feature of various cardiovascular diseases. However, the precise mechanisms controlling the intracellular distribution of Zn2+ and its variations during normal cardiac function and during pathological conditions are not fully understood. In this review, we consider the major pathways by which the concentration of intracellular Zn2+ is regulated in the heart, the role of Zn2+ in EC coupling, and discuss how Zn2+ dyshomeostasis resulting from altered expression levels and efficacy of Zn2+ regulatory proteins are key drivers in the progression of cardiac dysfunction.

The IP6K Inhibitor LI-2242 Ameliorates Diet-Induced Obesity, Hyperglycemia, and Hepatic Steatosis in Mice by Improving Cell Metabolism and Insulin Signaling

Obesity and nonalcoholic fatty liver disease (NAFLD) are global health concerns, and thus, drugs for the long-term treatment of these diseases are urgently needed. We previously discovered that the inositol pyrophosphate biosynthetic enzyme IP6K1 is a target in diet-induced obesity (DIO), insulin resistance, and NAFLD. Moreover, high-throughput screening (HTS) assays and structure-activity relationship (SAR) studies identified LI-2242 as a potent IP6K inhibitor compound. Here, we tested the efficacy of LI-2242 in DIO WT C57/BL6J mice. LI-2242 (20 mg/kg/BW daily, i.p.) reduced body weight in DIO mice by specifically reducing the accumulation of body fat. It also improved glycemic parameters and reduced hyperinsulinemia. LI-2242-treated mice displayed reduced the weight of various adipose tissue depots and an increased expression of metabolism- and mitochondrial-energy-oxidation-inducing genes in these tissues. LI-2242 also ameliorated hepatic steatosis by reducing the expression of genes that enhance lipid uptake, lipid stabilization, and lipogenesis. Furthermore, LI-2242 enhances the mitochondrial oxygen consumption rate (OCR) and insulin signaling in adipocytes and hepatocytes in vitro. In conclusion, the pharmacologic inhibition of the inositol pyrophosphate pathway by LI-2242 has therapeutic potential in obesity and NAFLD.

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.

Neuronal signalling of zinc: from detection and modulation to function

Zinc is an essential trace element that stabilizes protein structures and allosterically modulates a plethora of enzymes, ion channels and neurotransmitter receptors. Labile zinc (Zn²⁺) acts as an intracellular and intercellular signalling molecule in response to various stimuli, which is especially important in the central nervous system. Zincergic neurons, characterized by Zn²⁺ deposits in synaptic vesicles and presynaptic Zn²⁺ release, are found in the cortex, hippocampus, amygdala, olfactory bulb and spinal cord. To provide an overview of synaptic Zn²⁺ and intracellular Zn²⁺ signalling in neurons, the present paper summarizes the fluorescent sensors used to detect Zn²⁺ signals, the cellular mechanisms regulating the generation and buffering of Zn²⁺ signals, as well as the current perspectives on their pleiotropic effects on phosphorylation signalling, synapse formation, synaptic plasticity, as well as sensory and cognitive function.

An Endoplasmic Reticulum-Targeted Ratiometric Fluorescent Molecule Reveals Zn2+ Micro-Dynamics During Drug-Induced Organelle Ionic Disorder

The endoplasmic reticulum (ER) is the main storage site of Zn2+, and Zn2+ plays an important role in regulating ER homeostasis. Therefore, we designed and synthesized a ratiometric fluorescent Zn2+ probe ER-Zn targeting ER stress. The probe displayed a specific Zn2+ induced blue shift at the spectral maximum values of excitation (80 nm) and emission (30 nm). The ratio imaging capability of Zn2+ under dual excitation mode can be applied not only to quantitative and reversible detection of exogenous Zn2+, but also the observation of the Zn2+ level change under ER stress, elucidating the different behaviors of Zn2+ release in ER stimulated by tunicamycin and thapsigargin. Additionally, the NIR imaging capability of ER-Zn provides an important basis for further research on animal models and is expected to realize the visualization and treatment of ER stress-related diseases through the regulation of ER stress by Zn2+. We envision that this probe can be applied to screen drugs for diseases related to ER stress regulation.

Trace element homeostasis in the neurological system after SARS-CoV-2 infection: Insight into potential biochemical mechanisms

BACKGROUND

Several studies have suggested that COVID-19 is a systemic disease that can affect several organs, including the brain. In the brain, specifically, viral infection can cause dyshomeostasis of some trace elements that promote complex biochemical reactions in specialized neurological functions.

OBJECTIVE

Understand the neurovirulence of SARS-CoV-2 and the relationship between trace elements and neurological disorders after infection, and provide new insights on the drug development for the treatment of SARS-CoV-2 infections.

METHODS

The main databases were used to search studies published up September 2021, focusing on the role of trace elements during viral infection and on the correct functioning of the brain.

RESULTS

The imbalance of important trace elements can accelerate SARS-CoV-2 neurovirulence and increase the neurotoxicity since many neurological processes can be associated with the homeostasis of metal and metalloproteins. Some studies involving animals and humans have suggested the synapse as a vulnerable region of the brain to neurological disorders after viral infection. Considering the combined evidence, some mechanisms have been suggested to understand the relationship between neurological disorders and imbalance of trace elements in the brain after viral infection.

CONCLUSION

Trace elements play important roles in viral infections, such as helping to activate immune cells, produce antibodies, and inhibit virus replication. However, the relationship between trace elements and virus infections is complex since the specific functions of several elements remain largely undefined. Therefore, there is still a lot to be explored to understand the biochemical mechanisms involved between trace elements and viral infections, especially in the brain.

The Cardiac Ryanodine Receptor Provides a Suitable Pathway for the Rapid Transport of Zinc (Zn2+)

The sarcoplasmic reticulum (SR) in cardiac muscle is suggested to act as a dynamic storage for Zn²⁺ release and reuptake, albeit it is primarily implicated in the Ca²⁺ signaling required for the cardiac cycle. A large Ca²⁺ release from the SR is mediated by the cardiac ryanodine receptor (RYR2), and while this has a prominent conductance for Ca²⁺ in vivo, it also conducts other divalent cations in vitro. Since Zn²⁺ and permeant Mg²⁺ have similar physical properties, we tested if the RYR2 channel also conducts Zn²⁺. Using the method of planar lipid membranes, we evidenced that the RYR2 channel is permeable to Zn²⁺ with a considerable conductance of 81.1 ± 2.4 pS, which was significantly lower than the values for Ca²⁺ (127.5 ± 1.8 pS) and Mg²⁺ (95.3 ± 1.4 pS), obtained under the same asymmetric conditions. Despite similar physical properties, the intrinsic Zn²⁺ permeability (P_Ca/P_Zn = 2.65 ± 0.19) was found to be ~2.3-fold lower than that of Mg²⁺ (P_Ca/P_Mg = 1.146 ± 0.071). Further, we assessed whether the channel itself could be a direct target of the Zn²⁺ current, having the Zn²⁺ finger extended into the cytosolic vestibular portion of the permeation pathway. We attempted to displace Zn²⁺ from the RYR2 Zn²⁺ finger to induce its structural defects, which are associated with RYR2 dysfunction. Zn²⁺ chelators were added to the channel cytosolic side or strongly competing cadmium cations (Cd²⁺) were allowed to permeate the RYR2 channel. Only the Cd²⁺ current was able to cause the decay of channel activity, presumably as a result of Zn²⁺ to Cd²⁺ replacement. Our findings suggest that the RYR2 channel can provide a suitable pathway for rapid Zn²⁺ escape from the cardiac SR; thus, the channel may play a role in local and/or global Zn²⁺ signaling in cardiomyocytes.

Pharmacological tools to investigate inositol polyphosphate kinases - Enzymes of increasing therapeutic relevance

Inositol poly- and pyrophosphates (InsPs and PP-InsPs) are a group of central eukaryotic metabolites and signaling molecules. Due to the diverse cellular functions and widespread diseases InsPs and PP-InsPs are associated with, pharmacological targeting of the kinases involved in their biosynthesis has become a significant research interest in the last decade. In particular, the development of inhibitors for inositol hexakisphosphate kinases (IP6Ks) has leaped forward, while other inositol phosphate kinases have received scant attention. This review summarizes the efforts undertaken so far for discovering potent and selective inhibitors for this diverse group of small molecule kinases. The benefits of pharmacological inhibition are highlighted, given the multiple kinase-independent functions of inositol phosphate kinases. The distinct structural families of InsP and PP-InsP kinases are presented, and we discuss how compound availability for different inositol phosphate kinase families varies drastically. Lead compound discovery and optimization for the inositol kinases would benefit from detailed structural information on the ATP-binding sites of these kinases, as well as reliable biochemical and cellular read-outs to monitor inositol phosphate kinase activity in complex settings. Efforts to further tune well-established inhibitors, while simultaneously reviving tool compound development for the more neglected kinases from this family are indisputably worthwhile, considering the large potential therapeutic benefits.

The change of intracellular zinc distribution after strong acid challenge

Zinc (Zn2+) is stored in the nucleus, endoplasmic reticulum (ER), Golgi apparatus, mitochondria, lysosomes, and zinc-binding proteins. The acidity of the microenvironment affects the binding between zinc and proteins in which zinc become free or loosely bound. In this study, when cells were treated with an acidic medium, we started seeing free zinc 'hot spots' or zincosomes where we found bright zinc fluorescence. The rising free zinc quickly across whole cells with both intensity and distribution were pH-dependent. Interestingly, the nucleus was more sensitive to acidic treatment as the increase of nuclear zinc was faster and higher than the increase of cytosolic zinc. In addition, we re-cultured strong acid-challenged cells in a normal medium. Comparing to the control, these cells exhibited multiple zinc 'hot spots' beside the nucleus, suggesting that free zinc became more extensively distributed. To investigate further the function of zinc in cell shaping and morphological changes, we categorized strong acid-challenged cells into different shapes and found that the proportion of each cell shape had changed after the acid challenge. These acid-induced changes of the cell shape percentage were partially reversed by the reduction of zinc, suggesting that zinc participated in directing the cell shapes and morphologies during cell growth. Our findings reveal that acidic pH affects the dynamics of cellular zinc by making zinc more accessible to cellular compartments and zinc-binding proteins, which provided new insights into understanding the cellular behavior and the function of zinc in it.

Zinc cytotoxicity induces mitochondrial morphology changes in hela cell line

Zinc (Zn²⁺) is important in cellular processes. In the cell, free zinc is tightly regulated and found in minuscule amounts. However, in an unhealthy cellular environment, such as hypoxia, zinc increases in the cell and zinc overload may occur. Studies have shown that zinc overload causes cellular and mitochondrial stress. Mitochondrial stress affects mitochondrial morphology. In normal cells, mitochondrial morphology resembles a long, tubular shape. In unhealthy cells, mitochondrial morphology resembles fragmented, circular shape. To address whether zinc overload contributes directly to the abnormal changes of mitochondrial morphology, we imaged and analyzed mitochondria that were treated with the application of exogenous zinc. In the first part of the study, exogenous zinc was applied to HeLa cells at 1 µM, 10 µM, 50 µM, 100 µM, or 200 µM zinc chloride along with 10 µM pyrithione. Mitochondrial morphology was analyzed with Mito-Morphology micro in ImageJ. Mitochondrial morphology changed from a healthy tubular shape to an unhealthy circular shape and fragmentation. Mitochondrial morphology changes were observed in a dose-dependent fashion. The second part of the study involved applying the metal ion chelator TPEN after applying 50 µM zinc chloride along with 10 µM pyrithione. TPEN reduced zinc-induced abnormal mitochondrial morphology after zinc treatment. This present study supports that zinc overload may cause morphology changes induced by mitochondrial stress that may lead to cell death.

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.

Subcellular localised small molecule fluorescent probes to image mobile Zn2

Zn2+, as the second most abundant d-block metal in the human body, plays an important role in a wide range of biological processes, and the dysfunction of its homeostasis is related to many diseases, including Type 2 diabetes, Alzheimer's disease and prostate and breast cancers. Small molecule fluorescent probes, as effective tools for real-time imaging, have been widely used to study Zn2+ related processes. However, the failure to control their localisation in cells has limited their utility somewhat, as they are generally incapable of studying individual processes in a specific cellular location. This perspective presents an overview of the recent developments in specific organelle localised small molecule fluorescent Zn2+ probes and their application in biological milieu, which could help to extend our understanding of the mechanisms that cells use to respond to dysfunction of zinc homeostasis and its roles in disease initiation and development.

Galvanization of Protein-Protein Interactions in a Dynamic Zinc Interactome

The presence of Zn²⁺ at protein-protein interfaces modulates complex function, stability, and introduces structural flexibility/complexity, chemical selectivity, and reversibility driven in a Zn²⁺-dependent manner. Recent studies have demonstrated that dynamically changing Zn²⁺ affects numerous cellular processes, including protein-protein communication and protein complex assembly. How Zn²⁺-involved protein-protein interactions (ZPPIs) are formed and dissociate and how their stability and reactivity are driven in a zinc interactome remain poorly understood, mostly due to experimental obstacles. Here, we review recent research advances on the role of Zn²⁺ in the formation of interprotein sites, their architecture, function, and stability. Moreover, we underline the importance of zinc networks in intersystemic communication and highlight bioinformatic and experimental challenges required for the identification and investigation of ZPPIs.

Targeting the Inositol Pyrophosphate Biosynthetic Enzymes in Metabolic Diseases

In mammals, a family of three inositol hexakisphosphate kinases (IP6Ks) synthesizes the inositol pyrophosphate 5-IP7 from IP6. Genetic deletion of Ip6k1 protects mice from high fat diet induced obesity, insulin resistance and fatty liver. IP6K1 generated 5-IP7 promotes insulin secretion from pancreatic β-cells, whereas it reduces insulin signaling in metabolic tissues by inhibiting the protein kinase Akt. Thus, IP6K1 promotes high fat diet induced hyperinsulinemia and insulin resistance in mice while its deletion has the opposite effects. IP6K1 also promotes fat accumulation in the adipose tissue by inhibiting the protein kinase AMPK mediated energy expenditure. Genetic deletion of Ip6k3 protects mice from age induced fat accumulation and insulin resistance. Accordingly, the pan IP6K inhibitor TNP [N2-(m-trifluorobenzyl), N6-(p-nitrobenzyl)purine] ameliorates obesity, insulin resistance and fatty liver in diet induced obese mice by improving Akt and AMPK mediated insulin sensitivity and energy expenditure. TNP also protects mice from bone loss, myocardial infarction and ischemia reperfusion injury. Thus, the IP6K pathway is a potential target in obesity and other metabolic diseases. Here, we summarize the studies that established IP6Ks as a potential target in metabolic diseases. Further studies will reveal whether inhibition of this pathway has similar pleiotropic benefits on metabolic health of humans.

Endoplasmic reticulum targeting fluorescent probes to image mobile Zn2

Zn2+ plays an important role in the normal function of the endoplasmic reticulum (ER) and its deficiency can cause ER stress, which is related to a wide range of diseases. In order to provide tools to better understand the role of mobile Zn2+ in ER processes, the first custom designed ER-localised fluorescent Zn2+ probes have been developed through the introduction of a cyclohexyl sulfonylurea as an ER-targeting unit with different Zn2+ receptors. Experiments in vitro and in cellulo show that both probes have a good fluorescence switch on response to Zn2+, high selectivity over other cations, low toxicity, ER-specific targeting ability and are efficacious imaging agents for mobile Zn2+ in four different cell lines. Probe 9 has been used to detect mobile Zn2+ changes under ER stress induced by both tunicamycin or thapsigargin, which indicates that the new probes should allow a better understanding of the mechanisms cells use to respond to dysfunction of zinc homeostasis in the ER and its role in the initiation and progression of diseases to be developed.

The inositol pyrophosphate pathway in health and diseases

Inositol pyrophosphates (IPPs) are present in organisms ranging from plants, slime moulds and fungi to mammals. Distinct classes of kinases generate different forms of energetic diphosphate-containing IPPs from inositol phosphates (IPs). Conversely, polyphosphate phosphohydrolase enzymes dephosphorylate IPPs to regenerate the respective IPs. IPPs and/or their metabolizing enzymes regulate various cell biological processes by modulating many proteins via diverse mechanisms. In the last decade, extensive research has been conducted in mammalian systems, particularly in knockout mouse models of relevant enzymes. Results obtained from these studies suggest impacts of the IPP pathway on organ development, especially of brain and testis. Conversely, deletion of specific enzymes in the pathway protects mice from various diseases such as diet-induced obesity (DIO), type-2 diabetes (T2D), fatty liver, bacterial infection, thromboembolism, cancer metastasis and aging. Furthermore, pharmacological inhibition of the same class of enzymes in mice validates the therapeutic importance of this pathway in cardio-metabolic diseases. This review critically analyses these findings and summarizes the significance of the IPP pathway in mammalian health and diseases. It also evaluates benefits and risks of targeting this pathway in disease therapies. Finally, future directions of mammalian IPP research are discussed.

Cross talk between increased intracellular zinc (Zn2+) and accumulation of reactive oxygen species in chemical ischemia

Both zinc (Zn²⁺) and reactive oxygen species (ROS) have been shown to accumulate during hypoxic-ischemic stress and play important roles in pathological processes. To understand the cross talk between the two of them, here we studied Zn²⁺ and ROS accumulation by employing fluorescent probes in HeLa cells to further the understanding of the cause and effect relationship of these two important cellular signaling systems during chemical-ischemia, stimulated by oxygen and glucose deprivation (OGD). We observed two Zn²⁺ rises that were divided into four phases in the course of 30 min of OGD. The first Zn²⁺ rise was a transient, which was followed by a latent phase during which Zn²⁺ levels recovered; however, levels remained above a basal level in most cells. The final phase was the second Zn²⁺ rise, which reached a sustained plateau called Zn²⁺ overload. Zn²⁺ rises were not observed when Zn²⁺ was removed by TPEN (a Zn²⁺ chelator) or thapsigargin (depleting Zn²⁺ from intracellular stores) treatment, indicating that Zn²⁺ was from intracellular storage. Damaging mitochondria with FCCP significantly reduced the second Zn²⁺ rise, indicating that the mitochondrial Zn²⁺ accumulation contributes to Zn²⁺ overload. We also detected two OGD-induced ROS rises. Two Zn²⁺ rises preceded two ROS rises. Removal of Zn²⁺ reduced or delayed OGD- and FCCP-induced ROS generation, indicating that Zn²⁺ contributes to mitochondrial ROS generation. There was a Zn²⁺-induced increase in the functional component of NADPH oxidase, p47^phox, thus suggesting that NADPH oxidase may mediate Zn²⁺-induced ROS accumulation. We suggest a new mechanism of cross talk between Zn²⁺ and mitochondrial ROS through positive feedback processes that eventually causes excessive free Zn²⁺ and ROS accumulations during the course of ischemic stress.

Effect of tolbutamide on tetraethylammonium-induced postsynaptic zinc signals at hippocampal mossy fiber-CA3 synapses

The application of tetraethylammonium (TEA), a blocker of voltage-dependent potassium channels, can induce long-term potentiation (LTP) in the synaptic systems CA3-CA1 and mossy fiber-CA3 pyramidal cells of the hippocampus. In the mossy fibers, the depolarization evoked by extracellular TEA induces a large amount of glutamate and also of zinc release. It is considered that zinc has a neuromodulatory role at the mossy fiber synapses, which can, at least in part, be due to the activation of presynaptic ATP-dependent potassium (K_ATP) channels. The aim of this work was to study properties of TEA-induced zinc signals, detected at the mossy fiber region, using the permeant form of the zinc indicator Newport Green. The application of TEA caused a depression of those signals that was partially blocked by the K_ATP channel inhibitor tolbutamide. After the removal of TEA, the signals usually increased to a level above baseline. These results are in agreement with the idea that intense zinc release during strong synaptic events triggers a negative feedback action. The zinc depression, caused by the LTP-evoking chemical stimulation, turns into potentiation after TEA washout, suggesting the existence of a correspondence between the observed zinc potentiation and TEA-evoked mossy fiber LTP.

Involvement of intracellular Zn2+ signaling in LTP at perforant pathway-CA1 pyramidal cell synapse

Physiological significance of synaptic Zn²⁺ signaling was examined at perforant pathway-CA1 pyramidal cell synapses. In vivo long-term potentiation (LTP) at perforant pathway-CA1 pyramidal cell synapses was induced using a recording electrode attached to a microdialysis probe and the recording region was locally perfused with artificial cerebrospinal fluid (ACSF) via the microdialysis probe. Perforant pathway LTP was not attenuated under perfusion with CaEDTA (10 mM), an extracellular Zn²⁺ chelator, but attenuated under perfusion with ZnAF-2DA (50 μM), an intracellular Zn²⁺ chelator, suggesting that intracellular Zn²⁺ signaling is required for perforant pathway LTP. Even in rat brain slices bathed in CaEDTA in ACSF, intracellular Zn²⁺ level, which was measured with intracellular ZnAF-2, was increased in the stratum lacunosum-moleculare where perforant pathway-CA1 pyramidal cell synapses were contained after tetanic stimulation. These results suggest that intracellular Zn²⁺ signaling, which originates in internal stores/proteins, is involved in LTP at perforant pathway-CA1 pyramidal cell synapses. Because the influx of extracellular Zn²⁺ , which originates in presynaptic Zn²⁺ release, is involved in LTP at Schaffer collateral-CA1 pyramidal cell synapses, synapse-dependent Zn²⁺ dynamics may be involved in plasticity of postsynaptic CA1 pyramidal cells.

Significance of synaptic Zn2+ signaling in zincergic and non-zincergic synapses in the hippocampus in cognition

A portion of zinc concentrates in the synaptic vesicles in the brain and is released from glutamatergic (zincergic) neuron terminals. It serves as a signaling factor (in a form of free Zn²⁺). Both extracellular Zn²⁺ signaling, which predominantly originates in Zn²⁺ release from zincergic neuron terminals, and intracellular Zn²⁺ signaling, which is often linked to extracellular Zn²⁺ signaling, are involved in hippocampus-dependent memory. At mossy fiber-CA3 pyramidal cell synapses and Schaffer collateral-CA1 pyramidal cell synapses, which are zincergic, extracellular Zn²⁺ signaling leads to intracellular Zn²⁺ signaling and is involved in learning and memory. At medial perforant pathway-dentate granule cell synapses, which are non-zincergic, intracellular Zn²⁺ signaling, which originates in the internal stores containing Zn²⁺, is involved in learning and memory. The blockade of Zn²⁺ signaling with Zn²⁺ chelators induces memory deficit, while the optimal amount range of Zn²⁺ signaling is unknown. It is possible that the degree and frequency of Zn²⁺ signaling, which determine the increased Zn²⁺ levels, modulates learning and memory as well as intracellular Ca²⁺ signaling. To understand the precise role of synaptic Zn²⁺ signaling in the hippocampus, the present paper summarizes the current knowledge on Zn²⁺ signaling at zincergic and non-zincergic synapses in the hippocampus in cognition and involvement of zinc transporters and zinc-binding proteins in synaptic Zn²⁺ signaling.

Towards pharmacological intervention in inositol pyrophosphate signalling

To help define the molecular basis of cellular signalling cascades, and their biological functions, there is considerable value in utilizing a high-quality chemical 'probe' that has a well-defined interaction with a specific cellular protein. Such reagents include inhibitors of protein kinases and small molecule kinases, as well as mimics or antagonists of intracellular signals. The purpose of this review is to consider recent progress and promising future directions for the development of novel molecules that can interrogate and manipulate the cellular actions of inositol pyrophosphates (PP-IPs)--a specialized, 'energetic' group of cell-signalling molecules in which multiple phosphate and diphosphate groups are crammed around a cyclohexane polyol scaffold.

TNP [N2-(m-Trifluorobenzyl), N6-(p-nitrobenzyl)purine] ameliorates diet induced obesity and insulin resistance via inhibition of the IP6K1 pathway

Objective

Obesity and type 2 diabetes (T2D) lead to various life-threatening diseases such as coronary heart disease, stroke, osteoarthritis, asthma, and neurodegeneration. Therefore, extensive research is ongoing to identify novel pathways that can be targeted in obesity/T2D. Deletion of the inositol pyrophosphate (5-IP7) biosynthetic enzyme, inositol hexakisphosphate kinase-1 (IP6K1), protects mice from high fat diet (HFD) induced obesity (DIO) and insulin resistance. Yet, whether this pathway is a valid pharmacologic target in obesity/T2D is not known. Here, we demonstrate that TNP [N2-(m-Trifluorobenzyl), N6-(p-nitrobenzyl)purine], a pan-IP6K inhibitor, has strong anti-obesity and anti-diabetic effects in DIO mice.

Methods

Q-NMR, GTT, ITT, food intake, energy expenditure, QRT-PCR, ELISA, histology, and immunoblot studies were conducted in short (2.5-week)- and long (10-week)-term TNP treated DIO C57/BL6 WT and IP6K1-KO mice, under various diet and temperature conditions.

Results

TNP, when injected at the onset of HFD-feeding, decelerates initiation of DIO and insulin resistance. Moreover, TNP facilitates weight loss and restores metabolic parameters, when given to DIO mice. However, TNP does not reduce weight gain in HFD-fed IP6K1-KO mice. TNP specifically enhances insulin sensitivity in DIO mice via Akt activation. TNP decelerates weight gain primarily by enhancing thermogenic energy expenditure in the adipose tissue. Accordingly, TNP's effect on body weight is partly abolished whereas its impact on glucose homeostasis is preserved at thermoneutral temperature.

Conclusion

Pharmacologic inhibition of the inositol pyrophosphate pathway has strong therapeutic potential in obesity, T2D, and other metabolic diseases.

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Pharmacologic inhibition of IP6K by TNP, at the onset of high fat feeding, decelerates initiation of DIO and insulin resistance in mice.

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TNP, when treated to DIO mice, promotes weight loss and restores metabolic homeostasis.

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TNP does not reduce high fat diet induced weight gain in IP6K1-KO mice.

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TNP promotes insulin sensitivity by stimulating Akt activity, whereas it reduces body weight primarily by enhancing thermogenic energy expenditure.

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Long-term TNP treatment does not display deleterious side effects.

Intracellular zinc distribution in mitochondria, ER and the Golgi apparatus

Zinc (Zn(2+)) is required for numerous cellular functions. As such, the homeostasis and distribution of intracellular zinc can influence cellular metabolism and signaling. However, the exact distribution of free zinc within live cells remains elusive. Previously we showed the release of zinc from thapsigargin/IP3-sensitive endoplasmic reticulum (ER) storage in cortical neurons. In the present study, we investigated if other cellular organelles also contain free chelatable zinc and function as organelle storage for zinc. To identify free zinc within the organelles, live cells were co-stained with Zinpyr-1, a zinc fluorescent dye, and organelle-specific fluorescent dyes (MitoFluor Red 589: mitochondria; ER Tracker Red: endoplasmic reticulum; BODIPY TR ceramide: Golgi apparatus; Syto Red 64: nucleus). We examined organelles that represent potential storing sites for intracellular zinc. We showed that zinc fluorescence staining was co-localized with MitoFluor Red 589, ER Tracker Red, and BODIPY TR ceramide respectively, suggesting the presence of free zinc in mitochondria, endoplasmic reticulum, and the Golgi apparatus. On the other hand, cytosol and nucleus had nearly no detectable zinc fluorescence. It is known that nucleus contains high amount of zinc binding proteins that have high zinc binding affinity. The absence of zinc fluorescence suggests that there is little free zinc in these two regions. It also indicates that the zinc fluorescence detected in mitochondria, ER and Golgi apparatus represents free chelatable zinc. Taken together, our results support that these organelles are potential zinc storing organelles during cellular zinc homeostasis.

Zinc wave during the treatment of hypoxia is required for initial reactive oxygen species activation in mitochondria

Mitochondrial reactive oxygen species (ROS) are known to accumulate during chemical hypoxia, causing adverse effects on cell function and survival. Recent studies show important role zinc accumulation plays in dysfunction associated with hypoxia. It is well known that ROS accumulation also plays a major role in cellular damage by hypoxia. In this study, fluorescent imaging and pharmacological methods were used in live HeLa cells to determine role of zinc in initial ROS accumulation in mitochondria during chemical hypoxia (oxygen glucose depravation with 4 mM sodium dithionite). Accumulation of both was observed as a very rapid phenomenon with initial rapid zinc increase (zinc wave) within 60 seconds of hypoxia onset and ROS increase within 4.5 minutes. Zinc chelation with TPEN removed the initial zinc wave which in turn abolished ROS accumulation. Influx of exogenous zinc induced rapid ROS accumulation. Inhibition of NADPH oxidase with apocynin, a NADPH oxidase inhibitor, showed significant and prolonged reduction in zinc induced ROS accumulation. We proposed a novel mechanism of intracellular zinc increase that activates NADPH oxidase which in turn triggers mitochondrial ROS production.

Newport Green, a fluorescent sensor of weakly bound cellular Zn(2+): competition with proteome for Zn(2)

Newport Green (NPG) is a recognized sensor of cellular Zn(2+) that displays fluorescence enhancement upon binding to Zn(2+). Because of its modest affinity for Zn(2+), the extent of its capacity to bind cellular Zn(2+) is unclear. The present study investigated the range of reactivity of NPG(ESTER) with cells, isolated (Zn)-proteome, and model Zn-proteins. The sensor accumulated in pig kidney LLC-PK1 cells and was slowly (>40 min) hydrolyzed to the fluorescent, acid form, NPG(ACID). The powerful, cell permeant Zn(2+) chelator, N,N,N',N'-tetrakis(2-pyridylmethyl)-ethane-1,2-diamine (TPEN) failed to quench the growing fluorescence emission, indicating that Zn-NPG(ACID) had not formed and NPG-Zn-protein adduct species probably were not present. Furthermore, NPG(ACID) did not bind to Zn-carbonic anhydrase or Zn-alcohol dehydrogenase, two proteins that form adducts with some other sensors. Strikingly, most of the NPG(ACID) that had been converted from NPG(ESTER) was detected in the extracellular medium not the cells. As a result, after cells were incubated with NPG(ESTER) and then Zn-pyrithione to raise the internal concentration of mobile Zn(2+), Zn-NPG(ACID) was only observed in the external medium. Residual cellular NPG(ACID) was unable to bind extra intracellular Zn(2+) delivered by pyrithione. Proteome isolated from the sonicated cell supernatant was also unreactive with NPG(ACID). Titration of proteome or glutathione with Zn(2+) in the presence of NPG(ACID) revealed that NPG(ACID) only weakly competes for mobile Zn(2+) in the presence of these cellular components. In addition, when proteomic Zn(2+) was released by a nitric oxide donor or N-ethyl-maleimide, little Zn(2+) was detected by NPG(ACID). However, exposure to nitric oxide independently enhanced the fluorescence properties of NPG(ACID). Thus, the biochemical properties of NPG related to cellular Zn(2+) chelation deepen the question of how it functions as a Zn(2+) sensor.

Examining a new role for zinc in regulating calcium release in cardiac muscle

It is well established that mammalian cells contain a small but measurable pool of free or labile zinc in the cytosol that is buffered in the high picomolar range. Recent attention has focused on the fact that this pool of free zinc has signalling effects that can be evoked through extracellular stimuli posing the question as to whether zinc should be regarded as a second messenger. Our knowledge of the targets, the biological significance and the molecular mechanisms of zinc signalling is limited but recent evidence suggests that zinc homoeostasis may be intimately linked to intracellular calcium signalling. In this review, we discuss the role of zinc as an intracellular signalling molecule with an emphasis on the potential role of zinc in shaping calcium-dynamics in cardiac muscle. We also consider the evidence that the cardiac ryanodine receptor (RyR2) is a potential zinc signalling target.

Redistribution of subcellular calcium and its effect on apoptosis in primary cultures of rat proximal tubular cells exposed to lead

Previous studies have shown that cytosolic Ca(2+) ([Ca(2+)]c) overload was involved in Pb-induced apoptosis in primary cultures of rat proximal tubular (rPT) cells, but the source of elevated Ca(2+) and the effect of potential subcellular Ca(2+) redistribution on apoptosis are still unknown. In this study, variations of [Ca(2+)]c in two culture media (Ca(2+)-containing and Ca(2+)- free) were analyzed, indicating that Pb-induced elevation of [Ca(2+)]c was primarily generated intracellularly. Fluo-4-AM, dihydro-Rhod-2-AM and Mag-Fluo-4-AM was loaded to Pb-exposed rPT cells to monitor the imaging of Ca(2+) concentrations in the cytoplasm ([Ca(2+)]c), mitochondria ([Ca(2+)]mit) and endoplasmic reticulum (ER) ([Ca(2+)]ER), respectively, under the confocal microscope. Data indicate that elevations of [Ca(2+)]c and [Ca(2+)]mit with depletion of [Ca(2+)]ER were revealed in Pb-treated rPT cells, but this subcellular Ca(2+) redistribution could be significantly suppressed by 2-APB, a specific inhibitor of inositol 1,4,5-trisphosphate receptor (IP3R) that functions to release Ca(2+) from ER stores. Simultaneously, Pb-mediated mitochondrial Ca(2+) overload can be partially suppressed by the cytosolic Ca(2+) chelator BAPTA-AM, suggesting that Ca(2+) uptake into mitochondria occurs via diverse pathways and ER Ca(2+) storage was the chief source. Furthermore, Pb-induced apoptosis was markedly inhibited by 2-APB and BAPTA-AM, respectively. Additionally, elevated IP3 levels with up-regulated IP3R-1 and IP3R-2 (mRNA and protein) levels were revealed in Pb-exposed rPT cells. In summary, IP3R-mediated ER Ca(2+) release promoted the elevations of [Ca(2+)]c and [Ca(2+)]mit in Pb-exposed rPT cells, which played a chief role in apoptosis induced by impaired calcium homeostasis.

Zinc: indications in brain disorders

Zinc is the authoritative metal which is present in our body, and reactive zinc metal is crucial for neuronal signaling and is largely distributed within presynaptic vesicles. Zinc also plays an important role in synaptic function. At cellular level, zinc is a modulator of synaptic activity and neuronal plasticity in both development and adulthood. Different importers and transporters are involved in zinc homeostasis. ZnT-3 is a main transporter involved in zinc homeostasis in the brain. It has been found that alterations in brain zinc status have been implicated in a wide range of neurological disorders including impaired brain development and many neurodegenerative disorders such as Alzheimer's disease, and mood disorders including depression, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and prion disease. Furthermore, 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.

Caging metal ions with visible light-responsive nanopolymersomes

Polymersomes are bilayer vesicles that self-assemble from amphiphilic diblock copolymers, and provide an attractive system for the delivery of biological and nonbiological molecules due to their environmental compatibility, mechanical stability, synthetic tunability, large aqueous core, and hyperthick hydrophobic membrane. Herein, we report a nanoscale photoresponsive polymersome system featuring a meso-to-meso ethyne-bridged bis[(porphinato)zinc] (PZn2) fluorophore hydrophobic membrane solute and dextran in the aqueous core. Upon 488 nm irradiation in solution or in microinjected zebrafish embryos, the polymersomes underwent deformation, as monitored by a characteristic red-shifted PZn2 emission spectrum and confirmed by cryo-TEM. The versatility of this system was demonstrated through the encapsulation and photorelease of a fluorophore (FITC), as well as two different metal ions, Zn(2+) and Ca(2+).

Mitochondrial and ER-targeted eCALWY probes reveal high levels of free Zn2+

Zinc (Zn2+) ions are increasingly recognized as playing an important role in cellular physiology. Whereas the free Zn2+ concentration in the cytosol has been established to be 0.1-1 nM, the free Zn2+ concentration in subcellular organelles is not well-established. Here, we extend the eCALWY family of genetically encoded Förster Resonance Energy Transfer (FRET) Zn2+ probes to permit measurements in the endo(sarco)plasmic reticulum (ER) and mitochondrial matrix. Deployed in a variety of mammalian cell types, these probes reveal resting mitochondrial free [Zn2+] values of ∼300 pM, somewhat lower than in the cytosol but 3 orders of magnitude higher than recently reported using an alternative FRET-based sensor. By contrast, free ER [Zn2+] was found to be ≥5 nM, which is >5000-fold higher than recently reported but consistent with the proposed role of the ER as a mobilizable Zn2+ store. Treatment of β-cells or cardiomyocytes with sarco(endo)plasmic reticulum Ca2+-ATPase inhibitors, mobilization of ER Ca2+ after purinergic stimulation with ATP, or manipulation of ER redox, exerted no detectable effects on [Zn2+]ER. These findings question the previously proposed role of Ca2+ in Zn2+ mobilization from the ER and suggest that high ER Zn2+ levels may be an important aspect of cellular homeostasis.

TMC8 (EVER2) attenuates intracellular signaling by Zn2+ and Ca2+ and suppresses activation of Cl- currents

Eight paralogue members form the family of transmembrane channel-like (TMC) proteins that share considerable sequence homology to anoctamin 1 (Ano1, TMEM16A). Ano1 is a Ca(2+) activated Cl(-) channel that is related to head and neck cancer, often caused by human papilloma virus (HPV) infection. Mutations in TMC 6 and 8 (EVER1, EVER2) cause epidermodysplasia verruciformis. This rare skin disease is characterized by abnormal susceptibility to HPV infection and cancer. We found that in contrast to Ano1 the common paralogues TMC4-TMC8 did not produce Ca(2+) activated Cl(-) currents when expressed in HEK293 cells. On the contrary, TMC8 was found to be localized in the endoplasmic reticulum (ER), where it inhibited receptor mediated Ca(2+) release, activation of Ano1 and volume regulated LRRC8-related Cl(-) currents. Zn(2+) is co-released from the ER together with Ca(2+) and thereby further augments Ca(2+) store release. Because TMC8 is required to lower cytosolic Zn(2+) concentrations by the Zn(2+) transporter ZnT-1, we hypothesize that HPV infections and cancer caused by mutations in TMC8 are related to upregulated Zn(2+)/Ca(2+) signaling and activation of Ano1.

Dopamine- and zinc-induced autophagosome formation facilitates PC12 cell survival

Dopamine oxidation and divalent cations have been reported to induce neuronal cell death. Although autophagy is involved in neuronal cell death, it has also been suggested to facilitate cell survival. We sought to investigate the role of autophagy in PC12 cells and cultured neurons treated with dopamine and Zn2+. Cells expressing EGFP-LC3 were treated with high concentrations of dopamine and Zn2+, and the formation of EGFP-LC3 fluorescence aggregates was monitored. Our results showed a significant increase in the number of fluorescent puncta in the cytosol of PC12 cells treated with these chemicals. These treatments enhanced LC3 lipidation levels in PC12 cells. Decreasing the ATG7 protein level using specific small interference RNA (siRNA) and pretreating with phosphatidylinositol 3-phosphate kinase blockers, wortmannin and LY294002, inhibited puncta formation. Dopamine or Zn2+ treatment significantly elevated the intracellular Zn2+ concentration ([Zn2+] i ); however, inhibiting the [Zn2+] i elevation in dopamine-treated cells suppressed the puncta formation. LY294002 or siRNA-directed members of the autophagy pathway increased the fraction of phosphatidylserine present on the outer membrane leaflet in PC12 cells treated with dopamine or Zn2+, suggesting an increase in apoptosis. Primary embryonic midbrain neurons expressing EGFP-LC3 also displayed a significant increase in the number of fluorescent aggregates in cells upon treatment with dopamine or Zn2+. Dopamine or Zn2+ treatment significantly elevated the [Zn2+] i in neurons and caused neuronal death. Our results indicate that treating cells with dopamine and Zn2+ results in the activation of the autophagy pathway in an effort to enhance cell survival.

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.

Intracellular Zn(2+) signaling in cognition

Brain zinc homeostasis is strictly controlled under healthy conditions, indicating the importance of zinc for physiological function in the brain. A part of zinc in the brain exists in the synaptic vesicles, is released from a subclass of glutamatergic neurons (i.e., zincergic neurons), and serves as a signal factor (Zn(2+) signal) in the intracellular (cytosol) compartment as well as in the extracellular compartment. Zn(2+) signaling is dynamically linked to glutamate signaling and may be involved in synaptic plasticity, such as long-term potentiaion and cognitive activity. In zincergic synapses, intracellular Zn(2+) signaling in the postsynaptic neurons, which is linked to Zn(2+) release from zincergic neuron terminals, plays a role in cognitive activity. When nonzincergic synapses participate in cognition, on the other hand, it is possible that intracellular Zn(2+) signaling, which is due mainly to Zn(2+) release from the internal stores and/or metallothioneins, also is involved in cognitive activity, because zinc-dependent system such as zinc-binding proteins is usually required for cognitive process. Intracellular Zn(2+) dynamics may be modified via an endocrine system activity, glucocorticoid secretion in both zincergic and nonzincergic neurons, which is linked to a long-lasting change in synaptic efficacy. On the basis of the evidence of cognitive decline caused by the lack and/or the blockade of synaptic Zn(2+) signaling, this article summarizes the involvement of intracellular Zn(2+) signaling in zincergic synapses in cognition and a hypothetical involvement of that in nonzincergic synapses.

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.

Cognitive decline due to excess synaptic Zn(2+) signaling in the hippocampus

Zinc is an essential component of physiological brain function. Vesicular zinc is released from glutamatergic (zincergic) neuron terminals and serves as a signal factor (Zn²⁺ signal) in both the intracellular (cytosol) compartment and the extracellular compartment. Synaptic Zn²⁺ signaling is dynamically linked to neurotransmission and is involved in processes of synaptic plasticity such as long-term potentiation and cognitive activity. On the other hand, the activity of the hypothalamic–pituitary–adrenal (HPA) axis, i.e., glucocorticoid secretion, which can potentiate glutamatergic neuron activity, is linked to cognitive function. HPA axis activity modifies synaptic Zn²⁺ dynamics at zincergic synapses. An increase in HPA axis activity, which occurs after exposure to stress, may induce excess intracellular Zn²⁺ signaling in the hippocampus, followed by hippocampus-dependent memory deficit. Excessive excitation of zincergic neurons in the hippocampus can contribute to cognitive decline under stressful and/or pathological conditions. This paper provides an overview of the ``Hypothesis and Theory'' of Zn²⁺-mediated modification of cognitive activity.

Neuroprotective effect of zinc chelator DEDTC in a zebrafish (Danio rerio) Model of Hypoxic Brain Injury

A study was conducted using zebrafish as a model of hypoxic brain injury to investigate the potential neuroprotective effects of zinc (Zn(2+)) chelation. The accumulation of intracellular Zn(2+) is a significant causal factor of the neuronal injury, and has been implicated in cell death followed by ischemic stroke. In this study, the zebrafish was placed in the hypoxia chamber with an extremely low level of dissolved oxygen (less than 0.8 mg/L), which is similar to the conditions in a complete global ischemic stroke. Approximately 50% of zebrafish died after a short period (≈11 min) of hypoxic treatment, suggesting that this is a responsive model system for use in evaluating treatments for hypoxic brain damage. The application of DEDTC reduced intracellular Zn(2+) accumulation and produced a concentration-dependent effect by increasing the survival rate of zebrafish. Zn(2+) chelation also enhanced zebrafish tolerance for hypoxia. When the brain damages were evaluated with TTC staining, the zebrafish that were treated with DEDTC in hypoxic treatment yielded the improvement of TTC staining that was similar to the healthy zebrafish brain. The results support that rising intracellular Zn(2+) plays a critical role in the neuronal damages, and demonstrate the protective effects of Zn(2+) chelation in hypoxic-ischemic brain injury in zebrafish.

Rising intracellular zinc by membrane depolarization and glucose in insulin-secreting clonal HIT-T15 beta cells

Zinc (Zn²⁺) appears to be intimately involved in insulin metabolism since insulin secretion is correlated with zinc secretion in response to glucose stimulation, but little is known about the regulation of zinc homeostasis in pancreatic beta-cells. This study set out to identify the intracellular zinc transient by imaging free cytosolic zinc in HIT-T15 beta-cells with fluorescent zinc indicators. We observed that membrane depolarization by KCl (30–60 mM) was able to induce a rapid increase in cytosolic concentration of zinc. Multiple zinc transients of similar magnitude were elicited during repeated stimulations. The amplitude of zinc responses was not affected by the removal of extracellular calcium or zinc. However, the half-time of the rising slope was significantly slower after removing extracellular zinc with zinc chelator CaEDTA, suggesting that extracellular zinc affect the initial rising phase of zinc response. Glucose (10 mM) induced substantial and progressive increases in intracellular zinc concentration in a similar way as KCl, with variation in the onset and the duration of zinc mobilization. It is known that the depolarization of beta-cell membrane is coupled with the secretion of insulin. Rising intracellular zinc concentration may act as a critical signaling factor in insulin metabolism of pancreatic beta-cells.

Zinc enhances adiponectin oligomerization to octadecamers but decreases the rate of disulfide bond formation

Adiponectin, a hormone secreted from adipocytes, has been shown to protect against development of insulin resistance, ischemia-reperfusion injury, and inflammation. Adiponectin assembles into multiple oligomeric isoforms: trimers, hexamers and several higher molecular weight (HMW) species. Of these, the HMW species are selectively decreased during the onset of type 2 diabetes. Despite the critical role of HMW adiponectin in insulin responsiveness, its assembly process is poorly understood. In this report, we investigated the role of divalent cations in adiponectin assembly. Purified adiponectin 18mers, the largest HMW species, did not collapse to smaller oligomers after treatment with high concentrations of EDTA. However, treatment with EDTA or another chelator DTPA inhibited the oligomerization of 18mers from trimers in vitro. Zn(2+) specifically increased the formation of 18mers when compared with Cu(2+), Mg(2+), and Ca(2+). Distribution of adiponectin oligomers secreted from zinc chelator TPEN-treated rat adipocytes skewed toward increased proportions of hexamers and trimers. While we observed presence of zinc in adiponectin purified from calf serum, the role of zinc in disulfide bonding between oligomers was examined because the process is critical for 18mer assembly. Surprisingly, Zn(2+) inhibited disulfide bond formation early in the oligomerization process. We hypothesize that initial decreases in disulfide formation rates could allow adiponectin subunits to associate before becoming locked in fully oxidized conformations incapable of further oligomerization. These data demonstrate that zinc stimulates oligomerization of HMW adiponectin and possibly other disulfide-dependent protein assembly processes.

Insight into glutamate excitotoxicity from synaptic zinc homeostasis

Zinc is released from glutamatergic (zincergic) neuron terminals in the hippocampus, followed by the increase in Zn(2+) concentration in the intracellular (cytosol) compartment, as well as that in the extracellular compartment. The increase in Zn(2+) concentration in the intracellular compartment during synaptic excitation is mainly due to Zn(2+) influx through calcium-permeable channels and serves as Zn(2+) signaling as well as the case in the extracellular compartment. Synaptic Zn(2+) homeostasis is important for glutamate signaling and altered under numerous pathological processes such as Alzheimer's disease. Synaptic Zn(2+) homeostasis might be altered in old age, and this alteration might be involved in the pathogenesis and progression of Alzheimer's disease; Zinc may play as a key-mediating factor in the pathophysiology of Alzheimer's disease. This paper summarizes the role of Zn(2+) signaling in glutamate excitotoxicity, which is involved in Alzheimer's disease, to understand the significance of synaptic Zn(2+) homeostasis in the pathophysiology of Alzheimer's disease.