Inhibitory mechanism of store-operated Ca2+ channels by zinc

Zinc-sensing receptor activation induces endothelium-dependent hyperpolarization-mediated vasorelaxation of arterioles

BACKGROUND

The micronutrient zinc (Zn2+) is critical for cell function as intracellular signaling and endogenous ligand for Zn2+ sensing receptor (ZnR). Although cytosolic Zn2+ (cyt) signaling in the vascular system was studied previously, role of the ZnR has not been explored in vascular physiology.

METHODS

ZnR-mediated relaxation response of human submucosal arterioles and the mesenteric arterioles from wide-type (WT), ZnR-/- and TRPV4-/- mice were determined by a Mulvany-style wire myograph. The perfused vessel density (PVD) of mouse mesenteric arterioles was also measured in in vivo study. The expression of ZnR in arterioles and vascular endothelial cells (VEC) were examined by immunofluorescence staining, and its function was characterized in VEC by Ca2+ imaging and patch clamp study.

RESULTS

ZnR expression was detected on human submucosal arterioles, murine mesenteric arterioles and VEC but not in ZnR-/- mice. ZnR activation predominately induced endothelium-dependent hyperpolarization (EDH)-mediated vasorelaxation of arterioles in vitro and in vivo via Ca2+ signaling, which is totally different from endothelium-dependent vasorelaxation via Zn2+ (cyt) signaling reported previously. Furthermore, ZnR-induced vasorelaxation via EDH was significantly impaired in ZnR-/- and TRPV4-/- mice. Mechanistically, ZnR induced endothelium-dependent vasorelaxation predominately via PLC/IP3/IP3R and TRPV4/SOCE. The role of ZnR in regulating Ca2+ signaling and ion channels on VEC was verified by Ca2+ imaging and patch clamp techniques.

CONCLUSION

ZnR activation induces endothelium-dependent vasorelaxation of resistance vessels predominately via TRPV4/Ca2+/EDH pathway. We therefore not only provide new insights into physiological role of ZnR in vascular system but also may pave a potential pathway for developing Zn2+-based treatments for vascular disease.

Calcium's Role and Signaling in Aging Muscle, Cellular Senescence, and Mineral Interactions

Calcium research, since its pivotal discovery in the early 1800s through the heating of limestone, has led to the identification of its multi-functional roles. These include its functions as a reducing agent in chemical processes, structural properties in shells and bones, and significant role in cells relating to this review: cellular signaling. Calcium signaling involves the movement of calcium ions within or between cells, which can affect the electrochemical gradients between intra- and extracellular membranes, ligand binding, enzyme activity, and other mechanisms that determine cell fate. Calcium signaling in muscle, as elucidated by the sliding filament model, plays a significant role in muscle contraction. However, as organisms age, alterations occur within muscle tissue. These changes include sarcopenia, loss of neuromuscular junctions, and changes in mineral concentration, all of which have implications for calcium's role. Additionally, a field of study that has gained recent attention, cellular senescence, is associated with aging and disturbed calcium homeostasis, and is thought to affect sarcopenia progression. Changes seen in calcium upon aging may also be influenced by its crosstalk with other minerals such as iron and zinc. This review investigates the role of calcium signaling in aging muscle and cellular senescence. We also aim to elucidate the interactions among calcium, iron, and zinc across various cells and conditions, ultimately deepening our understanding of calcium signaling in muscle aging.

The ins and outs of mammary gland calcium and zinc transport: A brief review

Milk is an excellent source of all macrominerals and trace elements, which are essential for proper function of a wide variety of vital processes. The concentrations of minerals in milk are influenced by numerous factors, including stage of lactation, time of day, nutritional and health status of the mother, as well as maternal genotype and environmental exposures. Additionally, tight regulation of mineral transport within the secretory mammary epithelial cell itself is critical for the production and secretion of milk. In this brief review, we focus on the current understanding of how the essential divalent cations calcium (Ca) and zinc (Zn) are transported in the mammary gland (MG) with a focus on molecular regulation and the consequence of genotype. A deeper grasp of mechanisms and factors affecting Ca and Zn transport in the MG is important to understanding milk production, mineral output, and MG health to inform intervention design and novel diagnostic and therapeutic strategies in production animals and humans.

Zinc Signaling in the Mammary Gland: For Better and for Worse

Zinc (Zn²⁺) plays an essential role in epithelial physiology. Among its many effects, most prominent is its action to accelerate cell proliferation, thereby modulating wound healing. It also mediates affects in the gastrointestinal system, in the testes, and in secretory organs, including the pancreas, salivary, and prostate glands. On the cellular level, Zn²⁺ is involved in protein folding, DNA, and RNA synthesis, and in the function of numerous enzymes. In the mammary gland, Zn²⁺ accumulation in maternal milk is essential for supporting infant growth during the neonatal period. Importantly, Zn²⁺ signaling also has direct roles in controlling mammary gland development or, alternatively, involution. During breast cancer progression, accumulation or redistribution of Zn²⁺ occurs in the mammary gland, with aberrant Zn²⁺ signaling observed in the malignant cells. Here, we review the current understanding of the role of in Zn²⁺ the mammary gland, and the proteins controlling cellular Zn²⁺ homeostasis and signaling, including Zn²⁺ transporters and the Gq-coupled Zn²⁺ sensing receptor, ZnR/GPR39. Significant advances in our understanding of Zn²⁺ signaling in the normal mammary gland as well as in the context of breast cancer provides new avenues for identification of specific targets for breast cancer therapy.

Dietary and Physiological Effects of Zinc on the Immune System

Evidence for the importance of zinc for all immune cells and for mounting an efficient and balanced immune response to various environmental stressors has been accumulating in recent years. This article describes the role of zinc in fundamental biological processes and summarizes our current knowledge of zinc's effect on hematopoiesis, including differentiation into immune cell subtypes. In addition, the important role of zinc during activation and function of immune cells is detailed and associated with the specific immune responses to bacteria, parasites, and viruses. The association of zinc with autoimmune reactions and cancers as diseases with increased or decreased immune responses is also discussed. This article provides a broad overview of the manifold roles that zinc, or its deficiency, plays in physiology and during various diseases. Consequently, we discuss why zinc supplementation should be considered, especially for people at risk of deficiency. Expected final online publication date for the Annual Review of Nutrition, Volume 41 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Zinc transporters and their functional integration in mammalian cells

Zinc is a ubiquitous biological metal in all living organisms. The spatiotemporal zinc dynamics in cells provide crucial cellular signaling opportunities, but also challenges for intracellular zinc homeostasis with broad disease implications. Zinc transporters play a central role in regulating cellular zinc balance and subcellular zinc distributions. The discoveries of two complementary families of mammalian zinc transporters (ZnTs and ZIPs) in the mid-1990s spurred much speculation on their metal selectivity and cellular functions. After two decades of research, we have arrived at a biochemical description of zinc transport. However, in vitro functions are fundamentally different from those in living cells, where mammalian zinc transporters are directed to specific subcellular locations, engaged in dedicated macromolecular machineries, and connected with diverse cellular processes. Hence, the molecular functions of individual zinc transporters are reshaped and deeply integrated in cells to promote the utilization of zinc chemistry to perform enzymatic reactions, tune cellular responsiveness to pathophysiologic signals, and safeguard cellular homeostasis. At present, the underlying mechanisms driving the functional integration of mammalian zinc transporters are largely unknown. This knowledge gap has motivated a shift of the research focus from in vitro studies of purified zinc transporters to in cell studies of mammalian zinc transporters in the context of their subcellular locations and protein interactions. In this review, we will outline how knowledge of zinc transporters has been accumulated from in-test-tube to in-cell studies, highlighting new insights and paradigm shifts in our understanding of the molecular and cellular basis of mammalian zinc transporter functions.

Differential signaling patterns of stimulated bone marrow-derived dendritic cells under α1-antitrypsin-enriched conditions

Dendritic cells (DCs) mature upon an inflammatory trigger. However, an inflammatory trigger can lead to a semi-mature phenotype, allowing DCs to evoke tolerance and expedite the resolution of inflammation. This duality likely involves context-dependent modulation of inflammatory signaling. Human α1-antitrypsin (hAAT) promotes semimature DCs. We examined changes in a wide spectrum of signaling cascades in stimulated murine bone marrow-derived cells with hAAT. Upon stimulation by IL-1β+IFNγ, hAAT-treated cells depicted an attenuated calcium flux. Disrupting PKA or NF-κB pathways revoked only some hAAT-mediated outcomes. hAAT-treated cells exhibited a distict pattern of kinase phosphorylation. hAAT-mediated increase in Treg cells in-vitro required intact inflammatory signaling pathways. Taken together, hAAT appears to require a stimulated microenvironment to promote inflammatory resolution, setting it aside from classical anti-inflammatory agents. Further studies are required to identify the specific molecules targeted by hAAT that mediate these and other outcomes.

Excitotoxicity: Still Hammering the Ischemic Brain in 2020

Interest in excitotoxicity expanded following its implication in the pathogenesis of ischemic brain injury in the 1980s, but waned subsequent to the failure of N-methyl-D-aspartate (NMDA) antagonists in high profile clinical stroke trials. Nonetheless there has been steady progress in elucidating underlying mechanisms. This review will outline the historical path to current understandings of excitotoxicity in the ischemic brain, and suggest that this knowledge should be leveraged now to develop neuroprotective treatments for stroke.

Zn2+ ions inhibit gene transcription following stimulation of the Ca2+ channels Cav1.2 and TRPM3

Zinc, a trace element, is necessary for the correct structure and function of many proteins. Therefore, Zn²⁺ has to be taken up by the cells, using specific Zn²⁺ transporters or Ca²⁺ channels. In this study, we have focused on two Ca²⁺ channels, the L-type voltage-gated Ca_v1.2 channel and the transient receptor potential channel TRPM3. Stimulation of either channel induces an intracellular signaling cascade leading to the activation of the transcription factor AP-1. The influx of Ca²⁺ ions into the cytoplasm is essential for this activity. We asked whether extracellular Zn²⁺ ions affect Ca_v1.2 or TRPM3-induced gene transcription following stimulation of the channels. The results show that extracellular Zn²⁺ ions reduced the activation of AP-1 by more than 80% following stimulation of either voltage-gated Ca_v1.2 channels or TRPM3 channels. Experiments performed with cells maintained in Ca²⁺-free medium revealed that Zn²⁺ ions cannot replace Ca²⁺ ions in inducing gene transcription via stimulation of Ca_v1.2 and TRPM3 channels. Re-addition of Ca²⁺ ions to the cell culture medium, however, restored the ability of these Ca²⁺ channels to induce a signaling cascade leading to the activation of AP-1. Secretory cells, including neurons and pancreatic β-cells, release Zn²⁺ ions during exocytosis. We propose that the released Zn²⁺ ions function as a negative feedback loop for stimulus-induced exocytosis by inhibiting Ca²⁺ channel signaling.

Fine-Tuning of GPCR-Chemokine Interactions. Design and Identification of Chemokine Analogues as Receptor Agonists, Biased Agonists, and Antagonists

Chemokines play important roles in immune defense by directing migration of leukocytes and serve as key promoters of tumorigenesis and metastasis. This study explores the molecular mechanisms of recognition and activation of two homologous chemokine receptors, CXCR1 and CXCR2, using CXCL8 analogues with residue substitutions in the conserved Glu4Leu5Arg6 (ELR) triad. Analysis of the binding of CXCL8 analogues to CXCR1 is consistent with the two-site model for signal recognition of CXCR1, whereas analysis of the binding of CXCL8 analogues to CXCR2 supported a single-site model for signal recognition of CXCR2. The CXCL8-Arg6His analogue stimulated calcium release, phosphorylation of ERK1/2, and chemotaxis in cells expressing CXCR1. However, CXCL8-Arg6His failed to stimulate calcium release and chemotaxis in cells expressing CXCR2, although it stimulated phosphorylation of ERK1/2, indicating that CXCL8-Arg6His operated as a classical CXCR2 biased agonist. The CXCL8-Glu4AlaLeu5AlaArg6His analogue was inactive in cells expressing CXCR1 and CXCR2. These findings suggest that the Glu4Leu5 motif in CXCL8 is essential for activation of CXCR1 and CXCR2. Importantly, CXCL8-Glu4AlaLeu5AlaArg6His blocked specifically the calcium release and chemotaxis of cells expressing CXCR1 but not of cells expressing CXCR2. CXCL8-Glu4AlaLeu5AlaArg6His was identified as the first specific CXCR1 antagonist. The binding of CXCL8-ELR6H to CXCR1 created a Zn²⁺ coordination site at the receptor activation domain responsible for calcium release, as ZnCl₂ specifically blocked CXCL8-Arg6His-induced calcium release without affecting CXCL8-induced calcium release. This work provides the basis for further exploration of the activation mechanisms of chemokine receptors and will assist in the design of the next generation of modulators of CXCR1 and CXCR2.

The Zinc Sensing Receptor, ZnR/GPR39, in Health and Disease

A distinct G-protein coupled receptor that senses changes in extracellular Zn²⁺, ZnR/GPR39, was found in cells from tissues in which Zn²⁺ plays a physiological role. Most prominently, ZnR/GPR39 activity was described in prostate cancer, skin keratinocytes, and colon epithelial cells, where zinc is essential for cell growth, wound closure, and barrier formation. ZnR/GPR39 activity was also described in neurons that are postsynaptic to vesicular Zn²⁺ release. Activation of ZnR/GPR39 triggers Gαq-dependent signaling and subsequent cellular pathways associated with cell growth and survival. Furthermore, ZnR/GPR39 was shown to regulate the activity of ion transport mechanisms that are essential for the physiological function of epithelial and neuronal cells. Thus, ZnR/GPR39 provides a unique target for therapeutically modifying the actions of zinc in a specific and selective manner.

Selective inhibitory effects of zinc on cell proliferation in esophageal squamous cell carcinoma through Orai1

Zinc, an essential micronutrient, has a cancer preventive role. Zinc deficiency has been shown to contribute to the progression of esophageal cancer. Orai1, a store-operated Ca²⁺ entry (SOCE) channel, was previously reported to be highly expressed in tumor tissues removed from patients with esophageal squamous cell carcinoma (ESCC) with poor prognosis, and elevation of its expression contributes to both hyperactive intracellular Ca²⁺ oscillations and fast cell proliferation in human ESCC cells. However, the molecular basis of cancer preventive functions of zinc and its association with Orai1-mediated cell proliferation remains unknown. The present study shows that zinc supplementation significantly inhibits proliferation of ESCC cell lines and that the effect of zinc is reversible with N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine, a specific Zn²⁺ chelator, whereas nontumorigenic esophageal epithelial cells are significantly less sensitive to zinc treatment. Fluorescence live cell imaging revealed that extracellular Zn²⁺ exerted rapid inhibitory effects on Orai1-mediated SOCE and on intracellular Ca²⁺ oscillations in the ESCC cells. Knockdown of Orai1 or expression of Orai1 mutants with compromised zinc binding significantly diminished sensitivity of the cancer cells to zinc treatment in both SOCE and cell proliferation analyses. These data suggest that zinc may inhibit cell proliferation of esophageal cancer cells through Orai1-mediated intracellular Ca²⁺ oscillations and reveal a possible molecular basis for zinc-induced cancer prevention and Orai1-SOCE signaling pathway in cancer cells.-Choi, S., Cui, C., Luo, Y., Kim, S.-H., Ko, J.-K., Huo, X., Ma, J., Fu, L.-W., Souza, R. F., Korichneva, I., Pan, Z. Selective inhibitory effects of zinc on cell proliferation in esophageal squamous cell carcinoma through Orai1.

The direct modulatory activity of zinc toward ion channels

The divalent zinc ion is a cation that plays an indispensable role as a structural constituent of numerous proteins, including enzymes and transcription factors. Recently, it has been suggested that zinc also plays a dynamic role in extracellular and intracellular signaling as well. Ion channels are pore-forming proteins that control the flow of specific ions across the membrane, which is important to maintain ion gradients. In this review, we outline the modulatory effect of zinc on the activities of several ion channels through direct binding of zinc into histidine, cysteine, aspartate, and glutamate moieties of channel proteins. The binding of zinc to ion channels results in the activation or inhibition of the channel due to conformational changes. These novel aspects of ion-channel activity modulation by zinc provide new insights into the physiological regulation of ion channels.

Zn2+-stimulation of sperm capacitation and of the acrosome reaction is mediated by EGFR activation

Extracellular zinc regulates cell proliferation via the MAP1 kinase pathway in several cell types, and has been shown to act as a signaling molecule. The testis contains a relatively high concentration of Zn(2+), required in both the early and late stages of spermatogenesis. Despite the clinical significance of this ion, its role in mature sperm cells is poorly understood. In this study, we characterized the role of Zn(2+) in sperm capacitation and in the acrosome reaction. Western blot analysis revealed the presence of ZnR of the GPR39 type in sperm cells. We previously demonstrated the presence of active epidermal growth factor receptor (EGFR) in sperm, its possible transactivation by direct activation of G-protein coupled receptor (GPCR), and its involvement in sperm capacitation and in the acrosome reaction (AR). We show here that Zn(2+) activates the EGFR during sperm capacitation, which is mediated by activation of trans-membrane adenylyl cyclase (tmAC), protein kinase A (PKA), and the tyrosine kinase, Src. Moreover, the addition of Zn(2+) to capacitated sperm caused further stimulation of EGFR and phosphatydil-inositol-3-kinase (PI3K) phosphorylation, leading to the AR. The stimulation of the AR by Zn(2+) also occurred in the absence of Ca(2+) in the incubation medium, and required the tmAC, indicating that Zn(2+) activates a GPCR. The AR stimulated by Zn(2+) is mediated by GPR39 receptor, PKA, Src and the EGFR, as well as the EGFR down-stream effectors PI3K, phospholipase C (PLC) and protein kinase C (PKC). These data support a role for extracellular zinc, acting through the ZnR, in regulating multiple signaling pathways in sperm capacitation and the acrosome reaction.

Zn(2+) induces hyperpolarization by activation of a K(+) channel and increases intracellular Ca(2+) and pH in sea urchin spermatozoa

Zinc (Zn(2+)) has been recently recognized as a crucial element for male gamete function in many species although its detailed mechanism of action is poorly understood. In sea urchin spermatozoa, Zn(2+) was reported as an essential trace ion for efficient sperm motility initiation and the acrosome reaction by modulating intracellular pH (pHi). In this study we found that submicromolar concentrations of free Zn(2+) change membrane potential (Em) and increase the concentration of intracellular Ca(2+) ([Ca(2+)]i) and cAMP in Lytechinus pictus sperm. Our results indicate that the Zn(2+) response in sperm of this species mainly involves an Em hyperpolarization caused by K(+) channel activation. The pharmacological profile of the Zn(2+)-induced hyperpolarization indicates that the cGMP-gated K(+) selective channel (tetraKCNG/CNGK), which is crucial for speract signaling, is likely a main target for Zn(2+). Considering that Zn(2+) also induces [Ca(2+)]i fluctuations, our observations suggest that Zn(2+) activates the signaling cascade of speract, except for an increase in cGMP, and facilitates sperm motility initiation upon spawning. These findings provide new insights about the role of Zn(2+) in male gamete function.

Zinc inhibits osteoclast differentiation by suppression of Ca2+-Calcineurin-NFATc1 signaling pathway

BACKGROUND

Zinc, an essential trace element, inhibits osteoclast differentiation in vitro and in vivo. The molecular mechanism for the inhibitory effect of zinc, however, is poorly understood. The purpose of this study was to investigate the effect of zinc and determine its molecular mechanism on receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis in mouse bone marrow-derived monocyte cells (BMMs) and RAW264.7 cells.

RESULTS

In BMMs, zinc treatment during osteoclast differentiation decreased RANKL-induced osteoclast formation in a dose-dependent manner. We show that zinc suppressed the mRNA levels of nuclear factor of activated T-cells, cytoplasmic 1 (Nfatc1). Zinc also accumulated phospho-Nfatc1 (p-Nfatc1) in the cytosol in a dose-dependent manner and inhibited the translocation of Nfatc1 to the nucleus in RAW264.7 cells. Zinc suppressed the activities of Nfatc1 in the nucleus without changing the activities of NF-κB in RAW264.7 cells. In contrast, calcineurin activity decreased in response to zinc but its protein level was unchanged. RANKL-induced Ca2+ oscillations were inhibited by zinc treatment, but phospho-phospholipase Cγ1 (p-PLCγ1), the upstream signaling molecule of Ca2+ oscillations, was unaffected. Moreover, a constitutively active form of Nfatc1 obviously rescued suppression of osteoclastogenesis by zinc.

CONCLUSIONS

Taken together, these results demonstrate for the first time that the inhibitory effect of zinc during osteoclastogesis is caused by suppressing the Ca2+-Calcineurin-NFATc1 signaling pathway. Thus, zinc may be a useful therapeutic candidate for the prevention of bone loss caused by NFATc1 activation in osteoclasts.

hZip1 (hSLC39A1) regulates zinc homoeostasis in gut epithelial cells

Zinc is an essential trace element required for enzyme catalysis, gene regulation and signal transduction. Zinc absorption takes place in the small intestine; however, the mechanisms by which cells accumulate zinc are not entirely clear. Zip1 (SLC39A1) is a predicted transmembrane protein that is postulated, but not conclusively proven to mediate zinc influx in gut cells. The aim of this study was to investigate a role for hZip1 in mediating zinc uptake in human enterocytes. Both hZip1 mRNA and protein were detected in human intestinal tissue. In non-differentiated Caco-2 human gut cells, hZip1 was partially localised to the endoplasmic reticulum. In contrast, in differentiated Caco-2 cells cultured in extracellular matrix, the hZip1 protein was located in proximity to the apical microvilli. Lack of surface antibody binding and internalisation indicated that hZip1 was not present on the plasma membrane. Functional studies to establish a role for hZip1 in cellular zinc accumulation were carried out using (65)Zn. In Caco-2 cells harbouring an hZip1 overexpression construct, cellular zinc accumulation was enhanced relative to the control. Conversely, Caco-2 cells with an hZip1 siRNA construct showed reduced zinc accumulation. In summary, we show that the Caco-2 cell differentiation endorses targeting of hZip1 to a region near the apical domain. Given the absence of hZip1 at the apical plasma membrane, we propose that hZip1 may act as an intracellular sensor to regulate zinc homoeostasis in human gut cells.

Extracellular pH regulates zinc signaling via an Asp residue of the zinc-sensing receptor (ZnR/GPR39)

Zinc activates a specific Zn(2+)-sensing receptor, ZnR/GPR39, and thereby triggers cellular signaling leading to epithelial cell proliferation and survival. Epithelial cells that express ZnR, particularly colonocytes, face frequent changes in extracellular pH that are of physiological and pathological implication. Here we show that the ZnR/GPR39-dependent Ca(2+) responses in HT29 colonocytes were maximal at pH 7.4 but were reduced by about 50% at pH 7.7 and by about 62% at pH 7.1 and were completely abolished at pH 6.5. Intracellular acidification did not attenuate ZnR/GPR39 activity, indicating that the pH sensor of this protein is located on an extracellular domain. ZnR/GPR39-dependent activation of extracellular-regulated kinase (ERK)1/2 or AKT pathways was abolished at acidic extracellular pH of 6.5. A similar inhibitory effect was monitored for the ZnR/GPR39-dependent up-regulation of Na(+)/H(+) exchange activity at pH 6.5. Focusing on residues putatively facing the extracellular domain, we sought to identify the pH sensor of ZnR/GPR39. Replacing the histidine residues forming the Zn(2+) binding site, His(17) or His(19), or other extracellular-facing histidines to alanine residues did not abolish the pH dependence of ZnR/GPR39. In contrast, replacing Asp(313) with alanine resulted in similar Ca(2+) responses triggered by ZnR/GPR39 at pH 7.4 or 6.5. This mutant also showed similar activation of ERK1/2 and AKT pathways, and ZnR-dependent up-regulation of Na(+)/H(+) exchange at pH 7.4 and pH 6.5. Substitution of Asp(313) to His or Glu residues restored pH sensitivity of the receptor. This indicates that Asp(313), which was shown to modulate Zn(2+) binding, is an essential residue of the pH sensor of GPR39. In conclusion, ZnR/GPR39 is tuned to sense physiologically relevant changes in extracellular pH that thus regulate ZnR-dependent signaling and ion transport activity.

Pharmacological characterization of zinc and copper interaction with the human alpha(1A)-adrenoceptor

Metal ions have a major role in human health, and interact with many classes of receptors including the G-protein coupled receptors. In the peripheral system, zinc mainly accumulates in the soft prostate organ and, with copper, influences prostate disease progression, from normal to hypertrophic or cancerous states. The development of these pathologies may be influenced by the α(1A)-adrenoceptor, the principal regulator of prostate tonicity. There is currently no information on possible interactions between metals and the α(1A)-adrenoceptor. We therefore studied the effects of several mono- and divalent ions on this receptor subtype using binding and functional experiments performed on expressed cloned human α(1A)-adrenoceptor. Regardless of the counter anion used, Zn(2+) and Cu(2+) interact with α(1A)-adrenoceptor with apparent affinities in the low micromolar range. In addition, using specific binding experiments, we established that these ions acted as negative allosteric ligands on prazosin/α(1A)-adrenoceptor interaction, but in a different manner from the allosteric modulator 5-(N-ethyl-N-isopropyl)-amiloride, suggesting distinct mode of interaction. In addition, the presence of Cu(2+) weakly decreased epinephrine affinity, whereas the addition of Zn(2+) shifted to the left the epinephrine binding curve, revealing a positive allosteric effect but only on half of the binding site. Finally, cell-based functional experiments demonstrated that Zn(2+) and Cu(2+) antagonized epinephrine activation in an insurmountable manner, by reducing agonist efficacy without any shift in the epinephrine activation curves. This study shows the interactions between metal ions and the α(1A)-adrenoceptor with affinities compatible with physiological concentrations and suggests that zinc and copper may have a biological role in prostate function.

Stabilization and swelling of hagfish slime mucin vesicles

When agitated, Atlantic hagfish (Myxine glutinosa) produce large quantities of slime that consists of hydrated bundles of protein filaments and membrane-bound mucin vesicles from numerous slime glands. When the slime exudate contacts seawater, the thread bundles unravel and the mucin vesicles swell and rupture. Little is known about the mechanisms of vesicle rupture in seawater and stabilization within the gland, although it is believed that the vesicle membrane is permeable to most ions except polyvalent anions. We hypothesized that the most abundant compounds within the slime gland exudate have a stabilizing effect on the mucin vesicles. To test this hypothesis, we measured the chemical composition of the fluid component of hagfish slime exudate and conducted functional assays with these solutes to test their ability to keep the vesicles in a condensed state. We found K(+) concentrations that were elevated relative to plasma, and Na(+), Cl(-) and Ca(2+) concentrations that were considerably lower. Our analysis also revealed high levels of methylamines such as trimethylamine oxide (TMAO), betaine and dimethylglycine, which had a combined concentration of 388 mmol l(-1) in the glandular fluid. In vitro rupture assays demonstrated that both TMAO and betaine had a significant effect on rupture, but neither was capable of completely abolishing mucin swelling and rupture, even at high concentrations. This suggests that some other mechanism such as the chemical microenvironment within gland mucous cells, or hydrostatic pressure is responsible for stabilization of the vesicles within the gland.

Calcium entry is regulated by Zn2+ in relation to extracellular ionic environment in human airway epithelial cells

The extracellular pH, sodium and divalent cation concentrations influence the ATP-induced changes in cytosolic Ca(2+) concentration ([Ca(2+)](i)). This elevation of [Ca(2+)](i) and activation of Ca(2+)-dependent Cl(-) channels represent a possible therapeutic approach in cystic fibrosis (CF). We investigated the changes of [Ca(2+)](i) in different external ionic environment, and P2X purinergic receptors (P2XRs) expression in the control and CF airway epithelial cells. The parallel removal of Na(+) and alkalinization of the extracellular solution increased the amplitude of sustained ATP-induced Ca(2+) signals independent of wild-type or mutant CFTR expression. The ATP-induced Ca(2+) entry was either inhibited or stimulated by Zn(2+) depending on the extracellular Na(+) concentration. In Na(+)-free environment, Zn(2+) and other divalent cations elicited a biphasic Ca(2+) signal. Immunohistochemical data suggest that, multiple subtypes of P2XRs are expressed in these airway epithelial cells. In conclusion, Ca(2+) entry is finely regulated by external ionic environment. Therefore, we speculate that properly compiled aerosols could influence efficacy of zinc-based therapy in CF.

Both Ca2+ and Zn2+ are essential for S100A12 protein oligomerization and function

Background

Human S100A12 is a member of the S100 family of EF-hand calcium-modulated proteins that are associated with many diseases including cancer, chronic inflammation and neurological disorders. S100A12 is an important factor in host/parasite defenses and in the inflammatory response. Like several other S100 proteins, it binds zinc and copper in addition to calcium. Mechanisms of zinc regulation have been proposed for a number of S100 proteins e.g. S100B, S100A2, S100A7, S100A8/9. The interaction of S100 proteins with their targets is strongly dependent on cellular microenvironment.

Results

The aim of the study was to explore the factors that influence S100A12 oligomerization and target interaction. A comprehensive series of biochemical and biophysical experiments indicated that changes in the concentration of calcium and zinc led to changes in the oligomeric state of S100A12. Surface plasmon resonance confirmed that the presence of both calcium and zinc is essential for the interaction of S100A12 with one of its extracellular targets, RAGE – the Receptor for Advanced Glycation End products. By using a single-molecule approach we have shown that the presence of zinc in tissue culture medium favors both the oligomerization of exogenous S100A12 protein and its interaction with targets on the cell surface.

Conclusion

We have shown that oligomerization and target recognition by S100A12 is regulated by both zinc and calcium. Our present work highlighted the potential role of calcium-binding S100 proteins in zinc metabolism and, in particular, the role of S100A12 in the cross talk between zinc and calcium in cell signaling.

The genetics of essential metal homeostasis during development

The essential metals copper, zinc, and iron play key roles in embryonic, fetal, and postnatal development in higher eukaryotes. Recent advances in our understanding of the molecules involved in the intricate control of the homeostasis of these metals and the availability of natural mutations and targeted mutations in many of the genes involved have allowed for elucidation of the diverse roles of these metals during development. Evidence suggests that the ability of the embryo to control the homeostasis of these metals becomes essential at the blastocyst stage and during early morphogenesis. However, these metals play unique roles throughout development and exert pleiotropic, metal-specific, and often cell-specific effects on morphogenesis, growth, and differentiation. Herein, we briefly review the major players known to be involved in the homeostasis of each of these essential metals and their known roles in development.

The zinc sensing receptor, a link between zinc and cell signaling

Zinc is essential for cell growth. For many years it has been used to treat various epithelial disorders, ranging from wound healing to diarrhea and ulcerative colon disease. The physiological/molecular mechanisms linking zinc and cell growth, however, are not well understood. In recent years, Zn2+ has emerged as an important signaling molecule, activating intracellular pathways and regulating cell fate. We have functionally identified an extracellular zinc sensing receptor, called zinc sensing receptor (ZnR), that is specifically activated by extracellular Zn2+ at physiological concentrations. The putative ZnR is pharmacologically coupled to a Gq-protein which triggers release of Ca2+ from intracellular stores via the Inositol 1,4,5-trisphosphate (IP3) pathway. This, in turn results in downstream signaling via the MAP and phosphatidylinositol 3-kinase (PI3 kinase) pathways that are linked to cell proliferation. In some cell types, e.g., colonocytes, ZnR activity also upregulates Na+/H+ exchange, mediated by Na+/H+ exchanger isoform 1 (NHE1), which is involved in cellular ion homeostasis in addition to cell proliferation. Our overall hypothesis, as discussed below, is that a ZnR, found in organs where dynamic zinc homeostasis is observed, enables extracellular Zn2+ to trigger intracellular signaling pathways regulating key cell functions. These include cell proliferation and survival, vectorial ion transport and hormone secretion. Finally, we suggest that ZnR activity found in colonocytes is well positioned to attenuate erosion of the epithelial lining of the colon, thereby preventing or ameliorating diarrhea, but, by signaling through the same pathways, a ZnR may enhance tumor progression in neoplastic disease.

Targeting of the mouse Slc39a2 (Zip2) gene reveals highly cell-specific patterns of expression, and unique functions in zinc, iron, and calcium homeostasis

Fourteen members of the Slc39a superfamily of metal ion uptake transporters have been identified in mice and humans, but the physiological functions of most remain obscure. Herein, we created mice with Zip2 (Slc39a2) genes in which the open reading frame was replaced with that of the enhanced green fluorescent protein (EGFP), to study temporal and spatial patterns of Zip2 gene expression and examine the physiological roles of this transporter. Expression of this gene was remarkably cell-type specific and developmentally regulated in pericentral hepatocytes, developing keratinocytes, and a subset of immature dendritic cells in the immune system. In addition, the Zip2 gene was transiently expressed in giant trophoblast cells in the placenta. Although the Zip2 gene was not essential under conditions of normal dietary zinc, it played an important role in adapting to dietary zinc deficiency during pregnancy, and in the homeostasis of iron in the liver as well as iron and calcium in developing embryos. These studies suggest that active expression of the Zip2 gene in these few specific cell types, aforementioned, plays a particularly important role during zinc deficiency. These studies further reveal novel interactions between zinc transporter function and the homeostasis of other essential metals.

Zinc dynamics in the myocardial redox signaling network

Zinc plays a vital role in various cellular functions. Zinc deprivation is associated with severe disorders related to growth, maturation, and stress responses. In the heart, zinc affects differentiation and regeneration of cardiac muscle, cardiac conductance, acute stress responses, and recovery of heart transplants. Recent discoveries of the molecular players in zinc homeostasis revealed that the amount of intracellular free zinc is tightly controlled on the level of uptake, intracellular sequestration, redistribution, storage, and elimination, consequently creating a narrow window of optimal zinc concentration in the cells. Most of intracellular zinc is bound to numerous structural and regulatory proteins, with metabolically active, labile zinc present in picoto nanomolar concentrations. The central position of zinc in the redox signaling network is built on its unique chemical nature. The redox inert zinc creates a redox active environment when it binds to a sulfur ligand. The reversible oxidation of the sulfur ligand is coupled to the reversible zinc release from the protein, thereby executing the task of so-called protein "redox zinc switch." Clearly, the impairment of zinc homeostasis will have far reaching physiological consequences.

A store-operated Ca(2+) influx pathway in the bag cell neurons of Aplysia

Although store-operated Ca(2+) influx has been well-studied in nonneuronal cells, an understanding of its nature in neurons remains poor. In the bag cell neurons of Aplysia californica, prior work has suggested that a Ca(2+) entry pathway can be activated by Ca(2+) store depletion. Using fura-based imaging of intracellular Ca(2+) in cultured bag cell neurons, we now characterize this pathway as store-operated Ca(2+) influx. In the absence of extracellular Ca(2+), the endoplasmic reticulum Ca(2+)-ATPase inhibitors, cyclopiazonic acid (CPA) or thapsigargin, depleted intracellular stores and elevated intracellular free Ca(2+). With the subsequent addition of extracellular Ca(2+), a prominent Ca(2+) influx was observed. The ryanodine receptor agonist, chloroethylphenol (CEP), also increased intracellular Ca(2+) but did not initiate store-operated Ca(2+) influx, despite overlap between CEP- and CPA-sensitive stores. Bafilomycin A, a vesicular H(+)-ATPase inhibitor, liberated intracellular Ca(2+) from acidic stores and attenuated subsequent Ca(2+) influx, presumably by replenishing CPA-depleted stores. Store-operated Ca(2+) influx was partially blocked by low concentrations of La(3+) or BTP2, and strongly inhibited by either 1-[b-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole (SKF-96365) or a high concentration of Ni(2+). Regarding IP(3) receptor blockers, 2-aminoethyldiphenyl borate, but not xestospongin C, prevented store-operated Ca(2+) influx. However, jasplakinolide, an actin stabilizer reported to inhibit this pathway in smooth muscle cell lines, was ineffective. The bag cell neurons initiate reproductive behavior through a prolonged afterdischarge associated with intracellular Ca(2+) release and neuropeptide secretion. Store-operated Ca(2+) influx may serve to replenish stores depleted during the afterdischarge or participate in the release of peptide that triggers behavior.

Modulation of inhibitory glycine receptors in cultured embryonic mouse hippocampal neurons by zinc, thiol containing redox agents and carnosine

Modulation of inhibitory glycine receptors by zinc (Zn(2+)) and endogenous redox agents such as glutathione may alter inhibition in the mammalian brain. Despite the abundance of Zn(2+) in the hippocampus and its ability to modulate glycine receptors, few studies have examined Zn(2+) modulation of hippocampal glycine receptors. Whether redox agents modulate hippocampal glycine receptors also remains unknown. This study examined Zn(2+) and redox modulation of glycine receptor-mediated currents in cultured embryonic mouse hippocampal neurons using whole-cell recordings. Zn(2+) concentrations below 10 microM potentiated currents elicited by low glycine, beta-alanine, and taurine concentrations by 300-400%. Zn(2+) concentrations above 300 microM produced nearly complete inhibition. Potentiating Zn(2+) concentrations shifted the dose-response curves for the three agonists to the left and decreased the Hill coefficient for glycine and beta-alanine but not taurine. Inhibiting Zn(2+) concentrations shifted the dose-response curves for glycine and beta-alanine to the right but reduced the maximum taurine response. Histidine residues may participate in potentiation because diethyl pyrocarbonate and pH 5.4 diminished Zn(2+) enhancement of glycine currents. pH 5.4 diminished Zn(2+) block of glycine currents, but diethyl pyrocarbonate did not. These findings indicate that separate sites mediate Zn(2+) potentiation and inhibition. The redox agents glutathione, dithiothreitol, tris(2-carboxyethyl)phosphine, and 5,5'-dithiobis(2-nitrobenzoic acid) did not alter glycine currents by a redox mechanism. However, glutathione and dithiothreitol interfered with the effects of Zn(2+) on glycine currents by chelating it. Carnosine had similar effects. Thus, Zn(2+) and thiol containing redox agents that chelate Zn(2+) modulate hippocampal glycine receptors with the mechanism of Zn(2+) modulation being agonist dependent.

Zinc and copper: pharmacological probes and endogenous modulators of neuronal excitability

As well as being key structural components of many proteins, increasing evidence suggests that zinc and copper ions function as signaling molecules in the nervous system and are released from the synaptic terminals of certain neurons. In this review, we consider the actions of these two ions on proteins that regulate neuronal excitability. In addition to the established actions of zinc, and to a lesser degree copper, on excitatory and inhibitory ligand-gated ion channels, we show that both ions have a number of actions on selected members of the voltage-gated-like ion channel superfamily. For example, zinc is a much more effective blocker of one subtype of tetrodotoxin (TTX)-insensitive sodium (Na+) channel (NaV1.5) than other Na+ channels, whereas a certain T-type calcium (Ca2+) channel subunit (CaV3.2) is particularly sensitive to zinc. For potassium (K+) channels, zinc can have profound effects on the gating of certain KV channels whereas zinc and copper have distinct actions on closely related members of the 2 pore domain potassium channel (K2P) channel family. In addition to direct actions on these proteins, zinc is able to permeate a number of membrane proteins such as (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptors, Ca2+ channels and some transient receptor potential (trp) channels. There are a number of important physiological and pathophysiological consequences of these many actions of zinc and copper on membrane proteins, in terms of regulation of neuronal excitability and neurotoxicity. Furthermore, the concentration of free zinc and copper either in the synaptic cleft or neuronal cytoplasm may contribute to the etiology of certain disease states such as Alzheimer's disease (AD) and epilepsy.

Zinc as an ambivalent but potent modulator of murine hair growth in vivo- preliminary observations

Oral zinc (Zn(2+)) is often employed for treating hair loss, even in the absence of zinc deficiency, although its mechanisms of action and efficacy are still obscure. In the current study, we explored the in vivo effects of oral zinc using the C57BL/6 mouse model for hair research. Specifically, we investigated whether continuous administration of high-dose ZnSO(4) x 7H(2)O (20 mg/ml) in drinking water affects hair follicle (HF) cycling, whether it retards or inhibits chemotherapy-induced alopecia (CIA) and whether it modulates the subsequent hair re-growth pattern. Here, we show that high doses of oral zinc significantly inhibit hair growth by retardation of anagen development. However, oral zinc also significantly retards and prolongs spontaneous, apoptosis-driven HF regression (catagen). Oral zinc can also retard, but not prevent, the onset of CIA in mice. Interestingly, Zn(2+) treatment of cyclophosphamide-damaged HFs also significantly accelerates the re-growth of normally pigmented hair shafts, which reflects a promotion of HF recovery. However, if given for a more extended time period, zinc actually retards hair re-growth. Thus, high-dose oral zinc is a powerful, yet ambivalent hair growth modulator in mice, whose ultimate effects on the HF greatly depend on the timing and duration of zinc administration. The current study also encourages one to explore whether oral zinc can mitigate chemotherapy-induced hair loss in humans and/or can stimulate hair re-growth.

Role of GPR40 in fatty acid action on the beta cell line INS-1E

GPR40 is a G protein-coupled receptor expressed preferentially in beta cells, that has been implicated in mediating free fatty acid-stimulated insulin release. GPR40 RNAi impaired the ability of palmitic acid (PA) to increase both insulin secretion and intracellular calcium ([Ca2+]i). The PA-dependent [Ca2+]i increase was attenuated by inhibitors of Galphaq, PLC, and SERCA. Thus GPR40 activates the Galphaq pathway, leading to release of Ca2+ from the ER. Yet the GPR40-dependent [Ca2+]i rise was dependent on extracellular Ca2+ and elevated glucose, and was blocked by inhibition of L-type calcium channels (LTCC) or opening of the K(ATP) channel; this suggests that GPR40 promotes Ca2+ influx through up-regulation of LTCC pre-activated by glucose and membrane depolarization. Taken together, the data indicate that GPR40 mediates the increase in [Ca2+]i and insulin secretion through the Galphaq-PLC pathway, resulting in release of Ca2+ from the ER and leading to up-regulation of Ca2+ influx via LTCC.

The extracellular zinc-sensing receptor mediates intercellular communication by inducing ATP release

Taste and salivary secretion disorders have been linked to zinc deficiency, indeed zinc is found in secretory granules in the salivary gland. The signaling role for the zinc release in this tissue, however, is poorly understood. Here, we address the signaling pathways and physiological role of the zinc-sensing receptor, ZnR, in the ductal salivary gland cell line, HSY. Exposure of these cells to zinc triggered intracellular Ca2+ release from thapsigargin-sensitive stores. The G alpha q inhibitor, YM-254890 (1 microM), eliminated the Zn2+-dependent Ca2+ response, demonstrating that ZnR is a G alpha q-coupled receptor. Dose-response curves yielded an apparent K0.5 of 36 microM and a Hill coefficient of 7 in the absence of extracellular Ca2+, and K0.5 of 55 microM with a Hill coefficient of 3 in its presence. This indicates that although Zn2+ is essential for ZnR activation, Ca2+ may affect the receptor co-operativity. The homologous desensitization pattern of ZnR was characterized by pre-exposure of cells to Zn2+ at concentrations found to activate the receptor. Re-exposure of cells to Zn2+ elicited an attenuated Zn2+-dependent Ca2+ response for at least 3 h, indicating that the ZnR is strongly desensitized by Zn2+. Finally, we studied the paracrine affects of ZnR using a co-culture consisting of the HSY cells and vascular smooth muscle cells (VSMCs). While no Zn2+-dependent Ca2+ release was observed in VSMC alone, application of Zn2+ to the co-culture induced a Ca2+ rise in both HSY cells and VSMC. This Ca2+ rise was inhibited by the ATP scavenger, apyrase. Taken together, our results demonstrate that ZnR activity is monitored in salivary cells and is modulated by extracellular Ca2+. We further show that ZnR enhances secretion of ATP, thereby linking zinc to key signaling pathways involved in modification of salivary secretions by the ductal cells.

Zinc ions are endogenous modulators of neurotransmitter-stimulated capacitative Ca2+entry in both cultured andin situmouse astrocytes

Astrocytes express a variety of metabotropic receptors and their activation leads to a biphasic Ca2+ response due to Ca2+ release from intracellular stores and subsequent capacitative Ca2+ entry. We performed Ca2+ imaging with Fura-2 on cultured mouse astrocytes and showed that extracellular zinc reversibly blocks the capacitative Ca2+ entry following application of the metabotropic ligands ATP, glutamate and endothelin-1. Zinc blocked the plateau phase of the ligand-triggered Ca2+ responses. When ligands were repetitively applied in the presence of zinc the calcium responses progressively decayed and even disappeared, indicating that capacitative Ca2+ entry is required to refill the stores. Zinc inhibited the capacitative Ca2+ entry with a K(i) of approximately 6 microM, which is well within the physiological concentration range of zinc found in the brain. Application of the reducing agent DTT prevented the blocking effect by zinc ions but not the inhibition elicited by the nonphysiological metal ions Gd3+ and La3+, indicating that zinc has a distinct binding site. To monitor the capacitative Ca2+ entry in astrocytes in situ and to determine the effect of zinc on this pathway we utilized X-rhod-1 imaging in hippocampal slices of a transgenic mouse line with green fluorescent astrocytes. Zinc affected the repetitive metabotropic Ca2+ response in the following fashion: (i) after depleting stores in Ca(2+)-free solution, re-addition of Ca2+ led to an influx of Ca2+ via a zinc-sensitive Ca2+ entry route; (ii) with repetitive application of metabotropic ligands, Ca2+ responses became smaller and even disappeared in the presence of zinc. We conclude that zinc, which is co-released from glutamatergic synaptic vesicles upon neuronal activity, has a major impact on shaping the astrocytic calcium responses.