Fluorescent detection of Zn(2+)-rich vesicles with Zinquin: mechanism of action in lipid environments

Directed Neural Stem Cells Differentiation via Signal Communication with Ni-Zn Micromotors

Neural stem cells (NSCs), with the capability of self-renewal, differentiation, and environment modulation, are considered promising for stroke, brain injury therapy, and neuron regeneration. Activation of endogenous NSCs, is attracting increasing research enthusiasm, which avoids immune rejection and ethical issues of exogenous cell transplantation. Yet, how to induce directed growth and differentiation in situ remain a major challenge. In this study, a pure water-driven Ni-Zn micromotor via a self-established electric-chemical field is proposed. The micromotors can be magnetically guided and precisely approach target NSCs. Through the electric-chemical field, bioelectrical signal exchange and communication with endogenous NSCs are allowed, thus allowing for regulated proliferation and directed neuron differentiation in vivo. Therefore, the Ni-Zn micromotor provides a platform for controlling cell fate via a self-established electrochemical field and targeted activation of endogenous NSCs.

Assessing metal ion transporting activity of ZIPs: Intracellular zinc and iron detection

Zrt/Irt-like proteins (ZIPs or SLC39A) are a large family of metal ion transporters mainly responsible for zinc uptake. Some ZIPs have been shown to specifically transport zinc, whereas others have broader substrate specificity in divalent metal ion trafficking, notably those of zinc and iron ions. Measuring intracellular zinc and iron levels helps assess their molecular and physiological activities. This chapter presents step-by-step methods for evaluating intracellular metal ion concentrations, including direct measurement using inductively coupled plasma-mass spectrometry (ICP-MS), chemical staining, fluorescent probes, and indirect reporter assays such as activity analysis of enzymes whose activities are dependent on metal ion availability.

ZnT8 Loss of Function Mutation Increases Resistance of Human Embryonic Stem Cell-Derived Beta Cells to Apoptosis in Low Zinc Condition

The rare SLC30A8 mutation encoding a truncating p.Arg138* variant (R138X) in zinc transporter 8 (ZnT8) is associated with a 65% reduced risk for type 2 diabetes. To determine whether ZnT8 is required for beta cell development and function, we derived human pluripotent stem cells carrying the R138X mutation and differentiated them into insulin-producing cells. We found that human pluripotent stem cells with homozygous or heterozygous R138X mutation and the null (KO) mutation have normal efficiency of differentiation towards insulin-producing cells, but these cells show diffuse granules that lack crystalline zinc-containing insulin granules. Insulin secretion is not compromised in vitro by KO or R138X mutations in human embryonic stem cell-derived beta cells (sc-beta cells). Likewise, the ability of sc-beta cells to secrete insulin and maintain glucose homeostasis after transplantation into mice was comparable across different genotypes. Interestingly, sc-beta cells with the SLC30A8 KO mutation showed increased cytoplasmic zinc, and cells with either KO or R138X mutation were resistant to apoptosis when extracellular zinc was limiting. These findings are consistent with a protective role of zinc in cell death and with the protective role of zinc in T2D.

Comparative Response of Cardiomyocyte ZIPs and ZnTs to Extracellular Zinc and TPEN

Intracellular zinc concentrations are tightly regulated by the coordinated regulation of ZIPs and ZnTs. Very little is known about the regulation of these transporters in cardiomyocytes, in response to extracellular zinc. Adult rat cardiomyocytes express ZnTs 1, 2, 5, and 9, in addition to ZIPs 1, 2, 3, 6, 7, 9, 10, 11, 13, and 14. We have determined the intracellular free zinc levels using Zinpyr-1 fluorescence and studied response of ZIP and ZnT mRNA by real-time PCR to the changes in extracellular zinc and TPEN in adult rat ventricular myocytes. TPEN downregulated ZnT1, ZnT2, and ZIP11 mRNAs but upregulated ZnT5, ZIP2, ZIP7, ZIP10, ZIP13, and ZIP14 mRNAs. Zinc supplementation upregulated ZnT1, ZnT2 mRNA but downregulated ZnT5, ZIP1, ZIP2, ZIP3, ZIP7, ZIP9, and ZIP10 mRNA. The negative regulation of ZIPs by zinc excess can be explained in terms of zinc homeostasis as these transporters may act to protect cells from zinc over accumulation by reducing zinc influx when the extracellular concentration of zinc is high. Similarly, the ZnT expression appears to be regulated to avoid loss of zinc from the intracellular milieu, under zinc-deficient conditions.

Perfusion alters free zinc levels in the rodent brain

BACKGROUND

Fixation of brain tissue is a common practice which allows preservation of tissue and aids in preventing structural and chemical abnormalities. However, fixation procedures may disrupt the levels of biometals such as zinc when compared to tissue that is fresh-frozen. Thus, we sought to determine if any differences in free-zinc levels exist between perfused and fresh-frozen tissue. Zinc is an essential biometal critical for cellular communication and memory and exists in both bound and free forms; the latter playing critical roles in synaptic communication. New method: C57BL/6 J mice were divided into two water types: those given lab water and those given water supplemented with 10 ppm zinc carbonate. Perfusion was carried out with 4% paraformaldehyde on half of the animals in each water group to assess the impact on levels of free Zn as measured through Zinpyr-1 fluorescence.

RESULTS

There were significant differences in Zn fluorescence values between Zn-supplemented and lab water groups as well as between perfused and fresh-frozen tissues in the dentate gyrus and CA3 regions of the hippocampus, regions critical in learning & memory. Comparison with existing methods: These results show that when determining a method for euthanasia, any future histological techniques involving assessment of metal content should first be considered.

CONCLUSIONS

Researchers must be cautious with the way in which tissue is collected and treated since this can lead to misleading conclusions when linking changes in behavior and relative levels of trace metals.

Steric and Stereochemical Modulation in Pyridyl- and Quinolyl-Containing Ligands

Nitrogen-containing pyridine and quinoline are outstanding platforms on which excellent ionophores and sensors for metal ions can be built. Steric and stereochemical effects can be used to modulate the affinity and selectivity of such ligands toward different metal ions on the coordination chemistry front. On the signal transduction front, such effects can also be used to modulate optical responses of these ligands in metal sensing systems. In this review, steric modulation of achiral ligands and stereochemical modulation in chiral ligands, especially ionophores and sensors for zinc, copper, silver, and mercury, are examined using published structural and spectral data. Although it might be more challenging to construct chiral ligands than achiral ones, isotropic and anisotropic absorption signals from a single chiroptical fluorescent sensor provide not only detection but also differentiation of multiple analytes with high selectivity.

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

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

Selectivity and specificity of small molecule fluorescent dyes/probes used for the detection of Zn2+ and Ca2+ in cells

Fluorescent dyes are widely used in the detection of labile (free or exchangeable) Zn(2+) and Ca(2+) in living cells. However, their specificity over other cations and selectivity for detection of labile vs. protein-bound metal in cells remains unclear. We characterized these important properties for commonly used Zn(2+) and Ca(2+) dyes in a cellular environment. By tracing the fluorescence emission signal along with UV-Vis and size exclusion chromatography-inductively coupled plasma mass spectrometry (SEC-ICP-MS) in tandem, we demonstrated that among the dyes used for Zn(2+), Zinpyr-1 fluoresces in the low molecular mass (LMM) region containing labile Zn(2+), but also fluoresces in different molecular mass regions where zinc ion is detected. However, FluoZin™-3 AM, Newport Green™ DCF and Zinquin ethyl ester display weak fluorescence, lack of metal specificity and respond strongly in the high molecular mass (HMM) region. Four Ca(2+) dyes were studied in an unperturbed cellular environment, and two of these were tested for binding behavior under an intracellular Ca(2+) release stimulus. A majority of Ca(2+) was in the labile form as tested by SEC-ICP-MS, but the fluorescence from Calcium Green-1™ AM, Oregon Green® 488 BAPTA-1, Fura red™ AM and Fluo-4 NW dyes in cells did not correspond to free Ca(2+) detection. Instead, the dyes showed non-specific fluorescence in the mid- and high-molecular mass regions containing Zn, Fe and Cu. Proteomic analysis of one of the commonly seen fluorescing regions showed the possibility for some dyes to recognize Zn and Cu bound to metallothionein 2. These studies indicate that Zn(2+) and Ca(2+) binding dyes manifest fluorescence responses that are not unique to recognition of labile metals and bind other metals, leading to suboptimal specificity and selectivity.

Intracellular sequestration of zinc, cadmium and silver in Hebeloma mesophaeum and characterization of its metallothionein genes

Sequestration of intracellular heavy metals in eukaryotes involves compartmentalization and binding with cytosolic, cysteine-rich metallothionein (MT) peptides. We examined the roles of these processes in handling of zinc (Zn), cadmium (Cd) and silver (Ag) in sporocarps and a metal-exposed extraradical mycelium of Hebeloma mesophaeum, the Zn-accumulating ectomycorrhizal (EM) species frequently associated with metal disturbed sites. Size exclusion chromatography revealed that the majority of Zn and Cd in the sporocarps and mycelium was contained in a low molecular mass fraction attributable to compartmentalized metal. The staining of hyphal cells with the Zn-specific Zinquin and Cd-specific Leadmium fluorescent tracers labeled Zn and Cd in small, punctuated vesicles and vacuoles, respectively. By contrast, the sporocarp and mycelium Ag was associated with cysteine-rich, 5-kDa peptides. The peptides of the same size were also identified in minor Zn and Cd complexes from the metal-exposed mycelium. We have further isolated and characterized HmMT1, HmMT2 and HmMT3 genes coding for different 5-kDa MTs of H. mesophaeum collected at a lead smelter site. Heterologous complementation assays in metal-sensitive yeast mutants indicated that HmMTs encode functional, metal-specific peptides: only HmMT1 was able to complement sensitivity to Zn; HmMT1 conferred higher tolerance to Cd and Cu than HmMT2 or HmMT3; and both HmMT2 and HmMT3, but not HmMT1, conferred increased tolerance to Ag. The presence of HmMT1 and HmMT3, but not HmMT2, was also confirmed in a H. mesophaeum isolate from an unpolluted site. Gene expression analysis in the extraradical mycelium of this isolate revealed that the transcription of HmMT1 was preferentially induced in the presence of Zn and Cd, while Ag was a stronger inducer of HmMT3. Altogether, these results improve our understanding of the handling of intracellular Zn, Cd and Ag in Hebeloma and represent the first evidence suggesting involvement of MTs in sequestration of Zn in EM fungi.

Parkinson's disease-linked human PARK9/ATP13A2 maintains zinc homeostasis and promotes α-Synuclein externalization via exosomes

α-Synuclein plays a central causative role in Parkinson's disease (PD). Increased expression of the P-type ATPase ion pump PARK9/ATP13A2 suppresses α-Synuclein toxicity in primary neurons. Our data indicate that ATP13A2 encodes a zinc pump; neurospheres from a compound heterozygous ATP13A2(-/-) patient and ATP13A2 knockdown cells are sensitive to zinc, whereas ATP13A2 over-expression in primary neurons confers zinc resistance. Reduced ATP13A2 expression significantly decreased vesicular zinc levels, indicating ATP13A2 facilitates transport of zinc into membrane-bound compartments or vesicles. Endogenous ATP13A2 localized to multi-vesicular bodies (MVBs), a late endosomal compartment located at the convergence point of the endosomal and autophagic pathways. Dysfunction in MVBs can cause a range of detrimental effects including lysosomal dysfunction and impaired delivery of endocytosed proteins/autophagy cargo to the lysosome, both of which have been observed in cells with reduced ATP13A2 function. MVBs also serve as the source of intra-luminal nanovesicles released extracellularly as exosomes that can contain a range of cargoes including α-Synuclein. Elevated ATP13A2 expression reduced intracellular α-Synuclein levels and increased α-Synuclein externalization in exosomes >3-fold whereas ATP13A2 knockdown decreased α-Synuclein externalization. An increased export of exosome-associated α-Synuclein may explain why surviving neurons of the substantia nigra pars compacta in sporadic PD patients were observed to over-express ATP13A2. We propose ATP13A2's modulation of zinc levels in MVBs can regulate the biogenesis of exosomes capable of containing α-Synuclein. Our data indicate that ATP13A2 is the first PD-associated gene involved in exosome biogenesis and indicates a potential neuroprotective role of exosomes in PD.

Evidence for an extracellular zinc-veneer in rodent brains from experiments with Zn-ionophores and ZnT3 knockouts

Ionic zinc is found at a high concentration in some glutamatergic vesicles of the mammalian brain. Ionic zinc is also found chelated to macromolecules in the extracellular space, constituting what has been called the "zinc veneer". In this communication we show that the zinc ionophore, pyrithione, can be used to demonstrate the presence of the veneer. Application of pyrithione without added ionic zinc to rodent hippocampal slices mobilizes extracellular zinc, which can be detected intracellularly by the zinc probe FluoZin-3. In addition, we show that ZnT3 null mice, which lack the transporter responsible for stocking synaptic vesicles, nevertheless do have a zinc veneer, albeit diminished compared to wild type animals. The presence of the zinc veneer in ZnT3 null mice may account for the absence of any marked deficit in these animals.

Reactions of the fluorescent sensor, Zinquin, with the zinc-proteome: adduct formation and ligand substitution

Zinquin (ZQ) is a commonly used sensor for cellular Zn(2+) status. It has been assumed that it measures accessible Zn(2+) concentrations in the nanomolar range. Instead, this report shows a consistent pattern across seven mammalian cell and tissue types that ZQ reacts with micromolar concentrations of Zn(2+) bound as Zn-proteins. The predominant class of products were ZQ-Zn-protein adducts that were characterized in vivo and in vitro by a fluorescence emission spectrum centered at about 470 nm, by their migration over Sephadex G-75 as protein not low molecular weight species, by the exclusion of reaction with lipid vesicles, and by their large aggregate concentration. In addition, variable, minor formation of Zn(ZQ)(2) with a fluorescence band at about 490 nm was observed in vivo in each case. Because incubation of isolated Zn-proteome with ZQ also generated similar amounts of Zn(ZQ)(2), it was concluded that this species had formed through direct ligand substitution in which ZQ had successfully competed for protein-bound Zn(2+). Parallel studies with the model Zn-proteins, alcohol dehydrogenase (ADH), and alkaline phosphatase (AP) revealed a similar picture of reactivity: ZQ(ACID) (Zinquin acid, (2-methyl-8-p-toluenesulfonamido-6-quinolyloxy)acetate)) able to bind to one Zn(2+) and extract the other in Zn(2)-ADH, whereas it removed one Zn(2+) from Zn(2)-AP and did not bind to the other. Zinquin ethyl ester (ethyl(2-methyl-8-p-toluenesulfonamido-6-quinolyloxy)acetate); ZQ(EE)) bound to both proteins without sequestering Zn(2+) from either one. In contrast to a closely related sensor, 6-methoxy-8-p-toluenesulfonamido-quinoline (TSQ), neither ZQ(ACID) nor ZQ(EE) associated with Zn-carbonic anhydrase. A survey of reactivity of these sensors with partially fractionated Zn-proteome confirmed that ZQ and TSQ bind to distinct, overlapping subsets of the Zn-proteome.

A fluorescent probe for both pH and Zn2+ based on 2-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenol

A sensitive fluorescent probe 2-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenol (HBIZ) for pH and Zn2+ has been developed. Great changes have taken place in the UV-vis absorption and fluorescence spectra for HBIZ upon increasing pH of its aqueous solution, acting as a pH-induced emission "off-on-off" switch with large enhancement factors of ∼290 and ∼75 over the pH range of 1.00-5.40 and 5.20-10.40. A over 100-fold fluorescence enhancement was also observed after complexation of HBIZ to Zn2+ in N,N-dimethylformamide.

Dependence of the histofluorescently reactive zinc pool on zinc transporter-3 in the normal brain

In the brain, free zinc levels are under the exquisite control of a variety of zinc-regulating systems, in which zinc transporter (ZnT) proteins play a central role. ZnT3, which is prominently expressed in the brain, facilitates the concentration of free zinc in pre-synaptic vesicles. In addition to histochemical staining methods, a variety of zinc-specific fluorescence dyes has been developed to image or analyze zinc in brain tissue. In this study, we demonstrate the close correlations between histofluorescently reactive zinc and ZnT3. We examined the overlapping distribution of the zinc-specific fluorescent dye, N-(6-methoxy-8-quinolyl)-p-toluenesulfonamide (TSQ)-, and ZnT3-immunoreactive fluorescence throughout the normal brain. TSQ and ZnT3-antibody intensely stained the hippocampus, cortex and amygdala, highlighting the characteristic laminar organization of these regions by variably staining the different layers. TSQ fluorescence and ZnT3 immunoreactivity were roughly co-localized with synaptophysin along the neuropil, but were absent in the neuronal soma. However, albeit relatively faint, TSQ fluorescence was also found throughout the brains of ZnT3-knockout mice. Although these results may indicate the presence of very small cerebral free zinc pools distinct from synaptic vesicle zinc, the synaptic vesicle zinc pool is predominant, accounting for more than 95% of the entire histofluorescently reactive zinc pool in the hippocampus and cortex. Thus, the physiological activity of free zinc in the normal brain might largely depend on the pool of synaptic vesicle zinc that is determined by ZnT3.

Differential transition metal uptake and fluorescent probe localization in hippocampal slices

Metals are taken up by the combined action of metal transporters and ion channels. In this communication we have measured the uptake of the biologically important transition metals Mn, Fe, Co, Ni, Cu, Zn and Cd by rat and mouse hippocampal slices using the fluorescent probes FluoZin-3 (FZ3) and Newport Green (NPG), introduced by acetoxymethyl ester (AM) loading. The combination of metals and probes is also used to attempt to localize cellular sites into which metals translocate. We show that FZ3 and NPG partition into different cellular compartments; FZ3 into neuropil, whereas NPG localizes in neuropil and compartments within the cell bodies of neurons. Ni, Zn and Cd pass across the plasma membrane and then accumulate in intracellular vesicles and within intracellular membranes of cell bodies. The latter accumulate Cd, while synaptic vesicles take up Co. The passage of Mn, Cu and Fe into cells can be detected but there is some uncertainty about their disposition within the cell. All of our experiments are consistent with metals accumulating in intracellular compartments rather than the cytoplasm. Whether and to what extent there are transient elevations of free zinc levels in the cytoplasm remains unclear.

Cytosolic zinc release and clearance in hippocampal neurons exposed to glutamate--the role of pH and sodium

Although Zn(2+) homeostasis in neurons is tightly regulated and its destabilization has been linked to a number of pathologies including Alzheimer's disease and ischemic neuronal death, the primary mechanisms affecting intracellular Zn(2+) concentration ([Zn(2+) ](i)) in neurons exposed to excitotoxic stimuli remain poorly understood. The present work addressed these mechanisms in cultured hippocampal neurons exposed to glutamate and glycine (Glu/Gly). [Zn(2+)](i) and intracellular Ca(2+) concentration were monitored simultaneously using FluoZin-3 and Fura-2FF, and intracellular pH (pH(i)) was studied in parallel experiments using 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein. Glu/Gly applications under Na(+)-free conditions (Na(+) substituted with N-methyl-D-glucamine(+)) caused Ca(2+) influx, pH(i) drop, and Zn(2+) release from intracellular stores. Experimental maneuvers resulting in a pH(i) increase during Glu/Gly applications, such as stimulation of Na(+) -dependent pathways of H(+) efflux, forcing H(+) efflux via gramicidin-formed channels, or increasing extracellular pH counteracted [Zn(2+)](i) elevations. In the absence of Na(+), the rate of [Zn(2+)](i) decrease could be correlated with the rate of pH(i) increase. In the presence of Na(+), the rate of [Zn(2+) ](i) decrease was about twice as fast as expected from the rate of pH(i) elevation. The data suggest that Glu/Gly-induced cytosolic acidification promotes [Zn(2+) ](i) elevations and that Na(+) counteracts the latter by promoting pH(i)-dependent and pH(i)-independent mechanisms of cytosolic Zn(2+) clearance.

Insulin storage and glucose homeostasis in mice null for the granule zinc transporter ZnT8 and studies of the type 2 diabetes-associated variants

OBJECTIVE

Zinc ions are essential for the formation of hexameric insulin and hormone crystallization. A nonsynonymous single nucleotide polymorphism rs13266634 in the SLC30A8 gene, encoding the secretory granule zinc transporter ZnT8, is associated with type 2 diabetes. We describe the effects of deleting the ZnT8 gene in mice and explore the action of the at-risk allele.

RESEARCH DESIGN AND METHODS

Slc30a8 null mice were generated and backcrossed at least twice onto a C57BL/6J background. Glucose and insulin tolerance were measured by intraperitoneal injection or euglycemic clamp, respectively. Insulin secretion, electrophysiology, imaging, and the generation of adenoviruses encoding the low- (W325) or elevated- (R325) risk ZnT8 alleles were undertaken using standard protocols.

RESULTS

ZnT8(-/-) mice displayed age-, sex-, and diet-dependent abnormalities in glucose tolerance, insulin secretion, and body weight. Islets isolated from null mice had reduced granule zinc content and showed age-dependent changes in granule morphology, with markedly fewer dense cores but more rod-like crystals. Glucose-stimulated insulin secretion, granule fusion, and insulin crystal dissolution, assessed by total internal reflection fluorescence microscopy, were unchanged or enhanced in ZnT8(-/-) islets. Insulin processing was normal. Molecular modeling revealed that residue-325 was located at the interface between ZnT8 monomers. Correspondingly, the R325 variant displayed lower apparent Zn(2+) transport activity than W325 ZnT8 by fluorescence-based assay.

CONCLUSIONS

ZnT8 is required for normal insulin crystallization and insulin release in vivo but not, remarkably, in vitro. Defects in the former processes in carriers of the R allele may increase type 2 diabetes risks.

The ligand environment of zinc stored in vesicles

Zinc serves regulatory functions in cells and thus, several mechanisms exist for tight control of its homeostasis. One mechanism is storage in and retrieval from vesicles, so-called zincosomes, but the chemical speciation of zincosomal zinc has remained enigmatic. Here, we determine the intravesicular zinc-coordination in isolated zincosomes in comparison to intact RAW264.7 murine macrophage cells. In elemental maps of a cell monolayer, generated by microbeam X-ray fluorescence, zincosomes were identified as spots of high zinc accumulation. A fingerprint for the binding motif obtained by muXANES (X-ray absorption near edge structure) matches the XANES from isolated vesicles; zinc is not free, but present as a complexed form (average coordination; 1.0 sulfur, 2,5 histidines 30 and 1.0 oxygen), resembling regulatory or catalytic zinc sites in proteins. Such coordination enables reversible binding, acting as a 'zinc sink', facilitating the accumulation of high amounts of zinc against a concentration gradient.

The interaction of biological and noxious transition metals with the zinc probes FluoZin-3 and Newport Green

Zinc-sensitive fluorescent probes have become increasingly important in the investigation of the cellular roles of zinc. There is, however, little information on how the other transition metals in cells may influence the measurement of zinc. We have characterized in vitro the interaction of the nominal zinc indicators FluoZin-3 and Newport Green with all the cationic transition metals found within cells, Cr, Mn, Fe, Co, and Cu, as well as Ni and Cd, by measuring their dissociation constants. In addition, we have shown how FluoZin-3 can be used to quantify the concentration of copper in a cell-free assay and report that the fluorescence of Newport Green is boosted by both Cu(I) and Fe(II). Furthermore, we have introduced diagnostics for detecting the interference of metals other than zinc with its measurement within cells.

Ultraviolet-A irradiation but not ultraviolet-B or infrared-A irradiation leads to a disturbed zinc homeostasis in cells

Changes of the redox balance in cells alter the availability of intracellular free Zn(2+). Here, cells were exposed to ultraviolet (UV)-A, UV-B, or infrared (IR)-A light irradiation, and the intracellular free zinc pool was monitored. Under sublethal conditions only UV-A irradiation resulted in a transient cytoplasmic and nuclear increase of intracellular free Zn(2+). Likewise, tert-butyl hydroperoxide and singlet oxygen, but not H(2)O(2) or intracellular generation of O(2)(*-) by redox cyclers, mimicked the effects of UV-A irradiation, while disulfide stress by diamide only led to a transient cytoplasmic zinc release. These results show that only certain types of subtoxic cellular stress massively disturb the zinc homeostasis in cells.

Imaging zinc in brain slices

INTRODUCTIONThis protocol provides a practical guide to imaging zinc within synaptic vesicles and the extracellular space of brain slices. Certain glutamatergic synaptic terminals in the brain have high concentrations of exchangeable Zn(2+) within their synaptic vesicles. Classically, these terminals have been revealed by Timm's histochemical stain. More recently, a number of fluorimetric probes have become available that allow Zn(2+) to be detected in live preparations. This protocol describes two methods, one for visualizing Zn(2+) in synaptic vesicles, and another for detecting Zn(2+) in the extracellular space. These methods are applicable to any tissue with the high levels of loosely bound Zn(2+) that are typically found in vesicles, and tissue that might have Zn(2+) associated with macromolecules in the extracellular space.

The accumulation of potentially-toxic metals by grazing ruminants

The main factors affecting the accumulation of potentially-toxic metals (PTM) by grazing animals are the presence of the metal, its concentration in herbage and at the soil surface, and the duration of exposure to the contaminated pasture and soil. In addition, the elapsed time between the contamination of the pasture and grazing, the quantity of soil ingested together with herbage, the mechanism of absorption of the metal into blood and the presence or absence of antagonistic metals can interact to influence the rate and extent of accumulation of heavy metals in edible body tissues. Models of the accumulation of metals by grazing animals may be used to determine the statutory limits of radionuclides and PTM in soils under grazed pastures. Meta-analysis of existing data, using a random-effects model, is a useful approach to understanding the factors affecting the accumulation of some metals, e.g. Cd. The target edible body tissues for the accumulation of most PTM are the liver and kidneys, with the exception of radiocaesium, which accumulates in muscle to a greater extent than in other tissues. The livers and kidneys of mature livestock that have been grazed on areas of pasture at the legal limit of contamination by Cd for more than one grazing season should be removed from the human food chain in order to reduce the risk of intake of Cd by the human population.

Inhibition of cyclooxygenase-2 expression by zinc-chelator in retinal ischemia

The zinc ion (Zn2+) is abundant in neurons. However, excessive Zn2+ can induce neuronal cell death. This study examined the role of Zn2+ in transient retinal ischemia in adult male rats. The rats were sacrificed 4-24 h after retinal ischemia by high intra-ocular pressure, and the retinas were prepared for microscopic examination of retinal cell degeneration, and fluorescence microscopy using zinquin ethyl ester as the zinc ion-specific probe. Moreover, COX-2 expression was observed by Western blotting. In control retinas, there was a low Zn2+ concentration in the inner plexiform layer (IPL), a high Zn2+ concentration in the outer plexiform layer (OPL), and no detectable Zn2+ in either the ganglion cell layer (GCL) or the inner nuclear layer (INL). In contrast, in the retinas exposed to ischemia without the administration of the zinc ion chelators (Ca2+-EDTA and TPEN), Zn2+ deposits were found in the IPL and INL beginning 4 h after ischemia and degeneration of neurons was found in the GCL and INL. Less Zn2+ accumulation in the IPL and INL and less neuronal degeneration in the GCL and INL were found in the retinas treated with Ca2+-EDTA or TPEN before ischemia. Furthermore, the COX-2 protein levels increased 4-8 h after retinal ischemia, and chelation of zinc ion inhibited this effect. These results suggest that the accumulation of Zn2+ following an ischemic insult can cause retinal degeneration and induce abnormal COX-2 expression.

Imaging synaptic zinc: promises and perils

It is well established that some excitatory nerve terminals have high concentrations of Zn(2+) in their synaptic vesicles. For some time, it has been believed that synaptic Zn(2+) is released during neurotransmission and acts as a neuromodulator. Fluorescent Zn(2+) indicators that do not penetrate membranes offer the prospect of rendering the release of Zn(2+) visible. Here, I take a critical look at fluorimetric imaging experiments devised to determine whether Zn(2+) is released and show that they are particularly susceptible to artifacts. Moreover, I will argue that recent experiments suggest that, rather than being released, Zn(2+) is presented to the extracellular space firmly coordinated to presynaptic macromolecules.

Zinquin identifies subcellular compartmentalization of zinc in cortical neurons. Relation to the trafficking of zinc and the mitochondrial compartment

Zinquin (Zn(2+) selective fluorophore), when used to visualize intracellular Zn(2+), typically shows brightly fluorescent perinuclear endosome-like structures, presumably identifying Zn(2+) containing organelles. In this study, zinquin identified numerous and widespread sites of Zn(2+) compartmentalization in primary cultures of embryonic rat cortical neurons. Nuclear fluorescence, however, was absent. We labeled neuronal mitochondria with MitoTracker Green in the presence of zinquin and show that the fluorescent patterns of MitoTracker Green and zinquin were distinct and clearly different in both the perinuclear region and in processes. The mitochondrial compartment was much larger than the sum of the areas of zinquin fluorescence, as indicated by the small amount (<10% MitoTracker Green over zinquin) of overlap of MitoTracker Green on zinquin. Zinquin fluorescence was unaffected by carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) treatment. The zinquin fluorescent objects were generally spherical in shape with a average diameter of about 0.6 mum. Most fluorescent objects, nearly two thirds on average, appeared to be docked, but both anterograde and retrograde movements were observed by time lapse image analysis. Although some fluorescent objects moved as much as 1 mum in 5 min, typical movements were smaller, usually 0.5 mum or less. Colchicine treatment caused striking aggregation of MitoTracker Green most noticeable in the perinuclear region. Zinquin fluorescence similarly showed reduced distribution throughout the cytoplasm, suggesting that zinquin fluorescent structures were associated with microtubules. Treatment with cytochalasin D had little noticeable effect on either the pattern of zinquin and MitoTracker Green fluorescence or their coincidence. Thus, numerous Zn(2+) sequestering organelles/structures are present in perinuclear regions and processes of cultured neurons and are sometimes found coincident with mitochondria. We demonstrated real time trafficking of sequestered Zn(2+), using zinquin fluorescence, apparently associated with an endosome-like compartment or protein complexes in the cytosol.

Influence of Location of a Fluorescent Zinc Probe in Brain Slices on Its Response to Synaptic Activation

The precise role of the high concentration of ionic zinc found in the synaptic vesicles of certain glutamatergic terminals is unknown. Fluorescent probes with their ability to detect ions at low concentrations provide a powerful approach to monitoring cellular Zn2+ levels. In the last few years, a number of fluorescent probes (indicators) have been synthesized that can be used to visualize Zn2+ in live cells. The interpretation of data gathered using such probes depends crucially on the location of the probe. Using acutely prepared hippocampal slices, we provide evidence that the Zn2+ probes, ZnAF-2 and ZP4, are membrane permeant and are able to pass into synaptic vesicles. In addition, we show that changes in fluorescence of the Zn2+ probes can be used to monitor presynaptic activity; however, these changes are inconsistent with Zn2+ release.

Membrane-permeable and -impermeable sensors of the Zinpyr family and their application to imaging of hippocampal zinc in vivo

Esterification of fluorescent biosensors is a common strategy used to trap probes within the cell. Zinpyr-1 (ZP1) is a fluorescein-based bright fluorescent sensor for divalent zinc that is cell permeable without prior modification. We describe here the synthesis and characterization of ZP1 sensors containing a carboxylic acid or ethyl ester functionality at the 5 or 6 position of the fluorescein. The presence of an electronegative carboxylate decreases the proton-induced background fluorescence of the probe by lowering the pKa of the benzylic amines responsible for fluorescence quenching. The charged species ZP1(6-CO2-) is membrane-impermeant, whereas the permeability of the neutral ZP1(5/6-CO2Et) is similar to that of the parent sensor. Intracranial microinfusion of ZP1(6-CO2Et) into rat hippocampus produces reduced staining of vesicular zinc in neuropil and very clear delineation of zinc-positive injured neuronal somata and dendrites as compared with ZP1.

Measurement of presynaptic zinc changes in hippocampal mossy fibers

The hippocampal mossy fiber terminals of CA3 area contain high levels of vesicular zinc that is released in a calcium-dependent way, following high-frequency stimulation. However the properties of zinc release during normal synaptic transmission, paired-pulse facilitation and mossy fiber long-term potentiation are still unknown. Using the fluorescent zinc probe N-(6-methoxy-8-quinolyl)-para-toluenesulfonamide, we measured fast mossy fiber zinc changes indicating that zinc is released following single and low levels of electrical stimulation. The observed presynaptic zinc signals are maintained during the expression of mossy fiber long-term potentiation, assumed to be mediated by an increase in transmitter release, and are enhanced during paired-pulse facilitation. This zinc enhancement is, like paired-pulse facilitation, reduced during established long-term potentiation. The correlation between the paired-pulse evoked zinc and field potential responses supports the idea that zinc is co-released with glutamate.

Method for identifying neuronal cells suffering zinc toxicity by use of a novel fluorescent sensor

During excitotoxic brain damage, injured neurons accumulate an anomalous, pathological burden of weakly bound, rapidly exchangeable Zn(2+) that diffusely fills the soma, nucleus and proximal dendrites. Mounting evidence indicates that this Zn(2+) is a major contributing factor in the subsequent demise of the damaged neurons. Thus, identifying, imaging, and characterizing zinc-filled cells have become essential steps in understanding excitotoxicity. Here we demonstrate that a new fluorescent stain for zinc can rather selectively and quite vividly label zinc-filled neurons in frozen histologic sections. The method is more sensitive and selective than the existing stain TSQ, and simpler than the Timm-Danscher silver staining techniques. A previously unobserved population of apparently injured cells in the dentate gyrus has been discovered with the new reagent. Whereas cells viewed in situ in normal, healthy tissue virtually never display any perikaryal staining by histochemical methods for zinc, injured cells stain intensely for zinc in culture, acute slice preparations and in tissue harvested in vivo. Thus, the presence of rapidly-exchangeable, "stainable" perikaryal zinc may be taken as an indicator of cell injury.

Differential regulation of zinc efflux transporters ZnT-1, ZnT-5 and ZnT-7 gene expression by zinc levels: a real-time RT–PCR study

Intracellular zinc levels are strictly regulated by zinc channels and zinc-binding proteins to maintain cellular zinc-dependent functions. We demonstrated a correlation between extracellular zinc concentration and intracellular exchangeable zinc levels using the fluorescent zinc-specific probes zinquin and zinpyr-1. The effect of extracellular zinc status on the regulation of the two trans-Golgi network directed zinc transporters ZnT-5 and ZnT-7 was next studied by real-time RT-PCR in zinc supplemented or depleted HeLa cells. While sub-toxic extracellular zinc addition strongly induced the efflux transporter ZnT-1 gene expression, consistent with its activation by the transcription factor MTF-1, treated HeLa cells did not display any change in ZnT-5 and ZnT-7 mRNA levels compared to control cells. In contrast, zinc depletion induced by non-toxic doses of the zinc chelator TPEN (N,N,N',N' tetrakis-(2 pyridylmethyl) ethylene diamine) resulted in a up to eight-fold induction of transporters ZnT-5 and ZnT-7 mRNA levels, providing the first evidence of a transcriptional control of these two zinc efflux transporters by zinc deficiency in cultured cells.

Zn2+ transporters and Zn2+ homeostasis in neurons

Although the presence of Zn2+ in the brain has been known for nearly half a century, only recently has its precise location and potential roles as a neuromodulator and signaling molecule as well as neurotoxic agent come to the forefront. Unfortunately, our understanding of Zn2+ homeostatic mechanisms lags far behind. The recent identification of presumed Zn2+ transporters has opened new approaches to studying Zn2+ homeostatic mechanisms in neurons. Zn2+ transporters are involved in separate Zn2+ influx and efflux pathways in neurons. However, we are only beginning to understand the mechanism of Zn2+ transport and much more research needs to be done. We are only beginning to understand the transcriptional control and cellular location of Zn2+ transporters, as well. Finally, this review presents a working model of neuronal Zn2+ homeostasis and discusses the experimental evidence for the proposed roles that Zn2+ transporters might play.

Neuronal zinc exchange with the blood vessel wall promotes cerebral amyloid angiopathy in an animal model of Alzheimer's disease

Cerebral amyloid angiopathy (CAA) is common in Alzheimer's disease (AD) and may contribute to dementia and cerebral hemorrhage. Parenchymal beta-amyloid deposition is dependent on the activity of zinc transporter 3 (ZnT3), a neocortical synaptic vesicle membrane protein that causes enrichment of exchangeable Zn2+ in the vesicle, which is externalized on neurotransmission. However, the contribution of zinc to vascular beta-amyloid deposition remains unclear. Here, we identify for the first time an exchangeable pool of Zn2+ in the cerebrovascular wall of normal mice. This histochemically reactive Zn2+ is enriched in CAA in a transgenic mouse model of AD (Tg2576), and a dramatic reduction of CAA occurs after targeted disruption of the Znt3 gene in these mice. Also, in Znt3 knock-out mice, the amount of exchangeable Zn2+ [detected by N-(6-methoxy-8-quinolyl)-p-carboxybenzoylsulphonamide (TFL-Zn)] in the perivascular space was significantly decreased in the neocortex but not in peripheral organs. ZnT3 was not detected in the cerebral vessel walls or in blood components of wild-type mice. Thus, synaptic ZnT3 activity may promote CAA by indirectly raising exchangeable Zn2+ concentrations in the perivascular spaces of the brain.

Mechanism of apoptosis induced by S100A8/A9 in colon cancer cell lines: the role of ROS and the effect of metal ions

The protein complex S100A8/A9, abundant in the cytosol of neutrophils, is secreted from the cells upon cellular activation and induces apoptosis in tumor cell lines and normal fibroblasts in a zinc-reversible manner. In the present study, we present evidence that the S100A8/A9 also exerts its apoptotic effect by a zinc-independent mechanism. Treatment of the colon carcinoma cells with different concentrations of human S100A8/A9 or the metal ion chelator diethylenetriaminepentacetic acid (DTPA) resulted in a significant increase of cell death. Annexin V/phosphatidylinositol and Hoechst 33258 staining revealed that cell death was mainly of the apoptotic type. A significant increase in the activity of caspase-3 and -9 was observed in both cell lines after treatment. Caspase-8 activation was negligible in both cell lines. The cytotoxicity/apoptotic effect of human S100A8/A9 and DTPA was inhibited significantly (P<0.05) by Zn(+2) and Cu(+2), more effectively than by Ca(2+) and Mg(2+). The antioxidant N-acetyl-L-cysteine inhibited the cytotoxicity/apoptotic effect of S100A8/A9 and DTPA. However, as a result of the different time-courses of both agents and that the S100A8/A9-induced apoptosis was not completely reversed, we conclude that S100A8/A9 exerts its apoptotic effect on two colon carcinoma cell lines through a dual mechanism: one via zinc exclusion from the target cells and the other through a yet-undefined mechanism, probably relaying on the cell-surface receptor(s).

Detecting and minimizing zinc contamination in physiological solutions

Background

To explore the role of zinc (Zn) in cellular physiology it is important to be able to control and quantify the level of Zn contamination in experimental solutions. A technique that relies on a Zn-sensitive fluorimetric probe is introduced for measuring Zn concentrations as low as 100 pM. The method depends on the combination of the Zn-probe FluoZin-3 together with a slow Zn-chelator, Ca-EDTA, that reduces the background Zn levels and allows repeated measurements in the same solution.

Results

The method was used to determine which common labware items could leach Zn into solution. Contamination was predictably found to arise from stainless steel and glass. Perhaps less expectedly it was also introduced by methacrylate cuvettes, plastic tissue culture dishes and other plastic labware. The release of nickel from stainless steel electrodes was also imaged using the fluorescent probe Newport Green.

Conclusion

Zn contamination may arise from rather unexpected sources; it is important that all aspects and components used in the course of an experiment be analyzed for the possibility of introducing contaminants.

The accumulation of potentially-toxic metals by grazing ruminants

The main factors affecting the accumulation of potentially-toxic metals (PTM) by grazing animals are the presence of the metal, its concentration in herbage and at the soil surface, and the duration of exposure to the contaminated pasture and soil. In addition, the elapsed time between the contamination of the pasture and grazing, the quantity of soil ingested together with herbage, the mechanism of absorption of the metal into blood and the presence or absence of antagonistic metals can interact to influence the rate and extent of accumulation of heavy metals in edible body tissues. Models of the accumulation of metals by grazing animals may be used to determine the statutory limits of radionuclides and PTM in soils under grazed pastures. Meta-analysis of existing data, using a random-effects model, is a useful approach to understanding the factors affecting the accumulation of some metals, e.g. Cd. The target edible body tissues for the accumulation of most PTM are the liver and kidneys, with the exception of radiocaesium, which accumulates in muscle to a greater extent than in other tissues. The livers and kidneys of mature livestock that have been grazed on areas of pasture at the legal limit of contamination by Cd for more than one grazing season should be removed from the human food chain in order to reduce the risk of intake of Cd by the human population.

The zinc transporter ZnT3 interacts with AP-3 and it is preferentially targeted to a distinct synaptic vesicle subpopulation

Synaptic vesicles (SV) are generated by two different mechanisms, one AP-2 dependent and one AP-3 dependent. It has been uncertain, however, whether these mechanisms generate SV that differ in molecular composition. We explored this hypothesis by analyzing the targeting of ZnT3 and synaptophysin both to PC12 synaptic-like microvesicles (SLMV) as well as SV isolated from wild-type and AP-3-deficient mocha brains. ZnT3 cytosolic tail interacted selectively with AP-3 in cell-free assays. Accordingly, pharmacological disruption of either AP-2- or AP-3-dependent SLMV biogenesis preferentially reduced synaptophysin or ZnT3 targeting, respectively; suggesting that these antigens were concentrated in different vesicles. As predicted, immuno-isolated SLMV revealed that ZnT3 and synaptophysin were enriched in different vesicle populations. Likewise, morphological and biochemical analyses in hippocampal neurons indicated that these two antigens were also present in distinct but overlapping domains. ZnT3 SV content was reduced in AP-3-deficient neurons, but synaptophysin was not altered in the AP-3 null background. Our evidence indicates that neuroendocrine cells assemble molecularly heterogeneous SV and suggests that this diversity could contribute to the functional variety of synapses.

Evidence for chelatable zinc in the extracellular space of the hippocampus, but little evidence for synaptic release of Zn

Zinc colocalizes with glutamate in the synaptic vesicles of certain glutamatergic vesicles in the mammalian brain. Here, I introduce a method for detecting Zn in the extracellular space of brain slices and another method for detecting the passage of Zn out of the slice. In both cases, the fluorimetric Zn probe FluoZin-3 is used in conjunction with a slow Zn chelator, Ca-EDTA, to reduce background fluorescence. In addition, a new Zn chelator, ethylenediiminodi-2-pentanedioic acid, with little affinity for Ca or Mg is introduced. These tools are then used to show that little Zn (approximately 2 nm) is released during the course of synaptic transmission into the extracellular space. However, when hippocampal slices are subjected to a high potassium stimulus (50 mM) combined with an increase in osmolarity, Zn is externalized in the Timm's-stained areas (approximately 6 nm). This stimulus also leads to even greater Zn elevations in area CA1 that is only weakly stained by the Timm's method. Nevertheless, even under these conditions, little if any Zn makes its way out of the slices. I present evidence for a layer of Zn in the extracellular space that maps onto the Timm's stained region of the hippocampus. This Zn veneer appears to be loosely associated with molecules in the extracellular space and may be the raison d'être for vesicular Zn.