Measuring picomolar intracellular exchangeable zinc in PC-12 cells using a ratiometric fluorescence biosensor

Genetically encoded sensors to elucidate spatial distribution of cellular zinc

Transition metals are essential enzyme cofactors that are required for a wide range of cellular processes. Paradoxically, whereas metal ions are essential for numerous cellular processes, they are also toxic. Therefore cells must tightly regulate metal accumulation, transport, distribution, and export. Improved tools to interrogate metal ion availability and spatial distribution within living cells would greatly advance our understanding of cellular metal homeostasis. In this work, we present genetically encoded sensors for Zn2+ based on the principle of fluorescence resonance energy transfer. We also develop methodology to calibrate the probes within the cellular environment. To identify both sources of and sinks for Zn2+, these sensors are genetically targeted to specific locations within the cell, including cytosol, plasma membrane, and mitochondria. Localized probes reveal that mitochondria contain an elevated pool of Zn2+ under resting conditions that can be released into the cytosol upon glutamate stimulation of hippocampal neurons. We also observed that Zn2+ is taken up into mitochondria following glutamate/Zn2+ treatment and that there is heterogeneity in both the magnitude and kinetics of the response. Our results suggest that mitochondria serve as a source of and a sink for Zn2+ signals under different cellular conditions.

Essential role of p53 in TPEN-induced neuronal apoptosis

Depletion of intracellular zinc with N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) induces protein synthesis-dependent apoptosis. In this study, we examined the requirement for p53 as an upstream transcription factor in TPEN-induced neuronal apoptosis. Chemical or genetic blockade of p53 markedly attenuated TPEN-induced neuronal apoptosis, while the stability and activity of p53 were increased by TPEN. In addition, expression of proapoptotic genes, PUMA and NOXA, and activation of caspase-11 were increased by TPEN in a p53-dependent manner. Inhibition of p53 blocked cytochrome C release from mitochondria to cytosol and prevented caspase-3 activation. Therefore, p53 may be an essential regulatory factor for TPEN-induced neuronal apoptosis.

Energies, Geometries, and Charge Distributions of Zn Molecules, Clusters, and Biocenters from Coupled Cluster, Density Functional, and Neglect of Diatomic Differential Overlap Models

We present benchmark databases of Zn-ligand bond distances, bond angles, dipole moments, and bond dissociation energies for Zn-containing small molecules and Zn coordination compounds with H, CH3, C2H5, NH3, O, OH, H2O, F, Cl, S, and SCH3 ligands. The test set also includes clusters with Zn-Zn bonds. In addition, we calculated dipole moments and binding energies for Zn centers in coordination environments taken from zinc metalloenzyme X-ray structures, representing both structural and catalytic zinc centers. The benchmark values are based on relativistic-core coupled cluster calculations. These benchmark calculations are used to test the predictions of four density functionals, namely B3LYP and the more recently developed M05-2X, M06, and M06-2X levels of theory, and six semiempirical methods, including neglect of diatomic differential overlap (NDDO) calculations incorporating the new PM3 parameter set for Zn called ZnB, developed by Brothers and co-workers, and the recent PM6 parametrization of Stewart. We found that the best DFT method to reproduce dipole moments and dissociation energies of our Zn compound database is M05-2X, which is consistent with a previous study employing a much smaller and less diverse database and a much larger set of density functionals. Here we show that M05-2X geometries and single-point coupled cluster calculations with M05-2X geometries can also be used as benchmarks for larger compounds, where coupled cluster optimization is impractical, and in particular we use this strategy to extend the geometry, binding energy, and dipole moment databases to additional molecules, and we extend the tests involving crystal-site coordination compounds to two additional proteins. We find that the most predictive NDDO methods for our training set are PM3 and MNDO/d. Notably, we also find large errors in B3LYP for the coordination compounds based on experimental X-ray geometries.

A self-cleaving DNA enzyme modified with amines, guanidines and imidazoles operates independently of divalent metal cations (M2+)

The selection of modified DNAzymes represents an important endeavor in expanding the chemical and catalytic properties of catalytic nucleic acids. Few examples of such exist and to date, there is no example where three different modified bases have been simultaneously incorporated for catalytic activity. Herein, dCTP, dATP and dUTP bearing, respectively, a cationic amine, an imidazole and a cationic guanidine, were enzymatically polymerized on a DNA template for the selection of a highly functionalized DNAzyme, called DNAzyme 9-86, that catalyzed (M(2+))-independent self-cleavage under physiological conditions at a single ribo(cytosine)phosphodiester linkage with a rate constant of (0.134 +/- 0.026) min(-1). A pH rate profile analysis revealed pK(a)'s of 7.4 and 8.1, consistent with both general acid and base catalysis. The presence of guanidinium cations permits cleavage at significantly higher temperatures than previously observed for DNAzymes with only amines and imidazoles. Qualitatively, DNAzyme 9-86 presents an unprecedented ensemble of synthetic functionalities while quantitatively it expresses one of the highest reported values for any self-cleaving nucleic acid when investigated under M(2+)-free conditions at 37 degrees C.

Glutamate mobilizes [Zn2+] through Ca2+ -dependent reactive oxygen species accumulation

Liberation of zinc from intracellular stores contributes to oxidant-induced neuronal injury. However, little is known regarding how endogenous oxidant systems regulate intracellular free zinc ([Zn(2+)](i)). Here we simultaneously imaged [Ca(2+)](i) and [Zn(2+)](i) to study acute [Zn(2+)](i) changes in cultured rat forebrain neurons after glutamate receptor activation. Neurons were loaded with fura-2FF and FluoZin-3 to follow [Ca(2+)](i) and [Zn(2+)](i), respectively. Neurons treated with glutamate (100 microM) for 10 min gave large Ca(2+) responses that did not recover after termination of the glutamate stimulus. Glutamate also increased [Zn(2+)](i), however glutamate-induced [Zn(2+)](i) changes were completely dependent on Ca(2+) entry, appeared to arise entirely from internal stores, and were substantially reduced by co-application of the membrane-permeant chelator TPEN during the glutamate treatment. Pharmacological maneuvers revealed that a number of endogenous oxidant producing systems, including nitric oxide synthase, phospholipase A(2), and mitochondria all contributed to glutamate-induced [Zn(2+)](i) changes. We found no evidence that mitochondria buffered [Zn(2+)](i) during acute glutamate receptor activation. We conclude that glutamate-induced [Zn(2+)](i) transients are caused in part by [Ca(2+)](i)-induced reactive oxygen species that arises from both cytosolic and mitochondrial sources.

Structure of a 129Xe-cryptophane biosensor complexed with human carbonic anhydrase II

Cryptophanes represent an exciting class of xenon-encapsulating molecules that can be exploited as probes for nuclear magnetic resonance imaging. The 1.70 A resolution crystal structure of a cryptophane-derivatized benezenesulfonamide complexed with human carbonic anhydrase II shows how an encapsulated xenon atom can be directed to a specific biological target. The crystal structure confirms binding measurements indicating that the cryptophane cage does not strongly interact with the surface of the protein, which may enhance the sensitivity of 129Xe NMR spectroscopic measurements in solution.

Selective Labeling of Proteins with Chemical Probes in Living Cells

Selective labeling of proteins with small molecules introduces novel chemical and physical properties into proteins, enabling the target protein to be investigated or manipulated with various techniques. Different methods for labeling proteins in living cells have been developed by using protein domains, small peptides, or single amino acids. Their application in cells and in vivo has yielded novel insights into diverse biological processes.

His-tags as Zn(II) binding motifs in a protein-based fluorescent sensor

Fluorescent indicators that allow real-time imaging of Zn(II) in living cells are invaluable tools for understanding Zn(II) homeostasis. Genetically encoded sensors based on fluorescence resonance energy transfer between fluorescent protein domains have important advantages over synthetic probes. We discovered that hexahistidine tags have a strong tendency to dimerize upon binding of Zn(II) in solution and we used this principle to develop a new protein-based sensor for Zn(II). Enhanced cyan and yellow fluorescent proteins were connected by long flexible peptide linkers and His-tags were incorporated at both termini of this fusion protein. The resulting sensor CLY9-2His allows the ratiometric fluorescent detection of Zn(II) in the nanomolar range. In addition, CLY9-2His is selective over the physiologically relevant metal ions Fe(II), Mn(II), Ca(II) and Mg(II). Our approach demonstrates the potential of using small peptides as metal-binding ligands in chelating fluorescent protein chimeras.

Imaging the boundaries-innovative tools for microscopy of living cells and real-time imaging

Recently, light microscopy moved back into the spotlight, which is mainly due to the development of revolutionary technologies for imaging real-time events in living cells. It is truly fascinating to see enzymes “at work” and optically acquired images certainly help us to understand biological processes better than any abstract measurements. This review aims to point out elegant examples of recent cell-biological imaging applications that have been developed with a chemical approach. The discussed technologies include nanoscale fluorescence microscopy, imaging of model membranes, automated high-throughput microscopy control and analysis, and fluorescent probes with a special focus on visualizing enzyme activity, free radicals, and protein–protein interaction designed for use in living cells.

Design and synthesis of a caged Zn2+ probe, 8-benzenesulfonyloxy-5-N,N-dimethylaminosulfonylquinolin-2-ylmethyl-pendant 1,4,7,10-tetraazacyclododecane, and its hydrolytic uncaging upon complexation with Zn2+

8-Benzenesulfonyloxy-5- N,N-dimethylaminosulfonylquinolin-2-ylmethyl-pendant cyclen (BS-caged-L(4), BS = benzenesulfonyl) was designed and synthesized as a "caged" derivative of a previously described Zn(2+) fluorophore, 8-hydroxy-5- N,N-dimethylaminosulfonylquinolin-2-ylmethyl-pendant cyclen (L(4)) (cyclen = 1,4,7,10-tetraazacyclododecane). In the absence of metal ions and in the dark, BS-caged-L(4) (10 microM) showed negligible fluorescence emission at pH 7.4 (10 mM HEPES with I = 0.1 (NaNO3)) and 25 degrees C (excitation at 328 nm). Addition of Zn(2+) induced an increase in the UV/vis absorption of BS-caged-L(4) (10 microM) at 258 nm and a significant increase in fluorescence emission at 512 nm. These responses are results from the formation of Zn(H-1L(4)) by the hydrolysis of the sulfonyl ester at the 8-position of the quinoline unit promoted by the Zn(2+)-bound HO(-). Improvement of cell membrane permeation in comparison with L(4) is also described.

Insights into Zn2+homeostasis in neurons from experimental and modeling studies

To understand the mechanisms of neuronal Zn2+homeostasis better, experimental data obtained from cultured cortical neurons were used to inform a series of increasingly complex computational models. Total metals (inductively coupled plasma-mass spectrometry), resting metallothionein,65Zn2+uptake and release, and intracellular free Zn2+levels using ZnAF-2F were determined before and after neurons were exposed to increased Zn2+, either with or without the addition of a Zn2+ionophore (pyrithione) or metal chelators [EDTA, clioquinol (CQ), and N, N, N′, N′-tetrakis(2-pyridylmethyl)ethylenediamine]. Three models were tested for the ability to match intracellular free Zn2+transients and total Zn2+content observed under these conditions. Only a model that incorporated a muffler with high affinity for Zn2+, trafficking Zn2+to intracellular storage sites, was able to reproduce the experimental results, both qualitatively and quantitatively. This “muffler model” estimated the resting intracellular free Zn2+concentration to be 1.07 nM. If metallothionein were to function as the exclusive cytosolic Zn2+muffler, the muffler model predicts that the cellular concentration required to match experimental data is greater than the measured resting concentration of metallothionein. Thus Zn2+buffering in resting cultured neurons requires additional high-affinity cytosolic metal binding moieties. Added CQ, as low as 1 μM, was shown to selectively increase Zn2+influx. Simulations reproduced these data by modeling CQ as an ionophore. We conclude that maintenance of neuronal Zn2+homeostasis, when challenged with Zn2+loads, relies heavily on the function of a high-affinity muffler, the characteristics of which can be effectively studied with computational models.

Determination of zinc using carbonic anhydrase-based fluorescence biosensors

This chapter summarizes the use of carbonic anhydrase (CA)-based fluorescent indicators to determine free zinc in solution, in cells, and in subcellular organelles. Expression (both in situ and in vitro) and preparation of CA-based indicators are described, together with techniques of their use, and procedures to minimize contamination. Recipes for zinc buffers are supplied.

Thionein/metallothionein control Zn(II) availability and the activity of enzymes

Fundamental issues in zinc biology are how proteins control the concentrations of free Zn(II) ions and how tightly they interact with them. Since, basically, the Zn(II) stability constants of only two cytosolic zinc enzymes, carbonic anhydrase and superoxide dismutase, have been reported, the affinity for Zn(II) of another zinc enzyme, sorbitol dehydrogenase (SDH), was determined. Its log K is 11.2 +/- 0.1, which is similar to the log K values of carbonic anhydrase and superoxide dismutase despite considerable differences in the coordination environments of Zn(II) in these enzymes. Protein tyrosine phosphatase 1B (PTP 1B), on the other hand, is not classified as a zinc enzyme but is strongly inhibited by Zn(II), with log K = 7.8 +/- 0.1. In order to test whether or not metallothionein (MT) can serve as a source for Zn(II) ions, it was used to control free Zn(II) ion concentrations. MT makes Zn(II) available for both PTP 1B and the apoform of SDH. However, whether or not Zn(II) ions are indeed available for interaction with these enzymes depends on the thionein (T) to MT ratio and the redox poise. At ratios [T/(MT + T) = 0.08-0.31] prevailing in tissues and cells, picomolar concentrations of free Zn(II) are available from MT for reconstituting apoenzymes with Zn(II). Under conditions of decreased ratios, nanomolar concentrations of free Zn(II) become available and affect enzymes that are not zinc metalloenzymes. The match between the Zn(II) buffering capacity of MT and the Zn(II) affinity of proteins suggests a function of MT in controlling cellular Zn(II) availability.

Mobile trap algorithm for zinc detection using protein sensors

We present a mobile trap algorithm to sense zinc ions using protein-based sensors such as carbonic anhydrase (CA). Zinc is an essential biometal required for mammalian cellular functions although its intracellular concentration is reported to be very low. Protein-based sensors like CA molecules are employed to sense rare species like zinc ions. In this study, the zinc ions are mobile targets, which are sought by the mobile traps in the form of sensors. Particle motions are modeled using random walk along with the first passage technique for efficient simulations. The association reaction between sensors and ions is incorporated using a probability (p1) upon an ion-sensor collision. The dissociation reaction of an ion-bound CA molecule is modeled using a second, independent probability (p2). The results of the algorithm are verified against the traditional simulation techniques (e.g., Gillespie's algorithm). This study demonstrates that individual sensor molecules can be characterized using the probability pair (p1,p2), which, in turn, is linked to the system level chemical kinetic constants, kon and koff. Further investigations of CA-Zn reaction using the mobile trap algorithm show that when the diffusivity of zinc ions approaches that of sensor molecules, the reaction data obtained using the static trap assumption differ from the reaction data obtained using the mobile trap formulation. This study also reveals similar behavior when the sensor molecule has higher dissociation constant. In both the cases, the reaction data obtained using the static trap formulation reach equilibrium at a higher number of complex molecules (ion-bound sensor molecules) compared to the reaction data from the mobile trap formulation. With practical limitations on the number sensors that can be inserted/expressed in a cell and stochastic nature of the intracellular ionic concentrations, fluorescence from the number of complex sensor molecules at equilibrium will be the measure of the intracellular ion concentration. For reliable detection of zinc ions, it is desirable that the sensors must not bind all the zinc ions tightly, but should rather bind and unbind. Thus for a given fluorescence and with association-dissociation reactions between ions and sensors, the static trap approach will underestimate the number of zinc ions present in the system.

A water-soluble, small molecular fluorescent sensor with femtomolar sensitivity for zinc ion

A water-soluble fluorescent sensor, 1, based on the quinoline platform, demonstrates femtomolar sensitivity for zinc ion with a 14-fold enhanced quantum yield upon chelation to zinc ion and also exhibits high selectivity to zinc ion over other physiological relevant divalent metals in the presence of EDTA. X-ray crystal structure of zinc complex reveals that an acetic carboxylic group participates in coordination, which significantly enhances the affinity of 1 for zinc ion.

Mechanism and regulation of cellular zinc transport

Zinc is an essential cofactor for the activity and folding of up to ten percent of mammalian proteins and can modulate the function of many others. Because of the pleiotropic effects of zinc on every aspect of cell physiology, deficits of cellular zinc content, resulting from zinc deficiency or excessive rise in its cellular concentration, can have catastrophic consequences and are linked to major patho-physiologies including diabetes and stroke. Thus, the concentration of cellular zinc requires establishment of discrete, active cellular gradients. The cellular distribution of zinc into organelles is precisely managed to provide the zinc concentration required by each cell compartment. The complexity of zinc homeostasis is reflected by the surprisingly large variety and number of zinc homeostatic proteins found in virtually every cell compartment. Given their ubiquity and importance, it is surprising that many aspects of the function, regulation, and crosstalk by which zinc transporters operate are poorly understood. In this mini-review, we will focus on the mechanisms and players required for generating physiologically appropriate zinc gradients across the plasma membrane and vesicular compartments. We will also highlight some of the unsolved issues regarding their role in cellular zinc homeostasis.

Ratiometric fluorescent sensor proteins with subnanomolar affinity for Zn(II) based on copper chaperone domains

The ability to image the concentration of transition metals in living cells in real time is important for further understanding of transition metal homeostasis and its involvement in diseases. The goal of this study was to develop a genetically encoded FRET-based sensor for copper(I) based on the copper-induced dimerization of two copper binding domains involved in human copper homeostasis, Atox1 and the fourth domain of ATP7B (WD4). A sensor has been constructed by linking these copper binding domains to donor and acceptor fluorescent protein domains. Energy transfer is observed in the presence of Cu(I), but the Cu(I)-bridged complex is easily disrupted by low molecular weight thiols such as DTT and glutathione. To our surprise, energy transfer is also observed in the presence of very low concentrations of Zn(II) (10(-)(10) M), even in the presence of DTT. Zn(II) is able to form a stable complex by binding to the cysteines present in the conserved MXCXXC motif of the two copper binding domains. Co(II), Cd(II), and Pb(II) also induce an increase in FRET, but other, physiologically relevant metals are not able to mediate an interaction. The Zn(II) binding properties have been tuned by mutation of the copper-binding motif to the zinc-binding consensus sequence MDCXXC found in the zinc transporter ZntA. The present system allows the molecular mechanism of copper and zinc homeostasis to be studied under carefully controlled conditions in solution. It also provides an attractive platform for the further development of genetically encoded FRET-based sensors for Zn(II) and other transition metal ions.

Dual nanomolar and picomolar Zn(II) binding properties of metallothionein

Each of the seven Zn(II) ions in the Zn(3)S(9) and Zn(4)S(11) clusters of human metallothionein is in a tetrathiolate coordination environment. Yet analysis of Zn(II) association with thionein, the apoprotein, and analysis of Zn(II) dissociation from metallothionein using the fluorescent chelating agents FluoZin-3 and RhodZin-3 reveal at least three classes of sites with affinities that differ by 4 orders of magnitude. Four Zn(II) ions are bound with an apparent average log K of 11.8, and with the methods employed, their binding is indistinguishable. This binding property makes thionein a strong chelating agent. One Zn(II) ion is relatively weakly bound, with a log K of 7.7, making metallothionein a zinc donor in the absence of thionein. The binding data demonstrate that Zn(II) binds with at least four species: Zn(4)T, Zn(5)T, Zn(6)T, and Zn(7)T. Zn(5)T and Zn(6)T bind Zn(II) with a log K of approximately 10 and are the predominant species at micromolar concentrations of metallothionein in cells. Central to the function of the protein is the reactivity of its cysteine side chains in the absence and presence of Zn(II). Chelating agents, such as physiological ligands with moderate affinities for Zn(II), cause dissociation of Zn(II) ions from metallothionein at pH 7.4 (Zn(7)T <==> Zn(7-n)T + nZn(2+)), thereby affecting the reactivity of its thiols. Thus, the rate of thiol oxidation increases in the presence of Zn(II) acceptors but decreases if more free Zn(II) becomes available. Thionein is such an acceptor. It regulates the reactivity and availability of free Zn(II) from metallothionein. At thionein/metallothionein ratios > 0.75, free Zn(II) ions are below a pZn (-log[Zn(2+)](free)) of 11.8, and at ratios < 0.75, relatively large fluctuations of free Zn(II) ions are possible (pZn between 7 and 11). These chemical characteristics match cellular requirements for Zn(II) and suggest how the molecular structures and redox chemistries of metallothionein and thionein determine Zn(II) availability for biological processes.

Mechanism and regulation of cellular zinc transport

Zinc is an essential cofactor for the activity and folding of up to ten percent of mammalian proteins and can modulate the function of many others. Because of the pleiotropic effects of zinc on every aspect of cell physiology, deficits of cellular zinc content, resulting from zinc deficiency or excessive rise in its cellular concentration, can have catastrophic consequences and are linked to major patho-physiologies including diabetes and stroke. Thus, the concentration of cellular zinc requires establishment of discrete, active cellular gradients. The cellular distribution of zinc into organelles is precisely managed to provide the zinc concentration required by each cell compartment. The complexity of zinc homeostasis is reflected by the surprisingly large variety and number of zinc homeostatic proteins found in virtually every cell compartment. Given their ubiquity and importance, it is surprising that many aspects of the function, regulation, and crosstalk by which zinc transporters operate are poorly understood. In this mini-review, we will focus on the mechanisms and players required for generating physiologically appropriate zinc gradients across the plasma membrane and vesicular compartments. We will also highlight some of the unsolved issues regarding their role in cellular zinc homeostasis.

Midrange affinity fluorescent Zn(II) sensors of the Zinpyr family: syntheses, characterization, and biological imaging applications

The syntheses and photophysical characterization of ZP9, 2-{2-chloro-6-hydroxy-3-oxo-5-[(2-{[pyridin-2-ylmethyl-(1H-pyrrol-2-ylmethyl)amino]methyl}phenylamino)methyl]-3H-xanthen-9-yl}benzoic acid, and ZP10, 2-{2-chloro-6-hydroxy-5-[(2-{[(1-methyl-1H-pyrrol-2-ylmethyl)pyridin-2-ylmethylamino]methyl}phenylamino)methyl]-3-oxo-3H-xanthen-9-yl}benzoic acid, two asymmetrically derivatized fluorescein-based dyes, are described. These sensors each contain an aniline-based ligand moiety functionalized with a pyridyl-amine-pyrrole group and have dissociation constants for Zn(II) in the sub-micromolar (ZP9) and low-micromolar (ZP10) range, which we define as "midrange". They give approximately 12- (ZP9) and approximately 7-fold (ZP10) fluorescence turn-on immediately following Zn(II) addition at neutral pH and exhibit improved selectivity for Zn(II) compared to the di-(2-picolyl)amine-based Zinpyr (ZP) sensors. Confocal microscopy studies indicate that such asymmetrical fluorescein-based probes are cell permeable and Zn(II) responsive in vivo.

The zinc/thiolate redox biochemistry of metallothionein and the control of zinc ion fluctuations in cell signaling

Free zinc ions are potent effectors of proteins. Their tightly controlled fluctuations ("zinc signals") in the picomolar range of concentrations modulate cellular signaling pathways. Sulfur (cysteine) donors generate redox-active coordination environments in proteins for the redox-inert zinc ion and make it possible for redox signals to induce zinc signals. Amplitudes of zinc signals are determined by the cellular zinc buffering capacity, which itself is redox-sensitive. In part by interfering with zinc and redox buffering, reactive species, drugs, toxins, and metal ions can elicit zinc signals that initiate physiological and pathobiochemical changes or lead to cellular injury when free zinc ions are sustained at higher concentrations. These interactions establish redox-inert zinc as an important factor in redox signaling. At the center of zinc/redox signaling are the zinc/thiolate clusters of metallothionein. They can transduce zinc and redox signals and thereby attenuate or amplify these signals.

Chemical tools for biomolecular imaging

The visualization of biologically relevant molecules and activities inside living cells continues to transform cell biology into a truly quantitative science. However, despite the spectacular achievements in some areas of cell biology, the majority of cellular processes still operate invisibly, not illuminated by even our brightest laser beams. Further progress therefore will depend not only on improvements in instrumentation but also increasingly on the development of new fluorophores and fluorescent sensors to target these activities. In the following, we review some of the recent approaches to generating such sensors, the methods to attach them to selected biomolecules, and their applications to various biological problems.

Fluorescent dyes of the esculetin and alizarin families respond to zinc ions ratiometrically

Commercially available dyes of the esculetin and alizarin families are identified as lead structures for constructing ratiometric fluorescent probes for zinc ions.

Zinspy sensors with enhanced dynamic range for imaging neuronal cell zinc uptake and mobilization

Thiophene moieties were incorporated into previously described Zinspy (ZS) fluorescent Zn(II) sensor motifs (Nolan, E. M.; Lippard, S. J. Inorg. Chem. 2004, 43, 8310-8317) to provide enhanced fluorescence properties, low-micromolar dissociation constants for Zn(II), and improved Zn(II) selectivity. Halogenation of the xanthenone and benzoate moieties of the fluorescein platform systematically modulates the excitation and emission profiles, pH-dependent fluorescence, Zn(II) affinity, and Zn(II) complexation rates, offering a general strategy for tuning multiple properties of xanthenone-based metal ion sensors. Extensive biological studies in cultured cells and primary neuronal cultures demonstrate 2-{6-hydroxy-3-oxo-4,5-bis[(pyridin-2-ylmethylthiophen-2-ylmethylamino)methyl]-3H-xanthen-9-yl}benzoic acid (ZS5) to be a versatile imaging tool for detecting Zn(II) in vivo. ZS5 localizes to the mitochondria of HeLa cells and allows visualization of glutamate-mediated Zn(II) uptake in dendrites and Zn(II) release resulting from nitrosative stress in neurons.

Zinc-buffering capacity of a eukaryotic cell at physiological pZn

In spite of the paramount importance of zinc in biology, dynamic aspects of cellular zinc metabolism remain poorly defined at the molecular level. Investigations with human colon cancer (HT-29) cells establish a total cellular zinc concentration of 264 microM. Remarkably, about 10% of the potential high-affinity zinc-binding sites are not occupied by zinc, resulting in a surplus of 28 muM ligands (average Kd(c) = 83 pM) that ascertain cellular zinc-buffering capacity and maintain the "free" zinc concentration in proliferating cells at picomolar levels (784 pM, pZn = 9.1). This zinc-buffering capacity allows zinc to fluctuate only with relatively small amplitudes (DeltapZn = 0.3; below 1 nM) without significantly perturbing physiological pZn. Thus, the "free" zinc concentrations in resting and differentiated HT-29 cells are 614 pM and 1.25 nM, respectively. The calculation of these "free" zinc concentrations is based on measurements at different concentrations of the fluorogenic zinc-chelating agent and extrapolation to a zero concentration of the agent. It depends on the state of the cell, its buffering capacity, and the zinc dissociation constant of the chelating agent. Zinc induction of thionein (apometallothionein) ensures a surplus of unbound ligands, increases zinc-buffering capacity and the availability of zinc (DeltapZn = 0.8), but preserves the zinc-buffering capacity of the unoccupied high-affinity zinc-binding sites, perhaps for crucial physiological functions. Jointly, metallothionein and thionein function as the major zinc buffer under conditions of increased cellular zinc.

In Vivo Monitoring of Mercury Ions Using a Rhodamine-Based Molecular Probe

Exposure to mercury causes severe damage to various tissues and organs in humans. Concern over mercury toxicity has encouraged the development of efficient, sensitive, and selective methods for the in vivo detection of mercury. Although a variety of chemosensors have been exploited for this purpose, no in vivo monitoring systems have been described to date. In this report, we describe an irreversible rhodamine chemosensor-based, real-time monitoring system to detect mercury ions in living cells and, in particular, vertebrate organisms. The chemosensor responds rapidly, irreversibly, and stoichiometrically to mercury ions in aqueous media at room temperature. The results of experiments with mammalian cells and zebrafish show that the mercury chemosensor is cell and organism permeable and that it responds selectively to mercury ions over other metal ions. In addition, real-time monitoring of mercury-ion uptake by cells and zebrafish using this chemosensor shows that saturation of mercury-ion uptake occurs within 20-30 min in cells and organisms. Finally, accumulation of mercury ions in zebrafish tissue and organs is readily detected by using this rhodamine-based chemosensor.

Intracellular metal detectors

Metal ions play numerous crucial roles in biology, and there is great interest in obtaining an accurate measurement of the intracellular concentrations of both tightly bound and exchangeable metal ions. Measuring the concentration of readily exchangeable transition metal ions in a cell has been particularly difficult because of the extremely small concentrations involved, interference from other metal ions and biomolecules, and the challenge of introducing probes into the cell with minimal perturbations. Recent work has made quantification of the intracellular exchangeable zinc pool possible for the first time using a cell-permeable, ratiometric, fluorescence resonance energy transfer based zinc biosensor.

Single and three-color flow cytometry assay for intracellular zinc ion availability in human lymphocytes with Zinpyr-1 and double immunofluorescence: Relationship with metallothioneins

BACKGROUND

: The amount of available intracellular zinc is pivotal to regulate many cellular processes, including oxidative stress response and apoptotic mechanisms. Therefore it is not surprising that zinc homeostasis and dyshomeostasis is involved in many physiological and pathological states, respectively. Cell permeable zinc probes allow intracellular applications with microscopy technology, but flow cytometry (FC) applications have been scarcely explored, albeit they can be suited to study zinc homeostasis in different cell types, including rare cells.

METHODS

: We describe a FC method able to estimate intracellular zinc ion availability and the intracellular capability to activate a zinc signal after treatment with an NO-donor (AcOM-DEA/NO) in human PBMCs, using the fluorescent zinc-specific probe, Zinpyr-1 (ZP1), alone or in association with CD4-PE and CD8-Cychrome mAb.

RESULTS

: This method was able to detect an increase/decrease of intracellular zinc available in human fresh cultured PBMC and in immune subsets using AcOM-DEA/NO or TPEN, respectively. ZP1 mean fluorescence on gated histograms was sensitive to the amount of zinc added in the culture medium and significantly correlated to metallothioneins and total intracellular zinc.

CONCLUSIONS

: FC applications using ZP1 may be a fast and useful tool to study zinc homeostasis in immune cells.