Illuminating mobile zinc with fluorescence from cuvettes to live cells and tissues

Luminescence Probes in Bio-Applications: From Principle to Practice

Bioanalysis based on optical imaging has gained significant progress in the last few decades. Luminescence probes are capable of detecting, monitoring, and tracing particular biomolecules in complex biological systems to figure out the roles of these molecules in organisms. Considering the rapid development of luminescence probes for bio-applications and their promising future, we have attempted to explore the working principles and recent advances in bio-applications of luminescence probes, in the hope of helping readers gain a detailed understanding of luminescence probes developed in recent years. In this review, we first focus on the current widely used luminescence probes, including fluorescence probes, bioluminescence probes, chemiluminescence probes, afterglow probes, photoacoustic probes, and Cerenkov luminescence probes. The working principles for each type of luminescence probe are concisely described and the bio-application of the luminescence probes is summarized by category, including metal ions detection, secretion detection, imaging, and therapy.

Fluorescent Turn-On Sensing of Zinc(II) and Alkaline Phosphatase Activity Using a Pyridoxal-5'-Phosphate Derived Schiff Base

A novel Zn2+ ion and alkaline phosphatase (ALP) selective fluorescence turn-on sensor L was developed by reacting pyridoxal 5'-phosphate (PLP) with hydrazine. Sensor L shows significant flurescence enhancement at 476 nm due to the formation of a L-Zn2+ complex in 1:1 binding stoichiometry with the association constant of 3.1⋅104 M- 1. Using L, the concentration of Zn2+ can be detected down to 2.34 µM, and the practical utility of L was validated by quantifying Zn2+ in real water samples. Additionally, the receptor L was applied to mimic the dephosphorylation reaction catalysed by the enzyme ALP and the resulted fluorescence change was monitored to detect the ALP activity.

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

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

An optical keypad lock with high resettability based on a quantum dot-porphyrin FRET nanodevice

Due to their appealing properties, nanomaterials have become ideal candidates for the implementation of computing systems. Herein, an optical keypad lock based on a Förster resonance energy transfer (FRET) nanodevice is developed. The nanodevice is composed of a green-emission quantum dot with a thick silica shell (gQD@SiO2) and peripheric blue-emission quantum dots with ultrathin silica spacer (bQD@SiO2), on which 5,10,15,20-tetrakis(4-sulfophenyl)porphyrin (TSPP) is covalently linked. The nanodevice outputs dual emission-based ratiometric fluorescence, depending on the FRET efficiency of bQD-porphyrin pairs, which is highly sensitive to the metalation of TSPP: values are 59.7%, 44.8%, and 10.1% for bQD-Zn(ii)TSPP, bQD-TSPP, and bQD-Fe(iii)TSPP pairs, respectively. As such, by using the competitive chelation-induced transmetalation of TSPP, the nanodevice is capable of implementing a 3-input keypad lock that is unlocked only by the correct input order of Zn(ii) chelator, iron ions, and UV light. Interestingly, the reversible transmetalation of TSPP permits the reset (lock) operation of the keypad lock with the correct input order of ascorbic acid, Zn(ii), and UV light. Application of the nanodevice is exemplified by the construction of paper and cellular keypad locks, respectively, both of which feature signal readability and/or high resettability, showing high potential for personal information identification and bio-encryption applications.

Simple in vitro method to evaluate ZIP zinc transport ability through zinc transporter 1 and metallothionein expression measurements

Measuring the cellular zinc content and examining the alteration of zinc status are critical for investigating the cellular homeostasis and dynamics of zinc and its involvement in patho-physiological functions. Many Zrt- and Irt-related protein (ZIP) transporters uptake zinc from the extracellular space. Among Zn transporters (ZNTs), ZNT1 effluxes cytosolic zinc. As cytosolic zinc-binding proteins, metallothioneins (MTs) also contribute to the control of cellular zinc homeostasis. Systemic and cellular zinc homeostasis is considered to be maintained by balancing expression and functional activities of these proteins. The zinc transport ability of ZIPs is typically measured by evaluating cellular zinc content with various zinc-detection methods and systems. Many small-molecule fluorescent probes and fluorescence resonance energy transfer-based protein sensors have been exploited for this purpose. Although powerful analytical methods using special instruments have been developed to quantify zinc, they are often not easily accessible. Here, we present a simplified and inexpensive method to estimate the zinc transport ability of ZIP transporters using the expression responses of ZNT1 and MT. This protocol should be effective in several applications because ZNT1 and MT expression are easily evaluated by immunoblotting and immunofluorescence staining as basic biochemical techniques available in most laboratories. This method is advantageous for examining the relative zinc status or alterations mediated by expression changes of ZIPs in cells cultured in normal medium without zinc supplementation. As zinc is an essential micronutrient, extensive research is necessary to improve dietary zinc absorption to promote health. Therefore, we also propose a simple screening method of foods to improve zinc absorption as an application of measuring ZIP-mediated MT expression.

Comparison of Three Low-Molecular-Weight Fluorescent Probes for Measuring Free Zinc Levels in Cultured Mammary Cells

Free zinc is a critical regulator in signal transduction and affects many cellular processes relevant to cancer, including proliferation and cell death. Acting as a second messenger, altered free intracellular zinc has fundamental effects on regulating enzymes such as phosphatases and caspases. Therefore, the determination of free intracellular zinc levels is essential to assess its influence on the signaling processes involved in cancer development and progression. In this study, we compare three low-molecular-weight fluorescent probes, ZinPyr-1, TSQ, and FluoZin-3, for measuring free zinc in different mammary cell lines (MCF10A, MCF7, T47D, and MDA-MB-231). In summary, ZinPyr-1 is the most suitable probe for free Zn quantification. It responds well to calibration based on minimal fluorescence in the presence of the chelator TPEN (N,N,N',N'-Tetrakis(2-pyridylmethyl)ethylenediamine) and maximal fluorescence by saturation with ZnSO₄, resulting in the detection of free intracellular zinc in breast cancer subtypes ranging from 0.62 nM to 1.25 nM. It also allows for measuring the zinc fluxes resulting from incubation with extracellular zinc, showing differences in the zinc uptake between the non-malignant MCF10A cell line and the other cell lines. Finally, ZinPyr-1 enables the monitoring of sub-cellular distributions by fluorescence microscopy. Altogether, these properties provide a basis for the further exploration of free zinc in order to realize its full potential as a possible biomarker or even therapeutic target in breast cancer.

Mobile zinc as a modulator of sensory perception

Mobile zinc is an abundant transition metal ion in the central nervous system, with pools of divalent zinc accumulating in regions of the brain engaged in sensory perception and memory formation. Here, we present essential tools that we developed to interrogate the role(s) of mobile zinc in these processes. Most important are (a) fluorescent sensors that report the presence of mobile zinc and (b) fast, Zn-selective chelating agents for measuring zinc flux in animal tissue and live animals. The results of our studies, conducted in collaboration with neuroscientist experts, are presented for sensory organs involved in hearing, smell, vision, and learning and memory. A general principle emerging from these studies is that the function of mobile zinc in all cases appears to be downregulation of the amplitude of the response following overstimulation of the respective sensory organs. Possible consequences affecting human behavior are presented for future investigations in collaboration with interested behavioral scientists.

Development and Characterization of a Red Fluorescent Protein-Based Sensor RZnP1 for the Detection of Cytosolic Zn2

Fluorescent sensors have been developed to record Zn²⁺ dynamics and measure Zn²⁺ concentrations within the cell. Most previous efforts on developing single-wavelength sensors are focused on green sensors. Here, we engineer a genetically encoded, single red fluorescent protein-based Zn²⁺ sensor, Red Zinc Probe (RZnP1), which can detect intracellular concentrations of Zn²⁺. RZnP1 demonstrates a sensitive response to cytosolic Zn²⁺ (K_d = 438 pM), decent brightness (quantum yield (QY) = 0.15), good in situ dynamic range (F_max/F_min = 4.0), and specificity for Zn²⁺ over other biologically relevant metal cations. RZnP1 offers a way to image Zn²⁺ with multiple intracellular ions in tandem. We demonstrate the simultaneous recording of Zn²⁺ and Ca²⁺ using RZnP1 alongside the Ca²⁺ sensor GCaMP5G in HeLa cells. We also use RZnP1 with mito-GZnP2, a green fluorescent protein (GFP)-based mitochondrial Zn²⁺ sensor, to track Zn²⁺ dynamics in the cytosol and mitochondria concurrently in rat primary neuron culture. Our work not only expands the toolbox of Zn²⁺ sensors but also demonstrates techniques for imaging Zn²⁺ dynamics along with other cations and between multiple subcellular compartments simultaneously.

Neuronal signalling of zinc: from detection and modulation to function

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

Harnessing reaction-based probes to preferentially target pancreatic β-cells and β-like cells

Highly sensitive approaches to target insulin-expressing cells would allow more effective imaging, sorting, and analysis of pancreatic β-cells. Here, we introduce the use of a reaction-based probe, diacetylated Zinpyr1 (DA-ZP1), to image pancreatic β-cells and β-like cells derived from human pluripotent stem cells. We harness the high intracellular zinc concentration of β-cells to induce a fluorescence signal in cells after administration of DA-ZP1. Given its specificity and rapid uptake by cells, we used DA-ZP1 to purify live stem cell-derived β-like cells as confirmed by immunostaining analysis. We tested the ability of DA-ZP1 to image transplanted human islet grafts and endogenous mouse pancreatic islets in vivo after its systemic administration into mice. Thus, DA-ZP1 enables purification of insulin-secreting β-like cells for downstream applications, such as functional studies, gene-expression, and cell-cell interaction analyses and can be used to label engrafted human islets and endogenous mouse islets in vivo.

Metalation and Maturation of Zinc Ectoenzymes: A Perspective

Numerous zinc ectoenzymes are folded and activated in the compartments of the early secretory pathway, such as the ER and the Golgi apparatus, before reaching their final destination. During this process, zinc must be incorporated into the active site; therefore, metalation of the nascent protein is indispensable for the expression of the active enzyme. However, to date, the molecular mechanism underlying this process has been poorly investigated. This is in sharp contrast to the physiological and pathophysiological roles of zinc ectoenzymes, which have been extensively investigated over the past decades. This manuscript concisely outlines the present understanding of zinc ectoenzyme activation through metalation by zinc and compares this with copper ectoenzyme activation, in which elaborate copper metalation mechanisms are known. Moreover, based on the comparison, several hypotheses are discussed. Approximately 80 years have passed since the first zinc enzyme was identified; therefore, it is necessary to improve our understanding of zinc ectoenzymes from a biochemical perspective, which will further our understanding of their biological roles.

Optical Control of Metal Ion Probes in Cells and Zebrafish Using Highly Selective DNAzymes Conjugated to Upconversion Nanoparticles

Spatial and temporal distributions of metal ions in vitro and in vivo are crucial in our understanding of the roles of metal ions in biological systems, and yet there is a very limited number of methods to probe metal ions with high space and time resolution, especially in vivo. To overcome this limitation, we report a Zn²⁺-specific near-infrared (NIR) DNAzyme nanoprobe for real-time metal ion tracking with spatiotemporal control in early embryos and larvae of zebrafish. By conjugating photocaged DNAzymes onto lanthanide-doped upconversion nanoparticles (UCNPs), we have achieved upconversion of a deep tissue penetrating NIR 980 nm light into 365 nm emission. The UV photon then efficiently photodecages a substrate strand containing a nitrobenzyl group at the 2'-OH of adenosine ribonucleotide, allowing enzymatic cleavage by a complementary DNA strand containing a Zn²⁺-selective DNAzyme. The product containing a visible FAM fluorophore that is initially quenched by BHQ1 and Dabcyl quenchers is released after cleavage, resulting in higher fluorescent signals. The DNAzyme-UCNP probe enables Zn²⁺ sensing by exciting in the NIR biological imaging window in both living cells and zebrafish embryos and detecting in the visible region. In this study, we introduce a platform that can be used to understand the Zn²⁺ distribution with spatiotemporal control, thereby giving insights into the dynamical Zn²⁺ ion distribution in intracellular and in vivo models.

Characterization of Caco-2 cells stably expressing the protein-based zinc probe eCalwy-5 as a model system for investigating intestinal zinc transport

Intestinal zinc resorption, in particular its regulation and mechanisms, are not yet fully understood. Suitable intestinal cell models are needed to investigate zinc uptake kinetics and the role of labile zinc in enterocytes in vitro. Therefore, a Caco-2 cell clone was produced, stably expressing the genetically encoded zinc biosensor eCalwy-5. The aim of the present study was to reassure the presence of characteristic enterocyte-specific properties in the Caco-2-eCalwy clone. Comparison of Caco-2-WT and Caco-2-eCalwy cells revealed only slight differences regarding subcellular localization of the tight junction protein occludin and alkaline phosphatase activity, which did not affect basic integrity of the intestinal barrier or the characteristic brush border membrane morphology. Furthermore, introduction of the additional zinc-binding protein in Caco-2 cells did not alter mRNA expression of the major intestinal zinc transporters (zip4, zip5, znt-1 and znt-5), but increased metallothionein 1a-expression and cellular resistance to higher zinc concentrations. Moreover, this study examines the effect of sensor expression level on its saturation with zinc. Fluorescence cell imaging indicated considerable intercellular heterogeneity in biosensor-expression. However, FRET-measurements confirmed that these differences in expression levels have no effect on fractional zinc-saturation of the probe.

A new probe for detecting zinc-bound carbonic anhydrase in cell lysates and cells

We report the synthesis and application of a small molecule probe for carbonic anhydrase (CA) to track holo-CA in cell lysates and live-cell models of zinc dyshomeostasis. The probe displays a 12-fold increase in fluorescence upon binding to bovine CA and also responds to human CA isoforms.

Enhanced Detection of Sub-Retinal Pigment Epithelial Cell Layer Deposits in Human and Murine Tissue: Imaging Zinc as a Biomarker for Age-Related Macular Degeneration (An American Ophthalmological Society Thesis)

PURPOSE

Understanding the apparent paradoxical role of zinc in the pathogenesis and prevention of age-related macular degeneration (AMD) has been limited by the lack of animal models for its detection in sub-retinal epithelial deposits (drusen), a definitive early hallmark of AMD. In-vitro studies using Zinpyr-1 showed drusen contained high levels of zinc, but the probe was not suitable for in-vivo studies. This study compares Zinpyr-1 to ZPP1, a new fluorescein-based probe for zinc, to assess the potential of ZPP1 for in-vivo detection of zinc in drusen.

METHODS

Flat mounts of human sub-RPE tissue using the probes were analyzed by fluorescence and confocal microscopy. Flat mounts of sub-RPE tissue from mice deficient in superoxide dismutase isoform-1 (CuZn-SOD-KO) or isoform-2 (Mn-SOD-RPE-KO) were analyzed with sub-RPE deposits confirmed by histology.

RESULTS

Drusen are detected in greater numbers and intensity with ZPP1 compared to Zinpyr-1. Using ZPP1, drusen was detected in a sample from a 46-year old human donor without ocular history, suggesting that ZPP1 might be sensitive enough to detect drusen at an early stage. With CuZn-SOD KO mice, ZPP1 detected sub-RPE deposits at 10 months of age, whereas Zinpyr-1 required 14 months.

CONCLUSION

Detection of sub-RPE deposits by ZPP1 was greatly enhanced compared to Zinpyr-1. This enhanced sensitivity will allow for more insightful analysis of zinc in AMD using human specimens and mouse models. This could result in the development of a sensitive in-vivo probe to enhance research on the role zinc in drusen formation and the early clinical diagnosis of AMD.

Low-Affinity Zinc Sensor Showing Fluorescence Responses with Minimal Artifacts

The study of the zinc biology requires molecular probes with proper zinc affinity. We developed a low-affinity zinc probe (HBO-ACR) based on an azacrown ether (ACR) and an 2-(2-hydroxyphenyl)benzoxazole (HBO) fluorophore. This probe design imposed positive charge in the vicinity of a zinc coordination center, which enabled fluorescence turn-on responses to high levels of zinc without being affected by the pH and the presence of other transition-metal ions. Steady-state and transient photophysical investigations suggested that such a high tolerance benefits from orchestrated actions of proton-induced nonradiative and zinc-induced radiative control. The zinc bioimaging utility of HBO-ACR has been fully demonstrated with the use of human pancreas epidermoid carcinoma, PANC-1 cells, and rodent hippocampal neurons from cultures and acute brain slices. The results obtained through our studies established the validity of incorporating positively charged ionophores for the creation of low-affinity probes for the visualization of biometals.

Mobile zinc increases rapidly in the retina after optic nerve injury and regulates ganglion cell survival and optic nerve regeneration

Retinal ganglion cells (RGCs), the projection neurons of the eye, cannot regenerate their axons once the optic nerve has been injured and soon begin to die. Whereas RGC death and regenerative failure are widely viewed as being cell-autonomous or influenced by various types of glia, we report here that the dysregulation of mobile zinc (Zn²⁺) in retinal interneurons is a primary factor. Within an hour after the optic nerve is injured, Zn²⁺ increases several-fold in retinal amacrine cell processes and continues to rise over the first day, then transfers slowly to RGCs via vesicular release. Zn²⁺ accumulation in amacrine cell processes involves the Zn²⁺ transporter protein ZnT-3, and deletion of slc30a3, the gene encoding ZnT-3, promotes RGC survival and axon regeneration. Intravitreal injection of Zn²⁺ chelators enables many RGCs to survive for months after nerve injury and regenerate axons, and enhances the prosurvival and regenerative effects of deleting the gene for phosphatase and tensin homolog (pten). Importantly, the therapeutic window for Zn²⁺ chelation extends for several days after nerve injury. These results show that retinal Zn²⁺ dysregulation is a major factor limiting the survival and regenerative capacity of injured RGCs, and point to Zn²⁺ chelation as a strategy to promote long-term RGC protection and enhance axon regeneration.

Progressive structural modification to a zinc-actuated photoinduced electron transfer (PeT) switch in the context of intracellular zinc imaging

Zinc-targeting sensors that contain an anilino-based electron donor (aka, the PeT switch) are reported.

Zinc(II) Ion Sensing in Aqueous Micellar Solution Using Modified Bipyridine-Based "Turn-On" Fluorescent Probes and its Application in Bioimaging

The bipyridine-based constructs 4-(pyridine-2-yl)-3H-pyrrolo[2,3-c]quinoline (PPQ) and [6-(3H-pyrrolo[2,3-c]quinolin-4-yl)pyridin-2-yl]methanol (PPQ-OH) and their assemblies with surfactants are evaluated as turn-on fluorescent sensors for Zn²⁺ ions in aqueous solution. This study strives to overcome the problem of low water solubility of the hydrophobic PPQ and PPQ-OH by using micelles. Whereas the ligands show selective sensing behavior for Zn²⁺ over important biological cations including Na⁺ , K⁺ , Ca²⁺ , Mg²⁺ in anionic sodium dodecyl sulfate and non-ionic Tween 80 micelles, no Zn²⁺ sensing is observed in cationic cetyltrimethylammonium bromide micelles. Unlike in DMF, Cd²⁺ interference is observed in aqueous conditions, which can be avoided either by performing the study at pH≥9 or by carrying out a time-resolved fluorescence study. Analysis of the Job plot data, the fluorescence lifetimes, and experiments on varying micellar shape and pH, confirms that the coordination volume of the resulting octahedral metal complex and formation of a five-membered chelate ring are critical factors for Cd²⁺ interference. The described sensing systems are capable of detecting Zn²⁺ ions at the micromolar level. Additionally, it is shown that PPQ and PPQ-OH can be used to detect Zn²⁺ in HeLa cells under physiological conditions in bioimaging studies.

Single Fluorescent Protein-Based Indicators for Zinc Ion (Zn(2+))

Genetically encoded fluorescent Zn(2+) indicators (GEZIs) are highly attractive research tools for studying Zn(2+) homeostasis and signaling in mammalian cells. Most current GEZIs are based on Förster resonance energy transfer (FRET) between a select pair of fluorescent proteins (FPs) fused with Zn(2+)-binding motifs. One drawback of such FRET-based GEZIs is their broad spectral profile bandwidths, creating challenges when monitoring multiple targets or parameters. To address this issue, we have engineered a group of intensiometric GEZIs based on single teal and red FPs that can be utilized to monitor subcellular Zn(2+) diffusion and glucose-induced Zn(2+) secretion in pancreatic INS-1E β-cells. These GEZIs offer the simplicity of intensiometric measurements, compatibility in multicolor imaging, large dynamic ranges, and relatively small molecular sizes, making them valuable additions to the molecular toolbox for imaging Zn(2+).

Reaction-Based Probes for Imaging Mobile Zinc in Live Cells and Tissues

Chelatable, or mobile, forms of zinc play critical signaling roles in numerous biological processes. Elucidating the action of mobile Zn(II) in complex biological environments requires sensitive tools for visualizing, tracking, and manipulating Zn(II) ions. A large toolbox of synthetic photoinduced electron transfer (PET)-based fluorescent Zn(II) sensors are available, but the applicability of many of these probes is limited by poor zinc sensitivity and low dynamic ranges owing to proton interference. We present here a general approach for acetylating PET-based probes containing a variety of fluorophores and zinc-binding units. The new sensors provide substantially improved zinc sensitivity and allow for incubation of live cells and tissue slices with nM probe concentrations, a significant improvement compared to the μM concentrations that are typically required for a measurable fluorescence signal. Acetylation effectively reduces or completely quenches background fluorescence in the metal-free sensor. Binding of Zn(II) selectively and quickly mediates hydrolytic cleavage of the acetyl groups, providing a large fluorescence response. An acetylated blue coumarin-based sensor was used to carry out detailed analyses of metal binding and metal-promoted acetyl hydrolysis. Acetylated benzoresorufin-based red-emitting probes with different zinc-binding sites are effective for sensing Zn(II) ions in live cells when applied at low concentrations (∼50-100 nM). We used green diacetylated Zinpyr1 (DA-ZP1) to image endogenous mobile Zn(II) in the molecular layer of mouse dorsal cochlear nucleus (DCN), confirming that acetylation is a suitable approach for preparing sensors that are highly specific and sensitive to mobile zinc in biological systems.

5-Arylvinyl-2,2'-bipyridyls: Bright "push-pull" dyes as components in fluorescent indicators for zinc ions

This article reviews the zinc(II)-dependent photophysical properties of arylvinylbipyridines (AVBs), a class of fluoroionophores in which 2,2'-bipyridyl and an aryl moiety are electronically conjugated. Zinc(II) binding of an AVB may lead to an emission bathochromic shift of the fluoroionophore without diminishing its fluorescence quantum yield. This observation can be explained using the excited state model of electron donor-π bridge-electron acceptor "push-pull" fluorophores, in which the bipy moiety acts as an electron acceptor, and zinc(II)-coordination strengthens its electron affinity. The spectral sensitivity of bipy-containing fluoroionophores, such as AVBs, to zinc(II) can be exploited to prepare fluorescent indicators for this ion. In several cases, AVB moieties are incorporated in fluorescent heteroditopic ligands, so that the variation of zinc(II) concentration over a relatively large range can be correlated to fluorescence changes in either intensity or color. AVB fluoroionophores are also used to introduce an intramolecular Förster resonance energy transfer (FRET) strategy for creating zinc(II) indicators with high photostability and a narrow emission band, two desired characteristics of dyes used in fluorescence microscopy.

The Physiological, Biochemical, and Molecular Roles of Zinc Transporters in Zinc Homeostasis and Metabolism

Zinc is involved in a variety of biological processes, as a structural, catalytic, and intracellular and intercellular signaling component. Thus zinc homeostasis is tightly controlled at the whole body, tissue, cellular, and subcellular levels by a number of proteins, with zinc transporters being particularly important. In metazoan, two zinc transporter families, Zn transporters (ZnT) and Zrt-, Irt-related proteins (ZIP) function in zinc mobilization of influx, efflux, and compartmentalization/sequestration across biological membranes. During the last two decades, significant progress has been made in understanding the molecular properties, expression, regulation, and cellular and physiological roles of ZnT and ZIP transporters, which underpin the multifarious functions of zinc. Moreover, growing evidence indicates that malfunctioning zinc homeostasis due to zinc transporter dysfunction results in the onset and progression of a variety of diseases. This review summarizes current progress in our understanding of each ZnT and ZIP transporter from the perspective of zinc physiology and pathogenesis, discussing challenging issues in their structure and zinc transport mechanisms.

Peptide-based, two-fluorophore, ratiometric probe for quantifying mobile zinc in biological solutions

Small-molecule fluorescent sensors are versatile agents for detecting mobile zinc in biology. Capitalizing on the abundance of validated mobile zinc probes, we devised a strategy for repurposing existing intensity-based sensors for quantitative applications. Using solid-phase peptide synthesis, we conjugated a zinc-sensitive Zinpyr-1 derivative and a zinc-insensitive 7-hydroxycoumarin derivative onto opposite ends of a rigid P₉K peptide scaffold to create HcZ9, a ratiometric fluorescent probe for mobile zinc. A plate reader-based assay using HcZ9 was developed, the accuracy of which is comparable to that of atomic absorption spectroscopy. We investigated zinc accumulation in prostatic cells and zinc levels in human seminal fluid. When normal and tumorigenic cells are bathed in zinc-enriched media, cellular mobile zinc is buffered and changes slightly, but total zinc levels increase significantly. Quantification of mobile and total zinc levels in human seminal plasma revealed that the two are positively correlated with a Pearson’s coefficient of 0.73.

A fluorescent indicator for imaging lysosomal zinc(II) with Förster resonance energy transfer (FRET)-enhanced photostability and a narrow band of emission

We demonstrate a strategy to transfer the zinc(II) sensitivity of a fluoroionophore with low photostability and a broad emission band to a bright and photostable fluorophore with a narrow emission band. The two fluorophores are covalently connected to afford an intramolecular Förster resonance energy transfer (FRET) conjugate. The FRET donor in the conjugate is a zinc(II)-sensitive arylvinylbipyridyl fluoroionophore, the absorption and emission of which undergo bathochromic shifts upon zinc(II) coordination. When the FRET donor is excited, efficient intramolecular energy transfer occurs to result in the emission of the acceptor boron dipyrromethene (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene or BODIPY) as a function of zinc(II) concentration. The broad emission band of the donor/zinc(II) complex is transformed into the strong, narrow emission band of the BODIPY acceptor in the FRET conjugates, which can be captured within the narrow emission window that is preferred for multicolor imaging experiments. In addition to competing with other nonradiative decay processes of the FRET donor, the rapid intramolecular FRET of the excited FRET-conjugate molecule protects the donor fluorophore from photobleaching, thus enhancing the photostability of the indicator. FRET conjugates 3 and 4 contain aliphatic amino groups, which selectively target lysosomes in mammalian cells. This subcellular localization preference was verified by using confocal fluorescence microscopy, which also shows the zinc(II)-enhanced emission of 3 and 4 in lysosomes. It was further shown using two-color structured illumination microscopy (SIM), which is capable of extending the lateral resolution over the Abbe diffraction limit by a factor of two, that the morpholino-functionalized compound 4 localizes in the interior of lysosomes, rather than anchoring on the lysosomal membranes, of live HeLa cells.

A Fluorescent Indicator for Imaging Lysosomal Zinc(II) with Förster Resonance Energy Transfer (FRET)-Enhanced Photostability and a Narrow Band of Emission

We demonstrate a strategy to transfer the zinc(II) sensitivity of a fluoroionophore with low photostability and a broad emission band to a bright and photostable fluorophore with a narrow emission band. The two fluorophores are covalently connected to afford an intramolecular Förster resonance energy transfer (FRET) conjugate. The FRET donor in the conjugate is a zinc(II)-sensitive arylvinylbipyridyl fluoroionophore, the absorption and emission of which undergo bathochromic shifts upon zinc(II) coordination. When the FRET donor is excited, efficient intramolecular energy transfer occurs to result in the emission of the acceptor boron dipyrromethene (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene or BODIPY) as a function of zinc(II) concentration. The broad emission band of the donor/zinc(II) complex is transformed into the strong, narrow emission band of the BODIPY acceptor in the FRET conjugates, which can be captured within the narrow emission window that is preferred for multicolor imaging experiments. In addition to competing with other nonradiative decay processes of the FRET donor, the rapid intramolecular FRET of the excited FRET-conjugate molecule protects the donor fluorophore from photobleaching, thus enhancing the photostability of the indicator. FRET conjugates 3 and 4 contain aliphatic amino groups, which selectively target lysosomes in mammalian cells. This subcellular localization preference was verified by using confocal fluorescence microscopy, which also shows the zinc(II)-enhanced emission of 3 and 4 in lysosomes. It was further shown using two-color structured illumination microscopy (SIM), which is capable of extending the lateral resolution over the Abbe diffraction limit by a factor of two, that the morpholino-functionalized compound 4 localizes in the interior of lysosomes, rather than anchoring on the lysosomal membranes, of live HeLa cells.

SpiroZin1: a reversible and pH-Insensitive, reaction-based, red-fluorescent probe for imaging biological mobile zinc

A reversible, reaction-based sensor for biological mobile zinc was designed, prepared, and characterized. The sensing mechanism of this probe is based on the zinc-induced, ring-opening reaction of spirobenzopyran to give a cyanine fluorophore that emits in the deep-red region of the electromagnetic spectrum. This probe is not activated by protons and operates efficiently in aqueous solution at pH 7 and high ionic strength. The mechanism of this reaction was studied by using a combination of kinetics experiments and DFT calculations. The biocompatibility of the probe was demonstrated in live HeLa cells.

Zn(II)-coordination modulated ligand photophysical processes - the development of fluorescent indicators for imaging biological Zn(II) ions

Molecular photophysics and metal coordination chemistry are the two fundamental pillars that support the development of fluorescent cation indicators. In this article, we describe how Zn(II)-coordination alters various ligand-centered photophysical processes that are pertinent to developing Zn(II) indicators. The main aim is to show how small organic Zn(II) indicators work under the constraints of specific requirements, including Zn(II) detection range, photophysical requirements such as excitation energy and emission color, temporal and spatial resolutions in a heterogeneous intracellular environment, and fluorescence response selectivity between similar cations such as Zn(II) and Cd(II). In the last section, the biological questions that fluorescent Zn(II) indicators help to answer are described, which have been motivating and challenging this field of research.

Tris(2-pyridylmethyl)amine (TPA) as a membrane-permeable chelator for interception of biological mobile zinc

We report the characterization of tris(2-pyridylmethyl)amine (TPA) as a membrane-permeable zinc chelator for intercepting biological mobile zinc. Compared to N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), TPA chelates zinc with faster kinetics in cuvettes, live cells, and brain slices. TPA also is generally less toxic than TPEN in cell culture. Mechanistic analysis indicates that these improvements arise from both the electronic and steric properties of TPA including weaker metal-binding affinity, lower pKa, and smaller size. These results demonstrate that TPA chelation is a valuable addition to the methodologies available for investigating mobile zinc in biology.

Low-molecular-mass metal complexes in the mouse brain

The presence of labile low-molecular-mass (LMM, defined as <10 kDa) metal complexes in cells and super-cellular structures such as the brain has been inferred from chelation studies, but direct evidence is lacking. To evaluate the presence of LMM metal complexes in the brain, supernatant fractions of fresh mouse brain homogenates were passed through a 10 kDa cutoff membrane and subjected to size-exclusion liquid chromatography under anaerobic refrigerated conditions. Fractions were monitored for Mn, Fe, Co, Cu, Zn, Mo, S and P using an on-line ICP-MS. At least 30 different LMM metal complexes were detected along with numerous P- and S- containing species. Reproducibility was assessed by performing the experiment 13 times, using different buffers, and by examining whether complexes changed with time. Eleven Co, 2 Cu, 5 Mn, 4 Mo, 3 Fe and 2 Zn complexes with molecular masses <4 kDa were detected. One LMM Mo complex comigrated with the molybdopterin cofactor. Most Cu and Zn complexes appeared to be protein-bound with masses ranging from 4-20 kDa. Co was the only metal for which the "free" or aqueous complex was reproducibly observed. Aqueous Co may be sufficiently stable in this environment due to its relatively slow water-exchange kinetics. Attempts were made to assign some of these complexes, but further efforts will be required to identify them unambiguously and to determine their functions. This is among the first studies to detect low-molecular-mass transition metal complexes in the mouse brain using LC-ICP-MS.