Advances in Organic Fluorescent Probes for Intracellular Zn2+ Detection and Bioimaging

Zinc ions (Zn2+) play a key role in maintaining and regulating protein structures and functions. To better understand the intracellular Zn2+ homeostasis and signaling role, various fluorescent sensors have been developed that allow the monitoring of Zn2+ concentrations and bioimaging in live cells in real time. This review highlights the recent development of organic fluorescent probes for the detection and imaging of intracellular Zn2+, including the design and construction of the probes, fluorescent response mechanisms, and their applications to intracellular Zn2+ detection and imaging on-site. Finally, the current challenges and prospects are discussed.

Development and Validation of a Fluorogenic Probe for Lysosomal Zinc Release

Zinc homeostasis, which allows optimal zinc utilization in diverse life processes, is responsible for the general well-being of human beings. This paper describes developing and validating an easily accessible indole-containing zinc-specific probe in the cellular milieu. The probe was synthesized from readily available starting materials and was subjected to steady-state fluorescence studies. It showed selective sensing behavior towards Zn2+ with reversible binding. The suppression of PET (Photoinduced Electron Transfer) and ESIPT (Excited State Intramolecular Proton Transfer) elicited selectivity, and the detection limit was 0.63 μM (LOQ 6.8 μM). The zinc sensing capability of the probe was also screened in the presence of low molecular weight ligands [LMWLs] and showed interference only with GSH and ATP. It is non-toxic and can detect zinc in different cell lines under various stress conditions such as inflammation, hyperglycemia, and apoptosis. The probe could stain the early and late stages of apoptosis in PAN-2 cells by monitoring the zinc release. Most experiments were conducted without external zinc supplementation, showing its innate ability to detect zinc.

DIRECT, a low-cost system for high-speed, low-noise imaging of fluorescent bio-samples

A targeted imaging system has been developed for applications requiring recording from stationary samples at high spatiotemporal resolutions. It works by illuminating regions of interest in rapid sequence, and recording the signal from the whole field of view onto a single photodetector. It can be implemented at low cost on an existing microscope without compromising existing functionality. The system is characterized in terms of speed, spatial resolution, and tissue penetration depth, before being used to record individual action potentials from ASAP-3 expressing neurons in an ex vivo mouse brain slice preparation.

A novel strategy of engineering genetically encoded probe for ultrasensitive sensing Hg2+ with unusual planar trigonometric coordination configuration

At present, few genetically encoded fluorescent probes are currently available for the analysis of toxic heavy metal ions, and most have poor performance that cannot meet the requirements of sensitive and dynamic detection in living cells. In this study, we designed a single fluorescent protein-based probe sfGFP-MerBD, which can specifically response to Hg²⁺ with high binding affinity and wide dynamic range. More importantly, the developing probe can timely and reversibly monitor changes of Hg²⁺ concentration in living mammalian cells. The excellent performance of this probe is largely due to the recognition element of the probe, MerBD, which adopts an unusual planar trigonometric coordination configuration with Hg²⁺, and the coordination can cause enough conformational change to influence the fluorescence of skeleton protein sfGFP coupled with it. The small peptide MerBD was delicately designed based on the three-dimensional structure of metalloprotein MerR. This novel design strategy solves the challenging problems that there are few natural functional proteins in the process of constructing fluorescent probes for toxic metal ions and some functional proteins cannot be directly used as recognition elements. Based on the new strategy, more genetically encoded fluorescent probes of toxic heavy metal ions can be efficiently constructed and applied in the future.

A genetically encoded far-red fluorescent indicator for imaging synaptically released Zn2

Synaptic zinc ion (Zn²⁺) has emerged as a key neuromodulator in the brain. However, the lack of research tools for directly tracking synaptic Zn²⁺ in the brain of awake animals hinders our rigorous understanding of the physiological and pathological roles of synaptic Zn²⁺. In this study, we developed a genetically encoded far-red fluorescent indicator for monitoring synaptic Zn²⁺ dynamics in the nervous system. Our engineered far-red fluorescent indicator for synaptic Zn²⁺ (FRISZ) displayed a substantial Zn²⁺-specific turn-on response and low-micromolar affinity. We genetically anchored FRISZ to the mammalian extracellular membrane via a transmembrane (TM) ⍺ helix and characterized the resultant FRISZ-TM construct at the mammalian cell surface. We used FRISZ-TM to image synaptic Zn²⁺ in the auditory cortex in acute brain slices and awake mice in response to electric and sound stimuli, respectively. Thus, this study establishes a technology for studying the roles of synaptic Zn²⁺ in the nervous system.

Fluorescent proteins and genetically encoded biosensors

The genetically encoded fluorescent sensors convert chemical and physical signals into light. They are powerful tools for the visualisation of physiological processes in living cells and freely moving animals. The fluorescent protein is the reporter module of a genetically encoded biosensor. In this study, we first review the history of the fluorescent protein in full emission spectra on a structural basis. Then, we discuss the design of the genetically encoded biosensor. Finally, we briefly review several major types of genetically encoded biosensors that are currently widely used based on their design and molecular targets, which may be useful for the future design of fluorescent biosensors.

A near-infrared Fluorescent Probe Based on Dicyanisophorone for the Detection of Zinc Ions (Zn2+) in Water and Living Cells

As one of the important metal ions, zinc ions (Zn²⁺) are widely involved in various physiological and pathological processes, and play fundamental roles in neurotransmission, cell metabolism and apoptosis. However, the convenient monitor of Zn²⁺ in environmental and biological samples remains challenging. In this study, a small molecule dicyanoisophorone-based schiff base incorporating with o-phenylenediamine was synthesized. It can rapidly combine with Zn²⁺ to emit significant near-infrared fluorescence (maximum emission wavelength: 660 nm), so it can be used as a probe to quantitatively detect Zn²⁺ in the range of 0-10 μM, with a detection limit as low as 4.8 nM, showing the probe has high sensitivity for Zn²⁺. And the probe has a fast response time to Zn²⁺ (less than 30 s) and a large Stoke-shift (179 nm). In addition, the high recovery rates in practical water samples, and the clear fluorescent images in living A549 cells were obtained, which are of great significance for the detection of Zn²⁺ in the environment and biosystem. Due to its simple operation, good selectivity and anti-interference ability, short detection time and high sensitivity, this probe has great application potential as a fast detection tool for Zn²⁺ in environmental water and biological samples.

Mutant Flavin-Based Fluorescent Protein Sensors for Detecting Intracellular Zinc and Copper in Escherichia coli

Flavin-based fluorescent proteins (FbFPs) are a class of fluorescent reporters that undergo oxygen-independent fluorophore incorporation, which is an important advantage over green fluorescent proteins (GFPs) and mFruits. A FbFP derived from Chlamydomonas reinhardtii (CreiLOV) is a promising platform for designing new metal sensors. Some FbFPs are intrinsically quenched by metal ions, but the question of where metals bind and how to tune metal affinity has not been addressed. We used site-directed mutagenesis of CreiLOV to probe a hypothesized copper(II) binding site that led to fluorescence quenching. Most mutations changed the fluorescence quenching level, supporting the proposed site. One key mutation introducing a second cysteine residue in place of asparagine (CreiLOV_N41C) significantly altered metal affinity and selectivity, yielding a zinc sensor. The fluorescence intensity and lifetime of CreiLOV_N41C were reversibly quenched by Zn²⁺ ions with a biologically relevant affinity (apparent dissociation constant, K_d, of 1 nM). Copper quenching of CreiLOV_N41C was retained but with several orders of magnitude higher affinity than CreiLOV (K_d = 0.066 fM for Cu²⁺, 5.4 fM for Cu⁺) and partial reversibility. We also show that CreiLOV_N41C is an excellent intensity- and lifetime-based zinc sensor in aerobic and anaerobic live bacterial cells. Zn²⁺-induced fluorescence quenching is reversible over several cycles in Escherichia coli cell suspensions and can be imaged by fluorescence microscopy. CreiLOV_N41C is a novel oxygen-independent metal sensor that significantly expands the current fluorescent protein-based toolbox of metal sensors and will allow for studies of anaerobic and low oxygen systems previously precluded by the use of oxygen-dependent GFPs.

Albumin alleviated esketamine-induced neuronal apoptosis of rat retina through downregulation of Zn2+-dependent matrix metalloproteinase 9 during the early development

Aims

Esketamine upregulates Zn²⁺-dependent matrix metalloproteinase 9 (MMP9) and increases the neuronal apoptosis in retinal ganglion cell layer during the early development. We aimed to test whether albumin can alleviate esketamine-induced apoptosis through downregulating Zn²⁺-dependent MMP9.

Methods

We investigate the role of Zn²⁺ in esketamine-induced neuronal apoptosis by immunofluorescence. MMP9 protein expression and enzyme activity were investigated by zymography in situ., western blot and immunofluorescence. Whole-mount retinas from P7 Sprague-Dawley rats were used.

Results

We demonstrated that esketamine exposure increased Zn²⁺ in the retinal GCL during the early development. Zn²⁺-dependent MMP9 expression and enzyme activity up-regulated, which eventually aggravated apoptosis. Albumin effectively down-regulated MMP9 expression and activity via binding of free zinc, ultimately protected neurons from apoptosis. Meanwhile albumin treatment promoted activated microglia into multi-nucleated macrophagocytes and decreased the inflammation.

Conclusion

Albumin alleviates esketamine-induced neuronal apoptosis through decreasing Zn²⁺ accumulation in GCL and downregulating Zn²⁺-dependent MMP9.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12868-022-00753-5.

Dual-mode chemosensor for the fluorescence detection of zinc and hypochlorite on a fluorescein backbone and its cell-imaging applications

Fluorescein coupled with 3-(aminomethyl)-4,6-dimethylpyridin-2(1H)-one (FAD) was synthesized for the selective recognition of Zn²⁺ over other interfering metal ions in acetonitrile/aqueous buffer (1 : 1). Interestingly, there was a significant fluorescence enhancement of FAD in association with Zn²⁺ at 426 nm by strong chelation-induced fluorescence enhancement (CHEF) without interrupting the cyclic spirolactam ring. A binding stoichiometric ratio of 1 : 2 for the ligand FAD with metal Zn²⁺ was proven by a Jobs plot. However, the cyclic spirolactam ring was opened by hypochlorite (OCl^-) as well as oxidative cleavage of the imine bond, which resulted in the emission enhancement of the wavelength at 520 nm. The binding constant and detection limit of FAD towards Zn²⁺ were determined to be 1 × 10⁴ M^-1 and 1.79 μM, respectively, and the detection limit for OCl^- was determined as 2.24 μM. We introduced here a dual-mode chemosensor FAD having both the reactive functionalities for the simultaneous detection of Zn²⁺ and OCl^- by employing a metal coordination (Zn²⁺) and analytes (OCl^-) induced chemodosimetric approach, respectively. Furthermore, for the practical application, we studied the fluorescence imaging inside HeLa cells by using FAD, which demonstrated it can be very useful as a selective and sensitive fluorescent probe for zinc.

Ratiometric Bioluminescent Zinc Sensor Proteins to Quantify Serum and Intracellular Free Zn2

Fluorescent Zn²⁺ sensors play a pivotal role in zinc biology, but their application in complex media such as blood serum or plate reader-based cellular assays is hampered by autofluorescence and light scattering. Bioluminescent sensor proteins provide an attractive alternative to fluorescent sensors for these applications, but the only bioluminescent sensor protein developed so far, BLZinCh, has a limited sensor response and a suboptimal Zn²⁺ affinity. In this work, we expanded the toolbox of bioluminescent Zn²⁺ sensors by developing two new sensor families that show a large change in the emission ratio and cover a range of physiologically relevant Zn²⁺ affinities. The LuZi platform relies on competitive complementation of split NanoLuc luciferase and displays a robust, 2-fold change in red-to-blue emission, allowing quantification of free Zn²⁺ between 2 pM and 1 nM. The second platform was developed by replacing the long flexible GGS linker in the original BLZinCh sensor by rigid polyproline linkers, yielding a series of BLZinCh-Pro sensors with a 3–4-fold improved ratiometric response and physiologically relevant Zn²⁺ affinities between 0.5 and 1 nM. Both the LuZi and BLZinCh-Pro sensors allowed the direct determination of low nM concentrations of free Zn²⁺ in serum, providing an attractive alternative to more laborious and/or indirect approaches to measure serum zinc levels. Furthermore, the genetic encoding of the BLZinCh-Pro sensors allowed their use as intracellular sensors, where the sensor occupancy of 40–50% makes them ideally suited to monitor both increases and decreases in intracellular free Zn²⁺ concentration in simple, plate reader-based measurements, without the need for fluorescence microscopy.

Organelle-Level Labile Zn2+ Mapping Based on Targetable Fluorescent Sensors

Although many Zn²⁺ fluorescent probes have been developed, there remains a lack of consensus on the labile Zn²⁺ concentrations ([Zn²⁺]) in several cellular compartments, as the fluorescence properties and zinc affinity of the fluorescent probes are greatly affected by the pH and redox environments specific to organelles. In this study, we developed two turn-on-type Zn²⁺ fluorescent probes, namely, ZnDA-2H and ZnDA-3H, with low pH sensitivity and suitable affinity (K_d = 5.0 and 0.16 nM) for detecting physiological labile Zn²⁺ in various cellular compartments, such as the cytosol, nucleus, ER, and mitochondria. Due to their sufficient membrane permeability, both probes were precisely localized to the target organelles in HeLa cells using HaloTag labeling technology. Using an in situ standard quantification method, we identified the [Zn²⁺] in the tested organelles, resulting in the subcellular [Zn²⁺] distribution as [Zn²⁺]_ER < [Zn²⁺]_mito < [Zn²⁺]_cyto ∼ [Zn²⁺]_nuc.

Translocation of Foliar Absorbed Zn in Sunflower (Helianthus annuus) Leaves

Foliar zinc (Zn) fertilization is an important approach for overcoming crop Zn deficiency, yet little is known regarding the subsequent translocation of this foliar-applied Zn. Using synchrotron-based X-ray fluorescence microscopy (XFM) and transcriptome analysis, the present study examined the translocation of foliar absorbed Zn in sunflower (Helianthus annuus) leaves. Although bulk analyses showed that there had been minimal translocation of the absorbed Zn out of the leaf within 7 days, in situ analyses showed that the distribution of Zn in the leaf had changed with time. Specifically, when Zn was applied to the leaf for 0.5 h and then removed, Zn primarily accumulated within the upper and lower epidermal layers (when examined after 3 h), but when examined after 24 h, the Zn had moved to the vascular tissues. Transcriptome analyses identified a range of genes involved in stress response, cell wall reinforcement, and binding that were initially upregulated following foliar Zn application, whereas they were downregulated after 24 h. These observations suggest that foliar Zn application caused rapid stress to the leaf, with the initial Zn accumulation in the epidermis as a detoxification strategy, but once this stress decreased, Zn was then moved to the vascular tissues. Overall, this study has shown that despite foliar Zn application causing rapid stress to the leaf and that most of the Zn stayed within the leaf over 7 days, the distribution of Zn in the leaf had changed, with Zn mostly located in the vascular tissues 24 h after the Zn had been applied. Not only do the data presented herein provide new insight for improving the efficiency of foliar Zn fertilizers, but our approach of combining XFM with a transcriptome methodological system provides a novel approach for the study of element translocation in plants.

The Bioinorganic Chemistry of Mammalian Metallothioneins

The functions, purposes, and roles of metallothioneins have been the subject of speculations since the discovery of the protein over 60 years ago. This article guides through the history of investigations and resolves multiple contentions by providing new interpretations of the structure-stability-function relationship. It challenges the dogma that the biologically relevant structure of the mammalian proteins is only the one determined by X-ray diffraction and NMR spectroscopy. The terms metallothionein and thionein are ambiguous and insufficient to understand biological function. The proteins need to be seen in their biological context, which limits and defines the chemistry possible. They exist in multiple forms with different degrees of metalation and types of metal ions. The homoleptic thiolate coordination of mammalian metallothioneins is important for their molecular mechanism. It endows the proteins with redox activity and a specific pH dependence of their metal affinities. The proteins, therefore, also exist in different redox states of the sulfur donor ligands. Their coordination dynamics allows a vast conformational landscape for interactions with other proteins and ligands. Many fundamental signal transduction pathways regulate the expression of the dozen of human metallothionein genes. Recent advances in understanding the control of cellular zinc and copper homeostasis are the foundation for suggesting that mammalian metallothioneins provide a highly dynamic, regulated, and uniquely biological metal buffer to control the availability, fluctuations, and signaling transients of the most competitive Zn(II) and Cu(I) ions in cellular space and time.

Recent progress in developing fluorescent probes for imaging cell metabolites

Cellular metabolites play a crucial role in promoting and regulating cellular activities, but it has been difficult to monitor these cellular metabolites in living cells and in real time. Over the past decades, iterative development and improvements of fluorescent probes have been made, resulting in the effective monitoring of metabolites. In this review, we highlight recent progress in the use of fluorescent probes for tracking some key metabolites, such as adenosine triphosphate, cyclic adenosine monophosphate, cyclic guanosine 5'-monophosphate, Nicotinamide adenine dinucleotide (NADH), reactive oxygen species, sugar, carbon monoxide, and nitric oxide for both whole cell and subcellular imaging.

ZIGIR, a Granule-Specific Zn2+ Indicator, Reveals Human Islet α Cell Heterogeneity

Numerous mammalian cells contain abundant Zn²⁺ in their secretory granules, yet available Zn²⁺ sensors lack the desired specificity and sensitivity for imaging granular Zn²⁺. We developed a fluorescent zinc granule indicator, ZIGIR, that possesses numerous desired properties for live cell imaging, including >100-fold fluorescence enhancement, membrane permeability, and selective enrichment to acidic granules. The combined advantages endow ZIGIR with superior sensitivity and specificity for imaging granular Zn²⁺. ZIGIR enables separation of heterogenous β cells based on their insulin content and sorting of mouse islets into pure α cells and β cells. In human islets, ZIGIR facilitates sorting of endocrine cells into highly enriched α cells and β cells, reveals unexpectedly high Zn²⁺ activity in the somatostatin granule of some δ cells, and uncovers variation in the glucagon content among human α cells. We expect broad applications of ZIGIR for studying Zn²⁺ biology and Zn²⁺-rich secretory granules and for engineering β cells with high insulin content for treating diabetes.

Water-soluble fluorescent sensor for Zn2+ with high selectivity and sensitivity imaging in living cells

A new water-soluble 4-amino-1, 8-naphthalimide based fluorescent sensor, with iminoacetic acid and iminoethoxyacetic acid as receptor contained two different arms, was developed. Under physiological pH conditions, it demonstrates good water solubility, high selectivity and sensitivity for sensing Zn²⁺ with about 20-fold enhancement in aqueous solution, with a characteristic emission band of 4-amino-1, 8-naphthalimide with a green color centered at 550 nm. It was applied successfully to detect Zn²⁺ in living cells.

HaloTag-Based Hybrid Targetable and Ratiometric Sensors for Intracellular Zinc

We report a new series of small molecule-protein hybrid zinc sensors that combine genetic targetability with the spectroscopic profile of synthetic fluorophores. We functionalized the zinc sensor ZinPyr-1 (ZP1) with a chloroalkane linker (ZP1-12Cl) that reacts specifically with the engineered protein HaloTag. The resulting construct, ZP1-HaloTag, binds zinc ions with a threefold fluorescence enhancement. Through exploitation of the protein synthesis machinery of live cells, the HaloTag protein component was expressed, and the ZP1-HaloTag hybrid was assembled upon bath application of ZP1-12Cl. After fusion of HaloTag with targeting peptides or proteins, the resulting hybrid sensor could be directed to specific subcellular locales, including the nucleus, mitochondrial outer membrane, and endoplasmic reticulum. Furthermore, HaloTag was linked with the red fluorescent protein mCherry, permitting formation of a two-fluorophore system that provides not only targetable but also ratiometric sensing of cellular zinc. This system reversibly detects both exogenous and endogenous mobile Zn2+ in response to reactive nitrogen species in live HeLa cells. HaloTag-based hybrid zinc sensors offer new opportunities for visualizing and quantifying biological mobile zinc at discrete subcellular compartments.

DNAzyme-Mediated Genetically Encoded Sensors for Ratiometric Imaging of Metal Ions in Living Cells

Genetically encoded fluorescent proteins (FPs) have been used for metal ion detection. However, their applications are restricted to a limited number of metal ions owing to the lack of available metal-binding proteins or peptides that can be fused to FPs and the difficulty in transforming the binding of metal ions into a change of fluorescent signal. We report herein the use of Mg²⁺ -specific 10-23 or Zn²⁺ -specific 8-17 RNA-cleaving DNAzymes to regulate the expression of FPs as a new class of ratiometric fluorescent sensors for metal ions. Specifically, we demonstrate the use of DNAzymes to suppress the expression of Clover2, a variant of the green FP (GFP), by cleaving the mRNA of Clover2, while the expression of Ruby2, a mutant of the red FP (RFP), is not affected. The Mg²⁺ or Zn²⁺ in HeLa cells can be detected using both confocal imaging and flow cytometry. Since a wide variety of metal-specific DNAzymes can be obtained, this method can likely be applied to imaging many other metal ions, expanding the range of the current genetically encoded fluorescent protein-based sensors.

Sub-nanomolar sensitive GZnP3 reveals TRPML1-mediated neuronal Zn2+ signals

Although numerous fluorescent Zn²⁺ sensors have been reported, it is unclear whether and how Zn²⁺ can be released from the intracellular compartments into the cytosol due to a lack of probes that can detect physiological dynamics of cytosolic Zn²⁺. Here, we create a genetically encoded sensor, GZnP3, which demonstrates unprecedented sensitivity for Zn²⁺ at sub-nanomolar concentrations. Using GZnP3 as well as GZnP3-derived vesicular targeted probes, we provide the first direct evidence that Zn²⁺ can be released from endolysosomal vesicles to the cytosol in primary hippocampal neurons through the TRPML1 channel. Such TRPML1-mediated Zn²⁺ signals are distinct from Ca²⁺ in that they are selectively present in neurons, sustain longer, and are significantly higher in neurites as compared to the soma. Together, our work not only creates highly sensitive probes for investigating sub-nanomolar Zn²⁺ dynamics, but also reveals new pools of Zn²⁺ signals that can play critical roles in neuronal function.

Numerous fluorescent Zn²⁺ sensors are available but most are unsuitable to detect physiological dynamics of cytosolic Zn²⁺. In this study, the authors present a genetically encoded sensor with sub-nanomolar sensitivity and show that Zn^2 + is released from endolysosomal vesicles via TRPML1 in neurons.

Genetically Encoded, Photostable Indicators to Image Dynamic Zn2+ Secretion of Pancreatic Islets

As an essential element for living organisms, zinc (Zn2+) exerts its biological functions both intracellularly and extracellularly. Previous studies have reported a number of genetically encoded Zn2+ indicators (GEZIs), which have been widely used to monitor Zn2+ in the cytosol and intracellular organelles. However, it is challenging to localize existing GEZIs to the extracellular space to detect secreted Zn2+. Herein, we report two photostable, green fluorescent protein (GFP) based indicators, ZIBG1 and ZIBG2, which respond to Zn2+ selectively and have affinities suited for detecting Zn2+ secretion from intracellular vesicles. In particular, ZIBG2 can be effectively targeted to the extracellular side of plasma membrane. We applied cell surface-localized ZIBG2 to monitor glucose-induced dynamic Zn2+ secretion from mouse insulinoma MIN6 cells and primary mouse and human pancreatic islets. Because Zn2+ is co-released with insulin from β-cells, the fluorescence of cell surface-localized ZIBG2 was shown to be a strong indicator for the functional potency of islets. Our work here has thus expanded the use of GEZIs to image dynamic Zn2+ secretion in live tissue. Because it is convenient to use genetically encoded indicators for expression over extended periods and for in vivo delivery, we envision future applications of ZIBG2 in development of induced β-cells or islets to advance cell replacement therapies for diabetes and in direct imaging of Zn2+ secretion dynamics in vivo.

A highly selective and sensitive 1,8-naphthalimide-based fluorescent sensor for Zn2+ imaging in living cells

A new 4-amino-1,8-naphthalimide-based fluorescent sensor, with iminoacetic acid and iminoethoxyacetic acid as receptor, was developed. It was applied successfully to detect Zn²⁺ in aqueous solution and living cells. Under physiological pH conditions, it demonstrates high selectivity and sensitivity for sensing Zn²⁺ with about 7-fold enhancement in aqueous solution, with a characteristic emission band of 4-amino-1,8-naphthalimide with a green color centered at 550 nm.

Engineering genetically encoded FRET-based nanosensors for real time display of arsenic (As3+) dynamics in living cells

Arsenic poisoning has been a major concern that causes severe toxicological damages. Therefore, intricate and inclusive understanding of arsenic flux rates is required to ascertain the cellular concentration and establish the carcinogenetic mechanism of this toxicant at real time. The lack of sufficiently sensitive sensing systems has hampered research in this area. In this study, we constructed a fluorescent resonance energy transfer (FRET)-based nanosensor, named SenALiB (Sensor for Arsenic Linked Blackfoot disease) which contains a metalloregulatory arsenic-binding protein (ArsR) as the As³⁺ sensing element inserted between the FRET pair enhanced cyan fluorescent protein (ECFP) and Venus. SenALiB takes advantage of the ratiometic FRET readout which measures arsenic with high specificity and selectivity. SenALiB offers rapid detection response, is stable to pH changes and provides highly accurate, real-time optical readout in cell-based assays. SenALiB-676n with a binding constant (K_d) of 0.676 × 10⁻⁶ M is the most efficient affinity mutant and can be a versatile tool for dynamic measurement of arsenic concentration in both prokaryotes and eukaryotes in vivo in a non-invasive manner.

A unique benzimidazole-naphthalene hybrid molecule for independent detection of Zn2+ and N3- ions: Experimental and theoretical investigations

Single crystal X-ray structurally characterized benzimidazole-naphthalene hybrid (NABI) functions as a unique dual analyte sensor that can detect Zn²⁺ cation and N₃^- anion independently. The NABI forms chelate with Zn²⁺ to inhibit internal charge transfer (ICT) and CHN isomerisation resulting chelation enhanced fluorescence (CHEF). On the other hand, the sensing of N₃^- is based on formation of supramolecular H-bonded rigid assembly. The association constant of NABI for Zn²⁺ and N₃^- ions are 19 × 10⁴ M^-1 and 11 × 10² M^-1, respectively. Corresponding limit of detections (LOD) are 6.85 × 10^-8 and 1.82 × 10^-7 M, respectively. NABI efficiently detects intracellular Zn²⁺ and N₃^- ions with no cytotoxicity on J774A.1cells under fluorescence microscope. DFT studies unlock underlying spectroscopic properties of free NABI and Zn²⁺/N₃^- bound forms.

Sensors for measuring subcellular zinc pools

Zinc homeostasis is essential for normal cellular function, and defects in this process are associated with a number of diseases including type 2 diabetes (T2D), neurological disorders and cardiovascular disease. Thus, variants in the SLC30A8 gene, encoding the vesicular/granular zinc transporter ZnT8, are associated with altered insulin release and increased T2D risk while the zinc importer ZIP12 is implicated in pulmonary hypertension. In light of these, and findings in other diseases, recent efforts have focused on the development of refined sensors for intracellular free zinc ions that can be targeted to subcellular regions including the cytosol, endoplasmic reticulum (ER), secretory granules, Golgi apparatus, nucleus and the mitochondria. Here, we discuss recent advances in Zn²⁺ probe engineering and their applications to the measurement of labile subcellular zinc pools in different cell types.

Metal binding properties of zinc fingers with a naturally altered metal binding site

Zinc fingers (ZFs) are among the most abundant motifs found in proteins, and are commonly known for their structural role. Classical ZFs (CCHH) are part of the transcription factors that participate in DNA binding. Although biochemical studies of classical ZFs have a long history, there is limited knowledge about the sequential and structural diversity of ZFs. We have found that classical ZFs, with metal binding sites consisting of amino acids other than conserved Cys or His residues, are frequently encoded in the human genome, and we refer to these peptides as ZFs with a naturally altered metal binding site. The biological role of the altered ZFs remains undiscovered. In this study, we characterized nine natural XCHH, CXHH, CCXH and CCHX ZFs in terms of their Zn(ii) and Co(ii) binding properties, such as complex stoichiometry, spectroscopic properties and metal-to-peptide affinity. We revealed that XCHH and CXHH ZFs form ML complexes that are 4-5 orders of magnitude weaker in comparison to CCHH ZFs. Nevertheless, spectroscopic studies demonstrate that, depending on the altered position, they may adopt an open coordination geometry with one or two water molecules bound to a central metal ion, which has not been demonstrated in natural ZFs before. Stability data show that both CCXH and CCHX peptides have high Zn(ii) affinity (with a K_d of 10^-9 to 10^-11 M), suggesting their potential biological function. This study is a comprehensive overview of the relationship between the sequence, structure, and stability of ZFs.

Real-Time Imaging of Nitric Oxide Signals in Individual Cells Using geNOps

Nitric oxide (NO^•) is a versatile signaling molecule which regulates fundamental cellular processes in all domains of life. However, due to the radical nature of NO^• it has a very short half-life that makes it challenging to trace its formation, diffusion, and degradation on the level of individual cells. Very recently, we expanded the family of genetically encoded sensors by introducing a novel class of single fluorescent protein-based NO^• probes-the geNOps. Once expressed in cells of interest, geNOps selectively respond to NO^• by fluorescence quench, which enables real-time monitoring of cellular NO^• signals. Here, we describe detailed methods suitable for imaging of NO^• signals in mammalian cells. This novel approach may facilitate a broad range of studies to (re)investigate the complex NO^• biochemistry in living cells.

Ultra-fast zinc ion detection in living cells and zebrafish by a light-up fluorescent probe

As the second most abundant transition metal after iron in biological systems, Zn²⁺ takes part in various fundamental life processes such as cellular metabolism and apoptosis, neurotransmission. Thus, the development of analytical methods for fast detection of Zn²⁺ in biology and medicine has been attracting much attention but still remains a huge challenge. In this report, we develop a novel Zn²⁺-specific light-up fluorescent probe based on intramolecular charge transfer combined with chelation enhanced fluorescence induced by structural transformation. Addition of Zn²⁺ in vitro can induce a remarkable color change from colorless to green and a strong fluorescence enhancement with a red shift of 43 nm. Moreover, the probe shows an extremely low detection limit of 13 nM and ultra-fast response time of less than 1 s. The Zn²⁺ sensing mechanism was fully supported by TDDFT calculations as well as HRMS and ¹H NMR titrations. The recognition of Zn²⁺ in living Hela cells as well as the MTT assay demonstrate that the probe can rapidly light-up detect Zn²⁺ in vivo with low cytotoxicity and good cell-permeability. Furthermore, the probe can also be successfully applied to bioimaging Zn²⁺ in living zebrafish.

Subtle structural variation in azine/imine derivatives controls Zn2+ sensitivity: ESIPT-CHEF combination for nano-molar detection of Zn2+ with DFT support

Excited-state intra-molecular proton transfer (ESIPT)-active imine and azine derivatives, structurally characterised by XRD, and denoted L1, L2, L3 and L4, possess weak fluorescence. The interaction of these probes with Zn²⁺ turns ON the fluorescence to allow its nano-molar detection. Among the four ESIPT-active molecules, L2, L3 and L4 are bis-imine derivatives while L1 is a mono-imine derivative. Among the three bis-imine derivatives, one is symmetric (L3) while L2 and L4 are unsymmetrical. The lowest detection limits (DL) of L1, L2, L3 and L4 for Zn²⁺ are 32.66 nM, 36.16 nM, 15.20 nM and 33.50 nM respectively. All the probes bind Zn²⁺ (10⁵ M⁻¹ order) strongly. Computational studies explore the orbital level interactions responsible for the associated photo-physical processes.

Single crystal X-ray structurally characterised ESIPT-active weakly fluorescent imine and azine derivatives undergo Zn²⁺ assisted turn ON fluorescence.

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.

Cyclic AMP Pathway Activation and Extracellular Zinc Induce Rapid Intracellular Zinc Mobilization in Candida albicans

Zinc is an essential micronutrient, required for a range of zinc-dependent enzymes and transcription factors. In mammalian cells, zinc serves as a second messenger molecule. However, a role for zinc in signaling has not yet been established in the fungal kingdom. Here, we used the intracellular zinc reporter, zinbo-5, which allowed visualization of zinc in the endoplasmic reticulum and other components of the internal membrane system in Candida albicans. We provide evidence for a link between cyclic AMP/PKA- and zinc-signaling in this major human fungal pathogen. Glucose stimulation, which triggers a cyclic AMP spike in this fungus resulted in rapid intracellular zinc mobilization and this “zinc flux” could be stimulated with phosphodiesterase inhibitors and blocked via inhibition of adenylate cyclase or PKA. A similar mobilization of intracellular zinc was generated by stimulation of cells with extracellular zinc and this effect could be reversed with the chelator EDTA. However, zinc-induced zinc flux was found to be cyclic AMP independent. In summary, we show that activation of the cyclic AMP/PKA pathway triggers intracellular zinc mobilization in a fungus. To our knowledge, this is the first described link between cyclic AMP signaling and zinc homeostasis in a human fungal pathogen.

Chromis-1, a Ratiometric Fluorescent Probe Optimized for Two-Photon Microscopy Reveals Dynamic Changes in Labile Zn(II) in Differentiating Oligodendrocytes

Despite the significant advantages of two-photon excitation microscopy (TPEM) over traditional confocal fluorescence microscopy in live-cell imaging applications, including reduced phototoxicity and photobleaching, increased depth penetration, and minimized autofluorescence, only a few metal ion-selective fluorescent probes have been designed and optimized specifically for this technique. Building upon a donor-acceptor fluorophore architecture, we developed a membrane-permeant, Zn(II)-selective fluorescent probe, chromis-1, that exhibits a balanced two-photon cross section between its free and Zn(II)-bound form and responds with a large spectral shift suitable for emission-ratiometric imaging. With a Kd of 1.5 nM and wide dynamic range, the probe is well suited for visualizing temporal changes in buffered Zn(II) levels in live cells as demonstrated with mouse fibroblast cell cultures. Moreover, given the importance of zinc in the physiology and pathophysiology of the brain, we employed chromis-1 to monitor cytoplasmic concentrations of labile Zn(II) in oligodendrocytes, an important cellular constituent of the brain, at different stages of development in cell culture. These studies revealed a decrease in probe saturation upon differentiation to mature oligodendrocytes, implying significant changes to cellular zinc homeostasis during maturation with an overall reduction in cellular zinc availability. Optimized for TPEM, chromis-1 is especially well-suited for exploring the role of labile zinc pools in live cells under a broad range of physiological and pathological conditions.

Oxygen Sensing, Hypoxia Tracing and in Vivo Imaging with Functional Metalloprobes for the Early Detection of Non-communicable Diseases

Hypoxia has been identified as one of the hallmarks of tumor environments and a prognosis factor in many cancers. The development of ideal chemical probes for imaging and sensing of hypoxia remains elusive. Crucial characteristics would include a measurable response to subtle variations of pO2 in living systems and an ability to accumulate only in the areas of interest (e.g., targeting hypoxia tissues) whilst exhibiting kinetic stabilities in vitro and in vivo. A sensitive probe would comprise platforms for applications in imaging and therapy for non-communicable diseases (NCDs) relying on sensitive detection of pO2. Just a handful of probes for the in vivo imaging of hypoxia [mainly using positron emission tomography (PET)] have reached the clinical research stage. Many chemical compounds, whilst presenting promising in vitro results as oxygen-sensing probes, are facing considerable disadvantages regarding their general application in vivo. The mechanisms of action of many hypoxia tracers have not been entirely rationalized, especially in the case of metallo-probes. An insight into the hypoxia selectivity mechanisms can allow an optimization of current imaging probes candidates and this will be explored hereby. The mechanistic understanding of the modes of action of coordination compounds under oxygen concentration gradients in living cells allows an expansion of the scope of compounds toward in vivo applications which, in turn, would help translate these into clinical applications. We summarize hereby some of the recent research efforts made toward the discovery of new oxygen sensing molecules having a metal-ligand core. We discuss their applications in vitro and/or in vivo, with an appreciation of a plethora of molecular imaging techniques (mainly reliant on nuclear medicine techniques) currently applied in the detection and tracing of hypoxia in the preclinical and clinical setups. The design of imaging/sensing probe for early-stage diagnosis would longer term avoid invasive procedures providing platforms for therapy monitoring in a variety of NCDs and, particularly, in cancers.

Photoactivatable Sensors for Detecting Mobile Zinc

Fluorescent sensors for mobile zinc are valuable for studying complex biological systems. Because these sensors typically bind zinc rapidly and tightly, there has been little temporal control over the activity of the probe after its application to a sample. The ability to control the activity of a zinc sensor in vivo during imaging experiments would greatly improve the time resolution of the measurement. Here, we describe photoactivatable zinc sensors that can be triggered with short pulses of UV light. These probes are prepared by functionalizing a zinc sensor with protecting groups that render the probe insensitive to metal ions. Photoinduced removal of the protecting groups restores the binding site, allowing for zinc-responsive changes in fluorescence that can be observed in live cells and tissues.

Fabrication of a self-assembled supramolecular fluorescent nanosensor from functional graphene oxide and its application for the detection of Al3+

In this work, we present an excellent water-soluble supramolecular self-assembled fluorescent nanosensor toward Al³⁺based on graphene oxide and a β-cyclodextrin derivative.

Genetically encoded fluorescent sensors for measuring transition and heavy metals in biological systems

Great progress has been made in expanding the repertoire of genetically encoded fluorescent sensors for monitoring intracellular transition metals (TMs). This powerful toolkit permits dynamic and non-invasive detection of TMs with high spatial-temporal resolution, which enables us to better understand the roles of TM homeostasis in both physiological and pathological settings. Here we summarize the recent development of genetically encoded fluorescent sensors for intracellular detection of TMs such as zinc and copper, as well as heavy metals including lead, cadmium, mercury, and arsenic.

Development of a coumarin-furan conjugate as Zn2+ ratiometric fluorescent probe in ethanol-water system

In this study, a novel coumarin-derived compound bearing the furan moiety called 7-diethylamino-3-formylcoumarin (2'-furan formyl) hydrazone (1) has been designed, synthesized and evaluated as a Zn²⁺ ratiometric fluorescent probe in ethanol-water system. This probe 1 showed good selectivity and high sensitivity towards Zn²⁺ over other metal ions investigated, and a decrease in fluorescence emission intensity at 511nm accompanied by an enhancement in fluorescence emission intensity at 520nm of this probe 1 was observed in the presence of Zn²⁺ in ethanol-water (V : V=9 : 1) solution, which provided ratiometric fluorescence detection of Zn²⁺. Additionally, the ratiometric fluorescence response of 1 to Zn²⁺ was nearly completed within 0.5min, which suggested that this probe 1 could be utilized for sensing and monitoring Zn²⁺ in environmental and biological systems for real-time detection.