Fluorescence lifetime imaging of physiological free Cu(II) levels in live cells with a Cu(II)-selective carbonic anhydrase-based biosensor

Chemical background of silver nanoparticles interfering with mammalian copper metabolism

The rapidly increasing application of silver nanoparticles (AgNPs) boosts their release into the environment, which raises a reasonable alarm for ecologists and health specialists. This is manifested as increased research devoted to the influence of AgNPs on physiological and cellular processes in various model systems, including mammals. The topic of the present paper is the ability of silver to interfere with copper metabolism, the potential health effects of this interference, and the danger of low silver concentrations to humans. The chemical properties of ionic and nanoparticle silver, supporting the possibility of silver release by AgNPs in extracellular and intracellular compartments of mammals, are discussed. The possibility of justified use of silver for the treatment of some severe diseases, including tumors and viral infections, based on the specific molecular mechanisms of the decrease in copper status by silver ions released from AgNPs is also discussed.

Ratiometric Zinc Biosensor Based on Bioluminescence Resonance Energy Transfer: Trace Metal Ion Determination with Tunable Response

Determination of metal ions such as zinc in solution remains an important task in analytical and biological chemistry. We describe a novel zinc ion biosensing approach using a carbonic anhydrase-Oplophorus luciferase fusion protein that employs bioluminescence resonance energy transfer (BRET) to transduce the level of free zinc as a ratio of emission intensities in the blue and orange portions of the spectrum. In addition to high sensitivity (below nanomolar levels) and selectivity, this approach allows both quantitative determination of "free" zinc ion (also termed "mobile" or "labile") using bioluminescence ratios and determination of the presence of the ion above a threshold simply by the change in color of bioluminescence, without an instrument. The carbonic anhydrase metal ion sensing platform offers well-established flexibility in sensitivity, selectivity, and response kinetics. Finally, bioluminescence labeling has proven an effective approach for molecular imaging in vivo since no exciting light is required; the expressible nature of this sensor offers the prospect of imaging zinc fluxes in vivo.

PC-12 Cell Line as a Neuronal Cell Model for Biosensing Applications

PC-12 cells have been widely used as a neuronal line study model in many biosensing devices, mainly due to the neurogenic characteristics acquired after differentiation, such as high level of secreted neurotransmitter, neuron morphology characterized by neurite outgrowth, and expression of ion and neurotransmitter receptors. For understanding the pathophysiology processes involved in brain disorders, PC-12 cell line is extensively assessed in neuroscience research, including studies on neurotoxicity, neuroprotection, or neurosecretion. Various analytical technologies have been developed to investigate physicochemical processes and the biosensors based on optical and electrochemical techniques, among others, have been at the forefront of this development. This article summarizes the application of different biosensors in PC-12 cell cultures and presents the modern approaches employed in neuronal networks biosensing.

Selective Detection of Cu+ Ions in Live Cells via Fluorescence Lifetime Imaging Microscopy

Copper is an essential trace element in living organisms with its levels and localisation being carefully managed by the cellular machinery. However, if misregulated, deficiency or excess of copper ions can lead to several diseases. Therefore, it is important to have reliable methods to detect, monitor and visualise this metal in cells. Herein we report a new optical probe based on BODIPY, which shows a switch-on in its fluorescence intensity upon binding to copper(I), but not in the presence of high concentration of other physiologically relevant metal ions. More interestingly, binding to copper(I) leads to significant changes in the fluorescence lifetime of the new probe, which can be used to visualize copper(I) pools in lysosomes of live cells via fluorescence lifetime imaging microscopy (FLIM).

A novel near-infrared turn-on and ratiometric fluorescent probe capable of copper(ii) ion determination in living cells

A near-infrared ratiometric fluorescent probe CR-Ac based on a coumarin-benzopyrylium platform has been developed for selective detection of Cu2+. The cell imaging data revealed the capabilities of CR-Ac in monitoring the dynamic changes of subcellular Cu2+ and the quantification of Cu2+ levels in living cells.

Imaging hydroxyapatite in sub-retinal pigment epithelial deposits by fluorescence lifetime imaging microscopy with tetracycline staining

SIGNIFICANCE

Recent evidence suggests that hydroxyapatite (HAP) in sub-retinal pigment epithelial (sub-RPE) deposits in aged human eyes may act to nucleate and contribute to their growth to clinically detectable size. Sub-RPE deposits such as drusen are clinical hallmarks of age-related macular degeneration (AMD), therefore enhanced and earlier detection is a clinical need. We found that tetracycline-family antibiotics, long known to stain HAP in teeth and bones, can also label the HAP in sub-RPE deposits. However, HAP-bound tetracycline fluorescence excitation and emission spectra overlap with the well-known autofluorescence of outer retinal tissues, making them difficult to resolve.

AIM

In this initial study, we sought to determine if the HAP-bound tetracyclines also exhibit enhanced fluorescence lifetimes, providing a useful difference in lifetime compared with the short lifetimes observed in vivo in the human retina by the pioneering work of Schweitzer, Zinkernagel, Hammer, and their colleagues, and thus a large enough effect size to resolve the HAP from background by fluorescence lifetime imaging.

APPROACH

We stained authentic HAP with tetracyclines and measured the lifetime(s) by phase fluorometry, and stained aged, fixed human cadaver retinas with drusen with selected tetracyclines and imaged them by fluorescence lifetime imaging microscopy (FLIM).

RESULTS

We found that chlortetracycline and doxycycline exhibited substantial increase in fluorescence lifetime compared to the free antibiotics and the retinal background, and the drusen were easily resolvable from the retinal background in these specimens by FLIM.

CONCLUSIONS

These findings suggest that FLIM imaging of tetracycline (and potentially other molecules) binding to HAP could become a diagnostic tool for the development and progression of AMD.

Biophysical approaches for the study of metal-protein interactions

Protein-protein interactions play important roles for a variety of cell functions, often involving metal ions; in fact, metal-ion binding mediates and regulates the activity of a wide range of biomolecules. Enlightening all of the specific features of metal-protein and metal-mediated protein-protein interactions can be a very challenging task; a detailed knowledge of the thermodynamic and spectroscopic parameters and the structural changes of the protein is normally required. For this purpose, many experimental techniques are employed, embracing all fields of Analytical and Bioinorganic Chemistry. In addition, the use of peptide models, reproducing the primary sequence of the metal-binding sites, is also proved to be useful. In this paper, a review of the most useful techniques for studying ligand-protein interactions with a special emphasis on metal-protein interactions is provided, with a critical summary of their strengths and limitations.

The copper(II) binding centres of carbonic anhydrase are differently affected by reductants that ensure the redox intracellular environment

Copper is involved in several biological processes. The static and labile copper pools are controlled by means of a network of influx and efflux transporters, storage proteins, chaperones, transcription factors and small molecules as glutathione (GSH), which contributes to the cell reducing environment. To follow the fate of intracellular copper labile pool, a variant of human apocarbonic anhydrase has been proposed as fluorescent probe to monitor cytoplasmic Cu²⁺. Aware that in this cellular compartment copper ion is present as Cu⁺, electron spin resonance technique (ESR) was used to ascertain whether (bovine or human) carbonic anhydrase (CA) was able to accommodate Cu⁺ in the same sites occupied by Cu²⁺, in the presence of naturally occurring reducing agents such as ascorbate and GSH. Our ESR results on Cu²⁺ complexes with CA allow for a complete characterization of the two metal binding sites of the protein in solution. The use of the reported affinity constants of zinc in the catalytic site and of Cu²⁺ in the peripheral and catalytic site, allow us to obtain the speciation of copper species mimicking the spectroscopic study conditions. The different Cu²⁺ coordination features in the catalytic and the peripheral (the N-terminus cleft mouth) binding sites influence the chemical reduction effect of the two main naturally occurring reductants. Ascorbate reversibly reduces the Cu²⁺ complex with CA, while glutathione irreversibly induces the formation of Cu²⁺ complex with its oxidized form (GSSG). Our results questioned the use of CA as intracellular Cu²⁺ sensor. Furthermore, translating these findings to intracellular environment, the conversion of GSH in GSSG can significantly alter the metallostasis.

A high-affinity fluorescence probe for copper(II) ions and its application in fluorescence lifetime correlation spectroscopy

Copper is one of the most important transition metals in many organisms where it catalyzes a manifold of different processes. As a result of copper's redox activity, organisms have to avoid unbound ions, and a dysfunctional copper homeostasis may lead to multifarious pathological processes in cells with very severe ramifications for the affected organisms. In many neurodegenerative diseases, however, the exact role of copper ions is still not completely clarified. In this work, a high-affinity and highly selective copper probe molecule, based on the naturally occurring tetrapeptide DAHK is synthesized. The sensor (log K_D = - 12.8 ± 0.1) is tagged with a fluorescent BODIPY dye whose fluorescence lifetime distinctly decreases from 5.8 ns ± 0.2 ns to 0.4 ns ± 0.1 ns on binding to copper(II) cations. It is shown by using fluorescence lifetime correlation spectroscopy that the concentration of both probe and probe-copper complex can be simultaneously measured even at nanomolar concentration levels. This work presents a possible starting point for a new type of probe and method for future in vivo studies to further reveal the exact role of copper ions in organisms. Graphical abstract.

Copper signalling: causes and consequences

Copper-containing enzymes perform fundamental functions by activating dioxygen (O2) and therefore allowing chemical energy-transfer for aerobic metabolism. The copper-dependence of O2 transport, metabolism and production of signalling molecules are supported by molecular systems that regulate and preserve tightly-bound static and weakly-bound dynamic cellular copper pools. Disruption of the reducing intracellular environment, characterized by glutathione shortage and ambient Cu(II) abundance drives oxidative stress and interferes with the bidirectional, copper-dependent communication between neurons and astrocytes, eventually leading to various brain disease forms. A deeper understanding of of the regulatory effects of copper on neuro-glia coupling via polyamine metabolism may reveal novel copper signalling functions and new directions for therapeutic intervention in brain disorders associated with aberrant copper metabolism.

Picomolar zinc binding modulates formation of Bcl10-nucleating assemblies of the caspase recruitment domain (CARD) of CARD9

The caspase recruitment domain-containing protein 9 (CARD9)-B-cell lymphoma/leukemia 10 (Bcl10) signaling axis is activated in myeloid cells during the innate immune response to a variety of diverse pathogens. This signaling pathway requires a critical caspase recruitment domain (CARD)-CARD interaction between CARD9 and Bcl10 that promotes downstream activation of factors, including NF-κB and the mitogen-activated protein kinase (MAPK) p38. Despite these insights, CARD9 remains structurally uncharacterized, and little mechanistic understanding of its regulation exists. We unexpectedly found here that the CARD in CARD9 binds to Zn²⁺ with picomolar affinity-a concentration comparable with the levels of readily accessible Zn²⁺ in the cytosol. NMR solution structures of the CARD9-CARD in the apo and Zn²⁺-bound states revealed that Zn²⁺ has little effect on the ground-state structure of the CARD; yet the stability of the domain increased considerably upon Zn²⁺ binding, with a concomitant reduction in conformational flexibility. Moreover, Zn²⁺ binding inhibited polymerization of the CARD9-CARD into helical assemblies. Here, we also present a 20-Å resolution negative-stain EM (NS-EM) structure of these filamentous assemblies and show that they adopt a similar helical symmetry as reported previously for filaments of the Bcl10 CARD. Using both bulk assays and direct NS-EM visualization, we further show that the CARD9-CARD assemblies can directly template and thereby nucleate Bcl10 polymerization, a capacity considered critical to propagation of the CARD9-Bcl10 signaling cascade. Our findings indicate that CARD9 is a potential target of Zn²⁺-mediated signaling that affects Bcl10 polymerization in innate immune responses.

Photocontrollable fluorogenic probes for visualising near-membrane copper(ii) in live cells

Photocontrollable fluorogenic probes for detecting near-membrane copper(ii) after membrane anchoring using spatial and temporal controlled release.

Temporal binning of time-correlated single photon counting data improves exponential decay fits and imaging speed

Time-correlated single photon counting (TCSPC) enables acquisition of fluorescence lifetime decays with high temporal resolution within the fluorescence decay. However, many thousands of photons per pixel are required for accurate lifetime decay curve representation, instrument response deconvolution, and lifetime estimation, particularly for two-component lifetimes. TCSPC imaging speed is inherently limited due to the single photon per laser pulse nature and low fluorescence event efficiencies (<10%) required to reduce bias towards short lifetimes. Here, simulated fluorescence lifetime decays are analyzed by SPCImage and SLIM Curve software to determine the limiting lifetime parameters and photon requirements of fluorescence lifetime decays that can be accurately fit. Data analysis techniques to improve fitting accuracy for low photon count data were evaluated. Temporal binning of the decays from 256 time bins to 42 time bins significantly (p<0.0001) improved fit accuracy in SPCImage and enabled accurate fits with low photon counts (as low as 700 photons/decay), a 6-fold reduction in required photons and therefore improvement in imaging speed. Additionally, reducing the number of free parameters in the fitting algorithm by fixing the lifetimes to known values significantly reduced the lifetime component error from 27.3% to 3.2% in SPCImage (p<0.0001) and from 50.6% to 4.2% in SLIM Curve (p<0.0001). Analysis of nicotinamide adenine dinucleotide-lactate dehydrogenase (NADH-LDH) solutions confirmed temporal binning of TCSPC data and a reduced number of free parameters improves exponential decay fit accuracy in SPCImage. Altogether, temporal binning (in SPCImage) and reduced free parameters are data analysis techniques that enable accurate lifetime estimation from low photon count data and enable TCSPC imaging speeds up to 6x and 300x faster, respectively, than traditional TCSPC analysis.

Fluorescence imaging of metal ions implicated in diseases

Metal ions play an important role in various biological processes, their abnormal homeostasis in cells is related to many diseases, such as neurodegenerative disease, cancer and diabetes. Fluorescent imaging offers a unique route to detect metal ions in cells via a contactless and damage-free way with high spatial and temporal fidelity. Consequently, it represents a promising method to advance the understanding of physiological and pathological functions of metal ions in cell biology. In this highlight article, we will discuss recent advances in fluorescent imaging of metal ions by small-molecule sensors for understanding the role of metals in related diseases. We will also discuss challenges and opportunities for the design of small-molecule sensors for fluorescent detection of cellular metal ions as a potential method for disease diagnosis.

Sensing of intracellular environments by fluorescence lifetime imaging of exogenous fluorophores

Fluorescence lifetime imaging (FLIM) has been recognized as a powerful microscopy technique to examine environments in living systems. The fluorescence lifetime does not depend on the photobleaching and optical conditions, which allows us to obtain quantitative information on intracellular environments by analyzing the fluorescence lifetime. A variety of exogenous fluorophores have been applied in FLIM measurements to examine cellular processes. Information on the correlation between the fluorescence lifetime and the physiological parameters is essential to elucidate the cellular environments from the fluorescence lifetime measurements of exogenous fluorophores. In this review, exogenous fluorophores used for lifetime-based sensing are summarized, with the expectation that it becomes a basis for selecting the fluorophore used to investigate the intracellular environment with FLIM. Experimental results of the intracellular sensing of pH, metal ions, oxygen, viscosity, and other physiological parameters on the basis of the FLIM measurements are described along with a brief explanation of the mechanism of the change in the fluorescence lifetime.

Visualising coordination chemistry: fluorescence X-ray absorption near edge structure tomography

Here we develop a measurement scheme to determine the abundance, distribution, and coordination environment of biological copper complexes in situ, without need for complex sample preparation.

Analyzing free zinc(II) ion concentrations in cell biology with fluorescent chelating molecules

Essential metal ions are tightly controlled in biological systems. An understanding of metal metabolism and homeostasis is being developed from quantitative information of the sizes, concentrations, and dynamics of cellular and subcellular metal ion pools. In the case of human zinc metabolism, minimally 24 proteins of two zinc transporter families and a dozen metallothioneins participate in cellular uptake, extrusion, and re-distribution among cellular compartments. Significantly, zinc(ii) ions are now considered signaling ions in intra- and intercellular communication. Such functions require transients of free zinc ions. It is experimentally quite challenging to distinguish zinc that is protein-bound from zinc that is not bound to proteins. Measurement of total zinc is relatively straightforward with analytical techniques such as atomic absorption/emission spectroscopy or inductively coupled plasma mass spectrometry. Total zinc concentrations of human cells are 200-300 μM. In contrast, the pool of non-protein bound zinc is mostly examined with fluorescence microscopy/spectroscopy. There are two widely applied fluorescence approaches, one employing low molecular weight chelating agents ("probes") and the other metal-binding proteins ("sensors"). The protein sensors, such as the CALWY, Zap/ZifCY, and carbonic anhydrase-based sensors, can be genetically encoded and have certain advantages in terms of controlling intracellular concentration, localization, and calibration. When employed correctly, both probes and sensors can establish qualitative differences in free zinc ion concentrations. However, when quantitative information is sought, the assumptions underlying the applications of probes and sensors must be carefully examined and even then measured pools of free zinc ions remain methodologically defined. A consensus is building that the steady-state free zinc ion concentrations in the cytosol are in the picomolar range but there is no consensus on their concentrations in subcellular compartments. Applying the extensive toolbox of available probes/sensors in biological systems requires an understanding of the principles of cellular zinc homeostasis and the chemical biology of the probes and sensors. Regardless of limitations in specificity (for a particular metal ion), selectivity (for a particular metal pool), and sensitivity (detection limit), the technology is making remarkable contributions to imaging zinc with high spatiotemporal resolution in single cells and to defining the biochemical functions of zinc ions in cellular regulation.

The role of copper ions in pathophysiology and fluorescent sensors for the detection thereof

Copper ions are crucial to life, and some fundamental roles of copper in pathophysiology have been elucidated using fluorescent sensors.

A set of robust fluorescent peptide probes for quantification of Cu(ii) binding affinities in the micromolar to femtomolar range

A set of four fluorescent probes were developed and demonstrated to be capable of quantifying Cu(ii) binding affinities in the micromolar to femtomolar range and exploring Cu(ii) binding ligands in peptides and proteins.

Cucumber metal transport protein MTP8 confers increased tolerance to manganese when expressed in yeast and Arabidopsis thaliana

Cation diffusion facilitator (CDF) proteins are ubiquitous divalent cation transporters that have been proved to be essential for metal homeostasis and tolerance in Archaebacteria, Bacteria, and Eukaryota. In plants, CDFs are designated as metal tolerance proteins (MTPs). Due to the lack of genomic resources, studies on MTPs in other plants, including cultivated crops, are lacking. Here, the identification and organization of genes encoding members of the MTP family in cucumber are described. The first functional characterization of a cucumber gene encoding a member of the Mn-CDF subgroup of CDF proteins, designated as CsMTP8 based on the highest homology to plant MTP8, is also presented. The expression of CsMTP8 in Saccharomyces cerevisiae led to increased Mn accumulation in yeast cells and fully restored the growth of mutants hypersensitive to Mn in Mn excess. Similarly, the overexpression of CsMTP8 in Arabidopsis thaliana enhanced plant tolerance to high Mn in nutrition media as well as the accumulation of Mn in plant tissues. When fused to green fluorescent protein (GFP), CsMTP8 localized to the vacuolar membranes in yeast cells and to Arabidopsis protoplasts. In cucumber, CsMTP8 was expressed almost exclusively in roots, and the level of gene transcript was markedly up-regulated or reduced under elevated Mn or Mn deficiency, respectively. Taken together, the results suggest that CsMTP8 is an Mn transporter localized in the vacuolar membrane, which participates in the maintenance of Mn homeostasis in cucumber root cells.