A novel copper-chelating strategy for fluorescent proteins to image dynamic copper fluctuations on live cell surfaces

Surface display provides an efficient expression system for production of recombinant proteins and bacterial whole cell biosensor in E. coli

A novel bacterial display vector based on Escherichia coli has been engineered for recombinant protein production and purification. Accordingly, a construct harboring the enhanced green fluorescent protein (EGFP) and the ice nucleation protein (INP) was designed to produce EGFP via the surface display in E. coli cells. The fusion EGFP-expressed cells were then investigated using fluorescence measurement, SDS- and native-PAGE before and after TEV protease digestion. The displayed EGFP was obtained with a recovery of 57.7 % as a single band on SDS-PAGE. Next, the efficiency of the cell surface display for mutant EGFP (EGFP S202H/Q204H) was examined in sensing copper ions. Under optimal conditions, a satisfactorily linear range for copper ions concentrations up to 10 nM with a detection limit of 0.073 nM was obtained for cell-displayed mutant EGFP (mEGFP). In the presence of bacterial cell lysates and purified mEGFP, response to copper was linear in the 2-10 nM and 0.1-2 μM concentration range, respectively, with a 1.3 nM and 0.14 μM limit of detection. The sensitivity of bacterial cell lysates and surface-displayed mEGFP in the detection of copper ions is higher than the purified mEGFP.

Postassembly Modification of Peptides by Histidine-Directed β-C(sp3)-H Arylation of Alanine at the Internal Positions: Overcoming the Inhibitory Effect of Peptide Bonds

Peptide modification by C(sp3)-H functionalization of residues at the internal positions remains underdeveloped due to the inhibitory effect of backbone amides. In this study, using histidine (His) as an endogenous directing group, we developed a novel method for the β-C(sp3)-H functionalization of alanine (Ala) at diverse positions of peptides. Through this approach, a wide range of linear peptides were modified on the side-chain of Ala adjacent to His to afford the functionalized peptides in moderate to good yield and excellent position selectivity. Furthermore, conjugation of peptides with functional molecules such as glucuronide, oleanolic acid, dipeptide, and fluorophore derivatives was achieved.

Fluorescent Protein-Based Sensors for Detecting Essential Metal Ions across the Tree of Life

Genetically encoded fluorescent metal ion sensors are powerful tools for elucidating metal dynamics in living systems. Over the last 25 years since the first examples of genetically encoded fluorescent protein-based calcium indicators, this toolbox of probes has expanded to include other essential and non-essential metal ions. Collectively, these tools have illuminated fundamental aspects of metal homeostasis and trafficking that are crucial to fields ranging from neurobiology to human nutrition. Despite these advances, much of the application of metal ion sensors remains limited to mammalian cells and tissues and a limited number of essential metals. Applications beyond mammalian systems and in vivo applications in living organisms have primarily used genetically encoded calcium ion sensors. The aim of this Perspective is to provide, with the support of historical and recent literature, an updated and critical view of the design and use of fluorescent protein-based sensors for detecting essential metal ions in various organisms. We highlight the historical progress and achievements with calcium sensors and discuss more recent advances and opportunities for the detection of other essential metal ions. We also discuss outstanding challenges in the field and directions for future studies, including detecting a wider variety of metal ions, developing and implementing a broader spectral range of sensors for multiplexing experiments, and applying sensors to a wider range of single- and multi-species biological systems.

Genetically encoded protein sensors for metal ion detection in biological systems: a review and bibliometric analysis

Metal ions are indispensable elements in living organisms and are associated with regulating various biological processes. An imbalance in metal ion content can lead to disorders in normal physiological functions of the human body and cause various diseases. Genetically encoded fluorescent protein sensors have the advantages of low biotoxicity, high specificity, and a long imaging time in vivo and have become a powerful tool to visualize or quantify the concentration level of biomolecules in vivo and in vitro, temporal and spatial distribution, and life activity process. This review analyzes the development status and current research hotspots in the field of genetically encoded fluorescent protein sensors by bibliometric analysis. Based on the results of bibliometric analysis, the research progress of genetically encoded fluorescent protein sensors for metal ion detection is reviewed, and the construction strategies, physicochemical properties, and applications of such sensors in biological imaging are summarized.

A Portable, Encapsulated Microbial Whole-Cell Biosensing System for the Detection of Bioavailable Copper (II) in Soil

A portable, field deployable whole-cell biosensor was developed that can withstand the complex matrices of soil and requires minimal to no sample preparation to monitor bioavailable concentrations of the essential micronutrient copper (II). Conventional measurement of micronutrients is often complex, laboratory-based, and not suitable for monitoring their bioavailable concentration. To address this need, we developed a fluorescence based microbial whole-cell biosensing (MWCB) system encoding for a Cu2+-responsive protein capable of generating a signal upon binding to Cu2+. The sensing-reporting protein was designed by performing circular permutation on the green fluorescent protein (GFP) followed by insertion of a Cu2+ binding motif into the structure of GFP. The design included insertion of several binding motifs and creating plasmids that encoded the corresponding sensing proteins. The signal generated by the sensing-reporting protein is directly proportional to the concentration of Cu2+ in the sample. Evaluation of the resulting biosensing systems carrying these plasmids was performed prior to selection of the optimal fluorescence emitting Cu2+-binding protein. The resulting optimized biosensing system was encapsulated in polyacrylate-alginate beads and embedded in soil for detection of the analyte. Once exposed to the soil, the beads were interrogated to measure the fluorescence signal emitted by the sensing-reporting protein using a portable imaging device. The biosensor was optimized for detection of Cu2+ in terms of selectivity, sensitivity, matrix effects, detection limits, and reproducibility in both liquid and soil matrices. The limit of detection (LoD) of the optimized encapsulated biosensor was calculated as 0.27 mg/L and 1.26 mg/kg of Cu2+ for Cu2+ in solution and soil, respectively. Validation of the portable imaging tools as a potential biosensing device in the field was performed.

Designed Metal-ATCUN Derivatives: Redox- and Non-redox-Based Applications Relevant for Chemistry, Biology, and Medicine

The designed "ATCUN" motif (amino-terminal copper and nickel binding site) is a replica of naturally occurring ATCUN site found in many proteins/peptides, and an attractive platform for multiple applications, which include nucleases, proteases, spectroscopic probes, imaging, and small molecule activation. ATCUN motifs are engineered at periphery by conjugation to recombinant proteins, peptides, fluorophores, or recognition domains through chemically or genetically, fulfilling the needs of various biological relevance and a wide range of practical usages. This chemistry has witnessed significant growth over the last few decades and several interesting ATCUN derivatives have been described. The redox role of the ATCUN moieties is also an important aspect to be considered. The redox potential of designed M-ATCUN derivatives is modulated by judicious choice of amino acid (including stereochemistry, charge, and position) that ultimately leads to the catalytic efficiency. In this context, a wide range of M-ATCUN derivatives have been designed purposefully for various redox- and non-redox-based applications, including spectroscopic probes, target-based catalytic metallodrugs, inhibition of amyloid-β toxicity, and telomere shortening, enzyme inactivation, biomolecules stitching or modification, next-generation antibiotic, and small molecule activation.

Copper-mediated peptide arylation selective for the N-terminus

Polypeptides present remarkable selectivity challenges for chemical methods. Amino groups are ubiquitous in polypeptide structure, yet few paradigms exist for reactivity and selectivity in arylation of amine groups. This communication describes the utilization of boronic acid reagents bearing certain o-electron withdrawing groups for copper-mediated amine arylation of the N-terminus under mild conditions and primarily aqueous solvent. The method adds to the toolkit of boronic acid reagents for polypeptide modification under mild conditions in water that shows complete selectivity for the N-terminus in the presence of lysine side chains.

The effect of an engineered ATCUN motif on the structure and biophysical properties of the SH3 domain of c-Src tyrosine kinase

Metal binding to sites engineered in proteins can provide an increase in their stability and facilitate new functions. Besides the sites introduced in purpose, sometimes they are present accidentally as a consequence of the expression system used to produce the protein. This happens with the copper- and nickel-binding (ATCUN) motif generated by the amino-terminal residues Gly-Ser-His. This ATCUN motif is fortuitously present in many proteins, but how it affects the structural and biophysical characterization of the proteins has not been studied. In this work, we have compared the structure and biophysical properties of a small modular domain, the SH3 domain of the c-Src tyrosine kinase, cloned with and without an ATCUN motif at the N terminus. At pH 7.0, the SH3 domain with the ATCUN motif binds nickel with a binding constant K_a = 28.0 ± 3.0 mM^-1. The formation of the nickel complex increases the thermal and chemical stability of the SH3 domain. A comparison of the crystal structures of the SH3 domain with and without the ATCUN motif shows that the binding of nickel does not affect the overall structure of the SH3 domain. In all crystal structures analyzed, residues Gly-Ser-His in complex with Ni²⁺ show a square planar geometry. The CD visible spectrum of the nickel complex shows that this geometry is also present in the solution. Therefore, our results not only show that the ATCUN motif might influence the biophysical properties of the protein, but also points to an advantageous stabilization of the protein with potential biotechnological applications.

Circularly Permuted Fluorescent Protein-Based Indicators: History, Principles, and Classification

Genetically encoded biosensors based on fluorescent proteins (FPs) are a reliable tool for studying the various biological processes in living systems. The circular permutation of single FPs led to the development of an extensive class of biosensors that allow the monitoring of many intracellular events. In circularly permuted FPs (cpFPs), the original N- and C-termini are fused using a peptide linker, while new termini are formed near the chromophore. Such a structure imparts greater mobility to the FP than that of the native variant, allowing greater lability of the spectral characteristics. One of the common principles of creating genetically encoded biosensors is based on the integration of a cpFP into a flexible region of a sensory domain or between two interacting domains, which are selected according to certain characteristics. Conformational rearrangements of the sensory domain associated with ligand interaction or changes in the cellular parameter are transferred to the cpFP, changing the chromophore environment. In this review, we highlight the basic principles of such sensors, the history of their creation, and a complete classification of the available biosensors.

Click-Type Protein-DNA Conjugation for Mn2+ Imaging in Living Cells

A click-type protein-DNA conjugation, named as MnDDC (Mn²⁺-activated DCV-DNA conjunction), is presented, where DCV (rep protein of duck circovirus) and its target DNA work as the modular blocks to rapidly and effectively generate Mn²⁺-dependent and site-specific protein-DNA linkage. On the basis of MnDCC, a fluorescent Mn²⁺ biosensor composed of DCV and a molecular beacon, was developed for rapid sensing of Mn²⁺ within 2 min with nanomolar sensitivity. Using the proposed biosensor, not only analysis of Mn²⁺ in real samples (e.g., serum and food), but also wash-free fluorescent imaging of Mn²⁺ in extracellular environment and cytoplasm have been achieved. Moreover, the MnDDC-based sensor was proved to be a powerful tool for visualization of Mn²⁺ during exploration of the associated cytotoxicity in living neural cells, which is helpful to reveal the cellular responses toward the disordered homeostasis of Mn²⁺ in both extracellular and intracellular microenvironments.

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.

Ultrafast and Noninvasive Long-Term Bioimaging with Highly Stable Red Aggregation-Induced Emission Nanoparticles

Strongly red luminescent and water-soluble probes are very important for studying biological events and processes. Fluorescent nanoparticles (NPs) built from the aggregation-induced emission luminogen (AIEgen) and amphipathic polymeric matrixes have been considered as promising candidates for bioimaging. However, AIE NPs with long-wavelength absorption suitable for in vivo application are still scarce. In this work, three AIE-active red-emissive BODIPY derivatives with long-wavelength absorption were rationally designed and synthesized. Then three NPs based on these AIEgens exhibit bright red photoluminescence with high fluorescence quantum yield in aqueous media. These NPs uniformly dispersed in water and showed excellent stability and good biocompatibility. They can be readily internalized by HeLa cells, and the staining process is performed by simply shaking the culture with cells for just a few seconds at room temperature, which indicates an ultrafast and easy-to-operate staining protocol. More importantly, long-term tracing in living cells and mouse over 15 days is successfully achieved. The strong fluorescence signals, ultrafast staining procedure, and long-term tracing abilities indicate that these AIE NPs hold great potential for monitoring biological processes.

Tuning the Color Palette of Fluorescent Copper Sensors through Systematic Heteroatom Substitution at Rhodol Cores

Copper is an essential nutrient for sustaining life, and emerging data have expanded the roles of this metal in biology from its canonical functions as a static enzyme cofactor to dynamic functions as a transition metal signal. At the same time, loosely bound, labile copper pools can trigger oxidative stress and damaging events that are detrimental if misregulated. The signal/stress dichotomy of copper motivates the development of new chemical tools to study its spatial and temporal distributions in native biological contexts such as living cells. Here, we report a family of fluorescent copper sensors built upon carbon-, silicon-, and phosphorus-substituted rhodol dyes that enable systematic tuning of excitation/emission colors from orange to near-infrared. These probes can detect changes in labile copper levels in living cells upon copper supplementation and/or depletion. We demonstrate the ability of the carbon-rhodol based congener, Copper Carbo Fluor 1 (CCF1), to identify elevations in labile copper pools in the Atp7a-/- fibroblast cell model of the genetic copper disorder Menkes disease. Moreover, we showcase the utility of the red-emitting phosphorus-rhodol based dye Copper Phosphorus Fluor 1 (CPF1) in dual-color, dual-analyte imaging experiments with the green-emitting calcium indicator Calcium Green-1 to enable simultaneous detection of fluctuations in copper and calcium pools in living cells. The results provide a starting point for advancing tools to study the contributions of copper to health and disease and for exploiting the rapidly growing palette of heteroatom-substituted xanthene dyes to rationally tune the optical properties of fluorescent indicators for other biologically important analytes.

N-Terminal Cu-Binding Motifs (Xxx-Zzz-His, Xxx-His) and Their Derivatives: Chemistry, Biology and Medicinal Applications

Peptides and proteins with N-terminal amino acid sequences NH2 -Xxx-His (XH) and NH2 -Xxx-Zzz-His (XZH) form well-established high-affinity CuII -complexes. Key examples are Asp-Ala-His (in serum albumin) and Gly-His-Lys, the wound healing factor. This opens a straightforward way to add a high-affinity CuII -binding site to almost any peptide or protein, by chemical or recombinant approaches. Thus, these motifs, NH2 -Xxx-Zzz-His in particular, have been used to equip peptides and proteins with a multitude of functions based on the redox activity of Cu, including nuclease, protease, glycosidase, or oxygen activation properties, useful in anticancer or antimicrobial drugs. More recent research suggests novel biological functions, mainly based on the redox inertness of CuII in XZH, like PET imaging (with 64 Cu), chelation therapies (for instance in Alzheimer's disease and other types of neurodegeneration), antioxidant units, Cu transporters and activation of biological functions by strong CuII binding. This Review gives an overview of the chemical properties of Cu-XH and -XZH motifs and discusses the pros and cons of the vastly different biological applications, and how they could be improved depending on the application.

Exploration research on synthesis and application of a new dye containing di-2-picolyamine

A newly designed fluorescence dye L based on di-2-picolyamine (DPA) moiety as a chelator was obtained under the protection of N₂ at 120°C, and KI as catalyst with relatively better yield. More interestingly, L not only could selectively and sensitively detect Cu²⁺ ions in aqueous medium but also examine the Cu²⁺ ions of the actual water samples. Nevertheless, L could be visual in Hela cells with excellent cell permeability, viz, monitoring exogenous Cu²⁺ ions as well as realizing an "on-off-on" process.

Tailoring the morphology of AIEgen fluorescent nanoparticles for optimal cellular uptake and imaging efficacy

The rational design of robust fluorescent organic materials for long-term cell tracing is still challenging, and aggregation-caused quenching of emission is a big limitation of this strategy. Organic dyes with aggregation-induced emission (AIE) can effectively address this problem. Herein, AIEgen-containing nanoparticles, with different morphologies and emission, were prepared by assembling amphiphilic copolymers with an AIEgen. We compared the physical and chemical properties of rod-like and spherical nanoparticles, particularly investigating the effects of the shape on internalization and the imaging effect. The formulated nanoparticles exhibit advantageous features, such as a large Stokes shift, robust stability in physiological conditions, strong fluorescent emission, and photobleaching resistance. Interestingly, the rod-like nanoparticles were internalized more efficiently than their spherical counterparts, and their strong green fluorescence can still be clearly observed even after 15 days in vitro and in vivo. This work demonstrates the great potential of regulating the morphology of nanoparticles to obtain an ideal biological function.

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.

Designing metallodrugs with nuclease and protease activity

The accidental discovery of cisplatin some 50 years ago generated renewed interest in metallopharmaceuticals. Beyond cisplatin, many useful metallodrugs have been synthesized for the diagnosis and treatment of various diseases, but toxicity concerns, and the propensity to induce chemoresistance and secondary cancers make it imperative to search for novel metallodrugs that address these limitations. The Amino Terminal Cu(ii) and Ni(ii) (ATCUN) binding motif has emerged as a suitable template to design catalytic metallodrugs with nuclease and protease activities. Unlike their classical counterparts, ATCUN-based metallodrugs exhibit low toxicity, employ novel mechanisms to irreversibly inactivate disease-associated genes or proteins providing in principle, a channel to circumvent the rapid emergence of chemoresistance. The ATCUN motif thus presents novel strategies for the treatment of many diseases including cancers, HIV and infections caused by drug-resistant bacteria at the genetic level. This review discusses their design, mechanisms of action and potential for further development to expand their scope of application.

Engineering an FMN-based iLOV protein for the detection of arsenic ions

Over the past few decades, genetically encoded fluorescent proteins have been widely used as efficient probes to explore and investigate the roles of metal ions in biological processes. The discovery of small FMN-based fluorescent proteins, such as iLOV and FbFP, has enabled researchers to exploit these fluorescent reporter proteins for metal-sensing applications. In this study, we report the inherent binding properties of iLOV towards arsenic ions. The fluorescence quenching of iLOV was linearly related to the concentration of arsenic ions, and engineered proteins showed better sensitivity than the wild-type protein. Engineering key residues around the chromophore converted the iLOV protein into a highly sensitive sensor for As³⁺ ions. iLOV_N468S exhibited an improved binding affinity with a dissociation constant of 1.5 μM. Furthermore, the circular dichroism spectra indicated that the fluorescence quenching mechanism might be related to arsenic-protein complex formation. Thus, the reagentless sensing of arsenic can potentially be exploited to determine intracellular or environmental arsenic using a genetically encoded biosensing approach.

A reaction-based fluorescent turn-on probe for Cu2+ in complete aqueous solution

FP can detect Cu²⁺ in complete aqueous solution with a rapid response time, high sensitivity, and high selectivity.