Supramolecular metal displacement allows on-fluorescence analysis of manganese(II) in living cells

Small-Molecule Fluorescent Probes for Binding- and Activity-Based Sensing of Redox-Active Biological Metals

Although transition metals constitute less than 0.1% of the total mass within a human body, they have a substantial impact on fundamental biological processes across all kingdoms of life. Indeed, these nutrients play crucial roles in the physiological functions of enzymes, with the redox properties of many of these metals being essential to their activity. At the same time, imbalances in transition metal pools can be detrimental to health. Modern analytical techniques are helping to illuminate the workings of metal homeostasis at a molecular and atomic level, their spatial localization in real time, and the implications of metal dysregulation in disease pathogenesis. Fluorescence microscopy has proven to be one of the most promising non-invasive methods for studying metal pools in biological samples. The accuracy and sensitivity of bioimaging experiments are predominantly determined by the fluorescent metal-responsive sensor, highlighting the importance of rational probe design for such measurements. This review covers activity- and binding-based fluorescent metal sensors that have been applied to cellular studies. We focus on the essential redox-active metals: iron, copper, manganese, cobalt, chromium, and nickel. We aim to encourage further targeted efforts in developing innovative approaches to understanding the biological chemistry of redox-active metals.

Improving the Sensitivity of a Mn(II)-Specific DNAzyme for Cellular Imaging Sensor through Sequence Mutations

Detection of Mn2+ in living cells is important in understanding the roles of Mn2+ in cellular processes and investigating its potential implications in various diseases and disorders. Toward this goal, we have previously selected a Mn2+-specific 11-5 DNAzyme through an in vitro selection method and converted it into a fluorescence sensor for intracellular Mn2+ sensing. Despite the progress, the nucleotides responsible for the activity are unclear, and the performance of the DNAzyme needs to be improved in order for more effective applications in biological systems. To address these issues, we herein report site-specific mutations within the catalytic domain of the selected 11-5 DNAzyme. As a result, we successfully identified a variant DNAzyme, designated as Mn5V, which exhibited a twofold increase in activity compared to the original 11-5 DNAzyme. Importantly, Mn5V DNAzyme maintained its high selectivity for Mn2+ over other competing metal ions. Upon the addition of Mn2+, Mn5V DNAzyme exhibited a higher fluorescence signal within the tumor cells compared to that of the 11-5 DNAzyme. This study therefore provides a better understanding of how the DNAzyme functions and a more sensitive probe for investigating Mn2+ in biological systems.

A genetically encoded fluorescent sensor for manganese(II), engineered from lanmodulin

The design of selective metal-binding sites is a challenge in both small-molecule and macromolecular chemistry. Selective recognition of manganese (II)-the first-row transition metal ion that tends to bind with the lowest affinity to ligands, as described by the Irving-Williams series-is particularly difficult. As a result, there is a dearth of chemical biology tools with which to study manganese physiology in live cells, which would advance understanding of photosynthesis, host-pathogen interactions, and neurobiology. Here we report the rational re-engineering of the lanthanide-binding protein, lanmodulin, into genetically encoded fluorescent sensors for MnII, MnLaMP1 and MnLaMP2. These sensors with effective Kd(MnII) of 29 and 7 µM, respectively, defy the Irving-Williams series to selectively detect MnII in vitro and in vivo. We apply both sensors to visualize kinetics of bacterial labile manganese pools. Biophysical studies indicate the importance of coordinated solvent and hydrophobic interactions in the sensors' selectivity. Our results establish lanmodulin as a versatile scaffold for design of selective protein-based biosensors and chelators for metals beyond the f-block.

Selective detection of manganese(II) ions based on the fluorescence turn-on response via histidine functionalized carbon quantum dots

Herein, water-soluble emissive carbon quantum dots (His-CQDs) were synthesized from pyrolysis of sodium citrate in the presence of histidine under hydrothermal conditions. The as-synthesized His-CQDs were characterized using Fourier transform infrared (FT-IR), fluorescence spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM) techniques. The obtained His-CQDs display a strong emission peak at 534 nm when excited at 476 nm with a high quantum yield (61.8 %). The as-synthesized His-CQDs were applied as a new platform for highly selective determination of Mn(II) based on the fluorescence "turn-on" response with a limit of detection of 1.85 µg L^-1 (at 3σ) and a linear range of 3.50-35.5 µg L^-1 in aqueous solution. The sensing mechanism of the His-CQDs probe for the detection of Mn(II) was studied via density functional theory (DFT), FT-IR, and EDTA complexation methodology. In addition, His-CQDs were successfully applied to determine the accurate amounts of Mn(II) in whole blood control material. More importantly, the integrating such an efficient sensor with point-of-care technology can enable portable, easy-to-use, and rapid sensing systems for better biological and clinical applications.

Zinc(ii), copper(ii) and cadmium(ii) complexes as fluorescent chemosensors for cations

The fluorescence chemosensing behavior of Zn(ii), Cu(ii), and Cd(ii) based complexes toward cations has been described. Cation detection via conventional mechanisms, metal-metal exchange and chemodosimetric approaches along with the importance of metal ions and the scope, significance, and challenges with regard to the detection of cations by metal complex based probes will be discussed in detail. The fundamentals of photophysical behavior and mechanisms involved in the fluorescence detection of analytes will also be described. This article provides a detailed overview of Zn(ii), Cu(ii), and Cd(ii) based complexes as fluorescent probes for cations, together with essential discussions pertaining to detection mechanisms.

Manganese Mapping Using a Fluorescent Mn2+ Sensor and Nanosynchrotron X-ray Fluorescence Reveals the Role of the Golgi Apparatus as a Manganese Storage Site

Elucidating dynamics in transition-metal distribution and localization under physiological and pathophysiological conditions is central to our understanding of metal-ion regulation. In this Forum Article, we focus on manganese and specifically recent developments that point to the relevance of the Golgi apparatus in manganese detoxification when this essential metal ion is overaccumulated because of either environmental exposure or mutations in manganese efflux transporters. In order to further evaluate the role of the Golgi apparatus as a manganese-ion storage compartment under subcytotoxic manganese levels, we use a combination of confocal microscopy using a sensitive "turn-on" fluorescent manganese sensor, M1, and nanosynchrotron X-ray fluorescence imaging to show that manganese ions are stored in the Golgi apparatus under micromolar manganese exposure concentrations. Our results, along with previous reports on manganese accumulation, now indicate a central role of the Golgi apparatus in manganese storage and trafficking under subcytotoxic manganese levels and hint toward a possible role of the Golgi apparatus in manganese storage even under physiological conditions.

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.

Emerging chemical tools and techniques for tracking biological manganese

This frontier article discusses chemical tools and techniques for tracking and imaging Mn ions in biology.

Potentially toxic elements and environmentally-related pollutants recognition using colorimetric and ratiometric fluorescent probes

A safer detection or sensing of toxic pollutants is one among several environmental contamination issues, across the globe. The ever-increasing industrial practices and controlled or uncontrolled release of toxic pollutants from various industrial sectors is a key source of this environmental problem. Significant research efforts have been or being made to tackle this problematic issue to fulfill the growing needs of the modern world. Despite many useful aspects, heavy metals are posing noteworthy toxicological concerns and human-health related issues at various levels of the ecosystem. In this context, notable efforts from various regulatory authorities, the increase in the concentration of these toxic heavy metals in the environment is of serious concern, so real-time monitoring is urgently required. Herein, we reviewed fluorescent sensor based models and their potentialities to address the detection fate of hazardous pollutants including chromium, manganese, cobalt, nickel, copper, and zinc as model elements. The novel aspects of turn-on/off fluorescent sensors have also been discussed from a state of the art viewpoint. In summary, comprehensive literature regarding fluorescent sensor based models and their potentialities to detect various types of toxic pollutants is reviewed.

A conjugated BODIPY–triphenylamine multi-aldoxime: Sonogashira coupling, ratiometric chemodosimeter and rapid detection of hypochlorite with two-photon excited fluorescence

The chemodosimeter is an off–on TPEF probe for detecting NaOCl selectively with red emission.

Distinction between Mn(iii) and Mn(ii) by using a colorimetric chemosensor in aqueous solution

A new colorimetric chemosensor for Mn(iii) and Mn(ii) was developed and used to differentiate between Mn(iii) and Mn(ii).

Tuning macrocycles to design ‘turn-on’ fluorescence probes for manganese(ii) sensing in live cells

We report novel ‘turn-on’ fluorescence probes for imaging Mn²⁺ in live cells.

Fluorescent metal ion chemosensors via cation exchange reactions of complexes, quantum dots, and metal–organic frameworks

Overview of a new paradigm in the design of fluorescent chemosensors for detecting metal ions via cation exchange reactions of complexes, quantum dots, and metal–organic frameworks.

Engineering an effective Mn-binding MRI reporter protein by subcellular targeting

PURPOSE

Manganese (Mn) is an effective contrast agent and biologically active metal, which has been widely used for Mn-enhanced MRI (MEMRI). The purpose of this study was to develop and test a Mn binding protein for use as a genetic reporter for MEMRI.

METHODS

The bacterial Mn-binding protein, MntR was identified as a candidate reporter protein. MntR was engineered for expression in mammalian cells, and targeted to different subcellular organelles, including the Golgi Apparatus where cellular Mn is enriched. Transfected HEK293 cells and B16 melanoma cells were tested in vitro and in vivo, using immunocytochemistry, MR imaging and relaxometry.

RESULTS

Subcellular targeting of MntR to the cytosol, endoplasmic reticulum and Golgi apparatus was verified with immunocytochemistry. After targeting to the Golgi, MntR expression produced robust R1 changes and T1 contrast in cells, in vitro and in vivo. Co-expression with the divalent metal transporter DMT1, a previously described Mn-based reporter, further enhanced contrast in B16 cells in culture, but in the in vivo B16 tumor model tested was not significantly better than MntR alone.

CONCLUSION

This second-generation reporter system both expands the capabilities of genetically encoded reporters for imaging with MEMRI and provides important insights into the mechanisms of Mn biology which create endogenous MEMRI contrast.

Fluorescent, MRI, and colorimetric chemical sensors for the first-row d-block metal ions

Transition metals (d-blocks) are recognized as playing critical roles in biology, and they most often act as cofactors in diverse enzymes; however, improper regulation of transition metal stores is also connected to serious disorders. Therefore, the monitoring and imaging of transition metals are significant for biological research as well as clinical diagnosis. In this article, efforts have been made to review the chemical sensors that have been developed for the detection of the first-row d-block metals (except Cu and Zn): Cr, Mn, Fe, Co, and Ni. We focus on the development of fluorescent sensors (fall into three classes: "turn-off", "turn-on", and ratiometric), colorimetric sensors, and responsive MRI contrast agents for these transition metals (242 references). Future work will be likely to fill in the blanks: (1) sensors for Sc, Ti, and V; (2) MRI sensors for Cr, Mn, Co, Ni; (3) ratiometric fluorescent sensors for Cr(6+), Mn(2+), and Ni(2+), explore new ways of sensing Fe(3+) or Cr(3+) without the proton interference, as well as extend applications of MRI sensors to living systems.

Selectively sensing first-row transition metal ions through fluorescence enhancement

Fluorescence signaling systems that give enhancement in the presence of first-row transition metal ions are discussed.