A mitochondria-targeted ratiometric two-photon fluorescent probe for biological zinc ions detection

Cell metabolism: Functional and phenotypic single cell approaches

Several metabolic pathways are essential for the physiological regulation of immune cells, but their dysregulation can cause immune dysfunction. Hypermetabolic and hypometabolic states represent deviations in the magnitude and flexibility of effector cells in different contexts, for example in autoimmunity, infections or cancer. To study immunometabolism, most methods focus on bulk populations and rely on in vitro activation assays. Nowadays, thanks to the development of single-cell technologies, including multiparameter flow cytometry, mass cytometry, RNA cytometry, among others, the metabolic state of individual immune cells can be measured in a variety of samples obtained in basic, translational and clinical studies. Here, we provide an overview of different single-cell approaches that are employed to investigate both mitochondrial functions and cell dependence from mitochondria metabolism. Moreover, besides the description of the appropriate experimental settings, we discuss the strengths and weaknesses of different approaches with the aim to suggest how to study cell metabolism in the settings of interest.

Thermogenic adipocyte-derived zinc promotes sympathetic innervation in male mice

Sympathetic neurons activate thermogenic adipocytes through release of catecholamine; however, the regulation of sympathetic innervation by thermogenic adipocytes is unclear. Here, we identify primary zinc ion (Zn) as a thermogenic adipocyte-secreted factor that promotes sympathetic innervation and thermogenesis in brown adipose tissue and subcutaneous white adipose tissue in male mice. Depleting thermogenic adipocytes or antagonizing β3-adrenergic receptor on adipocytes impairs sympathetic innervation. In obesity, inflammation-induced upregulation of Zn chaperone protein metallothionein-2 decreases Zn secretion from thermogenic adipocytes and leads to decreased energy expenditure. Furthermore, Zn supplementation ameliorates obesity by promoting sympathetic neuron-induced thermogenesis, while sympathetic denervation abrogates this antiobesity effect. Thus, we have identified a positive feedback mechanism for the reciprocal regulation of thermogenic adipocytes and sympathetic neurons. This mechanism is important for adaptive thermogenesis and could serve as a potential target for the treatment of obesity.

A sensitive zinc probe operating via enhancement of excited-state intramolecular charge transfer

Novel highly sensitive fluorescent probes for zinc cations based on the diketopyrrolopyrrole scaffold were designed and synthesized. Large bathochromic shifts (≈80 nm) of fluorescence are observed when the Zn²⁺-recognition unit (di-(2-picolyl)amine) is bridged with the fluorophore possessing an additional pyridine unit able to participate in the coordination process. This effect originates from the dipolar architecture and the increasing electron-withdrawing properties of the diketopyrrolopyrrole core upon addition of the cation. The new, greenish-yellow emitting probes, which operate via modulation of intramolecular charge transfer, are very sensitive to the presence of Zn²⁺. Introduction of a morpholine unit in the diketopyrrolopyrrole structure induces a selective six-fold increase of the emission intensity upon zinc coordination. Importantly, the presence of other divalent biologically relevant metal cations has negligible effects and typically even at a 100-fold higher concentration of Mg²⁺/Zn²⁺, the effect is comparable. Computational studies rationalize the strong bathochromic shift upon Zn²⁺-complexation. Decorating the probes with the triphenylphosphonium cation and morpholine unit enables selective localization in the mitochondria and the lysosome of cardiac H9C2 cells, respectively.

Fluorescent Probes Design Strategies for Imaging Mitochondria and Lysosomes

Modern cellular biology faces several major obstacles, such as the determination of the concentration of active sites corresponding to chemical substances. In recent years, the popular small-molecule fluorescent probes have completely changed the understanding of cellular biology through their high sensitivity toward specific substances in various organisms. Mitochondria and lysosomes are significant organelles in various organisms, and their interaction is closely related to the development of various diseases. The investigation of their structure and function has gathered tremendous attention from biologists. The advanced nanoscopic technologies have replaced the diffraction-limited conventional imaging techniques and have been developed to explore the unknown aspects of mitochondria and lysosomes with a sub-diffraction resolution. Recent progress in this field has yielded several excellent mitochondria- and lysosome-targeted fluorescent probes, some of which have demonstrated significant biological applications. Herein, we review studies that have been carried out to date and suggest future research directions that will harness the considerable potential of mitochondria- and lysosome-targeted fluorescent probes.

Nitrogen and sulfur co-doped carbon dot-based ratiometric fluorescent probe for Zn2+ sensing and imaging in living cells

A near-infrared nitrogen and sulfur co-doped carbon dot (N,S-CD)-based ratiometric fluorescent probe is proposed that is synthesized via hydrothermal approach using glutathione and formamide as precursor for sensing and imaging of Zn²⁺. The prepared N,S-CDs facilitate binding with Zn²⁺ owing to N and S atom doping. The ratio (I₆₅₀/I₆₈₀) of fluorescence intensity at 650 nm and 680 nm increased with the concentrations of Zn²⁺ when the excitation wavelength was 415 nm. The linearity range was 0.01 to 1.0 μM Zn²⁺with a detection limit of 5.0 nM Zn²⁺. The proposed probe was applied to label-free monitoring of Zn²⁺ in real samples and fluorescent imaging of Zn²⁺ in living cells, which confirmed its promising applications.

A biocompatible two-photon absorbing fluorescent mitochondrial probe for deep in vivo bioimaging

We reported a mitochondria-targeted two-photon fluorescent dye with an excellent two-photon absorption cross-section. With this dye, we reached an imaging depth of ca. 640 μm during mitochondrial imaging of cortical cells in live animals.

Design, synthesis and imaging of a novel mitochondrial fluorescent nanoprobe based on distyreneanthracene-substituted triphenylphosphonium salt

Targeting and monitoring the dynamics of mitochondria are of great significance because mitochondria are involved in a variety of physiological and pathological processes. For achieving this purpose, highly sensitive, photostable, tolerance and specific fluorescent probe is necessary. To obtain a superior mitochondrial fluorescent probe, (4-distyreneanthracenoxybutyl) bis(triphenylphosphonium) bromide (DSA-TPP) with aggregation-induced emission (AIE) characteristic was designed and synthesized for mitochondrial targeting. DSA-TPP dots with high fluorescence quantum yield (Φ = 17.9) and small particle size (8 nm) can be easily prepared by self-assembly formation. DSA-TPP dots had the ability of lightning mitochondria in living cells with high brightness, superior photostability and strong tolerance to cell environment change.

A red-emissive two-photon fluorescent probe for mitochondrial sodium ions in live tissue

A cyclocyanine (CC)-based organic small molecule two-photon (TP) fluorescent probe (CCNa1) was developed for mitochondrial sodium ion sensing. CCNa1 exhibits a low solvatochromic shift and strong TP fluorescence enhancement at 575 nm upon binding to Na⁺ and is insensitive to other metal ions and to pH. CCNa1 demonstrated fast cell loading ability, biocompatibility, and sensitive response to mitochondrial Na⁺ influx in live cells and mouse brain tissue.

Bicolour fluorescent molecular sensors for cations: design and experimental validation

Molecular entities whose fluorescence spectra are different when they bind metal cations are termed bicolour fluorescent molecular sensors. The basic design criteria of this kind of compound are presented and the different fluorescent responses are discussed in terms of their chemical behaviour and electronic features. These latter elements include intramolecular charge transfer (ICT), formation of intramolecular and intermolecular excimer/exciplex complexes and Förster resonance energy transfer (FRET). Changes in the electronic properties of the fluorophore based on the decoupling between its constitutive units upon metal binding are also discussed. The possibility of generating fluorescent bicolour indicators that can capture metal cations in the gas phase and at solid-gas interfaces is also discussed.

Precise imaging of mitochondria in cancer cells by real-time monitoring of nitroreductase activity with a targetable and activatable fluorescent probe

An activatable and mitochondrial-targetable fluorescent probe was developed. This designed probe showed ratiometric fluorescence and light-up near-infrared emission responsiveness to nitroreductase, achieving precise imaging of mitochondria in cancer cells by real-time monitoring of nitroreductase activity.

Anti-tumour activity of zinc ionophore pyrithione in human ovarian cancer cells through inhibition of proliferation and migration and promotion of lysosome-mitochondrial apoptosis

Zinc pyrithione (ZPT) is widely used as an antimicrobial. Zinc is a necessary trace element of the human whose homeostasis associated with several cancers. However, the anticancer effect of increased Zinc in ovarian cancer is still unclear. This study focussed on the anti-tumour effects of ZPT combined with Zinc in SKOV3 and SKOV3/DDP cells. The cell viability, apoptosis, migration, and invasion assays were detected by CCK-8, flow cytometry, wound healing and transwell assay, respectively. The distribution of Zinc in cells was monitored by staining of Zinc fluorescent dye and lysosome tracker. The changes in lysosomal membrane stability were reflected by acridine orange fluorescence and cathepsin D reposition. Expression of the proteins about invasion and apoptosis was evaluated by western blot. The results indicated that ZPT combined with Zinc could notably reduce cell viability, inhibit migration and invasion in SKOV3 and SKOV3/DDP cells. Besides, ZPT performed as a Zinc carrier targeted lysosomes, caused the increase of its membrane permeability and the release of cathepsin D accompanied by mitochondrial apoptosis in SKOV3/DDP cells. In conclusion, our work suggests that ZPT combined with Zinc could inhibit proliferation, migration, invasion, and promote apoptosis by trigger the lysosome-mitochondrial apoptosis pathway in ovarian carcinoma.

Mitochondrial Targeting of Probes and Therapeutics to the Powerhouse of the Cell

Mitochondria, colloquially known as "the powerhouse of the cell", play important roles in production, but also in processes critical for cellular fate such as cell death, differentiation, signaling, metabolic homeostasis, and innate immunity. Due to its many functions in the cell, the mitochondria have been linked to a variety of human illnesses such as diabetes, cancer, and neurodegenerative diseases. In order to further our understanding and pharmaceutical targeting of this critical organelle, effective strategies must be employed to breach the complex barriers and microenvironment of mitochondria. Here, we summarize advancements in mitochondria-targeted probes and therapeutics.

A TAT peptide-based ratiometric two-photon fluorescent probe for detecting biothiols and sequentially distinguishing GSH in mitochondria

Although biothiols, including cysteine (Cys), glutathione (GSH), and homocysteine (Hcy) can be used to diagnose many diseases and research physiological metabolism in many physiological processes, in situ real-time detection and differentiation of biothiols is still challenging because their similar chemical properties and molecular structures. Herein, we utilized the native chemical ligation (NCL) reaction mechanism to develop a Förster resonance energy transfer (FRET) strategy for designing a cell penetration peptide TAT-modified ratiometric two-photon biothiols probe (TAT-probe). The TAT-probe can not only rapidly enter into mitochondria assisted by TAT peptide, but also simultaneously detect biothiols and sequentially distinguish GSH. When the TAT-probe was excited with 404/820 nm wavelength light, it showed a change in the ratio of fluorescence after adding biothiols, including a quenched red fluorescence intensity (λ_em = 585 nm) and an enhanced signal in green fluorescence intensity (λ_em = 520 nm). Excitingly, the TAT-probe excited at 545 nm could display a red fluorescence (λ_em = 585 nm) towards GSH and a quenched signal towards Hcy or Cys. This specific fluorescence response indicated the TAT-probe could effectively detect biothiols and differentiate GSH from Cys/Hcy in mitochondria. This work pioneered a new approach to design and synthesize biothiol-probes based on peptides and NCL reaction mechanism.

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.

Tracking mitochondrial viscosity in living systems based on a two-photon and near red probe

A two-photon fluorescence probe, Mito-V, with near-red emission to measure the viscosity changes of mitochondria in living cells and in vivo.

A mitochondria-targeting fluorescent probe for the selective detection of glutathione in living cells

We report a fluorescent probe for the selective detection of mitochondrial glutathione (GSH). The probe, containing triphenylphosphine as a mitochondrial targeting group, exhibited ratiometric and selective detection of GSH over Cys/Hcy. The probe was used for imaging mitochondrial GSH in living HeLa cells.

A two-photon fluorescent probe for detecting lipid droplet viscosity in living cells and zebra fish

A fluorescent probe CBA with high photostability to measure the viscosity changes of lipid drops.

A highly sensitive fluorescent probe for bioimaging zinc ion in living cells and zebrafish models

A simple OFF–ON fluorescent probe was prepared and successfully applied for bioimaging Zn²⁺ in living systems.

A novel turn-on fluorescent probe for the multi-channel detection of Zn2+ and Bi3+ with different action mechanisms

A novel multifunctional sensor, RSPT, was identified and developed for multichannel turn-on fluorescent responses to Zn²⁺ and Bi³⁺ in practice.

A nanosensor made of sulfur–nitrogen co-doped carbon dots for “off–on” sensing of hypochlorous acid and Zn(ii) and its bioimaging properties

In this study, we present a facile one-step solvothermal strategy for the fabrication of sulfur–nitrogen co-doped carbon dots (SNCDs) using p-phenylenediamine and cysteamine hydrochloride as the precursors.

A diketopyrrolopyrrole-based fluorescent probe for investigating mitochondrial zinc ions

We report a diketopyrrolopyrrole-based fluorescent probe DPP-Mito, which is low toxicity and mitochondrial-specific localization and could monitor the changes in the intracellular Zn²⁺ concentrations.

Steric paper based ratio-type electrochemical biosensor with hollow-channel for sensitive detection of Zn2

The construction of flexible platform possessing the functions of immobilizing, separating, rinsing, and high-throughput analysis plays a significant role in biological and clinical research. Herein, hollow-channel technique was integrated with lab-on-paper for the simultaneous determination of two different concentrations of Zn²⁺ based on the origami principle, in which microfluidic channels were first patterned on a cellulose paper using commercial solid-state wax printer. Hollow-channels were created by laser cutting method as the role of both injecting ending and reaction tank. After screen printing three electrodes system, the resulting planar paper sheets were then folded into steric structures and functionalized by in-situ synthesized reduced graphene oxide. As a proof-of-concept, such lab-on-paper device was employed in the ratiometric electrochemical monitoring of zinc ion from the environment and HepG2 cells extract, by combining with co-catalysis of porous metal-organic frameworks and hemin/G-quadruplex toward H₂O₂ in the linear range of 0.1-7,000nmol/L. The results indicated that integrating hollow-channel with steric lab-on-paper offered a new methodological approach for the development of metal ions monitoring research. It is believed that it could be useful for various point-of-care related research fields, such as, on-site environmental monitoring, food safety, and disease diagnosis.

A two-photon fluorescent probe for viscosity imaging in vivo

A quinoline-based two-photon fluorescent probe (MCN) was developed for viscosity imaging in vivo.

Novel mono-cationic fluorescent probes based on different central π-conjugated bridges for two-photon bioimaging of cellular nuclei

A series of novel pyridine mono-cationic two-photon fluorescent probes based on different central π-conjugated bridges, fluorenone (W-pyI), dibenzothiophene (S-pyI), and dibenzofuran (F-pyI), were prepared and studied for improving photostability in bioimaging applications.