Fluorescent and Luminescent Probes for Monitoring Hydroxyl Radical under Biological Conditions

Rational design of a ratiometric fluorescent nanoprobe for real-time imaging of hydroxyl radical and its therapeutic evaluation of diabetes

Hydroxyl radical (•OH), one of the most reactive and deleterious substances in organisms, belongs to a class of reactive oxygen species (ROS), and it has been verified to play an essential role in numerous pathophysiological scenarios. However, due to its extremely high reactivity and short lifetime, the development of a reliable and robust method for tracking endogenous •OH remains an ongoing challenge. In this work, we presented the first ratiometric fluorescent nanoprobe NanoDCQ-3 for •OH sensing based on oxidative C-H abstraction of dihydroquinoline to quinoline. The study mainly focused on how to modulate the electronic effects to achieve an ideal ratiometric detection of •OH, as well as solving the inherent problem of hydrophilicity of the probe, so that it was more conducive to monitoring •OH in living organisms. The screened-out probe NanoDCQ-3 exhibited an exceptional ratiometric sensing capability, better biocompatibility, good cellular uptake, and appropriate in vivo retention, which has been reliably used for detecting exogenous •OH concentration fluctuation in living cells and zebrafish models. More importantly, NanoDCQ-3 facilitated visualization of •OH and evaluation of drug treatment efficacy in diabetic mice. These findings afforded a promising strategy for designing ratiometric fluorescent probes for •OH. NanoDCQ-3 emerged as a valuable tool for the detection of •OH in vivo and held potential for drug screening for inflammation-related diseases.

Chemical Cyclic Amplification: Hydroxylamine Boosts the Fenton Reaction for Versatile and Scalable Biosensing

Nucleic acid detection is undoubtedly one of the most important research fields to meet the medical needs of genetic disease diagnosis, cancer treatment, and infectious disease prevention. However, the practical detection methods based on biological amplification are complex and time-consuming and require highly trained operators. Herein, we report a simple, rapid, and sensitive method for the nucleic acid assay by fluorescence or naked eye using chemical cyclic amplification. The addition of hydroxylamine (HA) during the Fenton reaction can continuously generate hydroxyl radicals (^•OH) via Fe³⁺/Fe²⁺ cycle, termed as "hydroxylamine boosts the Fenton reaction (Fenton-HA system)". Meanwhile, the reducing substances, such as terephthalic acid or o-phenylenediamine, react with ^•OH to generate oxidized substances that can be recognized by the naked eye or detected by fluorescence so as to realize the detection of Fe³⁺. The concentration of Fe³⁺ has a good linear relationship with fluorescence intensity in the range of 0.1 to 100 nM, and the limit of detection is calculated to be 0.03 nM (S/N = 3). Subsequently, Fe was introduced into the nucleic acid hybridization system after the Fe source was transformed into Fe³⁺, and the nucleic acids were indirectly determined by this method. This Fenton-HA system was used for sensing HIV-DNA and miRNA-21 to verify the validity of this method in nucleic acid detection. The detection limits were as low as 2.5 pM for HIV-DNA and 3 pM for miRNA-21. We believe that our work has unlocked an efficient signal amplification strategy, which is expected to develop a new generation of highly sensitive chemical biosensors.

Hydroxyl Radical Overproduction in the Envelope: an Achilles' Heel in Peptidoglycan Synthesis

While many mechanisms governing bacterial envelope homeostasis have been identified, others remain poorly understood. To decipher these processes, we previously developed an assay in the Gram-negative model Escherichia coli to identify genes involved in maintenance of envelope integrity. One such gene was ElyC, which was shown to be required for envelope integrity and peptidoglycan synthesis at room temperature. ElyC is predicted to be an integral inner membrane protein with a highly conserved domain of unknown function (DUF218). In this study, and stemming from a further characterization of the role of ElyC in maintaining cell envelope integrity, we serendipitously discovered an unappreciated form of oxidative stress in the bacterial envelope. We found that cells lacking ElyC overproduce hydroxyl radicals (HO^•) in their envelope compartment and that HO^• overproduction is directly or indirectly responsible for the peptidoglycan synthesis arrest, cell envelope integrity defects, and cell lysis of the ΔelyC mutant. Consistent with these observations, we show that the ΔelyC mutant defect is suppressed during anaerobiosis. HO^• is known to cause DNA damage but to our knowledge has not been shown to interfere with peptidoglycan synthesis. Thus, our work implicates oxidative stress as an important stressor in the bacterial cell envelope and opens the door to future studies deciphering the mechanisms that render peptidoglycan synthesis sensitive to oxidative stress. IMPORTANCE Oxidative stress is caused by the production and excessive accumulation of oxygen reactive species. In bacterial cells, oxidative stress mediated by hydroxyl radicals is typically associated with DNA damage in the cytoplasm. Here, we reveal the existence of a pathway for oxidative stress in the envelope of Gram-negative bacteria. Stemming from the characterization of a poorly characterized gene, we found that HO^• overproduction specifically in the envelope compartment causes inhibition of peptidoglycan synthesis and eventually bacterial cell lysis.

Imaging of Hydroxyl-Radical Generation Using Dynamic Nuclear Polarization-Magnetic Resonance Imaging and a Spin-Trapping Agent

Reactive oxygen species (ROS) play an important role in cell metabolism, but they can cause oxidative damage to biomolecules. Among ROS, the hydroxyl radical (·OH) is one of the most reactive molecules in biological systems because of its high reaction rate constant. Therefore, imaging of ·OH could be useful for evaluation of the redox mechanism and diagnosis of oxidative diseases. In vivo dynamic nuclear polarization-magnetic resonance imaging (DNP-MRI) is a noninvasive imaging method to obtain spatiotemporal information about free radicals with MRI anatomical resolution. In this study, we investigated the visualization of hydroxyl radicals generated from the Fenton reaction by combining DNP-MRI with a spin-trapping agent (DMPO: 5,5-dimethyl-1-pyrroline N-oxide) for ·OH. Additionally, we demonstrated the radical-scavenging effect using four thiol-related reagents by DNP-MRI. We demonstrated that DNP enhancement could be induced by the DMPO-OH radical using the DNP-MRI/spin-trapping method and visualized ·OH generation for the first time. Maximum DNP enhancement was observed at an electron paramagnetic resonance irradiation frequency of 474.5 MHz. Furthermore, the radical-scavenging effect was simultaneously evaluated by the decrease in the DNP image value of DMPO-OH. An advantage of our methods is that they simultaneously investigate compound activity and the radical-scavenging effect.

Developed a novel quinazolinone based turn-on fluorescence probe for highly selective monitoring hypochlorite and its bioimaging applications

A novel quinazolinone based turn-on fluorescence probe for sensitive monitoring hypochlorite was prepared using the mild condensation reaction between 2-(2'-hydroxyphenyl)-4(3H)-quinazolinone derivative and 4-methylbenzenesulfonyl hydrazide. The probe exhibited specific selectivity to ClO^- with obvious optical signal changes from weak fluorescence at 560 nm to a strong fluorescence emission at 520 nm and color changes from colorless to yellow, which could be noticed by the naked eye. The detection limit toward hypochlorite is as low as 11.4 nM. Moreover, the probe could sensitively response to ClO^- in living cells with satisfying imaging effect and has been successfully applied to the determination of ClO^- in practical water samples, which indicated that the probe has certain application potential for hypochlorite monitoring.

Association between oxidative stress and temporomandibular joint dysfunction: A narrative review

BACKGROUND

The role of oxidative stress in temporomandibular disorders (TMDs) has been studied using synovial fluid, to evaluate oxidative stress (im)balance and its potential role in the onset and/or progression of the disease.

OBJECTIVE

This review aimed to evaluate the association between oxidative stress markers and the etiopathogenesis of TMJ dysfunction by means of synovial fluid analysis.

METHODS

Two independent reviewers searched the electronic databases from inception to March 2019: PubMed/MEDLINE, LILACS, SciELO, EMBASE, TRIPDATABASE, SCOPUS, and Google Scholar. The following key search terms were used: Temporomandibular Joint Disorders OR Temporomandibular Joint Disc OR Temporomandibular Joint Dysfunction Syndrome OR Temporomandibular Joint OR Facial Pain AND Free Radicals OR Oxidative Stress. Data were extracted from the selected articles, including study design, sample profile, TMJ disease reported, diagnostic method, reactive oxygen and nitrogen species evaluated, enzymatic and non-enzymatic antioxidants evaluated, and techniques used to measure free radicals and antioxidants.

RESULTS

After title and abstract screening of 6974 results, and full-text reading, 19 studies were included. All selected articles were cross-sectional observational studies. Enzymatic and non-enzymatic antioxidant defences appeared to be reduced in these patients, resulting in the establishment of the oxidative stress process. In addition, the studies showed a positive correlation between the severity of the intra-articular TMD and the increase in oxidative damage.

CONCLUSION

The establishment of oxidative stress, whether by an increase in reactive oxygen/nitrogen species or by a decrease in antioxidant defences, or a combination of both, may be associated with the establishment and maintenance of intra-articular damage.

Detection and quantification of nitric oxide-derived oxidants in biological systems

The free radical nitric oxide (NO•) exerts biological effects through the direct and reversible interaction with specific targets (e.g. soluble guanylate cyclase) or through the generation of secondary species, many of which can oxidize, nitrosate or nitrate biomolecules. The NO•-derived reactive species are typically short-lived, and their preferential fates depend on kinetic and compartmentalization aspects. Their detection and quantification are technically challenging. In general, the strategies employed are based either on the detection of relatively stable end products or on the use of synthetic probes, and they are not always selective for a particular species. In this study, we describe the biologically relevant characteristics of the reactive species formed downstream from NO•, and we discuss the approaches currently available for the analysis of NO•, nitrogen dioxide (NO2•), dinitrogen trioxide (N2O3), nitroxyl (HNO), and peroxynitrite (ONOO-/ONOOH), as well as peroxynitrite-derived hydroxyl (HO•) and carbonate anion (CO3•-) radicals. We also discuss the biological origins of and analytical tools for detecting nitrite (NO2-), nitrate (NO3-), nitrosyl-metal complexes, S-nitrosothiols, and 3-nitrotyrosine. Moreover, we highlight state-of-the-art methods, alert readers to caveats of widely used techniques, and encourage retirement of approaches that have been supplanted by more reliable and selective tools for detecting and measuring NO•-derived oxidants. We emphasize that the use of appropriate analytical methods needs to be strongly grounded in a chemical and biochemical understanding of the species and mechanistic pathways involved.

Dihydroxy-Substituted Coumarins as Fluorescent Probes for Nanomolar-Level Detection of the 4-Amino-TEMPO Spin Label

This paper reports on dihydroxycoumarins as fluorescent probes suitable for the detection and determination of the nitroxide radical, namely 4-amino-TEMPO. Since 4-amino-TEMPO is used as a spin label for the detection of various radicals and damage caused by these species, its determination under physiological conditions might help us to understand the mechanism of the oxidative stress. Among different coumarins studied, only dihydroxy-substituted derivatives show high sensitivity, specificity, and selectivity for the nitroxide radical. In this assay, dihydroxy-substituted coumarins under the action of 4-amino-TEMPO show a very fast and significant increase in fluorescence intensity and lifetime. Among them 6,7-dihydroxycoumarin (esculetin) exhibits the strongest fluorescence enhancement (up to 40 times), with an estimated limit of detection equal to 16.7 nM—a significantly lower value when compared with UV-Vis or electron paramagnetic resonance (EPR) spectroscopy. The method is characterized by an easy procedure of sample preparation and very short time of analysis. The mechanism of the interaction between 6,7-dihydroxycoumarin and 4-amino-TEMPO has been examined with the use of a series of complementary techniques, such as steady-state and time-resolved fluorescence spectroscopy, UV-Vis spectroscopy, electron paramagnetic resonance spectroscopy, potentiometric titration, and high-performance liquid chromatography. It has been proven that the only route of the reaction in the system studied is a proton transfer from the molecule of esculetin to the amino group of the nitroxide. Biological studies performed on prostate cancer cells, breast cancer cells, and normal skin fibroblasts revealed significant anticancer properties of 6,7-dihydroxycoumarin, which caused a considerable decrease in the viability and number of cancer cells, and affected their morphology, contrary to normal fibroblasts. Furthermore, the experiment performed on prostate cancer cells showed that fluorescence emission of esculetin is closely related to intracellular pH—the higher pH, the higher observed fluorescence intensity (in accordance with a chemical experiment). On the other hand, the studies performed in different pH levels revealed that when pH of the solution increases, the observed fluorescence intensity enhancement under the action of 4-amino-TEMPO decreases (better sensing properties of esculetin towards the nitroxide in lower pH).

Versatile fluorophores for bioimaging applications: π-expanded naphthalimide derivatives with skeletal and appendage diversity

Four novel fluorescent cores bearing a transformable functional group based on a π-expanded naphthalimide including a fused pyranone or furan ring have been prepared.