Coumarin-based fluorescent probes toward viscosity in mitochondrion/lysosome

Design and Application of Fluorescent Probes to Detect Cellular Physical Microenvironments

The microenvironment is indispensable for functionality of various biomacromolecules, subcellular compartments, living cells, and organisms. In particular, physical properties within the biological microenvironment could exert profound effects on both the cellular physiology and pathology, with parameters including the polarity, viscosity, pH, and other relevant factors. There is a significant demand to directly visualize and quantitatively measure the fluctuation in the cellular microenvironment with spatiotemporal resolution. To satisfy this need, analytical methods based on fluorescence probes offer great opportunities due to the facile, sensitive, and dynamic detection that these molecules could enable in varying biological settings from in vitro samples to live animal models. Herein, we focus on various types of small molecule fluorescent probes for the detection and measurement of physical parameters of the microenvironment, including pH, polarity, viscosity, mechanical force, temperature, and electron potential. For each parameter, we primarily describe the chemical mechanisms underlying how physical properties are correlated with changes of various fluorescent signals. This review provides both an overview and a perspective for the development of small molecule fluorescent probes to visualize the dynamic changes in the cellular environment, to expand the knowledge for biological process, and to enrich diagnostic tools for human diseases.

A red-emitting mitochondria targetable fluorescent probe for detecting viscosity in HeLa, zebrafish, and mice

Viscosity, an essential parameter of the cellular microenvironment, has the ability to indicate the condition of living cells. It is closely linked to numerous diseases like Alzheimer's disease, diabetes, and cardiovascular disorders. Therefore, it is necessary to design tools to effectively monitor viscosity changes, which could provide promising avenues for therapeutic interventions in these diseases. Herein, we report a novel mitochondria-targeting fluorescent probe GX-VS which was suitable for the detection of viscosity changes in vivo and in vitro. The probe GX-VS had many advantages such as long emission wavelength (650 nm), large Stokes shift (105 nm), significant fluorescence enhancement (59-fold), high sensitivity, good biocompatibility and so on. Biological experiments showed that the probe could target mitochondria and detect viscosity alterations in HeLa cells. Moreover, it has been successfully utilized to monitor viscosity changes induced by lipopolysaccharides (LPS) in inflammatory zebrafishes and living mice, which further underscored the capacity of GX-VS to explore fluctuations in viscosity within living organisms.

Lysosome-targeted fluorescent probes: Design mechanism and biological applications

As an integral organelle in the eukaryote, the lysosome is the degradation center and metabolic signal center in living cells, and partakes in significant physiological processes such as autophagy, cell death and cellular senescence. Fluorescent probe has become a favorite tool for studying organelles and their chemical microenvironments because of its high specificity and non-destructive merits. Over recent years, it has been reported that increasingly new lysosome-targeted probes play a major role in the diagnosis and monitor of diseases, in particular cancer and neurodegenerative diseases. In order to deepen the relevant research on lysosome, it is challenging and inevitability to design novel lysosomal targeting probes. This review first introduces the concepts of lysosome and its closely related biological activities, and then introduces the fluorescent probes for lysosome in detail according to different detection targets, including targeting mechanism, biological imaging, and application in diseases. Finally, we summarize the specific challenges and discuss the future development direction facing the current lysosome-targeted fluorescent probes. We hope that this review can help biologists grasp the application of fluorescent probes and broaden the research ideas of researchers targeting fluorescent probes so as to design more accurate and functional probes for application in diseases.

Viscosity probes towards different organelles with red emission based on an identical hemicyanine structure

A series of viscosity probes targeting different organelles were obtained using a single hemicyanine dye as the matrix structure. Specifically, probes 1a-d were obtained by introducing four amines (6-amino-2H-chromen-2-one, N-(2-aminoethyl)-4-methylbenzenesulfonamide, dodecan-1-amine and N,N diphenylbenzene-1,4-diamine) into the indole hemicyanine dye of the carboxylic acid with a D-π-A structure. Their maximum absorption wavelengths were in the range 570-586 nm and they had relatively large molar absorption coefficients, while their maximum emission wavelengths in the red light region were in the range 596-611 nm. Moreover, their fluorescence intensity in glycerol was 35-184 times higher than that in phosphate buffer solution (PBS). The lg(Fl) and lg η of probes 1a-d showed good linearity with high correlation coefficients according to the Förster-Hoffman equation. In addition, cell staining experiments demonstrated that 1a-c could target lysosomes, endoplasmic reticulum and mitochondria, respectively. They could also undergo viscosity-detectable changes in the corresponding organelles under the action of the corresponding ion carriers.

Mitochondria-targeting fluorescent sensor with high photostability and permeability for visualizing viscosity in mitochondrial malfunction, inflammation, and AD models

Viscosity changes in mitochondria are closely associated with numerous cellular processes and diseases. Currently available fluorescence probes used in mitochondrial viscosity imaging are not very photostable or sufficiently permeable. Herein, a highly photostable and permeable mitochondria-targeting red fluorescent probe (Mito-DDP) was designed and synthesized for viscosity sensing. Viscosity was imaged in living cells using a confocal laser scanning microscope, and the results suggested that Mito-DDP penetrated the membrane and stained the living cells. Importantly, practical applications of Mito-DDP were demonstrated: viscosity visualization was realized for mitochondrial malfunction, cellular and zebrafish inflammation, and Drosophila Alzheimer's disease models, i.e., for subcellular organelles, cells, and organisms. The excellent analytical and bioimaging performance of Mito-DDP in vivo makes it an effective tool for exploring the physiological and pathological effects of viscosity.

Application of COSMO-RS-DARE as a Tool for Testing Consistency of Solubility Data: Case of Coumarin in Neat Alcohols

Coumarin is a naturally occurring lactone-type benzopyrone with various applications in the pharmaceutical, food, perfume, and cosmetics industries. This hydrophobic compound is poorly soluble in water but dissolves well in protic organic solvents such as alcohols. Despite the extensive use of coumarin, there are only a few reports documenting its solubility in organic solvents, and some reported data are incongruent, which was the direct impulse for this study. To resolve this problem, a theoretical congruency test was formulated using COSMO-RS-DARE for the determination of intermolecular interaction parameters, which allowed for the identification of outliers as suspicious datasets. The perfect match between back-computed values of coumarin solubility and the experimental ones confirms the reliability of the formulated theoretical approach and its adequacy for testing solubility data consistency. As the final approval, the temperature-related coumarin solubility in seven neat alcohols was determined experimentally. Four solvents (methanol, ethanol, 1-propanol, and 2-propanol) were used for reproducibility purposes, and an additional three (1-butanol, 1-pentanol, and 1-octanol) were used to extend the information on the homologous series. The consistency of this extended solubility dataset is discussed in terms of the comparison of remeasured solubility values with the ones already published and within the series of structurally similar solvents. The proposed procedure extends the range of applicability of COSMO-RS-DARE and provides a real and useful tool for consistency tests of already published solubility data, allowing for the approval/disapproval of existing data and filling gaps in datasets. Linear regressions utilizing a 2D molecular descriptor, SpMin2_Bhm, or the distance between solute and solvent in the Hansen solubility space, Ra, were formulated for the estimation of COMSO-RS-DARE integration parameters.