An Enumerated Outlook of Intracellular Micropolarity Using Solvatochromic Organic Fluorescent Probes

Abstract

The spatiotemporal distribution of intracellular physical parameters of a live cell is heterogeneous and complex. Measuring physical properties inside given cellular compartments (organelles) is challenging and important for therapy and diagnostics. The tiny volume of a single cell and even tinier organelles are not accessible by classical measuring devices. The microenvironment inside an organelle vastly controls the outcome of any biochemical and biophysical processes taking place inside it, which is crucial for the overall cellular health. Therefore, it is very important to understand the microenvironmental physical properties inside cellular organelles. Moreover, specific alterations of such microenvironmental properties were observed in the disease condition, making them a diagnostic hallmark. With this high demand, small-molecule organic fluorophores are emerging as the most successful tool due to their small relative size, bioavailability, and ease of functionalization. In this mini-review, the development of micropolarity-sensitive small organic fluorophore with the capability of targeting a specific cellular organelle has been discussed. Here, we have highlighted the strategies of targeting a specific organelle, the micropolarity, and the challenges and prospects of the field.

Figure 1

Concept of using intramolecular charge transfer (ICT) solvatofluorochromic dyes for measuring organelle micropolarity. ICT dye is comprised of a donor (D) and acceptor (A) connected with a π-conjugated linker (top left); it can be customized with an organelle-targeting group (TG) for specific intracellular localization. ICT dyes show solvatofluorochromic properties (bottom left, adapted with permission from [Pal, K.; Kumar, P.; Koner, A. L. Deciphering interior polarity of lysosome in live cancer and normal cells using spectral scanning microscopy. J. Photochem. Photobiol. B2020, 206, 111848–111854]. Copyright 2020. Elsevier) which can be implemented to understand the micropolarity inside a cellular organelle in live cells using spectral scanning techniques under a fluorescence microscope.

Figure 2

Chemical structures of the probes used for studying polarity of the plasma membrane.

Figure 3

Chemical structures of the probes used for studying the polarity of mitochondria.

Figure 4

Chemical structures of the lysosome-targeting probes reported for determining the polarity.

Figure 5

Chemical structures of the polarity-sensitive ER-targeting probes.

Figure 6

Schematic representation of the lipid droplet and chemical structures of the probes used for studying polarity inside lipid droplets; the micropolarity range is E_T(30) ∼ 32–52 kcal mol^–1.

Figure 7

Laser scanning confocal images of KB cells in the presence of 2 μM DAF (28). Image obtained with λ_ex= 405 (A) in the blue channel, E_m = 415–480 nm and (B) in the red channel, E_m = 500–600 nm. (C) Ratiometric image obtained from the ratio between the red and the blue channels for mapping the polarity distribution in live cells. The inset in C was zoomed-in for figure E, showing that LDs have heterogeneous polarity. Under identical conditions, D and F are ratiometric images of 10 μM 28 using pure ethanol and oil (Labrafac), respectively. The scale bar is 20 μm for A, B, and C images. Reprinted with permission from [Ashoka, A. H.; Ashokkumar, P.; Kovtun, Y. P.; Klymchenko, A. S. Solvatochromic Near-Infrared Probe for Polarity Mapping of Biomembranes and Lipid Droplets in Cells under Stress. J. Phys. Chem. Lett. 2019, 10 (10), 2414–2421]. Copyright 2019. American Chemical Society.