Targeted Solvatochromic Fluorescent Probes for Imaging Lipid Order in Organelles under Oxidative and Mechanical Stress

A lipid droplet-targeted probe for imaging of lipid metabolism disorders during mitochondrial myopathy

Lipid metabolism is closely related to various biological processes in cells. The accumulation of Lipid droplets (LDs) is a typical manifestation of certain metabolic diseases, such as mitochondrial myopathy, which shows a significant increase in LDs. The accumulation of LDs can exacerbate the progression of disease, and lysosomes selectively degrade LDs to cope with this phenomenon. Visualizing lipid metabolism disorders and the interaction between LDs and other organelles is of great significance for the diagnosis and understanding of various physiological processes within cells in diseases. In this work, we synthesized two novel LD fluorescent probes and screened the best PDM, which exhibited stable fluorescence performance and strong photobleaching resistance in complex environments. The dynamics of intracellular LDs were tracked using PDM, and abnormal lipid metabolism within mitochondrial myopathy cells was visualized. This provides new tools and perspectives for studying LD dynamics and diagnosing mitochondrial myopathy.

Rational development of Nile red derivatives with significantly improved specificity and photostability for advanced fluorescence imaging of lipid droplets

Since the first report of Nile Red as a fluorescent probe for lipid droplets (LDs) imaging was published in 1985, this fluorescent probe has been widely used for nearly 40 years, and so far, it is still one of the most commonly used probes for LDs imaging. Although Nile Red has achieved continuous success, it has gradually emerged two major limitations (poor LDs specificity and low photostability) which directly limit the study of LDs via advanced fluorescence imaging techniques. In this context, we have developed a new synthetic route to conveniently prepare a series of Nile Red derivatives (NR-1 to NR-15). With these 15 derivatives in hand, the relationships between molecular structures and their properties (LDs specificity, photostability) have been comprehensively investigated. Consequently, we have rationally designed a new Nile Red derivative, NR-11, which exhibits significantly improved LDs specificity and photostability. Utilizing this new LDs probe, we have successfully conducted various advanced fluorescence imaging, e.g. time-lapse three-dimensional (3D) confocal imaging of cells, time-lapse 3D dynamic tracking of a single LD, and two-photon 3D imaging of tissues. These advanced imaging results not only demonstrate the utility of this new fluorescent probe but also provide novel insights into the cell biology study of LDs.

Tiny but mighty! N,N-dimethyl-4-(5-nitrothiophen-2-yl)aniline, a push-pull fluorescent dye for lipid droplet imaging

Lipid droplets (LDs) are ubiquitous cellular organelles with a neutral lipid core containing triacylglycerols and cholesteryl esters surrounded by phospholipids. Recent findings indicate that LDs are intricately linked to diseases, such as cancer and neurological disorders, in addition to their roles in cellular senescence and immune responses. Herein, we describe a simple yet robust push-pull molecule, N,N-dimethyl-4-(5-nitrothiophen-2-yl)aniline (NiTA), as a versatile LD fluorescent probe. NiTA showed an absorption spectrum with a substantial bathochromic shift and a fluorescence spectrum with excellent solvatochromism. Leveraging the remarkable photophysical features of NiTA, we stained LDs in major immune cells, including T and B cells, and macrophages, and monitored the changes in LDs under oxidative and starvation conditions. Furthermore, we demonstrated the applicability of NiTA for visualizing the organization of medaka fish (Oryzias latipes) embryos during development. We expect the small yet powerful NiTA to be utilized in various applications, including fluorescence mapping to observe LD numbers, morphology, and polarity changes in animals and cells.

Spin Manipulation Engineering of Photodynamic Intermediates: Magnetic Amplification of Oxyradicals Generation for Enhanced Antitumor Phototherapeutic Efficacy

Improving the photosensitization efficiency represents a critical challenge in photodynamic therapy (PDT) research. While cyanines exhibit potential as photosensitizers (PSs) due to their large extinction coefficients and excellent biocompatibility, the inherent limitations in intersystem crossing severely affect therapeutic efficacy. Herein, we proposed a bottom-up magnetically enhanced photodynamic therapy (magneto-PDT) paradigm employing fluorobenzene-substituted pentamethine cyanine as type-I reactive oxygen species generators. Based on the radical pair mechanism and magnetic field effect, the notable difference in g-factors (Δg) between PSs and oxyradicals enabled magnetically responsive amplification of Cy5-3,4,5-3F-mediated hydroxyl radical (•OH) and superoxide anion radical (O2•-) production, achieving maximum yield enhancements of 66.9 and 28.0% respectively at 500 mT. This magnetically augmented oxyradicals generation exhibited universal cytotoxicity superiority over conventional PDT protocols in various cancer cell models. Notably, the semi-inhibitory concentration (IC50) of murine mammary carcinoma 4T1 cells demonstrated a remarkable reduction under both normoxic and hypoxic conditions, with the most pronounced decrease observed in normoxia from 0.91 μM (PDT alone) to 0.38 μM (magneto-PDT). The significantly magneto-enhanced therapeutic performance effectively inhibited orthotopic tumor growth. This magneto-PDT paradigm established a novel strategy for manipulating spin-dependent photosensitization processes in biological applications.

PSL Chemical Biology Symposia: The Increasing Impact of Chemistry in Life Sciences

This symposium is the 6th Paris Sciences & Lettres (PSL) Chemical Biology meeting (2015, 2016, 2019, 2023, 2024, 2025) being held at Institut Curie. This initiative originally started in 2013 at Institut de Chimie des Substances Naturelles (ICSN) in Gif-sur-Yvette and was mostly focused on organic synthesis. It was then exported at Institut Curie to cover a larger scope, before becoming the official French Chemical Biology meeting. This year, around 200 participants had the opportunity to meet world leaders in chemistry and biology who described their latest innovations and future trends covering topics as diverse as prebiotic chemistry, activity-based protein profiling, high-resolution cell imaging, nanotechnologies, bio-orthogonal chemistry, metal ion signaling, ferroptosis, and biocatalysis.

Improved Solvatochromism and Quantum Yields in Acridine through Polarity-Enhanced π-Conjugation

Fluorophores exhibiting strong solvatochromism are strongly desired for applications in cellular imaging, photodevices, and fluorescence sensing. However, achieving high quantum yields in increasingly polar environments remains a significant challenge due to the complex balance required between a high transition dipole moment and a low emission energy. In this study, we present an acridine derivative of pyrene, Ad-Py, which features a donor-π-acceptor (D-π-A) scaffold. Notably, this fluorophore demonstrated a strong solvatochromic property, exhibiting a red-shift of 113 nm, along with high quantum yields (Φf = 83.5-90.6%) from the less polar solvent n-hexane to the highly polar solvent dimethyl sulfoxide (DMSO). The incorporation of the pyrene moiety effectively extended the π-conjugation of Ad-Py in polar solvents and finely tuned the proportion of the charge transfer (CT) state, leading to enhanced quantum yields. Moreover, due to its sensitivity to microenvironmental changes, Ad-Py facilitated the visual measurement of SO2, achieving a detection limit of <10 ppb.

High-Resolution Correlative Microscopy Approach for Nanobio Interface Studies of Nanoparticle-Induced Lung Epithelial Cell Damage

Correlated light and electron microscopy (CLEM) has become essential in life sciences due to advancements in imaging resolution, sensitivity, and sample preservation. In nanotoxicology─specifically, studying the health effects of particulate matter exposure─CLEM can enable molecular-level structural as well as functional analysis of nanoparticle interactions with lung tissue, which is key for the understanding of modes of action. In our study, we implement an integrated high-resolution fluorescence lifetime imaging microscopy (FLIM) and hyperspectral fluorescence imaging (fHSI), scanning electron microscopy (SEM), ultrahigh resolution helium ion microscopy (HIM) and synchrotron micro X-ray fluorescence (SR μXRF), to characterize the nanobio interface and to better elucidate the modes of action of lung epithelial cells response to known inflammatory titanium dioxide nanotubes (TiO2 NTs). Morpho-functional assessment uncovered several mechanisms associated with extensive DNA, essential minerals, and iron accumulation, cellular surface immobilization, and the localized formation of fibrous structures, all confirming immunomodulatory responses. These findings advance our understanding of the early cellular processes leading to inflammation development after lung epithelium exposure to these high-aspect-ratio nanoparticles. Our high-resolution experimental approach, exploiting light, ion, and electron sources, provides a robust framework for future research into nanoparticle toxicity and its impact on human health.

HBimmCue: A Versatile Fluorescent Probe for Multi-Scale Imaging of Lipid Polarity and Membrane Order in Inner Mitochondrial Membrane

Mitochondrial membrane environmental dynamics are crucial for understanding function, yet high-resolution observation remains challenging. Here, HBimmCue is introduced as a fluorescent probe localized to inner mitochondrial membrane (IMM) that reports lipid polarity and membrane order changes, which correlate with cellular respiration levels. Using HBimmCue and fluorescence lifetime imaging microscopy (FLIM), IMM lipid heterogeneity is uncovered across scales, from nanoscale structures within individual mitochondria to mouse pre-implantation embryos. At the sub-organelle level, stimulated emission depletion (STED)-FLIM imaging highlights nanoscale polarity variations within the IMM. At the sub-cellular and cellular level, reduced IMM lipid polarity is observed in damaged mitochondria marked for lysosomal degradation and distinct IMM lipid distributions are identified in neurons and disease models. Additionally, metabolic dysfunction associated with oocytes aging and metabolic reprogramming from zygote to blastocyst is detected. Together, the work demonstrates the broad applicability of HBimmCue, offering a new paradigm for investigating lipid polarity and respiration level at multiple scales.

Mechanistic study of tumor fluorescence response signals based on a near-infrared viscosity-sensitive probe

Viscosity is an important physiological parameter closely associated with various cellular processes and diseases. Several fluorescence probes responsive to viscosity have been developed, demonstrating high sensitivity specifically towards tumor tissues. However, the underlying core mechanism of this highly potential responsive signal has been a subject of debate, as highly sensitive probes encounter excessive environmental interferences in complex tumor tissues. Therefore, we have developed a viscosity-responsive fluorescence probe based on the classical TICT mechanism (twisted intramolecular charge transfer) as a research tool. This probe features an ultra-wide emission range of 700-1200 nm in the near-infrared spectrum, strong photostability, and simultaneous targeting of mitochondria and lysosomes. Through in-depth analysis, we have revealed the intrinsic mechanisms underlying its functionality, demonstrating that the major contributor to the fluorescence change of responsive probes during imaging is the inherent state of cells rather than the tumor microenvironment or the cell type. Our findings provide a theoretical foundation for the continued exploration and application of viscosity-responsive probes.

Organelle-Targeted Photo-triggered Delivery of Acetylperoxyl Radicals for Redox Homeostasis Modulation

Dysfunction of subcellular organelles initiates complex pathophysiological cascades and underlies numerous diseases, underscoring the need for organelle-specific therapeutic interventions. Precise spatiotemporal control of reactive oxygen species (ROS) generation within organelles offers a promising intervention approach. Herein, we report the design and synthesis of a novel series of organelle-targeted, photoactivatable acetylperoxyl radical donors (ACR575s) based on an acetyl-caged rhodamine scaffold. Blue light irradiation triggered the release of highly oxidative acetylperoxyl radicals, concomitantly generating a rhodamine dye for real-time monitoring. In vitro studies demonstrated the organelle-specific delivery of acetylperoxyl radicals, which subsequently induced concentration-dependent oxidative stress within specific subcellular compartments. Notably, this resulted in membrane damage and the modulation of macrophage polarization, providing clear evidence of the therapeutic potential of acetylperoxyl radicals in regulating redox balance and inflammatory responses. The ACR575 series provides a novel toolset for acetylperoxyl radical biology and subcellular redox regulation, enabling precise spatiotemporal control of acetylperoxyl radical-mediated oxidative stress and showing potential for applications in precise cancer therapy.

A Solvatochromic and Photosensitized Lipid Droplet Probe Detects Local Polarity Heterogeneity and Labels Interacting Proteins in Human Liver Disease Tissue

The intricate morphology, physicochemical properties, and interacting proteins of lipid droplets (LDs) are associated with cell metabolism and related diseases. To uncover these layers of information, a solvatochromic and photosensitized LDs-targeted probe based on the furan-based D-D-π-A scaffold is developed to offer the following integrated functions. First, the turn-on fluorescence of the probe upon selectively binding to LDs allows for direct visualization of their location and morphology. Second, its solvatochromic fluorescence with linear correlation to polarity quantifies micro-environmental heterogeneity among LDs. Third, the unique photosensitized properties enable photocatalytic proximity labeling and enrichment of LDs-interacting proteins, ready for potential downstream proteomic analysis. These functions are exemplified using artificial LDs in buffer, stressed liver cell line, and diseased liver tissues biopsied from patients. While most LD sensors only offer fluorescence imaging functions, the multi-functional LD probe reported herein integrates both singlet fluorescence and triplet photosensitization properties for LDs studies.

Membrane Fusion-Based Drug Delivery Liposomes Transiently Modify the Material Properties of Synthetic and Biological Membranes

Many drug targets are located in intracellular compartments of cells but they often remain inaccessible to standard imaging and therapeutic agents. To aid intracellular delivery, drug carrier nanoparticles have been used to overcome the barrier imposed by the plasma membrane. The carrier must entrap large amounts of cargo, efficiently and quickly deliver the cargo in the cytosol or other intracellular compartments, and must be inert; they should not induce cellular responses or alter the cell state in the course of delivery. This study demonstrates that cationic liposomes with high charge density efficiently fuse with synthetic membranes and the plasma membrane of living cells. Direct fusion efficiently delivers large amounts of cargo to cells and cell-like vesicles within seconds, bypassing slow and often inefficient internalization-based pathways. These effects depend on liposome charge density, concentration, and the helper lipid. However, fusion-mediated cargo delivery results in the incorporation of large amounts of foreign lipids, causing changes to the material properties of these membranes, namely modifications in membrane packing and fluidity, induction of membrane curvature, decrease in surface tension, and the formation of (short-lived) pores. Importantly, these effects are transient and liposome removal allows cells to recover their state prior to liposome interaction.

Click and shift: the effect of triazole on solvatochromic dyes

Solvatochromic dyes are prime candidates for exploring complex polarity-dependent biological processes. The design of novel dyes for these applications typically presents long synthetic routines. Herein, we report a concise synthesis of azido-functionalised push-pull fluorenes, belonging to the most potent groups of fluorosolvatochromic compounds. These fluorenes can be attached to multiple alkynes via the highly versatile copper-catalysed azide-alkyne cycloaddition (CuAAC). Our study focuses on the beneficial effect of the triazole, formed by CuAAC, comparing the simple push-pull fluorene FR1 with the novel model compound FR1TP. While the triazole in FR1TP is not conjugated to the π-system of the dye, the heterocycle surprisingly produces a bathochromic emission shift of around 50 nm. This effect, caused by triazole-induced LUMO stabilisation, was also observed for two other model compounds: FR2TP and FR1TM. All compounds display photophysical properties that are highly desired for in vivo imaging dyes, including remarkable two-photon absorption properties. The experimental results are supported by theoretical calculations. We anticipate that our findings will enable synthesis of new high-performance fluorosolvatochromic dyes for diverse applications.

Synthetic Lipid Biology

Cells contain thousands of different lipids. Their rapid and redundant metabolism, dynamic movement, and many interactions with other biomolecules have justly earned lipids a reputation as a vexing class of molecules to understand. Further, as the cell's hydrophobic metabolites, lipids assemble into supramolecular structures─most commonly bilayers, or membranes─from which they carry out myriad biological functions. Motivated by this daunting complexity, researchers across disciplines are bringing order to the seeming chaos of biological lipids and membranes. Here, we formalize these efforts as "synthetic lipid biology". Inspired by the idea, central to synthetic biology, that our abilities to understand and build biological systems are intimately connected, we organize studies and approaches across numerous fields to create, manipulate, and analyze lipids and biomembranes. These include construction of lipids and membranes from scratch using chemical and chemoenzymatic synthesis, editing of pre-existing membranes using optogenetics and protein engineering, detection of lipid metabolism and transport using bioorthogonal chemistry, and probing of lipid-protein interactions and membrane biophysical properties. What emerges is a portrait of an incipient field where chemists, biologists, physicists, and engineers work together in proximity─like lipids themselves─to build a clearer description of the properties, behaviors, and functions of lipids and membranes.

Analyzing the Cholesteryl Ester Fraction in Lipid Droplets with a Polarity-Ultrasensitive Fluorescence Lifetime Probe

Quantifying the lipid composition of cellular lipid droplets (LDs) in situ is challenging but crucial for understanding lipid metabolic diseases. Here, we propose a fluorescence lifetime imaging method based on a polarity-sensitive probe (LD660) for analyzing the lipid composition of the LDs. The probe emits strong fluorescence at 660 nm only in apolar LD environments, with dielectric constants of 2-4, and outperforms Nile red in LD imaging. Importantly, the fluorescence lifetime of LD660 increases with the incremental fraction of cholesteryl ester in neutral lipid mixtures. Using fluorescence lifetime microscopy with LD660, we imaged and quantified the cholesteryl ester fractions of LDs in cells and tissues. It is found that macrophages and surrounding hepatocytes in fatty liver diseases show significantly higher cholesteryl ester contents than other hepatocytes. This finding suggests that cholesteryl ester may serve as a potential indicator of the degree of hepatic steatosis.

Lipid-Directed Covalent Labeling of Plasma Membranes for Long-Term Imaging, Barcoding and Manipulation of Cells

Fluorescent probes for cell plasma membranes (PM) generally exploit a noncovalent labeling mechanism, which constitutes a fundamental limitation in multiple bioimaging applications. Here, we report a concept of lipid-directed covalent labeling of PM, which exploits transient binding to the lipid membrane surface generating a high local dye concentration, thus favoring covalent ligation to random proximal membrane proteins. This concept yielded fluorescent probes for PM called MemGraft, which are built of a dye (cyanine Cy3 or Cy5) bearing a low-affinity membrane anchor and a reactive group: an activated ester or a maleimide. In contrast to specially designed control dyes and commercial Cy3-based labels of amino or thiol groups, MemGraft probes stain efficiently PM, revealing the crucial role of the membrane anchor combined with optimal reactivity of the activated ester or the maleimide. MemGraft probes overcome existing limitations of noncovalent probes, which makes them compatible with cell fixation, permeabilization, trypsinization, and the presence of serum. The latter allows long-term cell tracking and video imaging of cell PM dynamics without the signs of phototoxicity. The covalent strategy also enables staining and long-term tracking of cocultured cells labeled in different colors without exchange of probes. Moreover, the combination of MemGraft-Cy3 and MemGraft-Cy5 probes at different ratios enabled long-term cell barcoding in at least 5 color codes, important for tracking and visualizing multiple populations of cells. Ultimately, we found that the MemGraft strategy enables efficient biotinylation of the cell surface, opening the path to cell surface engineering and cell manipulation.

Spectrally Resolved Single-Molecule Orientation Imaging Reveals a Direct Correspondence between the Polarity and Microviscosity Experienced by Nile Red in Supported Lipid Bilayer Membranes

Molecular-level interactions among lipids, cholesterol, and water dictate the nanoscale membrane organization of lipid bilayers into liquid-ordered (Lo) and liquid-disordered (Ld) phases, characterized by different polarities and orders. Generally, solvatochromic dyes easily discriminate polarity difference between Lo and Ld phases, whereas molecular flippers and rotors show distinct photophysics depending on the membrane order. Despite progress in single-molecule spectral imaging and single-molecule orientation mapping, direct experimental proof linking polarity with microviscosity sensed by the same probe eludes us. Here, we demonstrate spectrally resolved single-molecule orientation localization microscopy to connect nanoscopic localization of a probe on a bilayer membrane with its emission spectra, three-dimensional dipole orientation, and rotational constraint offered by the local microenvironment and highlight the excellent correspondence between the polarity and order experienced by the same probe. This technique has the potential to address nanoscale heterogeneity and dynamics, especially in biology and material sciences.

Measuring plasma membrane fluidity using confocal microscopy

Membrane fluidity is a crucial parameter for cellular physiology. Recent evidence suggests that fluidity varies between cell types and states and in diseases. As membrane fluidity has gradually become an important consideration in cell biology and biomedicine, it is essential to have reliable and quantitative ways to measure it in cells. In the past decade, there has been substantial progress both in chemical probes and in imaging tools to make membrane fluidity measurements easier and more reliable. We have recently established a robust pipeline, using confocal imaging and new environment-sensitive probes, that has been successfully used for several studies. Here we present our detailed protocol for membrane fluidity measurement, from labeling to imaging and image analysis. The protocol takes ~4 h and requires basic expertise in cell culture, wet lab and microscopy.

Polarity-sensitive pyrene fluorescent probes for multi-organelle imaging in living cells

Polarity-sensitive probes (PAS) were synthesized through the attachment of azetidine and sulfonyl substituents to the pyrene fluorescent core. The emission peaks and fluorescence lifetimes of these PAS probes exhibit high sensitivity to polarity, enabling the visualization of microenvironmental characteristics and dynamics across multiple organelles.

Small-Molecule Benzo-Phenoselenazine Derivatives for Multi-Subcellular Biomolecule Profiling

Elucidating the subcellular localization of RNAs and proteins is fundamental to understanding their biological functions. Genetically encoded proteins/enzymes provide an attractive approach to target many proteins of interest, but are limited to specific cell lines. Although small-molecule-based methods have been explored, a comprehensive system for profiling multiple locations in living cells, comparable to fusion-protein techniques, is yet to be established. In this study, we introduce a novel proximity labeling strategy employing a suite of small molecules derived from benzo-phenoselenazine (e.g., selenium-containing Nile Blue [SeNB]), which achieves proximity labeling through singlet oxygen generation upon near-infrared light activation in the presence of propargylamine. These SeNB compounds allow for selective labeling of RNAs and proteins within living cells, exhibiting a distinct preference for organelle membranes, which are systematically investigated via in vitro, computational, and in cellulo examinations. Our findings highlight the capabilities of SeNB derivatives as wash-free and genetics-free approaches to illuminate the subcellular localization of biological molecules with deep penetration and high spatial resolution. Moreover, SeNB derivatives are capable of elucidating inter-organelle interactions at the molecular level, as evidenced by proteomic and transcriptomic analyses, thus holding significant potential for advancing our understanding of cellular processes related to disease progression and therapeutic development.

RNAi-based ALOX15B silencing augments keratinocyte inflammation in vitro via EGFR/STAT1/JAK1 signalling

Arachidonate 15-lipoxygenase type B (ALOX15B) peroxidises polyunsaturated fatty acids to their corresponding fatty acid hydroperoxides, which are subsequently reduced into hydroxy-fatty acids. A dysregulated abundance of these biological lipid mediators has been reported in the skin and blood of psoriatic compared to healthy individuals. RNAscope and immunohistochemistry revealed increased ALOX15B expression in lesional psoriasis samples. Using a cytokine cocktail containing IL-17A, interferon-gamma and tumour necrosis factor-alpha to produce a psoriasis-like phenotype, a role for ALOX15B in human epidermal keratinocyte inflammation was investigated. siRNA-mediated silencing of ALOX15B increased CCL2 expression and secretion. In addition to CCL2, secretion of CCL5 and CXCL10 were elevated in skin equivalents treated with lipoxygenase inhibitor ML351. Inhibition of the JAK1/STAT1 pathway reversed the enhanced CCL2 expression found with ALOX15B silencing. Previous studies have linked epidermal growth factor receptor (EGFR) inhibition with the upregulation of cytokines including CCL2, CCL5 and CXCL10. ALOX15B silencing reduced EGFR expression and inhibition of EGFR signalling potentiated the effect of ALOX15B silencing on increased CCL2, CCL5 and CXCL10 expression. Confirming previous findings, gene expression of cholesterol biosynthesis genes was reduced via reduced ERK phosphorylation. Reduced ERK phosphorylation was dependant on EGFR and NRF2 activation. Furthermore, plasma membrane lipids were investigated via confocal microscopy, revealing reduced cholesterol and lipid rafts. This study suggests a role for ALOX15B in keratinocyte inflammation through modulation of lipid peroxidation and the EGFR/JAK1/STAT1 signalling axis.

Szabo B, Kurtan K, Varga Z, Panyi G, Nagy P, Zakany F, Kovacs T. Look Beyond Plasma Membrane Biophysics: Revealing Considerable Variability of the Dipole Potential Between Plasma and Organelle Membranes of Living Cells

Due to the lack of measurement techniques suitable for examining compartments of intact, living cells, membrane biophysics is almost exclusively investigated in the plasma membrane despite the fact that its alterations in intracellular organelles may also contribute to disease pathogenesis. Here, we employ a novel, easy-to-use, confocal microscopy-based approach utilizing F66, an environment-sensitive fluorophore in combination with fluorescent organelle markers and quantitative image analysis to determine the magnitude of the molecular order-related dipole potential in the plasma membrane and intracellular organelles of various tumor and neural cell lines. Our comparative analysis demonstrates considerable intracellular variations of the dipole potential that may be large enough to modulate protein functions, with an inward decreasing gradient on the route of the secretory/endocytic pathway (plasma membrane >> lysosome > Golgi > endoplasmic reticulum), whereas mitochondrial membranes are characterized by a dipole potential slightly larger than that of lysosomes. Our approach is suitable and sensitive enough to quantify membrane biophysical properties selectively in intracellular compartments and their comparative analysis in intact, living cells, and, therefore, to identify the affected organelles and potential therapeutic targets in diseases associated with alterations in membrane lipid composition and thus biophysics such as tumors, metabolic, neurodegenerative, or lysosomal storage disorders.

Microsolvation-Driven Hours-Long Spectral Dynamics in Phenoxazine Dyes

The phenoxazine class of dyes has found widespread applications in chemistry and biology for more than a century, particularly for lipid membrane studies. Here, we report a general phenomenon on the ensemble spectral stability of traditional phenoxazine class of dyes (nile red, cresyl violet, and nile blue) that exhibit hours-long microstructural transitions reflected through systematic changes of electronic spectra over an hour. Mechanistic investigations reveal that such spectral dynamics of the dyes can be mitigated by tuning microenvironments, where microsolvation plays an underlying role. These microsolvation-induced microstructural changes in a single dye species tend to follow zeroth-order kinetics. The half-life values of such processes systematically vary with solvent hydrogen bonding strength and ionic radius of the dyes' counteranions. In so doing, using a model lipid membrane, we demonstrate that the spectral response of a phenoxazine dye must be utilized appropriately for studying membrane properties. These findings of the phenoxazine class of dyes are of high significance for their careful applications in chemistry and biology.

An exchangeable SIM probe for monitoring organellar dynamics of necrosis cells and intracellular water heterogeneity in kidney repair

Monitoring subcellular organelle dynamics in real time and precisely assessing membrane heterogeneity in living cells are very important for studying fundamental biological mechanisms and gaining a comprehensive understanding of cellular processes. However, there remains a shortage of effective tools for these purposes. Herein, we propose a strategy to develop the exchangeable water-sensing probeAPBD for time-lapse imaging of dynamics in cellular membrane-bound organelle morphology with structured illumination microscopy at the nanoscale. In this work, our results reveal mitochondria as the first organelle to undergo morphological changes through swelling, fission, and fusion in cell necrosis, leading to the rupture of the endoplasmic reticulum (ER) sheet adhered to the mitochondria. Meanwhile, the ER tubules are then reconstructed by stretching and fusion of autophagosomes. Moreover, APBD allows us to directly visualize spatially resolved distribution of biomembranes vs. water inside single mammalian cells. Our findings show that the renal ischemia-reperfusion injury (IRI) model results in the increased biomembrane to cytoplasmic water ratio in the tissue. This reveals intracellular water heterogeneity between the nucleus and the cytoplasm during the IRI process. Overall, this study presents a strategy for development of the molecular tools for cellular water heterogeneity and organelle dynamics.

Freeze response indicators in sugarcane (Saccharum spp. hybrids)

A recent series of extreme weather events in Southern U.S. (2022 winter freeze followed by 2023 summer drought) calls for quantitative markers to expedite the release of climate resilient sugarcane varieties. A cluster analysis revealed potential markers for freeze damage including fluorescent amino acids. Of 8 cultivars investigated, tolerant variety HoCP 04-838 sustained 3-5 fold lower stalk fracture from the freeze; contained among the highest fiber; and maintained the lowest particulate juice decomposition byproducts (p<0.05). Based on those observations, fluorescence cellular markers were developed in fiber components of sugarcane to assess cold tolerance. Fluorescence microscopy visualized a cluster of markers in lignin cells around the vascular bundles of HoCP 04-838, within the far-red emission ranges attributable to lipids and other hydrophobic components. Cellular distributions of markers were made visible using fluorescent nanoparticles designed to enhance cellular uptake and imaging at wider wavelengths. Developed chemical phenotyping approaches offer advantages over post-freeze damage assessment currently used in the breeding program, as no genetic marker exists for cold tolerance of sugarcane.

Covalent assembly-based two-photon fluorescent probes for in situ visualizing nitroreductase activities: From cancer cells to human cancer tissues

Nitroreductase (NTR) is widely regarded as a biomarker whose enzymatic activity correlates with the degree of hypoxia in solid malignant tumors. Herein, we utilized 2-dimethylamino-7-hydroxynaphthalene as fluorophore linked diverse nitroaromatic groups to obtain four NTR-activatable two-photon fluorescent probes based on covalent assembly strategy. With the help of computer docking simulation and in vitro assay, the sulfonate-based probe XN3 was proved to be able to identify NTR activity with best performances in rapid response, outstanding specificity, and sensitivity in comparison with the other three probes. Furthermore, XN3 could detect the degree of hypoxia by monitoring NTR activity in kinds of cancer cells with remarkable signal-to-noise ratios. In cancer tissue sections of the breast and liver in mice, XN3 had the ability to differentiate between healthy and tumorous tissues, and possessed excellent fluorescence stability, high tissue penetration and low tissue autofluorescence. Finally, XN3 was successfully utilized for in situ visualizing NTR activities in human transverse colon and rectal cancer tissues, respectively. The findings suggested that XN3 could directly identify the boundary between cancer and normal tissues by monitoring NTR activities, which provides a new method for imaging diagnosis and intraoperative navigation of tumor tissue.

NeoMProbe: a new class of fluorescent cellular and tissue membrane probe

The development of long-lasting plasma membrane (PM) and basement membrane (BM) probes is in high demand to advance our understanding of membrane dynamics during differentiation and disease conditions. Herein, we report that the microheterogeneity of heparan sulfate (HS) on fluorescent neo-proteoglycans backbone offers a facile platform for designing membrane probes. Confocal live-cell imaging studies of cancer and normal cell lines with a panel of Cy5 fluorescently tagged neo-proteoglycans confirmed that highly sulfated HS ligands with an l-iduronic acid component (PG@ID-6) induce a prolonged and brighter expression on the PM compared to low-sulfated and uronic acid counterparts. Mono- and multi-photon microscopic imaging of tissue sections with NeoMProbe (PG@ID-6) allowed mapping BM and demonstrated staining efficacy equivalent to antibodies against the BM components. Finally, in vivo, whole-body imaging of NeoMProbe and subsequent tissue section imaging confirmed versatile and efficient membrane mapping by the probe. Overall, NeoMProbe offers a novel toolkit for cell biology and tissue biomembrane imaging.

Permeability-Controlled Probe for Ratiometric Detection of Plasma Membrane Integrity and Late Apoptosis

The destruction of plasma membrane integrity is closely related to immune response, neuronal injury, cell apoptosis, and other pathological events. However, the construction of ratiometric fluorescent probes capable of detecting plasma membrane integrity remains a significant challenge, hindering in-depth studies on related biomedical areas. Herein, a polarity-responsive fluorescent probe was constructed for the ratiometric detection of cell membrane integrity for the first time. The probe targeted intact plasma membranes in healthy cells and relocated into the cytoplasm to give significantly red-shifted fluorescence after plasma membrane damage. Molecular simulations revealed that the high transmembrane barrier and amphipathic nature of the probe were responsible for its targeting ability. With the probe, the ratiometric detection of late apoptosis stage was realized for the first time, and the membrane damage of tumor cells induced by UV irradiation, toxins, and antitumor drugs was visualized. The effect of formaldehyde on membrane integrity was evaluated using a probe, and cysteine was proved to be a potential detoxifier to counteract the toxicity of formaldehyde.

AI-driven precision subcellular navigation with fluorescent probes

Precise navigation within intricate biological systems is pivotal for comprehending cellular functions and diagnosing diseases. Fluorescent molecular probes, designed to target specific biological molecules, are indispensable tools for this endeavor. This paper delves into the revolutionary potential of artificial intelligence (AI) in crafting highly precise and effective fluorescent probes. We will discuss how AI can be employed to: design new subcellular dyes by optimizing physicochemical properties; design prospective subcellular targeting probes based on specific receptors; quantitatively explore the potential chemical laws of fluorescent molecules to optimize the optical properties of fluorescent probes; optimize the comprehensive properties of the probe and guide the construction of multifunctional targeting probes. Additionally, we showcase recent AI-driven advancements in probe development and their successful biomedical applications, while addressing challenges and outlining future directions towards transforming subcellular research, diagnostics, and drug discovery.

Synergistically Activated Aggregation-Induced Emission Probe for Precise In Situ Staining of Lipids in Atherosclerotic Plaques

Visualizing the localization and distribution of lipids within arteries is crucial for studying atherosclerosis. However, existing lipid-specific probes face challenges such as strong hydrophobicity and nonspecific staining of lipophilic organelles or tissues, making them impractical for the precise identification of atherosclerotic plaques. To address this issue, we design a synergistically activated probe, Cbz-Lys-Lys-TPEB, which responds to cathepsin B (CTB) and H2O2 for the in situ generation of aggregation-induced emission luminogens (AIEgens). This enables specific staining of lipids within arteries and precise imaging of atherosclerotic plaques. The probe combines a tetraphenylethene building block with a hydrophilic peptide sequence (Cbz-Lys-Lys) and phenylboric acid module, providing excellent water solubility and fluorescence quenching in a molecular dispersion state. Upon interaction with H2O2 and CTB within plaques, the hydrophilic Cbz-Lys-Lys-TPEB probe is specifically cleaved and converted into hydrophobic AIEgens, leading to rapid aggregation and significant fluorescence enhancement. Interestingly, the in situ-liberated AIEgens display distinct lipid binding ability, effectively tracking the location and distribution of lipids in plaques. This synergistic target-activated AIEgen liberation strategy demonstrates significant feasibility for the reliable and accurate identification of atherosclerotic plaques, holding tremendous potential for clinical diagnosis and risk stratification of atherosclerosis.

A Novel Near-Infrared Tricyanofuran-Based Fluorophore Probe for Polarity Detection and LD Imaging

In this paper, LD-TCF, a targeting probe for lipid droplets (LDs) with a near-infrared emission wavelength and large Stokes shift, was fabricated for polarity detection by assembling a donor-π-acceptor (D-π-A) molecule with typical twisted intramolecular charge transfer (TICT) characteristics. Surprisingly, the fluorescence emission wavelength of the newly constructed probe LD-TCF was stretched to 703 nm, and the Stokes shift was amplified to 126 nm. Furthermore, LD-TCF could specifically answer the change in polarity efficiently and did not experience interference from other biologically active materials. Importantly, LD-TCF exhibited the ability to target lipid droplets, providing valuable insights for the early diagnosis and tracking of pathophysiological processes underlying LD polarity.

Flow-based bioconjugation of coumarin phosphatidylethanolamine probes: Optimised synthesis and membrane molecular dynamics studies

A series of phosphatidylethanolamine fluorescent probes head-labelled with 3-carboxycoumarin was prepared by an improved bioconjugation approach through continuous flow synthesis. The established procedure, supported by a design of experiment (DoE) set-up, resulted in a significant reduction in the reaction time compared to the conventional batch method, in addition to a minor yield increase. The characterization of these probes was enhanced by an in-depth molecular dynamics (MD) study of the behaviour of a representative probe of this family, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine labelled with 3-carboxycoumarin (POPE-COUM), in bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-stearoyl-2-linoleoyl-sn-glycero-3-phosphocholine (SLPC) 2:1, mimicking the composition of the egg yolk lecithin membranes recently used experimentally by our group to study POPE-COUM as a biomarker of the oxidation state and integrity of large unilamellar vesicles (LUVs). The MD simulations revealed that the coumarin group is oriented towards the bilayer interior, leading to a relatively internal location, in agreement with what is observed in the nitrobenzoxadiazole fluorophore of commercial head-labelled NBD-PE probes. This behaviour is consistent with the previously stated hypothesis that POPE-COUM is entirely located within the LUVs structure. Hence, the delay on the oxidation of the probe in the oxygen radical absorbance capacity (ORAC) assays performed is related with the inaccessibility of the probe until alteration of the LUV structure occurs. Furthermore, our simulations show that POPE-COUM exerts very little global and local perturbation on the host bilayer, as evaluated by key properties of the unlabelled lipids. Together, our findings establish PE-COUM as suitable fluorescent lipid analogue probes.

Dual-/multi-organelle-targeted AIE probes associated with oxidative stress for biomedical applications

In situ monitoring of biological processes between different organelles upon oxidative stress is one of the most important research hotspots. Fluorescence imaging is especially suitable for biomedical applications due to its distinct advantages of high spatiotemporal resolution, high sensitivity, non-invasiveness, and in situ monitoring capabilities. However, most fluorescent probes can only achieve light-up imaging of single organelles, thus the combined use of two or more probes is usually required for monitoring biological processes between organelles, which can suffer from tedious staining and washing procedures, increased cytotoxicity and poor photostability. Exogenetic oxidants can affect broad-spectrum subcellular organelles, which are not conducive to in situ monitoring of biological processes between specific organelles. To tackle these challenges, a series of dual-/multi-organelle-targeted aggregation-induced emission (AIE) probes associated with oxidative stress have been designed and developed in the past few years. Herein, the recent progress of these AIE probes is summarized in biomedical applications, such as apoptosis monitoring, interplay between organelles, microenvironmental changes of organelles, organelle morphology tracking, precise cancer therapy, and so forth. Moreover, the further outlook for dual-/multi-organelle-targeted AIE probes is discussed, aiming to promote innovative research in biomedical applications.

Fluorescent octahydrophenazines as novel inhibitors against herpes simplex viruses

A new series of racemic fluorescent octahydrophenazines (rac-PZ1-11) have been designed and synthesized via the efficient nucleophilic aromatic substitution (SNAr) of tetrafluorobenzenedinitriles (1a-c) and racemic cyclohexane-1,2-diamines (rac-2a and b). The bioactivities of these racemic rac-PZs (20 μM) against herpes simplex virus type-1 (HSV-1) were evaluated by the relative cell viability of Vero cells infected with HSV-1. It was found that rac-PZ3 shows much higher anti-HSV-1 activity than others, with EC50 = 9.2 ± 1.4 μM. Further investigation into the anti-HSV activities of rac-PZ3 and its enantiomers RR- and SS-PZ3 indicates that rac-PZ3 can also efficiently inhibit HSV-2 and even ACV-resistant HSV-2 (EC50 = 11.0 ± 2.3 and 14.9 ± 2.8 μM, respectively), SS-PZ3 has better activities against HSV-1, HSV-2 and ACV-resistant HSV-2 (EC50 = 4.1 ± 1.1, 5.8 ± 1.0 and 7.9 ± 1.2 μM, respectively), but RR-PZ3 has almost no antiviral activities. The primary mechanism study indicates that rac-PZ3 efficiently reverses the HSV-1/2-induced cytopathic effect and suppresses the expression of viral mRNA and proteins. In addition, rac-, RR- and SS-PZ3 possess excellent fluorescence properties with almost the same emission wavelength and high fluorescence quantum yields (ΦF = 90.3-92.3 % in cyclohexane solutions and 54.4-57.3 % in solids) and can target endoplasmic reticulum and cell membrane. The efficient anti-HSV bioactivities and excellent fluorescence of PZ3 prove its potential applications in antiviral therapy and biological imaging.

Selective delivery of imaging probes and therapeutics to the endoplasmic reticulum or Golgi apparatus: Current strategies and beyond

To maximize therapeutic effects and minimize unwanted effects, the interest in drug targeting to the endoplasmic reticulum (ER) or Golgi apparatus (GA) has been recently growing because two organelles are distributing hubs of cellular building/signaling components (e.g., proteins, lipids, Ca2+) to other organelles and the plasma membrane. Their structural or functional damages induce organelle stress (i.e., ER or GA stress), and their aggravation is strongly related to diseases (e.g., cancers, liver diseases, brain diseases). Many efforts have been developed to image (patho)physiological functions (e.g., oxidative stress, protein/lipid-related processing) and characteristics (e.g., pH, temperature, biothiols, reactive oxygen species) in the target organelles and to deliver drugs for organelle disruption using organelle-targeting moieties. Therefore, this review will overview the structure, (patho)physiological functions/characteristics, and related diseases of the organelles of interest. Future direction on ER or GA targeting will be discussed by understanding current strategies and investigations on targeting, imaging/sensing, and therapeutic systems.

Subcellular targeting strategies for protein and peptide delivery

Cytosolic delivery of proteins and peptides provides opportunities for effective disease treatment, as they can specifically modulate intracellular processes. However, most of protein-based therapeutics only have extracellular targets and are cell-membrane impermeable due to relatively large size and hydrophilicity. The use of organelle-targeting strategy offers great potential to overcome extracellular and cell membrane barriers, and enables localization of protein and peptide therapeutics in the organelles. Although progresses have been made in the recent years, organelle-targeted protein and peptide delivery is still challenging and under exploration. We reviewed recent advances in subcellular targeted delivery of proteins/peptides with a focus on targeting mechanisms and strategies, and highlight recent examples of active and passive organelle-specific protein and peptide delivery systems. This emerging platform could open a new avenue to develop more effective protein and peptide therapeutics.

Leveraging Chlorination-Based Mechanism for Resolving Subcellular Hypochlorous Acid

Hypochlorous acid (HOCl) is crucial for pathogen defense, but an imbalance in HOCl levels can lead to tissue damage and inflammation. Existing HOCl indicators employ an oxidation approach, which may not truly reveal the chlorinative stress environment. We designed a suite of indicators with a new chlorination-based mechanism, termed HOClSense dyes, to resolve HOCl in sub-cellular compartments. HOClSense dyes allow the visualization of HOCl with both switch-on and switch-off detection modes with diverse emission colors, as well as a unique redshift in emission. HOClSense features a minimalistic design with impressive sensing performance in terms of HOCl selectivity, and our design also facilitates functionalization through click chemistry for resolving subcellular HOCl. As a proof of concept, we targeted plasma membrane and lysosomes with HOClSense for subcellular HOCl mapping. With utilizing HOClSense, we discovered the STING pathway-induced HOCl production and the abnormal HOCl production in Niemann-Pick diseases. To the best of our knowledge, this is the first chlorination-based HOCl indicator series for resolving subcellular HOCl.

Organelle-Targeted Laurdans Measure Heterogeneity in Subcellular Membranes and Their Responses to Saturated Lipid Stress

Organelles feature characteristic lipid compositions that lead to differences in membrane properties. In cells, membrane ordering and fluidity are commonly measured using the solvatochromic dye Laurdan, whose fluorescence is sensitive to lipid packing. As a general lipophilic dye, Laurdan stains all hydrophobic environments in cells; therefore, it is challenging to characterize membrane properties in specific organelles or assess their responses to pharmacological treatments in intact cells. Here, we describe the synthesis and application of Laurdan-derived probes that read out the membrane packing of individual cellular organelles. The set of organelle-targeted Laurdans (OTL) localizes to the ER, mitochondria, lysosomes, and Golgi compartments with high specificity while retaining the spectral resolution needed to detect biological changes in membrane ordering. We show that ratiometric imaging with OTLs can resolve membrane heterogeneity within organelles as well as changes in lipid packing resulting from inhibition of trafficking or bioenergetic processes. We apply these probes to characterize organelle-specific responses to saturated lipid stress. While the ER and lysosomal membrane fluidity is sensitive to exogenous saturated fatty acids, that of mitochondrial membranes is protected. We then use differences in ER membrane fluidity to sort populations of cells based on their fatty acid diet, highlighting the ability of organelle-localized solvatochromic probes to distinguish between cells based on their metabolic state. These results expand the repertoire of targeted membrane probes and demonstrate their application in interrogating lipid dysregulation.

Push-Pull Fluorescent Dyes with Trifluoroacetyl Acceptor for High-Fidelity Sensing of Polarity and Heterogeneity of Lipid Droplets

Imaging and sensing of lipid droplets (LDs) attracted significant attention due to growing evidence for their important role in cell life. Solvatochromic dyes are promising tools to probe LDs' local polarity, but this analysis is biased by their non-negligible emission from intracellular membranes and capacity to emit from both the apolar core and polar interface of LDs. Here, we developed two push-pull solvatochromic dyes based on naphthalene and fluorene cores bearing an exceptionally strong electron acceptor, the trifluoroacetyl group. The latter was found to boost the optical properties of the dyes by shifting their absorption and emission to red and increasing their extinction coefficient, photostability, and sensitivity to solvent polarity (solvatochromism). In contrast to classical solvatochromic dyes, such as parent aldehydes and reference Nile Red, the new dyes exhibited strong fluorescence quenching by millimolar water concentrations in organic solvents. In live cells, the trifluoroacetyl dyes exhibited high specificity to LDs, whereas the parent aldehydes and Nile Red showed a detectable backgrounds from intracellular membranes. Experiments in model lipid membranes and nanoemulsion droplets confirmed the high selectivity of new probes to LDs in contrast to classical solvatochromic dyes. Moreover, the new probes were found to be selective to the LDs oil core, where they can sense lipid unsaturation and chain length. Their ratiometric imaging in cells revealed strong heterogeneity in polarity within LDs, which covered the range of polarities of unsaturated triglyceride oils, whereas Nile Red failed to properly estimate the local polarity of LDs. Finally, the probes revealed that LDs core polarity can be altered by fatty acid diets, which correlates with their chain length and unsaturation.

Dual-targeted fluorescent probe for tracking polarity and phase transition processes during lipophagy

Eukaryotic cells regulate various cellular processes through membrane-bound and membrane-less organelles, enabling active signal communication and material exchange. Lysosomes and lipid droplets are representative organelles, contributing to cell lipophagy when their interaction and metabolism are disrupted. Our limited understanding of the interacting behaviours and physicochemical properties of different organelles during lipophagy hinders accurate diagnosis and treatment of related diseases. In this contribution, we report a fluorescent probe, PTZ, engineered for dual-targeting of lipid droplets and lysosomes. PTZ can track liquid-liquid phase separation and respond to polarity shifts through ratiometric fluorescence emission, elucidating the lipophagy process from the perspective of organelle behavior and physicochemical properties. Leveraging on the multifunctionality of PTZ, we have successfully tracked the polarity and dynamic changes of lysosomes and lipid droplets during lipophagy. Furthermore, an unknown homogeneous transition of lipid droplets and lysosomes was discovered, which provided a new perspective for understanding lipophagy processes. And this work is expected to serve as a reference for diagnosis and treatment of lipophagy-related diseases.

Discovery of GPR84 Fluorogenic Probes Based on a Novel Antagonist for GPR84 Bioimaging

GPR84 is a promising therapeutic target and biomarker for a range of diseases. In this study, we reported the discovery of BINOL phosphate (BINOP) derivatives as GPR84 antagonists. By investigating the structure-activity relationship, we identified 15S as a novel GPR84 antagonist. 15S exhibits low nanomolar potency and high selectivity for GPR84, while its enantiomer 15R is less active. Next, we rationally designed and synthesized a series of GPR84 fluorogenic probes by conjugating Nile red and compound 15S. The leading hybrid, probe F8, not only retained GPR84 activity but also exhibited low nonspecific binding and a turn-on fluorescent signal in an apolar environment. F8 enabled visualization and detection of GPR84 in GPR84-overexpressing HEK293 cells and lipopolysaccharide-stimulated neutrophils. Furthermore, we demonstrated that F8 can detect upregulated GPR84 protein levels in mice models of inflammatory bowel disease and acute lung injury. Thus, compound F8 represents a promising tool for studying GPR84 functions.

Fluorescence Lifetime Imaging of Lipid Heterogeneity in the Inner Mitochondrial Membrane with a Super-photostable Environment-Sensitive Probe

The inner mitochondrial membrane (IMM) undergoes dynamic morphological changes, which are crucial for the maintenance of mitochondrial functions as well as cell survival. As the dynamics of the membrane are governed by its lipid components, a fluorescent probe that can sense spatiotemporal alterations in the lipid properties of the IMM over long periods of time is required to understand mitochondrial physiological functions in detail. Herein, we report a red-emissive IMM-labeling reagent with excellent photostability and sensitivity to its environment, which enables the visualization of the IMM ultrastructure using super-resolution microscopy as well as of the lipid heterogeneity based on the fluorescence lifetime at the single mitochondrion level. Combining the probe and fluorescence lifetime imaging microscopy (FLIM) showed that peroxidation of unsaturated lipids in the IMM by reactive oxygen species caused an increase in the membrane order, which took place prior to mitochondrial swelling.

Bora-Diaza-Indacene Based Fluorescent Probes for Simultaneous Visualisation of Lipid Droplets and Endoplasmic Reticulum

Organelle selective fluorescent probes, especially those capable of concurrent detection of specific organelles, are of benefit to the research community in delineating the interplay between various organelles and the impact of such interaction in maintaining cellular homeostasis and its disruption in the diseased state. Although very useful, such probes are synthetically challenging to design due to the stringent lipophilicity requirement posed by different organelles, and hence, the lack of such probes being reported so far. This work details the synthesis, photophysical properties, and cellular imaging studies of two bora-diaza-indacene based fluorescent probes that can specifically and simultaneously visualise lipid droplets and endoplasmic reticulum; two organelles suggested having close interactions and implicated in stress-induced cellular dysfunction and disease progression.

Fluorescent styrenes for mitochondrial imaging and viscosity sensing

Fluorophores bearing cationic pendants, such as the pyridinium group, tend to preferentially accumulate in mitochondria, whereas those with pentafluorophenyl groups display a distinct affinity for the endoplasmic reticulum. In this study, we designed fluorophores incorporating pyridinium and pentafluorophenyl pendants and examined their impact on sub-cellular localization. Remarkably, the fluorophores exhibited a notable propensity for the mitochondrial membrane. Furthermore, these fluorophores revealed dual functionality by facilitating the detection of viscosity changes within the sub-cellular environment and serving as heavy-atom-free photosensitizers. With easy chemical tunability, wash-free imaging, and a favorable signal-to-noise ratio, these fluorophores are valuable tools for imaging mitochondria and investigating their cellular processes.

Alared: Solvatochromic and Fluorogenic Red Amino Acid for Ratiometric Live-Cell Imaging of Bioactive Peptides

To fill the need for environmentally sensitive fluorescent unnatural amino acids able to operate in the red region of the spectrum, we have designed and synthesized Alared, a red solvatochromic and fluorogenic amino acid derived from the Nile Red chromophore. The new unnatural amino acid can be easily integrated into bioactive peptides using classical solid-phase peptide synthesis. The fluorescence quantum yield and the emission maximum of Alared-labeled peptides vary in a broad range depending on the peptide's environment, making Alared a powerful reporter of biomolecular interactions. Due to its red-shifted absorption and emission spectra, Alared-labeled peptides could be followed in living cells with minimal interference from cellular autofluorescence. Using ratiometric fluorescence microscopy, we were able to track the fate of the Alared-labeled peptide agonists of the apelin G protein-coupled receptor upon receptor activation and internalization. Due to its color-shifting environmentally sensitive emission, Alared allowed for distinguishing the fractions of peptides that are specifically bound to the receptor or unspecifically bound to different cellular membranes.

Chemical Control of Fluorescence Lifetime towards Multiplexing Imaging

Fluorescence lifetime imaging has been a powerful tool for biomedical research. Recently, fluorescence lifetime-based multiplexing imaging has expanded imaging channels by using probes that harbor the same spectral channels and distinct excited state lifetime. While it is desirable to control the excited state lifetime of any given fluorescent probes, the rational control of fluorescence lifetimes remains a challenge. Herein, we chose boron dipyrromethene (BODIPY) as a model system and provided chemical strategies to regulate the fluorescence lifetime of its derivatives with varying spectral features. We find electronegativity of structural substituents at the 8' and 5' positions is important to control the lifetime for the green-emitting and red-emitting BODIPY scaffolds. Mechanistically, such influences are exerted via the photo-induced electron transfer and the intramolecular charge transfer processes for the 8' and 5' positions of BODIPY, respectively. Based on these principles, we have generated a group of BODIPY probes that enable imaging experiments to separate multiple targets using fluorescence lifetime as a signal. In addition to BODIPY, we envision modulation of electronegativity of chemical substituents could serve as a feasible strategy to achieve rational control of fluorescence lifetime for a variety of small molecule fluorophores.

Ratiometric fluorescence nanoscopy and lifetime imaging of novel Nile Red analogs for analysis of membrane packing in living cells

Subcellular membranes have complex lipid and protein compositions, which give rise to organelle-specific membrane packing, fluidity, and permeability. Due to its exquisite solvent sensitivity, the lipophilic fluorescence dye Nile Red has been used extensively to study membrane packing and polarity. Further improvement of Nile Red can be achieved by introducing electron-donating or withdrawing functional groups. Here, we compare the potential of derivatives of Nile Red with such functional substitutions for super-resolution fluorescence microscopy of lipid packing in model membranes and living cells. All studied Nile Red derivatives exhibit cholesterol-dependent fluorescence changes in model membranes, as shown by spectrally resolved stimulated emission depletion (STED) microscopy. STED imaging of Nile Red probes in cells reveals lower membrane packing in fibroblasts from healthy subjects compared to those from patients suffering from Niemann Pick type C1 (NPC1) disease, a lysosomal storage disorder with accumulation of cholesterol and sphingolipids in late endosomes and lysosomes. We also find small but consistent changes in the fluorescence lifetime of the Nile Red derivatives in NPC1 cells, suggesting altered hydrogen-bonding capacity in their membranes. All Nile Red derivatives are essentially non-fluorescent in water but increase their brightness in membranes, allowing for their use in MINFLUX single molecule tracking experiments. Our study uncovers the potential of Nile Red probes with functional substitutions for nanoscopic membrane imaging.

Tracking Plasma Membrane Damage Using a Ruthenium(II) Complex Phosphorescent Indicator Paired with Cholesterol

Long-term in situ plasma membrane-targeted imaging is highly significant for investigating specific biological processes and functions, especially for the imaging and tracking of apoptosis processes of cells. However, currently developed membrane probes are rarely utilized to monitor the in situ damage of the plasma membrane. Herein, a transition-metal complex phosphorescent indicator, Ru-Chol, effectively paired with cholesterol, exhibits excellent properties on staining the plasma membrane, with excellent antipermeability, good photostability, large Stokes shift, and long luminescence lifetime. In addition, Ru-Chol not only has the potential to differentiate cancerous cells from normal cells but also tracks in real time the entire progression of cisplatin-induced plasma membrane damage and cell apoptosis. Therefore, Ru-Chol can serve as an efficient tool for the monitoring of morphological and physiological changes in the plasma membrane, providing assistance for drug screening and early diagnosis and treatment of diseases, such as immunodeficiency, diabetes, cirrhosis, and tumors.

Giant organelle vesicles to uncover intracellular membrane mechanics and plasticity

Tools for accessing and studying organelles remain underdeveloped. Here, we present a method by which giant organelle vesicles (GOVs) are generated by submitting cells to a hypotonic medium followed by plasma membrane breakage. By this means, GOVs ranging from 3 to over 10 µm become available for micromanipulation. GOVs are made from organelles such as the endoplasmic reticulum, endosomes, lysosomes and mitochondria, or in contact with one another such as giant mitochondria-associated ER membrane vesicles. We measure the mechanical properties of each organelle-derived GOV and find that they have distinct properties. In GOVs procured from Cos7 cells, for example, bending rigidities tend to increase from the endoplasmic reticulum to the plasma membrane. We also found that the mechanical properties of giant endoplasmic reticulum vesicles (GERVs) vary depending on their interactions with other organelles or the metabolic state of the cell. Lastly, we demonstrate GERVs' biochemical activity through their capacity to synthesize triglycerides and assemble lipid droplets. These findings underscore the potential of GOVs as valuable tools for studying the biophysics and biology of organelles.