Evaluation of intracellular lipid droplets viscosity by a probe with high fluorescence quantum yield

A non-solvatochromic fluorescent probe for imaging of lipid droplets in live cells and tissues

Lipid droplets (LDs) have recently attracted considerable attention owing to their crucial roles in both biological processes and disease pathogenesis. Visualization of LDs is fundamental for elucidating their roles in biological mechanisms and facilitating the early detection of diseases. Donor-acceptor (D-A) typed fluorescent probes have been extensively designed and utilized for the detection of LDs. However, such probes often exhibit a pronounced solvatochromic effect, leading to several limitations in detecting LDs, such as short excitation/emission wavelength, low specificity. Herein, we reported a non-solvatochromic D-A typed fluorescent probe S7 for LDs imaging in live cells and in vivo. S7 is polarity-dependent, which exhibits a very weak fluorescence in high-polar solvents owing to the photoinduced electron transfer (PET) mechanism but intense fluorescence in low-polarity environments without undergoing a solvatochromic blue shift. Except polarity, the fluorescent signal of S7 remains unaffected by factors such as viscosity, pH, ions, reactive oxygen species, reactive sulfur species, nucleic acids, proteins, and other biological molecules, allowing it to selectively light up LDs in live cells. Furthermore, this probe S7 exhibits an enhanced fluorescence intensity in tumor tissue when compared to normal tissue. This characteristic potentially provides an efficient and straightforward approach for tumor diagnosis.

Aggregation-induced emission-twisted intramolecular charge transfer-activated fluorescent probe for analyzing mitochondrial viscosity in cells and zebrafish

Mitochondria are crucial energy-supplying organelles that support cellular activities and play vital roles in cell metabolism, aging, autophagy, and apoptosis. Abnormal viscosity can alter the mitochondrial microenvironment, disrupt normal mitochondrial function, and lead to disease. To address this, we designed and developed two aggregation-induced emission-twisted intramolecular charge transfer fluorescent probes, namely, (E)-1,1,3-trimethyl-2-(4-(1,2,2-triphenylvinyl)styryl)-1H-benzo[e]indol-3-ium (HSL-1) and (E)-2-(4-(di-p-tolylamino)styryl)-1,3,3-trimethyl-1H-benzo[e]indol-3-ium (HSL-2). In vitro fluorescence detection revealed that both HSL-1 and HSL-2 were sensitive to viscosity and demonstrated a strong log-linear relationship, with linear coefficients of 0.982 and 0.980, respectively. Notably, the responses of HSL-1 and HSL-2 to viscosity changes were unaffected by pH, polarity, or interfering ions. HSL-1 exhibited stronger resistance to background interference than HSL-2 and significantly enhanced fluorescence intensity; thus, it was selected for cell experiments and animal fluorescence intensity assessments. Furthermore, HSL-1 showed excellent biocompatibility, enabling real-time detection of mitochondrial viscosity changes and identification of viscosity abnormalities triggered by mitophagy in HeLa cells. It could also monitor changes in mitochondrial viscosity in zebrafish. In conclusion, HSL-1 is a valuable tool for studying viscosity and understanding diseases associated with abnormal mitochondrial viscosity.

Twisted Molecular Core Conjugated Oxo-Ether as a Fluorescent Probe for Lipid-Droplets Bioimaging and Live Cancer Cell Discrimination

In quest of a new working design for a photostable lipid-droplets (LDs) bioimaging probe, we herein unveil and demonstrate a twisted donor(naphthalene)-π-acceptor(dicyano) architecture linked with oxo/thioether functionality, where the probes' emission, hydrophobicity, cytotoxicity, and cell permeability are altered by replacing the present chalcogen/s. In this class of molecules, an "oxanthrene"-based compound, "OXNCN", was realized as the noncytotoxic and cell-permeable probe, displaying intense fluorescence in a nonpolar solvent, aggregates, and viscous medium. Time-dependent density functional theory (TD-DFT) investigations revealed that OXNCN holds a favorable extent of excited-state planarity to bring out considerable emission only in a nonpolar solvent, resulting in polarity-dependent emission. Outcomes of the concentration- and time-dependent colocalization investigations, cholesterol depletion/repletion studies, and oleic acid treatment-based experiments validated its LD specificity. Strong twisted intramolecular charge transfer (TICT) culminated in weak emission in the polar medium, which helped the probe reduce the cytoplasmic signal. Moreover, the results of time-dependent kinetic acquisitional photophysical studies, fluorescence recovery after photobleaching (FRAP), and intracellular emission investigations testified to the probe's photostability. Assiduous analysis and quantification of confocal laser scanning microscopy (CLSM) images by two-way analysis of variance (ANOVA), followed by Sidak's multiple comparison statistics, could provide insights into the probe's better performance in robust cancer cells (FaDu) than in normal ones (HEK-293). A precise discrimination between oral and normal cancer cells could be established by quantifying the deposited lipid droplets from the CLSM-captured cellular images and applying Student's t test with the quantified values.

Mitochondria-targeted bifunctional probes for monitoring SO2 and viscosity in diverse environments

Sulfur dioxide (SO2) and mitochondrial viscosity are critical indicators of both environmental and physiological health. However, simultaneous detection of these parameters remains challenging due to limitations in probe sensitivity and spectral overlap. In this study, we report the design and synthesis of two mitochondrial-targeted fluorescent probes, HVTI and HVTB, capable of simultaneously detecting SO2 derivatives and viscosity both in vitro and in vivo. These bifunctional probes utilize the Michael addition reaction and the twisted intramolecular charge transfer (TICT) mechanism to achieve high selectivity and sensitivity. Upon reacting with SO2 derivatives, the probes display a pronounced fluorescence shift from red to blue-specifically from 614 nm to 456 nm for HVTI and from 642 nm to 463 nm for HVTB. In addition, increased viscosity results in a significant enhancement of red fluorescence intensity. eliminate spectral crosstalk and enable colorimetric detection discernible to the naked eye. Using advanced spectroscopic methods, the probes demonstrated robust performance in real water samples, achieving recovery rates of 94.29-104.30 % and detection limits as low as 0.25 µM. Moreover, imaging studies in RAW 264.7 cells and zebrafish validated their ability to visualize SO2 and viscosity changes in vivo, with excellent biocompatibility and low cytotoxicity. The probes exhibit exceptional functionality across diverse environments, offering unprecedented precision in monitoring cellular microenvironments and aquatic ecosystems. This study represents a significant advancement in fluorescence-based detection, positioning HVTI and HVTB as versatile tools for both environmental analysis and biomedical research.

Dual-locked NIR fluorescent probe for detection of GSH and lipid droplets and its bioimaging application in cancer model

Fluorescence probes with outstanding merits have wide applications in tumor diagnosis. However, most of these probes can only detect single tumor biomarker, potentially generating "false positive" signals within intricate biological systems. In contrast, the dual-locked fluorescent probes triggered by two response factors can effectively address the aforementioned limitations. In this work, we fabricated a novel coumarin-based NIR fluorescent probe (CP-GSH), demonstrating dual-responsiveness to high glutathione (GSH) concentrations and high viscosity. Specifically, the probe showed strong fluorescence enhancement at 675 nm ∼ 725 nm in the simultaneous presence of GSH and high viscosity, whereas the presence of either GSH or high viscosity alone could not induce a noticeable change in fluorescence intensity of CP-GSH. More importantly, the bioimaging experiments further validated CP-GSH triggered by endogenous GSH possessed excellent targeting capability towards lipid droplets (LDs), which could be utilized to effective discriminate between cancer cells and normal cells. This work proposes a promising strategy for the design of dual-locked probe for tumor imaging.

Enhanced Near-Infrared Fluorescence Emission near a Graphene-Metal Hybrid Structure

Plasmon resonance plays an important role in improving the detection of biomolecules, and it is one of the focuses of research to use metal plasmon resonance to achieve fluorescence enhancement and to improve detection sensitivity. However, the problems of nondynamic tuning and fluorescence quenching of metal plasmon resonance need to be solved. Graphene surface plasmon resonance can be dynamically controlled, and the graphene adsorption of fluorescent molecules can avoid fluorescence quenching and greatly improve the fluorescence emission intensity. The graphene-metal hybrid structure designed in this work can solve the above two problems well, and the plasmon resonance can improve the fluorescence emission efficiency of molecules on the surface of graphene and improve the sensitivity of biological detection. At the same time, graphene nanoribbons in our hybrid structure do not require patterning, which greatly lowers the threshold for graphene application in biosensing.

Dual-responsive two-photon probe for specific lipid droplets near-infrared fluorescence imaging in the brain of epileptic mice

Abnormal lipid metabolism in glial cells is a key pathological feature of epilepsy. The identification of lipid droplets (LDs) is essential for investigating lipid metabolism, disease progression, and potential therapeutic interventions. Two-photon imaging technology enables real-time visualization of the spatial distribution and temporal dynamics of LDs in epilepsy models. In this study, we developed a novel two-photon excited dual-responsive near-infrared fluorescent probe, CabA, based on viscosity and polarity, to monitor dynamic changes in LDs. The fluorescence of CabA at 670 nm exhibits a significant increase in response to low polarity and high viscosity due to the twisted intramolecular charge transfer and intramolecular charge transfer mechanisms. The LDs-targeting capability of CabA at the cellular level and the process of LDs generation between neurons and astrocytes during the pathological advancement of epilepsy have been validated. In situ synchronous imaging experiments in epileptic and normal mice using CabA revealed abnormal LDs accumulation in the brain during seizures. Two-photon fluorescence imaging further demonstrated LDs accumulation in the brains of epileptic mice at a penetration depth of 100 μm. This study offers a valuable tool for enhancing the understanding of LDs in physiological and pathological processes, potentially aiding in the early diagnosis of epilepsy.

A novel fluorescent probe for visualizing viscosity changes in lipid droplets during chemotherapy-induced ferroptosis

BACKGROUND

Ferroptosis, as a novel form of cell death, is becoming one of the hot topics in cancer treatment research. It differs from necrosis and autophagy in that it involves the accumulation of lipid peroxides and is triggered by iron dependency. Recent studies have suggested that this mechanism may alter the viscosity or structure of lipid droplets (LDs). The relationship between LDs viscosity and ferroptosis remains an active area of research with limited reports at present. Additionally, there is a lack of effective anticancer drugs targeting the ferroptosis pathway to promote ferroptosis in tumour cells. Therefore, the development of tools to detect viscosity changes during ferroptosis and targeted therapeutic strategies is of great significance.

RESULTS

By coupling 1,3-indandione with naphthalimide, including decamethylamine as a LDs recognition group, we designed and synthesized an environmental fluorescent probe that induces intramolecular charge transfer (TICT) effects. Notably, the diffusion and transport of intracellular substances may be affected in highly viscous environments. Under such conditions, intracellular iron ions may accumulate, leading to peroxide production and cellular damage, which can trigger ferroptosis. Therefore, WD-1 achieved excellent in situ bioimaging of LDs targeting and its viscosity during ferroptosis in HeLa cells and zebrafish. Furthermore, it was observed that WD-1 effectively differentiated between malignant and normal cells during this process, highlighting its potential significance in distinguishing cellular states. In addition, we used the antitumour drug paclitaxel to study ferroptosis in cancer cells. These findings not only provide an excellent tool for the development of the ferroptosis response, but also are crucial for understanding the biological properties of LDs during the ferroptosis response.

SIGNIFICANCE AND NOVELTY

Based on a powerful tool of fluorescent probe with in vivo bioimaging, we developed WD-1 to track the impact of paclitaxel on the process of ferroptosis in living cells. Therefore, we preliminarily believe that paclitaxel may affect the occurrence of ferroptosis and control apoptosis in cancer cells. These findings not only serve as an exceptional tool for advancing our understanding of the ferroptosis response, but furthermore play a vital role in comprehending the biological characteristics of LDs in relation to ferroptosis.

Unraveling Ferroptosis Mechanisms: Tracking Cellular Viscosity with Small Molecular Fluorescent Probes

Ferroptosis is a recently identified form of regulated cell death characterized by iron accumulation and lipid peroxidation. Numerous functions for ferroptosis have been identified in physiological as well as pathological processes, most notably in the treatment of cancer. The intricate balance of redox homeostasis is profoundly altered during ferroptosis, leading to alteration in cellular microenvironment. One such microenvironment is viscosity among others such as pH, polarity, and temperature. Therefore, understanding the dynamics of ferroptosis associated viscosity levels within organelles is crucial. To date, there are a very few reviews that detects ferroptosis assessing reactive species. In this review, we have summarized organelle's specific fluorescent probes that detects dynamics of microviscosity during ferroptosis. Also, we offer the readers an insight of their design strategy, photophysics and associated bioimaging concluding with the future perspective and challenges in the related field.

Promoting In Vivo NIR-II Fluorescent Imaging for Lipid in Lipid Metabolism Diseases Diagnosis

Lipid metabolism diseases have become a tremendous risk worldwide, along with the development of productivity and particular attention to public health. It has been an urgent necessity to exploit reliable imaging strategies for lipids and thus to monitor fatty liver diseases. Herein, by converting the NIR-I signal to the NIR-II signal with IR1061 for the monitoring of lipid, the in vivo imaging of fatty liver disease was promoted on the contrast and visual effect. The main advantages of the imaging promotion in this work included a long emission wavelength, rapid response, and high signal-background-ratio (SBR) value. After promoting the NIR-I signal to NIR-II signal, IR1061 achieved higher SBR value and exhibited a dose-dependent fluorescence intensity at 1100 nm along with the increase of the EtOH proportion as well as steady and selective optical responses toward liposomes. IR1061 was further applied in the in vivo imaging of lipid in fatty liver diseases. In spite of the differences in body weight gain and TC level between healthy mice and fatty liver diseases two models, IR1061 achieved high-resolution imaging in the liver region to monitor the fatty liver disease status. This work might be informatic for the clinical diagnosis and therapeutical treatments of fatty liver diseases.

A wash-free fluorescent probe with a large Stokes shift for the identification of NAFL through tracing the change of lipid droplets

As one of the important organelles in cells, lipid droplets (LDs) are involved in various physiological processes, especially affecting the occurrence and progression of non-alcoholic fatty liver (NAFL). Therefore, it is of great significance to develop LD-specific probes with excellent biocompatibility, deep penetration and bright fluorescence. Herein, a fluorescent probe LD-HWZ was designed and synthesized based on triphenylamine and the dicyanoisophorone group. It is found that probe LD-HWZ has a large Stokes shift (Δλ = 160 nm in DMSO) and exhibits bright fluorescence in a lipid environment. In addition, biological experiments showed that LD-HWZ can localize in lipid droplets, which can be used to detect the dynamic changes of LDs. Importantly, LD-HWZ has been successfully used to discriminate NAFL tissues from normal livers. The excellent properties of probe LD-HWZ in this work are expected to shed new light on the design of lipid droplet probes for the study of fatty liver diagnosis.