Long-Term Dynamic Imaging of Cellular Processes Using an AIE Lipid Order Probe in the Dual-Color Mode

Intracellularly manipulable aggregation of the aggregation-induced emission luminogens

Biophotonics has seen significant advancements with the development of optical imaging techniques facilitating the noninvasive detection of biologically relevant species. Aggregation-induced emission (AIE) materials have emerged as a novel class of luminogens exhibiting enhanced luminescence or photodynamic efficiency in the aggregated state, making them ideal for biomedical applications. The intracellularly controlled aggregation of aggregate-induced emission luminogens (AIEgens) enables high-resolution imaging of intracellular targets and diagnosis of related diseases, and enables disease therapy by exploiting the novel properties of aggregates. This review provides an in-depth analysis of the strategies employed to modulate the aggregation of AIEgens, focusing on the importance of molecular modifications to improve hydrophilicity and achieve precise control over the intercellular aggregation of AIEgens. Furthermore, the representative applications of AIEgens in bioimaging, such as enzyme activity monitoring, protein tracking, organelle function monitoring, and in vivo tumor-specific therapeutics, are reviewed. Additionally, we outline the challenges and future opportunities for AIE research, emphasizing the importance of the strategies for realizing the precisely controllable aggregation of AIEgens inside cells and the need for extending AIEgens' absorption and emission wavelengths. This review aims to elucidate the rational development of responsive AIEgens for advanced biomedical applications.

Molecular engineering of fluorescent dyes for long-term specific visualization of the plasma membrane based on alkyl-chain-regulated cell permeability

Long-term visualization of changes in plasma membrane dynamics during important physiological processes can provide intuitive and reliable information in a 4D mode. However, molecular tools that can visualize plasma membranes over extended periods are lacking due to the absence of effective design rules that can specifically track plasma membrane fluorescent dye molecules over time. Using plant plasma membranes as a model, we systematically investigated the effects of different alkyl chain lengths of FMR dye molecules on their performance in imaging plasma membranes. Our findings indicate that alkyl chain length can effectively regulate the permeability of dye molecules across plasma membranes. The study confirms that introducing medium-length alkyl chains improves the ability of dye molecules to target and anchor to plasma membranes, allowing for long-term imaging of plasma membranes. This provides useful design rules for creating dye molecules that enable long-term visualization of plasma membranes. Using the amphiphilic amino-styryl-pyridine fluorescent skeleton, we discovered that the inclusion of short alkyl chains facilitated rapid crossing of the plasma membrane by the dye molecules, resulting in staining of the cell nucleus and indicating improved cell permeability. Conversely, the inclusion of long alkyl chains hindered the crossing of the cell wall by the dye molecules, preventing staining of the cell membrane and demonstrating membrane impermeability to plant cells. The FMR dyes with medium-length alkyl chains rapidly crossed the cell wall, uniformly stained the cell membrane, and anchored to it for a long period without being transmembrane. This allowed for visualization and tracking of the morphological dynamics of the cell plasma membrane during water loss in a 4D mode. This suggests that the introduction of medium-length alkyl chains into amphiphilic fluorescent dyes can transform them from membrane-permeable fluorescent dyes to membrane-staining fluorescent dyes suitable for long-term imaging of the plasma membrane. In addition, we have successfully converted a membrane-impermeable fluorescent dye molecule into a membrane-staining fluorescent dye by introducing medium-length alkyl chains into the molecule. This molecular engineering of dye molecules with alkyl chains to regulate cell permeability provides a simple and effective design rule for long-term visualization of the plasma membrane, and a convenient and feasible means of chemical modification for efficient transmembrane transport of small molecule drugs.

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.

Charge-regulated fluorescent anchors enable high-fidelity tracking of plasma membrane dynamics during biological events

Many biological processes generally require long-term visualization tools for time-scale dynamic changes of the plasma membrane, but there is still a lack of design rules for such imaging tools based on small-molecule fluorescent probes. Herein, we revealed the key regulatory roles of charge number and species of fluorescent dyes in the anchoring ability of the plasma membrane and found that the introduction of multi-charged units and appropriate charge species is often required for fluorescent dyes with strong plasma membrane anchoring ability by systematically investigating the structure-function relationship of cyanostyrylpyridium (CSP) dyes with different charge numbers and species and their imaging performance for the plasma membrane. The CSP-DBO dye constructed exhibits strong plasma membrane anchoring ability in staining the plasma membrane of cells, in addition to many other advantages such as excellent biocompatibility and general universality of cell types. Such a fluorescent anchor has been successfully used to monitor chemically induced plasma membrane damage and dynamically track various cellular biological events such as cell fusion and cytokinesis over a long period of time by continuously monitoring the dynamic morphological changes of the plasma membrane, providing a valuable precise visualization tool to study the physiological response to chemical stimuli and reveal the structural morphological changes and functions of the plasma membrane during these important biological events from a dynamic perspective. Furthermore, CSP-DBO exhibits excellent biocompatibility and imaging capability in vivo such as labelling the plasma membrane in vivo and monitoring the metabolic process of lipofuscin as an aging indicator.

An Aggregation-induced Emission Probe to Detect the Viscosity Change in Lipid Droplets during Ferroptosis

Ferroptosis is a recently identified form of cell death characterized by iron-dependent lipid peroxidation. Understanding the effects of lipid peroxidation on cellular processes during ferroptosis requires insights into lipid droplets (LDs) and their viscosity changes. To gain further insights into the intricacies of ferroptosis, it is crucial to have a fluorescent probe that targets LDs and responds to changes in viscosity. In this study, we introduce a novel LD-targeting viscosity fluorescent probe named TQE, based on the principles of aggregation-induced emission (AIE). The probe displayed AIE characteristics in tetrahydrofuran, possessing a partition coefficient (logP) of 5.87. With increased viscosity, intramolecular rotation was restricted, leading to a remarkable 3.3-fold enhancement in emission. Notably, TQE exhibited robust resistance to photo-bleaching during cellular imaging, maintaining approximately 75% of its emission intensity even after 30 min of laser irradiation. Importantly, the AIEgen could not generate hydroxyl radicals when exposed to light for up to 3 h, suggesting the low photo-toxicity of TQE to cells. Leveraging these properties, we successfully employed the probe for fluorescent imaging of the viscosity change in LDs during ferroptosis.

A mitochondria-targeted far-red AIE fluorescent probe for distinguishing between mitophagy and ferroptosis in cancer cells

A mitochondria-targeted far-red fluorescent probe LY-1 with AIE character was formulated to track cell viscosity alterations with excellent sensitivity and selectivity, which was used to discriminate between mitophagy and ferroptosis in cancer cells. Probe LY-1 is expected to be an effective vehicle for the diagnosis of mitochondrial viscosity relevant diseases.

Aggregation-Induced Emission (AIE), Life and Health

Light has profoundly impacted modern medicine and healthcare, with numerous luminescent agents and imaging techniques currently being used to assess health and treat diseases. As an emerging concept in luminescence, aggregation-induced emission (AIE) has shown great potential in biological applications due to its advantages in terms of brightness, biocompatibility, photostability, and positive correlation with concentration. This review provides a comprehensive summary of AIE luminogens applied in imaging of biological structure and dynamic physiological processes, disease diagnosis and treatment, and detection and monitoring of specific analytes, followed by representative works. Discussions on critical issues and perspectives on future directions are also included. This review aims to stimulate the interest of researchers from different fields, including chemistry, biology, materials science, medicine, etc., thus promoting the development of AIE in the fields of life and health.

Fluorescent probes for lighting up ferroptotic cell death: A review

Ferroptosis is a newly discovered form of regulated cellular demise, characterized by the accumulation of intracellular oxidative stress that is dependent on iron. Ferroptosis plays a crucial role not only in the development and treatment of tumors but also in the pathogenesis of neurodegenerative diseases and illnesses related to ischemia-reperfusion injury. This mode of cell death possesses distinctive properties that differentiate it from other forms of cell death, including unique morphological changes at both the cellular and subcellular levels, as well as molecular features that can be detected using specific methods. The use of fluorescent probes has become an invaluable means of detecting ferroptosis, owing to their high sensitivity, real-time in situ monitoring capabilities, and minimal damage to biological samples. This review comprehensively elucidates the physiological mechanisms underlying ferroptosis, while also detailing the development of fluorescent probes capable of detecting ferroptosis-related active species across various cellular compartments, including organelles, the nucleus, and the cell membrane. Additionally, the review explores how the dynamic changes and location of active species from different cellular compartments can influence the ignition and execution of ferroptotic cell death. Finally, we discuss the future challenges and opportunities for imaging ferroptosis. We believe that this review will not only aid in the elucidation of ferroptosis's physiological mechanisms but also facilitate the identification of novel treatment targets and means of accurately diagnosing and treating ferroptosis-related diseases.

Recent advances in small-molecule fluorescent probes for studying ferroptosis

Ferroptosis is an iron-dependent, non-apoptotic form of programmed cell death driven by excessive lipid peroxidation (LPO). Mounting evidence suggests that the unique modality of cell death is involved in the development and progression of several diseases including cancer, cardiovascular diseases (CVDs), neurodegenerative disorders, etc. However, the pathogenesis and signalling pathways of ferroptosis are not fully understood, possibly due to the lack of robust tools for the highly selective and sensitive imaging of ferroptosis analytes in complex living systems. Up to now, various small-molecule fluorescent probes have been applied as promising chemosensors for studying ferroptosis through tracking the biomolecules or microenvironment-related parameters in vitro and in vivo. In this review, we comprehensively reviewed the recent development of small-molecule fluorescent probes for studying ferroptosis, with a focus on the analytes, design strategies and bioimaging applications. We also provided new insights to overcome the major challenges in this emerging field.

Polarity-Sensitive Cell Membrane Probe Reveals Lower Polarity of Tumor Cell Membrane and Its Application for Tumor Diagnosis

Cancer is a health threat worldwide, and it is urgent to develop more sensitive cancer detection methods. Herein, a polarity-sensitive cell membrane probe (named COP) was developed for detecting cancer cells and tumors sensitively and selectively at the cell membrane level. The probe shows a strong polarity-dependent fluorescence and excellent cell membrane targeting ability to visualize cell membrane with red fluorescence with a non-washing process. Notably, COP can selectively light up the tumor cell membranes, which reveals that cancer cell membranes have lower polarity than normal cell membranes. The giant unilamellar vesicle model and cell imaging studies proved this. Moreover, COP can effectively and selectively light up tumors. Overall, this work demonstrates that the polarity of the tumor cell membrane is quite different to normal cell membranes, and based on this, sensitive membrane probes can be developed to selectively visualize cancer cells and tumors, which opens up a new way for tumor diagnosis at the cellular level.

Polarity-Sensitive Probe: Dual-Channel Visualization of the "Chameleon" Migration with the Assistance of Reactive Oxygen Species

The real-time and differentiated visualization of the organelles is favorable for exploring the distribution and interaction. However, most visual probes emit monochromatic fluorescence and target a single organelle, which impedes the in-depth study of their interplay. To overcome this obstacle, we tactfully conceived a polarity-sensitive fluorescent DPDO-C that could accurately discriminate polarity changes in the cellular environment, exhibiting distinct fluorescence in lipid droplets (LDs) and mitochondria. Remarkably, the probe DPDO-C could migrate from mitochondria to LDs with the assistance of reactive oxygen species, which was conducive to further monitoring of the number and size of LDs as well as the interactions between LDs and other organelles. Moreover, the nuanced difference between normal and fatty liver tissues was also distinguished by two-color fluorescence imaging, which could act as a promising candidate for the early diagnosis of fatty liver.

Aggregation-Induced Emission Luminogens for Cell Death Research

Cell death is closely related to various diseases, and monitoring and controlling cell death is a promising strategy to develop efficient therapy. Aggregation-induced emission luminogens (AIEgens) are ideal candidates for developing novel theranostic agents because of their intriguing properties in the aggregate state. The rational application of AIE materials in cell death-related research is still in its infancy but has shown great clinical potential. This review discussed the research frontier and our understanding of AIE materials in various subroutines of cell death, including apoptosis, necrosis, immunogenic cell death, pyroptosis, autophagy, lysosome-dependent cell death, and ferroptosis. We hope that the new insights can be offered to this growing field and attract more researchers to provide valuable contributions.

Antipermeability Strategy to Achieve Extremely High Specificity and Ultralong Imaging of Diverse Cell Membranes Based on Restriction-Induced Emission of AIEgens

Long-term in situ cell membrane-targeted bioimaging is of great significance for studying specific biological processes and functions, but currently developed membrane probes are rarely simultaneously used to image the plasma membrane of animal and plant cells, and these probes lack sufficiently high long-term targeting ability. Herein, we proposed an antipermeability strategy to achieve highly specific and long-term imaging of plasma membranes of both human and plant cells using the steric hindrance effect and restriction-induced emission of AIE-active probes based on an updated membrane model. A certain degree of rigidity of plasma membrane containing a large ratio of rigid cholesterol molecules in the updated membrane model provides a promising opportunity to design antipermeable probes by introducing a rigid steric hindrance group in the probe. The designed antipermeable probes can anchor inside plasma membrane for a long term relying on the combination of the steric hindrance effect and the electrostatic and hydrophobic interactions between the probe and the membrane, as well as light up the membrane via the restriction-induced emission mechanism. The excellent performance in imaging completeness and specificity for both human cells and plant cells clearly shows that these designed probes possess outstanding antipermeability to achieve long-term specific imaging of membrane. These probes also show some advanced features such as ultrafast staining, wash-free merit, favorable biocompatibility, good photostability, and effective resistance to viscosity and pH alteration. This work also provides a valuable design principle for membrane probes of plant cells that the designed probes require a suitable molecular size favoring the penetration of small pores of cell walls.

HydroFlipper membrane tension probes: imaging membrane hydration and mechanical compression simultaneously in living cells

HydroFlippers are introduced as the first fluorescent membrane tension probes that report simultaneously on membrane compression and hydration. The probe design is centered around a sensing cycle that couples the mechanical planarization of twisted push–pull fluorophores with the dynamic covalent hydration of their exocyclic acceptor. In FLIM images of living cells, tension-induced deplanarization is reported as a decrease in fluorescence lifetime of the dehydrated mechanophore. Membrane hydration is reported as the ratio of the photon counts associated to the hydrated and dehydrated mechanophores in reconvoluted lifetime frequency histograms. Trends for tension-induced decompression and hydration of cellular membranes of interest (MOIs) covering plasma membrane, lysosomes, mitochondria, ER, and Golgi are found not to be the same. Tension-induced changes in mechanical compression are rather independent of the nature of the MOI, while the responsiveness to changes in hydration are highly dependent on the intrinsic order of the MOI. These results confirm the mechanical planarization of push–pull probes in the ground state as most robust mechanism to routinely image membrane tension in living cells, while the availability of simultaneous information on membrane hydration will open new perspectives in mechanobiology.

HydroFlippers respond to membrane compression and hydration in the same fluorescence lifetime imaging microscopy histogram: the responses do not correlate.

Novel Pyrazine-Bridged D-A-D Type Charge Neutral Probe for Membrane Permeable Long-Term Live Cell Imaging

A novel donor-acceptor-donor (D-A-D) type compound containing pyrazine as the acceptor and triphenylamine as the donor has been designed and synthesized. The photophysical properties and biocompatibility of this probe, namely (OMeTPA)2-Pyr for live cell imaging were systematically investigated, with observed large Stokes shifts, high photostability, and low cytotoxicity. Furthermore, we demonstrated that (OMeTPA)2-Pyr could permeate live cell membranes for labeling. The proposed mechanism of this probe was the binding and shafting through membrane integral transport proteins by electrostatic and hydrophobic interactions. These salient and novel findings can facilitate the strategic design of new pyrazine-fused charge-neutral molecular platforms as fluorescent probes, for long-term in situ dynamic monitoring in live cells.