Unique pH-Sensitive RNA Binder for Ratiometric Visualization of Cell Apoptosis

Unique fluorescent probe for the recognition of late apoptosis via translocation from plasma membrane to nucleus

Cell apoptosis is a crucial physiological process playing central roles in key biological and pathological activities. However, the current fluorescent probes for the detection of late apoptosis were "off-on" probes, which were facilely interfered by false positive signals caused by inhomogeneous staining and other factors. Herein, a unique fluorescent probe (NPn) discriminating late apoptosis from early apoptosis and heathy status with two different sets of fluorescent signals have been prepared, to overcome the possible false positive signals. NPn was designed impermeable to biomembranes and simultaneously with high affinity to DNA/RNA, which localized on the plasma membranes of living and early apoptotic cells, while relocated to the nucleus in late apoptotic cells. The hydrophilic amine unit and small ion radius were responsive for its membrane impermeability, which was confirmed with two control molecules without amine group. Using the probe, we have successfully evaluated the cell apoptosis induced by ultraviolet irradiation, rotenone, colchicine, and paclitaxel, demonstrating its potential application in biological researches.

Single Fluorescent Probe for Multiple Tasks: Illuminating Lipid Droplets and Lysosomes in Dual Channels and Distinguishing Autophagy and Apoptosis

Lipid droplets (LDs) and lysosomes play key roles in autophagy and cell apoptosis, and the discriminative visualization of the two organelles and simultaneously of autophagy and apoptosis is very helpful to understand their internal relationships. However, fluorescent probes that can concurrently achieve these tasks are not available currently. Herein, we delicately fabricate a robust probe CAQ2 for multiple tasks: illumination of LDs and lysosomes in dual emission colors as well as discriminative visualization of cell apoptosis and autophagy. The probe exhibited both lipophilic and basic properties and displayed different emission colors in neutral and protonated forms; thus, LDs and lysosomes emitted blue and red fluorescence colors, respectively. Because of the lysosomal acidification during autophagy, CAQ2 detected autophagy with evidently enhanced red emission. Because of the lysosomal alkalization during apoptosis, CAQ2 imaged apoptosis with a drastically decreased red fluorescence intensity. With the robust probe, the autophagy under starvation and lipidless conditions was visualized, and the apoptosis induced by H2O2, ultraviolet (UV) irradiation, and rotenone treatment was successfully observed. The efficient detoxification of Na2S against rotenone treatment was successfully revealed.

Conjugated Oligoelectrolyte with DNA Affinity for Enhanced Nuclear Imaging and Precise DNA Quantification

Precise DNA quantification and nuclear imaging are pivotal for clinical testing, pathological diagnosis, and drug development. The detection and localization of mitochondrial DNA serve as crucial indicators of cellular health. We introduce a novel conjugated oligoelectrolyte (COE) molecule, COE-S3, featuring a planar backbone composed of three benzene rings and terminal side chains. This unique amphiphilic structure endows COE-S3 with exceptional water solubility, a high quantum yield of 0.79, and a significant fluorescence Stokes shift (λex = 366 nm, λem = 476 nm), alongside a specific fluorescence response to DNA. The fluorescence intensity correlates proportionally with DNA concentration. COE-S3 interacts with double-stranded DNA (dsDNA) through an intercalation binding mode, exhibiting a binding constant (K) of 1.32 × 106 M-1. Its amphiphilic nature and strong DNA affinity facilitate its localization within mitochondria in living cells and nuclei in apoptotic cells. Remarkably, within 30 min of COE-S3 staining, cell vitality can be discerned through real-time nuclear fluorescence imaging of apoptotic cells. COE-S3's high DNA selectivity enables quantitative intracellular DNA analysis, providing insights into cell proliferation, differentiation, and growth. Our findings underscore COE-S3, with its strategically designed, shortened planar backbone, as a promising intercalative probe for DNA quantification and nuclear imaging.

Harnessing single fluorescent probe to image deoxyribonucleic acid and ribonucleic acid in cells

The roles of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) in cells are closely related. However, the absence of molecular tools for simultaneous imaging of the two nucleic acids has prevented scientists from elucidating the regulatory mechanisms of nucleic acid interaction. The nucleic acid probes developed in recent years have ignored the regulatory relationship between DNA and RNA. Simultaneously imaging RNA and DNA in cells through a single small-molecule fluorescent probe is important. In this study, we propose a strategy for developing fluorescent probes localized to DNA and RNA to investigate their detection and imaging characteristics. The novel probe Bptp-RD has been successfully used for DNA and RNA imaging in cells. We investigated the detection and imaging characteristics of this nucleic acid probe and discovered the following: 1) the differences in the detection results of this nucleic acid probe for DNA and RNA come from the structural differences of the nucleic acids rather than chemical composition differences; 2) through using small-molecule probes to image a nucleic acid in cells, another nucleic acid can be visualized by reducing the fluorescence signal caused by DNA or RNA; 3) the order of response of the small-molecule fluorescent probe with intercalation and binding mechanisms to the type of nucleic acid structure is single chain, double chain, and ring. This work will help improve the understanding of RNA and DNA probes, and the novel probe has high potential to explore the interaction between RNA and DNA in cells.

Organic Fluorescent Probes for Monitoring Micro-Environments in Living Cells and Tissues

As a vital parameter in living cells and tissues, the micro-environment is crucial for the living organisms. Significantly, organelles require proper micro-environment to achieve normal physiological processes, and the micro-environment in organelles can reflect the state of organelles in living cells. Moreover, some abnormal micro-environments in organelles are closely related to organelle dysfunction and disease development. So, visualizing and monitoring the variation of micro-environments in organelles is helpful for physiologists and pathologists to study the mechanisms of the relative diseases. Recently, a large variety of fluorescent probes was developed to study the micro-environments in living cells and tissues. However, the systematic and comprehensive reviews on the organelle micro-environment in living cells and tissues have rarely been published, which may hinder the research progress in the field of organic fluorescent probes. In this review, we will summarize the organic fluorescent probes for monitoring the microenvironment, such as viscosity, pH values, polarity, and temperature. Further, diverse organelles (mitochondria, lysosome, endoplasmic reticulum, cell membrane) about microenvironments will be displayed. In this process, the fluorescent probes about the "off-on" and ratiometric category (the diverse fluorescence emission) will be discussed. Moreover, the molecular designing, chemical synthesis, fluorescent mechanism, and the bio-applications of these organic fluorescent probes in cells and tissues will also be discussed. Significantly, the merits and defects of current microenvironment-sensitive probes are outlined and discussed, and the development tendency and challenges for this kind of probe are presented. In brief, this review mainly summarizes some typical examples and highlights the progress of organic fluorescent probes for monitoring micro-environments in living cells and tissues in recent research. We anticipate that this review will deepen the understanding of microenvironment in cells and tissues and facilitate the studies and development of physiology and pathology.

Novel Polarity Fluorescent Probe for Dual-Color Visualization of Lysosomes and Plasma Membrane during Apoptosis

Apoptosis plays a crucial role in the occurrence of cancer and other diseases. Real-time monitoring of the cell apoptosis process has great significance for cell viability and drug screening. Herein, a novel fluorescent probe was constructed based on the fluorescence resonance energy transfer mechanism, which track the sensitivity of polarity changes, as well as detect the drug-induced cell apoptosis process in a dual-color mode. Importantly, the change of cellular microenvironmental polarity makes it possible to dynamically visualize the process of drug-induced cell apoptosis. More significantly, the designed probe targeted the lysosomes in the living cells to give a blue emission, and it accumulated on the plasma membrane to display red fluorescence during the drug-induced cell apoptosis process. Thus, cell viability could be monitored by both the localization and emission colors of the robust probe. We expect that the unique probe can provide a new blueprint for evaluating and screening apoptosis-related drugs.

Permeability-Controlled Probe for Directly Visualizing the Opening of Mitochondrial Permeability Transition Pore in Native Status

The opening of mitochondrial permeability transition pore (mPTP) plays a fundamental role in cell apoptosis regulation, ischemia-reperfusion injury, and neurodegenerative disorders. However, the molecular tools for detecting mPTP open in cellular native status have not been reported yet. Herein, we de novo designed a robust fluorescent probe mPTP-F to monitor mPTP opening in cellular native status for the first time. The membrane-permeable probe could accumulate into mitochondria and convert to a product poorly permeable to biomembranes, which was trapped in mitochondria to form near-infrared (NIR)-emissive aggregates. After mPTP opening, the product was released from mitochondria through the pore to form green-emissive monomers. Significantly, with mPTP-F, we discovered that formaldehyde, a signaling molecule, could induce mPTP opening. Therefore, the new probe could serve as a desirable molecular tool for the study of ischemia-reperfusion injury, cell apoptosis, and relative areas.

Construction of fluorescent probes for ratiometric visualization of mitochondrial depolarization based on the FRET mechanism

Two fluorescent probes based on the FRET mechanism were reasonably fabricated for the visualization of mitochondrial depolarization in a ratiometric manner.

A red-emissive and positively charged RNA ligand enables visualization of mitochondrial depolarization and cell damage

In this work, a red-emissive RNA ligand bearing two positive charges were developed for the visualization of mitochondrial depolarization, via the subcellular localization of the ligand molecules. The ligand with quinolinium moiety and strong electronic donor displays red fluorescence peaked at 630 nm. Meanwhile, the probe is concentrated in mitochondria of live cells due to the high mitochondrial membrane potential, and re-localizes into nucleolus upon mitochondrial depolarization owing to the affinity to RNA. In this manner, the decrease of mitochondrial membrane potential could be real-timely and in-situ monitored with the red-emissive probe. Particularly, two cations were decorated on the probe, which enables the fast response to mitochondrial depolarization with elevated sensitivity. Cell damage induced by H₂O₂ was also successfully observed with the probe. We expect that the probe can promote researches on mitochondrial membrane potential, cell apoptosis, and relative areas.

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

Lipid order in the cytoplasm membrane of eukaryotic cells undergoes dynamic changes in almost all cellular processes. Dynamically monitoring these changes is of essential biological significance and remains challenging. This work provides the first aggregation-induced emission probe, TPNPDA-C15, with highly three-dimensional specificity to cell membranes for fluorescent imaging of lipid order of live cells. TPNPDA-C15 displays red fluorescence enhancement with the viscosity increase while emits yellow fluorescence when aggregates form. Imaging analyses of giant unilamellar vesicles and live cells under osmotic shock by the probe demonstrate its sensitive response to the degree of phospholipids packing on artificial and cell membranes. Taking advantage of its superior low photocytotoxicity and high photostability, TPNPDA-C15 is further applied for long-term dynamic imaging of entire live cell physiological processes including apoptosis, ferroptosis, and mitosis in the dual-color mode. With the analysis of fluorescence signal changes in the two fluorescence channels, TPNPDA-C15 serves as a robust fluorescent probe for the imaging study of cellular dynamics.

Light-Driven Cascade Mitochondria-to-Nucleus Photosensitization in Cancer Cell Ablation

Nuclei and mitochondria are the only cellular organelles containing genes, which are specific targets for efficient cancer therapy. So far, several photosensitizers have been reported for mitochondria targeting, and another few have been reported for nuclei targeting. However, none have been reported for photosensitization in both mitochondria and nucleus, especially in cascade mode, which can significantly reduce the photosensitizers needed for maximal treatment effect. Herein, a light-driven, mitochondria-to-nucleus cascade dual organelle cancer cell ablation strategy is reported. A functionalized iridium complex, named BT-Ir, is designed as a photosensitizer, which targets mitochondria first for photosensitization and subsequently is translocated to a cell nucleus for continuous photodynamic cancer cell ablation. This strategy opens new opportunities for efficient photodynamic therapy.

Intelligent Nanoprobe: Acid-Responsive Drug Release and In Situ Evaluation of Its Own Therapeutic Effect

The design of an intelligent nanoprobe capable of intracellular controlled release of apoptosis inducers and subsequent high-fidelity imaging of the drug-induced apoptosis is highly desirable for precise cancer treatment. Herein, we report an intelligent nanoprobe that combined therapeutic and imaging functions in one agent. Briefly, a gold nanoparticle is designed to be conjugated with acid-responsive DNA duplexes (Dox intercalates in this region) and caspase-3-specific cleavable peptides (labeled with fluorophore). We demonstrated that the nanoprobe could efficiently deliver an anticancer drug (Dox) into cancer cells and achieve acid-responsive drug release. Furthermore, the apoptotic process was in situ-monitored by detection of fluorescence through the cleavage of the peptide linker by caspase-3, which is one of the executioner caspases involved in apoptosis. This newly developed nanoprobe could serve as a theranostic agent for targeted responsive chemotherapy and also provide feedback apoptosis imaging of the self-therapeutic effect.

A NIR-I light-responsive superoxide radical generator with cancer cell membrane targeting ability for enhanced imaging-guided photodynamic therapy

Photodynamic therapy (PDT), as an emerging treatment modality, which takes advantage of reactive oxygen species (ROS) generated upon light illumination to ablate tumours, has suffered from a limited treatment depth, strong oxygen dependence and short ROS lifespan. Herein, we developed a highly efficient NIR-I light (808 nm laser) initiated theranostic system based on a fluorescent photosensitizer (EBD-1) with cancer cell membrane targeting ability, which can realize large penetration depth in tissue, generate superoxide radicals (O2 -˙) to relieve the oxygen-dependence, confine the ROS oxidation at the cell membrane, and self-report the cell viability during the PDT process. In vivo experiments demonstrated that EBD-1 under 808 nm light successfully accomplished remarkable cancer ablation. This work will be beneficial for the design of novel photosensitizers for PDT-based theranostic systems.

Fabrication of a fluorescent probe for in-situ and ratiometric visualization of autophagy

In situ and real-time visualization of autophagy process is of vital importance for fundamental researches in biology. However, fluorescent probes for autophagy were rarely reported, which greatly hindered the study on autophagy. In this work, a fluorescent probe was rationally designed and fabricated for the ratiometric visualization of autophagy. The probe targeted non-lysosome organelles in healthy cells, and gave blue emission. During autophagy, the organelles together with the probe were delivered into acid lysosomes, and gave red fluorescence. In this way, the autophagy process could be monitored in ratiometric manner. The starvation induced autophagy was successfully visualized by means of the probe, and the inhibition effect of chloroquine to autophagy was also observed. We expect that the probe can serve as a powerful tool for the investigation of autophagy and relative areas.

Time Evolution of Local pH Around a Photo-Acid in Water and a Polymer Hydrogel: Time Resolved Fluorescence Spectroscopy of Pyranine

In this work, we propose a new analysis of the time resolved emission spectra of a photo-acid, HA, pyranine (8-hydroxypyrene-1,3,6-trisulphonic acid, HPTS) based on time resolved area normalized emission spectra (TRANES). Presence of an isoemissive point in TRANES confirms the presence of two emissive species (HA and A^- ) inside the system in bulk water and inside a co-polymer hydrogel [F127, (PEO)₁₀₀ -(PPO)₇₀ -(PEO)₁₀₀ ]. We show that following electronic excitation, the local pH around HPTS, is much lower than the bulk pH presumably because of ejection of proton from the photo-acid in the excited state. With increase in time, the local pH increases and reaches the bulk value. We further, demonstrate that the excited state pKa of HPTS may be estimated from the emission intensities of HA and A^- at long time. The time constant for time evolution of pH is ∼630 ps in water, ∼1300 ps in F127 gel and ∼4700 ps in CTAB micelle. The location and local viscosity sensed by the probe is ascertained using fluorescence correlation spectroscopy (FCS) and fluorescence anisotropy decay. The different values of the local viscosity reported by these two methods are reconciled.