Fluorescent Probes for the Visualization of Cell Viability

Construction of a Dual-Emissive Probe for Discriminative Visualization of Lysosomal and Mitochondrial Dysfunction

Discriminatively visualizing mitochondrial and lysosomal dysfunction is crucial for an in-depth understanding of cell apoptosis regulation and relative biology. However, fluorescent probes for the separate visualization of lysosomal and mitochondria damages have not been reported yet. Herein, we have constructed a fluorescent probe [2-(4-hydroxystyryl)-1,3,3-trimethyl-3H-indol-1-ium iodide (HBSI)] for labeling mitochondria and lysosomes in dual emission colors and discriminatively imaging mitochondrial and lysosomal damage in two different sets of fluorescent signals. In living cells, HBSI targeted both lysosomes and mitochondria to give green and red emission, respectively. During mitochondrial damages, HBSI immigrated into lysosomes, and the red emission decreased. During lysosomal damage, HBSI immigrated into mitochondria, and the green emission decreased. With the robust probe, the different damaging sequences of mitochondria and lysosomes under different amounts of H2O2 and chloral hydrate have been revealed. The sequential damage of lysosomes and mitochondria during cell apoptosis induced by rotenone, paclitaxel, and colchicine has been discovered. Furthermore, the regulation of mitochondria, lysosome, and their interplay during autophagy was also observed with the probe.

Quantifying cell viability through organelle ratiometric probing

Detecting cell viability is crucial in research involving the precancerous discovery of abnormal cells, the evaluation of treatments, and drug toxicity testing. Although conventional methods afford cumulative results regarding cell viability based on a great number of cells, they do not permit investigating cell viability at the single-cell level. In response, we rationally designed and synthesized a fluorescent probe, PCV-1, to visualize cell viability under the super-resolution technology of structured illumination microscopy. Given its sensitivity to mitochondrial membrane potential and affinity to DNA, PCV-1's ability to stain mitochondria and nucleoli was observed in live and dead cells, respectively. During cell injury induced by drug treatment, PCV-1's migration from mitochondria to the nucleolus was dynamically visualized at the single-cell level. By extension, harnessing PCV-1's excellent photostability and signal-to-noise ratio and by comparing the fluorescence intensity of the two organelles, mitochondria and nucleoli, we developed a powerful analytical assay named organelle ratiometric probing (ORP) that we applied to quantitatively analyze and efficiently assess the viability of individual cells, thereby enabling deeper insights into the potential mechanisms of cell death. In ORP analysis with PCV-1, we identified 0.3 as the cutoff point for assessing whether adding a given drug will cause apparent cytotoxicity, which greatly expands the probe's applicability. To the best of our knowledge, PCV-1 is the first probe to allow visualizing cell death and cell injury under super-resolution imaging, and our proposed analytical assay using it paves the way for quantifying cell viability at the single-cell level.

Aerogel-Based Materials in Bone and Cartilage Tissue Engineering-A Review with Future Implications

Aerogels are fascinating solid materials known for their highly porous nanostructure and exceptional physical, chemical, and mechanical properties. They show great promise in various technological and biomedical applications, including tissue engineering, and bone and cartilage substitution. To evaluate the bioactivity of bone substitutes, researchers typically conduct in vitro tests using simulated body fluids and specific cell lines, while in vivo testing involves the study of materials in different animal species. In this context, our primary focus is to investigate the applications of different types of aerogels, considering their specific materials, microstructure, and porosity in the field of bone and cartilage tissue engineering. From clinically approved materials to experimental aerogels, we present a comprehensive list and summary of various aerogel building blocks and their biological activities. Additionally, we explore how the complexity of aerogel scaffolds influences their in vivo performance, ranging from simple single-component or hybrid aerogels to more intricate and organized structures. We also discuss commonly used formulation and drying methods in aerogel chemistry, including molding, freeze casting, supercritical foaming, freeze drying, subcritical, and supercritical drying techniques. These techniques play a crucial role in shaping aerogels for specific applications. Alongside the progress made, we acknowledge the challenges ahead and assess the near and far future of aerogel-based hard tissue engineering materials, as well as their potential connection with emerging healing techniques.

Functional Materials for Subcellular Targeting Strategies in Cancer Therapy: Progress and Prospects

Neoadjuvant and adjuvant therapies have made significant progress in cancer treatment. However, tumor adjuvant therapy still faces challenges due to the intrinsic heterogeneity of cancer, genomic instability, and the formation of an immunosuppressive tumor microenvironment. Functional materials possess unique biological properties such as long circulation times, tumor-specific targeting, and immunomodulation. The combination of functional materials with natural substances and nanotechnology has led to the development of smart biomaterials with multiple functions, high biocompatibilities, and negligible immunogenicities, which can be used for precise cancer treatment. Recently, subcellular structure-targeting functional materials have received particular attention in various biomedical applications including the diagnosis, sensing, and imaging of tumors and drug delivery. Subcellular organelle-targeting materials can precisely accumulate therapeutic agents in organelles, considerably reduce the threshold dosages of therapeutic agents, and minimize drug-related side effects. This review provides a systematic and comprehensive overview of the research progress in subcellular organelle-targeted cancer therapy based on functional nanomaterials. Moreover, it explains the challenges and prospects of subcellular organelle-targeting functional materials in precision oncology. The review will serve as an excellent cutting-edge guide for researchers in the field of subcellular organelle-targeted cancer therapy.

New Advances in the Exploration of Esterases with PET and Fluorescent Probes

Esterases are hydrolases that catalyze the hydrolysis of esters into the corresponding acids and alcohols. The development of fluorescent probes for detecting esterases is of great importance due to their wide spectrum of biological and industrial applications. These probes can provide a rapid and sensitive method for detecting the presence and activity of esterases in various samples, including biological fluids, food products, and environmental samples. Fluorescent probes can also be used for monitoring the effects of drugs and environmental toxins on esterase activity, as well as to study the functions and mechanisms of these enzymes in several biological systems. Additionally, fluorescent probes can be designed to selectively target specific types of esterases, such as those found in pathogenic bacteria or cancer cells. In this review, we summarize the recent fluorescent probes described for the visualization of cell viability and some applications for in vivo imaging. On the other hand, positron emission tomography (PET) is a nuclear-based molecular imaging modality of great value for studying the activity of enzymes in vivo. We provide some examples of PET probes for imaging acetylcholinesterases and butyrylcholinesterases in the brain, which are valuable tools for diagnosing dementia and monitoring the effects of anticholinergic drugs on the central nervous system.

Molecularly Targeted Fluorescent Sensors for Visualizing and Tracking Cellular Senescence

Specific identification and monitoring of senescent cells are essential for the in-depth understanding and regulation of senescence-related life processes and diseases. Fluorescent sensors providing real-time and in situ information with spatiotemporal resolution are unparalleled tools and have contributed greatly to this field. This review focuses on the recent progress in fluorescent sensors for molecularly targeted imaging and real-time tracking of cellular senescence. The molecular design, sensing mechanisms, and biological activities of the sensors are discussed. The sensors are categorized by the types of markers and targeting ligands. Accordingly, their molecular recognition and fluorescent performance towards senescence biomarkers are summarized. Finally, the perspective and challenges in this field are discussed, which are expected to assist future design of next-generation sensors for monitoring cellular senescence.

Experimental training in molecular pharmacology education based on drug-target interactions

INTRODUCTION

Drug development has been challenged by the dual drawbacks involving unpredictable disease outcomes and drug resistance, which has placed greater demands on pharmacology education. Molecular pharmacology, as a frontier crossover field of pharmacology, focuses on the research of new drugs and targets. However, due to the lack of a systematic experimental training system, molecular pharmacology has not made a corresponding contribution in promoting the training of innovative talent in pharmacology. We aim to establish an experimental training program suitable for molecular pharmacology to improve students' ability to engage in drug development in future.

METHODS

Based on the feasibility of drug-target projects, a comprehensive training program containing molecular docking, target stability experiment, and fluorescent probe detection of protein expression in living cells and mice was conducted among 20 pharmacy graduate students. The experimental training was assessed by the experimental training report and the student recognition questionnaires.

RESULTS

All 20 students mastered the experimental principles and operations required for the training program. The experimental reports proved that the students were in good command of the experimental principles, operations and applications. The results of the Likert questionnaire indicated that the training program promoted the understanding of the drug research process and increased motivation to learn.

CONCLUSION

The designed experimental training program has a positive effect on the training of pharmacology talents, and can be implemented as a part of molecular pharmacology education.

Recent Advances in Fluorescent Probes for Zinc Ions Based on Various Response Mechanisms

Zinc is a vital metal element with extensive applications in various fields such as industry, metallurgy, agriculture, food, and healthcare. For living organisms, zinc ions are indispensable, and their deficiency can lead to physiological and metabolic abnormalities that cause multiple diseases. Hence, there is a significant need for selective recognition and effective detection of free zinc ions. As a probe method with high sensitivity, high selectivity, real-time monitoring, safety, harmlessness and ease of operation, fluorescent probes have been widely used in metal ion identification studies, and many convenient, low-cost and easy-to-operate fluorescent probes for Zn2+ detection have been developed. This article reviews the latest research advances in fluorescent chemosensors for Zn2+ detection from 2019 to 2023. In particular, sensors working through photo-induced electron transfer (PET), excited state intramolecular proton transfer (ESIPT), intramolecular charge transfer (ICT), fluorescence resonance energy transfer (FRET), chelation-enhanced fluorescence (CHEF), and aggregation-induced emission (AIE) mechanisms are described. We discuss the use of various recognition mechanisms in detecting zinc ions through specific cases, some of which have been validated through theoretical calculations.

Quantifying cell viability through organelle ratiometric probing

Detecting cell viability is crucial in research involving the precancerous discovery of abnormal cells, the evaluation of treatments, and drug toxicity testing. Although conventional methods afford cumulative results regarding cell viability based on a great number of cells, they do not permit investigating cell viability at the single-cell level. In response, we rationally designed and synthesized a fluorescent probe, PCV-1, to visualize cell viability under the super-resolution technology of structured illumination microscopy. Given its sensitivity to mitochondrial membrane potential and affinity to DNA, PCV-1's ability to stain mitochondria and nucleoli was observed in live and dead cells, respectively. During cell injury induced by drug treatment, PCV-1's migration from mitochondria to the nucleolus was dynamically visualized at the single-cell level. By extension, harnessing PCV-1's excellent photostability and signal-to-noise ratio and by comparing the fluorescence intensity of the two organelles, mitochondria and nucleoli, we developed a powerful analytical assay named organelle ratiometric probing (ORP) that we applied to quantitatively analyze and efficiently assess the viability of individual cells, thereby enabling deeper insights into the potential mechanisms of cell death. In ORP analysis with PCV-1, we identified 0.3 as the cutoff point for assessing whether adding a given drug will cause apparent cytotoxicity, which greatly expands the probe's applicability. To the best of our knowledge, PCV-1 is the first probe to allow visualizing cell death and cell injury under super-resolution imaging, and our proposed analytical assay using it paves the way for quantifying cell viability at the single-cell level.

7-Geranyloxycinnamic Acid Isolated from Melicope lunu-ankenda Leaves Perturbs Colon Cancer and Breast Cancer Cell Lines' Growth via Induction of Apoptotic Pathway

Globally, breast cancer is the most prevalent form of cancer in women and there is a need for alternative therapies such as plant-derived compounds with low systemic toxicity and selective toxicity to cancer cells. The aim of this study is to assess the cytotoxicity effects of 7-geranyloxycinnamic acid isolated from leaves of Melicope lunu-ankenda, a traditional medicinal plant, on the human breast cancer cell lines. Dried leaf powder was used for the preparation of different crude extracts using different solvents of increasing order of polarity. The structure of the isolated compound from the petroleum ether extract was elucidated by ¹H and ¹³C NMR, LC-MS, and DIP-MS spectroscopy. The cytotoxic activity of the crude extract and 7-geranyloxycinnamic acid analyzed using MTT assay. Apoptotic analysis was evaluated using Annexin V-PI staining, AO/PI staining, intracellular ROS measurement, and measurement of activities of caspases 3/7, 8, and 9. Crude extracts and the isolated pure compound showed significant cytotoxicity against tested cancer cell lines. 7-geranyloxycinnamic acid was found to exert significant cytotoxic effects against breast cancer cell lines such as the MCF-7 and MDA-MB-231 cell lines. The cytotoxic effects are attributed to its ability to induce apoptosis via accumulation of ROS and activation of caspases in both breast cancer cell lines. The pure compound, 7-geranyloxycinnamic acid isolated from the leaves of M. lunu-ankenda, can exert significant cytotoxic effects against breast cancer cell lines without affecting the normal cells.

An aminopeptidase N-based color-convertible fluorescent nano-probe for cancer diagnosis

Specific cancer diagnosis at an early stage plays a significant role in preventing cancer metastasis and reducing cancer mortality. Thus, exploring specific and sensitive fluorescent probes to realize early cancer diagnosis is an urgent need in clinic. Aminopeptidase N (APN/CD13), overexpressed in numerous malignant tumors, is an important tumor biomarker associated with cancer progression, invasion, and metastasis. In this study, a novel fluorescent molecule APN-SUB, capable of monitoring APN in real time, is encapsulated in a pH-responsive block copolymer (termed APN-SUB nanoprobe) for cancer diagnosis. APN-SUB contains a fluorophore center and a trigger moiety (leucine group), which is covalently conjugated on the fluorophore with an amide bond. The hydrolysis of the amide bond in APN-SUB activated by APN leads to a red shift of maximum fluorescence emission wavelength from 495 nm to 600 nm, realizing dual-color transformation from green to red. Moreover, the APN-SUB nanoprobe with pH-responsiveness is prepared to improve the accumulation and the release rate in the tumor region. It is worth noting that the APN-SUB nanoprobe exhibits good performance for APN imaging, namely, superior limit of detection (0.14 nU mL^-1), excellent selectivity and strong photostability. More importantly, the APN-SUB nanoprobe can be successfully employed as a color-convertible fluorescent probe for cancer diagnosis by tracking the activity of APN with high specificity and sensitivity in vivo, demonstrating its potential value for cancer diagnosis.

Functionalized Fluorescent Nanostructures Generated from Self-Assembly of a Cationic Tripeptide Direct Cell-Selective Chemotherapeutic Drug Delivery

Nanodrug delivery systems (NDDs) capable of conveying chemotherapeutics directly into malignant cells without harming healthy ones are of significant interest in the field of cancer therapy. However, the development of nanostructures with the requisite biocompatibility, inherent optical properties, cellular penetration ability, encapsulation capability, and target selectivity has remained elusive. In an effort to develop cell-selective NDDs, we have synthesized a cationic tripeptide Boc-Arg-Trp-Phe-OMe (PA1), which self-assembles into well-ordered spheres in 100% aqueous medium. The inherent fluorescence properties of the peptide PA1 were shifted from the ultraviolet to the visible region by the self-assembly. These fluorescent nanostructures are proteolytically stable, photostable, and biocompatible, with characteristic blue fluorescence signals that permit us to monitor their intracellular entry in real time. We also demonstrate that these tripeptide spherical structures (TPSS) have the capacity to entrap the chemotherapeutic drug doxorubicin (Dox), shuttle the encapsulated drug within cancerous cells, and initiate the DNA damage signaling cascade, which culminates in apoptosis. Next, we functionalized the TPSS with an epithelial-cell-specific epithelial cell adhesion molecule aptamer. Aptamer-conjugated PA1 (PA1-Apt) facilitated efficient Dox delivery into the breast cancer epithelial cell line MCF7, resulting in cell death. However, cells of the human cardiomyocyte cell line AC16 were resistant to the cell killing actions of PA1-Apt. Together, these data demonstrate that not only can the self-assembly of cationic tripeptides like PA1 be exploited for efficient drug encapsulation and delivery but their unique chemistry also allows for functional modifications, which can improve the selectivity of these versatile NDDs.

Monitoring cell viability in N-nitrosodiethylamine induced acute hepatitis and detection of hydrazine in solution and gas phase with Dual-function fluorescent probes

The highly toxic N-nitrosodiethylamine (NDEA) and hydrazine (N₂H₄) caused severe environmental contamination and serious health risks. Herein, we designed the two-photon ratiometric fluorescent probe (Nap-2), emission maximum shifted from 466 nm to 571 nm, to monitor cell viability of NDEA induced acute hepatitis via esterase activity detection. Furthermore, the probe Nap-2 evaluate the hydrazine (N₂H₄) content in the solution and gas phase. It is worth mentioning that we used NDEA induced acute hepatitis in the mice and evaluated the negative correlation of esterase activity in the tissue cells and serum with Nap-2. The probe Nap-2 exhibited that acute hepatitis induced by NDEA decreased cell viability. Furthermore, we made convenient test papers using Nap-2 to detect N₂H₄ in solution and gas phase. After adding N₂H₄, the fluorescence color changed from blue to yellow and was visible to the naked eye. This work provides a convenient tool and method for evaluating the toxicity of NDEA induced acute hepatitis and detecting N₂H₄ in the environment.

Synthesis, Optical Properties, and Antiproliferative Evaluation of NBD-Triterpene Fluorescent Probes

A fluorescent labeling protocol for hydroxylated natural compounds with promising antitumor properties has been used to synthesize, in yields of 72-86%, 12 derivatives having fluorescent properties and biological activity. The reagent used for the synthesis of these fluorescent derivatives was 7-nitrobenzo-2-oxa-1,3-diazole chloride (NBD-Cl). The linkers employed to bind the NBD-Cl reagent to the natural compounds were ω-amino acids (Aa) of different chain lengths. The natural triterpene compounds chosen were oleanolic and maslinic acid, as their corresponding 28-benzylated derivatives. Thus, 12 NBD-Aa-triterpene conjugates have been studied for their optical fluorescence properties and their biological activities against cell proliferation in three cancer cell lines (B16-F10, HT-29, and HepG2), compared with three nontumor cell lines (HPF, IEC-18, and WRL68) from different tissues. The results of the fluorescence study have shown that the best fluorescent labels are those in which the ω-amino acid chain is shorter, and the carboxylic group is not benzylated. Analysis by confocal microscopy showed that these compounds were rapidly incorporated into cells in all three cancer cell lines, with these same derivatives showing the highest toxicity against the cancer cell lines tested. Then, the fluorescent labeling of these NBD-Aa-triterpene conjugates enabled their uptake and subcellular distribution to be followed in order to probe in detail their biological properties at the cellular and molecular level.

An ESIPT-based reversible ratiometric fluorescent sensor for detecting HClO/H2S redox cycle in living cells

HClO and H₂S, as two kinds of crucial small biomolecules, are endowed various roles in biological organisms. The redox balance between HClO and H₂S is closely related to the physiological and pathological processes. Thus, it is significant to monitor the redox process between HClO and H₂S. Inspired by the advantages of ratiometric fluorescent probes, we firstly developed a reversible ratiometric fluorescent probe (BT-Se) for HClO and H₂S via combination of phenyl selenide as the response group and 2-(2'-hydroxyphenyl)benzothiazole dye as the fluorophore. The proposed probe BT-Se could detect HClO with well-separated dual emission (110 nm), fast response, good selectivity and sensitivity owing to the oxidation reaction of the Se atom induced by HClO. Moreover, only H₂S could effectively recover the fluorescence of the detection system to the original state via H₂S induced-reduction of selenoxide. Cell imaging studies demonstrated that the probe BT-Se was capable of ratiometric monitoring the changes of intracellular HClO/H₂S, which suggested that it has great potential for researching the biological functions of HClO and H₂S.

Rational Design of Organelle-Targeted Fluorescent Probes: Insights from Artificial Intelligence

Monitoring the physiological changes of organelles is essential for understanding the local biological information of cells and for improving the diagnosis and therapy of diseases. Currently, fluorescent probes are considered as the most powerful tools for imaging and have been widely applied in biomedical fields. However, the expected targeting effects of these probes are often inconsistent with the real experiments. The design of fluorescent probes mainly depends on the empirical knowledge of researchers, which was inhibited by limited chemical space and low efficiency. Herein, we proposed a novel multilevel framework for the prediction of organelle-targeted fluorescent probes by employing advanced artificial intelligence algorithms. In this way, not only the targeting mechanism could be interpreted beyond intuitions but also a quick evaluation method could be established for the rational design. Furthermore, the targeting and imaging powers of the optimized and synthesized probes based on this methodology were verified by quantitative calculation and experiments.

Ratiometric and Discriminative Visualization of Autophagy and Apoptosis with a Single Fluorescent Probe Based on the Aggregation/Monomer Principle

Autophagy and apoptosis play a central role in maintaining homeostasis in mammals. Therefore, discriminative visualization of the two cellular processes is an important and challenging task. However, fluorescent probes enabling ratiometric visualization of both autophagy and apoptosis with different sets of fluorescence signals have not been developed yet. In this work, we constructed a versatile single fluorescent probe (NKLR) based on the aggregation/monomer principle for the ratiometric and discriminative visualization of autophagy and apoptosis. NKLR can simultaneously perform two-color imaging of RNA (deep red channel) and lysosomes (yellow channel) in aggregation and monomer states, respectively. During autophagy, NKLR migrated from cytoplasmic RNA and nuclear RNA to lysosomes, showing enhanced yellow emission and sharply decreased deep red fluorescence. Moreover, this migration process was reversible upon the recovery of autophagy. Comparatively, during apoptosis, NKLR immigrated from lysosomes to RNA, and the yellow emission decreased and even disappeared, while the fluorescence of the deep red channel slightly increased. Overall, autophagy and apoptosis could be discriminatively visualized via the fluorescence intensity ratios of the two channels. Meanwhile, the cells in three different states (healthy, autophagic, apoptotic) could be distinguished by three point-to-point fluorescence images via the localization and emission color of NKLR. Therefore, the probe NKLR can serve as a desirable molecular tool to reveal the in-depth relation between autophagy and apoptosis and facilitate the study on the two cellular processes.

Mitochondrion-Targeted NIR Therapeutic Agent Suppresses Melanoma by Inducing Apoptosis and Cell Cycle Arrest via E2F/Cyclin/CDK Pathway

Malignant melanoma is the most fatal form of skin cancer worldwide, and earlier diagnosis and more effective therapies are required to improve prognosis. As a possible solution, near-infrared fluorescent heptamethine cyanine dyes have been shown to be useful for tumor diagnosis and treatment. Here, we synthesized a novel theranostic agent, IR-817, a multifunctional bioactive small-molecule that has near-infrared emission, targets mitochondria in cancer cells, and has selective anti-cancer effects. In in vitro experiments, IR-817 preferentially accumulated in melanoma cells through organic anion transporting polypeptide transporters but also selectively inhibited the growth of tumor cells by inducing mitochondrial-dependent intrinsic apoptosis. Mechanistically, IR-817 caused G0/G1 cell cycle arrest by targeting the E2F/Cyclin/CDK pathway. Finally, IR-817 significantly suppressed the growth of xenograft tumors in zebrafish and mice. Immunohistochemical staining and hematoxylin and eosin staining revealed that IR-817 induced apoptosis and inhibited tumor cell proliferation without notable side effects. Therefore, mitochondrial-targeting theranostic agent IR-817 may be promising for accurate tumor diagnosis, real-time monitoring, and safe anti-cancer treatments.

Engineered Extracellular Vesicles with SHP2 High Expression Promote Mitophagy for Alzheimer's Disease Treatment

Mitochondrial dysfunction is a fundamental pathological feature of Alzheimer's disease (AD). However, toxicity and poor brain enrichment of existing mitophagy inducers limit their further applications. In this study, a platform for AD therapy is developed using nanosized mesenchymal-stem-cells-derived extracellular vesicles with tyrosine phosphatase-2 (SHP2) high-expression (MSC-EVs-SHP2). The high blood-brain barrier penetration ability of MSC-EVs-SHP2 is demonstrated in AD-mice, facilitating SHP2 delivery to the brain. In addition, MSC-EVs-SHP2 significantly induces mitophagy of neuronal cells, which alleviates mitochondrial damage-mediated apoptosis and NLRP3 inflammasome activation. Mitophagy further diminishes neuronal cells apoptosis and neuroinflammation, culminating with rescued synaptic loss and cognitive decline in an AD mouse model. The EV-engineering technology provides a potential platform for effective AD therapy by inducing mitophagy.

Three-dimensional imaging and analysis of pathological tissue samples with de novo generation of citrate-based fluorophores

Two-dimensional (2D) histopathology based on the observation of thin tissue slides is the current paradigm in diagnosis and prognosis. However, labeling strategies in conventional histopathology are limited in compatibility with 3D imaging combined with tissue clearing techniques. Here, we present a rapid and efficient volumetric imaging technique of pathological tissues called 3D tissue imaging through de novo formation of fluorophores, or 3DNFC, which is the integration of citrate-based fluorogenic reaction DNFC and tissue clearing techniques. 3DNFC markedly increases the fluorescence intensity of tissues by generating fluorophores on nonfluorescent amino-terminal cysteine and visualizes the 3D structure of the tissues to provide their anatomical morphology and volumetric information. Furthermore, the application of 3DNFC to pathological tissue achieves the 3D reconstruction for the unbiased analysis of diverse features of the disorders in their natural context. We suggest that 3DNFC is a promising volumetric imaging method for the prognosis and diagnosis of pathological tissues.

Super-multiplexing excitation spectral microscopy with multiple fluorescence bands

Fluorescence microscopy, with high molecular specificity and selectivity, is a valuable tool for studying complex biological systems and processes. However, the ability to distinguish a large number of distinct subcellular structures in a single sample is impeded by the broad spectra of molecular fluorescence. We have recently shown that excitation spectral microscopy provides a powerful means to unmix up to six fluorophores in a single fluorescence band. Here, by working with multiple fluorescence bands, we extend this approach to the simultaneous imaging of up to ten targets, with the potential for further expansions. By covering the excitation/emission bandwidth across the full visible range, an ultra-broad 24-wavelength excitation scheme is established through frame-synchronized scanning of the excitation wavelength from a white lamp via an acousto-optic tunable filter (AOTF), so that full-frame excitation-spectral images are obtained every 24 camera frames, offering superior spectral information and multiplexing capability. With numerical simulations, we validate the concurrent imaging of 10 fluorophores spanning the visible range to achieve exceptionally low (∼0.5%) crosstalks. For cell imaging experiments, we demonstrate unambiguous identification of up to eight different intracellular structures labeled by common fluorophores of substantial spectral overlap with minimal color crosstalks. We thus showcase an easy-to-implement, cost-effective microscopy system for visualizing complex cellular components with more colors and lower crosstalks.

Chameleon-Like Fluorescent Probe for Monitoring Interplays between Three Organelles and Reporting Cell Damage Processes through Dramatic Color Change

The in-depth study of the interplay and cooperation between multiple organelles is an important biological task. Single fluorescent probes for separate visualization of multiple organelles is a desirable molecular tool, but the construction of such a probe is extremely difficult owing to the lack of valid strategies. In this work, utilizing the reversible cyclization reaction and intermolecular π stacking mechanism, a robust fluorescent probe is constructed to discriminatively illuminate lipid droplets (LDs), mitochondria, and lysosomes with blue, green, and red emission colors, respectively. Using the probe, the interplays and cooperation between LDs, mitochondria, and lysosomes are successfully studied, and the critical roles of lysosomes and LDs during mitochondrial fission are successfully revealed. Furthermore, this unique probe reveals the sequential damage of mitochondria and lysosomes during apoptosis through the successive fading of green and red emission. Thereby, the probe enables the discrimination of health state, early apoptosis, and late apoptosis of cells with three different sets of fluorescent signals. Overall, the robust probe is a desirable molecular tool to reveal the interactions between the three organelles, and investigate cell apoptosis and relative areas.

Discrimination of Cell Death Types with an Activatable Fluorescent Probe through Visualizing the Lysosome Morphology

Cell death plays a vital role in body development, maintenance of tissue function, and homeostasis. Accurate evaluation of cell death types is of great importance for pharmacological and pathological research. However, there is a lack of efficient fluorescent probes to discriminate various cell states. Here, we design and synthesize a novel activatable fluorescent probe PNE-Lyso to detect intracellular pH and hexosaminidases with two kinds of fluorescence signals. PNE-Lyso could distinguish dead cells from healthy cells based on a dual-color mode by targeting the lysosome and evaluating lysosomal hexosaminidase activity. Significantly, PNE-Lyso could also discriminate apoptotic and necrotic cells through visualizing lysosome morphology that is adjusted by the integrity of the lysosome membrane. Moreover, probe PNE-Lyso was successfully applied to investigate the drug-induced cell death process. To the best of our knowledge, this work is the first time cell death types have been distinguished based on a single fluorescent probe.

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.

Behavioral disturbances induced by cyanobacterial oligopeptides microginin-FR1, anabaenopeptin-A and microcystin-LR are associated with neuromotoric and cytotoxic changes in Brachionus calyciflorus

Aquatic animals are exposed to various cyanobacterial products released concomitantly to the environment by decaying blooms. Although there exist results on the toxicity of cyanobacterial extracts little is known on the influence of pure oligopeptides or their mixtures and elucidated mechanisms of behavioral toxicity in zooplanktonic organisms. Therefore, the aim of the present study was to assess the effects of single and mixed pure cyanobacterial oligopeptides: microginin FR-1 (MG-FR1), anabaenopeptin-A (ANA-A) and microcystin-LR (MC-LR) at various concentrations on the swimming behavior and catecholamine neurotransmitter activity, muscular F-actin structure, DNA nuclear content and cell viability of a model rotifer Brachionus calyciflorus. Swimming behavior was analyzed with the use of video digital analysis. Fluorescent microscopy imaging was used to analyze neuromotoric biomarkers in the whole organisms: neuromediator release (by staining with EC517 probe), muscle F-actin filaments (by staining with blue phalloidin dye). DNA content and cytotoxicity was also determined by Hoechst 34580 and propidium iodide double staining, respectively. The results showed that single oligopeptides inhibited all the tested endpoints. The binary mixtures induced synergistic interaction on swimming speed except for MG-FR1 +MC-LR which was nearly additive. Both binary and ternary mixtures also synergistically degraded F-actin and triggered cytotoxic effects visible in the whole organisms. Antagonistic inhibitory effects of all the binary mixtures were found on catecholamine neurotransmitter activity, however the ternary mixture induced additive toxicity. Antagonistic effects of both binary and ternary mixtures were also noted on nuclear DNA content. The results of the study suggest that both depression of neurotransmission and impairment of muscle F-actin structure in muscles may contribute to mechanisms of Brachionus swimming speed inhibition by the tested single cyanobacterial oligopeptides and their mixtures. The study also showed that natural exposure of rotifers to mixtures of these cyanobacterial metabolites may result in different level of interactive toxicity with antagonistic, additive synergistic effects depending on the variants and concentrations present in the environment.

Probing cell membrane integrity using a histone-targeting protein nanocage displaying precisely positioned fluorophores

Cell membrane integrity is fundamental to the normal activities of cells and is involved in both acute and chronic pathologies. Here, we report a probe for analyzing cell membrane integrity developed from a 9 nm-sized protein nanocage named Dps via fluorophore conjugation with high spatial precision to avoid self-quenching. The probe cannot enter normal live cells but can accumulate in dead or live cells with damaged membranes, which, interestingly, leads to weak cytoplasmic and strong nuclear staining. This differential staining is found attributed to the high affinity of Dps for histones rather than DNA, providing a staining mechanism different from those of known membrane exclusion probes (MEPs). Moreover, the Dps nanoprobe is larger in size and thus applies a more stringent criterion for identifying severe membrane damage than currently available MEPs. This study shows the potential of Dps as a new bioimaging platform for biological and medical analyses.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (Figs. S1-S12 including distance information between neighboring fluorophores on Dps, TEM images, MALDI-TOF analysis, fluorescence spectra, confocal images, gel retardation analysis, tissue staining, and additional data) is available in the online version of this article at 10.1007/s12274-022-4785-5.

AIEgen-Peptide Bioprobes for the Imaging of Organelles

Organelles are important subsystems of cells. The damage and inactivation of organelles are closely related to the occurrence of diseases. Organelles’ functional activity can be observed by fluorescence molecular tools. Nowadays, a series of aggregation-induced emission (AIE) bioprobes with organelles-targeting ability have emerged, showing great potential in visualizing the interactions between probes and different organelles. Among them, AIE luminogen (AIEgen)-based peptide bioprobes have attracted more and more attention from researchers due to their good biocompatibility and photostability and abundant diversity. In this review, we summarize the progress of AIEgen-peptide bioprobes in targeting organelles, including the cell membrane, nucleus, mitochondria, lysosomes and endoplasmic reticulum, in recent years. The structural characteristics and biological applications of these bioprobes are discussed, and the development prospect of this field is forecasted. It is hoped that this review will provide guidance for the development of AIEgen-peptide bioprobes at the organelles level and provide a reference for related biomedical research.

Benzoselenazolium-based hemicyanine dye for G-Quadruplex detection

Benzothiazolium and benzoxazolium are common groups for the construction of hemicyanine dyes; however, their isosteric analogue benzoselenazolium have rarely been studied. Here, we report the development of the first benzoselenazolium-based hemicyanine dye for the selective detection of G-quadruplexes. This molecule, SEMA-1, was validated as a red-emitting and activatable fluorescent probe whose fluorescence would only be activated in the presence of G-quadruplexes in buffer solution. Consistent with this, SEMA-1 was found to accumulate in nucleoli and could be used to detect the high abundance of nucleolar rDNA and rRNA G-quadruplexes in fixed HeLa cells. On the other hand, due to the retained mitochondrial membrane potential in live HeLa cells, SEMA-1 was captured by mitochondria and had the potential to detect the mitochondrial G-quadruplexes. Collectively, this work demonstrates the value of developing G-quadruplex-specific fluorescent probes from novel benzoselenazolium-based hemicyanine scaffold.

Recent Progress in Fluorescent Probes for Diabetes Visualization and Drug Therapy

Diabetes has become one of the most prevalent endocrine and metabolic diseases that threaten human health, and it is accompanied by serious complications. Therefore, it is vital and pressing to develop novel strategies or tools for prewarning and therapy of diabetes and its complications. Fluorescent probes have been widely applied in the detection of diabetes due to the fact of their attractive advantages. In this report, we comprehensively summarize the recent progress and development of fluorescent probes in detecting the changes in the various biomolecules in diabetes and its complications. We also discuss the design of fluorescent probes for monitoring diabetes in detail. We expect this review will provide new ideas for the development of fluorescent probes suitable for the prewarning and therapy of diabetes in future clinical transformation and application.

Single Fluorescent Probe for Zero-Crosstalk Discrimination of Lipid Droplets and the Endoplasmic Reticulum Based on Reversible Cyclization Reaction

The interactions between different organelles are ubiquitous and crucial for life activities. Thus, development of a single fluorescent probe enabling the simultaneous two-color visualization of two organelles is of great significance for the study of organelle interplay. Herein, using the reversible ring-opening/closing reactions of rhodamine dyes, we have fabricated a robust fluorescent probe to distinguish lipid droplets (LDs) and the endoplasmic reticulum (ER) in dual-emission channels with negligible crosstalk. The probe 6'-(diethylamino)-4'-((7-(diethylamino)-2-oxo-2H-chromen-3-yl)methylene)-1',2',3',4'-tetrahydro-3H-spiro[isobenzofuran-1,9'-xanthen]-3-one, which was sensitive to the changes in the water content in the organism, displayed strong green fluorescence in the hydrophobic LDs from its ring-closed form, while it existed in a ring-opened form in the ER to illuminate a strong near-infrared emission. Importantly, the spectral difference was up to 320 nm, and thus the crosstalk between two channels was negligible. With the unique probe, the lipid accumulation in cells treated with different concentrations of oleic acid, cholesterol, and stearic acid has been successfully observed. The changes of LDs and the ER in living cells stimulated by temperature changes and hypoxia stimulation have also been revealed. Meanwhile, the different sizes and distribution of LDs and the ER in various tissues were also studied using the robust probe. This work provides a new approach to the design of dual-emissive probes and contributes to a significant molecular tool to promote the study of organelle interactions.

Near-Infrared Mitochondria-Targetable Single-Molecule probe for Dual-Response of viscosity and sulfur dioxide in vivo

Studies have demonstrated that the viscosity and SO₂ in mitochondria are involved in various physiological processes, which are both important for maintaining the normal function of mitochondria. In this research, we rationally designed and synthesized a novel mitochondria-targetable fluorescent probe Mito-MG with near-infrared absorption and emission for dual-response of viscosity and SO₂. Based on the TICT mechanism and Michael addition reaction, the probe Mito-MG responds to viscosity and SO₂ with different near-infrared fluorescence signals. Importantly, Mito-MG presented outstanding mitochondrial targeting ability (Pearson's colocalization coefficient is 0.8602) and has been successfully applied to imaging of the cells, zebrafish and mice. We believe that Mito-MG could be applied as a powerful tool for a deep understanding of the connection of viscosity and SO₂ in biological systems.

Recent progress in small-molecule fluorescent probes for endoplasmic reticulum imaging in biological systems

Endoplasmic reticulum (ER) is an indispensable organelle in eukaryotic cells involved in protein synthesis and processing, as well as calcium storage and release. Therefore, maintaining the quality of ER is of great importance for cellular homeostasis. Aberrant fluctuations of bioactive species in the ER will result in homeostasis disequilibrium and further cause ER stress, which has evolved to contribute to the pathogenesis of various diseases. Therefore, the real-time monitoring of various bioactive species in the ER is of high priority to ascertain the mysterious roles of ER, which will contribute to unveiling the corresponding mechanism of organism disturbances. Recently, fluorescence imaging has emerged as a robust technique for the direct visualization of molecular events due to its outstanding sensitivity, high temporal-spatial resolution and noninvasive nature. In this review, we comprehensively summarize the recent progress in design strategies, bioimaging applications, potential directions and challenges of ER-targetable small-molecular fluorescent probes.

Multifunctional Fluorescent Probe for Simultaneously Detecting Microviscosity, Micropolarity, and Carboxylesterases and Its Application in Bioimaging

Based on OR logic gate, we proposed a smart near-infrared (NIR) fluorescent probe, named VPCPP, for simultaneously monitoring local microviscosity, micropolarity, and carboxylesterases (CEs) in living cells through blue and red channels. This proposed probe was capable of distinguishing cancer cells from normal cells and had good potential for identifying living liver cell lines. Furthermore, the fluctuations of the three analytes of interest in different cell status was successfully explored. Particularly, facilitated with high-content analysis (HCA) and VPCPP, a simple and efficient high-throughput screening (HTS) platform was first constructed for screening antitumor drugs and studying their effect on the analytes. For the first time, we found that sorafenib-induced ferroptosis led to an increase in the microviscosity and up-regulation of CEs at the same time. Additionally, the procedure that aristolochic acid (AA) induced the overexpression of CEs was verified. Besides, VPCPP was utilized for imaging the variations of the two microenvironment parameters and CEs in the inflammation model. Finally, VPCPP was able to image the tumor ex vivo and in vivo through two channels and one channel separately, as well as to visualize the kidneys and liver ex vivo with dual emissions, which indicated that the probe had great potential for imaging applications such as medical diagnosis, preclinical research, and imaging-guided surgery.

A New Spectral Shift-Based Method to Characterize Molecular Interactions

There are many fluorescence-based applications that can be used to characterize molecular interactions. However, available methods often depend on site-specific labeling techniques or binding-induced changes in conformation or size of the probed target molecule. To overcome these limitations, we applied a ratiometric dual-emission approach that quantifies ligand-induced spectral shifts with sub-nanometer sensitivity. The use of environment-sensitive near-infrared dyes with the method we describe enables affinity measurements and thermodynamic characterization without the explicit need for site-specific labeling or ligand-induced conformational changes. We demonstrate that in-solution spectral shift measurements enable precise characterization of molecular interactions for a variety of biomolecules, including proteins, antibodies, and nucleic acids. Thereby, the described method is not limited to a subset of molecules since even the most challenging samples of research and drug discovery projects like membrane proteins and intrinsically disordered proteins can be analyzed.

Tracking Cell Viability for Adipose-Derived Mesenchymal Stem Cell-Based Therapy by Quantitative Fluorescence Imaging in the Second Near-Infrared Window

Cell survival rate determines engraftment efficiency in adipose-derived mesenchymal stem cell (ADSC)-based regenerative medicine. In vivo monitoring of ADSC viability to achieve effective tissue regeneration is a major challenge for ADSC therapy. Here, we developed an activated near-infrared II (NIR-II) fluorescent nanoparticle consisting of lanthanide-based down-conversion nanoparticles (DCNPs) and IR786s (DCNP@IR786s) for cell labeling and real-time tracking of ADSC viability in vivo. In dying ADSCs due to excessive ROS generation, absorption competition-induced emission of IR786s was destroyed, which could turn on the NIR-II fluorescent intensity of DCNPs at 1550 nm by 808 nm laser excitation. In contrast, the NIR-II fluorescent intensity of DCNPs was stable at 1550 nm by 980 nm laser excitation. This ratiometric fluorescent signal was precise and sensitive for tracking ADSC viability in vivo. Significantly, the nanoparticle could be applied to quantitively evaluate stem cell viability in real-time in vivo. Using this method, we successfully sought two small molecules including glutathione and dexamethasone that could improve stem cell engraftment efficiency and enhance ADSC therapy in a liver fibrotic mouse model. Therefore, we provide a potential strategy for real-time in vivo quantitative tracking of stem cell viability in ADSC therapy.

Revealing the Phase Separation in ER Membranes of Living Cells and Tissues by In Situ NIR Ratiometric Imaging

Biomembranes in the endoplasmic reticulum (ER) play indispensable roles in various bioactivities, and therefore, visualizing the phase separation in ER membranes is crucial for the studies on the fundamental biology of the ER. However, near-infrared (NIR) ratiometric imaging of the phase behaviors of the ER in living cells with different statuses and in diverse tissues has not been investigated. Herein, we developed a polarity-responsive NIR fluorescent probe (DCA) for the visualization of the phase behavior in ER membranes. The probe displayed a large Stokes shift and was highly sensitive to polarity. By direct and native fluorescence imaging at room temperature, the ER_o and ER_d biomembranes in the ER could be clearly distinguished by dual NIR emission colors. Oxidative damage by H₂O₂ and homocystein (Hcy)-induced ER stress can efficiently induce the formation of large-scale ER_o domains in ER membranes. Moreover, we have also revealed that different tissues exhibited diverse phase behaviors in the ER membranes. The ER membranes in cardiac and skeletal muscle tissues showed no evident phase separation, while large-scale ER_o domains existed in the ER of liver tissues and formed at the ER membranes adjacent to lipid droplets (LDs) in white adipose tissues. We expect that the probe could serve as a powerful molecular tool to promote fundamental research studies on ER membranes and relative biomedical areas.

Quantitative, traceable determination of cell viability using absorbance microscopy

Cell viability, an essential measurement for cell therapy products, lacks traceability. One of the most common cell viability tests is trypan blue dye exclusion where blue-stained cells are counted via brightfield imaging. Typically, live and dead cells are classified based on their pixel intensities which may vary arbitrarily making it difficult to compare results. Herein, a traceable absorbance microscopy method to determine the intracellular uptake of trypan blue is demonstrated. The intensity pixels of the brightfield images are converted to absorbance images which are used to calculate moles of trypan blue per cell. Trypan blue cell viability measurements, where trypan blue content in each cell is quantified, enable traceable live-dead classifications. To implement the absorbance microscopy method, we developed an open-source AbsorbanceQ application that generates quantitative absorbance images. The validation of absorbance microscopy is demonstrated using neutral density filters. Results from four different microscopes demonstrate a mean absolute deviation of 3% from the expected optical density values. When assessing trypan blue-stained Jurkat cells, the difference in intracellular uptake of trypan blue in heat-shock-killed cells using two different microscopes is 3.8%. Cells killed with formaldehyde take up ~50% less trypan blue as compared to the heat-shock-killed cells, suggesting that the killing mechanism affects trypan blue uptake. In a test mixture of approximately 50% live and 50% dead cells, 53% of cells were identified as dead (±6% standard deviation). Finally, to mimic batches of low-viability cells that may be encountered during a cell manufacturing process, viability was assessed for cells that were 1) overgrown in the cell culture incubator for five days or 2) incubated in DPBS at room temperature for five days. Instead of making live-dead classifications using arbitrary intensity values, absorbance imaging yields traceable units of moles that can be compared, which is useful for assuring quality for biomanufacturing processes.

A Fluorescent Probe Targeting Mitochondria and Lipid Droplets for Visualization of Cell Death

Subcellular organelles play an indispensable role in various biological process. Therefore, it is very important to develop fluorescent probe to identify different organelles and their dynamics in specific biological processes. Herein, a new fluorescent probe has been prepared, which can be used to visualize cell death via targeting mitochondria and lipid droplets (LDs) in dual-emission channels. The new probe appeared in the form of ring-open in mitochondria to emit strong yellow fluorescence in living cells, while it carried out intramolecular spiral cyclization reaction to target LDs and give a cyan emission in dead cells. The performance of cell death in the UV-exposure, lipopolysaccharide and hydrogen peroxide treatment is successfully revealed by the probe. The probe has great potential in dual colour biomedical imaging of dynamic changes of mitochondria and LDs in biological processes.

Distinguishing normal and inflammatory models by viscosity changes with sensitively mitochondrial-trackable fluorescent probe

Biological microenvironment plays a momentous role in the regulation of various vital activities, and its abnormal changes are often closely related to some diseases. Viscosity, as an indispensable part of microenvironment parameters, has always been one of the research hotspots of investigators. Herein, we constructed a new red-emitting fluorescent probe (HVM) to identify the abnormal situation of mitochondria through viscosity changes in the biological microenvironment. Interestingly, HVM has excellent optical properties such as large stokes shift (160 nm), viscosity sensitivity (195-fold), high photostability, and biochemical properties with low cytotoxicity and excellent biocompatibility. For these reasons, the novel probe could successfully be used to identify the normal and inflammatory models via viscosity changes in biological experiments. Therefore, we provided a convenient synthetic route to obtain viscosity sensor HVM with excellent application properties.