Fluorescent probes for targeting endoplasmic reticulum: design strategies and their applications

An endoplasmic reticulum-targeted and near-infrared fluorescent probe for determining hydrogen polysulfides derived from a naphthalimide

Hydrogen polysulfides (H2Sn, n > 1) was the oxidized form of H2S and plays significant roles in physiological and pathological processes. Abnormal levels of H2Sn is associated with a series of physiological diseases. The endoplasmic reticulum-targetable fluorescent probes are rare for determining H2Sn. Therefore, quantitative detection of H2Sn levels in the endoplasmic reticulum is extremely crucial. In this study, a naphthalimide-based near-infrared fluorescent probe was constructed for detecting H2Sn in the endoplasmic reticulum. 2-Fluoro-5-nitrobenzoate was chose as the H2Sn sensing element. Before the addition of H2Sn, the probe exhibited very weakly fluorescence at 700 nm. After adding H2Sn, the probe demonstrated intense red fluorescence at 700 nm. The probe responded rapidly and exhibited excellent selectivity and sensitivity. And the probe displayed negligible cytotoxicity and had been successfully utilized for near-infrared imaging of H2Sn within the endoplasmic reticulum of A549 cells. Additionally, the probe was also employed for imaging H2Sn in zebrafish and exhibited satisfied results.

Enhancing clinical precision in lung cancer tissue biopsy through elevated response-threshold of an endoplasmic reticulum-targeted fluorogenic probe

Lung carcinoma is the leading cause of mortality globally, posing a significant public health concern. Fluorescent-mediated tumor imaging is emerging as a novel diagnostic and therapeutic approach in clinical practice. Nevertheless, traditional probes lack accuracy in diagnosing tumors due to the overlap in baseline values of certain tumor biomarkers between normal and tumor cells as both exhibit turn-on fluorescence, rendering it impossible to distinguish tumor tissue from normal tissue with high resolution. We introduce a sensing strategy that constructs a probe with an elevated biomarker response-threshold and targeting ability for the endoplasmic reticulum (ER), enabling precise distinction between tumor and normal cells, and successfully develop such a probe. Elevating the response-threshold is advantageous in minimizing interference from baseline values of biomarkers in normal cells. Additionally, targeting the ER ensures that the probe's response range is consistent with the biomarker content in the ER, collectively enhancing differentiation between normal and cancer cells. Using this novel probe, a distinct bright fluorescence signal from tumors could be observed in confocal imaging of tumor tissues from tumor-bearing mice after intravenous injection, in stark contrast to the limited fluorescence emanating from normal tissues. Furthermore, this probe demonstrated exceptional precision in distinguishing clinical lung cancer tissue from para-cancer tissue. This work presents a more reliable tumor detection strategy, capable of accurate diagnosis even when the biomarker is highly expressed in both normal and tumor tissues. It promises to be a valuable tool for future clinical applications, particularly in intraoperative assisted resection.

Naphthalimide and Carbazole Based Mechanochromic Molecular Dyads and Triads for Selective Lysosome Imaging

In this study, we report the design and development of a stable fluorescent probe that is selectively localized in the cytosol of Hela cells. We designed two probes, 1 and 2, with D-π-A (carbazole (Cbz)-vinyl-naphthalimide (NPI)) and A-π-D-π-A (NPI-vinyl-Cbz-vinyl-NPI) architecture, respectively. Probes 1 and 2 exhibit broad photoluminescence (PL) spectra ranging from green (550 nm) to far-red (800 nm) in solutions and aggregated states. In the solid-state, the PL of these probes shows a bathochromic shift, which can be attributed to intermolecular interactions. In a water-rich medium, Probe 1, with a single NPI moiety, shows aggregation-caused quenching (ACQ) but retains a moderate quantum yield of 13.7 % (Φsoln=61.4 %). On the other hand, probe 2, with two NPI units, showed aggregation-induced enhanced emission AIEE, where the PLQY is increased nearly 4 times (Φsoln=3.5 %, Φagg=12.8 %). In-vitro cell studies revealed that these probes are non-toxic and effectively stain cells in green and red channels. Notably, Probe 1 demonstrated excellent cellular uptake and selectivity for lysosome, with a Pearson overlap coefficient of 0.91.

Lophine analogues as fluorophores for selective bioimaging of the endoplasmic reticulum

The design of small-molecule fluorescent probes for labelling the endoplasmic reticulum (ER) revolves around a well-established albeit limited group of structural architectures. Here, we synthesized new fluorescent lophines in one step in high-temperature water (HTW) and explored their application as dyes for selective bioimaging of the ER.

Exploiting Flavylium Merocyanine Dyes for Intrinsic, Multiplexed Labeling of the Endoplasmic Reticulum

Merocyanine dyes are a versatile class of donor-acceptor polymethine dyes that exhibit unique properties depending on their structural makeup and surrounding environment. Scaffolds that favor the cyanine state (i.e., narrow, red-shifted absorption and high fluorescence quantum yields) in biologically relevant settings are highly advantageous for multiplexed labeling experiments, but remain limited by their visible absorption. Herein, we synthesize a new class of far-red (650-700 nm) to near-infrared (NIR, 700-1000 nm) flavylium merocyanine dyes and demonstrate that, unlike conventional scaffolds, they favor the cyanine state with increasing solvent viscosity and hydrogen bond donation, rather than polarity. We leverage these advantageous properties for live cell labeling, where we observed intrinsic targeting to the endoplasmic reticulum (ER) and lipid droplets, and minimal crosstalk with commercial stains. We reveal that intrinsic ER labeling is achieved by the dipolarity in the cyanine state and lipophilicity (ClogP) of the merocyanine architecture, making this class of dyes a simple, red-shifted alternative to the more structurally complex ER stains currently available.

Endoplasmic Reticulum-Targeting Delayed Fluorescent Probe for Dual-mode Nitroreductase Sensing

Organic thermally activated delayed fluorescence (TADF) materials, known for their long-lived emission properties, are highly sought after for background-free imaging of selective analytes in time-resolved modes. However, their practical application faces significant challenges, including the air sensitivity of triplet states, lack of organelle specificity, and the absence of precise analyte recognition centres. These limitations hinder their effectiveness in detecting key cancer biomarkers such as nitroreductase (NTR). Herein, we present the development of donor (triphenylamine)-acceptor (quinoxaline)-based probes, TPQS and TPNS, which are functionalized with a sulphonamide unit to offer endoplasmic reticulum specificity. TPQS exhibits delayed fluorescence, attributed to a minimal singlet-triplet energy gap, as confirmed by time-resolved fluorescence measurements. Additionally, a nonfluorescent probe, TPNS, is synthesized by introducing a nitro group into the sulphonamide unit of the TPQS backbone, serving as a recognition centre for NTR. Upon reacting with NTR, TPNS displays a "turn-on" luminescence and delayed fluorescence, enabling dual-mode detection of NTR through both confocal fluorescence imaging and time-resolved fluorescence imaging (TRFI) in cancer cells. These findings underscore the potential of delayed fluorescent emitters for the sensitive and specific detection of cancer biomarkers in complex biological environments.

Multi-organelle imaging with dye combinations: targeting the ER, mitochondria, and plasma membrane

Multi-organelle imaging allows the visualization of multiple organelles within a single cell, allowing monitoring of the cellular processes in real-time using various fluorescent probes that target specific organelles. However, the limited availability of fluorophores and potential spectral overlap present challenges, and many optimized designs are still in nascency. In this work, we synthesized various sulfonamide-based organic fluorophores that emit in the blue, green, and red regions to target different sub-cellular organelles. By utilizing binary mixtures, we successfully demonstrated multiple imaging of the sub-cellular organelles, such as the endoplasmic reticulum, plasma membrane, and mitochondria in HeLa cells, and dual imaging of the endoplasmic reticulum and mitochondria in A549 lung carcinoma cells with the help of blue and red-emitting fluorophores without any spectral spillover. Additionally, these photostable probes allowed precise cell staining and differentiation, structural features, and live cell dynamics. This approach of utilizing fluorescent mixtures can gain traction for various cellular studies and investigations.

Tröger's Base as a Potential Bridge to Type-I Photosensitizers: Mechanism and Antitumor Applications

In contrast to Type-II photodynamic therapy (PDT), Type-I PDT with less oxygen consumption has shown great potential against tumor hypoxia. However, there are limited strategies available for designing Type-I photosensitizers (PSs). Herein, we present a promising strategy for synthesizing Type-I PSs (TBC-1-TBC-4) using Tröger's base (TB) framework. The TB framework can promote intersystem crossing efficiency and create an electron-rich environment, making it the most likely site for electron transfer to O2 to generate Type-I ROS. As anticipated, TBC-1-TBC-4 demonstrates Type-I ROS generation capability and their impressive visible light-harvesting ability significantly enhances this capability. Among them, TBC-1 demonstrates outstanding biocompatibility and PDT efficiency in vitro under both normoxia and hypoxia. Furthermore, TBC-1 effectively inhibits tumor growth in vivo, with negligible side effects. This is attributed to TBC-1's efficient generation of Type-I ROS and endoplasmic reticulum targeting ability. This study thus offers useful insights into developing Type-I PSs.

Design strategies for organelle-selective fluorescent probes: where to start?

Monitoring physiological changes within cells is crucial for understanding their biological aspects and pathological activities. Fluorescent probes serve as powerful tools for this purpose, offering advantageous characteristics over genetically encoded probes. While numerous organelle-selective probes have been developed in the past decades, several challenges persist. This review explores the strategies and key factors contributing to the successful rationale design of these probes. We systematically discuss the typical mode of cellular uptake generally adopted by fluorescent probes and provide a detailed examination of the key factors to consider in design rationale from two perspectives: the properties of the target organelle and the physicochemical properties of the probe itself. Additionally, recent examples of organelle-targeted probes are presented, along with a discussion of the current challenges faced by fluorescent probes in the field.

Endoplasmic reticulum-targeting fluorescence turn-on probe for nitric oxide detection in living cells and serum samples

Nitric oxide (NO) is an important gas signaling molecule, and endoplasmic reticulum (ER) stress induced by NO may be related to the pathogenesis of many diseases. Therefore, the development of ER-targeted fluorescent probes for NO is of great significance to investigate the relationship between ER stress and NO concentration changes in related diseases. Herein, an ER-targeted fluorescent probe (ER-Np) for sensing NO was constructed. ER-Np was served as an excellent tool for detection NO with high selectivity, sensitivity and ER-targetable ability. Moreover, fluorescence imaging experiments indicated that ER-Np is capable of imaging NO in living cells. Impressively, visualization of endogenous NO production during dithiothreitol (DTT)-induced ER stress in living cells was successfully observed. In addition, we found that serum NO levels were upregulated in epilepsy children, which opens up a new avenue for further understanding the relationship between the diagnostic of epilepsy.

A STEDable BF2-Azadipyrromethene Fluorophore for Nuclear Membrane and Associated Endoplasmic Reticulum Imaging

The endoplasmic reticulum and the internal nuclear compartments are intrinsically connected through the nuclear membrane, pores and lamina. High resolution imaging of each of these cellular features concurrently remains a significant challenge. To that end we have developed a new molecular nuclear membrane-endoplasmic reticulum (NM-ER) staining fluorophore with emission maxima at 650 nm. NM-ER is compatible with fixed and live cell imaging and stimulated emission depletion microscopy (STED) showing significant improvement in resolution when compared to comparable confocal laser scanning microscopy. The imaging versatility of NM-ER was illustrated through its compatible use with other fluorophores for co-imaging with DNA, nuclear pores and lamina allowing cellular abnormalities to be identified. NM-ER alone, or in use with other nuclear region labels could be an important tool for the investigation of nuclear transport and associated cellular processes.

Targeted Photoactivatable Green-Emitting BODIPY Based on Directed Photooxidation-Induced Activation and its Application to Live Dynamic Super-Resolution Microscopy

Photoactivatable fluorescent probes are valuable tools in bioimaging for tracking cells down to single molecules and for single molecule localization microscopy. For the latter application, green emitting dyes are in demand. We herein developed an efficient green-emitting photoactivatable furanyl-BODIPY (PFB) and we established a new mechanism of photoactivation called Directed Photooxidation Induced Activation (DPIA) where the furan is photo-oxidized in a directed manner by the singlet oxygen produced by the probe. The efficient photoconverter (93-fold fluorescence enhancement at 510 nm, 49 % yield conversion) is functionalizable and allowed targeting of several subcellular structures and organelles, which were photoactivated in live cells. Finally, we demonstrated the potential of PFB in super-resolution imaging by performing PhotoActivated Localization Microscopy (PALM) in live cells.

Efficient light-induced reactive oxygen species production from a far-red ER-targeting BODIPY dye

Generation of reactive oxygen species (ROS) within the ER evokes stress leading to immunogenic cell death. A red light activated BODIPY dye capable of subcellular localization within the ER producing high quantum yields of ROS is reported. The ability of this dye to act as a photodynamic therapy (PDT) agent in breast cancer cells suggests promising organelle-targeted therapeutics.

Diphenylbutadiene Fluorescent Analogues in Sub-Cellular Imaging and Monitoring Mitophagy

A series of donor-acceptor (D-π-A) substituted diphenylbutadienes exhibiting solvatochromic emission and a large Stokes shift (100-200 nm) were designed and synthesized for distinctive organelle labelling, enabling real-time monitoring of organelle behaviour such as lysosomal dynamics, mitophagy monitoring, and stress responses. The morpholine-substituted D-A-D diphenylbutadiene (M2) was employed to investigate selective imaging of lysosomes, the uptake of damaged mitochondria through mitophagy, and monitoring lysosomal viscosity or pH changes. Other diphenylbutadiene derivatives (M1, M3, M4) selectively accumulated in lipid droplets. All the synthesized derivatives demonstrated significant uptake in 5-day post-fertilization zebrafish larvae, with M2 showing maximum uptake in the enterocyte-containing heart and intestinal regions, which include the lysosomes.

Leveraging the Photofunctions of Transition Metal Complexes for the Design of Innovative Phototherapeutics

Despite the advent of various medical interventions for cancer treatment, the disease continues to pose a formidable global health challenge, necessitating the development of new therapeutic approaches for more effective treatment outcomes. Photodynamic therapy (PDT), which utilizes light to activate a photosensitizer to produce cytotoxic reactive oxygen species (ROS) for eradicating cancer cells, has emerged as a promising approach for cancer treatment due to its high spatiotemporal precision and minimal invasiveness. However, the widespread clinical use of PDT faces several challenges, including the inefficient production of ROS in the hypoxic tumor microenvironment, the limited penetration depth of light in biological tissues, and the inadequate accumulation of photosensitizers at the tumor site. Over the past decade, there has been increasing interest in the utilization of photofunctional transition metal complexes as photosensitizers for PDT applications due to their intriguing photophysical and photochemical properties. This review provides an overview of the current design strategies used in the development of transition metal complexes as innovative phototherapeutics, aiming to address the limitations associated with PDT and achieve more effective treatment outcomes. The current challenges and future perspectives on the clinical translation of transition metal complexes are also discussed.

Flaring Inflammation and ER Stress by an Organelle-Specific Fluorescent Cage

The endoplasmic reticulum (ER) plays an important role in protein synthesis and its disruption can cause protein unfolding and misfolding. Accumulation of such proteins leads to ER stress, which ultimately promotes many diseases. Routine screening of ER activity in immune cells can flag serious conditions at early stages, but the current clinically used bio-probes have limitations. Herein, an ER-specific fluorophore based on a biocompatible benzothiadiazole-imine cage (BTD-cage) with excellent photophysical properties is developed. The cage outperforms commercially available ER stains in long-term live cell imaging with no fading or photobleaching over time. The cage is responsive to different levels of ER stress where its fluorescence increases accordingly. Incorporating the bio-probe into an immune disorder model, a 6-, 21-, and 48-fold increase in intensity is shown in THP-1, Raw 246.7, and Jurkat cells, respectively (within 15 min). These results strongly support that this system can be used for rapid visual and selective detection of ER stress. It is envisaged that tailoring molecular interactions and molecular recognition using supramolecular improved fluorophores can expand the library of biological probes for enhanced selectivity and targetability toward cellular organelles.

Subcellular targeting strategies for protein and peptide delivery

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

Precise subcellular targeting approaches for organelle-related disorders

Pharmacological research has expanded to the nanoscale level with advanced imaging technologies, enabling the analysis of drug distribution at the cellular organelle level. These advances in research techniques have contributed to the targeting of cellular organelles to address the fundamental causes of diseases. Beyond navigating the hurdles of reaching lesion tissues upon administration and identifying target cells within these tissues, controlling drug accumulation at the organelle level is the most refined method of disease management. This approach opens new avenues for the development of more potent therapeutic strategies by delving into the intricate roles and interplay of cellular organelles. Thus, organelle-targeted approaches help overcome the limitations of conventional therapies by precisely regulating functionally compartmentalized spaces based on their environment. This review discusses the basic concepts of organelle targeting research and proposes strategies to target diseases arising from organelle dysfunction. We also address the current challenges faced by organelle targeting and explore future research directions.

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

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

Surface-Functionalized Halo-Tag Gold Nanoprobes for Live-Cell Long-Term Super-Resolution Imaging of Endoplasmic Reticulum Dynamics

Super-resolution fluorescence microscopy has emerged as a powerful tool for studying endoplasmic reticulum (ER) dynamics in living cells. However, the lack of high-brightness, high-photostability, and stable labeling probes makes long-term super-resolution imaging of the ER still challenging. Herein, we reported a surface-functionalized Halo-tag gold nanofluorescent probe (GNP-Atto565-fR8-CA) that exhibits excellent brightness, photostability, and biocompatibility. GNP-Atto565-fR8-CA can simultaneously load multiple Atto565 dye molecules, significantly improving its brightness. Modifying the cell-penetrating peptide fR8 enables GNP-Atto565-fR8-CA to be efficiently delivered into the cytoplasm, overcoming the challenge of their easy entrapment in vesicles. Fluorescent labeling of ER proteins via Halo tags enables high specificity and stable labeling of GNP-Atto565-fR8-CA to the ER. The SIM super-resolution imaging results showed that GNP-Atto565-fR8-CA can track and observe the long-term dynamic process of the ER, and can also be used for long-term super-resolution imaging of the dynamic interactions between the ER and other organelles. This work offers a practical tool to study live-cell ER ultrastructure and dynamics.

Fabrication of a Near-Infrared-Emissive Probe for Detecting Dipeptidyl Peptidase 4 in the Liver of Diabetic Mice and Clinical Serum

Dipeptidyl peptidase 4 (DPP4) plays a key role in glucose metabolism, which has been a close target for diabetes pathology and treatment. It is significant for the evaluation of cellular DPP4 activity in various biological systems. Fluorescence imaging technology is currently a popular method for detecting enzymes in living cells due to its advantages of high selectivity, high sensitivity, high spatiotemporal resolution, and real-time visualization. Herein, a near-infrared (NIR)-emissive probe NEDP with a large Stokes shift (153 nm) was developed for the assay of DPP4 activity. Upon addition of DPP4, NEDP can emit a significant turn-on NIR fluorescence signal (673 nm) with high sensitivity and specificity. Moreover, NEDP can successfully be used for imaging of intracellular DPP4, confirming the regulation of DPP4 expression in hyperglucose and its treatment in living cells. Most importantly, NEDP can not only monitor the changes of DPP4 in vivo but also show that DPP4 in diabetes is mainly up-regulated in the liver, and the level of DPP4 is positively correlated with the pathological damage of the liver. In addition, NEDP can identify the serum of diabetic patients from healthy people through the fluorescence response to DPP4. These results demonstrated that the designed probe NEDP provides a prospective visual tool to explore the relationship between DPP4 and diabetes and would be applied for detecting serum of diabetes in the clinic.

Surface functionalized perovskite nanocrystals: a design strategy for organelle-specific fluorescence lifetime multiplexing

Fluorescent molecules or materials with high photoluminescence quantum yields and stability towards photobleaching are ideally suited for multiplex imaging. Despite complying with such properties, perovskite nanocrystals (Pv-NCs) are rarely used for bioimaging owing to their toxicity and limited stability in aqueous media and towards human physiology. We aim to address these deficiencies by designing core-shell structures with Pv-NCs as the core and surface-engineered silica as the shell (SiO2@Pv-NCs) since silica is recognized as a biologically benign carrier material and is known to be excreted through urine. The post-grafting methodology is adopted for developing [SiO2@Pv-NCs]tpm and [SiO2@Pv-NCs]tsy (tpm: triphenylphosphonium ion, tsy: tosylsulfonamide) for specific imaging of mitochondria and endoplasmic reticulum (ER) of the live HeLa cell, respectively. A subtle difference in their average fluorescence decay times ([SiO2@Pv-NCs]tpm: tpm τ av = 3.1 ns and [SiO2@Pv-NCs]tsy: tsy τ av = 2.1 ns) is used for demonstrating a rare example of perovskite nanocrystals in fluorescence lifetime multiplex imaging.

Fluorescent probes for targeting the Golgi apparatus: design strategies and applications

The Golgi apparatus is an essential organelle constructed by the stacking of flattened vesicles, that is widely distributed in eukaryotic cells and is dynamically regulated during cell cycles. It is a central station which is responsible for collecting, processing, sorting, transporting, and secreting some important proteins/enzymes from the endoplasmic reticulum to intra- and extra-cellular destinations. Golgi-specific fluorescent probes provide powerful non-invasive tools for the real-time and in situ visualization of the temporal and spatial fluctuations of bioactive species. Over recent years, more and more Golgi-targeting probes have been developed, which are essential for the evaluation of diseases including cancer. However, when compared with systems that target other important organelles (e.g. lysosomes and mitochondria), Golgi-targeting strategies are still in their infancy, therefore it is important to develop more Golgi-targeting probes. This review systematically summarizes the currently reported Golgi-specific fluorescent probes, and highlights the design strategies, mechanisms, and biological uses of these probes, we have structured the review based on the different targeting groups. In addition, we highlight the future challenges and opportunities in the development of Golgi-specific imaging agents and therapeutic systems.

Modular development of organelle-targeting fluorescent probes for imaging formaldehyde in live cells

Formaldehyde (FA) is endogenously generated via fundamental biological processes in living systems. Aberrant FA homeostasis in subcellular microenvironments is implicated in numerous pathological conditions. Fluorescent probes for detecting FA in specific organelles are thus of great research interest. Herein, we present a modular strategy to construct diverse organelle-targeting FA probes by incorporating selective organelle-targeting moieties into the scaffold of a 1,8-naphthalimide-derived FA fluorescent probe. These probes react with FA through the 2-aza-Cope arrangement and exhibit highly selective fluorescence increases for detecting FA in aqueous solutions. Moreover, these organelle-targeting probes, i.e., FFP551-Nuc, FFP551-ER, FFP551-Mito, and FFP551-Lyso, allow selective localization and imaging of FA in the nucleus, endoplasmic reticulum, mitochondria, and lysosomes of live mammalian cells, respectively. Furthermore, FFP551-Nuc has been successfully employed to monitor changes of endogenous FA levels in the nucleus of live mammalian cells. Overall, these probes should represent new imaging tools for studying the biology and pathology associated with FA in different intracellular compartments.

A Universal and Programmable Platform based on Fluorescent Peptide-Conjugated Probes for Detection of Proteins in Organelles of Living Cells

Realizing protein analysis in organelles of living cells is of great significance for developing diagnostic and therapeutic methods of diseases. Fluorescent-labeled antibodies with well imaging performance and high affinity are classical biochemical tools for protein analysis, while due to the inability to effectively enter into cells, not to mention organelles and the uncontrollable reaction sites that might cause antibodies inactivation when chemically modification, they are hard to apply to living cells. Inspired by the structure of fluorescent-labeled antibodies, we designed as a universal detection platform that was based on the peptide-conjugated probes (PCPs) and consisted of three parts: a) a rotor type fluorescent molecular scaffold for conjugation and signal output; b) the cell penetration protein recognition unit; c) the subcellular organelle targeting unit. In living cells, PCPs could firstly localize at organelles and then proceed protein specific recognition, thus jointly leading to the restriction of twisted intramolecular charge transfer and activation of fluorescence signal. As a proof-of-concept, six different proteins in three typical intracellular organelles could be detected by our platform through simply replacing the recognition sequence of proteins and matching organelle targeting units. The position and intensity of fluorescence signals demonstrated specificity of PCPs and universality of the platform.

Organelle-targeted Laurdans measure heterogeneity in subcellular membranes and their responses to saturated lipid stress

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

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

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

An ER-targeted, Viscosity-sensitive Hemicyanine Dye for the Diagnosis of Nonalcoholic Fatty Liver and Photodynamic Cancer Therapy by Activating Pyroptosis Pathway

The concept of molecular design, integrating diagnostic and therapeutic functions, aligns with the general trend of modern medical advancement. Herein, we rationally designed the smart molecule ER-ZS for endoplasmic reticulum (ER)-targeted diagnosis and treatment in cell and animal models by combining hemicyanine dyes with ER-targeted functional groups (p-toluenesulfonamide). Owing to its ability to target the ER with a highly specific response to viscosity, ER-ZS demonstrated substantial fluorescence turn-on only after binding to the ER, independent of other physiological environments. In addition, ER-ZS, being a small molecule, allows for the diagnosis of nonalcoholic fatty liver disease (NAFLD) via liver imaging based on high ER stress. Importantly, ER-ZS is a type I photosensitizer, producing O2 ⋅- and ⋅OH under light irradiation. Thus, after irradiating for a certain period, the photodynamic therapy inflicted severe oxidative damage to the ER of tumor cells in hypoxic (2 % O2 ) conditions and activated the unique pyroptosis pathway, demonstrating excellent antitumor capacity in xenograft tumor models. Hence, the proposed strategy will likely shed new light on integrating molecular optics for NAFLD diagnosis and cancer therapy.

Ultrabright Green-Emissive Nanodots for Precise Biological Visualization

Developing general methods to fabricate water-dispersible and biocompatible fluorescent probes will promote different biological visualization applications. Herein, we report a metal-facilitated method to fabricate ultrabright green-emissive nanodots via the one-step solvothermal treatment of rose bengal, ethanol, and various metal ions. These metal-doped nanodots show good water dispersity, ultrahigh photoluminescence quantum yields (PLQYs) (e.g., the PLQY of Fe-doped nanodots (FeNDs) was ∼97%), and low phototoxicity. Owing to the coordination effect of metal ions, the FeNDs realize glutathione detection with outstanding properties. Benefiting from the high endoplasmic reticulum (ER) affinity of the chloride group, the FeNDs can act as an ER tracker with long ER imaging capacity (FeNDs: >24 h; commercial ER tracker: ∼1 h) and superb photostability and can achieve tissue visualization in living Caenorhabditis elegans. The metal-doped nanodots represent a general nanodot preparation method and may shed new light on diverse biological visualization uses.

A two-in-one probe: imaging lipid droplets and endoplasmic reticulum in tandem

The endoplasmic reticulum (ER) and lipid droplets (LDs) intricately interact in cellular processes, with the ER serving as a hub for lipid synthesis and LDs acting as storage organelles for lipids. Developing fluorescent probes that can simultaneously visualise the ER and LDs provides a means for real-time and specific visualisation of these subcellular organelles and elucidating their interaction. Herein, we present synthetically simple and novel donor-π-acceptor styryl fluorophores (PFC, PFN and PFB) incorporating pentafluorophenyl (PFP) to demonstrate exquisite discriminative imaging of ER and LD with a single excitation wavelength. The PFP moiety aids the ER selectivity, while the overall hydrophobicity of the molecule aids in the LD targeting. Furthermore, the fluorophores are utilised in studying the changes in size, distribution, and biogenesis of LDs within ER regions after treatment with oleic acid. Strong emission, lower concentrations ∼100 nM requirement, minimal cytotoxicity, and photostability make these fluorophores excellent tools for probing sub-cellular dynamics.

Chromene-based BioAIEgens: 'in-water' synthesis, regiostructure-dependent fluorescence and ER-specific imaging

Exploration of artificial aggregation-induced emission luminogens (AIEgens) has garnered extensive interest in the past two decades. In particular, AIEgens possessing natural characteristics (BioAIEgens) have received more attention recently due to the advantages of biocompatibility, sustainability and renewability. However, the extremely limited number of BioAIEgens extracted from natural sources have retarded their development. Herein, a new class of BioAIEgens based on the natural scaffold of chromene have been facilely synthesized via green reactions in a water system. These compounds show regiostructure-, polymorphism- and substituent-dependent fluorescence, which clearly illustrates the close relationship between the macroscopic properties and hierarchical structure of aggregates. Due to the superior biocompatibility of the natural scaffold, chromene-based BioAIEgens can specifically target the endoplasmic reticulum (ER) via the introduction of tosyl amide. This work has provided a new chromene scaffold for functional BioAIEgens on the basis of green and sustainable 'in-water' synthesis, applicable regiostructure-dependent fluorescence, and effective ER-specific imaging.

Rationally Engineered hCES2A Near-Infrared Fluorogenic Substrate for Functional Imaging and High-Throughput Inhibitor Screening

Human carboxylesterase 2A (hCES2A) is an important endoplasmic reticulum (ER)-resident enzyme that is responsible for the hydrolytic metabolism or activation of numerous ester-bearing drugs and environmental toxins. The previously reported hCES2A fluorogenic substrates suffer from limited emission wavelength, low specificity, and poor localization accuracy, thereby greatly limiting the in situ functional imaging of hCES2A and drug discovery. Herein, a rational ligand design strategy was adopted to construct a highly specific near-infrared (NIR) substrate for hCES2A. Following scaffold screening and recognition group optimization, HTCF was identified as a desirable NIR fluorophore with excellent photophysical properties and high ER accumulation ability, while several HTCF esters held a high potential to be good hCES2A substrates. Further investigations revealed that TP-HTCF (the tert-pentyl ester of HTCF) was an ideal substrate with ultrahigh sensitivity, excellent specificity, and a substantial signal-to-noise ratio. Upon the addition of hCES2A, TP-HTCF could be rapidly hydrolyzed to release HTCF, a chemically stable product that emitted bright fluorescent signals at around 670 nm. A TP-HTCF-based biochemical assay was then established for the high-throughput screening of potent and cell-active hCES2A inhibitors from an in-house compound library. Furthermore, TP-HTCF displayed high imaging resolution for imaging hCES2A in living cells as well as mouse liver slices and tumor-xenograft mice. Collectively, this study demonstrates a rational strategy for developing highly specific fluorogenic substrates for an ER-resident target enzyme, while TP-HTCF can act as a practical tool for sensing hCES2A in living systems.

N-Functionalized fluorophores: detecting urinary albumin and imaging lipid droplets

A series of novel N-sulfonyl pyridinium fluorophores were designed, synthesized, and explored in terms of their ability to bind with serum albumins. Upon binding the fluorophores with BSA, noticeable emission wavelength or intensity changes accompanied by color changes were observed. Competitive binding studies revealed that the fluorophore selectively binds to the warfarin site, but the binding affinity also depends on the nature of the scaffold. Additionally, the fluorophores were employed to detect spiked serum albumin in artificial urine. Cellular imaging experiments indicated that the fluorophores accumulate within lipid droplets (LDs), suggesting their potential as promising biomarkers for lipid droplets. Furthermore, the fluorescence intensity, number, and size of LDs increased upon serum starvation.

Design and synthesis of photostable triphenylamine based neutral AIE nano luminogens: specific and long-term tracking of mitochondria in cells

With the increasing dependence on fluorescence bioimaging, luminogens with aggregation-induced emission (AIE) properties have gained significant attention due to their excellent photostabilization, minimal photobleaching, high reliability, and superior biocompatibility. Since mitochondria are crucial subcellular organelles in eukaryotic cells with important biological functions, organelle-specific AIE emitters with distinct functions have been highly sought after, but with limited success using simple synthetic methods. Here, we describe a strategy for synthesizing two triphenylamine (TPA) based acrylonitriles, tethered to different donor groups, TPA and phenothiazine (PTZ), respectively, with superior AIE properties using Suzuki coupling. We conducted a systematic and detailed experimental analysis of the structural characteristics of both AIE luminogens, which exhibited excellent photostability, a large Stokes shift, and bright solid-state emission. A cell viability study carried out with F1 and F2 dyes revealed that both luminogens exhibited excellent biocompatibility. Based on fluorescence experiments, F2 displayed excellent AIE characteristics, permeability, biocompatibility, and photostability compared to rhodamine 123, allowing it to selectively stain and track mitochondria in cancer cells over an extended period of time. The Pearson correlation coefficient of F2 and rhodamine 123 was estimated to have an r-value of 0.99. Our findings are expected to provide insight into the synthesis of an extensive archive of AIE-based acrylonitriles with fascinating properties for mitochondrial staining.

Hyperbranched polysiloxane-based probe with enhanced lipophilicity for visualizing ONOO- fluctuations in endoplasmic reticulum

The endoplasmic reticulum, a cellular signaling regulator, participates in the synthesis and secretion of many proteins, glycogen, lipids and cholesterol substances. Peroxynitrite (ONOO^-) is a highly oxidative and nucleophilic agent. Abnormal fluctuations of ONOO^- induce oxidative stress in the endoplasmic reticulum, further disrupting the normal function of protein folding and transport and glycosylation modification, ultimately leading to neurodegenerative diseases, cancer and Alzheimer's disease. Up to now, most probes have tended to achieve targeting functions by introducing specific targeting groups. However, this approach increased the difficulty of the construction process. Therefore, a simple and efficient construction strategy for fluorescent probes with excellent specificity targeting the endoplasmic reticulum is lacking. To overcome this difficulty and put forward an efficient design strategy for the endoplasmic reticulum targeted probes, in this paper, we constructed alternating rigid and flexible polysiloxane-based hyperbranched polymeric probes (Si-Er-ONOO) by bonding perylenetetracarboxylic anhydride and silicon-based dendrimers for the first time. Efficient and specific targeting of the endoplasmic reticulum was successfully achieved by the excellent lipid solubility of Si-Er-ONOO. Furthermore, we observed different effects of metformin and rotenone on the changes of ONOO^- volatility in the cellular and zebrafish internal environment by Si-Er-ONOO. We believe that Si-Er-ONOO will expand the application of organosilicon hyperbranched polymeric materials in bioimaging and provide an excellent indicator of reactive oxygen species fluctuations in biological systems.

A review: Small organic molecule dual/multi-organelle-targeted fluorescent probes

In recent years, the dual/multi-organelle-targeted fluorescent probe based on small organic molecules has good biocompatibility and can visualize the interaction between different organelles, which has attracted much attention. In addition, these probes can also be used to detect small molecules in the organelle environment, such as active sulfur species (RSS), reactive oxygen species (ROS), pH, viscosity and so on. However, the review of dual/multi-organelle-targeted fluorescent probe for small organic molecules lacks a systematic summary, which may hinder the development of this field. In this review, we will focus on the design strategies and bioimaging applications of dual/multi-organelle-targeted fluorescent probe, and classify them into six classes according to different organelles targeted. The first class probe targeted mitochondria and lysosome. The second class probe targeted endoplasmic reticulum and lysosome. The third class probe targeted mitochondria and lipid droplets. The fourth class probe targeted endoplasmic reticulum and lipid droplets. The fifth class probe targeted lysosome and lipid droplets. The sixth class multi-targeted probe. The mechanism of these probes targeting organelles and the visualization of the interaction between different organelles are emphasized, and the prospect and future development direction of this research field are prospected. This will provide a systematic idea for the development and functional research of dual/multi-organelle-targeted fluorescent probe, and promote its research in related physiological and pathological medicine field in the future.

Recent Development of Advanced Fluorescent Molecular Probes for Organelle-Targeted Cell Imaging

Fluorescent molecular probes are very powerful tools that have been generally applied in cell imaging in the research fields of biology, pathology, pharmacology, biochemistry, and medical science. In the last couple of decades, numerous molecular probes endowed with high specificity to particular organelles have been designed to illustrate intracellular images in more detail at the subcellular level. Nowadays, the development of cell biology has enabled the investigation process to go deeply into cells, even at the molecular level. Therefore, probes that can sketch a particular organelle's location while responding to certain parameters to evaluate intracellular bioprocesses are under urgent demand. It is significant to understand the basic ideas of organelle properties, as well as the vital substances related to each unique organelle, for the design of probes with high specificity and efficiency. In this review, we summarize representative multifunctional fluorescent molecular probes developed in the last decade. We focus on probes that can specially target nuclei, mitochondria, endoplasmic reticulums, and lysosomes. In each section, we first briefly introduce the significance and properties of different organelles. We then discuss how probes are designed to make them highly organelle-specific. Finally, we also consider how probes are constructed to endow them with additional functions to recognize particular physical/chemical signals of targeted organelles. Moreover, a perspective on the challenges in future applications of highly specific molecular probes in cell imaging is also proposed. We hope that this review can provide researchers with additional conceptual information about developing probes for cell imaging, assisting scientists interested in molecular biology, cell biology, and biochemistry to accelerate their scientific studies.

Tracking endoplasmic reticulum viscosity during ferroptosis and autophagy using a molecular rotor probe

Ferroptosis is a unique non-apoptotic cell death process associated with endoplasmic reticulum (ER) stress-related diseases. We have designed and synthesized a far-red emitting and ER targetable viscosity-sensitive fluorophore to track ER-phagy. Furthermore, the ER viscosity alteration during the ferroptosis process was investigated via intensity and lifetime-based spectroscopy and microscopy.

Endoplasmic reticulum-targeted polymer dots encapsulated with ultrasonic synthesized near-infrared carbon nanodots and their application for in vivo monitoring of Cu2

Endoplasmic reticulum (ER) is the largest organelle in eukaryotic cells and plays a variety of functions in living cells include protein folding, calcium homeostasis, and lipid biosynthesis. Normal function of ER is crucial for cell survival, while disequilibrium of ER can cause misfolding of proteins and ER stress, leading to many serious diseases. It has been documented that ER stress is closely related to the metabolism of Cu²⁺, as ER is the main intracellular accumulation space of Cu²⁺ and toxic reactive oxygen species can be generated by Cu²⁺ via Fenton and Haber-Weiss reactions. In this context, developing a powerful tool capable of selective and sensitive monitoring of Cu²⁺ in ER and investigating its role in physiological and pathological processes is of great importance. Herein, we report the first ER targeted near infrared (NIR) nanosensor, polymer dots encapsulated with NIR hydrophobic carbon nanodots, for detecting Cu²⁺ in biosystems. This nanosensor with stable fluorescence showed a fast response toward Cu²⁺ (120 s) and can be used for the quantification of Cu²⁺ in a linear range covering from 0.25 to 9.0 μM with a detection limit of 13 nM. In addition, the fluorescence variations of the nanosensor are remarkably specific to Cu²⁺ in comparison with the other metal ions and amino acids. Moreover, the developed nanosensor exhibited low cytotoxicity, good biocompatibility, and ER targeting ability. Because of these excellent spectroscopic features, the nanosensor was successfully utilized for visualizing Cu²⁺ fluctuations at the living cell, zebrafish and mouse levels, which further proved its potential application in biological systems.

A dual-targeting fluorescent probe for simultaneous and discriminative visualization of lipid droplets and endoplasmic reticulum

A single fluorescent probe (SF-probe) that can simultaneously and discriminatively visualize two organelles is a powerful tool to investigate their interaction in cellular processes.

Lutidine Derivatives for Live-Cell Imaging of Mitochondria and Endoplasmic Reticulum

Mitochondria and endoplasmic reticulum (ER) are highly dynamic subcellular structures essential for several biological functions. The development of non-toxic, wash-free fluorophores to visualize these structures inside cells aid in understanding...