Targeting Fluorescent Sensors to Endoplasmic Reticulum Membranes Enables Detection of Peroxynitrite During Cellular Phagocytosis

Direct assessment of nitrative stress in lipid environments: Applications of a designer lipid-based biosensor for peroxynitrite

Lipid membranes and lipid-rich organelles are targets of peroxynitrite (ONOO-), a highly reactive species generated under nitrative stress. We report a membrane-localized phospholipid (DPPC-TC-ONOO-) that allows the detection of ONOO- in diverse lipid environments: biomimetic vesicles, mammalian cell compartments, and within the lung lining. DPPC-TC-ONOO- and POPC self-assemble to membrane vesicles that fluorogenically and selectively respond to ONOO-. DPPC-TC-ONOO-, delivered through lipid nanoparticles, allowed for ONOO- detection in the endoplasmic reticulum upon cytokine-induced nitrative stress in live mammalian cells. It also responded to ONOO- within lung tissue murine models upon acute lung injury. We observed nitrative stress around bronchioles in precision cut lung slices exposed to nitrogen mustard and in pulmonary macrophages following intratracheal bleomycin challenge. Results showed that DPPC-TC-ONOO- functions specifically toward iNOS, a key enzyme modulating nitrative stress, and offers significant advantages over its hydrophilic analog in terms of localization and signal generation.

A perylenemonoimide-based fluorescent probe: ultrasensitive and selective tracing of endogenous peroxynitrite in living cells

Peroxynitrite (ONOO-), a highly reactive species, plays a key role in various physiological and pathological processes. Herein, a red-emitting fluorescent reporter perylenemonoimide-boronate ester (PMI-BE) was synthesized and utilized for ultrasensitive detection of ONOO-. The unique feature of PMI-BE is its nanomolar sensitivity with high selectivity towards ONOO-. Moreover, PMI-BE also detects endogenously generated ONOO- in live cells.

BTK inhibition potentiates anti-PD-L1 treatment in murine melanoma: potential role for MDSC modulation in immunotherapy

Myeloid-derived suppressor cells (MDSC) have been linked to loss of immune effector cell function through a variety of mechanisms such as the generation of reactive oxygen and nitrogen species and the production of inhibitory cytokines. Our group has shown that signaling through Bruton's tyrosine kinase (BTK) is important for MDSC function. Ibrutinib is an orally administered targeted agent that inhibits BTK activation and is currently used for the treatment of B cell malignancies. Using a syngeneic murine model of melanoma, the effect of BTK inhibition with ibrutinib on the therapeutic response to systemic PD-L1 blockade was studied. BTK was expressed by murine MDSC and their activation was inhibited by ibrutinib. Ibrutinib was not directly cytotoxic to cancer cells in vitro, but it inhibited BTK activation in MDSC and reduced expression of inducible nitric oxide synthase (NOS2) and production of nitric oxide. Ibrutinib treatments decreased the levels of circulating MDSC in vivo and increased the therapeutic efficacy of anti-PD-L1 antibody treatment. Gene expression profiling showed that ibrutinib decreased Cybb (NOX2) signaling, and increased IL-17 signaling (upregulating downstream targets Mmp9, Ptgs2, and S100a8). These results suggest that further exploration of MDSC inhibition could enhance the immunotherapy of advanced melanoma.PrécisInhibition of Bruton's tyrosine kinase, a key enzyme in myeloid cellular function, improves therapeutic response to an anti-PD-L1 antibody in an otherwise fairly resistant murine melanoma model.

Recent Progress of Spectroscopic Probes for Peroxynitrite and Their Potential Medical Diagnostic Applications

Peroxynitrite (ONOO-) is a crucial reactive oxygen species that plays a vital role in cellular signal transduction and homeostatic regulation. Determining and visualizing peroxynitrite accurately in biological systems is important for understanding its roles in physiological and pathological activity. Among the various detection methods, fluorescent probe-based spectroscopic detection offers real-time and minimally invasive detection, high sensitivity and selectivity, and easy structural and property modification. This review categorizes fluorescent probes by their fluorophore structures, highlighting their chemical structures, recognition mechanisms, and response behaviors in detail. We hope that this review could help trigger novel ideas for potential medical diagnostic applications of peroxynitrite-related molecular diseases.

Fluorescent Detection of Peroxynitrite Produced by Myeloid-Derived Suppressor Cells in Cancer and Inhibition by Dasatinib

Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells that expand dramatically in many cancer patients. This expansion contributes to immunosuppression in cancer and reduces the efficacy of immune-based cancer therapies. One mechanism of immunosuppression mediated by MDSCs involves production of the reactive nitrogen species peroxynitrite (PNT), where this strong oxidant inactivates immune effector cells through destructive nitration of tyrosine residues in immune signal transduction pathways. As an alternative to analysis of nitrotyrosines indirectly generated by PNT, we used an endoplasmic reticulum (ER)-targeted fluorescent sensor termed PS3 that allows direct detection of PNT produced by MDSCs. When the MDSC-like cell line MSC2 and primary MDSCs from mice and humans were treated with PS3 and antibody-opsonized TentaGel microspheres, phagocytosis of these beads led to production of PNT and generation of a highly fluorescent product. Using this method, we show that splenocytes from a EMT6 mouse model of cancer, but not normal control mice, produce high levels of PNT due to elevated numbers of granulocytic (PMN) MDSCs. Similarly, peripheral blood mononuclear cells (PBMCs) isolated from blood of human melanoma patients produced substantially higher levels of PNT than healthy human volunteers, coincident with higher peripheral MDSC levels. The kinase inhibitor dasatinib was found to potently block the production of PNT both by inhibiting phagocytosis in vitro and by reducing the number of granulocytic MDSCs in mice in vivo, providing a chemical tool to modulate the production of this reactive nitrogen species (RNS) in the tumor microenvironment.

A Mitochondria-Targeted Fluorescent Probe for Monitoring NADPH Overproduction during Influenza Virus Infection

Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is an important cofactor in the progress of antioxidant synthesis and biosynthesis, and an abnormal NADPH level has been observed in many viral infection processes. However, efficient tools to monitor NADPH in living cells after viral infection have not been reported. In this work, we present a fluorescent probe, NAFP4, that could detect NADPH ex vivo with a low detection limit of 3.66 nM and image mitochondrial NADPH level changes in living cells. The probe exhibits excellent cell permeability, rapid reactivity, and high selectivity with minimal cytotoxicity. Using NAFP4, we reveal that the NADPH is overproduced in the host cells infected by influenza virus, which was caused by an elevated level of G6PDH during the virus infection. Moreover, there was positive association between the G6PDH level and virus replication. With the proposed probe NAFP4, our study highlights that the virus infection would influence the host metabolism in NADPH production and also suggests that G6PDH is expected to be a promising target for antiviral therapy.

A xanthene-based fluorescent probe for detection of peroxynitrite in living cells and zebrafish

Peroxynitrite (ONOO^-) is one of quite critical reactive oxygen species that acts critical roles in a number of diverse biological functions and pathological events. Notably, excessive ONOO^- will lead to sorts of diseases. Thus, monitoring of endogenous ONOO^- levels will be conducive to exploring the physiological activities and functions of ONOO^-. Here, a simple turn-on fluorescent probe named DMX is reported using CN bond as the ONOO^- recognition site and xanthene as the fluorophore. DMX possessed a good linear dependence with ONOO^- concentration (0-9 μM), highly sensitive detection (DL = 37 nM), and excellent selectivity towards ONOO^-. What is more, the biological experiments reveal that DMX is able to be utilized to track exogenous/endogenous ONOO^- employing confocal laser scanning microscopy. Visualization of ONOO^- in zebrafish was also successfully conducted, suggesting that DMX might be used to study ONOO^- roles in vivo. We believe that DMX will have potential for exploring the pivotal role of ONOO^- during all sorts of physiological and pathological activities.

Construction of a mitochondria-endoplasmic reticulum dual-targeted red-emitting fluorescent probe for imaging peroxynitrite in living cells and zebrafish

Peroxynitrite (ONOO - ) is one of the important reactive oxygen species, which plays a vital role in the physiological process of intracellular redox balance. Revealing the biological functions of ONOO - will contribute to further understanding of the oxidative process of organisms. In this work, we designed and synthesized a novel red-emitting fluorescent probe MCSA for the detection of ONOO - , which could rapidly respond to ONOO - within 250 s and exhibited high sensitivity to ONOO - with a low detection limit of 78 nM. Co-localization experiments demonstrated MCSA had the ability to localize into the mitochondria and endoplasmic reticulum. What's more, MCSA enabled monitoring ONOO - level changes during tunicamycin-induced endoplasmic reticulum stress. We have also successfully achieved the visual detection of exogenous and endogenous ONOO - in living cells and zebrafish. This work presented a chemical tool for imaging ONOO - in vitro and in vivo.

Metabolic, Pharmacokinetic, and Activity Profile of the Liver Stage Antimalarial (RC-12)

The catechol derivative RC-12 (WR 27653) (1) is one of the few non-8-aminoquinolines with good activity against hypnozoites in the gold-standard Plasmodium cynomolgi-rhesus monkey (Macaca mulatta) model, but in a small clinical trial, it had no efficacy against Plasmodium vivax hypnozoites. In an attempt to better understand the pharmacokinetic and pharmacodynamic profile of 1 and to identify potential active metabolites, we now describe the phase I metabolism, rat pharmacokinetics, and in vitro liver-stage activity of 1 and its metabolites. Compound 1 had a distinct metabolic profile in human vs monkey liver microsomes, and the data suggested that the O-desmethyl, combined O-desmethyl/N-desethyl, and N,N-didesethyl metabolites (or a combination thereof) could potentially account for the superior liver stage antimalarial efficacy of 1 in rhesus monkeys vs that seen in humans. Indeed, the rate of metabolism was considerably lower in human liver microsomes in comparison to rhesus monkey microsomes, as was the formation of the combined O-desmethyl/N-desethyl metabolite, which was the only metabolite tested that had any activity against liver-stage P. vivax; however, it was not consistently active against liver-stage P. cynomolgi. As 1 and all but one of its identified Phase I metabolites had no in vitro activity against P. vivax or P. cynomolgi liver-stage malaria parasites, we suggest that there may be additional unidentified active metabolites of 1 or that the exposure of 1 achieved in the reported unsuccessful clinical trial of this drug candidate was insufficient to kill the P. vivax hypnozoites.

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.

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

Advances in developing organic fluorescent probes and fluorescence imaging techniques have enhanced our understanding of cell biology. The endoplasmic reticulum (ER) is a dynamic structure that plays a crucial role in protein synthesis, post-translational modifications, and lipid metabolism. The malfunction of ER contributes to several physiological and pathological conditions. Therefore, the investigations on the imaging and role of ER have attracted a lot of attention. Due to their simplicity, synthetic tunability, photostability, high quantum yields, easier cellular uptake, and lower cytotoxicity, organic fluorophores offer invaluable tools for the precision targeting of various cellular organelles and probe ER dynamics. The precision staining is made possible by incorporating specific functional groups having preferential and local organelle biomolecular interactions. For instance, functional moieties such as methyl sulfonamide, sulfonylurea, and pentafluorophenyl assist in ER targeting and thus have become essential tools to probe a deeper understanding of their dynamics. Furthermore, dual-function fluorescent probes that simultaneously image ER and detect specific physiological parameters or biological analytes were achieved by introducing special recognition or chemically reactive sites. This article attempts to comprehensively capture various design strategies currently employed by researchers utilizing small organic molecules to target the ER and detect specific analytes.

Visualization of peroxynitrite in cyclophosphamide-induced oxidative stress by an activatable probe

Oxidative stress is considered to be one of the main contributors of cyclophosphamide (CP)-induced toxicity, and the generation of free radicals will cause the interruption of multiple signal transduction pathways. Peroxynitrite (ONOO^-) has strong oxidation and nitrification ability and is considered as an indirect indicator of oxidative stress. Therefore, it is necessary to design a fluorescent probe that can detect ONOO^- and monitor CP-induced oxidative stress during chemotherapy. Herein, we synthesized a lipid droplet targeting fluorescent probe SX-1 based on triphenylamine-benzoindocyanine. When ONOO^- is added to the probe SX-1, the CC bond in the probe is broken, thereby releasing fluorescence. The good spectral response characteristics enable SX-1 to successfully track the fluctuations of ONOO^- in living cells. Most importantly, we provided the first visual evidence that the level of ONOO^- in HeLa cells was up-regulated under CP induction. All results indicated that SX-1 has great potential in detecting drug-induced ONOO^-, and provided a new detection tool for a deeper understanding of drug-induced organism injury.

Carnosine Protects Macrophages against the Toxicity of Aβ1-42 Oligomers by Decreasing Oxidative Stress

Carnosine (β-alanyl-L-histidine) is a naturally occurring endogenous peptide widely distributed in excitable tissues such as the brain. This dipeptide has well-known antioxidant, anti-inflammatory, and anti-aggregation activities, and it may be useful for treatment of neurodegenerative disorders such as Alzheimer’s disease (AD). In this disease, peripheral infiltrating macrophages play a substantial role in the clearance of amyloid beta (Aβ) peptides from the brain. Correspondingly, in patients suffering from AD, defects in the capacity of peripheral macrophages to engulf Aβ have been reported. The effects of carnosine on macrophages and oxidative stress associated with AD are consequently of substantial interest for drug discovery in this field. In the present work, a model of stress induced by Aβ1-42 oligomers was investigated using a combination of methods including trypan blue exclusion, microchip electrophoresis with laser-induced fluorescence, flow cytometry, fluorescence microscopy, and high-throughput quantitative real-time PCR. These assays were used to assess the ability of carnosine to protect macrophage cells, modulate oxidative stress, and profile the expression of genes related to inflammation and pro- and antioxidant systems. We found that pre-treatment of RAW 264.7 macrophages with carnosine counteracted cell death and apoptosis induced by Aβ1-42 oligomers by decreasing oxidative stress as measured by levels of intracellular nitric oxide (NO)/reactive oxygen species (ROS) and production of peroxynitrite. This protective activity of carnosine was not mediated by modulation of the canonical inflammatory pathway but instead can be explained by the well-known antioxidant and free-radical scavenging activities of carnosine, enhanced macrophage phagocytic activity, and the rescue of fractalkine receptor CX3CR1. These new findings obtained with macrophages challenged with Aβ1-42 oligomers, along with the well-known multimodal mechanism of action of carnosine in vitro and in vivo, substantiate the therapeutic potential of this dipeptide in the context of AD pathology.

An inherently kidney-targeting near-infrared fluorophore based probe for early detection of acute kidney injury

Acute kidney injury (AKI) is common in hospital patients. Delayed diagnosis and treatment of AKI due to the lack of efficient early diagnosis is an important cause of its high mortality. While fluorescence imaging seems promising to non-intrusively interrogate AKI-related biomarkers, the low kidney contrast of many fluorophores conferred by their relatively low abundance of distribution in the kidney limits their application for AKI detection. Herein, we discovered a near-infrared fluorophore with inherent kidney-targeting ability. Based on this fluorophore, a fluorogenic probe (KNP-1) was developed by targeting peroxynitrite (ONOO^-), which is upregulated at the early onset of AKI. KNP-1 exhibits desirable kidney distribution after intravenous administration and is fluorescent only after activation by ONOO^-. These properties lead to excellent kidney contrast imaging results. KNP-1 is capable of detecting both nephrotoxin-induced and ischemia-reperfusion injury-induced AKI in live mice. Temporally resolved imaging of AKI-disease model mice with KNP-1 suggests a gradual increase in renal ONOO^- levels with disease progression. Notably, the upregulation of ONOO^- can be observed at least 24 h earlier than the clinically popular sCr and BUN methods. Blocking ONOO^- generation also proves beneficial. These results highlight the applicability of this inherently tissue targeting-based strategy for designing probes with desirable imaging contrast; potentiate ONOO^- as a biomarker and target for AKI early diagnosis and medical intervention; and imply the clinical relevance of KNP-1 for AKI early detection.

Visualizing Peroxynitrite in Microvessels of the Brain with Stroke Using an Engineered Highly Specific Fluorescent Probe

Stroke is one of the leading causes of death and disability in the world, which is associated with malfunction of reactive oxygen species and reactive nitrogen species (ROS/RNS) in cerebral microvessels. In vivo monitoring these species, such as ONOO^-, with high selectivity in stroke process is of great significance for early diagnoses and therapies of the disease. Herein, by engineering an indoline-2,3-dione moiety as the recognizing domain, we proposed a novel fluorescence probe Rd-PN2 with highly specific response toward ONOO^-, even in the coexistence of other ROS/RNS with high concentration. Rd-PN2 showed high sensitivity and reaction speed in response to ONOO^- and exhibited satisfying performances in tracking the endogenously generated ONOO^- in living cells and zebrafish. Accordingly, Rd-PN2 can furnish real-time and in vivo visualizing of ONOO^- in cerebral microvessels of mice with ischemic and hemorrhagic strokes under two-photon microscopy. This work presented a precisely modulated fluorescence probe for real-time visualizing of ONOO^- production in cerebral micovessels, which will also help to acquire more accurate information in the studies of ONOO^- functions in the future.

A sensitive and selective fluorescent probe for the detection of endogenous peroxynitrite (ONOO-) in living cells

Peroxynitrite (ONOO-) is one of reactive oxygen species, and plays a vital role in numeorus physiological and pathological processes. Given that the ONOO- level is closely related with various serious diseases, the in situ and real time detection of endogenous ONOO- is highly important for the in-depth study of its roles in living systems. Herein, we present a new fluorescent probe (RHPN) for the real-time detection of intracellular ONOO-. The probe RHPN consists of a rhodamine analogue and an arylhydrazide group as a response site for ONOO-. In response to ONOO-, the probe RHPN converts to an open-ring form and generates strong fluorescence. Moreover, the probe RHPN was successfully used for the imaging of the endogenous and exogenous ONOO- level changes in living cells.

Fluorescent detection of peroxynitrite during antibody-dependent cellular phagocytosis

Peroxynitrite (PNT) is a highly reactive oxidant that plays a key role in the destruction of foreign pathogens by specific phagocytic immune cells such as macrophages. However, when its production is dysregulated, this oxidant can contribute to cardiovascular disease, neurological diseases, and cancer. To facilitate the detection of PNT in living cells, we designed and synthesized a fluorescent sensor termed PS3 that accumulates in membranes of the endoplasmic reticulum (ER). This subcellular targeting enhances the proximity of PS3 to the phagosome of macrophages where PNT is generated. When PS3-treated macrophages are stimulated with 10 µm opsonized tentagel microspheres, antibody-dependent cellular phagocytosis (ADCP) of these particles results in production of endogenous PNT, oxidative cleavage of the fluorescence-quenching phenolic side chain of PS3, and increased fluorescence that can be detected by confocal laser scanning microscopy, flow cytometry, and other assays. We describe methods for the synthesis of PS3 and evaluation of its photophysical properties, selectivity, and reactivity. We further report differential production of PNT during ADCP by the phagocytic cell lines RAW 264.7, J774A.1, and THP-1, as detected by confocal microscopy and changes in fluorescence intensity on 96-well plates. This approach may be useful for identification of modulators of PNT and related studies of ADCP.

A novel pyridinium-based fluorescent probe for ratiometric detection of peroxynitrite in mitochondria

Peroxynitrite (ONOO^-) is a primary kind of reactive oxygen species. Excessive ONOO^- can induce oxidative damage to biomolecules and further results in various diseases. So, quantitative monitoring ONOO^- with excellent selectivity and sensitivity is imperative for elucidating its role in biological processes. In this study, a novel pyridinium fluorescent ONOO^- probe (CPC) has been constructed base on ICT-modulated by combining coumarin fluorophore and diphenylphosphinate recognition group. The fluorescence response of CPC for ONOO^- is realized via the removal of diphenylphosphinate group. The probe CPC shows prominent features for detection of ONOO^- including fast response rate (within 3 min), excellent selectivity and sensitivity, distinct colorimetric (red to green), and a large emission wavelength shift (105 nm). The emission intensity ration (I₅₃₈/I₆₄₃) exhibits 153-fold enhancement along with the increasing ONOO^- and the detection limit is as low as 1.60 × 10^-8 M. These good response properties make CPC possible to quantitative detection of ONOO^- concentration. By using the strategy, the ratiometric CPC has been employed to detection of mitochondrial ONOO^- in live cell successfully.

Ultrafast Detection of Peroxynitrite in Parkinson's Disease Models Using a Near-Infrared Fluorescent Probe

Parkinson's disease (PD) is a neurodegenerative disease that devastatingly affects people's lives. Numerous research studies have shown that peroxynitrite (ONOO^-) plays a pivotal role in the pathogenesis of PD. However, a suitable tool that could quickly and sensitively detect ONOO^- in various PD models is still lacking. To this end, we designed and synthesized a series of near-infrared probes that could detect ONOO^- within seconds by near-infrared fluorescent imaging in an ultrafast and highly selective manner. It is noteworthy that one of those developed probes, NIR-PN1, showed excellent sensing performance and blood-brain barrier penetrating ability. NIR-PN1 was successfully applied for imaging of ONOO^- fluxes in multiple PD models including PC12 cell, Drosophila, C. elegans, and mouse brain, indicating its great potential application not only for understanding the biological roles that ONOO^- played in PD but also for early PD diagnosis and treatment.

An endoplasmic reticulum-targeting fluorescent probe for imaging ˙OH in living cells

A new ER-targeting fluorescent probe for ˙OH is developed and applied to imaging ˙OH generation as well as lipid droplet formation in ER stress.

A time-resolved near-infrared phosphorescent iridium(iii) complex for fast and highly specific peroxynitrite detection and bioimaging applications

Peroxynitrite (ONOO^-), one of the reactive oxygen/nitrogen species (ROS/RNS) found in vivo, plays crucial roles in many physiological and pathological processes. The ability to selectively and sensitively determine ONOO^-in vivo is important for the understanding of its biological roles. Thus, by utilizing the excellent chemical stability and photostability, high luminescence efficiency, and long luminescence lifetime of iridium complexes, we developed a novel near-infrared (NIR) phosphorescent iridium(iii) complex (FNO2) probe to detect ONOO^- within seconds. The probe FNO2 showed better selectivity towards ONOO^- over other interfering biomolecules, including O₂^- and ClO^-. Moreover, it possessed a long luminescence lifetime, which enabled successful elimination of the interference from background fluorescence in vitro (simulated by Rhodamine B) in time-resolved emission spectra. Finally, in addition to its low cytotoxicity, the probe FNO2 showed emission wavelength in the NIR region and was able to specifically sense ONOO^- induced in living cells and inflamed mouse models.

Physicochemical-property guided design of a highly sensitive probe to image nitrosative stress in the pathology of stroke

In vivo real-time imaging of nitrosative stress in the pathology of stroke has long been a formidable challenge due to both the presence of the blood–brain barrier (BBB) and the elusive nature of reactive nitrogen species, while this task is also informative to gain a molecular level understanding of neurovascular injury caused by nitrosative stress during the stroke episode. Herein, using a physicochemical property-guided probe design strategy in combination with the reaction-based probe design rationale, we have developed an ultrasensitive probe for imaging nitrosative stress evolved in the pathology of stroke. This probe demonstrates an almost zero background fluorescence signal but a maximum 1000-fold fluorescence enhancement in response to peroxynitrite, the nitrosative stress marker. Due to its good physicochemical properties, the probe readily penetrates the BBB after intravenous administration, and quickly accumulates in mice brain to sense local vascular injuries. After accomplishing its imaging mission, the probe is easily metabolized and therefore won't cause safety concerns. These desirable features make the probe competent for the straightforward visualization of nitrosative stress progression in stroke pathology.

A brain–blood barrier permeable probe was developed for fluorogenically sensing nitrosative stress caused by brain vascular injury.

Visualizing Autophagic Flux during Endothelial Injury with a Pathway-Inspired Tandem-Reaction Based Fluorogenic Probe

Autophagy is a dynamic and complicated catabolic process. Imaging autophagic flux can clearly advance knowledge of its pathophysiology significance. While the most common way autophagy is imaged relies on fluorescent protein-based probes, this method requires substantial genetic manipulation that severely restricts the application. Small fluorescent probes capable of tracking autophagic flux with good spatiotemporal resolution are highly demanable.

Methods: In this study, we developed a small-molecule fluorogenic probe (AFG-1) that facilitates real-time imaging of autophagic flux in both intact cells and live mice. AFG-1 is inspired by the cascading nitrosative and acidic microenvironments evolving during autophagy. It operates over two sequential steps. In the first step, AFG-1 responds to the up-regulated peroxynitrite at the initiation of autophagy by its diphenylamino group being oxidatively dearylated to yield a daughter probe. In the second step, the daughter probe responds to the acidic autolysosomes at the late stage of autophagy by being protonated.

Results: This pathway-dependent mechanism has been confirmed first by sequentially sensing ONOO^- and acid in aqueous solution, and then by imaging autophagic flux in live cells. Furthermore, AFG-1 has been successfully applied to visualize autophagic flux in real-time in live mice following brain ischemic injury, justifying its robustness.

Conclusion: Due to the specificity, easy operation, and the dynamic information yielded, AFG-1 should serve as a potential tool to explore the roles of autophagy under various pathological settings.