Naphthalimide-4-(4-nitrophenyl)thiosemicarbazide: A Fluorescent Probe for Simultaneous Monitoring of Viscosity and Nitric Oxide in Living Cells.

Intracellular viscosity is a physicochemical factor that determines the outcomes of various biological processes, while nitric oxide (NO) is an essential signaling molecule that controls many cellular processes, including oxidative stress. Anticipating that both may be interrelated with a variety of pathologies, their simultaneous measurement would be highly valuable for the investigation of the pathological condition of cells. However, the development of a sensor for such simultaneous detection has not been attempted yet. Herein, we present the synthesis of naphthalimide-4-(4-nitrophenyl)thiosemicarbazide, probe 1, and its application to living cells under conditions of lipopolysaccharide or nystatin treatment, adopted as oxidative stress and altered intracellular viscosity models, respectively. The probe showed increased fluorescence in response to elevation of viscosity and NO levels at 470 and 550 nm, respectively, in the solution studies. When the probe was used for a confocal microscopic study of HeLa cells under stressed conditions, simultaneous monitoring of viscosity and NO level elevations was possible through fluorescence imaging using band-pass filters of 420-475 and 505-600 nm, respectively, upon excitation at a wavelength of 405 nm. Interestingly, both the cellular viscosity and NO levels increased together under lipopolysaccharide or nystatin treatment. Therefore, we suggest that probe 1 is a fluorescent chemical probe that enables the monitoring of alterations in intracellular viscosity and NO levels in living cells, which would be valuable in studies of various cellular damage models.

Emerging two-dimensional monoelemental materials (Xenes) for biomedical applications.

The emergence of novel two-dimensional (2D) monoelemental materials (Xenes) has shown remarkable potential for their applications in different fields of technology, as well as addressing new discoveries in fundamental science. Xenes (e.g., borophene, silicene, germanene, stanene, phosphorene, arsenene, antimonene, bismuthene, and tellurene) are of particular interest because they are the most chemically tractable materials for synthetic exploration. Owing to their excellent physical, chemical, electronic and optical properties, Xenes have been regarded as promising agents for biosensors, bioimaging, therapeutic delivery, and theranostics, as well as in several other new bio-applications. In this tutorial review, we summarize their general properties including the classification of Xenes according to their bulk properties. The synthetic and modification methods of Xenes are also presented. Furthermore, the representative Xene nanoplatforms for various biomedical applications are highlighted. Finally, research progress, challenges, and perspectives for the future development of Xenes in biomedicines are discussed.

Shedding light on tau protein aggregation: the progress in developing highly selective fluorophores.

Historically, in Alzheimer's disease research, a lot of attention has been paid to the development of highly selective fluorophores for beta amyloid plaques. With a shift in the understanding of the disease and the importance of a network of cross-talk interactions, the development of small-molecule fluorescent dyes with high selectivity for (hyperphosphorylated) tau protein aggregates in neurofibrillary tangles has been gaining increasing attention. Fluorescent dyes for the selective labelling of tau aggregates in histological AD brain sections have been described, spanning the entire visible range of the electromagnetic spectrum. Despite the relatively early stages of the development of the field, a large diversity in probe architectures has been reported. Importantly, a handful of near-infrared-emissive dyes have been described as well, and some of these have exhibited good pharmacological profiles, with a significant blood-brain-barrier permeability, and a demonstrated ability to label tau tangles in vivo in small-animal models of Alzheimer's disease and other tauopathies. The developments summarized in the current work are expected to aid the unravelling of the diverse set of players in the etiology of Alzheimer's disease. In this tutorial review, we seek to provide the reader with an overview of the most important recent developments and hope to provide some guidelines for the design of future probes.

Fluorogenic reaction-based prodrug conjugates as targeted cancer theranostics.

Theranostic systems are receiving ever-increasing attention due to their potential therapeutic utility, imaging enhancement capability, and promise for advancing the field of personalized medicine, particularly as it relates to the diagnosis, staging, and treatment of cancer. In this Tutorial Review, we provide an introduction to the concepts of theranostic drug delivery effected via use of conjugates that are able to target cancer cells selectively, provide cytotoxic chemotherapeutics, and produce readily monitored imaging signals in vitro and in vivo. The underlying design concepts, requiring the synthesis of conjugates composed of imaging reporters, masked chemotherapeutic drugs, cleavable linkers, and cancer targeting ligands, are discussed. Particular emphasis is placed on highlighting the potential benefits of fluorogenic reaction-based targeted systems that are activated for both imaging and therapy by cellular entities, e.g., thiols, reactive oxygen species and enzymes, which are present at relatively elevated levels in tumour environments, physiological characteristics of cancer, e.g., hypoxia and acidic pH. Also discussed are systems activated by an external stimulus, such as light. The work summarized in this Tutorial Review will help define the role fluorogenic reaction-based, cancer-targeting theranostics may have in advancing drug discovery efforts, as well as improving our understanding of cellular uptake and drug release mechanisms.

A Fluorescent Probe for Stimulated Emission Depletion Super-Resolution Imaging of Vicinal-Dithiol-Proteins on Mitochondrial Membrane.

Realizing the significant roles of vicinal-dithiol proteins (VDPs) in maintaining the cellular redox homeostasis and their implication in many diseases, we synthesized a smart arsenate based fluorescent probe 1 which can preferentially target the mitochondrial membrane-bound vicinal dithiol proteins (VDPs), especially voltage-dependent anion channel (VDAC2). The probe targetability was demonstrated by in vitro studies such as colocalization, stimulated emission depletion (STED) super-resolution imaging, proteomic MS/MS analysis, and Western blot analysis. The probe represents a rare example of fluorescence labeling of mitochondrial membrane-bound VDPs and can provide a new way to construct VDPs-specific fluorescent probes to gain deeper understanding of their roles in mitochondrial-related disorders.

A Mitochondria-Targeted Cryptocyanine-Based Photothermogenic Photosensitizer.

Cryptocyanine-based probes exhibit highly efficient photothermal conversion and represent a new class of photothermal agents for use in photothermal therapy (PTT). With the thermal susceptibility of mitochondria in mind, we have prepared a mitochondria-targeted, NIR-absorbing cryptocyanine probe (Mito-CCy) and evaluated its photophysical properties, photothermal conversion efficiency, biological compatibility, cytotoxicity, and mitochondrial localization in HeLa cells. Upon subjecting 0.5 mL of a PBS buffer solution (10 mM, pH 7.4, containing 50% DMSO) of Mito-CCy (0.5 mM) to 730 nm laser irradiation at 2.3 W/cm2, the temperature of the solution increased by 13.5 °C within 5 min. In contrast, the corresponding cryptocyanine (CCy) lacking the triarylphosphonium group gave rise to only an ∼3.4 °C increase in solution temperature under otherwise identical conditions. Mito-CCy also exhibited high cytotoxicity in HeLa cells when subject to photoirradiation. This light-induced cytotoxicity is attributed to the endogenous production of reactive oxygen species (ROS) induced under conditions of local heating. ROS are known to interfere with the mitochondrial defense system and to trigger apoptosis. By targeting the mitochondria, the present sensitizer-based photothermogenic approach is rendered more effective. As such, the system reported here represents the vanguard of what might be a new generation of organelle-targeted photothermal therapeutics.

Rational Design of in Vivo Tau Tangle-Selective Near-Infrared Fluorophores: Expanding the BODIPY Universe.

The elucidation of the cause of Alzheimer's disease remains one of the greatest questions in neurodegenerative research. The lack of highly reliable low-cost sensors to study the structural changes in key proteins during the progression of the disease is a contributing factor to this lack of insight. In the current work, we describe the rational design and synthesis of two fluorescent BODIPY-based probes, named Tau 1 and Tau 2. The probes were evaluated on the molecular surface formed by a fibril of the PHF6 (306VQIVYK311) tau fragment using molecular docking studies to provide a potential molecular model to rationalize the selectivity of the new probes as compared to a homologous Aß-selective probe. The probes were synthesized in a few steps from commercially available starting products and could thus prove to be highly cost-effective. We demonstrated the excellent photophysical properties of the dyes, such as a large Stokes shift and emission in the near-infrared window of the electromagnetic spectrum. The probes demonstrated a high selectivity for self-assembled microtubule-associated protein tau (Tau protein), in both solution and cell-based experiments. Moreover, the administration to an acute murine model of tauopathy clearly revealed the staining of self-assembled hyperphosphorylated tau protein in pathologically relevant hippocampal brain regions. Tau 1 demonstrated efficient blood-brain barrier penetrability and demonstrated a clear selectivity for tau tangles over Aß plaques, as well as the capacity for in vivo imaging in a transgenic mouse model. The current work could open up avenues for the cost-effective monitoring of the tau protein aggregation state in animal models as well as tissue staining. Furthermore, these fluorophores could serve as the basis for the development of clinically relevant sensors, for example based on PET imaging.

Enhanced NIR radiation-triggered hyperthermia by mitochondrial targeting.

Mitochondria are organelles that are readily susceptible to temperature elevation. We selectively delivered a coumarin-based fluorescent iron oxide nanoparticle, Mito-CIO, to the mitochondria. Upon 740 nm laser irradiation, the intracellular temperature of HeLa cells was elevated by 2.1 °C within 5 min when using Mito-CIO, and the treatment resulted in better hyperthermia and a more elevated cytotoxicity than HeLa cells treated with coumarin iron oxide (CIO), which was missing the mitochondrial targeting unit. We further confirmed these results in a tumor xenograft mouse model. To our knowledge, this is the first report of a near-infrared laser irradiation-induced hyperthermic particle targeted to mitochondria, enhancing the cytotoxicity in cancer cells. Our present work therefore may open a new direction in the development of photothermal therapeutics.

BODIPY-Coumarin Conjugate as an Endoplasmic Reticulum Membrane Fluidity Sensor and Its Application to ER Stress Models.

An endoplasmic reticulum (ER) membrane-selective chemosensor composed of BODIPY and coumarin moieties and a long alkyl chain (n-C18) was synthesized. The emission ratio of BODIPY to coumarin depends on the solution viscosity. The probe is localized to the ER membrane and was applied to reveal the reduced ER membrane fluidity under ER stress conditions.

Small polyanion recognition of a triazolium cyclodextrin click cluster in water.

In order to detect small polyanions (sPAs), which play important roles in many biological systems, a triazolium cyclodextrin click cluster (5, hexakis{6-(3-methyl-4-hydroxymethyl-1H-1,2,3-triazolium-1-yl)-6-deoxy}-α-cyclodextrin iodide) was synthesized and characterized. The competition binding to 5 occupied by 5-carboxyfluorescein of inositol-1,4,5-trisphosphate (IP3), phytic acid, adenosine triphosphate (ATP), ethylenediaminetetraacetic acid (EDTA), glucose, and glucose-6-phosphate was evaluated by UV/vis titration in HEPES (10 mM, pH 7.4) : methanol (1 : 1, v/v). We obtained the binding constants of IP3 and phytic acid to 5 (1.4 × 10(6) and 1.9 × 10(6) M(-1), respectively); however, the binding constants of ATP and EDTA were significantly lower (2.1 × 10(5) and 4.5 × 10(4) M(-1), respectively). Moreover, glucose and glucose-6-phosphate did not show any detectable binding. In addition, the sPA recognition of the triazolium cyclodextrin click cluster in water was confirmed by fluorescence titration.

Chemical sensing of neurotransmitters.

In the past few decades, the development of chemosensors for neurotransmitters has emerged as a research area of significant importance, which attracted a tremendous amount of attention due to its high sensitivity and rapid response. This current review focuses on various neurotransmitter detection based on fluorescent or colorimetric spectrophotometry published for the last 12 years, covering biogenic amines (dopamine, epinephrine, norepinephrine, serotonin, histamine and acetylcholine), amino acids (glutamate, aspartate, GABA, glycine and tyrosine), and adenosine.

Toward a chemical marker for inflammatory disease: a fluorescent probe for membrane-localized thioredoxin.

Thioredoxin (Trx) is a redox-active protein that plays a key role in mitigating the effects of oxidative stress. The secretion of Trx on the plasma membrane has been suggested as a distinctive feature of inflammation. However, selective monitoring of membrane-associated Trx activity has proved challenging because of the ubiquity of Trx action in cells. Here, we report a Trx-specific probe that allows visualization of Trx activity associated with the membranes via fluorescence microscopy. The ability of this probe to act as a possible screening tool for agents that modulate Trx secretion was demonstrated in HeLa cells under oxidative stress conditions and in a cellular hepatosteatosis model. Control experiments serve to confirm that the response seen for the present probe is due to Trx and that it is selective over various potentially competing metabolites, including thiol-containing small molecules and test proteins.

Mitochondria-immobilized pH-sensitive off-on fluorescent probe.

We report here a mitochondria-targetable pH-sensitive probe that allows for a quantitative measurement of mitochondrial pH changes, as well as the real-time monitoring of pH-related physiological effects in live cells. This system consists of a piperazine-linked naphthalimide as a fluorescence off-on signaling unit, a cationic triphenylphosphonium group for mitochondrial targeting, and a reactive benzyl chloride subunit for mitochondrial fixation. It operates well in a mitochondrial environment within whole cells and displays a desirable off-on fluorescence response to mitochondrial acidification. Moreover, this probe allows for the monitoring of impaired mitochondria undergoing mitophagic elimination as the result of nutrient starvation. It thus allows for the monitoring of the organelle-specific dynamics associated with the conversion between physiological and pathological states.

Gemcitabine-coumarin-biotin conjugates: a target specific theranostic anticancer prodrug.

We present here, the design, synthesis, spectroscopic characterization, and in vitro biological assessment of a gemcitabine-coumarin-biotin conjugate (5). Probe 5 is a multifunctional molecule composed of a thiol-specific cleavable disulfide bond, a coumarin moiety as a fluorescent reporter, gemcitabine (GMC) as a model active drug, and biotin as a cancer-targeting unit. Upon addition of free thiols that are relatively abundant in tumor cells, disulfide bond cleavage occurs as well as active drug GMC release and concomitantly fluorescence intensity increases. Confocal microscopic experiments reveal that 5 is preferentially taken up by A549 cells rather than WI38 cells. Fluorescence-based colocalization studies using lysosome- and endoplasmic reticulum-selective dyes suggest that thiol-induced disulfide cleavage of 5 occur in the lysosome possibly via receptor-mediated endocytosis. The present drug delivery system is a new theranostic agent, wherein both a therapeutic effect and drug uptake can be readily monitored at the subcellular level by two photon fluorescence imaging.

A self-calibrating bipartite viscosity sensor for mitochondria.

A self-calibrating bipartite viscosity sensor 1 for cellular mitochondria, composed of coumarin and boron-dipyrromethene (BODIPY) with a rigid phenyl spacer and a mitochondria-targeting unit, was synthesized. The sensor showed a direct linear relationship between the fluorescence intensity ratio of BODIPY to coumarin or the fluorescence lifetime ratio and the media viscosity, which allowed us to determine the average mitochondrial viscosity in living HeLa cells as ca. 62 cP (cp). Upon treatment with an ionophore, monensin, or nystatin, the mitochondrial viscosity was observed to increase to ca. 110 cP.

Folate-based near-infrared fluorescent theranostic gemcitabine delivery.

A series of heptamethine cyanine (1-3) derivatives bearing a carbamate ethyl disulfide group and gemcitabine, an anticancer drug, have been newly synthesized. Their disulfide bonds are readily cleaved by various thiols including glutathione, to result in a subsequent decomposition of the carbamate into amine followed by release of the active gemcitabine, which can be monitored by the fluorescence changes. In the biological experiment, prodrug 1 is preferentially up-taken by folate-positive KB cells over folate-negative A549 cells via receptor-mediated endocytosis to release gemcitabine causing cell death and to emit fluorescence in endoplasmic reticulum. Moreover, it is selectively accumulated in the KB cells which were treated to mice by dorsal subcutaneous injection. This drug delivery system is a new theranostic agent, wherein both therapeutic effect and drug uptake can be easily monitored at the subcellular level, by in vivo and in vitro fluorescence imaging.

Glycyrrhizin alleviates neuroinflammation and memory deficit induced by systemic lipopolysaccharide treatment in mice.

The present study investigated the effects of glycyrrhizin (GRZ) on neuroinflammation and memory deficit in systemic lipopolysaccharide (LPS)-treated C57BL/6 mice. Varying doses of GRZ was orally administered (10, 30, or 50 mg/kg) once a day for 3 days before the LPS (3 mg/kg) injection. At 24 h after the LPS injection, GRZ significantly reduced TNF-α and IL-1ß mRNA at doses of 30 and 50 mg/kg. COX-2 and iNOS protein expressions were significantly reduced by GRZ at doses of 30 and 50 mg/kg. In the Morris water maze test, GRZ (30 mg/kg) significantly prolonged the swimming time spent in the target and peri-target zones. GRZ also significantly increased the target heading and memory score numbers. In the hippocampal tissue, GRZ significantly reduced the up-regulated Iba1 protein expression and the average cell size of Iba1-expressing microglia induced by LPS. The results indicate that GRZ ameliorated the memory deficit induced by systemic LPS treatment and the effect of GRZ was found to be mediated through the inhibition of pro-inflammatory mediators and microglial activation in the brain tissue. This study supports that GRZ may be a putative therapeutic drug on neurodegenerative diseases associated with cognitive deficits and neuroinflammation such as Alzheimer's disease.

Mitochondrial thioredoxin-responding off-on fluorescent probe.

We synthesized a new probe, Mito-Naph, to visualize mitochondrial thioredoxin (Trx) activity in cells. A fluorescence off-on change is induced by disulfide cleavage of the probe, resulting from a reaction with Trx and subsequent intramolecular cyclization by the released thiolate to give a fluorescent product. By measuring the fluorescence at 540 nm, Trx activity can be detected at nanomolar concentrations (down to 50 nM) well below its physiological levels. The in vitro and in vivo Trx preference of Mito-Naph was demonstrated by fluorometric and confocal microscopic experiments. In vitro kinetic analysis of the disulfide bond cleavage revealed that the second-order rate constant for Trx is (4.04 ± 0.26) × 10(3) (M s)(-1), approximately 5000 times faster than that for GSH. The inhibition experiments involving PX-12, a selective inhibitor of Trx, also revealed that the emission from Mito-Naph significantly decreased in PX-12 dose-dependent manners, both in living cells and in cellular protein extracts. The Trx preference was further supported by an observation that the fluorescence intensity of rat liver extract was decreased according to the Trx depletion by immunoprecipitation. On the basis of these results, it is concluded that Mito-Naph preferentially reacts with Trx, compared with other biological thiols containing amino acids in vitro and in vivo.

Hepatocyte-targeting single galactose-appended naphthalimide: a tool for intracellular thiol imaging in vivo.

We present the design, synthesis, spectroscopic properties, and biological evaluation of a single galactose-appended naphthalimide (1). Probe 1 is a multifunctional molecule that incorporates a thiol-specific cleavable disulfide bond, a masked phthalamide fluorophore, and a single galactose moiety as a hepatocyte-targeting unit. It constitutes a new type of targetable ligand for hepatic thiol imaging in living cells and animals. Confocal microscopic imaging experiments reveal that 1, but not the galactose-free control system 2, is preferentially taken up by HepG2 cells through galactose-targeted, ASGP-R-mediated endocytosis. Probe 1 displays a fluorescence emission feature at 540 nm that is induced by exposure to free endogenous thiols, most notably GSH. The liver-specificity of 1 was confirmed in vivo via use of a rat model. The potential utility of this probe in indicating pathogenic states and as a possible screening tool for agents that can manipulate oxidative stress was demonstrated in experiments wherein palmitate was used to induce lipotoxicity in HepG2 cells.

Direct fluorescence monitoring of the delivery and cellular uptake of a cancer-targeted RGD peptide-appended naphthalimide theragnostic prodrug.

Presented here is a multicomponent synthetic strategy that allows for the direct, fluorescence-based monitoring of the targeted cellular uptake and release of a conjugated therapeutic agent. Specifically, we report here the design, synthesis, spectroscopic characterization, and preliminary in vitro biological evaluation of a RGD peptide-appended naphthalimide pro-CPT (compound 1). Compound 1 is a multifunctional molecule composed of a disulfide bond as a cleavable linker, a naphthalimide moiety as a fluorescent reporter, an RGD cyclic peptide as a cancer-targeting unit, and camptothecin (CPT) as a model active agent. Upon reaction with free thiols in aqueous media at pH 7.4, disulfide cleavage occurs. This leads to release of the free CPT active agent, as well as the production of a red-shifted fluorescence emission (λ(max) = 535 nm). Confocal microscopic experiments reveal that 1 is preferentially taken up by U87 cells over C6 cells. On the basis of competition experiments involving okadaic acid, an inhibitor of endocytosis, it is concluded that uptake takes place via RGD-dependent endocytosis mechanisms. In U87 cells, the active CPT payload is released within the endoplasmic reticulum, as inferred from fluorescence-based colocalization studies using a known endoplasmic reticulum-selective dye. The present drug delivery system (DDS) could represent a new approach to so-called theragnostic agent development, wherein both a therapeutic effect and drug uptake-related imaging information are produced and can be readily monitored at the subcellular level. In due course, the strategy embodied in conjugate 1 could allow for more precise monitoring of dosage levels, as well as an improved understanding of cellular uptake and release mechanisms.