Organelle-localizable fluorescent chemosensors for site-specific multicolor imaging of nucleoside polyphosphate dynamics in living cells

Boron-Salphen Conjugate based Molecular Probe Exhibiting Fluorescence On-Off-On Response in Detection of Cu2+ and ATP through Displacement Approach

Synthesis and photophysical properties of a fluorescent probe HBD is described. Probe upon interaction with metal ions, anions and nucleoside pyrophosphates (NPPs) showed fluorescence quenching with Cu2+ due to chelation enhanced quenching effect (CHEQ). Moreover, interaction of ensemble HBD.Cu2+ with anions and NPPs showed fluorescence "turn-On" response with ATP selectively. "On-Off-On" responses observed with Cu2+ and ATP is attributed to an interplay between ESIPT and TICT processes. Cyclic voltammogram of probe exhibited quasi-reversible redox behaviour with three oxidation and two reduction potentials and the change in band gaps of probe suggested the interaction with Cu2+ and ATP. The 2 : 1 and 1 : 1 binding stoichiometry for an interaction between probe and Cu2+ (LOD, 62 nM) and ensemble, HBD.Cu2+ with ATP (LOD, 0.4 μM) respectively are realised by Job's plot and HRMS data. Cell imaging studies carried out to detect Cu2+ and ATP in HeLa cells. Also, the output emission observed with Cu2+ and ATP is utilized to construct an implication (IMP) logic gate. Test paper strips showed naked-eye visible color responses to detect Cu2+ and ATP. In real water samples probe successfully detected copper (0.03 μM) between 5-6.5 ppb level (ICP-MS method).

Macrocyclic Conformational Switch Coupled with Pyridinium-Induced PET for Fluorescence Detection of Adenosine Triphosphate

The binding of nucleotides is crucial for signal transduction as it induces conformational protein changes, leading to downstream cellular responses. Synthetic receptors that bind nucleotides and transduce the binding event into global conformational rearrangements are highly challenging to design, especially those that operate in an aqueous solution. Much work is focused on evaluating functionalized dyes to detect nucleotides, whereas coupling of a nucleotide-induced conformational switching to a sensing event has not been reported to date. We disclose synthetic receptors that undergo a global conformational rearrangement upon nucleotide binding. Integrating naphthalimide and the pyridinium ion into the structure enables stabilization of the folded conformation and efficient fluorescence quenching. The binding of a nucleotide rearranges the receptor conformation and alters the strong fluorescence enhancement. The methylpyridinium-containing receptor demonstrated high sensing selectivity for adenosine 5'-triphosphate (ATP) and a record 160-fold fluorescence enhancement. It can detect fluctuations of ATP in HeLa cells and possesses low cytotoxicity. The developed systems present an attractive approach for designing ATP-responsive artificial molecular switches that operate in water and integrate a strong fluorescence response.

Supramolecular Recognition of Cytidine Phosphate in Nucleotides and RNA Sequences

Supramolecular recognition of nucleotides would enable manipulating crucial biochemical pathways like transcription and translation directly and with high precision. Therefore, it offers great promise in medicinal applications, not least in treating cancer or viral infections. This work presents a universal supramolecular approach to target nucleoside phosphates in nucleotides and RNA. The artificial active site in new receptors simultaneously realizes several binding and sensing mechanisms: encapsulation of a nucleobase via dispersion and hydrogen bonding interactions, recognition of the phosphate residue, and a self-reporting feature-"turn-on" fluorescence. Key to the high selectivity is the conscious separation of phosphate- and nucleobase-binding sites by introducing specific spacers in the receptor structure. We have tuned the spacers to achieve high binding affinity and selectivity for cytidine 5' triphosphate coupled to a record 60-fold fluorescence enhancement. The resulting structures are also the first functional models of poly(rC)-binding protein coordinating specifically to C-rich RNA oligomers, e.g., the 5'-AUCCC(C/U) sequence present in poliovirus type 1 and the human transcriptome. The receptors bind to RNA in human ovarian cells A2780, causing strong cytotoxicity at 800 nM. The performance, self-reporting property, and tunability of our approach open up a promising and unique avenue for sequence-specific RNA binding in cells by using low-molecular-weight artificial receptors.

Carrier-Free ATP-Activated Nanoparticles for Combined Photodynamic Therapy and Chemotherapy under Near-Infrared Light

The combination of photodynamic therapy (PDT) and chemotherapy (chemo-photodynamic therapy) for enhancing cancer therapeutic efficiency has attracted tremendous attention in the recent years. However, limitations, such as low local concentration, non-suitable treatment light source, and uncontrollable release of therapeutic agents, result in reduced combined treatment efficacy. This study considered adenosine triphosphate (ATP), which is highly upregulated in tumor cells, as a biomarker and developed ingenious ATP-activated nanoparticles (CDNPs) that are directly self-assembled from near-infrared photosensitizer (Cy-I) and amphiphilic Cd(II) complex (DPA-Cd). After selective entry into tumor cells, the positively charged CDNPs would escape from lysosomes and be disintegrated by the high ATP concentration in the cytoplasm. The released Cy-I is capable of producing single oxygen (¹ O₂ ) for PDT with 808 nm irradiation and DPA-Cd can concurrently function for chemotherapy. Irradiation with 808 nm light can lead to tumor ablation in tumor-bearing mice after intravenous injection of CDNPs. This carrier-free nanoparticle offers a new platform for chemo-photodynamic therapy.

A Small-Molecule Fluorescence Probe for Nuclear ATP

Fluorescence monitoring of ATP in different organelles is now feasible with a few biosensors developed, which, however, show low sensitivity, limited biocompatibility, and accessibility. Small-molecule ATP probes that alleviate those limitations thus have received much attention recently, leading to a few ATP probes that target several organelles except for the nucleus. We disclose the first small-molecule probe that selectively detects nuclear ATP through reversible binding, with 25-fold fluorescence enhancement at pH 7.4 and excellent selectivity against various biologically relevant species. Using the probe, we observed 2.1-3.3-fold and 3.9-7.8-fold higher nuclear ATP levels in cancerous cell lines and tumor tissues compared with normal cell lines and tissues, respectively, which are explained by the higher nuclear ATP level in the mitosis phase. The probe has great potential for studying nuclear ATP-associated biology.

A dual-emission ratiometric fluorescent sensor based on copper nanoclusters encapsulated in zeolitic imidazolate framework-90 for rapid detection and imaging of adenosine triphosphate

Adenosine triphosphate (ATP) is the primary energy carrier for intracellular metabolic processes. Accurate and rapid detection of ATP has important implications for clinical diagnosis. In this work, we reported a dual-emission ratiometric fluorescent probe Cu NCs-Al@ZIF-90 formed by encapsulating copper nanoclusters (Cu NCs) into zeolitic imidazolate framework-90 (ZIF-90) using a simple one-pot method. Cu NCs exhibited a remarkable fluorescence enhancement in the presence of aluminum ions due to the aggregation-induced emission (AIE) properties. When ATP existed, the Zn²⁺ nodes in the MOF material acted as selective sites for ATP recognition, resulting in the cleavage of Cu NCs-Al@ZIF-90. As a consequence, two reverse fluorescence changes were observed from released Cu NCs at 620 nm and imidazole-2-carboxaldehyde (2-ICA) at 450 nm, respectively. With the dual-emission ratiometric strategy, efficient and rapid determination of ATP was realized, giving a detection limit down to 0.034 mM in the concentration range of 0.2 mM to 0.625 mM. The convenient synthesis process and the rapid ATP-responsive ability made the proposed Cu NCs-Al@ZIF-90 probe highly promising in clinical and environmental analysis.

Improving the Brightness of Pyronin Fluorophore Systems through Quantum-Mechanical Predictions

The pyronin class of fluorophores serves a critical role in numerous imaging applications, particularly involving preferential staining of RNA through base pair intercalation. Despite this important role in molecular staining applications, the same set of century-old pyronins (i.e., pyronin Y (PY) and pyronin B (PB)), which possess relatively low fluorophore brightness, are still predominantly being used due to the lack of methodology for generating enhanced variants. Here, we use TD-DFT calculations of interconversion energies between structures on the S1 surface as a preliminary means to evaluate fluorophore brightness for a proposed set of pyronins containing variable substitution patterns at the 2, 3, 6, and 7 positions. Using a nucleophilic aromatic substitution/hydride addition approach, we synthesized the same set of pyronins and demonstrate that quantum-mechanical computations are useful for predicting fluorophore performance. We produced the brightest series of pyronin fluorophores described to date, which possess considerable gains over PY and PB.

Synthesis of a fluorinated pyronin that enables blue light to rapidly depolarize mitochondria

Fluorinated analogues of the fluorophore pyronin B were synthesized as a new class of amine-reactive drug-like small molecules. In water, 2,7-difluoropyronin B was found to reversibly react with primary amines to form covalent adducts. When this fluorinated analogue is added to proteins, these adducts undergo additional oxidation to yield fluorescent 9-aminopyronins. Irradiation with visible blue light enhances this oxidation step, providing a photochemical method to modify the biological properties of reactive amines. In living HeLa cells, 2,7-difluoropyronin B becomes localized in mitochondria, where it is partially transformed into fluorescent aminopyronins, as detected by spectral profiling confocal microscopy. Further excitation of these cells with the blue laser of a confocal microscope can depolarize mitochondria within seconds. This biological activity was only observed with 2,7-difluoropyronin B and was not detected with analogues such as pyronin B or 9-methyl-2,7-difluoropyronin B. This irradiation with blue light enhances the cellular production of reactive oxygen species (ROS), suggesting that increased ROS in mitochondria promotes the formation of aminopyronins that inactivate biomolecules critical for maintenance of mitochondrial membrane potential. The unique reactivity of 2,7-difluoropyronin B offers a novel tool for photochemical control of mitochondrial biology.

Complexes of Zn(II) with a mixed tryptanthrin derivative and curcumin chelating ligands as new promising anticancer agents

In this study, two novel curcumin (H-Cur)-tryptanthrin metal compounds-[Zn(TA)Cl₂], i.e., Zn(TA), and [Zn(TA)(Cur)]Cl, i.e., Zn(TAC)-were synthesized and investigated using 5-(bis-pyridin-2-ylmethyl-amino)-pentanoic acid (6,12-dioxo-6,12-dihydro-indolo[2,1-b]quinazolin-8-yl)-amide (TA) and H-Cur as the targeting and high-activity anticancer chemotherapeutic moieties, respectively. They were then compared with the di-(2-picolyl)amine (PA) Zn(II) complex [Zn(PA)Cl₂], i.e., Zn(PA). When compared with Zn(PA) and cisplatin, the IC₅₀ values of Zn(TA) and Zn(TAC) indicated that the compounds had high cytotoxicity against A549/DDP cancer cells, implying that the H-Cur-tryptanthrin Zn(II) compounds have the potential for use as anticancer drugs. We propose the use of synthesized theragnostic H-Cur-tryptanthrin Zn(II) complexes with nuclear-targeting and DNA-damaging capabilities as a simple therapeutic strategy against tumors. The Zn(TA) and Zn(TAC) complexes could be traced via red fluorescence and were found to accumulate in the cell nuclei and induce DNA damage, cell cycle arrest, mitochondrial dysfunction, and cell apoptosis both in vitro and in vivo. In addition, Zn(TAC) exhibited a higher antiproliferative effect on A549/DDP than Zn(TA) and Zn(PA), which was undoubtedly associated with the key roles of the novel tryptanthrin derivative TA and H-Cur in the Zn(TAC) complex.

Extracellular ATP Limits Homeostatic T Cell Migration Within Lymph Nodes

Whereas adenosine 5'-triphosphate (ATP) is the major energy source in cells, extracellular ATP (eATP) released from activated/damaged cells is widely thought to represent a potent damage-associated molecular pattern that promotes inflammatory responses. Here, we provide suggestive evidence that eATP is constitutively produced in the uninflamed lymph node (LN) paracortex by naïve T cells responding to C-C chemokine receptor type 7 (CCR7) ligand chemokines. Consistently, eATP was markedly reduced in naïve T cell-depleted LNs, including those of nude mice, CCR7-deficient mice, and mice subjected to the interruption of the afferent lymphatics in local LNs. Stimulation with a CCR7 ligand chemokine, CCL19, induced ATP release from LN cells, which inhibited CCR7-dependent lymphocyte migration in vitro by a mechanism dependent on the purinoreceptor P2X7 (P2X7R), and P2X7R inhibition enhanced T cell retention in LNs in vivo. These results collectively indicate that paracortical eATP is produced by naïve T cells in response to constitutively expressed chemokines, and that eATP negatively regulates CCR7-mediated lymphocyte migration within LNs via a specific subtype of ATP receptor, demonstrating its fine-tuning role in homeostatic cell migration within LNs.

Pyridinium-conjugated polynorbornenes for nanomolar ATP sensing using an indicator displacement assay and a PET strategy

An indicator displacement assay, namely polymeric PNPY-n/UD consisting of a cationic polynorbornene backbone with pyridinium functional groups (PNPY-1,2,3) and an anionic uranine dye (UD) as an indicator, has been developed for highly sensitive "turn-on" fluorescence sensing of ATP. While PNPY-1/UD itself is non-emissive, a bright green fluorescence signal was observed in the presence of ATP [K_a = 2.17 × 10⁵ M^-1, LOD = 5.7 nM]. The potential of a highly photostable system PNPY-1/UD was also validated in detecting ATP levels in live-cell imaging applications.

A Dual-Control Strategy by Phosphate Ions and Local Microviscosity for Tracking Adenosine Triphosphate Metabolism in Mitochondria and Cellular Activity Dynamically

Adenosine triphosphate (ATP) acts as the main energy source for growth and development in organisms, and the disorder reflects the mitochondrial damage to a large extent. Therefore, an efficient tool for the evaluation of the ATP metabolic level is important to track mitochondrial health, providing an additional perspective for an in-depth long-term study on living activities. Herein, a twisted intramolecular charge transfer (TICT) framework is utilized to build up a sensitive receptor, Mito-VP, with a negligible background to target mitochondrial ATP metabolism by monitoring the phosphate ion (Pi) level upon ATP hydrolysis under the overall consideration of the structural and functional features of mitochondria. The responsive fluorescence could be lighted on under the dual control of Pi and local microviscosity, and the two steps of ATP hydrolysis could be captured through fluorescence. In addition to the well-behaved mitochondrial targeting, the energy metabolism at cellular and organism levels has been clarified via mitosis and zebrafish development, respectively.

A highly emissive Zn(II)-pyridyl-benzimidazolyl-phenolato-based chemosensor: detection of H2PO4-via "use" and "throw" device fabrication

2-Ethoxy-6-[1-(phenyl-pyridin-2-yl-methyl)-1H-benzoimidazol-2-yl]-phenol (HL) selectively serves as a sensitive 'turn on' Zn²⁺ sensor in 9 : 1 (v/v) DMSO/H₂O (HEPES buffer, pH = 7.4) medium in the presence of sixteen other cations at the limit of detection (LOD) of 3.2 nM. The strong blue emission of the complex, {[Zn(L¹)OAc]} (HL¹ = benzimidazolyl ring-opening structure of HL) (λ_em, 461 nm), is quenched by H₂PO₄^- in the presence of eighteen other anions and the LOD is 0.238 μM. The emission of the complex is due to restricted intramolecular rotation (RIR) followed by chelation-enhanced fluorescence (CHEF). The quenching of the emission of [Zn(L¹)OAc] by H₂PO₄^- (in the presence of other P^Vs (inorganic and biological) as well as additional anions) is due to the 'turn off' fluorescence via the demetallation and release of the nonfluorescent ligand, HL, and [Zn(H₂PO₄)]⁺. An INHIBIT logic gate memory circuit of the probe HL was devised with Zn²⁺ and H₂PO₄^- as two consecutive inputs. The percentage of H₂PO₄^- recovery was excellent and was obtained from distilled, tap, and drinking water sources. The bright blue emission of [Zn(L¹)OAc] further triggered the fabrication of ready-made portable thin films of the Zn-complex, which executed a cost-effective 'on-site' solid-state contact mode detection of H₂PO₄^- with selectivity at the picogram level (10.97 pg cm^-2) by monitoring the intensities of quenched spots under UV light upon varying the analyte concentration from 10^-8 to 10^-3 M. Finally, taking advantage of reversible fluorescence switching, a simple and definite ion-responsive security feature was successfully embedded into a "use" and "throw" solution-coated paper strip of the Zn(II)-pyridyl-benzimidazolyl-phenolato-based chemosensor, which efficiently detected H₂PO₄^- in water by a successive 'ON-OFF' fluorescence switching-driven security activity without any exhaustion of the emission phenomenon.

Optical/electrochemical methods for detecting mitochondrial energy metabolism

This review highlights the biological importance of mitochondrial energy metabolism and the applications of multiple optical/electrochemical approaches to determine energy metabolites. Mitochondria, the main sites of oxidative phosphorylation and adenosine triphosphate (ATP) biosynthesis, provide the majority of energy required by aerobic cells for maintaining their physiological activity. They also participate in cell growth, differentiation, information transmission, and apoptosis. Multiple mitochondrial diseases, caused by internal or external factors, including oxidative stress, intense fluctuations of the ionic concentration, abnormal oxidative phosphorylation, changes in electron transport chain complex enzymes and mutations in mitochondrial DNA, can occur during mitochondrial energy metabolism. Therefore, developing accurate, sensitive, and specific methods for the in vivo and in vitro detection of mitochondrial energy metabolites is of great importance. In this review, we summarise the mitochondrial structure, functions, and crucial energy metabolic signalling pathways. The mechanism and applications of different optical/electrochemical methods are thoroughly reviewed. Finally, future research directions and challenges are proposed.

A Multivariate-Gated DNA Nanodevice for Spatioselective Imaging of Pro-metastatic Targets in Extracellular Microenvironment

Probing pro-metastatic biomarkers is of significant importance to evaluate the risk of tumor metastasis, but spatially selective imaging of such targets in extracellular microenvironment is particularly challenging. By introducing the bilinguality of PNA/peptide hybrid that can speak both peptide substrate and nucleobase-pairing languages to combine with aptamer technology, we designed a smart DNA nanodevice programmed to respond sequentially to dual pro-metastatic targets, MMP2/9 and ATP, in extracellular tumor microenvironment (TME). The DNA nanodevice is established based on the combination of an ATP-responsive aptamer sensor and a MMP2/9-hydrolyzable PNA/peptide copolymer with a cell membrane-anchoring aptamer module. Taking 4T1 xenograft as a highly aggressive tumor model, the robustness of the DNA nanodevice in spatioselective imaging of MMP2/9 and ATP in TME is demonstrated. We envision that this design will enable the simultaneous visualization of multiple pro-metastatic biomarkers, which allows to gain insights into their pathological roles in tumor metastasis.

Azulene-based fluorescent chemosensor for adenosine diphosphate

AzuFluor® 435-DPA-Zn, an azulene fluorophore bearing two zinc(II)-dipicolylamine receptor motifs, exhibits fluorescence enhancement in the presence of adenosine diphosphate. Selectivity for ADP over ATP, AMP and PPi results from appropriate positioning of the receptor motifs, since an isomeric sensor cannot discriminate between ADP and ATP.

Reductase and Light Programmatical Gated DNA Nanodevice for Spatiotemporally Controlled Imaging of Biomolecules in Subcellular Organelles under Hypoxic Conditions

Monitoring hypoxia-related changes in subcellular organelles would provide deeper insights into hypoxia-related metabolic pathways, further helping us to recognize various diseases on subcellular level. However, there is still a lack of real-time, in situ, and controllable means for biosensing in subcellular organelles under hypoxic conditions. Herein, we report a reductase and light programmatical gated nanodevice via integrating light-responsive DNA probes into a hypoxia-responsive metal-organic framework for spatiotemporally controlled imaging of biomolecules in subcellular organelles under hypoxic conditions. A small-molecule-decorated strategy was applied to endow the nanodevice with the ability to target subcellular organelles. Dynamic changes of mitochondrial adenosine triphosphate under hypoxic conditions were chosen as a model physiological process. The assay was validated in living cells and tumor tissue slices obtained from mice models. Due to the highly integrated, easily accessible, and available for living cells and tissues, we envision that the concept and methodology can be further extended to monitor biomolecules in other subcellular organelles under hypoxic conditions with a spatiotemporal controllable approach.

Structurally Compact, Blue-Green-Red Fluorescence Trackers for the Outer Cell Membrane: Zwitterionic (Naphthylvinyl)pyridinium Dyes

The cell membrane regulates the flux of materials in and out of cell, cell adhesion, and signaling. Fluorophores that selectively localize on it are in demand for investigations of the molecular events occurring on the outer cell membrane. Commercial membrane trackers based on phospholipids are structurally complex and difficult to modify further. We disclose the zwitterionic (naphthylvinyl)pyridinium dyes that selectively localize on the outer cell membrane and emit blue, green, and red fluorescence, respectively. Notably, they are structurally compact and provide bright fluorescence images of the cell membrane. By comparing with control compounds, we identified minimal structural elements for the "robust" localization of dye on the outer cell membrane. Further, the dyes are two-photon active, enabling high-resolution, deep-tissue imaging. One of the dyes was used to image a spleen tissue, which provided high-resolution fluorescent images with a distinct morphology. In addition, the materials and results disclosed are valuable for the development of membrane-targeting probes and structurally compact fluorophores.

AIEgen-based nanoprobe for the ATP sensing and imaging in cancer cells and embryonic stem cells

A turn-on fluorescent nanoprobe (named AAP-1), based on an aggregation-induced emission luminogen (AIEgen), is disclosed for the detection of adenosine triphosphate (ATP), which is an essential element in the biological system. Organic fluorophore (named TPE-TA) consists of tetraphenylethylene (TPE, sensing and signaling moiety) and mono-triamine (TA, sensing moiety), and it forms an aggregated form in aqueous media as a nanoprobe AAP-1. The nanoprobe AAP-1 has multiple electrostatic interactions as well as hydrophobic interactions with ATP, and it displays superior selectivity toward ATP, reliable sensitivity, with a detection limit around 0.275 ppb, and fast responsive (signal within 10 s). Such a fluorescent probe to monitor ATP has been actively pursued throughout fundamental and translational research areas. In vitro assay and a successful cellular ATP imaging application was demonstrated in cancer cells and embryonic stem cells. We expect that our work warrants further ATP-related studies throughout a variety of fields.

Advances in anion binding and sensing using luminescent lanthanide complexes

Luminescent lanthanide complexes have been actively studied as selective anion receptors for the past two decades. Ln(iii) complexes, particularly of europium(iii) and terbium(iii), offer unique photophysical properties that are very valuable for anion sensing in biological media, including long luminescence lifetimes (milliseconds) that enable time-gating methods to eliminate background autofluorescence from biomolecules, and line-like emission spectra that allow ratiometric measurements. By careful design of the organic ligand, stable Ln(iii) complexes can be devised for rapid and reversible anion binding, providing a luminescence response that is fast and sensitive, offering the high spatial resolution required for biological imaging applications. This review focuses on recent progress in the development of Ln(iii) receptors that exhibit sufficiently high anion selectivity to be utilised in biological or environmental sensing applications. We evaluate the mechanisms of anion binding and sensing, and the strategies employed to tune anion affinity and selectivity, through variations in the structure and geometry of the ligand. We highlight examples of luminescent Ln(iii) receptors that have been utilised to detect and quantify specific anions in biological media (e.g. human serum), monitor enzyme reactions in real-time, and visualise target anions with high sensitivity in living cells.

Turn-on detection of assorted phosphates by luminescent chemosensors

This review illustrates a variety of luminescent chemosensors for the selective detection of assorted phosphates via the “Turn-On” emission mechanism with focus on their design aspects, chemical structures and sensing mechanism.

Target-Triggered Assembly in a Nanopipette for Electrochemical Single-Cell Analysis

Engineered nanopipette tools have recently emerged as a powerful approach for electrochemical nanosensing, which has major implications in both fundamental biological research and biomedical applications. Herein, we describe a generic method of target-triggered assembly of aptamers in a nanopipette for nanosensing, which is exemplified by sensitive and rapid electrochemical single-cell analysis of adenosine triphosphate (ATP), a ubiquitous energy source in life and important signaling molecules in many physiological processes. Specifically, a layer of thiolated aptamers is immobilized onto a Au-coated interior wall of a nanopipette tip. With backfilled pairing aptamers, the engineered nanopipette is then used for probing intracellular ATP via the ATP-dependent linkage of the split aptamers. Due to the higher surface charge density from the aptamer assembly, the nanosensor would exhibit an enhanced rectification signal. Besides, this ATP-responsive nanopipette tool possesses excellent selectivity and stability as well as high recyclability. This work provides a practical single-cell nanosensor capable of intracellular ATP analysis. More generally, integrated with other split recognition elements, the proposed mechanism could serve as a viable basis for addressing many other important biological species.

Selective recognition of ATP by multivalent nano-assemblies of bisimidazolium amphiphiles through "turn-on" fluorescence response

Bisimidazolium receptors, tagged with chromophoric pyrene at one end and linked to an n-alkyl chain at the other, underwent self-assembly in aqueous media depending on the length of the alkyl segment. The amphiphilic derivatives having n-decyl or longer chains, formed nano-assemblies with cyanic-green emission resulting from the stacked pyrene chromophores in the aggregates. The presence of positive surface charges on the multivalent aggregates led to ATP binding which was accompanied by a significant increase in the excimeric emission intensity. This provided a convenient way of monitoring ATP binding in a "turn-on" mode and an efficient detection of ATP was achieved in aqueous buffer and also in buffer containing 150 mM NaCl at physiological pH value. Furthermore, the multivalent aggregates demonstrated a significant selectivity in ATP detection over ADP, AMP and pyrophosphate.

A Remarkable Fluorescence Quenching Based Amplification in ATP Detection through Signal Transduction in Self-Assembled Multivalent Aggregates

Signal transduction is essential for the survival of living organisms, because it allows them to respond to the changes in external environments. In artificial systems, signal transduction has been exploited for the highly sensitive detection of analytes. Herein, a remarkable signal transduction, upon ATP binding, in the multivalent fibrillar nanoaggregates of anthracene conjugated imidazolium receptors is reported. The aggregates of one particular amphiphilic receptor sensed ATP in high pm concentrations with one ATP molecule essentially quenching the emission of thousands of receptors. A cooperative merging of the multivalent binding and signal transduction led to this superquenching and translated to an outstanding enhancement of more than a millionfold in the sensitivity of ATP detection by the nanoaggregates; in comparison to the "molecular" imidazolium receptors. Furthermore, an exceptional selectivity to ATP over other nucleotides was demonstrated.

A nanoprobe based on molybdenum disulfide nanosheets and silver nanoclusters for imaging and quantification of intracellular adenosine triphosphate

Adenosine triphosphate (ATP), as a high-energy phosphate compound that stores and releases energy in living cells, has an irreplaceable role in many physiological processes and maintenance of biological functions, and can be used as an indicator of many diseases. In this work, a composite nanoprobe, silver nanocluster (AgNC) @ molybdenum disulfide (MoS₂), was designed to achieve in situ fluorescence imaging and quantitative analysis of intracellular ATP in HeLa cells by fluorescence spectrometry and inductively coupled plasma mass spectrometry (ICP-MS). The probe was constructed based on the adsorption of DNA-AgNCs by MoS₂ nanosheets, and the DNA-AgNCs were prepared with the ATP aptamer as a template, whose fluorescence was initially quenched by MoS₂. When the probe was incubated into the cells, intracellular ATP recognized the aptamer sequence and caused the DNA-AgNCs to fall off the MoS₂ nanosheets, resulting in fluorescence recovery. Here, AgNCs not only acted as a fluorescence label for imaging, but also as an element tag for quantitative analysis of intracellular ATP with the detection of ¹⁰⁷Ag by ICP-MS. The ATP in HeLa cells detected by this method was 24.6 ± 1.7 nmol L^-1, which was in good agreement with the test result of the ATP test kit (20.4 ± 0.8 nmol L^-1). The proposed method has potential application in medical clinical diagnosis and evaluation of the body's metabolic level via fluorescence imaging and ICP-MS detection of intracellular ATP.

Ratiometric Detection of ATP by Fluorescent Cyclophanes with Bellows-Type Sensing Mechanism

Pyrene-based cyclophanes have been synthesized with the aim to realize a bellows-type sensing mechanism for the ratiometric detection of nucleotide concentrations in a buffered aqueous solution. The sensing mechanism involves the encapsulation of a nucleobase between two pyrene rings, which affects the monomer-excimer equilibrium of the receptor in the excited state. The nature of the spacer and its connection pattern to pyrene rings have been varied to achieve high selectivity for ATP. The 1,8-substituted pyrene-based cyclophane with the 2,2'-diaminodiethylamine spacer demonstrates the best selectivity for ATP showing a 50-fold increase in the monomer-excimer emission ratio upon saturation with the nucleotide. The receptor can detect ATP within the biological concentrations range over a wide pH range. NMR and spectroscopic studies have revealed the importance of hydrogen bonding and stacking interactions for achieving a required receptor selectivity. The probe has been successfully applied for the real-time monitoring of creatine kinase activity.

Real-time in vivo imaging of extracellular ATP in the brain with a hybrid-type fluorescent sensor

Adenosine 5' triphosphate (ATP) is a ubiquitous extracellular signaling messenger. Here, we describe a method for in-vivo imaging of extracellular ATP with high spatiotemporal resolution. We prepared a comprehensive set of cysteine-substitution mutants of ATP-binding protein, Bacillus FoF₁-ATP synthase ε subunit, labeled with small-molecule fluorophores at the introduced cysteine residue. Screening revealed that the Cy3-labeled glutamine-105 mutant (Q105C-Cy3; designated ATPOS) shows a large fluorescence change in the presence of ATP, with submicromolar affinity, pH-independence, and high selectivity for ATP over ATP metabolites and other nucleotides. To enable in-vivo validation, we introduced BoNT/C-Hc for binding to neuronal plasma membrane and Alexa Fluor 488 for ratiometric measurement. The resulting ATPOS complex binds to neurons in cerebral cortex of living mice, and clearly visualized a concentrically propagating wave of extracellular ATP release in response to electrical stimulation. ATPOS should be useful to probe the extracellular ATP dynamics of diverse biological processes in vivo.

Tuning the anion binding properties of lanthanide receptors to discriminate nucleoside phosphates in a sensing array

The development of synthetic receptors for the selective binding and discrimination of anions in water requires an understanding of how anions interact with these synthetic receptors. Molecules designed to differentiate nucleoside phosphate anions (e.g. ATP, ADP, GTP, GDP, UDP) under physiological conditions could underpin exciting new sensing tools for biomedical research and drug discovery, but it is very challenging due to the similarities in anion structure, size and charge. We present a series of lanthanide-based anion receptors and establish key structural elements that impact on nucleoside phosphate anion binding and sensing. Structural evidence of anion binding using X-ray crystallographic and NMR data, supported by DFT calculations indicate the binding modes between the lanthanide complexes and certain phosphoanions, revealing a bidentate (α-, γ-) binding mode to ATP. We further use four of the receptors to allow discrimination of eight nucleoside phosphate anions in the first array-based assay using lanthanide complexes, taking advantage of the multiple emission bands and long emission lifetimes associated with luminescent lanthanide complexes.

Engineering a Reversible Fluorescent Probe for Real-Time Live-Cell Imaging and Quantification of Mitochondrial ATP

Real-time imaging and quantification of adenosine triphosphate (ATP) fluctuation in cells are significant for understanding the relationship between energy metabolism and cell functions. However, few synthetic fluorescent probes have been reported to tackle this challenge due to lack of accurate fluorescence readout and suitable response concentration. Herein we designed and synthesized a ratiometric fluorescent probe (Rh6G-ACFPN) for quantitatively detecting the fluctuation of mitochondrial ATP in living cells. Rh6G-ACFPN selectively and reversibly responds to ATP with an ideal dissociation constant (K_d) of 4.65 mM (3-10 mM: the range of mitochondrial ATP concentrations). Live-cell imaging allows us to directly monitor the dynamic changes of mitochondrial ATP in high temporal resolution. Moreover, for the first time, mitochondrial ATP in normal and cancer cells lines was successfully quantified and discriminated. These results demonstrate the versatility of Rh6G-ACFPN as a useful imaging tool to elucidate the function of mitochondrial ATP in living cells.

Fluorescence Differentiation of ATP-related Multiple Enzymatic Activities in Synovial Fluid as a Marker of Calcium Pyrophosphate Deposition Disease using Kyoto Green

Calcium pyrophosphate deposition disease (CPPD) is a crystal induced inflammation in joints, and causes severe pain in elderly people. The accumulation of pyrophosphate (PPi) in synovial fluid (SF) results from several enzymatic reactions, especially the highly activated e-NPPs, which catalyze the conversion of ATP to PPi. This study demonstrates the detection of relative catalytic activity of 3 enzymes—ecto-nucleotide pyrophosphatase/phosphodiesterases (e-NPPs), tissue nonspecific alkaline phosphatase (TNAP), and ecto-nucleoside triphosphate diphosphohydrolases (e-NTPDases)—using a single molecular sensor called Kyoto Green. Kyoto Green exhibits excellent performance in sensing the catalytic activity of the commercial representatives of the e-NPPs, TNAP, and e-NTPDases, which are ENPP1, PPase, and apyrase, respectively, in both single-enzyme and multi-enzyme assays. Analysis of SF enzymes in 19 SF samples from human and swine revealed moderate activity of e-NPPs, high activity of e-NTPDases, and low activity of TNAP. Our newly developed method for analysis of multiple enzymatic activities using Kyoto Green in biological SF will assist improvement in accuracy of the CPPD prognosis/diagnosis, which will minimize unnecessary medical procedures.

Toward a Rational Design of Polyamine-Based Zinc-Chelating Agents for Cancer Therapies

In vitro viability assays against a representative panel of human cancer cell lines revealed that polyamines L1a and L5a displayed remarkable activity with IC₅₀ values in the micromolar range. Preliminary research indicated that both compounds promoted G1 cell cycle arrest followed by cellular senescence and apoptosis. The induction of apoptotic cell death involved loss of mitochondrial outer membrane permeability and activation of caspases 3/7. Interestingly, L1a and L5a failed to activate cellular DNA damage response. The high intracellular zinc-chelating capacity of both compounds, deduced from the metal-specific Zinquin assay and ZnL²⁺ stability constant values in solution, strongly supports their cytotoxicity. These data along with quantum mechanical studies have enabled to establish a precise structure-activity relationship. Moreover, L1a and L5a showed appropriate drug-likeness by in silico methods. Based on these promising results, L1a and L5a should be considered a new class of zinc-chelating anticancer agents that deserves further development.

A new strategy to improve the water solubility of an organic fluorescent probe using silicon nanodots and fabricate two-photon SiND-ANPA-N3 for visualizing hydrogen sulfide in living cells and onion tissues

A water-soluble fluorescent probe based on SiNDs for H₂S detection can be used in both fully aqueous media and living cells.

Mitochondrion-Specific Blinking Fluorescent Bioprobe for Nanoscopic Monitoring of Mitophagy

Dynamic changes of mitochondrial morphology play an important role in cellular metabolism. Real-time monitoring mitochondrial ultrastructural dynamics at nanometer-scale resolution is crucially desired for further understanding of the mitochondria-based cellular function. In this work, we introduce a fluorescent carbon dot, which can selectively target mitochondria in live cells (named as MitoCD). MitoCD can effectively accumulate in mitochondria regardless of the decrease or vanishing of mitochondrial membrane potential (MMP), enabling the exploration of MMP-independent mitochondrial process. Moreover, the MitoCD is a thiol-based reaction-free probe that target mitochondria without consuming the thiol groups from mitochondrial proteins. Additionally, the MitoCD possesses good photophysical properties under physiological conditions, such as burst-like blinking, high photon counts, and low "on"/"off" ratio, which are specifically suitable for localization-based nanoscopic imaging. According to the optical microscopic imaging results, dynamical fission and fusion processes from mitochondria have been observed in live cells. During mitophagy, it is found that reticular formation of the mitochondria gradually collapsed, and then a portion of mitochondria split and vanished. Owing to the attractive biological and special photophysical properties, this probe displays promising application in a variety of super-resolution based biological studies and will provide deep insight in mitochondrial metabolism.

Frontline Science: Escherichia coli use LPS as decoy to impair neutrophil chemotaxis and defeat antimicrobial host defense

Bacterial infections and sepsis are leading causes of morbidity and mortality in critically ill patients. Currently, there are no effective treatments available to improve clinical outcome in sepsis. Here, we elucidated a mechanism by which Escherichia coli (E. coli) bacteria impair neutrophil (PMN) chemotaxis and we studied whether this mechanism can be therapeutically targeted to improve chemotaxis and antimicrobial host defense. PMNs detect bacteria with formyl peptide receptors (FPR). FPR stimulation triggers mitochondrial ATP production and release. Autocrine stimulation of purinergic receptors exerts excitatory and inhibitory downstream signals that induce cell polarization and cell shape changes needed for chemotaxis. Here we show that the bacterial cell wall product LPS dose-dependently impairs PMN chemotaxis. Exposure of human PMNs to LPS triggered excessive mitochondrial ATP production and disorganized intracellular trafficking of mitochondria, resulting in global ATP release that disrupted purinergic signaling, cell polarization, and chemotaxis. In mice infected i.p. with E. coli, LPS treatment increased the spread of bacteria at the infection site and throughout the systemic circulation. Removal of excessive systemic ATP with apyrase improved chemotaxis of LPS-treated human PMNs in vitro and enhanced the clearance of E. coli in infected and LPS-treated mice. We conclude that systemic ATP accumulation in response to LPS is a potential therapeutic target to restore PMN chemotaxis and to boost the antimicrobial host immune defense in sepsis.

An Acidic-Microenvironment-Driven DNA Nanomachine Enables Specific ATP Imaging in the Extracellular Milieu of Tumor

Extracellular ATP is an emerging target for cancer treatment because it is a key messenger for shaping the tumor microenvironment (TME) and regulating tumor progression. However, it remains a great challenge to design biochemical probes for targeted imaging of extracellular ATP in the TME. A TME-driven DNA nanomachine (Apt-LIP) that permits spatially controlled imaging of ATP in the extracellular milieu of tumors with ultrahigh signal-to-background ratio is reported. It operates in response to the mild acidity in the TME with the pH (low) insertion peptide (pHLIP) module, thus allowing the specific anchoring of the structure-switching signaling aptamer unit to the membrane of tumor cells for "off-on" fluorescence imaging of the extracellular ATP. Apt-LIP allows for acidity driven visualization of different extracellular concentrations of exogenous ATP, as well as the monitoring of endogenous ATP release from cells. Furthermore, it is demonstrated that Apt-LIP represents a promising platform for the specific imaging of the extracellular ATP in both primary and metastatic tumors. Ultimately, since diverse aptamers are obtained through in vitro selection, this design strategy can be further applied for precise detection of various extracellular targets in the TME.

Conformational Selection in Anion Recognition: cGMP-Selective Binding by a Naphthalimide-Functionalized Amido-Amine Macrocycle

Amido-amine macrocycles with two and four naphthalimide dyes were designed to bind nucleoside monophosphates and oligonucleotides in an aqueous buffered solution. Anion-templated synthesis was used to direct the macrocyclization reaction to the [2+2] product, while high dilution conditions favored the formation of the [4+4] macrocycle with an unprecedented geometry, as revealed from the X-ray analysis. The [2+2] product was found to exhibit a remarkable binding strength and fluorescence response for cyclic guanosine monophosphate (cGMP) in an aqueous solution. To our knowledge, this is the first synthetic receptor for cGMP, which also demonstrates a high preference to bind guanine-rich sequences accomplished by a strong fluorescence quenching. The receptor conformation is very sensitive to the guest structure in an aqueous solution, thus modeling the adaptive behavior of proteins. The study of synthetic systems with a detectable conformational equilibrium represents a great potential for understanding highly specific and tightly regulated interactions in biological systems.