Discovery of an F-actin-binding small molecule serving as a fluorescent probe and a scaffold for functional probes.

Actin is a ubiquitous cytoskeletal protein, forming a dynamic network that generates mechanical forces in the cell. There is a growing demand for practical and accessible tools for dissecting the role of the actin cytoskeleton in cellular function, and the discovery of a new actin-binding small molecule is an important advance in the field, offering the opportunity to design and synthesize of new class of functional molecules. Here, we found an F-actin­binding small molecule and introduced two powerful tools based on a new class of actin-binding small molecule: One enables visualization of the actin cytoskeleton, including super-resolution imaging, and the other enables highly specific green light­controlled fragmentation of actin filaments, affording unprecedented control of the actin cytoskeleton and its force network in living cells.

Quantitative modeling of regular retinal microglia distribution.

Microglia are resident immune cells in the central nervous system, showing a regular distribution. Advancing microscopy and image processing techniques have contributed to elucidating microglia's morphology, dynamics, and distribution. However, the mechanism underlying the regular distribution of microglia remains to be elucidated. First, we quantitatively confirmed the regularity of the distribution pattern of microglial soma in the retina. Second, we formulated a mathematical model that includes factors that may influence regular distribution. Next, we experimentally quantified the model parameters (cell movement, process formation, and ATP dynamics). The resulting model simulation from the measured parameters showed that direct cell-cell contact is most important in generating regular cell spacing. Finally, we tried to specify the molecular pathway responsible for the repulsion between neighboring microglia.

In vivo Fluorescence Imaging of Extracellular ATP in the Mouse Cerebral Cortex with a Hybrid-type Optical Sensor.

Adenosine 5'-triphosphate (ATP) works as an extracellular signaling molecule for cells in the brain, such as neurons and glia. Cellular communication via release of ATP is involved in a range of processes required for normal brain functions, and aberrant communication is associated with brain disorders. To investigate the mechanisms underlying these cellular processes, various techniques have been developed for the measurement of extracellular ATP. To monitor the dynamics of extracellular ATP signaling with high spatiotemporal resolution, we recently developed a hybrid-type ATP optical sensor (ATPOS) that enables in vivo fluorescence imaging of extracellular ATP dynamics in the brain. ATPOS is synthesized by labeling an ATP-binding protein, Bacillus FoF1-ATP synthase ε subunit, with a small-molecular fluorescent dye Cy3. Injection of ATPOS into the cerebral cortex of living mice enables visualization of the wave-like propagation of extracellular ATP release in response to electrical stimulation. The protocol described here should be useful for visualizing ATP signaling in diverse processes involved in intercellular communication in the brain.

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.

Biologists often refer to a small molecule called adenosine triphosphate ­ or ATP for short ­ as 'the currency of life'. This molecule carries energy all through the body, and most cells and proteins require ATP to perform their various roles. Nerve cells (also known as neurons) in the brain release ATP when activated, and use this molecule to send signals to other active neurons or other cells in the brain. But ATP can also signal danger in the brain. A molecule derived from ATP is involved in transmitting the pain signals of migraines and severe headaches; and ATP levels can become imbalanced after strokes, when parts of the brain are deprived of blood. Despite its importance, ATP remains difficult to visualize in the body, and monitoring the molecule in the active brain in real time is challenging. To address this issue, Kitajima et al. designed an optical sensor that could monitor ATP in the healthy brain, and was sensitive enough to detect when and where it was released. First, Kitajima et al. made several potential sensors by attaching various fluorescent tags to different locations on a protein that binds ATP. Next each sensor was tested to determine whether it could bind ATP tightly and get bright upon binding. This is important because previous sensors could not detect ATP release in the brains of living animals. To illustrate the new sensors' potential, Kitajima et al. used the sensor to image ATP in the brains of live mice. A 'wave' of ATP was seen spreading through the brain after neurons were stimulated with a small electric pulse, mimicking a sudden migraine or stroke. The results confirm that this new sensor is suitable for imaging how ATP signals in the brain, and it may help resolve the underlying mechanisms of migraines and strokes. This sensor could also be used to understand other cellular process which rely on ATP to carry out their role.

Synaptic weight set by Munc13-1 supramolecular assemblies.

The weight of synaptic connections, which is controlled not only postsynaptically but also presynaptically, is a key determinant in neuronal network dynamics. The mechanisms controlling synaptic weight, especially on the presynaptic side, remain elusive. Using single-synapse imaging of the neurotransmitter glutamate combined with super-resolution imaging of presynaptic proteins, we identify a presynaptic mechanism for setting weight in central glutamatergic synapses. In the presynaptic terminal, Munc13-1 molecules form multiple and discrete supramolecular self-assemblies that serve as independent vesicular release sites by recruiting syntaxin-1, a soluble N-ethylmaleimide-sensitive-factor attachment receptor (SNARE) protein essential for synaptic vesicle exocytosis. The multiplicity of these Munc13-1 assemblies affords multiple stable states conferring presynaptic weight, potentially encoding several bits of information at individual synapses. Supramolecular assembling enables a stable synaptic weight, which confers robustness of synaptic computation on neuronal circuits and may be a general mechanism by which biological processes operate despite the presence of molecular noise.

Novel Hexosaminidase-Targeting Fluorescence Probe for Visualizing Human Colorectal Cancer.

Precise tumor diagnosis and evaluation of disease extent are crucial for treatment of solid cancers. In order to complement the limited ability of the unaided human eye to discriminate tumor tissue and normal tissue, we have developed a series of fluorescence probes activatable specifically in cancer tissues. Here, we describe the design, synthesis, and application of a new fluorescence probe targeting hexosaminidase (HMRef-ßGlcNAc), which is located in lysosomes and is overexpressed in several carcinomas, including colorectal cancer. This probe could sensitively detect intracellular hexosaminidase activity in human colorectal cancer cell lines, and could visualize tiny metastatic nodules (smaller than 1 mm) in a mouse model of disseminated human peritoneal colorectal cancer (HCT116). In human colorectal cancer specimens obtained at surgery, the probe showed high tumor sensitivity/specificity, together with a high tumor-to-normal signal ratio. HMRef-ßGlcNAc is a promising candidate for clinical application during surgical or endoscopic procedures to treat colorectal cancer.

Protease-resistant modified human ß-hexosaminidase B ameliorates symptoms in GM2 gangliosidosis model.

GM2 gangliosidoses, including Tay-Sachs and Sandhoff diseases, are neurodegenerative lysosomal storage diseases that are caused by deficiency of ß-hexosaminidase A, which comprises an αß heterodimer. There are no effective treatments for these diseases; however, various strategies aimed at restoring ß-hexosaminidase A have been explored. Here, we produced a modified human hexosaminidase subunit ß (HexB), which we have termed mod2B, composed of homodimeric ß subunits that contain amino acid sequences from the α subunit that confer GM2 ganglioside-degrading activity and protease resistance. We also developed fluorescent probes that allow visualization of endocytosis of mod2B via mannose 6-phosphate receptors and delivery of mod2B to lysosomes in GM2 gangliosidosis models. In addition, we applied imaging mass spectrometry to monitor efficacy of this approach in Sandhoff disease model mice. Following i.c.v. administration, mod2B was widely distributed and reduced accumulation of GM2, asialo-GM2, and bis(monoacylglycero)phosphate in brain regions including the hypothalamus, hippocampus, and cerebellum. Moreover, mod2B administration markedly improved motor dysfunction and a prolonged lifespan in Sandhoff disease mice. Together, the results of our study indicate that mod2B has potential for intracerebrospinal fluid enzyme replacement therapy and should be further explored as a gene therapy for GM2 gangliosidoses.

High affinity receptor labeling based on basic leucine zipper domain peptides conjugated with pH-sensitive fluorescent dye: Visualization of AMPA-type glutamate receptor endocytosis in living neurons.

Techniques to visualize receptor trafficking in living neurons are important, but currently available methods are limited in their labeling efficiency, specificity and reliability. Here we report a method for receptor labeling with a basic leucine zipper domain peptide (ZIP) and a binding cassette specific to ZIP. Receptors are tagged with a ZIP-binding cassette at their extracellular domain. Tagged receptors expressed in cultured cells were labeled with exogenously applied fluorescently labeled ZIP with low background and high affinity. To test if ZIP labeling is useful in monitoring endocytosis and intracellular trafficking, we next conjugated ZIP with a pH-sensitive dye RhP-M (ZIP-RhP-M). ZIP binding to its binding cassette was pH-resistant and RhP-M fluorescence dramatically increased in acidic environment. Thus AMPA-type glutamate receptors (AMPARs) labeled by ZIP-RhP-M can report receptor endocytosis and subsequent intracellular trafficking. Application of ZIP-RhP-M to cultured hippocampal neurons expressing AMPARs tagged with a ZIP-binding cassette resulted in appearance of fluorescent puncta in PSD-95-positive large spines, suggesting local endocytosis and acidification of AMPARs in individual mature spines. This spine pool of AMPARs in acidic environment was distinct from the early endosomes labeled by transferrin uptake. These results suggest that receptor labeling by ZIP-RhP-M is a useful technique for monitoring endocytosis and intracellular trafficking. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Sensitive ß-galactosidase-targeting fluorescence probe for visualizing small peritoneal metastatic tumours in vivo.

Fluorescence-guided diagnostics is one of the most promising approaches for facile detection of cancer in situ. Here we focus on ß-galactosidase, which is overexpressed in primary ovarian cancers, as a molecular target for visualizing peritoneal metastases from ovarian cancers. As existing fluorescence probes are unsuitable, we have designed membrane-permeable HMRef-ßGal, in which the optimized intramolecular spirocyclic function affords >1,400-fold fluorescence enhancement on activation. We confirm that HMRef-ßGal sensitively detects intracellular ß-galactosidase activity in several ovarian cancer lines. In vivo, this probe visualizes metastases as small as <1 mm in diameter in seven mouse models of disseminated human peritoneal ovarian cancer (SHIN3, SKOV3, OVK18, OVCAR3, OVCAR4, OVCAR5 and OVCAR8). Because of its high brightness, real-time detection of metastases with the naked eye is possible. Endoscopic fluorescence detection of metastases is also demonstrated. The results clearly indicate preclinical potential value of the probe for fluorescence-guided diagnosis of peritoneal metastases from ovarian cancers.

High-throughput development of a hybrid-type fluorescent glutamate sensor for analysis of synaptic transmission.

Fluorescent sensors are powerful tools for visualizing cellular molecular dynamics. We present a high-throughput screening system, designated hybrid-type fluorescence indicator development (HyFInD), to identify optimal position-specific fluorophore labeling in hybrid-type sensors consisting of combinations of ligand-binding protein mutants with small molecular fluorophores. We screened sensors for glutamate among hybrid molecules obtained by the reaction of four cysteine-reactive fluorescence probes with a set of cysteine-scanning mutants of the 274 amino acid S1S2 domain of AMPA-type glutamate receptor GluA2 subunit. HyFInD identified a glutamate-responsive probe (enhanced glutamate optical sensor: eEOS) with a dynamic range >2400 %, good photostability, and high selectivity. When eEOS was specifically tethered to neuronal surfaces, it reliably visualized the spatiotemporal dynamics of glutamate release at single synapses, revealing synapse-to-synapse heterogeneity of short-term plasticity.

High-throughput screening system to identify small molecules that induce internalization and degradation of HER2.

Overexpression of growth factor receptors in cancers, e.g., human epidermal growth factor receptor 2 (HER2) in ovarian and breast cancers, is associated with aggressiveness. A possible strategy to treat cancers that overexpress those receptors is blockade of receptor signaling by inducing receptor internalization and degradation. In this study, we developed a cell-based high-throughput screening (HTS) system to identify small molecules that induce HER2 internalization by employing our recently developed acidic-pH-activatable probe in combination with protein labeling technology. Our HTS system enabled facile and reliable quantification of HER2 internalization with a Z' factor of 0.66 and a signal-to-noise ratio of 44.6. As proof of concept, we used the system to screen a ∼155,000 small-molecule library and identified three hits that induced HER2 internalization and degradation via at least two distinct mechanisms. This HTS platform should be adaptable to other disease-related receptors in addition to HER2.

Arrayed lipid bilayer chambers allow single-molecule analysis of membrane transporter activity.

Nano- to micron-size reaction chamber arrays (femtolitre chamber arrays) have facilitated the development of sensitive and quantitative biological assays, such as single-molecule enzymatic assays, digital PCR and digital ELISA. However, the versatility of femtolitre chamber arrays is limited to reactions that occur in aqueous solutions. Here we report an arrayed lipid bilayer chamber system (ALBiC) that contains sub-million femtolitre chambers, each sealed with a stable 4-µm-diameter lipid bilayer membrane. When reconstituted with a limiting amount of the membrane transporter proteins α-hemolysin or F0F1-ATP synthase, the chambers within the ALBiC exhibit stochastic and quantized transporting activities. This demonstrates that the single-molecule analysis of passive and active membrane transport is achievable with the ALBiC system. This new platform broadens the versatility of femtolitre chamber arrays and paves the way for novel applications aimed at furthering our mechanistic understanding of membrane proteins' function.

Acidic-pH-activatable fluorescence probes for visualizing exocytosis dynamics.

Live imaging of exocytosis dynamics is crucial for a precise spatiotemporal understanding of secretion phenomena, but current approaches have serious limitations. We designed and synthesized small-molecular fluorescent probes that were chemically optimized for sensing acidic intravesicular pH values, and established that they can be used to sensitively and reliably visualize vesicular dynamics following stimulation. This straightforward technique for the visualization of exocytosis as well as endocytosis/reacidification processes with high spatiotemporal precision is expected to be a powerful tool for investigating dynamic cellular phenomena involving changes in the pH value.

Fluorescence endoscopic detection of murine colitis-associated colon cancer by topically applied enzymatically rapid-activatable probe.

OBJECTIVES: Screening colonoscopy to monitor for early colitis-associated colon cancer (CAC) is difficult due to the aberrant mucosal patterns associated with long-standing colitis. The aim of this study was to develop a rapid fluorescent detection method for use during colonoscopy for improving the detection of CAC utilising a topically applied enzymatically activatable probe (gGlu-HMRG) which fluoresces in the presence of γ-glutamyltranspeptidase (GGT), an enzyme associated with cancer. METHODS: Expression of GGT in colon cell lines was examined with fluorescence microscopy and flow cytometry. A mouse model (azoxymethane/dextran sulphate sodium) of CAC was used and mice were examined with white light and fluorescence colonoscopy before and after topical gGlu-HMRG administration. RESULTS: Expression of GGT, although variable, was higher in human colon cancer cells than normal human colon cells. Using fluorescence colonoscopy in mice, gGlu-HMRG fluorescent lesions were detected 5 min after topical administration and fluorescence persisted for at least 30 min. Fluorescence guided biopsy revealed all fluorescent lesions that contained cancer or dysplasia (n=16), whereas three out of 12 non-fluorescent lesions contained low grade dysplasia and others did not contain neoplastic histology. Microscopic inflammatory infiltration also had variable fluorescence but in general was much lower (∼10-fold) in signal than cancer. Repeat fluorescence endoscopy allowed individual tumours to be monitored. CONCLUSION: These results suggest that gGlu-HMRG can improve endoscopic detection of CAC with a higher target to background ratio than conventional white light colonoscopy. This could be of benefit to patients with long-standing colitis who must undergo repeated screening colonoscopies.

Rational design of highly sensitive fluorescence probes for protease and glycosidase based on precisely controlled spirocyclization.

We have synthesized and evaluated a series of hydroxymethyl rhodamine derivatives and found an intriguing difference of intramolecular spirocyclization behavior: the acetylated derivative of hydroxymethyl rhodamine green (Ac-HMRG) exists as a closed spirocyclic structure in aqueous solution at physiological pH, whereas HMRG itself takes an open nonspirocyclic structure. Ac-HMRG is colorless and nonfluorescent, whereas HMRG is strongly fluorescent. On the basis of these findings, we have developed a general design strategy to obtain highly sensitive fluorescence probes for proteases and glycosidases, by replacing the acetyl group of Ac-HMRG with a substrate moiety of the target enzyme. Specific cleavage of the substrate moiety in the nonfluorescent probe by the target enzyme generates a strong fluorescence signal. To confirm the validity and flexibility of our strategy, we designed and synthesized fluorescence probes for leucine aminopeptidase (Leu-HMRG), fibroblast activation protein (Ac-GlyPro-HMRG), and ß-galactosidase (ßGal-HMRG). All of these probes were almost nonfluorescent due to the formation of spirocyclic structure, but were converted efficiently to highly fluorescent HMRG by the target enzymes. We confirmed that the probes can be used in living cells. These probes offer great practical advantages, including high sensitivity and rapid response (due to regulation of fluorescence at a single reactive site), as well as resistance to photobleaching, and are expected to be useful for a range of biological and pathological investigations.

Rapid cancer detection by topically spraying a γ-glutamyltranspeptidase-activated fluorescent probe.

The ability of the unaided human eye to detect small cancer foci or accurate borders between cancer and normal tissue during surgery or endoscopy is limited. Fluorescent probes are useful for enhancing visualization of small tumors but are typically limited by either high background signal or the requirement for administration hours to days before use. We synthesized a rapidly activatable, cancer-selective fluorescence imaging probe, γ-glutamyl hydroxymethyl rhodamine green (gGlu-HMRG), with intramolecular spirocyclic caging for complete quenching. Activation occurs by rapid one-step cleavage of glutamate with γ-glutamyltranspeptidase (GGT), which is not expressed in normal tissue, but is overexpressed on the cell membrane of various cancer cells, thus leading to complete uncaging and dequenching of the fluorescence probe. In vitro activation of gGlu-HMRG was evident in 11 human ovarian cancer cell lines tested. In vivo in mouse models of disseminated human peritoneal ovarian cancer, activation of gGlu-HMRG occurred within 1 min of topically spraying the tumor, creating high signal contrast between the tumor and the background. The gGlu-HMRG probe is practical for clinical application during surgical or endoscopic procedures because of its rapid and strong activation upon contact with GGT on the surface of cancer cells.

ß-Galactosidase fluorescence probe with improved cellular accumulation based on a spirocyclized rhodol scaffold.

We identified a rhodol bearing a hydroxymethyl group (HMDER) as a suitable scaffold for designing fluorescence probes for various hydrolases. HMDER shows strong fluorescence at physiological pH, but phenolic O-alkylation of HMDER results in a strong preference for the spirocyclic form, which has weak fluorescence. As a proof of concept, we utilized this finding to develop a new fluorescence probe for ß-galactosidase. This probe has favorable characteristics for imaging in biological samples: it has good cellular permeability, and its hydrolysis product is well-retained intracellularly. It could rapidly and clearly visualize ß-galactosidase activity in cultured cells and in Drosophila melanogaster tissue, which has rarely been achieved with previously reported fluorescence probes.

Intraperitoneal photodynamic therapy mediated by a fullerene in a mouse model of abdominal dissemination of colon adenocarcinoma.

Functionalized fullerenes represent a new class of photosensitizer (PS) that is being investigated for photodynamic therapy (PDT) of various diseases, including cancer. We tested the hypothesis that fullerenes could be used to mediate PDT of intraperitoneal (IP) carcinomatosis in a mouse model. In humans this form of cancer responds poorly to standard treatment and manifests as a thin covering of tumor nodules on intestines and on other abdominal organs. We used a colon adenocarcinoma cell line (CT26) stably expressing luciferase to allow monitoring of IP tumor burden in BALB/c mice by noninvasive real-time optical imaging using a sensitive low-light camera. IP injection of a preparation of N-methylpyrrolidinium-fullerene formulated in Cremophor-EL micelles, followed by white-light illumination delivered through the peritoneal wall (after creation of a skin flap), produced a statistically significant reduction in bioluminescence and a survival advantage in mice. FROM THE CLINICAL EDITOR: This team of investigators report on functionalized fullerenes, to be used as photosensitizer for photodynamic therapy and demonstrate the efficacy of this method in an intraperitoneal carcinomatosis mouse model.

Fluorescence imaging of tumors with "smart" pH-activatable targeted probes.

One goal of molecular imaging is to establish a widely applicable technique for specific detection of tumors with minimal background originated from non-target tissues. In this study, a "smart" activatable strategy for specific tumor imaging is proposed in which pH-activatable targeted probes specifically detect tumors after binding to the target cell surface proteins, internalization, and eventual acidic pH activation within the acidic organelles. We successfully visualized submillimeter-sized tumors using this strategy in two different tumor mouse models. Since the design of pH-activatable targeted probes can be applied to any target molecules on the cell surface that are to be internalized after ligand binding, this imaging strategy can afford a general and powerful method to diagnose and monitor the target tumors.