Advances in Synthetic Fluorescent Probe Labeling for Live-Cell Imaging in Plants.

Fluorescent probes are powerful tools for visualizing cellular and subcellular structures, their dynamics and cellular molecules in living cells and enable us to monitor cellular processes in a spatiotemporal manner within complex and crowded systems. In addition to popular fluorescent proteins, a wide variety of small-molecule dyes have been synthesized through close association with the interdisciplinary field of chemistry and biology, ranging from those suitable for labeling cellular compartments such as organelles to those for labeling intracellular biochemical and biophysical processes and signaling. In recent years, self-labeling technologies including the SNAP-tag system have allowed us to attach these dyes to cellular domains or specific proteins and are beginning to be employed in plant studies. In this mini review, we will discuss the current range of synthetic fluorescent probes that have been exploited for live-cell imaging and the recent advances in the application that enable genetical tagging of synthetic probes in plant research.

On-Site Monitoring of Postoperative Bile Leakage Using Bilirubin-Inducible Fluorescent Protein.

BACKGROUND: Bile leakage is the most common postoperative complication associated with hepatobiliary and pancreatic surgery. Until now, however, a rapid, accurate diagnostic method for monitoring intraoperative and postoperative bile leakage had not been established. METHOD: Bilirubin levels in drained abdominal fluids collected from 23 patients who had undergone hepatectomy (n = 22) or liver transplantation (n = 1) were measured using a microplate reader with excitation/emission wavelengths of 497/527 nm after applying 5 µM of UnaG to the samples. UnaG was also sprayed directly on hepatic raw surfaces in swine hepatectomy models to identify bile leaks by fluorescence imaging. RESULTS: The bilirubin levels measured by UnaG fluorescence imaging showed favorable correlations with the results of the conventional light-absorptiometric methods (indirect bilirubin: rs = 0.939, p < 0.001; direct bilirubin: rs = 0.929, p < 0.001). Approximate time required for bilirubin measurements with UnaG was 15 min, whereas it took about 40 min with the conventional method at a hospital laboratory. Following administration of UnaG on hepatic surfaces, the fluorescence imaging identified bile leaks not only on the resected specimens but also in the abdominal cavity of the swine hepatectomy models. CONCLUSION: Fluorescence imaging techniques using UnaG may enable real-time identification of bile leaks during hepatectomy and on-site rapid diagnosis of bile leaks after surgery.

Covalent Self-Labeling of Tagged Proteins with Chemical Fluorescent Dyes in BY-2 Cells and Arabidopsis Seedlings.

Synthetic chemical fluorescent dyes promise to be useful for many applications in biology. Covalent, targeted labeling, such as with a SNAP-tag, uses synthetic dyes to label specific proteins in vivo for studying processes such as endocytosis or for imaging via super-resolution microscopy. Despite its potential, such chemical tagging has not been used effectively in plants. A major drawback has been the limited knowledge regarding cell wall and membrane permeability of the available synthetic dyes. Of 31 synthetic dyes tested here, 23 were taken up into BY-2 cells, while eight were not. This creates sets of dyes that can serve to measure endocytosis. Three of the dyes that were able to enter the cells, SNAP-tag ligands of diethylaminocoumarin, tetramethylrhodamine, and silicon-rhodamine 647, were used to SNAP-tag α-tubulin. Successful tagging was verified by live cell imaging and visualization of microtubule arrays in interphase and during mitosis in Arabidopsis (Arabidopsis thaliana) seedlings. Fluorescence activation-coupled protein labeling with DRBG-488 was used to observe PIN-FORMED2 (PIN2) endocytosis and delivery to the vacuole as well as preferential delivery of newly synthesized PIN2 to the actively forming cell plate during mitosis. Together, the data demonstrate that specific self-labeling of proteins can be used effectively in plants to study a wide variety of cellular and biological processes.

Spectral-differential-based unmixing for multispectral photoacoustic imaging.

We propose the use of a spectral differential method (SDM) to emphasize the spectral peaks of multispectral photoacoustic images. Because contrast agent signals have spectral peaks at the contrast agent absorption peak, the SDM can selectively emphasize contrast agent signals. Unlike the conventional spectral fitting method (SFM), the SDM does not require reference background spectra and, consequently, does not suffer from separation error caused by the deviation of reference spectra from the measured spectra. We performed multispectral photoacoustic imaging of tissue-mimicking phantoms and subcutaneous tumors of mice injected with small organic molecule-based contrast agents. Contrast agent images obtained by the SDM were clearer than those obtained by SFM.

Silicon Rhodamine-Based Near-Infrared Fluorescent Probe for γ-Glutamyltransferase.

We designed and synthesized an activatable near-infrared (NIR) fluorescent probe for γ-glutamyltransferase, based on an asymmetric silicon rhodamine scaffold with an optimized equilibrium of intramolecular spirocyclization. The synthesized probe exhibits dramatic NIR fluorescence activation and, in combination with previously reported probes, enables discrimination of tumors with different enzymatic profiles.

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.

Asymmetric Rhodamine-Based Fluorescent Probe for Multicolour In Vivo Imaging.

To achieve rapid and sensitive detection of cancer, activatable fluorescent probes targeting proteases that are overexpressed in various types of cancer have been developed, based on the hydroxymethyl rhodamine green (HMRG) scaffold. However, to visualize altered activities of multiple enzymes in cancer sites, other scaffolds with distinct fluorescence properties from those of HMRG are needed. A novel asymmetrically modified rhodamine with suitable absorption/emission, brightness and equilibrium constant of intramolecular spirocyclization, working in the yellow/orange region, is introduced. As a proof of concept, a probe targeting γ-glutamyl transpeptidase (gGlu-HMJCR) was developed on the basis of the new scaffold. Simultaneous visualization and discrimination of tumours expressing γ-glutamyl transpeptidase (with gGlu-HMJCR) and cathepsins (with Z-Phe-Arg-HMRG) by colour were achieved in a mouse model in vivo.

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.