Self-assembled monolayer coating of biological probes to avoid protein adhesion

Probes designed to locally illuminate structures within plant cells are described. The probes studied are etch-sharpened single mode optical fibers, coated with aluminum, similar to probes used for near-field scanning optical microscopy. We find that cellular material adheres to the probes that are not coated with a self-assembled monolayer octadecyltrichlorosilane. The hydrophobic monolayer coating enabled these probes to be inserted into and removed from plant cells with no protein adhesion to the probes. This allows probe reinsertion and it causes less damage to the target cell, greatly facilitating in vivo optical study of cells.     

A laser spectrofluorimeter for studies of cultured cells on the growth surface.

We have developed a new method for direct measurement of fluorescent probes associated with undisturbed monolayers of cultured fibroblasts. A helium-cadmium continuous wave laser produced excitation light at 325 nm to illuminate the cell monolayer on the front inner surface of a quartz sample tube. The emitted light from the cell monolayer passed through a scanning monochrometer to a low noise photomultiplier tube and was amplified with a photon-counting system. The fluorescent probe, cholesteryl 4-(3′-pyrenyl)butanoate, was incorporated into normal human low density lipoproteins (LDL). The interaction between LDL containing the fluorescent probe and the cell surface of normal human fibroblasts was examined. The uptake of the fluorescent LDL was measured as a function of temperature, concentration, time, specificity, and ability to suppress 3-hydroxy-3-methyl-glutaryl-CoA reductase. In all respects, LDL containing the fluorescent probe and native LDL were comparable. Using this technique, cell-surface interactions can be studied in situ so that changes in structure and function caused by removal of the cells from the growth surface can be avoided.     

Functionalized quantum dots to quantify NADPH and their use for NADP+-dependent biocatalyzed transformations

Quantum dots are semiconducting nanoparticles that can be prepared with interesting optical properties. The fluorescent properties of quantum dots are one of the key advantages for their use as optical labels for biorecognition events and biocatalytic processes. We have prepared semiconductor quantum dots conjugated with Nile Blue (NB), and demonstrate that NB-functionalized quantum dots can act as versatile probes to analyze different biocatalyzed transformations, and can be used for the quantitative detection of NADPH as well as NADH. This approach provides a new path for the optical detection of NAD(P)H and for the quantitative analysis of NAD(P)(+)-dependent biotransformations.     

Comparative Assessment of Substrates and Activity Based Probes as Tools for Non-Invasive Optical Imaging of Cysteine Protease Activity

Recent advances in the field of non-invasive optical imaging have included the development of contrast agents that report on the activity of enzymatic targets associated with disease pathology. In particular, proteases have proven to be ideal targets for development of optical sensors for cancer. Recently developed contrast agents for protease activity include both small peptides and large polymer-based quenched fluorescent substrates as well as fluorescently labeled activity based probes (ABPs). While substrates produce a fluorescent signal as a result of processing by a protease, ABPs are retained at the site of proteolysis due to formation of a permanent covalent bond with the active site catalytic residue. Both methods have potential advantages and disadvantages yet a careful comparison of substrates and ABPs has not been performed. Here we present the results of a direct comparison of commercially available protease substrates with several recently described fluorescent ABPs in a mouse model of cancer. The results demonstrate that fluorescent ABPs show more rapid and selective uptake into tumors as well as overall brighter signals compared to substrate probes. These data suggest that the lack of signal amplification for an ABP is offset by the increased kinetics of tissue uptake and prolonged retention of the probes once bound to a protease target. Furthermore, fluorescent ABPs can be used as imaging reagents with similar or better results as the commercially available protease substrates.     

Illuminating intracellular signaling and molecules for single cell analysis

Fluorescent and bioluminescent proteins are now widely used for detection of small molecules and various intracellular events ranging from protein conformational change to cell death in living cells. To analyze the dynamics of molecular processes in real time at the level of single cells, engineered protein-based probes with higher sensitivity and selectivity are required. The probes can be entirely genetically encoded and can comprise fusions of different proteins or domains. This review specifically examines basic concepts of designing genetically encoded fluorescent and bioluminescent probes developed in the past decade, highlighting some potential applications for basic research and for drug discovery.     

High-Contrast Fluorescence Imaging in Fixed and Living Cells Using Optimized Optical Switches

We present the design, synthesis and characterization of new functionalized fluorescent optical switches for rapid, all-visible light-mediated manipulation of fluorescence signals from labelled structures within living cells, and as probes for high-contrast optical lock-in detection (OLID) imaging microscopy. A triazole-substituted BIPS (TzBIPS) is identified from a rational synthetic design strategy that undergoes robust, rapid and reversible, visible light-driven transitions between a colorless spiro- (SP) and a far-red absorbing merocyanine (MC) state within living cells. The excited MC-state of TzBIPS may also decay to the MC-ground state emitting near infra-red fluorescence, which is used as a sensitive and quantitative read-out of the state of the optical switch in living cells. The SP to MC transition for a membrane-targeted TzBIPS probe (C₁₂-TzBIPS) is triggered at 405 nm at an energy level compatible with studies in living cells, while the action spectrum of the reverse transition (MC to SP) has a maximum at 650 nm. The SP to MC transition is complete within the 790 ns pixel dwell time of the confocal microscope, while a single cycle of optical switching between the SP and MC states in a region of interest is complete within 8 ms (125 Hz) within living cells, the fastest rate attained for any optical switch probe in a biological sample. This property can be exploited for real-time correction of background signals in living cells. A reactive form of TzBIPS is linked to secondary antibodies and used, in conjunction with an enhanced scope-based analysis of the modulated MC-fluorescence in immuno-stained cells, for high-contrast immunofluorescence microscopic analysis of the actin cytoskeleton.     

Luminescent quantum dots for multiplexed biological detection and imaging

Recent advances in nanomaterials have produced a new class of fluorescent labels by conjugating semiconductor quantum dots with biorecognition molecules. These nanometer-sized conjugates are water-soluble and biocompatible, and provide important advantages over organic dyes and lanthanide probes. In particular, the emission wavelength of quantum-dot nanocrystals can be continuously tuned by changing the particle size, and a single light source can be used for simultaneous excitation of all different-sized dots. High-quality dots are also highly stable against photobleaching and have narrow, symmetric emission spectra. These novel optical properties render quantum dots ideal fluorophores for ultrasensitive, multicolor, and multiplexing applications in molecular biotechnology and bioengineering.     

Imaging of single molecules in live cells

Progress in optical microscopy, combined to the emergence of new fluorescent probes and advanced instrumentation, now permits the imaging of single molecules in fixed and live cells. This extreme detection sensitivity has opened new modalities in cellular imaging. On the one hand, optical images with an unprecedented resolution in the 10-50 nm range, well below the diffraction limit of light, can be recorded. These super-resolution images give new insights into the properties of cellular structures. On the other hand, proteins, either in the membrane or intracellular, can be tracked in live cells and in physiological conditions. Their individual trajectories provide invaluable information on the molecular interactions that control their dynamics and their spatial organization. Single molecule imaging is rapidly becoming a unique tool to understand the biochemical and biophysical processes that determine the properties of molecular assemblies in a cellular context.     

FLOW CYTOMETRY AND THE SINGLE CELL IN PHYCOLOGY

Flow cytometers measure light scattering and fluorescence characteristics from individual particles in a fluid stream as they cross one or more light beams at rates of up to thousands of events per second. Flow cytometrically detectable optical signals may arise naturally from algae, reflecting cell size, structure, and endogenous pigmentation, or may be generated by fluorescent stains that report the presence of otherwise undetected cellular constituents. Some flow cytometers can physically sort particles with desired optical characteristics out of the flow stream and collect them for subsequent culture or other analyses. The statistically rigorous, cell‐level perspective provided by flow cytometry has been advantageous in experimental investigations of phycological problems, such as the regulation of cell cycle progression. The capacity of flow cytometry to measure large numbers of cells in large numbers of samples rapidly and quantitatively has been used extensively by biological oceanographers to define the distributions and dynamics of marine picophytoplankton. Recent work has shown that flow cytometry can be used to elucidate relationships between the optical properties of individual cells and the bulk optical properties of the water they live in, and thereby may provide an explicit link between algal physiology and global biogeochemistry. Unfortunately, commercially available flow cytometers that are optimized for biomedical applications have a limited capacity to analyze larger phytoplankton. To circumvent these limitations, many investigators are developing flow cytometers specifically designed for analyzing the broad range of sizes, shapes, and pigments found among algae. These new instruments can perform some novel measurements, including simple fluorescence excitation spectra, detailed angular scattering measurements, and in‐flow digital imaging. The growing accessibility and power of flow cytometers may allow the technology to be applied to a wider array of problems in phycology, including investigations of nonplanktonic and multicellular algae, but also presents new challenges for effectively analyzing the large quantity of multiparameter data produced. Ultimately, the detection of molecular probes by flow cytometry may allow single‐cell taxonomic and physiological information to be garnered for a variety of algae, both in culture and in nature.     

A comparison of the emission efficiency of four common green fluorescence dyes after internalization into cancer cells

In vivo optical imaging to enhance the detection of cancer during endoscopy or surgery requires a targeted fluorescent probe with high emission efficiency and high signal-to-background ratio. One strategy to accurately detect cancers is to have the fluorophore internalize within the cancer cells permitting nonbound fluorophores to be washed away or absorbed. The choice of fluorophores for this task must be carefully considered. For depth of penetration, near-infrared probes are ordinarily preferred but suffer from relatively low quantum efficiency. Although green fluorescent protein has been widely used to image tumors on internal organs in mice, green fluorescent probes are better suited for imaging the superficial tissues because of the short penetration distance of green light in tissue and the highly efficient production of signal. While the fluorescence properties of green fluorophores are well-known in vitro, less attention has been paid to their fluorescence once they are internalized within cells. In this study, the emission efficiency after cellular internalization of four common green fluorophores conjugated to avidin (Av-fluorescein, Av-Oregon green, Av-BODIPY-FL, and Av-rhodamine green) were compared after each conjugate was incubated with SHIN3 ovarian cancer cells. Using the lectin binding receptor system, the avidin-fluorophore conjugates were endocytosed, and their fluorescence was evaluated with fluorescence microscopy and flow cytometry. While fluorescein demonstrated the highest signal outside the cell, among the four fluorophores, internalized Av-rhodamine green emitted the most light from SHIN3 ovarian cancer cells both in vitro and in vivo. The internalized Av-rhodamine green complex appeared to localize to the endoplasmic vesicles. Thus, among the four common green fluorescent dyes, rhodamine green is the brightest green fluorescence probe after cellular internalization. This information could have implications for the design of tumor-targeted fluorescent probes that rely on cellular internalization for cancer detection.     

In vivo fluorescence lifetime imaging monitors binding of specific probes to cancer biomarkers

One of the most important factors in choosing a treatment strategy for cancer is characterization of biomarkers in cancer cells. Particularly, recent advances in Monoclonal Antibodies (MAB) as primary-specific drugs targeting tumor receptors show that their efficacy depends strongly on characterization of tumor biomarkers. Assessment of their status in individual patients would facilitate selection of an optimal treatment strategy, and the continuous monitoring of those biomarkers and their binding process to the therapy would provide a means for early evaluation of the efficacy of therapeutic intervention. In this study we have demonstrated for the first time in live animals that the fluorescence lifetime can be used to detect the binding of targeted optical probes to the extracellular receptors on tumor cells in vivo. The rationale was that fluorescence lifetime of a specific probe is sensitive to local environment and/or affinity to other molecules. We attached Near-InfraRed (NIR) fluorescent probes to Human Epidermal Growth Factor 2 (HER2/neu)-specific Affibody molecules and used our time-resolved optical system to compare the fluorescence lifetime of the optical probes that were bound and unbound to tumor cells in live mice. Our results show that the fluorescence lifetime changes in our model system delineate HER2 receptor bound from the unbound probe in vivo. Thus, this method is useful as a specific marker of the receptor binding process, which can open a new paradigm in the "image and treat" concept, especially for early evaluation of the efficacy of the therapy.     

Targeted optical probing of neuronal circuit dynamics using fluorescent protein sensors

Interest in non-invasive methods for optical probing of neuronal electrical activity has been ongoing for several decades and methods for imaging the activity of single or multiple individual neurons in networks composed of thousands of neurons have been developed. Most widely used are techniques that use organic chemistry-based dyes as indicators of calcium and membrane potential. More recently a new generation of probes, genetically encoded fluorescent protein sensors, have emerged for use by physiologists studying the operation of neuronal circuits. In this review we describe the advance of these emerging optical techniques and compare them with more conventional approaches.     

SHG nanoprobes: advancing harmonic imaging in biology

Second harmonic generating (SHG) nanoprobes have recently emerged as versatile and durable labels suitable for in vivo imaging, circumventing many of the inherent drawbacks encountered with classical fluorescent probes. Since their nanocrystalline structure lacks a central point of symmetry, they are capable of generating second harmonic signal under intense illumination - converting two photons into one photon of half the incident wavelength - and can be detected by conventional two-photon microscopy. Because the optical signal of SHG nanoprobes is based on scattering, rather than absorption as in the case of fluorescent probes, they neither bleach nor blink, and the signal does not saturate with increasing illumination intensity. When SHG nanoprobes are used to image live tissue, the SHG signal can be detected with little background signal, and they are physiologically inert, showing excellent long-term photostability. Because of their photophysical properties, SHG nanoprobes provide unique advantages for molecular imaging of living cells and tissues with unmatched sensitivity and temporal resolution.     

A new optical multichannel microspectrofluorometer.

A new microspectrofluorometer has been developed that combines a photometric fluorescence microscope with an optical multichannel analyzer. This instrument provides fluorescence emission spectra of biological materials by detecting the entire spectrum simultaneously in real time. These spectra are subsequently recorded and corrected so as to identify the fluorescent reaction products or to test whether fluorescent cytochemical probes bind to the expected substrate within cells. The procedures and advantages of optical multichannel analysis are described, and an application of microspectrofluorometry to acriflavine-Feulgen cytochemistry is given.     

mEosFP-based green-to-red photoconvertible subcellular probes for plants

Photoconvertible fluorescent proteins (FPs) are recent additions to the biologists' toolbox for understanding the living cell. Like green fluorescent protein (GFP), monomeric EosFP is bright green in color but is efficiently photoconverted into a red fluorescent form using a mild violet-blue excitation. Here, we report mEosFP-based probes that localize to the cytosol, plasma membrane invaginations, endosomes, prevacuolar vesicles, vacuoles, the endoplasmic reticulum, Golgi bodies, mitochondria, peroxisomes, and the two major cytoskeletal elements, filamentous actin and cortical microtubules. The mEosFP fusion proteins are smaller than GFP/red fluorescent protein-based probes and, as demonstrated here, provide several significant advantages for imaging of living plant cells. These include an ability to differentially color label a single cell or a group of cells in a developing organ, selectively highlight a region of a cell or a subpopulation of organelles and vesicles within a cell for tracking them, and understanding spatiotemporal aspects of interactions between similar as well as different organelles. In addition, mEosFP probes introduce a milder alternative to fluorescence recovery after photobleaching, whereby instead of photobleaching, photoconversion followed by recovery of green fluorescence can be used for estimating subcellular dynamics. Most importantly, the two fluorescent forms of mEosFP furnish bright internal controls during imaging experiments and are fully compatible with cyan fluorescent protein, GFP, yellow fluorescent protein, and red fluorescent protein fluorochromes for use in simultaneous, multicolor labeling schemes. Photoconvertible mEosFP-based subcellular probes promise to usher in a much higher degree of precision to live imaging of plant cells than has been possible so far using single-colored FPs.     

Atomic force microscopy and scanning near-field optical microscopy studies on the characterization of human metaphase chromosomes

A better knowledge of biochemical and structural properties of human chromosomes is important for cytogenetic investigations and diagnostics. Fluorescence in situ hybridization (FISH) is a commonly used technique for the visualization of chromosomal details. Localizing specific gene probes by FISH combined with conventional fluorescence microscopy has reached its limit. Also, microdissecting DNA from G-banded human metaphase chromosomes by either a glass tip or by laser capture needs further improvement. By both atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM), local information from G-bands and chromosomal probes can be obtained. The final resolution allows a more precise localization compared to standard techniques, and the extraction of very small amounts of chromosomal DNA by the scanning probe is possible. Besides new strategies towards a better G-band and fluorescent probe detection, this study is focused on the combination of biochemical and nanomanipulation techniques which enable both nanodissection and nanoextraction of chromosomal DNA.     

Identification of single bacterial cells using digoxigenin-labelled, rRNA-targeted oligonucleotides.

Oligonucleotides were end-labelled with digoxigenin (DIG), chemically at the 5'-end or enzymically at the 3'-end. Following specific in situ hybridization of these probes to intracellular rRNA molecules, the hybrids were detected with anti-DIG Fab fragments labelled with fluorescent dyes. The antibody fragments penetrated through the bacterial cell periphery and specifically bound to their antigens. Probe-conferred and non-specific fluorescence per cell were quantified by flow cytometry and compared to values obtained with end-labelled fluorescent probes. The DIG reporter molecules could also be detected in whole fixed cells by antibodies labelled with either alkaline phosphatase or horseradish peroxidase. The penetration of the large antibody-enzyme complexes into the cells required lysozyme/EDTA treatment prior to the hybridization and has so far only been achieved for Gram-negative bacteria. This technique has the potential for significant signal amplification as compared to the fluorescently end-labelled oligonucleotides hitherto used for single cell identification in microbial ecology. Moreover, it can be used instead of fluorescent assays in natural samples showing autofluorescence.