Fluorescent protein biosensors: measurement of molecular dynamics in living cells

Fluorescent tags of protein function in living cells

A cell's biochemistry is now known to be the biochemistry of molecular machines, that is, protein complexes that are assembled and dismantled in particular locations within the cell as needed. One important element in our understanding has been the ability to begin to see where proteins are in cells and what they are doing as they go about their business. Accordingly, there is now a strong impetus to discover new ways of looking at the workings of proteins in living cells. Although the use of fluorescent tags to track individual proteins in cells has a long history, the availability of laser-based confocal microscopes and the imaginative exploitation of the green fluorescent protein from jellyfish have provided new tools of great diversity and utility. It is now possible to watch a protein bind its substrate or its partners in real time and with submicron resolution within a single cell. The importance of processes of self-organisation represented by protein folding on the one hand and subcellular organelles on the other are well recognised. Self-organisation at the intermediate level of multimeric protein complexes is now open to inspection. BioEssays 22:180-187, 2000.

Fluorescence tracing of intracellular proteins

Developments in fluorescence microscopy and the availability of fluorescently labeled antibodies and probes for localization of molecules and organelles have made the microscope an indispensable tool with which one can map specific molecules to subcellular loci allowing deep insight into cell and organelle biology. Furthermore, confocal microscopy permits analysis of the three dimensional architecture of cells that could not be accomplished by conventional light microscopy. The goal of fluorescence protein tracing by microscopy is to visualize cellular constituents and general cytoarchitecture as close to native organization as possible. To achieve this, and to preserve cellular structure in the best possible manner, the specimen is usually fixed chemically. Here I review several standard fixation, permeabilization and labeling schemes followed by examples of several standard imaging techniques.

Binding of 9-anthroylcholine monitors the interactions of adenosine cyclic 3',5'-phosphate-dependent protein kinase with MgATP, substrates, and regulatory subunits

The isolated catalytic subunit of cAMP-dependent protein kinase and smooth muscle myosin light chain kinase undergo interactions with the fluorescent dye 9-anthroylcholine (9AC) that are responsive to the two enzymes' associations with substrates and effectors. Additionally, the binding of 9AC is highly sensitive to subtle structural or functional differences among closely related protein kinases. Skeletal muscle myosin light chain kinase and the catalytically active chymotryptic fragment of the gamma-subunit of phosphorylase kinase do not associate with 9AC. The 1:1 fluorescent complex of the isolated catalytic subunit of cAMP-dependent protein kinase with 9AC exhibits a dissociation constant of 21 microM. The association of the catalytic subunit with either of the regulatory subunits, RI and RII, results in decreases in the observed 9AC fluorescence that are reversed upon the addition of cAMP. The effects of MgATP and of polypeptide substrates (Kemptide, troponin I, protamine) on the 9AC-catalytic subunit complex are consistent with a general noncompetitive model in which the interactions of 9AC and the other ligands with the enzyme are mutually antagonistic but not purely competitive.

Loading pyranine via purinergic receptors or hypotonic stress for measurement of cytosolic pH by imaging

Although used extensively for the measurement of intracellular pH, derivatives of fluorescein such as 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) have suboptimal sensitivity and can generate toxic photoproducts. These limitations can be overcome using the pH-sensitive fluorescent dye 8-hydroxypyrene-1,3,6-trisulfonic acid (pyranine), which has improved spectroscopic properties. However, the use of pyranine has been limited by the difficulties encountered in delivering this highly hydrophilic dye to the cell interior. We describe a strategy for intracellular delivery of pyranine based on the reversible activation of purinergic P2x7 receptors, which allow permeation of the dye into otherwise intact cells. When loaded into J774 or RAW cells by this method, pyranine is not only more sensitive than BCECF (the dynamic range is approximately 7-fold greater), but is retained better and is less toxic. Pyranine was distributed throughout the cytosol but was not detectable in endomembrane compartments. Repeated illumination resulted in blebbing and loss of functional responsiveness of cells loaded with BCECF, whereas comparably irradiated cells loaded with pyranine remained healthy and responsive. Pyranine can also be loaded into cells not expressing P2x7 receptors by brief exposure to a hypotonic solution. The properties of cells labeled by this method are similar to those loaded via purinergic receptors and compare favorably with those of BCECF-loaded cells. Pyranine thus provides a useful alternative to fluorescein derivatives for the measurement of intracellular pH, particularly when using the high excitation intensities required for microscopic digital imaging.

Fluorescent probes for living cells

The functional characteristics of fluorescent probes used for imaging and measuring dynamic processes in living cells are reviewed. Initial consideration is given to general design requirements for delivery, targeting, detectability and fluorescence readout, and current technologies for attaining them. Discussion then proceeds to the more application-specific properties of intracellular ion indicators, membrane potential sensors, probes for proteins and lipids, and cell viability markers.

Fluorescent-protein biosensors: new tools for drug discovery

Recent improvements in target discovery and high-throughput screening have increased the pressure at key points along the drug-discovery pipeline. High-content screening was developed to ease the bottlenecks formed at the target-validation and lead-optimization points, and a new generation of reagents that report on specific molecular processes in living cells (fluorescent-protein biosensors) have been important in its development. Creative designs of fluorescent-protein biosensors have emerged and been used to measure the molecular dynamics of macromolecules, metabolites and ions. Recent applications of fluorescent-protein biosensors to biological problems have provided a foundation for their use in biotechnology.

Localized sampling of cytoplasm from Xenopus oocytes for capillary electrophoresis

Continued progress in cellular physiology requires new measurement strategies which can be applied to solitary cells. Since many cellular signaling pathways act on time scales of a few seconds, there is a critical need for single-cell techniques with subsecond time resolution. Capillary electrophoresis shows great promise as a tool for the analysis of individual cells. In the present work, we describe a technique to load a capillary with picoliter to nanoliter volumes of cytoplasm and initiate electrophoresis in less than 500 ms. When cytoplasm was sampled from a Xenopus laevis oocyte previously loaded with fluorescein, calcium green, or a mixture of the two fluorophores, their fluorescent peaks were readily identifiable on the electropherogram. Since the volume of cytoplasm (< or = 30 nL) loaded into the capillary was much smaller than the 1 microL oocyte volume, spatially localized biochemical measurements were also possible. To demonstrate the utility of this new technique, the activity of the enzyme beta-galactosidase was measured in small regions of the Xenopus oocyte. Subcellular, subsecond sampling of oocyte cytoplasm will enable biochemical measurements with the resolution required to understand many cellular signal transduction pathways.

The phytofluors: a new class of fluorescent protein probes

BACKGROUND

Biologically compatible fluorescent protein probes, particularly the self-assembling green fluorescent protein (GFP) from the jellyfish Aequorea victoria, have revolutionized research in cell, molecular and developmental biology because they allow visualization of biochemical events in living cells. Additional fluorescent proteins that could be reconstituted in vivo while extending the useful wavelength range towards the orange and red regions of the light spectrum would increase the range of applications currently available with fluorescent protein probes.

RESULTS

Intensely orange fluorescent adducts, which we designate phytofluors, are spontaneously formed upon incubation of recombinant plant phytochrome apoproteins with phycoerythrobilin, the linear tetrapyrrole precursor of the phycoerythrin chromophore. Phytofluors have large molar absorption coefficients, fluorescence quantum yields greater than 0.7, excellent photostability, stability over a wide range of pH, and can be reconstituted in living plant cells.

CONCLUSIONS

The phytofluors constitute a new class of fluorophore that can potentially be produced upon bilin uptake by any living cell expressing an apophytochrome cDNA. Mutagenesis of the phytochrome apoprotein and/or alteration of the linear tetrapyrrole precursor by chemical synthesis are expected to afford new phytofluors with fluorescence excitation and emission spectra spanning the visible to near-infrared light spectrum.

FLUORESCENCE MICROSCOPY OF LIVING PLANT CELLS

Since its inception, light microscopy has shown the elegance and subtlety with which function is expressed in the form of the cells, tissues, and organs of the plant. Recently, light microscopy has seen a resurgence in use fueled by advances in microscope design and computer-based image analysis. The structural resolution afforded by static, fixed samples is being increasingly supplemented by approaches using fluorescent analogs and selective fluorescent indicators, which visualize the dynamic processes in living, functioning cells. This review describes some of these approaches and discusses how they are taking us a step closer to viewing the intricate complexity with which plants organize and regulate their functions down to the subcellular level.

Myosin II transport, organization, and phosphorylation: evidence for cortical flow/solation-contraction coupling during cytokinesis and cell locomotion

The mechanism of cytokinesis has been difficult to define because of the short duration and the temporal-spatial dynamics involved in the formation, activation, force production, and disappearance of the cleavage furrow. We have investigated the structural and chemical dynamics of myosin II in living Swiss 3T3 cells from prometaphase through the separation and migration of daughter cells. The structural and chemical dynamics of myosin II have been defined using the semiautomated, multimode light microscope, together with a fluorescent analogue of myosin II and a fluorescent biosensor of myosin II regulatory light chain (RLC) phosphorylation at serine 19. The correlation of image data from live cells using different modes of light microscopy allowed interpretations not possible from single-mode investigations. Myosin II transported toward the equatorial plane from adjacent regions, forming three-dimensional fibers that spanned the volume of the equator during anaphase and telophase. A global phosphorylation of myosin II at serine 19 of the RLC was initiated at anaphase when cortical myosin II transport started. The phosphorylation of myosin II remained high near the equatorial plane through telophase and into cytokinesis, whereas the phosphorylation of myosin II at serine 19 of the RLC decreased at the poles. The timing and pattern of phosphorylation was the same as the shortening of myosin II-based fibers in the cleavage furrow. Myosin II-based fibers shortened and transported out of the cleavage furrow into the tails of the two daughter cells late in cytokinesis. The patterns of myosin II transport, phosphorylation, and shortening of fibers in the migrating daughter cells were similar to that previously defined for cells migrating in a wound in vitro. The temporal-spatial patterns and dynamics of myosin II transport, phosphorylation at serine 19 of the RLC, and the shortening and disappearance of myosin II-based fibers support the proposal that a combination of the cortical flow hypothesis and the solation-contraction coupling hypothesis explain key aspects of cytokinesis and polarized cell locomotion.

Multiparameter fluorescence imaging microscopy: reagents and instruments

Fluorescence imaging microscopes and fluorescent reagents have both evolved greatly in the past 10 years. Sensitive imaging cameras developed over the last decade allow detection of fluorescence signals from even a single fluorescent tag on a protein. At the same time, numerous antibody markers for growth control proteins and DNA probes for chromosomal alterations have been discovered and these can be labeled with the new multicolor fluorescent dyes for detection by microscopy. Image analysis software automatically localizes and quantifies of as many as 5-6 of these different color fluorescent markers in the individual cells of a preparation. The results is that powerful methods for multiparameter analysis of molecular structures and dynamic processes are now accessible to the pathologist.

Dissecting the individuality of cancer cells: the morphological and molecular dynamics of single human glioma cells

A glioma produces some of the most heterogeneously growing, angiogenic, and invasive primary brain tumor cells known. To dissect cellular individuality, and therefore tumor heterogeneity, multiple morphological and molecular processes in single living human glioma cells were measured using multimode light microscopy. Feature extraction of time-lapse image series of spreading, locomoting, and interacting cells either in the presence or absence of physiological modulators was performed by defining five parameters that described cell shape, movement, and cell-cell contacts. Concurrent visualization of all five parameters with a scatterplot matrix revealed temporal as well as time-independent relationships between the parameters that were sufficient to define the individuality of normal and transformed glial cells. Because the actin-cytoskeleton plays a role in regulating the cellular processes described above, the dynamics of a fluorescent analog of non-muscle actin within motile glioma cells were measured in addition to the morphological parameters. The actin-cytoskeleton within the thin sweeping lamellipodia of a glioma exhibited a paucity of large stress fibers, a rich collection of microvillar structures containing actin, and dynamics that were distinct from those of normal motile cells. This approach can therefore potentially be used to dissect the molecular origins of transformation using a small number of representative tumor cells.

A fluorescent protein biosensor of myosin II regulatory light chain phosphorylation reports a gradient of phosphorylated myosin II in migrating cells

Phosphorylation of the regulatory light chain by myosin light chain kinase (MLCK) regulates the motor activity of smooth muscle and nonmuscle myosin II. We have designed reagents to detect this phosphorylation event in living cells. A new fluorescent protein biosensor of myosin II regulatory light chain phosphorylation (FRLC-Rmyosin II) is described here. The biosensor depends upon energy transfer from fluorescein-labeled regulatory light chains to rhodamine-labeled essential and/or heavy chains. The energy transfer ratio increases by up to 26% when the regulatory light chain is phosphorylated by MLCK. The majority of the change in energy transfer is from regulatory light chain phosphorylation by MLCK (versus phosphorylation by protein kinase C). Folding/unfolding, filament assembly, and actin binding do not have a large effect on the energy transfer ratio. FRLC-Rmyosin II has been microinjected into living cells, where it incorporates into stress fibers and transverse fibers. Treatment of fibroblasts containing FRLC-Rmyosin II with the kinase inhibitor staurosporine produced a lower ratio of rhodamine/fluorescein emission, which corresponds to a lower level of myosin II regulatory light chain phosphorylation. Locomoting fibroblasts containing FRLC-Rmyosin II showed a gradient of myosin II phosphorylation that was lowest near the leading edge and highest in the tail region of these cells, which correlates with previously observed gradients of free calcium and calmodulin activation. Maximal myosin II motor force in the tail may contribute to help cells maintain their polarized shape, retract the tail as the cell moves forward, and deliver disassembled subunits to the leading edge for incorporation into new fibers.

Measurement and manipulation of cytoskeletal dynamics in living cells

A new ear in cell biology is at hand with the development of tools for imaging molecular functions in living cells and tissues. Specific chemical and molecular events can now be measured and manipulated in cells in order to explore the mechanisms of cell functions. In particular, cytoskeletal processes are being dissected temporally and spatially in single cells from lower eukaryotes, plants, and animals using light-based reagents and electronic light microscopy.