Multicolor imaging of Ca(2+) and protein kinase C signals using novel epifluorescence microscopy

Multisolitons-like patterns in a one-dimensional MARCKS protein cyclic model

In this paper, we study the nonlinear dynamics of the MARCKS protein between cytosol and cytoplasmic membrane through the modulational instability phenomenon. The reaction-diffusion generic model used here is firstly transformed into a cubic complex Ginzburg-Landau equation. Then, modulational instability (MI) is carried out in order to derive the MI criteria. We find the domains of some parameter space where nonlinear patterns are expected in the model. The analytical results on the MI growth rate predict that phosphorylation and binding rates affect MARCKS dynamics in opposite way: while the phosphorylation rate tends to support highly localized structures of MARCKS, the binding rate in turn tends to slow down such features. On the other hand, self-diffusion process always amplifies the MI phenomenon. These predictions are confirmed by numerical simulations. As a result, the cyclic transport of MARCKS protein from membrane to cytosol may be done by means of multisolitons-like patterns.

Hippocampal pyramidal neurons switch from a multipolar migration mode to a novel "climbing" migration mode during development

The hippocampus plays important roles in brain functions. Despite the importance of hippocampal functions, recent analyses of neuronal migration have mainly been performed on the cerebral neocortex, and the cellular mechanisms responsible for the formation of the hippocampus are not yet completely understood. Moreover, why a prolonged time is required for hippocampal neurons to complete their migration has been unexplainable for several decades. We analyzed the migratory profile of neurons in the developing mouse hippocampal CA1 region and found that the hippocampal pyramidal neurons generated near the ventricle became postmitotic multipolar cells and accumulated in the multipolar cell accumulation zone (MAZ) in the late stage of development. The hippocampal neurons passed through the pyramidal layer by a unique mode of migration. Their leading processes were highly branched and made contact with many radial fibers. Time-lapse imaging revealed that the migrating cells changed their scaffolds from the original radial fibers to other radial fibers, and as a result they proceed in a zigzag manner, with long intervals. The migrating cells in the hippocampus reminded us of "rock climbers" that instead of using their hands to pull up their bodies were using their leading processes to pull up their cell bodies. Because this mode of migration had never been described, we called it the "climbing" mode. The change from the "climbing" mode in the hippocampus to the "locomotion" mode in the neocortex may have contributed to the brain expansion during evolution.

Simultaneous live cell imaging using dual FRET sensors with a single excitation light

Fluorescence resonance energy transfer (FRET) between fluorescent proteins is a powerful tool for visualization of signal transduction in living cells, and recently, some strategies for imaging of dual FRET pairs in a single cell have been reported. However, these necessitate alteration of excitation light between two different wavelengths to avoid the spectral overlap, resulting in sequential detection with a lag time. Thus, to follow fast signal dynamics or signal changes in highly motile cells, a single-excitation dual-FRET method should be required. Here we reported this by using four-color imaging with a single excitation light and subsequent linear unmixing to distinguish fluorescent proteins. We constructed new FRET sensors with Sapphire/RFP to combine with CFP/YFP, and accomplished simultaneous imaging of cAMP and cGMP in single cells. We confirmed that signal amplitude of our dual FRET measurement is comparable to of conventional single FRET measurement. Finally, we demonstrated to monitor both intracellular Ca(2+) and cAMP in highly motile cardiac myocytes. To cancel out artifacts caused by the movement of the cell, this method expands the applicability of the combined use of dual FRET sensors for cell samples with high motility.

Dynamic adhesions and MARCKS in melanoma cells

Cell motility necessitates the rapid formation and disassembly of cell adhesions. We have studied adhesions in a highly motile melanoma cell line using various biochemical approaches and microscopic techniques to image close adhesions. We report that WM-1617 melanoma cells contain at least two types of close adhesion: classic focal adhesions and more extensive, irregularly shaped adhesions that tend to occur along lamellipodial edges. In contrast to focal adhesions, these latter adhesions are highly dynamic and can be disassembled rapidly via protein kinase C (PKC) activation (e.g. by eicosanoid) and MARCKS phosphorylation. MARCKS overexpression, however, greatly increases the area of close adhesions and renders them largely refractory to PKC stimulation. This indicates that nonphosphorylated MARCKS is an adhesion stabilizer. Unlike focal adhesions, the dynamic adhesions contain alpha3 integrin and MARCKS, but they do not contain the focal adhesion marker vinculin. Overall, these results begin to define the molecular and functional properties of dynamic close adhesions involved in cell motility.

Fluorescence resonance energy transfer imaging of PKC signalling in living cells using genetically encoded fluorescent probes

Perception of ligands in the extracellular space by transmembrane receptors initiates signal transduction. The conformation change of the receptor induces changes of intracellular signalling components, including altered cellular concentration, altered subcellular location, altered conformation and altered interacting partners. Biochemical approaches have yielded a lot of information about these processes. However, methods that are compatible with analysis of single living cells are often preferred, since cells are highly organized and their response is usually spatially heterogeneous. In addition, the study of signalling cascades requires high temporal resolution. Fluorescence imaging approaches meet these requirements. Moreover, imaging approaches can be combined with genetically encoded green fluorescent protein-based probes that have a high selectivity and sensitivity for the process/molecule of interest. Nowadays, many genetically encoded probes are available for visualizing signalling in living cells. This review is centred on a key regulator of cellular signalling, protein kinase C (PKC). We will discuss imaging approaches that are used for analysing the molecules involved in activation of PKC, visualizing the dynamics of the location of PKC, measuring the conformation of PKC and quantifying the activity of PKC. These approaches are of general interest since they can be applied to study the dynamics, conformation and activity of any protein in living cells.

Particle size analysis of dispersed oil and oil-mineral aggregates with an automated ultraviolet epi-fluorescence microscopy system

This paper describes recent advances in microscopic analysis for quantitative measurement of oil droplets. Integration of a microscope with bright-field and ultraviolet epi-fluorescence illumination (excitation wavelengths 340-380 nm; emission wavelengths 400-430 nm) fitted with a computer-controlled motorized stage, a high resolution digital camera, and new image-analysis software, enables automatic acquisition of multiple images and facilitates efficient counting and sizing of oil droplets. Laboratory experiments were conducted with this system to investigate the size distribution of chemically dispersed oil droplets and oil-mineral aggregates in baffled flasks that have been developed for testing chemical dispersant effectiveness. Image acquisition and data processing methods were developed to illustrate the size distribution of chemically dispersed oil droplets, as a function of energy dissipation rate in the baffled flasks, and the time-dependent change of the morphology and size distribution of oil-mineral aggregates. As a quantitative analytical tool, epifluorescence microscopy shows promise for application in research on oil spill response technologies, such as evaluating the effectiveness of chemical dispersant and characterizing the natural interaction between oil and mineral fines and other suspended particulate matters.

The Atg5 Atg12 conjugate associates with innate antiviral immune responses

Autophagy is an essential process for physiological homeostasis, but its role in viral infection is only beginning to be elucidated. We show here that the Atg5-Atg12 conjugate, a key regulator of the autophagic process, plays an important role in innate antiviral immune responses. Atg5-deficient mouse embryonic fibroblasts (MEFs) were resistant to vesicular stomatitis virus replication, which was largely due to hyperproduction of type I interferons in response to immunostimulatory RNA (isRNA), such as virus-derived, double-stranded, or 5'-phosphorylated RNA. Similar hyperresponse to isRNA was also observed in Atg7-deficient MEFs, in which Atg5-Atg12 conjugation is impaired. Overexpression of Atg5 or Atg12 resulted in Atg5-Atg12 conjugate formation and suppression of isRNA-mediated signaling. Molecular interaction studies indicated that the Atg5-Atg12 conjugate negatively regulates the type I IFN production pathway by direct association with the retinoic acid-inducible gene I (RIG-I) and IFN-beta promoter stimulator 1 (IPS-1) through the caspase recruitment domains (CARDs). Thus, in contrast to its role in promoting the bactericidal process, a component of the autophagic machinery appears to block innate antiviral immune responses, thereby contributing to RNA virus replication in host cells.

Highlighted generation of fluorescence signals using simultaneous two-color irradiation on Dronpa mutants

Dronpa absorbs blue light and emits bright green fluorescence. It can also be converted by strong irradiation at 490 nm to a nonfluorescent state, which can then be switched back to the original emissive state with irradiation at 400 nm. Through semirandom mutagenesis studies, we have developed two mutants of Dronpa that show efficient photoswitching kinetics. Compared to Dronpa, the mutants can be turned off by blue light more efficiently. Thus, excitation with an argon laser line (488 nm) makes the mutants quickly become dark such that no substantial fluorescence signals can be observed. Excitation with a violet laser diode (405 nm) also produces no fluorescence signals. Simultaneous 488- and 405-nm irradiation, however, results in a rapid oscillation between the two states, thereby keeping the emissive state population large enough to produce sufficiently bright fluorescence signals.

Persistent activation of protein kinase Calpha is not necessary for expression of cerebellar long-term depression

Protein kinase Calpha (PKCalpha) plays a major role in the induction of long-term depression (LTD) in a cerebellar Purkinje cell (PC). The sequential activation model for classical PKC states that PKCalpha translocates to the plasma membrane by binding Ca(++) and then becomes fully activated by binding diacylglycerol (DAG), which enables estimation of the activity by monitoring its localization. Here, we performed simultaneous electrophysiological recording and fluorescence imaging in a cultured PC expressing GFP-tagged PKCalpha. When a PC was depolarized, PKCalpha transiently translocated to the plasma membrane in a Ca(++)-dependent manner. Application of membrane permeable DAG or the blocker of DAG lipase prolonged the translocation. These results suggest that the sequential activation model is applicable to PCs. Conjunctive applications of glutamate and depolarization pulse induced LTD, but did not prolong the translocation. Thus, our results imply that persistent activation of PKCalpha is not necessary for the expression of LTD.

Electrical stimulation modulates fate determination of differentiating embryonic stem cells

A clear understanding of cell fate regulation during differentiation is key in successfully using stem cells for therapeutic applications. Here, we report that mild electrical stimulation strongly influences embryonic stem cells to assume a neuronal fate. Although the resulting neuronal cells showed no sign of specific terminal differentiation in culture, they showed potential to differentiate into various types of neurons in vivo, and, in adult mice, contributed to the injured spinal cord as neuronal cells. Induction of calcium ion influx is significant in this differentiation system. This phenomenon opens up possibilities for understanding novel mechanisms underlying cellular differentiation and early development, and, perhaps more importantly, suggests possibilities for treatments in medical contexts.

Myristoylated, alanine-rich C-kinase substrate phosphorylation regulates growth cone adhesion and pathfinding

Repellents evoke growth cone turning by eliciting asymmetric, localized loss of actin cytoskeleton together with changes in substratum attachment. We have demonstrated that semaphorin-3A (Sema3A)-induced growth cone detachment and collapse require eicosanoid-mediated activation of protein kinase C epsilon (PKC epsilon) and that the major PKC epsilon target is the myristoylated, alanine-rich C-kinase substrate (MARCKS). Here, we show that PKC activation is necessary for growth cone turning and that MARCKS, while at the membrane, colocalizes with alpha3-integrin in a peripheral adhesive zone of the growth cone. Phosphorylation of MARCKS causes its translocation from the membrane to the cytosol. Silencing MARCKS expression dramatically reduces growth cone spread, whereas overexpression of wild-type MARCKS inhibits growth cone collapse triggered by PKC activation. Expression of phosphorylation-deficient, mutant MARCKS greatly expands growth cone adhesion, and this is characterized by extensive colocalization of MARCKS and alpha3-integrin, resistance to eicosanoid-triggered detachment and collapse, and reversal of Sema3A-induced repulsion into attraction. We conclude that MARCKS is involved in regulating growth cone adhesion as follows: its nonphosphorylated form stabilizes integrin-mediated adhesions, and its phosphorylation-triggered release from adhesions causes localized growth cone detachment critical for turning and collapse.

A fluorescent variant of a protein from the stony coral Montipora facilitates dual-color single-laser fluorescence cross-correlation spectroscopy

Dual-color fluorescence cross-correlation spectroscopy (FCCS) is a promising technique for quantifying protein-protein interactions. In this technique, two different fluorescent labels are excited and detected simultaneously within a common measurement volume. Difficulties in aligning two laser lines and emission crossover between the two fluorophores, however, make this technique complex. To overcome these limitations, we developed a fluorescent protein with a large Stokes shift. This protein, named Keima, absorbs and emits light maximally at 440 nm and 620 nm, respectively. Combining a monomeric version of Keima with cyan fluorescent protein allowed dual-color FCCS with a single 458-nm laser line and complete separation of the fluorescent protein emissions. This FCCS approach enabled sensitive detection of proteolysis by caspase-3 and the association of calmodulin with calmodulin-dependent enzymes. In addition, Keima and a spectral variant that emits maximally at 570 nm might facilitate simultaneous multicolor imaging with single-wavelength excitation.

Single molecular multianalyte (Ca2+, Mg2+) fluorescent probe and applications to bioimaging

Intracellular signal transduction relies on spatial and temporal signal transmitter dynamics. To clarify the correlations of these transmitter molecules, multicolor-imaging has been widely used. However, in the case of applying multiple indicators in a cell, spectral overlap of the indicators prevents accurate quantitative analysis. Moreover, the invasive (toxic) effect, the localization, the metabolism, as well as photobleaching of these indicators complicate the situation. Here, we show that single-molecular multifluorescent probes can overcome these problems. While intracellular calcium plays a critical role as a signal transmitter and magnesium acts as a cofactor in many situations, the correlations between the two cations are now the main issue. We designed and synthesized a Ca2+-Mg2+ responsive multifluorescent probe, KCM-1. KCM-1 shows a spectral blue shift upon complexation to Ca2+ and a red shift to the presence of Mg2+. With data analyzed at different excitation wavelengths, the concentrations of Ca2+ and Mg2+ are simultaneously quantified. Furthermore, by using the AM-ester method, intracellular Ca2+ and Mg2+ concentrations are simultaneously imaged. Such a type of intracellular multiple analyte imaging by a single-molecular multifluorescent probe is successfully demonstrated for the first time.

Microscopy and image analysis

This unit provides an overview of light microscopy, including objectives, light sources, filters, film, and color photography for fluorescence microscopy and fluorescence in situ hybridization (FISH). Computerized image-analysis systems currently used in clinical cytogenetics are also discussed.

Multiparameter Imaging for the Analysis of Intracellular Signaling

In biological experimentation and especially in drug discovery there is a trend towards more complex test systems. Cell-based assays are replacing conventional binding or enzyme assays more and more. This development is strongly driven by novel fluorescent probes that give insight into cellular processes. Target proteins are studied in their natural environment; this gives much more realistic test results, especially with respect to enzyme location and kinetics. However, in the complex environment of cells, many parameters contribute to the performance of the protein of interest. Therefore, it would be desirable to monitor simultaneously as many of the relevant cellular processes as possible. Here, we discuss the possibilities and limitations provided by multiparameter monitoring of cellular events with fluorescent probes. Some novel examples of the use of fluorescent probes and multiparameter imaging are shown.

Deoxycholic acid activates protein kinase C and phospholipase C via increased Ca2+ entry at plasma membrane

BACKGROUND & AIMS

Secondary bile acids like deoxycholic acid (DCA) are well-established tumor promoters that may exert their pathologic actions by interfering with intracellular signaling cascades.

METHODS

We evaluated the effects of DCA on Ca2+ signaling in BHK-21 fibroblasts using fura-2 and mag-fura-2 to measure cytoplasmic and intraluminal internal stores [Ca2+], respectively. Furthermore, green fluorescent protein (GFP)-based probes were used to monitor time courses of phospholipase C (PLC) activation (pleckstrin-homology [PH]-PLCdelta-GFP), and translocation of protein kinase C (PKC) and a major PKC substrate, myristolated alanine-rich C-kinase substrate (MARCKS).

RESULTS

DCA (50-250 micromol/L) caused profound Ca2+ release from intracellular stores of intact or permeabilized cells. Correspondingly, DCA increased cytoplasmic Ca2+ to levels that were approximately 120% of those stimulated by Ca2+-mobilizing agonists in the presence of external Ca2+, and approximately 60% of control in Ca2+-free solutions. DCA also caused dramatic translocation of PH-PLCdelta-GFP, and conventional, Ca2+/diacylglycerol (DAG)-dependent isoforms of PKC (PKC-betaI and PKC-alpha), and MARCKS-GFP, but only in Ca2+-containing solutions. DCA had no effect on localization of a novel (PKCdelta) or an atypical (PKCzeta) PKC isoform.

CONCLUSIONS

Data are consistent with a model in which DCA directly induces both Ca2+ release from internal stores and persistent Ca2+ entry at the plasma membrane. The resulting microdomains of high Ca2+ levels beneath the plasma membrane appear to directly activate PLC, resulting in modest InsP 3 and DAG production. Furthermore, the increased Ca2+ entry stimulates vigorous recruitment of conventional PKC isoforms to the plasma membrane.

Similar diffusibility of membrane proteins across the axon-soma and dendrite-soma boundaries revealed by a novel FRAP technique

In polarized mature neurons, the asymmetrical distribution of proteins between axonal and somatodendritic plasma membrane (PM) domains may be maintained by a diffusion barrier at the axon-soma boundary. At the boundary, a complex containing membrane-associated and cytoskeletal proteins is formed, anchoring axonal membrane proteins and indirectly hindering the diffusion of other membrane proteins. We examined the latter case, i.e., secondary diffusion impedance by comparing the mobility of fluorescently labeled membrane proteins within the axon-soma and dendrite-soma boundaries. We performed fluorescence recovery after photobleaching (FRAP) experiments using mature cultured hippocampal neurons that had been labeled specifically at their PMs with fluorescent proteins (FPs). The maturation of these neurons was confirmed by immunolocalization with Ankyrin-G, which is thought to participate in the creation of the diffusion barrier at the axon-soma boundary. We developed a wide-field microscope equipped with a device (digital micromirror device) composed of 1024 x 768 binary mirrors at the field-stop, allowing free control of the illumination area and intensity. After the FPs in peripheral processes were photobleached, nonbleached FPs diffused into all the processes at equivalent speeds. These results indicate that the secondary diffusion barrier to exogenously overexpressed membrane proteins is not specific to the axon-soma boundary.

Blind spectral decomposition of single-cell fluorescence by parallel factor analysis

Simultaneous measurement of multiple signaling molecules is essential to investigate their relations and interactions in living cells. Although a wide variety of fluorescent probes are currently available, the number of probes that can be applied simultaneously is often limited by the overlaps among their fluorescence spectra. We developed the experimental system to measure and analyze many overlapping fluorescent components in single cells. It is based on the recording of two-dimensional single-cell fluorescence spectra and on the blind spectral decomposition of fluorescence data by method of parallel factor analysis. Because this method does not require any preknowledge about the shapes of individual component spectra, it can be applied to the specimens that contain fluorescent components with unknown spectra. By examining the performance using the mixture solutions of fluorescent indicators, it was confirmed that >10 largely overlapping spectral components could be easily separated. The effectiveness in the physiological experiments was proven in the applications to the temporal analysis of intracellular Ca(2+) concentration and pH, as well as the intrinsic fluorescent components, in single mouse oocytes.

PKC Signaling Mediates Global Enhancement of Excitatory Synaptogenesis in Neurons Triggered by Local Contact with Astrocytes

Here we provide evidence that astrocytes affect neuronal synaptogenesis by the process of adhesion. Local contact with astrocytes via integrin receptors elicited protein kinase C (PKC) activation in individual dissociated neurons cultured in astrocyte-conditioned medium. This activation, initially focal, soon spread throughout the entire neuron. We then demonstrated pharmacologically that the arachidonic acid cascade, triggered by the integrin reception, is responsible for the global activation of PKC. Local astrocytic contact also facilitated excitatory synaptogenesis throughout the neuron, a process which could be blocked by inhibitors of both integrins and PKC. Thus, propagation of PKC signaling represents an underlying mechanism for global neuronal maturation following local astrocyte adhesion.

Whole-field fluorescence microscope with digital micromirror device: imaging of biological samples

We have developed a whole-field fluorescence microscope equipped with a Digital Micromirror Device to acquire optically sectioned images by using the fringe-projection technique and the phase-shift method. This system allows free control of optical sectioning strength through computer-controlled alteration of the fringe period projected onto a sample. We have employed this system to image viable cells expressing fluorescent proteins and discussed its biological applications.

Distinct developmental modes and lesion-induced reactions of dendrites of two classes of Drosophila sensory neurons

Little has been understood about the underlying mechanisms that generate the morphological diversity of dendritic trees. Dendritic arborization neurons in Drosophila provide an excellent model system to tackle this question, and they are classified into classes I-IV in order of increasing arbor complexity. Here we have developed transgenic green fluorescent protein markers for class I or class IV cells, which allowed time-lapse recordings of dendritic birth in the embryo, its maturation processes in the larva, and lesion-induced reactions. The two classes used distinct strategies of dendritic emergence from the cell body and branching, which contributed to differences in their basic arbor patterns. In contrast to the class I cells examined, one cell of class IV, which was a focus in this study, continued to elaborate branches throughout larval stages, and it was much more capable of responding to the severing of branches. We also investigated the cellular basis of field formation between adjacent class IV cells. Our results support the fact that class-specific inhibitory interaction is necessary and sufficient for tiling and confirmed that this intercellular communication was at work at individual dendrodendritic interfaces. Finally, this inhibitory signaling appeared to play a central role when arbors of adjacent cells started meeting midway between the cells and until the body wall became partitioned into abutting, minimal-overlapping territories.

Distinct developmental modes and lesion-induced reactions of dendrites of two classes of Drosophila sensory neurons

Little has been understood about the underlying mechanisms that generate the morphological diversity of dendritic trees. Dendritic arborization neurons in Drosophila provide an excellent model system to tackle this question, and they are classified into classes I-IV in order of increasing arbor complexity. Here we have developed transgenic green fluorescent protein markers for class I or class IV cells, which allowed time-lapse recordings of dendritic birth in the embryo, its maturation processes in the larva, and lesion-induced reactions. The two classes used distinct strategies of dendritic emergence from the cell body and branching, which contributed to differences in their basic arbor patterns. In contrast to the class I cells examined, one cell of class IV, which was a focus in this study, continued to elaborate branches throughout larval stages, and it was much more capable of responding to the severing of branches. We also investigated the cellular basis of field formation between adjacent class IV cells. Our results support the fact that class-specific inhibitory interaction is necessary and sufficient for tiling and confirmed that this intercellular communication was at work at individual dendrodendritic interfaces. Finally, this inhibitory signaling appeared to play a central role when arbors of adjacent cells started meeting midway between the cells and until the body wall became partitioned into abutting, minimal-overlapping territories.

Visualization of the spatial and temporal dynamics of intracellular signaling

In recent years, our ability to unravel the finer details of intracellular signaling has improved remarkably. Technological innovations resulting from the introduction of green fluorescent protein (GFP) have played a significant role in these advances. Fluorescent indicators allow us to visualize events within a cell in real-time and space. This review focuses on indicators that use GFP-based fluorescence resonance energy transfer (FRET) technologies and discusses how these methodologies have given insights into biological questions relating to the spatiotemporal patterns of signaling by cAMP, calcium, receptor tyrosine kinases, and other molecules.

Single-cell kinase assays: opening a window onto cell behavior

Recent advances in analytical techniques have made the performance of biochemical assays on individual mammalian cells possible. Of particular interest is the ability to measure the activation of kinases, enzymes with critical roles in virtually every aspect of cell physiology. Single-cell kinase assays promise to deliver a newfound understanding of the molecular mechanisms responsible for cellular control and behavior by revealing the dynamic nature of signal transduction networks in living cells. A recent exciting development is the potential to perform assays of multiple kinases simultaneously in a single cell.

Spectroscopic study and evaluation of red-absorbing fluorescent dyes

The spectroscopic characteristics (absorption, emission, and fluorescence lifetime) of 13 commercially available red-absorbing fluorescent dyes were studied under a variety of conditions. The dyes included in this study are Alexa647, ATTO655, ATTO680, Bodipy630/650, Cy5, Cy5.5, DiD, DY-630, DY-635, DY-640, DY-650, DY-655, and EVOblue30. The thorough characterization of this class of dyes will facilitate selection of the appropriate red-absorbing fluorescent labels for applications in fluorescence assays. The influences of polarity, viscosity, and the addition of detergent (Tween20) on the spectroscopic properties were investigated, and fluorescence correlation spectroscopy (FCS) was utilized to assess the photophysical properties of the dyes under high excitation conditions. The dyes can be classified into groups based on the results presented. For example, while the fluorescence quantum yield of ATTO655, ATTO680, and EVOblue30 is primarily controlled by the polarity of the surrounding medium, more hydrophobic and structurally flexible dyes of the DY-family are strongly influenced by the viscosity of the medium and the addition of detergents. Covalent binding of the dyes to biotin and subsequent addition of streptavidin results in reversible fluorescence quenching or changes in the relaxation time of other photophysical processes of some dyes, most likely due to interactions with tryptophan residues in the streptavin binding site.

An optical marker based on the UV-induced green-to-red photoconversion of a fluorescent protein

We have cloned a gene encoding a fluorescent protein from a stony coral, Trachyphyllia geoffroyi, which emits green, yellow, and red light. The protein, named Kaede, includes a tripeptide, His-Tyr-Gly, that acts as a green chromophore that can be converted to red. The red fluorescence is comparable in intensity to the green and is stable under usual aerobic conditions. We found that the green-red conversion is highly sensitive to irradiation with UV or violet light (350-400 nm), which excites the protonated form of the chromophore. The excitation lights used to elicit red and green fluorescence do not induce photoconversion. Under a conventional epifluorescence microscope, Kaede protein expressed in HeLa cells turned red in a graded fashion in response to UV illumination; maximal illumination resulted in a 2,000-fold increase in the ratio of red-to-green signal. These color-changing properties provide a simple and powerful technique for regional optical marking. A focused UV pulse creates an instantaneous plane source of red Kaede within the cytosol. The red spot spreads rapidly throughout the cytosol, indicating its free diffusibility in the compartment. The extensive diffusion allows us to delineate a single neuron in a dense culture, where processes originating from many different somata are present. Illumination of a focused UV pulse onto the soma of a Kaede-expressing neuron resulted in filling of all processes with red fluorescence, allowing visualization of contact sites between the red and green neurons of interest.

Lateral propagation of EGF signaling after local stimulation is dependent on receptor density

We analyzed lateral propagation of epidermal growth factor (EGF) signaling in single live COS cells following local stimulation, achieved by the use of laminar flows containing rhodamine-labeled EGF. The spatiotemporal pattern of EGF signaling was visualized by fluorescent indicators for Ras activation and tyrosine phosphorylation. Contrary to the findings in previous reports, both signals were localized to the stimulated regions in control COS cells expressing EGF receptor at the basal level. However, the signals spread over the entire cell when EGF receptors were overexpressed or when receptor/ligand endocytosis was blocked. We thus present evidence that ligand-independent propagation of EGF signaling occurs only when the receptor density on the plasma membrane is high, such as in carcinoma cells.