Lighting up the cell surface with evanescent wave microscopy

VISUALIZING Ca2+ SPARKS AND SUBSTRUCTURE OF Ca2+ WAVES BY TOTAL INTERNAL REFLECTION FLUORESCENCE MICROSCOPY (TIRFM)

Based on near-field optical theory, total internal reflection fluorescence microscope shows a novel character that its picture has great signal-to-noise ratio and high temporal resolution achieved by high quality CCD camera. This allows us to analyze the spatiotemporal details of local Ca 2+ dynamics within the nanoscale microdomain surrounding different Ca 2+ channels. We have recently constructed a versatile objective TIRFM equipped with a high numerical aperture (NA = 1.45) objective. Using fluo-4 as the Ca 2+ indicator, we visualized the near-membrane profiles of Ca 2+ waves and elementary Ca 2+ sparks generated by Ca 2+ release channels in rat ventricular myocytes. Different from those detected using conventional or confocal microscopy, Ca 2+ waves observed with TIRFM exhibited fine inhomogenous substructures. The propagation of Ca 2+ waves with anfractuous routes of spark recruitment is much more complicated than previously imagined. We believe that TIRFM will provide a unique tool for dissecting the microscopic mechanisms of intracellular Ca 2+ signaling.

Fibronectin matrix assembly requires distinct contributions from Rho kinases I and -II

Extracellular matrix is integral to tissue architecture and regulates many aspects of cell behavior. Fibronectin matrix assembly involves the actin cytoskeleton and the small GTPase RhoA, but downstream signaling is not understood. Here, down-regulation of either rho kinase isoform (ROCK I or -II) by small interfering RNA treatment blocked fibronectin matrix assembly, although the phenotypes were distinct and despite persistence of the alternate kinase. Remnant fibronectin on ROCK-deficient fibroblasts was mostly punctate and more deoxycholate soluble compared with controls. Fibronectin matrix assembly defects in ROCK-deficient cells did not result from decreased synthesis/secretion, altered fibronectin mRNA splicing, metalloproteinase activity, or alpha5beta1 integrin dysfunction. Rescue could be effected by ROCK protein restoration or phosphomimetic myosin light chain expression. However, the effect of ROCK I deficiency on fibronectin matrix assembly was secondary to altered cell surface morphology, rich in filopodia, resulting from high GTP-Cdc42 levels. Total internal reflection microscopy revealed that a submembranous pool of myosin light chain in control cells was missing in ROCK II-deficient cells and replaced by stress fibers. Together, two rho kinases contribute to fibronectin matrix assembly in a different manner and cortical myosin II-driven contractility, but not stress fibers, may be critical in this activity.

Fluorescence molecular imaging

There is a wealth of new fluorescent reporter technologies for tagging of many cellular and subcellular processes in vivo. This imposed contrast is now captured with an increasing number of available imaging methods that offer new ways to visualize and quantify fluorescent markers distributed in tissues. This is an evolving field of imaging sciences that has already achieved major advances but is also facing important challenges. It is nevertheless well poised to significantly impact the ways of biological research, drug discovery, and clinical practice in the years to come. Herein, the most pertinent technologies associated with in vivo noninvasive or minimally invasive fluorescence imaging of tissues are summarized. Focus is given to small-animal imaging. However, while a broad spectrum of fluorescence reporter technologies and imaging methods are outlined, as necessary for biomedical research, and clinical translation as well.

Enhanced live cell membrane imaging using surface plasmon-enhanced total internal reflection fluorescence microscopy

Using a total internal reflection fluorescence microscopy (TIRFM) technique to image live cells on a biosurface not only provides an enhanced understanding of cellular functions, but also improves the signal-to-noise ratio of the images. However, the intensity of the fluorescence signal must be increased if a more dynamic biomolecular imaging capability is required. Accordingly, this study presents a surface plasmon-enhanced TIRFM technique in which the fluorescence signals are enhanced via surface plasmons offered by a silver nanolayer. The developed microscopy technique is successfully applied to the real-time observation of the thrombomodulin proteins of live cell membranes. The experimental results and the simulation results demonstrate that the live cell membrane images obtained in the proposed surface plasmon-enhanced TIRFM technique are brighter by approximately one order of magnitude than those provided by conventional TIRFM.

Overlapping events with application to image sequences

Counting spatially and temporally overlapping events in image sequences and estimating their shape-size and duration features are important issues in some applications. We propose a stochastic model, a particular case of the nonisotropic 3D Boolean model, for performing this analysis: the temporal Boolean model. Some probabilistic properties are derived and a methodology for parameter estimation from time-lapse image sequences is proposed using an explicit treatment of the temporal dimension. We estimate the mean number of germs per unit area and time, the mean grain size and the duration distribution. A wide simulation study in order to assess the proposed estimators showed promising results. The model was applied on biological image sequences of in-vivo cells in order to estimate new parameters such as the mean number and duration distribution of endocytic events. Our results show that the proposed temporal Boolean model is effective for obtaining information about dynamic processes which exhibit short-lived, but spatially and temporally overlapping events.

Real-time observation of temperature-dependent protein–protein interactions using real-time dual-color detection system

This study examined the ability of a real-time dual-color detection system to allow direct observations of the kinetics of temperature-dependent protein-protein interaction at a single-molecule level. The primary target protein was an Alexa Fluor 488-labeled actin conjugate, which had been pre-incubated with an unlabeled rabbit anti-actin antibody (IgG). The complementary fluorescent protein was Alexa Fluor 633-labeled goat anti-rabbit IgG antibody, which interacts with the rabbit anti-actin antibody (IgG) bound to the Alexa Fluor 488-labeled actin conjugate. The individual protein molecules labeled with different fluorescent dyes in solution were effectively focused, interacted with the other protein molecules at 500 aM, and detected directly in real-time using the dual-wavelength (lambda(ex)=488 and 635 nm) laser-induced fluorescence detection system. The kinetics of the protein-protein interactions were examined at different temperatures (12-32 degrees C). At concentrations in the aM range, the number of bound complex molecules through the protein-protein interaction decreased gradually with time at a given temperature, and increased with decreasing temperature at a set time. A high concentration (above 500 pM) of the protein sample caused aggregation and nonspecific binding of the protein molecules, even though the protein molecules were not an example of complementary binding. The results demonstrated that the real-time kinetics of a protein-protein interaction could be analyzed effectively at the single-molecule level without any time delay using the real-time dual-color detection system.

Monitoring neurotransmitter release using surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a promising tool to monitor neurotransmitter release at the single-cell level: it is a sensitive technique that provides structural information of the released compounds and spatial information about their release sites. In this study we demonstrate that depolarization-evoked catecholamine secretion by rat phaeochromocytoma (PC12) cells can be spatially resolved by SERS using silver colloids. A suitable SERS substrate was created by adding silver colloids to the cell culture medium. Nomarski-DIC microscopy combined with reflection confocal laser scanning microscopy showed that the colloids were primarily present on top of the cell membrane. The SERS spectra were successfully corrected for the contribution of cell constituents. Dopamine and noradrenaline were localized by examining the correlation coefficient between spectra and reference catecholamine spectra. Potential improvements of the temporal resolution of the technique are discussed.

Epifluorescence, confocal and total internal reflection microscopy for single-molecule experiments: a quantitative comparison

Epifluorescence, confocal and total internal reflection microscopy are the most widely used techniques for optical single-molecule experiments. Employing these methods, we recorded the emission intensity of the same single molecule as a function of the excitation rate under otherwise identical experimental conditions. Evaluation of these data provides a quantitative comparison of the signal-to-background ratios that can be achieved for the three microscopic techniques.

T-Cell Antigen Receptor-Induced Signaling Complexes: Internalization Via a Cholesterol-Dependent Endocytic Pathway

T-cell antigen receptor engagement causes the rapid assembly of signaling complexes. The adapter protein SLP-76, detected as SLP-yellow fluorescent protein, initially clustered with the TCR and other proteins, then translocated medially on microtubules. As shown by total internal reflection fluorescence microscopy and the inhibition of SLP-76 movement at 16 degrees C, this movement required endocytosis. Immunoelectron microscopy showed SLP-76 staining of smooth pits and tubules. Cholesterol depletion decreased the movement of SLP-76 clusters, as did coexpression of the ubiquitin-interacting motif domain from eps15. These data are consistent with the internalization of SLP-76 via a lipid raft-dependent pathway that requires interaction of the endocytic machinery with ubiquitinylated proteins. The endocytosed SLP-76 clusters contained phosphorylated SLP-76 and phosphorylated LAT. The raft-associated, transmembrane protein LAT likely targets SLP-76 to endocytic vesicles. The endocytosis of active SLP-76 and LAT complexes suggests a possible mechanism for downregulation of signaling complexes induced by TCR activation.

Initiation of attachment and generation of mature focal adhesions by integrin-containing filopodia in cell spreading

Integrin receptors, and associated cytoplasmic proteins mediate adhesion, cell signaling and connections to the cytoskeleton. Using fluorescent protein chimeras, we analyzed initial integrin adhesion in spreading fibroblasts with Total Internal Reflection Fluorescence (TIRF) microscopy. Surprisingly, sequential radial projection of integrin and actin containing filopodia formed the initial cell-matrix contacts. These Cdc42-dependent, integrin-containing projections recruited cytoplasmic focal adhesion (FA) proteins in a hierarchical manner; initially talin with integrin and subsequently FAK and paxillin. Radial FA structures then anchored cortical actin bridges between them and subsequently cells reorganized their actin, a process promoted by Src, and characterized by lateral FA reorientation to provide anchor points for actin stress fibers. Finally, the nascent adhesions coalesced until they formed mature FAs.

Src SH2 arginine 175 is required for cell motility: specific focal adhesion kinase targeting and focal adhesion assembly function

Src kinase is a crucial mediator of adhesion-related signaling and motility. Src binds to focal adhesion kinase (FAK) through its SH2 domain and subsequently activates it for phosphorylation of downstream substrates. In addition to this binding function, data suggested that the SH2 domain might also perform an important role in targeting Src to focal adhesions (FAs) to enable further substrate phosphorylations. To examine this, we engineered an R175L mutation in cSrc to prevent the interaction with FAK pY397. This constitutively open Src kinase mediated up-regulated substrate phosphorylation in SYF cells but was unable to promote malignant transformation. Significantly, SrcR175L cells also had a profound motility defect and an impaired FA generation capacity. Importantly, we were able to recapitulate wild-type motile behavior and FA formation by directing the kinase to FAs, clearly implicating the SH2 domain in recruitment to FAK and indicating that this targeting capacity, and not simply Src-FAK scaffolding, was critical for normal Src function.

Dual-color total internal reflection fluorescence cross-correlation spectroscopy

We present the development and first application of a novel dual-color total internal reflection (TIR) fluorescence system for single-molecule coincidence analysis and fluorescence cross-correlation spectroscopy (FCCS). As a performance analysis, we measured a synthetic DNA-binding assay, demonstrating this dual-color TIR-FCCS approach to be a suitable method for measuring coincidence assays such as biochemical binding, fusion, or signal transduction at solid/liquid interfaces. Due to the very high numerical aperture of the epi-illumination configuration, our setup provides a very high fluorescence collection efficiency resulting in a two- to three-fold increase in molecular brightness compared to conventional confocal FCCS. Further improvements have been achieved through global analysis of the spectroscopic data.

Segmentation and 3D reconstruction of microtubules in total internal reflection fluorescence microscopy (TIRFM)

The interaction of the microtubules with the cell cortex plays numerous critical roles in a cell. For instance, it directs vesicle delivery, and modulates membrane adhesions pivotal for cell movement as well as mitosis. Abnormal function of the microtubules is involved in cancer. An effective method to observe microtubule function adjacent to the cortex is TIRFM. To date most analysis of TIRFM images has been done by visual inspection and manual tracing. In this work we have developed a method to automatically process TIRFM images of microtubules so as to enable high throughput quantitative studies. The microtubules are extracted in terms of consecutive segments. The segments are described via Hamilton-Jacobi equations. Subsequently, the algorithm performs a limited reconstruction of the microtubules in 3D. Last, we evaluate our method with phantom as well as real TIRFM images of living cells.

Synergistic control of protein kinase Cgamma activity by ionotropic and metabotropic glutamate receptor inputs in hippocampal neurons

Conventional protein kinase C (PKC) isoforms are abundant neuronal signaling proteins with important roles in regulating synaptic plasticity and other neuronal processes. Here, we investigate the role of ionotropic and metabotropic glutamate receptor (iGluR and mGluR, respectively) activation on the generation of Ca2+ and diacylglycerol (DAG) signals and the subsequent activation of the neuron-specific PKCgamma isoform in hippocampal neurons. By combining Ca2+ imaging with total internal reflection microscopy analysis of specific biosensors, we show that elevation of both Ca2+ and DAG is necessary for sustained translocation and activation of EGFP (enhanced green fluorescent protein)-PKCgamma. Both DAG production and PKCgamma translocation were localized processes, typically observed within discrete microdomains along the dendritic branches. Markedly, intermediate-strength NMDA receptor (NMDAR) activation or moderate electrical stimulation generated Ca2+ but no DAG signals, whereas mGluR activation generated DAG but no Ca2+ signals. Both receptors were needed for PKCgamma activation. This suggests that a coincidence detection process exists between iGluRs and mGluRs that relies on a molecular coincidence detection process based on the corequirement of Ca2+ and DAG for PKCgamma activation. Nevertheless, the requirement for costimulation with mGluRs could be overcome for maximal NMDAR stimulation through a direct production of DAG via activation of the Ca2+-sensitive PLCdelta (phospholipase Cdelta) isoform. In a second important exception, mGluRs were sufficient for PKCgamma activation in neurons in which Ca2+ stores were loaded by previous electrical activity. Together, the dual activation requirement for PKCgamma provides a plausible molecular interpretation for different synergistic contributions of mGluRs to long-term potentiation and other synaptic plasticity processes.

Light-induced exocytosis in cell development and differentiation

Calcium-dependent exocytosis of fluorescently labeled single secretory vesicles in PC12 cells and primary embryonic telencephalon cells can be triggered by illumination with visible light and imaged by TIRF or epifluorescence microscopy. Opsin 3 was identified by quantitative PCR expression analysis as the putative light receptor molecule for light-induced exocytosis. In primary chicken telencephalon cells, light-induced exocytosis is restricted to a specific period during embryonic development, and involves fusion of rather large vesicles. Strictly calcium-dependent exocytosis starts after a delay of a few seconds of illumination and lasts for up to 2 min. We analyzed the frequency, time course and spatial distribution of exocytotic events. Exocytosis in PC12 cells and telencephalon cells occurs at the periphery or the interface between dividing cells, and the duration of single secretion events varies considerably. Our observation strongly supports the idea that light induced exocytosis is most likely a mechanism for building plasma membrane during differentiation, development and proliferation rather than for calcium-dependent neurotransmitter release.

Investigation of cell adhesion to structured surfaces using total internal reflection fluorescence and confocal laser scanning microscopy

Adhesion of adherent cells on structured surfaces is influenced by the surface pattern given. Here, we designed a structured gold relief surface based on cell adhesion patterns we had previously observed. We analysed the geometric parameters and the overall distribution of focal adhesion kinase in focal adhesions on unstructured glass surfaces using optical microscopy. The basic structural elements obtained from this analysis were arranged in regular clusters that resembled the shape of a polarised migratory cell. In time-lapse studies we observed that the cells adhere preferentially to the gold pads and adopt the shape of the clusters. Staining of the actin cytoskeleton revealed that the actin filaments are aligned to the gold pads of the elementary structure.

Total internal reflection fluorescence imaging using an upconverting cover slip for multicolour evanescent excitation

Total internal reflection fluorescence microscopy is well known as a means of studying surface-bound structures in cell biology. It is usually measured either by coupling a light source to the sample using a prism or with a special objective where light passing through the periphery of the lens illuminates the contact region beyond the critical angle. In this study we present a new and simple approach to total internal reflection fluorescence microscopy where the sample is mounted on a cover slip prepared from a high-index upconverting glass-ceramic. Excitation of the cover slip with a low-cost near-infrared laser diode generates intense narrow-band visible emission within the cover slip, some of which is totally internally reflected. This emission gives rise to an evanescent wave at the interface and hence can excite surface-bound fluorescent species. Depending on the excitation conditions the cover slip can generate violet, green and red emission and hence can excite a wide range of fluorescent labels. Fluorescence emission from the sample can be detected in spectral regions where the direct emission from the cover slip is very weak. The advantages and limitations of the technique are discussed in comparison with conventional total internal reflection fluorescence microscopy measurements and prospects for novel total internal reflection fluorescence microscopy geometries are considered.

Lymphocyte transcellular migration occurs through recruitment of endothelial ICAM-1 to caveola- and F-actin-rich domains

During inflammation, leukocytes bind to the adhesion receptors ICAM-1 and VCAM-1 on the endothelial surface before undergoing transendothelial migration, also called diapedesis. ICAM-1 is also involved in transendothelial migration, independently of its role in adhesion, but the molecular basis of this function is poorly understood. Here we demonstrate that, following clustering, apical ICAM-1 translocated to caveolin-rich membrane domains close to the ends of actin stress fibres. In these F-actin-rich areas, ICAM-1 was internalized and transcytosed to the basal plasma membrane through caveolae. Human T-lymphocytes extended pseudopodia into endothelial cells in caveolin- and F-actin-enriched areas, induced local translocation of ICAM-1 and caveolin-1 to the endothelial basal membrane and transmigrated through transcellular passages formed by a ring of F-actin and caveolae. Reduction of caveolin-1 levels using RNA interference (RNAi) specifically decreased lymphocyte transcellular transmigration. We propose that the translocation of ICAM-1 to caveola- and F-actin-rich domains links the sequential steps of lymphocyte adhesion and transendothelial migration and facilitates lymphocyte migration through endothelial cells from capillaries into surrounding tissue.

Use of TIRF Microscopy to Visualize Actin and Microtubules in Migrating Cells

Total internal reflection fluorescence (TIRF) is the technique of choice to visualize and quantify cellular events localized at the basal plasma membrane of adherent cells. By selectively illuminating the first 200 nm above the basal membrane, it allows maximal resolution in the vertical z-axis. In this chapter, I describe a prism-based TIRF setup and the procedures to visualize the actin and microtubule cytoskeleton in migrating astrocytes. TIRF microscopy provides quantitative information on the organization of the cytoskeleton in both fixed and live migrating cells.

Sub-second cellular dynamics: time-resolved electron microscopy and functional correlation

Subcellular processes, from molecular events to organellar responses and cell movement, cover a broad scale in time and space. Clearly the extremes, such as ion channel activation are accessible only by electrophysiology, whereas numerous routine methods exist for relatively slow processes. However, many other processes, from a millisecond time scale on, can be "caught" only by methods providing appropriate time resolution. Fast freezing (cryofixation) is the method of choice in that case. In combination with follow-up methodologies appropriate for electron microscopic (EM) analysis, with all its variations, such technologies can also provide high spatial resolution. Such analyses may include, for example, freeze-fracturing for analyzing restructuring of membrane components, scanning EM and other standard EM techniques, as well as analytical EM analyses. The latter encompass energy-dispersive x-ray microanalysis and electron spectroscopic imaging, all applicable, for instance, to the second messenger, calcium. Most importantly, when conducted in parallel, such analyses can provide a structural background to the functional analyses, such as cyclic nucleotide formation or protein de- or rephosphorylation during cell stimulation. In sum, we discuss many examples of how it is practically possible to achieve strict function-structure correlations in the sub-second time range. We complement this review by discussing alternative methods currently available to analyze fast cellular phenomena occurring in the sub-second time range.

Spatio-temporal analysis of constitutive exocytosis in epithelial cells

Exocytosis is an essential cellular trafficking process integral to the proper distribution and function of a plethora of molecules, including transporters, receptors, and enzymes. Moreover, incorrect protein targeting can lead to pathological conditions. Recently, the application of evanescent wave microscopy has allowed us to image the final steps of exocytosis. However, spatio-temporal analysis of fusion of constitutive vesicular traffic with the plasma membrane has not been systematically performed. Also, the spatial sites and times of vesicle fusion have not yet been analyzed together. In addition, more formal tests are required in testing biological hypotheses, rather than visual inspection combined with statistical descriptives. Ripley K-functions are used to examine the joint and marginal behavior of locations and fusion times. Semiautomatic detection and mapping of constitutive fusion sites reveals spatial and temporal clustering, but no dependency between the locations and times of fusion events. Our novel approach could be translated to other studies of membrane trafficking in health and diseases such as diabetes.

Targeting and trafficking of the human thiamine transporter-2 in epithelial cells

Humans lack biochemical pathways for thiamine synthesis, so cellular requirements are met via specific carrier-mediated uptake pathways. Two proteins from the solute carrier SLC19A gene family have been identified as human thiamine transporters (hTHTRs), SLC19A1 (hTHTR1) and SLC19A2 (hTHTR2). Both of these transporters are co-expressed but are differentially targeted in polarized cell types that mediate vectorial thiamine transport (e.g. renal and intestinal epithelia). It is important to understand the domain structure of these proteins, namely which regions within the polypeptide sequence are important for physiological delivery to the cell surface, in order to understand the impact of clinically relevant mutations on thiamine transport. Here we have characterized the mechanisms regulating hTHTR2 distribution by using live cell imaging methods that resolve the targeting and trafficking dynamics of full-length hTHTR2, a series of hTHTR2 truncation mutants, as well as chimeras comprising the hTHTR1 and hTHTR2 sequence. We showed the following: (i) that the cytoplasmic COOH-tail of hTHTR2 is not essential for apical targeting in polarized cells; (ii) that delivery of hTHTR2 to the cell surface is critically dependent on the integrity of the transmembrane backbone of the polypeptide so that minimal truncations abrogate cell surface expression of hTHTR2; and (iii) video rate images of hTHTR2-containing intracellular vesicles displayed rapid bi-directional trafficking events to and from the cell surface impaired by microtubule-disrupting but not microfilament-disrupting agents as well as by overexpression of the dynactin subunit dynamitin (p50). Finally, we compared the behavior of hTHTR2 with that of hTHTR1 and the human reduced folate carrier (SLC19A1) to underscore commonalities in the cell surface targeting mechanisms of the entire SLC19A gene family.

Simultaneous multicolor detection system of the single-molecular microbial antigen with total internal reflection fluorescence microscopy

Immunological diagnostic methods have been widely performed and showed high performance in molecular and cellular biology, molecular imaging, and medical diagnostics. We have developed novel methods for the fluorescent labeling of several antibodies coupled with fluorescent nanocrystal QDs. In this study we demonstrated that two bacterial toxins, diphtheria toxin and tetanus toxin, were detected simultaneously in the same view field of a cover slip by using directly QD-conjugated antibodies. We have succeeded in detecting bacterial toxins by counting luminescent spots on the evanescent field with using primary antibody conjugated to QDs. In addition, each bacterial toxin in the mixture can be separately detected by single excitation laser with emission band pass filters, and simultaneously in situ pathogen quantification was performed by calculating the luminescent density on the surface of the cover slip. Our results demonstrate that total internal reflection fluorescence microscopy (TIRFM) enables us to distinguish each antigen from mixed samples and can simultaneously quantitate multiple antigens by QD-conjugated antibodies . Bioconjugated QDs could have great potentialities for in practical biomedical applications to develop various high-sensitivity detection systems.

An evanescent approach for mitochondrial function assay of living cells

Mitochondria are now known to function physiologically not only in the production of ATP as the major cellular energy source, but also in the regulation of intracellular signaling, in, for example, stress-induced apoptosis and buffering of cytosolic calcium. It should be noted, when interpreting mitochondrial studies in situ, that mitochondria within cells show heterogeneity in both function and location. We applied both conventional epifluorescence microscopy (EPIFM) and total-internal-reflection fluorescence microscopy (TIRFM) in this study. Image data taken from TIRFM are excellent and markedly different from those taken from EPIFM. We further investigated the physiological variations of mitochondrial functions using an EPIFM/TIRFM dual-imaging system. This system permits further analysis of functions of mitochondria and other organelles with more precision than is possible using a traditional platform.

Quantitative elucidation of a distinct spatial gradient-sensing mechanism in fibroblasts

Migration of eukaryotic cells toward a chemoattractant often relies on their ability to distinguish receptor-mediated signaling at different subcellular locations, a phenomenon known as spatial sensing. A prominent example that is seen during wound healing is fibroblast migration in platelet-derived growth factor (PDGF) gradients. As in the well-characterized chemotactic cells Dictyostelium discoideum and neutrophils, signaling to the cytoskeleton via the phosphoinositide 3-kinase pathway in fibroblasts is spatially polarized by a PDGF gradient; however, the sensitivity of this process and how it is regulated are unknown. Through a quantitative analysis of mathematical models and live cell total internal reflection fluorescence microscopy experiments, we demonstrate that PDGF detection is governed by mechanisms that are fundamentally different from those in D. discoideum and neutrophils. Robust PDGF sensing requires steeper gradients and a much narrower range of absolute chemoattractant concentration, which is consistent with a simpler system lacking the feedback loops that yield signal amplification and adaptation in amoeboid cells.

Cdc42 and Par6-PKCzeta regulate the spatially localized association of Dlg1 and APC to control cell polarization

Cell polarization is essential in a wide range of biological processes such as morphogenesis, asymmetric division, and directed migration. In this study, we show that two tumor suppressor proteins, adenomatous polyposis coli (APC) and Dlg1-SAP97, are required for the polarization of migrating astrocytes. Activation of the Par6–PKCζ complex by Cdc42 at the leading edge of migrating cells promotes both the localized association of APC with microtubule plus ends and the assembly of Dlg-containing puncta in the plasma membrane. Biochemical analysis and total internal reflection fluorescence microscopy reveal that the subsequent physical interaction between APC and Dlg1 is required for polarization of the microtubule cytoskeleton.

Optical sectioning microscopy

Confocal scanning microscopy, a form of optical sectioning microscopy, has radically transformed optical imaging in biology. These devices provide a powerful means to eliminate from images the background caused by out-of-focus light and scatter. Confocal techniques can also improve the resolution of a light microscope image beyond what is achievable with widefield fluorescence microscopy. The quality of the images obtained, however, depends on the user's familiarity with the optical and fluorescence concepts that underlie this approach. We describe the core concepts of confocal microscopes and important variables that adversely affect confocal images. We also discuss data-processing methods for confocal microscopy and computational optical sectioning techniques that can perform optical sectioning without a confocal microscope.

Expression and optical properties of green fluorescent protein expressed in different cellular environments

This study has investigated the expression of green fluorescent protein (GFP) variants in the cytosol and the endoplasmic reticulum (ER) of HeLa cells and evaluated the effects of the different cellular environments on the fluorescence properties of these GFP variants. Several GFP variants have been constructed by adding different N- or C-terminal signal sequences. These proteins were expressed and folded in distinct cellular compartments in HeLa cells. The localization of these GFP variants targeted to the endoplasmic recticulum was confirmed by the co-localization of DsRed2-ER as assessed by confocal microscopy. The addition of signal peptides targeting GFP variants to the ER or cytosol did not appear to alter the optical spectra of these GFP variants. However, the fluorescence intensity of these GFP variants in the ER was significantly less than that in the cytosol. Thus, the results clearly suggest that the cellular environment affects the formation and/or maturation of green fluorescence protein in vivo. These findings will be helpful in the future development and application of GFP technology aimed at investigating cellular functions performed in the ER and the cytosol.

The p85 regulatory subunit of phosphoinositide 3-kinase down-regulates IRS-1 signaling via the formation of a sequestration complex

Phosphoinositide (PI) 3-kinase is required for most insulin and insulin-like growth factor (IGF) 1–dependent cellular responses. The p85 regulatory subunit of PI 3-kinase is required to mediate the insulin-dependent recruitment of PI 3-kinase to the plasma membrane, yet mice with reduced p85 expression have increased insulin sensitivity. To further understand the role of p85, we examined IGF-1–dependent translocation of p85α by using a green fluorescence protein (GFP)–tagged p85α (EGFP–p85α). In response to IGF-1, but not to PDGF signaling, EGFP–p85α translocates to discrete foci in the cell. These foci contain the insulin receptor substrate (IRS) 1 adaptor molecule, and their formation requires the binding of p85 to IRS-1. Surprisingly, monomeric p85 is preferentially localized to these foci compared with the p85–p110 dimer, and these foci are not sites of phosphatidylinositol-3,4,5-trisphosphate production. Ultrastructural analysis reveals that p85–IRS-1 foci are cytosolic protein complexes devoid of membrane. These results suggest a mechanism of signal down-regulation of IRS-1 that is mediated by monomeric p85 through the formation of a sequestration complex between p85 and IRS-1.

N-WASP deficiency impairs EGF internalization and actin assembly at clathrin-coated pits

WASP and WAVE family proteins promote actin polymerization by stimulating Arp2/3-complex-dependent filament nucleation. Unlike WAVE proteins, which are known to drive the formation of protrusions such as lamellipodia and membrane ruffles, vertebrate cell functions of WASP or N-WASP are less well established. Recent work demonstrated that clathrin-coated pit invagination can coincide with assembly of actin filaments and with accumulation of N-WASP and Arp2/3 complex, but the relevance of their recruitment has remained poorly defined. We employed two-colour total internal reflection microscopy to study the recruitment and dynamics of various components of the actin polymerization machinery and the epidermal growth factor receptor signalling machinery during clathrin-coated pit internalization in control cells and cells genetically deficient for functional N-WASP. We found that clathrin-coated pit endocytosis coincides with the recruitment of N-WASP, Arp2/3 complex and associated proteins, but not of WAVE family members. Actin accumulation at clathrin-coated pits requires the Arp2/3 complex, since Arp2/3 complex sequestration in the cytosol abolished any detectable actin assembly. The absence of N-WASP caused a significant reduction in the frequencies of actin and Arp2/3 complex accumulations at sites of clathrin-coated pit invagination and vesicle departure. Although N-WASP was not essential for Arp2/3-complex-mediated actin assembly at these sites or for EGF receptor-mediated endocytosis, N-WASP deficiency caused a marked reduction of EGF internalization. We conclude that the assembly of WASP subfamily proteins and associated factors at sites of clathrin-coated pit invagination amplifies actin accumulations at these sites promoting efficient internalization of ligands via clathrin-mediated endocytosis.

Spatial analysis of 3' phosphoinositide signaling in living fibroblasts, III: influence of cell morphology and morphological Polarity

Activation of phosphoinositide (PI) 3-kinase is a required signaling pathway in fibroblast migration directed by platelet-derived growth factor. The pattern of 3' PI lipids in the plasma membrane, integrating local PI 3-kinase activity as well as 3' PI diffusion and turnover, influences the spatiotemporal regulation of the cytoskeleton. In fibroblasts stimulated uniformly with platelet-derived growth factor, visualized using total internal reflection fluorescence microscopy, we consistently observed localized regions with significantly higher or lower 3' PI levels than adjacent regions (hot and cold spots, respectively). A typical cell contained multiple hot spots, coinciding with apparent leading edge structures, and at most one cold spot at the rear. Using a framework for finite-element modeling with actual cell contact area geometries, we find that although the 3' PI pattern is affected by irregular contact area shape, cell morphology alone cannot explain the presence of hot or cold spots. Our results and analysis instead suggest that these regions reflect different local 3' PI dynamics, specifically through a combination of mechanisms: enhanced PI 3-kinase activity, reduced 3' PI turnover, and possibly slow/constrained 3' PI diffusion. The morphological polarity of the cell may thus bias 3' PI signaling to promote persistent migration in fibroblasts.

Green fluorescent proteins in receptor research: An emerging tool for drug discovery

In the last five years, green fluorescent protein (GFP) has emerged from being a mere curiosity to become a reliable tool for molecular pharmacological research. GFP produces an intense and stable green fluorescence noncatalytically by absorbing blue light maximally at 395 nm and emitting green light with a peak at 509 nm. It consists of 238 amino acids and its molecular mass is 27-30 kDa. GFP fluorescence occurs without cofactors and this property allows GFP fluorescence to be utilised in nonnative organisms, wherein it can be used as a reporter. This use of GFP permits real-time analysis of receptor dynamics. The emitted fluorescence can be used as a nontoxic marker and detected using fluorescence-activated cell sorting (FACS), thus avoiding any staining procedure, expensive mRNA analysis or hazardous radiolabeled binding assays. The potential value of GFP has also been recognized in orphan receptor research, where various GFP-tagged therapeutic proteins have been constructed in an attempt to identify the endogenous ligand(s). These chimeric proteins have been used to determine the site and time course of receptor expression and to relate receptor dynamics with therapeutic outcome. The preparation of new GFP constructs for identifying germ layer cells (endodermal, ectodermal, and mesodermal), as well as neuronal, haematopoietic, endothelial, and cartilage cells, has provided a useful battery of tissue/receptor-specific screening assays for new chemical entities. Genetically engineered cells with GFP expression have provided a valuable tool for automated analysis, and can be adapted for high-throughput systems. GFP is being increasingly utilised for the study of receptor dynamics, where, having already proved beneficial, it will likely continue to contribute towards the search for new classes of drugs, as well as to "de-orphaning" orphan receptors.

An optical microsensor to measure fluorescent light intensity in biofilms

We have developed an optical microsensor to quantify fluorescent light intensity distribution in biofilms. The optical system consisted of a beam splitter, light couplers, filters and a spectrophotometer able to accept the fiberoptic cable to measure fluorescent light intensity. The emitted light, fluorescence from the biofilm, was collected at the tip of the optical microsensor and was transferred to a spectrophotometer via a fiberoptic cable. The total fluorescent light intensity was evaluated from the emission spectrum by numerical integration. The newly developed fiberoptic microsensor was tested using a Staphylococcus aureus strain producing yellow fluorescent protein (YFP) grown as biofilm. We used a 405-nm violet laser diode for excitation, and measured the emission intensity between 480 nm and 540 nm. The optical microsensor that quantifies fluorescent light intensity is a promising tool in biofilm research which often requires detection and quantification of fluorescent light intensity distribution generated by various fluorescent proteins.

Spatial analysis of 3' phosphoinositide signaling in living fibroblasts: I. Uniform stimulation model and bounds on dimensionless groups

Fluorescent protein probes now permit spatial distributions of specific intracellular signaling molecules to be observed in real time. Mathematical models have been used to simulate molecular gradients and other spatial patterns within cells, and the output of such models may be compared directly with experiments if the binding of the fluorescent probe and the physics of the imaging technique are each incorporated. Here we present a comprehensive model describing the dynamics of 3' phosphoinositides (PIs), lipid second messengers produced in the plasma membrane in response to stimulation of the PI 3-kinase signaling pathway, as monitored in the cell-substratum contact area using total internal reflection fluorescence microscopy. With this technique it was previously shown that uniform stimulation of fibroblasts with platelet-derived growth factor elicits the formation of axisymmetric 3' PI gradients, which we now characterize in the context of our model. We find that upper and lower bounds on the relevant dimensionless model parameter values for an individual cell can be calculated from four well-defined fluorescence measurements. Based on our analysis, we expect that the key dimensionless group, the ratio of 3' PI turnover and diffusion rates, can be estimated within approximately 20% or less.

Spatial analysis of 3' phosphoinositide signaling in living fibroblasts: II. Parameter estimates for individual cells from experiments

Fibroblast migration is directed by gradients of platelet-derived growth factor (PDGF) during wound healing. As in other chemotactic systems, it has been shown recently that localized stimulation of intracellular phosphoinositide (PI) 3-kinase activity and production of 3' PI lipids in the plasma membrane are important events in the signaling of spatially biased motility processes. In turn, 3' PI localization depends on the effective diffusion coefficient, D, and turnover rate constant, k, of these lipids. Here we present a systematic and direct comparison of mathematical model calculations and experimental measurements to estimate the values of the effective 3' PI diffusion coefficient, D, turnover rate constant, k, and other parameters in individual fibroblasts stimulated uniformly with PDGF. In the context of our uniform stimulation model, the values of D and k in each cell were typically estimated within 10-20% or less, and the mean values across all of the cells analyzed were D = 0.37 +/- 0.25 microm2/s and k = 1.18 +/- 0.54 min(-1). In addition, we report that 3' PI turnover is not affected by PDGF receptor signaling in our cells, allowing us to focus our attention on the regulation of 3' PI production as this system is studied further.

Imaging the activity and localization of single voltage-gated Ca(2+) channels by total internal reflection fluorescence microscopy

The patch-clamp technique has enabled functional studies of single ion channels, but suffers limitations including lack of spatial information and inability to independently monitor currents from more than one channel. Here, we describe the use of total internal reflection fluorescence microscopy as an alternative, noninvasive approach to optically monitor the activity and localization of multiple Ca(2+)-permeable channels in the plasma membrane. Images of near-membrane Ca(2+) signals were obtained from >100 N-type channels expressed within restricted areas (80 x 80 micro m) of Xenopus oocytes, thereby permitting simultaneous resolution of their gating kinetics, voltage dependence, and localization. Moreover, this technique provided information inaccessible by electrophysiological means, demonstrating that N-type channels are immobile in the membrane, show a patchy distribution, and display diverse gating kinetics even among closely adjacent channels. Total internal reflection fluorescence microscopy holds great promise for single-channel recording of diverse voltage- and ligand-gated Ca(2+)-permeable channels in the membrane of neurons and other isolated or cultured cells, and has potential for high-throughput functional analysis of single channels.

5'-AMP-activated protein kinase controls insulin-containing secretory vesicle dynamics

Changes in 5'-AMP-activated protein kinase (AMPK) activity have recently been implicated in the control of insulin secretion by glucose (da Silva Xavier, G., Leclerc, I., Varadi, A., Tsuboi, T., Moule, S. K., and Rutter, G. A. (2003) Biochem. J. 371, 761-774). Here, we examine the possibility that activation of AMPK may regulate distal steps in insulin secretion, including vesicle movement and fusion with the plasma membrane. Vesicle dynamics were imaged in single pancreatic MIN6 beta-cells expressing lumen-targeted pH-insensitive yellow fluorescent protein, neuropeptide Y.Venus, or monomeric red fluorescent protein by total internal reflection fluorescence and Nipkow disc confocal microscopy. Overexpression of a truncated, constitutively active form of AMPK (AMPKalpha1, 1-312, T172D; AMPK CA), inhibited glucose-stimulated (30 versus 3.0 mM) vesicle movements, and decreased the number of vesicles docked or fusing at the plasma membrane, while having no effect on the kinetics of individual secretory events. Expression of the activated form of AMPK also prevented dispersal of the cortical actin network at high glucose concentrations. Monitored in permeabilized cells, where the effects of AMPK CA on glucose metabolism and ATP synthesis were bypassed, AMPK CA inhibited Ca2+ and ATP-induced insulin secretion, and decreased ATP-dependent vesicle movements. These findings suggest that components of the vesicle transport network, including vesicle-associated motor proteins, may be targets of AMPK in beta-cells, dephosphorylation of which is required for vesicle mobilization at elevated glucose concentrations.

Optical single-channel recording: imaging Ca2+ flux through individual N-type voltage-gated channels expressed in Xenopus oocytes

Functional studies of single membrane ion channels were made possible by the introduction of the patch-clamp technique, which allows single-channel currents to be measured with unprecedented resolution. Nevertheless, patch clamping has some limitations: including the need for physical access of the patch pipette, possible disruption of local cellular architecture, inability to monitor multiple channels, and lack of spatial information. Here, we demonstrate the use of confocal fluorescence microscopy as a non-invasive technique to optically monitor the gating of individual Ca2+ channels. Near-membrane fluorescence signals track the gating of N-type Ca2+ channels with a kinetic resolution of about 10ms, provide a simultaneous and independent readout from several channels, and allow their locations to be mapped with sub-micrometer spatial resolution. Optical single-channel recording should be applicable to diverse voltage- and ligand-gated Ca2+-permeable channels, and has the potential for high-throughput functional analysis of single channels.

Fluorescence imaging with two-photon evanescent wave excitation

We demonstrate broad-field, non-scanning, two-photon excitation fluorescence (2PEF) close to a glass/cell interface by total internal reflection of a femtosecond-pulsed infrared laser beam. We exploit the quadratic intensity dependence of 2PEF to provide non-linear evanescent wave (EW) excitation in a well-defined sample volume and to eliminate scattered background excitation. A simple model is shown to describe the resulting 2PEF intensity and to predict the effective excitation volume in terms of easily measurable beam, objective and interface properties. We demonstrate non-linear evanescent wave excitation at 860 nm of acridine orange-labelled secretory granules in live chromaffin cells, and excitation at 900 nm of TRITC-phalloidin-actin/GPI-GFP double-labelled fibroblasts. The confined excitation volume and the possibility of simultaneous multi-colour excitation of several fluorophores make EW 2PEF particularly advantageous for quantitative microscopy, imaging biochemistry inside live cells, or biosensing and screening applications in miniature high-density multi-well plates.

The AP-2 complex is excluded from the dynamic population of plasma membrane-associated clathrin

Numerous biologically relevant substrates are selectively internalized via clathrin-mediated endocytosis. At the plasma membrane the AP-2 complex plays a major role in clathrin coat formation, interacting with both cargo and clathrin. Utilizing simultaneous dual-channel total internal reflection fluorescence microscopy we have analyzed components of the AP-2 complex (alpha- and beta 2-adaptin) during clathrin-mediated endocytosis. Although in static images enhanced green fluorescent protein-tagged AP-2 markers significantly co-localized with clathrin and other components of clathrin-coated pits, AP-2 did not seem to be present in clathrin spots that appeared to undergo internalization or motility parallel to the plane of the plasma membrane. Two populations of clathrin at the plasma membrane seem to exist, the dynamic and the static, and AP-2 appears to be only found within the latter. These results suggest that colocalized clathrin/AP-2 puncta may represent loci for coated pit production and that previous models that assumed AP-2 was retained within clathrin coats during endocytosis may need to be re-evaluated.

Interaction of the actin cytoskeleton with microtubules regulates secretory organelle movement near the plasma membrane in human endothelial cells

The role of cytoskeletal elements in regulating transport and docking steps that precede exocytosis of secretory organelles is not well understood. We have used Total Internal Reflection Fluorescence (TIRF) microscopy to visualize the three-dimensional motions of secretory organelles near the plasma membrane in living endothelial cells. Weibel-Palade bodies (WPb), the large tubular storage organelles for von Willebrand factor, were labelled with Rab27a-GFP. By contrast, green fluorescent protein (GFP)-tagged tissue-type plasminogen activator (tPA-GFP) labelled submicron vesicular organelles. Both populations of GFP-labelled organelles underwent stimulated exocytosis. The movement of these morphologically distinct organelles was measured within the evanescent field that penetrated the first 200 nm above the plasma membrane. WPb and tPA-GFP vesicles displayed long-range bidirectional motions and short-range diffusive-like motions. Rotating and oscillating WPb were also observed. TIRF microscopy enabled us to quantify the contribution of actin and microtubules and their associated motors to the organelle motions close to the plasma membrane. Long-range motions, as well as WPb rotations and oscillations, were microtubule-and kinesin-dependent. Disruption of the actin cytoskeleton and inhibition of myosin motors increased the number of long-range motions and, in the case of WPb, their velocity. The actin and microtubules had opposite effects on the mobility of organelles undergoing short-range motions. Actin reduced the mobility and range of motion of both WPb and tPA vesicles, whereas microtubules and kinesin motors increased the mobility of WPb. The results show that the dynamics of endothelial secretory organelles close to the plasma membrane are controlled by the opposing roles of the microtubule and actin cytoskeletal transport systems.

Quantitative fluorescent speckle microscopy: where it came from and where it is going

Fluorescent speckle microscopy (FSM) is a technology for analysing the dynamics of macromolecular assemblies. Originally, the effect of random speckle formation was discovered with microtubules. Since then, the method has been expanded to other proteins of the cytoskeleton such as f-actin and microtubule binding proteins. Newly developed, specialized software for analysing speckle movement and photometric fluctuation in the context of polymer transport and turnover has turned FSM into a powerful method for the study of cytoskeletal dynamics in cell migration, division, morphogenesis and neuronal path finding. In all these settings, FSM serves as the quantitative readout to link molecular and genetic interventions to complete maps of the cytoskeleton dynamics and thus can be used for the systematic deciphering of molecular regulation of the cytoskeleton. Fully automated FSM assays can also be applied to live-cell screens for toxins, chemicals, drugs and genes that affect cytoskeletal dynamics. We envision that FSM has the potential to become a core tool in automated, cell-based molecular diagnostics in cases where variations in cytoskeletal dynamics are a sensitive signal for the state of a disease, or the activity of a molecular perturbant. In this paper, we review the origins of FSM, discuss these most recent technical developments and give a glimpse to future directions and potentials of FSM. It is written as a complement to the recent review (Waterman-Storer & Danuser, 2002, Curr. Biol., 12, R633-R640), in which we emphasized the use of FSM in cell biological applications. Here, we focus on the technical aspects of making FSM a quantitative method.

Nanometer targeting of microtubules to focal adhesions

Although cell movement is driven by actin, polarization and directional locomotion require an intact microtubule cytoskeleton that influences polarization by modulating substrate adhesion via specific targeting interactions with adhesion complexes. The fidelity of adhesion site targeting is precise; using total internal reflection fluorescence microscopy (TIRFM), we now show microtubule ends (visualized by incorporation of GFP tubulin) are within 50 nm of the substrate when polymerizing toward the cell periphery, but not when shrinking from it. Multiple microtubules sometimes followed similar tracks, suggesting guidance along a common cytoskeletal element. Use of TIRFM with GFP- or DsRed-zyxin in combination with either GFP-tubulin or GFP-CLIP-170 further revealed that the polymerizing microtubule plus ends that tracked close to the dorsal surface consistently targeted substrate adhesion complexes. This supports a central role for the microtubule tip complex in the guidance of microtubules into adhesion foci, and provides evidence for an intimate cross-talk between microtubule tips and substrate adhesions in the range of molecular dimensions.

Visualizing signals moving in cells

Cells display a highly complex spatiotemporal organization, required to exert a wide variety of different functions, for example, detection, processing, and propagation of nerve impulses by neurons; contraction and relaxation by muscle cells; movement by leukocytes; and adsorption and secretion of nutrients and metabolites by epithelial cells lining the gut. Successful execution of these complex processes requires highly dynamic information transfer between different regions and compartments within cells. Through the development of fluorescent sensors for intracellular signaling molecules coupled with improved microscopic imaging techniques, it has now become possible to investigate signal propagation in cells with high spatial and temporal resolution.

A sense of closeness: protein detection by proximity ligation

Highly specific and sensitive procedures will be required to evaluate proteomes. Proximity ligation is a recently introduced mechanism for protein analysis. In this technique, the convergence of sets of protein-binding reagents on individual target molecules juxtaposes attached nucleic acid sequences. Through a ligation reaction a DNA reporter sequence is created, which can be amplified. The procedure thus encodes detected proteins as specific nucleic acid sequences in what may be viewed as a reverse translation reaction.

Cell motility: insights from the backstage

In the true spirit of Michael Abercrombie's pioneering studies on cell locomotion, the Fifth Abercrombie's Symposium on Cell Behaviour--held in St Catherine's College at Oxford University (September 15-18, 2002)--celebrated the intricate beauty of cell motility with an explosion of new technologies that Abercrombie could only have dreamed of. Building on the complementary approaches of quantitative cell biology, biochemistry and genetics, the meeting provided new insights into the ever-growing complexity of the signal transduction pathways involved in cell movement.