Measurement and manipulation of cytoskeletal dynamics in living cells

Targeting phosphatidylinositol 3-kinase gamma (PI3Kγ): Discovery and development of its selective inhibitors

Phosphatidylinositol 3-kinase gamma (PI3Kγ) has been regarded as a promising drug target for the treatment of advanced solid tumors, leukemia, lymphoma, and inflammatory and autoimmune diseases. However, the high level of structural conservation among the members of the PI3K family and the diverse physiological roles of Class I PI3K isoforms (α, β, δ, and γ) highlight the importance of isoform selectivity in the development of PI3Kγ inhibitors. In this review, we provide an overview of the structural features of PI3Kγ that influence γ-isoform selectivity and discuss the structure-selectivity-activity relationship of existing clinical PI3Kγ inhibitors. Additionally, we summarize the experimental and computational techniques utilized to identify PI3Kγ inhibitors. The insights gained so far could be used to overcome the main challenges in development and accelerate the discovery of PI3Kγ-selective inhibitors.

High-Content Screening: A New Approach to Easing Key Bottlenecks in the Drug Discovery Process

Recent improvements in target discovery and high throughput screening (HTS) have increased the pressure at key points along the drug discovery pipeline. High-content screening (HCS) was developed to ease bottlenecks that have formed at target validation and lead optimization points in the pipeline. HCS defines the role of targets in cell functions by combining fluorescence-based reagents with the ArrayScan™ System to automatically extract temporal and spatial information about target activities within cells. The ArrayScan System is a tabletop instrument that includes optics for subcellular resolution of fluorescence signals from many cells in a field within a well of a microtiter plate. One demonstrated application is a high-content screen designed to measure the drug-induced transport of a green fluorescent protein-human glucocorticoid receptor chimeric protein from the cytoplasm to the nucleus of human tumor cells. A high-content screen is also described for the multiparametric measurement of apoptosis. This single screen provides measurements of nuclear size and shape changes, nuclear DNA content, mitochondrial potential, and actin-cytoskeletal rearrangements during drug-induced programmed cell death. The next generation HCS system is a miniaturized screening platform, the CellChip™ System, that will increase the throughput of HCS, while integrating HCS with HTS on the same platform.

Multiplexed high content screening assays create a systems cell biology approach to drug discovery

High content screening (HCS) has emerged as an important platform technology for early drug discovery from target identification through in vitro ADME/Tox. The focus is now on implementing multiplexed assays, developing and using advanced reagents and developing and harnessing more sophisticated informatics tools. Multiplexed HCS assays have the potential to dramatically improve the early drug discovery process by creating systems cell biology profiles on the activities of compounds. It is predicted that multiplexed HCS assays will accelerate the overall workflow and produce deeper functional knowledge, thereby permitting better decisions on what compounds to pursue.:

Actin pedestal formation by enteropathogenic Escherichia coli is regulated by IQGAP1, calcium, and calmodulin

During infection, enteropathogenic Escherichia coli (EPEC) injects effector proteins into the host cell to manipulate the actin cytoskeleton and promote formation of actin pedestals. IQGAP1 is a multidomain protein that participates in numerous cellular functions, including Rac1/Cdc42 and Ca(2+)/calmodulin signaling and actin polymerization. Here we report that IQGAP1, Ca(2+), and calmodulin modulate actin pedestal formation by EPEC. Infection with EPEC promotes both the interaction of IQGAP1 with calmodulin and the localization of IQGAP1 and calmodulin to actin pedestals while reducing the interaction of IQGAP1 with Rac1 and Cdc42. IQGAP1-null fibroblasts display a reduced polymerization of actin in response to EPEC. In addition, antagonism of calmodulin or chelation of intracellular Ca(2+) reduces EPEC-dependent actin polymerization. Furthermore, IQGAP1 specifically interacts with Tir in vitro and in cells. Together these data identify IQGAP1, Ca(2+), and calmodulin as a novel signaling complex regulating actin pedestal formation by EPEC.

Multiple proteins mediate IQGAP1-stimulated cell migration

Cell migration, a highly complex physiological phenomenon that requires the co-ordinated and tightly regulated function of several proteins, is mediated by a number of signalling pathways. Elucidation of the molecular mechanisms of cell migration impacts our comprehension of numerous cell functions, ranging from development and immune surveillance to angiogenesis and metastasis. The scaffold protein IQGAP1, which binds multiple proteins and regulates their functions, promotes cell motility. Many of the IQGAP1 binding proteins have been implicated in cell migration. In this study, we employed a multifaceted strategy to identify proteins that contribute to IQGAP1-stimulated cell migration. Using specific IQGAP1 point mutant constructs, an interaction with actin was shown to be essential for IQGAP1 to increase cell migration. In contrast, eliminating the binding of Ca(2+)/calmodulin, but not Ca(2+)-free calmodulin, augmented the ability of IQGAP1 to stimulate cell migration. Consistent with these findings, selective inhibition of calmodulin function at the plasma membrane with a specific peptide inhibitor enhanced cell migration mediated by IQGAP1. Interestingly, immunofluorescence staining and confocal microscopy suggest that localization of Cdc42 at the leading edge is not necessary for maximal migration of epithelial cells. Coupled with the observations that Cdc42 and Rac1 contribute to IQGAP1-stimulated cell migration, these data suggest that IQGAP1 serves as a junction to integrate multiple signalling molecules to facilitate cell migration.

Dynamics of beta1-integrins in living fibroblasts--effect of substratum wettability

The dynamics of integrin receptors mobility was studied in living human fibroblasts using fluorescence-labeled beta(1)-integrin monoclonal antibodies. Time-lapse image series were obtained by confocal laser scanning microscopy when cells were adhering on model hydrophilic (clean glass) and hydrophobic (octadecyl-silanized; i.e., ODS) surfaces coated with fibronectin. Direct measurements showed approximately twice-higher velocity of integrins on glass compared to ODS, and these velocities varied in different zones of the cells. A kinetic model and algorithm for quantification of images was developed, and the analysis identified three receptor populations on glass: immobilized (82.76% of all), slow (4.16%), and fast (13.08%), while, on ODS, only two were identified: immobilized (83.36%) and fast (16.64%). Fast integrins in the peripheral zone of cells have maximal velocities of 0.353 +/- 0.02 mum/min (n = 48, four cells) on hydrophilic and 0.218 +/- 0.02 mum/min (n = 30, three cells) on hydrophobic substrata. The slow population has a velocity of 0.114 mum/min (n = 48, four cells). Further analyses show that these velocities also differ significantly in the peripheral and middle zones of cells in a substrate-dependent fashion. A well-defined circular motion of receptors around the cell center expressed mainly on hydrophobic substrata was monitored and quantified as well.

Isolation and characterization of 190K protein from aorta smooth muscle

Molecular assemblies of actin and myosin for the contractility of smooth muscle are quite different from those of striated muscle. Another striking difference is that vascular smooth muscle has a potential to transform to migratory synthetic cell type. At this point of view, smooth muscle cell has properties similar to those of non-muscle. In fact, myosin Ic, a single headed unconventional myosin, was identified in aorta smooth muscle. During the studies on myosin Ic, we have found another calmodulin related 190kDa protein. This protein binds to calmodulin irrespective on calcium ion and to F-actin in an ATP independent manner. Furthermore, the F-actin binding stoichiometry diminished to half upon the addition of exogenous calmodulin. Partial amino acid sequence indicated a high homology to those of GRD (GTPase Related Domain) of human brain IQGAP1. Western blot analysis using anti-human IQGAP1 antibody also indicated a strong cross-reactivity with the protein. We have tested the protein with respect to the characteristic F-actin gelation by IQGAP1. In the presence of cdc42 and GTPgammaS, 190kDa protein could cause a high viscosity of F-actin. These data indicate a close similarity to human brain IQGAP1. The presence of IQGAP1 in aorta smooth muscle suggests contributions for cellular processes such as actin reorganization during contraction-relaxation cycle, association of cytoskeletal structure to cell membrane, organelle movement.

Rho-kinase inhibitors: pharmacomodulations on the lead compound Y-32885

In order to specify structure-activity relationships we have synthesized new series of analogues of the Rho-kinase inhibitor (R)-(+)-N-(4-pyridyl)-4-(1-aminoethyl)benzamide (Y-32885). The structural modifications concerned the 1-aminoethyl, the pyridyl and the amide groups which are the main features of this lead compound. Our analogue derivatives were evaluated on GTPgammaS-induced contraction in permeabilized smooth-muscle and on the actin cytoskeleton. All the modifications result in a diminution or a loss of activity showing that interactions of Y-32885 with the catalytic domain of Rho-kinase seem to be particularly definite and sensitive to structural variations. The presence of a pyridine moiety and a basic amine group separated by a spacer bearing an amide function are of utmost importance.

Application of natural receptors in sensors and assays

Biosensors are analytical devices that use a biological or biologically derived material immobilized at a physicochemical transducer to measure one or more analytes. Although there are a large number of reviews on biosensors in general, there has been little systematic information presented on the application of natural receptors in sensor technology. This perspective discusses broadly the fundamental properties of natural receptors, which make them an attractive option for use as biorecognition elements in sensor technology. It analyses the current situation by reference to typical examples, such as the application of nicotinic acetylcholine receptor and G protein-linked receptors in affinity sensors and analyses the problems that need to be resolved prior to any commercialization of such devices.

Regulation of the cytoskeleton and cell adhesion by the Rho family GTPases in mammalian cells

Members of the Rho family of small Ras-like GTPases--including RhoA, -B, and -C, Rac1 and -2, and Cdc42--exhibit guanine nucleotide-binding activity and function as molecular switches, cycling between an inactive GDP-bound state and an active GTP-bound state. The Rho family GTPases participate in regulation of the actin cytoskeleton and cell adhesion through specific targets. Identification and characterization of these targets have begun to clarify how the Rho family GTPases act to regulate cytoskeletal structure and cell-cell and cell-substratum contacts in mammalian cells. The Rho family GTPases are also involved in regulation of smooth muscle contraction, cell morphology, cell motility, neurite retraction, and cytokinesis. However, the molecular mechanisms by which the Rho family GTPases participate in the regulation of such processes are not well established.

The actin-based nanomachine at the leading edge of migrating cells

Two fundamental parameters of the highly dynamic, ultrathin lamellipodia of migrating fibroblasts have been determined-its thickness in living cells (176 +/- 14 nm), by standing-wave fluorescence microscopy, and its F-actin density (1580 +/- 613 microm of F-actin/microm(3)), via image-based photometry. In combination with data from previous studies, we have computed the density of growing actin filament ends at the lamellipodium margin (241 +/- 100/microm) and the maximum force (1.86 +/- 0.83 nN/microm) and pressure (10.5 +/- 4.8 kPa) obtainable via actin assembly. We have used cell deformability measurements (. J. Cell Sci. 44:187-200;. Proc. Natl. Acad. Sci. USA. 79:5327-5331) and an estimate of the force required to stall the polymerization of a single filament (. Proc. Natl. Acad. Sci. USA. 78:5613-5617;. Biophys. J. 65:316-324) to argue that actin assembly alone could drive lamellipodial extension directly.

Rho A regulates sustained smooth muscle contraction through cytoskeletal reorganization of HSP27

The ras-related protein Rho p21 regulates various actin-dependent functions, including smooth muscle contraction. However, the precise mechanism of action of Rho p21 is still not clear. We report here that Rho A is a key regulator of agonist-induced contractile effects in rabbit colonic smooth muscle. Endothelin-1 and C2 ceramide were used. Both seem to activate phosphoinositide 3-kinase (PI 3-kinase) through G protein and pp60(src), respectively. Immunoprecipitation and immunoblotting revealed one form of 21-kDa Rho A that translocated from the cytosol to the membrane in response to stimulation by either endothelin (10(-7) M) or ceramide (10(-7) M) ( approximately 30% increase at 30 s that was sustained at 4 min). The translocation of Rho A to the membrane was confirmed by immunostaining. The translocation of Rho A was inhibited by Clostridium botulinum C3 exoenzyme, which ADP ribosylated Rho A, but was not inhibited by the pp60(src) inhibitor herbimycin A or by the protein kinase C (PKC) inhibitor calphostin C, suggesting that Rho A may be upstream of pp60(src) and PKC or may belong to a different pathway than these proteins. Both ceramide- and endothelin-induced PI 3-kinase activation was inhibited by C3 exoenzyme pretreatment. However, the C3 exoenzyme inhibited endothelin- but not ceramide-induced mitogen-activated protein kinase phosphorylation, indicating that Rho regulates ceramide- and endothelin-induced contraction through different pathways. Furthermore, the dominant negative form of Rho (N19Rho) inhibited the actin binding protein, 27-kDa heat shock protein (HSP27), reorganization in response to ceramide and endothelin observed under confocal microscopy.

An infra-red light-transmitting aperture controller for use in single-cell fluorescence photometry

Photometric techniques are commonly used to monitor the output from fluorescent indicators during the study of cellular signalling. At the single-cell level, the region of interest is normally set by a variable aperture placed within the microscope emission pathway. The present study reports an improved aperture controller which adjusts the area for fluorescence measurement, whilst allowing objects throughout the field of view to be continuously monitored using infra-red illumination. A rectangular aperture is selected by four 715-nm long-pass glass filters which block > 99.9% of the fluorescence emission at 480-600 nm. A 780-nm long-pass glass filter is used to provide infra-red illumination which does not interfere with the fluorescence signal, yet is detectable by a standard CCD camera. This allows detection of morphological events throughout the field of view and facilitates manipulation of extracellular pipettes, without interruption to a single-cell fluorescence recording. The infra-red light-transmitting controller is suitable for use with a range of other fluorescent indicators, including those routinely used to detect Ca2+, Cl-, Na+ and pH. Data are presented which demonstrate the use of this controller to measure ADP-evoked [Ca2+]i increases in single human erythroleukaemia cells loaded with the Ca2+ indicator fura-2.

Association of the myosin-binding subunit of myosin phosphatase and moesin: dual regulation of moesin phosphorylation by Rho-associated kinase and myosin phosphatase

The small GTPase Rho is believed to regulate the actin cytoskeleton and cell adhesion through its specific targets. We previously identified the Rho targets: protein kinase N, Rho-associated kinase (Rho-kinase), and the myosin-binding subunit (MBS) of myosin phosphatase. We found that in MDCK epithelial cells, MBS accumulated at the tetradecanoylphorbol-13-acetate (TPA)-induced membrane ruffling area, where moesin, a member of the ERM (ezrin, radixin, and moesin) family, was localized. Neither membrane ruffling nor an accumulation of moesin and MBS at the free-end plasma membrane was induced when MDCK cells were stimulated with TPA after the microinjection of C3, which ADP-ribosylates and inactivates Rho. MBS was colocalized with moesin at the cell-cell contact sites in MDCK cells. We also found that moesin was coimmunoprecipitated with MBS from MDCK cells. Recombinant MBS interacted with the amino-terminal domains of moesin and ezrin. Myosin phosphatase composed of the catalytic subunit and MBS showed phosphatase activity toward moesin, which was phosphorylated by Rho-kinase. The phosphatase activity was inhibited when MBS was phosphorylated by Rho-kinase. These results suggest that MBS is recruited with moesin to the plasma membrane and that myosin phosphatase and Rho-kinase regulate the phosphorylation state of moesin downstream of Rho.

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.

Signaling pathways underlying eosinophil cell motility revealed by using caged peptides

Insights into structure-function relations of many proteins opens the possibility of engineering peptides to selectively interfere with a protein's activity. To facilitate the use of peptides as probes of cellular processes, we have developed caged peptides whose influence on specific proteins can be suddenly and uniformly changed by near-UV light. Two peptides are described which, on photolysis of a caging moiety, block the action of calcium-calmodulin or myosin light chain kinase (MLCK). The efficacy of theses peptides is demonstrated in vitro and in vivo by determining their effect before and after photolysis on activities of isolated enzymes and cellular functions known to depend on calcium-calmodulin and MLCK. These caged peptides each were injected into motile, polarized eosinophils, and when exposed to light promptly blocked cell locomotion in a similar manner. The results indicate that the action of calcium-calmodulin and MLCK, and by inference myosin II, are required for the ameboid locomotion of these cells. This methodology provides a powerful means for assessing the role of these and other proteins in a wide range of spatio-temporally complex functions in intact living cells.

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.

Fibroblasts contracting collagen matrices form transient plasma membrane passages through which the cells take up fluorescein isothiocyanate-dextran and Ca2+

When fibroblasts contract collagen matrices, the cells activate a Ca(2+)-dependent cyclic AMP signaling pathway. We have found that contraction also stimulates uptake of fluorescein isothiocyanate-dextran molecules from the medium. Our results indicate that fluorescein isothiocyanate-dextran enters directly into the cell cytoplasm through 3- to 5-nm plasma membrane passages. These passages, which reseal in less than 5 s in the presence of divalent cations, also are likely sites of Ca2+ uptake during contraction and the first step in contraction-activated cyclic AMP signaling. The formation of plasma membrane passages during fibroblast contraction may reflect a general cellular response to rapid mechanical changes.

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.

Regulation of myosin phosphatase by Rho and Rho-associated kinase (Rho-kinase)

The small guanosine triphosphatase Rho is implicated in myosin light chain (MLC) phosphorylation, which results in contraction of smooth muscle and interaction of actin and myosin in nonmuscle cells. The guanosine triphosphate (GTP)-bound, active form of RhoA (GTP.RhoA) specifically interacted with the myosin-binding subunit (MBS) of myosin phosphatase, which regulates the extent of phosphorylation of MLC. Rho-associated kinase (Rho-kinase), which is activated by GTP.RhoA, phosphorylated MBS and consequently inactivated myosin phosphatase. Overexpression of RhoA or activated RhoA in NIH 3T3 cells increased phosphorylation of MBS and MLC. Thus, Rho appears to inhibit myosin phosphatase through the action of Rho-kinase.

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.

Patterns of spontaneous motility in videomicrographs of human epidermal keratinocytes (HEK)

The subject of our observations was the spontaneous behaviour of normal and transfected human epidermal keratinocytes. Cell movements were recorded on video micrographs and analyzed by a mathematical approach, using new methods of image processing and statistical correlation analysis. Protrusive activity of single lamellae was examined using one-dimensional analysis of phase-contrast image sequences along section lines transversal to the cell edge. This method revealed high periodicity and correlation in the motility patterns of lamellae and ruffles. Two-dimensional correlation analysis of automatically digitized cell outlines was applied to detect spatiotemporal patterns and coordination of lamellar extension and retraction. Most cells showed regularly alternating pulsations of lamellar protrusions. In some extreme cases, extension waves rotating around the cell periphery were observed. The results were compared with computer simulations of two simple models for lamellar dynamics and shape deformation, based on few assumptions about chemical kinetics of F-actin and cytomechanical properties of the actin network, neglecting regulatory effects of actin-associated proteins or extracellular stimulations. The simulation results reproduced the main dynamical features of the observed real cells, indicating the possibility that the basic universal mechanism for lateral coordination of lamellipodial protrusion is the interplay between hydrostatic pressure and viscocontractile tension in the cortical F-actin-plasma membrane complex.