Actin machinery: pushing the envelope

Early molecular events in the assembly of matrix adhesions at the leading edge of migrating cells

Cellular locomotion is driven by repeated cycles of protrusion of the leading edge, formation of new matrix adhesions and retraction of the trailing edge. In this study we addressed the molecular composition and dynamics of focal complexes, formed under the leading lamellae of motile cells, and their maturation into focal adhesions. We combined phase-contrast and fluorescence microscopy approaches to monitor the incorporation of phosphotyrosine and nine different focal adhesion proteins into focal complexes in endothelial cells, migrating into an in vitro 'wound'. We show that newly formed complexes are located posterior to an actin-, VASP- and alpha-actinin-rich region in the lammelipodium. They are highly tyrosine phosphorylated, contain beta3-integrin, talin, paxillin and low levels of vinculin and FAK, but are apparently devoid of zyxin and tensin. The recruitment of these proteins into focal complexes occurs sequentially, so that their specific protein composition depends on their age. Interestingly, double color, time-lapse movies visualizing both paxillin and zyxin, indicated that the transition from paxillin-rich focal complexes to definitive, zyxin-containing focal adhesions, takes place only after the leading edge stops advancing or retracts. These observations illuminate, for the first time, early stages in focal complex assembly and the dynamic process associated with its transformation into focal adhesion.

Unconventional myosin Myo1c promotes membrane fusion in a regulated exocytic pathway

Glucose homeostasis is controlled in part by regulation of glucose uptake into muscle and adipose tissue. Intracellular membrane vesicles containing the GLUT4 glucose transporter move towards the cell cortex in response to insulin and then fuse with the plasma membrane. Here we show that the fusion step is retarded by the inhibition of phosphatidylinositol (PI) 3-kinase. Treatment of insulin-stimulated 3T3-L1 adipocytes with the PI 3-kinase inhibitor LY294002 causes the accumulation of GLUT4-containing vesicles just beneath the cell surface. This accumulation of GLUT4-containing vesicles near the plasma membrane prior to fusion requires an intact cytoskeletal network and the unconventional myosin motor Myo1c. Remarkably, enhanced Myo1c expression under these conditions causes extensive membrane ruffling and overrides the block in membrane fusion caused by LY294002, restoring the display of GLUT4 on the cell exterior. Ultrafast microscopic analysis revealed that insulin treatment leads to the mobilization of GLUT4-containing vesicles to these regions of Myo1c-induced membrane ruffles. Thus, localized membrane remodeling driven by the Myo1c motor appears to facilitate the fusion of exocytic GLUT4-containing vesicles with the adipocyte plasma membrane.

Experimental Evidence for the Limiting Role of Enzymatic Reactions in Chemoattractant-induced Pseudopod Extension in Human Neutrophils

Chemoattractant-stimulated pseudopod growth in human neutrophils was used as a model system to study the rate-limiting mechanism of cytoskeleton rearrangement induced by activated G-protein-coupled receptors. Cells were activated with N-formyl-Met-Leu-Phe, and the temperature dependence of the rate of pseudopod extension was measured in the presence of pharmacological inhibitors with known mechanisms of action. Three groups of inhibitors were used: (i) inhibitors sequestering substrates involved in F-actin polymerization (latrunculin A for G-actin and cytochalasin D for actin filament-free barbed ends) or sequestering secondary messengers (PIP-binding peptide for phosphoinositide lipids); (ii) competitively binding inhibitors (Akt-inhibitor for Akt/protein kinase B); and (iii) inhibitors that reduce enzyme activity (wortmannin for phosphoinositide 3-kinase and chelerythrine for protein kinase C). The experimental data are consistent with a model in which the relative involvement of a given pathway of F-actin polymerization to the measured rate of pseudopod extension is limited by a slowest (bottleneck) reaction in the cascade of reactions involved in the overall signaling pathway. The approach we developed was used to demonstrate that chemoattractant-induced pseudopod growth and mechanically stimulated cytoskeleton rearrangement are controlled by distinct pathways of F-actin polymerization.

Assembly and reorientation of stress fibers drives morphological changes to endothelial cells exposed to shear stress

Fluid shear stress greatly influences the biology of vascular endothelial cells and the pathogenesis of atherosclerosis. Endothelial cells undergo profound shape change and reorientation in response to physiological levels of fluid shear stress. These morphological changes influence cell function; however, the processes that produce them are poorly understood. We have examined how actin assembly is related to shear-induced endothelial cell shape change. To do so, we imposed physiological levels of shear stress on cultured endothelium for up to 96 hours and then permeabilized the cells and exposed them briefly to fluorescently labeled monomeric actin at various time points to assess actin assembly. Alternatively, monomeric actin was microinjected into cells to allow continuous monitoring of actin distribution. Actin assembly occurred primarily at the ends of stress fibers, which simultaneously reoriented to the shear axis, frequently fused with neighboring stress fibers, and ultimately drove the poles of the cells in the upstream and/or downstream directions. Actin polymerization occurred where stress fibers inserted into focal adhesion complexes, but usually only at one end of the stress fiber. Neither the upstream nor downstream focal adhesion complex was preferred. Changes in actin organization were accompanied by translocation and remodeling of cell-substrate adhesion complexes and transient formation of punctate cell-cell adherens junctions. These findings indicate that stress fiber assembly and realignment provide a novel mode by which cell morphology is altered by mechanical signals.

Forces generated during actin-based propulsion: a direct measurement by micromanipulation

Dynamic actin networks generate forces for numerous types of movements such as lamellipodia protrusion or the motion of endocytic vesicles. The actin-based propulsive movement of Listeria monocytogenes or of functionalized microspheres have been extensively used as model systems to identify the biochemical components that are necessary for actin-based motility. However, quantitative force measurements are required to elucidate the mechanism of force generation, which is still under debate. To directly probe the forces generated in the process of actin-based propulsion, we developed a micromanipulation experiment. A comet growing from a coated polystyrene bead is held by a micropipette while the bead is attached to a force probe, by using a specially designed "flexible handle." This system allows us to apply both pulling and pushing external forces up to a few nanonewtons. By pulling the actin tail away from the bead at high speed, we estimate the elastic modulus of the gel and measure the force necessary to detach the tail from the bead. By applying a constant force in the range of -1.7 to 4.3 nN, the force-velocity relation is established. We find that the relation is linear for pulling forces and decays more weakly for pushing forces. This behavior is explained by using a dimensional elastic analysis.

Cascade pathway of filopodia formation downstream of SCAR

The protrusion of two distinct actin-containing organelles, lamellipodia and filopodia, is thought to be regulated by two parallel pathways: from Rac1 through Scar/WAVEs to lamellipodia, and from Cdc42 through N-WASP to filopodia. We tested this hypothesis in Drosophila, which contains a single gene for each WASP subfamilies, SCAR and WASp. We performed targeted depletion of SCAR or WASp by dsRNA-mediated interference in two Drosophila cultured cell lines expressing lamellipodial and filopodial protrusion. Knockdown was verified by laser capture microdissection and RT-PCR, as well as western blotting. Morphometrical, kinetic and electron microscopy analyses of the SCAR-depleted phenotype in both cell types revealed strong inhibition of lamellipodial formation and cell spreading, as expected. More importantly, filopodia formation was also strongly inhibited, which is not consistent with the parallel pathway hypothesis. By contrast, depletion of WASp did not produce any significant phenotype, except for a slight inhibition of spreading, showing that both lamellipodia and filopodia in Drosophila cells are regulated predominantly by SCAR. We propose a new, cascade pathway model of filopodia regulation in which SCAR signals to lamellipodia and then filopodia arise from lamellipodia in response to additional signal(s).

Variations on a theme: the many modes of cytokinesis

Animal cell division is believed to be mediated primarily by the 'purse-string' mechanism, which entails furrowing of the equatorial region, driven by the interaction of actin and myosin II filaments within contractile rings. However, myosin II-null Dictyostelium cells on substrates divide efficiently in a cell cycle-coupled manner. This process, termed cytokinesis B, appears to be driven by polar traction forces. Data in the literature can be interpreted as suggesting that adherent higher animal cells also use a cytokinesis B-like mechanism for cytokinesis. An additional chemotaxis-based cytokinesis that involves a 'midwife' cell has also been reported. Collectively, these findings demonstrate an unexpected diversity of mechanisms by which animal cells carry out cytokinesis.

Vinculin, VASP, and profilin are coordinately regulated during actin remodeling in epithelial cells, which requires de novo protein synthesis and protein kinase signal transduction pathways

Transformation progression of epithelial cells involves alterations in their morphology, polarity, and adhesive characteristics, all of which are associated with the loss and/or reorganization of actin structures. To identify the underlying mechanism of formation of the adhesion-dependent, circumferential actin network, the expression and localization of the actin binding and regulating proteins (ABPs), vinculin, VASP, and profilin were evaluated. Experimental depolarization of epithelial cells results in the loss of normal F-actin structures and the transient upregulation of vinculin, VASP, and profilin. This response is due to the loss of cell-cell, and not cell-substrate interactions, since cells that no longer express focal adhesions or stress fibers are still sensitive to changes in adhesion and manifest this in the altered profile of expression of these ABPs. Transient upregulation is dependent upon de novo protein synthesis, and protein kinase-, but not phosphatase-sensitive signal transduction pathway(s). Inhibition of the synthesis of these proteins is accompanied by dephosphorylation of the ribosomal S6 protein, but does not involve inhibition of the PI3-kinase-Akt-mTOR pathway. Constitutive expression of VASP results in altered cell morphology and adhesion and F-actin and vinculin structures. V12rac1 expressing epithelial cells are constitutively nonadhesive, malignantly transformed, and constitutively express high levels of these ABPs, with altered subcellular localizations. Transformation suppression is accompanied by the restoration of normal levels of the three ABPs, actin structures, adhesion, and epithelial morphology. Thus, vinculin, VASP, and profilin are coordinately regulated by signal transduction pathways that effect a translational response. Additionally, their expression profile maybe indicative of the adhesion and transformation status of epithelial cells.

Multiple Parallelisms in Animal Cytokinesis

The process of cytokinesis in animal cells is usually presented as a relatively simple picture: A cleavage plane is first positioned in the equatorial region by the astral microtubules of the anaphase mitotic apparatus, and a contractile ring made up of parallel filaments of actin and myosin II is formed and encircles the cortex at the division site. Active sliding between the two filament systems constricts the perimeter of the cortex, leading to separation of two daughter cells. However, recent studies in both animal cells and lower eukaryotic model organisms have demonstrated that cytokinesis is actually far more complex. It is now obvious that the three key processes of cytokinesis, cleavage plane determination, equatorial furrowing, and scission, are driven by different mechanisms in different types of cells. In some cases, moreover, multiple pathways appear to have redundant functions in a single cell type. In this review, we present a novel hypothesis that incorporates recent observations on the activities of mitotic microtubules and the biochemistry of Rho-type GTPase proteins and postulates that two different sets of microtubules are responsible for the two known mechanisms of cleavage plane determination and also for two distinct mechanisms of equatorial furrowing.

Cytoskeletal remodeling in leukocyte function

PURPOSE OF REVIEW

This review focuses on recent developments in understanding the roles and regulation of the cytoskeleton in the function of leukocytes.

RECENT FINDINGS

New studies have shed light on the regulation and dynamics of actin and microtubules in leukocytes relevant both to cell motility generally and to immune function specifically. The roles of cytoskeletal dynamics in processes such as cell activation, cell migration, and phagocytosis are being elucidated. Dramatic progress has been made recently in understanding the mechanisms of leukocyte directional sensing, polarization, and chemotaxis.

SUMMARY

Leukocytes need to be activated, polarize, change shape, move, or phagocytose in response to their environment. Leukocytes accomplish these processes by remodeling their cytoskeleton, the active musculoskeletal system of the cell that is not just the ultimate effector of motile responses but is also a dynamic framework for subcellular organization and regional signaling. Active areas of research include the direct and indirect reciprocal interactions between the cytoskeleton and the membrane and among cytoskeletal elements. The pervasive and multi-layered roles played by small GTPases of the Rho family and phosphoinositides in leukocyte function are also becoming clearer.

Reciprocal interactions between cells and extracellular matrix during remodeling of tissue constructs

Cells remodel extracellular matrix during tissue development and wound healing. Similar processes occur when cells compress and stiffen collagen gels. An important task for cell biologists, biophysicists, and tissue engineers is to guide these remodeling processes to produce tissue constructs that mimic the structure and mechanical properties of natural tissues. This requires an understanding of the mechanisms by which this remodeling occurs. Quantitative measurements of the contractile force developed by cells and the extent of compression and stiffening of the matrix describe the results of the remodeling processes. Not only do forces exerted by cells influence the structure of the matrix but also external forces exerted on the matrix can modulate the structure and orientation of the cells. The mechanisms of these processes remain largely unknown, but recent studies of the regulation of myosin-dependent contractile force and of cell protrusion driven by actin polymerization provide clues about the regulation of cellular functions during remodeling.

Dissection of theAscarisSperm Motility Machinery Identifies Key Proteins Involved in Major Sperm Protein-based Amoeboid Locomotion

Although Ascaris sperm motility closely resembles that seen in many other types of crawling cells, the lamellipodial dynamics that drive movement result from modulation of a cytoskeleton based on the major sperm protein (MSP) rather than actin. The dynamics of the Ascaris sperm cytoskeleton can be studied in a cell-free in vitro system based on the movement of plasma membrane vesicles by fibers constructed from bundles of MSP filaments. In addition to ATP, MSP, and a plasma membrane protein, reconstitution of MSP motility in this cell-free extract requires cytosolic proteins that orchestrate the site-specific assembly and bundling of MSP filaments that generates locomotion. Here, we identify a fraction of cytosol that is comprised of a small number of proteins but contains all of the soluble components required to assemble fibers. We have purified two of these proteins, designated MSP fiber proteins (MFPs) 1 and 2 and demonstrated by immunolabeling that both are located in the MSP cytoskeleton in cells and in fibers. These proteins had reciprocal effects on fiber assembly in vitro: MFP1 decreased the rate of fiber growth, whereas MFP2 increased the growth rate.

Modulation of epithelial tubule formation by Rho kinase

We have developed a model system for studying integrin regulation of mammalian epithelial tubule formation. Application of collagen gel overlays to Madin-Darby canine kidney (MDCK) cells induced coordinated disassembly of junctional complexes that was accompanied by lamellipodia formation and cell rearrangement (termed epithelial remodeling). In this study, we present evidence that the Rho signal transduction pathway regulates epithelial remodeling and tubule formation. Incubation of MDCK cells with collagen gel overlays facilitated formation of migrating lamellipodia with membrane-associated actin. Inhibitors of myosin II and actin prevented lamellipodia formation, which suggests that actomyosin function was involved in regulation of epithelial remodeling. To determine this, changes in myosin II distribution, function, and phosphorylation were studied during epithelial tubule biogenesis. Myosin II colocalized with actin at the leading edge of lamellipodia thereby providing evidence that myosin is important in epithelial remodeling. This possibility is supported by observations that inhibition of Rho kinase, a regulator of myosin II function, alters formation of lamellipodia and results in attenuated epithelial tubule development. These data and those demonstrating myosin regulatory light-chain phosphorylation at the leading edge of lamellipodia strongly suggest that Rho kinase and myosin II are important modulators of epithelial remodeling. They support a hypothesis that the Rho signal transduction pathway plays a significant role in regulation of epithelial tubule formation.

Regulated exocytosis in neuroendocrine cells: a role for subplasmalemmal Cdc42/N-WASP-induced actin filaments

In neuroendocrine cells, actin reorganization is a prerequisite for regulated exocytosis. Small GTPases, Rho proteins, represent potential candidates coupling actin dynamics to membrane trafficking events. We previously reported that Cdc42 plays an active role in regulated exocytosis in chromaffin cells. The aim of the present work was to dissect the molecular effector pathway integrating Cdc42 to the actin architecture required for the secretory reaction in neuroendocrine cells. Using PC12 cells as a secretory model, we show that Cdc42 is activated at the plasma membrane during exocytosis. Expression of the constitutively active Cdc42(L61) mutant increases the secretory response, recruits neural Wiskott-Aldrich syndrome protein (N-WASP), and enhances actin polymerization in the subplasmalemmal region. Moreover, expression of N-WASP stimulates secretion by a mechanism dependent on its ability to induce actin polymerization at the cell periphery. Finally, we observed that actin-related protein-2/3 (Arp2/3) is associated with secretory granules and that it accompanies granules to the docking sites at the plasma membrane upon cell activation. Our results demonstrate for the first time that secretagogue-evoked stimulation induces the sequential ordering of Cdc42, N-WASP, and Arp2/3 at the interface between granules and the plasma membrane, thereby providing an actin structure that makes the exocytotic machinery more efficient.

Myosin IIA drives neurite retraction

Neuritic extension is the resultant of two vectorial processes: outgrowth and retraction. Whereas myosin IIB is required for neurite outgrowth, retraction is driven by a motor whose identity has remained unknown until now. Preformed neurites in mouse Neuro-2A neuroblastoma cells undergo immediate retraction when exposed to isoform-specific antisense oligonucleotides that suppress myosin IIB expression, ruling out myosin IIB as the retraction motor. When cells were preincubated with antisense oligonucleotides targeting myosin IIA, simultaneous or subsequent addition of myosin IIB antisense oligonucleotides did not elicit neurite retraction, both outgrowth and retraction being curtailed. Even during simultaneous application of antisense oligonucleotides against both myosin isoforms, lamellipodial spreading continued despite the complete inhibition of neurite extension, indicating an uncoupling of lamellipodial dynamics from movement of the neurite. Significantly, lysophosphatidate- or thrombin-induced neurite retraction was blocked not only by the Rho-kinase inhibitor Y27632 but also by antisense oligonucleotides targeting myosin IIA. Control oligonucleotides or antisense oligonucleotides targeting myosin IIB had no effect. In contrast, Y27632 did not inhibit outgrowth, a myosin IIB-dependent process. We conclude that the conventional myosin motor, myosin IIA, drives neurite retraction.

Rac2 specificity in macrophage integrin signaling: potential role for Syk kinase

Herein we report that, despite the similarity of Rac2 to Rac1 (92% amino acid identity), macrophages derived from Rac2-/- mice, which continue to express Rac1, display a marked defect in alphavbeta3/alphavbeta5 and alpha4beta1 integrin-directed migration measured on vitronectin and fibronectin fragments (FN-H296), respectively. In contrast, mouse embryo fibroblasts derived from the Rac2 knockout mice utilize Rac1 for migration via alphavbeta3/alphavbeta5 and alpha4beta1. The genetic reconstitution of bone marrow-derived macrophages (BMM) with Rac2 restores the integrin-dependent migration of Rac2-deficient macrophages on vitronectin (VN) and FN-H296. The levels of GTP-Rac2 generated upon specific integrin engagement in wild type macrophages parallels the phenotypic defect observed in Rac2-deficient macrophages; i.e. FN-H296, alpha4beta1 > VN, alphavbeta3/alphavbeta5 > FN-CH271, alpha5beta1 > intact FN. In a COS7 cell system, the expression of Syk kinase alone is sufficient to convert the alpha4beta1 migration response to Rac2 dependence. Therefore, we present the first evidence that the alpha4beta1 receptor in blood cells has evolved a Syk-Rac2 signaling axis to transmit signals required for integrin-directed migration suggesting that Syk kinase in part encodes myeloid Rac2 specificity in vivo.

Dissection of the Ascaris sperm motility machinery identifies key proteins involved in major sperm protein-based amoeboid locomotion

Although Ascaris sperm motility closely resembles that seen in many other types of crawling cells, the lamellipodial dynamics that drive movement result from modulation of a cytoskeleton based on the major sperm protein (MSP) rather than actin. The dynamics of the Ascaris sperm cytoskeleton can be studied in a cell-free in vitro system based on the movement of plasma membrane vesicles by fibers constructed from bundles of MSP filaments. In addition to ATP, MSP, and a plasma membrane protein, reconstitution of MSP motility in this cell-free extract requires cytosolic proteins that orchestrate the site-specific assembly and bundling of MSP filaments that generates locomotion. Here, we identify a fraction of cytosol that is comprised of a small number of proteins but contains all of the soluble components required to assemble fibers. We have purified two of these proteins, designated MSP fiber proteins (MFPs) 1 and 2 and demonstrated by immunolabeling that both are located in the MSP cytoskeleton in cells and in fibers. These proteins had reciprocal effects on fiber assembly in vitro: MFP1 decreased the rate of fiber growth, whereas MFP2 increased the growth rate.

Association of clathrin, AP-2 adaptor and actin cytoskeleton with developing interlocking membrane domains of lens fibre cells

Interlocking membrane domains are specialized membrane interdigitations in the form of ball-and-sockets and protrusions between lens fibre cells of all species. They are believed to play a key role in maintaining fibre-fibre stability and are therefore, important for normal lens function. Here we report the specific association of the clathrin/AP-2 adaptor complex and the branching F-actin network with the development of interlocking domains in rats and several other species. By thin-section electron microscopy we consistently observed a layer of distinct coating (approximately 25-nm thick) on the concave membrane surface of small and intermediate-sized developing interlocking domains. These membrane coats remarkably resembled the clathrin-coat of endocytic vesicles in which clathrin and the AP-2 adaptor are involved in the induction of coated pit formation during receptor-mediated endocytosis. We hypothesize that the clathrin/AP-2 complex is directly involved in the induction of interlocking domains in fibre cells. By immunoconfocal microscopy, co-labelling of a dotted-pattern of clathrin and AP-2 adaptor antibodies was seen along the cortical fibre cells. Immunoblot analysis further confirmed that clathrin and AP-2 adaptor antibodies specifically stained a polypeptide band of 180 and 106kD, respectively, in the membrane fractions prepared separately from the outer and inner cortical fibres where interlocking domains are abundant but endocytic vesicles are absent. Immunoelectron microscopy showed that the clathrin antibody was localized along the interlocking membrane. In addition, branching actin filament networks were frequently observed within the cytoplasmic compartment of developing interlocking domains by TEM, in consistent with the findings by fluorescence and immunogold labelling of the F-actin antibody in the domains. These results demonstrate for the first time that the clathrin/AP-2 complex plays a new role for the formation of interlocking domains in lens fibre cells. Branching actin networks and possibly other cytoskeletal components are also associated with the development and maintenance of these interlocking domains. The coordinated 'pulling and pushing' actions generated by the clathrin/AP-2 complex and branching actin networks during interlocking domain formation are discussed.

Orientational order of the lamellipodial actin network as demonstrated in living motile cells

Lamellipodia of crawling cells represent both the motor for cell advance and the primary building site for the actin cytoskeleton. The organization of actin in the lamellipodium reflects actin dynamics and is of critical importance for the mechanism of cell motility. In previous structural studies, the lamellipodial actin network was analyzed primarily by electron microscopy (EM). An understanding of lamellipodial organization would benefit significantly if the EM data were complemented and put into a kinetic context by establishing correspondence with structural features observable at the light microscopic level in living cells. Here, we use an enhanced phase contrast microscopy technique to visualize an apparent long-range diagonal actin meshwork in the advancing lamellipodia of living cells. Visualization of this meshwork permitted a correlative light and electron microscopic approach that validated the underlying organization of lamellipodia. The linear features in the light microscopic meshwork corresponded to regions of greater actin filament density. Orientation of features was analyzed quantitatively and compared with the orientation of actin filaments at the EM level. We infer that the light microscopic meshwork reflects the orientational order of actin filaments which, in turn, is related to their branching angle.

Biomimetic Systems for Studying Actin-Based Motility

Actin polymerization provides a major driving force for eukaryotic cell motility. Successive intercalation of monomeric actin subunits between the plasma membrane and the filamentous actin network results in protrusions of the membrane enabling the cell to move or to change shape. One of the challenges in understanding eukaryotic cell motility is to dissect the elementary biochemical and biophysical steps that link actin polymerization to mechanical force generation. Recently, significant progress was made using biomimetic, in vitro systems that are inspired by the actin-based motility of bacterial pathogens such as Listeria monocytogenes. Polystyrene microspheres and synthetic phospholipid vesicles coated with proteins that initiate actin polymerization display motile behavior similar to Listeria, mimicking the leading edge of lamellipodia and filopodia. A major advantage of these biomimetic systems is that both biochemical and physical parameters can be controlled precisely. These systems provide a test bed for validating theoretical models on force generation and polarity establishment resulting from actin polymerization. In this review, we discuss recent experimental progress using biomimetic systems propelled by actin polymerization and discuss these results in the light of recent theoretical models on actin-based motility.

Kette regulates actin dynamics and genetically interacts with Wave and Wasp

During development of the Drosophila nervous system, kette is required for axonal growth and pathfinding. It encodes a highly conserved homolog of the Nck-associated protein 1 (NAP1) that genetically interacts with the Drosophila homolog of Nck, dock. We show that in vivo as well as in tissue culture models most of the Kette protein is found in the cytoplasm where it colocalizes with F-actin to which it can bind via its N-terminal domain. Some Kette protein is localized at the membrane and accumulates at focal contact sites. Loss of Kette protein results in the accumulation of cytosolic F-actin. The kette mutant phenotype can be suppressed by reducing the wave gene dose, demonstrating that kette antagonizes wave function. Overexpression of the wild-type Kette protein does not interfere with normal development, whereas expression of an activated, membrane-tethered Kette protein induces the formation of large F-actin bundles in both, tissue culture cells and in vivo. This gain-of-function phenotype is independent of wave but can be suppressed by reducing the wasp gene dose, indicating that Kette is able to regulate Wasp, to which it is linked via the Abelson interactor (Abi). Our data suggest a model where Kette fulfils a novel role in regulating F-actin organization by antagonizing Wave and activating Wasp-dependent actin polymerization.

Analysis of actin dynamics at the leading edge of crawling cells: implications for the shape of keratocyte lamellipodia

Leading edge protrusion is one of the critical events in the cell motility cycle and it is believed to be driven by the assembly of the actin network. The concept of dendritic nucleation of actin filaments provides a basis for understanding the organization and dynamics of the actin network at the molecular level. At a larger scale, the dynamic geometry of the cell edge has been described in terms of the graded radial extension model, but this level of description has not yet been linked to the molecular dynamics. Here, we measure the graded distribution of actin filament density along the leading edge of fish epidermal keratocytes. We develop a mathematical model relating dendritic nucleation to the long-range actin distribution and the shape of the leading edge. In this model, a steady-state graded actin distribution evolves as a result of branching, growth and capping of actin filaments in a finite area of the leading edge. We model the shape of the leading edge as a product of the extension of the actin network, which depends on actin filament density. The feedback between the actin density and edge shape in the model results in a cell shape and an actin distribution similar to those experimentally observed. Thus, we explain the stability of the keratocyte shape in terms of the self-organization of the branching actin network.

Cortactin tyrosine phosphorylation requires Rac1 activity and association with the cortical actin cytoskeleton

Cortactin is an F-actin binding protein that activates actin-related protein 2/3 complex and is localized within lamellipodia. Cortactin is a substrate for Src and other protein tyrosine kinases involved in cell motility, where its phosphorylation on tyrosines 421, 466, and 482 in the carboxy terminus is required for cell movement and metastasis. In spite of the importance of cortactin tyrosine phosphorylation in cell motility, little is known regarding the structural, spatial, or signaling requirements regulating cortactin tyrosine phosphorylation. Herein, we report that phosphorylation of cortactin tyrosine residues in the carboxy terminus requires the aminoterminal domain and Rac1-mediated localization to the cell periphery. Phosphorylation-specific antibodies directed against tyrosine 421 and 466 were produced to study the regulation and localization of tyrosine phosphorylated cortactin. Phosphorylation of cortactin tyrosine 421 and 466 was elevated in response to Src, epidermal growth factor receptor and Rac1 activation, and tyrosine 421 phosphorylated cortactin localized with F-actin in lamellipodia and podosomes. Cortactin tyrosine phosphorylation is progressive, with tyrosine 421 phosphorylation required for phosphorylation of tyrosine 466. These results indicate that cortactin tyrosine phosphorylation requires Rac1-induced cortactin targeting to cortical actin networks, where it is tyrosine phosphorylated in hierarchical manner that is closely coordinated with its ability to regulate actin dynamics.

A conserved amphipathic helix in WASP/Scar proteins is essential for activation of Arp2/3 complex

Members of the Wiskott-Aldrich syndrome protein (WASP) family link Rho GTPase signaling pathways to the cytoskeleton through a multiprotein assembly called Arp2/3 complex. The C-terminal VCA regions (verprolin-homology, central hydrophobic, and acidic regions) of WASP and its relatives stimulate Arp2/3 complex to nucleate actin filament branches. Here we show by differential line broadening in NMR spectra that the C (central) and A (acidic) segments of VCA domains from WASP, N-WASP and Scar bind Arp2/3 complex. The C regions of these proteins have a conserved sequence motif consisting of hydrophobic residues and an arginine residue. Point mutations in this conserved sequence motif suggest that it forms an amphipathic helix that is required in biochemical assays for activation of Arp2/3 complex. Key residues in this motif are buried through contacts with the GTPase binding domain in the autoinhibited structure of WASP and N-WASP, indicating that sequestration of these residues is an important aspect of autoinhibition.

p116Rip is a novel filamentous actin-binding protein

p116Rip is a ubiquitously expressed protein that was originally identified as a putative binding partner of RhoA in a yeast two-hybrid screen. Overexpression of p116Rip in neuroblastoma cells inhibits RhoA-mediated cell contraction induced by lysophosphatidic acid (LPA); so far, however, the function of p116Rip is unknown. Here we report that p116Rip localizes to filamentous actin (F-actin)-rich structures, including stress fibers and cortical microfilaments, in both serum-deprived and LPA-stimulated cells, with the N terminus (residues 1-382) dictating cytoskeletal localization. In addition, p116Rip is found in the nucleus. Direct interaction or colocalization with RhoA was not detected. We find that p116Rip binds tightly to F-actin (Kd approximately 0.5 microm) via its N-terminal region, while immunoprecipitation assays show that p116Rip is complexed to both F-actin and myosin-II. Purified p116Rip and the F-actin-binding region can bundle F-actin in vitro, as shown by electron microscopy. When overexpressed in NIH3T3 cells, p116Rip disrupts stress fibers and promotes formation of dendrite-like extensions through its N-terminal actin-binding domain; furthermore, overexpressed p116Rip inhibits growth factor-induced lamellipodia formation. Our results indicate that p116Rip is an F-actin-binding protein with in vitro bundling activity and in vivo capability of disassembling the actomyosin-based cytoskeleton.

Sequential interaction of actin-related proteins 2 and 3 (Arp2/3) complex with neural Wiscott-Aldrich syndrome protein (N-WASP) and cortactin during branched actin filament network formation

The WASP and cortactin families constitute two distinct classes of Arp2/3 modulators in mammalian cells. Physical and functional interactions among the Arp2/3 complex, VCA (a functional domain of N-WASP), and cortactin were examined under conditions that were with or without actin polymerization. In the absence of actin, cortactin binds significantly weaker to the Arp2/3 complex than VCA. At concentrations of VCA 20-fold lower than cortactin, the association of cortactin with the Arp2/3 complex was nearly abolished. Analysis of the cells infected with Shigella demonstrated that N-WASP located at the tip of the bacterium, whereas cortactin accumulated in the comet tail. Interestingly, cortactin promotes Arp2/3 complex-mediated actin polymerization and actin branching in the presence of VCA at a saturating concentration, and cortactin acquired 20 nm affinity for the Arp2/3 complex during actin polymerization. The interaction of VCA with the Arp2/3 complex was reduced in the presence of both cortactin and actin. Moreover, VCA reduced its affinity for Arp2/3 complex at branching sites that were stabilized by phalloidin. These data imply a novel mechanism for the de novo assembly of a branched actin network that involves a coordinated sequential interaction of N-WASP and cortactin with the Arp2/3 complex.

The first three minutes: smooth muscle contraction, cytoskeletal events, and soft glasses

Smooth muscle exhibits biophysical characteristics and physiological behaviors that are not readily explained by present paradigms of cytoskeletal and cross-bridge mechanics. There is increasing evidence that contractile activation of the smooth muscle cell involves an array of cytoskeletal processes that extend beyond cross-bridge cycling and the sliding of thick and thin filaments. We review here the evidence suggesting that the biophysical and mechanical properties of the smooth muscle cell reflect the integrated interactions of an array of highly dynamic cytoskeletal processes that both react to and transform the dynamics of cross-bridge interactions over the course of the contraction cycle. The activation of the smooth muscle cell is proposed to trigger dynamic remodeling of the actin filament lattice within cellular microdomains in response to local mechanical and pharmacological events, enabling the cell to adapt to its external environment. As the contraction progresses, the cytoskeletal lattice stabilizes, solidifies, and forms a rigid structure well suited for transmission of tension generated by the interaction of myosin and actin. The integrated molecular transitions that occur within the contractile cycle are interpreted in the context of microscale agitation mechanisms and resulting remodeling events within the intracellular microenvironment. Such an interpretation suggests that the cytoskeleton may behave as a glassy substance whose mechanical function is governed by an effective temperature.

A 48 kDa integral membrane phosphoprotein orchestrates the cytoskeletal dynamics that generate amoeboid cell motility in Ascaris sperm

Protrusion of the lamellipod in the crawling sperm of Ascaris is tightly coupled to the localized vectorial assembly and bundling of the major sperm protein cytoskeleton. In cell-free extracts of sperm, vesicles derived from the leading edge membrane reconstitute protrusion by directing the assembly of columnar meshworks of major sperm protein filaments that push the vesicle forward as they elongate. Treatment with proteases or a tyrosine phosphatase abolished vesicle activity, suggesting the involvement of a membrane phosphoprotein. Fractionation of vesicle proteins by sequential detergent lysis, size exclusion chromatography and immunoprecipitation with antiphosphotyrosine antibody identified a 48 kDa integral membrane phosphoprotein as the only sperm membrane component required to nucleate major sperm protein polymerization under physiological conditions. Immunolabeling assays showed that this protein is distributed uniformly in the sperm plasma membrane, but that its active phosphorylated form is located only at sites of major sperm protein polymerization at the leading edge. Because this protein specifies sites of cytoskeletal assembly, we have named it major sperm protein polymerization organizing protein (MPOP). The phosphorylation of MPOP is pH sensitive and appears to require a soluble tyrosine kinase. Comparison of the activity of MPOP to that of analogous membrane proteins in actin-based systems emphasizes the importance of precise transmission of information from the membrane to the cytoskeleton in amoeboid cell motility.

A 48 kDa integral membrane phosphoprotein orchestrates the cytoskeletal dynamics that generate amoeboid cell motility in Ascaris sperm

Protrusion of the lamellipod in the crawling sperm of Ascaris is tightly coupled to the localized vectorial assembly and bundling of the major sperm protein cytoskeleton. In cell-free extracts of sperm, vesicles derived from the leading edge membrane reconstitute protrusion by directing the assembly of columnar meshworks of major sperm protein filaments that push the vesicle forward as they elongate. Treatment with proteases or a tyrosine phosphatase abolished vesicle activity, suggesting the involvement of a membrane phosphoprotein. Fractionation of vesicle proteins by sequential detergent lysis, size exclusion chromatography and immunoprecipitation with antiphosphotyrosine antibody identified a 48 kDa integral membrane phosphoprotein as the only sperm membrane component required to nucleate major sperm protein polymerization under physiological conditions. Immunolabeling assays showed that this protein is distributed uniformly in the sperm plasma membrane, but that its active phosphorylated form is located only at sites of major sperm protein polymerization at the leading edge. Because this protein specifies sites of cytoskeletal assembly, we have named it major sperm protein polymerization organizing protein (MPOP). The phosphorylation of MPOP is pH sensitive and appears to require a soluble tyrosine kinase. Comparison of the activity of MPOP to that of analogous membrane proteins in actin-based systems emphasizes the importance of precise transmission of information from the membrane to the cytoskeleton in amoeboid cell motility.

Dendritic cell regulation of immune responses: a new role for interleukin 2 at the intersection of innate and adaptive immunity

Dendritic cells are professional antigen-presenting cells able to initiate innate and adaptive immune responses against invading pathogens. In response to external stimuli dendritic cells undergo a complete genetic reprogramming that allows them to become, soon after activation, natural killer cell activators and subsequently T cell stimulators. The recent observation that dendritic cells produce interleukin 2 following microbial stimulation opens new possibilities for understanding the efficiency of dendritic cells in regulating immune system functions. This review discusses how dendritic cells control natural killer, T- and B-cell responses and the relevance of interleukin 2 in these processes.

The mechanisms of pollination and fertilization in plants

In flowering plants, pollen grains germinate to form pollen tubes that transport male gametes (sperm cells) to the egg cell in the embryo sac during sexual reproduction. Pollen tube biology is complex, presenting parallels with axon guidance and moving cell systems in animals. Pollen tube cells elongate on an active extracellular matrix in the style, ultimately guided by stylar and embryo sac signals. A well-documented recognition system occurs between pollen grains and the stigma in sporophytic self-incompatibility, where both receptor kinases in the stigma and their peptide ligands from pollen are now known. Complex mechanisms act to precisely target the sperm cells into the embryo sac. These events initiate double fertilization in which the two sperm cells from one pollen tube fuse to produce distinctly different products: one with the egg to produce the zygote and embryo and the other with the central cell to produce the endosperm.

Activation of Gbetagamma signaling downstream of Wnt-11/Xfz7 regulates Cdc42 activity during Xenopus gastrulation

Wnt-11/Xfz7 signaling plays a major role in the regulation of convergent extension movements affecting the dorsal marginal zone (DMZ) of gastrulating Xenopus embryos. In order to provide data concerning the molecular targets of Wnt-11/Xfz7 signals, we have analyzed the regulation of the Rho GTPase Cdc42 by Wnt-11. In animal cap ectoderm, Cdc42 activity increases as a response to Wnt-11 expression. This increase is inhibited by pertussis toxin, or sequestration of free Gbetagamma subunits by exogenous Galphai2 or Galphat. Activation of Cdc42 is also produced by the expression of bovine Gbeta1 and Ggamma2. This process is abolished by a PKC inhibitor, while phorbol esther treatment of ectodermal explants activates Cdc42 in a PKC-dependent way, implicating PKC downstream of Gbetagamma. In activin-treated animal caps and in the embryo, interference with Gbetagamma signaling rescues morphogenetic movements inhibited by Wnt-11 hyperactivation, thus phenocopying the dominant negative version of Cdc42 (N(17)Cdc42). Conversely, expression of Gbeta1gamma2 blocks animal cap elongation. This effect is reversed by N(17)Cdc42. Together, our results strongly argue for a role of Gbetagamma signaling in the regulation of Cdc42 activity downstream of Wnt-11/Xfz7 in mesodermal cells undergoing convergent extension. This idea is further supported by the observation that expression of Galphat in the DMZ causes severe gastrulation defects.

Contingent phosphorylation/dephosphorylation provides a mechanism of molecular memory in WASP

Cells can retain information about previous stimuli to produce distinct future responses. The biochemical mechanisms by which this is achieved are not well understood. The Wiskott-Aldrich syndrome protein (WASP) is an effector of the Rho-family GTPase Cdc42, whose activation leads to stimulation of the actin nucleating assembly, Arp2/3 complex. We demonstrate that efficient phosphorylation and dephosphorylation of WASP at Y291 are both contingent on binding to activated Cdc42. Y291 phosphorylation increases the basal activity of WASP toward Arp2/3 complex and enables WASP activation by new stimuli, SH2 domains of Src-family kinases. The requirement for contingency in both phosphorylation and dephosphorylation enables long-term storage of information by WASP following decay of GTPase signals. This biochemical circuitry allows WASP to respond to the levels and timing of GTPase and kinase signals. It provides mechanisms to specifically achieve transient or persistent actin remodeling, as well as long-lasting potentiation of actin-based responses to kinases.

Pointed-end capping by tropomodulin3 negatively regulates endothelial cell motility

Actin filament pointed-end dynamics are thought to play a critical role in cell motility, yet regulation of this process remains poorly understood. We describe here a previously uncharacterized tropomodulin (Tmod) isoform, Tmod3, which is widely expressed in human tissues and is present in human microvascular endothelial cells (HMEC-1). Tmod3 is present in sufficient quantity to cap pointed ends of actin filaments, localizes to actin filament structures in HMEC-1 cells, and appears enriched in leading edge ruffles and lamellipodia. Transient overexpression of GFP-Tmod3 leads to a depolarized cell morphology and decreased cell motility. A fivefold increase in Tmod3 results in an equivalent decrease in free pointed ends in the cells. Unexpectedly, a decrease in the relative amounts of F-actin, free barbed ends, and actin-related protein 2/3 (Arp2/3) complex in lamellipodia are also observed. Conversely, decreased expression of Tmod3 by RNA interference leads to faster average cell migration, along with increases in free pointed and barbed ends in lamellipodial actin filaments. These data collectively demonstrate that capping of actin filament pointed ends by Tmod3 inhibits cell migration and reveal a novel control mechanism for regulation of actin filaments in lamellipodia.

Probing polymerization forces by using actin-propelled lipid vesicles

Actin polymerization provides a powerful propulsion force for numerous types of cell motility. Although tremendous progress has been made in identifying the biochemical components necessary for actin-based motility, the precise biophysical mechanisms of force generation remain unclear. To probe the polymerization forces quantitatively, we introduce an experimental system in which lipid vesicles coated with the Listeria monocytogenes virulence factor ActA are propelled by actin polymerization. The polymerization forces cause significant deformations of the vesicle. We have used these deformations to obtain a spatially resolved measure of the forces exerted on the membrane using a model based on the competition between osmotic pressure and membrane stretching. Our results indicate that actin exerts retractile or propulsive forces depending on the local membrane curvature and that the membrane is strongly bound to the actin gel. These results are consistent with the observed dynamics. After a slow elongation of the vesicle from a spherical shape, the strong bonds between the actin gel and the membrane rupture if the retractile forces exceed a critical value, leading to a rapid release of the vesicle's trailing edge.

Microfilament and microtubule organization and dynamics in process extension by central glia-4 oligodendrocytes: evidence for a microtubule organizing center

Microfilaments in freshly adhering CG-4 cells and differentiated CG-4 oligodendrocytes are concentrated at the tips and edges of rapidly forming processes while microtubules are concentrated in new processes and extend from a concentrated spot of alpha-tubulin staining in the cell body to the cell periphery. In motile bipolar CG-4 cells, microfilaments are heavily concentrated at the flattened end of one process and along the rim of processes and the cell body: microtubules are concentrated along main processes and splay out into process tips and the cell body. In differentiated CG-4 oligodendrocytes, microfilaments are concentrated at the many process tips, in filopodia and in fine processes, but are not obvious in main processes where separate bundles of microtubules, which diverge at process branch points, are concentrated. gamma-tubulin, involved in microtubule nucleation, is concentrated at a small discrete area in the cell body, indicative of a microtubule organizing center. Polymerization of both actin and tubulin is required for initial process elaboration. Depolymerization of microtubules, but not of microfilaments, causes complete retraction of bipolar CG-4 cell processes. This process retraction does not occur if microfilaments are depolymerized first, indicating that process extension/retraction in motile bipolar CG-4 cells may occur by a balance of motor protein-driven forces as suggested for growth cone motility. Cytoskeleton organization in CG-4 cells is very similar to that reported for oligodendrocytes. CG-4 cells are thus a useful model for investigating the signals and mechanisms regulating oligodendrocyte process dynamics.

Mechanics of cell spreading: role of myosin II

As it migrates over a substratum, a cell must exert different kinds of forces that act at various cellular locations and at specific times. These forces must therefore be coordinately regulated. The Rho-family GTPases Rac1 and Cdc42 promote actin polymerization that drives extension of the leading cell edge. Subsequently, RhoA regulates myosin-dependent contractile force, which is required for formation of adhesive contacts and stress fibers. During cell spreading, however, the activity of RhoA is reduced by a mechanism involving the tyrosine kinases c-Src and focal adhesion kinase (FAK), and the p190RhoGAP. It has been proposed that this reduction of RhoA activity facilitates edge extension by reducing myosin-dependent contractile forces that could resist this process. We have directly tested this hypothesis by correlating myosin activity with the rate of cell spreading on a substratum. The rate of spreading is inversely related to the myosin activity. Furthermore, spreading is inhibited by low concentrations of cytochalasin D, as expected for a process that depends on the growth of uncapped actin filaments. Cell indentation measurements show that a myosin-dependent viscoelastic force resists cell deformation.

RhoA and ROCK promote migration by limiting membrane protrusions

Previously, we and others have shown that RhoA and ROCK signaling are required for negatively regulating integrin-mediated adhesion and for tail retraction of migrating leukocytes. This study continues our investigation into the molecular mechanisms underlying RhoA/ROCK-regulated integrin adhesion. We show that inhibition of ROCK up-regulates integrin-mediated adhesion, which is accompanied by both increased phosphotyrosine signaling through Pyk-2 and paxillin and inappropriate membrane protrusions. We provide evidence that inhibition of ROCK induces integrin adhesion by promoting remodeling of the actin cytoskeleton. Furthermore, we find that ROCK regulates membrane activity through a pathway involving cofilin. Inhibition of RhoA signaling allows the formation of multiple competing lamellipodia that disrupt productive migration of monocytes. Together, our results show that RhoA/ROCK signaling promotes migration by restricting integrin activity and membrane protrusions to the leading edge.

Probing the cell peripheral movements by optical trapping technique

Swiss 3T3 fibroblasts cultured on a poly-L-lysine-coated coverslip was stimulated with 0.5 micro M phorbol myristate acetate, and the movements of the peripheral membranes were probed with a 1- micro m polystyrene bead held in an optical trap. The bead brought into contact with the cell edge occasionally moved away from and returned to the original position. The movement ranged over 100 nm and occurred mainly in one direction, suggesting that the protruding cell membrane pushed the bead. The maximum velocities derived from individual pairs of protrusive and withdrawal movements exhibited a correlation, which is consistent with the previous reports. Acceleration and deceleration occurred both in the protrusive and withdrawal phases, indicating that the movements were regulated. Movement of the membrane occurred frequently with an ensemble-averaged maximum speed of 23 nm/s at the trap stiffness of 0.024 pN/nm, but it was strongly suppressed when the trap stiffness was increased to 0.090 pN/nm. Correlation of the protrusive and withdrawal velocities and the acceleration and deceleration both in the protrusive and withdrawal phases can be explained by the involvement of myosin motor at least in the withdrawal process. However, the fact that the movements were suppressed at higher trap stiffness implies a stochastic nature in the creation of the gap between the peripheral cell membrane and the actin network underlying it.

PACE-1, a novel protein that interacts with the C-terminal domain of ezrin

The ERM proteins (ezrin, radixin, moesin) together with merlin comprise a subgroup of the band 4.1 superfamily. These proteins act as membrane cytoskeletal linker proteins mediating interactions between the cytoplasmic domains of transmembrane proteins and actin. To better understand how the ERM proteins function to regulate these junctional complexes, a yeast 2-hybrid screen was undertaken using ezrin as a bait. We describe here the identification and cloning of a novel protein, PACE-1, which binds to the C-terminal domain of ezrin. Characterization of PACE-1 in human breast cancer cell lines demonstrates it to have two distinct intracellular localizations. A proportion of the protein is associated with the cytoplasmic face of the Golgi apparatus. This distribution is dependent upon the presence of the PACE-1 N-terminal myristoylation consensus sequence but is not dependent on an association with ezrin. In contrast, PACE-1 colocalises with ezrin in the lamellipodia, where ezrin has a role in cell spreading and motility. A notable feature of PACE-1 is the presence of a putative N-terminal kinase domain; however, in biochemical assays PACE-1 was shown to have associated rather than intrinsic kinase activity. Together these data suggest that PACE-1 may play a role in regulating cell adhesion/migration complexes in migrating cells.

Regulation of sodium channel activity by capping of actin filaments

Ion transport in various tissues can be regulated by the cortical actin cytoskeleton. Specifically, involvement of actin dynamics in the regulation of nonvoltage-gated sodium channels has been shown. Herein, inside-out patch clamp experiments were performed to study the effect of the heterodimeric actin capping protein CapZ on sodium channel regulation in leukemia K562 cells. The channels were activated by cytochalasin-induced disruption of actin filaments and inactivated by G-actin under ionic conditions promoting rapid actin polymerization. CapZ had no direct effect on channel activity. However, being added together with G-actin, CapZ prevented actin-induced channel inactivation, and this effect occurred at CapZ/actin molar ratios from 1:5 to 1:100. When actin was allowed to polymerize at the plasma membrane to induce partial channel inactivation, subsequent addition of CapZ restored the channel activity. These results can be explained by CapZ-induced inhibition of further assembly of actin filaments at the plasma membrane due to the modification of actin dynamics by CapZ. No effect on the channel activity was observed in response to F-actin, confirming that the mechanism of channel inactivation does not involve interaction of the channel with preformed filaments. Our data show that actin-capping protein can participate in the cytoskeleton-associated regulation of sodium transport in nonexcitable cells.

Formation of filopodia-like bundles in vitro from a dendritic network

We report the development and characterization of an in vitro system for the formation of filopodia-like bundles. Beads coated with actin-related protein 2/3 (Arp2/3)-activating proteins can induce two distinct types of actin organization in cytoplasmic extracts: (1) comet tails or clouds displaying a dendritic array of actin filaments and (2) stars with filament bundles radiating from the bead. Actin filaments in these bundles, like those in filopodia, are long, unbranched, aligned, uniformly polar, and grow at the barbed end. Like filopodia, star bundles are enriched in fascin and lack Arp2/3 complex and capping protein. Transition from dendritic to bundled organization was induced by depletion of capping protein, and add-back of this protein restored the dendritic mode. Depletion experiments demonstrated that star formation is dependent on Arp2/3 complex. This poses the paradox of how Arp2/3 complex can be involved in the formation of both branched (lamellipodia-like) and unbranched (filopodia-like) actin structures. Using purified proteins, we showed that a small number of components are sufficient for the assembly of filopodia-like bundles: Wiskott-Aldrich syndrome protein (WASP)-coated beads, actin, Arp2/3 complex, and fascin. We propose a model for filopodial formation in which actin filaments of a preexisting dendritic network are elongated by inhibition of capping and subsequently cross-linked into bundles by fascin.

Cortactin interacts with WIP in regulating Arp2/3 activation and membrane protrusion

BACKGROUND

Modulation of actin cytoskeleton assembly is an integral step in many cellular events. A key regulator of actin polymerization is Arp2/3 complex. Cortactin, an F-actin binding protein that localizes to membrane ruffles, is an activator of Arp2/3 complex.

RESULTS

A yeast two-hybrid screen revealed the interaction of the cortactin Src homology 3 (SH3) domain with a peptide fragment derived from a cDNA encoding a region of WASp-Interacting Protein (WIP). GST-cortactin interacted with WIP in an SH3-dependent manner. The subcellular localization of cortactin and WIP coincided at the cell periphery. WIP increased the efficiency of cortactin-mediated Arp2/3 complex activation of actin polymerization in a concentration-dependent manner. Lastly, coexpression of cortactin and WIP stimulated membrane protrusions.

CONCLUSIONS

WIP, a protein involved in filopodia formation, binds to both actin monomers and cortactin. Thus, recruitment of actin monomers to a cortactin-activated Arp2/3 complex likely leads to the observed increase in cortactin activation of Arp2/3 complex by WIP. These data suggest that a cortactin-WIP complex functions in regulating actin-based structures at the cell periphery.

Local force induced conical protrusions of phagocytic cells

Magnetic tweezers were used to study the passive and active response of macrophages to local centripetal nanonewton forces on beta1 integrins. Superparamagnetic beads coated with the beta1-integrin-binding protein invasin were attached to J774 murine macrophages to mimic phagocytosis of bacterial pathogens. Forces exceeding approximately 0.5 nN induce the active formation of trumpet-like protrusions resembling pseudopodia after an initial elastic deflection and a response time of approximately 30 seconds. The speed of advancement of the protrusion is <v>=0.065+/-0.020 micro m second(-1) and is force independent. After saturation (after about 100 seconds) the protrusion stops abruptly and is completely retracted again against forces exceeding 5 nN with an effective relaxation time of approximately 30 seconds. The active protrusion is tentatively attributed to the growth of the actin cortex in the direction of the force, and evidence for the involvement of actin is provided by the finding that Latrunculin A abolishes the activated cone growth. The growth is assumed to be activated by cell signaling mediated by the invasin-specific integrins (exhibiting beta1 chains) and could play a role in phagocytic and protrusive events during immune response by macrophages.

Force generation by actin polymerization II: the elastic ratchet and tethered filaments

The motion of many intracellular pathogens is driven by the polymerization of actin filaments. The propulsive force developed by the polymerization process is thought to arise from the thermal motions of the polymerizing filament tips. Recent experiments suggest that the nucleation of actin filaments involves a phase when the filaments are attached to the pathogen surface by a protein complex. Here we extend the "elastic ratchet model" of Mogilner and Oster to incorporate these new findings. We apply this "tethered ratchet" model to derive the force-velocity relation for Listeria and discuss relations of our theoretical predictions to experimental measurements. We also discuss "symmetry breaking" dynamics observed in ActA-coated bead experiments, and the implications of the model for lamellipodial protrusion in migrating cells.

A biomimetic motility assay provides insight into the mechanism of actin-based motility

Abiomimetic motility assay is used to analyze the mechanism of force production by site-directed polymerization of actin. Polystyrene microspheres, functionalized in a controlled fashion by the N-WASP protein, the ubiquitous activator of Arp2/3 complex, undergo actin-based propulsion in a medium that consists of five pure proteins. We have analyzed the dependence of velocity on N-WASP surface density, on the concentration of capping protein, and on external force. Movement was not slowed down by increasing the diameter of the beads (0.2 to 3 microm) nor by increasing the viscosity of the medium by 10(5)-fold. This important result shows that forces due to actin polymerization are balanced by internal forces due to transient attachment of filament ends at the surface. These forces are greater than the viscous drag. Using Alexa488-labeled Arp2/3, we show that Arp2/3 is incorporated in the actin tail like G-actin by barbed end branching of filaments at the bead surface, not by side branching, and that filaments are more densely branched upon increasing gelsolin concentration. These data support models in which the rates of filament branching and capping control velocity, and autocatalytic branching of filament ends, rather than filament nucleation, occurs at the particle surface.

Mechanism of filopodia initiation by reorganization of a dendritic network

Afilopodium protrudes by elongation of bundled actin filaments in its core. However, the mechanism of filopodia initiation remains unknown. Using live-cell imaging with GFP-tagged proteins and correlative electron microscopy, we performed a kinetic-structural analysis of filopodial initiation in B16F1 melanoma cells. Filopodial bundles arose not by a specific nucleation event, but by reorganization of the lamellipodial dendritic network analogous to fusion of established filopodia but occurring at the level of individual filaments. Subsets of independently nucleated lamellipodial filaments elongated and gradually associated with each other at their barbed ends, leading to formation of cone-shaped structures that we term Lambda-precursors. An early marker of initiation was the gradual coalescence of GFP-vasodilator-stimulated phosphoprotein (GFP-VASP) fluorescence at the leading edge into discrete foci. The GFP-VASP foci were associated with Lambda-precursors, whereas Arp2/3 was not. Subsequent recruitment of fascin to the clustered barbed ends of Lambda-precursors initiated filament bundling and completed formation of the nascent filopodium. We propose a convergent elongation model of filopodia initiation, stipulating that filaments within the lamellipodial dendritic network acquire privileged status by binding a set of molecules (including VASP) to their barbed ends, which protect them from capping and mediate association of barbed ends with each other.

Structural insights into actin-binding, branching and bundling proteins

Structural advances in our understanding of the functions of the actin cytoskeleton have come from diverse sources. On the one hand, the determination of the structure of a bacterial actin-like protein MreB reveals the prokaryotic origins of the actin cytoskeleton, whereas on the other, cryo-electron microscopy and crystallography have yielded reconstructions of many actin crosslinking, regulatory and binding proteins in complex with F-actin. Not least, a high-resolution structure of the Arp2/3 complex and a reconstruction with F-actin provides considerable insight into the eukaryotic machinery, vital for the formation of new F-actin barbed ends, a prerequisite for rapid actin polymerisation involved in cell shape change and motility.