Differential stimulation of monocytic cells results in distinct populations of microparticles

BACKGROUND

Microparticles (MPs), small vesicles shed from stimulated cells, permit cross-talk between cells within a particular environment. Their composition is thought to reflect their cell of origin, and differs according to whether they are produced by stimulation or by apoptosis. Whether MP properties vary according to stimulus is not yet known.

METHODS

We studied the characteristics of MPs produced from monocytic THP-1 cells upon stimulation with lipopolysaccharide or a soluble P-selectin chimera, using proteomics, flow cytometry, western blotting, and electron microscopy.

RESULTS

Utilizing a novel criterion of calcein-AM staining to define MPs, we found that MP populations were similar with respect to size, presence and organization of cytoskeleton, and expression of certain antigens. The MPs shared the same level of procoagulant activity. We found that MPs also have distinct characteristics, depending on stimuli. These include differences in phosphatidylserine expression and expression of proteins from specific subcellular locations such as the mitochondria, and of unique antigens such as leukocyte-associated immunoglobin-like-receptor (LAIR)-1, which was found only upon stimulation with the soluble P-selectin chimera.

CONCLUSION

We found that the properties of MPs depend on the stimulus that produced them. This supports the concept that monocytic MPs differentially modulate thrombosis, inflammation and immune regulation according to stimulus.

Actin filament architecture and movements in macrophage cytoplasm

Actin filaments are the predominant structural elements in macrophage cortical cytoplasm. These fibres form a unique orthogonal network that fills all lamellae extended from the cell and which, in the cell body, bifurcates to form layers 0.2-0.5 micron thick on the cell top and bottom. Single short filaments, 0.1 micron in length, intersect in space in either T-shaped or X-shaped overlaps to form this ultrastructure. Network assembly and pseudopod extension occur when actin filaments within the network elongate. This filament growth is driven by a large storage pool of actin bound to the sequestering protein, profilin. Elongation is regulated by acumentin, gelsolin and possibly severin, proteins that bind to the end of the filaments, preventing the addition of actin monomers to the filaments. The cytosolic concentration of calcium controls whether filaments assemble or disassemble. Filaments can assemble when the filament ends are not blocked by gelsolin, a condition predicted to occur when the calcium concentration is less than 0.1 micron. Orthogonality results when actin filaments are cross-linked by molecules of actin-binding protein.

Mechanics of proplatelet elaboration

The cellular and molecular basis of the intricate process by which megakaryocytes (MKs) form and release platelets remains poorly understood. Work has shown that proplatelets, long cytoplasmic extensions made by mature MKs, are essential intermediates in platelet biogenesis. Microtubules are the main structural component of proplatelets and it is microtubule sliding, driven by dynein motors within cortical bundles, which elongates and thins proplatelets. Kinesin motors carry their cargo of platelet-specific granules and organelles into the proplatelets using the microtubule bundles as tracks. Extension of proplatelets is associated with repeated actin-dependent bending and bifurcation, which results in considerable amplification of free proplatelet ends. Large proplatelets, dissociated from the residual MK cell body, have the capacity to mature platelets. Only the ends of proplatelets form marginal microtubule coils similar to that observed in mature platelets, demonstrating that platelet formation completes primarily at proplatelet ends. Understanding the molecular basis of platelet formation requires detailed knowledge of how the MK microtubule machinery interacts to generate proplatelets and release platelets.

Prestressed F-actin networks cross-linked by hinged filamins replicate mechanical properties of cells

We show that actin filaments, shortened to physiological lengths by gelsolin and cross-linked with recombinant human filamins (FLNs), exhibit dynamic elastic properties similar to those reported for live cells. To achieve elasticity values of comparable magnitude to those of cells, the in vitro network must be subjected to external prestress, which directly controls network elasticity. A molecular requirement for the strain-related behavior at physiological conditions is a flexible hinge found in FLNa and some FLNb molecules. Basic physical properties of the in vitro filamin-F-actin network replicate the essential mechanical properties of living cells. This physical behavior could accommodate passive deformation and internal organelle trafficking at low strains yet resist externally or internally generated high shear forces.

Cell permeant polyphosphoinositide-binding peptides that block cell motility and actin assembly

Polyphosphoinositides (PPIs) affect the localization and activities of many cellular constituents, including actin-modulating proteins. Several classes of polypeptide sequences, including pleckstrin homology domains, FYVE domains, and short linear sequences containing predominantly hydrophobic and cationic residues account for phosphoinositide binding by most such proteins. We report that a ten-residue peptide derived from the phosphatidylinositol 4,5-bisphosphate (PIP(2)) binding region in segment 2 of gelsolin, when coupled to rhodamine B has potent PIP(2) binding activity in vitro; crosses the cell membrane of fibroblasts, platelets, melanoma cells, and neutrophils by a process not involving endocytosis; and blocks cell motility. This peptide derivative transiently disassembles actin filament structures in GFP-actin-expressing NIH3T3 fibroblasts and prevents thrombin- or chemotactic peptide-stimulated actin assembly in platelets and neutrophils, respectively, but does not block the initial [Ca(2+)] increase caused by these agonists. The blockage of actin assembly and motility is transient, and cells recover motility within an hour after their immobilization by 5-20 microm peptide. This class of reagents confirms the critical relation between inositol lipids and cytoskeletal structure and may be useful to probe the location and function of polyphosphoinositides in vivo.

Filamin A, the Arp2/3 complex, and the morphology and function of cortical actin filaments in human melanoma cells

The Arp2/3 complex and filamin A (FLNa) branch actin filaments. To define the role of these actin-binding proteins in cellular actin architecture, we compared the morphology of FLNa-deficient human melanoma (M2) cells and three stable derivatives of these cells expressing normal FLNa concentrations. All the cell lines contain similar amounts of the Arp2/3 complex. Serum addition causes serum-starved M2 cells to extend flat protrusions transiently; thereafter, the protrusions turn into spherical blebs and the cells do not crawl. The short-lived lamellae of M2 cells contain a dense mat of long actin filaments in contrast to a more three-dimensional orthogonal network of shorter actin filaments in lamellae of identically treated FLNa-expressing cells capable of translational locomotion. FLNa-specific antibodies localize throughout the leading lamellae of these cells at junctions between orthogonally intersecting actin filaments. Arp2/3 complex-specific antibodies stain diffusely and label a few, although not the same, actin filament overlap sites as FLNa antibody. We conclude that FLNa is essential in cells that express it for stabilizing orthogonal actin networks suitable for locomotion. Contrary to some proposals, Arp2/3 complex-mediated branching of actin alone is insufficient for establishing an orthogonal actin organization or maintaining mechanical stability at the leading edge.

N-WASP deficiency reveals distinct pathways for cell surface projections and microbial actin-based motility

The Wiskott-Aldrich syndrome protein (WASP) family of molecules integrates upstream signalling events with changes in the actin cytoskeleton. N-WASP has been implicated both in the formation of cell-surface projections (filopodia) required for cell movement and in the actin-based motility of intracellular pathogens. To examine N-WASP function we have used homologous recombination to inactivate the gene encoding murine N-WASP. Whereas N-WASP-deficient embryos survive beyond gastrulation and initiate organogenesis, they have marked developmental delay and die before embryonic day 12. N-WASP is not required for the actin-based movement of the intracellular pathogen Listeria but is absolutely required for the motility of Shigella and vaccinia virus. Despite these distinct defects in bacterial and viral motility, N-WASP-deficient fibroblasts spread by using lamellipodia and can protrude filopodia. These results imply a crucial and non-redundant role for N-WASP in murine embryogenesis and in the actin-based motility of certain pathogens but not in the general formation of actin-containing structures.

A lineage-restricted and divergent beta-tubulin isoform is essential for the biogenesis, structure and function of blood platelets

BACKGROUND

Mammalian megakaryocytes release blood platelets through a remarkable process of cytoplasmic fragmentation and de novo assembly of a marginal microtubule band. Cell-specific components of this process include the divergent beta-tubulin isoform beta1 that is expressed exclusively, and is the predominant isoform, in platelets and megakaryocytes. The functional significance of this restricted expression, and indeed of the surprisingly large repertoire of metazoan tubulin genes, is unclear. Fungal tubulin isoforms appear to be functionally redundant, and all mammalian beta-tubulins can assemble in a variety of microtubules, whereas selected fly and worm beta-tubulins are essential in spermatogenesis and neurogenesis. To address the essential role of beta1-tubulin in its natural context, we generated mice with targeted gene disruption.

RESULTS

beta1-tubulin(-/-) mice have thrombocytopenia resulting from a defect in generating proplatelets, the immediate precursors of blood platelets. Circulating platelets lack the characteristic discoid shape and have defective marginal bands with reduced microtubule coilings. beta1-tubulin(-/-) mice also have a prolonged bleeding time, and their platelets show an attenuated response to thrombin. Two alternative tubulin isoforms, beta2 and beta5, are overexpressed, and the total beta-tubulin content of beta1-tubulin(-/-) megakaryocytes is normal. However, these isoforms assemble much less efficiently into platelet microtubules and are thus unable to compensate completely for the absence of beta1-tubulin.

CONCLUSIONS

This is the first genetic study to address the essential functions of a mammalian tubulin isoform in vivo. The results establish a specialized role for beta1-tubulin in platelet synthesis, structure, and function.

The neurofibromatosis 2 protein product merlin selectively binds F-actin but not G-actin, and stabilizes the filaments through a lateral association

The neurofibromatosis 2 protein product merlin, named for its relatedness to the ezrin, radixin and moesin (ERM) family of proteins, is a tumour suppressor whose absence results in the occurrence of multiple tumours of the nervous system, particularly schwannomas and meningiomas. Merlin's similarity to ERMs suggests that it might share functions, acting as a link between cytoskeletal components and the cell membrane. The N-terminus of merlin has strong sequence identity to the N-terminal actin-binding region of ezrin; here we describe in detail the merlin-actin interaction. Employing standard actin co-sedimentation assays, we have determined that merlin isoform 2 binds F-actin with an apparent binding constant of 3.6 microM and a stoichiometry of 1 mol of merlin per 11.5 mol of actin in filaments at saturation. Further, solid-phase binding assays reveal that merlin isoforms 1 and 2 bind actin filaments differentially, suggesting that the intramolecular interactions in isoform 1 might hinder its ability to bind actin. However, merlin does not bind G-actin. Studies of actin filament dynamics show that merlin slows filament disassembly with no influence on the assembly rate, indicating that merlin binds along actin filament lengths. This conclusion is supported by electron microscopy, which demonstrates that merlin binds periodically along cytoskeletal actin filaments. Comparison of these findings with those reported for ERM proteins reveal a distinct role for merlin in actin filament dynamics.

Mechanisms of cold-induced platelet actin assembly

Various agonists but also chilling cause blood platelets to increase cytosolic calcium, polymerize actin, and change shape. We report that cold increases barbed end nucleation sites in octyl glucoside-permeabilized platelets by 3-fold, enabling analysis of the intermediates of this response. Although chilling does not change polyphosphoinositide (ppI) levels, a ppI-binding peptide completely inhibits cold-induced nucleation. The C terminus of N-WASp, which inhibits the Arp2/3 complex, blocks nucleation by 40%; GDPbetaS, N17Rac and N17Cdc42 have no effects. Some gelsolin translocates to the detergent-insoluble cytoskeleton after cooling. Chilled platelets from gelsolin-deficient mice have approximately 50% fewer new actin nuclei compared with platelets from wild-type mice. EGTA completely inhibits gelsolin translocation into the cytoskeleton, and the small amount of gelsolin initially there becomes soluble. Chilling releases adducin from the detergent-resistant cytoskeleton. We conclude that platelet actin filament assembly induced by cooling involves ppI-mediated actin filament barbed end uncapping and de novo nucleation independently of surface receptors or downstream signaling intermediates besides calcium. The actin-related changes occur in platelets at temperatures below 37 degrees C, suggesting that the platelet may be more activable at temperatures at the body surface than at core temperature, thereby favoring superficial hemostasis over internal thrombosis.

WIP regulates N-WASP-mediated actin polymerization and filopodium formation

Induction of filopodia is dependent on activation of the small GTPase Cdc42 and on neural Wiskott-Aldrich-syndrome protein (N-WASP). Here we show that WASP-interacting protein (WIP) interacts directly with N-WASP and actin. WIP retards N-WASP/Cdc42-activated actin polymerization mediated by the Arp2/3 complex, and stabilizes actin filaments. Microinjection of WIP into NIH 3T3 fibroblasts induces filopodia; this is inhibited by microinjection of anti-N-WASP antibody. Microinjection of anti-WIP antibody inhibits induction of filopodia by bradykinin, by an active Cdc42 mutant (Cdc42(V12)) and by N-WASP. Our results indicate that WIP and N-WASP may act as a functional unit in filopodium formation, which is consistent with their role in actin-tail formation in cells infected with vaccinia virus or Shigella.

Filamins as integrators of cell mechanics and signalling

Filamins are large actin-binding proteins that stabilize delicate three-dimensional actin webs and link them to cellular membranes. They integrate cellular architectural and signalling functions and are essential for fetal development and cell locomotion. Here, we describe the history, structure and function of this group of proteins.

The mechanics of F-actin microenvironments depend on the chemistry of probing surfaces

To understand the microscopic mechanical properties of actin networks, we monitor the motion of embedded particles with controlled surface properties. The highly resolved Brownian motions of these particles reveal the viscoelastic character of the microenvironments around them. In both non-cross-linked and highly cross-linked actin networks, particles that bind F-actin report viscoelastic moduli comparable to those determined by macroscopic rheology experiments. By contrast, particles modified to prevent actin binding have weak microenvironments that are surprisingly insensitive to the introduction of filament cross-links. Even when adjacent in the same cross-linked gel, actin-binding and nonbinding particles report viscoelastic moduli that differ by two orders of magnitude at low frequencies (0.5-1.5 rad/s) but converge at high frequencies (> 10(4) rad/s). For all particle chemistries, electron and light microscopies show no F-actin recruitment or depletion, so F-actin microheterogeneities cannot explain the deep penetration (approximately 100 nm) of nonbinding particles. Instead, we hypothesize that a local depletion of cross-linking around nonbinding particles explains the phenomena. With implications for organelle mobility in cells, our results show that actin binding is required for microenvironments to reflect macroscopic properties, and conversely, releasing actin enhances particle mobility beyond the effects of mere biochemical untethering.

Roles of SLP-76, phosphoinositide 3-kinase, and gelsolin in the platelet shape changes initiated by the collagen receptor GPVI/FcR gamma-chain complex

How platelet shape change initiated by a collagen-related peptide (CRP) specific for the GPVI/FcR gamma-chain complex (GPVI/FcR gamma-chain) is coupled to SLP-76, phosphoinositide (PI) 3-kinase, and gelsolin is reported. As shown by video microscopy, platelets rapidly round and grow dynamic filopodial projections that rotate around the periphery of the cell after they contact a CRP-coated surface. Lamellae subsequently spread between the projections. All the actin-driven shape changes require SLP-76 expression. SLP-76 is essential for the Ca(++) mobilization induced by CRP, whereas PI 3-kinase only modulates it. The extension of lamellae requires net actin assembly and an exposure of actin filament barbed ends downstream of PI 3-kinase. Gelsolin expression is also required for the extension of lamellae, but not for the formation of filopodia. Altogether, the data describe the role of SLP-76 in the platelet activation initiated by GPVI/FcR gamma-chain and the roles of PI 3-kinase and gelsolin in lamellae spreading. (Blood. 2000;96:3786-3792)

Regulation of the actin cycle in vivo by actin filament severing

Cycling of actin subunits between monomeric and filamentous phases is essential for cell crawling behavior. We investigated actin filament turnover rates, length, number, barbed end exposure, and binding of cofilin in bovine arterial endothelial cells moving at different speeds depending on their position in a confluent monolayer. Fast-translocating cells near the wound edge have short filament lifetimes compared with turnover values that proportionately increase in slower moving cells situated at increasing distances from the wound border. Contrasted with slow cells exhibiting slow actin filament turnover speeds, fast cells have less polymerized actin, shorter actin filaments, more free barbed ends, and less actin-associated cofilin. Cultured primary fibroblasts manifest identical relationships between speed and actin turnover as the endothelial cells, and fast fibroblasts expressing gelsolin have higher actin turnover rates than slow fibroblasts that lack this actin-severing protein. These results implicate actin filament severing as an important control mechanism for actin cycling in cells.

P-Selectin glycoprotein ligand 1 (PSGL-1) is expressed on platelets and can mediate platelet-endothelial interactions in vivo

The platelet plays a pivotal role in maintaining vascular integrity. In a manner similar to leukocytes, platelets interact with selectins expressed on activated endothelium. P-selectin glycoprotein ligand 1 (PSGL-1) is the main P-selectin ligand expressed on leukocytes. Searching for platelet ligand(s), we used a P-selectin-immunoglobulin G (IgG) chimera to affinity purify surface-biotinylated proteins from platelet lysates. P-selectin-bound ligands were eluted with ethylenediaminetetraacetic acid. An approximately 210-kD biotinylated protein was isolated from both human neutrophil and platelet preparations. A band of the same size was also immunopurified from human platelets using a monoclonal anti-human PSGL-1 antibody and could be blotted with P-selectin-IgG. Under reducing conditions, both the predicted PSGL-1 approximately 210-kD dimer and the approximately 120-kD monomer were isolated from platelets. Comparative immunoelectron microscopy and Western blotting experiments suggested that platelet PSGL-1 expression is 25-100-fold lower than that of leukocytes. However, patients with chronic idiopathic thrombocytopenic purpura who harbor predominantly young platelets displayed greater expression, indicating that PSGL-1 expression may be decreased during platelet aging. By flow cytometry, thrombin-activated platelets from normal individuals exhibited greater expression than those unstimulated. An inhibitory anti-PSGL-1 antibody significantly reduced platelet rolling in mesenteric venules, as observed by intravital microscopy. Our results indicate that functional PSGL-1 is expressed on platelets, and suggest an additional mechanism by which selectins and their ligands participate in inflammatory and/or hemostatic responses.

Type Ialpha phosphatidylinositol-4-phosphate 5-kinase mediates Rac-dependent actin assembly

Action polymerization is essential for a variety of cellular processes including movement, cell division and shape change. The induction of actin polymerization requires the generation of free actin filament barbed ends, which results from the severing or uncapping of pre-existing actin filaments [1] [2], or de novo nucleation, initiated by the Arp2/3 complex [3] [4] [5] [6] [7]. Although little is known about the signaling pathways that regulate actin assembly, small GTPases of the Rho family appear to be necessary [8] [9] [10] [11]. In thrombin-stimulated platelets, the Rho family GTPase Rac1 induces actin polymerization by stimulating the uncapping of actin filament barbed ends [2]. The mechanism by which Rac regulates uncapping is unclear, however. We previously demonstrated that Rac interacts with a type I phosphatidylinositol-4-phosphate 5-kinase (PIP 5-kinase) in a GTP-independent manner [12] [13]. Because PIP 5-kinases synthesize phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)), a lipid that dissociates capping proteins from the barbed ends of actin filaments [14] [15] [16], they are good candidates for mediating the effects of Rac on actin assembly. Here, we have identified the Rac-associated PIP 5-kinase as the PIP 5-kinase isoforms alpha and beta. When added to permeabilized platelets, PIP 5-kinase alpha induced actin filament uncapping and assembly. In contrast, a kinase-inactive PIP 5-kinase alpha mutant failed to induce actin assembly and blocked assembly stimulated by thrombin or Rac. Furthermore, thrombin- or Rac-induced actin polymerization was inhibited by a point mutation in the carboxyl terminus of Rac that disrupts PIP 5-kinase binding. These results demonstrate that PIP 5-kinase alpha is a critical mediator of thrombin- and Rac-dependent actin assembly.

Activation of the small GTPases, rac and cdc42, after ligation of the platelet PAR-1 receptor

Stimulation of platelet PAR-1 receptors results in the rapid (10 to 30 seconds) and extensive (30% to 40% of total) guanosine triphosphate (GTP) charging of endogenous platelet rac, previously identified as a possible key intermediate in the signal pathway between PAR-1 and actin filament barbed-end uncapping, leading to actin assembly. During PAR-1-mediated platelet activation, rac distributes from the cell interior to the cell periphery, and this reorganization is resistant to the inhibition of PI-3-kinase activity. Rac, in resting or activated platelets, is Triton X-100 soluble, suggesting that it does not form tight complexes with actin cytoskeletal proteins, though its retention in octyl-glucoside-treated platelets and ultrastructural observations of activated platelets implies that rac binds to plasma membranes, where it can interact with phosphoinositide kinases implicated in actin assembly reactions. PAR-1 stimulation also rapidly and extensively activates cdc42, though, in contrast to rac, some cdc42 associates with the actin cytoskeleton in resting platelets, and the bound fraction increases during stimulation. The differences in subcellular distribution and previous evidence showing quantitatively divergent effects of rac and cdc42 on actin nucleation in permeabilized platelets indicate different signaling roles for these GTPases.

Three-dimensional reconstruction of the actin cytoskeleton from stereo images

In eucaryotic cells, actin filaments are abundant components in the cytoskeleton where they form a complex three dimensional (3D) structural network that provides the cell with its shape and mechanical properties. However, understanding the structural and mechanical properties of actin filaments composing the cell cytoskeleton is often hampered by the inability to faithfully reconstruct the three-dimensional geometric relationships. This paper presents a vision-based reconstruction approach that automatically reconstitutes the three-dimensional structures of cytoskeletal polymers from stereo image pairs taken at the different tilt angles. The approach finds corresponding points between two images and recovers the depth information about the structures. The computational process consists of three major procedures: feature representation, stereo matching, and disparity refinement, implemented in a multi-resolution manner based on a coarse-to-fine strategy. The reconstruction depicts the three-dimensional structure of cytoskeletal polymers and their geometric relationships. New and useful information becomes available and allows quantitative analysis of the structure. Measurement of the cytoskeleton geometrical properties and the filament concentration in a defined volume are obtained by direct calculation.

The elegant platelet: signals controlling actin assembly

In summary, the actin-based cytoskeleton of the human platelet has been assembled to serve two primary purposes. The first allows the platelet to circulate throughout the blood vascular system in near proximity to the endothelial cells. To accomplish this feat, a small discoid fragment has been meticulously assembled in the megakaryocyte. This released platelet is maintained in the discoid shape using a cytoplasmic scaffolding composed of rigidly crosslinked actin filament laminated to both a spectrin membrane skeleton and the cell surface by the alpha-chain of the vWFR. The small size and discoid shape of the platelet causes it to be pushed to the endothelial surface of the blood vascular system during blood flow. The second purpose of the cytoskeleton is to allow for a massive cell spreading to plug wounds. To accomplish this task, the cytoskeleton is quickly taken apart and reassembled. The temporal nature of this reaction makes the platelet one of the most useful cells in which to understand both the proteins and the signaling pathways that initiate cell shape changes mediated by the actin filament system of cells.

Cell crawling two decades after Abercrombie

In response to extracellular signals, cells remodel actin networks. Monomeric actin subunits at the cell's leading edge assemble into linear polymers that are cross-linked by accessory proteins into three-dimensional structures that are contracted by myosins to generate hydraulic force; elsewhere in the cell, actin networks dismantle. Actin subunit sequestering proteins prevent spontaneous actin nucleation, but not the growth of actin sub-units on to fast-growing filament ('barbed') ends, and at least half of the actin in most cells is filamentous. Therefore regulation of cellular actin assembly also requires proteins that block ('cap') actin filament barbed ends. Members of the capping protein gelsolin family also sever actin filaments mechanically. Calcium and protons activate gelsolin for severing and capping. Phosphoinositides reverse such capping, and a pathway has been defined in which receptor perturbation operates through GTP-Rac1 to stimulate the synthesis of endogenous phosphoinositides that uncap actin filaments. Other GTPases (and other signalling pathways) target phosphoinositide synthesis where other protrusions (e.g. filopodia) emerge. Cells maintain adequate, albeit compromised, locomotion in the absence of some, but not all, important machine parts. For example, gelsolin-null fibroblasts crawl using predominantly filopodia rather than lamellae. However, ABP-280 (actin-binding protein of 280 kDa), which promotes orthogonal branching of short actin filaments, seems to be necessary for membrane stability and translational locomotion. ABP-null cells hardly crawl at all, although they are viable and engage in surface movements.

The small GTPase RalA targets filamin to induce filopodia

The Ras-related small GTPases Rac, Rho, Cdc42, and RalA bind filamin, an actin filament-crosslinking protein that also links membrane and other intracellular proteins to actin. Of these GTPases only RalA binds filamin in a GTP-specific manner, and GTP-RalA elicits actin-rich filopods on surfaces of Swiss 3T3 cells and recruits filamin into the filopodial cytoskeleton. Either a dominant negative RalA construct or the RalA-binding domain of filamin 1 specifically block Cdc42-induced filopod formation, but a Cdc42 inhibitor does not impair RalA's effects, which, unlike Cdc42, are Rac independent. RalA does not generate filopodia in filamin-deficient human melanoma cells, whereas transfection of filamin 1 restores the functional response. RalA therefore is a downstream intermediate in Cdc42-mediated filopod production and uses filamin in this pathway.

Measuring actin dynamics in endothelial cells

Cytoplasmic actin distributes between monomeric and filamentous phases in cells. As cells crawl, actin polymerizes near the plasma membrane of expanding peripheral cytoplasm and depolymerizes elsewhere. Thus, the finite actin filament lifetime, the diffusivity of actin monomer, and the distribution of actin between the polymer and monomer phases are key parameters in cell motility. The dynamics of cellular actin can be determined by following the evolution of fluorescence in the techniques of photoactivated fluorescence (PAF) or fluorescence recovery after photobleaching (FRAP) of microinjected actin derivatives. A mathematical model is discussed that measures monomer diffusion coefficients, filament turnover rates, and the fraction of actin polymerized from measurements of the evolution of fluorescence from a photoactivated band [Tardy et al. (1995) Biophys. J., 69:1674-1682; McGrath et al. (1998) Biophys. J., in press]. Applying this model to subconfluent endothelial cells shows that approximately 40% of the actin is polymer and that these filaments turn over on average every 6 minutes. This report discusses how PAF and FRAP can be combined with more traditional biochemistry to probe actin cytoskeleton remodeling in endothelial cells.

Caspase-3-induced gelsolin fragmentation contributes to actin cytoskeletal collapse, nucleolysis, and apoptosis of vascular smooth muscle cells exposed to proinflammatory cytokines

Gelsolin, an 80 kDa actin-severing protein, has been recently identified as a substrate for the cell death-promoting cysteinyl protease caspase-3 (CPP32/apopain/YAMA). We investigated the role of gelsolin and its cleavage product in apoptosis of vascular smooth muscle cells (SMC) induced by the proinflammatory cytokines interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha). Treatment with a combination of IFN-gamma and TNF-alpha reduced viability of SMC in a time- and concentration-dependent manner. Immunoblotting revealed that SMC treated with the cytokines generated a 41 kDa gelsolin fragment. The gelsolin fragmentation required activation of caspase-3, as the caspase-3 inhibitor diminished cytokine-induced cell death as well as the fragmentation. Gelsolin cleavage was accompanied by a reduction in F-actin content and by a marked disruption of cell structure. Adenovirus-mediated transfection of this N-terminal gelsolin fragment into SMC altered cell morphology, reduced cell viability, increased the number of TUNEL-positive cells, and promoted internucleosomal DNA fragmentation. Compared to wild-type cells, gelsolin-deficient SMC showed resistance to apoptosis induced by the inflammatory cytokines. These results suggest a mechanistic role for gelsolin cleavage during SMC apoptosis, a process implicated in vessel development as well as stability of atherosclerotic plaque.

Simultaneous measurements of actin filament turnover, filament fraction, and monomer diffusion in endothelial cells

The analogous techniques of photoactivation of fluorescence (PAF) and fluorescence recovery after photobleaching (FRAP) have been applied previously to the study of actin dynamics in living cells. Traditionally, separate experiments estimate the mobility of actin monomer or the lifetime of actin filaments. A mathematical description of the dynamics of the actin cytoskeleton, however, predicts that the evolution of fluorescence in PAF and FRAP experiments depends simultaneously on the diffusion coefficient of actin monomer, D, the fraction of actin in filaments, FF, and the lifetime of actin filaments, tau (, Biophys. J. 69:1674-1682). Here we report the application of this mathematical model to the interpretation of PAF and FRAP experiments in subconfluent bovine aortic endothelial cells (BAECs). The following parameters apply for actin in the bulk cytoskeleton of subconfluent BAECs. PAF: D = 3.1 +/- 0.4 x 10(-8) cm2/s, FF = 0.36 +/- 0.04, tau = 7.5 +/- 2.0 min. FRAP: D = 5.8 +/- 1.2 x 10(-8) cm2/s, FF = 0.5 +/- 0.04, tau = 4.8 +/- 0.97 min. Differences in the parameters are attributed to differences in the actin derivatives employed in the two studies and not to inherent differences in the PAF and FRAP techniques. Control experiments confirm the modeling assumption that the evolution of fluorescence is dominated by the diffusion of actin monomer, and the cyclic turnover of actin filaments, but not by filament diffusion. The work establishes the dynamic state of actin in subconfluent endothelial cells and provides an improved framework for future applications of PAF and FRAP.

Gelsolin is a downstream effector of rac for fibroblast motility

Rac, a member of the rho family of GTPases, when activated transmits signals leading to actin-based membrane ruffling in fibroblasts. Compared with wild-type fibroblasts, gelsolin null (Gsn-) dermal fibroblasts have a markedly reduced ruffling response to serum or EGF stimulation, which signal through rac. Bradykinin-induced filopodial formation, attributable to activation of cdc42, is similar in both cell types. Wild-type fibroblasts exhibit typical lamellipodial extension during translational locomotion, whereas Gsn- cells move 50% slower using structures resembling filopodia. Multiple Gsn- tissues as well as Gsn- fibroblasts overexpress rac, but not cdc42 or rho, 5-fold. Re-expression of gelsolin in Gsn- fibroblasts by stable transfection or adenovirus reverts the ruffling response, translational motility and rac expression to normal. Rac migrates to the cell membrane following EGF stimulation in both cell types. Gelsolin is an essential effector of rac-mediated actin dynamics, acting downstream of rac recruitment to the membrane.

Cytoskeletal protein ABP-280 directs the intracellular trafficking of furin and modulates proprotein processing in the endocytic pathway

Furin catalyzes the proteolytic maturation of many proproteins within the trans-Golgi network (TGN)/endosomal system. Furin's cytosolic domain (cd) directs both the compartmentalization to and transit between its manifold processing compartments (i.e., TGN/biosynthetic pathway, cell surface, and endosomes). Here we report the identification of the first furin cd sorting protein, ABP-280 (nonmuscle filamin), an actin gelation protein. The furin cd was used as bait in a yeast two-hybrid screen to identify ABP-280 as a furin-binding protein. Binding analyses in vitro and coimmunoprecipitation studies in vivo showed that furin and ABP-280 interact directly and that ABP-280 tethers furin molecules to the cell surface. Quantitative analysis of both ABP-280-deficient and genetically replete cells showed that ABP-280 modulates the rate of internalization of furin but not of the transferrin receptor, a cycling receptor. However, although ABP-280 directs the rate of furin internalization, the efficiency of sorting of the endoprotease from the cell surface to early endosomes is independent of expression of ABP-280. By contrast, efficient sorting of furin from early endosomes to the TGN requires expression of ABP-280. In addition, ABP-280 is also required for the correct localization of late endosomes (dextran bead uptake) and lysosomes (LAMP-1 staining), demonstrating a pleiotropic role for this actin binding protein in the organization of cellular compartments and directing protein traffic. Finally, and consistent with the trafficking studies on furin, we showed that ABP-280 modulates the processing of furin substrates in the endocytic but not the biosynthetic pathways. The novel roles of ABP-280 and the cytoskeleton in the sorting of furin in the TGN/ endosomal system and the formation of proprotein processing compartments are discussed.

WIP, a protein associated with wiskott-aldrich syndrome protein, induces actin polymerization and redistribution in lymphoid cells

Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency caused by mutations that affect the WAS protein (WASP) and characterized by cytoskeletal abnormalities in hematopoietic cells. By using the yeast two-hybrid system we have identified a proline-rich WASP-interacting protein (WIP), which coimmunoprecipitated with WASP from lymphocytes. WIP binds to WASP at a site distinct from the Cdc42 binding site and has actin as well as profilin binding motifs. Expression of WIP in human B cells, but not of a WIP truncation mutant that lacks the actin binding motif, increased polymerized actin content and induced the appearance of actin-containing cerebriform projections on the cell surface. These results suggest that WIP plays a role in cortical actin assembly that may be important for lymphocyte function.

Mechanical remodeling of the endothelial surface and actin cytoskeleton induced by fluid flow

OBJECTIVE

The mechanism by which cultured endothelial cells respond to shear stress is controversial. The cell surface and cytoskeleton are involved, but their roles are undefined. In this study, previously unknown changes in the surface detail and actin cytoskeleton of bovine aortic endothelial cells were identified.

METHODS

Actin filament content and filament number in resting and flow-oriented cells were determined by biochemical assays. The three-dimensional organization of the actin cytoskeleton in cells was defined in the confocal microscope and in the electron microscope after rapid-freezing, freeze-drying, and metal coating of detergent-permeabilized cells.

RESULTS

Endothelial cells have smooth apical membranes in situ. However, cultured cells exhibit surface microvilli which increase the apical surface area, exposing the ruffled surface to forces from fluid flow and potentially enhancing cell interactions with blood-borne white cells. Stereoscopic micrographs show that stress fibers are integrated into a complex distributed cytoplasmic structural actin network (DCSA). This lattice is formed by actin filaments that frequently cross and connect to each other, stress fibers, and microfilaments and microtubules. The cytoskeletons of cells cultured in static media lack apparent order when compared to cytoskeletons from cells which have been exposed to 24 hours of laminar flow.

CONCLUSIONS

The DCSA physically connects the apical and basal cell membranes and fills the volume between nucleus and membrane, providing a mechanism for transmitting mechanical forces across cells and a signaling pathway from membrane to nucleus. Stress fibers increase the mechanical modulus of the DCSA, although this increase is probably unnecessary to withstand the increase in shear stress caused by blood flow in vivo. This implies that actin rearrangements are not required for mechanical integrity, but serve an alternate function.

Signal transduction pathways involving the small G proteins rac and Cdc42 and phosphoinositide kinases

We found that rac specifically binds to a type I PtdIns-4-P 5-kinase and that both rac and Cdc42 in the activated forms associate with PI 3-kinase. The association of PI 3-kinase with rac was stimulated by PDGF in vivo. Rac is constitutively associated with a PtdIns-4-P 5-kinase and stimulates PtdIns-4,5-P2 production in permeabilized platelets. These data suggest a model in which the initial step in the activation of rac is release from rho GDI (Fig. 7). Rac in the GDP bound form can associate with the PtdIns-4-P 5-kinase and also interact with an exchange factor. GTP bound rac may then localize to sites of actin reorganization, bringing the PtdIns-4-P 5-kinase with it. Locally synthesized PtdIns-4,5-P2 binds to actin capping proteins, leading to their release and the production of actin free ends. Actin polymerization can then occur from the free ends. Many other factors must be involved to regulate the type and extent of actin polymerization that is necessary in such complex processes as cell movement and membrane ruffling. The rac-associated PtdIns-4-P 5-kinase and its product PtdIns-4,5-P2 may act at a crucial regulatory point that permits polymerization to begin.

Polyphosphoinositide synthesis and platelet shape change

Polyphosphoinositides both integrate signaling pathways at the cytoplasmic surface of the plasma membrane and serve as the final messenger of signaling pathways. They have a well-defined role in the release of calcium through the degradative pathway. Recent work, however, has highlighted the role of the polyphosphoinositide synthetic pathways in activated cells. This review emphasizes the role of polyphosphoinositides in recruiting signaling proteins that have pleckstrin homology domains and in directly regulating actin assembly in the human blood platelet.

The lipid products of phosphoinositide 3-kinase increase cell motility through protein kinase C

Phosphoinositide 3-kinase has been implicated as an activator of cell motility in a variety of recent studies, yet the role of its lipid product, phosphatidylinositol 1,4,5-trisphosphate (PtdIns-3,4,5-P3), has yet to be elucidated. In this study, three independent preparations of PtdIns-3,4,5-P3 were found to increase the motility of NIH 3T3 cells when examined utilizing a microchemotaxis chamber. Dipalmitoyl L-alpha-phosphatidyl-D-myo-inositol 3,4,5-triphosphate (Di-C16-PtdIns-3,4,5-P3) also produced actin reorganization and membrane ruffling. Cells pretreated with 12-O-tetradecanoylphorbol-13-acetate to cause down-regulation of protein kinase C (PKC) exhibited complete inhibition of cell motility induced by Di-C16-PtdIns-3,4,5-P3. These results are consistent with previous observations that PtdIns-3,4,5-P3 activates Ca2+-independent PKC isoforms in vitro and in vivo and provide the first demonstration of an in vivo role for the lipid products of the phosphoinositide 3-kinase. PtdIns-3,4,5-P3 appears to directly initiate cellular motility via activation of a PKC family member.

Immunoelectron microscopy shows a clustered distribution of NADPH oxidase components in the human neutrophil plasma membrane

The NADPH oxidase that produces superoxide in professional phagocytic cells is a flavocytochrome b electron transport chain in the membrane, a heterodimer of gp91phox and p22phox, that is activated by a number of cytosolic proteins, including p47phox, p67phox, and the small GTP-binding protein p21rac, which translocate to the membrane and attach to the flavocytochrome on activation. The components of this oxidase were localized on the cytoplasmic surface of the plasma membrane of adherent unroofed neutrophils by immunolabeling. Components of the NADPH oxidase and p21rac were found together in punctate clusters occupying 0.03-0.1 microm2 of the cytoplasmic surface of the plasma membrane where the density of labeling of the cytosolic components was increased after stimulation with phorbol myristate acetate.

Identification of the region in actin-binding protein that binds to the cytoplasmic domain of glycoprotein IBalpha

Actin-binding protein (ABP-280) is a component of the submembranous cytoskeleton and interacts with the glycoprotein (GP) Ibalpha subunit of the GP Ib-IX complex in platelets. In the present studies, we have identified the binding site for GP Ibalpha in ABP-280. A melanoma cell line lacking ABP-280 was stably transfected with the cDNAs coding for GP Ib-IX, then transiently transfected with cDNA coding for various carboxyl-truncates of ABP-280. Immunocapture assays and co-immunoprecipitation experiments from detergent-lysed cells showed that deletion of the carboxyl-terminal repeats 20-24 of ABP-280 had no effect on GP Ib-IX binding, but deletion of residues 2099 through 2136 within repeat 19 abolished binding. In the yeast two-hybrid system, an ABP-280 fragment comprising repeats 17-19 bound GP Ibalpha. Deletion from either end abolished binding. Individual or multiple repeats of ABP-280 were expressed as fusion protein in bacteria and purified; structural folding was evaluated, and binding to GP Ib-IX was assessed. Binding depended on the presence of repeats 17-19. None of the individual repeats were able to bind to GP Ib-IX. These findings demonstrate that residues 1850-2136 comprising repeats 17-19 contain the binding site for GP Ib-IX.

D3 phosphoinositides and outside-in integrin signaling by glycoprotein IIb-IIIa mediate platelet actin assembly and filopodial extension induced by phorbol 12-myristate 13-acetate

Phorbol 12-myristate 13-acetate (PMA) uncaps a small number of the fast-growing (barbed) ends of actin filaments, thereby eliciting slow actin assembly and extension of filopodia in human blood platelets. These reactions, which also occur in response to immunologic perturbation of the integrin glycoprotein (GP) IIb-IIIa, are sensitive to the phosphoinositide 3-kinase inhibitor wortmannin. Platelets deficient in GPIIb-IIIa integrins or with GPIIb-IIIa function inhibited by calcium chelation or the peptide RGDS have diminished PMA responsiveness. The effects of PMA contrast with thrombin receptor stimulation by >/=5 microM thrombin receptor-activating peptide (TRAP), which causes rapid and massive wortmannin-insensitive actin assembly and lamellar and filopodial extension. However, we show here that wortmannin can inhibit filopod formation if the thrombin receptor is ligated using suboptimal doses (<1 microM) of TRAP. Phosphatidylinositol 3,4-bisphosphate inhibits actin filament severing and capping by human gelsolin in vitro. The findings implicate D3 polyphosphoinositides and integrin signaling in PMA-mediated platelet stimulation and implicate D3 containing phosphoinositides generated in response to protein kinase C activation and GPIIb-IIIa signaling as late-acting intermediates leading to filopodial actin assembly.

Coordinated regulation of platelet actin filament barbed ends by gelsolin and capping protein

Exposure of cryptic actin filament fast growing ends (barbed ends) initiates actin polymerization in stimulated human and mouse platelets. Gelsolin amplifies platelet actin assembly by severing F-actin and increasing the number of barbed ends. Actin filaments in stimulated platelets from transgenic gelsolin-null mice elongate their actin without severing. F-actin barbed end capping activity persists in human platelet extracts, depleted of gelsolin, and the heterodimeric capping protein (CP) accounts for this residual activity. 35% of the approximately 5 microM CP is associated with the insoluble actin cytoskeleton of the resting platelet. Since resting platelets have an F-actin barbed end concentration of approximately 0.5 microM, sufficient CP is bound to cap these ends. CP is released from OG-permeabilized platelets by treatment with phosphatidylinositol 4,5-bisphosphate or through activation of the thrombin receptor. However, the fraction of CP bound to the actin cytoskeleton of thrombin-stimulated mouse and human platelets increases rapidly to approximately 60% within 30 s. In resting platelets from transgenic mice lacking gelsolin, which have 33% more F-actin than gelsolin-positive cells, there is a corresponding increase in the amount of CP associated with the resting cytoskeleton but no change with stimulation. These findings demonstrate an interaction between the two major F-actin barbed end capping proteins of the platelet: gelsolin-dependent severing produces barbed ends that are capped by CP. Phosphatidylinositol 4,5-bisphosphate release of gelsolin and CP from platelet cytoskeleton provides a mechanism for mediating barbed end exposure. After actin assembly, CP reassociates with the new actin cytoskeleton.

Phosphorylation of actin-binding protein 280 by growth factors is mediated by p90 ribosomal protein S6 kinase

Although Ras-related small GTPases are believed to control cell proliferation and motility through activation of protein kinase cascades, little is known about the intracellular protein targets of activated kinases. Here we show that the p90 ribosomal S6 kinase 2 (RSK2) phosphorylates actin-binding protein (ABP-280) in intact rat 3Y1 fibroblasts. Growth factors such as fetal calf serum, epidermal growth factor, phorbol 12-myristate 13-acetate, and lysophosphatidic acid stimulate the phosphorylation of serine residues in ABP-280 in quiescent 3Y1 cells. Extracts from 3Y1 cells prepared after stimulation by lysophosphatidic acid, fetal calf serum, and epidermal growth factor retain activated protein kinase activity(s) toward ABP-280 in vitro. ABP kinase activities in lysates from lysophosphatidic acid-stimulated 3Y1 cells can be fractionated by MonoQ anion exchange column chromatography into three peaks having ABP kinase activities. One (ABP kinase peak 1) coelutes with the peak of RSK2 as judged by immunoblotting and S6 peptide kinase assays. Two-dimensional phosphopeptide maps show RSK2 phosphorylated ABP-280 to be phosphorylated at the same site(s) as those stimulated by growth factors in vivo. Incubation of ABP kinase peak 1 fractionated from unstimulated cells with activated ERK2 activates latent ABP kinase activity. These results show RSK2 to phosphorylate ABP-280 in vivo.

Thrombin-induced GPIb-IX centralization on the platelet surface requires actin assembly and myosin II activation

In resting platelets, the GPIb-IX complex, the receptor for the von Willebrand factor (vWF), is linked to underlying actin filaments by actin-binding protein (ABP-280). Thrombin stimulation of human platelets leads to a decrease in the surface expression of the GPIb-IX complex, which is redistributed from the platelet surface into the open canalicular system (OCS). Because the centralization of GPIb-IX is inhibited by cytochalasin, it is believed to be linked to actin cytoskeletal rearrangements that take place during platelet activation. We have further characterized the mechanism of GPIb-IX centralization in platelets in suspension. Following thrombin stimulation, GPIb-IX shifts from the membrane skeleton of the resting cell to the cytoskeleton of the activated cell in a reaction sensitive to cytochalasin B. The cytoskeletal association of GPIb-IX involves ABP-280, as it correlates with the incorporation of ABP-280 into the activated cytoskeleton and because no dissociation of the ABP-280/GPIb-IX complexes is detected after thrombin activation. However, the incorporation of GPIb-IX into the cytoskeleton is complete within 1 minute, whereas GPIb-IX centralization requires 5 to 10 minutes for completion. The movement of GPIb-IX to the cytoskeleton of activated platelets is therefore necessary, but not sufficient for GPIb-IX centralization. Blockage of cytosolic calcium increases induced by thrombin by loading with the cell permeant calcium chelator Quin-2 AM inhibited GPIb-IX centralization by 70%, but did not prevent its association with the activated cytoskeleton. Quin-2 loading did, however, decrease the incorporation of myosin II into the activated cytoskeleton. The role of myosin II was further probed using the myosin light chain kinase (MLCK) inhibitor wortmannin. Wortmannin prevents myosin II association to the activated cytoskeleton and inhibits GPIb-IX centralization by 50%, without affecting actin assembly or the association of GPIb-IX to the cytoskeleton. Only micromolar concentrations of wortmannin, high enough to inhibit MLCK, prevent GPIb-IX centralization. These results indicate that thrombin-induced GPIb-IX centralization requires a minimum of two steps, one associating GPIb-IX to the activated cytoskeleton and the second requiring myosin II activation. The involvement of myosin II implies that GPIb-IX/ABP-280 complexes, linked to actin filaments, are pulled into the cell center, and that platelets may exert contractile tension on vWF bound to its receptor.

Direct binding of F actin to the cytoplasmic domain of the alpha 2 integrin chain in vitro

The transmembrane integrins have been shown to interact with the cytoskeleton via noncovalent binding between cytoplasmic domains (CDs) of integrin beta chains and various actin binding proteins within the focal adhesion complex. Direct or indirect integrin alpha chain CD binding to the actin cytoskeleton has not been reported. We show here that actin, as an abundant constituent of focal adhesion complex proteins isolated from fibroblasts, binds strongly and specifically to alpha 2 CD, but not to alpha 1 CD peptide. Similar specific binding to alpha 2 CD peptide was seen for highly purified F actin, free of putative actin-binding proteins. The bound complex of actin and peptide was visualized directly by coprecipitation, and actin binding was abrogated by removal of a five amino acid sequence from the alpha 2 CD peptide. Our findings may explain the earlier observation that, while integrins alpha 2 beta 1 and alpha 1 beta 1 both bind to collagen, only alpha 2 beta 1 can mediate contraction of extracellular collagen matrices.

Phosphorylation of the platelet p47 phosphoprotein is mediated by the lipid products of phosphoinositide 3-kinase

Platelet stimulation by thrombin or the thrombin receptor activating peptide (TRAP) results in the activation of phosphoinositide 3-kinase and the production of the novel polyphosphoinositides phosphatidylinositol 3,4-bisphosphate (PtdIns-3,4-P2) and phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P3). We have shown previously that these lipids activate calcium-independent protein kinase C (PKC) isoforms in vitro (Toker, A., Meyer, M., Reddy, K. K., Falck, J. R., Aneja, R., Aneja, S., Parra, A., Burns, D. J., Ballas, L. M. and Cantley, L. C. (1994) J. Biol. Chem. 269, 32358-32367). Activation of platelet PKC in response to TRAP is detected by the phosphorylation of the major PKC substrate in platelets, the p47 phosphoprotein, also known as pleckstrin. Here we provide evidence for two phases of pleckstrin phosphorylation in response to TRAP. A rapid phase of pleckstrin phosphorylation (< 1 min) precedes the peak of PtdIns-3,4-P2 production and is unaffected by concentrations of wortmannin (10-100 nM) that block production of this lipid. However prolonged phosphorylation of pleckstrin (> 2 min) is inhibited by wortmannin concentrations that block PtdIns-3,4-P2 production. Phorbol ester-mediated pleckstrin phosphorylation was not affected by wortmannin and wortmannin had no effect on purified platelet PKC activity. Phosphorylation of pleckstrin could be induced using permeabilized platelets supplied with exogenous gamma-32P[ATP] and synthetic dipalmitoyl PtdIns-3,4,5-P3 and dipalmitoyl PtdIns-3,4-P2 micelles, but not with dipalmitoyl phosphatidylinositol 3-phosphate or phosphatidylinositol 4,5-bisphosphate. These results suggest two modes of stimulating pleckstrin phosphorylation: a rapid activation of PKC (via diacylglycerol and calcium) followed by a slower activation of calcium-independent PKCs via PtdIns-3,4-P2.

Interpreting photoactivated fluorescence microscopy measurements of steady-state actin dynamics

A continuum model describing the steady-state actin dynamics of the cytoskeleton of living cells has been developed to aid in the interpretation of photoactivated fluorescence experiments. In a simplified cell geometry, the model assumes uniform concentrations of cytosolic and cytoskeletal actin throughout the cell and no net growth of either pool. The spatiotemporal evolution of the fluorescent actin population is described by a system of two coupled linear partial-differential equations. An analytical solution is found using a Fourier-Laplace transform and important limiting cases relevant to the design of experiments are discussed. The results demonstrate that, despite being a complex function of the parameters, the fluorescence decay in photoactivated fluorescence experiments has a biphasic behavior featuring a short-term decay controlled by monomer diffusion and a long-term decay governed by the monomer exchange rate between the polymerized and unpolymerized actin pools. This biphasic behavior suggests a convenient mechanism for extracting the parameters governing the fluorescence decay from data records. These parameters include the actin monomer diffusion coefficient, filament turnover rate, and ratio of polymerized to unpolymerized actin.

The ligand-induced membrane IgM association with the cytoskeletal matrix of B cells is not mediated through the Ig alpha beta heterodimer

The B cell Ag receptor complex consists of membrane-associated Ig (mIg), Ig alpha, and Ig beta, associated molecules that have been implicated in transducing the activation signal that occurs upon receptor cross-linking. The role of the Ig alpha beta heterodimer in mediating binding to the cytoskeleton is unknown. We studied the ligand-induced association of mIgM with the cytoskeleton following receptor cross-linking in mIgM-expressing B lymphoma lines by biochemical assays, FACS analysis, and electron microscopy. Cytoskeletal association is not detected in unstimulated cells, but occurs rapidly upon anti-IgM-mediated cross-linking. Ig alpha is absent from the cytoskeleton-mIgM complex. To further analyze the possible role of Ig alpha beta in cytoskeletal binding, a surface Ig alpha beta-negative plasmacytoma line was transfected with a mutant form of mIgM (IgM-MutA). IgM-MutA is expressed on the surface despite the lack of Ig alpha beta, and the cytoskeletal binding occurred to a similar extent as in Ig-alpha-positive cell lines. In another transfectant expressing a mutated form of human mIgM (YS:VV), which does not have the capacity to bind to Ig alpha beta, the association of the receptor with the cytoskeleton appeared to be more extensive (100%) and faster than with mouse mIgM. These data indicate that Ig-associated Ig alpha beta proteins are not required for mIgM association with the cytoskeleton.

The ligand-induced membrane IgM association with the cytoskeletal matrix of B cells is not mediated through the Ig alpha beta heterodimer

The B cell Ag receptor complex consists of membrane-associated Ig (mIg), Ig alpha, and Ig beta, associated molecules that have been implicated in transducing the activation signal that occurs upon receptor cross-linking. The role of the Ig alpha beta heterodimer in mediating binding to the cytoskeleton is unknown. We studied the ligand-induced association of mIgM with the cytoskeleton following receptor cross-linking in mIgM-expressing B lymphoma lines by biochemical assays, FACS analysis, and electron microscopy. Cytoskeletal association is not detected in unstimulated cells, but occurs rapidly upon anti-IgM-mediated cross-linking. Ig alpha is absent from the cytoskeleton-mIgM complex. To further analyze the possible role of Ig alpha beta in cytoskeletal binding, a surface Ig alpha beta-negative plasmacytoma line was transfected with a mutant form of mIgM (IgM-MutA). IgM-MutA is expressed on the surface despite the lack of Ig alpha beta, and the cytoskeletal binding occurred to a similar extent as in Ig-alpha-positive cell lines. In another transfectant expressing a mutated form of human mIgM (YS:VV), which does not have the capacity to bind to Ig alpha beta, the association of the receptor with the cytoskeleton appeared to be more extensive (100%) and faster than with mouse mIgM. These data indicate that Ig-associated Ig alpha beta proteins are not required for mIgM association with the cytoskeleton.

Actin cytoskeleton. Setting the pace of cell movement

Eukaryotic cells have many proteins that cap the barbed ends of actin filaments. Manipulation of their cellular concentration leads to changes in cell motility rates, actin dynamics and signal transduction reactions.

Thrombin receptor ligation and activated Rac uncap actin filament barbed ends through phosphoinositide synthesis in permeabilized human platelets

Cells respond to diverse external stimuli by polymerizing cytoplasmic actin, and recent evidence indicates that GTPases can specify where this polymerization takes place. Actin assembly in stimulated blood platelets occurs where sequestered monomers add onto the fast-growing (barbed) ends of actin filaments (F-actin), which are capped in the resting cells. We report that D3 and D4 polyphosphoinositides, Pl(4)P, Pl(4,5)P2, Pl(3,4)P2, and Pl(3,4,5)P3, uncap F-actin in resting permeabilized platelets. The thrombin receptor-activating peptide (TRAP), GTP, and GTP gamma S, but not GDP beta S, also uncap F-actin in permeabilized platelets. GDP beta S inhibits TRAP-induced F-actin uncapping, and Pl(4,5)P2 overcomes this inhibition. Constitutively active mutant Rac, but not Rho, activates uncapping of F-actin. Pl(4,5)P2-binding peptides derived from gelsolin inhibit F-actin uncapping by TRAP, Rac, and GTP gamma S. TRAP and Rac induce rapid Pl(4,5)P2 synthesis in permeabilized platelets. The findings establish a signaling pathway for actin assembly involving Rac in which the final message is phosphoinositide-mediated F-actin uncapping.

Actin filament cross-linking by chicken gizzard filamin is regulated by phosphorylation in vitro

Filamin is a dimeric muscle phosphoprotein that cross-links actin filaments. We have found that purified chicken gizzard filamin is phosphorylated in vitro at serine residues by the Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). Up to 0.9 mol of phosphate can be incorporated into 1 mol of filamin dimer. Phosphorylation by CaM kinase II increases filamin's critical actin filament gelling concentration and diminishes the amount of actin sedimented by filamin at low G-force. The modulation of filamin function by CaM kinase II requires ATP, Ca2+, and calmodulin, and it is abolished when CaM kinase II is inactivated with heat. Protein phosphatase 2A removed the phosphate added by CaM kinase II and restored filamin's actin filament cross-linking activity to the untreated basal level. In cosedimentation experiments, phosphorylation reduces the binding of filamin to actin filaments. The Kd for binding of filamin to actin filaments increases approximately 2-fold, from 3.2 to 6.9 microM, following CaM kinase II-mediated phosphorylation. Phosphorylation by CaM kinase II, therefore, regulates the binding of filamin to actin filaments.