Coupling membrane protrusion and cell adhesion

IpaA targets beta1 integrins and rho to promote actin cytoskeleton rearrangements necessary for Shigella entry

Shigella invasion into the colonic epithelium involves many steps including the formation of large membrane protrusions by the epithelial cells that facilitate bacterial engulfment. IpaA, a Shigella protein secreted into target cells upon cell contact induces a loss of actin stress fibers in cells and promotes the reorganization of actin at the site of entry. The mechanism for this is not known but is thought to involve recruitment of the focal adhesion protein vinculin to IpaA. Here we have examined the mechanism for the effects of IpaA on the actin cytoskeleton. We show that IpaA-induced loss of actin stress fibers and cell rounding do not require vinculin expression or an intact vinculin binding site on IpaA. Rather, we find that cells expressing IpaA exhibited elevated Rho activity and increased myosin light chain phosphorylation. In addition, IpaA decreases integrin affinity for extracellular matrix ligands by interfering with talin recruitment to the integrin cytoplasmic tail. The combination of these two effects, namely weakened adhesion and increased contractility, account for the loss of actin stress fibers and cell rounding observed in cells exposed to IpaA.

ROCK signaling mediates the adoption of different modes of migration and invasion in human mammary epithelial tumor cells

For the invasive migration of tumor cells, at least two mechanisms are currently discussed: (1) the mesenchymal mode depending on extracellular proteolysis and (2) the proteolysis-independent amoeboid mode depending on the activity of the Rho kinase ROCK. The ability of tumor cells to switch between different modes of motility has been shown to limit the efficiency of agents aimed to reduce invasion. Here we show by combining 2D and 3D migration assays that human mammary tumor cells exhibited a strongly reduced migration velocity as compared to their normal counterparts indicating that high invasiveness is not necessarily correlated with high migratory capacity in 2D assays. This reduced migration was apparently due to significant differences in actin organization, decreased persistence of lamellipodia by 50% and increased cell substrate adhesion. These differences resulted from a 2.5-fold higher activity of ROCK and were mediated by its downstream effectors myosin light chain kinase and cofilin. Thus, inhibition of ROCK activity caused a marked increase in 2D migration efficiency by 40%, without, however, affecting 3D invasion. A massive reduction of invasion by 60% was achieved by the simultaneous inhibition of the ROCK-dependent amoeboid and the extracellular proteolysis-dependent mesenchymal mode. These results may point to a new efficient strategy for blocking tumor cell invasion in vivo.

Insights into allosteric control of vinculin function from its large scale conformational dynamics

Vinculin is an important constituent of both cell-cell and cell-matrix junctions, where it plays crucial roles in the regulation of cell adhesion and migration. When activated, it mediates the linkage between cadherins (cell-cell) or integrins (cell-matrix) and the actin cytoskeleton through interactions with various proteins. The activation of vinculin requires structural conversions from an autoinhibited conformation to the "open" conformations in which the occluded binding sites of its different ligands become exposed, while the structural dynamics underlying the vinculin activation remains largely unknown. Here we report the first computational study of large scale conformational dynamics of full-length vinculin. We find that the "holding" and "releasing" motions between vinculin tail and pincer-like structure formed by first three domains of vinculin are the dominant motions near the native state of vinculin, indicating that an inherent flexibility of vinculin has a large influence on its allostery. We also find a cooperative dissociation between the head and tail domains of vinculin with increasing temperature in both thermodynamic and kinetic simulations, implying that vinculin may function as an allosteric switch in response to external signals. We show that the kinetics of vinculin unfolding exhibits specific sequential patterns, suggesting that a sophisticated interplay between domains may synergistically contribute to vinculin activation. We further find that the interaction between vinculin-binding site peptide from talin and vinculin significantly destabilizes the intramolecular head-tail interactions, suggesting a direct role of talin binding in vinculin activation.

Distinct roles of nonmuscle myosin II isoforms in the regulation of MDA-MB-231 breast cancer cell spreading and migration

Initial stages of tumor cell metastasis involve an epithelial-mesenchyme transition that involves activation of amoeboid migration and loss of cell-cell adhesion. The actomyosin cytoskeleton has fundamental but poorly understood roles in these events. Myosin II, an abundant force-producing protein, has roles in cell body translocation and retraction of the posterior of the cell during migration. Recent studies have suggested that this protein may also have roles in leading edge protrusive events. The metastasis-promoting protein metastasin-1, a regulator of myosin II assembly, colocalizes with myosin IIA at the leading edge of cancer cells, suggesting direct roles for myosin II in metastatic behavior. We have assessed the roles of specific myosin II isoforms during lamellar spreading of MDA-MB-231 breast cancer cells on extracellular matrix. We find that the two major myosin II isoforms IIA and IIB are both expressed in these cells, and both are recruited dramatically to the lamellar margin during active spreading on fibronectin. There is also a transient increase in regulatory light chain phosphorylation that correlates the recruitment of myosin IIA and myosin IIB into this spreading margin. Pharmacologic inhibition of myosin II or myosin light chain kinase dramatically reduced spreading. Depletion of myosin IIA via small interfering RNA impaired migration but enhanced lamellar spreading, whereas depletion of myosin IIB impaired not only migration but also impaired initial rates of lamellar spreading. These results indicate that both isoforms are critical for the mechanics of cell migration, with myosin IIB seeming to have a preferential role in the mechanics of lamellar protrusion.

Polymer microarrays: identification of substrates for phagocytosis assays

A polymer microarray of 120 polyurethanes was used to identify polymers that promoted the adhesion of bone marrow dendritic cells (BMDC). Identified polymers were coated onto glass cover slips and shown to be efficient substrates for the immobilisation of these primary cells, which underwent efficient phagocytosis while still presumably maintaining their immature state.

Activation of myosin in HeLa cells causes redistribution of focal adhesions and F-actin from cell center to cell periphery

Activation of actomyosin II by phosphorylation of its regulatory light chain is one of the main factors involved in the regulation of cytoskeletal dynamics. Phosphorylation of myosin regulatory light chain may be mediated directly and indirectly by several kinases including myosin light chain kinase (MLCK) and kinases activated by small GTP-binding proteins. Most of the myosin kinases, including PAK, can also interact with other proteins through binding sites located outside of their catalytic domains. In an attempt to study the effects due only to phosphorylation of myosin light chain, we expressed the constitutively active catalytic domain of ameba PAK in HeLa cells. The catalytic domain phosphorylates myosin light chain in vitro with high specific activity but has none of the sequences that target mammalian PAK to other proteins and membranes. Expression of the catalytic domain caused disassembly of focal adhesions and stress fibers in the cell center and accumulation of focal adhesions and F-actin at the cell periphery. There was a twofold increase in the phosphorylation level of endogenous myosin light chain and changes in cell shape consistent with enhanced cell contractility. The phenotype was independent of MLCK, ROCK, MEK, Rac, and Rho activities but was abolished by blebbistatin, a specific inhibitor of myosin II activity. Our data are consistent with myosin being directly phosphorylated by the expressed catalytic domain of ameba PAK with the induced phenotype resulting from cell retraction driven by contraction of peripheral actomyosin. The phenotype induced by expression of the catalytic domain is reminiscent of that caused by expression of active mammalian PAK, suggesting that myosin phosphorylation may play an important role in PAK-induced cytoskeletal changes. The catalytic domain of ameba PAK may be a useful tool for studying the effects of myosin light chain phosphorylation in other cells.

ER-bound PTP1B is targeted to newly forming cell-matrix adhesions

Here, we define the mechanism through which protein tyrosine phosphatase 1B (PTP1B) is targeted to cell-matrix adhesion sites. Green fluorescent protein (GFP)-labeled PTP1B bearing the substrate-trapping mutation D181A was found in punctate structures in lamellae. The puncta co-localized with focal adhesion kinase (FAK) and Src, and defined the distal tips of cell-matrix adhesion sites identified with paxillin and vinculin. PTP1B is largely associated with the external face of the endoplasmic reticulum (ER) and the puncta develop from ER projections over cell-matrix adhesion sites, a process dependent on microtubules. Deletion of the ER-targeting sequence resulted in cytosolic localization and altered the distribution of PTP1B at cell-matrix foci, whereas mutations disrupting interactions with Src homology 3 (SH3) domains, and the insulin and cadherin receptors had no effect. PTP1B recognizes substrates within forming adhesion foci as revealed by its preferential association with paxillin as opposed to zyxin-containing foci. Our results suggest that PTP1B targets to immature cell-matrix foci in newly forming lamellae by dynamic extensions of the ER and contributes to the maturation of these sites.

Multifunctional Ti-(Ca,Zr)-(C,N,O,P) films for load-bearing implants

Films of Ti-Ca-P-C-O-(N), Ti-Ca-C-O-(N) and Ti-Zr-C-O-(N) were deposited by DC magnetron sputtering or ion implantation-assisted magnetron sputtering of composite targets TiC0.5 + 10%Ca10(PO4)6(OH)2, TiC0.5 + 20%(CaO + TiO2) and TiC0.5 + 10%ZrO2 in an Ar atmosphere or reactively in a gaseous mixture of Ar + 14%N2. The microstructure, elemental and phase composition of films were studied by means of X-ray diffraction, transmission electron microscopy, scanning force microscopy, X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. The films were characterized in terms of their hardness, Young's modulus, elastic recovery, adhesion strength, and friction and wear both in air and under physiological solution. Particular attention was paid to the analysis of deformation and fracture for various film/substrate systems during scratch testing. The biocompatibility of the films was evaluated by both in vitro and in vivo experiments. In vitro studies involved the investigation of adhesion, spreading, and proliferation of MC3T3-E1 osteoblasts and IAR-2 epithelial cells, morphometric analysis, actin cytoskeleton, focal contacts staining, alkaline phosphatase activity and von Kossa staining of osteoblastic culture. Cell culture experiments demonstrated an increase of osteoblastic proliferation on Ca- and P-incorporated films. In vivo studies were fulfilled by subcutaneous implantation of Teflon plates coated with the tested films in mice and analysis of the population of adherent cells on their surfaces. The results obtained show that multicomponent nanostructured Ti-(Ca, Zr)-(C, N, O, P) films possess a combination of high hardness, wear resistance and adhesion strength, reduced Young's modulus, low friction coefficient and high biocompatibility.

Regulation of sphingosine 1-phosphate-induced endothelial cytoskeletal rearrangement and barrier enhancement by S1P1 receptor, PI3 kinase, Tiam1/Rac1, and alpha-actinin

Endothelial cell (EC) barrier dysfunction results in increased vascular permeability observed in inflammation, tumor angiogenesis, and atherosclerosis. The platelet-derived phospholipid sphingosine-1-phosphate (S1P) decreases EC permeability in vitro and in vivo and thus has obvious therapeutic potential. We examined S1P-mediated human pulmonary artery EC signaling and barrier regulation in caveolin-enriched microdomains (CEM). Immunoblotting from S1P-treated EC revealed S1P-mediated rapid recruitment (1 microM, 5 min) to CEMs of the S1P receptors S1P1 and S1P3, p110 PI3 kinase alpha and beta catalytic subunits, the Rac1 GEF, Tiam1, and alpha-actinin isoforms 1 and 4. Immunoprecipitated p110 PI3 kinase catalytic subunits from S1P-treated EC exhibited PIP3 production in CEMs. Immunoprecipitation of S1P receptors from CEM fractions revealed complexes containing Tiam1 and S1P1. PI3 kinase inhibition (LY294002) attenuated S1P-induced Tiam1 association with S1P1, Tiam1/Rac1 activation, alpha-actinin-1/4 recruitment, and EC barrier enhancement. Silencing of either S1P1 or Tiam1 expression resulted in the loss of S1P-mediated Rac1 activation and alpha-actinin-1/4 recruitment to CEM. Finally, silencing S1P1, Tiam1, or both alpha-actinin isoforms 1/4 inhibits S1P-induced cortical F-actin rearrangement and S1P-mediated barrier enhancement. Taken together, these results suggest that S1P-induced recruitment of S1P1 to CEM fractions promotes PI3 kinase-mediated Tiam1/Rac1 activation required for alpha-actinin-1/4-regulated cortical actin rearrangement and EC barrier enhancement.

Integration of cell adhesion reactions—a balance of forces?

The rearrangement of receptors by oligomeric adhesion molecules constitutes a configurational mechanism able to sculpture membranes and dislocate receptors from cytoplasmic anchorage. This provides a conceptual framework for complex cellular processes in mechanical terms, as a dynamic balance between extracellular and intracellular driving forces.

Endothelial nitric oxide synthase is segregated from caveolin-1 and localizes to the leading edge of migrating cells

The enzyme endothelial Nitric Oxide Synthase (eNOS) is involved in key physiological and pathological processes, including cell motility and apoptosis. It is widely believed that at the cell surface eNOS is localized in caveolae, where caveolin-1 negatively regulates its activity, however, there are still uncertainties on its intracellular distribution. Here, we applied high resolution confocal microscopy to investigate the surface distribution of eNOS in transfected HeLa cells and in human umbilical vein endothelial cells (HUVEC) endogenously expressing the enzyme. In confluent and non-confluent HUVEC and HeLa cells, we failed to detect substantial colocalization between eNOS and caveolin-1 at the cell surface. Instead, in non-confluent cells, eNOS was concentrated in ruffles and at the leading edge of migrating cells, colocalizing with actin filaments and with the raft marker ganglioside G(M1), and well segregated from caveolin-1, which was restricted to the posterior region of the cells. Treatments that disrupted microfilaments caused loss of eNOS from the cell surface and decreased Ca(2+)-stimulated activity, suggesting a role of the cytoskeleton in the localization and function of the enzyme. Our results provide a morphological correlate for the role of eNOS in cell migration and raise questions on the site of interaction between eNOS and caveolin-1.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Migration of bone marrow stromal cells in 3D: 4 Color methodology reveals spatially and temporally coordinated events

The cytoskeleton plays a central role in many cell processes including directed cell migration. Since most previous work has investigated cell migration in two dimensions (2D), new methods are required to study movement in three dimensions (3D) while preserving 3D structure of the cytoskeleton. Most previous studies have labeled two cytoskeletal networks simultaneously, impeding an appreciation of their complex and dynamic interconnections. Here we report the development of a 4 color method to simultaneously image vimentin, actin, tubulin and the nucleus for high-resolution confocal microscopy of bone-marrow stromal cells (BMSCs) migrating through a porous membrane. Several methods were tested for structural preservation and labeling intensity resulting in identification of an optimized simultaneous fixation and permeabilization method using glutaraldehyde, paraformaldehyde and Triton X-100 followed by a quadruple fluorescent labeling method. This procedure was then applied at a sequence of time points to migrating cells, allowing temporal progression of migration to be assessed by visualizing all three networks plus the nucleus, providing new insights into 3D directed cell migration including processes such as leading edge structure, cytoskeletal distribution and nucleokinesis. Colocalization of actin and microtubules with distinct spatial arrangements at the cellular leading edge during migration, together with microtubule axial polarization supports recent reports indicating the pivotal role of microtubules in directed cell migration. This study also provides a foundation for 3D migration studies versus 2D studies, providing precise and robust methods to attain new insights into the cellular mechanisms of motility.

Keratinocyte Growth Factor Promotes Melanosome Transfer to Keratinocytes

Melanogenesis and melanosome transfer from the melanocytes to the neighboring keratinocytes are induced by ultraviolet radiation and modulated by autocrine and paracrine factors. Keratinocyte growth factor (KGF/fibroblast growth factor (FGF)7) is a paracrine mediator of human keratinocyte growth and differentiation. We evaluated the influence of KGF on melanosome transfer in co-cultures of keratinocytes and melanocytes. Immunofluorescence analysis using anti-tyrosinase and anti-human cytokeratin antibodies, phagocytic assays using fluorescent latex beads, and ultrastructural analysis indicated that KGF is able to induce melanosome transfer acting only on the recipient keratinocytes and as a consequence of a general role of KGF in the promotion of the phagocytic process. Inhibition of proteinase-activated receptor-2, to block the Rho-dependent phagocytic pathway, or of the Src family tyrosine kinases, to inhibit the Rac-dependent pathway, showed that KGF promotes phagocytosis through both mechanisms. Increased expression of the KGF receptor (KGFR) on the keratinocytes by transfection led to increased phagocytosis of latex beads following KGF treatment, suggesting that the KGF effect is directly mediated by KGFR expression and activation. Moreover, confocal microscopic analysis revealed that KGFR localize in phagosomes during KGF-induced phagocytosis, suggesting a direct role of the receptor in regulating both the early steps of uptake and the intracellular traffic of the phagosomes.

Interferon-gamma inhibits T84 epithelial cell migration by redirecting transcytosis of beta1 integrin from the migrating leading edge

Intestinal inflammation is associated with epithelial damage and formation of mucosal wounds. Epithelial cells migration is required for wound closure. In inflammatory status, migrating epithelial cells are exposed to proinflammatory cytokines such as IFN-gamma. However, influence of such cytokines on intestinal epithelial wound closure remains unknown. The present study was designed to investigate the effect of IFN-gamma on migration of model T84 intestinal epithelial cells and recovery of epithelial wounds. IFN-gamma significantly inhibited rate of T84 cell migration and closure of epithelial wounds. This effect was accompanied by the formation of large aberrant lamellipodia at the leading edge as well as significant decrease in the number of beta(1) integrin containing focal adhesions. IFN-gamma exposure increased endocytosis of beta(1) integrin and shifted its accumulation from early/recycling endosomes at the leading edge to a yet unidentified compartment at the cell base. This redirection in beta(1) integrin transcytosis was inhibited by depolymerization of microtubules with nocodazole and was unaffected by stabilization of microtubules with docetaxel. These results suggest that IFN-gamma attenuates epithelial wound closure by microtubule-dependent redirection of beta(1) integrin transcytosis from the leading edge of migrating cells thereby inhibiting adequate turnover of focal adhesion complexes and cell migration.

Sphingosine-1-phosphate mediates migration of mature dendritic cells

Sphingosine-1-phosphate (S1P) represents a potent modulator of diverse cellular activities, including lymphocyte trafficking and maintenance of lymphocyte homeostasis. The five known receptors for S1P (S1P(1-5)) belong to the family of G protein-coupled receptors. Upon binding S1P, they act downstream via heterotrimeric G proteins on members of the small GTPase family (Cdc42/Rac/Rho), evoking a S1P receptor-dependent activation pattern of Cdc42, Rac, and Rho, respectively. This, in turn, triggers cytoskeletal rearrangements determining cellular morphology and movement. In this study we investigated the effects of S1P on murine dendritic cells (DC). Mature DC, but not immature in vitro differentiated DC, were found to migrate to S1P, a phenomenon that correlated to the up-regulation of S1P1 and S1P3 in maturing DC. The same pattern of S1P receptor regulation could be observed in vivo on skin DC after their activation and migration into the lymph node. The migration-inducing effect of S1P could be severely hampered by application of the S1P analogon FTY720 in vitro and in vivo. A similar, yet more pronounced, block was observed upon preventing Cdc42/Rac and/or Rho activation by specific inhibitors. These results suggest that S1P-mediated signaling plays a pivotal role in the life cycle of DC.

Drebrin, an Actin-Binding, Cell-Type Characteristic Protein: Induction and Localization in Epithelial Skin Tumors and Cultured Keratinocytes

Isoform E2 of drebrin, an actin-binding protein originally identified in neuronal cells, has recently been identified in diverse non-neuronal cells, mostly in association with cell processes and intercellular junctions. Here, we report on the presence of drebrin in normal human skin, epithelial skin cancers, and cultured keratinocytes. Keratinocytes of normal epidermis contain almost no drebrin but the protein is readily seen in hair follicles. By immunohistochemistry and immunoblot, basal cell carcinomas (BCC) are rich in drebrin, and confocal laser scanning and immunoelectron microscopy show accumulation at adhering junctions, in co-localization with actin and partially with plaque proteins. In squamous cell carcinomas, keratoacanthomas, and in epidermal precancers, drebrin is heterogeneously distributed, appearing as mosaics. Primary keratinocyte cultures contain significant amounts of drebrin enriched at adhering junctions. When epithelium-derived cells devoid of drebrin are transfected with drebrin-enhanced green fluorescent protein, constructs accumulate in the cell periphery, and immunoprecipitation shows complexes with actin. During epidermal growth factor induced formation of cell processes, drebrin retains this junction association, as observed by live cell microscopy. Our results suggest novel functions of drebrin such as an involvement in cell-cell adhesion and tumorigenesis and a potential value in diagnosis of BCC.

The last but not the least: the origin and significance of trailing adhesions in fibroblastic cells

Mature adhesions in a motile fibroblast can be classified as stationary "towing" adhesions in the front and sliding trailing adhesions that resist the traction force. Adhesions formed at the front of motile fibroblasts rarely reach the trailing zone, due to disassembly promoted by intensive microtubule targeting. Here, we show that the majority of adhesions found at the trailing edge originate within small short-lived protrusions that extend laterally and backwards from the cell edge. These adhesions enlarge by sliding and by fusion with neighboring adhesions. A further subset of trailing adhesions is initiated at a novel site proximal to trailing stress fibre termini. Following tail retraction, trailing adhesions are actively regenerated and the stress fibre system is remodeled accordingly; the tensile forces elaborated by the contractile actin system are consequently redirected according to trailing adhesion location. We conclude that persistent and dynamic anchorage of the cell rear is needed for the maintenance of continuous unidirectional movement of fibroblasts.

FGF signal regulates gastrulation cell movements and morphology through its target NRH

We used cDNA microarray analysis to screen for FGF target genes in Xenopus embryos treated with the FGFR1 inhibitor SU5402, and identified neurotrophin receptor homolog (NRH) as an FGF target. Causing gain of NRH function by NRH mRNA or loss of NRH function using a Morpholino antisense-oligonucleotide (Mo) led to gastrulation defects without affecting mesoderm differentiation. Depletion of NRH by the Mo perturbed the polarization of cells in the dorsal marginal zone (DMZ), thereby inhibiting the intercalation of the cells during convergent extension as well as the filopodia formation on DMZ cells. Deletion analysis showed that the carboxyl-terminal region of NRH, which includes the "death domain," was necessary and sufficient to rescue gastrulation defects and to induce the protrusive cell morphology. Furthermore, we found that the FGF signal was both capable of inducing filopodia in animal cap cells, where they do not normally form, and necessary for filopodia formation in DMZ cells. Finally, we demonstrated that FGF required NRH function to induce normal DMZ cell morphology. This study is the first to identify an in vivo role for FGF in the regulation of cell morphology, and we have linked this function to the control of gastrulation cell movements via NRH.

Vinculin--a dynamic regulator of cell adhesion

The ability of cells to tightly adhere to one another and to the extracellular matrix is fundamentally important in numerous biological processes, including embryogenesis, wound healing and maintenance of tissue integrity. Vinculin, a protein localized at the cytoplasmic face of cell-matrix and cell-cell adhesions, is required for strong cell adhesion. Two new crystal structures reveal that vinculin exhibits a high degree of structural plasticity upon ligand binding that might promote rapid changes in cell adhesion.

Src regulates distinct pathways for cell volume control through Vav and phospholipase Cgamma

Cell volume recovery in response to swelling requires reorganization of the cytoskeleton and fluid efflux. We have previously shown that electrolyte and fluid efflux via K+ and Cl- channels is controlled by swelling-induced activation of phospholipase Cgamma (PLCgamma). Recently, integrin engagement has been suggested to trigger responses to swelling through activation of Rho family GTPases and Src kinases. Because both PLCgamma and Rho GTPases can be regulated by Src during integrin-mediated cytoskeletal reorganization, we sought to identify swelling-induced Src effectors. Upon hypotonic challenge, Src was rapidly activated in transient plasma membrane protrusions, where it colocalized with Vav, an activator of Rho GTPases. Inhibition of Src with PP2 attenuated phosphorylation of Vav. PP2 also attenuated phosphorylation of PLCgamma, and inhibited swelling-mediated activation of K+ and Cl- channels and cell volume recovery. These findings suggest that swelling-induced Src regulates cytoskeletal dynamics, through Vav, and fluid efflux, through PLCgamma, and thus can coordinate structural reorganization with fluid balance to maintain cellular integrity.

Functional role of syndecan-1 cytoplasmic V region in lamellipodial spreading, actin bundling, and cell migration

Cell protrusions contribute to cell motility and migration by mediating the outward extension and initial adhesion of cell edges. In many cells, these extensions are supported by actin bundles assembled by the actin cross-linking protein, fascin. Multiple extracellular cues regulate fascin and here we focus on the mechanism by which the transmembrane proteoglycan, syndecan-1, specifically activates lamellipodial cell spreading and fascin-and-actin bundling when clustered either by thrombospondin-1, laminin, or antibody to the syndecan-1 extracellular domain. There is almost no knowledge of the signaling mechanisms of syndecan-1 cytoplasmic domain and we have tested the hypothesis that the unique V region of syndecan-1 cytoplasmic domain has a crucial role in these processes. By four criteria--the activities of N-cadherin/V region chimeras, syndecan-1 deletion mutants, or syndecan-1 point mutants, and specific inhibition by a membrane-permeable TAT-V peptide--we demonstrate that the V region is necessary and sufficient for these cell behaviors and map the molecular basis for its activity to multiple residues located across the V region. These activities correlate with a V-region-dependent incorporation of cell-surface syndecan-1 into a detergent-insoluble form. We also demonstrate functional roles of syndecan-1 V region in laminin-dependent C2C12 cell adhesion and three-dimensional cell migration. These data identify for the first time specific cell behaviors that depend on signaling through the V region of syndecan-1.

Glycoprotein gp130 of dictyostelium discoideum influences macropinocytosis and adhesion

Glycoprotein gp130, found on the plasma membrane of Dictyostelium discoideum amoebae, was postulated previously to play a role in phagocytosis. The gene for gp130 was cloned and when translated, yielded a 768 amino acid preproprotein of 85.3 kDa. It had nearly 40% similarity to the 138 kDa family of glycoproteins implicated in sexual cell fusion during macrocyst formation in D. discoideum. The difference between the calculated size and observed M(r) of 130 kDa on protein gels likely was due to N-glycosylation that was confirmed by lectin blots. Consistent with its surface-exposure, an antibody raised against recombinant protein stained the plasma membrane of D. discoideum amoebae. Gp130 and its transcripts were high during axenic growth of cells, but relatively low during growth on bacteria. The gene for gp130 was disrupted and cell lines lacking the glycoprotein were efficient phagocytes, indicating that gp130 was dispensable for phagocytosis. Gp130-null cells were similar in size to parent DH1 cells, had enhanced macropinocytosis and grew faster to higher densities. They also exhibited weaker cell-substrate adhesion but displayed greater cell-cell cohesion. Collectively, the data indicated that gp130 influenced macropinocytosis and played a role in adhesion during vegetative growth.

Design, characterization and testing of Ti-based multicomponent coatings for load-bearing medical applications

A comparative investigation of multicomponent thin films based on the systems Ti-Ca-C-O-(N), Ti-Zr-C-O-(N), Ti-Si-Zr-O-(N) and Ti-Nb-C-(N) is presented. TiC(0.5) + 10%CaO, TiC0.5 + 20%CaO, TiC0.5 + 10%ZrO2, TiC0.5 + 20%ZrO2, Ti5Si3 + 10%ZrO2, TiC0.5 + 10%Nb2C and TiC0.5 + 30%Nb2C composite targets were manufactured by means of self-propagating high-temperature synthesis, followed by DC magnetron sputtering in an atmosphere of argon or in a gaseous mixture of argon and nitrogen. The films were characterized in terms of their structure, chemical composition, surface topography, hardness, elastic modulus, elastic recovery, surface charge, friction coefficient, and wear rate. The biocompatibility of the films was evaluated by both in vitro and in vivo experiments. In vitro studies involved the investigation of the proliferation of Rat-1 fibroblasts and IAR-2 epithelial cells on the tested films, morphometric analysis and actin cytoskeleton staining of the cells cultivated on the films. In vivo studies were fulfilled by subcutaneous implantation of Teflon plates coated with the tested films in mice and analysis of the population of cells on the surfaces. The films deposited under optimal conditions showed high hardness in the range of 30-37 GPa, significant reduced Young's modulus, low friction coefficient down to 0.1-0.2 and low wear rate in comparison with conventional magnetron-sputtered TiC and TiN films. The surface of all films was negatively charged with an outstanding shift between the Ar and Ar + N2 Zeta potential curves that reaches 5 mV at the highest pH values. We did not detect statistically significant differences in the attachment, spreading and cell shape of cultured IAR-2 and Rat-1 cells on the Ti-Ca-C-O-(N), Ti-Zr-C-O-(N) (TiC0.5 + 10%ZrO2 target), Ti-Si-Zr-O-(N) films and the uncoated substrata. The adhesion and proliferation of cultured cells in vitro was perfect at all investigated films. Assessment of the population of cells covering on the Teflon plates coated with the Ti-Ca-C-O-(N) and Ti-Zr-C-O-(N) films after 16 weeks of subcutaneous implantation revealed the high biocompatibility level of tested films and absence of inflammatory reactions in mice. Contrary, the epitheliocytes and fibroblasts cultivated on the Ti-Zr-C-O-(N) (TiC0.5 + 20%ZrO2 target) and Ti-Nb-C-(N) films had disturbing actin cytoskeleton.

Localized glucose and water influx facilitates Cryptosporidium parvum cellular invasion by means of modulation of host-cell membrane protrusion

Dynamic membrane protrusions such as lamellipodia and filopodia are driven by actin polymerization and often hijacked by intracellular microbes to enter host cells. The overall rate of membrane protrusion depends on the actin polymerization rate and the increase of localized cell volume. Although the signaling pathways involving actin polymerization are well characterized, the molecular mechanisms regulating local cell volume associated with membrane protrusion are unclear. Cryptosporidium parvum, an intracellular parasite, depends on host-cell membrane protrusion to accomplish cell entry and form the parasitophorous vacuole. Here, we report that C. parvum infection of cholangiocytes recruits host-cell SGLT1, a Na+/glucose cotransporter, and aquaporin 1 (AQP1), a water channel, to the attachment site. SGLT1-dependent glucose uptake occurs at the attachment site. Concordantly, the region of attachment displays localized water influx that is inhibited by either suppression of AQP1 by means of AQP1-small interfering RNA (siRNA) or inhibition of SGLT1 by a specific pharmacologic inhibitor, phlorizin. Inhibition of SGLT1 does not affect actin accumulation but decreases the membrane protrusion at the attachment site. Moreover, functional inhibition of host-cell AQP1 and SGLT1 hampers C. parvum invasion of cholangiocytes. Thus, glucose-driven, AQP-mediated localized water influx is involved in the membrane protrusion during C. parvum cellular invasion, phenomena that may also be relevant to the mechanisms of cell membrane protrusion in general.

Myosin VI: cellular functions and motor properties

Myosin motor proteins use the energy derived from ATP hydrolysis to move cargo along actin tracks. Myosin VI, unlike almost all other myosins, moves toward the minus end of actin filaments and functions in a variety of intracellular processes such as vesicular membrane traffic, cell migration, and mitosis. These diverse roles of myosin VI are mediated by interaction with a number of different binding partners present in multi-protein complexes. Myosin VI can work in vitro as a processive dimeric motor and as a nonprocessive monomeric motor, each with a large working stroke. The possibility that both monomeric and dimeric forms of myosin VI operate in the cell may represent an important regulatory mechanism for controlling the multiple steps in transport pathways where nonprocessive and processive motors are required.

Membrane ruffles in cell migration: indicators of inefficient lamellipodia adhesion and compartments of actin filament reorganization

During epithelial cell migration, membrane ruffles can be visualized by phase contrast microscopy as dark waves arising at the leading edge of lamellipodia that move centripetally toward the main cell body. Despite the common use of the term membrane ruffles, their structure, molecular composition, and the mechanisms leading to their formation remained largely unknown. We show here that membrane ruffles differ from the underlying cell lamella by more densely packed bundles of actin filaments that are enriched in the actin cross-linkers filamin and ezrin, pointing to a specific bundling process based on these cross-linkers. The accumulation of phosphorylated, that is, inactivated, cofilin in membrane ruffles suggests that they are compartments of inhibited actin filament turnover. High Rac1 and low RhoA activities were found under conditions of suboptimal integrin-ligand interaction correlating with low lamellipodia persistence, inefficient migration, and high ruffling rates. Based on these findings, we define membrane ruffles as distinct compartments of specific composition that form as a consequence of inefficient lamellipodia adhesion.

Small G-protein Rho is involved in the maintenance of cardiac myocyte morphology

The use of small membrane-permeable sequences or protein transduction domains (PTDs) can facilitate the transport of proteins into many cell types. In preliminary studies with the application of three PTDs (penetratin, modified penetratin, and the HIV TAT transduction domains) to cardiac myocytes, we found that the TAT and penetratin sequences showed high efficiency of uptake and low toxicity. Rho has been previously shown to be an important regulator of cytoskeletal organization and morphology in other non-cardiac cell types. To evaluate a role for Rho in cardiac myocyte morphology, we used the TAT-PTD to deliver a RhoA-specific inhibitor, the C3 exoenzyme, to cultured cardiac myocytes. We showed that this incubation with TAT-C3 abolished the basal levels of RhoA activity, demonstrating the efficacy of this treatment. Incubation with TAT-C3 also altered cardiac myocyte morphology so that TAT-C3-treated cells produced multiple projections from the major cell body. This was accompanied by a statistically significant increase in cell size, albeit to a lesser extent than the changes accompanying exposure to the hypertrophic agent, endothelin-1. Furthermore, the change in size of TAT-C3-treated cells was not accompanied by the induction of atrial natriuretic factor (ANF) expression that accompanies the hypertrophy of cardiac myocytes. These results reveal a role for RhoA in the maintenance of normal myocyte morphology.

Cellular mechanisms of bacterial internalization counteracted by Yersinia

Upon host-cell contact, human pathogenic Yersinia species inject Yop virulence effectors into the host through a Type III secretion-and-translocation system. These virulence effectors cause a block in phagocytosis (YopE, YopT, YpkA, and YopH) and suppression of inflammatory mediators (YopJ). The Yops that block phagocytosis either interfere with the host cell actin regulation of Rho GTPases (YopE, YopT, and YpkA) or specifically and rapidly inactivate host proteins involved in signaling from the receptor to actin (YopH). The block in uptake has been shown to be activated following binding to Fc, Complement, and beta1-integrin receptors in virtually any kind of host cell. Thus, the use of Yersinia as a model system to study Yersinia-host cell interactions provides a good tool to explore signaling pathways involved in phagocytosis.

alpha3beta1 integrin modulates neuronal migration and placement during early stages of cerebral cortical development

We show that alpha3 integrin mutation disrupts distinct aspects of neuronal migration and placement in the cerebral cortex. The preplate develops normally in alpha3 integrin mutant mice. However, time lapse imaging of migrating neurons in embryonic cortical slices indicates retarded radial and tangential migration of neurons, but not ventricular zone-directed migration. Examination of the actin cytoskeleton of alpha3 integrin mutant cortical cells reveals aberrant actin cytoskeletal dynamics at the leading edges. Deficits are also evident in the ability of developing neurons to probe their cellular environment with filopodial and lamellipodial activity. Calbindin or calretinin positive upper layer neurons as well as the deep layer neurons of alpha3 integrin mutant mice expressing EGFP were misplaced. These results suggest that alpha3beta1 integrin deficiency impairs distinct patterns of neuronal migration and placement through dysregulated actin dynamics and defective ability to search and respond to migration modulating cues in the developing cortex.

Paxillin: adapting to change

Molecular scaffold or adaptor proteins facilitate precise spatiotemporal regulation and integration of multiple signaling pathways to effect the optimal cellular response to changes in the immediate environment. Paxillin is a multidomain adaptor that recruits both structural and signaling molecules to focal adhesions, sites of integrin engagement with the extracellular matrix, where it performs a critical role in transducing adhesion and growth factor signals to elicit changes in cell migration and gene expression.

SHEP1 Function in Cell Migration Is Impaired by a Single Amino Acid Mutation That Disrupts Association with the Scaffolding Protein Cas but Not with Ras GTPases

SHEP1 is a signaling protein that contains a guanine nucleotide exchange factor-like domain, which binds Ras family GTPases and also forms a stable complex with the scaffolding protein Crk-associated substrate (Cas). SHEP1 and Cas have several common functions, such as increasing c-Jun N-terminal kinase activity, promoting T cell activation, and regulating the actin cytoskeleton. However, it is unclear whether a physical association between SHEP1 and Cas is required for these activities. We reported previously that SHEP1 is tyrosine-phosphorylated downstream of the EphB2 receptor; in this study, we further demonstrate that activated EphB2 inhibits SHEP1 association with Cas. To investigate whether phosphorylation negatively regulates the SHEP1-Cas complex, we have identified by mass spectrometry several SHEP1 tyrosine phosphorylation sites downstream of EphB2; of particular interest among them is tyrosine 635 in the Cas association/exchange factor domain. Mutation of this tyrosine to glutamic acid, but not to phenylalanine, disrupts Cas binding to SHEP1 without inhibiting Ras GTPase binding. The glutamic acid mutation also makes SHEP1 unable to promote Cas-Crk association, membrane ruffling, and cell migration toward epidermal growth factor (EGF), implying that these activities of SHEP1 depend upon a physical interaction with Cas. Association with Cas also seems to be necessary for EGF-induced SHEP1 tyrosine phosphorylation, which is mediated by a Src family kinase. It is noteworthy that EGF stimulation does not cause dissociation of SHEP1 from Cas. These data show that SHEP1 regulates membrane ruffling and cell migration and that binding to Cas is probably critical for these functions. Furthermore, the SHEP1-Cas complex may have different roles downstream of EphB2 and the EGF receptor.

Phosphorylation of actopaxin regulates cell spreading and migration

Actopaxin is an actin and paxillin binding protein that localizes to focal adhesions. It regulates cell spreading and is phosphorylated during mitosis. Herein, we identify a role for actopaxin phosphorylation in cell spreading and migration. Stable clones of U2OS cells expressing actopaxin wild-type (WT), nonphosphorylatable, and phosphomimetic mutants were developed to evaluate actopaxin function. All proteins targeted to focal adhesions, however the nonphosphorylatable mutant inhibited spreading whereas the phosphomimetic mutant cells spread more efficiently than WT cells. Endogenous and WT actopaxin, but not the nonphosphorylatable mutant, were phosphorylated in vivo during cell adhesion/spreading. Expression of the nonphosphorylatable actopaxin mutant significantly reduced cell migration, whereas expression of the phosphomimetic increased cell migration in scrape wound and Boyden chamber migration assays. In vitro kinase assays demonstrate that extracellular signal-regulated protein kinase phosphorylates actopaxin, and treatment of U2OS cells with the MEK1 inhibitor UO126 inhibited adhesion-induced phosphorylation of actopaxin and also inhibited cell migration.

Shape and motility of a model cell: A computational study

We have investigated the shape, size, and motility of a minimal model of an adherent biological cell using the Monte Carlo method. The cell is modeled as a two dimensional ring polymer on the square lattice enclosing continuously polymerizing and depolymerizing actin networks. Our lattice model is an approximate representation of a real cell at a resolution of one actin molecule, 5 nm. The polymerization kinetics for the actin network are controlled by appropriate reaction probabilities which correspond to the correct experimental reaction rates. Using the simulation data we establish various scaling laws relating the size of the model cell to the concentration of polymerized and unpolymerized actin molecules and the length of the enclosing membrane. The computed drift velocities, which characterize the motility of the cell, exhibit a maximum at a certain fraction of polymerized actin which agrees with physiological fractions observed in experiments. The appearance of the maximum is related to the competition between the polymerization-induced protrusion of the membrane and the concomitant suppression of membrane fluctuations.

Aggregation of Integrins and RhoA Activation Are Required for Thy-1-induced Morphological Changes in Astrocytes

Thy-1, a cell adhesion molecule abundantly expressed in mammalian neurons, binds to a beta(3)-containing integrin on astrocytes and thereby stimulates the assembly of focal adhesions and stress fibers. Such events lead to morphological changes in astrocytes that resemble those occurring upon injury in the brain. Extracellular matrix proteins, typical integrin ligands, bind to integrins and promote receptor clustering as well as signal transduction events that involve small G proteins and cytoskeletal changes. Here we investigated the possibility that the cell surface protein Thy-1, when interacting with a beta(3)-containing integrin on astrocytes, could trigger signaling events similar to those generated by extracellular matrix proteins. DI-TNC(1) astrocytes were stimulated with Thy-1-Fc immobilized on beads, and increased RhoA activity was confirmed using an affinity precipitation assay. The effect of various inhibitors on the cellular response was also studied. The presence of Y-27632, an inhibitor of Rho kinase (p160ROCK), a key downstream effector of RhoA, significantly reduced focal adhesion and stress fiber formation induced by Thy-1. Similar effects were obtained when astrocytes were treated with C3 transferase, an inhibitor of RhoA. Alternatively, astrocytes were transfected with an expression vector encoding fusion proteins of enhanced green fluorescent protein with either the Rho-binding domain of Rhotekin, which blocks RhoA function, or the dominant-negative N19RhoA mutant. In both cases, Thy-1-induced focal adhesion formation was inhibited. Furthermore, we observed that RhoA activity after stimulation with soluble Thy-1-Fc molecule was augmented upon further cross-linking using protein A-Sepharose beads. The same was shown by cross-linking beta(3)-containing integrin with anti-beta(3) antibodies. Together, these results indicate that Thy-1-mediated astrocyte stimulation depended on beta(3) integrin clustering and the resulting increase in RhoA activity.

Crystal structure of human vinculin

Alterations in the actin cytoskeleton following the formation of cell-matrix and cell-cell junctions are orchestrated by vinculin. Vinculin associates with a large number of cytoskeletal and signaling proteins, and this flexibility is thought to contribute to rapid dissociation and reassociations of adhesion complexes. Intramolecular interactions between vinculin's head (Vh) and tail (Vt) domains limit access of its binding sites for other adhesion proteins. While the crystal structures of the Vh and Vt domains are known, these domains represent less than half of the entire protein and are separated by a large central region of unknown structure and function. Here we report the crystal structure of human full-length vinculin to 2.85 A resolution. In its resting state, vinculin is a loosely packed collection of alpha-helical bundles held together by Vh-Vt interactions. The three new well ordered alpha-helical bundle domains are similar in their structure to either Vh (Vh2 and Vh3) or to Vt (Vt2) and their loose packing provides the necessary flexibility that allows vinculin to interact with its various protein partners at sites of cell adhesion.

The giant protein HERC1 is recruited to aluminum fluoride-induced actin-rich surface protrusions in HeLa cells

HERC1 is a very large protein involved in membrane traffic through both its ability to bind clathrin and its guanine nucleotide exchange factor (GEF) activity over ARF and Rab family GTPases. Herein, we show that HERC1 is recruited onto actin-rich surface protrusions in ARF6-transfected HeLa cells upon aluminum fluoride (AlF(4)(-)) treatment. Moreover, the fact that HERC1 overexpression does not stimulate protrusion formation in the absence of AlF(4)(-), in conditions where ARNO does, indicates that HERC1 is not acting as an ARF6-GEF in this system, but that instead its recruitment takes place downstream of ARF6 activation. Finally, we suggest a phosphoinositide-binding mechanism whereby HERC1 may translocate to these protrusions.