Molecular diversity of cell-matrix adhesions

The integrin-linked kinase regulates cell morphology and motility in a rho-associated kinase-dependent manner

The integrin-linked kinase (ILK) is a multidomain focal adhesion protein implicated in signal transmission from integrin and growth factor receptors. We have determined that ILK regulates U2OS osteosarcoma cell spreading and motility in a manner requiring both kinase activity and localization. Overexpression of wild-type (WT) ILK resulted in suppression of cell spreading, polarization, and motility to fibronectin. Cell lines overexpressing kinase-dead (S343A) or paxillin binding site mutant ILK proteins display inhibited haptotaxis to fibronectin. Conversely, spreading and motility was potentiated in cells expressing the "dominant negative," non-targeting, kinase-deficient E359K ILK protein. Suppression of cell spreading and motility of WT ILK U2OS cells could be rescued by treatment with the Rho-associated kinase (ROCK) inhibitor Y-27632 or introduction of dominant negative ROCK or RhoA, suggesting these cells have increased RhoA signaling. Activation of focal adhesion kinase (FAK), a negative regulator of RhoA, was reduced in WT ILK cells, whereas overexpression of FAK rescued the observed defects in spreading and cell polarity. Thus, ILK-dependent effects on ROCK and/or RhoA signaling may be mediated through FAK.

Calpain-mediated proteolysis of talin regulates adhesion dynamics

Dynamic regulation of adhesion complexes is required for cell migration and has therefore emerged as a key issue in the study of cell motility. Recent progress has been made in defining some of the molecular mechanisms by which adhesion disassembly is regulated, including the contributions of adhesion adaptor proteins and tyrosine kinases. However, little is known about the potential contribution of proteolytic mechanisms to the regulation of adhesion complex dynamics. Here, we show that proteolysis of talin by the intracellular calcium-dependent protease calpain is critical for focal adhesion disassembly. We have generated a single point mutation in talin that renders it resistant to proteolysis by calpain. Quantification of adhesion assembly and disassembly rates demonstrates that calpain-mediated talin proteolysis is a rate-limiting step during adhesion turnover. Furthermore, we demonstrate that disassembly of other adhesion components, including paxillin, vinculin and zyxin, is also dependent on the ability of calpain to cleave talin, suggesting a general role for talin proteolysis in regulating adhesion turnover. Together, these findings identify calpain-mediated proteolysis of talin as a mechanism by which adhesion dynamics are regulated.

Early molecular events in the assembly of the focal adhesion-stress fiber complex during fibroblast spreading

Cell adhesion to the extracellular matrix triggers the formation of integrin-mediated contact and reorganization of the actin cytoskeleton. Examination of nascent adhesions, formed during early stages of fibroblast spreading, reveals a variety of forms of actin-associated matrix adhesions. These include: (1). small ( approximately 1 microm), dot-like, integrin-, vinculin-, paxillin-, and phosphotyrosine-rich structures, with an F-actin core, broadly distributed over the ventral surfaces of the cells; (2). integrin-, vinculin-, and paxillin-containing "doublets" interconnected by short actin bundles; (3). arrays of actin-vinculin complexes. Such structures were formed by freshly plated cells, as well as by cells recovering from latrunculin treatment. Time-lapse video microscopy of such cells, expressing GFP-actin, indicated that long actin cables are formed by an end-to-end lining-up and apparent fusion of short actin bundles. All these structures were prominent during cell spreading, and persisted for up to 30-60 min after plating. Upon longer incubation, they were gradually replaced by stress fibers, associated with focal adhesions at the cell periphery. Direct examination of paxillin and actin reorganization in live cells revealed alignment of paxillin doublets, forming long and highly dynamic actin bundles, undergoing translocation, shortening, splitting, and convergence. The mechanisms underlying the assembly and reorganization of actin-associated focal adhesions and the involvement of mechanical forces in regulating their dynamic properties are discussed.

Illuminating adhesion complexes in migrating cells: moving toward a bright future

Cell migration is a complex, tightly regulated process that involves the continuous formation and disassembly of adhesions. Despite the importance of these processes, very little is known about the factors that regulate adhesion dynamics during migration. Recent advances in imaging technologies are allowing monitoring of these processes during migration and are providing insight into the mechanisms that regulate them.

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

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

Talin B is required for force transmission in morphogenesis of Dictyostelium

Talin plays a key role in the assembly and stabilisation of focal adhesions, but whether it is directly involved in force transmission during morphogenesis remains to be elucidated. We show that the traction force of Dictyostelium cells mutant for one of its two talin genes talB is considerably smaller than that of wild-type cells, both in isolation and within tissues undergoing morphogenetic movement. The motility of mutant cells in tightly packed tissues in vivo or under strong resistance conditions in vitro was lower than that of wild-type cells, but their motility under low external force conditions was not impaired, indicating inefficient transmission of force in mutant cells. Antibody staining revealed that the talB gene product (talin B) exists as small units subjacent to the cell membrane at adhesion sites without forming large focal adhesion-like assemblies. The total amount of talin B on the cell membrane was larger in prestalk cells, which exert larger force than prespore cells during morphogenesis. We conclude that talin B is involved in force transmission between the cytoskeleton and cell exterior.

Induction of apoptosis in cultured endothelial cells by a cadherin antagonist peptide: involvement of fibroblast growth factor receptor-mediated signalling

Cadherins are a family of transmembrane glycoproteins mediating calcium-dependent, homophilic cell-cell adhesion. In addition, these molecules are involved in signaling events, regulating such processes as cell motility, proliferation, and apoptosis. Members of the cadherin subfamily, called either classical or type I cadherins, contain a highly conserved sequence at their homophilic binding site consisting of the three amino acids--histidine-alanine-valine (HAV). Previous studies have shown that peptides containing the HAV motif inhibit cadherin-dependent events such as cell aggregation, compaction, and neurite outgrowth. We report here that a cyclic peptide, N-Ac-CHAVC-NH2 can perturb cadherin-mediated endothelial cell interactions, resulting in a progressive apoptotic cell death. This effect depends on cell density, as it is only observed when dense cultures are treated with the peptide. Adherens junction (AJ)-associated cadherin and catenins are differentially affected by the N-Ac-CHAVC-NH2 treatment, as judged by double immunofluorescence labeling followed by immunofluorescence-ratio imaging. However, cell-cell adhesions are largely retained during the first few hours after addition of the peptide. It was also observed that following treatment, actin filaments partially lose their plasma membrane anchorage at AJs and translocate towards the cell center. Interestingly, addition of basic fibroblast growth factor to confluent, peptide-treated, endothelial cell cultures, completely blocks apoptosis and the inhibitory peptide reduce the phosphorylation of the FGF receptor target protein FRS2, suggesting that the peptide exerts its effect by inhibiting cadherin-mediated activation of fibroblast growth factor receptor signaling. We propose that cadherin-mediated signaling is essential for maintaining viability of confluent endothelial cells, and that its perturbation by N-Ac-CHAVC-NH2 drives these cells to apoptosis.

A dynamic podosome-like structure of epithelial cells

Focal contacts and hemidesmosomes are cell-matrix adhesion structures of cultured epithelial cells. While focal contacts link the extracellular matrix to microfilaments, hemidesmosomes make connections with intermediate filaments. We have analyzed hemidesmosome assembly in 804G carcinoma cells. Our data show that hemidesmosomes are organized around a core of actin filaments that appears early during cell adhesion. These actin structures look similar to podosomes described in cells of mesenchymal origin. These podosome-like structures are distinct from focal contacts and specifically contain Arp3 (Arp2/3 complex), cortactin, dynamin, gelsolin, N-WASP, VASP, Grb2 and src-like kinase(s). The integrin alpha3beta1 is localized circularly around F-actin cores and co-distributes with paxillin, vinculin, and zyxin. We also show that the maintenance of the actin core and hemidesmosomes is dependent on actin polymerization, src-family kinases, and Grb2, but not on microtubules. Video microscopy analysis reveals that assembly of hemidesmosomes is preceded by recruitment of beta4 integrin subunit to the actin core before its positioning at hemidesmosomes. When 804G cells are induced to migrate, actin cores as well as hemidesmosomes disappear and beta4 integrin subunit becomes co-localized with dynamic actin at leading edges. We show that podosome-like structures are not unique to cells of mesenchymal origin, but also appear in epithelial cells, where they seem to be related to basement membrane adhesion.

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

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

Designing a hepatocellular microenvironment with protein microarraying and poly(ethylene glycol) photolithography

In this study, robotic protein printing was employed as a method for designing a cellular microenvironment. Protein printing proved to be an effective strategy for creating micropatterned co-cultures of primary rat hepatocytes and 3T3 fibroblasts. Collagen spots (ca. 170 microm in diameter) were printed onto amino-silane- and glutaraldehyde-modified glass slides. Groups of 15-20 hepatocytes attached to collagen regions in a highly selective manner forming cell clusters corresponding in size to the printed collagen domains. Fibroblasts, seeded onto the same surface, adhered and spread around arrays of hepatocyte islands creating a heterotypic environment. The co-cultured hepatocytes produced and maintained high levels of liver-specific biomarkers, albumin and urea, over the course of 2 weeks. In addition, protein printing was combined with poly(ethylene glycol) photolithography to define intercellular contacts within the clusters of hepatocytes residing on individual collagen islands. Glass slides, treated with 3-acryloxypropyl trichlorosilane and imprinted with 170 m diameter collagen spots, were micropatterned with a high-density array of 30 microm x 30 microm poly(ethylene glycol) (PEG) wells. As a result, discrete groups of ca. 9 PEG microwells became functionalized with the cell-adhesive ligand. When exposed to micropatterned surfaces, hepatocytes interacted exclusively with collagen-modified regions, attaching and becoming confined at a single-cell level within the hydrogel wells. Micropatterning strategies proposed here will lead to greater insights into hepatocellular behavior and will benefit the fields of hepatic tissue engineering and liver biology.

Membrane and acto-myosin tension promote clustering of adhesion proteins

Physicists have studied the aggregation of adhesive proteins, giving a central role to the elastic properties of membranes, whereas cell biologists have put the emphasis on the cytoskeleton. However, there is a dramatic lack of experimental studies probing both contributions on cellular systems. Here, we tested both mechanisms on living cells. We compared, for the same cell line, the growth of cadherin-GFP patterns on recombinant cadherin-coated surfaces, with the growth of vinculin-GFP patterns on extracellular matrix protein-coated surfaces by using evanescent wave microscopy. In our setup, cadherins are not linked to actin, whereas vinculins are. This property allows us to compare formation of clusters with proteins linked or not to the cytoskeleton and thus study the role of membrane versus cytoskeleton in protein aggregation. Strikingly, the motifs we obtained on both surfaces share common features: they are both elongated and located at the cell edges. We showed that a local force application can impose this symmetry breaking in both cases. However, the origin of the force is different as demonstrated by drug treatment (butanedione monoxime) and hypotonic swelling. Cadherins aggregate when membrane tension is increased, whereas vinculins (cytoplasmic proteins of focal contacts) aggregate when acto-myosin stress fibers are pulling. We propose a mechanism by which membrane tension is localized at cell edges, imposing flattening of membrane and enabling aggregation of cadherins by diffusion. In contrast, cytoplasmic proteins of focal contacts aggregate by opening cryptic sites in focal contacts under acto-myosin contractility.

Oxidative stress activates both Src-kinases and their negative regulator Csk and induces phosphorylation of two targeting proteins for Csk: caveolin-1 and paxillin

Csk negatively regulates Src family kinases (SFKs). In lymphocytes, Csk is constitutively active, and is transiently inactivated in response to extracellular stimuli, allowing activation of SFKs. In contrast, both SFKs and Csk were inactive in unstimulated mouse embryonic fibroblasts, and both were activated in response to oxidative stress. Csk modulated the oxidative stress-induced, but not the basal SFK activity in these cells. These data indicate that Csk may be more important for the return of Src-kinases to the basal state than for the maintenance of basal activity in some cell types. Csk must be targeted to its SFK substrates through an SH2-domain-mediated interaction with a phosphoprotein. Our data indicate that caveolin-1 is one of these targeting proteins. SFKs bind to caveolin-1 and phosphorylate it in response to oxidative stress and insulin. Csk binds specifically to the phosphorylated caveolin-1 and attenuates its stress-induced phosphorylation. Importantly, phosphocaveolin was one of two major phosphoproteins associated with Csk after incubation with peroxide or insulin. Paxillin was the other. Activation/rapid attenuation of SFKs by Csk is required for actin remodeling. Caveolin-1 is phosphorylated at the ends of actin fibers at points of contact between the actin cytoskeleton and the plasma membrane, where it could in part mediate this attenuation.

Tensin Stabilizes Integrin Adhesive Contacts in Drosophila

We report the functional characterization of the Drosophila ortholog of tensin, a protein implicated in linking integrins to the cytoskeleton and signaling pathways. A tensin null was generated and is viable with wing blisters, a phenotype characteristic of loss of integrin adhesion. In tensin mutants, mechanical abrasion is required during wing expansion to cause wing blisters, suggesting that tensin strengthens integrin adhesion. The localization of tensin requires integrins, talin, and integrin-linked kinase. The N-terminal domain and C-terminal PTB domain of tensin provide essential recruitment signals. The intervening SH2 domain is not localized on its own. We suggest a model where tensin is recruited to sites of integrin adhesion via its PTB and N-terminal domains, localizing the SH2 domain so that it can interact with phosphotyrosine-containing proteins, which stabilize the integrin link to the cytoskeleton.

Variation in cell–substratum adhesion in relation to cell cycle phases

The quantification of focal adhesion sites offers an assessable method of measuring cell-substrate adhesion. Such measurement can be hindered by intra-sample variation that may be cell cycle derived. A combination of autoradiography and immunolabelling techniques, for scanning electron microscopy (SEM), were utilised simultaneously to identify both S-phase cells and their focal adhesion sites. Electron-energy 'sectioning' of the sample, by varying the accelerating voltage of the electron beam, combined with backscattered electron (BSE) imaging, allowed for S-phase cell identification in one energy 'plane' image and quantitation of immunogold label in another. As a result, it was possible simultaneously to identify S-phase cells and their immunogold-labelled focal adhesions sites on the same cell. The focal adhesion densities were calculated both for identified S-phase cells and the remaining non-S-phase cells present. The results indicated that the cell cycle phase was a significant factor in determining the density of focal adhesions, with non-S-phase cells showing a larger adhesion density than S-phase cells. Focal adhesion morphology was also seen to correspond to cell cycle phase; with 'dot' adhesions being more prevalent on smaller non-S-phase and the mature 'dash' type on larger S-phase cells. This study demonstrated that when quantitation of focal adhesion sites is required, it is necessary to consider the influence of cell cycle phases on any data collected.

The influence of the extracellular matrix on the morphology and intracellular pH of cultured astrocytes exposed to media lacking bicarbonate

In previous work we showed that the polygonal shape of hippocampal astrocytes cultured on poly-L-lysine changes to a stellate morphology with loss of actinomyosin stress fibers on exchanging the culture medium for saline buffered with HEPES [Brain Res 946 (2002)12]. By contrast, in bicarbonate-buffered saline containing Ca(2+) astrocytes remained polygonal and continued to express stress fibers. Evidence suggests that stellation induced by saline buffered with HEPES is related to intracellular acidification due to the absence of bicarbonate. Here we studied the influence of the matrix used in preparing astrocyte cultures. Stellation in HEPES-saline occurred on a matrix of fibronectin, but not on matrices of collagen I or IV provided Ca(2+) was present. Laminin partially prevented stellation in HEPES-saline. Further, the intracellular acidification induced by HEPES-saline observed in astrocytes cultured on polylysine was abolished in cells cultured on collagens and was attentuated on a matrix of laminin. Two observations suggested the involvement of integrins and focal adhesions. (1) Treatment of cultures on collagens with a blocking antibody to the beta1 integrin subunit abolished protection against HEPES-induced stellation. (2) Compared with polylysine, astrocytes cultured on collagens expressed increased contents of phosphotyrosine proteins, focal adhesion proteins vinculin and paxillin, the beta1 integrin subunit and increased numbers of focal adhesions labelled with anti-vinculin. The observation that astrocytes cultured on collagen I or IV, in contrast to polylysine, express stress fibers and a constant intracellular pH in the absence of buffering by bicarbonate may be related to the fact that in the intact brain astrocytic processes (or end-feet) encounter and bind to collagen IV and laminin in the basement membrane of the endothelial cells which surround the cerebral capillaries. It is also possible that astrocytes retain this capacity from early development when fibrous matrix proteins are present.

The tissue engineeting puzzle: a molecular perspective

The inability of biomaterial scaffolds to functionally integrate into surrounding tissue is one of the major roadblocks to developing new biomaterials and tissue-engineering scaffolds. Despite considerable advances, current approaches to engineering cell-surface interactions fall short in mimicking the complexity of signals through which surrounding tissue regulates cell behavior. Cells adhere and interact with their extracellular environment via integrins, and their ability to activate associated downstream signaling pathways depends on the character of adhesion complexes formed between cells and their extracellular matrix. In particular, alpha5beta1 and alphavbeta3 integrins are central to regulating downstream events, including cell survival and cell-cycle progression. In contrast to previous findings that alphavbeta3 integrins promote angiogenesis, recent evidence argues that alphavbeta3 integrins may act as negative regulators of proangiogenic integrins such as alpha5beta1. This suggests that fibronectin is critical for scaffold vascularization because it is the only mammalian adhesion protein that binds and activates alpha5beta1 integrins. Cells are furthermore capable of stretching fibronectin matrices such that the protein partially unfolds, and recent computational simulations provide structural models of how mechanical stretching affects fibronectin function. We propose a model whereby excessive tension generated by cells in contact to biomaterials may in fact render fibronectin fibrils nonangiogenic and potentially inhibit vascularization. The model could explain why current biomaterials independent of their surface chemistries and textures fail to vascularize.

Magnetic Cellular Switches

This paper focuses on the development of magnetic cellular switches to enable magnetic control of intracellular functions in living mammalian cells, including receptor signal transduction and gene transcription. Our approach takes advantage of the mechanosensitivity of adenosine 3',5'-monophosphate (cAMP) induction and downstream transcription controlled by the cAMP regulatory element (CRE) to engineer gene constructs that optically report gene expression in living cells. We activate transcription of these gene reporters by applying magnetic (mechanical) stress to magnetic microbeads bound to cell surface integrin receptors. In these gene reporter constructs, CRE motifs drive the expression of fluorescent proteins or enzymes that produce fluorescent products, such as DsRed and β-lactamase (BLA), respectively. We demonstrate that a chemical inducer of cAMP (forskolin) increases expression of CRE-DsRed in living cells. More importantly, a threefold increase in CRE-BLA expression is induced by application of mechanical stress to magnetic microbeads (4.5 µm) bound to cell surface integrin receptors. Induction of cAMP could be detected within 5 min using a protein fragment complementation assay involving interactions between the KID and KIX domains of the CRE binding protein linked to complementary halves of the BLA enzyme. These studies confirm that application of magnetic stress to integrins induces gene transcription by activating the cAMP-dependent transcription factor CREB. Ongoing studies focus on optimizing sensitivity and reducing signal-to-noise by establishing stable cell lines that express these gene reporters. These studies collectively demonstrate the feasibility of using magnetic technologies to control function in living mammalian cells and, hence, support the possibility of developing magnetically-actuated cellular components for use in future micro- and nanotechnologies.

Tyrosine phosphatase epsilon is a positive regulator of osteoclast function in vitro and in vivo

Protein tyrosine phosphorylation is a major regulator of bone metabolism. Tyrosine phosphatases participate in regulating phosphorylation, but roles of specific phosphatases in bone metabolism are largely unknown. We demonstrate that young (<12 weeks) female mice lacking tyrosine phosphatase epsilon (PTPepsilon) exhibit increased trabecular bone mass due to cell-specific defects in osteoclast function. These defects are manifested in vivo as reduced association of osteoclasts with bone and as reduced serum concentration of C-terminal collagen telopeptides, specific products of osteoclast-mediated bone degradation. Osteoclast-like cells are generated readily from PTPepsilon-deficient bone-marrow precursors. However, cultures of these cells contain few mature, polarized cells and perform poorly in bone resorption assays in vitro. Podosomes, structures by which osteoclasts adhere to matrix, are disorganized and tend to form large clusters in these cells, suggesting that lack of PTPepsilon adversely affects podosomal arrangement in the final stages of osteoclast polarization. The gender and age specificities of the bone phenotype suggest that it is modulated by hormonal status, despite normal serum levels of estrogen and progesterone in affected mice. Stimulation of bone resorption by RANKL and, surprisingly, Src activity and Pyk2 phosphorylation are normal in PTPepsilon-deficient osteoclasts, indicating that loss of PTPepsilon does not cause widespread disruption of these signaling pathways. These results establish PTPepsilon as a phosphatase required for optimal structure, subcellular organization, and function of osteoclasts in vivo and in vitro.

β1integrins are distributed in adhesion structures with fibronectin and caveolin and in coated pits

Integrins are found in adhesion structures, which link the extracelullar matrix to cytoskeletal proteins. Here, we attempt to further define the distribution of β1integrins in the context of their association with matrix proteins and other cell surface molecules relevant to the endocytic process. We find that β1integrins colocalize with fibronectin in fibrillar adhesion structures. A fraction of caveolin is also organized along these adhesion structures. The extracellular matrix protein laminin is not concentrated in these structures. The α4β1integrin exhibits a distinct distribution from other β1integrins after cells have adhered for 1 h to extracellular matrix proteins but is localized in adhesion structures after 24 h of adhesion. There are differences between the fibronectin receptors: α5β1integrins colocalize with adaptor protein-2 in coated pits, while α4β1integrins do not. This parallels our earlier observation that of the two laminin receptors, α1β1and α6β1, only αaβ1integrins colocalize with adaptor protein-2 in coated pits. Calcium chelation or inhibition of mitogen-activated protein kinase kinase, protein kinase C, or src did not affect localization of α1β1and α5β1integrins in coated pits. Likewise, the integrity of coated-pit structures or adhesion structures is not required for integrin and adaptor protein-2 colocalization. This suggests a robust and possibly constitutive interaction between these integrins and coated pits.Key words: adhesion, endocytosis, extracellular matrix, microscopy, confocal, signalling.

Organization and adhesive properties of the hyaluronan pericellular coat of chondrocytes and epithelial cells

Hyaluronan is a megadalton glycosaminoglycan composed of repeating units of D-N-acetylglucosamine-beta-D-Glucuronic acid. It is known to form a highly hydrated pericellular coat around chondrocytes, fibrosarcoma, and smooth muscle cells. Using environmental scanning electron microscopy we detected fully hydrated hyaluronan pericellular coats around rat chondrocytes (RCJ-P) and epithelial cells (A6). Hyaluronan mediates early adhesion of both chondrocytes and A6 cells to glass surfaces. We show that chondrocytes in suspension establish early "soft contacts" with the substrate through a thick, hyaluronidase-sensitive coat (4.4 +/- 0.7 microm). Freshly-attached cells drift under shear stress, leaving hyaluronan "footprints" on the surface. This suggests that chondrocytes are surrounded by a multilayer of entangled hyaluronan molecules. In contrast, A6 cells have a 2.2 +/- 0.4- microm-thick hyaluronidase-sensitive coat, do not drift under shear stress, and remain firmly anchored to the surface. We consider the possibility that in A6 cells single hyaluronan molecules, spanning the whole thickness of the pericellular coat, mediate these tight contacts.

A role for alphabeta1 integrins in focal adhesion function and polarized cytoskeletal dynamics

alphabeta1 integrins have been implicated in the survival, spreading, and migration of cells and tissues. To explore the underlying biology, we identified conditions where primary beta1 null keratinocytes adhere, proliferate, and display robust alphavbeta6 integrin-induced, peripheral focal contacts associated with elaborate stress fibers. Mechanistically, this appears to be due to reduced FAK and Src and elevated RhoA and Rock activities. Visualization on a genetic background of GFPactin shows that beta1 null keratinocytes spread, but do so aberrantly, and when induced to migrate from skin explants in vitro, the cells are not able to rapidly reorient their actin cytoskeleton toward the polarized movement. As judged by RFPzyxin/GFPactin videomicroscopy, the alphavbeta6-actin network does not undergo efficient turnover. Without the ability to remodel their integrin-actin network efficiently, alphabeta1-deficient keratinocytes cannot respond dynamically to their environment and polarize movements.

The ins and outs of fibronectin matrix assembly

Cell phenotype is specified by environmental cues embedded in the architecture and composition of the extracellular matrix (ECM). Much has been learned about matrix organization and assembly through analyses of the ECM protein fibronectin (FN). FN matrix assembly is a cell-mediated process in which soluble dimeric FN is converted into a fibrillar network. Binding of cell surface integrin receptors to FN converts it to an active form, which promotes fibril formation through interactions with other cell-associated FN dimers. As FN fibrils form on the outside of the cell, cytoplasmic domains of integrin receptors organize cytoplasmic proteins into functional complexes inside. Intracellular connections to the actin cytoskeletal network and stimulation of certain key intracellular signaling pathways are essential for FN-integrin interactions and propagation of FN fibril formation. Thus, assembly of native functional ECM depends on exquisite coordination between extracellular events and intracellular pathways.

Targeting membrane-localized focal adhesion kinase to focal adhesions: roles of tyrosine phosphorylation and SRC family kinases

In the present study, we examined regulation of activated focal adhesion kinase localization in focal adhesions. By using focal adhesion kinase fused to an inert transmembrane anchor, we found that the focal contact targeting region within focal adhesion kinase was preserved in the membrane-targeted fusion protein. However, upon tyrosine phosphorylation, full-length focal adhesion kinase became excluded from focal adhesions. This negative regulation of localization could be abolished by mutating key amino acid residues of focal adhesion kinase shown previously to be involved in adhesion-mediated signal transduction. Hyper-phosphorylation of endogenous focal adhesion kinase induced by pervanadate resulted in a similar reduction of localization at focal adhesions. We also show here that Src family kinases are essential for the phosphorylation-dependent exclusion of focal adhesion kinase from focal adhesions. We propose here a molecular model for the tyrosine phosphorylation-dependent regulation of focal adhesion kinase organization involving Src kinases and an inhibitory phosphorylation of the C-terminal (Tyr-925) tyrosine residue.

Actin-based motility of Burkholderia pseudomallei involves the Arp 2/3 complex, but not N-WASP and Ena/VASP proteins

The facultative intracellular bacterium Burkholderia pseudomallei induces actin rearrangement within infected host cells leading to formation of actin tails and membrane protrusions. To investigate the underlying mechanism we analysed the contribution of cytoskeletal proteins to B. pseudomallei-induced actin tail assembly. By using green fluorescent protein (GFP)-fusion constructs, the recruitment of the Arp2/3 complex, vasodilator-stimulated phosphoprotein (VASP), Neural Wiskott-Aldrich syndrome protein (N-WASP), zyxin, vinculin, paxillin and alpha-actinin to the surface of B. pseudomallei and into corresponding actin tails was studied. In addition, antibodies against the same panel of proteins were used for immunolocalization. Whereas the Arp2/3 complex and alpha-actinin were incorporated into B. pseudomallei-induced actin tails, none of the other proteins were detected in these structures. The overexpression of an Arp2/3 binding fragment of the Scar1 protein, shown previously to block actin-based motility of Listeria, had no effect on B. pseudomallei tail formation. Infections of either N-WASP- or Ena/VASP-defective cells showed that these proteins are not essential for B. pseudomallei-induced actin polymerization. In conclusion, our results suggest that B. pseudomallei induces actin polymerization through a mechanism that differs from those evolved by Listeria, Shigella, Rickettsia or vaccinia virus.

Alpha-smooth muscle actin is crucial for focal adhesion maturation in myofibroblasts

Cultured myofibroblasts are characterized by stress fibers, containing alpha-smooth muscle actin (alpha-SMA) and by supermature focal adhesions (FAs), which are larger than FAs of alpha-SMA-negative fibroblasts. We have investigated the role of alpha-SMA for myofibroblast adhesion and FA maturation. Inverted centrifugation reveals two phases of initial myofibroblast attachment: during the first 2 h of plating microfilament bundles contain essentially cytoplasmic actin and myofibroblast adhesion is similar to that of alpha-SMA-negative fibroblasts. Then, myofibroblasts incorporate alpha-SMA in stress fibers, develop mature FAs and their adhesion capacity is significantly increased. When alpha-SMA expression is induced in 5 d culture by TGFbeta or low serum levels, fibroblast adhesion is further increased correlating with a "supermaturation" of FAs. Treatment of myofibroblasts with alpha-SMA fusion peptide (SMA-FP), which inhibits alpha-SMA-mediated contractile activity, reduces their adhesion to the level of alpha-SMA negative fibroblasts. With the use of flexible micropatterned substrates and EGFP-constructs we show that SMA-FP application leads to a decrease of myofibroblast contraction, shortly followed by disassembly of paxillin- and beta3 integrin-containing FAs; alpha5 integrin distribution is not affected. FRAP of beta3 integrin-EGFP demonstrates an increase of FA protein turnover following SMA-FP treatment. We conclude that the formation and stability of supermature FAs depends on a high alpha-SMA-mediated contractile activity of myofibroblast stress fibers.

Alphavbeta3 integrin signaling pathway is involved in insulin-like growth factor I-stimulated human extravillous trophoblast cell migration

IGF-I and -II provide paracrine and autocrine stimuli, respectively, for extravillous trophoblast (EVT) cell migration. This study examined the role of alpha(v)beta(3) integrin and its signaling pathway in IGF-I-stimulated migration. Migration assays were conducted using cultured EVT cells treated with or without IGF-I in the presence or absence of alphaIR3, Arg-Gly-Asp (RGD) hexapeptide, and antibody against alpha(v)beta(3) integrin. Morphological changes were studied using scanning electron microscopy. Colocalization of alpha(5)beta(1) alpha(v)beta(3) integrins, vinculin, focal adhesion kinase, and paxillin were determined by immuno-cytochemistry and immunoblotting. The results showed that IGF-I could stimulate EVT cell migration in a time- and dose-dependent manner and addition of alphaIR3, Arg-Gly-Asp hexapeptide, and antibody against alpha(v)beta(3) integrin attenuated the IGF-I migratory effect. Scanning electron microscopy images revealed that IGF-I promoted lamellipodia formation. Immunostaining and immunoblotting exhibited the colocalization of alpha(v)beta(3) integrin with phosphorylated focal adhesion kinase, paxillin, and vinculin at focal adhesions after IGF-I treatment. Immunoblotting demonstrated an increase in focal adhesion kinase and paxillin tyrosine phosphorylation followed by tyrosine phosphorylation of IGF-I receptor in a time- and dose-dependent manner. These findings indicated alpha(v)beta(3) integrin localization in the core of focal adhesions of EVT cells and that alpha(v)beta(3) integrin signaling pathways are activated in IGF-I-mediated migration of these cells.

Live-cell monitoring of tyrosine phosphorylation in focal adhesions following microtubule disruption

Tyrosine phosphorylation of focal adhesion components is involved in the regulation of focal adhesion formation and turnover, yet the underlying molecular mechanisms are still poorly defined. In the present study, we have used quantitative fluorescence microscopy to investigate the dynamic relationships between the incorporation of new components into growing focal adhesions and tyrosine phosphorylation of these sites. For this purpose, a new approach for monitoring phosphotyrosine levels in live cells was developed, based on a 'phosphotyrosine reporter' consisting of yellow fluorescent protein fused to two consecutive phosphotyrosine-binding Src-homology 2 (SH2)-domains derived from pp60(c-Src). This YFP-dSH2 localized to cell-matrix adhesions and its intensity was linearly correlated with that of an anti-phosphotyrosine antibody labeling. The differential increase in vinculin and phosphotyrosine levels was examined in live cells by two-color time-lapse movies of CFP-vinculin and YFP-dSH2. In this study, focal adhesion growth was triggered by microtubule disruption, which was previously shown to stimulate focal adhesion development by inducing cellular contraction. We show here that, 2 minutes after addition of the microtubule-disrupting drug nocodazole, the local densities of the focal adhesion-associated proteins vinculin, paxillin and focal adhesion kinase (FAK) are significantly elevated and the focal adhesion area is increased, whereas elevation in tyrosine phosphorylation inside the growing adhesions occurs only a few minutes later. Phosphotyrosine and FAK density reach their maximum levels after 10 minutes of treatment, whereas vinculin and paxillin levels as well as focal adhesion size continue to grow, reaching a plateau at about 30 minutes. Our findings suggest that protein recruitment and growth of focal adhesions are an immediate and direct result of increased contractility induced by microtubule disruption, whereas tyrosine phosphorylation is activated later.

RACK1 regulates integrin-mediated adhesion, protrusion, and chemotactic cell migration via its Src-binding site

Mammalian cDNA expression cloning was used to identify novel regulators of integrin-mediated cell-substratum adhesions. Using a focal adhesion morphology screen, we identified a cDNA with homology to a receptor for activated protein kinase C (RACK1) that induced a loss of central focal adhesions and stress fibers in CHO-K1 cells. The identified cDNA was a C-terminal truncated form of RACK1 that had one of the putative protein kinase C binding sites but lacked the region proposed to bind the beta integrin cytoplasmic domain and the tyrosine kinase Src. To investigate the role of RACK1 during cell spreading and migration, we tagged RACK1, a C-terminal truncated RACK1 and a point mutant that does not bind Src (RACK Y246F) with green fluorescent protein and expressed them in CHO-K1 cells. We found that RACK1 regulates the organization of focal adhesions and that it localizes to a subset of nascent focal complexes in areas of protrusion that contain paxillin but not vinculin. We also found that RACK1 regulates cell protrusion and chemotactic migration through its Src binding site. Together, these findings suggest that RACK1 regulates adhesion, protrusion, and chemotactic migration through its interaction with Src.

Hemidesmosome protein dynamics in live epithelial cells

Hemidesmosomes mediate stable anchorage of epithelial cells to laminin-5 in the basement membrane zone and have been likened to spot-welds. Indeed, it has been assumed that hemidesmosomes are not dynamic, at least when compared to other matrix adhesion sites including focal contacts. We tested this notion by monitoring the fate of green fluorescent protein (GFP)-tagged human integrin beta4 subunit (GFP-hbeta4) and GFP-tagged 180-kD human bullous pemphigoid (BP) autoantigen (GFP-BP180) in live cultures of 804G cells that assemble numerous mature hemidesmosomes. In subconfluent 804G cells, both GFP-hbeta4 and GFP-BP180 protein clusters are not stable but assemble into and disassemble out of cat paw-like arrays at a relatively rapid rate. In confluent populations of 804G cells, although some cat paw-like clusters of both GFP-hbeta4 and GFP-BP180 are stable over periods of >60 min, other GFP-hbeta4 and GFP-BP180 protein arrays form and/or disappear during the same time period. Moreover, individual labeled particles show considerable motility in the plane of the membrane. Fluorescence recovery after photobleaching analyses provide a further indication of the dynamics of hemidesmosome proteins. In particular, bleached GFP-hbeta4 protein clusters in confluent cells recover signal within about 30 min, indicating that there is a relatively rapid turnover of hemidesmosome components in protein arrays clustered along the substratum attached surface of a cell. The rate of recovery is dependent on an intact microfilament system. In sharp contrast, bleached GFP-BP180 protein clusters in confluent cells fail to recover signal even when observed for longer than 60 min. To evaluate hemidesmosome protein dynamics in motile cells, we monitored GFP-hbeta4 and GFP-BP180 in 804G cells populating scrape wound sites in vitro. In these migratory cells, which lack mature hemidesmosomes, integrin beta4 subunit and BP180 protein clusters progressively assemble and disassemble into linear and cat-paw arrays. In summary, hemidesmosome protein clusters, like their counterparts in focal contacts, are dynamic. We discuss these results in relation to hemidesmosome functions.

Calculation of forces at focal adhesions from elastic substrate data: the effect of localized force and the need for regularization

Forces exerted by stationary cells have been investigated on the level of single focal adhesions by combining elastic substrates, fluorescence labeling of focal adhesions, and the assumption of localized force when solving the inverse problem of linear elasticity theory. Data simulation confirms that the inverse problem is ill-posed in the presence of noise and shows that in general a regularization scheme is needed to arrive at a reliable force estimate. Spatial and force resolution are restricted by the smoothing action of the elastic kernel, depend on the details of the force and displacement patterns, and are estimated by data simulation. Corrections arising from the spatial distribution of force and from finite substrate size are treated in the framework of a force multipolar expansion. Our method is computationally cheap and could be used to study mechanical activity of cells in real time.

Regulation of focal complex composition and disassembly by the calcium-dependent protease calpain

Cell migration requires the regulated and dynamic turnover of adhesive complexes. We have previously demonstrated that the calcium-dependent protease, calpain, regulates the organization of adhesive complexes and cell detachment during cell migration. Evidence is now provided that inhibiting calpain through over-expression of the endogenous inhibitor of calpain, calpastatin, and pharmacological inhibitors results in an inhibition of adhesive complex disassembly with stabilization of GFP-vinculin and GFP/RFP-zyxin at the cell periphery. Calpain was also required for the microtubule-mediated turnover of adhesive complex sites after nocodazole wash-out, suggesting that calpain may mediate focal complex disassembly downstream of microtubules. Using dual imaging of RFP-zyxin and GFP-alpha-actinin, we observed a temporal and spatial relationship between alpha-actinin localization to focal contacts and the subsequent disassembly or translocation of RFP-zyxin containing focal complexes in areas of cell retraction. Calpain inhibition disrupted alpha-actinin localization to zyxin-containing focal contacts and focal complex disassembly or translocation to the cell center. In addition, disrupting alpha-actinin localization to focal complexes through expression of the alpha-actinin rod domain, but not the head domain, resulted in inhibition of focal adhesion disassembly similar to calpain inhibition. Our studies suggest a novel mechanism of action whereby calpain may modulate alpha-actinin localization into focal complexes and their subsequent disassembly or translocation.

Integrin-dependent regulation of gene expression in leukocytes

In addition to their role in strengthening intercellular adhesion, leukocyte integrins transduce signals which affect genetic programs, consequently defining cell phenotype and function. These signals can be independently sufficient, or can cooperate with other environmental stimuli to affect gene expression regulation. In the past several years, there has been an emergence of mechanistic data which contribute to our understanding of these critical integrin roles. In this review, we describe anchorage-dependent T lymphocyte proliferation and, in particular, how leukocyte integrin engagement overcomes the G1 to S cell cycle restriction point in antigen-activated T cells. The related role of alphaLbeta2 integrin (LFA-1) as a T cell co-stimulatory molecule is discussed. This includes defining mechanisms whereby LFA-1 engagement enhances transcriptional activation of numerous genes by regulating its association with transcription modulators such as JAB-1, and through interaction with other gene-activating signaling complexes such as JAK-STATs. Evidence is presented to support that leukocyte integrin engagement provides potent signals which stabilize otherwise labile activation mRNA transcripts, including those encoding cytokine and extracellular matrix degrading proteins. These integrin-dependent mechanisms, all described recently, play important roles in T cell differentiation and proliferation, immune surveillance and inflammatory responses.

Regulation of fibronectin matrix deposition and cell proliferation by the PINCH-ILK-CH-ILKBP complex

Alteration in renal glomerular mesangial cell growth and fibronectin matrix deposition is a hallmark of glomerulosclerosis, which ultimately leads to end-stage renal failure. We have previously shown that the expression of integrin-linked kinase (ILK), a cytoplasmic component of the cell-extracellular matrix contacts, is increased in mesangial cells in human patients with diabetic nephropathy. We show here that ILK forms a complex with PINCH and CH-ILKBP in primary mesangial cells, which are co-clustered at fibrillar adhesions, sites that are involved in fibronectin matrix deposition. To investigate functional significance of the PINCH-ILK-CH-ILKBP complex formation, we expressed the PINCH-binding N-terminal fragment and the CH-ILKBP-binding C-terminal fragment of ILK, respectively, in mesangial cells by using an adenoviral expression system. Overexpression of either the N-terminal fragment or the C-terminal fragment of ILK effectively inhibited the PINCH-ILK-CH-ILKBP complex formation. Inhibition of the PINCH-ILK-CH-ILKBP complex formation significantly reduced fibronectin matrix deposition and inhibited cell proliferation. These results indicate that the PINCH-ILK-CH-ILKBP complex is critically involved in the regulation of mesangial fibronectin matrix deposition and cell proliferation, and suggest that it may potentially serve as a useful target in the therapeutic control of progressive renal failure and other pathological processes involving abnormal cell proliferation and fibronectin matrix deposition.

Regulation of S33/S37 phosphorylated β-catenin in normal and transformed cells

A novel phosphorylation-specific antibody (αpβ-catenin) was generated against a peptide corresponding to amino acids 33-45 of humanβ-catenin, which contained phosphorylated serines at positions 33 and 37. This antibody is specific to phosphorylated β-catenin and reacts neither with the non-phosphorylated protein nor with phosphorylated or non-phosphorylated plakoglobin. It weakly interacts with S33Y β-catenin but not with the S37A mutant. pβ-catenin is hardly detectable in normal cultured cells and accumulates (up to 55% of total β-catenin) upon overexpression of the protein or after blocking its degradation by the proteasome. Inhibition of both GSK-3β and the proteasome resulted in a rapid (t1/2=10 minutes) and reversible reduction in pβ-catenin levels, suggesting that the protein can undergo dephosphorylation in live cells, at a rate comparable to its phosphorylation by GSK-3β. pβ-catenin interacts with LEF-1, but fails to form a ternary complex with DNA, suggesting that it is transcriptionally inactive. Immunofluorescence microscopy indicated that pβ-catenin accumulates in the nuclei of MDCK and BCAP cells when overexpressed and is transiently associated with adherens junctions shortly after their formation. pβ-catenin only weakly interacts with co-transfected N-cadherin, although it forms a complex with the ubiquitin ligase component β-TrCP. SW480 colon cancer cells that express a truncated APC, at position 1338, contain high levels of pβ-catenin,whereas HT29 cells, expressing APC truncated at position 1555, accumulate non-phosphorylated β-catenin, suggesting that the 1338-1555 amino acid region of APC is involved in the differential regulation of the dephosphorylation and degradation of pβ-catenin.

Regulation of substrate adhesion dynamics during cell motility

The movement of a metazoan cell entails the regulated creation and turnover of adhesions with the surface on which it moves. Adhesion sites form as a result of signaling between the extracellular matrix on the outside and the actin cytoskeleton on the inside, and they are associated with specific assembles of actin filaments. Two broad categories of adhesion sites can be distinguished: (1) "focal complexes" associated with lamellipodia and filopodia that support protrusion and traction at the cell front; and (2) "focal adhesions" at the termini of stress fibre bundles that serve in longer term anchorage. Focal complexes are signaled via Rac1 or Cdc42 and can either turnover on a minute scale or differentiate, via intervention of the RhoA pathway, into longer-lived focal adhesions. All classes of adhesion sites depend on the stress in the actin cytoskeleton for their formation and maintenance. Different cell types use different adhesion strategies to move, in terms of the relative engagement of filopodia and lamellipodia in focal complex formation and protrusion and the extent of focal adhesion formation. These differences can be attributed to variations in the relative activities of Rho family members. However, the Rho GTPases alone are unable to signal asymmetry in the actin cytoskeleton, necessary for polarisation and movement. Polarisation requires the collaboration of the microtubule cytoskeleton. Changes in the polymerisation state of microtubules influences the activities of both Rac1 and RhoA and microtubules interact directly with adhesion foci and promote their turnover. Possible mechanisms of cross-talk between the microtubule and actin cytoskeletons in determining polarity are discussed.

Microfilament-dependent movement of the beta3 integrin subunit within focal contacts of endothelial cells

To gain insight into the dynamic properties of focal contacts, we induced expression of green fluorescent protein-tagged beta3 integrin (GFP-beta3) and actinin-1 (GFP-actinin-1) in endothelial cells. Both tagged proteins localize with alpha(v)beta3 integrin in focal contacts distributed towards the periphery of transfected cells. Labeled focal contacts migrate at about 0.1 mm/min in stationary live endothelial cells. We compared beta3 integrin and actinin-1 dynamics in focal contacts by using fluorescence recovery after photobleaching. Recovery of signal in bleached focal contacts that have incorporated actinin-1 is rapid and occurs within less than 4 min. This recovery is energy-dependent. In contrast, recovery of bleached focal contacts that contain GFP-beta3 integrin takes longer than 30 min. Yet, when a narrow stripe of fluorescence is bleached across a beta3 integrin-labeled focal contact, recovery is complete within 16 min. The latter recovery is energy-dependent and is blocked not only by actin-filament disrupting drugs but also by a myosin light chain kinase inhibitor. Thus, integrins are not immobile when incorporated into focal contacts, as some have suggested. We propose that integrins are mobile within the confines of focal contacts and that this mobility is supported by an actin-associated molecular motor.

Different phenotypes in human prostate cancer: alpha6 or alpha3 integrin in cell-extracellular adhesion sites

The distribution of alpha6/alpha3 integrin in adhesion complexes at the basal membrane in human normal and cancer prostate glands was analyzed in 135 biopsies from 61 patients. The levels of the polarized alpha6/alpha3 integrin expression at the basal membrane of prostate tumor glands were determined by quantitative immunohistochemistry. The alpha6/alpha3 integrin expression was compared with Gleason sum score, pathological stage, and preoperative serum prostate-specific antigen (PSA). The associations were assessed by statistical methods. Eighty percent of the tumors expressed the alpha6 or alpha3 integrin and 20% was integrin-negative. Gleason sum score, but not serum PSA, was associated with the integrin expression. Low Gleason sum score correlated with increased integrin expression, high Gleason sum score with low and negative integrin expression. Three prostate tumor phenotypes were distinguished based on differential integrin expression. Type I coexpressed both alpha6 and alpha3 subunits, type II exclusively expressed alpha6 integrin, and type III expressed alpha3 integrin only. Fifteen cases were further examined for the codistribution of vinculin, paxillin, and CD 151 on frozen serial sections using confocal laser scanning microscopy. The alpha6/alpha3 integrins, CD151, paxillin, and vinculin were present within normal glands. In prostate carcinoma, alpha6 integrin was colocalized with CD 151, but not with vinculin or paxillin. In tumor phenotype I, the alpha6 subunit did not colocalize with the alpha subunit indicating the existence of two different adhesion complexes. Human prostate tumors display on their cell surface the alpha6beta1 and/or alpha3beta1 integrins. Three tumor phenotypes associated with two different adhesion complexes were identified, suggesting a reorganization of cell adhesion structures in prostate cancer.

Fibronectin extension and unfolding within cell matrix fibrils controlled by cytoskeletal tension

Evidence is emerging that mechanical stretching can alter the functional states of proteins. Fibronectin (Fn) is a large, extracellular matrix protein that is assembled by cells into elastic fibrils and subjected to contractile forces. Assembly into fibrils coincides with expression of biological recognition sites that are buried in Fn's soluble state. To investigate how supramolecular assembly of Fn into fibrillar matrix enables cells to mechanically regulate its structure, we used fluorescence resonance energy transfer (FRET) as an indicator of Fn conformation in the fibrillar matrix of NIH 3T3 fibroblasts. Fn was randomly labeled on amine residues with donor fluorophores and site-specifically labeled on cysteine residues in modules FnIII(7) and FnIII(15) with acceptor fluorophores. Intramolecular FRET was correlated with known structural changes of Fn in denaturing solution, then applied in cell culture as an indicator of Fn conformation within the matrix fibrils of NIH 3T3 fibroblasts. Based on the level of FRET, Fn in many fibrils was stretched by cells so that its dimer arms were extended and at least one FnIII module unfolded. When cytoskeletal tension was disrupted using cytochalasin D, FRET increased, indicating refolding of Fn within fibrils. These results suggest that cell-generated force is required to maintain Fn in partially unfolded conformations. The results support a model of Fn fibril elasticity based on unraveling and refolding of FnIII modules. We also observed variation of FRET between and along single fibrils, indicating variation in the degree of unfolding of Fn in fibrils. Molecular mechanisms by which mechanical force can alter the structure of Fn, converting tensile forces into biochemical cues, are discussed.

Microscope-based techniques to study cell adhesion and migration

Modern light microscopy has evolved to provide a variety of quantitative imaging techniques and also the capability to perturb structure-function relationships in living cells. The advances have been especially useful in the study of cell adhesion and migration. This review will focus on how such microscopy-based techniques can be useful in situ to study the molecular interactions and dynamics, to locally perturb actin-based structures and to measure the traction forces exerted by motile cells.

Enteropathogenic Escherichia coli induces modification of the focal adhesions of infected host cells

Enteropathogenic Escherichia coli (EPEC) is a human-specific pathogen that causes severe diarrhoea in young children. The disease involves intimate interaction between the pathogen and the brush border of enterocytes. During infection, EPEC uses a type III secretion system (TTSS) to inject several proteins into the infected cells, and these effector proteins modify specific processes in the host cell. We show that, upon infection, EPEC induces detachment of the infected host cells from the substratum, modification of focal adhesions (FA) in the infected cells and specific dephosphorylation of focal adhesion kinase (FAK). We also show that EPEC-induced cell detachment is dependent on FAK expression by the infected cells. Finally, we demonstrate that cell detachment, FA modification and FAK dephosphorylation are dependent on functional TTSS in the infecting EPEC. These results suggest that EPEC is using its TTSS to inject protein(s) into the infected cells, which can induce FAK dephosphorylation, as well as FAK-dependent FA modification and cell detachment. These processes are specific and probably play an important role in EPEC virulence.

Microvilli-like structures are associated with the internalization of virulent capsulatedNeisseria meningitidisinto vascular endothelial cells

Bacterial pathogens are internalized into non-phagocytic cells either by a zipper mechanism involving a direct contact between a bacterial ligand and a cellular receptor or a trigger mechanism secondary to the formation of membrane ruffles. Here we show that internalization of capsulated Neisseria meningitidis within endothelial cells following type IV pilus-mediated adhesion is associated with the formation of cellular protrusions at the site of bacterial attachment. These protrusions, like microvilli, are highly enriched in ezrin and moesin, two members of the ERM(ezrin/radixin/moesin) family, whereas vinculin and paxillin are absent. ERM-binding transmembrane proteins, such as CD44, and cortical actin polymerization colocalized within these membrane protrusions. Expression of dominant-negative ezrin largely prevented cortical actin polymerization, thus confirming the role of this molecule in bacteria-induced cytoskeletal modifications. Moreover, using selective inhibitors and dominant-negative mutants of the Rho family GTPases, we show that bacteria-induced actin polymerization required the activation of both Rho and Cdc42 but not of Rac1. Whereas GTPase inhibition dramatically reduced actin polymerization at the site of bacterial attachment, ezrin recruitment was not affected, indicating that bacterial adhesion promotes ezrin recruitment independently of the activity of the Rho-GTPases. Furthermore, GTPase inhibition largely reduced N. meningitidis entry into endothelial cells without affecting adhesion. We thus propose that following pilus-mediated adhesion, capsulated N. meningitidis recruit ERM-binding transmembrane proteins, as well as ezrin and moesin, and that both Rho and Cdc42 are critical for the subsequent cytoskeletal modifications responsible for the formation of microvilli-like cellular protrusions and bacterial internalization.

A structural model for force regulated integrin binding to fibronectin's RGD-synergy site

The synergy site on fibronectin's FN-III(9) module, located approximately 32 A away from the RGD-loop on FN-III(10), greatly enhances integrin alpha(5)beta(1) mediated cell binding. Since fibronectin is exposed to mechanical forces acting on the extracellular matrix in vivo, we used steered molecular dynamics to study how mechanical stretching of FN-III(9-10) affects the relative distance between these two synergistic sites. Our simulations predict the existence of an intermediate state prior to unfolding. In this state, the synergy-RGD distance is increased from 32 A to approximately 55 A, while the conformations of both sites remain unperturbed. This distance is too large for both sites to co-bind the same receptor, as indicated by experiments that confirm that increasing the length of the linker chain between FN-III(9) and FN-III(10) reduces alpha(5)beta(1) binding. Our simulations thus suggest that increased alpha(5)beta(1)-binding attributed to the synergy site, along with the associated downstream cell-signaling events, can be turned off mechanically by stretching FN-III(9-10) into this intermediate state. The potential physiological implications are discussed.

Flexible substrata for the detection of cellular traction forces

By modulating adhesion signaling and cytoskeletal organization, mechanical forces play an important role in various cellular functions, from propelling cell migration to mediating communication between cells. Recent developments have resulted in several new approaches for the detection, analysis and visualization of mechanical forces generated by cultured cells. Combining these methods with other approaches, such as green-fluorescent protein (GFP) imaging and gene manipulation, proves to be particularly powerful for analyzing the interplay between extracellular physical forces and intracellular chemical events.

A critical role of the PINCH-integrin-linked kinase interaction in the regulation of cell shape change and migration

The interaction of cells with extracellular matrix recruits multiple proteins to cell-matrix contact sites (e.g. focal and fibrillar adhesions), which connect the extracellular matrix to the actin cytoskeleton and regulate cell shape change, migration, and other cellular processes. We previously identified PINCH, an adaptor protein comprising primarily five LIM domains, as a binding protein for integrin-linked kinase (ILK). In this study, we show that PINCH co-localizes with ILK in both focal adhesions and fibrillar adhesions. Furthermore, we have investigated the molecular basis underlying the targeting of PINCH to the cell-matrix contact sites and the functional significance of the PINCH-ILK interaction. We have found that the N-terminal LIM1 domain, which mediates the ILK binding, is required for the targeting of PINCH to the cell-matrix contact sites. The C-terminal LIM domains, although not absolutely required, play an important regulatory role in the localization of PINCH to cell-matrix contact sites. Inhibition of the PINCH-ILK interaction, either by overexpression of a PINCH N-terminal fragment containing the ILK-binding LIM1 domain or by overexpression of an ILK N-terminal fragment containing the PINCH-binding ankyrin domain, retarded cell spreading, and reduced cell motility. These results suggest that PINCH, through its interaction with ILK, is crucially involved in the regulation of cell shape change and motility.

Regulation of articular chondrocyte phenotype by bone morphogenetic protein 7, interleukin 1, and cellular context is dependent on the cytoskeleton

Bone morphogenetic proteins (BMPs) induce cartilage differentiation and morphogenesis. There are profound changes in the cytoskeletal architecture during the morphogenesis of cartilage. To investigate the possibility that morphogenetic signals such as BMPs may regulate chondrocyte phenotype by modulation of cytoskeletal protein expression, we determined whether the expression and distribution of cytoskeletal proteins in chondrocytes are regulated by bone morphogenetic protein 7 (BMP 7), interleukin 1 (IL-1), and cellular context. Addition of BMP 7, a morphogen that induces chondrogenesis, to primary cultures of bovine and murine chondrocytes induced increased expression of four cytoskeletal proteins: tensin, talin, paxillin, and focal adhesion kinase (FAK). The expression of cytoskeletal proteins is dependent on cellular context; compared to monolayer, chondrocytes in suspension exhibited increased expression of cytoskeletal components. Conversely, addition of IL-1, a catabolic cytokine, induced loss of chondrocyte phenotype and decreased the expression of these cytoskeletal components. Treatment of chondrocytes with cytochalasin D (an agent that disrupts the actin cytoskeleton) inhibited BMP 7-induced upregulation of tensin, talin, paxillin, and FAK, and blocked the effect of BMP 7 on chondrocyte phenotype. Taken together these data demonstrate that cytoskeletal components play a critical role in the response to morphogens and cytokines in the regulation of chondrocyte phenotype. (c)2001 Elsevier Science.

Simulations of cell-surface integrin binding to nanoscale-clustered adhesion ligands

Clustering of ligated integrins strongly influences integrin signaling and mechanical linkages between integrins and intracellular structures. Extracellular spatial organization of integrin ligands in clusters may facilitate clustering of bound integrins and thus potentially regulate cellular responses to a defined average amount of ligand in the extracellular environment. The possible role of such ligand clustering effects in controlling overall receptor occupancy is studied here using a simple mass-action equilibrium model as well as a two-dimensional Monte Carlo lattice description of the cell-substrate interface, where cell surface receptors are free to diffuse in the plane of the interface and interact with the substrate-immobilized ligand. Results from the analytical treatment and simulation data indicate that for a single-state model in which receptor-ligand binding equilibria are not influenced by neighboring complexes, clustering of ligand does not enhance total receptor binding. However, if receptor binding energy increases in the presence of neighboring ligated receptors, strong ligand spatial distribution effects arise. Nonlinear responses to increasing ligand density are also observed even in the case of random ligand placement due to stochastic juxtaposition of ligand molecules. These results describe how spatial distribution of ligand presented by the extracellular matrix or by synthetic biomimetic materials might control cell responses to external ligands, and suggest a feedback mechanism by which focal contact formation might be initiated.