Cell-substratum Adhesion Sites Research Articles

Inhibition of glioblastoma dispersal by the MEK inhibitor PD0325901

BackgroundDispersal of glioblastoma (GBM) cells leads to recurrence and poor prognosis. Accordingly, molecular pathways involved in dispersal are potential therapeutic targets. The mitogen activated protein kinase/extracellular signal regulated kinase (MAPK/ERK) pathway is commonly dysregulated in GBM, and targeting this pathway with MEK inhibitors has proven effective in controlling tumor growth. Since this pathway also regulates ECM remodeling and actin organization − processes crucial to cell adhesion, substrate attachment, and cell motility – the aim of this study was to determine whether inhibiting this pathway could also impede dispersal.MethodsA variety of methods were used to quantify the effects of the MEK inhibitor, PD0325901, on potential regulators of dispersal. Cohesion, stiffness and viscosity were quantified using a method based on ellipsoid relaxation after removal of a deforming external force. Attachment strength, cell motility, spheroid dispersal velocity, and 3D growth rate were quantified using previously described methods.ResultsWe show that PD0325901 significantly increases aggregate cohesion, stiffness, and viscosity but only when tumor cells have access to high concentrations of fibronectin. Treatment also results in reorganization of actin from cortical into stress fibers, in both 2D and 3D culture. Moreover, drug treatment localized pFAK at sites of cell-substratum adhesion. Collectively, these changes resulted in increased strength of substrate attachment and decreased motility, a decrease in aggregate dispersal velocity, and in a marked decrease in growth rate of both 2D and 3D cultures.ConclusionsInhibition of the MAPK/ERK pathway by PD0325901 may be an effective therapy for reducing dispersal and growth of GBM cells.

Dynamics of leading lamellae of living fibroblasts visualized by high-speed scanning probe microscopy

In this study, we aimed at improving the temporal resolution of scanning probe microscopy (SPM) for observing living cells by introducing soft cantilevers, low feedback-gain operations, and cantilever deflection imaging. We achieved visualization of the mechanical architecture in leading lamellae of living fibroblasts at a temporal resolution of around 10 s, which is higher than that of conventional contact-mode SPM. Time-lapse SPM could be used to monitor not only cytoskeletal dynamics but also the dynamics of numerous microgranules. Statistical analysis of microgranular motion revealed that the microgranules have superdiffusive behaviors and significant directional order of motion. We also found that the direction of their motion is correlated with the direction of growing actin stress fibers. The combination of SPM with fluorescence microscopy showed that vinculin, a component of cell-substratum adhesion sites, localizes at the microgranules. Our experimental data provides a new insight into the intracellular mechanical architecture and its structural dynamics, suggesting that high-speed live-cell SPM has great potential for investigating the structural origin of cellular dynamics.

Members of the Zyxin Family of LIM Proteins Interact with Members of the p130Cas Family of Signal Transducers

Integrin binding to extracellular matrix proteins induces formation of signaling complexes at focal adhesions. Zyxin co-localizes with integrins at sites of cell-substratum adhesion and is postulated to serve as a docking site for the assembly of multimeric protein complexes involved in regulating cell motility. Recently, we identified a new member of the zyxin family called TRIP6. TRIP6 is localized at focal adhesions and overexpression of TRIP6 slows cell migration. In an effort to define the molecular mechanism by which TRIP6 affects cell migration, the yeast two-hybrid assay was employed to identify proteins that directly bind to TRIP6. This assay revealed that both TRIP6 and zyxin interact with CasL/HEF1, a member of the Cas family. This association is mediated by the LIM region of the zyxin family members and the SH2 domain-binding region of CasL/HEF1. Furthermore, the association between p130(Cas) and the two zyxin family members was demonstrated to occur in vivo by co-immunoprecipitation. Zyxin and Cas family members may cooperate to regulate cell motility.

Early stages of endothelial wound repair: conversion of quiescent to migrating endothelial cells involves tyrosine phosphorylation and actin microfilament reorganization.

Endothelial repair to reestablish structural integrity following wounding is a complex process. Since the actin cytoskeleton undergoes specific changes in distribution as quiescent endothelial cells switch to activated migrating cells over a 6-h period following wounding (Lee et al. 1996), we studied tyrosine phosphorylation in association with actin microfilaments and adhesion proteins using double immunofluorescent confocal microscopy. We showed that in a confluent monolayer phosphotyrosine localized at the periphery of the cell at vinculin cell-cell adhesion sites within the actin-dense peripheral band (DPB) and centrally at talin/vinculin cell-substratum adhesion sites at the ends of central microfilaments. Over a period of 6 h following in vitro wounding there was a reduction of peripheral phosphotyrosine associated with the loss of both cell-cell adhesion sites and the DPB (stage I). Concomitantly, an increase in central phosphotyrosine was associated with an increase in cell-substratum adhesion sites and central microfilaments parallel to the wound edge (stage II), which subsequently redistributed perpendicular to the wound edge (stage III). We also localized FAK and paxillin at the ends of parallel and perpendicular central microfilaments. Immunoprecipitation of paxillin showed increased phosphotyrosine and protein levels when prominent central microfilaments were present and underwent remodeling. Inhibition of tyrosine kinases by genistein and tyrosine phosphatases by sodium orthovanadate resulted in reduced endothelial repair associated with disruption of adhesion site formation and central microfilament formation/redistribution in each stage of repair. We suggest that tyrosine phosphorylation of adhesion proteins, such as paxillin, may be important in regulating the early stages of endothelial wound repair.

Biology and function of hemidesmosomes.

Hemidesmosomes are cell-substratum adhesion sites that connect the extracellular matrix to the keratin cytoskeleton. Our knowledge of the function of these structures has greatly increased as a result of studies on patients with aberrant expression of hemidesmosome components and studies using targeted inactivation of mouse genes encoding these components. Insight into the formation of hemidesmosomes, as well as into protein-protein interactions that occur in these junctional complexes, has recently been gained by in vitro cell transfections, blot overlay and yeast two-hybrid assays. In addition, recent results indicate that the alpha6 beta4 integrin is involved in the transduction of signals that are induced by the extracellular matrix and which modulate processes as diverse as cell proliferation, differentiation, apoptosis, migration and tissue morphogenesis. Thus it seems that hemidesmosomes do not merely maintain dermo-epidermal adhesion and tissue integrity, but that they are also implicated in intracellular signaling. Here we discuss recently published data on the biology and function of hemidesmosomes.

Zyxin: zinc fingers at sites of cell adhesion.

Zyxin is a low abundance phosphoprotein that is localized at sites of cell-substratum adhesion in fibroblasts. Zyxin displays the architectural features of an intracellular signal transducer. The protein exhibits an extensive proline-rich domain, a nuclear export signal and three copies of the LIM motif, a double zinc-finger domain found in many proteins that play central roles in regulation of cell differentiation. Zyxin interacts with alpha-actinin, members of the cysteine-rich protein (CRP) family, proteins that display Src homology 3 (SH3) domains and Ena/VASP family members. Zyxin and its partners have been implicated in the spatial control of actin filament assembly as well as in pathways important for cell differentiation. Based on its repertoire of binding partners and its behavior, zyxin may serve as a scaffold for the assembly of multimeric protein machines that function in the nucleus and at sites of cell adhesion.

Actin crosslinking proteins at the leading edge

The lamellar membrane at the leading edge of motile cells participates in a series of complex movements that involve the assembly and reorganization of actin bundles and networks, both structures formed by actin crosslinking proteins. Immunofluorescence miscroscopy localizes within lamellipodia and filopodia several crosslinking proteins including fascin, fimbrin, α-actinin and filamin. While these proteins may organize actin into bundles and networks, fimbrin and α-actinin may play an additional role of linking the cytoskeleton to cell-substratum adhesion sites.

The expression and localization of urokinase-type plasminogen activator and its type 1 inhibitor are regulated by retinoic acid and fibroblast growth factor in human teratocarcinoma cells.

Human Tera 2 embryonal carcinoma cells switch gradually from rapidly growing undifferentiated cells to almost nonproliferating cells during retinoic acid (RA)-induced neuronal differentiation. This process is associated with the increased expression of type 1 plasminogen activator inhibitor (PAI 1) mRNA, and the secreted inhibitor is immobilized to the pericellular area. Furthermore, the differentiation is accompanied by a decrease in the amount of both the secreted tissue-type PA (tPA) and the mainly cell-associated urokinase-type PA (uPA) activity. In RA-differentiated cells, uPA becomes localized at the vinculin-rich cell-substratum adhesion sites. Fibroblast growth factor activity has been associated with various events during embryonal growth and with the regulation of proteolytic enzymes. A short-term treatment of the undifferentiated Tera 2 cells with basic fibroblast growth factor (bFGF) increases uPA mRNA levels and the cell-associated uPA activity, whereas the secretory tPA activity decreases. bFGF induces PAI 1 mRNA expression in the undifferentiated cells, but unlike PAI 1 protein after RA-treatment, the inhibitor does not accumulate around the cells but is released in the medium. A similar exposure to bFGF has less effect on the RA-differentiated Tera 2 cells. Under these conditions bFGF treatment leads to an increase in the amounts of PAI 1 and uPA mRNAs, but no changes in the localization of these components can be seen. Differentiation of human embryonal carcinoma cells is thus connected with an altered response to bFGF.

Purification and characterization of zyxin, an 82,000-dalton component of adherens junctions.

We describe here the purification and characterization of a recently identified adherens junction protein that has an apparent molecular mass of 82 kDa on sodium dodecyl sulfate-polyacrylamide gels (Beckerle, M. C. (1986) J. Cell Biol. 103, 1679-1687). The 82-kDa protein was isolated from avian smooth muscle by a low ionic strength alkaline pH extraction followed by ammonium sulfate fractionation. Sequential chromatographic separation using DEAE-cellulose, phenyl-Sepharose CL-4B, and hydroxylapatite resins results in a purified 82-kDa protein. The 82-kDa protein has a Stokes radius of 5.6 nm and a relative sedimentation coefficient of 3.0 S. The calculated native molecular mass of the protein based on its hydrodynamic properties is 69 kDa, and the derived frictional ratio (f/fo) is 2.1. The protein does not focus discretely by isoelectric-focusing-sodium dodecyl sulfate-polyacrylamide gel electrophoresis; there are numerous isoelectric point variants in the range of 6.4-7.2, with the average isoelectric point being 6.9. The 82-kDa protein is phosphorylated in vivo and appears to be a cytoplasmic component of adherens junctions. The properties of the 82-kDa protein distinguish it from other known adherens junction proteins of this molecular mass. In fibroblasts, the 82-kDa protein is found in adhesion plaques as well as along actin-containing stress fibers near where they terminate at sites of cell-substratum adhesion. It is also found in the cell-cell adherens junctions of pigmented retinal epithelial cells and the dense plaques of smooth muscle cells. Since the 82-kDa protein is found at both cell-substratum and cell-cell adherens junctions, we propose to call it zyxin, meaning a joining, to indicate that it is found at regions where extracellular ligands are structurally and functionally joined to the cytoskeleton.

Localization of two fibronectin-binding glycoproteins in rat liver and primary hepatocytes: Co-distribution in vitro of integrin (α5β1) and non-integrin (AGp110) receptors in cell-substratum adhesion sites

We have compared the localization of integrin alpha 5 beta 1 and AGp110 (apical glycoprotein of Mr 110 x 10(3]] in rat liver parenchyma and in primary hepatocyte cultures. Integrin alpha 5 beta 1 is a heterodimeric fibronectin receptor. AGp110 is a newly described monomeric glycoprotein of the apical (bile canalicular) membrane domain of liver parenchyma that binds in an RGD-independent manner to fibronectin and mediates spreading of hepatocytes onto fibronectin-coated substrata. Using Western blotting of fractionated liver membranes and immunocytochemistry of liver sections at light- and electron-microscope levels, we have confirmed that AGp110 is a canalicular glycoprotein and have established that integrin is located in approximately equal proportions in the sinusoidal, lateral and canalicular membrane domains. In the canalicular surface domain both glycoproteins are associated with microvilli. Examination of immunolabelled primary hepatocytes spread on fibronectin-coated substrata by light and laser scanning confocal microscopy revealed colocalization of AGp110, integrin, actin and vinculin in substratum-attached microextensions at the periphery of the basal cell surface. Actin filaments that terminated at these cell processes originated from circular sub-cortical actin fibres. Interference reflection microscopy revealed focal adhesive contacts at the edge of the basal cell periphery at the same location where AGp110 and integrin were observed by immunofluorescence. In vitro, a proportion of the primary hepatocytes seeded onto fibronectin-coated substrata aggregated into colonies of several cells with intercellular contacts between neighbouring cells. Cell-substratum contacts containing integrin, AGp110, actin and vinculin followed the contours of these colonies in the same manner as they delineated the basal periphery of single, substratum-attached cells. We conclude that both integrin and AGp110 contribute to hepatocyte-fibronectin adhesive interactions and that intercellular adhesion and cooperation among hepatocytes in their response to fibronectin matrices leads to colony formation and morphological differentiation of parenchymal cell monolayers in vitro.

Talin: Role at sites of cell‐substratum adhesion

Talin: Role at sites of cell‐substratum adhesion

Multiple and masked pools of fibronectin in murine fibroblast cell-substratum adhesion sites

Substrate-attached material (SAM) prepared from murine BALB/c 3T3 cells and various derivatives contains adhesion sites which pinch off from the cell surface during EGTA-mediated detachment but which remain bound to the serum-coated tissue culture substratum. SAM contains the related adhesive glycoproteins cold-insoluble globulin (CIG) (from serum in the medium) and fibronectin (synthesized by the cells) as detected by immune staining of electrophoretically separated proteins, using antibodies of defined specificity. Serum and SAM contain cross-linked multimers of serum-derived CIG (not disulfide-mediated) but not of cell-derived fibronectin; therefore, thiol-resistant cross-linking between CIG and fibronectin is not involved in adhesion of these cells. Immunofluorescence microscopy of SAM from sparse cultures reveals fibrillar pools containing cellular fibronectin, although most retraction fibers seen on EGTA-treated cells do not stain, even after treatment with non-ionic detergent. Very little specific staining can be detected in SAM prepared from dense cultures, although gel electrophoretic analysis reveals proportionately as much murine fibronectin as is found in SAM from sparse cultures. Hyaluronidase digestion of SAM has no effect on the immunofluorescent staining, while gentle trypsin digestion completely abolishes staining without removing all biochemically detectable fibronectin. We conclude that some of the fibronectin and CIG in adhesion sites is masked and unavailable for antibody binding and that multiple pools of fibronectin exist in this adhesive material.

Glycosaminoglycans that bind cold-insoluble globulin in cell-substratum adhesion sites of murine fibroblasts.

Glycosaminoglycans (GAGs) and glycoprotein-derived glycopeptide from mouse BALB/c3T3 and simian virus 40-transformed 3T3 whole cells or their adhesion sites, which are left bound to the serum-coated tissue culture substratum after detachment of cells mediated by [ethylenebis-(oxyethylenenitrilo]tetraacetic acid (EGTA), were analyzed for specific binding to Sepharose columns derivatized with cold-insoluble globulin (CIg). CIg is the serum-contained form of fibronectin and is required for the adhesion of these fibroblasts to the substratum. Of the various GAGs present in these fractions of either cell type, only the highly N-sulfated sequences of heparan sulfate and a small subset of dermatan sulfate bind to CIg-Sepharose. There was no detectable binding of glycopeptide, undersulfated heparan sulfate, the various chondroitin species, or hyaluronate. Adhesion sites from newly attaching cells were greatly enriched in CIg-binding heparan sulfate when compared to long-term-growth adhesion sites or EGTA-detached cells. Various properties of binding were determined. The reference standard standard GAGs heparin (or heparan sulfate) and dermatan sulfate were able to displace bound radiolabeled adhesion site GAG from the column, whereas the other GAGs had no effect. CIg has been shown to be the only adhesion-promoting activity in the serum layer of this culture system. Because these fibroblast adhesion sites do not contain collagen, which could potentially mediate adhesion to the substratum-bound CIg, these data support other evidence that multivalent heparan sulfate proteoglycans mediate substratum adhesion of these cells by coordinate binding to fibronectin on the cell surface and CIg on the substratum.

Adhesion sites of murine fibroblasts on cold insoluble globulin-adsorbed substrata.

The attachment and detachment behavior of three mouse fibroblast cell lines adhering to plastic tissue culture substrata coated with the serum protein cold-insoluble globulin (CIg) resembles that seen on the usual serum-coated substrata. The transformed cell line SVT2 spreads more extensively on the CIg-coated than on the serum-coated substratum, while the nontransformed Balb/c 3T3 line and concanavalin A-selected "revertant" of SVT2 are equally well spread on both substrata. In all three cases, immunofluorescence microscopy using antibodies to CIg suggests that the cells are more tightly apposed to the CIg-coated substratum than to the serum-coated substratum. Substrate-attached material (SAM), which contains cell-substratum adhesion sites and which is left after EGTA-mediated detachment of cells, is enriched for cell surface fibronectin and glycosaminoglycans (GAG). When cells are seeded onto CIg-coated substrata rather than serum-coated substrata, there is an increased deposition of GAG but a comparable deposition of cellular proteins. The protein distribution of the two types of SAM are identical as analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, including fibronectin content. This indicates that substratum-bound CIg cannot functionally substitute for cell surface fibronectin in these adhesion sites. Analysis of the GAG deposited on CIg-coated substrata reveals that hyaluronate and the chondroitins are increased to a much greater extent than heparan sulfate; however, the ratio of hyaluronate to the various chondroitin species is invariant. These data provide further evidence that hyaluronate and the chondroitins are deposited in adhesion sites in well-defined stoichiometric proportions, possibly as supramolecular complexes, and that CIg may mediate adhesion of cells in the serum layer by binding to GAG-containing proteoglycans.

Phospholipid composition of substrate adhesion sites of normal, virus-transformed, and revertant murine cells

The phospholipid composition of cell-substratum adhesion sites, obtained after EGTA-mediated detachment of cells from the tissue-culture substratum, was determined for [32P]orthophosphate radiolabeled Balb/c 3T3, SV40-transformed (SVT2), and concanavalin A selected revertant variant cell lines. All of the major phospholipid classes were found in the substrate-attached material, but there was an enrichment for specific phospholipid species in this adhesive material as compared to whole-cell and surface-enriched membranes. The phospholipid composition was remarkable similar for the whole-cell and surface-enriched membrane fractions from the three cell lines. However, pronounced differences in the phospholipid composition of the adhesion sites were observed as a result of viral transformation--SVT2 sites were clearly enriched in phosphatidylethanolamine and depleted in phosphatidylcholine when compared to 3T3 sites. This alteration in adhesion site phospholipids of transformed cells reverted to 3T3-like values in the adhesive material of revertant cells. The composition of adhesive material of newly attaching cells was also examined to differentiate compositional differences between "footpad" adhesion sites and "footprints", adhesive material pinched off from the posterior of cells as they move across the substratum. Pulse and pulse-chase analyses of the [32P]phospholipids revealed some differences in synthesis and turnover rates in the three cell lines; in addition, altered rates of deposition of newly synthesized material into adhesion sites of transformed cells were observed. These data afford further evidence that the cell-substratum adhesion sites are highly specialized areas of the cell surface enriched in components which are intricately involved in the adhesive process. The transformation-dependent changes in adhesion site phospholipids may help to determine the basis for the altered adhesive properties of transformed cells.

Cell to substratum contacts of chick fibroblasts and their relation to the microfilament system. A correlated interference-reflexion and high-voltage electron-microscope study.

ABSTRACT The interference reflexion microscope may be used to examine the pattern of cell-substratum separation of cultured cells. Earlier studies on chick fibroblasts using this technique identified 3 types of separation: (1) focal contacts, representing approx. 10—14 nm separation and which are sites of cell-substratum adhesion and form at the leading edge of a cell during locomotion; (2) close contacts, representing approx. 30 nm separation; and (3) regions of greater separation, generally 100 – 140 nm. Using light microscopy these studies described the association of the focal contacts with the distal ends of cytoplasmic fibres and noted their similarity to the adhesion plaques described in EM studies of thin vertical sections of fibroblasts. In this investigation we have directly confirmed these earlier observations by using a correlated interference-reflexion and high-voltage stereo electron-microscopic study of cultured chick heart fibroblasts. The results show that focal contacts are always associated with the distal ends of linear bundles of microfilaments which pass centripetally and often obliquely through the cytoplasm. At their proximal ends the bundles insert into a loose matrix of microfilaments and organelles surrounding the nucleus. The close contacts are often associated with a meshwork of microfilaments. The evidence supports the hypothesis that the microfilament bundles of a fibroblast form synchronously with the focal adhesions to the substratum. Our results are discussed with reference to the current evidence for the role of contractile proteins in cell locomotion.