Cell-matrix Adhesive Interactions Research Articles

Expression of αV integrin and its potential partners in bull reproductive tissues, germ cells and spermatozoa

Integrins are transmembrane receptors expressed in all nucleated mammalian cells, critically involved in cell-matrix adhesion and cell-cell interactions that modulate many signalling cascades. It is assumed that integrins also provide essential functions of the reproductive system. In this study, we describe the detailed localization and distribution of αV integrin in the plasma membrane of bull sperm head and tail. Integrin αV was observed in the area of forming acrosome in developing sperm since the stage of round spermatids and persists in the acrosome during epididymal maturation and ejaculation till the acrosomal exocytosis. We detected CD9 and CD81 tetraspanins as the potential partners of αV integrin. Their similar staining pattern in testicular tissue suggested the involvement of these molecules in the tetraspanin web of “testisomes”. Moreover, the complex of αV with β1 and β3 integrin subunits cannot be excluded at least in sperm. The presented findings contribute to understanding the mutual action of integrins and tetraspanins during sperm development and maturation.

Switching between individual and collective motility in B lymphocytes is controlled by cell-matrix adhesion and inter-cellular interactions

Lymphocytes alternate between phases of individual migration across tissues and phases of clustering during activation and function. The range of lymphocyte motility behaviors and the identity of the factors that govern them remain elusive. To explore this point, we here collected unprecedented statistics pertaining to cell displacements, cell:matrix and cell:cell interactions using a model B cell line as well as primary human B lymphocytes. At low cell density, individual B lymphocytes displayed a high heterogeneity in their speed and diffusivity. Beyond this intrinsic variability, B lymphocytes adapted their motility to the composition of extra-cellular matrix, adopting slow persistent walks over collagen IV and quick Brownian walks over fibronectin. At high cell density, collagen IV favored the self-assembly of B lymphocytes into clusters endowed with collective coordination, while fibronectin stimulated individual motility. We show that this behavioral plasticity is controlled by acto-myosin dependent adhesive and Arp2/3-dependent protrusive actin pools, respectively. Our study reveals the adaptive nature of B lymphocyte motility and group dynamics, which are shaped by an interplay between and cell:matrix and cell:cell interactions.

Interleukin-1β induces intercellular adhesion molecule-1 expression, thus enhancing the adhesion between mesenchymal stem cells and endothelial progenitor cells via the p38 MAPK signaling pathway.

Endothelial progenitor cells (EPCs) are an important component of stem-cell niches, which are able to promote the self-renewal and pluripotency of mesenchymal stem cells (MSCs). The biological functions of these two cell types is dependent on adhesion, and the adhesion between MSCs and EPCs is important due to their critical role in neovascularization and bone regeneration in tissue engineering. Intercellular adhesion molecule-1 (ICAM-1, also known as cluster of differentiation 54), is a member of the immunoglobulin supergene family, which functions in cell-cell and cell-matrix adhesive interactions. Compared with other adhesion molecules, ICAM-1 is expressed in hematopoietic and nonhematopoietic cells, and can mediate adhesive interactions. The present study aimed to investigate the importance of ICAM-1 in the adhesion of MSCs and EPCs, and demonstrated that adhesion between these cells could be regulated by interleukin (IL)-1β via the p38 mitogen-activated protein kinase pathway. In addition, the results confirmed that ICAM-1 served a critical role in regulation of adhesion between MSCs and EPCs. ELISA, cell immunofluorescence, western blot analysis and adhesion assay were used to confirm our theory from phenomenon to essence. The present study provided evidence to support and explain the adhesion between MSCs and EPCs. Furthermore, the present findings provide a theoretical basis for further stem-cell niche transplantation to increase understanding of the function of MSCs and the crosstalk between MSCs and EPCs in the stem-cell niche.

Actin cytoskeleton regulates functional anchorage-migration switch during T-cadherin-induced phenotype modulation of vascular smooth muscle cells

ABSTRACTVascular smooth muscle cell (SMC) switching between differentiated and dedifferentiated phenotypes is reversible and accompanied by morphological and functional alterations that require reconfiguration of cell-cell and cell-matrix adhesion networks. Studies attempting to explore changes in overall composition of the adhesion nexus during SMC phenotype transition are lacking. We have previously demonstrated that T-cadherin knockdown enforces SMC differentiation, whereas T-cadherin upregulation promotes SMC dedifferentiation. This study used human aortic SMCs ectopically modified with respect to T-cadherin expression to characterize phenotype-associated cell-matrix adhesion molecule expression, focal adhesions configuration and migration modes. Compared with dedifferentiated/migratory SMCs (expressing T-cadherin), the differentiated/contractile SMCs (T-cadherin-deficient) exhibited increased adhesion to several extracellular matrix substrata, decreased expression of several integrins, matrix metalloproteinases and collagens, and also distinct focal adhesion, adherens junction and intracellular tension network configurations. Differentiated and dedifferentiated phenotypes displayed distinct migrational velocity and directional persistence. The restricted migration efficiency of the differentiated phenotype was fully overcome by reducing actin polymerization with ROCK inhibitor Y-27632 whereas myosin II inhibitor blebbistatin was less effective. Migration efficiency of the dedifferentiated phenotype was diminished by promoting actin polymerization with lysophosphatidic acid. These findings held true in both 2D-monolayer and 3D-spheroid migration models. Thus, our data suggest that despite global differences in the cell adhesion nexus of the differentiated and dedifferentiated phenotypes, structural actin cytoskeleton characteristics per se play a crucial role in permissive regulation of cell-matrix adhesive interactions and cell migration behavior during T-cadherin-induced SMC phenotype transition.

Measurement systems for cell adhesive forces.

Cell adhesion to the extracellular matrix (ECM) involves integrin receptor-ligand binding and clustering to form focal adhesion (FA) complexes, which mechanically link the cell's cytoskeleton to the ECM and regulate fundamental cell signaling pathways. Although elucidation of the biochemical events in cell-matrix adhesive interactions is rapidly advancing, recent studies show that the forces underlying cell-matrix adhesive interactions are also critical to cell responses. Therefore, multiple measurement systems have been developed to quantify the spatial and temporal dynamics of cell adhesive forces, and these systems have identified how mechanical events influence cell phenotype and FA structure-function relationships under physiological and pathological settings. This review focuses on the development, methodology, and applications of measurement systems for probing (a) cell adhesion strength and (b) 2D and 3D cell traction forces.

Western blot analysis of adhesive interactions under fluid shear conditions: the blot rolling assay.

Western blotting has proven to be an important technique in the analysis of receptor-ligand interactions (i.e., by ligand blotting) and for identifying molecules mediating cell attachment (i.e., by cell blotting). Conventional ligand blotting and cell blotting methods employ non-dynamic (static) incubation conditions, whereby molecules or cells of interest are placed in suspension and overlaid on membranes. However, many cell-cell and cell-matrix adhesive interactions occur under fluid shear conditions, and shear stress itself mediates and/or facilitates the engagement of these physiologically appropriate receptors and ligands. Notably, shear forces critically influence the adhesion of circulating cells and platelets to vessel walls in physiologic cell migration and hemostasis, as well as in inflammatory and thrombotic disorders, cancer metastasis, and atherosclerosis. Use of non-dynamic blotting conditions to analyze such interactions can introduce bias, overtly missing relevant effectors and/or exaggerating the relative role(s) of non-physiologic adhesion molecules. To address this shortfall, we have developed a new technique for identifying binding interactions under fluid shear conditions, the "blot rolling assay." Using this method, molecules in a complex mixture are resolved by gel electrophoresis, transferred to a membrane that is rendered semitransparent, and the membrane is then incorporated into a parallel-plate flow chamber apparatus. Under controlled flow conditions, cells or particles bearing adhesion proteins of interest are then introduced into the chamber and interactions with individual immobilized molecules (bands) can be visualized in real time. The substrate molecule(s) supporting adhesion under fluid shear can then be identified by staining with specific antibodies or by excising the relevant band(s) and performing mass spectrometry or microsequencing of the isolated material. This method thus allows for the identification, within a complex mixture and without prior isolation or purification, of both known and previously uncharacterized adhesion molecules operational under dynamic conditions.

The ECM-cell interaction of cartilage extracellular matrix on chondrocytes.

Cartilage extracellular matrix (ECM) is composed primarily of the network type II collagen (COLII) and an interlocking mesh of fibrous proteins and proteoglycans (PGs), hyaluronic acid (HA), and chondroitin sulfate (CS). Articular cartilage ECM plays a crucial role in regulating chondrocyte metabolism and functions, such as organized cytoskeleton through integrin-mediated signaling via cell-matrix interaction. Cell signaling through integrins regulates several chondrocyte functions, including differentiation, metabolism, matrix remodeling, responses to mechanical stimulation, and cell survival. The major signaling pathways that regulate chondrogenesis have been identified as wnt signal, nitric oxide (NO) signal, protein kinase C (PKC), and retinoic acid (RA) signal. Integrins are a large family of molecules that are central regulators in multicellular biology. They orchestrate cell-cell and cell-matrix adhesive interactions from embryonic development to mature tissue function. In this review, we emphasize the signaling molecule effect and the biomechanics effect of cartilage ECM on chondrogenesis.

Tetraspanins CD81 and CD82 Facilitate α4β1-Mediated Adhesion of Human Erythroblasts to Vascular Cell Adhesion Molecule-1

The proliferation and terminal differentiation of erythroid progenitors occurs in human bone marrow within erythroblastic islands, specialised structures consisting of a central macrophage surrounded by developing erythroid cells. Many cell-cell and cell-matrix adhesive interactions maintain and regulate the co-ordinated daily production of reticulocytes. Erythroid cells express only one integrin, α4β1, throughout differentiation, and its interactions with both macrophage Vascular Cell Adhesion Molecule-1 and with extracellular matrix fibronectin are critical for erythropoiesis. We observed that proerythroblasts expressed a broad tetraspanin phenotype, and investigated whether any tetraspanin could modulate integrin function. A specific association between α4β1 and CD81, CD82 and CD151 was demonstrated by confocal microscopy and co-immune precipitation. We observed that antibodies to CD81 and CD82 augmented adhesion of proerythroblasts to Vascular Cell Adhesion Molecule-1 but not to the fibronectin spliceoforms FnIII12-IIICS-15 and FnIII12–15. In contrast, different anti-CD151 antibodies augmented or inhibited adhesion of proerythroblasts to Vascular Cell Adhesion Molecule-1 and the fibronectin spliceoform FnIII12-IIICS-15 but not to FnIII12–15. These results strongly suggest that tetraspanins have a functional role in terminal erythropoiesis by modulating interactions of erythroblast α4β1 with both macrophages and extracellular matrix.

Degradation-mediated cellular traction directs stem cell fate in covalently crosslinked three-dimensional hydrogels

Although cell-matrix adhesive interactions are known to regulate stem cell differentiation, the underlying mechanisms, in particular for direct three-dimensional (3D) encapsulation within hydrogels, are poorly understood. Here, we demonstrate that in covalently crosslinked hyaluronic acid (HA) hydrogels, the differentiation of human mesenchymal stem cells (hMSCs) is directed by the generation of degradation-mediated cellular-traction, independent of cell morphology or matrix mechanics. hMSCs within HA hydrogels of equivalent elastic moduli that either permit (restrict) cell-mediated degradation exhibited high (low) degrees of cell spreading and high (low) tractions, and favoured osteogenesis (adipogenesis). In addition, switching the permissive hydrogel to a restrictive state via delayed secondary crosslinking reduced further hydrogel degradation, suppressed traction, and caused a switch from osteogenesis to adipogenesis in the absence of changes to the extended cellular morphology. Also, inhibiting tension-mediated signalling in the permissive environment mirrored the effects of delayed secondary crosslinking, whereas upregulating tension induced osteogenesis even in the restrictive environment.

Mimicking normal tissue architecture and perturbation in cancer with engineered micro-epidermis

Correct tissue architecture is essential for normal physiology, yet there have been few attempts to recreate tissues using micro-patterning. We have used polymer brush micro-engineering to generate a stratified micro-epidermis with fewer than 10 human keratinocytes. Epidermal stem cells are captured on 100 μm diameter circular collagen-coated disks. Within 24 h they assemble a stratified micro-tissue, in which differentiated cells have a central suprabasal location. For rings with a non-adhesive centre of up to 40 μm diameter, cell–cell and cell–matrix adhesive interactions together result in correct micro-epidermis assembly. Assembly requires actin polymerization, adherens junctions and desmosomes, but not myosin II-mediated contractility nor coordinated cell movement. Squamous cell carcinoma cells on micro-patterned rings exhibit disturbed architecture that correlates with the characteristics of the original tumours. The micro-epidermis we have generated provides a new platform for screening drugs that modulate tissue assembly, quantifying tissue stratification and investigating the properties of tumour cells.

Structure and function of ameloblastin as an extracellular matrix protein: adhesion, calcium binding, and CD63 interaction in human and mouse

The functional significance of extracellular matrix proteins in the life of vertebrates is underscored by a high level of sequence variability in tandem with a substantial degree of conservation in terms of cell-cell and cell-matrix adhesion interactions. Many extracellular matrix proteins feature multiple adhesion domains for successful attachment to substrates, such as integrin, CD63, and heparin. Here we have used homology and ab initio modeling algorithms to compare mouse ameloblastin (mAMBN) and human ameloblastin (hABMN) isoforms and to analyze their potential for cell adhesion and interaction with other matrix molecules as well as calcium binding. Sequence comparison between mAMBN and hAMBN revealed a 26-amino-acid deletion in mAMBN, corresponding to a helix-loop-helix frameshift. The human AMBN domain (174Q-201G), homologous to the mAMBN 157E-178I helix-loop-helix region, formed a helix-loop motif with an extended loop, suggesting a higher degree of flexibility of hAMBN compared with mAMBN, as confirmed by molecular dynamics simulation. Heparin-binding domains, CD63-interaction domains, and calcium-binding sites in both hAMBN and mAMBN support the concept of AMBN as an extracellular matrix protein. The high level of conservation between AMBN functional domains related to adhesion and differentiation was remarkable when compared with only 61% amino acid sequence homology.

Extracellular ATP induces CD44 shedding from macrophage‐like P388D1 cells via the P2X7 receptor

The P2X7 receptor (P2X7R) is a nucleotide receptor expressed predominantly on hemopoietic, bone, and epithelial cells. The P2X7R can be activated by extracellular ATP and induces the influx of calcium, releases cytokines, and participates in cell proliferation and apoptosis. CD44 is an adhesion molecule. The effects of CD44 include cell-cell and cell-matrix adhesion interactions, lymphocyte activation, and cell migration. Many studies have shown that P2X7R and CD44 play important roles in hematological malignancies, but no study exists regarding the relationship between P2X7R and CD44. In the present study, we characterized P388D1 cells for the surface expression of CD44 and analyzed ATP-induced shedding. The data showed that P388D1 cells express CD44. Incubation of P388D1 cells with ATP induced a rapid loss of CD44 from the P388D1 cell surface. In addition, using a receptor inhibitor and P2X7R short hairpin RNA, we showed that the loss of CD44 is mediated via the P2X7R. Finally, we demonstrated that activation of P2X7R by ATP induces CD44 shedding.

Lib, transcriptionally induced in senile plaque-associated astrocytes, promotes glial migration through extracellular matrix

In an effort to identify astrocyte-derived molecules that may be intimately associated with progression of Alzheimer’s disease (AD), Lib, a type I transmembrane protein belonging to leucine-rich repeat superfamily, has been identified as a distinctly inducible gene, responsive to β-amyloid as well as pro-inflammatory cytokines in astrocytes. To evaluate the roles of Lib in AD, we investigated Lib expression in AD brain. In non-AD brain, Lib mRNA has been detected in neurons but not in quiescent astrocytes. On the contrary, in AD brain, Lib mRNA is expressed in activated astrocytes associated with senile plaques, but not expressed in neurons around lesions. Lib-expressing glioma cells displayed promotion of migration ability through reconstituted extracellular matrix and recombinant Lib protein bound to constituents of extracellular matrix. These observations suggest that Lib may contribute to regulation of cell–matrix adhesion interactions with respect to astrocyte recruitment around senile plaques in AD brain.

Influence of β1 Integrin Intracytoplasmic Domains in the Regulation of VLA-4-Mediated Adhesion of Human T Cells to VCAM-1 under Flow Conditions

The VLA-4 integrin supports static cell-cell, cell-matrix adhesion, and dynamic interactions with VCAM-1. Although functions for well-conserved beta(1) integrin cytoplasmic domains in regulating static cell adhesion has been established, the molecular basis for beta(1) integrin-dependent arrest on VCAM-1 under flow conditions remains poorly understood. We have transfected the beta(1) integrin-deficient A1 Jurkat T cell line with beta(1) cDNA constructs with deletions of the NPXY motifs and specific mutations of tyrosine residues. Deletion of either NPXY motif impaired static adhesion induced by CD2 or CD47 triggering or direct beta(1) integrin stimulation. In contrast, PMA-induced adhesion to VCAM-1 was unaffected by deletion of the NPIY motif and only slightly impaired by deletion of NPKY. Moreover, deletion of the NPIY motif resulted in enhanced rolling and reduced arrest on VCAM-1 under shear flow conditions. In contrast, deletion of the NPKY motif did not alter arrest under flow. Although tyrosine to phenylalanine substitutions within two NPXY motifs did not alter static adhesion to VCAM-1, these mutations enhanced arrest on VCAM-1 under flow conditions. Furthermore, although deletion of the C'-terminal 5 AA of the beta(1) cytoplasmic domain dramatically impaired activation-dependent static adhesion, it did not impair arrest on VCAM-1 under flow conditions. Thus, our results demonstrate distinct structural requirements for VLA-4 function under static and shear flow conditions. This may be relevant for VLA-4 activity regulation in different anatomic compartments, such as when circulating cells arrest on inflamed endothelium under shear flow and when resident cells in bone marrow interact with VCAM-1- positive stromal cells.

Topographical significance of membrane skeletal component protein 4.1B in mammalian organs

The polarized architecture of epithelial cells is a fundamental determinant of cell structures and functions. Both formation and orientation of proper epithelial polarity are needed for cell-cell or cell-matrix adhesion, signal transduction and cytoskeletal interactions of multimolecular complexes at apical, lateral and basal cell membranes. These cell membrane domains are usually segregated by some junctional complexes. Recent molecular genetic studies on the anchor structure between myelin sheaths and axons have indicated the specific molecular organization for polarization of axolemma and the myelin sheaths at paranodes, termed 'septate-like junctions'. It was also speculated that other mammalian organs may use a similar junctional system. The protein 4.1 B was originally found to be localized in paranodes and juxtaparanodes of myelinated nerve fibers. Our recent immunohistochemical studies on protein 4.1B have indicated its significance for the cell-cell and/or cell-matrix adhesion in various rodent organs. The protein 4.1 family of proteins have been supposed to possess variable molecular domains relating to cell adhesion, ion balance, receptor responses and signal transduction. Therefore, more precise studies on the molecular structure and the functional domains of protein 4.1B, as well as on its changes under physiological and pathological conditions, may provide a clue for organogenesis in various mammalian organs.

Phagocytosis, an alternative model system for the study of cell adhesion

Cell adhesion encompasses a variety of cell-cell and cell-matrix adhesive interactions. Whereas ligation of most adhesion receptors activate Rho-family GTP-binding proteins and the subsequent reorganization of the actin cytoskeleton, the molecular mechanisms involved remain poorly understood. Because phagocytosis is a spatially restricted adhesion process, it represents a simplified model system to investigate the spatio-temporal regulation of the signalling pathways that link surface adhesion receptors, small GTPases and the actin cytoskeleton. This review highlights some of the similarities between the formation and maintenance of adhesive contacts and phagocytic uptake and discusses why the study of phagocytosis can help understand more complex adhesion processes.

Overexpression of the thymosin beta-4 gene is associated with increased invasion of SW480 colon carcinoma cells and the distant metastasis of human colorectal carcinoma.

Cell-matrix and cell-cell adhesive interactions play important roles in the normal organization and stabilization of the cell layer in epithelial tissue. Alterations in the expression and function of these adhesion systems that cause a switch to a migratory phenotype in tumor invasion and metastasis are critical for the malignant conversion of epithelial cells. Thymosin beta-4 (Tbeta-4) is the major actin-sequestering protein that has been shown to be upregulated in a wide variety of human carcinomas and has been implicated to be involved in altering the motility of certain tumors. We have recently demonstrated that the growth rate, colony formation in soft agar, and motility, all good indicators for malignant progression, of SW480 colon carcinoma cells are dramatically increased by enforced Tbeta-4 expression. To test the hypothesis that overexpression of this G-actin sequestering peptide also promotes tumor invasion, we examined not only the invasion capability of Tbeta-4-overexpressing SW480 cells, but also the expression levels of Tbeta-4 as well as several proteins that participate in different stages of tumor progression in matched samples of human primary colorectal adenocarcinoma and liver metastases from several patients. A marked increase on the invasiveness in Tbeta-4-overexpressing SW480 cells with increased levels and activity of matrix metalloproteinase-7 (MMP-7) was observed. Furthermore, the levels of Fas as well as the susceptibility to Fas ligand-mediated apoptosis in Tbeta-4-overexpressing cells were significantly decreased. Interestingly, the levels of Tbeta-4 mRNA, beta-catenin, c-Myc, and MMP-7 in metastatic liver lesions were relatively higher, whereas the levels of E-cadherin and Fas were significantly lower than those in the matched primary colorectal tumors. These results suggest that upregulation of Tbeta-4, by promoting the disruption of cell-cell adhesion and a consequential activation of the beta-catenin signaling, could be a key event in the acquisition of growth advantages as well as invasive phenotypes in human colorectal carcinomas.

Fascin, an Actin-Bundling Protein, Modulates Colonic Epithelial Cell Invasiveness and Differentiation in Vitro

In epithelial tissue, cell-matrix and cell-cell adhesive interactions have important roles in the normal organization and stabilization of the cell layer. The malignant conversion of epithelial cells involves alterations in the expression and function of these adhesion systems that enable a switch to a migratory phenotype in tumor invasion and metastasis. Fascin is an actin-crosslinking protein that is found in the core actin bundles of cell-surface spikes and projections that are implicated in cell motility. We demonstrate that fascin is not detectable in normal colonic epithelium, but is dramatically up-regulated in colorectal adenocarcinoma. To test the hypothesis that fascin could participate in tumor invasive behavior, we developed a cell culture model to examine the effect of fascin expression on the adhesive interactions, invasiveness, and differentiation of colonic epithelial cells. We report marked effects on the organization of cell-surface protrusions, actin cytoskeleton, and focal adhesions in the absence of alterations in the protein levels of the major components of these structures. These effects correlate with alterations in cell movements on two-dimensional matrix, and increased invasiveness in three-dimensional matrix. The cells also show increased proliferation and decreased capacity for normal glandular differentiation in collagen gels. We propose that up-regulation of fascin, by promoting the formation of protrusive, actin-based, cell-motility structures, could be a significant component in the acquisition of invasive phenotype in colonic carcinoma.

Dynamic aspects of platelet adhesion under flow.

1. Cell-cell and cell-matrix adhesive interactions are critical for a wide range of physiological processes, including embryogenesis, inflammation, immunity and haemostasis. 2. The ability of circulating blood cells, such as platelets and leucocytes, to adhere to sites of vascular injury is complicated by the presence of blood flow, which imposes hydrodynamic forces on adhesion contacts. 3. To overcome this problem, platelets and leucocytes have evolved specific adhesion receptors with unique biomechanical properties that enable these cells to adhere to the vessel wall under flow conditions. 4. Platelet adhesion in the normal circulation appears to be a multiple-step process involving an initial reversible interaction between the platelet adhesion receptor glycoprotein Ib-IX-V and the vascular adhesion protein von Willebrand factor. Once tethered to the vessel wall, platelets form irreversible adhesion contacts through the binding of one or more platelet integrins to specific subendothelial matrix proteins. 5. There is now a wealth of evidence demonstrating that these receptors not only mediate platelet adhesion, but also transduce signals leading to platelet activation. 6. In the present review, we will briefly discuss the current understanding of the specific roles of individual platelet receptors in supporting the haemostatic function of platelets and discuss mechanisms by which these receptors induce platelet activation.

肥満細胞におけるインテグリン接着性の制御機構

Cell-cell and cell-matrix adhesive interactions mediated by integrins play crucial roles in leukocyte migration to inflamed tissues, and also in cell migration during embryogenesis.Mast cells are known to play a central role in regulating inflammatory responses of allergic and chronic immune diseases. The localization of mast cells in normal and inflamed tissues is therefore, an important regulatory process to influence intensity and duration of inflammatory responses. However, little is known about mechanisms that control localization of mast cells.Here we examined adhesion of extracellular matrix proteins and mast cells stimulated with steel factor (stem cell factor, kit ligand) and FcERI crosslinking. We showed that steel factor and FcεRI crosslinking were potent stimulators of avidity of β1 integrin VLA-5, which mediated a rapid and transient adhesion to fibronectin.