Integrin (alpha) and beta subunit contribution to the kinetic properties of (alpha)2beta1 collagen receptors on human keratinocytes analyzed under hydrodynamic conditions

Distinct Binding Interactions of α5β1-Integrin and Proteoglycans with Fibronectin

Dynamic single-molecule force spectroscopy was performed to monitor the unbinding of fibronectin with the proteoglycans syndecan-4 (SDC4) and decorin and to compare this with the unbinding characteristics of α₅β₁-integrin. A single energy barrier was sufficient to describe the unbinding of both SDC4 and decorin from fibronectin, whereas two barriers were observed for the dissociation of α₅β₁-integrin from fibronectin. The outer (high-affinity) barriers in the interactions of fibronectin with α₅β₁-integrin and SDC4 are characterized by larger barrier heights and widths and slower dissociation rates than those of the inner (low-affinity) barriers in the interactions of fibronectin with α₅β₁-integrin and decorin. These results indicate that SDC4 and (ultimately) α₅β₁-integrin have the ability to withstand deformation in their interactions with fibronectin, whereas the decorin-fibronectin interaction is considerably more brittle.

Modeling the Kinetics of Integrin Receptor Binding to Hepatic Extracellular Matrix Proteins

The composition of the extracellular matrix (ECM) proteins and the expression of their cognate receptors dictate cell behavior and dynamics. In particular, the interactions of ECM proteins with integrin receptors are key mediators of these cellular processes, playing a crucial role in the progression of several diseases of the liver, including inflammation, fibrosis/cirrhosis and cancer. This study establishes a modeling approach combining computation and experiments to evaluate the kinetics of integrin receptor binding to hepatic ECM proteins. ECM ligand concentration was derived from LC-MS/MS quantification of the hepatic ECM from mice exposed to chronic carbon tetrachloride (CCl4); receptor density was derived from published literature. Mathematical models for ECM-integrin binding kinetics that were developed incorporate receptor divalence and an aggregation scheme to represent clustering. The computer simulations reproduced positive cooperativity in the receptor aggregation model when the aggregation equilibrium constant (Ka) was positive and greater than Keq for divalent complex formation. Importantly, the modeling projected an increase in integrin binding for several receptors for which signaling is known to be increased after CCl4 exposure in the liver. The proposed modeling approach may be of use to elucidate the kinetics of integrin receptor binding to ECM proteins for homeostatic and diseased livers.

DISSECTING INDIVIDUAL LIGAND–RECEPTOR BONDS WITH A LAMINAR FLOW CHAMBER

The most important function of proteins may well be to bind to other biomolecules. It has long been felt that kinetic rates of bond formation and dissociation between soluble receptors and ligands might account for most features of the binding process. Only theoretical considerations allowed to predict the behaviour of surface-attached receptors from the properties of soluble forms. During the last decade, experimental progress essentially based on flow chambers, atomic force microscopes or biomembrane force probes allowed direct analysis of biomolecule interaction at the single bond level and gave new insight into previously ignored features such as bond mechanical properties or energy landscapes. The aim of this review is (i) to describe the main advances brought by laminar flow chambers, including information on bond response to forces, multiplicity of binding states, kinetics of bond formation between attached structures, effect of molecular environment on receptor efficiency and behaviour of multivalent attachment, (ii) to compare results obtain by this and other techniques on a few well defined molecular systems, and (iii) to discuss the limitations of the flow chamber method. It is concluded that a new framework may be needed to account for the effective behaviour of biomolecule association.

Studying Molecular Interactions at the Single Bond Level with a Laminar Flow Chamber

During the last decade, many investigators developed new methodologies allowing to study ligand-receptor interactions with unprecedented accuracy, up to the single bond level. Reported results include information on bond mechanical properties, association behaviour of surface-attached molecules, and dissection of energy landscapes and reaction pathways. The purpose of the present review is to discuss the potential and limitations of laminar flow chambers operated at low shear rates. This includes a brief review of basic principles, practical tips and problems associated with data interpretation. It is concluded that flow chambers are ideally suited to analyze weak interactions between a number of biomolecules, including the main families of adhesion receptors such as selectins, integrins, cadherins and members of the immunoglobulin superfamily. The sensitivity of the method is limited by the quality of surfaces and efficiency of the studied ligand-receptor couple rather than the hardware. Analyzing interactions with a resolution of a piconewton and a few milliseconds shows that ligand-receptor complexes may experience a number of intermediate binding states, making it necessary to examine the definition of association and dissociation rates. Finally, it is emphasized that association rates measured on surface-bound molecules are highly dependent on parameters unrelated to binding surfaces.

What is the biological relevance of the specific bond properties revealed by single-molecule studies?

During the last decade, many authors took advantage of new methodologies based on atomic force microscopy (AFM), biomembrane force probes (BFPs), laminar flow chambers or optical traps to study at the single-molecule level the formation and dissociation of bonds between receptors and ligands attached to surfaces. Experiments provided a wealth of data revealing the complexity of bond response to mechanical forces and the dependence of bond rupture on bond history. These results supported the existence of multiple binding states and/or reaction pathways. Also, single bond studies allowed us to monitor attachments mediated by a few bonds. The aim of this review is to discuss the impact of this new information on our understanding of biological molecules and phenomena. The following points are discussed: (i) which parameters do we need to know in order to predict the behaviour of an encounter between receptors and ligands, (ii) which information is actually yielded by single-molecule studies and (iii) is it possible to relate this information to molecular structure?

Single-molecule dynamic force spectroscopy of the fibronectin-heparin interaction

The integrity of cohesive tissues strongly depends on the presence of the extracellular matrix, which provides support and anchorage for cells. The fibronectin protein and the heparin-like glycosaminoglycans are key components of this dynamic structural network. In this report, atomic force spectroscopy was used to gain insight into the compliance and the resistance of the fibronectin-heparin interaction. We found that this interaction can be described by an energetic barrier width of 3.1+/-0.2A and an off-rate of 0.2+/-0.1s(-1). These dissociation parameters are similar to those of other carbohydrate-protein interactions and to off-rate values reported for more complex interactions between cells and extracellular matrix components. Our results indicate that the function of the fibronectin-heparin interaction is supported by its capacity to sustain significant deformations and considerable external mechanical forces.

Beta-1 integrin-mediated adhesion may be initiated by multiple incomplete bonds, thus accounting for the functional importance of receptor clustering

The regulation of cell integrin receptors involves modulation of membrane expression, shift between different affinity states, and topographical redistribution on the cell membrane. Here we attempted to assess quantitatively the functional importance of receptor clustering. We studied beta-1 integrin-mediated attachment of THP-1 cells to fibronectin-coated surfaces under low shear flow. Cells displayed multiple binding events with a half-life of the order of 1 s. The duration of binding events after the first second after arrest was quantitatively accounted for by a model assuming the existence of a short-time intermediate binding state with 3.6 s(-1) dissociation rate and 1.3 s(-1) transition frequency toward a more stable state. Cell binding to surfaces coated with lower fibronectin densities was concluded to be mediated by single molecular interactions, whereas multiple bonds were formed <1 s after contact with higher fibronectin surface densities. Cell treatment with microfilament inhibitors or a neutral antiintegrin antibody decreased bond number without changing aforementioned kinetic parameters whereas a function enhancing antibody increased the rate of bond formation and/or the lifetime of intermediate state. Receptor aggregation was induced by treating cells with neutral antiintegrin antibody and antiimmunoglobulin antibodies. A semiquantitative confocal microscopy study suggested that this treatment increased between 40% and 100% the average number of integrin receptors located in a volume of approximately 0.045 microm(3) surrounding each integrin. This aggregation induced up to 2.7-fold increase of the average number of bonds. Flow cytometric analysis of fluorescent ligand binding showed that THP-1 cells displayed low-affinity beta-1 integrins with a dissociation constant in the micromolar range. It is concluded that the initial step of cell adhesion was mediated by multiple incomplete bonds rather than a single equilibrium-state ligand receptor association. This interpretation accounts for the functional importance of integrin clustering.

Direct quantification of the modulation of interaction between cell- or surface-bound LFA-1 and ICAM-1

The functional activity of leukocyte integrins is highly regulated by several mechanisms related to intrinsic molecular properties and receptor interaction with the cell membrane. Here, we present a microkinetic study of the lymphocyte function-associated antigen-1-mediated interaction between flowing Jurkat cells and surface- or cell-bound intercellular adhesion molecule-1 (ICAM-1). We conclude that adhesion is initiated by the formation of a single bond with approximately 0.3 s(-1) dissociation rate, and attachment is subsequently strengthened by the formation of additional bonds during the next 10 s; exposing cells to Mg2+ or Mn2+ resulted in up to a 16-fold increase of the binding frequency, in line with reported measurements performed on isolated molecules with surface plasmon resonance methodology; cell-bound ICAM-1 molecules were more efficient in mediating adhesion than Fc-ICAM-1, properly oriented and bound by surface-adsorbed protein A; and quantitative analysis of binding frequency suggested that adhesion efficiency was ten- to 100-fold lower than the maximum value allowed by previously determined association rates of soluble molecules. It is concluded that the presented methodology provides a simple and unique way of dissecting the initial step of cell adhesion and discriminating between affinity and avidity modulation of adhesion receptors.

Transgene expression of alpha(1,2)-fucosyltransferase-I (FUT1) in tumor cells selectively inhibits sialyl-Lewis x expression and binding to E-selectin without affecting synthesis of sialyl-Lewis a or binding to P-selectin

During inflammation, E- and P-selectins appear on activated endothelial cells to interact with leukocytes through sialyl-Lewis x and sialyl-Lewis a antigens (sLe(x/a)). These selectins can also interact with tumor cells in a sialyl-Lewis-dependent manner and for this reason, they are thought to play a key role in metastasis. Diverting the biosynthesis of sialyl-Lewis antigens toward nonadhesive structures is an attractive gene therapy for preventing the hematogenous metastatic spread of cancers. We have previously shown that transfection of alpha(1,2)-fucosyltransferase-I (FUT1) in Chinese hamster ovary (CHO) cells had a slight effect on the overall sialylation while the synthesis of sLE(x) was dramatically prevented. We herein delivered the gene of FUT1 by a human immunodeficiency virus-derived lentiviral vector to three human cancer cell lines including pancreatic (BxPC3), hepatic (HepG2), and colonic (HT-29) cancer cells. We found that on FUT1 transduction, all cells exhibited a dramatic decrease in sLe(x) synthesis with a concomitant increase in Le(y) and Le(b) expression, without any detectable effect on the level of cell surface sLe(a) antigens. In parallel, FUT1-transduced HT-29 and HepG2 cells, but not BxPC3 cells, failed to interact with E-selectin as assessed by E-selectin-binding assay or dynamic adhesion to activated endothelial cells. We show also that transduced FUT1 efficiently fucosylates the P-selectin ligand PSGL-1 without altering P-selectin binding. These results have important implications for understanding cell-specific reactions underlying the synthesis of selectin ligands in cancer cells and may provide a basis for the development of anti-metastatic gene therapy.

Expression, regulation, and function of alpha V integrins in hepatocellular carcinoma: an in vivo and in vitro study

The expression of alpha V integrins by neoplastic cells contributes to the promotion of local invasion and metastasis. The most characteristic extracellular ligands of alpha V integrins are vitronectin and fibronectin. Hepatocytes are the main source of vitronectin, and the capacity to synthesize and secrete vitronectin is usually retained in hepatocellular carcinoma. The aim of this study was to explore the expression, regulation, and functional role of alpha V integrins in hepatocellular carcinoma. We first analyzed the expression of alpha V integrins and their ligands fibronectin and vitronectin in 80 cases of hepatocellular carcinoma. alpha V integrin chain was detected in 44 cases and vitronectin in 50. Twenty-four of the 44 alpha V-positive tumors contained large amounts of vitronectin. These cases presented more frequently with adverse histoprognostic factors, including infiltrative growth pattern (62.5%), lack of capsule (71%), presence of capsular invasion (57%), and satellite nodules (50%). We then used HepG2 and Hep3B cell lines as in vitro models to study alpha V integrin regulation and function. HepG2 and Hep3B cells expressed alpha V integrin chain and used alpha V beta 1 and alpha V beta 5 for adhesion and migration on vitronectin. Tumor necrosis factor (TNF) alpha and transforming growth factor (TGF) beta significantly increased the expression levels of alpha V integrins and stimulated the adhesion and migration of both HepG2 and Hep3B cell lines on vitronectin. The effects of growth factors on cell adhesion and migration were reproduced by incubation with conditioned medium from rat liver myofibroblasts. In conclusion, our results support the existence of an alpha V integrin/vitronectin connection in hepatocellular carcinoma and suggest that this connection may be an adverse prognostic factor.

Identification of self through two-dimensional chemistry and synapses

Cells in the immune and nervous systems communicate through informational synapses. The two-dimensional chemistry underlying the process of synapse formation is beginning to be explored using fluorescence imaging and mechanical techniques. Early analysis of two-dimensional kinetic rates (k(on) and k(off)) and equilibrium constants (K(d)) provides a number of biological insights. First, there are two regimes for adhesion-one disordered with slow k(on) and the other self-ordered with 10(4)-fold faster k(on). Despite huge variation in two-dimensional k(on), the two-dimensional k(off) is like k(off) in solution, and two-dimensional k(off) is more closely related to intrinsic properties of the interaction than the two-dimensional k(on). Thus difference in k(off) can be used to set signaling thresholds. Early signaling complexes are compartmentalized to generate synergistic signaling domains. Immune antigen receptor components have a role in neural synapse editing. This suggests significant parallels in informational synapse formation based on common two-dimensional chemistry and signaling strategies.

Role of the cytoskeleton in rapid activation of CD11b/CD18 function and its subsequent downregulation in neutrophils

When rolling adherent neutrophils are stimulated, they rapidly immobilize through activation of integrin CD11b/CD18, and then modulate attachment through this integrin to allow migration. We investigated links between cytoskeletal rearrangement and changes in function of integrin CD11b/CD18 in neutrophils stimulated with formyl peptide (fMLP). Neutrophils treated with the actin-polymerizing agent jasplakinolide became rolling adherent on monolayers of activated platelets, but could not use CD11b/CD18 to become immobilised when fMLP was perfused over them. If treated with jasplakinolide after fMLP, the cells stopped migrating but could not detach when fMLP was removed. Jasplakinolide did not inhibit changes in intracellular Ca(2+) seen after fMLP treatment, or inhibit neutrophil immobilisation induced by externally added Mn(2+). Thus cytoskeletal rearrangement was directly implicated in upregulation and, later, downregulation of CD11b/CD18 binding. Inhibition of RhoA with C3-transferase caused a dose-dependent reduction of initial rolling adhesion of neutrophils, and reduced the rate of migration after stimulation; however, neither the conversion of rolling to stationary adhesion, nor the ability of neutrophils to detach on removal of the stimulus, were inhibited. Thus, Rho may regulate actin polymerisation and motility in neutrophils, but did not appear to control integrin-mediated adhesion itself. Integrin binding may be promoted by disruption of links to the cytoskeleton, effected through depolymerisation of actin or cleavage of linking protein talin by calpain. Disruption of actin filaments with cytochalasin D did not, however, cause integrin-mediated immobilisation of rolling neutrophils. Although the calpain inhibitor calpeptin did inhibit the adhesion response to fMLP, this was only at doses where actin polymerisation was also ablated. We suggest that the cytoskeleton actively regulates binding conformation of CD11b/CD18 as well as its mobility in the membrane.