Molecular mechanisms determining the strength of receptor-mediated intermembrane adhesion

The magnitude of LFA-1/ICAM-1 forces fine-tune TCR-triggered T cell activation

T cells defend against cancer and viral infections by rapidly scanning the surface of target cells seeking specific peptide antigens. This key process in adaptive immunity is sparked upon T cell receptor (TCR) binding of antigens within cell-cell junctions stabilized by integrin (LFA-1)/intercellular adhesion molecule-1 (ICAM-1) complexes. A long-standing question in this area is whether the forces transmitted through the LFA-1/ICAM-1 complex tune T cell signaling. Here, we use spectrally encoded DNA tension probes to reveal the first maps of LFA-1 and TCR forces generated by the T cell cytoskeleton upon antigen recognition. DNA probes that control the magnitude of LFA-1 force show that F>12 pN potentiates antigen-dependent T cell activation by enhancing T cell-substrate engagement. LFA-1/ICAM-1 mechanical events with F>12 pN also enhance the discriminatory power of the TCR when presented with near cognate antigens. Overall, our results show that T cells integrate multiple channels of mechanical information through different ligand-receptor pairs to tune function.

Mechanics of antigen extraction in the B cell synapse

B cell encounter with antigen displayed on antigen-presenting cells leads to B cell immune synapse formation, internalisation of the antigen, and stimulation of antibody responses. The sensitivity with which B cells detect antigen, and the quality and quantity of antigen that B cells acquire, depend upon mechanical properties of the immune synapse including interfacial tension, the strength of intermolecular bonds, and the compliance of the molecules and membranes that participate in antigen presentation. In this review, we discuss our current understanding of how these various physical parameters influence B cell antigen extraction in the immune synapse and how a more comprehensive understanding of B cell mechanics may promote the development of new approaches to stimulate the production of desired antibodies.

Supported lipid bilayer platforms to probe cell mechanobiology

Mammalian and bacterial cells sense and exert mechanical forces through the process of mechanotransduction, which interconverts biochemical and physical signals. This is especially important in contact-dependent signaling, where ligand-receptor binding occurs at cell-cell or cell-ECM junctions. By virtue of occurring within these specialized junctions, receptors engaged in contact-dependent signaling undergo oligomerization and coupling with the cytoskeleton as part of their signaling mechanisms. While our ability to measure and map biochemical signaling within cell junctions has advanced over the past decades, physical cues remain difficult to map in space and time. Recently, supported lipid bilayer (SLB) technologies have emerged as a flexible platform to mimic and perturb cell-cell and cell-ECM junctions, allowing one to study membrane receptor mechanotransduction. Changing the lipid composition and underlying substrate tunes bilayer fluidity, and lipid and ligand micro- and nano-patterning spatially control positioning and clustering of receptors. Patterning metal gridlines within SLBs confines lipid mobility and introduces mechanical resistance. Here we review fundamental SLB mechanics and how SLBs can be engineered as tunable cell substrates for mechanotransduction studies. Finally, we highlight the impact of this work in understanding the biophysical mechanisms of cell adhesion. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

Targeted Tuning of Interactive Forces by Engineering of Molecular Bonds in Series and Parallel Using Peptide-Based Adhesives

Polymer-mediated adhesion plays a major role for both technical glues and biological processes like self-assembly or biorecognition. In contrast to engineering systems, adhesive strength in biological systems is precisely tuned via well-adjusted arrangement of individual bonds. How adhesion may be engineered by arrangement of individual bonds is however not yet well-understood. Here we show how the number of bonds in series and parallel can significantly influence adhesion forces using specifically designed surface-bridging peptides. We directly measure how adhesion forces between -COOH and -NH2 functionalized surfaces across aqueous media vary as a function of the number of bonds in parallel. We also introduce surface bridging peptide sequences that are similarly end-functionalized with amines and carboxylic acid. Compared to single molecular junctions, adhesive strength mediated by these surface bridging peptides decreases by a factor of 2 for adhesive junctions that consist of two acid/base bonds in series. Furthermore, adhesive strength varies with the density of bonds in parallel. For dense systems, we observe that the formation of a bridging peptide monolayer is sterically hindered and therefore adhesion is further reduced significantly by 20%. Our results unravel how the arrangement of individual bonds in an adhesive junction allows for a wide tuning of adhesive strength on the basis of utilizing just one single specific bond. As such, for peptide adhesives it is essential to consider bonds in parallel in a wide range of applications where both high adhesion and triggered release of adhesive bonds is essential.

Scaling from single molecule to macroscopic adhesion at polymer/metal interfaces

Understanding the evolution of macroscopic adhesion based on fundamental molecular interactions is crucial to designing strong and smart polymer/metal interfaces that play an important role in many industrial and biomedical applications. Here we show how macroscopic adhesion can be predicted on the basis of single molecular interactions. In particular, we carry out dynamic single molecule-force spectroscopy (SM-AFM) in the framework of Bell-Evans' theory to gain information about the energy barrier between the bound and unbound states of an amine/gold junction. Furthermore, we use Jarzynski's equality to obtain the equilibrium ground-state energy difference of the amine/gold bond from these nonequilibrium force measurements. In addition, we perform surface forces apparatus (SFA) experiments to measure macroscopic adhesion forces at contacts where approximately 10(7) amine/gold bonds are formed simultaneously. The SFA approach provides an amine/gold interaction energy (normalized by the number of interacting molecules) of (36 ± 1)k(B)T, which is in excellent agreement with the interaction free energy of (35 ± 3)k(B)T calculated using Jarzynski's equality and single-molecule AFM experiments. Our results validate Jarzynski's equality for the field of polymer/metal interactions by measuring both sides of the equation. Furthermore, the comparison of SFA and AFM shows how macroscopic interaction energies can be predicted on the basis of single molecular interactions, providing a new strategy to potentially predict adhesive properties of novel glues or coatings as well as bio- and wet adhesion.

Deciphering the scaling of single-molecule interactions using Jarzynski's equality

Unravelling the complexity of the macroscopic world relies on understanding the scaling of single-molecule interactions towards integral macroscopic interactions. Here, we demonstrate the scaling of single acid-amine interactions through a synergistic experimental approach combining macroscopic surface forces apparatus experiments and single-molecule force spectroscopy. This experimental framework is ideal for testing the well-renowned Jarzynski's equality, which relates work performed under non-equilibrium conditions with equilibrium free energy. Macroscopic equilibrium measurements scale linearly with the number density of interfacial bonds, providing acid-amine interaction energies of 10.9 ± 0.2 kT. Irrespective of how far from equilibrium single-molecule experiments are performed, the Jarzynski's free energy converges to 11 ± 1 kT. Our results validate the applicability of Jarzynski's equality to unravel the scaling of non-equilibrium single-molecule experiments to scenarios where large numbers of molecules interacts simultaneously in equilibrium. The developed scaling strategy predicts large-scale properties such as adhesion or cell-cell interactions on the basis of single-molecule measurements.

Exchangeable colloidal AFM probes for the quantification of irreversible and long-term interactions

An original method is presented to study single-colloid interaction with a substrate in liquid environment. Colloids, either in solution or adsorbed on a surface, are fixed by suction against the aperture of a microchanneled atomic force microscopy cantilever. Their adhesion to the substrate is measured, followed by their release via a short overpressure surge. Such colloid exchange procedure allows for 1), the quick variation of differently functionalized colloids within the same experiment; 2), the investigation of long-term interactions by leaving the colloids on a surface for a defined time before detaching them; and 3), the inspection of irreversible interactions. After validation of the method by reproducing literature results obtained with traditional colloidal atomic force microscopy, the serial use of colloids with different surface functionalization was shown on a micropatterned surface. Finally, concanavalin A-coated colloids were allowed to adsorb on human embryonic kidney cells and then detached one by one. The adhesion between cells and colloids was up to 60 nN, whereas individual cells adhered with 20 nN to the glass substrate. A cellular elastic modulus of 0.8 kPa was determined using the attached colloid as indenter.

Biophysics of cadherin adhesion

Since the identification of cadherins and the publication of the first crystal structures, the mechanism of cadherin adhesion, and the underlying structural basis have been studied with a number of different experimental techniques, different classical cadherin subtypes, and cadherin fragments. Earlier studies based on biophysical measurements and structure determinations resulted in seemingly contradictory findings regarding cadherin adhesion. However, recent experimental data increasingly reveal parallels between structures, solution binding data, and adhesion-based biophysical measurements that are beginning to both reconcile apparent differences and generate a more comprehensive model of cadherin-mediated cell adhesion. This chapter summarizes the functional, structural, and biophysical findings relevant to cadherin junction assembly and adhesion. We emphasize emerging parallels between findings obtained with different experimental approaches. Although none of the current models accounts for all of the available experimental and structural data, this chapter discusses possible origins of apparent discrepancies, highlights remaining gaps in current knowledge, and proposes challenges for further study.

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

Geometry and adhesion of extracellular domains of DC-SIGNR neck length variants analyzed by force-distance measurements

Force–distance measurements have been used to examine differences in the interaction of the dendritic cell glycan-binding receptor DC-SIGN and the closely related endothelial cell receptor DC-SIGNR (L-SIGN) with membranes bearing glycan ligands. The results demonstrate that upon binding to membrane-anchored ligand, DC-SIGNR undergoes a conformational change similar to that previously observed for DC-SIGN. The results also validate a model for the extracellular domain of DC-SIGNR derived from crystallographic studies. Force measurements were performed with DC-SIGNR variants that differ in the length of the neck that result from genetic polymorphisms, which encode different numbers of the 23-amino acid repeat sequences that constitute the neck. The findings are consistent with an elongated, relatively rigid structure of the neck repeat observed in crystals. In addition, differences in the lengths of DC-SIGN and DC-SIGNR extracellular domains with equivalent numbers of neck repeats support a model in which the different dispositions of the carbohydrate-recognition domains in DC-SIGN and DC-SIGNR result from variations in the sequences of the necks.

Strength of adhesion clusters under shared linear loading

A cluster of N ligand-receptor pairs between two parallel surfaces under an applied force F=Γt with a constant loading rate Γ is considered. Our theoretical and numerical studies show that there is a characteristic force f(c) and a characteristic loading rate Γ(c). At Γ<Γ(c), the mean rupture force F(r) of the cluster is close to but lower than Nf(c). In this regime, cluster dissociation can be modeled as a one-dimensional barrier crossing process and F(r) scales like Nf(c)-F(r)~N(1/3)[ln(Γ(c)/Γ)](2/3). At Γ=Γ(c), the cluster dissociation occurs at F(r)=Nf(c). At Γ>Γ(c), F(r) for clusters with large N is well predicted by the rate equation because the fluctuations of the number of closed bonds are unimportant. Our study shows that f(c) and Γ(c) are important emergent properties for understanding the mechanical response of adhesion clusters.

Allosteric cross talk between cadherin extracellular domains

Atomic force microscopy and surface force apparatus measurements determined the functional impact of the cadherin point mutation W2A and domain deletion mutations on C-cadherin binding signatures. Direct comparison of results obtained using both experimental approaches demonstrates that C-cadherin ectodomains form multiple independent bonds that require different structural regions. The results presented reveal significant interdomain cross talk. They further demonstrate that the mutation W2A not only abolishes adhesion between N-terminal domains, but allosterically modulates other binding states that require functional domains distal to the N-terminal binding site. Such allosteric effects may play a prominent role in modulating adhesion by Type I classic cadherins, cadherin oligomerization at junctional contacts, and propagation of binding information to the cytoplasmic region.

Monitoring ligand-receptor interactions by photonic force microscopy

We introduce a method for the acquisition of single molecule force measurements of ligand-receptor interactions using the photonic force microscope (PFM). Biotin-functionalized beads, manipulated with an optical trap, and a streptavidin-functionalized coverslip were used to measure the effect of different pulling forces on the lifetime of individual streptavidin-biotin complexes. By optimizing the design of the optical trap and selection of the appropriate bead size, pulling forces in excess of 50 pN were achieved. Based on the amplitude of three-dimensional (3D) thermal position fluctuations of the attached bead, we were able to select for a bead-coverslip interaction that was mediated by a single streptavidin-biotin complex. Moreover, the developed experimental system was greatly accelerated by automation of data acquisition and analysis. In force-dependent kinetic measurements carried out between streptavidin and biotin, we observed that the streptavidin-biotin complex exhibited properties of a catch bond, with the lifetime increasing tenfold when the pulling force increased from 10 to 20 pN. We also show that silica beads were more appropriate than polystyrene beads for the force measurements, as tethers, longer than 200 nm, could be extracted from polystyrene beads.

Models of dynamic extraction of lipid tethers from cell membranes

When a ligand that is bound to an integral membrane receptor is pulled, the membrane and the underlying cytoskeleton can deform before either the membrane delaminates from the cytoskeleton or the ligand detaches from the receptor. If the membrane delaminates from the cytoskeleton, it may be further extruded and form a membrane tether. We develop a phenomenological model for this process by assuming that deformations obey Hooke's law up to a critical force at which the cell membrane locally detaches from the cytoskeleton and a membrane tether forms. We compute the probability of tether formation and show that tethers can be extruded only within an intermediate range of force loading rates and pulling velocities. The mean tether length that arises at the moment of ligand detachment is computed as are the force loading rates and pulling velocities that yield the longest tethers.

Design rules for biomolecular adhesion: lessons from force measurements

Cell adhesion to matrix, other cells, or pathogens plays a pivotal role in many processes in biomolecular engineering. Early macroscopic methods of quantifying adhesion led to the development of quantitative models of cell adhesion and migration. The more recent use of sensitive probes to quantify the forces that alter or manipulate adhesion proteins has revealed much greater functional diversity than was apparent from population average measurements of cell adhesion. This review highlights theoretical and experimental methods that identified force-dependent molecular properties that are central to the biological activity of adhesion proteins. Experimental and theoretical methods emphasized in this review include the surface force apparatus, atomic force microscopy, and vesicle-based probes. Specific examples given illustrate how these tools have revealed unique properties of adhesion proteins and their structural origins.

Ligand-receptor interactions between surfaces: the role of binary polymer spacers

The interactions between a receptor-modified planar surface and a surface grafted with a bimodal polymer layer, where one of the polymer species is ligand functionalized, are studied using a molecular theory. The effects of changing the binding energy of the ligand-receptor pair, the polymer surface coverage, the composition, and molecular weight of both the unfunctionalized and ligand functionalized polymers on the interactions between the surfaces are investigated. Our findings show that bridging exists between the surfaces including when the molecular weight of the ligand-bearing polymer is smaller than that of the unfunctionalized polymer, even though the ligand is initially buried within the polymer layer. The distance at which the surfaces bind depends only on the molecular weight of the ligand-modified polymer, while the strength of the interaction at a given surface separation can be tuned by changing the molecular weight of the polymers, the total polymer surface coverage, and the fraction of ligated polymers. The composition of the bimodal layer alters the structure of the polymer layer, thereby influencing the strength of the steric repulsions between the surfaces. Our theoretical results show good agreement with experimental data. The present theoretical study can be used as guidelines for the design of surfaces with tailored abilities for tunning the binding strength and surface-ligand separation distances for polymer-grafted surfaces bearing specific targeting ligands.

Beyond structure: mechanism and dynamics of intercellular adhesion

This review summarizes findings from multiple complementary quantitative investigations of adhesion by classical cadherins. The systems investigated range from single molecules to cells, and the approaches used quantify the kinetics, energetics and mechanical strengths of cadherin bonds. The cumulative results demonstrate that cadherins adhere via a multistage binding mechanism that involves multiple extracellular domains. In kinetic measurements of cell adhesion, cell pairs first form a low-probability-binding state with fast kinetics. This is followed by a lag and a slow transition to a second, high-probability, binding state. This two-stage process is independent of the cytoplasmic domain. Studies with domain-deletion mutants demonstrate that the N-terminal domains are required for the first, fast, weak binding. However, the full-ectodomain and EC3 (extracellular repeat 3), in particular, are required to form the second, high-probability, binding state, which is characterized by slow dissociation kinetics and much stronger adhesive bonds. Together, these different studies reveal a more complex multistage binding mechanism than was predicted by structural models.

Adhesion from tethered ligand-receptor bonds with microsecond lifetimes

According to classical thermodynamics, biological ligand-receptor bonds should have a median lifetime of about 2 ms, and nearly half should have lifetimes of nanoseconds to microseconds. As a result, it is clear that many "weak" bonds are indispensable for cellular adhesion, signaling, and other critical events. However, the forces required to rupture such weak bonds and the adhesion they provide between surfaces are largely unknown because of their propensity to dissociate rapidly from a measuring probe. To measure such weak bond forces quantitatively, we followed nature's example of adhering surfaces with many weak ligand-receptor bonds. Analogously to how multiplicity promotes stronger adhesion between cellular membranes, multiple bonds created significant adhesion between model cellular surfaces. Specifically, we used an automated surface forces apparatus to measure the adhesion between complementary surfaces bearing dense populations of streptavidin receptors and flexible PEG tethers that each anchored a weakly binding ligand (HABA, or 2-(4-hydroxyphenylazo) benzoic acid). We show that this short-lived bond (<100 mus) leads to low forces of dissociation and only a small fraction being simultaneously bound. These results are significant because the HABA-streptavidin bond energy ( approximately 10.5kBT) is similar to the average found in nature (14.7kBT). The measurements exemplify how a single ligand-receptor bond may fall apart and rejoin many times before completing a cellular function yet can still exhibit strength in numbers.

Dynamics of membrane adhesion: the role of polyethylene glycol spacers, ligand-receptor bond strength, and rupture pathway

Biological adhesion typically occurs through discrete cross bridges between complementary molecules on adjacent membranes. Here we report quantitative measurements of the binding distance between a lipid membrane functionalized with ligands on flexible polymer tether chains and a second membrane bearing complementary receptors using the surface force apparatus technique. The binding distance is shown to increase as a function of polymer tether length. Upon separation, adhesive failure occurs not at the strong ligand-receptor bond but primarily through the mechanical pullout of cross-bridging polymer tethers from the membrane. We summarize these measurements of complementary membrane adhesion dynamics using an energy-state diagram that encompasses the energetics of the polymer tether, ligand-receptor bond strength, and number of cross bridges formed.

Synthesis of a Reversible Streptavidin Binder for Biomimetic Assemblies

The biotin/streptavidin ligand/receptor pair is used extensively in biotechnology. However, less is known about HABA (2-(4-hydroxyphenylazo)benzoic acid), which binds to streptavidin with a bond energy and dissociation constant that more closely mimics antibody/antigen interactions. In this work we demonstrate some of HABA’s useful properties that may make it a good substitute for biotin in a broad range of biochemical research. Specifically, we investigate its ease of conjugation to an anchoring pegylated lipid, characterization with MALDI, NMR, and visible-wavelength spectroscopies, and incorporation into lipid vesicles.

Bimodal polymer mushrooms: compressive forces and specificity toward receptor surfaces

End-grafted poly(ethylene glycol) (or PEG) polymer chains are used to extend the in vivo circulation time of targeted liposomes and nanoparticles; however, the most efficacious structure for also imparting high target specificity remains unknown. Using the surface force apparatus, we have measured the specific and nonspecific forces between bimodal mixtures of grafted polymer mushrooms and model receptor surfaces. Specifically, supported lipid membranes anchoring 2000 or 5000 Da PEG with a controlled fraction of PEG(2000) bearing biotin ligands were compressed against opposing streptavidin surfaces. The presence of the longer 5000 Da chain increased the steric repulsion of the bimodal mushroom layer and thus decreased the net adhesive force when shorter chains were ligated. However, the 5000 Da chain did not detectably alter the distance where ligand-receptor binding occurs and adhesion begins. This latter result is in good agreement with theoretical predictions based on summing the repulsive steric and attractive bridging forces. Further, all ligated structures adhered to receptors under both static and dynamic fluid flow conditions. The dynamic movement of the flexible PEG tethers permitted ligand-receptor bonds to form far beyond the equilibrium edge of the bimodal mushroom layer. This work demonstrates that liposome targeting should be enhanced by grafting ligands to liposomes with a tether that has a contour length longer than the equilibrium height of the bimodal mushroom layer.

The role of flexible tethers in multiple ligand-receptor bond formation between curved surfaces

Ligands mounted to surfaces via extensible tethers are present in nature and represent a growing class of molecules used to engineer adhesion in drug targeting, biosensing, self-assembling nanostructures, and in other biophysical research. Using a continuum approach with geometric and thermodynamic arguments, we derive a number of analytical expressions that relate key properties of single-tethered ligand-receptor interactions to multiple bond formation between curved surfaces. The theoretical predictions are in good agreement with measurements made with the surface forces apparatus. We establish that, when ligated, many tethers commonly used in biophysical research exhibit a discrete binding range that can be accurately measured with force spectroscopy. The distribution of bound ligated tethers is independent of the surfaces' interaction radius, R. The bridging force scales linearly with R, the tether's effective spring constant and grafting density, and with the ligand-receptor bond energy when the surfaces are in direct contact. These results are contrasted to bridging forces that evolve between plane-parallel geometries. Last, we show how our simple analytical reductions can be used to predict adhesive forces for STEALTH liposomes and other targeted and self-assembled nanoparticles.

Probing the mechanical folding kinetics of TAR RNA by hopping, force-jump, and force-ramp methods

Mechanical unfolding and refolding of single RNA molecules have previously been observed in optical traps as sudden changes in molecular extension. Two methods have been traditionally used: "force-ramp", with the applied force continuously changing, and "hopping". In hopping experiments the force is held constant and the molecule jumps spontaneously between two different states. Unfolding/refolding rates are measured directly, but only over a very narrow range of forces. We have now developed a force-jump method to measure the unfolding and refolding rates independently over a wider range of forces. In this method, the applied force is rapidly stepped to a new value and either the unfolding or refolding event is monitored through changes in the molecular extension. The force-jump technique is compared to the force-ramp and hopping methods by using a 52-nucleotide RNA hairpin with a three-nucleotide bulge, i.e., the transactivation response region RNA from the human immunodeficiency virus. We find the unfolding kinetics and Gibbs free energies obtained from all three methods to be in good agreement. The transactivation response region RNA hairpin unfolds in an all-or-none two-state reaction at any loading rate with the force-ramp method. The unfolding reaction is reversible at small loading rates, but shows hysteresis at higher loading rates. Although the RNA unfolds and refolds without detectable intermediates in constant-force conditions (hopping and force-jump), it shows partially folded intermediates in force-ramp experiments at higher unloading rates. Thus, we find that folding of RNA hairpins can be more complex than a simple single-step reaction, and that application of several methods can improve understanding of reaction mechanisms.

Measurement of the anchorage force between GPI-anchored alkaline phosphatase and supported membranes by AFM force spectroscopy

Mammalian alkaline phosphatases (AP) are glycosylphosphatidylinositol (GPI) anchored proteins that are localized on the outer layer of the plasma membrane. The GPI anchors are covalently attached to the C-termini of proteins and consist of a glycan chain bonded to phosphatidylinositol with two acyl chains anchored into the membrane bilayer. Force spectroscopy, based on atomic force microscope (AFM) technology, was used to determine the adhesion of alkaline phosphatase in the absence and presence of anchors. The GPI anchors increase markedly the adhesion frequency (i.e., the protein affinity for the membrane). An adhesion force of 350 +/- 200 pN is measured between GPI-anchored AP (AP(GPI)) and supported phospholipid bilayers of dipalmitoylphosphatidylcholine (DPPC) presenting structural defects (holes). In the absence of defects, the adhesion force (103 +/- 17 pN) and the adhesion frequency are reduced. These results indicate that AP(GPI) poorly spontaneously insert into membranes in vivo and open new perspectives for the characterization of the interactions between GPI proteins and membranes.

How T cells 'see' antigen

T lymphocytes bearing alphabeta T cell receptors are pivotal in the immune response of most vertebrates. For example, helper T cells orchestrate antibody production by B cells as well as stimulating other cells, whereas cytotoxic T cells kill virally infected or abnormal cells. Regulatory T cells act to dampen responsiveness, and natural killer-like T cells monitor lipid metabolism. The specificity of these cells is governed by the alphabeta T cell receptors - antibody-like heterodimeric receptors that detect antigenic fragments (peptides) or lipids bound to histocompatibility molecules. Intriguing clues as to how these peculiar ligands are recognized have gradually emerged over the years and tell a remarkable story of biochemical and cellular novelty. Here we summarize some of the more recent work on alphabeta T cell receptor recognition and discuss the implications for activation.

Effect of streptavidin RGD mutant on the adhesion of endothelial cells

Adhesion of endothelial cells (EC) to surfaces can be enhanced by supplementing the integrin-mediated adhesion with high-affinity streptavidin (SA) that links a biotinylated EC to a biotinylated surface. Biotin pullout from the EC membrane limits the effectiveness of this treatment, leading to a predominance of EC detachment by cohesive failure. In this study we investigated whether a RGD-SA mutant that links SA to EC integrin receptors, and eliminates EC biotinylation, improves EC adhesion. Suspended EC were incubated with the RGD-SA mutant prior to cell seeding, primarily via attachment to the RGD binding site on alpha(v)beta(3) integrin. RGD-SA-incubated EC were subsequently seeded onto a surface preadsorbed with a mixture of fibronectin (Fn) and biotinylated bovine serum albumin (b-BSA). Results showed EC adhesion supplemented with the RGD-SA-biotin system significantly increased cell retention under flow, critical shear stresses for detachment, focal contact area, and force per bond relative to SA used with biotinylated EC. These increases were accompanied by significant reductions in membrane fragments left behind following EC detachment, which suggested cohesive failure via cell membrane rupture was significantly reduced, and enhanced phosphorylation of focal adhesion kinase, which suggested activation and clustering of integrin receptors. Together, these results show that the integrin-independent augmentation of EC adhesion using SA-biotin can be further improved through use of an RGD-SA mutant.

Mechanism of homophilic adhesion by the neural cell adhesion molecule: use of multiple domains and flexibility

The extracellular regions of adhesion proteins of the Ig superfamily comprise multiple, tandemly arranged domains. We used directforce measurements to investigate how this modular architecture contributes to the adhesive interactions of the neural cell adhesion molecule (NCAM), a representative of this protein class. The extracellular region of NCAM comprises five immunoglobulin and two fibronectin domains. Previous investigations generated different models for the mechanism of homophilic adhesion that each use different domains. We use force measurements to demonstrate that NCAM binds in two spatially distinct configurations. Igdomain deletion mutants identified the domains responsible for each of the adhesive bonds. The measurements also confirmed the existence of a flexible hinge that alters the orientation of the adhesive complexes and the intermembrane distance. These results suggest that a combination of multiple bound states and internal molecular flexibility allows for sequentially synergistic bond formation and the ability to accommodate differences in intercellular space.

Functional analysis of the structural basis of homophilic cadherin adhesion

The structures of many cell surface adhesion proteins comprise multiple tandem repeats of structurally similar domains. In many cases, the functional significance of this architecture is unknown, and there are several cases in which evidence for individual domain involvement in adhesion has been contradictory. In particular, the extracellular region of the adhesion glycoprotein cadherin consists of five tandemly arranged domains. One proposed mechanism postulated that adhesion involves only trans interactions between the outermost domains. However, subsequent investigations have generated several competing models. Here we describe direct measurements of the distance-dependent interaction potentials between cadherin mutants lacking different domains. By quantifying both the absolute distances at which opposed cadherin fragments bind and the quantized changes in the interaction potentials that result from deletions of individual domains, we demonstrate that two domains participate in homophilic cadherin binding. This finding contrasts with the current view that cadherins bind via a single, unique site on the protein surface. The potentials that result from interactions involving multiple domains generate a novel, modular binding mechanism in which opposed cadherin ectodomains can adhere in any of three antiparallel alignments.

Direct measurements of heterotypic adhesion between the cell surface proteins CD2 and CD48

Direct force measurements were used to investigate the molecular mechanism of heterophilic adhesion between the murine T-cell adhesion glycoprotein CD2 and its ligand CD48. From the distance dependence of the protein-protein interaction potential, we demonstrate directly that the full-length extracellular domains adhere in a head-to-head orientation. The absence of long-range electrostatic protein-protein attraction further indicates that the salt bridges between the binding surfaces only influence the interaction at short range. Despite the loss of a stabilizing disulfide bond in domain 1 (D1) of CD2, adhesive failure occurs abruptly with no evidence of partial protein unfolding during detachment. Finally, these measurements between extended membrane surfaces directly confirm that the low-affinity CD2-CD48 bond generates weak adhesion and that lateral receptor mobility is required for the development of appreciable adhesion. This is the first direct measurement of the range and magnitude of the forces governing heterotypic adhesion mediated by cell surface proteins. These results both verified the head-to-head CD2-CD48 docking alignment and demonstrated the ability to elucidate the structure-function relationships of adhesion proteins from the measured distance dependence of their interaction potentials.

Measurement of membrane binding between recoverin, a calcium-myristoyl switch protein, and lipid bilayers by AFM-based force spectroscopy

Myristoyl switch is a feature of several peripheral membrane proteins involved in signal transduction pathways. This unique molecular property is best illustrated by the "Ca(2+)-myristoyl switch" of recoverin, which is a Ca(2+)-binding protein present in retinal rod cells of vertebrates. In this transduction pathway, the Ca(2+)-myristoyl switch acts as a calcium sensor involved in cell recovery from photoactivation. Ca(2+) binding by recoverin induces the extrusion of its myristoyl group to the solvent, which leads to its translocation from cytosol to rod disk membranes. Force spectroscopy, based on atomic force microscope (AFM) technology, was used to determine the extent of membrane binding of recoverin in the absence and presence of calcium, and to quantify this force of binding. An adhesion force of 48 +/- 5 pN was measured between recoverin and supported phospholipid bilayers in the presence of Ca(2+). However, no binding was observed in the absence of Ca(2+). Experiments with nonmyristoylated recoverin confirmed these observations. Our results are consistent with previously measured extraction forces of lipids from membranes.

Molecular properties in cell adhesion: a physical and engineering perspective

The past several years have seen accelerating growth in research directed towards the understanding and control of cell adhesion processes, from a spectrum of disciplinary approaches including molecular cell biology, biochemistry, biophysics and bioengineering. Consequently, our understanding of the mechanisms involved in cell adhesion has increased substantially. Corresponding quantitative analysis and modeling of the key molecular properties governing their action in regulating dynamic cell attachment and detachment events is crucial for advancing conceptual insight along with technological applications.

Direct measurements of multiple adhesive alignments and unbinding trajectories between cadherin extracellular domains

Direct measurements of the interactions between antiparallel, oriented monolayers of the complete extracellular region of C-cadherin demonstrate that, rather than binding in a single unique orientation, the cadherins adhere in three distinct alignments. The strongest adhesion is observed when the opposing extracellular fragments are completely interdigitated. A second adhesive alignment forms when the interdigitated proteins separate by 70 +/- 10 A. A third complex forms at a bilayer separation commensurate with the approximate overlap of cadherin extracellular domains 1 and 2 (CEC1-2). The locations of the energy minima are independent of both the surface density of bound cadherin and the stiffness of the force transducer. Using surface element integration, we show that two flat surfaces that interact through an oscillatory potential will exhibit discrete minima at the same locations in the force profile measured between hemicylinders covered with identical materials. The measured interaction profiles, therefore, reflect the relative separations at which the antiparallel proteins adhere, and are unaffected by the curvature of the underlying substrate. The successive formation and rupture of multiple protein contacts during detachment can explain the observed sluggish unbinding of cadherin monolayers. Velocity-distance profiles, obtained by quantitative video analysis of the unbinding trajectory, exhibit three velocity regimes, the transitions between which coincide with the positions of the adhesive minima. These findings suggest that cadherins undergo multiple stage unbinding, which may function to impede adhesive failure under force.

Measuring the forces that control protein interactions

Although the force fields and interaction energies that control protein behavior can be inferred indirectly from equilibrium and kinetic measurements, recent developments have made it possible to quantify directly (a) the ranges, magnitudes, and time dependence of the interaction energies and forces between biological materials; (b) the mechanical properties of isolated proteins; and (c) the strength of single receptor-ligand bonds. This review describes recent results obtained by using the atomic force microscope, optical tweezers, the surface force apparatus, and micropipette aspiration to quantify short-range protein-ligand interactions and the long-range, nonspecific forces that together control protein behavior. The examples presented illustrate the power of force measurements to quantify directly the force fields and energies that control protein behavior.

Mechanism of homophilic cadherin adhesion

Direct measurements of the distance-dependent forces between membrane-bound cadherins were used to test current models of homophilic cadherin interactions. The results reveal a complex binding mechanism in which the proteins adhere in multiple alignments that involve more than the amino-terminal domains.

Affinity imaging of red blood cells using an atomic force microscope

We used an atomic force microscope (AFM) to produce an image of a mixed layer of group A and O red blood cells with a contrast based only on the measured strength of a specific receptor-ligand pair. The image was obtained by measuring and plotting for each image pixel the adhesion force between the mixed RBC layer and the AFM tip functionalized with Helix pomatia lectin. The high specificity of that lectin for the N -acetylgalactosamine-terminated glycolipids present in the membrane of group A RBCs enabled us to discriminate between the two cell populations and to produce an image based on affinity contrast. The rupture force of the adhesion events leading to the image formation were quantitatively analyzed and compared to rupture forces measured with the same AFM tip on N-acetylgalactosamine tethered to agarose beads. The mean rupture force was found to be 65 pN when measured on the group A RBCs and 35 pN on the agarose beads. These results show that the adhesion, mediated by only a few receptor-ligand pairs, produces sufficient contrast for the affinity image formation.

Quantitative evaluation of cell attachment to glass, polystyrene, and fibronectin- or collagen-coated polystyrene by measurement of cell adhesive shear force and cell detachment energy

Quantitative evaluation of a material's affinity for cells is essential to understanding cell-material interaction inside a body and it is also necessary for the development of new biomaterials with superior biocompatibility. In the present study, the shear force and the total energy necessary to detach a single murine fibroblast L929 adhering to glass, polystyrene, and fibronectin- or collagen-coated polystyrene were measured directly by applying a lateral force, using a cantilever, to the cell. The projected area of the cell was also measured, and then cell adhesive shear strength and cell detachment surface energy were determined by dividing the shear force and the total energy by the area. Among these four materials, the cells on collagen-coated polystyrene have the highest cell adhesive shear strength and cell detachment surface energy (1500 Pa and 29 pJ on average, respectively), followed by the cells on fibronectin-coated polystyrene (1000 Pa and 16 pJ, respectively). The cells on glass and polystyrene had almost the same cell adhesive shear strength and cell detachment surface energy (420-670 Pa and 7-11 pJ, respectively). These observations suggest that cell adhesive shear strength and cell detachment surface energy depend on the number of the bindings between the cell and a material's surface rather than on the strength of each binding.

Force probe measurements of antibody-antigen interactions

The surface force apparatus has been used to quantify directly the forces that govern the interactions between proteins and ligands. In this work, we describe the measured interactions between the antigen fluorescein and the Fab' fragment of the monoclonal 4-4-20 anti-fluorescyl IgG antibody. Here we first describe the use of the surface force apparatus to demonstrate directly the impact of the charge composition in the region of the antibody binding site on the antibody interactions. Several approaches are described for immobilizing antigens, antibodies, and proteins in general for direct force measurements. The measured force profiles presented are accompanied by an extensive discussion of protocols used to analyze the force-distance curves and to interpret them in terms of the antibody structure. In addition to long-range electrostatic forces, we also consider short-range forces that can affect the strength of adhesion between the Fab' and immobilized fluorescein. The latter investigations demonstrate the influence of interfacial properties on the recognition of surface-bound antigens.

Direct force measurements of the streptavidin-biotin interaction

The interaction between streptavidin and its ligand, biotin, were studied by direct force measurements. The complimentary approaches of surface force apparatus (SFA) and atomic force microscopy (AFM) were used to elucidate both long-range and short-range adhesive interactions of the streptavidin biotin interaction. The high spatial resolution of the SFA provided a detailed profile of the intersurface forces of apposing surfaces functionalized with streptavidin and biotin. Measurements obtained by the SFA corresponded to long and intermediate-range forces that are important in determining ligand receptor association. AFM was used to measure the unbinding force of individual streptavidin biotin complexes. These measurements revealed the short-range interactions (i.e. hydrophobic and hydrogen bonding forces) that stabilize the intermolecular bond.

Role of endothelial cell-substrate contact area and fibronectin-receptor affinity in cell adhesion to HEMA/EMA copolymers

The objective of this study was to examine the effect of substrate hydrophobicity on cell-substrate contact area and the affinity between adsorbed fibronectin (Fn) and its receptor. Homo- and copolymer films of hydrophobic ethyl methacrylate (EMA) and hydrophilic hydroxyethyl methacrylate (HEMA) were spun-cast onto glass slides. Bovine aortic endothelial cells (BAEC) were plated for 2 h in serum-free medium onto polymers preadsorbed with Fn. Cells were fixed, labeled, and examined by total internal reflection fluorescence microscopy (TIRFM) to determine the topography of the basal surface as a function of distance from the substrate. Phase contrast microscopy was used to examine the total projected area of adherent cells. The cumulative contact area was greatest on cells attached to surfaces prepared from 0% HEMA and lowest on surfaces with the highest HEMA content. An equilibrium adhesion model used these data together with the critical force for detachment and the Fn density (Burmeister et al., J Biomed Mater Res 1996;30:13-22) to determine the affinity between Fn and its receptor and the bond strength. The affinity and force per bond decreased with increasing HEMA content. These results indicate that differences in the strength of endothelial cell adhesion to polymers are influenced by the conformation of the adsorbed adhesion proteins.

Direct molecular force measurements of multiple adhesive interactions between cadherin ectodomains

Direct-force measurements of the interactions between recombinant C-cadherin from Xenopus demonstrated that the ectodomain of cadherin exhibits multiple adhesive contacts that involve successive domains along the extracellular region of the protein. Contacts between the fully interdigitated antiparallel proteins form the strongest adhesive interaction. A second weaker minimum was measured when the interdigitated proteins were separated by a distance equal to the length of one domain of the extracellular (EC) fragment and corresponding to the antiparallel alignment of domains one through four (EC1 through EC4). The successive rupture of these interactions generates an unbinding force profile that may be optimized to impede the abrupt failure of cadherin-mediated junctions under force.

Feasibility of measuring antigen-antibody interaction forces using a scanning force microscope

The molecular affinity scanning force microscopy (MASFM) described in this study was developed in an effort to test the possibility of antigen-antibody binding measurement using force-separation distance profiles. The MASFM configuration was comprised of a spherical glass bead as an MASFM probe, to which the fluorescein antigen has been covalently attached, and a silicon dioxide-based substrate, to which the antifluorescyl IgG antibody was covalently bound. The bead was glued to the tip of a commercial SFM cantilever. Adhesion forces have been measured between two different specific antigen-antibody pairs and between nonspecific surfaces bearing only glycidoxypropylsilane immobilization chemistry. In force-separation (F-s) measurements, nonspecific forces displayed relatively few force discontinuities and mean adhesion forces lower than those found for specific antigen-antibody measurements. Force-separation profiles measured between specific antigen-antibody pairs showed many discontinuities and had higher mean forces. Positive controls revealed that the mean forces were slightly reduced by the addition of free ligand. The magnitude of mean forces did not correlate with the respective activation enthalpies of the proteins, as would be expected. At lower force values the force histograms for the specific pairs and for positive controls were indistinguishable. None of the force-separation data sets could fit a Poisson discrete-force model. This statistical analysis showed a large relative contribution from nonspecific interactions. It is concluded that the use of the large sphere as an SFM probe is counterproductive: while the large sphere does sample a larger number of specific interactions during each measurement, it also samples at the same time a large proportion of nonspecific forces. The presence of the nonspecific force contributions is likely due to the deformation of the polymerized GPS spacer layer which is thought to be delaminated from the surface upon the application of tension across the specific antigen-antibody bonds.

The cytoskeletal association of CD11/CD18 leukocyte integrins in phorbol ester-activated cells correlates with CD18 phosphorylation

Leukocyte adhesion is a regulated process, which involves CD11/CD18 leukocyte integrins. CD11/CD18 acidity may be regulated intracellularly, and the CD18 polypeptide has previously been shown to become phosphorylated on serine and threonine after phorbol ester activation of T cells. Increased adhesiveness is believed to be mediated by regulating the overall avidity of cellular contact. CD11/CD18 integrins have earlier been reported to interact with several cytoskeletal proteins. We have now studied the involvement of the CD18 phosphorylation in cytoskeletal associations. We have investigated the distribution of phosphorylated CD18 between soluble, cytoskeletal and nuclear fractions of T cell detergent lysates. A significant amount of phosphorylated CD18 polypeptides was observed to fraction along with the cytoskeleton, while the majority of the cell surface CD18 molecules remained in the soluble fraction. Putative candidates for this altered cytoskeletal binding of CD11/CD18 were shown to be talin and filamin, which were observed to bind to CD18 cytoplasmic peptides and co-precipitate with CD18. The importance of the CD18 cytoplasmic domain in the regulation of the leukocyte adhesion was further strengthened by inhibition of phorbol ester-induced T cell adhesion with a phosphorylated lipopeptide corresponding to the cytoplasmic portion of the CD18. These results indicate that the induced CD18 phosphorylation and the altered cytoskeletal binding of the phosphorylated integrin complex may contribute to the increased avidity of CD11/CD18-mediated leukocyte adhesion.

CD2 and the nature of protein interactions mediating cell-cell recognition

Rapid progress has recently been made in characterising the structures of leukocyte cell-surface molecules. Detailed analyses of the structure and interactions of CD2 were the first involving a molecule that has not been directly linked to antigen recognition in the manner of antigen receptors or co-receptors. It seems highly likely that the properties of ligand binding by CD2 are relevant to the general mechanisms of cell-cell recognition. As an example of biological recognition, the defining characteristic of cell-cell contact is that it involves the simultaneous interaction of hundreds, if not thousands, of molecules. Affinity and kinetic analyses of ligand binding by CD2 indicated that the protein interactions mediating cell-cell contact, whilst highly specific, are much weaker than initially anticipated, probably due to the requirement that such contacts be easily reversible. Simultaneously, in addressing the mechanism of this mode of recognition, structural and mutational studies focussed on the role of charged residues clustered in the ligand-binding face of CD2, yielding the concept that electrostatic complementarity, rather than surface-shape complementarity, is the dominant feature of specific, low-affinity protein recognition at the cell surface by CD2. The crystallographic analysis of the CD2-binding domain of CD58 strongly supports this concept.