Measuring bonds between surface-associated molecules

Understanding How Cells Probe the World: A Preliminary Step towards Modeling Cell Behavior?

Cell biologists have long aimed at quantitatively modeling cell function. Recently, the outstanding progress of high-throughput measurement methods and data processing tools has made this a realistic goal. The aim of this paper is twofold: First, to suggest that, while much progress has been done in modeling cell states and transitions, current accounts of environmental cues driving these transitions remain insufficient. There is a need to provide an integrated view of the biochemical, topographical and mechanical information processed by cells to take decisions. It might be rewarding in the near future to try to connect cell environmental cues to physiologically relevant outcomes rather than modeling relationships between these cues and internal signaling networks. The second aim of this paper is to review exogenous signals that are sensed by living cells and significantly influence fate decisions. Indeed, in addition to the composition of the surrounding medium, cells are highly sensitive to the properties of neighboring surfaces, including the spatial organization of anchored molecules and substrate mechanical and topographical properties. These properties should thus be included in models of cell behavior. It is also suggested that attempts at cell modeling could strongly benefit from two research lines: (i) trying to decipher the way cells encode the information they retrieve from environment analysis, and (ii) developing more standardized means of assessing the quality of proposed models, as was done in other research domains such as protein structure prediction.

Early T cell activation: integrating biochemical, structural, and biophysical cues

T cells carry out the formidable task of identifying small numbers of foreign antigenic peptides rapidly and specifically against a very noisy environmental background of endogenous self-peptides. Early steps in T cell activation have thus fascinated biologists and are among the best-studied models of cell stimulation. This remarkable process, critical in adaptive immune responses, approaches and even seems to exceed the limitations set by the physical laws ruling molecular behavior. Despite the enormous amount of information concerning the nature of molecules involved in the T cell antigen receptor (TCR) signal transduction network, and the description of the nanoscale organization and real-time analysis of T cell responses, the general principles of information gathering and processing remain incompletely understood. Here we review currently accepted key data on TCR function, discuss the limitations of current research strategies, and suggest a novel model of TCR triggering and a few promising ways of going further into the integration of available data.

Multiscale evaluation of cellular adhesion alteration and cytoskeleton remodeling by magnetic bead twisting

Cellular adhesion forces depend on local biological conditions meaning that adhesion characterization must be performed while preserving cellular integrity. We presently postulate that magnetic bead twisting provides an appropriate stress, i.e., basically a clamp, for assessment in living cells of both cellular adhesion and mechanical properties of the cytoskeleton. A global dissociation rate obeying a Bell-type model was used to determine the natural dissociation rate ([Formula: see text]) and a reference stress ([Formula: see text]). These adhesion parameters were determined in parallel to the mechanical properties for a variety of biological conditions in which either adhesion or cytoskeleton was selectively weakened or strengthened by changing successively ligand concentration, actin polymerization level (by treating with cytochalasin D), level of exerted stress (by increasing magnetic torque), and cell environment (by using rigid and soft 3D matrices). On the whole, this multiscale evaluation of the cellular and molecular responses to a controlled stress reveals an evolution which is consistent with stochastic multiple bond theories and with literature results obtained with other molecular techniques. Present results confirm the validity of the proposed bead-twisting approach for its capability to probe cellular and molecular responses in a variety of biological conditions.

Biophysical description of multiple events contributing blood leukocyte arrest on endothelium

Blood leukocytes have a remarkable capacity to bind to and stop on specific blood vessel areas. Many studies have disclosed a key role of integrin structural changes following the interaction of rolling leukocytes with surface-bound chemoattractants. However, the functional significance of structural data and mechanisms of cell arrest are incompletely understood. Recent experiments revealed the unexpected complexity of several key steps of cell-surface interaction: (i) ligand-receptor binding requires a minimum amount of time to proceed and this is influenced by forces. (ii) Also, molecular interactions at interfaces are not fully accounted for by the interaction properties of soluble molecules. (iii) Cell arrest depends on nanoscale topography and mechanical properties of the cell membrane, and these properties are highly dynamic. Here, we summarize these results and we discuss their relevance to recent functional studies of integrin-receptor association in cells from a patient with type III leukocyte adhesion deficiency. It is concluded that an accurate understanding of all physical events listed in this review is needed to unravel the precise role of the multiple molecules and biochemical pathway involved in arrest triggering.

The Serial Engagement Model 17 Years After: From TCR Triggering to Immunotherapy

More than 15 years ago the serial engagement model was proposed as an attempt to solve the low affinity/high sensitivity paradox of TCR antigen recognition. Since then, the model has undergone ups and downs marked by the technical and conceptual advancements made in the field of T lymphocyte activation. Here, I describe the development of the model and survey recent literature providing evidence either for or against the idea that serial TCR/pMHC engagement might contribute to T lymphocyte activation. I also discuss how the concept of serial TCR engagement might be useful in the design of immunotherapeutic approaches aimed at potentiating T lymphocyte responses in vivo.

Kinetics and mechanics of two-dimensional interactions between T cell receptors and different activating ligands

Adaptive immune responses are driven by interactions between T cell antigen receptors (TCRs) and complexes of peptide antigens (p) bound to Major Histocompatibility Complex proteins (MHC) on the surface of antigen-presenting cells. Many experiments support the hypothesis that T cell response is quantitatively and qualitatively dependent on the so-called strength of TCR/pMHC association. Most available data are correlations between binding parameters measured in solution (three-dimensional) and pMHC activation potency, suggesting that full lymphocyte activation required a minimal lifetime for TCR/pMHC interaction. However, recent reports suggest important discrepancies between the binding properties of ligand-receptor couples measured in solution (three-dimensional) and those measured using surface-bound molecules (two-dimensional). Other reports suggest that bond mechanical strength may be important in addition to kinetic parameters. Here, we used a laminar flow chamber to monitor at the single molecule level the two-dimensional interaction between a recombinant human TCR and eight pMHCs with variable potency. We found that 1), two-dimensional dissociation rates were comparable to three-dimensional parameters previously obtained with the same molecules; 2), no significant correlation was found between association rates and activating potency of pMHCs; 3), bond mechanical strength was partly independent of bond lifetime; and 4), a suitable combination of bond lifetime and bond strength displayed optimal correlation with activation efficiency. These results suggest possible refinements of contemporary models of signal generation by T cell receptors. In conclusion, we reported, for the first time to our knowledge, the two-dimensional binding properties of eight TCR/pMHC couples in a cell-free system with single bond resolution.

Determination of single cell surface protein expression using a tagless microfluidic method

We describe a method to detect the expression of a surface protein in single cells without prior labeling or manipulation using a microfluidic device. When the protein is expressed on a cell surface, it undergoes transient bond formation with an immobilized ligand as the cell is pumped through a microfluidic channel, resulting in a specific decrease in the cell's velocity. We were able to detect the expression of interleukin 13 receptor alpha 2 (IL13Rα2) differentially expressed on LM2 cells, a subline of MDA-MB-231 human breast cancer cells with a unique lung metastatic capability. The detection of cells with high expression of the protein was near 100% sensitive and 100% specific. We also provided proof of principle of multiplexing by tracking the same cell over two, differentially-coated patches. The method is non-destructive and cells can be collected for reanalysis. The system can identify positive cells in a cell mixture. This method will have a potential impact in analyzing cancer cells when only a few are available, such as the case with needle aspirates and when labeling and manipulation result in cell loss.

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.

A novel leukocyte adhesion deficiency III variant: kindlin-3 deficiency results in integrin- and nonintegrin-related defects in different steps of leukocyte adhesion

Leukocyte adhesion deficiency type III is a recently described condition involving a Glanzmann-type bleeding syndrome and leukocyte adhesion deficiency. This was ascribed to a defect of the FERMT3 gene resulting in abnormal expression of kindlin-3, a protein expressed in hematopoietic cells with a major role in the regulation of integrin activation. In this article, we describe a patient with a new mutation of FERMT3 and lack of kindlin-3 expression in platelets and leukocytes. We assayed quantitatively the first steps of kindlin-3-defective leukocyte adhesion, namely, initial bond formation, bond strengthening, and early spreading. Initial bond formation was readily stimulated with neutrophils stimulated by fMLF, and neutrophils and lymphocytes stimulated by a phorbol ester or Mn(2+). In contrast, attachment strengthening was defective in the patient's lymphocytes treated with PMA or Mn(2+), or fMLF-stimulated neutrophils. However, attachment strengthening was normal in patient's neutrophils treated with phorbol ester or Mn(2+). In addition, the patient's T lymphocytes displayed defective integrin-mediated spreading and a moderate but significant decrease of spreading on anti-CD3-coated surfaces. Patient's neutrophils displayed a drastic alteration of integrin-mediated spreading after fMLF or PMA stimulation, whereas signaling-independent Mn(2+) allowed significant spreading. In conclusion, the consequences of kindlin-3 deficiency on β(2) integrin function depend on both cell type and the stimulus used for integrin activation. Our results suggest looking for a possible kindlin-3 involvement in membrane dynamical event independent of integrin-mediated adhesion.

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.

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?

Two stage cadherin kinetics require multiple extracellular domains but not the cytoplasmic region

Micropipette manipulation measurements quantified the pre-steady state binding kinetics between cell pairs mediated by Xenopus cleavage stage cadherin. The time-dependence of the intercellular binding probability exhibits a fast forming, low probability binding state, which transitions to a slower forming, high probability state. The biphasic kinetics are independent of the cytoplasmic region, but the transition to the high probability state requires the third extracellular domain EC3. Deleting either EC3 or EC3-5, or substituting Trp(2) for Ala reduces the binding curves to a simple, monophasic rise in binding probability to a limiting plateau, as predicted for a single site binding mechanism. The two stage cadherin binding process reported here directly parallels previous biophysical studies, and confirms that the cadherin ectodomain governs the initial intercellular adhesion dynamics.

Single particle tracking across sequences of microscopical images: application to platelet adhesion under flow

A versatile and automated image processing technique and data extraction procedure from videomicroscopic data is presented. The motivation is a detailed quantification of blood platelet adhesion from laminar flow onto a surface. The characteristics of the system under observation (type of cells, their speed of movement, and the quality of the optical image to analyze) provided the criteria for developing a new procedure enabling tracking for long image sequences. Specific features of the novel method include: automatic segmentation methodology which removes operator bias; platelet recognition across the series of images based on a probability density function (two-dimensional, Gaussian-like) tailored to the physics of platelet motion on the surface; options to automatically tune the procedure parameters to explore different applications; integrated analysis of the results (platelet trajectories) to obtain relevant information, such as deposition and removal rates, displacement distributions, pause times and rolling velocities. Synthetic images, providing known reference conditions, are used to test the method. The algorithm operation is illustrated by application to images obtained by fluorescence microscopy of the interaction between platelets and von Willebrand factor-coated surfaces in parallel-plate flow chambers. Potentials and limits are discussed, together with evaluation of errors resulting from an inaccurate tracking.

Synthesis and surface engineering of iron oxide nanoparticles for biomedical applications

Superparamagnetic iron oxide nanoparticles (SPION) with appropriate surface chemistry have been widely used experimentally for numerous in vivo applications such as magnetic resonance imaging contrast enhancement, tissue repair, immunoassay, detoxification of biological fluids, hyperthermia, drug delivery and in cell separation, etc. All these biomedical and bioengineering applications require that these nanoparticles have high magnetization values and size smaller than 100 nm with overall narrow particle size distribution, so that the particles have uniform physical and chemical properties. In addition, these applications need special surface coating of the magnetic particles, which has to be not only non-toxic and biocompatible but also allow a targetable delivery with particle localization in a specific area. To this end, most work in this field has been done in improving the biocompatibility of the materials, but only a few scientific investigations and developments have been carried out in improving the quality of magnetic particles, their size distribution, their shape and surface in addition to characterizing them to get a protocol for the quality control of these particles. Nature of surface coatings and their subsequent geometric arrangement on the nanoparticles determine not only the overall size of the colloid but also play a significant role in biokinetics and biodistribution of nanoparticles in the body. The types of specific coating, or derivatization, for these nanoparticles depend on the end application and should be chosen by keeping a particular application in mind, whether it be aimed at inflammation response or anti-cancer agents. Magnetic nanoparticles can bind to drugs, proteins, enzymes, antibodies, or nucleotides and can be directed to an organ, tissue, or tumour using an external magnetic field or can be heated in alternating magnetic fields for use in hyperthermia. This review discusses the synthetic chemistry, fluid stabilization and surface modification of superparamagnetic iron oxide nanoparticles, as well as their use for above biomedical applications.

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.

A Parallel-Plate Flow Chamber to Study Initial Cell Adhesion on a Nanofeatured Surface

Cells in the human body come across many types of information, which they respond to. Both material chemistry and topography of the surface where they adhere have an effect on cell shape, proliferation, migration, and gene expression. It is possible to create surfaces with topography at the nanometric scale to allow observation of cell-topography interactions. Previous work has shown that 100-nm-diameter pits on a 300-nm pitch can have a marked effect in reducing the adhesion of rat fibroblasts in static cultures. In the present study, a flow of cell suspension was used to investigate cell adhesion onto nanopits in dynamic conditions, by means of a parallel-plate flow chamber. A flow chamber with inner nanotopography has been designed, which allows real-time observation of the flow over the nanopits. A nanopitted pattern was successfully embossed into polymethylmethacrylate to meet the required shape of the chamber. Dynamic cell adhesion after 1 h has been quantified and compared on flat and nanopitted polymethylmethacrylate substrates. The nanopits were seen to be significantly less adhesive than the flat substrates (p < 0.001), which is coherent with previous observations of static cultures.

Developments for inverted atomic force microscopy

Atomic force microscopy (AFM) has been used to study a wide range of systems. Chemically and biologically modified probes have extended AFM by coupling chemical and biological information with the physical measurements. In an effort to further expand the capabilities of modified AFM probes, previous studies investigated the use of an inverted AFM design (i-AFM), wherein a microfabricated tip array is used to image a cantilever-supported sample. This report details developments in cantilever and tip array fabrication which are aimed at improving the applicability and performance of this i-AFM design. Using an epoxy-based procedure, commercial cantilevers were modified with a series of standard substrates, including template-stripped gold, highly oriented pyrolytic graphite, and mica. The samples on these cantilevers were imaged with i-AFM, and lateral force images are obtained. This paper demonstrates the first use of i-AFM for measuring friction.

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.

Scanning force microscopy in the applied biological sciences

Fifteen years after its invention, the scanning force microscope (SFM) is rooted deep in the biological sciences. Here we discuss the use of SFM in biotechnology and biomedical research. The spectrum of applications reviewed includes imaging, force spectroscopy and mapping, as well as sensor applications. It is our hope that this review will be useful for researchers considering the use of SFM in their studies but are uncertain about its scope of capabilities. For the benefit of readers unfamiliar with SFM technology, the fundamentals of SFM imaging and force measurement are also briefly introduced.

Diffusion of microspheres in shear flow near a wall: use to measure binding rates between attached molecules

The rate and distance-dependence of association between surface-attached molecules may be determined by monitoring the motion of receptor-bearing spheres along ligand-coated surfaces in a flow chamber (Pierres et al., Proc. Natl. Acad. Sci. U.S.A. 95:9256-9261, 1998). Particle arrests reveal bond formation, and the particle-to-surface distance may be estimated from the ratio between the velocity and the wall shear rate. However, several problems are raised. First, data interpretation requires extensive computer simulations. Second, the relevance of standard results from fluid mechanics to micrometer-size particles separated from surfaces by nanometer distances is not fully demonstrated. Third, the wall shear rate must be known with high accuracy. Here we present a simple derivation of an algorithm permitting one to simulate the motion of spheres near a plane in shear flow. We check that theoretical predictions are consistent with the experimental dependence of motion on medium viscosity or particle size, and the requirement for equilibrium particle height distribution to follow Boltzman's law. The determination of the statistical relationship between particle velocity and acceleration allows one to derive the wall shear rate with 1-s(-1) accuracy and the Hamaker constant of interaction between the particle and the wall with a sensitivity better than 10(-21) J. It is demonstrated that the correlation between particle height and mean velocity during a time interval Deltat is maximal when Deltat is about 0.1-0.2 s for a particle of 1.4-microm radius. When the particle-to-surface distance ranges between 10 and 40 nm, the particle height distribution may be obtained with a standard deviation ranging between 8 and 25 nm, provided the average velocity during a 160-ms period of time is determined with 10% accuracy. It is concluded that the flow chamber allows one to detect the formation of individual bonds with a minimal lifetime of 40 ms in presence of a disruptive force of approximately 5 pN and to assess the distance dependence within the tens of nanometer range.

Single molecule force spectroscopy in biology using the atomic force microscope

The importance of forces in biology has been recognized for quite a while but only in the past decade have we acquired instrumentation and methodology to directly measure interactive forces at the level of single biological macromolecules and/or their complexes. This review focuses on force measurements performed with the atomic force microscope. A general introduction to the principle of action is followed by review of the types of interactions being studied, describing the main results and discussing the biological implications.

Evaluation of the energy of red blood cell agglutination by monoclonal antibodies

The knowledge of the energy involved in cell-cell interactions has significant implications in biological and medical sciences. Red blood cells (RBC) become mutually adhered when specific (agglutinins) or nonspecific macromolecules bind adjacent cells in an irreversible or reversible form. Flow chamber technique with digital image processing was successfully applied to determine the partial separation, by shear stress, of two RBC agglutinated face to face (doublet) by specific monoclonal antibodies (MAb) and also to evaluate the shear stress required to attain this separation. The work done in separating adhered cells is then easily calculated and serves to evaluate the antibody affinity. In this work, this technique was applied to evaluate the affinity of a monoclonal antibody specific to the blood group A antigen. The specific disaggregation energy gamma (i.e., the work done by the shear stress) required to disrupt a unit of adhered membrane areas between agglutinated cells was calculated. On play back of the image analysis, measurements were taken to determine the force applied to the doublet and the relative separation between both RBC. Values of gamma(d) (from 2.70 to 4.61 x 10(-9) N/cm) were found to be proportional to the density (D = 27 to 55 molecules/microm(2)) of MAb molecules bound on the RBC membrane.

Spatially resolved force spectroscopy of biological surfaces using the atomic force microscope

The spatial distribution of intermolecular forces governs macromolecular interactions. The atomic force microscope, a relatively new tool for investigating interaction forces between nanometer-scale objects, can be used to produce spatially resolved maps of the surface or material properties of a sample; these include charge density, adhesion and stiffness, as well as the force required to break specific ligand-receptor bonds. Maps such as these will provide fundamental insights into biological structure and will become an important tool for characterizing technologically important biological systems.

A biosensor based on magnetoresistance technology

We are developing a biosensor that will measure, at the level of single molecules, the forces that bind DNA-DNA, antibody-antigen, or ligand-receptor pairs together. The Bead Array Counter (BARC) will use these interaction forces to hold magnetic microbeads to a solid substrate. Microfabricated magnetoresistive transducers on the substrate will indicate whether or not the beads are removed when pulled by magnetic forces. By adapting magnetoresistive computer memory technology, it may be possible to fabricate millions of transducers on a chip and detect or screen thousands of analytes. The multi-analyte capability of this portable sensor would be ideal for on-site testing, while the potential to directly gauge intermolecular interaction strengths suggests drug discovery applications.

Measuring two-dimensional receptor-ligand binding kinetics by micropipette

We report a novel method for measuring forward and reverse kinetic rate constants, kf0 and kr0, for the binding of individual receptors and ligands anchored to apposing surfaces in cell adhesion. Not only does the method examine adhesion between a single pair of cells; it also probes predominantly a single receptor-ligand bond. The idea is to quantify the dependence of adhesion probability on contact duration and densities of the receptors and ligands. The experiment was an extension of existing micropipette protocols. The analysis was based on analytical solutions to the probabilistic formulation of kinetics for small systems. This method was applied to examine the interaction between Fc gamma receptor IIIA (CD16A) expressed on Chinese hamster ovary cell transfectants and immunoglobulin G (IgG) of either human or rabbit origin coated on human erythrocytes, which were found to follow a monovalent biomolecular binding mechanism. The measured rate constants are Ackf0 = (2.6 +/- 0.32) x 10(-7) micron 4 s-1 and kr0 = (0.37 +/- 0.055) s-1 for the CD16A-hIgG interaction and Ackf0 = (5.7 +/- 0.31) X 10(-7) micron 4 s-1 and kr0 = (0.20 +/- 0.042) s-1 for the CD16A-rIgG interaction, respectively, where Ac is the contact area, estimated to be a few percent of 3 micron 2.

Studying receptor-mediated cell adhesion at the single molecule level

Cell adhesion is essentially mediated by specific interactions between membrane receptors and ligands. It is now apparent that the mere knowledge of the on- and off-rate of association of soluble forms of these receptors and ligands is not sufficient to yield accurate prediction of cell adhesive behavior. During the last few years, a variety of complementary techniques relying on the use of hydrodynamic flow, atomic force microscopy, surface forces apparatus or soft vesicles yielded accurate information on i) the dependence of the lifetime of individual bonds on applied forces and ii) the distance dependence of the association rate of bound receptors and ligands. The purpose of this review is, first to recall the physical significance of these parameters, and second to describe newly obtained results. It is emphasized that molecular size and flexibility may be a major determinant of the efficiency of receptor mediated adhesion, and this cannot be studied by conventional methods dealing with soluble molecules.

Atomic force microscopy detects changes in the interaction forces between GroEL and substrate proteins

The structure of the Escherichia coli chaperonin GroEL has been investigated by tapping-mode atomic force microscopy (AFM) under liquid. High-resolution images can be obtained, which show the up-right position of GroEL adsorbed on mica with the substrate-binding site on top. Because of this orientation, the interaction between GroEL and two substrate proteins, citrate synthase from Saccharomyces cerevisiae with a destabilizing Gly-->Ala mutation and RTEM beta-lactamase from Escherichia coli with two Cys-->Ala mutations, could be studied by force spectroscopy under different conditions. The results show that the interaction force decreases in the presence of ATP (but not of ATPgammaS) and that the force is smaller for native-like proteins than for the fully denatured ones. It also demonstrates that the interaction energy with GroEL increases with increasing molecular weight. By measuring the interaction force changes between the chaperonin and the two different substrate proteins, we could specifically detect GroEL conformational changes upon nucleotide binding.

Interest of image processing in cell biology and immunology

Microscopy is a basic tool for cell biologists. Recent progress of electronics and computer science made powerful methodologies for digital processing of microscopic images easily available. These methods allowed impressive increase of the power of conventional microscopy. Dramatic image enhancement may be achieved by combination of filtering techniques, computer-based deblurring and contrast enhancement. Quantitative treatment of digitized images allows absolute determination of the density of different components of the observed sample, including antigens, intracellular calcium and pH. Morphometric studies are also greatly facilitated by image processing techniques. The capture of fast phenomena may be performed by transfer of small portion of microscopic images into computer memory as well as particular use of confocal microscopy. Finally, improved display of experimental data through coded colors or other procedures may enhance the amount of information that can be conveyed by visual examination of microscopical images. The purpose of the present review is to describe the basic principles of image processing and exemplify the power of this approach with a variety of illustrated applications to conventional, fluorescence or electron microscopy as well as confocal microscopy.