Quantification of cell surface roughness; a method for studying cell mechanical and adhesive properties

Fluorescence-Based Measurements of Two-Dimensional Affinity in Membrane Interfaces

Binding between ligands and receptors across cell contacts influences a range of biological processes including the formation of the immune synapse. The dissociation constant (K_d = 1/affinity) of the interaction corresponds to the concentration of ligands where half of the receptors in the contact have bound a ligand. In this chapter, we outline how to measure this two-dimensional affinity using model cell membranes called supported lipid bilayers (SLBs) functionalized with fluorescently labeled ligands that bind to cells containing the corresponding receptor. The affinity is calculated from the accumulation of ligands at the cell-SLB interface, while the use of different fluorescent tags, and/or unlabeled molecules, makes it possible to include various binding pairs in the contact to better mimic the conditions of binding in vivo.

Interleukin 1β and lipopolysaccharides induction dictate chondrocyte morphological properties and reduce cellular roughness and adhesion energy comparatively

Osteoarthritis (OA) is a whole joint disease marked by the degradation of the articular cartilage (AC) tissue, chronic inflammation, and bone remodeling. Upon AC's injury, proinflammatory mediators including interleukin 1β (IL1β) and lipopolysaccharides (LPS) play major roles in the onset and progression of OA. The objective of this study was to mechanistically detect and compare the effects of IL1β and LPS, separately, on the morphological and nanomechanical properties of bovine chondrocytes. Cells were seeded overnight in a full serum medium and the next day divided into three main groups: A negative control (NC) of a reduced serum medium and 10 ng/ml IL1ß or 10 ng/ml LPS-modified media. Cells were induced for 24 h. Nanomechanical properties (elastic modulus and adhesion energy) and roughness were quantified using atomic force microscopy. Nitric oxide, prostaglandin 2 (PGE2), and matrix metalloproteinases 3 (MMP3) contents; viability of cells; and extracellular matrix components were quantified. Our data revealed that viability of the cells was not affected by inflammatory induction and IL1ß induction increased PGE2. Elastic moduli of cells were similar among IL1β and NC while LPS significantly decreased the elasticity compared to NC. IL1ß induction resulted in least cellular roughness while LPS induction resulted in least adhesion energy compared to NC. Our images suggest that IL1ß and LPS inflammation affect cellular morphology with cytoskeleton rearrangements and the presence of stress fibers. Finally, our results suggest that the two investigated inflammatory mediators modulated chondrocytes' immediate responses to inflammation in variable ways.

Supported Lipid Bilayers and the Study of Two-Dimensional Binding Kinetics

Binding between protein molecules on contacting cells is essential in initiating and regulating several key biological processes. In contrast to interactions between molecules in solution, these events are restricted to the two-dimensional (2D) plane of the meeting cell surfaces. However, converting between the more commonly available binding kinetics measured in solution and the so-called 2D binding kinetics has proven a complicated task since for the latter several factors other than the protein-protein interaction per se have an impact. A few important examples of these are: protein density, membrane fluctuations, force on the bond and the use of auxiliary binding molecules. The development of model membranes, and in particular supported lipid bilayers (SLBs), has made it possible to simplify the studied contact to analyze these effects and to measure 2D binding kinetics of individual protein-protein interactions. We will in this review give an overview of, and discuss, how different SLB systems have been used for this and compare different methods to measure binding kinetics in cell-SLB contacts. Typically, the SLB is functionalized with fluorescently labelled ligands whose interaction with the corresponding receptor on a binding cell can be detected. This interaction can either be studied 1) by an accumulation of ligands in the cell-SLB contact, whose magnitude depends on the density of the proteins and binding affinity of the interaction, or 2) by tracking single ligands in the SLB, which upon interaction with a receptor result in a change of motion of the diffusing ligand. The advantages and disadvantages of other methods measuring 2D binding kinetics will also be discussed and compared to the fluorescence-based methods. Although binding kinetic measurements in cell-SLB contacts have provided novel information on how ligands interact with receptors in vivo the number of these measurements is still limited. This is influenced by the complexity of the system as well as the required experimental time. Moreover, the outcome can vary significantly between studies, highlighting the necessity for continued development of methods to study 2D binding kinetics with higher precision and ease.

New Insights into Hemolytic Anemias: Ultrastructural and Nanomechanical Investigation of Red Blood Cells Showed Early Morphological Changes

Several diseases are characterized by changes in the mechanical properties of erythrocytes. Hemolytic anemias are an example of these diseases. Among the hemolytic anemias, Sickle Cell Disease and Thalassemia are the most common, characterized by alterations in the structure of their hemoglobin. Sickle cell disease has a pathological origin in synthesizing abnormal hemoglobin, HbS. In contrast, thalassemia results in extinction or decreased synthesis of α and β hemoglobin chains. This work presents a detailed study of biophysical and ultrastructural early erythrocytes membrane alterations at the nanoscale using Atomic Force Microscopy (AFM). Cells from individuals with sickle cell anemia and thalassemia mutations were studied. The analysis methodology in the AFM was given by blood smear and exposure of the inner membrane for ghost analysis. A robust statistic was used with 65,536 force curves for each map, ten cells of each type, with three individuals for each sample group. The results showed significant differences in cell rigidity, adhesion, volume, and roughness at early morphological alterations, bringing new perspectives for understanding pathogenesis. The sickle cell trait (HbAS) results stand out. Significant alterations were observed in the membrane properties, bringing new perspectives for the knowledge of this mutation. This work presents ultrastructural and biomechanical signatures of sickle cell anemia and thalassemia genotypes, which may help determine a more accurate biophysical description and clinical prognosis for these diseases.

Single-cell measurements of two-dimensional binding affinity across cell contacts

The two-dimensional (2D) affinity between protein molecules across contacting cells is a key parameter regulating and initiating several cellular processes. However, measuring 2D affinity can be challenging, and experimental data are limited. In addition, the obtained 2D affinities are typically averaged over the cell population. We here present a method to measure 2D affinity on single cells binding to polyhistidine-tagged fluorescent ligands anchored to a supported lipid bilayer (SLB). By decreasing the density of ligands in the SLB using imidazole, a new steady-state accumulation in the contact is obtained, and from this change, both the 2D affinity and the number of receptors on the cell can be determined. The method was validated on an SLB containing rat CD2 binding to the rat CD48 mutant T92A expressed on Jurkat T cells. The addition of imidazole did not influence the average 2D affinity (1/Kd), and the spread in affinities within the cell population was low, Kd = 4.9 ± 0.9 molecules/μm2 (mean ± SD), despite an order of magnitude spread in ligand accumulation because of differences in receptor density. It was also found that cell contact size increased both with ligand density and with the number of receptors per cell but that the contact size stayed approximately constant when lowering the ligand density, above a density of around 10 rat CD2 molecules/μm2, after the contact first had formed, indicative of a heterogeneous process. In summary, this method not only allows for single-cell affinities to be measured, but it can also reduce measurement and analysis time and improve measurement accuracy. Because of the low spread in 2D Kd within the cell population, the analysis can further be restricted to the cells showing the strongest binding, paving the way for using this method to study weak binding events.

Effects of a local auxiliary protein on the two-dimensional affinity of a TCR-peptide MHC interaction

The affinity of T-cell receptors (TCRs) for major histocompatibility complex molecules (MHCs) presenting cognate antigens likely determines whether T cells initiate immune responses, or not. There exist few measurements of two-dimensional (2D) TCR-MHC interactions, and the effect of auxiliary proteins on binding is unexplored. Here, Jurkat T-cells expressing the MHC molecule HLA-DQ8-glia-α1 and the ligand of an adhesion protein (rat CD2) were allowed to bind supported lipid bilayers (SLBs) presenting fluorescently labelled L3-12 TCR and rat CD2, allowing measurements of binding unconfounded by cell signaling effects or co-receptor binding. The 2D Kd for L3-12 TCR binding to HLA-DQ8-glia-α1, of 14±5 molecules/μm2 (mean±s.d.), was only marginally influenced by including CD2 up to ∼200 bound molecules/μm2 but higher CD2 densities reduced the affinity up to 1.9-fold. Cell-SLB contact size increased steadily with ligand density without affecting binding for contacts at up to ∼20% of total cell area, but beyond this lamellipodia appeared, giving an apparent increase in bound receptors of up to 50%. Our findings show how parameters other than the specific protein-protein interaction can influence binding behavior at cell-cell contacts.

Three-Dimensional Super-Resolution in Eukaryotic Cells Using the Double-Helix Point Spread Function

Single-molecule localization microscopy, typically based on total internal reflection illumination, has taken our understanding of protein organization and dynamics in cells beyond the diffraction limit. However, biological systems exist in a complicated three-dimensional environment, which has required the development of new techniques, including the double-helix point spread function (DHPSF), to accurately visualize biological processes. The application of the DHPSF approach has so far been limited to the study of relatively small prokaryotic cells. By matching the refractive index of the objective lens immersion liquid to that of the sample media, we demonstrate DHPSF imaging of up to 15-μm-thick whole eukaryotic cell volumes in three to five imaging planes. We illustrate the capabilities of the DHPSF by exploring large-scale membrane reorganization in human T cells after receptor triggering, and by using single-particle tracking to image several mammalian proteins, including membrane, cytoplasmic, and nuclear proteins in T cells and embryonic stem cells.

Force and affinity in ligand discrimination by the TCR

T cell recognition of antigen is a physical process that requires formation of a cell-cell junction that is rich in active force generation. Recently a biomolecular force probe was used to examine how the T cell receptor (TCR)-pMHC interaction responds to force and the consequences of force-dependent interactions for T cell activation. While adhesion and costimulatory molecules in the immunological synapse impact upon the overall force of the interaction, these results suggest that the TCR uses a force-dependent bond - a catch bond - and that it may therefore be important to consider the TCR-pMHC interaction in isolation in the early phases of the decision process. We discuss here these findings in the context of other work on the impact of forces on the TCR and the quantification of interaction in interfaces.

In vitro membrane reconstitution of the T-cell receptor proximal signaling network

T-cell receptor (TCR) phosphorylation is controlled by a complex network that includes Lck, a Src family kinase (SFK), the tyrosine phosphatase CD45, and the Lck-inhibitory kinase Csk. How these competing phosphorylation and dephosphorylation reactions are modulated to produce T-cell triggering is not fully understood. Here we reconstituted this signaling network using purified enzymes on liposomes, recapitulating the membrane environment in which they normally interact. We demonstrate that Lck's enzymatic activity can be regulated over a ~10-fold range by controlling its phosphorylation state. By varying kinase and phosphatase concentrations, we constructed phase diagrams that reveal ultrasensitivity in the transition from the quiescent to the phosphorylated state and demonstrate that coclustering TCR-Lck or detaching Csk from the membrane can trigger TCR phosphorylation. Our results provide insight into the mechanism of TCR signaling as well as other signaling pathways involving SFKs.

An improved strip FRAP method for estimating diffusion coefficients: correcting for the degree of photobleaching

Fluorescence recovery after photobleaching is a widely established method for the estimation of diffusion coefficients, strip bleaching with an associated recovery curve analysis being one of the simplest techniques. However, its implementation requires near 100% bleaching in the region of interest with negligible fluorescence loss outside, both constraints being hard to achieve concomitantly for fast diffusing molecules. We demonstrate that when these requirements are not met there is an error in the estimation of the diffusion coefficient D, either an under- or overestimation depending on which assumption is violated the most. We propose a simple modification to the recovery curve analysis incorporating the concept of the relative bleached mass m giving a revised recovery time parametrization tau=m(2)w(2)/4piD for a strip of width w. This modified model removes the requirement of 100% bleaching in the region of interest and allows for limited diffusion of the fluorophore during bleaching. We validate our method by estimating the (volume) diffusion coefficient of FITC-labelled IgG in 60% glycerol solution, D= 4.09 +/- 0.21 microm(2) s(-1), and the (surface) diffusion coefficient of a green-fluorescent protein-tagged class I MHC protein expressed at the surface of a human B cell line, D= 0.32 +/- 0.03 microm(2) s(-1) for a population of cells.

Early contacts between T lymphocytes and activating surfaces

Cells continually probe their environment to adapt their behaviour. A current challenge is to determine how they analyse nearby surfaces and how they process information to take decisions. We addressed this problem by monitoring human T lymphocyte attachment to surfaces coated with activating anti-CD3 or control anti-HLA antibodies. Interference reflection microscopy allowed us to monitor cell-to-surface apposition with a few nanometre vertical resolution during the first minutes following contact. We found that (i) when a cell fell on a surface, contact extension was preceded by a lag of several tens of seconds. (ii) During this lag, vertical membrane undulations seemed to generate transient contacts with underlying surfaces. (iii) After the lag period, the contact area started increasing linearly with a rate of about 1.5 µm(2) s(-1) on activating surfaces and about 0.2 µm(2) s(-1) on control surfaces. (iv) Concomitantly with lateral surface extension, the apparent distance between cell membranes and surfaces steadily decreased. These results are consistent with the hypothesis that the cell decision to spread rapidly on activating surfaces resulted from the integration of information yielded by transient contacts with these surfaces generated by membrane undulations during a period of about 1 min.

Mechanisms of Cellular Avidity Regulation in CD2-CD58-Mediated T Cell Adhesion

The CD2 receptor on T lymphocytes is essential for T cell adhesion and stimulation by antigen presenting cells (APCs). Blockade of CD2 function is immunosuppressive in both model systems and humans, indicating the importance of CD2 for the cellular immune response. Although the affinity of the molecular interaction between CD2 and its counter-receptor, CD58, is relatively low when measured in solution, this interaction mediates tight adhesion within the 2D cell-cell interface. To understand the mechanisms responsible for regulating the avidity of the CD2-CD58 interaction, we measured the number, affinity, and lateral mobility of CD2 molecules on resting and activated T cells. Cell activation caused a 1.5-fold increase in the number of CD2 sites on the cell surface, and the 2D affinity of CD2 for CD58 increased by 2.5-fold. The combination of T cell activation and CD2 ligation to CD58 decreased the laterally mobile fraction of the ligated CD2. Together, these changes would substantially enhance CD2 avidity and strengthen T cell-APC adhesion. The change in CD2 mobile fraction suggests that the cell uses cytoskeletal regulators to immobilize the receptor selectively at the site of contact with surfaces expressing CD58. Our observations are consistent with a model in which T cell activation initially induces increased CD2 2D affinity, cell surface receptor expression, and lateral mobility, allowing the CD2 molecules to diffuse to sites of contact with CD58-bearing APCs. Subsequently, T cell activation causes the CD58-bound CD2 to be recognized and immobilized at sites of cell-cell contact, thereby strengthening T cell-APC adhesion.

Analysis of two-dimensional dissociation constant of laterally mobile cell adhesion molecules

We formulate a general analysis to determine the two-dimensional dissociation constant (2D Kd), and use this method to study the interaction of CD2-expressing T cells with glass-supported planar bilayers containing fluorescently labeled CD58, a CD2 counter-receptor. Both CD2 and CD58 are laterally mobile in their respective membranes. Adhesion is indicated by accumulation of CD2 and CD58 in the cell-bilayer contact area; adhesion molecule density and contact area size attain equilibrium within 40 min. The standard (Scatchard) analysis of solution-phase binding is not applicable to the case of laterally mobile adhesion molecules due to the dynamic nature of the interaction. We derive a new binding equation, B/F=[(Ntxf)/(KdxScell)]-[(Bxp)/Kd], where B and F are bound and free CD58 density in the contact area, respectively; Nt is CD2 molecule number per cell; f is CD2 fractional mobility; Scell is cell surface area; and p is the ratio of contact area at equilibrium to Scell. We use this analysis to determine that the 2D Kd for CD2-CD58 is 5.4-7.6 molecules/microm2. 2D Kd analysis provides a general and quantitative measure of the mechanisms regulating cell-cell adhesion.

Feasibility of probing boundary morphology of structured materials by 2D NMR q-space imaging

It is well known that one-dimensional (1D) q-space imaging allows retrieval of structural information at cellular resolution. Here we demonstrate by simulation that boundary morphology of structured materials can be derived from 2D q-space mapping. Based on a finite-difference model for restricted diffusion, 2D q-space maps obtained from water diffusion inside apertures at various levels of asperity were simulated. The results indicate that the observed ring patterns (diffraction minima) reveal the boundary profiles of the apertures but become blurred in the case of significant variation in aperture size. For uniform size distribution of apertures, a quantitative measure of surface roughness can be established by means of spatial autocorrelation analysis. The results suggest that 2D q-space imaging may allow probing of the boundary morphology of structured materials and possibly biological cells.

Low affinity interaction of human or rat T cell adhesion molecule CD2 with its ligand aligns adhering membranes to achieve high physiological affinity

The mechanism by which low affinity adhesion molecules function to produce stable cell-cell adhesion is unknown. In solution, the interaction of human CD2 with its ligand CD58 is of low affinity (500 mM-1) and the interaction of rat CD2 with its ligand CD48 is of still lower affinity (40 mM-1). At the molecular level, however, the two systems are likely to be topologically identical. Fluorescently labeled glycosylphosphatidylinositol-anchored CD48 and CD58 were prepared and incorporated into supported phospholipid bilayers, in which the ligands were capable of free lateral diffusion. Quantitative fluorescence imaging was used to study the binding of cell surface human and rat CD2 molecules to the fluorescent ligands in contact areas between Jurkat cells and the bilayers. These studies provide two major conclusions. First, CD2/ligand interactions cooperate to align membranes with nanometer precision leading to a physiologically effective two-dimensional affinity. This process does not require the intact cytoplasmic tail of CD2. Second, the degree of membrane alignment that can be achieved by topologically similar receptors deteriorates with decreasing affinity. This suggests an affinity limit for the ability of this mode of cooperativity to achieve stable cell-cell adhesion at approximately 10 mM-1.

Use of cell contour analysis to evaluate the affinity between macrophages and glutaraldehyde-treated erythrocytes

Recently, several authors evaluated the affinity between lipid bilayers or erythrocyte membranes by analyzing the deformation of cells or vesicles they brought into close contact using micromanipulators. In the present report, we extend this approach in a study of the adhesive properties of rough nucleated cells. Rat peritoneal macrophages were made to bind human red cells modified with glutaraldehyde or glutaraldehyde and polylysine. Conjugates were examined with electron microscopy, and photomicrographs were digitized for quantification of cell surface roughness in and out of adhesion areas. Also, macrophages were subjected to micropipette aspiration to find a relationship between apparent surface tension and area increase. Assuming that this increase was a direct consequence of a smoothing of the cell surface on the submicrometer scale, the actual affinity between macrophages and erythrocytes was estimated. The obtained values ranged between 8.4 X 10(-5) and 18.2 X 10(-5) J/m2. It is concluded that cell surface roughness may be an important parameter of cell adhesion and perhaps deformation. This is made amenable to experimental study by the present approach.

Study of CTL and LAK contacts to target cells after treatment with mitomycin C and adriamycin

We tried to understand the role of Mitomycin C and Adriamycin in the increased killing of target cells by Cytotoxic T lymphocytes (CTL) and lymphokine activated killers (LAK). For this purpose, we used an objective method allowing quantitative evaluation of the roughness of cell contours on electron micrographs. We compared the folding of the membranes of LAK and CTL as well as conjugated targets exposed to different treatments. We demonstrated first that CTL and LAK displayed similar morphological patterns: the killer cells were more villous than targets in the free areas, and second that the former cells exhibited significant smoothing on the areas of contact with these targets. These results suggest that the binding process (as distinct from the recognition step) is dependent on killer properties which are the same in CTL, LAK and probably NK cells.