We have used a modified, dual pipette assay to quantify the strength of cadherin-dependent cell–cell adhesion. The force required to separate E-cadherin–expressing paired cells in suspension was measured as an index of intercellular adhesion. Separation force depended on the homophilic interaction of functional cadherins at the cell surface, increasing with the duration of contact and with cadherin levels. Severing the link between cadherin and the actin cytoskeleton or disrupting actin polymerization did not affect initiation of cadherin-mediated adhesion, but prevented it from developing and becoming stronger over time. Rac and Cdc42, the Rho-like small GTPases, were activated when E-cadherin–expressing cells formed aggregates in suspension. Overproduction of the dominant negative form of Rac or Cdc42 permitted initial E-cadherin–based adhesion but affected its later development; the dominant active forms prevented cell adhesion outright. Our findings highlight the crucial roles played by Rac, Cdc42, and actin cytoskeleton dynamics in the development and regulation of strong cell adhesion, defined in terms of mechanical forces.

Membrane fusion occurs when SNAREpins fold up between lipid bilayers. How much energy is generated during SNAREpin folding and how this energy is coupled to the fusion of apposing membranes is unknown. We have used a surface forces apparatus to determine the energetics and dynamics of SNAREpin formation and characterize the different intermediate structures sampled by cognate SNAREs in the course of their assembly. The interaction energy-versus-distance profiles of assembling SNAREpins reveal that SNARE motifs begin to interact when the membranes are 8 nm apart. Even after very close approach of the bilayers (approximately 2-4 nm), the SNAREpins remain partly unstructured in their membrane-proximal region. The energy stabilizing a single SNAREpin in this configuration (35 k(B)T) corresponds closely with the energy needed to fuse outer but not inner leaflets (hemifusion) of pure lipid bilayers (40-50 k(B)T).

Oil-in-water emulsions are currently being investigated to facilitate the transport of viscous heavy oils. The behavior of these emulsions is largely controlled by the interfaces between oil drops and water. The surface-active components of crude oil, such as asphaltenes and naphthenic acids, compete among themselves at these interfaces and also with possibly added synthetic surfactant emulsifier. Here, we present a study of dynamic interfacial tension of interfaces between water and a model oil (toluene) in which variable amounts of asphaltenes are solubilized. We show that pH has a strong influence on interfacial properties of asphaltenes at the oil/water interface. At high or low pH, asphaltenes functional groups become charged, enhancing its surface activity. The influence of lower-molecular-weight surface-active species, such as the natural naphthenic acids contained in maltenes (crude oil without asphaltenes), has been investigated, and an interaction between asphaltenes and maltenes that facilitates molecular arrangement at the interface was detected. Several micropipette experiments, in which micrometric drops have been manipulated, are also described and indicate that very little coalescence of water droplets is observed at high or low pH. The microscopic properties of the interface and the macroscopic behavior of the emulsion are determined to be correlated.

Johnson-Kendall-Roberts (JKR) theory is an accurate model for strong adhesion energies of soft slightly deformable material. Little is known about the validity of this theory on complex systems such as living cells. We have addressed this problem using a depletion controlled cell adhesion and measured the force necessary to separate the cells with a micropipette technique. We show that the cytoskeleton can provide the cells with a 3D structure that is sufficiently elastic and has a sufficiently low deformability for JKR theory to be valid. When the cytoskeleton is disrupted, JKR theory is no longer applicable.

2014 On peut obtenir des renseignements sur la structure d'un film de cristal liquide qui sépare deux surfaces solides en mesurant les variations, en fonction de leur séparation, de la force qui s'exerce entre elles. Ici, ces mesures sont faites en utilisant des surfaces de mica moléculairement planes séparées par un cristal liquide, le 4'-n-pentyl 4-cyanobiphényle (5CB). L'orientation du 5CB est soit planaire soit homéotrope. La force est déterminée par la mesure de la flexion d'un ressort qui supporte une des feuilles de mica, et une technique optique est simultanément utilisée pour mesurer l'épaisseur du film avec une précision de ± (0,1-0,2) nm. Cette technique permet également de mesurer les indices de réfraction du film de cristal liquide et donc de déterminer la densité et le paramètre d'ordre moyens en fonction de son épaisseur. On met en évidence trois types de forces, chacun reflétant un mode de structuration du cristal liquide près des surfaces de mica. Le premier provient d'une déformation élastique dans le cristal liquide ; il est uniquement observé dans les films planaires twistés où les molécules de 5CB sont orientées dans des directions différentes sur les deux surfaces de mica. Le second, mesuré à la fois dans les structures planaires et homéotropes, est attribué à une augmentation du paramètre d'ordre près des surfaces. Ces deux forces sont répulsives et monotones, mesurables en dessous de 80 nm. Enfin, il y a une force de courte portée (jusqu'à six couches moléculaires) qui oscille entre l'attraction et la répulsion en fonction de l'épaisseur. Ceci provient de la structuration des molécules en couches près de la surface solide. On observe ce phénomène dans les structures planaires, homéotropes, et également dans des liquides isotropes. Abstract. 2014 Measurements of the force as a function of distance between two solids separated by a liquid crystal film give information on the structure of the film. We report such measurements for two molecularly smooth surfaces of mica separated by the nematic liquid crystal 4'-n-pentyl 4-cyanobiphenyl (5CB) in both the planar and

Poly-12-hydroxystearic acid ͑PHSA͒ is widely used as a coating on colloidal spheres to provide a ''hardsphere-type'' interaction. These hard spheres have been widely used in fundamental studies of nucleation, crystallization, and glass formation. Most authors describe the interaction as ''nearly'' hard sphere. In this paper we directly measure this interaction, using layers of PHSA adsorbed onto mica sheets in a surfaces force apparatus. We find that the layers, in appropriate solvents, have no long-range interaction. When the solvent is decahydronaphthalene ͑decalin͒, the repulsion rises from zero to the maximum measurable over a distance range of 15-20 nm. The data is converted to equivalent forces between spheres of different diameters, and modeled using a hard core potential. Using zeroth-order perturbation theory and computer simulation, we demonstrate that the equation of state does not deviate from that of a perfect hard-sphere system under any relevant experimental conditions.

Using a dual pipette assay that measures the force required to separate adherent cell doublets, we have quantitatively compared intercellular adhesiveness mediated by Type I (E-or N-cadherin) or Type II (cadherin-7 or -11) cadherins. At similar cadherin expression levels, cells expressing Type I cadherins adhered much more rapidly and strongly than cells expressing Type II cadherins. Using chimeric cadherins, we found that the extracellular domain exerts by far the dominant effect on cell adhesivity, that of E-cadherin conferring high adhesivity, and that of cadherin-7 conferring low adhesivity. Type I cadherins were incorporated to a greater extent into detergent-insoluble cytoskeletal complexes, and their cytoplasmic tails were much more effective in disrupting strong adherent junctions, suggesting that Type II cadherins form less stable complexes with ␤-catenin. The present study demonstrates compellingly, for the first time, that cadherins are dramatically different in their ability to promote intercellular adhesiveness, a finding that has profound implications for the regulation of tissue morphogenesis.Adhesive interactions, so essential to multicellular life, are mediated by a diversity of cell surface receptors. Prominent among them are the cadherins, calcium-dependent adhesion molecules central to tissue development and morphogenesis (1-3). The growing superfamily of cadherins is subdivided into five families: classical Type I cadherins, atypical Type II cadherins, desmosomal cadherins, protocadherins, and seven-pass transmembrane cadherins (4, 5). Classical Type I and desmosomal cadherins are found primarily in tissues where a high degree of cell cohesion is required for tissue integrity. Other types of cadherins are expressed in situations where cells are more motile, and intercellular interactions are more transitory (6 -9). It is increasingly clear that cadherins contribute to other cellular functions, including cell signaling, proliferation, differentiation, segregation, and migration (10 -17).Predictably, the pattern of cadherin expression during development is complex. For example, development of the neural crest involves epithelial to mesenchymal transitions, cell migration, cell aggregation, and cell differentiation (18), each of which is associated with tightly regulated, differential expression of Type I and II cadherins. Premigratory cells of the avian neural crest express first N-cadherin, and then they down-regulate N-cadherin and express Type II cadherin-6B, but later down-regulate it and induce expression of the Type II cadherin-7 as they migrate throughout the embryo (19,20). Another Type II cadherin, cadherin-11, is similarly induced in migrating neural crest cells of rat and Xenopus embryos (21-23). Cell grafting experiments in vivo verify that expression of cadherin-7 correlates with cell dispersion and migration along migratory pathways, whereas that of N-cadherin fosters strong intercellular cohesivity and failure to migrate (6).Regulation of cellular adhesion can be achieved in a v...

The core mechanism of intracellular vesicle fusion consists of SNAREpin zippering between vesicular and target membranes. Recent studies indicate that the same SNARE-binding protein, Complexin (CPX), can act either as a facilitator or as an inhibitor of membrane fusion, giving rise to a major controversy. Here, we employ energetic measurements using the Surface Forces Apparatus which reveal that CPX acts sequentially on assembling SNAREpins, first facilitating zippering by nearly doubling the distance at which v- and t-SNAREs can engage, and then by clamping them into a half-zippered fusion-incompetent state. Specifically, we find that the central helix of CPX allows SNAREs to form this intermediate energetic state at 9–15 nm, but not when the bilayers are closer than 9 nm. Stabilizing the activated-clamped state at separations < 9 nm requires the accessory helix of CPX, which prevents membrane-proximal assembly of SNAREpins.