Tight vesicles: Two glycosphingolipids, CerLLe^X (see picture) and CerLLe^a (Cer=ceramide, L=lactose, Le^X=Lewis^X determinant, Le^a=Lewis a determinant) are used to study the Ca²⁺-mediated specific adhesion of natural Le^X-bearing molecules inserted in fluid bilayer membranes. Vesicle adhesion energy experiments show that Le^X–Le^X recognition is highly sensitive to molecular structure.

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 When the cytoskeleton is disrupted, JKR theory is no longer applicable.

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.

The interactions of supramolecular systems often depend on small and complex molecules. It is tempting though dangerous to apply polymer theory to these molecules that are normally considered to be too small to be polymers and too large to be rigid. Here, forces and adhesions between surfaces bearing several types of such molecules with both flexible and rigid parts are measured. The force/distance profiles follow closely the description given by polymer theory. It is shown for a wide variety of systems containing these molecules that if one obtains an effective radius of gyration R_g of the molecules, polymer theory can be used to predict their adhesion energy. Conversely, if the adhesion energy for bilayers containing such small and complex molecules is measured, polymer theory allows to deduce the effective R_g of the molecule.