Cross-linking of cell surface receptors enhances cooperativity of molecular adhesion

Cooperativity of molecular adhesion has been proposed as a mechanism for en hanced binding strength of adhesion molecules on the cell surface. Direct e vidence for its mechanism, however, has been lacking until now, Atomic forc e microscopy (AFM) was used to measure the adhesive strength between concan avalin A (Con A) coupled to an AFM tip and Con A receptors on the surface o f NIH3T3 fibroblast cells. Cross-linking of receptors with either glutarald ehyde or 3,3'-dithio-bis(sulfosuccinimidylproprionate) (DTSSP) led to an in crease in adhesion that could be attributed to enhanced cooperativity among adhesion complexes. An increase in loading rate due to greater stiffness o f fixed cells also contributed to the twofold increase in binding strength. These results show that receptor cross-linking can greatly contribute to a total increase in cell adhesion by creating a shift toward cooperative bin ding of receptors.