Selective interactions of cells with crystal surfaces. Implications for the mechanism of cell adhesion.

In this study we have characterized the mode of cell adhesion to calcite and calcium (R,R)-tartrate tetrahydrate crystals. The use of crystals as adhesion substrata was motivated by their well-established chemical nature and structurally defined surfaces. We show that calcite binds A6 Xenopus laevis epithelial cells rapidly and efficiently, most likely via surface-adsorbed proteins. Surface topology had only a limited effect on the adhesive interactions. Calcium (R,R)-tartrate tetrahydrate crystals exhibits two chemically equivalent, yet structurally distinct faces that differ mainly in the surface distribution of their lattice water molecules and charges. However, despite the gross similarity between the two faces striking differences were noted in their adhesive behavior. One of the faces was highly adhesive for cells, leading to protein-independent attachment and spreading followed by cell death. In contrast, cell adhesion to the other surface of tartrate was slow (> 24 h) and apparently mediated by RGD-containing protein(s). It was further shown that the latter face of tartrate crystals could be "conditioned" by long (24 h) incubation with serum-containing medium, after which it becomes highly adhesive. The results presented here indicate that crystal surfaces may serve as excellent, structurally defined, substrata for cell adhesion, that cell binding may occur directly or via RGD-containing proteins and that cell adhesion may be dramatically modulated by variations in surface structure. The implications of the results to the mechanism of cell-substratum adhesion are discussed.