Neuronal cell–cell adhesion depends on interactions of N-CAM with heparin-like molecules

Abstract

Cell–cell interactions are of critical importance during neural development, particularly since the migration of neural cells and the establishment of functional interactions between growing axons and their target cells^1,2 has been suggested to depend upon cell recognition processes. Neurone–neurone adhesion has been well studied in vitro, and is mediated in part by the neural cell adhesion molecule N-CAM^3–7. N-CAM-mediated cell–cell adhesion has been postulated to occur by a homophilic binding mechanism⁸, in which N-CAM on the surface of one cell binds to N-CAM on a neighbouring cell. Studies in our laboratory have identified a cell surface glycoprotein, now known to be N-CAM⁹, which participates in cell–substratum interactions in the developing chicken nervous system^10–12. Although this adhesion involves a homophilic binding mechanism, the binding of the cell surface proteoglycan heparan sulphate to the glycoprotein is also required¹³. This raises the question of whether the binding of heparan sulphate to N-CAM is also required for cell-cell adhesion. Here we show that the binding of retinal probe cells to retinal cell monolayers is inhibited by heparin, a functional analogue of heparan sulphate, but not by chondroitin sulphate. Monoclonal antibodies that recognize two different domains on N-CAM, the homophilic-binding and heparin-binding domains, inhibit cell–cell adhesion. The heparin-binding domain isolated from N-CAM by selective proteolysis also inhibits cell-cell adhesion when bound to the probe cells.