INTERACTIONS OF THE CHONDROITIN SULFATE PROTEOGLYCAN PHOSPHACAN, THE EXTRACELLULAR DOMAIN OF A RECEPTOR-TYPE PROTEIN-TYROSINE-PHOSPHATASE, WITH NEURONS, GLIA, AND NEURAL CELL-ADHESION MOLECULES

Phosphacan is a chondroitin sulfate proteoglycan produced by glial cel ls in the central nervous system, and represents the extracellular dom ain of a receptor-type protein tyrosine phosphatase (RPTP zeta/beta). We previously demonstrated that soluble phosphacan inhibited the aggre gation of microbeads coated with N-CAM or Ng-CAM, and have now found t hat soluble I-125-phosphacan bound reversibly to these neural cell adh esion molecules, but not to a number of other cell surface and extrace llular matrix proteins. The binding was saturable, and Scatchard plots indicated a single high affinity binding site with a K-d of similar t o 0.1 nM. Binding was reduced by similar to 15% after chondroitinase t reatment, and free chondroitin sulfate was only moderately inhibitory, indicating that the phosphacan core glycoprotein accounts for most of the binding activity. Immunocytochemical studies of embryonic rat spi nal cord and early postnatal cerebellum demonstrated that phosphacan, Ng-CAM, and N-CAM have overlapping distributions. When dissociated neu rons were incubated on dishes coated with combinations of phosphacan a nd Ng-CAM, neuronal adhesion and neurite growth were inhibited. I-125- phosphacan bound to neurons, and the binding was inhibited by antibodi es against Ng-CAM and N-CAM, suggesting that these CAMs are major rece ptors for phosphacan on neurons. C6 glioma cells, which express phosph acan, adhered to dishes coated with Ng-CAM, and low concentrations of phosphacan inhibited adhesion to Ng-CAM but not to laminin and fibrone ctin. Our studies suggest that by binding to neural cell adhesion mole cules, and possibly also by competing for ligands of the transmembrane phosphatase, phosphacan may play a major role in modulating neuronal and glial adhesion, neurite growth, and signal transduction during the development of the central nervous system.