Glial cells express N-CAM/D2-CAM-like polypeptides in vitro

Abstract

The joining together of neurites to form fascicles and the growth of axons along glial surfaces^1,2 during early development suggest that neurone–neurone and neurone–glial adhesion interactions are of considerable importance for defining nerve tracts. In vitro studies have indicated that adhesion between neurones^3–5 involves a glycoprotein that has been independently studied under the names of N-CAM (for neural cell adhesion molecule^6–8), D2-CAM⁹ and BSP-2 (refs 10, 11). As N-CAM/D2-CAM appears to be a homophilic ligand¹² that binds to N-CAM/D2-CAM polypeptide on adjacent cells, this glycoprotein is potentially important in adhesion interactions between any two N-CAM/D2-CAM-expressing cells. While it has been suggested that neurone–glial adhesion involves molecules other than N-CAM/D2-CAM^13–15, it is known that N-CAM/D2-CAM antigenic determinants are expressed by glial cells in vivo^16,17 and that injection of anti-N-CAM antibodies into the eye-cup of chick embryos disrupts normal patterns of neuritic apposition to glial endfeet in the developing optic stalk¹⁷. Do the molecules expressed by glia share restricted antigenic determinants, or binding domains, with N-CAM/D2-CAM, or are N-CAM/D2-CAM polypeptides expressed by glia? Here we present immunocytochemical evidence which suggests that all classes of macroglia express N-CAM/D2-CAM antigenic determinants on their surfaces and immunochemical analyses which indicate that the molecules expressed by purified astrocytes are closely similar, or identical, to at least some forms of N-CAM/D2-CAM obtained from whole brain or purified neurones. However, our results also suggest that different N-CAM/D2-CAM polypeptides may be separately expressed by neurones and astrocytes.