Polypeptide components and binding functions of neuron-glia cell adhesion molecules.

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Abstract
Neuron-glia cell adhesion molecule (Ng-CAM) was previously shown to be present exclusively on neurons and to mediate adhesion between neuronal membranes and glial cells. Its chain structure, binding functions and relation to N-CAM (the other known CAM on neurons) were investigated further. Three polypeptide components of chicken Ng-CAM (MW 200,000, 135,000 and 80,000) were isolated. By using specific antisera against each component, the MW 135,000 and MW 80,000 components were found to cross-react antigenically with the MW 200,000 component but not with each other. The conclusion that the MW 135,000 and 80,000 components are structurally related to different regions of the MW 200,000 component was further supported by the finding that 32P could be incorporated in vitro into the MW 200,000 and 80,000 components but not into the MW 135,000 component. Ng-CAM appears to be involved in both neuron-glia adhesion and neuron-neuron adhesion by distinguishable mechanisms that appear to involve different sites or conformations of the molecule. Polyclonal antibodies and a monoclonal antibody against Ng-CAM both inhibited adhesion between glia and neurons derived from brain, cerebellum and retina. Antibodies against N-CAM (which inhibit neuron-neuron adhesion) did not inhibit neuron-glia adhesion. These findings confirm the proposed function of Ng-CAM in neuron-glia adhesion. Ng-CAM was found to be involved directly or indirectly in neuron-neuron adhesion. Non-cross-reactive polyclonal anti-Ng-CAM and anti-N-CAM antibodies each inhibited the aggregation of neurons from whole brain and cerebellum and the inhibition was greater when both antibodies were present together. Monoclonal anti-Ng-CAM antibodies were found that inhibited neuron-glia adhesion but did not inhibit neuronal cell aggregation. The amount of Ng-CAM expressed on neurons was not directly predictive of the effect of anti-Ng-CAM antibodies on their homotypic aggregation. Although Ng-CAM and N-CAM can be expressed simultaneously on individual neurons, the ratio of N-CAM to Ng-CAM ranged from 1.5 for cerebellar cells to 10.0 for retinal cells. While, as expected, retinal cell aggregation was inhibitable only by anti-N-CAM, cerebellar cells, which expressed at least as much Ng-CAM as brain cells, showed significantly less inhibition by anti-Ng-CAM antibodies. These findings raise the possibility that Ng-CAM may actually interact with N-CAM to yield non-linear effects. That Ng-CAM and N-CAM may function differently in vivo was suggested by their distribution in sections of brain regions. Within the cerebellum, for example, immunofluorescent anti-N-CAM staining was relatively uniform in all layers; anti-Ng-CAM staining was absent on dividing external granule cells and was present in greatest abundance on processes of post-mitotic migratory cells in the molecular layer. These observations are consistent with the hypothesis that Ng-CAM mediates neuron-glia adhesion and is thereby also involved in neuronal migration along radial glial cells.