Fast track - Critical and optimal ig domains for promotion of neurite outgrowth by L1/Ng-CAM

Mammalian LI and avian Ng-CAM arc homologous neural cell adhesion molecules (CAMs) that promote neurite outgrowth and cell adhesion in most neurons. Previous attempts to map these activities to discrete regions in the CAMs have suggested the involvement of a variety of different domains. However, these studies mainly used bacterially expressed proteins that were much less active on a molar basis than the native molecules. To define regions that are critical for maximal neurite outgrowth, we constructed and tested a panel of eukaryotically expressed proteins containing various extracellular segments of human LI (hLl) or Ng-CAM. Our results indicate that Ig domains 1-4 of hLl are critical for homophilic binding and neurite outgrowth; however this segment is less potent than the entire extracellular region. Optimal neurite outgrowth activity was seen with proteins containing all six Ig domains of hLl or Ng-CAM. The adhesive properties of hLl fragments correlated tightly with their neurite outgrowth activities, suggesting that these two processes are closely linked. These results suggest that Ig domains 1-4 form a structural cassette responsible for hLl homophilic binding, while Ig domains 1-6 represent a functional region for optimal promotion of neurite out-growth in vitro and possibly in vivo.