Abstract
Adhesive contacts made by filopodia of neuronal growth cones are essential for proper neurite elongation and may have a role in the formation of synaptic junctions. Previously we described the appearance of filamentous materials extending from growth cone surfaces that seem to be associated with the strongly adhesive behavior of filopodia (Tsui, H.-C., K. L. Lankford, and W. L. Klein. 1985. Proc. Natl. Acad. Sci. USA. 82:8256-8260). Here, we have used immunogold labeling to determine whether known adhesive molecules might be localized at points of adhesion and possibly be constituents of the filamentous material. Antibodies to an adhesive molecule (neural cell adhesion molecule [N-CAM]) and to an adhesive macromolecular complex of proteins and proteoglycans (adheron) were localized at the EM level in whole mounts of cultured avian retina cells. Labeling of fixed cells showed that N-CAM and adheron molecules were both present on growth cones and on filopodia. However, filamentous materials extending from the cell surface were labeled with anti-adheron but not with anti-N-CAM. If cells were labeled before fixation, patches of anti-N-CAM labeling occurred in random areas over the growth cones, but adheron antibodies concentrated at points of apparent adhesion. Particularly dense clustering of anti-adheron occurred at individual filopodial tips and at points of contact between pairs of filopodia. The different patterns of labeling imply that N-CAMS do not associate with the main antigenic components of adheron on the membrane surface. Most importantly, the data indicate the N-CAMs were mobile in the membrane but that constituents of adherons were anchored at adhesive loci. An appealing hypothesis is that molecules found in adheron preparations have an important role in establishing the adhesive junctions formed by growth cone filopodia.
Highlights
M ECHANISMS which underly axonal guidance and synaptic target recognition are critical to the generation of proper neuronal connections
The effects of complexes from non-neuronal cells are mediated by components of the extracellular matrix, including laminin, fibronectin, and heparan sulphate proteoglycan, that bind to specific receptors on the neuronal cell surfaces (Tomaselli et al, 1986; Bozyczko and Horwitz, 1986; Schubert et al, 1983)
The current data show that the filamentous extensions between filopodia are labeled with anti-adheron but not anti-neural cell adhesion molecule (N-CAM), and indicate that adhesive interactions of growth cones and filopodia are mediated by adheron constituents
Summary
M ECHANISMS which underly axonal guidance and synaptic target recognition are critical to the generation of proper neuronal connections. The role of specific adhesive interactions between growth cones and their environment have been of particular interest (Letourneau, 1975; Silver and Rutishauser, 1984; Bentley and Caudy, 1983; Berlot and Goodman, 1984) These studies of adhesion have focused on membrane-associated proteins and extracellular matrix molecules (Tomaselli et al, 1986; Rogers et al, 1986; Tosney and Landmesser, 1985). For cultured chick retina neurons, such complexes can be isolated by high speed centrifugation of conditioned media (Schubert et al, 1983) These complexes, termed adherons, contain many proteoglycans and proteins, including a 20-kD retinol-binding protein that promotes cell-adheron adhesion (Schubert et al, 1986). The current data show that the filamentous extensions between filopodia are labeled with anti-adheron but not anti-N-CAM, and indicate that adhesive interactions of growth cones and filopodia are mediated by adheron constituents
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