Neurite outgrowth on muscle cell surfaces involves extracellular matrix receptors as well as Ca2+-dependent and -independent cell adhesion molecules.

To identify the molecules on the neuronal surface that mediate axonal growth on myotubes, we have examined neurite formation by ciliary neurons grown on myotubes in the absence or presence of specific antibodies. Dramatic inhibition of neurite outgrowth was seen only when antibodies blocked simultaneously the functions of two cell adhesion molecules--neural cell adhesion molecule (N-CAM) and neural Ca2+-dependent CAM (N-Cal-CAM)--and the neuronal receptors for several extracellular matrix (ECM) proteins. Although the antibody used to block ECM receptors (JG22) has been shown to eliminate almost all neurite growth on ECMs, it had only small effects on neurite growth on myotubes, reducing somewhat the length of neurites. Similarly, antibodies to the two CAMs, when used alone, had no detectable effects on neurite length and, when used together, had only small inhibitory effects on neurite growth. Combination of anti-ECM receptor (JG22) with antibodies to either CAM, however, greatly shortened the length of neurites. These results imply that ECM receptors and the CAMs N-CAM and N-Cal-CAM are major macromolecules used by neuronal growth cones for interactions with myotubes. Each provides a distinct mechanism for regulating growth cone motility.