Surface-Bound and Released Neuronal Glycoconjugates

Abstract

The establishment of synaptic arrays during vertebrate neural development is the result of a series of cellular interactions between neurons and their targets. These interactions occur throughout the stages of cell migration, axonal outgrowth, recognition of target cells, differentiation of pre- and postsynaptic elements, competition between and reorganization of connections, and stabilization of the final connections in maturity (cf. Patterson and Purves, 1982). A number of these stages, particularly the later ones, are controlled primarily by synaptic transmission. For example, distribution of acetylcholine receptors (AChR) in adult skeletal muscle is determined largely by the activity induced in the muscle by synaptic transmission (Lømo and Westgaard, 1976). On the other hand, a number of interactions, particularly during the early phases of synapse formation, do not appear to be mediated by the transmitter. The initial organization of muscle AChR under the nerve, for example, can occur in the absence of ACh binding to its receptor (Anderson et al., 1977). Cellular interactions in this case must involve other secreted or cell-surface molecules. The identification, purification, and characterization of these molecules are essential for at least two reasons: (1) purification of such molecules and production of antibodies against them will demonstrate the physical reality of the often formal concept of “trophic” interactions, and (2) manipulation of these molecules is essential for studies of the mechanisms that underlie such interactions.