Formation of postsynaptic specializations induced by latex beads in cultured muscle cells

An early event in the formation of neuromuscular junctions is an accumulation of acetylcholine receptors (AChRs) in the postsynaptic membrane. This concentration of receptors is essential for the establishment of the synaptic transmission. Previously, we showed that latex beads coated with positively charged polypeptide molecules can induce the formation of AChR clusters at the bead-muscle contacts (Peng, H. B., P.-C. Cheng, and P. W. Luther (1981) Nature 292: 831-834). In this study, we demonstrate that, in addition to AChR clusters, a set of postsynaptic specializations normally present at the adult neuromuscular junction also develops at the bead-muscle contacts in the absence of nerve. These specializations include the membrane-associated cytoplasmic density and its associated thin (6- to 8-nm) filaments, clusters of intramembranous particles, membrane invaginations, the basal lamina, and 60- to 100-nm coated vesicles. Our results indicate that the mechanism for the formation of the postsynaptic specializations is preprogramed in the muscle and can be triggered by an interaction with polypeptide-coated latex beads. Thus, this bead-muscle system can be used as a simple model for understanding the postsynaptic development during synaptogenesis.