Reorganization and stabilization of acetylcholine receptor aggregates on rat myotubes.

Aggregation of acetylcholine receptors (AChRs) is an important early feature of the postsynaptic development of the vertebrae neuromuscular junction. At later stages of differentiation, aggregates are remodeled and stabilized. Aggregation of AChRs can be induced on rat myotubes in culture within 4 hr by treatment with embryonic pig brain extract (EBX). In this study, further sequential changes in the distribution of AChRs were followed by video-intensified fluorescence microscopy. These studies have revealed that groups of AChR aggregates that have formed after 4 hr in EBX are reorganized during the exposure to EBX for 20 additional hr to form a smaller number of larger, oval-shaped aggregates. We have named these two types of aggregates "4-hr aggregates" and "24-hr aggregates". This reorganization occurs by the expansion and merging of individual aggregates within a group, and by the incorporation of newly inserted AChRs. The 24-hr aggregates are an average of 15 times greater in area than 4-hr aggregates, and contain regions with an apparent AChR site density (fluorescence intensity) that is more than twice that of 4-hr aggregates. Electron microscopy of mapped 24-hr aggregates revealed that folded plasma membrane is associated with these regions, probably accounting for the elevated fluorescence. The 24-hr aggregates are more stable than 4-hr aggregates, as determined by their significantly slower disassembly after removal of EBX, elevation of temperature (38 degrees C), reduction of extracellular calcium levels (0.1 mM), or the addition of sodium azide (7 mM). This was determined by following disassembly both statistically (using fixed cultures) and by direct observations of living myotubes. These findings were confirmed by measuring the sequential changes in relative AChR site density over time in individual living myotubes. Thus, 24-hr aggregates form by the reorganization of 4-hr aggregates; exhibit a more regular, compact shape; and are more stable than 4-hr aggregates. These changes in AChR organization and aggregate stability resemble the changes occurring after the initial formation of junctional AChR aggregates during embryonic development, demonstrating additional similarities between this model system and the developing neuromuscular junction.