Myoblast fusion and innervation with rat motor nerve alter distribution of acetylcholinesterase and its mRNA in cultures of human muscle

To elucidate the mechanisms underlying acetylcholinesterase (AChE) localization, we analyzed the distribution of AChE and Ache mRNA during myogenesis in cocultures of human muscle and fetal rat spinal cord. We observed a temporal coincidence in alterations of AChE localization and nuclei expressing the message, suggesting developmental regulation at the mRNA level. Nonuniform mRNA staining among nuclei suggests asynchronous regulation, also supporting an earlier proposal that transcription proceeds intermittently. Asynchrony seems to be overridden by generally acting factors during myoblast fusion, when message is up-regulated, and at the onset of muscle contractions, when it becomes restricted to some nuclei in the junctional region and focal patches of AChE appear near nerve contacts. Coincidence of mRNA down-regulation and synthesis of stable basal lamina-bound AChE suggests coordinated adaptation, so that sufficient enzyme may be derived from low message levels.