DEVELOPMENTAL REGULATION OF MEMBRANE TRAFFIC ORGANIZATION DURING SYNAPTOGENESIS IN MOUSE DIAPHRAGM MUSCLE

In innervated adult skeletal muscles, the Golgi apparatus (GA) display s a set of remarkable features in comparison with embryonic myotubes. We have previously shown by immunocytochemical techniques, that in adu lt innervated fibers, the GA is no longer associated with all the nucl ei, but appears to be concentrated mostly in the subneural domain unde r the nerve endings in chick (Jasmin, B. J., J. Cartaud, M. Bornens, a nd J.-P. Changeux. 1989. Proc. Natl. Acad. Sci. USA. 86:7218-7222) and rat (Jasmin, B. J., C. Antony, J.-P. Changeux, and J. Cartaud. 1995. fur. J. Neurosci. 7:470-479). In addition to such compartmentalization , biochemical modifications take place that suggest a functional speci alization of the subsynaptic GA. Here, we focused on the developmental regulation of the membrane traffic organization during the early step s of synaptogenesis in mouse diaphragm muscle. We investigated by immu nofluorescence microscopy on cryosections, the distribution of selecte d subcompartments of the exocytic pathway, and also of a representativ e endocytic subcompartment with respect to the junctional or extrajunc tional domains of developing myofibers. We show that throughout develo pment the RER, the intermediate compartment, and the prelysosomal comp artment (mannose 6-phosphate receptor-rich compartment) are homogeneou sly distributed along the fibers, irrespective of the subneural or ext ra-junctional domains. In contrast, at embryonic day E17, thus 2-3 d a fter the onset of innervation, most GA markers become restricted to th e subneural domain. Interestingly, some Golgi markers (e.g., alpha-man nosidase II, TGN 38, present in the embryonic myotubes) are no longer detected in the innervated fiber even in the subsynaptic GA. These dat a show that in innervated muscle fibers, the distal part of the biosyn thetic pathway, i.e., the GA, is remodeled selectively shortly after t he onset of innervation. As a consequence, in the innervated fiber, th e GA exists both as an evenly distributed organelle with basic functio ns, and as a highly differentiated subsynaptic organelle ensuring matu ration and targeting of synaptic proteins. Finally, in the adult, dene rvation of a hemidiaphragm causes a burst of reexpression of all Golgi markers in extrasynaptic domains of the fibers, hence showing that th e particular organization of the secretory pathway is placed under ner ve control.