Nerve-dependent plasticity of the Golgi complex in skeletal muscle fibres: compartmentalization within the subneural sarcoplasm.

Several recent reports have highlighted the plasticity of the Golgi apparatus during myogenesis, yet the organization of this specialized organelle in innervated skeletal muscle fibres remains poorly understood. Using four bona fide anti-Golgi antibodies, directed against a 210 kDa protein, a 160 kDa sialoglycoprotein, the small GTP-binding protein rab6p, and TGN38, the localization of which covers the various compartments of the Golgi complex, we show by immunofluorescence microscopy that the Golgi complex undergoes considerable reorganization in the course of myogenic differentiation and motor endplate formation in the rat. Unlike the typical perinuclear distribution of the Golgi stacks associated with every nucleus in myotubes, a striking subneural compartmentalization is observed in adult innervated myofibres. In short-term denervated adult muscle fibres, we noticed the presence of the perinuclear Golgi apparatus in extrajunctional regions, a pattern reminiscent of that of developing myotubes. At variance with anti-Golgi antibodies, antibodies to the rough endoplasmic reticulum label structures dispersed throughout the entire sarcoplasm, hence suggesting that it is not the entire membrane/secretory protein synthesis machinery which is compartmentalized, but only the Golgi apparatus. Also, an unexpected lack of immunoreactivity with the TGN38 and alpha-mannosidase II antibodies points to biochemical differentiation of the subneural Golgi apparatus at the adult motor endplate. These new data extend our previous observations on the compartmentalization of the Golgi apparatus in the postsynaptic sarcoplasm of chick muscle fibres, and further illustrate the plasticity of the Golgi apparatus in muscle cells. The specialization of the Golgi apparatus within the subneural compartment provides this particular region with a compartmentalized secretory pathway, and these observations highlight the notion that the level of differentiation of this domain is not only maintained via transcriptional regulation but also by post-translational control mechanisms.