NERVE-DEPENDENT PLASTICITY OF THE GOLGI-COMPLEX IN SKELETAL-MUSCLE FIBERS - COMPARTMENTALIZATION WITHIN THE SUBNEURAL SARCOPLASM

Several recent reports have highlighted the plasticity of the Golgi ap paratus during myogenesis, yet the organization of this specialized or ganelle in innervated skeletal muscle fibres remains poorly understood . Using four bona fide anti-Golgi antibodies, directed against a 210 k Da protein, a 160 kDa sialoglycoprotein, the small GTP-binding protein rab6p, and TGN38, the localization of which covers the various compar tments of the Golgi complex, we show by immunofluorescence microscopy that the Golgi complex undergoes considerable reorganization in the co urse of myogenic differentiation and motor endplate formation in the r at, Unlike the typical perinuclear distribution of the Golgi stacks as sociated with every nucleus in myotubes, a striking subneural compartm entalization is observed in adult innervated myofibres. In short-term denervated adult muscle fibres, we noticed the presence of the perinuc lear Golgi apparatus in extrajunctional regions, a pattern reminiscent of that of developing myotubes, At variance with anti-Golgi antibodie s, antibodies to the rough endoplasmic reticulum label structures disp ersed throughout the entire sarcoplasm, hence suggesting that it is no t the entire membrane/secretory protein synthesis machinery which is c ompartmentalized, but only the Golgi apparatus. Also, an unexpected la ck of immunoreactivity with the TGN38 and a-mannosidase II antibodies points to biochemical differentiation of the subneural Golgi apparatus at the adult motor endplate. These new data extend our previous obser vations on the compartmentalization of the Golgi apparatus in the post synaptic 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 the se 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.