Reorganization of microtubule nucleation during muscle differentiation.

Skeletal muscle differentiation involves a complete reorganization of the microtubule network. Nearly 20 years ago, Tassin et al. [1985: J Cell Biol 100:35-46] suggested a mechanism for this reorganization by showing a redistribution of the microtubule organizing center from the centrosome to the nuclear membrane. Little progress has been made since. It is still not clear whether centrosomal proteins are redistributed together, whether microtubules are nucleated at the nuclear membrane or transported there post-nucleation, and whether gamma-tubulin (gammatub) remains necessary for nucleation in myotubes. To investigate these questions, we have examined the redistribution of the centrosomal proteins pericentrin (PC), gammatub, and ninein in the C2 muscle cell line. Immunofluorescence of differentiated myotubes shows PC along the nuclear membrane whereas gammatub is only detected there after pre-fixation detergent extraction. After expression of a GFP-tagged gammatub, we observe a weak fluorescence along the nuclear membrane, confirming the presence of gammatub at a low concentration relative to PC. Microinjection of anti-gammatub antibodies into myotubes blocks microtubule growth from both nuclear membranes and centrosomal sites. The centrosomal microtubule-anchoring protein, ninein, is found at the nuclear membrane as well and its distribution appears independent of microtubule integrity. We conclude that centrosomal proteins are redistributed independently during muscle differentiation, to sites that nucleate microtubules both along the nuclear membranes and through the cytoplasm.