The targeted, differential localization of specific mRNAs and proteins is important for the functional specialization of cells in the nervous system. In muscle, it may contribute to the maintenance of the neuromuscular junction, whereas in neurons, it may play a role into the establishment and maintenance of axonal-dendritic polarity. The goal of this project is to understand how mRNAs, proteins, and subcellular organelles are distributed and organized in muscle and nerve cells. In order to understand how translation and stability of an mRNA influences its localization, we have been following the distribution, in the mouse muscle cell line C2, and in primary cultures of rat hippocampal neurons, of two endogenous mRNAs whose stability and translation can be specifically controlled by iron: the ferritin and transferrin receptor (TfR) mRNAs. Earlier results suggested that the nonuniform distribution of TfR mRNA may reflect translational localization to the rough endoplasmic reticulum (RER). We have now shown, by simultaneous detection of TfR mRNA and of a RER protein, that TfR mRNA has a narrower distribution than the RER, whereas the mRNA for another transmembrane protein, GLUT4, has a wider distribution in the myotubes. These results suggest that mRNAs can be retained very near their source nucleus, even when they are associated with a subcellular organelle that forms a continuous network through the muscle and may help us to understand the mechanism of AChR mRNA retention near the synaptic nuclei at the neuromuscular junction. We are also continuing to investigate the mechanism of vesicle and protein traffic in muscle by studying the localization of the glucose transporter GLUT4 in vitro and in vivo. The observation of GLUT4 by immunofluorescence in different muscles of the rat has brought to light pattern differences that appear related to fiber types. We have now shown that these differences result from variations in the organization of the Golgi complexes, for which GLUT4 is a marker. This work has been reinforced by the identification of several antibodies to Golgi complex proteins that recognize their antigen in skeletal muscle. With these valuable new tools, we are now studying factors affecting the distribution of the Golgi complex in muscle and trying to clarify the differences between the Golgi complexes at the neuromuscular junction and those elsewhere in the muscle, a point of debate.
