Ultrastructural localization of cellular prion protein (PrPc) at the neuromuscular junction.

We examined the localization of the normal cellular isoform of prion protein (PrPc) in mammalian skeletal muscle. Using two anti-PrP antibodies, the neuromuscular junction (NMJ) was preferentially stained after immunohistofluorescence. The mouse, hamster, and human NMJ displayed a fluorescent signal specific for PrPc. Postembedding immunoelectron microscopy analysis performed in the mouse muscle showed that the PrPc-specific colloidal gold immunolabelling was concentrated over the sarcoplasmic cytoplasm. The membrane of the postsynaptic domain was devoid of gold particles, while a weak signal was occasionally observed close to the presynaptic vesicles of the terminal axons. These results indicate that the PrP gene is expressed in mammalian muscle at the NMJ. The subsynaptic sarcoplasm of the NMJ appears to be the privileged site where PrPc presumably associated with endosome membrane may play a role in either physiological activity or maintenance of the morphological integrity of the synapse.