Skeletal muscle expression of clathrin and mannose 6-phosphate receptor in experimental chloroquine-induced myopathy

Previous studies suggest that the muscle fiber lysosome system plays a central role in the increased formation of autophagosomes and autolysosomes that occurs in the context of chloroquine-induced myopathy. The goal of this study was to characterize the contribution of receptor-mediated intracellular transport, particularly the endosomal pathway, to the abnormal accumulation of vacuoles in experimental chloroquine myopathy. Expression of the mannose 6-phosphate receptor (M6PR) and clathrin were analyzed in innervated and denervated rat soleus muscles after treatment with either saline or chloroquine. Accumulation of vacuoles was observed only in chloroquine-treated denervated muscles. Further, clathrin immunostaining and M6PR messenger ribonucleic acid (mRNA) were significantly increased in denervated soleus muscle from saline- and chloroquine-treated rats compared to contralateral, innervated muscles. However, there was no difference in clathrin levels when comparing saline- and chloroquine-treated denervated muscles. These data suggest that chloroquine activates the transport of newly synthesized lysosomal enzymes from the secretory pathway via the trans-Golgi network of the Golgi apparatus (an endosomal pathway) as well as autophagosome formation (an autophagic process) in skeletal muscles. Vacuoles may subsequently accumulate secondary to abnormal formation or turnover of autolysosomes at or after fusion of autophagosomes with early endosomes.