Expression of lysosome-related proteins and genes in the skeletal muscles of inclusion body myositis
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Despite the unknown etiology and pathogenesis of sporadic inclusion body myositis (s-IBM), investigators have speculated that the lysosome system in muscle fiber plays a central role in rimmed vacuole formation, a hallmark of s-IBM. We explored the role of receptor-mediated intracellular transport and autophagy in the lysosomal system in the abnormal accumulation of rimmed vacuoles in s-IBM. Expressions of mannose 6-phosphate receptor (M6PR), clathrin and hApg5 and hApg12 were analyzed in muscle biopsy specimens from patients with s-IBM, amyotrophic lateral sclerosis (ALS) or peripheral neuropathy and in normal human muscle specimens by means of immunohistochemistry and reverse transcriptase-polymerase chain reaction (RT-PCR). Most muscle fibers in control specimens showed little or no immunoreactivity for clathrin and M6PR, which are involved in the receptor-mediated intracellular transport. Abnormal increases in both proteins were observed mainly in the sarcoplasm of atrophic fibers in all diseased specimens. In s-IBM muscles in particular, clathrin and M6PR were often observed inside rimmed vacuoles and in the sarcoplasm of vacuolated or non-vacuolated fibers. mRNA levels of hApg5 and hApg12, which are involved in autophagic vacuole formation, as well as of M6PR and clathrin were significantly increased in s-IBM muscles in comparison to levels in normal and ALS/peripheral neuropathy muscles. Our results suggest that the transport of newly synthesized lysosomal enzymes from the secretory pathway via the trans-Golgi network of the Golgi apparatus and autophagic vacuole formation (i.e., autophagy) in the lysosome system are activated in s-IBM muscles. Remarkable accumulation of rimmed vacuoles is thought to occur because of abnormal lysosome function, especially the formation or turnover of autolysosomes after the fusion of autophagic vacuoles with the early endosomes or because of the increase in the rate of muscle fiber breakdown.