Impaired autophagy in sporadic inclusion-body myositis and in endoplasmic reticulum stress-provoked cultured human muscle fibers.
The hallmark pathologies of sporadic inclusion-body myositis (s-IBM) muscle fibers are autophagic vacuoles and accumulation of ubiquitin-positive multiprotein aggregates that contain amyloid-beta or phosphorylated tau in a beta-pleated sheet amyloid configuration. Endoplasmic reticulum stress (ERS) and 26S proteasome inhibition, also associated with s-IBM, putatively aggrandize the accumulation of misfolded proteins. However, autophagosomal-lysosomal pathway formation and function, indicated by autophagosome maturation, have not been previously analyzed in this system. Here we studied the autophagosomal-lysosomal pathway using 14 s-IBM and 30 disease control and normal control muscle biopsy samples and our cultured human muscle fibers in a microenvironment modified to resemble aspects of s-IBM pathology. We report for the first time that in s-IBM, lysosomal enzyme activities of cathepsin D and B were decreased 60% (P < 0.01) and 40% (P < 0.05), respectively. We also detected two indicators of increased autophagosome maturation, the presence of LC3-II and decreased mammalian target of rapamycin-mediated phosphorylation of p70S6 kinase. Moreover, in cultured human muscle fibers, ERS induction significantly decreased activities of cathepsins D and B, increased levels of LC3-II, decreased phosphorylation of p70S6 kinase, and decreased expression of VMA21, a chaperone for assembly of lysosomal V-ATPase. We conclude that in s-IBM muscle, decreased lysosomal proteolytic activity might enhance accumulation of misfolded proteins, despite increased maturation of autophagosomes, and that ERS is a possible cause of s-IBM-impaired lysosomal function. Thus, unblocking protein degradation in s-IBM muscle fibers may be a desirable therapeutic strategy.
Nogalska
Anna
A
USC Neuromuscular Center, Department of Neurology, University of Southern California Keck School of Medicine, Good Samaritan Hospital, Los Angeles, CA 90017-1912, USA.
D'Agostino
Carla
C
Terracciano
Chiara
C
Engel
W King
WK
Askanas
Valerie
V
D000368:Aged; D001343:Autophagy / Q000502:physiology*; D015153:Blotting, Western; D002401:Cathepsin B / Q000378:metabolism; D002402:Cathepsin D / Q000378:metabolism; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum / Q000378:metabolism* / Q000473:pathology; D006801:Humans; D007150:Immunohistochemistry; D008247:Lysosomes / Q000378:metabolism / Q000473:pathology; D008875:Middle Aged; D018485:Muscle Fibers, Skeletal / Q000378:metabolism*; D018979:Myositis, Inclusion Body / Q000378:metabolism* / Q000473:pathology
D002401:Cathepsin B; D002402:Cathepsin D