β-amyloid is a substrate of autophagy in sporadic inclusion body myositis

Objective: Sporadic Inclusion Body Myositis (sIBM) is the most common acquired muscle disease in patients above 50 years of age. Apart from inflammation in the skeletal muscle, overexpression of amyloid precursor protein (APP) and intracellular accumulation of its proteolytic fragment fi-amyloid play a central role in the pathogenesis of sIBM. In neurodegenerative disorders, similar aggregations of aberrant proteins have recently been shown to be susceptible to autophagic degradation. Therefore, we analyzed macroautophagy of APP in human muscle cell lines and sIBM muscle biopsies. Methods: Colocalization of APP with the essential autophagy protein Atg8/LC3, which associates with preautophagosomal and autophagosomal membranes via lipidation, was analyzed in the CCL-136 muscle cell line and muscle biopsies by immunofluorescence. While APP was visualized with specific antibodies in the muscle cell line and in tissue sections. Atg8/LC3 localization was analyzed after GFP-Atg8/LC3 transfection or with an Atg8/LC3 specific antiserum, respectively. Results: We demonstrate here that Atg8/LC3 colocalizes with APP in cultured human muscle cells. In addition, APP/beta-amyloid-containing autophagosomes can be observed at increased frequency in muscle fibers of sIBM muscle biopsies, but not in non-myopathic muscle or non-vacuolated myopathic controls. APP/beta-amyloid and Ata8/LC3 double-positive compartments were almost exclusively observed in degenerating muscle fibers of the type II (fast-twitching) and were in part associated with overexpression of major histocompatibility complex (MHC) class I and II on myofibers and invasion by CD4(+) and CD8(+) cells. Interpretation: These findings indicate that APP/beta-amyloid is targeted for lysosomal degradation via macroautophagy and suggest that the autophagy pathway should be explored for its potential therapeutic merit in sIBM.