Abnormalities of NBR1, a novel autophagy-associated protein, in muscle fibers of sporadic inclusion-body myositis

Abstract

Intra-muscle fiber accumulation of ubiquitinated protein aggregates containing several conformationally modified proteins, including amyloid-β and phosphorylated tau, is characteristic of the pathologic phenotype of sporadic inclusion-body myositis (s-IBM), the most common progressive degenerative myopathy of older persons. Abnormalities of protein-degradation, involving both the 26S proteasome and autophagic-lysosomal pathways, were previously demonstrated in s-IBM muscle. NBR1 is a ubiquitin-binding scaffold protein importantly participating in autophagic degradation of ubiquitinated proteins. Whereas abnormalities of p62, a ubiquitin-binding protein, were previously described in s-IBM, abnormalities of NBR1 have not been reported in s-IBM. We have now identified in s-IBM muscle biopsies that NBR1, by: (a) immunohistochemistry, was strongly accumulated within s-IBM muscle-fiber aggregates, where it closely co-localized with p62, ubiquitin, and phosphorylated tau; (b) immunoblots, was increased threefold (p < 0.001); and (c) immunoprecipitation, was associated with p62 and LC3. By real-time PCR, NBR1 mRNA was increased twofold (p < 0.01). None of the various disease- and normal-control muscle biopsies had any NBR1 abnormality. In cultured human muscle fibers, NBR1 also physically associated with both p62 and LC3, and experimental inhibition of either the 26S proteasome or the lysosomal activity resulted in NBR1 increase. Our demonstration of NBR1 abnormalities in s-IBM provides further evidence that altered protein degradation pathways may be critically involved in the s-IBM pathogenesis. Accordingly, attempts to unblock defective protein degradation might be a therapeutic strategy for s-IBM patients.