Endoplasmic reticulum stress and unfolded protein response in inclusion body myositis muscle

Proteins in the endoplasmic reticulum (ER) require an efficient system of molecular chaperones whose role is to assure their proper folding and to prevent accumulation of unfolded proteins. The response of cells to accumulation of unfolded proteins in the ER is termed "unfolded protein response" (UPR). UPR is a functional mechanism by which cells attempt to protect themselves against ER stress, resulting from the accumulation of the unfolded/misfolded proteins. Because intracellular inclusions, containing either amyloid-beta (Abeta) or phosphorylated tau, are the characteristic feature of sporadic inclusion body myositis (s-IBM) muscle biopsies, we studied expression and immunolocalization of five ER chaperones, calnexin, calreticulin, GRP94, BiP/GRP78, and ERp72, in s-IBM and control muscle biopsies. Physical interaction of the ER chaperones with amyloid-beta precursor protein (AbetaPP) was studied by a combined immunoprecipitation/immunoblotting technique in s-IBM and control muscle biopsies, and in AbetaPP-overexpressing cultured human muscle fibers. In all s-IBM muscle biopsies, all five of the ER chaperones were immunodetected in the form of inclusions that co-localized with amyloid-beta. By immunoblotting, expression of ER chaperones was greatly increased as compared to the controls. By immunoprecipitation/immunoblotting experiments, ER chaperones co-immunoprecipitated with AbetaPP. Our studies provide evidence of the UPR in s-IBM muscle and demonstrate for the first time that the ER chaperones calnexin, calreticulin, GRP94, BiP/GRP78, and ERp72 physically associate with AbetaPP in s-IBM muscle, suggesting their playing a role in AbetaPP folding and processing.